Rapid and ultrasensitive Salmonella Typhimurium quantification using positive dielectrophoresis driven on-line enrichment and fluorescent nanoparticleslabel.

This paper describes a convenient and sensitive methodology for Salmonella Typhimurium (S. Typhimurium) detection in microfluidic channels using positive dielectrophoresis (pDEP) driven on-line enrichment and fluorescent nanoparticles label. In the protocol, S. Typhimurium was first incubated with the anti-S. Typhimurium antibody conjugated RuBpy-doped fluorescent nanoparticles (Ab(S. Typhimurium)-FNPs). After incubation, the reacted mixture of S. Typhimurium and Ab(S. Typhimurium)-FNPs bioconjugates was directly introduced into the pDEP-based microfluidic system. On applying an AC voltage to the electrodes in a pDEP frequency region, the FNPs labeled S. Typhimurium cells moved to the electrodes and accumulated in the electrode gap, while the free Ab(S. Typhimurium)-FNPs were flowed away. The enriched FNPs labeled S. Typhimurium were detected by fluorescence microscopy at the focused detection areas. This new method constructed a simultaneous on-line enrichment and detection platform, eliminated the separation and washing-out steps usually required for FNPs label involved bioassay. Utilizing signal amplification effects produced by pDEP on-line enrichment and FNPs label, the detection limit for S. Typhimurium in deionized water is 56 colony forming units per milliliter (cfu/mL). We applied the established approach to detecting artificially contaminated bottled mineral water samples; S. Typhimurium of 110 cfu/mL was accurately detected. It is believed that the proposed method will find wide applications in biomedicine fields demanding higher sensitive bacterial identification.