Particles and Electrodes

The reconstitution of an apo-flavoenzyme (apo-GOD) on a 1.4 nm gold nano colloidal particle functionalized with the cofactor FAD has been done recently by Xiao et al. The electron transfer rate observed in this system was up to 5000 1 and thus significantly high than ever observed with the natural substrate oxygen (700 s-1). The noble metal particle acted as an electronic relay wiring the charge from the active site of the enzyme to the supporting electrode. 

One of the outstanding issues is the use of colloids to follow DNA4iybridization on electrodes via conductivity changes. First experiments had been done by the group of Mirkin et al. intercoimecting oligonucleotides of 24 to 72 bases length. The assemblies of nano-particles behave like a semiconductor and retain their discrete multiparticle structure. 

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A coloured charge transfer complex formed by absorption of ferrocyanide at the surface of Ti02 particles and electrodes, on photo-excitation injects electrons into the conduction band of Ti02 the conduction band electrons can be used to generate a photocurrent (Gratzel, 1987)... 

Albery et al. suggest that solvent shell reorganisation may not be relevant to electron transfer between two massive entities, implying that the charge transfer between particle and electrode should have been rapid. They further suggest that the observed irreversibility must therefore arise from the mutual repulsion of the particle and electrode double layers. 

Sandwich Formation between Different Particles and between Particle and Electrode... 

The maximum coincided with a minimum of the distance between particle and copper surface calculated from capacitance measurements. Maximum and minimum also coincided with the potential of zero charge of the copper electrode. To explain maximum and minimum, a new force was postulated that depended on the reorientation of the solvation shell around particles and electrode surface. The orientation of the solvent molecules on the electrode surface should have a minimum at the pzc. On the particle side, formation of a close contact with the metal surface would be easier for hydrophobic particles with only a weak interaction with solvent molecules. A maximum co-deposition should be observed for hydrophobic particles at the pzc. A number of papers published experimental results that support this concept. Dedeloudis and Fransaer even measured the attraction of hydrophilic and hydrophobic particles to a metal surface by placing the particles on the tip... 

Figure 12.10 Adhesion force (O) and equilibrium distance ( ) between particle and electrode surface determined by rotating disc experiments with polystyrene particle co-deposition and copper. (Reproduced with permission from Ref. [15], 1992, The Electrochemical Society.)...

Figure 12.12 Electrostatic interaction between particles and electrode surface. Particles can be charged positively and negatively. For attraction of particles by the electrode surface the effective charge of the electrode surface must be considered.

This limiting form can be used for calculating electrostatic interactions between particles and electrodes in the case of adsorption from gases or xmpolar solvents. 

Since the conduction of ACAs is based on mechanical particle-electrode contacts, pressure is a requisite to form qualified joints. A typical ACA assembly is shown in Fig. 2. After alignment, pressure is applied on the backside of the chip. The adhesive resin is squeezed out and conductive particles are trapped and deformed between opposing electrodes. Once electrical continuity is generated, the adhesive resin is cured with heat or UV. The intimate particle-electrode contacts are maintained by the cured matrix, and the elastic deformation of particles and electrodes exerts a continuous contact pressure. 

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