{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T09:29:28Z","timestamp":1785403768057,"version":"3.56.0"},"reference-count":44,"publisher":"MDPI AG","issue":"14","license":[{"start":{"date-parts":[[2026,7,20]],"date-time":"2026-07-20T00:00:00Z","timestamp":1784505600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100000780","name":"European Union","doi-asserted-by":"publisher","award":["Contract No. PVU-16 dated 04.06.2024 IFTT\/BG-RRP-2.012-0003-C01"],"award-info":[{"award-number":["Contract No. PVU-16 dated 04.06.2024 IFTT\/BG-RRP-2.012-0003-C01"]}],"id":[{"id":"10.13039\/501100000780","id-type":"DOI","asserted-by":"publisher"}]},{"award":["Contract No. PVU-16 dated 04.06.2024 IFTT\/BG-RRP-2.012-0003-C01"],"award-info":[{"award-number":["Contract No. PVU-16 dated 04.06.2024 IFTT\/BG-RRP-2.012-0003-C01"]}],"id":[{"id":"https:\/\/ror.org\/019w4f821","id-type":"ROR","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Microplastic monitoring needs methods that operate directly in water with minimal sample handling. Conventional techniques such as infrared and Raman spectroscopy and pyrolysis\u2013GC\/MS provide polymer-specific information but require sample preparation and delayed laboratory analysis. We propose an optical sensor concept for real-time, in situ microplastic assessment, based on multispectral pulsed transmission in the visible range using synchronized laser-diode lines and the directly transmitted signal through an active sensor volume. After calibration on particle-free water, each particle event reduces to a water-normalized transmission whose deficit is set by geometrical beam\u2013particle overlap and the wavelength-dependent extinction efficiency. The weak polymer absorption is represented by the Urbach-tail formalism, the refractive-index-related redirection of light by a Fresnel-based, surface- and orientation-averaged probability of direct transmission, and particle size and shape are decoupled through an effective optical length. The coupled nonlinear system is solved for the bounds of the polymer absorption coefficient per candidate geometry. Because each polymer occupies a bounded region in multi-wavelength absorption space fixed by its band gap and structural state, the method can, in principle, separate structural modifications of identical composition, such as low- and high-density polyethylene. This is a sensor concept with a model-based proof of concept, not full environmental validation. Experimental verification on real reference particles is reported separately; the present article establishes the measurement model and inversion scheme that this verification builds on.<\/jats:p>","DOI":"10.3390\/s26144594","type":"journal-article","created":{"date-parts":[[2026,7,20]],"date-time":"2026-07-20T13:18:31Z","timestamp":1784553511000},"page":"4594","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["A Multispectral Pulsed-Transmission Laser-Diode Sensor Concept for Real-Time In Situ Assessment of Microplastics in Water"],"prefix":"10.3390","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0009-0003-2392-170X","authenticated-orcid":false,"given":"Georgi V.","family":"Vladimirov","sequence":"first","affiliation":[{"name":"Georgi Nadjakov Institute of Solid-State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ekaterina","family":"Iordanova","sequence":"additional","affiliation":[{"name":"Georgi Nadjakov Institute of Solid-State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Georgi","family":"Yankov","sequence":"additional","affiliation":[{"name":"Georgi Nadjakov Institute of Solid-State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Victoria","family":"Atanassova","sequence":"additional","affiliation":[{"name":"Georgi Nadjakov Institute of Solid-State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Dimitar","family":"Filipov","sequence":"additional","affiliation":[{"name":"AquaLID Ltd., 1784 Sofia, Bulgaria"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,7,20]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1596","DOI":"10.1016\/j.marpolbul.2011.05.030","article-title":"Microplastics in the marine environment","volume":"62","author":"Andrady","year":"2011","journal-title":"Mar. Pollut. Bull."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"838","DOI":"10.1126\/science.1094559","article-title":"Lost at sea: Where is all the plastic?","volume":"304","author":"Thompson","year":"2004","journal-title":"Science"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"eadl2746","DOI":"10.1126\/science.adl2746","article-title":"Twenty years of microplastic pollution research\u2014What have we learned?","volume":"386","author":"Thompson","year":"2024","journal-title":"Science"},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"Ghosh, S., Sinha, J.K., Ghosh, S., Vashisth, K., Han, S., and Bhaskar, R. (2023). Microplastics as an emerging threat to the global environment and human health. Sustainability, 15.","DOI":"10.3390\/su151410821"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"249","DOI":"10.1021\/envhealth.3c00052","article-title":"Potential health impact of microplastics: A review of environmental distribution, human exposure, and toxic effects","volume":"1","author":"Li","year":"2023","journal-title":"Environ. Heal."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"672","DOI":"10.1126\/science.abe5041","article-title":"Microplastics and human health","volume":"371","author":"Vethaak","year":"2021","journal-title":"Science"},{"key":"ref_7","unstructured":"Europe, P. (2023). Plastics\u2014The Fast Facts 2023, Plastics Europe AISBL. Available online: https:\/\/plasticseurope.org\/knowledge-hub\/plastics-the-fast-facts-2023\/."},{"key":"ref_8","unstructured":"OECD (2022). Global Plastics Outlook: Policy Scenarios to 2060, OECD Publishing."},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Przygoda-Ku\u015b, P., Kosiorowska, K.E., Urbanek, A.K., and Miro\u0144czuk, A.M. (2025). Current approaches to microplastics detection and plastic biodegradation. Molecules, 30.","DOI":"10.3390\/molecules30112462"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"3060","DOI":"10.1021\/es2031505","article-title":"Microplastics in the marine environment: A review of the methods used for identification and quantification","volume":"46","author":"Gutow","year":"2012","journal-title":"Environ. Sci. Technol."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"383","DOI":"10.1007\/s10311-022-01525-7","article-title":"Analytical methods for microplastics in the environment: A review","volume":"21","author":"Huang","year":"2023","journal-title":"Environ. Chem. Lett."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"113101","DOI":"10.1016\/j.marpolbul.2021.113101","article-title":"A comparison of spectroscopic analysis methods for microplastics: Manual, semi-automated, and automated Fourier transform infrared and Raman techniques","volume":"173","author":"Song","year":"2021","journal-title":"Mar. Pollut. Bull."},{"key":"ref_13","doi-asserted-by":"crossref","unstructured":"Mariano, S., Tacconi, S., Fidaleo, M., Rossi, M., and Dini, L. (2021). Micro and nanoplastics identification: Classic methods and innovative detection techniques. Front. Toxicol., 3.","DOI":"10.3389\/ftox.2021.636640"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1099","DOI":"10.1177\/0003702820945713","article-title":"Sampling and quality assurance and quality control: A guide for scientists investigating the occurrence of microplastics across matrices","volume":"74","author":"Brander","year":"2020","journal-title":"Appl. Spectrosc."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"e20119","DOI":"10.1016\/j.heliyon.2023.e20119","article-title":"It matters how we measure\u2014Quantification of microplastics in drinking water by \u03bcFTIR and \u03bcRaman","volume":"9","author":"Maurizi","year":"2023","journal-title":"Heliyon"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"17","DOI":"10.1186\/s43591-023-00065-3","article-title":"Microplastics analytics: Why we should not underestimate the importance of blank controls","volume":"3","author":"Noonan","year":"2023","journal-title":"Microplastics Nanoplastics"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"105771","DOI":"10.1016\/j.jwpe.2024.105771","article-title":"A critical analysis on the limits and possibilities of the \u03bc-Raman as a routine method for microplastics determination in drinking water","volume":"65","author":"Barchiesi","year":"2024","journal-title":"J. Water Process Eng."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"4","DOI":"10.1186\/s43591-024-00081-x","article-title":"Value for money: A cost-effectiveness analysis of microplastic analytics in seawater","volume":"4","author":"Meyers","year":"2024","journal-title":"Microplastics Nanoplastics"},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Campanale, C., Savino, I., Pojar, I., Massarelli, C., and Uricchio, V.F. (2020). A practical overview of methodologies for sampling and analysis of microplastics in riverine environments. Sustainability, 12.","DOI":"10.3390\/su12176755"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"5800","DOI":"10.1021\/acs.est.5b05416","article-title":"Wastewater treatment works (WWTW) as a source of microplastics in the aquatic environment","volume":"50","author":"Murphy","year":"2016","journal-title":"Environ. Sci. Technol."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"487","DOI":"10.1002\/lol2.10387","article-title":"Measurements of the inherent optical properties of aqueous suspensions of microplastics","volume":"9","author":"Koestner","year":"2024","journal-title":"Limnol. Oceanogr. Lett."},{"key":"ref_22","doi-asserted-by":"crossref","unstructured":"Asamoah, B.O., Uurasj\u00e4rvi, E., R\u00e4ty, J., Koistinen, A., Roussey, M., and Peiponen, K.-E. (2021). Towards the development of portable and in situ optical devices for detection of micro- and nanoplastics in water: A review on the current status. Polymers, 13.","DOI":"10.3390\/polym13050730"},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"173111","DOI":"10.1016\/j.scitotenv.2024.173111","article-title":"In-situ detection of microplastics in the aquatic environment: A systematic literature review","volume":"934","author":"Abimbola","year":"2024","journal-title":"Sci. Total Environ."},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Nicolai, E., Pizzoferrato, R., Li, Y., Frattegiani, S., Nucara, A., and Costa, G. (2022). A new optical method for quantitative detection of microplastics in water based on real-time fluorescence analysis. Water, 14.","DOI":"10.3390\/w14203235"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"161","DOI":"10.1016\/j.chemosphere.2019.05.114","article-title":"A prototype of a portable optical sensor for the detection of transparent and translucent microplastics in freshwater","volume":"231","author":"Asamoah","year":"2019","journal-title":"Chemosphere"},{"key":"ref_26","doi-asserted-by":"crossref","unstructured":"Campos-L\u00f3pez, M., Aguilar-Garay, R., Gonz\u00e1lez-Rodriguez, D.E., Mej\u00eda-Lopez, V.I., Gamboa-Lugo, M.M., Garibay-Febles, V., Reyes-Guzm\u00e1n, M.A., Gordillo-Sol, \u00c1., and Mendoza-P\u00e9rez, J.A. (2025). A portable optical sensor for microplastic detection: Development and calibration. Appl. Sci., 15.","DOI":"10.3390\/app15094757"},{"key":"ref_27","doi-asserted-by":"crossref","unstructured":"Gl\u00f6ckler, F., Foschum, F., and Kienle, A. (2023). Continuous sizing and identification of microplastics in water. Sensors, 23.","DOI":"10.3390\/s23020781"},{"key":"ref_28","doi-asserted-by":"crossref","unstructured":"Kumar, M., Khamis, K., Stevens, R., Hannah, D.M., and Bradley, C. (2024). In-situ optical water quality monitoring sensors\u2014Applications, challenges and future opportunities. Front. Water, 6.","DOI":"10.3389\/frwa.2024.1380133"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"012027","DOI":"10.1088\/1742-6596\/3194\/1\/012027","article-title":"Multiwavelength device based on metal vapor lasers for application in green technology","volume":"3194","author":"Kostadinov","year":"2026","journal-title":"J. Phys. Conf. Ser."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"410","DOI":"10.1016\/j.watres.2019.02.054","article-title":"Microplastics in freshwaters and drinking water: Critical review and assessment of data quality","volume":"155","author":"Koelmans","year":"2019","journal-title":"Water Res."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"8710","DOI":"10.1364\/AO.36.008710","article-title":"Absorption spectrum (380\u2013700 nm) of pure water. II. Integrating cavity measurements","volume":"36","author":"Pope","year":"1997","journal-title":"Appl. Opt."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"1324","DOI":"10.1103\/PhysRev.92.1324","article-title":"The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids","volume":"92","author":"Urbach","year":"1953","journal-title":"Phys. Rev."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"3988","DOI":"10.1038\/s41467-021-24202-9","article-title":"A universal Urbach rule for disordered organic semiconductors","volume":"12","author":"Kaiser","year":"2021","journal-title":"Nat. Commun."},{"key":"ref_34","doi-asserted-by":"crossref","unstructured":"Bohren, C.F., and Huffman, D.R. (1998). Absorption and Scattering of Light by Small Particles, Wiley-VCH.","DOI":"10.1002\/9783527618156"},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"230586","DOI":"10.1098\/rsos.230586","article-title":"Numerical analysis and experimental verification of optical scattering from microplastics","volume":"10","author":"Genc","year":"2023","journal-title":"R. Soc. Open Sci."},{"key":"ref_36","doi-asserted-by":"crossref","unstructured":"Jonasz, M., and Fournier, G.R. (2007). Light Scattering by Particles in Water: Theoretical and Experimental Foundations, Academic Press\/Elsevier.","DOI":"10.1016\/B978-012388751-1\/50004-1"},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"204","DOI":"10.1364\/OME.434715","article-title":"Determination of optical constants and scattering properties of transparent polymers for use in optoelectronics","volume":"12","author":"Bauer","year":"2022","journal-title":"Opt. Mater. Express"},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"2337","DOI":"10.1364\/AO.383831","article-title":"Complex refractive indices measurements of polymers in visible and near-infrared bands","volume":"59","author":"Zhang","year":"2020","journal-title":"Appl. Opt."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"2400162","DOI":"10.1002\/smtd.202400162","article-title":"A full-scale analysis of absorption edges in dye-doped nonlinear optical polymers","volume":"8","author":"Chen","year":"2024","journal-title":"Small Methods"},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"225","DOI":"10.1134\/1.1604429","article-title":"Optical characterization of weakly absorbing PP, PE, and PP\/PE films","volume":"95","author":"Mamedov","year":"2003","journal-title":"Opt. Spectrosc."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"6128","DOI":"10.1038\/s41598-017-06357-y","article-title":"Electronic structure of polyethylene: Role of chemical, morphological and interfacial complexity","volume":"7","author":"Chen","year":"2017","journal-title":"Sci. Rep."},{"key":"ref_42","doi-asserted-by":"crossref","unstructured":"Donges, A., and Noll, R. (2015). Laser Measurement Technology: Fundamentals and Applications. Springer Series in Optical Sciences, Springer.","DOI":"10.1007\/978-3-662-43634-9"},{"key":"ref_43","unstructured":"Filipov, D.B., and Popov, P.G. (2025). Method and Device for Evaluating the Concentration, Material and Size of Microplastics in Aqueous Medium. (European Patent EP4196764)."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"100478","DOI":"10.1016\/j.emcon.2025.100478","article-title":"Atomic force microscopy (AFM) nanomechanical characterization of micro- and nanoplastics to support environmental investigations in groundwater","volume":"11","author":"Galluzzi","year":"2025","journal-title":"Emerg. Contam."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/26\/14\/4594\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,7,23]],"date-time":"2026-07-23T04:13:45Z","timestamp":1784780025000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/26\/14\/4594"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,7,20]]},"references-count":44,"journal-issue":{"issue":"14","published-online":{"date-parts":[[2026,7]]}},"alternative-id":["s26144594"],"URL":"https:\/\/doi.org\/10.3390\/s26144594","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,7,20]]}}}