Abstract
The integration of graphene materials into electrochemical biosensing platforms has gained significant interest in recent years. Bulk quantities of graphene can be synthesized by oxidation of graphite to graphite oxide and subsequent exfoliation to graphene oxide (GO). However, the size of the resultant GO sheets changes from the parent graphite yielding a polydispersed solution of sizes ranging from a few nanometers to tens of micrometers. Here, we investigate the direct effect of GO sheets sizes on biosensor performance. We separated different GO sheets sizes, and we characterized them via atomic force, scanning electron, Raman and X-ray photoelectron spectroscopies and solid state nuclear magnetic resonance (NMR). As proof of concept, the sensing performance of these GO samples was probed using a well-known ssDNA aptasensor against microcystin-LR toxin and an immunosensor against β-lactoglobulin. The resulting aptasensors and immunosensors are fabricated by using covalent attachment and physical adsorption. We found that the aptasensors fabricated using physical adsorption, the binding signal variation was dramatically increased with increasing the GO sheet size. In contrast, for the aptasensor fabricated using covalent immobilization, the binding signal variation decreased with increasing GO sheet size. However, for the β-lactoglobulin immunosensors, the optimum signals were observed at intermediate GO sheet size. GO sheet size could enhance or inhibit the sensitivity of the graphene-based electrochemical sensors. Our results demonstrate that controlling the size of GO sheets may have a profound impact in specific biosensing applications.
Highlights
The integration of graphene materials into electrochemical biosensing platforms has gained significant interest in recent years
The immunosensors fabricated on the larger sized graphene oxide (GO) sheets does not display satisfactory selectivity towards β-LG compared to the smaller sheets likely due to the non-specific adsorption of the protein on the large sheets
The smallest graphene oxide sheets showed higher defect density and degree of oxidation based on the Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results
Summary
Since the production of monodisperse GO remains a major challenge, a post-processing approach has been performed after GO synthesis for the flake’s separation. An increase of the signal suppression of the β-LG immunosensor will occur if the electron transfer was more retarded after analyte binding or if the fraction of bound antibody on the GO surface increased The opposite trend is observed for the Apt/Cov (Fig. 6B) where the smallest sized-GO sheets show the highest signals and the signal decreases with increasing the sheet size with almost comparable signals obtained for the larger sizes (> 2.5 μm) This signal enhancement in the Apt/Phys case at larger GO sheet size seems to be associated with the increased efficiency of the electron transfer on the larger sheets that showed less degree of oxidation as confirmed by the XPS and NMR results. Negative control and cross reactivity experiments were performed by incubating the aptasensors with MC-LA, OA in binding buffer and the immunosensors with OVA, BSA in PBS
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