Time-dependent SERS and LC-MS for molecular-level monitoring of methyl orange photodegradation over plasmonic ZnO/Ag nanohybrids.

The persistent presence of azo dyes, such as methyl orange (MO), in industrial wastewater represents a major environmental concern due to their chemical stability, toxicity, and the potential formation of hazardous transformation products during treatment processes. Consequently, decolorization alone is not a reliable indicator for environmental remediation. Unlike previous studies that rely on individual techniques, in this work a multimodal analytical approach combining UV-Vis spectroscopy, time-dependent surface-enhanced Raman scattering (SERS), and liquid chromatography-mass spectrometry (LC-MS) is employed to simultaneously monitor chromophore removal and molecular transformation during photocatalytic oxidation. As a photocatalyst model system, ZnO multipods functionalized with Ag nanoparticles (ZnO/Ag nanohybrids) were used under simulated solar irradiation, showing rapid MO degradation. Importantly, time-dependent SERS provides direct insight into the evolution of molecular structure, enabling identification of key processes such as azo-bond cleavage, N-demethylation, and modification of aromatic moieties. Complementary LC-MS analysis enables the separation and identification of intermediate species, including demethylated and hydroxylated species, allowing reconstruction of a multistep oxidative transformation pathway. The combined approach further reveals the temporal evolution and persistence of intermediates, with extended irradiation leading to disappearance of detectable aromatic species. Overall, the integration of SERS and LC-MS provides a robust framework for correlating spectroscopic signatures with molecular-level transformations, offering a comprehensive assessment of photocatalytic processes beyond conventional decolorization metrics.