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Selective photocatalysis by means of molecular recognition.

Heterogeneous photocatalysis, using titanium dioxide as the photocatalyst of choice, is a useful technique for the degradation of many contaminants in air, in water, or on solid surfaces. 1 The process, initiated by the generation of electron -hole pairs following absorption of supra-bandgap photons, continues, in most cases, through oxidation by hydroxyl radicals formed when the photogenerated holes interact with adsorbed water. As such, the process is nonselective, although some selectivity can be obtained by altering the charge on the surface of the photocatalyst, for example by controlling the pH. 2 Since many contaminants can be degraded readily by biological methods, while others are nonbiodegradable, it would be of great advantage if one could use photocatalysis and biological treatment sequentially, such that photocatalysis will be used to degrade nonbiodegradable contaminants to simpler molecules that can be handled biologically. In such a scenario, the need for selectivity is obvious, since contaminated media often contains a large variety of contaminants. Furthermore, since the allowable concentration of a hazardous substance is lower than that of a less hazardous material, it would be beneficial to have a mechanism that would increase the degradation rate of the former, even at the expense of some reduction in the degradation rates of the latter. In this communication we report on an approach to achieve selectivity, based on molecular recognition sites constructed on inert domains located in the vicinity of titanium dioxide. Although the method is demonstrated hereby for one pair of contaminants, namely benzene and 2-methyl-1,4-naphthoquinone (2MNQ), it can easily be extended to other systems. Likewise, although the samples presented here were prepared on solid films, it should not be too complicated to extend this attitude to powders or the like. The basic concept is portrayed in Figure 1. Molecular recognition sites (MRS), in this example thiolated, are constructed on inert sites located in the vicinity of titanium dioxide. Due to the affinity between the MRS and the target molecules (here, 2MNQ), the target molecules adsorb on the MRS, from where they surfacediffuse to the photocatalytic sites. Since the molecular recognition sites are organic in nature, care has to be taken to prevent the degradation of the MRS. Constructing the MRS on inert sites is therefore necessary, but is not sufficient, since oxidizing species can travel on inert surfaces, such as silicon, for quite a long distance. 3,4 The observation that self-assembled monolayers on gold and platinum, located near titanium dioxide domains, are not prone to an attack by oxidizing species formed on titanium dioxide4b enables one to construct such arrays which serve to “shuttle” contaminants of interest. It should be mentioned that although the surface diffusion of contaminants from adsorptive, inert sites to photocatalytic sites is documented, 5 we are not aware of any attempts to achieve selectivity by combining surface diffusion and molecular recognition. Thiolatedâ-cyclodextrin (TCD) was chosen as the molecular recognition site in this work, based on the well-known affinity between this molecule and a variety of hazardous contaminants. 6