Preparation of gold nanoparticle coated NaYF4:Yb,Er,Gd nanoparticles and their application for AFP detection in the red region

A novel surface plasmon resonance (SPR) enhanced upconversion (UC) luminescence system was designed for the detection of α-fetoprotein (AFP) by using gold nanoparticle (NP) coated NaYF4:Yb,Er,Gd upconversion nanoparticles (UCNPs) as probes. The gold NP coated NaYF4:Yb,Er,Gd UCNPs had a strong emission at 665 nm and were modified with poly(acrylic acid) (PAA) before being conjugated with anti-AFP (Ab1). When the AFP antigens were added to the system, the binding affinity between AFP and Ab1 resulted in a reduced distance between the gold NP coated UCNPs. The UC luminescence intensity of this system was enhanced because of the SPR properties of the gold NPs. The sensing system exhibited a high sensitivity, good specificity, and wide linear range, 0.098–15.0 ng mL−1 for the detection of AFP (R = 0.99), with a detection limit as low as 0.095 ng mL−1 under optimal conditions. The method was successfully applied to AFP sensing in human serum samples.