Shape dependent sensing potential of gold nanoparticles in etching based multicolorimetric plasmonic-ELISA

In the present study, a systematic investigation has been carried out for the first time to assess the potential of three different shapes of gold nanoparticles (AuNPs), viz. nanorods (AuNRs), nanotriangles (AuNTs), and nanospheres (AuNSs), to develop a horseradish peroxidase (HRP) enzyme-mediated etching-based plasmonic ELISA (p-ELISA) strategy. The etching of the AuNPs in ELISA is achieved by 3'-3-5'-5-tetramethylbenzidine (TMB²⁺), which is produced by the biocatalytic conversion of chromogenic TMB via HRP. All three types of AuNPs were interacted with varying concentrations of TMB²⁺ (7-131 μM) (product of HRP enzyme reaction) and characterized for visible color change and by UV-Vis spectroscopy and transmission electron microscopy (TEM). From the comparative analysis of all three shapes of AuNPs, AuNRs exhibited vivid visible color change and absorbance intensity change compared to spherical and triangle-shaped nanoparticles. The TEM analysis of the etched nanoparticles revealed the gradual etching pattern of AuNRs compared to AuNTs which resulted in multicolor generation as opposed to AuNTs where the etching was relatively very fast and thus shows a faster shape transformation and poor color discrimination. Further, the potential of the AuNR etching-based optimized strategy was successfully demonstrated to develop an indirect competitive p-ELISA for human IgG detection. The developed p-ELISA showed an ultra-low visual limit of detection of 1 fg mL^-1 (∼6.54 aM) without the aid of any sophisticated instruments. In the future, the developed competitive p-ELISA strategy can be easily employed to develop cost-effective, portable, and point-of-care assays for the detection of various disease biomarkers with ultra-high sensitivity.