Avoiding the self-nucleation interference: a pH-regulated gold growth strategy to enable ultrasensitive immunochromatographic diagnostics.

Gold nanoparticle-based immunochromatographic assay (AuNP-ICA) has insufficient sensitivity due to its inherent colorimetric signal intensity and low capture efficiency of AuNPs. The metal growth is a common strategy to enhance the sensitivity of AuNP-ICA due to its superior signal amplification potential and simple operation. However, the detection distortion caused by metal self-nucleation during the growth process can seriously affect the accuracy and reproducibility of the strips. We present a pH-regulated gold growth (GISG) strategy to amplify the colorimetric signal and demonstrate its application in improving the performance of traditional AuNP-ICA. The controllable growth signal amplification is achieved by lowering the pH of the growth solution to weaken the reducibility of hydroxylamine (HA), thus urging the crystallization and growth of Au on the AuNP surface instead of free reduction and self-nucleation. In addition, the mechanism of pH regulation on HA reducibility is elucidated by introducing an electron-donating or electron-withdrawing group to affect the electron density of hydroxyl group. The proposed GISG strategy shows improved sensitivity, low background, robust operation, and good reproducibility. The LOD values of the designed GISG-amplified AuNP-ICA are as low as 0.0198 ng mL for hepatitis B surface antigen and 0.0125 ng mL for HIV-1 capsid p24 antigen, which are lower by about 500- and 70-fold, respectively, than those of the unamplified AuNP-ICA. This method is extended to enable ultrasensitive and rapid diagnosis of viral infections, and has potential as a general signal amplification platform to redefine immunochromatographic diagnostics.