Investigating the applicability of HEPES–AuNPs for SERS measurements on individual functional red blood cells in saline

Applying plasmonic metal nanoparticles (PMNPs) to perform surface-enhanced Raman spectroscopy (SERS) of live cells in physiological media, such as saline, has been challenging, as they generally aggregate in these media. However, PMNPs synthesized using physiological media, such as 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), are stable in saline. However, evaluating the role of HEPES-PMNPs in SERS of cell components is essential. This study uses near-infrared (NIR) vortex beam excitation to investigate the influence of PMNPs on the Raman spectra of cells in functional RBCs decorated with HEPES-reduced gold nanoparticles (AuNPs) in saline. The RBC spectra include contributions from membrane lipids, transmembrane proteins, and carbohydrate extensions. Higher plasmonic enhancements are seen in amide III bands at 1240, 1244, and 1268 cm^-1, a few amino acid frequencies, and CH₂ deformations at 1447 and 1455 cm^-1 from proteins and lipids, respectively. Hemoglobin Raman peaks showed relatively less plasmonic enhancement. Furthermore, HEPES-AuNPs showed good stability in saline, and the FESEM and flow cytometry measurements on control and NPs-decorated RBCs confirmed the adherence of AuNPs on RBCs.