Redox‐Active Hybrid Polyoxometalate‐Stabilised Gold Nanoparticles

Abstract 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 nanocomposites that are stable at temperatures and pH values which destroy analogous electrostatically functionalised nanocomposites. Photoelectrochemical analysis revealed the unique photochemical and redox properties of these systems.

S4 X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos AXIS Ultra DLD instrument. The chamber pressure during the measurements was 5 × 10−9 Torr. Wide energy range survey scans were collected at pass energy of 80 eV in hybrid slot lens mode and a step size of 0.5 eV. High-resolution data on the C 1s, O 1s, S 2p, P 2p, Au 4f and W 4f photoelectron peaks was collected at pass energy 20 eV over energy ranges suitable for each peak, and collection times of 5 min, step sizes of 0.1 eV. The charge neutraliser filament was used to prevent the sample charging over the irradiated area. The X-ray source was a monochromated Al Kα emission, run at 10 mA and 12 kV (120 W). The energy range for each 'pass energy' (resolution) was calibrated using the Kratos Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 three-point calibration method. The transmission function was calibrated using a clean gold sample method for all lens modes and the Kratos transmission generator software within Vision II. The data were processed with CASAXPS (Version 2.3.17). The high resolution data was charge corrected to the reference carbon adventitious signal at 284.8 eV.
XANES spectroscopy measurements were performed on the I20-scanning beamline at the Diamond Light Source (Harwell Campus, UK). I20 is equipped with a Si(111) four bounce monochromator for selecting the incident X-ray energy, a multi-element Ge (fluorescence) detector was used to record the X-ray absorption spectra. The measurements were performed in air at room temperature at the W L3 edge. Multiple scans were taken to improve signal-to-noise ratio. All data were analysed using the Athena software package.

Synthesis of Au@POM nanocomposites NP-1 and NP-P2W18 by ligand exchange.
A standard procedure to prepare both nanocomposites was as follows: 0.02 mmol of polyoxometalate (around 100 mg of 1 for NP-1 or P2W18 for NP-P2W18) were placed in a flask and dissolved in 5 mL of milli-Q water. Then, 25 mL of citrate stabilized AuNPs (from Sigma Aldrich, 10 nm) were added. The mixture was stirred for 2 days at 25 °C in the absence of light. Afterwards, the red solutions containing a solid in suspension were centrifuged at 7000 rpm for 5 min to separate the waste solid from the solution containing AuNPs. Next, the solvent was concentrated using a rotary evaporator to give a deep red solution (around 1-2 mL volume). 3 mL of methanol was added to precipitate the AuNPs which were then separated from the supernatant by centrifugation. AuNPs were washed with a mixture of methanol:acetonitrile (1:1, 4 mL in total). This process was repeated three more times before drying under vacuum. Although NP-1 and NP-P2W18 are stable at ambient temperature, both samples were stored in the dark at 6 °C.   Figure S1. 1 H NMR spectrum of (4-((11-(thio)undecyl)oxy)phenyl)phosphonic acid (E) in DMSO-d6 (500 MHz, RT). Figure S2. 13 C NMR spectrum of (4- ((11-(thio)

S22
Ultramicroelectrode cyclic voltammetry measurements. A three-component system consisting of a Pt working microelectrode (d = 25µm), a Ag-wire reference electrode and Pt wire counter electrode was used. All potentials are reported against ferrocene as an internal reference. Experiments were performed under N2 atmosphere at r.t. and in dry DMF solutions 0.1 M of tetrabutylammoniumhexafluorophosphate (NBu4PF6) was employed as supporting electrolyte. All measurements were performed at a scan rate of 10 mV s -1 .   S25 Figure S26. TEM micrograph of NP-1. Figure S27. TEM micrograph of NP-1.

CALCULATION OF AVERAGE NUMBER OF POMs PER NANOPARTICLE
If we assume spherical and uniform nanoparticles, the average number of gold atoms per nanoparticles (N) can be calculated by the following equation: 5 where r is the density of fcc gold (19.3 g/cm 3 ), M is the atomic weight of gold (197 g/mol) and D is the average core diameter of AuNPs in nm obtained by HR TEM.
With this data in hand, the average number of POM ligands per nanoparticle may be calculated by the used of ICP-OES analysis and by UV-vis spectroscopy (colorimetry).

ICP-OES ANALYSIS
The Gold to Tungsten ratio for NP-1 was determined by ICP-OES, and is calculated to be in the proportion of 34:1 gold atoms to POM clusters (each POM containing 17 tungsten atoms).
If the average number of gold atoms per nanoparticles (N) is 14105 and the Gold:POM ratio is 34:1, the average number of POMs per nanoparticles can be estimated as 414.

COLORIMETRY
The average number of POMs per nanoparticle can be also calculated by UV-vis spectroscopy (colorimetry). In this experiment, the reduced hybrid POM (1red) is used as analyte, with a strong characteristic absorbance at 820 nm. A calibration plot (Figures S28 and S29) were prepared using different solutions with concentrations of 1 between 1-40 µM.
Solutions were irradiated for 45 min with a solar simulator (200 W) fitted with a 395 nm cut-off filter ( Figure S28). Similarly, a solution of 0.25 mg of NP-1 in 3 mL DMF was was irradiated for 45 min under the same conditions and its UV-vis spectrum was also recorded (black trace in Figure S28). The POM concentration in this sample was obtained by comparison to the calibration profile shown in Figure S29: Abs(NP-1) = 0.0548 → [POMs] = 8.4 µM → Au-to-POM ratio is 25:1 → 564 Here, if the average number of gold atoms per nanoparticles (N) is 14105 and the gold-to-POMs ratio is 25:1, the average number of POMs per nanoparticles can be estimated as 564.

S27
We attribute the difference between the Gold-to-POM ratio and the number of POMs per nanoparticle calculated by both methods to be due to systematic error in the colorimetry analysis, since the Au plasmon resonance band and POMs intervalence charge transfer bands somewhat overlap in the UV-vis spectrum of NP-1.   irradiated without cut-off filters during 45 mins (green trace) and photoreduced NP-P2W18 (green trace) oxidized by bubbling O2 for 60 mins (pink trace). In comparison to Figure 3 in the manuscript, this clearly shows both the markedly lower photoreactivity and instability of NP-P2W18 in comparison to NP-1 (shown by the shift in the gold surface plasmon upon successful reduction of the POM).