Surrounding media sensitive photoluminescence of boron-doped graphene quantum dots for highly fluorescent dyed crystals, chemical sensing and bioimaging

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Surrounding media sensitive photoluminescence of boron-doped graphene quantum dots for highly fluorescent dyed crystals, chemical sensing and bioimaging Zetan Fan a, Yunchao Li a, Xiaohong Li a, Louzhen Fan Decai Fang a,*, Shihe Yang c,* a b c

a,* ,

Shixin Zhou b,

Department of Chemistry, Beijing Normal University, Beijing 100875, China Department of Cell Biology, School of Basic Medicine, Peking University Health Science Center, Beijing 100191, China Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

A R T I C L E I N F O

A B S T R A C T

Article history:

We demonstrate that boron-doping of graphene quantum dots (B-GQDs) give rise to rich

Received 10 November 2013

fluorescence owing to their peculiar interaction with the surrounding media. A B-GQDs

Accepted 24 December 2013

borax (Na2B4O7Æ10H2O) solution was shown to be highly fluorescent in green, and upon

Available online 5 January 2014

evaporation, formed bright green fluorescent crystals. However, removing the borax left behind only blue and yellow luminescent solutions. These intriguing findings are rationalized by the insight that B-GQDs associate in the absence of borax whereas they form dative valence bonds with the bridgehead O atoms of borax in the presence of borax. We further demonstrate the use of the B-GQDs, for example, in chemosensor for detecting Al3+ and biomarker for cellular imaging.  2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene is a zero-bandgap semiconductor, so no optical luminescence is observed in pristine form. A bandgap, however, can be engineered into graphene quantum dots (GQDs) due to quantum confinement and edge effects. As a result, luminescent GQDs have attracted great attention as highly promising bionanomaterials due to their exceptional advantages of low cytotoxicity, excellent solubility, stable photoluminescence (PL), biocompatibility as well as low-cost [1–6]. These fascinating merits distinguish the GQDs from traditional fluorescent materials, making them suitable alternatives for numerous exciting applications: bioimaging [7,8], fluorescence sensors [9–12], fuel cells [13,14], photovoltaic devices [15,16]. Most of the efforts on GQDs have been focused on theoretical predictions, and the experimental synthesis

and characterization of GQDs are only recent efforts. Blue and green fluorescence with a relatively narrow bandwidth when excited with UV irradiation has been detected from GQDs [17–20]. Recently, we reported an approach of electrochemical reaction followed by room temperature reduction for synthesizing water-soluble GQDs with strong yellow PL and established their use in imaging stem cells for exploring their native development and tissue regeneration [21]. Doping carbon materials with heteroatoms can effectively tune their intrinsic properties, thus producing new phenomena and unexpected characteristics. Although various heteroatom-doped carbon materials (e.g., P-doped graphite layer [22], B-doped CNTs [23], B (S or I)-doped graphene [24,25]) have been reported, doped GQDs have been much less studied. To date, only N doped GQDs have been reported by Qu’s group [14], which was demonstrated to possess unique

* Corresponding authors: Fax: +86 10 5880 2075. E-mail address: [email protected] (L. Fan). 0008-6223/$ - see front matter  2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.12.085

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optoelectronic properties due to the N-doping-induced modulation of the chemical and electronic characteristics of the GQDs. Herein, we report an electrochemical approach for the synthesis of B doped GQDs (B-GQDs) with B content of 5.24 at %, in which only borax was used as the electrolyte. The water-soluble B-GQDs feature intriguing rich fluorescence.

2.

Experimental section

2.1.

Preparation of B-GQDs

High purity graphite rods (99.9%) were purchased from Shanghai Carbon Co., Ltd. All solvents and reagents were purchased from J&K and used without further purification. The electrolysis of graphite rod was performed on CHI 705 electrochemical workstation with a current intensity in the range of 80–160 mA cm2. The graphite rod was inserted into 10 mL 0.1 M borax (99.5%) aqueous solution, placed parallel to a Pt foil as counter electrode. After 2 h electrochemical reaction, the water soluble B-GQDs were collected by filtering the resulting solution by 0.22 lm filter membrane to remove the precipitated graphite oxide and graphite particles. Then the obtained solution was dialyzed over deionized water in a dialysis bag (nominal 3500 Da) for one day to remove the electrolyte of borax. About 2 mg B-GQDs were obtained after 94 mg graphite exfoliated from graphite rod. The QY of the B-GQDs was determined by comparing the integrated PL intensities (excited at 400 nm) and the absorbance values (at 400 nm) using rhodamine 6G in ethanol (QY = 95%) as a reference.

2.2.

Preparation of green PL crystals

Green PL crystals were obtained by evaporating B-GQDs borax aqueous solution at room temperature.

2.3.

Characterization

Transmission electron microscopy (TEM) was taken on JEOL JEM 2100. Sample solution was drop-cast from solution onto a carbon-coated TEM grid and the solvent was evaporated at room temperature. X-ray diffraction (XRD) patterns were obtained by using Cu Ka radiation (XRD, PANalytical X’Pert Pr MPD). Fourier transform X-ray photoelectron spectroscopy (XPS) were carried out by an ESCAlab 250Xi electro spectrometer from Thermo Scientific using 300 W Al Ka radiation. The base pressure was about 3 · 109 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The UV–vis absorption and the PL spectra were measured with a UV-2450 spectrometer and a Cary Eclipse fluorimeter, respectively. An Olympus fluorescence microscope was used to obtain fluorescence microscopy images, with excitation wavelengths at 405 nm. The Raman spectra were taken with Laser Confocal Micro-Raman Spectroscopy (LabRAM Aramis). The atomic force microscope (AFM) images were obtained by MultiMode V SPM (VEECO). Intensity data for crystal was collected on a Bruker Smart Apex G CCD diffractometer with graphite–monochromated Mo–Ka radiation

˚ ) at 110 K. The structure was solved by direct meth(0.71073 A ods and refined with the full-matrix least-squares technique based on F2 using the SHELXL program with anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms of the water molecules were located from the difference Fourier maps and refined with restraint of the O–H and H. . .H dis˚ , respectively). Other hydrogen atoms tances (0.96 and 1.52 A were placed by calculated positions.

2.4.

Computational methods

The ground state and the first excited state of one luminescent unit of pure B-GQDs in the presence and absence of borax were obtained from theoretical calculation with density function theory (B3LYP/6-31G(d)). The geometric parameters of the ground state were optimized and verified at B3LYP/631G(d) leve and the geometric parameters of the first excited state were optimized with TD-B3LYP/6-31G(d). The absorption spectrum of B-GQDs in the presence of borax was obtained from theoretical calculation with density function theory (B3LYP/6-311++G(d,p)). The absorption spectra of B-GQDs without borax was obtained from theoretical calculation with density function theory (B3LYP/6-31G(d)). Based on the optimized geometric parameters, a time-dependent density function theory, called TD-B3LYP/6-31G(d) was performed to get the absorption spectra.

3.

Results and discussion

The electrochemical preparation of B-GQDs was performed in 0.1 M borax aqueous solution with graphite rod as anode and Pt foil as cathode at a potential of +3.0 V with electrolysis for approximately 2 h. It is expected that a 3 V potential in the current would effectively drive the electrolyte ions into the carbon materials and oxide the C–C bonds as reported previously [14]. Fig. 1a and b display TEM images of B-GQDs, exhibiting a size of 3–7 nm in diameter. The lattice spacing is 0.240 nm (the inset of Fig. 1a), corresponding to the lattice constant in the plane of graphite. The AFM image (Fig. 1d and e) demonstrates that the B-GQDs have a thickness <1 nm, i.e., the B-GQDs are mostly single layered or bi-layered [26]. As shown in Fig. 2a, the XRD pattern of the pristine graphite reflects one prominent peak centered at around 26 corresponding to the (0 0 2) planes of graphite. But the as prepared B-GQDs exhibit a weak broad peak, which is attributed to the presence of reduced graphene oxide.[27] Raman spectrum of the B-GQDs (Fig. 2b) shows a G band at 1591 cm1 and a D band at 1342 cm1 with a large intensity ratio ID/IG of 1.23, indicating numerous defects on the B-GQDs [8,28]. XPS of BGQDs (Fig. S1 in Supporting Information) shows the presence of B, C and O with atomic percentages of 5.24%, 62.93%, 31.83%, and the corresponding B1s, C1s, O1s peaks are located at ca. 191, 284 and 531 eV, respectively. This confirmed the successful incorporation of B atoms into the GQDs by the electrochemical process from the Na2B4O7 electrolyte. In the highresolution B1s spectrum (Fig. 2c), the peak centered at 189.8 eV can be assigned to sp2 C–B bonds present in B-GQDs [29]. The peaks at 190.7 and 191.4 eV correspond to the

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Fig. 1 – (a) TEM, HRTEM (inset) images and (b) size distribution of B-GQDs. (c) AFM image and (d) height profile along the line from A to B (inset) of the B-GQDs deposited on freshly cleaved mica substrates. (A color version of this figure can be viewed online.)

Fig. 2 – (a) XRD patterns of the pristine graphite and B-GQDs. (b) Raman spectrum of B-GQDs. B1s (c) and C1s (d) XPS spectra of B-GQDs. (A color version of this figure can be viewed online.)

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Fig. 3 – UV–vis absorption and PL spectra of the B-GQDs aqueous solution in the absence (a, b) and presence (c, d) of borax. The insets in (b) and (d) show photographs of the B-GQDs solution under 254 and 365 nm illumination (b), and under 365 nm UV illumination (d). (A color version of this figure can be viewed online.)

structure of B atoms bonding to C and O atoms (BC2O) and B atoms surrounded by C and O atoms (BCO2) [30–32], respectively. In addition to the C–B bond (283.8 eV) [25], the high-resolution C1s spectrum (Fig. 2d) further confirmed the O-rich groups, such as C–O (286.0 eV), C@O (288.3 eV) and OAC@O (290.0 eV) [33]. All these results indicate the formation of C–B bonds in the electrochemical reaction. The most distinctive feature of B-GQDs which sets them apart from other previously reported GQDs is their specific PL behavior. From the UV–vis absorption spectrum of pure B-GQDs aqueous solution (pH = 7) (Fig. 3a), a typical absorption peak at ca. 230 nm is assigned to the p–p* transition of aromatic sp2 domains [34]. Besides, two n–p* absorption bands at ca. 300 and 360 nm were also observed [35]. As shown in Fig. 3b, the PL spectra feature prominently two wavelength regions peaking at 460 and 535 nm. When excited at wavelengths from 300 to 340 nm, the first peak dominates the second and the 460 nm PL intensity increases to the maximum without shifting. The brightness of this PL peak was quantified in terms of the QY: at 340 nm excitation, it was determined to be only about 3.6%. For the second yellow fluorescence centered at 535 nm, the intensity decreases when the excitation wavelength is varied from 360 to 420 nm (the QY here is 3.2%). It should be noted that all the aforementioned PL characteristics result from the B-GQDs solution after borax was removed by dialyzing at pH = 7. However, when borax remains in the solution at pH = 9, we observe distinctly different PL features. Fig. 3c shows the UV–vis absorption spectrum of the B-GQDs in borax aqueous solution. Compared with Fig. 3a, the p–p* absorption peak at ca. 230 nm and n–p* absorption band at ca. 300 nm was still observed while the

360 nm n–p* absorption band disappeared. Correspondingly, only one strong emission peak at 490 nm was observed, resulting in excitation-independent PL behaviors (Fig. 3d). When excited at the wavelengths from 340 to 440 nm, the intensity of the PL increased to the maximum, then decreased, but the emission peak at 490 nm is almost unshifted. In the insert to Fig. 3d, a photograph of the aqueous solution illuminated under UV light (365 nm) is shown. The bright green PL is so strong that it is easily visible to the naked eyes. The QY was determined to be about 13%, which is much higher than that of B-GQDs in the absence of borax. Obviously, the specific interaction between B-GQDs and borax is responsible for the bright green PL. This is further supported by the fact that the PL intensity is enhanced with the increase of borax concentration (Fig. S2). In addition to the strong down-conversion PL properties, it also shows a clear up-converted emission located at 490 nm (Fig. S3) [36,37]. Taken together, it is evident that the unique optoelectronic properties of B-GQDs should be associated with the doping of B into the GQDs structures. We further deduce that the bright green fluorescence arises from the B doping as well as the interaction between B-GQDs and borax. Since B3+ is bonded to the sp2 clusters of GQDs, the short distance and energy level overlap could lead to effective energy transfer from B3+ to the sp2 cluster. As a result, the radiative recombination rate is increased and the PL emissions are consequently enhanced. On the other hand, the special electron deficiency of B atom gives rise to the interaction between B-GQDs and O atom of Na2B4O7 nearby, which should be also responsible for the bright green fluorescence. Following this rational line of thinking, the fluorescent emission peaks at 460 and 535 nm (Fig. 3b) stem from the association of B-GQDs in the absence

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Fig. 4 – The ground state structures of one luminescent unit of B-GQDs in the presence (a) and absence (b) of borax obtained by theoretical calculation with density function theory. (c) The size distribution of B-GQDs in the presence (left) and absence (right) of borax determined by DLS. The structure illustration of B-GQDs in the presence (d) and absence (e) of borax, respectively. (A color version of this figure can be viewed online.)

of borax, leading mainly to the weaker and longer wavelength emissions. Our theoretical calculation (density function theory B3LYP/ 6-31G(d)) has clearly revealed the detailed structures responsible for the special PL of B-GQDs in the absence and presence of borax in the solution. Fig. 4b, e and Fig. S4 display the optimized structure of pure B-GQDs without borax, which is thought to be responsible for the PL. The structure stems from the association of B-GQDs by a [2+2] cycloaddition reaction over two C@B double bonds. The calculated absorptions are mainly on 336 and 350 nm, which lead to two luminescence peaks at 435 and 542 nm (Fig. S5), in good agreement with the results in Fig. 3a and b. In contrast, in the presence of Na2B4O7, a dative valence bond is formed between B atom of C@B double bond in B-GQDs and bridgehead O atoms bonded to sp3 B atoms in Na2B4O7 as shown in Fig. 4a, d and Fig. S6. The fluorescence arises from the transition between C@B p* and p (LUMO!HOMO) at 483 nm with an oscillator strength of 0.17 (Fig. S7, Table S1), which is much stronger than those of the self-association complexes of B-GQDs in good agreement with the results in Fig. 3. This is also confirmed by the dynamic light scatting (DLS) data of the B-GQDs in the absence and presence of borax (Fig. 4e). It can be clearly seen that the size of former is larger than that of the latter, indicating that the association happens for the pure B-GQDs.

On the basis of the results presented above, it is evident that the introduction of the B atoms produced p-type GQDs and brought about the rich interactions with surrounding media, which are further demonstrated by the following interesting phenomena. While evaporating B-GQDs borax aqueous solution at room temperature, bright green fluorescent crystals were obtained as shown in Fig. 5. From Fig. 5a–f, it can be seen clearly that the green rectangle crystals with lengths of more than several hundred microns and widths of dozens of microns could be obtained on a large scale. These crystals were brown (Fig. S8a) and single crystal X-ray diffraction (SXRD) showed that they were mainly borax decahydrate monoclinic crystals (Table S2). However, it was found that pristine borax decahydrate crystals obtained by evaporating borax aqueous solution were transparent and colorless and did not emit PL as shown in Fig. S8b, indicating that the green emitting crystals contained different components. Furthermore, on comparing the XRD pattern of these two kinds of crystals (Fig. S9), it can be found that for the green PL crystals, apart from the prominent peaks of borax crystals, a weak broad peak centered at around 26 is observed, demonstrating the existence of B-GQDs in the crystals. In other words, the green PL crystals contain mainly borax crystals but with B-GQDs decorated on the relevant surfaces. Interestingly, it appears that not all surfaces of a crystal emit green PL (Fig. 5a and b).

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Fig. 5 – Fluorescence microscopy images (at the excitation wavelength of 405 nm) of B-GQDs crystals obtained by evaporating B-GQDs borax aqueous solution at room temperature (a), and a single B-GQDs crystal taken at different rotation directions (b, c, d, e and f). (A color version of this figure can be viewed online.)

Owing to the large size, the crystals could be readily rotated and cut with tweezers or a blade (Fig. 5b–f, Fig. S10 and S11). It is clearly observed that only four of the six surfaces emit bright green PL. If a pristine borax crystal was put in pure B-GQDs solution and taken out quickly, a green PL crystal as shown in Fig. 5 could be obtained. SEM images of B-GQDs show that there is obvious accessory of substances which emit green PL on the four surfaces of the crystal. The green PL part can be even cut and redissolved into water. XRD pattern and fluorescent spectrum display that it contains mainly B-GQDs (Fig. S9 and S13). The lattice structure of Na2B4O7 is then referred to understand the remarkable results. From Fig. S14, it can be seen that there are four planes, where exhibit bridgehead O atoms bonded to sp3 B atoms and they are all or partly exposed to the outside of the planes. In contrast, there is almost no such kind of O atom in the other two planes. Thus it can be imagined that B-GQDs connect to the four surfaces of borax crystal by the dative valence bond between B atoms in B-GQDs and bridgehead O atoms bonded to sp3 B atoms in Na2B4O7, generating bright green PL crystal as shown in Fig. 5. Since the fluorescent crystals were easily obtained on a large scale, we used them to demonstrate patterned fluorescent display from templates. To do that, we etched a piece of copper etched into patterns and immersed it into a green fluorescent aqueous solution. When the solution was dried after several minutes, bright green fluorescent patterns could be observed clearly under a UV lamp (Fig. S15, Fig. 6). The patterns retained their stability when exposed to the air in an indoor environment after one month, a feature which is important for practical applications.

Fig. 6 – Graphic patterns on a copper piece (under 365 nm UV illumination). (A color version of this figure can be viewed online.)

Al3+ has been found to exert several neurotoxic effects in organisms, which links to Alzheimer’s disease, Parkinson’s disease, bone softening, chronic renal failure and smoking-related diseases [38,39]. Since there is a close relationship between Al3+ and human health, the accurate determination of Al3+ is of great importance. The water soluble, strongly and stably fluorescent GQDs, coupled with their special properties of interaction with surrounding medium are compelling and have promoted us to explore their potential in sensing Al3+. Fig. 7a depicted the emission spectra of aqueous B-GQDs upon titration with Al3+. With increasing Al3+ concentration from 0 to 100 lM, the fluorescent intensity was obviously enhanced with the emission band at 535 nm stayed steady. The dependence of the changes in emission intensity on the Al3+ concentration was shown in the inset of Fig. 7a. Notably, it exhibited a good linear relationship between the changes in fluorescent intensity and the concentration of Al3+ (R2 = 0.9989, 0–100 lM). The detection limit of B-GQDs towards Al3+ was estimated as 3.64 lM, which meets the limit for drinking water according to the World Health Organization standard (7.41 lM). To evaluate the specificity of B-GQDs towards Al3+, we carried out fluorescence titration with various metal ions, to make sure of its maximum fluorescent response. It is clearly seen that B-GQDs has a positive response towards Ag+, Na+, Zn2+, as well as a negative response towards Ba2+, Pb2+, Fe2+ and Cr3+. However, the most striking effect is observed for Al3+ (Fig. 7b). That is, B-GQDs show good selectivity and high sensibility to Al3+. To further testify single selectivity of BGQDs for Al3+ in practical application, we choose Na+, K+, Mg2+, Ca2+, Ba2+ and Fe3+ as the interfering ions, some of which have a relatively high concentration in biological tissue and drinking water. As shown in Fig. 7b, the fluorescent intensities of B-GQDs enhanced by Al3+ are not much affected in the background of the interfering ions, thus providing a potential application for biological detection and the water quality monitoring. Like other GQDs reported previously [21], B-GQDs also possess low cytotoxicity, which was evaluated by the viability of Hela cells after incubation with B-GQDs for 24 h by MTT

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Fig. 7 – (a) PL emission spectra of B-GQDs aqueous solution upon addition of Al3+ at 405 nm excitation. The Al3+ concentrations are 0, 10, 20, 40, 60, 80 and 100 lM, from bottom to top. The inset is the relationship between (I–I0)/I0 and Al3+ from 0 to 100 lM. I and I0 are the PL intensities of B-GQDs at 400 nm excitation in the presence and absence of Al3+, respectively. (b) Maximum fluorescent response of B-GQDs upon addition of different metal ions (100 lM) listed from left to right: (A) Al3+, (B) Ag+, (C) Na+, (D) K+, (E) Mg2+, (F) Ca2+, (G) Ba2+, (H) Cd2+, (I) Pb2+, (J) Fe2+, (K) Zn2+, (L) Fe3+, (M) Cr3+ and (N) Al3+ + Na+ + K+ + Mg2+ + Ca2+ + Ba2+ + Fe3+. (A color version of this figure can be viewed online.)

Fig. 8 – (a) Confocal fluorescence microscopy images of Hela cells with B-GQDs coated with liposomes at the excitation wavelength of 405 nm and corresponding image under bright field (b). (A color version of this figure can be viewed online.)

(tetrazolium salt reduction) assays. As can be seen from Fig. S16, no significant loss of cell viability was observed with the concentration of incubated B-GQDs 200 lg mL1, indicating the excellent biocompatibility and low cytotoxicity of the B-GQDs. B-GQDs uptake and bioimaging experiments were performed by the confocal fluorescence microscope as shown in Fig. 8a and b. Observation of the bright yellow area inside the cells indicates translocation of B-GQDs through the cell membrane (405 nm excitation). All these results indicate that the as prepared B-GQDs can be used in bioimaging and other biomedical applications with little cytotoxicity. Furthermore, due to the doping of B into GQDs, B-GQDs also exhibited good electrocatalytic activity towards oxygen reduction reaction (ORR) (Fig. S17).

of 13%. A large scale of green fluorescent crystals can be obtained by evaporating B-GQDs borax solution, which have shown the ability to clearly display patterns. After removing the borax, blue and yellow fluorescent solutions could be obtained. Our theoretical computation has clearly explained the interactions both between B-GQDs themselves and between B-GQDs and Na2B4O7. B-GQDs were demonstrated to be a chemosensor for detecting Al3+, biomarker for cellular imaging and electrocatalysis for oxygen reduction. To the best of our knowledge, this is the first time that the B-GQD dyed crystals are obtained and the rich interactions between the B-GQDs and surrounding media studied. Such B-GQDs and crystals with accessory of B-GQDs are expected to have wide practicable applications on optical device. Detailed follow-up work is underway in our laboratory and will be reported in due course.

Acknowledgments This work is supported by NSFC (21073018), the Major Research Plan of NSFC (21233003), Key Laboratory of Theoretical and Computational Photochemistry and RGC of Hong Kong (GRF No. 605710).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.12.085.

R E F E R E N C E S

4.

Conclusions

We have prepared water-soluble B-GQDs by electrochemical exfoliation of graphite in borax electrolyte. As a result of the introduction of the highly electron deficient B atoms, B-GQDs have demonstrated intriguing rich properties. In the presence of borax, the B-GQDs are highly green-fluorescent with a QY

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