Fluorescence Correlation Spectroscopy (FCS) Unlocks the Presence of Microcystin-LR in Water

Water is essential for human life, and access to clean water is considered a basic human right by the United Nations. Around the world, a high proportion of the population still does not have access to safe fresh water, with high impact on health. This situation perpetuates a cycle of poverty, hindering economic development and exacerbating inequality. Water is considered unsafe to drink if it is contaminated. The contamination can be categorized into three types: physical, chemical, and biological. Biological contamination arises from the presence in water of living organisms such as bacteria, viruses, algae, fungi, and parasites. Recently, the scientific community has raised the alarm on contamination caused by a large group of bacteria known as <i>Cyanobacteria</i>, which can release harmful toxins into water, including cyanotoxins like microcystin-LR (MC-LR). Currently, the standard analytical procedure for the detection and quantification of MC-LR relies on chromatography-based techniques (HPLC, LC/MS, GC/MS), immunological assays (ELISA), or protein phosphatase inhibition assays (PPIAs). In this study, we used fluorescence correlation spectroscopy (FCS) methodology to develop a competitive assay for detection of traces of the MC-LR toxin in a water solution. A conjugated form of bovine serum albumin (BSA) with MC-LR was labeled with the fluorescence dye CF488 (MC-LR BSA 488). The diffusion coefficient values of the MC-LR BSA 488 complex were investigated in the absence and in the presence of MC-LR. The change in the value of the diffusion coefficient was correlated with the concentration value of MC-LR in solution. The results showed a limit of detection (LoD) of the assay of 0.18 nM (0.18 µg/L), a value lower than the limit value (1.0 μg/L) established by the World Health Organization (WHO).