High sensitivity simultaneous determination of myricetin and rutin using a polyfurfural film modified glassy carbon electrode

Abstract

In this work, we successfully develop a simple electrochemical sensor based on a polyfurfural film modified glassy carbon electrode to realize the simultaneous electrochemical determination of myricetin and rutin. The proposed sensor exhibits superior electrocatalytic activity to both myricetin and rutin, simultaneously demonstrating a wide linear detection range of 0.05–10 μmol dm⁻³ with a low detection limit of 10 ± 0.5 nmol dm⁻³ (S/N = 3, α = 0.05, n = 3, type I error) for myricetin, and linear detection range of 0.001–10 μmol dm⁻³ with a low detection limit of 0.025 ± 0.004 nmol dm⁻³ (S/N = 3, α = 0.05, n = 3, type I error) for rutin. With favorable selectivity, stability and reproducibility, the proposed sensor was satisfied by applying for simultaneous determination of myricetin and rutin in real samples. It may provide a new strategy to simultaneously determine different kinds of polyphenolic compounds with polyfurfural film modified glassy carbon electrode in application of pharmaceutical and biological analysis in the future.

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Introduction

Natural flavonoid products like myricetin and rutin are commonly contained in tea,¹ vegetables² and fruits³ and extensively used in therapeutic agents as the active components due to their positive anti-oxidative,⁴ anti-bacterial,⁵ anti-carcinogenic⁶ and anti-aging properties.⁷ As polyphenolic compounds, myricetin and rutin possess comparable structures (Chart 1), resulting in similar chemical and physical properties. Therefore, it is significant to develop a strategy which can realize the determination of myricetin and rutin at the same panel at the same time.

Chart 1 Chemical structure of myricetin (A) and rutin (B).

Simultaneous determination,^8,9 quantitatively determining two or more analytes at the same time, have attracted enormous attention in analytical chemistry.^10,11 By contrast to assays only suitable for a single analyte, it possesses the advantages of high test efficiency, low cost of analytical procedure and short analysis time.^12–15 Some reported techniques such as spectrophotometry,¹⁶ high performance liquid chromatography,¹⁷ capillary electrophoresis¹⁸ and chemiluminescence¹⁹ offer available opportunities for the simultaneous determination of the analytes of myricetin and rutin, but these techniques show the disadvantages of high cost of instruments and time-consuming experimental process, finally limit their extensive use in practical application. By contrast, sensors based on electrochemical methods exhibit some distinctive properties of economical analysis,^20,21 fast respond,^22,23 high sensitive^24,25 and ease of use,^26,27 making them become a promising alternative for the determination of myricetin and rutin. Surprisingly, a considerable number of studies has been only conducted on the electrochemical determination of the single target, myricetin^28,29 or rutin.^30–33 However, the work for simultaneously electrochemical determination of myricetin and rutin has been paid less attention, probably due to the fact of similar electrochemical properties of flavonoid components causing significant potential overlap. As reported recently, functionalized carbon nanomaterials^34,35 seem to become favored candidates in this kind of applications. For a fine example, Ran and co-works used β-cyclodextrin-gold@3,4,9,10-perylene tetracarboxylic acid functionalized single-walled carbon nanohorns as the sensing material to successfully achieve simultaneous determination of myricetin and rutin.³⁴ In another illustration, Yola and co-workers developed an electrochemical sensor based on gold nanoparticles involved p-aminothiophenol functionalized multi-walled carbon nanotubes modified glassy carbon electrode for the simultaneous determination of quercetin and rutin.³⁵ However, the synthesis of these nanomaterials and the formation of these sensors are usually involving multiple-step processes and it would complicate the preparation of electrodes, potentially reducing the stability and reproducibility of the proposed sensors.

In order to achieve the goal of simultaneous determination of myricetin and rutin, a simple electrochemical sensor based on polyfurfural film modified glassy carbon electrode was developed in this work by a one-step electropolymerization process. In particular, the utilization of polyfurfural film in this sensing application is attractive for several reasons. First, the synthesis of polyfurfural can be electrochemically controlled at an electrode surface.³⁶ Second, polyfurfural film introduces multiple active sites with high stability.^37,38 Finally, polyfurfural exhibits excellent electrocatalytic activity to the reduction of nitro groups^37,38 and the oxidation of hydroxyl groups³⁶ in aromatic compounds. Benefited from these superiorities,^36–38 the proposed polyfurfural electrochemical sensor is highly sensitive to both myricetin and rutin with wide linear detection ranges and low detection limits. This sensor also possesses high stability, selectivity and reproducibility and successfully applied in practical applications of the real sample analysis.

Experimental

Reagents

Furfural (GC, 98%) and myricetin (97%, analytical grade) were purchased from TCI Ltd. Rutin (98%, analytical grade) was purchased from J&K Chemical (Beijing, China). Absolute alcohol, sodium perchlorate and acetonitrile were purchased from DaMao Chemical Reagent Company (Tianjin, China). All chemicals were of analytical grade and used without further purification. The stock solutions of 2.0 mmol dm⁻³ myricetin and 2.0 mmol dm⁻³ rutin were prepared by dissolving in 50 mL ethanol and then diluting with doubly distilled water to 100 mL, respectively. The working solutions were prepared by diluting the stock solutions with BR (pH = 6) buffer solution. Britton–Robinson (BR) buffer solution was prepared by mixing 0.04 mol dm⁻³ of boric acid, phosphoric acid and acetic acid, and then adjusted to the required pH with 0.2 mol dm⁻³ of NaOH solution. All aqueous solutions were prepared with doubly distilled water.

Fabrication of polyfurfural film/GCE

Before the fabrication of the modified electrode, a bare glassy carbon electrode (GCE, 3 mm diameter) was firstly polished with 0.3 μm alumina powders, and then successively polished with 0.05 μm alumina powders. After polishing, the bare glassy carbon electrode was rinsing with absolute alcohol and doubly distilled water in an ultrasonic bath, and then was dried with nitrogen stream for further modification. The fabrication of polyfurfural film/GCE was according to our previous works^36–38 and the schematic representation of preparation of polyfurfural film/GCE was shown in Scheme 1.

Scheme 1 Schematic representation of fabrication of polyfurfural film/GCE.

Apparatus and method

Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and square wave voltammetry (SWV) experiments were performed on a CHI 660D electrochemical workstation (Chenhua, Shanghai, China) with a three-electrode configuration in all electrochemical experiments. A polyfurfural film/GCE working electrode, a platinum wire counter electrode and a saturated calomel reference electrode (SCE) were applied in the determination. The potential range for SWV is from −0.1 V to 0.5 V with an amplitude of 0.025 V. Electrochemical impedance spectroscopy was obtained in 5 mmol dm⁻³ of K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution containing 0.1 mol dm⁻³ of KCl with 50 mV amplitude and a frequency range from 0.1 Hz to 100 kHz.

For the determination of myricetin and rutin, the detection limit, C_m, was obtained using equation eqn (1):

C_m = 3S_b/m (1)

where m is the slope of the calibration plot in the linear range, and S_b is the standard deviation of the blank response which is obtained from 20 replicate measurements of the blank BR buffer solution.

Results and discussion

Electrochemical behavior of myricetin and rutin at the polyfurfural film/GCE

In our previous work,³⁷ the polyfurfural film/GCE was characterized by both SEM and EIS. The SEM image of the polyfurfural film/GCE shows a clear view of the polymer layer with a rough surface. The EIS plot shows the resistance increased significantly when the polyfurfural film modified to the surface of a bare glassy carbon electrode.

The electrochemical behaviors of both myricetin and rutin at the polyfurfural film/GCE were investigated by CV and SWV. As shown in Fig. 1A, myricetin and rutin as a single component present a pair of obvious redox peaks on the polyfurfural film/GCE by CV at E_pa = 0.150 V, E_pc = 0.123 V (curve a) and E_pa = 0.326 V, E_pc = 0.295 V (curve b), respectively, which due to the reversible oxidation of phenolic hydroxyl groups. These two pairs of redox peaks still remain at approximately same potentials and can be significantly distinguished (curve c) when myricetin and rutin were mixed together in the same solution, indicating the possibility of simultaneous determination for these two components. Moreover, due to the excellent electrocatalytic activity of polyfurfural film, the observed peak currents for both myricetin and rutin are drastically enhanced by comparing to those obtained on a bare GCE (curve d). In addition, no redox peaks were observed on the polyfurfural film/GCE in BR (pH 6) buffer solution containing 0.5% ethanol without myricetin or rutin (curve e). Similar observations have been gained by SWV in Fig. 1B, where oxidations for myricetin and rutin on the polyfurfural film/GCE occur at 0.120 V and 0.284 V, respectively. With increased sensitivity by SWV, the oxidation processes of myricetin and rutin in the mixed solution became much sharper, resulting in less overlap between these two oxidation peaks and it is very much in favor of the simultaneous determination of myricetin and rutin.

Fig. 1 CVs (A) and SWVs (B) on polyfurfural film/GCE of 10 μmol dm⁻³ myricetin (a), rutin (b), myricetin and rutin (c), without myricetin or rutin (e) in BR (pH 6) buffer solution containing 0.5% ethanol. CVs (A) and SWVs (B) on bare GCE of 10 μmol dm⁻³ myricetin and rutin (d) in BR (pH 6) buffer solution containing 0.5% ethanol. Scan rate was 50 mV s⁻¹.

Effect of scan rate

In Fig. 2, both oxidation and reduction peak currents of myricetin and rutin obtained at the polyfurfural film/GCE by CV showed linearity with scan rates, where the linear regression equations were expressed for myricetin as I_pa (μA) = 0.00628ν (mV s⁻¹) + 0.12523 (R = 0.9985) and I_pc (μA) = −0.00534ν (mV s⁻¹) − 0.10165 (R = 0.9989) and for rutin as I_pa (μA) = 0.00678ν (mV s⁻¹) + 0.1404 (R = 0.9992) and I_pc (μA) = −0.00485ν (mV s⁻¹) + 0.00991 (R = 0.9993), respectively. These results indicated that both the oxidation and reduction processes of both myricetin and rutin are typical adsorption controlled process.

Fig. 2 CVs (A) of 10 μmol dm⁻³ myricetin and rutin at the polyfurfural film/GCE in BR (pH 6) buffer solution containing 0.5% ethanol at different scan rates (20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280 mV s⁻¹). The relationship between the oxidation peak currents (a), reduction peak currents (b) of 10 μmol dm⁻³ myricetin (B), rutin (C) and scan rates.

Effect of pH

The redox properties of myricetin and rutin were also influenced by pH of buffer solutions. As the consequence, the oxidation peak potentials of myricetin and rutin obtained by SWV change to be more negative when pH values increased from 4 to 8 (Fig. 3A). However, a fact should not be overlooked that the variations of the oxidation peak potentials (E_pa) for both myricetin and rutin are linearly with the changes of pH values (Fig. 3B and C). In particular, the linear regression equations were expressed as: E_pa = −0.0684pH + 0.5264 (R = 0.9977) and E_pa = −0.0600pH + 0.652 (R = 0.9999) and the slopes of the E_pa–pH plots are −68.4 mV per pH and −60.0 mV per pH for myricetin and rutin, respectively. According to theoretical value of −59.2 mV per pH, these observations reveal that the numbers of transferring proton and electron involved in this electrochemical oxidation reaction are same. Therefore, the reaction mechanisms for both myricetin and rutin can be proposed as Scheme 2.^30,35 Moreover, the results appeared in the insets of Fig. 3B and C clearly demonstrate that the electrochemical responses for both myricetin and rutin are most sensitive at pH of 6, which was thus chosen as an optimum condition in the subsequent quantification.

Fig. 3 CVs (A) of 10 μmol dm⁻³ myricetin and rutin in BR buffer solution containing 0.5% ethanol at different pH values on the polyfurfural film/GCE. The relationship of pH vs. oxidation peak potential and pH vs. oxidation peak current (inset) for myricetin (B) and rutin (C).

Scheme 2 The reaction mechanisms of myricetin (A) and rutin (B).

The simultaneously quantitative determination of myricetin and rutin

The simultaneously quantitative determination of myricetin and rutin at the polyfurfural film/GCE was successfully achieved by SWV. The oxidation peak currents of myricetin and rutin are proportional to their concentrations (Fig. 4A). Particularly, the detection ranges of myricetin are obtained as 0.05–0.5 μmol dm⁻³ and 0.5–10 μmol dm⁻³ (Fig. 4B), giving linear equations as I_pa (μA) = 0.53088c (μM) + 0.10295 (R = 0.9962, F = 394.75636, F(0.05, 1, 2) = 18.51, F > F(0.05, 1, 2), indicated significant linear relationship) and I_pa (μA) = 0.24709c (μM) + 0.27235 (R = 0.9990, F = 2408.75062, F(0.05, 1, 4) = 7.71, F > F(0.05, 1, 4), indicated significant linear relationship), respectively with a detection limit of 10 ± 0.5 nmol dm⁻³ (S/N = 3, α = 0.05, n = 3). For rutin (Fig. 4C), the oxidation peak currents change linearly with the proportion of the concentrations over the ranges of 0.001–0.05 μmol dm⁻³ and 0.05–10 μmol dm⁻³ with linear equations: I_pa (μA) = 3.85418c (μM) + 0.25104 (R = 0.9997, F = 6338.48512, F(0.05, 1, 3) = 10.13, F > F(0.05, 1, 3), indicated significant linear relationship) and I_pa (μA) = 0.24007c (μM) + 0.46416 (R = 0.9983, F = 2293.78734, F(0.05, 1, 7) = 5.59, F > F(0.05, 1, 7), indicated significant linear relationship), respectively. A lower detection limit for rutin is also determined as 0.025 ± 0.004 nmol dm⁻³ (S/N = 3, α = 0.05, n = 3).

Fig. 4 (A) SWVs of myricetin and rutin at the polyfurfural film/GCE in BR (pH 6) buffer solution containing 0.5% ethanol. Concentrations: 0.001, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 8 and 10 μmol dm⁻³ (from a to m). Linear relationship between oxidation peak currents and concentrations of myricetin (B) and rutin (C).

As shown in Table 1, in contrast with the reported electrochemical sensors for single^28,30–33 or bi-component³⁴ of myricetin and rutin, the proposed sensor in this work exhibits at least comparable features of wide linear detection range and low detection limit for both myricetin and rutin, simultaneously. However, one point has to be addressed in that the preparation of the polyfurfural film/GCE sensor is much more straightforward since the polyfurfural film was formed on the surface of GCE just by a one-step electropolymerization process and all parameters can be fully controlled by electrochemistry. The proposed sensor would therefore be considered more reliable and stable to achieve the best performance for the simultaneous determination of myricetin and rutin.

Table 1 Comparison of performances of the polyfurfural film/GCE with other modified electrodes

Modified electrode	Performance	Myricetin	Rutin	References
AuNPs/en/MWCNTs/GCE	Linear range (μM)	0.05–40	—	28
	LOD (nM)	12	—
ASiG/G/GCE	Linear range (μM)	—	0.001–1.2	30
	LOD (nM)	—	3.3
PdNPs/GO/GCE	Linear range (μM)	—	0.005–6	31
	LOD (nM)	—	1
Graphene/GCE	Linear range (μM)	—	0.1–2	32
	LOD (nM)	—	23.2
AgNPs/PMB-GR/Au electrode	Linear range (μM)	—	0.01–10	33
	LOD (nM)	—	10
β-CD-Au@PTCA-SWCNHs/GCE	Linear range (μM)	0.01–10	0.01–10	34
	LOD (nM)	3.8	4.4
Polyfurfural film/GCE	Linear range (μM)	0.05–10	0.001–10	This work
	LOD (nM)	10 ± 0.5	0.025 ± 0.004

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Selectivity of the polyfurfural film/GCE

Interferences arising from 100-fold concentrations of some organic compounds (such as citric acid, tartaric acid, cystine, glucose, dopamine, ascorbic acid and oxalic acid) and 1000-fold concentrations of some inorganic ions (such as PO₄³⁻, F⁻, NO₃⁻, SO₄²⁻, Cl⁻, Ac⁻, K⁺, NH₄⁺, Na⁺, Mg²⁺, Ca²⁺ and Zn²⁺) were used to evaluate the selectivity of the polyfurfural film/GCE by SWV. In Table 2, the results showed that the effects of various possible interferences for the simultaneous determination of myricetin and rutin were negligible, proving favorable selectivity of the proposed sensor.

Table 2 Effects of various interferences on the oxidation peak current signal of 10 μmol dm⁻³ myricetin and rutin at the polyfurfural film/GCE in BR (pH 6) buffer solution containing 0.5% ethanol (α = 0.05, n = 3)

Interference	Concentration (mol L^−1)	Signal change (%)
		Myricetin	Rutin
K3PO4	0.01	−0.87 ± 0.12	3.86 ± 0.05
NH4F	0.01	0.33 ± 0.08	−0.88 ± 0.04
NaNO3	0.01	−2.86 ± 0.27	1.81 ± 0.16
MgSO4	0.01	2.10 ± 0.10	4.32 ± 0.09
CaCl2	0.01	1.08 ± 0.11	3.68 ± 0.19
Zn(Ac)2	0.01	3.00 ± 0.56	2.96 ± 0.41
Citric acid	0.001	−1.84 ± 0.47	−3.10 ± 0.54
Tartaric acid	0.001	−0.54 ± 0.04	−4.20 ± 0.37
Cystine	0.001	−1.08 ± 0.22	−2.63 ± 0.30
Glucose	0.001	5.57 ± 0.72	−3.86 ± 0.22
Dopamine	0.001	4.84 ± 0.39	−5.38 ± 0.14
Ascorbic acid	0.001	−4.09 ± 0.41	5.01 ± 0.23
Oxalic acid	0.001	5.35 ± 0.68	5.19 ± 0.03

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Reproducibility and stability of the polyfurfural film/GCE

Reproducibility of the electrochemical sensors is an important factor in their practical application and development. In this work, five modified electrodes were fabricated to evaluate the sensor's reproducibility, the relative standard deviation (RSD) of the oxidation peak currents of myricetin and rutin were calculated to be 3.72 ± 0.32% and 2.64 ± 0.45% for myricetin and rutin (α = 0.05, n = 3), respectively, demonstrated that the proposed sensor possesses high reproducibility. In addition, the stability of the electrochemical sensor is another important factor in their practical application. After the polyfurfural film/GCE was stored at 4 °C in a refrigerator for 1 month, the oxidation peak currents of myricetin and rutin retained 97.64 ± 1.37% and 92.66 ± 2.40% of their original values respectively (α = 0.05, n = 3), suggested long-term stability and excellent sensor life. Therefore, from the results above, with excellent selectivity, reproducibility and stability, the polyfurfural film/GCE is promising for the simultaneous determination of myricetin and rutin.

Real samples detection

As active components, myricetin and rutin are often added into juice to make them possess healthy function. In order to evaluate the analytical reliability and application potential of the proposed sensor, orange juice samples were chosen as real samples for simultaneously quantitative determination of myricetin and rutin. Three parallel experiments were performed at each concentration by the standard-addition technique. As shown in Table 3, the results showed the recoveries for myricetin from the orange juice samples range from 99.20% to 106.00% and the RSD value was less than 6.80%, and the recoveries for rutin from the orange juice samples range from 100.20% to 105.00% and the RSD value was less than 3.52%, closeness of the results to 100.00% indicated that recovery of the sensor was favorable for real samples analysis and without significant interference from other components contained in the commercial orange juice samples. Therefore, the polyfurfural film/GCE can be successfully used for the simultaneously quantitative determination of myricetin and rutin in real samples.

Table 3 Simultaneous determination of myricetin and rutin in orange juice samples

	Added (μmol dm^−3)	Found^a (μmol dm^−3)	Recovery (%)	RSD (%)
a Sample responses are expressed as a confidence interval of 95% probability (n = 3).
Myricetin	5	5.30 ± 0.08	106.00 ± 1.60	5.85
	1	1.03 ± 0.05	103.00 ± 5.00	6.80
	0.25	0.248 ± 0.017	99.20 ± 6.80	2.82
	0.05	0.0506 ± 0.0022	101.20 ± 4.40	1.78
Rutin	5	5.01 ± 0.07	100.20 ± 1.40	0.60
	1	1.05 ± 0.02	105.00 ± 2.00	0.95
	0.25	0.256 ± 0.022	102.40 ± 8.80	3.52
	0.05	0.0502 ± 0.0037	100.40 ± 7.40	2.99

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Conclusions

In summary, we have demonstrated a high sensitivity electrochemical sensor for simultaneous determination of myricetin and rutin based on the polyfurfural film modified glassy carbon electrode. Compared with the reported electrochemical sensors in this field, the proposed sensor in this contribution can be prepared in a much easier way by a simple and controllable one-step electropolymerization process. It demonstrates a wide linear range of 0.05–10 μmol dm⁻³ for myricetin and 0.001–10 μmol dm⁻³ for rutin, and it also had a low detection limit of 10 ± 0.5 nmol dm⁻³ (S/N = 3, α = 0.05, n = 3) for myricetin and 0.025 ± 0.004 nmol dm⁻³ (S/N = 3, α = 0.05, n = 3) for rutin. Moreover, the proposed sensor exhibited excellent selectivity, reproducibility and stability, which successfully applied to simultaneous determine myricetin and rutin in real samples. Therefore, this study would provide a new and simple strategy for simultaneous determination of different kinds of polyphenolic compounds in application of real samples analysis.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant No. 21475046, 21427809) and State Key Laboratory of Pulp and Paper Engineering (201623) are gratefully acknowledged. We also acknowledge the Fundamental Research Funds for the Central Universities (No. 2015ZM050 and 2015ZP028).

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