J. Chem. Sci. (2020) 132:107 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12039-020-01804-2Sadhana(0123456789().,-volV)FT3](0123456789().,-volV) REGULAR ARTICLE

Cationic dye by phosphomolybdate immobilised on polyelectrolyte matrix

K SHAKEELAb, SRUTHI GURUa and G RANGA RAOa,* aDepartment of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India bDepartment of Chemistry, B.S. Abdur Rahman Crescent Institute of Science and Technology, Vandalur, Chennai 600048, India E-mail: [email protected]

MS received 31 January 2020; revised 9 April 2020; accepted 19 April 2020

Abstract. A polyelectrolyte complex (PEC) made up of poly(di-allyl di-methylammonium chloride) and poly(styrene sulphonate) polyelectrolytes has been synthesised and used for the immobilisation of phos- 3 phomolybdate (PMo),½ PMo12O40 nanoclusters. The material is characterised by FTIR, powder XRD, SEM, TEM and UV-Visible DRS spectroscopy. It was found that the PMo clusters are intact in the hybrid and are present as nanoaggregates within the layers of PEC. The amount of PMo present in the hybrid is estimated from thermal measurements and the optical band gap energy of the material is found to be in the semi- conductor region. The presence of negatively charged PMo clusters helped in almost complete adsorption of cationic dyes, methylene blue and rhodamine B. The PMo/PEC hybrid has shown to follow Langmuir adsorption isotherm in case of both the dye molecules.

Keywords. Polyelectrolyte; phosphomolybdate nanoclusters; dye adsorption; Langmuir adsorption.

1. Introduction sulphonate) (PSS) is one of the well-studied sys- tem.9,10 Although this PEC dried at 60 °C is very hard, containing polyelectrolyte complexes brittle and difficult to process,11 it can absorb 1-butyl- (PECs) have always occupied an important place in 3-methylimidazolium chloride [BmimCl] ionic liquid 1–11 materials chemistry. When two oppositely charged leading to glassy to rubbery PEC transition.12 polymers called polyelectrolytes or polysalts are Polyoxometalates (POMs) are a class of oxide mixed stoichiometrically, PECs are formed. Such an nanoclusters with high negative charges.13 They can ionically bonded polymeric network of PECs offers interact with various cations to produce hybrid improved physicochemical properties compared to salts.14–17 POMs can also interact with substrates like 1 individual polyelectrolytes. Generally, PECs are zeolites,18 conducting polymers,19,20 graphene21–23 formed by simultaneous mixing of cationic and anio- and carbon nanotubes,24,25 where the surface area of nic polyelectrolytes which can result in a tangled PEC the active groups is enhanced. POM-supported zeolites network or alternate layer-by-layer assembly of poly- are used as heterogeneous catalysts for dehydration of 2 chains. Biocompatible chitosan-based ethanol, esterification and other organic transforma- PECs are found to be good carriers of drug tions.18,26,27 In electrochemistry, it is important to 3,4 molecules. For instance, the PEC formed by layer- develop materials where the electrochemically active by-layer assembly of levofloxacin (anti-bacterial drug) species are electrically wired through conductive incorporated chitosan and TiO2 incorporated substrates. For instance, POMs interact with conduc- gellan gum composite can serve as a potential material tive polymers like polyaniline (PANI), poly(3,4- 5 for wound dressing. Other applications of PECs ethylenedioxythiophene) (PEDOT), carbon nanotubes 6 7 include anticorrosion, electrochromism and oil and grapheme to form hybrid materials. These mate- 8 repellency. The PEC made of linear cationic poly- rials exhibit sensing, energy conversion and storage electrolyte, poly(di-allyl-di-methylammonium) chlo- properties.28–34 Other materials like POM-based ionic ride (PDDA) and anionic polyelectrolyte, poly(styrene liquids were found to be efficient in removal of heavy

*For correspondence 107 Page 2 of 10 J. Chem. Sci. (2020) 132:107 metal from water35 and POM-based showed interaction between the negatively charged surface of cytotoxic activity on cancer cells.36 PMo and positively charged functional groups of RhB POMs immobilized on polymer matrices are insol- and MB can contribute to proficient removal of the uble in aqueous solutions besides being highly nega- dye.45 tively charged. Thus, they can be the potential materials for removing hazardous metal ions and dyes, especially from industrial effluents. The process of adsorption of 2. Experimental dyes on solid supports is advantageous over other methods including precipitation and coagulation.37 For 2.1 Materials and characterisation techniques instance, heteropoly blue, a reduced form of POM, intercalated in layered double hydroxide clay is a good The polyelectrolytes (PDDA and PSS) of molar masses * -1 * -1 adsorbent material for methylene blue dye.38 Similarly, 100 000 g mol and 70 000 g mol , respectively, were purchased from Sigma-Aldrich. Phosphomolybdic POM-based materials,ðÞ 4 aminopyridine ½Mo O 4 8 26 and sodium chloride were bought from Fisher scien- and Cs4H2PMo11FeO40 6H2O show selective adsorp- tific, Mumbai. Rhodamine B and Methylene blue dyes were tion of methylene blue over rhodamine B and methyl purchased from Loba Chemie and Central Drug House, orange dyes.39,40 Recently, POMs encapsulated in respectively. cationic surfactant dimethyldioctadecylammonium A JASCO FT-IR-4100 spectrophotometer was employed bromide are loaded in polyvinylidene fluoride matrix to to record FT-IR spectra of materials. The X-ray diffraction prepare composite membrane which is very efficient in pattern of the materials was recorded with a Bruker AXS ° removing Reactive Black 5 and Safranine T dye mole- D8 Advance diffractometer, at a scan rate of 0.05 per second, using Cu Ka (k = 1.5414 A˚ ) radiation generated at cules.41 In another study, the Keggin-based POM 0 40 kV and 30 mA. Thermogravimetric analyses (TGA) of encapsulated in N-decyl-N -carboxymethylimida- the samples were performed on a TA make TGA (Perkin- zolium bromide surface active ionic liquid responds Elmer) Q500 instrument under nitrogen flow at a linear sensitively to the change in pH and is found to be a heating rate of 10 °C per minute, from room temperature to renewable catalyst for methyl orange degradation in 900 °C. The morphology of the compounds synthesized was aqueous medium.42 Chandhru et al., demonstrated that studied by using an FEI Quanta 200 Scanning electron POM-based ionic liquids are very effective in removing microscope (SEM). The samples were spread on a con- more than 92% of dye molecules and the dye removal ducting carbon tape and mounted on the sample holder for rate strongly depends on the cationic alkyl chain length SEM analysis. The high resolution transmission electron microscopy (HRTEM) studies were performed on a JEOL in the ionic liquid.43 3010 HRTE microscope operated at 300 kV. The phase In the present work, we have prepared polyelec- transitions of the materials were tested on a TA make DSC trolyte complex by using poly(di-allyl-di-methylam- (Perkin-Elmer) Q200. The adsorption studies of dyes by the monium) chloride and sodium poly(styrenesulphonate). materials were analysed by UV–Visible spectra recorded on 3 Phosphomolybdate (PMo),½ PMo12O40 nanoclusters a JASCO V-660 spectrometer using Millipore water as a are immobilised on the polyelectrolyte complex to reference solution. Milli-Q millipore water was used to prepare PMo/PEC composite. The adsorption beha- prepare the aqueous solutions. viour of the composite is studied for cationic dyes, methylene blue (MB) and rhodamine B (RhB). The adsorption process studied is a chemical interaction of 2.2 Synthesis of polyelectrolyte complex (PEC) negatively charged POMs on PEC matrix with cationic and phosphomolybdate immobilized polyelectrolyte dyes. It is confirmed by Langmuir adsorption beha- complex (PMo/PEC) viour. The adsorption process has been carried out at pH 5.5 for RhB and 3 for MB. The PMo on PEC is The PEC was synthesized from their linear polyelectrolyte 11 stable at these pH values. There are specific interac- precursors, PDDA and PSS, as reported in the literature. tions which govern the adsorption of the dye molecules Typically, equal volumes of 0.125 M in units of on the surface of a sorbent during the adsorption pro- PDDA and PSS aqueous solutions were prepared and their ionic strengths were adjusted to 0.25 M by NaCl. Both the cess.44 The hydrogen bonds amongst the p electron solutions were poured simultaneously into another beaker, cloud of benzene ring, the lone pair of electrons on under constant stirring. An immediate off-white coloured nitrogen and oxygen atoms of RhB and nitrogen and PEC was precipitated out, which was collected by filtration sulphur atoms of MB molecules and various functional and rinsed with copious NaCl solution. The product active sites on the surface of PMo/PEC play significant obtained soon after preparation is soft and rubbery role during the adsorption. Also, the electrostatic (Scheme 1). This fresh PEC was then chopped into small J. Chem. Sci. (2020) 132:107 Page 3 of 10 107

Scheme 1. Schematic representation of PMo/PEC formation from PEC and PMo. pieces and a part of it was dried overnight in an oven at 60 °C. The PEC obtained after drying becomes very hard. The remaining part of PEC which was not kept for drying was then soaked in 50 mM PMoA solution for 2–3 days. The deep yellow coloured PMoA solution turned pale and the PEC attained yellow colour of PMoA solution, indi- cating the formation of PMo/PEC, as shown in Scheme 1. The hybrid complex (PMo/PEC) was collected and washed with water thrice or till the pH of washings became neutral and finally dried. Both the dried PEC and PMo/PEC materials were finely ground and used for further study.

3. Results and Discussion

3.1 Material characterisation

The polyelectrolyte complex formed by stoichiometric Figure 1. FTIR spectra of (a) polyelectrolyte complex mixing of PDDA and PSS becomes a hard solid after (PEC); (b) PMo/PEC and (c) PMoA. drying overnight at 60 °C. Immobilization of PMo nanoclusters on PEC weakens the interactions amongst the polyelectrolyte chains and hence shows plasti- (Figure 1b). This shows that the PMoA clusters are cization effect. The FTIR spectra of the materials are interacting with PEC through their bridging oxygen shown in Figure 1. The peaks corresponding to the atoms. Although the materials are well dried, due to functional groups present in PEC (Figure 1a) are also their hydrophilic nature, a strong peak at 3450 cm-1 is -1 seenÀÁ in PMo/PEC (Figure 1b) at 1183 cm due to present in the IR spectra corresponding to the m -1 as SO3 of PSS group, 1468 cm due to aromatic stretching vibrations of water molecules. -1 benzene ring marðÞC ¼ C of PSS group, 1635 cm The materials are further characterised by powder -1 due to mbðÞOH of water molecules and 2920 cm XRD, as shown in Figure 2a. The XRD pattern of PEC due to mðÞCH2 of PDDA and PSS groups present in showed two broad peaks, indicating low degree of PEC.46 The signature peaks of PMoA in the range of crystallinity, at a scattering angle of 18.4° 700–1100 cm-1 (Figure 1c) can also be noticed in the (d = 0.47 nm) and 23° (d = 0.38 nm), as shown in spectrum of PMo/PEC (Figure 1b).47 The peaks at Figure 2a(i). The powder XRD spectra of pure phos- -1 -1 963 cm due to mðMo ¼ OtÞ and at 1063 cm due to phomolybdic acid are shown in Figure 2a(iii). The mðÞP ¼ O correspond well with those of PMo/PEC. powder XRD of PMoA (JCPDS No. 43-0316) shows However, the peaks at 787 and 870 cm-1 due to sharp peaks at 6.5° (111), 10.7° (220), 15.2° (400), ° ° ° ° mðÞMo-Oe Mo and mðÞMo-Oc -Mo stretches in 16.6 (331), 18.6 (422), 19.8 (511), 22.6 (531), PMoA are slightly blue shifted in PMo/PEC 26.5° (444). The powder XRD pattern of PMo/PEC 107 Page 4 of 10 J. Chem. Sci. (2020) 132:107

water which is lost up to 150 °C. The PEC shows single step decomposition in the range of 360–460 °C (Figure 2b(i)). The little residue left is possibly the NaCl present as the counter ions in the uninteracted positions of the polyelectrolyte chains.11 The PMo/ PEC also shows single major decomposition step starting at 360 °C which indicates the decomposition of PEC. The decomposition of PMo in the hybrid occurs beyond 700 °C. From TGA curves, 30% of PMo is estimated to be present in the PMo/PEC composite. Thermal stability of PEC in the hybrid is not affected and PMo moieties are also stable at higher temperatures due to their entanglement in the poly- electrolyte matrix. The PEC is a very hard material after drying and its microscopic morphology shows rough surface, as seen from the SEM images in Figure 3a(i). However, the material undergoes plasticization when the PMo clusters are introduced into it. The SEM image in Figure 3a(ii) shows that material becomes softer and its surface is smoother. Further, the materials are also analysed by TEM images, shown in Figure 3b. The high-resolution TEM images of PEC (Figure 3b(i)), shows two types of lattice spacing of 0.47 nm and 0.38 nm, which is in agreement with d-spacing cal- culated in powder XRD, from Figure 2a(i). The TEM images of PMo/PEC, shown in Figure 3b(ii), confirms that the PMo clusters enter the PEC matrix as nanoaggregates. The PMo clusters are pointed with red Figure 2. (a) Powder XRD; (b) TGA of (i) polyelectrolyte arrows in Figure 3b(ii). The EDX spectrum in complex (PEC), (ii) PMo/PEC, (iii) PMoA. Figure 3c shows the presence of the elements C, N, O, S, P, Mo in PMo/PEC hybrid material. ° showed amorphous nature, with peaks at 6.5 and The UV-Visible diffuse reflectance spectra of PMo/ ° 18.6 due to PMo nanoclusters, and broad peaks at PEC and their precursors are shown in Figure 4. The ° ° 18.4 and 23 due to the PEC matrix (Figure 2a (ii) PMo/PEC hybrid (Figure 4b) shows a hypsochromic and inset). The intensities of PMo peaks in PMo/PEC blue shift when compared to pure PMoA (Figure 4c). are largely reduced, indicating that PMo is immobi- This can be attributed to the reduction in particle size lized onto the PEC matrix. The broadness of the peaks due to uniform distribution of PMo clusters in PEC in PMo/PEC indicates that the PMo clusters are dis- matrix. The oxygen 2p and molybdenum d orbitals tributed in the PEC matrix as small agglomerates and 29 form the highest occupied molecular orbital (HOMO) form nanocrystalline POM phase within the matrix. and the lowest unoccupied molecular orbital (LUMO), These aggregates of PMo nanoclusters are stabilized in respectively. The HOMO-LUMO gap is determined the PEC matrix by strong interactions between bridged : by the Tauc plot, which is ðahmÞ0 5 versus energy (eV), oxygen atoms of PMo and ammonium cations of a ¼ ðÞ1R 2 PDDA polyelectrolyte in PEC, as concluded from where 2R ;(R is the reflectance of an infinitely FTIR analysis. thick layer at a given wavelength), as shown in the The TGA measurements reveal the thermal stability inset of Figure 4.49,50 The optical band gap of PMo/ of these materials, as shown in Figure 2b. For pure PEC was found to be 2.55 eV which is slightly higher PMoA, most of the water of crystallization is released than pure PMoA (2.28 eV) and falls in the semicon- below 100 °C, and decomposition into P2O5, MoO3 ductor range. The electronic interactions between PEC 48 and H2O takes place beyond 700 °C. The PEC and and PMo cluster can affect the overall electronic PMo/PEC samples are dried well, but due to their structure of the Keggin clusters attached to the poly- strong hydrophillicity, they have some amount of electrolyte complex. This causes variations in the J. Chem. Sci. (2020) 132:107 Page 5 of 10 107

Figure 3. (a) SEM; (b) TEM images of (i) PEC, (ii) PMo/PEC and (c) EDX of PMo/PEC.

electronic spectra from which band gap values are estimated.51–55 The PMo/PEC nanohybrid materials are stable in and PMo units do not leach out from the polymeric membrane.

3.2 Dye adsorption studies

The PMo/PEC material is suitable for the adsorption studies on chosen cationic dyes, methylene blue and rhodamine B, due to the presence of negatively charged phosphomolybdate clusters in the PEC matrix. It can be argued that PSS being negatively charged can also interact with cationic dye molecules. However, all negatively charged species cannot adsorb or remove the cationic dye molecules. When PSS and PDDA Figure 4. UV-DRS spectra of (a) polyelectrolyte complex interact by electrostatic forces to form PEC, there are (PEC), (b) PMo/PEC and (c) PMoA. Inset shows the Tauc plots of the materials with calculated optical band gap some decoupled regions present in both PSS and values. PDDA where interaction between the two moieties is 107 Page 6 of 10 J. Chem. Sci. (2020) 132:107

Figure 5. UV-Visible spectra of 15 ppm of MB dye using 10 mg of (a) PEC and (b) PMo/PEC; and 15 ppm of RhB dye using 10 mg of (c) PEC and (d) PMo/PEC. Insets in (b and d) show the respective dye adsorption capacity of the material at different intervals of time. not established. At the decoupled regions of PDDA, of MB adsorption in 1.5 h (Figure 5b) and 99% of the PMo anions interact with PDDA cations. Similarly, RhB adsorption in 12 h (Figure 5d). The insets in in the decoupled regions of PSS, PMo anions with Figure 5b,d show the maximum time taken to reach either stabilize with Na? or H? ions. Now, when the equilibrium, which is much faster for MB com- cationic dye molecules are present in solution, PSS, pared to RhB. The amount of dye adsorbed at equi- although being negatively charged, cannot interact librium is calculated using equation (1).56 with them efficiently. This is because PMo anions have a discrete molecular structure with negative ðC C ÞV q ¼ 0 e ð1Þ charge delocalized over them, which helps them e m interact with their counter cations strongly. Hence, PEC cannot adsorb cationic dye molecules, whereas where qe is the quantity of dye adsorbed over PMo/ -1 PMo/PEC does so in an efficient manner. The mate- PEC (mmol g ), C0 and Ce are the initial and equi- rials are used as such without any activation since they librium concentrations of dye in aqueous solution - have negligible porosity as per BET studies. The BET (mmol L 1), V is the volume of the dye solution (L), surface areas are 0.4 m2 g-1 for PEC and 0.6 m2 g-1 and m is the mass of the PMo/PEC adsorbent (g). for PMo/PEC. All the adsorption studies have been Figure 6 shows the effect of PMo/PEC dose ranging carried out using 10 ml of 15 ppm dye solutions at from 0 to 10 mg in 10 ml of 15 ppm MB and RhB ambient conditions and the changes in dye concen- solutions. trations are monitored by using a UV-Visible spec- Further, the surface properties and the affinity of the trophotometer. The maximum adsorption capacity of dye molecules towards PMo/PEC adsorbent are also PEC is just 53% for MB (Figure 5a) and 57% for RhB investigated from the parameters of adsorption iso- (Figure 5c) per 10 mg of material. The PMo/PEC is therms. Langmuir and Freundlich isotherm models are more efficient in dye adsorption where it shows 97% most commonly used and the parameters are deduced J. Chem. Sci. (2020) 132:107 Page 7 of 10 107

Figure 6. UV-Visible spectra showing the effect of PMo/PEC adsorbent dose on 15 ppm of (a) MB and (b) RhB.

 from their linear expressions. The linear form of 1 log q ¼ log K þ log C ð3Þ Langmuir adsorption isotherm is shown in the fol- e f n e lowing equation (2)57 and the plot is shown in Figure 7: where K ðmmol1ng1LnÞ and n are the Freundlich  f C 1 1 constants and they are related to the adsorption e ¼ þ C ð2Þ capacity of the material. q bq q e e 0 0 The relative coefficient (R2) values obtained from where Ce and qe are equilibrium concentration of dyes Figure 7, suggest that the adsorption of both the dye (mmol L-1) in solution and the amount of dye molecules on PMo/PEC follow Langmuir model, adsorbed on PMo/PEC (mmol g-1), respectively, where adsorption is monolayer and the surface of the 41,56 whereas q0 and b are the Langmuir constants. A plot material is homogeneous. The material shows -1 between Ce and Ce/qe gives the adsorption capacity maximum adsorption capacity of 16 mg g for MB -1 (q0) and affinity of binding sites (b). Freundlich in 1.5 h and 10 mg g for RhB in 10 h at room adsorption isotherm model explains multilayer temperature. The adsorption parameters are tabulated adsorption on a heterogeneous surface and its linear in Table 1. expression is shown below in equation (3):58 Further, the reversible nature of the adsorption process can be determined by the dimensionless parameter (RL) obtained from Langmuir isotherm and is expressed as shown in equation (4):56,59  1 RL ¼ ð4Þ 1 þ bC0

where b is a constant and C0 is the highest initial dye -1 concentration (mmol L ). The experimental RL val- ues obtained for MB and RhB dye adsorptions are 0.01 and 0.02, respectively. The RL values are between 0 and 1 (0 \ RL \ 1), which essentially imply that this

Table 1. Adsorption parameters obtained from the iso- therms for the adsorption of methylene blue and rhodamine B dyes by PMo/PEC hybrid material. Parameters MB RhB

-1 Langmuir adsorption model q0 (mg g )1610 b (L mg-1)7 4 Figure 7. Plots showing linear fitting of Langmuir R2 0.99 0.96 adsorption isotherm: (i) MB and (ii) RhB. 107 Page 8 of 10 J. Chem. Sci. (2020) 132:107 type of adsorption isotherm is favourable in both the complex materials consisting of antibacterial and cell- cases.60 supporting layers Macromol. Biosci. 12 374 6. Andreeva D V, Skorb E V and Shchukin D G 2010 Layer-by-layer polyelectrolyte/inhibitor nanostructures 4. Conclusions for metal corrosion protection ACS Appl. Mater. Interfaces 2 1954 7. Cui M, Ng W S, Wang, Darmawan P and Lee P S 2015 Polyelectrolyte complex of poly(di-allyl di-methy- Enhanced electrochromism with rapid growth layer- lammonium chloride) and poly(styrene sulphonate) bylayer assembly of polyelectrolyte complexes Adv. has been used to immobilize the Keggin-type phos- Funct. Mater. 25 401 phomolybdate nanoclusters forming PMo/PEC hybrid 8. Liu X, Leng C, Yu L, He K, Brown L J, Chen Z, Cho J material. 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