Spectrophotometric Determination of Trace Concentrations of Copper In

applied sciences

Article Spectrophotometric Determination of Trace Concentrations of Copper in Waters Using the Chromogenic Reagent 4-Amino-3-Mercapto-6- [2-(2-Thienyl)Vinyl]-1,2,4-Triazin-5(4H)-One: Synthesis, Characterization, and Analytical Applications

Salman. S. Alharthi 1 and Hamed. M. Al-Saidi 2,*

1 Department of Chemistry, Faculty of Science, Taif University, P.O. BOX 7685, Taif 21944, Saudi Arabia; [email protected] 2 Department of Chemistry, University College in Al-Jamoum, Umm Al-Qura University, Makkah 21955, Saudi Arabia * Correspondence: [email protected] or [email protected]

Received: 19 May 2020; Accepted: 2 June 2020; Published: 4 June 2020

Abstract: A simple, selective, and inexpensive spectrophotometric method is described in the present study for estimation of trace concentrations of Cu2+ in water based on its reaction with chromogenic reagent namely 4-amino-3-mercapto-6-[2-(2-thienyl)vinyl]-1,2,4-triazin-5(4H)-one (AMT). The reaction between copper(II) ions and AMT reagent gives [Cu(L)(NO )(H O) ] H O complex, where L 3 2 2 • 2 represents AMT molecule with NH group. The formed complex exhibits a sharp, and well-deﬁned 4 1 1 peak at λmax = 434 nm with a molar absorptivity (ε) of 1.90 10 L mol− cm− , and Sandell’s 2 × factor of 0.003 µg mL− . Absorbance of the [Cu(L)(NO3)(H2O)2] H2O follows Beer’s law over a 1 1 • 0.7–25 µg mL− range with a detection limit of 0.011 µg mL− . Validation of the submitted method was established by estimating Cu2+ in certiﬁed reference materials and actual sea and tap water samples. The results are compared with data obtained from copper concentration measurements using ICP-OES. The chemical structure of the Cu(II)-AMT complex was fully characterized by FT-IR, SEM, EDX, TGA, and ESR techniques.

Keywords: copper ions; spectrophotometry; characterization; water samples

1. Introduction Copper is a biologically-active element, whose compounds have a strong inﬂuence on the vital activities of organisms. Copper plays an essential role in biological processes including metabolism, nerve function, hemoglobin synthesis, and bone development [1–3]. However, an increase in the amount of copper in the human body may lead to serious problems such as heart disease, anemia, and liver damage [4].Therefore, the copper concentration in environmental samples—especially water—should be monitored continuously, preferably by simple and eﬀective analytical methods. Many analytical techniques including microwave plasma—AES [5], AAS [6], potentiometry [7,8], voltammetry [9], ICP-OES [10], and ICP-MS [11] have been developed and used to monitor copper concentration levels in a variety of samples. Actually, most AAS and ICP-OES methods recently developed use preconcentration methodology to improve sensitivity and selectivity. However, such methodology is highly costly, and time-consuming. Spectrophotometric techniques generally are preferred for metal ion determinations in aqueous media, because of their simplicity, low cost, and availability [12]. Among the many

Appl. Sci. 2020, 10, 3895; doi:10.3390/app10113895 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 3895 2 of 17

spectrophotometric reagents used to determine copper are 1,4-dioxane [13], 4-(20-benzothiazolylazo) salicylic acid (BTAS) [14], 2,4-dinitro APTPT [15], diethyldithiocarbamate (DDTC) [16,17], 2-amino-4- (m-tolylazo)pyridine-3-ol (ATAP) [18], 2-(5-Bromo-2-Oxoindolin-3-ylidene) hydrazine carbothioamide (HBITSC) [19], 4-(40-nitrobenzylideneimino)-3-methyl-5-mercapto-1,2,4-triazole (NBIMMT) [20], 2-acetylpyridine thiosemicarbazone (2-APT) [21], N,Nbissalicylidene-2,3-diaminopyridine(H2IF) [22], and rhodamin derivative with 5-(α´-methyl-3 hydoxybenzylidene) (5M, 3H-BR) [23]. However, most of these reagents suﬀer from shortcomings such as a narrow pH range, interference from common ions, the need to heat the reaction mixture, and lengthy color development [24]. Thus, there is an urgent need to prepare selective and sensitive chromogenic reagents that can operate over a wide range of concentrations and pH values. Although reagent 4-amino-3-mercapto-6-[2-(2-thienyl)vinyl]-1,2,4-triazin-5(4H)-one (AMT) has been previously reported [25–28], as far as we know, there is no analytical use of this reagent yet. Therefore, the present work will discuss the preparation and characterization of a copper(II) complex with AMT and the use of this ligand as a spectrophotometric reagent for developing an analytical method for determining copper in certiﬁed reference materials and water samples.

2. Experimental

2.1. Chemicals and Reagents All materials employed in the study were of analytical reagent grade and no further purification was required. Alkali, alkaline earth, and heavy metal nitrates were obtained from Sigma-Aldrich, St. Louis, MO, USA. All solvents were purchased from Fisher Scientific. Deionized water was used exclusively in the experiments. A weighed amount of CuNO3 3H2O was dissolved in an appropriate volume of • 2+ 1 deionized water for preparation of stock solution of Cu (50 µg mL− )[29]. Appropriate weights of nitrate salts of some metal ions were dissolved in deionized water for obtaining stock solutions of these ions. The pH of aqueous solutions was adjusted to 4.5 with acetate buffer (CH3COOH/CH3COONa).

2.2. Apparatus Electronic spectra and absorbance values of the ligand and its Cu complex were recorded using a HACH LANGE (Model DR-6000) spectrophotometer. The elemental compositions of the ligand and its complex with copper were determined using a Perkin Elmer CHNS/O analyzer model 2400 Series II. 1 The ligand and its Cu complex were characterized by FT-IR spectroscopy at 400–4000 cm− with an ALPHA attenuated FT-IR spectrometer (Bruker, Billerica, MA, USA). 1H NMR of AMT compound was recorded by FT-NMR spectrometer (A Bruker DRX-500). A JEOL JEM-6390 scanning electron microscope combined with a unit of energy dispersive X-ray spectroscopy was used to record morphological images and to determine elemental distributions in the samples. Measurements of electron spin resonance (ESR) were carried out using a SPINSCAN X spectrometer from ADANI with operating frequency of 9.2–9.55 GHz. Diphenylpicrylhydrazyl (DPPH) was used as a reference to calculate the g-values. The g factor of DPPH is 2.0036. Copper complex with AMT was grounded to get ﬁne particles, then the power was placed in 1 mm (ID) sealed capillary tube. All ESR measurements were carrying out at 300 K. Magnetic susceptibility of Cu(II)-AMT complex was measured at 25 0.35 C by ± ◦ a magnetic susceptibility balance (Johnson Matthey, Alfa product, Model No. (MKI)) calibrated using Hg[Co(NCS)4]. Determination of metal ion concentrations in aqueous solution was carried out by ICP-OES spectrometry (Optima 2100 DV, Perkin Elmer, Waltham, MA, USA). pH measurements were made with a Jenway 3305 pH meter. Thermogravimetric measurements were performed by use of a 1 Mettler-Toledo TGA/DSC1 calorimeter under a nitrogen ﬂow of 60 mL min− at 25–900 ◦C.

2.3. Preparation of the AMT Ligand AMT ligand was synthesized by the method reported in [26]. Scheme1 summarizes the 1 procedure. H NMR (DMSO-d6): δ 6.48 (s, 2H, NH2), 6.90 (d, 1H, thiophene-CH=CH-triazine), Appl. Sci. 2020, 10, 3895 3 of 17

Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 17 7.06 (t, 1H, Hb of thiophene), 7.36 (d, 1H, Hc of thiophene), 7.59 (d, 1H, Ha of thiophene), 8.11 (d, 13 1H, thiophene-CH=CH-triazine), 14.04 (s, 1H, SH). CNMR (DMSO-d6): δ 118.0 (thiophene-CH=CH- thiophene-CH=CH-triazine), 14.04 (s, 1H, SH). 13CNMR (DMSO-d6): δ 118.0 (thiophene-CH=CH- triazine), 126.7 (thiophene-CH=CH-triazine), 127.7 (C4 of thiophene), 128.7 (C3 of thiophene), 129.3 triazine), 126.7 (thiophene-CH=CH-triazine), 127.7 (C4 of thiophene), 128.7 (C3 of thiophene), 129.3 (C5 of thiophene), 141.0 (C2 of thiophene), 141.2 (C6 of triazine), 147.7 (C-SH), 166.8 (C=O). Elemental (C5 of thiophene), 141.0 (C2 of thiophene), 141.2 (C6 of triazine), 147.7 (C-SH), 166.8 (C=O). Element1 al analysis(%) of AMT calculated for C9H8N4OS2 (252.31) is displayed in Table 2, while, the HNMR analysis(%) of AMT calculated for C9H8N4OS2 (252.31) is displayed in Table 2, while, the 1HNMR and and 13CNMR spectra of AMT ligand are shown in Supplementary Materials (Figures S1 and S2, 13CNMR spectra of AMT ligand are shown in Supplementary Materials (Figures S1 and S2, respectively). respectively).

Scheme 1. Synthesis of AMT. Scheme 1. Synthesis of AMT. 2.4. Preparation of [Cu(L)(NO )(H O) ] H O 3 2 2 • 2 2.4. Preparation of [Cu(L)(NO3)(H2O)2].H2O 1.00 mmol of AMT and 1.00 mmol of CuNO 3H O were refluxed in methanol at 90 C for 3• 2 ◦ three1.00 hours. mmol After of AMT cooling and the 1.00 reaction mmol vesselof CuNO to room3.3H2O temperature were refluxed (25 in 0.4),methanol the brown at 90 °C precipitate, for three ± [Cu(L)(NOhours. After3)(H cooling2O)2] Hthe2O, reaction was obtained vessel toby room filtration temperature in non-pure (25 form.± 0.4), However, the brown the precipitate, obtained 3 2 2 . • 2 precipitate[Cu(L)(NO was)(H purifiedO) ] H O, by recrystallizationwas obtained by from filtration methanol, in and non the-pure pure form. product However, was dried the at 80–90 obtained◦C. Yield:precipitate 78%. was Elemental purified analysis by recrystallization (%) of [Cu(L)(NO from methanol,)(H O) ] andH O the calculated pure product for C wasH driedN O Sat-Cu 80– 3 2 2 • 2 9 14 5 7 2 (431.90)90 °C. Yield: is displayed 78%. Elemental in Table 2.analysis (%) of [Cu(L)(NO3)(H2O)2].H2O calculated for C9H14N5O7S2-Cu (431.90) is displayed in Table 2. 2.5. Estimation of Cu(II) in Waters Using Recommended Method

2.5. Estimation of Cu(II) in Waters Using Recommended Method 1 Different volumes of standard copper(II) solutions (0.7–25.0 µg mL− ) were added to 8 mL of a 0.042%Different (w/v) AMT volumes solution of standard in 10 mL ofcopper(II) acetate bu solutionsffer (pH (0.7 4–6).–25.0 Deionized μg mL− water1) were was added used to to 8 dilute mL of all a solutions0.042% (w/v) to 50 AMT mL. solution Absorbance in 10 was mL measuredof acetate atbuffer 434 nm(pH against 4–6). Deionized a reagent water blank. was 100 used mL ofto tapdilute or seaall solutions water was to sampled,50 mL. Absorbance filtered, and was transferred measured intoat 434 clean nm against polyethylene a reagent bottles. blank. All 100 samples mL of tap were or analyzedsea water within was sampled, 6 h of collection. filtered, Appropriateand transferred quantities into clean were polyethylene treated according bottles. to theAll recommendedsamples were procedureanalyzed within before 6 and h of after collection. spiking Appropriate with known quantities concentrations were treated of Cu(II). according to the recommended procedure before and after spiking with known concentrations of Cu(II). 2.6. Evaluation of Proposed Method Using Certified Reference Materials 2.6. EvaluationMultielement of Proposed standard Method solution Using (# 90243)Certified purchased Reference fromMaterials Sigma-Aldrich was used as a certified referenceMultielement material standard to evaluate solution the proposed (# 90243) method.purchased According from Sigma to- productAldrich was catalog, used thisas a solutioncertified 1 1 containsreference 10 material mg L− of to Cu, evaluate in addition the proposed to K, Bi, and method. Pb, with According concentration to product of 100 catalog, mg L− ; this Al, B, solution Cr, Li, 1 Mo,contains Na, Ni,10 mg and L Tl−1 atof concentrationCu, in addition level to K, of Bi, 50 and mg LPb,− ;with while, concentration Ba, Ca, Cd, Co,of 100 Fe, mg Mg, L Mn,−1; Al, Sr, B, and Cr, ZnLi, areMo, in Na, concentrations Ni, and Tl at equal concentration to Cu. An level aliquot of of50 themgsolution L−1; while, was Ba, analyzed Ca, Cd, byCo, recommended Fe, Mg, Mn, Sr, procedure and Zn forare estimating in concentrations Cu after addition equal to of Cu. few An drops aliquot of KF of2H the2O solution (0.1% m /v was) as a analyzed masking by agent. recommended • procedureThe steel for sampleestimating (No.21899) Cu after obtained addition from of few Analytical drops of ChemistryKF.2H2O (0.1% Laboratory m/v) as Service, a masking (München, agent. Germany)The steel was sample also used (No.21899) as a certified obtained reference from material Analytical to evaluate Chemistry the Laboratory proposed method. Service, The(München, copper contentGermany) in the was sample also used is 0.519% as a certified (w/w). The reference main components material to are evaluate Cr, Ni, the Mn, proposed and Zn,in method. addition The to ultra-tracescopper content concentrations in the sample of Mo, is 0.519% P, C, S, Si, (w/w). Tl, V, The and main Co. components are Cr, Ni, Mn, and Zn, in addition0.2 g to of ultra steel-traces sample concentrations was heated with of Mo, 10 mL P, C, of S, aqua Si, Tl regia, V, toand near Co. dryness. The sample was then mixed0. with2 g of 5 steel mL of sample concentrated was heated H2SO with4 and 10 heated mL of foraqua 30 regia min. Theto near resulting dryness. solution The sample was neutralized was then bymixed NaOH with and 5 mL diluted of concentrated using doubly H2SO distilled4 and heated water for in 30 a 100min. mL The calibrated resulting flask.solution A partwas neutralized of aqueous solutionby NaOH was and subjected diluted tousing recommended doubly distilled procedure water to in estimate a 100 mL copper calibrated in steel flask. sample. A part of aqueous solution was subjected to recommended procedure to estimate copper in steel sample.

3. Results and Discussion A brown product was observed immediately upon mixing the aqueous copper(II) and AMT solutions. The electronic spectrum of the free ligand solution (1.2 × 10−4 mol L−1) recorded against deionized water as a blank showed two peaks at 254, and 376 nm assigned to π–π* and n → π* transitions, respectively (Figure 1a). The peak at 376 nm shifted to 434 nm after reaction of the AMT Appl. Sci. 2020, 10, 3895 4 of 17

3. Results and Discussion A brown product was observed immediately upon mixing the aqueous copper(II) and AMT Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 17 solutions. The electronic spectrum of the free ligand solution (1.2 10 4 mol L 1) recorded against × − − deionized water as a blank showed two peaks at 254, and 376 nm assigned to π–π* and n π* reagent with copper(II) (Figure 1b–d). Figure 1 also demonstrates the increase in absorbance with → increasingtransitions, Cu2+ concentration. respectively (Figure Moreover,1a). Thethe electronic peak at 376 spectrum nm shifted of Cu(II)-AMT to 434 nm complex after reaction recorded of the AMT in solidreagent state with using copper(II) the Nujol (Figure mulls 1methodb–d). Figure and shown1 also demonstratesin Supplementary the increaseMaterials in (Figure absorbance S3) with 2+ exhibitedincreasing a broad Cu bandconcentration. at 630 nm corresponding Moreover, theto T electronic2g(2D) ← E spectrumg in octahedral of Cu(II)-AMT environment complex [30]. The recorded in effectivesolid magnetic state using moment the Nujol (μeff) mulls of copper method complex and shown with AMT in Supplementary calculated from Materials the Equation (Figure (1) S3)was exhibited 2 1.82 aB.M broad confirming band at the 630 distorted nm corresponding octahedral tosymmetry T ( D) of thisEg incomplex. octahedral environment [30]. The effective 2g ← magnetic moment (µeff) of copper complex with AMT1 calculated from the Equation (1) was 1.82 B.M μeff = 2.828 (χM.T) /2 (1) confirming the distorted octahedral symmetry of this complex. where, χM is molar susceptibility modified using Pascal’s constants of all atoms diamagnetism, while 1/2 T is absolute temperature. µeff = 2.828 (χM.T) (1) The stoichiometry of Cu(II)-AMT complex was estimated using the mole ratio method, wherein χ the absorbancewhere, M isat molar434 nm susceptibility was plotted versus modified the using[AMT]/[Cu Pascal’s2+] ratio constants as shown of all in atoms Figure diamagnetism, 2. The results while T indicateis absolute that the temperature. AMT ligand and Cu2+ reacted in a 1:1 ratio.

148000

a b 98000

c ) -1 d cm 3 dm -1 48000 ε (mol

-2000 200 300 400 500 600 700 800

Wavelength, nm

FigureFigure 1. Electronic 1. Electronic spectra spectra recorded recorded in aqueousaqueous mediamedia of (ofa) 4-amino-3-mercapto-6-[2-(2-thienyl)vinyl]-(a) 4-amino-3-mercapto-6-[2-(2- thienyl)vinyl]-1,2,4-triazin-5(4H)-one1,2,4-triazin-5(4H)-one (AMT) ligand (AMT) at concentrationligand at concentration of 1.2 10 of4 mol1.2 L× 110and−4 mol following L−1 and addition of × − − followingthe Cu(II)-AMT addition of complex the Cu(II)-AMT at Cu2+ concentrationscomplex at Cu2+ of concentrations (b) 0.70 µg/mL, of ( c()b 3.50) 0.70µg μ/mL,g/mL, and (c) ( d3.50) 5.50 µg/mL. μg/mL, and (d) 5.50 μg/mL. The stoichiometry of Cu(II)-AMT complex was estimated using the mole ratio method, wherein the absorbance at 434 nm was plotted versus the [AMT]/[Cu2+] ratio as shown in Figure2. The results indicate that the AMT ligand and Cu2+ reacted in a 1:1 ratio.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 17

reagent with copper(II) (Figure 1b–d). Figure 1 also demonstrates the increase in absorbance with increasing Cu2+ concentration. Moreover, the electronic spectrum of Cu(II)-AMT complex recorded in solid state using the Nujol mulls method and shown in Supplementary Materials (Figure S3) exhibited a broad band at 630 nm corresponding to T2g(2D) ← Eg in octahedral environment [30]. The effective magnetic moment (µ eff) of copper complex with AMT calculated from the Equation (1) was 1.82 B.M confirming the distorted octahedral symmetry of this complex.

1 µ eff = 2.828 (χM.T) /2 (1)

where, χM is molar susceptibility modified using Pascal’s constants of all atoms diamagnetism, while T is absolute temperature. The stoichiometry of Cu(II)-AMT complex was estimated using the mole ratio method, wherein the absorbance at 434 nm was plotted versus the [AMT]/[Cu2+] ratio as shown in Figure 2. The results indicate that the AMT ligand and Cu2+ reacted in a 1:1 ratio.

148000

a b 98000

)

1 - d

1 - 48000

(mol

-2000 200 300 400 500 600 700 800

Wavelength, nm (nm) Figure 1. Electronic spectra recorded in aqueous media of (a) 4-amino-3-mercapto-6-[2-(2- thienyl)vinyl]-1,2,4-triazin-5(4H)-one (AMT) ligand at concentration of 1.2 × 10−4 mol L−1 and following addition of the Cu(II)-AMT complex at Cu2+ concentrations of (b) 0.70 µg/mL, (c) 3.50 Appl. Sci.µg/mL2020,, and10, 3895 (d) 5.50 µg/mL. 5 of 17

0.06

0.05

0.04

Abs 0.03

0.02

0.01

0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

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Figure 2. Determination of the stoichiometry of the Cu(II)–AMT complex. Blue lines represent the Figure 2. Determination of the stoichiometry of the Cu(II)–AMT complex. Blue lines represent the tangentstangents of of the the linear linear portions portions of of the the mole mole ratio ratio curve, curve, while while red red dashed dashed line line shows shows the the molar molar ratio. ratio.

3.1. Characterization 3.1. Characterization Techniques including FT-IR, SEM, EDX, TGA, and ESR were used to characterize the complex Techniques including FT-IR, SEM, EDX, TGA, and ESR were used to characterize the complex formed between Cu(II) and AMT. formed between Cu(II) and AMT. 3.1.1. FT-IR Technique 3.1.1. FT-IR Technique The FT-IR spectra of AMT and the [Cu(L)(NO3)(H2O)2] H2O complex are demonstrated by traces The FT-IR spectra of AMT and the [Cu(L)(NO3)(H2O)•2].H2O complex are demonstrated by traces1 A and B, respectively, in Figure3. For AMT, a peak due to NH 2 was observed at 3291–3195 cm− , A and B, respectively, in Figure 3. For AMT, a peak due 1to NH2 was observed at 3291–3195 cm−1, aromatic C–H stretching peak was located at 3064–3091 cm− , and the aliphatic C–H stretching peak aromatic C–H stretching1 peak was located at 3064–3091 cm−1, and the aliphatic C–H1 stretching peak was observed at 2936 cm− . A peak corresponding to S–H was observed at 2665 cm− . Peaks of C=O, was observed at 2936 cm−1. A peak corresponding to S–H was1 observed at 2665 cm−1. Peaks of C=O, C=C, and C=N were observed at 1660, 1593, and 1618 cm− , respectively. Following differences C=C, and C=N were observed at 1660, 1593, and 1618 cm−1, respectively. Following2+ differences were were observed between the spectrum of AMT and that of its complex with Cu . The NH2 peak in observed between the spectrum of AMT and1 that of its complex with Cu2+. The NH2 peak in Cu(II)- Cu(II)-AMT shifted to approximately 3142 cm− and decreased significantly in intensity. This indicates AMT shifted to approximately 3142 cm−1 and decreased significantly in intensity. This indicates that that the NH2 group participated in complex formation. The C=O frequency shifted from 1660 to the NH12 group participated in complex formation. 2The+ C=O frequency shifted from 1660 to 1705 cm−1 1705 cm− in reference to coordination of AMT to Cu through the carbonyl oxygen atom. Peaks due in reference to coordination of AMT to Cu2+ through the carbonyl1 oxygen atom. Peaks due to ν(Cu– to ν(Cu–N), and ν(Cu–O) were observed at 530, and 518 cm− , respectively. The coordination behavior −1 ofN), the and nitrate ν(Cu ion–O) was were confirmed observed through at 530, and the 518 appearance cm , respectively. of characteristic The coordination frequencies behavior of bidentate of the nitrate ion was confirmed through the1 appearance of characteristic frequencies of bidentate nitrate nitrate group at 1545, 1353, and 820 cm− and the absence of the distinctive band of ionic nitrate group group at 1545,1 1353, and 820 cm−1 and the absence of the distinctive band of ionic nitrate group at at 1766 cm− [31]. Therefore, the nitrate group served as a bidentate ligand with C2v symmetry. Table1 1766 cm−1 [31]. Therefore, the nitrate group served as a bidentate ligand with C2v symmetry. Table 1 lists the different FT-IR frequencies of AMT and the [Cu(L)(NO3)(H2O)2] H2O complex. lists the different FT-IR frequencies of AMT and the [Cu(L)(NO3)(H2O)•2].H2O complex. 100 A 90 B 80 70 T 60 50 40 30 20 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber, Cm-1

Figure 3. FT-IR spectra of (A) AMT and (B) [Cu(L)(NO3)(H2O)2] H2O. •

Figure 3. FT-IR spectra of (A) AMT and (B) [Cu(L)(NO3)(H2O)2].H2O.

Table 1. FT-IR frequencies (cm−1) and tentative assignments of AMT and the Cu(II)-AMT complex.

AMT Reagent Tentative Assignments Cu(II)-AMT Complex Tentative Assignments 3291, 3195 strong ν(NH2) 3142 weak ν(NH) 3064–3091 weak ν(aromatic C–H) 3066 weak ν(aromatic C–H) 2936 weak ν(aliphatic C–H) 2901 weak ν(aliphatic C–H) 2665 weak ν(SH) 2667 weak ν(SH) 1660 very strong ν(C=O) 1705 very strong ν(C=O) 1593 strong ν(C=C) 1554 strong ν(C=C) 1618 strong ν(C=N) 1618 strong ν(C=N) 1401 strong δ(N–C–S) 1421 strong δ(N-C-S) 1420 medium δ(aliphatic C–H) 1412 medium δ(aliphatic C–H) Appl. Sci. 2020, 10, 3895 6 of 17

1 Table 1. FT-IR frequencies (cm− ) and tentative assignments of AMT and the Cu(II)-AMT complex.

AMT Reagent Tentative Assignments Cu(II)-AMT Complex Tentative Assignments

3291, 3195 strong ν(NH2) 3142 weak ν(NH) 3064–3091 weak ν(aromatic C–H) 3066 weak ν(aromatic C–H) 2936 weak ν(aliphatic C–H) 2901 weak ν(aliphatic C–H) 2665 weak ν(SH) 2667 weak ν(SH) 1660 very strong ν(C=O) 1705 very strong ν(C=O) 1593 strong ν(C=C) 1554 strong ν(C=C) 1618 strong ν(C=N) 1618 strong ν(C=N) 1401 strong δ(N–C–S) 1421 strong δ(N-C-S) 1420 medium δ(aliphatic C–H) 1412 medium δ(aliphatic C–H) 530 medium ν(Cu–N) 518 medium ν(Cu–O) 1545, 1353, 820 Nitrate group

Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 17 3.1.2. SEM and EDX Analysis 530 medium ν(Cu–N) The SEM images of AMT and its complex with518 medium copper(II) shownν(Cu–O) in Figure 4 provide useful information on surface morphology and particles 1545, size. 1353, The 820 surfacemorphology Nitrate group of AMT changed 2+ dramatically3.1.2. after SEM its and reaction EDX Analysis with Cu . Free AMT molecules are visualized as platelet–like particles with a clear, andThe smoothSEM images surface of AMT (Figure and its complex4A–D). with The copper(II) AMT particlesshown in Figure size 4 calculated provide useful using image J program rangedinformation from on4.6 surface to 8.2morphologyµm. Agglomerated and particles size. particles The surface with morphology a rough of AMT surface changed are observed in the SEM imagedramatically of Cu(II)-AMT after its reaction as with shown Cu2+. inFree Figure AMT molecules4E–H. Theare visualiz particlesed as platelet size of–like Cu(II)-AMT particles complex with a clear, and smooth surfaceµ (Figure 4A–D). The AMT particles size calculated using image J were in theprogram range ofranged 18.8–31.25 from 4.6 to 8.2m. μm. However, Agglomerated the particles SEM images with a rough of Cu(II)-AMTsurface are observed complex in the displayed a few particlesSEM with image size of Cu(II)-AMT less than as 10 shownµm in that Figure may 4E–H. have The particles combined size of toCu(II)-AMT form agglomerates complex were with larger size. The EDXin the analysis range of 18.8–31.25 of the AMT μm. However, ligand the revealed SEM images three of Cu(II)-AMT signals corresponding complex displayed to a few carbon, oxygen, particles with size less than 10 μm that may have combined to form agglomerates with larger size. and sulfur elementsThe EDX analysis (Figure of the5A), AMT while ligand the revealed EDX th spectrumree signals corresponding of copper complexto carbon, oxygen, withAMT and conﬁrmed the presencesulfur of copperelements in(Figure the 5A), complex while the as EDX indicated spectrum byof copper the signals complex correspondingwith AMT confirmed to the this element in Figure5B. Suchpresence signals of copper were in the not complex found as inindicated the EXD by the spectrum signals corresponding of free AMT to this (Figure element in5A). Figure The elemental 5B. Such signals were not found in the EXD spectrum of free AMT (Figure 5A). The elemental analysis analysis of AMTof AMT ligand ligand and and its itscomplex complex with Cu(II) with is Cu(II)displayed is in displayed Table 2. in Table2.

(A) (B)

Figure 4. Cont. Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 17

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(E) (F)

360

SKa 330

300 MoLa 270

240 (G) (H) (G) (H) 210 Figure 4. SEM images at a different magnification of (A–D) AMT and (E–H) Cu(II)-AMT. FigureCPS 180 4. SEMFigure images4. SEM images at a at di aﬀ differenterent magniﬁcation magnification of (A of–D (A) AMT–D) and AMT (E–H and) Cu(II)-AMT. (E–H) Cu(II)-AMT. 150

OKa 120 150

CKa 150 360 360 90

SKb

SKa SKa SKa 360 SKa SKa SKa 135 135 330 330 60 MoLl 330

300 MoLa 120 300 30 MoLa 120 300 270 MoLa 270 0 105 270 240 105 240 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.0090 240 210 90 210 CPS 180 keV 75 210 CPS

CPS

75 CuLa 180 150 CPS

OKa 60 CPS 180

CuLa

150 120 CKa CuLl OKa 60

CKa 45 150 90

CKa 120 OKa SKb SKb

CuLl OKa CuKa CKa 45 120 60 MoLl 30

90 CuKb

SKb

CKa

SKb

30 OKa

15 CuKa 90 MoLl 30

60 SKb 0 CuKb 6030 MoLl 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 15 30 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 keV 0 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 keV 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 (A)keV (B) keV keV . 360 Figure 5. EDX spectra of (A) AMT and (B) [Cu(L)(NO3)(H2O)2] H2O.

(ASKa ) (B) 330 Table 2. The elemental analysis of AMT ligand and Cu(II)-AMT complex..

300 FigureMoLa 5. EDX spectra of (A) AMT and (B) [Cu(L)(NO3)(H2O)2] H2O. Figure 5. EDX spectra of (A) AMT and (B) [Cu(L)(NO3)(H2O)2] H2O. 270 Weight % Weight• % Element 240 Calculated Found TableTable 2. The 2. The elemental elemental analysis analysisof of AMTAMT ligand ligand and and Cu(II) Cu(II)-AMT-AMT complex complex.. 210 C 42.85 42.90

CPS H Weight3.21 % 3.16 Weight % 180 Element Weight % Weight % 150 AMT N Calculated22.22 22.03 Found

OKa Element C Calculated42.85 Found 42.90 120 O 6.34 6.56 360 90 CKa HS 25.423.21 25.81 3.16

SKa 360 SKb C 42.85 42.90

MoLl SKa Weight % Weight % 330 60 AMT N 22.22 22.03 330 HCalculated 3.21 3.16 Found

MoLa 300 30 O 6.34 6.56 300 270 0 MoLa AMT N 22.22 22.03 S 25.42 25.81 270 240 0.00 1.00 2.00 3.00 4.00 5.00 6.00 O7.00 8.00 9.00 6.3410.00 6.56 Weight % Weight % 240 210 S 25.42 25.81 keV Calculated Found 210CPS 180

CPS 180 150 Weight % Weight %

OKa 150 120 Calculated Found

OKa 120 90 CKa

SKb

CKa C 25.06 25.76 90 60 MoLl

SKb

60 30 MoLl H 3.27 2.45 0 N 16.22 15.36 30 Cu(II)-AMT 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 O9.00 10.00 25.93 24.50 0.00 1.00 2.00 3.00 4.00keV 5.00 6.00 S7.00 8.00 14.819.00 10.00 14.44

keV Cu 14.59 14.36 *

* Cu content in Cu(II)-AMT was determined from EDX measurements.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 17 C 25.06 25.76 C H 25.063.27 25.762.45 H N 3.2716.22 2.4515.36 Cu(II)-AMT N 16.22 15.36 Cu(II)-AMT O 25.93 24.50 O S 25.9314.81 24.5014.44 S Cu 14.8114.59 14.4414.36 * * Cu content inCu Cu(II) -AMT was determined14.59 from EDX measurements. 14.36 * * Cu content in Cu(II)-AMT was determined from EDX measurements. Appl. Sci. 2020, 10, 3895 8 of 17 3.1.3. Thermal Analysis 3.1.3. Thermal Analysis Thermogravimetric analysis of the ligand occurred in two stages as shown in Figure 6A. The 3.1.3.Thermogravimetric Thermal Analysis analysis of the ligand occurred in two stages as shown in Figure 6A. The first stage which was because of the loss of one molecule each of NH3 and CO covered the range 60– first200 stage Thermogravimetric°C. Completewhich was decomposition because analysis of the of ofloss the ligand ligandof one occur occurredmoleculered throughout in each two of stages NH the3 asand second shown CO coverstage in Figureed at the2006A. –range600 The °C. first60 –No 200 °C. Complete decomposition of ligand occurred throughout the second stage at 200–600 °C. No stageresidue which was was observed. because Scheme of the loss 2 shows of one the molecule proposed each path ofNH of ligand3 and COthermolysis. covered the Thermogravimetric range 60–200 ◦C. residue was observed. Scheme 2 shows the proposed path of ligand thermolysis. Thermogravimetric Completeanalysis of decomposition Cu(II)-AMT involve of ligandd four occurred stages throughout as shown thein Figure second 6B stage and atScheme 200–600 3.◦ TheC. No loss residue of one analysiswasmolecule observed. of Cu(II)of uncoordinated Scheme-AMT2 involve shows water thed four proposedwas stages in the path asfirst shown ofstage ligand inand Figure thermolysis. occur red6B and at 27 Thermogravimetric Scheme–104 °C 3.with The a 4.45%loss analysis of weight one moleculeofloss. Cu(II)-AMT Two of uncoordinatedmolecul involvedes of coordinated four water stages was asinwater the shown first were instage lost Figure andin 6the Boccur andsecondred Scheme at stage 27–3104. at The 120 °C loss –with200 of a°C one4.45% with molecule weighta weight loss. Two molecules of coordinated water were lost in the second stage at 120–200 °C with a weight ofloss uncoordinated of 8.25%. The water third wasstage in at the 200 first–528 stage °C involve and occurredd partial atfragmentation 27–104 ◦C with of the a 4.45% ligand weight moiety loss. and loss of 8.25%. The third stage at 200–528 °C involved partial fragmentation of the ligand moiety and Twothe moleculesrelease of nitrogen of coordinated oxides water amounting were lostto a in 63.02% the second weight stage loss. at Complet 120–200e◦ Cdecomposition with a weight of loss the the release of nitrogen oxides amounting to a 63.02% weight loss. Complete decomposition of the ofcomplex 8.25%. Theleaving third a stageresidue at of 200–528 copper◦ Coxide involved occur partialred in the fragmentation final stage at of 528 the– ligand1000 °C moiety (24.64% and weight the complexreleaseloss). of leaving nitrogen a residue oxides amountingof copper oxide to a 63.02% occurred weight in the loss. final Complete stage at decomposition528–1000 °C (24.64% of the complexweight loss). leaving a residue of copper oxide occurred in the final stage at 528–1000 ◦C (24.64% weight loss).

100 100

80 80

60 60

40 40

20 20

0 0

-20 -20 0 200 400 600 800 1000 0 200 400 600 800 1000

(A) (B) (A) (B) Figure 6. TGA of AMT (A) and [Cu(L)(NO3)(H2O)2].H2O (B). Figure 6. TGA of AMT ( A)) and [Cu( L)(NO3)(H)(H22O)O)22].]HH2O2 O((B).B ). •

Scheme 2. Suggested thermal decomposition of AMT. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 17

Scheme 2. Suggested thermal decomposition of AMT.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 17 Appl. Sci. 2020, 10, 3895 9 of 17 Scheme 2. Suggested thermal decomposition of AMT.

Scheme 3. Suggested thermal decomposition of Cu(II)-AMT.

3.1.4. Electron Spin Resonance of [Cu(L)(NO3)(H2O)2].H2O

The powdered ESRScheme spectrum 3. Suggested of the Cu thermal-AMT decompositioncomplex measured of Cu(II)-AMT. at 300 K showed two bands at Scheme 3. Suggested thermal decomposition of Cu(II)-AMT. 330 and 310 mT with g⊥ = 2.08306 and gıı = 2.2175, respectively (Figure 7). The g-values confirmed that3.1.4. the Electron unpaired Spin electron Resonance of Cu(II) of [Cu(L)(NO occupies the3)(H orbital2O)2] ofH2 dOx2−y2, where gıı is greater than both g⊥ and 3.1.4. Electron Spin Resonance of [Cu(L)(NO3)(H2O)2].H• 2O g of freeThe powderedelectron (2.0023) ESR spectrum [32,33]. ofThe the shape Cu-AMT of the complex ESR spectrum measured suggests at 300 Ka showeddistorted two octahedral bands at symmetryThe powdered for the Cu ESR-ATM spectrum complex of the [32] Cu in-AMT excellent complex agreement measured with at 300 µ eff K value. show ed The two g avbands value at 330 and 310 mT with g = 2.08306 and gıı = 2.2175, respectively (Figure7). The g-values conﬁrmed that calculated330 and 310 from mT Eq withuation ⊥g⊥ (=2 )2.08306 was equal and to g 2.1279.ıı = 2.2175, Deviation respectively of gav from(Figure g of 7). free The electron g-values was confirm becauseed the unpaired electron of Cu(II) occupies the orbital of dx2 y2, where gıı is greater than both g and g of − ⊥ offreethat the electron the covalent unpaired (2.0023) proper electron [32ty, 33of ].bondingof The Cu(II) shape inoccupies Cu of- theATM ESRthe [33]. orbital spectrum of d suggestsx2−y2, where a distorted gıı is greater octahedral than both symmetry g⊥ and g of free electron (2.0023) [32,33]. The shape of the ESR spectrum suggests a distorted octahedral for the Cu-ATM complex [32] in excellentg agreementav = 1/3(gıı + with2 g⊥) µeﬀ value. The gav value calculated from(2) symmetry for the Cu-ATM complex [32] in excellent agreement with µ eff value. The gav value Equation (2) was equal to 2.1279. Deviation of gav from g of free electron was because of the covalent A molar conductivity of Λm = 8.2 Ω−1 cm2 mol−1 measured at 1.00 × 10−3 M in DMF solvent at room propertycalculated of from bonding Equation in Cu-ATM (2) was [ 33equal]. to 2.1279. Deviation of gav from g of free electron was because temperatureof the covalent indicate properd thatty of the bonding Cu-ATM in Cu complex-ATM [33].is a non -electrolyte [34]. Full characterization of the 2+ complex formed between Cu andg = AMT1/3(g + suggests2 g ) the chemical formula to be(2) avgav = 1/3(gıııı + 2 g⊥) (2) [Cu(L)(NO3)(H2O)2].H2O. Figure 8 shows the proposed structure⊥ of the complex. A molar conductivity of Λm = 8.2 Ω−1 cm2 mol−1 measured at 1.00 × 10−3 M in DMF solvent at room temperature indicated that the Cu-ATM complex is a non-electrolyte [34]. Full characterization of the complex formed between Cu2+ and AMT suggests the chemical formula to be [Cu(L)(NO3)(H2O)2].H2O. Figure 8 shows the proposed structure of the complex.

Figure 7. Electron spin resonance of [Cu(L)(NO3)(H2O)2] H 2O. Two arrows indicate the signiﬁcant • peaks in spectrum. Fig. ESR spectrum of Cu(II)-AMT complex.

1 2 1 3 A molar conductivity of Λm = 8.2 Ω− cm mol− measured at 1.00 10− M in DMF solvent at room × temperature indicated that the Cu-ATM complex is a non-electrolyte [34]. Full characterization of the Fig. ESR spectrum of Cu(II)-AMT complex. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 17

Appl. Sci. 2020, 10, 3895 10 of 17 Appl. Sci.Figure 2020 , 7.10 ,Electron x FOR PEER spin REVIEW resonance of [Cu(L)(NO3)(H2O)2].H2O. Two arrows indicate the significant10 of 17 peaks in spectrum. . complexFigure formed 7. Electron between spin Cu resonance2+ and AMT of [Cu( suggestsL)(NO the3)(H chemical2O)2] H2O formula. Two arrows to be [Cu(L)(NO indicate the )(HsignificantO) ] H O. 3 2 2 • 2 Figurepeaks8 shows in spectrum the proposed. structure of the complex.

Figure 8. Chemical structure of [Cu(L)(NO3)(H2O)2].H2O complex. L represent s AMT molecules with NH group. Figure 8. Chemical structure of [Cu(L)(NO3)(H2O)2] H2O complex. L represents AMT molecules with Figure 8. Chemical structure of [Cu(L)(NO3)(H2O)2]•.H2O complex. L represents AMT molecules with NH group. 3.2. AnalyticalNH group. A pplication 3.2. Analytical Application The brown complex formed between Cu2+ and AMT was used to develop an analytical 3.2. Analytical Application procedureThe brown for monitoring complex formed the copper between concentration Cu2+ and AMT in environmental was used to develop samples. an analytical procedure for monitoringThe brown the complex copper concentration formed between in environmental Cu2+ and AMT samples. was used to develop an analytical procedure3.2.1. Optimization for monitoring of the Recommendedthe copper concentration Procedure in environmental samples. 3.2.1. Optimization of the Recommended Procedure The relationship between solution pH and the absorbance of [Cu(L)(NO3)(H2O)2].H2O at λmax = 3.2.1. Optimization of the Recommended Procedure The relationship between solution pH and the absorbance of [Cu(L)(NO )(H O) ] H O at 434 nm was examined at various pH values using HCl/NaOH and3 acetate2 2 • buffer2 λ . (CHmax3The=COOH/CH434 relationship nm was3COONa). examined between The at solution various results pH pH in and valuesFigure the using absorbance9 show HCl/ NaOH thatof [Cu( the andL )(NO acetatemaximum3)(H bu2O)ff er2 absorbance] H (CH2O 3atCOOH λmax of=/ CH434 COONa). nm was The examined results. in Figure at various9 show that pH the values maximum using absorbance HCl/NaOH of [Cu(L)(NO and acetate)(H O) ] bufferH O [Cu(3 L)(NO3)(H2O)2] H2O occurred at pH 4–6 in acetate buffer. The absorbance is 3 less 2 at pH2 • <2 3, occurred(CH3COOH/CH at pH3 4–6COONa). in acetate The bu resultsffer. The in absorbanceFigure 9 isshow less thatat pH the 3, maximum because the absorbance protonation of because the protonation equilibrium of the reagent shifted to the left≤ as displayed in Scheme 4 . equilibrium[Cu(producingL)(NO 3protonated)(H of the2O) reagent2] H2 Oform occurshifted (Hred2L). to Therefore, theat pH left 4as–6 displayed the in acetatedeprotonated in Schemebuffer. form 4 The producing of absorbance the reagent protonated is (HL) less form aist required pH (H <2 L).3, Therefore,becausefor complex the the formation. protonation deprotonated The equilibrium formabsorbance of the reagent of of thethe Cu(II) reagent (HL) is-AMT required shift complexed forto complexthe decrease left as formation. d displayed dramatically The in absorbance Schemeabove pH 4 ofproducing6, thea behavior Cu(II)-AMT protonated most complex likely form attributed decreased (H2L). Therefore, to dramatically the hydrolysis the deprotonated above of pHCu(II) 6, a- behaviorformAMT ofand the most the reagent formation likely (HL) attributed ofis requiredcolorless to the hydrolysisforforms complex of copper(II) of formation. Cu(II)-AMT such The as and absorbance hydroxo the formation species. of theof Cu(II) Thus, colorless- AMTthe formsacetate complex of buffer copper(II) decrease withd suchpH dramatically 4 as–6 hydroxo was chosen above species. pHfor Thus,6,further a behavior the experiments. acetate most bu likelyff er with attributed pH 4–6 to was the chosen hydrolysis for further of Cu(II) experiments.-AMT and the formation of colorless forms of copper(II) such as hydroxo species. Thus, the acetate buffer with pH 4–6 was chosen for further experiments.0.6

0.60.5

0.50.4

0.40.3 Abs.

0.30.2 Abs. 0.20.1

0.10 0 2 4 6 8 10 0 pH 0 2 4 6 8 10 pH . FigureFigure 9.9. InﬂuenceInfluence ofof solution solution pH pH on on the the absorbance absorbance of of [Cu(L)(NO [Cu(L)(NO)(H3)(HO)2O)]2]HH2O atat λλmaxmax = 434 nm. 3 2 2 • 2 Copper(II)Copper(II) concentrationconcentration == 2 µμgg mL −11;; [AMT] [AMT] == 1.201.20 × 1010−4 M.4 M. − × − Figure 9. Influence of solution pH on the absorbance of [Cu(L)(NO3)(H2O)2].H2O at λmax = 434 nm. Copper(II) concentration = 2 μg mL−1; [AMT] = 1.20 × 10−4 M. Appl.Appl. Sci. Sci.2020 2020, 10, 10, 3895, x FOR PEER REVIEW 1111 of of 17 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 17

Scheme 4. Chemical equilibrium of AMT reagent. SchemeScheme 4. 4. ChemicalChemical equilibrium equilibrium of of AMT AMT reagent reagent..

The absorbance of [Cu(L)(NO3)(H2O)2].H2O at λmax was significantly influenced by a change in The absorbance of [Cu(L)(NO3)(H2O)2] H2O at λmax was significantly influenced by a change 3 2 2 . 2 max AMTThe concentration absorbance of from [Cu( 4.0L)(NO × 10)(H−4 to4O) 6.7] H×• 10O −at5 M λ5 at wasconstant significantly Cu2+ concentration2+ influenced by(Figure a change 10). Thein in AMT concentration from 4.0 −410− to 6.7 −5 10− M at constant2+ Cu concentration (Figure 10). AMTabsorbance concentration of the complexfrom 4.0 was× 10× constant to 6.7 ×at 10 [AMT]× M at = constant6.5 × 10−5 Cu5to 2.2 concentration × 10−4 mol4 L− 1(,Figure 1but increase 10). Thed at The absorbance of the complex was constant at [AMT] = 6.5 10−5 − to 2.2 −410− mol−1 L− , but increased absorbanceconcentrations of the above complex 2.2 × was 10−4 molconstant 4L−1. A at small [AMT]1 decrease = 6.5 × in ×10 absorbanceto 2.2 × 10× was mol seen L above, but increase 2.8 × 10−4d molat at concentrations above 2.2−4 10− −1 mol L− . A small decrease in absorbance was seen above−4 concentrationsL−1, possibly4 above because1 2.2 excess × 10 × mol chromogenic L . A small reagent decrease reduce in absorbanced the extent was ofseen complex above 2.8 formation × 10 mol by 2.8−1 10− mol L− , possibly because excess chromogenic reagent reduced the extent of complex L , × possibly because excess chromogenic reagent reduced the extent of complex formation−4 by formationincreasing by the increasing acidity of the the acidity solution of the[35]. solution Thus, the [35 AMT]. Thus, concentration the AMT concentration was fixed at 2.8 was × fixed10 mol at increasing the acidity of the solution [35]. Thus, the AMT concentration was fixed at 2.8 × 10−4 mol 2.8L−1 10for 4allmol measurements. L 1 for all measurements. L−1× for all− measurements.− 0.25 0.25

0.2 0.2

. 0.15

Abs 0.15

Abs

0.1 0.1

0.05 0.05

0 0 0 0.0001 0.0002 0.0003 0.0004 0.0005 0 0.0001 0.0002 0.0003 0.0004 0.0005 AMT Concentration (mol·L-1) AMT Concentration (mol·L-1) Figure 10. The relationship between the concentration of AMT reagent and the absorbance of [Cu(FigureL)(NO 10.)(H TheO) relationship] H O at 434 between nm. Cu(II) the concentrationconcentration= of2 µ AMTg mL reagent1. and the absorbance of 3 2 2 • 2 − Figure[Cu(L )(NO 10. The3)(H 2 relationshipO)2].H2O at 434 between nm. Cu(II) the concentration = of 2 μg AMT mL reagent−1. and the absorbance of 3.2.2.[Cu( InvestigationL)(NO3)(H2O) of2] Method.H2O at 434 Selectivity nm. Cu(II) concentration = 2 μg mL−1. 3.2.2.The Investigation potential impact of Method of common Selectivity foreign ions on the estimation of copper was tested at optimized 3.2.2. Investigation of Method Selectivity 1 conditionsThe potential and in the impact presence of common of 2 µg mL foreign− Cu(II). ions The on the tolerance estimation limit of of copperinterfering was ions tested was at expressedoptimizedThe potential by conditions the concentration impact and in of the common of presence these foreign ions of or2 μg species ions mL on−1 Cu(II). that the makes estimation The tolerance the relative of copperlimit error of wasinterfering in the tested copper ions at optimizeddetermination conditions greater and than in the5%. presence Al3+ and of 2 Mn μg2 +mLseriously−1 Cu(II). interfered—probablyThe tolerance limit of dueinterfering to complex ions was expressed by the concentration± of these ions or species that makes the relative error in the copper wasformationdetermination expressed with by AMT—as greater the concentration than shown ±5%. in Tableof Al these3+ 3and. However, ions Mn or2+ species seri theseously that interferences interferemakes thed were— relativeprobably eliminated error du ine by the to addition complexcopper determinationof 0.5 mL KF 2H greaterO (0.1 than mol ±5%. L 1), whichAl3+ and reduced Mn2+ theseri relativeously interfere error tod an—probably acceptable due5% to level complex at an formation with• 2 AMT—as shown− in Table 3. However, these interferences were± eliminated by µ 1 formationinterferingaddition of with ion 0.5 concentration mL AMT KF—.2Has 2O shown of(0.1 100 mol ing L Table mL−1), −which . 3. However, reduced the these relative interferences error to an were acceptable eliminated ±5% level by . −1 additionat an interfering of 0.5 mL ion KF con2H2centrationO (0.1 mol ofL 100), which µg mL reduce−1. d the relative error to an acceptable ±5% level at an interfering ion concentration of 100 µg mL−1.

Appl. Sci. 2020, 10, 3895 12 of 17

Table 3. Inﬂuence of some foreign ions when applying the developed spectrophotometric procedure for the estimation of 2 µg mL 1 Cu(II). [AMT] = 1.665 10 3 mol L 1; relative error: 5%. − × − − ± Coexisting Ion Diverse Ion/Copper(II) Coexisting Ion Diverse Ion/Copper (II) Na+ 1200 Co2+ 100 K+ 1000 Ni2+ 100 Li+ 1000 Ba2+ 100 Zn2+ 1000 Cr3+ 100 Ca2+ 1000 Pb2+ 100 Ag+ 1000 Sr2+ 100 F− 500 NO2− 200 2+ Cl− 500 Mn * 5 Bi3+ 300 Al3+ * 5 * The interference was eliminated by use of KF 2H O as a masking agent. • 2

3.2.3. Analytical Performance of the Recommended Procedure

1 The plot of Cu(II) concentration versus absorbance is linear over a range of 0.7–25 µg mL− . The linear equation that describes the calibration curve of our developed method is:

A = 0.18 C 0.009 (R2 = 0.992, n = 10) (3) − where, A and C are absorbance, and concentration of copper(II) solution, respectively. Sandell’s factor, 2 4 1 1 and the molar absorptivity (ε) of the proposed method are 0.003 µg cm− , and 1.9 10 L mol− cm− , 1 × 1 respectively. LOD calculated using the equations in [21] is 0.011 µg mL− (11.00 µg L− ). Relative standard deviation (RSD) and relative error (RE) of the spectrophotometric method calculated from 1 ten replicate measurements of copper recovery at 2 µg mL− in distilled water were 1.4% and 1.2%, respectively. Variables demonstrating the eﬀectiveness of the analytical method are listed in Table4. Comparison of these analytical features with those of previously-published spectrophotometric methods in Table5 establishes the good sensitivity and wide linear dynamic range of our method. Moreover, the analytical performance of the proposed method, in terms of selectivity, sensitivity, and dynamic concentration range, was compared with a wide variety of analytical techniques [5–9,36–39]. The spectrophotometric method developed in the present study provides better sensitivity and selectivity than some the methods mentioned in Table6, without the need to use the extraction or preconcentration methodology. The dynamic concentration range of the proposed method covers a wide domain of concentrations including the maximum values of copper in drinking waters recommended 1 1 by the U.S. Environmental Protection Agency [EPA] (1 mg L− ) and the WHO (2 mg L− ). Moreover, the proposed chromogenic reagent can operate at a wide range of pH values. Therefore, the suggested spectrophotometric method is appropriate for rapid and routine analyzes in many laboratories.

Table 4. Summary of the analytical performance of the spectrophotometric method.

Variable Value

Wavelength, λmax (nm) 434 pH 4.5 Temperature ( C) 25 2 ◦ ± Linear dynamic range (µg/mL) 0.7–25 LOD, µg/mL 0.011 RSD,% (n = 10) 1.4 RE,% (n = 10) 1.2 Slope 0.18 Intercept 0.009 ε, (L/mol.cm) 1.9 104 2 × Sandell’s factor (µg cm− ) 0.003 Correlation factor (R2) 0.999 Appl. Sci. 2020, 10, 3895 13 of 17

Table 5. Comparison between the proposed method and previously-published spectrophotometric methods to estimate Cu(II) *.

λ Linear Dynamic Range Chromogenic max pH Molar Absorptivity, Detection Limit, (LOD), Reagent (nm) L/mol.cm µg/mL (µg/mL) Interferences Ref. Ceﬁxime 336 4.68 8.29 103 1.015–8.122 0.032 Hg2+,Al3+,Zn2+,Fe3+,Mn2+ [13] × BTAS 485 5.0 2.35 104 0.63–5.04 7 10 3 Sn2+,Cd2+,Co2+ [14] × × − 2,4-dinitro APTPT 445 8.7 0.87 103 10–80 1.72 Sb3+,Mn2+,Ag+,Pb2+,Co2+,Hg2+ [15] × DDTC 435 5 2.86 105 0.2–12 0.2 Fe2+,Ni2+,Pb2+,Mn2+ [16] × DDTC 435 5 3.61 105 0.02–12.0 0.029 Ag+,Pb2+ [17] × ATAP 608 4.5 4.37 105 4 10 3–0.115 1.2 10 3 Fe3+,Sb3+,Mo5+,NO - [18] × × − × − 2 HBITSC 510 4 2.5 103 1–8 NM Co2+,Fe2+,Fe3+,Ni2+,Cr3+ [19] × NBIMMT 470 6.2 2.8 103 4.75–16.13 NM Pb2+,Cd2+,Sn2+,Fe2+,Zn2+,Ag+,Al3+ [20] × 2-APT 370 8 2.14 104 0.16–1.3 0.053 NM [21] × H2IF 414 4.8 1.46 105 6.35 10 3–0.318 6.38 10 3 There is no interference at1:10 ratio [22] × × − × − 5M, 3H-BR 430 5.5 0.603 104 0.05–13 NM There is no interference at1:10 ratio [23] × Rubeanic acid 380 3.5 1.01 104 0.65–2.65 NM Co2+,Ni2+ [4] × CBIMMT 414 4.2 3.38 103 5–17.5 NM Ni2+,Pb2+,Al3+,Bi3+ [37] × AMT 434 4–6 1.9 104 0.7–25 0.011 Mn2+,Al3+ PW × * CBIMMT: 4-(40-chlorobenzylideneimino)-3-methyl-5-mercapto-1,2,4-triazole; NM: not mentioned; PW: present work.

Table 6. Comparison between the proposed method and various previously published analytical techniques to estimate Cu(II).

Analysis Technique pH LOD, µg/mL Concentration Range, µg/mL Interferences Ref. Micro SPE-AAS 3 0.022 0.05–1 Fe3+,Ni2+,Co2+,Zn2+ [5] AAS with high resolution continuum source Acidic medium 0.021–1.4 * 0.07–100 * NM [6] Potentiometry using modified carbon paste electrode 3 9.2 10 5 2.22 10 4–635 Hg2+,Cd2+,Ag+ [7] × − × − Potentiometry using modified carbon paste electrode 4.5–8.5 0.057 0.081–812 Fe3+,Cd2+,Al3+,Ca2+,Cr3+ [8] Voltammetry using modified carbon paste electrode 1.4 0.017 0.1–1 Hg2+,Cd2+,Pb2+ [9] AAS combined with SPE 5 NM 0.15–2 Al3+,Sn2+,Ni2+,Mn2+ [37] AAS combined with SPE 5 0.01 0.01–0.34 There is no interference at1Cu:10M ratio [38] AAS combined with SPE 5.6 0.3 NM Fe3+,Co2+ [39] Direct spectrophotometry 4–6 0.011 0.7–25 Mn2+,Al3+ PW * Depending on the used atomic lines. Appl. Sci. 2020, 10, 3895 14 of 17

3.2.4. Evaluation of the Recommended Procedure Our spectrophotometric procedure was effectively employed for the analysis of Cu in multielement standard solution (# 90243), and steel sample (No.21899) as certified reference materials, in addition to tap water (Taif City, Saudi Arabia) and sea water (Red Sea, Jeddah Governorate, Saudi Arabia). Copper concentrations in multielement solution and steel sample estimated by the proposed spectrophotometric procedure were 9.91 0.27 mg L 1 and 0.475% (w/w), respectively, with small differences from certified ± − values. However, Student’s t-test revealed no significant differences between the concentrations determined by spectrophotometric method and certified values at the 95% confidence level since the tabulated t–value (2.78) was always greater than the calculated values (2.61, 2.55) for five replicate measurements. Water samples were spiked with a known concentration of copper as shown in Table7, and the recovery method was determined. Acceptable recoveries were obtained, which confirms the accuracy and applicability of the method for copper determination in water samples. A comparison of the results of our spectrophotometric method with those of the standard ICP-OES indicates analytically-acceptable agreement between these procedures. Therefore, the proposed method can be used for the rapid and sensitive detection of Cu(II) in water samples. The rapid color development due to reaction of AMT with copper(II) can be detected by the naked eye, which makes the proposed method suitable for the qualitative analysis of copper(II) in a variety of samples.

Table 7. Evaluation of proposed spectrophotometric method by determining copper in water samples *.

Spectrophotometric Method ICP-OES

Sample Amount of Added Found Amount of Added Found Cu(II) 1 Recovery % Cu(II) 1 Recovery % 1 (µg mL− ) 1 (µg mL− ) (µg mL− ) (µg mL− ) Red Sea water ___ ND ______ND ___ Red Sea water 4 3.79 0.31 95.0 4 3.62 0.22 90.5 Tap water ___ ND± ______ND± ___ Tap water 4 3.95 0.23 98.7 4 3.78 0.15 95.0 ± ± * ND: not detected.

4. Conclusions The present study describes the first synthesis and characterization of the [Cu(L)(NO )(H O) ] H O 3 2 2 • 2 complex. The AMT ligand is a bidentate N,O donor, which coordinates with Cu2+ in 1:1 ratio. Full characterization of the [Cu(L)(NO )(H O) ] H O complex suggests the octahedral structure shown in 3 2 2 • 2 Figure8. The reaction between AMT and copper(II) was used to develop an e ﬀective spectrophotometric procedure for the determination of copper in water samples. The developed method is simple, low cost, and sensitive with LOD of 11.00 ppb. The proposed chromogenic reagent operates at a wide range of concentrations and pH values. On the other hand, the developed method can work as a sensitive chemo sensor for copper monitoring in water samples since color change due to reaction of AMT with Cu(II) appears rapidly within less than 10 s and remains stable for up to 2 h.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/10/11/3895/s1, Figure S1: 1H NMR spectrum of AMT ligand, Figure S2: 13C NMR spectrum of AMT ligand, Figure S3: The electronic spectrum of [Cu(L)(NO )(H O) ] H O. 3 2 2 • 2 Author Contributions: Investigation, methodology, validation and writing: S.S.A. and H.M.A.-S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. Appl. Sci. 2020, 10, 3895 15 of 17

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