5 Covalent Assemblies of Metal Nanoparticles—Strategies …
151
From this analysis of the literature dealing with direct cross-linking methods, it is
clear that the direct covalent bonding of metal NP into assemblies is highly impacted
by the choice of the multidentate ligand. The chain length, the binding group, and
middle backbone tune the properties of the final material at different levels. Deep
insight into the self-assembly process was obtained at both the nano- and atomic
levels by a vast panel of characterization techniques, as well as modeling. A large
proportion of work involved Au and Ag, particularly because of potential applications
in optics, sensors, and electronics. Less work has been devoted to other noble metals
of interest for catalysis, such as Pt, Pd, or Ru, meaning that additional efforts are
needed to develop reliable methods to produce covalent networks of NP with these
metals.
5.2.2 Indirect Cross-Linking Methods
Besides the direct cross-linking methods that often involved the use of ditopic ligands
to directly link metal NP, several indirect methods have also been reported. These
methods involved the reaction between NP stabilized by specific ligands, which allow
a coupling reaction between them to create the metal NP network. Various reactions
can occur (click chemistry, Diels–Alder reaction, nucleophilic substitution…) either
under thermal- or photo-activation (see Table 5.3 for representative examples).
5.2.2.1 Click Chemistry
Click chemistry, a group of chemical reactions with favorable reaction rate and
orthogonality, is an efficient way for NP modification [100], including their assembly.
Trials with NP have been reported based on click chemistry, including coppercatalyzed cycloaddition, strain-promoted azide–alkyne cycloaddition [101], and
inverse-demand Diels–Alder reaction [102].
Metallic NP can be modified by azide- and/or alkyne-functional groups, and then
assembled into organized arrays through the basic Cu(I)-catalyzed azide/alkyne“click” chemistry approach (Scheme 5.2).
Two procedures have been followed, the use of a mixture of NP (one population being azide-functionalized and the other one alkyne-functionalized) and the
reaction of azide- or alkyne-functionalized NP with a dialkyne or a diazide (triand tetra-azides have also been used), respectively. The Huisgen 1,3-cycloaddition
reaction between azide and ethynyl groups has been employed to obtain assemblies
(nanochains) of Au nanorods [103]. Using a 1:1 mixture of the nanorods stabilized
by azidoalkane- and alkyne-thiols allows the formation of chain-like assemblies. The
preferred end-to-end assembly of the Au nanorods could be attributed to the preferential ligand displacement at the (111) faces at the end of the nanorods. In the case
of Au NP (2 nm), Au NP networks with an inter-particle distance of 2 nm (which fits
with the molecular length of 1.8 nm for the rigid azobenzene unit) were obtained by
151
From this analysis of the literature dealing with direct cross-linking methods, it is
clear that the direct covalent bonding of metal NP into assemblies is highly impacted
by the choice of the multidentate ligand. The chain length, the binding group, and
middle backbone tune the properties of the final material at different levels. Deep
insight into the self-assembly process was obtained at both the nano- and atomic
levels by a vast panel of characterization techniques, as well as modeling. A large
proportion of work involved Au and Ag, particularly because of potential applications
in optics, sensors, and electronics. Less work has been devoted to other noble metals
of interest for catalysis, such as Pt, Pd, or Ru, meaning that additional efforts are
needed to develop reliable methods to produce covalent networks of NP with these
metals.
5.2.2 Indirect Cross-Linking Methods
Besides the direct cross-linking methods that often involved the use of ditopic ligands
to directly link metal NP, several indirect methods have also been reported. These
methods involved the reaction between NP stabilized by specific ligands, which allow
a coupling reaction between them to create the metal NP network. Various reactions
can occur (click chemistry, Diels–Alder reaction, nucleophilic substitution…) either
under thermal- or photo-activation (see Table 5.3 for representative examples).
5.2.2.1 Click Chemistry
Click chemistry, a group of chemical reactions with favorable reaction rate and
orthogonality, is an efficient way for NP modification [100], including their assembly.
Trials with NP have been reported based on click chemistry, including coppercatalyzed cycloaddition, strain-promoted azide–alkyne cycloaddition [101], and
inverse-demand Diels–Alder reaction [102].
Metallic NP can be modified by azide- and/or alkyne-functional groups, and then
assembled into organized arrays through the basic Cu(I)-catalyzed azide/alkyne“click” chemistry approach (Scheme 5.2).
Two procedures have been followed, the use of a mixture of NP (one population being azide-functionalized and the other one alkyne-functionalized) and the
reaction of azide- or alkyne-functionalized NP with a dialkyne or a diazide (triand tetra-azides have also been used), respectively. The Huisgen 1,3-cycloaddition
reaction between azide and ethynyl groups has been employed to obtain assemblies
(nanochains) of Au nanorods [103]. Using a 1:1 mixture of the nanorods stabilized
by azidoalkane- and alkyne-thiols allows the formation of chain-like assemblies. The
preferred end-to-end assembly of the Au nanorods could be attributed to the preferential ligand displacement at the (111) faces at the end of the nanorods. In the case
of Au NP (2 nm), Au NP networks with an inter-particle distance of 2 nm (which fits
with the molecular length of 1.8 nm for the rigid azobenzene unit) were obtained by