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Summer Research Fellowship Programme of India's Science Academies

Designed Synthesis of an Azomethine based Covalent Organic Polymer for Selective Detection of As3+

Oshal Dsouza

Student, Dept. of Postgraduate Studies Research in Chemistry, St. Aloysius College (Autonomous), St. Aloysius College Road, Light House Hill, P.B. No. 720, Mangaluru-575 003, Karnataka, India.

Guided by:

Dr. Priyabrata Banerjee

Senior Scientist, Surface Engineering and Tribology, CSIR- Central Mechanical Engineering Research Institute, Mahatma Gandhi Avenue, Durgapur-713 209, West Bengal, India.

Abstract

Environmental pollution by several anthropogenic activities has become a threat to public health and is also considered as a serious global issue. Heavy metals like arsenic, lead, mercury and cadmium are commonly associated with poisoning of living organism. Hence, the detection of these heavy metals is of high necessity in the realm of scientific research. Arsenic is one of the toxic metals which an individual can be exposed to through routine activities. Being overexposed to this metal may even cause life threatening problems. Therefore, to address this issue we have successfully constructed an azomethine based COP (ThPA) at ambient temperature and have utilized it towards sensing of analytes. keeping in mind the advantages of Schiff base condensation and covalent organic polymers (COPs). COPs have a large number of active sites and it is easy to modify the structural features of this polymer. This Schiff base condensation reaction is cost effective and most reliable method of synthesis. Organic struts of COPs are connected to each other by strong covalent bonds. Its crystalline and porous nature provides enough thermal as well as chemical stability that offers a splendid platform for hosting the metal ion detection. The conjugation within the molecule may act as a signal transducer which may act as an amplifier of fluorescence signal. Furthermore, colorimetric detection is the simplest and a straightforward way for the detection of analytes. Herein, the newly synthesized COP (ThPA) can detect Arsenic from aqueous solution colorimetrically, showing a visible colour change from pale yellow to brick red. Therefore, further characterizations have to be performed in order to get a clear idea about the structural features of the polymer and insightfulness of the sensing phenomenon.

Keywords: COPs, Heavy Metals, Colorimetric and Fluorimetric Detection.

Abbreviations

 AAS Atomic Absorption Spectroscopy
 ICP-MSInductively Coupled Plasma- Mass Spectroscopy 
 AFS Atomic Fluorescence Spectroscopy
 PLPhotoluminescence 
 ACNAcetonitrile 
 BET Brunauer Emmett Teller
 BJHBarrett-Joyner-Halenda 
 POP Porous Organic Polymer

INTRODUCTION

Background

Heavy metals among all environmental pollutants, have gained massive importance due to their nature and extent of toxicity. These natural components are mainly present in asthenosphere in trace amounts and unsafe when comes in contact with the mankind. Certain metals like lead, arsenic, cadmium, selenium, mercury, zinc, chromium and nickel are poisonous even at very low concentration.1 Arsenic is present in both organic as well as in inorganic form, out of which the inorganic arsenic is said to be more toxic compared to the other.2,3 It is termed as the 26th abundant element in earth’s crust.4 Inorganic arsenic exits in ground water in the form of arsenate or arsenite, binding to oxygen atoms. This semi-metalloid element is naturally present in various parts of world. 200 million people all over the world are exposed to unsafe level of arsenic contaminated water. Highly affected region in India includes Jharkhand, Bihar, Assam, Uttar Pradesh, Manipur, West Bengal which contains more than 50µg/L,5 whereas the given limit of 10µg/L by Environmental Protection Agency (EPA) and World Health Organization (WHO).6,7 Out of 20 districts in West Bengal 9 districts are tremendously affected by ground water arsenic contamination and about 26 million people are at risk8 due to prolonged exposure through consumption of crops which are irrigated using contaminated water, smoking, breathing of industrial toxins, intake of food cooked using filthy water etc.6 which can malfunction human cells by displacing elements present within cell. Arsenicosis affects the lungs, kidney, liver and skin hence leading unto various complications including deadly diseases such as cancer, diabetes, dysfunction of sense organs etc.9 Therefore detection of this toxic arsenic becomes mandatory and a primary challenging task for chemists.

IMAGE.png
Advantages of Schiff base COFs
    IMAGE2.png
    Features and applications of porous materials
      Various applications of COPs.

      Covalent organic polymers (COPs) on the other hand are most prominent approach of organic synthesis because they are extremely hydrophobic10 and robust in nature11 and their aptness of recoupment once the task is complete. COPs are covalently bonded to each other through building-blocks of organic materials11 which give tunable porosity,13 topology,14 large surface area15-17 and crystallinity17 to the polymeric structure. They contain light elements18 like N, O, C, B enabling it to contain low density. π –conjugation is extended within the COP which inherits microporosity to the structure.19 We make use of Schiff- base organic molecules which are fundamental and highly potential struts for scheming a COP and also known for the purpose of sensing analytes. They typically involve one step and high yield reaction which makes it more preferable. There are considered to be highly versatile and promising way of approach rather than complex strategies and exclusive techniques which could make the procedure tedious. These polymers are facile, economically affordable and most importantly they give a promising instant outcome20. Various analytical methods like AAS, ICP-MS, AFS can detect arsenic but they are laborious and exorbitant hence they lag in their application.21 COPs have diverse applications in various fields such as heterogeneous catalysis,22-24 sensors,25 gas adsorption and storage,26-28 optoelectronic devices29 etc. due to their large number of active sites which could selectively act as signal detectors. Colorimetric chemosensors gain attention due to swift visible response and simple analysis.30

      Aim & Objective

      · Design and development of some decisive and cost effective covalent organic polymers (COPs) with the aid of some simplest synthetic procedures.

      · Utilization of those polymers for detection of Heavy Metals particularly in aqueous media.

      · Exploitation of several sophisticated analytical instrumentations to get insightfulness of the sensing mechanism.

      LITERATURE REVIEW

      In this vibrant field of current research the development of chemosensors to selectively detect heavy metals has been an innovative as well as a trending approach.31,32 Substantial efforts have been put to detect these heavy metals.15,33 The journey of crystalline POPs using one step reactions to generate covalently bonded cross linked polymers34 paved a way for promoting further design and development of such porous crystalline materials. Therefore in order to detect these heavy metals, we have taken advantage of COPs which are highly stable in aqueous media so that selective detection becomes more competent. COPs gain interest due to the distinctive features of Schiff base chemistry as mentioned by Jose´ L. Segura.14 Literature reveals that introduction of an electron rich site such as N, O and S in the precursor molecule augmented the capability of the material to act as an effectual chemosensor towards As3+.7 Therefore, before commencement, the starting material which possesses active sites were preferred so that they can easily condense to produce a long polymeric chain that contain signal receptors that can easily interact with the analyte species. Comparing synthetic strategies previously executed, room temperature synthesis was found to be more superficial. Thus implementing the former methodologies, an initiative to develop COP in a simplistic and cost effective way was put into effect.

      We have designed an azomethine based COP (ThPA) as a novel and an innovative approach towards schiff base condensation reaction and meanwhile to selectively detect Arsenic in aqueous media. This COP (ThPA) was synthesized at room temperature which is the most proficient, straightforward and an apt route adapted instead of using solvothermal technique35 which uses circumstances that are callous and solvent assisted. This newly developed arsenic sensing COP (ThPA) is cost effective and facile, making it more reliable for the exploitation which could aid the civic who are prey to arsenic contaminated water source. This facile construction was performed using monomers 1 and 2 which yielded a long polymeric chain. Characterizations using sophisticated instruments like ultraviolet visible spectroscopy (UV-Vis), photoluminescence (PL), Fourier Transform infrared spectroscopy (FT-IR), were performed to show the interaction of host and the guest. The chemoreceptor ThPA showed colorimetric sensing upon addition of arsenic in its solution. PL revealed that ThPA exhibited intense emission indicating the drastic quench in the fluorescence in the presence of arsenic ions, signifying the sensing ability of the developed chemosensor specimen when in contact with the analyte.

      OSHAL.png
        Synthetic scheme of ThPA using 1 and 2 at ambient condition.

        METHODOLOGY

        Synthesis of azomethine based COP (ThPA):

        p-phenylenediamine (PhDA) (1mmol) was taken in 15ml methanol in a 100ml round bottom flask and dissolved well till the mixture becomes homogeneous. Then 2,5-thiophenedicarboxaldehyde (ThAld) (1mmol) is dissolved in 10ml methanol and added dropwise to the round bottom flask upon continuous stirring. Instantly, the colour changed from colourless to orange on the addition of ThAld and gradually the material starting precipitating out. After 6 hours of stirring, the material was completely formed. It was then filtered, washed well for 3 times with methanol and dried completely to obtain orange coloured compound.

        CHARACTERISATION RESULTS

        Fourier-Transform Infrared spectroscopy (FT-IR):

        FT-IR Spectra was obtained by Spectrum 65, Perkin Elmer FT-IR Spectrometer. Figure 1 shows the FT-IR spectrum of PhDA, ThAld and ThPA. The two peaks of PhDA at 3214 cm-1 and 3302 cm-1 were assigned to N-H stretching band and the sharp peak of ThAld at 1635 cm-1 was ascribed to the C=O Stretching vibration. The ThPA spectra showed an absorption peak at 1635 cm-1 which was attributed to C=N and peaks at 3214 cm-1 and 3302cm-1 were diminished revealing the absence of N-H bond, thus indicating successful condensation of the amine group of PhDA and formyl group of ThAld into newly formed imine bond.

        IR.png
          Comparative FT-IR spectrum of PhDA, ThAld and COP(ThPA).

          UV-Spectrocopy:

           Solutions of PhDA and ThAld of concentration 1×10-4 M in ACN and dispersed solution of 0.1 mg ThPA in 10ml ACN was prepared and was taken in a quartz cuvette individually. Figure 1 displays UV-Vis absorption spectra of ThPA and monomers PhDA and ThAld. The PhDA shows absorption at 250 nm and 325 nm. ThAld shows the absorption at 292 nm whereas, ThPA displays a strong UV absorption band at 425 nm which indicates a bathochromic shift in peak position from the peak values of the precursors viz. PhDA and ThAld signifying absence of monomers. The occurrence of this red shift is a clear indication of the extended π-conjugation existence within the ThPA polymer.

          UV.png
            UV-Vis spectra of PhDA, ThAld and ThPA

            UV Titration:

            A dispersion of ThPA (0.1 mg in 10 ml ACN) and solution of As3+ (2×10-4M in H2O) were used to perform UV-Vis titration. The absorption maxima of ThPA was observed at 425 nm and upon gradual addition of 10µL of As3+each time into ACN dispersion of ThPA, peaks at 292 nm and 363 nm has been arrived. When comparing with the UV data obtained, the wavelength of ThAld matches with the wavelength 292 nm after the addition of analyte, which clearly infers that the afresh polymer breaks down giving back to monomer ThAld and peak at 325 nm of PhDA has been red shifted to 363 nm which implies that the addition of As3+ may causes an interaction of As3+ with the nitrogen atom of PhDA.

            UV T.png
              UV-Vis Titration of ThPA upon As3+ addition.

              Fluorimetric Titration:

               The sensitivity of ThPA towards As3+ detection was analyzed by fluorescence titration. A dispersion of ThPA in ACN and As3+ in H2O (2×10-4M) was prepared. Upon excitation at 530 nm the ThPA exhibited an intense and stable emission peak at 572 nm. Gradual addition of 10µL of As3+ into the dispersion , resulted in lowering of the peak intensity which implies that the fluorescence intensity of ThPA was quenched in the presence of As3+ ions under UV lamp. A comparative study with other cations along with targeted analyte was been performed which showed no quenching of fluorescence of ThPA, signifying that the ThPA selectively senses only As3+ ions in the aqueous medium.

              PL image.jpg
                Fluorescence titration of ThPA with As3+

                Colorimetric Response of ThPA:

                The sensing study has been performed using ACN dispersion of ThPA. As3+ and other cations were used in same concentration. ThPA showed a distinct colour change from yellow to brick red in the presence of the As3+ in aqueous medium whereas color of ThPA remained unchanged in the presence of other cations like Zn2+, Hg2+, Co3+, Cu2+, Cd2+, Ni2+, Pb2+, Fe3+ and Al3+.

                SENSING.png
                  Colorimetric response of ThPA in different cations (2×10-4M) in H2O at room temperature, 1. ThPA dispersion , 2. Hg2+, 3. Zn2+, 4. As3+, 5. Co2+, 6. Cu2+, 7. Cd2+, 8. Ni2+, 9. Pb2+, 10. Fe3+ and 11. Al3+.

                  Surface Area and Pore Size:

                  The Nitrogen adsorption and desorption analysis was carried out using Quantachrome iQ2 with ThPA at 77K to get informtaion about the surface area and porosity of the material. Prior to experiment degassing of ThPA was performed at 120º for 3 days, for elimination of any trapped solvent appropriately, present in the pores of the polymer. The surface area obtained using BET method is 64.056m2/g. The pore size distribution data was obtained from BJH method. The pore radius of the polymer is 307.0Å and pore volume was found to be 0.903cc/g. ThPA is a polymer that contains mesopores as well as macropores. These various pore sizes allow all the other cations passing through this COP and selectively binds the As3+ due to its functionality.

                  Graph1_1.JPG
                  Nitrogen adsorption/desorption isotherm at 77K of ThPA
                    Graph2.JPG
                    Plot of BET surface area of ThPA.
                      Surface area measurement of ThPA

                      Other Reactions:

                      Similar kind of synthesis was done with 1,5-diaminonaphthalene (1mmol) in 100ml round bottom flask dissolved in 15ml methanol in and 2,5-thiophenedicarboxaldehyde (1mmol) was dissolved in 15ml methanol and then added dropwise to it with stirring. 0.1% of acetic acid (v/v) was added to the solution to trigger the reation. Colour changed from pinkish violet to dark orange. After 5 hours of continous stirring, as obtined precipitate was filtered and residue obtained was washed well with methanol for 6 times and dried.

                      OSHAL D.png
                        Similar synthesis like ThPA

                        CONCLUSION

                        In conclusion, an azomethine based COP, ThPA has been successfully designed and synthesized at ambient condition for selective detection of As3+ ions in aqueous medium. ThPA has showed a visible colorimetric change from yellow to brick red upon addition of arsenic. Various characterizations by the aid of sophisticated instruments like UV-Vis, FT-IR and PL was performed which gives the detailed spectroscopic data of ThPA. UV-Vis and FT-IR reveals an insight of effectual condensation and formation of C=N bond. Moreover, ThPA has an exclusive capability of breaking down into its constituent monomers in the presence of arsenic in the aqueous media and it remained undisturbed when sensing study was performed with other cations, which makes it exceptional. Fluorimetric study shows the fluorescence quenching of ThPA on arsenic addition. A comparative study of ThPA using UV-Vis and PL with As3+ and other cations was investigated and was found that there was no noticeable absorbance or fluorescence changing of various cations except arsenic. Nevertheless it possesses all characteristics of an ideal chemosensor and makes it accessible for potential practical appliances. The present porous ThPA gave us an insight to synthesize and investigate many more COPs which can act as superior chemosensors towards different cations and find ways for potent removal of heavy metal ions fruitfully.

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                        ACKNOWLEDGEMENTS

                        My heartfelt thanks to Indian Academy of Sciences for providing me with such a wonderful opportunity to enhance my research skills.

                        My immense gratitude to CSIR-Central Mechanical Engineering Research Institute, Durgapur for giving me an opportunity to work as Summer Research fellow in the wonderful campus.

                        I would like to express my sincerest gratitude to my mentor Dr Priyabrata Banerjee (Senior Scientist), Surface Engineering and Tribology group, CSIR-CMERI for his encouragement and guidance during the project. A big thanks to you, sir for being an amazing guide and a motivator throughout the journey.

                        I owe a debt of gratitude to Mr. Debanjan Dey and Mr. Subhajit Maity for their constant help in carrying out the research. Thanks a lot for guiding and being with me through thick and thin during the project. Their immense support helps me a lot to know the tricky steps of synthesis protocol. I am grateful to Ms. Suparna Paul for helping me in best possible way. I thank all my lab mates for their continuing support throughout.

                        I am very much grateful to my parents for their constant love and support. Special thanks to Dr Ronald Nazareth and Ms. Preema Pais, Department of Chemistry, St. Aloysius College, Mangaluru for being my motivators and support system.

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