Surface modifications of functional electrodes of a light addressable potentiometric sensor (LAPS): Non-dependency of pH sensitivity on the surface functional group

To achieve a functional surface design on the electrode of a light addressable potentiometric sensor (LAPS), two different surface modification techniques were adopted to determine the effect of surface functional groups on the sensitivity and reliability of the LAPS. One was chemisorption of alkanethiols (HS-(CH₂)_n-X, n is 2 or 11, X is CH₃, COOH or NH₂) on a gold-deposited electrode, and the other was photochemical immobilization of copolymers containing benzophenone and functional groups of CON(CH₃)₂, COOH and N(CH₃)₂ in their side chains on a propyltrimethoxysilane-treated electrode. Linearity was observed between the potentials of inflection points on photocurrent-potential curves (V_pip) and the pH of the contacting solution: V_pip = a - b · pH. Gold deposition on the electrode reduced both the pH sensitivity of the V_pip and the degree of linearity of relationship between V_pip and pH. The alkanethiol chemisorption on the gold surface hardly changed the V_pip-pH relationship irrespective of the terminal functional groups of the alkanethiols, Neither the propyltrimethoxysilane treatment nor the photogel immobilizations changed the pH sensitivities of the V_pip or the linearities between V_pip and at pH 3-8. In conclusion, gold deposition is not suitable for modification of the LAPS electrode, while surface modification with photogel immobilization of a propyltrimethoxysilane-treated electrode can be used without reducing the pH sensitivity or reliability.