Simple and reusable picoinjector for liquid delivery via nanofluidics approach
© Li et al.; licensee Springer. 2014
Received: 12 February 2014
Accepted: 14 March 2014
Published: 25 March 2014
Precise control of sample volume is one of the most important functions in lab-on-a-chip (LOC) systems, especially for chemical and biological reactions. The common approach used for liquid delivery involves the employment of capillaries and microstructures for generating a droplet which has a volume in the nanoliter or picoliter range. Here, we report a novel approach for constructing a picoinjector which is based on well-controlled electroosmotic (EO) flow to electrokinetically drive sample solutions. This picoinjector comprises an array of interconnected nanochannels for liquid delivery. Such technique for liquid delivery has the advantages of well-controlled sample volume and reusable nanofluidic chip, and it was reported for the first time. In the study of the pumping process for this picoinjector, the EO flow rate was determined by the intensity of the fluorescent probe. The influence of ion concentration in electrolyte solutions over the EO flow rate was also investigated and discussed. The application of this EO-driven picoinjector for chemical reactions was demonstrated by the reaction between Fluo-4 and calcium chloride with the reaction cycle controlled by the applied square waves of different duty cycles. The precision of our device can reach down to picoliter per second, which is much smaller than that of most existing technologies. This new approach, thus, opens further possibilities of adopting nanofluidics for well-controlled chemical reactions with particular applications in nanoparticle synthesis, bimolecular synthesis, drug delivery, and diagnostic testing.
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Precise control of the sample volume is the first prerequisite in high-resolution micro total analysis systems (μTAS) and microreactors[1–3]. Nanopipettes and picoinjectors are major ways to achieve this aim. However, the existing techniques utilizing either carbon nanotubes or electromicrofluidics are cumbersome to fabricate and difficult to operate. Chen et al. developed a nanoinjector based on atomic force microscopy (AFM). This technique is limited by the throughput and difficulty in control of liquid volume. Seger et al. demonstrated single-cell surgery by a nanopipette. It is applied to penetrate the cell membrane by mechanical force. Sometimes, one has to adjust the surrounding medium outside of cells for biochemical reactions. The embedded pumps are regarded as portable and stand-alone systems for this application. Yokokawa et al. invented an on-chip syringe pump for picoliter liquid manipulation by integrating sliders of an electrostatically controlled linear inchworm actuator made by a piezoelectric material. However, the drawback of the on-chip syringe pump is the complex fabrication method involving a multistructured MEMS procedure. Unlike traditional micropipette injection and on-chip syringe pump methods which rely on pressure differences, we proposed direct delivery of liquid using an electrical signal in μTAS. This is another novel approach for constructing a picoinjector with high precision and without mechanical movements. This technique is based on the fact that fluid and nanoparticles have interesting properties in nanoscaled pores or channels[9, 10]. It is due to the large effect of the electrical double layer which is comparable to the pore or channel size. Electrokinetic phenomena[11, 12] generated by an electric field in microscopic scale are most promising to deliver pure water, pure polar organic solvents, inorganic buffer, and bio-macromolecules in nanochannels. Electroosmotic pumps, based on electrokinetics and operated with no moving part, are a better way for liquid delivery since they are much easier to integrate in μTAS than the piezoelectric method. They are driven by electroosmosis (EO) which arises from the existence of an electrical double layer at the solid-liquid interface and holds great promise in generating fluid flow in nanochannels under the influence of an electric field. Transport of analytes in nanochannels has been well studied by Pennathur and Santiago, and the concept can be conveniently adopted in our picoinjector. The electroosmosis-based picoinjector possesses an array of one-dimensional (1D) nanochannels for precise fluid transfer under the condition of applying the controlling signal. Potential applications based on this picoinjector include precisely controlled chemical reactions, drug delivery, as well as biomolecular translocation. All of these applications are based on the variation of the applied voltage bias across nanopores or nanochannels.
In this paper, we reported a new approach of a picoinjector by means of 1D nanochannels which offers precise control of solution volume on the scale of picoliter. The injection rate or pumping rate was determined by measuring the fluorescent intensity subsequent to the injection of the fluorescent solution into the connected microchannel. Solutions of different ion concentrations were also utilized for simulating various scenarios. Moreover, microreaction between Fluo-4 and calcium ions was successfully demonstrated by our picoinjector to show the capability of our device in terms of its controllability of chemical reaction in a continuous phase.
where 1/k is the Debye length.
It is concluded that the ion concentration in the filled solution will affect the EOF velocity by altering the zeta potential of EDL as suggested by Equations 1 and 2. A higher ion concentration of the solution results in lower EOF velocity due to the larger capability to balance the negative charges at the channel wall, and thus, the EDL will be narrowed. This character of variation of EDL can also be expressed by the Debye length which is closely related to the zeta potential as seen in Equation 3. A larger Debye length means a higher zeta potential of EDL and larger EOF velocity. It was reported that the Debye length of silica filled with a 10 μM monovalent ion solution was 100 nm, compared to 0.3 nm when silica was immersed in a 1 M monovalent ion solution.
A two-step deep reactive ion etching (DRIE) was performed to achieve a microreactor chip containing a picoinjector based on a 1D nanochannel. The first step of DRIE was conducted to fabricate the 1D nanochannel junction for liquid delivery. Then, the second step of DRIE was conducted to form microchannels for the chemical reaction. Conventional photolithography and photoresist stripping processes were employed to construct channels with the desired depth. A silicon (Si) wafer was cleaned in H2SO4:H2O2 solution (volume ratio of 10:1) at 120°C for 10 min, followed by deionized water (DI) for 4 cycles, then HF:H2O solution (1:50) at 22°C for 1 min and DI water for 4 cycles, and finally spin-dried in hot N2 gas for 15 min. Then, the Si wafer was processed by hexamethyldisiloxane (HMDS) coating and positive photoresist HPR 504 spin-coated at 4,000 rpm for 30 s. The wafer was soft-baked on a hot plate at 110°C for 60 s before exposing to UV via the Mask Aligner (SUSS Microtec MA6-2, Garching Germany) for 5 s. The photoresist was developed using FHD-5 for 60 s and post-baked on a hot plate at 120°C for 60 s. The micropatterns were successfully defined at this stage. The Si wafer was then etched by a DRIE machine (Surface Technology Systems, Newport, UK) and followed by photoresist stripping in PS210 Photoresist Asher (PVA Tepla AG, Kirchheim, Germany) for 25 min. After constructing the microchannels, 10 nm of thermal oxide was grown using a diffusion furnace to form silica on the channel wall.
After drilling the inlets and outlets on the Si chip by a mechanical driller, the chip has to be sealed to form a closed channel. A thin film of polydimethylsiloxane (PDMS) was applied for such purpose due to the good adhesion between PDMS and the Si chip. PDMS was formulated from Sylgard 184 silicone elastomer mixture (Dow Corning Corporation, Midland, MI, USA) at a weight ratio of base:curing agent = 10:1. Then, it was poured onto a Si wafer with saline coating on the surface and pressed against a cleaned glass slide. After curing PDMS in an oven at 60°C for 2 h, the microchip was constructed by pressing the Si chip against the glass slide with the thin layer of PDMS on its surface.
Materials and methods
A fluorescent dye solution was used in our experiment for the determination of the pumping rate from one microchannel to another. A pH 7.0 phosphate buffer solution (PBS) with a K2HPO4 concentration of 27.5 mM and a KH2PO4 concentration of 20.0 mM was prepared as the standard solution since many biochemical reactions are conducted in this buffer solution. Then, analyte solutions with specific ion concentrations were prepared by diluting the standard PBS. The dilution of the standard PBS is denoted by ‘a × PBS,’ where ‘1/a’ denotes the dilution factor, e.g., ‘0.1× PBS’ stands for a dilution of 10×, while 1× PBS stands for the standard solution concentration. Fluorescein isothiocyanate isomer I (FITC) (Sigma-Aldrich Co., St. Louis, MO, USA) with a concentration of 50 nM was dissolved in the solutions for visualization. To demonstrate the controlled chemical reaction using our device, the binding reaction between Fluo-4 and calcium chloride was performed. Fluo-4 (Invitrogen, Carlsbad, CA, USA) solution was prepared by dissolving the Fluo-4 powder in DI water to obtain a final concentration of 10.8 μM, while calcium chloride solution was prepared with a concentration of 5 mM.
The square waves were generated by a direct current (DC) power supply (HP Hewlett Packard 6653A, Palo Alto, CA, USA) which supplied an output voltage of 0 to 35 V, with the duty cycle controlled by LabVIEW (version 8.2, National Instruments, Austin, TX, USA). The dynamic process of the fluidic flow was monitored using an inverted optical microscope (Olympus IX71, Tokyo, Japan), and the motion was recorded by a charge-coupled device (CCD) camera (Olympus DP73, Tokyo, Japan). The exposure time was fixed at 200 ms, the magnification was set at × 6.4, and the acquired image size was 2,400 × 1,800 pixels.
Results and discussion
Calibration of fluorescent intensity as a function of dye concentration
Here, the unit of dye concentration is nanomolar. It is noted that the interception of the fitted line is not ideally zero due to the systematic error from the CCD in detecting a very weak light signal as shown by the fluctuation in the measured intensity in Figure 3 when the dye concentration is very low (lower than 5 nM). However, the fluorescent intensity of the dye concentration greater than 5 nM indicates a good linear relation.
Pumping rate vs. applied electric voltage
Effect of ion concentration
Program-controlled reaction in continuous flow
Calcium ion (Ca2+) is an important intracellular information transfer substance. Intracellular regulation of calcium is an important second messenger, which is widely involved in cell motility, secretion, metabolism, and differentiation of a variety of cellular functions. An accurate control of the extracellular calcium concentration is significant in many biological studies. Therefore, a real-time system with dynamic control of the calcium concentration is of great significance. We herein demonstrated the capability of our nanofluidic device for precise control of calcium concentration for biological systems.
We have demonstrated that a simple nanofluidic device fabricated on a Si wafer with a thin layer of SiO2 and then sealed by a PDMS thin film has its potential for constructing a picoinjector. The bonding between the Si wafer and PDMS relies on the adhesion force other than chemical bonding. Therefore, it is easy to separate them, and the silicon chip could be cleaned to use repeatedly. The injection process is based on the electroosmotic flow generated by the voltage bias across the nanochannels. The EO pumping rate was measured by analyzing the fluorescent intensity when the fluorescent probe (FITC) was used in PBS as an indicator. The variations in EO flow rate at different DC voltages and different analyte concentrations were investigated, and the results exhibited good agreement with the existing theory. The precisely controlled reaction between Fluo-4 and calcium ions was used to demonstrate our device's potential application in electrochemical reaction, biochemical reaction, DNA/protein analysis, drug delivery, and drug screening. The electroosmotic effect dominates the fluid transport in our picoinjector, and electroosmosis allows our device to attain precision in fluid transport for chemical reaction on a nanoscopic scale using low DC bias voltage. The advantages of our device are its being simple, reusable, and low-cost with high precision of delivering water-based solutions.
The authors acknowledge the financial support provided by the Hong Kong Research Grants Council Grant No. HKUST604710, 605411 and National Natural Science Foundation of China (Grant No. 11290165). This publication is based on work partially supported by Award No. SA-C0040/UK-C0016 made by King Abdullah University of Science and Technology (KAUST).
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