Hua-Ji Liu

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.

Gold nanoparticles immobilized in hyperbranched polyethylenimine modified polyacrylonitrile fiber as highly efficient and recyclable heterogeneous catalysts for the reduction of 4-nitrophenol

Hyperbranched polyethylenimine (HPEI) modified polyacrylonitrile fiber (PANF) was prepared through a water mediated hydrolysis and amidation reaction in an autoclave. The grafting amount of HPEI onto PANF could be modulated conveniently by varying the preparation conditions, such as reaction temperature, reaction time and the feed ratio of HPEI to PANF. The Youngs modulus of the PANF decreased with the grafting of HPEI, especially when more HPEIs were grafted. As for the PANF-g-HPEI with low HPEI content, the Youngs moduli were similar before and after loading of AuNPs, whereas the loading of AuNPs obviously deteriorated the strength of the fibers with high HPEI content. From the nitrogen adsorption and desorption isotherms, it could be seen that PANF contained nanometer sized pores, and the grafting with HPEI did not affect the pore size, but did reduce the surface area. Moreover, the loading of AuNPs into PANF-g-HPEI also did not influence the pore size, but decreased the surface area. FTIR and XPS analyses demonstrated that the obtained PANF-g-HPEI not only contained a large amount of amino groups from the HPEI moiety, but also many carboxylate ions due to the hydrolysis of the cyano groups of PANF. XRD characterization proved that the inner crystal region of PANF was partially broken by the introduction of HPEI moieties. SEM showed that the PANFs swelled up after grafting with HPEI, and the increase of the grafting efficiency led to a larger average diameter of the fibers. When the grafting amount of HPEI onto PANF reached as high as 97%, the surface of the fibers was severely impaired. The obtained PANF-g-HPEIs could be successfully used as supporters and stabilizers in the preparation of small-sized AuNPs. TEM characterization showed that the mixing time of PANF-g-HPEIs with HAuCl4 aqueous solution affected the size and size distribution of the formed AuNPs, and the optimal mixing time was around 0.5 h. The average diameter of the obtained AuNPs was around 3.0 nm at a feed ratio of amino groups of PANF-g-HPEI to Au atoms ([N] : [Au]) of 200, independent of HPEI content of the PANF-g-HPEIs used. Reducing the [N] : [Au] feed ratio increased the average size of the obtained AuNPs. The AuNPs supported by PANF-g-HPEIs could be used as efficient catalysts for the heterogeneous catalytic reduction of 4-nitrophenol by NaBH4. The PANF-g-HPEI with lower HPEI content endowed the supported AuNPs with a slightly higher catalytic rate. These heterogeneous AuNP catalysts could be conveniently recovered and reused many times, especially the AuNPs supported by the PANF-g-HPEIs containing a low content of HPEI. The turnover number (TON) values of the AuNPs supported by PANF-g-HPEI0.31 and PANF-g-HPEI0.58 could reach more than 5 × 104, which is unprecedented in the catalytic reduction of 4-nitrophenol.

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.

Hyperbranched Polymers with Thermoresponsive Property Highly Sensitive to Ions

The salt effects on the water solubility of thermoresponsive hyperbranched polyethylenimine and polyamidoamine possessing large amounts of isobutyramide terminal groups (HPEI-IBAm and HPAMAM-IBAm) were studied systematically. Eight anions with sodium as the counterion and ten cations with chloride as the counterion were used to measure the anion and cation effects on the cloud point temperature (T(cp)) of these dendritic polymers in water. It was found that the T(cp) of these dendritic polymers was much more sensitive to the addition of salts than that of the traditional thermoresponsive linear polymers. At low anion concentration, the electrostatic interaction between anions and the positively charged groups of these polymers was dominant, resulting in the unusual anion effect on the T(cp) of these polymers in water, including (1) T(cp) of these dendritic polymers decreasing nonlinearly with the increase of kosmotropic anion concentration; (2) the chaotropic anions showing abnormal salting-out property at low salt concentration and the stronger chaotropes having much pronounced salting-out ability; (3) anti-Hofmeister ordering at low salt concentration. At moderate to high salt concentration, the specific ranking of these anions in reducing the T(cp) of HPEI-IBAm and HPAMAM-IBAm polymers was PO(4)(3-) > CO(3)(2-) > SO(4)(2-) > S(2)O(3)(2-) > F(-) > Cl(-) > Br(-) > I(-), in accordance with the well-known Hofmeister series. At moderate to high salt concentration, the specific ranking order of inorganic cations in reducing the T(cp) of HPEI-IBAm polymer was Sr(2+) ≈ Ba(2+) > Na(+) ≈ K(+) ≈ Rb(+) > Cs(+) > NH(4)(+) ≈ Ca(2+) > Li(+) ≈ Mg(2+). This sequence was only partially similar to the typical Hofmeister cation series, whereas at low salt concentration the cation effect on T(cp) of the dendritic polymer was insignificant and no obvious specific ranking order could be found.

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.

Synthesis of thermoresponsive hyperbranched polyamidoamine and the molecular weight, pH, and anion sensitive thermoresponsive properties thereof

Hyperbranched polyamidoamine (HPAMAM) polymers, chemically analogous to the commercially available PAMAM dendrimer, were modified with isobutyric anhydride to result in isobutyramide (IBAm) terminated HPAMAMs (HPAMAM-IBAm). The aqueous solutions of HPAMAM-IBAm polymers had the lower critical solution temperature (LCST). The lower molecular-weight HPAMAM-IBAm exhibited higher LCST and the LCST difference was around 18 °C for one pseudo-generation variation. Further, the hyperbranched thermoresponsive polymers exhibited much lower LCSTs than the corresponding dendrimers with similar molecular weight. The LCST of HPAMAM-IBAm was pH sensitive. At pH below 10, the LCST increased significantly upon decreasing the pH, whereas, at pH above 10, the LCST decreased slowly with an increasing pH value. Nine sodium salts were used to measure the anion effect on the LCST of HPAMAM-IBAm. It was found that the LCST could also be modulated up or down in a broad range by simply adding a small amount of different kinds of inorganic anions. The specific ranking of inorganic anions in salting-out HPAMAM4-IBAm polymer was in accordance with the well-known Hofmeister series.

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.

A biocompatible poly(N-vinylimidazole)-dot with both strong luminescence and good catalytic activity

Poly(N-vinylimidazole) (PVIm) that contains a large amount of bio-active imidazole units was used as the sole carbon source to synthesize PVIm-dot through a one-pot hydrothermal method without any further modification and surface passivation. The measurements of X-ray photoelectron spectroscopy, dynamic light scattering, transmission electron microscopy and X-ray diffraction proved that only a slight carbonization occurred during the hydrothermal treatment of PVIm. The characterizations of 1H NMR, FTIR and thermogravimetric analysis verified that the obtained PVIm-dot well inherited the chemical structure of its precursor PVIm. Unlike PVIm, the obtained PVIm-dot showed an obvious excitation-dependent photoluminescence (PL) behavior, and its PL features were quite stable at different pH values and ionic strength. The PVIm-dot possessed low cytotoxicity and could enter cancer cells, making it a suitable candidate for bio-imaging. Moreover, the PVIm-dot still kept the catalytic activity of its imidazole units. With the catalytic hydrolysis of p-nitrophenyl acetate as the model reaction, it was found that the PVIm-dot showed good catalytic activity in this reaction and its catalytic efficiency was better than PVIm. Whats more, the variation of PL intensity during the reaction could be used as a luminescent sensor to monitor the progress of the hydrolysis reaction.

Thermoresponsive poly(vinyl alcohol) derivatives: preparation, characterization and their capability of dispersing gold nanoparticles

Three amino acid derivatives, GI, AI and VI resulted from the corresponding isobutyryl chloride modified Gly, Ala and Val, were each conjugated with some of the hydroxyl groups of poly(vinyl alcohol) (PVA) through a one-step esterification reaction, resulting in PVA-GI, PVA-AI and PVA-VI, respectively. FTIR and 1H NMR verified the successful conjugation of these amino-acid derivative units onto PVA. X-ray diffraction characterization demonstrated that the introduction of these units led to a decrease in the degree of crystallinity. Turbidity measurement showed that GI, AI and VI were all effective molecular units to make such PVA-derivatives thermoresponsive, and the phase transition temperature could be modulated in a wide range by varying the degree of substitution or altering the type of amino-acid derivative. The efficiency of these three molecular units to make PVA thermoresponsive was as follows: VI > AI ∼ GI. Dynamic light scattering (DLS) measurement demonstrated that these thermoresponsive PVA derivatives experienced a conformation transition from the loose coil to the compact and crumbled state, till the aggregated state was attained following the temperature increase. Such thermoresponsive PVA derivatives were good dispersing agents for colloidal gold nanoparticles (AuNPs) in water when enough polymers were used. A core–shell structure was proved by DLS and TEM measurements where the AuNP was covered by a thick organic shell formed by the thermoresponsive PVA derivatives. Turbidity measurement showed that the composites of such PVA derivatives and AuNPs were also thermoresponsive, and they had a little lower phase transition temperature than those of the corresponding neat thermoresponsive PVA derivatives. DLS measurement demonstrated that such thermoresponsive AuNP composites experienced a shell shrinkage and subsequent aggregation of nanoparticles during the temperature increase. Moreover, AuNPs stabilized by such thermoresponsive PVA derivatives showed better salt-resistance than those stabilized by neat PVAs.