Rare-Earth Doping Graphitic Carbon Nitride Endows Distinctive Multiple Emissions with Large Stokes Shifts

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE6 Jun 2022Rare-Earth Doping Graphitic Carbon Nitride Endows Distinctive Multiple Emissions with Large Stokes Shifts Xun Liu†, Shangqing Zhang†, Jinhui Liu, Xing Wei, Ting Yang, Mingli Chen and Jianhua Wang Xun Liu† Research Center for Analytical Sciences, Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819 †X. Liu and S. Zhang contributed equally to this work.Google Scholar More articles by this author , Shangqing Zhang† Research Center for Analytical Sciences, Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819 †X. Liu and S. Zhang contributed equally to this work.Google Scholar More articles by this author , Jinhui Liu Research Center for Analytical Sciences, Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819 Google Scholar More articles by this author , Xing Wei Research Center for Analytical Sciences, Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819 Google Scholar More articles by this author , Ting Yang Research Center for Analytical Sciences, Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819 Google Scholar More articles by this author , Mingli Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Research Center for Analytical Sciences, Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819 Google Scholar More articles by this author and Jianhua Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Research Center for Analytical Sciences, Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101104 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Two-dimensional (2D) graphitic carbon nitride (g-C3N4) possesses a unique geometric configuration featuring a superimposed heterocyclic sp2 carbon and nitrogen framework. Its fluorescence may be attributed to π–π*, lone pair (LP)-π*, or LP-δ* transitions. So far, the manipulation of its fluorescence emissions is largely unexploited and remains challenging. Herein, for the first time, rare-earth doping into the backbone structure of a g-C3N4 framework under microwave agitation endows unprecedented fluorescence nature, with the emergence of two exceptional new fluorescence emissions in the 450–700 nm range. With terbium-doped g-C3N4:Tb as a representative, these emissions exhibit distinctive features, that is, very sharp fluorescence peaks with narrow full width at half maximum (FWHM) (peak width at half-height) of <12 nm, quantum yields of 2.3 ± 0.0% and 7.6 ± 0.1% for the new emissions at λex/λem = 290/490 nm, and 290/545 nm, respectively; and a large Stokes shift of >200 nm. These features of g-C3N4:Tb are most advantageous for applications in various fields, as demonstrated by (1) tracking biodistribution of g-C3N4 in vivo with mass spectrometric imaging where the doped terbium serves as a tag, (2) a biometrics study facilitating the identification of an individual through fingerprint, and (3) anti-counterfeiting with g-C3N4∶Tb as a dual-functional marker to facilitate fluorescence and mass spectrometric imaging. Download figure Download PowerPoint Introduction Graphitic carbon nitride (g-C3N4) is a fascinating organic semiconductive material, with special-layered structure and visible-light activity, as well as chemical stability.1–4 g-C3N4 has become attractive in photocatalysis,3 solar energy conversion,5 electronic devices,6 and chemical sensors.7 The bulk g-C3N4 is not suitable to serve as a fluorescence sensor and bioimaging agent,8 while better fluorescence performance may be achieved by ultrathin g-C3N4 nanosheets derived by liquid-phase exfoliation,9 chemical oxidation,10 and hydrothermal protocols,11 with strong acids and time-consuming preparation protocols. Nevertheless, spectral interference due to its broad emission in the 320–450 nm range and small Stokes shift significantly limits biosensing applications. A series of modification protocols were used for tailoring the performance of g-C3N4. Nonmetal doping alters the energy band structure and electronic structure of g-C3N4.12 Boron- and fluoride-doping promoted cyclohexane oxidation,13 while sulfur doping enhanced photocatalytic hydrogen evolution from water.14 g-C3N4 can easily capture metal cations due to the strong interactions between the cations and the negatively charged nitrogen atoms.15,16 Metal doping reduces the band gap, increases the light absorption, and prolongs the lifetime of charge carriers and so greatly enhances photocatalytic activity.17–19 Metal ions may be doped into the frameworks of g-C3N4 by heating the aqueous mixture with metal salts at 500–600 °C under the protection of argon.20,21 The above studies were mainly focused on the improvement of the catalytic performance of g-C3N4. There was no effort devoted to the regulation of its optical/fluorescent features with simple strategies, which is still quite challenging. The mechanism for fluorescence emission of g-C3N4 is not yet fully clear. The possible approaches for triggering the emission may include π–π*,22–24 lone pair (LP)-π*,25,26 or LP-δ* transitions.6,27 Rare-earth metals exhibit electron configurations of [Xe] 4fn−15d0,16s2 (n = 1–15).28 They are generally preferred as doping elements. Their incompletely occupied 4f and empty 5d orbitals make them the center of electron capture and increase their optical absorption capability.29 Thus, rare-earth doping g-C3N4 may significantly enhance photocatalytic activity. In addition, the coordination of organic ligand with rare earth may lead to fluorescence emission in the visible region.30 In this study, for the first time we attempt to dope g-C3N4 with rare-earth metals in a one-pot approach, that is, under microwave agitation, to alter and regulate its fluorescence nature. We take advantage of various substrates, that is, thiourea, dicyandiamide, and guanidine isothiocyanate, and rare earths, that is, terbium, europium, and erbium, to manipulate the emission features of g-C3N4. This approach is not only simple and time-saving but also universal. With respect to native g-C3N4, terbium-doped g-C3N4 (g-C3N4∶Tb) exhibits two exceptional narrow emissions in the region of 450–700 nm with large Stokes shifts of >200 nm. This is highly beneficial for the applications in biological samples, as demonstrated by fingerprint identification on the surface of various matrixes and biological tissue imaging applications with fluorescence and inductively coupled plasma mass spectrometry (ICP-MS). Experimental Methods Preparation of terbium-doped g-C3N4 and native g-C3N4 nanosheets The bulk Tb-doped g-C3N4:Tb was prepared by direct polymerization of thiourea and terbium chloride (TbCl3) under microwave agitation. 0.7 g of thiourea was introduced into a 200-mL beaker and dissolved in 10 mL of deionized (DI) water. 75 mg of TbCl3·6H2O was then added in the aqueous solution with 10 μL of nitric acid (16 M). The mixture was immediately irradiated for 3 min in a microwave oven [G80F23CN3L-Q6 (p0), Galanz, Foshan, China] at an output power of 800 W. Cream-white powder was obtained and washed with DI water three times to remove the residual acid. The bulk g-C3N4:Tb was exfoliated by ultrasonic agitation for 8 h to obtain suspension of the final product, which was then centrifuged at 4000 rpm for 5 min to achieve uniform g-C3N4:Tb nanosheets by removing the large-sized portion. The preparation of native g-C3N4 was performed by following exactly the same procedures as described above, in the absence of terbium chloride. The Dicy-/Guan-g-C3N4∶Tb were prepared by following the same procedure, except for the replacement of thiourea by dicyandiamide and guanidine isothiocyanate. In addition, g-C3N4:Er and g-C3N4:Eu were obtained by substitution of TbCl3·6H2O with ErCl3·6H2O and EuCl3·6H2O, respectively, and the same preparation procedure was followed. Preparation of anti-counterfeiting labels 2 mg g-C3N4:Tb and 5 g agar powder were dissolved in 20 mL DI water by heating them to 90 °C and producing a homogeneous solution. 100 μL of the above solution was used to stamp “NEU” on a glass surface of 0.75 cm2. The anti-counterfeiting labels were obtained after the stamp was naturally cooled and solidified. LA-ICP-MS imaging in animal Male CD-1(ICR) mice (7 weeks old, ∼20 g) were received from Beijing Vital River Laboratory Experimental Animal Technology Co. Ltd. (Beijing, China). All the animal study protocols were reviewed and approved by the Animal and Medical Ethics Committee of Northeastern University (NEU). The mice were housed in an air-conditioned room at 20 °C with a regular 12 h light/dark cycle. Sterilized chow and DI water were provided ad libitum. After 1 week of acclimatization, the mice were intravenously injected with g-C3N4∶Tb solution at a dose of 5 mg mL−1, 100 μL. The mice were sacrificed after 24 h, and the organic tissues, including lung, liver, spleen, and kidney, were collected, followed by immediately freezing them and cutting them into 10 μm thick slices. The slices were attached to a regular glass slide, and the distribution of Tb was imaged by laser ablation-ICP-MS (LA-ICP-MS). The laser spot size and scan were typically set at 70 μm and 100 μm s−1. Results and Discussion Synthesis and characterization In this work, rare-earth-doped g-C3N4 was obtained under microwave in a flask. Considering the similar chemical properties among rare-earth metals, we took terbium-doped graphitic carbon nitride (g-C3N4∶Tb) as representative to illustrate their morphological and fluorescence characteristics as well as fluorescence mechanisms. Pristine and Tb-modified g-C3N4 were achieved under microwave in air-atmosphere (Figure 1a and Experimental). Transmission electron microscopy (TEM) images (Figures 1b and 1c) provided a magnified view of the morphologies of g-C3N4 and g-C3N4∶Tb. For the identification of the graphene-like structure of g-C3N4∶Tb, it was ultrasonically treated for 24 h. By spherical aberration-corrected TEM (ACTEM; Supporting Information Figure S1), the film of g-C3N4∶Tb may be clearly observed. The ultrasonic treatment results in smaller sized g-C3N4∶Tb with thinner lamellar structure ( Supporting Information Figure S1a), and individual Tb spots can clearly be identified on the surface of g-C3N4 ( Supporting Information Figure S1b). Scanning electron microscopy (SEM) images in Figures 1d and 1e clearly showed that g-C3N4 exhibits as lumps or block masses, and virtually no morphological change can be observed after doping with terbium (Figures 1f and 1g). This observation indicated that the doping of terbium atoms posed no effect on the shape or morphology of g-C3N4. In addition, the compositions of C, N, and Tb for g-C3N4 by elemental mapping in Figure 1h illustrated that Tb may be clearly observed and homogeneously distributed over the entire carbon nitride framework. Figure 1 | (a) Schematic illustration for the preparation of terbium-doped g-C3N4∶Tb. (b and c). TEM images of g-C3N4 and its terbium-doped counterpart g-C3N4∶Tb. (d and e). SEM images of g-C3N4 and its terbium-doped counterpart g-C3N4∶Tb. (f and g). The molecular structure diagram of g-C3N4 and its terbium-doped counterpart g-C3N4∶Tb. (h) The images for elemental compositions of g-C3N4∶Tb, including C, N, and Tb. Download figure Download PowerPoint X-ray diffraction (XRD) patterns in Figure 2a provided further information on structural variation of the layered crystal structures of g-C3N4 and g-C3N4∶Tb. It was obvious that g-C3N4 and g-C3N4∶Tb exhibited diffraction peaks at 26.8° (3.32 Å) and 26.9° (3.31 Å), respectively. These peaks matched with the (002) reflection of g-C3N4 arising from the interlayer stacking (∼0.32 nm) of the conjugated heptazine units, which clearly illustrated that the distance (Figure 2b, dlam) between g-C3N4∶Tb layers exhibited no obvious change. This further illustrated that terbium atoms were not intercalated in carbon nitride layers and posed no interference with the layer stacking. Figure 2 | (a) XRD patterns of the native C3N4 and terbium-doped C3N4∶Tb. The close-up in the inset illustrates the main diffraction peaks for C3N4 and C3N4∶Tb. (b) Illustration for the molecular lamellar structure of C3N4∶Tb with an interlamellar spacing dlam. (c and d) XPS survey and high-resolution XPS spectra for the C1s, N1s, and Tb 3d3/2 regions in the native C3N4 and terbium-doped C3N4∶Tb. Download figure Download PowerPoint X-ray photoelectron spectroscopy (XPS) helped to explore the states of Tb atoms in the carbon nitride framework. For native C3N4 (Figure 2c), the C1s spectrum was fitted with four components at binding energies of 284.7, 285.5, 288.0, and 289.0 eV. 284.7 eV is typical for graphitic carbon or adventitious carbon contamination commonly seen on surfaces by XPS. 285.5 eV may be assigned to terminal sp carbon in C≡N groups, while that at 288.0 eV is contributed to by sp2 carbon ring (=N−C=N−). The peak at 289.0 eV was associated with surface C−O species due to surface oxidation. The N1s peak was fitted with two components at 398.8 eV (Nα) and 400.1 eV (Nβ), which were assigned to the sp2-bound −C−N=C−, for example, those expected for triazine-based or related CxNy rings and sp3 nitrogen joined to three aromatic rings, respectively. XPS spectra of terbium-doped C3N4∶Tb in Figure 2d identified all the above peaks, which clearly indicated that terbium incorporation caused no destruction on the local structure of carbon nitride. More importantly, the obvious response signal of Tb3d was diagnostic for the formation of C3N4∶Tb. For further elucidation of the structure and bonding information of C3N4∶Tb, curve fitting of N1s and C1s was performed by adopting Gaussian–Lorentzian components (GL ratio fixed at 80%:20%) after subtraction of the Shirley background. Full details of the results are given in Supporting Information Tables S1 and S2, including the number of adopted components, binding energy (BE), the percentage of each component, and the full width at half maximum (FWHM). It is obvious that the introduction of terbium enhanced the rigidity of carbon nitride, which led to the shift of the C1s peak to the stronger BE region with respect to native C3N4. A reduction in the proportion of Cγ peak area indicated the decrease on the number of =N−C=N−, due to the breakage of a C=N π bond and the formation of a new σ bond with Tb (N-Tb). In C3N4∶Tb, coordination of N atoms with Tb resulted in an increase of sp3 hybridization of C, as reflected by an increase in the proportion of Cα. This conclusion is well consistent with XPS spectra of N1s, where N atoms coordinated with Tb and led to the conversion of sp2 to sp3 hybridization, as reflected by the decrease of Nα moiety and the increment of Nβ moiety. The Fourier transform infrared (FT-IR) spectrum of g-C3N4∶Tb was very similar to that of native g-C3N4 ( Supporting Information Figure S2). They both exhibited broad bands for stretching and deformation of −OH groups at 3150 cm−1, the absorption for =N−C=N− at 2075 cm−1, a group of multiple bands as characteristics for triazine ring vibrations, that is, 1668 cm−1 for quadrant stretching, 1398 cm−1 for semicircle stretching, and 803 cm−1 for out-of-plane ring bending by sextants. Although no bonding information was identified by Tb3+ doping, the absence of significant changes between FT-IR spectra of g-C3N4 and g-C3N4∶Tb well indicated that the structure of carbon nitride was maintained after Tb3+ doping. To the best of our knowledge, this is the first time that rare-earth-doped g-C3N4 has been prepared via the microwave-assisted one-pot approach in 3 min without nitrogen protection and high temperature. The above characterizations well demonstrated the doping of terbium in the framework of the carbon nitride structure rather than in the lamellae or simply adsorbed on its surface. Study of optical properties The optical properties of g-C3N4∶Tb were exploited by studying its fluorescence features with the variation of the content of doped Tb3+ in the final product. The contents of Tb in g-C3N4∶Tb were determined by ICP-MS as given in Supporting Information Figure S3 and Table S3. The three-dimensional (3D) fluorescence patterns of the obtained g-C3N4∶Tb in Figure 3a illustrated a maximum excitation wavelength at 290 nm. With the increase of the Tb3+ doping amount in g-C3N4∶Tb, the characteristic emission of Tb3+ was gradually identified under the excitation at 290 nm. The fluorescence spectra of g-C3N4∶Tb (Figure 3b) illustrated the three main emissions at 370, 490, and 545 nm, corresponding to the intrinsic emission of g-C3N4 (due to π–π* transition31), 5D4→7F6 and 5D4→7F5 transitions of Tb3+, respectively. A low mass fraction of doped terbium in g-C3N4∶Tb (<0.21%) gave rise to a limited transition (energy transfer) efficiency to the terbium ion. Thus, the intrinsic emission at 370 nm from g-C3N4 exhibited virtually no change, along with very weak Tb3+-related luminescence at 490 and 545 nm. At a Tb3+ mass fraction of 0.90%, an effective “antenna effect” was formed between g-C3N4 and Tb3+, which led to remarkable increment of the sharp fluorescence emission of g-C3N4∶Tb. However, further increase of the doping amount of Tb3+in g-C3N4∶Tb caused significant decrease of the emissions at 490 and 545 nm due to the self-absorption of Tb3+. The principle for the variation of the fluorescence spectrum was illustrated in Figure 3c. The above observations clearly demonstrated that the variation of the doping amount of Tb3+ in g-C3N4∶Tb may be used to regulate the intensity of fluorescence emissions at 490 and 545 nm. It was noticeable that the intrinsic emission of g-C3N4 at 370 nm was maintained during the entire experimental process. This observation further proved that terbium doping did not destroy the primary structure of carbon nitride. The fluorescence quantum yields for the emissions at λex/λem = 290/370, 290/490, and 290/545 nm were derived to be 2.8 ± 0.1%, 2.3 ± 0.0%, and 7.6 ± 0.1%, along with fluorescence lifetime of 3.04 ns, 1.58 ms, and 8.28 ms, respectively ( Supporting Information Figure S4). The influence of irradiation time on the fluorescence property of g-C3N4∶Tb is illustrated in Supporting Information Figure S5. It can be seen that the fluorescence of g-C3N4∶Tb tends to be stable as the irradiation time exceeds 1 min. The reaction was completed at an irradiation time of >3 min. Thus, 3 min was adopted for the ensuing studies. Figure 3 | (a) The 3D fluorescence patterns of native g-C3N4 and terbium-doped g-C3N4∶Tb with various doping amounts (mass ratio) of Tb3+ (0.21, 0.46, 0.90, and 1.99%). The concentration of mg is adopted for g-C3N4 and g-C3N4∶Tb. (b) The fluorescence spectra of native g-C3N4 and terbium-doped g-C3N4∶Tb with various doping amount of Tb3+. The concentration of mg is adopted for g-C3N4 and g-C3N4∶Tb. Schematic illustration for the transition of 5D4→7F6 and 5D4→7F5 in Tb3+ ions and the corresponding fluorescence Download figure Download PowerPoint In the above terbium-doped g-C3N4∶Tb was obtained by microwave-assisted polymerization of thiourea and Tb3+. further studies demonstrated that it is a for the preparation of rare-earth-doped carbon nitride. the g-C3N4∶Tb may be prepared by of an thiourea was by dicyandiamide and guanidine isothiocyanate, respectively. In both g-C3N4∶Tb may be achieved as indicated in Supporting Information Figure where emission were observed at 490 and 545 nm. the the doping of g-C3N4 with rare-earth metals than terbium, for example, and led to remarkable of the fluorescence nature. Supporting Information Figure illustrates emission and fluorescence intensity for g-C3N4:Er and with respect to that of g-C3N4∶Tb. The observations clearly indicated that it is to regulate the emission or the fluorescence of carbon nitride by doping with rare-earth metal ions into the framework of g-C3N4. This is for the of on the of various biological applications. It is highly to the of g-C3N4∶Tb. were Supporting Information Figure of for the at a concentration of g-C3N4∶Tb. This well the of g-C3N4∶Tb. biological In biological the luminescence of rare-earth metals has for example, large Stokes shifts and characteristic sharp For the ensuing biological the doping amount of Tb3+ was fixed at g-C3N4∶Tb in the of both powder ( Supporting Information Figure and aqueous solution ( Supporting Information Figure exhibit a strong fluorescence with respect to the native g-C3N4. For the of applications of g-C3N4∶Tb, small amounts were first to followed by the treated on the surfaces of substrates, glass and in this The of the was then by the details of the were also by fluorescence The images in Figure clearly typical fingerprint patterns with to the identification of an Figure | of the fingerprint after small amount of g-C3N4∶Tb in and The full details in the two that is, 1 and were by the fluorescence Download figure Download PowerPoint In to the applications in biometrics that take advantage of the fluorescence performance of terbium-doped carbon nitride, g-C3N4∶Tb may also serve as a marker for glass surfaces by amounts of g-C3N4∶Tb in agar solution. to the chemical of carbon nitride, the g-C3N4∶Tb marker is to and 90 The doped rare earth a suitable for its imaging by mass The marker surface was by of the with Figure 5 illustrates the mass spectrometric imaging for the of along with the imaging achieved by fluorescence at These results clearly illustrate that the terbium-doped carbon nitride g-C3N4∶Tb, or its by doping with rare-earth metals, is expected to a new of anti-counterfeiting Figure 5 | g-C3N4∶Tb serves as anti-counterfeiting marker on the glass followed by its fluorescence imaging by fluorescence and its mass spectrometric imaging obtained by 2 mg g-C3N4∶Tb was with g of agar to make a solution of g mL−1, and 100 μL was used to stamp “NEU” on the glass surface of 0.75 cm2. Download figure Download PowerPoint For biological graphitic carbon nitride is among the due to its low as well as In Supporting Information Figure the effect can be clearly which illustrates that both g-C3N4 and g-C3N4∶Tb typical is is also a of g-C3N4∶Tb may the biological and applications. In addition, rare-earth metals are in biological or which very low for ICP-MS and high and an 100 μL of g-C3N4∶Tb solution mg was injected into the of The doped terbium ions Tb3+ the tracking of the biodistribution of g-C3N4 in Figure indicated the of the liver, lung, and after the of g-C3N4∶Tb for 24 h. of Tb distribution in these by LA-ICP-MS imaging indicated high local concentration of which demonstrated that g-C3N4 is by the in these However, it may be clearly observed in Figure that g-C3N4 was in the spleen, while it was to be the in the kidney, with a concentration in the only of that in the This is the first attempt to LA-ICP-MS for the of carbon nitride in its Thus, this rare-earth doping greatly the applications of carbon nitride and its in biological by as an anti-counterfeiting marker to identification or a to facilitate in imaging and tracking in Figure | LA-ICP-MS images of terbium in liver, spleen, lung, and of mice after 24 h of amount of g-C3N4∶Tb into the via Download figure Download PowerPoint In this work, rare-earth metals, that is, terbium, europium, and erbium, were into the framework of graphitic carbon nitride by a and preparation under microwave without high and With respect to the native graphitic carbon nitride g-C3N4, terbium-doped g-C3N4∶Tb exhibited fluorescence at λex/λem = 290/490 and 290/545 nm. More the of emission and the fluorescence be by adopting rare-earth metals or their doping and doping This approach greatly the of biological applications of carbon nitride. 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