Portable photonic immunosensor for the simultaneous detection of Escherichia coli and Salmonella typhimurium in milk samples

Abstract

Milk can be contaminated with pathogenic bacteria such as Salmonella typhimurium and Escherichia coli, which can cause acute foodborne illnesses. In this study, we present an optical immunosensor designed for the simultaneous detection of these two bacteria in milk samples. The sensor is based on a silicon chip that incorporates two U-shaped silicon nitride waveguides configured as Mach-Zehnder Interferometers (MZIs). The sensing windows of the MZIs, located at one end of the chip, differ in length and allow for sequential immersion into reagent solutions during assay procedures. At the opposite end, the chip is optically coupled to a broadband white LED and a spectrophotometer via a bifurcated optical fiber and a dedicated coupler. To enable selective detection, the sensing windows of the MZIs are functionalized with lipopolysaccharides from the outer membranes of S. typhimurium and E. coli, respectively. A competitive immunoassay, completed in just 15 min, enabled detection limits of 45 cfu/mL for S. typhimurium and 125 cfu/mL for E. coli in milk. With a pre-enrichment step of approximately 8 h, single-cell detection became possible for both bacteria. The sensor excellent performance, quantitative determinations capability, and compact design, achieved by eliminating the need for microfluidics or pumps, make it a powerful and practical tool for bacterial detection in milk.