Cost-Effective Spectrophotometer for Real-Time Monitoring of Microbial Growth, Analysis of Soil Nutrients, and Detection of Microplastics

Presently employed high-end instruments designated for spectroscopic applications are quite precise but are encumbered by their exorbitant cost and confined operability solely within sophisticated laboratory settings. In this study, we present the design and development of a cost-effective and reliable visible range spectrophotometer utilising commonplace materials and simplistic construction techniques. The proposed system is evaluated for colorimetric assessment of bacterial contamination in fluid samples, quantification of soil nutrients, and detection of microplastics. The approach pivots on monitoring variations in the color profile and relative spectral absorbance of the samples for all three test cases. The system was calibrated using the samples of known safranine (safranin-O) concentrations, the absorbance of which was measured utilizing a commercial-grade UV-visible spectrophotometer. The level of accuracy of the proposed system was comparable to that of high-end commercial-grade instruments within the visible spectrum range. After proper calibration and validation of the system through safranine samples, the system was tested for real-time monitoring of microbial growth in the broth media, quantification of NPK constituents in soil samples collected from varied locations, and detection of microplastics in water samples. In the context of microbial growth monitoring, the controlled experiment featured herein incorporated chromogenic media that functions dually as an indicator and selective growth medium for E. coli cells. We used generic colour-altering chemical reagents for the assessment of soil NPK and further discussed a proof-of-concept on how the Tauc-plot extrapolation method applied to absorption spectra can be used for microplastic detection in water samples. Beyond its cost-effectiveness, the proposed system is highly portable and can be customized for specific spectroscopic applications, making it a promising solution for reliable on-site testing in resource-limited settings.