Adsorption of rhodamine 6G and choline on gold electrodes: a molecular dynamics study

Abstract

The adsorption of analyte molecules on nano-optoelectrodes (e.g., a combined nanoantenna and nanoelectrode device) significantly affects the signal characteristics in Surface-Enhanced Raman Scattering (SERS) measurements. Understanding how different molecules adsorb on electrodes and their modulation of electrical potential help better interpret SERS measurements. This work investigates the adsorption of prototypical analyte molecules, Rhodamine 6G and choline, on gold electrodes with negative, neutral, and positive surface charges using molecular dynamics simulations. We show that both molecules can readily adsorb on gold surfaces at all surface charge densities considered here. Nevertheless, the configurations of adsorbed molecules can differ for different surface charge densities, and adsorption can also change a molecule’s conformation. Rhodamine 6G molecules adsorb more strongly than choline molecules, and the adsorption of both molecules is affected by electrode charge in 0.25 M NaCl solutions. The mechanisms of these observations are elucidated, and their implications for SERS measurements are discussed.

Fabrication of Large-Area Ordered Au Nano-Ring Arrays for the Electrochemical Removal and Sensing of Rhodamine 6G Molecules

“…Apart from the Au•••H-X bonding reported in the previous work [22], the adsorption could also occur by building noncovalent Au-N interactions featuring an ultrafast charge transfer [46], in which the dative Au←:N bond can be electrochemically converted to an electronically transparent Au-N bond [47]. As supported in another study, the Au-N or Au-O anchor bonds can also be reinforced by N-H•••Au bonds [48], thus with the NaCl-rich nature of the electrolyte [49] and rough surface of the Au NRA [50], R-6G could be easily adsorbed during the electrochemical removal process. The difference in the detection results might origin from the mechanisms of the respective techniques; SERS detects the analytes through their adsorption on and interactions with the substrates [51], while UV-Vis detection follows the Beer-Lambert law through optical absorption and subsequent calculated concentrations of the analytes [52].…”

Impact of the Crystal Structure of Silica Nanoparticles on Rhodamine 6G Adsorption: A Molecular Dynamics Study

“…The charges in CHARMM− GUI-generated topology files for R6G were manually corrected to match restrained electrostatic potential partial (RESP) atomic charges obtained from the highly accurate DFT B3LYP/6-31G** calculations reported by Chuichay et al 25 Finally, the generated force field (FF) parameters, including the chemical properties of R6G atoms, were compared with corresponding values obtained by Vaiana et al using automated frequency matching 32 for cross-validation. Furthermore, the R6G structure was previously used in other MD studies involving adsorption on gold electrodes 33 and TiO 2 hydroxylated surfaces. 34 SNPs were built using the Nanomaterial Modeler extension in CHARMM−GUI.…”

Graphene Oxide and Its Derivatives as Adsorbents for PFOA Molecules

“…In our study, a PFOA molecule is initially adsorbed on the surface. Then, its center of mass (COM) is pulled through a spring (spring constant: 5000 kJ mol –1 nm –2 ) in the z -direction at a speed of 0.2 nm/ns toward the bulk solution until it reaches z = 1.86 nm . The pulling speed is selected as a balance between reducing the time needed to reach the desired sampling positions and maintaining the stability of the simulation.…”

“…Then, its center of mass (COM) is pulled through a spring (spring constant: 5000 kJ mol −1 nm −2 ) in the zdirection at a speed of 0.2 nm/ns toward the bulk solution until it reaches z = 1.86 nm. 42 The pulling speed is selected as a balance between reducing the time needed to reach the desired sampling positions and maintaining the stability of the simulation. A total of 20 initial configurations of the system are generated with window sizes of 0.05 nm near the surface and 0.1 nm near the bulk solution.…”