Detection of folate binding protein with enhanced sensitivity using a functionalized quartz crystal microbalance sensor

In this report, we describe the development of a quartz crystal microbalance biosensor for detection of folate binding protein (FBP). Using a simple folate-BSA conjugate adsorbed onto a Au-coated quartz sensor, a detection limit of 30 nM was achieved. Binding of FBP to the sensor surface could be blocked at concentrations as high as 1 μM 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. Moreover, capture could be achieved in the presence of blood serum, making the assay amenable to the analysis of bodily fluids. Further signal enhancement based on an anti-FBP antibody and protein-A-coated gold nanosphere sandwich assay extended the detection limit to 50 pM (∼3 orders-of-magnitude improvement). Given the overexpression of FBP in certain malignancies and inflammatory disorders, we expect the methodology described here to be useful to detect FBP as a possible biomarker for disease diagnosis.