An "OFF-ON" quantum dot-graphene oxide bioprobe for sensitive detection of micrococcal nuclease of Staphylococcus aureus.

Micrococcal nuclease (MNase) is an extracellular endonuclease of Staphylococcus aureus (S. aureus). It digests single stranded nucleic acid. The presence of MNase is the gold standard to identify S. aureus and its content. The present study reports the ultrahigh sensitive and selective fluorescence platform for MNase detection, designed and developed based on the surface energy transfer mechanism. A "proof of concept" is being developed based on monoclonal antibody-conjugated quantum dots (mAb-QDs), wherein mAb-QDs act as donors and graphene oxide (GO) acts as an acceptor. mAb-QDs in close proximity to GO undergo adsorption due to weak affinity between them and this results in fluorescence quenching by the transfer of surface energy from mAb-QDs to GO. During sensing, a much stronger affinity of mAb-QDs towards MNase inhibits the energy transfer to GO and this allows the regaining of fluorescence. Immobilized mAb-QDs on nitrocellulose membrane strips were fabricated and tested for "ON-OFF-ON" sensing of MNase. The limit of detection for fluorescence based assay and strips is found to be 0.3 ng mL-1 and 0.5 ng mL-1, respectively. The developed strips were applied on real samples for the detection of S. aureus.

Surface modified glass substrate for sensing E. coli using highly stable and luminescent CdSe/CdS core shell quantum dots.

CdSe/CdS core shelled quantum dots (QDs) were prepared by colloidal synthesis using a binary ligand system and a non-coordinating, reusable solvent n-octadecane (nOD). Both the synthesis of CdSe and CdSe/CdS core shelled quantum dots were achieved by hot injection technique at much lower temperatures than reported earlier. The use of binary ligand facilitated enough nucleation and growth. Red shift in absorption spectra, an enhanced crystallite and particle size is evidenced by XRD and TEM respectively, confirming the formation of core shell structure of CdSe/CdS. The synthesized core shells exhibited high fluorescence intensity, long term stability and good mono dispersion, making it a potential material for bio-imaging and sensing. Core shell QDs were modified with mercapto propionic acid (MPA) to impart aqueous solubility. Studies on cytotoxicity of shelled QDs reveal good bio compatibility with a very minimum toxicity of IC50 = 20 µg/L. These QDs were used for sensing E. coli. Ordinary glass slide, modified using plasma etching is surface modified through APTES aiding conjugation of antibodies. Anti- E. coli polyclonal antibody on glass matrix (slide) and antibody conjugated QDs were used for detection of E. coli in a typical sandwich model. The excellent optical transparency of glass and high emission of QDs lead to detection of E.coli with a limit of detection of 50 CFU/mL.