Chemical Patterns by Molecular Design

Spontaneous formation of stationary chemical patterns through reaction-diffusion processes, first proposed by Alan Turing, is central to understanding biological morphogenesis. However, most existing synthetic pattern-forming systems rely on inorganic reactions with limited molecular tunability, posing challenges for exploring evolutionary and design aspects of pattern formation. Here, we developed an organic reaction-diffusion system based on a thiol-based chemical reaction network (CRN), rationally designed to generate stationary patterns. The CRN features autocatalysis coupled with both rapid direct inhibition and a negative feedback loop, employing azocarboxamides as thiol oxidants. We used disulfide-crosslinked polyacrylamide hydrogels to modulate thiol diffusion and optimized the molecular structures of reactants to finely tune their reactivity and diffusivity. Patterns formed within a 12-mm hydrogel disk supplied continuously with reactants from a well-mixed reservoir through a nanoporous membrane. The resulting dot, line, and net patterns exhibited characteristic feature sizes around 1 mm. While the membrane permeability primarily influenced the pattern type, the reactivity and diffusivity of reactants determined feature sizes. Experimental results were further validated by numerical modeling. Our findings illustrate that molecular-level design can yield complex pattern-forming CRNs from organic building blocks. By providing a tunable platform that bridges inorganic and biological systems, this study opens avenues to systematically explore the principles governing formation, evolution, and robustness of reaction-diffusion patterns.