Near-Ultraviolet to Near-Infrared Fluorescent Nitrogen Doped Carbon

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Near-Ultraviolet to Near-Infrared Fluorescent Nitrogen-Doped Carbon Dots with Two-Photon and Piezochromic Luminescence Yan Zhan,†,‡ Ting Geng,§ Yingliang Liu,∥ Chaofan Hu,∥ Xuejie Zhang,∥ Bingfu Lei,∥ Jianle Zhuang,∥ Xu Wu,*,† Di Huang,*,‡ Guanjun Xiao,*,§ and Bo Zou§ College of Chemistry and Chemical Engineering and ‡Department of Biomedical Engineering, Research Center for Nano-Biomaterials & Regenerative Medicine, College of Mechanics, Taiyuan University of Technology, Taiyuan 030024, China § State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China ∥ College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 08/20/18. For personal use only.

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ABSTRACT: Carbon dots (CDs) have gained intensive interests owing to their unique structure and excellent optoelectronic performances. However, to acquire CDs with a broadband emission spectrum still remains an issue. In this work, nitrogen-doped CDs (N-CDs) with near-ultraviolet (NUV), visible, and nearinfrared (NIR) emission were synthesized via one-pot solvothermal strategy, and the excitation-independent NUV and NIR emission and excitation-dependent visible emission were observed in the photoluminescence (PL) spectra of NCDs. Moreover, the as-synthesized N-CDs displayed two-photon fluorescence emission. It is important to note that N-CDs also exhibited piezochromic luminescence with reversibility, in which the red- and blue-shifted PL with increasing applied pressure (0.07−5.18 GPa) and the red- and blue-shifted PL with releasing applied pressure (5.18 GPa to 1 atm) were developed for the first time. Combined with good hydrophilicity, high photobleaching resistance, and low toxicity, the piezochromic luminescence would greatly boost the valuable applications of N-CDs. KEYWORDS: solvothermal strategy, nitrogen-doped carbon dots, near-ultraviolet to near-infrared fluorescence, two-photon luminescence, piezochromic luminescence

1. INTRODUCTION Carbon dots (CDs), as new emerging fluorescent nanomaterials, have been extensively used in biosensing, drug delivery, optoelectronic devices, and energy storage fields for their unique nanostructures, excellent optoelectronic properties, low toxicity, good hydrophilicity, and high chemical stability.1−6 To date, tremendous efforts have been expended to the extraction of CDs with fascinating optical performances, such as CDs doped with heteroatoms can enhance the quantum yield (QY),7,8 CDs with red, green, and blue luminescence were synthesized using o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine as sources,9 CDs obtained from dopamine and o-phenylenediamine can exhibit nearinfrared (NIR) and two-photon emission,10 full-color emissive CDs were obtained from citric acid, urea,11 etc. The performances of materials mainly depend on the generation form and microstructure.12,13 The possible intrinsic structures of CDs can be classified into carbon nanodots, carbon quantum dots, and polymer dots.9,14−18 The CDs prepared from assorted carbon precursors and synthetic methods may display different nanostructures, which result in their complicated optical performances. The CDs with broadband emission including ultraviolet (UV), visible, and NIR spectra © XXXX American Chemical Society

have great valuable applications in biomedicine and optoelectronic devices.19 Wang et al., have reported the near-ultraviolet (NUV) fluorescent polymer carbon nanosheets,20 CDs with NIR excitation/emission were synthesized by Qu et al.,21 and the deep ultraviolet to NIR emissive N-doped graphene quantum dots under different excitation wavelengths have been obtained by Lau et al.19,22 Although great achievements have been gained, fewer literatures related to CDs with NUV to NIR emission under fixed excitation region have been reported. As detection probes, fluorescent materials sensitive to environment stimuli are of technological and scientific importance due to their extensive applications in optical devices and luminescent switches.23,24 Piezochromic materials display color changes in responding to external pressure or mechanical grinding stimuli, which facilitates their potential applications in mechanosensors, data storage, security ink, optoelectronic devices, and indicators of mechanohistory.25−30 Recently, piezochromic materials covering organic fluorophore, Received: May 7, 2018 Accepted: July 26, 2018 Published: July 26, 2018 A

DOI: 10.1021/acsami.8b07498 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces organic−inorganic compounds, metal nanoclusters, and inorganic nanocrystals have been reported;23,31−36 however, compared to light-, pH-, and temperature-sensitive materials, research on the piezochromic materials is still in its infancy, and the blur fluorescence mechanism may inevitably restrict their valuable applications. The red-shifted photoluminescence (PL) spectra of Si nanocrystals with increasing pressure were found by Schaller et al.36 CDs with blue-shifted PL as applied pressure increases from 10.42 to 22.84 GPa were reported by Yang et al.26 Zhang et al. have fabricated the mechanofluorochromic CDs with tunable blue- and red-shifted PL under 0.07−1.70 GPa pressure.27 All the above findings indicate that Si nanocrystals and CDs can be employed as piezochromic materials for mechanosensors applications. Controlled nanostructure and surface or molecular state may play a dominant role in piezochromic CDs with monoshifting or bidirectional shifting PL. Unfortunately, to actualize tunable red- and blue-shifted CDs with reversible changes in fluorescence still remains a challenge. Herein, a facile and one-step solvothermal approach was proposed to fabricate nitrogen-doped carbon dots (N-CDs). The possible generation mechanism was systemically introduced by two carbonization and polymerization procedures. Encouragingly, the as-synthesized N-CDs displayed the excitation-independent NUV and NIR emission with 280− 350 nm excitation and excitation-dependent visible emission PL spectra with 280−450 nm excitation. Compared with CDs, the oxygen-rich functional groups of N-CDs were obviously increased, and the QY was up to 56.1%, far higher than that of CDs (26.9%). Moreover, two-photon emission spectrum was observed in N-CDs. It is important to note that N-CDs also exhibited piezochromic luminescence with reversibility. When the applied pressure was increased from 0.07 to 5.18 GPa, the red- and blue-shifted PL of N-CDs were monitored; accordingly, the color of PL changed from bright blue to dark green, and the red- and blue-shifted PL as the applied pressure releases from 5.18 GPa to 1 atm could also be tested for the first time. Benefiting from good water solubility, low toxicity, and high photobleaching resistance, it will be interesting to apply the piezochromic N-CDs in the sensitive detection of external pressure stimuli.

Figure 1. Schematic diagram of facile synthesis of N-CDs via one-step ethanol-thermal strategy treated with H2O2 and NH3·H2O.

and NIR emission and excitation-dependent visible emission PL spectra. The morphology and microstructure of N-CDs were tested by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and atomic force microscopy (AFM) measurements. As shown in Figure 2a, a TEM image of NCDs demonstrates a well-monodispersed N-CDs with uniform size, and the average diameter of N-CDs is about 2.15 nm by counting 200 individual nanoparticles (Figure 2b). The HRTEM image of N-CDs shown in Figure 2c revels the high crystallinity of N-CDs, and the spacing with the lattice fringe of 0.34 nm corresponds to the (002) plane of graphitic carbon.43 The AFM image of N-CDs (Figure 2d) demonstrates that the average height is approximately 1.8 nm, ranging from 1.68 to 2.2 nm, suggesting that the as-synthesized N-CDs comprise several layers of carbon nanosheets. These results above indicate that the nanostructured N-CDs with uniform size were prepared from ethanol treated with H2O2 and NH3· H2O. To verify the crystalline structure and the surface groups of the as-synthesized samples, X-ray diffraction (XRD) pattern, Raman spectrum, and Fourier transform infrared (FTIR) spectroscopy were measured. Figure 3a shows the XRD pattern of N-CDs with a broad diffraction centered at 26° (0.34 nm), which is attributed to the d-spacing of the graphitic carbon [002] plane.43−45 A broad diffraction peak in the XRD pattern of N-CDs may result from the small/thin nanostructures and weak layer−layer interactions.46,47 The sp2-hybridized D band (1325 cm−1) and sp3-hybridized G band (1640 cm−1) are the two obvious peaks presented in the Raman spectra of N-CDs (Figure S1),21,48,49 and the intensity ratio (0.75) of the D to G band (ID/IG) indicates moderate amount of graphitic carbon inside the N-CDs. The FTIR spectra of N-CDs (Figure 3b) display that the absorption bands at 1012 and 1175 cm−1 correspond to the vibrational absorption of C−O and the strong absorption band at 1601 cm−1 may be attributed to the aromatic CC or CO stretching vibrations.48 The two weak absorption bands at 2821 and 2987 cm−1 are assigned to the vibrational absorption of C−H. The broad absorption peak at 3429 cm−1 originates from the O−H stretching vibrations.50 The analogous band stretching vibrations can also be characterized in the FTIR spectra of CDs (Figure S2a). In addition, the FTIR spectra of N-CDs demonstrates two

2. RESULTS AND DISCUSSION Inspired by our previous experiment, hydrogen peroxide (H2O2), ethanol, and ammonia (NH3·H2O) were usually used as oxidant, solvent, and nitrogen source for the synthesis of fluorescent nanomaterials.13,37,38 N-CDs were fabricated via a facile and one-step solvothermal strategy (illustrated in Figure 1) in which low-cost and easily available ethanol, H2O2, and NH3·H2O were chosen as carbon source, oxidant, and nitrogen precursor, respectively. The synthetic procedure was accomplished in two steps. In the first stage, ethanol was carbonized in the presence of H2O2, accordingly, some intermediates including acetic acid, acetaldehyde, glycollic acid, and glyoxal might be produced, and these oxidized molecules could react with each other to generate unsaturated aldehyde.39−41 With increasing heating time, the unsaturated aldehyde as the new carbon precursor was then polymerized and aromatized for the growth of carbogenic nuclei under high temperature and pressure.42 Compared to CDs without nitrogen doping, the PL intensity of N-CDs was highly increased, and the QY of N-CDs was up to 56.1%. The assynthesized N-CDs displayed excitation-independent NUV B

DOI: 10.1021/acsami.8b07498 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Typical TEM image, (b) size distribution, (c) HRTEM image, and (d) AFM image of N-CDs. Inset of (d) represents the corresponding height profile along the line.

Figure 3. (a) XRD pattern and (b) FTIR spectra of N-CDs.

suggesting that the N-CDs with sufficient oxygen-rich functional groups have been fabricated. To investigate the spectral performances of N-CDs, the ultraviolet−visible (UV−vis) absorption and photoluminescence (PL) spectra were measured. As shown in Figure 5a, two absorption peaks located at 268 and 300 nm are characterized, which are attributed to the π−π* transition of the aromatic CC band and the n−π* transition of the CO/CN groups, and two small peaks at 262 and 278 nm may be caused by energy-level transition of the π−π* transition of the aromatic CC or some small molecules.26 Different from most CDs, the as-synthesized N-CDs exhibited a broadband emission covering NUV, visible, and NIR emissions. Interestingly, the PL spectra of N-CDs (Figure 5b,c) display the excitation-independent NUV (355 nm) and NIR (728 nm) emissions with 280−350 nm excitation and the excitationdependent visible (410−525 nm) emission PL spectra under 280−450 nm excitation. The excitation-independent/excitation-dependent emission spectra may be attributed to the free zigzag sites, emissive traps, and electronic conjugate structures of N-CDs.54 The CDs prepared in the absence of NH3·H2O exhibited similar optical properties to that of N-CDs (Figure S3); however, from Figure S4, the as-synthesized N-CDs show

characteristic vibrational absorption of CN in the region of 1357−1492 cm−1 and N−H located at 3429 cm−1,20,51 indicating the successful introduction of N in the in-plane structure of N-CDs. For chemical composition, X-ray photoelectron spectroscopy (XPS) of the samples was characterized. As shown in Figure 4a, three obvious binding energy peaks including C 1s (284.5 eV), N 1s (400.3 eV), and O 1s (532.2 eV) can be tested in the XPS survey spectrum of N-CDs.48 However, the XPS survey spectrum (Figure S2b) of CDs presents only two binding energy peaks of C 1s and O 1s. For the C 1s spectra of N-CDs and CDs (Figures 4b and S2c), there are three binding energy peaks of CC (284.8 eV), C−O (286.1 eV), and C O/CN (288.8 eV).27,52 The O 1s spectra of N-CDs (Figure 4d) can be deconvoluted into three doublets of CO (531.1 eV), C−O/O−H (532.0 eV), and OC−O (532.9 eV),20,53 whereas the O 1s of spectra of CDs (Figure S2d) can be deconvoluted into two doublets of CO and C−O/O−H. Moreover, the N 1s spectra of N-CDs (Figure 4c) present two binding energy peaks located at 399.8 and 402.2 eV, corresponding to the pyridinic/pyrrolic N and amino N groups, respectively.26 The functional groups found in the XPS spectrum are consistent with that of the FTIR spectra, C

DOI: 10.1021/acsami.8b07498 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) XPS survey spectra, (b) C 1s, (c) N 1s, and (d) O 1s spectra of N-CDs.

Figure 5. (a) UV−vis absorption spectra, (b) NUV (λex = 280−340 nm) and visible PL spectra (λex = 280−450 nm), (c) NIR emissive PL spectra (λex = 280−350 nm), (d) fluorescence excitation−emission map (λex = 280−340 nm), and (e, f) mechanism for NUV, visible, and NIR emission of N-CDs. D

DOI: 10.1021/acsami.8b07498 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (a) Two-photon fluorescence spectra N-CDs with different laser excitation power of 800 nm. (b) Relationship plotted on logarithmic scales of the two-photon emission intensity against the laser excitation power. Inset of (a) is the optical photograph of N-CDs with 800 nm excitation.

water as a pressure-transmitting medium, and the widths and separation of R1 and R2 lines of ruby fluorescence were monitored to detect the actual pressure.26 A semiconductor laser with 355 nm excitation was employed to record the PL spectra of N-CDs. As presented in Figure 7a, with the applied pressure increasing from 0.07 to 0.88 GPa, the PL spectra of N-CDs initially display a gradual red shift from 516 to 558 nm. As the pressure further increases to 5.18 GPa, the PL spectra then exhibit a gradual blue-shift, changing from 558 to 520 nm. Accordingly, the color of N-CDs dispersion is gradually darkened as the applied pressure increases (Figure 7e). Encouragingly, with the release in pressure from 5.18 to 3.74 GPa, the PL spectra of the as-synthesized N-CDs first show a gradual red-shift from 517 to 553 nm. The blue-shift of the PL peaks of N-CDs is then observed, changing from 553 to 510 nm as the pressure releases from 3.1 GPa to ambient conditions (Figure 7b), indicating the partial reversible piezochromic luminescence of N-CDs. These results are similar to those of the traditional piezochromic materials. Compared to the piezochromic CDs with nearly ignorable PL reversibility after blue- and blue/red-shifts reported by Yang et al. and Zhang et al., the PL spectra of the as-prepared N-CDs exhibit red- and blue-shift with increasing pressure;26,27 accordingly, as the pressure releases, the red- and blue-shift PL peaks are also monitored (Figure 7c). This finding represents a great progress in piezochromic CDs, and the hybridized carbon core and surface states of N-CDs may be responsible for the piezochromic fluorescence.26,27 According to the piezochromic Si nanocrystals, the red-shifted PL spectra with increasing pressure may be caused by the Xconduction-toΓvalence transition of bulk crystalline Si. Together with the calculation of molecular dynamics for pressure, the efficient PL of Si NCs was identified from core states.36 On the basis of the above results, the piezochromic N-CDs may be used as a hoping candidate of the traditional piezochromic materials for sensitive detection of external pressure stimuli.

stronger PL intensity with 340 and 350 nm excitation, and the strongest PL intensity is the N-CDs prepared with 0.4 mL NH3·H2O. The visible PL QY of N-CDs was calculated to be 56.1% excited at 360 nm with quinine sulfate as a standard, far higher than that of CDs (26.9%). Accordingly, the twodimensional-fluorescence topographical map shown in Figure 5d demonstrates that the emission band of N-CDs includes NUV, visible, and NIR regions with 280−350 nm excitation, and the corresponding optimal excitation and emission wavelengths are 340 and 355, 360 and 435, and 340 and 728 nm, respectively (Figure S5). Based on the structure and optical performances, the possible mechanism for the broadband emission spectra of N-CDs can be interpreted in Figure 5e,f. For NUV, visible, and NIR emissions, an electron−hole pair (exciton) after electronic transition (V3−C3) can be produced by the absorption of UV photons in the range of 280−340 nm through the localized π electrons in the double bonds, which leads to the NUV light (C3−V3) emission through radiative recombination after vibrational relation, and the excited electron can also go through interband transition from a higher conduction band to a lower conduction band, which results in emission of visible (C2−V2) and NIR (C1−V1) light through radiative recombination. The absorption of visible photons (V2−C2) in the range of 360−450 nm through the partial conjugated π-electrons may be responsible for the excitons that may emit visible light (C2−V2) after vibrational relation, and the wavelengths of emitting photons are longer than that of absorbed photons.19,22 This may be the reason why the UV absorption results in emission of NUV, visible, and NIR light, and visible absorption leads to visible emission. To further study the optical performances of N-CDs, twophoton fluorescence spectra were measured using a NIR femtosecond pulsed laser. As shown in Figure 6a, a representative two-photon fluorescence spectra display green fluorescence with 800 nm excitation, which is similar to the spectra of the single-photon fluorescence. Figure 6b demonstrates the quadratic relationship of the two-photon emission intensity against the laser excitation power, and the slope (1.8) of the plot conforms to the occurrence of a two-photon excitation process, confirming the two-photon fluorescence of N-CDs under NIR femtosecond pulsed laser excitation.21 Similar to previous reported CDs, two-photon fluorescence of N-CDs may be assigned to the multiphoton active process.55,56 To gain an insight into the piezochromic luminescence of NCDs, high-pressure experiments were measured with a symmetric diamond anvil cell apparatus using deionized

3. CONCLUSIONS In summary, N-CDs were fabricated by a green and one-step ethanol-thermal approach treated with H2O2 and NH3·H2O. The as-synthesized N-CDs exhibited the excitation-independent NUV and NIR emission and excitation-dependent visible emission PL spectra. The QY of N-CDs was determined to be 56.1%, far higher than that of CDs (26.9%). Interestingly, the unexpected but important two-photon luminescence of N-CDs was also observed. Moreover, the piezochromic fluorescence of E

DOI: 10.1021/acsami.8b07498 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 7. (a) PL spectra of N-CDs as the pressure increases. (b) PL spectra of N-CDs as the pressure releases to the ambience. (c) PL emissive peaks changes of N-CDs with increasing and releasing pressure. (d) Schematic diagram of the piezochromic fluorescence of N-CDs. (e) Optical photographs of N-CDs with increasing pressure.

N-CDs with partial reversibility was developed for the first time. With applied pressure increasing from 0.07 to 5.18 GPa, the red- and blue-shifted PL of N-CDs were monitored; accordingly, its color of PL changed from blue to green, and the red- and blue-shifted PL with the applied pressure releasing from 5.18 GPa to 1 atm could also be tested. Combined with good hydrophilicity, high photobleaching resistance and low toxicity, the piezochromic N-CDs can be used for applications in pressure-sensing and optical-recording systems.

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Experimental procedures, Raman spectra of N-CDs, XPS, and FTIR spectra of CDs, UV−vis and PL spectra of CDs, PL spectra of CDs and N-CDs with 340 and 350 nm excitation, PL spectra of N-CDs treated with different amounts of NH3·H2O, excitation and emission PL spectra of N-CDs (PDF)

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Corresponding Authors

*E-mail: [email protected] (X.W.). *E-mail: [email protected] (D.H.). *E-mail: [email protected] (G.X.).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b07498.

Chaofan Hu: 0000-0003-2311-8733 F

DOI: 10.1021/acsami.8b07498 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Xuejie Zhang: 0000-0002-4029-7000 Bingfu Lei: 0000-0002-6634-0388 Jianle Zhuang: 0000-0002-4649-1939 Guanjun Xiao: 0000-0002-7013-1378 Bo Zou: 0000-0002-3215-1255 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (Grant Nos. 51541210, 51402207, 21725304, 21571067, 11774125, 21401028, 51372091, and 11502158), the Natural Science Foundation of Shanxi Province (201601D102007), Scientific Research Planning Project of the Education Department of Jilin Province (JJKH20180118KJ), National Defense Science and Technology Key Laboratory Fund (6142A0306010917), State Key Laboratory of Chemical Resource Engineering (CRE-2015-C106).

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DOI: 10.1021/acsami.8b07498 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b07498 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX