Ultrasensitive immunoassay of Staphylococcus aureus based on colorimetric and fluorescent responses of 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole to L-cysteine

We report here a sensitive method for immunoassay of Staphylococcus aureus (S. aureus) based on colorimetric and fluorescent responses of 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) to L-cysteine (Cys). S. aureus cells were separated from samples by immunomagnetic nanoparticles and labeled with Cys-encapsulated liposome (Cys@liposome) immuno-nanocapsules. After mangnetic separation, the Cys@liposome immuno-nanocapsules linked to bacterial cells were destructed by Tween-20, and then the released Cys molecules reacted with NBD-Cl to generate colorimetric and fluorescent signals. Due to the amplification effect of liposome nanocapsules, the high capture efficiency of immunomagnetic nanoparticles, and the high responses of NBD-Cl to Cys, the detection limits for colorimetric and fluorescent detection of S. aureus were as low as 10 and 1 CFU mL(-)(1), respectively. The proposed method was successfully used to detect S. aureus in milk samples. This work has provided a novel NBD-Cl based colorimetric and fluorescent platform for immunoassay.