English translation of the original Chinese article. Publication dates and the extent of recovered text are preserved. Figures retain their original labels. Read the Chinese original.
University of Waterloo
Keywords: DNA, gold nanoparticles
As a precious metal, gold has long served as an outward symbol of wealth and power, while the inward symbol of nobility was a continuous family lineage. With advances in science and technology, gold is now far more than a precious metal used as currency, and our understanding of lineage has reached the level of DNA. To chemists, AuNP@DNA—combining gold nanoparticles with special quantum effects and highly biocompatible nucleic-acid polymers—already represents the “noble lineage” of a new era [1]. This article discusses surface and interface problems encountered when synthesizing AuNP@DNA, particularly how to overcome electrostatic repulsion between gold nanoparticles and DNA.
Depending on how they are made, gold nanoparticle surfaces can carry positive charges, negative charges or even no charge. Gold nanoparticles produced by citrate reduction are widely used because the process is simple and yields particles with reasonably uniform sizes and good stability. Nanoparticles produced in this way carry a negative charge because their surfaces are coated with citrate ions. DNA also carries a negative charge in a neutral environment because of its phosphate backbone. Thus, when we try to attach DNA to gold nanoparticle surfaces, the two repel each other. On the other hand, DNA and gold nanoparticles can also experience other interactions, such as attractive van der Waals forces. These act over shorter distances than electrostatic forces: electrostatic repulsion therefore takes effect before these attractive interactions can act. Solving this problem is a crucial step in synthesizing AuNP@DNA.
Chemists have offered a very simple solution: add salt [2]. For a charged surface, electrical double-layer theory predicts that its electric field in solution changes exponentially with distance from the surface, rather than linearly. This exponential behavior can be understood as a particularly rapid initial decay, followed by a gradual approach to a steady value, as shown in Figure 1. The characteristic length marking this change is called the Debye length (the λD in the figure). Adding salt can greatly reduce the Debye length. The range over which electrostatic repulsion acts is consequently much shorter—that is, electrostatic interactions are screened—and may even become shorter than the range of attraction between DNA and gold nanoparticles. DNA can then attach to the nanoparticle surface. There is, however, an important practical detail: the salt concentration must be increased slowly. Adding salt screens not only the repulsion between DNA and gold nanoparticles but also the repulsion between gold nanoparticles themselves, which can destabilize them and cause aggregation. The salt concentration must therefore be raised step by step. First, some DNA is allowed to attach to the nanoparticle surfaces, increasing their stability. More salt is then added to enable additional DNA to attach.
Figure 1: Electrical double-layer theory and the Debye length.
Chemists are no longer satisfied merely to attach DNA to gold nanoparticle surfaces; they hope to incorporate DNA's functionality into this technology. That functionality arises from its spatial configuration. How to control DNA's configuration precisely on gold nanoparticle surfaces is attracting increasing research interest.
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References:[1] Eustis, S.; El-Sayed, M. A., Why gold nanoparticles are more precious than pretty gold: Noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes. Chem. Soc. Rev. 2006, 35, (3), 209-217.
[2] Zhang, X.; Servos, M. R.; Liu, J., Instantaneous and Quantitative Functionalization of Gold Nanoparticles with Thiolated DNA Using A pH-assisted and Surfactant-free Route. J. Am. Chem. Soc. 2012, 134, (17), 7266-9.
