A label-free AuNPs-based ultrasensitive plasmonic ELISA by alkaline phosphatase and ferrous ion-initiated cascade amplification

Conventional ELISA methods have been widely used for disease diagnosis, yet the sensitivity and accuracy are still to be improved. Based on our label-free AuNPs-mediated plasmonic sensor, we developed an ultrasensitive plasmonic ELISA by integrating the plasmonic sensor and cascade amplification. In principle, alkaline phosphatase (ALP)-conjugated antibodies captured targets, and the ALP hydrolyzed 2-phospho-ascorbic acid (AA2P) into ascorbic acid (AA). Then, the released AA changed Fe3+ into Fe2+, which further triggered Fe2+-catalyzed H2O2-preventing aggregation of AuNPs by oxidizing Cysteine (FeHOAuC). Finally, the target content was negatively related with the AuNPs aggregation and the subsequent color variation. As a proof-of-concept, this novel plasmonic ELISA exhibited an ultralow LOD (0.1 ng/mL, around 4.4 pM) and a detection range (0.25–8.0 ng/mL) on detecting neutrophil gelatinase-associated lipocalin (NGAL) within 120 min. The sensitivity was superior to those of conventional ELISA and plasmonic ELISA. In summary, our study provided a novel and sensitive plasmonic ELISA for proteinic biomarker diagnosis.