bacterial and magnetic nanoparticles (MNP) – polystyrene (PS) microsphere, the second

component is for the catalyst and AuNPs cross-linking mixing channel, and the third

component is for the detection chamber. The detection mechanism of the colorimetric

biosensor for microfluidic LOC was based on the aggregation of AuNPs, which were

then evaluated using a smartphone for imaging (Figure 5.2b). In the presence of E. coli

O157:H7, aggregation of AuNPs was avoided, and the solution color remained red.

WithoutE. coli, AuNPs aggregation occurred due to the MNP-bacteria-PS complexes not

forming thus causing H2O2 not to catalyze. The AuNPs solution changed color from red

to blue. The color changes from the reaction were evaluated using a Hue-Saturation-

Lightness–based imaging app using an Android smartphone imaging to quantify the

concentration of the target bacteria. The colorimetric biosensor for microfluidic LOC

showed the linear detection of 5.0 × 10¹ to 5.0 × 10⁸ cfu/mL, and lower LOD of 50 cfu/mL

for detection of E. coli O157:H7 in chicken samples.

Recently, the application of AuNPs and multi-walled carbon nanotubes (MWCNT) as a

label for E. coli O157:H7 detection was developed and evaluated using the smartphone-

based colorimetric device [8]. The colorimetric device was made of acrylic plates, a

light-emitting diode, and a mobile power box. With the presence of E. coli O157:H7, the

AuNP-Aptamer that binds with E. coli O157:H7 remained red because of no aggregation of

AuNPs. However, without E. coli O157:H7, the AuNP-Aptamer aggregated and appeared

in blue. The colorimetric aptasensor showed LOD of 430 cfu/mL and 524 cfu/mL of E. coli

O157:H7 when tested in pure culture and artificially contaminated milk, respectively.

Another important application of AuNPs is in the SPR biosensor application. AuNPs

act as signal amplification and provide a large biocompatible surface area for binding

with high-affinity biomolecules (such as an antibody, aptamer, and enzyme), which are

specific towards analyte molecules to be detected. The isotropic structure of AuNPs

FIGURE 5.2

(a) The colorimetric biosensor microfluidic LOC components, and (b) mechanism of the colorimetric biosensor

microfluidic LOC based on AuNPs aggregation and smartphone imaging for rapid detection of E. coli O157:H7.

Adapted with permission [ 7]. Copyright (2019) Elsevier.

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