# A Portable Optical Sensor for Microplastic Detection: Development and Calibration
## 1 Introduction  
Pollution caused by human activities has severely damaged ecosystems and public health. Microplastics (MPs) are among the most concerning pollutants, and their long-term effects are still uncertain. The term \"microplastics\" was first used in 2004 to describe plastic particles smaller than 5 mm. However, the scientific community has yet to agree on a clear definition. Understanding their long-term effects is becoming increasingly important [1]. Since 1950, global plastic production has skyrocketed, reaching a total of 8.3 billion metric tons [2]. Microplastics pollute the oceans, with industries releasing approximately 1.3 million metric tons of microplastics annually from sources such as tire dust, plastic pellets, textiles, and personal care products. Projections indicate that this figure will rise to 3 million metric tons by 2040. The fishing and tourism industries suffer massive economic losses, incurring costs of between USD 500 billion and USD 2.5 trillion annually due to this pollution [3]. Microplastics (MPs) disrupt aquatic ecosystems when crustaceans and mollus consume them, resulting in damage to their digestive, reproductive, and immune systems [4]. The impact of microplastics extends up the food chain, accumulating in key food sources, including seafood, milk, honey, and beverages. Ingesting these particles may cause toxic, allergenic, and inflammatory reactions, as well as intestinal blockages, reduced mucus secretion, intestinal barrier damage, gut microbiota imbalance, and altered metabolism [5]. Bioplastics, such as polylactic acid (PLA), are gaining popularity for their biodegradability and renewable origins. However, under specific conditions, they can fragment into microplastics, thereby exacerbating pollution. [PERSON] et al. analyze the environmental consequences of PLA production and call for vigilant monitoring of its degradation products [6]. [PERSON] et al. (2020) urge scientists to thoroughly evaluate the environmental footprint of PLA and its breakdown products [7]. Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy (RMS) are the dominant primary techniques for identifying microplastics. These methods determine the polymeric composition of microplastics in water-based environments, thereby enhancing our understanding of their prevalence and impact on aquatic ecosystems [8; 9; 10]. FTIR identifies polymer composition by studying how light interacts with matter, but RMS detects particles as small as 1 micron and provides detailed morphological and chemical analysis. RMS is particularly effective for compounds with aliphatic, aromatic, or carbon-carbon double bonds. To ensure accurate Raman spectra and avoid fluorescence interference, it is important to purify samples [11]. Although these methods prove effective, they require sample purification to avoid interference and are not always portable or cost-efficient for large-scale monitoring [12; 13; 14]. RMS is especially valuable for detecting MPs in diverse matrices, including human samples, as demonstrated by [PERSON] et al. [15], for the identification of microplastics in water bodies. They demonstrated its capability to detect particles as small as 1 \(\upmu\)m and provide detailed morphological and chemical analysis. Similarly, [PERSON] et al. [16] highlighted its effectiveness in detecting MPs in human tissue samples, highlighting its potential for studying the impact of microplastics on human health. Despite its high accuracy, Raman spectroscopy's limitations include its susceptibility to fluorescence interference and the need for extensive sample preparation. FTIR, on the other hand, has been widely used for polymer identification and analysis of contamination. For example, the work by [PERSON] et al. [17], which developed standardized FTIR methodologies to detect microplastics in drinking water, employed cellulose nitrate filters and Nile red staining to enhance accuracy. [PERSON] et al.