Sensors play a significant role in the detection of toxic species and explosives, and in the remote control of chemical processes. In this work, we report a single-molecule-based pH switch/sensor that exploits the sensitivity of dye molecules to environmental pH to build metal-molecule-metal (m-M-m) devices using the scanning tunneling microscopy (STM) break junction technique. Dyes undergo pH-induced electronic modulation due to reversible structural transformation between a conjugated and a nonconjugated form, resulting in a change in the HOMO-LUMO gap. The dye-mediated m-M-m devices react to environmental pH with a high on/off ratio (≈100:1) of device conductivity. Density functional theory (DFT) calculations, carried out under the non-equilibrium Green's function (NEGF) framework, model charge transport through these molecules in the two possible forms and confirm that the HOMO-LUMO gap of dyes is nearly twice as large in the nonconjugated form as in the conjugated form. Molecular on/off switching: A single-molecule-based pH switch/sensor was fabricated exploiting the sensitivity of the dye molecules malachite green and pararosaniline (see picture). The high on/off ratio (ca. 100:1) of the device conductivity was confirmed by electron-transport calculations.
- density functional calculations
- molecular electronics
- pH sensors
- single-molecule devices
ASJC Scopus subject areas