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Rapid and sensitive detection of carminic acid based on pH studies in food and cosmetic samples
*Corresponding author: E-mail address: musharraf1977@yahoo.com (S.G. Musharraf)
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Received: ,
Accepted: ,
Abstract
Carminic acid (CA), a natural red colorant derived from insects, is classified as a xenobiotic. It is commonly used as a colorant and additive in food, cosmetic, pharmaceutical, and textile industries due to its origin, color, and stability. Here, this current method provides a simple, quick, cost-effective method for the quantitative detection of CA in food and cosmetic samples based on pH studies. Intense color change was observed from orange to purple with a bathochromic shift at the basic pH range of pH 11-12, which confirmed the presence of CA. However, pH 11 (wavelength 567 nm) was finally used for the quantitative detection of CA. Nuclear magnetic resonance (1H-NMR) spectroscopy was used to understand the effect of pH on the structure of CA. The limit of detection (LOD) and limit of quantification (LOQ) for the described method were 0.0000497 and 0.000151 mg∙ml-1, respectively. The method was also validated through inter-and intra-day analysis, with % RSD values ranging from 0.06 to 0.39%. Spiking experiment was conduct for the matrix analysis with a percentage recovery of 97-107%. Other dyes were also analyzed over the pH range from 1-12; However, none exhibited purple color (bathochromic shift) at pH 11 as observed in the case of CA, which demonstrates the selectivity of the method. Several commercially available food and cosmetics samples were successfully analyzed to quantitatively detect the presence of CA. This current colorimetric method has the advantages of simplicity and efficiency over traditional methods by producing a simple, low-cost, and rapid quantitative detection of CA in food and cosmetic samples.
Keywords
Bathochromic shift
Carminic acid (CA)
Color change
Developed method
Dyes
Quantification
pH
1. Introduction
Color has great importance in food sciences and is used as one of the elements for food attraction. Based on sources, natural dyes/colors are broadly categorized as plants, animals, and minerals, although plants are the major source of the natural colors [1]. Natural colors are safer than artificial colorants and have documented nutritional and pharmacological effects [2]. Carminic acid (CA; E120; C1. 75470), a β-D-glucopyranosyl, is a natural red colorant extracted from the female insect called Dactylopius coccus Costa. [3,4]. The chemical formula of CA is C22H20O13, while the IUPAC name is 7-a-D-glycopyranosyl-9,10-dihydro-3,5,6,8-tetrahydroxy-1-methyl-9,10-dioxo-2-antracenecarboxylicacid [5]. Commercially, it is also known as carmine (calcium and aluminum lake of CA) [6].
CA is classified as a xenobiotic. It has been defined as any substance, such as drugs, environmental pollutants, food additives, and industrial chemicals, to which an organism is exposed that is extrinsic to the normal metabolism of that organism [7]. CA, a xenobiotic because it is foreign to the human body’s metabolic pathway and does not serve any physiological or nutritional roles.
CA is extensively used as a food additive and colorant in diverse applications in various industries, including cosmetics, textiles, pharmaceuticals, plastics, food, and beverages due to its stability against heat and oxygen-induced decomposition [8]; however, stability under light exposure is debatable [8]. The presence of CA in foodstuffs should be monitored not just by religious beliefs, dietary trends, or environmental considerations, but also the content of CA in commercial food products can cause potential health problems, e.g., anaphylactic shock, angioedema, rhinorrhea, urticaria, and dyspnea [3]. The acceptable daily intake of CA established by the European Food Safety Authority (EFSA) is 2.5 mg/kg of body weight/day [3].
Many methods have been introduced for detecting CA in food samples, such as high-performance liquid chromatography (HPLC) [9,10], colorimetric method [1], different pulse photography [11], spectrophotometry [12,13], capillary electrophoresis [14], etc. In recent years, significant progress has been made in the development of colorimetric and fluorescent sensors for detecting small molecules and xenobiotics with high sensitivity and selectivity. For example, He et al. developed a ratiometric and colorimetric fluorescent probe for detecting hypochlorite in living systems [15]. Similarly, Xue et al. introduced an AIE-based photosensitizer for creating enhanced antibacterial interfaces [16] and Chen et al. reported a quantum dot-based hydrogel sensor for on-site pesticide degradation monitoring in plants [17]. These studies highlight the growing interest in designing low-cost, portable, and efficient sensors for environmental and food safety monitoring. Considering the increasing importance of CA as a natural food colorant and avoiding using high-tech and expensive instruments and time-consuming methods to detect CA, a simple and cost-effective method for detecting CA in various food and cosmetic samples is needed. However, so far, only one colorimetric method has been reported for the qualitative detection of CA.
This current colorimetric method is novel because it is a simple, rapid, selective, and cost-effective method for the quantitative detection of CA in various food and cosmetics products without using expensive instruments and time-consuming processes. This method is based on pH studies using a cheap and common lab chemical, sodium hydroxide (NaOH), as a base. A visible colorimetric change (bathochromic shift) due to a change in pH of the CA solution indicates the presence of CA in the samples, which was fully verified by the UV-visible spectrophotometric analysis. The structural changes that occur due to a change in pH are also confirmed by the nuclear magnetic resonance (1H-NMR) spectroscopy. The wavelength 567 nm at pH-11 was used for the quantitatively detection of CA.
2. Materials and Methods
2.1. Chemicals and reagents
Pure purity of CA (95.78%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Hydrochloric acid (36.5%) and sodium hydroxide (98-100.5% pellets) were purchased from E. Merck (Kenilworth, NJ, USA). Deionized water was made available using the Direct 16 Milli-Q purification system (Millipore Co., Bedford, MA, USA). Eleven dyes including Acid Red 14 (1), Dark Red (2), Direct Red 17 (3), Rose Pink (4), Strawberry Red (5), Carminic Acid (6), Rusbery Red H-9193 (7), Acid Red 1 (8), Sun Set Yellow (9), Erythrosine B (10) and Allura Red (11) were purchased from Sigma-Aldrich (St. Louis, MO, USA) while Rose Pink (4) and Strawbery Red (5) which were purchased from the local market.
2.2. Preparation of standard solutions
A stock solution of CA (0.197 mg∙mL-1) was prepared (pH of 4) in DI water and was kept in darkness at a temperature less than 4°C. To study CA at various pH conditions (pH 1-12), the pH of the solution was adjusted with the help of HCl and NaOH. However, pH-11 was finally used to detect CA in various food and cosmetics samples quantitatively. Deionized water was used to fix the final volume of a solution.
To obtain the calibration curve, the initial stock solution of CA (0.1969 mg∙mL-1) was diluted to seven different concentrations (0.0098, 0.0197, 0.0295, 0.0492, 0.0591, 0.0689, and 0.0788 mg∙mL-1). However, the pH of the solution was adjusted to pH-11 before fixing the final volume by adding DI water.
To observe the change in UV-visible absorption spectra and color of other red dyes at pH-11, different dye solutions were prepared by dissolving 1 mg of each dye in 2 mL of deionized water. The pH of each dye solution was adjusted to pH 11 by using 0.5 M sodium hydroxide (NaOH) as a base. All solutions were subjected to spectrophotometric analyses.
2.3. Sample preparation and analyses
For this, 15 samples, comprising three categories of food (9), cosmetics (4), and pharmaceuticals (2) were purchased from local supermarkets. The samples were stored at 4°C until further processing. CA was extracted from the samples by using simple hydrolysis to convert the carmine into CA [1]. For a solid sample, a critical pretreatment step involves grinding the sample using liquid nitrogen to achieve fine homogenization. This facilitates more efficient hydrolysis and extraction of CA. After grinding, 1 g of the solid sample was treated with 3 mL of HCl (3M) followed by sonication at 60°C for 30 min. After sonication, the solid samples were centrifuged at 8000 rpm for 10 min, and the supernatant was filtered. To extract CA from liquid samples, 1 mL of the liquid sample was directly treated with 2 mL of HCl (3M), and then the samples were sonicated for 30 min at 60°C and cooled the samples at room temperature. The remaining steps were consistent across both samples. The extraction of CA was then confirmed by the UV-visible spectrophotometer. After the extraction of CA, 14 M of NaOH solution was used to adjust the pH of the solution to pH 11. The solution was diluted with deionized water to make a final volume of 3.5 mL. The amount of CA was quantitatively detected using spectrophotometric analysis.
2.4. 1H-NMR spectroscopy analysis
The 1H-NMR of CA was recorded in DMSO-d6 as a solvent on Bruker Advance NEO 400 MHz NMR spectrometers (Switzerland). The chemical shift (δ) is in ppm, and the coupling constant (J) is in Hz. To record the 1H NMR spectra of CA at pH-4, 1 mg of standard CA was dissolved in 1 mL of DMSO-d6. To record the spectra, 120 µL of 4.16 M NaOH (100 mg of NaOH dissolved in 0.6 mL of DMSO-d6) was added to the standard solution of CA to adjust the pH of the solution to pH 11; this was followed by 1H-NMR analysis.
2.5. UV-visible spectroscopic analysis
All absorption spectra were recorded on a multimode Thermo Scientific 300 evolution UV-visible spectrophotometer (UK, Cambridge). All samples were analyzed using quartz cuvettes of 1 cm in width, and absorbance was measured at a wavelength of 350 to 750 nm. The radiation source was a xenon flash lamp.
2.6. Method performance
The validation of the developed method was checked at different concentration levels. Different concentration ranges of CA solution from 0.0098-0.0837 mg∙mL-1 were prepared for the calibration curve. LOD and LOQ for the analyte were calculated through standard deviation (σ) and slope (s) by employing formulae: (LOD = 3.3 σ/S, LOQ = 10 σ/S). The accuracy (%Bias) and precision (%RSD) were checked by intraday and interday analysis. For the matrix effect, the recovery from the samples was analyzed using the spiking method. For recovery, three known concentrations of 0.0148 mg∙mL-1(51.7 µg), 0.0246 mg∙mL-1 (86.16 µg), and 0.0443 mg∙mL-1 (155.10 µg) solutions of CA were spiked and extracted with 0.5 g of sample and analyzed by the UV-visible spectrophotometer.
3. Results and Discussion
3.1. Method optimization
The objective of the current study was to develop a rapid, sensitive, and cost-effective method to quantitatively analyze CA in various commercial food and cosmetic samples. Therefore, a simple strategy based on the pH studies was employed to monitor the color change of CA in the solution. A standard solution of CA was subjected to various pH conditions, ranging from 1-12, using HCl and NaOH as an acid and base, respectively. Calorimetric changes were monitored by the human naked eye, followed by UV-visible spectrophotometric analysis to monitor the changes in the wavelength. The color of the CA solution, concerning the pH of the solution, was changed from reddish orange (strongly acidic) to dark purple (strongly basic). In addition to the color change, the UV-visible absorbance spectra of the CA solution were also changed. The color and UV-visible spectra of CA at different pH values have been illustrated in Figure 1. The actual pH of the CA solution when dissolved in deionized water ranges from 3 to 4, depending on the concentration of the CA in the solution. However, a significant color change was observed by changing the pH of the CA solution from pH 4 to 11, where the color of the CA solution turned from orange to purple. The UV-visible absorption peak of CA shifted from 491 nm to 567 nm, respectively, as illustrated in Figure 2. The change in the UV-visible absorption peak and the color change confirm the presence of CA in the solution. The wavelength 567 nm at pH 11 was further used for the quantitative analysis of CA. The stability of the color of the CA solution at pH 11 was from 8 to 10 h at room temperature and pressure conditions.
- (a) UV-visible spectra of CA at different pH (1-12) values. (b) Represent the color of the CA solution at different pH (1-12) values.
- (a) The color and spectrum of CA at pH 4 (orange, λmax 491 nm). (b) Color and spectrum of CA at pH 11 (purple, λmax 567 nm).
3.2. Effect of pH on CA structure
A pH-based colorimetric method for the quantitative detection of CA was developed, exhibiting notable enhancements in sensitivity and safety. The method optimization was carried out by evaluating the color and UV-visible absorbance spectra of CA across different pH conditions. It was observed that the color of the CA solution, concerning the pH of the solution, was changed from reddish orange (strongly acidic) to dark purple (strongly basic). In addition to the color change, the UV-visible absorbance spectra of the CA solution were also changed. CA has 4 ionizable hydrogens of the hydroxyl group at carbon 3, 6, 8, and one proton of COOH with pKa values of 8.7, 5.4, 12.2, and 2.9, respectively [18]. At basic pH (pH-11), the hydrogen of the hydroxyl group at position 5 is more basic and cannot be deprotonated [18]. The spectral and color changes were caused by structural changes occurring in the structure of CA under different pH conditions. In CA, where several acidic centers are present, upon treatment with sodium hydroxide (NaOH), a tetraionic species may be formed due to the deprotonation of four acidic protons, further confirmed by 1H-NMR spectroscopy. The first deprotonation may have occurred at the carboxylic group (COOH), a more acidic site with a pKa value of 2.9. However, deprotonation at the COOH group does not produce appreciable spectral changes in the molecule; the hydroxyanthraquinone chromophore is essentially determined by the chromatic properties [18]. After deprotonating COOH, the second deprotonation may occur at position 6 (pK2 =5.4). The more acidic character of the 6-hydroxy group is probably due to the less hydrogen bonding between the hydroxyl group of position 6 and the negatively charged site COO- [18]. The di-anionic form of CA is relatively stable at pH 6. However, at basic conditions (pH 11-12), two further deprotonations may be observed at position 3 (pKa 8.7) and 8-hydroxyl (pKa 12.2) groups and form a tetra-ionic species [18]. The tetra-anionic species of CA is known to be more stable at basic pH and found to be similar to the structure of di-anionic purpurin [19]. Figure 3(a) represents the structure of CA, while Figure 3(b) represents the tetra-ionic structure of CA.
- (a) Chemical structure of CA. (b) Tetra-ionic structure of CA at pH 11-12.
3.3. Fluorescence response of carminic acid at pH-11
To further validate this tetraionic structure at pH 11, the fluorescence study of CA was conducted using a 0.0246 mg∙mL-1 solution. At pH 4, a strong fluorescence signal was observed with a maximum excitation wavelength at 310 nm, as illustrated in Figure S1. However, upon increasing the pH to pH 11 (maintaining the same concentration), a significant quenching of fluorescence was detected, as illustrated in Figure S2. This quenching further supports the formation of the tetraanionic specie. The tetraionic specie is more stable in the ground state at high pH and resulting in the bathochromic shift in UV-Vis absorbance due to an increase in conjugation. However, it increases intermolecular electron density and removes stabilizing hydrogen bonds, leading to excited-state destabilization. Increased electron density facilitates intermolecular photo-induced electron transfer (PET) process, where electrons from the anionic groups transfer to the excited chromophore (anthraquinone core), which is a non-radiative mechanism and leads to internal quenching.
3.4. 1H-NMR analysis
1H-NMR spectroscopy was performed to understand the structural changes that occurred in the structure of CA under acidic and basic pH conditions. 1H-NMR spectra of CA at both acidic and basic conditions were recorded in DMSO-d6 Figure 3a and 3b respectively. In acidic conditions (pH-4), the lH-NMR spectrum showed the most upfield signal of methyl protons appearing as a singlet at δ 2.75 and the aromatic methine proton appearing as a singlet at δ 7.64. These signals were characteristic of the CA of the anthraquinone skeleton [20]. Additionally, three hydroxyl groups were detected at different chemical shifts. The most downfield C-5 hydroxyl group appeared as a broad singlet at δ 14.59, while the C-8 hydroxyl resonated as a singlet at δ 13.15. Another hydroxyl group at C-6 was observed at δ 10.58. However, the signals corresponding to the hydroxyl group of carboxylic acid and other hydroxyl groups at C-3 did not appear in the 1H-NMR spectrum due to the rapid exchangeable protons/ionization. Furthermore, the remaining signals of CA exhibited slight shifts compared to previously reported data [20].
In basic conditions (pH-11), the peaks corresponding to the hydroxyl groups at C-6, C-8, and C-3, and the hydroxyl group of carboxylic acid, completely disappeared compared to CA in acidic conditions. Specifically, the C-5 hydroxyl group exhibited a more downfield chemical shift of the proton at δ 16.84 ppm because this hydroxyl group has a high pKa value (more basic) in basic conditions, as given in the previous literature [18]. Consequently, the hydroxyl group at the C-5 position cannot be deprotonated by the sodium hydroxide (NaOH). In basic conditions (pH-11), the most stable tetra-ionic structure of CA is formed, which is dually verified through literature data [18]. 1H-NMR spectral data of CA and CA-Na1+ salt have been given in Table 1.
| Position | CA δH (multiplicity, J in Hz) | CA-Na1+ salt δH (multiplicity, J in Hz) |
|---|---|---|
| 1 | 2.75 (3H, s, CH3) | 2.76 (3H, s, CH3) |
| 2 | - | - |
| 3 | - | - |
| 4 | 7.64 (1H, s, 4-H) | 7.29 (1H, s, 4-H) |
| 4a | - | - |
| 4b | - | - |
| 5 | 14.59 (1H, br s, 8-OH) | 16.84 (1H, br s, 8-OH) |
| 6 | 10.58 (1H, br s, 5-OH) | - |
| 7 | - | - |
| 8 | 13.15 (1H, br s, 6-OH) | - |
| 8a | - | - |
| 8b | - | - |
| 9 | - | - |
| 10 | - | - |
| C=O | - | - |
| C-glucose | - | - |
| 1′ | 4.72 (1H, d, J = 9.8 Hz) | 4.55 (1H, d, J = 8.8 Hz) |
| 2′ | 4.03 (1H, br t, J = 7.0 Hz) | 4.25 (1H, br t, J = 7.6 Hz) |
| 3′ | 3.22 (1H, br t) | 3.16 (1H, br t) |
| 4′ | 3.16 (1H, br t) | 3.11 (1H, br t) |
| 5′ | 3.20 (1H, m) | 3.14 (1H, m) |
| 6′ |
3.66 (1H, br dd) 3.69 (1H, br d) |
3.59 (1H, br dd) 3.62 (1H, br d) |
3.5. Method validation
The UV-visible absorption spectrum of the CA calibration curve solutions was found to be linear with the concentration range from 0.0098-0.0788 mg∙mL-1 with a linear regression coefficient value >0.9991, as illustrated in Figures 4(a, b). LOD and LOQ were found to be 0.0000497 and 0.000151 mg∙mL-1, respectively. The intr-a and inter-day analyses were also conducted. The percentage accuracy for inter and intra-day analysis was 107.8 and 109.7, while the % RSD was 0.133 and 0.141, respectively. The data of inter- and intra-day analysis have been given in Table 2. The percentage recovery obtained for 0.0148, 0.0246, and 0.0443 mg∙mL-1 was 102.86, 97.27%, and 106.89%, respectively, as represented in Table S1. The stability of the color of the CA solution at pH 11 was 8 to 10 h at room temperature and pressure conditions.
- (a) UV-visible spectrum of calibration curve solution. (b) A graph of calibration solution with a concentration range of 0.0098 - 0.0788 mg∙mL-1.
| Concentration (mg∙mL-1) | Intra-day | Inter-day | ||||
|---|---|---|---|---|---|---|
| Found (mg∙mL-1) | RSD (%) | Accuracy (%) | Found (mg∙mL-1) | RSD (%) | Accuracy (%) | |
| 0.0197 | 0.0181×10-2 ± 6.89×10-5 | 0.380 | 108.74 | 0.0169± 3.45×10-5 | 0.21 | 116.14 |
| 0.0591 | 0.0552 ± 3.45×10-5 | 0.064 | 106.97 | 0.0579 ± 1.77×10-4 | 0.30 | 102.24 |
| 0.0837 | 0.0752 ± 5.91×10-5 | 0.080 | 111.29 | 147.69 | 0.00 | 115.10 |
3.6. Analysis of other red dyes
To make the procedure more selective, the other 11 dyes at pH-11 were also analyzed by the developed method. It was observed that at pH-11, only the CA solution changes its color from orange at pH-4 to purple at pH-11, with the UV-visible peak shifted from 491 nm to 567 nm. This visible color change was observed due to the formation of a stable tetra-anionic specie at pH-11, confirmed by 1H-NMR spectroscopy. No color and spectral changes were observed at pH-11 for other dyes (Figure 5). The change in the color and the change in the UV-visible absorption spectrum at pH-11 selectively occur for the quantitative detection of CA, which makes the method more specific and selective for the monitoring of CA in food and cosmetic samples.
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(a) UV-visible spectrum of all 11 different dyes at pH 4. Acid red 14 (1), Dark red (2), Direct red 17 (3), Rose pink (4), Strawberry red (5), CA (6), Rusbery red H-9193 (7), Acid red 1 (8), Sun set yellow (9), Erythrosine B (10) and Allura red (11). (b) UV-visible spectrum of different dyes at pH 11.
3.7. Real sample analysis
To detect CA in commercially available food and cosmetic samples, 15 products were analyzed. Of 15, eight products mentioned the presence of CA in their products. However, CA was quantitatively detected only in five samples using the developed method. The remaining three samples in which CA was mentioned but not detected may be due to lower than the LOD of the given method. The maximum quantification of CA was 0.24 mg∙g-1 in the candy sample, while the lowest quantity of CA was detected at 0.05 mg∙g-1 in bubble gum. The quantitative data of CA in real samples have been presented in Table 3.
| S. No | Type of sample analyzed | Amount taken | Nature of sample | Quantitative detection of CA in a real sample (mg∙g-1) |
|---|---|---|---|---|
| 1 | Food (Chocolates) | 1 g | Solid | 0.118 |
| 2 | Food (Pomegranate juice) | 1 mL | Liquid | 0.177 |
| 3 | Cosmetics (Lipstick) | 1 g | Solid | 0.074 |
| 4 | Food (Bubble gum) | 1 g | Solid | 0.052 |
| 5 | Food (Candy) | 1 g | Solid | 0.237 |
3.8. Comparison with the recently reported colorimetric method
Several analytical methods have been reported for detecting CA in various food and cosmetic samples. A detailed comparison of the reported method has already been done in previous publications [1]. However, this paper compares with the most recent colorimetric method to detect CA [1]. The previous colorimetric method used lead for the qualitative detection of CA. The method was less sensitive for the qualitative detection of CA, characterized by a higher value of the LOD and LOQ. This indicates that the method can only be used for the qualitative detection of CA at relatively higher concentrations. In contrast, the developed method utilizes a cheap and nonhazardous chemical, sodium hydroxide, and adopts a quantitative approach rather than a qualitative one. This developed method has a lower LOD and LOQ, which further enhances the sensitivity of the developed method for the quantitative detection of CA. This implies that the developed colorimetric method is more suitable for application and requires high sensitivity and accuracy. It also eliminates the potential hazards, making the method safer and more environmentally friendly. The comparison data have been illustrated in Table 4. However, the comparison of the developed method with the previously reported analytical method has also been illustrated in Table 5 [21-27].
| S.No | Samples | Method | Reagent used | LOD (mg∙mL-1) | LOQ (mg∙mL-1) | LR |
RSD (%) Inter/Intra |
Recovery (%) | Reference |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Various food and non-food samples | Qualitative colorimetric method for the qualitative detection of CA | Lead (Pb) |
a) 0.0025 b) 0.0136 |
a) 0.0076 b) 0.0415 |
a).0.073-0.492 b).0.031-0.207 |
a) 0.1-2.70 b) 0.0-9.90 |
95-128% | [1] |
| 2 | Various food and cosmetic samples | Quantitative colorimetric method for the detection of CA | Sodium hydroxide (NaOH) | 0.0000497 | 0.000151 | 0.0098 - 0.0788 | 0.06-0.39 | 97-107% | Current method |
| S. No | Samples | Method | LOD | LOQ | LR | %RSD | %Recovery | Reference |
|---|---|---|---|---|---|---|---|---|
| 1 | Food samples | Spectrophotometry | 0.4 μg∙L-1 | - |
1.5-350 μg∙L-1 |
3.7 | 94.8-104.7 | [21] |
| 2 | Lipstics products | RP-HPLC-PDA | 6.428 ng∙mL-1 |
21.78 ng∙mL-1 |
1.0 ng∙mL-1 |
1.25 | 80-91 | [22] |
| 3 | Saffron sample | FT-IR/UV–Visible/RP-HPLC-DAD | 0.03 μg∙mL-1 |
0.10 μg∙mL-1 |
0.01-5.00 μg∙mL-1 |
2.80-6.80 | 96-101 | [23] |
| 4 | Food samples | Cloud point extraction/Spectroscopy |
0.012 μg∙mL-1 |
0.04 μg∙mL−1 |
0.04-5.0 μg∙mL−1 |
4.8 | 93.7-105.8 | [23]. |
| 5 | Candy and milk | Pulse polarography | 0.16 μM | - |
1-90 μM |
2-7 | 87-105 | [24] |
| 6 | Food stuffs |
Stripping voltammetry |
1.43 109 mol∙L-1 |
- | - | 2.2 | 97.9 | [25] |
| 7 | Food samples | HPLC |
0.4 μg∙mL−1 |
1.0 μg∙mL−1 | 1.0–100 μg∙mL-1 | 2.8-6.8 | 90.4-96.2 | [26] |
| 8 | Food samples | HPLC-DAD and LC- | 0.05 mg∙kg-1 | 0.15 mg∙kg-1 | 0.2-50 mg∙mL-1 | 0.48-8.90 | 87.3-97.1 | [27] |
| 9 | Various food and cosmetic samples | Quantitative-colorimetric method for the detection of CA |
0.0000497 mg∙mL-1 |
0.000151 mg∙mL-1 |
0.0098 - 0.0788 mg∙mL-1 | 0.06-0.39 | 97-107 | Current method |
4. Conclusions
A quantitative colorimetric method was developed for the rapid, sensitive, and selective detection of CA in various foods and cosmetics samples. The detection of CA is based on a visible color change from orange at pH-4 to purple at pH-11 with a UV shift from 491 nm to 567 nm. The wavelength 567 nm at pH 11 was further used for the quantitative analysis of CA. The change in the color and the UV-visible absorption peak of the CA solution was due to the change in the structure at different pH conditions. A structure of tetra-ionic species (salt) is formed at higher pH and is supported by the 1H-NMR spectroscopic data. The color and structural changes were confirmed by UV-visible and 1H-NMR spectroscopy, respectively. This method is selective for the quantitative detection of CA, because no other red dyes give a bathochromic shift (purple color) at pH 11. The LOD and LOQ of the developed method were 0.0000497 and 0.000151 mg∙mL-1, respectively. Percentage recovery of CA was 97-107%. However, CA is used in a variety of industrial products; hence, its recovery might be affected from sample to sample. Future studies will be done to analyze a broad range of samples for matrix effect.
Acknowledgment
This work was supported by the Higher Education Commission, Pakistan, under the Center of Excellence (CoE-75) funding program.
CRediT authorship contribution statement
Umat Khan: Formal analysis, Data curation, Methodology, Visualization, Writing–Original draft. Azra Akbar: Data curation, Methodology, Writing–Original draft, Visualization. Amna Jabbar Siddiqui: Investigation, Validation, Visualization, Writing–review and editing. Tooba Ali: Help in samples preparation, Graphs designing, and editing. Dilshad Hussain: Visualization. Writing–review and editing and Hesham R. El-Seedi: Writing–review and editing, Syed Ghulam Musharraf: Supervision, Conceptualization, Writing–review and editing. All authors have read and agreed to the published version of the manuscript.
Declaration of competing interest
The authors declare no competing interest for this study.
Declaration of generative AI and AI-assisted technologies in the writing process
The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.
Supplementary data
Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_233_2025.
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