Ion Recognition : Synthesis of 2-Methyl-2 , 4-di ( 2-thienyl )-2 , 3-dihydro-1 H-1 , 5-benzodiazepine and its Application in Construction of a Highly Selective and Sensitive Ag + Membrane Sensors

Uma membrana carregadora plastificada foi preparada a partir de 2-metil-2,4-di(2-tienil)2,3-diidro-1H-1,5-benzodiazepina (MTHB). Esta membrana é altamente sensível e seletiva a íons Ag. O sensor mostra um intervalo dinâmico linear de 1,0 × 10 –1,0 × 10 mol L, com inclinação Nernstiniana de (58,5±0,3) mV per década, e limite de detecção 5,0 × 10 mol L (~54 ppb). Apresenta rápido tempo de resposta, 10s, e pode ser usado por 8 semanas no mínimo, sem alterações consideráveis no seu potencial. O sensor proposto pode ser usado no intervalo de pH entre 4,0 e 8,0 e foi utilizado com sucesso, como eletrodo indicador, na titulação potenciométrica de Ag com NaCl.


Introduction
Silver plays no known natural biological role in humans, and possible health effects of silver are subject of dispute.Silver itself is not toxic but most of its salts are, and some may be carcinogenic.The toxicity of silver is characterized by a severe pulmonary edema, hemorrhage and necrosis of bone marrow, liver, and kidney.Long-term human exposure to silver salts or colloidal silver may cause argyria.Repeated exposure of animals to silver may produce anemia, cardiac enlargement, growth retardation and degenerative changes in the liver.Silver compounds were used successfully to prevent infection in World War I before the advent of antibiotics, and Silver compounds are still widely used externally today to accelerate healing in burn victims.Very low concentrations of silver are present in various complex samples like drinking water (5 μg L -1 , soil, rock, coal fly ash, air, cigarettes, alloys, plants, sea water etcetera). 1,2me techniques such as atomic absorption spectroscopy, 3 inductively coupled plasma, 4 spectroscopy with complexation agents 5 and Rayleigh light-scattering 6 have been used to determine amount of silver.
Because of the increasing use of silver compounds in industry and medicine the quick determination of trace quantities of Ag + ion by simple methods is important in chemical, clinical and environmental analysis.
Potentiometer monitoring based on ion-selective membrane sensor, as a simple method, offers several advantages such as speed and ease of preparation and procedures, simple instrumentation, relatively fast response, wide dynamic range, reasonable selectivity, and low cost.These characteristics have inevitably led to sensors for several ionic species, and the list of available electrodes has grown substantially over the last few years. 7][10][11][12][13] Chen and coworkers 8,10 reported polymeric membranes based on nitrogen containing calixarene derivatives; Mahajan et al. 9 used 2,6bis-methylsulfanyl- [1,3,5]thiadiazine-4-thione as a Ag +seleetive ionophore; Wrbblewski and Brzdzka 11 reported an Ion Recognition J. Braz.Chem.Soc.

Reagents
Reagent grade o-nitrophenyloctyl ether (NPOE), dibutylphthalate (DBP), benzyl acetate (BA), sodium tetraphenylphenyl borate (NaTPB), tetrahydrofuran (THF), and high relative molecular weight PVC were purchased from Merck and Aldrich, used as received.The nitrate and chloride salts of all cations used (all from Merck and Aldrich) were of the highest purity available and used without any further purification except for vacuum drying over P 2 O 5 .Triply distilled de-ionized water was used throughout.

Electrode preparation
The general procedure to prepare the PVC membrane was thoroughly mixing 30 mg of powdered PVC, and 63 mg of DBP in 3 mL of THF.To the mixture was added, 2 mg NaTPB and 5 mg MTHB and the solution was mixed well.The resulting mixture was transferred into a glass dish of 2 cm in diameter.The THF content of the mixture was evaporated slowly, until an oily concentrated mixture was obtained.A Pyrex tube (3-5 mm o.d.) was dipped into the mixture for about 10 s, so that a transparent membrane of about 0.3 mm thickness was formed. 27,28The tube was then pulled out of the solution and kept at room temperature for 10 h.The tube was then filled with an internal filling solution (1.0 × 10 -3 mol L -1 AgNO 3 ).The electrode was finally conditioned for 24 h by soaking in a 1.0 × 10 -3 mol L -1 solution of AgNO 3 .A silver/silver chloride coated wire was used as an internal reference electrode.

EMF measurements
Potential measurements were carried out by means of a Corning ion analyzer 250 pH/mV meter at the room temperature.
A cell assembly is:

Conductance study of complexation of MTHB with metal ions in propylene carbonate solution
At first, the interactions between ion-carrier (MTHB) and different cations were tested.To the best of our knowledge, there is no report on the stability of MTHB complexes with different cations.Thus, the complexation of MTHB with a number of common cations was conductometricaly investigated in a propylene carbonate solution, at (25±0.05) °C, in order to obtain a clue about the stability and selectivity of the resulting complexes.15 mL of vanadyl solution was titrated with 0.01 mol L -1 of MTHB solution. 29he resulting molar conductance vs.
[MTHB]/[M n+ ] mole ratio plots showed the formation of 1:1 complexes between MTHB and silver ions.The complex formation constants, K f , were evaluated by computer fitting of the molar conductance/mole ratio data with appropriate equations, 30 and the results are summarized in Table 1.As it is obvious, the stability constant of the Ag + -MTHB complex is higher than other cations-MTHB complexes tested.Thus, MTHB may be used as a suitable selective ionophore in construcrtion of a silver ion-selective membrane electrode.
Thus, in the next step, MTHB was used as a potentially suitable neutral carrier in the fabrication of a number of PVC membrane ion-selective electrodes for Ag + ion and common mono and di-valent metal ions.The potential responses for these metal ions used are depicted in Figure 2. As can be seen from Figure 2, among the metal ions examined, only the resulting Ag + -selective sensor possesses a Nernstian behavior over a wide concentration range.

Effect of membrane composition on the potential response of the Ag + sensor based on MTHB
2][33][34][35][36] Thus, the influences of membrane compositions on the potential responses of the Ag + sensor were studied, and the results are given in Table 2.
Using 5% of MTHB and 2% of NaTPB in the membrane electrode displays Nernstian slope towards silver ion (membrane No. 4).As it is obvious from Table 2 DBP is a more effective solvent mediator than BA and NPOE in preparing the silver ion-selective electrode.It should be noted that the nature of the plasticizer influences both the dielectric constant of the membrane and the mobility of the ionophore and its complex.
Table 2 revealed that addition of 2% NaTPB will increase the sensitivity of the electrode response considerably so that the membrane sensor (No. 4) displays a nice Nernstian behavior.It is well known that the presence of lipophilic anions in cation-selective membranes based on neutral carrier not only diminishes the ohmic resistance and enhances the response behavior and selectivity but also, in cases where the extraction capability is poor, increases the sensitivity of the membrane electrodes. 35,36However, the membrane with the composition of 30% PVC, 5% MTHB, 2% NaTPB, and 63% DBP displays a very nice Nernstian behavior.

Calibration curve and statistical data
The plot of EMF vs. pAg + obtained under optimal membrane ingredients for the sensor (Figure 3), indicate that it has a Nernstian behavior over a very wide concentration ranges of Ag + ion (1.0 × 10 -6 -1.0 × 10 -1 mol L -1 ).The slope and linear range of the resulting calibration graph was (58.5±0.3)mV decade -1 and 1.0 × 10 -6 -1.0 × 10 -1 mol L -1 , respectively.The limit of detection (LOD), defined as the concentration of Ag + ion obtained when the linear regions of the calibration graph extrapolated to the base line potential, is (5.0±0.7)× 10 "7 mol L -1 (~54 ppb).The standard deviation of 8 replicate potential measurements for proposed sensor was at the most ± 0.3 mV.

Effect of pH
The pH dependence of the Ag + membrane sensor was tested over a pH range of 3.5-10.5 in a 1.0 × 10 -3 mol L -1 Ag + solution, and the results are illustrated in Figure 4.As it can be seen, the potential remains fairly constant in a pH range of 4.0-8.0.Beyond this range, a gradual change in the potential was detected.This is most probably due to the protonation of two nitrogen atoms of ionophore in some extent.In this case, protonated nitrogen atoms cannot contribute to the interaction of donor atoms of ionophore with silver ions, and so the response of the sensor to silver ions decreases drastically.The main reason for the observed potential drift at the higher pH values could be due to the responds of the membrane sensor to hydroxyl ions by the deprotonation of -NH group of the ionophore and decreasing of silver ion concentration in some extent by formation of insoluble AgOH (with a Ksp of 1.6 ×10 -8 ).
The potential response of the sensor to the proton ions in the absence of silver ion was also investigated.The results showed that in the pH range of 4.0-7.5 the sensor has not any response to the proton ions.This is due to the stability of the ionophore in the pH range of 4.0-7.5.At acidic pH values (2.0-3.5), the sensor response was shown a selective behavior to the proton ions with a near-Nernstian slope.In this case due to the protonation of nitrogen atoms of ionophore and formation of a watersoluble ionophore, the stability of the membrane sensor was drastically reduced.The pH of silver ion (as nitrate salt) at very low concentration (10 -7 mol L -1 or higher) is lower than 7.0 (due to the acidic property of silver ion) and therefore, detection limit of the proposed sensor can be determined without adjusting of pH.

Dynamic response time
For analytical purposes, response time is one of the most important factors that are taken into account.In this work, the practical response time was recorded by immediate and successive changing of Ag + concentration from 1.0 × 10 -6 to 10 × 10 -1 mol L -1 and the results are shown in Figure 5.As it can be seen, in the whole concentration range, the electrode reaches to its equilibrium response in a very short time (10 s).

Life time study
For investigation the stability and lifetime of Ag + membrane sensor, three sensors were tested over a period of ten weeks.During this period, the sensors were in daily used over extended period of time (one hour per day) and their slopes and detection limits were measured.The results are summarized in Table 3.As can be seen from Table 3, after eight weeks slight changes in the slopes and detection limits (from 58.5 and 5.0 × 10 -7 to 55.6 mV per decade and 8.0 × 10 -6 mol L -1 , respectively) was observed.It should be mentioned that due to the   independency of the sensor to proton ions in the range of 4.0-7.5, and pH range of 5.5-6.8 of the silver solutions (with concentration range of 10 -1 -10 -7 mol L -1 ), life time study was done without adjusting of pH.

Evaluation of selectivity coefficients
The potentiometric selectivity coefficients, which reflected the relative response of the membrane sensor towards the primary ion, over other ions present in solution, perhaps are the most important characteristics of an ion-selective electrode.In this work, the potential responses of the proposed Ag + membrane sensor to a wide variety of cations were investigated through the matched potential (MPM) 37 and separation solution method (SSM). 38The resulting selectivity coefficient values obtained for the Ag + membrane electrode are summarized in Table 4.As it is seen, for alkali and alkali earth metal ions tested (Na + , K + , Ca 2+ ) the selectivity coefficients are less than 7.9 × 10 -6 .Selectivity coefficients obtained for other cations tried (Cd 2+ , Pb 2+ , Hg 2+ , Cu 2+ ) are also very small, and are smaller than 1.9 × 10 -5 .
The obtained selectivity coefficients indicate that the disturbance produced by these cations in the functioning of the proposed Ag + membrane sensor is negligible.
][10][11][12][13] From the data given in Table 5, it is immediately obvious that the selectivity coefficients of the sensor are superior to those reported by other researchers.

Analytical application
The Ag + sensor was successfully used as an indicator electrode in the titration of 25.0 mL of 1.0 × 10 -4 mol L -1 Ag + solution with a 1.0 × 10 -2 mol L -1 NaCl, and the resulting titration curve is given in Figure 6.As it is seen, due to the sharp end-point of the titration, the amount of Ag + ion in the solution can be determined with the sensor with good accuracy.

Figure 3 .
Figure 3. Calibration curve of the Ag + membrane sensor based on MTHB.

Figure 4 .
Figure 4.The effect of the pH of the test solution (1.0 × 10 -3 mol L -1 ) on the potential response of the Ag + membrane sensor.

Table 1 .
The formation constants of MTHB __ M n+ complexes

Table 2 .
The optimization of the membrane ingredients

Table 5 .
Comparison of the selectivity coefficients of different Ag + sensors

Table 4 .
Selectivity coefficients of various interfering cations for the best composition membrane

Table 3 .
The lifetime behavior of the Ag + membrane sensor