Improve the Plasmonic Spectral Detection of Alpha-Fetoprotein: the Effect of Branch Length on the Coagulation of Gold Nanostars

A sensitive spectral sensing method for detection of alpha-fetoprotein (AFP) was proposed based on the intense aggregation of colloidal gold nanostars. It has been found that the intensity of the infrared plasmonic absorption peak of the gold nanostar colloid is very sensitive to the addition of AFP. Because of the electrostatic attraction-induced particle aggregation and coagulation, the plasmonic absorbance corresponding to the suspending nanostars fades down rapidly as the AFP concentration is increased. This spectral sensitivity based on the particle aggregation could be effectively improved by increasing the branch length of the gold nanostars, and the limit of detection reaches 2 pg/mL. The corresponding physical mechanism has been illuminated by the tip effect-enhanced surface charge concentration and electrostatic attraction.