Metallamacrocycle-modified gold nanoparticles: A new pathway for surface functionalization

Article abstract of DOI:10.1039/c3cc47722c

We report herein the designed synthesis of four silver(i)- or gold(i)-bridged flexible metalla-macrocycles (MMCs) and their distinct performance in the surface modification of gold nanoparticles (AuNPs). The resulting gold(i)-MMCs-modified AuNPs were found to be resistant to pH variation and capable of binding with metal ions. This journal is

Full text of DOI:10.1039/c3cc47722c

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Metallamacrocycle-modified gold nanoparticles:
a new pathway for surface functionalization†‡
Cite this:DOI: 10.1039/c3cc47722c
Hai-Xia Liu,§ Xin He§ and Liang Zhao*
Received 9th October 2013,
Accepted 29th October 2013
DOI: 10.1039/c3cc47722c
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We report herein the designed synthesis of four silver(I)- or gold(I)- atoms bridged by thiolate ligands to form a series of Aum(SR)n
bridged flexible metalla-macrocycles (MMCs) and their distinct flexible semi-ring structures.12 These peripheral Au atoms ultimately
performance in the surface modification of gold nanoparticles bind with the nanocluster core via significant aurophilic inter-
(AuNPs). The resulting gold(I)-MMCs-modified AuNPs were found to action.13 Inspired by this [Aux]central–[Aum(SR)n]peripheral core–
be resistant to pH variation and capable of binding with metal ions.
shell structural model, we envision that gold(I)-contained flexible
MMCs may be capable of modifying the surface of AuNPs by
Surface chemical modification of gold nanoparticles (AuNPs), one aurophilic interactions.
of the most popular and extensively used metal nano-materials, is
To achieve stable gold-bridged MMCs, we selected N-heterocyclic
of fundamental importance in preventing aggregation, realizing carbenes (NHCs) as organic donor species for binding to gold(I)
hierarchical assembly and affecting properties of AuNPs.1 atoms in view of the high bond dissociation energy of Au–C(NHC)
In this regard, organic macrocyclic ligands, such as cyclodextrins,2 (calculated De = 80 kcal molÀ1),14 which is 420 kcal molÀ1 larger
cucurbiturils3 and calixarenes,4 have recently attracted intense than the Au–S bond in common thiolate-protected AuNPs.15 The
interests since the endowment of abundant molecular recognition pyridine oligomer species (Scheme 1) were introduced as not only a
properties of macrocyclic host molecules to AuNPs can largely linking unit but also a stabilizing one to fix the conformation of the
expand potential applications of AuNPs as nanosensors,5 and flexible polypyridine arm by use of a possible chelation based on
drug delivery vehicles6 etc. During the past two decades, carbene and pyridyl nitrogen coordination sites. Two bridging metal
metalla-macrocycles (MMCs)7 constructed by metal–ligand atoms (Ag(I) and Au(I)) were adopted to probe the influence of
coordination self-assembly have emerged as a new type of different metallophilic interactions (Ag–Au and Au–Au, respectively)
macrocyclic compounds, which can serve as precursors of func- in the surface modification of AuNPs.
tional materials,8 reagents for molecular recognition,9 or cavity
As shown in Scheme 1, two diimidazolium salts (H2-1)(PF6)2
controlled catalysts.10 Despite such versatile functionalities, however, and (H2-2)(PF6)2 were first prepared by the reaction of imidazole
MMCs have been scarcely employed in the functionalization of with dibromo-substituted pyridine trimer or pentamer followed
AuNPs by surface modification possibly due to their moderate by methylation and anion exchange with NH4PF6. Treatment of
stability and rigid skeletons.
(H2-1)(PF6)2 and (H2-2)(PF6)2 by silver oxide in dichloromethane–
Recent single-crystal X-ray analysis of some reported gold– acetonitrile yielded two silver-bridged organometallic macro-
thiolate nanoclusters11 has shown that while the cores of these cycles MMC-1 and MMC-2. The silver atoms in these two
nanoclusters are composed of Au atoms in high symmetry,
their surface layers usually consist of a number of outermost Au
Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry &
Chemical Biology (MOE), Tsinghua University, Beijing 100084, China.
E-mail: zhaolchem@mail.tsinghua.edu.cn
† This work is dedicated to Professor James Trotter in celebration of his 80th
birthday.
‡ Electronic supplementary information (ESI) available: Synthetic procedures and
characterization data for MMC-1–4. CCDC 964573 and 964574. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc47722c
§ These authors contributed equally.
Scheme 1 Synthetic procedures for MMC-1–4.
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complexes can be substituted by gold(I) atoms to produce two
gold(I)-bridged macrocycles MMC-3 and MMC-4.
Formation of flexible macrocyclic MMC-1–4 was confirmed
by NMR, mass spectrometry and single-crystal X-ray crystallo-
graphy. A simple one set of NMR signals corresponding to the
protons at asymmetrical positions in the molecular structures
of MMC-1–4 suggests a highly flexible conformation of MMC-1–4
in solution at ambient temperature. After transmetalation from
MMC-1–2 to MMC-3–4, the proton signals corresponding to the
imidazolium rings undergo a downfield shift (0.1–0.2 ppm),
implying a stronger electron-withdrawing ability of the gold(I)
atom. ESI-MS spectra of MMC-1–4 clearly showed the peaks
corresponding to the M2L2 stoichiometric complexes.
Fig. 2 UV-vis spectra of CA-AuNPs and MMC-modified AuNPs (A–D) in
DMF–H2O (v/v = 9 : 1) solution. (inset) Photographs of solution samples of
CA-AuNPs and A–D.
An X-ray diffraction study with crystals of MMC-1ÁCH3OCH2-
CH2OCH3 confirmed the 2 : 2 metal : ligand composition and
macrocyclic structure of MMC-1 (Fig. 1a). The metric para-
meters found in this crystal structure [Ag–Ccarbene = 2.049(17)–
2.139(14) Å; +Ccarbene–Ag–Ccarbene = 171.6(7)–177.1(7)1] fall in
the range previously described for linear silver dicarbene com-
plexes.16 The close contacts (B2.8 Å) between two silver(I)
atoms (Ag1 and Ag2) and two pyridyl nitrogen atoms (N7 and
N16) substantiate our design that the coordination of adjacent
pyridyl nitrogen atoms indeed influences the conformational
fixation of the resulting flexible MMCs. Crystallization of MMC-4
is more challenging due to the existence of two long pyridine
pentamers. We supposed that intramolecular hydrogen bonding
within the pyridine pentamer species may be helpful in rigidifying
the conformation of MMC-4. Upon the addition of triflate acid
into an acetonitrile solution of MMC-4 in the crystallization
process, pale-yellow crystals of a protonated MMC-4 complex
[H2Au2(2)2(H2O)2](CF3SO3)4ÁH2O were finally acquired. X-Ray
crystallographic analysis revealed that each gold(I) ion is linearly
coordinated by two NHC donors with Au–C distances in the
range of 2.012–2.033 Å (Fig. 1), consistent with values in other
Au–CNHC complexes.17 The whole structure can be divided into a
helical unit and a pincer-like Py–NHC–Au–NHC–Py (Py = pyridine)
species. In the helical unit, two protonated pyridine rings (N9 and
N18 in Fig. 1b) constitute hydrogen-bonding with their adjacent
pyridine rings. Noticeably, two NHC moieties around the same
gold(I) center are almost coplanar with each other, consequently
leaving the gold(I) center laterally reachable.
Employment of MMC-1–4 to modify the surface of AuNPs
was implemented by a ligand-exchange reaction. The citrate-
protected AuNPs (CA-AuNPs) were first synthesized by a literature
method.18 Addition of the N,N-dimethylformamide (DMF) solution
of MMC-1–4 into the red aqueous solution of CA-AuNPs generated
distinct results. Upon addition of the silver(I)-bridged macrocycles
MMC-1 or -2, the AuNPs solution samples (abbreviated as A and B,
respectively) became gray within a few minutes. The UV-vis spectra
of A and B (Fig. 2) revealed that the surface plasmon resonance
peak of AuNPs at around 530 nm decreased dramatically relative to
CA-AuNPs, suggesting possible aggregation and precipitation of
AuNPs. In contrast, the mixed solution samples of AuNPs and
MMC-3 or -4 (C and D, respectively) remained the same red color
as CA-AuNPs. The surface plasmon resonance peak in C and D
both underwent a slight red shift from 530 (CA-AuNPs) to 535 nm.
Transmission electron microscope (TEM) images provided
further evidence for the distinct performance of MMC-1–4 in
samples A–D. As shown in Fig. 3, the images of freshly prepared
CA-AuNPs displayed some small aggregates composed of several
spherical particles (size: 41.7 Æ 8.7 nm). These small aggregates were
found to densely assemble into large networks in samples A and B
upon the addition of the silver-bridged macrocycles MMC-1 and -2.
This observation is in good agreement with the UV-vis results.
Fig. 1 Crystal structures of (a) MMC-1ÁCH3OCH2CH2OCH3 and (b)
[H2Au2(2)2(H2O)2](CF3SO3)4ÁH2O (a protonated MMC-4). Uncoordinated
À
À
water and CH3OCH2CH2OCH3 molecules, PF6 and CF3SO3 anions,
and hydrogen atoms are omitted for clarity. Color scheme for atoms:
Ag, purple; Au, cyan; C, black; N, blue; O, red.
Fig. 3 TEM images of CA-AuNPs and MMC-modified AuNPs (A–D). Scale bar =
500 nm. The high resolution-TEM image shows a gold nanoparticle in D
protected by a monolayer.
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In contrast, the TEM images of samples C and D showed that in
The coordination ability of MMC-3 and -4 in samples C and
these two samples the AuNPs were mostly well-dispersed as D was further evaluated with [Cu(CH3CN)4](PF6) as a probe
discrete species. The reason for such good dispersity may be (Fig. S14, ESI‡). Mixing a DMF solution of [Cu(CH3CN)4](PF6)
ascribed to the surface decoration by a number of positively with CA-AuNPs led to the disappearance of the characteristic
charged gold(I)-MMCs, which increase the surface positive red color of well-dispersed AuNPs. This is probably because
charge density of each AuNPs and make them electrostatically of the coordination-based cross-linkage between citrates and
repel to each other. Considering the gold(I)-containing molecular copper(I) ions. However, adding Cu(I) ions (up to 5 fold relative
structures of MMC-3 and -4, it is possible to obtain direct imaging to MMCs) into the MMC-3 or -4 modified AuNPs solution did
of the MMCs-based monolayer around the periphery of AuNPs.19 not change its red color. UV-vis spectra revealed that the surface
As shown in Fig. 3, high-resolution TEM revealed that most AuNPs plasma resonance peak in these two samples kept its intensity
in sample D indeed possess a discernible monolayer shell. The but red-shifted 2–3 nm. In addition, the UV-vis absorption peak
thickness of this monolayer is around 0.8 Æ 0.1 nm, comparable at around 325 nm due to the p - p* transition of pyridine
to the dimension of MMC-4 measured in its crystal structure.
oligomer species were located at identical positions with the
Samples A–D were then purified by centrifugation to remove MMC–[Cu(CH3CN)4](PF6) complexes. This suggests the MMCs
unbound MMCs and citrate ligands. In the Fourier transform in samples C and D behave like free macrocycles and their
infrared (FT-IR) spectra of the centrifuged samples (Fig. S1–S9 coordination with copper(I) ions did not influence the stability
in ESI‡), the typical absorptions at ca. 2924 cmÀ1 arising from of AuNPs. We thus concluded that aurophilic interaction rather
the C–H stretching of the methyl groups in the pyridine than coordination may play a significant role in the binding
oligomers were observed, indicating the inclusion of the between MMC-3–4 and AuNPs.
organic NHC species (1 and 2 in Scheme 1) in these nanoparticle
samples.
In summary, we have described the designed synthesis of four
MMCs by a side-chain-assisted approach. The silver(I)-bridged
The different performance of MMC-1–2 and MMC-3–4 in the MMCs can undergo transmetalation to form a network aggregation
surface modification of AuNPs was then investigated. It is well of AuNPs, while the gold(I)-bridged ones were found to anchor onto
known that silver(I)–NHC compounds can undergo a Ag-transfer the surface of AuNPs. The coordinative pyridyl nitrogen atoms of
process to synthesize other metal-centered NHC complexes,20 MMCs in these Au(I)-MMC-modified AuNPs can further bind with
driven by the formation of insoluble silver salts (such as AgCl in protons and metal ions, indicating their future application in
the transmetalation process from MMC-1–2 to MMC-3–4). We molecular recognition and hierarchical assembly. The herein
hypothesized that a similar transmetalation may take place as well demonstrated use of MMCs to decorate the surface of metal
between MMC-1 or -2 and the surface gold atoms of CA-AuNPs. The nanoparticles is an important step forward in efforts to provide a
formation of silver citrate, an undissolved polymeric precipitate, new stabilizing effect as well as establish a promising method to
may drive the occurrence of this reaction, consequently resulting in access structurally and functionally diverse modified nanoparticles.
the cross-linking and aggregation of AuNPs.
Financial support by the NSFC (21002057, 91127006,
In order to clarify the interaction between gold(I) macro- 21132005, 21121004), MOST (2013CB834501, 2011CB932501),
cycles MMC-3–4 and AuNPs, the following characterization and MOE (NCET-12-0296) and Tsinghua University (2011Z02155) is
control experiments were carried out. X-Ray photoelectron gratefully acknowledged. We are grateful to Prof. Zhong-Qun
spectroscopy (XPS) measurement of the centrifuged sample D Tian and Mr. Chao-Yu Li at Xiamen University for helpful
(Fig. S10, ESI‡), which excludes the unbound MMC-4, revealed discussions.
two Au 4f7/2 peaks with binding energies of 88.22 and 84.52 eV.
These values are different from CA-AuNPs (87.47 and 83.77 eV)
but very close to the values of MMC-4 (88.44 and 84.97 eV).
Notes and references
We speculated this may result from the abundant distribution
of MMC-4 on the surface of AuNPs. Attempts to characterize
aurophilic interaction between the surface gold atoms of AuNPs
and the gold(I) atoms in MMC-3 or -4 by surface-enhanced
Raman spectroscopy (SERS) were unsuccessful. Therefore, we
tried to perform the following contrast experiments to elucidate
the interaction between MMCs and AuNPs. In contrast to the
CA-AuNPs that produced severe aggregation of AuNPs upon the
addition of only a small amount of HBF4 (pH value is lowered to 2),
the MMC-3 or -4 modified AuNPs can preserve their coloration and
surface plasma resonance peak intensity at pH = 2 (Fig. S11–S13,
ESI‡). Such acid resistance arising from the proton binding by
free pyridyl nitrogen atoms of MMC-3 or -4 indicates that the
coordination of pyridyl nitrogen atoms with the surface gold
atoms of AuNPs has small contribution in the ligation between
MMCs and AuNPs.
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