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

Technical Summary: Picogram-Level Nanoplastic Analysis with NEMS-FTIR

Problem Statement
Nanoplastics pose significant environmental and health risks due to their ubiquity, high reactivity, and ability to penetrate deep into tissues. However, their routine chemical characterization and monitoring remain challenging. Existing analytical techniques face critical limitations: mass spectrometry methods like Py-GC/MS are complex, costly, and often lack the sensitivity required for routine monitoring; while FTIR microscopy (µ-FTIR, FPA-FTIR) is widely used, it is diffraction-limited for particles below 10 µm. Other advanced methods, such as QCL-IR, O-PTIR, AFM-IR, and Raman spectroscopy (including SERS and SRS), offer improved resolution or sensitivity but suffer from narrow spectral ranges, coherence artifacts, slow imaging speeds, reliance on engineered substrates, or prohibitive instrumentation costs. Furthermore, many techniques struggle with spectral artifacts (e.g., Mie scattering, ATR anomalies) and often require extensive sample pre-concentration or pre-treatment, which can introduce errors or lose volatile components.

Methodology
The authors introduce NEMS-FTIR, a technique that integrates the high sensitivity of nanoelectromechanical systems (NEMS) with the broad spectral range and accessibility of commercial Fourier Transform Infrared (FTIR) spectrometers.

• Core Mechanism: The system utilizes NEMS resonators (chips) consisting of a pre-stressed, 50 nm thick silicon nitride (SiN) membrane with a central circular perforation (600 µm diameter). The chip serves as both the sample carrier and the detector. When IR light from a standard FTIR source passes through the membrane and is absorbed by the sample deposited on the surface, local heating occurs. This induces thermal expansion and a reduction in tensile stress, causing a frequency detuning of the resonator proportional to the absorbed power.
• Detection: The frequency shift is monitored via a closed-loop oscillation scheme. Because the readout is purely photothermal, the method is inherently immune to common IR spectral artifacts such as Mie scattering, ATR-related anomalies, and coherence issues.
• Sample Deposition: To enable quantitative analysis, the authors developed two drop-casting methods to confine non-volatile analytes strictly within the perforated sensing area:
  1. Piezoelectric Nanodroplet Dispensing: For precise deposition of nanoliter volumes (e.g., 20 nL).
  2. Pervaporation-assisted Drop Casting: For larger volumes (up to 500 nL), where a humidity gradient drives solvent evaporation preferentially through the membrane perforation, concentrating the analyte in the center.
• Calibration and Quantification: The SiN membrane exhibits a broad absorption band at 835 cm⁻¹, which serves as an intrinsic internal standard to normalize chip-to-chip variability. The measured absorptance is converted to absolute sample mass using the material's attenuation coefficient derived from refractive index data.

Key Contributions

1. Picogram Sensitivity without Cryogenics: The system achieves limits of detection (LoDs) in the picogram range (101–353 pg) for nanoplastics, operating at room temperature without the need for cryogenic cooling required by some high-sensitivity FTIR detectors.
2. Broad Spectral Fingerprinting: Unlike tunable laser sources (e.g., QCLs) that cover limited windows, NEMS-FTIR utilizes the full spectral range of commercial FTIR (4000–400 cm⁻¹). This allows for the identification of diverse polymer types and the resolution of complex mixtures via chemometric deconvolution.
3. Artifact-Free Spectra: The transmission-based photothermal detection avoids the spectral distortions (e.g., peak shifts, intensity variations) associated with ATR-FTIR and the coherence artifacts of QCL-IR.
4. Direct Analysis of Complex Matrices: The method demonstrates the ability to analyze real-world samples, specifically tea brewing water, without pre-concentration, digestion, or ultrafiltration, even in the presence of a complex organic matrix.

Results

• Model Nanoplastics: The system successfully analyzed polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC) nanoparticles with nominal diameters ranging from 54 to 262 nm.
  • LoDs: 101 pg for PP, 351 pg for PS, and 353 pg for PVC. These values are approximately one order of magnitude lower than typical Py-GC/MS LoDs (1–10 ng) and comparable to state-of-the-art TD-PTR-MS.
  • Quantification: A linear relationship was established between the measured absorbance and the deposited mass of PS nanoparticles, allowing for mass estimation.
  • Mixtures: NEMS-FTIR successfully identified and distinguished PS, PP, and PVC in a 1:1:1 mass ratio mixture, with characteristic peaks clearly visible despite low mass loads (5 ng per component).
• Real-World Application (Nylon Teabags):
  • The method identified nylon-based polyamide (PA) leachates released from a single nylon teabag during brewing in 200 mL of water.
  • Sensitivity Comparison: NEMS-FTIR detected distinct nylon spectral features (Amide I at 1642 cm⁻¹, Amide II at 1553 cm⁻¹) in 100 nL and 500 nL aliquots without pre-concentration. In contrast, ATR-FTIR analysis of a 500 nL aliquot yielded only a faint, barely distinguishable signal, even after amplification.
  • Matrix Effects: In complex samples containing lemon balm tea leaves, the nylon signal was successfully extracted via spectral subtraction, identifying both polymer fragments and smaller oligomers.
  • Accelerated Aging: Monitoring teabags under accelerated aging (UV radiation and heat) revealed a time-dependent increase in the release of nylon oligomers and fragments, detectable via the increasing intensity of characteristic peaks.

Significance and Claims
The paper claims that NEMS-FTIR offers a time-efficient, cryogen-free, and routine-ready solution for the chemical characterization and quantification of nanoplastics. By combining the sensitivity of nanomechanical detection with the spectral comprehensiveness of FTIR, the technique overcomes the size limitations of microscopy and the complexity/cost barriers of mass spectrometry.

The authors emphasize that the method is particularly significant for:

• Routine Monitoring: Its compatibility with commercially available FTIR spectrometers makes it suitable for widespread adoption by water suppliers and control laboratories.
• Minimal Sample Preparation: The ability to detect nanoplastics in aqueous samples without pre-concentration or extensive matrix removal (as demonstrated with tea leachates) reduces analysis time and potential for error.
• Versatility: The non-destructive nature of the measurement allows for subsequent analysis by complementary techniques (e.g., SEM, EDX, O-PTIR) on the same sample chip.
• Reliability: The generation of spectra comparable to transmission-FTIR facilitates the use of standard spectral libraries and chemometric tools, while the intrinsic SiN standard ensures measurement consistency.

The study concludes that NEMS-FTIR is a promising tool for environmental monitoring and nanomaterial analysis, capable of detecting nanoplastics at levels previously difficult to achieve with routine, accessible instrumentation.