A ‘one–tube’ synthesis of a selective fluorescence ‘turn off/on’ DNA probe based on a C-phycocyanin-graphene oxide (CPC-GO) bio composite

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A naturally fluorescent protein, C-phycocyanin (CPC), was used as a fluorophore to study the effect of graphene oxide (GO) as a quencher. The protein was purified using established procedures and titrated with increasing GO concentrations. UV–visible titration showed a minor effect on the phycocyanobilin absorbance but significant interactions with the amino acid backbone. Fluorescence titration showed notable CPC quenching upon increasing GO concentration to 30 μg ml−1; the corresponding fluorescence dropped by ~97%. A non–linear Stern Volmer curve showed that the fluorophores did not interact directly with the quencher. Powder X-ray diffraction studies showed that the bio-composite lost the crystalline arrangement of GO and became amorphous, akin to CPC. SEM analysis showed GO sheets enfolding a protein nucleus with an increase in oxygen after the interaction compared to CPC. A 20 min incubation of the bio-composite with various biomolecules including amino acids, sugars, polydispersed exopolysaccharides (EPS), other proteins and DNA showed that only DNA could recover the CPC fluorescence. The ‘turn on’ effect of DNA was distinguishable even when all the other molecules were in the same sample matrix. These results showed that CPC GO could be a fluorescence ‘turn off/on’ DNA probe.