5 Covalent Assemblies of Metal Nanoparticles—Strategies …
175
[173, 174, 177], which, in some cases are reversible, thus allowing one to turn on/off
the catalytic reaction at will [202].
As stated in the previous sections, a large volume of scientific contributions in
the field of covalent assemblies of metal NP is devoted to Au and at certain extent
to Ag, mainly because of their possible applications in several fields such as optics,
sensors, and electronics [204, 205]. Au assembled NP have also found applications
as catalysts, mainly in reduction [52, 130, 202, 206, 207] and oxidation reactions [45,
70, 71, 111, 126, 208]. Even if predominately metal NP assemblies pivot around Au
NP, other metals have been discussed and investigated. Metals such as Pd, Pt, and Ru
that possess excellent catalytic properties have also been studied for the production
of NP assemblies, which in turn can be used as catalysts [205, 209]. Some examples
of the use of metal NP assemblies as catalyst in reduction, oxidation, water splitting
as well as in other reactions are summarized in Tables 5.5, 5.6, and 5.7, respectively,
and representative systems are discussed below.
Reduction Reactions
Thin films of Au NP prepared by means of the LBL method using O-carboxymethyl
chitosan as stabilizer were used as catalysts for the reduction 4-nitrophenol by NaBH 4
[52]. The layers of the catalysts deposited on glass plates (10, 20, or 40 layers) and
the size of the Au NP (5 nm or 10–20 nm) were crucial for the measured catalytic
activity. The rate of reaction was higher with catalysts with less layers (ten layers) and
smaller NP size (5 nm), hence proportional to the available catalytic metal surface,
related to the porosity and NP size. SEM analyses were performed after catalysis,
showing that the most active catalyst was the least stable, but no leaching tests were
reported.
Au NP inside DNA hydrogel were synthesized by reduction of HAuCl 4 adsorbed
in the DNA hydrogel by NaBH 4 (Fig. 5.33) [130]. The Au nanocomposite displayed
a weak absorbance band at λ = 550 nm, with an average NP size of 2.8 ± 1.0 nm.
Reduction of 4-nitrophenol to 4-aminophenol by NaBH 4 was monitored spectroscopically. The calculated rate constant was 1.5 × 10
−3 s
−1 , which according to the
authors was faster than the rates of already reported analogous systems. However, no
recycling test or characterization of the catalysts after catalysis was given to evaluate
the robustness of this Au nanocomposite.
Two-dimensional self-assembled AuCu NP were synthesized from CuCl 2 and
HAuCl 4 in the presence of hexadecylamine by reduction with glucose, to give
well-organized assemblies into highly flexible ribbon-like structures [206]. The
morphology and the self-assembly were highly dependent on the synthetic procedure.
These assemblies were active in the photocatalytic degradation of 4-nitrophenol by
NaBH 4 , which was carried out with a 515 nm continuous laser.
1-D or 3-D assemblies of Au NP were synthesized in a controlled manner by
adjusting the pH in the reduction of HAuCl 4 by α-cyclodextrin. While alkaline solutions promoted the synthesis of isolated Au NP, higher pH produced the Au NP
assemblies. 1-D assemblies could also be obtained by the host–guest interaction
between α-cyclodextrin capped Au NP and toluene [210]. The Au NP were tested
as catalyst in the reduction of 4-nitrophenol by NaBH 4 . An effect of the size on
175
[173, 174, 177], which, in some cases are reversible, thus allowing one to turn on/off
the catalytic reaction at will [202].
As stated in the previous sections, a large volume of scientific contributions in
the field of covalent assemblies of metal NP is devoted to Au and at certain extent
to Ag, mainly because of their possible applications in several fields such as optics,
sensors, and electronics [204, 205]. Au assembled NP have also found applications
as catalysts, mainly in reduction [52, 130, 202, 206, 207] and oxidation reactions [45,
70, 71, 111, 126, 208]. Even if predominately metal NP assemblies pivot around Au
NP, other metals have been discussed and investigated. Metals such as Pd, Pt, and Ru
that possess excellent catalytic properties have also been studied for the production
of NP assemblies, which in turn can be used as catalysts [205, 209]. Some examples
of the use of metal NP assemblies as catalyst in reduction, oxidation, water splitting
as well as in other reactions are summarized in Tables 5.5, 5.6, and 5.7, respectively,
and representative systems are discussed below.
Reduction Reactions
Thin films of Au NP prepared by means of the LBL method using O-carboxymethyl
chitosan as stabilizer were used as catalysts for the reduction 4-nitrophenol by NaBH 4
[52]. The layers of the catalysts deposited on glass plates (10, 20, or 40 layers) and
the size of the Au NP (5 nm or 10–20 nm) were crucial for the measured catalytic
activity. The rate of reaction was higher with catalysts with less layers (ten layers) and
smaller NP size (5 nm), hence proportional to the available catalytic metal surface,
related to the porosity and NP size. SEM analyses were performed after catalysis,
showing that the most active catalyst was the least stable, but no leaching tests were
reported.
Au NP inside DNA hydrogel were synthesized by reduction of HAuCl 4 adsorbed
in the DNA hydrogel by NaBH 4 (Fig. 5.33) [130]. The Au nanocomposite displayed
a weak absorbance band at λ = 550 nm, with an average NP size of 2.8 ± 1.0 nm.
Reduction of 4-nitrophenol to 4-aminophenol by NaBH 4 was monitored spectroscopically. The calculated rate constant was 1.5 × 10
−3 s
−1 , which according to the
authors was faster than the rates of already reported analogous systems. However, no
recycling test or characterization of the catalysts after catalysis was given to evaluate
the robustness of this Au nanocomposite.
Two-dimensional self-assembled AuCu NP were synthesized from CuCl 2 and
HAuCl 4 in the presence of hexadecylamine by reduction with glucose, to give
well-organized assemblies into highly flexible ribbon-like structures [206]. The
morphology and the self-assembly were highly dependent on the synthetic procedure.
These assemblies were active in the photocatalytic degradation of 4-nitrophenol by
NaBH 4 , which was carried out with a 515 nm continuous laser.
1-D or 3-D assemblies of Au NP were synthesized in a controlled manner by
adjusting the pH in the reduction of HAuCl 4 by α-cyclodextrin. While alkaline solutions promoted the synthesis of isolated Au NP, higher pH produced the Au NP
assemblies. 1-D assemblies could also be obtained by the host–guest interaction
between α-cyclodextrin capped Au NP and toluene [210]. The Au NP were tested
as catalyst in the reduction of 4-nitrophenol by NaBH 4 . An effect of the size on