pH-Sensitive phosphate lipid self-assembled monolayers on gold

Self-assembled monolayers (SAMs) of a phosphate lipid, bis(11-mercaptoundecyl)phosphoric acid, were formed on gold electrodes via chemisorption from ethanol solutions by altering the immersion time. The electrochemistry of the lipid monolayers on gold electrodes was examined by using methylviologen and dipropanesulphonate-4,4′-bipyridinium as a cationic and a zwitterionic redox-active marker, respectively. SAMs of n-dodecylmercaptan, 11-mercaptoundecane-1-ol and 11-mercaptoundecane-1-oic acid on gold electrodes which were fabricated by 24 h adsorption from ethanolic solutions of each thiol compound, could not block the redox reaction of methylviologen. However, SAMs of the phosphoric acid lipid on gold which were prepared by adsorption over 6 h exhibited pH-responsive ion-gating phenomena; at lower pHs (<pH 4), penetration of both methylviologen and its reduced form was almost inhibited (gate closed) and at higher pH, incorporation of the redox compound into the SAMs was possible (gate open). By using SAMs of the lipid fabricated by 40 min adsorption from ethanolic solution, similar gating behavior was observed for the oxidation reaction of the reduced form of 1,1′-dipropanesulphonate-4,4′-bipyridinium. In this case, the monolayer gate was closed at higher pH and opened at lower pH. The electrochemical response of SAMs of the lipid using methylviologen as a marker toward Ca²⁺ ion is also described.