Disulfide Incorporated Pyridine and Pyridinium Push–PullDyes for Optical Sensing and Antifouling Electrodes

A new push–pull dye has been developed as a pH indicator when covalently copolymerized in a lipoic acid–based polymer (LA-Py). Furthermore, various charged forms of the dyes are investigated as antifouling diluents in self-assembled monolayers (SAMs) on gold electrodes. The dyes contain an amino donor incorporated into a seven-membered heterocyclic disulfide ring, a pyridine or pyridinium acceptor moiety, and an oligo-phenylene ethylene (OPE) conjugated bridge. A common characteristic of a push–pull dye is the high sensitivity to solvent dipoles such as the Stokes shifts of 176 and 211 nm for pyridine (Py) in acetonitrile and pyridinium (MePy) in chloroform, respectively. This is coupled with a decrease in quantum yields with increasing polarity of the solvents. The LA-Py has an apparent p<i>K</i>_a of 3.5 and showed high sensitivity in acidic conditions with robust cycling between pH 7 and −0.08, producing an average contrast of 36% over 10 cycles. Incorporated into a SAM, the zwitterionic pyridinium sulfonate (SPy) derivative demonstrated antifouling to BSA by continuous monitoring of impedance with electrochemical impedance spectroscopy and ionic permeability of K₃[FeCN₆] by cyclic voltammetry. When “backfilled” with mercaptohexadecane (MHD), an enhanced SAM was formed (SPy + MHD), which showed resistance to ion penetration and improved antifouling.