Progress in Solid State Chemistry 34 (2006) 161e169 www.elsevier.com/locate/pssc
Core-shell copper hydroxide-polysaccharide composites with hierarchical macroporosity Franc¸oise Quignard a,*, Didier Cot b, Francesco Di Renzo a, Corine Ge´rardin a a
Laboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618-CNRS-ENSCM-UM1, Institut C. Gerhardt, FR 1878, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France b Institut Europe´en des Membranes, Universite´ Montpellier II, CC047, 2 Place Euge`ne Bataillon, 34095 Montpellier Cedex 5, France
Abstract A new method of preparation of polymeric-inorganic porous nanocomposites with core-shell morphology allowed to stabilise copper hydroxide salts in water. A shell of inorganic salts was formed around microspheres of alginate gel by migration of the metal ions from the core of the particles. The formation of the copper hydroxide nitrate shell was controlled by the kind of mineralisation treatment. The natural polysaccharide was an effective agent of inorganic growth control. The mineral shell stabilised the hydrated polymer gel by considerably limiting the drying and induced the formation of a hierarchical macroporosity when drying the microspheres under supercritical CO2 conditions. Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction Nanomaterials of controlled composition, chemical structure, size and shape characteristics have raised a widespread interest in the recent literature [1]. The use of organic agents has allowed to modify the synthesis pathway of inorganic phases and helped in controlling the properties of the mineral materials. Organic modifying agents are most commonly surfactants or
* Corresponding author. Tel.: þ33 4 67 16 34 60; fax: þ33 4 67 16 34 70. E-mail address:
[email protected] (F. Quignard). 0079-6786/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.progsolidstchem.2005.11.016
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polymers which can be either synthetic macromolecules with well-controlled architecture or natural polymers. The use of amphiphilic molecules has led to the preparation of nanostructured phases presenting a highly ordered organisation, and textures as different as those of mesoporous materials, nanoparticules or complex hierarchical structure [2]. Hydrophilic molecules have also been extensively used to control the growth and stability of mineral phases. More precisely, the use of double-hydrophilic block copolymers (DHBCs) has allowed to prepare either crystals of inorganic salts such as CaCO3 [3e6], BaSO4 with tunable shapes [7,8], or colloids of metal hydrous oxides with adjustable size [9]. Hydrophilic polymers have been used to stabilise mineral suspensions, as in the case of the preparation of copper hydrosols in the presence of poly(vinyl alcohol) [10]. K-carrageenans, a natural sulphated polysaccharide, has been used to prepare Ni, Co and iron hydroxides particles in aqueous suspension [11]. It was shown that the polysaccharide acts as an efficient stabiliser and prevents the precipitation of iron oxides and hydroxides at high pH. Nickel hydroxide nanoparticles were also well stabilised, while Co2þ hydroxide progressively transformed into Co3þ oxide by alkaline oxidation. The good stabilisation properties of the carrageenan molecules are due to the possible interaction of the sulphate and the hydroxy groups in the macromolecules with the mineral species. Alginate, another natural polysaccharide, was used in the preparation of several silica-based hybrid materials by polycondensation reactions of different siliceous precursors. Alginate/silica biocomposites were prepared by using sodium silicate [12], aminopropyl silicate [13e16], and tetra-alkoxysilane [17]. as precursors of silica. Alginates are easily available polysaccharides produced by brown algae. They are block copolymers of (1e4) linked b-D-mannuronic (M) and a-L-guluronic (G) residues (Fig. 1). Several alginates exist and differ by their M/G ratio and their gelation behaviour [18]. Alginates are extensively used for the entrapment of biologically active materials for applications as different as the controlled release of drug or cosmetics, biological catalysis and the transport of enzymes for detergency [19e25]. The use of alginates as immobilising agents in different applications lies in their ability to form heat-stable strong gels with divalent cations. Most generally, Ca2þ ions are used for the above mentioned applications. Gelling of alginate occurs when the divalent cations take part in the interchain binding between the G blocks affording a three-dimensional network [26]. In the present work, alginate gels represent the organic part of composites whose inorganic component is a copper hydroxide phase. Alginate was chosen due to the high affinity of its COO- functional groups for Cu2þ [27], copper was chosen due to its known bactericide and fungicide properties. A major problem in the industrial preparation of fungicide and bactericide products based on copper hydroxysalts is the instability of copper hydrolysis products in water. In fact, copper hydroxide Cu(OH)2 is metastable. It easily transforms into copper oxide CuO
OH -OOC
O
OH
O
m
(M)
-OOC
OH
OH COO-
O O
O O
OH
O
HO n
(G) Fig. 1. Alginate structure.
(M)
p
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either in the solid state at relatively low temperature, or at room temperature in aqueous basic solutions [28]. The fungicide properties of copper salts were discovered in the late 19th century when Pr Alexis Millardet of the University of Bordeaux (France) successfully used copper sulphate mixed with lime for the control of downy mildew on grapes [29]. Due to the unstability of copper hydroxide, this mixture, known as Bordeaux Mixture, had to be mixed in the field prior to each use. Presently, several fungicide products are based on surfactant-stabilised copper hydroxysulphate granules. In this paper, the study of the hydrolysis of the copper-alginate system allows investigating the effect of the biodegradable polymeric matrix on the chemical stabilisation of the metal hydroxide phase.
2. Experimental section 2.1. Microspheres preparation Sodium alginate (Sigma; medium molecular weight) was dissolved in distilled water at a concentration of 1% (w/w). The polymer solution was added dropwise at room temperature to a Cu(NO3)2 (Aldrich) solution (0.24 M) under stirring using a syringe with a 0.8-mm diameter needle. Gel microspheres were thus formed, cured in the Cu2þ solution for 3.5 h, then separated from the cationic solution and washed with distilled water. Mineralisation of the copper alginate gel spheres was induced by addition drop by drop of a 0.48 M NaOH (0.48 M KOH) solution to an aqueous suspension of the gel microspheres. Addition of the alkali solution was continued until pH 9. Aerogels were prepared by supercritical drying of the gel microspheres. CO2 supercritical drying was preceded by a step of dehydration of the gel microspheres; they are successively immersed in a series of ethanolewater baths of increasing alcohol concentration (10, 30, 50, 70, 90, and 100%) during 15 min each for the copper alginate beads and 10 h each for the mineralised beads [30]. The gel microspheres were then dried under supercritical CO2 conditions (74 bars, 31.5 C) in a Polaron 3100 apparatus. Xerogel spheres were prepared by evaporative drying of the hydrogel water at room temperature. The samples studied in this paper are named according to the kind of cation used in the mineralisation step and the number of days elapsed between mineralisation and drying. For instance, sample Na-1d is a sample mineralised with NaOH and dried 1 day after the mineralisation treatment.
2.2. Characterisation of materials Scanning electron micrographs (SEM) of the supercritically dried microspheres were obtained using a Hitachi S-4500 apparatus after platinum metallisation. The local chemical composition was measured on cross-sections of dried microspheres by using an EDX microprobe on a Cambridge Stereoscan 260 apparatus. Nitrogen adsorption/desorption isotherms on supercritically dried samples were recorded using a Micromeritics ASAP 2010 apparatus at 77 K after outgassing the sample at 323 K under a vacuum until a stable 3 105 Torr pressure was obtained without pumping. Surface areas were evaluated by the BET method assuming that a monolayer N2 molecule covers 0.162 nm2.
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Thermogravimetric analyses were performed with a Netzsch TG 209 C apparatus in air flow at a heating rate of 5 C/min. The amount of organics in the composite was evaluated from the weight loss at a temperature lower than 250 C. X-ray diffraction patterns of crushed xerogel spheres were recorded on a Brucker D8 diffractometer with Cu Ka radiation. 3. Results and discussion This investigation was perfomed starting with well-shaped copper alginate gel microspheres as precursors for the mineralisation step. Formation of alginate gel microspheres was obtained by dropping a water solution of sodium alginate into an aqueous solution of Cu2þ ions. This method leads to the spontaneous formation of well-shaped blue microspheres. In order to form metal hydroxide phases from metal cations in solution, hydroxide ions are added and strong bases such as sodium hydroxide can be used. As Naþ ions are known to inhibit alginate gel formation, mineralisation was performed in this work by using NaOH and KOH. The phases formed during the mineralisation treatment were identified by X-ray diffraction analysis. The diffraction patterns showed that the Cu hydrolysis product was mainly gerhardtite, Cu2(NO3)(OH)3, when mineralisation was performed by adding either NaOH or KOH. In the case of NaOH mineralisation, no crystalline phases were observed for samples Na-1d. The mass ratio between organic and inorganic components was deduced from thermogravimetric analysis. In Fig. 2, the thermal degradation patterns of the mineralised composites Na-1d and K-1d are compared to that of the precursor Cu-alginate microspheres. For all samples, a first loss of nearly 20% weight took place up to 130 C, corresponding to the amount of water adsorbed after the supercritical CO2 treatment. A second weight loss occurred suddenly at 170 C. Such a rapid phenomenon, corresponding to the ignition of the organic component, is not usually observed in the calcination of alginate gels. In the same conditions, a calcium alginate gel decomposes at a leisurely pace between 170 and 420 C 100 90 80 70
% wt
60 50 40 30 20 10 0
0
200
400
600
800
1000
T(°C) Fig. 2. Thermogravimetric analyses of the precursor Cu-alginate (plain line) and of the samples mineralised with KOH (K-1d) (triangles) and NaOH (Na-1d) (diamonds).
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(not shown). In the case of copper alginate samples, it can be assumed that a copper phase catalysed the combustion of the organics and dramatically speeded up the decomposition. In the case of the presursor Cu-alginate, after the rapid combustion step, the decomposition of the organics continued more slowly and some weight loss continued up to 800 C. On the contrary, in the case of the most mineralised sample, K-1d, the decomposition of the organics was nearly complete at 180 C. This sample presented another sharp weight loss at 360 C, which corresponds to the thermal degradation of the gerhardtite. The less efficiently mineralised sample, Na-1d, presented an intermediate behaviour, with some decomposition of the organics after the rapid combustion step and before a sudden weight loss at 330 C. The presence of this weight loss indicates that copper hydroxide nitrate is present in this sample even though the XRD pattern of gerhardtite was not observed. A small crystal size of the copper phase allows for both the absence of XRD lines and the low degradation temperature observed by TG. The TG data indicate that the inorganic part represents nearly 30% weight of the composite materials. Textural properties [31,32] were obtained by studying nitrogen adsorptionedesorption isotherms at 77 K on the microspheres dried under supercritical CO2 conditions. This method has proved to be efficient for revealing the textural properties of the gel in Ca-alginate microspheres [33]. All isotherms are at the borderline between type II and type IV in the IUPAC classification, indicating the presence of large mesopores with a size distribution which continues into the macropore domain. Surface areas for all samples are very high: 670 m2 g1 for the precursor spheres, 574 m2 g1 for sample K-1d, and 496 m2 g1 for sample Na-1d. For the three samples, the energetic parameter of the BET equation, C(BET), is nearly 145, a quite high value often observed for alginate gels [28]. The adsorption data indicate that the texture of the alginate gel was scantily affected by the mineralisation treatment. A decrease of surface area was indeed observed for the mineralised samples. This effect is largely due to the contribution of the non-porous mineral phase to the mass of the mineralised samples. Observation by SEM provides a clear representation of the structure of the composite materials (Fig. 3). In (K-1d) sample, the outer shell of the microsphere was formed by a layer of plate-like particles characteristic of Cu(OH)1.5(NO3)0.5 (Fig. 3c). Observation of crosssections of the microspheres indicates that the core of the bead is totally occupied by a radial pattern of channels with a diameter of 5e10 mm, lined by a macroporous alginate matrix, as shown in Fig. 3a. Some crystalline particles of Cu(OH)1.5(NO3)0.5 are still present in the inner part of the bead, as evidenced in the cross-section detail in Fig. 3b. In sample Na-1d, there was no evidence of the presence of crystalline particles, in agreement with the absence of diffraction lines. However, the same radial pattern of channels was present (Fig. 3d). The radial distribution of the cations in the mineralised beads was determined by EDX microprobe analysis of a microsphere cross-section of sample K-1d (Fig. 4). The results indicated that the copper concentration is virtually nil at the centre of the bead and rapidly increases towards the outer shell. This confirmed that, during the mineralisation treatment, Cu2þ cations partially moved from the core to the surface and are replaced by Kþ ions uniformly distributed inside the microsphere. The difference between Na and K treatments disappears when a longer time elapsed between mineralisation and drying. For both Na-10d or K-10d samples, the outer shell is formed by a composite constituted of a shrunk alginate structure and a copper hydroxy-nitrate phase located at the outer surface of the microsphere (Fig. 5: sample Na-10d). An image of the
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Fig. 3. SEM of Cu-alginate aerogel microspheres after mineralisation with (aec) KOH (sample K-1d) or (d) NaOH (Na1d). (a and b) Cross-section parallel to the macroporous channels; (c) details of the mineral shell.
plate-like particles characteristic of Cu(OH)1.5(NO3)0.5 is given in Fig. 5c. As in sample K-1d, the core of the bead is totally occupied by radial pattern channels, whose diameter is close to 10 mm as shown in Fig. 5d. No crystallised particles were observed in the inner part of the beads. In all cases, the channels have formed during the drying process by partial syneresis of the polysaccharide gel. It is clear that the migration of copper towards the periphery of the beads and the formation of a crystalline outer shell takes place at a different rate depending on the kind of alkali cation
16 14
% mass
12 10 8 6 4 2 0
0
200
400
600
micrometer Fig. 4. Radial distribution of the K (empty squares) and Cu (filled squares) ions by EDX analysis of a cross-section of the aerogel sample K-1d. The X-axis is the distance to core centre.
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Fig. 5. SEM of a cross-section of a Cu-alginate aerogel microsphere after mineralisation with NaOH (sample Na-10d). (a) Cross section of the alginate bead; (b) macroporous core; (c) details of the composite shell; (d) details of the mineral shell.
used. In the case of Kþ, the phenomenon is virtually complete after one day, while in the case of Naþ, some days are needed to achieve the same texture. A remarkable result is that the presence of this mineral shell had a dramatic effect on the behaviour of the gel towards drying. While the non-mineralised spheres dried to small shrunken particles (Fig. 6d) when left in air for 1 day, the mineralised beads retained the original size and water content after 10 days in air. Thermogravimetric analyses revealed that, while the initial hydrogel contains 96.6% water (mass loss up to 120 C), the hydrogels after NaOH or KOH treatment and 10 days storage in air still contain 94.4% and 94.8% water, respectively. Some bead shrinking was observed only after 20 days at 20 C. Moreover, it was observed that the mineral phase was very stable with time, in the dry state, as well as in aqueous solution. Similar chemical stability of the basic salt, Cu hydroxy-nitrate, was previously observed in the case of the use of other complexing polymers such as doublehydrophilic block copolymers [17]. 4. Conclusion This paper describes an original method to obtain new organiceinorganic composites formed by controlled mineral growth. The presence of the mineral shell at the surface of the microsphere is at the origin of the stabilisation of the gel in its hydrated form. The mineral phase also presents a very high stability with time and its transformation to copper oxide is prevented, in water as well as in a dry state. The stability together with the high porosity of the material should lead to the development of a range of applications profiting from both the
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Fig. 6. Effect of mineralisation of Cu-alginate hydrogel microspheres on water retention. (a) Precursor spheres, spheres exposed to air for 6 days after mineralisation with (b) KOH (sample K-6d) or (c) NaOH (sample Na-6d); (d) precursor spheres exposed to air for 1 day.
bactericide or fungicide properties of the copper based mineral part and the biodegradability of the organic part. Moreover, the peculiar macroscopic porosity can provide a large volume available for the storage of biological molecules such as drugs. In this field of application, these core-shell systems can usefully complement the properties of the microcapsules commonly used in drug delivery systems.
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