Communication pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Hexaniobate Cluster Anion Monolayers on Gold Nanoparticles: A New Structural Role for Alkali Metal Countercations Shelly Sharet Leiser,† Libi Polin,† Gal Gan-Or,† Manoj Raula,§ 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 § Amity Institute of Applied Sciences, Amity University, Noida 201313, India
Downloaded via RUTGERS UNIV on January 2, 2019 at 22:17:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
The crystallographically larger cations possess smaller hydrated radii, resulting in shorter cation−anion distances and larger Coulombic driving forces for monolayer formation.7 In general, polyoxoniobates, including hexaniobate (Nb6O198−, with the Lindqvist structure; Figure 1C), have larger charge densities than iso-structural polyoxotungstates. This renders their bridging and terminal oxo atoms8 much more electron dense and better able to serve as oxo-donor ligands for coordinating alkali metal9 and other cations.10−16 Hence, while alkali metal cation salts of polyoxotungstates dissolve as dissociated cations and anions, hexaniobates are present in water as contact ion pairs, with numbers of bound cations (and solubility) increasing with size in the order: Li+ < Na+ < K+ < Cs+. Notably, numbers of bound cations increase from two for Li+ to eight for Cs+.17 As such, the more extensively ion-paired clusters possess smaller (net) anionic charges. For tungsten-based polyoxometalates (POMs), stabilized by electrostatic interactions with their hydrated (solvent-separated) countercations, larger cluster anion charges result in more stable monolayers.2,3 This raised an intriguing question regarding how the formation of contact ion pairs between alkali metal cations and hexaniobate anionsand the corresponding attenuation of cluster anion chargemight influence (or even preclude) the formation of hexaniobate monolayers on Au NPs. We now report the first examples of hexaniobate monolayers on Au NPs in water (Figure 1D), along with formation constants, K, for four alkali metal cations (Li+, Na+, K+, and Cs+) and the tetramethylammonium cation (TMA+). Taken together, the findings point to an entirely new structural motif for POM monolayer formation. Hexaniobate monolayers for cryo-TEM imaging (Figure 1D) were formed on 14 nm Au NPs by reacting aqueous cesium hexaniobate (Cs8Nb6O19) with cesium-citrate-protected Au NPs,2,7 adjusted to pH values of between 10 and 11. At this pH range, hexaniobate exists as a mixture of monoand diprotonated anions, [H2Nb6O19]6− and [HNb6O19]7− (general formula, [HxNb6O19]8−x, designated as 1). Comparing the two types of POM monolayers ([α-SiW11O39]8− and 1 (Figure 1, panels B and D), the hexaniobate protecting layer is thinner and of lower contrast (see Figures S1 and S2 for more images and related discussion). This is because the atomic
ABSTRACT: Monolayer shells of polyoxotungstate cluster anions on gold nanoparticles in water were electrostatically stabilized by structurally integrated countercations, with formation constants, K, increasing in the order: Li+ < Na+ < K+ < TMA+ < Cs+ (TMA+ = tetramethylammonium). We now report that for hexaniobate cluster anions, K values increase in the same order, with the notable exception of TMA+, which is effectively unable to induce monolayer formation. These findings point to a new structural model in which hexaniobate anions form a spherical coordination polymer at the gold surface with alkali metal countercations serving as single-atom structural building units between hexaniobate linkers.
I
n water, polyoxotungstate cluster anions (Figure 1A) and their structurally integrated alkali metal countercations form electrostatically stabilized monolayers1−5 on 14 nm diameter gold nanoparticles (Au NPs: Figure 1B)6 with stability constants, K, increasing in the order: Li+ < Na+ < K+ < Cs+.7
Figure 1. Polyhedral representations of (A) [α-SiW11O39]8− and (C) hexaniobate; [Nb6O19]8−. (B and D) Cryo-TEM images of [αSiW11O39]8− (K+ salt) and hexaniobate (Cs+ salt), respectively, on ca. 14 nm diameter Au NPs. Scale bar = 10 nm. © XXXX American Chemical Society
Received: November 7, 2018
A
DOI: 10.1021/acs.inorgchem.8b03139 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
the organic cation, TMA+, is entirely opposite to the quite large stability constants observed for its effect on polyoxotungstates such as [α-SiW11O39]8− (Table 1). While TMA+ is structurally unique relative to the other cations, it leads to highly stable polyoxotungstate monolayers,7 with K values between those for K+ and Cs+. This served as a definitive indication that, while parallel trends in K values are observed for the effects of alkali metal cation size, the structural roles of the cations differ in a fundamental way for polyoxoniobate versus polyoxotungstate cluster anions. This was understood by considering the aqueous solutionstate structures of the two classes of POM salts. Alkali metal cation salts of polyoxotungstates dissolve in water as extensively dissociated cations and anions. As noted above, polyoxoniobates, including hexaniobate, form contact ion pairs with alkali metal cations, with numbers of bound cations increasing in the order: Li+ < Na+ < K+ < Cs+, from two for Li+ to eight for Cs+.17 The larger Cs+ cations are directly coordinated by bridging oxo ligands on the eight tri-Nb faces of the Oh-symmetry hexaniobate cluster (Figure 3A).17 Upon
number of Nb (Z = 41) is smaller than that of W (Z = 74), combined with the smaller number of Nb atoms in each hexaniobate cluster, as compared to the 11 W atoms in each molecule of [α-SiW11O39]8−. Having demonstrated the formation of hexaniobate monolayers, the next step was to determine the constants, K, associated with their formation as a function of alkali metal cation, M+, and of TMA+. This was done by quantifying the change in the UV−vis absorbance (Figure 2A) of the surface
Figure 2. (A) UV−visible absorbance spectra of citrate-protected Au NPs before (black curve) and after addition of the Cs+ salt of 1 (0.02 mM; red curve). (B) Adsorption isotherms for reactions of citrateprotected Au NPs with 1 in the presence of identical concentrations18 of the monovalent cations: Li+ (▲ green), Na+ (● orange), K+ (■ blue), Cs+ (⧫ red), and TMA+ (× purple). The curves were used to obtain the K values reported in Table 1. During alkali metal cation induced monolayer formation, the ζ-potential of the particles changed from −32 (citrate) to ca. −40 (hexaniobate).
plasmon resonance (SPR) of the gold cores, as M+ salts18 of 1 (0.0−0.05 mM) were added to Au-NP solutions prepared from the M+ and TMA+ salts of citrate. Before mixing with 1, cation concentrations were adjusted to 4.8 mM. In each case, absorbance maxima (all between 521 and 523 nm) were plotted as a function of [1] (Figure 2B). Absorbance values increased with [1] to plateaus that indicated the completion of monolayer formation. Stability constants, K (Table 1), were obtained by fitting the absorbance plots to a
Figure 3. Cs+ salt of hexaniobate in water and in the solid state. (A) Solution-state structure of [Nb6O19]8−, with all eight Cs+ ions (purple spheres) bound to the hexaniobate cluster (in turquoise). (B) A fragment of crystalline solid Cs 6[H2 Nb 6O19] showing three [H2Nb6O19]6− clusters (in ball and stick notation) linked by direct coordination to a Cs+ cation by one face (three bridging O atoms) of one cluster and terminal oxo ligands of two adjacent clusters (Cs: purple, O: red, Nb: green; generated using a published crystallographic information file (CIF)).23
Table 1. POM Monolayer Formation Constants dissolution of the crystalline Cs+ salt, the loss of lattice enthalpy is attenuated by retention of these close-contact Cs+hexaniobate interactions, leading to enhanced solubility.22 In the solid state as well, crystallographic data23,24 show that direct contacts between oxo ligands of hexaniobate anions and their alkali metal countercations increase with size, in the order: Li+ < Na+ < K+ < Cs+. Importantly, even in the solid state, Li+ remains hydrated,24 while Cs+ (with a smaller enthalpy of hydration) is directly coordinated to the oxo ligands of hexaniobate (Figure 3B). Additional insight was provided by the seemingly contradictory behavior of TMA+. When combined with tungstenbased cluster anions such as [α-SiW11O39]8−, TMA+ gives highly stable monolayers, while with 1, TMA+ is effectively incapable of promoting monolayer formation (Figure 2B and Table 1; analogous results were obtained using tetraethylammonium, TEA+: see Figure S3). This is entirely consistent with the (obvious) lack of coordination sites on TMA+, as a result of which it is incapable of direct binding to oxo ligands of 1.22,25 Interestingly, with all the alkali metal cations, there is some hydrogen bonding in the solid state, which varies with the protonation state of the POM/pH of crystallization.25 For the TMA salt, this hydrogen bonding leads to organized chains in
Langmuir stability constants, K (mM−1) cation
ionic radiusa (Å)
hydrated radiusa (Å)
Cs+ K+ Na+ Li+ TMA+
1.69 1.33 0.95 0.60 3.22
2.28 2.32 2.76 3.40 ∼6.3
[αSiW11O39]8−b 750 79 10.3 7.4 138
± ± ± ± ±
260 13 0.6 1.6 5
[HxNb6O19]8−x (1) 2343 1006 945 351
