Identification and visualisation of microplastics/ nanoplastics by Raman imaging (ii): Smaller than the diffraction limit of laser?

Design of a confocal micro-Raman spectroscopy system and research on microplastics detection

Traditional micro-Raman spectroscopy technology has the disadvantages of a weak signal and low signal-to-noise ratio. To fix these issues, a cost-effective and rigorous design method is proposed in this paper, whereby a confocal micro-Raman spectroscopy system is designed and built, and a low-cost reflector and high-pass filter are introduced into the Raman signal-receiving module. The Raman light incident is fully perpendicular to the coupling lens by adjusting the reflection angle of the mirror, making the focus of the coupling lens highly conjugate with the focus of the microscope objective, to enhance the intensity of the Raman signal and improve the signal-to-noise ratio. In order to better apply this technology to the detection and study of microplastics in offshore sediments, a reflective illumination light path is used to avoid the visual interference caused by the capillary structure and opacity of the glass cellulose filter membrane. The detection and analysis of the microplastics on the glass cellulose filter membrane have been carried out by the confocal micro-Raman system designed, which is low cost and capable of obtaining good detection results and meeting the requirements of microplastics detection. The system designed in this paper is expected to be applied to the research and development of Raman detection equipment for microplastics in marine sediments, which is beneficial to promote the development of marine microplastic monitoring technology in the world.

Separation and enrichment of nanoplastics in environmental water samples via ultracentrifugation

Nanoplastics are an emerging contaminant in aquatic environments. However, analytical methods for the separation, concentration, and identification of nanoplastics, which are essential to assess nanoplastic presence in the environment, are lacking. Here, we developed a new and easy-to-use method to separate and enrich nanoplastics in field water samples with ultracentrifugation. River water was spiked with polystyrene fragments (< 1000 nm) at an environmentally relevant concentration (10⁸-10⁹ particles/L). The polystyrene fragments were successfully separated and enriched by a factor of nearly 50 times with a high recovery rate (87.1%) after undergoing our process. Particles were then characterized using UV-vis spectroscopy, scanning electron microscopy (SEM), and enhanced darkfield microscopy with a hyperspectral imaging (HSI) spectrometer. These techniques are non-destructive and allow the assessment of plastic concentration, morphology, and polymer type. Our method can potentially be applied to other water samples to supply clean, enriched nanoplastic samples that can facilitate their identification in environmental samples.

Interaction of nanoplastics with extracellular polymeric substances (EPS) in the aquatic environment: A special reference to eco-corona formation and associated impacts

Nanoplastics (NPs) are plastic particles with sizes ranging between 1 and 1000 nm, exhibiting exceptional qualities such as large surface area, lightweight, durability; therefore, are widely used in cosmetics, paints, electronics, etc. NPs are inevitability released into the aquatic environment where they tend to interact with both, the extracellular polymeric substances (EPS) and other fractions of natural organic matter (NOM), respectively secreted by organisms (e.g., DNA, proteins, and carbohydrates) and degradation byproducts of organic materials (e.g., humic acid and fulvic acid) fluxed into the water bodies. These biomolecules robustly encapsulate NPs to develop an eco-corona layer that alters not only the physicochemical properties but also the fate, bioreactivity, and ecological impacts of NPs. Therefore, this review summarized the documented studies highlighting the eco-corona formation on NPs and associated ecological implications in the aquatic environment. After presenting the precise background information on the occurrence of NPs and EPS in the aquatic environment, we demonstrated the basic difference between eco-corona and bio-corona formation. The reviewed studies showed that the eco-corona formed on NPs have varying sizes and composition, mainly depending on the properties of parent biomolecules, characteristics of NPs, and physicochemical parameters of the aquatic environment. Further, the potential methods for characterization and quantification of eco-corona and its composition have been also highlighted. Moreover, the ecological implications (both toxic and non-toxic) of eco-corona formation on NPs in marine and freshwater environments have been also summarized. Last but not the least, challenges and future research directions are also given, e.g., conducting field studies on eco-corona formation in the aquatic environment, optimizing methods for its characterization and quantification, and considering eco-corona concept in the future toxicity studies on NPs. Finally, understanding eco-corona formation will be critical to unveil the complex NP interactions occurring in natural aquatic systems.

Analysis of microplastics in drinking water and other clean water samples with micro-Raman and micro-infrared spectroscopy: minimum requirements and best practice guidelines

Microplastics are a widespread contaminant found not only in various natural habitats but also in drinking waters. With spectroscopic methods, the polymer type, number, size, and size distribution as well as the shape of microplastic particles in waters can be determined, which is of great relevance to toxicological studies. Methods used in studies so far show a huge diversity regarding experimental setups and often a lack of certain quality assurance aspects. To overcome these problems, this critical review and consensus paper of 12 European analytical laboratories and institutions, dealing with microplastic particle identification and quantification with spectroscopic methods, gives guidance toward harmonized microplastic particle analysis in clean waters. The aims of this paper are to (i) improve the reliability of microplastic analysis, (ii) facilitate and improve the planning of sample preparation and microplastic detection, and (iii) provide a better understanding regarding the evaluation of already existing studies. With these aims, we hope to make an important step toward harmonization of microplastic particle analysis in clean water samples and, thus, allow the comparability of results obtained in different studies by using similar or harmonized methods. Clean water samples, for the purpose of this paper, are considered to comprise all water samples with low matrix content, in particular drinking, tap, and bottled water, but also other water types such as clean freshwater.

Which particles to select, and if yes, how many? : Subsampling methods for Raman microspectroscopic analysis of very small microplastic

Micro- and nanoplastic contamination is becoming a growing concern for environmental protection and food safety. Therefore, analytical techniques need to produce reliable quantification to ensure proper risk assessment. Raman microspectroscopy (RM) offers identification of single particles, but to ensure that the results are reliable, a certain number of particles has to be analyzed. For larger MP, all particles on the Raman filter can be detected, errors can be quantified, and the minimal sample size can be calculated easily by random sampling. In contrast, very small particles might not all be detected, demanding a window-based analysis of the filter. A bootstrap method is presented to provide an error quantification with confidence intervals from the available window data. In this context, different window selection schemes are evaluated and there is a clear recommendation to employ random (rather than systematically placed) window locations with many small rather than few larger windows. Ultimately, these results are united in a proposed RM measurement algorithm that computes confidence intervals on-the-fly during the analysis and, by checking whether given precision requirements are already met, automatically stops if an appropriate number of particles are identified, thus improving efficiency. To provide quality control in the MP quantification by Raman microspectroscopy, a window subsampling and bootstrap protocol is presented, which can provide confidence intervals that enable the assessment of the reliability of the data. This is brought together with a proposed on-the-fly algorithm that assesses the precision during the measurement and stops at the optimal point.

Protein Corona-Mediated Extraction for Quantitative Analysis of Nanoplastics in Environmental Waters by Pyrolysis Gas Chromatography/Mass Spectrometry

There is a growing concern about the effects of nanoplastics on biological safety and human health because of their global ubiquity in the environment. Methodologies for quantitative analysis of nanoplastics are important for the critical evaluation of their possible risks. Herein, a sensitive yet simple and environmentally friendly extraction approach mediated by protein corona is developed and coupled to pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) for nanoplastic determination in environmental waters. The developed methodology involved the formation of protein corona by addition of bovine serum albumin (BSA) to samples and protein precipitation via salting out. Then, the resulting extract was directly introduced to Py-GC/MS for nanoplastic mass quantification. Taking 50 nm polystyrene (PS) particles as a model, the highest extraction efficiency for nanoplastics was achieved under the extraction conditions of BSA concentration of 20 mg/L, equilibration time of 5 min, pH 3.0, 10% (w/v) NaCl, incubation temperature of 80 °C, and incubation period of 15 min. The extraction was confirmed to be mediated by the protein corona by transmission electron microscopy (TEM) analysis of the extracted nanoplastics. In total, 1.92 and 2.82 μg/L PS nanoplastics were detected in river water and the influent of wastewater treatment plant (WWTP), respectively. Furthermore, the feasibility of the present methodology was demonstrated by applying to extract PS and poly(methyl methacrylate) (PMMA) nanoplastics from real waters with recoveries of 72.1-98.9% at 14.2-50.4 μg/L spiked levels. Consequently, our method has provided new insights and possibilities for the investigation of nanoplastic pollution and its risk assessment in the environment.

Identification and visualisation of microplastics / nanoplastics by Raman imaging (iii): algorithm to cross-check multi-images

We recently developed the Raman mapping image to visualise and identify microplastics / nanoplastics (Fang et al. 2020, Sobhani et al. 2020). However, when the Raman signal is low and weak, the mapping uncertainty from the individual Raman peak intensity increases and may lead to images with false positive or negative features. For real samples, even the Raman signal is high, a low signal-noise ratio still occurs and leads to the mapping uncertainty due to the high spectrum background when: the target plastic is dispersed within another material with interfering Raman peaks; materials are present that exhibit broad Raman peaks; or, materials are present that fluoresce when exposed to the excitation laser. In this study, in order to increase the mapping certainty, we advance the algorithm to combine and merge multi-images that have been simultaneously mapped at the different characteristic peaks from the Raman spectra, akin imaging via different mapping channels simultaneously. These multi-images are merged into one image via algorithms, including colour off-setting to collect signal with a higher ratio of signal-noise, logic-OR to pick up more signal, logic-AND to eliminate noise, and logic-SUBTRACT to remove image background. Specifically, two or more Raman images can act as "parent images", to merge and generate a "daughter image" via a selected algorithm, to a "granddaughter image" via a further selected algorithm, and to an "offspring image" etc. More interestingly, to validate this algorithm approach, we analyse microplastics / nanoplastics that might be generated by a laser printer in our office or home. Depending on the toner and the printer, we might print and generate millions of microplastics and nanoplastics when we print a single A4 document.

Quantitative Analysis of Polystyrene and Poly(methyl methacrylate) Nanoplastics in Tissues of Aquatic Animals

Micro- and nanoplastics unavoidably enter into organisms and humans as a result of widespread exposures through drinking waters, foods, and even inhalation. However, owing to the limited availability of quantitative analytical methods, the effect of nanoplastics inside animal bodies is poorly understood. Herein, we report a sensitive and robust method to determine the chemical composition, mass concentration, and size distribution of nanoplastics in biological matrices. This breakthrough is based on a novel procedure including alkaline digestion and protein precipitation to extract nanoplastics from tissues of aquatic animals, followed by quantitative analysis with pyrolysis gas chromatography-mass spectrometry. The optimized procedure exhibited good reproducibility and high sensitivity with the respective detection limits of 0.03 μg/g for polystyrene (PS) nanoplastics and 0.09 μg/g poly(methyl methacrylate) (PMMA) nanoplastics. This method also preserved the original morphology and size of nanoplastics. Furthermore, to demonstrate the feasibility of the proposed method, 14 species of aquatic animals were collected, and PS nanoplastics in a concentration range of 0.093-0.785 μg/g were detected in three of these animals. Recovery rates of 73.0-89.1% were further obtained for PS and PMMA nanospheres when they were spiked into the tissues of Zebra snail and Corbicula fluminea at levels of 1.84-2.12 μg/g. Consequently, this method provides a powerful tool for tracking nanoplastics in animals.

Dark-field hyperspectral microscopy for label-free microplastics and nanoplastics detection and identification in vivo: A Caenorhabditis elegans study

Microplastics pollution is a serious ecological threat, severely affecting environments and human health. Tackling microplastics pollution requires an effective methodology to detect minute polymer particles in environmental samples and organisms. Here were report a novel methodology to visualise and identify nanoscale (down to 100 nm) and microscale synthetic commercially-available uniform spherical polymer particles using dark-field hyperspectral microscopy in visible-near infrared (400-1000 nm) wavelength range. Polystyrene particles with diameters between 100 nm-1 μm, polymethacrylate 1 μm and melamine formaldehyde 2 μm microspheres suspended in pure water samples were effectively imaged and chemically identified based on spectral signatures and image-assisted analysis. We succeeded in visualisation and spectral identification of pure and mixed nano- and microplastics in vivo employing optically-transparent Caenorhabditis elegans nematodes as a model to demonstrate the ingestion and tissue distribution of microplastics. As we demonstrate here, dark-field hyperspectral microscopy is capable for differentiating between chemically-different microplastics confined within live invertebrate intestines. Moreover, this optical technology allows for quantitative identification of microplastics ingested by nematodes. We believe that this label-free non-destructive methodology will find numerous applications in environmental nano- and microplastics detection and quantification, investigation of their biodistribution in tissues and organs and nanotoxicology.

Microplastics contamination in the soil from Urban Landfill site, Dhaka, Bangladesh

Microplastics (MP) pollution has become a matter of global concern because of its several deleterious effects on environmental health, especially on the terrestrial environment. The evidence of MP contamination in terrestrial environment is less explored compared to aquatic bodies. However, in Bangladesh despite having high possibility of MP contamination, there is lacking of available research-based evidence. Urban areas soil is subjected to act as a major environmental reservoir for MPs. Thus, this study was carried out to investigate the presence of MP contamination in constructed landfill sites near Dhaka city, Bangladesh. Ten unmixed soil samples were collected from the Aminbazar Sanitary landfill sites, from that thirty replicated samples were investigated via Fourier Transform Infrared Spectroscopy (FT-IR) analysis and Stereomicroscope. The range of physicochemical parameters were found in the soil samples as follows: moisture content; 15.84%-56.54%; soil pH; 5.76-6.02, electric conductivity; 0.1 μs/cm - 2.43 μs/cm, alkalinity; 6.7 ± 1.528-14.33 ± 0.577, TOC; 0.18% ± 0.02-1.09 ± 0.03. Among the ten samples, 3 samples were identified to have the presence of MP in the form of Low density polyethylene (LDPE), High density polyethylene (HDPE), and Cellulose acetate (CA) respectively. The detection limit ranged from 1 - 2000 μm. Hence, the results show that the procurement and discharge of MPs in the landfills is an overlong process. The results of this study provide an initial evidence and affirm that landfill can be a potential source of MPs. This study indicates that MPs are comparatively overlong outcome of human induced activities which can significantly cause changes in terrestrial ecosystems.