Methods for preparing DNA-functionalized gold nanoparticles, a key reagent of bioanalytical chemistry

Gold nanoparticles (AuNPs) have excellent optical properties such as high extinction coefficients, distance-dependent color, strong fluorescence quenching, and localized surface plasmon resonance. At the same time, DNA has both programmable structures and molecular recognition functions. Functionalizing AuNPs with DNA has produced a diverse range of useful biosensors and stimuli-responsive materials. This article reviews a critical step required for all applications: conjugation of thiolated DNA to AuNPs. Since both DNA and AuNPs are negatively charged, a challenge is to overcome their charge repulsion while still maintaining the colloidal stability of AuNPs. Over the past twenty years, various strategies have been developed to achieve this goal, starting from the salt-aging method, where NaCl is added gradually over 1-2 days. The salt-aging method was accelerated by using acidic conditions, adding surfactants, and sonication. Depletion stabilization with a concentrated polymer solution was also tested as a new mechanism of avoiding AuNP aggregation. Finally, modified DNA was used to minimize charge repulsion. These research efforts have not only advanced the technology, but also allowed fundamental insights into the colloidal property of this system. For example, the three main colloidal stabilization mechanisms: charge, steric, and depletion have all been explored. This article describes both the experimental details as well as fundamental surface and colloid science.