Plasmonic cysteine nanoparticles for studying fluctuations in 5-hydroxytryptamine levels of spiked human serum

This study reports the fabrication of a plasmonic cysteine nanosubstrate (cys-AuNS) for studying fluctuations in the levels of the neurotransmitter 5-hydroxytryptamine (5HT) in aqueous media and in spiked human serum by surface-enhanced Raman scattering (SERS). The sensing strategy is based on hot spot generation resulting in amplification of the SERS intensity. This is primarily due to the interaction of cys-AuNS with 5HT in solution and in spiked human serum. The enhancement in SERS intensities was probed by recording the time-dependent SERS measurements using 632 nm excitation. The results suggest an initial increase in the intensity, followed by signal saturation within ∼600 s. The evolution in SERS signal was found to be in synchronization with the time-dependent hot spot generation and aggregation of cys-AuNS, providing mechanistic insight into the plasmonic enhancement. Owing to the biocompatibility of cys-AuNS, it holds potential for both in vitro and in vivo SERS based monitoring of fluctuations in 5HT levels of human serum. The procedure to monitor fluctuations in 5HT levels involves an ultrafiltration assisted approach for effective extraction of 5HT from the complex biological matrix. The proposed methodology detects traces of spiked 5HT in human serum with a limit of detection (LOD) of 3.83 ng mL^-1 corresponding to the Raman marker band (indole in-phase ring breathing) at 745 cm^-1. The LOD is well below the normal physiological range of 5HT in human serum (101-283 ng mL^-1). Such high sensitivity is particularly advantageous for identifying small but biologically meaningful fluctuations in the 5HT levels. This study thus highlights the practical applicability of the proposed nanosubstrate for biomedical analysis with successful validation in spiked human serum along with satisfactory recovery and precision.