Supporting Information
Role of Alkali-Metal Cation Size in the Self-Assembly of Polyoxometalate-Monolayer Shells on Gold Nanoparticles Yifeng Wang, Offer Zeiri, Shelly Sharet and Ira A. Weinstock* Department of Chemistry and the Ilse Katz Institute for Nanoscale Science & Technology, Ben Gurion University of the Negev, Beer Sheva, 84105, Israel
Table of contents I. Materials and methods II. Figure S1. Method used to obtain Figure 1D of the text. III. Stabilities of citrate protecting anion “layers” on the gold nanoparticles IV. Figure S2. Adsorption isotherm for reactions of the tetramethylammonium (TMA+) salt of 1 with TMA3Ct-protected Au NPs. V. Figure S3. Values of K as functions of pH in relation to the protonation states of K81 and citrate. VI. Table S1. DLS and zeta potential data, and electric-double layer parameters of citrate- and 1stabilized Au NPs. VII. Method used to determine the effective charges, q, in Table 2 of the text
S1
I. Materials and methods Materials. The polyoxometalate (POM) salts, α-K8SiW11O39·13H2O (K81),1 α-K9AlW11O39·13H2O (K92),2-4 and K12.5Na1.5[NaP5W30O110·15H2O]5 (K12.5Na1.53), were prepared as previously described. Trisodium citrate, C6H5Na3O7·2H2O (AG; Na3Ct) was obtained from Sigma-Aldrich, and citric acid, C6H8O7 (AG) was purchased from Frutarom LTD. The citrate salts, Li3Ct·4H2O, K3Ct·H2O, Cs3Ct·~4H2O, and TMA3Ct·~4H2O, were prepared by neutralizing citric acid solutions using hydroxides of the corresponding cations, monitored using a pH meter, and evaporated to dryness. The purities of the POM and citrate salts were checked (as appropriate) by FTIR, cyclic voltammetry (CV), and by 27Al and 31 P NMR spectroscopy.4 All solutions were prepared using high-purity Millipore® water (17 ± 1 MΩ) and all glassware used for synthesis and storage of gold nanoparticles was pre-treated with fresh aqua regia (3:1 v:v ratio of HCl to HNO3). Instruments. UV-visible spectra were obtained using a Hewlett-Packard 8453 spectrophotometer. Cryogenic sample preparation for transmission electron spectroscopy (cryo-TEM), and image acquisition, were as previously described.6-8 TEM and cryo-TEM images were captured on a FEI Tecnai 12 G2 instrument (120 kV) using a Gatan slow-scan camera. Kinetic data were obtained using an SX20 stopped-flow spectrometer (Applied Photophysics Ltd., UK) equipped with a photomultiplier and a cell path-length of 10 mm. The reaction temperature was maintained by a JULABO F12-ED circulating-bath at 25.0 ± 0.1 °C. Zeta potential data were obtained using a particle electrophoresis instrument (ZEM 3600, Zetasizer, Malvern Instruments Ltd). Dynamic light scattering (DLS) data were collected at 25 °C on an ALV-CGS-8F instrument (ALV-GmbH, Germany). FTIR spectra were obtained using a Nicolet 380 FT-IR Spectrometer (Thermo Scientific Inc.) Syntheses of POM salts containing different counter cations. Na81, Li81 and TMA81 (TMA = (nC4H9)4N), were prepared by cation-exchange. For that, ca. 2 mM solutions of K81 were passed very slowly (approx. 4 mL h-1) through a 60-cm length, 2-cm diameter column packed with ca. 160 mL of Amberlite 200 resin, Na+, Li+, and TMA+ forms, respectively. The eluent solutions were evaporated to dryness under reduced pressure. The amount of residual K+ ions in Li81 and Na81 was < 0.03% (sodium tetraphenylborate test9). Proton NMR was used to determine the number (n) of TMA cations in TMAn1, using methanesulfonic acid as an internal standard. The signal intensities indicated gave n = 8.0 ± 0.3. Cs81 was prepared by addition of 2 M CsCl to a 10 mM solution of Li81, with the precipitate recrystallized and dried in air. Using Li-7 NMR (with LiCl as reference), no residual Li+ was detected in D2O solutions of the as-prepared Cs81. The number of waters of crystallization in the polyoxometalate salts was assumed to be 13 ± 5 similar to the published1 value of 13 for K81·13H2O. Error analysis confirmed that small variations waters of crystallization resulted in negligibly small uncertainties in Langmuir constants (see below). The structural integrity of each POM salt was confirmed by FTIR spectroscopy and/or NMR spectroscopy and by cyclic voltammetry (CV). Two additional mono-defect Keggin-anion salts, Na92 and Li92, and the acid form of the Preyssler anion, H143·58H2O, were prepared by cation exchange as described above. For the Na+ and Li+ salts of 2, mild heating during solvent evaporation gave mixtures of alpha and beta isomers.3-4 TMA92 (mixture of alpha and beta isomers) was prepared by a method analogous to that reported for the synthesis of K92·13H2O3-4, except that tetramethylammonium hydroxide (TMAOH) was used in place of K2CO3. The Li+, Na+, K+, TMA+ and Cs+ salts of 3 were synthesized by neutralization of H143 by the hydroxides of the corresponding cations, followed by evaporation to dryness by rotary evaporation. Preparation of gold nanoparticles (Au NPs). Citrate-protected Au nanoparticles (Au NPs) were prepared by minor modification (published elsewhere)7 of the Turkevich method.10 In order to control and vary the cations present in solutions of the gold nanoparticles, Li+, Na+, K+, Cs+ and TMA+ salts of
S2
citrate were used during gold-nanoparticle (Au-NP) syntheses, and the corresponding solutions are here referred to as: Li-, Na-, K-, Cs- and TMA-Au NPs, respectively. The Na-Au NPs were close to spherical, and relatively monodisperse in size, with an average diameter of 13.8 ± 0.9 nm, based on TEM images7 and DLS data. DLS, zeta-potential and UV-vis data showed no distinct differences between citrate-stabilized Au NPs samples prepared using Li+, Na+, K+, Cs+ or TMA+ citrate counter cations. All nanoparticle samples have average hydrodynamic radii of 8.3 nm and narrow size distributions of ± 0.4 nm based on relative intensity-peak widths (see data in Table S1). For a 13.8 ± 0.9-nm diameter sample (size from TEM data), the calculated nanoparticle concentration (0.5 mM in Au) is 6.2 ± 1.3 × 10-9 M. Reactions of citrate-stabilized Au NPs with polyoxometalate salts. One mL of the polyoxometalate-salt solution was added to an equal volume of citrate-protected Au NPs. Unless indicated, the pH values of the Au-NP solutions were 6.1 ± 0.1. The mixtures were kept in the dark at constant temperature (25.0 ± 0.1 °C) for 24 h. To study the effects of pH, the pH values of both the POM and Au-NPs solutions were adjusted prior to mixing. Notably, 1 is stable to acid condensation or base hydrolysis at pH values of from 3 to 7,1, 11 and absorbance due to the surface-plasmon resonance (SPR) of citrate-protected Au NPs is insensitive to pH.7 DLS size distributions and zeta potentials were measured after POM-monolayer formation (Table S1). Surface-coverage calculations. At a given surface coverage Θ, the absorbance, A, at a wavelength λ (where λ >> particle size) can be expressed as shown in eq S1 (see ref. 7 for a full description). A = A0 + (A1 – A0) Θ
(S1)
Here, A0 and A1 refer to full coverage of citrate ions and polyoxometalate anions, respectively. According to eq S1, the absorbance, A, increases linearly with the fractional extent of citrate displacement by the POM ligands. Rearrangement of eq S1 gives eq S2, which is a structurally based relationship between changes in the SPR and degrees of surface coverage. Θ = (A – A0) / (A1 - A0)
(S2)
Langmuir isotherms for POM-monolayer formation. Langmuir isotherms were used to quantify degrees of surface coverage (Θ) as functions of POM concentration in solution. This was done using eq S3, where C is the bulk-solution concentration of the POM anion and K is a constant. Θ = KC / (1 + KC)
(S3)
Combining eqs S2 and S3 gives eq S4, which is used for fitting A versus C plots to obtain values of K. A = (A0 + KA1C) / (1 + KC)
(S4)
To obtain A versus C plots, a series of vials containing constant Au-NP concentrations and incrementally larger POM concentrations, were prepared and stored in the dark at room temperature (25 ± 0.1 ºC). UV-visible spectra were obtained after one day. Concentrations of POMs in the bulk solutions, C, were at least an order of magnitude larger than the adsorbed (monolayer) concentrations. Hence, C in eq S4 was set equal to the added [POM] values. In cases where values of K are very large, e.g., for TMA+ and Cs+ salts of the POMs, values of A reach a plateau at very small [POM], such that fewer data points fulfill the condition that [POM] ≈ C. In these cases, calculated K values possess larger uncertainties. Kinetic studies. Time-dependent SPR absorbance data were obtained at 24 ± 1 °C using a stoppedflow apparatus to mix POM and the Au-NPs solutions in 1:1 (v:v) ratio. Longer time-scale data obtained after mixing provide an overview of the entire self-assembly process. For that, concentrations of all the POM solutions were 2.0 mM before mixing. Initial rates (ri) for the gold-surface coverage of less than 3%, i.e., < 10 POM anions per gold NP, were calculated by linear fitting of the early parts (typically Δt ≤ 10 s) of the A versus t traces (eq S5).
S3
ri = Γ /Δt
(S5)
In eq S5, Γ is the molar concentration of adsorbed POM molecules. The adsorbed POM concentration corresponding to full monolayer formation (Γm for Θ = 1) is estimated to be 1.0 × 10-6 M, based on 3.1 × 10-9 M Au NPs and 330 POM molecules per monolayer.7 Smaller concentrations of adsorbed POMs, Γ, corresponding to partial coverage of the gold surface, are calculated as products of Γm × Θ. Previous report showed that association of the POM anions onto Au surface sites is first order dependence on the concentrations of both the POM and the Au sites for binding POMs.7 (The latter is equal to [Au NP] × number of sites on each particle, and for the present case, the averaged diameter of Au NP is 14 nm and each monolayer contains 330 POMs, thus [Au site] = 1.0 × 10-6 M.) The apparent rate constant of the early stages of self-assembly, ki, is defined by eq S6. ri = ki [1] [Au site]
(S6)
Values of ki were obtained by dividing the initial rates with the concentrations of 1 and that of the Au site. These values are summarized in Table 2 of the text.
II. Method used to obtain Figure 1D of the text A
B
C
+
+
D
+
E
+
F
=
Figure S1. Original images used to obtain Figure 1D in the text. A general method of combining exposure from several images of identical objects, used routinely in transmission electron microscopy, was used here, but with more care. For this, sets of images of individual particles were acquired in rapid succession with short exposure times under low dose conditions. In cryo, the matrix (and hence structure of the object studied) may change under the electron beam. Therefore, many images of numerous particles were obtained at the same magnification (265K) using the same degree of phase contrast or “under focus”, and inspected carefully to identify the above set of 5 images (A-E) in which (effectively) no changes in the object or matrix were observed. The five images were then aligned so that their gold cores were at the same location. The gold core in each image was graphically removed, after which, the images (now of the POM shells and background) were merged to selectively increase the intensity of the POM-monolayer shell. The gold core was then added back to give the final gold-core POM-shell image shown in Figure 1D of the text and reproduced here as panel F. Bar = 10 nm.
S4
III. Stabilities of citrate protecting anion “layers” on the gold nanoparticles The observed trend of K values cannot be attributed to ion pairing with citrate. If the cation had a significant effect, the stability of citrate-protected Au NPs would increase in following order: Li3Ct < Na3Ct < K3Ct < TMA3Ct < Cs3Ct, based on electrostatic considerations, and consistent with ion-pairing of alkali metal cations with carbonate12 and some carboxylic acids13 which form M···O bonds (O = carboxylate oxygen atom) in aqueous solutions. And, if this occurred, the increase in K values with alkali-metal size would be even more pronounced than that reported in Table 1 (text). Hence, ionic stabilization of the citrate-protected Au NPs would not change the ranking assigned to the effects of cations on the stabilities of the POM monolayers.
0.98
Abs
0.94 0.90 0.86 0.82 0.0
0.5
1.0
1.5
2.0
Conc., mM
IV. Figure S2. Adsorption isotherms for reactions of the tetramethylammonium (TMA+) salt of 1 with TMA3Ct-protected Au NPs. The red curve is a fit to eq S4. [Au NP] ≈ 3.1 × 10-9 M.
S5
8-
6-
20
7-
H21
H1
0.6
15
0.4
10
0.2
5
0.0
1
2
3
4
5
pH
6
7
8
0
-1
Fraction
1
H31
K, mM
5-
0.8
Fraction
25
Fraction
1.0
1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0
H2Ct
H3Ct
5-
H31
1
3
HCt
2-
7-
4
5
Ct
3-
8-
H1
6-
H21
2
-
1
6
7
8
9
pH
V. Figure S3. Values of K as function of pH in relation to the protonation states of K81 and citrate. Values of K are obtained by reactions of the K-salt of 1 with Na3Ct-protected Au NPs. The fractional protonation states of citrate (Ct; left and upper right) and 1 (left and lower right) are calculated based on published pKa values.11, 14-15
The close comparison (in the text) between the K values of mono-protonated [α-HSiW11O39]7- (pKa = 5.2), with that for un-protonated [α-PW11O39]7– is made using date from ref. 7 (below), and normalized using data provided in the present work to obtain K values at effectively identical cation compositions and concentrations. The comparable K values imply that the proton remains bound to 1 after monolayer formation, which, while consistent with the electrostatic model developed here, might inhibit binding of the defect-site oxide ligands to the gold surface. An addition, the proton might be lost during monolayer formation, with the energy cost of deprotonation from the basic oxide-ligands decreasing the overall binding constant, K, for monolayer formation. The observed K values might thus be interpreted in several ways, and don’t prove that the defect structure is not involved in binding to the gold surface.
S6
VI. Table S1. DLS and Zeta Potential Data, and Electric-Double Layer Parameters of Citrate- and 1Stabilized Au NPs estimated net Debye length, protecting Zeta potential / a Entry Rh / nm charge / electrons b ligands mV κ -1 / nm per NPc 1
Li3Ct
8.1 ± 1.1
-40.7 ± 6.4
18.4
4.5
2
Na3Ct
8.0 ± 0.8
-41.2 ± 6.1
19.4
4.5
3
K3Ct
7.9 ± 0.8
-33.2 ± 3.5
14.9
4.5
4
TMA3Ct
8.5 ± 1.7
-40.9 ± 2.5
19.4
4.5
5
Cs3Ct
8.6 ± 1.1
-39.5 ± 1.4
17.4
4.5
6
Li81
8.4 ± 1.7
-52.2 ± 10.0
25.1
1.1
7
Na81
8.5 ± 1.7
-52.0 ± 12.4
24.7
1.1
8
K8 1
8.6 ± 2.4
-48.4 ± 3.3
23.3
1.1
9
TMA81
8.6 ± 1.9
-54.7 ± 2.4
26.3
1.1
10
Cs81
8.7 ± 1.3
-61.9 ± 5.9
30.1
1.1
a
Error refers to relative intensity-peak widths. bError refers to the standard deviation. cCalculation based on κ-1 = (2000F2µ/εRT)-0.5, where F is the Faraday’s constant (96480 C mol-1), ε is the permittivity of water (7.12 × 10-10 F m-1 at 298K), R is the universal gas constant (R = 8.314 J K-1 mol-1), T is the temperature (298 K), and µ is the ionic strength of the solution (ca. 4.6 mM for citrate-Au NPs and 76.6 mM for 1-protected Au NPs), respectively. dCalculation based on q = 4πεRζ / e, where ε is the permittivity of water, and R is the radius of the particle (values of Rh are used here), ζ is the zeta potential, and e refers to the charge of an electron (-1.6 × 10-19 C).7, 16-17
VII. Method used to obtain the effective charges, q, in Table 2 of the text The kinetic barrier for penetration of an ion through the electric double layer is proportional to exp(qζ/0.37RT),18-20 where q is the charge of the approaching anion and ζ is the zeta potential of the nanoparticle. The rate of adsorption, in the absence of other barriers, is given by eq S7. ki = k0 exp(-qζ/0.37RT)
(S7)
In this equation, k0 is the rate constant in the absence of a double-layer potential-energy barrier, R is the universal gas constant (R = 8.314 J mol-1 K-1) and T is the temperature (T = 298 K). Since the ζ potentials of citrate-protected Au NPs don’t change with the nature of the cations (ζ ≈ -39 mV for all the citrate-protected Au NPs; see Table S1), the logarithm of ki is linearly dependent on the effective charge of 1, with a slope of -4.1 (eq S8). ln(ki) = lnk0 – 4.1q
(S8)
S7
Values of ki are determined by stopped flow kinetic measurements (see Figure 3 and Table 2 in the text). To obtain relative charge, q, values as a function of cation, the charge q for 1 in the presence of Li+ was set to -8. This á priori assignment is needed to find the relative charges (q) for the other cations, but is probably not far from physical reality as minmal ion pairing between Li+ and 1 is expected under the conditions used (5.4 mM Li+ and 0.3 mM 1). This assumption makes it possible to calculate the constant, lnk0. Using this constant, and the experimental ki values, values of q for other cations were obtained (see the right-most column in Table 2 of the text).
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