Detection of cancer markers via nanoparticle-based mass enhancement

We present a method for improving the sensitivity of label-free biosensors for detection of disease-specific proteins in low quantities. In this method, the signal generated by target binding is amplified by subsequent binding of antibodies and protein A-conjugated gold nanoparticles. We discuss the application of the method to detection of folate-binding protein (FBP), a potential cancer marker, using the quartz crystal microbalance (QCM) as a model platform. Direct capture of FBP using a folate-BSA conjugate adsorbed onto an Au coated quartz sensor revealed a detection limit of 30 nM. Binding of FBP to the sensor surface could be blocked at concentrations as high as 1μM with a 100-fold excess of folic acid, indicating the specificity of the folate-FBP interaction and the absence of nonspecific binding to the functionalized surface. Detection was also possible in the presence of blood serum making the assay amenable to the analysis of bodily fluids. Signal enhancement based on an anti-FBP antibody and protein A-coated gold nanosphere sandwich assay extended the detection limit to 50 pM (∼3 order-of-magnitude improvement). Since FBP is overexpressed in certain malignancies and inflammatory disorders such as rheumatoid arthritis, we expect the methodology described here to be useful to detect this potential disease marker.