Hydrazinylpyridine based highly selective optical sensor for aqueous source of carbonate ions: Electrochemical and DFT studies

Accepted Manuscript Hydrazinylpyridine based highly selective optical sensor for aqueous source of carbonate ions: Electrochemical and DFT studies

Vikram Thimaradka, Srikala Pangannaya, Makesh Mohan, Darshak R. Trivedi PII: DOI: Reference:

S1386-1425(17)31014-4 https://doi.org/10.1016/j.saa.2017.12.041 SAA 15686

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

4 October 2017 11 December 2017 13 December 2017

Please cite this article as: Vikram Thimaradka, Srikala Pangannaya, Makesh Mohan, Darshak R. Trivedi , Hydrazinylpyridine based highly selective optical sensor for aqueous source of carbonate ions: Electrochemical and DFT studies. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), https://doi.org/10.1016/j.saa.2017.12.041

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ACCEPTED MANUSCRIPT Pyridylhydrazine Hydrazinylpyridine Based Highly Selective Optical Sensor for Aqueous Source of Carbonate Ions: Electrochemical and DFT studies Vikram Thimaradka a$, Srikala Pangannaya a$, Makesh Mohanb, Darshak R. Trivedi a* a

Supramolecular Chemistry Laboratory, Department of Chemistry, National Institute of Technology Karnataka (NITK), Surathkal-575025, Karnataka, India b

Department of Physics, National Institute of Technology Karnataka (NITK), Surathkal575025, Karnataka, India *

both the authors have contributed equally

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$

Author to whom correspondence should be addressed e-mail: [email protected],

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Tel.: +91-824-2473205; fax: +91 824 2474033.

series

of

new

receptors

PDZ1-3

based

on

2-Pyridylhydrazine

2-

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A

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Abstract

(arylidenehydrazinyl)pyridines have been designed and synthesized for the detection of

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biologically and environmentally important ions. The colorimetric detection of CO32‒ using neutral organic receptor PDZ-1 has been achieved with characteristic visual colour change

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from yellow to green accompanied by a large redshift of 215 nm in absorption maxima. UV-

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Vis spectroscopic and cyclic voltammetric studies reveal the stoichiometry of binding and electrochemistry of host-guest complex formation. The binding constant was found to be

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0.77×104 M-2. In addition, electrochemical studies provide an insight into the stability of the complex. DFT studies performed on the PDZ-1 and PDZ-1-CO32‒ complex reveal the

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appropriate binding mechanism involved in the anion detection process. PDZ-1 is highly selective for carbonate and does not show any colorimetric response towards any other anions or cations, while PDZ-2 and PDZ-3 remain inactive for in the ion detection process. The limit of detection (LOD) and limit of quantification (LOQ) of PDZ-1 for carbonate was found to be 0.11 mM and 0.36 mM respectively. Considerable binding constant and limit of detection make PDZ-1 to be used as a real time sensor for the detection of carbonate in environmental and biological samples.

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ACCEPTED MANUSCRIPT Keywords: Pyridylhydrazine 2-(arylidenehydrazinyl)pyridines, Colorimetric, carbonate ion sensor, electrochemical study, DFT, LOD 1. Introduction Nature encompasses numerous chemical species which play vital role in biological and

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environmental processes. Among them, anions belong to a group of vital species having prime role in the scientific arena with their intriguing physiological applications. Subsequently,

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detection of anions have emerged as vibrant area of research. The reason for the burgeoning

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interest is related to the ubiquitous nature of anions.[1–3] Anions being ecologically important have been an essential part of the biochemical processes. For instance, phosphates make up

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genes[4] whereas, carbonates are main component of shells and skeleton in aquatics. Similarly,

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fluoride in toothpaste prevent dental caries[5–7] and acetate help in the transmission of nerve impulses. Anions beyond an optimum limit can lead to health deteriorating, hazardous effect.

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The interference of anion flux across cell membranes is increasingly recognized as being the

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primary factor of many diseases that includes cystic fibrosis, Bartter’s syndrome, Pendred’s syndrome and osteoporosis.[8] Chloride ion exists as a sea water pollutant and moreover,

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contact with anions like cyanide can lead to death within few minutes. Therefore, qualitative and quantitative determination is much desired in terms of both biological and environmental

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aspects.

In this regard, development of optical sensors through host-guest chemistry principle is increasingly appreciated due to their inherent simplicity and naked eye response. However, selective sensing of oxy anions such as AcO− (acetate), HCO3− (bicarbonate), CO32− (carbonate) are rather difficult owing to their similar basicity and high hydration energy that counters the recognition forces.[9] Although CO32− ion is equally important, only few literatures have been reported till date.[10–18] Selective optical sensing of CO32− ions is

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ACCEPTED MANUSCRIPT important due to several facts. Existence of CO32− ions is important in earth’s crust as CO32− ion is a soil-aggregating agent. CO32− ions are responsible for maintaining pH in seawater. Further, CaCO3 is used as an abrasive in toothpaste. Food additives from E501 to E505 consisting of various metal carbonates are essential for food manufacturing industries. It is evident that, CO32− ions play a key role in buffering action of blood in human physiology.

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Moreover, carbonates are important candidates for electric vehicle and hybrid electric vehicle

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power sources where vinylene carbonate are used as additive electrolytes for rechargeable Liion batteries.[19] In addition, carbonate compounds are extensively used in the manufacture of

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glass, rayon, rubber, plastics and cosmetics.[16] Despite the fact that CO32− ions are beneficial

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in many aspects, material safety data sheet states that their strong caustic effect to the gastrointestinal tract may cause severe abdominal pain, vomiting, diarrhoea, collapse and even death

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if ingested in excess. CO32− ions are common adulterant found in milk, which can cause disruption in hormone signalling that regulate development and reproduction.[20] These

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factors has intrigued researchers to develop numerous techniques such as continuous-flow

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method[21], acoustic method[22], gas chromatography[23], pH ion-sensitive field effect transistor[24,25], ion selective electrodes[26,27] and FT-IR[28] method to quantify CO32− ions

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in real time. These techniques are either expensive or cumbersome. To address the issue, there has been incipient attempts from researchers to develop

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chromogenic/ fluorogenic chemo sensors for CO32− ions. Hennrich and co-workers[10] in 2001 were first to report fluorogenic sensor for the detection of CO32− ions. In the later years, few more attempts were made to develop chromogenic and fluorogenic sensors by various groups[13,15,17,18]. However, the results suffered from either insignificant visual changes or poor selectivity amidst other anions/cations. Recently, Patra and co-workers developed chromogenic and fluorogenic sensor for CO32− ions[18]. In addition to poor visual response, their recent report[29] shows the affinity of receptor towards Pb2+ ions and this raises question 3

ACCEPTED MANUSCRIPT about the activity of sensor towards CO32− ions in the presence of Pb2+ ions. Concurrently, electro optical response of sensors have attracted special interest among scientists over the past few decades. Having an electrochemical response along with chromogenic output will be a reliable way to develop future generation highly reliable sensors.[30]

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Considering aforementioned challenges and with a view to overcome the shortcomings, we have developed first ever highly selective, aqueous compatible, 2-hydrazinopyridine 2-

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(arylidenehydrazinyl)pyridine based small molecule colorimetric chemosensor PDZ-1 for the

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detection of CO32− ions in aqueous media. PDZ-1 has single –NH functionality as binding site. The high acidity of –NH proton due to the presence of adjacent pyridine moiety is further

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enhanced by introducing –NO2 functionality para to it. The –NO2 functionality also acts as a signalling subunit and on the whole develops a binding site-signalling subunit approach

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towards the sensing of CO32− ions. PDZ-1 shows remarkable visual response from yellow to green in the presence of CO32− ions. UV-Vis spectrophotometric titrations validate the high

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selectivity of PDZ-1 towards CO32− ions amidst other test anions and cations. In the present

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study, an attempt has been made to study the stability of PDZ-1- CO32− ion ion complex using electrochemical method. Further, the results obtained corroborate the findings of UV-Vis

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spectrophotometric titrations. Density functionality theory (DFT) has been at the forefront in

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validating the experimental findings and known to be helpful to arrive at a binding mechanism. With this in view, an attempt has been made to analyse the electron density distribution on the receptor and receptor-anion complex to arrive at the energy optimized geometries. Further, to envisage the effect of substituents on chromogenic response, receptors PDZ2 was synthesized by introducing electron releasing –NMe2 (dimethylamino) and PDZ-3 without any substituent at para position to –NH proton bearing hydrazino pyridine group. In contrast to the multi-ion sensing properties of receptors reported so far [31-35], PDZ-1 could be successfully called as a selective sensor for aqueous source of carbonate ions. Consequently, 4

ACCEPTED MANUSCRIPT no chromogenic response was observed for any of the test anions /cations, demonstrating the role of –NO2 functionality in inducing the selective colorimetric response. 2. Experimental 2.1 Materials and Methods

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All chemicals and analytical grade reagents were used as bought without any further

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purification unless otherwise mentioned. Reactions sensitive to air and moisture were carried out under nitrogen atmosphere. Thin layer chromatography was performed using Merck TLC

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Silica Gel F254 plates. Melting point was measured on Stuart SMP3 melting-point apparatus in

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open capillaries. Infrared spectra were recorded on Bruker alpha FTIR spectrometer. 1H NMR was performed using Bruker-400 AV-400 spectrometer. Chemical shift values are reported in

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ppm scale (in DMSO-d6 with Tetramethylsilane as internal standard). HRMS-ESI was obtained from Agilent 6538 UHD/Q-TOF high resolution spectrometer. UV-Vis experiments were

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carried out using Thermo Scientific Genysys 10S spectrophotometer in standard 3.0 mL quartz

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cuvette having 1 cm path length. Electrochemical studies were performed using Ivium (vertex) Electrochemical workstation using 50 mV scan rate, potential window −1.5 V to +1.5 V.

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2.1.1 UV–Visible spectroscopic methods The solution of receptors as 1×10

−4

M were made up with dry DMSO solvent. UV–visible

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absorption titrations were performed by adding 1 eq. of the titrant as sodium carbonate (1×10−2 M in distilled water). The UV–visible spectra were recorded with the incremental addition of anion to the receptor solution. 2.1.2 Cyclic voltammetric studies The solution of receptor as 5×10−5 M was made up with dry DMSO solvent. Tetrabutylammonium perchlorate has been used as a supporting electrolyte. Cyclic

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ACCEPTED MANUSCRIPT voltammogram were recorded with the incremental addition of the titrant as sodium carbonate (1×10−2 M in distilled water). 2.2 Synthesis of receptors PDZ-1, PDZ-2 and PDZ-3 PDZ-1: (E)-2-(2-(4-nitrobenzylidene)hydrazinyl)pyridine

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To a solution of 4 nitrobenzaldehyde (100 mg, 0.65 mmol) in ethanol (12mL) added 2hydrazinopyridine (71.7 mg, 0.65 mmol). Acetic acid (0.5 mL) was used as catalyst and

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reaction mixture was refluxed at 80 oC for 3 hours. The reaction completion was monitored by

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TLC method and the reaction mixture was filtered. Residue was washed with water (5×10 mL) and minimum amount of cold ethanol. It was dried at 65 oC for next 1 hour to obtain pure

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product as yellow powder. The pure compound was characterized by FTIR, 1HNMR and HRMS studies. Yield 83%. Melting point: 243 ◦C. FT-IR (KBr pellet): 3198 cm-1 (N-H stretch),

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2997 cm-1 (C-H stretch), 1564 cm-1 (C=N stretch). HRMS (ESI- MS): Found. 243.08 [M+H]+ clcd. 243.08 for C12H11N4O2+. 1H NMR (DMSO-d6, 400 MHz) δ 11.3 (s, 1H) 8.2 (dd, J = 9

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Hz, 2H) 8.15 (ddd, J = 5 Hz, 1.9 Hz, 1H), 8.1 (s, 1H), 7.9 (dd, J = 9Hz, 2H), 7.69 (ddd, J = 8.6 Hz, 6.9 Hz, 1.8 Hz, 1H), 7.32 (ddd, J = 8.4 Hz, 1.3 Hz, 1H) 6.84 (ddd, J = 7.3 Hz, 4.8 Hz, 1

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Hz, 1H).

PDZ-2: (E)-N,N-dimethyl-4-((2-(pyridin-2-yl)hydrazono)methyl)aniline

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Compound 4-(dimethylamino)benzaldehyde (100 mg, 0.67 mmol) was dissolved in 12 mL of ethanol in a 25 mL round bottomed flask. To this solution 2-hydrazinopyridine (73.15 mg, 0.67 mmol) and acetic acid (0.5 mL) was added and reaction was kept under reflux condition for next 3 hours. The reaction progress was monitored by TLC. The precipitated crude product was filtered off. It was washed with water (5×10 mL) and minimum amount of cold ethanol solution and dried to obtain pure product as a pale brown crystalline solid. The pure compound was characterized by FTIR, 1HNMR and HRMS studies. Yield 89%. Melting point 187 ◦C. FT-

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ACCEPTED MANUSCRIPT IR (KBr pellet): 3193 cm-1 (N-H stretch), 3061 cm-1 (C-H stretch), 1601 cm-1 (C=N stretch). HRMS (ESI- MS): Found. 241.14 [M+H]+ clcd. 241.14 for C14H17N4+ 1H NMR (DMSO-d6, 400 MHz) δ 10.48 (s, 1H), 8.05 (ddd, J = 4.8 Hz, 2.05 Hz, 1H), 7.9 (s, 1H), 7.58 (ddd, J = 7.82 Hz, 2Hz, 1H), 7.46 (dd, J = 8.8 Hz, 4.6 Hz, 2H), 7.15 (dd, J = 8 Hz, 1.8Hz, 1H), 6.72 (dd, J =

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8.9 Hz, 2H), 6.67 (J = 7.9 Hz 4.9 Hz, 0.9 Hz, 1H). PDZ-3: (E)-2-(2-benzylidenehydrazinyl)pyridine

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2-hydrazinopyridine (102.83 mg, 0.94 mmol) was dissolved in 12 mL ethanol in a 25 mL round

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bottomed flask. To this solution, benzaldehyde (96 uL, 0.94 mmol) was added followed by glacial acetic acid (0.5 mL). The reaction mixture was refluxed for next 3 hours and reaction

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progress was monitored by TLC. Later, it was cooled and poured into 250 mL ice-cold water.

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The white solid precipitated out which was filtered, washed with minimum amount of ice cold ethanol and dried at 65 ◦C for 1 hour. The pure compound was characterized by FTIR, 1HNMR

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and HRMS studies. Yield 78%. Melting point 154 ◦C. FT-IR (KBr pellet): 3200 cm-1 (N-H

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stretch), 3005 cm-1 (C-H stretch), 1603 cm-1 (C=N stretch), 1140 cm-1 (C-N stretch). HRMS (ESI- MS): Found. 198.10 [M+H]+ clcd. 198.10 for C12H12N3+ 1H NMR (DMSO-d6, 400 MHz)

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δ 10.84 (s, 1H), 8.1 (ddd, J = 5 Hz, 2 Hz, 0.7 Hz 1H) 8.0 (s, 1H), 7.65 (ddd, J = 7.9 Hz, 1.5 Hz, 0.25 Hz, 2H), 7.6 (ddd, J = 8.4 Hz, 1.8 Hz, 1H), 7.4 (ddd, J = 8 Hz, 7.45 Hz, 2H) 7.3 (dddd, J

1H).

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= 7.4 Hz, 1.3 Hz, 1H) 7.2 (ddd, J = 7.85 Hz, 1.2 Hz, 1H) 6.7 (ddd, J = 7.2 Hz, 4.9 Hz, 1 Hz,

3. Results and Discussion The receptors have been synthesized by simple Schiff base condensation reaction between 2hydrazinopyridine and different substituted aldehydes as represented in Scheme 1 and characterized by standard spectroscopic techniques.

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ACCEPTED MANUSCRIPT 3.1 Colorimetric studies Initially, the receptors PDZ-1, PDZ-2 and PDZ-3 were investigated for the colorimetric detection of response towards anions in dry DMSO solvent. In this experiment, each time to 1×10-4 M receptor solution, 1×10-2 M of different anions such as F−, Cl−, Br−, H2PO4−, HPO42−,

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ClO4−, CO32−, HCO3−, SO42−, HSO4−, NO3−, AcO− as sodium salts were added. as sodium salts. Solution of free receptor PDZ-1 in DMSO was pale yellow in colour while PDZ-2 and PDZ-

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3 were colourless. Interestingly, Receptor PDZ-1 showed instantaneous colour change from

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pale yellow to green in the presence of CO32− ion whereas remained inactive for other test anions used in the present study. However, PDZ-2 and PDZ-3 did not exhibit any colour

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change in the presence of test anions. Fig.1 shows the change in the colour of receptor PDZ-1,

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PDZ-2 and PDZ-3 on addition of different anions as sodium salts. 3.2 UV-Vis spectroscopic studies

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To validate the selectivity of PDZ-1 towards CO32− ion, comparative study for PDZ-1

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has been performed with the addition of different anions. To 2 mL of 1×10-4 M solution of PDZ-1 taken in quartz cuvette, 18 equiv. of 1×10-2 M sodium salts of different test anion

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solutions were added individually. UV-Vis absorption spectra was recorded immediately. Free receptor exhibited an absorption band at 408 nm corresponding to pale yellow coloration of

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PDZ-1. As illustrated in Fig. 2, PDZ-1 exhibited instantaneous red shift in the absorption maxima in the presence of CO32− ions. PDZ-1 was inactive towards all other anions used in the present study, which is supported as revealed by the absence of red shift band. It is evident that, the λ max of PDZ-1-CO32− complex being 629 nm falls within the red region of electromagnetic spectrum, the PDZ-1+CO32− solution appears green in colour. PDZ1 was insusceptible to the interference from alkaline anions like CH3COO− AcO− and H2PO4−,

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ACCEPTED MANUSCRIPT consequently it displayed high selectivity toward CO32− ion. Moreover, PDZ-1 could readily differentiate HCO3‒ from CO32− ions. Anion binding ability of PDZ-2 and PDZ-3 has been studied to understand the influence of electron withdrawing group on the chromogenic response. PDZ-2 and PDZ-3 exhibited absorption bands at 361 and 334 nm respectively corresponding to localized

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transitions of CH=N group and –NH functionality. From Fig. S1 and S2 (ESI) , it is evident

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that, the receptors PDZ-2 and PDZ-3 neither showed any colour changes nor displayed any

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UV-Vis spectral changes even with the addition of 18 equivalents of anions. This finding clearly highlights the role of electron withdrawing nitro group aiding the charge transfer

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interaction leading to chromogenic signal output in CO32− ion detection process.

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Encouraged by the results, UV-Vis titration experiment of PDZ-1 with CO32‒ ions has been performed (Fig. 3). To 2 mL of PDZ-1 as 1×10-4 M in dry DMSO, 1 equiv. of 1 × 10-2 M

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CO32− ion was added each time. UV-Vis titration study results indicated that, below seven

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equiv. of CO32− ion, PDZ-1 was able to show instantaneous colour change from yellow to green. However, the colour change diminishes quickly. With the addition of seventh equiv. of

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CO32− ion and further incremental addition, the absorption band of PDZ-1 centered at 408 nm weakened gradually, with the emergence of new absorption band at 629 nm. New band at 629

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nm intensified concurrently, resulting in a bathochromic shift differing by 221 units corresponding to original band of free receptor. An isosbestic point indicating the formation of anion-receptor complex has been observed at 486 nm. The absorption peak at 408 nm is attributed to n-π* transition which is of lower energy. Upon binding with CO32− ion, the electron density in the system tends to increase leading to electronic transitions with more ease. Concomitantly, it leads to the appearance of new absorption band at longer wavelength. The receptor attained saturation level with the addition of 18 equiv. of CO32− ions. Interestingly, upon with increasing concentration of CO32− ions, the bands at 408 nm and 629 nm experienced 9

ACCEPTED MANUSCRIPT a hypsochromic shift of 5 nm and 13 nm respectively. Hypsochromic shift could be accounted for the formation of higher order complexes. With the incremental addition of CO32− ions, two PDZ-1 preferred to bind CO32− ion through hydrogen bonding interactions between –NH proton and oxygen atom of CO32− ion. At higher equiv., the diminution of colorimetric response lead to hypsochromic shift of band centered at 629 nm. In order to confirm the preference of

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hydrogen bonding over deprotonation with PDZ-1 in the presence of carbonate, OH– ion

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sensing properties have been investigated. Upon addition of 10 equiv. of NaOH, there was no

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significant colorimetric response confirming the hydrogen bond interaction existing between PDZ-1 and the active anion. (ESI Fig. S17)

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The binding stoichiometry between receptor PDZ-1 and CO32− ions was determined by the Benesi–Hildebrand method using UV spectrophotometric titration data at 629 nm. The

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linearity of the graph (Fig.S3 (ESI)) confirms the formation of a stable 2:1 PDZ-1:CO32− stoichiometric complex. Binding constant (K) has been calculated using B-H equation (Eqn.1

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(ESI)). Binding constant (K) value was found to be 0.77×104 M−2. For real time sensing of CO32− ions, it is essential to know the limit of detection (LOD)

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and limit of quantification (LOQ) of PDZ-1. LOD and LOQ values were calculated using Eqn.2 (ESI) and Eqn. 3 (ESI) respectively. The LOD value was found to be 0.11 mM while

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LOQ was 0.36 mM.

With a view to highlight the selective response of PDZ-1 towards CO32− ion, bar graph has been presented and shown in Fig. S4 (ESI). It is worth mentioning that, PDZ-1 showed activity towards CO32− ion in DMSO alone. PDZ-1 remained inactive towards CO32− ion in solvents of varying polarity such as dichloromethane, tetrahydrofuran, ethanol, methanol, acetone and acetonitrile. The reason for such a cause can be attributed to the higher order of

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ACCEPTED MANUSCRIPT dielectric constant of DMSO solvent, which aided the stabilization of excited state of the PDZ1 - CO32−complex leading to a chromogenic output. Comparative study of PDZ-1, PDZ-2 and PDZ-3 (1×10−4 M in dry DMSO) with the addition of different cations (1×10−3 M in distilled water) as their nitrate salts (except for Hg2+

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where HgCl2 was used due to poor solubility of HgNO3) Hg(NO3)2) has been performed. Fig S5, S6 and S7 (ESI) show that, none of the cations induced any shift in absorption spectrum of

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PDZ-1, PDZ-2 and PDZ-3. This proves the utility of PDZ-1 as a selective sensor for CO32−

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ions devoid of activity towards cations.

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3.3 1H-NMR titration

In order to validate proposed recognition mechanism, 1H-NMR titration has been

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performed by adding incremental amount of CO32− ions (in D2O) to a solution of PDZ-1 in DMSO-d6. Upon adding 0.5 equiv. of CO32− ions, no observable change in chemical shift was

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observed.

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Although –NH proton at 11.3 ppm from the above figure (Fig. 4) appears to be diminished, a closer look revealed the presence even after addition of one equivalent of CO32−

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ions. In addition, there was no any new peak observed during the course of titration. It could be justified that the basicity of CO32− ions is not sufficient enough to deprotonate the –NH

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proton of PDZ-1 but was only able to cause redshift in the UV-Vis spectrum due to 2:1 complex formation.[36,37]

Based on the UV-Vis titration and 1H-NMR titration of PDZ-1 and PDZ-1+ CO32− ion, binding mechanism has been proposed and is shown in Fig. 5. 3.4 Electrochemical study Recently, supramolecular electrochemistry is gaining more research interest for its utility in studying the stability and binding mechanism of host-guest complex. The presence of 11

ACCEPTED MANUSCRIPT electroactive groups such as –NH and NO2 in PDZ-1 hydrogen bonding in PDZ-1-CO32− complex triggered us to perform electrochemical study on PDZ-1 and PDZ-1+CO32− complex. Cyclic voltammetric (CV) studies have been performed utilizing the three electrode configuration consisting of a platinised platinum (working), platinum wire (auxiliary), and

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saturated calomel (reference) electrodes. The experiments have been performed in DMSO using 0.01 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. Owing to the

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activity of PDZ-1 towards CO32− ions in DMSO which is highly polar, aprotic in nature has

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been used as solvent medium. To 5×10−4 M solution of in DMSO, incremental addition of

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PDZ-1 0.5 equiv. of 1×10-2 M CO32− ions lead to electrochemical response. Cyclic voltammogram of PDZ-1 alone showed an insignificant oxidation peak at

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potential +0.16 V and the corresponding reduction peak was not observed (Fig. 6). This observation depicts the stability of neutral PDZ-1 towards oxidation and reduction. However,

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upon the addition of 0.5 equiv. CO32− ions, there was a remarkable rise and shift in the oxidation

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peak (E pc= +0.46 V) towards positive potential. Further, with incremental addition of CO32− ions, a steady increase and shift was observed (Fig. 7). It could be justified that, as the anion

).

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(guest) concentration increases, the E1/2 of CV wave shifts from EPDZ-1 to E(PDZ-1-Carbonate complex Surprisingly, a shift in pattern of cyclic voltammogram curve towards the positive potential

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was observed after the addition of two equiv. of CO32− ions which resulted in a set of two symmetric CV curves obtained as shown in the Fig. 6. and Fig. 8. With the addition of 4.5 equiv. of CO32− ions a saturation potential was obtained (E pc= +0.73 V). Qualitatively, if a guest binds more strongly to the oxidized form of the host, the oxidized host will be stabilized and it will be harder to reduce the host in the presence of the guest. This means that EHG is negative of EH. On the other hand, if the guest binds more strongly

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ACCEPTED MANUSCRIPT to the reduced form, it will be easier to reduce the host in the presence of the guest and EHG is positive of EH.[38] In the present study, reduction peak was entirely absent. A steady increase in oxidation peak may be attributed to the increase in electron density over the complex due to binding of

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di-negative carbonate ion. The oxidation increases the positive charge on nitrogen, which becomes a better hydrogen bond donor and thus enhancing the strength of hydrogen bonding

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resulting in more stable host-guest complex (Scheme 2). The shift in oxidation peak towards

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positive potential substantiates the comparatively lower binding constant obtained from UVVis absorption spectra and electrostatic nature of hydrogen bonding involved in the process.

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Reversing the scan after the electrochemical generation of a species is a direct and

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straightforward way to probe its stability. The absence of reduction peak indicates the inability to reduce electron rich PDZ-1+CO32− complex.

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Interestingly, during oxidation interval, there was appearance of a green coloured

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complex on the surface of the working right at one equiv. of CO32− ion where the entire solution retained yellow colour. A possible explanation for this observation is that, more stable complex

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is formed at the electrode surface upon oxidation leading to a green coloration observed at vicinity of the working electrode.

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3.5 DFT studies

The geometrical and electronic properties of molecule and complex were performed using Gaussian 09 package.[39] Ground state optimized geometry of the receptor and complex were achieved by means of the B3LYP (Becke three parameters hybrid functional with LeeYang-Perdew correlation functionals)[40,41] with the 6-31+G (d, p) basis set.[42,43] Occupied and unoccupied molecular orbitals of both receptor and complex were visualized using Avogadro software.[44,45] The excited state properties of the molecule were computed using 13

ACCEPTED MANUSCRIPT time-dependent DFT method (TD-DFT) at the same level of calculation to estimate vertical transition energies. Vertical transition energies of up to first 10 singlet excited states for the receptor and complex in DMSO solvent was performed. This effect of solvent on the energy parameters has been incorporated by self-consistent reaction field using inbuilt conductor polarizable continuum model (SCRF-CPCM) as implemented in Gaussian09 software.[46]

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The ground state optimized geometry of receptor and molecular orbitals of unoccupied

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higher excited states and lower occupied ground states of PDZ-1 and PDZ-1 + CO32− complex

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are as shown in the Fig 9. The receptor PDZ-1 exhibits a planar structure while that of the complex with 2:1 binding ration exhibits a structural twist. Owing to the existence of planarity

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in the receptor structure, there is a decent amount of electron delocalization in the network in HOMO and LUMO states. The complex formed with the carbonate ion have their HOMO and

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lower ground states and LUMO with their corresponding higher excited states are greatly red shifted in comparison with the free receptor PDZ-1. The HOMO of the complex is completely

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localized on the carbonate moiety, whereas LUMO and LUMO+1 are localized on

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nitrobenzene and imine linkage on one of the receptor PDZ-1. Further, the HOMO-1 and HOMO-2 confines electron cloud on pyridine moiety of the PDZ-1 and partly on carbonate

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ion. The band gap of the free receptor PDZ-1 is 2.84 eV and it verily reduces to a value of 1.90 eV upon complex formation. Vertical singlet transitions obtained computationally for the

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receptor and the complex agrees well with the experimentally obtained UV-Vis spectra as shown in the Fig 10. Vertical transitions with their oscillator strength for the complex is as shown in the Fig 11. Charge transfer transition can be ascribed to very closely spaced energy at 643 nm and 641 nm occurring between carbonate and one of the receptor PDZ-1 moiety. Further, π-π* can be attributed to transition occurring between HOMO-1 and HOMO-2 to the LUMO and LUMO+1 viz. 563 nm and 554 nm respectively. The UV-Vis spectral data

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ACCEPTED MANUSCRIPT corroborates well with the experimentally derived results confirming the associated color change in the colorimetric experiment.

4. Conclusions Conclusively, PDZ-1 stands superior among all the colorimetric receptors reported till

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date in terms of; (i) being highly selective with significant colour change under visible light and redshift in UV absorption spectra, (ii) synthetically important as it is possible to detect

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CO32− ion with one potent binding site and (iii) first ever a simple pyridine derivative has been

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explored for sensing of CO32− ions. (iv) differentiating CO32− ions in the presence of HCO3−

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ions. The binding constant was calculated and found to be 0.77×10−4 M-2. PDZ-1 could successfully detect CO32− ions at a concentration as low as 0.11 mM. Without any tedious

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sample preparation, quantification of CO32− ions in aqueous media up to 0.36 mM level has been achieved. DFT studies performed on the PDZ-1 and PDZ-1+CO32− complex provide an

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insight into the binding mechanism. Currently, there is a rising need for highly selective

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fluorogenic sensor, which can be used for in vivo applications. The present work provides a pathway for arriving at new strategy to develop fluorescent sensors for carbonate recognition

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in the near future.

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4.1 Electronic supplementary information Appendix A: UV-Vis graphs, B-H plot, FT-IR, HRMS-ESI and 1H-NMR spectrum are available.

Acknowledgement Authors express their gratitude to NITK Surathkal for the providing the research infrastructure. SP and MM are thankful to NITK for the research fellowship. DRT thanks DST (SB/FT/CS137/2012) for the financial support of this work. We are thankful to MIT Manipal for the NMR

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ACCEPTED MANUSCRIPT analysis. MM is thankful to Dr. Ajith K. M., Assistant professor, Department of Physics, NITK, Surathkal for his support in providing access to Gaussian 09 package. Notes and References [1] P.A. Gale, Anion receptor chemistry, Chem. Commun. 47 (2011) 82–86. [2] P.A. Gale, E.N. Howe, X. Wu, Anion receptor chemistry, Chem. 1 (2016) 351–422.

PT

[3] J.L. Sessler, P.A. Gale, W.-S. Cho, Anion receptor chemistry, Royal Society of Chemistry, 2006.

RI

[4] M.S. Han, D.H. Kim, Naked-Eye Detection of Phosphate Ions in Water at Physiological

Probe, Angew. Chem. 114 (2002) 3963–3965.

SC

pH: A Remarkably Selective and Easy-To-Assemble Colorimetric Phosphate-Sensing

[5] V.C. Marinho, J. Higgins, S. Logan, others, Fluoride toothpastes for preventing dental

NU

caries in children and adolescents, Cochrane Libr. (2003).

[6] T. Walsh, H.V. Worthington, A.-M. Glenny, P. Appelbe, V.C. Marinho, X. Shi, Fluoride

MA

toothpastes of different concentrations for preventing dental caries in children and adolescents, Cochrane Database Syst. Rev. (2010). [7] R.H. Selwitz, A.I. Ismail, N.B. Pitts, Dental caries, The Lancet. 369 (2007) 51–59.

D

[8] P. Bose, Development of Receptors for sensing of Fluoride, Acetate and Phosphate:

PT E

Recognition and Selectivity Studies, 2012. [9] Y. Marcus, Thermodynamics of solvation of ions. Part 5.—Gibbs free energy of hydration at 298.15 K, J. Chem. Soc. Faraday Trans. 87 (1991) 2995–2999.

CE

[10] G. Hennrich, H. Sonnenschein, U. Resch-Genger, Fluorescent anion receptors with iminoylthiourea binding sites—selective hydrogen bond mediated recognition of CO3 2-,

AC

HCO3- and HPO42-, Tetrahedron Lett. 42 (2001) 2805–2808. [11] Y. Guo, S.-J. Shao, J. Xu, Y.-P. Shi, S.-X. Jiang, Selective colorimetric sensing of PO43and CO32- based on the assembly of dihydroxymethyl-di-(2-pyrrolyl) methane and TCNQ, Inorg. Chem. Commun. 7 (2004) 333–336. [12] P. Kaur, S. Kaur, K. Singh, A selective and sensitive “naked eye”anion detector based on an imine-π-TCNQ assembly, Tetrahedron Lett. 48 (2007) 7191–7193. [13] C. Han, Z. Cui, Z. Zou, D. Tian, H. Li, others, Urea-type ligand-modified CdSe quantum dots as a fluorescence “turn-on” sensor for CO32- anions, Photochem. Photobiol. Sci. 9 (2010) 1269–1273.

16

ACCEPTED MANUSCRIPT [14] J. Vaněk, P. Lubal, P. Hermann, P. Anzenbacher, Luminescent sensor for carbonate ion based on lanthanide (III) complexes of 1, 4, 7, 10-tetraazacyclododecane-1, 4, 7-triacetic acid (DO3A), J. Fluoresc. (2013) 1–13. [15] L. He, C. Liu, J.H. Xin, A novel turn-on colorimetric and fluorescent sensor for Fe 3+ and Al 3+ with solvent-dependent binding properties and its sequential response to carbonate, Sens. Actuators B Chem. 213 (2015) 181–187. [16] M. Saleem, N.G. Choi, K.H. Lee, Facile synthesis of an optical sensor for CO32- and

PT

HCO3- detection, Int. J. Environ. Anal. Chem. 95 (2015) 592–608.

[17] H. Tavallali, G. Deilamy-Rad, A. Parhami, S. Lohrasbi, A novel and simple fluorescent

RI

and colorimetric primary chemosensor based on Congo-Red for sulfite and resultant

SC

complex as secondary fluorescent chemosensor towards carbonate ions: Fluorescent probe mimicking INHIBIT logic gate, Talanta. 149 (2016) 168–177.

NU

[18] A. Ghorai, J. Mondal, R. Chandra, G.K. Patra, A reversible fluorescent-colorimetric chemosensor based on a novel Schiff base for visual detection of CO3 2- in aqueous solution, RSC Adv. 6 (2016) 72185–72192.

MA

[19] H. Zhao, S.-J. Park, F. Shi, Y. Fu, V. Battaglia, P.N. Ross, G. Liu, Propylene carbonate (PC)-based electrolytes with high coulombic efficiency for lithium-ion batteries, J.

D

Electrochem. Soc. 161 (2014) A194–A200.

[20] T. Azad, S. Ahmed, Common milk adulteration and their detection techniques, Int. J.

PT E

Food Contam. 3 (2016) 22.

[21] M. Zougagh, A. Rı́os, M. Valcárcel, Direct determination of total carbonate salts in soil samples by continuous-flow piezoelectric detection, Talanta. 65 (2005) 29–35.

CE

[22] O. Dóka, D. Bicanic, M. Szücs, M. Lubbers, Direct measurement of carbonate content in soil samples by means of CO laser infrared photoacoustic spectroscopy, Appl. Spectrosc.

AC

52 (1998) 1526–1529.

[23] R.G. Amundson, J. Trask, E. Pendall, A rapid method of soil carbonate analysis using gas chromatography, Soil Sci. Soc. Am. J. 52 (1988) 880–883. [24] J.H. Shin, H.J. Lee, C.Y. Kim, B.K. Oh, K.L. Rho, H. Nam, G.S. Cha, ISFET-based differential pCO2 sensors employing a low-resistance gas-permeable membrane, Anal. Chem. 68 (1996) 3166–3172. [25] K. Tsukada, Y. Miyahara, Y. Shibata, H. Miyagi, An integrated chemical sensor with multiple ion and gas sensors, Sens. Actuators B Chem. 2 (1990) 291–295. [26] H.K. Lee, H. Oh, K.C. Nam, S. Jeon, Urea-functionalized calix [4] arenes as carriers for carbonate-selective electrodes, Sens. Actuators B Chem. 106 (2005) 207–211. 17

ACCEPTED MANUSCRIPT [27] R.K. Meruva, M.E. Meyerhoff, Catheter-type sensor for potentiometric monitoring of oxygen, pH and carbon dioxide, Biosens. Bioelectron. 13 (1998) 201–212. [28] E.E. Burt, A.H. Rau, The determination of the level of bicarbonate, carbonate, or carbon dioxide in aqueous solutions, Drug Dev. Ind. Pharm. 20 (1994) 2955–2964. [29] A. Ghorai, J. Mondal, R. Saha, S. Bhattacharya, G.K. Patra, A highly sensitive reversible fluorescent-colorimetric azino bis-Schiff base sensor for rapid detection of Pb 2+ in

PT

aqueous media, Anal. Methods. 8 (2016) 2032–2040. [30] P. Anzenbacher, M.A. Palacios, K. Jursíková, M. Marquez, Simple electrooptical sensors for inorganic anions, Org. Lett. 7 (2005) 5027–5030.

RI

[31] Q. Lin, T.-T. Lu, X. Zhu, T.-B. Wei , H. Li, Y.-M. Zhang, Rationally introduce multi-

SC

competitive binding interactions in supramolecular gels: a simple and efficient approach to develop multi-analyte sensor array, Chem. Sci., 7 (2016) 5341-5346.

NU

[32] Q. Lin, T.-T. Lu, X. Zhu, B. Sun, Q.-P. Yang, T.-B. Wei, Y.-M. Zhang, A novel supramolecular metallogel-based high-resolution anion sensor array, Chem. Commun. 51 (2015) 1635-1638.

Controlled

MA

[33] Q. Lin, K.-P. Zhong, J.-H. Zhu, L. Ding, J.-X. Su, H. Yao, T.-B. Wei, Y.-M. Zhang, Iodine Pillar[5]arene-Based

Multiresponsive

Supramolecular

Polymer

for

D

Fluorescence Detection of Cyanide, Mercury, and Cysteine. Macromolecules 50 (2017) 7863–7871.

PT E

[34] Q. Lin, P.-P. Mao, Y.-Q. Fan, P.-P. Jia, J. Liu, Y.-M. Zhang, H. Yao, T.-B. Wei, Novel multi-analyte responsive ionic supramolecular gels based on pyridinium functionalized-

CE

naphthalimide, Soft Matter 13 (2017) 7360-7364. [35] Q. Lin, P.-P. Mao, Y.-Q. Fan, L. Liu, J. Liu, Y.-M. Zhang, H. Yao and T.-B. Wei, A novel supramolecular polymer gel based on naphthalimide functionalized-pillar[5]arene

AC

for the fluorescence detection of Hg2+ and I−and recyclable removal of Hg2+via cation–π interactions, Soft Matter, 13 (2017) 7085-7089. [36] F. Zapata, A. Caballero, A. Espinosa, A. Tárraga, P. Molina, Cation coordination induced modulation of the anion sensing properties of a ferrocene- imidazophenanthroline dyad: multichannel recognition from phosphate-related to chloride anions, J. Org. Chem. 73 (2008) 4034–4044. [37] C. Pérez-Casas, A.K. Yatsimirsky, Detailing hydrogen bonding and deprotonation equilibria between anions and urea/thiourea derivatives, J. Org. Chem. 73 (2008) 2275– 2284. 18

ACCEPTED MANUSCRIPT [38] A.E. Kaifer, M. Gómez-Kaifer, Supramolecular electrochemistry, John Wiley & Sons, 2008. [39] R.A. Gaussian09, 1, MJ Frisch, GW Trucks, HB Schlegel, GE Scuseria, MA Robb, JR Cheeseman, G. Scalmani, V. Barone, B. Mennucci, GA Petersson et al., Gaussian, Inc Wallingford CT. (2009). [40] C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy

PT

formula into a functional of the electron density, Phys. Rev. B. 37 (1988) 785. [41] A.D. Becke, Density‐functional thermochemistry. III. The role of exact exchange, J.

RI

Chem. Phys. 98 (1993) 5648–5652. doi:10.1063/1.464913.

[42] W.J. Hehre, R. Ditchfield, J.A. Pople, Self—consistent molecular orbital methods. XII.

SC

Further extensions of gaussian—type basis sets for use in molecular orbital studies of organic molecules, J. Chem. Phys. 56 (1972) 2257–2261.

NU

[43] P.C. Hariharan, J.A. Pople, The influence of polarization functions on molecular orbital hydrogenation energies, Theor. Chem. Acc. Theory Comput. Model. Theor. Chim. Acta. 28 (1973) 213–222.

MA

[44] M.D. Hanwell, D.E. Curtis, D.C. Lonie, T. Vandermeersch, E. Zurek, G.R. Hutchison, Avogadro: an open-source molecular builder and visualization tool. Version 1.1. 0, 2012.

D

[45] M.D. Hanwell, D.E. Curtis, D.C. Lonie, T. Vandermeersch, E. Zurek, G.R. Hutchison,

PT E

Avogadro: an advanced semantic chemical editor, visualization, and analysis platform, J. Cheminformatics. 4 (2012) 17.

[46] J. Tomasi, M. Persico, Molecular interactions in solution: an overview of methods based

AC

CE

on continuous distributions of the solvent, Chem. Rev. 94 (1994) 2027–2094.

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ACCEPTED MANUSCRIPT Highlights 

Compounds 2-(arylidenehydrazinyl)pyridines were synthesized by varying the substituents on



the aromatic ring. Effect of substituents on molecular recognition has been visualized.

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PDZ-1 is the first ever selective probe towards aqueous source of CO3

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Limit of detection was as low as 0.11 mM.

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The receptor PDZ-1 was able to differentiate CO3

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− ions from HCO3 ions. 2− Development of selective fluorescent probe for CO3 ions can be achieved in future.

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Graphics Abstract

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