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Amphipathic carbon dots with solvent-dependent optical properties and sensing application
Optical Materials 89 (2019) 224–230
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Amphipathic carbon dots with solvent-dependent optical properties and sensing application
T
Shiliang Meia,1, Xian Weia,1, Zhe Hua, Chang Weia, Danlu Sua, Dan Yanga, Guilin Zhanga, Wanlu Zhanga,∗∗, Ruiqian Guoa,b,∗ a b
Engineering Research Center of Advanced Lighting Technology, Ministry of Education, Institute for Electric Light Sources, Fudan University, Shanghai, 200433, China Institute of Future Lighting, Academy for Engineering and Technology, Fudan University, Shanghai, 200433, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon dots Optical properties Solvent-dependent Cu2+ detection
Carbon dots (CDs) have been regarded as novel heavy-metal-free fluorescent materials because of their prominent optical features. In this work, one type of amphipathic CDs is prepared by facile one-step solvothermal treatment of p-Phenylenediamine. The obtained CDs own numerous surface function groups which endow them prominent dispersibility in different solvents. 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. Strong solvatochromic behavior with tunable emission from blue to green and strict excitation-independent emission characteristic can be observed when the as-prepared CDs are dissolved in different solvents, suggesting their feasible sensing applications as an alternative of solvatochromic dye molecules. Furthermore, highly selective detection of Cu2+ ions using the as-synthesized CDs as sensing probes is achieved. It is expected that the investigated CDs with solvent-dependent optical properties as well as selective Cu2+ detection may have broad application prospects in bioimaging and biodetecting.
1. 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 [1–4]. These characteristics endow CDs with the great potential applications in sensing, bioimaging, photocatalysis, LEDs and solar cells [5–9]. 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 [20–22]. 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
∗ Corresponding author. Engineering Research Center of Advanced Lighting Technology, Ministry of Education, Institute for Electric Light Sources, Fudan University, Shanghai, 200433, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (W. Zhang), [email protected] (R. Guo). 1 Authors contributed equally.
https://doi.org/10.1016/j.optmat.2019.01.021 Received 22 December 2018; Received in revised form 17 January 2019; Accepted 20 January 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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Teflon autoclave and heated at 180 °C in an electric oven for 6 h. The dark brownish solution was obtained and precipitated with the excess mixed solvent solution of petroleum ether and ethyl acetate with the volume ratio of 1/4, and then centrifuged at 8000 rpm for 10 min to separate the solvent. The final products were dried and collected in vacuum and stored in the dark for future using.
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 [26–28]. 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 pPhenylenediamine. 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.
2.3. Characterization The obtained CDs were characterized by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL), X-ray diffractometer (XRD, D8 Advance, Bruker), X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo) and Fourier transform-infrared spectroscopy (FT-IR, Nicolet 6700, ThermoFisher). Binding energy calibration was based on C 1s at 284.6 eV. The PL spectra were recorded using a fluorescence spectrophotometer (F97XP, Shanghai Lengguang). The absorption spectra were collected with an ultraviolet–visible (UV–vis) spectrophotometer (759S, Shanghai Lengguang). The fluorescence lifetime of the as-prepared CDs in different solvents were measured on a time-resolved spectrofluorometer (FLS 920, Edinburgh Instruments). The quantum yield (QY) of the CDs was measured using quinine sulfate dissolved in a 0.1 M H2SO4 aqueous solution as the standard, comparing the absorbance (less than 0.1 at 360 nm) and the integrated PL intensity (excited at 360 nm) of the CDs with those of the standard. The QY was calculated according to:
Φ = Φst (I / Ist )(Ast / A)(η2 / ηst2 ) Where φ is the QY, Ι is the measured integrated PL intensity, A is the absorbance and η is the refractive index of the solvent. The subscript “st” refers to the standard.
2. Experimental 2.4. Procedure for ion sensing 2.1. 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.
The fluorescence detection of Cu2+ ions was implemented by using the as-prepared CDs dissolved in phosphate buffer solution at room temperature. Concretely, a given concentration of Cu2+ ions was added to 10 mL aqueous solution of CDs, which were shaken well and kept for 5 min. The PL spectrum was then collected at the excitation wavelength of 380 nm. Some kinds of other metal ion aqueous solution (Ag+, Cd2+, Cs2+, Fe2+, Fe3+, In3+, K+, Mg2+, Mn2+, Na+, Pb2+, Zn2+) were selected to detect the selectivity of the CDs.
2.2. Synthesis of CDs
3. Results and discussion
p-Phenylenediamine (1 g) and urea (1 g) were added into 10 mL DMSO in a glass beaker containing 5 mL of PEG. After being stirred for about 5 min, the mixed solution was clear and then sealed in a 25 mL
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. 1. TEM (a), size distribution (b), and XRD pattern (c) images of resultant fluorescent CDs. The inset in (a) shows HRTEM image of the CDs with the lattice parameter of 0.21 nm. 225
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ethanol and water, respectively. Normalized PL spectra excited by 380 nm shown in Fig. 4f further depict that the emission peaks show a red shift with the solvent polarity increasing, thus leading to an emission color change from blue to green (Fig. 4g). Interestingly, the full width at half maximum (FWHM) is also found to be solvent-dependent (Table S1), indicating that solvent fluctuations have a non-negligible effect on expanding the emission. The changes in PL spectra were also investigated when different amount of DMSO is mixed with water. It can be seen that the maximum emission peak presents a red shift with the volume ratio of DMSO to water decreases (Fig. S1). To a certain extent, this solvent-dependent phenomenon is very analogous to the “solvatochromism” originated from fluorescent organic dyes [34]. Furthermore, the solvent-dependent emission behavior can also be efficiently described by using the Reichardt's polarity parameter ETN , a common polarity parameter. The value of ETN stands for the overall solvation capability of the solvents to form nonspecific (e.g., dipoledipole) and specific (hydrogen bond; HB) solute-solvent interaction [35]. The emission peak position of the CDs in different solvents presents a linear correlation (R2 = 0.975) to ETN (Fig. 5a), confirming the trend in Fig. 4f. This result approves the potential employment of the as-synthesized CDs as the fluorescent probes to discriminate the local environments with different polarities. In addition, the emission bands (Fig. 4a–e) of the CDs in any solvent remain unchanged under different excitation wavelengths, manifesting the strict excitation-independent emission feature of the CDs. QYs of the CDs in the five solvents were calculated via a relative method using quinine sulfate as the standard, and the results are shown in Table S1 for more details. Obviously, the QYs in other solvents are higher than that in water and become lower with the longer emission peak. Fig. 4h shows UV–vis absorption spectra of the CDs in the five solvents. Obviously, each absorption spectrum shows two characteristic absorption bands and the positions of the absorption bands present solvent-dependent, manifesting that the energy gaps of CDs differ with solvents. According to previous reports, the higher energy absorption band is commonly assigned to intrinsic state (π-π*) transitions within the sp2-hybridized carbon core of the CDs [36–38]. The π-π* core band derives from π-conjugation of aromatic carbon and thus has much relevance with the (crystalline) carbon core. While, the lower energy absorption band is assigned to defect states (n-π*) transitions resulting from the edge states between the sp2 and sp3 hybridized carbons [36–38]. The peak of high-energy core band shifts from 286 to 242 nm, depicting a blue shift with the solvent polarity increasing. And the peak of low-energy edge band also shows gradual hypsochromic shift from 328 to 294 nm expect in toluene. The inconsistency in toluene could be attributed to the relatively poor solubility which leads to the aggregation of the CDs. Generally, the core state leading to the π-π* transition is buried inside the CDs structure and not exposed to solvent, implying that the spectral position shows independence on solvents [38]. However, it is inconsistent with our result. Some groups have also noticed similar spectral shift for this band [36,38]. Such result reveals that the carbon core could be attached with massive surface/edge states to sample the variations in external environment. Moreover, spectral shift for the edge band is also observed, revealing that the interaction between the solvent molecules and CDs plays a major role in affecting nπ* transitions which are usually referred to the major fluorescent centers for the bright luminescence in the CDs [36], and thus results in the observed emission spectra shifts. Time resolved fluorescence decay curves of the CDs dispersed in five solvents are shown in Fig. 4i excited by 380 nm. For comparison with the previous reports of multiexponential kinetics [37,39], the fitted decay curves of the five samples are found to be monoexponential, suggesting a single, well-defined electronic transition accountable for the observed emission and implying that the excitation independence of the emission behavior is relevant with its uniform surface states. The lifetime results have been tabulated in Table S1. It can be observed that the fluorescence lifetimes depict the solvent dependence which agrees
Fig. 2. FT-IR spectrum of the as-synthesized CDs.
(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 CDs. As depicted in Fig. 1c, the diffraction peak located at about 23° for its (002) plane, suggesting that the as-prepared CDs consist of graphene-like structures [29,30]. These results are similar to those of many other reported CDs. The surface-related functional groups and elementary compositions of the CDs were studied by FT-IR and XPS. In the FT-IR spectrum shown in Fig. 2, the existence of absorption bands can be obviously observed, which resulted from several functional groups like OeH, NeH, C]O and C]C. Remarkably, the broad band at 3640-3109 cm−1 is assigned to the OeH and NeH vibrations [23,29]. The peak emerging at ∼2890 cm−1 derived from the CeH stretching vibration [28]. The stretching mode of vibration resulting from C]O and aromatic C]C bonds lead to the relatively weak absorption band in 1720–1570 cm−1 [31]. The bands at 1460 cm−1 and 1245 cm−1 could be attributed to C]N and aromatic ether, respectively [32]. The stretching vibration of CeOeC bond arises at the sharp absorption band from 990 to 1170 cm−1 [27]. These observations reveal that the CDs surface is rich in hydrophilic groups, endowing them with prominent dispersibility in different solvents like water, ethanol and DMSO, and capability of forming intermolecular hydrogen bonds. XPS was further employed to analyze the chemical bond composition of the CDs. Three peaks at 284.6, 399.7 and 532 eV can be observed in the XPS spectrum (Fig. 3a), indicating the presence of C, N and O elements in the CDs. The highresolution XPS spectrum of C 1s (Fig. 3b) is deconvoluted into four peaks, contributed to the sp2 CeC/C]C bond at 284.6 eV, sp3 CeN bond at 285.4, CeO bond at 286.0 eV, and C]N bond at 287.8 eV [21,31]. In Fig. 3c, the high-resolution N 1s spectrum exhibits three peaks at 398.5, 399.3, 400.7 eV, attributed to pyridinic N, amino N and pyrrolic N [28,33]. Besides, the high-resolution O 1s spectrum in Fig. 3d could be divided into two peaks at 531.8 and 532.9 eV, corresponding to C]O/OeH and CeO/eNO2, respectively [30]. The steady-state and absorption spectroscopy were adopted to study the optical properties of the as-synthesized CDs via dissolving the CDs into different solvents (toluene, acetone, DMSO, ethanol and water). PL spectra of the CDs dispersed in five solvents under various excitation wavelength are presented in Fig. 4a–e. The emission peak positions are observed at 454, 461, 476, 487 and 505 nm in toluene, acetone, DMSO,
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Fig. 3. XPS survey (a) and high-resolution C 1s (b), N 1s (c) and O 1s (d) peaks of the as-synthesized CDs.
dependent properties of the CDs in the protic solvents [40,42]. And, stronger HB may quench the fluorescence of the CDs more actively since stronger depletion of upper excited levels of surface defect states will be induced by uniform robust hydrogen-bonded network of water than unordered weaker hydrogen-bonded network of ethanol [43], which could explain why the QY of the CDs dispersed in water is the lowest. Recently, CDs have served to detect the neighbouring molecules and ions as unique fluorescent probes, which propelled us to inquire into the capacity of the as-prepared CDs as probes for metal ion detection. Herein the fluorescence properties of the CDs were measured after adding various metal ions (0.5 mM). To confirm the selectivity of the CDs to various metal ions, thirteen kinds of metal cations were applied in the experiment, including Ag+, Cd2+, Cs2+, Cu2+, Fe2+, Fe3+, In3+, K+, Mg2+, Mn2+, Na+, Pb2+ and Zn2+ (Fig. S2). Upon the addition of each kind of cation with a concentration of 0.5 mM to the CDs in phosphate buffer solution, almost no obvious fluorescence changes are observed for most of these metal ions in the same measuring condition. However, the fluorescence of the CDs is markedly quenched after adding Cu2+ and Ag+. Apparently, the quenching effect caused by Cu2+ is stronger than that caused by Ag+, indicating that the synthesized luminescent CDs are selective for the detection of Cu2+ ions. Furthermore, the CDs solution was treated with various concentrations of Cu2+ ions ranging from 0 to 100 μM to investigate the quenching degree (Fig. 6a). With the Cu2+ concentration increasing, a continual quenching of fluorescence intensity can be observed. However, the emission peak position does not change even in the presence of the highest Cu2+ concentration. Fig. 6b further shows the relationship between the relative fluorescence response of CDs (F0/F) and Cu2+ concentration ([Cu2+]). It can be observed that the quenching of
well with the emission and absorption spectra results, nevertheless, there is no monotonic change in fluorescence lifetimes with the variation of solvent polarity. Based on the values of the QY and lifetime (Table S1), the radiative (KR) and nonradiative (KNR) rates can be estimated using following expressions: KR = QY/τ and QY = KR/ (KR + KNR) [40]. The nonradiative rates in four solvents (toluene, acetone, DMSO and ethanol) are found to be similar (Fig. 5b), while it almost doubles in water, explaining why there is a drastic reduction of the lifetime and the QY of the CDs when dispersed in water. As discussed above, the interaction between the solvent molecules and the surface groups on the as-synthesized CDs plays a significant role in affecting the optical properties including PL emission, QY, absorption and fluorescence lifetime. In fact, both dipole-dipole and HB interaction can result in the observed solvent-dependent emission behavior (Fig. 5a). Based on the previous reports, the origin of the solvent-dependent properties of the CDs in protic solvents is mainly attributed to HB interaction between the protic solvent and the CDs [37,41]. In aprotic solvents, the solvent-dependent emission behavior is considered to be a change in surface electronic state caused by dipole-dipole interaction [21,42]. On the basis of the FT-IR and XPS results shown in Figs. 2 and 3, the as-synthesized CDs consist of abundant nitrogen and oxygen forming amine and carboxylic surface functional groups which could enhance the charge carrier density and give rise to new charge transfer of electrons to the edge. In aprotic solvents, the dipole moment has more influence on the surface electronic structure with the solvent polarity increasing, and then reduces the energy gap, which leads to the emission peak position shifting towards longer wavelength [21]. While, the amount of red shift in emission becomes highest when the CDs are dissolved in water, revealing that the donation of HB from the protic solvents to the CDs acts as a predominant contributor for the solvent-
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Fig. 4. PL spectra of the same CDs dispersed in the solvents of toluene (a), acetone (b), DMSO (c), ethanol (d) and water (e) excited by different excitation wavelength. Normalized PL spectra (f) (λex = 380 nm), corresponding photographs (g) under UV irradiation (λ = 365 nm), UV–vis absorption spectra (h) and fluorescence decay curves (i) of the CDs dispersed in five solvents.
fluorescence intensity caused by Cu2+ at low concentrations conforms to a typical Stern-Volmer equation [27,42]: F0/F = 1+ Ksv [Cu2+], where F0 represents the PL intensity of the CDs, F represents the PL intensity when adding Cu2+ and Ksv is the static Stern-Volmer constant. The Stern-Volmer plot depicts an excellent linear behavior (linear correlation coefficient of 0.9908) within the concentration range of 0–60 μM with a Ksv = 2.793 × 10 4 M−1. Fluorescence decay curve analysis was done to gain an insight into the fluorescence quenching mechanism of CDs (Fig. S3). It can be seen that the decay curves of CDs in the absence and presence of Cu2+ are overlapped, suggesting negligible change in the fluorescence lifetime of CDs after introducing Cu2+ ions. As a result, the mechanism of the quenching process could be ascribed to the formation of a stable non-fluorescent complex between Cu2+ and CDs (static quenching) rather than collisional deactivation (dynamic quenching) [44]. Plentiful hydroxy, carboxyl and amino groups distributing on the surface of the CDs could react with Cu2+ and form complexes through coordinating or chelating
interactions [28]. The complex may alter the electronic structure of the CDs and influence the distribution of excitons, which promotes the nonradiative recombination and thus leads to the fluorescence quenching [28,44,45]. 4. 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 ability of sensitive Cu2+ detection. It is expected that the investigated CDs with solvent-dependent optical properties as well as selective Cu2+ detection may have broad application prospects 228
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Fig. 5. (a) Emission peak in Fig. 4f as a function of ETN . (b) Nonradiative decay rates in five solvents as a function of ETN . Solvents used are as follows 1: toluene, 2: acetone, 3: DMSO, 4: ethanol, 5: water.
Fig. 6. (a) PL spectra of the CDs solution in the presence of different Cu2+ concentration. (b) The Stern-Volmer plot of F0/F versus [Cu2+], where F0 represents the PL intensity of the CDs, F represents the PL intensity when adding Cu2+.
in bioimaging and biodetecting. [3]
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.
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Acknowledgements
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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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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.01.021.
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Amphipathic carbon dots with solvent-dependent optical properties and sensing application
T
Shiliang Meia,1, Xian Weia,1, Zhe Hua, Chang Weia, Danlu Sua, Dan Yanga, Guilin Zhanga, Wanlu Zhanga,∗∗, Ruiqian Guoa,b,∗ a b
Engineering Research Center of Advanced Lighting Technology, Ministry of Education, Institute for Electric Light Sources, Fudan University, Shanghai, 200433, China Institute of Future Lighting, Academy for Engineering and Technology, Fudan University, Shanghai, 200433, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon dots Optical properties Solvent-dependent Cu2+ detection
Carbon dots (CDs) have been regarded as novel heavy-metal-free fluorescent materials because of their prominent optical features. In this work, one type of amphipathic CDs is prepared by facile one-step solvothermal treatment of p-Phenylenediamine. The obtained CDs own numerous surface function groups which endow them prominent dispersibility in different solvents. 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. Strong solvatochromic behavior with tunable emission from blue to green and strict excitation-independent emission characteristic can be observed when the as-prepared CDs are dissolved in different solvents, suggesting their feasible sensing applications as an alternative of solvatochromic dye molecules. Furthermore, highly selective detection of Cu2+ ions using the as-synthesized CDs as sensing probes is achieved. It is expected that the investigated CDs with solvent-dependent optical properties as well as selective Cu2+ detection may have broad application prospects in bioimaging and biodetecting.
1. 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 [1–4]. These characteristics endow CDs with the great potential applications in sensing, bioimaging, photocatalysis, LEDs and solar cells [5–9]. 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 [20–22]. 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
∗ Corresponding author. Engineering Research Center of Advanced Lighting Technology, Ministry of Education, Institute for Electric Light Sources, Fudan University, Shanghai, 200433, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (W. Zhang), [email protected] (R. Guo). 1 Authors contributed equally.
https://doi.org/10.1016/j.optmat.2019.01.021 Received 22 December 2018; Received in revised form 17 January 2019; Accepted 20 January 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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Teflon autoclave and heated at 180 °C in an electric oven for 6 h. The dark brownish solution was obtained and precipitated with the excess mixed solvent solution of petroleum ether and ethyl acetate with the volume ratio of 1/4, and then centrifuged at 8000 rpm for 10 min to separate the solvent. The final products were dried and collected in vacuum and stored in the dark for future using.
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 [26–28]. 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 pPhenylenediamine. 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.
2.3. Characterization The obtained CDs were characterized by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL), X-ray diffractometer (XRD, D8 Advance, Bruker), X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo) and Fourier transform-infrared spectroscopy (FT-IR, Nicolet 6700, ThermoFisher). Binding energy calibration was based on C 1s at 284.6 eV. The PL spectra were recorded using a fluorescence spectrophotometer (F97XP, Shanghai Lengguang). The absorption spectra were collected with an ultraviolet–visible (UV–vis) spectrophotometer (759S, Shanghai Lengguang). The fluorescence lifetime of the as-prepared CDs in different solvents were measured on a time-resolved spectrofluorometer (FLS 920, Edinburgh Instruments). The quantum yield (QY) of the CDs was measured using quinine sulfate dissolved in a 0.1 M H2SO4 aqueous solution as the standard, comparing the absorbance (less than 0.1 at 360 nm) and the integrated PL intensity (excited at 360 nm) of the CDs with those of the standard. The QY was calculated according to:
Φ = Φst (I / Ist )(Ast / A)(η2 / ηst2 ) Where φ is the QY, Ι is the measured integrated PL intensity, A is the absorbance and η is the refractive index of the solvent. The subscript “st” refers to the standard.
2. Experimental 2.4. Procedure for ion sensing 2.1. 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.
The fluorescence detection of Cu2+ ions was implemented by using the as-prepared CDs dissolved in phosphate buffer solution at room temperature. Concretely, a given concentration of Cu2+ ions was added to 10 mL aqueous solution of CDs, which were shaken well and kept for 5 min. The PL spectrum was then collected at the excitation wavelength of 380 nm. Some kinds of other metal ion aqueous solution (Ag+, Cd2+, Cs2+, Fe2+, Fe3+, In3+, K+, Mg2+, Mn2+, Na+, Pb2+, Zn2+) were selected to detect the selectivity of the CDs.
2.2. Synthesis of CDs
3. Results and discussion
p-Phenylenediamine (1 g) and urea (1 g) were added into 10 mL DMSO in a glass beaker containing 5 mL of PEG. After being stirred for about 5 min, the mixed solution was clear and then sealed in a 25 mL
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. 1. TEM (a), size distribution (b), and XRD pattern (c) images of resultant fluorescent CDs. The inset in (a) shows HRTEM image of the CDs with the lattice parameter of 0.21 nm. 225
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ethanol and water, respectively. Normalized PL spectra excited by 380 nm shown in Fig. 4f further depict that the emission peaks show a red shift with the solvent polarity increasing, thus leading to an emission color change from blue to green (Fig. 4g). Interestingly, the full width at half maximum (FWHM) is also found to be solvent-dependent (Table S1), indicating that solvent fluctuations have a non-negligible effect on expanding the emission. The changes in PL spectra were also investigated when different amount of DMSO is mixed with water. It can be seen that the maximum emission peak presents a red shift with the volume ratio of DMSO to water decreases (Fig. S1). To a certain extent, this solvent-dependent phenomenon is very analogous to the “solvatochromism” originated from fluorescent organic dyes [34]. Furthermore, the solvent-dependent emission behavior can also be efficiently described by using the Reichardt's polarity parameter ETN , a common polarity parameter. The value of ETN stands for the overall solvation capability of the solvents to form nonspecific (e.g., dipoledipole) and specific (hydrogen bond; HB) solute-solvent interaction [35]. The emission peak position of the CDs in different solvents presents a linear correlation (R2 = 0.975) to ETN (Fig. 5a), confirming the trend in Fig. 4f. This result approves the potential employment of the as-synthesized CDs as the fluorescent probes to discriminate the local environments with different polarities. In addition, the emission bands (Fig. 4a–e) of the CDs in any solvent remain unchanged under different excitation wavelengths, manifesting the strict excitation-independent emission feature of the CDs. QYs of the CDs in the five solvents were calculated via a relative method using quinine sulfate as the standard, and the results are shown in Table S1 for more details. Obviously, the QYs in other solvents are higher than that in water and become lower with the longer emission peak. Fig. 4h shows UV–vis absorption spectra of the CDs in the five solvents. Obviously, each absorption spectrum shows two characteristic absorption bands and the positions of the absorption bands present solvent-dependent, manifesting that the energy gaps of CDs differ with solvents. According to previous reports, the higher energy absorption band is commonly assigned to intrinsic state (π-π*) transitions within the sp2-hybridized carbon core of the CDs [36–38]. The π-π* core band derives from π-conjugation of aromatic carbon and thus has much relevance with the (crystalline) carbon core. While, the lower energy absorption band is assigned to defect states (n-π*) transitions resulting from the edge states between the sp2 and sp3 hybridized carbons [36–38]. The peak of high-energy core band shifts from 286 to 242 nm, depicting a blue shift with the solvent polarity increasing. And the peak of low-energy edge band also shows gradual hypsochromic shift from 328 to 294 nm expect in toluene. The inconsistency in toluene could be attributed to the relatively poor solubility which leads to the aggregation of the CDs. Generally, the core state leading to the π-π* transition is buried inside the CDs structure and not exposed to solvent, implying that the spectral position shows independence on solvents [38]. However, it is inconsistent with our result. Some groups have also noticed similar spectral shift for this band [36,38]. Such result reveals that the carbon core could be attached with massive surface/edge states to sample the variations in external environment. Moreover, spectral shift for the edge band is also observed, revealing that the interaction between the solvent molecules and CDs plays a major role in affecting nπ* transitions which are usually referred to the major fluorescent centers for the bright luminescence in the CDs [36], and thus results in the observed emission spectra shifts. Time resolved fluorescence decay curves of the CDs dispersed in five solvents are shown in Fig. 4i excited by 380 nm. For comparison with the previous reports of multiexponential kinetics [37,39], the fitted decay curves of the five samples are found to be monoexponential, suggesting a single, well-defined electronic transition accountable for the observed emission and implying that the excitation independence of the emission behavior is relevant with its uniform surface states. The lifetime results have been tabulated in Table S1. It can be observed that the fluorescence lifetimes depict the solvent dependence which agrees
Fig. 2. FT-IR spectrum of the as-synthesized CDs.
(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 CDs. As depicted in Fig. 1c, the diffraction peak located at about 23° for its (002) plane, suggesting that the as-prepared CDs consist of graphene-like structures [29,30]. These results are similar to those of many other reported CDs. The surface-related functional groups and elementary compositions of the CDs were studied by FT-IR and XPS. In the FT-IR spectrum shown in Fig. 2, the existence of absorption bands can be obviously observed, which resulted from several functional groups like OeH, NeH, C]O and C]C. Remarkably, the broad band at 3640-3109 cm−1 is assigned to the OeH and NeH vibrations [23,29]. The peak emerging at ∼2890 cm−1 derived from the CeH stretching vibration [28]. The stretching mode of vibration resulting from C]O and aromatic C]C bonds lead to the relatively weak absorption band in 1720–1570 cm−1 [31]. The bands at 1460 cm−1 and 1245 cm−1 could be attributed to C]N and aromatic ether, respectively [32]. The stretching vibration of CeOeC bond arises at the sharp absorption band from 990 to 1170 cm−1 [27]. These observations reveal that the CDs surface is rich in hydrophilic groups, endowing them with prominent dispersibility in different solvents like water, ethanol and DMSO, and capability of forming intermolecular hydrogen bonds. XPS was further employed to analyze the chemical bond composition of the CDs. Three peaks at 284.6, 399.7 and 532 eV can be observed in the XPS spectrum (Fig. 3a), indicating the presence of C, N and O elements in the CDs. The highresolution XPS spectrum of C 1s (Fig. 3b) is deconvoluted into four peaks, contributed to the sp2 CeC/C]C bond at 284.6 eV, sp3 CeN bond at 285.4, CeO bond at 286.0 eV, and C]N bond at 287.8 eV [21,31]. In Fig. 3c, the high-resolution N 1s spectrum exhibits three peaks at 398.5, 399.3, 400.7 eV, attributed to pyridinic N, amino N and pyrrolic N [28,33]. Besides, the high-resolution O 1s spectrum in Fig. 3d could be divided into two peaks at 531.8 and 532.9 eV, corresponding to C]O/OeH and CeO/eNO2, respectively [30]. The steady-state and absorption spectroscopy were adopted to study the optical properties of the as-synthesized CDs via dissolving the CDs into different solvents (toluene, acetone, DMSO, ethanol and water). PL spectra of the CDs dispersed in five solvents under various excitation wavelength are presented in Fig. 4a–e. The emission peak positions are observed at 454, 461, 476, 487 and 505 nm in toluene, acetone, DMSO,
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Fig. 3. XPS survey (a) and high-resolution C 1s (b), N 1s (c) and O 1s (d) peaks of the as-synthesized CDs.
dependent properties of the CDs in the protic solvents [40,42]. And, stronger HB may quench the fluorescence of the CDs more actively since stronger depletion of upper excited levels of surface defect states will be induced by uniform robust hydrogen-bonded network of water than unordered weaker hydrogen-bonded network of ethanol [43], which could explain why the QY of the CDs dispersed in water is the lowest. Recently, CDs have served to detect the neighbouring molecules and ions as unique fluorescent probes, which propelled us to inquire into the capacity of the as-prepared CDs as probes for metal ion detection. Herein the fluorescence properties of the CDs were measured after adding various metal ions (0.5 mM). To confirm the selectivity of the CDs to various metal ions, thirteen kinds of metal cations were applied in the experiment, including Ag+, Cd2+, Cs2+, Cu2+, Fe2+, Fe3+, In3+, K+, Mg2+, Mn2+, Na+, Pb2+ and Zn2+ (Fig. S2). Upon the addition of each kind of cation with a concentration of 0.5 mM to the CDs in phosphate buffer solution, almost no obvious fluorescence changes are observed for most of these metal ions in the same measuring condition. However, the fluorescence of the CDs is markedly quenched after adding Cu2+ and Ag+. Apparently, the quenching effect caused by Cu2+ is stronger than that caused by Ag+, indicating that the synthesized luminescent CDs are selective for the detection of Cu2+ ions. Furthermore, the CDs solution was treated with various concentrations of Cu2+ ions ranging from 0 to 100 μM to investigate the quenching degree (Fig. 6a). With the Cu2+ concentration increasing, a continual quenching of fluorescence intensity can be observed. However, the emission peak position does not change even in the presence of the highest Cu2+ concentration. Fig. 6b further shows the relationship between the relative fluorescence response of CDs (F0/F) and Cu2+ concentration ([Cu2+]). It can be observed that the quenching of
well with the emission and absorption spectra results, nevertheless, there is no monotonic change in fluorescence lifetimes with the variation of solvent polarity. Based on the values of the QY and lifetime (Table S1), the radiative (KR) and nonradiative (KNR) rates can be estimated using following expressions: KR = QY/τ and QY = KR/ (KR + KNR) [40]. The nonradiative rates in four solvents (toluene, acetone, DMSO and ethanol) are found to be similar (Fig. 5b), while it almost doubles in water, explaining why there is a drastic reduction of the lifetime and the QY of the CDs when dispersed in water. As discussed above, the interaction between the solvent molecules and the surface groups on the as-synthesized CDs plays a significant role in affecting the optical properties including PL emission, QY, absorption and fluorescence lifetime. In fact, both dipole-dipole and HB interaction can result in the observed solvent-dependent emission behavior (Fig. 5a). Based on the previous reports, the origin of the solvent-dependent properties of the CDs in protic solvents is mainly attributed to HB interaction between the protic solvent and the CDs [37,41]. In aprotic solvents, the solvent-dependent emission behavior is considered to be a change in surface electronic state caused by dipole-dipole interaction [21,42]. On the basis of the FT-IR and XPS results shown in Figs. 2 and 3, the as-synthesized CDs consist of abundant nitrogen and oxygen forming amine and carboxylic surface functional groups which could enhance the charge carrier density and give rise to new charge transfer of electrons to the edge. In aprotic solvents, the dipole moment has more influence on the surface electronic structure with the solvent polarity increasing, and then reduces the energy gap, which leads to the emission peak position shifting towards longer wavelength [21]. While, the amount of red shift in emission becomes highest when the CDs are dissolved in water, revealing that the donation of HB from the protic solvents to the CDs acts as a predominant contributor for the solvent-
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Fig. 4. PL spectra of the same CDs dispersed in the solvents of toluene (a), acetone (b), DMSO (c), ethanol (d) and water (e) excited by different excitation wavelength. Normalized PL spectra (f) (λex = 380 nm), corresponding photographs (g) under UV irradiation (λ = 365 nm), UV–vis absorption spectra (h) and fluorescence decay curves (i) of the CDs dispersed in five solvents.
fluorescence intensity caused by Cu2+ at low concentrations conforms to a typical Stern-Volmer equation [27,42]: F0/F = 1+ Ksv [Cu2+], where F0 represents the PL intensity of the CDs, F represents the PL intensity when adding Cu2+ and Ksv is the static Stern-Volmer constant. The Stern-Volmer plot depicts an excellent linear behavior (linear correlation coefficient of 0.9908) within the concentration range of 0–60 μM with a Ksv = 2.793 × 10 4 M−1. Fluorescence decay curve analysis was done to gain an insight into the fluorescence quenching mechanism of CDs (Fig. S3). It can be seen that the decay curves of CDs in the absence and presence of Cu2+ are overlapped, suggesting negligible change in the fluorescence lifetime of CDs after introducing Cu2+ ions. As a result, the mechanism of the quenching process could be ascribed to the formation of a stable non-fluorescent complex between Cu2+ and CDs (static quenching) rather than collisional deactivation (dynamic quenching) [44]. Plentiful hydroxy, carboxyl and amino groups distributing on the surface of the CDs could react with Cu2+ and form complexes through coordinating or chelating
interactions [28]. The complex may alter the electronic structure of the CDs and influence the distribution of excitons, which promotes the nonradiative recombination and thus leads to the fluorescence quenching [28,44,45]. 4. 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 ability of sensitive Cu2+ detection. It is expected that the investigated CDs with solvent-dependent optical properties as well as selective Cu2+ detection may have broad application prospects 228
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Fig. 5. (a) Emission peak in Fig. 4f as a function of ETN . (b) Nonradiative decay rates in five solvents as a function of ETN . Solvents used are as follows 1: toluene, 2: acetone, 3: DMSO, 4: ethanol, 5: water.
Fig. 6. (a) PL spectra of the CDs solution in the presence of different Cu2+ concentration. (b) The Stern-Volmer plot of F0/F versus [Cu2+], where F0 represents the PL intensity of the CDs, F represents the PL intensity when adding Cu2+.
in bioimaging and biodetecting. [3]
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.
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Acknowledgements
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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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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.01.021.
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