Professor Compton is a fellow of the Royal Society of Chemistry and Professor of Chemistry at Oxford University. He is also Editor-in-Chief of Electrochemistry Communications, Fellow of the International Society of Electrochemistry. In 2011 Professor Compton was awarded the RSC Sir George Stokes Award.

Electrochemistry: Volume 11 Nanosystems Electrochemistry

A Review of Recent Literature

By R. G. Compton, J. D. Wadhawan

The Royal Society of Chemistry

Copyright © 2013 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-401-1

Contents

Preface Richard Compton and Jay Wadhawan, v,
Electrochemistry to record single events Xiao-Shun Zhou and Emmanuel Maisonhaute, 1,
Electrocatalysis at nanoparticles Carlos M. Sánchez-Sánchez, Jose Solla-Gullón and Vicente Montiel, 34,
Bipolar electrochemistry in the nanosciences Gabriel Loget and Alexander Kuhn, 71,
Nanocarbon electrochemistry Martin Pumera, 104,
Electrochemistry within template nanosystems Mathieu Etienne and Alain Walcarius, 124,
Electrochemistry within liquid nanosystems Jonathan E. Halls and Jay D. Wadhawan, 198,

CHAPTER 1

Electrochemistry to record single events

Xiao-Shun and Emmanuel Maisonhaute

DOI:10.1039/9781849734820-00001

1 Introduction

From the nineteenth’s century pioneers such as Faraday, electrochemists have been tracking or exploiting the consequences of charge transfer. In the early 20th century, the polarography-derived methods allowed the first microelectroanalysis and the rise of the mechanism notion. In molecular electrochemistry, very complex paths could be (and still are) elucidated by cyclic voltammetry. The main advantage of electrochemical techniques is that the diffusion rate can be adjusted and thus used as a reference toward the timescale of the event to be studied. The present temporal resolution is about 10 ns, which amounts to disturb the concentration profiles near the electrode only over about one nanometre. Concomitantly, for sensor or more fundamental applications, the size of the electrodes is decreased gradually to attain presently a few nanometres.

Nevertheless, whatever the approach, the thermodynamic and kinetic information derived from these approaches still represent an average over a large number charge transfer events from individual structures. There would be of course a great interest to get the possibility to track individual electron transfer in electrochemical systems, as has been realised by solid-state physicists in very specific systems, but it is presently impossible in electrochemistry.

In solution, spectroscopic tools have been the first to demonstrate that a signal coming from individual molecules could be collected. Here, the high number of excitation/fluorescence cycles are occurring while a molecule travels through the confocal volume of a microscope furnishes a measurable flux of photons. More information such as fate due to chemical reactions or photobleaching can be obtained relying on correlation measurements.

In electrochemistry, for amplifiers used in classical electronics, increasing the gain induces a decrease in the bandwidth (in the simplest theory the gain × bandwidth product is constant). The quantity to consider is thus the minimum number of electrons that can be detected. With the best commercial systems available at present, this number is about 1000, which already allows to monitor the activity of solely a few tens of enzymes acting on a nanoelectrode. One physical reason for such limitation is that to be detected one electron should induce a significant perturbation on one observable, for example the current ?owing through a transistor. The electrochemical limitation is that the double layer capacity at electrode/ solution interface induces a noise. This is one of the reasons for the rise of ultramicroelectrodes. Unfortunately, stray capacitance takes over when interfacial capacitance reaches subpicofarad value, a limit easily attained corresponding to electrode diameter around 1 mm in standard conditions. Thus, at present only multiple electron transfers have been detected in electrochemical systems, and reaching the single electron is still a great challenge. This may induce the same revolution than when individual photons could be detected in photochemistry. Last, but not least, when individual events are probed, some special precautions should be taken when analysing the data, and statistical analysis is mandatory.

There are however still several scientific purposes to get information from individual systems. Indeed, the statistical analysis gives access to the fluctuations of the system, which is another way to access the information using the fluctuation-dissipation relationship. Second, several populations may be identified from the average information, which helps rationalising the global response observed onto large systems, for example in view of optimising catalytic systems. This review will highlight recent developments where electrochemistry is demonstrated to be a great tool to investigate single systems behaviour. The first section will be devoted to individual systems dispersed in solution and detected individually at small electrodes. Next, we will focus onto the contribution of electrochemistry in break junction experiments, thus with a focus onto molecular electronics. Last, we will underline that a good temporal resolution allows visualisation of nanometric interfaces evolution with the example of acoustic cavitation.

2 Individual systems explored with nanoelectrodes

2.1 Redox cycling

The first way to detect single molecules is to provoke a redox cycling of the same entity between two electrodes polarised so as to induce reduction on the first and oxidation on the second. The same molecule then causes a large number of electron transfer within the response time of the electronics. The pioneer approach of Bard consisted in fully insulating a Pt/Ir tip with Apiezon wax, and then provoke a controlled crash so as to get an electrode shape about such as the one depicted in Fig. 1. This tip was approached towards an electrode so as to produce a gentle crash that opens a hole of 10 to 20 nm diameter. In this peculiar utilisation of the Scanning Electrochemical Microscope, the electrode was thus slightly recessed in the insulator, as can be assessed by performing an approach curve on an insulating substrate (or an electrode polarised so as to avoid redox cycling). Then, at a constant distance of about 2 nm with positive feedback, the current is monitored. A simple calculation for a 2 mM solution of a redox probe and a volume delimited by a 20 nm diameter tip recessed by 10 nm gives that there should be only a few molecules processing the feedback. The time corresponding to travel from the UME to the substrate is then about 50 nanoseconds. If one electron is exchanged, this gives a current of about 1.6 pA per molecule (considering 100 ns for one complete cycle and taking D=10-5 cm2 s-1), which is a measurable current. A quantified change in the current reflects the arriving or departure of limited number (and often a single) of molecules between the electrodes. This configuration has been revisited recently by Mirkin.

This pioneer experiment is however not adapted for further analytical tools developments. Indeed, in the SECM configuration, drifts may occur substantially so that the precise distance (already not measured directly) may shift. Moreover, the Apiezon wax coating of the electrode is not reproducible. Recently, the Lemay group has revisited this concept but relying on devices produced by nanolithography methods. This led to the invention of electrochemical correlation spectroscopy. In these systems (see Fig. 2) the gap width is perfectly controlled and is about 50–100 nanometres. Furthermore, they can be stored easily and the chromium layer protecting the electrodes can be etched just before the experiment. The redox cycling can be clearly visualized on both electrodes as fluctuations of the same amplitude but opposite signs of the currents. In order to get more information, the power spectral density (or equivalently the autocorrelation function) of the signal can be analysed. Additional fluctuations were then observed and attributed to adsorption events. There is no doubt that this approach is very promising. Further refinements and modification of electrode surfaces will even induce more specificity and sensitivity in the near future.

Another promising alternative developed by Demaille et al. is to attach the molecules on the tip or on the surface with a polymer linker (Fig. 3).

There is a sufficient chain flexibility so that redox cycling occurs efficiently. Here, the authors control the distance by performing simultaneously an AFM measurement. At present, a few hundreds of molecules can be detected, and further developments may allow reaching the single molecule level. A key point to unlock is to favour the redox cycling. Biological discrimination between protein arrays has already been performed by this approach.

2.2 Molecules flowing through a nanopore

Another trick is to rely on conductivity measurements. The measurable current corresponds then to an ionic flux. In the conductivity cell, a nanometric constriction is present, so that the major part of the potential drop occurs in this region. As a consequence, any large molecule that can obstruct the ionic transport through the pore will be detected. This strategy has been nicely pioneered by several groups to study transport through a biomimetic membrane. The White group first made nanoelectrodes, and further etched the metal to produce a hole having a few tens to a few hundreds nanometres. Then, a bilayer is deposited so as to fully cover the hole. In this configuration, the resistance is extremely high if there is no leakage. A great advantage of relying onto low-diameter membrane is that they are very stable compared to other systems, and also induce less noise. Then, insertion of α-hemolysin, an ion-channel protein, in the membrane can be detected by molecule as current steps (Fig. 4). Last, but not least, when a single protein is inserted, and that one compartment contains DNA (a negatively charged backbone), each DNA strand that passes through the protein is detected. The current focus is to reach a submolecular recognition, ideally in view of DNA sequencing. The problem to unravel in this approach is then to take into account the random walk and global motion of the DNA bases so as to avoid to count the same base several times. For uncharged particles (vesicles or nanoparticles), the same method can be used, but the translocation of the structure needs to be induced by a pressure difference between the two compartments. Other phenomena can be studied with similar setup, for example protein unfolding as a function of temperature, that limits incorporation of the protein in the membrane.

Nanopores can also be produced by advanced lithography methods. One aim is to get the thinnest possible hole to be more sensitive to the submolecular variations. Graphene has been proposed as a “one-atom” (or at least a few for multilayer graphene) thick membrane. To refine the analysis, a tunnel junction can be incorporated in the hole, requiring a four-electrode configuration. A full analysis of the electrical cross talk has been recently proposed by Albrecht. A specific receptor can also be incorporated in the pore to enhance the selectivity. The temporal resolution of these devices is very good, and recent progress is allowed to reach a sub-microsecond resolution.

2.3 Detection of nanoparticles

The exponential interest onto nanoparticles chemistry and electrochemistry has recently also been focused onto individual behaviour quantification. We artificially split this section in electractive and electrocatalytic structures.

2.3.1 Electrocatalytic nanostructures. Recently, the Bard group has produced an innovative method based on SECM to screen the catalytic activity of individual clusters. Using microdispensers, nanostructures of different composition can be deposited on an electrode. The reactant to test is then locally generated on the SECM tip in the Tip Generation-Substrate Collection mode. The efficiencies of the different catalysts is directly visualized in the resulting image (Fig. 5). This method is particularly efficient to probe multicomponent systems since a single experiment can help choosing between a large range of compositions.

Another currently active field consists to monitor nanoparticle collisions with an electrode by the intermediate of the electrocatalytic reaction they provoke. Here, as soon as the nanoparticle contacts the electrode, the reaction starts, which amplifies greatly the current. Due to the large turnover, particles as small as 3 nm diameter could be detected. Moreover, the nanoparticles may stick on the electrode or have several collisions before travelling back to the bulk depending on the electrode treatment. In the first case, a stepwise behaviour is observed, while a blip response is observed in the second case. Diffusion and probability to collide to an active site have been considered in an analytical theory. Random walk simulations also helped the interpretation.

2.3.2 Electroactive nanoparticles. However, when nanoparticles are large enough to incorporate enough redox centers, they may be individually detected directly. This has been pioneered by the Compton group, starting with silver components. Here, the number of electrons available is easily computed knowing the radius and density of the system. In Fig. 6, the blips correspond to individual particle disintegration to Ag+. For each event, integration of charge gives the radius. When concentration is small enough (in the picomolar/nanomolar range), so that each event can occur separately, this experiment can help determining the size dispersity or following the coalescence of these nanoparticles in solution. This has been recently extended to non destructive measurements by tagging the nanoparticles with redox centers.

3 Single molecules for molecular electronics

The dream of molecular electronics would be to build an entire signal processing unit only with molecules. But to be competitive with top down methodologies, several problems need to be unravelled. First, the molecular design should allow a function. It has been now demonstrated that molecules indeed are able to treat electrical, optical or even magnetic information. This has been demonstrated for long relying onto experiments with collection of molecules. Another problem to encompass will be to organise individually each structure. In this aspect it is now common to get organisation of some nanoobjects over several hundreds of nanometers (the limitation often comes from the substrate) but usually the same entity is assembled. Moreover, these organisations often rely on weak interactions and more robust structures need to be produced for practical applications.

The first property to be tested was the ability to propagate a signal, i.e. to act as a molecular wire. This can be performed by placing an electron donor and an acceptor at each end of a linear entity and performing a photochemical activation. For example, this was thoroughly studied in DNA by Barton.

In electrochemistry, the donor or the acceptor can be replaced by an electrode. Then the measured quantity is the heterogeneous rate constant kET. Several experimental protocols are available to deduce this parameter. First, transient methods such as chronoamperometry and ultrafast voltammetry are rather direct. However, a great care should be paid to take into account ohmic losses and the inherent time constant of the electrode. Usually, when rather fast systems are measured, ultra-microelectrodes should be used to encompass these problems. The second approach was pioneered by Feldberg et al. and consists in illuminating an electrode so as to produce a temperature jump. The local equilibrium is then disturbed, and the faradaic impedance discharges in the double layer capacitance. This produces a small shift of the equilibrium potential that can be measured by an appropriate follower. The main advantage is that since there is no net current flux, ohmic losses are absent. This is however a very indirect method and several assumptions about heat dissipation in the assembly cannot be verified.

These different methods confirmed that most often the current decays exponentially with the electrode distance, kET being given by the following equation:

kET = k0exp(-βd) (1)

The ideal molecular wire would then have a very small β. This indeed occurs with conjugated backbones whereas for alkyl chains β is on the order of 10 nm-1. Departures from this law stem from a modification of the reorganisation energy with the distance or a change in the coupling element due to geometrical distortions. For long molecular wires, or when several redox centers are present, electron hopping needs also to be considered. These macroscopic experiments kept the dream alive, but the properties need to be tested at the individual level. To that respect, electrochemical concepts are useful, either to produce devices onto which the molecules can be studied, or to use the reference electrode as a gate to trigger the current inside the nanogap. These two ideas are depicted sequentially below.

(Continues…)Excerpted from Electrochemistry: Volume 11 Nanosystems Electrochemistry by R. G. Compton, J. D. Wadhawan. Copyright © 2013 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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