Promoting DNA adsorption by acids and polyvalent cations: Beyond charge screening

Adsorbing DNA oligonucleotides onto nanoparticles is the first step in developing DNA-based biosensors, drug delivery systems, and smart materials. Since DNA is a polyanion, it is repelled by negatively charged nanoparticles, which constitute the majority of commonly used nanomaterials. Adding salt such as NaCl to screen charge repulsion is a standard method of promoting DNA adsorption. However, Na⁺ does not supply additional attractive forces. In addition, adding a high concentration of NaCl can cause the aggregation of nanomaterials. In this feature article, we mainly summarize the methods developed in our laboratory to promote DNA adsorption by lowering the pH and by adding polyvalent metal ions, especially transition-metal ions. Various materials including noble metals (gold, silver, and platinum), 2D materials (graphene oxide, MoS₂, WS₂, and MXene), polydopamine, and several metal oxides are discussed. In general, low pH can protonate DNA bases and nanoparticle surfaces, reducing charge repulsion and even leading to attraction, although DNA folding at low pH can sometimes be detrimental to adsorption. Polyvalent metal ions can bridge additional interactions to achieve otherwise impossible adsorption. On the basis of the current understanding, a few future research directions are proposed to further improve DNA adsorption.