We report here a surface plasmon resonance (SPR) method for detection of cell membrane binding proteins with high degree signal amplification carried out in an all-aqueous condition. Ultrahigh detection sensitivity was achieved for a membrane-based biosensing interface through the use of functional gold nanoparticles (AuNP) in combination with in situ atom transfer radical polymerization (ATRP) reaction. Fusion of phosphatidylcholine vesicles on a calcinated SPR gold chip established a supported bilayer membrane in which cell receptor monosialoganglioside GM1 was embedded for capture of bacterial cholera toxin (CT). The surface-bound CT was recognized with biotinylated anti-CT, which was linked to the biotin-AuNP through an avidin bridge. The biotin-AuNP surface was functionalized with ATRP initiator that triggers localized growth of poly(hydroxyl-ethyl methacrylate) (PHEMA) brush, contributing to marked SPR signal enhancement and quantitative measurement of CT at very low concentrations. The resulting polymer film has been characterized by optical and atomic force microscopy. A calibration curve for CT detection has been obtained displaying a response range from 6.3 × 10–16 to 6.3 × 10–8 M with a detection limit of 160 aM (equivalent to ∼9500 molecules in 100 μL sample solution). Sensitive detection of biomolecules in complex medium has been conducted with CT-spiked serum, and the detection limit can be effectively improved by 6 orders of magnitude compared to direct measurement in serum. The combined AuNP/ATRP method reported here opens new avenues for ultrasensitive detection of proteins on delicate sensor interfaces constructed by lipid membranes or cell membrane mimics.