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Original article
2020
:14;
202101
doi:
10.1016/j.arabjc.2020.11.017

A photo-induced electron transfer based reversible fluorescent chemosensor for specific detection of mercury (II) ions and its applications in logic gate, keypad lock and real samples

, , ,
a Department of Chemistry, Karpagam Academy of Higher Education, Eachanari, Coimbatore 641 021, India
b Department of Applied Chemistry, Karunya Institute of Technology and Sciences (Deemed-to-be University), Karunya Nagar, Coimbatore 641 114, India

⁎Corresponding authors. ravisubban@rediffmail.com (S. Ravi), nandhakumar@karunya.edu (R. Nandhakumar)

Received: , Accepted: ,
© The Author(s) 2020
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Abstract

  • A novel bis-rhodanine derived fluorescent receptor R2 design and synthesis have been described.

  • The ‘turn-on’ fluorescence response was exhibited specifically toward Hg2+ over other metal ions by the receptor R2.

  • The complexing stoichiometry of receptor R2 to Hg2+ was estimated to be 1:1 by Job’s plot experiment.

  • The limit of detection of the receptor R2 toward Hg2+ was 7.33 × 10−7 M.

  • The receptor R2 was successfully applied to molecular logic and keypad lock applications and real water samples.

Abstract

A quantitative mercuric detection is very important in the environmental and biological systems. In this paper, we report a novel bis-rhodanine derived fluorescent chemosensor for recognition of Hg2+ ion in DMSO-H2O (1:1) medium. The ‘turn-on’ fluorescence response was exhibited specifically toward Hg2+ over other metal ions by receptor R2 at 410 nm, which could be ascribed to the restriction of photo-induced electron transfer (PET) process and chelation-enhanced fluorescence (CHEF) effect upon the complexation with mercury ions. The complexing stoichiometry of receptor R2 to Hg2+ was estimated to be 1:1 according to Job’s plot experiments. The limit of detection was calculated to be 7.33 × 10−7 M based on the concentration dependent emission changes with good association constant of 8.97 × 104 M−1. As a result, we express that receptor R2 is a promising candidate for Hg2+ recognition without any significant interference of other co-existing cations. The receptor R2 was successfully applied to molecular logic and keypad lock applications and real water samples to determine its potential applications.

Keywords

Fluorescence
Rhodanine
Chemosensor
Mercury recognition
Reversible
Molecular mimicking
1

1 Introduction

Mercury (II) is the one of the largely dispensed toxic metal ions of any foreign species in the ecosystem and environment area (Renzoni et al., 1998; Benoit et al., 1998; Burg, 1995; Clarkson et al., 2003; Harada, 1995; Joshi et al., 2012). It has been widely employed for the different industrial objectives, considerably enlarging the feasibility of atmospheric pollution and the likelihood of human exposure (Zahir et al., 2005., Grandjean et al., 1998; Matsumoto et al., 1965). Through the skin, respiratory or digestive system, mercury and its derivatives can invade living system and could create acute poisoning, respiratory tract infections, infectious pneumonia, chronic mercury poisoning, chronic mental illness, etc. (Nolan and Lippard, 2008; D’ltri et al., 1978; Czarnik, 1993; Kim et al., 2012; Behta and Ahmed, 2018). The Hg2+ accumulation can trigger cell dysfunction which eventually leads to miscellaneous health effects such as neurotoxicity, nephrotoxicity, hepatotoxicity, etc., in human and animal biological system due to the thiophilic nature of Hg2+ upon complexation with sulphur containing amino acids in proteins and enzymes (Silbergeld et al., 2005; Hoyle and Handy, 2005; Boening, 2000). In the 1950′s, the fatal Chisso-Minamata disease (a methylmercury poisoning) was caused by the daily consumption of fish and shell fish from the mercury contaminated waters (Ha et al., 2017; Eto et al., 2002; Mahaffey et al., 2004; Raju et al., 2020). Moreover, mercury pollution has become a global concern owing to its non-biodegradable and bio-accumulative character in the nature and living environment (Wang et al., 2020; Prabhu et al., 2014). According to the world health organization (WHO) report, the maximum permissible limit of Hg2+ in drinking water is 6 ppb (WHO., 2008). Therefore, the selective, reliable, and rapid recognition procedure for Hg2+ ions is of important challenge to the scientific and environmental bodies (Zhang et al., 2018; Jiang et al., 2018).

A variety of analytical techniques have been followed to determine the metal ion concentrations including, voltammetry, atomic absorption spectroscopy, plasma-atomic emission spectrometry, potentiometry, inductively coupled plasma mass spectroscopy, electrothermal atomic absorption spectroscopy, and fluorescence spectroscopy (Prabhu et al., 2019; Suresh et al., 2019; Lum and Leung, 2019; Feichtmeier and Leopold, 2014; Omidi et al., 2015; Bhuvanesh et al., 2018a, 2018b; Tan et al., 2012; Wygladacz et al., 2005; Liang et al., 2004; Chen and Zhu, 2005; Baron et al., 2000; Garcia et al., 2003; Singh and Pambid, 1990; Ndung’u et al., 2006; Yang and Sturgeon, 2002; Katarina et al., 2006; Bothra et al., 2017; Shamsipur et al., 2002). Unfortunately, fluorescent based chemosensors have captivated most of the researchers because of the accuracy in measurement, ease experimentation, rapid responses, and real-time monitoring in the practical area of recognition of metal ions (Li et al., 2018; Hu et al., 2016; Chen et al., 2019; Yang et al., 2015; Nandre et al., 2017; Goyal et al., 2010; Hu et al., 2020; Sahu et al., 2020). Despite, there are some limitations still occur in fluorescent based chemosensors such as tedious synthetic procedure, incapable detection in various solvent medium, turn-off response, longer detection time span, irreversible feature, and weak fluorescence in different pH ranges which predominantly influences the chemosensor applications (Ding et al., 2019; Huang et al., 2019; Liu et al., 2018; Ding et al., 2017). Under this context, developing a turn-on fluorescence responsive, profoundly sensitive and selective, cheap, and convenient fluorescent based chemosensor is vital.

Herein, we describe a novel fluorescent “turn-on” response chemosensor (R2) based on rhodanine derivative and 4-nitrobenzaldehyde synthesized via Knoevenagel condensation reaction as depicted in Scheme. 1. Rhodanine has significant number of heteroatoms (S, N, and O) in their structures which can chelate with metal ions very easily to provide some interesting electronic properties. These properties are helpful for the development of fluorescent based chemosensor for different analytes (Chen et al., 2016; Kundu et al., 2013). The sensing behavior of receptor (R2) was monitored by photoluminescence spectroscopy, surprisingly toward Hg2+ ion over other metal ions through photo-induced electron transfer (PET) and chelation-enhanced fluorescence (CHEF) mechanisms with the complete reversibility upon EDTA in DMSO-H2O (1:1) medium. Accordingly, the reversibility fluorescence switching mechanism of receptor R2 from Hg2+ using EDTA was utilized auspiciously to construct the potential molecular logic mimicking and keypad lock activities. Furthermore, the designed receptor R2 has specific nature to scavenge the Hg2+ ions in DMSO-H2O (1:1) medium without any disturbance of other cations and can be perturbed to detect trace amount of Hg2+ in real water samples.

Synthesis of receptor R2.
Scheme 1 Synthesis of receptor R2.

2

2 Experimental section

2.1

2.1 Apparatus and materials

All reagents and solvents were purchased from commercial suppliers in analytical and spectroscopic grade used without any purification process. 1H NMR and 13C NMR spectra were obtained on Bruker Avance III 400 MHz and 100 MHz spectrometer with TMS as internal standard. UV–vis absorption spectra and fluorescence emission spectra were recorded on Jasco V-730 spectrophotometer and Jasco FP-8200 spectrophotometer at 24 ± 1 °C. A solution of metal ions was prepared from the chloride and nitrate salts of Ag+, Al3+, Ba2+, Bi3+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li2+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sr2+, Zn2+ and Zr2+ in DMSO-H2O, 1:1 (v/v) HEPES buffered solution (50 mM) at pH = 7.4. The receptor R2 was prepared in the concentration of 2 × 10−5 M and 2 × 10−6 M in DMSO-H2O, 1:1 (v/v) HEPES buffered solution (50 mM) at pH = 7.4 for UV–vis absorption studies. And the concentration of receptor R2 was 4 × 10−6 M in DMSO-H2O, 1:1 (v/v) HEPES buffered solution (50 mM) at pH = 7.4 for overall the fluorescence spectral measurements.

2.2

2.2 Synthesis and characterization of receptor R2

The receptor R2 was synthesized via a two-step process, as illustrated in Scheme. 1. 1,2-diamino propane (0.74 g, 0.01 mol) and carbon disulphide (1.52 g, 0.02 mol) were dissolved in the presence of NaOH (0.16 g, 0.04 mol) and sodium chloroacetate (2.4 g, 0.02 mol) and stirred at room temperature for 4 h to yield 3,3′-(propane-1,2-diyl)bis(2-thioxothiazolidin-4-one) or bis-rhodanine as intermediate (3.1 g). It was followed by the Knoevenagel condensation, where the intermediate bis-rhodanine (2.5 g, 0.01 mol) with p-nitro benzaldehyde (4 g, 0.025 mol) in presence of sodium acetate (0.85 g, 0.01 mol) and acetic acid (0.6 g, 0.01 mol) was stirred at room temperature for 6 h. The reaction mixture was then cooled, and the resulted precipitate was filtered and washed with water, dried and recrystallised (ethanol) to afford the corresponding product receptor R2 as a yellow color solid in 65% yield. All the reaction conditions were optimized to afford a good yield. M. p.: 290 °C. 1H NMR (400 MHz, CDCl3, ppm): δ 1.15 (d, J = 6.8 Hz, 3H), 4.07 (d, 1H), 4.8 (d, 1H), 5.4 (m, 1H), 7.9 (m, 2H), 8.1 (m, 8H). 13C NMR (100 MHz, DMSO, ppm): δ 203.3, 201.3, 174.87, 174.62, 151.14, 149.94, 145.7, 143.9, 134.7, 125.38, 51.02, 44.84 and 14.8. Elemental analysis: C23H16N4O6S4; calcd.; C, 48.24; H, 2.82; N, 9.78. found.; C, 48.13; H, 2.92; N, 9.69. (Fig. S1 and S2)

3

3 Results and discussion

3.1

3.1 Fluorescence emission studies

The receptor R2 was made completely soluble in DMSO-H2O, 1:1 (v/v) HEPES buffered solution (50 mM) at pH = 7.4 for overall fluorescence emission studies. The excitation wavelength was fixed at 385 nm for the metal binding fluorescence emission studies, which was confirmed by the absorption spectrum of receptor R2 at various concentrations in DMSO-H2O, 1:1 (v/v) HEPES buffered solution (50 mM) at pH = 7.4 solution (Fig. S3). The fluorometric selectivity experiment was carried out using receptor R2 for the recognition of various metal ions in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) and excited at 385 nm. The miscellaneous metal ions such as Ag+, Al3+, Ba2+, Bi3+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li2+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sr2+, Zn2+ and Zr2+ were employed to investigate the sensing property of the synthesized receptor R2. The receptor R2 (4 × 10−6 M) showed low or non-fluorescence emission above 400 nm (excited at 385 nm) (Fig. 1a). However, upon the addition of various metal ions to the receptor R2 remarkably displayed excellent “turn-on” response toward only Hg2+ ions at 410 nm and the other co-existing metal ions didn’t induce any significant changes in fluorescence emission nature (Fig. 1b). The selective experimental results clearly showed that the receptor R2 can potentially detect Hg2+ ions with good selectivity. In addition, the sensing ability of the receptor R2 utilizing UV–Vis spectroscopy (colorimetric studies) have also been studied in presence of various cations. The experimental results showed no considerable absorbance changes in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) (Fig. S4).

a) Emission spectra of receptor R2 (4 × 10−6 M) b) Emission response of receptor R2 in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) in the presence and absence of 100 equivalents of various metal ions at λem = 410 nm (λex = 385 nm). Error bars indicate standard deviations (5%) from three repeated experiments.
Fig. 1 a) Emission spectra of receptor R2 (4 × 10−6 M) b) Emission response of receptor R2 in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) in the presence and absence of 100 equivalents of various metal ions at λem = 410 nm (λex = 385 nm). Error bars indicate standard deviations (5%) from three repeated experiments.

3.2

3.2 Competition experiment studies

The specific detection of Hg2+ was subsequently performed using the receptor R2 with the excess equivalents of other aggressive metal ions due to the importance in practical applications. To form the system R2 + Mn+, various metal ion solutions (150 equivalents) were added to receptor R2 (4 × 10−6 M) and then the system R2 + Mn++Hg2+ was formed by adding Hg2+ (4 × 10−6 M) into the mixture. As a result, the fluorescence emission intensity has not changed with any notable increment/decrement by all the other background metal ions (Fig. 2). The combined cross-contamination experimental outcomes, further openly explained that the receptor R2 possesses great Hg2+ discriminating character in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) with high specificity and interference-free Hg2+ chemosensor even in the presence of biologically and environmentally dominating metal cations.

Anti-interference studies of receptor R2 (4 × 10−6 M) with Hg2+ ion and other metal ions (150 equivalents) in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm). Error bars indicate standard deviations (5%) from three repeated experiments.
Fig. 2 Anti-interference studies of receptor R2 (4 × 10−6 M) with Hg2+ ion and other metal ions (150 equivalents) in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm). Error bars indicate standard deviations (5%) from three repeated experiments.

3.3

3.3 The binding of receptor R2 with Hg2+

A quantitative experimental study was investigated to understand the fluorescence detection properties of receptor R2 to Hg2+ ions using fluorescence titration profiles. As shown in Fig. 3a, the emission intensity was gradually increased with the increasing concentration (0–100 equivalents) of Hg2+ at 410 nm (λex = 385 nm). The fluorescence emission reached the maximum level, when the concentration of Hg2+ increased to five times higher than the concentration of receptor R2. Moreover, the fluorescence emission of receptor R2 showed good linear relationship in the increasing addition of Hg2+, as shown in Fig. 3b. The limit of detection (calculated using 3δ/S, where δ is the standard deviation of the blank measurements, and S is the slope of linear calibration curve, reported by previous report Immanuel David et al., 2020; Saravanan et al., 2019; Bhuvanesh et al., 2020; Tekuri et al., 2019) was calculated to be 7.33 × 10−7 M, which was lower than the many recently reported chemosensors (Table 1). From this characteristic linear relationship in different concentration of Hg2+, the receptor R2 explore its potential to detect Hg2+ ions quantitatively. The remarkable fluorescence enhancement upon addition of Hg2+ into receptor R2 was attributed to the binding of receptor R2 with Hg2+ through restriction of photo induced electron transfer (PET) process causing the chelation-enhanced fluorescence (CHEF) effect, which results in the stronger emission in fluorescence spectrum.

a) Fluorescence emission spectra of receptor R2 (4 × 10−6 M) upon addition of Hg2+ (0–100 equivalents) in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm) b) Calibration plot of fluorescence intensity vs concentration of Hg2+ at 410 nm upon gradual addition of Hg2+. Error bars indicate standard deviations (5%) from three repeated experiments.
Fig. 3 a) Fluorescence emission spectra of receptor R2 (4 × 10−6 M) upon addition of Hg2+ (0–100 equivalents) in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm) b) Calibration plot of fluorescence intensity vs concentration of Hg2+ at 410 nm upon gradual addition of Hg2+. Error bars indicate standard deviations (5%) from three repeated experiments.
Table 1 Recent literature report on detection of Hg2+.
Receptor type Mode of detection Testing media Response time Ka (M−1) LOD (M) Ref.
Perylene based Turn-on DMSO-H2O (5:1) 1 min – 0.35 × 10−6 M Liu et al., 2020
Anthracene based Turn-on DMSO – 5.35 × 103 M−1 6.34 × 10−6 M Li et al., 2019
Quinoline based Turn-on Aqueous – 6.56 × 103 M−1 1.01 × 10−6 M Paisuwan et al., 2019
Naphthalimide based Turn-off MeCN- H2O (4:1) Less than a minute 3.76 × 104 M−1 0.83 × 10−6 M Ye et al., 2019
Rhodanine based Turn-on MeCN- H2O (9:1) – 2.15 × 104 M−1 3.36 × 10−6 M Bayindir (2019)
Rhodanine based Turn-on DMSO-H2O (1:1) 2.5 min 8.97 × 104 M−1 7.33 × 10−7 M This work

Job’s plot variation technique (Bhuvanesh et al., 2019; Velmurugan et al., 2018) was experimented to confirm the binding stoichiometric interaction of receptor R2 + Hg2+ complex, which displayed maximum emission inflection point at 0.5 mol fraction. This clearly indicates that the coordination stoichiometry between receptor R2 and Hg2+ is 1:1 formation as shown in Fig. 4. The binding constant (Ka) of receptor R2 + Hg2+ complex was estimated to be 8.97 × 104 M−1 and R2 value of 0.9849 by the Benesi-Hildebrand analysis (Velmurugan et al., 2020; Bhuvanesh et al., 2018a, 2018b) based on the fluorescence emission results (Fig. 5). The binding result designated the efficient binding of receptor R2 with Hg2+ in DMSO-H2O media.

Job’s plot of the receptor R2 + Hg2+ complex in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm). Error bars indicate standard deviations (5%) from three repeated experiments.
Fig. 4 Job’s plot of the receptor R2 + Hg2+ complex in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm). Error bars indicate standard deviations (5%) from three repeated experiments.
Binding constant (Ka) of receptor R2 with Hg2+ for 1:1 binding stoichiometry in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm). Error bars indicate standard deviations (5%) from three repeated experiments.
Fig. 5 Binding constant (Ka) of receptor R2 with Hg2+ for 1:1 binding stoichiometry in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm). Error bars indicate standard deviations (5%) from three repeated experiments.

3.4

3.4 Response of time and pH

The response time for the detection of Hg2+ by receptor R2 in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) was examined as illustrated in Fig. 6. It reveals that the fluorescence emission intensity of receptor R2 attained the maximum influence upon binding of 100 equivalents of Hg2+ within 2.5 min and kept not fluctuate for 60 min. Therefore, the satisfactory outcome indicated that the receptor R2 is highly stable and efficient for appropriate and rapid detection of Hg2+ ions in real sample testing. Moreover, to understand the photophysical properties of receptor R2, the effect of pH using fluorescence spectral measurements on receptor R2 in absence and presence of Hg2+ ions were investigated in DMSO-H2O, 1:1 (v/v) medium at 1.0–12.0 pH ranges (Fig. 7). Hydrochloric acid and Sodium hydroxide solutions were used to adjust the acidic and basic pH ranges. The receptor R2 displays low and relatively stable fluorescence emission in acidic, basic and neutral pH ranges. Eventually, the low fluorescence was observed for R2 + Hg2+ complex about 1.0 and 2.0 pH ranges. The emission gradually increased when basicity increases in the system and at the neutral point (7.0), the strongest emission was obtained. Afterwards, the system again started resuming toward weak fluorescence between the pH ranges of 9.0–12.0 due to the lack of stability in complexation. Based on the above pH effect results, the fluorescence detection of Hg2+ and further subjecting the biological assays could be supported by the receptor R2 in a wide range of pH.

The response time study of receptor R2 to Hg2+. Error bars indicate standard deviations (5%) from three repeated experiments.
Fig. 6 The response time study of receptor R2 to Hg2+. Error bars indicate standard deviations (5%) from three repeated experiments.
The fluorescence emission changes of receptor R2 in the absence and presence of Hg2+ at various pH ranges (1.0–12.0).
Fig. 7 The fluorescence emission changes of receptor R2 in the absence and presence of Hg2+ at various pH ranges (1.0–12.0).

3.5

3.5 Reversibility of receptor R2 by EDTA

The reversibility of complexation of receptor R2 toward Hg2+ was analysed by using chelating agent ethylene diamine tetra acetic acid (EDTA) as complexing ligand to determine the reusable property of receptor in the fluorescence detection. The fluorescence emission disappeared and obtained very weak at 410 nm after the addition of EDTA to the receptor R2 + Hg2+ complex solution in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4). This implied that the receptor R2 was regenerated to its original position (fluorescence-off) from receptor R2 + Hg2+ complex formation. In addition, the fluorescence intensity was recovered stronger (fluorescence-on) upon the addition of excess amount of Hg2+ into the R2 + Hg2++EDTA mixture (Fig. 8a), and the combined results demonstrated that the reversible cycle could be unaffected for 5 more alternative additions of Hg2+ and EDTA (Fig. 8b). Hence, this reversible performance of receptor R2 can be applied in the molecular logic gate construction.

a) Fluorescence emission spectra of reversibility of receptor R2 experiment using EDTA b) Reversibility cycle upon alternative addition of Hg2+ and EDTA in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm).
Fig. 8 a) Fluorescence emission spectra of reversibility of receptor R2 experiment using EDTA b) Reversibility cycle upon alternative addition of Hg2+ and EDTA in DMSO-H2O, 1:1 (v/v) HEPES buffer solution (50 mM, pH = 7.4) at λem = 410 nm (λex = 385 nm).

3.6

3.6 Proposed sensing mechanism of receptor R2 to Hg2+

On the basis of analytical and spectroscopic studies, a sensing mechanism of receptor R2 to Hg2+ ion is proposed and shown in Scheme 2. The both carbonyl C⚌O groups of the receptor R2 are involved in the complexation with Hg2+ ions. Therefore, the receptor R2 acts as a fluorescence “Turn-off–on” sensor with respect to Hg2+. The weak fluorescence of the receptor R2 may be largely due to the photo-induced electron transfer (PET) process from the rhodanine moiety to phenyl derivative. Moreover, the receptor R2 was freely movable along the side and unrestricted within the system in connection with C-C covalent linkage between two rhodanine moieties. Since, the absence of structural rigidity in the receptor molecule because of the intramolecular rotation, therein the charge transfer-excited singlet state is abruptly deactivated. However, the strong fluorescence exhibited upon the complexation of receptor R2 with Hg2+ (1:1) due to the chelation-enhanced fluorescence (CHEF) effect, which resulted in the arrest of both PET process as well as C-C rotation that further increases the rigidity of the molecular assembly (Zhou et al., 2012; Maity and Govindaraju, 2011; Velmurugan et al., 2015; Mahajan et al., 2015). Additionally, the proposed coordination is also supported with Job’s plot nonlinear curve fitting analysis with 1:1 complexing stoichiometry for receptor R2 + Hg2+ complex. The color changes of the receptor R2 in presence of various metal ions have been monitored by both naked eye and long UV–Vis (365 nm) light and are shown in Fig. S5.

Proposed sensing mechanism of receptor R2 with Hg2+.
Scheme 2 Proposed sensing mechanism of receptor R2 with Hg2+.

3.7

3.7 Determination of fluorescence quantum yield

The fluorescence quantum yield (Φ) of the receptor R2 and receptor R2 + Hg2+ were derived based on the equation (Chae et al., 2019; Mittal et al., 2016) Φ U = Φ R × F U / F R × A R / A U × η U 2 / η R 2

Here, FU and FR indicate the integrated fluorescence intensity of unknown and reference samples, respectively. AU and AR represent the absorbance of unknown and reference samples, respectively. ηU2 (1.479) and ηR2 (1.33) are refractive indexes of the solvents used to dissolve unknown and reference samples. Quinine sulphate (Φ = 0.54 in water) used as the reference for the measurement. The quantum yield (Φ) of receptor R2 is 0.02, and for receptor R2 + Hg2+ complex, it is found to be 0.26 at 410 nm.

3.8

3.8 IR spectral studies

The IR spectra of receptor R2 and R2 + Hg2+ complex was recorded separately. The receptor R2 exhibits characteristic absorption bands at 1701 cm−1 ascribable to C⚌O stretching and 1195 cm−1 to C⚌S, respectively. The medium absorption band at 2376 cm−1 is due to the C—S stretching vibration. The strong absorptions at 1519 cm−1 (asym), 1257 cm−1 and 1323 cm−1 (sym) corresponding to N⚌O stretching. Upon addition of Hg2+ ion to the receptor R2, the absorption bands acquired at 1701 cm−1 is shifted to 1710 cm−1, which indicates there is a possible coordination between the receptor R2 and Hg2+ ions. (Fig. S6).

4

4 Applications

4.1

4.1 Molecular logic gate

The literature report says, the molecular logic function is the most conducting application and also has captivated great attention among researchers (Erbas-Cakmak et al., 2018; Bai et al., 2020; Ma et al., 2019). The fluorescent reversible switching process of receptor R2 was studied in molecular logic gate mimicking behavior. An INHIBIT logic gate could be built up based on the output of the reversible fluorescence signaling process in receptor R2 + Hg2+ complex upon the addition of strong chelating agent EDTA. Therefore, the logic gate circuit was constructed by using Hg2+ and EDTA as two chemically encoded inputs and the fluorescence response at 410 nm as output (Fig. 9a). Based on the fluorescence spectroscopic data the truth table was constructed. The absence and presence of the chemical inputs (Hg2+ and EDTA) were considered as the binary code ‘0’ and ‘1’ respectively. For the output, the strong fluorescence was indicated as ‘1’ or ON state and the weak fluorescence was indicated as ‘0’ or OFF state during the signaling process (Fig. 9b). Based on the truth table the INHIBIT logic gate circuit was designed. In this system, the strong fluorescence ‘1’ or ON state was generated in the output only with Hg2+ input alone (1,0). While all the other three input combinations were showed exceedingly weak fluorescence signals ‘0’ or OFF state. Hence, the above results plainly proved the formation of INHIBIT logic gate for the robust reversible receptor R2 (Fig. 9c).

a) Fluorescence intensity changes at 410 nm in presence four input combinations. b) Truth table for INHIBIT logic behavior. c) The INHIBIT molecular logic gate of receptor R2.
Fig. 9 a) Fluorescence intensity changes at 410 nm in presence four input combinations. b) Truth table for INHIBIT logic behavior. c) The INHIBIT molecular logic gate of receptor R2.

4.2

4.2 Molecular keypad lock

The proposed molecular model can be used to construct sequence dependent molecular keypad lock based on the specific selectivity and excellent reversibility of receptor R2, Hg2+, and EDTA. Here, receptor R2, Hg2+, and EDTA as the three various chemical inputs and also labeled as ‘R’, ‘H’, and ‘E’ respectively. The probable six input combinations are RHE, REH, HRE, HER, ERH, and EHR. The combination RHE produced maximum fluorogenic output signal, whereas minimum output was unveiled by REH, HER, ERH, and EHR amidst these six input combinations (Fig. 10). Even though output HRE shows moderate fluorescence enhancement, it couldn’t attain the complete “turn-on” mode to unlock the system. As an electronic keypad contains various keys (A-Z), the molecular keypad also restrains different keys and allows the accurate password (RHE) to unlock, wherein, all the other combinations are incorrect to open the locked system. For that reason, this type of sensing-based security systems may be accessible to keep the molecular level information safe (Wei et al., 2016; Vinoth Kumar et al., 2019).

Output for receptor R2, corresponding to probable six chemical input combinations at 410 nm. Inset: A molecular keypad lock triggering fluorescence emission at 410 nm when a correct password is entered, that is, RHE. The keys R, H, and E typify input receptor R2, Hg2+, and EDTA, respectively.
Fig. 10 Output for receptor R2, corresponding to probable six chemical input combinations at 410 nm. Inset: A molecular keypad lock triggering fluorescence emission at 410 nm when a correct password is entered, that is, RHE. The keys R, H, and E typify input receptor R2, Hg2+, and EDTA, respectively.

4.3

4.3 Real sample detection

To investigate the practical applicability of the designed receptor R2 for real sample detection, we evaluated the determination of Hg2+ ions in tap water, drinking water, and sewage water from different sources by using spike and recovery method. For this experiment, water samples were collected from Hepzibah Residence, KITS, Cbe, Tamil Nadu, India (Tap water); and Department of Applied Chemistry, KITS (Drinking water and sewage water). The test was conducted by spiking the known concentration of Hg2+ solution and calculating the recovery. The experimental results for the analytical measurements are given in Table 2. The obtained results were taken from each three repeated measurements in spiked real samples show massive recovery. Therefore, the results noticeably express the practical utility of the receptor R2 for Hg2+ ion detection in real water samples.

Table 2 Detection of Hg2+ in real samples using receptor R2.
Sample Hg2+ spiked (µM) Hg2+ recovered (µM) mean[a] ± SD[b] Recovery (%) Relative error (%)
Tap water-1 5.0 4.85 ± 0.09 97.0 −3.0
Tap water-2 10.0 9.85 ± 0.04 98.5 −1.5
Drinking water-1 5.0 5.04 ± 0.03 100.8 0.8
Drinking water-2 10.0 9.48 ± 0.16 94.8 −5.2
Sewage water-1 5.0 4.92 ± 0.01 98.4 −1.6
Sewage water-2 10.0 9.81 ± 0.19 98.1 −1.9
a Mean of three measurement.
b Standard deviation.

5

5 Conclusion

In summary, we have developed a new fluorescent based chemosensor (R2) for the selective detection of Hg2+ ion over other metal ions with a ‘turn-on’ fluorescence response at 410 nm. The strong fluorescence enhancement noticed was due to the formation of chelation-enhanced fluorescence (CHEF) which blocks the photo-induced electron transfer (PET) process in receptor R2 upon binding with Hg2+. The experimental outcomes demonstrated the 1:1 binding stoichiometry for receptor R2 + Hg2+ complex with low detection limit. Furthermore, the reversible switching process of receptor R2 toward Hg2+ was examined by using EDTA with upto 10 cycles. Finally, based on these experimental results it is concluded that receptor R2 can act as a highly selective and sensitive probe for detection of Hg2+ in logical mimicking and environmental monitoring works. The molecular logic and keypad lock applications along with the real water samples determination were demonstrated as the probe’s applications. Further derivatives of the chemosensor R2 for various other applications such as bioimaging, electrochemical detection along with their possible modes of binding using the Density Functional Theory studies are currently underway in our laboratory.

Acknowledgement

The authors gratefully thank the financial support of the SERB-EMR grant by the DST under sanction number SERB-EMR/2016/005692.

Declaration of Competing Interest

There are no conflicts of interest to declare.

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Appendix A

Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2020.11.017.

Appendix A

Supplementary data

The following are the Supplementary data to this article:

Supplementary data 1


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