223
The same authors studied the synthesis of palladium nanoparticles stabilized by
β-cyclodextrin by warming an aqueous solution of Na 2 PdCl 4 containing
β-cyclodextrin (1% in weight) at 60 °C via the classical polyol process (Willner and
Mandler 1989). The non-reduced palladium was removed by addition of an
Amberlyst
®
resin, and the resulting colloids were centrifuged to separate the precipitated palladium colloids. This preparation resulted in an active catalyst for the
photosensitized reduction of sodium bicarbonate to sodium formate by visible light
in the presence of deazariboflavin as photosensitizer, N, N′-dimethyl-4,4′bipyridinium as the first electron donor, and sodium oxalate as the sacrificial electron donor. After kinetic studies, the authors emphasized the biomimetic character
of these colloids acting as artificial enzymes. By comparing their Pd nanoparticles
to other Pd nanoparticles stabilized by classical agents such as glucose or poly(Nvinyl- 2-pyrrolidone), the authors clearly showed that the best catalytic activity was
obtained with their colloids.
Despite numerous studies concerning the synthesis of gold nanoparticles in the
presence of cyclodextrins, only a few of them was dedicated to catalytic applications. The first catalytic application was reported in 2009, when the group of Qi
synthesized α-cyclodextrin-capped gold nanoparticles by reduction of HAuCl 4 in
alkaline solution at 60 °C where α-cyclodextrin was playing the dual role of reducing agent of the metal precursor and stabilizing agent of Au nanoparticles (Huang
et al. 2009). As in the case of palladium, Au(III) can be reduced via the polyol process in alkaline medium using the cyclodextrin hydroxyls as reductant. The concentration of α-cyclodextrin was the key parameter to get the smallest particles with a
narrow size distribution; the higher the α-cyclodextrin concentration, the smaller the
nanoparticles. The sodium hydroxide concentration played also an important role.
For pH values lower than 10.5, α-cyclodextrin failed to reduce the gold precursor,
but too high pH (pH = 12) led to the irreversible agglomeration of Au nanoparticles.
These colloidal suspensions were effective hydrogen activators for the reduction of
4-nitrophenol to 4-aminophenol with a large excess of sodium borohydride. The
conversion was determined by the time-dependent decay of the 4-nitrophenol absorbance at 400 nm. The kinetic reaction rate constants were inversely proportional to
the particle size. These results pointed out that these α-cyclodextrin-capped Au
nanoparticles showed catalytic activity and that cyclodextrin did not disturb the
metal surface.
More recently, monodisperse Au nanoparticles with diameter of 15–20 nm were
synthesized by the polyol process (100 °C in a phosphate buffer solution) using
native cyclodextrins (α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin) as both
reducing agents of metal precursor and protective agents of Au colloidal suspensions. These Au nanoparticles were used for applications in fluorescent sensing,
self-assembly, and cascade catalysis (Zhao et al. 2016). The FTIR analysis clearly
showed the decrease of the intensity of the hydroxyl group absorbance band. In the
same time, the appearance of carboxyl groups was observed by X-ray photoelectron
spectroscopy due to the oxidation of the cyclodextrins. All of these analyses respectively justified the reduction of the gold precursor in the presence of cyclodextrins
and the stabilization of the resulting Au nanoparticles through the carboxylate
5 Metal Nanoparticles and Cyclodextrins for Catalytic Applications
The same authors studied the synthesis of palladium nanoparticles stabilized by
β-cyclodextrin by warming an aqueous solution of Na 2 PdCl 4 containing
β-cyclodextrin (1% in weight) at 60 °C via the classical polyol process (Willner and
Mandler 1989). The non-reduced palladium was removed by addition of an
Amberlyst
®
resin, and the resulting colloids were centrifuged to separate the precipitated palladium colloids. This preparation resulted in an active catalyst for the
photosensitized reduction of sodium bicarbonate to sodium formate by visible light
in the presence of deazariboflavin as photosensitizer, N, N′-dimethyl-4,4′bipyridinium as the first electron donor, and sodium oxalate as the sacrificial electron donor. After kinetic studies, the authors emphasized the biomimetic character
of these colloids acting as artificial enzymes. By comparing their Pd nanoparticles
to other Pd nanoparticles stabilized by classical agents such as glucose or poly(Nvinyl- 2-pyrrolidone), the authors clearly showed that the best catalytic activity was
obtained with their colloids.
Despite numerous studies concerning the synthesis of gold nanoparticles in the
presence of cyclodextrins, only a few of them was dedicated to catalytic applications. The first catalytic application was reported in 2009, when the group of Qi
synthesized α-cyclodextrin-capped gold nanoparticles by reduction of HAuCl 4 in
alkaline solution at 60 °C where α-cyclodextrin was playing the dual role of reducing agent of the metal precursor and stabilizing agent of Au nanoparticles (Huang
et al. 2009). As in the case of palladium, Au(III) can be reduced via the polyol process in alkaline medium using the cyclodextrin hydroxyls as reductant. The concentration of α-cyclodextrin was the key parameter to get the smallest particles with a
narrow size distribution; the higher the α-cyclodextrin concentration, the smaller the
nanoparticles. The sodium hydroxide concentration played also an important role.
For pH values lower than 10.5, α-cyclodextrin failed to reduce the gold precursor,
but too high pH (pH = 12) led to the irreversible agglomeration of Au nanoparticles.
These colloidal suspensions were effective hydrogen activators for the reduction of
4-nitrophenol to 4-aminophenol with a large excess of sodium borohydride. The
conversion was determined by the time-dependent decay of the 4-nitrophenol absorbance at 400 nm. The kinetic reaction rate constants were inversely proportional to
the particle size. These results pointed out that these α-cyclodextrin-capped Au
nanoparticles showed catalytic activity and that cyclodextrin did not disturb the
metal surface.
More recently, monodisperse Au nanoparticles with diameter of 15–20 nm were
synthesized by the polyol process (100 °C in a phosphate buffer solution) using
native cyclodextrins (α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin) as both
reducing agents of metal precursor and protective agents of Au colloidal suspensions. These Au nanoparticles were used for applications in fluorescent sensing,
self-assembly, and cascade catalysis (Zhao et al. 2016). The FTIR analysis clearly
showed the decrease of the intensity of the hydroxyl group absorbance band. In the
same time, the appearance of carboxyl groups was observed by X-ray photoelectron
spectroscopy due to the oxidation of the cyclodextrins. All of these analyses respectively justified the reduction of the gold precursor in the presence of cyclodextrins
and the stabilization of the resulting Au nanoparticles through the carboxylate
5 Metal Nanoparticles and Cyclodextrins for Catalytic Applications