Dialysis assisted ligand exchange on gold nanorods: Amplification of the performance of a lateral flow immunoassay for E. coli O157:H7.

Ligand exchange on the surface of gold nanorods (AuNRs) is widely used, but conventional methods usually require multiple centrifugation cycles to completely remove cetyltrimethylammonium bromide (CTAB). This can lead to undesired aggregation of AuNRs. A dialysis-assisted protocol is described here for ligand exchange on AuNRs. Dialysis driven by a concentration gradient is shown to be a powerful tool to separate CTAB from aqueous solutions. The concentration gradient of CTAB in a dialysis bag can avoid the possible aggregation of AuNRs that can be caused by drastic environmental changes. It also supports the rate of ligand exchange on the surfaces of the AuNRs. The modified AuNRs were employed in a lateral-flow test strip immunoassay (LFTS-IAs) for the food pathogen E. coli O157:H7 in order to study of efficiency of ligand exchange. Compared to AuNRs where ligand exchange was performed via multiple centrifugation cycles, the AuNRs prepared by dialysis-assisted ligand exchange show improved conjugation to antibody and enhanced visual signals in the test line of the LFTS-IAs. A portable strip reader (absorption wavelength = 525 nm) is used to records the testing results. The sensitivity of AuNRs modified by dialysis has been achieved even as low as 1 × 10 cfu·mL in a short time (within 15 min), and the working range is 1 × 10 to 1 × 10 cfu·mL, which is superior over the detection performance of conventional test strip using AuNRs modified by centrifugation. Graphical abstract Schematic presentation of the ligand exchange of AuNRs. The AuNRs were dialysed in water to decrease the CTAB concentration. Then, 11-mercaptoundecanoic acid (MUA) replaces the CTAB capped on the surface of AuNRs. The modified AuNRs were employed in a lateral flow immunoassay for E. coli O157:H7.