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Highly sensitive and selective detection of Fe3+ by utilizing carbon quantum dots as fluorescent probes
Chemical Physics Letters 705 (2018) 1–6
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Highly sensitive and selective detection of Fe3+ by utilizing carbon quantum dots as fluorescent probes Yanan Li a,b, Yingbo Liu b, Xiaohong Shang a,⇑, Daiyong Chao b, Liang Zhou b,⇑, Hongjie Zhang b a b
College of Chemistry and Life Science, Changchun University of Technology, Changchun 130012, People’s Republic of China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 17 March 2018 In final form 21 May 2018 Available online 22 May 2018 Keywords: Carbon quantum dots Fluorescent probe Selective detection Fe3+
a b s t r a c t The carbon quantum dots (CDs) based on carbonization of citric acid and tris(hydroxymethyl)aminome thane (Tris) in glycerinum as fluorescent probes for highly sensitive and selective detection of Fe3+ have been developed. The obtained CDs exhibit an excitation-dependent fluorescence behavior. The most intense fluorescence appears under 340 nm excitation and has a maximum emission peak at 414 nm. The CDs can keep good fluorescence intensity in a wide pH values from 2 to 11. The CDs can be used as highly sensitive and selective detection fluorescent probe of Fe3+, even in real sample analysis the CDs fluorescent probes can be used. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Fe3+ is an essential trace element in human body and plays an important role in many biological processes, for instance, cellular metabolism, enzyme catalysis, and electron-transfer processes, DNA and RNA synthesis [1–3]. Therefore, Fe3+ is indispensable for most organisms and both its excess and deficiency will result in various pathological disorders with iron transport, storage and balance [4,5]. Thus, the diversity of Fe3+ function, both beneficial and otherwise, makes the detection of Fe3+ important. During the last few years analytical techniques like colorimetry, atomic absorption spectroscopy, spectrophotometry, and voltammetry have been used for both qualitative and quantitative detection of Fe3+ ions [6–9]. But these techniques require sophisticated equipments, tedious sample preparation procedures, time-consuming processes, high cost and trained analysts [10]. The development of highly efficient detection of Fe3+ by a simple method is truly meaningful. Recently, carbon quantum dots (CDs) have received much attention due to its advantages such as low toxicity, good stability, biocompatibility, high resistance to photobleaching, low cost and environmental friendliness [11–13]. Compared with traditional semiconductor quantum dots (e.g. CdS, CdTe) [14,15], CDs not only show biocompatibility and avoid the environmental and health concerns arising from the toxicity of heavy metal ions, but they also exhibit high sensitivity and selectivity of some metal ions (e.g. Cu2+ [16,17], Fe3+ [10,18,19], Hg2+ [20], Ag+ [21]. So despite a ⇑ Corresponding authors. E-mail addresses: [email protected] (X. Shang), [email protected] (L. Zhou). https://doi.org/10.1016/j.cplett.2018.05.048 0009-2614/Ó 2018 Elsevier B.V. All rights reserved.
short history, CDs have revealed a cornucopia of both novel photophysical properties and potential applications. CDs bearing carbon and oxygen containing groups are biocompatible, and thus they are regarded as potential candidates for biochemical analysis and tissue engineering. In this paper, we report a facile, economic and green synthesis of CDs as a fluorescent probe for the highly sensitive and selective detection of Fe3+. The CDs prepared based on carbonization of citric acid and tris(hydroxymethyl)aminomethane (Tris) are monodisperse with the average particle size of 3.65 nm. Using quinine sulfate (54%) as reference the relative fluorescence quantum yield (QY) of the obtained CDs was estimated to be 12.54%. The CDs displayed highly sensitive and selective detection of Fe3+, even in real sample analysis the CDs fluorescent probes can be used. The sensing mechanism for the fluorescence quenching can be attributed to the electron transfer between the CDs and Fe3+ and forming nonradiative recombination, thus resulting in fluorescence quenching.
2. Experimental section 2.1. Materials All chemical reagents were purchased from commercial suppliers and were used as received without further purification unless otherwise noted. Citric acid monohydrate was purchased from Beijing Chemical Works. Tris(hydroxymethyl)aminomethane (Tris) and glycerinum were obtained from Aladdin Industrial Corporation. Deionized (DI)-water was supplied by Changchun Institute of Applied Chemistry Chinese Academy of Chinese.
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Y. Li et al. / Chemical Physics Letters 705 (2018) 1–6
Scheme 1. Schematic representation of the formation of CDs.
2.2. Preparation of CDs Tris (0.5 g) were dissolved in 15 mL glycerinum and then the solution was transferred into a three-neck flask and heated to 176 °C. Then, 5 mL glycerinum with 1.0 g citric acid dissolved was quickly injected into the three-neck flask and maintained at 176 °C for 3 h. During the experimental processes, argon gas was straightly passed. After the reaction, the reactor was naturally cooled to room temperature. To purify the as-prepared CDs and remove the excess molecular precursors, the crude product was subjected to dialysis against de-ionized water using dialysis bag of molecular weight cut-off (MWCO) 500.
various ion sources. All chemicals used were analytical graded reagents. The ion solution (0.01 mol/L, 1 mL) was added into the CDs solution (160 lL original fluid and 10 mL H2O) and detected
2.3. Metal ion detection For the detection of various metal ions, FeCl3, NaCl, KCl, MgCl6H2O, Zn(CH3COO)2, Mn(CH3COO)2, Cd(CH3COO)2, Pb(CH3 COO)2, CoCl26H2O, FeCl2H2O and CuCl22H2O have been used as
Fig. 1. (a) Photoluminescence and absorbance of the Quinine sulfate (used as a reference). (The slope is 5.50 * 106.) (b) Photoluminescence and absorbance of the CDs. (The slope is 1.28 * 106.)
Fig. 2. (a) TEM image of the as-prepared CDs. (b) XRD image of the as-prepared CDs. (c) FTIR image of the as-prepared CDs.
Y. Li et al. / Chemical Physics Letters 705 (2018) 1–6
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fluorescence emission spectra. Mixing 160 lL original fluid and 10 mL H2O and detecting its fluorescence emission spectra offered contrast. The PL spectra were recorded after reaction for 2 min. All the PL spectra were recorded at the excitation wavelength of 340 nm.
of solvent. The subscripts ‘x’ and ‘Ref’ refer to the tested sample and reference fluorescence dye, respectively. To minimize the reabsorption effects, the absorbance of the as-prepared CDs aqueous and reference fluorescence at excitation wavelength were kept under 0.1. For these aqueous solutions, gx/gRef = 1.
2.4. Characterization
3. Results and discussion
Transmission electron microscope (TEM) images were taken from FEI Tecnai G2 F20 microscope. X-ray diffraction (XRD) measurement was performed on a Bruker D8 Focus powder diffractometer with Cu Ka radiation. Fourier transform infrared (FT-IR) spectra were recorded on a VERTEX 70 FT-IR spectrometer. The absorbance spectra were collected on a Shimadzu UV-3600 UV– Vis spectrometer. Fluorescence emission spectra were collected on a Hitachi F-4500 fluorescence spectrophotometer.
where / is the fluorescence quantum yield, I is the integrated emission intensity, A is the optical density, and g is the refractive index
In this work, the CDs were synthesized by carbonization of citric acid and Tris at 176 °C in the medium of glycerinum (Scheme 1). Citric acid is an important starting material for preparation of CDs because it has abundant active groups (ACOOH and AOH), which not only benefit for water-solubility but also provide many reactivity sites [22–24]. Doping or surface passivation will introduce different surface functional groups which would create new surface electronic energy levels and various characteristics. Meanwhile, the Tris with both abundant hydrophilic groups (AOH) was chosen as nitrogen-doping agent to increase the fluorescence quantum yield (QY) and to modify the photoelectric properties [25]. The as-prepared CDs aqueous solution was collected from the sample in the dialysis tube. The relative fluorescence QY of the obtained CDs was calculated to be 12.54% by using quinine sulfate (54%) as reference (Fig. 1). TEM image of the obtained CDs are shown in Fig. 2(a). It can be clearly identified that the obtained CDs are well-dispersion without aggregation. The partical size distribution shows that the diameters of the CDs are within the range of 1.76–5.72 nm, with an average value of 3.65 nm. The XRD pattern of the obtained CDs in Fig. 2(b) exhibits a broad peak centered at 2h = 22.47°, due to the highly disorder graphitic structure [26].
Fig. 3. The optical properties of the as-prepared CDs. (a) UV–Vis absorption spectra of the as-prepared CDs. (b) Fluorescence emission spectra of the as-prepared CDs.
Fig. 4. (a) Fluorescence emission spectra of the as-prepared CDs of different pH value. (b) The effect of pH on the fluorescence intensity of CDs. All experimental results were obtained based on three independent measurements.
2.5. Fluorescence QY measurement Fluorescence QYs of the as-prepared CDs were determined using quinine sulfate in 0.1 M H2SO4 as a reference (QY = 54% at 350 nm excitation) The QY of a sample was calculated according to the following equation:
/x ¼ /Ref
Ix
IRef
ARef Ax
g2x g2Ref
! ð1Þ
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Y. Li et al. / Chemical Physics Letters 705 (2018) 1–6
Fig. 2(c) has given the FT-IR spectra used to identify the chemical composition and surface groups on the obtained CDs. It can be observed that the obtained CDs and precursors have obvious distinct FTIR spectra. The broad absorption band at 3300 and 1660 cm1 are consistent with the stretching vibration of OAH and C@O groups. In addition, an obvious feature absorption peak at 1058 cm1 is associated with CAO stretching vibration revealing the existence of abundant CAOH groups on the surface of obtained CDs. The absorption peak at 1570 cm1 is assigned to NAH vibration, while the stretching vibration of CH3 and CH2 are observed at 2946 and 2886 cm1, respectively. This result indicates that
Fig. 5. (a) Fluorescence emission spectra of the CDs in the presence of different concentration Fe3+. (b) The relationship between [(F0 F)/F0] and the concentration of Fe3+. F0 and F are fluorescence intensities without and with Fe3+, respectively. The inset shows a linearity relationship in the concentration range from 0.05 to 3.1 mM. (c) Rapid diagnosis strip using CDs. Left: Rapid diagnosis strip using CDs without Fe3+. Right: Rapid diagnosis strip using CDs with Fe3+.
there are carboxyl and hydroxy groups on the surface of CDs, which derives from the precursors of citric acid and Tris. Fig. 3 displayed the optical properties of obtained CDs. Fig. 3(a) gave the UV–vis absorption spectra of the CDs, obviously, wavelength of absorption onset at 400 nm was found, and its absorption peaked at 290 nm. Fig. 3(b) displayed the fluorescence emission spectra for the CDs at different excitation wavelengths, it was found that the obtained CDs exhibit an excitation-dependent fluorescence behavior. With the increasing excitation wavelength from 300 to 380 nm, the fluorescence peak shifts gradually from 408 to 431 nm. The most intense fluorescence appears under 340 nm excitation and has a maximum emission peak at 414 nm. This excitation-dependent fluorescent behavior was extensively reported in fluorescent CDs, and it may result from optical selection of differently sized CDs, different surface defects and different surface states of CDs. Then, we tested the fluorescence stability of the obtained CDs in different pH values from 1 to 12. As shown in Fig. 4, the change of the fluorescence intensity is small from pH of 2 to 11, only strong acid and alkali (pH = 1 or 12) lead to strong quenching of fluorescence. This result indicates that the CDs can keep good fluorescence intensity in a wide pH values. From Fig. 4 (a) we can see that in the acidic conditions, fluorescence spectrum generated red shift. Next, we have also investigated the sensing performance via the titration of CDs (160 lL original fluid and 10 mL phosphatic buffer solution (pH = 7)) with different concentrations of Fe3+. As shown in Fig. 5(a), the PL intensity of the obtained CDs decreased gradually with the increasing concentration of Fe3+. Because pH of phosphatic buffer solution was relatively steady, so the emission peak kept stability. When the concentration of Fe3+ was 13 mM, there was little PL intensity. Fig. 5(b) shows the dependence of (F0 F)/F0 on the concentration of Fe3+, where F and F0 stand for the PL intensities at 414 nm in the presence and absence of Fe3+, respectively. Importantly, a good linear correlation (R2 = 0.99648) with the regression equation of y = 0.02163 + 0.10495x is observed over the concentration range of 0.05–3.1 mM, where x and y denote the concentration of Fe3+ and (F0 F)/F0, respectively. Fig. 5(c) is the rapid diagnosis strip using CDs. Under ultraviolet light we can see blue light from the filter paper which had adsorbed CDs and dried naturally. After the filter paper immersing into Fe3+ solution (0.01 mol/L), the fluorescence was quenched. So the CDs can be used as the rapid diagnosis strip which can detect Fe3+ simply and rapidly. In addition to the high sensitivity for Fe3+, the response of the assay toward other different metal ions was also taken into
Fig. 6. The different PL intensity ratios (F/F0) of the CDs solutions in the presence and absence of various metal ions. All experimental results were obtained based on three independent measurements. F0 and F are fluorescence intensities without and with various metal ions, respectively.
Y. Li et al. / Chemical Physics Letters 705 (2018) 1–6
Fig. 7. (a) Fluorescence emission spectra of the CDs in multi-ion-mixed solution with different concentration Fe3+. The inset shows a linearity relationship in the concentration range from 0.2 to 2.6 mM. (b) Fluorescence emission spectra of the CDs in the water of South Lake with different concentration Fe3+. The inset shows a linearity relationship in the concentration range from 0.2 to 2.6 mM.
account. Fig. 6 given the PL fluorescence in the absence and presence of various metal ions, including Cd2+, Cu2+, Fe3+, K+, Co2+, Pb2+, Fe2+, Mg2+, Mn2+, Na+ and Zn2+. As shown in Fig. 6, Fe3+ shows the lowest PL fluorescence, and other ions show slight changes of fluorescence intensity compared with no metal ion, which indicated that Fe3+ had the obvious quenching effect on the PL intensity. In order to test the performance of the CDs in detecting Fe3+, the interference test was also carried out. In the presence of
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multi-ion-mixed solution (Cd2+, Cu2+, K+, Co2+, Pb2+, Mg2+, Mn2+, Na+ and Zn2+), Fe3+ was added into the CDs gradually. The concentration of various metal ions in multi-ion-mixed solution was 0.001 mol/L. Mixing 160 lL CDs original fluid, 9 mL H2O and 1 mL multi-ion-mixed solution serve as contrast. From Fig. 7(a) we can see the fluorescence intensity decreased gradually with the increasing concentration of Fe3+. As shown in the inset of Fig. 7 (a), there are a good linear correlation (R2 = 0.99663) between the concentration of Fe3+ and (F0 F)/F0, and the regression equation is y = 0.00826 + 0.19437x over the concentration range of 0.2– 2.6 mM, where x and y denote the concentration of Fe3+ and (F0 F)/F0, respectively. This result indicated that the obtained CDs possess high selectivity and specificity for Fe3+, and the other metal ions show small influence on the sensing system. To demonstrate the performance of the fluorescent probes in real sample analysis, we took the water from the South Lake (Jilin Province, China) and demonstrate the detection of Fe3+ in this environment. The fluorescence intensity decreased gradually when concentration of Fe3+ increased from 0.2 mM to 9.6 mM (Fig. 6(b)). The linear relationship is y = 0.04964 + 0.08089x between the concentration of Fe3+ and (F0 F)/F0. The linear range is 0.2–2.6 mM, and R2 is 0.93865. Although there are many other ions and different microorganisms, the CDs showed favourable quenching performance for Fe3+. Therefore, these result indicated that the CDs fluorescent probes can be used in real sample analysis. Because of the hydrolysis reaction of Fe3+, the acidity of this solution was enhanced gradually, which slightly makes the emission peak red shift. Then, the fluorescence quenching mechanism introduced by Fe3+ was discussed. When there is no Fe3+, carbon quantum dots absorb the energy of the incident light and electrons on the ground state transition to the excited state; when the electrons on the excited state fall back to the ground state, electrons and holes form radiative recombination and emit fluorescence. As shown in Scheme 2, the obtained CDs have a low electronegativity, because there are plentiful carboxyl and hydroxy groups on the surfaces of the CDs. Meanwhile, outer electron structure of Fe3+ was 3d5, and five d orbits were half-full; so electrons on the excited state of CDs are prone to transition to the d orbit of Fe3+ and form nonradiative recombination, thus result in fluorescence quenching.
4. Conclusions In conclusion, CDs with uniform size have been successfully prepared by a green and facil method using citric acid and Tris as source. The emission spectra of the obtained CDs exhibited an excitation-dependent fluorescence behavior with broad range from 408 to 431 nm. The obtained CDs displayed highly sensitive and
Scheme 2. Schematic representation of fluorescent CDs for detection of Fe3+.
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selective detection of Fe3+, and the degree of fluorescence quenching and concentration of Fe3+ are a good linear correlation when the concentration range of Fe3+ was 0.05–3.1 mM. This may be attributed to electron transfer between the CDs and Fe3+ and forming nonradiative recombination, thus resulting in fluorescence quenching. This fluorescent probes have potential application in the detecting of Fe3+. Acknowledgements The authors are grateful to the financial aid from the Science and Technology Research Project for the Thirteenth Five-year Plan of Education Department of Jilin Province of China (Grant No. JJKH20181021KJ), Research Equipment Development project of Chinese Academy of Sciences (Grant No. YZ201562), Youth Innovation Promotion Association CAS (Grant No. 2013150), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), National Natural Science Foundation of China (Grant Nos. 51502285, 21521092, 21590794 and 21210001), and National Key Basic Research Program of China (Grant Nos. 2014CB643802 and 2013CB922101). References [1] F.A. Tezcan, J.T. Kaiser, D. Mustafi, M.Y. Walton, J.B. Howard, D.C. Rees, Science 309 (2005) 1377. [2] G. Li, N. Lv, W. Bi, J. Zhang, J. Ni, New J. Chem. 40 (2016) 10213.
[3] S.H. Yang, X.H. Sun, Z.Y. Wang, X.Y. Wang, G.S. Guo, Q.S. Pu, Nano Res. 11 (2018) 1369. [4] D. Touati, Arch. Biochem. Biophys. 373 (2000) 1. [5] E. Beutler, V. Felitti, T. Gelbart, N. Ho, Drug Metab. Dispos. 29 (2001) 495. [6] Z. Liang, C. Wang, J. Yang, H. Gao, Y. Tiang, X. Tao, M. Jiang, New J. Chem. 31 (2007) 906. [7] A. Ohashi, H. Ito, C. Kanai, H. Imura, K. Ohashi, Talanta 65 (2005) 525. [8] D.M.C. Gomes, M.A. Segundo, J.L.F.C. Lima, A.O.S.S. Rangel, Talanta 66 (2005) 703. [9] A. Bobrowski, K. Nowak, J. Zarebski, Anal. Bioanal. Chem. 382 (2005) 1691. [10] N.R. Chereddy, S. Thennarasu, A.B. Mandal, Analyst 138 (2013) 1334. [11] Y.B. Liu, L. Zhou, Y.N. Li, R.P. Deng, H.J. Zhang, Nanoscale 9 (2017) 491. [12] R. Singh, S. Kashayap, V. Singh, A.M. Kayastha, H. Mishra, P.S. Saxena, A. Srivastava, R.K. Singh, Biosens. Bioelectron. 101 (2018) 103. [13] X. Liu, J. Li, Y. Huang, X. Wang, X. Zhang, X. Wang, Environ. Sci. Technol. 51 (2017) 6156. [14] M. Muruganandham, Y. Kusumoto, C. Okamoto, A. Muruganandham, Md. Abdulla-Al-Mamun, B. Ahmmad, J. Phys. Chem. C 113 (2009) 19506. [15] Y. Li, L. Jing, R. Qiao, M. Gao, Chem. Commun. 47 (2011) 9293. [16] S. Liu, J. Tian, L. Wang, Y. Zhang, X. Qin, Y. Luo, A.M. Asiri, A.O. Al-Youbi, X. Sun, Adv. Mater. 24 (2012) 2037. [17] Y. Dong, R. Wang, G. Li, C. Chen, Y. Chi, G. Chen, Anal. Chem. 84 (2012) 6220. [18] S. Qu, H. Chen, X. Zheng, J. Cao, X. Liu, Nanoscale 5 (2013) 5514. [19] S. Zhang, J. Li, M. Zeng, J. Xu, X. Wang, W. Hu, Nanoscale 6 (2014) 4157. [20] H. Li, J. Zhai, J. Tian, Y. Luo, X. Sun, Biosens. Bioelectron. 26 (2011) 4656. [21] H. Li, J. Zhai, X. Sun, Langmuir 27 (2011) 4305. [22] Y. Song, S. Zhu, S. Zhang, Y. Fu, L. Wang, X. Zhao, B. Yang, J. Mater. Chem. C 3 (2015) 5976. _ [23] W. Kasprzyk, S. Bednarz, P. Zmudzki, M. Galica, D. Bogdał, RSC Adv. 5 (2015) 34795. [24] X. Zhai, P. Zhang, Ch. Liu, T. Bai, W. Li, L. Dai, W. Liu, Chem. Commun. 48 (2012) 7955. [25] Y.A. Wang, J.J. Li, H. Chen, X. Peng, J. Am. Chem. Soc. 124 (2002) 2293. [26] J. Hou, W. Wang, T. Zhou, B. Wang, H. Li, L. Ding, Nanoscale 8 (2016) 11185.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Highly sensitive and selective detection of Fe3+ by utilizing carbon quantum dots as fluorescent probes Yanan Li a,b, Yingbo Liu b, Xiaohong Shang a,⇑, Daiyong Chao b, Liang Zhou b,⇑, Hongjie Zhang b a b
College of Chemistry and Life Science, Changchun University of Technology, Changchun 130012, People’s Republic of China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 17 March 2018 In final form 21 May 2018 Available online 22 May 2018 Keywords: Carbon quantum dots Fluorescent probe Selective detection Fe3+
a b s t r a c t The carbon quantum dots (CDs) based on carbonization of citric acid and tris(hydroxymethyl)aminome thane (Tris) in glycerinum as fluorescent probes for highly sensitive and selective detection of Fe3+ have been developed. The obtained CDs exhibit an excitation-dependent fluorescence behavior. The most intense fluorescence appears under 340 nm excitation and has a maximum emission peak at 414 nm. The CDs can keep good fluorescence intensity in a wide pH values from 2 to 11. The CDs can be used as highly sensitive and selective detection fluorescent probe of Fe3+, even in real sample analysis the CDs fluorescent probes can be used. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Fe3+ is an essential trace element in human body and plays an important role in many biological processes, for instance, cellular metabolism, enzyme catalysis, and electron-transfer processes, DNA and RNA synthesis [1–3]. Therefore, Fe3+ is indispensable for most organisms and both its excess and deficiency will result in various pathological disorders with iron transport, storage and balance [4,5]. Thus, the diversity of Fe3+ function, both beneficial and otherwise, makes the detection of Fe3+ important. During the last few years analytical techniques like colorimetry, atomic absorption spectroscopy, spectrophotometry, and voltammetry have been used for both qualitative and quantitative detection of Fe3+ ions [6–9]. But these techniques require sophisticated equipments, tedious sample preparation procedures, time-consuming processes, high cost and trained analysts [10]. The development of highly efficient detection of Fe3+ by a simple method is truly meaningful. Recently, carbon quantum dots (CDs) have received much attention due to its advantages such as low toxicity, good stability, biocompatibility, high resistance to photobleaching, low cost and environmental friendliness [11–13]. Compared with traditional semiconductor quantum dots (e.g. CdS, CdTe) [14,15], CDs not only show biocompatibility and avoid the environmental and health concerns arising from the toxicity of heavy metal ions, but they also exhibit high sensitivity and selectivity of some metal ions (e.g. Cu2+ [16,17], Fe3+ [10,18,19], Hg2+ [20], Ag+ [21]. So despite a ⇑ Corresponding authors. E-mail addresses: [email protected] (X. Shang), [email protected] (L. Zhou). https://doi.org/10.1016/j.cplett.2018.05.048 0009-2614/Ó 2018 Elsevier B.V. All rights reserved.
short history, CDs have revealed a cornucopia of both novel photophysical properties and potential applications. CDs bearing carbon and oxygen containing groups are biocompatible, and thus they are regarded as potential candidates for biochemical analysis and tissue engineering. In this paper, we report a facile, economic and green synthesis of CDs as a fluorescent probe for the highly sensitive and selective detection of Fe3+. The CDs prepared based on carbonization of citric acid and tris(hydroxymethyl)aminomethane (Tris) are monodisperse with the average particle size of 3.65 nm. Using quinine sulfate (54%) as reference the relative fluorescence quantum yield (QY) of the obtained CDs was estimated to be 12.54%. The CDs displayed highly sensitive and selective detection of Fe3+, even in real sample analysis the CDs fluorescent probes can be used. The sensing mechanism for the fluorescence quenching can be attributed to the electron transfer between the CDs and Fe3+ and forming nonradiative recombination, thus resulting in fluorescence quenching.
2. Experimental section 2.1. Materials All chemical reagents were purchased from commercial suppliers and were used as received without further purification unless otherwise noted. Citric acid monohydrate was purchased from Beijing Chemical Works. Tris(hydroxymethyl)aminomethane (Tris) and glycerinum were obtained from Aladdin Industrial Corporation. Deionized (DI)-water was supplied by Changchun Institute of Applied Chemistry Chinese Academy of Chinese.
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Y. Li et al. / Chemical Physics Letters 705 (2018) 1–6
Scheme 1. Schematic representation of the formation of CDs.
2.2. Preparation of CDs Tris (0.5 g) were dissolved in 15 mL glycerinum and then the solution was transferred into a three-neck flask and heated to 176 °C. Then, 5 mL glycerinum with 1.0 g citric acid dissolved was quickly injected into the three-neck flask and maintained at 176 °C for 3 h. During the experimental processes, argon gas was straightly passed. After the reaction, the reactor was naturally cooled to room temperature. To purify the as-prepared CDs and remove the excess molecular precursors, the crude product was subjected to dialysis against de-ionized water using dialysis bag of molecular weight cut-off (MWCO) 500.
various ion sources. All chemicals used were analytical graded reagents. The ion solution (0.01 mol/L, 1 mL) was added into the CDs solution (160 lL original fluid and 10 mL H2O) and detected
2.3. Metal ion detection For the detection of various metal ions, FeCl3, NaCl, KCl, MgCl6H2O, Zn(CH3COO)2, Mn(CH3COO)2, Cd(CH3COO)2, Pb(CH3 COO)2, CoCl26H2O, FeCl2H2O and CuCl22H2O have been used as
Fig. 1. (a) Photoluminescence and absorbance of the Quinine sulfate (used as a reference). (The slope is 5.50 * 106.) (b) Photoluminescence and absorbance of the CDs. (The slope is 1.28 * 106.)
Fig. 2. (a) TEM image of the as-prepared CDs. (b) XRD image of the as-prepared CDs. (c) FTIR image of the as-prepared CDs.
Y. Li et al. / Chemical Physics Letters 705 (2018) 1–6
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fluorescence emission spectra. Mixing 160 lL original fluid and 10 mL H2O and detecting its fluorescence emission spectra offered contrast. The PL spectra were recorded after reaction for 2 min. All the PL spectra were recorded at the excitation wavelength of 340 nm.
of solvent. The subscripts ‘x’ and ‘Ref’ refer to the tested sample and reference fluorescence dye, respectively. To minimize the reabsorption effects, the absorbance of the as-prepared CDs aqueous and reference fluorescence at excitation wavelength were kept under 0.1. For these aqueous solutions, gx/gRef = 1.
2.4. Characterization
3. Results and discussion
Transmission electron microscope (TEM) images were taken from FEI Tecnai G2 F20 microscope. X-ray diffraction (XRD) measurement was performed on a Bruker D8 Focus powder diffractometer with Cu Ka radiation. Fourier transform infrared (FT-IR) spectra were recorded on a VERTEX 70 FT-IR spectrometer. The absorbance spectra were collected on a Shimadzu UV-3600 UV– Vis spectrometer. Fluorescence emission spectra were collected on a Hitachi F-4500 fluorescence spectrophotometer.
where / is the fluorescence quantum yield, I is the integrated emission intensity, A is the optical density, and g is the refractive index
In this work, the CDs were synthesized by carbonization of citric acid and Tris at 176 °C in the medium of glycerinum (Scheme 1). Citric acid is an important starting material for preparation of CDs because it has abundant active groups (ACOOH and AOH), which not only benefit for water-solubility but also provide many reactivity sites [22–24]. Doping or surface passivation will introduce different surface functional groups which would create new surface electronic energy levels and various characteristics. Meanwhile, the Tris with both abundant hydrophilic groups (AOH) was chosen as nitrogen-doping agent to increase the fluorescence quantum yield (QY) and to modify the photoelectric properties [25]. The as-prepared CDs aqueous solution was collected from the sample in the dialysis tube. The relative fluorescence QY of the obtained CDs was calculated to be 12.54% by using quinine sulfate (54%) as reference (Fig. 1). TEM image of the obtained CDs are shown in Fig. 2(a). It can be clearly identified that the obtained CDs are well-dispersion without aggregation. The partical size distribution shows that the diameters of the CDs are within the range of 1.76–5.72 nm, with an average value of 3.65 nm. The XRD pattern of the obtained CDs in Fig. 2(b) exhibits a broad peak centered at 2h = 22.47°, due to the highly disorder graphitic structure [26].
Fig. 3. The optical properties of the as-prepared CDs. (a) UV–Vis absorption spectra of the as-prepared CDs. (b) Fluorescence emission spectra of the as-prepared CDs.
Fig. 4. (a) Fluorescence emission spectra of the as-prepared CDs of different pH value. (b) The effect of pH on the fluorescence intensity of CDs. All experimental results were obtained based on three independent measurements.
2.5. Fluorescence QY measurement Fluorescence QYs of the as-prepared CDs were determined using quinine sulfate in 0.1 M H2SO4 as a reference (QY = 54% at 350 nm excitation) The QY of a sample was calculated according to the following equation:
/x ¼ /Ref
Ix
IRef
ARef Ax
g2x g2Ref
! ð1Þ
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Y. Li et al. / Chemical Physics Letters 705 (2018) 1–6
Fig. 2(c) has given the FT-IR spectra used to identify the chemical composition and surface groups on the obtained CDs. It can be observed that the obtained CDs and precursors have obvious distinct FTIR spectra. The broad absorption band at 3300 and 1660 cm1 are consistent with the stretching vibration of OAH and C@O groups. In addition, an obvious feature absorption peak at 1058 cm1 is associated with CAO stretching vibration revealing the existence of abundant CAOH groups on the surface of obtained CDs. The absorption peak at 1570 cm1 is assigned to NAH vibration, while the stretching vibration of CH3 and CH2 are observed at 2946 and 2886 cm1, respectively. This result indicates that
Fig. 5. (a) Fluorescence emission spectra of the CDs in the presence of different concentration Fe3+. (b) The relationship between [(F0 F)/F0] and the concentration of Fe3+. F0 and F are fluorescence intensities without and with Fe3+, respectively. The inset shows a linearity relationship in the concentration range from 0.05 to 3.1 mM. (c) Rapid diagnosis strip using CDs. Left: Rapid diagnosis strip using CDs without Fe3+. Right: Rapid diagnosis strip using CDs with Fe3+.
there are carboxyl and hydroxy groups on the surface of CDs, which derives from the precursors of citric acid and Tris. Fig. 3 displayed the optical properties of obtained CDs. Fig. 3(a) gave the UV–vis absorption spectra of the CDs, obviously, wavelength of absorption onset at 400 nm was found, and its absorption peaked at 290 nm. Fig. 3(b) displayed the fluorescence emission spectra for the CDs at different excitation wavelengths, it was found that the obtained CDs exhibit an excitation-dependent fluorescence behavior. With the increasing excitation wavelength from 300 to 380 nm, the fluorescence peak shifts gradually from 408 to 431 nm. The most intense fluorescence appears under 340 nm excitation and has a maximum emission peak at 414 nm. This excitation-dependent fluorescent behavior was extensively reported in fluorescent CDs, and it may result from optical selection of differently sized CDs, different surface defects and different surface states of CDs. Then, we tested the fluorescence stability of the obtained CDs in different pH values from 1 to 12. As shown in Fig. 4, the change of the fluorescence intensity is small from pH of 2 to 11, only strong acid and alkali (pH = 1 or 12) lead to strong quenching of fluorescence. This result indicates that the CDs can keep good fluorescence intensity in a wide pH values. From Fig. 4 (a) we can see that in the acidic conditions, fluorescence spectrum generated red shift. Next, we have also investigated the sensing performance via the titration of CDs (160 lL original fluid and 10 mL phosphatic buffer solution (pH = 7)) with different concentrations of Fe3+. As shown in Fig. 5(a), the PL intensity of the obtained CDs decreased gradually with the increasing concentration of Fe3+. Because pH of phosphatic buffer solution was relatively steady, so the emission peak kept stability. When the concentration of Fe3+ was 13 mM, there was little PL intensity. Fig. 5(b) shows the dependence of (F0 F)/F0 on the concentration of Fe3+, where F and F0 stand for the PL intensities at 414 nm in the presence and absence of Fe3+, respectively. Importantly, a good linear correlation (R2 = 0.99648) with the regression equation of y = 0.02163 + 0.10495x is observed over the concentration range of 0.05–3.1 mM, where x and y denote the concentration of Fe3+ and (F0 F)/F0, respectively. Fig. 5(c) is the rapid diagnosis strip using CDs. Under ultraviolet light we can see blue light from the filter paper which had adsorbed CDs and dried naturally. After the filter paper immersing into Fe3+ solution (0.01 mol/L), the fluorescence was quenched. So the CDs can be used as the rapid diagnosis strip which can detect Fe3+ simply and rapidly. In addition to the high sensitivity for Fe3+, the response of the assay toward other different metal ions was also taken into
Fig. 6. The different PL intensity ratios (F/F0) of the CDs solutions in the presence and absence of various metal ions. All experimental results were obtained based on three independent measurements. F0 and F are fluorescence intensities without and with various metal ions, respectively.
Y. Li et al. / Chemical Physics Letters 705 (2018) 1–6
Fig. 7. (a) Fluorescence emission spectra of the CDs in multi-ion-mixed solution with different concentration Fe3+. The inset shows a linearity relationship in the concentration range from 0.2 to 2.6 mM. (b) Fluorescence emission spectra of the CDs in the water of South Lake with different concentration Fe3+. The inset shows a linearity relationship in the concentration range from 0.2 to 2.6 mM.
account. Fig. 6 given the PL fluorescence in the absence and presence of various metal ions, including Cd2+, Cu2+, Fe3+, K+, Co2+, Pb2+, Fe2+, Mg2+, Mn2+, Na+ and Zn2+. As shown in Fig. 6, Fe3+ shows the lowest PL fluorescence, and other ions show slight changes of fluorescence intensity compared with no metal ion, which indicated that Fe3+ had the obvious quenching effect on the PL intensity. In order to test the performance of the CDs in detecting Fe3+, the interference test was also carried out. In the presence of
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multi-ion-mixed solution (Cd2+, Cu2+, K+, Co2+, Pb2+, Mg2+, Mn2+, Na+ and Zn2+), Fe3+ was added into the CDs gradually. The concentration of various metal ions in multi-ion-mixed solution was 0.001 mol/L. Mixing 160 lL CDs original fluid, 9 mL H2O and 1 mL multi-ion-mixed solution serve as contrast. From Fig. 7(a) we can see the fluorescence intensity decreased gradually with the increasing concentration of Fe3+. As shown in the inset of Fig. 7 (a), there are a good linear correlation (R2 = 0.99663) between the concentration of Fe3+ and (F0 F)/F0, and the regression equation is y = 0.00826 + 0.19437x over the concentration range of 0.2– 2.6 mM, where x and y denote the concentration of Fe3+ and (F0 F)/F0, respectively. This result indicated that the obtained CDs possess high selectivity and specificity for Fe3+, and the other metal ions show small influence on the sensing system. To demonstrate the performance of the fluorescent probes in real sample analysis, we took the water from the South Lake (Jilin Province, China) and demonstrate the detection of Fe3+ in this environment. The fluorescence intensity decreased gradually when concentration of Fe3+ increased from 0.2 mM to 9.6 mM (Fig. 6(b)). The linear relationship is y = 0.04964 + 0.08089x between the concentration of Fe3+ and (F0 F)/F0. The linear range is 0.2–2.6 mM, and R2 is 0.93865. Although there are many other ions and different microorganisms, the CDs showed favourable quenching performance for Fe3+. Therefore, these result indicated that the CDs fluorescent probes can be used in real sample analysis. Because of the hydrolysis reaction of Fe3+, the acidity of this solution was enhanced gradually, which slightly makes the emission peak red shift. Then, the fluorescence quenching mechanism introduced by Fe3+ was discussed. When there is no Fe3+, carbon quantum dots absorb the energy of the incident light and electrons on the ground state transition to the excited state; when the electrons on the excited state fall back to the ground state, electrons and holes form radiative recombination and emit fluorescence. As shown in Scheme 2, the obtained CDs have a low electronegativity, because there are plentiful carboxyl and hydroxy groups on the surfaces of the CDs. Meanwhile, outer electron structure of Fe3+ was 3d5, and five d orbits were half-full; so electrons on the excited state of CDs are prone to transition to the d orbit of Fe3+ and form nonradiative recombination, thus result in fluorescence quenching.
4. Conclusions In conclusion, CDs with uniform size have been successfully prepared by a green and facil method using citric acid and Tris as source. The emission spectra of the obtained CDs exhibited an excitation-dependent fluorescence behavior with broad range from 408 to 431 nm. The obtained CDs displayed highly sensitive and
Scheme 2. Schematic representation of fluorescent CDs for detection of Fe3+.
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selective detection of Fe3+, and the degree of fluorescence quenching and concentration of Fe3+ are a good linear correlation when the concentration range of Fe3+ was 0.05–3.1 mM. This may be attributed to electron transfer between the CDs and Fe3+ and forming nonradiative recombination, thus resulting in fluorescence quenching. This fluorescent probes have potential application in the detecting of Fe3+. Acknowledgements The authors are grateful to the financial aid from the Science and Technology Research Project for the Thirteenth Five-year Plan of Education Department of Jilin Province of China (Grant No. JJKH20181021KJ), Research Equipment Development project of Chinese Academy of Sciences (Grant No. YZ201562), Youth Innovation Promotion Association CAS (Grant No. 2013150), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), National Natural Science Foundation of China (Grant Nos. 51502285, 21521092, 21590794 and 21210001), and National Key Basic Research Program of China (Grant Nos. 2014CB643802 and 2013CB922101). References [1] F.A. Tezcan, J.T. Kaiser, D. Mustafi, M.Y. Walton, J.B. Howard, D.C. Rees, Science 309 (2005) 1377. [2] G. Li, N. Lv, W. Bi, J. Zhang, J. Ni, New J. Chem. 40 (2016) 10213.
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