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A simple approach for optical transparent nanochannel device prototyping Fupeng Liang, An Ju, Yi Qiao, Jing Guo, Haiqing Feng, Junji Li, Na Lu, Jing Tu, Zuhong Lu*

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Abstract Comparing with microfluidic devices, fabrication of structurecontrollable and designable nanochannel devices has been considered to have high cost and complex procedures, which requires expensive equipments and high-quality raw materials. Exploring fast, simple and inexpensive approaches in nanochannel fabrication will be greatly helpful to speed up the laboratory studies of the nanofluidics. Here we developed a simple and inexpensive approach to fabricate a nanochannel device with glass/epoxy resin/glass structure. The grooves were engraved with UV laser on an aluminum sacrificial layer on the substrate glass, and epoxy resin was coated on the substrate and stuffed fully into the grooves. Another glass plate with holes of fluidic inlets and outlets was bonded on the top of the resin layer. The nanochannels were formed by etching the thin sacrificial layers electrochemically. Meanwhile, the microstructures of fluidic outlets and inlets could be simultaneously fabricated on the nanochannel. The total processing time for simple nanochannel device took less than 10 hours. The optical transparent nanochannels with the depth down to 20nm were achieved. The nanofluidic behaviors in the nanochannels were observed under both optical and fluorescent microscopes. KEYWORDS: Nanofluidics, Nanochannel, Optical Transparency, Epoxy Resin, Sacrificial Layer, Electrochemical Etching

1. Introduction One-dimensional nanochannel devices are nanofluidic channels with the size less than 100 nm1, 2 on one dimension, which attracted attentions3 in many fields, such as single molecule manipulation4, 5 , biological sensing and detection6-8 , nanoscale bioreactions 9 , purification and separation in biomaterials10-12 , nanofluidic studies of liquid or gas13, and nanofluidic studies of ion14. Since nanofluidic behaviors are observed under optical microscopes, it is essential to develop completely optical transparent nanochannel devices for both fundamental studies and practical applications15. Nanochannel devices can be fabricated by various different methods, such as direct etching, molding, self-sealing, sacrificial layer, nanometer step, and direct stretching et al. Direct etching16 is the most widely used method, which makes nanostructures firstly on the surface of a substrate by bulk lithographic technology, and then bonds a cover plate on the top of the substrate to form the nanochannels or nano-gaps. The method of molding mainly includes soft-lithographical methods17,such as nanoimprinting18-20 and polymer inking

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TECHNICAL INNOVATION

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process21, 22. Both direct etching and molding can be used to fabricate designed nanostructures in a well-controlled manner and are suitable for a large scale production. But such methods require precise bonding of two ultra-flat nano-scale surfaces under super-clear environment in order to form aligned and enclosed internal channels.The nanochannel devices fabricated by soft-lithographical methods could not endure the liquid pressure driven by high-pressure method. Nanometer step23, direct stretching24, self-sealing25, 26, and nanotube methods27 take advantages of the internal features of specific materials, and fabricate nanochannels directly. However, due to the limitations of the material’s structure, the size and shape of the fabricated nanochannels are not easy to design and control. Most materials for those nanochannels are opaque and will limit the optical observation in nanofluidic detection. Sacrificial layer methods28, 29 pre-fabricate removable nanostructures inside the devices and form nanochannels by removing sacrificial structures. These sacrificial layer methods can easily design and control the size and shape of nanochannels without the requirements of highly precise bonding process. However, the nanochannels made in this way are usually too fragile to endure the high pressure of the nanofluidic flow and easy to collapse due to the change of the internal stress after the removal of sacrificial layer. High-pressure is used in common nanofluidic applications and some special applications such as high-pressure nanochromatography30 . Both nano- and microfluidic structures are usually involved in a practical nanochannel device. How to fabricate the interface between the two kind structures simultaneously is another bottleneck in nanochannel fabrication, which limits the laboratory studies of nanofluidic systems. The conventional methods normally construct the nanochannel structures at first and then connect micro-scale structures to them. For example, it is difficult to simultaneously fabricate inlet and outlet holes with nanochannels in sacrificial layer methods. More complicated fabricating steps are required after the formation of nanochannels. Comparing with the microfluidic device, fabrication of structure-controllable and designable nanochannel device is considered to have high cost and complex procedures1, which requires expensive equipments31 and high-quality raw materials. Therefore, exploring fast, simple and inexpensive approaches in nanochannel fabrication will be greatly helpful to speed up the laboratory studies of the nanofluidics. Here we developed a new method to fabricate the completely transparent nanochannel device prototyping with the structure of substrate glass/cured epoxy resin/cover glass. An aluminum nanofilm was evaporated on the substrate glass plate and lithographed to form sacrificial layer according to the designed nanochannel by UV laser. The deep grooves around the sacrificial layer were engraved by UV laser through the aluminum layer into the substrate glass. The epoxy resin was coated on the top of sacrificial layer, and stuffed fully into the grooves on the boundaries of the sacrificial layer. The cover glass plate with inlet and outlet holes was bonded on the top surface of the epoxy resin layer, which also acted as a supporting structure of the prototyping to protect structures of nanochannels from collapse after removing the sacrificial layers. The resin in the holes were cleaned to expose the sacrificial layer outside device. Electrode leads were mounted into some inlet or outlet holes and electrically connected to the aluminum sacrificial layer. The sacrificial layer can be removed by electrolyzing process to form the nanochannels. This nanochannel device has a firm structure with optical transparence. The inlets and outlets of the nanofluidic device can be fabricated simultaneously during nanochannel formation.

The fabrication process can be done with conventional glass slides in a relatively simple condition, such as ordinary chemistry laboratory.

Fig. 1 Schematic of the fabrication process of the nanochannel device. Aluminum film was coated on a substrate glass by vacuum evaporation with the thickness of 20-200 nm (a→b); UV laser beam was used to engrave the boundary of the nanochannel patterns (b→c), and sacrificial layer (the middle bright structure in c ) was formed; Epoxy resin filled into the grooves and coated the sacrificial layer(c→d); Cover glass plate was bonded onto the epoxy resin (e); The resins were removed from the holes of the cover glass (f); Electric wire was fixed in one of the hole with conductive resin to connect sacrificial layer (g); The sacrificial layer was etched through electrochemical reactions and the wire was removed after electrochemical etching (h) ; Fluidic pipes for the inlet and outlet of the device were connected and fixed with UV adhesive (i and j), and another glass plate could be bonded on the top to fasten the pipes (k).

2. Methods and materials 2.1 Fabrication scheme The technological process of the fabrication for the nanochannel device can be simply described in Fig. 1. Aluminum film in a nanometer scale was evaporated on a substrate glass plate. The profile of the sacrificial layer were made by UV laser engraving. The whole engraving process of a single straight nanochannel took less than 20 min, which can be shown in PartⅠ of Video S1 in the supporting information. The substrate was pretreated with silane coupling agent. Twocomponent epoxy resin covered the engraved aluminum layer, and the middle aluminum sacrificial region for electrochemical etching was separated from the peripheral sacrificial layer by the resin which stuffed into the grooves. A cover glass plate with pre-drilled holes as inlets and outlets of fluids was bonded on the epoxy resin layer to form the glass/resin/glass structure. The resin in the holes of the cover plate was removed afterwards. The structure was solidified by heating the device, and electric wire was mounted on some holes of the cover plate and connected to the sacrificial layer electrically by conductive resin. Other holes remained empty as etching ones for

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electrolyte penetration to the aluminum sacrificial layer. The sacrificial layer was removed by electrochemical reactions to form the nanochannels and the electric wire in glass hole was removed after the electrochemical etching process. Finally, stainless steel pipes were mounted into the holes as fluidic inlets and outlets of the device with UV adhesive. An additional glass plate was bonded to the cover plate to reinforce the pipes, since the inlets or outlets should be strong enough to endure the high driving pressure of the fluid through nanochannel. The animation of the whole fabrication process can be shown in Part Ⅱ of Video S1 in the supporting information. The cross-section structure of the whole device under Scanning Electron Microscope (SEM) is given in Fig. S1 in the supporting information, which clearly showed the structures of the substrate glass, the nanochannel, the engraved grooves, the epoxy resin layer, and the cover glass. 2.2 Preparation of the substrate plate

cleaned with ethyl alcohol. The epoxy resin was cured for 120 minutes at 65℃ and then 30 minutes at 115℃. 2.5 Making electric conducting holes Conductive epoxy adhesive (Model Cu-211, Erbond Trading Co., Shanghai, China) was stuffed in part of the holes on the cover plate, and punctured by pins to make it fully electrically contacted to the aluminum sacrificial layer. Electric wires were inserted into the holes and completely contacted to the conductive epoxy adhesive. Then the conductive epoxy adhesive was cured for 5 min at 110℃. The cured conductive epoxy adhesive and the wires were packaged with epoxy resin. The remaining holes of the cover plate were for etching, through which the aluminum layer would be electrolyzed and removed out during the electrolyzing etching process. 2.6 Electrolyzing etching process

A glass slide was cut into the required shape. Holes were drilled on it as inlets and outlets of the designed nanochannel device by the CNC engraving machine (Model JH3030, Jinheng CNC Equipment Co., Shanghai, China). The locations of the throughholes were related to the position of the sacrificial layer on the substrate plate, which were used as the electrolyte infusing holes or electric connection holes for electrolyzing corrosion of aluminum sacrificial layer.

The sacrificial layers were removed through the etching holes by the electrochemical etching process. The packed device with open etching holes was immersed into the aqueous electrolyte of electrochemical etching system. The electrolyte replenished the holes and contacted the aluminum sacrificial layer. The electrochemical etching system was made of three cavities separated by a piece of anion-exchange membrane and a piece of cation-exchange membrane respectively, shown as Fig. 4. One cavity contained hydrochloric acid solution with the concentration of 1:4 (v/v, 36% hydrochloric acid: deionized water = 1:4), while others contained deionized water. The sacrificial layer acted as anode, while two graphite electrodes acted as cathodes. The applied voltage is about 1.5~2V in the process of electrochemical etching. The aluminum was dissolved and drawn out from nanochannel gradually. The etching process could be shown in the video S3 in the supporting information. After the aluminum sacrificial layer was removed, the solid conductive resin with the electric wires could be pulled out, making the holes connected directly to nanochannels. The structure of nanochannel was observed by field emission scanning electron microscope (Model Ultra Plus, Carl Zeiss Microscopy GmbH, Germany).

2.4 Packaging devices

2.7 Mounting pipes to the inlets and outlets

A ordinary glass slide was immersed into the ethyl alcohol and cleaned for 30 minutes by the ultrasonic cleaner, and then rinsed for 1 minute using deionized water. The slide was blowdried with nitrogen and put in the drying oven for 10 minutes at 70℃. Aluminum film was coated on the glass slide by vacuum evaporation coating machine (Model ZZS500-3/G, Nanguang Machinery Co., Chengdu, China). The boundary of the sacrificial aluminum regions for nanochannel was engraved by UV laser with the wavelength of 355nm and output power of 2W (Model SK-UV-3,Sanke Laser Technology Co., Shanghai, China), with grooves deeply down into the glass substrate. 2.3 Preparation of the cover plate

The substrate plate and cover plate were treated with O2 plasma Stainless steel capillary were mounted into the holes on the with output power of 100 W for 30 s (Model WH-1000Z, cover plate as the fluid pipes of nanochannel device. In order to Suzhou Wenhao Chip Technology Co., Jiangsu, China), and further strengthen the fluid pipes, another glass slide was immersed in a 1 wt% isopropyl alcohol solution of 3- bonded to the cover glass to reinforce the pipes with UV glycidoxypropyltrimethoxysilane (KH560) (KH560: isopropyl adhesive (Model 7104LV, One-Power New Materials alcohol: H2O: acetic acid=1: 88.8: 10: 0.2) for 30 min in order Technology Co., Suzhou, China), shown as Fig. S5 in the to graft epoxy-silane onto the surfaces of the substrate and supporting information. The nanochannel could be connected to cover plates. Unreacted KH560 on the surfaces was washed high-pressure pump or other driving source by stainless steel away with deionized water. The substrate and cover plates were capillary. But mounting stainless steel capillary is an optional heated 15 min at 135℃ in order to form epoxy-silane films on step, if high-pressure driving method is not used in the experiment. Instead, electric field can be applied between inlets the surfaces of substrate and cover plates. The mixing ratio of component A and component B of the and outlets to drive the electric matter in the solution. transparent epoxy resin (Model GOET-1080, Tonglian 2.8 Observation of the nanofluidic flow Hengxing Technology Co., Beijing, China) was 2:1 by weight. The epoxy resin spread evenly on the engraved substrate plate. The nanofluidic behaviors were observed in the nanochannel The substrate was then placed in the vacuum thermal devices with optical inverted microscope (Model IX2-UCB, evaporator for 5 min under negative-pressure of 30 mmHg at 50℃ Olympus Corp, Japan) with CCD camera (Model GO-3-CLRto drive out the air from the epoxy resin on the substrate. The 10,QImaging Corporation, Surrey, BC, Canada). Aqueous cover plate was aligned and bonded to the substrate plate for 1 nanofluid labeled with fluorescent tracer was observed with min at 60℃. The pressure was required to be less than 50 KPa laser scanning confocal microscope system (Model Andor for flattening the epoxy resin and driving out the air between Revolution XD, Andor Technology Ltd, UK). Fluorescein the cover and substrate plates. The liquid epoxy resin was sodium salt (Model F6377-100G, Sigma-Aldrich Chemie removed from the holes of the cover plate by syringe and Gmbh, Munich, Germany) was used as fluorescent tracer to indicate the flow behaviors in nanochannel.

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A nanochannel device was fabricated with the nanochannel 20 nm in depth, 180µm in width, and 400µm in length, which was connected to microchannels with the depth of 1.5 µm. Two kinds of aluminum layers with different thickness were evaporated to the surface of an ordinary glass slide. 20 nmthick sacrificial layers (the bright region of the top photograph in Fig. 2A) were fabricated for nanochannels, and 1.5 µm-thick sacrificial layers (the dark regions of the top photograph in Fig. 2A) were fabricated for the microchannels which connect the nanochannels to the inlets or outlets of the device. The device was formed after bonding the cover glass plate. The sacrificial layer (the bright region in Fig. 2B) was retained between the substrate glass slide and epoxy resin, and 4 holes on the cover glass could be observed in Fig. 2B. Two pairs of microchannels with inlets and outlets were connected by a nano-scale channel. Electric wires were mounted into one pair of holes of the cover glass to connect the aluminum layer, and another pair of holes remained empty to let electrolyte contact the aluminum layer which acted as anode of electrochemical etching (one pair of graphite electrodes in the electrochemical etching system acted as the cathode of the etching process). The bright aluminum layer was removed and the nanochannel in the middle of the pattern was formed, shown in Fig. 2C.

Fig. 2 Photos of different processing stages of nanochannel device. (A) Aluminum films with nano-thickness and microthickness regions coated on a glass slide. (B) The nanochannel device before etching. The enlarged picture on the top right corner is the nano-thickness region under optical microscope. (C) The nanochannel device after etching.

The depth of the nanochannel could be observed in crosssection of the device with the scanning electron microscopy(SEM). In Fig. 3A, the 20nm aluminum layer between the glass (bottom) and the epoxy resin (top) could be seen. The dark gap in Fig. 3B was the nanochannel after etching. The depth of the nanochannel was consistent with the thickness of the aluminum layer on the substrate which can be designed according to specific requirements. We also made several nanochannel devices with different depths. SEM pictures of the nanochannel devices with the depth of 30, 40, 160 and 300 nm could be shown in Fig. S2 in supporting information.

Fig. 3 SEM pictures of the nanochannel’s cross-section with the depth of 20 nm. The middle structures were sacrificial layer before etching in (A) and nanochannel after etching in (B). The

bottom structures were substrate glass and the top structures were epoxy resin layers in both pictures. 3.1 Effect of the experimental conditions In our fabrication process, especially the laser engraving process, we did not control the experimental environment strictly. The dust particles would not influence the nanochannels of the device, since the liquid epoxy resin could infiltrate the surface of the sacrificial aluminum and clean the surface automatically by wrapping the dust particles into the bulk epoxy resin. We observed the images of scattering particles in resin by optical microscope (Fig. S3 A and B in the supporting information). But obviously, those particles did not affect the fluid flow in the nanochannel device. The SEM picture of the cross-section of nanochannel (Fig. S3 C in the supporting information) showed that the dust particles appeared in the bulk resin, but did not remain on the channel surface and affect the function of nanochannel device. The fluidic epoxy resin could self-clean the dust particles from the surface of the aluminum sacrificial layer, which will greatly reduce the requirements of clear environment (e.g. ultra-clean lab). 3.2 Effect of the smoothness of the glass slide We used the ordinary microscope glass slides for the nanochannel device. The planeness on the slide is about 20µm. In our experiments, the smoothness of the surface of the glass slide would not affect nanochannel-forming process, because the aluminum layer was uniformly generated by evaporation and fit perfectly with the curve of the substrate slide. The liquid epoxy resin could package the sacrificed layer completely. Unlike the requirement of ultra-flat surfaces in solid phase bonding of conventional nanofabrication, the nanochannel of cured epoxy resin could be accurately shaped by sacrificial aluminum layer even if there was slight roughness on the surface of the slide. 3.3 Strength of the nanochannel structure The nanochannel was simply composed of the substrate glass slide and cured epoxy resin with hardness of Shore D85. The structure of nanochannel was highly stable, and did not collapse mainly due to the strong solid device structure with high adhesion and hardness of the cured epoxy resin fixed between the substrate and cover glass plates. The resin adhered to the substrate glass by infiltrating into the engraved grooves inside the glass. The thickness of the epoxy resin layer between the substrate and cover glass slides varied from 3 to 40 µm, depending on the temperature and the pressure applied to cover glass during the bonding process. The boundaries of the nanochannel remained intact under optical microscope when a pressure of 12MPa was applied by pumping fluid into the nanochannel through the inlet (with the outlet blocked by epoxy resin). The results of the destruction tests for 10 devices were shown in Fig. S4 in the supporting information with destruction pressures varying from 12.5 to 16.0 MPa. Only the inlet regions of the devices were damaged due to their relatively larger structures with the diameter of 1.2mm, while the nanochannel structure still remained intact. The epoxy resin used in this work had a linear shrinking rate of less than 1% in vertical direction to glass surface. The shrinkage would spontaneously change the distance between the substrate and the cover plates, but would not influence the size and shape of the nanochannel, since it was formed after the epoxy’s solidification by corrosion of the sacrificial layer, which could be proved by the SEM pictures of Fig. 3. The

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3. Results and discussion

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△P LP=4

 



≈ 5515.5kPa=5.5155MPa

(1)

where γ is the surface tension between gas and water (~72 mN m-1), θ is the contact angle (~64o for the epoxy resin surface of the nanochannel, and ~40o for the glass surface of the nanochannel), and d is the diameter of a circular cross section. (in our case, a hydraulic diameter DH can be assumed as32 DH=2wh/(w+h)=2×180µm×20 nm/(180µm+20nm)≈40nm for each rectangular cross-sectional nanochannel with w as width and h as height). According to the Bending of a Beam Method(a kind of testing method of Young's modules): 

E=



of the electrochemical etching was much faster than the ordinary chemical etching. In our experiments, we spent 2 days etching 500µm length of the aluminum sacrificial layer of 20nm thickness with the ordinary chemical etching method, while it only took less than 10 min to remove the same sacrificial layer with the electrochemical etching method. The electrochemical etching process was shown in the Video S3 in the supporting information.

(2)

λ ≈ 0.02nm,  where λ represents the deflection, F represents the pressure force generated by capillary pressure at the region of air-liquid meniscus, l represents the width of the nanochannel(180μm), E represents Young's modules of the ordinary glass(71.7GPa), a represents the width of the region of air-liquid meniscus, h represents the thickness of the glass slides(1.2mm). The deformation of the glass at the region of air-liquid meniscus can be estimated as 0.02nm. Therefore, our device simply solved the problem of the nanochannel collapse with the high Young's modules supporting material.

Fig. 4 The electrochemical etching system. (A) Schematic of the system strucuture. (B) Photo of the practical system. In the system, the aluminum sacrificial layer acted as anode, and two cathodes of graphite electrodes were isolated by an anion exchange membrane and a cation-exchange membrane respectively. Hydrochloric acid was used as the electrolyte in the cathode region with anion-exchange membrane. The deionized water was used as the electrolyte in the anode region and the cathode region with cation-exchange membrane.

3.4 Electrochemical etching process of the sacrificial layer There were several reports on the electrochemical etching process for removing sacrificial layer made of various kinds of metal, such as aluminum28 and chromium/gold29. Chromium sacrificial layer29 had much low etching rate due to its relatively low activity. Eijkel et al used an ordinary chemical etching method to form a polyimide nanochannel with aluminum sacrificial layer28. The device with the sacrificial layer was immersed into a standard aluminum etchant, and the Al3+ ions diffused away through the polyimide layer and two ends of the channel. In our device, solid and dense material was used to form the nanochannel. It was impossible for aluminum ions to penetrate across the epoxy resin layer and the glass, so we had to remove Al3+ ions through the nanochannel itself. We chose the electrochemical etching method, that is, the aluminum sacrificial layer acted as anodes and contacted to the electrochemical etching solution through the empty holes of the cover glass. Two cathodes of graphite electrodes were used in our electrochemical etching system, and were separated by an anion exchange membrane and a cation-exchange membrane respectively. The schematic structure of the electrochemical etching system was shown in Fig. 4. Hydrochloric acid was used as the electrolyte in the cathode region with anionexchange membrane, which prevented the H+ ions of hydrochloric acid from reaching the anodes of sacrificial layer and generating hydrogen bobbles by ordinary chemical reaction. The deionized water was used as the electrolyte in the other regions, including the cathode region with cation-exchange membrane and the anode region. The anode aluminum lost electrons and generated Al3+ ions into electrolyte, diffused away through the empty holes of the cover glass, reached and accumulated at the cathode region separated by anion-exchange membrane. From Fig. 5, the speed

Fig. 5 Comparison of the electrochemical etching and ordinary chemical etching of aluminium sacrificial layer of our nanochannels. The curves indicated the relationship between etching length and etching speed. The red curve stood for ordinary chemical etching, and the blue curve for electrochemical etching. The top right photos were taken by optical microscope during the etching process. The red curve corresponded to photo (a), ordinary chemical etching using hydrochloric acid or aluminum etchant (H3PO4 : HNO3 : CH3COOH = 60-80 %: 1-5 % : 5-15%), etching deficiently. The blue curve corresponded to photo (b) , the electrochemical etching (the etching voltage was 1.5~2V) with rapid etching speed and clear boundary. The thickness and width of the aluminium film were 20nm and 200µm.

3.5 Water evaporation in the nanochannel device

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cover glass adhered to epoxy resin could also support the nanochannel structure. The air-liquid meniscus is formed in the nanochannel and the capillary pressure can be estimated as follows 32:

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The nanochannel in depth of 20nm was filled with water firstly, and all the outlet or inlet holes of the device were open to the air. Thus the water in the nanochannel could evaporate out at room temperature under 1 atm. The fast movement of the boundary line between the water and the air could be observed under the optical microscopy, shown in Fig. 6 and Part Ⅰ of video S2 in the supporting information. The bright area stood for water region, while the gray area for the air region. The small bright dots in the background were the dust particles buried in the bulk resin body. The whole evaporation process took several minutes. Fig. 7 Observation of the deionized water flowing through the nanochannel under optical microscope. The water was driven by capillary force. The nanochannel had the depth of 20 nm and the width of 140µm. The air firstly filled the channel and was then driven out by the water flow.The numbers on the top-left stood for the time series during the experiment.

Fig. 6 Observation of water evaporation process in the nanochannel with the depth of 20nm and the width of 140µm under optical microscope at room temperature and 1 atm. The numbers on the top-left stood for the time series during the experiment.

3.6 Capillary effect of liquid flow in the nanochannel

Furthermore, we observed the capillary flow labeled with fluorescein sodium salt in nanochannel by the laser scanning confocal microscope system, which was shown in Fig. 8 and Part Ⅲ of video S2 in the supporting information. The fluorescence signal was rather weaker at the nanochannel region than the microchannel region, because the depth of microchannel was about 75 times more than that of the nanochannel. The experimental results above showed that the aqueous solution could easily flow through the nanochannel devices. The work provides a practical research platform of nanofluidics.

The nanochannel device was dried for 2 hours at room temperature. Then we wetted the inlet holes with a small amount of water, and observed the fluidic flow through the nanochannel under optical microscope, shown in Fig. 7 and Part Ⅱ of video S2 in the supporting information. The same as Fig. 6, the bright area stood for the water-occupied region and the gray area for the air region. The water was driven by capillary force, and the flow speed at nanochannel was estimated to 2 mm/s according to the Part Ⅱ of video S2 in the supporting information. According to the modified Washburn's equation in the nanochannel 33, the position shift of the moving meniscus x is a function of time t: 

   

(3)

where θ is the contact angle of the water to the channel glass walls(θ =40°), h the channel height(h =20nm), γ the surface tension of the water in air(γ≈72mN/m), and μ the viscosity of the water at room temperature(µ=1.0050mPa·s). In Fig.7,we can get that the air-water meniscus took 120ms to move about 200μm in the nanochannel. From theoretical calculation with the equation (3),we obtain that the air-water meniscus can move 209μm in 120ms,which is close to our experimental observation.

Fig. 8 Observation of the 10-4mol/L aqueous solution of fluorescein sodium salt flowing through the nanochannel between two microchannels under laser scanning confocal microscope system. The nanofluidic flow was driven by capillary force. The depth and the width of the nanochannel were 20nm and 180µm, respectively. The depth and the width of the microchannel were 1.5µm and 140µm, respectively.

3.7 The perm-selectivity of the nanochannel device We had tested the I-V curve by picoammeter/voltage source (Keithley 6487, Keithley Instruments Inc., Ohio, USA), The ohmiclimiting-overlimiting current response has been obtained, and the typical I-V curve was shown in the Fig. 9, which indicated the permselectivity of the nanochannel device. The drifting of limiting region was found under the present experimental conditions in our lab. The

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current declined holistically with the increasing of experiment times, which could be due to changing the surface states of the glass and the epoxy resin of which the nanochannel was made up. The further investigations should be carried out in the future.

and Dr. Hui Jiang were acknowledged with their kind helps in the experiments of testing I-V curve.

One Word file and three Video files have been added as the supporting information within this submission.

Corresponding Author Professor Zuhong Lu e-mail: [email protected] State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing, 210096, China.

Author Contributions Fig. 9 Ohmic-limiting-overlimiting current measured by sweep voltage from 0V to 40V in the nanochannel with the depth of 20nm and the width of 180µm. The nanochannel was placed between two microchannels with the depth of 1.5µm and the width of 140µm.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

References 1. 2. 3.

4. Conclusions We have successfully developed a simple method to fabricate the prototyping of nanochannel device, in which the flow of aqueous solution can be driven by capillary force. The device is made by three-layer structure of ordinary substrate glass /cured epoxy resin/cover glass, in which the nanofluidic flow can be easily studied under optical observation. The nanochannel was formed between the surface of glass substrate and cured epoxy resin by electrochemical etching of the aluminum sacrificial layer. There are many remarkable advantages of this nanochannel device. Firstly, the optical transparency enables researchers to directly observe the nanofluidic behaviors in various nanofluidic studies. Secondly, the robust and stable structures could endure the high driving force due to the high flow resistance in the conventional nanochannels and prevent structural collapse. Moreover, low cost of raw materials and simple equipments, and relatively low environmental requirements make the fabrication of nanochannel devices no longer difficult in ordinary laboratories. The sacrificial layer can also be simply engraved in laboratory with a graver by hand, instead of UV laser machine station. The method would be important to popularize nanofluidics studies as like the PDMS technology for microfluidics34, 35 . For more rigorous precision requirements, the lithographic technology can replace the UV laser engraving process, which can make the width of the nanochannel much smaller.

Acknowledgements

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National Natural Science Foundation of China No. 61227803 and No. 61571121. Prof. Zhongdang Xiao and Mr. Ke Yang were acknowledged with their kind helps in the experiments of laser scanning confocal microscope system. Prof. Qunjun Liu

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A simple approach for an optically transparent nanochannel device prototype.

Compared with microfluidic devices, the fabrication of structure-controllable and designable nanochannel devices has been considered to have high cost...
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