Organic–inorganic hybrid: a novel template for synthesis of nanostructured Ag

Abstract

We herein demonstrate that the amino acid arginine (Arg) can interact with phosphotungstate (Keggin) anions generating an excellent template for the synthesis of nanostructured silver wherein the Arg–PTA hybrid host plays the role of a UV-switchable reducing agent. The binding strength of the complex between arginine and phosphotungstate (PW₁₂O₄₀)³⁻ i.e. the Keggin ions in aqueous medium has been assessed by the isothermal titration calorimetric technique. It is shown that the Arg–PTA complex may be used as a new class of organic–inorganic scaffolds in the synthesis of nanostructured silver. The crystalline Arg–PTA host can be photochemically reduced, resulting in electron transfer to the entrapped silver ions to form nanoparticle assemblies on the underlying colloidal template.

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1. Introduction

There is increasing scientific interest in the growth of ordered two-dimensional (2D) and three-dimensional (3D) assembled structures of nanoparticles. They have potential uses in advanced technologies such as photonics and plasmonics. The different techniques used for the formation of inorganic nanomaterials with higher-order architecture are collectively termed as nanotectonics. These include the shape-directed assembly¹ and programmed assembly² of nanoparticles comprising surface attached molecules, ligands and recognition sites, as well as formation of complex hybrid nanostructures by in situ transformation of nanoparticle-based precursors.³ The template-directed assemblies include nanomaterials, which are spatially confined within organized interiors such as tobacco mosaic virus (TMV),⁴ carbon nanotubes,^5,6 colloidal crystals,⁷ coated polymer beads⁸ and bacterial membranes.⁹ Moreover, synthesis of hierarchical ordered inorganic framework materials using different templates are of great importance because of their potential applications in catalysis,¹⁰ separation techniques¹¹ and material chemistry. Over the last decade researchers have also employed organic compounds as templates for the generation of inorganic structures and materials.^12,13

There has been considerable current interest in the production of inorganic framework materials containing well defined networks like microporous¹⁴ and mesoporous materials.¹⁵ In addition to both organic and inorganic templates have been used in the formation of inorganic materials.^16–18 The process of transcription can be divided into several steps: at first an organic or inorganic template (consisting of either preformed or self-assembled entities) is brought into contact with an inorganic precursor (or small particles) of the actual inorganic material that will be formed. Deposition of the inorganic material on the inner or outer surface of the template will then result in the formation of an organic–inorganic or different inorganic hybrid material. The organic or inorganic template material can then be removed by heat treatment,¹⁹ microwave irradiation,²⁰ washing with organic solvents,²¹ calcinations²² and simple dissolution of the core particles,²³ which finally resulting in the formation of an inorganic material possessing a morphology directly related to the organic or inorganic template.

Matijevic et al. have shown that salts of Keggin ions with cesium,²⁴ and thorium and zirconium²⁵ cations can form uniform micron-sized colloidal particles in aqueous medium. Keggin ions form a subset of polyoxometalates with the general formula (XM₁₂O₄₀)⁽⁸⁻ⁿ⁾⁻, where ‘M’ stands for W or Mo and ‘X’ stands for heteroatoms viz. P, Si, Ge of n valency.²⁶ The Keggin ions, accompanying cations and other components such as water are arranged in a well-defined secondary three-dimensional structure, the stability of which depends on the nature of counterions, amount of water etc.²⁷ The polyoxometalates such as Keggin ions can undergo stepwise multielectron redox processes without involving into a structural change.²⁸ They may be reduced electrolytically, photochemically and with suitable reducing agents. Recently, Troupis et al. have shown that photochemically reduced polyoxometalates of the Keggin structure [(PW₁₂O₄₀)³⁻ and (SiW₁₂O₄₀)⁴⁻] when exposed to aqueous metal ions viz. Ag⁺, AuCl₄⁻, Pd²⁺ and PtCl₆²⁻ resulted in the formation of the corresponding metal nanoparticles of reasonable monodispersity.²⁹ Very recently, Sanyal et al.³⁰ showed that Keggin ion can complex with various amino acids to form interesting templates which can assemble Au nanoparticles on them.

In this report, we show that Keggin ions, more specifically phosphotungstic acid (PTA) can complex with L-arginine (Arg), an amino acid having multiple amine groups. Presence of multiple amine groups results in stronger complexation and exquisite structures which can be exploited as scaffolds to generate Ag nanoparticles. In fact UV irradiated Arg–PTA hybrid is found to be a pertinent template cum reducing agent to synthesize nanostructured Ag. Interaction of Keggin with Arginine has been thermodynamically assessed doing isothermal titration calorimetry (ITC) measurements. Though ITC is a well known technique to understand bio molecular interaction, it has been scarcely used to estimate interactions between inorganic precursors.^31–33

2. Experimental section

Reagents and materials

L-Arginine (C₆H₁₄N₄O₂, Arg), phosphotungstic acid (H₃PW₁₂O₄₀, PTA) and silver sulphate (Ag₂SO₄) were obtained from Aldrich and were used as-received.

Methods

In a typical experiment, 20 ml of aqueous solution of phosphotungstic acid was mixed in a test tube with 20 ml of aqueous solution of the amino acid L-arginine, and allowed to react for 3 hours. The concentration of the precursors was varied and the samples were marked as follows:

Sample Name	[PTA] (M)	[Arg] (M)
Sample 1	4 × 10^−5	4 × 10^−5
Sample 2	7 × 10^−4	7 × 10^−4
Sample 3	1 × 10^−3	1 × 10^−3
Sample 4	1 × 10^−2	1 × 10^−2

All the Samples were subjected to scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR) and isothermal titration calorimetric (ITC) measurements and X-ray diffraction (XRD). Samples for FTIR and SEM and XRD were prepared by drop-coating the Arg–PTA solutions onto Si (111) wafers and allowing the aqueous component to dry. SEM measurements were performed with a Zeiss EVO 18, Special Edition microscope whereas FTIR spectra were recorded on a Jasco, FTIR-6300 operated in the diffuse reflectance mode at a resolution of 4 cm⁻¹. The diffraction measurements were carried out on a Philips PW 1830 instrument operating at 40 kV and a current of 30 mA with Cu Ka radiation. ITC measurements were taken at 303 K in an OMEGA ITC, of Microcal, Northampton (MA, USA) instrument. The calorimeter consists of two cells: a sample titration cell filled with 1.325 ml of desired aqueous phosphotungstic acid (PTA) solution and a reference cell filled with 1.8 ml of the same PTA solution. Concentrated aqueous amino acid solution was added from the syringe into the titration cell containing the PTA solution in small steps of 8–10 μl per injection and the time between successive injections being 3.5 min. The heat evolved/absorbed during reaction of the amino acid with PTA anions was measured. In order to correct for calorimetric response arising from dilution of the amino acids in water, control experiments were performed using the same amino acid solution in the binding experiment. The associate heats in these dilution experiments were then subtracted from the heats related to the amino acid–PTA binding experiments, and the results were analyzed for the thermodynamic information. The pH of PTA solution was 3.5, two more pH values of 7.0 and 11.5 (adjusted by adding NaOH solution in the PTA and Arg solutions) were used in the study.

The Arg–PTA hybrid was utilized as the template for the formation of Ag nanoparticles after assessing the binding strength. 20 ml of aqueous solution of PTA taken in a test tube was mixed along with 20 ml of aqueous solution of Arg taken in another test tube and was kept for 1 hour (step ‘A and B’ in Scheme 1). To 30 ml deaerated solution of step ‘B’, 2 ml of isopropyl alcohol (IPA) was added, and the mixture was irradiated by UV light for 3 h using Hanovia medium pressure lamp with pyrex filter, generating wavelength >280 nm, at 450 W (step ‘B’ in the Scheme 1). IPA here acted as the chain initiator in the process of generation of the reduced form (PW₁₂O₄₀)⁴⁻. The color of the solution turned blue due the presence of reduced Keggin ions in Arg–PTA complex (step ‘C’ in Scheme 1).^34,35

Scheme 1 Schematic representation of different steps involved in the synthesis of Ag nanoparticles on Arg–PTA hybrid.

To 5 ml of 10⁻⁴ M Ag₂SO₄ solution, 5 ml of this irradiated Arg–PTA solution was added (step ‘C’ in Scheme 1) under continuous stirring for 10 minutes and then allowed to age for 2 h. The color of the solution changed from blue to greenish yellow indicating formation of Ag nanoparticles (step ‘D’ in Scheme 1). Samples for various characterization techniques like SEM, FTIR, TEM (transmission electron microscopy) and XRD were prepared by drop-coating films on Si (111) wafers, copper grids and glass slides from the Arg–PTA solutions at different stages, viz. before UV-irradiation, after UV-irradiation and after formation of Ag nanoparticles on the Arg–PTA template.

3. Results and discussion

The chosen amino acid i.e. L-arginine (isoelectric pH = 11.2) has multiple amino groups in its structure (Fig. 1A); the measured pH of the PTA aqueous solution was 3.5, full protonation of Arg was expected at this pH. Under such a condition it should naturally interact with PTA to form Arg–PTA complex.

Fig. 1 (A) Structure of arginine. Scanning electron micrographs of Arg–PTA complex for Sample 1 (B), Sample 2 (C), Sample 3 (D), Sample 4 (E).

It was noticed that the morphology of the complex was dependent on the concentration of the precursors at pH 3.5 and the exquisite morphologies could be best discerned from SEM images (Fig. 1B–E). The complex looked like pseudo spherical in the lowermost concentration. At some parts of the grid, elongated structures were observed to grow from those pseudo spherical particles as shown by arrows (Fig. 1B). Arg–PTA complex were found to be as well defined spheres in Sample 2, the average size of which could be estimated as ∼500 nm with few smaller spheres here and there (Fig. 1C). Spherical morphology changed to fine and long fibers when precursors' concentration was kept at 10⁻³ M i.e. for Sample 3, though some spherical structures were present within the mesh of the fibers, (Fig. 1D). The appearance changed drastically on increasing the concentration to 10⁻² M, Sample 4. The flowerlike structures obtained at higher concentration were quite massive in size along with few minor and seemed to be composed of layers of star shaped flat plates (Fig. 1E).

The interaction between Arg and PTA was attempted to understand from FTIR data analysis. The FTIR spectra of the pristine PTA and Arg are given in ESI S-1.† The peak positions for pure PTA were given in Table 1 as obtained during our measurement along with the literature values in the bracket.^36–38 Only few systematic studies on interaction between Keggin and amino acids had been reported till date.³⁶ Our systematic FTIR analysis showed that such interaction led to a change in frequencies of the metal-oxygen stretching bands (Fig. 2). Structure of [PW₁₂O₄₀]³⁻ has one tetrahedron PO₄ surrounded by four W₃O₁₃ units which are linked together through W–O_b–W oxygen atoms.³⁷ PO₄ unit was found to vibrate quite independently as reported earlier,^36,37 but any kind of interaction with PTA would lead to change in the band position or might result in split band structures for P–W, W–O_d, W–O_b–W and W–O_c–W.^36,37 The FTIR spectra for the samples with higher concentration showed gradual shift in most of the bands with splitting for a few. A blue shift in the position of the bands particularly W–O_c–W and W–O_d definitely indicated stronger bonding which might lead to hierarchical complex structures as concentration changed from Sample 1 to Sample 4.

Table 1 Comparison of the band position of Sample 1, 2, 3 and 4 with pure PTA.^a The spectra of pure PTA is given in ESI S-1.

Bands	H3PW12O40	Sample 1	Sample 2	Sample 3	Sample 4
a The values are in wave number (cm^−1).
P–W	1080 (1080)	1097	1076	1104 and 1077 (split)	1104 and 1077 (split)
W–Od	982 (926–995)	Not observed	980 and 945 (split)	982 and 945 (split)	977
W–Ob–W	891 (890–900)	871	878	880	882
W–Oc–W	783 (805–810)	801	780	771	Not observed

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Fig. 2 FTIR spectra for Sample 1 (Curve 1), Sample 2 (Curve 2), Sample 3 (Curve 3) and Sample 4 (Curve 4) respectively.

Amino acid Arg showed its characteristic bands at 1562 cm⁻¹ for C=O stretching, 1685 cm⁻¹ due to out of plane bending of –NH₂, and –NH stretching at 3297 cm⁻¹.³⁹ These bands were also shifted due to the interaction between Arg and PTA. The results obtained were tabulated in Tables 1 and 2.

Table 2 Comparison of the band position of Sample 1, 2, 3 and 4 with pristine Arg.^a The spectrum of pure L-Arg is given in ESI S-1.

Bands	L-Arg	Sample 1	Sample 2	Sample 3	Sample 4
a The units are same as in Table 1.
C=O stretching	1562	1539	1542	1543	1546
–NH stretching	3297	3198	3198	3205	3208
–CH2-stretching	2925	2932	2925	2927	2925

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The interesting alteration in morphology due to change in concentration as observed in SEM images in Fig. 1 and the indication obtained from FTIR regarding stronger binding for the Samples having relatively higher concentration of the precursors, prompted to understand the thermodynamics of binding between arginine and PTA. Detailed ITC experiments were done at different [PTA] and pH. [PTA] of 1.0, 0.85, 0.7, 0.5, 0.06 and 0.04 mM and pH of 3.5, 7.0 and 11.5 were used. The nature of isotherms at the above conditions was markedly different. All ITC isotherms presented were the resultant isotherms obtained after subtracting the heat of dilution of L-Arg in water under identical conditions.

The mode of interaction between Arg–PTA at concentration 1.0 mM (Sample 3) and 0.7 mM (Sample 2) at pH 3.5 and temperature 303 K were presented in Fig. 3. The isotherm at [PTA] = 0.85 mM was very close to that of 0.7 mM and hence not shown. The raw data in Fig. 3A with [PTA] = 1.0 mM depicted fairly strong exothermic interaction that varied after 8 molar ratio ([PTA]/[Arg]) non-systematically tending towards stabilization at molar ratio ≥14. The isotherms in Fig. 3B showed four-step enthalpy change profile in each (marked only on the isotherm of 0.7 mM PTA as ‘a’, ‘b’, ‘c’, and ‘d’). At 0.7 mM PTA, the course shifted to the left relative to that of 1.0 mM PTA. Such was the trend of the isotherm of 0.5 mM PTA relative to that of 0.7 mM PTA. Meaningful deciphering of these complex curves was beyond our understanding. On the isotherms we have only marked different stages of heat change in each profile. There were four stages in each isotherm i.e. a, b, c, and d. For the illustrated isotherms the values of the four stages were close: their average values were a = 3.7, b = 12.0, c = −3.2, and d = 5.3 kcal mol⁻¹.

Fig. 3 (A) Raw data of ITC recorded during injection of L-Arg in 1.0 mM PTA at pH 3.5 and temperature 303 K. (B) Binding isotherm at [PTA] = 1.0, 0.7, 0.5 mM obtained from integration of the raw data (as shown in A) plotted against molar ratio.

Since the interaction profile by ITC showed very complex nature for Sample 2 and Sample 3, the further higher concentration (Sample 4) was not included in this experiment.

The unusual nature of isotherm prompted to carry out experiments by lowering the concentrations and pH. At a much lower concentration i.e. [PTA] = 0.1 mM at pH 3.5, the interaction profile (isotherm) was quite different; a nearly sigmoidal profile was observed which instead of stabilizing tended to increase (ESI S-2†). This result also could not be analyzed by the Origin software. The ITC isotherm at 0.7 mM [PTA] and pH = 7.0 (ESI S-3†), was smooth but not useful to derive thermodynamic information as discussed above. The ITC isotherms with reduced [PTA] of 0.06 mM (Fig. 4A and B) and 0.04 mM (Fig. 4C) at pH 3.5 (Sample 1) and 7.0 (Fig. 4D) were sigmoidal in nature and were amenable to analysis using the software program to evaluate the stoichiometry of the interaction (n), the association constant (K) and the enthalpy (ΔH), free energy (ΔG) and entropy (ΔS). The data tallied with a single set of equal site rationale. The results are presented in Table 3.

Fig. 4 (A) Raw data of ITC recorded during injection of L-Arg in 0.06 mM PTA at pH 3.5 and temperature 303 K. (B) Binding isotherm obtained from integrated data of (A) plotted against molar ratio. (C) Binding isotherm of L-Arg in 0.04 mM PTA experimented in the same condition as above i.e. at pH 3.5 and temperature 303 K. (D) Binding isotherm of L-Arg in 0.04 mM PTA at pH 7.0 and temperature 303 K plotted in the same fashion as above.

Table 3 ITC derived thermodynamics parameters of Arg–PTA interaction at 303 K

[PTA] mM	pH	n	10^−5 K/M^−1	−ΔG/kcal mol^−1	−10^−4ΔH/kcal mol^−1	−ΔS/kcal mol^−1 K^−1
0.06	3.5	5.3	1.94	7.3	1.38	45.5
0.04	3.5	6.3	0.31	6.3	1.20	39.6
0.04	7.0	4.1	0.41	6.4	1.05	34.6

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The n values (bound L-Arg per PTA molecule) were found to be moderately pH dependent and modestly concentration dependent. K values fairly depended on concentration but weakly on pH. ΔH also fairly depended on pH and weakly on concentration; similar was the behavior of ΔS. Exothermicity produced overall ordering in the system. Interestingly, the ΔG values were like normal reactions, whereas both ΔH and ΔS were very large. Large value of ΔH was also reported earlier on Keggin–lysine interaction.³⁰ Large exothermic enthalpy change was associated with large negative entropy change. Such a behavior was not commonly observed. An excellent compensation between ΔH and ΔS had been observed (ESI S-4†) with a compensation temperature of 302.8 K found from the slope of the linear plot whereas experimental temperature was 303 K. Difference in concentration and pH hardly affected the energetics of the process. On further increasing the pH to 11.5 for Sample 1 the isotherm was again different with a wavy nature and fluctuating points (ESI S-5†). Here L-Arg was close to its isoelectric pH = 11.2, the interaction profile of the almost neutral substrate was thus much different. The interaction was totally destroyed at alkaline pH which could be speculated since the amine groups of arginine would not be available for protonation, and would not interact with Keggin anions.

The combined study of FTIR and ITC furnished a clear picture of the interaction. Most importantly ITC gave a clear idea of pH dependent interaction between Arg–PTA just like many other proteins.⁴⁰ Higher pH decreased the interaction and near the isoelectric pH of Arg, there was basically no interaction between the two. A systematic SEM (Fig. 5) study was carried out selecting the samples with the same concentration and pH which were considered for ITC. Sample 1 (Fig. 5A–C) and Sample 2 (Fig. 5D and E) showed the similar trend of deformation on increasing the pH from 3.5 to pH 7. In both cases hardly any structures was found on the substrate at higher pH. Sample 3 was also found to be deformed considerably, Fig. 5G did not show any of the elongated fibers on increasing the pH.

Fig. 5 SEM images at varied pH for Sample 1 (A–C), Sample 2 (D and E) and Sample 3 (F and G). The concentration and pH were selected in accordance with ITC results.

The organic-inorganic hybrid structures were quite stable in solution to be utilized as the template for the generation of nanostructured assembly of Ag at the normal pH of the solution i.e. pH ∼3.5. It was reported that UV irradiated PTA could reduce Au³⁺ and Ag⁺ to their respective nanoparticles.^29,41–43 The same principle was herein utilized to prepare Ag nanoparticle using UV irradiated Arg–PTA complex. The formation of Ag nanoparticles was ensured from UV-Vis spectra.⁴⁴

The Arg–PTA complex (step B, Scheme 1) formed a colorless solution and had no absorption in the visible region, Curve 1, Fig. 6. UV irradiation for 3 h changed this colorless solution to a deep blue one (step C, Scheme 1) which showed a broad absorption peak at 760 nm, characteristic of one-electron reduced PTA, [PW₁₂O₄₀]⁴⁻. The absorption was due to the forbidden d–d transition and also due to the Intervalent Charge Transfer (IVCT) band that gives the solution a deep blue color, Curve 2, Fig. 6. Addition of Ag⁺ solution to this deep blue colored solution generated Ag nanoparticles (step D, Scheme 1), characterized by a strong SPR (surface plasma resonance) absorption peak at 420 nm, Curve 3, Fig. 6.

Fig. 6 UV-Vis spectra for Arg–PTA complex before (Curve 1) and after (Curve 2) UV irradiation. Curve 3 is the same after the formation of Ag nanoparticles.

The XRD pattern recorded from a drop-coated film of Arg–PTA complex before UV irradiation was shown in Fig. 7, Curve 1 whereas Curve 2 was the same after the formation of Ag nanoparticles by UV irradiated Arg–PTA complex. Bragg reflections corresponding to the PTA were mostly intact in Curve 2 indicating that the formation of silver nanoparticles on the Arg–PTA colloidal template did not disrupt its structure. The (111), (200) and (220) Bragg reflections of fcc silver could be clearly observed (which were not present in Curve 1) along with some of the peaks which were present in Curve 1. The Ag peaks were relatively broad indicating that the Ag particles were quite small in size.^38,45 UV-Vis spectra and XRD recorded from all the samples were avoided due to repetitive nature. The spectra presented here were from Sample 3.

Fig. 7 X-ray diffractograms of Arg–PTA hybrid (Curve 1) and after the formation of Ag nanoparticles on the template (Curve 2). The planes mentioned in Curve 2 correspond to fcc Ag.

The formation of Ag nanoparticles on Arg–PTA hybrid template was best observed in TEM micrographs, Fig. 8. Sample 1 showed the formation of small Ag nanoparticles in clusters on the pseudo spherical structures, Fig. 8A. Sample 2 which formed spherical Arg–PTA, produced Ag nanoparticles mainly at the central part of the spheres. Sample 3 showed the Ag nanoparticles all over the fibrous template whereas Sample 4 maintained the flowerlike morphology after the formation of Ag nanoparticles on it. The overall size of the template matched well with the SEM images in Fig. 1, even after the synthesis of Ag nanoparticles on them. The crystalline nature of the Ag particles was confirmed by the selected area diffraction (SAED) from the part of the samples where the (111) and (200) planes were identified corresponding to fcc Ag. The energy dispersive analysis of X-ray (EDAX) obtained from the same region as shown in the images of Fig. 8, also showed the presence of Ag, Fig. 9. The SAED and EDAX together confirmed the formation of Ag selectively on the template and the crystalline nature of the metal. The generation of nanoparticles only on the Arg–PTA hybrids validated the success of using the hybrid as template.

Fig. 8 TEM images after the synthesis of Ag nanoparticles on Arg–PTA template for Sample 1 (A), Sample 2 (B), Sample 3 (C) and Sample 4 (D). Insets show the diffraction patterns of Ag nanoparticles taken from these regions.

Fig. 9 Spot EDAX analysis of Sample 1 (A), Sample 2 (B) and Sample 3 (C), obtained from the regions which are shown in the previous figure.

The strong interaction between Arg and Keggin (PTA) led to the formation of different morphologies for the Arg–PTA hybrid (Fig. 1) which acted as a suitable scaffold for the site selective generation of Ag nanoparticles (Fig. 8). The interaction weakened considerably as predicted from ITC and SEM results when the pH of the solution changed from 3.5 to 11.5: it was expected due to the absence of protonated amine groups of Arg. At high pH, Arg–PTA did not form a complex of well defined morphology and a trial to use it as scaffold led to the formation of discrete Ag nanoparticles as observed from TEM of Sample 2 (Fig. 10A) and Sample 3 (Fig. 10B). The diffraction pattern in the inset of Fig. 10B showed formation of fcc Ag. This conclusively proved our speculation that stronger interaction occurred at lower pH; increase in concentration keeping pH at 3.5 accentuated the Arg–PTA interaction leading to the formation of flowerlike morphology for the highest concentration used in this study.

Fig. 10 TEM image of Ag nanoparticles formed on the Arg–PTA hybrids when the initial pH was adjusted to 11.5 for Sample 2 (A) and Sample 3 (B). Diffraction pattern of Ag nanoparticle (inset, B).

4. Conclusion

It has been shown that L-arginine (Arg) could strongly interact with the Keggin (PTA) producing hybrid structures, the morphology of which was found to be highly dependent on concentration and pH. Only moderate PTA concentration could yield a defined interaction model to derive thermodynamic information of the process. The extent of interaction was understood by a detailed FTIR and ITC study. It was also shown that highly organized assemblies of silver nanoparticles could be formed by using crystalline Arg–PTA colloidal particles as template. The Ag nanoparticles synthesized on the template was highly site selective and well crystalline as observed from microscopic studies and XRD. The salient feature of this approach was that the Keggin ions played a multifunctional role – they provided the framework to support the generation of Arg–PTA complex which in turn acted as a UV-switchable matrix to participate in the reduction of the metal ions without disruption of the well-defined Arg–PTA structure. The possibility of using the Arg–PTA framework as a UV-switchable reducing scaffold has novelty and could definitely be extended to the formation of other highly organized hybrid structures with possible applications in catalysis and as potential optical materials.

Acknowledgements

DS and TB acknowledge the financial support (project no. Conv/162/Nano Pr 2011) from Centre for Research in Nanoscience and Nanotechnology, University of Calcutta. The same is also acknowledged for SEM, TEM and FTIR instrumental facilities. BN thanks UGC, Govt. of India for Senior Research Fellowship. SPM thankfully acknowledges the research support from Indian National Science Academy and Jadavpur University. We thank A. Pan of CSS, JU for assistance in the calorimetric measurements.

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Footnote

† Electronic supplementary information (ESI) available: FTIR of pure PTA and arginine and ITC results of more experiments. See DOI: 10.1039/c3ra43378a

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