Deltamethrin pesticides can cause inflammation, nephrotoxicity and hepatotoxicity as well as affect the activity of antioxidant enzymes in tissues. As a result of this concern, there is a rising focus on the development of fast and reliable pesticide residue testing to minimise potential risks to humans. The goal of this study is to use Au-Ag colloid nanoparticles as liquid surface-enhanced Raman spectroscopy (SERS) to improve the Raman signal in the detection of deltamethrin pesticide in a brewed tea. The liquid SERS system is fascinating to study due to its ease of use and its unlikeliness to cause several phenomena, such as photo-bleaching, combustion, sublimation and even photo-catalysis, which can interfere with the Raman signal, as shown in the SERS substrate. Our liquid SERS system is simpler than previous liquid SERS systems that have been reported. We performed the detection of pesticide analyte directly on brewed tea, without diluting it with ethanol or centrifuging it. Femtosecond laser-induced photo-reduction was employed to synthesise the liquid SERS of Au, Au-Ag, and Ag colloidal nanoparticles. The SERS was utilised to detect deltamethrin pesticide in brewed tea. The result showed that liquid SERS-based Ag NPs significantly enhance the Raman signal of pesticides compared with liquid SERS-based Au NPs and Au-Ag Nanoalloys. The maximum residue limits (MRLs) in tea in Indonesia are set at 10 ppm. Therefore, this method was also utilised to detect and improve, to 0.01 ppm, the deltamethrin pesticide Limit of Detection (LOD).

Surface-enhanced Raman spectroscopy (SERS) has attracted considerable research interest over the last four decades because of its rapid vibrational spectroscopic detection, high sensitivity, and nondestructive technique for enhancing the generally weak signal from Raman scattering. Here, SERS substrates were fabricated by drop-casting Au nanoparticles (NPs) onto two substrates (Cu and Si wafers). The AuNPs (diameter = 7.3 nm) were synthesized from an Au metal ion solution with a concentration of 4.22 × 10−4 M via photochemical reduction using a femtosecond laser. The SERS substrates were tested for their ability to enhance the Raman signal of paraquat pesticides at 10 ppm. Six vibration peaks of the paraquat pesticides at 671, 838, 1187, 1294, 1530, and 1643 cm−1 were successfully detected and enhanced. The results showed that the SERS substrate on the Si wafer increased the Raman signal more than the Cu wafer.

Raman spectroscopy identifies materials non-invasively at the molecular level by molecular vibrations induced by laser interaction with the material. Electrons in a molecular material are raised from their ground state into a virtual state by a laser’s interaction, then return to the ground state and release a photon. Since the incident and emitted photons have different energies, the molecules vibrate, which Raman scattering detects. Nevertheless, the intrinsic weakness of Raman signals, particularly at low analyte concentrations, poses a major limitation. Therefore, metal nanoparticles with plasmonic properties are crucial for increasing the Raman signal mainly due to plasmonic effects, not because of chemical interactions that involve charge transfer. Plasmonic phenomena, such as surface plasmon polaritons (SPPs) and localized surface plasmon resonance (LSPR), are significantly influenced by essential factors including the metal nanoparticles’ size, shape, and classification, together with the choice of substrate for surface-enhanced Raman spectroscopy (SERS). These phenomena collectively play pivotal roles in enhancing Raman signals. Moreover, the creation of “hot spots”—nanoscale gaps between adjacent nanoparticles—further intensifies local electromagnetic fields, significantly boosting sensitivity. Thus, systematic investigations into the interplay between nanoparticle morphology, plasmonic and polaritonic properties, and the resulting impact on Raman signal enhancement are essential for advancing the development of highly sensitive Raman-based detection platforms.