Hucheng Zhang

Simple and green synthesis of nitrogen-, sulfur-, and phosphorus-co-doped carbon dots with tunable luminescence properties and sensing application

A simple and green approach to prepare water-soluble, nitrogen-, sulfur-, and phosphorus-co-doped carbon dots (N/S/P-CDs) by one step hydrothermal treatment of cucumber juice is reported. The resultant N/S/P-CDs possess a uniform morphology, a size of less than 10 nm, plentiful O and N functional groups as well as a limited amount of P and S functional groups, and exhibit good luminescence stability, high resistance to photobleaching, high solubility, and excitation-dependent emission behavior. It was also found that a higher reaction temperature favors the formation of N/S/P-CDs with smaller size and longer wavelength of photoluminescence emissions. Moreover, the N/S/P-CDs can be applied as a fluorescent probe for the detection of Hg2+ ions.

The interaction of sodium alginate with univalent cations

The M/G ratio, dyad and triad frequencies in the sodium alginate chain, were determined from 13C-nmr spectra. The interactions of sodium alginate in solution with the univalent cations K+ ion and Na+ ion have been investigated by viscometry and membrane osmometry. The dependencies of intrinsic viscosity, Huggins constant, and second virial coefficient on ionic strength were observed, and the maximums in reduced viscosity were obtained in low KCl and NaCl concentrations, respectively. These show that the electroviscous effects play an important role in polyelectrolyte solution, and the effect of the Na+ ion on aqueous solution of sodium alginate is greater than the K+ ion. The experimental observations are interpreted in terms of ion-pair formation with carboxyl groups of mannuronate and isolated guluronate residues and cooperation “egg-box” binding between polyguluronate chain sequence. The difference of interaction between univalent cations and alginate chains in solution is attributed to the ability of their binding with the polyion, which depends on the properties of ions itself.

Critical behavior of viscosity for alginate solutions near the gelation threshold induced by cupric ions

The critical behavior of relative viscosity and intrinsic viscosity near the gelation point was experimentally examined for 1 g/100 ml alginate solution induced by cupric ions. The critical exponent of relative viscosity was found to be 1.27, which is in good agreement with the prediction of the percolation model based on a Rouse cluster approximation gives the exponent as 1.3. Critical behavior of intrinsic viscosity reveals that the constant of Huggins equation k′ for alginate clusters formed near the gelation point was considerably higher than that of linear alginate polychains in aqueous solution. Typically intramolecular crosslinking between polychains in the low cupric concentration could be detected in the plots of [η] vs molar fraction of cupric ions for the system. The weak divergence of intrinsic viscosity resembles the critical behavior of Zimm clusters predicted by the three‐dimensional percolation theory rather than clusters of classical theory.

Salt effects on the cross-linking mechanism of cupric-induced sol-gel transition in alginate solutions

The viscosity behaviour of alginate-Cu2+-NaCl systems has been experimentally examined at various concentrations of cupric and sodium salts. Dependence of the intrinsic viscosity of alginate as a function of NaCl concentration is discussed to supplement the previous study which shows a similar behaviour to that found for other polyelectrolytes in aqueous solution in the presence of an added salt. The effects of sodium ions on the cupric association in cupric-induced alginate solutions were investigated by means of viscosity measurements. The mechanisms of complex formation in the presence of the simple added salt were studied. It was found that, at a given NaCl concentration, the viscosity of the mixture will pass through a maximum with increasing cupric concentration. The amounts of cupric cations corresponding to the maximum depends on the concentration of NaCl in the solution. Comparison of salt effects on the viscosity behaviour of alginate solutions during sol—gel transition reveals that an optimum NaCl concentration of 10−2 mol 1−1 exists where the viscosity of the mixture gives a maximum value at a certain cupric amount. This result indicates that salt effects play an important role in the sol—gel transition of the polyelectrolyte solutions. The observed phenomenon was interpreted in terms of conformational change of polyelectrolyte chain due to the addition of salt resulting in a different cross-linking mode in the system.

pH triggered self-assembly structural transition of ionic liquids in aqueous solutions: smart use of pH-responsive additives

The creation of smart self-assembling fluids that undergo a morphological transition in response to a specific pH value can allow for the enhanced accumulation of drug delivery agents. In this work, we developed a series of pH-responsive fluids composed of 1-alkyl-3-methylimidazolium bromide [Cnmim]Br (n = 12, 14) and one of the pH-responsive hydrotropes of potassium hydrogen phthalate ([C6H4COOKCOOH]), sodium sulfosalicylate ([C6H3OHCOOHSO3Na]), or m-carboxylbenzenesulfonate sodium ([C6H4COOHSO3Na]). The self-assembled structures of these ILs in aqueous hydrotrope solutions were investigated by surface tension, dynamic light scattering, cryogenic-transmission electron microscopy, small-angle X-ray scattering, polarized optical microscopy, and nuclear magnetic resonance spectroscopy. It was found that the ionic liquids, [Cnmim]Br (n = 12, 14), could self-assemble into vesicles with the addition of the hydrotrope, and a reversible transition between spherical micelles and vesicles was observed with the change of solution pH value. The transition in the self-assembled structures of the ILs is suggested to be driven by the change in the molecular structure and hydrophilicity/hydrophobicity of the hydrotrope.

Constructing nanosized CdTe nanocrystal clusters with thermo-responsive photoluminescence characteristics

Assembling ultra-small nanoparticles into nanosized colloidal nanocrystal clusters (CNCs) to create novel collective properties still poses tremendous challenges. This work reports the fabrication of photoluminescent CdTe CNCs on the nanoscale and their thermo-responsive properties. Diblock copolymers of poly(N-(2-aminoethyl) acrylamide-b-N-isopropylacrylamide) (PNAEAM-b-PNIPAM) were synthesized and employed as self-assembling actuators of CNCs. The side chains of PNAEAM blocks act as efficient anchors to capture CdTe nanocrystals via surface ligand identification. The thermo-sensitive PNIPAM blocks serve as the protection layer of CNCs and the trigger to turn off/on the photoluminescence CNCs during heating/cooling cycles. The [HS–C10mim]+ ligands with smart noncovalent interactions on the as-prepared nanocrystals render the CNCs rapid and reversible thermo-response performances. These make the CNCs an excellent thermo-responser, and offer a new controllable self-assembly route for designing and engineering multifunctional nanosized CNCs.

Interaction modes in sol–gel transition of alginate solutions by the addition of cupric ions

The interaction mode between cupric ions and alginate in aqueous solutions during sol–gel transition has been experimentally examined at different temperatures. Some clusters were found to pass through a conformational transformation as the temperature increased, i.e. clusters formed by intramolecular crosslinks turned into intermolecular crosslinking clusters. This transformation of the interaction mode is not reversible as the temperature decreases. The interaction mode between cupric ions and alginate was re-examined by optical absorption techniques, and results suggest that the coordination of cupric ions mainly occurs to carboxyl groups and that the stoichiometric ratio f is proportional to the fraction of crosslinks formed.