Monitoring the Growth of Polyoxomolybdate Nanoparticles in Suspension by Flow Field-Flow Fractionation

Abstract

Reduction reactions of molybdate in acidic, aqueous solution lead to colored, mixed-valent Mo V/VI polyoxomolybdate (POM) species. [1][2][3][4][5] The two-century-old mystery of the "molybdenum blues" 6 exemplifies the difficulty in characterizing suspensions of POM particles. Müller, Liu, and co-workers revealed the fundamental compositions of the related "molybdenum browns", including I h symmetrical keplerates. In general, reduced molybdates assemble into nanometric species (Figure 1a). Light-scattering, small-angle X-ray scattering (SAXS), and microscopy show slow aggregation of Na 15 The key methodology of our study is field-flow fractionation (FFF), a flow-based, chromatography-like separation and sizing technique that can monitor changes in the particle-size distribution of a suspension in situ. FFF is conducted in a thin, empty channel, with separation based on relative hydrodynamic behavior. [21][22][23] In flow field-flow fractionation (FlFFF), a cross-flow perpendicular to the direction of laminar channel flow forces dissimilar particles to different levels in the channel, where they are eluted at different flow velocities. FlFFF fractionates particles between 1 and 1000 nm and allows analysis of the eluate. 24 We used FlFFF to monitor the evolution of molybdate nanoparticles in a {Mo 132 } suspension prepared by Müller's procedure. 7,15,16 Under our FlFFF conditions, 25 soluble species and particles smaller than 3 nm are swept to the waste stream by the cross-flow, while larger particles are separated and eluted through a UV-visible detector. FlFFF fractograms monitoring the absorbance of the eluate at 455 nm are shown in Figure 1b.The particle-size distributions calculated 26 from the data in Figure 1b have diameters ranging from 3 to 75 nm, roughly distributed in three populations with maxima about 3.4, 12, and 25 nm. The mean diameter is 31((1) nm (confidence level 95%), averaged over 24 FFF runs from 8 days. The baseline fractogram at 10 min indicates that no particles over ca. 3 nm had yet formed. At 2 h, the 3.4-nm population is largest. The total area under the fractogram increases rapidly from 2 to 8 h, at which time the 12-and 25-nm populations are larger. We suggest that the 3.4-nm particles, decreasing after 2 h but observed consistently throughout the entire period, are the {Mo 132 } with their associated counterions and solvation shell. The overall particle population reaches a maximum at ca. 8 h, decreases rapidly for about 2 days and more slowly thereafter (Figure 1b). The decrease parallels the precipitation of large particles that are not sampled by FlFFF shown in the SEM 27 image in Figure 2a. The precipitate is in dynamic equilibrium with species still in solution or suspension, yielding after several days ca. 0.1-mm octahedral crystals of {Mo 132 }, Fm3 with a ≈ 46 Å, 15 shown in Figure 2b.The particle-size distribution of the molybdate suspension was corroborated by AFM, SEM, and HRTEM. Good agreement between particle-size distributions obtained by FFF and microscopy suggests t...

Polyoxometalates: Building Blocks for Functional Nanoscale Systems

“…Apart from these assembly structures, which are stabilized by organic cations (Table 1), pure inorganic salts of POM and small inorganic cations can form self‐assembly structures that comprise a spherical layer, which were named blackberries by Liu et al. :60 nanoscale, water‐soluble macro‐ions, such as {Mo 132 }, {Mo 154 }, {Mo 72 Fe 30 }, and others, tend to form stable, uniform, single‐layer blackberry structures 20–1000 nm in size in dilute solutions by noncovalent interactions 104. 105 Two interesting features of Keplerate {Mo 72 Fe 30 } blackberries are that 1) they create a microscaled, relatively isolated water environment (containing over three million water molecules) that has different properties from the bulk water, and 2) the blackberry membrane is permeable to small cations, but not to anions.…”

Polyoxometallate als Bausteine für funktionelle Nanosysteme

“…Nanometergroße wasserlösliche Makroionen wie {Mo 132 }, {Mo 154 }, {Mo 72 Fe 30 } und andere neigen in verdünnten Lösun-gen zur Bildung von stabilen gleichmäßigen, einschichtigen "Brombeer"-Strukturen von 20-1000 nm Größe über nichtkovalente Wechselwirkungen. [104,105] Für "Brombeer"-Strukturen aus dem Keplerat {Mo 72 Fe 30 } sind zwei interessante Eigenschaften zu beobachten: 1) sie erzeugen eine mikroskopische, vergleichsweise isolierte Wasserumgebung (mit über 3 Millionen Wassermolekülen), und 2) die "Brombeer"-Membran ist für kleine Kationen durchlässig, nicht jedoch für Anionen. Der passive Transport von Kationen durch die Membran ist vergleichsweise langsam, er benötigt aber weder einen Träger noch zusätzliche Energie.…”

Investigation of Intermediates Involved in the Photochemical Formation of Mo‐Blue Nanoring by Capillary Electrophoresis–Mass Spectrometry

“…Lower-nuclearity polyoxometalates have been characterized by ESI-and CSI-MS. [9][10][11][12][13][14][15][16][17][18][19][20][21] Because the response factor for ionization differs among each species and because ionization may be prevented by coexisting different species, [7,22] it is necessary to separate the species in the reaction solution in advance. We employed capillary electrophoresis (CE) [23,24] for ESI-MS analysis of the solution of polyoxometalates instead of chromatographic separation [25][26][27] to avoid their irreversibly strong adsorption on packing materials. This CE procedure is quite compatible with ESI-MS, and CE-MS has been successfully applied to many biological/pharmaceutical systems.…”