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An electrochemical sensor based on [Ru(bpy)2dpp]2+/SMWCNTs/Au modified glassy carbon electrode for the detection of 5ʹ-GMP

Abstract

A sensitive electrochemical sensor for the selective detection of 5ʹ-guanylic acid (5ʹ-GMP) was prepared by combining sulfonated-multiwalled carbon nanotubes (SMWCNTs) and [Ru(bpy)2dpp]Cl2, which were dripped on the surface of a glass carbon electrode (GCE) immobilized with gold nanoparticles. The 5ʹ-GMP electrochemical biosensor was fabricated using [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE as working, Ag/AgCl as reference and Pt as auxiliary electrode connected by an electrochemical workstation. The modified electrode was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed the sensor’s response current had the best peak shape and maximum peak when the pH of electrolyte was 3, scan speed of CV was in the range of 100 to 180 mV/s, and the enrichment time was in the range of 200 to 300 s. Under the optimum conditions, a linear analytical curve was obtained for 5ʹ-GMP concentrations in the range of 0.01 to 0.5 mmol L−1, with a detection limit of 0.0014 mmol L−1. The analytical results of the 5ʹ-GMP sensor were exhibited good consistent with the data from liquid chromatography. The sensor has good reproducibility, long-term stability and strong immunity to interference, and may be a powerful device for 5ʹ-GMP detection, with great advantages such as simple preparation and operation, low equipment cost.

Introduction

The umami taste of broth is an important index to evaluate the quality of actual products. The important taste substances in the soup include amino acids and nucleotides, among which inosinic acid and guanosinic acid in nucleotides are important umami flavoring substances, which can give the broth a delicious taste. 5ʹ-GMP as one of the main sources of umami that is widely distributed in foods, especially in broths, braised brines and meat products, and is also a common food additive in food processing [1, 2]. 5ʹ-GMP consists of ribose, phosphate and guanine. It has a fresh taste and can act synergistically with glutamate to greatly improve the flavor of food [3]. As a representative nucleotide for umami, 5ʹ-GMP plays an important role in the overall flavor of foods, but it is difficult to detect by convenient techniques. Therefore, there is a need to establish a simple, accurate, and efficient method to detect and analyze 5ʹ-GMP in food products.

So far, the techniques used to detect 5ʹ-GMP are mainly ion chromatography, capillary electrophoresis and high-performance liquid chromatography [4,5,6]. Compared with traditional instrumental analytical techniques, electrochemical techniques offer attractive advantages such as higher sensitivity, faster response time, simpler instrumentation, and easier on-line detection without complicated pretreatment, expensive equipment, skilled personnel, and long analysis times. In recent years, electrochemical detection methods have been widely used for the detection and analysis of pesticides, amino acids, heavy metals and other chemical substances [7,8,9,10].

[Ru(bpy)2dpp]Cl2 is a polypyridyl ruthenium (II) complex with unique photophysical activity and excellent electrochemical properties [11]. Has been widely used in DNA structural probes, molecular optical switches and anti-cancer drugs [12, 13]. The ligand of dpp in [Ru(bpy)2dpp]Cl2 contains two uncoordinated N atoms and thus can be firmly attached to the surface of the glassy carbon electrode. [Ru(bpy)2dpp]Cl2 are excellent mediators for catalytic oxidation of guanine. 5ʹ-GMP oxidation is related to the sensitive substance and catalytically active compound of the modified electrode, and SMWCNTs can be effectively oxidized in the presence of polypyridyl ruthenium (II) complexes [14,15,16,17].

In this work, we constructed the [Ru(bpy)2dpp]2 + /SMWCNTs/Au/GCE sensor by coating Au/GCE with a coating made from a mixture of SMWCNT and [Ru(bpy)2dpp]Cl2. The electrochemical behavior of its detection of 5ʹ-GMP was characterized as well as the optimization of the effective experimental variables on the modified electrode. The experimental results demonstrate that the method is applicable to liquid samples with high accuracy and reliability and has potential applications for food quality control.

Experimental

Reagents and apparatus

Cis-Bis (2,2′-bipyridine) dichlororuthenium (II) hydrate (≥ 99.0%), SMWCNTs (2–5 nm of inner diameter, 10–30 μm of length), and N, N-Dimethylformamide (99.8%, DMF) were from Sigma-Aldrich Co. Ltd. (America). The standard substances such as 5ʹ-GMP, L-glutamic acid (L-Glu), aspartic acid (Asp), inosine 5ʹ-monophosphate (5ʹ-IMP), and adenosine 5ʹ-monophosphate (5ʹ-AMP) were from Shanghai Aladdin Co. Ltd. (Shanghai, China). The phosphate buffer solutions (PBS) were prepared by Na2HPO4·12H2O (AR) and C6H8O7·H2O (AR). Solutions of 5ʹ-GMP were prepared in PBS. Other chemical reagents are analytical grade pure reagents on the market.

Apparatus and measurements

All electrochemical experiments and electrochemical impedance spectroscopic studies were carried out by CHI660E Electrochemical Workstation from Shanghai CH Co. Ltd. (Shanghai, China). Electrochemical measurements were performed using a conventional three-electrode system consisting of KCl-saturated Ag/AgCl as the reference electrode, Pt wire as the counter electrode and a modified GCE of 3 mm diameters as the working electrode. Separation was performed using a high-performance liquid chromatograph (HPLC) (Agilent 1260, USA) with a binary mobile phase and a column (Tosoh ODS-80TM, Japan) with gradient elution.

Construction of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE modified electrode

To prepare [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE, the GCE surface was polished with 1 μm, 0.3 μm, and 0.05 μm alumina slurries on a polishing cloth to prior to immobilization. The polished electrodes were cleaned in ethanol (75%) to remove adsorbed particles and then activated in a solution of 0.5 mol L−1 H2SO4 using the CV method. During activation, the potential range was -1 to 1 V and the scan rate was 100 mV s−1. The activation of the electrode was carried out until a stable and reproducible CV curve appeared [18]. After activation, the electrode was removed, rinsed with deionized water and blown dry with N2 gas.

The pretreated bare GCE was scanned in 5 mmol L−1 chloroauric acid solution using the CV method. Parameters were set as follows: the scan potential range was -0.2 to 0.5 V, the scan rate was 50 mV s−1, and the scan segments was 20. Removed the GCE and rinsed repeatedly with deionized water. The electrode of Au/GCE was obtained until its surface appeared rose-red [19].

The preparation of [Ru(bpy)2dpp]Cl2 was known as Bhuiyan's method [20]. 0.2 mmol of 4,7-diphenyl-1,10-phenanthroline and 0.2 mmol of Ru(bpy)2Cl2 were dissolved in 30 mL of aqueous 75% ethanol. The mixed solution was heated to reflux at 85 ℃ for 0.5 h, cooled to room temperature, and then mixed with the 100 to 200 mesh pure silica gel powder. The solvent of the mixture was evaporated by rotary evaporator at 85 ℃ until the blood-red silica gel powder was obtained. The silica gel powder with the color of blood-red color was extracted by chromatography. After further separation and purification, orange [Ru(bpy)2dpp]Cl2 solid was obtained.

DMF was used as a dispersant to dissolve [Ru(bpy)2dpp]Cl2 and SMWCNTs in a mass ratio of 4:25. 10 μL of the mixed solution was dropped on the surface of Au/GCE to make an electrode of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE.

Testing and optimization of electrochemical sensor

To test the electrochemical sensor, experimental conditions such as the immobilization method of gold nanoparticles, pH of electrolyte, CV scan rate and enrichment time were optimized using a completely randomized design approach.

The CV method was used to detect 5ʹ-GMP with the following parameter settings: Einit was 0.6 V, Ehigh was 1.5 V, Elow was 0.6 V, Efinal was 1.5 V, initial scan polarity was positive, scan rate was 100 mV s−1, sensitivity was 1.0 × 10–4 A and scan segments was 4. Simultaneously, the preconditioned potential and time were set to 0.6 V and 300 s, respectively. The content of 5ʹ-GMP in the sample was determined by substitution method. Electrochemical impedance spectroscopy tests were conducted at steady-state potentials.

CV data were analyzed by Origin software. The 5ʹ-GMP oxidation peak current values were collected and linearly fitted to observe the different responses of the 5ʹ-GMP sensor to different solutions. Finally, the CV plots and the fitted plots were combined into one image for a more visual observation.

Determination of 5ʹ-GMP in broth by electrochemical sensor

The broth sample was provided by Shandong Dezhou Braised Chicken Co., LTD. The sauce marinated broth used for the actual sample assay was obtained from three different cooking pots. The broth was filtered through 200 mesh gauze and centrifuged with ethyl acetate (V: V = 1:1) at 10,000 r/min for 10 min at 4 °C. The supernatant was collected and adjusted to pH = 3 with HCl. All measurements were repeated three times at room temperature of 25 °C. To determine the 5ʹ-GMP in the sauce marinated broth, the same procedure as for sensor response measurements was used under optimal working conditions.

Results

Surface characterization of modified electrodes

The morphology of Au NPs, SMWCNTs and Au NPs/SMWCNTs was characterized by SEM technique. In Fig. 1A, the SEM images show that each Au NPs had a three-dimensional structure and well-distributed state, which not only maintains its large specific surface area and structural advantages, but also has good electrical properties [21]. Figure 1B shows the SEM images obtained for SMWCNTs powder that were entangled and interconnected in a mesh-like porous structure [22]. And as presented in Fig. 1C, a lot of Au NPs are tightly and dispersedly decorated on the SMWCNTs surface without aggregation status, which clearly indicates that Au NPs/SMWCNTs nanocomposite have been obtained and can be a good platform for sensing applications [23]. Figure 1D showed the XRD patterns for Au, Au/SMWCNTs and [Ru(bpy)2dpp]2+/SMWCNTs/Au. The XRD of Au exhibited diffraction peak at 24.8°and 37.62°, and a new peak of Au/SMWCNTs weas observed at 44.36°. The peak at 24.8 was assigned and enhanced the 64.52 and 77.58 diffraction peak in [Ru(bpy)2dpp]2+/SMWCNTs/Au. The results imply that the successful introduction of the Au/SMWCNTs and the synthesis of [Ru(bpy)2dpp]2+/SMWCNTs/Au nanocomposite.

Fig. 1
figure 1

SEM images obtained for A Au NPs, B SMWCNTs powder and C Au NPs/SMWCNTs. DXRD patterns of [Ru(bpy)2dpp]2+/SMWCNTs/Au

CV and EIS characterizations of modified electrodes

CV measurements were performed to assess the electrochemical behavior of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE on 5ʹ-GMP solutions. The voltametric behavior of bare GCE, Au/GCE and [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in the absence of 0.1 mmol L−1 5ʹ-GMP is shown in Fig. 2. As shown in Fig. 2, these three CV curves were obtained from three parallel experimental studies with deviation values of the upper and lower oxidation peak currents ranging from 0 to 11 μA. The response current of bare GCE in the presence of 5ʹ-GMP was 0 V. The response current of Au/GCE showed a weak oxidation peak corresponding to the oxidation reaction of 5ʹ-GMP with a peak potential of 1.2 V. The response current of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE exhibited two well-characteristic oxidation peaks with peak potentials of 0.85 V and 1.25 V, respectively. The oxidation peak at 0.85 V corresponded to the reaction of [Ru(bpy)2dpp]2+ →[Ru(bpy)2dpp]3+  + e and the oxidation peak at 1.25 V corresponded to the reaction of [Ru(bpy)2dpp]3+  + 5ʹ-GMP →[Ru(bpy)2dpp]2+  + 5ʹ-GMPOX. The appearance of each oxidation peak in the figure was consistent with the 5ʹ-GMP oxidation law [24]. Previous studies have shown that 5ʹ-GMP is most likely to be oxidized in four nucleotides (adenine nucleotides, guanylate, cytidylate, and thymidine-5ʹ-monophosphoric) [25, 26]. In addition, [Ru(bpy)2dpp]2+ could efficiently and specifically oxidize 5ʹ-GMP in the presence of SMWCNTs [27]. The electrocatalytic kinetics of [Ru(bpy)2dpp]2+ on 5ʹ-GMP was strongly dependent on the presence of SMWCNTs. The reaction equation is shown in Fig. 3. The redox of 5ʹ-GMP on [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE made by a simple method of cyclic voltametric deposition and drop coating produces a highly reversible redox peak that is very sensitive to changes in 5ʹ-GMP independent of other interfering substances. The activity of the catalytic oxidation reaction decreased with the reduction of [Ru(bpy)2dpp]Cl2 molecules, resulting in a significant decrease in the 5ʹ-GMP oxidation peak current [28].

Fig. 2
figure 2

CV curves of GCE, Au/GCE, and [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in the solution of 0.1 mmol L−1 5ʹ-GMP. Parameters: v = 100 mV s−1, Erange = from 0.6 to 1.5 V, the sensitivity was 1e−4 A, and the sweep segments were 4. Simultaneously, the precondition potential and time was set to 0.6 V, 300 s, respectively

Fig. 3
figure 3

The equation of the oxidation of [Ru(bpy)2dpp]2+ and 5ʹ-GMP

The immobilization method of gold nanoparticles affects the sensitivity and accuracy of the electrode in the preparation of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE. The loose connection between gold nanoparticles and bare GCE surface would directly lead to the decrease of sensor sensitivity. Two Au/GCEs obtained by two different immobilization methods (cyclic voltametric deposition and drop coating methods) were tested using electrochemical impedance spectroscopy method. The Nyquist diagram of the impedance spectra includes a semicircle part and a linear part, with the former at higher frequencies corresponding to the electron transfer limited process and the latter at lower frequencies corresponding to the diffusion process. The radius of the semicircle part is positively correlated with the resistance of the working electrode [29, 30]. The Nyquist plots of Au/GCE for the two immobilization methods of gold nanoparticles are shown in Fig. 4. The results show that the immobilization method of cyclic voltammetry deposition has the smallest impedance arc and the lowest resistance. This phenomenon implied that Au/GCE obtained by cyclic voltammetry deposition method has lower resistance, stronger electrical conductivity and higher sensitivity.

Fig. 4
figure 4

The Nyquist plot of the impedance spectra of Au/GCE obtained by two different fixation methods (the methods of cyclic voltammetry deposition and drip-coating). Parameters: Einit = 0.255 V, high frequency = 1e+5 Hz, low frequency = 0.01 Hz, amplitude = 0.005 V

Optimization experiment

Optimization analysis of the pH of electrolyte

The pH of the electrolyte plays an important role in the proton transfer at the electrode-solution interface and can change the adsorption phenomenon and kinetics of charge transfer process on the electrode surface. Therefore, the effect of various PBS with pH ranging from 3 to 9 on the redox process of 5ʹ-GMP (0.1 mmol L−1) on [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE electrode was investigated. Disodium hydrogen phosphate and citric acid buffer solutions with 7 different pH values of 0.1 mmol/L 5ʹ-GMP standard solution were used as the electrolyte for detection by cyclic voltammetry.

As shown in Fig. 5, the oxidation peak current of [Ru(bpy)2dpp]2+ →[Ru(bpy)2dpp]3+  + e remained constant with pH. On the contrary, the oxidation peak current of 5ʹ-GMP decreased significantly with increasing pH, which was consistent with studies related to the greater susceptibility of 5ʹ-GMP to oxidation under acidic conditions [31]. The oxidation peak current of 5ʹ-GMP was maximum at pH 3, which is consistent with the choline monolayer carrier and gold nanocavity functionalized carbon nanotube sensing interface (pH 4) and carboxylated multi-walled carbon nanotubes/AuNPs modified glassy carbon electrode (pH 3) in comparison [32, 33].

Fig. 5
figure 5

CV curves of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in the solution of 0.1 mmol L−1 5ʹ-GMP with different pH (in the range of 3 to 9). Parameters: v = 100 mV s−1, Erange = from 0.6 to 1.5 V, precondition potential = 0.6 V, precondition time = 300 s

Consequently, PBS at pH 3 was chosen as the most suitable electrolyte in this experiment to generate a larger response oxidation peak current of 5ʹ-GMP.

Optimization analysis of the scan rate of CV

The effect of scan rate on the oxidation peak current of 5ʹ-GMP was investigated to study the reaction kinetics of 5ʹ-GMP on [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE. The CV curves of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in the 0.1 mmol L−1 5ʹ-GMP standard solution at pH = 3.0, with a preconditioning time of 300 s and scanning rates ranging from 20 to 260 mV s−1 are shown in Fig. 6. According to the figure, the oxidation peak current of 5ʹ-GMP increased linearly with increasing concentration at scan rate from 20 to 260 mV s−1 (i (μA) = 1.1398v (mV s−1) + 26.311, R2 = 0.9905). The results indicated that 5ʹ-GMP and SMWCNT control the redox reaction mainly by adsorption under the catalytic effect of [Ru(bpy)2dpp]2+ [34]. The peak current shapes and values at each scan rate were compared with each other to clearly depict the CV curve of 5ʹ-GMP oxidation at the electrode. A scan rate of 100 to 180 mV s−1 was chosen as the most appropriate scan rate. The peak current of 5ʹ-GMP oxidation was weak when the scan rate was below 100 mV s−1, but the peak current of oxidation was significantly shifted when the scan rate exceeded 180 mV s−1.

Fig. 6
figure 6

CV curves of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in the solution of pH = 3 0.1 mmol L−1 5ʹ-GMP with different scan rate (in the range of 20 to 260 mV s−1). Parameters: Erange = from 0.6 to 1.5 V, precondition potential = 0.6 V, precondition time = 300 s

Optimization analysis of the precondition time

The oxidation peak current of 5ʹ-GMP was affected by the precondition time of 5ʹ-GMP on [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE. Molecules of 5ʹ-GMP accumulated on the electrode surface with sufficient pretreatment time but wasted time and too long precondition time wasted standard products [35]. Therefore, this experiment investigated the effect of pretreatment time of 5ʹ-GMP on the response peak current using the CV method. The CV curves of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in 0.1 mmol L−1 5ʹ-GMP solution at pH = 3.0 with pretreatment time from 0 to 400 s and scan rate of 100 mV s−1 are shown in Fig. 7. When the pretreatment time was 0 s, the response current of [Ru(bpy)2dpp]2+ to [Ru(bpy)2dpp]3+ transition was 0 μA and the 5ʹ-GMP oxidation response current was − 97.8 μA. The peak oxidation current of 5ʹ-GMP was positively correlated with the precondition time, but when the precondition time was in the range of 300 to 400 s, the peak oxidation current grew slowly. This phenomenon may be due to the dense aggregation of particles in the electrolyte on the electrode surface, and the long pretreatment time would increase the resistance of the electrode. In conclusion, for the 5ʹ-GMP assay, it is more appropriate to set the preconditioning time to 300 s. The response current for the transition from [Ru(bpy)2dpp]2+ to [Ru(bpy)2dpp]3+ was about − 120 μA at a preconditioning time of 300 s. The response current for 5ʹ-GMP oxidation also had a large value (− 387 μA) and a slight skew.

Fig. 7
figure 7

CV curves of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in the solution of pH = 3 0.1 mmol L−1 5ʹ-GMP with different preconcentration time (in the range of 0 to 400 s). Parameters: v = 100 mV s−1, Erange = from 0.6 to 1.5 V, precondition potential = 0.6 V

The linear range and detection limit of [Ru(bpy) 2 dpp] 2+ /SMWCNTs/Au/GCE

The detection performance of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE for 5ʹ-GMP was investigated under optimized conditions using the CV method. The inset in Fig. 8 highlights the linear response of the 5ʹ-GMP oxidation peak current with respect to the 5ʹ-GMP concentration. The linear equation was y = 0.6707x + 136.95, (R2 = 0.9914), where “x” represented the concentration of 5ʹ-GMP, and “y” represented the absolute value of the 5ʹ-GMP oxidation peak current. 5ʹ-GMP concentration could be determined by substituting the absolute value of peak current detected in the 5ʹ-GMP solution with the unknown concentration into the linear equation above. The linear range was from 0.01 to 0.5 mmol L−1 and the detection limit was 0.0014 mmol L−1.

Fig. 8
figure 8

CV curves of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in the solution of pH = 3 5ʹ-GMP with different concentration (in the range of 0.01 to 0.5 mmol L−1). Parameters: v = 100 mV s−1, Erange = from 0.6 to 1.5 V, precondition potential = 0.6 V, precondition time = 300 s

The detection of selectivity

To investigate the selectivity of the [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in more complex solution systems, interfering substances were selected according to 1) high levels of umami free amino acids in foods [36]. 2) substances with a structure similar to 5ʹ-GMP [37, 38]. Therefore, we investigated the effects of some umami components, especially L-Glutamic acid (L-Glu), aspartic acid (Asp), inosine 5ʹ-monophosphate (5ʹ-IMP) and adenosine 5ʹ-monophosphate (5ʹ-AMP) as possible interfering compounds [39, 40]. Interfering substances such as L-Glu, Asp, 5ʹ-IMP and 5ʹ-AMP were added sequentially to 0.2 mmol L−1 5ʹ-GMP were added sequentially to 0.2 mmol L−1 5ʹ-GMP solution, and the changes of 5ʹ-GMP oxidation peak currents are shown in Fig. 9. The rates of change of oxidation peak currents caused by L-Glu, Asp, 5ʹ-IMP and 5ʹ-AMP were 3.8%, 2.5%, 7.2% and 1.8%, respectively. The results showed that [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE had high selectivity and specificity for 5ʹ-GMP, and interfering substances such as L-Glu, Asp, 5ʹ-IMP and 5ʹ-AMP had no effect on the determination of 5ʹ-GMP.

Fig. 9
figure 9

Interference of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE by L-Glu, Asp, 5ʹ-IMP and 5ʹ-AMP

The detection of reproducibility and stability

Reproducibility and stability are two important markers for measuring the performance of electrochemical sensors [41]. To investigate the reproducibility and stability of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE, experiments were performed in a solution of 0.1 mmol/L 5ʹ-GMP. Three bare GCE were modified with gold nanoparticles, [Ru(bpy)2dpp]2+ and SMWCNTs to make three [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE electrodes. Perform multiple CV tests on 0.1 mmol/L 5ʹ-GMP, and the sensor should remain stable for 1 min after each data acquisition. Variations in the 5ʹ-GMP oxidation peak current and relative standard deviation were recorded and calculated. Table 1 indicates that the value of CV1 is between 0.6 and 1.6, and the value of CV2 is in the range of 0.74 to 1.92, indicating that the measurement performance difference between the three 5ʹ-GMP sensors is not obvious. Sensor 3 performed better than Sensors 1 and 2, and this difference might be due to differences in polish intensity of exposed GCE. The signal deviation of the 5ʹ-GMP sensor was within 5 times of detection. Therefore, tests within 5 times could be used as a standard for one test. The cause of signal attenuation might be that [Ru(bpy)2dpp]Cl2 partially shed from GCE during repeated detection with a catalytic oxidation reaction.

Table 1 Reproducibility test of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE in the solution of 0.1 mmol L−1 5ʹ-GMP

[Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE stability reference Nie’s method with some modifications [42]. [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCEs were placed at 4 ℃ for 15 d and detected at 5 d, 10 d and 15 d, respectively. Values of the oxidation peak current of 5ʹ-GMP were 96.3%, 93.1%, 89.7% of the electrode just made respectively. The decrease in the peak oxidation current of 5ʹ-GMP might be caused by the oxidation of [Ru(bpy)2dpp]2+ on the electrode surface [43]. Above results indicated that the good stability and reproducibility observed for the electrode of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE used for the quantitative detection of 5ʹ-GMP.

The detection of accuracy

Finally, in order to further prove the practicability and accuracy of present method, the 5ʹ-GMP concentration detection of three different cooking pot sauce soups was calculated by using the developed [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE, and the calibration curve was obtained by using HPLC as a reference method. The results of these two methods are shown in Table 2. The 5ʹ-GMP sensor determines the detection result values were well consistent with the data provided by HPLC. However, the results of 5ʹ-GMP sensor were generally higher than those of HPLC. The reason for this deviation might be that [Ru(bpy)2dpp]2+ caused an oxidation reaction of part of the guanine or guanosine on the electrode surface in the broth, thereby increasing the response current. From the results above, the electrochemical modified sensor has the potential to effectively determine 5ʹ-GMP in real samples. Comparison of the detection results with high performance liquid chromatography verified the practicality of the 5ʹ-GMP electrochemical sensor. Table 3 brings together a comparison between several other electrochemical sensors of 5’-GMP detection.

Table 2 The content of 5ʹ-GMP in different cooking pots was detected by two methods ([Ru(bpy)2dpp]2 + /SMWCNTs/Au/GCE by electrode and HPLC method)
Table 3 Performance comparison of [Ru(bpy)2dpp]2+/SMWCNTs/Au/GCE for 5ʹ-GMP detection with other electrochemical sensors

Availability of data and materials

The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.

Abbreviations

5ʹ-GMP:

5ʹ-Guanylic acid

SMWCNTs:

Sulfonated-multiwalled carbon nanotubes

GCE:

Glass carbon electrode

CV:

Cyclic voltammetry

EIS:

Electrochemical impedance spectroscopy

DMF:

N, N-Dimethylformamide

L-Glu:

L-Glutamic acid

Asp:

Aspartic acid

5ʹ-IMP:

Inosine 5ʹ-monophosphate

5ʹ-AMP:

Adenosine 5ʹ-monophosphate

PBS:

Phosphate buffer solutions

HPLC:

High-performance liquid chromatograph

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Acknowledgements

The authors thank the support of this work by Bohai University and Jilin University, and this work was supported by the Natural Science Foundation of Liaoning province [Grant Number 2019-MS-006].

Funding

Natural Science Foundation of Liaoning Province, 2019-MS-006, Dengyong Liu.

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YY analyzed the data and writing-reviewed and edited, SH performed the experiments, XW and DL conceived and designed the experiments, CY performed a grammar check. All authors read and approved the final manuscript.

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Correspondence to Dengyong Liu.

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Yu, Y., Huan, S., Wang, X. et al. An electrochemical sensor based on [Ru(bpy)2dpp]2+/SMWCNTs/Au modified glassy carbon electrode for the detection of 5ʹ-GMP. Appl Biol Chem 65, 51 (2022). https://doi.org/10.1186/s13765-022-00721-x

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Keywords

  • 5ʹ-Guanosine monophosphate
  • Polypyridyl ruthenium (II) complexes
  • Electrochemical sensor
  • Quantitative detection