A simple and rapid colorimetric detection of Staphylococcus aureus relied on the distance-dependent optical properties of silver nanoparticles.

The quick and accurate diagnosis of pathogens has appeared as a pressing issue in clinical diagnostics, environmental monitoring, and food safety. The available assays are suffering from limited capacities in simple, fast, low-cost, and on-site detection to increase prevention and proper treatment. Herein, we address these challenges by developing a simple, speedy, affordable, and ultrasensitive nanoplasmonic biosensor for colorimetric detection of cDNA from staphylococcal RNA relying on the distance-dependent optical features of silver nanostructures for the measurement of color variations and spectral shifts owing to the plasmon coupling generated by the cross-linking accumulation of AgNPs. The method described utilizes silver nanoparticles (AgNPs) immobilized with two different single-stranded oligonucleotides (ssDNA1 and ssDNA2 ) that specifically recognize the target DNA. Sandwich hybridization of target DNA with ssDNA1 and ssDNA2 induced color variations and spectral shifts of AgNPs, whereas test samples without the target DNA remained yellow as the initial color of colloidal silver. The designed nanoplasmonic biosensor demonstrated high specificity with the detection limit (LOD) of ∼1.8 amol target DNA (∼106 molecules per test) in the broad linear dynamic range from 0.01 to 100 nM, and LOD down to a few cells was attained for amplified bacterial nucleic acids and a linear range from 102 CFU mL-1 to 107 CFU mL-1 . The sensing approach showed great potential for the timely diagnosis of pathogens in low-density samples, and it has considerable merits over traditional culture approaches and qPCR techniques.