Fa Cheng

Preparation and characterization of novel thermoresponsive gold nanoparticles and their responsive catalysis properties

Novel thermoresponsive gold nanoparticles (AuNPs) with lower critical solution temperature (LCST) were obtained through the non-covalent interaction between a thermoresponsive hyperbranched polyethylenimine with isobutyramide groups (HPEI-IBAm) and citrate-protected AuNPs. The LCSTs of the thermoresponsive AuNPs could be conveniently modulated over a broad range by altering the molecular weight of the HPEI core, the degree of substitution (DS) of the IBAm groups of the HPEI-IBAm polymers or the pH of the solution. The obtained thermoresponsive AuNPs could be used as recyclable responsive catalysts for the reduction reaction of 4-nitrophenol by NaBH4. As far as the thermoresponsive catalysts were concerned, reducing the molecular weight of the HPEI core, lowering the DS values and increasing the concentrations of the capping HPEI-IBAm polymers or the gold resulted in the acceleration of the reaction. By choosing the right capping HPEI-IBAm polymers, the reaction was faster than that catalyzed by AuNPs without capping polymers. The reaction rate was accelerated by elevating the reaction temperature at first, but reached a plateau or decelerated upon raising the temperature close to the LCSTs of the thermoresponsive AuNPs catalysts. Moreover, the obtained thermoresponsive AuNP catalysts could be recovered by heating the temperature above their LCSTs and be recycled at least six times with more than 95% conversion.

Thermoresponsive gold nanoparticles with adjustable lower critical solution temperature as colorimetric sensors for temperature, pH and salt concentration

Thermoresponsive gold nanoparticles (AuNPs) with lower critical solution temperature (LCST) adjustable over a broad range were explored to be potentially used as colorimetric sensors. Upon raising the temperature above the LCST the surface plasmon resonance (SPR) peaks of the obtained thermoresponsive AuNPs red-shifted sharply in a narrow temperature range, accompanied by a color transition from transparent red to transparent purple–red until turbid red, which made them suitable to be used as sensitive colorimetric sensors for detecting environmental temperature variation. Moreover, the temperature range of sensitivity of the obtained thermoresponsive AuNPs could be tuned by modulating the molecular weight of core or degree of substitution of the thermoresponsive polymers employed. Furthermore, the solution colors of the thermoresponsive AuNPs were also sensitive to pH and NaCl concentration variation, as a result of which they could also be used as colorimetric sensors for detecting the variation of pH and salt concentration.

Unusual salt effect on the lower critical solution temperature of hyperbranched thermoresponsive polymers

Compared with linear thermoresponsive polymers hyperbranched ones having spheroid-like structure exhibited an unusual salt effect: a non-linear LCST decrease upon increasing the concentration of various salts such as NaCl, KCl or Na2SO4 has been observed. The LCST variation of such polymers was more sensitive to the presence of salt.

Comparative study of thiol-free amphiphilic hyperbranched and linear polymers for the stabilization of large gold nanoparticles in organic solvent

Amphiphilic hyperbranched and linear polymers based on the respective palmitic acid modified hyperbranched and linear polyethylenimines have been successfully employed to transfer the citrate-protected 17-nm gold nanoparticles (AuNPs) from water into chloroform without the aid of other compounds. Compared with their corresponding linear analog, the amphiphilic hyperbranched polymers exhibited higher efficiency in transferring the large AuNPs. The chloroform solutions of AuNPs were characterized by UV-vis spectrometry and dynamic light scattering. It was found that aggregated AuNPs existed in the system with the amphiphilic linear polymer as stabilizer, whereas much less aggregated AuNPs could be detected in the system with the amphiphilic hyperbranched polymer as stabilizer. Furthermore the amphiphilic hyperbranched polymers could form relatively homogeneous and densely packed shell around the gold core revealed by transmission electron microscopy. Stability experiments showed that the solution of AuNPs coated with the amphiphilic hyperbranched polymers were more stable than those coated with their linear analogs. Moreover, the AuNPs capped with the amphiphilic hyperbranched polymers could be also stored in dryness for long time.

Amphiphilic hyperbranched copolymers bearing a hyperbranched core and dendritic shell: synthesis, characterization and guest encapsulation performance

The 2,2-bis(hydroxymethyl)propionic acid (BHP)-based generation 1 dendron with two palmitate tails (D1-C16) and the generation 2 dendron with four palmitate tails (D2-C16) were synthesized. The coupling of D1-C16 or D2-C16 with hyperbranched polyethylenimine (PEI) through the amidation reaction resulted in amphiphilic hyperbranched copolymers bearing a hyperbranched PEI core and a dendritic D1-C16 shell or dendritic D2-C16 shell. The structure of the obtained copolymers was verified through Fourier transform infrared (FTIR) and 1H nuclear magnetic resonance (NMR) characterization. Differential scanning calorimetry (DSC) measurement demonstrated that the existence of the branching units in the shell pronouncedly reduced the crystallinity of the hyperbranched copolymers, and the copolymers with less branched shells had a higher melting temperature and melting enthalpy. These novel amphiphilic hyperbranched copolymers could be used as nanocarriers to efficiently accommodate the hydrophilic guests, including Methyl Orange (MO), Congo Red (CR) and Direct Blue 15 (DB), into the hydrophilic amidated PEI core. Each nanocarrier with a branched shell could accommodate a much higher number of guests than the corresponding nanocarriers with linear shells, which indicated that the dendritic structure of the shell played a key role in significantly enhancing the encapsulation capacity of the nanocarriers. As far as the weight ratio of the encapsulated guests to the nanocarriers was concerned, the nanocarriers with branched shells could be modulated to have a similar encapsulation capacity for the small MO with a mono-sulfonate group, but a much superior encapsulation capacity for the large CR and DB guests with multi-sulfonate groups to the nanocarriers with linear shells.

Self‐Assembled Supramolecular Nanocarrier Hosting Two Kinds of Guests in the Site‐Isolation State

Hyperbranched polyethylenimine (HPEI) was simply mixed with a solution of amphiphilic calix[4]arene (AC4), which possesses four phenol groups and four aliphatic chains, in chloroform. This resulted in the novel supramolecular complex HPEI-AC4 through the noncovalent interaction of the amino groups of HPEI with the phenol groups of AC4. The formed HPEI-AC4 supramolecular complexes were characterized by 1H NMR spectroscopy and dynamic light scattering. The cationic water-soluble dye methyl blue (MB) and the anionic water-soluble dye methyl orange (MO) were used as the model guests to test the performance of HPEI-AC4 as a supramolecular nanocarrier. It was found that HPEI-AC4 could accommodate the anionic water-soluble MO guests into the HPEI core. The MO encapsulation capacity of HPEI-AC4 was pH sensitive, which reached maximum loading under weakly acidic conditions. The loaded MO molecules could be totally released when the pH value was reduced to be around 4.5 or raised to be around 9.5, and this process was reversible. HPEI-AC4 could not only accommodate the anionic MO with the HPEI core but could also simultaneously load the cationic MB molecules using the formed AC4 shell, thereby realizing the site isolation of the two kinds of functional units. The amount of MO and MB encapsulated by HPEI-AC4 could be controlled by varying the ratio of hydroxyl groups of AC4 to amino groups of HPEI.

Systhesis of double-hydrophilic core-shell type multiarm star copolymer polyethylenimine-block-poly(2-hydroxyethyl methacrylate

Multiarm star block copolymers hyperbranched polyethylenimine-b-poly(2-hydroxyethyl methacrylate) (HPEI-b-PHEMA) with average 28 PHEMA arms have been prepared by atom transfer radical polymerization (ATRP) of HEMA in a mixed solvent of methanol and water using a core-first strategy. The hyperbranched macroinitiator employed was prepared on the basis of well-defined hyperbranched polyethylenimine with Mw/Mn of 1.04 by amidation with 2-bromo-isobutyryl bromide. The polymerization condition was optimized to prepare star copolymers with narrow dispersity, and the variables included the volume ratio of methanol to water, the molar ratio of initiating site to CuCl and the molar ratio of [CuCl]:[CuBr2]. Under the optimized polymerization condition, the lowest Mw/Mn value of the obtained star copolymers was around 1.3. Kinetic analysis showed that an induction period existed in the polymerization of HEMA. After this induction period, a linear dependence of ln([M]0/[M]t) on time was observed. The obtained HPEI-b-PHEMA could adsorb hydrophilic molecules. The comparison with the star copolymer with hydrophobic core and hydrophilic PHEMA shell verified that both the hydrophilic core and shell could host the hydrophilic guests, but the amidated HPEI core was more effective than the PHEMA shell.