Assembly of N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate monolayer films with catechol sensing selectivity

Abstract

The highly water insoluble N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate is synthesised and investigated for sensor applications. This amphiphilic molecule is immobilised by evaporation of an acetonitrile solution at a basal plane pyrolytic graphite (HOPG) electrode surface and is shown to provide a monolayer film. By varying the amount of deposit partial or full coverage can be achieved. The N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate monolayer acts as an active receptor for 1,2-dihydroxy-benzene (catechol) derivatives in aqueous media. The ability to bind alizarin red S is investigated and the Langmuirian binding constant determined as a function of pH. It is shown that the immobilised boronic acid monolayer acts as sensor film for a wider range of catechols. A comparison of Langmuirian binding constants for alizarin red S (1.4 × 10⁵ mol⁻¹ dm³), catechol (8.4 × 10⁴ mol⁻¹ dm³), caffeic acid (7.5 × 10⁴ mol⁻¹ dm³), dopamine (1.0 × 10⁴ mol⁻¹ dm³), and L-dopa (8 × 10³ mol⁻¹ dm³) reveals that a combination of hydrophobicity and electrostatic interaction causes considerable selectivity effects.