Self-assembly, disassembly, and reassembly of gold nanorods mediated by bis(terpyridine)-metal connectivity

Article abstract of DOI:10.1002/chem.201000040

"Figure Presentation" End-to-end connectivity of Au nanorods (NRs) employing bis(terpyridine)-metal (Fe^II or Cd^II) complexes has been demonstrated, along with their facile disassembly and reassembly. Biased surfactant adsorption at t

Full text of DOI:10.1002/chem.201000040

DOI: 10.1002/chem.201000040
Self-Assembly, Disassembly, and Reassembly of Gold Nanorods Mediated by
Bis(terpyridine)–Metal Connectivity
Yi-Tsu Chan,[a] Sinan Li,[b] Charles N. Moorefield,[c] Pingshan Wang,[d]
Carol D. Shreiner,[e] and George R. Newkome*[a]
Since the advent of nanotechnology, a primary challenge
has been to assemble materials from predesigned building
blocks for the fabrication of nanometer-sized materials.
Metal nanocrystals (NCs) have attracted significant atten-
tion due to their unique shape and size, as well as their tune-
able electrical, optical and catalytic properties.[1] A bottom-
up approach has generally been used for constructing the
desired multi-unit composites from individual components.
Although extensive studies have been performed on spheri-
cally shaped NCs, research on anisotropic nanoparticles, for
example, nanorods (NRs)[2] with a distinct 1D or linear
structure, is limited. Since the properties of NCs are mostly
size- and shape-dependent, NRs have the potential to show
novel as well as improved properties, such as in surface-en-
hanced Raman scattering (SERS) effects or optical and flu-
orescent properties,[2a] when compared to isotropic nano-
spheres (NSs).[1b,c,3] Geometrically, the aspect ratio (the
length of major axis divided by the width of minor axis) of
NRs is >1 and their cofacial assembly produces more angu-
larly complex structures than that of the NSs.[4] Side-to-side
assemblies of NRs have been achieved by introducing active
sites on the sides of the rods and subsequently connecting
via DNA, electrostatic interaction,[5] and more recently with
click chemistry;[6] however, arrangement of NRs in an end-
to-end orientation, for example, forming chains or branched
structures, requires a regiospecific functionalization. Toward
this goal, the tips of NRs have been modified with different
metals, which were subsequently termed nanodumbbells, for
participation in direct end-to-end assembly.[7]
It is well-known that Au NRs have different crystallo-
graphic facets, which comprise the ends and side surfaces.[8]
Recently, it was found that these side facets have a higher
surface energy, and in turn, adsorb more bilayer surfactant,
for example, cetyltrimethylammonium bromide (CTAB);[9]
notably, the yield of these surface-modified nanorods has
been shown to be supplier (CTAB) dependent.[10] Current
progress on direct end-to-end Au NR assembly has focused
on using biocompatible connectors,[11] a,w-dithiols,[12] and
synthetic polymers.[13] As well, organometallic connections,
such as bis(terpyridine)-transition metal complexes, are also
of interest due to their potential to impart unique electrical,
optical, and photovoltaic properties.[14] For example, hybrid
Au NCs and bis(terpyridine)–RuII complexes have been fab-
ricated and shown to exhibit enhanced electrochemical
properties.[15] Herein, we report the first example of a
bottom-up approach for the end-to-end linear and branched
assembly of Au NRs into multicomponent structures using
[(disulfide-modified terpyridine)2–MII] (M=Fe or Cd) inter-
connectors and their facile disassembly and reassembly.
The synthesis of disulfide-modified monoterpyridine (SS-
tpy) 2, the [(SS-tpy)2–MII] complexes 3 (M=Fe), and 4 (M=
Cd) is shown in Scheme 1. In a typical Gabriel synthesis, 4’-
(4-hydroxyphenyl)-2,2’:6’2’’-terpyridine[16] was treated with
N-(3-bromopropyl)phthalimide to afford the intermediate
protected amine, which was subsequently deprotected
(H2NNH2·H2O) to give the amine-modified monoterpyridine
1. The structure was supported (1H NMR) by signals for the
new OCH2 (4.17 ppm) and CH2NH2 (3.20 ppm) moieties; a
dominate peak (ESI-MS) at m/z 383.3 [M+H]+ further con-
firms the structure. Disulfide-modified monoterpyridine 2
[a] Y.-T. Chan, Prof. Dr. G. R. Newkome
Departments of Polymer Science and Chemistry
The University of Akron, Akron, OH 44325-4717 (USA)
Fax : (+1)330-972-2413
E-mail: newkome@uakron.edu
[b] Dr. S. Li
Excel Polymers, 14330 Kinsman Rd.
Burton, OH 44325 (USA)
[c] Dr. C. N. Moorefield
Maurice Morton Institute for Polymer Science
The University of Akron, Akron, OH 44325 (USA)
[d] Dr. P. Wang
ChemNano Materials, 2220 High St. Suite 605
Cuyahoga Falls, OH 44221 (USA)
[e] Prof. Dr. C. D. Shreiner
Chemistry Department, Hiram College, P.O. Box 67
Hiram, OH 44234 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000040.
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COMMUNICATION
was prepared via diimide-mediated amidation of monoter-
pyridine 1 using thioctic acid (TA). The transformation was
indicated by the appearance of a new signal (13C NMR) at
173.0 ppm attributed to the amide carbonyl carbon; peaks
(ESI-MS) at m/z 571.4 amu [M+H]+ and 593.3 amu [M
+Na]+ provided further confirmation. Reaction of terpyri-
NR connectivity (Scheme 2) was effected by adding
100 mL of a stock solution [27 mm in MeOH/MeCN/H2O
1:1:1 v/v/v] of the iron-complexed connector 3 to 1 mL of
Au NR aqueous solution, sonicated for 10 min, and left un-
disturbed at 258C for two days to afford a purple <(Au
NR)ACHTNUTGRNEUNG
[(SS-tpy)2-FeII]n >m precipitate that was collected by
dine
ACHTUNGTRENNUNG
2
with 0.5 equiv of either FeCl2·4H2O or Cd-
centrifuging (3000 rpm, 5 min) and removal of the superna-
tant.
linkers 3 (M=Fe) and 4 (M=Cd), respectively. Their
1H NMR, 13C NMR, and ESI-MS spectra agreed with the
structures given in Scheme 1 (see the Supporting Informa-
tion).
The seed-mediated growth method[17] was applied to syn-
thesize the Au NRs, whereby spherical Au NPs were pro-
duced as seeds by chemical reduction of HAuCl4·3H2O, fol-
lowed by the addition of a surfactant (CTAB) template. The
Au NRs were purified by centrifuging the mixture at
7000 rpm for 10 min and possessed an average aspect ratio
of about 4.2, as determined by TEM imaging (Figure 1A).
UV/Vis spectroscopy was used to characterize the two ab-
sorption bands of the Au NRs, which consisted of the trans-
verse and longitudinal plasmon bands (512 and 716 nm, re-
spectively) (Figure 3A). In distilled water, the Au NRs are
stable for months.
Scheme 2. Idealized <(Au NR)ACHTNUGRTNEUNG
[(SS-tpy)2-FeII]n >m formation.
X-ray photoelectron spectros-
copy (XPS, monochromatic
MgKa
radiation,
250 W,
93.90 eV) of the Au NRs (Fig-
ure S1A) revealed typical gold
peaks at Au 4f7 (84.0 eV) and
Au 4f5 (87.7 eV), which were
also used as reference peaks.
Bromine
peaks
(3p3
at
180.4 eV and 3p1 at 187.0 eV),
generated exclusively from
CTAB, were also detected.
Since the <(Au NR)ACTHUNRGTNEUNG[(SS-tpy)2-
FeII]n >m hybrid only possesses
small portions of the [(SS-tpy)2-
FeII] connectors, the XPS spec-
trum exhibited the appearance
of new peaks at 708.7 and
721.4 eV, which were attributed
to Fe 2p3 and Fe 2p1, respec-
tively (Figure S1B), as well as
new
S 2p peaks (163.7 and
168.9 eV).
TEM was used to image the
structures and shapes of the un-
complexed Au NRs and their
subsequent assemblies. It is
well-known that Au NRs can
appear to be physically bound
with no added connecting
agent[1d] under TEM imaging:
Scheme 1. Synthesis of complexes 3 and 4: i) K2CO3, MeCN, reflux, N2, 10 h; ii) EtOH, reflux, 4 h; iii) DCC,
HOBT, CH2Cl2, 258C, 18 h; iv) FeCl2·4H2O for 3 and Cd
G
À
as the PF6 salt; see the Supporting Information).
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G. R. Newkome et al.
tpy)2-FeII] complex (complete extended length: 4.6 nm and
central rigid section length: 2 nm).
Figure 2. A high magnification TEM image of end-to-end NR connectivi-
ty showing a separation that corresponds well with the calculated length
of the extended, space-filling representation of the [(SS-tpy)2-FeII] com-
plex.
Further confirming the assembly of the <(Au NR)ACHTUNGTRENNUNG[(SS-
tpy)2-FeII]n >m hybrids, UV/Vis absorption data for the
transverse plasmon bands at ~512 nm in Figure 3A and B
showed no significant difference. However, a new absorp-
tion band at 572 nm was observed and assigned as the
metal–ligand charge-transfer (MLCT) transition,[14a] which
was attributed to the promotion of an electron from the
FeII-centered d orbitals to unfilled ligand p* orbitals of the
[(SS-tpy)2-FeII] complex (the MLCT band for individual
[(SS-tpy)2-FeII] in MeOH at lmax =573 nm). An intense
ligand-centered p–p* transition of the terpyridine compo-
nents in the [(SS-tpy)2-FeII] can be clearly seen as the ab-
sorption peaks at lmax =286 and 323 nm. Furthermore, the
longitudinal plasmon band shows a red-shift (from 716 to
770 nm) and a broadened shape, accompanied by a decrease
in the absorption intensity. This spectral change is interpret-
ed as the interplasmon coupling of Au NR assembly in the
longitudinal direction and indicates an orientation of the Au
NRs into linear chains via the end-to-end connectivity.[11c,d]
Since [(tpy)2-FeII]-based complexes have been demon-
strated[18] to be unstable under basic conditions, an NaOH
solution (1 mm) was added to the hybrid causing a rapid dis-
assembly to generate individual Au NRs (Figure S2A). The
disassembled Au NRs were collected by centrifuging at
7000 rpm for 10 min, followed by removal of the supernate
and then redispersed into 1 mL distilled water. The XPS
Figure 1. Representative TEM images for the native, untreated Au NRs
(A), and <(Au NR)
[(SS-tpy)2-FeII]n >m hybrids (B–F): B) in low mag-
nification; C) linear end-to-end connectivity; D) a branched structure; E)
end-to-end as well as end-to-wall connectivity; F) a cyclic structure.
ACHTUNGTRENNUNG
in our case, >95% of the freshly prepared Au NRs were
found to be monomeric and <5% were dimers (i.e., Au
NR-Au NR). After assembly using the [(SS-tpy)2-FeII] com-
plex, few individual Au NRs were detected in the TEM
(Figure 1B). Since the side faces of the Au NRs are densely
protected with a CTAB bilayer, the disulfide groups of the
[(SS-tpy)2-FeII] connector preferentially, but not exclusively,
attack the tips of the Au NRs, leading to predominately
end-to-end assemblies.
A linear polymeric (Figure 1C)
structure of the hybrid was clearly observed in the TEM
images. Branched structures
were also observed (Figure 1D)
and attributed to multiple func-
tionalities distributed on the
tips of the rods. Although end-
to-end assembly predominates,
some connections showed end-
to-wall (arrows in Figure 1E
and F) attachment, which can
be attributed to side-wall
CTAB displacements by con-
nectors. The average distance
(Figure 2) between gold nano-
rods is less than 3 nm which is
consistent with the calculated
Figure 3. UV/Vis spectra of A) Au NRs, B) <(Au NR)ACTHNUTRGNEUNG
[(SS-tpy)2-FeII]n >m hybrids, and C) disassembled SS-
molecular length of the [(SS- tpy-modified Au NRs.
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Gold Nanorods
COMMUNICATION
spectrum (Figure S1C) showed small portions of S 2p peak,
and the total disappearance of Fe peaks, which proves that
the disassembled Au NRs possess the metal (iron)-free, tpy-
modified end-groups. Comparison of the UV spectrum of
the free Au NRs (Figure 3A) and the Au NR hybrids (Fig-
ure 3B), the disassembled material (Figure 3C) showed the
disappearance of the MLCT band at lmax =572 nm for the
[(SS-tpy)2-FeII] connectivity; whereas, the absorption bands
at lmax =254 and 277 nm for the free SS-tpy moieties were
still present. This facile disassembly showed the reversibility
NR)ACTHUNGETRNNUG
[(SS-tpy)2-CdII]n >m hybrid was obtained using the
labile linker complex 4. Reversibility of the tpy–CdII–tpy
linker 4 was subsequently demonstrated by adjusting the
ligand-to-metal ratio. Treatment of terpyridine
2 with
0.5 equiv of CdACTHNUTRGNE(UNG NO3)2·4H2O gave rise to 4 with intermolec-
ular connectivity, which exhibited an upfield shift (1H NMR;
Figure 4B) from 8.74 to 8.07 ppm of the doublet assigned to
the 6,6’’-tpyHs due to the octahedral geometry. Upon addi-
tion of another 0.5 equiv of CdACTHNURGTNE(UNG NO3)2·4H2O, the bis(terpyri-
dine) complex 4 dissociated into the SS-monotpy–CdII ad-
ducts that were verified by a comparable downfield shift of
the 6,6’’-tpyHs from 8.07 to 8.87 ppm (Figure 4C). More-
of the <(Au NR)ACHTUNGTRENNUNG
[(SS-tpy)2-FeII]n >m composite and the re-
tention of the free terpyridine ligands on the surface of the
gold nanorods. In order to reconnect these terpyridine-
modified gold nanorods, a FeCl2 solution (100 mL, 27 mm in
H2O) was added into the disassembled Au NRs. After soni-
cation for 10 min, transverse and longitudinal plasmon UV/
Vis bands showed no obvious difference (Figure S3) and
only individual nanorods were observed in the TEM images
(Figure S2B); however, a band at 572 nm was again ob-
served in UV/Vis. These results suggest a facile intramolecu-
lar complexation[19] of FeII ions between two terpyridines on
the same nanorod tip (Scheme 3A; notably, the number of
end-bound terpyridines varies) enhanced by a rapid com-
plexation rate[20] and a longer incubation period for nano-
chain formation.[11c]
over, the mono-terpyridine–Cd
ACHUTGTNRENNUG(H2O)ACHTUGNTRENNNUG
been reported in the presence of excess CdACHTUNGTRENNNUG
Reassembly of the hybrid with added additional CdII was
not feasible; however, since CdII-based complexes are de-
monstrably more labile than the corresponding FeII-based
complexes, the (Au NR)-(SS-monotpy-CdII)n hybrids (Sche-
me 3B) were obtained by disassembling the <(Au NR)-
ACHUNTGREUNNG HCAUTNGTREN(NUNG NO3)2·4H2O
[(SS-tpy)2-CdII]n >m chain with an excess of Cd
(unreacted CdII was removed by centrifugation), and used to
inhibit the intramolecular complexation on the surface of
Au NRs. Reassembly was achieved by mixing the free ter-
pyridine-modified (donor) Au NRs and the tpy-CdII-modi-
fied (acceptor) Au NRs. A red-shift (from 717 to 783 nm)
was again observed in the UV/Vis spectrum of the reassem-
bled Au NRs (Figure S4).
We have synthesized [(disulfide-terminated tpy)2-MII]
complexes and employed them in the predominately end-to-
end assembly of Au NRs; facile disassembly occurred upon
treatment with aqueous NaOH solution for the FeII linker
Scheme 3. Idealized representations of intramolecular vs. intermolecular
complexations: A) Complexation reaction of terpyridine-modified Au
NRs and FeCl2; B) disassembled and reassembled reactions of terpyri-
dine-modified Au NRs.
Attempts to verify the FeII-based connectivity of the hy-
brids by disassembly and reassembly were unsuccessful due
to the strong FeII–terpyridine coordination (K1 =1.3ꢁ
107 mÀ1)[14a] that is not easily dissociated without harsh condi-
tions or an added strong nucleophile (i.e., HOÀ). Investiga-
tions using CdII, a weaker coordinating species (K1 =1.3ꢁ
105 mÀ1),[14a] were subsequently undertaken. Thus, the <(Au
Figure 4. Aromatic region of 1H NMR spectra for the A) SS-tpy 2 in
CDCl3, B) bis(terpyridine) complex 4 in CD3CN, and C) SS-monotpy–
CdII adduct in CDCl3/CD3OD 1:1. The marked peak * corresponds to
CHCl3.
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G. R. Newkome et al.
and excess CdACHTUNGTRENNUNG
(NO3)2·4H2O for the CdII linker, respectively.
10007; d) >Y. Wang, Y. F. Li, Y. Song, C. Z. Huang, Chem.
Commun. 2010, 1332–1334.
XPS, UV/Vis, and TEM experiments were used to confirm
the predominately end-to-end assembly, as well as structural
disassembly. Attempted inter-NR reassembly of the tpy-Fe-
tpy connector was not realized but rather intra-NR com-
plexation occurred. The reassembly was achieved by mixing
the donor-modified and the acceptor-modified Au NRs. This
simple method can be used to construct more complicated
multicomponent nanoarchitectures and it gives rise to the
incorporation of organometallic species that facilitates
access to their unique properties.
[8] a) Z. L. Wang, M. B. Mohamed, S. Link, M. A. El-Sayed, Surf. Sci.
1999, 440, L809 L814; b) Z. L. Wang, R. P. Gao, B. Nikoobakht,
M. A. El-Sayed, J. Phys. Chem. B 2000, 104, 5417–5420.
[9] a) L. Gou, C. J. Murphy, Chem. Mater. 2005, 17, 3668–3672; b) B.
Nikoobakht, M. A. El-Sayed, Langmuir 2001, 17, 6368–6374.
[10] D. K. Smith, B. A. Korgel, Langmuir 2008, 24, 644–649.
[11] a) K. K. Caswell, J. N. Wilson, U. H. F. Bunz, C. J. Murphy, J. Am.
Chem. Soc. 2003, 125, 13914–13915; b) J.-Y. Chang, H. Wu, H.
Chen, Y.-C. Ling, W. Tan, Chem. Commun. 2005, 1092–1094;
c) S. T. S. Joseph, B. I. Ipe, P. Pramod, K. G. Thomas, J. Phys. Chem.
B 2006, 110, 150–157; d) K. G. Thomas, S. Barazzouk, B. I. Ipe, S. T.
Joseph, P. V. Kamat, J. Phys. Chem. B 2004, 108, 13066–13068.
[12] a) P. Pramod, K. G. Thomas, Adv. Mater. 2008, 20, 4300–4305;
b) S. T. S. Joseph, B. I. Ipe, P. Pramod, K. G. Thomas, J. Phys. Chem.
B 2005, 109, 150–157.
Acknowledgements
[13] a) Z. Nie, D. Fava, E. Kumacheva, S. Zou, G. C. Walker, M. Rubin-
stein, Nat. Mater. 2007, 6, 609–614; b) Z. Nie, D. Fava, M. Rubin-
stein, E. Kumacheva, J. Am. Chem. Soc. 2008, 130, 3683–3689; c) D.
Fava, Z. Nie, M. A. Winnik, E. Kumacheva, Adv. Mater. 2008, 20,
4318–4322; d) D. Fava, M. A. Winnik, E. Kumacheva, Chem.
Commun. 2009, 2571–2573.
We thank the National Science Foundation for generous financial support
(DMR-0705015).
Keywords: cadmium · iron · nanoparticles · self-assembly ·
[14] a) U. S. Schubert, H. Hofmeier, G. R. Newkome, Modern Terpyri-
dine Chemistry, Wiley-VCH, Weinheim, 2006; b) T. B. Norsten, B. L.
Frankamp, V. M. Rotello, Nano Lett. 2002, 2, 1345–1348; c) R.
Shenhar, E. Jeoung, S. Srivastava, T. B. Norsten, V. M. Rotello, Adv.
Mater. 2005, 17, 2206–2210; d) P. Arumugam, D. Patra, B. Samanta,
S. S. Agasti, C. Subramani, V. M. Rotello, J. Am. Chem. Soc. 2008,
130, 10046–10047; e) W. R. McNamara, R. C. Snoeberger, G. Li,
J. M. Schleicher, C. W. Cady, M. Poyatos, C. A. Schmuttenmaer,
R. H. Crabtree, G. W. Brudvig, V. S. Batista, J. Am. Chem. Soc.
2008, 130, 14329–14338.
tridentate ligands
[1] a) R. Shenhar, V. M. Rotello, Acc. Chem. Res. 2003, 36, 549–561;
b) M. A. El-Sayed, Acc. Chem. Res. 2001, 34, 257–264; c) K. G.
Thomas, P. V. Kamat, Acc. Chem. Res. 2003, 36, 888–898; d) J.
Perez-Juste, I. Pastoriza-Santos, L. M. Liz-Marzan, P. Mulvaney,
Coord. Chem. Rev. 2005, 249, 1870–1901.
[2] a) C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L.
Gou, S. E. Hunyadi, T. Li, J. Phys. Chem. B 2005, 109, 13857–
13870; b) C. J. Murphy, A. M. Gole, S. E. Hunyadi, C. J. Orendorff,
Inorg. Chem. 2006, 45, 7544–7554.
[15] M. Ito, T. Tsukatani, H. Fujihara, J. Mater. Chem. 2005, 15, 960–
964.
[16] Y.-T. Chan, C. N. Moorefield, G. R. Newkome, Chem. Commun.
2009, 6928–6930.
[3] S. K. Ghosh, T. Pal, Chem. Rev. 2007, 107, 4797–4862.
[4] C. J. Murphy, N. R. Jana, Adv. Mater. 2002, 14, 80–82.
[17] B. Nikoobakht, M. A. El-Sayed, Chem. Mater. 2003, 15, 1957–1962.
[18] P. Wang, C. N. Moorefield, G. R. Newkome, Angew. Chem. 2005,
117, 1707–1711; Angew. Chem. Int. Ed. 2005, 44, 1679–1683.
[19] G. R. Newkome, K. S. Yoo, C. N. Moorefield, Chem. Commun. 2002,
2164–2165.
[5] a) E. Dujardin, L.-B. Hsin, C. R. C. Wang, S. Mann, Chem.
Commun. 2001, 1264–1265; b) A. Gole, C. J. Murphy, Langmuir
2005, 21, 10756–10762; c) A. Gole, C. J. Orendorff, C. B. Murphy,
Langmuir 2004, 20, 7117–7122; d) C. J. Orendorff, P. L. Hawkins,
C. J. Murphy, Langmuir 2005, 21, 2022–2026.
[20] R. H. Holyer, C. D. Hubbard, S. F. A. Kettle, R. G. Wilkins, Inorg.
Chem. 1966, 5, 622–625.
[6] A. Gole, C. J. Murphy, Langmuir 2008, 24, 266–272.
[7] a) A. K. Salem, M. Cgen, J. Hayden, K. W. Leong, P. C. Searson,
Nano Lett. 2004, 4, 1163–1165; b) M. Chen, L. Guo, R. Ravi, P. C.
Searson, J. Phys. Chem. B 2006, 110, 211–217; c) A. Salant, E.
Amitay-Sadovsky, U. Banin, J. Am. Chem. Soc. 2006, 128, 10006–
[21] J. Granifo, M. Garland, R. Baggio, Inorg. Chem. Commun. 2004, 7,
77–81.
Received: January 8, 2010
Published online: March 16, 2010
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