Protein immunosensor using single-wall carbon nanotube forests with electrochemical detection of enzyme labels

Abstract

Vertically aligned arrays of single-wall carbon nanotubes (SWNT forests) on pyrolytic graphite surfaces were developed for amperometric enzyme-linked immunoassays. Improved fabrication of these SWNT forests utilizing aged nanotube dispersions provided higher nanotube density and conductivity. Biosensor performance enhancement was monitored using nanotube-bound peroxidase enzymes showing a 3.5-fold better sensitivity for H₂O₂ than when using fresh nanotubes to assemble the forests, and improved detection limits. Absence of improvements by electron mediation for detection of H₂O₂ suggested very efficient electron exchange between nanotubes and enzymes attached to their ends. Protein immunosensors were made by attaching antibodies to the carboxylated ends of nanotube forests. Utilizing casein/detergent blocking to minimize non-specific binding, a detection limit of 75 pmol mL⁻¹ (75 nM) was achieved for human serum albumin (HSA) in unmediated sandwich immunosensors using horseradish peroxidase (HRP) labels. Mediation of the immunosensors dramatically lowered the detection limit to 1 pmol mL⁻¹ (1 nM), providing significantly better performance than alternative methods. In the immunosensor case, the average distance between HRP labels and nanotube ends is presumably too large for efficient direct electron exchange, but this situation can be overcome by electron mediation.