Behavior of ultramicroelectrodes

National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Ireland 

In Sections 6.1 and 6.2, we look in detail at the fundamental properties of microelectrodes, consider some of the practical aspects of using microelectrodes and discuss the experimental factors that influence the choice of electrode size. 

---

In this section, selected studies are presented in which self-assembled monolayers have been used to address topics such as transition-state structures and sequential electron transfer. These studies were selected because they address fundamental mechanistic processes. SAMs have also been used to investigate such basic electrochemical phenomena as the potential profile near an electrode [134, 135], interfacial capacitance [136], the influence of redox [134] or polarizable [137] moieties on double-layer structure and the behavior of ultramicroelectrodes approaching molecular size [138]. These important topics are beyond the scope of this chapter, and the interested reader is directed to the literature for more information. 

An understanding of the operation of the SECM and an appreciation of the quantitative aspects of measurements with this instrument depends upon an understanding of electrochemistry at small electrodes. The behavior of ultramicroelectrodes in bulk solution (far from a substrate) has been the subject of a number of reviews (17-21). A simplified experimental setup for an electrochemical experiment is shown in Figure 1. The solution contains a species, O, at a concentration, c, and usually contains supporting electrolyte to decrease the solution resistance and insure that transport of O to the electrode occurs predominantly by diffusion. The electrochemical cell also contains an auxiliary electrode that completes the circuit via the power supply. As the power supply voltage is increased, a reduction reaction, O + ne — R, occurs at the tip, resulting in a current flow. An oxidation reaction will occur at the auxiliary electrode, but this reaction is usually not of interest in SECM, since this electrode is placed sufficiently far from the UME... 

Bond, A.M., Fleischmann, M. and Robinson, J. (1984) The construction and behavior of ultramicroelectrodes - investigations of novel electrochemical systems. Journal of the Electrochemical Society, 131, C109-C109. 

SECM involves the measurement of the current through an ultramicroelectrode (UME) (an electrode with a radius, a, of the order of a few nm to 25 (zm) when it is held or moved in a solution in the vicinity of a substrate. Substrates, which can be solid surfaces of different types (e.g., glass, metal, polymer, biological material) or liquids (e.g., mercury, immiscible oil), perturb the electrochemical response of the tip, and this perturbation provides information about the nature and properties of the substrate. The development of SECM depended on previous work on the use of ultramicroelectrodes in electrochemistry and the application of piezoelectric elements to position a tip, as in scanning tunneling microscopy (STM). Certain aspects of SECM behavior also have analogies in electrochemical thin-layer cells and arrays of interdigitated electrodes. 

To investigate the film permeability in its neutral form, we performed CV with a mediator in solution over bare FTO and PPTZPQ/ITO. The redox potential of the mediator was chosen within die potential window where die polymer does not show an electrochemical response, and therefore, behaves solely as a blocking layer on the ITO electrode. If the thickness of the film is of the order of the diameter of the hole, the behavior of the electrode can be treated as that of ultramicroelectrode arrays and the response is modulated by the polymer film thickness, the size and distribution of the pores, and the time scale of die experiment. The reduction behavior of die me yl viologen (MV ) on... 

The behavior of array electrodes depends on the ratio of the diameter of the individual electrode to the spacing between electrode features. High diffusion current densities conditioned by radial flows and low values of ohmic potential drop are most prominent in the case of ultramicroelectrodes [13, 14]. By replacing a single macroelectrode by an array of ultramicroelectrodes, the current density can be increased by orders of magnitude as well as the ratio of faradaic to capacitive currents. 

In most electrochemical studies, one employs solutions where the concentration of the electroactive species, i, is 1 mM. With these concentrations, the diffusion flux of electroactive species to the electrode, /, is of the order of otjC, where OTj is the mass transfer coefficient (cm/s) and C is the bulk concentration (mol/cm ). With m, 10 cm/s, this produces fluxes of the order of 10 mol/s/ cm or 6 X10 " molecules/s/cm, producing currents of 10 " A/cm. Under these conditions, even with very small electrodes, one measures the behavior of large ensembles of molecules. However, if the concentration of electroactive species is dropped to 1 pM, these fluxes drop to/j=10 mol/s/ cm or 6 X10 molecules/s/cm with a current density of 10 A/cm. Thus, with an ultramicroelectrode (UME) with about a 10 pm size or area of about 10 cm, the number of molecules arriving by diffusion to the electrode is about 1/s. In our previous work, we showed that by using very small ( pm) nanoelectrocatalytic C or Au electrodes with relatively small background currents, that nanometer-size electrocatalytic NPs, for example, of Pt, amplify the current of an appropriate inner-sphere (IS) reaction (e.g., hydrazine oxidation or proton reduction) to the pA level, and the frequency, size, and shape of collision events could be investigated. More recent work with different approaches has shown that interactions of the NPs with the electrode can be detected, even for outer-sphere (OS) reactions, as described in later sections of this chapter. 

Amatore et al. developed a theoretical framework to describe the electrochanical responses of ultramicroelectrode ensemble and NEEs by considering mass transport for assemblies of microdisk and microband electrodes. Lee et al. used finite element simulation to solve 3D diffusion equations and found that a collection of 10 pm diameter microdisk electrodes required a separation distance of more than 40R to exhibit a sigmoidal simulated CV response typical for radial diffusion. " CV response typical of reversible linear diffusion at macroelectrodes was observed when the separation distance was less than 6R . Assemblies of microelectrodes for which the separation distances were between 6R and 4QR exhibited peak-shaped simulated CVs indicative of a mixture of radial and linear diffusion behavior. Thus, 12/ seems to be too small a separation distance for the design of ideal microelectrode arrays. 

The particular behavior of a single miaoelectrode or an ensemble of millions of microelectrodes is discussed in Section 14.3. Since a nanoparticle is the ultimate case of an ultramicroelectrode, it is appropriate to discuss some of the properties of nanoparticles employing the equations developed for microelectrodes, in order to calculate the increased rate of diffusion towards an isolated nanoparticle and the corresponding decrease in solution resistance. 

Selzer Y and Manler D 2000 Scanning electrochemical microscopy. Theory of the feedback mode for hemispherical ultramicroelectrodes steady-state and transient behavior Anal. Chem. 72 2383... 

The influence of the nonlinearity of diffusion on the observed complex plane plots is shown in Fig. 13. Spherical mass transfer causes the formation of a depressed semicircle at low frequencies instead of the linear behavior observed for linear semi-infinite diffusion. For very small electrodes (ultramicroelectrodes) or low frequencies, the mass-transfer impedances become negligible and the dc current becomes stationary. On the Bode phase-angle graph, a maximum is observed at low frequencies. 

A combination of an STM with an SECM (see also below for this method) has been described [70] for details, see above. The SECM can also be used for surface structuring. In order to deposit gold on a surface that is spatially resolved, the experimental setup schematically depicted in Fig. 7.15 was used. The current flowing between the ultramicroelectrode and the surface is displayed in Fig. 7.16. Its distance dependence resembles exactly the behavior observed with a conductive surface, as discussed above. The deposited gold microdots are shown in Fig. 7.17. 

Finally, to conclude this introduction, and to avoid any possible confusion in the terminology, we wish to define briefly what is an ultramicroelectrode (at least in our sense ). When their interfacial properties are to be considered identical with those of any other electrode of a larger dimension, ultramicroelectrodes must remain much larger than the double layer thickness. This sets a lower dimension of a few tens of A for ultramicroelectrodes.On the other hand, if diffusional steady state voltammetry has to be observed without significant interference of convection, they must be smaller than convective layers, which sets an upper limit of a few tens of /im. Between these limits, all ultramicroelectrodes possess identical intrinsic physico-chemical properties. However, their behavior (viz. ohmic drop, steady state or transient currents, etc) obviously depends on the medium and the time-scale considered. ... 

---