Quantum dots in living cells
Quantum dots in vesicles can be a base for fast signal system in brain.
Generally accepted that human information processing is going via neural networks. But it's difficult to believe that so slow system (1 ms cycle) can solve complex problems. The hypothesis about fast long distance electromagnetic interaction of bioactive molecules or structures can help. Such interaction also could implement quantum calculations.
The interaction to take place the mobile electric charges are necessary. Nanoscale vesicles filled with conducting substance are ideally suited for the purpose. Such vesicles (liposomes) are known in living cells.
The most promising are vesicles in neurons filled with neurotransmitters, because they can take part in both: usual and fast information processing and connect these two signal systems.
The problem is to find objects with all required properties. The method can be used: collecting large amount of vesicles, study of electromagnetic properties of substance in vesicles.
Let's put together some already known facts.
1. Semiconductor nanocrystals, also called quantum dots (QDs),
are a new class of fluorescent biological labels. Originating from
quantum confinement of electrons and holes within the nanocrystal
core material, the fluorescence from QDs is unique compared with that from traditional organic fluorophores. For example, QDs exhibit high photostability, broad absorption, narrow and symmetric emission spectra, slow excited state
decay rates and large absorption cross sections.
Their emission color can be tuned from ultraviolet to visible and infrared wavelengths by changing the size and chemical composition. Growing a semiconductor shell with a larger band gap improves the quantum confinement, resulting in very bright and highly stable, chemically as well as optically, semiconductor fluorophores.
Typical excitation and emission spectra of water-soluble QDs. Their emission wavelength can be continuously tuned from 400 nm to 2000 nm.
2. Colloidal semiconductor nanocrystals consist of an inorganic, crystalline core surrounded by a monolayer of organic ligands. Ligands can be modified or exchanged for others, they provide a convenient way to give the quantum dots functionality. There is technique for the fabrication of nanocrystals of organic molecules and polymers. Using the liquid-phase technique, it's possible to fabricate organic nanocrystals ranging in size from 10 nm to 1000 nm.
In particular, nanocrystals of poly(4-BCMU) ranging from 20 nm to 350 nm were prepared.
To prepare the nanocrystals, the investigators mixed a common fluorescent dye with a water-soluble polymer. The researchers then coated the nanoparticles with the molecule streptavidin, which forms a tight molecular coupling with a second
3. Organic semiconductors are now-used as active elements in optoelectronic devices such as organic light-emitting diodes (OLED), organic solar cells, organic field effect transistors (OFET), electrochemical transistors and recently also in biosensing applications.
Several kinds of carriers mediate conductivity in organic semiconductors. These include π-electrons and unpaired electrons.
Almost all organic solids are insulators. However, when their constituent molecules have π-conjugate systems, electrons can move via π-electron cloud overlaps.
Polycyclic aromatic hydrocarbons and phthalocyanine salt crystalsare examples of these semiconductors.
In some organic molecules, even unpaired electrons can stay stable for a long time. In such cases, unpaired electrons will be the carriers. This type of semiconductor is also obtained by pairing an electron donor molecule and an electron acceptor
molecule and is called a charge transfer complex.
two major classes of organic semiconductors.
1) the organic charge-transfer complexes.
2) various derivatives of polyacetylene.
charge-transfer complexes often exhibit similar conduction mechanisms to inorganic semiconductors. the presence of a hole and electron conduction layer and a band gap. As with inorganic amorphous semiconductors, tunneling, localized states, mobility gaps, and phonon-assisted hopping also contribute to conduction,
particularly in the polyacetylenes. Like inorganic semiconductors, organic semiconductors can be doped. Highly doped organic semiconductors,
for example Polyaniline (Ormecon) and PEDOT:PSS,
are also known as organic metals.
4. Neurotransmitters and Neuromodulators.
Within the cells, small-molecule neurotransmitter molecules are usually packaged in vesicles. Classical NTs are in vesicles of 40-50 nM diameter.
Synaptic vesicles are abundant organelles of uniform size. Their diameter is ~40 nm as judged by electron microscopy.
As relatively small organelles, synaptic vesicles can accommodate only a limited number of proteins and phospholipids. This restricts
the number of molecules present on synaptic vesicles to the number that can be fit into a sphere of 40 nm and the complexity of synaptic vesicles. Calculations indicate that synaptic vesicles contain approximately 10,000 molecules of phospholipids per vesicle and maximum of 200 protein molecules.
Neurotransmission involves 3 main steps:
1. Synthesis and loading of NT into vesicles
2. Release of NT (by fusion of vesicles)
3. Binding of NT to receptors on postsynaptic membrane, leading to ion influx and depolarization of post synaptic membrane.
Acetylcholine - major neurotransmitter.
Glutamate – main neurotransmitter in the brain.
Biogenic amines are neurotransmitters containing an amino group.
Catecholamines such as dopamine, norepinephrine and epinephrine, serotonin.
Amino Acid Neurotransmitters.
Amino acid neurotransmitters are the most prevalent neurotransmitters in CNS.
Glutamate, aspartate GABA (gamma aminobutyric acid), glycine.
Neuropeptides are composed of two or more amino acids. Neurons releasing neuropeptides are called peptidergic. Beta-endorphin, dynorphin, enkephalins.
Nitric oxide, ATP, adenine also act as neurotransmitters.
5. Nanocrystals conjugated with peptides, antibodies, or other small molecules recognize their target cell surface receptors. In addition, nanocrystal probes, after binding to their targets, can modulate receptor functions by either inhibiting ligand
transportation or activating downstream signaling pathways.
Nanocrystals conjugated with the neurotransmitter serotonin were used to target serotonin transporters on transfected cells. Serotonin-labeled nanocrystals
specifically interacted with the serotonin receptor, and inhibited the transportation of free serotonin in amanner similar to that of an antagonist.
Semiconductor nanocrystal linked antibodies and peptides. Investigations were made whether QDs can function as fluorescent nano-devices to evoke specific cell physiological responses. The tracking and imaging of nanocrystals in live animals has been achieved.
Intravenous injection of peptide conjugated QDs.
6. Melanin is a semiconducting polymer currently of high interest to researchers in the field of organic electronics in both its organic and synthesized forms. Melanin is a biopolymer and a neuropeptide. Melanins are particles not molecules. Molecular weight between 500 and 30000 Da.
Melanin influences neural activity and mediates the conduction of radiation, light, heat and kinetic energy. It is the subject of intense interest in biotech research and nanotechnology, where dopants are used to boost melanin conductivity.
Neuromelanin is the dark pigment present in neurons of four deep brain nuclei: the substantia nigra - Pars Compacta part, the locus ceruleus ("blue spot"), the dorsal motor nucleus of the vagus nerve (cranial nerve X) and the median raphe nucleus of the pons. In humans, these nuclei are not pigmented at the time of birth, but develop pigmentation during maturation to adulthood.
Neuromelanin has been detected in primates and in carnivores such as cats and dogs.
Many pigments are candidates to semiconductivity or superconductivity like biline derivatives, metalloporphyrins, ommins, pterins, pheomelanins, pheochromes, benzothiazines, benzothiazinones, dibenzothiazinones indigoids, cyanines, humic acids, fulvic acids. All the pigments are radical-polarones with the characteristic polyconjugate chain of acetilene-black.
7. “Caged” neurotransmitters are molecules that are transformed to a neuroactive state by exposure to light of an appropriate wavelength and intensity. Use of these substances has centered on in vitro bath application and subsequent activation using light from lasers or flashlamps that is delivered into the preparation through microscope optics. The degree and timing of uncaging could be controlled by manipulating the wavelength, intensity and timing of the light projected into the optical fiber. Because of the small size of the light guide and the ability to control light exposure at the source, this new method promises greater control over the spatial and temporal delivery of neuroactive substances than simple bath or iontophoretic application, and enables delivery of conventional neurotransmitters with a spatial and temporal resolution closer to that of the natural neuronal circuitry. This new method allows the application of normally labile substances, such as the free radical gas nitric oxide, by the photoconversion of photosensitive precursors.
Newly synthesized photolabile derivatives of glutamate, caged glutamate, that release free glutamate on a microsecond time scale after a pulse of UV laser light are described. 2-Nitrobenzyl derivatives were attached to the amino or carboxyl groups of glutamate. Substitution with a -CO-2 group at the benzylic carbon accelerates the photolysis reaction when compared to -H and -CH3 substituents. -O-(-Carboxy-2-nitrobenzyl) glutamate is stable at neutral pH.
8. Holographic recording is based on the use of a photoactive material that records a hologram upon exposure to light.
A photoactive material is capable of forming a spatially defined difference in refraction within a matrix material upon exposure to IR, Visible or UV light.
A photoactive material includes monomer, recording initiator, and an inhibitor.
Recording is based on the use of photoactive dyes as a method for media protection, inhibitor removal, and gaugable pre-exposure.
Photoactive dyes are chemicals that upon absorption of light produce reactive species (such as a radical, a cation, an anion, an oxidizer, a reducer, an excited molecule, or a ground state molecule) that directly or indirectly reacts with oxygen and other inhibitory agents. A reaction inhibitor prevents sustained reactions that consume photoactive material. An inhibitor typically include oxygen.
9. Specific optical properties of glial cells in the retina.
Individual Müller cells, which are radial glial cells spanning the retina
(about 150 µm in length) are oriented along the direction of light propagation.
It was demonstrated that individual Müller cells act as optical conduits or fibers. Their parallel array in the retina is reminiscent of fiberoptic plates used for low-distortion image transfer. Müller cells seem to mediate the image transfer through the retina with minimal distortion and low loss.
The way in which Müller cells transport light is similar to the mechanism by which the optical fibres in fibre optic plates carry light. Fibre optic plates consist of optic fibres bundled together, and are used instead of lenses to transfer images between distant locations; this occurs without distortion of the image or loss of image detail.
This finding ascribes a new function to glial cells.
It's very interesting to search for other glia cells acting as optical fibers in brain.
10. Types and functions of glia - candidates for light transmitting fibers :
Astrocytes (Astroglia): Star-shaped cells.
The most abundant type of glial cell, have numerous projections.
In the mature brain, the cerebellum and retina retain characteristic radial glial cells. In the cerebellum, these are Bergmann glia. In the retina, the radial Müller cells.
Cortical ladders: long glial cells in the cortex.
Non-mammalian species: birds, lizards, and amphibians have radial glia in the CNS. The velum medulare of adult monkeys also contains radial glial cells.
The cerebellar cortex plays a critically important role in normal learning. The role of the cerebellum in classical conditioning of discrete behavioral responses, a basic form of associative learning and memory is essential. This is perhaps the clearest evidence for the localization of a memory trace to a particular brain region in mammals (cerebellum) that exists at present. A closely related and increasingly definitive literature supports the view that the cerebellum learns and stores complex, multijoint movements.
There is an evidence that the cerebellum may be critically involved in many other forms of learning and memory, including cardiovascular conditioning, discrete response instrumental avoidance learning, maze learning, spatial learning and memory, adaptive timing. There is a growing literature implicating the cerebellum in complex cognitive processes.
After GFAP knockout (glial fibrillary acidic protein) memory is injured. GFAP is not present in neurons, only in glial cells. In the cerebellum it is normally present in substantial amounts in the Bergmann glia that surround the parallel fiber and climbing fiber–Purkinje neuron dendrite synapses. Although the Bergman glia appear morphologically normal in the GFAP knockout, they have no GFAP.
The key point here is that an abnormality limited to glial cells markedly impairs a form of synaptic plasticity (LTD) and a form of basic associative learning and memory. This may be the first direct evidence for a key role of glia in processes of learning and memory.
12. Quantum dot laser that is insensitive to temperature fluctuation for use in optical data communications and optical networks has been developed.
Laser realized high-speed operation of 10Gbps at wavelengths of 1300 nm for a temperature range from 20°C to 50°C . The optical output characteristics were nearly stable independent of temperature. Quantum dot lasers surpass conventional lasers due to their higher speeds, lower power consumption, and greater range of operating temperatures.
13. About absorption. Effective solution is to move from visible light to near infrared (NIR), because most tissue chromophores absorb light weakly at such long wavelengths. Advantage of NIR imaging is deeper penetration than that achieved with visible light.
Research is being performed to create nanoshells with high absorptions at biologically useful wavelengths by altering the thickness of the shells.
Particularly, the Near Infra Red region, which corresponds with low absorption by tissue, may be useful.
Visible Light wavelength 450-760 nm. Infrared wavelength type A 760-1,200 nm. Infrared wavelength type B 1,200-3,000 nm. Light attenuation in a live biological specimen reaches a minimum around 1200~1350-nm wavelength.
In the near-infrared region (800~2500nm wavelength), light absorption of organic molecules are dominated by the anharmonic vibrational modes referred to as 'overtone' or 'combination'. The absorptions of those modes are much weaker than that of the fundamental vibrations, near-infrared spectroscopy can
penetrate deeper. Visible red light, at wavelengths from 630 to 700 nm penetrates tissue to a depth of about 10 mm. Infrared light (800 to 1000nm) penetrates to a depth of about 40mm.
14. Organic electrically pumped injection lasers.
An active region includes organic injector layer and organic emitter layer. Emitter layer has a first energy level and a second energy level on a same side of an energy gap defined by a conduction band and a valance band. Charge carriers are injected through the injector layer into the first energy level of the emitter layer when a voltage is applied across active region. The difference in energy between the first and second energy levels produces radiative emissions when charge carriers transit from the first energy level to the second energy level. Population inversion is maintained between the first and second energy levels, producing stimulated emission and lasing.
Advantages to using organic materials (including both small molecule and polymer materials) as lasing media:
1. the linewidths are extremely narrow, 2. the lasing wavelength is tunable by chemically modifying the lasing species, 3. the lasing wavelength is independent of temperature over wide ranges.
15. Organic light-emitting diodes (OLED) are composed of an emissive layer,
a conductive layer, anode and cathode. The layers are made of organic polymer molecules that conduct electricity. When a voltage is applied current of electrons flows through the device from cathode to anode. The cathode gives electrons to the emissive layer and the anode withdraws electrons from the conductive layer
giving holes. Soon, the emissive layer becomes negatively charged, while the conductive layer becomes rich in holes. Electrons and holes meet each other and recombine emitting radiation. OLEDs enable a range of colors and have a fast response time, can be less than 0.01ms.
16. Bioluminescence resonance energy transfer converts chemical energy into photon energy, resulting in dramatic increases in fluorophore excitation and dramatic reductions in the effects of tissue autofluorescence. The technology eliminates the need for fluorescent excitation light. It reduces the high background caused by surface illumination and tissue autofluorescence and exploits tissue-penetrating near-infrared wavelengths. It efficiently couples chemical energy with light energy. Because energy transfer is resonant, and nonradiative, absorption of excitation photons by hemoglobin and other tissue pigments is eliminated.
The use of near-infrared wavelengths (typically 700–900 nm) for excitation and emission helps to reduce absorption and scatter. Even in the near-infrared range tissue autofluorescence remains a major problem.