Flower-like MoS 2 screen-printed electrode based sensor for the sensitive detection of sunset yellow FCF in food samples

In this study, synthesis and electrochemical sensor application of flower-like MoS 2 for sunset yellow FCF sensing were evaluated. Linear sweep voltammetry (LSV), chronoam-perometry and differential pulse voltammetry (DPV) were used to determine flower-like MoS 2 electrochemical sensor performances, which had LOD of 0.04 μM in the linear working range of 0.1 –150.0 μM calculated from DPV. The sensor was successfully applied for the determination of sunset yellow FCF in real samples


Introduction
Over the past centuries, food and beverage products used different colors to maintain nutriational and health-related advantages.These compounds actively participate in various biological processes.Nowadays, color additives are commonly applied in a variety of foods, including dairy products, beverages, cereals, snack foods and ice creams, to make them more attractive and appetitive.Among various types of food additives, food colorants are played an important role in foodstuff due to their physical appearance and consumer acceptance.Food colorants are categorized into natural and synthetic dyes.However, synthetic food dyes are widely used in the food industry compared to natural colorants due to higher light, oxygen, and pH stability.In addition, they provide a uniform color and a strong tinting ability.Synthetic color additives are organic pigments using artificial synthesis methods and are generally made from coal tar from aniline dyes as raw material [1].Among the most used dyes by the food industry are those that contain an azo group (-N=N-), including the sunset yellow (SY), the Bordeaux red, and the tartrazine yellow.These dyes are extensively used in cereals, candies, dairy products, jellies, ice creams, fillings, liqueurs, powdered juices, soft drinks, and yogurts [2][3][4].Sunset yellow (SY), also known as evening yellow, E110 or edible yellow 3, is one of the most used azo dyes.It has an orange color and is used in a great number of fruit products, like sodas, juices, candies, and ice creams.Usually, it is the only artificial dye present in soft orange beverages.SY also has a large use in the pharmaceutical industry and in cosmetics.Nevertheless, it also causes some side effects in humans, and its consumption has been related to renal failure and hepatocellular damage [5][6][7].
Until now, different methods such as high-performance liquid chromatography, near-infrared spectroscopy and interval random forest, surface-enhanced Raman spectroscopy and fluorescence spectroscopy have been reported for the determination of sunset yellow [8][9][10][11].In addition, voltammetry was also used to determine sunset yellow dye based on their electrochemical activity [12][13][14].
Electrochemical sensor systems do not require particular expertise or expensive apparatus for operation.As a result, miniaturization of these systems for in-situ analysis of ecological contaminants is enabled.The advent of screen-printed electrodes (SPEs) has prospects for direct electrochemical mechanisms outside centralized laboratories.Features like rapid response, mass production, low power requirement, and compactness have reinforced SPEs as ideal transducers for environmental assays.Further, the sample volume required for an SPE is as low as 50 μL.Additionally, the surface of the SPEs' working electrode may be readily modified to improve the sensibility, lowering the detection limit, and increasing the selectivity of the electrochemical methods [15,16].The chemical modification of inert substrate electrodes offers significant advantages in the design and development of electrochemical sensors [17][18][19][20][21][22][23][24][25][26][27][28][29][30].In operations, the redox-active sites shuttle electrons between a solution of the analyte and the substrate electrodes often along with a significant reduction of the activation overpotential.A further advantage of chemically modified electrodes is that they are less prone to surface fouling and oxide formation compared to inert substrate electrodes [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45].
MoS 2 obtained by different preparation methods have different morphology, such as 2D-layered structures, nanorods, nanotubes and three-dimensional structures.Among them, flower-like MoS 2 has a unique three-dimensional structure.Because of its large specific surface area, ultra-short transmission path, high interfacial transfer, and excellent catalytic activity as a matrix, it is widely used in energy storage, catalysis, sensing and other fields.MoS 2 can increase electrocatalytic activity.Molybdenum disulfide (MoS 2 ), which is a two-dimensional (2D) transition metal dichalcogenides (TMDs), has gained immense attention over the past few years.As a semiconductor with an indirect energy band of 1.29 eV, MoS 2 has a typical 2D layered structure, and each layer is composed of two sulfur atoms and a molybdenum (Mo) atom, forming an S-Mo-S sandwich structure.MoS 2 shows tunable electric properties, excellent electrocatalytic activity, electrochemiluminescent (ECL) signal amplification, photochemical reactivity, fluorescence quenching ability, and adsorption performance and hence is a candidate for catalysis, biology, medicine, sensor monitoring, energy storage and conversion [70,71].The aim of this study was to synthesize flower-like MoS 2 (FL-MoS 2 ) to modify the screen-printed electrodes for the detection of sunset yellow FCF food dye.

Apparatus and chemicals
All the electrochemical measurements were carried out on a PGSTAT302N potentiostat/galvanostat Autolab.Screen-printed electrodes (SPE) (DropSens; DRP-110: Spain) consisted of a graphite counter electrode, a graphite working electrode, and a silver pseudo-reference electrode.Solution pH values were determined using a 713 pH meter combined with glass electrode (Metrohm, Switzer-land).Sunset yellow and other chemicals used were of analytical grade and were purchased from Merck.Flower-like MoS 2 was synthesized in our laboratory [72].A typical SEM is shown in Figure 1.

Figure 1. SEM image of flower-like MoS 2 Preparation of FL-MoS 2 /SPE
First, 1 mg of prepared FL-MoS 2 was added into an aqueous solution (1 ml), followed by sonication for 30 min to give a homogeneous solution.Then, 4 μL of FL-MoS 2 was dispersed on the surface of SPE dropwise.Following the solvent's evaporation, the sensor's surface was washed several times with deionized water to clean free modifier molecules and subsequently air-dried.The obtained electrode was noted as FL-MoS 2 /SPE.
The surface areas of the FL-MoS 2 /SPE and the bare SPE were obtained by cyclic voltammetry using 1 mM K 3 Fe(CN) 6 at various scan rates.Using the Randles-Ševčik equation for FL-MoS 2 /SPE, the electrode surface was found to be 0.11 cm 2 which was about 3.5 times greater than bare SPE.

Electrochemical behavior of sunset yellow at the surface of various electrodes
The effect of the electrolyte pH on the oxidation of 60.0 μM sunset yellow was investigated at FL-MoS 2 /SPE using DPV measurements in the phosphate buffer solution (PBS) in the pH range from 2.0 to 9.0.According to the results, the oxidation peak current of sunset yellow depends on the pH value and increases with increasing pH until it reaches the maximum at pH 7.0, and then decreases with higher pH values.The optimized pH corresponding to the higher peak current was 7.0, indicating that protons are involved in the reaction of sunset yellow oxidation.This result indicates the oxidation mechanism of sunset yellow involving electron transfers coupled with proton exchange.It is well-known that the hydroxyl group of sunset yellow undergoes a 1e -/1H + oxidation mechanism at the surface of reported electrodes.
The electrochemical behavior of sunset yellow was investigated by LSV.The linear sweep voltammetry was obtained using the bare SPE (curve b) and FL-MoS 2 /SPE (curve a) in 0.1 M PBS (pH 7.0) in the presence of 100.0 μM sunset yellow are shown in Figure 2. On a bare SPE, a signal with a low oxidation current of 3.3 μA was obtained with a peak potential of 760 mV.In contrast, FL-MoS 2 /SPE exhibited an enhanced sharp anodic peak current (I pa = 14.0 μA) at much lower overpotential E p = 740 mV.These results confirmed that the FL-MoS 2 /SPE improved the sensitivity of the modified electrode by enhancing peak current and decreasing the overpotential of the oxidation of sunset yellow.

Effect of scan rate on the determination of sunset yellow at FL-MoS 2 /SPE
The influence of the scan rate () on the peak currents (I pa ) of sunset yellow at FL-MoS 2 /SPE was investigated by LSV (Figure 3).

Chronoamperometric analysis
The analysis of chronoamperometry for sunset yellow samples was performed by use of FL-MoS 2 /SPE at 0.77 V.The Chronoamperometric results of different concentrations of sunset yellow in PBS (pH 7.0) are demonstrated in Figure 5.The Cottrell equation for the chronoamperometric analysis of electroactive moieties under mass transfer limited conditions is as in eq. ( 1) where D is diffusion coefficient (cm 2 s -1 ), and C b is the applied bulk concentration (mol cm -3 ).Experimental results of I vs. t -1/2 were plotted in Figure 6A, with the best fits for different concentrations of sunset yellow.The resulting slopes corresponding to straight lines in Figure 6A, were then plotted against the concentration of sunset yellow (Figure 6B).The mean value of D was determined to be 1.1×10 -5 cm 2 /s according to the resulting slope and Cottrell equation.

Calibration curve
Because DPV commonly has a higher sensitivity than cyclic voltammetry technology, the DPV technique was applied for the quantitative detection of sunset yellow.Figure 7 shows the differential pulse voltammograms of sunset yellow at various concentrations using FL-MoS 2 /SPE (Step potential = 0.01 V and pulse amplitude = 0.025 V).As seen, the oxidation peak currents of sunset yellow enhance gradually by increasing its concentration.The oxidation peak currents show a good linear relationship with the concentrations of sunset yellow ranging from 0.1 M to 150.0 μM.The linear equation is I pa = 0.1315C sunset yellow + + 1.0472 (R 2 = 0.9994) (Figure 8).Also, the limit of detection (LOD) was estimated to be 0.04 μM.The performance of this sensor is compared with some of the recently reported electrodes for sunset yellow quantification (see Table 1) [73][74][75][76].

Stability and repeatability
The long-term stability test of the FL-MoS 2 /SPE sensor using DPV was performed at room temperature.The results exhibited that the peak current of 35.0 μM sunset yellow at the FL-MoS 2 /SPE stayed at 93.2 % of its primary current after 7 days, 90.8 % after 14 days, and 89.97 % after 21 days, indicating the superior long-term stability of the proposed sensor.
The oxidation of sunset yellow (35.0 μM) on the same FL-MoS 2 /SPE sensor was performed by ten repeated voltammetric measurements, the results of which confirmed the superior repeatability of the fabricated sensor with the relative standard deviation (RSD) of 2.9 %.

Analysis of real samples
The real samples for the analysis were prepared and quantified by DPV method.The developed sensor was applied to detect sunset yellow in orange and apple juice samples.The results are summarized in Table 2.Each measurement was repeated five times.The recovery and relative standard deviation (RSD) values confirmed that the FL-MoS 2 /SPE sensor has a great potential for analytical application.

Conclusion
In the present study, sensing of sunset yellow FCF was attempted and implemented successfully with flower-like MoS 2 (FL-MoS 2 ) modified screen-printed electrodes.The morphology of flower-like MoS 2 was characterized by SEM.Using linear sweep voltammetry and differential pulse voltammetry, the electrochemical oxidative sensing of sunset yellow FCF was investigated.The FL-MoS 2 /SPE showed a linear response of 0.1 μM to 150.0 μM for sunset yellow FCF, respectively, and their detection limit was equal to 0.04 μM.The stability of the modified electrode was indeed excellent.

Figure 2 .
Figure 2. Linear sweep voltammograms of (a) FL-MoS 2 /SPE and (b) bare SPE in 0.1 M PBS (pH 7.0) in the presence of 100.0 μM sunset yellow at the scan rate 50 mVs -1

Figure 4 .
Figure 4. Plot of anodic peak current vs.  1/2 at different scan rates in the range of 5 to 200 mV/s.

c 8 .
sunset yellow / M Figure Plot of the electrocatalytic peak current as a function of sunset yellow concentration in the range of 0.1-150.0µM

Table 1 .
Comparison the sensing performances toward the detection of sunset yellow between the existing modified electrodes and the proposed FL-MoS 2 /SPE

Table 2 .
The application of FL-MoS 2 /SPE for the determination of sunset yellow in real samples (n=3)