Introduction

Sunset Yellow is an orange-yellow powder azo-food dye with the chemical formula C16H10N2Na2O7S2 and with the molar mass of 452.37 g mol−1 [1]. It is a biologically active material, with limited absorption in the gastrointestinal tract, while the free sulphonated aromatic amines may reach the systemic circulation [1,2,3]. According to International Numbering System (INS), it has received the code 110 (E110 in Europe) [4], especially when used as a colouring agent of foods and pharmaceuticals [3], and the code CI 15,985 when used as a cosmetic ingredient [4]. Its full chemical name is disodium 2-hydroxy-1-(4-sulphonatophenylazo) naphthalene-6-sulphonate. Acceptable daily intake (ADI) of this dye is 4 mg/kg body weight/day [2].

Sunset Yellow is regulated by the Food and Agriculture Organization and World Health Organization (FAO/WHO), Joint FAO/WHO Expert Committee on Food Additives (JECFA) [5], EFSA (European Food Safety Authority) [1] and is defined by the European Commission (EC) Regulation 1333/2008 [6]. Its structural formula is presented in Fig. 1 [7].

Fig. 1
figure 1

Structural formula of Sunset Yellow

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Until now, the main method for the determination of Sunset Yellow in foods was the high-performance liquid chromatography (HPLC) [8, 9], while other techniques did not led to the elucidation of their physical–chemical properties [10]; however, many studies of on such azo dyes involving thermal analysis and calorimetry methods proved to be prodigious [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36].

Here, many other properties of Sunset Yellow, such as the spectral, morphological, structural and compositional characteristics, are investigated. Also, the thermal behaviour of Sunset Yellow was studied in correlation with its physical (refractive index, electric susceptivity, optical anisotropy) and chemical (acidity) properties. Moreover, the biophysical properties (interaction with proteins) and biological activity (antioxidative and phytotoxicity properties) of Sunset Yellow were studied.

Experimental

Materials

Sunset Yellow was purchased from S.C. ORIETTA IMPEX S.R.L., the supplier guaranteeing a 100% content of powdered food dye. Sunset Yellow consists essentially of disodium 2-hydroxy-1-(4-sulphonatophenylazo) naphthalene-6-sulphonate and subsidiary colouring matters, together with sodium chloride and/or sodium sulphate as the principal uncoloured components. Sunset Yellow is described as the disodium salt. The calcium and the potassium salts are also permitted (Directive 2008/128/EC) [1, 6].

The purity is specified as not less than 85% total colouring matters, calculated as the disodium salt. The remaining 15% may be accounted for by sodium chloride or sodium sulphate, ≤ 5% subsidiary colouring matters and ≤ 0.5% 4-aminonaphthalene-1-sulphonic acid, 7-hydroxynaphthalene-1,3-disulphonic acid, 3-hydroxynaphthalene-2,7-disulphonic acid, 6-hydroxynaphthalene-2-sulphonic acid, 4,4′-diazoaminodi(benzene sulphonic acid) and 6,6′-oxydi(naphthalene-1,3-disulphonic acid), originating from the manufacturing process [1]. If the existing specifications could be extended to include ≤ 15.0% sodium chloride and/or sodium sulphate as the principal uncoloured components, 99.9% of the material would be accounted for [1]. The specifications for Sunset Yellow according to Commission Directive 2008/128/EC and JECFA (JECFA, 2006) allowed the presence of up to 5% subsidiary colouring matters.

Used methods and techniques

The refractive index of the Sunset Yellow’s solutions was obtained with the RA-520N refractometer which allows the samples to be heated on the temperature range 15–50 °C.

The measurement precision of the refractive index is ± 0.00002.

The Sunset Yellow’s optical anisotropy was performed with the polarized light microscope LEICA DM 2500 P equipped with a video-recorder camera (Linkam Scientific Instruments Ltd.) at room temperature (RT) [13]. Sunset Yellow was examined between the cross-polarizers of the polarizing light microscope. The samples were exposed on COVN-024-200 (24 × 24 mm, 0.13–0.16 mm thick) COVER GLASS from Labbox and exhibits the phenomenon of birefringence obtained in resulted crystallites by the drying at RT of the Sunset Yellow’s aqueous solutions with concentrations c = 1% and c = 5% [37, 38].

Infrared spectral measurements were performed with a Spectrum 100 PerkinElmer spectrometer which explores the range of 4000–400 cm−1 wavenumbers with a precision of 0.50 cm−1. For each spectrum, the average of 10 acquired spectra was performed. The Sunset Yellow’s spectrum was obtained using universal attenuated total reflectance (UATR) accessory, at a resolution of 4 cm−1, and CO2/H2O correction [12, 16, 21].

The Raman spectra were obtained with RENISHAW in Via Raman Microscope which monitors shifts in Raman band’s positions (down to as low as 0.02 cm−1), resolves spectral features narrower than 0.5 cm−1, measures Raman bands down to below 10 cm−1 (0.3 THz), acquires spectra from Raman bands down to 5 cm−1 (0.15 THz).

UV–Vis measurements were performed with the Ocean Optics S2000 UV–Vis Spectrophotometer using the Ar+ laser radiation with a wavelength of 476.5 nm [39].

Sunset Yellow’s atomic fluorescence was obtained with the Ar+ INOVA 308C laser from Coherent. Excitation radiation had the power of 440 mW and was measured with a Max Field Top 2 power meter from Coherent [40].

Sunset Yellow’s THz spectrum was obtained with the experimental set-up of THz-TDS system (an Ekspla THz kit with a FemtoFiber laser produced by TOPTICA). The system can measure a transmission spectrum in the THz domain (0.2–4.5 THz), by measuring the signal detected synchronous with a lock-in amplifier. The samples were measured on the same type of holder, made from medium density (MD) polyethylene [41].

The Sunset Yellow’s morphology and its chemical composition were studied with an scanning electron microscope (SU8010 from Hitachi) equipped with an energy-dispersive X-ray spectroscopy unit (EDXS) from Oxford Instruments, which used a beam energy of 15 keV. The scanning electron microscope (SEM) has registered the Sunset Yellow’s images at an accelerating voltage of 2 kV and 8200–9800 nA intensity of the emission electric current, allowing for magnifications between 1000× and 5000× and a scale detail of 10–50 μm.

Thermal analysis measurements of Sunset Yellow were performed in air dynamic atmosphere (150 cm3 min−1), at 10 °C min−1 heating rate, up to 1000 °C using alumina crucibles, with PerkinElmer DIAMOND TG/DTA device.

The pH measurement of the Sunset Yellow’s aqueous solution was carried out with a pH and Temperature Sensor PasPort, with Data Studio soft PS-2102, from Pasco Scientific, which can measure pH from 0 to 14 pH in a temperature range from − 4 °C up to 80 °C, with the resolution of 0.01 pH, gel-filled Ag–AgCl electrode, maximum sample rate 50 Hz.

The biophysical properties of Sunset Yellow were analysed through its interaction with proteins from the bovine serum albumin (BSA) by UV–Vis spectroscopy and with the collagen by FTIR spectroscopy. The biological activity of Sunset Yellow was observed in its antioxidative and phytotoxicity activities.

Results and discussion

Refractive indices of Sunset Yellow (E110)

It can be observed that for both concentrations of 1% and 5%, refractive indices of aqueous Sunset Yellow solutions decrease with increasing temperature (Fig. 2a):

Fig. 2
figure 2

a The refractive index as function of temperature and b the electric susceptivity represented as function of inverse square temperature (T−2), (sigmoidal fit) for the aqueous solutions of Sunset Yellow with concentrations of 1% and 5%, and for H2O

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Figure 2b represents the electric susceptivity according to the inverse of the temperature square, for the aqueous solution 1% and 5% of Sunset Yellow and, respectively, for H2O.

The linear dependence of the electric susceptivity (χ) on the inverse of the temperature square [22, 42, 43], for aqueous solutions of Sunset Yellow with concentrations of 0% (H2O, distilled water), 1% and 5% is exemplified through relationships (1–3):

$$\mathop \chi \nolimits_{{{\text{H}}_{2} {\text{O}}}} = {0}{\text{.72124}} + 4794\left( {\frac{1}{{{\text{T}}^{2} }}} \right)$$
(1)
$$\mathop \chi \nolimits_{{{\text{1}}\% \;{\text{Sunset}}\;{\text{Yellow}}}} = {0}{\text{.732312}} + 4631\left( {\frac{1}{{{\text{T}}^{2} }}} \right)$$
(2)
$$\mathop \chi \nolimits_{{{\text{5}}\% \;{\text{Sunset}}\;{\text{Yellow}}}} = {0}{\text{.776069}} + 3836\left( {\frac{{1}}{{{\text{T}}^{2} }}} \right)$$
(3)

The refractive index and the electric susceptibility of Sunset Yellow’s aqueous solutions are correlated with dye’s purity (the purity is specified as not less than 85% total colouring matters) [1].

Using the refractive indices of the Sunset Yellow solutions, the dielectric constant (εr), εr = n2, and the electric susceptivity (χe), χe = εr − 1, were calculated.

The electric susceptivity is expressed by Eq. 4:

$$\chi_{{\text{e}}} = \varepsilon_{{\text{r}}} - 1 = N_{1} \left( {\alpha_{1} + \frac{{p_{0}^{2} }}{{3\varepsilon_{0} kT}}} \right) + N_{2} \left( {\alpha_{ + } + \alpha_{ - } } \right)$$
(4)

In Eq. (4), N1 is the number of water molecules, which is temperature dependent and have the electric polarizability α1, \(N_{2}^{ + }\) is the number of positive ions of aqueous Sunset Yellow solutions with electric polarizability α+, and \(N_{2}^{ - }\) is the number of negative ions of aqueous Sunset Yellow solutions with electric polarizability α-, where \(N_{2}^{ + }\) = \(N_{2}^{ - }\) = N2 [22, 42, 43]. Also in Eq. (4), p0 is the electric dipole moment of Sunset Yellow, ε0 = 8.854 × 10–12 F m−1 is absolute permittivity of vacuum, and k = 1.38·10–23 J K−1 is Boltzmann constant.

Optical anisotropy of Sunset Yellow

When drying at room temperature of the Sunset Yellow aqueous solutions with concentrations of c = 1% and c = 5% were obtained the crystals which were analyzed with the polarized light microscope. Results indicated that Sunset Yellow is an anisotropic azo dye (Figs. 3, 4). It has non-uniform spatial distribution of properties depend of the direction of the incident light. During a 360° rotation of the rotating holder of the microscope (α is the angle of the microscope’s rotating holder), the section alternately shows four extinction positions and four of illumination, so the sample is anisotropic dye [13, 26, 34, 37, 38, 43].

Fig. 3
figure 3

Images of Sunset Yellow crystallites obtained by drying a solution with c = 1%; segment value = 10 μm; the sample holder was rotated in trigonometric sense with a step of 45°

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Fig. 4
figure 4

Images of Sunset Yellow crystallites obtained by drying a solution with c = 5%; segment value = 10 μm; the sample holder was rotated in trigonometric sense with a step of 45°

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The phenomenon of birefringence or double refraction, which is based on the laws of electromagnetism, is obtained in Sunset Yellow’s crystals. Light switch-off positions and positions in which the crystals have a maximum brightness are obtained [37, 38]. Optical anisotropy is a phenomenon correlated with the light absorption, with the development of the colour [26, 34], and probably with the Sunset Yellow’s intense the antioxidant activity, with he’s low level of phytotoxicity and with an fertilization effect at wheat grains growth, how we observed in the study of biological activity of Sunset Yellow (antioxidant activity and its phytotoxicity) from this paper.

Fourier transform infrared (FTIR) spectroscopy of Sunset Yellow

FTIR measurements of Sunset Yellow were effectuated on the wavenumbers domain: 4000–550 cm−1 with a resolution of 4 cm−1 (Fig. 5). The representation of the Sunset Yellow spectrum is in the absorbance mode.

Fig. 5
figure 5

FTIR spectrum of Sunset Yellow powder material

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Table 1 shows the wavenumbers of the Sunset Yellow absorption maxima, with the assignment of the bonds to which they refer [16, 17, 42,43,44,45,46,47,48,49,50]:

Table 1 Main infrared absorption maxima of Sunset Yellow and their assignments
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The FTIR spectrum of Sunset Yellow [16, 17, 42,43,44,45,46,47,48,49,50] reveals intense specific bands at 1504, 1176, 1117, 1030, 1005, 984, 898, 832, 801, 745, 707, 693, 669, 641, 633, 599, 573 and 562 cm−1 which are related to N=N, sodium 1-naphthalenesulfonate (C10H7NaO3S), SO3−M+ (Na+), C6H5−, −C6H4−, C10H7–R, > S=O, C–S, C−H and R−C6H4–R1 groups.

The typical bands are related to azo group (–N=N–), which gives signals at 1504 cm−1, 1476 cm−1 and at 1415 cm−1 (Table 1).

The Raman spectrum of Sunset Yellow

The Raman spectrum of Sunset Yellow powder is shown in Fig. 6. The positions of the intensities of the Raman displacements for Sunset Yellow are shown in Table 2, where an assignment of the most characteristic vibrations is also presented:

Fig. 6
figure 6

The Raman spectrum of Sunset Yellow

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Table 2 Raman shift of Sunset Yellow and assignments [42,43,44,45, 48,49,50]
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UV–Vis absorption spectrum of Sunset Yellow

The UV–Vis absorption spectrum of Sunset Yellow was obtained by using the Ocean Optics S2000 UV–Vis spectrophotometer. The sample was a Sunset Yellow’s aqueous solution of 0.0025% concentration and used a 10 mm thick cuvette. In UV–Vis absorption spectrum of Sunset Yellow, an absorption band with a peak at λ = 482.47 nm is observed (Fig. 7).

Fig. 7
figure 7

UV–Vis spectrum of Sunset Yellow

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The absorbance at this wavelength is 0.785. The experimentally established wavelength (located in the absorption band of E110), which is closest to the wavelength of λ = 476.50 nm (radiation emitted by the Ar + laser), was λ = 476.48 nm with the absorbance of 0.770. For this absorbance, which is about 98% of the absorbance at λ = 482.47 nm, it can be concluded that Sunset Yellow has a good quantum efficiency for laser fluorescence at λ = 476.50 nm [34].

Fluorescence of Sunset Yellow

Sunset Yellow shown a laser fluorescence spectrum. The power of the excitation radiation source (λ = 476.5 nm) is 440 mW and was measured using a Coherent Max Field Top 2 power meter. The cuvete is made of quartz and has a thickness of 10 mm. The fluorescence spectra were recorded with an optical fibre positioned perpendicular to the excitation beam. The optical fibre is coupled to the spectrometer, which is connected to the computer. From these graphs, using the NIST Atomic Database [51, 52], in the wavelengths corresponding to de-excitation radiation, respectively, the spectral lines of the transitions between the energy levels of the chemical elements in the studied Sunset Yellow, were determined. Determining of the wavelengths of the fluorescence radiation has the accuracy of ± 0.31 nm.

Sunset Yellow is an azo dye for which, in the aqueous solution with c = 5% concentration, the laser fluorescence spectrum of Fig. 8 was obtained.

Fig. 8
figure 8

Fluorescence spectrum of aqueous of Sunset Yellow’s solution with concentration c = 5%

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The wavelengths of de-excitation radiation for Sunset Yellow, respectively the energies corresponding to the energy levels involved in the transitions and the electronic configurations related to them [51, 52], are summarized in the Tables 3–7.

Table 3 Atomic fluorescence of the carbon from 5% Sunset Yellow solution in water [51, 52]
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Table 4 Atomic fluorescence of the oxygen from 5% Sunset Yellow solution in water [51, 52]
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Table 5 Atomic fluorescence of the sulphur from 5% Sunset Yellow solution in water [51, 52]
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Table 6 Atomic fluorescence of the nitrogen from 5% Sunset Yellow solution in water [51, 52]
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Table 7 Atomic fluorescence of the sodium from 5% Sunset Yellow solution in water [51, 52]
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The energies corresponding to the energy levels involved in the transitions and the electronic configurations corresponding to these levels, respectively, the radiation wavelengths at the transition from the superior level, to the inferior level (of de-excitation) [42, 43, 51, 52], are summarized in Tables 3–7. Sometimes electronic transitions occur at the same wavelength, of several elements (ions). Therefore, the electronic transitions between the energy levels in Tables 3–7 are systematized in Table 8, according to the wavelength of the emitted radiation.

Table 8 Systematization of laser electronic fluorescence spectral lines of Sunset Yellow elements (ions)
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The laser fluorescence lines of sunset yellow are intense and are in agreement with those of databases and experimental ones obtained by other authors [51, 52]. Table 8 confirms the finding that some laser fluorescence spectral lines may belong to the fluorescence of two or more chemical elements. There are also fluorescence spectral lines that are emitted by a single element, after which the element can be identified: carbon at 706.49 (w) and at 827.09 (w), oxygen at 738.34 (s), sulphur at 827.09 (w). The electronic transitions from the wavelengths of 612.14 (s), 617.51 (vs), 640.21 (w), 664.36 (vs), 686.98 (s), 697.08 (w), 750.77 (vs), 611.80 nm (s), 617.51 nm (m) and 750.77 nm (w) are thus attributed to the fluorescence of several elements (ions), which explains the high intensity of certain spectral lines of fluorescence.

The THz spectrum of Sunset Yellow

The THz spectroscopy investigation of the Sunset Yellow compound was conducted in a LabVIEW program. The experiments were recorded at ambient conditions of temperature and humidity; also, the sample temperature was monitored [41].

By identifying the characteristic absorption frequencies of the investigated substances, in different environments, a “signature” of the substance can be established, according to the characteristic frequencies of the radiation [26, 34, 41, 53,54,55,56,57,58,59,60]. The absorption spectra in the 0.2–4.5 THz range of Sunset Yellow were acquired for three forms: solid powder, 5% Sunset Yellow in aqueous solution, and the same solution impregnated in a porous paper that was subsequently dried at 105 °C.

Because water and paper absorb incident radiation over the entire frequency range used, water and paper absorption spectra were also recorded, which were removed by the software from the total spectra in order to only obtain curves for azo food dye Sunset Yellow.

The THz spectral signature of Sunset Yellow, obtained by processing numerical THz spectroscopy data, is shown in Table 9, where ν (THz) is the frequency of the Sunset Yellow sample transmission spectrum (0.2–4.5 THz) and the absorbance is the value for the corresponding frequency, for the three forms indicated above.

Table 9 THz spectral signature of Sunset Yellow
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In Table 9, the bold digits represent the values obtained by THz spectroscopy, corresponding to the absorbance of the common peaks for the three forms of the Sunset Yellow (E110, A-E110, DP-E110), observed at the same frequency ν/THz; the other values in Table 9 are the significant values of the absorbances corresponding to the peaks in the three forms of the Sunset Yellow, in the proximity of the common peaks, ordered in increasing frequency. The THz spectral signature of Sunset Yellow consists of the absorption frequencies: 0.76, 1.61, 1.67, 1.74, 1.80, 2.01, 2.65 and 3.54 Tz.

Morphological characterization of Sunset Yellow

  1. (a)

    Energy-dispersive X-ray spectroscopy (EDXS)

EDXS at beam energy of 15 keV was used to evaluate the chemical compositions of the Sunset Yellow’s powder in 10 points (Fig. 9).

Fig. 9
figure 9

SEM image of the Sunset Yellow’s sample with the positions where the EDX spectra were acquired

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In interpreting the results in the points of Fig. 9, the placement of Sunset Yellow powder on the carbon support must be taken into account. The particularly high value of the average percentage of carbon identified experimentally (66.98% by mass), compared to the value of the percentage of carbon calculated from the chemical formula of Sunset Yellow of 42.44% (by mass), is explained by the uneven distribution of crystals on the carbon support (see Table 10).

Table 10 The composition of the Sunset Yellow’s powder in different areas of the surface, analysed by EDX spectroscopy
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In Table 10, in addition to the elements contained in Sunset Yellow, in spectrum 1 appears the aluminum, from the production phase. Chlorine and potassium also appear in all 10 spectra. These elements come from sodium and potassium chlorides, which are allowed in small amounts [1].

  1. (b)

    Scanning electron microscopy (SEM)

The SEM images of Sunset Yellow are presented in Fig. 10a–g; the scanning electron microscope has registered the images at an accelerating voltage of 2 kV and 8200–9800 nA intensity of the emission electric current, allowing for magnifications between 1000× and 5000× and a scale detail of 10–50 μm.

Fig. 10
figure 10

SEM images of Sunset Yellow at magnifications of: a 1000×, b 1800×, c 4000×, d 1000×, e 2000×, f 2500×, g 5000×

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From the SEM images, it is found that the Sunset Yellow powder contains well-formed crystallites, with the visible crystallization planes and with the dispersion between 5 and 20 μm. Figure 10a–c shows the same points from the surface, at different magnifications and the same in Fig. 10d, e—from other points on the surface of the sample.

Thermal analysis and calorimetry study of Sunset Yellow

The thermal investigation of Sunset Yellow was performed in the temperature range RT–1000 °C [11, 26, 34, 42, 43, 61,62,63,64,65]. The Sunset Yellow powder was heated-up in dynamic air atmosphere (150 cm3 min−1) with a heating rate of 10 °C min−1; the thermoanalytic curves (TG, DTG and DSC) are shown in Fig. 11.

Fig. 11
figure 11

Thermoanalytical curves (TG, DTG, DSC) of Sunset Yellow in air atmosphere, with 10 ºC min−1

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These curves indicate a 4-step oxidative behaviour: 3 steps of oxidative decomposition and 1 step of burning; the results are summarized in Table 11.

Table 11 Thermal parameters of Sunset Yellow determined by thermal analysis and calorimetry methods
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Thermal analysis and calorimetry study performed in air indicates the removal of absorbed water molecules up to 188 °C, after which the Sunset Yellow compound exhibits a good thermal stability up to 330 °C.

In the temperature range 330–406 °C, a moderate exothermic effect is observed (thermal effect 1, from Fig. 12), which consists in the loss of C3H2 group, while between 460 and 510 °C, the weak exothermic effect (thermal effect 2, from Fig. 12) stands for the loss of the HCN group.

Fig. 12
figure 12

Thermal effects 1 and 2 of the decomposition of Sunset Yellow in air atmosphere with 10 °C min−1

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In the temperature range 512–616 °C, a major exothermic effect was observed (effect 3, from Fig. 13), with the loss of C10H, HCl and SO3 groups, of ΔH = − 4519.6 J g−1, while between 827 and 920 °C a major thermal exothermic effect (effect 4, from Fig. 14) happens with the burning of the formed polycondensed carbonaceous groups (tars), of ΔH = − 1453.41 J g−1.

Fig. 13
figure 13

Thermal effect 3 of the decomposition of Sunset Yellow in air atmosphere with 10 °C min−1

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Fig. 14
figure 14

Thermal effect 4 of the decomposition of Sunset Yellow in air atmosphere with 10 °C min−1

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The exact mass loss stages for the water removal and the oxidative decompositions of Sunset Yellow are shown in Fig. 15.

Fig. 15
figure 15

Thermogravimetric details of the behaviour Sunset Yellow when decomposing in the air atmosphere, with 10 °C min−1

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The residue is Na2SO4 (sodium sulphate) and Na2C2 (disodium acetylide/sodium carbide), with a theoretical percentage of 31.77% and practically 31.51% (Fig. 15). The groups of evolved/formed atoms were assumed, based on the experimental mass losses.

The results of the thermal analysis confirm the chemical formula and composition of the Sunset Yellow food azo dye: C16H10N2Na2O7S2, with NaCl traces and absorbed H2O.

Following the thermal and calorimetric study of Sunset Yellow, it was established that the maximum temperature at which a food—containing this dye—can be thermally processed is 330 °C. However, between 188 and 330 °C, it is recommended for the foods containing Sunset Yellow, to thermally process those in an oxygen-poor atmosphere, preferably in closed containers.

Acidity study of the Sunset Yellow

The pH measurements of Sunset Yellow, as a function of temperature, were made using distilled water as reference, starting from room temperature (RT), to approximately 55 °C [66].

The temperature-dependent pH measurements for the aqueous Sunset Yellow solution, with 1% concentration, are shown in Fig. 16a.

Fig. 16
figure 16

a PH variation with temperature and b proton concentration [H.+] in the aqueous solution of c = 1% Sunset Yellow (E110)

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The Sunset Yellow solution has, at temperature of 24 °C, an acid character, corresponding to a pH = 6.28, and as the temperature increases, the acidity of the sample increases, the pH reaching a value of 6.15 at temperature of 57 °C. From the pH value of the Sunset Yellow solution, the concentration of hydrogen ions is calculated Fig. 16b.

The concentration of hydrogen ions [H+] increases with increasing temperature, from the value of 5.22·10–7 mol L−1, corresponding to the temperature of 23.7 °C, to the value of 7.16·10–7 mol L−1 at the temperature of 57.42 °C.

Thus, by heating the liquids containing the food dye Sunset Yellow, their pH reveal a slightly increase in their acid character.

Biophysical properties and biological activity of Sunset Yellow

  1. (a)

    UV–Vis absorption spectra of bovine serum albumin (BSA) in interaction with Sunset Yellow

The interaction of food azo-dye E110 (Sunset Yellow) with BSA was analysed by UV–Vis spectroscopy with the UV–Vis Varian Cary 50 spectrophotometer (Fig. 17).

Fig. 17
figure 17

Absorption spectrum of Sunset Yellow-BSA system at pH = 7.4, cBSA = 5 × 10–6 M and cSunset Yellow = 0, 1, 2, 3, 4 and 5 × 10–5 M (b → g), curve a corresponds to the UV–Vis spectrum of BSA at cBSA = 5 × 10–6 M, and curve g corresponds to the UV–Vis spectrum of Sunset Yellow, cSunset Yellow = 5 × 10–5 M

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The electronic spectrum of bovine serum albumin shows a maximum at 280 nm due to the π → π* transitions in the aromatic nuclei of the amino acid residues (tryptophan, tyrosine)—Fig. 17a. In the BSA—Sunset Yellow system, in the presence of increasing dye concentrations, from 1 × 10–5 M to 5 × 10–5 M, it is observed that the absorbance increases. The absorbance maximum has a hyperchromic effect and a hypochromic effect and shifts to shorter wavelengths, from 278 nm, in the case of the dye-free solution (Fig. 17a), to 275 nm in the case of the dye-containing solution in concentration of 5 × 10–5 M (Fig. 17g). Also in UV, absorbance maxima were observed at approximately 262 nm and 320 nm for curves b–f, and a displacement at 315 nm for curve g, and in the visible range, a maximum was observed at approximately 482 nm (Fig. 17). The displacement is due to the deployment of the protein and the change in the polarity of the environment around the aromatic amino acid residues in the protein structure [67, 68]. These observations prove that the protein can bind the dye to form a BSA-dye complex.

The absorption maximum at 482 nm obtained for Sunset Yellow is in good agreement with those mentioned in other works [50, 69], where in aqueous solution, a maximum was obtained at about 485 nm [69].

Shifting the maximum absorbance towards shorter wavelengths, when the concentration of Sunset Yellow increases, demonstrates that BSA can bind the dye and that bovine serum albumin can form a complex with Sunset Yellow [67, 68]. This can disrupt the normal biological function of bovine serum albumin.

  1. (b)

    FTIR spectrum of collagen in interaction with Sunset Yellow

The interaction of food dyes with collagen [45, 70] can be studied by FTIR spectroscopy. Sunset Yellow belongs to the azoic group of dye compounds, from which, by the interaction with the environment where they are processed and employed, is possible formation of carcinogenic aromatic amines and amides [1, 71,72,73,74,75,76,77,78,79,80].

In this paper, the interaction of skin collagen with Sunset Yellow was studied, through the FTIR spectrum of natural skin samples, treated or untreated with food azo dye solutions with 5% concentration. Figure 18 shows the FTIR spectrum of treated (a) and untreated (b) skin with 5% aqueous Sunset Yellow solution. The FTIR Bruker ATR Zn-Se spectrometer was used.

Fig. 18
figure 18

FTIR spectrum of a treated and b untreated skin with Sunset Yellow solution (c = 5%)

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The characteristic vibrations of the amide bands I and II appear at 1651 cm−1 and 1549 cm−1, respectively, which is in agreement which other papers [26, 34, 42, 43, 70]. The molecular arrangement and folding of proteins depend on the nature of the hydrogen bonds, and the interaction of collagen with Sunset Yellow is evidenced in its FTIR spectrum [45, 70]. In the presence of Sunset Yellow, the position of amide I band (C=O stretching) is at 1651.35 cm−1, while the position of amide II band (NH bending and CN stretching) moved from 1540.17 to 1555.51 cm−1. The spectral images show the changes produced by the interaction of collagen with Sunset Yellow and the influence of hydrogen bonds on the characteristics of NH peptide bonds [78,79,80]. These changes can have an undesirable effect on the structure of collagen and pose a potential risk to the skin.

  1. (c)

    Determination of the antioxidant activity of Sunset Yellow, by the Folin–Ciocâlteu method

Folin–Ciocâlteu (FC) reagent reacts with the reducing substance (antioxidant) in an alkaline medium (sodium carbonate), with the formation of chromogens (oxides of blue tungsten—W8O23 and molybdenum—Mo8O23), which can be detected by spectrophotometry at 660 nm or 750 nm [81,82,83,84].

The antioxidant activity of Sunset Yellow is shown in Table 12:

Table 12 Antioxidant activity of Sunset Yellow (E110) solution of different concentrations [85, 86]
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The GAE values in the table indicate the antioxidant activity of Sunset Yellow at the three concentrations. Sunset Yellow has antioxidant activity at all three concentrations, the highest value being at the concentration c = 0.25%, which is probably due to the phenolic hydroxyl group.

  1. (d)

    Phytotoxicity of Sunset Yellow

Sunset Yellow’s phytotoxicity [87, 88] was studied using 4 samples. For each sample, five wheat grains are displayed in four Petri plates. Three samples were treated daily with 10 mL of Sunset Yellow aqueous solution with concentrations of 0.01, 0.05 and 0.25%. One of the Petri dishes contained the witness sample (W)—five wheat grains, treated daily with 10 mL of ultrapure water. The roots of the five wheat seeds in each Petri dish were measured daily [89] for 4 days and an arithmetic mean was made to find out the mean value of the roots length of sample W and the mean value of the roots length of samples treated with Sunset Yellow at the three concentrations (Table 13).

Table 13 Average length of the roots, depending on the growth time and concentration of the Sunset Yellow’s solutions
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By measuring the length of the roots and observing the appearance of the wheat stalks (Triticum aestivum), in the case of wheat treated with E110 (Sunset Yellow), an additional increase was observed compared to the control sample, as seen in Fig. 19 and as shown in Table 13. A fertilization effect for wheat grains growth was observed for all three concentrations.

Fig. 19
figure 19

The phytotoxicity experiment for samples containing Sunset Yellow aqueous solutions at three concentrations (0.01, 0.05 and 0.25%) and witness sample containing 0.00% Sunset Yellow, a on the first day, and b on the fourth day

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There was also a slight bending of the wheat strains treated with Sunset Yellow (at 0.01% and 0.05% concentrations), compared to the straight strains of the control sample W, and a discolouration of the strains (at concentrations of 0.05% and 0.25%), compared to the green stems of sample W, as shown in Fig. 19. During days three and four with of Sunset Yellow treatment, it was observed that wheat developed new, vigorous roots, probably in correlation with its intense antioxidant activity. Treating wheat with aqueous solutions of Sunset Yellow at concentrations of 0.01–0.05% could increase its resistance to drought conditions.

It can be concluded that at low concentrations, Sunset Yellow does not harm the growth of wheat, but is even a growth stimulant, but at higher concentrations (0.25%), there are slowdowns in wheat growth compared to the control sample W.

Conclusions

As the temperature increases in the range of 15–50 °C, the values of the refractive indices of Sunset Yellow solutions are decreasing, the behaviour depending on the concentration of the dye. The acidity and electrical susceptibility of aqueous Sunset Yellow solutions also depend on concentration; therefore, the Sunset Yellow content of the food can be identified.

The FTIR, Raman, UV–Vis and THz spectroscopic analyses have determined the radiation absorption lines and bands based on which the atoms, groups of atoms and the bonds responsible for these absorptions were identified. The THz absorption spectrum of Sunset Yellow obtained in the frequency range 0.2–4.5 THz allows the identification of the presence of the dye in any food using the specific absorption frequencies of Sunset Yellow which constitutes the THz spectral signature of the dye (0.76, 1.61, 1.67, 1.74, 1.80, 2.01, 2.65 and 3.54 Tz).

By laser irradiation with wavelength 476.5 nm, fluorescence of the Sunset Yellow was produced, establishing the transitions at the electron de-excitation of all elements. The laser fluorescence spectral lines may belong to the fluorescence of two or more chemical elements, which explains the high intensity of certain spectral lines of fluorescence, but there are also fluorescence spectral lines that are emitted by a single element.

Sunset Yellow is a crystalline substance at room temperature, with crystallites between 5 and 20 μm. Thermal analysis and calorimetry study performed in air indicate the evaporation of adsorbed water up to 188 °C, after which the material is thermally stable up to 330 °C. Oxidative decomposition takes place in four exothermic stages, of which the main one is between 510 and 643 °C with heat release of 4519.6 J g−1. At 913 °C, the residue of 31.77% is obtained. Sunset Yellow, used for colouring processed foods, can be safely used at temperatures below 330 °C (it is recommended that foods containing this azo dye be thermally processed in an oxygen-poor atmosphere—preferably in closed containers.

Sunset Yellow bioactivity was highlighted by its interaction with proteins (collagen and bovine serum albumin—BSA), by its antioxidant activity and its phytotoxicity on wheat. The results indicated that as the concentration of Sunset Yellow in the solution increases, BSA can bind the food dye azo forming a complex with the dye. This can disrupt the normal biological function of BSA. Sunset Yellow has an undesirable effect on collagen and poses a potential risk to the skin. The antioxidant activity of Sunset Yellow increases with increasing its concentration, the highest value of GAE equivalent gallic acid (458.1 mg L−1), being recorded for the concentration of 0.25%, most likely due to the presence of hydroxyl group. Treating wheat with aqueous Sunset Yellow solutions, at concentrations of 0.01–0.05%, could increase its resistance to drought conditions. At low concentrations, Sunset Yellow does not harm wheat, but at concentrations above 0.25%, there are negative changes in wheat growth.