All the chemicals were of analytical grade. 1-(2-pyridylazo)-2-naphthol )PAN), Low molecular weight poly vinyl chloride (LPVC, mol wt ~ 48000 g mol^-1), dioctyladipate (DOA) and sodium tetraphenylborate (NaTPB) were used without purification. Nickel solutions were prepared from a 17.04 mmol L^-1 standard solution. A buffer of pH 6.0 was prepared from sodium hydroxide (0.046 M) and potassium hydrogenphthalate (0.05 M) solutions ³⁹.

Absorption measurements were carried out on a Hitachi-U 3310 model Lambada-25 double beam UV-Vis spectrophotometer. The pH of the solutions was measured by a Metrohm model 691 pH/Ion Meter using a combined glass electrode. 

The membrane consisted appropriate amounts of active components. 30.0 mg of PVC, 75.0 mg of DOA, 8.0 mg of PAN and 5.0 mg of NaTPB were transferred in a glass vial and dissolved into 1 ml THF. The solution was immediately shaken vigorously to achieve complete homogeneity. A glass plate (1×9×50 mm³) was cleaned with pure THF and then placed in the spin-on device. Ninety micro liters of the above solution was injected to the glass plate. After 30 s spinning, at rotation frequency of 600 rpm, the membrane was located in ambient air and allowed to dry in air for few minutes.

The sensing membrane (optode) was placed in a beaker filled with 20 ml of the test solutions containing EDTA (1.0×10^-2 mol L^-1) and different concentration of Ni²⁺ (1.70 - 85.20 µmol L^-1) at pH 6.0. After 10 min the optode was mounted into the spectrophotometer directly and its net absorbance was recorded at 570 nm against a blank membrane.

Water samples (Mineral and river) were collected in 1 L amber glass bottles from Haraz re(Anahita, Polur) and Jajrud (Lavasan) rivers in Iran. After sample pretreatments ³⁴, 3 ml of each sample was spiked with appropriate amount of Ni (II) and was subjected to the above procedure.

Hydrogen peroxide (10-20%) samples were prepared freshly from commercial hydrogen peroxide (30%) solution after filtration. Then, each of samples after Ni (II) spiking was subjected to the membrane methodology.

1-(2-pyridylazo)-2-naphthol )PAN) was recommended as a spectrophotometric reagent by Cheng and Bray where it gives a red dish colored chelates with metal ions as an example of Ni (II) ⁴⁰. Because PAN is organic, it is efficiently immobilized in the hydrophobic part of the membrane without prior lyophilization ⁴¹. It was observed that by optimized fabricating a sensor from incorporation of PAN in a plasticized PVC membrane containing DOA, the determination of nickel could be practicable spectrophotometrically.

When nickel ions diffused into the membrane, they formed a complex with PAN so the membrane colors changed from yellow to red. The absorption spectra of the PAN optodes in different concentrations of Ni (λ_max=570 nm) are shown in Figure 1

Figure 1. Absorption spectra of the proposed optode at various concentrations of Ni2+: (1) 1.70, (2) 3.41, (3) 8.52, (4) 13.64, (5) 17.04, (6) 25.56, (7) 51.12, (8) 85.20 µmol L-1; Conditions: pH=6.0; T=25◦C; response time=10 min; membrane layer containing 25.42% PVC, 63.56% DOA, 6.78% PAN and 4.24% NaTPB.

The response characteristics and working concentration range of each optical sensor depends significantly on the different ingredients such as base matrix, solvent mediator, ionophore and additive used in the membrane structure. Therefore the sensor matrix should be selected, firstly. In comparison of different polymers, low molecular weight PVC was observed as the best selection for the membrane base. This choice was due to several parameters such as appropriate transmittance, suitable immobilization of PAN without any leakage, good mechanical stability and reliable permeability to Ni²⁺ ions.

The nature of plasticizer is important and must be physically adaptable with polymer. In order to have a homogenous organic phase, several plasticizers such as dibutylphthalate (DBP), tributylphosphate (TBP), dioctylphthalate (DOP), ortho-nitrophenyloctyl ether (O-NPOE) and DOA were tested as potential plasticizers. From the graphs in Figure 2 (a and b), it was concluded that the membranes containing DBP, TBP, DOP and O-NPOE did not produced a suitable signals because of either improper physical properties with LPVC or leakage of PAN from the membrane. We found that DOA was the best selection with respect to good physical properties, the highest sensitivity and the lowest leakage.

Figure 2. Effect of plasticizer nature on the response of the membrane after 10 min (a) and on the membrane leakage% after 30 min (b). Conditions: 34.08 µmol L-1 Ni2+; T=25◦C; membrane layer containing 30.0 mg of PVC, 75.0 mg of each plasticizers, 8.0 mg PAN.

As shown in Table 1

(membrane no. 1-5), the sensors with a weight ratio of DOA to LPVC as 2.5 provided best absorbance. Thus, 30 mg of PVC and 75 mg of DOA were selected as the optimum values. A decrease of the Ni²⁺ uptaking efficiency, at values lower than 75 mg, is explained by improper solidity of the optode that led to low diffusion of analyte cations into the membrane. At quantities more than 75 mg, flexibility of the optode increased, led to ionophore leakage into the test solution.

Effect of different amounts of PAN on the membrane response is observed in Table 1. As seen in membrane no. 6-10, the absorbance increased by increasing amounts of PAN up to 8 mg and decreased at higher amounts that were resulted from membrane leakages. Therefore, 8 mg PAN was selected as optimum value.

Due to the complete mass transfer of Ni2+ ions into the membrane and decreasing of response time, the presence of an anionic additive such as NaTPB facilitates the ion-exchange equilibrium ⁴². The effect of NaTPB was investigated in the range 3.0-7.0 mg (membrane no. 11-15 of Table 1). It is shown that the highest absorbance is recorded by using 5.0 mg of NaTPB.

The influence of media pH on the sensor response was studied in the range 4-8. As it is shown in Figure 3 optode absorbance increased at pH 6.0 and then decreased. At pH<6, protonation of the ligand prevents its reaction with Ni²⁺ ions and at pH>6.0, the response decreasing could be due to the hydrolysis of Ni²⁺ ions that is caused to incomplete diffusion of Ni²⁺ cations into the membrane. Therefore, a buffer with pH 6.0 was chosen in all experiments. In comparison, the optimum pH for Ni-PAN complex in aqueous solution has been reported as 6.5 ⁴³.

Figure 3. Effects of pH on the absorbance of the proposed optode; conditions: 34.08 µmol L-1 Ni2+; T=25◦C; response time=10 min; membrane layer containing 25.42% PVC, 63.56% DOA, 6.78% PAN and 4.24% NaTPB.

Response time of optodes is defined as the diffusion time of the metal ions from solution into the membrane (slowest step in complexation process) ⁴⁴. The effect of this parameter on the optode response was studied (Table 1 membrane no. 16-20). As seen, at least a time interval of 10 min is required for quantitative uptake at room temperature. It was observed that the optode response remained constant for more than 2 hours.

The properties of the optode membrane were measured by recording absorbance changed at 570 nm from individual solutions of 8.52, 17.04 and 51.12 µmol L^-1 of Ni. As it is seen in Figure 4 all of the three cases, the optodes reached to 98% absorbances after 10 minutes.

The stability of membranes was tested for 2 hours and during this period a mean difference of absorbances for the mentioned solutions was ±0.007. Also the membrane responses were stable for one month in air.

Figure 4. Response time of the optode membranes for different concentrations of nickel, Conditions: pH=6.0; T=25◦C; membrane layer containing 25.42% PVC, 63.56% DOA, 6.78% PAN and 4.24% NaTPB

The salting-out phenomenon on the optode response was investigated by adding different amounts of sodium nitrate. The results indicated that this parameter had no effect on the membrane response, up to 0.04 mol L^-1 of NaNO₃ and above this concentration it was reduced slightly. This is due to a decrease in the activity of Ni²⁺ ions at higher concentration of electrolyte which reduces the interaction of nickel (II) cation with PAN in the membrane.

The sensor regeneration was studied by using of different compounds such as hydrochloric acid, nitric acid, sulfuric acid, sodium fluoride and oxalic acid in different concentrations. It was found that all of the reagents could not regenerate the optode membrane thoroughly and thus the membrane could be used as a probe (single test).

Table 2 summarizes the analytical characteristics of the optimized membrane. In this manner dynamic linear range was resulted within 1.70–85.20 µmol L^-1 of nickel and detection limit was 0.17 µmol L^-1. Also the relative standard deviation (RSD %) for 7 replicate determinations of 17.10 µmol L^-1 of Ni⁺² in various membranes was 1.55% and so the method is reproducible during the experiments.

Table 3 presents a comparison between the proposed optode and the other sensors for determination of nickel previously ^{43, 45, 46, 47, 48}. It is obvious that the obtained results of this work are comparable with these existing sensors. In some cases, it provides better linearity range and detection limit.

The selectivity of this sensor for determination of 8.52 µmol L^-1 of Ni⁺² was summarized in Table 4. The tolerance limit was defined as the concentration of added ion causing less than ±5 ٪relative error.

From Table 4, presence of alkaline metals and anions such as sulfate, chloride and others did not have adverse effects on nickel uptake. As seen, bivalent and some trivalent cations can interfere at different ratios. For elimination of these interferences, EDTA was selected as a masking agent. The effect of EDTA interfering was investigated firstly. It was resulted that EDTA^2- did not affect (causing less than 4% negative deviation) on Ni measurement up to 1.0×10^-2 mol L^-1. Thus, the presence of EDTA in the test solution can effectively mask 0.2 mmol of any interfering cations. Hence, the proposed membrane is a selective sensor for nickel monitoring in water and H₂O₂ samples.

The results of applicability of the membrane methodology are presented in Table 5. As shown, the mean recoveries for the addition of different concentrations of nickel to the water samples and H₂O₂ solutions were in the range of 98-104% and 98-105% respectively. Therefore, the proposed sensor can be successfully applied for the determination of nickel in the mentioned samples.