29 August 2005: Increased sensitivity with nano detection of clinical markers
A biosensor that incorporates carbon nanotubes (CNTs) and metal oxide nanowires (NWs) can sensitively detect the main clinical marker for prostate cancer, report researchers in the US.
Raised levels of prostate-specific antigen (PSA) in the blood are indicative of prostate cancer, and doctors conduct PSA tests as a first stage in diagnosis. As part of an ongoing study into developing nanoscale biosensors, a team of researchers from the University of Southern California, Los Angeles, led by Chongwu Zhou, compared the PSA detection abilities of CNTs and metal oxide NWs.
The researchers connected individual CNTs and NWs up to electrodes and then synthesised succinimidyl molecules on their surfaces. These molecules readily bind with the amines that exist on most proteins and allowed Zhou and his team to attach anti-PSA antibodies to the CNTs and metal oxide NWs. The idea being that the capture of any passing PSAs by the antibodies would cause a detectable change in the electrical conductivity of the CNTs and NWs.
When the researchers exposed the two devices to PSAs, they detected a change in electrical conductivity, but in opposite directions. This can be explained by the fact that metal oxide NWs are n-type semiconductors (electrons are the major charge carriers), while CNTs are p-type semiconductors (positively-charged holes are the major carriers). The capture of PSAs by the antibodies causes electrons to be injected into the devices, increasing the conductance in metal oxide NWs but decreasing it in CNTs, where the electrons simply combine with the holes.
Comparing the responses of both devices allows PSA to be detected with great sensitivity. The researchers found they could confidently detect the antigen at levels as low as 5ng/mL. They are now working on adapting this biosensor technology to detect other medically-important proteins. 'We are looking into the detection of oxidised low-density lipoproteins, which are related to cardiovascular disease,' Zhou told Chemistry World. 'We are also studying the detection of certain hormones related to cancer research.'
Zhou and his team are also looking at extending this technology to develop a single biosensor to detect a variety of different biomarkers by monitoring patterns of electrical responses. Jon Evans
C Li et al, J. Am. Chem. Soc. (DOI: 10.1021/ja053761g)