Langmuir monolayer flow across hydrophobic surfaces. 2. Sensor development using Langmuir monolayer flow

Two sensing mechanisms utilizing Langmuir monolayer flow, the pressure-driven movement of a Langmuir film into a hydrophobic surface-liquid interface, have been investigated and involve analyte interaction with either the flowing monolayer or stationary film. The sensing aspect is achieved through modulation of the bilayer formation rate. A sensor using this novel transduction mechanism was developed for Cd2+, based on the interaction of divalent cations with the carboxylic headgroup of the flowing monolayer. Aqueous divalent cations associate with an oleic acid monolayer, increasing its viscosity. As a result, the flow rate of the associated monolayer is slower than for the unassociated monolayer. A rudimentary pH sensor was also developed to exemplify flow-based sensing, in which oleyl alcohol flows into the interface between a mixed methyl-carboxylic acid terminated self-assembled monolayer modified gold electrode and water. The flow rate for this system was sensitive to subphase pH. Attenuation of the monolayer flow rate is dependent upon the relative surface concentration of the carboxylic acid moiety in the multicomponent surface monolayer. The major contribution to the modulation of the monolayer flow rate in this system is attributed to solvation thermodynamics of the carboxylate anion macroscopically averaged across the modified electrode surface.