J. Phys. Chem. B 2009, 113, 13901–13909
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ARTICLES Electrolytical Entrapment of Organic Molecules within Metals Ofer Sinai and David Avnir* Institute of Chemistry, the Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: June 26, 2009; ReVised Manuscript ReceiVed: August 5, 2009
An electrolytical method is presented for the doping of metal with organic molecules (0.1-1% by weight). Several representative organic molecules, including dyes and polymers, have been entrapped in copper or silver using this procedure. The resulting doped metals have been characterized using TGA, UV-vis, SEM, EDS, and XRD. The dynamics of dye extraction from composites has been modeled, offering a means to quantify the leaching behavior of organics@metals. Typically, entrapped molecules are shown to be heterogeneously distributed within the composites, including a population which is entrapped tighter than with nonelectrolytic methods. Background In recent years, we developed a new family of composite materials, namely, metal matrixes in which various organic molecules, polymers, and enzymes have been irreversibly entrapped.1-12 The methodology utilizes a room-temperature, one-pot synthesis, based on reduction of metal cations in the presence of the desired dopant molecule, thereby leading to its entrapment. The resulting doped metal matrixes are generally denoted as dopant@metal composites. These new hybrid materials are characteristically mesoporous, where the porosity is due to the interstitial voids between tightly aggregated nanocrystals, and with the entrapped dopant molecules residing in cages and narrow entrance pores of the metallic structure. A characteristic metallic sheen appears when the powder of the composite is pressed into coins, and the crystalline structure matches that of pure bulk metal. Interestingly, it has been found that the dopant molecules, though unextractable by the original solvent of the synthesis in which it is soluble, are nevertheless accessible to reaction with reagent molecules diffusing into the metal matrix pore system.1 This accessibility is also the basis for partial extraction with a solvent more efficient than the synthesis solvent. Such experiments have proved that the organic molecule is typically extracted intact.1,4 Due to the accessibility to reaction with an external reagent and to the intimate interaction achieved between the metal matrix and the dopant molecule, the resulting composites possess unorthodox, novel properties, such as induction of chirality in metals,5,10 heterogenization of a homogeneous catalyst (introducing an entirely new family of heterogeneous catalysts with demonstrably superior performance),6,8 and biologically active metal composites.9 Such diverse applications demonstrate the versatility and customizability of the organics@metal composites, rooted in the ability to tailor metals with new properties by a suitable choice from a wide variety of organic dopants. Not only the choice of metal and dopant, but the synthesis conditions, such as synthesis solvent and reductant, influence the characteristic * To whom correspondence should be addressed. E-mail: david@ chem.ch.huji.ac.il.
features of the hybrid metal. In our first report, we employed a room-temperature homogeneous synthesis in an aqueous solution, in which dissolved silver cations were reduced using the water-soluble reductant hypophosphite in the presence of various soluble organic dyes.1 Recently, we have extended this approach to DMF solutions, enabling the entrapment of hydrophobic polymers.7 A heterogeneous entrapment method has been developed as well, in which the reductant is a sacrificial metal powder dispersed within the solution.4 Here, we extend the arsenal of entrapment methodologies to include the electrosynthesis of organics@metal composites. In this method, metal cation reduction is effected by the application of an electrical potential difference to the solution containing the cations and dopants. The feasibility of electrolytical entrapment is demonstrated using several dopant@metal combinations involving copper and silver, doped with various organic molecules, small and polymeric: Thionin@Cu, Thionin@Ag, Congo red@Cu, Crystal violet@Cu, Safranin-O@Cu, 1,10-phenanthroline@Cu, Nafion@Cu, Nafion@Ag, and poly(allylamine)@Cu have all been successfully prepared and characterized (see Scheme 1). It is noteworthy that the electrochemical doping of metals is in fact a reversal of common practice: electrolytic metal deposition is traditionally used for the purpose of metal purification or refinement rather than the purpose of introducing impurities (see, for example, any textbook on extractive metallurgy or electroplating13). Indeed, electrodeposition reports have mentioned as a typically undesired or, at best, harmless outcome the occlusion of organic additives from electrolytic baths in deposited metals (see Vermilyea,14 Bard and Faulkner,15 as well as the Encyclopedia of Electrochemistry16 and references therein). Other studies relevant to this report have been carried out: the electroless deposition of silver thin films has been suggested as a means for controlled delivery of drugs;17 codeposition of metals with organic or inorganic particles has been performed (see, for example, the review by Hovestad and Janssen18); however, the only reports we could find on intentional electrolytic entrapment involve the fabrication of glucoseoxidase biosensors, employing hybrids with a large excess of the enzyme.19-21 These works hint at the potential applicability
10.1021/jp906003m CCC: $40.75 2009 American Chemical Society Published on Web 09/24/2009
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SCHEME 1: Structures of the Dopants Used in This Study
of this field and the advantages in developing a general approach for electrolytical doping of metals with organic molecules. Experimental Section Chemicals. Dyes and Dopants. Thionin acetate (certified, dye content 86%, Aldrich), Congo red (certified, dye content 97%, Aldrich), Crystal violet (ACS reagent, Aldrich), Safranin-O (certified, dye content 99%, Aldrich), 1,10-phenanthroline monohydrate (reagent grade, Sigma), Nafion (5 wt % in mixture of lower aliphatic alcohols and water, hydrogen ion form, repeating unit 1100 g/mol, Aldrich), and poly(allylamine hydrochloride) (repeating unit 93.5 g/mol, Aldrich) were used. Copper(II) sulfate (98%) was obtained from Aldrich. Silver nitrate (ACS reagent) was from Acros Organics. Copper wires were taken from electrical cables (Ø 0.25, 2.0, and 3.0 mm, estimated purity >99.9%; according to the Copper Development Association, “The most popular form of pure copper is the standard electrical wire grade of copper (C11000) contains 99.95% Cu, 0.03% O2, and less than 50 ppm metallic impurities.”22). Silver wire (99.99%, Ø 0.8 mm) was purchased via Holland-Moran Ltd. Acetate buffer was prepared from glacial acetic acid (CP, Frutarom) and pH-adjusted with 1 mol/L NaOH (J.T. Baker). Electrolytical Entrapment Procedures. Several procedures were developed and different variations were carried out in search of optimal entrapment conditions. Procedure A. The synthesis of Crystal violet@Cu (CrV@Cu), is described as a representative case for this procedure: 3.3 mg of CrV was dissolved in 8.0 mL of 0.10 mol/L CuSO4 and 0.005 mol/L acetate buffer at pH 5.5 (the dopant concentration was 0.001 M). Thin (Ø 0.25 mm) and thick (Ø 3.0 mm) copper wire electrodes were immersed in the solution and a DC power supply was connected, with the thin and thick wires serving as the cathode and anode, respectively. The solution was stirred at 990 rpm. A constant potential difference of -1.2 V was then applied to the cell, and periodic measurements of the current passing between the electrodes were taken. The measured currents typically rise with time as the cathode’s surface area increases (in this example, from 5 to 18 mA). The anode visibly dissolves during synthesis as a deposit forms on the cathode. After 21 h, the power supply was disconnected, and the cathode wire with
Sinai and Avnir the resulting deposit (weighing ∼350 mg, in good agreement with an estimation from the measured currents) was removed from the solution. It was then rinsed for over a minute in a continuous stream of distilled water and then washed and drained an additional five times. Finally, the sample was dried overnight under a vacuum and then stored under an inert dry nitrogen atmosphere. UV-vis analysis of the supernatant solution after synthesis revealed a 94% drop in CrV concentration, indicating that the large majority of dopant molecules was incorporated into the metal deposit. The process occurring is the sum of the anodic and cathodic reactions:
In this notation, “dopant” is of course not a stochiometrically determined quantity; ratios of entrapment vary (see below). Note that charge conservation requires that Cu be oxidized at the anode at the same rate that it is reduced at the cathode, so in fact the copper solution concentration remains constant (in this case, 0.10 mol/L). Other Cu composites as well as Ag composites were synthesized using similar procedures with variations with regard to amounts used, voltage applied, etc. The specific conditions used to synthesize the composites described in this report are summarized in Table 1. As a supporting experiment, Crystal violet (CrV), Thionin (Th), and Congo red (CR) were tested for their electrochemical stability by cyclic voltammetry. CrV and Th proved to be stable, and CR appeared to undergo some reaction in this system; however, UV-vis analysis of the extracted dye (see below) provided no corroborating evidence of irreversible dye deterioration. Procedure B. An important variation on this procedure uses water oxidation as the anodic reaction. In this variant, acetate buffer was included in the synthesis solution in order to prevent dramatic pH changes. The synthesis of Nafion@Ag is described: A solution of 2.9 mg of Nafion, in 8.0 mL of 0.164 mol/L AgNO3 and 0.20 mol/L acetate buffer, pH 5.75, was prepared. Silver acetate precipitates but is subsequently dissolved as the synthesis proceeds. In the solution were immersed a silver wire cathode and a platinum wire anode, and the DC power supply was connected. The solution was stirred at 250 rpm, and a constant potential difference of -1.0 V was applied (the initial current measured was 1-2 mA). The synthesis was run for 73 h to completion, i.e., negligible current. Silver typically forms loose deposits, which are easily removed from the wire in the form of a composite powder. The resulting powder, weighing 139.9 mg (dry) of the theoretical maximum 144.2 mg (Ag + Nafion), was rinsed and dried overnight under a vacuum. In this methodology, the reaction occurring is 1 1 + + Ag(aq) + dopant + H2O(aq) f dopant@Ag(s) + H(aq) + O2(g) 2 4
Some oxygen evolution is visible at the anode, but this is not expected to affect the cathodic deposit. Background and Characterization Experiments. Entrapment Ws Adsorption Comparison Synthesis Procedures. Comparisons were made to discern the process of entrapment from the process of adsorption of the organic molecule on the metal (denoted dopant/metal). For this purpose, metal systems were synthesized using the same procedures described above but
Entrapment of Organic Molecules within Metals
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TABLE 1: The Doped Metals Prepared in This Report and Variations in Synthesis Solution Composition and Conditionsa composite
dopant mass (mg)
metal ion salt
metal ion conc. (M)
solution volume (mL)
buffer
CrV@Cu CR@Cu Th@Cu SaO@Cu Phen@Cub Naf@Cu PAA@Cub Th@Ag Th@Ag Naf@Agc
3.3 3.7 0.5-3.0 75.0 5.0 12.0 5.0 0.3 1.4 2.9
CuSO4 CuSO4 CuSO4 CuSO4 CuSO4 CuSO4 CuSO4 AgNO3 AgNO3 AgNO3
0.10 0.10 0.05-0.50 0.10 0.10 0.50 0.10 0.05-0.10 0.10 0.16
8.0 5.0 5.0 5.0 5.0 5.0 5.0 3.2 25.0 8.0
0.005 M acetate, pH 5.5
0.2 M acetate, pH 5.75
composite
stirring rate (rpm)
potential difference (V)
initial current (mA)
final current (mA)
synthesis time (h)
resulting deposit mass (mg)
CrV@Cu CR@Cu Th@Cu SaO@Cu Phen@Cub Naf@Cu PAA@Cub Th@Ag Th@Ag Naf@Agc
990 990 high high 990 high 990 NM medium 250
1.2 2.0 0.5-2.0 1.5 2.0 0.5 2.0 1.4-2.0 1.5 1.0
5 10 5-15
