A silver nanoparticle-enhanced paper sensor for Escherichia coli detection in water quality monitoring

ABSTRACT Graphical abstract showing the detection workflow of the paper-based sensor. An enriched water sample is applied as a drop onto a silver nanoparticle-Kovac strip. Following application, a distinct pink-coloured ring develops at the sample application site, indicating a positive colour response. We present a paper-based, silver nanoparticle-enhanced colourimetric sensor for screening water samples for Escherichia coli based on the detection of indole production. The classical indole test was translated into a portable paper assay by impregnating cellulose filter paper with silver nanoparticles functionalised with Kovac's reagent. The silver nanoparticles exhibited a strong surface plasmon resonance around 420 nm and a hydrodynamic diameter of approximately 99 nm, as determined by dynamic light scattering. The silver nanoparticle–Kovac complex was immobilised on the paper matrix to form test strips. When tested with bacterial cultures, the strips developed an immediate, intense pink ring in the presence of indole-producing E. coli, whereas indole-negative Klebsiella pneumoniae showed no colour change. Image analysis showed stronger red signals with silver nanoparticle-enhanced strips than with Kovac's reagent alone. The signal remained visually distinguishable and quantifiable for at least two weeks of ambient storage. The sensor was further evaluated using environmental water samples following an enrichment step in peptone-based media. Serial dilution experiments showed that 30 CFU/mL was the lowest tested concentration producing a visually detectable response. The assay is low-cost and user-friendly, providing a simple approach to water quality screening for E. coli.