Electrochemical response selectivity

While the variety of NPs used in catalytic and sensor applications is extensive, this chapter will primarily focus on metallic and semiconductor NPs. The term functional nanoparticle will refer to a nanoparticle that interacts with a complementary molecule and facilitate an electrochemical process, integrating supramolecular and redox function. The chapter will first concentrate on the role of exo-active surfaces and core-based materials within sensor applications. Exo-active surfaces will be evaluated based upon their types of molecular receptors, ability to incorporate multiple chemical functionalities, selectivity toward distinct analytes, versatility as nanoscale receptors, and ability to modify electrodes via nanocomposite assemblies. Core-based materials will focus on electrochemical labeling and tagging methods for biosensor applications, as well as biological processes that generate an electrochemical response at their core. Finally, this chapter will shift its focus toward the catalytic nature of NPs, discussing electrochemical reactions and enhancement in electron transfer. 

One approach utilizes the polymer-modified surface as a preconcentrating surface in which the analyte or some reaction product is collected and concentrated by reactive groups attached to the electrode. The preconcentrated analyte is then measured electro-chemically. Ideally, the collection process will be selective for the analyte species of interest. If not, the analyte must be elec-troanalytically discriminated from other collected species. The capacity of the polymer film on the modified electrode should be sufficient to prevent saturation by the analyte, and the electrochemical response during measurement should provide good sensitivity to the collected analyte [9]. 

Immobilization of bioactive material on/in the electrode allows combining bio-reaction selectivity with sensitivity of electrochemical detection. Irrespective of reaction in the biosensor, the electrochemical response is measured, in particular, as current at the given potential (amperometric sensor) or electrode potential (potentiometric sensor). 

It should be noted that chemical modification of the mesoporous host—for instance, by thiol groups—can provide not only functional attachment to electrode surfaces but also specific binding sites for preconcentration of selected analytes. The modified electrode configuration (monolayer, multilayer, etc.) can significantly influence the final electrochemical response. 

In the example just considered, the important feature was a shift in the wave position caused by selective chemical stabilization of one of the redox forms. In a reversible system the potential axis is a free energy axis, and the magnitude of the shift is a direct measure of the free energy involved in the stabilization. These concepts are quite general and can be used to understand many chemical effects on electrochemical responses. Any equilibrium in which either redox species participates will help to determine the wave position, and changes in concentrations of secondary participants in those equilibria (e.g., ammonia in the example above) will cause an additional shift in the half-wave potential. This state of affairs may seem confusing at first, but the principles are not complicated and are very valuable ... 

Voltammetric sensors based on chemically modified electrodes (conducting polymers, phthalocyanine complexes) with improved cross-selectivity were developed for the discrimination of bitter solutions [50], The performance and capability were tested by using model solutions of bitterness such as magnesium chloride, quinine, and four phenolic compounds responsible for bitterness in olive oils. The sensors gave electrochemical responses when exposed to the solutions. A multichannel taste sensor was constructed using the sensors with the best stabilities and cross-selectivities and PCA of the signals allowed distinct discrimination of the solutions. 

The objective of this review has been to show how supramolecular chemistry has made, and is continuing to make, an important contribution to the area of electrochemical sensing. From an applications point of view, future developments will continue to be directed toward recognition and read out at a surface, so that robust voltammetric sensors that are viable alternatives to other sensors (e.g., biosensors) can be produced. However, fundamental studies are required to further our understanding of how to maximize substrate selectivity, while maintaining an effective electrochemical response to... 

Meyerhoff et al have reported the use of a non-metallated porphyrin film as an ion-selective electrode. They have shown that the electropolymerized films offer significant selectivity for the detection of iodide over a wide range of anions. The lifetime of these electrodes was estimated by analyzing some of their characteristics such as sensitivity and detection limits. The effect of pH on the detection of iodide by the film-supported electrodes was also established. For pH values from 3 to 8, there is relatively little effect of the pH on the electrochemical response of the electrodes. At pH values higher than 8, the electrode response is affected by the interference by hydroxide ions. 

Effective complexation of Cu + ions was achieved with L-cysteine SAMs [49,50,54]. It was assumed that the Cu " " ion interacts with both the amino and carboxylate groups to form a stable surface complex with a 2 1 cysteine-to-Cu ratio [49]. Complex formation was pH-dependent, reaching a maximum at pH 5-6 [49, 54], where the zwitterionic form of the amino acid is stable. The Cu + surface complex was exceptionally stable, with the electrochemical response remaining unchanged upon repetitive cycling between the Cu + and Cu" " forms. The high selectivity of Cu + binding was seen as nearly no interference by the common cations... 

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