Boron Nitride Quantum Dots with Solvent‐Regulated Blue...

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Boron Nitride Quantum Dots with Solvent-Regulated Blue/Green Photoluminescence and Electrochemiluminescent Behavior for Versatile Applications

Mengli Liu, Yuanhong Xu,* Yao Wang, Xu Chen, Xuqiang Ji, Fushuang Niu, Zhongqian Song, and Jingquan Liu*

M. Liu, Prof. Y. H. Xu, Dr. Y. Wang, Dr. X. Ji, F. Niu, Z. Song, Prof. J. Q. LiuCenter for Micro/Nano Luminescent and Electrochemical MaterialsCollege of Materials Science and EngineeringInstitute for Graphene Applied Technology InnovationLaboratory of Fiber Materials and Modern Textilethe Growing Base for State Key LaboratoryCollaborative Innovation Center for Marine Biomass FibersMaterials and Textiles of Shandong ProvinceQingdao UniversityQingdao 266071, ChinaE-mail: [email protected]; [email protected]. X. ChenState Key Laboratory of Chemical Resource EngineeringBeijing University of Chemical TechnologyBeijing 100029, China

DOI: 10.1002/adom.201600661

such as layered transition metal dichal-cogenides (TMDs),[2] hexagonal boron nitride (h-BN),[3] metal halides,[4] and black phosphorus,[5] which opened up a new horizon for a novel class of low-dimen-sional systems with superior properties for applications in optoelectronics,[6] energy conversion,[7] and catalysis.[8] Among these, h-BN nanosheet has its peculiar and fascinating properties such as good electrical insulation,[9] high-temperature stability,[10] high mechanical strength,[11] large thermal conductivity,[12] low tox-icity and chemical stability,[13] leading to a variety of potential applications as both structural and electronic materials.[14] With further reducing the size of the lay-ered BN sheets to less than 10 nm, 0D BN quantum dots (BNQDs) with excellent fluorescence properties and good dispers-ibility can be endowed due to the quantum confinement, edge effects, and defect centers.[13–15] Combining with the intrinsic properties of BN, BNQDs were sup-

posed to be promising agents in biological and optoelectronic applications.[15b] However, compared with the widely studied 2D BN nanosheet,[9–11,16] BNQDs were much less reported than expected.[13–15]

First, further exploration is required for the preparation of desired BNQDs products. BNQDs were initially prepared via high intensity ultrasound and subsequent refluxing.[14a] Allwood et al. have also successfully fabricated monolayer BNQDs via three steps of potassium-intercalation, deintercalation, and disintegration of BN edges, but only low quantum yield (QY) of 2.5% was achieved.[15a] Recently, sonication–solvothermal strategy was found to be a facile and universal method for the generation of BNQDs with relatively high QY of 8.6% or 19.5%.[13,15b] Although the filling factor of the autoclave, syn-thesis temperature and reaction time in the solvothermal pro-cess have been investigated,[13] little attention has been paid to how different solvents affect the properties of the BNQDs, which is usually an important parameter to influence the optical properties and QYs of the zero-dimensional QDs.[17] Second, besides the most frequently studied PL properties, electro-chemiluminescence (ECL) is also an interesting characteristic

Exploration of novel optical features, generation mechanisms, and versa-tile applicability of boron nitride quantum dots (BNQDs) is still in nascent stage. Herein, BNQDs are prepared using liquid exfoliation–solvothermal treatment of bulk BN in three different solvents. The photoluminescence of BNQDs is blue in ethanol (or N,N-dimethylformamide (DMF)) and green in N-methyl-2-pyrrolidone (NMP) under the same UV-irradiation, respectively. The quantum yields (QYs) and average lateral sizes of the BNQDs are 12.6% and 4.1 ± 0.2 nm, 16.4%, and 2.8 ± 0.3 nm, as well as 21.3% and 2.0 ± 0.2 nm in solvents of ethanol, DMF, NMP, respectively. The distinct sizes, QYs, and optical properties of the BNQDs are found to depend on the polarity of these solvents. Different BNQDs can be chosen on-demand for versatile applica-tions. For example, the green BNQDs are much more competitive than the blue ones when serving as a fluorescent sensing platform for label-free spe-cific detection of ferric ions, while the blue ones in ethanol are much handier for cell imaging and fiber staining. More importantly, electrochemilumines-cence (ECL) property of the BNQDs is observed and studied for the first time using cysteine as a co-reactant. The possible ECL response mechanism of the BNQDs system is also proposed.

1. Introduction

In recent years, the discovery and extensive investigation of gra-phene[1] have encouraged exploration of other 2D nanomaterials

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to evaluate the significative functions and broaden the versatile applicability of the QDs,[15a] however, the ECL features of the BNQDs was seldom reported. Last but not least, the application of the BNQDs was only limited to cell imaging[14a,15b] and composite for proton exchange membrane[15b] till now, thus there remain great chances and chal-lenges for their wide applications.

The gaps and chances existing in this nas-cent field of BNQDs motivated us to explore more efficient synthesis method, fully under-stand the PL properties and discover the new ECL features as well as promote multiple applications. Herein, BNQDs with tunable blue/green photoluminescence (PL) can be generated through the sonication–solvo-thermal treatment of bulk BN in three dif-ferent organic solvents including ethanol, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP). Accordingly, the influ-ence of different solvents on the sizes and PL properties of the as-prepared BNQDs were investigated. The as-prepared BNQDs serving as fluorescent sensing agent for label-free detection of ferric ions, HeLa cells bioimaging, and fiber staining were studied systematically. More importantly, ECL prop-erties were observed with the BNQDs using cysteine as a coreactant for the first attempt. The possible ECL response mechanism of the BNQDs system was also proposed.

2. Results and Discussion

2.1. Synthesis and Characterization of the BNQDs

As shown in Scheme 1, bulk BN powder was first exfoliated to nanosheets via liquid exfoliation by sonication for 3 h. The obtained suspension of BN sheets were first degassed with N2 for 30 min to remove the oxygen of the dispersion, then the dispersion was solvothermally treated at 180 °C for 10 h to fur-ther incise the nanosheets into BNQDs.[13,15b] In this sonica-tion–solvothermal process, the solvent possessing appropriate surface energy to overcome the weak van der Waals forces between the BN layers has to be used, thus three solvents with different polarity including ethanol, DMF and NMP were tested, respectively.[13,15b,17] Thereinto, only different solvents were used for the preparation of BNQDs, other reaction con-ditions such as temperature, time, and filling factors were all kept the same. The as-prepared products were centrifuged for 10 min to obtain the transparent BNQDs suspensions, which were colorless obtained using ethanol or DMF as solvent, and light yellow using NMP (more synthesis details were shown in the Experimental Section). Besides, the as-prepared BNQDs in these solvents were all highly dispersible with good stability, which was owing to the surface bound functionalities compared with pristine BN.[13,15b] Interestingly, under the irradiation with 365 nm UV light, the as-prepared BNQDs exhibited strong blue

photoluminescence for those using ethanol or DMF as solvent, but greenish one for that obtained in NMP, respectively. The BNQDs were named as b1-BNQDs, b2-BNQDs, and g-BNQDs for those obtained using ethanol, DMF, and NMP as solvent, respectively. In addition, the yields of BNQDs were measured to be 33%, 36%, and 41% for b1-BNQDs, b2-BNQDs, and g-BNQDs, respectively, which confirmed the high efficiency of the proposed methods for BNQDs generation. Using quinine bisulfate as the reference, whose quantum yield (QY) is 0.54 in 0.1 m H2SO4, the quantum yields (QYs) were measured to be 12.6%, 16.4%, and 21.3% for b1-BNQDs, b2-BNQDs, and g-BNQDs, respectively. This two-color phenomenon and varied QYs are probably attributed to the synergetic effect of size, sur-face chemistry, and edge defects of the BNQDs resulted from different solvents.

To confirm the BNQDs formation and further explore the effects of solvents on the BNQDs formation, a variety of tech-niques were performed to characterize the three different BNQDs. First, the morphologies of the as-prepared BNQDs were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Figure 1A–F shows their TEM images and corresponding size distributions, which indi-cated that all the as-prepared BNQDs were well dispersed with uniform lateral sizes. The average lateral sizes were 4.1 ± 0.2 nm (Figure 1A,D), 2.8 ± 0.3 nm (Fs,E), 2.0 ± 0.2 nm (Figure 1C,F) for b1-BNQDs, b2-BNQDs, and g-BNQDs, respectively. AFM images (Figure 1G–I) further confirm the uniformity of the as-synthesized BNQDs. The AFM height profile (Figure 1J–L) pre-sents the typical topographic height of the BNQDs ranging from 4.0 ± 0.4, 2.6 ± 0.3, 1.3 ± 0.1 nm for the b1-BNQDs, b2-BNQDs,




Scheme 1. Schematic illustration for the generation of BNQDs with different color under day-light and UV light illumination at 365 nm through the sonication–solvothermal process using ethanol, DMF, or NMP as solvent, respectively.

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and g-BNQDs, respectively. Furthermore, dynamic light scat-tering (DLS) was carried out to characterize the hydrated size of the BNQDs (Figure S1, Supporting Information). Com-paring the three main particle size distributions, the radius of the hydrated BNQDs increased gradually from b1-BNQDs, b2-BNQDs to g-BNQDs, which was consistent with the varia-tion tendency of the lateral sizes obtained from TEM and AFM characterization. The solvent polarity, which is usually directly proportional to the surface energy[18] to overcome the weak van der Waals forces between the BN layers in solvothermal pro-cess, should be one important parameter in controlling the size of the BNQDs.[13,15b,17] It was known that the polarities were 4.3, 6.4, 10.2 for ethanol, DMF, and NMP, respectively,[19] thus it can be supposed that both the lateral diameter and height of

the as-prepared BNQDs should decrease with the increasing polarity of the organic solvents. The higher the polarity of the solvent, the interaction between BN nanosheets and organic solvents will be stronger, causing an easier exfoliation of bulk BN to nanosheets and then to BNQDs.

Accordingly, the solvent with higher polarity would result in smaller lateral size and lower height of BNQDs. High-resolu-tion TEM (HRTEM) images (insets of Figure 1A–C) revealed that all the three kinds of BNQDs have good crystallinity with lattices of 0.21–0.22 nm, which agrees well with the (100) face of the BN crystal.[15a] In addition, control experiments were car-ried out to exclude the possibility of generating carbon nano-dots (CNDs) from carbonization of solvents.[20] 15 mL of pure solvents including ethanol, DMF, and NMP were added in




Figure 1. A–C) TEM images and D–F) size distributions as well as G–I) the AFM topography images of the b1-BNQDs, b2-BNQDs, and g-BNQDs, respectively. J–L) The height profiles performed along the white line plotted in AFM images (G–I), respectively. Insets of (A–C) show the corresponding HRTEM images.

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different beaker (20 mL), respectively, and kept sonication for 3 h. Then the solvents were degassed with N2 for 30 min to remove the oxygen, subsequently they were decanted into auto-clave and under solvothermal treatment at 180 °C for 10 h in vacuum drying oven. However, no quantum dots were obtained, which should be because the participation of O2 is critical for the CNDs generation but was removed using pretreatment of N2 in our present experimental conditions.[20] These again con-firmed the fluorescent QDs were derived from the exfoliation of the bulk BN as starting materials.

The crystal structure of these as-prepared BNQDs was also characterized by XRD measurements. The main peak at 2θ = 26° should correspond to the (002) face of bulk h-BN (curve a in Figure 2A). The peak position corresponds to the d-spacing of crystals, theoretically, if the materials are mon-olayer and have no interaction between layers, there would be low or no signal peak on the XRD pattern.[17] After ultrasonica-tion and subsequent solvothermal treatment of the bulk h-BN, the signal of the (002) reflection was broadened and obviously decreased, accompanying with the disappearance of most other peaks for all the three BNQDs (curves b–d in Figure 2A), being ascribed to the lack of interlayer action and reduced crystallinity of the layered materials upon the exfoliation process. The XRD results confirmed the formation of these BNQDs with highly exfoliated structure.[15b,17]

The surface functional groups on BNQDs were analyzed by Fourier transform infrared (FTIR). As can be seen in Figure 2B, the three BNQDs exhibited similar FTIR spectra, revealing their similar chemical compositions. The as-prepared b1-BNQDs, b2-BNQDs, g-BNQDs, and bulk BN all showed absorption at around 1390 and 784 cm−1, which should be attributed to B–N stretching and B–N bending modes, respectively.[15a] Compared with the bulk BN starting materials, new characteristic peaks in the FTIR spectrum of BNQDs were observed at 2931, ≈1637, and ≈1098 cm−1, corresponding to –CH3, C–(BN), and N–B–O vibrations, respectively, which demonstrated the attachment of the solvent molecules and oxygen-containing functional groups onto the BNQDs surface.[13,21] In addition, the relative intensity of N–B–O to B–N was the highest in g-BNQDs and the smallest in b1-BNQDs, suggesting the oxygen-containing functional groups attached to the surface of g-BNQDs were more than those to other two. It can be seen that the oxygenous content in the three respective BNQDs was proportional to the polarity

of the used solvents. Since the oxygen-containing functional groups were suggested to play essential roles in the exfoliation of the bulk BN, it was anticipated that the exfoliation efficiency of the bulk BN into BNQDs was dependent on the polarity of the solvents.

X-ray photoelectron spectroscopy (XPS) was further carried out to monitor the composition and functional groups on the BNQDs. The three BNQDs all exhibited the same elemental species as indicated from the four main peaks at 535.2, 401.2, 285.0, and 193.1 eV in the survey spectra corresponding to the elemental species of C, N, O, and B (Figure 3A–C), respectively, which again confirmed the BNQDs can attach solvent molecules during the sonication–solvothermal process. Meanwhile, similar chemical bonds but with certain difference were observed for the respective BNQDs (Figure 3D–I). Figure 3D–F showed the spectra of B 1s, which could be deconvoluted into two peaks, respectively. Peaks centered at 190.7 (190.3 or 190.6) eV and 191.3 (191.1 or 191.5) eV should be attributed to BN and BO bonding, respectively.[15a,22] It was observed that the rela-tive intensity ratio of BO to BN increased gradually from ethanol to DMF and then to NMP. While the peaks at 398.9 (398.0 or 398.7 eV) and 400.7 (400.0 or 399.9) eV in the high-resolution spectrum of N should be assigned to NB and NO/NC, respectively.[15a] The relative intensities of NO to NB have the same changing trend as those of BO to BN with the increasing solvent polarity. The oxygen contents were estimated to be 15.21%, 22.67%, 23.43% for b1-BNQDs, b2-BNQDs, and g-BNQDs, respectively. In addition, the surface charge properties of the BNQDs were further investigated by measuring the respective Zeta potentials, which were −17.6, −29.3, and −49.9 mV for b1-BNQDs, b2-BNQDs, and g-BNQDs, respectively, indicating the presence of negatively charged oxygen-containing groups on the surface of the BNQDs. The Zeta potential results also evidenced that the stronger polarity of the used solvents will result in more oxygen-containing groups on the BNQDs. All these results suggested more solvent molecules and oxygenous groups were attached on the surface of BNQDs resulting from the higher polarity of the solvents,[15a] which were consistent with the FTIR results.

Based on the above-mentioned characterization results and analysis, reasonable mechanism for BNQDs generation can be proposed as follows: (1) Upon the ultrasonic treatment in the presence of solvent with appropriate surface energy, the bulk




Figure 2. A) XRD patterns and B) FTIR spectra of the as-prepared a) bulk BN, b) b1-BNQDs, c) b2-BNQDs, and d) g-BNQDs, respectively.

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BN can be exfoliated into BN sheets with surface defects at the edge and on the basal plane. (2) Further solvothermal treatment could lead to the attachment of solvent molecules and forma-tion of oxygen-containing functional groups on the BN sheets, which could create defects on BN sheets and serve as chemically reactive sites that allow BN to be cleaved into smaller sheets and then to QDs. This is consistent with most top-down strate-gies for QDs generation from layered starting materials.[23]

Increasing polarity of the solvent led to higher surface energy to overcome the van der Waals forces between the BN layers, resulting in higher exfoliation of the BN and formation of the more chemically reactive sites, which benefit for more efficient cutting of BN nanosheets. As a result, the higher cut-ting-off efficiency could result in much more efficient prepara-tion of BNQDs with smaller size and more chemically reactive sites. And so forth, the higher the polarity of the solvent, the smaller the size of the as-prepared BNQDs. Combining the best dispersive capacity with the higher cutting ability of NMP, the as-prepared g-BNQDs exhibited the highest QY.[17] In addition, the color difference among the BNQDs should be attributed not only to the particle size, but also to the different oxygen contents, since energy difference via moderate reduction may contribute to the blue-shift.[23] Thus, the higher the polarity of the solvent was, the more oxygen content will be obtained with the BNQDs. Accordingly, the color of the BNQDs was blue

in ethanol and DMF but green in NMP that is with relatively higher polarity.

To further explore the optical properties of the BNQDs, PL and UV–vis absorption spectra were measured (Figure 4). Figure 4A–C depicts the UV–vis spectra of the resultant BNQDs in the three different solvents. As observed, all the BNQDs exhibited typical UV–vis absorption at about 310 nm. While another much stronger absorption at 255 nm was obtained for the b1-BNQDs.[13] In the optimal excitation and emission PL spectra (Figure 4D–F), g-BNQDs had maximum excitation and emission wavelengths at 380 and 505 nm and showed bright green fluorescence under 365 nm UV irradiation (Figure 4F), while these peaks blue-shifted to 373 and 445 nm for b2-BNQDs as well as 327 and 420 nm for b1-BNQDs, respectively. Accord-ingly, both the b1-BNQDs and b2-BNQDs exhibited distinct blue fluorescence under the same UV irradiation (Figure 4D,E). Moreover, there were two sharp excitation peaks located at 270 and 327 nm for the PL spectra of b1-BNQDs, which were dif-ferent from those of b2-BNQDs, g-BNQDs, and previously reported BNQDs.[13,15] The different UV–vis adsorptions and PL phenomena for the three BNQDs should be resulted from the different sizes and surface chemical functionalities such as oxygeneous groups content in the respective BNQDs.[24] All in all, the higher the polarity of the solvent was, the smaller the size of the as-prepared BNQDs was, subsequently the more




Figure 3. The survey XPS spectra A–C), narrow scan spectra of B 1s D–F), and N 1s G–I) of the as-prepared b1-BNQDs, b2-BNQDs, and g-BNQDs, respectively.

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red-shift of the emission peak was obtained in the PL spectra. This is because the emission peak of BNQDs with smaller size is red shifted in comparison with that of the BNQDs with larger size as confirmed in the previous reports.[13] These results also confirmed the deduction that the higher oxygen contents in the BNQDs resulting from the higher solvent polarity would lead to red shift of the PL emission of the BNQDs.[23]

The typical PL emissions of the BNQDs under different excitation wavelengths are shown in Figure 4G–I. The intensi-ties of the three kinds of BNQDs’ suspensions exhibited exci-tation-dependent PL behaviors, which first increased and then decreased with the increasing excitation wavelength. Inter-estingly, the maximum emission wavelength of b1-BNQDs suspension remained constant with the changing excita-tion wavelength (Figure 4G), showing its λex-independent emission behavior. While the other two BNQDs showed λex-dependent one. As shown in Figure 4H,I, upon increasing the excitation wavelength, the PL peaks shifted gradually from 400 to 500 and 450 to 575 nm for b2-BNQDs and g-BNQDs, respectively. As far as we know, λex-independent emission behavior of the BNQDs in ethanol was observed for the first time.[13,15a,25] Through careful evaluation of the characteriza-tion data, only the particle sizes and element contents of the three BNQDs were found to be different from each other,

which were resulted from different solvents used for the son-ication-solvothermal process. These variations should be, at least partly, responsible for the different PL characteristics of the three BNQDs. The PL results again confirmed the impor-tant role of the solvent for BNQDs generation and the corre-sponding properties. In addition, the excitation wavelength of 327 nm for b1-BNQDs, 373 nm for b2-BNQDs, and 380 nm for g-BNQDs (Figure 4D–F), at which the maximum fluores-cence emission intensities were obtained, were selected for the following experiments.

2.2. Applications

As demonstrated above, the as-prepared BNQDs possess attrac-tive features including the small size, good stability, and excel-lent optical properties, thus the applicability of the BNQDs was investigated in the following experiments. Since various pH environments are often encountered in practical applications, the effects of pH on the PL intensity of the BNQDs were inves-tigated. It can be seen that no significant changes of the fluores-cence response were observed for the BNQDs upon changing the suspension pH ranging between 2.0 and 12.0 (Figure S2, Supporting Information). These results indicated that the




Figure 4. A–C) UV–vis absorption spectra, D–F) optimal excitation, and G–I) emission PL spectra and emission (Em) spectra at progressively increasing excitation wavelengths from 250 to 380, 270 to 420, or 280 to 490 nm for the as-prepared BNQDs, respectively. A,D,G) b1-BNQDs, (B,E,H) b2-BNQDs, and C,F,I) g-BNQDs aqueous dispersion.

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as-prepared BNQDs showed excellent stability against pH varia-tion, which is favorable for further applications.

2.2.1. Fluorescent Staining and Bioimaging

First, the cotton and silk fibers were stained with the BNQDs dispersion, rinsed by water, and then dried under air at 50 °C. As can be seen in Figure 5 and Figure S3 in the Supporting Information, excitation wavelength-dependent fluorescence was observed for both the silk (Figure 5) and cotton fiber (Figure S3, Supporting Information) stained with the three BNQDs, respectively. Both the silk and cotton fibers exhibited green and red staining when exposed to blue (450–480 nm) and green (510–550 nm) light excitation. This result confirmed that the as-prepared BNQDs can all be applied as valuable fluoro-chromes for chemical or biocompatible staining.

2.2.2. Cell Imaging and Cytotoxicity Measurement

Based on the benign nature of BN, the as-prepared BNQDs have also been tested in cancer cell imaging applications.

Since the cell imaging test should be conducted in aqueous medium, the solvents should be removed and then redis-persed in water to obtain the final BNQDs aqueous suspen-sion. Compared with the boiling points of DMF (152.8 °C) and NMP (202 °C), the boiling point of ethanol (78.4 °C) was much lower, which makes ethanol much easier to be removed from the BNQDs at a moderate condition. Accordingly, the as-prepared b1-BNQDs could be more handy to be transferred to aqueous medium and therefore, were chosen for the following bioimaging test using the HeLa cells as a model. After being incubated with b1-BNQDs for 1 h at 37 °C and 5% CO2, the images were collected with a laser scanning confocal micro-scope (LSCM). As illustrated in Figure 6A, the HeLa cells treated with b1-BNQDs showed bright green fluorescence under 488 nm. Furthermore, we performed the costaining experiments with a typical fluorescent markers (LysoTracker Red) and the b1-BNQDs. As can be seen in Figure S4 in the Supporting Information, the BNQDs also showed strongly fluorescent spots that colocalized with the LysoTracker dyes within the cells, which further confirmed the successful fluo-rescent imaging ability of the BNQDs onto the cells.[26] The easy cellular uptake of the fluorescent BNQDs for efficient bioimaging should be ascribed to the small size of BNQDs




Figure 5. A,D,G) Bright field and B,C,E,F,H,I) fluorescence images of silk fiber stained with B,C) b1-BNQDs, E,F) b2-BNQDs, and H,I) g-BNQDs, respectively. The fluorescent images were obtained at the excitation wavelengths of (B,E,H) 510–550 nm and (C,F,I) 450–480 nm. Scale bar: 50 µm.

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(<5 nm), which can be internalized into the cells with excel-lent cytocompatibility.[27]

The cytotoxicity of BNQDs was further tested by MTT assay (Figure 6D). The cell viability has the trend to be slightly decreased with the increasing concentrations of the BNQDs, taking 0, 50 100, 150, 200, 250, 300, and 350 µg mL−1 as exam-ples, respectively. Survival rates of the cells reached 93% in the culture medium containing BNQDs at the maximum tested concentrations of 350 µg mL−1. These MTT assay confirmed the excellent biocompatibility of the as-prepared BNQDs toward the cells. Moreover, with a prolonged irradiation time of 4 h, only slight fading was observed with the green fluorescence signals (see Figure S5 in the Supporting Information). These results indicated that the as-prepared BNQDs could be used as an excellent fluorescent bioimaging agent owning to their high stability, good biocompatibility, low toxicity, and excellent resist-ance to photobleaching.

2.2.3. Specific Ferric Ion (Fe3+) Detection

Due to the different optical properties of the BNQDs resulted from different tested solvents, the fluorescent response to metal ion should also be variable with different BNQDs. The impact of different metal ions (all at 320 × 10−6 m) including Ni2+, Fe2+, Ag+, Fe3+, Al3+, Cd2+, Co2+, Cu2+, Mg2+, Hg2+, Mn2+, Pb2+, Zn2+

on the PL intensity of the three BNQDs at the same concentration was studied, respectively. It was observed that all the PL intensities of BNQDs can be obviously quenched upon the addition of Fe3+ with the quenching degree in the order of b1-BNQDs > b2-BNQDs > g-BNQDs. Meanwhile, the addition of Cu2+ can only result in PL intensity decrease of b1-BNQDs and b2-BNQDs, while bring no obvious influence to that of g-BNQDs. It can also be obtained that the quenching trend of the respective BNQDs PL inten-sity upon Cu2+ addition was the same as that of Fe3+. Moreover, other metal ions dis-played slight or even negligible effects on the PL intensities of all the three BNQDs (Figure 7A–C). Similarly to other QDs, the PL intensity quenching of BNQDs upon the addition of Fe3+ or Cu2+ should be due to the coordination between Fe3+ or Cu2+ and the oxygeneous groups on the BNQDs sur-faces,[28] leading to decreased dispersibility and stability of the BNQDs suspension and subsequent BNQDs aggregation and fluo-rescence quenching of the BNQDs.[29] As demonstrated above, the oxygen content in the BNQDs was in the order of b1-BNQDs < b2-BNQDs < g-BNQDs, thus the ratio of the oxygen-containing groups bound with Fe3+ (or Cu2+) upon addition of the same concen-tration of Fe3+ (or Cu2+) was in the order of b1-BNQDs > b2-BNQDs > g-BNQDs, which meant the quenching effect of Fe3+ (or Cu2+)

addition on the PL intensity of BNQDs would also be in the sequence of b1-BNQDs > b2-BNQDs > g-BNQDs.

As observed from the above comparison results, Fe3+ has highly selective quenching effect toward the PL intensity of the g-BNQDs, thus the specific assay for Fe3+ can be established using g-BNQDs as fluorescent probe. As shown in Figure 7D, the fluorescence quenching of BNQDs upon the addition of dif-ferent concentrations of Fe3+ were dosage dependent (Figure 7D and Figure S6 in the Supporting Information). Figure 7E plots the quenched PL intensity (ΔI = I0 − Ii) of g-BNQDs against the Fe3+ concentrations. Linear dependence of ΔI on the con-centration of Fe3+ from 10 × 10−6 to 300 × 10−6 and 340 × 10−6 to 900 × 10−6 m was derived, respectively, with linear regres-sion equations of ΔI = 0.75C − 3.96 (correlation coefficient (R2) = 0.993) and ΔI = 0.43C + 191.9 (R2 = 0.994), respectively. The limit of detection for Fe3+ was found to be 3.0 × 10−6 m (S/N = 3), which is more superior or comparable to those pre-vious reported.[30] Obviously, this method had the advantages of wider linear ranges and simple process procedures compared with previous ones for Fe3+ detection such as atomic absorption spectrophotometry analysis.[31]

2.2.4. ECL Response of the BNQDs

ECL is one means to convert electrical energy into radiative energy, by which electroactive species were transferred to those




Figure 6. LSCM images of HeLa cells incubated with BNQDs of 100 µg mL−1 for 2 h at 37 °C. Fluorescence images A) and bright field images B); excitation wavelength was set at 488 nm. The image C) is the overlapped image of (A) and (B). D) Cell viability of HeLa cells (n = 3, mean ± S.D.) after being treated with BNQDs suspensions of various final concentrations (0, 50, 100, 150, 200, 250, 300, and 350 µg mL−1, respectively). Untreated cells were served as the control, whose viability was set as 100%.

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emitting measurable luminescent signals at electrodes via an applied voltage.[32] The QDs are suggested to be a promising alternative as ECL luminophores to traditional ones because of their excellent chemical inertness, easy labeling and low toxicity. Whether the QDs possess ECL features is determined by their surface states and the ECL generation mechanism is distinct from that of PL. Thus ECL is another important way to broaden the applicability of QDs. Accord-ingly, whether the BNQDs possess ECL behaviors was investigated.

ECL behaviors of the obtained BNQDs were studied in the presence of 5 × 10−3 m coreactant, such as L-cysteine (RSH), K2S2O8, H2O2, glutathione, and citric acid, etc. The ECL intensities of the three kinds of the as-prepared BNQDs were all obviously increased in the presence of RSH, while the other core-actants displayed weak or even negligible effects on the ECL intensities of the BNQDs. These observations reflected that RSH was the most efficient coreactant for the ECL systems of BNQDs. As shown in Figure 8A, taking b1-BNQDs as an example, when the potential was cycled between −2.0 and 0 V, an obvious ECL emission at −1.91 V was observed. Upon addition of 5 × 10−3 m RSH as coreactant, the ECL intensity was about five times of that for the b1-BNQDs. No obvious ECL was observed for RSH alone. All these results confirmed the irreplaceable roles of b1-BNQDs as ECL luminophores and RSH as coreactant in the present ECL systems. The possible cathodic ECL mechanism of the BNQDs/RSH system was intuitively illustrated in Figure 8B.

The possible mechanism can also be described with the fol-lowing equations.[33] Generally, the thiol group of RSH was primarily electrochemically oxidized to RS• radical, which was further oxidized to RSO2

• one through the participation of dis-solved oxygen (O2). The produced RSO2

• radical reacted with RSH to form RSO• and RSOH. Then RSO• was further oxidized to RSO3

• by the dissolved O2. Subsequently, RSO3• was reduced

Figure 7. Remained percentage of the PL intensity of b1-BNQDs, b2-BNQDs, g-BNQDs upon addition of different metal ions (all at 320 × 10−6 m) excited at 270 nm A), 373 nm B), and excited at 380 nm C). D) PL intensity of g-BNQDs excited at 380 nm in the presence of Fe3+ ions at different concentrations from 10 to 900 × 10−6 m. E) Plot of quenched PL intensity (ΔI = I0 − Ii) of g-BNQDs against the concentration of Fe3+. F) Schematic illustration of PL quenching mechanism of BNQDs by Fe3+.

Figure 8. A) ECL intensity–potential curves of 100 mg mL−1 b1-BNQDs in 100 × 10−3 m PBS (pH 7.0) a) with and b) without 5 × 10−3 m l-cysteine (RSH) and, c) 5 × 10−3 m RSH alone. B) Schematic illustration of the ECL mechanism of the BNQDs/RSH system. C) Influences of buffer pH on the ECL intensity of 100 mg mL−1 b1-BNQDs. D) ECL intensity–time curve under continuous CV scanning between −2.0 and 0 V for 600 s for b1-BNQDs/RSH system. The scan rate is 100 mV s−1.

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by RSH to produce RSO2• and RSOH. As a result, abundant

free radical intermediates including RS•, RSO•, RSO2•, and

RSO3• were produced during the anodic oxidation of RSH. On

the other hand, electrons were transferred from Pt electrode to the BNQDs, resulting in the generation of BNQDs•−. Then, the free radicals (RSO•, RSO2

•, and RSO3•) could react with

BNQDs•− via electron transfer to produce BNQDs in excited state which finally emitted measurable luminescent signals.[33]

RSH e RS HRS O RSORSO RSH RSO RSOHRSO O RSORSO RSH RSO RSOHBNQDs+e BNQDsBNQDs RSO /RSO /RSO BNQDs RSO /RSO /RSOBNQDs BNQDs h

*

*

2 2

2

2 3

3 2

2 3 2 3

υ

− → ++ →

+ → ++ →+ → +

→+ → +→ +

− • +

• •

• •

• •

• •

− •−

•− • • • − − −

Since the buffer pH is a crucial factor to influence the ECL response of the QDs, the influence of the buffer pH on the ECL behaviors of BNQDs was also investigated. As shown in Figure 8C, the ECL intensity increased with the increasing buffer pH from 3.0 to 7.0, then began to decrease when the pH was over 7.0. The effect of the pH on the ECL intensity may be ascribed to the instability of the free radicals (RSO•, RSO2

•, and RSO3

•) in acidic or alkaline conditions.[33] Therefore, pH 7.0 was chosen as the optimized pH value of the background buffer for this ECL system. The dependence of the ECL inten-sity on the RSH concentration was also studied. Figure S7 in the Supporting Information shows that the ECL intensity grad-ually increased with the increasing concentration of RSH, and tended to reach a plateau after 5 × 10−3 m. Therefore, 5 × 10−3 m was chosen as the optimal concentration of the coreactant RSH. Figure 8D and Figure S9 (Supporting Information) show the ECL emissions of b1-BNQDs (Figure 8D), b2-BNQDs (Figure S8A, Supporting Information) and g-BNQDs (Figure S8B, Sup-porting Information) in the presence of 5 × 10−3 m RSH under continuous 14 cyclic voltammetric sweeping, respectively. All the ECL emissions were highly repeatable when the potential was cycled between −2.0 to 0 V, indicating the good stability of the proposed ECL system for BNQDs. Furthermore, the ECL spectra of the BNQDs system were also recorded by employing a series of optical filters, taking b1-BNQDs as an example, a distinguished ECL spectral peak at ≈555 nm was observed, which fell into the wavelength region between green and yellow (Figure S9, Supporting Information). Compared with the PL spectrum, the ECL emission peak of b1-BNQDs was red shifted, which was in accordance with the previous reports.[34] This indicated that the BNQDs possessed stable and intense ECL properties and could be envisioned for broader potential applications in sensing and imaging.

3. Conclusion

In summary, BNQDs exhibiting green and blue photolu-minescence under the same ultraviolet irradiation have

been successfully prepared through the liquid exfoliation–solvothermal treatment of bulk BN in three different organic solvents. The quantum yields and average lateral sizes of the as-prepared BNQDs reached 12.6% and 4.1 ± 0.2 nm, 16.4% and 2.8 ± 0.3 nm, as well as 21.3% and 2.0 ± 0.2 nm in ethanol, DMF and NMP, respectively. The varied sizes and optical fea-tures of the as-prepared BNQDs were ascribed to the different polarity of the solvents. The green BNQDs were found to be much more competitive than the blue ones as a competitive fluorescent sensing probe for label-free specific detection of Fe3+. Moreover, the BNQDs could be successfully used as fluo-rescent probes for cancer cell imaging and fiber staining, con-firming the low cytotoxicity and excellent biocompatibility of the BNQDs. More importantly, ECL property is also observed for the first time from the BNQDs using cysteine as a coreac-tant, suggesting the promising applications of BNQDs in ECL sensing and imaging. The possible ECL response mechanism of the BNQDs system was also proposed. The present work not only enriched the fundamental study about the optical prop-erties of BNQDs but also promoted the wide applications of BNQDs based on the excellent PL and ECL features.

4. Experimental SectionReagents: Ethanol (AR), DMF (AR), NMP (AR), potassium phosphate

monobasic (99.5%, AR), potassium phosphate dibasic (99%, AR), and boron nitride (98.5%, AR) were obtained from Shangdong Laiyang Economic and Technological Development Zone Chemical Plant (Shandong, China). Silver nitrate, mercuric nitrate, 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT), dimethyl sulfoxide (DMSO), and quinine sulfate were purchased from Aladdin Reagent Co., Ltd (Shanghai, China). Aluminum chloride, sodium sulfate, chromium acetate, ferric chloride, copper chloride, cobalt chloride, ferrous chloride, magnesium chloride, lead acetate, manganese chloride, zinc chloride, and nickel nitrate were obtained from Tianjin Reagent Co., Ltd (Tianjin, China). The platinum foil electrodes and Ag/AgCl (3.0 m NaCl) electrode were purchased from Tianjin Aida Hengsheng Technology Co. Ltd (Tianjin, China). Ultrapure water was produced using a Flom ultrapure water system (Qingdao, China).

Apparatus: All electrochemical measurements were performed with a conventional three-electrode cell on a CHI 760D electrochemical workstation (CH Instruments, Shanghai, China). Two platinum foil electrodes and an Ag/AgCl (3.0 m NaCl) electrode were used as the working, counter, and reference electrodes, respectively. All potentials were carried out in a one-compartment electrochemical cell at room temperature. ECL signals were collected by a computer controlled chemiluminescence system (Xi’an Remex Electronics Co. Ltd. Xi’an, China). The voltage of photomultiplier tube (PMT) for collecting the ECL signal was set at 1000 V in the process of detection. The UV–vis spectra of samples were recorded on a UV–vis–near infrared reflection spectrophotometer (Lambda 950, Perkin Elmer Instruments). FTIR spectra were collected on a Nicolet Nexus 5700 (Thermo Electron Corporation, USA) using KBr pellets. The fluorescence was measured on a Cary Eclipse Fluorescence spectrophotometer (Varian Co., Australia). The excitation/emission slits were set at 5.0 nm × 5.0 nm. XPS data were obtained on an ESCALab220i-XL electron spectrometer (VG Scientific, West Sussex, UK) using 300 W Al Kα radiation. TEM images were obtained with a FEI TECNAI G2 transmission electron microscope (Eindhoven, Netherlands) operating voltage of 120 kV. AFM images were obtained using a SPI3800N microscope (Seiko Instruments Inc.) operating in the tapping mode. DLS analysis and zeta potential were measured using a Nano Zetasizer (Malvern, England).

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Preparation of BNQDs: Bulk BN powder was first exfoliated to nanosheets by solvent exfoliation method. 0.1 g BN powder and 15 mL of ethanol, DMF, and NMP were added in different beaker (20 mL), respectively, and kept sonication for 3 h to exfoliate BN powder in to nanosheets by ultrasonic cell crusher with an output power of 400 W. The obtained BN nanosheets were first degassed with N2 for 30 min to remove the oxygen of the dispersion, then the dispersions were decanted into autoclave with the filling factor of 2/3 and subsequently treated under solvothermal conditions of 180 °C for 10 h in vacuum drying oven, followed by being cooled down naturally to room temperature. Afterward, the resulted suspensions were centrifuged at 10 000 rpm for 5 min to remove the precipitation and the supernatant containing the as-prepared BNQDs were collected.

Quantum Yield Measurements: Quantum yield of the BNQDs was determined on the basis of previously established procedure.[35] Typically, quinine sulfate (literature quantum yield: 0.54) in 0.1 m H2SO4 was chosen as a standard.[35b,36] In order to minimize the reabsorption effects, the absorbance of the BNQDs dispersion and reference sample should be kept below 0.10 and 0.05 when excited at 335, 350, and 360 nm using ethanol, DMF, NMP as solvent, respectively. Quinine sulfate was dissolved in 0.1 m H2SO4 while the BNQDs were dissolved in ethanol, DMF, and NMP, respectively. The quantum yield of the BNQDs was calculated using the equation below[37]

STGradGradX

X

ST

X2

ST2ϕ ϕ η

η=

(1)

where the subscripts ST and X refer to quinine sulfate and BNQDs, respectively, ϕ represents the fluorescence QY. Grad represents the gradient from the plot of integrated fluorescence intensity versus absorbance, and η is the refractive index of the solvent.

The Intracellular Uptake of BNQDs, Bioimaging and MTT Assays: HeLa cells (106 cells per sample) were plated onto 35 mm glass chamber slides. The as-prepared BNQDs dispersion was first distilled to remove ethanol and diluted in deionized water to reach the storage concentration of 500 µg mL−1. BNQDs dispersion at concentration of 60 µg mL−1 in DMEM were then freshly prepared and placed over the cells for 24 h at 37 °C. Subsequently the cells were washed thoroughly three times with PBS to remove the free and physically absorbed BNQDs. Finally, the cellular images were taken by a Leica TCS SP2 confocal laser scanning microscope (Leica Microsystems Heidelberg GmbH, Germany) with excitation wavelength of 488 nm from the Ar laser.

MTT assays were used to evaluate the BNQDs doses on the viability of the HeLa cells. The cells were treated with various concentrations of BNQDs (0, 50, 100, 150, 200, 250, 300, 350 µg mL−1) in fresh DMEM for 24 h. Treated cells were mixed with DMEM containing MTT (10 mL, 5 mg mL−1 in PBS solution) and further incubated at 5% CO2, 37 °C for 4 h. Then the MTT containing medium was added to each well with 100 µL DMSO to solubilize the formazan crystals precipitate. Viability of untreated control cells was arbitrarily defined as 100%. Finally, the absorption at 490 nm of each well was measured by an EL808 ultramicroplate reader (Bio-TEK Instrument, Inc., Winooski, VT, USA).

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsM.L. and Y.X. contributed equally to this work. This work was supported by the National Nature Science Foundation of China (Nos. 21305133, 21575071, and 31301177), Qingdao Innovation Leading Expert Program, Qingdao Basic & Applied Research project (15-9-1-100-jch),

Open Funds of the State Key Laboratory of Electroanalytical Chemistry (SKLEAC201601), and Science & Technology Fund Planning Project of Shandong Colleges and Universities (J16LA13).

Received: August 12, 2016Revised: October 23, 2016

Published online: December 27, 2016

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