One-step microwave-assisted synthesis of a MoO _{3− <i>x</i>} -covalent organic framework (COF)-based nanocomposite for amplified surface-enhanced Raman spectroscopy in environmental applications

This study introduces a novel one-step microwave-assisted (MW-AT) synthesis strategy for a TpTTA COF and demonstrates the fabrication of a MoO_3-<i>x</i>-TpTTA COF-based nanocomposite (NC) for the first time. This method significantly reduces synthesis time (∼1 h) while achieving a high yield at a moderate temperature (120 °C). In contrast, traditional solution-based COF synthesis methods may take several days with continuous heating at higher temperatures (≤200 °C). The as-synthesized MoO_3-<i>x</i>-COF-based NC was shown to serve as an affordable and straightforward substrate for surface-enhanced Raman scattering (SERS) detection of an organic dye, such as methylene blue (MB), achieving a maximum enhancement factor (EF) of 7.76 × 10⁴. The incorporation of MoO_3-<i>x</i> into the COF provides several advantages, including plasmonic enhancement by nonstoichiometric transition metal oxide (TMO) without requiring the addition of noble nanocrystals (<i>e.g.</i>, Ag, Au, <i>etc</i>.), improved stability of the SERS substrate, comparable SERS sensitivity, and reduced costs. Furthermore, the as-synthesized MoO_3-<i>x</i>-TpTTA COF-based SERS substrate demonstrated exceptional detection capabilities with high reproducibility, achieving a limit of detection (LOD) as low as 12.5 ppm for Mancozeb (Mz), a common commercially used dithiocarbamate insecticide used on various vegetables, fruits, nuts, and field crops. Notably, Mz is classified as a category III carcinogen by the International Agency for Research on Cancer. This highlights the potential for developing TMO-COF-based NCs for next-generation SERS-based applications in environmental and food safety monitoring.