Novel cookie-with-chocolate carbon dots displaying extremely acidophilic high luminescence

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1 PAPER View Article Online View Journal View Issue Cite this:, 2014, 6, Received 14th July 2014, Accepted 7th September 2014 DOI: /c4nr03965c 1. Introduction Fluorescent carbon-based materials have recently drawn much attention owing to their stable photoluminescence (PL), low cytotoxicity, excellent biocompatibility, and environmental compatibility, as well as their potential applications in photocatalysis, energy conversion, sensing, and bio-based fields. These fluorescent carbon-based materials are generally small nanoparticles of diameter below 10 nm. 1 In the research of such a carbon nanoparticle (carbon dots, CDs) field, numerous novel structures appeared, which enhance the concepts for CDs. 2 There are different kinds of CDs in terms of intrinsic structures, 3 such as carbon quantum dots (CQDs), 4,5 carbon nanodots (CNDs) 6 14 and polymer dots (PDs) Their fluorescence is generally quenched at low ph, which may limit their practical applications. The synthesis of CDs that show high luminescence in an acid has not been well studied, despite the importance of developing improved fluorescent carbon materials We report here the development of a facile, lowcost, and green route to synthesize highly fluorescent carbonbased materials with a novel structure by the hydrothermal treatment of L-serine and L-tryptophan at a low temperature (200 C), 26 as shown in Scheme 1. A cookie-like CD film resulted; it emitted bright fluorescence in an acid (ph 1, 2, even <1) with a quantum yield that was increased to almost quadrupled that was observed in a neutral solution. Therefore, State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, , P. R. China. byangchem@jlu.edu.cn Electronic supplementary information (ESI) available. See DOI: / c4nr03965c These authors contributed equally to this work. Novel cookie-with-chocolate carbon dots displaying extremely acidophilic high luminescence Siyu Lu, Xiaohuan Zhao, Shoujun Zhu, Yubin Song and Bai Yang* A fluorescent carbon dot with a cookie-with-chocolate film structure (about 5 5 μm 2 ) showed a high fluorescence quantum yield (61.12%) at low ph. It was hydrothermally synthesized from L-serine and L-tryptophan. The formation mechanism of the film with carbon dots (CDs) was investigated. The film structure was formed by hydrogen bonding and π π stacking interactions between aromatic rings. The strong blue fluorescence of the CDs increased under strong acidic conditions owing to the changes in the N-groups. These cookie-like CDs are attractive for their potential use as effective fluorescent probes for the sensitive detection of aqueous H + and Fe 3+. Scheme 1 A schematic illustration of the preparation procedure of a cookie with a chocolate carbon dots structure by hydrothermal carbonization of L-serine and L-tryptophan. the as-prepared material could sensitively and selectively detect H + and be used under extremely acidic conditions. Furthermore, the PL of the cookie-like CD film could be quenched by Fe 3+. The reaction and the PL mechanism were also investigated through comparisons with other materials made with a different carbon source and at a different reaction temperature. 2. Results and discussion 2.1. Characterization of the CD structure and morphology The chemical structure and morphology of the CDs were investigated by several characterization methods. TEM images (recorded on carbon grids with holes) of the CD film (CDF-S) show that it was composed of small carbon cores which have an average diameter of 3.5 nm embedded in a large, smooth carbon-based film (Fig. 1). High-resolution TEM images show most of the particles to be amorphous carbon particles with no lattice structure (Fig. 1d). The addition of an acid to the system produced numerous holes in the CD film (CDF-SA), as shown in Fig. S1. FT-IR analysis of CDF-S and CDF-SA This journal is The Royal Society of Chemistry 2014,2014,6,

2 Fig. 1 (a) (c) TEM images of as-prepared CDs, (d) high-resolution TEM image of CDs, (e) the corresponding size distribution of CDs. Fig. 2 XPS data for model CDs and model with CD. XPS C 1s spectra of (a) model, (b) model with an acid. XPS N 1s spectra of (c) model and (d) model with an acid. Fig. 3 (a) PL spectra of model CDs in aqueous solution at different excitation wavelengths (in 20 nm increment starting from 300 nm to 420 nm). (b) UV-vis absorption of model CDs dispersed in aqueous solution. (c) PL spectra of model with acid CDs in aqueous solution at different excitation wavelengths (in 20 nm increment starting from 300 nm to 420 nm). (d) UV-vis absorption of model with acid CDs dispersed in aqueous solution. (e) PL spectra of model with alkalis CDs. (f) Photographs of the as-prepared model CDs, model with alkalis CDs, model with acid CD solution under a visible light (up) and a UV beam of 365 nm (down). revealed the presence of OH and COOH in CDF-S, although when an acid was added to the system, peaks attributable to COOR and C N also appeared, and the intensity of the OH, COOH, and NH 3 + peaks increased (Fig. S2 ). 27 These changes indicate the increase in the number of amidocyanogen groups in CDF-SA in comparison with CDF-S. XPS results reveal the differences in the surfaces between the two films (Fig. 2), although both consisted of C, N, and O. Deconvolution of the C 1s spectrum of CDF-S indicates the presence of three types of carbon bonds: sp 2 CvC ( ev), C N ( ev), and COOH (288.6 ev). The N 1s spectrum of CDF-S shows two types of N bonds: C N (399.3 ev) and N H (401.6 ev). Compared with CDF-S, the percentage of nitrous C increased in CDF-SA, the percentage of oxygenated C decreased, and the percentage of N H significantly increased. 18 The optical properties of CDF-S and CDF-SA were measured to reveal the effect of an acid on the PL (Fig. 3). The UV-vis spectra of both samples in aqueous solutions show peaks at 223 and 271 nm (Fig. 3b and d). The acidified sample showed a new peak at 368 nm that was not shown by the original film. This new absorption peak may be ascribed to the increased number of amino-functional groups and to electron enrichment. 28,29 The fluorescence spectra of the two samples show optimal excitation wavelengths of 309, 351, and 379 nm for CDF-S, and 276, 305, and 378 nm for CDF-SA. However, both samples possessed the same maximum emission wavelength of 448 nm. The fluorescence quantum yield (FLQYs) of CDF-S and CDF-SA excited at 360 nm were calculated to be 16.23% and 34.8%, respectively, using quinine sulfate as a standard. However, the maximum excitation wavelength for both samples was 380 nm. To further study the FLQY of the samples, they were also measured directly by an absolute PL quantum yield (internal quantum efficiency) measurement system. The FLQYs of CDF-S and CDF-SA excited at 380 nm are 18.25% and 61.12%, respectively. Strong PL enhancement in a 13940,2014,6, This journal is The Royal Society of Chemistry 2014

3 Fig. 5 (a) TEM images of as-prepared G-CDs, (b) high-resolution TEM image of G-CDs, and (c) the corresponding size distribution of CDs. Fig. 4 The time-correlated single-photon counting (TCSPC) of model CDs and model with acid CDs (up). The average lifetime of two kinds of CDs with two lifetime components (down) (360 nm excitation, delay time at 443 nm emission). UV beam of 365 nm is visible to the naked eye, as shown in Fig. 3f. The two samples were tested in a detailed PL study using different excitation wavelengths ranging from 300 to 420 nm. CDF-S showed a strong peak at 361 nm (purple emission) as well as a shoulder peak at 448 nm (blue emission) when excited by a 300 nm light (Fig. 3a). Increasing the excitation wavelength decreased the peak at 361 nm to nothing, whereas the peak at 448 nm increased considerably. CDF-SA excited at 300 nm showed a minimal peak at 361 nm and a strong blue emission at 448 nm (Fig. 3c). However, the emissions of both samples were nearly excitation-independent, and the organic fluorophores were assigned as the PL centers. 30 The origin of the PL was investigated by time-correlated single-photon counting (TCSPC), an important technique for clarifying the behavior of excited electrons Fig. 4 and Table S1 show that the long PL lifetimes over the whole emission spectra of both samples can be fitted to two exponential components. Both PL lifetimes are similar, and both behaviors are on a nanosecond time scale, indicating that both samples had only fluorescent properties. The addition of a base decreased the lifetime to some degree, indicating that the organic fluorophores were strongly affected by the surrounding environment. 34, Formation and the PL mechanism Two comparison tests were conducted to explore the formation and the PL mechanism of the new structure further. In the first test, the carbon source was changed from L-tryptophan to L-glycine, and all other conditions were kept constant. Fig. 5 shows that the as-prepared glycine-cds (G-CDs) are monodispersed nanocrystals with near-spherical morphology and an average diameter of 6.1 nm. They all had a clear lattice spacing of nm. Their FLQY under excitation at 360 nm was calculated to be 10% with quinine sulfate as the standard; the excitation spectra showed only one peak (at 409 nm). This suggests that intrinsic emission dominates over molecular emission. These particles behave very differently from CDF-S, and show obvious excitation-dependent PL, which can be attributed to their complete crystal structure (Fig. S4a ). The substitution of L-tryptophan with L-glycine is equivalent to the replacement of indolyl by hydrogen. The hydrogen of glycine can easily interact with the hydroxyl of L-serine to form a very stable, highly crystalline structure. In contrast to the behavior of CDF-S, the fluorescence of G-CDs in aqueous solution was quenched by an acid. These results indicate that indolyl is significant in the improvement of the emission efficiency and film structure. 36 In the second comparative experiment, the reaction temperature was changed from 200 to 300 C, and the other reaction conditions were unaltered. Fig. S3 shows that the hotter synthesis also produced a film-like morphology; however, a clear lattice emerged, which was very different from CDF-S. The resulting sample showed an obvious excitation-dependent PL behavior, which was also attributed to its complete crystal structure (Fig. S4c ). The resulting FLQY under 360 nm UV light was calculated to be 24.08% by selecting quinine sulfate as a standard. This value is much higher than that shown by CDF-S. These results imply that the formation of carbogenic cores with a clear lattice structure, which can act as an antenna, is favored at higher temperatures. 37,38 Therefore, the FLQY of this system was increased as a result of the higher synthetic temperature. Bimolecular films were easily formed via hydrogen bonding, and π π stacking interactions between the aromatic moieties played an important role in the formation of the films (Fig. 6). The dots in the film can be attributed to local overheating, which led to single carbogenic domains (cores). When an acid was add to CDF-S, the high concentration of H + destroyed the hydrogen bonding and released the amino groups and indolyl. It also led to the protonation of the amino groups and indolyl, which caused the π π stacking interactions between the aromatic moieties to change to a stronger Coulombic repulsive force, which formed rigid structures, This journal is The Royal Society of Chemistry 2014, 2014,6,

4 Fig. 6 The proposed cookie with a chocolate carbon dots microstructure. which could develop into the backbones of 3D superstructures. The resulting proton transfer between the pyrrole N and carbon chains of the film structure would make its greatest contribution in the excited state, and would prevent charge transfer, leading to the recovery of fluorescence. These two factors account for the enhanced fluorescence emission. This mechanism explains how CDF-SA shows stronger fluorescence enhancement than CDF-S or the G-CDs. To study the relationship between protonation and fluorescence further, we calculated the absorption spectrum of indolyl with respect to its protonation. Fig. S5 shows the emergence of a new peak in the protonated absorption spectrum, which is consistent with the UV-vis spectra of CDF-SA. This implies that the protonated CDs could absorb more light than the non-protonated CDs. All calculations were performed using the GAUSSIAN 09 software package at the B3LYP/6-31+G (d,p) level and the solvent is water. The effects of ionic strength and UV exposure on the stability of the PL were also investigated. There were no changes in the PL intensity or its peak characteristics at different ionic strengths (Fig. S6 ). This stability is significant and a necessary property if CDF-S is to be used practically in the presence Fig. 8 The different PL intensities of the CDF-S solution and the absence of various metal ions. of varying salt concentrations. No dramatic photo bleaching was found after 8 h of continuous UV exposure (Fig. S7 ). This may be a consequence of the stable film structure of the CDF-S. 39 This observed ph-dependence is very different from other reported behaviors. The FLQY is the highest in a strong acid; it is the lowest in a strong base, with the emission peak red shifted (Fig. 7) and can shift to 487 nm when we add extra NaOH (Fig. 3e). The fluorescence of the CDs was quenched by Fe 3+ ions (Fig. 8, Fig. S8 ), due to the special coordination interaction between the Fe 3+ ions and the phenolic hydroxy groups of the CDs. This interaction has been widely used for the detection of Fe 3+ ions or colored reactions in traditional organic chemistry. 40,41 Fluorescence quenching may contribute to non-radiative electron-transfer that involves the partial transfer of an electron in the excited state to the d orbital of Fe Conclusions We prepared cookie-like CDs that exhibit fluorescence, the quantum yield of which is enhanced in a strong acid and easily quenched by Fe 3+. The presence of indolyl groups and the synthesis temperature were significant in determining the emission efficiency and the formation of the film structure. The present results enriched the ph-sensitive CDs with different modifications, which will be also important for understanding the PL mechanism of fluorescent carbon based Fig. 7 (a) ph-dependent PL spectra for CDF-S excited at 380 nm. (b) Normalized ph-dependent PL spectra for CDF-S excited at 380 nm. (c) Relative intensity variation of PL spectra with the increase of ph value from 1 to ,2014,6, This journal is The Royal Society of Chemistry 2014

5 materials We believe that these cookie-like CDs have many potential applications. 4. Experimental section 4.1. Preparation of carbon dots The typical experimental procedure is described as follows: L-serine (0.157 g) and L-tryptophan ( g) were dissolved in 10 ml weak alkaline water (about ph 8.1) adjusted by adding NaOH. Then the mixture was transferred to a poly(tetrafluoroethylene) (Teflon)-lined autoclave (30 ml) and heated at 200 C for 8 h. The reactor was automatically cooled to room temperature. After this reaction, a clear golden yellow solution was obtained. Remarkably, the as-synthesized sample is found to exhibit favorable aqueous dispersion Characterization High-resolution transmission electron microscopy (HTEM) was performed on an FEI Tecnai F20. TEM was conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kv with a CCD cinema. Fluorescence spectroscopy was performed using a Shimadzu RF-5301 PC spectrophotometer. UV-vis absorption spectra were obtained using a Shimadzu 3100 UV-vis spectrophotometer. IR spectra were recorded on a Nicolet AVATAR 360 FT-IR spectrophotometer. The confocal microscopy images were recorded using an Olympus Fluoview FV1000. X-ray Photoelectron Spectroscopy (XPS) was performed using an ESCALAB 250 spectrometer with a mono X-ray source Al Kα excitation ( ev). Binding energy calibration was based on C 1s at ev Quantum yield (QY) measurements 1. Quinine sulfate (0.1 M H 2 SO 4 as a solvent; QY = 0.54) was chosen as a standard. The QYs of CDs (in water) were calculated accordingly by two kinds of methods. ϕ x ¼ ϕ st ði x =I st Þðη x 2 =η st 2 ÞðA st =A x Þ where ϕ is the QY, I is the measured integrated emission intensity, η is the refractive index of the solvent, and A is the optical density. The subscript st refers to the standard with the known QY and x for the sample. In order to minimize reabsorption effects, absorption in the 10 mm fluorescence cuvette was kept below 0.10 at the excitation wavelength (360 nm). 2. Photoluminescence QYs were also measured directly by the absolute PL quantum yield (internal quantum efficiency) measurement system (C , Hamamatsu Photonics K. K., Japan), which comprises an excitation light source, monochromator, an integrating sphere capable of nitrogen gas flow and a CCD spectrometer for detecting the whole spectral range simultaneously (Jun Lin s group, State Key Laboratory of Rare Earth Resource Utilization, Changchun, Institute of Applied Chemistry, Chinese Academy of Sciences). Acknowledgements This work was financially supported by the National Science Foundation of China (NSFC) under grant no , , and the National Basic Research Program of China (973 Program) under grant no. 2012CB Notes and references 1 S. N. Baker and G. A. Baker, Angew. Chem., Int. Ed., 2010, 49, H. Li, Z. Kang, Y. Liu and S.-T. Lee, J. Mater. Chem., 2012, 22, L. Cao, M. J. Meziani, S. Sahu and Y. P. Sun, Acc. Chem. Res., 2013, 46, S. Zhu, S. Tang, J. Zhang and B. Yang, Chem. Commun., 2012, 48, X. Wang, L. Cao, S. T. Yang, F. Lu, M. J. Meziani, L. Tian, K. W. Sun, M. A. Bloodgood and Y. P. Sun, Angew. Chem., Int. Ed., 2010, 49, Y. Song, S. Zhu and B. Yang, RSC Adv., 2014, 4, D. Pan, J. Zhang, Z. Li, C. Wu, X. Yan and M. Wu, Chem. Commun., 2010, 46, Y. Wang, Y. Li, Y. Yan, J. Xu, B. Guan, Q. Wang, J. Li and J. Yu, Chem. Commun., 2013, 49, Y. Yang, D. Wu, S. Han, P. Hu and R. Liu, Chem. Commun., 2013, 49, C. Guo, D. Zhao, Q. Zhao, P. Wang and X. Lu, Chem. Commun., 2014, 50, C. Guo, Y. Dong, H. Yang and C. Li, Adv. Energy Mater., 2013, 3(8), C. Guo, J. Xie, B. Wang, X. Zhang, H. Yang and C. Li, Sci. Rep., 2013, 3, Z.-A. Qiao, Y. Wang, Y. Gao, H. Li, T. Dai, Y. Liu and Q. Huo, Chem. Commun., 2010, 46, P. Mirtchev, E. J. Henderson, N. Soheilnia, C. M. Yip and G. A. Ozin, J. Mater. Chem., 2012, 22, Z. A. Qiao, Q. Huo, M. Chi, G. M. Veith, A. J. Binder and S. Dai, Adv. Mater., 2012, 24, S. Zhu, J. Zhang, L. Wang, Y. Song, G. Zhang, H. Wang and B. Yang, Chem. Commun., 2012, 48, S. Zhu, J. Zhang, Y. Song, G. Zhang, H. Zhang and B. Yang, Acta Chim. Sin., 2012, 70, T. Lai, E. Zheng, L. Chen, X. Wang, L. Kong, C. You, Y. Ruan and X. Weng,, 2013, 5, J. Gu, W. Wang, Q. Zhang, Z. Meng, X. Jia and K. Xi, RSC Adv., 2013, 3, S. Zhu, L. Wang, B. Li, Y. Song, X. Zhao, G. Zhang, S. Zhang, S. Lu, J. Zhang, H. Wang, H. Sun and B. Yang, Carbon, 2014, 77, Y. Song, W. Shi, W. Chen, X. Li and H. Ma, J. Mater. Chem., 2012, 22, W. Kwon, J. Lim, J. Lee, T. Park and S.-W. Rhee, J. Mater. Chem. C, 2013, 1, This journal is The Royal Society of Chemistry 2014,2014,6,

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