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“OFF-ON” sensor for detecting heparin based on Hg2+-quenching of photoluminescence nitrogen-rich polymer carbon nanoribbons
Sensors and Actuators B 242 (2017) 412–417
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
“OFF-ON” sensor for detecting heparin based on Hg2+ -quenching of photoluminescence nitrogen-rich polymer carbon nanoribbons Zhong-Xia Wang a , Fen-Ying Kong a , Wen-Juan Wang a , Rui Zhang a , Wei-Xin Lv a , Xian-He Yu a , Hong-Cheng Pan b , Wei Wang a,∗ a
School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, PR China Guangxi Key Laboratory of Electrochemistry and Magnetiochemistry Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, 12 Jiangan Road, Guilin 541004, PR China b
a r t i c l e
i n f o
Article history: Received 5 August 2016 Received in revised form 14 November 2016 Accepted 14 November 2016 Available online 15 November 2016 Keywords: Carbon nanoribbons “OFF-ON” sensor Photoluminescence Heparin detection Anion-cation interactions
a b s t r a c t A new strategy for the detection of heparin is developed by utilizing photoluminescence nitrogen-rich polymer carbon nanoribbons (NRCNRs) and Hg2+ ion. The emission of NRCNRs is found to be quenched in the presence of Hg2+ ion by energy and electron transfer. Upon addition of the polyanionic heparin, the quencher (Hg2+ ion) has been removed from the surface of NRCNRs owing to the stronger anion-cation interactions between heparin and Hg2+ ion, which leads to significant photoluminescence recovery of NRCNRs, allowing analysis of heparin in a very simple method. By employing this sensor, excellent linear relationship exists between the recovery degree of the NRCNRs and the concentration of heparin in the range of 0.05–50 g mL−1 . The detection limit for heparin is 0.013 g mL−1 , which is comparable to other sensors. The simultaneous possession of high sensitivity and selectivity, convenience, rapidity, and visualization could enable this sensor to be potentially applicable for ultrasensitive and rapid on-site detection toward trace heparin. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Heparin, an anionic rodlike and unbranched polydispersed polysaccharide (Fig. 1a), has an average of 70 negative charges per one molecule [1,2]. It is commercially used as an anticoagulant in surgical procedures for the prevention of blood clotting [3,4]. However, overdose and prolonged use of heparin often induce potentially fatal bleeding complication such as hemorrhages and thrombocytopenia [3,5]. So it is of crucial importance to monitor closely heparin levels for the sake of health. Up to date, various traditional techniques and methods, including activated partial thromboplastin time, activated clotting time, potentiometric assays and ion pair high performance liquid chromatography, have been developed to detect the heparin [6–9]. However, most of these methods show some disadvantages, which include timeconsuming analysis, complicated procedures, and the lack of specificity or potential interference from other factors. Recently, to overcome these drawbacks, various sensor platform systems for the detection of heparin have been developed, including colorimet-
∗ Corresponding author. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.snb.2016.11.075 0925-4005/© 2016 Elsevier B.V. All rights reserved.
ric, photoluminescence (PL) and electrochemical methods [10–14]. Among these methods, the PL sensors for heparin detection have attracted more attention due to its high-accuracy, sensitivity, lowcost, operational simplicity, fast analysis, and being nonsample destructing or less cell damaging. PL carbon-based nanomaterials (PCMs) are getting more and more attention because of their wide applications in daily life and specialized fields such as bioimaging, catalysis, sensors, and photoelectronics. As a consequence of their special surface state and unique physical and chemistry properties, PCMs are at the center of significant research efforts in the development of alternatives that have both the desirable optical characteristics and optical applications of PCMs. However, to date, nearly all intension on PCMs has focused on their synthesis, the investigation of their applications for determination biomolecule is still in its initial stages. Now, to the best of our knowledge, only a few approaches have been reported that can be used to detect heparin by PCMs sensing platforms [13,15]. Herein, we present the development of PL sensor for determination of heparin with the use of nitrogen-rich carbon nanoribbons (NRCNRs) as an effective sensing platform. The addition of trace amount of Hg2+ ion could effectively quench the PL of the NRCNRs probe by specific affinity between Hg2+ ion and oxygen/nitrogen
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Fig. 1. Molecular structure of heparin (a) and schematic diagram of the mechanism of the detection of heparin by the photoluminescence NRCNRs (b).
elements of the surface of NRCNRs. And in the presence of heparin, Hg2+ ion preferred to bind to heparin instead of NRCNRs due to the stronger electrostatic attraction of heparin to Hg2+ ion, and the PL of the NRCNRs could be restored, which allow repeated detection of Hg2+ ion and heparin. The principle of the proposed heparin sensing concept is shown in Fig. 1b. To the best of our knowledge, this is a representative example of the construction of PL sensing platform for heparin based on highly efficient NRCNRs.
suspension solution by sonication. Then, 25 mL of as-prepared precursor solution was transferred into an autoclave and heated at 180 ◦ C for 4.5 h, and hereafter cooled to room temperature naturally. Next, the obtained bright yellow solution was extracted with dichloromethane, and the water phase solution was allowed to place in the refrigerator for about one week to remove all large NRCNRs. Finally, the bright yellow solution was centrifuged at 8000 rpm for 15 min, and stored at 4 ◦ C.
2. Experimental
2.4. Optimizing experimental conditions
2.1. Reagents and chemicals
To obtain a highly sensitive response for the detection of heparin, the optimization of the conditions, including the pH value and concentration of Hg2+ ion, were carried out in our experiment. That is, 5 L NRCNRs solution (∼2 mg mL−1 ) and 50 L Hg2+ ion (1 mM) were reacted for 10 min in different pH values of 50 L HEPES (50 mM) buffer solutions, then, 25 L heparin solution (1.0 mg mL−1 ) were added to the above mixture solution, and the final volume of the mixture was adjusted to 500 L with double distilled water. The resulting solutions were studied by PL spectroscopy at room temperature with excitation at 355 nm, both the excitation and emission slit widths were 3 nm. In the same way, the optimization of concentration of Hg2+ ion was carried out in HEPES buffer solution with pH 7.4.
Heparin sodium salt was obtained from Shanghai Solarbio Co. Ltd. Uric acid (UA), protamine (PTM), hyaluronic acid (HA), adeno-sine-5 -triphosphate disodium salt (ATP) and Nhydroxysuccinimide (NHS) were purchased from Sigma-Aldrich. Mercuric nitrate (Hg(NO3 )2 ) was purchased from Taixing Chemical Reagent Co. Ltd. (Jiangsu, China). Glucose oxidase (GOD) and bovine serum albumin (BSA) were purchased from Shanghai Sangon Biological Co. Ltd. (Shanghai, China). Glycine was obtained from Shanghai Sinopharm Chemical Reagent (Shanghai, China). 4-(2Hydroxyethyl)-1-piperazinee-thanesulfonic acid buffer solutions (HEPES, pH 6.0–8.5, 50 mM) were prepared by varying the ratio of HEPES to NaOH. All other reagents were of analytical grade and were used without further purification. All solutions were prepared in deionized water under ambient conditions. 2.2. Apparatus UV–vis spectra were performed on a Shimadzu UV-2550 spectrophotometer (Tokyo, Japan). The photoluminescence (PL) measurements and spectra were obtained on a Fluoromax-4 fluorescence spectrofluorometer (Horiba, USA). Transmission electron microscopy (TEM) images were obtained by a JEM-2100 transmission electron microscope (JEOL Ltd.). PL photographs were obtained from the Apple iPhone 6S.
2.5. Photoluminescence assay of heparin For the heparin sensing experiments, solutions of HEPES buffer (50 L, 50 mM, pH 7.4), NRCNRs (5 L, ∼2 mg mL−1 ), Hg2+ ion (50 L, 1 mM), and different amounts of a 1.0 mg mL−1 heparin solution were added in turn to a centrifuge tube (1.5 mL). Then, the mixture solution was diluted with deionized water to a final volume of 500 L and immediately mixed thoroughly on the vortex mixer. After equilibration for 20 min, the PL spectrum of the resulting solution was recorded on the PL spectrophotometer by excitation with 355 nm; both the excitation and emission slit widths were 3 nm. 2.6. Sensor selectivity investigation
2.3. Preparation of photoluminescence carbon nanoribbons PL nitrogen-rich carbon nanoribbons (NRCNRs) were synthesized from UA by a hydrothermal approach as described in the literature with little modification [16]. In brief, the precursor solution was prepared by mixing 1.1 g of UA, 25 mL of deionized water and 25 mL of ethanol, and resulting formation of a homogeneous
In the selectivity experiment, 5 L NRCNRs and 50 L Hg2+ ion (1 mM) were incubated for a few minutes in 50 mM HEPES buffer (pH 7.4). Next, a series of competitive molecules, including ATP, BSA, GOD, HA, PTM, NHS, and glycine were mixed with this solution and incubated at room temperature for another 5 min. The final volume of the mixture was adjusted to 500 L with double distilled
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Fig. 2. A) The UV–vis spectra of (a) heparin, (b) NRCNRs, (c) NRCNRs + Hg2+ ion, (d) NRCNRs + heparin, (e) NRCNRs + Hg2+ ion + heparin; B) PL spectra of (a) NRCNRs, (b) NRCNRs + Hg2+ ion, (c) NRCNRs + heparin, (d) NRCNRs + Hg2+ ion + heparin. Inset (from left to right): photographs of an aqueous solution of (a) NRCNRs, (b) NRCNRs + Hg2+ ion, (c) NRCNRs + heparin and (d) NRCNRs + Hg2+ ion + heparin taken under visible light and 365 nm UV light, respectively.
water, and the mixtures were equilibrated at room temperature for 30 min before the PL spectrum measurements were recorded. The concentration of Hg2+ ion was 100 M; the concentration of heparin was 50 g mL−1 ; and other interference molecules were 1 mg mL−1 . The resulting solutions were studied by PL spectroscopy at room temperature with excitation at 355 nm; both the excitation and emission slit widths were 3 nm. 3. Results and discussion 3.1. Sensing principle for heparin and characterization As heparin is a highly negatively charged oligosaccharide mainly composed of repeating disaccharide units of iduronic acid and glucosamine [2,10,12], and Hg2+ ion (4f14 5d10 ) possessing both cationic of two valences and empty orbits binding sites might be a potential receptor for heparin and other oligomeric anionic saccharides. [17–19] Accordingly, the polyanionic heparin could easily combined with Hg2+ ions through anion-cation interaction [20]. With reference to our previous work and other related studies (Fig. 2) [16,21], the emergence of such absorption peak changes at ∼330 nm and PL peak at 420 nm is originated from aromatic polyimides and amide intramolecular charge transfer states of NRCNRs transition [22,23]. That is, the binding of NRCNRs with Hg2+ ion causes a disappearance around 330 nm in its absorbance spectra (Fig. 2A, curves b, c) and decreasing at 420 nm in its PL spectra (Fig. 2B, curves a, b), and PL intensity change from strong blue to much low PL, but when the presence of polyanionic heparin in the mixture solution, a typical absorption peak located at ∼330 nm and emission peak at 420 nm of NRCNRs can again appear and increase in the UV–vis and PL spectra, respectively. Meanwhile, the UV–vis and PL spectra of NRCNRs have not any changes in the presence of heparin (Fig. 2A, curve d; Fig. 2B, curve c), indicating that heparin can not affect molecular structure of NRCNRs. On the other hand, it is well-known that Hg2+ ion is a strong Lewis acid and, therefore, has a strong affinity for oxygen and nitrogen donor atoms [24,25], as result of forming the stable metal complexes between Hg2+ ion and oxygen/nitrogen atoms of the surface of NRCNRs by Hg2+ -O or Hg2+ -N bonds, and leading to formed nonfluorescent Hg2+ -NRCNRs metal complexes by the electron or energy transfer mechanism [26,27], thereby decreasing its luminescence intensity. However, when adding stronger Hg2+ ion electrostatic binding agent, polyanionic heparin, Hg2+ ions are easily removed from the surface of NRCNRs by forming metal chelates with polyanionic heparin, which induces the PL recovery of NRCNRs (Fig. 2B, curves d). Meanwhile, the turn-on effect of polyanionic heparin on PL of NRCNRs can be easily observed visually (inset of Fig. 2B). Thus, these results demonstrate that Hg2+ ion possesses more strongly electro-
static binding reaction for polyanionic heparin than PL NRCNRs, and anion-cation interaction plays a dominant role for heparin detection. In short, these observations support our suggestion that the PL increasing should be ascribed to the repeated formation of NRCNRs dispersion as a result of the much stronger and specific interaction between Hg2+ ion and heparin, offering strong support to the proposed working mechanism shown in Fig. 1. To observe the morphology of the prepared NRCNRs and to identify the formation of the Hg2+ -NRCNRs nanocomposites and the resulting Hg2+ -NRCNRs nanocomposites in the presence of heparin, TEM was performed. TEM images of NRCNRs show that they are mainly distributed ribbon structure in the range of 100–120 nm length (Fig. 3A). However, when Hg2+ ions are introduced, large irregular NRCNRs aggregates are formed with almost unchanged NRCNRs morphology (Fig. 3B), and the aggregates formed are redispersed and the average length decreases to 100–120 nm on addition of heparin (Fig. 3C). That is, the surface of the NRCNRs possesses a large amount of nitrogen and oxygen elements [16], where they could act as a bridge for the induction of NRCNRs aggregation in the presence of Hg2+ ion, as a consequence, the PL of NRCNRs is quenched through the formation of Hg2+ -O or Hg2+ -N bonds (switched off, Fig. 2B), and meanwhile, which should have much stronger and specific interactions with heparin by stronger electrostatic adsorption, as a result of formation of the more stable metal compound between Hg2+ ion and heparin. Then, the aggregated NRCNRs may be re-dispersed after the introduction of heparin because Hg2+ ion displays a higher affinity for heparin than for the carboxylate/amino groups on the NRCNRs surface. In this case, the subsequent re-dispersion of NRCNRs results in the restoration of PL (switched on, Fig. 2C). This further confirm that the introduction of heparin disrupts the Hg2+ -induced NRCNRs aggregation because Hg2+ ion of positive charges and empty molecular orbital have a higher affinity for polyanionic heparin of negative charges. Based on the fact of the above discussions, the proposed “OFF-ON” PL process is verified to correspond to the evolution of aggregation and subsequent dispersion. 3.2. Sensor optimization It should be noted that this “OFF-ON” PL process is pHdependent because of the nature of the carboxylate and sulfate groups on the surface of the heparin [11]. The pH of the reaction solution could greatly affect the interaction between heparin and Hg2+ ion. Therefore, the pH of the reaction solution is an important parameter for the kinetic binding between Hg2+ ion and the heparin. As shown Fig. 4A, it can clearly be seen that the PL of NRCNRs recovery efficiency increased gradually as the pH increased from 6.0 to 7.4, then, the PL recovery efficiency decreased gradually as
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Fig. 3. TEM images of A) the NRCNRs, B) the Hg2+ -NRCNRs and C) the Hg2+ -NRCNRs in the presence of heparin.
Fig. 4. A) The PL intensity of the NRCNRs (20 g mL−1 ) in different pH values of HEPES buffer solution in the absence (a) or presence of Hg2+ ion (b) or Hg2+ and heparin (c). The concentration of Hg2+ ion and heparin is 100 M and 50 g mL−1 , respectively. B) PL quenching of the NRCNRs in the presence of different concentrations of Hg2+ ion.
the pH increased from 7.4 to 8.5. This can be explained by the fact that the carboxylate and sulfate groups on the surface of the heparin will be deprotonated in a basic solution and, therefore, the results will make them loading lots of negative charges on the heparin surfaces. This would mean that there would be stronger electrostatic interactions between the Hg2+ ion and the heparin at alkaline solution, meaning that heparin would restore the PL of the NRCNRs efficiently. On the other hand, Hg2+ ion can complex with OH− to form the insoluble hydrated oxide Hg(OH)2 under strong alkaline conditions, preventing coordination of Hg2+ ion to the heparin negative charges, leading to incomplete PL quenching and restoration due to the suppression of Hg2+ -NRCNRs aggregate formation. Considering the protonation-deprotonation of the heparin and the stable effect of the Hg2+ ion for pH values, pH 7.4 HEPES buffer was selected as the optimum solvent for sensor performance. The Hg2+ ion concentration affected not only the PL intensity but also the sensitivity of the assay. To obtain high sensitivity for heparin, the concentration of Hg2+ ion was optimized. Titration of Hg2+ ion into the NRCNRs solution results in PL quenching, which can be ascribed to the aggregation of NRCNRs as a result of the coordination of Hg2+ ion to the oxygen and nitrogen groups on the surfaces of the NRCNRs. Fig. 4B shows the PL quenching of NRCNRs at various concentrations of Hg2+ ion, and the PL intensity at 420 nm decreases as the concentration of Hg2+ ion increases. Nearly 90% of the PL is quenched by the addition of 100 M Hg2+ ions. And requiring a high Hg2+ ion concentration could be unfavorable because it will lead to a poorer detection limit for heparin. After taking the aspect into consideration, therefore, the concentration of 100 M of Hg2+ ions was selected as intermediate mediator for detection heparin. 3.3. Selectivity of the detection system Considering the promise of the Hg2+ -NRCNRs nanocomposites sensor system for application in biological and environmental fields, the selectivity of the PL sensor for heparin was evaluated. Under the optimal conditions, we tested the PL intensity changes of
Fig. 5. Selectivity of the PL assay for heparin over other biomolecules. The concentration of heparin was 50 g mL−1 and that of other biomolecules was 1.0 mg mL−1 . The excitation wavelength was 355 nm and the emission at 420 nm was monitored.
the Hg2+ -NRCNRs nanocomposites in the presence of competitive biomolecules under the same conditions, respectively, including ATP, BSA, GOD, HA, PTM, NHS and glycine, as shown in Fig. 5. It is seen that the much higher relative PL intensity (PL-PL0 )/PL0 was enhanced the most by heparin, and the presence of the other biomolecules, in contrast, caused the relative PL intensity to change very little (The PL intensities of the Hg2+ -NRCNRs in the absence and presence of other biomolecules are denoted by PL0 and PL, respectively), suggesting an excellent enhancing behavior of heparin to the PL of Hg2+ -NRCNRs nanocomposites. The Hg2+ /heparin metal complex formed because of stronger electrostatic attraction between the negatively charged heparin and the positively charged Hg2+ ion. And the heparin possesses many more negatively charged side groups than do other biomolecules [13], so it will have much stronger electrostatic interactions than other biomolecules with Hg2+ cation. Therefore, these observations suggest that the proposed method is capable of discriminating between heparin and the interference biomolecules.
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Fig. 6. A) PL emission spectra of Hg2+ -NRCNRs nanocomposites in the presence of different concentrations of heparin (0, 0.05, 0.1, 0.5, 1.0, 5.0, 10.0, 20.0, 30.0, 40.0, 50.0, 70.0, 100.0, 150.0, 200 g mL−1 , top to bottom, excitation at 355 nm). B) Plot of the enhanced PL signals [(PL0 -PL)/PL0 ] versus heparin concentration. Table 1 Comparison of the linear range and detect limit for heparin using different detection probes. Method
Linear range (g mL−1 )
Detect limit(g mL−1 )
reference
Positively-charged gold nanoparticles Gold nanorods and graphene oxide Adenosine-based molecu- lar beacons Our method
0.09–3.12 0.02–0.28 0.18–1.8 0.05–50
0.03 0.005 0.06 0.013
11 28 29 ∼
Table 2 Results of heparin determination in real samples. Sample a
1 2b
Standard value (g mL−1 )
Heparin founded (g mL−1 )
Recovery (%)(n = 5)
RSD (%)(n = 5)
20 50
18.96 54.78
94.80 109.56
4.67 3.56
The real samples were prepared to the linear range of 0.05–50 g mL−1 before determination. a Produced by Cisen Pharmaceutical Co., Ltd. b Produced by Yangtze River Pharmaceutical Group Co., Ltd.
3.4. Sensitivity of the detection system The PL response of the nanoprobe to heparin at varied concentrations was investigated under the optimized conditions (HEPES buffer of pH 7.4; Hg2+ ion: 100 M), as shown in Fig. 6. Fig. 6A showed the PL responses of Hg2+ -NRCNRs nanocomposites when different concentrations of heparin were added into the system, and sequential increases of the PL emission at 420 nm were observed with increasing amount of heparin. Upon adding heparin (75 g mL−1 ) to the Hg2+ -NRCNRs nanocomposites solution, the blue PL of the NRCNRs was nearly complete restoration, revealing that the sensing system is sensitive to heparin concentration. In the range of 0.05-50.0 g mL−1 , the increased PL of NRCNRs showed a good linear relationship to the concentration of heparin with a correlation coefficient of 0.9913 (n = 9) (Fig. 6B), and the detection limit for heparin calculated as 0.013 g mL−1 (3/k) was comparable to other sensors (Table 1) [11,28,29]. It is also worth noting that the PL restoring phenomena might be observed by the naked-eye under UV illumination (inset photograph in Fig. 2B), in which the disappear blue PL of Hg2+ -NRCNRs nanocomposites became strong with heparin added, confirming the recovery of Hg2+ -NRCNRs by heparin.
the measured recoveries were between 94.80 and 109.56% with less than 5.0% RSD under the optimal conditions, which indicates the Hg2+ -NRCNRs probe offers advantages of simplicity, speed, and precision for determining heparin in real sample analysis.
4. Conclusions In summary, a simple PL sensor was successfully developed for ultrasensitive detection of heparin based on Hg2+ ion quenching the PL of self-assembly NRCNRs. We took full advantage of the high binding affinity between NRCNRs and Hg2+ ion as well as stronger electrostatic interaction of Hg2+ ion with heparin, and therefore successfully attained the ultrasensitive detection of heparin with the detection limit of 13 ng mL−1 . Meanwhile, the PL sensor showed a number of attractive analytical features in the following terms: high sensitivity, simple operation process, rapid detection, good reproducibility and environmentally friendly. And under optimal conditions, this method displayed a wide linear range and good selectivity over other biomolecules. The proposed system has also been successfully applied for the determination of heparin in commercial heparin injection samples, further demonstrating its value in practical applications.
3.5. Real sample analysis The excellent specificity combined with high sensitivity and fast response of the Hg2+ -NRCNRs complex to heparin suggests that our method might be directly applied to detecting heparin in real samples. Therefore, the proposed method was applied to the analysis of heparin in two real samples of heparin injection by the standard addition method, and the results are shown in Table 2. From Table 2,
Acknowledgments We greatly appreciate the support of the National Natural Science Foundation of China (21575123, 21305122, 21675139, 21603184) and the Industry-University- Research Cooperative Innovation Foundation of Jiangsu Province (BY2015057-17).
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Biographies Zhong-Xia Wang obtained her Ph.D. degree in 2016 from Southeast University, China. She is a Lecturer in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. Her current fields of interest are photoluminescent carbon-based nanomaterials and bio-analytical chemistry. Fen-Ying Kong is a lecturer in Yancheng Institute of Technology, China. She received her Ph.D. degree in analytical chemistry from Nanjing University in 2012. Her research is focused on the preparation of new functional nanomaterial and electrochemical sensing application. Wen-Juan Wang obtained her Ph.D. degree in 2014 from Nanjing University of Science and Technology, China. She is a Lecturer in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. Her current fields of interest is electrocatalysis. Rui Zhang obtained her Ph.D. degree in 2016 from Southeast University, China. He is a Lecturer in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. His current fields of interest is electrocatalysis. Wei-Xin Lv obtained her Ph.D. degree in 2014 from Southeast University, China. She is a Lecturer in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. Her current fields of interest is electrocatalysis. Xian-He Yu received his B.S. degrees in Applied Chemistry from Nanjing Teach University, China in 2014. Now he is a joint training Master of Organic Chemistry in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. His research interests are in the areas of fluorescence probe and electrochemical analysis. Hong-Cheng Pan received his Ph.D. degrees in analytical chemistry from Nanjing University, China in 2008, Now he is a professor of Guilin University of Technology, China. His research interests are in the areas of optoelectronic nanomaterials and bio-analytical chemistry. Wei Wang received his B.S. and Ph.D. degrees in analytical chemistry from Nanjing University, China in 1991 and 2007, respectively. Now he is a professor of Yancheng Institute of Technology, China. His research interests are in the areas of microfluidics and electrochemical detection.
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“OFF-ON” sensor for detecting heparin based on Hg2+ -quenching of photoluminescence nitrogen-rich polymer carbon nanoribbons Zhong-Xia Wang a , Fen-Ying Kong a , Wen-Juan Wang a , Rui Zhang a , Wei-Xin Lv a , Xian-He Yu a , Hong-Cheng Pan b , Wei Wang a,∗ a
School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, PR China Guangxi Key Laboratory of Electrochemistry and Magnetiochemistry Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, 12 Jiangan Road, Guilin 541004, PR China b
a r t i c l e
i n f o
Article history: Received 5 August 2016 Received in revised form 14 November 2016 Accepted 14 November 2016 Available online 15 November 2016 Keywords: Carbon nanoribbons “OFF-ON” sensor Photoluminescence Heparin detection Anion-cation interactions
a b s t r a c t A new strategy for the detection of heparin is developed by utilizing photoluminescence nitrogen-rich polymer carbon nanoribbons (NRCNRs) and Hg2+ ion. The emission of NRCNRs is found to be quenched in the presence of Hg2+ ion by energy and electron transfer. Upon addition of the polyanionic heparin, the quencher (Hg2+ ion) has been removed from the surface of NRCNRs owing to the stronger anion-cation interactions between heparin and Hg2+ ion, which leads to significant photoluminescence recovery of NRCNRs, allowing analysis of heparin in a very simple method. By employing this sensor, excellent linear relationship exists between the recovery degree of the NRCNRs and the concentration of heparin in the range of 0.05–50 g mL−1 . The detection limit for heparin is 0.013 g mL−1 , which is comparable to other sensors. The simultaneous possession of high sensitivity and selectivity, convenience, rapidity, and visualization could enable this sensor to be potentially applicable for ultrasensitive and rapid on-site detection toward trace heparin. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Heparin, an anionic rodlike and unbranched polydispersed polysaccharide (Fig. 1a), has an average of 70 negative charges per one molecule [1,2]. It is commercially used as an anticoagulant in surgical procedures for the prevention of blood clotting [3,4]. However, overdose and prolonged use of heparin often induce potentially fatal bleeding complication such as hemorrhages and thrombocytopenia [3,5]. So it is of crucial importance to monitor closely heparin levels for the sake of health. Up to date, various traditional techniques and methods, including activated partial thromboplastin time, activated clotting time, potentiometric assays and ion pair high performance liquid chromatography, have been developed to detect the heparin [6–9]. However, most of these methods show some disadvantages, which include timeconsuming analysis, complicated procedures, and the lack of specificity or potential interference from other factors. Recently, to overcome these drawbacks, various sensor platform systems for the detection of heparin have been developed, including colorimet-
∗ Corresponding author. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.snb.2016.11.075 0925-4005/© 2016 Elsevier B.V. All rights reserved.
ric, photoluminescence (PL) and electrochemical methods [10–14]. Among these methods, the PL sensors for heparin detection have attracted more attention due to its high-accuracy, sensitivity, lowcost, operational simplicity, fast analysis, and being nonsample destructing or less cell damaging. PL carbon-based nanomaterials (PCMs) are getting more and more attention because of their wide applications in daily life and specialized fields such as bioimaging, catalysis, sensors, and photoelectronics. As a consequence of their special surface state and unique physical and chemistry properties, PCMs are at the center of significant research efforts in the development of alternatives that have both the desirable optical characteristics and optical applications of PCMs. However, to date, nearly all intension on PCMs has focused on their synthesis, the investigation of their applications for determination biomolecule is still in its initial stages. Now, to the best of our knowledge, only a few approaches have been reported that can be used to detect heparin by PCMs sensing platforms [13,15]. Herein, we present the development of PL sensor for determination of heparin with the use of nitrogen-rich carbon nanoribbons (NRCNRs) as an effective sensing platform. The addition of trace amount of Hg2+ ion could effectively quench the PL of the NRCNRs probe by specific affinity between Hg2+ ion and oxygen/nitrogen
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Fig. 1. Molecular structure of heparin (a) and schematic diagram of the mechanism of the detection of heparin by the photoluminescence NRCNRs (b).
elements of the surface of NRCNRs. And in the presence of heparin, Hg2+ ion preferred to bind to heparin instead of NRCNRs due to the stronger electrostatic attraction of heparin to Hg2+ ion, and the PL of the NRCNRs could be restored, which allow repeated detection of Hg2+ ion and heparin. The principle of the proposed heparin sensing concept is shown in Fig. 1b. To the best of our knowledge, this is a representative example of the construction of PL sensing platform for heparin based on highly efficient NRCNRs.
suspension solution by sonication. Then, 25 mL of as-prepared precursor solution was transferred into an autoclave and heated at 180 ◦ C for 4.5 h, and hereafter cooled to room temperature naturally. Next, the obtained bright yellow solution was extracted with dichloromethane, and the water phase solution was allowed to place in the refrigerator for about one week to remove all large NRCNRs. Finally, the bright yellow solution was centrifuged at 8000 rpm for 15 min, and stored at 4 ◦ C.
2. Experimental
2.4. Optimizing experimental conditions
2.1. Reagents and chemicals
To obtain a highly sensitive response for the detection of heparin, the optimization of the conditions, including the pH value and concentration of Hg2+ ion, were carried out in our experiment. That is, 5 L NRCNRs solution (∼2 mg mL−1 ) and 50 L Hg2+ ion (1 mM) were reacted for 10 min in different pH values of 50 L HEPES (50 mM) buffer solutions, then, 25 L heparin solution (1.0 mg mL−1 ) were added to the above mixture solution, and the final volume of the mixture was adjusted to 500 L with double distilled water. The resulting solutions were studied by PL spectroscopy at room temperature with excitation at 355 nm, both the excitation and emission slit widths were 3 nm. In the same way, the optimization of concentration of Hg2+ ion was carried out in HEPES buffer solution with pH 7.4.
Heparin sodium salt was obtained from Shanghai Solarbio Co. Ltd. Uric acid (UA), protamine (PTM), hyaluronic acid (HA), adeno-sine-5 -triphosphate disodium salt (ATP) and Nhydroxysuccinimide (NHS) were purchased from Sigma-Aldrich. Mercuric nitrate (Hg(NO3 )2 ) was purchased from Taixing Chemical Reagent Co. Ltd. (Jiangsu, China). Glucose oxidase (GOD) and bovine serum albumin (BSA) were purchased from Shanghai Sangon Biological Co. Ltd. (Shanghai, China). Glycine was obtained from Shanghai Sinopharm Chemical Reagent (Shanghai, China). 4-(2Hydroxyethyl)-1-piperazinee-thanesulfonic acid buffer solutions (HEPES, pH 6.0–8.5, 50 mM) were prepared by varying the ratio of HEPES to NaOH. All other reagents were of analytical grade and were used without further purification. All solutions were prepared in deionized water under ambient conditions. 2.2. Apparatus UV–vis spectra were performed on a Shimadzu UV-2550 spectrophotometer (Tokyo, Japan). The photoluminescence (PL) measurements and spectra were obtained on a Fluoromax-4 fluorescence spectrofluorometer (Horiba, USA). Transmission electron microscopy (TEM) images were obtained by a JEM-2100 transmission electron microscope (JEOL Ltd.). PL photographs were obtained from the Apple iPhone 6S.
2.5. Photoluminescence assay of heparin For the heparin sensing experiments, solutions of HEPES buffer (50 L, 50 mM, pH 7.4), NRCNRs (5 L, ∼2 mg mL−1 ), Hg2+ ion (50 L, 1 mM), and different amounts of a 1.0 mg mL−1 heparin solution were added in turn to a centrifuge tube (1.5 mL). Then, the mixture solution was diluted with deionized water to a final volume of 500 L and immediately mixed thoroughly on the vortex mixer. After equilibration for 20 min, the PL spectrum of the resulting solution was recorded on the PL spectrophotometer by excitation with 355 nm; both the excitation and emission slit widths were 3 nm. 2.6. Sensor selectivity investigation
2.3. Preparation of photoluminescence carbon nanoribbons PL nitrogen-rich carbon nanoribbons (NRCNRs) were synthesized from UA by a hydrothermal approach as described in the literature with little modification [16]. In brief, the precursor solution was prepared by mixing 1.1 g of UA, 25 mL of deionized water and 25 mL of ethanol, and resulting formation of a homogeneous
In the selectivity experiment, 5 L NRCNRs and 50 L Hg2+ ion (1 mM) were incubated for a few minutes in 50 mM HEPES buffer (pH 7.4). Next, a series of competitive molecules, including ATP, BSA, GOD, HA, PTM, NHS, and glycine were mixed with this solution and incubated at room temperature for another 5 min. The final volume of the mixture was adjusted to 500 L with double distilled
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Fig. 2. A) The UV–vis spectra of (a) heparin, (b) NRCNRs, (c) NRCNRs + Hg2+ ion, (d) NRCNRs + heparin, (e) NRCNRs + Hg2+ ion + heparin; B) PL spectra of (a) NRCNRs, (b) NRCNRs + Hg2+ ion, (c) NRCNRs + heparin, (d) NRCNRs + Hg2+ ion + heparin. Inset (from left to right): photographs of an aqueous solution of (a) NRCNRs, (b) NRCNRs + Hg2+ ion, (c) NRCNRs + heparin and (d) NRCNRs + Hg2+ ion + heparin taken under visible light and 365 nm UV light, respectively.
water, and the mixtures were equilibrated at room temperature for 30 min before the PL spectrum measurements were recorded. The concentration of Hg2+ ion was 100 M; the concentration of heparin was 50 g mL−1 ; and other interference molecules were 1 mg mL−1 . The resulting solutions were studied by PL spectroscopy at room temperature with excitation at 355 nm; both the excitation and emission slit widths were 3 nm. 3. Results and discussion 3.1. Sensing principle for heparin and characterization As heparin is a highly negatively charged oligosaccharide mainly composed of repeating disaccharide units of iduronic acid and glucosamine [2,10,12], and Hg2+ ion (4f14 5d10 ) possessing both cationic of two valences and empty orbits binding sites might be a potential receptor for heparin and other oligomeric anionic saccharides. [17–19] Accordingly, the polyanionic heparin could easily combined with Hg2+ ions through anion-cation interaction [20]. With reference to our previous work and other related studies (Fig. 2) [16,21], the emergence of such absorption peak changes at ∼330 nm and PL peak at 420 nm is originated from aromatic polyimides and amide intramolecular charge transfer states of NRCNRs transition [22,23]. That is, the binding of NRCNRs with Hg2+ ion causes a disappearance around 330 nm in its absorbance spectra (Fig. 2A, curves b, c) and decreasing at 420 nm in its PL spectra (Fig. 2B, curves a, b), and PL intensity change from strong blue to much low PL, but when the presence of polyanionic heparin in the mixture solution, a typical absorption peak located at ∼330 nm and emission peak at 420 nm of NRCNRs can again appear and increase in the UV–vis and PL spectra, respectively. Meanwhile, the UV–vis and PL spectra of NRCNRs have not any changes in the presence of heparin (Fig. 2A, curve d; Fig. 2B, curve c), indicating that heparin can not affect molecular structure of NRCNRs. On the other hand, it is well-known that Hg2+ ion is a strong Lewis acid and, therefore, has a strong affinity for oxygen and nitrogen donor atoms [24,25], as result of forming the stable metal complexes between Hg2+ ion and oxygen/nitrogen atoms of the surface of NRCNRs by Hg2+ -O or Hg2+ -N bonds, and leading to formed nonfluorescent Hg2+ -NRCNRs metal complexes by the electron or energy transfer mechanism [26,27], thereby decreasing its luminescence intensity. However, when adding stronger Hg2+ ion electrostatic binding agent, polyanionic heparin, Hg2+ ions are easily removed from the surface of NRCNRs by forming metal chelates with polyanionic heparin, which induces the PL recovery of NRCNRs (Fig. 2B, curves d). Meanwhile, the turn-on effect of polyanionic heparin on PL of NRCNRs can be easily observed visually (inset of Fig. 2B). Thus, these results demonstrate that Hg2+ ion possesses more strongly electro-
static binding reaction for polyanionic heparin than PL NRCNRs, and anion-cation interaction plays a dominant role for heparin detection. In short, these observations support our suggestion that the PL increasing should be ascribed to the repeated formation of NRCNRs dispersion as a result of the much stronger and specific interaction between Hg2+ ion and heparin, offering strong support to the proposed working mechanism shown in Fig. 1. To observe the morphology of the prepared NRCNRs and to identify the formation of the Hg2+ -NRCNRs nanocomposites and the resulting Hg2+ -NRCNRs nanocomposites in the presence of heparin, TEM was performed. TEM images of NRCNRs show that they are mainly distributed ribbon structure in the range of 100–120 nm length (Fig. 3A). However, when Hg2+ ions are introduced, large irregular NRCNRs aggregates are formed with almost unchanged NRCNRs morphology (Fig. 3B), and the aggregates formed are redispersed and the average length decreases to 100–120 nm on addition of heparin (Fig. 3C). That is, the surface of the NRCNRs possesses a large amount of nitrogen and oxygen elements [16], where they could act as a bridge for the induction of NRCNRs aggregation in the presence of Hg2+ ion, as a consequence, the PL of NRCNRs is quenched through the formation of Hg2+ -O or Hg2+ -N bonds (switched off, Fig. 2B), and meanwhile, which should have much stronger and specific interactions with heparin by stronger electrostatic adsorption, as a result of formation of the more stable metal compound between Hg2+ ion and heparin. Then, the aggregated NRCNRs may be re-dispersed after the introduction of heparin because Hg2+ ion displays a higher affinity for heparin than for the carboxylate/amino groups on the NRCNRs surface. In this case, the subsequent re-dispersion of NRCNRs results in the restoration of PL (switched on, Fig. 2C). This further confirm that the introduction of heparin disrupts the Hg2+ -induced NRCNRs aggregation because Hg2+ ion of positive charges and empty molecular orbital have a higher affinity for polyanionic heparin of negative charges. Based on the fact of the above discussions, the proposed “OFF-ON” PL process is verified to correspond to the evolution of aggregation and subsequent dispersion. 3.2. Sensor optimization It should be noted that this “OFF-ON” PL process is pHdependent because of the nature of the carboxylate and sulfate groups on the surface of the heparin [11]. The pH of the reaction solution could greatly affect the interaction between heparin and Hg2+ ion. Therefore, the pH of the reaction solution is an important parameter for the kinetic binding between Hg2+ ion and the heparin. As shown Fig. 4A, it can clearly be seen that the PL of NRCNRs recovery efficiency increased gradually as the pH increased from 6.0 to 7.4, then, the PL recovery efficiency decreased gradually as
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Fig. 3. TEM images of A) the NRCNRs, B) the Hg2+ -NRCNRs and C) the Hg2+ -NRCNRs in the presence of heparin.
Fig. 4. A) The PL intensity of the NRCNRs (20 g mL−1 ) in different pH values of HEPES buffer solution in the absence (a) or presence of Hg2+ ion (b) or Hg2+ and heparin (c). The concentration of Hg2+ ion and heparin is 100 M and 50 g mL−1 , respectively. B) PL quenching of the NRCNRs in the presence of different concentrations of Hg2+ ion.
the pH increased from 7.4 to 8.5. This can be explained by the fact that the carboxylate and sulfate groups on the surface of the heparin will be deprotonated in a basic solution and, therefore, the results will make them loading lots of negative charges on the heparin surfaces. This would mean that there would be stronger electrostatic interactions between the Hg2+ ion and the heparin at alkaline solution, meaning that heparin would restore the PL of the NRCNRs efficiently. On the other hand, Hg2+ ion can complex with OH− to form the insoluble hydrated oxide Hg(OH)2 under strong alkaline conditions, preventing coordination of Hg2+ ion to the heparin negative charges, leading to incomplete PL quenching and restoration due to the suppression of Hg2+ -NRCNRs aggregate formation. Considering the protonation-deprotonation of the heparin and the stable effect of the Hg2+ ion for pH values, pH 7.4 HEPES buffer was selected as the optimum solvent for sensor performance. The Hg2+ ion concentration affected not only the PL intensity but also the sensitivity of the assay. To obtain high sensitivity for heparin, the concentration of Hg2+ ion was optimized. Titration of Hg2+ ion into the NRCNRs solution results in PL quenching, which can be ascribed to the aggregation of NRCNRs as a result of the coordination of Hg2+ ion to the oxygen and nitrogen groups on the surfaces of the NRCNRs. Fig. 4B shows the PL quenching of NRCNRs at various concentrations of Hg2+ ion, and the PL intensity at 420 nm decreases as the concentration of Hg2+ ion increases. Nearly 90% of the PL is quenched by the addition of 100 M Hg2+ ions. And requiring a high Hg2+ ion concentration could be unfavorable because it will lead to a poorer detection limit for heparin. After taking the aspect into consideration, therefore, the concentration of 100 M of Hg2+ ions was selected as intermediate mediator for detection heparin. 3.3. Selectivity of the detection system Considering the promise of the Hg2+ -NRCNRs nanocomposites sensor system for application in biological and environmental fields, the selectivity of the PL sensor for heparin was evaluated. Under the optimal conditions, we tested the PL intensity changes of
Fig. 5. Selectivity of the PL assay for heparin over other biomolecules. The concentration of heparin was 50 g mL−1 and that of other biomolecules was 1.0 mg mL−1 . The excitation wavelength was 355 nm and the emission at 420 nm was monitored.
the Hg2+ -NRCNRs nanocomposites in the presence of competitive biomolecules under the same conditions, respectively, including ATP, BSA, GOD, HA, PTM, NHS and glycine, as shown in Fig. 5. It is seen that the much higher relative PL intensity (PL-PL0 )/PL0 was enhanced the most by heparin, and the presence of the other biomolecules, in contrast, caused the relative PL intensity to change very little (The PL intensities of the Hg2+ -NRCNRs in the absence and presence of other biomolecules are denoted by PL0 and PL, respectively), suggesting an excellent enhancing behavior of heparin to the PL of Hg2+ -NRCNRs nanocomposites. The Hg2+ /heparin metal complex formed because of stronger electrostatic attraction between the negatively charged heparin and the positively charged Hg2+ ion. And the heparin possesses many more negatively charged side groups than do other biomolecules [13], so it will have much stronger electrostatic interactions than other biomolecules with Hg2+ cation. Therefore, these observations suggest that the proposed method is capable of discriminating between heparin and the interference biomolecules.
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Fig. 6. A) PL emission spectra of Hg2+ -NRCNRs nanocomposites in the presence of different concentrations of heparin (0, 0.05, 0.1, 0.5, 1.0, 5.0, 10.0, 20.0, 30.0, 40.0, 50.0, 70.0, 100.0, 150.0, 200 g mL−1 , top to bottom, excitation at 355 nm). B) Plot of the enhanced PL signals [(PL0 -PL)/PL0 ] versus heparin concentration. Table 1 Comparison of the linear range and detect limit for heparin using different detection probes. Method
Linear range (g mL−1 )
Detect limit(g mL−1 )
reference
Positively-charged gold nanoparticles Gold nanorods and graphene oxide Adenosine-based molecu- lar beacons Our method
0.09–3.12 0.02–0.28 0.18–1.8 0.05–50
0.03 0.005 0.06 0.013
11 28 29 ∼
Table 2 Results of heparin determination in real samples. Sample a
1 2b
Standard value (g mL−1 )
Heparin founded (g mL−1 )
Recovery (%)(n = 5)
RSD (%)(n = 5)
20 50
18.96 54.78
94.80 109.56
4.67 3.56
The real samples were prepared to the linear range of 0.05–50 g mL−1 before determination. a Produced by Cisen Pharmaceutical Co., Ltd. b Produced by Yangtze River Pharmaceutical Group Co., Ltd.
3.4. Sensitivity of the detection system The PL response of the nanoprobe to heparin at varied concentrations was investigated under the optimized conditions (HEPES buffer of pH 7.4; Hg2+ ion: 100 M), as shown in Fig. 6. Fig. 6A showed the PL responses of Hg2+ -NRCNRs nanocomposites when different concentrations of heparin were added into the system, and sequential increases of the PL emission at 420 nm were observed with increasing amount of heparin. Upon adding heparin (75 g mL−1 ) to the Hg2+ -NRCNRs nanocomposites solution, the blue PL of the NRCNRs was nearly complete restoration, revealing that the sensing system is sensitive to heparin concentration. In the range of 0.05-50.0 g mL−1 , the increased PL of NRCNRs showed a good linear relationship to the concentration of heparin with a correlation coefficient of 0.9913 (n = 9) (Fig. 6B), and the detection limit for heparin calculated as 0.013 g mL−1 (3/k) was comparable to other sensors (Table 1) [11,28,29]. It is also worth noting that the PL restoring phenomena might be observed by the naked-eye under UV illumination (inset photograph in Fig. 2B), in which the disappear blue PL of Hg2+ -NRCNRs nanocomposites became strong with heparin added, confirming the recovery of Hg2+ -NRCNRs by heparin.
the measured recoveries were between 94.80 and 109.56% with less than 5.0% RSD under the optimal conditions, which indicates the Hg2+ -NRCNRs probe offers advantages of simplicity, speed, and precision for determining heparin in real sample analysis.
4. Conclusions In summary, a simple PL sensor was successfully developed for ultrasensitive detection of heparin based on Hg2+ ion quenching the PL of self-assembly NRCNRs. We took full advantage of the high binding affinity between NRCNRs and Hg2+ ion as well as stronger electrostatic interaction of Hg2+ ion with heparin, and therefore successfully attained the ultrasensitive detection of heparin with the detection limit of 13 ng mL−1 . Meanwhile, the PL sensor showed a number of attractive analytical features in the following terms: high sensitivity, simple operation process, rapid detection, good reproducibility and environmentally friendly. And under optimal conditions, this method displayed a wide linear range and good selectivity over other biomolecules. The proposed system has also been successfully applied for the determination of heparin in commercial heparin injection samples, further demonstrating its value in practical applications.
3.5. Real sample analysis The excellent specificity combined with high sensitivity and fast response of the Hg2+ -NRCNRs complex to heparin suggests that our method might be directly applied to detecting heparin in real samples. Therefore, the proposed method was applied to the analysis of heparin in two real samples of heparin injection by the standard addition method, and the results are shown in Table 2. From Table 2,
Acknowledgments We greatly appreciate the support of the National Natural Science Foundation of China (21575123, 21305122, 21675139, 21603184) and the Industry-University- Research Cooperative Innovation Foundation of Jiangsu Province (BY2015057-17).
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Biographies Zhong-Xia Wang obtained her Ph.D. degree in 2016 from Southeast University, China. She is a Lecturer in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. Her current fields of interest are photoluminescent carbon-based nanomaterials and bio-analytical chemistry. Fen-Ying Kong is a lecturer in Yancheng Institute of Technology, China. She received her Ph.D. degree in analytical chemistry from Nanjing University in 2012. Her research is focused on the preparation of new functional nanomaterial and electrochemical sensing application. Wen-Juan Wang obtained her Ph.D. degree in 2014 from Nanjing University of Science and Technology, China. She is a Lecturer in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. Her current fields of interest is electrocatalysis. Rui Zhang obtained her Ph.D. degree in 2016 from Southeast University, China. He is a Lecturer in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. His current fields of interest is electrocatalysis. Wei-Xin Lv obtained her Ph.D. degree in 2014 from Southeast University, China. She is a Lecturer in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. Her current fields of interest is electrocatalysis. Xian-He Yu received his B.S. degrees in Applied Chemistry from Nanjing Teach University, China in 2014. Now he is a joint training Master of Organic Chemistry in the School of Chemistry and Chemical Engineering, Yancheng Institute of Technology. His research interests are in the areas of fluorescence probe and electrochemical analysis. Hong-Cheng Pan received his Ph.D. degrees in analytical chemistry from Nanjing University, China in 2008, Now he is a professor of Guilin University of Technology, China. His research interests are in the areas of optoelectronic nanomaterials and bio-analytical chemistry. Wei Wang received his B.S. and Ph.D. degrees in analytical chemistry from Nanjing University, China in 1991 and 2007, respectively. Now he is a professor of Yancheng Institute of Technology, China. His research interests are in the areas of microfluidics and electrochemical detection.