Galvanic engineering of interior hotspots in 3D Au/Ag bimetallic SERS nanocavities for ultrasensitive and rapid recognition of phthalate esters

The ecological and health risks of plasticizer accumulation have reached the global public agenda. For early screening of phthalate esters (PAEs), in the present study, we developed ultrasensitive and reliable bimetallic surface-enhanced Raman spectroscopy (SERS) platforms with Ag nanocavities galvanically replaced by Au (Au/AgNCs) at room temperature. A galvanic reaction (GR) coupled with the Kirkendall effect led to a large population of nanoscale hollow regions within 60 s. The interstitials at the bimetallic shells were activated as both interior hotspots for field confinement and diffusion pathways for small target molecules. The GR based on the intrinsic material properties resulted in an Au/AgNC platform with reliable sensing operation, including a relative standard deviation of <10 % at the individual analytic Raman peaks. The qualified and densified interior hotspots of the Au/AgNC platform led to sub-ppm sensitivity (i.e., the average limit of detection of ∼2.1 × 10−2 ppm) for four different PAEs. As a demonstration of the Au/AgNC platform's performance in actual sample environments, the detection and identification of various PAE combinations were also investigated. Subtle differences in the PAEs' spectral characteristics (i.e., peak positions and intensity ratios) were successfully classified using a machine learning algorithm based on a principal component analysis–linear discriminant analysis model. Furthermore, the amount of PAE released from the real sample and evaluated by SERS was consistent with that evaluated by gas chromatography–mass spectrometry. These results indicate that the SERS platforms with Au/AgNC substrates and portable Raman instruments are suitable for on-site safety monitoring of toxic substances in food and the environment.