Chitosan hydrogels were prepared and imprinted by Ag I with different mass ratios. Compared with the native chitosan hydrogel sample, the optimally imprinted sample (Agim75) presents a significantly enhanced Ag I adsorption capacity as well as a high selectivity. This is attributed to the specific cavities formed by the template Ag I ions in the polymer matrix. The effect of the adsorption conditions and desorption process were also studied.
Self-assembled crystals (helical crystalline ribbons and tubes with high axial ratio) and their applications have been reported in recent years.[1] Unlike the helical crystalline ribbons and tubes that have been obtained from the lamellarforming molecules, helical nanofibers are less prone than flat objects to form thick bundles that would eventually precipitate because the contact surface in a stack of columnar objects is limited compared with that of a stack of flat ones.[1b] A screw axial model has been proposed for some self-assembled helical nanofibers.[2] It seems that, so far, there only exists lamellar structure in the reported helical crystals. Here we report the discovery of double-stranded helical crystals with high axial ratio. The X-ray diffraction (XRD) results prove that there exists a sixfold screw axis in the helical crystals of a cholic acid ester from aqueous medium. The left-handed helical crystalline tubular model (diameter of ca. 2.6 nm) was constructed, which may serve as a model channel for the transport of cholesterol across membranes. The cholic acid esters (2, 3, and 4) with a hydrophilic tail of different lengths were synthesized by a simple esterification of the carboxyl group of cholic acid (1) with di(ethylene glycol), tri(ethylene glycol), and tetra(ethylene glycol), respectively (Scheme 1 and the Supporting Information, Scheme S1). To test the self-assembling potential, the cholic acid esters were each dissolved in a small amount of organic solvents (methanol, ethanol, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, acetone, etc.), followed by the addition of water. The turbid solutions (1 % w/v) were placed in a water bath at 55°C for 4 h and were cooled down to room temperature within 12 h. 2 precipitated in aqueous solutions, while 3 and 4 formed hydrogels (see Supporting Information, Fig. S1 and Table S1). Crystals appeared in the hydrogel of 4 within three months. Finally, the gel was broken and the crystals precipitated. The crystals were observed by means of laser confocal scanning microscopy (LCSM) (Fig. 1) and polarizing microscopy.Figure 1 (left) shows a left-handed double-stranded helical self-assembled crystal with high axial ratio. The pitch and to-COMMUNICATION 462
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