Research article Received: 15 May 2014,
Revised: 11 June 2014,
Accepted: 20 June 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2744
Preparation of carbon quantum dots based on starch and their spectral properties Zhengyu Yan, Juan Shu, Yan Yu, Zhengwei Zhang, Zhen Liu and Jianqiu Chen* ABSTRACT: A simple method for the synthesis of water-soluble carbon quantum dots (CQDs) has been developed based on chemical oxidation of starch. The structures and optical properties of the CQDs were characterized by ultraviolet–visible (UV–Vis) spectroscopy, photoluminescence spectroscopy (PL) and transmission electron microscopy. The CQDs were found to emit bright blue fluorescence and disperse uniformly. The effects of ambient temperature, light and pH on the properties of CQDs were studied. The CQDs exhibited good chemical stability, good photostability and pH sensitivity. Furthermore, the interaction between CQDs and bovine serum albumin (BSA) was investigated. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: starch; carbon quantum dots; chemical oxidation; characterization; spectral properties; BSA
Introduction
Luminescence 2014
Experimental Apparatus and reagents Soluble starch, potassium periodate, hydrochloric acid (36–38%) and acetic acid were all purchased from the Nanjing Chemical Reagent Co.; 30% (w/w) hydrogen peroxide was purchased from Sinopharm Chemical Reagent Co. Ltd. Tris was purchased from Sinopharm Chemical Reagent Co., Ltd. BSA was purchased from Nanjing Zhu Yan Biotechnology Co., Ltd. All reagents used were of analytical grade without further treatment. Water used throughout was ultrapure water. Soluble starch was digested to be CQDs in a high pressure digestion tank. A RF-5301 fluorescence spectrophotometer (Shimadzu Corporation, Japan) and a UV-2100 ultraviolet spectrophotometer (Shimadzu Corporation, Japan) were used to record fluorescence spectra and absorption spectra of CQDs respectively. A JEM-2100 transmission electron microscope (TEM) (HITACHI, Japan) was used to observe the size and morphology
* Correspondence to: J. Chen, Department of Analytical Chemistry, China Pharmaceutical University, Nanjing 210009, China. E-mail: [email protected] Department of Analytical Chemistry, China Pharmaceutical University, Nanjing 210009, China Abbreviations: BSA, bull serum albumin; CQD, carbon quantum dots; TEM, transmission electron microscope.
Copyright © 2014 John Wiley & Sons, Ltd.
1
Carbon quantum dots (CQDs) are a new class of zero-dimensional nanomaterials in the carbon family (1). Carbon is one of the basic elements present in biological organisms, so carbon nanomaterials have generally no or low toxicity to living organisms and low cost. Additionally, CQDs exhibit bio-compatibility, high chemical stability are environmentally friendly, as well as having good photostability and easy bioconjugation (2). More importantly, the CQDs preparation method has many advantages such as the use of simple equipment, mild reaction conditions, low cost, and the possibility of large-scale production. Because of these advantageous features, CQDs have exciting prospects in biomedical and clinical applications (3–7). To date, various methods have been developed to produce CQDs. Based on their synthetic sequence, they can be subdivided into a top-down and a bottom-up approach (8). A top-down approach means that CQDs are etched from a carbon target or peeled from carbon nanoparticles, including by laser ablation (9,10), electrochemical oxidation (11) and arc discharge (12). However, it is difficult to completely break the carbon skeleton into carbon nanoparticles. A bottom-up approach means that burning, heating or a microwave method are used to obtain firstly the carbon precursor, then CQDs are generated by a chemical oxidation method (13), pyrolysis (14), a microwaveassisted method (15), or vector synthesis (16). The chemical oxidation method does not require any special equipment and the carbon sources required are diverse, simple and easy to obtain, so this method is an effective and mass-produced synthesis approach. Candle gray ash (17) and phenolic resin (18) can be used as carbon sources. We prepared CQDs from starch which is an inexpensive carbon source. Water-soluble CQDs prepared by chemical oxidation have excellent fluorescence characteristics, high quantum yield and good stability. Prepared CQDs were characterized and analyzed. The spectral properties of the synthesized CQDs were investigated. Bull serum albumin (BSA), one example of an important carrier protein, which mainly functions to balance osmotic pressure and
contains nutrients, is a significant model protein in biology. It contains 581 amino acid residues, among which there are 35 cysteine residues that form 17 disulfide bonds. Meanwhile, there is a free sulfhydryl at section 34 of the peptide chain (19). Thus, BSA can act easily with small molecules. Interactions between BSA and CQDs were investigated under physiological conditions, constituting a preliminary exploration for the application of CQDs in cell labeling and bio-imaging.
Z. Yan et al.
Figure 1. Processing routing of CQDs (a: starch; b: carbon; c: CQDs).
of CQDs. Elemental analysis was conducted using a Vario EL Cube CHNOS Elemental Analyzer (Elementar, Germany). A rotary evaporator, ultrasonic instrument, digital constant temperature water bath and a pH meter were the common apparatus used. An illumination incubator was used to study the effect of illumination on stability of CQDs.
Preparation of CQDs The preparation process of CQDs is shown in Fig. 1. Firstly, starch was carbonized to prepare the carbon source in a high pressure digestion tank for 4 h at 300 °C. Next, the obtained product was dispersed in mixed oxidant containing 15 ml of distilled water, 15 ml of HAc and 30 ml of H2O2, and sonicated for 15 min at 25 °C and refluxed for 2 h at 120 °C and then filtered. The filtrate was neutralized by NaOH (0.5 mol/L) and concentrated to 10 ml. The concentrated solution was dialyzed in a dialysis bag for 24 h and then evaporated. A light yellow CQD powder was obtained after vacuum drying.
Figure 2. UV absorption (a) and FL spectrum (b) of the CQds in aqueous solution.
Characterization methods The luminescence properties of samples were determined by a fluorescence spectrophotometer and an ultraviolet spectrophotometer. The morphology and size of the samples were observed and determined using a TEM. The fluorescence quantum yield of synthetic CQDs was measured using quinine sulfate as a standard. Figure 3. Transmission electron microscopy micrograph of CQDs.
Results and discussion The choice of synthesis method
Characterization of CQDs A prominent and wide absorption peak (Fig. 2a) was observed in the ultraviolet range and the characteristic absorption peak was observed around 220 nm (20,21). The emission band (Fig. 2b) with high fluorescence intensity was observed around 450 nm with the optimal excitation wavelength at 330 nm, which corresponds to that of CQD’s emitted bright blue fluorescence (Fig. 2c) under a camera obscura ultraviolet lamp. The CQDs were spheroidal particles and dispersed uniformly under the TEM with a diameter of 5–8 nm (Fig. 3). Elemental analysis of the sample conducted by a CHNOS Elemental Analyzer revealed that the composition was C 95.8%, O 3.1% and N 1.1%. The fluorescence quantum yield was calculated according to a method in the literature (22), and was about 11.4%.
wileyonlinelibrary.com/journal/luminescence
The effect of reaction time. Next, 0.2 g of carbon source and 60 ml of mixed oxidant (VH2O:VHAc:VH2O2 = 1:1:2) were mixed. The above mixture was refluxed at 100 °C. Subsequently, 6 ml of the solution was taken to record its fluorescence spectrum at 1, 2, 4, 6, 8, 10, and 12 h. As shown in Fig. 4, the fluorescence intensity was highest at 2 h and weakened with time from 2–12 h. The effect of reaction temperature. The reaction solution was refluxed at 60, 80, 100, 120, and 140 °C for 2 h respectively; 10 ml of the solution was taken out to record its fluorescence spectrum (Fig. 5) showing that fluorescence intensity enhanced and blue shift occurred with temperature increase in the range 60–120 °C. Fluorescence intensity weakened with time when the temperature was higher than 120 °C.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Preparation of carbon quantum dots
Figure 4. Fluorescence spectrum of CQDs prepared at different reaction times.
Figure 6. Fluorescence spectrum of CQD purification.
Figure 7. Influence of pH on CQDs. Figure 5. Fluorescence spectrum of CQDs prepared at different reaction temperatures.
The choice of purification method Prepared CQDs might be not uniform in size due to the various diameters of the carbon source and due to the complex interaction between carbon source and mixed oxidant. Moreover, the reaction solution might contain incompletereacting activated carbon which might weaken the fluorescence intensity and broaden the emission band. Dialysis and extraction were chosen to purify the raw CQDs. Ether, ethyl acetate and n-butyl alcohol were used in the extraction process. As shown in Fig. 6, dialysis was highly efficient but the extraction method did not enhance the fluorescence intensity.
Spectral properties of CQDs
Luminescence 2014
The effect of temperature. Prepared CQDs were treated at 25, 35, 45, 55, 65, or 75 °C and their fluorescence spectra were recorded. As shown in Fig. 8, fluorescence intensity did not alter markedly when the temperature increased from 25 °C to 75 °C. Results showed that temperature had a small effect on fluorescence intensity. So CQDs stored at room temperature had fluorescent stability, which could meet the need for the application of CQDs as biological fluorescent markers in biological imaging and other areas. The effect of illumination. The prepared CQD solution was placed in an illumination incubator (T: 25 °C, illumination intensity: 50.12 μmol/m2 · s). Samples were taken at 0, 2, 4, 6, 8, 10, and 12 h and their fluorescence spectra were recorded. As shown in Fig. 9, the fluorescence intensity did not markedly alter after continuous illumination for 12 h, indicating that the CQDs synthetized had high optical stability.
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
3
The effect of pH value. A specified amount of prepared CQDs powder was dissolved in phosphate-buffered saline (PBS) and the pH value was adjusted to 1.0, 3.0, 5.0, 7.0, 9.0 or 11.0. Their fluorescence spectra were recorded respectively with an excitation wavelength of 330 nm. As shown in Fig. 7, the fluorescence
intensity was highest when the pH value was 7, which has important value for CQDs being used as a fluorescent label in the medical field and with biological cells.
Z. Yan et al. Interaction between BSA and CQDs BSA was dissolved in Tris–HCl buffer (pH 7.4) to simulate physiological conditions; 1 ml of BSA solution was reacted with CQD solution at different volumes in a cuvette and then emission spectra were recorded under 278 nm excitation. As shown in Fig. 10, the fluorescence intensity of BSA decreased with increasing volume of CQD solution, but the maximum emission wavelength remained at 349 nm, indicating that interaction and then energy transfer between BSA and CQDs might occur (23).
Conclusions
Figure 8. Influence of temperature on CQDs.
A simple method for the synthesis of water-soluble CQDs has been developed based on chemical oxidation of starch. The CQDs have small particle diameter and are dispersed uniformly. Additionally, they have excellent properties, such as resistance to photobleaching, high optical stability and biocompatibility. The interaction between BSA and CQDs was studied and it was found that CQDs could quench the fluorescence of BSA. Acknowledgements This work was supported by the National Special Purpose on Public Welfare of Environmental Protection Foundation (200809016), Jiangsu Key Laboratory of Environmental Engineering Open Foundation (KF2012008), Students Research Fund (No. 201210316099) and the Fundamental Research Funds for the Central Universities (JKY2011083).
References
Figure 9. Influence of light on CQDs.
Figure 10. Fluorescence spectra of BSA in the presence of different volumes of 6 3 CQDs (CBSA = 5 × 10 mol/L; CCQDs = (×10 mol/L; increasing volumes 0, 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 μl).
wileyonlinelibrary.com/journal/luminescence
1. Mao XJ, Zheng HZ, Long YJ, Du J, Hao JY, Wang LL, Zhou DB. Study on the fluorescence characteristics of carbon dots. Spectrochim Acta, Part A 2010;75:553–7. 2. Zhao LX, Di F, Wang DB, Guo LH, Yang Y, Wan B, Zhang H. Chemiluminescence of carbon dots under strong alkaline solutions: a novel insight into carbon dot optical properties. Nanoscale 2013;5:2655–8. 3. Cao L, Wang X, Meziani MJ, Lu F, Wang H, Luo PG, Lin Y, Harruff BA, Veca LM, Murray D. Carbon dots for multiphoton bioimaging. J Am Chem Soc 2007;129:11318–9. 4. da Silva J, Goncalves HMR. Analytical and bioanalytical applications of carbon dots. Trends Anal Chem 2011;30:1327–36. 5. Dong Y, Wang R, Li H, Shao J, Chi Y, Lin X, Chen G. Polyaminefunctionalized carbon quantum dots for chemical sensing. Carbon 2012;50:2810–5. 6. Li Q, Ohulchanskyy TY, Liu RL, Koynov K, Wu DQ, Best A, Kumar R, Bonoiu A, Prasad PN. Photoluminescent carbon dots as biocompatible nanoprobes for targeting cancer cells in vitro. J Phys Chem C 2010;114:12062–8. 7. Yang ST, Cao L, Luo PG, Lu F, Wang X, Wang H, Meziani MJ, Liu Y, Qi G, Sun YP. Carbon dots for optical imaging in vivo. J Am Chem Soc 2009;131:11308–9. 8. Li HT, Kang ZH, Liu Y, Lee ST. Carbon nanodots: synthesis, properties and applications. J Mater Chem 2012;22:24230–53. 9. Hu SL, Niu KY, Sun J, Yang J, Zhao NQ, Du XW. One-step synthesis of fluorescent carbon nanoparticles by laser irradiation. J Mater Chem 2009;19:484–8. 10. Sun YP, Zhou B, Lin Y, Wang W, Fernando KS, Pathak P, Meziani MJ, Harruff BA, Wang X, Wang H. Quantum-sized carbon dots for bright and colorful photoluminescence. J Am Chem Soc 2006;128:77567. 11. Zhou J, Booker C, Li R, Zhou X, Sham TK, Sun X, Ding Z. An electrochemical avenue to blue luminescent nanocrystals from multiwalled carbon nanotubes (MWCNTs). J Am Chem Soc 2007;129:744–5. 12. Xu X, Ray R, Gu Y, Ploehn HJ, Gearheart L, Raker K, Scrivens WA. Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments. J Am Chem Soc 2004;126:12736–7.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Preparation of carbon quantum dots 13. Zhang J, Shen W, Pan D, Zhang Z, Fang Y, Wu M. Controlled synthesis of green and blue luminescent carbon nanoparticles with high yields by the carbonization of sucrose. New J Chem 2010;34:591–3. 14. Bourlinos AB, Stassinopoulos A, Anglos D, Zboril R, Karakassides M, Giannelis EP. Surface functionalized carbogenic quantum dots. Small 2008;4:455–8. 15. Zhu H, Wang X, Li Y, Wang Z, Yang F, Yang X. Microwave synthesis of fluorescent carbon nanoparticles with electrochemiluminescence properties. Chem Commun 2009;4:5118–20. 16. Bourlinos AB, Stassinopoulos A, Anglos D, Zboril R, Georgakilas V, Giannelis EP. Photoluminescent carbogenic dots. Chem Mater 2008;20:4539–41. 17. Tian L, Ghosh D, Chen W, Pradhan S, Chang X, Chen S. Nanosized carbon particles from natural gas soot. Chem Mater 2009;21:2803–9. 18. Liu R, Wu D, Liu S, Koynov K, Knoll W, Li Q. An aqueous route to multicolor photoluminescent carbon dots using silica spheres as carriers. Angew Chem 2009;121:4668–71.
19. Cheng ZJ, Zhang YT. Fluorometric investigation on the interaction of oleanolic acid with bovine serum albumin. J Mol Struct 2008;879:81–7. 20. Wang Q, Huang X, Long Y, Wang X, Zhang H, Zhu R, Liang L, Teng P, Zheng H. Hollow luminescent carbon dots for drug delivery. Carbon 2013;59:192–9. 21. Xu Z, Yu J, Liu G. Fabrication of carbon quantum dots and their application for efficient detecting Ru(bpy)32+ in the solution. Sens Actuators B 2013;181:209–14. 22. Grabolle M, Spieles M, Lesnyak V, Gaponik N, Eychmüller A, Resch-Genger U. Determination of the fluorescence quantum yield of quantum dots: suitable procedures and achievable uncertainties. Anal Chem 2009;81:6285–94. 23. Hu YJ, Liu Y, Wang JB, Xiao XH, Qu SS. Study of the interaction between monoammonium glycyrrhizinate and bovine serum albumin. J Pharm Biomed Anal 2004;36:915–9.
5
Luminescence 2014
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
Revised: 11 June 2014,
Accepted: 20 June 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2744
Preparation of carbon quantum dots based on starch and their spectral properties Zhengyu Yan, Juan Shu, Yan Yu, Zhengwei Zhang, Zhen Liu and Jianqiu Chen* ABSTRACT: A simple method for the synthesis of water-soluble carbon quantum dots (CQDs) has been developed based on chemical oxidation of starch. The structures and optical properties of the CQDs were characterized by ultraviolet–visible (UV–Vis) spectroscopy, photoluminescence spectroscopy (PL) and transmission electron microscopy. The CQDs were found to emit bright blue fluorescence and disperse uniformly. The effects of ambient temperature, light and pH on the properties of CQDs were studied. The CQDs exhibited good chemical stability, good photostability and pH sensitivity. Furthermore, the interaction between CQDs and bovine serum albumin (BSA) was investigated. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: starch; carbon quantum dots; chemical oxidation; characterization; spectral properties; BSA
Introduction
Luminescence 2014
Experimental Apparatus and reagents Soluble starch, potassium periodate, hydrochloric acid (36–38%) and acetic acid were all purchased from the Nanjing Chemical Reagent Co.; 30% (w/w) hydrogen peroxide was purchased from Sinopharm Chemical Reagent Co. Ltd. Tris was purchased from Sinopharm Chemical Reagent Co., Ltd. BSA was purchased from Nanjing Zhu Yan Biotechnology Co., Ltd. All reagents used were of analytical grade without further treatment. Water used throughout was ultrapure water. Soluble starch was digested to be CQDs in a high pressure digestion tank. A RF-5301 fluorescence spectrophotometer (Shimadzu Corporation, Japan) and a UV-2100 ultraviolet spectrophotometer (Shimadzu Corporation, Japan) were used to record fluorescence spectra and absorption spectra of CQDs respectively. A JEM-2100 transmission electron microscope (TEM) (HITACHI, Japan) was used to observe the size and morphology
* Correspondence to: J. Chen, Department of Analytical Chemistry, China Pharmaceutical University, Nanjing 210009, China. E-mail: [email protected] Department of Analytical Chemistry, China Pharmaceutical University, Nanjing 210009, China Abbreviations: BSA, bull serum albumin; CQD, carbon quantum dots; TEM, transmission electron microscope.
Copyright © 2014 John Wiley & Sons, Ltd.
1
Carbon quantum dots (CQDs) are a new class of zero-dimensional nanomaterials in the carbon family (1). Carbon is one of the basic elements present in biological organisms, so carbon nanomaterials have generally no or low toxicity to living organisms and low cost. Additionally, CQDs exhibit bio-compatibility, high chemical stability are environmentally friendly, as well as having good photostability and easy bioconjugation (2). More importantly, the CQDs preparation method has many advantages such as the use of simple equipment, mild reaction conditions, low cost, and the possibility of large-scale production. Because of these advantageous features, CQDs have exciting prospects in biomedical and clinical applications (3–7). To date, various methods have been developed to produce CQDs. Based on their synthetic sequence, they can be subdivided into a top-down and a bottom-up approach (8). A top-down approach means that CQDs are etched from a carbon target or peeled from carbon nanoparticles, including by laser ablation (9,10), electrochemical oxidation (11) and arc discharge (12). However, it is difficult to completely break the carbon skeleton into carbon nanoparticles. A bottom-up approach means that burning, heating or a microwave method are used to obtain firstly the carbon precursor, then CQDs are generated by a chemical oxidation method (13), pyrolysis (14), a microwaveassisted method (15), or vector synthesis (16). The chemical oxidation method does not require any special equipment and the carbon sources required are diverse, simple and easy to obtain, so this method is an effective and mass-produced synthesis approach. Candle gray ash (17) and phenolic resin (18) can be used as carbon sources. We prepared CQDs from starch which is an inexpensive carbon source. Water-soluble CQDs prepared by chemical oxidation have excellent fluorescence characteristics, high quantum yield and good stability. Prepared CQDs were characterized and analyzed. The spectral properties of the synthesized CQDs were investigated. Bull serum albumin (BSA), one example of an important carrier protein, which mainly functions to balance osmotic pressure and
contains nutrients, is a significant model protein in biology. It contains 581 amino acid residues, among which there are 35 cysteine residues that form 17 disulfide bonds. Meanwhile, there is a free sulfhydryl at section 34 of the peptide chain (19). Thus, BSA can act easily with small molecules. Interactions between BSA and CQDs were investigated under physiological conditions, constituting a preliminary exploration for the application of CQDs in cell labeling and bio-imaging.
Z. Yan et al.
Figure 1. Processing routing of CQDs (a: starch; b: carbon; c: CQDs).
of CQDs. Elemental analysis was conducted using a Vario EL Cube CHNOS Elemental Analyzer (Elementar, Germany). A rotary evaporator, ultrasonic instrument, digital constant temperature water bath and a pH meter were the common apparatus used. An illumination incubator was used to study the effect of illumination on stability of CQDs.
Preparation of CQDs The preparation process of CQDs is shown in Fig. 1. Firstly, starch was carbonized to prepare the carbon source in a high pressure digestion tank for 4 h at 300 °C. Next, the obtained product was dispersed in mixed oxidant containing 15 ml of distilled water, 15 ml of HAc and 30 ml of H2O2, and sonicated for 15 min at 25 °C and refluxed for 2 h at 120 °C and then filtered. The filtrate was neutralized by NaOH (0.5 mol/L) and concentrated to 10 ml. The concentrated solution was dialyzed in a dialysis bag for 24 h and then evaporated. A light yellow CQD powder was obtained after vacuum drying.
Figure 2. UV absorption (a) and FL spectrum (b) of the CQds in aqueous solution.
Characterization methods The luminescence properties of samples were determined by a fluorescence spectrophotometer and an ultraviolet spectrophotometer. The morphology and size of the samples were observed and determined using a TEM. The fluorescence quantum yield of synthetic CQDs was measured using quinine sulfate as a standard. Figure 3. Transmission electron microscopy micrograph of CQDs.
Results and discussion The choice of synthesis method
Characterization of CQDs A prominent and wide absorption peak (Fig. 2a) was observed in the ultraviolet range and the characteristic absorption peak was observed around 220 nm (20,21). The emission band (Fig. 2b) with high fluorescence intensity was observed around 450 nm with the optimal excitation wavelength at 330 nm, which corresponds to that of CQD’s emitted bright blue fluorescence (Fig. 2c) under a camera obscura ultraviolet lamp. The CQDs were spheroidal particles and dispersed uniformly under the TEM with a diameter of 5–8 nm (Fig. 3). Elemental analysis of the sample conducted by a CHNOS Elemental Analyzer revealed that the composition was C 95.8%, O 3.1% and N 1.1%. The fluorescence quantum yield was calculated according to a method in the literature (22), and was about 11.4%.
wileyonlinelibrary.com/journal/luminescence
The effect of reaction time. Next, 0.2 g of carbon source and 60 ml of mixed oxidant (VH2O:VHAc:VH2O2 = 1:1:2) were mixed. The above mixture was refluxed at 100 °C. Subsequently, 6 ml of the solution was taken to record its fluorescence spectrum at 1, 2, 4, 6, 8, 10, and 12 h. As shown in Fig. 4, the fluorescence intensity was highest at 2 h and weakened with time from 2–12 h. The effect of reaction temperature. The reaction solution was refluxed at 60, 80, 100, 120, and 140 °C for 2 h respectively; 10 ml of the solution was taken out to record its fluorescence spectrum (Fig. 5) showing that fluorescence intensity enhanced and blue shift occurred with temperature increase in the range 60–120 °C. Fluorescence intensity weakened with time when the temperature was higher than 120 °C.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Preparation of carbon quantum dots
Figure 4. Fluorescence spectrum of CQDs prepared at different reaction times.
Figure 6. Fluorescence spectrum of CQD purification.
Figure 7. Influence of pH on CQDs. Figure 5. Fluorescence spectrum of CQDs prepared at different reaction temperatures.
The choice of purification method Prepared CQDs might be not uniform in size due to the various diameters of the carbon source and due to the complex interaction between carbon source and mixed oxidant. Moreover, the reaction solution might contain incompletereacting activated carbon which might weaken the fluorescence intensity and broaden the emission band. Dialysis and extraction were chosen to purify the raw CQDs. Ether, ethyl acetate and n-butyl alcohol were used in the extraction process. As shown in Fig. 6, dialysis was highly efficient but the extraction method did not enhance the fluorescence intensity.
Spectral properties of CQDs
Luminescence 2014
The effect of temperature. Prepared CQDs were treated at 25, 35, 45, 55, 65, or 75 °C and their fluorescence spectra were recorded. As shown in Fig. 8, fluorescence intensity did not alter markedly when the temperature increased from 25 °C to 75 °C. Results showed that temperature had a small effect on fluorescence intensity. So CQDs stored at room temperature had fluorescent stability, which could meet the need for the application of CQDs as biological fluorescent markers in biological imaging and other areas. The effect of illumination. The prepared CQD solution was placed in an illumination incubator (T: 25 °C, illumination intensity: 50.12 μmol/m2 · s). Samples were taken at 0, 2, 4, 6, 8, 10, and 12 h and their fluorescence spectra were recorded. As shown in Fig. 9, the fluorescence intensity did not markedly alter after continuous illumination for 12 h, indicating that the CQDs synthetized had high optical stability.
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
3
The effect of pH value. A specified amount of prepared CQDs powder was dissolved in phosphate-buffered saline (PBS) and the pH value was adjusted to 1.0, 3.0, 5.0, 7.0, 9.0 or 11.0. Their fluorescence spectra were recorded respectively with an excitation wavelength of 330 nm. As shown in Fig. 7, the fluorescence
intensity was highest when the pH value was 7, which has important value for CQDs being used as a fluorescent label in the medical field and with biological cells.
Z. Yan et al. Interaction between BSA and CQDs BSA was dissolved in Tris–HCl buffer (pH 7.4) to simulate physiological conditions; 1 ml of BSA solution was reacted with CQD solution at different volumes in a cuvette and then emission spectra were recorded under 278 nm excitation. As shown in Fig. 10, the fluorescence intensity of BSA decreased with increasing volume of CQD solution, but the maximum emission wavelength remained at 349 nm, indicating that interaction and then energy transfer between BSA and CQDs might occur (23).
Conclusions
Figure 8. Influence of temperature on CQDs.
A simple method for the synthesis of water-soluble CQDs has been developed based on chemical oxidation of starch. The CQDs have small particle diameter and are dispersed uniformly. Additionally, they have excellent properties, such as resistance to photobleaching, high optical stability and biocompatibility. The interaction between BSA and CQDs was studied and it was found that CQDs could quench the fluorescence of BSA. Acknowledgements This work was supported by the National Special Purpose on Public Welfare of Environmental Protection Foundation (200809016), Jiangsu Key Laboratory of Environmental Engineering Open Foundation (KF2012008), Students Research Fund (No. 201210316099) and the Fundamental Research Funds for the Central Universities (JKY2011083).
References
Figure 9. Influence of light on CQDs.
Figure 10. Fluorescence spectra of BSA in the presence of different volumes of 6 3 CQDs (CBSA = 5 × 10 mol/L; CCQDs = (×10 mol/L; increasing volumes 0, 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 μl).
wileyonlinelibrary.com/journal/luminescence
1. Mao XJ, Zheng HZ, Long YJ, Du J, Hao JY, Wang LL, Zhou DB. Study on the fluorescence characteristics of carbon dots. Spectrochim Acta, Part A 2010;75:553–7. 2. Zhao LX, Di F, Wang DB, Guo LH, Yang Y, Wan B, Zhang H. Chemiluminescence of carbon dots under strong alkaline solutions: a novel insight into carbon dot optical properties. Nanoscale 2013;5:2655–8. 3. Cao L, Wang X, Meziani MJ, Lu F, Wang H, Luo PG, Lin Y, Harruff BA, Veca LM, Murray D. Carbon dots for multiphoton bioimaging. J Am Chem Soc 2007;129:11318–9. 4. da Silva J, Goncalves HMR. Analytical and bioanalytical applications of carbon dots. Trends Anal Chem 2011;30:1327–36. 5. Dong Y, Wang R, Li H, Shao J, Chi Y, Lin X, Chen G. Polyaminefunctionalized carbon quantum dots for chemical sensing. Carbon 2012;50:2810–5. 6. Li Q, Ohulchanskyy TY, Liu RL, Koynov K, Wu DQ, Best A, Kumar R, Bonoiu A, Prasad PN. Photoluminescent carbon dots as biocompatible nanoprobes for targeting cancer cells in vitro. J Phys Chem C 2010;114:12062–8. 7. Yang ST, Cao L, Luo PG, Lu F, Wang X, Wang H, Meziani MJ, Liu Y, Qi G, Sun YP. Carbon dots for optical imaging in vivo. J Am Chem Soc 2009;131:11308–9. 8. Li HT, Kang ZH, Liu Y, Lee ST. Carbon nanodots: synthesis, properties and applications. J Mater Chem 2012;22:24230–53. 9. Hu SL, Niu KY, Sun J, Yang J, Zhao NQ, Du XW. One-step synthesis of fluorescent carbon nanoparticles by laser irradiation. J Mater Chem 2009;19:484–8. 10. Sun YP, Zhou B, Lin Y, Wang W, Fernando KS, Pathak P, Meziani MJ, Harruff BA, Wang X, Wang H. Quantum-sized carbon dots for bright and colorful photoluminescence. J Am Chem Soc 2006;128:77567. 11. Zhou J, Booker C, Li R, Zhou X, Sham TK, Sun X, Ding Z. An electrochemical avenue to blue luminescent nanocrystals from multiwalled carbon nanotubes (MWCNTs). J Am Chem Soc 2007;129:744–5. 12. Xu X, Ray R, Gu Y, Ploehn HJ, Gearheart L, Raker K, Scrivens WA. Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments. J Am Chem Soc 2004;126:12736–7.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Preparation of carbon quantum dots 13. Zhang J, Shen W, Pan D, Zhang Z, Fang Y, Wu M. Controlled synthesis of green and blue luminescent carbon nanoparticles with high yields by the carbonization of sucrose. New J Chem 2010;34:591–3. 14. Bourlinos AB, Stassinopoulos A, Anglos D, Zboril R, Karakassides M, Giannelis EP. Surface functionalized carbogenic quantum dots. Small 2008;4:455–8. 15. Zhu H, Wang X, Li Y, Wang Z, Yang F, Yang X. Microwave synthesis of fluorescent carbon nanoparticles with electrochemiluminescence properties. Chem Commun 2009;4:5118–20. 16. Bourlinos AB, Stassinopoulos A, Anglos D, Zboril R, Georgakilas V, Giannelis EP. Photoluminescent carbogenic dots. Chem Mater 2008;20:4539–41. 17. Tian L, Ghosh D, Chen W, Pradhan S, Chang X, Chen S. Nanosized carbon particles from natural gas soot. Chem Mater 2009;21:2803–9. 18. Liu R, Wu D, Liu S, Koynov K, Knoll W, Li Q. An aqueous route to multicolor photoluminescent carbon dots using silica spheres as carriers. Angew Chem 2009;121:4668–71.
19. Cheng ZJ, Zhang YT. Fluorometric investigation on the interaction of oleanolic acid with bovine serum albumin. J Mol Struct 2008;879:81–7. 20. Wang Q, Huang X, Long Y, Wang X, Zhang H, Zhu R, Liang L, Teng P, Zheng H. Hollow luminescent carbon dots for drug delivery. Carbon 2013;59:192–9. 21. Xu Z, Yu J, Liu G. Fabrication of carbon quantum dots and their application for efficient detecting Ru(bpy)32+ in the solution. Sens Actuators B 2013;181:209–14. 22. Grabolle M, Spieles M, Lesnyak V, Gaponik N, Eychmüller A, Resch-Genger U. Determination of the fluorescence quantum yield of quantum dots: suitable procedures and achievable uncertainties. Anal Chem 2009;81:6285–94. 23. Hu YJ, Liu Y, Wang JB, Xiao XH, Qu SS. Study of the interaction between monoammonium glycyrrhizinate and bovine serum albumin. J Pharm Biomed Anal 2004;36:915–9.
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Luminescence 2014
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
