Picogram-Level Nanoplastic Analysis with Nanoelectromechanical System Fourier Transform Infrared Spectroscopy: NEMS-FTIR

TL;DR

Nanoplastics are challenging to analyze at the nanoscale, especially in water, due to their small size and low concentrations. The authors introduce NEMS–FTIR, a cryogen-free photothermal infrared platform that integrates nanoelectromechanical sensing with Fourier transform infrared spectroscopy to enable bulk, single-particle, and mixed-polymer analysis with picogram-level detection. They demonstrate quantitative mass estimation from absorptance for PS, PP, and PVC and show reliable mixture identification; they also detect nylon teabag leachates directly in brewing water without preconcentration, with aging revealing increased leachate production. The approach offers broad spectral coverage, compatibility with standard FTIR libraries, reduced spectral artifacts via a vacuum path and an internal SiN standard, and potential for routine environmental monitoring of nanoplastics in simple and complex matrices, including aging-related releases.

Abstract

We present a photothermal infrared spectroscopy-based approach for the chemical characterization and quantification of nanoplastics. By combining the high sensitivity of nanoelectromechanical systems (NEMS) with the wide spectral range and ubiquity of commercially available Fourier transform infrared (FTIR) spectrometers, NEMS-FTIR offers a time-efficient and cryogen-free option for the rapid, routine analysis of nanoplastics in aqueous samples. Polypropylene, polystyrene, and polyvinyl chloride nanoplastics with nominal diameters ranging from 54 to 262 nm were analyzed by NEMS-FTIR with limits of detection ranging from 101 pg to 353 pg, one order of magnitude lower than values reported for pyrolysis-gas chromatography-mass spectrometry of nanoplastics. The absorptance measured by NEMS-FTIR could be further converted to absolute sample mass using the attenuation coefficient, as demonstrated for polystyrene. Thanks to the wide spectral range of NEMS-FTIR, nanoplastic particles from different polymers could be readily identified, even when present in a mixture. The potential of NEMS-FTIR for the analysis of real samples was demonstrated by identifying the presence of nanoplastics released in water during tea brewing. Polyamide leachates in the form of fragments and smaller oligomers could be identified in the brewing water without sample pre-concentration, even in the presence of an organic matrix. Accelerated aging of the nylon teabags under elevated temperature and UV radiation showed further release of polyamide over time.

Figures (19)

• Figure 1: Experimental setup for NEMS-FTIR analysis. (A) An FTIR spectrometer (Vertex 70, Bruker Corporation, MA, USA) equipped with the nanoelectromechanical IR analyzer (EMILIE$^{\text{TM}}$, Invisible-Light Labs GmbH, Austria) for NEMS-FTIR analysis. The inset illustrates the optical path of the IR beam from the spectrometer output to the NEMS chip. (B) A NEMS sampling and sensing chip (Invisible-Light Labs GmbH, Austria). The central square membrane on the NEMS resonator is made of SiN and has a lateral size of $1\times 1$ mm$^2$ and a thickness of $\sim$ 50 nm. It features a central circular perforated area with a diameter of approximately 600 $\mu$m, consisting of 6 $\mu$m holes spaced 3 $\mu$m apart. (C) 20 nL drop of PS dispersion, corresponding to a deposited mass of 15 ng, deposited and confined within the perforated membrane area using a piezoelectric nanodroplet dispenser. (D) Schematic of drop casting combined with the pervaporation method, where a humidity gradient causes solvent evaporation through the perforation to collect the sample in the membrane's central region. (E) 500 nL drop of PS dispersion, corresponding to a deposited mass of 15 ng, drop casted with a micropipette on the membrane area and dried using the pervaporation method, and (F) without using the pervaporation method.
• Figure 2: Characterization and quantification of model nanoplastics. (A) SEM image of PS nanoparticles located on the perforated membrane area of the NEMS chip. (B) Absorbance spectrum of 20 ng PS measured via NEMS-FTIR compared to a reference spectrum calculated with Eq. (\ref{['eq:absorbance-att coeff']}) and (\ref{['eq:dec lin att coeff']}) from PS refractive index data. Myers2018 (C) NEMS-FTIR spectra of varying mass loads of PS nanoparticles deposited on the NEMS chips. The inset highlights the 1452 cm$^{-1}$ peak, which was used to construct the calibration curve and determine the LoD. (D) Calibration curves for the 1452 $\text{cm}^{-1}$ PS peak, 1377 $\text{cm}^{-1}$ PP peak, and 1427 $\text{cm}^{-1}$ PVC peak, with the inset showing a zoomed-in view of the region corresponding to lower mass loads (error bars N=3, 95% C.L.).
• Figure 3: Characterization of nanoplastic mixture. (A) SEM image of a mixture containing PS, PP, and PVC. (B) Magnified view of the membrane area highlighting PS ( 100 nm, blue square), PP ( 54 nm, green square), and PVC nanoparticles ( 262 nm, red square). (C) Stacked NEMS-FTIR spectra of pure PS, PP, and PVC nanoparticles and their 1:1:1 mixture. Vertical lines mark the characteristic peaks of each polymer.
• Figure 4: Characterization of leachates released from nylon teabags in water. (A) Schematic illustration of the sample preparation process. (B) SEM image showing fragments released into water by a nylon teabag. (C) NEMS-FTIR spectra of nylon teabag leachates (100 nL, and 500 nL aliquots deposited onto NEMS chips without pre-concentration) compared to ATR-FTIR spectrum of 500 nL of teabag leachate and the ATR-FTIR spectrum of the bulk nylon teabag (Reference). Vertical lines indicate characteristic IR peaks associated with nylon-based PA. (D) NEMS-FTIR spectra of the leachates from nylon teabags subjected to accelerated weathering for 25 consecutive days, simulating one year of aging under adjusted conditions. The right inset graph highlights the characteristic nylon-based PA peak at 1642 cm$^{-1}$, corresponding to the amide I band. The left inset graph represents the dependence of signal intensity at 1642 cm$^{-1}$ on the number of days during which the nylon teabag was subjected to accelerated aging.
• Figure 5: Characterization of leachates released from nylon teabags in a complex matrix. (A) NEMS-FTIR spectra of brewed lemon balm tea with (red trace) and without (blue trace) the nylon teabag, together with the corresponding difference spectrum. Nylon-related peaks (vertical lines) remain distinguishable despite the complex matrix. (B) NEMS-FTIR spectra of the retentate, and permeate obtained after oxidation and ultrafiltration of the same samples. Nylon-based PA peak positions are marked with vertical lines. The permeate shows well-resolved nylon features, while the retentate spectrum is dominated by signals from the lemon balm tea matrix. All spectra are shown in arbitrary units.