Enhanced mid-IR detection characteristics of microplastics and nanoplastics using gold nanorods cluster at microneedle tips

In this study, characterization of surface enhanced infrared absorption (SEIRA) spectroscopy under attenuated total reflection (ATR) mode was provided to effectively identify irregularly shaped microplastics (MPs) through mid-IR plasmon and phonon resonance effect. Here, MPs specific binding microneedle array was fabricated and examined for the mid-IR detection through surface plasmon and phonon effects out of aggregates or clusters of gold nanorods (Au NRs), which were short-range-ordered among the Au NRs spaced within sub-wavelength scale. The densely packed Au NRs clusters were embedded at microneedle’s tips, which were conjugated by a short amino acid oligo-peptide of polystyrene binding peptide (PSBP) having a strong selectivity toward PS MPs and PS nanoplastics for selective capturing or binding. For comparison, Raman spectroscopies were also adopted for accomplishment of surface enhanced Raman spectroscopy (SERS) peaks for the PS MPs. The microneedle arrays were fabricated by poly dimehtylsiloxane (PDMS) molded stamp or imprint method with commercial adhesive polymer of Norland optical adhesive (NOA). The resonant couplings between the PS MPs and the short-range-ordered Au NRs clusters were confirmed by the SEIRA peaks under both conical and pyramidal shaped microneedle formats to identify a low concentration of MPs (0.1 mg/mL) sample in PS aqueous solution. In addition, SEM images could also confirm existences of PS MPs specifically bound with PSBP conjugated Au NRs at microneedle tips. Through this study, efficient MPs detection platforms based on plasmon and phonon SEIRA effects could be newly provided for small quantity identification of MPs samples to ensure spatial resolution for many applications.