Safe One-Pot Synthesis of Fluorescent Carbon Quantum Dots from

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Safe One-Pot Synthesis of Fluorescent Carbon Quantum Dots from Lemon Juice for a Hands-On Experience of Nanotechnology Elia M. Schneider, Amadeus Bärtsch, Wendelin J. Stark, and Robert N. Grass* ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland

J. Chem. Educ. Downloaded from pubs.acs.org by MCMASTER UNIV on 02/08/19. For personal use only.

S Supporting Information *

ABSTRACT: A simple synthesis of fluorescent carbon quantum dots from lemon juice is described to introduce advanced high-school students and undergraduate college students to nanoparticle synthesis and quantum dots. The synthesis is based on the carbonization of lemon juice using only a hot plate stirrer. Column chromatography is used to separate different carbon quantum dots according to their size. This laboratory experiment can be carried out within a 2 h laboratory course and introduces the students to (1) nanotechnology and nanoparticle synthesis, using safe and commonly available chemicals. Furthermore, (2) the concept of fluorescence can be visualized in an intriguing manner using a pocket UV lamp. (3) This experiment serves as an introduction into size-exclusion chromatography. (4) An insight into possible sensing applications is given by the specific fluorescence quenching with an iron(III) solution. The experiment has been tested with 80 students in 4 Swiss high schools, and the knowledge of the students was tested before and after the experiment with a questionnaire. The performance increased by 0.42 ± 0.39 on a grading scale of 1 to 6. The calculated average effect size was 0.76, which is in the range of a medium- to large-effect size, indicating a favorable effect of the experiment on the nanoparticle knowledge of the students. KEYWORDS: High School/Introductory Chemistry, Organic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Fluorescence Spectroscopy, Nanotechnology, Surface Science, Synthesis, Chromatography

■

Carbon Quantum Dots

INTRODUCTION Nanotechnology is everywhere, including in food products,1 medicinal applications,2 industrial processes,3 and electronic products, such as television displays (QLEDs).4 The range of different materials and forms, shapes, and sizes of nanoparticles is broad: From hollow spheres made of silica,5 over catalytically active palladium nanoparticles6 to magnetic nanoparticles used in biochemistry,7 the diversity is astonishing. Overall, the relevance of the subject in current research and industry is substantial. From a teacher’s perspective, nanoparticles and related concepts (colloidal dispersions, surface-to-volume ratio, fluorescence, size-exclusion chromatography) offer a great variety of new approaches to chemical education and can be integrated in the current curriculum (e.g., via related concepts). Furthermore, most of the concepts and materials can be explained and visualized with vivid experiments (e.g., gold nanoparticles). Nevertheless, there are few school laboratory experiments that give students insight into the world of nanotechnology. Articles about laboratory courses with polymeric, ZnO,8 silver, or gold nanoparticles9 have been published. Even a laboratory experiment with CdSe quantum dots has been published,10 but the toxicity of the used substances is problematic for a high-school laboratory experiment. Therefore, the need for safe, elaborate, intriguing, and feasible nanotechnology-related quantum-dot laboratory experiment procedures for chemistry teachers and their students remains. © XXXX American Chemical Society and Division of Chemical Education, Inc.

One particularly active field in nanotechnology research is the field of carbon quantum dots (CQDs). These small nanoparticles (160 °C. Both methods mostly apply inexpensive, widely available, nontoxic precursors. Because of their relatively facile synthesis, many potential applications for carbon quantum dots have emerged, such as chemical, biosensing, bioimaging, photocatalysis, and optoelectronic applications.18 Future Received: February 19, 2018 Revised: January 21, 2019

A

DOI: 10.1021/acs.jchemed.8b00114 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. One-pot synthesis of photoluminescent carbon quantum dots from lemon juice and poly(ethylenimine) in an open beaker at 100 °C. After three cycles of evaporation, subsequent carbonization (i.e., sticky gel-like organic residuals), and resolution in water (1 mL), fluorescent carbon quantum dots are obtained after approximately 50 min.

applications also include photovoltaic devices19 and displays of mobile phones or television. Herein, we developed a simple and low-cost one-pot synthesis method to produce highly fluorescent carbon quantum dots in a short time (Figure 1) using only a basic hot plate. The main novelty is that this procedure is not a hydrothermal synthesis, thus avoiding elaborate equipment. The procedure can be employed in high-schools and undergraduate university laboratories, since the equipment and the chemicals used are safe and readily available. Therefore, students can synthesize fluorescent nanoparticles easily and analyze them using a UV pocket lamp. Furthermore, the differently sized particles can be separated via column chromatography or even by a silica Pasteur-pipet plug, resulting in fractions that have different fluorescent properties (i.e., excitation and emission spectra). The whole experiment can be performed in a 2 h laboratory exercise.

■

• Silica gel (pore size = 60 Å, 230−400 mesh, SigmaAldrich No. 60752) Synthesis of Carbon Quantum Dots from Lemon Juice

A hot plate stirrer was preheated to 100 °C. Alternatively, an oil bath preheated to 150 °C can be used. Lemon juice (3 mL) was transferred to a 25 mL beaker equipped with a stirring bar. Branched poly(ethylenimine) (70−100 mg, Mw = 600) was added, and the beaker was placed on the hot plate at 100 °C. After ca. 16 min, most of the water had evaporated, and the solution had transformed into a brownish, sticky gel (this was when the stirrer was not moving anymore because of the viscosity of the gel). Then, 1 mL of distilled water was added. This procedure was repeated three times at intervals of ca. 6 min, each time resulting in a nearly complete evaporation of the water. Thus, the solution darkened gradually. The whole reaction was finished after approximately 45 min. Finally, 1 mL of water was added to the solution, and the resulting nanoparticle solution was transferred to a vial or Eppendorf tube.

PROCEDURE

Equipment and Chemicals

The necessary infrastructure and materials for this experiment can be found in most undergraduate chemistry laboratories (see the Instructor and Student Notes), besides maybe the following equipment and chemicals: • Basic hot plate stirrer • 25 mL beakers with a magnetic stirrer • Pocket UV lamp (excitation wavelength = 370−420 nm) • Commercially available lemon juice from concentrate (2.5 g of carbohydrates per 100 mL; pulp-free, noncarbonated, juice of ca. 4−5 lemons per 100 mL) • Poly(ethylenimine), branched, 99%, Mw = 600 (bPEI, ABCR No. AB209301) • FeCl3 (97%, Sigma-Aldrich No. 157740)

Separation Procedure via Chromatography

The nanoparticle solution in the vial (approximately 1 mL) was then transferred to a Pasteur pipet plugged with glass wool and filled with ca. 4 cm of silica. This is a mini-version of sizeexclusion column chromatography, which works well with small volumes and is solvent and waste efficient. Then, pressure was applied using a Pasteur-pipet rubber bulb, and the first fraction emerged. The liquid was collected in Eppendorf tubes, and five to six fractions of 1 mL each (solvent: 1 mL of water has to be added for every fraction) were obtained, ranging from very dark to light yellow. B




Figure 2. Transmission electron microscopy (TEM) image (left) of the obtained sample before chromatography and size distribution of the carbon quantum dots (right).

Figure 3. Absorbance (black line, left) and excitation-dependent photoluminescence of the carbon quantum dots before size-exclusion chromatography (1−8, colored lines, different excitation wavelengths range from 360 to 500 nm). Images depicting the visible color at the corresponding wavelength on top. Reaction solutions before and after reaction under visible and UV light (right).

■

Analysis of the Carbon Quantum Dots and Iron(III) Sensing

The main carbon containing ingredients of lemon juice are citric acid (∼48 mg/mL),20 ascorbic acid (∼47 mg/mL),21 and maleic acid (∼2.3 mg/mL).22 Carbonization of these compounds mixed with branched polyethylenimine results in fluorescent carbon quantum dots. This one-pot CQD synthesis method usually affords around 300 mg of highly fluorescent nanoparticles (using 3 mL of lemon juice). Figure 2 shows transmission electron microscopy (TEM) images of the synthesized parent solution. By measuring the quantum dots from the TEM micrographs, a size distribution with nanoparticles from 2 to 30 nm was obtained with an average carbon quantum-dot size of 5.7 ± 4.0 nm. Elemental analysis (CHN analyzer, Microcube ELEMENTAR) of the dried particles resulted in a carbon content of 38.9 ± 0.3%, a hydrogen content of 5.8 ± 0.02%, and a nitrogen content of 4.5 ± 0.1%. Another publication of a synthesis of CQDs by hydrothermal treatment, also using lemon juice as a carbon source, assumed porphyrin-like carbon quantum-dot structures coming from ascorbic acid, while citric acid generates graphitic islands.16 After Ogi et al., the photoluminescence emission mechanism mainly depends on sp2 clusters, which are isolated within an

The fluorescence of the carbon quantum-dot fractions was then investigated (usually, fractions 3−4 exhibit the strongest fluorescence at excitation at 370 nm, emission at 480 nm) using a TECAN SPARK 10 M Multimode Reader. Alternatively, the CQD solution can be analyzed by eye using a UV pocket lamp (excitation = 360−400 nm) in the dark. Ideally, the solutions are put into UV cuvettes or dropped onto a silica-aluminum plate (see instructor notes and Figure 1). FeCl3 solution (0.1 mL, 0.1 M) was added to a vial containing 1 mL of CQDs (preferably a fraction that has strong fluorescence intensity with the UV lamp), resulting in an immediate quenching of the fluorescence of the CQD.

■

RESULTS AND DISCUSSION

HAZARDS

Safety goggles must be worn at all times, especially when working with poly(ethylenimine). The oil bath at 150 °C has to be treated with caution (e.g., no drops of water should get into the hot bath). The synthesized nanoparticles should neither be dried and inhaled nor consumed. C




Figure 4. 2D excitation/emission spectra of the different fractions obtained by column chromatography reveal the presence of different carbon quantum-dot species, where green corresponds to low, and red corresponds to high fluorescence intensity (left). Pictures of the cuvettes and drops on silica plates below show the corresponding fraction under UV light excitation (360 nm). A shift of the maximum excitation/emission intensity can be observed (dotted arrow) from fraction 1 to 5. Bar plot depicting the absorbance values of the five fractions at two different excitation/ emission values (right).

same concentration, no fluorescence quenching occurs. Another possible explanation for this quenching can be found in the electronic structure of iron(III): It has 5d orbitals that are half-filled, and the electrons at the conduction band of the CQD can be transferred there quickly.27 Furthermore, iron(III) ions can coordinate very strongly to amines and carboxylic acids, thus possibly deactivating the photoluminescence of the CQDs.28 In this experiment, 0.1 mL of a 0.2 M FeCl3 solution was added to a vial containing CQDs, which resulted in an immediate and almost complete loss of fluorescence. With this simple additional experiment, students obtain an insight into selective chemical sensing of metal ions. This facile carbon quantum-dot synthesis has been performed by over 80 high-school students in a 2 h laboratory experiment lesson at 4 different high schools in Switzerland. The feedback of the teachers improved the compatibility of the synthesis with high-school lab equipment and was used to render the experiment as simple as possible with a minimal amount of chemicals. Generally, the teachers gave positive feedback; in particular, the connection to up-to-date research and the simple procedure were appreciated. However, in our opinion, an introduction in nanotechnology should be included in the chemistry curriculum of a high-school student, given the relevance of this topic in our daily lives and the applications connected to it. Experiments that are close to current research and usually not part of the standard curriculum also give the opportunity to motivate students to study chemistry and interest them in natural science. A solid nanotechnology frame will provide the background to understand and embed the experiment in class. To gain insight into the knowledge of nanotechnology of the students, they were given a defining features matrix (nongraded formative assessment, see Supporting Information) to fill out. The same test was carried out before and after the experiment, and an increase in correct answers of 8.5 ± 7.8% was observed. This resulted in an increase in grades of 0.42 ± 0.39 on a grading scale of 1 to 6. The calculated average effect size was 0.76, which is in the range of a medium- to large-effect size after Cohen.29 It can be concluded that the practical synthesis of these nanoparticles improves the understanding of theoretical nanoparticle concepts of the students. However, in our opinion, it would be beneficial to include a theoretical lesson with an introduction into nanoparticles before the experiment.

sp3 C−O matrix, combined with various C−N configurations (pyridinic-N and pyrrolic-N).23 Absorbance measurements (Figure 3) executed using the synthesized CQDs showed increase absorbance starting from 380 nm, which can be attributed to sp2-hybridized graphene flakes and carboxylic acid.24 However, the reaction mechanism for this specific mixture is part of ongoing research in the field. Furthermore, the photoluminescent properties of the prepared CQDs were analyzed (Figure 3). Depending on the excitation wavelength, the emission maximum shifted to the right, from the UV into the visible spectrum. Since the human eye is very sensitive in the region from 400 to 700 nm, a slight shift in the emission maximum changes the perception of the color greatly. In the fluorescence spectra, the optimal excitation/emission wavelength was detected at 420/540 nm (Figure 3, green line 4, corresponding to a very light green color). However, also at an excitation wavelength of 360 nm, a strong emission at 460 nm could be observed. Two different scenarios can be taken into account to explain the finding: (1) The carbon quantum dots are uniform and exhibit excitationdependent photoluminescence behavior, which is common in fluorescent carbon nanomaterials,25 or (2) a mixture of different carbon quantum dots with different properties was synthesized. The second scenario is considered more likely considering the differently sized carbon quantum dots (Figure 2). In order to separate CQDs according to size, small-scale size-exclusion column chromatography was conducted using silica gel in a Pasteur pipet. This size-exclusion chromatography setup allowed the largest particles to pass fast through the silica, while smaller particles and molecules are retarded. This resulted in fractions with low absorbance but high photoluminescence (Figure 4). Additionally, a shift in the excitation and emission spectra was observed, from an emission maximum of 540 nm (fraction 1) to 480 nm (fraction 5). This could be attributed to the different size of the nanoparticles. Large CQDs absorb at visible wavelengths, while smaller CQDs absorb in the UV range, as reported by Kang et al.14 Sensing metal ions is also possible with CQDs. Iron(III) in CQD solutions has been reported to result in strong fluorescence quenching,16,26 making CQDs a cheap sensor for the concentration of iron(III) in solution. A hypothesis was that the addition of iron ions resulted in agglomeration and thus quenching of the photoluminescence. However, if aluminum(III) ions or sodium chloride are added in the D



■


(3) Stark, W. J.; Stoessel, P. R.; Wohlleben, W.; Hafner, A. Industrial applications of nanoparticles. Chem. Soc. Rev. 2015, 44 (16), 5793− 5805. (4) Ganesh, N.; Zhang, W.; Mathias, P. C.; Chow, E.; Soares, J. A. N. T.; Malyarchuk, V.; Smith, A. D.; Cunningham, B. T. Enhanced fluorescence emission from quantum dots on a photonic crystal surface. Nat. Nanotechnol. 2007, 2 (8), 515−520. (5) Schneider, E. M.; Taniguchi, S.; Kobayashi, Y.; Hess, S. C.; Balgis, R.; Ogi, T.; Okuyama, K.; Stark, W. J. Efficient Recycling of Poly(lactic acid) Nanoparticle Templates for the Synthesis of Hollow Silica Spheres. ACS Sustainable Chem. Eng. 2017, 5 (6), 4941−4947. (6) Cheong, S.; Watt, J. D.; Tilley, R. D. Shape control of platinum and palladium nanoparticles for catalysis. Nanoscale 2010, 2 (10), 2045−2053. (7) (a) Schneider, E. M.; Zeltner, M.; Zlateski, V.; Grass, R. N.; Stark, W. J. Click and release: fluoride cleavable linker for mild bioorthogonal separation. Chem. Commun. 2016, 52 (5), 938−941. (b) Zwyssig, A.; Schneider, E. M.; Zeltner, M.; Rebmann, B.; Zlateski, V.; Grass, R. N.; Stark, W. J. Protein Reduction and Dialysis-Free Work-Up through Phosphines Immobilized on a Magnetic Support: TCEP-Functionalized Carbon-Coated Cobalt Nanoparticles. Chem. Eur. J. 2017, 23 (36), 8585−8589. (8) Reid, P. J.; Fujimoto, B.; Gamelin, D. R. A Simple ZnO Nanocrystal Synthesis Illustrating Three-Dimensional Quantum Confinement. J. Chem. Educ. 2014, 91 (2), 280−282. (9) (a) Ramalho, M. J.; Pereira, M. C. Preparation and Characterization of Polymeric Nanoparticles: An Interdisciplinary Experiment. J. Chem. Educ. 2016, 93 (8), 1446−1451. (b) Sharma, R. K.; Gulati, S.; Mehta, S. Preparation of Gold Nanoparticles Using Tea: A Green Chemistry Experiment. J. Chem. Educ. 2012, 89 (10), 1316−1318. (c) Van fraeyenhoven, P.; Glorie, N.; Chiaverini, N.; Mortier, T. An Exploration of the “Sweet Nanochemistry” Synthesis for Silver and Gold Colloids. J. Chem. Educ. 2016, 93 (4), 787−789. (10) Landry, M. L.; Morrell, T. E.; Karagounis, T. K.; Hsia, C.-H.; Wang, C.-Y. Simple Syntheses of CdSe Quantum Dots. J. Chem. Educ. 2014, 91 (2), 274−279. (11) Lim, S. Y.; Shen, W.; Gao, Z. Carbon quantum dots and their applications. Chem. Soc. Rev. 2015, 44 (1), 362−381. (12) Sierański, K.; Szatkowski, J. Substitutional impurity in the graphene quantum dots. Phys. E 2015, 73, 40−44. (13) Liu, H.; Ye, T.; Mao, C. Fluorescent Carbon Nanoparticles Derived from Candle Soot. Angew. Chem., Int. Ed. 2007, 46 (34), 6473−6475. (14) Li, H.; He, X.; Kang, Z.; Huang, H.; Liu, Y.; Liu, J.; Lian, S.; Tsang, C. H. A.; Yang, X.; Lee, S.-T. Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design. Angew. Chem., Int. Ed. 2010, 49 (26), 4430−4434. (15) Hess, S. C.; Permatasari, F. A.; Fukazawa, H.; Schneider, E. M.; Balgis, R.; Ogi, T.; Okuyama, K.; Stark, W. J. Direct synthesis of carbon quantum dots in aqueous polymer solution: one-pot reaction and preparation of transparent UV-blocking films. J. Mater. Chem. A 2017, 5 (10), 5187−5194. (16) Mondal, T. K.; Gupta, A.; Shaw, B. K.; Mondal, S.; Ghorai, U. K.; Saha, S. K. Highly luminescent N-doped carbon quantum dots from lemon juice with porphyrin-like structures surrounded by graphitic network for sensing applications. RSC Adv. 2016, 6 (65), 59927−59934. (17) Sahu, S.; Behera, B.; Maiti, T. K.; Mohapatra, S. Simple onestep synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents. Chem. Commun. 2012, 48 (70), 8835−8837. (18) Wang, R.; Lu, K.-Q.; Tang, Z.-R.; Xu, Y.-J. Recent progress in carbon quantum dots: synthesis, properties and applications in photocatalysis. J. Mater. Chem. A 2017, 5 (8), 3717−3734. (19) Gupta, V.; Chaudhary, N.; Srivastava, R.; Sharma, G. D.; Bhardwaj, R.; Chand, S. Luminscent Graphene Quantum Dots for Organic Photovoltaic Devices. J. Am. Chem. Soc. 2011, 133 (26), 9960−9963.

CONCLUSION This work presents a novel laboratory experiment for the synthesis of carbon quantum dots. The procedure is remarkably safe and simple and can be carried out in almost any laboratory in the world, including high-school teaching laboratories. Additionally, the experiment is also open to other carbon sources for CQD synthesis, for example, orange juice. The fluorescence intensity and the size-dependent photoluminescence offer an attractive basis to introduce the diversity and abilities of nanosized particles. Iron(III) sensing can be visualized without elaborate equipment. This protocol is suitable to teach high-school students concepts such as nanotechnology, colloidal dispersions, sensing, absorbance, and fluorescence. The experiment has been executed in four different Swiss high schools and received generally positive feedback from the teachers and the students. Moreover, an increase in knowledge about the concepts of nanotechnology could be observed using a defining features matrix before and after the laboratory course.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00114. Notes for students (PDF, DOCX) Notes for instructors (PDF, DOCX) Method description; formative assessment data and results (PDF, DOCX)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Elia M. Schneider: 0000-0003-2307-7521 Wendelin J. Stark: 0000-0002-8957-7687 Robert N. Grass: 0000-0001-6968-0823 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank ETH Zurich kindly for their financial support. We gratefully acknowledge Raphael Sigrist (Kantonsschule im Lee, Wintherthur), Lorenzo Vela (Kantonsschule Alpenquai, Luzern), and Christian Ammann (MNG Rämibühl, Zürich) for their effort in verifying the school laboratory suitability of the laboratory experiment on a high-school level. Furthermore, we kindly acknowledge Urs Lustenberger and ScopeM of ETH Zurich for TEM imaging.

■

REFERENCES

(1) Peters, R.; Dam, G. t.; Bouwmeester, H.; Helsper, H.; Allmaier, G.; Kammer, F. v.; Ramsch, R.; Solans, C.; Tomaniová, M.; Hajslova, J.; Weigel, S. Identification and characterization of organic nanoparticles in food. TrAC, Trends Anal. Chem. 2011, 30 (1), 100−112. (2) (a) Herrmann, I. K.; Schlegel, A. A.; Graf, R.; Stark, W. J.; BeckSchimmer, B. Magnetic separation-based blood purification: a promising new approach for the removal of disease-causing compounds? J. Nanobiotechnol. 2015, 13, 49. (b) Murthy, S. K. Nanoparticles in modern medicine: State of the art and future challenges. Int. J. Nanomed. 2007, 2 (2), 129−141. E




(20) Penniston, K. L.; Nakada, S. Y.; Holmes, R. P.; Assimos, D. G. Quantitative Assessment of Citric Acid in Lemon Juice, Lime Juice, and Commercially-Available Fruit Juice Products. J. Endourol. 2008, 22 (3), 567−570. (21) Ballentine, R. Determination of Ascorbic Acid in Citrus Fruit Juices. Ind. Eng. Chem., Anal. Ed. 1941, 13 (2), 89−89. (22) Sinclair, W. B.; Eny, D. M. The Organic Acids of Lemon Fruits. Bot. Gaz. 1945, 107 (2), 231−242. (23) (a) Ogi, T.; Aishima, K.; Permatasari, F. A.; Iskandar, F.; Tanabe, E.; Okuyama, K. Kinetics of nitrogen-doped carbon dot formation via hydrothermal synthesis. New J. Chem. 2016, 40 (6), 5555−5561. (b) Ogi, T.; Iwasaki, H.; Aishima, K.; Iskandar, F.; Wang, W.-N.; Takimiya, K.; Okuyama, K. Transient nature of graphene quantum dot formation via a hydrothermal reaction. RSC Adv. 2014, 4 (99), 55709−55715. (c) Permatasari, F. A.; Aimon, A. H.; Iskandar, F.; Ogi, T.; Okuyama, K. Role of C−N Configurations in the Photoluminescence of Graphene Quantum Dots Synthesized by a Hydrothermal Route. Sci. Rep. 2016, 6, 21042. (24) Wang, L.; Zhu, S.-J.; Wang, H.-Y.; Qu, S.-N.; Zhang, Y.-L.; Zhang, J.-H.; Chen, Q.-D.; Xu, H.-L.; Han, W.; Yang, B.; Sun, H.-B. Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots. ACS Nano 2014, 8 (3), 2541−2547. (25) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. Angew. Chem., Int. Ed. 2013, 52 (14), 3953−3957. (26) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. Angew. Chem. 2013, 125 (14), 4045−4049. (27) Dhenadhayalan, N.; Lin, K.-C. Chemically Induced Fluorescence Switching of Carbon-Dots and Its Multiple Logic Gate Implementation. Sci. Rep. 2015, 5, 10012. (28) Qian, Z.; Ma, J.; Shan, X.; Feng, H.; Shao, L.; Chen, J. Highly Luminescent N-Doped Carbon Quantum Dots as an Effective Multifunctional Fluorescence Sensing Platform. Chem. - Eur. J. 2014, 20 (8), 2254−2263. (29) Cohen, J. Statistical Power Analysis for the Behavioral Sciences, 2nd ed.; Taylor & Francis: New York, 1988.

F


Safe One-Pot Synthesis of Fluorescent Carbon Quantum Dots from

Recommend Documents