**6. Pyridine-based tetradentate platinum(II) complexes**

2-Phenylpyridine has been widely used as ligand for the iridium(III)- and platinum(II) based phosphorescent complexes, like Ir(ppy)3 , due to its high stability and easy preparation. However, owing to the low T1 state level, the emitting colors are usually from green to red. So far, various types of pyridine-based tetradentate platinum(II) complexes have been reported; importantly, most of them are highly efficient, and some complexes are so stable that they can achieve the early stage of commercial applications. The pyridine-based tetradentate platinum(II) complexes are illustrated in **Figure 7**, their photophysical properties are summarized in **Table 8**, and some of the device performances based on these complexes are showed in **Table 9**.

In 2010, Huo's group reported three pyridine-based platinum complexes (**43**–**45**) using phenylamine as linking group [24]. Complex **43** exhibits a dominant emission peak at 512 nm in diluted 2-MeTHF solution, and the excimer emission at about 740 nm was observed at elevated concentration, due to the planar molecular configuration. The HOMO level can be stabilized by introducing fluorine atoms into the phenyl rings; thus, complex **44** has a blueshift of 24 nm with a peaking emission at 488 nm. Because of the electron-donating character of the phenylamine, complex **45** has a shallower HOMO level of −4.56 eV compared to that of complex **43** of −5.27 eV; therefore, significant redshift of 100 nm was observed for complex **45**. All the three complexes show strongly luminescence with ϕ of 14–75% in solution, and device doped with **43** achieved a peak EQE of 14.7% with coordinates of (0.32, 0.62). Unfortunately, the device stability was not studied in the literature.

performance was compared with that of *fac*-Ir(ppy)3

peak EQE could be increased to 21.5% if using Bebq2

OLEDs with an estimated operational lifetime LT97 of 62 h at 1000 cd/m2

**Figure 7.** Molecular structures of pyridine-based tetradentate platinum(II) complexes.

ptb (**50**), and an estimated operational lifetime LT97 of 638 h at 1000 cd/m2

A great progress has been made for the development of stable and efficient platinum(II) based red OLEDs in the past several years. In 2014, Li′s group demonstrated a stable red

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes

of (0.60, 0.36) by employing PtON11Me (**49**) as emitter [48]. One year later, tenfold increase in the operational lifetime was achieved using more stable carbazole-linked emitter, PtN3N-

of 10.8%, which used CBP, TrisPCz, and BPyTP as host, electron-blocking, and transporting materials, respectively. Also, the turn on voltage was as low as about 2.0 eV. Moreover, the

adopting a bilayer emitting material layer (EML) with different dopant concentrations in the same device structure, PtN3N (**51**)-based orange OLED, could achieve a superior operational

the excision formation zone deep into the EML to suppress the potential fast degradation of

EQE of 22.3% could be achieved [23].

lifetime LT97 of 2057 h at 1000 cd/m2

in the same device structure, and peak

http://dx.doi.org/10.5772/intechopen.76346

91

as host material [49]. What's more is that

and peak EQE of 16.9%. This could be attributed to shift

with CIE coordinates

with peak EQE

In 2012, Fukagawa et al. developed two modified complexes TLEC-025 (**46**) and TLEC-027 (**47**) by incorporating δ-donating groups on the phenylamine to further destabilize the HOMO levels, resulting in redshift to about 620 nm, which were ideal emitters for red OLEDs [47]. TLEC-025-based device demonstrated an operational lifetime LT80 of 1290 h with a peak EQE of 18.5% and power efficiency (PE) of 20.7 lm/W at 100 cd/m2 and still remained 14.4% and 25.2 lm/W at 1000 cd/m2 . Encouragingly, device doped with TLEC-027 achieved further long operational lifetime LT80 of 3330 h with similar EQEs and even higher PEs of 25.5 and 30.3 lm/W at 100 and 1000 cd/m2 , respectively. Unfortunately, the molecular structures of the hole-injecting material ND-1501 and electron-transporting material ETM-143 were unknown. Anyway, this was the first time to demonstrate that the platinum(II)-based devices could be as efficient and stable as the iridium(II)-based ones, opening a door for the development of efficient and stable OLEDs by employing platinum(II) complexes.

Pyridine-based tetradentate platinum(II) complex PtOO3 (**48**) with luminescent quantum efficiency of up to 97% in thin film was developed by Li′s group in 2013. PtOO3-based device

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes http://dx.doi.org/10.5772/intechopen.76346 91

**Figure 7.** Molecular structures of pyridine-based tetradentate platinum(II) complexes.

a color rendering index (CRI) of up to 80 and peak EQE of 12.5% [46]. Due to the strong emis-

was twice as long as that of Pt2O2 in the same device setting, and this could be attributed to

2-Phenylpyridine has been widely used as ligand for the iridium(III)- and platinum(II)-

red. So far, various types of pyridine-based tetradentate platinum(II) complexes have been reported; importantly, most of them are highly efficient, and some complexes are so stable that they can achieve the early stage of commercial applications. The pyridine-based tetradentate platinum(II) complexes are illustrated in **Figure 7**, their photophysical properties are summarized in **Table 8**, and some of the device performances based on these complexes are

In 2010, Huo's group reported three pyridine-based platinum complexes (**43**–**45**) using phenylamine as linking group [24]. Complex **43** exhibits a dominant emission peak at 512 nm in diluted 2-MeTHF solution, and the excimer emission at about 740 nm was observed at elevated concentration, due to the planar molecular configuration. The HOMO level can be stabilized by introducing fluorine atoms into the phenyl rings; thus, complex **44** has a blueshift of 24 nm with a peaking emission at 488 nm. Because of the electron-donating character of the phenylamine, complex **45** has a shallower HOMO level of −4.56 eV compared to that of complex **43** of −5.27 eV; therefore, significant redshift of 100 nm was observed for complex **45**. All the three complexes show strongly luminescence with ϕ of 14–75% in solution, and device doped with **43** achieved a peak EQE of 14.7% with coordinates of (0.32, 0.62). Unfortunately,

In 2012, Fukagawa et al. developed two modified complexes TLEC-025 (**46**) and TLEC-027 (**47**) by incorporating δ-donating groups on the phenylamine to further destabilize the HOMO levels, resulting in redshift to about 620 nm, which were ideal emitters for red OLEDs [47]. TLEC-025-based device demonstrated an operational lifetime LT80 of 1290 h with a peak

long operational lifetime LT80 of 3330 h with similar EQEs and even higher PEs of 25.5 and

hole-injecting material ND-1501 and electron-transporting material ETM-143 were unknown. Anyway, this was the first time to demonstrate that the platinum(II)-based devices could be as efficient and stable as the iridium(II)-based ones, opening a door for the development of

Pyridine-based tetradentate platinum(II) complex PtOO3 (**48**) with luminescent quantum efficiency of up to 97% in thin film was developed by Li′s group in 2013. PtOO3-based device

. Encouragingly, device doped with TLEC-027 achieved further

, respectively. Unfortunately, the molecular structures of the

operational lifetime LT80 could achieve over 400 h at an initial luminance of 1000 cd/m2

90 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

, due to its high stability and easy prepara-

and still remained 14.4%

state level, the emitting colors are usually from green to

, which

sion of the excimer, 12% Pt1O2me2

tion. However, owing to the low T1

showed in **Table 9**.

the lack of high-energy blue emitters in the Pt1O2me2

based phosphorescent complexes, like Ir(ppy)3

the device stability was not studied in the literature.

and 25.2 lm/W at 1000 cd/m2

30.3 lm/W at 100 and 1000 cd/m2

EQE of 18.5% and power efficiency (PE) of 20.7 lm/W at 100 cd/m2

efficient and stable OLEDs by employing platinum(II) complexes.

**6. Pyridine-based tetradentate platinum(II) complexes**

performance was compared with that of *fac*-Ir(ppy)3 in the same device structure, and peak EQE of 22.3% could be achieved [23].

A great progress has been made for the development of stable and efficient platinum(II) based red OLEDs in the past several years. In 2014, Li′s group demonstrated a stable red OLEDs with an estimated operational lifetime LT97 of 62 h at 1000 cd/m2 with CIE coordinates of (0.60, 0.36) by employing PtON11Me (**49**) as emitter [48]. One year later, tenfold increase in the operational lifetime was achieved using more stable carbazole-linked emitter, PtN3Nptb (**50**), and an estimated operational lifetime LT97 of 638 h at 1000 cd/m2 with peak EQE of 10.8%, which used CBP, TrisPCz, and BPyTP as host, electron-blocking, and transporting materials, respectively. Also, the turn on voltage was as low as about 2.0 eV. Moreover, the peak EQE could be increased to 21.5% if using Bebq2 as host material [49]. What's more is that adopting a bilayer emitting material layer (EML) with different dopant concentrations in the same device structure, PtN3N (**51**)-based orange OLED, could achieve a superior operational lifetime LT97 of 2057 h at 1000 cd/m2 and peak EQE of 16.9%. This could be attributed to shift the excision formation zone deep into the EML to suppress the potential fast degradation of

b Doped in Bebq2

c Estimated from the emission spectrum.

.

**Table 8.** Photophysical properties of pyridine-based tetradentate platinum(II) complexes.

the device [50]. These device performances indicate that the platinum(II)-based complexes are more appealing as phosphorescent emitters in the display applications.

In fact, a synthetic challenge still remains for the gram-scale preparation of the PtNON and the PtON1 series complexes. Recently, our group developed an efficient approach for the CuCl-catalyzed C-N band cross coupling of carbazoles and 2-bromopyridine derivatives to synthesize 2-bromo-*N*-(hetero)arylcarbazoles. It was found that base t-BuOLi could accelerate the reaction significantly and just a few hours needed to complete the reaction [55]. However,

**Dopant λmax/nm CIE CRI ηEQE Device LT**

[24] 512 (0.32, 0.62) — 14.7 — —

 [23] 500 — — 22.3 17.6 — 6% **49**d [48] — (0.60, 0.36) — 4.7 4.6 LT97: 62

[47] — (0.662, 0.337) — 18.5 14.4 LT80: 1920

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes

[47] — (0.657, 0342) — 18.2 14.5 LT80: 3330

[49] — (0.63, 0.37) — 10.8 7.8 LT97: 638

[50] — (0.55, 0.45) — 16.9 15.3 LT97: 2057

[49] — (0.58, 0.42) — 21.5 13.5 LT97: 25

6% **52**h [33] — (0.17, 0.32) — 10.7 9.1 LT70: 624

[52] — (0.41, 0.44) 75 11.6 5.5 —

[52] — (0.41, 0.45) 74 17.0 12.4 —

[52] — (0.41, 0.45) 76 9.6 8.4 —

[52] — (0.29, 0.63) — 9.7 9.5 —

[53] 555 (0.44, 0.55) — 26.0 23.1 —

[53] — (0.31, 0.64) — 27.6 25.6 —

/ETM-143/LiF/Al.

/BAlq/BPyTP/LiF/Al.

Device structure: ITO/CFx/NBP/TCTA/dopant: TPBI:TCTA/TPBI/Alq/LiF/Mg:Ag.

Device structure: ITO/PEDOT:PSS/TAPC/dopant: 26mCPy/PO15/BmPyPB/LiF/Al. dDevice structure: ITO/HATCN/NPD/dopant:mCBP:BAlq/BAlq/Alq/LiF/Al.

Device structure: ITO/HATCN/NPD/TrisPCz/dopant: CBP/BAlq/BPyTP/LiF/Al.

hDevice structure: ITO/HATCN/NPD/dopant:mCBP/mCBT/BPyTP/LiF/Al.

Device structure: ITO/PEDOT:PSS/dopant:PVK:OXD-7/TmPyPb/TPBi/LiF/Al.

**Table 9.** Device performances of pyridine-based tetradentate platinum(II) complexes.

Device structure: ITO/HATCN/NPD/TrisPCz/20 wt%**51**: CBP/6 wt%**51**:CBP/BAlq/BPyTP/LiF/Al.

/TAPC/dopant:TCTA/TmPyPB/LiF/Al.

Device structure: ITO/ND-1501/α-NPD/dopant:Bebq2

Device structure: ITO/HATCN/NPD/TrisPCz/dopant:Bebq2

29]. Moreover, a directly hydroxylation of the 2-bromo-*N*-(hetero)arylcarbazoles catalyzed by CuCl was also developed [56]. Both of the approaches are suitable for large-scale synthesis and have been successfully applied in the gram-scale synthesis of PtNON and PdNON, demonstrating its practicability in organic synthesis methodology and materials science [56]. Early in 2013, Che's group had developed a series of symmetric (**53**) [51] and asymmetric (**54–62**) [52–54, 57] phenoxyl-pyridine containing tetradentate platinum(II) complexes. All asymmetric

as base according to the previous reported method [28,

**1000 cd/m2 Peak (%) 1000 cd/m (h) 2 (%)**

http://dx.doi.org/10.5772/intechopen.76346

93

CO3

3–6 days were needed if using K2

Device structure: ITO/MoO3

4% **43**a

6% **46**b

6% **47**b

8% **48**c

10% **50**e

2% **50**f

10–6% **51**g

10% **54**i

16% **55**i

20% **56**i

4% **57**i

10% **58**j

10% **59**j

**a**

b

c

e

f

g

i

j

For the development of iridium(III)-based blue emitters containing phenylpyridine moiety, generally, electron-withdrawing groups, like fluorine, are needed to be introduced into the phenyl group to stabilize the HOMO level [21]. However, this would result in electrochemical stability problems to accelerate the device degradation, which was unfavorable to the development of the stable blue OLEDs. In 2016, Li′s group developed a new rout for stable and efficient blue OLEDs through breaking the conjugation of the phenylpyridine with a sixmembered chelating rings (**52**, PtNON) [33]. Therefore, an operational lifetime LT70 of 624 h at 1000 cd/m2 with peak EQE of 10.7% and CIE coordinates of (0.17, 0.32) was achieved for the PtNON-based blue OLED. This device performance was comparable to the best iridium(III) based blue OLEDs reported in literatures [34].

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes http://dx.doi.org/10.5772/intechopen.76346 93

c Device structure: ITO/PEDOT:PSS/TAPC/dopant: 26mCPy/PO15/BmPyPB/LiF/Al.

dDevice structure: ITO/HATCN/NPD/dopant:mCBP:BAlq/BAlq/Alq/LiF/Al.

e Device structure: ITO/HATCN/NPD/TrisPCz/dopant: CBP/BAlq/BPyTP/LiF/Al.

f Device structure: ITO/HATCN/NPD/TrisPCz/dopant:Bebq2 /BAlq/BPyTP/LiF/Al.

g Device structure: ITO/HATCN/NPD/TrisPCz/20 wt%**51**: CBP/6 wt%**51**:CBP/BAlq/BPyTP/LiF/Al.

hDevice structure: ITO/HATCN/NPD/dopant:mCBP/mCBT/BPyTP/LiF/Al.

i Device structure: ITO/PEDOT:PSS/dopant:PVK:OXD-7/TmPyPb/TPBi/LiF/Al.

j Device structure: ITO/MoO3 /TAPC/dopant:TCTA/TmPyPB/LiF/Al.

the device [50]. These device performances indicate that the platinum(II)-based complexes are

For the development of iridium(III)-based blue emitters containing phenylpyridine moiety, generally, electron-withdrawing groups, like fluorine, are needed to be introduced into the phenyl group to stabilize the HOMO level [21]. However, this would result in electrochemical stability problems to accelerate the device degradation, which was unfavorable to the development of the stable blue OLEDs. In 2016, Li′s group developed a new rout for stable and efficient blue OLEDs through breaking the conjugation of the phenylpyridine with a sixmembered chelating rings (**52**, PtNON) [33]. Therefore, an operational lifetime LT70 of 624 h at

PtNON-based blue OLED. This device performance was comparable to the best iridium(III)-

with peak EQE of 10.7% and CIE coordinates of (0.17, 0.32) was achieved for the

more appealing as phosphorescent emitters in the display applications.

**Table 8.** Photophysical properties of pyridine-based tetradentate platinum(II) complexes.

**Comp. In solution at RT In PMMA at RT**

92 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

**Solvent λmax/nm ϕ/% τ/μs λmax/nm ϕ/% τ/μs**

Cl2 512 63 2.0 — 97 4.5

Cl2 614 — 3.6 — 40 —

Cl2 502 34 — — — —

Cl2 582 63 7.3 — — —

Cl2 508 31 2.6 474 83 3.8

Cl2 595 12 1.9 — — —

Cl2 479, 510, 624 60 5.8 — — —

Cl2 480, 510, 616 66 5.4 — — —

Cl2 482, 512, 624 75 17.7 — — —

Cl2 503 76 4.1 — — —

Cl2 551 90 4.3 — 74 —

Cl2 517 80 5.1 — 91 —

Cl2 553, 587 86 6.6 — — —

Cl2 526 47 5.9 — — —

Cl2 527 49 8.8 — — —

 [24] 2-MeTHF 512, 548 74 7.6 514, 551, 595a — — [24] 2-MeTHF 488, 523 75 11.4 541, 583a — — [24] 2-MeTHF 613 14 7.6 741, 782a — — [47] — — — — 621b 58 — [47] — — — — 620b,c — —

1000 cd/m2

**48** [23] CH2

**49** [48] CH2

**50** [49] CH2

**51** [50] CH2

**52** [33] CH2

**53** [51] CH2

**54** [52] CH2

**55** [52] CH2

**56** [52] CH2

**57** [52] CH2

**58** [53] CH2

**59** [53] CH2

**60** [54] CH2

**61** [54] CH2

**62** [54] CH2

Doped in Bebq2

.

Estimated from the emission spectrum.

a Solid state.

b

c

based blue OLEDs reported in literatures [34].

**Table 9.** Device performances of pyridine-based tetradentate platinum(II) complexes.

In fact, a synthetic challenge still remains for the gram-scale preparation of the PtNON and the PtON1 series complexes. Recently, our group developed an efficient approach for the CuCl-catalyzed C-N band cross coupling of carbazoles and 2-bromopyridine derivatives to synthesize 2-bromo-*N*-(hetero)arylcarbazoles. It was found that base t-BuOLi could accelerate the reaction significantly and just a few hours needed to complete the reaction [55]. However, 3–6 days were needed if using K2 CO3 as base according to the previous reported method [28, 29]. Moreover, a directly hydroxylation of the 2-bromo-*N*-(hetero)arylcarbazoles catalyzed by CuCl was also developed [56]. Both of the approaches are suitable for large-scale synthesis and have been successfully applied in the gram-scale synthesis of PtNON and PdNON, demonstrating its practicability in organic synthesis methodology and materials science [56].

Early in 2013, Che's group had developed a series of symmetric (**53**) [51] and asymmetric (**54–62**) [52–54, 57] phenoxyl-pyridine containing tetradentate platinum(II) complexes. All asymmetric complexes have high ϕ of 49–90% and τ of 4.1–17.7 μs in CH2 Cl2 solutions, and the OLEDs doped these complexes that showed very high brightness, even up to 66,000 cd/m2 at 10.5 eV. Moreover, the planar rigid molecular configuration enabled the fluorine-containing complexes **54**–**56** to have strong excimer emission at 616–624 nm, making them serve as ideal emitters for singledoped white OLEDs with CRI of up to 76 [52]. However, after introducing sterically bulky 3,5-di-*tert*-butylphenyl group to the pyridine ring, excimer formation was suppressed for the complexes **57**–**62**. Devices doped with complex **58** bridging phenylamine or complex **59** with a spiro linkage demonstrated peak EQEs over 25% and maximal PEs up to 109.4 lm/W using TmPyPB as ETL. The maximal PE of complex **59** could further be improved to 126.0 lm/W if Tm3PyBPZ as ETL, which were the highest among the reported platinum(II)-based OLEDs [53]. In the same year, Che's group developed another series of tetradentate platinum(II) complexes containing carbazole (**60**), phenoxazine (**61**), and phenothiazine (**62**) moieties, which served as yellow phosphorescent emitters combined with blue emitter to make white OLEDs [54].

potential applications for the next-generation full-color display and solid-state lighting. Owing to the square planar and rigid molecular configurations, platinum(II) complexes have many unique and exciting photophysical properties. On the one hand, easy molecular modification enables tunable emission spectra, and the FWHM of the pyrazole- or NHC-carbene-based complexes can achieve no more than 20 nm and can be as narrow as 15 nm. This facilitates them to serve as efficient deep-blue emitters, and device-doped NHC-carbene-based complex successfully realized "pure" blue emitting with CIE coordinates of (0.148, 0.079) and peak EQE of 24.8%.

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes

to achieve 18e structure in their excited state, making them serve as single-doped white OLEDs with high CRI values. Besides, tetradentate platinum(II)-based green, especially for the red OLEDs, demonstrated superlong operational lifetime and satisfied the requirements of the initial commercialization. What's more is that sky-blue OLEDs also achieved encouraging performances, indicating their bright future for the development of the efficient and stable blue OLEDs. Despite great progress that has been made for the tetradentate platinum(II) complexes, a challenge remains for the development of the stable deep-blue OLEDs, and more work still be needed. To overcome this challenge, it is important to develop stable host materials with a

tinued efforts of the academia and industry, we believe that these critical issues can be solved and the platinum(II)-based OLEDs will be one candidate for display and lighting applications.

The authors thank the National Natural Science Foundation of China (21602198, 21776259, 21476270), the "Qianjiang Talents Plan" (QJD1602017), and AAC Technologies for their financial support. The authors also thank Dr. Tyler Fleetham from the University of Southern California for the measurements of the quantum efficiency and luminescent lifetime of the PtON3.

state level and highly balanced charge carrier ability. However, through con-

platinum(II) complexes can form intermolecular Pt-Pt bond

http://dx.doi.org/10.5772/intechopen.76346

95

On the other hand, some planar d8

high enough T1

**Acknowledgements**

**Abbreviations**

Alq3 tris(8-hydroxyquinolinato)aluminium

BmPyPB 1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene

DPPS diphenyl-bis[4-(pyridin-3-yl)phenyl]-silane

BPyTP 2,7-di(2,2′-bipyridin-5-yl)triphenylene

CBP 4,4′-bis(*N*-carbazolyl) biphenyl

BAlq bis(2-methyl-8-quinolinolato) (biphenyl-4-olato)aluminum

CzSi 9-(4-(*tert*-butyl)phenyl)-3,6-bis(triphenylsilyl)-9*H*-carbazole

Bebq2 bis(benzo[h]quinolin-10-olato-κN,κO)beryllium(II)


**1.1 Malaria - a global infectious disease**

Malaria is a global deadly communicable disease [1], caused by plasmodium species, an apicomplexan microbe transmitted by the mosquito vector. Five major plasmodia parasites (*P falciparum, P vivax, P ovale, P malariae* and *P knowlesi*) have been implicated in malaria infections [2]. Of these, *Plasmodium falciparum* (*P. falciparum*) and *Plasmodium vivax (P. vivax)* are more widely distributed [3]. In sub-Saharan Africa, *P. falciparum* is the cause of most malaria cases while *P. vivax* is reported to cause most of the malaria cases in Asia; *P. falciparum* cause more fatal disease [4].

In 2019 alone, about 229 million positive malaria infections were reported globally, mortality from was estimated at about 409,000, with children under 5 years accounting for about 67% of death [5]. The disease is common among in poor communities [6], especially rural communities of the underdeveloped/ developing countries of the world. Economic, social and health importance of this disease in terms of loss of life (particularly; young children), reduction in productivity of affected adult population, and negative social and health implication of the disease makes it one of the high ranking microbial, infectious disease in the world.

#### **1.2 Malaria elimination agenda**

Among the diseases which have great public health impact, malaria is a significant public health concern [7–9]. The fight to eliminate malaria is an aged long battle globally. Elimination programmes were launched after the Second World War [6], with chloroquine as the major frontline anti-malaria drug [10] and Dichlorodiphenyltrichloroethane DDT used for vector control [11]. Malaria eliminations efforts in Africa began with the World Health Organisation (WHO) roll back malaria initiatives started in 1998 [1]; these efforts focused on entomology control (to reduce transmitting vectors) using indoor insecticides, and treated mosquito nets introduced for protection and prevention [6].

The success of these elimination programmes is why over one hundred countries have been awarded malaria free status [12] and thirty-four others accorded elimination status [13], and most malaria endemic countries working very hard towards the attainment of elimination status. Today the disease burden continues to decrease across the world [14] relative to the era, before the launch of the global elimination programmes.

Success stories leading to this elimination stage in malaria control had been heavily dependent on traditional entomology surveillance and drug use. However, the plasmodium is a ubiquitous parasites that has evolved, complex systems of survival [15]. Among these survival strategies is the development of drug resistance to nearly all know malaria drugs. Resistance to chloroquine (the major frontline medicine for the treatment of malaria) was reported and widespread [13] long before the roll-back-malaria initiatives. Better understanding of the biology of the parasites and the life cycle led to the development of a range of other anti-malaria drugs some of which are still actively being used, but resistance to almost all know malaria drugs have been reported [16–19].

Drug resistance vary, and is transferable from pathogen to host [20]. Of the different types of drug resistance, adaptive drug resistance is usually not permanent, but is capable of producing strains of parasites not targeted by known drugs. This chapter aims at reviewing the different types of drug resistance with focus on adaptive drug resistance in plasmodium and the implication to malaria elimination programme.

#### **2. Brief history of anti-malaria drug resistance**

The fight against drug resistance in pathogenic microbes is global. The life of these microbes are so inter-twined with human wellness that if overlooked could be very costly in terms of treatment cost and loss of life. As efforts are being up-scaled towards malaria elimination, the issue of drug resistance continues to surface as a major challenge to cope with. This is because the malaria parasite continues to evolve and regularly develop mechanisms for surviving the toxic effect of drugs. These mechanisms result with fixed mutations in the genetic architecture that confers fitness and resistance to withstand or evade targeting drugs, thereby hindering or completely preventing binding between drug compounds and their target.

The history of evolution of drug resistance in plasmodium dates back to the 1930s when chloroquine (CQ ) resistance in *P. falciparum* arose independently in Columbia and Thailand [21], and rapidly spread throughout the world. Research efforts to truncate this spread led to the development of different variants of malaria drugs to replace CQ. However, the plasmodium in it's unique way continue to adapt and evolve new mutations for survival and resistance to drugs which are harmful to it [22]. Advances in molecular technology has made it possible to

*Adaptive Drug Resistance in Malaria Parasite: A Threat to Malaria Elimination Agenda? DOI: http://dx.doi.org/10.5772/intechopen.98323*

#### **Table 1.**

*Common mutations associated with* P. falciparum *malaria drug resistance.*

uncover different mutations in the plasmodium parasites associated with drug resistance (**Table 1**). Evolution of these mutations are dynamic and difficult could be difficult to track and eliminate, especially when novel parasite results, against which known anti-malaria drugs is ineffective.


**5.3 The mtDNA hypothesis for schizophrenia and its predictions**

Thus we propose here a new hypothesis which insists that the risk loci for schizophrenia are in the mtDNA.

Mitochondria are involved in a variety of major cellular events such as oxidative phosphorylation, free radical production and Ca2+ buffering, and play an active role in apoptosis. They possess two classes of antioxidant defense system (non-enzymatic and enzymatic), and structurally and functionally intact mitochondria serve as a *net sink* rather than a *net source* of reactive oxygen species (ROS) (Andreyev et al., 2005). ROS-defenses are severely undermined in structurally compromised mitochondria (Andreyev et al., 2005). Thus, mitochondrial dysfunction, presumably through imbalance of ROS production and removal (Andreyev et al., 2005), raises ROS emission (Esposito et al., 1999; Senoo-Matsuda et al., 2001) and causes intracellular oxidative stress.

Because abnormal mtDNA may cause mitochondrial dysfunction, the hypothesis predicts: (1) enhanced oxidative stress and disturbed energy metabolism in predisposed individuals, which may cause various pathogenic alterations such as genomic instability, aberrations in neurodevelopment, and the brain dysfunction. Furthermore, because mtDNA can be transmitted only through females and there is no link between the nuclear genome and the

rates at those loci are near the upper limit in the autosomes. All of them are located on 1q, while 4 resistance genes (*MHTFR*, *GRIK3*, *PDE4B*, *GSTM1*) are on 1p. \* schizophreniaassociated alleles; variants that could meet the criterion are shown in bold characters

rs1801133 T\*/C 0.3532 0.3211 1.15 0.032

rs6691840 G\*/T 0.2600 0.2226 1.25 0.037

rs910694 C/T\* 0.5780 0.5477 1.30 0.030

**rs2661319** A/**G\*** 0.4920 0.4744 **1.08 0.0176**

rs1800896 G\*/A 0.3056 0.2657 1.42 0.040

rs841865 A/G\* 0.8434 0.8001 1.32 0.043 **rs1327175** G/**C\*** 0.92840 0.91243 1.32 **0.016**

**rs3737597 A\***/G 0.03069 0.01735 1.80 **0.013 rs999710 A\***/G 0.3989 0.3819 1.07 **0.0170**

Table 2. Schizophrenia-associated genes on the chromosome 1 that could meet the criterion

Thus we propose here a new hypothesis which insists that the risk loci for schizophrenia are

Mitochondria are involved in a variety of major cellular events such as oxidative phosphorylation, free radical production and Ca2+ buffering, and play an active role in apoptosis. They possess two classes of antioxidant defense system (non-enzymatic and enzymatic), and structurally and functionally intact mitochondria serve as a *net sink* rather than a *net source* of reactive oxygen species (ROS) (Andreyev et al., 2005). ROS-defenses are severely undermined in structurally compromised mitochondria (Andreyev et al., 2005). Thus, mitochondrial dysfunction, presumably through imbalance of ROS production and removal (Andreyev et al., 2005), raises ROS emission (Esposito et al., 1999; Senoo-Matsuda

Because abnormal mtDNA may cause mitochondrial dysfunction, the hypothesis predicts: (1) enhanced oxidative stress and disturbed energy metabolism in predisposed individuals, which may cause various pathogenic alterations such as genomic instability, aberrations in neurodevelopment, and the brain dysfunction. Furthermore, because mtDNA can be transmitted only through females and there is no link between the nuclear genome and the

**5.3 The mtDNA hypothesis for schizophrenia and its predictions**

**(minor/major)** *mA mU OR d*

allele\* 0.7546 0.7140 1.35 0.041

**Genes and**

*MHTFR* 1p36.22

*GRIK3* 1p34.3

*PDE4B* 1p31.3

*GSTM1* 1p13.3

*RGS4* **1q23.3**

*IL10* 1q32.1

*DISC1* **1q42.3**

in the mtDNA.

**SNPs Location Allele**

GSTM1\*0 ins-allele/del-

*PLXNA2* **1q32.2** A/G

et al., 2001) and causes intracellular oxidative stress.

mitochondrial genome, the mtDNA hypothesis predicts: (2) a higher maternal transmission of schizophrenia, and (3) positive associations between resistance genes and schizophrenia as well as negative associations between facilitating genes and schizophrenia (see 5.2). These predictions seem to be consistent with other major epidemiological findings and the results of the genetic and the pathophysiological studies to date.


**Abstract**

Astroglia or astrocytes, the most abundant cells in the brain, are interposed between neuronal synapses and the microvasculature in the brain's gray matter. This unique anatomical location allows astroglia to play pivotal roles in brain metabolism as well as in the regulation of cerebral blood flow. In particular, astroglial cellular metabolic compartmentation exerts supportive roles in dedicating neurons to the generation of action potentials and protects neurons against the oxidative stress associated with their high energy consumption. Key products of astroglia include lactate and ketone bodies (beta-hydroxybutyrate and acetoacetate), which can also be produced avidly by muscle and liver, respectively. Therefore, brain cells, skeletal muscles, and hepatocytes constitute a metabolic compartmentation in the whole body. In this chapter, I will focus on brain cells, especially astroglia, since the impairment of normal astroglial function can lead to numerous neurological disorders including stroke, neurodegenerative diseases, and neuro-immunological diseases. I will also discuss the metabolic responses of brain cells in terms of food consumption and exercise. A better understanding of the astroglial metabolic response is expected to lead to the development of novel therapeutic strategies for diverse neurological diseases.

**Keywords:** astrocyte, astroglia, brain-derived neurotrophic factor, beta-hydroxybutyrate, fatty acids, glucose


*Method of Measurement*

The rheological parameters of hydromixtures used in this study were performed by an Anton Paar MCR 302 rotational viscometer. A coaxial double-gap cylinder system was chosen to measure the shear stresses generated in the hydromixture. In this measuring system, the outer cylinder, which is also the vessel into which the sample is applied, is a stationary element that allows for easy temperature control and stabilization. The inner cylinder (rotor) is driven by a motor for which the speed can be determined and the torque measured. The torque is proportional to the power of the current supplying the motor. It is converted to shear stress, and the rotor speed to the shear rate. In the performed measurements, the value of the shear rate was set, while the value of the shear stress was measured by the device. The measurements were carried out at 20 ◦C in the range of a shear rate from about 8–10 s−1 to 100 s−1.


**7. Results**

According to the studies so far discussed, different techniques have proven to be useful in the identification of endophenotypes or potentially promising biomarkers. Social deficits, as well as cognitive, anatomical, and/or neurophysiological findings are encouraging and in some cases are associated with candidate genes. Endophenotypes may help detect children who are at higher risk for ASD. This could promote better and earlier diagnosis and treatment, as well as the establishment of prevention strategies. It is also probable that new drug targets can be identified and that personalized treatments can be available.

The precise genetic, environmental, and epigenetic factors that increase susceptibility to ASD may vary in several ways from one affected child to another. For this reason, it is very important to continue the search for means that allow us to form less heterogenous groups of patients. Endophenotypes, at least in some cases, could contribute to achieve this goal. They could help recognize risk factors that participate or affect common pathways or processes, such as chromatin remodeling and transcription that may affect neurodevelopment.

In some instances, de novo mutations associated with ASD may be highly penetrant, and in absence of a family history of ASD, endophenotypes may not be observed in relatives, but could still contribute to an earlier identification of symptoms in the carrier of the mutation; the next generations could also benefit from early assessments and prevention strategies. On the other hand, there will be families in which relatives will probably share enough risk factors with the patient, as to exhibit one or more endophenotypes or ASD.

### **8. Future research**

Further studies in patients and relatives, preferably of first and second degree, will help determine if biomarkers fulfill the criteria of heritability and cosegregation.

Knowledge about the neuroetiology of ASD will be enriched by research in areas such as neuroanatomy, physiology, and social behavior. Multiple investigations are underway, and risk factors for ASD will continue to be identified, possibly in more homogenous groups of patients.

Future research should ideally involve longitudinal research in large samples that include twin pairs, nuclear families and/or extended pedigrees. These conditions may facilitate the identi‐ fication or confirmation of endophenotypes, in which the criteria of heritability and cosegre‐ gation are fulfilled. Whenever possible, it will be useful to obtain detailed information about clinical and demographic aspects, not only regarding the individual with ASD, but about first and second degree relatives. While the grouping of individuals in a dichotomous way, as affected or unaffected will continue to be evaluated, and offers advantages, this approach can be complemented with the evaluation of endophenotypes [46].


*Review* **An Up-to-Date Review of Natural Nanoparticles for Cancer Management**

**Daniel Ion 1,2,†, Adelina-Gabriela Niculescu 3, Dan Nicolae Păduraru 1,2,†, Octavian Andronic 1,2,†, Florentina Mus, at 1,2,†, Alexandru Mihai Grumezescu 3,4,5,\* and Alexandra Bolocan 1,2,†**

**Abstract:** Cancer represents one of the leading causes of morbidity and mortality worldwide, imposing an urgent need to develop more efficient treatment alternatives. In this respect, much attention has been drawn from conventional cancer treatments to more modern approaches, such as the use of nanotechnology. Extensive research has been done for designing innovative nanoparticles able to specifically target tumor cells and ensure the controlled release of anticancer agents. To avoid the potential toxicity of synthetic materials, natural nanoparticles started to attract increasing scientific interest. In this context, this paper aims to review the most important natural nanoparticles used as active ingredients (e.g., polyphenols, polysaccharides, proteins, and sterol-like compounds) or as carriers (e.g., proteins, polysaccharides, viral nanoparticles, and exosomes) of various anticancer moieties, focusing on their recent applications in treating diverse malignancies.

**Keywords:** natural nanoparticles; natural anticancer compounds; natural cancer therapies; novel cancer treatment alternatives; natural nanocarriers; chemotherapeutic agents targeted delivery

#### **1. Introduction**

Cancer has long been a critical threat worldwide, imposing a global health and economic burden. Cancer cells can evade the immune system, multiply indefinitely, and perform angiogenesis, leading to challenging malignancies that directly damage human life [1–6].

The most frequently employed treatment option in fighting cancer is chemotherapy, which can be used either alone or in combinatorial approaches with radiotherapy, surgery, or adjuvant therapies (e.g., immunotherapy, hormone therapy, photothermal therapy, photodynamic therapy, and ablative techniques) to produce effective responses depending on the cancer stage [7–10]. However, the dissatisfying specificity coupled with poor aqueous solubility and short blood circulation of conventional anticancer drugs leads to low concentrations of drugs at the tumor site and the requirement of high doses [2,8,11–13]. In addition, the therapeutic efficacy of administered drugs diminishes over time due to the development of drug resistance [14,15]. Despite the growing number of anticancer agents developed in the last few decades, their severe toxicity, high production cost, and low patient compliance demand better antitumor alternatives [16].

To improve treatment outcomes, radiotherapy can be used complementarily to destroy cancer cells sensitized by chemotherapeutics. However, radiation also affects normal tissues,

**Citation:** Ion, D.; Niculescu, A.-G.; P ˘aduraru, D.N.; Andronic, O.; Mus,at, F.; Grumezescu, A.M.; Bolocan, A. An Up-to-Date Review of Natural Nanoparticles for Cancer Management. *Pharmaceutics* **2022**, *14*, 18. https://doi.org/10.3390/ pharmaceutics14010018

Academic Editor: Mazzucchelli Serena

Received: 27 November 2021 Accepted: 20 December 2021 Published: 22 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

leading to side effects occurrence immediately or soon after radiotherapy treatment [17,18]. Thus, the lack of specificity of conventional therapies results in negative effects upon rapidly multiplying normal cells (e.g., bone marrow, gastrointestinal tract, and hair follicles) [13] or other healthy tissues, leading to multiple off-target adverse effects, including appetite loss, anemia, internal bleeding, fatigue, and hair loss [2,12,19]. Some of the most important disadvantages associated with the classic trio of cancer therapeutic options are summarized in Figure 1.

**Figure 1.** Limitations of conventional cancer treatment strategies. Created based on information from [13,20–25].

Nanotechnology has appeared as a promising solution to overcome the most pressing challenges of current cancer treatments [2,26,27]. The use of nanocarriers is a viable option for enhancing solubility and bioavailability of anticancer compounds of both natural and synthetic origin [28], delivering drugs across traditional biological barriers in the body [29], and combining therapeutic agents with imaging techniques towards achieving synergic results [12]. In more detail, anticancer drug delivery via nanoparticles (NPs) is influenced by biological barriers, counting tumor microenvironment (TME) and vasculature, reticuloendothelial system, blood–brain barrier (BBB), and kidney filtration [30]. Thus, special attention must be given to overcoming these barriers and ensuring tumor uptake of NPs.

Specifically, by taking advantage of the newly accumulated knowledge TME [15,31,32], multifunctional NPs can be designed to deliver bioactive agents directly to the tumor, reducing systemic side effects (Figure 2). This can be done either by surfaced functionalization with ligands specific for receptors expressed by tumor cells or other cells in TME or by functionalization with chemical groups that can respond to TME signals (e.g., secreted molecules, acidic pH, and hypoxic conditions) [11,29,33].

**Figure 2.** NPs roles in regulating TME and improving tumor immunotherapy. Created based on information from [1,30,34–36]. Abbreviations: APCs—antigen-presenting cells; DCs—dendritic cells; TME—tumor microenvironment; ECM—extracellular matrix; MHC—major histocompatibility complex; CTL—cytotoxic T lymphocyte.

To date, a wide array of materials have been investigated for producing effective anticancer NPs. Researchers developed different nanostructures of different shapes, sizes, architectures, and compositions using various materials, including lipids, proteins, polysaccharides, synthetic polymers, and inorganic materials [2,8,9,11,29].

In particular, the beneficial physicochemical and biological properties of natural materials have recently rendered these NPs among the most promising base materials for cancer therapy. As compared to most synthetic NPs, nanostructures of natural origin have been noticed to have better safety profiles, enhanced biocompatibility, biodegradability, and non-immunogenicity; they also present functional groups that facilitate their chemical modification towards obtaining even more performant formulations [8,11,37]. Moreover, as detailed toxicology assessments are fundamental for the clinical translation of nanoparticulate nanomedicines [38], the favorable biological behavior of natural NPs may represent an opportunity for their faster introduction into clinical trials and consequently into medical practice as compared to synthetic alternatives.

In this respect, the present paper further discusses the most recent advances in natural NPs for cancer management, including both natural molecules with inherent anticancer properties loaded into NPs and natural NPs used as carriers of various freights in cancer therapies. More specifically, in this review are considered "natural" the compounds and materials that can be obtained from biological sources.

#### **2. Natural Compound-Based NPs with Intrinsic Anticancer Activity**

Attempting to avoid the side effects and downsides of chemotherapeutic drugs, researchers have tackled the anticancer potential of natural anticancer agents from a variety of plants and organisms [39,40]. Some of the most relevant examples of nanoparticles of natural origin with intrinsic anticancer activity are further described in this section.

#### *2.1. Polyphenols*

Polyphenols are well recognized for their health benefits, showing biomedical potential in various diseases, such as tumors, inflammatory diseases, and cardiovascular diseases [41,42]. They can be easily included in the daily diet, as they can be found in diverse natural sources (e.g., tea, red wine, cocoa, fruits, and olive oil), or they can be extracted and processed for developing nutraceutical and pharmaceutical formulations with specific and enhanced activity [43].

Polyphenols represent a broad class of bioactive compounds, comprising a variety of chemical structures (Figure 3). Their molecular structures allow polyphenols to combine with other materials, such as proteins, metal ions, polymers, and nucleic acids, creating better delivery strategies [41]. In this respect, the following subsections discuss the newest approaches for polyphenols delivery that are relevant for alternative or complementary cancer therapies.

**Figure 3.** Polyphenols classification and examples. Created based on information from [44–46].

#### 2.1.1. Flavonoids

Nanoparticles obtained from flavonoids have promising anticancer effects [28,47]. One such useful compound is chrysin, which has proven advantageous characteristics, including anti-inflammatory, antioxidant, antiallergic, and cancer chemopreventive properties [2,48,49]. Nonetheless, the poor water solubility and bioavailability of chrysin limit its use as an anticancer drug [48,50]. In this context, special attention has been drawn

to investigate chrysin in nanoparticulate form. Mutha et al. [49] have developed chrysin NPs by probe sonication technique. The authors used small amounts of sodium dodecyl sulfate to enhance aqueous solubility of NPs and mannitol as a bulking agent. The as-such obtained NPs demonstrated potential cytotoxicity and significant control on the growth of MCF-7 breast cancer cell line. Another convenient strategy to overcome pure chrysin limitations is to encapsulate this compound in PLGA-PEG nanoparticles [48,50]. For instance, Mohammadian et al. [48] have evaluated chrysin-loaded PLGA-PEG NPs against a gastric cancer cell line. The researchers observed a significant decrease in IC50 value of nanocapsulated chrysin as compared to free flavonoid and a decline in miR-18a, miR-21, and miR-221 gene expression. Another example is offered by Tavakoli et al. [50], who have used PLGA-PEG NPs to co-deliver chrysin and curcumin. The scientists reported enhanced antiproliferative and anti-metastatic effects on melanoma cancer when encapsulating these polyphenols than when using them in free form.

Quercetin is another flavonoid with potential applications in cancer therapy as it exhibits a strong inhibitory effect on the growth of several cancer cell lines, including nasopharyngeal, lung, prostate, ovarian, breast, leukemic, skin, bone, and colon cancer cells [51–53]. Nonetheless, the application of quercetin in anticancer treatments is limited by low aqueous solubility, bioavailability, and chemical instability in neutral and alkaline media [54]. To overcome these drawbacks, quercetin can be delivered via different nanoplatforms. For instance, Rezaei-Sadabady et al. [55] have encapsulated quercetin into liposomes to enhance the hydrophilicity and deliverability of this flavonoid. The authors obtained promising results in terms of quercetin solubility and bioavailability, but the types of cancer most likely to benefit from this non-toxic therapy were yet to be determined. Another delivery possibility is proposed by Sadhukhan et al. [56], who have loaded quercetin into phenylboronic acid conjugated zinc oxide NPs. The scientists observed that their nanosystem was able to enhance oxidative stress and mitochondrial damage, leading to apoptotic cell death in human breast cancer cells. Moreover, tumor-associated toxicity in the liver, kidney, and spleen was reportedly reduced. Quercetin was also prepared in combination with other nanomaterials, including chitosan NPs [57,58], PEGylated-PLGA nanocapsules [59], pluronic-grafted gelatin copolymers [60], mesoporous silica NPs [61], and metal–organic frameworks [54].

Baicalein has also been extensively studied as an alternative to synthetic chemotherapeutic agents [2]. It has been proven as an anti-inflammatory, antioxidant, and antitumor agent for different types of cancers, including lung, breast, skin, and gastric cancers [62,63] (Figure 4). To overcome its poor bioavailability caused by its hydrophobic nature, scientists started to explore various baicalein-based nanoparticulate combinations. For instance, Wang et al. [62] developed self-assembled NPs containing dual-targeted ligands of folate and hyaluronic acid for co-delivery of baicalein and paclitaxel. The as-described system was proven efficient for targeted drug delivery, leading to synergistic anticancer effects and overcoming multidrug resistance in human lung cancer cells. A different strategy is offered by Joshi et al. [64], who have fabricated solid lipid nanoparticles of baicalein that had an increase of over ~300% in relative oral bioavailability as compared to free flavonoid administration. The researchers also reported better radioprotection to healthy cells and sensitized cancer cells to radiation-induced killing, concluding that these novel nanoparticles can be employed as an adjuvant in cancer radiotherapy.

**Figure 4.** Baicalein anticancer activity by binding to and interacting with specific cellular targets. Reprinted from an open-access source [63].

Green tea catechins were also noted for their suppressive effects on cancer cell progression, metastasis, and angiogenesis [65]. The major bioactive constituent of green tea, (−)-epigallocatechin-3-gallate (EGCG), has been intensively studied for its chemopreventive and chemotherapeutic activity. However, its lack of target specificity, short half-life, low stability, and low bioavailability limit its free use and request for special delivery approaches [66,67] (Figure 5). EGCG-based nanosystems showed promising synergistic results by conjugation with gold NPs [68–70], encapsulation in synthetic [71–73] and natural [74–77] polymeric NPs, liposome delivery [78–81], and combination with various other anticancer agents [79,81–83].

**Figure 5.** EGCG delivery possibilities for cancer therapy: (**1**) incorporation of ligands on NPs surface for specific targeting of cancer cell receptors or antigens; (**2**) EGCG used as a capping agent; (**3**) surface functionalization with polymers for improving drug release, cellular uptake, and intestinal absorption; (**4**) co-encapsulation with conventional chemotherapeutic agents. Reprinted from an open-access source [66].

#### 2.1.2. Tannins

Condensed tannin extracts have also been remarked as promising anticancer candidates due to their antitumor activity and potential of inducing apoptosis in cancer cells via enzymes regulation [37,84,85]. Other anticancer mechanisms of tannins include negative regulation of transcription factors, growth factors, receptor kinases, and various oncogenic molecules [86].

Recently, scientists have moved from condensed tannins extracts to their nanoparticles' counterparts, as is the case of AlMalki et al. [37], who have synthesized NPs from a commercial product extracted from the bark of Pine trees. The researchers obtained potential anticancer effects against MCF-7 cells, concluding that tannin nanoparticles are promising candidates for treating breast cancer either alone or in combination with low doses of tamoxifen.

#### 2.1.3. Resveratrol

Resveratrol is the most studied stilbene due to its large availability, antioxidant properties, and potential induction of cancer chemopreventive and therapeutic responses [87,88]. Nonetheless, similar to other polyphenols, resveratrol's direct use is hindered by its low bioavailability and rapid metabolism [88].

Thus, the development of nanoparticulated formulations has become a convenient solution for overcoming these drawbacks. Conjugating resveratrol to gold NPs was seen to improve its bioavailability, leading to optimal cellular uptake, and enhanced antitumor efficacy against breast, prostate, and pancreatic cancer cells [89]. Other studies emphasized the potent anticancer activity of resveratrol (RES) when loaded in solid lipid NPs (SLNs [90], functionalized mesoporous silica NPs (MSNs) [91], gelatin NPs [92,93], and more (Table 1).

**Table 1.** Examples of anticancer nanoparticle formulation based on resveratrol.

**Table 1.** *Cont.*

#### 2.1.4. Curcumin

Curcumin is one of the most studied herbal anticancer compounds, especially due to its multiple-fold action (Figure 6). Curcumin can inhibit carcinogenesis, angiogenesis, and tumor growth [97] by modulating or interacting with growth factors, interleukins, and protein kinases, thus being a promising alternative to conventional chemotherapeutic agents [39,40].

However, curcumin has low water solubility and is unstable in physiological conditions, drawbacks that further lead to poor bioavailability and unfavorable biodistribution [39,98]. To overcome these challenges and enhance anticancer bioactivity, scientists employed curcumin (CUR) in the development of composite nanoparticles by conjugation with human serum albumin (HSA) [99], encapsulation in chitosan [40,100], silk fibroin [101], soybean polysaccharide [102], and more (Table 2).

**Figure 6.** Anticancer effects of curcumin. Created based on information from [39,97].

#### **Table 2.** *Cont.*

**Table 2.** *Cont.*

#### *2.2. Proteins*

Increasing interest has been directed towards using ovalbumin (OVA) protein antigen in developing novel anticancer nanoparticles. This non-toxic, temperature- and pHsensitive, and economical material is convenient for use in cancer immunotherapy as it elicits cellular and humoral immune responses [108,109]. Habibi et al. [108] have created different OVA-based NPs by chemically linking individual OVA molecules via PEG units. The scientists noticed that by controlling the PEG/OVA ratio, the physicochemical characteristics of the protein NPs, such as size, elasticity, and network structure, can be tailored. The authors reported better results as the PEG/OVA ratio decreased, i.e., increased uptake by lymph node macrophages, dendritic cells, and B cells, more effective processing by dendritic cells, enhanced lymphatic drainage, higher anti-OVA antibody titers in vivo, and overall improved humoral responses. Moreover, when compared to solute OVA, NPs significantly increased the median survival rate in a mouse model of B16F10-OVA melanoma.

Silk sericin is another protein of interest for improving cancer treatment strategies. Sericin's biocompatibility, non-immunogenicity, and antioxidant properties are the main factors contributing to its research for anticancer purposes. It has been proven that this natural material can reduce oxidative stress or suppress cancer cytokines for skin and colon cancer. Moreover, silk sericin has a self-assembling capacity and unique chemistry that favors surface modifications, thus also being a promising nanocarrier for anticancer drugs [110,111].

Keratin may also be employed in designing NPs for cancer therapies. This protein is particularly appealing due to its unique amino acid sequences that can specifically bind vitronectin integrin receptors overexpressed by several cancer cells [112]. Nonetheless, this inherent anticancer potential has been mostly exploited for delivery purposes, to create stimuli-responsive nanocarriers for chemotherapeutics [112–115], or to enhance the effects of phototherapies [116–119].

#### *2.3. Polysaccharides*

Fucoidan is a marine polysaccharide that can be extracted from different species of brown algae. As this polymer exhibited great promise in treating several types of cancer, including colon, bladder, liver, lymphoma, and gastric cancers, researchers started to investigate its effects in the nanoscale form [120,121]. For instance, Etman et al. [120] have

developed fucoidan-based NPs by polyelectrolyte interaction with lactoferrin targeted ligand. The researchers have reported cytotoxic properties against pancreatic cancer cells, enhanced ability to prevent tumor cells migration and invasion, with a 2.3-fold decreased IC50 value for fucoidan NPs than for fucoidan solution.

Chitosan, a polymeric biomaterial found in shellfish exoskeletons, can also be used in developing novel anticancer formulations. This polysaccharide manifests its antitumor activity by affecting cell digestion and inhibiting cell development. However, most of the studies approach chitosan as an assistant agent and nanocarrier rather than for its intrinsic anticancer properties [122].

#### *2.4. Sterol-Like Compounds*

Sterol-like natural compounds have also been shown to have promising antitumor activity, gaining interest in developing innovative nanoparticulate cancer therapeutics. For instance, Qiu et al. [123] have loaded 20(S)-ginsenoside (Rg3) into pH-sensitive polymeric NPs that can target cancer cells and prolong circulation time. It was reported that Rg3 could be released rapidly at the tumor site, significantly inhibiting tumor proliferation. Specifically, the sterol-like compound decreased the expressions of proliferating cell nuclear antigen, producing the apoptosis of colorectal cancer cells through the increased expressions of caspase-3.

A different approach was used by Kim et al. [124], who have prepared flower-shaped nanocomposites based on zinc oxide and hyaluronic acid, which they functionalized with ginsenoside Rh2. This complex nanosystem proved successful against lung, colon, and breast cancer cells, exerting its anticancer activity through several mechanisms (e.g., ROS production, upregulation of p53 and BAX, downregulation of BCL2, induction of morphological changes in the nucleus of tumor cells).

#### **3. Natural Nanoparticles for Anticancer Drug and Gene Delivery**

A lot of research effort has been put into developing biocompatible targeted delivery vehicles from materials of natural origin. Numerous studies have been published evaluating the anticancer action of various natural-based nanosystems able to effectively and efficiently deliver different cargos to the tumor site. Several recently developed nanoformulations based on proteins, polysaccharides, viral NPs, exosomes, and other natural materials are further presented to make an overview of current progress in the field and emphasize the versatility of these nanoparticles for cancer management.

#### *3.1. Protein-Based NPs*

#### 3.1.1. Albumin

Human serum albumin (HSA) is an appealing material for developing novel nanomedicines as it has a high drug loading ability, self-assembling properties, long half-life, and preferential uptake by tumor and inflamed tissues [11,125]. Moreover, increasing interest has been drawn to using HSA-based NPs in cancer management after the FDA approval of paclitaxel-bound albumin NPs (AbraxaneTM) [7,126]. Albumin NPs also show a good affinity for other cancer drugs, such as doxorubicin, curcumin, and tacrolimus [109].

For instance, Chaiwaree et al. [127] have successfully encapsulated doxorubicin in HSA submicron particles. The researchers reported excellent A549 cell uptake (up to 98%) and localization of drug nanoformulation within the cell lysosomal compartment. These observations were also reflected in the reduction of cancer cell metabolic activities after 72 h and less than 1% drug release within 5 h at physiological pH. Thus, it can be concluded that this delivery system is a potential candidate for cancer therapy.

Another promising strategy was developed by Yu et al. [128], who have fabricated albumin NPs loaded with docetaxel and functionalized with nucleolin-targeted aptamers. Their targeted drug delivery system was preferentially ingested by nucleolin-expressing CT26 colon cancer cells, leading to enhanced antitumor efficacy, low systemic toxicity, and prolonged survival of CT26-bearing mice.

#### 3.1.2. Keratin

The targeting potential and ease of functionalization of keratin nanoparticles have been exploited by Avancini et al. [116], who have used them as nanocarriers for salinomycin, chlorin e6 photosensitizer, and vitamin E acetate. The researchers tested this novel drug delivery system in vitro on breast cancer cell lines and cancer stem cell (CSC)-enriched mammospheres, reporting synergistic cell killing, limited self-renewal capacity, and eradication of CSCs. Further in vivo tests on zebrafish embryos confirmed the results and revealed that keratin encapsulation of the drug does not alter its CSC-specific cytotoxicity.

Lu et al. [117] have also investigated keratin-based NPs for photodynamic therapy against breast cancer. The complete nanosystem comprised keratin as nitric oxide generator, phenylboronic acid (PBA)-modified d-α-tocopherol polyethylene glycol 1000 succinate (TPGS) as targeting ligand, and methylene blue as photosensitizer. In vivo studies showed that the developed constructs induced extensive cell apoptosis, leading to significant inhibition of in situ tumor growth and lung metastases.

#### 3.1.3. Silk Sericin and Fibroin

The two proteins of silk (i.e., sericin and fibroin) can be used for engineering silk-based nanoparticles suitable for drug delivery and cancer treatment [129–131]. For instance, Huang et al. [111] have fabricated folate-conjugated sericin nanoparticles for tumor targeting and pH-responsive delivery of doxorubicin. The as-described NPs possessed good cytotoxicity and hemocompatibility, specifically releasing the encapsulated drug freight into the lysosomes of folate receptor-rich KB cells.

Silk sericin has also found use as a photosensitizer carrier. More specifically, Gao et al. [132] have used this protein to create NPs for chlorin e6 delivery. These nanosystems achieved superior accumulation in tumor sites compared to with free photosensitizer agents, suppressing tumor growth and avoiding side effects occurrence.

Pandey et al. [133] have developed silk fibroin NPs coated with hydrophilic stabilizers to allow longer circulation times and facilitate their uptake by low-density lipoprotein receptors. These nanocarriers loaded with doxorubicin showed a proinflammatory response, sustained drug release, and better cytotoxicity than the free drug, being a potential candidate for glioblastoma treatment.

Silk fibroin was also proven useful in the delivery of drugs for colorectal cancer treatment. Particularly, Hudita et al. [134] have designed silk fibroin-based NPs for the delivery of 5-fluorouracil (5-FU). The scientists reported great antitumor efficacy in vitro and promising results in vivo, as the proposed system was able to ameliorate mucositis induced during 5-FU treatment.

#### 3.1.4. Ferritin

Ferritin is a convenient protein for drug encapsulation as it self-assemblies naturally into a hollow nanocage with an inner diameter of 7–8 nm. This structure is composed of 24 subunits of either heavy chain ferritin (HFn), light chain ferritin (LFn), or a mix between the two types. In particular, HFn has been noted to bind human cells by interacting with transferrin receptor 1 (TfR1), which is highly expressed on cancer cells and is commonly used as a targeting marker for tumor diagnosis and therapy [135,136].

In this respect, Liang et al. [136] have investigated HFn nanocages for the delivery of doxorubicin. The researchers demonstrated that these ferritin nanostructures were able to transport high drug doses for tumor-specific targeting and killing without requiring any additional functionalization or property modulation. Doxorubicin-loaded HFn internalized into tumor cells via interaction with overexpressed TfR1, releasing the drug in lysosomes and significantly inhibiting tumor growth after a single dose injection with minimum healthy organ drug exposure. A similar approach was followed by Jiang et al. [137], who have used doxorubicin-loaded ferritin nanosystems against hepatocellular carcinoma. The scientists also used a targeted ligand (i.e., GRP78-targeted peptide SP94) to improve release selectivity. The as-such designed platform was able to encapsulate a high amount of drug, ensuring a lower dosage of carrier and fewer adverse effects. Moreover, a better therapeutic effect was observed as compared to currently reported nanomedicines.

A different strategy is offered by Sitia et al. [138], who have encapsulated navitoclax and functionalized HFn nanocages with fibroblast activation protein (FAP) antibody fragments aiming to create cancer-associated fibroblast (CAF)-targeted drug delivery agents. The obtained results are promising as there were reported significantly higher drug release only in FAP<sup>+</sup> cells, and considerably higher cytotoxicity than non-functionalized systems. The researchers concluded it would be interesting to study if this nanoplatform is also able to reduce metastases formation.

#### *3.2. Polysaccharide-Based NPs*

#### 3.2.1. Chitosan

Chitosan has many advantageous properties recommending it as a suitable delivery vehicle for a variety of biomolecules [139]. Being non-toxic, biodegradable, and biocompatible, chitosan became the material of choice for developing many nanoparticulate formulations for biomedical applications in general, and cancer management in particular [40].

For instance, chitosan has been demonstrated successful in improving the efficacy of photodynamic therapy. Ding et al. [140] have prepared chitosan NPs encapsulated with chlorin e6 photosensitizer by a nonsolvent-aided counterion complexation method, noting enhanced biocompatibility and dramatically increased therapy efficiency compared with free photosensitizer administration.

Chitosan NPs have also been remarked as a promising anticancer drug delivery platform [141]. Gounden et al. [142] have developed silver NPs conjugated with chitosan and loaded with cisplatin. The authors reported specificity towards breast cancer cells with minimal cytotoxicity towards normal cells, as the drug was efficiently and rapidly released from the nanosystem at low pH.

#### 3.2.2. Fucoidan

Etman et al. [143] have used fucoidan extracted from *Undaria pinnatifida* to encapsulate quinacrine. Fucoidan acted as both delivery vehicle and active targeting ligand, while for some particles lactoferrin was added as a second active targeting ligand. The researchers obtained promising results for both single- and dual-targeting particles against pancreatic cancer, with a higher animal survival rate and no hepatotoxicity. In particular, dualtargeted particles were reported to enhance quinacrine activity 5.7-fold compared to free drug solution, having a higher ability in inhibiting cancer migration and invasion.

Jafari et al. [144] have also taken advantage of fucoidan's targeting ability. The authors used fucoidan-conjugated doxorubicin nanoparticles to target P-selectin overexpressed by malignant cells, obtaining enhanced cellular uptake and cytotoxicity against the MDA-MB-231 cell line.

Alternatively, Coutinho et al. [145] have combined fucoidan and chitosan to create an oral delivery system for methotrexate. The nanoplatform was reported safe towards fibroblasts but hindered lung cancer cell proliferation via an apoptotic process, being 7-times more effective than the free drug.

#### 3.2.3. Alginate

Alginate has attracted research interest especially due to its biocompatibility, biodegradability, ease of production, and functionalization [146]. This natural polymer has been used as a base material for several novel anticancer formulations. As an example, Pourjavadi et al. [147] have employed this polysaccharide in the development of multifunctional nanocarriers. The researchers fabricated a magnetic core nanocarrier with a lipophilic surface based on oleic acid chains onto which paclitaxel and doxorubicin were adsorbed and covered by a smart pH-sensitive alginate shell. The system exhibited increased stability, enhanced biocompatibility, faster drug release in the acidic medium than at physiological pH, and even higher toxicity toward MCF-7 and HeLa cells than the free drugs.

#### 3.2.4. Hyaluronic Acid

Hyaluronic acid (HA) is another polysaccharide [148] of interest for cancer drug delivery especially due to its ability to specifically bind CD44 receptor overexpressed by cancer cells [11]. For instance, Gaio et al. [149] have proposed the co-delivery of docetaxel and meso-tetraphenyl chlorine disulfonate (TPCS2a) photosensitizer via HA-coated NPs. Combining chemotherapy and photodynamic therapy, their nanosystem demonstrated superior efficacy over monotherapies in lowering self-renewal capacity and inhibiting the growth of breast cancer CSCs.

Moreover, thiolated HA (HA-SS) is sensitive to glutathione [19], and is thus a potential targeting agent for TME-responsive delivery. In this respect, Debele et al. [150] have created a dual-sensitive HA-SS conjugated with 6-mercaptopurine for doxorubicin-targeted delivery to parental colon cancer and colon cancer CSCs. The synthesized nanoformulation was uptaken via CD44 receptor, accumulating more in the tumor region than in any other organ.

#### *3.3. Viral NPs*

The use of plant viruses in humans is considered a safe and promising alternative for intravital imaging and drug delivery [151]. Particularly, cowpea mosaic virus (CPMV) was reported advantageous due to its capsid's icosahedral shape, which allows enhanced multifunctional group display and endows the viral NP with the ability to carry specific cargos. In what concerns cancer therapies, CPMV was noted to have enhanced permeability and retention effect, allowing these viral NPs to preferentially extravasate from tumor neovasculature and efficiently penetrate tumors [152].

For instance, Lam et al. [153] have used CPMV for the delivery of mitoxantrone (MTO) antineoplastic chemotherapeutic to treat glioblastoma multiforme (GBM). The researchers reported CPMV-MTO uptake in glioma cells and significant in vitro cytotoxic effects both as a solo therapy and in combination with tumor necrosis factor-related apoptosis-inducing ligand, concluding that these plant virus-based NPs are promising platforms for GBM treatment.

Cowpea chlorotic mottle virus (CCMV) has also been exploited in developing innovative cancer treatments. Cai et al. [154] have employed this plant virus in the targeted delivery of oligodeoxynucleotides (ODN) with CpG motifs to tumor-associated macrophages (TAMs). This nanoformulation promoted ODN uptake by TAMS, enhancing their phagocytic activity. Moreover, the direct injection of these engineered NPs into tumor tissues induced a robust antitumor response increasing the phagocytic activity in the TME.

Another plant-based virus reported for anticancer drug delivery is tobacco mosaic virus (TMV). Franke et al. [155] used TMV as a nanocarrier of cisplatin as a potential treatment for platinum-resistant ovarian cancer. The scientists observed more efficient cell uptake, superior cytotoxicity and DNA double-strand breakage in both platinum-sensitive and platinum-resistant cancer cells than for free cisplatin, concluding that their newly developed nanoplatform may be a powerful tool in combating ovarian cancer.

Other viruses reported for cancer applications include potato virus X, red clover necrotic mosaic virus, papaya mosaic virus and physalis mottle virus [151].

#### *3.4. Exosomes*

Exosomes have recently emerged as potential nanocarriers of anticancer therapeutics due to their biocompatibility, low immunogenicity, long circulation time, and high loading capacity. These nanoscale extracellular vesicles benefit from an excellent tumor cell uptake and high specificity to tumor-associated cells [19,29].

Aqil et al. [156] have used bovine milk exosomes as nanocarriers for siRNA delivery. The scientists reported a dose-dependent antiproliferative activity against A549 cells treated with folic acid-functionalized exosomes loaded with siKRASG12S gene, concluding that this nanocarrier is suitable for siRNA delivery and effective for tumor growth inhibition.

Another application of bovine milk exosomes is offered by Li et al. [157], who have encapsulated doxorubicin in these vesicles and decorated them with HA as a targeting ligand for the selective delivery into overexpressed CD44 tumor cells. Munagala et al. [158] have also developed drug-loaded exosomes for cancer therapy. The authors loaded various chemopreventive and chemotherapeutic agents (i.e., withaferin A, bilberry-derived anthocyanidins, curcumin, paclitaxel, and docetaxel) into exosomes and used folic acid to achieve tumor targeting, reporting enhanced biological efficacy, improved specificity, and elimination of off-target side effects of encapsulated drugs.

Furthermore, cancer cell-derived exosomes have unique characteristics that can be exploited in early cancer diagnosis, detecting highly metastatic cancer cells, and assessing cancer heterogeneity [159].

#### *3.5. Other Natural NPs*

Studies have also evaluated drug delivery nanoplatforms that do not fit under any of the above-presented categories. One such example is proposed by Carvalho et al. [160], who have encapsulated *Solanum lycocarpum* alkaloidic extract in natural lipid-based NPs with the aim of creating a better bladder cancer treatment strategy. The authors reported a sustained release profile 36 h after administration, antitumor activity in targeted cancer cells, and high antiproliferative activity, with a 5.4 times lower cell viability than with free extract.

Olive oil nanocapsules have also been described in the literature as efficient delivery vehicles for anticancer drugs. For instance, Galisteo-Gonzalez et al. [161] have developed nanocarriers with an olive oil core covered by a cross-linked HSA shell loaded with curcumin. The as described nanoplatforms had a similar IC50 value to that of free curcumin, but also avoided issues associated with excipients and displayed an excellent uptake performance (entered human breast cancer cells massively in less than one minute). Another approach of using olive oil nanocapsules was taken by Navarro-Marchal et al. [162], who have surrounded them with a different shell containing phospholipids, a nonionic surfactant and deoxycholic acid molecules, further coated with an αCD44 antibody. This complex nanosystem was loaded with paclitaxel, leading to high targeted uptake and increased antitumor efficacy (up to four times compared to free drug in pancreatic CSCs).

As it has been recently suggested that hydroxyapatite NPs have selective anticancer activity for lung cancer cells, this natural inorganic material can also be employed in designing targeted delivery vehicles [163]. Thus, Chen et al. [164] have developed hafniumdoped hydroxyapatite NPs able to enhance photodynamic therapy efficiency and tumor growth when bombarded with ionizing radiation. The authors concluded that their newly developed nanoplatform is suitable for palliative treatment after lung surgical intervention.

Recent progress has been reported in developing DNA nanocages for delivery purposes. For instance, Tam et al. [113] have designed self-assembled DNA nanocages functionalized with or without blood–brain barrier (BBB)-targeting ligands. Their nanocarriers were able to carry anticancer drugs and penetrate BBB to inhibit the tumor growth in a U-87 MG xenograft mouse model, being a safe and cost-effective targeted delivery platform for brain tumors. DNA nanocages were also proposed as multifunctional vehicles, such as platforms for the co-delivery of anti-miR-21 and doxorubicin [165], or fluorometric detection of human 8-oxoG DNA glycosylase 1 and doxorubicin delivery [166].

Another highly researched recent delivery concept in cancer therapy is the use of bioinspired nanoparticles mimicking natural components in the body [7,167]. More specifically, cell membrane coating technology has emerged as a promising strategy to deliver drugs into tumors as it endows nanomaterials with functions and properties that are inherent to source cells [29,168]. In this respect, researchers started to disguise nanoparticles using the plasma membranes of various cell types, including erythrocytes [168–171], leukocytes [172,173], platelets [174,175], macrophages [176,177], bacterial cells [178], stem cells [179,180], cancer cells [181,182], and different hybrid cell coatings [183–186].

#### *3.6. Summative Discussion*

A variety of natural materials can be employed in developing nanoconstructs for safe and efficient anticancer therapies. Incorporating a wide range of either conventional drugs or natural bioactive molecules, natural nanocarriers have been studied against diverse cancer cell lines in vitro or as in vivo therapeutic approaches in animal models. Summarizing the above-discussed research studies, Figure 7 overviews the four main categories of nanocarriers in a clear and concise manner in relation to their delivered moieties and cancer types for which they have been investigated.

**Figure 7.** Overview of the main discussed categories of natural nanocarriers for cancer management.

Proteins and polysaccharides can be considered dual-role biomaterials as some of them can act both as delivery vehicles and active ingredients. Moreover, several exponents of these categories may be concomitantly used as active targeting ligands (e.g., fucoidan, hyaluronic acid, ferritin, and keratin). Nonetheless, additional research is needed for improving the yields of synthesis methods, reducing inter-batch variability, and reducing the costs implied by the design of precisely tailored NPs.

Viral NPs represent an interesting alternative as the use of plant viruses is considered harmless for drug delivery in humans. Moreover, these nanoplatforms have been reported successful against drug-resistant tumors. Nevertheless, their in vivo behavior is still difficult to predict, demanding an individual investigation of each viral-based delivery system.

Exosomes and biomimetic NPs have recently gained interest in the scientific world, being highly promising natural carriers. However, these nanostructures are still in their infancy in cancer management, requiring more research, especially concerning optimizing their preparation processes.

In addition, comparative data on the performance and treatment efficiency of various natural-origin-based NPs are still scarce in the literature [187]. In this context, further research should also be focused on testing several types of nanocarriers in the same experimental conditions in order to create a basis of understanding of which is the best material and design option for a given type of tumor.

In an attempt to compare at least from a qualitative point of view the discussed biomaterials, Table 3 summarizes the advantages and disadvantages of each class of the presented nanocarriers.

**Table 3.** Advantages and disadvantages of main discussed natural nanocarriers for cancer management.

Moreover, natural NPs are considered attractive for developing theranostic systems. Through an interdisciplinary approach, multifunctional and multivalent nanostructures can be created towards generating innovative and performant cancer therapies [7,152]. Specifically, several of the discussed nanocarriers have been reported able to encapsulate and co-deliver imaging moieties, drugs, and genes, and even detect cancer cells by binding

to specific receptors. Thus, complex natural-based nanoparticulated formulations may hold the answer for effective cancer management.

#### **4. Conclusions**

To summarize, cancers remain a global health concern, requiring urgently improved therapeutic strategies. As the current treatment options are limited by invasiveness, severe adverse effects, and poor patient compliance, increasing scientific interest has been shifted to developing carrier systems able to ensure targeted delivery and controlled drug release. Numerous studies have revolved around various synthetic and natural nanomaterials as delivery platforms, yet, in recent years, the latter started to gain more ground. Alternatively, several natural anticancer compounds have been reported to be fabricated into nanoparticles, eliminating the need for synthetic chemotherapeutic agents.

Thus, through an interdisciplinary approach, better treatment strategies can be envisaged in the foreseeable future. Nonetheless, before applying the described nanoformulations in the clinic, more detailed research has to be performed concerning their efficacy and safety in human use. Moreover, further progress in drug encapsulation optimization, ligand conjugation efficiency, and high reproducibility fabrication with low costs is needed prior to undergoing mass production.

**Author Contributions:** All authors participated in the review, writing, and revision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**1.8 Soil**

Prunus

**1.3 Morphological characters of plant**

Clingstones: *Persica vulgaris*, Risso. Freestones: *Persica domestica*, Risso.

a.*Prunus persica var. nucipersica*, Schneid: the nectarine.

b.*Prunus persica* var*.* platycarpa, Bailey: saucer peach.

Usually small, smooth-skinned fruit, and leaves strongly serrate but not

Flattened, scarcely thick greenish fruit with a red cheek, medium in size with

The flowers are perfect, solitary, sessile and pink coloured. Flowering starts in the first week of February and continues till the end of the month. Pollination is aided by insects, and the mode of pollination is homogamy. The pollen of peach is highly viable. Commercial peach varieties are self-fruitful and set good crops without cross-pollination. J.L. Hale is the only variety which is self-unfruitful and requires to be pollinated by other varieties. Fruit setting starts in the beginning of March. The fruits are borne after 1-year growth. A small proportion is borne on short-lived spurs also. The fruit is drupe, and the edible portion is the mesocarp [2].

In India, peaches are mainly grown in midhills at a height ranging from 1000 to 1600 m. They also do well in wet and humid climate with cold winter and

**1.4 Ornamental forms of peach**

always so.

good flavour.

**1.5 Other species**

1.Prunus davidiana

2.Prunus *mira.* Small, bushy tree. Late blooming habit.

**1.6 Flowering and fruiting**

Slender, willow-like tree. Smaller than common peach.

Peach is a small- to medium-sized upright spreading, open topped deciduous tree. The trunk bark is dark brown and rough, and young shoots are smooth and pinkish in colour. The leaves are simple, large, oblong lanceolate, glabrous above and pubescent beneath. Vegetative and flower buds are borne in the axil of the leaves. Its flowers are numerous, sessile, white or pink appearing before the leaves. The flower is of perigynous type as the perianth surrounds the pistil but is not fused to it. The floral configuration is five sepals, five petals, thirty stamens and a single ovary. Fruits are fuzzy with free- or clingstone; however, the nectarines are fuzzless peaches. The stone is deeppitted and very hard. There are two well marked horticultural forms [1]. These include:

**82**

**1.7 Climate**

Peach thrives well on light sandy soils. Deep fertile loam or sandy loam with good drainage is considered to be the best. The pH ranges between 5.8 and 6.8. It cannot tolerate imperfect drainage. Fertile and heavy soils are hazardous.

## **1.9 Planting and aftercare**

Peaches are planted in the end of January in 1 m3 pits at a distance of 6.5 m apart. In hills, the spacing adopted is 5 × 4 m. A spacing of 3 × 3 m is practised under high-density orchards. Immediately after planting, staking is provided. The support should be firm, preferably of bamboo or other wooden logs. Young plants should be watered at frequent intervals. Stock sprouts should be removed carefully.

### **1.10 Propagation**

The peach is commercially propagated by means of budding and grafting. T-budding is the common method, and the season of budding is from April to September. Tongue grafting and wedge grafting are also done. Grafting is done in December–January.

#### **1.11 Raising of rootstocks**

Rootstock for peaches is raised from seeds of desi peach trees. The seeds of commercial cultivars like Sharbati and Khurmani are also used. Peach plants can also be raised on peach-almond hybrid, apricot, almond seedling, plum and behmi (*Prunus mira*). Most commonly used rootstock are the seedlings of wild peach. Peach seeds need chilling in hours to germinate. The process of meeting the cold requirement is called stratification. The stratification of peach seeds can be done under natural conditions and cold storage conditions at 10°C or below.

## **1.12 Grafting operations**

Peach seedlings from stratified seeds planted in the nursery beds during January become buddable in June. The grafting can be done when the seedlings are still in the nursery beds, or these can be uprooted and brought at one place for grafting.

#### **1.13 Training**

The peach is trained to the modified leader system. Plants should be headed back to a height of 90–100 cm at the time of planting. All the branches on the plants are also cut back to two buds. Three to five laterals which are well spaced are allowed to develop around the trunk. The lowest branch should not be below 40–50 cm from the ground level.

#### **1.14 Open-centre system**

After planting, the plant is cut back to 40–60 cm above the ground level. During the growing season, about three to six laterals, in addition to the central leader, are produced on the tree. In the first winter pruning, three to four scaffold branches

which are well located and have wide angle should be selected, and the remaining unwanted branches are removed. The central leader is also completely removed. The selected branches are headed back to ¼ to ½ of the growth. During the second dormant pruning, two to three secondary branches are selected on the primary branches. The major consideration in selecting secondary branches should be their location so that after pruning, the tips of the primary and secondary leaders are about 30–40 cm apart from each other. The height of secondary branches is staggered in different years by pruning all branch leaders more severely. The vertical ones are pruned more severely. This will produce branch leaders at different heights and prevent overcrowding when the tree is mature. In the following years, the head should be fully formed, and selection of secondary branches is completed.

#### **1.15 Tatura trellis system**

In high-density planting, this system of training of plants is very popular due to yield efficiency. Trees are planted at a spacing of 5 × 1 m or 6 × 1 m. At the time of planting, a 1-year-old plant is headed back to 20 cm above the ground level. In the next growing season, two limbs or branches are selected in opposite directions, and these branches are trained across the interrow space at an angle of 60° from the horizontal, forming a V-shaped canopy. The canopy is supported by a permanent trellis constructed of high-tensile galvanised steel fence posts. The secondary branches are developed along each primary branch forming a fruiting canopy.

#### **1.16 Pruning**

The peach fruits are borne after 1-year growth. A small proportion is borne on short-lived spurs also. Pruning should be done so as to produce 50–100 cm of growth in young trees and 30–70 cm in old trees annually. About 40% of 1-year-old branches should be thinned out to ensure proper tree growth and improve fruit size and quality. The pruning of peaches is carried out in early January.

The main objective of pruning is to maintain balance between vegetative growth and fruiting. Bearing peach requires heavy and regular pruning because it bears fruits laterally on the previous season growth. It is known that once the tree bears fruits, it will never bear again throughout its life. Therefore, pruning is done to remove the unproductive parts which in turn will form new fruiting branches in the following season. In peach pruning, thinning and heading back of shoots are two basic components. Pruning should be done so as to produce 30–70 cm of growth under subtropical conditions and 25–30 cm under midhills, annually, which is sufficient for optimum fruit production. For good-quality fruit production, 40–50% of thinning out and 75% heading back of shoots are suggested under midhill conditions. At the time of pruning, dead, diseased and broken branches should be pruned off.

#### **1.17 Orchard floor management**

In the initial years of plantation, the intercrops like peas, beans, tomato, cabbage, cauliflower and ginger are grown in the vacant area in between the trees but not in the basin area. Besides these, some green manuring crops like bean, peas and gram should be grown which helps in improving soil texture and nutrient status. In bearing orchard, the basin area of trees should be kept clean either by manual weeding or the use of weedicides. Sod grasses like white clover, red clover, orchard grass and rye grass are grown in the vacant area between the trees. Basins are mulched with 10-cm-thick dry grass mulch or black alkathene mulch. The mulching helps to conserve soil moisture and efficiently control the weeds in the basin area.

**85**

*Production Technology of Peach, Plum and Apricot in India*

average number of leaves per fruit should be 20–25.

when they are still hard as they ripen in storage or transit.

**1.19 Manuring and fertiliser application**

Thinning is more desirable on mature trees making small annual growth than on young vigorous plants. Thinning may also be another resort when good-sized uniform fruits are needed for canning and fancy trade. Hand thinning and chemical thinning can be done. While doing hand thinning, first shake the branches in order to dislodge the fruits which are likely to drop off naturally. If there is still surplus fruit, then start thinning from top to bottom of the branches. The distance from fruit to fruit after thinning on the shoots should be between 10 and 15 cm, and the

Peach fruits are harvested quickly when ready for harvest. Fruits are picked

**Particulars Himachal Pradesh Punjab Uttaranchal** FYM (kg/tree) 60 25 10 N (g/tree) 500 500 300 P2O5 (g/tree) 250 120 300 K2O (g/tree) 600 500 500

Irrigation is very essential for harvesting the peaches of better size and quality.

A sufficient moisture in the soil before the emergence of leaves and flowers is required for proper fruit set and growth. Frequent irrigations are needed during the fruit development. Lack of irrigation, particularly, during dry and hot summer, results in fruit drop and reduced fruit size and quality. In the hills, at least two to three irrigations and, in plains, weekly irrigation should be given during the fruit development period. In general, for quality fruit production, irrigation at 80% of field capacity is recommended. Orchard soil management and weed management are done during the initial 3–4 years after planting; the intercrops like peas, beans, tomato, cabbage, zinger and *Colocasia* are grown in between the peach trees; and basin area is mulched with hay or alkathene mulch. In fully grown tees, sod grasses are grown in vacant areas, and basin area is mulched with suitable mulch materials. Weedicides like simazine and atrazine at 2.0 kg/ha, terbacil at 0.8 kg/ha as pre-emergence and paraquat at 4.0 litre/ha and glyphosate 4.32 kg/ha as post-emergence herbicide proved to be most effective to control the

Heavy flowering and fruiting are the characteristic features of peach trees resulting in small-sized, poor-quality fruits and reduction of flowering in the subsequent season. Hence for production of quality fruits, crop regulation through thinning is essential in peach. The criteria for fruit thinning in peach are based on leaf to fruit ratio and spacing between fruits per tree. Generally 30:40 leaves per

*DOI: http://dx.doi.org/10.5772/intechopen.92884*

**1.18 Thinning of fruits**

**1.20 Harvesting**

**1.21 Irrigation**

weeds in peach orchards.

**1.22 Crop regulation**


**3. Sparse Visibility Graph-Based Path Planner**

Define Q ⊂ R3 as the robot navigation space, and S⊂Q as the sensor data from obstacles. A down-sample strategy is used to update and maintain the v-graph, denoted as G, and the grid to store pointcloud is denoted as L. Define the position of robot as *Probot* ∈ Q, the goal *Pgoal* ∈ Q.

The flow chart of the path planner proposed in the paper is shown in Figure 2. And the process consists of three parts: (1) generating the geometric contours of obstacles by LiDAR-to-plane mapping; (2) aggregating and simplifying complex obstacle information to maintain the v-graph at a low cost; and (3) searching for nodes and edges to generate the path from the start point to the goal through the v-graph.

**Figure 2.** The main flow chart of the path planner based on the v-graph.


**6. Concluding remarks**

This chapter has introduced the utility of mass spectrometry imaging (MSI) for small molecules, with a specific focus on neurotransmitters (NTs). Methods that have resulted in enhanced signals of NTs were highlighted, with alternative matrix materials and chemical derivatization of analytes the two main points of discussion. Future research is needed in both of these areas to determine optimum conditions and applications, as well as establishing standard procedures so that broad application of MSI can continue. Finally, an area not discussed here that is relevant to these

**109**

*Mass Spectrometry Imaging of Neurotransmitters DOI: http://dx.doi.org/10.5772/intechopen.93835*

The author declares no conflict of interest.

DESI desorption electrospray ionization

CHCA α-cyano-4-hydroxy cinnamic acid

LC-MS liquid chromatography mass spectrometry MALDI matrix-assisted laser desorption ionization

qMSI quantitative mass spectrometry imaging NEDC N-(1-napthyl)ethylenediamine dihydrochloride

nation of small molecules.

**Acknowledgements**

**Conflict of interest**

**Nomenclature**

DAG diacylglycerol

DAN 1,5-diaminonapthalene

Glucose 6-P glαucose 6-phosphate HTP high throughput ITO indium tin oxide

DHB 2,5-dihydroxybenzoic acid DPP 2,4-diphenyl pyranylium

LATT laser-assisted tissue transfer

MSI mass spectrometry imaging

PCA principal component analysis

PE phospatidylethanolamine PEP phosphoenolpyruvic acid PG phosphatidylglycerol PI phosphatidylinositol PC phosphatidylcholine SA sinapic (or sinapinic) acid

THAP trihydroxyacetophenone

NT neurotransmitter NP nanoparticle PA phosphatidic acid

PChol phosphocholine

TAG triacylglycerol

techniques and that will likely be explored in the future is the quantitative determi-

KS thanks the University of Scranton and the Chemistry Department for providing facilities, equipment, and financial support for any referenced experiments. KS thanks the Johns Hopkins Applied Imaging Mass Spectrometry (AIMS) Core Facility at the Johns Hopkins University School of Medicine for undertaking the imaging experiments referenced in this chapter. Lastly, KS thanks Nolan

McLaughlin and Tyler Bielinski for tagging along for the science journey.

techniques and that will likely be explored in the future is the quantitative determination of small molecules.