Dyes and Pigments 173 (2020) 107952
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A facile synthesis of high-efficient N,S co-doped carbon dots for temperature sensing application Ziying Guo a, 1, Jiabao Luo a, 1, Zhenpeng Zhu a, Zishan Sun a, Xinguo Zhang a, *, Zhan-chao Wu b, Fuwang Mo c, **, Anxiang Guan d a
Guangdong Provincial Key Laboratory of New Drug Screening, Guangzhou Key Laboratory of Drug Research for Emerging Virus Prevention and Treatment, School of Pharmaceutical Sciences, Southern Medical University, Guangzhou, 510515, China State Key Laboratory Base of Eco-chemical Engineering, Laboratory of Inorganic Synthesis and Applied Chemistry, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China c Guangxi Key Laboratory of Calcium Carbonate Resources Comprehensive Utilization, College of Materials & Chemical Engineering, Hezhou University, Hezhou, 542899, China d Laboratory of Advanced Materials, Fudan University, Shanghai, 200438, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon dots Luminescence Turn-off fluorescence Temperature sensing Quenching mechanism
A high-efficient nitrogen and sulfur-doped carbon dots (CDs) were prepared from L-Cysteine (Cys) and trisodium citrate dihydrate using a hydrothermal method. The Cys-CDs were completely water-soluble and remarkably stable under various pH and ionic strength conditions. Cys-CDs show a absorption maximum at 350 nm and PL maximum at 450 nm with high fluorescence quantum yield of 68%. It is found that Cys-CDs exhibit linear temperature-dependent emission intensity responses at in the 283–343 K range, along with strongly temperaturedependent monoexponential decay. The mechanism of temperature-dependent fluorescence is confirmed as temperature enhanced population of non-radiative channels by comparing radiative and nonradiative recom bination rates at different temperature. All results indicate that Cys-CDs could be a promising material for fluorescent temperature-sensing application.
1. Introduction Carbon dots (CDs) have emerged as a novel fluorescent nano material, which own a lot of merits like color-tunable, low toxicity, high quantum efficiency, photo-stabilization and good biocompatibility. CDs have attracted increasing interest in many fields such as disease diag nosis, heavy metal detection, photoelectric equipment, drug loading and bioimaging [1–3]. The luminescent properties of CDs are closely related to the quantum size effect, surface state and excitation’ radiative recombination, which affect the HOMO, LUMO and bandgap of CDs [4, 5]. Thus, the interaction between substance/environment and CDs could often result in gradually and stepwise fluorescence quenching, which make CDs of great application potential in detection and sensing fields [6,7]. In view of CDs’ fluorescence sensing applications, they had been used as probes to identify and monitor many chemical substances. CDs-
based fluorescence turn off detection of various metal ions like Hg2þ, Fe3þ, Co2þ, Cr6þ [8,9], biomolecules like glucose, heparin, cysteine [10, 11], and medicine molecules like bilirubin, curcumin [12,13] are re ported. Temperature is a physical quantity indicating the degree of heat and cold of an object, which is closely related to human life [14–17]. Accurate determination of temperature is of great importance due to the widespread applications [18]. The optical temperature sensing could be realized via down-conversion and up-conversion process. Many luminescence-based temperature sensors which take the change of emission intensity to the temperature as the temperature parameter have been reported, such as up-converting nano-particles [19,20], su pramolecular coordination complexes [21,22] and rare-earth doped phosphors [23–26]. However, studies for temperature-dependent opti cal properties of CDs are relatively rare compared with these for chemical-sensing properties of CDs [27,28]. Besides, the investigation of corresponding mechanism of CDs’ fluorescence thermal-quenching
* Corresponding author. ** Corresponding author. E-mail addresses:
[email protected] (X. Zhang),
[email protected] (F. Mo). 1 Ziying Guo and Jiabao Luo contributed equally to this work. https://doi.org/10.1016/j.dyepig.2019.107952 Received 22 August 2019; Received in revised form 2 October 2019; Accepted 3 October 2019 Available online 3 October 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. TEM, XRD (a), AFM image (b) and FTIR spectrum (c) of Cys-CDs.
process is lacked. In the present work, Cys-CDs are demonstrated to emit bright-blue fluorescence, which are stable under various pH and ionic strength conditions. Besides, the temperature-dependent fluorescence was measured in range of 283–343 K, showing decrease of emission intensity with rising temperature. Besides, the emission intensity of Cys-CDs so lution was reversible during consecutive heating and cooling cycles, showing its good reversibility and restorability with a relative sensitivity of 0.64% K 1. Because of the accurate linear response of emission in tensity with temperature, the Cys-CDs could act as a good luminescent thermometer. The mechanism of this temperature-dependent fluores cence was investigated thoroughly using PL spectra and decay lifetime data, and is confirmed to be the temperature enhanced population of non-radiative channels. Temperature-dependent changes of the spectral intensity of Cys-CDs can serve as a probe for temperature detection scheme, i.e. luminescence intensity thermometry, which is a versatile optical technique for the noninvasive, noncontact estimation of local temperature.
2.2. Preparation and characterizations of Cys-CDs Cys-CDs were synthesized via hydrothermal method. 0.2 g trisodium citrate dihydrate was mixed with 0.2 g L-Cysteine in 10 ml deionized water. After 5 min magnetic stirring, 500 μl ethylenediamine was added. The solution was transferred to stainless steel autoclave (50 ml) and heated at 200 � C for 4 h. The obtained yellow-brown solution was pu rified using dialysis bag (Mw ¼ 1000) for 24 h when cool down to room temperature. The final product stored at below 4 � C for further uses. 2.3. Calculation of the fluorescence quantum yield Quantum yield refers to the ratio of the quantum number of the product to the absorption quantum number, which is an important in dicator of the fluorescence intensity. The reference method is a commonly used method for quantum yield testing, with quinine sulfate as standard. The quinine sulfate was dissolved in 0.1 M H2SO4 (λex ¼ 360 nm, QY ¼ 54%), and the UV absorbance of the quinine sulfate solution and the Cys-CDs solution were kept below 0.1 to reduce the impact of other factors. The quantum yield of the Cys-CDs can be calculated by the following formula: � � � 2� � � Ix η Ast QYx ¼ QYst ⋅ ⋅ 2x ⋅ Ist ηst Ax
2. Experimental section 2.1. Material Trisodium citrate dihydrate was purchased from Guangzhou Jin huada chemical reagent Co. Ltd, L-Cysteine and dialysis bag were pur chased from MYM Biological Technology company. The ethylenediamine was obtained from Tianjin Damao Chemical Reagent Factory. All reagents were of analytical grade and used as received without further purification. The water used in the experiments was deionized water.
Where QY represents the quantum yield and I is the integrated intensity of the fluorescence. η is the solvent refractive index, and A is the absorbance of UV. The solvent for this examination is water. So ηx/ ηst ¼ 1. In addition,“st” and “x” represent the standard quinine sulfate and analyte, respectively.
2
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Fig. 2. (a) XPS spectra of Cys-CDs. (b–d) High-resolution XPS data of C1s, N1s and S2p of Cys-CDs.
2.4. Characterization
ethylenediamine was used as nitrogen source as well as surface passiv ation agent to increase fluorescence intensity of CDs. The field emission scanning electron microscopy (TEM) image was gained to know the size and morphology of Cys-CDs for further investigate. As shown in Fig. 1a, the Cys-CDs are spherical and uniformly dispersed with an average diameter of 3.6 nm. Such a small diameter may be suitable for biological research. Atomic force microscopy (AFM) is also used to probe Cys-CDs deposited on a mica substrate (Fig. 1b). These results show that Cys-CDs own the spherical morphology and the corresponding heights are in the range of 4–6 nm with average heights of 4 nm, which indicates that the Cys-CDs primarily consisted of eight or nine layers of graphene-like sheets, and is consistence with the TEM results. Thus, we infer that the Cys-CDs are monodispersed nanospheres with an average size of about 4.0 nm. The XRD diffraction (Fig. 1a inset) pattern exhibited a broad peak at 2θ ¼ 23.34� , corresponding to the (002) interlayer spacing of graphitic structure. The chemical structure of Cys-CDs was further characterized by fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). FT-IR spectrum of the Cys-CDs was given in Fig. 1c. The band in the region of 2800–3440 cm 1 aroused from stretching vi brations of O–H and N–H. The peak at 2343 cm 1 and 1686 cm 1 is – O bond, attributed to C–N, S–H bonds. and the stretching vibration of C– respectively. The absorption peak at 1548 cm 1 could be attributed to – C, and the peak at 1420 cm 1 could be the stretching vibration of C– identified as those of C–H, N–H and –COO groups, respectively [29]. The 1190 cm 1could be ascribed to the C–O, C–N and C–S bonds and 1036 cm 1 could be assigned to the –SO3-, C–O–C, and C–O bonds [30]. This result demonstrates that the surface of prepared Cys-CDs is rich in numerous oxygen-containing groups, e.g. carboxyl, carbonyl and hy droxyl. All of these functional groups make the Cys-CDs highly hydrophilic. The elemental composition and its chemical states of Cys-CDs were
The morphology and size of Cys-CDs were observed via the field emission scanning electron microscopy (Tecnai G2 F20). The FT-IR spectra of Cys-CDs was obtained on PerKinEImer FT-IR spectrometer Frontier in the range of 400–4000 cm 1 using KBr tableting technology. Atomic force microscopy (AFM) analysis was carried out in the acoustic AC mode on a Bruker BioScope Resolve AFM system equipped with a microcamera of Leica DFC3000G and inverted camera of Leica DMi8. The height profile analysis was assisted by using the NanoScope Analysis software distributed by Bruker. X-ray photoelectron spectroscopy (XPS) analysis of sample was recorded using Kratos Axis Ultra Dld photo electron spectrometer with a mono X-Ray source Al Kα excitation (1486.6 eV). UV–Vis absorption spectrum was collected by UV-5500PC spectrophotometer. The photoluminescence excitation (PLE) and pho toluminescence (PL) spectra of sample were measured on FLS980 fluo rescence spectrometer (Edinburgh, UK) with 150 W Xe lamp and the measured spectra were corrected for PMT sensitivity, while the fluo rescence lifetime cures were measured with 320 nm laser on the same instrument. For temperature-dependent spectra measurement, the sample was installed in a cryostat with a controllable temperature be tween 273 and 343 K using closed cycle water/coolant bath with waterglycol-cooled sample holders. The maximum measured temperature is limited to 343 K to avoid the coolant from being boiled. 3. Results and discussion 3.1. Morphology and structure of Cys-CDs In our work, Cys-CDs were synthesized by the facile and effective hydrothermal method. Briefly, Trisodium citrate dihydrate and LCysteine were used as carbon and sulfur resource, while 3
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Fig. 3. UV–Vis absorption spectrum of Cys-CDs (inset is the (αhν)1/2 vs hν curve).
Fig. 5. Emission spectra of Cys-CDs at various pH conditions (a) and fluores cence response of Cys-CDs with increasing ionic strength (b, λex ¼ 350 nm).
consisted of multiple functional groups with oxygen, nitrogen and sulfur elements, which results in the excellent water solubility and promising optical properties. 3.2. Optical properties of Cys-CDs The optical properties of Cys-CDs were first studied by UV–vis ab sorption spectroscopy at room temperature (Fig. 3). The absorption spectrum displays a dominant peak around 230 nm, which is ascribed to – C bond. The shoulder absorption peak at the π-π* transition of the C– – O bond. The 325 nm can be attributed to the n-π* transition of the C– optical band gap (Eg) of the Cys-CDs can be estimated from the (αhν)1/2 vs hν curve for direct band gap semiconductors, where α, h, and ν are the absorption coefficient, Planck constant, and light frequency, respec tively. By extrapolating the linear fit to the intercept of the energy axis (Fig. 3 inset), Eg value of Cys-CDs is estimated to be 4.6 eV. The PL and PLE spectra of Cys-CDs were evaluated in Cys-CDs aqueous solution state (Fig. 4a). The PLE band of Cys-CDs covers 240–400 nm rage with the maximum at 350 nm, which originates from the trapping of excited state energy by the surface states. Under 350 nm excitation, Cys-CDs exhibit a blue emission band with peak at 450 nm and the FWHM value of ~75 nm. The Stokes shift value could be esti mated to the difference between excitation maximum (350 nm, 28571 cm 1) and emission maximum (450 nm, 22223 cm 1), and found to be 6348 cm 1. The quantum yield (QY) of Cys-CDs is up to 68% with quinine sulfate (0.1 M H2SO4 as solvent; QY ¼ 54%) as the standard. The QY of prepared Cys-CDs was far higher than many already reported carbon dots [33]. Fig. 4a inset displays the digital photographs of Cys-CDs under sunlight and 365 nm UV light, which shows light yellow color and bright-blue fluorescence. The result demonstrates that the Cys-CDs can be readily dispersed in water and exhibit efficient blue emission at UV excitation. The PL spectra at different excitation wave lengths are depicted in Fig. 4b. The excitation-dependent variation of emission wavelength and intensity is a common phenomenon observed in carbon dots materials, which could be attributed to the existence of different particle size and multiple surface states in CDs [34].
Fig. 4. (a) Optimal excitation and emission spectra (inset: photographs taken under natural light and 365 nm UV). (b) excitation-dependent PL behaviors of Cys-CDs.
provided by XPS analysis. Binding energy calibration was based on C1s at 284.8 eV. Total XPS spectrum in Fig. 2a reveals the existence of sulfur (~167 eV), carbon (~285 eV), nitrogen (~398.5 eV), and oxygen (~531 eV) atoms. The high-resolution C1s, N1s and S2p of Cys-CDs are demonstrated in Fig. 2b-d, separately. The C1s spectrum can be resolved – C at 283 and 284 eV, C–N/C–O at 286 286 eV as follows: C–C and C– (Fig. 2b). As shown in Fig. 2c, the N1s spectrum have two peaks at 398 eV and 399 eV, which could be attributed to Pyridinic N and Amino N, respectively [31]. The S2p spectrum in Fig. 2d shows an asymmetric peak at 167.6 eV, which can be fit with three components at 167.6, 168.5, and 169.3 eV, arising from a -C-SOx- (x ¼ 2, 3, 4) species, such as sulfate or sulfonate [32]. The N and S contained in the Cys-CDs come from the precursor ethylenediamine and L-Cysteine, respectively. Both the spectrum of FT-IR and XPS indicates that the as-prepared Cys-CDs 4
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Fig. 6. The effect of various metal ions on the fluorescence response of Cys-CDs (a, Inset is the normalized emission intensity with addition of different metal ions); The decay curves of Cys-CDs with and without Cu2þ/Pb2þ addition.
Surprisingly, with the increase of excitation wavelength from 300 to 360 nm, the emission intensity of Cys-CDs increases remarkably with negligible shift of emission wavelength. The excitation-independent PL behavior indicates that both the size and the surface state of Cys-CDs should be uniform, which makes the Cys-CDs suitable for possible cell imaging application where the tissue autofluorescence can be avoided.
Fig. 7. PL spectra of Cys-CDs solution at various temperatures (a) and revers ible temperature-dependence of the PL of the Cys-CDs solution (b).
3.3. pH and ionic strength stability of Cys-CDs
ionic strength is much greater than the typical physiological value of around ~100 mM. The result indicates that CDs could be stable and do not aggregate in a medium with high ionic strength. These stability studies suggest that the Cys-CDs have good potential for practical applications. The effects of different metal ions on Cys-CDs fluorescence including Agþ, Cu2þ, Mn2þ, Pb2þ, Naþ were tested. The metal ions solutions were prepared with 200 μM concentration. In our assay, 400 μl metal ion solution mixed with 3.0 ml of Cys-CDs at pH ¼ 7 in room temperature before spectra measurement. The result was shown in Fig. 6a. The emission intensity drops to 89% of the blank when Agþ was adding. Similarly, the fluorescence intensity was about 90% of the blank after adding the Co2þ and Mo2þ. Moreover, Naþ and Kþ were added, the emission intensity only quenched 5% of original intensity, Meanwhile, it is found that Cu2þ and Pb2þ have significant quenching effect on the Cys-CDs’ emission, i.e. not only the emission intensity has obvious quenching (20% of original intensity), but also an emission red-shift of ~10 nm is observed. In order to shed light on the quenching mechanisms, the decay lifetime of Cys-CD with and without Cu2þ/Pb2þ addition is measured and depicted in Fig. 6b. Generally, the possible quenching mechanisms €rster reso of carbon dots are static quenching, dynamic quenching, Fo nance energy transfer (FRET), photo induced electron transfer (PET), inner filter effect (IFE) and aggregation induced quenching [38]. It is known that the static quenching, PET, IFE and aggregation induced
Many factors might affect the fluorescence property of Cys-CDs such as pH, NaCl strength, metal ions, temperature [35,36]. A series of fluorescent CDs solutions with pH ranging systematically from 1 to 12 are prepared, by mixed 3 ml PBS buffer with different pH and 100 μl Cys-CDs solution. Then, the pH-dependent emission spectra of the Cys-CDs was were measured upon λex ¼ 350 nm. As shown in Fig. 5a. The PL intensity of N-CDs remains remarkably stable over a wide range of pH between 3.5 and 9.5, while exhibits an obvious decrease at a lower pH (pH < 3) and changes slightly at higher pH values (pH > 10). Under a strong acidic or alkaline condition, the emission intensity was greatly reduced. The possible reason for the quenching is that the carboxyl and hydroxyl functional groups on the surface of N-CDs are prone to ag gregation due to strong intermolecular hydrogen bonding between carboxyl and hydroxyl groups under an extreme pH [37]. This result confirms that the Cys-CDs were stable in a broad pH range. The ion-stability of Cys-CDs emission is investigated by the inter ference of different physical salt (NaCl) concentrations (0.2–1.2 mol/L) with the Cys-CDs solution at room temperature (Fig. 5b). The relative fluorescence intensity is determined by calculating the ratio of the emission intensities of Cys-CDs solution in the presence and absence of the interference ions as shown in Fig. 5b inset. The result reveals that almost no variation of emission intensity or emission wavelength could be found with increasing ioninc strength, and the emission intensity of Cys-CDs remains unchangeable even in a 1.2 M NaCl solution, whose 5
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Fig. 8. Arrhenius plot revealing the linearity of temperature-dependent emis sion ratio in heating (a) and cooling process (b).
quenching have neglectable effect on decay time variation, while FRET process, which results in shorten decay lifetime, requires spectral overlap with CD’s emission and quencher’s absorption. In present case, the original decay curve of Cys-CDs in aqueous so lution could be well-fitted with bi-exponential decay kinetics consisting of 2.48 ns (2.48%) and 14.86 ns (96.52%) contributing components. Since one of the components shared as high as 96.52%, it indicates that only one radiative transition channel is mostly responsible for the PL emission of the Cys-CDs. The value of average decay life (τ*) for Cys-CDs is found to be 14.44 ns, which implies the singlet state nature of emission of the Cys-CDs. The result suggests that radiative recombination of the excitons is the most likely reason of the blue light with such short decay lifetimes, matching with the excitation and emission spectra of Cys-CDs (Fig. 3a). An addition of Cu2þ and Pb2þ causes a declining in Cys-CDs lifetime, which is 10.40 ns and 9.75 ns, respectively. It is reported that Cu2þ aqueous solution has no absorption in 350–600 nm range with a major absorption peak at 810 nm [39], and the similar scenario could be applied in Pb2þ, which has no obvious spectral overlap with Cys-CDs’ emission. The reduced lifetime indicates an ultrafast electron-transfer process in the Cu2þ/Pb2þ-Cys-CDs system and implying its dynamic quenching. Thus, it is reasonable to conclude that dynamic quenching is the corresponding mechanism for Cys-CDs’ emission quenching by Cu2þ and Pb2þ.
Fig. 9. Temperature-dependent decay curves of Cys-CDs solution (a, λex ¼ 350 nm, λem ¼ 450 nm); the dependence of ln[(I0/IT)-1] on 1/kT Cys-CDs solution (b).
intensity was measured over three successive cycles of heating and cooling (Fig. 7b). In each measurement cycle, the PL intensity was measured after 3 min thermal equilibration. The emission variation of Cys-CDs solution was reversible during consecutive heating and cooling cycles. The result demonstrates that the Cys-CDs own good reversibility and restorability, and the temperature (283–343 K) cannot cause per manent destruction on the surface fluorescent structure of the Cys-CDs. Since the emission intensity of Cys-CDs exhibits temperaturesensitivity, its relative fluorescence intensity (I/I0) as a function of temperature is depicted in Fig. 8a, where I0 and I are the PL intensities at the initial (283 K) and elevated temperatures, respectively. The linear regressive equation can be described as: I/I0 ¼ 0.0047[T] þ 2.3404, where, [T] represent the temperature (K). The relative sensitivity of CysCDs is determined to be 0.64% K 1 from the slope of the linear fit. The coefficient of determination (R2) of the regression fitting is 0.999. The increase of fluorescence intensity with temperature decreasing during the cooling process also exhibits the great linear relationship, and the correlation coefficient (R2) is reached to 0.993 (Fig. 8b). The result suggested that Cys-CDs shows high accuracy for temperature detection. The temperature-dependent decay lifetimes of Cys-CDs were measured and shown in Fig. 9a. The data were collected by monitoring emission maximum as a function of the temperature under the 320 nm laser excitation. The result shows that the PL lifetime drops from 15.03 to 11.70 ns with the temperature increasing from 283 K to 343 K, which could be ascribed to the occurrence of nonradiative decay processes. Besides, the PL relaxation dynamics of Cys-CDs reveal multiexponential decay with temperature increasing, which suggests the photoexcited carriers following the complicated relaxation processes. Many researches suggested that the thermal quenching of CDs emission is related to temperature enhanced population of non-radiative channels of surface (trap/defect) states [40]. The occurrence of non-radiative trapping will be increased with rising temperature, and this could be quantitively analyzed by Arrhenius plot of the integrated PL intensities as:
3.4. Temperature-dependent fluorescence of Cys-CDs To investigate the temperature-dependent fluorescent property of Cys-CDs, the emission spectra of Cys-CDs aqueous solution at tempera tures between 283 K and 343 K (increasing by 5 K per step) are shown in Fig. 7a. Upon the excitation at 350 nm, the Cys-CDs all yield an emission peak centered at 450 nm at diverse temperature, both emission wave length and FWHM exhibits negligible shift or broaden over the measured temperature range (Fig. 3a), indicating that the effect of thermal broadening is tiny. Meanwhile, it is found that the emission intensity decreases about 46% with the temperature increasing from 283 K to 343 K, which indicates the visually distinguishable temperaturedependent PL intensity variation. As seen in Fig. 7a, it is obvious that the emission intensity exhibits a gradually decrease at higher tempera ture and recovered when temperature falls back to 283 K. To illustrate the reusability of Cys-CDs based luminescence thermometers, the PL
I ¼ I0 =½1 þ a expð
6
Ea = kT�
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influencing the position presented in, or the review of, the manuscript entitled, “A facile synthesis of high-efficient N,S co-doped carbon dots for temperature sensing application”.
Table 1 The values of quantum yield, radiative and nonradiative recombination rate at different temperature T (K)
τ* (ns)
QY (%)
Vr (*107 s 1)
Vnr (*107 s 1)
283 288 293 298 303 308 313 318 323 328 333 338 343
15.03 14.82 14.63 14.44 14.21 14.12 13.93 13.48 13.10 12.77 12.46 12.15 11.70
73.29 71.45 69.46 68.00 66.16 64.13 62.62 61.08 59.37 57.54 55.71 54.86 53.50
4.876 4.821 4.748 4.708 4.656 4.542 4.495 4.531 4.532 4.506 4.476 4.515 4.472
1.777 1.926 2.087 2.216 2.381 2.540 2.683 2.887 3.102 3.325 3.555 3.715 3.974
Acknowledgement The work was supported by National Natural Science Foundation of China (No. 21601081), Guangxi Natural Science Foundation (No. 2018GXNSFBA281209) and Guangzhou Scientific planning program (No. 201804010260). The corresponding author thanks Dr. Anxiang Guan (Laboratory of Advanced Materials, Fudan University, China) and Dr. Ping Yang (Guangdong Institute of Microbiology, China) for help in XPS and TEM measurement. References [1] Gong X, Lu W, Paau M, Hu Q, Wu X, Shuang S, Dong C, Choi M. Facile synthesis of nitrogen-doped carbon dots for Fe3þ sensing and cellular imaging. Anal Chim Acta 2015;861:74–84. [2] Yuan F, Li S, Fan Z, Meng X, Fan L, Yang S. Shining carbon dots: synthesis and biomedical and optoelectronic applications. Nano Today 2016;11:565–86. [3] Zhao P, Xu Q, Tao J, Jin Z, Pan Y, Yu C, Yu Z. Near infrared quantum dots in biomedical applications: current status and future perspective. Wires Nanomed Nanobiotechnol 2018;10:e1483. [4] Shamsipur M, Barati A, Karami S. Long-wavelength, multicolor, and white-light emitting carbon-based dots: achievements made, challenges remaining, and applications. Carbon 2017;124:429–72. [5] Song Y, Zhu S, Xiang S, Zhao X, Zhang J, Zhang H, Yang Y Fum B. Investigation into the fluorescence quenching behaviors and applications of carbon dots. Nanoscale 2014;6:4676–82. [6] Namdari P, Nagahdari B, Eatemadi A. Synthesis, properties and biomedical applications of carbon-based quantum dots: an updated review. Biomed Pharmacother 2017;87:209–22. [7] Han M, Wang L, Li S, Bai L, Zhou Y, Sun Y, Huang H, Li H, Liu Y, Liu Y, Kang Z. High-bright fluorescent carbon dot as versatile sensing platform. Talanta 2017;174: 265–73. [8] Liu W, Diao H, Chang H, Wang H, Li T, Wei W. Green synthesis of carbon dots from rose-heart radish and application for Fe3þ detection and cell imaging. Sens Actuators B Chem 2017;241:190–8. [9] Ma Y, Zhang Z, Xu Y, Ma M, Chen B, We L, Xiao L. A bright carbon-dot-based fluorescent probe for selective and sensitive detection of Hg2þ ions. Talanta 2016; 161:476–81. [10] Qu F, Guo X, Liu D, Chen G, You J. Dual-emission carbon nanodots as a ratiometric nanosensor for the detection of glucose and glucose oxidase. Sens Actuators B Chem 2016;233:320–7. [11] Liao S, Zhao X, Zhu F, Chen M, Wu Z, Song X, Yang H, Chen X. Novel S, N-doped carbon quantum dot-based "off-on" fluorescent sensor for silver ion and cysteine. Talanta 2018;180:300–8. [12] Anjana R, Devi J, Jayasree M, Aparna R, Aswathy B, Praveen G, Lekha G, Sony G. S. N-doped carbon dots as a fluorescent probe for bilirubin. Microchim. Acta. 2018; 185:11. [13] Guo Z, Zhu Z, Sun Z, Zhang X, Chen Y. Synthesis of dual-emitting (Gd,Eu)2O3-PEI@ CD composite and its potential as ratiometric fluorescent sensor for curcumin. Mater Res Bull 2018;108:83–8. [14] Huang K, Wang Z, Gu Y, Hu Y, Ji Z, Wang S, Li X, Xie Z, Pan S. Glibenclamide is comparable to target temperature management in improving survival and neurological outcome after asphyxial cardiac arrest in rats. J Am Heart Assoc 2016; 5:e003465. [15] Fu F, Xin S, Chen W. Temperature- and frequency-dependent dielectric properties of biological tissues within the temperature and frequency ranges typically used for magnetic resonance imaging-guided focused ultrasound surgery. Int J Hyperth 2014;30:56–65. [16] Zeng Y, Feng W, QI X, Li J, Chen J, Lu L, Deng P, Zeng J, Li F. Differential knee skin temperature following total knee arthroplasty and its relationship with serum indices and outcome: a prospective study. J Int Med Res 2016;44:1023–33. [17] Yang J, Yin P, Zhou M, et al. Cardiovascular mortality risk attributable to ambient temperature in China. Heart 2015;101:1966–72. [18] Zhang X, Huang Y, Gong M. Dual-emitting Ce3þ, Tb3þ co-doped LaOBr phosphor: luminescence, energy transfer and ratiometric temperature sensing. Chem Eng J 2017;307:291–9. [19] Li X, Sun X, Shahzad M, Liu L. Facile preparation of upconversion microfibers for efficient luminescence and distributed temperature measurement. J Mater Chem C 2019;7:7984–92. [20] Mikalauskaite I, Pleckaityte G, Skapas M, Zarkov A, Katelnikovas A, Beganskiene A. Emission spectra tuning of upconverting NaGdF4:20% Yb, 2% Er nanoparticles by Cr3þ co-doping for optical temperature sensing. J Lumin 2019;213:210–7. [21] Chen Z, Liang J, Nie Y, Xu X, Yu G, Yin J, Liu S. A novel carbazole-based gold(i) complex with interesting solid-state, multi stimuli-responsive characteristics. Dalton Trans 2015;44:17473–7.
Where, Ea is the activation energy; k is the Boltzmann constant; and a is a constant. Fig. 9b displays the plotting of the emission intensity with respect to 1/T, where the value of activation energy (Ea) is calculated to be 0.329 � 0.02 eV. In order to probe the reason for thermal quenching of Cys-CDs emission process, the radiative (Vr) and nonradiative (Vnr) recombina tion rates of Cys-CDs were determined from the lifetime (τ*) and quantum yield (QY) as:
τ* ¼
1 Vr ; QE ¼ Vr þ Vnr Vr þ Vnr
The QY of Cys-CDs at various temperatures was calculated from their temperature-dependent absorption and integrated PL intensity together with the QY determined at room temperature. Both radiative and non radiative recombination rates are listed in Table 1. The radiative rates have a slightly decline when temperature rises from 283 K to 343 K, still maintaining in the range of (4.4–4.8) � 107 s 1. At the same time, the nonradiative recombination rates have gradually increasing about 2fold, i.e. 1.777 � 107 s 1 at 283 K and 3.974 � 107 s 1 at 343 K. These results further indicate that the temperature-activated PL quenching in Cys-CDs is mainly due to the activation of nonradiative relaxation channels. 4. Conclusion In conclusion, a novel N,S co-doped blue-emitting carbon dots was synthesized by a facile one-pot hydrothermal method using L-Cysteine as raw material. The Cys-CDs not only possess a high luminescent quantum yield (QY) (68%), but also present unique optical properties of stable emission and excitation-independent characteristics. It is found that the Cys-CDs own excellent pH and ionic strength stability, while exhibit rapid temperature-dependent PL variation at temperatures in the range of 283–343 K. Temperature-dependent changes of the emission intensity of Cys-CDs indicate that it could serve as a luminescence intensity thermometry with relative sensitivity of 0.64% K 1. The activation en ergy of thermal quenching and radiative and nonradiative recombina tion rates are obtained by analyzed the temperature-dependent QY and decay lifetime. The thermal-quenching mechanism for Cys-CDs is the thermal activation of nonradiative relaxation channels. The results indicate that Cys-CDs could be applied as the luminescent temperature sensor with significant reversibility, sensitivity and linearity. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as 7
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Dyes and Pigments 173 (2020) 107952
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