In this regard, this report shows how the portable Raman device i-Raman Plus 785 can be used in combination with modified gold nanotriangles to develop an alternative quantification method for trigonelline. This alkaloid is a biomarker present in different food items, such as coffee and quinoa, that provides potential health benefits and whose thermal degradation (e.g., during the roasting process of the green coffee beans) makes the formation of different flavor and aroma compounds possible. For example, a coffee brew could contain around 2.3 mM of trigonelline, and there could be around 30-65 μmol of trigonelline in one gram of green coffee beans, which would be an indicator of quality and could be tested using this technique.

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.