Field-Deployable and Ultrasensitive Surface-Enhanced Raman Spectroscopy for Waterborne Arsenic Monitoring Based on Headspace Formation of Arsenic–Silver Nanocomposites

Millions of people worldwide are exposed to arsenic-contaminated water, driving an urgent need for field-deployable analytical tools. However, direct, label-free surface-enhanced Raman spectroscopy (SERS) detection of arsenic is hindered by its weak Raman activity and complex matrix interference. Herein, we report an ultrasensitive SERS platform that addresses these issues by integrating hydride generation, headspace solid-phase extraction, and in situ substrate formation into a single step. The target arsenic is converted to volatile AsH3, which simultaneously reduces Ag+ to silver nanoparticles (AgNPs) and codeposits with its oxidation products to form arsenic-associated AgNP composites (As-AgNPs). This design eliminates the need for external Raman reporters or prefabricated SERS substrates and, critically, enables direct fingerprint identification of arsenic via characteristic peaks at 425 and 790 cm-1 under the tested conditions. The in situ generated dense AgNP hotspots, synergized with headspace preconcentration, afford a 2.6% signal relative standard deviation at 10 ppb As(III) (n = 21) and a detection limit as low as 0.085 ppb, alongside tolerance to the coexisting ions under the tested conditions. Field testing at the Minjiang River using a portable Raman spectrometer yielded results in agreement with atomic fluorescence spectrometry (t-test, 95% confidence), with a total assay time of 30 min for six parallel samples. This work provides a robust, field-deployable solution for direct trace arsenic monitoring by coupling analyte collection with reaction-synchronized SERS substrate formation.