Light-triggered covalent assembly of gold nanoparticles in aqueous solution

Article abstract of DOI:10.1039/c0cc03361h

UV light irradiation triggers Au NPs that are respectively functionalized on the surface by o-nitrobenzyl alcohol and benzylamine to proceed with a covalent ligation reaction, which leads to assembling of Au NPs into anisotropic one-dimensional (1D) array

Full text of DOI:10.1039/c0cc03361h

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COMMUNICATION
Light-triggered covalent assembly of gold nanoparticles in aqueous
solutionw
Jinping Lai, Yunyan Xu, Xue Mu, Xiaoli Wu, Chong Li, Jinsheng Zheng, Chuanliu Wu,
Jianbin Chen and Yibing Zhao*
Received 19th August 2010, Accepted 9th February 2011
DOI: 10.1039/c0cc03361h
UV light irradiation triggers Au NPs that are respectively
functionalized on the surface by o-nitrobenzyl alcohol and
benzylamine to proceed with a covalent ligation reaction, which
leads to assembling of Au NPs into anisotropic one-dimensional
(1D) arrays in aqueous solution via indazolone linkages.
Scheme 1 Light-triggered reaction of o-nitrobenzyl alcohol with
benzylamine in aqueous solution.
Assemblies of inorganic nanoparticles (NPs) in aqueous solution
into one-, two- or three-dimensional architectures on the
nanoscale have received considerable recent attention because
of their potential applications in fields like electronics, photonics,
catalysis and sensors.1 Self-assembly provides a simple and
low-cost means for producing ensembles of NPs in a controllable
manner. A variety of approaches have been developed for
assembling of NPs that are based on noncovalent interactions
such as hydrogen bonding, electrostatic interaction and
van der Waals interaction, specific biorecognition, chemical
conjugation, polar interactions or employing polymer and
biomacromolecules as binders.1,2 Among them, approaches
involving covalent interactions are attractive as they produce
NPs assemblies of higher stability and not susceptible to
disassembly under various solution conditions, e.g., salt,
pH and temperature.3 However, only a few covalent reactions,
i.e. gold–thiol bonding,3a–d amide chemistry,3e,f Diels–Alder
reaction3g and click reaction4,5 have been employed for covalent
self-assembling of NPs. A challenge which remains in NPs
assembling by most of the available approaches is that they
usually lead to spherical or irregularly shaped aggregates,
rather than specific architectures such as anisotropic chains,
sheets, vesicles or three-dimensional (3D) crystals that are more
valuable for the fabrication of optoelectronics and biosensor
devices.1 Therefore, approaches for covalently fabricating NPs
in aqueous solution into anisotropic architectures are of
particular interest and highly demanded.
irradiation (Scheme 1).6 The photolysis of o-nitrobenzyl alcohol
rapidly generates o-nitrosobenzaldehyde in a quantum yield of
0.45, which subsequently condenses with benzylamine to lead
to a thermally stable 2-(N-benzyl)indazolone with a reaction
rate constant of 370 MÀ1 sÀ1 and a yield of 98%.
Herein reported is the preparation of 1D arrays of gold nano-
particles (Au NPs) in aqueous solutions via this light-triggered
coupling reaction. We show that the photolysis of light-
responsive o-nitrobenzyl derivative thioctic acid 3-(3-hydroxy-
methyl-4-nitrophenoxy)-propyl ester (TNB) functionalized Au
NPs (TNB–Au NPs) by UV light triggers the condensa-
tion with benzylamine functionalized neighboring Au NPs
[thioctic acid 4-aminomethylbenzylamide (TBA) functionalized
Au NPs, TBA–Au NPs], that couples these NPs into assemblies
by the indazolone linkages (Scheme 2). The topological
features of the assemblies were found to be dependent on
the irradiation time and the concentration of NPs. With
increasing time of irradiation on dilute solution, anisotropic
short discrete chains, long branched chains, looped chains
and a chain network were progressively produced, which is
driven by the anisotropic electrostatic repulsions during the
In a previous study, we have shown that o-nitrobenzyl
alcohol and benzylamine in aqueous solution can undergo
an aldehyde–amine ligation-like coupling reaction under UV
Department of Chemistry and the Key Laboratory of Analytical
Sciences of Xiamen University, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China.
E-mail: ybzhao@xmu.edu.cn
w Electronic supplementary information (ESI) available: Experimental
and spectroscopic data. See DOI: 10.1039/c0cc03361h
Scheme 2 Schematic illustration of light-triggered covalent assembling
of Au NPs.
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self-assembling of the charged NPs.7 With solutions containing
relatively high concentration of NPs, however, complicated
aggregates were found to be dominant. Unlike available
fabricating methods for 1D NPs arrays,2,3,5 the light-triggered
assembling strategy does not need any cross-linking ligand or
template, thereby representing a spatiotemporally controllable
means of covalently assembling NPs in aqueous solutions
because of the non-invasive and high spatiotemporal resolu-
tion character of light.
Further irradiation produced a continuous increase in the
absorbance of the low energy band and the spectrum finally
shifted to the red to 675 nm, indicating a lengthening of the Au
chains.10 Consistent with the absorption spectral change, the
solution turned to blue from the original deep red colour.
TEM images corresponding to the above spectral changes
clearly reveal the formation of highly anisotropic 1D Au
nanochains. Au NPs were found well dispersed in aqueous
solutions as isolated NPs before being exposed to UV light
(Fig. 2a). By increasing irradiation time from 5 to 15 min,
tri- and tetra-NPs chain-like structures (Fig. 2b), short discrete
chains (Fig. 2c) and complex chains of increasing length
and branching (Fig. 2d) were developed in the solution in
sequence. The Au chains seem to form networks by loop upon
a further irradiation (Fig. 2e). It is worth noting from the
TEM images that after 30-min irradiation almost all the
Au NPs were assembled together with negligible discrete
NPs remaining, demonstrating a rapid and highly efficient
assembling of the functionalized Au NPs under UV light.
Fig. 2f shows a linear chain 200 nm long consisting of 12 NPs.
The distance between successive NPs revealed by high resolution
transmission electron microscopy (HRTEM) (Fig. 2g) is
1.5 Æ 0.4 nm, which is smaller than the size of the formed
indazolone linker that is 3.64 nm in length (calculated by
gaussian view). This suggests that the observed chain is brought
together by the molecular linkers, which are in a molecular
folding status.11
To apply the light-triggered coupling reaction in the assembling
of Au NPs, Au NPs were functionalized by thiol ligands, TNB
and TBA that were obtained from reactions of o-nitrobenzyl
alcohol and benzylamine respectively with thioctic acid (TA).
The light-triggered crosslinking property of these thiol ligands
in aqueous solutions was monitored by UV-visible absorption
spectra and APCI-MS analysis (Fig. S1 and S2, ESIw). Au NPs
with a diameter of 23 nm were obtained by a sodium citrate
reduction method.8 TNB–Au NPs and TBA–Au NPs were
prepared by refluxing the citrate-stabilized Au NPs with the
corresponding thiol ligands TNB and TBA, respectively. TA
was employed as a stabilizing agent together with TNB and
TBA to allow the Au NPs to be stably dispersed. The molar
ratios of TA to the functionalized thiols loaded on nano-
particles were estimated to be 100 : 1 (for details see ESIw). The
Au NPs were further purified by repeated centrifugations and
redispersions in H2O to remove the excess thiols. The as
prepared TNB–Au NPs and TBA–Au NPs were well dispersed
in aqueous solution and both showed an intense surface
plasmon resonance at 520 nm.
We next examined the light-triggered assembling of Au NPs
in relatively concentrated solution. It was found that, with
increasing concentration, the formation of linear super-
structures was inhibited and aggregates were instead preferred.
In a solution of Au NPs with the absorbance at 520 nm of
0.18, a 30-min UV irradiation led to chain assemblies together
with irregular aggregates (Fig. 3a). At a higher concentration
of Au NPs (A520 nm = 0.49), only aggregates were observed
after irradiation (Fig. 3b). In control experiments with the
solutions containing only the TNB–Au NPs or TBA–Au NPs,
no notable spectral change was observed after 45-min irradia-
tion (Fig. S3), indicating that Au NPs do not assemble in these
cases. These observations demonstrate that the functionalized
The light-triggered assembling of Au NPs was investigated
by absorption spectroscopy and transmission electron micro-
scopy (TEM). Assembling of Au NPs in a dilute solution
(A520 nm = 0.074, concentration of Au NPs was indicated by
the absorbance at 520 nm) was investigated first. The mixture
of the two kinds of functionalized Au NPs in pH 7.4 PBS
showed the isotropic surface plasmon band at 520 nm (Fig. 1)
and no change was observed after allowing the solution to
stand in the dark for 24 h. Irradiation of the solution by UV
light (l 4 310 nm) for 5 min induced a slight increase in the
absorbance beyond 550 nm. However, a 10-min exposure led
to a significant reduction in the absorbance of the 520 nm
band as well as the development of a new band at 650 nm that
is characteristic of the Au nanochains due to excitation of the
plasmon resonance along the long axis of the nanochains.9
Fig. 2 TEM micrographs of the Au NPs after UV irradiation
of increasing time: (a) 0, (b) 5, (c) 10, (d) 15 and (e) 30 min. Images
(f) and (g) are the HRTEM characterizations of an obtained Au NPs
nanochain.
Fig. 1 UV-visible spectra of TNB–Au NPs and TBA–Au NPs in
pH 7.4 PBS after irradiation with increasing time.
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the TNB–Au NPs in dilute solution under UV light tend to
couple TBA–Au NPs in the form of chains and networks while
aggregates were preferred in the solutions of Au NPs of higher
concentration. This strategy represents a spatiotemporally
controllable means for covalently assembling Au NPs into
1D architectures in aqueous solutions and can in principle be
extended to other applications in material sciences and
biochemistry.
Fig. 3 TEM micrographs of the Au NPs solutions of different concen-
We thank Prof. Yunbao Jiang for suggestions and editing of
the English while revising the paper. This work was supported
by the National Natural Science Foundation of China
(20875079, 20835005), The Planned Science and Technology
Project of Xiamen, China (3502z20080011) and the Specialized
Research Fund for Doctoral Program of Higher Education of
China (200803840007).
tration after 30-min UV irradiation: A520 nm = 0.18 (a) and 0.49 (b).
Notes and references
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Scheme 3 Electrostatic repulsions between NPs during the assembling.
Au NPs self-assemble only under UV irradiation when a
covalent reaction takes place and the superstructure of assemblies
can be facilely regulated by varying UV irradiation time and/or
the concentration of NPs.
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The formation of anisotropic 1D Au NPs arrays can be
rationally attributed to the anisotropic electrostatic repulsions
during the self-assembling of the charged NPs. The mixture of
the two kinds of Au NPs is stable in PBS solution because
of the strong electrostatic repulsion among the NPs that
were negatively charged from the TA ions. Photolysis of
the TNB–Au NPs generates NPs counterparts bearing now
o-nitrosobenzaldehyde groups that act as reactive sites towards
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In conclusion, covalent fabrication of 1D arrays of Au NPs
in aqueous solution was achieved by using a light-triggered
coupling reaction between o-nitrobenzyl alcohol and benzyl-
amine moieties respectively functionalized on the surface of
NPs. Photolysis of TNB functionalized Au NPs surface
under UV light generates reactive sites on the surface of NPs
towards TBA–Au NPs. Balanced by electrostatic repulsion,
c
3824 Chem. Commun., 2011, 47, 3822–3824
This journal is The Royal Society of Chemistry 2011