Development of a specific and highly sensitive optical chemical sensor for determination of Hg(II) based on a new synthesized ionophore

Materials Science and Engineering C 33 (2013) 4167–4172

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Development of a specific and highly sensitive optical chemical sensor for determination of Hg(II) based on a new synthesized ionophore Ali R. Firooz a,⁎, Ali A. Ensafi b,⁎, K. Karimi a, H. Sharghi c a b c

Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Department of Chemistry, University of Shiraz, Shiraz, Iran

a r t i c l e

i n f o

Article history: Received 8 February 2013 Received in revised form 11 April 2013 Accepted 6 June 2013 Available online 13 June 2013 Keywords: Optode Cyclopentadecine derivative ETH5294 Hg(II)

a b s t r a c t A novel optode for determination of Hg(II) ions is developed based on immobilization of a recently synthesized ionophore, 7-(1H-imidazol-1-ylmethyl)-5,6,7,8,9,10-hexahydro-2H-1,13,4,7,10 benzodioxatriaza cyclopentadecine-3, 11(4H,12H)-dione, in a PVC membrane. Dioctyl sebacate was used as a plasticizer, sodium tetraphenylborate as an anionic additive and ETH5294 as a chromoionophore. The response of the optode was based on the complexation of Hg(II) with the ionophore in the membrane phase, resulting an ion exchange process between Hg(II) in the sample solution and H+ in the membrane. The effects of pH and amounts of the ionophore, chromoionophore, ionic additive and type of plasticizer on the optode response were investigated. The selectivity of the optode was studied in the present of several cations. The optode has a linear response to Hg(II) in the range of 7.2 × 10−13–4.7 × 10−4 mol L−1 with detection limit of 0.18 pmol L−1. The optode was successfully applied to the determination of Hg(II) in real samples. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, because of some human activities such as combustion of fossil fuels, mining, combustion and fusion of solid wastes, usage of agricultural fertilizers and industrial extracts, mercury directly enters to weather, soil or water. Mercury found in water can enter the body of marines, and then nutritious chain of human beings. Because of high poisonousness of Hg(II), identification of ultra trace amounts of mercury ions in environmental water and in the body of aquatics like fishes is of great importance and worthy of high attention to chemists [1–3]. Additionally, wide application of mercury in production of chemicals, its electric and electronic usage causes more importance of its measurement. Different methods such as atomic absorption spectrometry (AAS) [4], inductively coupled plasma (ICP) [5], X-ray fluorescence spectrometry [6], anodic striping voltammetry [7], neutron activation analysis [8] and chromatography [9] have been reported for the Hg(II) measurement. Because of difficulties coming along with these methods, a fast, cheap and low detection limit method is necessary. For this reason, applications of optical chemical sensors (optodes) for the determination of metal ions have been widely developed [10–30]. The advantages of the optical sensors are their small size, low price and simplicity of their production, no need to complex methods and instrumentation, no need to preconcentration, and ⁎ Corresponding authors. Tel.: +98 311 3913269; fax: +98 311 3912350, +98 311 7932749. E-mail addresses: a.fi[email protected] (A.R. Firooz), Ensafi@cc.iut.ac.ir (A.A. Ensafi). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.06.009

their wide linear range [12,3]. Stabilizing a reacting phase on an optically transparent support produces the sensor. Optical properties of an optode change by interacting with the analyte. Usually the sensing layer (membrane) is commonly made of polymeric or porous material containing active compounds, which present a favorite optical response to analyte [13–15]. In this work we introduced a new optode for determination of Hg(II) ions in aqueous solution, which can selectively measure ultra trace amounts of Hg(II) with no need to any preconcentration. A recently synthesized ionophore [16] (Fig. 1) is used for preparation of the optical membrane. As shown on Table 1, this optode has a very lower detection limit and a wider linear range relative to other reported optode for mercury ions [1,3,10,12,17,23–37].

2. Experimental 2.1. Reagents and solutions 7-(1-H-Imidazol-1-ylmethyl)-5,6,7,8,9,10-hexahydro-2H-1,13,4,7,10 benzodioxatriaza cyclopentadecine-3,11(4H,12H)-dione, C18H23N5O4 (MW: 373.406), used as an ionophore, which was prepared according to the reported method [16]. The ionophore is a white powder with m.p. of 172–173 °C with IR (KBr) bands of: 756(s), 814(m), 1061(s), 1126(s), 1215(s), 1261(s), 1439(m), 1504(s), 1539(s), 1593(w), 1678(vs), 2851(w), and 3402(s) cm−1. 1H-NMR (CDCl3, 250 MHz) bands showed: δ = 2.79(t, 4H, J = 5.3 Hz), 3.62(t, 4H, J = 5.3 Hz), 4.49(s, 4H), 4.90(s, 2H), 6.86–7.03(m, 5H), 7.08(s, 1H), 7.43(s, 2H),

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0.010 mol L−1 L-histidine solution was prepared by dissolving 0.310 g of L−1 histidine in distilled water in a 100-mL volumetric flask. 2.2. Apparatus UV–vis spectra were obtained with a UV–vis spectrophotometer (Cary 500 Scan, Varian, Palo Alto, CA, USA) with 1.0 cm quartz cells. pH measurements were taken by pH/ion meter (Metrohm, Herisau, Switzerland), Model692 that was equipped with a combined glass electrode. Atomic emission measurements were taken by inductively coupled plasma (OPTIMA7300DV). Spin–coater instrument, Model HAB-500, was used for coating the membrane solution on Plexiglas-made support. 2.3. Preparation of the optode

Fig. 1. Structure of 7-(1H-imidazol-1-ylmethyl)-5,6,7,8,9,10-hexahydro-2H-1,13,4,7,10benzodioxatriaza cyclopentadecine-3,11(4H,12H)-dione.

7.53(s,1H). 13C-NMR (CDCl3, 62.9 MHz) showed δ = 34.8, 49.5, 60.4, 67.3, 113.2, 119.7, 122.4, 129.6, 137.7, 146.3, and 167.5 ppm. All chemicals were of analytical-reagent grade (with the highest degree of purity available and free of Hg ions). Polyvinyl chloride (PVC), high molecular weight and chromoionophore I (ETH5294) were purchased from Fluka, dioctyl phthalate (DOP), dioctyl sebacate (DOS) and dibuthyl phthalate (DBP) were from Aldrich. Sodium tetraphenylborate (NaTPB), tetrahydrofuran (THF), Hg(NO3)2.2H2O, L-histidine and other reagent were purchased from Merck. Doubly distilled water was used throughout. Phosphate buffer (0.05 mol L− 1) solution was used. Stock solution of 0.01 mol L− 1 Hg(II) was prepared by dissolving 0.3420 g Hg(NO3)2H2O in 0.1 mol L−1 nitric acid and diluting it to 100 mL in a 100-mL volumetric flask with water. Lower concentrations of Hg(II) ions were prepared by dilution of the stock solution with water.

The membrane components involving 32.0 mg PVC as a support, 64.0 mg DOS as a plasticizer, 2.0 mg NaTPB as an anionic additive, 1.5 mg ETH5294 as chromoionophore and 1.5 mg of the synthesized ionophore were dissolved in 1.0 mL THF and mixed well to homogenized. Plexiglas films (9 mm × 50 mm) were used as support. The membranes were cast by placing 20 μL of the homogenized membrane solution on to the Plexiglas slide, and spread quickly using spin-on device (1500 rpm rotation frequency). The prepared membranes were dried in ambient air for 15 min and then in a clean place for 2 h in a dark place to completely evaporate the solvent. Blank (reference) membranes were prepared in a similar way excluding chromoionophore I from the membrane solution. 2.4. Analytical procedure The prepared membrane was placed in a buffer solution at pH 8.0 for 100 s to reach equilibrium. Then, the membrane was placed in a quartz cell containing 2.0 mL of phosphate buffer (pH 8.0) in the sample path of the spectrophotometer. The quartz cell in the reference path of the spectrophotometer consisted of a film without chromoionophore I.

Table 1 Comparison of the figures of merit of recently reported optode with the proposed optode for determination of Hg(II). Ionophore

Response Dynamic range time(s) (mol L−1)

Hexathia cyclooctadecane 4-Phenyl-2,6-bis(2,3,5,6-tetrahydro benzo [b][1,4,7]trioxononin-9-yl)pyrylium 4-(2-Pyridylazo) resorcinol Tetrathia-12-crown-4 2-[(2-Sulfanylphenyl) ethanimidoyl]phenol Dithizone Trityl-picolinamide 1-(2-Pyridylazo)-2-naphthol 4-(2-Pyridylazo)-resorcinol 4-Hydroxy salophen Tetra(p-dimethylaminophenyl) porphyrin Indigo carmine; N-cetylpyridinium chloride (IC-N-CPC) Bis(benzoyl acetone) diethylenetriamine (1Z,2Z)-N′1,N′2-dihydroxy-N1,N2dipyridin-2-ylethanediimidamidedecine-3,11(4H,12H)- dione 1,3-Di(2-methoxyphenyl)triazene 4-Ethyl-5-hydroxy-5,6-di-pyridin-2-yl-4,5dihydro-2H-[1,2,4]triazine-3-thione 2-Mercaptopyrimidine (2-MP) 2-Mercapto-2-thiazoline (MTZ) Dithiacyclooctadecanederivative 7-(1H-Imidazol-1-ylmethyl)-5,6,7,8,9,10-hexahydro-2H-1,13,4,7,10 benzodioxatriaza-cyclopentadecine-3,11(4H,12H)-dione

300 180

Detection limit (mol L−1)

2.0 × 10−7 2.1 × 10−7–1.2 × 10−4 1.52 × 10−9–6.6 × 10−2 1.11 × 10−9 × × × × × × × × × ×

10−6–6.6 10−9–9.5 10−6–1.0 10−7–9.7 10−7–5.0 10−5–1.0 10−6–3.4 10−6–1.0 10−8–4.0 10−5–4.7

× × × × × × × × × ×

10−6 10−5 10−2 10−6 10−4 10−3 10−3 10−2 10−6 10−4

1.1 8.1 1.0 1.0 5.0 5.5 1.5 1.3 8.0 7.2

× × × × × × × × × ×

10−6 10−10 10−6 10−7 10−7 10−7 10−6 10−7 10−9 10−6

300 100 180–240 180–560 300–600 – 1200 360 300 480–600

1.1 1.0 1.0 7.5 5.0 1.0 5.0 1.0 4.0 2.4

60 b120

3.7 × 10−7 1.0 × 10−6–1.0 × 10−1 5.8 × 10−9–1.0 × 10−3 1.71 × 10−9

300 360

9.0 × 10−9–2.5 × 10−7 5.0 × 10−10–5.0 × 10−5

480 600 80 100

2.0 × 10−9–2.0 2.0 × 10−10–1.5 1.0 × 10−12–8.6 7.2 × 10−13–4.7

× × × ×

2.0 × 10−10 1.8 × 10−10

10−5 4.0 10−5 5.0 −4 10 5.3 −4 10 1.85

× × × ×

10−10 10−11 10−13 10−13

Ref. [1] [3] [12] [17] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [33] [36] [37] This work

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Then, buffer solutions containing different concentrations of Hg(II) ions (1.0 × 10−3–1.0 × 10−13 mol L−1) and 0.010 mol L−1 L-histidine, which work as a masking agent for Cu(II), were injected into the cell. The absorbance was measured at 661 nm. 2.5. Sample preparation After opening the tuna can, its oil was completely separated and homogenized completely. Then, 1000 g of the sample was weighed and transferred into a 100-mL Erlenmeyer flask containing 2 mL concentrated HCl. After 10 min, 5.0 mL of conc. HNO3 was gradually added. The flask was swirled calmly and 2.0 mL of (1:1) H2SO4 was added. The flask was capped with a watch glass and it was put on a steam bath until the dissolution was completed [2]. After cooling the mixture, the pH of digested tuna solution was adjusted to nearly 8.0 by addition of small sodium hydroxide solution. The result solution filtered through a filter paper. 0.30 g L-histidine and 5 mL of phosphate buffer (pH 8.0) were added to 5.0 mL of the filtrate in a 25–mL volumetric flask. The solution was diluted to the mark with water. 3. Results and discussion 3.1. Principle of operation 7-(1H-Imidazol-1-ylmethyl)-5,6,7,8,9,10-hexahydro-2H-1,13,4,7,10benzodioxatriaza-cyclopentadecine-3,11(4H,12H)-dione has a ring, which seems to be suitable to trap Hg2+ ion. The existence of \NH group in the ring well assists the Hg(II) ions to be trapped so, this molecule can be a good ionophore for Hg(II) ions in PVC membrane. By entering Hg(II) ions, the change of the membrane gets positive; so, the chromoionophore (HIn+) existed in the membrane deprotonates, and H+ enters into the aqueous solution to neutralize the membrane. The following reaction clarifies the matter [17]: 2þ

þ

2þ

þ

HgðaqÞ þ 2HInðorgÞ þ mLðorgÞ ⇔HgLmðorgÞ þ 2InðorgÞ þ 2HðaqÞ :

ð1Þ

In this reaction, L is the ionophore, HIn+ and In are the protonated and deprotonated form of the chromoionophore, respectively and m is the stoichiometric coefficient of the ionophore in complex form. The deprotonating degree of chromoionophore is calculated from the following relationship:  h i þ α ¼ ½In= ½In þ HIn :

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plasticizer to PVC is 2.0 [19,20]. We used this ratio for the optimization. To select the best plasticizer, three plasticizers such as DOP, DBP and DOS were tested as potential plasticizers for preparing the membrane. Since DOS containing membrane has the best response toward Hg(II) ions against the other plasticizer (Table 2), we selected DOS as a plasticizer. To optimize the amount of the ionophore, several membranes were prepared containing 1.0 to 2.0 mg of the ionophore in the presence of other additives. The higher percentage of the ligand was not selected due to the non-transparency of the produced membrane containing more than 2.0 mg of the ionophore. Then, the absorbance change of the optodes were measured for 1.0 × 10−14 to 1.0 × 10−2 mol L−1 Hg(II) at 661 nm and the correspondence (1-α) was calculated. The results are presented in rows 1–6 (Table 2). The best response was observed for a membrane incorporating 1.5 mg of the ionophore. Anionic additives are usually used to allow ionophore extraction of cationic analyte (Hg2+) and inhibition of extraction of anions. To optimize the composition of the optode with respect to NaTPB, several membranes with different amounts of NaTPB at a constant amount of the ionophore (1.5 mg) plus 64.0 mg DOS, 32.0 mg PVC and 1.5 mg of chromoionophore I, were prepared. Then, the absorbance change of the optodes were measured for 1.0 × 10−14 to 1.0 × 10−2 mol L−1 Hg(II) at 661 nm and the correspondence (1-α) was calculated. The results given in Table 2, show that 2.0 mg of NaTPB is the optimum amount of the anionic additive and was used to make the membrane solution. The amount of chromoionophore I on the optode response was also studied. To optimized the composition of the optode with respect to chromoionophore I, several membranes with 1.0 to 2.5 mg of chromoionophore I at a constant amount of the ionophore (1.5 mg) plus 64.0 mg DOS, 32.0 mg PVC and 2.0 mg of NaTPB, were prepared. Then, the absorbance change of the optodes were measured for 1.0 × 10−14 to 1.0 × 10−2 mol L−1 Hg(II) at 661 nm and the correspondence (1-α) was calculated. The results of our study showed that the optode with 1.5 mg of chromoionophore I in the presence of the optimized percent of the other additives has the maximum sensitivity and linear dynamic range. Increasing chromoionophore decreases the detection limit (Table 2, row no. 12) of the optical sensor. This is due to the fact that the higher concentration of the dye caused the dimerization of the indicator [19]. As can be seen in Table 2 the optimized membrane composition contains 64.0 mg DOS, 32.0 mg PVC, 2.0 mg NaTPB, 1.5 mg of chromoionophore I and 1.5 mg of the ionophore.

ð2Þ

By measuring the absorbance at wavelength 661 nm for fully protonated of the chromoionophore (HIn+, Ap), and in fully deprotonated form of the chromoionophore (In, AD), the absorbance of the membrane in each moment (when the two species HIn+ and In are in equilibrium, A) can be directly related to α [18]:     α ¼ Ap –A = Ap –AD :

ð3Þ

Fig. 2 shows the effect of Hg(II) concentration on the absorbance of chromoionophore I (ETH5294). It can be seen that as the concentration of Hg(II) ion increases, the absorbance of the membrane at 661 nm (related to HIn+) decreases, and the absorbance in 546 nm (related to In) increases. 3.2. Optimizing of the membrane composition In order to improve the sensitivity, detection limit and response time of the optode, the amount of the components existed in the membrane involving anionic additive, ionophore, chromoionophore I, and also the type of plasticizer were optimized. The optimized ratio of

Fig. 2. UV–vis absorption spectra of the optode in the presence of different concentrations of Hg(II) (mol L−1) in a phosphate buffer, pH 8.0; (1) blank solution (buffer); (2) 1.0 × 10−13; (3) 1.0 × 10−12; (4) 1.0 × 10−11; (5) 1.0 × 10−10; (6) 1.0 × 10−9; (7) 1.0 × 10−8; (8) 1.0 × 10−7; (9) 1.0 × 10−6; (10) 1.0 × 10−5; (11) 1.0 × 10−4; and (12) 1.0 × 10−3 mol L−1.

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Table 2 Effect of composition of the membrane components (as mg) on the response of the optode. No.

PVC

DOP

DOS

DBP

Ionophore

(ETH5294)

NaTPB

Linear dynamic range (mol L−1)

1 2 3 4 5 6 7 8 9 10 11 12 Optimum

32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0

– – 64.0 – – – – – – – – – –

– 64.0 – 64.0 64.0 64.0 64.0 64.0 64.0 64.0 64.0 64.0 64.0

64.0 – – – – – – – – – – – –

1.5 1.5 1.5 1.0 1.5 2.0 1.5 1.5 1.5 1.5 1.5 1.5 1.5

1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.0 1.5 2.0 1.5

2.0 2.0 2.0 2.0 2.0 2.0 1.0 2.0 3.0 2.0 2.0 2.0 2.0

1.28 7.19 1.18 2.69 7.19 1.32 2.82 7.19 7.46 6.87 7.19 1.40 7.2

3.3. Effect of pH The response of the optode is based on the Hg(II)–H+ exchange rate between the membrane and the solution phases. According to Eq. (1), which shows the reaction of the membrane when it is immersed in Hg(II) solution, the reaction equilibrium depends not only on the concentration of Hg(II), but also on the concentration of H+ of the sample solution. So, the pH of the solution is of great importance and should be kept constant. To optimize the pH, phosphate solutions in the range of 5.0–9.0 were prepared and the response of the optode was determined for Hg(II) concentration range of 1.0 × 10− 3–1.0 × 10− 13 mol L− 1 for each pH. The results are shown in Fig. 3. The results showed that the optimized pH was 8.0. At low pHs, as H+ enters the membrane (according to Eq. (1)) the equilibrium tends to reactants and the complex formation decreases. At higher pH values (> 8) Hg(II) ions convert to mercury hydroxide and detectability of the optode decreases.

× × × × × × × × × × × × ×

10−11–1.89 × 10−4 10−13–4.66 × 10−4 10−10–2.27 × 10−4 10−12–2.32 × 10−5 10−13–4.66 × 10−4 10−11–6.35 × 10−4 10−12–3.44 × 10−6 10−13–4.66 × 10−4 10−12–3.13 × 10−5 10−12–7.64 × 10−6 10−13–4.66 × 10−4 10−12–2.47 × 10−4 10−13–4.7 × 10−4

1.0 × 10− 8, and 1.0 × 10− 12 mol L− 1 Hg(II) solutions, and the absorbance changes at 661 nm were determined. As can be seen in Fig. 4 the response time is lower than 100 s. 3.5. Reproducibility and repeatability In order to evaluate the reproducibility and repeatability of the produced optode, 1.0 × 10−6 and 1.0 × 10−8 mol L−1 Hg(II) ions at phosphate buffer solution (pH 8.0) were prepared and measurement was performed 10 times for each concentration. The relative standard deviations (RSDs) for 1.0 × 10−6 and 1.0 × 10−8 mol L−1 were 1.6% and 1.2%, respectively. These results show that this method has a good precision. In addition, six membranes were prepared and their absorbance were recorded at 661 nm for 1.0 × 10−6 and 1.0 × 10−8 mol L−1 Hg(II) at pH 8.0 (the absorbance of each membrane was read 5 times). The results confirm that the RSDs were 3.8% and 3.5%, respectively.

3.4. Equilibrium response time

3.6. Regeneration of the optode

The response time of an optode in higher concentration of analyte controlled by the time requested for the analyte diffusion from bulk of the solution toward the membrane interface to associate with the ligand. In order to obtain the suitable response time of optode to solution of the species, the optode was immersed in 1.0 × 10− 4,

Different solutions of KCI, KI, HNO3, H2SO4, HCl, H3PO4 and EDTA (all in 0.1 mol L−1) were used to regenerate the optode. The results showed that the best regeneration was obtained by 0.1 mol L−1 H3PO4 in 120 s, because in acidic solution, H+ causes dissociation of

Fig. 3. Response of the optode for different Hg(II) concentrations at different pH values (the membrane consists of 64.0 mg DOS, 32.0 mg of PVC, 2.0 mg of NaTPB, 1.5 mg of ETH5294 and 1.5 mg of the ionophore).

Fig. 4. Response time of the optode for different concentrations of Hg(II) ions at pH 8.0.

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Table 3 Interference study of potential interfering ions. Species

Relative error %a

Relative error %b

Species

Relative error %a

Relative error %b

Al3+ Ba2+ Zn2+ Mg2+ (0.5 mol L−1) Li+ Ca2+ (0.5 mol L−1) Pb2+ Cu2+ Cu2+ (masked with L−1 histidine) Ag+

2.0 Not interfered 1.1 Not interfered 1.2 1.2 1.3 25.0 Not interfered Not interfered

3.3 Not interfered 2.9 1.6 2.1 1.6 2.1 25.1 Not interfered Not interfered

Cd2+ Ni2+ Co2+ Cr3+ Ti+ Fe3+ Mn+ Cs+ NH4NO3

1.8 3.0 2.1 Not interfered Not interfered Not interfered 2.6 2.6 1.6

3.6 3.4 3.4 Not interfered Not interfered Not interfered 2.6 3.2 2.2

a b

1000-Fold of the potential interfering ions were used. 10 000-Fold of the potential interfering ions were used.

Hg(II)-ionophore complex. To know how many times could the membrane be regenerated, we used a single optode to measure a solution of 1.0 × 10−8 mol L−1 Hg(II). After each measurement, the optode regenerated was used one more time for measuring Hg(II) in the solution. The results showed that the optode could be used 10 times after its regeneration before the RSD% reaches to 5%.

The detection limit of the method was calculated (3Sb/m) as 0.18 pmol L−1, which shows that the proposed optode is very efficient for quantitative analysis of Hg(II) ions. Table 1 demonstrates a comparison between the figures of merit of the proposed optode and the other reported optode for Hg(II) detection. 6. Application

4. Selectivity In order to study the effect of different cations on the determination of Hg(II) ions by the proposed membrane, its absorbance at λ = 661 nm was measured for a solution of 1.0 × 10−8 mol L−1 Hg(II). Then, a series of solutions containing the same amount of Hg(II) ions and 1.0 × 10−4 mol L−1 of the intended cations were prepared (the concentration interfering ion was 1000–time as much as Hg(II)) and the relative errors were determined for each of them. Tolerance limit was defined as the concentration of added ions causing less than ± 5% relative error [21]. As can be seen in Table 3 and Fig. 5, only cation having relative error of more than 5% was Cu(II). For repelling the interference of copper ions, a solution of 0.01 mol L-histidine as a masking agent was used. L-Histidine forms a stable complex with Cu(II). 5. Analytical figures of merit Fig. 2 shows absorption spectra of the optode in solutions containing different Hg(II) concentrations. A decreasing in absorption at 661 nm and an increasing at 555 nm were observed. Under the optimized conditions, the response function (1–α) vs. log[Hg(II)] was plotted and revealed that it is linear in the range of 7.2 × 10−13–4.7 × 10−4 mol L−1 Hg(II) according to the equation (1–α) = −0.1083log[Hg(II)]−0.3608 (R2 = 0.9984, n = 9). Fig. 6 shows a response curve of the optode with different Hg(II) concentrations at the optimum conditions.

Fig. 5. Response and calibration curves of the optode for different concentrations of interfering metal ions at pH 8.0.

To check the efficiency of the produced optode to analyze real samples, the quantity of Hg(II) in real samples such as tap water, canned tuna and Zayanderood river water (Isfahan city, Iran) were measured. The accuracy of the optode was also confirmed by inductively coupled plasma (ICP) atomic emission spectrometric method as a standard method. Table 4 shows the comparison between the result obtained by the proposed optode and by ICP method. It can be observed that the results data have no significant difference according to t-test. 7. Conclusion The new optode can analyze Hg(II) ions in a wide range of 4.7 × 10−4–7.2 × 10−13 mol L−1 with a detection limit of 1.85 × 10−13 mol L−1. The effects of different cations on the detection of Hg(II) ions were studied too. Cu(II) was the only interfering ion, which was masked with by L-histidine. The sensor can operate specifically and with no interfering ions for Hg(II). The new optode was used for analyzing Hg(II) ions in real samples. The optode has several advantages such as wide dynamic range, a reproducible response, a short response time, highly selectivity, and easy and low-cost methodology and provides an inexpensive and quick method for the determination of Hg(II).

Fig. 6. Response curve of the optode with different Hg(II) concentrations at the optimum conditions.

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Table 4 Determination of Hg(II) ions in real samples (n = 5). Sample Tap water Tap water Tap water Tap water Tap water Canned tuna (I) Canned tuna (II) Zayanderood river water

Hg(II) added (mol L−1) 2.00 2.00 1.55 2.50 – – –

× × × ×

10−5 10−7 10−7 10−9

Hg(II) found by the optode (mol L−1)

Recovery (%)

Hg(II) found by ICP (mol L−1)

tcal (ttable95%,8 = 2.31)

bDetection limit (2.02 ± 0.05) × 10−5 (1.98 ± 0.08) × 10−7 (1.57 ± 0.04) × 10−7 (2.48 ± 0.06) × 10−9 (1.32 ± 0.06) × 10−9 (2.38 ± 0.03) × 10−9 (1.18 ± 0.06) × 10−8

– 102 98 102 98 – – –

bDetection limit (1.8 ± 0.10) × 10−5 (2.33 ± 0.40) × 10−7 (1.6 ± 0.02) × 10−7 (2.6 ± 0.03) × 10−9 (2.40 ± 0.20) × 10−9 (2.40 ± 0.01) × 10−9 (1.40 ± 0.20) × 10−8

– 2.52 2.71 1.16 0.66 2.62 1.13 1.83

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