Instrumentation: i-Raman Plus portable spectrometer with 785 nm laser excitation, Raman shift range 150-2800 cm^-1, 50 sec integration time, 10 scans, and liquid cuvette holder with 10 mm optical path.

Samples: Standard trigonelline aqueous solutions ranging from 10.0 mM to 0.5 mM. Gold nanotriangles modified with mercaptopropionic acid and suspended in deionized water (AuNTs).

A 250 mM solution of trigonelline was analyzed using conventional Raman spectroscopy. The spectrum in Figure 1 shows an intense signal at 1034 cm^-1, corresponding to the pyridine ring breathing mode, that could be used to monitor the concentration of this compound in water.

Four independent sets at five different concentrations were analyzed within the 0.5 mM and 10 mM range by conventional Raman spectroscopy and by SERS. The latter one requires an extra step where the modified gold nanotriangles are mixed with the trigonelline solutions (trigonelline: gold nanotriangles = 15:2) before samples are scanned. In all cases, the strong signal observed at 1034 cm^-1 was monitored and the peak area within 1010-1045 cm^-1 spectral window was used to determine the concentration of the alkaloid. Based on the results and the calibration curves (Figure 2) it was possible to observe an improvement on the signal to noise ratio of SERS over the conventional Raman spectra under the same experimental conditions. The results show it is possible to detect concentrations below 0.5 mM using this method.

In summary, we describe a simple method to quantify the presence of diluted trigonelline in solutions using surface enhanced Raman spectroscopy as a tool could potentially improve the quality control process of food items such as coffee and quinoa.

We would like to thank Angeline Saldaña Ramos, Yulán Hernandez, and Prof. Betty C. Galarreta of the Departamento de Ciencias – Sección Química, Pontificia Universidad Católica del Perú for sharing their research results.