Engineered optoplasmonic core-satellite microspheres for SERS determination of methamphetamine derivative and its precursors

In this work, the core-satellite optoplasmonic particle comprised of the central dielectric silica sphere and the surrounding gold nanorods (AuNRs) with varied aspect ratio (AR) were fabricated through electrostatic-assisted self-assembly. The manipulation over the optoplasmonic resonance of the hybrid material was accomplished through tuning the AR of AuNRs at the perimeter of the silica microsphere, which enables the optimization of local plasmonic-photonic electric field enhancement in accordance with the incident wavelength. The monolayer structure assembled by the hybrid particles, termed as optoplasmonic surface, serves as a robust surface-enhanced Raman spectroscopy (SERS) sensing substrate because the building block is surprisingly agglomeration-resistive under severe conditions including the dryness and salination. The optimal SERS excitation plane of the optoplasmonic surface was found to be the equatorial one due to the geometrical factor holding the higher density of AuNRs, the plasmonic-photonic interaction between adjacent optoplasmonic particles and the light guiding effect. The fabricated optoplasmonic surface (silica sphere diameter = 4 µm, AR of AuNR = 3.2) was implemented for the SERS sensing of 3,4-methylenedioxymethamphetamine (MDMA) and the corresponding precursors including safrole and piperonal, achieving a limit of detection (LOD) down to the magnitude of 5 nM with the excitation wavelength at 785 nm. Practical SERS tests on the glassware surface and mimic drug demonstrate the compatibility of the optoplasmonic sensing platform in cooperation with the portable Raman spectrometers. This hybrid probe with engineerable resonance and elevated stability is a promising candidate for the application of SERS inspection in forensic activities.