Stability of omega-functionalized self-assembled monolayers as a function of applied potential

dIon penetration into a series of omega-functionalized X(CH2)(n)S/Au (X = C H3, OH, or CO2H, and n = 15) self-assembled monolayers (SAMs) and a partial ly fluorinated (CF3(CF2)(7)(CH2)(2)S/Au) SAM has been investigated by elect rochemical ac impedance spectroscopy in the absence of a redox probe. SAM p ermeability is revealed by the behavior of the phase angle at frequencies o f less than similar to 50 Hz, the frequency domain characteristic of diffus ion processes. The permeability of these omega-functionalized SAMs, as a fu nction of an applied potential, falls into two regimes. One regime correspo nds to a state where the SAM is an ionic insulator and is well described by the Helmholtz capacitor model. The second regime begins when a critical ap plied potential, V-c, is exceeded. V-c corresponds to the applied potential at which ion penetration into the SAM is activated. For a chain length of 15 carbon atoms, the chemical nature of the terminal group X greatly influe nces the value of V-c, where V-c is +0.25 V (vs Ag/AgCl) for X = OH, +0.15 V for X = CO2H, -0.35 V for X = CH3, and -0.25 V for the fluorinated SAM. A hydrophilic SAM/electrolyte interface, rather than a hydrophobic one, is m ore readily transformed into a form which favors ion/water penetration into the SAM. The potential-induced transformation described here is of importa nce to the application of SAMs in biosensors and molecular electronic devic es.