Blue-emitting carbon quantum dots: Ultrafast microwave synthesis, purification and strong fluorescence in organic solvents

https://doi.org/10.1016/j.colsurfa.2021.126673Get rights and content

Highlights

  • Dialysis and phase-transfer were introduced to purify CQDs prepared by microwave method

  • Dialyzed CQDs display excitation-dependent blue fluorescence maximally around 450 nm.

  • Extracted CQDs in organic solvents show strong blue fluorescence at ~425 nm and ~405 nm.

  • Quantum yields of the dialyzed and extracted CQDs are respectively 45.1% and 57.5%.

  • Surface state and solvent environment contribute to the different fluorescence property.

Abstract

The impurity of unpurified carbon quantum dots (CQDs) severely limits their application. In this paper, CQDs synthesized by ultrafast microwave hydrothermal method from L-cystine were purified by both dialysis and direct phase-transfer. The as-obtained CQDs in basic aqueous reaction mixture show excitation-independent fluorescence around 505 nm with a maximum excitation at 421 nm and pH-responsive emission intensity. However, after purification by dialysis the CQDs display excitation-dependent blue fluorescence from 410 to 500 nm with respect to the excitation from 310 to 430 nm. Particularly, the CQDs purified by direct phase-transfer into various organic solvents show two strong blue fluorescence peaks respectively focusing on 422–434 nm and 400–410 nm with a maximum excitation at 370–372 nm. The quantum yield of the CQDs decreases to 45.1% after a purification by dialysis from the original 82.7%, while it decreases to 57.5% after a purification of phase-transfer. The CQDs extracted in organic solvents are demonstrated to be pure small crystalline-carbon nanoparticles with chemically surface-grafted groups, while the dialyzed CQDs will physically adsorb additional L-cystine, sodium and by-product nitrate ions. The difference of surface state and solvent environment of these CQDs shall contribute to the different fluorescence property. Such strong luminescent, pure CQDs in organic solvents should have obvious advantages in further applications.

Graphical Abstract

Carbon quantum dots (CQDs) synthesized by microwave hydrothermal method from L-cystine were purified by dialysis in water and direct phase-transfer into organic solvents. The as-obtained CQDs in the basic reaction mixture show green excitation-independent fluorescence around 505 nm with a maximum excitation wavelength of 421 nm. However, the CQDs after purification by dialysis display an excitation-dependent blue fluorescence, the maximum emission intensity focuses on the position of 365 nm excitation and 450 nm emission. Particularly, the CQDs purified by phase-transfer show two strong blue fluorescence peaks respectively focusing around 422–434 nm and 400–410 nm in organic solvents with a maximum excitation at 370–372 nm.

ga1
  1. Download : Download high-res image (379KB)
  2. Download : Download full-size image

Introduction

In recent years, carbon quantum dots (CQDs) have attracted great interest due to their outstanding advantages such as photo stability, chemical stability, biocompatibility, low toxicity and low cost as compared with other optical materials [1], [2], [3], [4], [5], [6], [7], [8], [9], so they are extensively studied in the fields of light-emitting materials [10], [11], [12], [13], sensors [14], [15], [16], [17], [18], bio-imaging [19], [20], [21] and labeling [22], as well as promoters for solar cells [23].

Currently, multiple color CQDs have been synthesized by “top-down” and “bottom-up” strategies. In the former case, CQDs are produced from large precursors such as nanosized carbon [24], active carbon [25], [26], [27] and powder graphite [28]. The “bottom-up” strategies have more selections in carbon resources such as amino acid [29], [30], proteins [31], [32], peptides [29], sugars [33], [34], fruit juices [35], [36], [37], plant gums [38], [39] or hydrophilic polymers [40]. Among the “bottom-up” strategies pyrolysis carbonization has exhibited advantages of operation simplicity, high efficiency and so on [14], [34], [41], [42], [43]. For example, high-quality CQDs can be synthesized by hydrothermal methods at 300 °C from hydrophilic compounds [29], at 160–220 °C from orange pericarp extraction [44], or at 120 °C from orange juice [35]; meanwhile, hydrophobic CQDs can be obtained by pyrolysis of ascorbic acid in ethanol under reflux [45]. Besides, Xiong and his coworkers reported a solvent-controlled synthesis method for CQDs with a wide color gamut and a narrow emission peak in solvothermal reactions at 210 °C [46]. Particularly, the microwave hydrothermal carbonization can provide series of CQDs toward miscellaneous applications by low cost, simple reaction equipments [30], [31], [32], [33], [38], [47], so they are considered as fast and efficiency techniques to face the challenge of large-scale producing [37], [38], [48], [49], [50], [51]. Many carbon-containing small molecules and polymers can be used as presursors in the microwave methods including citric acid [48], [52], glycerol [50], [53], polyethylene glycols [54], amino acids [30], [55], [56], [57], saccharides [33], [58], [59], [60], nature polysaccharides [38], [49], [61], [62], [63], peptide and polyamine [64], [65]. Yu and Wang prepared highly green-fluorescent CQDs from phthalic acid and triethylenediamine [66]; Liu and Wang et al. reported water-soluble luminescent carbon dots as a biocompatible fluorescent ink from citric acid and urea [67]; Zhang and his coworkers synthesized crystalline red- or green- emitting CQDs in glycerol or ethylene glycol from ammonium citrate tribasic and formamide [68].

However, the purification of the obtained CQDs is still a critical problem till now. First, filtration and centrifugation can only remove aggregates or large particles and cannot provide a really pure sample [38], [39], [48], [49], [60], [61], [62], [63]. Second, though dialysis [30], [50], [57], [64], [65], [66] and silica gel chromatography [68] have been used for purification, they cannot adopt to a scalable amount. So that, large-scale producing of the CQDs were usually not purified [37], [51], and in this case the CQDs factually were suspended in a mixture solution or dispersed in a mixture powder of raw materials and by-products, and such impurity will severely limit their application. In this paper, CQDs were synthesized by microwave hydrothermal method from L-cystine, then dialysis and direct phase-transfer were introduced to purify the as-obtained CQDs, respectively. The CQDs before purification showed excitation-independent green fluorescence with pH-responsive emission intensity. However, after purification by dialysis in water or phase-transfer into organic solvents (also can be named as extraction) they emitted blue fluorescence: (1) the purified CQDs by dialysis showed excitation-dependent fluorescence from 410 nm to 500 nm as the excitation wavelength changed from 310 to 430 nm; (2) the purified CQDs by phase-transfer showed two blue emission peaks respectively around 422–434 nm and 400–410 nm in different organic solvents with largely enhanced emission intensity. The surface states of the different CQDs were analyzed to explain the difference of fluorescence property.

Section snippets

Synthesis of the CQDs

CQDs were synthesized from L-cystine by microwave hydrothermal method [30]. In detail, 1.0 g L-cystine (99.5%, Aladdin, China) was dissolved in 10 mL NaOH solution (0.95 mol/L), followed by an ultrasonication of the system to be clear. Then, the reaction system was controlled at 150 °C for 30–90 s under stirring in an intelligent microwave chemical synthesizer (XH-200A, 800 W, Xianghu Technologies, Beijing, China). The samples were obtained after a naturally cooling to room temperature.

pH-dependent fluorescence measurement

The

Synthesis of CQDs by microwave hydrothermal carbonization

In this work, CQDs were synthesized by microwave carbonization of a carbon source L-cystine [30]. The reactant concentration was relatively high: approximately, L-cystine was 0.417 mol/L and NaOH as a catalyst was 0.95 mol/L. The color of the obtained systems changed from light yellow to dark brown as prolonging the carbonization time in the microwave synthesizer [30]. The obtained samples showed green fluorescence as irradiated by 365 nm light, so the fluorescence emission spectra of these

Conclusions

In this work, CQDs were synthesized by the ultrafast microwave hydrothermal carbonization. The as-obtained CQDs in an aqueous basic mixture solution display pH response, excitation-independent fluorescence with the maximum excitation/emission wavelengths of 421/505 nm. After purification by dialysis in water, the emission intensity dramatically decrease, and the maximum excitation/emission wavelengths shift to 365/450 nm with excitation-dependent fluorescence. Whereas, after purification by

CRediT authorship contribution statement

Jie Zhu: Data curation, Investigation, Methodology, Writing - original draft. Chunxing Wu: Investigation, Methodology, Writing - original draft. Yongmei Cui: Formal analysis, Software. Dongxiang Li: Conceptualization, Funding acquisition, Supervision, Writing - original draft, Writing - review & editing. Yaojun Zhang: Data curation, Software, Investigation. Jie Xu: Methodology, Resources. Chunfang Li: Project administration, Supervision, Validation, Resources, Writing - review & editing. Shahid

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (NNSFC 21975139), and the Natural Science Foundation of Shandong Province (ZR2017MB042), China.

References (82)

  • M.I.S. Dela Cruz et al.

    Preparation of highly photoluminescent carbon dots from polyurethane: Optimization using response surface methodology and selective detection of silver (I) ion

    Colloids Surf. A

    (2019)
  • D. Lu et al.

    Facile synthesis of alkylated carbon dots with blue emission in halogenated benzene solvents

    Colloids Surf. A

    (2021)
  • X. Li et al.

    Engineering surface states of carbon dots to achieve controllable luminescence for solid-luminescent composites and sensitive Be2+ detection

    Sci. Rep.

    (2014)
  • V. Nguyen et al.

    Electron–hole recombination dynamics in carbon nanodots

    Carbon

    (2015)
  • Y. Wang et al.

    Carbon quantum dots: synthesis, properties and applications

    J. Mater. Chem. C

    (2014)
  • Z.L. Wu et al.

    Carbon dots: materials, synthesis, properties and approaches to long-wavelength and multicolor emission

    J. Mater. Chem. B

    (2017)
  • U. Abd Rani et al.

    A review of carbon quantum dots and their applications in wastewater treatment

    Adv. Colloid Interface Sci.

    (2020)
  • Y.-P. Sun et al.

    Quantum-sized carbon dots for bright and colorful photoluminescence

    J. Am. Chem. Soc.

    (2006)
  • H. Li et al.

    Carbon nanodots: synthesis, properties and applications

    J. Mater. Chem.

    (2012)
  • R. Wang et al.

    Recent progress in carbon quantum dots: synthesis, properties and applications in photocatalysis

    J. Mater. Chem. A

    (2017)
  • L. Zhao et al.

    Kinetically controlled self-assembly of phthalocyanine-peptide conjugate nanofibrils enabling superlarge redshifted absorption

    CCS Chem.

    (2019)
  • F. Zu et al.

    The quenching of the fluorescence of carbon dots: a review on mechanisms and applications

    Microchim. Acta

    (2017)
  • X. Ren et al.

    The dominant role of oxygen in modulating the chemical evolution pathways of tyrosine in peptides: dityrosine or melanin

    Angew. Chem. Int. Ed.

    (2019)
  • S.N. Baker et al.

    Luminescent carbon nanodots: emergent nanolights

    Angew. Chem. Int. Ed.

    (2010)
  • X. Guo et al.

    Facile access to versatile fluorescent carbon dots toward light-emitting diodes

    Chem. Commun.

    (2012)
  • F. Yuan et al.

    Bright high-colour-purity deep-blue carbon dot light-emitting diodes via efficient edge amination

    Nat. Photon.

    (2020)
  • F. Yuan et al.

    Engineering triangular carbon quantum dots with unprecedented narrow bandwidth emission for multicolored LEDs

    Nat. Commun.

    (2018)
  • S. Zhu et al.

    Highly photoluminescent carbon dots for multicolor patterning, sensors, and bioimaging

    Angew. Chem. Int. Ed.

    (2013)
  • X. Gao et al.

    Carbon quantum dot-based nanoprobes for metal ion detection

    J. Mater. Chem. C

    (2016)
  • A.H. Loo et al.

    Carboxylic carbon quantum dots as a fluorescent sensing platform for DNA detection

    ACS Appl. Mater. Interfaces

    (2016)
  • Z. Yan et al.

    Glycine-functionalized carbon quantum dots as chemiluminescence sensitization for detection of m-phenylenediamine

    Anal. Methods

    (2015)
  • S.C. Ray et al.

    Fluorescent carbon nanoparticles: synthesis, characterization, and bioimaging application

    J. Phys. Chem. C

    (2009)
  • L. Chai et al.

    Functionalized carbon quantum dots with dopamine for tyrosinase activity monitoring and inhibitor ccreening: in vitro and intracellular investigation

    ACS Appl. Mater. Interfaces

    (2015)
  • X. Gao et al.

    Carbon-dot-based ratiometric fluorescent probe for imaging and biosensing of superoxide anion in live cells

    Anal. Chem.

    (2014)
  • J.-H. Liu et al.

    Carbon “quantum” dots for fluorescence labeling of cells

    ACS Appl. Mater. Interfaces

    (2015)
  • J. Pan et al.

    Photovoltaic conversion enhancement of a carbon quantum dots/p-type CuAlO2/n-type ZnO photoelectric device

    ACS Appl. Mater. Interfaces

    (2015)
  • X. Li et al.

    Preparation of carbon quantum dots with tunable photoluminescence by rapid laser passivation in ordinary organic solvents

    Chem. Commun.

    (2011)
  • H. Liu et al.

    Fluorescent carbon nanoparticles derived from candle soot

    Angew. Chem. Int. Ed.

    (2007)
  • Y. Dong et al.

    Extraction of electrochemiluminescent oxidized carbon quantum dots from activated carbon

    Chem. Mater.

    (2010)
  • S.-L. Hu et al.

    One-step synthesis of fluorescent carbon nanoparticles by laser irradiation

    J. Mater. Chem.

    (2009)
  • P.-C. Hsu et al.

    Synthesis of high-quality carbon nanodots from hydrophilic compounds: role of functional groups

    Chem. Commun.

    (2012)
  • Cited by (0)

    View full text