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Quasi-Reference Electrodes in Confined Electrochemical Cells Can www.nature.com/scientificreports OPEN Quasi-reference electrodes in confned electrochemical cells can result in in situ production of Received: 13 September 2017 Accepted: 15 January 2018 metallic nanoparticles Published: xx xx xxxx Rukshan T. Perera & Jacob K. Rosenstein Nanoscale working electrodes and miniaturized electroanalytical devices are valuable platforms to probe molecular phenomena and perform chemical analyses. However, the inherent close distance of metallic electrodes integrated into a small volume of electrolyte can complicate classical electroanalytical techniques. In this study, we use a scanning nanopipette contact probe as a model miniaturized electrochemical cell to demonstrate measurable side efects of the reaction occurring at a quasi-reference electrode. We provide evidence for in situ generation of nanoparticles in the absence of any electroactive species and we critically analyze the origin, nucleation, dissolution and dynamic behavior of these nanoparticles as they appear at the working electrode. It is crucial to recognize the implications of using quasi-reference electrodes in confned electrochemical cells, in order to accurately interpret the results of nanoscale electrochemical experiments. New insights into nanoscale chemical systems have come hand in hand with experimental techniques that can probe ever-smaller volumes, but traditional physiochemical models cannot necessarily be applied to systems with nanoscale dimensions. Tis fact, combined with the availability of new fabrication methods to prepare nano- scale electrodes and nanoconfned electrochemical devices, has inspired a new branch of study referred to as “nanoelectrochemistry”1–6. Physical size reductions yield important benefts such as reduced background noise, reduced iR drop, and fast mass transport rates, and these features can be used to study phenomena at length scales approaching the dimensions of single molecules7–15. In addition to fundamental studies of nanoparticle electrocatalysis16–18, single nanoparticle detection19–22, single-molecule sensing23–26, and electrochemical imaging27–30, miniaturized electro- chemical systems have found applications in energy conversion and storage31–33. However, the extreme sensitivity required by nanoscale electrochemical measurements can also open the door to new complications. Numerous experimental hazards must be carefully managed in order to acquire meaning- ful data sets which are large enough to have sufcient statistical power. Tese include challenges with substrate cleanliness, background noise, sample purity, and nanoelectrode fouling, among others. It is important to under- stand the mechanisms behind these complications, in order to be in a better position to avoid them. In an ideal three-electrode electrochemical cell, the reference electrode is isolated from the bulk solution using a glass frit or salt bridge, and the counter electrode is positioned far from the working electrode. However, for reasons of size, cost, and complexity, miniaturized analytical devices ofen do not have this luxury. Instead, many systems use a non-isolated “quasi-reference” electrode, such as a simple Ag/AgCl wire. In some cases it is even common to eliminate the third electrode, and to instead use a simpler 2-electrode cell which balances the working electrode with a single “quasi-reference counter electrode” (QRCE). Such arrangements can be justifed if the current measured is very small and the surface area of the QRCE is relatively large compared to the WE. However, as the distance between the WE and QRCE decreases, or the duration of an experiment increases, the difusion of redox species generated at the QRCE to the working electrode may not be negligible. For example, a recent article reported artifacts observed in a lab-on-a-chip microelectrode arrays when non isolated Ag/AgCl reference electrodes are used34. It would be valuable to understand the origin of such artifacts, and the physical processes that guide them. School of Engineering, Brown University, 184 Hope Street, Providence, RI, 02912, USA. Correspondence and requests for materials should be addressed to J.K.R. (email: [email protected]) SCIENTIFIC REPORTS | (2018) 8:1965 | DOI:10.1038/s41598-018-20412-2 1 www.nature.com/scientificreports/ Figure 1. Schematic representation of the experiment (not to scale). A glass capillary is carefully positioned so that its tip forms a nanometer-scale liquid meniscus with a working electrode. In this study, we use a scanning nanopipette contact35–40 to show that a QRCE in a miniaturized electro- chemical cell can lead to in situ nanoparticle generation even in the absence of any precursor. We explore the key experimental factors responsible for this phenomenon. Tis is a critical practical observation that should not be ignored, as it will ultimately determine the achievable detection limits in nanoelectrochemical experiments. Results and Discussion All experiments were performed using a scanning nanopipette contact setup (Fig. 1). Briefy, a quartz capillary with ~40 nm diameter is fabricated with laser-assisted pipette puller. Te pipette is flled with electrolyte and held by a nanopositioning stage directly above a working electrode. A QRCE is inserted into the pipette, and a bias voltage (−0.2 V) is applied between the QRCE and the working electrode. Te pipette is lowered in 100-nm increments until its tip forms a liquid meniscus contact with the working electrode. Tis meniscus is detected by a characteristic current spike observed during the contact due to electric double layer charging at the interface. Once a meniscus is established, the pipette movement is halted. All potentials are reported as V vs QRCE used in the specifc experiment. First, we utilized cyclic voltammetry to evaluate the interference of Ag/AgCl QRCE electrodes commonly used for scanning pipette measurements. No electroactive species were present in the electrochemical cell, and we selected 0.1 M LiCl as representative of chloride electrolytes commonly used with an Ag/AgCl QRCE (to obtain a − stable potential by maintaining the reaction: AgCl(s) + e Ag(s) + Cl (aq)). A glassy carbon (GC) electrode was used as the working electrode. Once a meniscus contact was established, cyclic voltammograms were performed continuously at 0.1 V/s for Ag/AgCl QRCE for a period of 130 min, as shown in Fig. 2. Initially, the i-V curve did not show any signifcant oxidation or reduction current. However, afer 40 min of continuous measurements (Fig. 2b), signs of redox activity were observed. Tis redox behavior increased in prominence until 110 min had elapsed, with an oxidation peak near −0.05 V and crossover point ~0.2 V. Te shapes of these cyclic voltammo- grams are reminiscent of metal deposition and stripping41, and sharp current transients just above afer bulk oxidation potential suggest the presence of discrete oxidizable nanoparticles. Tese transients will be explored in more detail in later sections. We analyzed the total oxidation charge transferred during a voltammetry sweep, which exhibits a sigmoidal curve as time elapses (Fig. 3). Here we performed experiments for 160 min, and similarly to Fig. 2, no elec- trochemical oxidation is visible until afer approximately 40 min. Te calculated oxidation charge in each scan increases steadily over the time until settling at ~60 pC (~0.6 femto mols) afer 130 min. We hypothesize that this asymptotic curve is due to a reduced nucleation rate caused by depletion of metal ions in the vicinity of the work- ing electrode, leading to difusion-limited conditions for forming nanoparticles from dissolved ions. Stripping behavior depends on the QRCE. To test how this stripping behavior at the working electrode may change with other QRCEs, we used another commonly used Cu/CuCl2 QRCE with the same experimental set up under similar experimental conditions. Te observed results are compared with Ag/AgCl in Fig. 4. As indicated in Fig. 4(a), neither QRCE shows visible oxidation or reduction immediately afer contact. However, as the time progressed, similar deposition-stripping behavior was observed with the Cu/CuCl2 QRCE, as shown in Fig. 4(c). Proposed mechanism for the in situ generation of nanoparticles. It is worthwhile to emphasize once again that the electrolyte used in these experiments did not contain any electroactive species or added metal ions. Te redox activity is also not due to dissolved gas and is instead characteristic of metal ion dissolution and deposition. Terefore, we hypothesize that silver and copper ions are released from the QRCE, and are responsible for the observed redox activity. Te reversible reactions at the QRCE are as follows, SCIENTIFIC REPORTS | (2018) 8:1965 | DOI:10.1038/s41598-018-20412-2 2 www.nature.com/scientificreports/ Figure 2. Cyclic voltammograms showing progress of deposition-dissolution behavior over the time. Representative CVs are shown afer (a) 10 min (b) 40 min (c) 45 min (d) 50 min (e) 65 min (f) 80 min (g) 95 min (h)110 min (i) 130 min afer initial contact on GC electrode vs Ag/AgCl QRCE. All voltammograms were recorded at 0.1 V/s. Figure 3. Total oxidation charge for a single voltammetry scan, calculated over the course of 160 min for Ag/ AgCl QRCE and a GC working electrode. (a) Integrated oxidation charge versus time elapsed. Te data is ft to a Hill-1 sigmoidal model. (b) An example i-t trace from a voltammogram afer 110 min. Inset: expanded sections of the oxidation current peaks, showing the integrated charge area. Additional i-t traces used to plot (a) are shown in Fig. S2. ++− AgCl(s)(eA gCs) l (aq) − CuCl2(s) ++2e Cu(s)(2Cl aq) At negative applied potentials, reduction current at the working electrode is balanced by an oxidation reac- tion at the QRCE. At sufcient reduction potentials vs QRCE, oxidized Ag/Cu formation is favorable. Te fate of SCIENTIFIC REPORTS | (2018) 8:1965 | DOI:10.1038/s41598-018-20412-2 3 www.nature.com/scientificreports/ Figure 4. Cyclic voltammograms recorded with a GC working electrode afer ~110 min vs Ag/AgCl and Cu/ CuCl2 are indicative of deposition and dissolution.
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