Influence of droplet drying configuration on surface-enhanced Raman scattering performance

When colloidal noble metal nanoparticles are used as substrates for surface-enhanced Raman scattering (SERS), they are simply mixed with an analyte and a small volume is placed on a surface to dry before analysis. Depending on the charge properties of the nanoparticles and analyte, they show enormous variations in their distribution pattern in the droplet area. This uncontrolled distribution of the nanoparticles and analyte is the major source of variation in a SERS measurement. In this study, the SERS performance of citrate reduced silver nanoparticles (AgNPs) as the substrate at two droplet configurations, sessile and suspended from a hydrophobic surface, was tested using rhodamine 6G, adenine, a 10-base polyadenine oligonucleotide, and catalase as model analytes. It was found that the reproducibility and quality of the spectra obtained from the area of the suspended configuration easily outperformed the spectral reproducibility and quality obtained from the area of a sessile dried droplet due to improved AgNP aggregation control. The enhancement factor was improved up to one order of magnitude by simply drying the droplet in the suspended position.