Amphipathic carbon dots with solvent-dependent optical properties and sensing application
Graphical abstract
Introduction
Carbon dots (CDs), a novel type of heavy-metal-free fluorescent materials, have attracted extensive attention during the past few years owing to their appealing features, such as rich raw materials, easy and budget synthesis procedures, low toxicity and excellent biocompatibility [[1], [2], [3], [4]]. These characteristics endow CDs with the great potential applications in sensing, bioimaging, photocatalysis, LEDs and solar cells [[5], [6], [7], [8], [9]]. To date, a great many of studies have focused on the synthesis and optical properties of CDs. A diverse range of synthesis routes have been developed to synthesize luminescent CDs, such as oxygen plasma treatment [10], laser ablation [11], microwave irradiation [12], electrochemical oxidation [13], hydrothermal carbonization [14], solvothermal treatment [15] and organic synthetic route [16], which can be classified into “top-down” and “bottom-up” types [17]. Among them, hydrothermal and solvothermal carbonization approaches are most commonly used because of their ease, high effectiveness, and inexpensiveness. Interestingly, though the sizes, shapes and surfaces of CDs obtained in different approaches vary considerably, unlike traditional semiconductor quantum dots, the vast majority of them show similar photoluminescence (PL) properties dependent on excitation wavelength, which means the emission can be simply tuned by changing the excitation wavelength. By contrast, excitation-independent PL less appears in CDs. So far, the PL mechanism still remains an open question.
Besides, the external parameters such as dopants and solvents can also affect the emission wavelength of CDs. Heteroatom like nitrogen and sulphur doping can not only manipulate the band gap of CDs but also impact the sensitivity of CDs towards the external environment [18,19]. Moreover, when CDs are dissolved in different solvents, complex interactions between the surface groups and solvent molecules, namely solvent effect, exert a vital influence on the optical properties of the CDs [[20], [21], [22]]. The comprehension of the solvent effect is essential for understanding the fluorescence mechanisms of CDs. In fact, only a few studies briefly mentioned solvent-dependent emission behavior in carbon-based nanomaterials. For instance, Zhu et al. reported one type of graphene quantum dots with solvent-dependent emission which could be induced by different emissive traps on the surface [20]. Wang and coworkers adopted a hot-injection method in diphenyl ether to prepare solvent-dependent CDs with various emission depending on the solvent polarity [21]. A research on hybrid carbon nanosheet also described tunable emission in a series of solvents with different polarities [22]. While, Zhou's group reported that their CDs prepared from natural polysaccharide displayed solvent-independent emission behavior in diverse organic solvents [23]. These results reveal that the solvent effect might be different for various functionalized CDs. Despite significant progress in the development of carbon-based materials exhibiting solvent-dependent PL, more research is needed to understand the effect of solute-solvent interaction on the optical properties of the CDs.
It is well known that copper is a critical cofactor in a variety of biological processes and plays a crucial role in clinic and environment [24,25]. Therefore, the development of Cu2+ concentration monitor approaches is essential. Currently, there is great interest in employing CDs as platform for the development of Cu2+ sensor [[26], [27], [28]]. Nevertheless, how to improve the fluorescent Cu2+ detection performance with high sensitivity and selectivity remains to be further investigated.
Herein, we report one type of amphipathic CDs prepared by facile one-step solvothermal method with the treatment of p-Phenylenediamine. The obtained CDs show strong solvatochromic behavior with tunable emission from blue to green in various solvents, and manifest strict excitation-independent emission feature. Absorption, steady-state and time-resolved spectroscopy have been adopted to investigate the mutual influence between the surface groups and different solvent molecules on the optical properties of the CDs. In addition, the ability of the as-synthesized CDs as sensing probes for selective Cu2+ detection has also been studied.
Section snippets
Materials
p-Phenylenediamine, urea, quinine sulfate, poly (ethylene glycol) (PEG, MW = 400) and all the organic solvents of toluene, acetone, dimethyl sulfoxide (DMSO) and ethanol were of analytical grade and purchased from different commercial companies. The ultrapure water used throughout this work had a resistivity higher than 18 MΩ/cm3. All chemicals were utilized directly as received without further purification.
Synthesis of CDs
p-Phenylenediamine (1 g) and urea (1 g) were added into 10 mL DMSO in a glass beaker
Results and discussion
The size and morphology of the as-synthesized CDs dissolved in water were investigated by TEM, as shown in Fig. 1a. It is apparent that the CDs have a uniform dispersion with the average size of 4.22 nm (Fig. 1b). Furthermore, the HRTEM image (Fig. 1a, inset) exhibits the well-resolved lattice fringes with an average interplanar lattice spacing of 0.21 nm, which could be attributed to the (100) diffraction facet of graphite [15,21]. The XRD measurement was adopted to further characterize the
Conclusions
In conclusion, we have reported one type of amphipathic CDs synthesized via a facile one-step solvothermal route. The obtained CDs exhibit strong solvatochromic behavior with tunable emission from blue to green and strict excitation-independent emission feature when dissolved in different solvents. In addition, the relative fluorescence response of the CDs as a function of Cu2+ concentration have presented a good linear relationship within the concentration range from 0 to 60 μM, suggesting the
Declaration of interests
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.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC, No. 61675049, NSFC, No. 61377046, and NSFC, No. 61177021) and Fudan University-CIOMP Joint Fund (FC2017-004).
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Authors contributed equally.