Highly Stable Colorimetric Sensing by Assembly of Gold Nanoparticles with SYBR Green I: From Charge Screening to Charge Neutralization

Aggregation of plasmonic gold nanoparticles (AuNPs) has been widely used for label-free colorimetric sensing. Adding salt is a commonly used method to induce a color change, but salt-induced aggregation by charge screening is a slow kinetic process, making quantitative and reproducible measurement difficult. Herein, we explored an alternative method by charge neutralization using positively charged molecules, such as a DNA-staining dye, SYBR Green I (SG), for efficient and stable assembly of AuNPs. Even submicromolar SG effectively triggered the aggregation of AuNPs in less than 1 min, and the resulting intense blue color was stable for over 10 h. The charge neutralization effect of SG was confirmed with zeta potential measurements. Besides, pH-dependent aggregation of AuNPs by SG agreed well with the pKa of the nitrogen atoms responsible for the positive charges in the SG structure. Moreover, the charge neutralization effect was also observed from polycations such as polyamines. The SG-induced assembly of AuNPs was developed into a label-free colorimetric sensing platform. Benefiting from the intense blue color, this new sensing strategy allowed visual detection of as low as 0.1 nM DNA, 50-fold lower than that by salt. Such a charge neutralization strategy may inspire further thinking on fundamental colloidal science and its applications in analytical chemistry. This work itself significantly improved AuNP-based colorimetric sensors in terms of detection time, signal stability, and sensitivity.