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Cite This: Anal. Chem. XXXX, XXX, XXX−XXX
Ionic Current Behaviors of Dual Nano- and Micropipettes Shudong Zhang,† Xiaohong Yin,† Mingzhi Li, Xianhao Zhang, Xin Zhang, Xiaoli Qin, Zhiwei Zhu, Shuang Yang,* and Yuanhua Shao* Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
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S Supporting Information *
ABSTRACT: Ionic current rectification (ICR) phenomena within dual glass pipettes are investigated for the first time. We demonstrate that the ionic flow presents different behaviors in dual nano- and micropipettes when the two channels are filled with the same electrolyte KCl and hung in air. Bare dual nanopipettes cannot rectify the ionic current because of their geometric symmetry, but the ICR can be directly observed based on bare dual micropipettes. The phenomena based on dual micropipettes could be explained by the simulation of the Poisson-Nernst-Plank equation. After modification with different approaches, the dual nanopipettes have asymmetric charge patterns and show various ICR behaviors. They have been successfully employed to fabricate various nanodevices, such as ionic diodes and bipolar junction transistors. Due to the simple and fast fabrication with high reproducibility, these dual pipettes can provide a novel platform for controlling ionic flow in nano- and microfluidics, fabrication of novel nanodevices, and detection of biomolecules.
I
also useful tools because they have two channels, which make it possible to finish multitasks at one time. Recently, they have attracted much attention and been applied in several areas. Shao et al. combined dual pipettes and mass spectrometry to study in situ electrochemical reactions.32 Zhang et al. used dual nanopipettes to build extended field-effect transistors for selective single-molecule biosensing,33 and Edel et al. developed zeptoliter nanobridges for single molecule trapping and sensing.34 The nonlinear current−voltage (I−V) responses of nanostructures, referred to as an ionic current rectification, have long been observed using bionanopores.35 Since Bard et al. reported the first study of ICR based on nanopipettes in 1997,36 the ICR has been intensively investigated using various artificial systems including silicon-based nanopores, graphene nanopores, polymer nanochannels, and glass nanopipettes.37−47 Several theoretical models have also been established to elucidate the phenomena and mechanisms of ICR, such as the perm-selectivity at the tip,36 the model of membrane with narrow pores,48 the ratchet model,49 and the Poisson-Nernst-Planck (PNP) equations.50 Many reports also explained the influence of pH, geometry, and other factors on ICR.51−53 However, most of the models and experimental
nspired by the functionalities of ion channels on biomembranes, the behaviors of ion transport within nanodevices, such as nanopores, nanochannels, nanopipettes, nanofluidics, and so on, have received considerable attention over the past few decades because they have several unique properties, such as double layer overlap, high surface to volume ratios, surface charge, ionic current rectification (ICR), and entropic barriers. These phenomena have potential applications in mimic biological ion channels, developing novel molecular delivery devices and chemical/biosensors, detection of ionic species, and DNA sequencing.1−19 Glass pipettes are common tools in laboratories for manipulation of different types of liquids. A new technique called patch clamping was invented when the sizes of pipettes were in micrometer and has been employed to measure the membrane potentials.20 Later on, micro- and nanoscopic liquid/liquid interfaces were supported at micro- and nanopipettes.21 Ionic generation/collection reactions could also be studied by a dual (θ) micro- or nanopipet. 22 The miniaturization of liquid/liquid interfaces has brought important impact on electrochemistry at a liquid/liquid interface, such as the fast kinetics of charge (electron and ion) transfer reactions could be measured and complicated coupling reactions between ion transfer and electron transfer might be evaluated.23−30 These pipettes have also been widely employed as probes for scanning electrochemical microscopy22 and scanning ion conductance microscopy.31 Dual pipettes are © XXXX American Chemical Society
Received: April 20, 2018 Accepted: June 13, 2018
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DOI: 10.1021/acs.analchem.8b01765 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry work have ignored the effect of electroosmotic flow (EOF), because it has minor influence on ICR when the diameter of pore is of dozens of nanometers which is comparable to the Debye length. In 2010, Mayer et al. demonstrated that EOF could also generate ICR within micropores, which extended the study of ICR to much broader scale.54−57 Recently, several experiments have demonstrated that the ICR could also be observed even with micropipettes.55,58 These observations were obtained at micrometer scale in symmetric electrolyte solution with polymers modified micropipettes.55 They lay the foundation for developing novel devices based on surface chemistry and proving possibility of ICR at microscale. Micro- and nanodevices based on ICR have been widely studied as they can be used to control ion flow without implementation of mechanical forces.59 Ionic diodes, whose behaviors are analogous to semiconductor ones, are the most fabricated ionic current devices and have already reached several hundreds of rectification ratio.6,60,61 Fabrication of ionic transistors has attracted much attention lately. Siwy et al. reported the first nanofluidic bipolar transistor based on an hourglass-shaped nanopore.45 The double-conical shaped nanopores could be modified to specific patterns. Both the experimental results and theoretical modeling showed that they actually acted as pnp bipolar junction transistors through changing pH and concentration of bulk electrolyte. However, the fabrication of such nanopores was elaborate, and they only worked when the radii are pretty small. In this work, we demonstrated for the first time that ionic current rectification could be observed with easily fabricated dual micro- and nanopipettes. Through different types of surface modifications and controlling solution pH or electrolyte concentration alone, ionic current rectification can occur under applied voltage, and several devices have been developed. These devices are analogous to nanoresistor, diodes, and bipolar junction transistors in many ways except for controlling ion flux instead of electrons or holes. Bare dual micropipettes based on the specific geometry can directly rectify current and could be further developed into novel molecular concentration devices and a simple model, mainly considering the effect of thickness of aqueous film on the band, was proposed to explain the behaviors observed based on simulation of the Poisson-Nernst-Plank equation.
Fabrication and Characterization of Dual Micro/ Nanopipettes. Dual pipettes are made from quartz (0.9 mm I.D. and 1.2 mm O.D.) or borosilicate capillaries (1.0 mm I.D. and 1.5 mm O.D.) by using different pulling programs based on a Sutter model P-2000 laser puller (Sutter Instrument Co.).63 The radii depend on different parameters of programs. The detail pulling programs can be found in Supporting Information (SI). One pulling operation can produce two almost identical dual pipettes.64 The radii of them are estimated by analysis of the respective SEM images and steady-state voltammograms. Due to the poor conductive properties of glass, dual pipettes are coated with gold layers in order to obtain clear SEM images. The SEM (S-4800, Hitachi, Japan) is employed and Figure 1a,b show images of the dual
Figure 1. (a) SEM image of a dual nanopipet. (b) SEM image of a dual micropipet. (c) Cyclic voltammogram of the transfer of K+ at the W/DCE interface facilitated by DB18C6 obtained with the same sized dual nanopipet in (a). (d) Cyclic voltammogram of the transfer of K+ at W/DCE interface facilitated by DB18C6 obtained with the same sized dual micropipet in (b). Scan rates for (c) and (d) are 50 mV/s.
pipettes used in the experiments. The radius and bandwidth shown in Figure 1a are 19 and 13 nm, while in Figure 1b are 521 and 125 nm correspondingly (the radius is approximately equal to (R + r)/2, see Figure 2). The effective sizes of dual pipettes can be alternatively evaluated through the voltammetry of classical facilitated ion transfer (FIT).65 High quality steady-state voltammograms are critical to calculate radii of dual pipettes with electrochemical methods. Here we employed the system of potassium ion transfer at the water/ 1,2-dichloroethane (W/DCE) interface facilitated by dibenzo18-crown-6 (DB18C6). The electrochemical cell employed is listed as follows: Ag|AgTPBCl|2 mM DB18C6 + 2 mM BTPPATPBCl (DCE)||100 mM KCl (W)|AgCl|Ag. The radii of dual pipettes are calculated (Figure 1c,d) by the empirical equation65
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EXPERIMENTAL SECTION Chemicals. Potassium chloride (>99.5%), potassium hydroxide (>99.5%), hydrochloric acid (>99.5%), and 1,2dichloroethane (>99%) were obtained from Beijing Chemical Co. Polyethylenimine (linear, M.W. 25000) and dibenzo-18crown-6 (>98%) were purchased from Alfa Aesar. Potassium tetrakis(4-chlorophenyl)-borate (>98%) and bis(triphenylphosphoranylidene)-ammonium chloride (>98%) were purchased from Sigma-Aldrich. Trimethylchlorosilane was obtained from Acros Organics. 1,2-Dichloroethane (DCE) was distilled before use. Bis(triphenylphosphoranylidene)ammonium tetrakis(4-chlorophenyl) borate (BTPPATPBCl) was synthesized by metathesis of equimolar solutions of potassium tetrakis(4-chlorophenyl)-borate (KTPBCl) and bis(triphenylphosphoranylidene)-ammonium chloride (BTPPACl), which has been reported in previous articles.26,62 The salts were recrystallized from acetone and then dried in an oven at 95 °C for 24 h. All aqueous solutions were prepared with triply distilled water.
iss = 3.35πnFDCR *
(1)
where iss is the steady-state current, R* is the effective radius of one orifice of the pipet employed, D and C are the respective diffusion coefficient and concentration of species in the outer solution responsible for the interfacial charge-transfer reaction, F is the Faraday constant, and n is the transferred charge.63 The cyclic voltammograms are obtained with only one channel filled with aqueous solution and put into the DCE phase. The B
DOI: 10.1021/acs.analchem.8b01765 Anal. Chem. XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION Rectification Behaviors of Bare Different Radii of Dual Pipettes. Dual pipettes with different radii present discrepant current−voltage responses (Figures 3 and S1). At
Figure 2. (Left) Schematic experimental setup. (Right) Bottom view (top) and front view (bottom) of a magnified part in the left. The thicknesses of the band (d) and the pipet radius (R) are not in proportional scale.
out surfaces and bands of dual pipettes are silanized to avoid that solution diffuses from one channel to the other.64 The radii can be calculated from the steady-state currents using eq 1, and results are 22 and 498 nm, which are essentially close to the values measured by the SEM images. Modification of Micro/Nano Dual Pipettes. Due to dissociation of silanol group in aqueous solution, the surfaces of glass dual pipettes carry negative charges. Linear polyethyleneimines (PEIs) are used to modify inner surfaces of dual pipettes through electrostatic force as they carry a bunch of amines and are protonated at pH lower than 10. First, about 0.6 μL of 0.1% PEIs solution is backfilled into channels of dual pipettes. Then they are kept for 30 min in order that PEIs have enough time to interact with inner surfaces of dual pipettes. Finally, dual pipettes are baked at 120 °C for 2 h to remove water. Silanization is another method to modify inner surfaces of dual pipettes. Silane reagents, such as trimethylchlorosilane (TMCS), can form covalent bonds with dissociated silanol group. TMCS blended with DCE is injected into channels of dual pipettes. The reaction is fast and silanization solution is removed after 3 min. Inner surfaces carry no charges after silanization. These methods can be used to modify one or both of channels of dual pipettes to change properties of surface charges. Silanization of the bands in the middle of the dual pipettes takes advantage of low saturated vapor pressure of silane reagents. Dual pipettes are hung above TMCS and DCE mixed solution about 1 cm with argon flow passing from back to prevent silanization of channels. Standing for about 3 min, bands of dual pipettes are silanized and carry no charges. At the same time, both channels are aerating with argon. Electrochemical Measurements. The experimental setup is shown in Figure 2. Nanoscale or microscale dual pipettes filled with KCl solution are exposed in the air. The aqueous film on the band causing by its hydrophilic nature can connect the two channels. Two Ag/AgCl electrodes placed in each channel are used respectively as working electrode (WE) and reference electrode (RE), and different bias potentials can be controlled using an electrochemical workstation (CHI 760E). High scanning speed has great influence on current−voltage curves,66,67 so it is set up at 100 mV/s.
Figure 3. Different ionic current behaviors based on various modified dual nanopipettes. (a) Bare dual nanopipettes show ohmic property. (b) One-channel-modified dual nanopipettes act as ionic diodes. (c) Two-channels-modified dual nanopipettes rectify ionic current just like a bipolar junction transistor (R* = 20 nm, c (KCl) = 10 mM, v = 100 mV/s).
nanometer scale, dual pipettes show no rectification behavior without modification (Figure 3a). With increasing radii, rectification behaviors occur. Figures S1a have obvious nonlinear current−voltage responses. With larger radii, for example 10 μm, rectification behavior disappears (Figure S1d). These behaviors will be discussed later. Modification of Dual Nanopipettes and Construction of Nano Rectification Devices. The ICR behavior of a bare nanopipette with radius of 20 nm is essentially the ohmic response (Figure 3a), because under the experimental condition (10 mM KCl and pH is about 7), the charge pattern on two channel and the band is negative (−), negative (−), and negative (−) (“---”). The charge and geometry of the bare dual nanopipet are both symmetric. Chemically modifying C
DOI: 10.1021/acs.analchem.8b01765 Anal. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Function of dual nanopipettes was determined by surface charge pattern and solution pH. (a) Positive, neutral, and positive pattern at pH 2.5. (b) Whole surface was positively charged at pH 1.5 and the dual nanopipet was on-state. (c) Neutral, negative, and neutral pattern obtained with silanization at pH 7. (d) Negative, neutral, and negative pattern with dilaniztion of the band at pH 7 (R* = 20 nm, c (KCl) = 10 mM, v = 100 mV/s).
plateaus indicates that ion flux could be restricted within a specific range by the dual nanopipettes after these modifications. Obviously, the surface charge pattern is one of the crucial elements influencing rectification behavior for a dual nanopipet. Surface charge patterns are changeable with different modification methods and various pH of the solution. The isoelectric point of silanol groups on the pipet surfaces is about 336 and that of PEIs is about 10.58 At pH 2.5, the inner walls modified with PEIs carry positive charges because amine groups can be pronated at pH lower than 10 whereas the bare portion at the tip is neutral. Therefore, the surface charge pattern is formed as positive (one channel), neutral (the band), and then positive (the other channel; the “+0+” pattern). The ion transport properties of this pattern are shown in Figure 4a and it resembles a bipolar junction transistor. The whole surfaces of a dual pipet will be positively charged (the “+++” pattern) if the pH of solution is controlled to 1.5, so that the dual nanopipet operates predominantly in the open state and the I−V curve is nearly linear, as shown in Figure 4b. Using silanization, except for the tip of band, the inner surfaces turn into a neutral, negative, and then neutral pattern under pH 7 (the “0-0” pattern). This pattern acts as a bipolar junction transistor as well (Figure 4c). Only modifying the band creates the “-0-” pattern at pH 7. It also has features of bipolar junction transistors (Figure 4d). These experimental results demonstrate that modification and controlling pH of the solution significantly affect surface charges and charge patterns of the tip, thus, controlling ionic flow under different circumstances. The beauty of a dual pipet is that it has three parts, which can be chemically modified accordingly, that is, two channels and a band (Figures 1 and 2). It is convenient to change charge patterns of pipet surfaces and build different kinds of nanodevices.
only one channel of a dual nanopipet with PEIs could alter the inner surface charges from negative to positive. In order to keep the band from modification, PEIs solution is backfilled into dual pipettes. The tiny bubble at the tip prevents PEIs solution contacting with the band. After modification, a dual nanopipet has the “+--” charge pattern, which controls ionic flux under one direction, for example, only at negative voltages, where it shows ionic current in Figure 3b. This behavior is very similar to a diode that electrons only pass at one direction of voltage. This phenomenon occurs mostly due to formation of a depletion zone at the junction between positively and negatively charged surfaces.68 The radius of nanoscale tip is comparable to the electrical double layer (EDL), which is also called the Debye length (about 3 nm, approximately occupies 1 − (10−3)2/102 = 51% of the cross-sectional area in this case29), and the EDLs caused by surface charges of dual pipettes are overlapped. When the surface is positively charged, the channel is primarily filled with anions and vice versa. At negative potential, both Cl− and K+ could be accumulated at the junction driven by the externally applied potential, which results in increasing solution conductance. In contrast, both counterions are migrated away from the junction at positive potential. As a result, a depletion zone is formed at the junction and solution conductance decreases significantly. Based on different surface charge patterns, dual nanopipettes are switchable between on and off states under different externally applied potentials. Subsequent modification of another channel can change the property of surface charges using the same method, but it will rebuild the symmetric structure and charges (“+-+” pattern). The current−voltage responses for such case have typical features of a bipolar junction transistor (Figure 3c). Because of formation of depletion zone at one of the two junctions between opposite charged surfaces, current increases in a range at low potentials, and becomes independent of the applied potential out of that ranges. The occurrence of current D
DOI: 10.1021/acs.analchem.8b01765 Anal. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Influence of electrolyte concentration to a dual nanopipet with asymmetric charge pattern (“+--”). Concentration of KCl was (a) 1 M, (b) 100 mM, (c) 1 mM, and (d) 0.1 mM (R* = 20 nm, v = 100 mV/s).
Figure 6. Rectification behaviors of dual micropipettes at pH (a) 7, (b) 3, and (c) 1.5. (d) Rectification behavior of a dual micropipet modified with PEIs (R* = 500 nm, c (KCl) = 10 mM, v = 100 mV/s).
Influences of Electrolyte Concentration and Electrolyte pH to Rectification Behaviors of Dual Pipettes. The rectification behaviors of the modified nano dual pipettes with asymmetric and symmetric charge patterns are also found to be dependent upon electrolyte concentration. When dual pipettes are chemically modified only one channel, their surface charge patterns are asymmetric. Figure 3b shows the current−voltage curve under 10 mM KCl solution. Increasing concentration of solution to 100 mM, as shown in Figure 5b, rectification behavior weakens, and it disappears with 1 M KCl solution
(Figure 5a). Meanwhile, rectification degrees decrease a little with dilute electrolyte like 1 mM and 0.1 mM KCl solution (Figures 5c,d). These phenomena could be explained by the changes of EDLs under different concentration of solution within dual pipettes. Thickness of electrical double layers decreases along with increasing concentration of electrolyte solutions, which leads to weak rectification behavior under high KCl concentration. The EDLs overlap when the electrolyte concentration is lower than 10 mM.29 The fixed number of charge carrying ions in the pore at low E
DOI: 10.1021/acs.analchem.8b01765 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry
equal to kBT. ε is the dielectric constant of solvent. In an aqueous solution at room temperature, lB = 0.7 nm. The Poisson-Nernst-Plank (PNP) equations are solved numerically for our systems under certain experimental conditions. To simplify the problem, we model this system as a twodimensional ionic channel (Figures 7a and S5). The channels
concentration results in lower rectification degrees at much lower concentration.69 Besides, increasing of KCl concentration also causes reduction in ionic selectivity of the symmetric charge pattern (like the “+-+”).45 At high electrolyte concentration (c (KCl) = 1 M), the dual nanopipettes are not capable of controlling ion flux, and the current−voltage curves will be restored to the ohmic features (Figure S2). These experimental results further confirm that the ion transport behavior within the dual nanopipettes is mostly controlled by the surface charges and electrolyte properties, thus can provide a convenient way for manipulation flux of ion and charged molecules. With increase of radii to submicrometer and micrometer ranges, the EDLs become thinner and the unexpected rectification is observed. However, bare dual micropipettes only rectify ionic current under a certain range of electrolyte concentration. As shown in Figure S3, when the concentration of KCl is above 1 M or under 1 mM, rectification behavior becomes weak or disappears. As EDLs are negligible comparing with radii of dual micropipettes, the reason for these phenomena will be explained in the theoretical part. Obviously, the electrolyte pH also can influence the surface charges of dual micropipettes. With the decrease of negative charges on the channels and band, the ICR becomes weak. Rectification behaviors of dual micropipettes under different electrolyte pH are shown in Figure 6. Rectification occurs using original KCl solution (Figure S3a, 10 mM and pH is about neutral), and it disappears with a decrease of solution pH. Figure 6b shows the I−V curve at pH 3, which is close to the isoelectric point of silanol group, so that the rectification is nearly unobserved. The I−V curve is almost linear at pH 1.5 (Figure 6c; the pH is controlled by addition of HCl). But the ionic current magnifies several times, which is caused by high concentration of H+. In order to obtain positively charged surfaces of dual micropipettes without influence of H+ concentration, the inner surfaces of dual micropipettes are modified with PEIs. The result shows (Figure 6d) that the rectification occurs just like the negatively charges surfaces. Theoretical Model for Dual Micropipettes. It clearly indicates that the bare dual micropipettes only rectify ionic current under a certain range of electrolyte concentration. The rectification disappeared when the bare dual micropipette was put directly into a 10 mM KCl solution but not exposed in air (Figure S4). This shows that the thickness of aqueous film on the band might play an important role for such rectification of dual micropipettes. Here we propose a simple method to modeling such influence. In order to simplify the simulation, the electroosmotic effect was ignored. The ion flux Ji under applied potential ϕ (in unit of kBT/e) can be described by the Nernst-Plank equation:70,71 Ji = −Di(∇ci + zici∇ϕ),
i = +, −
Figure 7. (a) Schematic modeling of dual pipettes. The pipette walls carry negative charges. The button denotes the aqueous film with thickness of Ly. A potential is applied to both side of pipettes, and a positive ion flux pass through the channel shown as arrows. (b) Theoretical current−voltage curves of dual micropipettes with different thicknesses of aqueous layers. The pore size of pipettes is 1 μm while the width of middle band is 200 nm. The bulk salt concentration is 10 mM. With increasing the thickness of water layer Ly, the rectification effect becomes weak.
contain water and ions, and the bottom part is the aqueous film that can connect the two channels. The potential is applied on both channels of the dual pipettes. The positive ion flux passes through the channels from left side to right side, denoted as the arrows shown in Figure 7a. The whole dual pipet (two channels and a band) is the system that needs to be considered. A nonuniformed two-dimensional mesh is applied to the system and eqs 3 and 4 are discretized accordingly. For a given potential bias, the discretized equations with appropriate boundary conditions are solved numerically. Starting from an initial guest of potential and ion concentrations, one can iterate the equations and obtain the convergent solutions (steady potential distribution and ion fluxes) in terms of some iterative techniques. Consequently, the current curves at different potentials can be constructed. The detailed modeling and the iteration method in the simulation are provided in the Supporting Information. Figure 7b displays the main results obtained by the simulation. It can be found clearly that the relative width of band between two channels with respect to channel size, as well as the thickness of aqueous layer, plays key roles in determining the ion transport properties. When the aqueous layer is rather thin (200 nm) for a given system with diameter of the channel R* = 1000 nm (Lx = 1100 nm), one can find the abrupt reduction of the I−V curve slope once the potential exceeds some values, a similar rectification behavior as those shown in Figures S1a,b and S3. Furthermore, with increasing the thickness of aqueous layer, the ionic current curve tends to become more linear and the rectification effect disappears. Decreasing the channel size or width of the band leads to the same tendency (see Figure S9 for details). For smaller dual pipettes with channel size of 20 nm, the I−V curve still exhibits a tiny rectification effect. However, it almost approaches linear behavior (Figure S10). All of our simulation is in accord with the experimental phenomena observed (see Figures 3 and S3).
(2)
where ci and zi refer to the local concentration and the charge of ionic species i (i = + for cations and i = − for anions) in the solution. The system satisfies steady-state continuity equations, ∇·Ji = 0,
i = +, −
(3)
and the Poisson equation ∇2 ϕ = 4πlB(c − − c+)
(4)
here lB = βe2/4πεkBT is the Bjerrum length, and at this distance, the Coulomb energy of two elementary charges is F
DOI: 10.1021/acs.analchem.8b01765 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry Notes
The specific geometry of dual pipettes is also responsible for the unusual ionic current rectification. Further analysis (see the Supporting Information) shows that the partly contribution to the ion flux is from the migration of mobile ions (the second term on the right side of the eq 2). For large pores with thin aqueous layer, the potential varies strongly near the charged band. Especially, if the applied potential is large enough, the narrow water channel restricts the special potential distribution under the influence of charged band, and results in the distinctively lower ion concentrations as one can expect. Therefore, although high voltage leads to stronger electric field, the obvious reduction of ion concentration within thin water layer at high potential produces the nonlinear rectification effect.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC; Nos. 21335001, 21575006, and 21674005) and the National Key Research and Development Program of China (No. 2016YFA0201300).
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CONCLUSIONS In summary, we have fabricated novel ionic current devices based on glass dual nano- and micropipettes, and studied their ion transport behaviors electrochemically and theoretically. Instead of controlling ionic flow with mechanical methods, these dual pipettes can regulate ionic flow with various chemically modified methods such as silanization, PEIs modification and controlling solution pH, and electrolyte properties. The size of dual pipettes determines the way of controlling ion flux. Nanoscale dual pipettes based on the principle of electrical double layers overlapping can switch between the on and off states by changing pH and surface charge pattern, as well as concentration of electrolyte. Meanwhile, microscale dual pipettes are also influenced by electrolyte pH and concentration and might be used to develop molecular concentration devices. These simple ionic devices can be used to control ion flux within nanometer-sized and micrometer-sized dual pipettes, and they have promising applications in understanding the nano- and microfluidics, in fabrication of novel nanodevices, and design novel platforms for sensing of biomolecules.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b01765. Detailed description of dual micro/nanopipettes fabrication programs and rectification behaviors of different radii of dual pipettes under various concentrations of electrolyte solution. The detailed simulation methodology is also presented (PDF).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Tel.: +86-10-62759394. Fax: +86-10-62751708. *E-mail:
[email protected]. ORCID
Shuang Yang: 0000-0002-5573-5632 Yuanhua Shao: 0000-0003-3922-6229 Author Contributions †
These authors contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. G
DOI: 10.1021/acs.analchem.8b01765 Anal. Chem. XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.analchem.8b01765 Anal. Chem. XXXX, XXX, XXX−XXX
