Selective and sensitive colorimetric detection of Hg(II) in aqueous solution by quinone-diimidazole ensemble with mimicking YES-OR-INHIBIT logic gate operation

Sensors and Actuators B 237 (2016) 284–290

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Selective and sensitive colorimetric detection of Hg(II) in aqueous solution by quinone-diimidazole ensemble with mimicking YES-OR-INHIBIT logic gate operation C. Parthiban, Kuppanagounder P. Elango ∗ Department of Chemistry, Gandhigram Rural Institute, Deemed University, Gandhigram, 624302, India

a r t i c l e

i n f o

Article history: Received 20 April 2016 Received in revised form 11 June 2016 Accepted 14 June 2016 Available online 16 June 2016 Keywords: Mercury Sensors Colorimetric Quinone Complex

a b s t r a c t A quinone-diimidazole ensemble (R1) has been prepared and characterized. The receptor exhibits a striking colour change from brown to blue instantaneously with Hg(II) ion in DMF:Water (1:9 v/v) selectively and sensitively. Electronic, fluorescence and 1 H NMR titration experiments show that the mechanism of sensing involves the formation of [Hg(R1)2 Cl2 ] complex having binding constant 6.31 × 105 M−1 . The product of the reaction is characterized using LCMS and FT-IR spectroscopy which suggests the coordination of R1 to Hg(II) ion through imidazole N-atom. The addition of Hg(II) to R1 leads to fluorescence quenching with limits of detection 10 nM. The concatenation of YES-OR-INHIBIT combinational logic circuit has also been simulated and its truth table is verified successfully. © 2016 Elsevier B.V. All rights reserved.

1. Introduction During recent past, the development of sensors for the recognition of biologically and environmentally important metal ions has attracted much attention from environmentalists and chemists [1,2]. Among the ions, mercury is one of the most important analyte because of its toxicity and harmful effects on human health as it arises from a variety of natural sources [3,4]. It is well known that mercury can be absorbed through the skin, mucous membrane and respiratory system and cause serious health problems including DNA impairment, dysfunction of liver and kidney and permanent lesions of the central nervous system [5,6]. According to the World Health Organization (WHO) the permissible limit of mercury in drinking water is 0.001 mg/L [7]. Hence, development of simple, selective and sensitive colorimetric sensors for Hg(II) ion in aqueous solution is very much necessary as they would be required for rapid and on-site detection of this metal ion. Review of the literature revealed that a wide variety of fluorogenic and chromogenic receptors has been published for the recognition and sensing of Hg(II) ion based on chemodosimeters [8,9], chemical bond formation [10], H-bonding interaction [11], complex formation [12] and organometallic compound formation

∗ Corresponding author. E-mail address: [email protected] (K.P. Elango). http://dx.doi.org/10.1016/j.snb.2016.06.085 0925-4005/© 2016 Elsevier B.V. All rights reserved.

[13] etc. In continuation of our recent efforts in the development of quinone based receptors for sensing anions [14–16] and cations [13,17], here in this article we report a quinone-diimidazole ensemble (R1) as a selective and sensitive sensor for Hg(II) ion in aqueous solution. The selection of R1 for the study is based on the facts that: i) imidazole N-atom is a well-known ligating atom in the formation of metal complexes [18,19], ii) recently few reports have also been published in which the second N-atom (N-H) of the imidazole ring coordinated to Hg(II) ion after deprotonation [20,21] and iii) any small electronic perturbation that occurs on these two Natoms as a result of coordination to the metal ion would alter the intramolecular charge transfer (ICT) transition, that exists between the N-atoms and the electron deficient quinone moiety [22], significantly and consequently impart striking colour change that can be seen visually. Further, in the absence of d–d transitions in Hg(II) complexes, the colour of them is mainly due to intra ligand charge transfer and/or metal-to-ligand charge transfer transitions, for which this type of ligands will serve the purpose. The main objective, therefore, of the present endeavor is to investigate the Hg(II) ion sensing behavior of R1 in aqueous solution using several spectral techniques such as UV–vis, fluorescence, 1 H NMR and ESImass. Molecular logic gates are an interesting and hopeful research topic in chemistry for further contraction in information technology. In recent years, various molecular logic devices, such as logic gates (AND, OR, XOR, INHIBIT, NAND, half-adder, half subtractor)

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Scheme 1. Synthesis of receptor R1.

[23,24] and information storage devices [25,26], have been widely explored. Thus, in the present study, we have made an attempt to utilize YES logic gate combined with OR-INHIBIT logic gate functions for the detection of Hg(II) ion using R1.

practical application of R1 in the detection of Hg(II) ion real water samples.

2. Materials and methods

The metal ion recognition capability of R1 was carried out for diverse metal ions in DMF:Water (1:9 v/v) solution. Upon the addition of Hg(II) ion, the colour of the solution of R1 changed remarkably from brown to blue instantaneously (Fig. 1). However, the colour of the solution of R1 remained unchanged after the addition of other common cations such as Na(I), K(I), Mg(II), Ca(II), Ba(II), Fe(III), Co(II), Ni(II), Cu(II), Pb(II), Cd(II), Zn(II), Al(III), Fe(III), Cr(III) and Sn(IV) even in large excess (10 equivalents). It is interesting to note that similar colour change was observed upon addition of Hg(II) ions to R1 in DMSO:Water (1:9 v/v) also (Fig. S5). However, all the further woks were carried out only in DMF:Water (1:9 v/v) solution. This indicated the high selectivity of R1 towards Hg(II) ion. This prompted us to further investigate the selective sensing of Hg(II) ion by R1 using various spectral techniques. The colour changes observed upon addition of one equivalent of Hg(II) ion to R1 at different pH values in 90% aq. DMF is depicted in Fig. S6. As seen from the figure that at pH 4 the receptor failed to impart any colour change upon addition of Hg(II) ion. This may be due to the protonation of the imidazole N-atom which in turn prevents the coordination of the receptor to Hg(II) ion through the N-atom. In the pH range 5–7, the receptor exhibited a clear colour change from brown to blue with Hg(II) ions. While, in the basic pH range (8–9) the receptor itself is blue in colour which is due to the H-bond formation between the imidazole N-H and hydroxide ions [27]. However, upon addition of excess water to the solution of R1 in pH 8–9 range the blue colour disappeared, as expected. Whereas in the case of R1-Hg(II) mixture, in the basic medium, the colour was not decolorized. These observations indicated that the receptor senses Hg(II) ions in a wide pH range (5–9).

2.1. Chemicals All the chemicals were purchased from Sigma-Aldrich and were used as received. Commercially available spectroscopic grade solvents (Merck, India) were used as received. The solutions of cations were prepared from their analytical grade chloride salts. Double distilled water was used throughout the work and the second distillation was carried out using alkaline permanganate. 2.2. Instrumentation UV–vis spectral studies (V 630 JASCO, Japan) were carried out in a double beam spectrophotometer using 1 cm matched quartz cells. Steady state fluorescence spectra was obtained on a Cary eclipse fluorescence spectrophotometer (Agilent technologies). Nuclear magnetic resonance spectra was recorded in DMSO-d6 (Bruker, 1 H NMR 300 MHz & 13 C NMR 75 MHz). The 1 H NMR spectra data are expressed in the form: chemical shift in units of ppm (normalized integration, multiplicity and the value of J in Hz). 2.3. Synthesis of the receptor (R1) The receptor R1 was synthesized and characterized as reported by us earlier [27]. A mixture of the amine (1 mmol) and the aldehyde (2 mmol) in DMSO (5 mL) was heated at 80◦ C with stirring for 12 h. After cooling to room temperature, the precipitate obtained from the reaction mixture was filtered through a filter paper and washed with cold ethanol to obtain the pure product (Scheme 1). The receptor R1 was characterized using 1 H NMR and LCMS spectral techniques. The results obtained are: Brown solid (0.4 g, yield = 42.8%). ␦H (300 MHz; DMSO-d6 ; Me4 Si): 7.09-7.23 (m, 8H), 7.42-7.56 (m, 6H), 7.78 (s, 2H), 7.93-7.95 (d, 2H, J = 7.5 Hz), 14.32 (s, 2H); (Fig. S1), 13 C NMR (DMSO-d6 , 75 MHz) ı 181.20, 155.50, 150.52, 140.14, 135.22, 134.44, 133.16, 130.58, 127.62, 126.68, 124.95, 124.22, 114.13, 113.53, 112.55; (Fig. S2), LCMS (ESI-APCI) m/z: Calcd. for C32 H20 N4 O4 [M-H]+ : 524.15; Found: 523.20 (Fig. S3), HRMS (ESI, m/z): [M + H]+ = 525.1543 (calculated as 525.1557 for [C32 H20 N4 O4 ]) (Fig. S4), m.p. 215–217 ◦ C. 3. Results and discussion The quinone-diimidazole ensemble (R1) was prepared and characterized. The Hg(II) ion sensing property of R1 was investigated in DMF:Water (1:9 v/v) solution using various spectral techniques such as UV–vis, fluorescence and 1 H NMR. The product of the reaction was isolated and characterized using ESI-Mass and FT-IR spectral techniques with an aim to validate the proposed mechanism of sensing. Test strips were also prepared to demonstrate the

3.1. Visual detection

3.2. UV–vis spectral studies The electronic spectrum of R1 was recorded in DMF:Water (1:9 v/v) and is depicted in Fig. 2. The peak at 495 nm corresponds to the n → ␲* type ICT transition from N-atom (donor) to quinone moiety (acceptor) [27]. With an aim to confirm the selectivity of the receptor R1 towards Hg(II) ion, the electronic spectra of R1 was also recorded in the presence of various metal ions (Fig. S7). It is evident from the spectra that a dramatic red shift of the absorption maximum (␭ICT ) was observed only in the presence of Hg(II) ion, which is consistent with the results of the visual detection experiments. The binding properties of R1 with Hg(II) ion were further studied using UV–vis titration experiments. As shown in Fig. 2, with the addition of incremental amounts of Hg(II) ions into the DMF:Water (1:9 v/v) solution of R1, the peak at 495 nm diminished gradually, while the absorbance of a new peak at 614 nm increased with an isosbestic point at 544 nm. The observed spectral change is accompanied by the change of colour of the solution from brown to blue. The occurrence of an isosbestic point indicated the formation of a complex between R1 and Hg(II) ion [15]. The association constant

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Fig. 1. Colour change of R1 in DMF: Water (1:9 v/v) solution in presence of various metal ions.

Fig. 2. UV–vis absorbance changes of R1 (6.25 × 10−4 M) upon addition of Hg(II) ions (0–6.25 × 10−5 M) in DMF: Water (1:9 v/v).

Fig. 3. UV–vis absorbance intensity response of R1 upon addition of Hg(II) in the presence of other interfered metal ions in DMF: Water (1:9 v/v).

for the complex was determined using Scott equation (Fig. S8) [28] and it was found to be 5.56 × 104 M−1 . Competitive experiments were conducted in the presence of Hg(II) mixed with 10 equivalents of other chosen metal ions. The results shown in Fig. 3 indicated that no significant variation in the absorbance of Hg(II) solution was found in the presence and absence of other metal ions. These results indicated that the recognition of Hg(II) by R1 is not influenced by the coexisting metal ions and R1 exhibited high selectivity towards Hg(II) ion. The stoichiometry of the receptor-Hg(II) ion complex was ascertained using the continuous variation method (Job’s plot) and it strongly suggested a 2:1 [R1:Hg(II)] complex formation between the receptor and Hg(II) ion (Fig. 4) [29]. 3.3. Fluorescence spectral studies The fluorescence response of R1 in DMF:Water (1:9 v/v) solution towards increasing amounts of Hg(II) ion was also examined and it is given in Fig. 5. The fluorescence spectrum of R1 showed a strong emission band at 629 nm when excited at ␭ICT (495 nm) with a large Stoke’s shift (134 nm), which indicated the existence

Fig. 4. Job’s plot of R1 upon addition of Hg(II) ions.

Fig. 5. Fluorescence changes of R1 upon addition of Hg(II) ion in DMF: Water (1:9 v/v).

of a very competent ICT transition in the molecule as discussed in the electronic spectral studies [13]. Upon the addition of incremental amounts of Hg(II) to the solution of R1, the fluorescence at 629 nm was quenched indicating the formation of a complex between the receptor and Hg(II) ion [22]. The binding constant (Ka ) for the R1-Hg(II) complexation was determined using the fluorescence titration data [30] and it was found to be 6.31 × 105 M−1 (Fig. S9). To investigate the practical applicability of R1, the detection limit of this sensor system was also studied. The fluorescence titration data indicated that there exists a good linear correlation (r = 0.99) between the fluorescence intensity and [Hg(II)] (Fig. S10). The detection limit (S/N = 3) was calculated to be 10 nM [31]. Thus, the receptor R1 can be used to detect Hg(II) ions in aqueous solution at a concentration lower than the permissible limit of Hg(II) ion in drinking water fixed by the WHO (0.001 mg/L). The quenching effect of Hg(II) ion on the emission intensity of the receptor was also confirmed by determining the fluorescence quantum yield (Ф) of the receptor in the absence and presence of Hg(II) ion. The Ф values of the free receptor determined using quinine sulfate as standard were found to be 0.4083. The quantum

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Fig. 6. Partial 1 H NMR spectra of R1 with (a) 0, (b) 0.25, (c) 0.5, (d) 1 equiv. of Hg(II) in DMSO-d6 .

yield was found to decrease from 0.4083 to 0.2722 upon addition of one equivalent of Hg(II) ion to R1 (Fig. S11–S13). Parallel observations were made by Srivastava et al. [32], Zhou et al. [33], Li et al. [34] and Srivastava et al. [35] in similar Hg(II) sensing studies. 3.4.

1H

NMR titration study

As spelt in the introduction section Hg(II) ion can coordinate to either of the two imidazole N-atoms. Recently, Navneet et al. [20] and Hu et al. [21] have reported that the Hg(II) ion coordinated to the N-atom of the imidazole N-H group after deprotonation in similar Hg(II) ion sensing studies. In the present case, in R1, since the imidazole N-H group is directly attached to an electron deficient quinone moiety, the intensity of ICT transition would be higher and consequently the N-H proton is more acidic (␦H = 14.32 ppm) than that of benzimidazole (␦H = 12.53 ppm) [36]. Hence, it is expected that such an N-H group can be deprotonated by Hg(II) ion relatively more easily. Thus, to elucidate the mechanism of binding of Hg(II) ion with R1, 1 H NMR titration experiments in DMSO-d6 were performed and the results obtained are depicted in Fig. 6. It is evident from the figure that the addition of one equivalent of Hg(II) ion to R1, the signal corresponds to the N-H proton does not disappear but rather experience a significant downfield shift with broadening (␦H ∼ 0.52 ppm) i.e. it became relatively more acidic [37]. This may be due to the fact that coordination of Hg(II) to the bare Natom might remove the electron density on that N-atom and thus reduced the ICT transition between it and the quinone. This would enhance the ICT transition from the other N-atom (N-H) to the quinone and consequently make it relatively more acidic. Thus, the mechanism of sensing of Hg(II) ion by R1 involves the coordination of R1 to Hg(II) ion through the bare N-atom of the imidazole ring with a 2:1 [R1:Hg(II)] stoichiometry (Scheme 2). The change of colour from brown to blue observed upon the complex formation between the R1 and Hg(II) ion may be due to the ligand-to-ligand charge transfer (LLCT) transition in the complex [13]. In order to support the proposed mechanism of sensing, the product of the interaction, the R1-Hg(II) complex, was isolated and characterized using FT-IR and ESI-mass spectral techniques. The FTIR spectrum of free R1 showed characteristic bands for imidazole C N and quinone C O at 1587 and 1665 cm−1 , respectively (Fig. 7 and S14). In the FT-IR spectrum of the complex, the band corre-

Fig. 7. Partial IR spectra of R1 and Hg(II) complex.

sponds to the imidazole C N was shifted to 1615 cm−1 indicating coordination of the N-atom (C N) to the metal ion [38]. In free R1, since the two imidazole rings are attached to the benzoquinone ring in a symmetrical manner, the band corresponds to the two C O stretching appeared as a single band. However, in the complex, after coordination of Hg(II) to one of the imidazole N-atoms, the band corresponds to (C O) undergoes splitting [39]. The ESIMass spectrum obtained for the product (Fig. 8) confirmed that the complex formed during the sensing process is [Hg(R1)2 Cl2 ] with a mass Cal. for C64 H40 Cl2 HgN8 O8 , 1320.56 and found, 1321.32 for [M + H]+ . It is interesting to note that the complex prepared from R1 and Hg(II) in ethanol (Scheme S1) also exhibited similar FT-IR and ESI-Mass spectral behavior (Fig. S15). 3.5. Combinational logic circuit Recently, molecular logic gates and its integrated operations have attracted remarkable attention due to its wide applications such as molecular switches, keypad devices, sensing, diagnostics, information processing and storage. A simple molecule with its interesting spectroscopic results mimics the logic operations (YESOR-INHIBIT). In particular, a system has some chemically encoded information as input and UV–vis or fluorescence as output [40,41]. In the present study, different modes were spotted when the receptor R1 was treated with various cations such as Hg(II), Zn(II) and

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Scheme 2. Mechanism of sensing.

Fig. 8. ESI–Mass spectrum of Hg(II) complex.

Cd(II). These chemical inputs and optical outputs were coded as binary digits (0, 1) which are denoted as ‘off’ and ‘on’ respectively. The chemical inputs were designated as In Hg(II), In Zn(II) and In Cd(II). For the receptor R1, the absorbance and emission were recorded at 614 and 629 nm, respectively. The absorbance threshold was set as 0.38 and fluorescence threshold was 405. When the optical output was higher than the threshold value, the output was recorded as ‘1’ (ON state). Otherwise it was accounted as ‘0’ (OFF state). The optical outputs were obtained (in the form of truth table) upon eight combinations (23 = 8) of three inputs (In Hg(II), In Zn(II) and In Cd(II)). In the absence of chemical input both absorbance (Output-1, ␭abs = 614 nm) and fluorescence (Output-2, ␭em = 629 nm) were accounted as ‘0’ because both absorbance band and fluorescence peak were recorded below the threshold value. However, the addition of Hg(II) to the receptor R1 produced the absorbance band at 614 nm (output-1) in the formation of complex between R1 and Hg(II). As the significance of the UV–vis behavior of R1 towards the chemical inputs rendered the molecular system of R1. The output 1 provides a structure that mimicking YES logic function. On the other hand, the addition of chemical input Hg(II) to the R1 was quenched the fluorescence intensity below the threshold level and the output −2 (␭em = 629 nm) were credited as ‘0’ (i.e. OFF state). Furthermore the addition of Zn(II) and Cd(II) were displayed a fluorescence peak above the threshold level and recorded as ‘1’. The fluorescence behavior of R1 mimic the OR logic gate with INHIBIT logic function. From the obtained optical outputs (truth table), we have constructed a combinational logic circuit with three inputs [In Hg(II), In Zn(II) and In Cd(II)] and two outputs (Output-1, ␭abs = 614 nm, Output-2, ␭em = 629 nm) (Fig. 9). The addition of Hg(II) monitors the output-1 (␭abs ) and leads to a YES logic function. However, output-

2 (␭em ) realizes that the OR logic gate with INHIBIT logic function. The concatenation of YES-OR-INHIBIT combinational logic circuit has been simulated and its truth table is verified successfully (Fig. S16). 4. Practical application A simple and selective colorimetric receptor R1 for Hg(II) ion was developed and the practical application of the receptor was investigated by preparing easy-to-use test strips. The strips were prepared by immersing Whatman filter paper into DMF:Water (1:9 v/v) solution of R1 and dried in air. As shown in Fig. 10 when the test strips were dipped in solutions of different concentrations of Hg(II) ion prepared using deep well waters. The colour of the strips was instantaneously changed from brown to blue. Thus, the test strips proved the conceivable sensing of Hg(II) ion visually and colorimetrically in real life applications. 5. Conclusion A quinone-diimidazole ensemble has been prepared and characterized using 1 H NMR and LCMS spectral techniques. The cation sensing property of R1 was investigated using visual detection experiments and various spectral techniques (UV–vis, fluorescence and 1 H NMR). Addition of Hg(II) ion to R1 in DMF:Water (1:9 v/v) was found to change the colour of the receptor solution from brown to blue instantaneously and selectively with a concomitant red-shift in the absorbance maximum. Incremental addition of Hg(II) ions to R1 quenched the fluorescence the later indicating the formation of a complex between them. The binding constant of the 2:1 complex was found to be 6.31 × 105 M−1 . The observed

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Fig. 9. (A) Truth table and (B) Combinational Logic Circuit (YES-OR-INHIBIT).

Acknowledgement The authors are highly thankful to the Council of Scientific and Industrial Research, New Delhi for financial support (CSIR No. 02 (0118)/13/EMR-II).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.06.085.

Fig. 10. Colour changes of test papers for detecting Hg(II) ion in aqueous solution with different concentrations.

detection limit of the receptor (10 nM) displayed the high sensitivity of R1 towards Hg(II) ion. 1 H NMR titration indicated that the receptor coordinated to Hg(II) ion through the bare imidazole N-atom and not through the other N-atom (i.e. N-H). The composition of the formed complex was identified to be [Hg(R1)2 Cl2 ] by product analysis using ESI-Mass and FT-IR spectral techniques. A YES-OR-INHIBIT combinational logic circuit has been simulated using absorption and emission spectral data and verified successfully.

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Biographies K.P. Elango received his PhD degree in the area of Coordination Complexes. He is now working as a Professor of Chemistry in Gandhigram Rural Institute-Deemed University. His area of research interests includes development of chemosensors and coordination complexes. C. Parthiban has obtained his Master Degree in Chemistry in 2010 and now is pursuing for his Doctoral degree.