Boron Nitride Quantum Dots as Efficient Coreactant for Enhanced

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Article Cite This: Anal. Chem. 2018, 90, 2141−2147

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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*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China ‡ University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

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 had a 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 an 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 coreactants for ECL of Ru(bpy)32+, and the possible mechanism of the 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, the 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.

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reaction at a 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” mechanism.16 Tripropylamine (TPA) as a coreactant proposed by Leland and Powell was the successful example in commercial ECL immunoassays.17 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 background.18 In addition, the stability of the Ru(bpy)32+/TPA system need to be improved for the ECL cycle; therefore, much effort has 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 ECL.19 With the development of the nanotechnology, some nanomaterials (NMs) with their unique feature entered people’s vision as a novel coreactant by utilizing the unique surface capped groups, especially carbonbased NMs.20,21 For instance, Pang’s group has proved the CNDs as coreactant of the anodic ECL of Ru(bpy)32+ owing to

uantum dots (QDs) with size-dependent optical and electronic feature1 have appealed many researchers to explore their application in bioimaging,2,3 sensor,4,5 photocatalysis,6 energy conversion.7 To further expand the application of QDs, more and more attention has been moved from toxic heavy-metal elements based QDs to 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 stability,9 excellent mechanical strength,10 high thermal conductivity,11 low toxicity, and chemical stability12 endowed them as attractive candidates for popular semiconductor-based quantum dots. Notably, research on BNQDs-based application was still at the initial stage and most of works were mainly focused on the superior optical properties using different synthesis strategies. Other novel features related to the surface-state have few researchers involved. Electrochemiluminescence (ECL) as a powerful analytical tool has been widely used in the clinical detection due to high sensitivity, low background, simple setup, and good spatial and temporal resolution, especially Ru(bpy)32+ and its derivativesbased ECL with outstanding efficiency.13 ECL was often generated via the coreactant pathway, where the excited state of Ru(bpy)32+ was generated from two different precursors (emitter and coreactant) via a high-energy electron transfer © 2017 American Chemical Society

Received: October 26, 2017 Accepted: December 22, 2017 Published: December 22, 2017 2141

DOI: 10.1021/acs.analchem.7b04428 Anal. Chem. 2018, 90, 2141−2147

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Figure 1. (a) HRTEM image of BNQDs, (b) size distributions of BNQDs, (c) AFM image of BNQDs, and (d) height profile performed along the white line in part c.

the novel feature of BNQDs, especially for the surface-related properties.

the oxidization of benzylic alcohol units on the surface of CNDs.22 Recently, Cola et al. successfully used nitrogen-doped carbon nanodots (NCNDs) as coreactant to achieve the selfenhanced ECL platform using Ru-NCNDs hybrid due to the primary or tertiary amino groups on NCNDs.23 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 the 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 bottomup hydrothermal method. To our surprise, an intense and stable anodic ECL signal was observed from a suspension of BNQDs and Ru(bpy)32+. About a 400-fold enhancement was observed when a 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 coreactants 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), the ECL sensing platform was successfully constructed for DA analysis. On the basis of 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 into exploring

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EXPERIMENTAL 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, Lcysteine, 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 workstation (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 Jobin Yvon 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 threeelectrode 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 2142

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Figure 2. (a) Excitation (black line) and emission (red and blue lines) fluorescence spectra of as-prepared BNQDs. Inset: photos of BNQDs (left) under the visible light and (right) UV lamp at 365 nm. (b) Fluorescence emission spectrum of BNQDs at different excitation wavelengths.

Figure 3. 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.

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 to 1.5 V with a scanning rate of 0.1 V/s. The 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. Preparation of BNQDs. The BNQDs were synthesized using a simple bottom-up hydrothermal method.25 Generally, 1.2 g of boric acid was dissolved in 30 mL of deionized water to obtain a homogeneous solution, then 2.4 mL of concentrated ammonia was added into the above solution and degassed with nitrogen for 10 min. Then the precursor was heated at 200 °C for 12 h in an autoclave. The obtained BNQDs were stored in 4 °C before use.

nm, which was higher than the most of the used CdSe/ZnS nanocrystal.26 Moreover, BNQDs exhibited the excitationindependent emission with the changing of excitation wavelength, which was evoked by the surface state and chemical environment rather than the morphology which was confirmed by other groups.27−29 This unique behavior of BNQDs was beneficial for distinguishing cancerous tissues from the normal one in the organic system by avoiding the autofluorescence phenomenon.30 All the above optical and morphology characterization confirmed the successful preparation of BNQDs. 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 work,22 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-fold) 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 system.24 However, in a recent work,31 the cathodic ECL of BNQDs was reported in the presence of L-cysteine where BNQDs served as the 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,

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RESULTS AND DISCUSSION Characterization of BNQDs. The synthesis of BNQDs was conducted via a simple one-step bottom-up hydrothermal method.25 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 a monolayered structure, and the height of BNQDs was in the range of 0.35 to 1.25 nm with an average thickness of 0.91 nm (in Figure 1c,d). The typical fluorescence spectra were presented in Figure 2. Two sharp excitation peaks at 230 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 and 90 2143

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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 1.43−11.4 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 DBAE.19 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 reaction.15 On the basis of 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 a rich amino group were electrochemically oxidized to produce a BNQDs-NH+•, which underwent a deprotonation process to generate a reductive intermediate BNQDs-N• in alkaline conditions and then reacted with Ru(bpy)33+ to form the excited state Ru(bpy)32+* emitting an anodic optical signal. Moreover, the effect of the 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 solution.22 Different ECL performance was obtained with different electrode materials and the maximum ECL intensity was obtained with GCE, as

suggesting BNQDs could not produce ECL under this condition. On the basis of 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 Figure 4, the individual BNQDs showed no ECL

Figure 4. 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.

emission in the range from 260 to 830 nm and pure Ru(bpy)32+ had a very weak ECL signal at around maximum emission wavelength 620 nm. However, in the coexistence of Ru(bpy)32+ and BNQDs, an 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 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 (220−800 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 the excited state of Ru(bpy)32+ leading to the decrease of fluorescence intensity.24 The above discussion confirmed the ECL luminophor was indeed from the Ru(bpy)32+. As for the

Figure 5. Influence of different concentration of BNQDs (a) and pH (b) on the ECL intensity of Ru(bpy)32+/BNQDs. 2144

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Scheme 1. Schematic Illustration and Proposed Pathway of BNQDs as Novel Coreactant for Enhancing the ECL of Ru(bpy)32+

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

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 carbon.23 However, good biocompatibility, good solubility, high chemical stability, low cost, and easy synthesis of BNQDs made them the ideal coreactants. 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 the brain was closely related to the various diseases such as schizophrenia, depression, and Parkinson’s disease and played an important role in the human physiological system.34,35 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, a 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 surface.37 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 ECL.37,38 The specific reaction process was presented in Scheme S1, the concentration-dependent ECL performance of DA was collected in Figure 8, and the ECL intensity decreased with the increasing concentration of DA ranging from 500 nM to 10 mM (in Figure 8a). Interestingly, the ECL quenched efficiency (denoted as Y = (I0 − I)/I0)

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

Figure 7. 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.

change for ECL intensity, indicating the high stability and good application potential of BNQDs in the 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 coreactants. The ECL intensity enhancement using amines as coreactant follows the order: 2145

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

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(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 to 1 mM with a detection limit of 500 nM (S/N = 3). The linear regression equation was Y = 0.23129 + 0.0903 log CDA (in Figure 8b). The selectivity was a key criterion for the biosensor performance to investigate whether the detection of DA was selective or not, and 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 a 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 a real sample to further verify the practical application of proposed sensor platform. First, 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 to 109.5%, indicating the designed sensor could be available for the detection of DA in real serum sample.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b04428. Electrochemical behavior of BNQDs; UV−vis absorption and fluorescence spectra of BNQDs; comparison of ECL intensity enhanced by BNQDs, precursor, and TPA as coreactants; feasibility of ECL sensor based on Ru(bpy)32+/BNQDs for detecting dopamine; selectivity and recovery ananysis for dopamine based on the proposed sensor; and quenching mechanism of dopamine for the ECL (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Phone: +86-431-85262003. Fax: +86-431-85689711. E-mail: [email protected]. ORCID

Erkang Wang: 0000-0001-9843-1834 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from The National Natural Science Foundation of China (Grant No. 21427811), MOST, China (Grant No. 2016YFA0203200), Youth Innovation Promotion Association CAS (Grant No. 2016208), Jilin Province Science Technology Development Plan Project 20170101194JC, Cooperation Foundation (Grant No. 16YFXTNC00080) is greatly apppreciated. The authors provide additional thanks to Professor Guizheng Zou at Shandong University for his support from the National Natural Science Foundation of China (Grant No. 21427808).

CONCLUSION

In summary, BNQDs were proposed for the first time as novel and efficient coreactants of the Ru(bpy)32+ system for enhancing ECL and confirmed by ECL spectra. A 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 the 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.

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