Amperometric feedback mode

In scanning electrochemical microscopy (SECM) a microelectrode probe (tip) is used to examine solid-liquid and liquid-liquid interfaces. SECM can provide information about the chemical nature, reactivity, and topography of phase boundaries. The earlier SECM experiments employed microdisk metal electrodes as amperometric probes [29]. This limited the applicability of the SECM to studies of processes involving electroactive (i.e., either oxidizable or reducible) species. One can apply SECM to studies of processes involving electroinactive species by using potentiometric tips [36]. However, potentio-metric tips are suitable only for collection mode measurements, whereas the amperometric feedback mode has been used for most quantitative SECM applications. 

In addition to the amperometric feedback mode described above, other amperometric operation modes are also possible. For example, in the substrate generation/tip collection (SG/TC) mode, ip is used to monitor the flux of electroactive species from the substrate and vice versa for the tip generation/substrate collection (TG/SC) mode. These operation modes will be described in Section 12.3.1.3 and are useful in studies of homogeneous reactions that occur in the tip-substrate gap (see Section 12.4.2) and also in the evaluation of catalytic activities of different materials for useful reactions, e.g., oxygen reduction and hydrogen oxidation (see Section 12.4.3). In addition to the amperometric methods, other techniques, e.g., potentiometric method is also applicable for SECM and will be discussed in Section 12.3.2. We will also update the techniques suitable for the preparation of SECM amperometric tips in Section 12.3.1.1 and potentiometric probes in Section 12.3.2.2. 

Fig. 37.11. Use of an NO microsensor for detection of the NO release from cultured endothelial cells. The sensor is a dual probe microsensor. The small sensor is a bare Pt UME used to position the sensor in the feedback mode. Onto the larger Pt electrode a polymer was deposited from an acrylic resin containing Ni(4-lV-tetramethyl) pyridyl porphyrin and served as amperometric NO sensor, (a) Schematic of the sensor, (b) optical microphotograph of the sensor surface, (c) Response of the NO sensor to the stimulation of the cells with bradykinin at different distances of the sensor to the surface of the cells. Reprinted with permission from Ref. [104], Copyright 2004, American Chemical Society.

It should be noted that GC mode experiments with amperometric tips may contain a feedback component to the current if the electrochemical process at the tip is reversible and the tip-to-specimen distance is less than about 5a. However, at greater distances or when employing a potentiometric tip, the tip acts approximately as a passive sensor, i.e., one that does not perturb the local concentration. This situation is quite distinct from feedback mode, where the product of the electrolysis at the tip is an essential reactant in the process at the specimen surface. This interdependence of tip and specimen reactions in feedback mode ensures that the biochemical process is confined to an area under the tip defined by the tip radius and diffusional spreading of the various reagents (20). In contrast, the biochemical process in GC mode is independent of the presence of the tip and may therefore occur simultaneously across the whole surface. In addition, the tip signal often does not directly provide information on the height of the tip above the surface methods to overcome this limitation are described in Sec. I.D. Finally, since the tip process and the biochemical reaction at the specimen are independent, a wide range of microsensors may be employed as the tip, e.g., ion-selective microelectrodes, which are not applicable in feedback experiments. 

FIG. 6 Schematic illustrating the difference between (A) GC mode and (B) feedback mode detection of immobilized enzyme activity. The example enzyme is glucose oxidase (GOx) with a ferrocenyl redox mediator (Fc) detected at a Pt tip. (A) The bulk solution contains oxidized mediator (Fc+) and the tip detects the reduced form amperometrically which is generated continually wherever active enzyme is present on the specimen. (B) The bulk solution contains reduced mediator, the tip generates ferrocenium locally and detects the enhanced flux due to feedback when there is active enzyme on the portion of the specimen beneath the tip. , Glucose o, gluconolactone. 

SECM amperometric methods are based on the measurement of electrode currents (tip and substrate, (p and ig, respectively) as a function of various parameters, including tip-substrate distance (d) and tip or substrate potentials (Ej or Eg). Irrespective of the system studied (e.g., an electrode, an inert surface, a catalyst, or a living organism), the probe tip is a necessary component to perform any SECM experiment. In all cases, the amperometric tip is a UME that can be positioned in close proximity to another surface. There are two amperometric operation modes feedback and generation/collection modes. The preparation and characterization of commonly used amperometric tips of different geometries as well as their operation modes will be described in this section. 

The generation/collection (G/C) modes constitute a different SECM procedure that expands the applicability of the technique to a wide range of situations, hi these modes, the collector (either tip or substrate) works as an amperometric sensor that collects the products produced at the generator surface (either substrate or tip, respectively). Thus, the collector potential is controlled to electrochemically reaet with the generator-produced species. Typical collector responses used in G/C experiments are (a) voltammetric curves, where the collector potential is swept, and (b) diffusion-controlled limiting current vs. time curves. In contrast to the feedback mode where steady-sate responses are monitored, in G/C experiments, the current-time dependence is an important set of data to evaluate. The timescale of most of G/C transient experiments is much wider, possibly up to 100 sec. Moreover, as the tip-substrate distances increase, typical coupling and distortion of transient responses are not significant. 

The lateral resolution of SECM is defined by the ability to resolve two nearby objects and is different from the ability to detect an isolated small object, that is, detectability. The lateral resolution and detectability of SECM in the feedback mode were assessed theoretically, when a disk-shaped tip was positioned over the disk-shaped conductive spot embedded in an insulating substrate (Figure 1.1a) [18]. In the feedback mode, a redox mediator in solution, 0, is electrolyzed amperometrically at the tip (0 + e R) to yield steady-state diffusion-limited current. When the tip is positioned within a tip diameter from the conductive spot, tip current is enhanced... 

At smaller values of the tip-substrate separation distance, d, feedback and hindering effects, as those observed in the feedback modes at amperometric tips, perturb the transport processes. The tip then starts to interfere with the source, which complicates the quantitative data analysis without numerical modeling.Such perturbing effects are not observed when passive probes such as potentiometric " or biosensor microelectrodes are used, but then the positioning of these substance-selective sensors is difficult. An alternative would be to consider the use of the scanning ion conductance microscopy (SICM). Indeed, recently, the SICM afforded the opportunity to image and quantify precisely local K+ and Cl ionic fluxes. ... 

SECM. In conventional amperometric SECM an eleetro-ehemical probe, typically a glass-encapsulated microdisk electrode, is brought into close proximity to the interface of interest, and electroehemical reaetions are driven in the small gap between the tip and the surface. Current can be measured at the tip and/or at the substrate and the response is highly sensitive to both the nature of the surface and the tip-surface separation. There are various modes of SECM, such as feedback mode, generation-collection mode and redox competition mode, each of which can... 

The instrumentation and theory for the basic SECM experiment are discussed in detail elsewhere in this book, so only a very brief description is provided here. SECM involves the movement of a very small amperometric microelectrode (usually a disk microelectrode of a few micrometers or smaller radius, referred to as the tip or the probe) near the surface of a substrate immersed in an electrolyte containing at least one redox-active species (a mediator). The two most common modes of operation are the feedback mode and the generator-collector mode. 

There are many possible scanning modes in SECM three of the most common are shown in Fig. 18B. In feedback mode, an amperometric probe scans the sample under an electrolyte solution containing a redox mediator, i.e., a soluble molecule that can exist in two states of charge (R, reduced, and O, oxidized). A potential is applied to the probe such that mediator molecules that contact the probe are electrochemically oxidized,... 

The transport of molecules across biological cell membranes and biomimetic membranes, including planar bilayer lipid membranes (BLMs) and giant liposomes, has been studied by SECM. The approaches used in those studies are conceptually similar to generation-collection and feedback SECM experiments. In the former mode, an amperometric tip is used to measure concentration profiles and monitor fluxes of molecules crossing the membrane. In a feedback-type experiment, the tip process depletes the concentration of the transferred species on one side of the membrane and in this way induces its transfer across the membrane. 

While most of the SECM work has been carried out with amperometric tips for measuring feedback current or for use in the generation/collection mode, other types of tips such as potentiometric and enzymatic tips are also possible but are only briefly described in Section II.D. The readers who are interested in these tips are encouraged to refer to the appropriate chapters in this monograph. Finally, Section III deals with an approach to determining the shape of an ultramicroelectrode from its SECM response. 

In different publications the term generation/collection mode has been applied to several substantially different SECM experiments. The two main types are tip generation/substrate collection (TG/SC) and substrate genera-tion/tip collection (SG/TC) modes. The modifications of the latter type of G/C experiments include potentiometric G/C mode, where the tip is a po-tentiometric sensor, and two amperometric modes. In the first the tip/sub-strate separation is so large that no feedback effect is observed, and the amperometric tip only detects the species produced at the substrate. At much shorter distances the species reoxidized (or rereduced) at the tip can return to the substrate producing a feedback effect. 

The substrate generation/tip collection (SG/TC) mode with an ampero-metric tip was historically the first SECM-type measurement performed (32). The aim of such experiments was to probe the diffusion layer generated by the large substrate electrode with a much smaller amperometric sensor. A simple approximate theory (32a,b) using the well-known c(z, t) function for a potentiostatic transient at a planar electrode (33) was developed to predict the evolution of the concentration profile following the substrate potential perturbation. A more complicated theory was based on the concept of the impulse response function (32c). While these theories have been successful in calculating concentration profiles, the prediction of the time-de-pendent tip current response is not straightforward because it is a complex function of the concentration distribution. Moreover, these theories do not account for distortions caused by interference of the tip and substrate diffusion layers and feedback effects. 

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