Tuning the Surface Chemistry of Chiral Cu(531)^<i>S</i> for Enhanced Enantiospecific Adsorption of Amino Acids

Amino acids are important bioorganic compounds composed of amine and carboxylic acid because they are the main building blocks of many biomolecules. All of them are chiral except glycine. Thus, they have two enantiomers which provide dramatically different biological effects, thereby requiring their separation. High Miller index metal surfaces often define intrinsically chiral structures. A number of previous studies have proved the enantiospecific adsorption difference of chiral molecules on those surfaces. To further enhance the enantiospecificity, step decoration, which is doping the kink site of chiral metal surface with a second metal, can be one route. It may induce one enantiomer adsorbed on the surface to become more stable than the other, inducing the larger enantiospecific energy difference. In this study, we performed density functional theory (DFT) calculations to systemically examine the adsorption geometries and energetics of each enantiomer of alanine, serine, and cysteine, and their enantiospecific energy differences on pure, Pd-, Pt-, and Au-decorated Cu(531)^<i>S</i>, respectively. By decorating the kinked site with an Au atom, the enantiospecificity of adsorbed cysteine was meaningfully enhanced by 0.08 eV, in the case when the side chain has a high affinity with the surface. Our results provide useful insight of how to tune chiral metal surfaces to enlarge the enantiospecificity of chiral molecules

Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl alcohol) as Specialized Microfibrous Li⁺ Adsorbents

Crown ether (CE)-based Li⁺ adsorbent microfibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol cross-linking. The 14- to 16-membered CEs, with varied ring subunits and cavity dimensions, have two hydroxyl groups for covalent attachments to poly­(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) cross-linking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in cross-linking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g^–1 MF). GA cross-linking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE–GA–PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through Fourier transform infrared-attenuated total reflection, ¹³C cross-polarization magic angle spinning NMR, field emission scanning electron microscope, N₂ adsorption/desorption, and universal testing machine. The MFs exhibited pseudo-second-order rate and Langmuir-type Li⁺ adsorption. At their saturated states, the MFs were able to use 90–99% CEs for 1:1 Li⁺ complexation, suggesting favorability of their microfibrous structures for CE accessibility to Li⁺. The MFs were highly Li⁺-selective in seawater. Neopentyl-bearing CE was most effective in blocking larger monovalents Na⁺ and K⁺, whereas the dibenzo CE was best in discriminating divalents Mg²⁺ and Ca²⁺. Experimental selectivity trends concur with the reaction enthalpies from density functional theory calculations, confirming the influence of CE structures and cavity dimensions in their “size-match” Li⁺ selectivity