Surface bound molecules

The background problem can be further overcome when using a surface-confined fluorescence excitation and detection scheme at a certain angle of incident light, total internal reflection (TIR) occurs at the interface of a dense (e.g. quartz) and less dense (e.g. water) medium. An evanescent wave is generated which penetrates into the less dense medium and decays exponentially. Optical detection of the binding event is restricted to the penetration depth of the evanescent field and thus to the surface-bound molecules. Fluorescence from unbound molecules in the bulk solution is not detected. In contrast to standard fluorescence scanners, which detect the fluorescence after hybridization, evanescent wave technology allows the measurement of real-time kinetics (www.zeptosens.com, www.affinity-sensors.com). 

An interesting approach to measuring rates of electron transfer reactions at electrodes is through the study of surface bound molecules (43-451. Molecules can be attached to electrode surfaces by irreversible adsorption or the formation of chemical bonds (461. Electron transfer kinetics to and from surface bound species is simplified because there is no mass transport and because the electron transfer distance is controlled to some degree. 

Theoretically, only adsorbed, i.e. surface-bound, molecules can be directly available for microbial uptake. As pointed out before, molecules which are absorbed, i.e. dissolved in a solid, are unlikely to be contacted by microorganisms unless they appear at the surface of the sorbent, i.e. they change from the absorbed to an ephemeral adsorbed state. Therefore, the discussion of the direct availability of sorbed molecules will be restricted to adsorbed molecules. 

Measurement of intermolecular distances between fluorescent surface-bound molecules in the presence of a large excess of fluorophore or background fluorescence in the bulk. 

The general electrochemical behavior of surface-bound molecules is treated in Sect. 6.4. The response of a simple electron transfer reaction in Multipulse Chronoamperometry and Chronocoulometry, CSCV, CV, and Cyclic Staircase Voltcoulometry and Cyclic Voltcoulometry is also presented. Multielectronic processes and first- and second-order electrocatalytic reactions at modified electrodes are also discussed extensively. 

This equivalence between the charge of surface-bound molecules and the current of solution soluble ones is due to two main reasons first, in an electro-active monolayer the normalized charge is proportional to the difference between the total and reactant surface excesses ((QP/QP) oc (/> — To)), and in electrochemical systems under mass transport control, the voltammetric normalized current is proportional to the difference between the bulk and surface concentrations ((///djC) oc (c 0 — Cq) [49]. Second, a reversible diffusionless system fulfills the conditions (6.107) and (6.110) and the same conditions must be fulfilled by the concentrations cQ and cR when the process takes place under mass transport control (see Eqs. (2.150) and (2.151)) when the diffusion coefficients of both species are equal. 

Chronoamperometric curves have been used as a standard tool to obtain values of the rate constants of surface-bound molecules and they prove as very useful for validating the Marcus-Hush s formalist, and, indeed, the experimental application of the MH theory to electrode processes has been mainly carried out with surface-bound redox systems. Thus, Chidsey studied the oxidation of ferrocene groups connected to a gold electrode by means of a long alkylthiol chain by using Single Potential Pulse Chronoamperometry (see examples of the experimental responses in Fig. 6.21) [43]. 

This technique is of special interest in the case of charge transfer processes at surface-bound molecules since it allows a simple and more effective correction of the non-faradaic components of the response than Cyclic Voltammetry. Moreover, this technique presents an intense peak-shaped signal for fast charge transfer, whereas other multipulse techniques give rise to nonmeasurable currents under these conditions and it is necessary to use short potential pulses to transform the response to quasi-reversible, which is much more difficult to analyze [4, 6, 10]. 

In the case of surface-bound molecules, due to the characteristics of the current obtained when a sequence of potential pulses is applied (see Sect. 6.4.1.2), the use of DSCVC is only recommended for the analysis of non-reversible electrochemical reactions, since for very fast electrochemical reactions (i.e., for values of the dimensionless rate constant which fulfill log( 0r) >0.5), the current becomes negligible, in accordance with Eq. (6.132). The response obtained in DSCVC when non-reversible electrochemical reactions are considered presents two peaks, one maximum positive fV dscvc) an(J one minimum negative (v Sdscvc) which appear for values of the applied potentials EMax and Emin, respectively (with i// >scvc = / >scvc/The cross potential value, at which dscvc = 0,... 

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