Smartphone-based digitized recognition of As³⁺ along with its effectual mitigation in water using a benzothiazole-functionalized molecular scaffold

Abstract

In this work, an innovative azomethine-functionalized chromogenic and electrochemical chemosensor ABH [2-((acridin-9-yl)methylene)-1-(benzo[d]thiazol-2-yl)hydrazine] was explored, which was synthesized via a one-pot synthetic route with the aim of prolific and sequential aqueous-phase detection of As³⁺ and F⁻. The chemosensor exhibited distinct colorimetric transformation from yellow to burgundy upon addition of As³⁺, which upon subsequent addition of F⁻ rejuvenated, suggesting a strategic application for designing three-input logic gate imitative electronic circuits derived from Boolean algebra. Several sophisticated wet chemical experiments, for instance, UV-Vis spectroscopy, ESI-MS, cyclic voltammetry, ¹H-NMR spectroscopy, and FTIR spectroscopy along with theoretical computations substantiated the sensing phenomena. Furthermore, the development of an inexpensive portable arsenic sensor kit (ASK) in addition to a RGB-based smartphone-coupled diagnostic sensor station for rapid on-field detection of As³⁺ made the chemosensor an exclusive one as a value-added product in our society. Furthermore, the complexation ability of the sensor with toxic As³⁺ introduced it as a smart material for the aqueous-phase elimination of As³⁺.

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1. Introduction

Arsenic (As³⁺), one of the most abundant minerals in the earth's crust, causes worldwide intimidation to the ecosystem and human health due to its severe toxicological effects. Drinking water with an elevated concentration of As³⁺ causes adverse health effects including arsenicosis, black foot disease, hyperkeratosis, fatigue, genotoxic and mutagenic effects and even cancer. As stated by the World Health Organization (WHO) and the US Environmental Protection Agency (USEPA), the acceptable level for As³⁺ in drinking water is 10 ppb.¹ According to the reports, 20 countries worldwide suffer from exceeding As³⁺ concentration in drinking water.² As a consequence, low-level efficacious recognition of As³⁺ in water is of considerable importance in recent times. The conventional As³⁺ detection techniques include atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), atomic fluorescence spectrometry (AFS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), graphite furnace-atomic absorption spectrometry (GF-AAS), and hydride generation-atomic absorption spectrometry (HG-AAS), which are highly expensive, time-consuming and require trained person.^3,4 Moreover, the conventional colorimetric methods for As³⁺ recognition, based on the Gutzeit reaction, suffer from fewer optimistic response and lethal by-products such as AsH₃ gas and mercury compounds.^5a Therefore, introducing proficient chemical sensors,^5b–d capable of detecting As³⁺ in water is of immense importance to the scientific community. In this perspective, metal nanoparticles have drawn considerable attention over conventional probes, which also suffer from various limitations such as lack of matrix interference, poor stability, high cost and the requirement of special agents to discriminatively detect As³⁺.⁶ Conversely, premeditated benzothiazole-functionalized azomethine-based derivatives are acquiring noteworthy consideration owing to their interesting nonlinear optical properties for naked eye detection of As³⁺ below the WHO permissible limit.⁷

Therefore, tailor-made synthesis of an inimitable azomethine-functionalized chemosensor, 2-((acridin-9-yl)methylene)-1-(benzo[d]thiazol-2-yl)hydrazine [ABH], was explored in this work, which showed unprecedented selectivity and sensitivity towards As³⁺ in a water medium. This benzothiazole-functionalized Schiff-base chemosensor was synthesized via a one-step condensation reaction by a simple wet chemical method and characterized by several analytical techniques such as ESI-MS, FTIR spectroscopy, SCXRD, and ¹H-NMR spectroscopy. The presence of aqueous-phase As³⁺ can be selectively detected by the rapid colorimetric alteration of the chemosensor from yellow to burgundy even in the presence of other cations, owing to its bis-coordination to the N,N-chelate of the benzothiazole derivative, which was further confirmed by ¹H-NMR titration studies along with density functional theory calculations.⁸ Interestingly, the ABH–As³⁺ adduct was found to be highly efficient to regenerate ABH selectively in the presence of F⁻, one of the most perilous anions due to its various fatal effects such as dental and skeletal fluorosis, and kidney failure in humans,⁹ which can further mimic an explicit progression of numerous logical functions, competent of performing molecular-level arithmetic computations.¹⁰ The pH of the ground water As³⁺ ranges from pH 5 to pH 6. Consequently, there exists a tendency of interference between the sensing response of As³⁺ and sensor solution in acidic pH. After synthesizing the sensor molecule, the halochromic behaviour was investigated. The ABH was found to exhibit colorimetric alteration at an extremely lower pH (pH 2), which signified that the sensing response of As³⁺ will not interfere with the working pH. Furthermore, to examine any interference caused by the solvatochromic alteration of ABH and sensing response of ABH with As³⁺, different solvatochromic responses (nonpolar to polar) have been considered and a subtle red-shift in the UV-Vis spectra with the increase in solvent polarity was found. However, in MeOH, like a low-polar protic solvent, compared to highly polar DMSO, such type of chromogenic alteration was not observed. Moreover, MeOH has been selected as sensing media for the high solubility of the sensor. Overall, these solvatochromic and halochromic properties of the sensor strengthen the recognition aptitude of ABH towards As³⁺. To the best of our knowledge, this type of sensory probe capable of consecutively detecting both As³⁺ and F⁻ is yet to be explored in the literature.

However, updated strategies are required in clinical, food, and environmental realms for rapid analysis using sophisticated instrumentation. In this context, both Analytical Chemistry and Data Management are disciplines of paramount importance. Small, low-cost and portable analytical devices that can be used by inexperienced personnel to acquire accurate and precise data are of significant importance for the betterment of the society.¹¹ This analytical device should be compatible with resource-limitations such as power supply, laboratory setup and skilled operators. At present, smartphones can be an ideal platform for monitoring contamination in ground water in large areas and the most suitable standpoint for providing device portability, analysis power, data processing and excellent communication abilities. Therefore, in this work, a portable smartphone-based archetype was fabricated in order to have a clear perception about the concentration of As³⁺ in groundwater based on the colorimetric alterations of ABH in the presence of different concentrations of As³⁺. This Bluetooth-operated smartphone-based prototype will also be effective to provide contactless data to minimize the risk of direct contact with the contaminants. As per our knowledge, a mobile platform for arsenic detection from ground water with systematized digital response is still less explored in the pertinent research territory.

Moreover, complex-based aqueous-phase mitigation of As³⁺ along with its effectual recognition using ABH is another advantageous key feature of this probe. Thus, in this work, our prime proposition is based on the exploration of a new economically viable and quick responsive colorimetric chemosensor (ABH) for the detection of As³⁺ in groundwater with high sensitivity and exceptional selectivity along with a smartphone-based digital response, which was further employed for the sequential recognition of F⁻ and complexation-based mitigation of As³⁺.

2. Experimental

2.1. Materials

The requisite precursors and other solvents were used without further modification. All the cationic and anionic salts were purchased from Sigma Aldrich and Alfa Aesar Company. Solvents such as acetonitrile, dimethyl sulfoxide, dimethyl formamide, tetrahydrofuran, dichloromethane, hexane, methanol, acetone and water of spectroscopic grade were procured from Merck (India) Pvt. Ltd.

2.2. Synthesis and characterization

ABH was synthesized via a straightforward condensation reaction of 2-hydrazinobenzothiazole and acridine-9-carboxaldehyde in MeOH as a solvent medium along with the catalytic amount of acetic acid (Fig. S1, ESI†). The reaction takes place via a condensation reaction of aldehyde with a primary amine. After 12 hours of constant stirring, a yellow colored compound was obtained as the sole and end product of the reaction. The product was then collected by solvent evaporation. Yellow colored crystals of ABH were obtained by sluggish evaporation of a THF–HXN (1 : 1) solution. ABH was thereafter characterized by sophisticated analytical techniques such as CHNS analysis, ESI-MS, HR-MS, FT-IR spectroscopy, ¹H-NMR spectroscopy (1D and 2D-COSY),¹³C-NMR spectroscopy and UV-Vis spectroscopy to confirm the structure and purity, (m/z) calculated 354 for C₂₁H₁₄N₄S; found, 354.8 (ABH + H⁺) (Fig. S2, ESI†). CHN anal. calcd for C₂₁H₁₄N₄S: C: 71.16%; H: 3.98%; N: 15.81%; S: 9.05%; found. C: 71.05%; H: 3.13%; N: 15.6%; S: 9%.¹H-NMR (400 MHz, solvent: DMSO d⁶): δ = 9.4(s, 1H), δ = 8.8 (d, 2H, J = 8.87 Hz), δ = 8.2 (d,2H, J = 8.21 Hz), δ = 7.9(t, H, J = 7.92 Hz), δ = 7.7(t, 3H, J = 7.73), δ = 7.5 (d, 2H, J = 8 Hz), δ = 7.35 (t, 1H, J = 7.36), δ = 7.15 (t, 1H, J = 7.4), δ = 6.95 (s, 1H) (Fig. S3a, ESI†), 2D COSY ¹H-NMR (Fig. S3b and Table S9, ESI†) and ¹³CNMR (Fig. S3c, ESI†).

2.3. Instrumentation

FT-IR spectroscopy (Spectrum 65, PerkinElmer) was used to confirm the functional groups present within the newly synthesized ABH moiety. The mass of ABH was corroborated using a mass spectrometer (Advion Make; Serial no. 3013-0140). A CARY60 spectrophotometer was used to perform the absorbance spectroscopic studies. X-Ray crystallography (Bruker D8 Venture (APEX-III) with photon detector) and ¹H-NMR spectroscopy (Bruker 400 MHz NMR spectrometer) were performed to identify the exact structure and purity of ABH. Cyclic voltammetry was carried out using a CH instrument to monitor the electrochemical activity of ABH. Atomic absorption spectroscopy (iCE 3500 AAS atomic absorption spectrometer) was accomplished in acquaintance with complex-based As³⁺ removal.

2.4. X-Ray crystallographic discussion

The crystals of ABH (CCDC No. 2107203) were acquired owing to the sluggish evaporation of the THF–HXN solvent (1 : 1) mixture (Fig. 1 and Tables S1–S4, ESI†). The probe was revealed to be asymmetric in nature. In the thiazole part, the endo-cyclic –C=N, C–S bond distance and <CNC bond angle were found to be 1.29 Å, 1.75 Å and 109.5° respectively, which are almost similar to the conventional benzothiazole moiety with regard to endo-cyclic –C=N (1.31 Å), C–S (1.733 Å) bond length and <CNC (110.62°) bond angle.¹² Moreover, exo-cylic –C–N, –N–N, –C=N and C–C bond distances were found to be 1.362 Å, 1.345 Å, 1.29 Å and 1.466 Å respectively. In the acrydine part, the endo-cyclic –C=N bond distance and the <CNC bond angle were found to be 1.35 Å and 118° respectively. Additionally, two ABH molecules were interconnected via extensive H-bonding interactions between endo-cyclic N (–C=N) and exo-cyclic –NH groups, which formed a closely packed rectangular dimeric assembly.

Fig. 1 (a) H-bonding interaction within ABH between N of –C=N (ring) and H of –NH; (b) closely packed rectangular shaped structure of ABH; and (c) ORTEP image of ABH (50% probability).

3. Results and discussions

3.1. Solvatochromism and halochromism exhibited by ABH

Interesting solvatochromic behavior was noticed for ABH in different solvents, wherein bathochromic shifts were observed with the increment in solvent polarity. To confirm these findings, the UV-Vis absorption spectra of ABH in both nonpolar and polar solvents (DCM, THF, ACN, MeOH, DMF and DMSO) were investigated [Fig. 2(a)]. The studies revealed that with the increment in solvent polarity from DCM to DMSO, the absorption spectra of ABH showed substantial red-shifting from 410 to 430 nm. Basically, enhancement of solvent polarity can stabilize the excited state of molecules and, therefore, diminish the energy gap among ground and excited states, facilitating the charge transfer from the ground state to the excited state at a higher wavelength (i.e. lower energy) [Fig. 2(c)].^13,14 In addition, this newly synthesized azomethine-based chemosensor, ABH, was professed to specify a particular pH region, establishing the superiority of this chemosensor over previously reported sensory probes. The nature has preserved certain pH-dependent environments to maintain proficient functionalism. Each cell of our body works like a tiny molecular machine and has its own pH-dependent working environment. Alkalinity keeps these tiny machines healthy, whereas an acidic environment spawns an alarm indicating disease.^15–20 In this relevance, the halochromic nature of ABH can play a feasible role. The initial base peak of ABH appearing at 430 nm remained constant at a biological pH of 6 to 8. However, at pH 2, the peak at 430 nm diminished through the generation of a new peak at 525 nm, indicated by a sharp color change from yellow (color of at biological pH) to violet red, while at pH 4, the peak broadened and ABH turned into firebrick color, which could be attributed to the acid-mediated charge transfer within the molecular scaffold. Therefore, these types of pH-dependent colorimetric alterations can further be a footprint towards the development of small molecule–based effectual colorimetric pH sensors [Fig. 2(b and d)].

Fig. 2 Pictorial representation of (a) solvatochromism and (b) halochromism exhibited by (1 × 10⁻⁴ M) ABH. UV-Vis spectral evidence of (c) solvatochromism and (d) halochromism produced by ABH.

3.2. Absorbance spectral outcomes of ABH with cations

The chromogenic response of ABH in the presence of several analytes is shown in Fig. 3(a), where it can be revealed that the sensor color changed from yellow to burgundy owing to the addition of As³⁺, while it remained unaltered in the presence of other metal ions. The optical response of the probe with As³⁺ was validated by UV-Vis spectroscopy. The free sensor showed a peak at 425 nm, which diminished in intensity with the generation of a new peak at 540 nm by the gradual addition of As³⁺, suggesting the charge transfer within the sensor and As³⁺ [Fig. 3(b)].^21,22

Fig. 3 (a) Visual colorimetric responses of ABH (1 × 10⁻⁴ M) in MeOH towards different metal ions (1 × 10⁻⁴ M) in water and (b) UV-Vis absorption spectra of ABH (10⁻⁴ M, MeOH) upon sequential addition of As³⁺ (10⁻⁴ M, water).

The plausible stoichiometric interaction between ABH and As³⁺ was initially acquainted from Job's plot [Fig. S4, ESI†], which revealed the probable binding mode of ABH and As³⁺ to be 1 : 1. Thenceforward, this possibility of interaction has been well supported by upshot from ESI-MS of ABH–As³⁺ adducts [Fig. S5, ESI†]. The binding constant was also calculated using the Benesi–Hildebrand equation [Table 1 and Fig. S6(a), ESI†], which revealed that the binding constant was 0.47 × 10³ M⁻¹, indicating the effective interaction of the targeted analyte with the sensor moiety. The LOD (limit of detection) was also found to be 5.4 ppb, i.e. 72.1 nM [Fig. S6(b), ESI†]. Moreover, to get an insight into the requisite time of interaction of ABH with As³⁺, the kinetic study was performed, which showed that the mechanistic interaction pathway of ABH with As³⁺ followed a zero-order reaction kinetics with a rate constant of 2.6 × 10⁻³ mol L⁻¹ s⁻¹, signifying the independence of rate constant on the concentration of both of the sensor and the analyte [Table 1 and Fig. S7, ESI†].²³

Table 1 Interaction of ABH with As³⁺ in terms of LOD, binding constant and reaction kinetics

Interaction between sensor and incoming analyte	Limit of detection (LOD in ppb)	Binding constant (K in M^−1)	Order and rate of the reaction (mol L^−1 s^−1)
ABH–As^3+	5.4 ppb	0.47 × 10^3	2.6 × 10^−3

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3.3. Interference studies of ABH with different cationic analytes

The selectivity of ABH towards As³⁺ was affirmed from the interference studies, which were conducted by introducing As³⁺ into the ABH solution containing other competitive cations such as Zn²⁺,Cd²⁺, Pb²⁺, Hg²⁺, Mn²⁺, Mg²⁺, Ni²⁺, Co²⁺, Cu²⁺, Fe³⁺, La³⁺, Nd³⁺, Gd³⁺, Sb³⁺, Sm³⁺, Al³⁺, Cr³⁺, Au³⁺, +Bi³⁺, and Ca²⁺.²⁴ The UV-Vis spectral responses of the ABH solution upon addition of As³⁺ in the presence of various other cations showed that As³⁺ induced an absorbance enhancement at 540 nm, while the other metal ions did not produce any significant variation even if the concentration of interfering ions was much higher than that of As³⁺, indicating high selectivity of ABH towards As³⁺ [Fig. S8(a–c), ESI†]. This excellent selectivity of ABH towards As³⁺ along with its anti-interference potentiality provides a prospective application to make a scrutiny of As³⁺ in groundwater.

3.4. Real-world application of ABH in determining As³⁺ in water

The present work was envisaged to detect As³⁺ from contaminated ground water samples. Therefore, aliquots of 300 μL of As³⁺ of different concentrations (10 ppb, 20 ppb, 50 ppb, 100 ppb and 1 ppm) were added to 500 μL of ABH solutions. Immediately, the yellow-colored sensor solution turned into pale yellow, yellowish orange, orange, reddish orange and burgundy as per the incremental concentration of As³⁺ [Fig. 4 and Fig. S9(a), ESI†]. Hence, ABH was perceived to be an efficient chemosensor towards detection of As³⁺ in water at both its nontoxic and toxic levels by varying the colorimetric responses, which is of utmost importance in recent times to defeat the worldwide As³⁺ contamination.

Fig. 4 Pictorial view of the quantitative detection of As³⁺ in terms of RGB values.

3.5. Development of an As³⁺ Sensor Kit (ASK) and a Smartphone-coupled diagnostic sensor station for As³⁺ quantification in water

The concentration-dependent chromogenic sub-micro molar prompt response of ABH in the presence of As³⁺ instigated us to develop an As³⁺ sensor kit (ASK) [Fig. 5(a and b)] and a smartphone-coupled, RGB-based point-of-care testing (POCT) device [Fig. 6(a, b) and Fig. S9(b), ESI†] in order to acquire a coherent acuity regarding the concentration of As³⁺ in water samples [Fig. S10(a–f), ESI†]. Herein, the chemosensor solution (0.5 mL 10⁻⁴ M) was taken in a vial followed by the addition of 0.3 mL contaminated ground water sample. Instantaneous concentration–dependent chromogenic change has been observed upon shaking the vial for 3–5 seconds. The concentration-specific colors of the sensor-analyte solutions were thereafter employed to create a color chart, so that common people can tally the chromogenic change of the sensor after the addition of As³⁺ in order to be acquainted with the concentration of As³⁺. Accordingly, a sachet pack of arsenic sensor kit (ASK) was developed containing vial 1 (containing a chemosensor solution), vial 2, color chart, working methodology, tissue and soap paper. First, 0.3 mL contaminated water sample will have to be taken within the vial 2 up to the mark. Thereafter, the sensor solution will have to be added from vial 1 to vial 2 to mix the sensor-analyte solution and the observed color change will have to be tallied with the provided color chart. Therefore, rural people can have a preliminary idea about the concentration of As³⁺ in water.

Fig. 5 (a) Arsenic sensor kit (ASK) and (b) different components of ASK.

Fig. 6 (a) Smartphone-coupled diagnostic sensor station for As³⁺ detection; (b) physical image of the fabricated device for the quantitative detection of As³⁺; (c) 3D CAD model of the circuit of the fabricated device; (d) 3D CAD model of the fabricated device; and (e) X-ray view of the fabricated device.

The ASK has further been employed towards the development of a Bluetooth-operated potable smartphone-based POCT device depending upon the RGB color code for rapid and on-field recognition of As³⁺. Unambiguously, the fabricated archetype comprises various unique features such as (i) straight measurement of photophysical parameters using integrated sensors, (ii) wireless connectivity with additional devices and (iii) easy operation. The entire prototype is allied with a TCS sensor and a HC-05 Bluetooth SPP module mounted on a designed circuitry board entrenched with a microcontroller along with the boot loader [Fig. S9(b), ESI† and Fig. 6]. The electronic circuitry was positioned in an acrylic-made black box [Fig. 6(b)] to inhibit the infiltration of the surrounding light towards the color sensor. In this device, the concomitant colorimetric responses of ABH in the presence of diverse concentrations of As³⁺ are recognized by the TCS color sensor and transpired via the integrated circuitry and the final output is observed in terms of RGB values on the smartphone via Arduino Bluetooth Controller by Giumig Apps. The different RGB values corresponding to different concentrations of As³⁺ clearly illustrate the concentration of As³⁺ within a specific water solution [Table 2]. Moreover, this Bluetooth-operated module will further take a cutting-edge outline towards the contactless analysis of As³⁺-like toxic analytes for the sustainability of our society. Therefore, this RGB-based Bluetooth-operated smartphone-coupled POCT device can be deployed along with the sachet kit and the table containing the experimentally observed RGB values of different concentration of As³⁺ in order to batch scale investigation of As³⁺ water samples.

Table 2 Obtained RGB values in the Smartphone-coupled device after consecutive addition of As³⁺ in the sensor

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3.6. Proposition of electronic circuit fabrication

The attention-grabbing reversible outcomes of ABH upon sequential addition of As³⁺ and F⁻ [Fig. S12(a and b), ESI†] were employed to design a specific logical circuitry, which imitates a particular sequence of molecular-level arithmetic calculations. Moleculators can perform logical operations based on one or more chemical inputs and a single output. In this case, numerous spectroscopic upshots of the chemical inputs produce a molecular-level complex logic gate circuitry, which can efficiently resolve assorted computational intricacies. The spectroscopic upshots of ABH with inward guests, As³⁺ and F⁻, are tabulated in Table 3. Several logic functions such as NAND, NOT, and NOR can be obtained by solving the truth table. A permutation of these logic functions formulates fascinating multifaceted circuits [Fig. 7 and Fig. S11, ESI†]. It was evidently observed from the UV-Vis spectra that by the concurrence of F⁻ to the ABH–As³⁺ solution, the peak at 525 nm was omitted with the arrival of the sensor's initial peak at 425 nm, while further addition of As³⁺ again produced the response at 525 nm. Therefore, the inputs are as follows: In1: ABH; In2: As³⁺; and In3: F⁻. In brief, output Y2 will be ON if both inputs 1 (i.e.; sensor) and 2 (i.e.; As³⁺) are present. Thus, in the presence of successful input 3 (i.e.; F⁻) as the inhibitor, it will inhibit the interaction between the sensor and As³⁺ by forming AsF₃: achievement of the inverter, i.e., NOT gate. Truth Table 2 successfully mimicked the AND–NOT–OR for ABH. Hence, the replication of the truth table input along with the output on the suggested logic circuit was accomplished productively.^25,26

Table 3 Truth table for ABH with As³⁺ and F⁻

Input			Output
In1 (ABH)	In2 As^3+	In3 (F^−)	Output Y1 (425 nm)	Output Y2 (525 nm)
0	0	0	0	0
1	0	0	1	0
0	1	0	0	0
0	0	1	0	0
1	1	0	0	1
1	1	1	1	0
0	1	1	0	0
1	0	1	1	0

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Fig. 7 Fabrication of the three-input logic gate mimic ensemble based on NAND–NOT–NOR logic functions for ABH with As³⁺ and F⁻.

3.7. Cyclic voltammetric titration of ABH with As³⁺

The electrochemical experimentation of ABH with As³⁺ was accomplished in MeOH in an ambience with TBAPF₆ as the supporting electrolyte, which showed noticeable alteration of the chemosensor's inherent oxidation-reduction potential upon addition of As³⁺. In the absence of As³⁺, the sensor showed quasi-reversibility, while upon addition of As³⁺ (120 μL), the anodic potential at 0.85 V (current height 0.013 mA) and 0.35 V (current height 0.0072 mA) was diminished with the generation of a new peak at 0.52 V (current height 0.0165 mA), indicating the oxidation of the probe upon addition of As³⁺. Additionally, the cathodic potential at 0.7 V (current height 0.0031 mA) and 0.35 V was obtained, signifying the feasibility of the corresponding reduction of the system in the cathode after binding As³⁺ [Fig. S13 and Table S5, ESI†].

3.7. Infrared spectroscopic experimentation of ABH with As³⁺

The infrared spectroscopy of ABH was performed in order to have an inclination regarding the host–guest interaction mode. The sensor exhibited a peak at 3408 cm⁻¹, owing to the N–H stretching vibration of secondary amines and the appearance of a sharp peak at 3074 cm⁻¹ may be attributed to the Ar–CH stretching vibration,²⁷ while the peaks at 1627 cm⁻¹, 1557 cm⁻¹ and 750 cm⁻¹ were attributed to the stretching vibration of –C=C, –C=N and C–S bonds respectively [Fig. S14, ESI†]. With the addition of As³⁺ to ABH, the characteristic peaks in the aromatic region (1450–1650 cm⁻¹) were diminished due to interaction with As³⁺via N and S atoms of the probe. Moreover, the reduction of peak intensity in the specific region (1700 cm⁻¹–1000 cm⁻¹) signified the electronic allocation in the host–guest system with the addition of As³⁺ [Fig. S15, ESI†].^8,28,29

3.8. ¹H-NMR titration of ABH with AsI₃

The ¹H-NMR experiments of ABH with As³⁺ were performed in MeOD and D₂O solvents with 10⁻⁴ M of the sensor and 10⁻⁴ M targeted analyte solutions in order to affirm the plausible mode of interaction involving ABH and As³⁺. The outcome of this study remains in a close-knit with all the spectroscopic and electrochemical experiments [Fig. 8]. With the addition of As³⁺, benzothiazole-functionalized aromatic ring protons (δ 8.85 ppm and δ 9.35 ppm) started shifting towards the downfield region, signifying the movement of charge density from ABH towards As³⁺, while the decrease in the –NH proton peak signified the interaction between As³⁺ with the nearby imine N and benzothiazole ring N centres.^30,31

Fig. 8 ¹H-NMR titration of ABH with As³⁺ (molar ratio of L to As³⁺; 5 : 3) in MeOD-D₂O.

3.9. Theoretical substantiations

DFT computation was accomplished in the Turbomole (v7.0) in order to expand an obvious perception concerning the plausible mechanistic interaction among ABH and As³⁺, which were completely in proportion to the experimental findings. In the case of 1 : 1 ABH: As³⁺ adducts, the energies and the HOMO–LUMO gap of the host–guest moiety (1.579 eV) seemed to be inferior than those of the sensor molecule (3.012 eV), suggesting a strong host–guest interaction [Fig. 9]. The interaction energy was evaluated using the following equation:

ΔE_int = E (A, B) − [E(A) + E(B)]

where E(A, B) is the energy of the ABH: As³⁺ adduct, E(A) is the energy of ABH and E(B) is the energy of AsI₃. At first, the geometry of ABH was optimized having an energy of −2258.23 kJ mol⁻¹ prior to interaction with As³⁺ and the optimized energy of AsI₃ was found to be −2270.07 kJ mol⁻¹. However, after interaction of ABH with As³⁺, the interaction energy (ΔE_int) was found to be −1412.62 kJ mol⁻¹, implying a spontaneous chemical interaction [ESI† and Table 4]. Moreover, the interaction of ABH with As³⁺ and other interfering lethal trivalent cations was executed. ABH:As³⁺ adduct formation was obtained as energetically the most promising among other trivalent cationic adducts with ABH [Fig. S16, ESI†], which indicated the selectivity of As³⁺ among other trivalent congeners. The Lowdin population analysis in the DFT study reflected that addition of 1 equivalent As³⁺ resulted in diminution in electron density on nitrogen atoms of exo-cyclic and endo-cyclic (benzothiazole part) –C=N units, and the electron density on S atoms of the benzothiazole unit remained slightly rehabilitated, reflecting the feasibility of bis-coordinated N,N-chelate formation of the benzothiazole derivative with As³⁺. Furthermore, electron density on the nitrogen atom of acrydine part slightly altered due to the drifting of the electron cloud towards arseno-chelate [Table S8, ESI†].

Fig. 9 Geometrically optimized structures of ABH with As³⁺.

Table 4 Theoretical upshot of ABH: As³⁺ interaction

	Total energy (KJ mol^−1)	HOMO−LUMO gap (eV)	ΔEint = E (A, B) – [E(A) + E(B)] (KJ mol^−1)
ABH	−2837.8307	3.012	—
ABH:As^3+	−3694.4551	1.579	−1412.62

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3.10. Riddance of As³⁺ in adulterated water

The complexation capability of ABH with As³⁺ instigated us to check its mitigation potential of As³⁺ in contaminated water. Initially, ABH resulted in a colorimetric change from yellow to burgundy upon addition of As³⁺ in water, while after a few hours nucleation followed by precipitation was observed at the vial, keeping the upper surface of the solution colorless. The UV-Vis and ESI-MS spectroscopic studies of the complex confirmed the ABH–As³⁺ complexation. Therefore, As³⁺ ions can be removed by filtering the As³⁺ complex from the solution. Furthermore, atomic absorption spectroscopy of the filtrate confirmed that 90% subsidence of As³⁺ occurred after separating the ABH: As³⁺ adduct [Fig. 10].

Fig. 10 Removal of As³⁺ in water by ABH.

3.11. Comparative literature survey

In order to explore the advantage of this chemosensor, a thorough literature study was carried out. From the hitherto explored reports, it is observed that most of the probes showed selectivity towards As⁵⁺ and remained silent towards As³⁺, while As³⁺ is more toxic than As⁵⁺. Moreover, some of the synthesized probes showed equal selectivity towards trivalent cations, not specific towards As³⁺. Though plethora of As³⁺ detecting probes have been synthesized till date, aqueous-phase detection of As³⁺ still can be regarded as a challenge. Ground water contains AsO₂⁻, AsO₄⁻³, As³⁺ and As⁵⁺. Previously, some probes were designed in order to detect AsO₂⁻ (anionic form), which failed to detect As³⁺ (cationic form). Somewhere, expensive precursors such as gold salt and environmentally toxic materials were used in order to develop target-specific chemosensors. The newly synthesized chemosensor ABH (i) can detect As³⁺ from the aqueous phase using a smartphone-coupled device, (ii) can act as an efficient pH sensor by showing halo-chromism and (iii) can detect the solvent purity due to the solvatochromic behavior. Moreover, the newly synthesized chemosensor is nontoxic, inexpensive and shows almost 5 cycles of reversibility with F⁻ [Table S9, ESI†].^31–39

Conclusion

Briefly, the newly synthesized inexpensive chemosensor ABH can detect highly toxic As³⁺ in water with a detection limit (LOD) of 5.4 ppb. The solvatochromic behavior of the newly synthesized chemosensor makes it more efficient to verify the solvent purity, while the halochromism of the probe permits us to treat it as a pH sensor for lower pH values. Furthermore, a logic gate imitating electronic circuitry has been formulated by exploiting the successive reversibility of ABH with As³⁺ and F⁻, which further attributes the effective detection efficiency of the probe towards venomous As³⁺ and F⁻ reversibly. In addition, the fabricated arsenic sensor kit (ASK) and smartphone-coupled diagnostic sensor station introduced the chemosensor as a value-added product in our society, which is explicitly unique of its kind. Moreover, this inimitable probe possesses the integral efficiency towards simultaneous detection and removal of toxic As³⁺ as a result of its complexation ability with As³⁺, which made it a smart and efficient material for aqueous-phase elimination of As³⁺.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors sincerely acknowledge DST-DAAD funded project GAP-228512. Authors are thankful to Mr Abhijit Hazra, Research scholar, CSIR-CMERI, for his scientific inputs in designing the sensor molecule. AM is thankful to UGC for her UGC sponsored NFSC fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2107203. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ma00309k

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