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J. Phys. Chem. C 2009, 113, 14838–14843
Controllable Synthesis and Formation Mechanism Investigation of Prussian Blue Nanocrystals by Using the Polysaccharide Hydrolysis Method Ya Ding,†,‡ Yu-Lin Hu,‡ Gang Gu,‡ and Xing-Hua Xia*,‡ Department of Pharmaceutical Analysis, School of Pharmacy, China Pharmaceutical UniVersity, Nanjing 210009, China, and Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: June 17, 2009; ReVised Manuscript ReceiVed: July 8, 2009
Very controllable synthesis of cyanometallate-based coordination nanomaterials has been achieved by the hydrolysis reaction of polysaccharide (e.g., chitosan) in an acidic single precursor (i.e., potassium ferricyanide) solution. By controlling the hydrolysis rate of chitosan, Prussian blue (PB) nanoparticles and nanocubes with different sizes have been prepared via the rapid-seeding creation and long-time growth processes, respectively. The PB formation mechanism via the reducing effect of the hydrolysis product of chitosan and oxidation of the Prussian white intermediate in the solution was confirmed by Ultraviolet-visible (UV-vis), Fourier transform infrared (FT-IR), and transmission electron microscopy (TEM) measurements. The size- and structuredependent magnetic property of PB was also investigated. The present approach allows simple and controllable preparation of transition metal hexacyanoferrate nanocrystals and is promising for the study of unique shape-, size-, and structure-dependent properties for optoelectronic, magnetic, and electrocatalytic applications. Introduction In recent years, transition metal hexacyanoferrates (MHCFs), such as Prussian blue (PB) and its analogs, have received renewed interest1-3 due to their important properties of magnetism,4-6 electrochromism,7-9 and potential applications in sensors10,11 and batteries.12,13 As the first approved drug product for treatment of thallium and radiocesium poisoning by the FDA in the US,14 PB has been widely used in the field of biomedicine. Perl’s method using Prussian blue staining is commonly accepted in labeling cells containing iron ions.15,16 And recently, PB has been applied to determine the medicaments (e.g., isoniazid) among pharmaceutical samples as a spectroscopic probe reagent.17 It has been noted that the properties of nanoparticles are usually related to the structure, morphology, and compositions of the synthesized materials. Thus, shape- and size-controllable synthesis is of great importance, which provides effective means to exploit novel properties of these coordination polymers on the nanoscale. In order to achieve finely controllable synthesis of MHCFs, many synthetic methodologies have been advocated. Of these methods, the reverse microemulsion/micelle approach turns out to be successful in the controllable synthesis of MHCFs nanomaterials, due to the well-defined volume of the micro- or nanoreactors. Nanocubes (NCs),18 nanoparticles (NPs),19 nanorods, nanobelts,20 and other nanocrystals21 have been obtained by using the microemulsion technique. Alternatively, biocompatible polymeric molecules and a biomolecular matrix, such as poly(vinylpyrrolidone) (PVP),22-25 poly(diallyldimethylammonium chloride) (PDDA),23 Apoferritin,26 soluble starch,27 and chitosan (CS),28 have also been used to prepare PB and other MHCFs nanoparticles. In these reports, polymers play an important role as the separator and protector in the preparation of monodispersed nanoparticles. In the case of PB formation, * E-mail:
[email protected]. † China Pharmaceutical University. ‡ Nanjing University.
two strategies of double and single precursor methods are usually used. The former includes two routes. In one route, equimolar aqueous FeCl2 and K3Fe(CN)6 solutions were mixed in the presence of protector that provides chemically and spatially confined environments for the formation of NPs. In another route, the reaction mixture is composed of a soft template and equimolar FeCl3 and K3Fe(CN)6. It has also been reported that controllable synthesis of MHCFs can be achieved by applying external fields of sonochemistry,29,30 photosynthesis,31-33 and electrochemistry34-36 to the reaction system. With the intensive investigation of PB formation mechanism, the latter strategy of a single precursor method is desirable and has been proposed recently. This approach is based on dissociation of the singlesource precursor of either K3[M(CN)6] or K4[M(CN)6] (M ) Co, Fe, etc.) in the surfactant-formed microemulsion systems.37-40 The formation of MHCF nanoparticles can be induced by hydrothermal,37,39 sonochemical,38 and illumination40 processes, and the control of the shape and size of products was realized by varying the molar ratio of protector to precursor. In this paper, we report a simple, controllable, and green synthesis route for the formation of PB nanocrystals, in which K3[Fe(CN)6] has been chosen as the only precursor for PB in the protection of chitosan. The driving force for the particle formation comes from the system interior, i.e. the reducing agent provided by the hydrolysis product of chitosan in an acidic condition. Therefore, PB nanoparticles with different shapes and sizes can be prepared by controlling the hydrolysis rate of chitosan. Making use of pollution-free reactants and avoiding organic and surfactant agents, the synthesis of PB can be carried out in aqueous solutions. Transmission electron microscopy (TEM), Ultraviolet-visible/Fourier transform infrared (UV-vis/ FT-IR) absorption spectroscopy, and X-ray diffraction (XRD) experiments were used to characterize the resultant nanomaterials. In this report, we focus on finely tuning the shape and size of PB nanomaterials, revealing the formation mechanism and studying their magnetic properties.
10.1021/jp905704c CCC: $40.75 2009 American Chemical Society Published on Web 07/28/2009
Investigation of Prussian Blue Nanocrystals
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Figure 1. Four possible routes for the formation of PB in the acidic solution of ferricyanide and chitosan.
Experimental Section Materials. Chitosan has a deacetylation degree of 99.5% and average molecular weight of 70 000 D. Potassium ferricyanide and other chemicals were of analytical reagent grade and used as received. Solutions were prepared with deionized water (>18.2 MΩ, PureLab Classic Corp., USA). Synthesis of PB Nanoparticles. Stock solution of CS (35 mM) was prepared by dissolving purified CS in HCl solution (0.2%, v/v) under stirring until a clear and transparent solution was obtained. Prior to use, the stock solution was diluted to designed concentrations using HCl solution (0.2%, v/v). For Prussian blue nanoparticle preparation, K3Fe(CN)6 solution (in 0.2% HCl, v/v) was mixed with the diluted CS-HCl solution to the final concentration of [Fe(CN)63-] ) 1 mM and [saccharide unit] ) 0.5, 1.5, 5, 10, 20, and 35 mM. The mixture solutions were stirred in the dark and at 60 °C for 3 h to obtain PB NPs with different size. Synthesis of PB Nanocubes. To synthesize PB NCs, one mixture solution with the concentration of [Fe(CN)63-] ) 1 mM and [saccharide unit] ) 1.5 mM was stored in the dark and at room temperature. Small portion of this storage solution was taken out for characterization at different reactions duration. All the samples were collected by using centrifuge method, and the solid powder of PB-CS composites were obtained by a freezedry procedure. Characterizations. A single drop of reaction solution was deposited on the TEM grid immediately after reaction and dried in the dark. All samples of PB-CS composites were examined on a JEM-200CX TEM (JEM-100s JEOL, Japan) at an acceleration voltage of 200 kV. UV-vis spectra were collected on an UV-2401 PC spectrophotometer (Shimadzu, USA) immediately after reaction. X-ray diffraction patterns were obtained on an X’Pert X-ray diffraction spectrometer (Philips, USA). The structure information of CS and PB-CS in KBr discs were obtained by collecting infrared spectra on a Tensor27 Fourier IR spectrometer (Bruker, USA) equipped with liquid nitrogen cooled MCT detector. The magnetizm of the PB samples was measured on a magnetic property measurement system (Quantum Design MPMS-XL, USA). The measurements of zeta potential were performed by dispersing PB powder in water on a Malvern Instruments (UK). Results and Discussion For the purpose of controlling the shape and size of PB nanomaterials, two synthesis strategies including rapid seeding creation and slow crystal growth processes were employed. Under proper conditions, PB NPs and NCs with different size can be prepared. The PB formation mechanism in the present single precursor method can be summarized into four possible routes (Figure 1(1-4)).36 The key steps in these routes are the providing of
Figure 2. TEM images of the rapid seeding PB NPs stirred in the dark at 60 °C for 3 h with the [saccharide unit]/[Fe(CN)63-] molar ratio of (A) 0.5, (B) 1.5, (C) 5, and (D) 10.
TABLE 1: PB NPs Size Tuned by the Molar Ratio of [Saccharide Unit]/[Fe(CN)63-] molar ratio of [saccharide unit]/ 0.5 1.5 5.0 10.0 20.0 35.0 [Fe(CN)63-] PB NPs size (nm) 4 5 8 10
the electron source and the formation process of PB nanomaterials. For eliminating the interference from the exterior (such as daylight), we stored the reaction solution in the darkness. With the help of FT-IR investigations, the electron source provided by the hydrolysis product of CS in the rapid seeding creation method has been confirmed, and the PB formation in the slow crystal growth process has been proved via the mechanism (2) as illustrated in Figure 1. Rapid Seeding Creation Process. First, PB NPs with different diameters were synthesized in the K3Fe(CN)6-CS mixture solution (pH 1.6) by a rapid seeding creation method with stirring in the dark and at 60 °C for 3 h. To finely tune the size of PB NPs, the molar ratio of a saccharide unit to Fe(CN)63in the reaction mixtures changed from 1:2 to 35:1. As shown in Figure 2, many highly dispersed PB NPs with uniform size were formed. Estimated from the TEM images (Figure 2A-D), the average particle size of PB NPs with different [saccharide unit]/[Fe(CN)63-] molar ratios has been calculated and listed in Table 1. It is interesting that 1D PB chains appeared at low CS concentration (Figure 2A, inset), implying that the PB NPs were initially formed on or had molecular interaction with the CS chains. It is commonly accepted that higher concentrations of the reducing agent induces a faster nucleation rate, which would result in smaller particles size.41 However, it was found that increased concentration of CS resulted in larger particle sizes of PB as indicated in Table 1 ([saccharide unit]/[Fe(CN)63-] molar ratio from 0.5 to 10.0), which implies that concentrated CS inhibits the reduction of PB precursor and, thus, slows down the particles formation rate. This phenomenon leads us to make a conclusion that the CS molecule itself is not the reducing agent but plays an important role in the formation of PB. In addition, the slow formation rate of PB resulted in nanocubic morphology
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Figure 4. Hydrolysis process of chitosan and the chemical structures of its hydrolysis products.
Figure 5. FT-IR spectra of chitosan (dashed curve) vs PB-CS composite prepared by the rapid seeding creation method at the [saccharide unit]/[Fe(CN)63-] molar ratio of 1.5 (solid curve).
Figure 3. Plots of the intensity of PB UV-vis spectra. (A and B) UV-vis absorbance intensity at 422 and 680 nm of PB samples with [saccharide unit]/[Fe(CN)63-] molar ratios from 0.5 to 35.0, respectively. (inset) Typical spectrum of PB sample with [saccharide unit]/ [Fe(CN)63-] molar ratio of 1.5.
in the typical samples as shown in Figure 2C and D. This indicates that the slow reduction rate of PB precursor favors PB cube generation. TEM images of the samples prepared from more highly concentrated CS (i.e., the molar ratio of 20:1 and 35:1) were difficult to investigate due to the thick CS film and having more large PB NPs on the Cu grids. Furthermore, although great efforts have been made on the selected area electron diffraction (SAED) of PB nanostructures, no characteristic crystalline ED pattern was obtained, which may imply a complete coating of CS around the surface of PB crystals. Therefore, all samples can be collected by using centrifuge method. The resultant solid of PB-CS composites were obtained by a freeze-dry procedure and stored for a long time. Due to the diluting and separating effect of polymer molecules (chitosan), PB nanomaterials can be separated well again in water by ultrasonication. In addition, the zeta potential value for the redispersed PB nanoparticles in water shows positive results, for example, the ζ-potential of 3.23 for the sample with [saccharide unit]/[Fe(CN)63-] molar ratio of 5, which indicates again that the electrostatic interaction offered by the CS coating makes the PB nanocrystals dispersed stably in the solution. The TEM characterization of the redispersed samples is similar to those in the reaction solution (not shown here). The UV-vis spectrum of a typical PB sample with a [saccharide unit]/[Fe(CN)63-] molar ratio of 1.5 is displayed as inset in Figure 3A. The continuous decrease in the band intensity of Fe(CN)63- at 422 nm and the increase of the PB absorbance at ca. 680 nm corresponding to the {FeIII[(t2g)3(eg)2]FeII[(t2g)6]} f {FeII[(t2g)4(eg)2]FeIII[(t2g)5]}* electronic transition42 provide a clue to reveal the formation amount of PB NPs. After the same reaction time, the maximum absorption intensity at 422 nm (Figure 3A) showed that, the higher the CS concentration, the lower the ferricyanide absorbance, which indicates that more
CS molecules consume more Fe(CN)63- ions. However, the formed amount of PB is not proportional to the consumed precursor. The formation rate of PB NPs as indicated by the absorption band at 680 nm (Figure 3B) shows that the PB formation rate reaches a maximum at a [saccharide unit]/ [Fe(CN)63-] molar ratio ) 1.5. In the present method, potassium ferricyanide is the only precursor for the formation of PB. Since there are no other reducing agents available in the present system, electrons for the formation of PB can only come from CS molecules. On the basis of the results of the rapid seeding creation method, the phenomena can be well-interpreted by the CS hydrolysis. As illustrated in Figure 4, a CS molecule is composed of many D-glucosamine units connected via a β 1-4 linkage extending along the polysaccharide long chain. In an acidic solution (pH 1.6), the free aldehyde groups (the reducing end of a sugar chain) will be gradually released by the hydrolysis of the β 1-4 linkage from the CS structure.43 The released aldehyde group is an mild reducing agent and can chemically reduce the dissociated ferric or ferricyanide ion to ferrous or ferrocyanide ion, respectively. Small PB NPs are obtained due to the rapid PB formation rate, which can be understood by the fast hydrolysis rate of CS at an elevated temperature and from a diluted solution. Meanwhile, the large particles formation in the case of higher CS concentration is due to the inhibited hydrolysis rate at increased viscosity of the polymer solution.44-46 In addition, the maximum PB formation rate at a [saccharide unit]/[Fe(CN)63-] molar ratio ) 1.5 can be explained by the deficient electrons due to either reducing agent insufficiency at a low CS content or the hydrolysis inhibition at a high concentration. Direct evidence of the hydrolysis process of CS taking part in the formation PB can be obtained from the FT-IR spectroscopy analysis of PB-CS composites. As shown in Figure 5, the FTIR spectrum of PB NPs prepared by the rapid seeding creation method with the [saccharide unit]/[Fe(CN)63-] molar ratio of 1.5 is totally different from that of CS. It exhibits a major band at 2061 cm-1 due to the C-N stretching mode in the Fe2+-CN-Fe3+ in the ferrocyanide and Prussian white (PW, the reducing state of PB) cyanometallic lattice. The absorption band at 1595 cm-1 due to the bending vibration of the amino unit in CS disappeared, which indicates the coordination
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Figure 7. FTIR spectra of K3Fe(CN)6-CS mixture sample at different reaction durations: (A) 1 and (B) 20 days. Figure 6. TEM images of cubic PB nanocrystals synthesized in the dark and at room temperature for 2 (A), 6 (B), 10 (C), 15 (D), 20 (E), and 30 days (F).
interactions between amine groups and ferric/ferrous ions both freely in the solution and coordinated on the surface of PB particles. It is fully consistent with the featureless SAED results we discussed in TEM measurements, implying the protection effect of CS molecules on PB nanocrystals. The decrease in the band intensities at 1156, 1075, and 1033 cm-1 confirms the partial loss of the asymmetric stretching of the C-O-C bridge in CS,47 which is attributed to the degradation of chitosan structure, as illustrated in Figure 4, and further oxidation of the reducing ends during the PB formation.48 Slow Crystal Growth Process. We used the reaction mixture with a [saccharide unit]/ [Fe(CN)63-] molar ratio of 1.5 as a demonstration to investigate the growth procedure of PB nanocrystals in the slow growth process. The reaction mixture was stored in the dark and at room temperature. In this long time growth method, the crystal growth process can be easily terminated at any time by separating the particle-polymer colloids from the reaction solution using the centrifuge and subsequently freeze-dry method. Figure 6 shows the TEM images of PB crystals obtained at different growth duration. The sample with reaction time of 2 days gave small dots with the average diameter of several nanometers (Figure 6A). At a reaction time of 6 days, discrete dots and cubes (∼10 nm in diameter or side length) were formed (Figure 6B), which shows the initial formation of PB nanocubes (NCs). With the reaction time increasing, all PB NPs transformed into PB NCs and their sizes increased to about 56, 80, and 90 nm at reaction times of 10, 15, and 20 days, respectively (Figures 6C-E). Interestingly, when the reaction time was longer than 20 days, i.e., at a reaction time of 30 days, hemispherically structured PB with the size about 700 nm in diameter instead of larger PB NCs (Figure 6F) was formed. The magnified TEM image of a hemispherical particle (inset in Figure 6F) shows a much roughened surface, indicating that further nucleus creation occurs again possibly due to the surface energy change or the existence of defects on the surface of NCs. We believed that the slow PB
growth process produced a hill-like change in the system energy, which could be related to the preferential binding of organic molecules on crystallographic planes.40 When the NPs or NCs are small in size (
