Picking up a grain of rice with one chopstick can be a futile task. But a pair of chopsticks, held at just the right distance, gives the precise two-pronged grip needed to move that grain exactly where you want it. Some of Nature’s most important enzymes work on a similar principle—with two precisely spaced active sites that grip reactant molecules in tandem to steer them through complex, multi-step chemical transformations known as redox reactions.
From living cells to batteries, redox reactions underpin many natural and manmade systems, making customised catalysts for them a matter of great industrial interest. However, recreating Nature’s cooperative strategy in the laboratory has proved challenging. Synthetic catalysts that attempt dual-site cooperation often lack the structural precision to position their active sites consistently, and most depend on metal-centred sites, which—while offering versatile redox abilities—can be unstable and difficult to fine-tune for selective reactions.
A recent team-up between the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and the National University of Singapore (NUS) set out to change that. “We wanted to ask whether Nature’s ‘two-hand’ strategy could be recreated without using metals,” explained Le Yang, Head of A*STAR IMRE’s Sensors and Flexible Electronics Department.
Yang, A*STAR IMRE Scientist Jun Zhu and colleagues worked with NUS groups led by Jishan Wu and Pengfei Ou to design a family of metal-free molecular catalysts called bicarbeniums for redox reactions. Each bicarbenium molecule features a rigid xanthene bridge: a molecular scaffold that holds two carbon-based active sites face-to-face at a fixed distance of roughly 0.43 nm. When an electric current supplies the molecule with electrons, its active sites transform into a pair of radicals.
“This creates a snug pocket perfectly sized to capture and activate small diatomic molecules such as oxygen gas (O₂) and nitric oxide (NO) in a controlled way,” said Zhu.
In a series of experimental and computational studies, the team prepared three catalyst variants (C-C₂⁺, O-C₂⁺ and S-C₂⁺), swapping heterocyclic groups on their scaffolds to tune their stability and reactivity. The sulphur-containing variant, S-C₂⁺, proved the standout performer: in oxygen reduction tests, it achieved 99.3 percent selectivity for hydrogen peroxide (H₂O₂) with a Faradaic efficiency of 96.8 percent. Turned toward NO, S-C₂⁺ steered redox reactions along a three-electron pathway to produce hydroxylamine (NH₂OH) with 87.2 percent Faradaic efficiency.
“Sulphur acts like a better ‘shock absorber’ for the highly reactive intermediates formed during catalysis,” said Yang. “This stabilisation helps S-C₂⁺ avoid unwanted side reactions and maintain high selectivity and durability.”
Both H₂O₂ and NH₂OH are industrially valuable chemicals, but their production often requires harsh conditions or generates significant waste. “Electrocatalysts offer a cleaner possibility because they can use electricity from renewable sources to drive these reactions under milder conditions,” noted Zhu.
Preliminary tests showed the team’s bicarbenium platform could also activate more ‘stubborn’ molecules such as carbon dioxide and nitrogen gas. “We see this platform as a way to design metal-free molecular catalysts for challenging small-molecule transformations relevant to energy, environment and sustainable chemical production,” added Yang.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
