A bimodal electrochemiluminescence method based on dual-enhancement Ru(bpy)32+/CQDs/AA system combined with magnetic field enhanced solid-phase microextraction for the direct determination of ascorbic acid
Graphical abstract
Introduction
Carbon quantum dots (CQDs), a new photoluminescent nanomaterial, was found accidentally by Xu et al. in the process of separating and purifying single walled carbon nanotubes by the method of arc discharge [1] for the first time in 2004. CQDs is expected as a powerful tool for biomedicine because of its exceptional characteristics including excellent chemical stability, hypotoxicity, satisfactory biocompatibility, small size, highly tunable emission and so on [[2], [3], [4]], which is widely applied in the various fields such as bioimaging, targeted drug delivery [5,6], photocatalysis [7] and biosensors [[8], [9], [10], [11], [12], [13], [14]]. In recent years, CQDs becomes a promising material for ECL co-reactant, which has received widespread attention [[15], [16], [17], [18], [19]].
Ascorbic acid (AA) is also known as Vitamin C, which is an indispensable water-soluble vitamin in the human body, and which plays an important role in regulating various redox metabolic reactions. AA naturally exists in various agricultural products such as vegetables and fruits, and cannot be produced directly by the human body itself. In addition, AA, as an antioxidant was widely used in pharmaceutical, food, biological and other fields, helps lower blood pressure in patients with hypertension, plays the important roles in preventing neurodegenerative diseases and cancer caused by free radicals, and can also effectively prevent common cold, scurvy and so on [[20], [21], [22]]. Researches found that deficiency of AA results in serious diseases such as anemia [23] and mental illness [24], but excessive intake of AA may cause diarrhea, urinary stones and stomach cramps [25]. Therefore, the development of an accurate and sensitive method for the detection of AA is of great significance for food security and medical detection [26]. Up to now, various traditional methods for AA detection have been designed, including high performance liquid chromatography [[27], [28], [29]], colorimetry [30,31], fluorescence spectroscopy [32] and electrophoresis [33]. However, these detection methods are limited to some extent due to the shortcomings of expensive equipment, complicated technology and consuming time. As we all know, the electrochemiluminescence (ECL) method has attracted wide attention because of its advantages such as high sensitivity, fast response, low cost for equipment and simple operation, which is an essential method for detecting AA in real samples.
Electrochemiluminescence (ECL), as a practical analytical technique, combines the advantages of electrochemical and chemiluminescence methods including high sensitivity and low background, which has been used widely in various research fields. ECL analysis generally uses a three-electrode system, which applies a certain voltage to the chemical system containing the luminophore through the working electrode, causing that chemical substances gain or lose electrons in the solution. And the excited state of the luminophor is formed during the electrochemical reactions [34]. When the luminophor returns from the excited state to the ground state, the energy is released with the form of light. The production of ECL is mainly explained by two reaction mechanisms. One is the ion annihilation reaction mechanism at the double-step alternating positive and negative pulse potentials [35]. When the alternating potentials are applied to the electrode surface, the electroactive materials are oxidized at the anode and reduced at the cathode to obtain radical cations and radical anions, respectively. Above two free radical ions react with each other to produce the excited state and emit ECL. The other is co-reactant electrochemiluminescence reaction mechanism. ECL emission is generated by the introduction of the co-reactant in the presence of the electroluminescent reagent under the unidirectional potential. The co-reactant can become the intermediate species with the strong reducibility or oxidizability when the oxidation or reduction reaction occurs. And the produced intermediate species unceasingly react with the electroluminescent agent to engender the excited state molecule. Therefore, the selection of efficient co-reactants is extremely necessary for improving the performance of ECL. Tris(2,2′-bipyridine)dichlororuthenium(II) (Ru(bpy)32+) as a typical ECL emitter has been extensively applied in highly sensitive ECL sensors [[36], [37], [38]] because of its good electrochemical stability, satisfactory luminescent efficiency and favorable sensitivity [39,40]. Although Tripropylamine (TPA) is a typical co-reactant of Ru(bpy)32+, which is limited in the application of sensors due to the volatility, corrosivity and toxicity [41]. Therefore, there is an urgent need to exploit a new alternative co-reactant which is expected to be low-cost, environmentally friendly and nontoxic. The low-toxicity CQDs is preferred [42,43]. For instance, CQDs was reported for the first time by Long et al. [19] as an efficient and nontoxic co-reactant for Ru(bpy)32+. Li et al. [44] had also demonstrated that nitrogen-doped carbon dots (N-CDs) could effectively enhance the ECL intensity. And Cola et al. utilized amine-rich N-CDs successfully as a novel co-reactant to facilitate ECL emission [41]. Most researches about ECL systems were based on the single signal of ECL, which would lead to false positive analysis results [45] because of the interference by external factors. In order to improve the reliability of the analysis results, the ratio ECL sensor is proposed using a linear relationship between the ratio of dual ECL signals and the concentration of the target analyte. Xu et al. [46], Hu et al. [47] and Wang et al. [35] had constructed the ratio ECL sensor using two ECL emitters or two co-reactants. However, the bimodal ECL system with single ECL emitter and single co-reactant can reduce environmental pollution and save experimental costs, which is very necessary for the development of ECL sensors.
And how to extract AA from agricultural products or food is also a key technical issue which has been widely concerned. Extraction is widely used to extract heavy metal ion and organic compounds [48] in view of its excellent efficiency and simple process. Recently, magnetic field enhanced solid-phase microextraction (MFE-SPME), as a sample pretreatment technique, has attracted the attention of researchers [49], which is based on the target analytes being extracted by the magnetic adsorbent dispersed in the sample and separated by the external magnet. This technique not only has a fast and simple separation process, but also can improve extraction efficiency to a certain extent based on the differences of the magnetic susceptibility between the target analyte and the medium where it is dispersed [50,51]. In order to obtain higher extraction performance, the promising magnetic adsorbents are usually prepared by combining magnetic materials and adsorbent materials. As we all know, chitosan (CS) is a naturally-occurring biopolymer with abundant hydroxyl and amino groups in its molecule, which is an excellent adsorbent for heavy metal ions in aqueous solution and drug molecules in the environment. Graphene oxide (GO) is proven to be a satisfactory adsorbent [52,53] because it has a relatively large specific surface area and rich oxygen-containing groups including carboxyl, epoxy and hydroxyl groups [[54], [55], [56], [57]]. However, as adsorbents, CS and GO have the drawbacks with the poor solubility and easy agglomeration, respectively, when they exist alone. Therefore, the magnetic composite material is prepared by combining CS, GO and Fe3O4, which will reduce the aggregation of GO, increase the solubility of CS and greatly improve extraction performance compared with each component. Since AA is a water-soluble target analyte, it can be successfully extracted by the magnetic adsorbent Fe3O4@CS@GO. According to the above introduction, it will be widely concerned if the bimodal ECL method utilizing CQDs as a co-reactant combines with MFE-SPME.
In this study, the magnetic adsorbent of Fe3O4@CS@GO was prepared to directly extract and separate AA under the external magnetic field from the magnetic glassy carbon electrode (MGCE) and the method of bimodal ECL in dual-enhancement Ru(bpy)32+/CQDs/AA system combined with MFE-SPME was designed for the direct determination of AA. When the potential was cyclically scanned between −3.5 and 2.0 V, the dual ECL signals were generated in the Ru(bpy)32+/CQDs/AA system based on CQDs as the co-reactant. As AA concentration increased, the bimodal ECL intensities were improved correspondingly. Therefore, this method can be developed to accurately detect AA.
Section snippets
Reagents and materials
All reagents used in the experiments were of analytical grade and were not further purified. Sodium hydroxide, aqueous ammonia (28% ~ 30%), hydrochloric acid and sodium acetate anhydrous were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tris(2,2′-bipyridine)dichlororuthenium(II) hexahydrate was provided by ADA Reagent Co., Ltd. Ethanol, chitosan, graphene oxide, (±)-Epichlorohydrin, iron chloride hexahydrate (FeCl3·6H2O), ascorbic acid (AA), β-Alanine, iminodiacetic
Comparison of ECL intensity of different CQDs
Three different CQDs were prepared by hydrothermal method using β-Alanine, iminodiacetic acid and N,N-Dimethylglycine as three different raw materials. The ECL intensities of the above three CQDs were studied, as shown in Fig. 1. It could be observed that the ECL intensity of the CQDs synthesized from N, N-Dimethylglycine as raw material was the strongest and thus the above CQDs was selected as co-reactant for Ru(bpy)32+ in all tests and characterizations.
Characterization of the CQDs
The size and morphology of the CQDs
Conclusions
A method was proposed by combining bimodal ECL of dual-enhancement Ru(bpy)32+/CQDs/AA system and MFE-SPME for detecting AA. The magnetic adsorbent of Fe3O4@CS@GO under the external magnetic field from the MGCE could efficiently extract AA and the extraction efficiency could be improved to 89.2%. When the potential was cyclically scanned between −3.5 and 2.0 V, the bimodal ECL emissions were produced in the Ru(bpy)32+/CQDs/AA system based on CQDs as the co-reactant for Ru(bpy)32+. When AA
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Financial support from the National Natural Science Foundation of China (41576098, 81773483), the Science and Technology Department of Zhejiang Province of China (2016C33176, LGF18B070002), Natural Science Foundation of Ningbo (2017A610231, 2017A610228) and State Key Laboratory for Quality and Safety of Agro-products (ZS20190101) are gratefully acknowledged. This research was also sponsored by K.C. Wong Magna Fund in Ningbo University.
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