Polyamine ligand-mediated self-assembly of gold and silver nanoparticles into chainlike structures in aqueous solution: Towards new nanostructured chemosensors

Article abstract of DOI:10.1002/open.201300023

1D Nanochain formation: The binding ability of a polyamine molecular linker (L)^2- bearing different functional groups, which favors the self-assembling of silver (AgNPs) and gold nano-particles (AuNPs) into 1D nanochains in aqueous solution was explored. UV/Vis spectrophotometry and TEM were used to determine time-dependent structural changes associated with these 1D structure formations. Sensing of Hg²⁺ using AgNPs@ (L) ^2- and AuNPs@ (L)^2- assemblies was also carried out in aqueous solution.

Full text of DOI:10.1002/open.201300023

DOI: 10.1002/open.201300023
Polyamine Ligand-Mediated Self-Assembly of Gold and
Silver Nanoparticles into Chainlike Structures in Aqueous
Solution: Towards New Nanostructured Chemosensors
Adriꢀn Fernꢀndez-Lodeiro,[a] Javier Fernꢀndez-Lodeiro,[a] Cristina NfflÇez,*[a, b] Rufina Bastida,[c]
JosØ Luis Capelo,[a] and Carlos Lodeiro*[a]
Polyamine ligands are very versatile compounds due to their
water solubility and flexibility. In the present work, we have ex-
ploited the binding ability of a polyamine molecular linker
(L2ꢀ) bearing different functional groups, which favors the self-
assembling of silver nanoparticles (AgNPs) and gold nanoparti-
cles (AuNPs) into 1D nanochains in aqueous solution. The
chainlike assemblies of AuNPs and AgNPs were structurally
stable for a long period of time, during which their characteris-
tic optical properties remained unchanged. The mechanism of
AuNPs and AgNPs chain assembly associated with the induc-
tion of electric dipole–dipole interactions arising from the par-
tial ligand exchange of surface-adsorbed citrate ions by (L2ꢀ)
was investigated. UV/Vis spectrophotometry and transmission
electron microscopy (TEM) were used to determine time-
dependent structural changes associated with formation of the
1D nanoparticle structures. Finally, the sensing of Hg2+ in
aqueous solution using AgNPs@(L)2ꢀ and AuNPs@(L)2ꢀ assem-
blies was also carried out in aqueous solution.
Introduction
Noble metal nanoparticles (MNPs) have been intensively pur-
sued in recent years, not only for their fundamental scientific
interest[1] but also for their technological applications, ranging
from analytical sensors to catalysis and fuel cells.[2] Recently,
the attention paid to 1D nanomaterials has been increasing
significantly, because of the need to fabricate alternative func-
tional 1D nanostructures for applications in the fields of nano-
electronics and nanobiotechnology,[3] due to the fact that they
can act as interconnects between functional nanoscale compo-
nents.[4]
Several experimental routes have been recently proposed to
efficiently self-assemble preformed MNPs into 1D chains:
methods involving hard,[5] polymeric[6] or surfactant-based[7]
templates, molecular recognition,[8] specific functionalization,[9]
and surface- or solvent-induced phase separation[10] have been
successfully demonstrated.[11] Chains of MNPs also have been
prepared by using linear macromolecular templates, such as,
DNA,[12,13] peptide,[14,15] insulin fibrils,[16] protein fibrils[17,18] or
carbon nanotubes.[19]
The growth mechanism of self-assembled metal nanostruc-
tures using (macro)molecular ligands has been also reported
to exhibit common features with molecular step-growth poly-
merization.[20] Similar to functional monomers, metal nano-
structures assemble to form chains. The assembly was per-
formed by small molecules (<2 nm), called molecular linkers,
that contain at least two reactive ending groups, capable of at-
taching to a solid surface by chemisorption (thiol, amine) or in-
teracting electrostatically with other functional groups (hy-
droxy, carboxyl, amine) present on the surface of nanoparticles
(NPs).[20] The governing factor in linker-mediated assembly of
MNPs is the equilibrium between the attractive and repulsive
forces.[21] In particular, fabrication of anisotropic 1D noble MNP
chains to obtain integrated optics operating below the diffrac-
tion limit of light has attracted much attention.[22]
Stellaci and co-workers[10a] have introduced anisotropic prop-
erties on ligand-stabilized AuNPs. Face-centered cubic (fcc)
metallic NPs exhibit no intrinsic electric dipole, however, heter-
ogeneities in surface charge and polarity, associated for exam-
ple with the non-uniform spatial distribution of capping li-
gands on different crystal faces,[23] or nanophase separation in
mixed-ligand stabilization layers,[24] are possible driving forces
for anisotropic self-assembly.[25] In the case of spherical NPs,
[a] A. Fernꢀndez-Lodeiro, Dr. J. Fernꢀndez-Lodeiro, Dr. C. NfflÇez,
Dr. J. L. Capelo, Dr. C. Lodeiro
BIOSCOPE Group, REQUIMTE-CQFB, Chemistry Department
Faculty of Science and Technology, University NOVA of Lisbon
2829-516, Monte da Caparica (Portugal)
E-mail: cristina.nunez@fct.unl.pt
cle@fct.unl.pt
[b] Dr. C. NfflÇez
Ecology Research Group, Department of Geographical and Life Sciences
Canterbury Christ Church University
CT1 1QU, Canterbury (UK)
[c] Prof. R. Bastida
Inorganic Chemistry Department, Faculty of Chemistry
University of Santiago de Compostela
15782 Santiago de Compostela (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/open.201300023.
ꢁ 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
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Results and Discussion
controlling the surface chemistry of the fabricated NPs allows
the creation of an anisotropic ligand organization.[26] Enthalpy
minimization, is obtained by promoting dipole alignment and
reducing interdipole distances through the formation of linear
chains of single NPs. This facilitates the orientation of specific
interactions in one direction, which helps directing the self-
assembly into 1D arrays. The self-assembly of the NPs into
a well-defined 1D array is also in-
Formation of chainlike structures from AgNPs and AuNPs
1D metallic silver or gold nanostructures, can be obtained by
exploiting the binding ability of the linear polyamine molecular
probe L in water (see chemical structure in Scheme 1). The
fluenced by interparticle chemi-
cal bonding, hydrogen bonding,
van der Waals interactions, elec-
trostatic forces, or any combina-
tion of these forces. In addition,
entropy can be maximized at
Scheme 1. Synthetic route to compound L.
finite temperature by introduc-
ing some disorder in the linear
chain, which corresponds to the incorporation of branching
junctions and to chain reticulation that should be favored at
elevated temperature.
donor atoms presented in the structure of compound L could
be responsible for the formation of NP chainlike aggregates
and the partial removal of the citrate ion from the starting
metal nanoparticle (MNP) surface. This method is similar to
that reported by Zhang et al.[36,37] and it is different to that in
which the assembly was induced by electrostatic interactions
and the disassembly was labile in the presence of stronger
chelating agents.[38]
Aggregation of NPs induces variations in absorption spectra
accompanied by significant color changes of solutions.[27] Simi-
lar color changes can be observed upon the addition of an an-
alyte, which initiates the aggregation of noble MNPs, and this
feature can be used for permitting their industrial application
in biosensing, immunological, and biochemical investiga-
tions.[28,29] In the particular case of AgNPs, the geometrical
shape also plays an important role in determining plasmon res-
onance properties.[30] For example, triangular, pentagonal, and
spherical silver particles are colored red, green, and blue, re-
spectively. Consequently, it is important to develop approaches
that can manipulate NPs into different shapes and dimensions.
While many studies have tackled the synthesis and charac-
terization of gold dimers and networks with peculiar plasmon
resonance behavior,[31] organization of AgNPs in 2D superlatti-
ces is less common.[32,33] For example, Chang et al. reported
a variety of 1D- and 2D-nanostructured assemblies formed
from AgNPs by variations in pressure, temperature, and time in
supercritical water (SCW) without the need for any external
linking agents.[34]
In that case, firstly, we assumed that the level of ionization
(protonation-deprotonation of functional groups) of potential
modifying compounds, might play a role in the process of
MNP functionalization, and consequently, could affect the sta-
bility of their dispersions. Therefore, the pH of the mixture was
modified with a NaOH solution, to an approximate value of
pH 12, which leads to the change in the linker ionization
degree. After the addition of base to a solution of L, this com-
pound was dissociated bearing negative charges L2ꢀ because
of deprotonation of the amine groups. Second, we assumed
that the chemisorption of the linker on the AuNP surface could
take place through the sulfur atom and/or by deprotonated
amine groups, leading to the modification of the z potential
from z0 (initial potential) to z (potential that determines the
equilibrium between attractive and repulsive forces).
To follow our interest in new emissive materials, and func-
tionalized NPs and to explore their applications,[35] herein, we
investigate the mechanism of AuNP and AgNP chain assembly
associated with the induction of electric dipole–dipole interac-
tions. The nanoassembly capacity arises from the partial ligand
exchange of surface-adsorbed negatively charged citrate ions,
by covalently bound neutral molecular ligand L to produce
a final mixed-ligand surface layer.
Finally, it is well known that changing the medium surround-
ing the nanoparticles (NPs) for another medium, having a mark-
edly different refractive index, strongly alters the surface plas-
mon resonance (SPR) band of the NPs in the UV/Vis spectrum.
The position, intensity, and shape of SPR band strongly de-
pends on the dielectric constant of surrounding medium, the
size and shape of NPs as well as the electronic interaction be-
tween the stabilizing ligand and NP.[39] Therefore, UV/Vis ab-
sorption spectroscopy is an important analytical tool to probe
the stability, surface chemistry, and aggregation behavior of
AgNPs and AuNPs.
We show that exchange of surface adsorbed citrate with L,
results in the formation of chain-like superstructures with topo-
logical features that are dependent on the extent of surface-
ligand substitution. We determine the time-dependent struc-
tural changes associated with the formation of 1D NP super-
structures. Morphological and optical characteristics of various
nanostructures were investigated by TEM and UV/Vis.
Formation of chainlike structures from AgNPs
Spherical shape
Initial characterization of spherical AgNPs prepared by citrate
reduction of a silver nitrate solution revealed an absorption
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The replacement of citrate
ions most probably takes place
after the addition of the nega-
tively charged ligand L2ꢀ. In that
case, the amount of negative
charge on the surface of NPs
does not decrease significantly,
and as a result, a slight change
of the z potential was observed
from z0 ꢁꢀ33.5 mVcmꢀ1 to z0 ꢁ
ꢀ27.8 mVcmꢀ1. The formations
of chain-like superstructures
with topological features are de-
pendent on the extent of sur-
face-ligand substitution. The re-
placement of citrate ions of the
AgNPs surface by L2ꢀ probably
induces less electric dipole–
dipole interactions, observing
that the 1D NP assembly takes
place less prominently (Fig-
ure 1F,G) compared with that
observed with L (Figure 1D,E).
In that case, IR spectra were
recorded to demonstrate the re-
placement of citrate ions of the
AgNP surface by polyamine
ligand L. The IR spectrum of
compound L shows peaks char-
Figure 1. UV/Vis absorption spectra of spherical AgNPs@citrate during titration to A) AgNPs@L and
B) AgNPs@(L)2ꢀ. C) Size distribution diagram for AgNPs@(L)2ꢀ. TEM images of silver nanowire formed from
D,E) AgNPs@L and F,G) AgNPs@(L)2ꢀ. Visual color changes and TEM images obtained H) before and I) after the ad-
dition of L2ꢀ to an aqueous solution of spherical AgNPs@citrate.
maximum (lmax) of the SPR peak for single particles at
ꢁ420 nm (transverse SPR band; Figure 1A,B). Analyses of TEM
imaging shows that the prepared citrate-capped silver NPs
(AgNPs@citrate) are nearly monodisperse spheres with an aver-
age size of about (25ꢂ3) nm. The AgNP concentration was es-
timated to be 7.8”10ꢀ10 in terms of molar concentration. This
value was obtained taking into account that the entire mass of
silver in AgNPs employed for colloidal dispersion preparation
was fully transformed into NPs.
acteristic of the carbonyl group at 1676 cmꢀ1 and the n˜(C=C)
stretching mode at 1436 cmꢀ1 (see Figure S1, Supporting Infor-
mation). In addition, a peak at 3262 cmꢀ1 is observed due to
the n˜(NꢀH) stretching mode. A decrease in the intensity of the
n˜(C=O) band was observed in the IR spectra of AgNPs@L in
comparison with L. The most interesting part of the spectrum
is the region from 3000 to 3200 cmꢀ1 due to n˜(NꢀH) stretching
modes. The IR spectrum for AgNPs@L is very different from the
spectrum of L in the same region. These results suggest the in-
teraction of L with the MNP surface through the carbonyl and
amine groups, confirming the replacement of the citrate ions
from the NP surface.
Figure 1A and 1B show the absorption titration of spherical
AgNPs@citrate with compound L and L2ꢀ, respectively. As men-
tioned above, because AgNPs@citrate are highly dispersed in
solution, the spectrum is characterized by a single SPR band at
ꢁ420 nm (transverse SPR band). AgNPs@L and AgNPs@(L)2ꢀ
exhibit two absorption bands, at 420 nm and a new band at
ꢁ580 nm, respectively. In both cases, the appearance of the
second band (longitudinal SPR band) is a clear evidence of the
assembly of AgNPs in solution, and a color change from yellow
(Figure 1H) to deep red (Figure 1I) was also observed.
Triangular shape
Because better results were obtained after titration of spherical
AgNPs@citrate with the deprotonated ligand L2ꢀ, we used the
same method with the triangular AgNPs@citrate. The forma-
tion of triangular AgNPs@citrate in aqueous solution with sizes
in the range of (64ꢂ10) nm (see Figure S2, Supporting Infor-
mation) was confirmed by the presence of the blue SPR band
at ꢁ700 nm in the UV/Vis spectrum (Figure 2A). As shown in
Figure 2C, the triangular AgNPs@citrate were well dispersed in
milli-Q water before adding L2ꢀ, and they remained well sepa-
rated on the TEM grid. After the addition of 5 mL of L2ꢀ
(1.10ꢀ3 m) to a solution of triangular AgNPs@citrate in aqueous
solution, a redshift in the absorption band was observed to
ꢁ780 nm, due to the formation of chains in which L2ꢀ ions
Chainlike structure formations of AgNPs in aqueous solution
were induced in both cases by L and L2ꢀ, with the contact be-
tween AgNPs being easier at higher concentrations of L and
L2ꢀ. The z potential distribution plays the major role in linker-
mediated self-assembly of MNPs, and the development of
linear chains and branched chain network is directed by the
fact that the electrostatic double layer is rearranged around
the dimers and becomes anisotropic.
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fects, loop domains) in place of
a strictly linear assembled super-
structure. The second band
shifts with time toward higher
wavelengths and its intensity
(Il2) increases. As the interparticle
spacing decreases, the first peak
becomes weaker, while the
second peak intensifies and
shifts to longer wavelengths.
The wide range of different
chain morphologies observed in
the extended networks accounts
for the broadness of the emerg-
ing longitudinal plasmon band
and the absence of isosbestic
points in the time-dependent
spectra.[15a]
The spectral change associat-
ed with the progressive aggre-
gation of the NPs into chains
and branched networks was also
demonstrated by TEM (Fig-
Figure 2. A) UV/Vis absorption spectra of triangular citrate@AgNPs during titration to AgNPs@(L)2ꢀ and B) size dis-
tribution diagram for AgNPs@(L)2ꢀ. Visual color changes and TEM images obtained C) before and D) after the addi-
tion of L2ꢀ to an aqueous solution of triangular AgNPs@citrate.
played the role of connectors between the AgNPs (Figure 2D).
As shown in Figure 2C,D, a slight color change from intense to
pale blue was also observed.
ure 3C,D). Consequently, the number of single NPs progres-
sively decreases, leading to a decrease in the intensity of the
first peak (Il1).
The z potential of the original AuNPs@citrate solution was
z0 ꢁꢀ33.5 mVcmꢀ1, which is high enough (in absolute value)
to keep the NPs electrostatically stable, and avoid aggregation
due to repulsive forces between the negatively charged citrate
ions. The use of citrate permits a controlled ligand exchange
of the ions adsorbed on the surface. This spontaneous assem-
bly is attributed to the electric dipole formed by the anisotrop-
ic organization of the ligands on the surface of the NP.[41] After
the addition of L2ꢀ, a destabilization of the system was ob-
Formation of chainlike structures from AuNPs
As a model experiment, we also used citrate-capped spherical
gold NPs (AuNP@citrate) with a hydrodynamic diameter of
(20ꢂ5) nm (see Figure S3, Supporting Information). The self-
assembly was performed using a AuNP@citrate solution with
a CNP =3.7”10ꢀ9 m (2.2”1015 particleLꢀ1) and a concentration
of 1.6”10ꢀ6 m for L2ꢀ (ꢁpH 12) (CL/CNP ꢁ4.5”102).
As we mentioned above, the
assembly of AuNPs has a signifi-
cant effect on the optical prop-
erties of the NPs, reflected by
a dramatic change of the UV/Vis
extinction value of the SPR
band. As shown in Figure 3A,
single spherical AuNPs were
characterized by an extinction
transverse plasmon band at l1
ꢁ520 nm.[40] After addition of
the linker L2ꢀ, a second low-
energy longitudinal surface plas-
mon band (l2) appears at higher
wavelengths
(630–710 nm),
which is a result of the plasmon-
ic coupling of linearly assembled
NPs. The position of this longitu-
dinal surface plasmon band (l2)
could be modified in function of
the topological distortions in the
Figure 3. A) UV/Vis absorption spectra recorded in real-time (one spectrum per minute) during the self-assembly
of AuNPs@citrate by L2ꢀ to form AuNPs@(L)2ꢀ. B) Size distribution diagram for AuNPs@(L)2ꢀ. Visual color changes
chains (Y-junction, zigzag de- and TEM images of C) AgNPs@citrate and D) gold nanowire formed from AuNPs@(L)2ꢀ
.
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tained with a value of potential zꢁꢀ27.0 mVcmꢀ1
.
The effect of amine functionality on binding to the
surface of AuNPs has been investigated,[42] and re-
cently Chegel et al. reported experimental and theo-
retically studies about the cooperative functionalities
of amine and thiol groups for aggregation of
AuNPs.[43] However, some authors have claimed that
one type of amine group can readily bind to Au col-
loids, whereas others cannot.[44] In that case, an effi-
cient surface displacement of citrate ions on the
presence of L2ꢀ could take place due to a cooperative
functionality of the deprotonated amine groups, and
the sulfur atom capable of forming stable chemical
bonds with gold atoms.
In the absence of linker molecules, the aggregation
of gold nanospheres could be mainly caused by van
der Waals attraction, yielding random and irregular
geometries and usually leading to rapid precipitation.
The presence of a functional linker as L2ꢀ in the sur-
face of NPs minimizes the effect of van der Waals at-
tractions by significantly increasing long-range inter-
actions represented by electrostatic forces.[45] Experi-
mentally speaking, the z potential distribution seems
to play the major role in linker-mediated self-assem-
bly of AuNPs. As shown in Figure 3C,D, a color
change from intense red to blue was also observed.
In this particular case, the chainlike-structured
AuNPs@(L)2ꢀ were observed after no longer reaction
time (45 min) due to connection between one parti-
cles with other NPs from solution. These superstruc-
tures remained unchanged even for prolonged incu-
bation times such as two weeks.
Figure 4. A) Spectrophotometric titration of spherical AgNPs@(L)2ꢀ with the addition of
increasing amounts of HgCl2 in aqueous solution. B) Modification with time in the ab-
sorption spectra of AgNPs@(L)2ꢀ with the addition of 6 mL of HgCl2 (1:1 L/M). TEM images
of an aqueous solution of spherical AgNPs@(L)2ꢀ C) before and D,E) after the addition of
HgCl2 (1equiv, [HgCl2]=1.10”10ꢀ3 m).
On the other hand, different results were obtained for linker
L2ꢀ adsorbed on the AuNPs surface because a more stable
bond is formed compared with that obtained for L2ꢀ and Hg2+
, explaining that the AuNPs@(L)2ꢀ assembly remains un-
Interaction of AgNPs@(L)2ꢀ and AuNPs@(L)2ꢀ with Hg2+
Trying to apply the obtained nanostructures as optical effec-
tive nanochemosensors, the sensing of Hg2+ using the colloi-
dal systems AgNPs@L2ꢀ and AuNPs@L2ꢀ was carried out in
aqueous solution. Changes in the absorption spectra of the
colloidal systems AgNPs@(L)2ꢀ with spherical (Figure 4A,B) and
triangular (see Figure 5) shapes were observed after the addi-
tion of increasing amounts of Hg(NO3)2. In both cases, a blue-
shift of the SPR bands in the UV/Vis spectra suggest a continu-
ous deformation of the chains, that is confirmed by TEM mi-
croscopy (Figure 4D,E and Figure 5D,E). The z potentials of the
changed after the interaction with this metal ion.
The sensitivity of this nanoassembly system AgNPs@(L)2ꢀ
(spherical shape) toward Hg2+ was found to be comparable to
the small fluorescent molecular system L. The value for the
limit of detection (LOD) shows that the best candidate for the
detection of this metal ion is system AgNPs@(L)2ꢀ, with the
minimum amount of Hg2+ detectable being 8.3 ppm, whereas
the LOD value with compound L was 19.0 ppm. The selectively
of system L towards Hg2+ was also explored as shown in Fig-
ure S4 and S5.
spherical and triangular AgNPs@(L)2ꢀ solutions were zspherical
ꢁ
ꢀ27.8 mVcmꢀ1 and ztriangular ꢁꢀ19.1 mVcmꢀ1, respectively. In
both cases, after the addition of Hg2+, a destabilization of the
systems was observed, and the value of z potentials changes
Conclusions
This study of 1D self-assembly of silver nanoparticles (AgNPs)
and gold nanoparticles (AuNPs) demonstrates direct evidence
of the cooperative interaction between the metal nanoparticles
(MNPs) and the deprotonated polyamine compound L2ꢀ at the
nanoscale in aqueous solution.
The unexpected symmetry breaking that occurs when L2ꢀ is
added to an aqueous suspension of citrate-capped isotropic
AgNPs and AuNPs was attributed to the ligand-mediated in-
duction of a surface electrical dipole. At least three features of
to zspherical ꢁꢀ22.5 mVcmꢀ1 and ztriangular ꢁꢀ17.0 mVcmꢀ1
.
In view of these results, we can conclude that the destabili-
zation of the spherical and triangular AgNPs@(L)2ꢀ systems and
the loss of the assembly, could be due to the more favorable
interaction of L2ꢀ with Hg2+ to that observed for L2ꢀ with
AgNPs. The exchange of L2ꢀ adsorbed on the surface produces
a decrease of the negative net charge in the NP surface, which
causes an increase of the z potentials.
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L2ꢀ with Hg2+ is more favorable
to that observed for L2ꢀ with
AgNPs. Linker L2ꢀ binds to AuNP
surfaces and forms
stable bond compared with that
obtained between L2ꢀ and Hg2+
a more
,
explaining that the AuNPs@(L)2ꢀ
assembly remains unchanged
after the interaction with this
metal ion.
Experimental Section
General
Instrumentation: Elemental analy-
ses were carried out with Fisons In-
struments EA1108 microanalyzer
(Ipswich, UK) at the University of
Vigo (CACTI), Spain. Infrared spec-
tra were recorded in KBr windows
using a JASCO FT/IR-410 spectro-
photometer (Spain). 1H and
13C NMR were carried out on
a Bruker Avance III400 at an oper-
ating frequency of 400 MHz for
1H NMR
and
100.6 MHz
for
13C NMR using the solvent peak as
an internal reference at 258C.
MALDI-MS analyses were per-
formed with a MALDI-TOF/TOF MS
model Ultraflex II (Bruker, Germa-
ny) equipped with nitrogen from
the BIOSCOPE group. Each spec-
trum represents accumulations of
5”50 laser shots. The reflection
Figure 5. A) Modifications with time in the absorption spectra of triangular AgNPs@(L)2ꢀ with the addition of
HgCl2 (22 mL, 1:1 L/M). B) Size distribution diagram for an aqueous solution of triangular AgNPs@(L)2ꢀ after the ad-
dition of HgCl2 (1 equiv, [HgCl2]=1.10”10ꢀ3 m). TEM images of an aqueous solution of triangular AgNPs@(L)2ꢀ
C) before and D, E) after the addition of HgCl2 (1 equiv, [HgCl2]=1.10”10ꢀ3 m).
potentially modifying L2ꢀ could influence citrate-stabilized
mode was used, and the ion source and flight tube pressure were
less than 1.80”10ꢀ7 and 5.60”10ꢀ8 Torr, respectively. The MALDI
mass spectra of the soluble samples (1 or 2 mgmLꢀ1) were recorded
using the conventional sample preparation method for MALDI MS.
One microliter was put on the sample holder on which the ligand
had been previously spotted. The sample holder was inserted in
the ion source. UV/Vis absorption spectra (220–900 nm) were per-
formed using a JASCO-650 UV/Vis spectrophotometer (Oklahoma
City, OK, USA) and fluorescence spectra on a HORIBA JOVIN-IBON
Spectramax 4 (Irvine, CA, USA). All measurements were performed
at 298 K.
AgNPs and AuNPs: (1) the presence of a sulfur atom, which
can form covalent bonds with silver and gold atoms; (2) the
presence of ionizable functional groups (amine); and (3) the
charge (+/ꢀ) of ionizable functional groups. Electrostatic re-
pulsion between AgNPs and AuNPs was progressively reduced,
and the stability of the electric dipole associated with charge
separation on the nanocrystal surface was potentially en-
hanced by spatial partitioning of L2ꢀ and citrate-capping li-
gands. As a consequence, highly extended 1D NP assemblies
in the form of discrete chains, bifurcated and looped chains, or
interconnected chain networks are assembled spontaneously
as the concentration of surface-adsorbed L2ꢀ molecules in-
creases.
Monoanionic compound L2ꢀ causes shifts to the initial ab-
sorption spectra of AgNPs and AuNPs, and can be used for de-
velopment of a surface plasmon resonance (SPR) chemical and
biomolecular sensing platform because the interaction of cit-
rate-stabilized AgNPs and AuNPs with the aforementioned
compound is a very sensitive easy-to-visualize process.
Limit of detection: The limit of detection (LOD) for Hg2+ with the
small fluorescent molecular systems L and the nanoassembly
system AgNPs@(L)2ꢀ (spherical) were performed having in mind
their use for real ion detection and for analytical applications. The
LOD was obtained using Equation 1:
ydl ¼ yblank þ 3std
ð1Þ
where ydl=signal detection limit and std=standard deviation.
The sensing of Hg2+ in aqueous solution using AgNPs@(L)2ꢀ
and AuNPs@(L)2ꢀ was carried out. A destabilization of the
system AgNPs@(L)2ꢀ (spherical and triangular shape) and the
loss of the assembly were observed due to the interaction of
Concentration determination: Assuming a spherical shape and
uniform face centered cubic (fcc) structure, the molar concentra-
tions of the silver nanoparticle (AgNP) and gold nanoparticle
(AuNP) solutions was calculated using Equations 2 and 3.[46]
ꢁ 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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p 1D3
6 M
C28H30N4O2S: C 69.1, N 11.5, S 6.6, H 6.2, found: C 69.3, N 11.2, 6.4,
H 6.6.
ð2Þ
N ¼
where N is the number of atoms per AgNP or AuNP, 1 [gcmꢀ3] is
Preparation of AgNPs: Citrate-stabilized AgNPs of different shapes
(spherical and triangular) were synthesized in aqueous solution fol-
lowing the Frank methodology.[48] For the synthesis of AgNPs with
spherical shape, sodium citrate (2.0 mL, 1.25”10ꢀ2 m), AgNO3
(5.0 mL, 3.75”10ꢀ4 m), and H2O2 (5.0 mL, 5.0”10ꢀ2 m) were. After
that, freshly prepared NaBH4 (2.5 mL, 5.0”10ꢀ3 m) was added. To
obtain AgNPs with triangular shape, before the addition of NaBH4,
KBr (40 mL, 1.0”10ꢀ3 m) was added to the solution. Once all re-
agents were combined, the resulting solutions were carefully
swirled to fully mix the reactants. Almost immediately, the progres-
sion of the reaction becomes evident through the visual changes
consistent with the growth of silver nanoprisms. Yellow and blue
colors were observed for the spherical and triangular AgNPs, re-
spectively (see Figure 1S, Supporting Information). Using Equa-
tions 2 and 3, N=30.70D3, and the resulting spherical AgNPs solu-
tion had a concentration of CAgNP =7.8”10ꢀ10 m with AgNPs of
(25ꢂ3) nm size.
the density of face centered cubic (fcc) silver (10.5 gcmꢀ3) of gold
(19.3 gcmꢀ3), and M [gmolꢀ1
] is the atomic mass of sliver
(107.86 gmolꢀ1) or gold (196.97 gmolꢀ1).
NT
C ¼
ð3Þ
NVNA
where C is the molar concentration of AgNPs or AuNPs, NT is the
total number of silver atoms added as AgNO3 or gold atoms added
as HAuCl4, N is the number of NPs, V is the volume of the reaction
solution in L, NA is Avogadro’s constant (number of atoms per
mole).
Characterization of the assemblies of AgNPs and AuNPs: We
characterized the AgNPs, AuNPs and the chainlike assemblies of
AgNPs and AuNPs using a number of optical tools including, trans-
mission electron microscopy (TEM) and dynamic light scattering
(DLS). To collect TEM images, the samples were prepared dropping
1 mL of the colloidal suspension onto a copper grid coated with
a continuous carbon film and allowing the solvent to evaporate.
TEM images were obtained using a JEOL JEM 1010F TEM operating
at 200 kV. To perform the Fourier transformations, we used the Dig-
ital Micrograph (Gatan) software.[47] The NP size distributions were
measured using the DLS system, Malvern Nano ZS instrument
(Worcestershire, UK) with a 633 nm laser diode. We investigated
the optical properties of these structures using a JASCO-650
UV/Vis spectrophotometer.
Preparation of AuNPs: Preparation of AuNPs was performed fol-
lowing the Turkevish method[49] through reduction of tetrachlor-
oaurate ions (AuCl4ꢀ) by boiling in aq sodium citrate solution.
HAuCl4·3H2O (49.5 mg, 0.125 mmol) dissolved in nanopure H2O
(125 mL; 18.2 MW cm) was added to a preheated solution of
sodium citrate (12.5 mL, 1 wt%). The resulting solution was heated
to 1008C for 60 min and turned colorless before changing to violet
and finally to ruby red. AuNPs obtained using this method appear
as almost monodispersed globular structures with a size of (20ꢂ
5) nm, which are stabilized by weakly bound citrate anions. Using
Equations 2 and 3, N=30.896D3 for AuNPs and the resulting
AuNPs solution was found to have a of CAuNPs =3.7”10ꢀ9 m.
Chainlike assemblies of AgNPs and AuNPs: We used the poly-
amine molecular probe L2ꢀ to investigate the effect of the pres-
ence of different donor atoms in the AuNP assemblies and their
optical properties. The formation of chainlike assemblies of AuNPs
was controlled and modulated observing that the total formations
were obtained by adding an acetonitrile solution of L2ꢀ (5 mL, 1”
10ꢀ3 m) into a suspension of AuNPs and AgNPs (circular and trian-
gular) in nanopure H2O (ꢁ10ꢀ8–10ꢀ9 m in 3 mL).
Synthesis
2,2’-((Thiobis(ethane-2,1-diyl))bis(azanediyl))bis(N-(naphthalen-1-
yl)acetamide) (L): A solution of 20% NaOH (3.4 g, 0.085 mol) was
added to a stirred solution of 1-naphthylamine (9.34 g, 0.051 mol)
in CH2Cl2 (30 mL). The mixture was cooled to 08C and chloroacetyl
chloride (9.29 g, 0.083 mol) was added dropwise for 45 min. After
stirring at 08C for 100 min, the mixture was allowed to warm to RT.
The aqueous layer was separated and extracted with CH2Cl2 (2”
25 mL). The combined organic phases were washed with HCl (5%
v/v), NaHCO3 (5% v/v) and H2O, dried over MgSO4 and filtered to
obtain a white solid. The crude product was purified by silica
column chromatography and characterized as 2-chloro-N-(1-naph-
thyl)acetamide (74%).
Acknowledgements
We are grateful to the Scientific Association ProteoMass (Portu-
gal) for financial support. C.N. thanks Xunta de Galicia (Spain)
for her postdoctoral contract (I2C program).
A
solution of 2-chloro-N-(1-naphthyl)acetamide (439.34 mg,
2 mmol) in tetrahydrofuran (THF; 25 mL) was added dropwise to
a solution of 2,2’-thiobis(ethylamine) (120 mg, 1 mmol) and tri-
ethylamine (202.24 mg, 2 mmol) dissolved in THF (50 mL) over 1 h
in an ice bath. After the addition was completed, the reaction mix-
ture was kept at reflux for 4 h. The solvent was removed in vacuo,
and the residue was washed with H2O/CHCl3 (1:3 v/v; 4”20 mL).
The resulting organic phase was dried in vacuo to give L as a pink
powder (407.05 mg, 84%): 1H NMR (500 MHz, CDCl3): d=2.81 (t,
4H), 3.12 (t, 4H), 3.72 (m, 4H), 5.84 (s, 2H), 7.41–7.59 (m, 2H), 7.72
(m, 2H), 7.80 (m, 2H), 7.92 (m, 4H), 8.10 (m, 4H), 10.35 ppm (s,
2H); 13C NMR (500 MHz, CDCl3) d=30.32, 54.85, 58.37, 120.69,
124.80, 125.73, 125.90, 127.20, 128.05, 128.19, 133.03, 133.56,
169.83 ppm; IR (KBr): n˜ =1436 ((C=C)ar), 1676 (C=O), 3262 cmꢀ1 (Nꢀ
H); MALDI-TOF MS: m/z 487.21 [M+H]+; Anal. calcd for
Keywords: 1D nanochains · self-assembly · gold · mercury ·
nanoparticles · silver
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Received: May 10, 2013
Published online on && &&, 0000
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FULL PAPERS
1D Nanochain formation: The binding
ability of a polyamine molecular linker
(L)2ꢀ bearing different functional
A. Fernꢀndez-Lodeiro,
J. Fernꢀndez-Lodeiro, C. NfflÇez,*
R. Bastida, J. L. Capelo, C. Lodeiro*
groups, which favors the self-assem-
bling of silver (AgNPs) and gold nano-
particles (AuNPs) into 1D nanochains in
aqueous solution was explored. UV/Vis
spectrophotometry and TEM were used
to determine time-dependent structural
changes associated with these 1D struc-
ture formations. Sensing of Hg2+ using
AgNPs@(L)2ꢀ and AuNPs@(L)2ꢀ assem-
blies was also carried out in aqueous so-
lution.
&& – &&
Polyamine Ligand-Mediated Self-
Assembly of Gold and Silver
Nanoparticles into Chainlike
Structures in Aqueous Solution:
Towards New Nanostructured
Chemosensors
ꢁ 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 2013, 00, 1 – 9
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9
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ÞÞ
These are not the final page numbers!