the 
 emerging field of optofluidics explores new possibilities to 
 combine optical detection techniques with micro- and nanofluidics to develop miniaturized optical sensors with improved performance . 

the analytical techniques commonly used include surface plasmon resonance 
 ( spr ) , surface - enhanced raman spectroscopy ( sers ) , and fluorescence 
 imaging .
sers , a powerful technique for 
 biosensing and chemical identification , largely depends on the drastic 
 enhancement of weak raman signal obtained from analyte molecules , 
 upon their interaction with optical near - fields of metallic nanostructures . as an analytical technique , sers combines the benefits of label - free 
 sensing , high sensitivity , and chemical specificity .
an important 
 aim is to further improve the capabilities of sers for diverse applications 
 such as healthcare and environmental monitoring using substrates that 
 can be fabricated reproducibly and have a uniform geometry .
as the enhancement of electromagnetic fields 
 occurs in nanoscale regions , termed as plasmonic hotspots , integration 
 of microfluidics capable of directing the analyte flow and resultant 
 concentration toward these hotspots can improve the sensitivity and 
 reduce the detection time for sers . among various options ,
we explore 
 metallic nanohole arrays fabricated in suspended silicon nitride ( si3n4 ) membranes because they provide a unique option 
 to integrate sers with flow - through sensing , which has yet to be shown 
 experimentally .
nanohole arrays in thin metallic films can act 
 as plasmonic substrates 
 with multifarious applications .
enhancement of the electromagnetic 
 field and extraordinary optical transmission ( eot ) through the nanoholes 
 has led to their application in plasmonic sensing .
primarily , they have been used 
 as a platform for spr refractive - index sensing . while groups have demonstrated nanohole - enhanced 
 raman spectroscopy , the reported raman enhancement factors ( below 10 ) are weaker than that of other sers substrates .
these previous experiments 
 relied on low - throughput fabrication techniques such as focused ion 
 beam ( fib ) or electron - beam lithography to pattern nanoholes in a 
 metal film .
suspended nanohole arrays have been utilized for spr sensing , 
 but have not been utilized for sers .
flow - through suspended nanoholes 
 have been utilized for overcoming diffusion - limited transport in spr 
 sensing .
most of these methods , however , require external sources for generating 
 pressure gradients , which can often damage the fragile membrane , and 
 microfluidic tubing or an external power supply for creating an electric 
 field .
a simple and robust sample delivery mechanism is needed for 
 flow - through sensing . in this work , nanoimprint
lithography 
 ( nil ) is used to fabricate periodic 
 ag nanohole arrays over a millimeter - sized suspended silicon nitride 
 ( si3n4 ) membrane with high throughput and reproducibility . 
 to improve the raman enhancement factor ( ef ) on these large - area substrates , 
 plasmon resonances of suspended nanohole arrays
we report a high raman ef of up to 10 for silver - coated suspended nanoholes using this technique . 

furthermore , suspended nanohole arrays are capable of fast flow through 
 the nanoholes driven by surface tension forces , which promote adsorption 
 of the analytes toward the plasmonic hotspot and further boost the 
 sers signal by 50 times compared to diffusion - limited transport . using 
 this optofluidic sers substrate , we detect a 100 pm 4-mercaptopyridine 
 ( 4-mp ) sample .
suspended nanohole 
 arrays were fabricated over a region of 1 mm in size using 
 nanoimprint lithography ( nil ) , as shown in 
 figure 1a  c .
low - stress silicon nitride 
 ( 200 nm ) was deposited on both sides of single - sided polished ( 100 ) 
 silicon wafers using low - pressure chemical vapor deposition ( lpcvd ) . 

these wafers were then patterned using photolithography and dry etching 
 such that some regions on the back of the wafers had nitride removed , 
 exposing the silicon .
a nanoimprint mold was carefully placed on the 
 resist so that it aligned well with patterns etched on the back of 
 the wafer .
the mold used for nanoimprinting had pillars with a 200 
 nm diameter , 500 nm periodicity , and 300 nm pillar height over a 1 
 cm  1 cm area .
the sample was further processed to etch the nanohole 
 arrays into the top nitride layer .
the etching was done for a fixed 
 time period such that the holes go about two - thirds of the way into 
 the nitride .
the nanoimprint resist was then cleaned off , and samples 
 were placed in a koh bath for anisotropic etching .
the samples were 
 further dry - etched to remove the remaining nitride from the bottom 
 of the nanoholes and obtain suspended open - ended nanoholes .
silver 
 ( 120 nm ) was then deposited on the top of the samples using an electron - beam 
 evaporator .
( a ) a silicon chip 
 after 200 nm low - stress nitride deposition and photolithography to 
 expose desired regions on the backside .
( b ) nanoimprinting and dry 
 etching was performed to transfer the nanohole array pattern to the 
 top nitride membrane .
( c ) 
 anisotropic koh etching of silicon was used to obtain the suspended 
 nitride membrane .
a final dry etch removed the remaining nitride from 
 the bottom of the holes .
the 
 nanoimprinted region further has a 1 mm  1 mm suspended membrane 
 in the center ( circled ) .
for raman measurements of benzenethiol 
 ( bzt ) and 4-mp solutions , a quartz cuvette with a 1 mm path length 
 was used . a 785 nm diode laser with an incident power of 10 mw was 
 loosely focused onto the sample using a 10 objective .
the raman 
 scattered light was collected in transmission mode using a 50 
 objective ( na 0.5 ) and passed onto a spectrometer ( ocean optics qe65000 ) 
 through a multimode fiber after removing the excitation light with 
 a notch filter .
the acquisition time was 30 s. neat bzt ( 10 
 m ) and 0.6 m 4-mp solution were used to acquire the raman spectra . 
 for formation of a self - assembled monolayer ( sam ) , chips were placed 
 in 1 mm bzt ( eth . ) or 1 mm 4-mp ( aq . ) overnight .
they were then rinsed 
 thrice with ethanol ( bzt samples ) or deionized ( di ) water ( 4-mp samples ) 
 over a period of 30 min .
the parameters used for calculation of the 
 enhancement factor are described in the supporting 
 information .
the chips were 
 placed on a nikon eclipse 
 lv 100 upright microscope stage with a 10 objective , and a fiber - optic 
 spectrometer was used to record transmission spectra through the nanoholes . 

sucrose solutions of desired concentrations ( 01.5 m ) were 
 used to tune the transmission spectra .
data plots were prepared using matlab ( mathworks , inc . ) and graphpad 
 prism version 5.04 ( graphpad software , inc . ) .
three - dimensional ( 3d ) finite - difference 
 time - domain ( fdtd ) simulations around the suspended nanoholes were 
 performed using fullwave simulation software .
a single 
 hole was simulated with periodic boundary conditions on the faces 
 intersected by the metal film to represent an infinite hole array 
 and absorbing boundary conditions on the final two faces .
a grid size 
 of 3 nm was used around the hole in all dimensions .
the index of refraction 
 for the nitride was set at 2 , the optical constants for silver were 
 measured via ellipsometry , and the refractive indices of materials 
 surrounding the hole array were varied .
a 30 nm thick sio2 film was deposited on 
 the back of the samples using an electron - beam 
 evaporator . using this directional deposition technique ,
the sidewalls 
 of the nanoholes , which lie parallel to the direction of evaporation , 
 were coated with a much thinner layer of silica .
the desired volume 
 of aqueous solution was then added to the top surface of the chips , 
 as shown in figure 4 .
fluorescent polystyrene 
 beads were obtained from bangs laboratories ( poly(styrene/2%divinylbenzene / vinyl - cooh ) , 
 mean diameter : 2.19 m ) and were used at a final concentration 
 of 10 beads / ml in di water .
a 10 l solution containing 
 beads was added on the top of the chips for the three cases i , ii , 
 and iii , as shown in figure 5 . the samples 
 were left undisturbed until the solution was consumed through evaporation 
 or flow .
the top was then imaged using a fluorescence microscope to 
 monitor accumulation of particles . for sers measurements ,
a 4-mp solution 
 of desired concentration and volume was added to the top of the chips 
 and allowed to completely flow through .
sucrose solution was used 
 to obtain the optimized sers signal from the chips , as discussed earlier . 
 in the case of loss in the volume of sucrose from the top of the chip 
 due to flow - through ,
suspended 
 metallic nanohole arrays were fabricated on low - stress 
 silicon nitride ( si3n4 ) membranes using nil 
 ( figure 1a  c ) .
one such chip with a 
 1 cm  1 cm area of imprinted nanohole arrays is shown in figure 1d .
the suspended membrane region , approximately 
 1 mm  1 mm defined by koh wet etching of si , consisting of open - ended 
 nanoholes is also indicated . a scanning electron micrograph ( sem ) 

an sem image at higher magnification shows individual nanoholes 
 in the silicon nitride membrane ( figure 1f ) . 

the holes are approximately 200 nm in diameter , and the array has 
 a periodicity of 500 nm .
silver ( thickness 120 nm ) was deposited over 
 the chips using electron - beam evaporation .
optimization of sers 
 signal requires tuning of the plasmon resonances 
 of the metallic substrate such that , for a given laser wavelength , 
 there is maximum enhancement of electromagnetic field around nanoholes . 

generally , modification of the design of the plasmonic substrate such 
 as changing the periodicity of nanohole arrays or altering the wavelength 
 of the laser source is used for this tuning . here
, we present a simple method to achieve 
 desired plasmon resonance conditions on suspended nanohole arrays 
 using refractive - index - based tuning .
it has been established that 
 surface - plasmon - mediated transmission can be modulated by the refractive 
 index at the metal 
in fact , the shift 
 in transmission peaks of nanohole arrays due to change in refractive 
 index close to the sensor surface has been used for biosensing .
we utilized this intrinsic property of metallic nanohole arrays 
 to tune the resonance peaks . here
, we used sucrose solutions of appropriate 
 concentration on the nanohole array surface to shift the transmission 
 spectra as desired and tested the evolution of the sers signal . sugars 
 like glucose and sucrose have been reported to be very difficult to 
 detect using sers owing to their small normal cross section and negligible 
 adsorption to bare metal surfaces .
this 
 property makes them suitable to be used on a sers substrate without 
 adding unwanted raman peaks .
the schematic for sers measurement 
 and the steps for signal optimization 
 from the suspended nanohole array chips are illustrated in figure 2a .
bzt and 4-mp were used to characterize the sers 
 response of the chips as these molecules are known to have large scattering 
 cross sections and can easily form self - assembled monolayers ( sams ) 
 owing to thiol  metal bonding on gold or silver .
chips with 
 4-mp sam layers were used for the results shown in figure 2 . by increasing the concentration of the added sucrose 
 solution ,
upon addition of approximately 1 m sucrose solution , the transmission 
 peak at 600 nm corresponding to the ( 1,0 ) silver  air interface 
 was shifted to 785 nm ( figure 2b ) .
( a ) schematics showing 
 the process of obtaining a sers signal from chips with air as dielectric 
 media , resonance tuning using sucrose to improve the signal , and index 
 matching to further optimize the signal .
( b ) figure showing normalized 
 transmission maxima at around 600 nm corresponding to ( 1,0 ) metal 
air 
 interface shifts to 785 nm after addition of the desired concentration 
 of sucrose .
( c ) sers signal obtained from the same spot on the chip 
 with the transmission peak tuned to different wavelengths .
( d ) sers 
 signal obtained from the same spot before and after index matching . 

( e ) sers signal obtained from 4-mp sam layers on suspended nanohole 
 arrays as compared to fib milled dead - ended nanoholes .
the signal 
 from suspended nanoholes is about 50 times higher than that obtained 
 from dead - ended samples .
sers spectra were obtained from the sample with variation 
 in the 
 position of the transmission peak of the nanoholes ( figure 2c ) .
these measurements were taken from the same 
 spot on the sample to avoid any other sources of variation in signal . 

as expected , the maximum sers signal was obtained when the peak transmission 
 wavelength of the nanoholes matched the laser wavelength at 785 nm . 
 upon adding the same sucrose solution to the backside cavity of the 
 suspended nanohole array membrane ,
a further increase ( up to 300% ) 
 in the sers signal was observed ( figure 2d ) . 

it has been known that surface - plasmon - enhanced transmission through 
 nanoholes can be enhanced up to an order of magnitude by matching 
 the refractive index on either side of the metal film .
three - dimensional ( 3d ) finite - difference 
 time - domain ( fdtd ) simulations 
 were performed to demonstrate the increase in the plasmonic field 
 around the nanohole arrays with refractive - index - based tuning ( figure 3 ) . in the first case ,
air was used as the refractive 
 index medium and weak plasmonic hotspots can be observed at the edges 
 of the nanoholes ( figure 3a ) . with the addition 
 of sucrose and filling of the nanoholes such that the transmission 
 maxima is in resonance with the excitation wavelength ( 785 nm ) , more 
 than an order of magnitude enhancement of the plasmonic field at the 
 hotspots can be observed ( figure 3b , c ) .
a further 
 increase in electric field was obtained with sucrose present on both 
 sides of the sample as well as inside the nanoholes ( figure 3d ) .
the increase in electric field observed in these 
 fdtd simulations correlates qualitatively with the measured increase 
 in sers signal ( figure 2c , d ) . for further comparison
, 
 samples with dead - ended hole arrays were prepared using fib milling ; 
 120 nm thick silver was deposited on suspended nitride membranes and 
 nanoholes with diameter 200 nm and periodicity 500 nm were milled 
 through the silver but not through the nitride membrane .
these samples 
 were also placed in a solution of 4-mp overnight to form a sam layer . 
 to measure the sers signal from the dead - ended hole array samples , 
 a drop of sucrose solution
was added to the substrate such that the 
 transmission peak matches the laser wavelength .
the sers signal obtained 
 from an fib milled dead - ended hole array sample as compared to the 
 signal obtained from the suspended nanohole array under optimized 
 transmission conditions is shown in figure 2e .
the signal obtained from suspended nanoholes was approximately 
 50 times higher as compared to dead - ended samples ( figure 2e ) .
this difference could be attributed to a combination 
 of a number of factors , including refractive index matching between 
 the top and bottom sides of the suspended nanoholes , improved transmission , 
 and nanoscale variations in geometry of the nanohole edges ( hotspots ) 
 obtained via nanoimprinting as compared to ion milling .
three - dimensional 
 fdtd simulations for suspended nanohole arrays 
 with an excitation wavelength of 785 nm .
electric field intensity 
 with ( a ) air as the dielectric medium around and inside the suspended 
 nanoholes .
( c ) sucrose present 
 on the top silver surface and filling top half of the nanoholes , whereas 
 air is present elsewhere .
concentration of sucrose is adjusted such 
 that the transmission maxima through the nanoholes matches the excitation 
 wavelength ( 785 nm ) .
raman signals obtained from neat solutions of 4-mp and bzt 
 were 
 used to calculate enhancement factors for the chips .
the vibration 
 bands used for this calculation correspond to the in - plane ring breathing 
 mode coupled to the c  s stretching for the bzt and 4-mp samples . when bzt adsorbed to the silver surface , the 
 vibration band was observed to shift from 1092 to 1072 cm .
similarly for 4-mp , the vibration band shifted from 1115 cm in aqueous solution to 1099 cm upon adsorbing to the silver surface .
enhancement factors of 10 were obtained for the suspended nanoholes for both 4-mp and 
 benzenethiol samples .
these numbers were obtained repeatedly over 
 multiple spots on different samples using both bzt and 4-mp as analytes . 
 in comparison , previous studies on nanohole array sers have reported efs in the range 
 of 1010 .
this improvement in the ef 
 values ( 100 times ) obtained by our system confirms that suspended 
 nanohole arrays can be used as excellent sers substrates .
nanohole - based 
 biosensors with integrated flow - through fluidics 
 can provide one route to overcome diffusion - limited transport of analytes , 
 improving the detection time and detection limit of the sensor . to facilitate the flow - through process , silica ( 30 nm ) , which is 
 hydrophilic ,
as this deposition technique is highly directional , the 
 sidewalls of the nanoholes , which lie parallel to the direction of 
 evaporation , were coated with a much thinner layer of silica .
the 
 addition of the silica layer on the opposite side of sample injection 
 drastically altered the flow properties of the chip .
figure 4 shows the schematic 
 of chips with silica on the backside , and a hydrophobic top surface . 

when an aqueous solution of analyte or particles was placed on the 
 top of the chip , flow through the nanoholes was initiated as soon 
 as the solution came into contact with the hydrophilic inner walls . 

figure 4c  e shows snapshots of the flow 
 in process , where a 2 l drop of water disappears through the 
 nanohole membrane within 2 min . for membranes of size 1 mm , we obtained an approximate flow rate of 1 l / min .
the top metal surface 
 is hydrophobic , whereas the silica layer within the holes and on the 
 backside of the chip is hydrophilic .
( b ) hydrophilic nanohole sidewalls 
 suck in the solution , driving the flow toward the back of the chip 
 ( shown as red arrows ) .
( c  e ) bright - field images showing flow 
 of a 2 l drop of water through the suspended nanohole array 
 within 2 min .
the water drop shrinks as it is sucked in through the 
 hydrophilic nanohole sidewalls .
the effect of flow - through concentration as compared to diffusion - limited 
 adsorption or evaporation - driven concentration over suspended nanoholes 
 was visualized using fluorescent 2 m polystyrene beads . in 
 figure 5
, we compare 
 the accumulation of polystyrene beads ( 10 beads / ml ) in 
 three cases : ( i ) where the holes were dead - ended ; ( ii ) where the holes 
 were open and concentration was driven by evaporation through the 
 open - end of the nanoholes ; and ( iii ) holes were open and silica was 
 deposited on the backside , leading to flow and concentration .
schematics 
 for the cross section of a single nanohole accompany the fluorescence 
 images for each of the three cases in figure 5(a  c ) . during the evaporation of a liquid drop on a surface , 
 evaporation of solution from the pinned outer edges generates an outward 
 flow and concentrates particles at the edges .
this phenomenon , known 
 as the  coffee stain effect  , is clearly visible in images ( a ) and ( b ) in figure 5 .
the particles were randomly distributed within the area 
 of the drop , as shown in figure 5a , as their 
 position was determined by diffusion and the outer flow . in case ii , 
 open - ended holes with no silica also promoted evaporation - based localized 
 flow and accumulation of particles over the nanohole array was obtained . 

there was no significant difference in the time scales of evaporation 
 for cases i and ii .
for case iii ( open - ended holes with silica ) , the 
 flow was based on surface tension and the time scale was an order 
 of magnitude less than evaporation - based methods .
this case demonstrates 
 a significantly higher concentration as most of the solution was directed 
 to flow through the nanoholes , improving particle aggregation over 
 the holes ( figure 5c ) .
the absence of a prominent 
 outer ring for the drop indicates the dominance of nanohole directed 
 flow and the relatively shorter time scale of the process .
surface 
 plots revealing the intensity of fluorescence for these three cases 
 are shown in figure 5(d  f ) .
the average 
 fluorescence from the accumulated beads over the suspended membrane 
 area was calculated over three rounds of experiments and is shown 
 in figure 5 g . bead concentration and time taken for 
 consumption of solution over 
 nanohole arrays for three cases .
( a ) case i : accumulation of 2 m 
 polystyrene beads after evaporation of a 10 l drop over dead - ended 
 nanohole arrays .
the schematic shows cross section of a single dead - ended 
 nanohole for this case .
( b ) case ii : the same experiment performed 
 over suspended nanohole arrays with metal - coated ( hydrophobic ) sidewalls . 

( c ) case iii : these samples had silica deposited on 
 the back surface and sidewalls , resulting in surface - tension - directed 
 flow .
( d  f ) surface plots corresponding to ( a  c ) representing 
 the intensity of fluorescence over the suspended membrane . (
g ) average 
 fluorescence intensity calculated from beads accumulated over the 
 suspended membranes after 3 rounds of experiments .
these results demonstrate 
 that case iii surface - tension - induced flow - through promotes the most 
 effective concentration of particles over the nanoholes and is an 
 order of magnitude faster than purely evaporation - driven concentration .
these results illustrate that 
 surface - tension - induced flow through 
 nanoholes promotes the most efficient concentration of particles and 
 is at least an order of magnitude faster as compared to evaporation - based 
 techniques .
this principle can also be utilized for overcoming diffusion - limited 
 interaction of analytes with the nanoholes for plasmonic sensing . 
 for small analyte molecules ,
the passive flow generated by surface 
 tension forces drives them to pass within less than 100 nm ( radius 
 of the nanoholes ) of the hotspot , i.e. , edge of the nanoholes and 
 the nanohole sidewalls .
this short distance can be easily overcome 
 by diffusion of analyte molecules increasing the probability of the 
 analytes adsorbing on or close to the plasmonic hotspots , which can 
 lead to enhancement of the raman signal of analytes .
the 4-mp molecules , 
 which have a thiol group , can be captured on the nanohole array substrate 
 through formation of covalent thiol  silver bonds .
the molecules 
 may also get physically adsorbed to silica layered regions inside 
 the nanoholes through van der waals forces . to demonstrate the 
 advantage of flow through the nanoholes over 
 diffusion - based transport for sers sensing , we performed experiments 
 where the nanohole substrates were prepared in two different ways . 
 in the first case ,
20 l of 100 nm 4-mp solution was allowed 
 to completely flow through suspended nanoholes in about 20 min . for 
 diffusion - based adsorption ,
substrates were submerged in a reservoir 
 containing identical 4-mp solution for the same amount of time .
samples with surface - tension - induced flow gave 
 about 50 times higher signal as compared to diffusion - limited adsorption . 

the experiment was repeated over multiple sets of samples and similar 
 results were observed ( figure s3 , supporting information ) .
to demonstrate detection of low concentration samples , 50 l 
 of 4-mp at a concentration of 100 pm ( i.e. , 5 fmol of 4-mp ) was allowed 
 to flow through the sample for about an hour .
the sers signal collected 
 from this sample shown in figure 6b demonstrates 
 the capability of the method to detect low concentrations of analyte 
 molecules .
the high sensitivity in this case results from the combination 
 of two factors : large raman enhancement factor of the nanohole array 
 substrate and flow - through - based concentration of molecules at the 
 hot spot .
( a ) comparison 
 of sers spectra obtained from flow - through of 100 nm 4-mp ( red spectrum ) 
 as compared to diffusive transport ( blue ) .
( b ) sers spectra obtained 
 after flow - through of 100 pm 4-mp for an hour .
the limit of detection of the setup , in terms of 4-mp concentration , 
 would depend on the duration of time the solution is allowed to flow 
 through the nanoholes .
however , it can be estimated in terms of number 
 of molecules of 4-mp in solution . using the peak at 1099 cm in the sers signal shown in figure 6b , the 
 signal - to - noise ( s / n ) ratio was calculated to be approximately 55 . 
 assuming a linear concentration vs signal response ( figure s2 , supporting information ) , and calculating for 
 a final s / n ratio of 3 , gave us a limit of detection of approximately 
 272 amol of 4-mp molecules in solution .
we 
 have demonstrated the fabrication of millimeter - sized suspended 
 metallic nanohole arrays using nil and their potential for flow - through 
 sers .
refractive - index - based tuning with sucrose solution was implemented 
 on this suspended nanohole array substrate for optimized sers .
this 
 tuning method was utilized to match the plasmon resonance peaks with 
 the excitation laser wavelength .
sucrose solution was used because 
 it has a very weak intrinsic raman signal and , therefore , does not 
 contribute detectable background to the measurements .
this simple 
 tuning method can also be utilized for plasmon - enhanced fluorescence where the plasmon resonance can be tuned based on the excitation 
 or emission spectra of the fluorescent species . furthermore , this 
 technique can easily be translated to other plasmonic substrates utilizing 
 the principle that , by changing the refractive index of the dielectric 
 medium around the metal , plasmon resonance conditions can be changed 
 to optimize the detection of the optical signal from analyte molecules . 

the suspended ag nanohole arrays have a sers enhancement factor on 
 the order of 10 , which is 2 orders of magnitude greater 
 than those reported previously for nanohole arrays and compares well 
 with other commonly used sers substrates .
our demonstration of high 
 sers enhancement factor from nanohole arrays can open up many possibilities , 
 because the nanohole geometry enables trapping of biomolecules and 
 small particles , formation of model membranes 
 such as suspended lipid bilayers , as well as flow - through sensing to overcome the diffusion limit . to combine 
 nanohole - enhanced sers with flow - through sensing ,
we have demonstrated 
 surface - tension - induced passive flow and concentration of particles 
 over the nanohole arrays .
passive flow rates of around 1 l / min 
 were obtained using this technique .
this passive flow allowed us to 
 overcome diffusion - limited transport of analytes and reduced the time 
 required to consume the solution while bringing the analyte molecules 
 closer to the plasmonic hotspots .
our device design utilizing passive 
 flow obviates the use of a bulky external pump and fluidic interconnects , 
 thus improving the simplicity and portability of the process .
thus , 
 the simple and novel resonance tuning as well as passive flow - through 
 technique discussed here on a suspended nanohole array platform can 
 help alleviate the mass transport limitations and benefit wide applications 
 in optofluidics .
 we 
 present metallic nanohole arrays fabricated on suspended membranes 
 as an optofluidic substrate . millimeter 
 - sized suspended nanohole arrays 
 were fabricated using nanoimprint lithography . 
 we demonstrate refractive - index - based 
 tuning of the optical spectra using a sucrose solution for the optimization 
 of sers signal intensity , leading to a raman enhancement factor of 
 107 . 
 furthermore , compared to dead - ended nanohole arrays , 
 suspended nanohole arrays capable of flow - through detection increased 
 the measured sers signal intensity by 50 times . for 
 directed transport 
 of analytes , we present a novel methodology utilizing surface tension 
 to generate spontaneous flow through the nanoholes with flow rates 
 of 1 l / min , obviating the need for external pumps or microfluidic 
 interconnects . using this method for sers 
 , we obtained a 50 times 
 higher signal as compared to diffusion - limited transport and could 
 detect 100 pm 4-mercaptopyridine . 
 the suspended nanohole substrates 
 presented herein possess a uniform and reproducible geometry and show 
 the potential for improved analyte transport and sers detection . 
Introduction
Experimental Methods
Results and Discussion
Conclusion