We report complementary detection of prostate-specific antigen (PSA) using n-type In2O3 nanowires and p-type carbon nanotubes. Our innovation involves developing an approach to covalently attach antibodies to In2O3 NW surfaces via the onsite surface synthesis of phosphonic acid-succinylimide ester. Electronic measurements under dry conditions revealed complementary response for In2O3 NW and SWNT devices after the binding of PSA. Real-time detection in solution has also been demonstrated for PSA down to 5 ng/mL, a benchmark concentration significant for clinical diagnosis of prostate cancer, which is the most frequently diagnosed cancer.

A strategy to covalently attach biological molecules to the electrochemically active surface of indium oxide nanowire (In2O3 NW) mat devices is presented. A self-assembled monolayer (SAM) of 4-(1,4-dihydroxybenzene)butyl phosphonic acid (HQ-PA) was generated on an indium tin oxide (ITO)-coated glass and In2O3 NWs surface. The chemical steps required for surface derivatization were optimized on an ITO surface prior to modifying the In2O3 NWs. The hydroquinone group contained in the HQ-PA SAM was electrochemically oxidized to quinone (Q-PA) at +330 mV. The monolayer of Q-PA was allowed to react with a thiol-terminated DNA. The DNA was paired to its complementary strand tagged with a fluorescence dye. Attachment of DNA was verified using fluorescence microscopy. A device was subsequently prepared on a SiO2-supported mat of In2O3 NWs by depositing gold electrodes on the mat surface. The reaction strategy optimized on ITO was applied to this In2O3 NW-based device. Arrays of In2O3 NWs on a single substrate were electrochemically activated in a selective manner to Q-PA. Activated In2O3 NWs underwent reaction with HS-DNA and gave a positive fluorescence response after pairing with the dye-DNA. The unactivated In2O3 NWs gave no response, thus demonstrating selective functionalization of an In2O3 NW array. This can be considered a key step for the future fabrication of large-scale, inexpensive, nanoscale biosensors.

Antibody mimic proteins (AMPs) are poly-peptides that bind to their target analytes with high affinity and specificity, just like conventional antibodies, but are much smaller in size (2-5 nm, less than 10kDa). In this report, we describe the first application of AMP in the field of nanobiosensors. In 2 O 3 nanowire based biosensors have been configured with an AMP (Fibronectin, Fn) to detect nucleocapsid (N) protein, a biomarker for severe acute respiratory syndrome (SARS). Using these devices, N protein was detected at sub-nanomolar concentration in the presence of 44 µM bovine serum albumin as a background. Furthermore, the binding constant of the AMP to Fn was determined from the concentration dependence of the response of our biosensors.

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