Novel Electrochemical Sensor for Sunset Yellow Based on a Platinum Wire-Coated Electrode.
In this research, the construction and general performance characteristics of a sunset yellow sensor based on sunset yellow–cetyl pyridinum (SY-CPY) ion pair as an ion exchanger were described. A coated platinum wire electrode (CPE) was prepared and compared with coated graphite (CGE) and membrane electrode (PME). The CPE exhibited a rapid and Nernstian response (−29.77 ± 0.2 mV decade−1) to SY concentration range from 3.16 × 10−7 to 3.16 × 10−3 mol dm−3 within pH 4.5–9.5. Interfering effects of some foreign substances were reported. The optimized matrix was successfully applied to the determination of SY in artificial mixtures and commercial soft drinks. The results showed good agreements with the determination made by use of high-performance liquid chromatography.
Keywords: Potentiometer; sensor; sunset yellow; synthetic food colorant
INTRODUCTION
Synthetic colorants are commonly used as additives in food, drugs, and soft drinks to get suitable and natural-looking colors. Because of their high brightness, stability, low cost, and wide range of shades, their use has extensively increased in the past two decades in comparison with natural colorants (Macdougall [22]; Zollinger [34]; Ames and Hofman [4]). Studies by Sasaki et al. ([25]) showed the dyes induced DNA damage in the gastrointestinal organs at a low dose. Boris and Mandel ([10]) studied young children with attention deficit hyperactivity disorder (ADHD) and found that an elimination diet can lead to a statistically significant decrease in symptoms. Abdel Aziz et al. ([1]) showed sperm abnormalities increase with dye consumption. Consumption of red no. 3 (erythrosine), which has estrogen-like growth stimulatory properties, could be a significant risk factor in human breast carcinogenesis (Dees et al. [14]). Abdel Aziz et al. ([1]) also showed that it reduced mobility of sperm in mice.
1-p-Sulfophenylazo-2-naphthol-6-sulfonic acid disodium salt (C.I. 15958), also named sunset yellow FCF (SY), is a synthetic colorant, widely used as an additive in soft drinks and other nonalcoholic drinks. The highest dose of it is in soft drinks: 70.0 mg l−1 (Capitán-Vallvey et al. [11]). It may cause asthma, rashes, and hyperactivity, and aspirin-sensitive people must also avoid it. The concentration of these additives must be carefully controlled as they may have various harmful effects on human health.
A number of spectrophotometric (Berzas Nevado et al. [9]; Berzas Nevado, Cabanillas, and Contento Salcedo 1985), chromatographic (Patel et al. [23]; Pnina, Chain, and Cais [24]; Greenway, Kometa, and Macrae [16]; Lawrence, Lancaster, and Conacher [21]), and electroanalytical (Barros [7]; Fogg, Barros, and Cabral [15]) methods for determination of food colorants have been proposed. Chromatographic techniques have the disadvantage of requiring expensive equipment and expert operators. The other methods require long analytical times and are overly complex. Spectrophotometric methods have been more widely used, but overlapping of the spectral absorption bands of the colorants and the influence of matrix effects on the measurement of the analytical signal are two significant drawbacks. These problems can be overcome by using an appropriate derivative technique and/or a statistical method for the manipulation of the obtained results.
In recent years, polymeric membrane–based potentiometric sensors that are responsive to a variety of ionic species have attracted increasing interest because of their potential use in biological, chemical, and pharmaceutical analysis (Cosofret [12]; Cosofret and Buck [13]). The inherent advantages of potentiometric electrodes are simplicity, short times, low cost, adequate precision and accuracy, adequate detection limit, wide analytical range, and particularly the ability to measure the activity of various species selectively, in most cases without prior separation of the target molecule from the formulation matrix (Bakker, Buhlman, and Pretsch [6]; Valsami, Koupparis, and Macheras [33]; Hassan et al. [17]). These make them very attractive for chemical and pharmaceutical analysis (Aboul-Enein and Sun [2]; Aboul-Enein, Sun, and Sun [3]; Shamsipur, Jalili, and Haghgoo [28]; Shamsipur et al. 2000). To the best of our knowledge, there is no previous literature report on development of a potentiometric sensor for determination of artificial food colorants.
In this article, simple potentiometric PVC membrane (PME), graphite (CGE), and platinum-coated (CPE) electrodes for the determination of sunset yellow in food matrix are described. The membrane used in these electrodes was made from liquid-plasticized PVC and is based on a water-insoluble sunset yellow–cetyl pyridinum (SY-CPY) ion pair as an ion exchanger.
EXPERIMENTAL
Reagents
The following materials were purchased from Merck chemical company and were used as received: reagent-grade dioctyl phthalate (DOP), benzyl acetate (BAC), dibutyl sebssate (DBS), dibutyl phthalate (DBP), 2-nitrophenyl octyl ether (NPOE), cetyl pyridinum bromide (CPYB), high-relative-molecular-weight PVC, tetrabutyl ammonium bromide (TBAB), N-hexadecyl,N,N,N-trimethyl ammonium bromide (CTAB), and tetrahydrofuran (THF). Sunset yellow (E110) FD&C yellow no. 6. was obtained from the Food and Drug Quality Control Laboratory (Ministry of Health and Medical Education) (Fig. 1). The reagent-grade sodium salts of the anions (all from Merck) were of the highest possible purity and were used without any further purification except for vacuum drying over P2O5. Triply distilled deionized water was used throughout.
Graph: Figure 1 Sunset yellow structure.
The ion pair was prepared by shaking 5.0 cm3 of a 0.01 mol dm−3 SY solution with an equal volume of a 0.03 mol dm−3 of CPYB. The resulting SY-CPY ion pair precipitates were centrifuged, filtered, thoroughly washed with water, and dried under vacuum over P2O5 for 72 h.
Electrode Preparation
The general procedure to prepare the PVC membrane was to mix thoroughly the required amounts of powdered PVC, SY-CPY, plasticizer, and additive. Then the mixture was dissolved in 5 ml of THF. The obtained mixture was transferred into a glass dish 2 cm in diameter. Then the solvent was evaporated slowly until an oily concentrated mixture was obtained. A Pyrex tube (3 mm) was dipped into the mixture for ∼10 s so that a nontransparent membrane of ∼0.3 mm thickness was formed. Next the tube was pulled out from the mixture and kept at room temperature for ∼1 h. Then the tube was filled with internal filling solution (1.0 × 10−3 M SY). Finally the electrode was conditioned for 4 h by soaking in a 1.0 × 10−3 M solution of SY. Also a silver/silver chloride electrode was used as an internal reference electrode.
Spectroscopic-grade graphite rods 10 mm long and 3 mm in diameter were used to prepare CGEs. A shielded copper wire was glued to one end of the graphite, and the electrode was sealed into the end of a PVC tube of about the same diameter with epoxy resin. The working surface of the electrode was polished with fine alumina slurries on a polishing cloth, sonicated in distilled water, and then dried in the air. The polished graphite electrode was dipped into the membrane solution mentioned previously, and the solvent was evaporated. A membrane was formed on the graphite surface and the electrode was allowed to stabilize overnight. The electrode was finally conditioned by soaking in a 1 × 10−2 M solution of SY for 48 h.
Platinum wires with 8 mm long and 2.5 mm in diameter were used to prepare coated wire electrodes. A shielded copper wire was glued to one end of the platinum wires, and the electrode was sealed into the end of a glass tube of about the same diameter with epoxy resin. The working surface of the wire was polished with fine alumina slurries on a polishing cloth, sonicated in distilled water, and then dried in the air. The polished platinum wire was dipped into the membrane solution mentioned previously, and the solvent was evaporated. A membrane was formed on the platinum surface, and the electrode was allowed to stabilize overnight. The electrode was finally conditioned as CGE.
Emf Measurements
All emf measurements were carried out with the following group:
- Ag-AgCl, KCl (3 M) | internal solution, 1.0 × 10−3 M NaNO3| PVC membrane | test solution || Hg-Hg2Cl2, KCl (satd.): PME
- Graphite surface | PVC membrane | sample solution || Hg-Hg2Cl2, KCl (satd.): CGE
- Platinum surface | PVC membrane | sample solution || Hg-Hg2Cl2, KCl (satd.): CPE
A 713 Metrohm pH/mV meter was used for potential measurements at 25.0 ± 0.1°C. The emf observations were related to a double-junction saturated calomel electrode (SCE, Metrohm) with the chamber filled with an ammonium nitrate solution. Activities were calculated according to the Debye-Huckel procedure (Kamata et al. 1988).
Food Sample Treatment
The method was applied to determine SY in a commercial soft drink. For this purpose, an appropriate amount of soft drink powder (commercial product, TONG drink powder [orange taste], Sunich, Alifard Co. Ind.) was transferred to a 25-ml calibrated flask and dissolved in 0.1 M HCl. The solution was centrifuged 20 min to 5000 rpm, and supernatant was transferred to a 25-ml volumetric flask.
RESULTS AND DISCUSSION
Some important features of PVC membrane, such as nature and amount of the plasticizer, the additives, and the membrane composition are reported to significantly influence the sensitivity and selectivity of potentiometric electrode (Shamsipur, Jalili and Haghgoo 2002; Shamsipur et al. [30]). Thus the different aspects of membranes were optimized; the results are summarized in Table 1. As is obvious among the different solvent mediators tested, DBS exhibited a proper response near to the expected Nernstian slope. It seems that the most effective additive was CPYB with 6% w/w. The optimum amount of ion pair was obtained from the data in Table 1 (5% w/w). Thus the optimal membrane composition obtained was PVC/DBS/CPYB/SY-CPY ratio 30:59:6:5 for PME, CGE, and CPE electrodes.
Table 1. Optimization of membrane ingredients
| Composition (% W/W) | Slope (mV/decade) |
| No. | PVC | Plasticizer | Additive | SY-CPY | PME* | CGE* | CPE* |
| 1 | 30 | DOP,67 | 0 | 3 | −9.0 ± 0.3 | −10.3 ± 0.2 | −12.2 ± 0.2 |
| 2 | 30 | DBP,67 | 0 | 3 | −11.4 ± 0.4 | −12.0 ± 0.4 | −13.0 ± 0.3 |
| 3 | 30 | BAC,67 | 0 | 3 | −0.5 ± 0.2 | −0.5 ± 0.2 | −0.5 ± 0.2 |
| 4 | 30 | NPOE,67 | 0 | 3 | −12.0 ± 0.4 | −14.0 ± 0.3 | −15.1 ± 0.3 |
| 5 | 30 | DBS,67 | 0 | 3 | −19.5 ± 0.3 | −21.1 ± 0.2 | −23.2 ± 0.3 |
| 6 | 30 | DBS,65 | TBAB,2 | 3 | −20.6 ± 0.3 | −23.0 ± 0.3 | −24.0 ± 0.2 |
| 7 | 30 | DBS,65 | CTAB,2 | 3 | −15.0 ± 0.4 | −14.3 ± 0.3 | −16.0 ± 0.3 |
| 8 | 30 | DBS,65 | CPYB,2 | 3 | −23.5 ± 0.3 | −27.0 ± 0.2 | −27.9 ± 0.2 |
| 9 | 30 | DBS,62 | CPYB,4 | 3 | −23.5 ± 0.2 | −27.5 ± 0.2 | −27.8 ± 0.1 |
| 10 | 30 | DBS,61 | CPYB,6 | 3 | −24.0 ± 0.3 | −27.0 ± 0.2 | −28.5 ± 0.2 |
| 11 | 30 | DBS,59 | CPYB,8 | 3 | −24.0 ± 0.4 | −26.0 ± 0.2 | −27.2 ± 0.1 |
| 12 | 30 | DBS,63 | CPYB,6 | 1 | −22.5 ± 0.3 | −24.2 ± 0.4 | −25.0 ± 0.3 |
| 13 | 30 | DBS,61 | CPYB,6 | 3 | −25.0 ± 0.2 | −26.0 ± 0.2 | −28.0 ± 0.1 |
| 14* | 30 | DBS,59 | CPYB,6 | 5 | −25.6 ± 0.2 | 28.3 ± 0.2 | −29.8 ± 0.2 |
| 15 | 30 | DBS,56 | CPYB,6 | 8 | −25.0 ± 0.3 | −23.6 ± 0.4 | −27.8 ± 0.1 |
| 16 | 30 | DBS,54 | CPYB,6 | 10 | −22.0 ± 0.4 | −21.1 ± 0.3 | −20.1 ± 0.3 |
|
The electrodes prepared based on the optimum conditions for PME, CGE, and CPE showed high sensitivity with the Nernstian slope over a wide dynamic range (Fig. 2).
Graph: Figure 2 Calibration graphs for the CPE (•), CGE (▴), and PME (♦) at pH 6, 25°C.
The critical response characteristics of the proposed electrodes were assessed according to IUPAC recommendations (IUPAC Analytical Chemistry Division 1976). The slopes and linear ranges of the resulting emf-pSY graphs are given in Table 2. The limits of detection, defined as the concentration of SY that is obtained when the linear ranges of the calibration graphs are extrapolated to the baseline potentials, are also included in Table 2. It is obvious that the performance characteristics of the CPE improved respect to those of CGE and PME. It presumably originated from the coated electrode technology, where an internal 1.0 × 10−3 M SY solution, in the case of PME, has been replaced by graphite and platin of much higher electrical conductivity (Amini, Shahrokhian, and Tangestaninejad [5]; Shahrokhian et al. [26]; Shamsipur et al. [31]).
Table 2. Characteristics response of the SY electrodes
| Electrode | Slope (mV decade−1) | Linear range | Limit of detection (M) | Response time (s) | Working pH range | Life time (day) |
| PME | −25.6 ± 0.2 | 1.99 × 10−5 to 1.0 × 10−1 | 1.8 × 10−5 | 15 | 4.5–9.5 | 45 |
| CGE | −28.2 ± 0.2 | 3.16 × 10−6 to 1.0 × 10−3 | 3.16 × 10−6 | 10 | 4.5–9.5 | 45 |
| CPE | −29.8 ± 0.2 | 3.16 × 10−7 to 3.16 × 10−3 | 3.16 × 10−7 | <10 | 4.5–9.5 | 45 |
The influence of the pH of the test solution on the potential response of the PME, CGE, and CPE in the presence of 1.0 × 10−4 M SY ion is shown in Fig. 3. As it can be seen, the potential remains constant from pH 4.5 to 9.5, beyond which it changes considerably. The observed drift at lower and higher pH values could be due to the protonation of SY in acidic media, which decreases its ion exchange properties and also for alkali media. It seems that OH– will strongly compete with analyte ion for exchange in the membrane.
Graph: Figure 3 Variation of a CPE potential in solutions with various buffered pHs and containing 1.0 × 10−4 M SY, 25°C.
The average time required for the electrodes to reach a potential response within ±1 mV of the final equilibrium value after successive immersion in a series of SY solutions was studied, each having a 10-fold difference in concentration. The typical static response time of CPE is shown in Fig. 4. The static response time of the electrodes is also listed in Table 2. The prepared CPE could be used frequently for 45 days, without any measurable divergence.
Graph: Figure 4 Variation of a CPE potential during SY concentration change from 1.0 × 10−5 M to 1.0 × 10−4 M, 25°C.
The repetition of the potential reading for electrodes were examined by subsequent measurements in 5.0 × 10−4 M of SY solution immediately after measuring the first set of solutions at 5.0 × 10−3 M of SY solution. The typical dynamic response curves of CPE for several high-to-low sample cycles are shown in Fig. 5. The standard deviation of measuring emf for five replicate measurements was 0.20 mV. This means that the repeatability of potential response of the electrode was high.
Graph: Figure 5 Variation of a CPE potential during successive immersion in a 5.0 × 10−3 M and 5.0 × 10−4 M solution of SY, 25°C.
The influences of interfering ions on the potential response behavior of the membrane electrodes are usually described in terms of the selectivity coefficients. In this work, the matched potential method (MPM), which is totally independent of the Nicolski–Eiseman equation, was used for determination of selectivity coefficients (Umezawa, Umezawa, and Sato [32]). According to the MPM, the selectivity coefficients are defined as the activity ratio of the primary ion (A) and the interfering ion (B), which gives the same potential change in a reference solution. Thus, one should measure the change in potential upon changing of the primary ion activity. Then the interfering ion would be added to an identical reference solution until the same potential change is obtained. The selectivity coefficient is determined as
Graph
where ΔaA = aA′ − aA and where aA′ and aA are the initial primary ion activity and the primary ion activity in the presence of the interfering ion aB, respectively. The resulting values are summarized in Table 3. The basic ingredients of soft drinks are always a sweetener, acid (generally ascorbic acid, citric acid), carbon dioxide, and preservative (generally benzoic acid). For all diverse ions that were used, the selectivity coefficients were on the order of 10−3 or smaller, indicating they would not significantly disturb the electrode behavior except for nitrate and nitrite ions. Quinoline yellow, carmozine, and indigothin also cause significant interface. However, nitrate and nitrite ions are generally not used in soft drink formulation. Qunoline yellow is generally not mixed with SY in food products, because of the same color. Ingredients in the soft drinks such as citrate, ascorbate, carbonate, benzoate ions, Ponceau 4R, and brilliant blue did not influence the SY determination.
Table 3. Selectivity coefficients of various interfering ions for SY-CPE by MPM method (1.0 × 10−4 M SY initial concentration)
| Interfering ion | Selectivity coefficient |
| 8.0 × 10−3 |
| 6.8 × 10−3 |
| 5.8 × 10−5 |
| 5.0 × 10−5 |
| 3.0 × 10−5 |
| 7.0 × 10−4 |
| 3.5 × 10−4 |
| Citrate | 1.2 × 10−4 |
| Benzoate | 3.1 × 10−3 |
| Fructose | 2.6 × 10−6 |
| Maltose | 3.3 × 10−6 |
| Glucose | 4.1 × 10−6 |
| Acetate | 3.7 × 10−4 |
| Carmoisine | 8.9 × 10−3 |
| Ponceau 4R | 1.2 × 10−4 |
| Brilliant blue | 1.6 × 10−4 |
| Indigotine | 1.2 × 10−2 |
| Quinoline yellow | 8.9 × 10−2 |
The reliability of the proposed SY electrode (CPE) for quantification of SY was assessed by determining the synthetic samples of SY solutions using the direct potential method, and the data obtained are shown in Table 4. The average of six determinations was founded, and the mean of recovery was 100.08%.
Table 4. Recovery of SY for SY-CPE (three replicates)
| SY added (µg ml−1) | SY found (µg ml−1) | Recovery (%) |
| 2.00 | 2.22 ± 0.05 | 100.22 |
| 4.00 | 4.25 ± 0.04 | 100.25 |
| 6.00 | 5.87 ± 0.05 | 99.87 |
| 8.00 | 8.15 ± 0.03 | 100.15 |
| 10.00 | 10.14 ± 0.05 | 100.14 |
| 20.00 | 19.85 ± 0.04 | 99.85 |
|
The direct potential method was applied to the determination of SY in soft drinks and compared with the high-performance liquid chromatographic (HPLC) method. As a result, the mean value and the relative standard deviation (RSD) obtained using SY sensor was 51.55 ± 0.03 µg/gand 1.4% (n = 5). This method is in good agreement with the values obtained using the HPLC technique (51.12 ± 0.01 µg/g).
CONCLUSION
Novel CPE, CGE, and PME for the determination of SY were prepared based on SY-CPY ion pair as an ion exchanger. CPE showed a wider dynamic range and lower detection limit. The electrodes prepared have been successfully applied to the determination of SY in commercial food products. The proposed method possesses many advantages such as fast response, low detection of limit (<10−6 M), good accuracy, adequate selectivity, and simple analytical procedures for the determination of SY in food without any sample preparation. Because of the importance of the determination of synthetic food colorants in a complex matrix, the construction of sensors for other artificial food colorants is under study in our laboratory.
Acknowledgments
The author thanks the Institute for Colorants, Paint, and Coatings (ICPC) for supporting this work.
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Footnotes
*Variation of potentiometric responses were measured at the concentration range of 1.0 × 10−1 to 5.0 × 10−6 mol of SY solutions in phosphate buffers (pH 7.0), 25°C.
a
Mean ± standard deviation (n = 3).
]
By Shohre Rouhani
Reported by Author
