Efficient Chemisorption of Organophosphorous Redox
Probes on Indium Tin Oxide Surfaces under Mild Conditions
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Abstract
We report a mild and straightforward
one-step chemical surface
functionalization of indium tin oxide (ITO) electrodes by redox-active
molecules bearing an organophosphoryl anchoring group (i.e., alkyl
phosphate or alkyl phosphonate group). The method takes advantage
of simple passive adsorption in an aqueous solution at room temperature.
We show that organophosphorus compounds can adsorb much more strongly
and stably on an ITO surface than analogous redox-active molecules
bearing a carboxylate or a boronate moiety. We provide evidence, through
quantitative electrochemical characterization (i.e., by cyclic voltammetry)
of the adsorbed organophosphoryl redox-active molecules, of the occurrence
of three different adsorbate fractions on ITO, exhibiting different
stabilities on the surface. Among these three fractions, one is observed
to be strongly chemisorbed, exhibiting high stability and resistance
to desorption/hydrolysis in a free-redox probe aqueous buffer. We
attribute this remarkable stability to the formation of chemical bonds
between the organophosphorus anchoring group and the metal oxide surface,
likely occurring through a heterocondensation reaction in water. From
XPS analysis, we also demonstrate that the surface coverage of the
chemisorbed molecules is highly affected by the degree of surface
hydroxylation, a parameter that can be tuned by simply preconditioning
the freshly cleaned ITO surfaces in water. The lower the relative
surface hydroxide density on ITO, the higher was the surface coverage
of the chemisorbed species. This behavior is in line with a chemisorption
mechanism involving coordination of a deprotonated phosphoryl oxygen
atom to the non-hydroxylated acidic metal sites of ITO