  We developed a novel detection method for osteopontin (OPN), a new biomarker
for prostate cancer, by attaching a genetically engineered single chain
variable fragment (scFv) protein with high binding affinity for OPN to a carbon
nanotube field-effect transistor (NTFET). Chemical functionalization using
diazonium salts is used to covalently attach scFv to NT-FETs, as confirmed by
atomic force microscopy, while preserving the activity of the biological
binding site for OPN. Electron transport measurements indicate that
functionalized NT-FET may be used to detect the binding of OPN to the
complementary scFv protein. A concentration-dependent increase in the
source-drain current is observed in the regime of clinical significance, with a
detection limit of approximately 30 fM. The scFv-NT hybrid devices exhibit
selectivity for OPN over other control proteins. These devices respond to the
presence of OPN in a background of concentrated bovine serum albumin, without
loss of signal. Based on these observations, the detection mechanism is
attributed to changes in scattering at scFv protein-occupied defect sites on
the carbon nanotube sidewall. The functionalization procedure described here is
expected to be generalizable to any antibody containing an accessible amine
group, and to result in biosensors appropriate for detection of corresponding
complementary proteins at fM concentrations.
