Redox-Active Hybrid Polyoxometalate-Stabilised Gold Nanoparticles

Article abstract of DOI:10.1002/anie.202005629

We report the design and preparation of multifunctional hybrid nanomaterials through the stabilization of gold nanoparticles with thiol-functionalised hybrid organic–inorganic polyoxometalates (POMs). The covalent attachment of the hybrid POM forms new na

Full text of DOI:10.1002/anie.202005629

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Accepted Article
Title: Redox-active hybrid polyoxometalate-stabilised Au nanoparticles
Authors: Carmen Martin, Katharina Kastner, Jamie M Cameron,
Elizabeth Hampson, Jesum Alves Fernandes, Emma K
Gibson, Darren A Walsh, Victor Sans, and Graham N. Newton
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202005629
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Redox-active hybrid polyoxometalate-stabilised Au nanoparticles
Carmen Martin,[a,b] Katharina Kastner,[a] Jamie M. Cameron,[a] Elizabeth Hampson,[a] Jesum Alves
Fernandes,[c] Emma K. Gibson,[d] Darren A. Walsh,[a]* Victor Sans,[a,e]* and Graham N. Newton[a]*
Abstract: We report the design and preparation of multifunctional
hybrid nanomaterials through the stabilization of gold nanoparticles
with thiol-functionalised hybrid organic-inorganic polyoxometalates.
The covalent attachment of the hybrid-POM forms new
nanocomposites which are stable at temperatures and pH values,
a new family of amphiphilic hybrid POM surfactants.[10] The
surfactant molecules self-assemble in solution to form micellar
aggregates that retain the (modified) redox properties of the
molecular building blocks and similar nanoscale systems have
been proposed as charge carriers for use in solution-phase
energy-storage systems.[11] Hybrid POMs have also been used as
capping agents for Au NPs,[12] where thiol-terminated
organosilane functionalised clusters are used to covalently
stabilise the NP surface. This strategy is notable in that it
minimises desorption of the POM capping groups and presents
additional opportunities for synthetic control over the system. For
instance, Polarz et al. have recently shown how synergistic
interactions between appended hybrid-POM and the surface-
plasmon resonance of AuNPs can enhance photocatalytic
activity.[13] Whilst these studies point to the considerable promise
of such materials, our understanding of the stability and emergent
properties of these systems remains very much in its infancy,
limiting our ability to rationally prepare new nanomaterials with
properties tailored towards specific, novel applications.
Here, we describe the synthesis and properties of AuNPs
stabilised by a thiol-bearing hybrid organophosphoryl-modified
Wells-Dawson POM, K6[P2W17O61(PO2C17H26-SH)2] (1),[14] and
report a detailed comparison of the new Au@hybrid-POM
nanoparticle composite (NP-1) with its electrostatically-stabilised
analogue (NP-P2W18), comprising AuNPs non-covalently capped
with the conventional K6[P2W18O62] ({P2W18}) Wells-Dawson
cluster.[15] Transmission electron microscopy (TEM), X-ray
photoelectron spectroscopy (XPS), X-ray absorption near-edge
structure (XANES), UV-visible absorption, and FT-IR analyses
have been used to probe the structure and surface interactions of
both nanoparticle composites, whilst spectroscopic and
voltammetric analyses have been used to compare their photo--
and electro-chemical properties, respectively. As such, this
represents the first detailed analysis of the vital role played by
direct chemical attachment of the multi-functional POM capping
agent to the nanoparticle surface.
which
destroy
analogous
electrostatically-functionalised
nanocomposites. Photoelectrochemical analyses reveal the unique
photochemical and redox properties of these systems.
Due to their unique catalytic, electronic and optical properties,
gold nanoparticles (AuNPs) are used in a wide range of areas,
including healthcare, sensing, catalysis, and photonics.[1] To
prevent agglomeration of AuNPs during nucleation and growth, a
range of surfactants and capping agents are often used in their
synthesis. These capping agents can also be designed to impart
added functionality, such as redox activity.[2] In this regard, the
use of redox-active polyoxometalates (POMs) offers a range of
material-design opportunities owing to their stability and
enormous structural and compositional diversity. POMs are
anionic metal oxide clusters that exhibit rich electrochemical and
photochemical properties,[3] and are increasingly-popular
reducing agents[4] and electrostatic capping agents[5] for the
synthesis of metal NPs and nanocomposites.[6]
In parallel to the development of NP@POM composites, the
past decade has seen a rapid increase in the development of
organic/inorganic hybrid-POMs, in which organic groups are
covalently tethered to the metal oxide cluster.[7] The properties of
such materials can be finely tuned by changing the nature of the
POM and the organic modifier,[8] paving the way for the rational
molecular design of new multifunctional supramolecular
materials.[7,9] For instance, it was recently shown that POMs
could be functionalised with organophosphoryl groups to prepare
[a]
Dr. C. Martin, Dr. K. Kastner, Dr. J. M. Cameron, E. Hampson, Dr.
Dr. D. A. Walsh, and Dr. G N. Newton
Nottingham Applied Materials and Interfaces (NAMI) Group,
The GSK Carbon Neutral Laboratories for Sustainable Chemistry,
University of Nottingham, Nottingham, NG7 2TU, UK
E-mail: graham.newton@nottingham.ac.uk
darren.walsh@nottingham.ac.uk
[b]
[c]
[d]
[e]
Dr Carmen Martin
Universidad de Sevilla, Departamento de Quimica Fisica, Facultad
de Quimica, 41012 Sevilla, Spain
Dr. J. Alves Fernandes
School of Chemistry, University of Nottingham.
University Park, Nottingham, NG7 2RD, UK
Dr. E. K. Gibson
School of Chemistry, University of Glasgow,
Glasgow, G12 8QQ, UK
Dr. V. Sans
Institute of Advanced Materials (INAM)
University Jaume I, 12006 Castellon, Spain
E-mail: sans@uji.es
Scheme 1. A route to stable, redox and photo-active NP@POM composites.
Supporting information for this article is given via a link at the end of
the document.
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No such effect was observed for NP-P2W18, where agglomerated
groups of NPs (d = 7.2 nm) with a similar shape to the Au@citrate
nanoparticle precursors were observed (Figure S22-24). This
suggests that weaker or unstable coverage of the nanoparticle
surface resulted when te electrostatic capping approach was
used (agreeing with the aggregation observed in TEM imaging of
these materials).
FTIR spectroscopy of NP-1 showed similar features to that
of the hybrid-POM 1, while NP-P2W18 more closely resembles
those of citrate-capped Au NPs (Figures S11 and S12).[16] This
can be attributed to the excellent surface coverage of 1 on the Au
NPs (see SI). The UV-visible spectrum of NP-1 also exhibits
features consistent with successful formation of the
nanocomposite:
a strong absorption centred at 335 nm
corresponding to the ligand-to-metal charge transfer (LMCT)
band of the hybrid-POM and a surface-plasmon resonance (SPR)
peak at 531 nm (Figure S15).[17] The SPR peak is centred at a
slightly longer wavelength than that of the citrate-stabilised Au NP
precursors (527 nm) which is due to the different refractive indices
of the capping agents, confirming the successful capping of the
Au NPs with 1. The SPR peak of NP-P2W18 was found to be
broader than that of the citrate-capped NPs and was red-shifted
by about 15 nm (Figure S18), consistent with agglomeration of the
NPs as revealed previously by TEM.
The stability of both composites was compared against a
range of different conditions. The thermal stability of both
materials was studied via UV-vis spectroscopic analysis. Where
the plasmon resonance band of NP-1 showed no change after
heating at 80 °C for 24h, that of NP-P2W18 disappeared (Figures
S32 and S33). Furthermore, NP-1 was stable upon storage for 1
month at –6 oC, and for at least 24h in both 0.5 % H2O2 and buffer
solutions (pH 5 and 7; measured at 37 oC). Conversely, NP-P2W18
degraded under all of these conditions (Figures S34-S38). The
excellent stability of NP-1 in a range of environments (including
under biologically relevant conditions) suggests that NP-1 and
similar covalently-stabilised NP@POM materials could find use in
a range of potentially demanding applications such as sensing or
catalysis.
Figure 1. a) Schematic showing the synthesis of NP-1 (colour code: {WO6} =
blue polyhedra; {PO4} = purple polyhedra; Au nanoparticles = red spheres;
hybrid-POM 1 = blue spheres, cations omitted for clarity); b) TEM micrograph of
NP-1 highlighting the surface-bound monolayer of POMs (inset); c) size
distributions for NP-1 determined by TEM analysis.
The synthesis of the hybrid-POM capping ligand, 1, was
carried out using a modified version of the synthesis previously
described by our group.[14] Briefly, 4-((11-(thio)undecyl)-oxy)
phenyl-phosphonic acid and the mono-lacunary precursor
K10[P2W17O61] were combined via an acid-mediated condensation
reaction in acetonitrile (see Supporting Information). The hybrid-
Au@POM composite NP-1 was then synthesised by mixing 1 with
a solution of 10 nm citrate-stabilised Au NPs (Au@citrate) in H2O
and stirring for 2 days at room temperature. The resulting
composite nanoparticles had an Au-to-W ratio of 34:1 and the
average number of POMs per Au NP was found to be ca. 400
(according to inductively-coupled plasma optical emission
spectroscopy – refer to Section 8 in the SI). NP-P2W18 was
synthesised according to the same method, but using {P2W18} in
place of 1.
TEM of both nanocomposites showed that NP-1 comprised
monodisperse and well-separated NPs with an average diameter
(dall) of 11.0 nm (Figure 1) Owing to the excellent contrast of W in
TEM imaging, a clear surface layer of POMs could be resolved as
a ring around the surface of the AuNPs in NP-1. This layer is
estimated to have a thickness of 1.7 nm, corresponding closely to
the expected dimensions of a monolayer of hybrid-POM clusters.
Following basic characterisation and comparison of our
systems, we probed the interactions between the capping POM
groups and nanoparticle surfaces. XPS was performed as a
means to investigate the Au-POM interaction in both systems
(Figure 2a). The Au 4f core XPS spectrum of NP-P2W18 shows
significantly lower binding energies (83.3 and 87.00 eV) when
compared to NP-1 (84.0 and 87.7 eV). This is further evidence
Figure 2. a) XPS spectra of NP-1 and NP-P2W18 showing the Au 4f7/5 and 4f7/2 absorptions, respectively; b) XANES spectra of the hybrid POM 1 compared to
the nanocomposite NP-1, showing a decrease in the intensity of the W L3 ‘white line’ absorption edge upon formation of the hybrid nanomaterial, and; c)
XANES spectra of {P2W18} compared to NP-P2W18, showing an increase in the intensity of the W L3 absorption edge upon interaction of the POMs with Au.
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Figure 3. UV-Vis absorption spectra of NP-1 in DMF showing spectrum after
photoreduction of capping POMs with visible light (blue) and after subsequent
aerobic oxidation and recovery of the native state (green).
Figure 4. Cyclic voltammograms of 0.125 mM solutions of 1 and NP-1 in DMF
at a scan rate of 100 mV s-1. Initial scan was to negative potentials. Potentials
have been reported relative to that of the Fc/Fc+ redox couple.
that the capping agents in NP-P2W18 have a strong electrostatic
interaction with the Au surface, and similar binding energies were
also recently found by Weinstock et al. for analogous
electrostatically-decorated Au@{P2W18} NPs (83.5 and 87.20
eV).[6d] Typically, lower binding energies point towards decreased
oxidation states or, more accurately in this case, a higher negative
surface polarisation/charge density resulting from the capping of
the Au NPs by the negatively charged POM in NP-P2W18.[6d,18] It is
therefore interesting that the binding energies found in NP-1 are
comparatively higher and, in fact, are an excellent match to the
values expected for both thiol-functionalised Au0 surfaces,[19] and
bulk Au0 (84.0 and 87.70 ± 0.2 eV). This suggests that the POM
clusters are shielded from the Au surface by the ligand groups
which helps to minimise the electrostatic interaction between
them. A similar effect has been previously reported by Cronin and
Kadodwala et al. for POM monolayers on gold surfaces.[20]
To further probe the surface interactions of the POM-
capped nanomaterials, XANES analysis was performed on both
NP-1 and NP-P2W18 and compared to the corresponding spectra
for the native POM cluster in each case (Figure 2b-c). The broad
‘white line’ W-L3-edge observed in all four spectra corresponds
well to the expected peak shape for distorted {WO6} octahedra.[21]
More interestingly, the intensity of the white line signal can also
provide an indication of the charge density associated with the W-
centres. Here, a clear difference can be observed between the
covalently-bound species in NP-1 and the electrostatically-bound
clusters in NP-P2W18. The white line intensity falls in the case of
the former, suggesting an increase in the effective charge density
of the W centres in the hybrid-POM, whereas it rises in the case
of the latter, suggesting a reduction in the charge density. This
supports the findings of the XPS analysis, suggesting that each
POM interacts differently with the Au surface. In the case of the
non-covalent composites, {P2W18} donates electron density in a
manner similar to that previously discussed by Weinstock.[6d] As
discussed above however, the covalently-bound POMs in NP-1
are effectively shielded from the gold surface and do not
electronically interact with it. Instead, one possibility is that the
tight-packing of the POM monolayer around the NPs (which have
a notably high POM-loading) increases the coulombic interactions
between neighbouring anions, significantly modifying the local
environment of each cluster. This agrees with previous
observations we have made on micellar systems,14] and also with
previous studies of POM monolayers formed on Au NPs, where
even the extensive integration of counter-cations into the
monolayer does not fully attenuate the high negative charge at
the composite surface.[6c] This is likely to have profound
implications for the photochemical and redox properties of the
composite materials (vide infra).
The photochemical properties of the new nanocomposites
were evaluated by irradiating a de-aerated sample of NP-1 in
N,N’-dimethylformamide (DMF) using a solar simulator (200 W)
fitted with a 395 nm cut-off filter. After a 30 minutes of visible-light
irradiation, the absorption spectrum of NP-1 exhibited the typical
intervalence charge transfer (IVCT) band expected for a mono-
reduced organophosphoryl hybrid Wells-Dawson ion at 820
nm,[8a] while the SPR band from the Au NP core at 531 nm
remained intact (Figure 3). Interestingly, the sample could be
easily and cleanly re-oxidised by bubbling O2 into the solution,
and several reduction-oxidation cycles could be performed
without any evidence of decomposition (Figure S30), indicating
that these materials may be useful for photometric or
photocatalytic applications.[22] In contrast, the photoreduced state
of NP-P2W18 could only be accessed using harder UV irradiation
(i.e. < 395 nm), under which conditions NP-P2W18 decomposed
(Figure S31). Whilst organophosphonate hybrid-POM clusters in
particular are known to be photoactive in the near-visible regime,
their photoreduction can be difficult to reverse in O2 due to their
low lying LUMO levels and correspondingly more positive redox
potentials.[23] One possibility is that the tightly packed structure of
1 on the NP surface may destabilise the reduced state of the
POM, as seen previously in micellar systems,[14] facilitating its re-
oxidation by a relatively mild oxidant (O2).
Owing to the highly reversible, rich multi-electron redox
chemistry conferred by the POM capping groups, we also studied
the electrochemical properties of both composites. Cyclic
voltammetry of 0.125 mM NP-1 in DMF shows two quasi-
reversible redox processes, centred at –0.76 V (peak to peak
separation, ∆Ep = 90 mV) and –1.15 V (∆Ep = 140 mV) vs. Fc/Fc+.
This cyclic voltammogram is similar to that of the discrete hybrid-
POM 1 (E1/2 = -0.74 V (∆Ep = 70 mV) and -1.12 V (∆Ep = 100 mV),
see Figure 4). The larger ∆Ep values and negatively shifted redox
potentials observed for NP-1 indicates that reduction/oxidation is
more kinetically and thermodynamically hindered in this system.
This agrees with both the XANES analysis, which indicates that 1
is more electron-rich when bound to the NP surface (Fig. 2b), and
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our previous discussion around the aerobic reoxidation of the
photoreduced state. NP-1 is also shown to be stable to repeated
potential cycling (Figure S20), highlighting the strong attachment
of the POM to the Au surface. In contrast, the behaviour of NP-
P2W18 is identical to that of the {P2W18} capping groups (Figure
S19), indicating that the kinetics of electron transfer to and from
the weakly bound POMs appear unaffected by their electrostatic
association to the AuNP surface. The electrochemical properties
of 1 and NP-1 were also compared via voltammetry using a Pt
ultramicroelectrode (see SI for details). 1 yielded a clear steady-
state voltammogram due to efficient, convergent diffusion of the
redox species to the electrode surface from the expanding
diffusion field. In contrast, NP-1 yielded a transient (peak-shaped)
response due to depletion of the slowly-diffusing nanocomposite
within the diffusion field next to the electrode surface, consistent
with the retention of stable POM-NP interactions in NP-1.
Keywords: redox chemistry • gold nanoparticle • hybrid material
• photochemistry • polyoxometalate
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The authors thank the University of Nottingham Propulsion
Futures Beacon of Excellence. KK thanks the Deutscher
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thanks
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A new material based on gold
nanoparticles stabilised by
organic-inorganic hybrid
Carmen Martin, Katharina
Kastner, Jamie M. Cameron,
Elizabeth Hampson, Jesum
Alves Fernandes Emma K.
Gibson, Darren A, Walsh,* Victor
Sans,* and Graham N. Newton*
polyoxometalates is described.
The new nanomaterial exhibits
unprecedented stability and
photoactivity under visible light
and can be reversibly reduced
and oxidised at electrode
Page No. – Page No.
Redox-active
hybrid
surfaces, making it very
polyoxometalate-stabilised Au
nanoparticles
promising for future photonic
and electrochemical systems.
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