Ultrasensitive Plasmonic Sensor for Detecting Sub-PPB Levels of Alachlor

Alachlor is a globally used pesticide that has a significant impact on our worldwide ecological system due to its penetration into soil and drinking water; thus, it can greatly affect human health. Because of the increasing demand for sensor optimization, along with significant advances in nanotechnology, we present a one-micrometer-scale, label-free optical sensor based on a plasmonic structure for detecting a minute amount of this highly toxic pesticide. The detection is achieved by surface-enhanced Raman scattering; our plasmonic device is designed systematically to overlap energetically with the Raman laser in order to maximize the signal enhancement. Furthermore, the studied device differs from conventional plasmonic systems because the electromagnetic field is confined to a flat surface between triangular nanocavital features with a specific orientation, providing an area for deposition of molecules. An enhancement factor of ∼1 × 10⁶ allows for detection of trace amounts of alachlor, down to 0.4 parts-per-billion and to differentiate it from a similar pesticide. Consequently, our device could potentially serve as a general, low-cost analyte sensing detector. The plasmonic sensors were characterized by linear transmission measurements, cathodo-luminescence, and surface-enhanced Raman spectroscopy.