Mechanochemical Green Synthesis of Exfoliated Edge-Functionalized

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Functional Nanostructured Materials (including low-D carbon)

Mechanochemical Green Synthesis of Exfoliated EdgeFunctionalized Boron Nitride Quantum Dots: Application to Vitamin C Sensing through Hybridization with Gold Electrodes Shayan Angizi, Amir Hatamie, Hajar Ghanbari, and Abdolreza Arash Simchi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07332 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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ACS Applied Materials & Interfaces

Mechanochemical Green Synthesis of Exfoliated Edge-Functionalized Boron Nitride Quantum Dots: Application to Vitamin C Sensing through Hybridization with Gold Electrodes Shayan Angizi, a Amir Hatamie, a Hajar Ghanbari , b and Abdolreza Simchi a,c* a

Department of Material Science and Engineering, Sharif University of Technology, PO Box:

11365-9466, Tehran, Iran a

School of Metallurgy and Materials Engineering, Iran University of Science and Technology,

PO Box: 163- 16765, Tehran, Iran a,c

Institute for Nanoscience and Nanotechnology, Sharif University of Technology, 14588

Tehran, Iran [email protected] (Sh. Angizi) [email protected] (A. Hatamie) [email protected] (H. Ghanbari) Corresponding author: Tel: +98 (21) 6616 5226; Fax: +98 (21) 6600 5717; [email protected] (A. Simchi)

KEYWORDS Green chemistry, Exfoliation; Electrocatalytic activity, Biosensing; Ascorbic acid

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ABSTRACT Two-dimensional boron nitride quantum dots (2D BNQD) with excellent chemical stability, high photoluminescence efficiency, and low toxicity are a new class of advanced materials for biosensing and bioimaging applications. To overcome the current challenge about lack of facile, scalable and reproducible synthesis approach of BNQD, we introduce a green and facile approach based on mechanochemical exfoliation of bulk h-BN particles in ethanol. Few-layered hydroxylated-functionalized QDs with a thickness of 1-2 nm and a lateral dimension of 2- 6 nm have been prepared. The synthesized nanocrystals exhibit a strong florescence emission at 407 nm and 425 nm with a quantum efficiency of ~6.2%. Spectroscopic analyses determine that interactions between oxygen groups of the solvent with boron sites occur, which along with the mechanical forces, lead to efficient exfoliation of the hexagonal structure and surface functionalization with –OH groups. We also demonstrate that orbitals interaction between BNQD and gold surface result in profound electrochemical catalytic activity toward oxidation of vitamin C. It is shown that BNQD-modified screen-printed gold electrode exhibit a decreased onset oxidation potential for about 0.37 V/AgCl. In addition to high catalytic activity, electrochemical studies also reveal that this electrode show selectively and sensitively detection of vitamin C with a good response over a wide range from 0.80 µM to 5.0 mM with a detection limit of 0.45 µM (S/N = 3) and sensitivity of 1.3 µA. µM-1 cm-2. Finally, the potential application of the hybrid sensor for detecting vitamin C in commercial drinks is demonstrated.


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1. INTRODUCTION Two-dimensional quantum dots, particularly heavy-metal free nanocrystals, have recently experienced increasing attention due to their unique and fascinating physico-chemical properties.1-3 Synergetic effects originated from different shapes, sizes and lattice defects provide tailorable properties which are crucial for many applications. Recent studies have shown that carbon quantum dots may show unique optoelectronic properties which makes them suitable for sensors and photovoltaic applications. 4-7 Their strong photoluminescence emission also makes them applicable for bioimaging as a non-toxic dying. 8-11 Two-dimensional boron nitride quantum dots are relatively a new member of this category, which are much less explored, besides their advantages with regard to nontoxicity, chemical stability (thermodynamic stability), and long-lasting PL emission. Little efforts have recently been attempted to synthesis BNQD. Although the analogous structure of h-BN to graphite provide an opportunity to employ a similar procedure to exfoliate the layered structure, the partial ionic bond between B-N atoms compared to pure covalent C-C bonds makes the procedure less effective. 12 Besides few attempts on the bottom-up synthesis of BNQD, the major route is top-down approach based on exfoliation of bulk h-BN particles. In a pioneering work, Lin et al.13 successfully synthesized monolayer of BNQD by potassium intercalation of h-BN and utilization of sonication to induce the boron nitride nanosheets to quantum dots. However, the product quantum yield was low (2.5%). In following, researchers worked on size-controllable methods for processing of BNQD through sonication or two-step sonication-solvothermal procedures 14-16. Li et al.17 synthesized BNQD with a quantum yield of 19.5% through sonication-assisted liquid exfoliation and solvothermal processes at the optimized condition. Fan et al.18 used a sonication-microwave method to produce highly efficient BNQD. A facile one-step synthesis of BNQD was introduced by Liu et


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al.19, which was based on the bottom-up method and interaction of boric acid and ammonia in a sealed vessel. Their experiment was followed by Huo et al.20, who was able to produce a high yield BNQD in a similar way using alternative precursors (melamine and boric acid). Generally, among different employed methods, the solvothermal process is remained the most effective route due to its simplicity, low-cost equipment, and high efficiency17, 21. Based on a recent report 17

, solvothermal parameters such as temperature, processing time and filling factor can affect the

size and surface chemistry of QDs. Comprehending the mechanism and optimizing the synthesis to attain large-scale processing of BNQD with controllable size and surface functional groups have still remained challenging. Besides, in many developed procedures, toxic solvents and precursors are utilized which their residue on products may restrict their applications in biorelated fields. In this study, we have employed a green and facile mechanochemical process to prepare BNQD. The process begins with high energy ball milling in wet condition for grinding and exfoliating of bulk h-BN particles into nanosheets. Ethanol was chosen as the solvent due to its low toxicity, ease of removability, and its adequate polarity (4.3). Also, it has enough surface energy to predominate on Van der Walls bonds between BN atomic layers.22 We have found that the time of high-energy milling is a crucial parameter determining the success of the process, as presented in Supplementary Electronic Material (Section S1, Figure S1). To obtain hydroxylatedfunctionalized QD, the nanosheets are subjected to solvothermal treatment in ethanol at 250ºC for 24 h. This process enhances the interaction between the solvent and BN nanosheets and induces pill-off events. The advantages of the utilized procedures relied on the fact that no hazardous chemicals such as hydrazine, n-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), aggressive acids and bases are utilized. Additionally, ethanol can easily be removed and


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recycled without the need to severe raising and drying steps. Moreover, no cation ions such as K+ and Li+ remains after exfoliation. Furthermore, the processing has the potential to be scaled up at a reasonable cost and time along with the possibility of surface functionalization of BNQD in order to affect their colloidal stability and electronic structure (bandgap energy).23 Aiming to demonstrate the potential applications of the prepared BNQD for biosensing, we have modified screen-printed gold electrode by the functionalized QDs and utilized them for sensitive and selective detection of vitamin C, which has an essential role in immune system function and also in food (soft drinks) and drug (cough syrups, and vitamin tablets) industries. To the best of our author’s knowledge, this is the first time that the application of BNQDmodified Au electrode for vitamin C detection is shown. In comparison with many electrodes based on carbon nanostructures (graphene and nanotubes), our electrode shows superior properties with regard to the linear range of sensing and limit of detection (see Table S1). Here, it is worthy to mention that BNQD supported on a conducting substrates (specially gold) possess high electrocatalytic activity for oxygen reduction reactions.24-25 This activity is due to the interaction between the N-pz and B-pz orbitals of BN and the dz2 metal orbitals.24, 26 These binds are responsible for remarkable amendment of the electronic properties of the BN supported on 3d, 4d, and 5d transition metal surfaces.27 It has also been reported that the N-p states of BN are placed close to the bottom of the d band, and conversely the B-p levels will remain mostly over the Fermi level .27 Therefore the BN π band is mostly created by N-p states, while the π* band formation is based on B-p states. No additional bonding is formed when the π bands interact with metal-d states. However, the interaction of d bands and Au surface with π* band “pushes” some B-p-metal bonding and N-p-metal antibonding states below the Fermi level.24 So, the interaction of both B-pz and N-pz as attraction and repulsion, have a significant role in this binding.


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2. MATERIALS and METHODS 2.1 Materials. Ethanol was purchased from Merck KGaA (Darmstadt, Germany). Hexagonal boron nitride powder (99.8%) was supplied from Sigma Aldrich (USA). For electrochemical analyses, gold screen-printed electrodes with 3 mm diameter were purchased from Metrohm (Netherlands). Commercial orange and lemon juices were purchased from a local supermarket. 2.2 Preparation of BN nanosheets: To prepare BN nanosheets, bulk BN particles were subjected to high-energy mechanical milling in ethanol. 0.1 mg BN powder was added to 10 mL ethanol, and the suspension was milled in a SPEX 8000 machine (USA) in a zirconia cup having a ball to powder ratio on 10. The time of milling varied up to 16 h. After milling, the suspension was quenched in a mixture of ice and water and residual h-BN removed from suspension by centrifuging at 1500 rpm for 20 min. The supernatant suspension containing BN nanosheets was used for the preparation of BNQD. 2.3 Synthesis of BN Quantum Dots. Solvothermal methods were employed to prepare BNQD. The supernatant suspension was poured in an autoclave with PTFE lining (100ml) and heated for 18 h at 250°C. After cooling, the product was centrifuged at 13000 rpm for 30 min to separate large fractions. The supernatant suspension was filtered by a 0.22um porous size PTFE-filter. Afterwards, the BNQD were separated by rotary evaporation (Heidolph, Germany) followed by freeze-drying (Alpha 2-4 LDplus, Germany). 2.4 Materials Characterizations. The UV-Vis spectra were recorded on a Rey Leigh UV-2601 spectrophotometer. Fluorescence spectroscopy was carried out with an Avantes Avaspec 2048 TEC spectrophotometer. X-ray Diffraction (XRD) was performed in X’Pert PRO MP with Cu


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Kα radiation. Dynamic Light size results were achieved using Nano S (red badg-632.8 nm). XPS spectra were acquired by a RBD upgraded PHI- 5000C ESCA system (Perkin Elmer) with Mg Ka radiation (hv = 1253.6 eV). The topography of samples were measured by VEECO on mica substrate. Raman spectra were recorded on XploRA. FTIR spectra were recorded on PerkinElmer Spectrum RX I spectrometer on KBr plates. TEM studies were done by a Philips CM20 instrument. Field-emission scanning electron microscopy (FESEM; TESCAN–Mira3) was used to investigate the BNQD on the surface of Au electrode. The instrument was equipped with an energy- dispersive X-ray (EDX) spectrometer which was used for compositional analysis and elemental distribution mapping in the regions of interest across the sample. For measurement of quantum yield, the plot of integrated fluorescence vs. absorbance of Quinine sulfate (in sulfuric acid 0.1 M) as reference and BNQD as the sample was plotted (Section S4, Figure S7). Five samples with different concentration were chosen from BNQD and reference and then absorbance and fluorescence spectra at excitation wavelength were achieved. The gradient of these linear plots were used for calculation of Quantum yield of BNQD from this equation:

Grad X ηX2 )( 2 ) Φ X = Φ ST ( Grad ST ηST ST and X are related to standard and sample respectively. φ is the Quantum yield, and η the refractive indices of the solvents. 2.5 Fabrication of BNQD-modified gold sensor. The gold screen-printed electrode (GSPE) was modified with BNQD via drop casting methods. An aliquot volume (5 µL) of the BNQD


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suspension was dropped onto the GSPE electrode and kept in a desiccator overnight to be dried at room temperature. 2.6 Electrochemical measurements. Voltammetric measurements were carried out by employing a “Autolab PGSTAT 101” (Metrohm Autolab, The Netherlands) potentiostat. Cyclic voltammetric (CV) experiment was carried out with the scan rate of 50 mV s-1 in 5 mM [Fe(CN)6]3-/[Fe(CN)6]4- containing 0.1 M KCl. Electrochemical impedance spectroscopy (EIS) measurement were performed in the range of 0.01-100 kHz at open circuit potential. Details of electrochemical analyses are reported in Section 3 of Electronic Supplementary Materials (ESM). To examine commercial drinks, a small volume was utilized to fall in the regime of linear detection range of the electrode. The so-called “standard addition method” was utilized. At first, an aliquot amount of the juices (100-500.0 µL) with unknown ascorbic acid concentration was added. Then, the triple standard addition of VC (here, 20, 60, 120 µM) was carried out. The analytical signals were recorded and the calibration curve was constructed. The concentration of VC in the juices was then determined by taking into account small change in the volume of the testing solution.

3. RESULTS and DISCUSSION 3.1 Characterizations of BN nanocrystals 2D BNQD were prepared by a green and facile mechanochemical method. The nanocrystals were characterized by various characterization techniques including microscopic (TEM and AFM) and spectroscopic (XPS, Raman, UV-Vis, and PL) techniques. Figure 1a shows the absorption spectrum of the synthesized BNQD. A strong absorption peak at 255 nm is observed,


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which indicates a significant red-shift relative to the bulk BN (203.5 nm28). As we have recently shown by density functional simulations23, this bandgap shift could be related to surface functional groups and/or defects formation in the edges of QDs. Another absorption peak at 310 nm might be assigned to π electron transition in the edges of oxygen-containing BNQD due to their high interaction with the solvent (Ethanol).29 As shown in the inset, a blue luminescence emission is observed by the excitation of QDs by 356 nm wavelength of light. PL spectroscopy shows that the maximum emission occurs at 407 nm and 425 nm with a shoulder, which may indicate the presence of three active luminescence centers (Figure 1b). It is also found that the PL emission wavelength is independent on the excitation wavelength in the range of 250 nm to 390 nm. The presence of three active luminescence centers in BNQD have been reported by others13, 17, 29-30 as well. For instance, 1-B and 3-B centres could be formed by replacing carbon atoms instead of nitrogen and making vacancy point defects; carbene structure at zigzag edges and BOx- (x=1 and 2) could also be centers of emission. Since ethanol was used in the processing steps, it is likely that interactions between oxygen sides of the solvent and boron sides occur leading to the formation of oxygen-based centers despite the difficulty in the distinction between the emission from the BO2- and 1-B, 3-B sites (2.7-3 and 3-3.4 eV, respectively), it is suggestible that PL peaks and the shoulder are related to BO2-, carbene zigzag edge, and 1-B, 3-B centers, respectively.


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Figure 1. Characterizations of the prepared BNQD. a) UV-Vis spectrum of BNQD shows an absorption peak at 255 nm. The inset shows blue emission of the dots under UV excitation at 356 nm light. b) PL spectra of BNQD at different wavelength lengths indicates triple luminescence centers. PLE spectrum exhibits the effect of surface functional groups. c) Raman and d) XRD spectra support the formation of high-exfoliated BNQD. The effect of surface functional groups on the PL emission can be deduced from the PL excitation spectrum shown in Figure 1b (at a detection wavelength of 425 nm). This spectrum exhibits a maximum peak at around 360 nm (3.4 eV) and two weak peaks at ~341 nm (3.6 eV), 370 nm (3.4 eV) and a shoulder at ~280 nm (4.4 eV). Similar results have been observed for hydroxylate-functionalized BNQD. 30 Raman spectroscopy was employed to study the structural


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fingerprint of BNQD. Figure 1c compares the Raman spectrum of bulk BN and BNQD. A weaker E2g phonon mode at around 1360 cm-1 is noticed while the half width in half maximum (FWHM) is wider for the quantum dots. 13, 17 This observation is a good indicator of few-layer formation after the two-step exfoliation process. Meanwhile, no peak was detected at 1587 cm-1 which commonly ascribed to carbon doping into the BN lattice structure.13, 31 XRD studies also support highly-exfoliated structure of BNQD (Figure 1d), because the intensity of the main peak at 26o (corresponding to the (002) planes of h-BN) is significantly decreased and the other peaks are disappeared. Although intensive Scherer broadening is noticed, the presence of the broadened peak signs the formation of few-layered structure 14. The size and morphology of the nanocrystals were investigated by TEM, AFM and DLS studies. Figures 2a shows a representative TEM image of the prepared BNQD. More images at lower magnifications are provided in Figure S2. TEM studies show that the QDs have an average diameter of ~4 nm with a narrow size distribution (±2 nm). Despite the aggregation of quantum dots during sample preparation, AFM studies indicate that the dots mostly have a thickness of 1 to 2 nm (Figures 2b,c). Therefore, the QDs seem to have a disc-like morphology and are comprised of 2 to 3 atomic layers.


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Figure 2. Size and morphology analysis of BNQD. a) Representative bright-field TEM image and size distribution plot. (b,d) Representative AFM image and the height profile show the formation of disc-like particles with a thickness of 1-2 nm. (c) Size distribution plot obtained by DLS measurement. In good agreement with TEM studies, size distribution analysis by DLS (Figure 2d) indicates that QDs are narrow-sized and well dispersed in Ethanol (not agglomerated). We have found that the colloidal QDs are stable up to 4 weeks; afterwards, sight coagulation of the nanoparticles


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occurs and the solution becomes turbid. In FTIR spectra (Figure 3a), the characteristic vibrational lines located around 1387 and 800 cm-1 of bulk pristine h-BN and BNQD are attributed to the B-N stretching and B-N bending mode, respectively.15 The peak coming from OH is observed at 3500 cm-1, which could be attached to the boron sites of the surface.32 The OH ions could be originated from Ethanol degeneration during processing caused by mechanical forces exerted by milling balls and/or high temperature/pressure of the solvothermal treatment32. Therefore, it is deemed that BNQD are functionalized with –OH through covalent bonds with B sites. It is noteworthy that N-OH bonds are not stable from the perspective of energy and degenerate or replace with other groups easily 33. The other vibrations at 1750-1550, 1150-850 and 750 cm-1 are attributed to C-BN (C-B or C-N as a result of the interaction of carbons atoms with boron nitride), N-B-O and O-B-O, respectively.15, 17 The chemical composition of BNQD and their functional groups were analyzed using XPS. The survey spectrum (Figure S3) indicates the presence of boron (B), carbon (C), nitrogen (N) and oxygen (O) atoms. The existence of C and O binding energies supports interactions of BN with the solvent molecules during the synthesis process. High-resolution spectra of B1s is deconvoluted in two peaks coming from B-N and B-O at around 190.3 eV and 191.2 eV, respectively (Figure 3b). The peaks centered in highresolution spectra of N1s are located around 398.1 eV and 400.1 eV (Figure 3c) that highlight NB and N-C/N-O bonds.13, 17, 19 The bonds between N and C are probably related to the attachment of ethanol molecules on the surface of BN. Figure 3d shows the high-resolution spectrum of C1s deconvoluted in three minor peaks at 285.3, 286, and 288.9 eV which are ascribed to C-C/C=C ,C-N and C-N/C-O-B, respectively.15, 20 The absence of C-B bonding energy in C1s spectrum supports the formation of C-N bonds, which is also detected in FTIR spectrum. On the other hand, the presence of B-O and N-O bonds reveals the possible oxygen doping in the BN


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structure. The other evidence for O doping may be deduced from the absorption and emission responses of BNQD related to the oxygen-based centers (Figures 2a and 2b). If so, oxygen bonded to the edges affecting the exfoliation process facilitates cutting of the planes and promotes edge interactions.

Figure 3. FTIR and high-resolution XPS spectra of BNQD. (a) FTIR spectrum shows that the synthesized nanocrystals are functionalized with hydroxylate groups. Additional surface bonds including N-B-O and O-B-O are visible. Deconvoluted high-resolution XPS spectra of (b) B1s, (c) N1s and (d) C1s reveal oxygen doping in the BN structure.


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3.2 Electrocatalytic activity of hybrid BNQD/Au electrodes The electrocatalytic activity of BNQD on gold screen-printed electrodes (GSPE) was studied. The dots were drop-cast on the gold surface. Representative FESEM images of drop-cast QDs on the electrode surface are shown in Figure S4 (some of them are circled). Well dispersed particles on the surface are visible. It seems that the nanocrystals might be agglomerated in some part to form larger particles of 10 nanometers. The high surface area of the dots and solvent evaporation upon drying steps exerts high forces (capillary, liquid drag, Brownian, etc) on the particles on the substrate to form agglomerates. 34 EDS supports that BN are decorated the Au substrate, bearing in mind that detection of B cannot be detected by this method. The eelectrocatalytic activity of the hybrid electrode was then examined. At first, cyclic voltammetric (CV) studies using Fe(CN)63−/4− (5 m0M) ions as a reversible redox probe were performed. As shown in Figure 4a, BNQD decrease forward and backward current intensities of Fe(CN)63−/4− ions. The difference between the two peak potentials (∆Ep) for the hybrid BNQD/GSPE electrode is increased (∆Ep =0.404 V/ AgCl) in comparison with GSPE (∆Ep =0.369 V/ AgCl) that could be related to the Fe(CN)63−/4− diffusion rate.35-36 The CV graphs clearly indicate that the electroactivity of the gold electrode surface is influenced by BNQD. In order to determine the active electrochemical surface area of the hybrid electrode, the Randles-Sevcik equation was utilized according to the procedure explained in ESM, S3.1. In this experiments, CVs were recorded at different scan rates in the range of 50-350 mV.s-1 (Figure 4b). As shown in the inset, the peak current oxidation of K3Fe(CN)6 and the square root of scan rate flow a linear relationship with R2=0.996. Therefore, the active electrochemical surface area of the modified electrode could be 0.3 mm2. To determine the effect of BNQD on the electron transfer process, EIS was employed. As shown in Figure 4c, BNQD operate as a barrier against


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the diffusion of Fe(CN)63−/4− ions to the electrode surface. This could be in part due to the negative surface charge of the nanocrystals (-6.0 mV) as determined by zeta potential measurement (Figure S5). From the equivalent circuit diagrams (Figure 4d) of the EIS spectrum, it is deduced that BNQD increase the semicircle’s diameter (Rct) and constant phase element (CPE) component of GSPE. From ZW (Warburg short-circuit term coupled to Rct), which accounts for the Nernstian diffusion (ZCPE , impedance of constant phase element), the dimensionless parameter (n) for GSPE and the BNQD-modified electrode was found to be 0.95 and 0.85, respectively. Noted that a lower n value reveals a rougher microscopic surface (n = 1 stands for an ideally flat electrode surface).37-38 Further information about EIS characterization are provided in Section 3.2 of ESM.


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Figure 4. Effect of BNQD on the electroactivity of gold screen printed electrode. a) Cyclic voltagrams in Fe (CN)63−/4− (5.0 mM). b) Effect of scan rate on cyclic voltammograms of BNQD/GSPE electrode in 5 mM (K3Fe(CN)6)/0.1M KCl solution. The inset shows the anodic peak current versus the square root of scan rate. c) Nyquist diagrams in the presence of Fe(CN)63−/4− (5 mM) in KCl (0.1 M). d) Equivalent circuits of with the Nyquist diagrams. 3.3 Detection of vitamin C by BNQD-modified Au electrodes The applicability of the purposed probe to catalyze oxidation and analysis of vitamin C (VC) as an essential nutrient for body functioning as well as important elements in food and drug industries was investigated. For the evaluation of the electrochemical behavior of VC on modified and unmodified gold electrodes, the electro-oxidation of VC at both electrodes were recorded (Figure 5a). A remarkable decrease in the peak potential of VC oxidation on hybrid


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BNQD/SPE as compared with GSPE (about 0.37 V/AgCl) is observed, which shows catalytic activity of BN nanocrystals. The mechanism of electro-oxidation of VC involves different steps including oxidation of deprotonated anion followed by transforming the anion to dehydroascorbic acid, which is rapidly protonated and then dehydrated to 2,3-diketogluconic acid. 39-40 The decreased onset potential might be as a result of two main events: 1) the high surface energy of nanomaterials in general, and quantum dots, in particular, will lead to an extensive tendency to reduce its energy by adsorbing the VC molecules onto the surface; 2) active catalytic sites of BN such as B atoms can adsorb the VC from O sides due to the Lewis interaction41-42 and probably trapping them to their defects and vacancies.43 Ultimately, two paths might be suggestible for transferring the produced electron from catalytic sites to the surface, which includes electron tunneling in ultrathin BN and electron transferring by hybridization of surface and BN orbitals. 44 The effect of scan rate (ν) on the oxidation peak potential of VC was also investigated. As shown in Figure 5b, the peak potential shifts in the positive direction with the scan rate. This behavior reveals the irreversible nature of the electrochemical process. A linear change in the anodic current intensity with the square root of the potential sweep rate (ν0.5) is noticed (inset of Figure 5b). This observation indicates a mass transfer-controlling process for the VC oxidation.45-46 It has also been found that the oxidation peak potential (Ep) varies linearly with the logarithm of the scan rate (Figure S6). Based on these variations, the Tafel slope (b) is 0.362 V, so that the electron transfer coefficient should be α=0.91 based on the equation: b=2.303RT/(1-α)mF, m is the number of electrons involved in the rate-determining step of the electrochemical reaction, R the gas constant, T temperature, and F the Faraday constant. The electron transfer coefficient


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shows a highly asymmetrical kinetic curve. This behavior also confirms that the oxidation of VC is an irreversible process. Figure 5c shows amperometric curves of BNQD/Au electrode at the potential of +0.4 V for VC detection. These experiments were carried out in PBS (pH 7.4, 25 mL) under stirring condition at 100 rpm. After successive VC standard addition, the amperometric response of the electrode was recorded. We have found that after each addition, current reached to steady-state response mostly within ~5 s. The fabricated sensor exhibits a good linear response to VC concentrations in the ranges of 0.80 to 84.8 µM (I (µA) = 0.004C + 0.151 (R2=0.991)) and 100.0 µM to 5.0 mM (I (µA) = 0.011C-0.552 (R2=0.992)). The limit of detection (LOD) was determined as three times of standard deviation (SD) of blank (here PBS solution without VC). Based on the slope of the calibration curve and LOD=3Sb/m equation, the LOD is 0.45 µM. Table S1 shows that the electrode performance as compared with some reported VC sensors is superior. Figure 5d shows the selectivity of the BNQD/Au sensor. It is seen that 2-fold concentration of common interfering substances only interfere slightly (lower than 5%) the detection. Thus, the purposed sensor has high selectivity for the determination of VC. In order to demonstrate the potential application of the hybrid electrode for VC detection, two commercial drinks (orange and lemon juices) were bought from the local supermarket and used as real samples. A small volume of the lemon (500.0 µL) and orange (100 µL) juices was added to the supporting buffer solution in a voltammetric cell. Then, the standard amounts of VC were added and analyzed with the standard addition method and recovery tests. The recorded chronoamperograms of the electrode responses are shown in Figure S7. Using the intercept point of the calibration curve with the concentration axis, the concentration of VC in the lemon (Figure S7a) and orange (Figure S7b) juice samples were obtained as 7.5±1 µmol L-1 (~1.32 mg mL-1)


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and 2.5±1 µmol L-1 (~0.44 mg mL-1), respectively. As seen in Table S2, the recoveries of the spiked VC are 95% to 104%. Therefore, BNQD could successfully be utilized for sensing applications, as their application for VC detection was shown by modifying the gold screenprinted electrode. Finally, the stability of the electrode was checked by performing 300 consecutive potential scans (0.01 V.s-1) in PBS pH 7.4 containing 0.1 mM VC. As shown in Figure S8, the SPE/BNQD electrode exhibited a 5.9 % decrease in the signal after this test that is acceptable and obviously shows the electrode is stable for long time scanning.


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Figure 5. Electrocatalytic response of BNQD/Au electrode toward VC detection. a) Cyclic voltagrams of GSPE electrode at the VC concentration of 80.0 µM (red) as compared with BNQD-modified GSPE electrode at different VC concentrations of 0.0 µM (green), 80.0 µM (blue) and 240.0 µM (yellow). The solvent was PBS (1.0 mM) and the scan rate was 0.10 Vs−1. b) Effect of scan rate (0.05- 0.30 V.s−1) on the electroactivity of BNQD/GSPE electrode in 1.0 mM PBS containing 0.5 mM VC. The insert shows the anodic peak current versus the square root of scan rate. c) Typical amperometric responses of BNQD/GSPE electrode towards VC detection in the concentration range of 100.0 µM to 5.0 mM. The response of the electrode at


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lower VC concentrations (0.80 to 84.8 µ) is shown in the upper inset. The downside inset shows the variation of current density with the VC concentration. These experiments were carried out in PBS (pH 7.4, 25 mL) under stirring condition at 100 rpm. After successive VC standard addition, the amperometric response of the electrode was recorded. d) Selective detection of VC by BNQD/GSPE electrode through successive addition of 40 µM interfering species and 20 µM of VC in 1.0 mM PBS. A constant potential of +0.4 V was applied.

4. CONCLUSIONS A novel approach to prepare functionalized-BNQD based on two steps mechanochemical approach was introduced. The BN nanocrystals had an average lateral size of about 4 nm with a thickness of ~2 nm. FTIR and XPS studied determined the critical role of oxygen atoms presented in the solution (ethanol) on the formation of N-O and B-O bonds, which facilitated the exfoliation process. The dots showed blue PL emissions linked with oxygen- and carbon-based emission centers. In order to demonstrate potential applications of BNQD, the functionalized products were utilized for VC detection. Hybrid BNQD/Au sensors were fabricated and their high electrocatalytic activity for electro-oxidation of VC was shown. Interestingly, the oxidation occurred at the surface of BNQD/Au electrode a lower positive potential (about 0.37 V) than the GSPE. This electrode could be used for VC detection in a wide linear range with good sensitivity and relatively low detection limit. Although BNQD were utilized for VC detection as a model analyte, the material has a great potential to be employed for fabrication of various type of sensors and biosensors, which will be examined in future work.

Conflicts of interest The authors declare that there is no conflict of interest regarding the publication of this paper.


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Acknowledgments This work was supported by the Grant Program of Sharif University of Technology (Grant No. G930305) and Iran National Science Foundation (INSF, Grant No. 95-S-48740).

Associate Content Supporting Information. Experimental section and time optimization related to the fabrication of BN nanosheets by mechanical milling, structural characterizations by FE-SEM and TEM, determination of quantum yield efficiency, and details of electrochemical studies of the composite electrode including amperometric analysis and stability tests are available in the Electronic Supplementary Materials (ESM). REFERENCES (1) Xu, Y.; Wang, X.; Zhang, W. L.; Lv, F.; Guo, S. Recent progress in two-dimensional inorganic quantum dots. Chemical Society Reviews 2018. DOI:10.1039/C7CS00500H (2) Wang, X.; Sun, G.; Li, N.; Chen, P. Quantum dots derived from two-dimensional materials and their applications for catalysis and energy. Chemical Society Reviews 2016, 45 (8), 22392262. (3) Wang, L.; Zhu, S.-J.; Wang, H.-Y.; Qu, S.-N.; Zhang, Y.-L.; Zhang, J.-H.; Chen, Q.-D.; Xu, H.-L.; Han, W.; Yang, B. Common origin of green luminescence in carbon nanodots and graphene quantum dots. ACS nano 2014, 8 (3), 2541-2547. (4) Li, Y.; Hu, Y.; Zhao, Y.; Shi, G.; Deng, L.; Hou, Y.; Qu, L. An electrochemical avenue to green‐luminescent graphene quantum dots as potential electron‐acceptors for photovoltaics. Advanced Materials 2011, 23 (6), 776-780. (5) Sun, H.; Wu, L.; Wei, W.; Qu, X. Recent advances in graphene quantum dots for sensing. Materials Today 2013, 16 (11), 433-442. (6) Tian, X.; Peng, H.; Li, Y.; Yang, C.; Zhou, Z.; Wang, Y. Highly sensitive and selective paper sensor based on carbon quantum dots for visual detection of TNT residues in groundwater. Sensors and Actuators B: Chemical 2017, 243, 1002-1009. (7) Zhang, X.; Liu, C.; Li, Z.; Guo, J.; Shen, L.; Guo, W.; Zhang, L.; Ruan, S.; Long, Y. An easily prepared carbon quantum dots and employment for inverted organic photovoltaic devices. Chemical Engineering Journal 2017, 315, 621-629. (8) Pan, Y.; Yang, J.; Fang, Y.; Zheng, J.; Song, R.; Yi, C. One-pot synthesis of gadoliniumdoped carbon quantum dots for high-performance multimodal bioimaging. Journal of Materials Chemistry B 2017, 5 (1), 92-101.


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(42) Lyalin, A.; Nakayama, A.; Uosaki, K.; Taketsugu, T. Functionalization of monolayer h-BN by a metal support for the oxygen reduction reaction. The Journal of Physical Chemistry C 2013, 117 (41), 21359-21370. (43) Khan, A. F.; Brownson, D. A.; Randviir, E. P.; Smith, G. C.; Banks, C. E. 2D Hexagonal boron nitride (2D-hBN) explored for the electrochemical sensing of dopamine. Analytical chemistry 2016, 88 (19), 9729-9737. (44) Britnell, L.; Gorbachev, R. V.; Jalil, R.; Belle, B. D.; Schedin, F.; Katsnelson, M. I.; Eaves, L.; Morozov, S. V.; Mayorov, A. S.; Peres, N. M. Electron tunneling through ultrathin boron nitride crystalline barriers. Nano letters 2012, 12 (3), 1707-1710. (45) Bard, A. J.; Faulkner, L. R., Fundamentals and Applications, New York: Wiley, 2001. Springer: 2002. (46) Shahrokhian, S.; Asadian, E. Simultaneous voltammetric determination of ascorbic acid, acetaminophen and isoniazid using thionine immobilized multi-walled carbon nanotube modified carbon paste electrode. Electrochimica Acta 2010, 55 (3), 666-672.


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Figures:

Figure 1. Characterizations of the prepared BNQD. a) UV-Vis spectrum of BNQD shows an absorption peak at 255 nm. The inset shows blue emission of the dots under UV excitation at 356 nm light. b) PL spectra of BNQD at different wavelength lengths indicates triple luminescence centers. PLE spectrum exhibit the effect of surface functional groups. c) Raman and d) XRD spectra support the formation of high-exfoliated BNQD.


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Figure 2. Size and morphology analysis of BNQD. a) Representative bright-field TEM image and size distribution plot. (b,d) Representative AFM image and the height profile show the formation of disc-like particles with a thickness of 1-2 nm. (c) Size distribution plot obtained by DLS measurement.


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Figure 3. FTIR and high-resolution XPS spectra of BNQD. (a) FTIR spectrum shows that the synthesized nanocrystals are functionalized with hydroxylate groups. Additional surface bonds including N-B-O and O-B-O are visible. Deconvoluted high-resolution XPS spectra of (b) B1s, (c) N1s and (d) C1s reveal oxygen doping in the BN structure


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Figure 4. Effect of BNQD on the electroactivity of gold screen printed electrode. a) Cyclic voltagrams in Fe (CN)63−/4− (5.0 mM). b) Effect of scan rate on cyclic voltammograms of BNQD/GSPE electrode in 5 mM (K3Fe(CN)6)/0.1M KCl solution. The inset shows the anodic peak current versus the square root of scan rate. c) Nyquist diagrams in the presence of Fe(CN)63−/4− (5 mM) in KCl (0.1 M). d) Equivalent circuits of with the Nyquist diagrams


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Figure 5. Electrocatalytic response of BNQD/Au electrode toward VC detection. a) Cyclic voltagrams of GSPE electrode at the VC concentration of 80.0 µM (red) as compared with BNQD-modified GSPE electrode at different VC concentrations of 0.0 µM (green), 80.0 µM (blue) and 240.0 µM (yellow). The solvent was PBS (1.0 mM) and the scan rate was 0.10 Vs−1. b) Effect of scan rate (0.05- 0.30 V s−1) on the electroactivity of BNQD/GSPE electrode in 1.0 mM PBS containing 0.5 mM VC. The insert shows the anodic peak current versus the square root of scan rate. c) Typical amperometric responses of BNQD/GSPE electrode towards VC detection in the concentration range of 100.0 µM to 5.0 mM. The response of the electrode at


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lower VC concentrations (0.80 to 84.8 µ) is shown in the upper inset. The downside inset shows the variation of current density with the VC concentration. These experiments were carried out in PBS (pH 7.4, 25 mL) under stirring condition at 100 rpm. After successive VC standard addition, the amperometric response of the electrode was recorded. d) Selective detection of VC by BNQD/GSPE electrode through successive addition of 40 µM interfering species and 20 µM of VC in 1.0 mM PBS. A constant potential of +0.4 V was applied.


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Figure 1. Characterizations of the prepared BNQD. a) UV-Vis spectrum of BNQD shows an absorption peak at 255 nm. The inset shows blue emission of the dots under UV excitation at 356 nm light. b) PL spectra of BNQD at different wavelength lengths indicates triple luminescence centers. PLE spectrum exhibit the effect of surface functional groups. c) Raman and d) XRD spectra support the formation of high-exfoliated BNQD. 158x124mm (150 x 150 DPI)



Figure 2. Size and morphology analysis of BNQD. a) Representative bright-field TEM image and size distribution plot. (b,d) Representative AFM image and the height profile show the formation of disc-like particles with a thickness of 1-2 nm. (c) Size distribution plot obtained by DLS measurement. 127x115mm (300 x 300 DPI)


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Figure 3. FTIR and high-resolution XPS spectra of BNQD. (a) FTIR spectrum shows that the synthesized nanocrystals are functionalized with hydroxylate groups. Additional surface bonds including N-B-O and O-BO are visible. Deconvoluted high-resolution XPS spectra of (b) B1s, (c) N1s and (d) C1s reveal oxygen doping in the BN structure. 127x111mm (300 x 300 DPI)



Figure 4. Effect of BNQD on the electroactivity of gold screen printed electrode. a) Cyclic voltagrams in Fe (CN)63−/4− (5.0 mM). b) Effect of scan rate on cyclic voltammograms of BNQD/GSPE electrode in 5 mM (K3Fe(CN)6)/0.1M KCl solution. The inset shows the anodic peak current versus the square root of scan rate. c) Nyquist diagrams in the presence of Fe(CN)63−/4− (5 mM) in KCl (0.1 M). d) Equivalent circuits of with the Nyquist diagrams 127x109mm (300 x 300 DPI)


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Figure 5. Electrocatalytic response of BNQD/Au electrode toward VC detection. a) Cyclic voltagrams of GSPE electrode at the VC concentration of 80.0 µM (red) as compared with BNQD-modified GSPE electrode at different VC concentrations of 0.0 µM (green), 80.0 µM (blue) and 240.0 µM (yellow). The solvent was PBS (1.0 mM) and the scan rate was 0.10 Vs−1. b) Effect of scan rate (0.05- 0.30 V s−1) on the electroactivity of BNQD/GSPE electrode in 1.0 mM PBS containing 0.5 mM VC. The insert shows the anodic peak current versus the square root of scan rate. c) Typical amperometric responses of BNQD/GSPE electrode towards VC detection in the concentration range of 100.0 µM to 5.0 mM. The response of the electrode at lower VC concentrations (0.80 to 84.8 µ) is shown in the upper inset. The downside inset shows the variation of current density with the VC concentration. These experiments were carried out in PBS (pH 7.4, 25 mL) under stirring condition at 100 rpm. After successive VC standard addition, the amperometric response of the electrode was recorded. d) Selective detection of VC by BNQD/GSPE electrode through successive addition of 40 µM interfering species and 20 µM of VC in 1.0 mM PBS. A constant potential of +0.4 V was applied.

127x106mm (300 x 300 DPI)



Graphical Abstract 127x90mm (300 x 300 DPI)


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Mechanochemical Green Synthesis of Exfoliated Edge-Functionalized

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