Photocatalytic activity of AgInS2 quantum dots upon visible light irradiation for melatonin determination through its reactive oxygen species scavenging effect

In this work, the ability of thiomalic acid (TMA)-capped AgInS2 quantum dots to generate reactive oxygen species (ROS) upon irradiation with visible light was investigated, which led to the subsequent development of a luminol-based chemiluminometric method for the monitoring of melatonin (MLT) in commercial dietary supplements. The developed approach was based on the antioxidant properties of melatonin conferred by the electron-rich aromatic indole ring which functions as an electron-donor moiety making it a potent direct scavenger of ROS. This capacity to scavenge free radicals was explored as a quantification process relying on MLT quenching effect on the CL emission of the luminol probe produced upon oxidation by ROS in alkaline medium. The short-lived chemiluminescent species resulting from this reactional scheme were detected by means of a multi-pumping flow system (MPFS) coupled to a photo-irradiation unit in order to allow an efficient control of the irradiation process, to improve the mixture between solutions and to reduce reagents' consumption. Upon optimization, the obtained results showed a linear relationship between the ratio of CL signal, in the absence (blank) and presence of the analyte (CL0/CL), and melatonin concentration for values of up to 100 mg L-1. The limit of detection was about 0.44 mg L-1.