Substrates, electroactive layers

To assess the electrochromic response of the bipyridinium dications embedded into multilayers of 7, we envisaged the possibility of assembling these films on optically transparent platinum electrodes.27d f Specifically, we deposited an ultrathin platinum him on an indium-tin oxide substrate and then immersed the resulting assembly into a chloroform/methanol (2 1, v/v) solution of 7. As observed with the gold electrodes (Fig. 7.5), the corresponding cyclic voltammograms show waves for the reversible reduction of the bipyridinium dications with a significant increase in 2p with the immersion time. In fact, is 0.8,1.5, and 3.1 nmol/cm2 after immersion times of 1, 6, and 72 h, respectively. Furthermore, the correlation between ip and v is linear after 1 h and deviates from linearity after 6 and 72 h. Thus, the bisthiol 7 can indeed form multiple electroactive layers also on platinum substrates. 

Yeo WS, Hodneland CD, Mrksich M (2001) Electroactive mono-layer substrates that selectively release adherent cells. Chem-biochem 2(7-8) 590-593... 

Case II Reversible or Ouasi-Reversible Redox Species. If the tip-sample bias is sufficient to cause the electrolysis of solution species to occur, i.e., AEt > AEp, ev, the proximity of the STM tip to the substrate surface (d < 10 A) implies that the behavior of an insulated STM tip-substrate system may mimic that of a two-electrode thin-layer cell (TLC)(63). At the small interelectrode distances required for tunneling, a steady-state concentration gradient with respect to the oxidized (Ox) and and reduced (Red) electroactive species should be established between the tip and the substrate, and the resulting steady-state current will augment that present as a result of the convection of electroactive species from the bulk solution. In many cases, this steady state current is predicted to overwhelm the convective currents, so this situation is of concern when STM imaging under electrochemical conditions (64). 

The lure of new physical phenomena and new patterns of chemical reactivity has driven a tremendous surge in the study of nanoscale materials. This activity spans many areas of chemistry. In the specific field of electrochemistry, much of the activity has focused on several areas (a) electrocatalysis with nanoparticles (NPs) of metals supported on various substrates, for example, fuel-cell catalysts comprising Pt or Ag NPs supported on carbon [1,2], (b) the fundamental electrochemical behavior of NPs of noble metals, for example, quantized double-layer charging of thiol-capped Au NPs [3-5], (c) the electrochemical and photoelectrochemical behavior of semiconductor NPs [4, 6-8], and (d) biosensor applications of nanoparticles [9, 10]. These topics have received much attention, and relatively recent reviews of these areas are cited. Considerably less has been reported on the fundamental electrochemical behavior of electroactive NPs that do not fall within these categories. In particular, work is only beginning in the area of the electrochemistry of discrete, electroactive NPs. That is the topic of this review, which discusses the synthesis, interfacial immobilization and electrochemical behavior of electroactive NPs. The review is not intended to be an exhaustive treatment of the area, but rather to give a flavor of the types of systems that have been examined and the types of phenomena that can influence the electrochemical behavior of electroactive NPs. 

Historically, the first SECM-type experiments were carried out to measure concentration profiles in the diffusion layer generated by a macroscopic substrate [3, 26]. This type of measurement represents substrate generation/tip collection (SG/TC) mode. When the tip is moved through the thick diffusion layer produced by the substrate, the changes in iT reflect local variations of concentrations of redox species (Fig. 3b). Ideally, the tip should not perturb the diffusion layer at the substrate. This is easier to achieve with a potentiometric tip, which is a passive sensor and does not change concentration profiles of electroactive species. 

Another alternative that would help to raise catalyst utilization would be to make CLs of extremely thin two-phase composites. Electroactive Pt (eventually deposited on a substrate) should form the electronically conductive phase. The remaining volume should be filled with liquid water. Since the layer is only a two-phase composite, not impregnated with ionomer, the problem of the protonic contact resistance at the PEM CL interface could be mitigated, making the CCL insensitive to the type of PEM. Using Pois-son-Nemst-Planck theory, it could be shown that close to 100% of the catalyst would be utilized, since transport of oxygen and protons would be unproblematic for such thicknesses ( 100 nm) [129],... 

With the water-soluble poly(o-methoxyaniline) (POMA), Mattoso and coworkers162 have reported the self-assembly of multilayer conducting polymer films by depositing alternating layers of the POMA cation and polyanionic dopants such as poly(styrenesulfonate) and poly(vinylsulfonate) onto a glass substrate. This concept has been further developed with POMA by employing the anion of poly(3-thiopheneacetic acid) as the polyanionic dopant, giving novel self-assembled films in which both the cationic and anionic components are electroactive polymers.163... 

Unlike the feedback mode of the SECM operation where the overall redox process is essentially confined to the thin layer between the tip and the substrate, in SG/TC experiments the tip travels within a thick diffusion layer produced by the large substrate. The theoretical treatment is easier when the tip is a potentiometric sensor. Such a passive sensor does not change the concentration profile of electroactive species generated (or consumed) chemically or electrochemically at the substrate. Still, a consistent theoretical treatment was proposed only for a steady-state situation when a small substrate (a microdisk or a spherical cap) generates stable species. The concentration of such species can be measured by an ion-selective microtip as a function of the tip position. The concentration at any point can be related to that at the source surface. For a microdisk substrate the dimensionless expression is (24,25)... 

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