A Review of Spectroscopic Techniques used for the Quantification and Classification of Microplastics and Nanoplastics in the Environment

Spectroscopic Measurements of Microplastics and Nanoplastics in Our Environment © trattieritratti - stock.adobe.com

Introduction

Microplastics (MPs) and nanoplastics (NPs) have emerged as significant environmental contaminants, posing risks to ecosystems and human health. Their identification and quantification in various environmental matrices, including natural waters, air, and biological tissues, require robust analytical techniques. This report outlines various spectroscopy techniques and complementary methods for characterizing and quantifying these plastic particles in different matrices. For each analytical technique, details are given for the methodology, sample handling and presentation, applications and research. Methods covered include inductively coupled plasma–mass spectrometry (ICP-MS), inductively coupled plasma–optical emission spectrometry (ICP-OES), Raman spectroscopy, X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), near-infrared spectroscopy (NIR), ultraviolet–visible spectroscopy (UV-vis), fluorescence spectroscopy, and nuclear magnetic resonance spectroscopy (NMR) are all diverse analytical techniques used to characterize chemical and physical properties of MPs and NPs in various sample types.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Technique Overview: ICP-MS is primarily used for elemental analysis but can aid in the characterization of microplastics by determining the metal content within them.

Sample Handling:

Sample Preparation: Samples must be digested or dissolved in suitable solvents to release metals.

Filtration: Pre-filtration may be required to remove larger particles before analysis.

Sample Presentation:

Samples are typically presented as solutions in vials.

Solid samples may need to be converted to a liquid phase for ICP-MS analysis.

Applications:

ICP-MS can quantify metals adsorbed onto microplastics or contained within their structure, helping assess their potential toxicity and environmental impact.

ICP-MS Research:

Nanoplastics (NPs, <1 µm) are recognized as a key environmental contaminant due to their extensive environmental presence, high mobility, and significant bioavailability. Detection and quantification challenges arise from their carbon-rich composition, variable sizes, polymer types, surface characteristics, and shapes. To address this, metal-tagged NPs were synthesized by cryo-milling lab-prepared plastics incorporating 1% w/w of an organometallic additive, resulting in sub-micron, irregularly shaped particles (1). These metal-tagged NPs were successfully measured by single-particle inductively coupled plasma mass spectrometry (spICP–MS), which enables precise particle size distribution (PSD) and particle number concentration (PNC) measurements at low concentrations (µg/L) (1).

This method allows for creating diverse metal-tagged NPs from commercially significant polymers, such as polystyrene (PS), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), low-density polyethylene (LDPE), and polyvinylpyrrolidone (PVP), using distinct metal-polymer pairings (for example, Sn in PS, Ta in PMMA) to explore the role of polymer composition in NP behavior. The technique’s high sensitivity allows low metal loadings, preserving surface properties of the NPs akin to unmodified forms. Comparisons with existing NP labeling methods highlight the advantages of this approach, which enables the study of NP formation through abrasion, photodegradation, and biological uptake in controlled laboratory settings (1).

A published review examines 44 case studies employing ICP-MS for the detection, characterization, and quantification of submicron- and nanoplastics (SMNPs) (2). It provides a detailed account of analytical approaches, experimental design considerations, quality checkpoints, and the current trajectory of SMNP research. ICP-MS plays a pivotal role in method development, environmental fate studies, and exposure assessment of SMNPs, with a focus on SMNP functionalization through metal probes or intrinsic metal composition. Among functionalization methods, palladium and gold are notably effective as probes. While spherical, laboratory-synthesized polystyrene SMNPs dominate in studies, interest is expanding to irregularly shaped and fiber-like SMNPs. Advances in ICP-MS technology now permit SMNP analysis in complex real-world samples, though data on environmental SMNP prevalence remains sparse. Additionally, a framework is proposed for guiding early-career researchers in SMNP research via ICP-MS techniques (2).

Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)

Technique Overview: Like ICP-MS, ICP-OES is used for elemental analysis but provides different sensitivity and detection limits.

Sample Handling:

Sample Preparation: Samples need to be dissolved in appropriate solvents.

Matrix Matching: Calibration standards must match the matrix of the environmental sample for accurate quantification.

Sample Presentation:

Samples are presented in glass (preferred) or plastic vials, similar to ICP-MS.

Applications:

Useful for quantifying heavy metals in microplastics, which can provide insights into the degradation and weathering processes affecting plastic materials.

ICP-OES Research:

A published research article investigates the potential impact of polystyrene nanoplastics (PS-NPs) on fetal brain development in rats, focusing on NP distribution and effects on myelination (3). Pregnant rats were administered PS-NPs at varying concentrations (50, 10, 2.5, and 0.5 mg/kg) with particle sizes of 25 nm and 50 nm. Analytical findings confirmed transplacental transfer of PS-NPs, showing a preferential accumulation in specific fetal brain regions—particularly the cerebellum, hippocampus, striatum, and prefrontal cortex. Notably, the cerebellum exhibited the highest PS-NP load, prompting further analysis of its myelination. Compared to controls, PS-NP exposure reduced myelin basic protein (MBP) and myelin oligodendrocyte glycoprotein (MOG) expression, decreased myelin thickness, increased apoptosis, and lowered oligodendrocyte counts. These changes correlated with observable motor deficits in offspring, indicating that prenatal PS-NP exposure hinders cerebellar myelination in the fetal brain (3).

The morphology of polystyrene nanoplastics (PS-NPs) was examined via transmission electron microscopy (TEM; FEI Tecnai G2 F30 S-TWIN). Hydrodynamic diameter and zeta potential measurements were conducted using a Zetasizer Nano ZS (Malvern Instruments), providing insights into particle size and stability (3). Elemental analysis for metals and non-metals, including Al, Cd, Cr, and Cu, was performed using ICP-OES. Surface functional groups on PS-NPs were identified through Fourier transform infrared spectroscopy (FT-IR; Thermo Nicolet 6700). Additionally, the green fluorescence of PS-NPs was confirmed to remain stable for at least three months (3).

A 2024 study examines microplastic (MP) contamination in (sub)urban Serbian soils and its influence on the mobility of Cd, As, and Pb within the soil and medicinal plant system, specifically Capsella bursa-pastoris (4). Soil physicochemical characteristics (pH, Eh, SOM, texture) and pseudo-total (aqua regia) and phytoavailable (EDTA) concentrations of Cd, As, and Pb were measured. Elemental concentrations in soil and plant tissues were analyzed using ICP-OES. MP presence was quantified using density separation with 30% H₂O₂ and 5% NaClO, yielding an average of 489 MPs/kg soil (4). Attenuated total reflection-Fourier transform infrared spectroscopy (ATR FT-IR) confirmed seven polymer types, with polystyrene (28.57%) and cardanol prepolymer (23.81%) as the primary contributors. Spearman correlation showed a strong positive association between MP abundance and phytoavailable Cd, As, and Pb concentrations (ρ = 0.82, 0.95, and 0.63, respectively), as well as Cd accumulation in plant roots (ρ = 0.61) and shoots (ρ = 0.65) (4). These results suggest synergistic interactions between MPs and toxic metals, highlighting potential risks for human health due to increased metal mobility in MP-contaminated soils.

Elemental analysis was performed using ICP-OES (Thermo Fisher iCAP 6500), with each sample analyzed in triplicate. Operating parameters for ICP-OES included a nebulizer gas flow rate of 0.5 L/min, axial gas flow of 0.5 L, cooling flow at 12 L/min, and RF power set at 1150 W. The limits of detection (LOD) and quantification (LOQ) were calculated by analyzing 10 blank solutions, setting LOD and LOQ as 3 and 10 times the standard deviation of blanks, respectively. Method detection limits (MDL) and quantification limits (MQL) were then adjusted by the sample dilution factor (4).

Raman Spectroscopy

Technique Overview: Raman spectroscopy is a powerful technique for identifying and characterizing microplastics due to its molecular specificity.

Sample Handling:

Samples “as is” or in solution on microscope slide

Sample Preparation:

Minimal preparation: samples can often be analyzed in their native state.

Microscopy:

Microplastics may be visualized using optical microscopy before Raman analysis.

Sample Presentation:

Samples are placed on a glass slide or within a Raman-compatible cell for analysis.

Applications:

Raman spectroscopy can provide detailed information about the chemical structure of microplastics, allowing for their identification and classification based on polymer types.

Raman Research:

The characterization of microplastics (< 5 mm) and nanoplastics (< 1000 nm) poses significant analytical challenges due to their small sizes, varied shapes, and complex chemical compositions, compounded by high background interference. Traditional methods for microplastics detection include microscopy techniques (optical microscopy, scanning electron microscopy) and various spectroscopy methods (FT-IR, Raman, fluorescence), as well as mass spectrometry (MS) approaches (pyrolysis–gas chromatography–mass spectrometry and matrix-assisted laser desorption/ionization–mass spectrometry). Among these, Raman imaging has emerged as a powerful technique, offering advantages such as higher resolution due to the shorter wavelength of laser light compared to infrared methods, nondestructive analysis, minimal sample preparation, and quick results, making it suitable for real-time applications (5).

A 2024 published paper presents a comprehensive protocol for employing confocal Raman imaging specifically to identify and quantify microplastics and nanoplastics (5). The paper details the process of generating a hyperspectral matrix by scanning the sample, which allows for the visualization and chemical characterization of the plastics. Despite the advancements in hyperspectral imaging, particularly in satellite applications, the adaptation of these techniques for microscale analysis is relatively recent. This work aims to standardize the Raman imaging analysis protocol, addressing the lack of established methodologies and optimizing imaging parameters to enhance signal-to-noise ratios (5).

Additionally, this study investigates the fate of microplastics and nanoplastics released from burning bioplastics, which are considered more environmentally friendly alternatives to traditional plastics. However, some bioplastics may still contribute to plastic contamination if they do not fully degrade. This research provides insights into the implications of burning bioplastics, which can generate both microplastics and harmful emissions. By integrating Raman imaging with laser diffraction and a statistical approach for quantification, the method offers a novel framework for characterizing plastic contaminants, contributing valuable knowledge to environmental research and the impact of bioplastics on plastic pollution (5).

Microplastic pollution represents a significant environmental challenge, necessitating effective detection and identification methods. One paper published in 2024 introduces a line scan confocal Raman micro-spectroscopy tool designed to enhance the speed and efficiency of microplastic particle analysis (6). Traditional point confocal Raman micro-spectroscopy relies on single-point detection, which can lead to prolonged measurement times when scanning larger sample areas. In contrast, the line scan configuration significantly accelerates the imaging process, improving detection speed by approximately one to two orders of magnitude (6).

The presented system demonstrates high sensitivity, enabling the identification of microplastic particle sizes down to 200 nm, with a spatial resolution limited by diffraction to 500 nm. This advancement facilitates comprehensive imaging and compositional analysis across extensive areas, making it a powerful tool for the rapid detection of microplastic pollution. The efficacy of this technology positions it as a promising approach for addressing the pressing environmental concerns associated with microplastics (6).

X-Ray Fluorescence (XRF) Spectroscopy

Technical Overview: XRF is a non-destructive technique used to determine elemental composition, providing insights into the contaminants associated with microplastics.

Sample Handling:

Minimal Preparation: Samples can often be analyzed in solid form.

Calibration: Standards of known composition are essential for quantitative analysis.

Sample Presentation:

Samples can be analyzed directly in their native state or mounted on a substrate.

Applications:

XRF can identify and quantify metals in microplastics, aiding in understanding their environmental and biological interactions.

XRF Research:

Microplastics (MPs) and nanoplastics (NPs) have emerged as significant global pollutants, necessitating effective methods to predict and assess their toxicity. A 2024 published study utilizes non-targeted metallomics, integrating synchrotron radiation X-ray fluorescence (SRXRF) with deep learning algorithms, to screen the phytotoxicity of nano polyethylene terephthalate (nPET) and micro polyethylene terephthalate (mPET) (7).

The toxicity assessment involved evaluating seed germination, seedling growth, photosynthetic performance, and antioxidant activity. Results indicated that nPET at a concentration of 10 mg/L exhibited greater toxicity to rice seedlings than mPET, significantly inhibiting growth, chlorophyll content, malondialdehyde (MDA) levels, and superoxide dismutase (SOD) activity.

Subsequently, rice seedling leaves exposed to nPET and mPET were analyzed using SRXRF, and the resultant data were processed through deep learning algorithms. The implementation of a one-dimensional convolutional neural network (1D-CNN) model yielded an impressive accuracy of 98.99% in distinguishing between mPET and nPET exposures without the need for data preprocessing (7).

This published research demonstrates that non-targeted metallomics, in conjunction with SRXRF and 1D-CNN, can effectively screen for the exposure and phytotoxicity of nPET and mPET, highlighting its potential applicability for evaluating other emerging pollutants. Future studies are recommended to expand the investigation of phytotoxicity across various types of MPs and NPs using this analytical approach (7).

The increasing recognition of the toxicity of micro- and nanoplastics (MNPs) necessitates a better understanding of their biodistribution within organisms. One recent study introduces X-ray fluorescence imaging (XFI) as a promising technique for elucidating MNP bioavailability, facilitating precise measurements of uptake across biological barriers and bioaccumulation in various organs (8). XFI is highlighted for its capability to conduct quantitative analyses with high spatial resolution, making it suitable for longitudinal studies (8).

The research focuses on a numerical investigation of the detection limits for a palladium (Pd) marker in XFI, designed to support future preclinical in vivo studies (8). Utilizing Monte Carlo simulations with a three-dimensional voxel mouse model, the study estimates palladium detection thresholds across different organs under in vivo conditions. The results indicate that the minimum detectable Pd mass in abdominal organs is less than 20 ng/mm², while the brain exhibits a lower detection threshold of less than 16 μg/mm². Consequently, MNPs labeled with Pd, if uniformly distributed within an organ, can be detected at concentrations ranging from less than 1 μg/mL to less than 2.5 mg/mL in vivo (8). These findings suggest that long-term studies involving chronic MNP exposure at low concentrations are feasible, positioning XFI measurements as a valuable tool for assessing the health risks of MNPs in small animals and potentially humans (8).

X-Ray Photoelectron Spectroscopy (XPS)

Technical Overview: XPS is a surface-sensitive quantitative spectroscopic technique used to determine the elemental composition and chemical state of materials, making it valuable for analyzing the surface chemistry of microplastics.

Sample Handling:

Minimal Preparation: Samples typically require minimal preparation and can be analyzed in their solid state.

Calibration: Calibration with standards of known composition is crucial for accurate quantitative analysis.

Sample Presentation:

Samples are often analyzed directly in their native state or can be mounted onto substrates for enhanced stability and measurement accuracy.

Applications:

XPS is utilized to investigate the elemental composition and chemical states of elements in microplastics, providing insights into their reactivity and potential environmental impacts.

XPS Research:

A novel approach for determining the size of polystyrene (PS)-based nanoplastics, addressing the urgent need for effective environmental monitoring has been published (9). The method employs a composite material, benzoic acid-doped NH₂–UIO-66 (designated as BA-NU), which enhances the radiative transition efficiency through the incorporation of benzoic acid as an auxiliary ligand (9).

The doping of benzoic acid serves to cap defect sites on the NU framework, leading to an increased nonradiative transition efficiency. Consequently, the BA-NU composite exhibits heightened sensitivity to interactions between neighboring NU sites and nanoplastics. This interaction restricts the rotation and vibration of the benzene ring within the BA-NU structure, resulting in enhanced fluorescence (9).

The study demonstrates that the sensitivity of the BA-NU composite towards nanoplastics can be finely tuned by adjusting the doping levels of benzoic acid, allowing for precise control over its physicochemical properties. This nanostructure serves as an ultrasensitive turn-on probe for discriminating different sizes of PS nanoplastics. Overall, this work emphasizes the potential of enhancing detection capabilities by modulating the primary structure with guest molecules at the molecular level (9).

Fourier Transform Infrared (FT-IR) Spectroscopy

Technical Overview: FT-IR is widely used for the identification of organic compounds, making it suitable for characterizing microplastics.

Sample Handling:

Sample Preparation: Samples may require grinding or mixing with potassium bromide (KBr) to form pellets.

Sampling Techniques: Attenuated total reflectance (ATR) can be used for solid or liquid samples without extensive preparation.

Sample Presentation:

Samples can be presented as KBr pellets, films, or in ATR crystals.

Applications:

FT-IR provides spectra that can be matched to polymer libraries, enabling identification of the plastic type.

FT-IR Research:

The rising concern over micro- and nanoplastics (MNPs) as significant anthropogenic pollutants has prompted extensive research into their environmental impacts and potential health risks. A review highlights the recent advancements in infrared (IR) spectroscopic techniques and instrumentation applied to the analysis of MNPs, based on a comprehensive literature survey of articles published in the last three years (10).

FT-IR spectroscopy emerged as the predominant technique utilized in MNP analysis, with focal plane array FT-IR (FPA-FT-IR) representing the forefront of this technology. Additionally, quantum cascade laser infrared (QCL-IR) spectroscopy has enabled rapid analysis of plastic particles, while optical photothermal infrared (O-PTIR) spectroscopy provides submicron spatial resolution. Atomic force microscopy-based infrared (AFM-IR) spectroscopy allows for the analysis of MNPs at the nanoscale level (10).

The review compares the most advanced IR instruments identified in the literature, evaluating metrics such as substrates and filters, data quality, spatial resolution, data acquisition speed, data processing capabilities, and associated costs. Limitations of the current IR instrumentation are also discussed, with recommendations provided to overcome these challenges. This review serves as a valuable resource for researchers studying MNPs, guiding them in the selection of appropriate instrumentation to advance understanding of the environmental and health risks associated with these pollutants (10).

The contamination of natural water sources by microplastics (MPs) highlights an urgent environmental concern, necessitating the monitoring of these emerging contaminants in water for human consumption (WHC). A study on optimizing and validating a FT-IR spectroscopy method coupled with optical microscopy (micro-FTIR) in transmission mode for the detection of MPs in WHC has been published (11).

Water samples (250 mL) were filtered through 5 µm silicon filters without pre-treatment. The identification of infrared spectra was conducted using OMNIC mathematical correlation, employing various polymer spectra libraries, including an in-house IR library, alongside a background reading on a clean silicon filter. The method demonstrated robust validation, achieving an average recovery of 91% for representative polymers, a relative standard deviation of 13%, and a reporting limit (RL) of 44 MPs/L (11).

Analysis of 60 WHC samples from the Lisbon water supply revealed MPs concentrations ranging from below the reporting limit (<RL) to 934 MPs/L, with an average of 309 MPs/L. The predominant polymers identified were polyethylene (PE, 76.8%), polyethylene terephthalate (PET, 6.9%), polypropylene (PP, 6%), polystyrene (PS, 4%), and polyamide (PA, 4%). The microplastic particles exhibited an average length and width of 76 µm and 39 µm, respectively. This validated micro-FT-IR method serves as a reliable approach for assessing the presence of MPs in WHC, contributing to environmental and human health risk evaluations (11).

Near-Infrared (NIR) Spectroscopy

Technical Overview: NIR spectroscopy is effective for quantifying organic compounds, including many polymers, elastomers, and plastics, based on overtones and combinations of fundamental vibrations.

Sample Handling:

Minimal Preparation: Direct analysis of solid particles or aqueous suspensions is possible.

Calibration: Use of standard curves based on known concentrations is essential.

Sample Presentation:

Samples can be presented in cuvettes or directly on an NIR spectrometer.

Applications:

NIR can be used to quantify microplastics based on their concentration in environmental samples.

NIR Research:

Microplastic detection, particularly in food substrates, poses significant challenges despite advancements in environmental analysis. One study explores the feasibility of employing Fourier near-infrared (FT-NIR) spectroscopy for the quantitative detection of polystyrene (PS) microplastics in flour (12). A FT-NIR spectrometer was utilized to obtain spectral data from flour samples with varying PS concentrations (12).

To enhance the detection capabilities, four variable selection methods were implemented: iterative variable subset optimization (IVSO), bootstrapping soft shrinkage (BOSS), interval variable iterative space shrinkage approach (IVISSA), and variable-dimensional particle swarm optimization movement window (VDPSO-CMW). Detection models were constructed using partial least squares (PLS) regression to evaluate the effectiveness of these feature selection techniques in quantifying PS content (12).

The VDPSO-CMW-PLS model exhibited superior generalization performance, achieving a coefficient of determination (Rp²) of 0.9810, and a root mean square error of prediction (RMSEP) of 0.0462%, with a relative percent deviation (RPD) of 7.3890. These results indicate that the optimized PLS detection model can rapidly and accurately identify PS microplastics in flour (12). The study introduces a novel technical approach for the swift quantitative analysis of microplastics in food, advancing the capabilities of FT-NIR spectroscopy in food safety assessments (12).

Microplastics are emerging environmental pollutants, and their detection in waste incineration ash remains underexplored, particularly due to the lengthy duration of traditional testing methods. A 2024 study addresses this gap by employing a portable near-infrared spectroscopy (NIR or NIRS) spectrometer for the qualitative and quantitative analysis of microplastics in ash (13).

A total of 84 simulated ash samples, containing various types of microplastics—specifically polypropylene (PP), polystyrene (PS), polyethylene (PE), and polyvinyl chloride (PVC)—at concentrations ranging from 2.4 wt% to 20 wt%, were analyzed (13). The qualitative discrimination model utilized a support vector machine (SVM) approach with multiplicative scatter correction (MSC) preprocessing, achieving 100% accuracy in identifying the different microplastic types present in the ash (13).

For quantitative predictions, partial least squares regression (PLSR) models were developed, demonstrating robust performance. The coefficient of determination (Rp²) values for PP, PS, PE, and PVC were 0.95, 0.93, 0.89, and 0.95, respectively. Corresponding relative percent deviation (RPD) values were 3.97 for PP, 3.96 for PS, 2.89 for PE, and 5.02 for PVC, indicating high predictive accuracy across the models (13). Overall, this study illustrates that portable NIR/NIRS technology can rapidly and accurately detect microplastics in waste incineration ash, offering a promising alternative to traditional methods (13).

Ultraviolet-Visible (UV-vis) Spectroscopy

Technical Overview: UV-vis spectroscopy is primarily used for quantifying compounds based on their light absorption properties.

Sample Handling:

Sample Preparation: Filtration to isolate microplastics may be necessary, and samples should be diluted to within the detector’s range.

Sample Presentation:

Samples are typically analyzed in quartz cuvettes.

Applications:

While not specific to microplastics, UV-vis can detect certain dyes and additives commonly found in plastics.

UV-vis Research:

A straightforward method for quantifying nano/microplastics (N/MPs) in soil, addressing the challenges posed by filter mesh size and measurement resolution was published (14). In this study UV-vis spectrophotometer was utilized to measure polystyrene particles as representative N/MP samples.

To account for interference from soil particles and leachates during N/MP absorbance measurements, ultraviolet (UV) absorbance was recorded at two selected wavelength ranges: 220–260 nm and 280–340 nm. This combination of spectral regions was identified as effective across various soil types (14). Given that N/MPs tend to adsorb onto soil particle surfaces and co-precipitate within soil suspensions, a calibration curve was established correlating the concentration of N/MPs in the suspension to their concentration in the soil (14).

The resulting calibration curve exhibited a linear relationship, facilitating the estimation of N/MP concentrations in soil samples. Although the methodology demonstrates potential for application across different soil types, further investigation into other N/MP materials, such as polyethylene and polyethylene terephthalate, is warranted. Overall, this method offers a promising approach for the measurement of N/MPs in soil environments (14).

Fluorescence Spectroscopy

Technical Overview: Fluorescence spectroscopy can be employed to detect and quantify specific dyes or fluorescent markers associated with microplastics.

Sample Handling:

Sample Preparation: Similar to UV-Vis; samples may require filtering and dilution.

Sample Presentation:

Samples are presented in cuvettes or other suitable containers.

Applications:

Effective for identifying fluorescent microplastics and quantifying them based on their emission intensity.

Fluorescence Research:

Plastic pollution poses significant environmental challenges with implications for ecosystem and human health. A published review article focuses on fluorescence-based detection techniques for identifying MNPs in various environmental samples, addressing the complexities of detecting plastic debris in the submicron and nanoscale range (15). Despite increased scientific scrutiny, the detection of MNPs remains difficult due to inconsistencies in approaches, which often lack standardization and are tailored to specific cases. This variability adversely affects the reproducibility, accuracy, and reliability of data (15).

The review article provides an overview of existing and emerging fluorescence-based methods for MNP detection, highlighting the most used fluorescent dyes, staining protocols, and analytical instruments (15). The strengths and weaknesses of these methodologies are critically evaluated, emphasizing their potential and limitations in advancing our understanding of the distribution and fate of MNPs in the environment. Overall, fluorescence-based techniques, with their high sensitivity, are recognized for their promising capabilities and unique analytical opportunities in the detection and quantification of MNPs, which are crucial for addressing the ongoing challenges of plastic pollution (15).

The detection of micro- and nanoplastics in aquatic environments is a pressing global issue, yet current methodologies for identifying such particles in liquid samples are limited. A recent study introduces a fluorescence lifetime analysis system as a novel technique for detecting micro- and nanoplastics in water, leveraging the inherent endogenous fluorescence of plastic materials (16). The system employs a pulsed laser beam (40 MHz repetition frequency) directed at the sample solution, where the emitted single photons from plastic fragments are collected and analyzed (16).

The emitted light undergoes filtering and is processed to trace the time distributions of photons with high temporal resolution. Fluorescence lifetimes are measured using fitting procedures alongside phasor analysis, which is a fit-free method that requires no assumptions about the sample's decay pattern. The developed system was validated using unlabeled micro- and nano-scale particles, successfully detecting polystyrene in water with a remarkable sensitivity and a detection limit of 0.01 mg/mL, all without requiring sample pre-treatment or visual inspection (16). While further research is needed to improve detection limits and differentiate between various plastic materials, this proof-of-concept study demonstrates the potential of fluorescence lifetime analysis as a rapid, robust, and cost-effective method for the early detection and identification of plastic contaminants in aquatic environments (16).

Nuclear Magnetic Resonance (NMR) Spectroscopy

Technical Overview: NMR spectroscopy can provide detailed structural information about the polymer backbone of microplastics.

Sample Handling:

Sample Preparation: Requires dissolution of microplastics in a deuterated solvent, which can be complex and time-consuming.

Sample Presentation:

Samples are placed in NMR tubes for analysis.

Applications:

NMR is less common for environmental samples but can be used for detailed characterization of specific types of plastics and additives.

NMR Research:

A 2024 published study explores the application of nuclear magnetic resonance (NMR) spectroscopy for the identification and quantification of microplastics, a crucial step for assessing environmental concentrations and associated risks (17). The investigation focused on dilution series of various polymers, including polystyrene (PS), polyisoprene-cis (PI), polybutadiene-cis (PB), polylactic acid (PLA), polyvinyl chloride (PVC), and polyurethane (PU) (17). Each polymer was dissolved in an appropriate solvent, and an internal standard was employed for quantification (17).

Detection and quantification limits for each polymer type were determined using two distinct methods: method 1—a proton signal-based equation utilizing the internal standard with a known concentration, and method 2—the limit of quantification (LOQ) based on the signal-to-noise ratio (SNR) (17). The results demonstrated that method 1, which incorporated the internal standard, yielded more accurate and lower concentration limits ranging from 0.2 to 8 µg/mL across all polymer types. In contrast, method 2 produced consistently higher concentration limits, between 1 and 10 µg/mL (17).

The findings affirm the accuracy, efficacy, and reliability of quantitative NMR spectroscopy for analyzing microplastics at these concentration levels, positioning it as a competitive alternative to established methods such as pyrolysis gas chromatography–mass spectrometry (Py-GC–MS), FT-IR, and Raman spectroscopy for polymer quantification (17).

A novel methodology for the recovery and quantification of low-molecular-weight polyethylene (PE) and polydimethylsiloxane (PDMS) in seawater, marking the first demonstration of their presence in this environment was published in 2024 (18). The methodology effectively addresses the challenges posed by synthetic polymer debris (SPD) with very low molecular weights and nano- to micro-meter sizes, which have eluded conventional analytical techniques (18).

The recovery process involved filtering 2 L of seawater through a nitrocellulose membrane filter with a 0.45 μm pore size, enabling efficient capture of the SPD. The filter was then dissolved in acetone, allowing for the isolation of particulates via centrifugation and subsequent drying. The recovered SPD were analyzed using proton nuclear magnetic resonance spectroscopy (¹H NMR), successfully identifying the presence of PE and PDMS (18).

The method provided high sensitivity and rapid results, with polymer concentrations quantified in mg/m³. In a practical application, 120 surface seawater samples were collected across two sampling campaigns in the Mediterranean Sea, specifically from the Gulf of Salerno to the Gulf of Policastro in Southern Italy. ¹H and ¹³C NMR analyses of the PE debris revealed oxidized polymer chains with low molecular weights (18).

Further investigation into the origins of these low molecular weight polymers was conducted by analyzing influents and effluents from a wastewater treatment plant (WWTP) in Salerno, identified as a hotspot for SPD release. The findings indicated the presence of low molecular weight polymers compatible with wax-PE, commonly used in coatings, food applications, cosmetics, and detergents. Additionally, the PDMS found in seawater was linked to silicone-based antifoam agents and emulsifiers (18).

Conclusion

The characterization and quantification of microplastics and nanoplastics in various environmental matrices requires a combination of analytical techniques. Each method has unique strengths, and the choice depends on the specific characteristics of the samples and the information needed. The integration of these techniques can provide comprehensive insights into the identity, quantity, and potential impacts of microplastics on the environment and human health.

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