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Fabrication of Metal Nanoelectrodes by Interfacial Reactions Xinyu Zhu, Yonghui Qiao, Xin Zhang, Sensen Zhang, Xiaohong Yin, Jing Gu, Ye Chen, Zhiwei Zhu, Meixian Li, and Yuanhua Shao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac501119z • Publication Date (Web): 23 Jun 2014 Downloaded from http://pubs.acs.org on June 26, 2014

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Fabrication of Metal Nanoelectrodes by Interfacial Reactions Xinyu Zhu, Yonghui Qiao, Xin Zhang, Sensen Zhang, Xiaohong Yin, Jing Gu, Ye Chen, Zhiwei Zhu, Meixian Li and Yuanhua Shao* College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China To whom correspondence should be addressed. Email: [email protected]. Tel: +86-10-62759394. Fax: +86-10-62751708. RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

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ABSTRACT

Despite great improvements in the past decades, the controllable fabrication of metal nanoelectrodes still remains very challenging. In this work, a simple and general way to fabrication of metal nanoelectrodes (Ag, Au and Pt) is developed. Based on interfacial reactions at nano-liquid/liquid interfaces supported at nanopipettes, the nanoparticles can be formed in situ and have been used to block the orifices of pipettes to make nanoelectrodes. The effect of the driving force for interfacial reaction at the liquid/liquid interface, the ratio of redox species in organic and aqueous phases, and the surface charge of inner wall of a pipette have been studied. The fabricated nanoelectrodes have been characterized by scanning electron microscopy (SEM) and electrochemical techniques. A silver electrode with about 10 nm in radius has been employed as the scanning electrochemical microscopy (SECM) probe to explore the thickness of a water/nitrobenzene (W/NB) interface and this value is equal to 0.8±0.1 nm (n=5). This method of fabrication of nanoelectrodes can be extended to other metal or semiconductor electrodes.

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INTRODUCTION Since the first inception at about 30 years ago, ultramicroelectrodes (UMEs) have tremendously extended the electrochemistry both spatially and temporally.1-8 Due to the small size, these UMEs can significantly reduce iR drop, charging current and enhance mass transport.9 Therefore, they have been frequently employed to explore events in no or little supporting electrolyte media and various microenvironments, such as monitoring chemical and biological processes near a living cell in vivo.10-13 In addition, UMEs have been implemented as the probes for scanning electrochemical microscopy (SECM) to measure heterogeneous electron transfer (ET) rate constants for the reactions at various surfaces and interfaces, and high spatial resolution imaging and microfabrications.14,15 Nanoelectrodes are such electrodes of which at least one dimension is smaller than 100 nm, further reinforce above advantages and provide the possibility to study the electrochemistry and electrocatalysis of single nanoparticle, single molecule detection, single enzyme reaction and monitoring chemical and biological processes within a living cell.16-19 For example, Zhang et al. utilized single gold nanoparticle electrode to examine the oxygen-reduction reaction in KOH solution, and the electrocatalytic activity was proved to be dependent on the size of the gold nanoparticles. This remarkable study explored the structure-function relationship in nanoparticle-level electrocatalysis.16 Sun and Mirkin introduced a nanometer-sized electrochemical thin layer cell with zeptoliter volume to detect individual molecules, as well as studying physicochemical processes at the single molecule level.17 Lemay et al. used scaling down protein film voltammetry to study the electrochemical behavior of a redox-active protein. This method provides a new tool for studying behaviors of few redox enzymes.18 Amatore et al. used the Pt/Pt black nanoelectrodes to determine reactive oxygen and nitrogen species inside macrophages, which could not be accomplished by the previous micrometer-sized electrodes.19 The first nanoelectrode was a nano-band electrode and reported in the 1987 By White et al. 20 This type of electrode was fabricated by three steps: deposition of a thin metal film onto a flat substrate, sealing by the insulation material and the final exposure. Thereafter, many nanoelectrodes with different geometries including inlaid disk, hemisphere, sphere, ring, conical and nanopore, and more fabrication ACS Paragon Plus Environment

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approaches have been reported.21-25 Fabrication of solid nanoelectrodes generally rely on either topdown or bottom-up methods. A representative top-down method is insulating and subsequently exposing the electrochemically etched thin metal (or carbon) films or wires. The insulation materials used include glass, wax, polymer and electrophoretic paint.26-29 Laser-assisted wire pulling is another popular topdown fabrication method. By this way silver and platinum nanoelectrodes have been fabricated respectively.30-32 One of the bottom-up fabrication approaches is based on the pyrolysis of butane or other gases. This method is popularly used to fabricate carbon electrodes.33, 34 Confining nanoparticles in a limited space is another promising bottom-up method to make metal nanoelectrodes. Bard et al. used a dithiol as linking agent to assemble gold nanoparticles at the orifice of a pipette to make microelectrodes.35 Zhang et al. and Mirkin et al. employed metal nanoparticles to block nanopores to form nanoelectrodes.36,

37

An advantage of this bottom-up method is that nanoparticles are not

expensive, and they can be formed in smaller dimensions in the aqueous solution under controllable experimental conditions.38 Furthermore, compared to polish and etch the fragile nanowires, the use of metal nanoparticles to block pipettes is easier to operation and can be possibly high-throughput. Despite vast improvements in the past decades, the controllable fabrication of metal nanoelectrodes still remains very challenging even today, and one has to develop different method for different metal. Charge (electron and ion) transfer reactions at a liquid/liquid (L/L) interface are closely related to drug delivery, mimicking biological membranes and phase transfer catalysis.39 The miniaturization of a L/L interface was pioneered by Girault et al. in 1986 through supporting it at a micropipette,40 which is actually the extension of patch clamping technique in studying of charge transfer process at soft interfaces. In 1997, the sizes of L/L interfaces have been further decreased to nanometers.41 The functionality of micro and nano-L/L interfaces for investigation of charge transfer processes at soft interfaces is similar to that of an UME for a solid/liquid interface. The interfacial formation of nanoparticles at a L/L interface has been attracted much attention recently.42 Different metal nanoparticles, such as Cu, Pd, Pt, Ag and Au, have been successfully synthesized.43, 44 In this work, we report a novel and general way to fabricate metal nanoelectrodes. By using a glass ACS Paragon Plus Environment

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nanopipette to provide a limited space, a nanoelectrode can be made by the self-assembled nanoparticles which are produced in-situ by the interfacial reactions at a liquid/liquid interface.45-47 By this approach, Ag, Au and Pt electrodes with radii from nano to micrometers have been fabricated. This method is simple, controllable and can be extended to other metal or semiconductor electrodes. In addition, using a silver nanoelectrode with the radius of about 10 nm as a scanning electrochemical microscopy (SECM) probe, the thickness of a water/nitrobenzene (W/NB) interface has been extracted from the approach curve and is less than 1 nm.

EXPERIMENTAL SECTION Chemicals. Silver nitrate (AgNO3, ≥99.8), sodium perchlorate (NaClO4, ≥99.5%), potassium chloride (KCl, ≥99.5%), lithium chloride (LiCl) and 1,2-dichloroethane (DCE, ≥99%) were purchased from Beijing Chemical Co. Nitrobenzene (NB, ≥99%, J&K), decamethylferrocene (DMFc, ≥95%, Fluka), dibenzo-18-crown-6 (DB18C6, ≥98%, Alfa Aesar) were used as received. Tetraphenylarsonium chloride (TPAsCl, 97%), sodium tetraphenylborate (NaTPB, 99.5%), potassium tetrakis (4chlorophenyl) borate (KTPBCl, 98.0%), bis (triphenylphosphoranylidene)ammonium chloride (BTPPACl, 98.0%) and hexaammineruthenium (III) chloride (Ru(NH3)6Cl3, ≥98%) were from Aldrich. Tetraphenylarsonium tetraphenylborate (TPAsTPB) and bis(triphenylphosphoranylidene)ammonium tetrakis(4-chlorophenyl)borate (BTPPATPBCl) were prepared using the method reported previously.48 All aqueous solutions were prepared from triply distilled water. Fabrication of Nanopipettes. Nanopipettes of different sizes were fabricated from quartz capillaries (1 mm outer diameter, 0.7 mm inner diameter) using a laser-based P-2000 pipette puller (Sutter Instrument Co.). All capillaries were thoroughly cleaned with a fresh piranha solution (Caution! Piranha solution is highly corrosive and reactive. Extreme care should be taken when handling Piranha solution. Glass or pyrex containers should be used. Appropriate gloves, eye protection and body protection are necessary.) and rinsed thoroughly with water before use. The outer walls of the nanopipettes were silanized prior to use by the following method: Fixing a nanopipette 1 cm above the solution of 5 ACS Paragon Plus Environment

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chlorotrimethylsilane and DCE (volume ratio with 1:5) for 2 min, while passing a flow of argon through the pipette to avoid silanizing the inner wall. The aqueous solution was filled from the back of the nanopipette by a small syringe (10 µL). Prior to each experiment, the pipettes were inspected by an Olympus BX-51optical microscope to ensure that no air bubbles were trapped inside. Hitachi S-4800 scanning electron microscope (SEM) was used to inspect the nanopipettes and nanoelectrodes. About 3 nm thick Au layer was coated on the insulating nanopipettes and by this way they could be imaged without obvious charging. Preparation of Nanoelectrodes. 10 mM AgNO3 and 100 mM NaClO4 were injected into the pulled pipettes. To fabricate different sized electrodes, the pipettes were immersed into the NB solutions with 100 mM TBAClO4 and concentrations of DMFc ranging from 2 mM for an about 20 nm radius electrode to 20 mM for an about 190 nm radius electrode. After 12 h reaction, the electrodes were sonicated in the ethanol for 5 minutes gently. The tip end of the pipette was densely blocked by the silver nanoparticles, and the impurities were thoroughly removed. Because of the fragile nature of the tips, much care must be taken in this step. Finally, the electrical connection between the silver electrode and external circuits was accomplished by inseting one silver wire into inner solution. Most steps were the same for fabrication of Au and Pt nanoelectrodes. Some differences were: 1) 4.8 mM HAuCl4 and 100 mM NaClO4, 5 mM H2PtCl6 and 100 mM NaClO4 were employed to form the corresponding nanoparticles. 2) 20 mM HAuCl4 and saturated KCl, 19 mM H2PtCl6 and saturated NaCl were used to conduct the systems, respectively. Electrochemical Instruments. A BAS-100B electrochemical workstation (Bioanalytical Systems, Inc.) was used to characterize the electrodes and pipettes. One silver, gold or platinum wire was inserted into the nanopipette to conduct the corresponding system. Another Ag/AgCl wire was placed in bulk aqueous solution to be an auxiliary/reference electrode. Measurements of the Thickness of a Water/Nitrobenzene Interface by SECM. The electrochemical cell used in the experiments has been described in our previous work.49 A two-electrode setup was employed with two 0.25 mm silver wires. One Ag/AgCl immersed in the solution inside the 6 ACS Paragon Plus Environment

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pipette served as an aqueous reference electrode, and the other Ag/AgTPB acted as an organic reference electrode in the top NB solution. The two liquid phases in all experiments were kept for 3 h or more before measurements to get a relatively stable interface. The phase contained 1 mM TPAsTPB in NB, and the bottom phase contained 10 mM LiCl and 3 mM Ru(NH3)63+ in water. This cell was mounted on a vibration isolation table (Technical Manufacturing Co.) and shielded in a Faraday cage. SECM measurements were performed using a ScanIC scanning ion conductance microscope (SICM, Ionscope). The tip electrode was biased at a potential (-0.5 V) where the reduction of Ru(NH3)63+ was diffusioncontrolled. The approach curves were obtained by moving the tip across the liquid/liquid interface. All experiments were carried out at room temperature (23±2oC). All precautions were considered in order to minimize the problem related to precisely control.

RESULTS AND DISCUSSION Interfacial Reactions at the Liquid/liquid Interface and the Fabrication of Nanoelectrodes. Figure 1A shows the principle of how can the metal nanoparticles form at a liquid/liquid interface, and block the nanopipettes. When a metal precursor (Ag+ as an example) located in the water phase meets a reductant (decamethylferrocene, DMFc) in the organic phase, and there is an enough interfacial potential drop (driving force), metal nanoparticles will be spontaneously formed at the interface.50, 51 The process can be briefly described as: Ag + (w) + DMFc(o) → Ag (s) + DMFc+ (o)

(1)

This electroless deposition process is mainly controlled by the driving force. For the heterogeneous ET reaction, it is determined approximately by the difference of potentials of the aqueous and organic redox couples (∆E = Eacceptor - Edonor, where Ag+ is the acceptor and DMFc is the donor) and the interfacial potential:52 ∆ G ≈ − F ( ∆ E + ∆ ow φ )

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o ∆E = ∆ OW EET +

W O RT  aO1 aR 2 ln  W O F  aR1 aO2

W

o ∆OW EET =  EOo1 R1  −  EOo 2 SHE

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   

(3)

O

R2

 SHE

(4)

o where ∆owφ is the Galvani potential drop across the interface;  EO1

W

R1

 and  EOo 2 SHE

O

R2

 are the standard SHE

potentials of each couple in its respective phase. ∆owφ is a Nernstian function of the concentration ratio of common ion in both phases:53 o w

o o w ClO -4

∆ φ=∆ φ

 ClO 4 -  w - 0.059log ClO 4 - 

(5)

o

All the experiments have been done with [ClO4-]w = [ClO4-]o, so that the ∆owφ at the W/NB interface, is o o equal to ∆ w φClO-4 , keeps at 80 mV during the experiments.54 As the standard potentials for the redox

o couples are constant value (  EAg +

Eo +  DMFc

 

W

Eo  Ag  SHE = 0.80V,  AuCl 4

W

W

 E o 2-   Au  SHE = 0.99 V,  PtCl6 Pt  SHE = 0.74 V,

O

DMFc SHE

=0.016 V) ,55 and the activities of them slightly affect the potential, therefore the ∆E is

controllable. In addition, the big differences between the standard potentials of metal (or complex) ions and DMFc suggest a large driving force which promises that the heterogeneous ET reactions can spontaneously happen, and the aqueous metal precursors Ag+, AuCl4- and PtCl62- can be easily reduced to corresponding nanoparticles. Figure 1B shows the optical image of a fabricated silver electrode. The black part at the tip end is the densely aggregated silver nanoparticles. This approach allows fabrication of Ag electrodes with the radii from 10 nm to a few micrometers. The sizes of Ag electrodes directly depend on the radii of initial pulled pipettes. An empty nanopipette with high quality is also a prerequisite for the subsequent good electrode reproducibility. Therefore, high quality pipettes are really needed. Based on our previous work,56, 57 they can be made by a laser puller with carefully choosing pulling parameters. The radii of the empty quartz nanopipettes can be electrochemically characterized by steady-state voltammetry of the facilitated K+ transfer at the ACS Paragon Plus Environment

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water/1,2-dichloroethane (W/DCE) interface by DB18C6 and SEM (for characterization details, see Figure S1 in the supporting information, SI) . Clearly different sizes of high quality nanopipettes can be made. Besides the radius of a nanopipette, the property of pipette wall also affects the size of nanoelectrode. Due to the hydrophilic nature, a thin aqueous film can be formed at the outer wall of a bare pipette. Such a film will appear when the pipette is immersed into an organic phase and leads to the true contact area of the liquid/liquid interface much bigger than the geometrical area of the pipette (Figure 2A).57 Metal nanoparticles will be randomly formed around the pipette orifice, and this brings much difficulty to control the size and geometry of the electrodes (Figure 2C). This aqueous film can be efficiently avoided by silanizing the outer pipette wall, and a confirmed flat interface can form at the orifice (Figure 2B).5759

As shown in Figure 2D, the nanoparticles will be orderly formed and well assembled at the orifice of

a silanizied pipette. Figure 2E shows the steady-state currents before and after silanization. The steadystate current before silanization is about 5-6 times larger than the one after silanization. This value is even bigger than what we obtained for the cases of charge transfer at a L/L interface.57 Clearly, it is necessary to silanize the pipettes in order to controllably and successfully fabricate the smaller electrode, and all the pipettes used in the further experiments have been silanized. Characterization of Metal Nanoelectrodes. Using a fabricated silver nanoelectrode as example, Figure 3 shows the electrochemical and SEM characterizations. The size and geometry of an electrode and the geometry of the insulator are three key parameters. For electrochemical characterization, the steady-state current can be used to calculate the size of a nanoelectrode when we know the geometries of the electrode and surrounding insulator. Because different shaped electrodes have different calculation equations, it is necessary to do SEM characterization to obtain the electrode geometry. From the SEM side image (Figure 3C), we can clearly see the electrode does not protrude out of the pipette orifice. The exposed electrode surface is almost in the same plane with the surrounding insulator. From the SEM top view (Figure 3B), we can also easily judge that the fabricated electrode has a disk shape and does not recess into the pipette. Meanwhile, we can directly see the metal part is pinning onto the glass and ACS Paragon Plus Environment

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densely aggregates. No obvious leakage can be estimated by the cyclic voltammograms (see Figure S2). The ratio of insulator thickness to radius (RG) also affects the mass transport. Thus, the equation used to calculate the electrode radius can be expressed as following:58, 60 iss = AnFDca A = 4.00+ 4B(RG-C)d

(6) (7)

where D and c are the diffusion coefficient and the bulk concentration of the electroactive specie, F is the faraday constant, n is the number of transferred electrons, and a is the effective radius of the electrode. RG is the ratio of the radius of the pipette glass sheath (rg) to the orifice raduis (a). From Figure 3A, we can estimate the RG is equal to 1.2. According to Ref. 58, B, C and d are equal to 0.1380, 0.6723 and -0.8686. Consequently A in equation 6 is equal to 4.96. The radius of the electrode in Figure 3D is calculated to be 91 nm. From the top SEM view (Figure 3B), the radius is estimated to be about 80 nm. The difference from two characterization methods might be attributed to the rough electrode surface. The electrodes here are fabricated by the aggregated metal nanoparticles, which should have rough and larger electrochemical active area. We have found that the effect of the concentration ratio of redox couples in both phases on controllable fabrication process is significant. Figures 4A and 4B show the SEM diagrams of the silver electrodes made by this approach using the same experimental conditions, that is, the same concentration ratio of redox couples. Clearly, it is hard to completely block the orifice of a pipette when its radius is larger. Based on equations 1-5, the concentrations of them will influence the thermodynamic parameters for this interfacial reaction. However, the effect of concentration variations of redox species on the driving force is limited compared with the differences of standard potentials. Another possibility is that kinetics of the interfacial reaction is also playing an important role. If the concentration of Ag+ is fixed, the interfacial reaction can be considered as a first-order ET process. The rate constant k can be described by the following equation:61

k = KcDMFc exp ( −∆G / RT )

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where K is a constant, cDMFc is the concentration of DMFc in the organic phase; ∆G is the Gibbs energy for this ET reaction which have been discussed in eq 2. Thus, the concentration of DMFc will directly control the ET rate for this interfacial reaction. For the different sized pipettes, different cDMFc is necessary. Figures 4C, D and E are the silver electrodes with different sizes made by this approach. They have been fabricated by changing the concentration ratio of the two redox couples, i.e., ρ = cAg+/cDMFc. Here, a set of ρ values of 10:20, 10:5 and 10:2 are designed to get the different sized electrodes. From the top SEM views of the electrodes, we can find that silver nanoparticles densely aggregated at the orifice of the nanopipette, and the size distribution of them is relatively large due to the fast interfacial reaction. All of those fabricated electrodes have well-defined voltammograms. The morphology of the silver nanoparticles are in good agreement to the nucleation and growth theory proposed lately by Ustarroz et al.62 Figure 5 depicts the possible mechanism of the nanoparticles formation by this electroless deposition process. Silver nuclei first form at the liquid/liquid interface, and then they aggregate to the silver nanoclusters.63 These nanoclusters diffuse together along the interface, and coalescence to form the bigger aggregation with different sizes.64 Because of fast nucleation and growth rate of the silver nuclei,65 the initial cluster grows with increasing deposition time, and becomes larger. Then, these Ag clusters turn to be Ag nanoparticles. The formed silver nanoparticles have large size distribution and tend to form spherical shape due to the silver nanoclusters with full recrystallization.62 These aggregated nanoparticles are not equal to a nanoelectrode yet. Only when the nanoparticles can densely block the orifice of a pipette and connect to a conductive wire with outside, and then the electrode can be made successfully. Therefore, the surface property of the inner wall of a pipette is crucial for this step. When pH value of the aqueous solution inside the pipette is larger than 3,66, 67 the Si-OH will be ionized to Si-O-, and the inner wall will have negative charge which is easy to attract the positive charged Ag+ (see Figure 5B). In addition, there is a strong interaction between O-Ag.68 Multiple interactions between the inner wall and the silver are stable enough for the pinning of Ag nanoparticles onto the pipette wall. After the interfacial reaction terminate, 5 min ultrasonic treatment in the ethanol has been carried out to ensure the nanoparticles densely block the ACS Paragon Plus Environment

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orifice of pipette. This method has resulted in rather high successful rate (see Figure S2). The stability of the fabricated Ag nanoelectrodes has also been tested and the results show good performance (Figure S2). Similar to the fabrication process for Ag electrodes, Au and Pt nanoelectrodes have also been successfully made. However, there are also some differences. 4.8 mM HAuCl4 and 100 mM NaClO4 (aqueous phase), 20 mM DMFc,100 mM TBAClO4 (organic phase) were used to fabricate gold nanoelectrode as described in Figure S3A. 5 mM H2PtCl6 and 100 mM NaClO4 (aqueous phase), 20 mM DMFc,100 mM TBAClO4 (organic phase) were used to fabricate platinum nanoelectrode as described in Figure S3B. As the HAuCl4 and H2PtCl6 are both strong acid, the pH values of used solutions are 2.3 and 2.0 respectively, which are both smaller than the Si-OH deionization point. Under this experimental condition, the protons will concentrate near the inner wall, and they will provide a whole positive environment for the interaction with negative metal precursors (AuCl4- and PtCl62-). Beside the electrostatic force, interactions of the Au-O and Pt-O also help the nanoparticles growing on the inner wall. Additionally, different from the silver nanoparticles with large size distribution, gold and platinum nanoparticles are more uniform, which can be attributed to the slower reduction, clustering rates and the less degree of coalescence.62, 69 Evaluation of the Thickness of a W/NB Interface by SECM. We also demonstrate that the fabricated nanoelectrodes can be employed as SECM probes to explore the thickness of a liquid/liquid interface. Here, the SECM measurement is performed by a SICM setup with a 10 nm silver nanoelectrode as the probe. The thermodynamics and kinetics of a charge transfer reaction at soft interfaces depend directly upon the interfacial structure. Bard et al. have demonstrated that the thickness of a W/NB interface is less than 4 nm using SECM with a Pt tip of 25 nm in radius.70 Later on we have used a nanopipette with about 4 nm in radius filled with KCl solution as the SICM probe to investigate the thickness of a W/NB interface and ion distribution near the interface.49 The improvements both on vertical resolution of SICM and the smaller size of the probe have resulted in much less thickness of the W/NB interface (less than 1 nm) which is in good agreement with theoretical predication and ACS Paragon Plus Environment

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spectroscopic results.71-73 Figure 6 shows the approach curve obtained by a 10 nm Ag electrode across the interface from top NB contained 1 mM TPAsTPB into the bottom aqueous solution with 3 mM Ru(NH3)6Cl3 and 10 mM LiCl. Positive horizontal coordinate means the nanoelectrode is in NB, and negative is in the aqueous solution. The tip current shows a gradually decreasing trend as the ET occurring when it touches the aqueous phase. Since Ru(NH3)63+ species hardly penetrate the interface and its distribution between NB and water is negliglible,74 only the reaction layer of Ru(NH3)63+ in water needs to be considered. The assumption for the thickness of the reacting diffuse layer can be extracted from the decrease region in tip current when the nanoelectrode goes through the L/L interface.70 From the current transition region in Figure 6, the thickness of the W/NB interface can be estimated and is equal to 0.8±0.1 nm (n=5). This value is very close to what was obtained by SICM with a nanopipette.49

CONCLUSIONS In summary, we have developed a simple, general and high-throughput method for fabrication of Ag, Au and Pt nanoelectrodes. This is the first report that a nanoelectrode can be made by the selfassembled of nanoparticles which are produced in-situ by the interfacial reactions at a liquid/liquid interface supported at a nanopipette. Several efforts are employed to control the fabrication process which including the thermodynamic (by proper selection of redox couples in both phases) and kinetic (by controlling reactant concentrations) means of the interfacial reactions. Another important point is that the property of outer wall of the pipette which affects the controllable fabrication of nanoelectrodes. The outer wall of the pipette has to be silanized in order to achieve such purpose. These fabricated electrodes have been characterized by SEM and cyclic voltammetry in detail. The fabricated silver nanoelectrodes with radii of about 10 nm are successfully applied to evaluate the interfacial thickness of a W/NB interface. The obtained thickness is less than 1 nm which is in good agreement with theoretical predication. This fabrication method has the significant potential to be a general approach to make other metal or semiconductor nanoelectrodes.

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ACKNOWLEDGEMENT We thank the financial support from the National Natural Science Foundation (NSFC) (21335001, 21075004).

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FIGURE CAPTIONS Figure 1. Principle of fabrication nanoelectrodes. (A) Schematic diagram for formation of silver nanoparticles at the liquid/liquid interface supported at a nanopipette. (B) Optical microscopy image of an Ag nanoelectrode.

Figure 2. Comparison of the fabricated silver electrodes using bare and outer wall silanized pipettes. Schematic diagrams for the interfaces supported at the bare pipette (A) and the silanized pipette (B). SEM images of the electrodes based on the bare pipette (C) and silanized one (D). (E) Cyclic voltammograms for two electrodes in the solution with Ru(NH3)63+ (5 mM) and KCl(100 mM). Curve 1 stands for the voltammetric curve obtained by the electrode made from the silanized pipette. Curve 2 represents one obtained by the electrode made from the bare pipette.

Figrue 3. Characterization of a representative silver nanoelectrode made by this method. (A) A SEM image of the initial empty pipette. a is the orifice radius, rg is the radius of pipette glass sheath. (B) A SEM top view of the electrode. This silver nanoelectrode was fabricated based on the nanopipette which is almost identical with the one shown in A. These two nanopipettes were pulled from one quartz pipette at the same time. (C) A SEM side view of the electrode. (D) Steady-state voltammogram of the same electrode in a solution with Ru(NH3)63+ (5 mM) and KCl (100 mM). The scan rate was 100 mV/s.

Figure 4. SEM and electrochemical characterizations for different sized electrode fabricated with the matched experimental conditions. SEM images of the different sized pipettes blocked by the same experimental condition. (A) SEM image of 2.5 µm radius pipette partially blocked by the 10 mM Ag+ and 20 mM DMFc. (B) SEM image of 200 nm radius pipette densely blocked by the same condition as shown in (A). (C) ~190 nm radius electrode by 20 mM DMFc. (D) ~50 nm radius electrode by 5 mM DMFc. (E) ~25 nm radius electrode by 2 mM DMFc. The steady state voltammograms of electrodes

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were obtained in a aqueous solution with Ru(NH3)63+ (5 mM) and KCl (100 mM). The scan rate = 100 mV·s-1

Figure 5. A) Possible mechanism of silver nanoparticles formation by the interfacial reaction at the liquid/liquid interface. I, silver ions are reduced to silver atoms; II, silver atoms aggregate to be clusters; III, silver clusters further coalencesene to be particles. B) Processes of the silver nanoparticles densely blocking the nanopipettes. First, silver ions absorb onto the negative charge site of the inner quartz wall. Second, these binding silver ions are reduced to silver nanoparticles, while other sized silver nanoparticles form at the liquid/liquid interface supported at the nanopipette orifice. Third, all silver nanoparticles aggregate to block the orifice of pipette.

Figure 6. SICM approach curve for a 10 nm Ag nanoelectrode tip moving perpendicularly across the W/NB interface: 1 mM TPAsTPB (NB) || 3 mM Ru(NH3)63+,10 mM LiCl (W). The tip potential was held at -0.5 V vs. Ag quasi-reference electrode. Scan rate was 50 nm/s.

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Fabrication of metal nanoelectrodes by interfacial reactions.

Despite great improvements in the past decades, the controllable fabrication of metal nanoelectrodes still remains very challenging. In this work, a s...
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