Structural control of the monolayer stability of water-soluble gold nanoparticles

Article abstract of DOI:10.1039/b711434f

The thermodynamic and kinetic stability of three structurally related monolayer-protected gold clusters have been systematically investigated, revealing that the nanoparticles display significantly different stability against thermo- and cyanide-induced decomposition and external thiol agents. The Royal Society of Chemistry.

Full text of DOI:10.1039/b711434f

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COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
Structural control of the monolayer stability of water-soluble gold
nanoparticles{
Sarit S. Agasti, Chang-Cheng You, Palaniappan Arumugam and Vincent M. Rotello*
Received 26th July 2007, Accepted 3rd October 2007
First published as an Advance Article on the web 15th October 2007
DOI: 10.1039/b711434f
by varying the chain length of the surrounding alkanethiol
ligands.13 Various peptide ligands14 and dendritic ligands15 have
also been used to tune the MPC stability. In a recent study, we
have investigated the in situ place-exchange reactions of gold
MPCs with a variety of alkanethiols and found that the reaction
kinetics are critically dependent on the structure of the external
alkanethiols.16 From this result, we envision that minor alterations
of ligand structures should allow tuning of MPC stability diversity.
To address the issues regarding the monolayer stability in terms of
subtle changes in the ligand structure, we prepared three struc-
turally related MPCs (Fig. 1) and investigated their thermo-
dynamic and kinetic stability. We have demonstrated that the
steric arrangement of substituents as well as the interaction
between adjacent ligands govern the MPC stability.
The thermodynamic and kinetic stability of three structurally
related monolayer-protected gold clusters have been system-
atically investigated, revealing that the nanoparticles display
significantly different stability against thermo- and cyanide-
induced decomposition and external thiol agents.
Monolayer protected clusters (MPCs) represent hybrid systems
intermediate between molecular systems and macroscopic mate-
rials and surfaces. In these systems, the nanometre-scale metal
cores of polyhedral shape are covered by a self-assembled
monolayer of organic ligands. Stimulated by the pioneering work
of Brust et al. in 1994,1 thiol-protected gold clusters (Au-MPCs)
have evolved to the most widely studied particle–ligand systems so
far.2 The optical, electronic and bioinert properties of the gold core
combined with the versatile surface functionalization facilitate the
applications of this kind of material in both fundamental research
and industrial development.3 These functional nanomaterials have
shown promising potential in electronics,4 catalysis,5 biosensing,6
and nanomedicine.7
The three nanoparticles were prepared through a single-phase
reduction of chloroauric acid by sodium borohydride in the
presence of the corresponding thiol ligands according to a
procedure described by Huang et al.17 Transmission electron
microscopy (TEM) revealed that the three Au-MPCs have the
same average core diameter of y3 nm (see ESI{). As shown in
Fig. 1, the three nanoparticles are capped with the respective thiol
ligands of the same backbone, where a three-carbon alkyl chain is
followed by a tri(ethylene glycol) segment. The tri(ethylene glycol)
moiety endows the resultant nanoparticles with good water
solubility. For the three ligands, the only structural difference lies
in the presence/absence of a methyl group on the alkyl chain and
its substituted position. A small amount of L-tryptophan moieties
(ca. 10%) were introduced onto the terminus of the thiol ligands to
The functionalization of gold clusters is commonly achieved
through Brust–Schiffrin reduction1 followed by Murray place-
exchange reaction8 with appropriate functional ligands. The
incorporation of multiple functionality within the scaffold of a
single cluster coupled with a scale comparable with biomacro-
molecules (e.g. proteins and DNA) provides multiple applications
at the interface of biotechnology and nanomaterials. For instance,
some recent studies have shown that proper surface functionaliza-
tion holds great promise in disrupting protein–protein interaction,9
regulating DNA transcription,10 gene transfection,11 etc.
Particle stability is of crucial importance for biological applica-
tions. On one hand, surface functionality of particles needs to be
preserved for proper function, e.g. imaging agents. Under some
circumstances, however, the MPC ligands need to be readily
replaceable by other ligands for the transformation of surface
functionality, for example in drug and gene delivery where the
carrier releasing can be triggered by certain cellular agents
(e.g. glutathione).12 In this regard, the modulation of monolayer
stability of MPCs is central to their environmentally dependent
applications.
Densely packed organic ligands confer stability to MPC
monolayers. Murray et al. have differentiated the cluster stability
Department of Chemistry, University of Massachusetts, 710 North
Pleasant Street, Amherst, MA, 01003, USA.
E-mail: rotello@chem.umass.edu; Fax: +1-413-5454490;
Tel: +1-413-5452058
{ Electronic supplementary information (ESI) available: Synthesis and
characterization of the ligands, the preparation of nanoparticles, and
experimental procedures. See DOI: 10.1039/b711434f
Fig. 1 Monolayer structure of three structurally related Au-MPCs.
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facilitate monitoring of the ligand-exchange reactions by harnes-
sing their inherent fluorescence properties (for the fluorescence
spectra, see Fig. S1, S2 and S3 in ESI{).
The thermal stability of the MPCs was studied by using
thermogravimetric analysis (TGA). The mass change was plotted
against temperature to generate the thermogravimetric combustion
profiles. Such mass loss reflects the decomposition of thiol ligands
from the gold surface. Fig. 2 shows the derivative curves of TGA
mass loss for the three MPCs. As can be seen, three MPCs exhibit
different thermal decomposition profiles, indicating the diverse
thermal stability provided by the surface ligands.18 From the
positions of the derivative peaks,{ it is clear that the well-defined
mass loss step for the cluster protected with iso-thiol (NP_Iso) is
comparatively higher than its linear analog (NP_Nor). By contrast,
NP_Sec shows the least thermal stability although it differs from
NP_Iso only by the position of the methyl substituent. These
results are consistent with our previous studies on alkyl-func-
tionalized particles,16 indicating that the ligand substituent position
distinctly affects the packing performance of ligands on the gold
surface and, thereby, the thermal stability of the resulting MPCs.
The chemical stability of the clusters in water was further tested
by cyanide-induced decomposition of the gold core. Previous
studies on self-assembled monolayers (SAMs) of thiols on a 2D
gold surface have shown that the gold underlayer can be corroded
by cyanide anions.19 In a similar fashion, thiol ligand protected 3D
gold clusters can also be decomposed into a colourless mixture
Fig. 3 UV/vis absorption changes of phosphate buffered saline (PBS)
solution of MPCs (0.2 mmol dm23) at 520 nm in the presence of potassium
cyanide (3 mmol dm23) at 37 uC. The lines represent the best fitting of the
data according to the kinetics of first-order reactions. The inset shows the
UV/vis spectrum change of NP_Iso during the time course of cyanide
decomposition.
absorption change profiles for three MPCs are depicted in Fig. 3.
As the cyanide concentration is 15 times higher than that of gold
atoms and is essentially constant during the reaction course, the
absorption change data was fitted to a general first-order reaction
function. As can be seen in Fig. 3, excellent curve-fitting results are
obtained for all three MPC systems, confirming the first-order
kinetics. The kinetic constants, including pseudo-first-order and
second-order rate constants, are compiled in Table 1.
2
comprising an Au(CN)2 complex, dialkyl disulfides and alkyl
cyanides.13 As the accessibility of cyanide anions to the gold core
is dependent on the protection of SAMs, the decay rate of
gold clusters in the presence of cyanide is correlated with the
particle stability.
In our experiments, the nanoparticle surface plasmon band
decreased gradually and the colour of the nanoparticle solution
changed from red-brown to colourless (for UV/vis spectral changes,
see inset in Fig. 3). The absorption changes of 0.2 mmol dm23
MPCs at 520 nm were monitored over time in the presence of an
excess amount of sodium cyanide (3 mmol dm23). Representative
The rate constants in Table 1 reveal that the decomposition
rates of MPCs increase in the order of NP_Iso , NP_Nor ,
NP_Sec. Accordingly, the chemical stability of the MPCs follows
an inverse order, which is governed by the protection of the gold
core against cyanide attack. Such a chemical stability sequence
displays the same trend as that of MPC thermal stability
(vide supra). As cyanide anions need first to access the surface
gold atoms to destroy the MPCs, the chemical stability of MPCs is
obviously related to the organization of ligands on the cluster
surface. In the case of NP_Sec, the methyl group adjacent to the
mercapto group provides steric hindrance upon interaction with
the nanoparticle surface, resulting in the relatively loose packing
of ligands within the monolayer.20 Indeed, TGA studies have
revealed that NP_Sec possesses the lowest ligand loading among
the three MPCs, while NP_Nor and NP_Iso have comparable
ligand loadings (see inset in Fig. 2).§ Therefore, the surface gold
atoms in NP_Sec are the most accessible by cyanide anions and its
decomposion rate is twice that of its linear analog. Interestingly,
in comparison with NP_Nor that is covered by linear ligands,
the branched ligand-functionalized NP_Iso shows additional
Table 1 The pseudo-first-order (k1) and second-order rate constants
(k2) for the decomposition of MPCs by potassium cyanide at 37 uC
Fig. 2 Derivative thermogravimetric curves of three MPCs with mixed
monolayers. The particles were heated from room temperature (ca. 25 uC)
to 500 uC at a rate of 10 uC min21 under a nitrogen atmosphere and the
mass change was recorded as a function of temperature. The inset shows
the weight loss versus the temperature.
Au-MPCs
k1/1023 s21
k2/dm3 mol21 s21
NP_Sec
NP_Nor
NP_Iso
3.58 ¡ 0.59
1.84 ¡ 0.20
0.84 ¡ 0.13
1.19 ¡ 0.19
0.61 ¡ 0.06
0.28 ¡ 0.04
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stabilization and its decomposition is only half that of NP_Nor.
This protection effect should arise from the methyl group in the
alkyl chain. It has been well demonstrated that a splay angle exists
between the adjacent ligands on spherical nanoclusters.20,21 The
methyl group in NP_Iso is expected to block such a splaying space
between the adjacent ligands. The methyl group is separated from
the mercapto group by two carbon atoms and thereby the steric
hindrance as in NP_Sec is significantly reduced. Moreover, the
methyl groups in the adjacent ligands can produce favourable van
der Waals and hydrophobic interactions, which further improve
the chemical stability of NP_Iso. In this context, it is not surprising
that NP_Iso shows the lowest cyanide decomposition rate.
The above studies establish the stability of MPCs in both solid
and solution states by regarding the favorable and unfavorable
steric arrangement of the substituents. Another important issue
that needs to be addressed is the MPC stability in the presence of
external thiol ligands, which is central to the applications of such
kinds of MPCs in biological systems. Therefore, we further
explored the in situ place-exchange reaction of the MPCs with
dithiothreitol (DTT), dihydrolipoic acid (DHLA), and glutathione
(GSH). The latter two compounds are particularly attractive since
they are the most abundant thiols in living cells. In the ligand
exchange experiments, the reaction kinetics were followed by using
a fluorophore displacement approach.22 The tryptophan fluores-
cence is initially quenched by gold clusters through energy/electron
transfer, while fluorescence recovery occurs once the ligands are
replaced by other thiols. To avoid the possible interference from
precipitation of the clusters or aggregation of hydrophobic HSTrp
in aqueous solution, a two-phase system was employed to extract
the released tryptophan-containing ligands into the organic phase
for signal detection (Fig. 4a).
Fig. 4b illustrates a representative fluorescence recovery curve of
NP_Sec in the presence of DHLA. As can be seen, the fluorescence
intensity at 350 nm increases exponentially overtime. Moreover,
the kinetic analysis of the parameter of ln[(If 2 I)/(If 2 I0)] as a
function of time shows a linear dependence on time (inset in
Fig. 4b), indicating the first-order kinetics. The pseudo-first-order
and second-order rate constants obtained are listed in Table 2.
While not as marked as for the cyanide decomposition, these rates
increase substantially in the order of NP_Iso , NP_Nor ,
NP_Sec. In other words, the kinetics of the ligand displacement
correlates again with the monolayer structure of MPCs. As the
ligand exchange reaction takes place first at the edges and
corners in the clusters,21 this result indicates that the branched
ligand in NP_Iso provides also a certain level of protection over
these regions.
Fig. 4 (a) Schematic depiction of surface monolayer exchange experi-
ments in a two-phase system and chemical structure of thiol ligands used
in ligand exchange. The released fluorophores were transferred into the
toluene phase and the fluorescence intensity of the toluene phase was
recorded over time. (b) The normalized fluorescence intensity at 350 nm
(lex = 295 nm) for the ligand exchange of NP_Sec (0.2 mmol dm23) with
DHLA (1 mmol dm23) in PBS at 37 uC. The inset shows the logarithm of
fluorescence intensity changes versus time and corresponding linear
regression plot (see ESI{ for details).
Structurally, DTT and DHLA carry two mercapto groups but
GSH possesses only one. The interaction of the former two thiols
with the target gold surface can be stabilized by synergetic
bonding, or namely a chelation effect. Therefore, DTT and DHLA
It is interesting to compare the impact of incoming thiols on the
place-exchange kinetics. In the reaction with MPCs, the reaction
rates decrease in the order of DTT . DHLA & GSH.
Table 2 The pseudo-first-order (k1) and second-order rate constants (k2) for the ligand exchange of MPCs (0.2 mmol dm23) with various thiol
ligands at 37 uCa
DTT
DHLA
GSH
Au-MPC
k1/1022 min21
k2/dm3 mol21 min21
k1/1022 min21
k2/dm3 mol21 min21
k1/1022 min21
k2/dm3 mol21 min21
NP_Sec
NP_Nor
NP_Iso
a
2.86 ¡ 0.25
2.38 ¡ 0.13
1.76 ¡ 0.09
28.6 ¡ 2.5
23.8 ¡ 1.3
17.6 ¡ 0.9
2.19 ¡ 0.02
1.96 ¡ 0.11
1.57 ¡ 0.07
21.9 ¡ 0.2
19.6 ¡ 1.1
15.7 ¡ 0.7
0.33 ¡ 0.02
0.18 ¡ 0.02
0.11 ¡ 0.04
0.66 ¡ 0.04
0.36 ¡ 0.04
0.22 ¡ 0.08
[DTT] = [DHLA] = 1 mmol dm23, [GSH] = 5 mmol dm23
.
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2005, 17, 163–171; (c) S. Eustis and M. A. El-Sayed, Chem. Soc. Rev.,
2006, 35, 209–217; (d) C.-C. You, A. Verma and V. M. Rotello, Soft
Matter, 2006, 2, 190–204.
4 (a) D. I. Gittins, D. Bethell, D. J. Schiffrin and R. J. Nichols, Nature,
2000, 408, 67–69.
5 (a) H. Li, Y. Y. Luk and M. Mrksich, Langmuir, 1999, 15, 4957–4959;
(b) P. Pengo, L. Baltzer, L. Pasquato and P. Scrimin, Angew. Chem., Int.
Ed., 2007, 46, 400–404.
6 C.-C. You, O. R. Miranda, B. Gider, P. S. Ghosh, I. B. Kim,
B. Erdogan, S. A. Krovi, U. H. F. Bunz and V. M. Rotello, Nat.
Nanotechnol., 2007, 2, 318–323.
show higher activity against ligand exchange. On the other hand,
the steric hindrance in close proximity to the thiol group in the case
of GSH drastically reduces its activity in ligand-exchange
reactions. DTT displays slightly higher activity over DHLA. One
plausible explanation is that the molecular size of the former is
smaller and it can penetrate the monolayer more easily during
the ligand-exchange reaction. Taken together, the three MPCs
show interesting activity and/or inertness against ligand exchange,
confirming the important role of monolayer packing in cluster
stability.
7 (a) I. H. El-Sayed, X. H. Huang and M. A. El-Sayed, Nano Lett., 2005,
5, 829–834; (b) P. K. Jain, I. H. El-Sayed and M. A. El-Sayed, Nano
Today, 2007, 2, 18–29.
In summary, we have explored the effect of subtle structural
changes of surface ligands on the cluster stability. These systematic
investigations on thermodynamic and kinetic stability of three
structurally related Au-MPCs reveal the modulation of surface
ligand packing through the introduction of substituents into the
side chain of ligands. The control over monolayer stability by
properly tuning ligand structures represents an important step
towards successful design of similar systems for future nanotech-
nology applications. The kinetic studies have demonstrated that
the ligand displacement rates by intracellular thiols (e.g. DHLA
and GSH) are critically dependent on the monolayer composition.
Such activity diversity can be utilized to achieve temporal release of
drug molecules from MPC-based drug delivery systems. Tunable
access of MPCs through chemical modification of monolayer
structures and incorporation of targeting functionalities will
further lead to controlled delivery in vivo.
8 A. C. Templeton, M. P. Wuelfing and R. W. Murray, Acc. Chem. Res.,
2000, 33, 27–36.
9 H. Bayraktar, P. S. Ghosh, V. M. Rotello and M. J. Knapp, Chem.
Commun., 2006, 1390–1392.
10 (a) C. M. McIntosh, E. A. Esposito, A. K. Boal, J. M. Simard,
C. T. Martin and V. M. Rotello, J. Am. Chem. Soc., 2001, 123,
7626–7629; (b) G. Han, C.-C. You, B. J. Kim, R. S. Turingan,
N. S. Forbes, C. T. Martin and V. M. Rotello, Angew. Chem., Int. Ed.,
2006, 45, 3165–3169; (c) C. Agbasi-Proter, J. Ryman-Rasmussen,
S. Franzen and D. Feldheim, Bioconjugate Chem., 2006, 17, 1178–1183.
11 (a) K. K. Sandhu, C. M. McIntosh, J. M. Simard, S. W. Smith and
V. M. Rotello, Bioconjugate Chem., 2002, 13, 3–6; (b) N. L. Rosi,
D. A. Giljohann, C. S. Thaxton, A. K. R. Lytton-Jean, M. S. Han and
C. A. Mirkin, Science, 2006, 312, 1027–1030.
12 (a) G. Han, N. S. Chari, A. Verma, R. Hong, C. T. Martin and
V. M. Rotello, Bioconjugate Chem., 2005, 16, 1356–1359; (b) R. Hong,
G. Han, J. M. Fernandez, B. J. Kim, N. S. Forbes and V. M. Rotello,
J. Am. Chem. Soc., 2006, 128, 1078–1079.
13 A. C. Templeton, M. J. Hostetler, C. T. Kraft and R. W. Murray, J. Am.
Chem. Soc., 1998, 120, 1906–1911.
14 L. Fabris, S. Antonello, L. Armelao, R. L. Donkers, F. Polo, C. Toniolo
and F. Maran, J. Am. Chem. Soc., 2006, 128, 326–336.
15 C. S. Love, I. Ashworth, C. Brennan, V. Chechik and D. K. Smith,
J. Colloid Interface Sci., 2006, 302, 178–186.
16 R. Hong, J. M. Ferna´ndez, H. Nakade, R. Arvizo, T. Emrick and
V. M. Rotello, Chem. Commun., 2006, 2347–2349.
Acknowledgements
This research is supported by the Center for Hierarchical
Manufacturing (NSEC, DMI-0531171), MRSEC facilities,
and National Institute of Health (NIH, GM077173).
17 M. Zheng, F. Davidson and X. Y. Huang, J. Am. Chem. Soc., 2003,
125, 7790–7791.
18 R. H. Terrill, T. A. Postlethwaite, C. H. Chen, C. D. Poon, A. Terzis,
A. D. Chen, J. E. Hutchison, M. R. Clark, G. Wignall, J. D. Londono,
R. Superfine, M. Falvo, C. S. Johnson, E. T. Samulski and
R. W. Murray, J. Am. Chem. Soc., 1995, 117, 12537–12548.
19 (a) A. Kumar and G. M. Whitesides, Appl. Phys. Lett., 1993, 63,
2002–2004; (b) C. S. Weisbecker, M. V. Merritt and G. M. Whitesides,
Langmuir, 1996, 12, 3763–3772.
Notes and references
{ Repeated experiments indicate a reproducibility of ¡3 uC.
§ According to the TGA and TEM results, it is estimated that ligand
densities of NP_Sec, NP_Nor and NP_Iso are 3.1, 4.5, and 4.3 ligand nm22
,
respectively. The relatively lower ligand density of NP_Sec is not surprising
as the steric repulsion between the methyl groups adjacent to the surface
bound sulfur atom would lead to a bigger footprint of the ligands.
20 J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and
G. M. Whitesides, Chem. Rev., 2005, 105, 1103–1170.
21 A. M. Jackson, Y. Hu, P. J. Silva and F. Stellacci, J. Am. Chem. Soc.,
2006, 128, 11135–11149.
1 M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman,
J. Chem. Soc., Chem. Commun., 1994, 801.
2 M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.
3 (a) K. G. Thomas and P. V. Kamat, Acc. Chem. Res., 2003, 36,
888–898; (b) L. Pasquato, P. Pengo and P. Scrimin, Supramol. Chem.,
22 (a) M. J. Hostetler, A. C. Templeton and R. W. Murray, Langmuir,
1999, 15, 3782–3789; (b) M. Montalti, L. Prodi, N. Zaccheroni,
R. Baxter, G. Teobaldi and F. Zerbetto, Langmuir, 2003, 19, 5172–5174.
This journal is ß The Royal Society of Chemistry 2008
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