A smartphone-based optical detection for rapid and reliable quantification of bacterial contamination on stainless-steel surfaces.

Surface bacterial monitoring is critical for verifying sanitation effectiveness and preventing cross-contamination in food processing and related industries. However, current methods have notable limitations: culture-based plate counts are time-consuming, and ATP swab tests lack microbial specificity due to interference from non-bacterial ATP. We developed a smartphone-based optical detection method for rapid quantification of bacteria swabbed from surfaces. The method employs 3-mercaptophenylboronic acid (3-MPBA)-coated gold chips to capture bacterial cells, which are visualized using a commercially available $30 smartphone microscope. Automated image processing and quantification are performed using a custom smartphone application based on ImageJ particle analysis. IMPORTANCE: Rapid assessment of surface bacterial contamination is essential for sanitation verification in food processing and environmental hygiene, yet existing tools remain limited by long turnaround times or readouts that cannot distinguish bacterial signals from non-bacterial residues. This study presents a smartphone-based, culture-independent method for quantifying surface-associated bacterial cells, providing a proof-of-concept optical approach for routine hygiene monitoring under defined conditions. The approach enables rapid visualization and enumeration of bacterial contamination while reducing interference from common non-microbial residues compared with ATP-based assays. By demonstrating consistent performance across multiple bacterial species, surface materials, and selected real-world environments, this work supports the utility of low-cost optical tools for surface hygiene monitoring. Overall, the method contributes an accessible framework for improving on-site assessment of bacterial surface contamination in applied microbiology settings.