Nanocavity-Based “Turn-Off” Plasmonic Metasurface Sensing Platform for the Detection of Trace Cadmium Ions in Complex Matrices

Plasmonic metasurfaces (PMs) are emerging as a powerful platform for optical biosensing due to their unique sensing capabilities. However, in practical applications involving complex matrices, nonspecific adsorption at the sensing interface generally causes false-positive signals, which considerably compromise the accuracy of detection. To address this limitation, we propose a cadmium ion (Cd²⁺)-driven "turn-off" biosensing strategy by leveraging the precise closing of plasmonic nanocavities constructed via the coupling of nanohole array-based PMs and gold nanoparticles (AuNPs), enabling the detection of trace Cd²⁺ in both serum and urine samples. Upon Cd²⁺ binding, the aptamers (Cd²⁺-specific single-stranded DNA) undergo a conformational change to a stem-loop structure, which dislodges the AuNPs from the PM surface, thereby inducing a blueshift in the resonance dip of the PM chip. This sensing mechanism allows for ultrasensitive Cd²⁺ detection in complex matrices while effectively mitigating signal errors arising from nonspecific adsorption. Crucially, the "turn-off" sensing mechanism endows the sensor with excellent chip regeneration capability. Experimental results demonstrate a wide linear detection range from 0.05 pg/L to 50 μg/L, a high correlation coefficient of <i>R</i>² = 0.999, and an ultra-low detection limit of 3.55 fg/L. This strategy allows extension to other heavy metal ions or biomolecules via tailored aptamer and metasurface design.