TY - JOUR
T1 - Characterizing Chelation at Surfaces by Charge Tunneling
AU - Li, Yuan
AU - Root, Samuel E.
AU - Belding, Lee
AU - Park, Junwoo
AU - Rawson, Jeff
AU - Yoon, Hyo Jae
AU - Baghbanzadeh, Mostafa
AU - Rothemund, Philipp
AU - Whitesides, George M.
N1 - Funding Information:
This work was supported by the National Science Foundation (NSF, CHE-1808361). We acknowledge the Materials Research and Engineering Center (MRSEC, DMR-1420570) at Harvard University for supporting XPS measurements and providing access to the clean room facilities. Sample preparation and characterization was performed in part at the Center for Nanoscale Systems (CNS) at Harvard University, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation (ECS0335765). L.B. acknowledges salary support from the Simons Foundation (award 290364FY18). J.P. acknowledges fellowship support from the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education of Korea (2018R1A6A3A03013079). H.J.Y. acknowledges the support from the NRF of Korea (NRF-2019R1A2C2011003 and NRF-2019R1A6A1A11044070). We thank Darrell Collison, Brian Cafferty, and Victoria E. Campbell for their discussions and preliminary experiments.
Publisher Copyright:
©
PY - 2021/4/21
Y1 - 2021/4/21
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=85105094415&partnerID=8YFLogxK
U2 - 10.1021/jacs.1c01800
DO - 10.1021/jacs.1c01800
M3 - Article
C2 - 33834784
AN - SCOPUS:85105094415
VL - 143
SP - 5967
EP - 5977
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 15
ER -