Until recently, attaching DNA strands to the surface of gold nanoparticles was slow and incomplete. But a breakthrough finding in 2012 by Waterloo scientists resolved many long-standing questions and paved the way for advances in biosensor technology.
By going back to the beginning, chemist Juewen Liu, post-doctoral fellow Xu Zhang and Canada Research Chair Mark Servos were able to understand the fundamental interactions happening at the particle surface.
Xu Zhang and I met by chance,” said Liu, a professor in the Department of Chemistry and a member of the Waterloo Institute for Nanotechnology. “I had just started at the University and Mark encouraged his post-doc to do some exploratory research with me in my lab. We didn’t expect so much would come out of this one project.”
At one-millionth of a centimetre in diameter, gold nanoparticles have some surprising features: stable, non-toxic, easy to manufacture, excellent size and shape control with vibrant colours in solution. Add strands of DNA to the surface and the particles become “functionalized” ̵ able to visibly change colour in the presence of target molecules without the need for expensive lab analysis.
Gold nanoparticles change colour in solution when they aggregate into larger particles. Separated, the nanoparticles appear bright red; aggregated, the nanoparticles shift to deep blue.
Controlling exactly when aggregation and dispersion take place is key to designing an accurate colour test. By adsorbing DNA to the surface, the nanoparticles can be programmed to detect metals, enzymes, or nucleic acids, which cause the DNA strands to link and the nanoparticles to aggregate.
But researchers have struggled to explain why DNA strands adsorb to the particle surface and under what conditions.
Zhang et al.’s paper demonstrates that DNA adsorption is impeded by electrostatic forces while enabled by strong DNA base coordination with the gold surface. Their findings refute previous theories that weaker hydrophobic or van der Waals forces were responsible. Their paper also reports under what chemical conditions DNA adsorption can be sped up.
Understanding the fundamental chemistry at work means researchers can control these interactions at the molecular level, leading to additional applications like delivering therapeutic drugs to target cells, improving the specificity of polymerase chain reactions (PCR) and catalyzing other biochemical reactions.
The authors subsequently published ten more highly cited articles on this subject, including “Instantaneous and Quantitative Functionalization of Gold Nanoparticles with Thiolated DNA Using a pH-Assisted and Surfactant-Free Route” in the Journal of the American Chemical Society, which has been cited over 100 times
Dr. Xu Zhang is currently a Research Chair in Applied Nanotechnology at the Verschuren Centre for Sustainability in Energy & the Environment at Cape Breton University.
Juewen Liu is also a member of the Centre for Bioengineering and Biotechnology and the Water Institute. Mark Servos is a professor in Waterloo’s Department of Biology and a member of the Water Institute.
The project was funded by the Canadian Foundation for Innovation, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Ontario Ministry of Research and Innovation. Xu Zhang was supported by a Canadian Institutes of Health Research (CIHR) Fellowship.