Tight-Anchoring of Gold Nanoparticles to Polymer-Wrapped Semiconducting Single-Walled Carbon Nanotubes for Biosensor Applications

Conjugated polymer-sorted semiconducting single-walled carbon nanotubes (sc-SWCNTs) are excellent materials for electronic biosensors due to their high electrical conductivity, specific surface area and sensitivity, and can be integrated into electrolyte-gated field-effect transistor (EGFET) sensors with very high semiconducting purities. Operation of electronic biosensors in bodily fluids is challenging because of the short Debye length (λD) in high ionic strength media. The receptor unit attached to the sc-SWCNT is a crucial component of the biosensor, and the sensor performance depends on efficient signal transduction between the receptor and the sc-SWCNT. In this work, we grew Au nanoparticles (NPs) on sc-SWCNTs sorted using a copolymer of fluorene and 2,2’-bipyridine (BPy), poly(9,9-di-n-dodecylfluorenyl-2,7-diyl-alt-2,2′-bipyridine-5,5′) (PFBPy-5,5’). Au NPs were anchored to the BPy ligands in the polymer backbone, facilitating efficient electrical connection between the Au NPs and the sc-SWCNTs. Label-free cortisol sensors were prepared by coupling a thiol-terminated cortisol aptamer to the Au NPs and integrating the Aptamer/AuNP/sc-SWCNT@PFBPy-5,5’ complex into an EGFET. The sensor exhibited a concentration-dependent increase in source-drain current upon increasing cortisol concentration from 1 – 1000 nM. A control sensor was prepared using sc-SWCNTs sorted with a fluorene homopolymer, poly(9,9-dodecylfluorene) (PFDD), which does not have a ligand to which Au NPs can anchor. The control Aptamer/AuNP/sc-SWCNT@PFDD cortisol sensor only showed a weak concentration-dependent response to cortisol. A further control was prepared using sc-SWCNTs@PFDD to which the aptamer was covalently attached via defect carboxylic acids on the nanotube’s sidewall, without Au NPs. The Covalent-Aptamer/sc-SWCNT@PFDD control sensor did exhibit a concentration-dependent response to cortisol, albeit to a lesser extent than the Aptamer/AuNP/sc-SWCNT@PFBPy-5,5’ system. The results indicate that the tight-anchoring of the Au NPs in the Aptamer/AuNP/sc-SWCNT@PFBPy-5,5’ system provides a crucial contribution to the proposed sensing mechanism: (1) upon cortisol recognition the aptamer undergoes a structure-switch where the negatively charged backbone is brought within or near the Debye length at the Au NP surface, altering the interfacial capacitance and electrostatically gating the Au NP, and (2) efficient signal transfer between the Au NPs and sc-SWCNTs@PFBPy-5,5’.