- Email: [email protected]
Synthesis of mechanical responsive carbon dots with fluorescence enhancement
Journal of Colloid and Interface Science 560 (2020) 85–90
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Synthesis of mechanical responsive carbon dots with fluorescence enhancement Yuru Wang, Yuhan Li, Yang Xu ⇑ National-Local Joint Engineering Laboratory for Energy Conservation in Chemical Process Integration and Resources Utilization and Tianjin Key Laboratory of Chemical Process Safety, School of Chemical Engineering and Technology, Hebei University of Technology, Guangrong Dao No.8, Hongqiao District, Tianjin 300130, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 9 August 2019 Revised 10 October 2019 Accepted 11 October 2019 Available online 13 October 2019 Keywords: Carbon dots Mechanical response Fluorescence enhancement Structure changes
a b s t r a c t Mechanical responsive carbon dots (CDs) is a rising star in the family of carbon materials. However, high pressure devices are essential for observing mechanical responses of carbon dots, but they limit the widespread application of mechanical responsive CDs. Two kinds of CDs were prepared using pyrene derivative (pyrene-1-butyric acid, PyBA) as carbon source, and had fluorescence enhancement via simply grinding. An apparent 20–30 times emission enhancement can be observed after the treatment of grinding because of CDs packed structure and morphology changes. To the best of our knownledge, it is the first example of mechanical responsive CDs with fluorescence enhancement without the stimulation of any complex pressure instrumentation. This simple approach could be extended to other carbon sources with p-conjugated structure for building new intelligentized CDs, and provide new routes for the applications of CDs toward mechanical sensing. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Mechanical responsive fluorescence materials, as a rising star of stimuli-responsive materials, have switchable fluorescence properties in colour or intensity via external mechanical stimuli. Therefore, they have attracted significant attention due to their potential for use as sensors, memory devices, motion systems, and security systems [1–4]. Recently, many different types of ⇑ Corresponding author. E-mail address: [email protected] (Y. Xu). https://doi.org/10.1016/j.jcis.2019.10.039 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
mechanical responsive fluorescent materials have been developed [5,6]. In these reports, the fluorescent properties changed due to the molecular orientation and intermolecular interactions, being disrupted by mechanical forces, including shearing, grinding, tension, and hydrostatic pressure. Li et al. prepared an ultrathin film from polyacrylate modified styrylbiphenyl derivative and layered double hydroxide nanosheet [4]. They found that an enhanced luminescence can be observed in this ultrathin film under pressure stimuli. Mechanical responsive fluorescent materials typically rely on changes in physical molecular packaging patterns or molecular design [7–9]. To date, a number of organic mechanical responsive
86
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
fluorescent materials with promising mechanochromic fluorescence have been developed and the corresponding mechanism of mechanochromic fluorescence has been investigated [10–12]. In general, the intermolecular interaction is important parameters for molecular stacking and fluorescence properties changes [13,14]. These principles are still applicable for mechanochromic phenomenon if the molecular materials are carbonized by high temperature. Carbon dots (CDs) have emerged as an outstanding nanomaterial because of their unique properties, such as excellent fluorescence, low toxicity, high photostability, and good biocompatibility [15–18]. Nowadays, various fluorescent CDs have been prepared from a number of carbon sources, such as citric acid, glucose, graphite, and carbon fiber [19–21]. However, only a few examples of CDs with intelligentized response ability, particularly mechanical responsive CDs have been reported. Lu et al. prepared a piezochromic CDs with a fluorescence color shift from yellow to blue via additional pressure of 0–22.83 GPa [22]. Liu et al. found that the fluorescence of CDs can be controlled by high external pressure device due to the influence of p conjugated system and carbonyl group [23]. Wang et al. prepared a red-emission CDs with fluorescence enhancement property via pressure [24]. Nevertheless, it required additional high pressure devices, which limit the widespread application of mechanical responsive CDs. Therefore, to fabricate mechanical responsive CDs without the stimulation of any complex pressure instrumentation is highly desired. In this work, two CDs were prepared using pyrene-1-butyric acid (PyBA) as carbon precursor. These CDs had a pyrene-based core, which had a merit of mechanical response. An apparent fluorescence enhancement can be observed with treatment of simply grinding. The mechanism of mechanical response was investigated that the transition between crystallization and amorphous structure play vital roles in this process. Notably, these CDs are the first example for mechanical response based on CDs without the stimulation of any complex pressure instrumentation. These CDs not only provide a new route for preparing intelligentized responsive CDs, but also promote the extensive applications of carbon materials. 2. Experimental section 2.1. Reagents and chemicals Pyrene-1-butyric acid (PyBA) with a purity of 97% and qphenylenediamine were purchased from J&K Co. (Beijing, China), and used without further purification. Ethanol in analytical pure degree as reagent was purchased from Tianjin Chemical Co. Ltd. (Tianjin, China). The water used in the experiment was deionized water. 2.2. Instrumentation and characterization The morphology characterization of mechanical responsive CDs was performed using a Tecnai 20 high resolution transmission electron microscope. The accelerating voltage of TEM was 200 KeV. A Bruker D8 Focus X-ray powder diffract instruction with Cu Ka radiation (k = 1.5418 Å) and an operating condition of 40 KV was used for investigation of the XRD patterns of mechanical responsive CDs. X-ray photoelectron spectroscopy (XPS) data including CDs and grinding process, were collected by a Thermo SCIENTIFIC ESCALAB250 Xi. Fourier transform infrared (FT–IR) spectroscopy of CDs and precursors were performed using a Bruker Vector 22 spectrophotometer, and treatment with KBr. The FT–IR was detected from 400–4000 cm 1 with a resolution of 4 cm 1. A fluorescence spectrometer (Edinburgh Instruments, FS920P) was
used to investigate the steady-state fluorescence spectra of two CDs. UV–visible spectra of two CDs were performed by an Agilent Carry 100 UV–vis spectrometer, with a scan range of 200–800 nm. 2.3. Preparation of CDs-1 144 mg of PyBA (0.5 mmol) was dispersed in 10 mL of ethanol, and the mixed solution was transfer to culture dish, and heated in a drying oven at 180 °C for 4 h to obtain powder of as-prepared CDs-1. 2.4. Preparation of CDs-2 144 mg of PyBA (0.5 mmol) and 27 mg of q-phenylenediamine (0.25 mmol) were dispersed in 10 mL of ethanol, and the mixed solution was transfer to culture dish, and heated in a drying oven at 180 °C for 4 h to obtain powder of CDs-2. 3. Results and discussion Two CDs (CDs-1 and CDs-2) powder was obtained by scraping from the culture dish with a medicine spoon. In excitation spectra, both as-prepared CDs and post grinding CDs showed an excitation band from 250 to 400 nm, and had a maximum excitation wavelength at 365 nm (Fig. S1). Spectroscopic measurements were performed to further characterize the fluorescence changes of CDs-1 in the solid state. Fluorescence spectra showed three peaks at 445, 471, and 524 nm for the as-prepared CDs-1 powder (Fig. 1A, and Table S1). The emission at 445 nm arises from the pyrene units, similar to other pyrene derivatives. The peak at 471 nm was notably red shifted compare to the pyrene monomer emission, caused by intramolecular charge transfer species [25,26]. After mechanical grinding in an agate, both CDs had two same peaks at 446 and 473 nm, meanwhile the fluorescence intensity was approximately 30 times enhanced under the UV light excitation (Fig. 1A, and Table S1). The fluorescence properties of CDs-2 were similar with CDs-1, but a slightly red-shift from 471 to 475 nm can be observed after the treatment of grinding (Fig. 1B, and Table S1). The pyrene core induced fluorescence change due to p–p* interaction, while surface passivation will provide a way to change the luminescence properites similar to the previous reports on CDs [27,28]. Further, a slight fluorescence emitting color changes can be observed in both CDs with grinding time increases (Fig. 1C and D). The influence of the morphology of two CDs on their fluorescence emission was investigated by the TEM analysis. The asprepared powder of CDs-1 and CDs-2 exhibited a well-defined morphology. The size distribution of both CDs was in the range of 2–6 nm, and average particle size was 3 nm (Fig. 2A and C). Both CDs had similar microcrystalline structure, and the lattice spacing was 0.24 nm, which was related to the (1 0 0) facet of graphite [29]. The morphology of CDs with the treatment of grinding was also investigated. The microcrystalline state became more amorphous after mechanical grinding (Fig. 2B and D). To further investigate the CDs formation, a series of the structure and properties analysis of the samples after grinding was performed. As shown as Fig. 3A and B, FT-IR spectroscopy of CDs and carbon source are performed. Four apparent peaks at 3446, 3020, 1680, and 1620 cm 1 can be founded in CDs, which were corresponded to the OAH, CAH, C@O, and C@C, respectively. It demonstrated the functional groups of two CDs were retained after grinding (Fig. 3A and B). For CDs-2, a peak at 1388 cm 1 in Fig. 3B correspond to the stretching vibrations of CAN in the amide bond formed during the hydrothermal process. A peak at 3280 cm 1 corresponded to amide NAH stretching were masked by the broad peaks ascribed to OAH (3446 cm 1).
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
87
Fig. 1. Fluorescence spectra of (A) CDs-1 and (B) CDs-2 as prepared and after grinding for 30 min. Photographs of (C) CDs-1 and (D) CDs-2 as prepared and after grinding for 10 min, 30 min and 60 min under UV irradiation (kex = 365 nm). All emission spectra and luminescence photos were obtained at room temperature.
Fig. 2. TEM images and size distributions of as-prepared CDs-1 (A and a), grinding CDs-1 (B and b), as-prepared CDs-2 (C and c), and grinding CDs-2 (D and d).
Some previous studies relationship between p-conjugated and their optical properties based on theoretical calculations and related experimental results [30,31]. Fluorescence changes maybe related to different supramolecular packing modes in the molecular level and the morphology of CDs [7,32]. However, it remains insufficient how the external force causes the fluorescence enhancement for the CDs. It is well known that fluorescence changes are attributed to the materials structure in crystal or amorphous states. To investigate the structure changes of CDs during grinding process, XRD patterns of PyBA, CDs-1, and CDs-2 were measured before and after grinding, respectively (Fig. 3C and D). Various sharp peaks were observed in the sample without grinding, indicating that the as-prepared CDs was in the state of highly ordered structures before grinding (Fig. 3C and D). However,
significant differences in the XRD patterns appeared after grinding (Fig. 3C and D). Initially, no apparent change was observed in the XRD pattern after grinding 10 min. Further, the peak in XRD pattern after grinding for 30 min became broaden, indicating that the structures of CDs was changed and gradually became amorphous. XPS is an effective technique to characterize the surface and component of two CDs. Three peaks can be observed in the XPS spectra at 285, 400 and 533 eV, which was ascribed to C1s, N1s, and O1s, respectively (Fig. S2) [33]. The C1s narrow scan in Fig. 4 was fitted with two components centered at 283.3 and 287.3 eV, assigned to C@C/CAC and C@O/COOH, respectively [34]. Meanwhile, two peaks centered at 532.0 and 532.7 eV in the O1s narrow scan were assigned to C@O and
88
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
Fig. 3. FT-IR spectra of (A) CDs-1 and (B) CDs-2 as prepared (black curve) and after grinding for 30 min (red curve). Powder XRD patterns of PyBA, (C) CDs-1, and (D) CDs-2 asprepared and after mechanical grinding for 10, 30, and 60 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. XPS analyses of the C 1s and O 1s spectra of (A) CDs-1 and (B) CDs-2 as prepared and after grinding for 30 min.
CAO, respectively. In CDs-2, the appearances of CAN (286.3 eV) was observed, indicating the formation of the hybrid between PyBA and q-phenylenediamine. Notably, the spectra of C1s reveal an increasing signal ratio for C@C/CAC to the C@O/COOH signal in samples before and after grinding (from 0.56 to 0.92 for CDs-1 and from 0.94 to 1.09 for CDs-2 in Table S2). It indicated that the composition was changed during the grinding process. Moreover, the change of signal ratio for C@C/CAC to the C@O/ COOH in CDs-1 sample were larger than CDs-2. It inferred that the passivation agent may play an important role in the change of signal ratio. Through the above characterizations, the structure of CDs was changed during grinding treatment from crystallization to amorphous state, along with composition changes. Liu et al. found that p-conjugated system and functional groups can
have a vital impact on the fluorescence properties of CDs [23]. Thus, the relationship between structure and fluorescence property is essential for fluorescence investigation. Previous studies have shown that adjacent pyrene groups move closer vertically and undergo a slight slippage under external stress stimuli [35]. The as-prepared powder samples were well-dispersed with weak interactions between neighboring nanoparticles. It implied that the structure configure can be induced by coplanarization structure [36]. The core structure of pyrene will endow stimuli response to as-prepared CDs. The CDs nanoparticles after grinding is the primary driver of the fluorescence change. The reconfiguration of pyrene core in CDs was originated from stack structure change, which induced fluorescent intensity enhancement. (Scheme 1).
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
89
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Scheme 1. Schematic illustration of the structure changes of CDs by grinding.
Table 1 The fluorescence lifetimes of CDs-1 and CDs-2 before and after grinding for 30 min (Excitation at 365 nm, data collected at 446 nm for CDs-1 and 447 nm for CDs-2). As-Prepared
CDs-1 CDs-2
Acknowledgements Financial support by the National Natural Science Foundation of China, china (No. 21605036), and the Natural Science Foundation of Hebei Province, China (No. B2017202068) is acknowledged.
After Grinding (30 min)
s(ns)
Percentage (%)
s(ns)
Percentage (%)
3.5 21.2 4.7 21.8
79.2 20.8 70.3 29.7
4.4 25.3 4.1 23.0
28.4 71.6 25.3 74.7
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.039. References
The excimers of pyrene had two types: partial overlap and sandwich-like packing, which exhibited short and long lifetime, respectively. Hence, time-resolved fluorescence spectra of CDs were used to confirm the pyrene excimers types and intermolecular packing modes based on their fluorescence lifetimes (Fig. S3). The as-prepared CDs-1 sample showed two types of fluorescence lifetimes with 3.5 ns (79.2%) and 21.2 ns (20.8%). The lifetime and the proportion changed upon grinding for 30 min (4.4 ns (28.4%) and 25.3 ns (71.6%), Table 1). The percentage of short lifetimes was decreased from 79.2% to 28.4% with the treatment of grinding. The short lifetimes reflected the excimer emission of the partial overlap stacking, which was usually founded in highly ordered structures. It demonstrated that the crystalline structure of CDs was partially destroyed during the grinding process. Simultaneously, the proportion of the long lifetimes increased from 20.8 to 71.6%, because the CDs were in the state of amorphous and sandwich-type excimer played a dominant role in fluorescence emission. Similar results were obtained for CDs-2 and the specific data is shown in Table 1. Thus, it can be concluded that structure stacking among the CDs nanoparticles are changed after sufficient grinding, achieving a response to external mechanical stimuli. With increasing grinding time, the morphology of the powder changes from crystalline to amorphous, leading to fluorescence intensity enhancement [37]. Further, other CDs were prepared in similar experiment method by using bovine serum albumin (BSA) as precursor, denoted as CDs-BSA. No apparent fluorescence enhancement can be observed for CDs-BSA (Fig. S4). It demonstrated that the stacking structure plays a crucial role in fluorescence enhancement, and will be an ideal precursor for preparing mechanical responsive materials. 4. Conclusion In summary, two CDs with mechanical-stimuli fluorescence enhancement were prepared. The mechanism of both CDs is the same that the transition between crystallization and amorphous structure induce mechanical responses. Compared with traditional fluorescent CDs, these CDs have apparent mechanical responses, and complex pressure devices are not required. Most importantly, this strategy provides an innovative route to prepare mechanical responsive CDs and carbon-based nanomaterials. These mechanical responsive CDs will have potential for mechanical applications.
[1] D. Yan, J. Lu, J. Ma, S. Qin, M. Wei, D.G. Evans, X. Duan, Layered host-guest materials with reversible piezochromic luminescence, Angew. Chem. 123 (2011) 7175–7178. [2] J. Zhao, S. Ji, Y. Chen, H. Guo, P. Yang, Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials, Phys. Chem. Chem. Phys. 14 (2012) 8803–8817. [3] Y. Sagara, S. Yamane, M. Mitani, C. Weder, T. Kato, Mechanoresponsive luminescent molecular assemblies: an emerging class of materials, Adv. Mater. 28 (2016) 1073–1095. [4] M. Li, R. Tian, D. Yan, R. Liang, M. Wei, D.G. Evans, X. Duan, A luminescent ultrathin film with reversible sensing toward pressure, Chem. Commun. (Camb) 52 (2016) 4663–4666. [5] F.M. Winnik, Photophysics of preassociated pyrenes in aqueous polymer solutions and in other organized media, Chem. Rev. 93 (1993) 587–614. [6] X. Sun, X. Zhang, X. Li, S. Liu, G. Zhang, A mechanistic investigation of mechanochromic luminescent organoboron materials, J. Mater. Chem. 22 (2012) 17332–17339. [7] S. Varghese, S. Das, Role of molecular packing in determining solid-state optical properties of p-conjugated materials, J. Phys. Chem. Lett. 2 (2011) 863– 873. [8] M. Sase, S. Yamaguchi, Y. Sagara, I. Yoshikawa, T. Mutai, K. Araki, Piezochromic luminescence of amide and ester derivatives of tetraphenylpyrene—role of amide hydrogen bonds in sensitive piezochromic response, J. Mater. Chem. 21 (2011) 8347–8354. [9] K. Nagura, S. Saito, H. Yusa, H. Yamawaki, H. Fujihisa, H. Sato, Y. Shimoikeda, S. Yamaguchi, Distinct responses to mechanical grinding and hydrostatic pressure in luminescent chromism of tetrathiazolylthiophene, J. Am. Chem. Soc. 135 (2013) 10322–10325. [10] Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. Lv, S. Wen, B. Li, L. Ye, B. Zou, W. Tian, Piezochromic luminescence based on the molecular aggregation of 9, 10-bis ((E)-2-(pyrid-2-yl) vinyl) anthracene, Angew. Chem. Int. Ed. 51 (2012) 10782–10785. [11] Y. Sagara, T. Komatsu, T. Ueno, K. Hanaoka, T. Kato, T. Nagano, A water-soluble mechanochromic luminescent pyrene derivative exhibiting recovery of the initial photoluminescence color in a high-humidity environment, Adv. Funct. Mater. 23 (2013) 5277–5284. [12] M.J. Fuller, L.E. Sinks, B. Rybtchinski, J.M. Giaimo, X. Li, M.R. Wasielewski, Ultrafast photoinduced charge separation resulting from self-assembly of a green perylene-based dye into p-stacked arrays, J. Phys. Chem. A 109 (2005) 970–975. [13] X. Luo, J. Li, C. Li, L. Heng, Y.Q. Dong, Z. Liu, Z. Bo, B.Z. Tang, Reversible switching of the emission of diphenyldibenzofulvenes by thermal and mechanical stimuli, Adv. Mater. 23 (2011) 3261–3265. [14] X. Zhang, J.-Y. Wang, J. Ni, L.-Y. Zhang, Z.-N. Chen, Vapochromic and mechanochromic phosphorescence materials based on a platinum (II) complex with 4-trifluoromethylphenylacetylide, Inorg. Chem. 51 (2012) 5569–5579. [15] P.G. Luo, S. Sahu, S.-T. Yang, S.K. Sonkar, J. Wang, H. Wang, G.E. LeCroy, L. Cao, Y.-P. Sun, Carbon ‘‘quantum” dots for optical bioimaging, J. Mater. Chem. B 1 (2013) 2116–2127. [16] B.C. Martindale, G.A. Hutton, C.A. Caputo, E. Reisner, Solar hydrogen production using carbon quantum dots and a molecular nickel catalyst, J. Am. Chem. Soc. 137 (2015) 6018–6025. [17] K. Hola, Y. Zhang, Y. Wang, E.P. Giannelis, R. Zboril, A.L. Rogach, Carbon dots— emerging light emitters for bioimaging, cancer therapy and optoelectronics, Nano Today 9 (2014) 590–603. [18] X.-T. Tian, X.-B. Yin, Carbon dots unconventional preparation strategies, and applications beyond photoluminescence, Small (2019) 1901803.
90
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
[19] S. Qu, X. Wang, Q. Lu, X. Liu, L. Wang, A biocompatible fluorescent ink based on water-soluble luminescent carbon nanodots, Angew. Chem. Int. Ed. 51 (2012) 12215–12218. [20] H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C.H.A. Tsang, X. Yang, S.T. Lee, Water-soluble fluorescent carbon quantum dots and photocatalyst design, Angew. Chem. Int. Ed. 49 (2010) 4430–4434. [21] L. Wang, S.-J. Zhu, H.-Y. Wang, S.-N. Qu, Y.-L. Zhang, J.-H. Zhang, Q.-D. Chen, H.-L. Xu, W. Han, B. Yang, H.-B. Sun, Common origin of green luminescence in carbon nanodots and graphene quantum dots, ACS Nano 8 (2014) 2541–2547. [22] S. Lu, G. Xiao, L. Sui, T. Feng, X. Yong, S. Zhu, B. Li, Z. Liu, B. Zou, M. Jin, J.S. Tse, H. Yan, B. Yang, Piezochromic carbon dots with two-photon fluorescence, Angew. Chem. Int. Ed. 56 (2017) 6187–6191. [23] C. Liu, G. Xiao, M. Yang, B. Zou, Z.L. Zhang, D.W. Pang, Mechanofluorochromic carbon nanodots: controllable pressure-triggered blue-and red-shifted photoluminescence, Angew. Chem. Int. Ed. 57 (2018) 1893–1897. [24] Q. Wang, S. Zhang, B. Wang, X. Yang, B. Zou, B. Yang, S. Lu, Pressure-triggered aggregation-induced emission enhancement in red emissive amorphous carbon dots, Nanoscale Horiz. 4 (2019) 1227–1231. [25] Y. Suzuki, T. Morozumi, H. Nakamura, M. Shimomura, T. Hayashita, R.A. Bartsh, New fluorimetric alkali and alkaline earth metal cation sensors based on noncyclic crown ethers by means of intramolecular excimer formation of pyrene, J. Phys. Chem. B 102 (1998) 7910–7917. [26] D. Jiao, J. Geng, X.J. Loh, D. Das, T.C. Lee, O.A. Scherman, Supramolecular peptide amphiphile vesicles through host–guest complexation, Angew. Chem. Int. Ed. 51 (2012) 9633–9637. [27] S. Zhu, J. Zhang, S. Tang, C. Qiao, L. Wang, H. Wang, X. Liu, B. Li, Y. Li, W. Yu, X. Wang, H. Sun, B. Yang, Surface chemistry routes to modulate the photoluminescence of graphene quantum dots: from fluorescence mechanism to up-conversion bioimaging applications, Adv. Funct. Mater. 22 (2012) 4732–4740. [28] H. Tetsuka, R. Asahi, A. Nagoya, K. Okamoto, I. Tajima, R. Ohta, A. Okamoto, Optically tunable amino-functionalized graphene quantum dots, Adv. Mater. 24 (2012) 5333–5338.
[29] J. Lu, J.-X. Yang, J. Wang, A. Lim, S. Wang, K.P. Loh, One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids, ACS Nano 3 (2009) 2367–2375. [30] W. Pisula, M. Kastler, D. Wasserfallen, M. Mondeshki, J. Piris, I. Schnell, K. Müllen, Relation between supramolecular order and charge carrier mobility of branched alkyl Hexa-p eri-hexabenzocoronenes, Chem. Mater. 18 (2006) 3634–3640. [31] K. Li, W. Qin, D. Ding, N. Tomczak, J. Geng, R. Liu, J. Liu, X. Zhang, H. Liu, B. Liu, B.-Z. Tang, Photostable fluorescent organic dots with aggregation-induced emission (AIE dots) for noninvasive long-term cell tracing, Scient. Rep. 3 (2013) 1150. [32] G. Zhang, J. Lu, M. Sabat, C.L. Fraser, Polymorphism and reversible mechanochromic luminescence for solid-state difluoroboron avobenzone, J. Am. Chem. Soc. 132 (2010) 2160–2162. [33] A. Adenier, M.-C. Bernard, M.M. Chehimi, E. Cabet-Deliry, B. Desbat, O. Fagebaume, J. Pinson, F. Podvorica, Covalent modification of iron surfaces by electrochemical reduction of aryldiazonium salts, J. Am. Chem. Soc. 123 (2001) 4541–4549. [34] M. Shi, G. Dunham, M. Gross, G. Graff, P. Martin, Plasma treatment of PET and acrylic coating surfaces-I. In-situ XPS measurements, J. Adhesion Sci. Technol. 14 (2000) 1485–1498. [35] Y. Li, Z. Ma, A. Li, W. Xu, Y. Wang, H. Jiang, K. Wang, Y. Zhao, X. Jia, A single crystal with multiple functions of optical waveguide, aggregation-induced emission, and mechanochromism, ACS Appl. Mater. Interfaces 9 (2017) 8910– 8918. [36] M. Levitus, K. Schmieder, H. Ricks, K.D. Shimizu, U.H. Bunz, M.A. GarciaGaribay, Steps to demarcate the effects of chromophore aggregation and planarization in poly (phenyleneethynylene) s. 1. Rotationally interrupted conjugation in the excited states of 1, 4-bis (phenylethynyl) benzene, J. Am. Chem. Soc. 123 (2001) 4259–4265. [37] P. Sudhakar, T.P. Radhakrishnan, Stimuli responsive and reversible crystalline– amorphous transformation in a molecular solid: fluorescence switching and enhanced phosphorescence in the amorphous state, J. Mater. Chem. C 7 (2019) 7083–7089.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Synthesis of mechanical responsive carbon dots with fluorescence enhancement Yuru Wang, Yuhan Li, Yang Xu ⇑ National-Local Joint Engineering Laboratory for Energy Conservation in Chemical Process Integration and Resources Utilization and Tianjin Key Laboratory of Chemical Process Safety, School of Chemical Engineering and Technology, Hebei University of Technology, Guangrong Dao No.8, Hongqiao District, Tianjin 300130, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 9 August 2019 Revised 10 October 2019 Accepted 11 October 2019 Available online 13 October 2019 Keywords: Carbon dots Mechanical response Fluorescence enhancement Structure changes
a b s t r a c t Mechanical responsive carbon dots (CDs) is a rising star in the family of carbon materials. However, high pressure devices are essential for observing mechanical responses of carbon dots, but they limit the widespread application of mechanical responsive CDs. Two kinds of CDs were prepared using pyrene derivative (pyrene-1-butyric acid, PyBA) as carbon source, and had fluorescence enhancement via simply grinding. An apparent 20–30 times emission enhancement can be observed after the treatment of grinding because of CDs packed structure and morphology changes. To the best of our knownledge, it is the first example of mechanical responsive CDs with fluorescence enhancement without the stimulation of any complex pressure instrumentation. This simple approach could be extended to other carbon sources with p-conjugated structure for building new intelligentized CDs, and provide new routes for the applications of CDs toward mechanical sensing. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Mechanical responsive fluorescence materials, as a rising star of stimuli-responsive materials, have switchable fluorescence properties in colour or intensity via external mechanical stimuli. Therefore, they have attracted significant attention due to their potential for use as sensors, memory devices, motion systems, and security systems [1–4]. Recently, many different types of ⇑ Corresponding author. E-mail address: [email protected] (Y. Xu). https://doi.org/10.1016/j.jcis.2019.10.039 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
mechanical responsive fluorescent materials have been developed [5,6]. In these reports, the fluorescent properties changed due to the molecular orientation and intermolecular interactions, being disrupted by mechanical forces, including shearing, grinding, tension, and hydrostatic pressure. Li et al. prepared an ultrathin film from polyacrylate modified styrylbiphenyl derivative and layered double hydroxide nanosheet [4]. They found that an enhanced luminescence can be observed in this ultrathin film under pressure stimuli. Mechanical responsive fluorescent materials typically rely on changes in physical molecular packaging patterns or molecular design [7–9]. To date, a number of organic mechanical responsive
86
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
fluorescent materials with promising mechanochromic fluorescence have been developed and the corresponding mechanism of mechanochromic fluorescence has been investigated [10–12]. In general, the intermolecular interaction is important parameters for molecular stacking and fluorescence properties changes [13,14]. These principles are still applicable for mechanochromic phenomenon if the molecular materials are carbonized by high temperature. Carbon dots (CDs) have emerged as an outstanding nanomaterial because of their unique properties, such as excellent fluorescence, low toxicity, high photostability, and good biocompatibility [15–18]. Nowadays, various fluorescent CDs have been prepared from a number of carbon sources, such as citric acid, glucose, graphite, and carbon fiber [19–21]. However, only a few examples of CDs with intelligentized response ability, particularly mechanical responsive CDs have been reported. Lu et al. prepared a piezochromic CDs with a fluorescence color shift from yellow to blue via additional pressure of 0–22.83 GPa [22]. Liu et al. found that the fluorescence of CDs can be controlled by high external pressure device due to the influence of p conjugated system and carbonyl group [23]. Wang et al. prepared a red-emission CDs with fluorescence enhancement property via pressure [24]. Nevertheless, it required additional high pressure devices, which limit the widespread application of mechanical responsive CDs. Therefore, to fabricate mechanical responsive CDs without the stimulation of any complex pressure instrumentation is highly desired. In this work, two CDs were prepared using pyrene-1-butyric acid (PyBA) as carbon precursor. These CDs had a pyrene-based core, which had a merit of mechanical response. An apparent fluorescence enhancement can be observed with treatment of simply grinding. The mechanism of mechanical response was investigated that the transition between crystallization and amorphous structure play vital roles in this process. Notably, these CDs are the first example for mechanical response based on CDs without the stimulation of any complex pressure instrumentation. These CDs not only provide a new route for preparing intelligentized responsive CDs, but also promote the extensive applications of carbon materials. 2. Experimental section 2.1. Reagents and chemicals Pyrene-1-butyric acid (PyBA) with a purity of 97% and qphenylenediamine were purchased from J&K Co. (Beijing, China), and used without further purification. Ethanol in analytical pure degree as reagent was purchased from Tianjin Chemical Co. Ltd. (Tianjin, China). The water used in the experiment was deionized water. 2.2. Instrumentation and characterization The morphology characterization of mechanical responsive CDs was performed using a Tecnai 20 high resolution transmission electron microscope. The accelerating voltage of TEM was 200 KeV. A Bruker D8 Focus X-ray powder diffract instruction with Cu Ka radiation (k = 1.5418 Å) and an operating condition of 40 KV was used for investigation of the XRD patterns of mechanical responsive CDs. X-ray photoelectron spectroscopy (XPS) data including CDs and grinding process, were collected by a Thermo SCIENTIFIC ESCALAB250 Xi. Fourier transform infrared (FT–IR) spectroscopy of CDs and precursors were performed using a Bruker Vector 22 spectrophotometer, and treatment with KBr. The FT–IR was detected from 400–4000 cm 1 with a resolution of 4 cm 1. A fluorescence spectrometer (Edinburgh Instruments, FS920P) was
used to investigate the steady-state fluorescence spectra of two CDs. UV–visible spectra of two CDs were performed by an Agilent Carry 100 UV–vis spectrometer, with a scan range of 200–800 nm. 2.3. Preparation of CDs-1 144 mg of PyBA (0.5 mmol) was dispersed in 10 mL of ethanol, and the mixed solution was transfer to culture dish, and heated in a drying oven at 180 °C for 4 h to obtain powder of as-prepared CDs-1. 2.4. Preparation of CDs-2 144 mg of PyBA (0.5 mmol) and 27 mg of q-phenylenediamine (0.25 mmol) were dispersed in 10 mL of ethanol, and the mixed solution was transfer to culture dish, and heated in a drying oven at 180 °C for 4 h to obtain powder of CDs-2. 3. Results and discussion Two CDs (CDs-1 and CDs-2) powder was obtained by scraping from the culture dish with a medicine spoon. In excitation spectra, both as-prepared CDs and post grinding CDs showed an excitation band from 250 to 400 nm, and had a maximum excitation wavelength at 365 nm (Fig. S1). Spectroscopic measurements were performed to further characterize the fluorescence changes of CDs-1 in the solid state. Fluorescence spectra showed three peaks at 445, 471, and 524 nm for the as-prepared CDs-1 powder (Fig. 1A, and Table S1). The emission at 445 nm arises from the pyrene units, similar to other pyrene derivatives. The peak at 471 nm was notably red shifted compare to the pyrene monomer emission, caused by intramolecular charge transfer species [25,26]. After mechanical grinding in an agate, both CDs had two same peaks at 446 and 473 nm, meanwhile the fluorescence intensity was approximately 30 times enhanced under the UV light excitation (Fig. 1A, and Table S1). The fluorescence properties of CDs-2 were similar with CDs-1, but a slightly red-shift from 471 to 475 nm can be observed after the treatment of grinding (Fig. 1B, and Table S1). The pyrene core induced fluorescence change due to p–p* interaction, while surface passivation will provide a way to change the luminescence properites similar to the previous reports on CDs [27,28]. Further, a slight fluorescence emitting color changes can be observed in both CDs with grinding time increases (Fig. 1C and D). The influence of the morphology of two CDs on their fluorescence emission was investigated by the TEM analysis. The asprepared powder of CDs-1 and CDs-2 exhibited a well-defined morphology. The size distribution of both CDs was in the range of 2–6 nm, and average particle size was 3 nm (Fig. 2A and C). Both CDs had similar microcrystalline structure, and the lattice spacing was 0.24 nm, which was related to the (1 0 0) facet of graphite [29]. The morphology of CDs with the treatment of grinding was also investigated. The microcrystalline state became more amorphous after mechanical grinding (Fig. 2B and D). To further investigate the CDs formation, a series of the structure and properties analysis of the samples after grinding was performed. As shown as Fig. 3A and B, FT-IR spectroscopy of CDs and carbon source are performed. Four apparent peaks at 3446, 3020, 1680, and 1620 cm 1 can be founded in CDs, which were corresponded to the OAH, CAH, C@O, and C@C, respectively. It demonstrated the functional groups of two CDs were retained after grinding (Fig. 3A and B). For CDs-2, a peak at 1388 cm 1 in Fig. 3B correspond to the stretching vibrations of CAN in the amide bond formed during the hydrothermal process. A peak at 3280 cm 1 corresponded to amide NAH stretching were masked by the broad peaks ascribed to OAH (3446 cm 1).
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
87
Fig. 1. Fluorescence spectra of (A) CDs-1 and (B) CDs-2 as prepared and after grinding for 30 min. Photographs of (C) CDs-1 and (D) CDs-2 as prepared and after grinding for 10 min, 30 min and 60 min under UV irradiation (kex = 365 nm). All emission spectra and luminescence photos were obtained at room temperature.
Fig. 2. TEM images and size distributions of as-prepared CDs-1 (A and a), grinding CDs-1 (B and b), as-prepared CDs-2 (C and c), and grinding CDs-2 (D and d).
Some previous studies relationship between p-conjugated and their optical properties based on theoretical calculations and related experimental results [30,31]. Fluorescence changes maybe related to different supramolecular packing modes in the molecular level and the morphology of CDs [7,32]. However, it remains insufficient how the external force causes the fluorescence enhancement for the CDs. It is well known that fluorescence changes are attributed to the materials structure in crystal or amorphous states. To investigate the structure changes of CDs during grinding process, XRD patterns of PyBA, CDs-1, and CDs-2 were measured before and after grinding, respectively (Fig. 3C and D). Various sharp peaks were observed in the sample without grinding, indicating that the as-prepared CDs was in the state of highly ordered structures before grinding (Fig. 3C and D). However,
significant differences in the XRD patterns appeared after grinding (Fig. 3C and D). Initially, no apparent change was observed in the XRD pattern after grinding 10 min. Further, the peak in XRD pattern after grinding for 30 min became broaden, indicating that the structures of CDs was changed and gradually became amorphous. XPS is an effective technique to characterize the surface and component of two CDs. Three peaks can be observed in the XPS spectra at 285, 400 and 533 eV, which was ascribed to C1s, N1s, and O1s, respectively (Fig. S2) [33]. The C1s narrow scan in Fig. 4 was fitted with two components centered at 283.3 and 287.3 eV, assigned to C@C/CAC and C@O/COOH, respectively [34]. Meanwhile, two peaks centered at 532.0 and 532.7 eV in the O1s narrow scan were assigned to C@O and
88
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
Fig. 3. FT-IR spectra of (A) CDs-1 and (B) CDs-2 as prepared (black curve) and after grinding for 30 min (red curve). Powder XRD patterns of PyBA, (C) CDs-1, and (D) CDs-2 asprepared and after mechanical grinding for 10, 30, and 60 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. XPS analyses of the C 1s and O 1s spectra of (A) CDs-1 and (B) CDs-2 as prepared and after grinding for 30 min.
CAO, respectively. In CDs-2, the appearances of CAN (286.3 eV) was observed, indicating the formation of the hybrid between PyBA and q-phenylenediamine. Notably, the spectra of C1s reveal an increasing signal ratio for C@C/CAC to the C@O/COOH signal in samples before and after grinding (from 0.56 to 0.92 for CDs-1 and from 0.94 to 1.09 for CDs-2 in Table S2). It indicated that the composition was changed during the grinding process. Moreover, the change of signal ratio for C@C/CAC to the C@O/ COOH in CDs-1 sample were larger than CDs-2. It inferred that the passivation agent may play an important role in the change of signal ratio. Through the above characterizations, the structure of CDs was changed during grinding treatment from crystallization to amorphous state, along with composition changes. Liu et al. found that p-conjugated system and functional groups can
have a vital impact on the fluorescence properties of CDs [23]. Thus, the relationship between structure and fluorescence property is essential for fluorescence investigation. Previous studies have shown that adjacent pyrene groups move closer vertically and undergo a slight slippage under external stress stimuli [35]. The as-prepared powder samples were well-dispersed with weak interactions between neighboring nanoparticles. It implied that the structure configure can be induced by coplanarization structure [36]. The core structure of pyrene will endow stimuli response to as-prepared CDs. The CDs nanoparticles after grinding is the primary driver of the fluorescence change. The reconfiguration of pyrene core in CDs was originated from stack structure change, which induced fluorescent intensity enhancement. (Scheme 1).
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
89
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Scheme 1. Schematic illustration of the structure changes of CDs by grinding.
Table 1 The fluorescence lifetimes of CDs-1 and CDs-2 before and after grinding for 30 min (Excitation at 365 nm, data collected at 446 nm for CDs-1 and 447 nm for CDs-2). As-Prepared
CDs-1 CDs-2
Acknowledgements Financial support by the National Natural Science Foundation of China, china (No. 21605036), and the Natural Science Foundation of Hebei Province, China (No. B2017202068) is acknowledged.
After Grinding (30 min)
s(ns)
Percentage (%)
s(ns)
Percentage (%)
3.5 21.2 4.7 21.8
79.2 20.8 70.3 29.7
4.4 25.3 4.1 23.0
28.4 71.6 25.3 74.7
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.039. References
The excimers of pyrene had two types: partial overlap and sandwich-like packing, which exhibited short and long lifetime, respectively. Hence, time-resolved fluorescence spectra of CDs were used to confirm the pyrene excimers types and intermolecular packing modes based on their fluorescence lifetimes (Fig. S3). The as-prepared CDs-1 sample showed two types of fluorescence lifetimes with 3.5 ns (79.2%) and 21.2 ns (20.8%). The lifetime and the proportion changed upon grinding for 30 min (4.4 ns (28.4%) and 25.3 ns (71.6%), Table 1). The percentage of short lifetimes was decreased from 79.2% to 28.4% with the treatment of grinding. The short lifetimes reflected the excimer emission of the partial overlap stacking, which was usually founded in highly ordered structures. It demonstrated that the crystalline structure of CDs was partially destroyed during the grinding process. Simultaneously, the proportion of the long lifetimes increased from 20.8 to 71.6%, because the CDs were in the state of amorphous and sandwich-type excimer played a dominant role in fluorescence emission. Similar results were obtained for CDs-2 and the specific data is shown in Table 1. Thus, it can be concluded that structure stacking among the CDs nanoparticles are changed after sufficient grinding, achieving a response to external mechanical stimuli. With increasing grinding time, the morphology of the powder changes from crystalline to amorphous, leading to fluorescence intensity enhancement [37]. Further, other CDs were prepared in similar experiment method by using bovine serum albumin (BSA) as precursor, denoted as CDs-BSA. No apparent fluorescence enhancement can be observed for CDs-BSA (Fig. S4). It demonstrated that the stacking structure plays a crucial role in fluorescence enhancement, and will be an ideal precursor for preparing mechanical responsive materials. 4. Conclusion In summary, two CDs with mechanical-stimuli fluorescence enhancement were prepared. The mechanism of both CDs is the same that the transition between crystallization and amorphous structure induce mechanical responses. Compared with traditional fluorescent CDs, these CDs have apparent mechanical responses, and complex pressure devices are not required. Most importantly, this strategy provides an innovative route to prepare mechanical responsive CDs and carbon-based nanomaterials. These mechanical responsive CDs will have potential for mechanical applications.
[1] D. Yan, J. Lu, J. Ma, S. Qin, M. Wei, D.G. Evans, X. Duan, Layered host-guest materials with reversible piezochromic luminescence, Angew. Chem. 123 (2011) 7175–7178. [2] J. Zhao, S. Ji, Y. Chen, H. Guo, P. Yang, Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials, Phys. Chem. Chem. Phys. 14 (2012) 8803–8817. [3] Y. Sagara, S. Yamane, M. Mitani, C. Weder, T. Kato, Mechanoresponsive luminescent molecular assemblies: an emerging class of materials, Adv. Mater. 28 (2016) 1073–1095. [4] M. Li, R. Tian, D. Yan, R. Liang, M. Wei, D.G. Evans, X. Duan, A luminescent ultrathin film with reversible sensing toward pressure, Chem. Commun. (Camb) 52 (2016) 4663–4666. [5] F.M. Winnik, Photophysics of preassociated pyrenes in aqueous polymer solutions and in other organized media, Chem. Rev. 93 (1993) 587–614. [6] X. Sun, X. Zhang, X. Li, S. Liu, G. Zhang, A mechanistic investigation of mechanochromic luminescent organoboron materials, J. Mater. Chem. 22 (2012) 17332–17339. [7] S. Varghese, S. Das, Role of molecular packing in determining solid-state optical properties of p-conjugated materials, J. Phys. Chem. Lett. 2 (2011) 863– 873. [8] M. Sase, S. Yamaguchi, Y. Sagara, I. Yoshikawa, T. Mutai, K. Araki, Piezochromic luminescence of amide and ester derivatives of tetraphenylpyrene—role of amide hydrogen bonds in sensitive piezochromic response, J. Mater. Chem. 21 (2011) 8347–8354. [9] K. Nagura, S. Saito, H. Yusa, H. Yamawaki, H. Fujihisa, H. Sato, Y. Shimoikeda, S. Yamaguchi, Distinct responses to mechanical grinding and hydrostatic pressure in luminescent chromism of tetrathiazolylthiophene, J. Am. Chem. Soc. 135 (2013) 10322–10325. [10] Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. Lv, S. Wen, B. Li, L. Ye, B. Zou, W. Tian, Piezochromic luminescence based on the molecular aggregation of 9, 10-bis ((E)-2-(pyrid-2-yl) vinyl) anthracene, Angew. Chem. Int. Ed. 51 (2012) 10782–10785. [11] Y. Sagara, T. Komatsu, T. Ueno, K. Hanaoka, T. Kato, T. Nagano, A water-soluble mechanochromic luminescent pyrene derivative exhibiting recovery of the initial photoluminescence color in a high-humidity environment, Adv. Funct. Mater. 23 (2013) 5277–5284. [12] M.J. Fuller, L.E. Sinks, B. Rybtchinski, J.M. Giaimo, X. Li, M.R. Wasielewski, Ultrafast photoinduced charge separation resulting from self-assembly of a green perylene-based dye into p-stacked arrays, J. Phys. Chem. A 109 (2005) 970–975. [13] X. Luo, J. Li, C. Li, L. Heng, Y.Q. Dong, Z. Liu, Z. Bo, B.Z. Tang, Reversible switching of the emission of diphenyldibenzofulvenes by thermal and mechanical stimuli, Adv. Mater. 23 (2011) 3261–3265. [14] X. Zhang, J.-Y. Wang, J. Ni, L.-Y. Zhang, Z.-N. Chen, Vapochromic and mechanochromic phosphorescence materials based on a platinum (II) complex with 4-trifluoromethylphenylacetylide, Inorg. Chem. 51 (2012) 5569–5579. [15] P.G. Luo, S. Sahu, S.-T. Yang, S.K. Sonkar, J. Wang, H. Wang, G.E. LeCroy, L. Cao, Y.-P. Sun, Carbon ‘‘quantum” dots for optical bioimaging, J. Mater. Chem. B 1 (2013) 2116–2127. [16] B.C. Martindale, G.A. Hutton, C.A. Caputo, E. Reisner, Solar hydrogen production using carbon quantum dots and a molecular nickel catalyst, J. Am. Chem. Soc. 137 (2015) 6018–6025. [17] K. Hola, Y. Zhang, Y. Wang, E.P. Giannelis, R. Zboril, A.L. Rogach, Carbon dots— emerging light emitters for bioimaging, cancer therapy and optoelectronics, Nano Today 9 (2014) 590–603. [18] X.-T. Tian, X.-B. Yin, Carbon dots unconventional preparation strategies, and applications beyond photoluminescence, Small (2019) 1901803.
90
Y. Wang et al. / Journal of Colloid and Interface Science 560 (2020) 85–90
[19] S. Qu, X. Wang, Q. Lu, X. Liu, L. Wang, A biocompatible fluorescent ink based on water-soluble luminescent carbon nanodots, Angew. Chem. Int. Ed. 51 (2012) 12215–12218. [20] H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C.H.A. Tsang, X. Yang, S.T. Lee, Water-soluble fluorescent carbon quantum dots and photocatalyst design, Angew. Chem. Int. Ed. 49 (2010) 4430–4434. [21] L. Wang, S.-J. Zhu, H.-Y. Wang, S.-N. Qu, Y.-L. Zhang, J.-H. Zhang, Q.-D. Chen, H.-L. Xu, W. Han, B. Yang, H.-B. Sun, Common origin of green luminescence in carbon nanodots and graphene quantum dots, ACS Nano 8 (2014) 2541–2547. [22] S. Lu, G. Xiao, L. Sui, T. Feng, X. Yong, S. Zhu, B. Li, Z. Liu, B. Zou, M. Jin, J.S. Tse, H. Yan, B. Yang, Piezochromic carbon dots with two-photon fluorescence, Angew. Chem. Int. Ed. 56 (2017) 6187–6191. [23] C. Liu, G. Xiao, M. Yang, B. Zou, Z.L. Zhang, D.W. Pang, Mechanofluorochromic carbon nanodots: controllable pressure-triggered blue-and red-shifted photoluminescence, Angew. Chem. Int. Ed. 57 (2018) 1893–1897. [24] Q. Wang, S. Zhang, B. Wang, X. Yang, B. Zou, B. Yang, S. Lu, Pressure-triggered aggregation-induced emission enhancement in red emissive amorphous carbon dots, Nanoscale Horiz. 4 (2019) 1227–1231. [25] Y. Suzuki, T. Morozumi, H. Nakamura, M. Shimomura, T. Hayashita, R.A. Bartsh, New fluorimetric alkali and alkaline earth metal cation sensors based on noncyclic crown ethers by means of intramolecular excimer formation of pyrene, J. Phys. Chem. B 102 (1998) 7910–7917. [26] D. Jiao, J. Geng, X.J. Loh, D. Das, T.C. Lee, O.A. Scherman, Supramolecular peptide amphiphile vesicles through host–guest complexation, Angew. Chem. Int. Ed. 51 (2012) 9633–9637. [27] S. Zhu, J. Zhang, S. Tang, C. Qiao, L. Wang, H. Wang, X. Liu, B. Li, Y. Li, W. Yu, X. Wang, H. Sun, B. Yang, Surface chemistry routes to modulate the photoluminescence of graphene quantum dots: from fluorescence mechanism to up-conversion bioimaging applications, Adv. Funct. Mater. 22 (2012) 4732–4740. [28] H. Tetsuka, R. Asahi, A. Nagoya, K. Okamoto, I. Tajima, R. Ohta, A. Okamoto, Optically tunable amino-functionalized graphene quantum dots, Adv. Mater. 24 (2012) 5333–5338.
[29] J. Lu, J.-X. Yang, J. Wang, A. Lim, S. Wang, K.P. Loh, One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids, ACS Nano 3 (2009) 2367–2375. [30] W. Pisula, M. Kastler, D. Wasserfallen, M. Mondeshki, J. Piris, I. Schnell, K. Müllen, Relation between supramolecular order and charge carrier mobility of branched alkyl Hexa-p eri-hexabenzocoronenes, Chem. Mater. 18 (2006) 3634–3640. [31] K. Li, W. Qin, D. Ding, N. Tomczak, J. Geng, R. Liu, J. Liu, X. Zhang, H. Liu, B. Liu, B.-Z. Tang, Photostable fluorescent organic dots with aggregation-induced emission (AIE dots) for noninvasive long-term cell tracing, Scient. Rep. 3 (2013) 1150. [32] G. Zhang, J. Lu, M. Sabat, C.L. Fraser, Polymorphism and reversible mechanochromic luminescence for solid-state difluoroboron avobenzone, J. Am. Chem. Soc. 132 (2010) 2160–2162. [33] A. Adenier, M.-C. Bernard, M.M. Chehimi, E. Cabet-Deliry, B. Desbat, O. Fagebaume, J. Pinson, F. Podvorica, Covalent modification of iron surfaces by electrochemical reduction of aryldiazonium salts, J. Am. Chem. Soc. 123 (2001) 4541–4549. [34] M. Shi, G. Dunham, M. Gross, G. Graff, P. Martin, Plasma treatment of PET and acrylic coating surfaces-I. In-situ XPS measurements, J. Adhesion Sci. Technol. 14 (2000) 1485–1498. [35] Y. Li, Z. Ma, A. Li, W. Xu, Y. Wang, H. Jiang, K. Wang, Y. Zhao, X. Jia, A single crystal with multiple functions of optical waveguide, aggregation-induced emission, and mechanochromism, ACS Appl. Mater. Interfaces 9 (2017) 8910– 8918. [36] M. Levitus, K. Schmieder, H. Ricks, K.D. Shimizu, U.H. Bunz, M.A. GarciaGaribay, Steps to demarcate the effects of chromophore aggregation and planarization in poly (phenyleneethynylene) s. 1. Rotationally interrupted conjugation in the excited states of 1, 4-bis (phenylethynyl) benzene, J. Am. Chem. Soc. 123 (2001) 4259–4265. [37] P. Sudhakar, T.P. Radhakrishnan, Stimuli responsive and reversible crystalline– amorphous transformation in a molecular solid: fluorescence switching and enhanced phosphorescence in the amorphous state, J. Mater. Chem. C 7 (2019) 7083–7089.