Multiplex enumeration of Escherichia coli and Salmonella enteritidis in a passive capillary microfluidic chip
Multiplex detection and quantification of bacteria in water by using portable devices are particularly essential in low and middle-income countries where access to clean drinking water is limited. Addressing this crucial problem, we report a highly sensitive immunoassay sensor system utilizing fluorescence technique with magnetic nanoparticles (MNPs) to separate target bacteria and two different types of quantum dots (CdTe and Ni doped CdTe QDs) incorporated into a passive microfluidic chip to transport and to form sandwich complexes for the detection of two target bacteria, namely Escherichia coli (E. coli) and Salmonella enteritidis (S. enteritidis) in less than 60 min. The assay is carried out on a capillary driven microfluidic chip that can be operated by merely pipetting the samples and reagents, and fluorescent measurements are done by using a handheld fluorescent spectrophotometer, which renders the system portable. The linear range of the method was found as 101–105 cfu/mL for both E. coli and S. enteritidis. The limit of detection (LOD) was calculated as 5 and 3 cfu/mL for E. coli and S. enteritidis, respectively. The selectivity of the method was examined by testing Enterobacter dissolvens (E. dissolvens) and Staphylococcus aureus (S. aureus) samples, and no significant interference has been observed. The method was also demonstrated to detect bacteria in tap water and lake water samples spiked with target bacteria.