Plasmonic ‘Light Tweezers’ Use Solar Energy To Move Molecules

26th May 2020

In a scientific breakthrough, Dr Sarina Sarina from the QUT Centre for Materials Science has used sunlight to power a targeted and reversible process for chemical reactions.

Molecules are 10,000 times smaller than a cell — too small for a mechanical tweezer to move.

Sarina’s research group optimised a process called ‘plasmonic optical trapping’ — dubbed light tweezers — which uses visible light and nanoparticles to create a plasmonic optical force capable of moving molecules through a solution.

The process could be used to capture and convert molecules from raw materials into ingredients for medicines, intermediates and other valuable products.

“Until now, it was believed that intense optical force could only be generated by lasers,” Sarina said.

“We discovered how to use low-intensity visible light to generate enough plasmonic optical force to move molecules by introducing metallic nanoparticles to the solution.”

Plasmonic optical force is created when gold, silver and copper nanoparticles absorb light and dissipate excess energy as an electromagnetic field strong enough to attract or repel molecules based on polarity — the positive and negative charge.

Lamps are used in lab experiments to irradiate nanoparticles with visible light. Different light intensities determine the strength of the nanoparticle’s electromagnetic field. Called ‘plasmonic optical trapping’ the force generated by irradiated nanoparticles can attract or repel molecules from the catalyst surface. Image credit: Dr Sarina Sarina.

“We can target molecules of interest by their polarity strength and draw them to the catalyst surface, then convert them into important products of interest or remove them from the solution,” Sarina said

“This process lets us control a chemical reaction better and avoid by-products.”

“It is a greener process because photocatalysts accelerate a chemical reaction with light, not with fossil fuels.”

“Light-induced reactions are also about 1000 times faster than thermal processes in a solution with very low concentration of molecules in a solution, which means we see results in less time using less energy.”

Using the visible light spectrum, which accounts for 47 per cent of solar energy, is also what allows Sarina to tune the electromagnetic field to target specific molecules.

“Each molecule has a different polarity. Changing the lumens, the intensity of light, into the nanoparticle will change the strength of its plasmonic optical force — attracting different molecules by polar strength,” Sarina said.

“However, not all metal nanoparticles can work this way with light. Only gold, silver and copper have plasmonic properties.”

A chemical reaction with an on-off switch

Unlike thermal processes that affect all molecules in a solution, light-induced trapping is not only selective but reversible.

“Molecules are trapped when exposed to light, but released when the light is turned off, or vice versa depending on the polarity.”

Irradiated plasmonic nanoparticles attract molecules when the light is on and release them when the light is off. Illustration by Dr Sarina Sarina.

This light-switching process has wide potential applications in chemical synthesis design and product control, according to Sarina.

“The control of product selectivity in organic reactions has always been, and still remains, an ongoing challenge.

“But plasmonic optical trapping can address the challenge in a smart way. We can trap a molecule and control its reaction ratio to achieve various products from the same solution.

In a solution containing alkyne and amine molecules, for example, Sarina can switch reaction pathways by turning the light on or off to create both imines and diyenes.

“Light-on will create imines from the reaction of alkyne and amine, while light-off will achieve diyenes from the reaction between alkyne and other alkyne molecules,” Sarina said.

“Imines are intermediate molecules used to synthesise alkaloids, which are used to make quinine for antimalarial drugs, ephedrine for antiasthma treatment, and homoharringtonine for anticancer drugs.

“Diynes are used to produce polymer materials that are widely applied to sensors.”

The process could also be applied to create more accurate and sensitive sensor or gas detector technology, according to Sarina.

The next steps

While Sarina and her team have proven the optimised plasmonic optical trapping process in bench-scale studies of solutions <100ml, scaling up the process will be the focus of future research.

Findings were published in the Angewandte Chemie International Edition as Plasmonic Switching of Reaction Pathway: Visible-Light Irradiation Varies Reactant Concentration at the Solid-Solution Interface of a Gold-Cobalt Catalyst, and in Chem as Promoting Ni (II) catalysis with plasmonic antennas.

Her research builds on her previous work using light, instead of fossil fuels, to drive chemical reactions at room temperature, a focus which won her the prestigious Alexander von Humboldt fellowship in Germany.

Sarina’s current project is part of her Australian Research Council Discovery Early Career Researcher Award grant to use plasmonic metal photocatalysts for selective organic reaction. She continues to collaborate with senior QUT researchers Professor Huai Yong Zhu, Associate Professor Eric Waclawik, and Professor Godwin A. Ayoko.

Also published in The LABS.

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