title: Single-molecule sensing of environmental pH--an STM break junction and NEGF-DFT approach.
abstract: 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.
authors: Department of Chemistry, Temple University, 1901 North 13th Street, Philadelphia, PA 19122 (USA).
authors: Zhihai
authors: Z
authors: Li
authors: Manuel
authors: M
authors: Smeu
authors: Sepideh
authors: S
authors: Afsari
authors: Yangjun
authors: Y
authors: Xing
authors: Mark A
authors: MA
authors: Ratner
authors: Eric
authors: E
authors: Borguet
keywords: density functional calculations; dyes/pigments; molecular electronics; pH sensors; sensing; single-molecule devices