Effect of particle concentration at outer coffee stain edge on the limit of analyte detection via surface-enhanced Raman spectroscopy

Developing highly sensitive and selective sensors for molecule detection is a critical objective in advancing various technological fields. Numerous methodologies have been employed to create nanomaterial-based sensors that detect molecules at very low concentrations. Among these, colloidal droplet evaporation stands out as a straightforward and efficient technique for developing distinct nanomaterial deposition morphologies, thereby enhancing the sensitivity and selectivity of the sensor. In this study, tunable, multilayer, smectic assemblies of gold nanorods (Au-NRs) constituting coffee stain patterns on a hydrophilic silicon substrate are demonstrated to facilitate reproducible and ultra-low detection of analyte molecules through surface-enhanced Raman spectroscopy (SERS). Smectic arrangements of Au-NRs with side-to-side and end-to-end orientations occur at the outer coffee stain edge after the evaporation of a sessile Au-NR dispersion droplet. The thickness and height of the outer coffee stain edge—and consequently the self-assembled structures of Au-NRs—can be finely controlled by varying the particle concentrations in the evaporating colloidal droplet. We utilized the smectic structures of Au-NRs as a template for detecting very minute amounts of an analyte molecule, Rhodamine B (RhB), known to be toxic to humans and the environment. We demonstrate here that when the metallic nanorod concentration increased by only two decades, the limit of detection of the analyte molecules improved by six orders of magnitude. This work demonstrates the direct correlation between the minimum detection limit of the analyte molecule using SERS and the extent of the self-assembled anisotropic nanoparticle structures, quantified using microscopic techniques, formed at the outer coffee stain edge. (Left) FE-SEM image of a coffee stain obtained after the complete evaporation of the Au-NR dispersion droplet on a piranha-treated silicon substrate. (Middle-top) enhanced deposition of Au-NRs at the outer coffee stain edge after evaporation of the sessile Au-NR dispersion droplet with high Au-NR and low RhB concentrations. (Right-top) the corresponding detection of very low concentration RhB using surface-enhanced Raman scattering (SERS). (Middle-bottom) reduced deposition of Au-NRs at the outer coffee stain edge after the evaporation of sessile Au-NR dispersion droplet with low Au-NR and high RhB concentrations. (Right-bottom) corresponding detection of high concentration RhB using SERS. • Deposited colloidal gold nanorod particles at the coffee stain edge are used for SERS detection. • Analyte detection limit varies with particle concentration at the coffee stain edge. • At the lowest particle concentrations, 10–6 M Rhodamine was detected. • At the highest particle concentrations, 10–12 M Rhodamine was detected.