Single-molecule sensing of environmental pH - An STM break junction and NEGF-DFT approach

Zhihai Li, Manuel Smeu, Sepideh Afsari, Yangjun Xing, Mark A. Ratner*, Eric Borguet

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

48 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)1098-1102
Number of pages5
JournalAngewandte Chemie - International Edition
Issue number4
StatePublished - Jan 20 2014


  • density functional calculations
  • dyes/pigments
  • molecular electronics
  • pH sensors
  • sensing
  • single-molecule devices

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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