Quantitative surface-enhanced Raman spectroscopy chemical analysis using citrate as an in situ calibrant.

Direct detection, or inferring the presence of illicit substances, is of great forensic and toxicological value. Surface-enhanced Raman spectroscopy (SERS) has been shown capable of detecting such molecules in a quick and sensitive manner. Herein we describe an analysis strategy for quantitation of low concentrations of three analytes (methamphetamine, cocaine, and papaverine) by SERS analysis using the citrate capping agent that initially saturates the silver nanoparticles' surface as an in situ standard. The citrate is subsequently displaced by the analyte to an extent dependent on the analyte's concentration in the analyte solution. A general model for the competitive adsorption of citrate and a target analyte was developed and used to determine the relative concentrations of the two species coexisting on the surface of the silver nanoparticles. To apply this model, classical least squares (CLS) was used to extract the relative SERS contribution of each of the two species in a given SERS spectrum, thereby accurately determining the analyte concentration in the sample solution. This approach, in essence, transforms citrate into a local standard against which the concentration of an analyte can be reliably determined.

Dielectrophoretic Nanoparticle Aggregation for On-Demand Surface Enhanced Raman Spectroscopy Analysis.

Rapid chemical identification of drugs of abuse in biological fluids such as saliva is of growing interest in healthcare and law enforcement. Accordingly, a label-free detection platform that accepts biological fluid samples is of great practical value. We report a microfluidics-based dielectrophoresis-induced surface enhanced Raman spectroscopy (SERS) device, which is capable of detecting physiologically relevant concentrations of methamphetamine in saliva in under 2 min. In this device, iodide-modified silver nanoparticles are trapped and released on-demand using electrodes integrated in a microfluidic channel. Principal component analysis (PCA) is used to reliably distinguish methamphetamine-positive samples from the negative control samples. Passivation of the electrodes and flow channels minimizes microchannel fouling by nanoparticles, which allows the device to be cleared and reused multiple times.

Screening for canine transitional cell carcinoma (TCC) by SERS-based quantitative urine cytology.

Canine lower urinary tract neoplasia is a clinically important disease process that has high mortality due to late stage diagnosis and poorly durable response to treatment. Non-invasive diagnostic techniques (e.g. dipstick test, urine cytology) currently have poor diagnostic value, while more invasive tests (e.g. cystoscopy and biopsy) are costly and often require general anesthesia. We have developed and herein describe a quantitative cytological analysis method based on the use of surface-enhanced Raman spectroscopy (SERS), for identifying cancerous transitional cells in urine using SERS biotags (SBTs) carrying the peptide PLZ4 (amino acid sequence cQDGRMGFc) that targets malignant transitional cells. By analyzing the ratio of the PLZ4-SBTs to an on board control we were able to show that transitional cells had significantly higher ratios (P < 0.05) in patients diagnosed with transitional cell carcinoma (TCC) than in healthy samples.

Detection of Papaverine for the Possible Identification of Illicit Opium Cultivation.

Papaverine is a non-narcotic alkaloid found endemically and uniquely in the latex of the opium poppy. It is normally refined out of the opioids that the latex is typically collected for, hence its presence in a sample is strong prima facie evidence that the carrier from whom the sample was collected is implicated in the mass cultivation of poppies or the collection and handling of their latex. We describe an analysis technique combining surface-enhanced Raman spectroscopy (SERS) with microfluidics for detecting papaverine at low concentrations and show that its SERS spectrum has unique spectroscopic features that allows its detection at low concentrations among typical opioids. The analysis requires approximately 2.5 min from sample loading to results, which is compatible with field use. The weak acid properties of papaverine hydrochloride were investigated, and Raman bands belonging to the protonated and unprotonated forms of the isoquinoline ring of papaverine were identified.