Boron Nitride Quantum Dots as Efficient Coreactant for Enhanced

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Boron Nitride Quantum Dots as Efficient Coreactant for Enhanced Electrochemiluminescence of Ruthenium(II) Tris(2,2’-bipyridyl) Huanhuan Xing, Qingfeng Zhai, Xiaowei Zhang, Jing Li, and Erkang Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04428 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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Analytical Chemistry

Boron Nitride Quantum Dots as Efficient Coreactant for Enhanced Electrochemiluminescence of Ruthenium(II) Tris(2,2’-bipyridyl) Huanhuan Xing,a,b Qingfeng Zhai,a Xiaowei Zhang,a Jing Li,a,* Erkang Wang a,*

a. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China. b. University of Science and Technology of China, Hefei, Anhui, 230029, P. R. China. Corresponding

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[email protected]; [email protected]

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

In the present work, an enhanced and stable anodic electrochemiluminescence (ECL) was observed from a suspension of boron nitride quantum dots (BNQDs) and Ru(bpy)32+, which behaved 400-fold enhancement compared with individual Ru(bpy)32+. Interestingly, different from the previous research on BNQDs as a type of optical probe, BNQDs were demonstrated as efficient coreactant of Ru(bpy)32+ based ECL for the first time and confirmed by collecting the ECL spectra. The amino-bearing groups and the electrocatalytic effect of the BNQDs endowed them as potential coreactant for ECL of Ru(bpy)32+ and the possible mechanism of electrode surface reaction was discussed. Several factors including electrode material, the pH of the buffer solution and the amount of BNQDs were investigated and also further confirmed the role of the BNQDs in the proposed Ru(bpy)32+/BNQDs system. On the basis of the quenching effect between the excited state of Ru(bpy)32+ and the oxidation form of DA in the ECL system of Ru(bpy)32+/BNQDs, ECL sensing platform for DA was successfully established. The proposed ECL system with the outstanding ECL efficiency may hold great potential in the bioanlysis because of the biocompatibility and good stability of BNQDs. INTRODUCTION Quantum dots (QDs) with size-dependent optical and electronic feature1 have appealed many researchers to explore their application in bioimaging2,3, sensor4,5, photocatalysis6, energy conversion7. To further expand the application of QDs, more and more attention has been moved from toxic heavy-metal elements based QDs to


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environmentally friendly inorganic elements based QDs, such as carbon nitride quantum dots (CNQDs), carbon quantum dots (CQDs), graphene quantum dots (GQDs) and boron nitride quantum dots (BNQDs). Recently, BNQDs with the large intrinsic band gap (5.7 eV)8, thermal stability9, excellent mechanical strength10, high thermal conductivity11, low toxicity and chemical stability12 endowed them as attractive candidate for popular semiconductor-based quantum dots. Notably, researches on BNQDs based application were still at the initial stage and most of works were mainly focused on the superior optical properties using different synthesis strategies, other novel feature related to the surface-state has few been involved. Electrochemiluminescence (ECL) as a powerful analytical tool has been widely used in the clinical detection due to high sensitivity, low background, simple set-up and good spatial and temporal resolution, especially Ru(bpy)32+ and its derivatives based ECL with the outstanding efficiency13. ECL was often generated via coreactant pathway, where excited state of Ru(bpy)32+ was generated from two different precursors (emitter and coreactant) via high-energy electron transfer reaction at particular potential14,15 and gave rise to the light emission. The choice of the effective coreactant was very important for the enhanced ECL performance. Alkyl amines was the typical and efficient coreactant of Ru(bpy)32+ in the “oxidative-reduction” mechanism16. Tripropylamine (TPA) as a coreactant proposed by Leland and Powell was the successful example in commercial ECL immunoassays17. However, TPA itself was toxic, corrosive and volatile. Moreover, to achieve good sensitivity, high concentration was often used (up to 100 mM), which led to the high background18. In



addition, the stability of the Ru(bpy)32+/TPA system need to be improved for ECL cycle, therefore much efforts have been devoted to exploiting new coreactants to enhance the ECL intensity of Ru(bpy)32+. For example, Xu’s group proposed that the suitable substituent of alkyl amines would catalyze the oxidation of amines and lead to the enhanced ECL, such as the use of 2-(dibutylamino)ethanol (DBAE), which provided a facile strategy for high efficiency ECL19. With the development of the nanotechnology, some nanomaterials (NMs) with their unique feature entered people’s vision as novel coreactant by utilizing the unique surface capped groups, especially carbon based NMs20,21. For instance, Pang’s group has proved the CNDs as coreactant of the anodic ECL of Ru(bpy)32+ owing to the oxidization of benzylic alcohol units on the surface of CNDs22. Recently, Cola et al successfully used nitrogen-doped carbon nanodots (NCNDs) as coreactant to achieve the self-enhanced ECL platform using Ru-NCNDs hybrid due to the primary or tertiary amino groups on NCNDs23. Besides, the oxygenous units of alcohols on GQDs were also demonstrated as the coreactant sites for enhancing the ECL of Ru(bpy)32+24. All the above work indicated that the functional groups on the surface of QDs could act as active sites for catalyzing the reaction of coreactant in ECL process. Inspired by the abundant hydroxyl and amino functional groups of BNQDs, the unrevealed property of BNQDs related to the surface state was explored in this work using ECL as the detection tool. BNQDs were synthesized via a simple bottom-up hydrothermal method. To our surprise, an intense and stable anodic ECL signal was observed from a suspension of BNQDs and Ru(bpy)32+. About 400 folds enhancement


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was observed when glassy carbon electrode was used, which endowed BNQDs as one of the most effective coreactant for enhanced ECL of Ru(bpy)32+. By collecting the ECL spectra, BNQDs were confirmed as efficient coreactant of Ru(bpy)32+ based ECL for the first time and the possible catalytic mechanism was discussed. The effect of electrode material, the pH of buffer solution and the amount of BNQDs on the ECL performance of Ru(bpy)32+/BNQDs system were investigated detailedly and also proved the enhanced ECL was from the catalytic oxidation of the surface groups of BNQDs. On the basis of the quenching effect between the excited state of Ru(bpy)32+ and the oxidation form of dopamine (DA), ECL sensing platform was successfully constructed for DA analysis. Based on the biocompatibility and good stability of BNQDs, the proposed ECL system with the excellent ECL efficiency may pave a new way in the bioanalysis. Furthermore, this work has offered a new insight to explore the novel feature of BNQDs, especially for the surface-related properties. EXPERIMENT SECTION

Chemicals and Reagents. All the chemicals were used as received without any further purification. Tris(2,2’-bipyridyl)dichlororuthenium(II) hexahydrate, ascorbic acid, glucose, L-cysteine, Nafion were purchased from Sigma-Aldrich Chemical Co.(Milwaukee, WI) and H3BO3 was bought from Xilong Chemical Co.,Ltd (Guangdong). Lactic acid, NH3, KCl, HCl, K3[Fe(CN)6], K4[Fe(CN)6], NaOH, NaH2PO4.2H2O and Na2HPO4.12H2O were bought from Beijing Chemical Reagent Company (Beijing, China). Dopamine was obtained from Alfa Aesar Chemical Company (Tianjin). The deionized water (18.2 MΩ·cm−1 ) purified by a Milli-Q



system (Millipore, Bedford, MA) was used to prepare all the solutions. Instrument. The CHI660D electrochemical work station (Shanghai Chenhua Instrument Corporation, China) was used to carry out all the electrochemical experiments. The fluorescence spectra were obtained from a Fluoromax-4 spectrofluorometer (Horiba JobinYvon Inc., France) and visual photos were acquired by a hand-held UV lamp (365 nm, 8.0 W). The Cary 500 Scan UV-vis spectrometer (Varian) was performed to record absorption spectra. The Bruker atomic force microscope (AFM) was used to characterize thickness of BNQDs by the tapping mode on the mica substrate. The specific morphology and size of BNQDs were acquired by the JEOL 2100F transmission electron microscope (TEM) with an accelerating voltage of 200 kV. The ECL experiments were performed on a MPI-A capillary electrophoresis ECL analytical system (Xi’an Remax Electronics Science & Technology Co. Ltd) using a traditional three-electrode system composed of a glassy carbon working electrode (GCE, 3 mm diameter), an Ag/AgCl reference electrode and a platinum wire counter electrode. The GCE was polished with 0.1 µm Al2O3 powder and cleaned in an ultrasonic cleaner with double-distilled water and ethanol sequentially, then dried with nitrogen flow prior to use. The ECL measurements were recorded by potential scanning from 0 V to 1.5 V with a scanning rate of 0.1 V/s. 0.1 M PBS containing 0.1 M NaCl (pH = 8.5) was chosen as supporting electrolyte. ECL spectra were recorded in Guizheng Zou’s group (Shandong University) with a homemade ECL spectra system consisting of a monochromator and an electrochemical analyzer.


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Preparation of BNQDs. The BNQDs were synthesized using a simple bottom-up hydrothermal method25. Generally, 1.2 g boric acid was dissolved in 30 mL deionized water to obtain homogeneous solution, then 2.4 mL concentrated ammonia was added into the above solution and degassed with nitrogen for ten minutes. Then the precursor was heated at 200℃ for 12 hours in an autoclave. The obtained BNQDs were stored in 4℃ before use. RESULTS AND DISCUSSION

The characterization of BNQDs. The synthesis of BNQDs was conducted via a simple one-step bottom-up hydrothermal method25. It could be found that BNQDs were uniformly dispersed with a mean size of 1.71 nm as estimated from the TEM image. AFM measurements showed mono-layered structure and the height of BNQDs was in the range of 0.35 nm to 1.25 nm with an average thickness of 0.91 nm (in Figure 1c and 1d). The typical fluorescence spectra were presented in Figure 2. Two sharp excitation peaks at 230 nm and 320 nm were observed (Figure 2a) and the maximum emission peak of BNQDs occurred at 410 nm with the Stokes shifts of λ=180 nm and 90 nm, which was higher than the most of the used CdSe/ZnS nanocrystal26. Moreover, BNQDs exhibited the excitation-independent emission with the changing of excitation wavelength, which was evoked by the surface state and chemical environment rather than the morphology which confirmed by other groups27-29. This unique behavior of BNQDs was beneficial for distinguishing cancerous tissues from normal one in the organic system by avoiding the auto-fluorescence phenomenon30. All the above



optical and morphology characterization confirmed the successful preparation of BNQDs. The ECL behavior of BNQDs as the novel coreactant of Ru(bpy)32+. The ECL performance of Ru(bpy)32+ in the presence of BNQDs was studied. Similar to the previous work22, the anodic ECL of 100 µM Ru(bpy)32+ at the GCE was extremely weak without the participation of coreactant. Surprisingly, intense ECL emission (400 folds) was observed in the anodic polarization process upon the addition of BNQDs (in Figure 3a). CV curves in Figure 3b were also recorded for demonstrating the role of BNQDs. A typical reversible redox peak of Ru(bpy)32+ was observed in the absence of BNQDs. Upon the addition of BNQDs, the oxidation potential shifted more negatively and the oxidation current increased obviously, manifesting the catalytic effect of BNQDs on the oxidation of Ru(bpy)32+, which was similar to the most coreactant in ECL system24. However, in a recent work31, the cathodic ECL of BNQDs was reported in the presence of L-cysteine where BNQDs were served as ECL probe. To further verify the original of the above ECL, the anodic ECL behavior of individual BNQDs was investigated as a control. As shown in the Figure S1, there was no ECL emission for the individual BNQDs, suggesting BNQDs could not produce ECL under this condition. Based on the above results, we preliminarily inferred that the enhanced ECL may be attributed from BNQDs as the coreactant of Ru(bpy)32+. In order to confirm the above hypothesis, the ECL spectra were collected. As depicted in the Figure 4, the individual BNQDs showed no ECL emission in the range from 260 nm to 830 nm and pure Ru(bpy)32+ had a very weak


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ECL signal at around maximum emission wavelength 620 nm. However, in the co-existence of Ru(bpy)32+ and BNQDs, obvious ECL emission peak at 620 nm was obtained, which was in accordance with FL spectra of Ru(bpy)32+ but distinguished from the FL spectra of BNQDs (λ=410 nm). These spectra confirmed the ECL of the Ru(bpy)32+/BNQDs was originated from the Ru(bpy)32+ and the intense ECL may be attributed to the coreactant mechanism of BNQDs. To precisely understand the mechanism of BNQDs as coreactant of Ru(bpy)32+, the UV-vis absorbance and FL emission were recorded (as depicted in Figure S2). The absorbance spectra of mixture of Ru(bpy)32+/BNQDs in a wide range (220 nm - 800 nm) was a merging of absorptions from separated Ru(bpy)32+ and BNQDs, indicating that BNQDs could not react with Ru(bpy)32+ under the ground state in 0.1 M PBS buffer solution (pH = 8.5). However, the FL emission of BNQDs at 410 nm declined significantly upon the addition of Ru(bpy)32+, while the fluorescence intensity of Ru(bpy)32+ at 609 nm decreased slightly compared with the pure Ru(bpy)32+, revealing the excited state of BNQDs could react with excited state of Ru(bpy)32+ leading to the decrease of fluorescence intensity24. The above discussion confirmed the ECL luminophor was indeed from the Ru(bpy)32+. As for the coreactant pathway, the concentration of coreactant played an important role and the concentration-dependent ECL was monitored. It could be seen the ECL intensity increased linearly with the logarithm concentration of BNQDs in a range of 1.43 mg/mL - 11.4 mg/mL with R2 =0.984 in Figure 5a, with the further increase of concentration of BNQDs (more than 11.4 mg/mL), the ECL intensity decreased, which was coincident with the behavior of



DBAE19. To make sure what kind of group was oxidized in this ECL system, the ECL from the unreacted precursor of BNQDs was recorded as a control. The ECL intensity of Ru(bpy)32+ with the same volume of precursor (black line) and BNQDs (red line) was shown in Figure S3. It could be seen that the ECL was only enhanced dozens of times with precursor as coreactant compared with individual Ru(bpy)32+, it was caused by the ammonia of the precursor which could also be used as coreactant of Ru(bpy)32+32,33. However, the enhanced trend was inferior to that obtained with BNQDs as coreactant. As we know, BNQDs had similar structure to the graphite and had rich amino group, therefore the enhanced mechanism may be from the catalytic oxidation of surface functional group of BNQDs (denoted as amino) to the Ru(bpy)32+. The effect of buffer pH on ECL indicated the deprotonation of BNQDs was also involved in the ECL reaction process. As shown in Figure 5b, the ECL intensity was sensitive to pH and gradually increased in alkaline condition. When the pH value of solution was higher than 8.5, the ECL decreased slightly due to the reaction between Ru(bpy)33+ and hydroxide ion causing the consumption of Ru(bpy)33+ in the ECL reaction15. Based on the above discussion, the possible mechanism was described in the following scheme 1. The Ru(bpy)32+ was electrochemically oxidized to Ru(bpy)33+ via one-electron oxidation on the electrode surface. Meanwhile, BNQDs with rich amino group were electrochemically oxidized to produce a BNQDs-NH+., which underwent deprotonation process to generate a reductive intermediate BNQDs-N. in alkaline condition and then reacted with Ru(bpy)33+ to form the excited state Ru(bpy)32+* emitting an anodic optical signal.


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Moreover, the effect of atmosphere and electrode materials on the ECL were also investigated. As seen from Figure 6a, the ECL intensity under saturated nitrogen was higher than that obtained in the air and oxygen atmosphere. The suppressed ECL was attributed to the energy transfer quenching effect between the dissolved oxygen and the excited state Ru(bpy)32+* in the aqueous solution22. Different ECL performance was obtained with different electrode materials and the maximum ECL intensity was obtained with GCE, as shown in Figure 6b. This difference was attributed to the lower evolution potential of oxygen at Pt and Au, which could promote the generation of oxygen by electrochemical oxidation and inhibited the ECL at Pt and Au electrodes. All these results were in accordance with the behavior of the Ru(bpy)32+ as the ECL regents. Under the optimal condition, the stability of the proposed Ru(bpy)32+/BNQDs system was tested. Under 25 repetitive CV cycles (in Figure 7), there was no detectable change for ECL intensity, indicating the high stability and good application potential of BNQDs in ECL bioassay field. Furthermore, a comparison between BNQDs and TPA was executed as coreactant Figure S4. It is found that the enhanced ratio caused by the 11.4 mg/mL BNQDs was equal to that obtained using 1.43 mg/mL TPA. The different ECL enhancement efficiency may be attributed to the different functional groups when used as co-reactants. The ECL intensity enhancement using amines as co-reactant follows the order: primary < secondary < tertiary. Therefore, TPA as the coreactant with tertiary amine was superior to the BNQDs, which was similar to the results of the nitrogen-doped carbon23. However, good biocompatibility, good solubility, high chemical stability, low cost and easy synthesis of BNQDs made



them as the ideal coreactants. The application of ECL sensor for dopamine To demonstrate the application of the prepared system, DA was chosen for the analysis. DA as a neurotransmitter of brain was closely related to the various diseases such as schizophrenia, depression, and Parkinson’ s disease and played an important role in human physiological system34,35. Thus the monitoring of DA was critical for the early diagnosis of some diseases. Here, DA was analyzed based on the quenching effect from the formation of o-benzoquinone (BQ) in the ECL reaction. To improve the analytical efficiency, modified electrode was used by mixing the Ru(bpy)32+ and DA with the assistance of Nafion based on the high ion-exchange capacity to the Ru(bpy)32+36. The ECL curves of modified GCE with 10 µM Ru(bpy)32+ were obtained in 0.1 M PBS (in Figure S5). In the presence of 11.4 mg/mL BNQDs, Nafion-Ru(bpy)32+ modified electrode exhibited intense ECL emission due to the effective enhancement of BNQDs (in Figure S5b). While introducing 1 mM DA into the above mixture, the ECL intensity decreased greatly and an irreversible oxidation peak at 0.67 V was observed in the CV curve (in Figure S5c ) due to the oxidation of DA to BQ on the GCE surface37. BQ as an effective quencher of ECL could react with the excited state Ru(bpy)32+＊ intermediate formed in the ECL reaction and led to the quenching of ECL37,38. The specific reaction process was presented in scheme S1 and the concentration-dependent ECL performance of DA was collected in the Figure 8, the ECL intensity decreased with the increasing concentration of DA ranging from 500 nM - 10 mM (in Figure 8a). Interestingly, the ECL quenched efficiency (denoted


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as Y=(I0-I)/I0 ) (here, I0 and I represented the ECL intensity in the absence and presence of DA, respectively) was in a linear relationship with the natural logarithm of the DA concentration ranging from 1 µM - 1 mM with a detection limit of 500 nM (S/N = 3). The linear regression equation was Y = 0.23129 + 0.0903 logCDA (in Figure 8b). The selectivity was a key criterion for the biosensor performance, to investigate whether the detection of DA was selective or not, several small biomolecules including ascorbic acid (AA), glucose, L-cysteine (L-Cys) and lactic acid were chosen as the interference substance with the concentration of 1 mM. It could be seen from Figure S6 the interference substance caused slight change of ECL meanwhile the 100 µM DA could trigger obvious quenching of ECL, which may be ascribed to the energy transfer between the excited state Ru(bpy)32+* and oxidation product BQ on the surface of GCE. The recovery analysis was performed using human serum as real sample to further verify the practical application of proposed sensor platform. Firstly, the healthy human serum sample from local hospital was diluted with deionized water by 40-fold and centrifuged at 3500 rpm for 25 min to get the purified serum for dissolution of different concentration of DA. The recovery analysis was listed in Table S1 and the recovery ranged from 93.28% - 109.5%, indicating the designed sensor could be available for the detection of DA in real serum sample. CONCLUSION In summary, BNQDs were proposed for the first time as novel and efficient



coreactant of Ru(bpy)32+ system for enhancing ECL and confirmed by ECL spectra. 400-fold enhancement was obtained compared with the individual Ru(bpy)32+ using GCE. The possible mechanism from the catalytic routes due to the amino-bearing groups on the surface of BNQDs was proposed and the corresponding influence factors were discussed, which would provide a new route for improving the stability and sensitivity of ECL system. Besides, a signal-off sensor based on the system of Ru(bpy)32+/BNQDs was constructed for the determination of DA with good stability as well as excellent selectivity. This work would open a new opportunity for the wide application of BNQDs in immunoassay and various sensors. ACKNOWLEDGMENT The National Natural Science Foundation of China (Grant No. 21427811), MOST, China (No. 2016YFA0203200), Youth Innovation Promotion Association CAS (No.2016208), Jilin province science technology development plan Project 20170101194JC and additional thanks to Professor Guizheng Zou at Shandong University for his support of the National Natural Science Foundation of China (Grant No. 21427808). Supporting Information: The electrochemical behavior of BNQDs; The UV/vis absorption and fluorescence spectra of BNQDs; The comparison of ECL intensity enhanced by BNQDs, precursor and TPA as coreactants; The feasibility of ECL sensor based on Ru(bpy)32+/BNQDs for detecting dopamine; The selectivity and recovery ananysis for dopamine based on the proposed sensor; The quenching mechanism of dopamine for the ECL (PDF)


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Figure 1. (a) The HRTEM image of BNQDs; (b) The size distributions of BNQDs ; (c) The AFM image of BNQDs; (d) Height profile performed along the white line in c).

Figure 2. (a) The excitation (black line) and emission (red and blue line) fluorescence spectrum of as-prepared BNQDs. Inset: photos of BNQDs (left)


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under the visible light and (right) UV lamp at 365 nm. (b) Fluorescence emission spectrum of BNQDs at different excitation wavelengths.

Figure 3. The ECL (a) and CV (b) behaviors of 100 µM Ru(bpy)32+ (black line), 100 µM Ru(bpy)32+ with 11.4 mg/mL BNQDs (red line) in 0.1 M pH = 8.5 PBS solution. The scanning rate was 100 mV/s.

Figure 4. The ECL spectra of BNQDs (blue line), Ru(bpy)32+ (red line), and Ru(bpy)32+ /BNQDs (black line) in 0.1 M pH=8.5 PBS solution.



Figure 5. The influence of different concentration of BNQDs (a) and pH (b) on the ECL intensity of Ru(bpy)32+/BNQDs.

Figure 6. The ECL intensity of Ru(bpy)32+/BNQDs with different atomosphere (a) and working electrodes (b).

Figure 7. The ECL response of 100 µM Ru(bpy)32+ with 11.4 mg/mL BNQDs in 0.1 M PBS solution (pH = 8.5) after 25 repetitive CV scans.


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Figure 8. a: The normalized ECL intensity of sensor with different concentration of DA (0, 0.5, 1, 10, 100, 300, 500, 800, 1000, 5000, 10000 µM); b: the linear relationship between (I0 – I)/I0 and the natural logarithm concentration of DA in the range of 1 - 1000 µM.

Scheme 1. Schematic illustration and proposed pathway of BNQDs as novel coreactant for enhancing the ECL of Ru(bpy)32+.



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Boron Nitride Quantum Dots as Efficient Coreactant for Enhanced

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