The authors have declared that no competing interests exist.
Application of nickel in different industries has been developed and so contamination of natural water is a great concern due to its potentially toxic effects on living beings. Therefore, fast monitoring of Ni2+ in aqueous samples is important. In this work, we fabricated a sensitive optical sensor for determination of nickel in mineral water samples and hydrogen peroxide solutions. The optode was prepared by incorporation of 1-(2-pyridylazo)-2-naphthol and sodium tetraphenylborate in a plasticized poly (vinyl chloride) membranes containing dioctyladipate as a plasticizer. The influence of several parameters such as pH, base matrix, solvent mediator and ligand concentration were optimized. Comparison the obtained results with previously reported sensors revealed that the proposed method, in addition to fast and simplicity, provided good linear range (1.70–85.20 µmol L-1) and low detection limit (0.17 µmol L-1). The precision (relative standard deviation) was better than 1.55% for 7 replicate determinations of 17.10 µmol L-1 of Ni in various membranes.
Nowadays, the pollution of natural water by heavy metals is a great concern due to their potentially toxic effects on living beings, therefore the detection and monitoring of toxic metals in water samples is necessary and very important
Due to wide applications of nickel in different industries
Hydrogen peroxide is used as a disinfectant, oxidizer and so forth in usual applications
Determination of nickel is performed by several techniques such as x-ray absorption spectroscopy
Recently, an interest has been increased on the development of optical sensors compared to electrochemical sensors
The most widely used polymers in optical sensors are poly (vinyl chloride) groups. They have many desirable features and compare well with sol–gel matrices for most applications
1-(2-pyridylazo)-2-naphthol) PAN) is a red solid which is readily soluble in common organic solvents such as methanol, ethanol etc. It forms coloured complexes with a large number of metal ions
In this research, we introduced a selective, sensitive and rapid method based on PVC sensor for spectrophotometric determination of nickel by using PAN in water and hydrogen peroxide samples. According to the best of our knowledge, although analysis of Ni2+ was reported by this method previously but there is no information for monitoring of this cation in H2O2 media.
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
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
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)
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 (λmax=570 nm) are shown in
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
As shown in
(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
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
Membrane | LPVC (mg) | DOA (mg) | PAN (mg) | NaTPB (mg) | Response time (min) | Absorbance (570 nm)a |
1 | 30 | 65 | 8 | 5 | 10 | 0.197 ± 0.021 |
2 | 30 | 70 | 8 | 5 | 10 | 0.285 ± 0.012 |
3 | 30 | 75 | 8 | 5 | 10 | 0.357 ± 0.005 |
4 | 30 | 80 | 8 | 5 | 10 | 0.315 ± 0.009 |
5 | 30 | 85 | 8 | 5 | 10 | 0.177 ± 0.018 |
6 | 30 | 75 | 4 | 5 | 10 | 0.214 ± 0.017 |
7 | 30 | 75 | 6 | 5 | 10 | 0.303 ± 0.011 |
8 | 30 | 75 | 8 | 5 | 10 | 0.357 ± 0.005 |
9 | 30 | 75 | 10 | 5 | 10 | 0.348 ± 0.008 |
10 | 30 | 75 | 12 | 5 | 10 | 0.325 ± 0.010 |
11 | 30 | 75 | 8 | 3 | 10 | 0.231 ± 0.024 |
12 | 30 | 75 | 8 | 4 | 10 | 0.314 ± 0.011 |
13 | 30 | 75 | 8 | 5 | 10 | 0.358 ± 0.006 |
14 | 30 | 75 | 8 | 6 | 10 | 0.326 ± 0.017 |
15 | 30 | 75 | 8 | 7 | 10 | 0.280 ± 0.032 |
16 | 30 | 75 | 8 | 5 | 2 | 0.132 ± 0.023 |
17 | 30 | 75 | 8 | 5 | 6 | 0.300 ± 0.011 |
18 | 30 | 75 | 8 | 5 | 10 | 0.358 ± 0.004 |
19 | 30 | 75 | 8 | 5 | 12 | 0.359 ± 0.006 |
20 | 30 | 75 | 8 | 5 | 14 | 0.358 ± 0.005 |
a Mean absorbance ± SD (n=3) of each parameter is recorded from three solutions of 34.08 µmol L-1 Ni+2 (pH 6.0)
The influence of media pH on the sensor response was studied in the range 4-8. As it is shown in
Response time of optodes is defined as the diffusion time of the metal ions from solution into the membrane (slowest step in complexation process)
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
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.
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).
Regression equation (n=12) | A = 0.1721C + 0.018, r = 0.9991 |
Linear range (µmol L-1) | 1.70-85.20 |
Limit of detection (µmol L-1) |
0.17 |
Reproducibility (RSD%) |
1.55 |
For seven replicate determinations of the sensor in the absence of nickel (n=7).
For seven replicate determinations of Ni: 17.10 µmol L-1
Membrane type | Ionophore | pH | LDR |
LOD |
Ref. |
Nafion | 1-(2-pyridylazo)-2- naphthol | 6.5 | 2×10-5-12×10-5 | 1.7×10-5 | 43 |
″ | 2-(5-bromo-2-pyridylazo)-5-(diethylamino) phenol | 6.5 | 5. 1×10-6-3.4×10-2 | 8.5×10-6 | 45 |
PVC | 2-amino-1-cyclopentene-1-dithiocarboxylic acid | 4-6.5 | 5.0×10-6 -1.0×10-3 | 5.2×10-7 | 46 |
″ | 1,2-di( |
6 | 1.0 × 10−5-5.0 × 10−3 | 8.5×10−6 | 47 |
Triacetyl cellulose-optical | triazene-1-oxide derivative | 5.7 | 1.18×10−9-7.34×10−5 | 1.0×10−9 | 48 |
PVC | 1-(2-pyridylazo)-2- naphthol | 6 | 1.70×10-6-8.52×10-5 | 1.70×10-7 | This work |
Linear Dynamic Range; Limit of Detection
The selectivity of this sensor for determination of 8.52 µmol L-1 of Ni+2 was summarized in
From
Foreign ions | Tolerance Ratio (M:[Ni+2]) |
Cations | |
Na+, K+, Li+ and NH4+ | 83.33403 |
Ca2+, Mg2+, Ba2+, Sr2+, Cr3+, UO22+ and ZrO22+ | 4.167361 |
Mn2+, Fe2+, Fe3+, Al3+, Zn2+, Ga3+ and Cu2+ | 2.084028 |
Co2+, Cd2+ and Pb2+ | 0.417361 |
Anions | |
SO42-, CO32-, NO3-, NO2-, BO3- and HCO3- | 83.33403 |
I-, HPO42- and PO43- | 41.66736 |
CN-, Cl- and F- | 0.834028 |
a ≤ 5.0% Deviation in the absence of masking agent.
The results of applicability of the membrane methodology are presented in
Samples |
Ni2+ (µmol L-1) | ||
Added | Found |
Recovery (%) | |
Mineral | - | n.d. |
- |
1.7 | 1.77 ± 0.34 | 104 | |
13.63 | 13.46 ± 0.17 | 99 | |
River | - | n.d. c | - |
6.82 | 6.64 ± 0.34 | 98 | |
17.04 | 17.38 ± 0.17 | 102 | |
H2O2 (10%) | - | 1.87 ± 0.51 | - |
5.11 | 7.16 ± 0.34 | 103 | |
11.93 | 13.63 ± 0.17 | 99 | |
H2O2 (20%) | - | 2.90 ± 0.51 | - |
3.41 | 6.47 ± 0.34 | 105 | |
13.63 | 16.19 ± 0.34 | 98 |
Sample sources as described in the text.
Mean value of three replicate determination ± Standard deviation.
Not detected.
The proposed optode is a precise, low cost and sensitive device for determination of nickel, based on PVC membrane. Also the proposed method, in addition to fast and simple, provides a wide dynamic range, reliable reproducibility and a good limit of detection. EDTA was used as masking agent and the method could be made selective in this way. A comparison of the proposed optode with the previously reported sensors indicates that the proposed method in some cases provides wider linear range and lower detection limit. Finally, the fabricated sensor can be successfully applied to nickel monitoring in water and hydrogen peroxide samples. According to the best of our knowledge no manufactured optode has been reported in the literature for the determination of Ni in H2O2 solutions.