Fig. 2.

Schematic illustration of the SERS detection protocol. The sample having the featured peak at 1030 cm⁻¹ was considered positive for nicotine. Those samples that failed to exhibit a positive signal in the first-round SERS detection were subjected to a second round of measurements after adding 100 ppm exogenous nicotine to the samples. If the spiked nicotine could be revealed by the second round of SERS detection, the initial negative response revealed by the first round of SERS detection was correct and the original e-liquid was considered nicotine-free. On the other hand, if the second round of detection failed to reveal the spiked nicotine, the sample might have contained ingredients that interfered with SERS detection, and in such cases, other methods, such as GC–MS, should be employed.