253
Taking advantage of CuI-catalyzed azide-alkyne cycloaddition, also known as
click reaction, Wang et al. (2016a) developed a series of ligands containing bifunctional 1,2,3-triazole for stabilization of water-solubilized gold nanoparticles of
3.0–11.2 nm. In one designed ligand, β-cyclodextrin and biocompatible polyethylene glycol were substituted on the triazole ring. The authors believed that the synthesized gold nanoparticles could have potential applications for encapsulation,
catalysis, and sensing and confirmed the catalytic activity of the gold nanoparticles
for reduction of 4-nitrophenol in the presence of NaBH 4 . Noteworthy the catalytic
activity of the Au nanoparticles stabilized by triazoles was superior (k = 7.0 × 10
−3
s
−1
,
when 0.5% Au nanoparticles is used) to that of the thiolate Au nanoparticles. This
observation was attributed to the easier removal of the triazole ligands by the substrate from the fine gold nanoparticles.
Nanoparticles Stabilized by Cyclodextrin-Based
Supramolecular Hydrogels
Moreover, more and more sophisticated catalytic systems were developed in order
to furnish active and selective catalysts playing on the confinement of the metal
nanoparticles with the substrate. Among the reported systems, hydrogel applications have become popular in a wide range of applications such as medicine, materials, and catalysis. The embedment of metal nanoparticles into the
supramolecular-structured hydrogels based on host-guest interactions in the presence of cyclodextrins was investigated.
The first example had been developed by the group of Zhang in 2009. Silver
nanoparticles were embedded in a supramolecular hydrogel made from Pluronic
®
F-68, an amphiphilic block copolymer of poly(oxyethylene)-poly(oxypropylene)poly(oxyethylene) and α-cyclodextrin (Ma et al. 2009). These colloidal suspensions
were finally used for the catalytic reduction of methylene blue in the presence of
sodium borohydride. The colloidal suspension was synthesized within two successive steps. The first step consisted into the reduction of a silver nitrate in an aqueous
solution of Pluronic
®
F-68 which played the role of reducing agent leading to the
formation of Ag nanoparticles. The second step evolved the addition of an
α-cyclodextrin solution to the pre-synthesized colloidal suspension conducting to
the formation of a gel due to the supramolecular self-assembly between
α-cyclodextrin and the block copolymer. These colloidal suspensions in the gel state
were fully characterized by viscosimetry measurements, wide-angle X-ray diffraction, and scanning electron microscopy. The authors clearly showed that gelation
time decreased with the increase of block copolymer concentration and that the
supramolecular hybrid hydrogels demonstrated to have predominantly a solid-like
behavior. The beneficial effect of α-cyclodextrin was evidenced in the catalytic
reduction of methylene blue. Indeed, when Ag nanoparticles were used in an aqueous solution of Pluronic
®
F-68, the relative absorbance of methylene blue decreased
very slowly, and when Ag nanoparticles were used in a hybrid hydrogel, the
5 Metal Nanoparticles and Cyclodextrins for Catalytic Applications
Taking advantage of CuI-catalyzed azide-alkyne cycloaddition, also known as
click reaction, Wang et al. (2016a) developed a series of ligands containing bifunctional 1,2,3-triazole for stabilization of water-solubilized gold nanoparticles of
3.0–11.2 nm. In one designed ligand, β-cyclodextrin and biocompatible polyethylene glycol were substituted on the triazole ring. The authors believed that the synthesized gold nanoparticles could have potential applications for encapsulation,
catalysis, and sensing and confirmed the catalytic activity of the gold nanoparticles
for reduction of 4-nitrophenol in the presence of NaBH 4 . Noteworthy the catalytic
activity of the Au nanoparticles stabilized by triazoles was superior (k = 7.0 × 10
−3
s
−1
,
when 0.5% Au nanoparticles is used) to that of the thiolate Au nanoparticles. This
observation was attributed to the easier removal of the triazole ligands by the substrate from the fine gold nanoparticles.
Nanoparticles Stabilized by Cyclodextrin-Based
Supramolecular Hydrogels
Moreover, more and more sophisticated catalytic systems were developed in order
to furnish active and selective catalysts playing on the confinement of the metal
nanoparticles with the substrate. Among the reported systems, hydrogel applications have become popular in a wide range of applications such as medicine, materials, and catalysis. The embedment of metal nanoparticles into the
supramolecular-structured hydrogels based on host-guest interactions in the presence of cyclodextrins was investigated.
The first example had been developed by the group of Zhang in 2009. Silver
nanoparticles were embedded in a supramolecular hydrogel made from Pluronic
®
F-68, an amphiphilic block copolymer of poly(oxyethylene)-poly(oxypropylene)poly(oxyethylene) and α-cyclodextrin (Ma et al. 2009). These colloidal suspensions
were finally used for the catalytic reduction of methylene blue in the presence of
sodium borohydride. The colloidal suspension was synthesized within two successive steps. The first step consisted into the reduction of a silver nitrate in an aqueous
solution of Pluronic
®
F-68 which played the role of reducing agent leading to the
formation of Ag nanoparticles. The second step evolved the addition of an
α-cyclodextrin solution to the pre-synthesized colloidal suspension conducting to
the formation of a gel due to the supramolecular self-assembly between
α-cyclodextrin and the block copolymer. These colloidal suspensions in the gel state
were fully characterized by viscosimetry measurements, wide-angle X-ray diffraction, and scanning electron microscopy. The authors clearly showed that gelation
time decreased with the increase of block copolymer concentration and that the
supramolecular hybrid hydrogels demonstrated to have predominantly a solid-like
behavior. The beneficial effect of α-cyclodextrin was evidenced in the catalytic
reduction of methylene blue. Indeed, when Ag nanoparticles were used in an aqueous solution of Pluronic
®
F-68, the relative absorbance of methylene blue decreased
very slowly, and when Ag nanoparticles were used in a hybrid hydrogel, the
5 Metal Nanoparticles and Cyclodextrins for Catalytic Applications