Assembling Prussian Blue Nanoclusters along Single Polyelectrolyte

C h a p t e r 35

Assembling Prussian Blue Nanoclusters along Single Polyelectrolyte Molecules Downloaded by NORTH CAROLINA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch035

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Anton Kiriy , Vera Bosharova , Ganna Gorodyska , Paul Simon , Ingolf Mönch , Dieter Elefant , and Manfred Stamm 3

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Leibniz Institute of Polymer Research, Hohe Strasse 6, 01069 Dresden, Germany Max Planck Institut for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany Leibniz Institute for Solid State and Materials Research Dresden, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany 3

A simple method for the preparation of charge-stabilized Prussian Blue nanocrystals (PBNs) of readily adjustable size is reported. PBNs have been purified by addition of non-solvents and redispersed in water without aggregation. PBNs may be electrostatically arranged along isolated polycation (PC) chains adsorbed onto flat surfaces. PC-PBNs nanohybrids constitute useful materials for the manufacture of electrooptical devices. PBNs can be also used as contrasting reagent to improve AFM visualization of positively charged polymer chains deposited on substrates of relatively high roughness.

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© 2006 American Chemical Society

In Metal-Containing and Metallosupramolecular Polymers and Materials; Schubert, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Downloaded by NORTH CAROLINA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch035

Background The design of new materials with desired properties from organic macromolecules and inorganic building blocks involving non-covalent interactions is a newly emerging and rapidly developed area of research. The properties of thus fabricated materials (e.g., optical, electronic, mechanical, etc.) are defined by the properties of the components used and can be easily tuned by altering the composition (varying the kind, number and ratio of the components). Along this line various macromolecules containing chelating ligands were already utilized to immobilize different metallic ions via complexation reactions. On the other hand, combination of polyelectrolytes with oppositely charged metal ions or clusters constitutes an alternative and highly universal route to a number of metal-containing nanostructural materials. Prussian Blue (PB), an old pigment, is a coordination polymer formed by reaction of either hexacyanoferrate(II) (HCF-II) anions with ferric (Fe(HI)) cations, or hexacyanoferrate(IH) (HCF-III) anions with ferrous (Fe(II)) cations. According to X-ray diffraction analysis, PB is a three-dimensional crystal of ferric and ferrous ions which alternate at the sites of a cubic lattice. The ferric ion is coordinated to the nitrogen atoms, and the ferrous ion to the carbon atoms, of the bridging cyanide ligands. The remaining charge is balanced either by potassium ions in the so-called "soluble" PB, or by ferric ions in the "insoluble" PB. The term "soluble", however, does not refer to the true solubility but only to the tendency of PB to form colloidal solutions. A large family of cyano-bridged compounds with a cubic structure (PB analogues; PBs) are known for interesting physical properties. Neff et al. reported that the oxidation state o f the iron centers could be controlled electrochemically, making dramatic color changes possible that could be used in electrochromic devices. PB shows a long-range ferromagnetic ordering at 5.6 K , whereas few PBs undergo magnetization at room temperature and even higher. Hashimoto et al. showed that the magnetic properties of PBs could be modulated not only by the chemical composition but also by an optical or electrical stimulation (photomagnetism and electromagnetism, respectively). " Finally, PBs exhibit remarkable ion-sieving properties as result of an open pore zeolite-like structure. For the unique properties of PBs to be exploited, PBs must be deposited properly onto a solid support. It is highly desirable to prepare mechanically robust PBs films with controlled thickness, chemical composition and crystallinity, having ion-sieving membranes and electrochromic devices in mind, or to create regular patterns of PB-based single molecule magnets. Classical methods of PB immobilization by casting, dip-coating or electrochemical deposition do not allow film thickness, composition and/or mechanical properties to be controlled accurately. This problem has recently been overcome by the Langmuir-Blodget technique and multiple sequential 1

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adsorption techniques based on the stepwise adsorption of H C F anions and ferric (or ferrous) cations. On the other hand, utilization of larger size PB nanoparticles instead of small precursor ions would have a favorable effect on film assembly. However, only few methods to produce well-defined P B nanoparticles have been reported. Uemura et. al succeeded in the preparation of P B nanoparticles stabilized by polyvinylpyrrolidone). Mann et al. have developed the approach to crystalline nanoparticles of some PB analogues combining water-in-oil microemulsions of appropriate reactants. In that case growth of the nanoparticles occurred through the aggregation of primary clusters inside micelles. Thus, all of the methods mentioned above lead to PB particles stabilized by either polymeric or small-molecule surfactants. 16,17

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Objective Here we report on the preparation of surfactant-free, water-dispersible Prussian Blue nanoparticles (PBNs) by mixing solutions of ferric chloride and excess of potassium ferrocyanide. PBNs display a remarkable stability against aggregation because of an uncompensated negative charge. The average size of PBNs was readily controlled by the molar ratio of the two reagents. Thus formed P B nanoparticles are crystalline and display optical and magnetic properties similar to the properties of PB bulk. PBNs have been electrostatically arranged along the individual polycation molecules. We also developed a simple contrasting procedure to improve the A F M visualization of positively charged polymer chains deposited on substrates of relatively high roughness via counter ion exchange between small CI" anions and bulky H C F anions or negatively charged nanoclusters of Prussian Blue.

Experimental Synthesis of poly(methacryloyloxyethyl dimethylbenzyl) ammonium chloride ( P M B ) ( M = 6000 kg/mol, polydispersity index (PDI) = 1.6) was reported elsewhere. Substrates. Si-wafers (Wacker-Chemitronics), patterned Si-wafers and glasses were first cleaned with dichloromethane in a ultrasonic bath for 5 min (3 times), followed by cleaning with a solution of N H O H and H 0 at 60 °C for one hour. This N H O H : H 0 solution must be handled cautiously because of violent reaction with organic compounds. Samples were finally exposed to 30% sulfuric acid for 15 min and then rinsed repeatedly with water purified through a Millipore (18 MQxcm) filter. w

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503 Layer-by-layer assembly on isolated polyelectrolyte molecules. P M B molecules were deposited onto freshly cleaned Si-wafer in a stretched conformation by spin-coating of a 0.005 g/L solution in acidified water (pH 2, HCI) at 10000 rpm. The substrate with the P M B molecules was then immersed in solutions of K F e ( C N ) (or K Fe(CN) ) solutions (0.5-15 g/L) and KC1 (0-50 g/L) at the same pH and for the same period of time as in the first part of the cycle. The substrate was analyzed by A F M after either half of cycle, a complete cycle or several cycles. Deposition of PB clusters. Dispersion of PB clusters was prepared by mixing vigorously a solution of K F e ( C N ) · 3 H 0 (HCF-II, 1.18 mmol/L) in acidified water (HCI, p H 2.0) and an equal volume of a solution of FeCl at the same pH. Concentration of the FeCl solution was either 0.148 mmol/L for the preparation of smaller PB1 clusters (3.7 nm), or 0.296 mmol/L for larger PB2 clusters (4.8 nm). A small amount of tetrahydrofuran (THF, Aldrich) was added to the freshly prepared dispersions of P B in order to separate the PB clusters from the unreacted HCF-II and KC1. P B clusters were collected by filtration, washed with a water-THF solution (2:1), and redispersed in acidified water (HCI, p H 2). A Si-wafer onto which P M B molecules were deposited was dipped in the freshly prepared dispersion of PB clusters for 3 min at 25 °C, followed by washing with water and drying under an argon flow. AFM measurements. A multimode A F M instrument and a NanoScope IVD3100 (Digital Instruments, Santa Barbara) were operated in the tapping mode. Silicon tips with a radius of 10-20 nm, a spring constant of 30 N/m and a resonance frequency of250-300 kHz were used. TEM measurements. T E M images were recorded with a Philips C M 200 F E G at 200 kV. They were processed by the Digital Micrograph program (Gatan, U S A ) . Magnetisation measurements. The magnetisation measurements were performed in a SQUID magnetometer (MPSM-Quantum Design). 4

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Results & Discussion Layer-by-layer (LBL) deposition of the Prussian Blue precursors on isolated polyelectrolyte molecules. Prussian Blue was tentatively deposited onto isolated polycations (PC) by the well-known L B L (or multiple sequential adsorption) technique. The stepwise adsorption of H C F anions and ferric (or ferrous) cations was actually effective in fabricating P B films. In this work, the first adsorption step of H C F anions along positively charged P M B chains was successful as confirmed by the increase of the chain thickness by approximately 0.7 nm (Figure la). However, whenever the H C F pre-adsorbed P M B chains on the Si-substrate were dipped into the ferric chloride solution, the H C F anions 11


504 were completely removed leaving bare P M B molecules undetectable at the rough Si-wafer surface. The complete removal o f the previously deposited H C F layer was systematically observed whatever the salt, e.g., FeCl , N i C l , C o C l , C u C l , PdCl , and A u C l , in a broad range of pH and ionic strength. When the deposition cycle of H C F and F e C l onto pre-adsorbed P M B molecules was repeated several times, local growth of relatively big clusters was occasionally observed (Figure lb). 2

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Figure 1. (a) AFM image of PMB molecules adsorbed onto Si-wafer after dipping into a 5 g/L solution of K Fe(CN) in acidic water and washing with water (the height of the worm-like structure is about 0.6 nm). Further dipping into a FeCU solution removes the HCF anions and the PMB molecules are unobserved (image not shown), (b) AFM image of the sample shown in (a) after the fifth cycle of the sequential dipping into K Fe(CN) andFeCl solutions (FeCli being the outermost deposit layer): PB clusters (10 nm height) are randomly located and the PMB molecules are unobserved. 4

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The failure of the L B L deposition of P B clusters can be explained as follows. The H C F anions interact expectedly with the positively charged units of the P M B molecules. In the next step, an excess of ferric cations interact with the pre-adsorbed H C F particles and overcharge them. The accordingly formed P B nanoparticles are positively charged by a shell of ferric cations, which facilitate their detachment from the similarly charged P M B chains. The observations in this work are consistent with Tieke et al. who reported on the non-linear increase of the PB film thickness with the number of dipping cycles. This irregular growth was pronounced up to the 6-th cycle and whenever the dipping time was relatively long. Thus, although the L B L method is useful to prepare P B films 11


505 onto polyelectrolyte multilayers, it is ineffective in the case of deposition onto isolated polycationic molecules. Deposition of water-dispersible Prussian Blue nanoclusters onto polycations. In order to prepare clear dispersions of PB nanoclusters stable for several months, the method of mixing diluted acidic solutions (pH = 2) of F e C l and K F e ( C N ) (used in excess) proved to be effective. The characteristic blue color and broad signal in the UV-vis spectrum with λ , ^ at 695 nm are consistent with an intermetal charge-transfer band from F e to F e and reflect the formation of PB (Figure 2). 3

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Wavelength, nm Figure 2. UV-vis spectrum of Prussian Blue dispersion (prepared at molar ratio K Fe(CN)

Assembling Prussian Blue Nanoclusters along Single Polyelectrolyte

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