Colorimetric biosensing of nopaline synthase terminator using Fe3O4@Au and hemin-functionalized reduced graphene oxide

In this paper, we present a simple and label-free colorimetric biosensor for detection of the nopaline synthase (NOS) terminator in genetically modified (GM) plants. The “signal on” colorimetric biosensor was developed using a nanocomposite consisted of gold nanoparticles doped magnetic Fe 3 O 4 nanoparticles (Fe 3 O 4 @Au NP), capture probe DNA (cDNA), and hemin-functionalized reduced graphene oxide nanosheets (H-GN). The nanocomposite was successfully prepared by means of Au–S bonds and the strong π interactions between cDNA and H-GN. The sensing approach is based on the excellent peroxidase-mimicking activity of H-GN and its different electrostatic interactions with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). In presence of the target NOS, the cDNA in the nanocomposite will hybridize with its complementary sequence, and form dsDNA structure. Due to the weak π interactions between dsDNA and H-GN, a portion of H-GN will be released from the surface of Fe 3 O 4 @Au NPs and transferred into solution. After magnetic separation was performed, the supernatant was incubated with 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H 2 O 2 . The released H-GN can catalyze the oxidation reaction of TMB and turn the colorless solution blue. This “signal-on” colorimetric biosensor shows a broad linear range of 0.5–100 nM for the target NOS, with a 0.19 nM detection limit. The application of the biosensor for determination of NOS segments in samples of GM and non-GM tomatoes shows that it can discriminate between GM and non-GM plants. The reliability of the method for samples of NOS-spiked GM tomato suggests satisfactory recoveries in the range of 93.6%–94.2%.