This study presents a rapid microfluidic paper-based
analytical
device (μPAD) capable of simultaneously monitoring Gram-negative
bacteria and nitrite ions (NO2
–) for
water quality monitoring. We utilize gold nanoparticles (AuNPs) functionalized
with polymyxin molecules (AuNPs@polymyxin) to cause color change due
to aggregation for the detection of Gram-negative bacteria, and antiaggregation
in the presence of o-phenylenediamine (OPD) for NO2
– detection. In this study, Escherichia coli (E. coli) serves as the model of a Gram-negative bacterium. Using the developed
μPADs, the color changes resulting from aggregation and antiaggregation
reactions are measured using a smartphone application. The linear
detection ranges from 5.0 × 102 to 5.0 × 105 CFU/mL (R
2 = 0.9961) for E. coli and 0.20 to 2.0 μmol/L (R
2 = 0.995) for NO2
–. The
detection limits were determined as 2.0 × 102 CFU/mL
for E. coli and 0.18 μmol/L for
NO2
–. Notably, the newly developed assay
exhibited high selectivity with no interference from Gram-positive
bacteria. Additionally, we obtained acceptable recovery for monitoring E. coli and NO2
– in
drinking water samples with no significant difference between our
method and a commercial assay by t test validation.
The sensor was also employed for assessing the quality of the pond
and environmental water source. Notably, this approach can also be
applied to human urine samples with satisfactory accuracy. Furthermore,
the assay’s stability is extended due to its reliance on AuNPs
rather than reagents like antibodies and enzymes, reducing costs and
ensuring long-term viability. Our cost-effective μPADs therefore
provide a real-time analysis of both contaminants, making them suitable
for assessing water quality in resource-limited settings.