Ionic Current Rectification in Organic Solutions with Quartz

Jul 28, 2015 - The study of behaviors of ionic current rectification (ICR) in organic solutions with quartz nanopipettes is reported. ICR can be obser...
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Ionic Current Rectification in Organic Solutions with Quartz Nanopipettes Xiaohong Yin, Shudong Zhang, Yitong Dong, Shujuan Liu, Jing Gu, Ye Chen, Xin Zhang, Xianhao Zhang, and Yuanhua Shao* Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The study of behaviors of ionic current rectification (ICR) in organic solutions with quartz nanopipettes is reported. ICR can be observed even in organic solutions using quartz pipettes with diameters varied from several to dozens of nanometers, and the direction of ICR is quite different from the ICR observed in aqueous phase. The influences of pore size, electrolyte concentration, and surface charge on the ICR have been investigated carefully. Water in organic solutions affects the direction and extent of ICR significantly. Mechanisms about the formation of an electrical double layer (EDL) on silica in organic solutions with different amount of water have been proposed. An improved method, which can be employed to detect trace water in organic solutions, has been implemented based on Au ultramicroelectrodes with cathodic differential pulse stripping voltammetry.

T

et al.,13 and White et al.19 have studied the effects of electroosmotic flow (EOF) on ionic transport in glass pipettes. Zhang et al.20 and Girault et al.21 have used nanopipettes to evaluate the influence of potential scan rates on ICR, which correlated to ionic redistribution. Baker et al.22 have suggested that ICR could be enhanced or reduced through the electrostatic interactions between a neutral or a charged nanopipette and a charged substrate. In addition, many others have modified the surface of pipettes to explore the ICR phenomenon with different types of polymers, such as polylysine,23 poly(acrylic acid),24 chitosan,25 and polymine.26 Currently, almost all of the studies about ICR have been carried out in aqueous solutions, and mostly using KCl as the electrolyte;26 only a few solvents other than water have been involved. Yusko et al.27 and White et al.35 added dimethyl sulfoxide (DMSO) into KCl solution to change the conductance of the solution, which caused the liquid-phase negative differential resistance to appear. This mixture still interacted with the surface of pore and functioned like aqueous solutions. Davenport et al.28 investigated the i−V response in ionic liquids (ILs) using nanopores. Their results have shown that the mechanism is similar to that of ICR observed in aqueous solutions. Clearly, current research for ICR is still focus on hydrophilic solutions. In an organic solution, the interface between the quartz and organic solution is rather different from that in aqueous solutions. Therefore, the current theory for ICR in aqueous

he nonlinear current−voltage (i−V) response of artificial nanochannels or nanopores, referred to as an ionic current rectification (ICR), has attracted considerable attention over about the past two decades.1 ICR is of great importance in understanding the mechanisms of biological ion channels and has promise in developing nanodevices, for instance, as a platform for chemical and biological nanosensors, for the manipulation of ions and molecules in a nanofluidic setup.2−7 The ICR phenomenon has long been observed using bionanopores. However, in 1997, Bard et al.8 reported the first observation of ICR based on glass nanopipettes. Since then, the ICR has been investigated in various artificial systems including silicon-based nanopores,9,10 polymer nanochannels,11,12 and glass pipettes.8,13 Generally, ICR is assumed as the consequence of asymmetric geometry and distribution of surface charge in the nanopores. ICR usually occurs when the electrical double layer (EDL) thickness is comparable to the diameter of a nanochannel. There have been some reports concerning the origin of ICR with computational simulations, mainly include the perm-selective region theory,8 the ratchet model,11 the three-regions theory,14 the quantitative description theory,15,16 and the space-charge model.17 Compared with other nanopores and nanochannels made by different materials, glass nanopipettes are easy to be fabricated and can be conveniently operated. Here, we differentiate nanopore and nanochannel by their shapes. If the orifice diameter is far less than its length, we call it a nanochannel. Otherwise, it can be considered to be a nanopore. Obviously, a glass nanopipette can be classified as a nanochannel. There have been increasingly reports on the mechanisms and applications of ICR by using nanopipettes. Daiguji et al.,18 Ai © XXXX American Chemical Society

Received: June 22, 2015 Accepted: July 28, 2015

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Analytical Chemistry solutions might not be perfectly fitted for the situation with organic solutions any more. The questions of does ICR still exist and how does ICR work in organic solutions are what we are trying to answer here. We select N,N-dimethylformamide (DMF), nitrobenzene (NB), 1,2-dichloroethane (DCE), 2nitrophenyl octyl ether (NPOE), and benzene as the solvents. We believe that the findings will possibly provide insight into the mechanism of ICR from a new point of view. Using these organic solvents and tetraphenylarsonium tetraphenylborate (TPAsTPB) as the supporting electrolyte, we have studied the ICR behaviors by quartz nanopipettes. The influences of orifice size of a pipette, electrolyte concentration, and surface charges on ICR have been investigated. The experimental results show that ICR can still be observed, even in organic solutions with nanopipettes. We have improved the electrochemical method for determination of trace water in DMF and DCE based on gold ultramicroelectrodes (UMEs) and cathodic differential pulse stripping voltammetry. The effect of water content in organic solution on ICR has also been explored, and the mechanisms about the formation of EDL on silica in organic solutions have been proposed.

pipette.26,32−34 The chemical system for characterization of nanopipettes can be represented as Cell 1: Ag/AgTPBCl/2 mM DB18C6 + 2 mM BTPPATPBCl(DCE)//100 mM KCl(W)/AgCl/Ag

Because the concentration of K+ inside the pipette is much higher than that of DB18C6 in the DCE phase outside the pipette, the steady-state current is limited by the diffusion of DB18C6 to the interface at the tip. The effective radii of the nanopipettes can be calculated from the steady-state current, according to the following empirical equation proposed by Girault et al.30 Iss = 3.35πnFDca

(1)

Here Iss is the steady-state current, n the charge of the transferred ion, F the Faraday constant, D the diffusion coefficient, c the bulk concentration of DB18C6 in the DCE phase, and a the inner pipette radius. For thin-wall pipettes on the condition of RG ≤ 2 (RG = rg/a, where rg is the outer radius of the pipettes), eq 1 can be employed to estimate the effective radii of the pipettes.32 Current−voltage curves were obtained with an electrochemical workstation (Model BAS 100B, Bioanalytical System, Inc.). The organic solution was backfilled into the pipettes using a 10 μL syringe, and then the pipettes were tapped to drive out the air bubbles and checked under an optical microscope (Model BX-51, Olympus). An Ag/AgTPB electrode was inserted into the pipette and the other Ag/ AgTPB as a reference electrode was placed in bulk solution. In all cases, the nanopipettes were filled with the same electrolyte as the bulk solution. The experimental setup is shown in Figure S2 in the SI. The potential was swept from −1 V to 1 V at a scan rate of 50 mV/s. Fabrication and Characterization of Gold UMEs. Au UME could be fabricated from ∼2−4 cm length of gold wire, then heat-sealed in a glass capillary, and polished to obtain a disk UMEs.35 The Au microelectrodes were characterized by recording the cyclic voltammograms (Figure S3 in the SI) in a 0.5 M H2SO4 solution and 2 mM Fe(CN)63− solution, respectively.36 Preparation of Water Standard Solutions in DMF and DCE. We added water into the organic solvents with a microsyringe after the solvents were dried with molecular sieves. Karl Fischer titration measurements were performed with a Model DL31 KF Coulometer (Mettler Toledo, Schweiz, Switzerland). Cathodic stripping voltammetric experiments were carried out with a Model CHI 900 electrochemical workstation (CH Instruments, Inc.) in a glovebox (Model VG1200/750TS, Vigor). The electrochemical cell used in the experiments contains a two-electrode setup and an Ag/AgTPB electrode was used as the reference electrode.

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EXPERIMENTAL SECTION Chemicals and Materials. Potassium chloride (KCl, 99.5%, Beijing Chemical Co.), dibenzo-18-crown-6 (DB18C6, 98%, Alfa Aesar), N,N-dimethylformamide (DMF, 99.9%, Tianjin Siyou Chemical Co.), nitrobenzene (NB, 99%, Sinopharm Chemical Reagent Co.), 2-nitrophenyl octyl ether (NPOE, 99%, Aldrich), chlorotrimethylsilane (98%, Acros Organics), potassium tetrakis(4-chlorophenyl)-borate (KTPBCl, 98%, Aldrich), bis(triphenylphosphoranylidene) ammonium chloride (BTPPACl, 98%, Aldrich), sodium tetraphenylborate (NaTPB, 99.5%, Aldrich), tetraphenylarsonium chloride (TPAsCl, 97%, Aldrich), 3-aminopropyl triethoxysilane (98%, Aldrich) were used as received without purification. 1,2-Dichloroethane (DCE, 99%, Beijing Chemical Co.) was washed with triply distilled water before use. Bis(triphenylphosphoranylidene)-ammonium tetrakis (4-chlorophenyl) borate (BTPPATPBCl) was synthesized by metathesis of equimolar solutions of BTPPACl and KTPBCl,29 a lipophilic salt used to provide ionic conductivity for the DCE phase. Tetraphenylarsonium tetraphenylborate (TPAsTPB) was also synthesized by metathesis of equimolar solutions of TPAsCl and NaTPB. These salts were recrystallized from methanol and then dried in an oven at 95 °C for 24 h.30,31 KCl solution was prepared from triply distilled water. All organic solutions were dried with molecular sieves (4 Å, Sinopharm Chemical Reagent Co.) before use. Gold wires with the diameters of 15 μm (99.99%) for fabricating the microelectrodes were obtained from Institute of Precious Metals (Kunming, Yunnan, China). Fabrication and Characterization of Quartz Nanopipettes and Electrochemical Measurements. A CO2laser-based pipette puller (Model P-2000, Sutter Instrument Co.) was used to fabricate nanopipettes with quartz capillaries (0.7/1.0 mm inner and outer diameters, also from Sutter Instrument Co.). The radii of the quartz nanopipettes pulled were evaluated by steady-state voltammetry of facilitated K+ transfer at the water/1,2-dichloroethane (W/DCE) interface by DB18C6 (see Figures S1A−D in the Supporting Information (SI)). The electrochemical methodology has been proved to be an effective way to determine the radius of a nano-

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RESULTS AND DISCUSSION ICR Behaviors in Different Organic Solutions Based on Bare Nanopipettes. In order to compare with the ICR obtained in aqueous solutions, we choose TPAsTPB as the electrolyte. TPAs+ and TPB− have similar molecular structure and size, and they are assumed to have the same transfer Gibbs energy (TATB assumption),37 which is analogous to the function of KCl in aqueous solutions. We first study the influence of different organic solutions on ICR. DMF, NB, DCE, NPOE, and benzene have been chosen because B

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DCE are rather large, and DMF is hydrophilic but DCE is hydrophobic;38,39 therefore, we choose these two as the model organic solvents in the following study. The two main reasons resulting in the difference may be (1) the different concentrations of free ions (TPAS+ and TPB−) and (2) the different water content in the solvents. These will be discussed later. Figure 2A shows a typical i−V relationship of a submicropipette (a = 500 nm) used in the DCE solution with 10 μM TPAsTPB. Under this condition, the current is a linear function of applied potential. However, the i−V curves become nonlinear with smaller radii of nanopipettes. Figures 2B (a = 100 nm), 2C (a = 20 nm), and 2D (a = 4 nm) all exhibit asymmetric i−V curves, in which the currents under positive voltage are larger than that under negative voltage. These results are similar to that observed in an aqueous solution: the smaller the nanopipettes used and the more asymmetric features that could be obtained, the larger difference of conductance inside and outside of the tip will be. Figure S4 in the SI shows similar results that were obtained in the DMF solution, but with even larger R values when nanopipettes in the similar sizes are employed. However, the direction of ICR observed is just opposite that obtained in aqueous solutions under similar experimental conditions.8 The simplified theory models,14,15,17,19,40 usually employed in aqueous environments for ICR are not perfectly suitable to explain the situation in organic solutions. According to the previous report,25 EOF may play important roles for these cases, as a simplified definition is that electro-osmosis is the converse of electrophoresis.17 In aqueous solutions, EOF collapses in smaller channels, where the EDL overlap significantly.41,42 Therefore, EOF contributes very little to

TPAsTPB can be dissolved to some extent in these organic solvents. From the current−voltage curves in Figure 1, the

Figure 1. Current−voltage curves obtained using nanopipettes in different organic solutions which had been dried with molecular sieves. Legend: (1) DMF, (2) NB, (3) DCE, (4) NPOE, and (5) benzene. [Conditions: a = 4 nm, c(TPAsTPB) = 10 μM. Scan rate, 50 mV/s.]

difference among various solvents on ICR is clearly rather big, but the direction is consistent, which is opposite to the case of that in an aqueous solution. We employ a rectification factor (R) to quantify the extent of ICR, which is defined as the absolute ratio of positive current value (i+) to negative current value (i−) at the potentials of ±1 V. The bigger the R value is, the more significant the ICR will be. The R values are 94.72 ± 4.80, 24.33 ± 3.23, 16.29 ± 1.25, 10.34 ± 0.52, and 6.28 ± 0.88, corresponding to DMF, DCE, NB, NPOE, and benzene, respectively. Obviously, the extents of ICR from DMF and

Figure 2. Current−voltage (i−V) curves obtained using nanopipettes with different tip radii in the DCE solution which had been dried with molecular sieves: (A) a = 500 nm, (B) a = 100 nm, (C) a = 20 nm, and (D) a = 4 nm. [Conditions: c(TPAsTPB) = 10 μM. Scan rate, 50 mV/s.] C

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in the nanopipettes. Thus, EOF also collapses, leading to indecisive effects on ICR in organic solutions. Effect of the Supporting Electrolyte Concentration. When the concentration of TPAsTPB in DCE is in millimolar range, it has an i−V curve that is almost a straight line (see Figures 3A and 3C). In contrast, the ICR becomes obvious with lower TPAsTPB concentrations (Figures 3B and 3D). The lower the concentration of TPAsTPB, the larger the R value of ICR that will be observed (see Table 2). This is similar to the

rectification in aqueous solutions, and it can be normally neglected in the simulations of the conical nanopores. In organic environments and in our experimental conditions (supporting electrolyte is rather small), the Debye−HückelOnsager theory is applicable.43 Therefore, the Debye length (λD), which is approximately equal to EDL, can be estimated from the following equation:17 ⎛ ε ε k T ⎞1/2 1 λD = = ⎜⎜ 2 0 r B∞ 2 ⎟⎟ κ ⎝ e ∑i ni zi ⎠

(2)

Table 2. Values of R, 1 − θ and K for Different Concentrations of TPAsTPB in the Dried DCE Solutionsa

where κ is called the Debye−Hückel parameter and is mainly dependent on the electrolyte concentration (free ions) in the bulk solution n∞ i . ε0 is the vacuum permittivity and εr is the relative dielectric constant of the medium, kB is the Boltzmann constant, T is the temperature, e is the elementary charge, and zi is the charge number of ion i. The values of λD corresponds to different organic solutions are presented in Table 1 (with the Table 1. EDL Thickness λD for Various Concentrations of TPAsTPB in the DMF and DCE Solutions c(TPAsTPB) [μM]

λD (DMF) [nm]

λD (DCE) [nm]

1 10 100 500 1000

319.27 163.34 89.46 53.68 49.01

166.68 85.27 46.71 28.02 25.58

a

a [nm]

c (TPAsTPB) [μM]

20 20 20 20 20 100 100 100 100 100

1 10 100 500 1000 1 10 100 500 1000

1−θ

R 15.32 6.25 2.91 2.18 1.29 5.65 3.38 2.88 1.89 1.01

± ± ± ± ± ± ± ± ± ±

0.77 0.31 0.15 0.11 0.06 0.28 0.17 0.14 0.09 0.05

0.424 0.162 0.054 0.030 0.018 0.424 0.162 0.054 0.030 0.018

K [M−1]

3207

The repeat number (n) is 5.

ICR observed in aqueous solutions. However, the TPAsTPB in organic solutions is more complicated than that of KCl in aqueous solutions, because it might form ion pairs. Based on the Bjerrum theory that ion pairs can be formed when ions with opposite charges approach each other close enough for the static attractive force to become stronger than

concentrations corrected by the ion-pair formation) (see next section). The Debye length is comparable to the diameter of the nanopipettes, where the EDL has developed fully or partly

Figure 3. Dependence of ion current rectification on TPAsTPB concentration in the dried DCE solution with quartz nanopipettes (∼20 nm): (A, C), c(TPAsTPB) = 1 mM; (B, D) c (TPAsTPB) = 1 μM (curve 1), 10 μM (curve 2), 100 μM (curve 3), and 500 μM (curve 4). [Conditions: scan rate = 50 mV/s; (A, B) a = 20 nm; (C, D) a = 100 nm.] D

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Analytical Chemistry the kinetic energy of thermal agitation. The ion-pair formation can be treated by a spherical shell of radius r and thickness dr around the cation, a Boltzmann distribution is defined as the number of anion charges dna(r):44,45 ⎛ ⎞ e2 dna(r ) = 4πr 2Na(r )dr = 4πr 2drNa∞ exp⎜ ⎟ ⎝ 4πε0εr rkBT ⎠ (3) −19

where Na(r) is the density of anions, e = 1.6 × 10 C, kB = 1.38 × 10−23 J/K, and T is 298 K, and εr = 10 and 36.7 for DCE and DMF, respectively. By differentiating eq 3, the minimum is defined as the Bjerrum distance q,46

r=q=

e2 8πε0εr k BT

Figure 4. Relationship of fraction of free ions (1 − θ) and R values with different TPAsTPB concentrations in the dried DCE. Curve 1 (20 nm radius) and curve 2 (100 nm radius) pipettes have been used to investigate the relations of R values and TPAsTPB concentrations.

(4)

only the distance between the positive and negative ions is less than q, such that an ion pair can be possibly formed (see Figure S5 in the SI). The value of q is 0.76 and 2.8 nm for DMF and DCE.45 The fraction of ion pairs (θ) is θ=

∫S

q

dna(r )dr = 4πNa∞

∫S

q

r 2 e 2q / r d r

pairs have almost dissociated with very low concentration, the very big conductance difference in and out of the nanopipette tip would cause larger R value. Moreover, with smaller radii of nanopipettes (20 nm), the tendency of R−c is almost the same with (1 − θ)−c (see curve 2 and the inset in Figure 4). Effect of Water in the Organic Solutions on ICR. During the experiments, we have found that the amount of water in the organic solutions has a crucial impact on the ICR. The i−V curves will change gradually to the opposite direction, along with the increase of water in the organic solutions absorbed from the atmosphere. Figure 5A shows the influence of water on ICR. Curve a is obtained in the DMF solution without any pretreatments, while curve b is operated in the same DMF solution but has been dried with molecular sieves. Obviously, the water content in solution a is much higher than that in solution b. The direction of curve a is downward, whereas that of curve b is upward. The reason for this change is that the increasing the amount of water will change the properties of the interface where organic solutions come into contact with quartz nanopipettes. We further investigate i−V responses during the drying process of the DMF solution inside a glovebox (Figure 5B). Compared to the saturating process of a dry solution with water vapor, the drying process is easy to be controlled. In the experiments, after added molecular sieves in the untreated DMF solution, the potential scanning process is carried on continually until the curves no longer change. At first, the ICR phenomenon is the same as that in aqueous solutions. Along with the decreasing of water, the i−V curve changes gradually from downward to be a straight line, and finally to upward one. The ICR behaviors might be controlled by different types of mechanisms, as discussed subsequently here: during the drying process, surface charge changes from a negative value to a neutral value and to a positive value. That might be the reason why a gradual changing process can be observed when the solution is losing water. According to the work of Zhao et al.,47 the water in ionic liquids can be determined by cathodic differential pulse stripping voltammetry (CDPSV) with gold electrodes. When a positive potential is applied, the gold electrode can be electrooxidized by vestigial water in organic solutions, which leads to accumulation of an oxide film. Then, a negative potential is applied, and the gold oxide can be reduced back to gold. Similarly, considering the high iR drop in organic solutions, the water in DMF might also be detected using Au UMEs (Figure

(5)

where s is the minimum approach distance, which is the sum up of the ionic radius. For TPAs+TPB−, the value of s is 0.659 nm.37 The equilibrium regarding the association of the ion pair of TPAs+TPB− in organic solutions is written as K

TPAs+ + TPB− ⇄ TPAs+TPB−

(6)

The association constant (K) is written as K= ≈

θcγTPAsTPB (1 − θ )cγTPAs+(1 − θ )cγTPB−

=

γ θ · TPAsTPB 2 (1 − θ ) c γ±2

θ (1 − θ )2 c

(7)

where c is the concentration of TPAsTPB, γTPAsTPB is the activity of the ion pair, and γ± is the average activity coefficient of the salt (γ± = γTPAs+γTPB−). Since the ion pair is neutral, γTPAsTPB is taken to be 1. In the experiments, the solutions are very dilute to micromolar to millimolar ranges (γ± ≈ 1). Using eq 5 to replace θ yields K = 4000πNA

∫S

q

r 2 e 2q / r d r

(8) −1

where NA is the Avogadro’s constant (NA = 6.02 × 10 mol ). The value of K can be calculated using the integral of eq 8, and is equal to 3.32 M−1 for DMF and 3207 M−1 for DCE, which are in good agreement with previous report.45 Thereby, the values of θ with different concenntration of TPAsTPB can also be obtained and listed in Table 2. From Table 2, we can see that the fraction of free ions is dependent upon changes of the TPAsTPB concentration in the dried DCE solutions. Clearly, the lower the concentration of TPAsTPB, the larger the change of ICR will be. Similar results have also been obtained in the DMF solution (see Figure S6 in the SI). In Figure 4, when the value of fraction of ions (1 − θ) is higher (more free ions), the extent of ICR is larger. According to the analysis of the TPAsTPB concentration data, the correlation among the parameters (e.g., free ions and R) has been found (see the inset and curves 1 and 2). When the ion 23

E

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Figure 6. (A) Cathodic differential pulse stripping voltammograms obtained at the Au microelectrodes in the DMF with increasing water contents; (B) dependence of stripping peak current on concentration of water added into the DMF dried with molecular sieves. Cathodic differential pulse stripping voltammetry parameters are as follows: initial potential, 1.6 V; final potential, −0.5 V; increase potential, 0.004 V; amplitude, 0.025 V; pulse width, 0.06 s; pulse period, 0.2 s; deposition quiet time, 120 s; and sensitivity, 1× 10−10 A/V.

Figure 5. (A) Current−voltage response of a 20-nm-radius nanopipette in the DMF solution containing 10 μM TPAsTPB. For curve a, the DMF solution is tested without any pretreatments; for curve b, the same DMF solution is dried with molecular sieves. (B) The drying process and the arrows indicate the changing i−V curves with time. [Scan rate = 50 mV/s.]

the DMF, but the difference between before and after the drying process is not as much as that in DMF (see Figures S7 and S8 in the SI). This is because DCE is a hydrophobic solvent, the water content in DCE is much lower than that in DMF, which contains 8 ± 5 ppm water determined using the same standard addition method (Figure S9 in the SI), while the data obtained from the Karl Fischer titration method is 12 ± 8 ppm of water. Thus, even in the untreated DCE solution, the direction of ICR is downward, by virtue of lower water content. In Figure S7B, the ICR behavior in the DCE solution with 100 nm nanopipettes is also discussed. The progress of water taken up from atmosphere in the DCE is shown in Figure S8. As the amount of water in the DCE increases up to 190 ± 40 ppm, the direction of ICR will be from upward to downward. In order to explain the effects of water on ICR in organic solutions, we consider the possibility whether the silica surface can have different charges under different amount of water in organic solutions. Based on the donor−acceptor behaviors of silanol group in organic liquids,48,49 the mechanisms about the formation of EDL on silica in organic solutions are assumed as following: (I) It is very hard to completely eliminate the water in DMF and DCE. The surface silanol groups can be ionized according to the following manner where greater amounts of water exist:

6A). The magnitude of the peak current derived from the stripping process is a function of the water concentration in the DMF solutions. In the dried DMF solution, the shift of reductive stripping peak potential is ∼600 mV, which is consistent with the growth of gold oxide film. In contrast, the potential shift is relatively smaller. The main cause is the different water contents of the organic solvents. The stripping peak potentials shift with increasing water concentration, in accordance with the formation of thicker oxide films.47 The experiments have been done within a glovebox in order to eliminate the effect of water and oxygen in the air. Quantification of water in DMF was carried out using the standard addition method. The standard curve is shown in Figure 6B. It suggests that the DMF dried with molecular sieves still contains 180 ± 70 ppm of water, which is in agreement with the data obtained from the Karl Fischer titration method as 215 ± 50 ppm of water. Moreover, we estimate that the amount from the experiments of water in DMF exceeds 440 ± 60 ppm, the direction of ICR will be downward, which indicates that a negatively charged nanopipette might be present in organic solutions. The ICR phenomena in the DCE solution without pretreatments and using dried method are also studied (see Figure S7A in the SI). The effect of water is similar to that in F

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The silica surface should have negative charges, so that the ICR behavior should be similar to that in aqueous environments. The dashed line indicates that they then form a double layer with TPAs+ in the organic solutions. (II) When the organic solution is dried and the water content is lower, the water layer is probably too thin to distribute in the EDL. We have carried out a quantitative analysis of water in a rectangle with dimensions of 20 nm × 20 nm × 20 nm (which is approximately equal to the top 20 nm for a 20-nm-diameter nanopipette, and most of the ICR is probably resulted in this part). In the dried DMF solution which contains 180 ppm water (1 ppm water in DMF corresponds to 0.053 mM water), only ∼40 water molecules existed. Therefore, the solvent molecules can replace water and adsorb on the silica surface; there, a solvation surface appeared. Under this consequence, the surfaces could be positively charged. That is the possible reason why the direction of ICR is upward in dried organic solutions. Here, the dashed line has a meaning similar to that given above, and a double layer forms between the mixed solution and the organic phase.

Figure 7. Current−voltage response of a 20-nm-radius nanopipette in the DMF solution containing 10 μM TPAsTPB with the addition of trace amount of (A) HCl and (B) KOH. [Scan rate = 50 mV/s.]

voltage response is almost linear at a certain range. It indicates that near the tip of a nanopipette is totally silanized, and water does not have an impact on the ICR; thus, the ICR disappears. Then, away from the tip, the ICR emerges because of the incomplete silanization. The existance of water promotes the dissociation of silanol groups.

(III) From Figure 5 and Figures S7 and S8, we know that there is a critical point where R is ∼1. That might be because the above mechanisms work together on the silica surface. Under this situation, the number of water molecules in the rectangle is ∼110 in the DMF solution, and the surface is neutral.

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CONCLUSIONS In summary, the ionic current rectification (ICR) phenomena in organic solutions have been studied for the first time using quartz nanopipettes. The following conclusions can be drawn from this work: (1) ICR indeed exists in organic environments with quartz nanochannels but with opposite direction, compared with that observed in aqueous solutions; (2) the pore size, electrolyte concentration, and surface charge have influence on ICR in the organic phase (particularly, when the value of fraction of ions (1 − θ) is higher, the ICR is bigger, because of larger conductance difference into and out of the orifice); (3) water in organic solutions plays a very important role, relative to ICR, and we proposed the mechanisms about the formation of EDL on silica in organic solutions; and (4) an improved method to detect the water in organic solutions based on Au microelectrode with cathodic differential pulse stripping voltammetry has been implemented. We consider these findings to be an extension of the current exploration of ICR from a new point of view, and these are significant in understanding the mechanism of ICR and have

In order to test the above proposed mechanisms, we have carried out the following experiments: a tiny drop of acid (e.g., HCl) or base (e.g., KOH) has deliberately added to the organic solutions, and then the inner walls of pipettes will have a positively or negatively charged surface. Figure 7 shows the corresponding current−voltage curves (Figure 7A shows data for a positively charged surface and Figure 7B shows data for a negatively charged surface), which clearly demonstrate that the ICR is dominated by surface charges. We have also used trimethylchlorosilane (TMSCl) to silanize the inner walls of nanopipettes. Figure S10 in the SI shows that the current− G

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Article

Analytical Chemistry

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potential application in the development of novel nanodevices. More organic solutions are undertaken in our laboratory.

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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.5b02337. Experimental setup and data regarding ICR in various organic solutions, the determination of water in organic solutions, and further experimental tests on the mechanisms proposed (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-62759394. Fax: +86-10-62751708. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from the National Natural Science Foundation of China (NSFC) (Nos. 21335001 and 21075004).

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REFERENCES

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DOI: 10.1021/acs.analchem.5b02337 Anal. Chem. XXXX, XXX, XXX−XXX