One-step and label-free detection of alpha-fetoprotein based on aggregation of gold nanorods

In this study, a novel one-step label-free immunoassay using gold nanorods (GNRs) for sensitive detection of alpha-fetoprotein (AFP), one of the major markers for hepatocellular tumors, was successfully developed. GNRs were fabricated through seed-mediated growth procedure, and their surface was activated by complete replacement with alkanethiols for the attachment of antibodies to obtain GNR-antibody conjugates with special space conformation. Dramatic aggregation of GNRs was induced by the interaction between target proteins and capture antibodies, and confirmed by transmission electron microscopy (TEM) and Ultraviolet–visible (UV–vis) analysis. Associated wavelength red-shifts and intensity decreases in the absorption peak of GNRs were observed with increasing concentrations of target AFP. Therein the absorption intensity as sensing response showed excellent linearity with the concentration of AFP in logarithmic scale. It was illustrated that this method can be used reliably to detect AFP protein in phosphate-buffered saline (PBS) buffer at concentrations of picomolar level with a wide quantification range from 0.25 to 4.0 nM. The complication of interfering proteins in the sample showed negligible effects on the detection results. Furthermore, the simple procedure, cost-effective and label-free features of this method offer advantages over common immunoassays. It is also expected that this new method would be applicable for detection of other tumor markers at low concentrations.