Noble metal nanochains through helical self-assembly

Article abstract of DOI:10.1039/b924724f

Silver nanochains composed of nanoparticles were prepared by photoreduction in situ under assistance of a helical self-assembly originating from coordination of Congo red and Ag⁺. The special hierarchical nanostructures with different surface plasma resonance compared with silver nanoparticles have potential applications for electrical nanodevices.

Full text of DOI:10.1039/b924724f

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Noble metal nanochains through helical self-assemblyw
Jinbo Fei, Yue Cui, Anhe Wang, Pengli Zhu and Junbai Li*
Received (in Cambridge, UK) 24th November 2009, Accepted 4th January 2010
First published as an Advance Article on the web 23rd January 2010
DOI: 10.1039/b924724f
Silver nanochains composed of nanoparticles were prepared by
photoreduction in situ under assistance of a helical self-assembly
originating from coordination of Congo red and Ag+. The
special hierarchical nanostructures with different surface plasma
resonance compared with silver nanoparticles have potential
applications for electrical nanodevices.
Congo red might have interacted by coordination effect
between Ag+ and –NQN–, –NH2, –SO3À. A drop of the
mixture (Fig. 1Sbw) was put onto silicon plates and dried
under vacuum at room temperature (25 1C). The experiment
was operated in a dark room. Meanwhile, after 24 h, the
solution (Fig. 1Sbw) was under vis-light for 48 h. At this time
the colour had disappeared and there was a yellow substance
at the bottom of the flask. After concentration and purifi-
cation, the samples were characterized by scanning electron
microscopy (SEM), transmission electron microscopy (TEM),
atom force microscopy (AFM), X-ray diffraction (XRD) and
UV-vis spectroscopy.
Over the past decade, one-dimensional nanostructures have
attracted much attention because of the beneficial influence
of their dimensionality on electronic and optical materials
properties.1–6 Exciting examples involve the chemical vapor
deposition preparation of carbon nanotubes (CNTs),1 semi-
conductor nanowires, nanotubes and nanobelts,2,3 and the
self-assembly of cyclindrical micelles,4,5 ribbons,6 and peptide
nanofibers and nanobelts.7,8 Particularly, self-assembly of
small molecules offers many opportunities for obtaining one-
dimensionally hierarchical nanostructures.9–18 Pioneered by
Braun, Belcher and their coworkers, there have been numerous
examples of DNA or viruses as scaffolds for complex nano-
structured inorganic materials.19
From Fig. 1a, large-scale helical precursors were shown.
The diameter of the precursor was 100–200 nm and the width
of them was about 40 nm (Fig. 1b). Some of them had
branched structures, shown in Fig. 1c. TEM image also
showed the helical nanobelt microstructures (Fig. 1d). Our
understanding of CR ligation was further complicated by the
fact that CR might exist in two different forms: as unimolecular
and supramolecular ligand species.29 In our case, in the presence
of Ag+, elongated symmetric polyaromatic ring molecules
with planar structure forming the hydrophobic center and
charged terminals make CR prone to self-aggregation in helical
aggregates.30 The result of the energy dispersive X-ray (EDX)
analysis (Fig. S3w) confirmed that the helical precursor consisted
of the elements of C, N, O, S and Ag, a chemical composition
that agreed with the C32H22N6Ag3O6S2 circuitously. It indicated
that in the precursor, the ratio of Congo red and Ag+ was
about 1 : 3.
Noble metal nanostructures, such as nanoparticles, nano-
plates, nanowires and nanobelts, have attracted extensive
research interest due to their potential applications in catalysis,
electronics and biology.20–23 As a typical noble metal, various
silver nanostructures are obtained by different methods.24–27
Special structures such as helical nanostructures could result in
interest and unique optical and electromagnetic properties.28
In this paper, Ag chainlike hierarchical nanostructures based
on helical small-molecular self-assembly of AgNO3 and Congo
red (CR, C32H22N6Na2O6S2) were introduced.
After photo-induced reduction under visible light for 48 h, a
typical TEM image (Fig. 2a) shows that there still was similar
morphology of the precursor. In detail, compared with
In a typical experiment, 10 mL of 2.5 mM AgNO3 was
mixed with 20 mL of Congo red (100 mg mLÀ1) in a flask. As
shown in Fig. S1w, obviously, the colour of the solution was
changed from red to purple. Fig. S2w presents the solution
Uv-vis spectra before and after mixing. There was only little
difference in the ultraviolet region, indicating that Ag(I) and
Beijing National Laboratory for Molecular Sciences CAS Key Lab of
Colloid and Interface Science, Institute of Chemistry Chinese
Academy of Sciences, Beijing 100190, P.R. China.
E-mail: jbli@iccas.ac.cn; Fax: (+86)10 82612629
w Electronic supplementary information (ESI) available: Experimental
details and relevant characterizations of the precursor. See DOI:
10.1039/b924724f
Fig. 1 (a) Low-magnification SEM image of the precursor; (b) higher-
magnification SEM image of single precursor; (c) SEM image of
branched microstructure; (d) TEM image of the precursor.
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Fig. 2 (a) TEM image of the final product; (b) relevant SAED
Fig.
4
AFM images of Ag nanochains (a) low-magnification;
pattern.
(b) higher-magnification.
Fig. 1d, in the terms of contrast between the substrate
(Cu grid) and the products, it could be induced that there
was more heavy element in the final product than that in
the precursor. Selected area electron diffraction (SAED) of the
final product was shown in Fig. 2b. It indicates that the
product was cubic single crystalline. From the XRD pattern
(Fig. 3), the peaks of the product can be attributed to the bulk
cubic Ag patterns. The diffraction features appearing at about
38.11, 44.11 and 64.41, correspond to the (111), (200) and (220)
planes of the cubic phase of Ag, respectively.24 Based on the
pattern, the lattice constant was 0.4091 nm, which is very close
to the reported data (JCPDS File 04-0783).
the short (transverse band) and long (longitudinal band) axes of
these systems.31,32 Spectrum a in Fig. 5 shows the UV-vis
absorption spectrum of silver nanochains suspended in water.
One peak corresponding to the short axes of the nanochains
appears at about 420 nm; the other peak occurs at about
650 nm, which can be attributed to the short aspect ratio of
the Ag nanochains. As a comparison, for Ag nanoparticles with
a diameter of approximately 40 nm synthesized in the presence
of citrate acid, the characteristic surface plasmon band is
observed at 420 nm (spectrum b in Fig. 5). Thus, the appearance
of the absorption band actually mirrors the appearance of the
different nanostructures of silver.
In order to obtain more details, atom force microscopy
(AFM) was employed to investigate the morphology of the Ag
nanochains prepared. As shown in Fig. 4a, there were many
Ag nanochains with the length of 300–400 nm on the surface
of the Si substrate, which were shorter than the initial helical
organic template nanofibres. The reason may be the structural
shrinkage and partial fracture after photoreduction in situ. In
detail, Ag nanochains were composed of caved Ag nano-
particles with the diameter of about 40 nm (shown Fig. 4b),
which was near to the width of the Ag–Congo red com-
plex nanohelix (Fig. 1). So, it could be proposed that Ag
nanochains were evolved from the nanohelices by reduction
under vis-light.
It is further noted that metal ions in the subphase can affect
the precursor helical formation. Among Cu(II), Ni(II), Cd(II),
Ca(II), Zn(II), and Ag(I), only in the case of Ag(I) did we get the
helical nanostructures (data not shown). This suggested that
all these metal ions except Ag(I) block the propagation of the
assembly backbone in a helical sense. Furthermore, no helical
precursors can be obtained from Congo red solution. After all,
a proposed mechanism was as follows (Fig. 6): Ag(I)–Congo
red helical nanobelts were formed by small molecular self-
assembly. Then, under visible light, Ag nanochains were
reduced from the helical nanobelts.
In conclusion, we have successfully synthesized silver nano-
chains based on supramolecular co-assembly. This method is
simple, convenient and economical and can be performed
under ambient conditions. The Ag nanochains have the
potential for incorporation into functional electronic, opto-
electronic, or sensing devices, which could lead to extreme
miniaturization and enhanced performance. In addition, such
Silver nanomaterials show unique optical properties as a
result of surface plasmon resonance. It is well known that
spherical silver nanoparticles exhibit a single absorption band
at 420 nm, whereas anisotropic silver particles show two surface
plasmon resonance absorption peaks that are characteristic of
Fig. 5 The UV-vis spectra of (a) Ag nanochains and (b) Ag nano-
Fig. 3 XRD pattern of the final product prepared.
particles dispersed in water.
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We acknowledge the financial support of this research by the
National Nature Science Foundation of China (No. 20833010),
the Chinese Academy of Sciences as well as the German
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