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

² = 0.9947) between surface bacterial load and particle counts over 10⁴-10⁷ cells per 100 cm². The detection limit was 1,978 cells/100 cm², providing approximately twofold greater sensitivity than ATP. Comparable detection performance was observed across multiple bacterial species and strains and across different surface materials, including stainless steel and high-density polyethylene. The method maintained performance in the presence of some common food residues (1% vol/vol or wt/vol), while ATP readings were significantly affected. The 3-MPBA-coated gold chips remained stable for 6 months at room temperature, and the method was validated on a public water fountain surface. Overall, this smartphone-based method provides a rapid, cost-effective optical alternative to conventional ATP-based surface hygiene monitoring tools. 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.