Article Cite This: Langmuir 2018, 34, 12845−12852
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Matrix-Free and Highly Efficient Room-Temperature Phosphorescence of Nitrogen-Doped Carbon Dots Yifang Gao,† Hui Han,† Wenjing Lu,† Yuan Jiao,† Yang Liu,† Xiaojuan Gong,† Ming Xian,†,‡ Shaomin Shuang,† and Chuan Dong*,† †
Institute of Environmental Science, and School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China Department of Chemistry, Washington State University, Pullman, Washington 99164, United States
‡
Langmuir 2018.34:12845-12852. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/29/19. For personal use only.
S Supporting Information *
ABSTRACT: Efficient room-temperature phosphorescence (RTP) of carbon dots (CDs) usually is seriously limited to appropriate solid matrix or introduced heavy atoms responsible for promoting intersystem crossing and suppress vibrational dissipation between singlet and triplet states. So, facile preparation efficient RTP of CDs with nonmatrix is still a highly difficulty and challenging task. Here, we first reported a subtle strategy to induce highly efficient free-matrix RTP of nitrogen-doped CDs (NCDs). The NCDs are composed of a core and hydrophilic surface of polyaspartic acid chains arising from high-temperature polymerization by a one-pot heating treatment of Laspartic acid and D-glucose. The obtained NCDs have an ultralong phosphorescence lifetime of 747 ms and a high phosphorescence quantum yield (PQY) of 35% under 320 nm excitation in air. To the best of our knowledge, the PQY is currently the highest values recorded for RTP of CDs. The facile preparation and unique optical features offer these NCDs potential application in numerous applications, such as anticounterfeiting and white light-emitting diodes.
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for the common human to recognize the afterglow emission.12 Therefore, it is still highly desirable to explore facile preparation, environment-friendly RTP materials with a long afterglow emission. Carbon dots (CDs), as an important class of photoluminescent (PL) nanomaterials, have attracted significant attention due to their low toxicity, excellent optical properties, environmental friendliness, and high photostability. Their optical applications frequently focus on biological imaging,13 printing inks,4,14,15 and light-emitting devices2,3 from their excellent fluorescence properties. However, the rare attention is paid to the phosphorescent phenomenon and their related applications. Zhao et al.16 first observed RTP of CDs by introducing a polymer poly(vinyl alcohol) as a protective matrix. The generation of CDs RTP was from the hydrogen bonding interaction between hydroxyl groups on polymer and carbonyl groups on CDs, which effectively limited the intramolecular motions and preventing the nonradiative relaxation of CDs. This research released the possibility of CDs phosphorescence creation for the matrix selection concern. Numerous polymers, such as polyurethane,17 silica gel,18 and polymer composite matrix (urea and biuret),1 as matrix materials have been used for RTP of CDs. In addition,
INTRODUCTION Room-temperature phosphorescence (RTP) have aroused considerable interest for their attracting applications in security aspects,1 optoelectronics devices2−4 and biological imaging5,6 based on their much longer lifetime and higher internal quantum efficiency than fluorescence. However, the observation of phosphorescence phenomenon is in principle extremely difficult because it involves spin-forbidden transitions, which are responsible for low probability of intersystem crossing (ISC) and radiative pathway between singlet and triplet states.7,8 These factors seriously limit the appearance of phosphorescence (Figure 1A). To date, the effective routes to promote intersystem crossing are to incorporate heavy metals, halogen bonding, carbonyl bonds, siloxy groups or C− N/CN bonds, etc.9 In another case, it can be realized by embedding a luminescent species in appropriate solid matrices, e.g., filter paper, polymers, or silica, which can offer dense hydrogen bonding with strong rigidity and good oxygen-barrier rigidly to suppress vibrational dissipation.10,11 However, regardless of the methods of introduced heavy atoms or embedded appropriate solid matrices, these methods all involved complicated preparation process, toxicity, high cost, and potential environmental hazards. In addition, most materials of RTP have relatively short emission lifetimes (i.e., several microseconds to a few milliseconds), and such short decay times cannot meet the requirement for the persistent luminescence because tens of milliseconds is generally required © 2018 American Chemical Society
Received: April 23, 2018 Revised: August 31, 2018 Published: October 10, 2018 12845
DOI: 10.1021/acs.langmuir.8b00939 Langmuir 2018, 34, 12845−12852
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Figure 1. (A) Energy-level diagram of the relevant photophysical processes (S0 = ground state, S1 = singlet excited state, T1 = triplet excited state; ISC = intersystem crossing; Abs. = absorption; Non Rad. = nonradiative). (B) Synthesis of nitrogen-doped CDs (NCDs).
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inorganic matrices (KAl(SO4)2·xH2O19 or NaCl hybrid crystals20) were also found as substrate to induce RTP of CDs, which were able to protect their energy from rotational or vibrational loss by rigidifying these aromatic carbonyl groups, so as to suppress the nonradiative deactivation pathways.21 However, relatively deficient dispersion medium and complicated manipulated process have confined further utilization to some extent. The RTP CD-based matrix systems are facing a great challenge. Recently, Liu et al.9 induced RTP of CDs based on the aggregation-induced effect from PVAconjugated CDs. The CDs themselves serve as both solidified host and luminescent guest without introducing any heavy atoms and additional supporting matrices. The strategy could effectively prevent oxygen from permeating the aggregates and quenching the triplet and simplified the required conditions for CDs RTP. This study inspired us to develop the facile preparation of CDs with long RTP lifetime for practical application purpose. In this work, a highly efficient and matrix-free RTP of novel NCDs was prepared with the one-step high-temperature polymerization method22 by conjugating polyaspartic acid chains onto the surface of NCDs. The NCDs themselves serve as both solidified host and luminescent guest without any supporting matrix. The polyaspartic acid chains could play dual roles, which both efficiently formed the hydrogen bond framework on the surface of NCDs for stabilizing the triplet states of NCDs and strongly hindered oxygen quenching to present RTP behavior at air atmosphere. Moreover, the NCDs possess the intriguing features of ultralong phosphorescence lifetime of 747 ms and a high phosphorescence quantum yield (PQY) of 35%, which are favorable for the purpose of visible to naked eye. To the best of our knowledge, the PQY value is currently the highest record for RTP of NCDs. It will be of great potential for anticounterfeiting and white light-emitting diodes.
EXPERIMENTAL SECTION
Materials. D-Glucose and L-aspartic acid were obtained from Aladdin. Other reagents were acquired from Aladdin Ltd. (Shanghai, China). All chemicals were of analytical reagent grade and used as received without any further purification. Ultrapure water (≥18.25 MΩ cm) from a molecular purification system (Shanghai, China) was used in all experiments. Apparatus. The transmission electron microscopic (TEM) images were acquired on a JEOL JEM-2100 transmission electron microscopy (Tokyo, Japan) with an accelerating voltage of 300 kV. The elemental analysis was conducted on an Elementar Analysen systemevario EL cube elemental analyzer (Hanau, Germany). The Xray photoelectron spectra (XPS) were acquired on an AXIS ULTRA DLD X-ray photoelectron spectrometer (Kratos, Tokyo, Japan) with Al Kα radiation operating at 1486.6 eV. Spectra were processed by Case XPS v.2.3.12 software using a peak-fitting routine with symmetrical Gaussian−Lorentzian functions. The Fourier transform infrared spectra (FTIR) were recorded on a Bruker Tensor Π FTIR spectrometer (Bremen, German). Absorption spectra were recorded using a UV-2450-PC spectrophotometer (Shimadzu, Japan). Steadystate PL and phosphorescence spectra were collected on a Hitachi F4500 fluorescence spectrophotometer. Time-resolved luminescence spectra were performed using an Edinburgh FLS920 fluorescence spectrophotometer. The absolute quantum efficiency was obtained on an Edinburgh FLS920 spectrophotometer equipped with an integrating sphere from the reconvolution fit analysis. Synthesis of NCDs. The synthesis of NCDs was carried out, as shown in Figure 1B. A typical synthetic route is described as follow. Glucose (0.11 g, 0.62 mmol) and L-aspartic acid (0.33 g, 2.5 mmol) were sufficiently mixed via sonication with 3 mL of aqueous sodium hydroxide solution, and then the mixture was heated to 150 °C by an oil bath (Figure 1B). The reaction was completed when the mixture became a pale-yellow solid. The products were completely dissolved into 5 mL of water and freeze-dried to obtain solid. It only takes a few minutes. The product was cooled to room temperature and was completely dissolved into 20 mL water and then placed in a dialysis bag (cutoff Mn: 500−1000 Da) and dialyzed against water for 1 day to remove small molecules. The water was replaced every 4 h and finally NCDs in the dialysis bag was freeze-dried to obtain light yellow solid. 12846
DOI: 10.1021/acs.langmuir.8b00939 Langmuir 2018, 34, 12845−12852
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Figure 2. (A) SEM images of NCDs powder. (B, C) TEM and HRTEM images of NCDs, and (D) size distribution of NCDs powder in ethanol. (E) X-ray photoelectron spectra (XPS) of NCDs. (F) FTIR spectra of NCDs.
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RESULTS AND DISCUSSION Characterization of NCDs. As shown in Figure 2A, SEM images of NCDs display powder-like microsheets, which can redissolve in water without aggregation. The TEM image of the nanoparticles is displayed in Figure 2B, which indicates that the nanoparticles are uniformly dispersed without any aggregation. The statistical particle-size distribution, as shown in Figure 2D, is calculated using 100 nanoparticles. The size distribution ranges between 5.5 and 8.5 nm, with an average particle size of about 6.7 nm. The high-resolution transmission electron microscopy (HRTEM) image shows that most of the NCDs are amorphous in structure without any lattices. To confirm the particle organization, we took NCDs powder/ethanol suspension (the NCDs do not dissolve in ethanol so that their aggregated morphologies may mostly retain) for HRTEM image of typical NCDs and expectedly observed that the NCDs particles are composed of a core and a shell (Figure 2C). NCDs and polyaspartic acid (without adding D-glucose) powder are similar in infrared absorbance to polyaspartic acid chains (Figure S1), which is a clue to existence of polyaspartic acid chains even though after hydrothermal treatment of 150 °C. The above statement indicates that incomplete carbonizing lead to a more disorder surface. These TEM of the NCDs and CDs in the absence of Laspartic acid are somewhat different, that is, CDs remain high crystallinity (Figure S3), but NCDs have hardly any high crystallinity. Therefore, we predict that NCDs particles may be depicted as a graphitizing core in amorphous surface. This similar structure is also suggested by previous study,9 which revealed that numerous polyaspartic acid chains radially wrap the carbon cores. Thus, every nanoparticle may tightly crosslink with each other via hydrogen bonds after removing the water, just like the formation of solid film.23 Surface analysis was carried out using X-ray photoelectron spectroscopy (Figure 2E). The C 1s, N 1s, and O 1s peaks are observed at 284.6, 399.4, and 530.9 eV, respectively. The high-resolution spectrum of C 1s (Figure S4.A) shows three peaks at 284.8,
286.5, and 288.6 eV for C−C/CC, C−N/C−O, and COOH, respectively. The N 1s spectrum (Figure S4.B) shows the characteristic peaks of a pyrrole-like N and N−H groups at about 399.9 and 401.7 eV, respectively. The O 1s XPS spectrum of CDs (Figure S4.C) can be deconvoluted into two peaks at 531.7 and 533.1 eV, indicating the existence of CO and C−OH, respectively.24 These XPS results indicate that NCDs are rich in oxygen and nitrogen atoms. The FTIR spectrum of NCDs gave the further structure information (Figure 2F). The broad and pronounced absorption peaks at 3409 and 3161 cm−1 indicate the existence of large amounts of O−H and N−H groups. Peaks at 1397 cm−1 could be ascribed to the bending vibration of C−H, while those at 2976, 2934, and 2875 cm−1 could be assigned to the stretching vibration of C−H. The strong stretching vibration at 1625 cm−1 corresponded to the CO of aromatic carbonyl. The appearance of characteristic peaks at 1339, 1306, and 1074 cm−1 originated from the C−N, C−O−C, and C−O−H stretching vibrations, respectively. For further exploration of the source of surface functional groups on the surface of the NCDs, FTIR spectra of L-aspartic acid, polyaspartic acid, glucose and polyglucose have been revealed in Figure S1. By contrast, CO, O−H, and COOH on the surface of the NCDs can be derived from polyglucose and polyaspartic acid. The TGA analyses curves of NCDs, polyaspartic acid, and polyglucose have been conducted to investigate the surface functional groups of NCDs ( Figure S2). A comparative TGA curves of NCDs and other materials (polyaspartic acid and polyglucose) related to weight loss against temperature were analyzed and are displayed in Figure S2. All of the profiles showed a little weight loss below 150 °C due to loss of absorbed water. 25 The main step of the co-polymer degradation starts at 303 °C with the mass loss of 31%, which probably corresponds to the decomposition of amide and ester bonds.26 The decomposition temperature of NCDs, polyaspartic acid, and polyglucose were at 303, 315, and 272 °C, respectively. The difference in thermal degradation 12847
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Figure 3. (A) Photographs of NCDs before and after turning off the UV excitation (365 nm). (B) UV−vis absorption spectrum. (C) The fluorescence (black lines) and phosphorescence (red lines) emission spectra of NCDs with excitation at 320 nm. (D) Time-resolved phosphorescence spectra of NCDs.
behaviors may be attributed to attached carboxylate groups.27 The polyaspartic acid, polyglucose, and NCDs decomposed with the weight losses up to 66.66, 71.83, and 79.85% of their actual weight at 900 °C, respectively. TGA data clearly demonstrate more weight loss in the case of NCDs over the polyaspartic acid and polyglucose, which could be related to the weight contribution originated from hydrogen bonds.28 The present results indicate that there exist strong hydrogen bonds among NCDs. The TGA identify surface functional groups consistent to XPS and FTIR. The XPS, FTIR, and TGA results confirmed that the NCDs contain abundant functional groups (CO, C−N, NH, OH, etc.), which impart excellent water solubility without further chemical modification. Photoluminescence Properties of NCDs. The photoluminescence properties of NCDs are the basis of their applications. We had studied their spectroscopic properties by UV−visible absorption and photoluminescence (fluorescence and phosphorescence) emission spectra. The UV−visible absorption spectra of NCDs (Figure 3B) exhibit absorption peaks at 297 nm, which is ascribed to the n−π* transition of the CO bond.29,30 Surprisingly, we found that the NCDs not only have intense fluorescence under UV light (365 nm) excitation, but also an obvious afterglow when the UV light is turned off. The digital photographs of the afterglow are demonstrated at time 0, 1, and 2 s in air (Figure 3A), and the corresponding video is included in Supporting Information (SI), which is visible to the naked eye for seconds (a corresponding video is provided in the SI). Figure 3C shows the corresponding fluorescence spectrum (black line) and phosphorescence spectrum (red line) of NCDs at an excitation
wavelength 320 nm. There is an approximately stokes shift of 42 nm. This implies that the difference between singlet−triplet energy level is about 0.21 eV. According to previous reports,16,17,19,31,32 the emergence of phosphorescence is mainly due to the aromatic carbonyl groups formed at the surfaces of CDs. In the phosphorescence excitation spectrum with a broader band emission at 515 nm, only a broad band at 240−340 nm appears and overlaps the absorption band of C O bonds, suggesting that the phosphorescence may come from the CO bonds on NCDs (Figure S5). The absolute PQY (excitation at 320 nm) of the NCDs is about 35%, which is a relatively high PQY for NCDs (Table S2). Compared with the PQY based on carbon dots, the NCDs possess the highest value recorded for RTP of CDs (Table S3). The high PQY is mainly due to the existence of O and N atoms. These heteroatoms in the NCDs lattice can disrupt the sp2 hybridization of carbon atoms, altering the electronic structures and increasing the PL intensity of CDs.33,34 The time-resolved phosphorescence spectra with an excitation wavelength of 320 nm and the emission wavelengths of 340 and 650 nm are shown in Figure 3D. The results of fitting of the two spectra with biexponential functions are presented with an average lifetime of 747 ms in Table S2. The two phosphorescence lifetimes of 250.50 ms (47.56%) and 1198.14 ms (52.44%) imply that the NCDs surface have diverse energy levels.35 The multiple phosphorescence lifetimes imply the presence of various electronic transition processes, which originate from the various chemical environments of the carbonyls on the surface of CDs.36,37 12848
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Figure 4. (A) Phosphorescence spectra of NCDs-1, NCDs-2, and NCDs-3 excited at 320 nm. (B) Phosphorescence spectra of NCDs at temperatures from 78 to 301 K. (C) The effect of exposure time under UV light on the intensity of phosphorescence. (D) The effect of exposure time in air on the intensity of phosphorescence.
Figure 5. (A) Security protection application. (B) Phosphorescence mechanism of NCDs. 12849
DOI: 10.1021/acs.langmuir.8b00939 Langmuir 2018, 34, 12845−12852
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between carboxylic acids is also a key factor for RTP. Moreover, the rigidity of hydrogen bonding plays a key role in rigidifying the CO bonds on the surface of NCDs, minimizing the nonradiation transitions of triplet excitons. In addition, the long chain of the NCDs surface forms a carboxylic acid dimer has a good oxygen-barrier performance, which can hinder the direct collisions between CO and oxygen molecules and promote the phosphorescence.
In a further set of experiment, three kinds of NCDs-1, NCDs-2, and NCDs-3 (molar ratio D-glucose: L-aspartic acid (1:4, 1:2, and 1:1)) were synthesized, via the same method. The elemental analysis indicates (Table S1) that the NCDs contain carbon, hydrogen, nitrogen, and oxygen elements. The element analysis from XPS and total organic carbon can be analyzed in Table S1, and results are consistent to elemental analyzer. In addition, we found that phosphorescence intensity (Figure 4A), the absolute PQY, and the lifetimes (Table S2) are enhanced with the increase of nitrogen content. The digital photos of NCDs-1, NCDs-2, and NCDs-3 of the afterglow are demonstrated at time 0, 1, 2, and 3 s in air (Figure S6). These results confirm that the intensities of phosphorescence and PQY are enhanced with the increase of nitrogen content. We infer that more nitrogen content favors n−π transition and hence facilitates the spin-forbidden transfer of singlet-to-triplet excited states through intersystem crossing to populate triplet excitons.38,39 In addition, the intensity of phosphorescence decreases as the temperature increases with the range of 78− 301 K (Figure 4B), confirming major contribution from RTP. The PL stability of NCDs was also investigated under UV exposure time and kept in air conditions. At different UV exposure times (Figure 4C), the PL intensities increase and eventually reach a steady state, and peak characteristics show no prominent changes, which indicates that NCDs are beneficial for practical applications in optoelectronics and data security. In addition, no significant changes were detected when the NCDs were exposed in air for a month (Figure 4D) and further proved that the as-prepared NCDs themselves provided unique rigid shape based on the hydrogen bond framework from polyaspartic acid chains on the surface of NCDs and can be an obstacle in the penetration of moisture and oxygen. Application of NCDs. Taking advantages of the abovementioned properties (the solution of NCDs has no phosphorescence phenomenon, and the solid has phosphorescence phenomenon), the NCDs could also be developed as promising applications in data encryption.23 The letters s” have been written on Chinese character “ nonfluorescent paper using NCDs mixed solution as the ink, which could not be clearly seen in visible light. After drying, the letters be observed when the UV light is turned off. When the paper was wetted via water-spraying, the letters gradually became dark when the UV light is turned off (Figure 5A). These observations demonstrate that the NCDs aqueous dispersion could potentially be employed as an advanced anticounterfeiting ink for advanced authentication or hiding important information from unanticipated people. Phosphorescence Mechanism of NCDs. To further understand the phosphorescence mechanism of NCDs, we investigate the RTP emission of an aqueous solution of NCDs, but no RTP can be detected when NCDs are dissolved in the water (WNCDs). We compared the FTIR spectra of NCDs and WNCDs (Figure S7). In the FTIR analysis, the appearance of characteristic peaks at 2732, 2664, 2515, 1330, 1305, 1248, and 925 cm−1 originate from the carboxylic acid dimer vibrations.40 It has been confirmed that the NCDs are immobilized on the surface of nCOOH through the formation of CO···H−O bonds, but not the formation of CO···H− O bonds when the NCDs are dissolved in the water. The reason may be that intra-NCDs hydrogen bonds are disrupted by the solvent effect of water molecules (Figure 5B). So, we think that the formation of intra-NCDs hydrogen bonds
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CONCLUSIONS In summary, we provided a kind of highly efficient RTP of NCDs by one step with nonmatrices. The obtained NCDs have an ultralong phosphorescence lifetime of 747 ms and a high PQY of 35% under 320 nm excitation. It is found that polyaspartic acid chains on the surface of NCDs gave much advantage to the moisture resistance that makes NCDs powder stand rigidly under ambient condition. And this polyaspartic acid chains structure also acts as tight barrier that effectively prevents oxygen from permeating the aggregates and quenching the triplet. Its potential applications include security aspects and optoelectronics devices. This discovery of a new class of CDs-based RTP materials with ultralong lifetimes and high efficiency is developed as a proof-of-concept, which we believe that will promote both fundamental understanding and future practical applications of RTP materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b00939. FTIR spectra of NCDs; polyaspartic acid and L-aspartic acid; TEM and HRTEM (inset) images of CDs; highresolution C 1s XPS; high-resolution N 1s XPS; highresolution O 1s XPS spectra of the NCDs; UV−vis absorption and excitation spectra of the as-prepared NCDs; the digital photos of NCDs-1, NCDs-2, and NCDs-3 of the afterglow are demonstrated at time 0, 1, 2, and 3 s in air, FTIR spectra of NCDs and WNCDs; elemental analysis of the NCDs; lifetime data obtained from the time-resolved decay curves of NCDs at λex/λem of 320/515 nm and the absolute PQY (PDF)
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NCDs of fluorescence and phosphorescence (ZIP)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +86-351-7018613. ORCID
Xiaojuan Gong: 0000-0002-2152-2639 Ming Xian: 0000-0002-7902-2987 Chuan Dong: 0000-0002-1827-8794 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21705101, 21475080, and 21575084) and Shanxi Province Hundred Talents Project. 12850
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DOI: 10.1021/acs.langmuir.8b00939 Langmuir 2018, 34, 12845−12852
