Fabrication of prime number checkers based on colorimetric responses of gold nanoparticles
In this work, we demonstrated the fabrication of molecular prime number checkers based on the concentration- and sequence-dependent colorimetric responses of citrate-capped gold nanoparticles (Au NPs) to two simple model chemicals, i.e. cysteine (Cys) and Hg2+ ions. After being added into the dispersions of Au NPs, Cys could be adsorbed either in a monolayer or in a bilayer fashion on the surface of the NPs, dependent on the concentration of Cys added. The monolayer Cys-modified Au NPs maintained the original red color, whereas the bilayer Cys-modified Au NPs experienced a color change from red to purple or blue, attributed to their aggregation, in a wide range of Hg2+ concentration. When the amount of Cys introduced was in excess of that required for adsorption of bilayered Cys on the Au NP surface, Au NPs could only experience aggregation and color change at a high Hg2+ concentration. Likewise, Au NPs covered by various amounts of Hg2+ ions also presented concentration-dependent colorimetric responses upon the subsequent addition of Cys. Inspired by these facts, prime number checkers were designed and fabricated using Cys and Hg2+ with the adequate concentrations and different adding sequences as inputs and the colorimetric responses of Au NPs as outputs. For example, the as-designed 3, 4- and 5-bit prime number checkers were qualified to encode natural numbers 0–6, 0–14 and 0–31, respectively, and identify prime numbers among them based on the colorimetric responses. The successful fabrication of prime number checkers shows the significance of precise control over the surface properties of Au NPs in the design of logic devices, which should be profitable to promote their applications in chemical sensing and information processing.