Chirality plays a key role in modern science and technology. Here, we report a simple and effective sensing platform for visual chiral recognition of enantiomers. In this sensing platform, gold nanorods (AuNRs) prepared through a common synthesis route are used as colorimetric probes for visual recognition of glutamine (Gln) enantiomers. D-Gln could rapidly induce the aggregation of AuNRs, thereby resulting in appreciable blue-to-gray color change of AuNRs solution; however, L-Gln could not induce color change of AuNRs. This distinct color change can be easily distinguished by the naked eyes; as a result, a visual method of chiral recognition was suggested. The method was applied to determine the enantiometric excess of D-Gln through the whole range of −100% ~ 100%. The chiral assay can be performed with a simple UV-vis spectrometer or the naked eyes. Notably, the AuNRs do not need any chiral labeling or modification, and the chiral recognition is based on the inherent chirality of AuNRs. This chiral assay method is simple, sensitive, cheap and easy to operate. This study is the first example using AuNRs for direct visual recognition of enantiomers, and will open new opportunity to construct more chiral recognition methods for some important compounds.
Materials with negative thermal expansion (NTE), which contract upon heating, are of great interest both technically and fundamentally. Here, we report giant NTE covering room temperature in mechanically milled antiperovksite GaN x Mn 3 compounds. The micrograin GaN x Mn 3 exhibits a large volume contraction at the antiferromagnetic (AFM) to paramagnetic (PM) (AFM-PM) transition within a temperature window (ΔT) of only a few kelvins. The grain size reduces to ~ 30 nm after slight milling, while ΔT is broadened to 50K. The corresponding coefficient of linear thermal expansion (α) reaches ~ -70 2 ppm/K, which is almost two times larger than those obtained in chemically doped antiperovskite compounds. Further reducing grain size to ~ 10 nm, ΔT exceeds 100 K and α remains as large as -30 ppm/K (-21 ppm/K) for x = 1.0 (x = 0.9). Excess atomic displacements together with the reduced structural coherence, revealed by high-energy Xray pair distribution functions, are suggested to delay the AFM-PM transition. By controlling the grain size via mechanically alloying or grinding, giant NTE may also be achievable in other materials with large lattice contraction due to electronic or magnetic phase transitions.
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