Monolayers Based on Oligomers and Polymers

All the strategies discussed so far suffer from a potential drawback, i.e., phase segregation. It has been reported that, in a mixed monolayer, the single components can be localized within domains, leading to a patchwork surface [85, 86]. This arrangement of the molecules often minimizes the overall energy of the system, thanks to the maximization of the attractive interaction between similar molecules within each domain. 

Oligomeric and polymeric species adsorbed on a substrate play a crucial role in many systems devoted to amperometric sensing. When considering multilayers possessing a thickness not exceeding a few nanometers, their nature is conditioned, in one way or another, by the closeness to the substrate. As a consequence, the properties of the resulting thin films resemble those of monolayers. 

Three major classes of monolayers based on An, Py, and Th derivatives can be identified (1) monolayers possessing one An, Py, or Th moiety as the head group (Fig. 5.14a) (2) monolayers possessing one An, Py, or Th moiety as the tail group 

The interaction of An, Py, and Th derivatives with metal or metal oxide surfaces may also lead to chemical reactions that differ from the simple anchoring of the single ring to the electrode surface [76], As an example, the formation of oligomeric and polymeric species from the relevant monomers, as a consequence of the adsorption on the metal surface, has been reported (Fig. 5.16a) [45, 90]. Surface polymerization may involve the reduction of the metal substrate with concomitant oxidation of the monomer and consequent oUgo- or polymerization reaction. The addition of a proper oxidizing species has also been experienced [91]. Unfortunately, the relevant mechanism is stUl unknown. 

Surface polymerization can also take place on metal oxides. The reaction may occur thanks to the oxidation of the heterocyclic monomeric head, by possible concomitant reduction of metal cations in the oxides. Zeolites are the most widely studied systems suitable for this purpose monomers diffuse from gas or solution phase into the pores, where polymerization takes place [92]. Similar phenomena also occur in the case of layered inorganic oxides, such as clays. An example is given by the polymerization of Th confined within adjacent montmoiihonite layers [93]. 

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