Gold nanorods as SERS substrate for the ultratrace detection of cocaine in non-pretreated oral fluid samples

Abstract

This work shows the capability of surface-enhanced Raman spectroscopy (SERS) to detect ultratraces of cocaine(COC) in oral fluid (OF). It proposes a new solid substrate made of gold nanorods (Au NRs) to perform sensitiveanalyses of this complex matrix without any sample pretreatment. The Au NRs were synthetized optimizing thesynthesis conditions, and then characterized and tested as SERS substrate. The presented results demonstratedthat the SERS methodology was able to detect COC in OF with a limit of detection (LOD) as low as 10 ng/mL.This value was five orders of magnitude smaller than the one obtained with Raman spectroscopy (RS) and in theorder of the cut-off value (8 ng/mL) established for confirmatory test of COC in OF. Besides, a multivariateanalysis (OPLS-DA) applied on the samples analysed by SERS evidenced that it was possible to discriminateamong various COC concentrations. These are quite positive results because even the 1 ng/mL COC concentration sample could be identified as different from the OF sample. In addition, the absence of samplepretreatment and the ease of the proposed method, allowed performing rapid and non-destructive analyses,making the method suitable for in situ forensic analyses.