Plasmonic ELISA for the ultrasensitive detection of Treponema pallidum

In this report, we have developed a plasmonic ELISA strategy for the detection of syphilis. Plasmonic ELISA is an enzyme-linked immunoassay combined with enzyme-mediated surface plasmon resonance (SPR) of gold nanoparticles (AuNPs). Immune response of the Treponema pallidum (T. pallidum) antibodies triggers the acetylcholinesterase-catalyzed hydrolysis of acetylthiocholine to produce abundant thiocholine. The positive charged thiol, in turn, alters the surface charge distribution the AuNPs and leads to the agglomeration of the AuNPs. The induced strong localized SPR effect of the agglomerate AuNPs can, thus, allow the quantitative assay of T. pallidum antibodies due to the remarkable color and absorption spectral response changes of the reaction system. The plasmonic ELISA exhibited a quasilinear response to the logarithmic T. pallidum antibody concentrations in the range of 1 pg/mL-10 ng/mL with a detection limit of 0.98 pg/mL. Such a low detection limit was 1000-fold improvements in sensitivity over a conventional ELISA. The results of plasmonic ELISA in syphilis assays of serum specimens from 60 patients agreed with those obtained using a conventional ELISA method. The plasmonic ELISA has characteristics (analyte specific, cost-effective, ease of automatic, low limit of detection) that provide potential for diagnosis and therapeutic monitoring of syphilis.