Spontaneously adsorbed monolayers

Widrig CA, Alves CA, Porter MD (1991) Scanning tunneling microscopy of ethanthiolate and n-octadecanethiolate monolayers spontaneously adsorbed at gold surfaces. J Am Chem Soc 113 2805-2810... 

Widrig C A, Alves C A and Porter M D 1991 Scanning tunneling microscopy of ethanethiolate and normal-octadecanethiolate monolayers spontaneously adsorbed at gold surfaces J. Am. Chem. Soc. 113 2805-10... 

Spontaneously adsorbed monolayers possessing sulfonate terminal groups on gold electrodes can enhance the lead UPD current, for example [280], see also Chapter Electrochemistry of Gold (24.4) in the present book. 

Self-assembled and spontaneously adsorbed monolayers offer a facile means of controlling the chemical composition and physical structure of a surface. As discussed later in Chapter 5, applications of these monolayers include modeling election transfer reactions, biomimetic membranes, nano-scale photonic devices, solar energy conversion, catalysis, chemical sensing and nano-scale lithography. 

The Langmuir isotherm describes equilibrium adsorption when there are no lateral interactions between the adsorbed molecules, the limiting surface coverage is dictated simply by the size of the adsorbate, all adsorption sites on the surface are equivalent and adsorption is fully reversible. As discussed in Section 5.5.1 below, spontaneously adsorbed monolayers that fulfill these criteria will exhibit ideal voltammetric responses at slow scan rates, thus including a linear dependence of the peak current, zp, on the scan rate, v, an FWHM of 90.6In mV, where n is the number of electrons transferred, and a formal potential that is independent of the surface coverage. 

A wide variety of quinones spontaneously adsorb onto various electrodes, including gold, platinum, carbon, and especially mercury. On mercury electrodes, these quinonoid monolayers often exhibit nearly ideal electrochemical responses in low-pH electrolytes, so making them attractive model systems for probing the thermodynamics of adsorption. In low-pH electrolytes, both the oxidized and... 

Figure 5.3 Cyclic voltammogram of a spontaneously adsorbed [Os(bpy)2py(p3p)]2+ monolayer, obtained by using a scan rate of 50 V s 1, with a surface coverage of 9.5 x 10 n mol cnr2. The supporting electrolyte is 0.1 M TBABF4 in acetonitrile, and the radius of the platinum microelectrode is 25 xm. The cathodic currents are shown as up, while the anodic currents are shown as down. The complex is in the [Os(bpy)2py(p3p)]2+ form between +0.400 and —1.200 V the initial potential was 1.000 V. Reprinted with permission from R. J. Forster, Inorg. Chem., 35, 3394 (1996). Copyright (1996) American Chemical Society...

Figure 5.6 Cyclic voltammograms obtained for a spontaneously adsorbed [Os(bpy)2 4-tet Cl]+ monolayer on a 5 pm radius gold microdisk electrode where the scan rate is 1333 V s 1. The theoretical fits to the data, using a non-adiabatic electron tunneling model at electrolyte pH values of 0.9 and 6.0, are denoted by O and , respectively. In both cases, k is 27 kj mol-1, while k° is 1.1 x 103 and 1.1 x 104 s 1 at pH values of 0.9 and 6.0, respectively. Reprinted with permission from D. A. Walsh, T. E. Keyes, C. F. Hogan and R. J. Forster, ]. Phys. Chem., 105, 2792 (2000). Copyright (2000) American Chemical Society...

Figure 5.8 Cyclic voltammograms for the Os2+/3+ redox reaction within spontaneously adsorbed [Os(OMebpy)2(p3p)Cl]+ monolayers. From right to left, the electrode materials are platinum, gold, carbon and mercury. The scan rate is 50 Vs-1, with a surface coverage of 1.0 0.1 x 10-1° mol cm-2 the supporting electrolyte is aqueous 1.0 M NaC104. Reprinted with permission from R. J. Forster, P. J. Loughman and T. E. Keyes,/. Am. Chem. Soc., 122,11948 (2000). Copyright (2000) American Chemical Society...

The development of spontaneously adsorbed and self-assembled monolayers has greatly facilitated investigations into those factors which influence the rate of heterogeneous electron transfer. However, important issues such as the effect of switching... 

Spontaneously adsorbed monolayers of the dimeric complex (Figure 5.11) [(pOp) Os(bpy)2 (4-tet) Os(bpy)2 Cl]3+, where pOp is 4,4 -bipyridyl, bpy is 2,2/-bipyridyl and 4-tet is 3,6-bis(4-pyridyl)-l,2,4,5-tetrazine, have been assembled on platinum microelectrodes in an attempt to address these issues [33]. Significantly, as illustrated in Figure 5.11, the voltammetric response associated with the Osn/m reaction is unusually ideal for both metal centers. Studies using mononuclear model compounds reveal that the redox responses centered at approximately 0.620 and 0.300 V correspond to the inner [(pOp) Os(bpy)2 (4-tet)]2+ and outer [(4-tet) Os(bpy)2 Cl]+ moieties, respectively. The observation of two well-defined voltammetric waves indicates that electron transfer can occur across the [(pOp) Os(bpy)2 (4-tet)]2+ bridge to the outer [Os(bpy)2 Cl]+ moiety, i.e. charge trapping does not occur. 

Figure 5.12 Structures of someanthroquinones used to form spontaneously adsorbed monolayers on mercury electrodes...

The presence of these spontaneously adsorbed (i.e. adsorbed during minimum exposure times), thin films on polymeric substrates such as polystyrene culture dishes and glass plates usually cannot be demonstrated by other spectroscopic methods. For example, the modified internal reflection spectroscopic technique, in which an auxiliary salt prism (usually the malleable salt KRS-5) is pressed against the demonstrably (by other techniques) protein-coated substrates, always fails this technique is not sensitive enough for the near monolayer ranges required for this demonstration. The required sensitivity is achieved only when adsorption occurs directly on the face of a clean, or thin film-coated, internal reflection element. 

Another variation involves the stripping or electrolysis of species that have spontaneously adsorbed on the surface of an electrode without the preelectrolysis step. This technique, called adsorptive stripping voltammetry, can be applied, for example, to sulfur-containing species, organic compounds, and certain metal chelates that adsorb on Hg and Au (74, 81). Examples include cysteine (and proteins that contain this amino acid), dissolved titanium in the presence of the chelator solochrome violet RS, and the drug diazepam. The amounts found by this method would necessarily be limited to monolayer levels. However, similar approaches can be employed with thicker polymer layers that can interact with solution species. Related experiments are described in Chapter 14. 

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