Surface-Enhanced Raman Spectroscopy Facilitates the Detection of Microplastics <1 μm in the Environment

Abstract

Micro- and nanoplastics are considered one of the top pollutants that threaten the environment, aquatic life, and mammalian (including human) health. Unfortunately, the development of uncomplicated but reliable analytical methods that are sensitive to individual microplastic particles, with sizes smaller than 1 μm, remains incomplete. Here, we demonstrate the detection and identification of (single) micro- and nanoplastics by using surface-enhanced Raman spectroscopy (SERS) with Klarite substrates. Klarite is an exceptional SERS substrate; it is shaped as a dense grid of inverted pyramidal cavities made of gold. Numerical simulations demonstrate that these cavities (or pits) strongly focus incident light into intense hotspots. We show that Klarite has the potential to facilitate the detection and identification of synthesized and atmospheric/aquatic microplastic (single) particles, with sizes down to 360 nm. We find enhancement factors of up to 2 orders of magnitude for polystyrene analytes. In addition, we detect and identify microplastics with sizes down to 450 nm on Klarite, with samples extracted from ambient, airborne particles. Moreover, we demonstrate Raman mapping as a fast detection technique for submicron microplastic particles. The results show that SERS with Klarite is a facile technique that has the potential to detect and systematically measure nanoplastics in the environment. This research is an important step toward detecting nanoscale plastic particles that may cause toxic effects to mammalian and aquatic life when present in high concentrations.

Detection of Sub-Micro- and Nanoplastic Particles on Gold Nanoparticle-Based Substrates through Surface-Enhanced Raman Scattering (SERS) Spectroscopy

“…For all PS particles, the primary peak of interest used for their detection is present at 1002 cm −1 and at the higher sample concentrations (e.g., 100 µg/mL and 40 µg/mL of 161 nm PS), a weak peak at 1032 cm −1 can also be observed ( Figure 2 ). Such findings are in good agreement with Raman control data ( Figure A4 ) and findings reported in the literature [ 16 , 18 ].…”

“…Prior to characterizing the plastic particles by SERS, the Raman spectra of both PS and PET were taken by drop casting the stock solutions on a glass slide ( Figure A4 ; Table A2 ). For PS, key peaks of interest are present as a result of ring breathing vibrations (i.e., v(C–C) near 1002 cm −1 and β(C–H) near 1032 cm −1 ) of the benzenes within the polymer backbone [ 16 ]. Key peaks of interest for PET are known to be present at 1615–1620 cm −1 and 1730 cm −1 as a result of the ring breathing and carbonyl stretching, respectively [ 9 , 31 , 32 ].…”

“…Despite such promising signal improvements being reported for other systems, to date relatively little literature exists which discusses the use of the SERS for the detection of sub-micro- or nanoplastic particles. Xu et al have reported the use of a commercially available Klarite substrate to detect particles composed of PET, PS, and poly(methyl methacrylate) (PMMA) with signal enhancements of up to 2 orders of magnitude [ 16 ]. However, the particles analyzed in this study were all reported to be >360 nm in size [ 16 ]; thus exceeding the maximum size (i.e., 100 nm) defined for a nanomaterial by the European Commission [ 17 ].…”

“…Xu et al have reported the use of a commercially available Klarite substrate to detect particles composed of PET, PS, and poly(methyl methacrylate) (PMMA) with signal enhancements of up to 2 orders of magnitude [ 16 ]. However, the particles analyzed in this study were all reported to be >360 nm in size [ 16 ]; thus exceeding the maximum size (i.e., 100 nm) defined for a nanomaterial by the European Commission [ 17 ]. In a study conducted by Lv et al it was reported that the detection of PS particles with a diameter of 100 nm was possible, and that the enhancement factors observed could be as much as 500 times greater than a regular Raman spectroscopy signal [ 18 ].…”

Single-particle Raman spectroscopy for studying physical and chemical processes of atmospheric particles

“…(Kunihisa et al, 2020) The wide analytical range of particle size enables SERS to be a potentially powerful tool in studying ambient particulate matters. (Vejpongsa et al, 2017;Steer et al, 2016;Chen et al, 2021) Some species including components of microorganisms (Tahir et al, 2020), microplastics (Xu et al, 2020a), crystalline silica (Zheng et al, 2018), the fine structure of organics (Craig et al, 2015;Gen et al, 2018;Fu et al, 2017), and even some Raman-active inorganic salts at very low concentration (Saniel et al, 2019) are invisible in normal Raman due to their weak Raman-activities. However, they could be detected with a SERS substrate.…”