Organic Anion Detection with Functionalized SERS Substrates via Coupled Electrokinetic Preconcentration, Analyte Capture, and Charge Transfer

Abstract

Detecting ultralow concentrations of anionic analytes in solution by surface-enhanced Raman spectroscopy (SERS) remains challenging due to their low affinity for SERS substrates. Two strategies were examined to enable in situ, liquid phase detection using 5(6)-carboxyfluorescein (5(6)-FAM) as a model analyte: functionalization of a gold nanopillar substrate with cationic cysteamine self-assembled monolayer (CA-SAM) and electrokinetic preconcentration (EP-SERS) with potentials ranging from 0 to +500 mV. The CA-SAM did not enable detection without an applied field, likely due to insufficient accumulation of 5(6)-FAM on the substrate surface limited by passive diffusion. 5(6)-FAM could only be reliably detected with an applied electric field with the charged molecules driven by electroconvection to the substrate surface and the SERS intensity following the Langmuir adsorption model. The obtained limits of detection (LODs) with an applied field were 97.5 and 6.4 nM on bare and CA-SAM substrates, respectively. For the CA-SAM substrates, both the ligand and analyte displayed an ∼15-fold signal enhancement with an applied field, revealing an additional enhancement due to charge-transfer resonance taking place between the metal and 5(6)-FAM that improved the LOD by an order of magnitude.

TiO2 Thickness-Dependent Charge Transfer in an Ordered Ag/TiO2/Ni Nanopillar Arrays Based on Surface-Enhanced Raman Scattering

“…Another CT path that may exist in the system is that electron transfer from VB of TiO 2 to the surface state energy levels (E SS ) and transfer to LUMO of PATP, which is considered to increase the amounts of electrons transferred to the PATP molecule. The whole charge transfer process results in the enhanced CT resonance of the PATP/Ag/TiO 2 /Ni system, and the degree of CT resonance enhancement depends on the amounts of electrons participating in the charge transfer process in the system [ 43 ]. The photon of the 532 nm excitation wavelength possesses higher excitation energy, which can theoretically result in a stronger CT resonance enhancement.…”

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

“…Its signal is usually weak and difficult to separate from other stronger signals, such as elastic (Rayleigh) scattering and other possible fluorescent signals, which prevents the further interfacial applications of Raman spectroscopy . Therefore, surface-enhanced Raman spectroscopy (SERS) has been applied to characterize SAMs deposited on rough metal surfaces or metal nanoparticles benefiting from their capability in amplifying signals. , SERS has long been employed to characterize a variety of molecules on gold and silver surfaces as shown in Figure b. These studies allow the identification of chemical species, structures, and adsorption directions, as well as the monitoring of anions, surfactants, environmental contaminants, biomolecules, and dye molecules when they participate in a chemical or electrochemical reaction.…”

Strongly coupled plasmonic metal nanoparticles with reversible pH-responsiveness and highly reproducible SERS in solution

“…PAA-grafted metal NPs are negatively charged; therefore, they can bind with positively charged Raman reporters as well. 61,62 The electrostatic interaction between PAA and charged substrates provides alternative ways to board the target molecules. We chose rhodamine 6G (R6G, Fig.…”