Characterizing Chelation at Surfaces by Charge Tunneling

Yuan Li, Samuel E. Root, Lee Belding, Junwoo Park, Jeff Rawson, Hyo Jae Yoon, Mostafa Baghbanzadeh, Philipp Rothemund, George M. Whitesides

Research output: Contribution to journalArticlepeer-review

Abstract

This paper describes a surface analysis technique that uses the "EGaIn junction"to measure tunneling current densities (J(V), amps/cm2) through self-assembled monolayers (SAMs) terminated in a chelating group and incorporating different transition metal ions. Comparisons of J(V) measurements between bare chelating groups and chelates are used to characterize the composition of the SAM and infer the dissociation constant (Kd, mol/L), as well as kinetic rate constants (koff, L/mol·s; kon, 1/s) of the reversible chelate-metal reaction. To demonstrate the concept, SAMs of 11-(4-methyl-2,2′-bipyrid-4′-yl (bpy))undecanethiol (HS(CH2)11bpy) were incubated within ethanol solutions of metal salts. After rinsing and drying the surface, measurements of current as a function of incubation time and concentration in solution are used to infer koff, kon, and Kd. X-ray photoelectron spectroscopy (XPS) provides an independent measure of surface composition to confirm inferences from J(V) measurements. Our experiments establish that (i) bound metal ions are stable to the rinsing step as long as the rinsing time, τrinse 1koff; (ii) the bound metal ions increase the current density at the negative bias and reduce the rectification observed with free bpy terminal groups; (iii) the current density as a function of the concentration of metal ions in solution follows a sigmoidal curve; and (iv) the values of Kd measured using J(V) are comparable to those measured using XPS, but larger than those measured in solution. The EGaIn junction, thus, provides a new tool for the analysis of the composition of the surfaces that undergo reversible chemical reactions with species in solution.

Original languageEnglish
Pages (from-to)5967-5977
Number of pages11
JournalJournal of the American Chemical Society
Volume143
Issue number15
DOIs
Publication statusPublished - 2021 Apr 21

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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