Spectral and electrochemical studies of methylene blue and thionine ...

J Porous Mater (2008) 15:343–349 DOI 10.1007/s10934-006-9087-x

Spectral and electrochemical studies of methylene blue and thionine encapsulated in zeolite-Y S. Easwaramoorthi Æ P. Natarajan

Received: 26 July 2006 / Revised: 11 October 2006 / Published online: 29 March 2007 Springer Science+Business Media, LLC 2007

Abstract Investigation on the spectral and electrochemical properties of methylene blue and thionine encapsulated in zeolite-Y is carried out. Both the dyes exist as monomer and H-aggregates in zeolites and the ratios for the aggregates of the dyes are different at the zeolite surface even though the dyes have similar basic structure. The electrochemical behaviour of thionine and methylene blue at zeolite modified electrode shows that both the dyes experience different environment at zeolite modified electrode and the presence of methyl groups in methylene blue plays a vital role in stabilizing the electroactive species at the electrode surface. The concentration of the supporting electrolyte is found to influence the nature of the redox process. Keywords Zeolite modified electrodes Methylene blue Thionine Zeolite-Y

1 Introduction Chemically modified electrodes offer improved selectivity and sensitivity over conventional electrodes [1, 2]. Several methods and materials are known for the modification of the electrode surfaces. Among the many solid materials such as silica [3], clays [4] and S. Easwaramoorthi P. Natarajan (&) National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai 600113, India e-mail: [email protected] S. Easwaramoorthi Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600025, India

zeolites [5–7] studied for the modification of the electrode surfaces, zeolites are proven to be efficient due to the high porosity, chemical stability and one dimensional to three-dimensional structure of the zeolites. The zeolite modified electrodes find applications in sensors, batteries and electrocatalysis [8]. The electron transfer process of the electroactive species encapsulated inside the zeolite cavities at the zeolite modified electrode [9] is an important phenomena and understanding of this is vital for improving the efficiencies of these devices for different applications. Three types of mechanisms are proposed for the electrodic process: extrazeolitic, intrazeolitic and surface mediated electron transfer reactions. The electrochemical behaviour of modified zeolite electrodes using metal complexes, counter ions and organic molecules is known to be influenced by various parameters such as charge balancing cations, supporting electrolyte and immersion time of the electrode into the solution [10, 11] among others. In particular, incorporation of luminescent organic dye molecules into the micro and nanoporous molecular sieves leads to new kind of advanced materials for application in microlasers, asymmetric catalysts, artificial antenna systems and nano-scale advanced materials [12] and chemically modified electrodes. Methylene blue and thionine dyes belong to thiazine family and are widely used as photosensitisers [13] and redox mediators in electrocatalytic systems. Incorporation of methylene blue and thionine in various zeolitic host materials [14–16] have been studied in detail. It has been reported that methylene blue encapsulated mordenite finds potential application in optical humidity sensor [17] and data storage devices [18]. Furthermore methylene blue loaded mordenite [15] and zeolite-b [19]

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modified electrode act as amperometric sensors for ascorbic acid. Gorton and co-workers have studied the electrochemical oxidation of NADH by methylene blue and thionine adsorbed on graphite and zirconium phosphate modified carbon paste electrodes [20]. These studies show that the dye molecule has been located between the layers of the zirconium phosphate and are insensitive to the pH of the electrolyte solution, a highly desired property for sensor applications. However methylene blue adsorbed on the silica and modified silica surfaces [21] is highly sensitive to pH as in homogeneous solution. In this paper, we report the spectral and electrochemical studies carried out with thionine, 1, and methylene blue, 2, encapsulated into Na+-zeolite-Y. The stability and the influence of electrolyte concentration on the redox properties of the encapsulated dyes are discussed in detail.

2 Experimental Methylene blue and thionine are purified by column chromatography over neutral alumina using ethanol as an eluent. Zeolite-Y (Suid Chemie India) was stirred with 1 M sodium chloride solution for 2 h to remove the extra framework impurities like iron and was then washed with copious amount of triply distilled water until the filtrate shows negative test for chloride ions with silver nitrate solution. The sodium ion exchanged zeolite-Y sample was calcined at 530C in a muffle furnace for 12 h to remove organic templates added during synthesis and it was then stored in an airtight container. Methylene blue and thionine loaded zeolites were prepared by ion exchange method. Aqueous solution of the dye was stirred with zeolite for about 12 h and the resulting solid was washed with water until the filtrate shows no absorbance at 660 nm and 600 nm respectively for methylene blue and thionine. The concentration of the dyes in zeolite-Y was maintained at 2.5 lM per gram of zeolite. The dye loaded zeolites kept as such after the incorporation of dye are termed as hydrated and remaining part of the sample heated at 125C in vacuum are indicated as dehydrated samples. All the samples were stored in a vacuum dessicator. Titanium dioxide loaded zeolite samples were prepared using the reported procedure [22]. Briefly the particular zeolite host material was stirred with potas-

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sium titanooxalate in aqueous solution for 24 h, which resulted in the formation of TiO2+-zeolite. Heating at 150C for 2–3 h results in the formation of titanium dioxide encapsulated zeolite samples. The samples were repeatedly washed with water to remove any titanium dioxide adsorbed on to the external surface of the zeolite powder. The samples prepared thus had no titanium dioxide on the external surface of the powder as evidenced by the EDAX measurements. Different loading of titanium dioxide was achieved by successive ion exchange followed by heat treatment. Electrochemical experiments were carried out with triple distilled water at 25C under nitrogen atmosphere. The home made Teflon sheathed graphite electrode was used as a working electrode, 1 cm2 platinum foil is used as the counter electrode and the saturated calomel electrode (SCE) was used as the reference electrode. Zeolite modified electrodes were prepared by the following procedure: 10 mg of zeolite/ dye–zeolite samples were mixed with 3 mg of graphite powder and was suspended in 0.4 ml of tetrahydro furan containing 1.4 mg of polystyrene. The aliquot was sonicated for about 1 min and 9 ll of the aliquot was applied to the freshly polished graphite electrode surface and was then allowed to dry for about 15 min using this procedure. A uniform coating of zeolite particles on the surface of the graphite electrode was obtained. The composition of methylene blue and thionine loaded zeolite modified electrode is 0.2 mg of dye-loaded zeolite containing 0.75 mg of graphite and 0.03 mg of polystyrene on the graphite electrode surface. UV-visible diffuse reflectance spectral studies were carried out using Agilent 8453 diode array spectrophotometer equipped with labsphere RSA-HP-8453 reflectance accessory. Cyclic voltammetric measurements were performed with CH 620B electrochemical analyser.

3 Results and discussion 3.1 Diffuse reflectance spectral studies The spectra of methylene blue (MB) and thionine (TH) loaded zeolites (MBY and THY respectively) are shown in Fig. 1. Methylene blue in the monomeric form of the dye absorbs at 657 nm and monomeric thionine shows absorption at 598 nm. Both the dye molecules exist as monomeric and dimeric forms in zeolite as indicated by the distinct absorption maxima. The spectral characteristics of the monomeric and dimeric forms in homogeneous solution are well studied [23– 25]. However the ratios of the monomeric and higher

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Fig. 1 Diffuse reflectance spectra of (i) MBY [hydrated form (a), dehydrated form (b), dehydrated form with encapsulated titanium dioxide nanoparticles (c)], (ii) THY hydrated form (a), dehydrated form (b) hydrated form with encapsulated titanium dioxide nanoparticles (c) dehydrated form with encapsulated titanium dioxide nanoparticles (d). Inset - deconvuluted peaks

aggregates of the dyes in the zeolite matrix is found to be different for thionine and methylene blue. The dimeric or H-aggregated form predominates over the monomeric species for the thionine while monomeric form predominates over dimeric or H-aggregated form for methylene blue. This is further supported by the observation that the absorption maxima of H-aggregated form of THY are blue shifted with the appearance of broad peak indicating the involvement of more than two monomeric units in forming the H-aggregated forms in zeolite matrix. The broad absorbance peak has been deconvoluted into three Gaussian peaks (Microcal Origin version 5) with maxima at 500, 530 and 565 nm. The peak at 565 nm is known to be due to dimeric thionine and the rest of them are presumably

corresponding to higher aggregates. Thionine also exist as H-aggregates formed from two or more monomers when encapsulated in AlMCM-48 [26]. The width of the dimeric peak is decreased when another guest molecule is included in the cavity of the zeolite as shown in Fig. 1b; in this case titanium dioxide nanoparticles are encapsulated in the cavities of the zeolites along with the dye molecules. The absorption peak of H-aggregates becomes sharp with the increase in the concentration of titanium dioxide nanoparticles in zeolite. The molecular size of MB and TH are smaller as compared to the pore opening of the zeolite and are therefore encapsulated in the cavities of the zeolite-Y. The locus of MB in zeolite-Y is at the middle of the supercages as revealed from the powder X-ray crystallography [27] and recently, occupational sites of methylene blue [28] and thionine [29] encapsulated in mordenite have been studied through single crystal X-ray study. Molecular modelling studies of thionine in zeolite suggest that the dye is present both at the middle of the cage and in between the two supercages [25] as well. Due to the larger molecular size of the dimeric or higher aggregated forms, thionine and methylene blue aggregates are not entrapped into the cavities and possibly adsorbed on the external surfaces of the zeolite as well which is also supported by the molecular modelling studies [25]. The dimer and higher aggregates are stabilised by the presence of water molecules in the host material. Removal or decreasing the water content by means of incorporating guest molecules such as titanium dioxide nanoparticles results in the decrease in the concentration of aggregates as compared to the monomeric species. Dehydration of the THY samples leads to the exclusive formation of monomeric thionine. Exposing the samples to the atmosphere results in the formation of dimers, which is clearly seen by the sample turning from pink to blue colour. Dehydration of MBY results in the red shift in the absorption maximum with the formation of new shoulder at 770 nm corresponding to the protonated methylene blue. The concentration of protonated form of MB is higher in presence of encapsulated titanium dioxide nanoparticles, as higher concentration of protons has been released to the framework during the synthesis of TiO2 encapsulated zeolite samples by ion exchange method [22]. However in the case of THY dehydration at elevated temperature (125C) in vacuum under the experimental condition does not result in the formation of protonated thionine and only deaggregation occurs during dehydration. The differences in the spectral behaviour of structurally similar MB and TH may originate from the different environments of dimeric and monomeric

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forms of the dyes in zeolite matrix. Heating the THY samples at 125C in vacuum leads to the dissociation of aggregated form while for MBY protonation occurs predominantly since major proportion of methylene blue dye exists in monomeric form. 3.2 Electrochemical studies The cyclic voltammograms of methylene blue (MBYME) and thionine (THYME) encapsulated modified zeolite electrodes with well-defined reduction and oxidation peaks are shown in Fig. 2. The E1/2 values observed for MB (–265 ± 5 mV) and for TH (–240 ± 5 mV) Vs SCE in homogeneous solution are shifted by ~70 mV towards negative potential in 20

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Fig. 2 Multiple scan cyclic voltammogram of (i) MBYME at 25 mV s–1 and (ii) THYME at 100 mV s–1 for continuous 100 cycles (some of them are omitted for clarity) in 0.1 M KCl solution; inset- cycle numbers

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modified zeolite electrodes (ZME). The observed negative shift in E1/2 value of methylene blue and thionine at ZME is due to the interaction of the positively charged dye molecules with the negatively charged framework of the zeolite, which prevents the facile reduction of the dye. The cyclic voltammogram for the dyes in ZME indicates that the oxidation potential shows a higher shift (~100 mV) as compared to the reduction potential (~35 mV) towards the negative direction; encapsulation of the dye on the zeolite surface results in the facile oxidation of the dye molecule. The shift in the oxidation and reduction potentials is attributed to the preferential interaction between the redox species and the surface of the zeolite host. The negative shift in the E1/2 value is observed when the oxidised form of the redox species interacts more strongly with the electrode surface. In this case the shift towards negative potential in the redox cycle indicates that the oxidised form interacts strongly with the host surface as compared to the reduced form [20]. Stability of the zeolite-modified electrode was examined by carrying out the continuous cyclic voltammetric experiments using the modified zeolite electrodes as shown in Fig. 2. The peak current decreases during the continuous cycle experiments indicating that the concentration of dye molecules at the electrode surface decreases after each continuous cycle. The decrease in the concentration of TH from the electrode surface shows a higher value as compared to MB and the redox peak of TH appears as shoulder after ~50 continuous cycles. Even though both the dyes are structurally similar, the difference in the stability of the dye at the electrode surface is indicative of the influence of N-methyl groups present in MB. Observations similar to these were made by several authors using nafion [30] and zirconium phosphate [20] modified electrodes with the same dye systems. The size of MB and TH enables the dyes to be encapsulated in the cages and are subjected to move rather freely within and out of the zeolite cavity. In the case of MB bulkier N-methyl groups somewhat sterically hinder the free movement of the dye within and out of the zeolite pores as compared to TH leading to significantly higher stability of MB at the modified electrode surface. Redox peaks of MB/ZME observed in the cyclic voltammograms appear broad, as the dye in the zeolite host exists in different chemical environments. According to the model proposed by Rolison and co-workers [31], the encapsulated guest molecules exist in at least four possible occupational sites as depicted in the Scheme 1 and are termed as topological isomers. These topological isomers manifest different type of interaction with the host matrix, which results in the

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Scheme 1 Pictorial representation of the various topological isomers of dye in zeolite matrix

broadening of the redox peaks in cyclic voltammograms. It may be noted that all the topological isomers may not equally contribute to the observed redox peak; some of them may be inaccessible during the course of experiment. Identification of the topological isomers, which undergo redox reactions at zeolite modified electrodes helps to elucidate the electron transfer mechanism. The possible isomers are, (1) electroactive species dissolved in the electrolyte solution and has no association with zeolite (2) outer isomer, those which are adsorbed on the external surfaces of the zeolite (3) those encapsulated in the supercages in the first layer of the zeolite crystals and (4) those present in the interior cages of the zeolite crystals. The more negatively shifted redox potential observed for both MB and TH at ZME as compared to that in homogeneous solution excludes the contribution of dissolved species to the redox peak. As both MB and TH are encapsulated in the cages and are susceptible to move within and out of the zeolite in presence of electrolyte solution it would be very difficult to precisely pinpoint the involvement of other topological isomers in the redox process. However from the observed broad redox peaks it is inferred that the dye molecules present in different chemical environments contribute to the shift in the observed redox peak. It is worth noting that continuous cyclic voltammetric measurements show a decrease in the peak current. The observed shift in the redox potential of MB towards more negative region indicated in Fig. 2 shows the appearance of a sharp peak. As discussed earlier, MB present at the external surface or in the broken supercages are more vulnerable to ion exchange with the cations present in the supporting electrolyte. This ion exchange process weakens the interaction between

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MB and zeolite surface leading to the observation of relatively more positive redox potential at initial cycles of the cyclic voltammograms. The more negative redox potential observed at later cycles is due to the MB adsorbed strongly at the zeolite surface. However the changes observed for thionine were insignificant even though the dye exists in different environments. The observations indicated for thionine are attributed to the more facile movement of the dye within and out of the zeolite matrix as compared to methylene blue. In the case of MB, N-methyl groups restrict the free movement of the dye inside the cavity, which leads to different redox potential with respect to the different chemical environment exerted by the host matrix. The anodic and cathodic peak separation (DEp) for both MBYME and THYME is observed to be 110 ± 10 mV when the concentration of the supporting electrolyte is 0.1 M. When the concentration of the supporting electrolyte is increased to 0.5 M, DEp changes to 150 ± 10 mV. In general the DEp value is known to decrease with increasing concentration of the supporting electrolyte as the movement of ions facilitate charge transport. The difference in the observed behaviour probably arises from the enhanced ion exchange between the dye and the electrolyte cation at higher concentration of the electrolyte solution, which eventually weakens the host-guest interaction and results in higher DEp value similar to that observed in homogeneous solution. However the DEp value decreases to 90 ± 10 mV for MB and 60 ± 10 mV for TH in the continuous cyclic voltammetric measurement in subsequent cycles as shown in Fig. 3 and the original DEp value could be regained when the voltammetric measurement is repeated after a few minutes interval between subsequent set of cycles. This electrodic property is also due to the ion exchange reaction between the electroactive dye molecules with the cations present in the electrolyte. Reports dealing with the redox properties of methylene blue adsorbed on various zeolite host materials [17, 19, 32] do not show this type of behaviour. The difference in the observed electrodic behaviour in the present studies may be due to the fact that most of the earlier electrochemical studies with this dye were performed in buffered solution. Further we have also noticed two electron two proton redox process for MB & TH/ZME at pH = 5.5 phosphate buffer solution where the redox potential remains unaltered throughout the study. From these results it is concluded that the two electron two proton redox process of MB and TH occurs presumably at the interface between the zeolite and the solution. The Ipa or Ipc for both MB and TH do not show linearity with either scan or square root of scan

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Dehydration of the dye loaded zeolite results in the formation of protonated dye in the case of methylene blue whereas for thionine deaggregation of H-aggregates occurs on heating. The stability of methylene blue at the electrode surface is higher than thionine due to the restricted movement of methylene blue within and out of the cavities of zeolite. The concentration of the supporting electrolyte affects the redox property of the dyes as it influences the ion exchange between the electrolyte cation and electroactive species.

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Acknowledgements The investigations reported here are supported by the SERC and IRHPA programmes of Department of Science and Technology, India. Fellowship received by S.E. from Council of Scientific and Industrial Research, India is gratefully acknowledged.

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rate corresponding to surface adsorbed and diffusion controlled redox processes respectively in the range of scan rate 25–1000 mV s–1. The dependence of Ip with m was linear in the logarithmic plot i.e., logIpc vs. logm and from the slope n values were calculated [33]. The n values for MB (0.85 ± 0.05, R2 = 0.999) and TH (0.72 ± 0.04, R2 = 0.99) show a mixed behaviour of surface controlled and diffusion controlled redox processes. This observation is explained to be due to the fact that both the dyes are subjected to free movement within and out of the zeolite and ion exchange reaction with the cations of the electrolyte leads to the both surface and diffusion controlled redox processes.

4 Conclusion Methylene blue and thionine dyes encapsulated in zeolite-Y exist as monomers and H-aggregated forms.

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