168
Y. Min et al.
with a monofunctional catechol makes it possible to break the covalent bonds between
NP and form free NP in solution. This phenomenon is nevertheless slow (5 days for
aggregation and 42 days for disaggregation). Qu and coworkers also took advantage
of the reversibility of boron chemistry to produce Au NP networks that are composed
of covalent and readily reversible spiroborate diester linkages [182]. Au NP stabilized
with thioctic amides terminated by d-lactose (d-Lac) were cross-linked with borate
ions by forming dynamic covalent spiroborates between the cis-vicinal diol sites of
d-Lac under alkaline conditions. In the presence of external chemical stimulus, such
as cis-vicinal diols that compete for bonding with the borate, the spiroborate linkages
between the Au NP can be broken. TEM micrographs of the initial D-Lac stabilized
Au NPs and of their assembly and disassembly are shown on Fig. 5.26c–e.
The thermally reversible Diels–Alder reaction between furan and maleimides
has been also investigated to connect and disconnect Au NP (Fig. 5.27a) [102].
Maleimide-modified monolayer-protected Au NP (2-Au) were produced from
protected furan-maleimide NP (1-Au) via a thermally reversible Diels–Alder reaction. These maleimide-NP served as a general platform for a Diels–Alder reaction with furan-modified Au NP (3-Au) to prepare 3D networks reversibly. A
similar strategy was followed by Xia and coworkers, but in that case the reaction occurred through mild Diels–Alder cross-linking between maleimide bearing
oligo(p-phenylenevinylene)- OPV- and 2 nm furan-functionalized Au NP (Au-f) as
depicted in Fig. 5.27b [183].
5.2.3.2 Metal Ions as Chemical Stimuli
The interest of assembling (plasmonic) NP using metal ions comes initially from the
development of sensors to detect these often-toxic ions.
The self-assembly, disassembly, and reassembly of Au nanorods mediated by
[(disulfide-terminated tpy) 2 -M
II ] complexes (M = Fe, Cd) was investigated by
Newcome et al. [184]. The side faces of the Au nanorods are protected more
strongly by the stabilizer (cetyl trimethylammonium bromide, CTAB) than their
tips, where the ligand exchange reaction occurs preferentially with the M
II -based
cross-linker. Facile disassembly occurred upon NaOH addition for the Fe
II linker
and Cd(NO 3 ) 2 .4H 2 O addition for the Cd
II linker. The process was not reversible in
the case of iron since subsequent Fe
II addition results in the chelation of Fe
II by two
terpyridine units on the same nanorod. Reversibility was achieved using Cd
II , since
cadmium complexes with terpyridine ligands are weaker and more labile.
Metal-ion-induced reversible self-assembly of carboxylated peptidefunctionalized gold NP was reported by Mandal et al. [185]. The extent of
assembly (2-D and 3-D structures) is dependent on the amount of metal ions
(Pb
II , Cd
II , Cu
II , and Zn
II ) present in aqueous solution. The process is completely
reversible by addition of alkaline ethylenediaminetetraacetic acid (EDTA) solution
(Fig. 5.28).
In order to achieve the formation of stable NP aggregation, which is difficult to
control by binding divalent metal ions with carboxylate ligands because of charge
Y. Min et al.
with a monofunctional catechol makes it possible to break the covalent bonds between
NP and form free NP in solution. This phenomenon is nevertheless slow (5 days for
aggregation and 42 days for disaggregation). Qu and coworkers also took advantage
of the reversibility of boron chemistry to produce Au NP networks that are composed
of covalent and readily reversible spiroborate diester linkages [182]. Au NP stabilized
with thioctic amides terminated by d-lactose (d-Lac) were cross-linked with borate
ions by forming dynamic covalent spiroborates between the cis-vicinal diol sites of
d-Lac under alkaline conditions. In the presence of external chemical stimulus, such
as cis-vicinal diols that compete for bonding with the borate, the spiroborate linkages
between the Au NP can be broken. TEM micrographs of the initial D-Lac stabilized
Au NPs and of their assembly and disassembly are shown on Fig. 5.26c–e.
The thermally reversible Diels–Alder reaction between furan and maleimides
has been also investigated to connect and disconnect Au NP (Fig. 5.27a) [102].
Maleimide-modified monolayer-protected Au NP (2-Au) were produced from
protected furan-maleimide NP (1-Au) via a thermally reversible Diels–Alder reaction. These maleimide-NP served as a general platform for a Diels–Alder reaction with furan-modified Au NP (3-Au) to prepare 3D networks reversibly. A
similar strategy was followed by Xia and coworkers, but in that case the reaction occurred through mild Diels–Alder cross-linking between maleimide bearing
oligo(p-phenylenevinylene)- OPV- and 2 nm furan-functionalized Au NP (Au-f) as
depicted in Fig. 5.27b [183].
5.2.3.2 Metal Ions as Chemical Stimuli
The interest of assembling (plasmonic) NP using metal ions comes initially from the
development of sensors to detect these often-toxic ions.
The self-assembly, disassembly, and reassembly of Au nanorods mediated by
[(disulfide-terminated tpy) 2 -M
II ] complexes (M = Fe, Cd) was investigated by
Newcome et al. [184]. The side faces of the Au nanorods are protected more
strongly by the stabilizer (cetyl trimethylammonium bromide, CTAB) than their
tips, where the ligand exchange reaction occurs preferentially with the M
II -based
cross-linker. Facile disassembly occurred upon NaOH addition for the Fe
II linker
and Cd(NO 3 ) 2 .4H 2 O addition for the Cd
II linker. The process was not reversible in
the case of iron since subsequent Fe
II addition results in the chelation of Fe
II by two
terpyridine units on the same nanorod. Reversibility was achieved using Cd
II , since
cadmium complexes with terpyridine ligands are weaker and more labile.
Metal-ion-induced reversible self-assembly of carboxylated peptidefunctionalized gold NP was reported by Mandal et al. [185]. The extent of
assembly (2-D and 3-D structures) is dependent on the amount of metal ions
(Pb
II , Cd
II , Cu
II , and Zn
II ) present in aqueous solution. The process is completely
reversible by addition of alkaline ethylenediaminetetraacetic acid (EDTA) solution
(Fig. 5.28).
In order to achieve the formation of stable NP aggregation, which is difficult to
control by binding divalent metal ions with carboxylate ligands because of charge