Sorption of Metal Ions by Poly(ethylene glycol) - American Chemical

Jun 19, 2013 - †“Ilie Murgulescu” Institute of Physical Chemistry of the Romanian ... of Chemistry, University of York, York YO10 5DD, United Ki...
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Sorption of Metal Ions by Poly(ethylene glycol)/β-CD Hydrogels Leads to Gel-Embedded Metal Nanoparticles Gabriela Ionita,† Gabriela Marinescu,† Cornelia Ilie,† Dan F. Anghel,† David K. Smith,‡ and Victor Chechik*,‡ †

“Ilie Murgulescu” Institute of Physical Chemistry of the Romanian Academy, 202 Spl. Independentei, Bucharest, 060021, Romania Department of Chemistry, University of York, York YO10 5DD, United Kingdom

‡

S Supporting Information *

ABSTRACT: Inorganic nanoparticles can be embedded within gels by selectively preloading them with suitable molecular precursors followed by reduction or another suitable reaction. Here, we exploit the selective sorption properties of cross-linked β-cyclodextrin/poly(ethylene glycol) hydrogels, in analogy with polyurethane foams, to preconcentrate metal salts (HAuCl4 and K2[Co(SCN)4]) and subsequently generate gelembedded metal nanoparticles (10−50 nm). The nanoparticles are shown to be immobilized within the gel network as a consequence of their large dimensions in comparison to the gel network pore size. We suggest this is a useful approach for the generalized synthesis of hybrid soft−hard nanomaterials.

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INTRODUCTION Gels represent a class of materials of great interest due to their scientific significance and technological applications in the fields of pharmacy, cosmetics, pollutant capture and removal, and catalysis.1−5 They are considered as smart materials capable of responding to environmental changes such as temperature, pH, light, electric/magnetic fields, and chemical/biological species.6,7 Apart from everyday applications such as hair gel, gelatin desserts, and soft contact lenses, these materials can find advanced biological and biomedical applications in drug delivery, cell growth, and regenerative medicine.7,8 Gels can also act as reaction media or can be used to control the crystal growth of inorganic or organic compounds.9−12 On a molecular level, gels are an elastic cross-linked network embedded in a solvent.1 They are structurally similar to many other materials such as foams and porous resins; however, gels usually have very large internal volumes. They are also typically characterized by a very small cross section of the fibers (100− 500 nm) which leads to very large internal surface areas. Functionalized porous materials have long been used for absorption of different types of guest molecules. For instance, chelator-functionalized resins have been developed for selective absorption of transition metal ions; this is often exploited in the purification of waste streams.13 A similar approach can be used with chelator-modified gels. Fast diffusion in gels and high capacity due to large internal surface area make these materials attractive for developing selective absorbents. For example, thioether-derivatized gels have been used for extraction of soft metal ions such as Ag(I), Au(III), and Pd(II).14 Cellulose-based hydrogels have been used for absorption of metal ions such as Cd(II), Pb(II), and Ni(II).15 Sahiner and co-workers has © 2013 American Chemical Society

investigated selectivity of metal ion absorption by a number of chelator-functionalized gels.16,17 In some cases, strong and selective absorption of metal ions has been reported for materials lacking obvious chelating groups. This is usually driven by either electrostatic interactions (e.g., for polyelectrolyte gels18,19) or hydrophobic interactions. For example, polyether-containing polyurethane foams are wellknown to absorb anionic complexes of many transition metal ions with suitable ligands.20,21 The absorption is usually explained by the water structure enforced ion pairing (WSEIP).22,23 Large and fairly hydrophobic anionic metal complexes disrupt the hydrogen-bonded water structure. This forces the formation of ion pairs in the aqueous solution in order to maximize water−water interactions. The ion pairs can then be extracted from water into an organic solvent or solid absorbent. For example, this mechanism is usually used to explain the extraction of ion pairs into liquid poly(ethylene glycols).24 In the case of polyether-containing polyurethane foams, such extraction is facilitated by the interaction between positively charged counterions (usually alkali metals) and poly(ethylene oxide) groups, and hydrogen bonding between anionic metal complexes and urea functionalities.21 Apart from removal of the metal ions from the waste streams, selective absorption of metal complexes by the porous materials such as gels can be used for sensing or preconcentration of metal ions (e.g., for ion-exchange chromatography, or metal ion recovery applications). Additionally, the absorption results in Received: September 21, 2012 Revised: June 11, 2013 Published: June 19, 2013 9173

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accumulation of significant amount of metal ions. These metal ions can then be reduced with an appropriate reducing agent to form entrapped metal nanoparticles. Sahiner and co-workers used this strategy extensively; hydrogels functionalized with phosphonic, sulfonic, glycolic acids, and thiourea groups have been used to preconcentrate metal ions and prepare nanoparticles.25−27 Inorganic nanoparticles embedded in gel structures represent good candidates for applications in several diverse fields including catalysis,28,29 sensing,30,31 electronics,32 and developing biomedical devices.33−35 These applications benefit from the highly porous three-dimensional organization of gels. Although gel/metal nanoparticle composites can be prepared by simply impregnating the gel with the precursor salt prior to reduction,28,36 selective absorption of metal ions offers several advantages, including uniform distribution of nanoparticles throughout the gel, and high loading. The alternative method of preparing gel/nanoparticle composites by diffusing presynthesized nanoparticles into the gel structures is often limited by the permeability of the gel.36−38 Here, we report on the selective uptake of metal salts into polymer gels structurally similar to polyurethane foams in order to generate nanoparticle-studded gels. We have recently become interested in exploiting covalent polymeric hydrogels formed by cross-linking of diisocyanate-terminated poly(ethylene glycol) (PEG; average molecular weight 900) with β-cyclodextrin (β-CD) in a 10:1 ratio (PEG900/β-CD).39,40 The gel contains solvent pools surrounded by the PEG chains, with the cyclodextrin units acting as cross-linking groups (Figure 1). These gels are easily synthesized and can be readily tuned by simple structural modifications.

Article

EXPERIMENTAL SECTION

Materials. The PEG/β-CD and PEG/glycerol gels were prepared as previously described and the synthesis is reported in the Supporting Information. Methods. FTIR spectra of gels were recorded on Thermo Scientific Nicolet iS10 FT-IR spectrometer. UV−vis spectra of gels were recorded on JASCO V-670 spectrophotometer and those of Co2+ and its thiocyanate complex solutions were recorded on U-3000 Hitachi spectrometer. For transmission electron microscopy (TEM), the gel sections were prepared as follows: The gel was immersed in 2.3 M sucrose solution overnight and sectioned at 178 K, using the Leica FCS attachment on the Leica Ultracut ultramicrotome. The sections were examined using the Tecnai 12 BioTWIN TEM at 120 kV and imaged using a SIS Megaview III camera. The microtome was set to 100 nm (however it did not cut a section at every pass). The loading capacity of the gels toward H[AuCl4] was calculated as follows: aqueous H[AuCl4] (3 mL, 10−2 M) was mixed with wet gel (0.5 g). The change in UV absorption of the supernatant solution was monitored at 215 nm. Upon equilibration, the UV intensity was reduced by 56% for the PEG/β-CD gel and by 51% for the PEG/ glycerol gel.

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RESULTS AND DISCUSSION Interactions of Alkali Metal Salts with the Gels. We first monitored the uptake of some simple potassium and sodium salts by the PEG900/β-CD gel by measuring the conductivity of a 10−2 M solution of each salt (10 mL) in contact with 2 g of water-swollen gel (20 wt %). In these experiments, the carbamate groups were in ca. 8-fold excess compared to the metal salts, while the metal/cyclodextrin ratio was approximately 1:1. Upon interaction with a gel, the conductivity of salt solutions can change due to a dilution effect and also specific interactions between the ions and the gel fibres. Assuming the gel only causes a dilution effect, the relative decrease in conductivity should be 13% at equilibrium, regardless of the nature of the salt. The relative decrease in conductivity after equilibration with the gel (Table 1), however, was somewhat less than the value attributed to a dilution factor for all salts except KSCN. All salts could be released from the gels by washing with distilled water. Table 1. Relative Decrease of Conductivity of 10−2 M Salt Solutions after Equilibration with PEG900/β-CD Gel salt crel (%)a

NaCl 9.2

KCl 10.5

KNO3 10.3

KSCN 14.2

KBr 10.5

K4[Fe(CN)6] 5.9

a crel is a relative decrease in conductivity of solution upon addition of the water-swollen gel.

The small changes in conductivity reported in Table 1 clearly show the absence of any strong interactions between the gel network and alkali metal salts. We noted however that the thiocyanate salt appeared to show somewhat stronger interactions with the gel network than the other anions. Thiocyanates are often used as a ligand to facilitate the extraction of transition metal ions, such as Zn2+, Cd2+, and Co2+, by polyurethane foams.20,21,41 Polyethylene glycols and their derivatives can also extract anionic thiocyanate complexes.21 As the PEG/β-CD gel contains both carbamate and oligoethylene glycol units, we reasoned that it might show a similar behavior. We were therefore interested to see if the PEG/β-CD gels would show strong uptake of metal thiocyanates such as Co2+ thiocyanate. As a control experiment, conductivity measurements showed the absence of any specific

Figure 1. Schematic representation of the gel network.

Given the structural similarity between gels and foams, we wondered whether these gels might behave somewhat like the polyurethane foams described above. The aim of this work was therefore to capitalize on the ability of PEG900/β-CD gel components to form intermolecular interactions and explore the sorption properties of this gel for metal ions. We hoped that the ability of gels to preconcentrate metal ions could be exploited for the preparation of metal nanostructures within the nanostructured network of the gel itself. 9174

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as the cross-linking points of the gel network play an important role in the uptake, we prepared a cyclodextrin-free gel by replacing cyclodextrin with glycerol. The uptake of the cobalt thiocyanate complex was the same as for the cyclodextrincontaining gel, thus confirming that cyclodextrin does not influence the uptake in this case, unlike the uptake of organic guest species such as adamantane, which is controlled by hydrophobic binding within the cyclodextrin nodal points. We therefore needed to propose an alternative uptake mechanism for cobalt thiocyanate. It was possible, if unlikely, that the thiocyanate might interact with the carbamate groups via hydrogen bond interactions, but we noted no change in the stretches of CO or bends of N−H from the ATR-FTIR spectrum. Indeed, the only changes in the IR were in the 2800−2900 cm−1 region attributed to symmetric and asymmetric stretches of CH2 groups (Figure S2 in Supporting Information). We noted that polyurethane foams can take up this kind of salt and, therefore, reasoned that the mechanism of the selective uptake by this gel might be similar and, as such, driven by the water structure enforced ion-pairing as described in the Introduction and consistent with the IR shifts at 2800−2900 cm−1. The interaction of cobalt thiocyanate complex with the gel fibres is reversible. When a sample of the gel saturated with [Co(SCN)4]2‑ was added to distilled water, the equilibrium was shifted toward [Co(H2O)6]2+ which then diffused out of the gel into solution. This experiment suggests that this type of material can be used to extract Co2+ as a thiocyanate complex which could subsequently be recovered through repeated washing. Encouraged by the successful and selective uptake of cobalt thiocyanate complexes, we went on to explore the absorption of other metal complexes. We found that, just like with polyurethane foams, [AuCl4]− is rapidly taken up by the gel.45,46 The capacity of the PEG/β-CD and PEG/glycerol gels for the uptake of [AuCl4]− can be evaluated from the changes in absorbance of this salt by UV−vis spectroscopy. Thus, it was found that both gels have similar capacities to load gold salt (3.7 × 10−5 mol HAuCl4/g for PEG/β-CD gel and 3.1 × 10−5 mol HAuCl4/g for PEG/glycerol gel; the weights refer to the wet gel which is 20% gelator and 80% water). The absorption was independent of the nature of the cation, with sodium and potassium salts and tetrachloroauric acid all being absorbed by the gels to a similar extent. The mechanism of the [AuCl4]− absorption in the gel is not clear. Absorption of this salt into polyurethane foams is thought to be facilitated by the interaction between the alkali metal countercation and the polyether chain, and the interaction between the [AuCl4]− with the NH group in the urethane functionality.46 We noticed however that absorption of the gold salt into the PEG900/β-CD gel is accompanied by an increase in conductivity of the surrounding solution. This was contrary to the expectation that the concentration of the salt would be reduced due to its uptake in the gel, and would hence lead to a decrease in conductivity. We believe that the increase in conductivity is due to the loss of Cl− from the [AuCl4]− upon absorption. As such, the diffusion of Cl− out of the gel into the surrounding solution would lead to the increased conductivity. The leaching of Cl− into surrounding solution was confirmed by addition of AgNO3 and subsequent precipitation of AgCl. Uptake of [AuCl4]− by PEG/glycerol gel also did not show a decrease in conductivity thus confirming that Cl− leaching is

interactions between CoCl2 and the gels, similar to the alkali metal salts. At high concentration, the uptake of CoCl2 can be conveniently monitored by UV−vis measurements. Following the change in absorbance of a 0.1 M solution of CoCl2 at 510 nm, we observed that the CoCl2 uptake is simply determined by the dilution effect. The same result was obtained using methanol as a solvent (i.e., if the gel was swollen with methanol prior to the addition of CoCl2). Uptake of Anionic Metal Complexes by the Gels. The addition of KSCN to solution of CoCl2 leads to formation of an anionic thiocyanate complex which is in equilibrium with the aqua complex: [Co(H 2O)6 ]2 + + 4SCN− ⇌ [Co(SCN)4 ]2 − + 6H 2O

Interestingly, addition of an equilibrium mixture of the aqua and thiocyanate cobalt complexes to the gel led to the highly selective uptake of the blue [Co(SCN)4]2‑ as could be clearly seen from the color of the gel and the solution (Figure 2a).

Figure 2. (a) [Co(SCN)4]2‑ absorbed by the PEG900/β-CD gel from a solution of CoCl2 in the presence of KSCN; (b) [Co(SCN)4]2‑ formed in PEG900/β-CD gel in the presence of a 0.6 M solution of KSCN.

Moreover, the decrease in absorbance of the solution containing Co2+ was about 50% more than that predicted by a simple dilution effect (Figure S1 in Supporting Information). Alternatively, if a sample of gel saturated with CoCl2 was immersed in a solution of 0.6 M KSCN, the blue complex could be clearly seen to form initially at the interface between the gel and solution and then spread throughout the gel (Figure 2b). There was no visible leaching of the blue color of the complex from the gel into the 0.6 M. KSCN solution, which confirms the strong and selective interactions between the gel and the [Co(SCN)4]2‑ complex. Due to the presence of different functional groups in the gel network, all of which are capable of noncovalent interactions, it is difficult to establish the precise factors that determine the capacity to uptake metal complexes. In particular, the crosslinked gel network includes cyclodextrin units, poly(ethylene glycol) chains, and carbamate groups. The presence of cyclodextrin cavities in the gel network enhances the ability of the gel to incorporate certain hydrophobic guest molecules. For instance, the absorption of organic molecules by the cyclodextrin-containing gels has been reported.42,43 We also previously reported on the ability of PEG/β-CD hydrogel to take up organic molecules with a known affinity for the cyclodextrin cavity, such as adamantane derivatives, with the uptake being followed by EPR spectroscopy using spin labeling.44 In order to check if the cyclodextrin cavities present 9175

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observation which has been previously noted as one of the potential advantages of direct formation of nanoparticles within a gel.50 Following reduction, the gel can take up even more [AuCl4]−. When the nanoparticle-studded gel was mixed with 10−3 M solution of [AuCl4]− (1 mL), rapid and nearly quantitative uptake was again observed. After the mixture equilibrated for 10 min, the reduction of Au(III) by citrate was carried out which increased the loading of Au nanoparticles in the gel. Further exposure to [AuCl4]−, however, resulted in only partial uptake of the gold salt by the gel. This is an unusual observation. Au nanoparticles of 10 nm contain over 100 000 Au atoms but only occupy a small volume. Therefore, when the Au(III) salt is reduced and the nanoparticles are formed, nearly all binding sites in the gel that were interacting with the salt should become free. Hence the reduced uptake cannot be explained by blocking some binding sites in the gel by the Au nanoparticles. We propose therefore that the reduced uptake is due to the nanoparticles partially blocking the pores in the gel. The gel contains ca. 20% fibers, and we found that even before nanoparticle formation it was not permeable for particles (metal particles or proteins) larger than ca. 4 nm. A simple calculation shows that the concentration of nanoparticles in the gel is ca. 0.3 μM. Assuming a random distribution and 10 nm particle size, the gaps between adjacent particles are ca. 90 nm.51 Although the diffusion of [AuCl4]− and reducing agent is fast compared to the rate of reduction, and hence the distribution of nanoparticles in the gel after first loading is likely to be quite uniform, we believe it is not unreasonable to suggest that the nanoparticles partially block the pores in the gel to restrict the access to large [AuCl4]− ions (ca. 1.2 nm in diameter) to some of the solvent voids within the gel.52 Partial blocking of the pores results in the shortening of the percolation length thus making the interior of the gel inaccessible to the [AuCl4]− ions. These arguments are confirmed by the visual inspection of the nanoparticle-studded gels. While after the first loading the distribution of nanoparticles throughout the gel is quite uniform as judged from the color, further loadings shows that the concentration of nanoparticles is highest near the surface of the gel (Figure 4

not limited to the cyclodextrin-containing gels. The dissociation of Cl− upon interaction of [AuCl4]− with polysaccharides has very recently been suggested, and this mechanism may be operating here.47 Preparation of Metal Nanoparticles in the Gel. The experiments described above confirm that PEG-based hydrogels can be used to absorb and preconcentrate anionic metal complexes. We noticed that, upon storage, the PEG900/β-CD gels containing [AuCl4]− turned pink, indicating partial reduction of Au(III) and the formation of Au nanoparticles. Although the ability of PEG900/glycerol and PEG900/β-CD gels to uptake H[AuCl4] was quite similar, this spontaneous reduction was only observed for the cyclodextrin-containing gel. This suggests that unreacted hydroxyl groups of the cyclodextrin are involved in the reduction. Hydroxyl groups are known to reduce Au(III) under certain conditions. For instance, spontaneous formation of gold nanoparticles from an α-cyclodextrin/Au salt complex was recently reported in the absence of extra reducing agents.48,49 Interestingly, spontaneous reduction of Au(III) by the PEG900/β-CD gel occurs only with the [AuCl4]− salts. The iodo derivative [AuI4]− is also rapidly taken up by the gels but this uptake is not accompanied by the spontaneous reduction, presumably due to the strength of the Au−I bond. Gel-embedded Au(III) can also be converted to Au nanoparticles by the addition of a suitable reducing agent. Addition of a substoichiometric amount of H[AuCl4] (10−6 mol) to the PEG900/β-CD gel (0.5 g wet gel, corresponding to 0.1 g dry gelator) led to fast, quantitative absorption of Au(III). The mixture was left to equilibrate for 10 min. Addition of sodium citrate (3 mL of 10−1 M solution) followed by heating at 80 °C resulted in the formation of a pink color, demonstrating the formation of nanoparticles inside the gel. The surrounding solution remained colorless and diffusion of the nanoparticles out of the gel was not observed. The UV−vis spectrum of the [AuCl4]− loaded gel after reduction shows a strong surface plasmon band at 538 nm, confirming the formation of nanoparticles. The TEM image of a gel section shows that the particle size is in the range 10−50 nm (Figure 3). The lack of nanoparticle diffusion indicates that these materials are trapped within the gel network, an

Figure 4. Image of a section of PEG900/β-CD gel after repeated loading with [AuCl4]− and reduction with citrate.

represents a cross section of the gel loaded with nanoparticle three times), suggesting that the interior of the gel becomes inaccessible to further nanoparticle precursor on repeated loading. Further uploading/reduction of [AuCl4]− into the gel leads to the formation of a metallic layer at the gel surface. The formation of nanoparticles from metal complexes absorbed within the gel is not restricted to Au salts. Although the gel loaded with cobalt thiocyanate complex did not show spontaneous reduction, addition of sodium borohydride led to the formation of Co nanoparticles inside the gel (Figure S3 in Supporting Information); as such, we suggest this is a general

Figure 3. TEM image of a PEG900/β-CD gel section with Au nanoparticles prepared by citrate reduction. The inset shows the UV− vis spectrum of the same gel. 9176

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approach to preloading gel-phase materials with heavy metals appropriate for reduction to functional and gel-immobilized nanoparticles.

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CONCLUSION In summary, we have shown that poly(ethylene oxide) hydrogels cross-linked by β-cyclodextrin or glycerol through polyurethane groups show similar metal uptake properties to polyurethane foams. Cobalt thiocyanate and tetrachloroaurate were absorbed into the gels. The addition of a reducing agent to the metal ion-loaded gel resulted in the formation of gelentrapped metal nanoparticles. We thus believe that this method can be used as a generic approach to the synthesis of inorganic nanoparticles embedded and trapped within soft materials. As such, the selective absorption and preconcentration step within the gel plays a vital role in the fabrication of these hybrid materials. Given the high permeability and solvent compatibility of gel networks, we suggest that such materials may have applications as novel catalytic materials; studies of the catalytic properties of these novel hybrid materials are currently in progress. In summary, the approach reported in this paper therefore offers a simple, effective, and controllable approach to hybrid soft/hard nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

Gel synthesis and additional UV and FT-IR data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the International Joint Project funded by Royal Society. G.I. gratefully acknowledges the support of the EU (ERDF) and Romanian Government, which allowed for acquisition of the research infrastructure under POS-CCE O 2.2.1 scheme INFRANANOCHEM - Nr. 19/ 01.03.2009.

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dx.doi.org/10.1021/la401541p | Langmuir 2013, 29, 9173−9178