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Journal of Photochemistry & Photobiology A: Chemistry

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A simple rhodanine-based fluorescent sensor for mercury and copper: Therecognition of Hg2+ in aqueous solution, and Hg2+/Cu2+ in organic solvent

Sinan BayindirDepartment of Chemistry, Faculty of Sciences and Arts, Bingol University, Bingol, 12000, Turkey

A R T I C L E I N F O

Keywords:RhodanineOxindoleChemosensorCopperColorimetricMercuryTurn-on sensor

A B S T R A C T

Detection of copper and mercury attracts important in most environmental and biological systems. In this study,the simple probe 2-OxI-Rh containing rhodanine core was synthesized by a green approach and sensing prop-erties were studied using colorimetric and fluorometric detection. The research indicated that the specific ionaffinity for Hg2+ ions in aqua systems and the multi-ion affinity for Hg2+ and Cu2+ in organic solvent results indrastic color and spectral changes. According to the data obtained, while the peak intensity increases at 390 nm,the peak intensity decreased at 272 nm in the absorption spectrum of 2-OxI-Rh and an increase in fluorescenceintensity of 2-OxI-Rh were observed in the presence of Hg2+ and Cu2+ ions. The binding ratio of 2-OxI-Rh toHg2+ and Cu2+ were found to be 1:1 according to Job's plot experiments. The binding constants were calculatedusing the Benesi-Hildebrand equation and found to be 2.15×104 M−1 for Hg2+ and 1.21×104 M−1 for Cu2+.Based on these concentration dependent fluorescence changes, the limit of detection (LOD) values were alsocalculated and found to be 3.36 μM for Hg2+ and 2.31 μM for Cu2+, which is the range of copper that should bein the blood (11.8–23.6 μM). As a result of all these studies, we can understand that prove 2-OxI-Rh, which isnon-toxic, is a good selective candidate turn-on sensor that can be used for Hg2+ and Cu2+ detection in differentsolvent systems.

1. Introduction

Ions play an essential role in many chemical processes and livingsystems [1–5]. Among the various transition metal ions, copper andmercury are essential metal elements for human beings, plants, andmicroorganisms, playing a very important role in various fundamentalbiological functions [6]. However, deficiency of these ions may lead tohematologic disorders, and influence the normal metabolism of or-ganisms, and corresponding enzyme activity, whereas excess Cu2+ canexhibit important toxicity and cause neurodegenerative diseases such asAlzheimer's and Wilson's diseases [7–9]. Similarly, among all the heavymetals mercury is one of the most toxic and dangerous metal because ofits strong affinity to S-containing ligands and results in the blocking ofsulfhydryl (–SH) groups in proteins and enzymes [10]. Moreover, Hg2+

is not biodegradable and accumulate in the living bodies, which causesthe dysfunction of cells and results in a wide variety of diseases relatedto the central nervous system [11–13]. Hence, World Health Organi-zation (WHO) and European Union (EU) have kept the strict ban on therelease of mercury into the environment [14–17]. Summarize, copperand mercury ions are considered important environmental pollutantbecause of its widespread use, such as waste from thermal power plants,cement kilns, fossil fuels, agriculture, the paper industry, gold mining

and trash incinerators (hazardous waste, medical waste, etc.) [18–20].Therefore, the environmental monitoring of the mercury and copper isvery much important to avoid further damage to the ecosystem [21]. Avariety of methods for determining metal ion concentrations have beenused including inductively coupled plasma mass spectroscopy, plasma-atomic emission spectrometry, voltammetry, emission, and atomic ab-sorption spectroscopy [22]. Unfortunately, most of these methods arenot appropriate for on-site assays and need expensive devices. There-fore, the design and synthesis of selective fluorescent chemosensorsbased on organic derivatives for the Hg2+ and Cu2+ ions detectionwere attention due to the low cost and high applicability in industrialand biological processes. [23–31]

On the other hand, the class of heterocyclic compounds known asrhodanine and oxindole derivatives are found in many drugs, naturaland synthetic products [32–34]. Different rhodanine and oxindole ringbearing compounds are used in the colorimetric sensor, fluorescent dye,and pharmacological studies [35–38]. Among the reported colorimetricstudies, chemosensors are attracted pretty attention due to specificcolor change by interaction with ions which can be detected by thenaked eye. The heteroaromatic compounds can interact with metal ionsquite easily thanks to the heteroatoms present in the structures [39].Rhodanine and oxindole compounds have the considerable number of

https://doi.org/10.1016/j.jphotochem.2018.12.021Received 23 September 2018; Received in revised form 15 December 2018; Accepted 20 December 2018

E-mail address: sbayindir@bingol.edu.tr.

Journal of Photochemistry & Photobiology A: Chemistry 372 (2019) 235–244

Available online 21 December 20181010-6030/ © 2018 Elsevier B.V. All rights reserved.

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heteroatom (S, N, and O) to may interact easily with the metals. Theseparticular features provide to the rhodanine and oxindole moleculessome interesting electronic properties such as fluorescent and colori-metric [40]. These properties of rhodanine and oxindole enable them tobe useful for the preparation of sensors and imaging agents. Here in,rhodanin-based simple ligands (Z)-3-amino-5-(2-oxoindolin-3-ylidene)-2-thioxothiazolidin-4-one [41] (2-OxI-Rh) and (E)-3-((2-oxoindolin-3-ylidene)amino)-2-thioxothiazolidin-4-one [42] (3) were synthesized byan effective, easy, and green approach and the obtained molecule 2-OxI-Rh was evaluated in terms of their abilities in cation recognitionand sensing.

2. Experimental

2.1. Material and apparatus

All chemicals, reagents, and solvents were commercially availablefrom Sigma-Aldrich or Merck. The ethanol-4-(2-hydroxyethyl) piper-azine-1-ethanesulfonic acid (HEPES) buffer (pH range 6.8–8.2) wasprepared by dissolving 2.38 g of pure HEPES in deionized water(100mL) and adding one NaOH pellet to raise the pH towards 7.4. ThepH is modulated by adding 75% HClO4 or NaOH solution. Meltingpoint was determined on a Buchi 539 capillary melting apparatus andare uncorrected. Infrared spectra were recorded on a Mattson 1000 FT-IR spectrophotometer. 1H NMR and 13C NMR spectra were recorded ona 400 (100)-MHz Varian and Bruker spectrometer and are reported interms of chemical shift (δ, ppm) with SiMe4 as an internal standard.Data for 1H NMR are recorded as follows: chemical shift (δ, ppm),multiplicity (s: singlet, d: doublet, t: triplet, q: quartet, p: pentet, m:multiplet, bs: broad singlet, bd: broad doublet, qd: quasi doublet) andcoupling constant (s) in Hz, integration. Elemental analyses were car-ried out on a LECO CHNS-932 instrument. Column chromatographywas carried out on silica gel 60 (230–400 mesh ASTM). The reactionprogress was monitored by thin-layer chromatography (TLC) (0.25-mm-thick precoated silica plates: Merck Fertigplatten Kieselgel (60F254)). ESR spectra was recorded on JEOL JES-FA300 ESR spectro-meter. UV–vis absorption and fluorescence spectra of samples wererecorded on a Shimadzu UV-3101PL UV–vis-NIR spectrometer andPerkin–Elmer (Model LS 55) Fluorescence Spectrophotometer, respec-tively.

2.2. The synthesis of 2-OxI-Rh

2.2.1. Procedure AThe 3-amino-2-thioxothiazolidin-4-one (2, 3-amino-rhodanine,

302.2 mg, 2.04mmol) in ethanol (15mL) was heated to boiling for10min, and the solution of indoline-2,3-dione (1, isatin, 300.0 mg,2.04mmol) in ethanol (15mL) was added slowly using dropwise. Thereaction mixture was stirred for 5 h at 100 °C without catalyst, and wasmonitored by TLC. After the completion of the reaction, the red productformed was recrystallized from ethanol, filtered, and dried in vacuo.After recrystallization, 2-OxI-Rh (407mg, 72%), which is Z isomer, wasobtained as red solid (m.p. > 300 °C). 1H-NMR (400MHz, DMSO-d6):δ 11.25 (s, NH, 1 H), 8.82 (d, J =7.7 Hz, =CH, 1 H), 7.42 (t, J=7.7 Hz, =CH, 1 H), 7.10 (t, J =7.7 Hz, =CH, 1 H), 6.97 (d, J=7.7 Hz, =CH, 1 H), 5.97 (bs, NH2, 2 H); 13C-NMR (100MHz, DMSO-d6): δ 191.9, 168.0, 163.5, 144.8, 133.2, 128.6, 127.8, 125.5, 122.2,119.9, 110.7 (Fig. S1); IR (KBr, cm–1): 3410 cm−1 (NH2, stretch pri-mary), 3146 cm−1 (]CeH, isatin H), 1713 cm−1 (C]O), 1632 cm−1

(O]C-NeC]S), 1595 cm−1 (CeC, stretch in ring), 1458, 1335 cm−1

(C]S), 1189 cm−1 (CeN, stretch peak), 742 cm−1 (NH2 wag) (Fig. S2);Anal. Calcd. for C11H7N3O2S2: C, 47.64; H, 2.54; N, 15.15; O, 11.54; S,23.12; found: C, 47.57; H, 2.31; N, 15.07.

2.2.2. Procedure BMixture of isatin (1, 500mg, 3.40mmol), 3-amino-rhodanine (2,

503.3 g, 3.40mol) and concentrated ammonia (0.4 mL) in 15mL ofethanol was heated to boiling. Ammonium chloride (0.4 g) dissolved in5mL of water heated to 80 °C was added slowly to the solution. Thereaction mixture was refluxed for 9 h, and was monitored by TLC. Afterthe completion of the reaction, the mixture was cooled to room tem-perature. The dark red crude washed with 30mL mixture of EtOH/H2O(v/v:1/1), filtered, and dried in vacuo. After recrystallization, 2-OxI-Rh(480mg, 51%) was obtained as red solid.

2.3. Synthesis of 3

To a solution of isatin (1, 100.0 mg, 0.68mmol) in ethanol (10mL)was added slowly to the solution of 3-amino-rhodanine (2, 100,7 mg,0.68mmol) using dropwise. The reaction mixture was stirred forovernight at room temperature without catalyst, and was monitored byTLC. After the completion of the reaction, the red product formed wasrecrystallized from ethanol, filtered, and dried in vacuo. After re-crystallization, 3 (162mg, 86%), which is E isomer, was obtained asdark yellow solid (m.p. > 300 °C). 1H-NMR (400MHz, DMSO-d6): δ11.34 (s, NH, 1 H), 8.68 (d, J =7.6 Hz, =CH, 1 H), 7.83 (d, J=7.6 Hz, =CH, 1 H), 7.62 (t, J =7.6 Hz, =CH, 1 H), 7.18 (t, J=7.6 Hz, =CH, 1 H), 4.22 (s, CH2, 2 H); 13C-NMR (100MHz, DMSO-d6): δ 193.4, 169.5, 165.0, 146.3, 134.7, 130.1, 129.3, 123.7, 121.9,34.2 (Fig. S3).

2.4. UV-Vis and fluorescence studies of 2-OxI-Rh with various cations

The solution of probe 2-OxI-Rh (1× 10−2 M) and cations (chloridesalt, 1× 10−2 M) were prepared in CH3CN/H2O (v/v:9/1) and H2O,respectively. A solution of 2-OxI-Rh (2.5 μM) was placed in a quartzcell and the UV–vis and fluorescence spectrums were recorded. Afterintroduction of the solution of cations (5 equiv.), the changes in ab-sorbance intensity were recorded at room temperature each time. As aresult of pH studies, all measurements were carried out in CH3CN/H2O(v/v:9/1) with HEPES buffer solutions (pH 7.4).

2.5. UV-Vis and fluorescence titration of 2-OxI-Rh with ions (HgCl2 andCuCl2)

The solution of probe 2-OxI-Rh (1× 10−2 M) and ions (HgCl2 andCuCl2, 1× 10−2 M) were prepared in CH3CN and H2O, respectively.The concentration of probe 2-OxI-Rh used in the experiments was 2.5μM. The UV–vis and fluorescence titration spectra were recorded byadding corresponding concentration of ions (HgCl2 and CuCl2) to asolution of 2-OxI-Rh in CH3CN/H2O (v/v:9/1) with HEPES buffer so-lutions (pH 7.4). Each titration was repeated at least twice until con-sistent values were obtained.

2.6. Job’s plot measurement

Probe 2-OxI-Rh was dissolved in CH3CN to make the concentrationof 1×10−2 M. 5.00, 4.50, 4.00, 3.50, 3.00, 2.50, 2.00, 1.50, 1.00, 0.50and 0.0mL of the ligand solution were taken and transferred to vials.HgCl2 and CuCl2 were dissolved in H2O to make the concentration of1× 10−2 M. 0.0, 0.50, 1.00, 1.50, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50,and 5mL of the HgCl2 and CuCl2 solutions were added to each ligandsolution in CH3CN/H2O (v/v:9/1) with HEPES buffer solutions (pH 7.4).Each vial had a total volume of 5mL. After shaking the vials for a fewseconds, fluorescence spectra were taken at room temperature.

3. Results and discussion

3.1. Chemistry

This research purposes were to synthesize rhodanine derivatives bya green approach and to investigate the ion sensor properties of these

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ligands. In this context, initially, the reaction of isatin (1, indoline-2,3-dione) with 3-amino-rhodanine (2, 3-amino-2-thioxothiazolidin-4-one)was carried out in ethanol at reflux temperature without a catalyst forsynthesis of 3. The no-catalysts reaction of 3-amino-rhodanine (2) withisatin (1) in ethanol at reflux temperature gave terminal oxindole de-rivative 2-OxI-Rh and expected product 3 in 52%, and 34% yields,respectively. Whereupon, the same reaction was carried out at highertemperatures (100 °C) by increasing the amount of solvent (30mL) andwithout catalyst, and the unexpected product 2-OxI-Rh, which is Zisomer, was obtained in 72% yields. Additionally, a very complexmixture was obtained while CH3COOH was used as catalysts at refluxtemperature. It is supported by the literature that 3-amino-rhodanine(2 A) is in balance with the taotomerization product 2B at high tem-peratures [43]. It may be thought that the 2C, which is an enol form of2A, was obtained by the reaction of 2B as a base with 2A, and there-upon, it can be said that the unexpected product 2-OxI-Rh was ob-tained as a result of the Knoevenagel condensation reaction of 2C withisatin (1). (Scheme 1A). The 2-OxI-Rh was also synthesized by thereaction of isatin (1) with 3-amino-rhodanine (2) using NH4OH/NH4Clcatalyst in ethanol at reflux temperature (Scheme 1B). The rhodaninederivative 3, which is E isomer, was obtained in 86% yields as a resultof slow addition of 3-amino-rhodanine (2) on the isatin using a drop-ping funnel at room temperature and without catalyst (Scheme 1C).

3.2. Colorimetric sensing, UV–vis and fluorescence spectral recognitions

The interaction of 2-OxI-Rh with a wide range of cations, includingMn2+, Cu2+, Co2+, Cr3+, Ni2+, Zn2+, Mg2+, Ca2+, Hg2+, Fe2+ andFe3+ (as their chloride salts) was studied in CH3CN/H2O (v/v:9/1) withHEPES buffer solutions (pH 7.4). Studies with 10 equivalent of thecations revealed that 2-OxI-Rh responded selectively to an increasedconcentration of Hg2+ and Cu2+, a response characterized by a distinctcolor change from colorless to orange and pale green, respectively(Fig. 1A). On the other hand, while the studies of 2-OxI-Rh with cations(10 equivalent) in aqua systems (CH3CN/H2O, v/v:1/1) revealed that 2-OxI-Rh responded selectively to an increased concentration of Hg2+

characterized by a response with a distinct color. nteraction of 2-OxI-Rh with Cu2+ did not cause any discoloration (Fig. 1B). Unfortunately,studies of 3 revealed that it has affinities toward multiple ions, whichmakes it improper for chemosensor applications.

In addition to colorimetric sensing, to clearly investigate the inter-action of 2-OxI-Rh (2.5 μM) with metal ions, the ultraviolet-visible(UV–vis) absorption spectra of 2-OxI-Rh was first studied in CH3CN/H2O (v/v:9/1) at room temperature. Spectra for all metal ions wererecorded after 10min of the addition of 5 equiv. of each mentionedions, and the UV–vis spectra was obtained as given in Fig. 2. As dis-played in Fig. 2A, the absorption spectrum of 2-OxI-Rh exhibited thetypical oxindole and rhodanine absorption bands, such as a strong bandat 272 nm and also a weak broad band at 390 nm. The (SeS]CeNeC]O) bonds attributed that conjugation of oxindole group with (HNeC]

O) bonds shifted the n-π* and π-π* transmissions towards the longerwavelength. Upon the interaction of 2-OxI-Rh with 5 equiv. of transi-tion metal ions (Mn2+, Cu2+, Co2+, Ni2+, and Zn2+ as their chloridesalts), and heavy metal ions (Cr3+, Fe2+, Mg2+, Ca2+, Hg2+, and Fe3+

as their chloride salts), distinct changes in the absorption spectrum of 2-OxI-Rh were observed for Hg2+ and Cu2+. According to experiments inCH3CN/H2O (v/v:9/1), in the presence of Hg2+ and Cu2+, the ab-sorption band of 2-OxI-Rh at 272 nm decreased to 270 nm (for Cu2+)and increased to 275 nm (for Hg2+) with blue/red-shift. In the presenceof Hg2+ and Cu2+, the absorption band at 389 nm red-shifted to 415and 392 nm with a substantial increase in the absorbance, respectively.On the other hand, according to experiments in CH3CN/H2O (v/v:1/1)with HEPES buffer solutions (pH 7.4), it was observed that the ab-sorption band of 2-OxI-Rh at 272 nm decreased to 275 nm, and theabsorption band at 389 nm increased to 415 nm with red-shift in thepresence of Hg2+ ions, but the absorbance change of 2-OxI-Rh in thepresence of Cu2+ ions was not observed (Fig. S4).

The UV–vis titration experiments were performed to understand thebinding rationale of probe 2-OxI-Rh with Hg2+ and Cu2+ ions with thegradual addition of HgCl2 and CuCl2 to 2-OxI-Rh (2.5 μM) in CH3CN/H2O (v/v:9/1) with HEPES buffer solutions (pH 7.4) (Fig. 3A and S5).According to experiments in presence of HgCl2 and CuCl2, while theUV–vis absorption peaks at 272 nm decreased to 270 nm (for Cu2+) andincreased to 275 nm (for Hg2+), the absorption band at 389 nm in-creased to 415 and 392 nm, respectively, with a substantial increaseduntil 21 equivalents of ions (Hg+2 and Cu+2) were added. Thesechanges, the decrease at 272 nm and increase at 376 nm peak of 2-OxI-Rh, corresponding to the 2-OxI-Rh-metal complexes, could be due tothe interaction of mercury and copper with rhodanine-based probe 2-OxI-Rh. The increase of the peak at 389 nm, and the decrease of thepeak at 272 nm began when the ions (Hg2+ and Cu2+) concentrationwere greater than 1 equivalent, and it reached its saturation after theaddition of 21 equivalents of Cu2+ and 18 equivalents of Hg2+ (Fig. S5and 3B).

Colorimetric change from colorless to dark-green was observed afteradding Cu2+ ions in acetonitrile solution. Similarly, when the experi-ments were carried out in aqueous solutions of acetonitrile (CH3CN/H2O, v/v:1/1) with HEPES buffer solutions (pH 7.4), no color changewas observed. When the previous studies were examined, it is under-stood that the cause of these colorimetric changes may be the matchlessradical formation. [44] A number of experiments have been carried outto confirm these obtained and suggested results. According to experi-ments, the UV–vis spectra displayed the formation of unique radicalcation in CH3CN. On addition of Cu2+ to a solution of 2-OxI-Rh inCH3CN, the new bands at 307 and 460 nm are attributable to a radicalcation appeared with the color change from colorless to green. Then thesame experiment was carried out in aqueous solutions of acetonitrile.Upon addition of Cu2+ to the solution of 2-OxI-Rh in CH3CN/H2O (v/v:9/1), new interactions bands at 272/392 nm and 460 nm are attri-butable to a radical cation were observed with the color change from

Scheme 1. Synthesis of 2-OxI-Rh and 3.

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colorless to pale green, but no interaction peaks were observed except2-OxI-Rh's characteristic peaks from the studies in CH3CN/H2O (v/v:1/1) (Fig. 4). This phenomenon was explained based on the absorption,and fluorescence investigations and further confirmed by ESR to ob-serve the formation of cation radicals upon the addition of CuCl2 inCH3CN, CH3CN/H2O (v/v:9/1) or CH3CN/H2O (v/v:1/1). According tothe result, the paramagnetic Cu2+ ion displayed a typical ESR signalaround 2110–2130 G. [23] The signal completely disappeared uponaddition of 1 equiv. of 2-OxI-Rh in CH3CN or CH3CN/H2O (v/v:9/1)(Fig. S6). This clearly shows that paramagnetic Cu2+ is reduced todiamagnetic Cu+ in consequence of electron transfer from 2-OxI-Rh toCu2+ in both solvent systems. Tthe radical cations of arylamines couldbe prepared by treating amine derivatives with Cu(ClO4)2 in CH3CNwas reported. [23] A light blue color, which is a feature of the radicalcation product, was observed by naked-eye upon addition of CuCl2 to acolorless solution of 2-OxI-Rh in CH3CN, then, as a result of the addi-tion of water to the CH3CN, the light blue color disappeared quickly.The rapid disappearance of the color of the radical cation and the ab-sence of ESR signal belong to radical cation indicate that these radicalsforms may be unstable in the aqueous solvent systems (Fig. S6).

In addition to UV–vis studies, fluorescence spectroscopy experi-ments were also carried out in order to measure the ability of 2-OxI-Rhas a fluorescent cation sensor. Thus to gain further insight into the se-lective and sensitive Hg2+ binding ability of 2-OxI-Rh toward a series

Fig. 1. Colorimetric screening of 2-OxI-Rh (2.5× 10−6 M) in CH3CN/H2O (v/v: 9/1, A and v/v: 1/1, B) in the presence of 10 equiv. of cations.

Fig. 2. (A) UV-vis spectrum of probe 2-OxI-Rh (2.5 μM) in CH3CN/H2O (v/v:9/1 with HEPES buffer) with various cations. (B) Bar chart for absorbance re-sponse of various cations with 2-OxI-Rh at 272 and 390 nm.

Fig. 3. (A) UV-vis titration of probe 2-OxI-Rh (2.5 μM) in CH3CN/H2O (v/v: 9/1 with HEPES buffer) solution with HgCl2. (B) Absorption of 2-OxI-Rh at272 nm and 415 nm vs. equivalent of HgCl2.

Fig. 4. The UV–vis spectra of 2-OxI-Rh (2.5 μM) in the absence and presence ofCuCl2 (25 μM) in CH3CN and aqueous solutions of CH3CN.

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of cation were evaluated by observing changes in their fluorescenceemission spectra in CH3CN/H2O (v/v:9/1 with HEPES buffer) solution.As shown in Fig. 5, 2-OxI-Rh alone exhibited weak fluorescence peak at515 nm in CH3CN/H2O (v/v:9/1) with an excitation of 390 nm.Similarly, upon addition of Cu2+ to a solution of 2-OxI-Rh at sameconditions the emission bands was also observed at 498 nm. When theamount of water in the solution (CH3CN/H2O; v/v, 1/1 with HEPESbuffer) was increased, it was seen that the 2-OxI-Rh-Hg2+ interactionpeak slightly decreased and the 2-OxI-Rh-Cu2+ interaction peak wasdisappeared (Fig. 5B). Analogs investigation were also carried out byfluorescence spectroscopy with other series of cations, but in all situa-tion, very small increasing occurred on the addition of cations to theprobe 2-OxI-Rh (Fig. 5A). Namely, the 2-OxI-Rh can be used as a se-lective “turn –on” colorimetric sensor for Hg2+ in the aqua solventsystems.

After fluorescence studies of the ligand with cations, to study thesensitivity of 2-OxI-Rh toward mercury and copper ion sensing, fluor-escence response of the interaction of 2-OxI-Rh with increasing Hg2+

and Cu2+ with excitation at 390 nm in CH3CN/H2O (v/v:9/1) was in-vestigated. As shown in Figs. 6 and S7, upon the progressive addition ofHg2+ and Cu2+, the fluorescence intensity gradually increased. In thepresence of 20 equiv. of Hg2+ and Cu2+ ions, the fluorescence differ-ence between 2-OxI-Rh and 2-OxI-Rh–ions were ∼24.5 and ∼14.4times greater than that of other cations, respectively. Fig. 6 and S7 showthe fluorescent intensity as an increasing function of the equivalent ofCu2+ and Hg2+ ions, respectively. As more Hg2+ and Cu2+ ions were

added, the emission intensity increased with the concentration of ions.In the range of 1–20 equiv. of ions, the emission intensity increasedsubstantiallt, and after more addition 21 equivalent of ions, the rate ofincrease was reduced.

To study the influence of other metal ions on Hg2+ ions bindingwith 2-OxI-Rh, competitive experiments in the presence of Hg2+ (10μM) with other metal ions (10 μM) was performed in CH3CN/H2O (v/v:1/1 or v/v:9/1). The results of studies showed that emission en-hancement induced by the mixture of Hg2+ with most metal ions wassimilar to that induced by Hg2+ alone in CH3CN/H2O (v/v:1/1 or v/v:9/1) (Figs. S8 and S9). However, none of the other tested metal ions werefound to not interfere with the interaction of 2-OxI-Rh with Hg2+.According to our results shown in Fig. 7, no significant enhancement inthe fluorescence emission in the presence of some metal cations exceptCu2+ was observed. On the other hand, lower fluorescence enhance-ment was observed when Hg2+ was mixed with Cu2+. Only a mixtureof Hg+2 and Cu+2 in CH3CN/H2O (v/v:1/1 with HEPES buffer) hascaused a small reduction and a red-shift in emissions, which may in-dicate that Cu2+ competes with Hg2+ for binding with 2-OxI-Rh (Fig. 7and S9B). To verify this phenomenon, the influence of other metal ionson Hg2+ binding with 2-OxI-Rh was also performed in the presence ofHg2+ (10 μM) with other metal ions (10 μM) in CH3CN/H2O (v/v:9/1with HEPES buffer). Similarly, the study of a mixture of Hg+2 and Cu+2

Fig. 5. Fluorescence spectra of probe 2-OxI-Rh (2.5 μM) in CH3CN/H2O (A: v/v: 9/1, and B: v/v: 1/1 with HEPES buffer) with various cations(λexc= 390 nm).

Fig. 6. Fluorescence spectra of the probe 2-OxI-Rh (2.5 μM) in CH3CN/H2O (v/v: 9/1 with HEPES buffer) with the increasing concentration of HgCl2(λexc = 390 nm).

Fig. 7. Metal-ion selectivity of 2-OxI-Rh (2.5 μM) with λexc= 390 nm in thepresence of Hg2+ (10 μM) and other tested metal ions (10 μM).

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with 2-OxI-Rh in CH3CN/H2O (v/v:9/1) has resulted in a reduction inemissions (Fig. 7 and S9A). These results show that probe 2-OxI-Rh hasa property of application for quantitatively detecting Hg2+ in aqua andorganic solvent systems.

In order to study the effect of incubation temperature on thefluorescence response of 2-OxI-Rh in the absence and presence ofHg2+, experiments were carried out in the temperature range 25–70 °C(Fig. 8 and S10). From Fig. 8, it was found that temperature had noobvious interference on the fluorescence response of 2-OxI-Rh-Hg2+

complex. In the presence of Hg2+, while the fluorescence intensity re-mained nearly constant from 25 to 60 °C, the fluorescence intensitybegan to decrease with further increase in temperature.

3.3. Job’s plot using the fluorescence method

In order to determine the binding modes of 2-OxI-Rh with Hg2+

and Cu2+, the Job plot analysis were carried out. For this purpose, aseries of samples containing ions (Hg2+ and Cu2+) and 2-OxI-Rh, inwhich their total concentrations were constants, were prepared. Thefluorescence intensity of those mixtures of ions and 2-OxI-Rh invarying molar ratios (XM

2+/X2-OxI-Rh; 1/9, 2/8, 3/7, 4/6, 5/5, 6/4, 7/3,8/2 and 9/1) were measured and the results indicated that the inter-action ratios between the probe 2-OxI-Rh and ions (Hg2+ and Cu2+)were 1:1 (Fig. 9A and B).

3.4. Binding constant (Ks) and LOD using the fluorescence method

Binding constant (KS) and limit of detection (LOD) of the 2-OxI-Rhfor Cu2+ and Hg2+ ions were obtained by the fluorescence titrationresults and relevant equations. The binding constants (Ks) of Cu2+ andHg2+ ions with probe 2-OxI-Rh were calculated from the slope of thegraphs drawn using the data obtained from using fluorescence titrationand the following Benesi-Hildebrand Eq. (1). More importantly, the plotof 1/F-F0 versus 1/[Cu2+] and 1/[Hg2+] were found to be linear(R2= 0.9964 for Cu2+ and R2 = 0,9979 for Hg2+) in this range, in-dicating that probe 2-OxI-Rh can be used to determine Cu2+ and Hg2+

concentration. Using the fluorescence titration data, the binding con-stant of 2-OxI-Rh with Cu2+ and Hg2+ in CH3CN/H2O (v/v:9/1) so-lution were calculated to be 1.21×104 M−1 and 2.15×104 M-1, re-spectively (Fig. 10A and S11A).

−=

−+

−−F F K F F X F F1 1

( )[ ]1

s maxn

max0 0 0 (1)

The LOD and LOQ values are calculated from the following formulas(2 and 3, The N is total experiments number). Based on these

concentration dependent fluorescence changes, the LOD and LOQ va-lues for Cu2+ ions were calculated as 2.31 μM and 7,01 μM in CH3CN/H2O (v/v:9/1) solution, which is in the range of concentration of copperin the blood (11.8–23.6 μM) (Fig. S7B), respectively. The LOD for Hg2+

ions were calculated to be 3.36 μM and 10.2 μM in CH3CN/H2O (v/v:9/1) solution, respectively (Fig. 10B and S11B).

=× × √

LODStandard Error SE N

Slope3,3 ( )

(2)

=× × √

LOQStandard Error SE N

Slope10 ( )

(3)

A comparison of the applicability and analytical part of the presentprobe with some of the previous reports on the Hg2+ sensors in terms oftheir Ks, LOD, and LOQ in the presence of other interfering ions wasgiven in Table 1. It can be said that this values were acceptable valuesas much as the values obtained using for copper and mercury detectionligands in the literature [45–54].

Although effective results were obtained with studies of 2-OxI-Rhwith HgCl2, it can not be assumed that in wastewater Hg2+ would bejust in the form of chlorides. Therefore, in addition to this interaction ofthe ligand against chloride salts of mercury, absorbance studies werecarried out with mercury salts in which the anion structure was variedsuch as Hg(OAc)2, Hg(NO3)2, and HgI2. While Hg(NO3)2, and HgI2 gavesimilar results with mercury (II) chloride salt, different interaction withthe Hg(OAc)2 was observed (Fig. S12). Although some different

Fig. 8. Effect of temperature on the fluorescence response of 2-OxI-Rh in theabsence (blue line) or presence (red line) of HgCl2 (10 μM) in CH3CN/H2O (v/v:9/1 or v/v: 1/1 with HEPES buffer).

Fig. 9. Job’s plot of the probe 2-OxI-Rh with Cu2+ (A) and Hg2+ (B) inCH3CN/H2O (v/v:9/1).

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absorbance behaviors with other mercury salts are observed, it can besaid that probe 2-OxI-Rh is selective against mercury ions in general.

3.5. Mercury and copper sensing mechanism

The signal changes observed are owing to the formation of a 1:1complex between 2-OxI-Rh and ions. The interaction between theprobe 2-OxI-Rh and ions (Cu2+ and Hg2+) were determined using FT-IR measurements in solution. The FT-IR spectra was taken with only theprobe 2-OxI-Rh (black color, Fig. 11) and it was also taken with the

solution having the probe 2-OxI-Rh and ions (red and pink color,Fig. 11). According to FT-IR spectrums of 2-OxI-Rh, the pure probe hadthe peaks at 3410 cm−1 (NH, stretch primary amine), at 3176 cm−1

(]CeH, phenyl H), at 1712 cm−1 (C]O), 1632 cm−1 (0eC-NeC]S),at 1458, 1339 cm−1 (C]S), at 1189 cm−1 (CeN, stretch peak) and at742 cm−1 (NH2 wag, primary) (Fig. 10). The coordination of 2-OxI-Rhwith Cu2+ and Hg2+ were evidenced in the FT-IR spectrum of the 2-OxI-Rh-Cu2+ and 2-OxI-Rh-Hg2+ complexes. When ions (Cu2+ andHg2+) were added to the solution of 2-OxI-Rh, it was observed that thecharacteristic peaks of the probe 2-OxI-Rh preserved except for somechanging. The 2-OxI-Rh which contain thiosemicarbazone groups areknown to coordinate as five-membered chelate rings in two differentmodes, involving either a neutral thione form or an anionic thiol form.[55] Recently, the results of sensor studies with ligands containinghydrazinecarbothioamide groups show that binding via NH, C]S,CeSeC, C]O or C]N. [10,55–58] FT-IR spectra of 2-OxI-Rh-Cu2+

and 2-OxI-Rh-Hg2+ suggest that the N∩S chelating ligands adopt thethione form, because of the presence of NH stretching (at 3429 cm-1),C]O (1713 cm−1) and NH (741 cm−1) molecular vibration bands, andalso the FT-IR spectrum of the complexes are similarities to those foundin complexes showing the thione (C]S) form of coordination. The mostimportant indicator of the interaction of the 2-OxI-Rh to the ions (Hg2+

and Cu2+) via the thione group is that the intensity of the interactionpeaks belonging to the group C]S (1458 cm−1 and 1335 cm−1) is thefall of the severity and has not undergone any changes except the shifts.

To have a deeper insight into the Hg2+ binding properties of the 2-OxI-Rh, 1H NMR experiments were carried out in the absence andpresence of Hg2+ in CD3CN at room temperature as shown in Fig. 12.The mercury, which is a heavy metal ion, may affect the proton re-sonance signals that are close to Hg2+ binding. In the 1H NMR spectraof 2-OxI-Rh, the NH proton (NH, oxindole) signal at 11.25 ppm showeddown-field shifts (Δδ=+0.22) upon the addition of Hg2+. The 1H NMRspectrum is showing that the NH peak appears to down-field shifts butdoes not disappear. This indicates that there is no tautomerization inthe oxindole core. The NMR spectrum of the interaction of probe 2-OxI-Rh with Hg2+ ions shows that most of the shift from 5.97 ppm to 6.58(Δδ=+0.61) is in the NH2 group. These results imply that Hg2+

binding occurs mainly from the rhodanine ring. The proton signals (Ha

and Hc, oxindole) showed up-and down-field shifts upon the addition ofHg2+, respectively. This indicated that Hg2+ binds to the rhodaninering (S]CeNeNH2) attached to the oxindole ring and Hg2+ bindingaffects the ring current at the phenyl ring. The other proton signals (Hb

and Hd) at the oxindole ring were slightly influenced by Hg2+ binding.From these shifts observed, it can be proposed that complexation exerts

Fig. 10. Benesi-Hildebrand plot based on a 1:1 association stoichiometry be-tween the 2-OxI-Rh with Hg2+ (A), and the change fluorescence intensity ofthe 2-OxI-Rh with the increasing concentration of HgCl2 (B) (λexc= 390 nm).

Table 1Comparison of some Hg2+ selective chemosensors.

Ref. Binding Constant(M−1)

Detection of Limit Sensing Ions

[3] 8.06×104 14.5× 10−6 M Hg2+

[45] – 4.60× 10−6 M Hg2+

[46] 1.12×108 2.20× 10−6 M Hg2+

[47] – 3.85× 10−10 M Hg2+

[48] 4.18×104 1.78× 10−8 M Hg2+, S2−

[49] – 1.2× 10−7 M Hg2+

[50] – 2.2× 10−6 M Hg2+

[51] – 8.7× 10−7 M Hg2+

[52] 9.0× 10−9 M Hg2+, F¯

2-OxI-Rh (Thiswork)

2.15×104 3.36× 10−6 M Hg2+, *Cu2+

* No interaction of 2-OxI-Rh with Cu2+ in CH3CN/H2O (v/v: 1/1, withHEPES buffer).

Fig. 11. FTIR spectra of probe 2-OxI-Rh (black line), 2-OxI-Rh-Cu2+ (red line)and 2-OxI-Rh -Hg2+ (pink line) in solution.

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an effect on the protons of the oxindole moiety of the 2-OxI-Rh. 1H-NMR spectroscopy of the solution obtained using 2-OxI-Rh with Cu2+

ions was not informative owing to obtained an extensive spectrumcaused by the paramagnetic nature of the complexed Cu2+ ions.

Although it is understood by the 1H-NMR experiments that theconnection occurs through the rhodanine ring, it is not understood thatthe connection occurs through the carbonyl group or the thionyl group.To support to 1H NMR studies and to understand the exact morphologyof the connection was carried out to 13C NMR experiments. As can beseen in Fig. 13, the peak of thionyl carbon (eC]S) substantial up-shifted from 191.9 ppm to 194.7 ppm (Δδ=+3.8). On the other hand,the peak of ketone group carbon (eC]O) slightly up-shifted from167.9 ppm to 168.6 ppm ((Δδ=+0.7). The other carbon signals (car-bonyl and aromatic) at the structure of the probe 2-OxI-Rh wereslightly influenced by Hg2+ binding. These results indicated that Hg2+

binds to the hydrazine group (C]NeNH2) and thionyl (-C]S) groupson the rhodanine ring.

As a result of all these experiments, the suggested mechanism forthe interaction of the probe 2-OxI-Rh with Hg2+ and Cu2+ ions werepresented in Scheme 2. These observations imply that probe 2-OxI-Rhcomplexes with ions through thionyl and hydrazine groups of rhoda-nine core.

3.6. pH profiles of 2-OxI-Rh

The effect of different pH environment (range of 2–12) was studiedfor the practical application of the probe 2-OxI-Rh (2.5 μM) in theabsence and presence of Cu2+ and Hg2+ (10 equiv.) and are describedin Fig. 14. The pH is modulated by adding 75% HClO4 or NaOH solu-tion. According to studies done, the fluorescence intensity of the 2-OxI-Rh was not sensitive to pH, except for pH=2–3. However, upon ad-dition of the Cu2+ and Hg2+ ions, it has been identified that a distinctincrease in the emission bands of 2-OxI-Rh at 492 and 515 nm betweenpH 2 and 12. As a result of all these studies, considering the applicationof samples in physiological cases, pH 7.0 was thus chosen as the ex-periment condition. Such broad pH spans in an aqueous environmentcan offer a great application opportunity very useful in several appli-cations, such as Cu2+ and Hg2+ detection in wastewater, blood, in-dustrial trade analysis, and physiological treatment (Fig. S13A andS13B).

4. Conclusion

In conclusion, it has been reported the synthesis of rhodanine-based2-OxI-Rh by a new green approach from the reaction of indoline-2,3-dione (1) with 3-amino-2-thioxothiazolidin-4-one (2). In addition to

Fig. 12. 1H NMR (400MHz) spectra in CD3CN of the probe 2-OxI-Rh (1× 10−2 M) with presence of HgCl2.

Fig. 13. 13C NMR (100MHz) spectra in DMSO-d6 of the probe 2-OxI-Rh (1× 10−2 M) with presence of HgCl2.

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synthesis, the chemosensor properties of this non-toxic probe was stu-died. The 2-OxI-Rh showed mercury and copper ion sensing cap-abilities via naked-eye detection of color changes, absorption andfluorescence signals. The absorption slider and fluorescence intensityincreasing effects of the Hg2+ and Cu2+ ions were in a concentrationdependent manner. The interaction ratios of 2-OxI-Rh with Hg2+ andCu2+ were found by Job’s method as 1:1. Using Benesi-Hildebrandequation, the binding constants of Hg2+ and Cu2+ to 2-OxI-Rh weredetermined as 2.15×104 M−1 and 1.21× 104 M−1, respectively. LODvalues of Hg2+ and Cu2+ were calculated as 3.36 μM and 2.31 μM,respectively. In addition, the interaction mechanism of ions with 2-OxI-Rh was proposed by using 1H NMR and FT-IR measurements. Finally, itcan be concluded that 2-OxI-Rh which is non-toxic, seems like a goodand selective candidate turn-on sensor for mercury detection in aquasystems, and turn-on sensor for mercury and copper detection in or-ganic solvent systems.

Acknowledgement

Authors are indebted to Department of Chemistry at BingolUniversity for their financial support for this study.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in theonline version, at doi:https://doi.org/10.1016/j.jphotochem.2018.12.021.

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