Colorimetric Detection of Multiple Metal Ions Using Schiff Base 1-(2-Thiophenylimino)-4-(N-Dimethyl)Benzene

chemosensors

Article Colorimetric Detection of Multiple Metal Ions Using Schiff Base 1-(2-Thiophenylimino)-4-(N-dimethyl)benzene

Ali Q. Alorabi 1,*, Mohamed Abdelbaset 1,2 and Sami A. Zabin 1

1 Chemistry Department, Faculty of Science, Albaha University, Albaha (P.O. Box 1988), Saudi Arabia; [email protected] (M.A.); [email protected] (S.A.Z.) 2 Chemistry Department, Faculty of Science, Al Azhar University, Assiut Branch, Assiut 71524, Egypt * Correspondence: [email protected]; Tel.: +966-55-6101-076

Received: 1 November 2019; Accepted: 16 December 2019; Published: 18 December 2019

Abstract: In this paper, a Schiff base ligand 1-(2-thiophenylimino)-4-(N-dimethyl)benzene (SL1) bearing azomethine (>C=N-) and thiol (-SH) moieties capable of coordinating to metals and forming colored metal complexes was synthesized and examined as a colorimetric chemosensor. The sensing 2+ 3+ 2+ 2+ 2+ 2+ 2+ 2+ 3+ 5+ ability toward the metal ions of Cu , Cr , Fe Ni , Co , Mg , Zn , Fe , Fe , NH4VO3 (V ), 2+ 2+ 2+ 3+ Mn , Hg , Pb , and Al was investigated in a mixture of H2O and dimethylformamide (DMF) solvent using the UV–Visible spectra monitoring method. The synthesized Schiff base ligand showed colorimetric properties with Cr3+, Fe2+, Fe3+, and Hg2+ ions, resulting in a different color change for each metal that could be identified easily with the naked eye. The UV–Vis spectra indicated a significant red shift (~69–288 nm) from the origin after the addition of the ligand to these metal ions, which may be due to ligand-to-metal charge-transfer (LMCT). On applying Job’s plot, it was indicated that the ligand binds to the metal ions in a 2:1 ligand-to-metal molar ratio. SL1 behaves as a bidentate ligand and binds through the N atom of the imine group and the S atom of the thiol group. The results indicate that the SL1 ligand is an appropriate coordination entity and can be developed for use as a chemosensor for the detection of Cr3+, Fe2+, Fe3+, and Hg2+ ions.

Keywords: colorimetric detection; metal ions; Schiﬀ bases; chemosensor

1. Introduction The detection of transition metal ions as pollutants has gained extreme importance in chemical, biological, and environmental sciences because of the toxic impact of such ions on the environment and human health. Numerous analytical techniques have been employed for the determination of toxic transition metal elements including atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry, inductively coupled plasma (ICP), atomic emission spectrometry, and fluorescence spectroscopy. The results obtained from analyses with these modern instruments are very sensitive, leading to trace analysis at ppt. or even at the femto mole level. However, these techniques are very expensive to purchase and maintain, and the analysis cost per sample is typically high. In addition, they need a complicated procedure for ion analysis, which makes them inconvenient for routine analysis [1,2]. Therefore, recent analytical interest has focused on exploring different organic molecules with optical properties and employing them as chemosensors for the determination of metal ions [3–6] so that the detection of metal ions can be observed directly with the naked eye and without the need for expensive and complicated equipment. This visualization is based on the coordination between organic molecules, having lone pair electrons (acting as donors), and the metals (acting as acceptors). Much effort has been made to design and synthesize organic molecules with colorimetric and fluorescence properties for metal ion sensing with high sensitivity and selectivity [6,7]. Various

Chemosensors 2020, 8, 1; doi:10.3390/chemosensors8010001 www.mdpi.com/journal/chemosensors Chemosensors 2020, 8, 1 2 of 11 Chemosensors 2020, 8, 1 2 of 10 properties for metal ion sensing with high sensitivity and selectivity [6,7]. Various molecular structures ofmolecular sensors comprising structures ofa crown sensors ether, comprising pyridines, a crownand quinolines ether, pyridines, have been and synthesized quinolines for have binding been withsynthesized metal ions for [7–10]. binding with metal ions [7–10]. AmongAmong all all the the identified identified organic organic sensors, sensors, Schiff Schiff basebase ligands ligands are are one one of of the the chemosensors chemosensors that that have have beenbeen extensively extensively studied studied for for sensing sensing both both cations cations and and anions anions [6]. [Schiff6]. Schi basesff bases are able are to able coordinate to coordinate with nearlywith nearly all metal all metal ions ionsthrough through the thenitrogen nitrogen atom atom of ofthe the azomethine azomethine group group and and any any other other donating donating atomsatoms adjacent adjacent to to the the azomethine azomethine group group such such as as oxygen, nitrogen, nitrogen, or sulfur atoms. Many Many Schiff Schiff basebase ligandsligands have have been been used used as as chemosenso chemosensorsrs for for various various metal metal ions ions such such as Zn as2+ Zn [11,12],2+ [11 ,Hg12],2+Hg [13–16],2+ [13 Cu–162+], [17–19],Cu2+ [17 Fe–19+3 ],[19–21], Fe+3 [19 and–21 Cr], and3+ [19,22]. Cr3+ [19,22]. KeepingKeeping the above facts in mind, thisthis presentpresent workwork aimedaimed toto synthesizesynthesize a a Schi Schiffff base base ligand ligand with with a anitrogen nitrogen of of the the azomethine azomethine group group as as the the donordonor atom,atom, supportedsupported withwith a soft dono donorr sulfur atom adjacent adjacent toto it it at at the the orthoortho positionposition to to enhance the binding ability towa towardrd the metal ions. The The research research attempted attempted toto examine examine this this Schiff Schiff basebase ligand ligand as as a a chemosenso chemosensorr for for the the detection detection of of sele selectedcted heavy heavy metals metals through through colorimetriccolorimetric and and visible visible color color changes.

2.2. Materials Materials and and Methods Methods

2.1.2.1. Materials Materials and and Instrumentation Instrumentation AllAll of of the the precursor precursor chemicals, chemicals, namely namely 2-aminot 2-aminothiophenol,hiophenol, 4-(dimethylamino)benzaldehyde, 4-(dimethylamino)benzaldehyde, metal nitratemetal salts, nitrate and salts, solvents, and solvents,used in this used study in were this of study analytical were grade of analytical purchased grade from purchased Sigma-Aldrich from (St.Sigma-Aldrich Louis, MO, (St.USA) Louis,and BDH MO, (UK) USA) and and used BDH directly (UK) without and used purification. directly withoutDimethylformamide purification. (DMF)Dimethylformamide solvent used in (DMF) the experiments solvent used was in the of experiments UV HPLC wasspectroscopic of UV HPLC grade spectroscopic (99.7%). UV–Vis grade absorption(99.7%). UV–Vis spectra absorption were recorded spectra with were a recorded Thermo withScientific a Thermo Evolution Scientific 300 EvolutionUV–Visible 300 double UV–Visible beam spectrophotometer.double beam spectrophotometer. An FT-IR spectrum An FT-IR for the spectrum ligand forwas the recorded ligand wason a recorded Thermo onScientific a Thermo Nicolet Scientific iS50 FT-IRNicolet spectrophotometer iS50 FT-IR spectrophotometer using the attenuated using the to attenuatedtal reflection total (ATR) reflection method (ATR) for method solid powder. for solid Elementalpowder. Elementalanalysis was analysis performed was performedusing a Thermo using Fisher a Thermo Scientific Fisher CHN/S/O Scientific analyzer CHN/S /instrumentO analyzer (Lecoinstrument Model (Leco VTF-900 Model CHN-S-O VTF-900 CHN-S-O932 version 932 1.3x, version Waltham, 1.3x, Waltham, MA, USA). MA, USA).The mass The massspectrum spectrum was recordedwas recorded on a onThermo a Thermo Scientific-LCQ Scientific-LCQ fleet fleetion trap ion trapmass mass spectrometer spectrometer with with high high resolution resolution using using the electrospraythe electrospray ionization ionization (ESI) (ESI) method. method.

2.2.2.2. Preparation Preparation of of 1-(2-Thiophenylimino)-4-(N-dimethyl)benzene 1-(2-Thiophenylimino)-4-(N-dimethyl)benzene The Schi base ligand 1-(2-thiophenylimino)-4-(N-dimethyl)benzene (SL , Scheme1) was prepared The Schiffff base ligand 1-(2-thiophenylimino)-4-(N-dimethyl)benzene (SL11, Scheme 1) was prepared byby following the the reported procedure [23]. [23]. It It was was prepared prepared by mixing the solution of 2-aminothiophenol (0.02(0.02 mol, mol, 2.08 2.08 mL) mL) in in 25 25 mL mL ethanol ethanol and and the the solution solution of of 4-(dimethylamino)benzaldehyde 4-(dimethylamino)benzaldehyde (0.02 (0.02 mol, mol, 2.982.98 gm) gm) in in 25 25 mL mL ethanol ethanol in a in round-bottomed a round-bottomed flask. flask. The mixture The mixture was refluxed was refluxed with continuous with continuous stirring forstirring nearly for one nearly hour. The one yellow hour. Theprecipitate yellow was precipitate filtered, washed was filtered, with hot washed ethanol with several hot times ethanol and severalfinally, withtimes ether, and and finally, then with dried ether, in the and open then air. driedThe product in the compound open air. The was productpurified compoundby recrystallization was purified using anby ethanol recrystallization and DMF using mixture. an ethanolA yellow-colored, and DMF mixture.stable solid A yellow-colored, was obtained with stable a yield solid of was 93.40%, obtained and thewith melting a yield point of 93.40%, was measured and the meltingat 177 °C. point The wasmolecular measured formula at 177 was◦C. C The15H16 molecularN2S; m/z: 255.08 formula [L–H] was+ + 1 (Mol.C15H 16Wt.N 2=S; 256 m /gz: mol 255.08−1). The [L–H] elemental(Mol. analysis Wt. = 256was gas mol follows:− ). The %C elemental70.31 calculated analysis (70.84 was found), as follows: %H 6.25%C 70.31(5.96), calculated%N 10.93 (11.13), (70.84 found),and %S %H12.50 6.25 (12.22), (5.96), and %N the 10.93important (11.13), IR spectra and %S bands 12.50 were (12.22), 1606 and cm the−1ν 1 1 (Cimportant = N) and IR 2560 spectra cm−1 bandsν (SH). were 1606 cm− ν (C = N) and 2560 cm− ν (SH).

CH3 N SH CH3 SH CH3 N + OHC N NH2 CH3

Scheme 1. Synthesis of Schiﬀ base ligand (SL ). Scheme 1. Synthesis of Schiff base ligand (SL1).

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2.3. UV–Visible Absorption Spectra for SL1

UV–Visible maximum absorption spectra (λmax) for the free ligand SL1 were recorded in DMF at room temperature with 0.001 M concentration over the wavelength range of 200–800 nm.

2.4. Detection of Metal Ions with SL1—Competition Experiments Stock solutions for transition metal ions (Cu2+, Cr3+, Fe2+, Ni2+, Co2+, Mg2+, Zn2+, Fe2+, Fe3+, NH VO , (V5+), Mn2+, Hg2+, Pb2+, and Al3+) were prepared with a 1 10 2 M concentration using 4 3 × − metal nitrates in deionized water, and the pH was 6.45. The solution of the Schiﬀ base ligand SL1 sensor was prepared with a concentration of 1 10 2 M in DMF, and the pH of the solution was 7.84. × − The advantage of SL1 is that it is soluble in DMF solvent, which is miscible with water. For the color change analysis, UV–Visible spectra were recorded in the range of 200–800 nm using an Evolution 300 UV–Visible double beam spectrophotometer instrument using a quartz cell with a 1 cm path length. The colorimetric detection of various transition metal ions was carried out at room temperature using 1 mL volume of each metal ion solution of a 1.0 10 2 M concentration and 2 mL of × − the Schiﬀ base ligand (1.0 10 2 M), and the solution mixture was diluted to 10 mL by adding the × − DMF solvent. The pH of the solution mixture was 7.37. After mixing properly, the UV–Visible spectra of the mixtures were recorded.

2.4.1. UV–Visible Titration Experiment To perform the titration experiment for each of the Cr3+, Fe2+, Fe3+, and Hg2+ metal ions, a solution (ﬁxed volume = 1 mL) of the ligand SL1 of concentration 0.01 M in DMF was taken in a test tube and increasing volumes (0.05–1.5 mL) of 0.01 M of each metal ion in deionized water were added. After mixing well, the UV–Visible spectra were recorded at room temperature in the range of 200–800 nm.

2.4.2. Job Plot Quantification Using the solutions of the metal ions Cr3+, Fe2+, Fe3+, and Hg2+ (1 10 2 M) and the solution × − of the ligand (1 10 2 M), a fixed volume of the metal ions (1 mL) was placed into the cuvette and × − then an incremental amount of the ligand SL1 was added. The molar ratio of the Schiff base ligand ([M]/[M] + [L]) was changed from 0.1 to 0.9 M. The maximum absorbance intensity for each metal at its particular wavelength was observed and recorded.

3. Results and Discussion During the past few years, several chemosensors have evolved for the selective qualitative analysis of different metal ions based on metal–ligand coordination as the host–guest interaction principle [7,24]. Our aim in this research work was to develop ligand bearing coordination sites that had a binding ability and selectivity toward specific transition metal ions in aqueous systems. The ligand chosen in this investigation was the Schiff base compound named 1-(2-thiophenylimino)-4-(N-dimethyl)benzene containing the functional moieties of the azomethine (CH=N-) group and the thiol (-SH) group at the ortho position to the azomethine group. This compound is considered a bidentate ligand capable of coordinating to the metal ions through the nitrogen atom of the azomethine group and the soft sulfur atom of the thiol group [25]. It is reported that compounds containing amine, mercapto, hydroxyl, or carboxyl groups adjacent to the azomethine group have high affinity and coordination capability to the heavy metal ions [24]. This selected Schiff base ligand, as a chromogenic system that has binding and signaling components, acts as a signal transduction bond with the analyte (metal ion), forms a metal complex, and generates electronic modulation, causing a color change that can be detected easily by the naked eye [26]. Chemosensors 2020, 8, 1 4 of 10

3.1. Preparation of the Schiﬀ Base Sensor Compound

The Schiff base ligand (SL1) was synthesized using the precursor 2-aminothiophenol as the primary amine and condensed with the aldehydic compound 4-(dimethylamino)benzaldehyde in a 1:1 molar ratio according to the reported procedure [23]. The synthesized SL1 compound was purified by recrystallization, and the yield was in good quantity. It was a solid, stable, yellow compound and soluble in hot ethanol, DMF, and DMSO. The purity of the compound was checked using thin layer chromatography (TLC), and the structure of the compound was characterized using UV–Vis, IR, and mass spectra. The mass spectra of the prepared Schiff base showed a peak at 255.08, which is in 1 agreement with the theoretically calculated molecular weight (256 g mole− ) of the Schiff base ligand. The elemental analysis was in good agreement with the theoretically calculated percentage values for Chemosensors 2020, 8, 1 5 of 11 the proposed molecular formulae and, hence, confirmed and proved the expected structures of the synthesized Schiratio ffaccordingbase ligand to the reported SL1. Moreover, procedure [23]. the The IR spectrasynthesized showed SL1 compound the characteristic was purified by bands for the azomethine grouprecrystallization, (1606 cm and1 )the and yield for was the in thiolgood quantity. group (2560It was cma solid,1). stable, Therefore, yellow compound the SL ligandand structure soluble in hot ethanol,− DMF, and DMSO. The purity of the compound− was checked using thin1 layer was confirmedchromatography before proceeding (TLC), and tothe the structure sensing of the experimentation. compound was characterized using UV–Vis, IR, and It is wellmass known spectra. that The Schi massff basespectra ligands of the prepared are usually Schiff suitablebase showed sensors a peakfor at 255.08, both cationswhich is andin anions [6]. −1 Thus, a synthesizedagreement Schiwith theff base theoretically (Scheme calculated1) ligand molecular was weight investigated (256 g mole for) of the its Schiff ability base to ligand. sense multiple The elemental analysis was in good agreement with the theoretically calculated percentage values for metal ions. Here,the proposed the cations molecular could formulae coordinate and, hence, withconfirmed the and prepared proved the ligand expected through structures the of the nitrogen atom of the azomethinesynthesized group Schiff and base the ligand adjacent SL1. Moreover, soft sulfur the IR spectra atom showed of the the mercapto characteristic group, bandsforming for the a colored −1 −1 solution dueazomethine to the formation group (1606 of cm metal) and complexesfor the thiol group that (2560 can cm be). recognized Therefore, the bySL1 theligand naked structure eye. was confirmed before proceeding to the sensing experimentation. It is well known that Schiff base ligands are usually suitable sensors for both cations and anions 3.2. Transition[6]. Metal Thus, a Ions synthesized Sensing Schiff with base Colorimetry (Scheme 1) ligand Analysis was investigated for its ability to sense multiple metal ions. Here, the cations could coordinate2 with the prepared ligand through the nitrogen atom The sensingof the azomethine ability of group SL1 and(1 the10 adjacent− M, soft prepared sulfur atom in ofDMF) the mercapto toward group, various forming a metal colored ions such as 2+ 3+ 2+ 2+ 2+ 2+× 2+ 2+ 3+ 5+ 2+ 2+ 2+ 3 Cu , Cr , Fesolution, Ni due, to Co the formation, Mg , of Zn metal, complexes Fe , Fe that, NHcan be4VO recognized3 (V ),by Mn the naked, Hg eye. , Pb , and Al was monitored by the naked eye experiment. When the metal ion solution in H O (1 10 2 M) was added 3.2. Transition Metal Ions Sensing with Colorimetry Analysis 2 − × 2+ to the solution of the SL1 compound, the color changed from light yellow to light brown for Fe and The sensing ability of SL1 (1 × 10−2 M, prepared in DMF) toward various metal ions such as Cu2+, 3+ 3+ +2 Fe , to purpleCr3+ for, Fe2+ Cr, Ni2+, Co and2+, Mg to2+ indigo, Zn2+, Fe blue2+, Fe3+ for, NH Hg4VO3 (V, as5+), Mn shown2+, Hg2+ in, Pb Figure2+, and Al1.3 Thiswas monitored observation by indicates that these fourthe metal naked eye ions experiment. successfully When the formed metal ion metal solution complexes in H2O (1 × with10−2 M) the was Schi addedff baseto the solution ligand, which may be due to theof high the SL a1ffi compound,nity of thethe color azomethine changed from moiety light yellow and to the light adjacent brown for mercapto Fe2+ and Fe3+ group, to purple to these metal for Cr3+, and to indigo blue for Hg+2, as shown in Figure 1. This observation indicates that these four ions. The SL1metalsolution ions successfully did not formed show metal any significantcomplexes with color the Schiff change base ligand, upon which the addition may be due of to thethe other metal 2+ 2+ 2+ 2+ 2+ 2+ 5+ 2+ solutions of Cuhigh affinity, Co of, Nithe azomethine, Zn , Mg moiety, and Mn the,V adjacent, and mercapto Pb group. This to could these metal be explained ions. The SL by1 the Gibb’s solution did not show any significant color change upon the addition of the other metal solutions of free energy of the reaction for the formation of the metal–ligand (SL1) complex for these metals might Cu2+, Co2+, Ni2+, Zn2+, Mg2+, Mn2+, V5+, and Pb2+. This could be explained by the Gibb’s free energy of the not be sufficientreaction for for the the randomness formation of the of metal–ligand the reaction (SL1 and,) complex therefore, for thesehas metals no might influence not be sufficient on the formation of the metal–SLfor1-ligand the randomness complex of the and reaction no and, colors therefore, were has seen no influence [24]. Therefore,on the formation SL of1 thewas metal–SL only1 able- to sense Fe2+, Fe3+, Crligand3+, and complex Hg +and2 ions no colors in aqueous were seen [24]. solutions Therefore, by SL giving1 was only a able diff toerent sense colorFe2+, Fe for3+, Cr each3+, and metal ion. Hg+2 ions in aqueous solutions by giving a different color for each metal ion.

Figure 1. Color change of SL1 (1 ×2 10−2 M in DMF) before and after the addition of the metal ions of Figure 1. Color change of SL1 (1 10− M in DMF) before and after the addition of the metal ions of Cu2+, Cr3+, Fe2+, Ni2+, Co2+, Mg2+, Zn2+, Fe2+, Fe3+, NH4VO3 (V5+), Mn2+, Hg2+, Pb2+, and Al3 (1 × 10−2 M 2+ 3+ 2+ 2+ 2+ 2×+ 2+ 2+ 3+ 5+ 2+ 2+ 2+ 3 Cu , Cr , Fesolution, Ni in H,2O). Co , Mg , Zn , Fe , Fe , NH4VO3 (V ), Mn , Hg , Pb , and Al (1 2 × 10− M solution in H2O).

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Chemosensors 2020, 8, 1 6 of 11 3.3. UV–Visible Monitoring 3.3. UV–Visible Monitoring The Schiff base’s (SL1) behavior as a chemosensor was examined by monitoring the UV–Visible absorptionThe Schiff spectra base’s upon (SL1 the) behavior addition as ofa chemosensor the different metalwas examined ions under by monitoring investigation the in UV–Visible a distilled water–DMFabsorption spectra (1:1) solvent upon mixturethe addition at room of the temperature different (Figuremetal ions2). First,under we investigation recorded the in UV–Visible a distilled spectrawater–DMF of the (1:1) free solvent Schiff basemixture ligand at room in DMF, temperatur which exhibitede (Figure a2). broad First, bandwe recorded absorption the UV–Visible centered at 365 nm (Figure2a), assigned to n π* transition due to the azomethine moiety [27]. Upon the addition spectra of the free Schiff base ligand→ in DMF, which exhibited a broad band absorption centered at of365 the nm examined (Figure 2a), metal assigned ion solutions to n→π* transition to the SL 1dueligand, to the the azomethine absorbance moiety intensity [27]. Upon of SL 1theat addition 365 nm decreased,of the examined a red shift metal (~69–288 ion solutions nm) was to observed, the SL1 andligand, new the bands absorbance appeared intensity at 434, 447, of 570, SL1and at 365 653 nm +3 +2 +3 +2 afterdecreased, the addition a red shift of Fe (~69–288, Fe , nm) Cr was, and observed, Hg solutions and new to thebands SL1 appearedsolution, at respectively 434, 447, 570, (Figure and2 653b). Thisnm after bathochromic the addition shift of mayFe+3, beFe+2 due, Cr to+3, theand ligand-to-metal Hg+2 solutions to charge-transfer the SL1 solution, (LMCT). respectively The appearance (Figure 2b). of newThis bathochromic peaks was due shift to themay coordination be due to the between ligand-to the-metal Schi ffcharge-transferbase ligand and (LMCT). the metal The ionsappearance that may of takenew placepeaks [ 16was]. Thedue azomethineto the coordination moiety increasesbetween thethe withdrawingSchiff base ligand character and the of themetal ligand, ions causingthat may a strongertake place intramolecular [16]. The azomethine charge transfer moiety from increases the electron-donating the withdrawing group character (-SH) of to the the ligand, metal ions causing in the a complexstronger intramolecular [27]. This is the charge charge transfer transfer from from the the el ligandectron-donating molecular group orbitals (-SH) to the to partiallythe metal filled ions metalin the d-orbitals,complex [27]. and This this is results the charge in a reduction transfer from of the the metal ligand ions. molecular The diff orbitalserence in to the thered partially shift for filled diff metalerent metalsd-orbitals, may and be duethis results to the diinff aerence reduction in metal of the ion me sizetal andions. the The charge difference densities in the [27 red,28 shift]. For for other different ions, themetals UV–Vis may absorptionbe due to the spectra difference of SL in1 exhibited metal ion nosize change and the because charge no densities complexes [27,28]. were For formed other ions, in these the +3 +2 +3 2+ cases.UV–Vis These absorption observations spectra of suggest SL1 exhibited that SL no1 canchange only because act as ano sensor complexes for Fe were, Feformed, Cr in ,these and cases. Hg metalThese ions.observations suggest that SL1 can only act as a sensor for Fe+3, Fe+2, Cr+3, and Hg2+ metal ions.

3 Figure 2. (a) UV–Vis spectra of SL (1 10−3 M, in DMF) and (b) UV–Vis spectra of SL upon titration Figure 2. (a) UV–Vis spectra of SL11 (1 ×× 10− M, in DMF) and (b) UV–Vis spectra of SL11 upon titration with aqueous solution of cations (SL + Al3+, SL + Cr3+, SL + Cu2+, SL + Fe2+, SL + Fe3+, SL + with aqueous solution of cations (SL11 + Al3+, SL1 +1 Cr3+, SL1 + Cu1 2+, SL1 + Fe2+1 , SL1 + Fe3+, 1SL1 + Hg2+, SL1 1 Hg2+, SL + Mg2+, SL + Mn2+,R + Ni2+, SL + Pb2+, SL + Zn2+, and R + V5). + Mg2+, SL1 1 + Mn2+, R +1 Ni2+, SL1 + Pb2+, SL1 + Zn1 2+, and R +1 V5). 3.4. Job’s Plot and Detection Limit 3.4. Job’s Plot and Detection Limit In order to study the sensitivity and the binding mechanism of Fe2+, Fe3+, Cr3+, and Hg2+ metal In order to study the sensitivity and the binding mechanism of Fe2+, Fe3+, Cr3+, and Hg2+ metal ions ions to the SL1 ligand, UV–Vis titration experiments were performed in DMF solution. For the to the SL1 ligand, UV–Vis titration experiments were2+ perf3+ormed3+ in DMF solution.2+ For the stoichiometric stoichiometric ratio between the ligand SL1 and Fe , Fe , Cr , and Hg metal ions for the formation ratio between the ligand SL1 and Fe2+, Fe3+, Cr3+, and Hg2+ metal ions for the formation of metal complexes, of metal complexes, the Job’s plot continuous variation method was utilized (Figure3)[ 29–31]. the Job’s plot continuous variation method was utilized (Figure 3) [29–31]. The method kept the total The method kept the total concentration of SL1 and metal ions at 0.01 M and changed the molar ratio concentration of SL1 and metal ions at 0.01 M and changed the molar ratio of the metal ions from 0.1 of the metal ions from 0.1 to 1.0 M. to 1.0 M. It can be seen from Figure3 that the maximum absorbance was observed when the molar ratio of the ligand to metal was 0.66, indicating the formation of a 1:2 (Metal:Ligand) complex [6]. The possible binding mode of the ligand with the metal ions is through the coordination of the nitrogen atom (N) of the azomethine moiety and the sulfur atom (S) of the thiol group. Therefore, SL1 is a bidentate ligand and forms metal complexes with a possible structure as shown in Scheme2.

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Figure 3. Job’s plot of (a) SL1 and Fe2+ complex, (b) SL1 and Fe3+ complex, (c) SL1 and Cr3+ complex,

and (d) SL1 and Hg2+ complex.

It can be seen from Figure 3 that the maximum absorbance was observed when the molar ratio of the ligand to metal was 0.66, indicating the formation of a 1:2 (Metal:Ligand) complex [6]. The possible binding mode of the ligand with the metal ions is through the coordination of the nitrogen atom (N) of the azomethine moiety and the sulfur atom (S) of the thiol group. Therefore, SL1 is a bidentate Figure 3. Job’s plot of (a) SL1 and 2Fe+ 2+ complex, (b) SL1 and Fe3+3 +complex, (c) SL1 and Cr3+ complex,3+ Figure 3. Job’s plot of (a) SL1 and Fe complex, (b) SL1 and Fe complex, (c) SL1 and Cr complex, ligand and formsand metal (d) SL 1 complexesand 2Hg+ 2+ complex. with a possible structure as shown in Scheme 2. and (d) SL1 and Hg complex. It can be seen from Figure 3 that the maximum absorbance was observed when the molar ratio of the ligand to metal was 0.66, indicating the formation of a 1:2 (Metal:Ligand) complex [6]. The possible binding mode of the ligand with the metal ions is through the coordination of the nitrogen atom (N) of the azomethine moiety and the sulfur atom (S) of the thiol group. Therefore, SL1 is a bidentate ligand and forms metal complexes with a possibleN structure as shown in Scheme 2. H3C S CH3 N M N N H3C S CH3 H3C S CH3 N MN N H3C S CH3 N

Scheme 2. Possible metal complex structure. Scheme 2. Possible metal complex structure. The Job’s plots (Figure3) showScheme some 2. significantPossible metal deviation complex structure. from linearity, which may be due to the The formationJob’s plots of (Figure weak complexes 3) show and, some hence, significant the stability deviation constants from of the lin complexesearity, which are a ffmayected be [32 due]. to the Moreover,The some Job’s mixtures plots (Figure of complex 3) show so speciesme significant cause displacementdeviation from oflinearity, maximum which plots. may be due to the formation of formationweak complexes of weak complexes and, and,hence, hence, the the stab stability constants constants of the of complexes the complexes are affected are [32]. affected [32]. The UV–Vis spectrum of SL upon the addition of Fe2+, Fe3+, Cr+3, and Hg2+ is shown in Moreover, someMoreover, mixtures some mixtures of complex of complex1 specie species causes cause displacement of maximum of maximum plots. plots. Figure4a–d, respectively It can be seen that increasing the concentration of the metal ions that were added to SL resulted in an increased absorbance intensity at the isosbestic points (i.e., specific 1 wavelength) of 434 nm (Fe+3), 447 nm (Fe+2), 570 nm (Cr+3), and 653 nm (Hg+2). The detection limit of +3 +2 +3 2+ metal ions (Fe , Fe , Cr , and Hg ) by SL1 was calculated from the plot of absorbance intensity Chemosensors 2020, 8, 1 8 of 11

The UV–Vis spectrum of SL1 upon the addition of Fe2+, Fe3+, Cr+3, and Hg2+ is shown in Figure 4a–d, Chemosensorsrespectively2020, 8, 1 It can be seen that increasing the concentration of the metal ions that were added to SL71 of 10 resulted in an increased absorbance intensity at the isosbestic points (i.e., specific wavelength) of 434 nm (Fe+3), 447 nm (Fe+2), 570 nm (Cr+3), and 653 nm (Hg+2). The detection limit of metal ions (Fe+3, Fe+2, Cr+3, 2+ 5 versusand the ionHg concentrations.) by SL1 was calculated Therefore, from the the detection plot of absorban limit force theintensity metal versus ions was the ation 10 concentrations.− M. According to the WorldTherefore, Health the detection Organization limit for (WHO-1984), the metal ions 0.1 mgwas/ Lat of10 iron,−5 M. 0.05According mg/L ofto chromiumthe World Health (III), and 0.001 mgOrganization/L of mercury (WHO-1984), are present 0.1 inmg/L drinking of iron, water0.05 mg/L [19]. of Consequently, chromium (III), ourand ligand0.001 mg/L can of be mercury employed are present in drinking water [19]. Consequently,+3 +2 + 3our ligand +can2 be employed5 to recognize the tested to recognize the tested metal ions of Fe , Fe , Cr , and Hg at 10− M. metal ions of Fe+3, Fe+2, Cr+3, and Hg+2 at 10−5 M.

Figure 4. (a) UV–Vis absorption spectroscopic titration of SL1 (1 × 10−22 M) with various concentrations Figure 4. (a) UV–Vis absorption spectroscopic titration of SL1 (1 10− M) with various concentrations 3+of Cr3+ in DMF solution (λmax = 570 nm)—inset: changes of UV–Vis× absorption intensity upon the of Cr in DMF solution (λmax = 570 nm)—inset: changes of UV–Vis absorption intensity upon the addition 3of+ Cr3+ at 570 nm; (b) UV–Vis absorption spectroscopic titration of SL1 (1 × 10−2 M)2 with addition of Cr at 570 nm; (b) UV–Vis absorption spectroscopic titration of SL1 (1 10− M) with various concentrations of Fe2+ in DMF solution (λmax = 447 nm)—inset: changes of UV–Vis× absorption various concentrations of Fe2+ in DMF solution (λ = 447 nm)—inset: changes of UV–Vis absorption 2+ max intensity upon the addition of 2Fe+ at 447 nm; (c) UV–Vis absorption spectroscopic titration of SL1 (1 × intensity upon the addition of Fe at 447 nm; (c) UV–Vis absorption spectroscopic titration of SL1 10−2 M) with various concentrations of Fe3+ in DMF solution (λmax = 434 nm)—inset: changes of UV– 2 3+ λ (1 10Vis− M) absorption with various intensity concentrations upon the addition of Feof Fe3+in at DMF 434 nm; solution and (d) (UV–Vismax = absorption434 nm)—inset: spectroscopic changes × 3+ of UV–Vistitration absorption of SL1 (1 intensity× 10−2 M) with upon various the addition concentrations of Fe of atHg 4342+ in nm;DMF andsolution (d) UV–Vis(λmax = 653 absorption nm)— 2 2+ spectroscopicinset: changes titration of UV–Vis of SL1 (1 absorption10− M) intensity with various upon the concentrations addition of Cr3+ of at Hg 653 nm.in DMF solution (λmax × = 653 nm)—inset: changes of UV–Vis absorption intensity upon the addition of Cr3+ at 653 nm. 3.5. Comparison with Other Studies 3.5. Comparison with Other Studies Alizadeh et al. (2011) reported the successful use of the Schiff base 2-[(2- Alizadehsulfanylphenyl)ethanimidoyl] et al. (2011) phenol, reported a tridentate the ligand successful with N, S, and use O donor of atoms, the as Schi a sensorff base 2-[(2-sulfanylphenyl)ethanimidoyl]for the selective monitoring of the phenol,Hg2+ ion by a tridentate the chemical ligand immobilization with N, S,of andthe ligand O donor on an atoms, agarose as a sensorfilm for themembrane. selective The monitoring study revealed of the the Hg successful2+ ion by application the chemical of immobilizationa selective sensor of for the Hg ligand2+ ions onin an agarosean film amalgam membrane. alloy and The spiked study water revealed samples the without successful any applicationsignificant interference of a selective from sensor other formetal Hg 2+ ions inions. an amalgam alloy and spiked water samples without any significant interference from other metal ions. A novel Schiff base containing a pyrene ring with the thiol group adjacent to the azomethine group has been reported and used as an effective fluorescent probe for detecting Hg2+ in living cells via a chelation mechanism [33]. New rhodamine Schiff base sensors carrying the dithiocarbonate group have been shown to respond selectively to Hg2+ by showing a strong colorimetric change and intense fluorescence [29]. This was explained in part to be due to the preferential metal bonding of two sulfur atoms of the dithiocarbonate groups regardless of the main structure of rhodamine. Chemosensors 2020, 8, 1 8 of 10

The reported Schiff base ligands previously mentioned have similar binding sites to our Schiff base ligand. Based on hard–soft trends, they are similar in having the soft sulfur atom of the thiol group and the intermediate hardness nitrogen atom of the azomethine group as binding sites to the metal ions. It seems to be that this system with a soft sulfur atom adjacent to the azomethine group demonstrated a high binding affinity to metal ions via a strong S–Mn+ interaction, resulting in unstable complexes and, hence, a color change [33].

4. Conclusions In conclusion, a Schiff base ligand named 1-(2-thiophenylimino)-4-(N-dimethyl)benzene was prepared and used for the detection of multiple metal ions in solution. The synthesized Schiff base ligand exhibited a remarkable selectivity and sensitivity response toward four metal cations: Fe2+, Fe3+, Cr3+, and Hg2+. The investigation method used was UV–Visible spectra. A new peak was observed at 447 nm with a red shift of 82 nm in the case of Fe2+, a peak at 434 nm in the case of Fe3+ with a red-shift value of 69 nm, a peak at 570 nm with a red shift of 205 nm in the case of Cr3+, and a peak at 653 nm with a red shift of 288 nm in the case of Hg2+, accompanied by a color change that could be distinguished by the naked eye. The investigation of the Schiff base ligand binding to the metal ions was found to be a 2:1 ligand-to-metal ratio, according to Job’s plot and using a UV–Visible method as a direct approach to monitoring ligand–metal interactions.

Author Contributions: The three authors worked as a team and contributed equally throughout the research work including the design of the study; the experimentation, analysis and interpretation of data; preparation of the manuscript; writing, editing, and revising. The corresponding author A.Q.A. obtained funding from the Research Deanship at Al Baha University, and is the project administrator. All authors have read and agreed the published version of the manuscript. Funding: The Research Deanship at Al Baha University funded this research (grant number 1439/20). Acknowledgments: We gratefully acknowledged the financial support by Al Baha University (Project No: 1439/20), and are grateful to the Scientific Research Deanship and the Dean of the Faculty of Science at Al Baha University for their encouragement in our research and for the use of the laboratory facilities. Conflicts of Interest: The funder, Al Baha University, had no role in the design of the study, in the execution of the research, in the interpretation or analysis of data, in the writing of the manuscript, or in the decision to publish the results.

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