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The Eindhoven team’s prototype, leaf-inspired photochemical reactor—fancifully shaped, in this example, as an actual maple leaf—included a reactant-containing microfluidic channel, embedded in a doped polymer that acted as a both a wavelength converter and a light concentrator. In lab and outdoor tests, the scheme provided a significant boost in reaction rates. [Image: Bart van Overbeeke]

Chemists have long sought approaches that would allow them to leverage power from the sun in energy-intensive reactions such as those used in drug development. One problem, though, has been getting enough solar photons to the reactants of interest to do any good.

A research team in the Netherlands, drawing on the complex natural example of tree leaves, has now put together recent advances in flow photochemistry and solar concentration to create a prototype artificial leaf that can act as a solar-powered, micro chemical reactor (Angew. Chem. Intl. Ed., doi: 10.1002/anie.201611101). The scientists found that the prototype could churn out chemical reactions at accelerated rates even on cloudy days with diffuse light. And they believe the technique could ultimately allow for the sustainable production of drugs in low-resource field settings, at the point of use.

Beyond the “flask in the sun”

The traditional approach to photochemistry is the proverbial “flask in the sun”—a slow technique with limited efficiency for getting chemical work out of solar photons. In recent years, chemists have made substantial advances over this inefficient approach using continuous-flow microchannels as reaction chambers. These microreactors allow dramatically accelerated reaction times and homogenous irradiation of reactants. But for use in solar-powered reactions, there’s still the problem of actually getting enough photons to the reactants to enable operation at production levels.

To address the photon-concentration problem, the team at Eindhoven University of Technology in the Netherlands, led by Michael Debije and Timothy Noël, looked for inspiration to nature’s most evolved light harvesters, the leaves of deciduous trees. In particular, they noted the leaves’ use of networks of light-absorbing molecules, or chromophores, in their light-harvesting “antenna complexes,” coupled with the architecture of fluid flow in the leaves’ vein systems. They reasoned that an analogous artificial system could be developed by combining the increasingly well-established techniques of microchannel flow chemistry with the light-concentrating power of luminous solar concentrators (LSCs).

Dyes and dots

LSCs use luminescent materials, such as fluorescent dyes or quantum dots, dispersed in a glassy matrix or waveguide to capture photons, convert them to a specific wavelength, and direct them within the device. Such concentrators have found use in switchable windows, solar power and other applications. One particularly attractive feature of LSCs is that they can effectively capture and concentrate light in both direct sunlight and the more diffuse illumination of a cloudy day.

To create an artificial-leaf chemical-reactor prototype, the Eindhoven team began by doping a transparent polymer, polydimethylsiloxane (PDMS), with a fluorescent dye, Lumogen F red 305 (LR305), and molding a rectangular slab of the doped material. The fluorescent dye acted as a luminophore that could both capture solar photons and down-convert the light into a specific wavelength—a wavelength tuned to trigger the photocatalyst driving the reaction used to test the setup.

Within the doped PDMS slab, the team embedded a thin silicon channel, through which the researchers pumped the reactant-containing liquids. The LR305-doped PDMS slab, owing to total internal reflection, acted as a light guide, concentrating and channeling the down-converted photons to the microchannel to provide energy to drive the chemical reaction.

Paracetamol on Mars?

Testing the setup under a variety of conditions, the team found that putting microflow chemistry together with solar concentration significantly boosted reaction rates. For example, in cloudy-day conditions, the LR305-doped reactor converted some 96 percent of the reactants, versus 57 percent for a non-doped equivalent setup.

Particularly because of the compactness of the prototype design, the team believes that such solar-powered microfluidic reactors could have applications well beyond the research lab. “Using a reactor like this means you can make drugs anywhere, in principle, whether malaria drugs in the jungle or paracetamol on Mars” says team leader Noël. “All you need is sunlight and this mini-factory.”