Carbon dots with concentration-modulated fluorescence: Aggregation-induced multicolor emission
Graphical abstract
Introduction
In recent years, carbon dots (CDs) have been widely explored due to their superior optical properties, ultra-small size, excellent biocompatibility and low toxicity [1], [2], [3], [4], [5], [6], [7], [8]. However, most of CDs have blue or green emissions, which severely limits their widespread application. Thus, developing full-color emissive CDs is considerably interesting and highly desired [9], [10], [11], [12], [13], [14]. Usually, there are two strategies to prepare multicolor fluorescent CDs: (1) Modulating the fluorescence of CDs by way of changing solvents, carbon sources, passivating agents, temperature and reaction time. For example, Cai et al. synthesized blue, green and red emissive CDs from p-phenylenediamine and different modifying agents [15]. Lin et al. synthesized CDs with multiple-mode emissions by varying precursors and solvents [16]; (2) Tuning the optical behaviors of CDs via post-modification. For instance, our group developed a simple strategy to tune the photoluminescence of CDs by conjugation with vitamin C and acetaldehyde and successfully obtained green and red fluorescent CDs [17].
Recently, several groups [18], [19], [20], [21], [22] have discovered an exciting fact that tuning the fluorescence across the entire visible spectrum can be readily achieved by controlling the concentrations of CDs. Shao et al. prepared polymer CDs (PCDs) with multi-color luminescence by modulating the concentration of the PCDs [23]. Zhang et al. reported that CDs obtained from citric acid, l-cysteine and KCl exhibit concentration-tunable luminescence [24]. Chen et al. tuned the emission wavelength of CDs from 514 nm to 585 nm through increasing CDs concentrations [25]. Nevertheless, the mechanisms of concentration-adjustable fluorescence of CDs are not intensely investigated and clearly illustrated [26], [27], [28], [29], [30], [31]. Therefore, systematic study on modulating the optical properties of CDs by varying concentrations is of great concern for revealing their fluorescence mechanism and expanding the practical applications of CDs.
Motivated by all the aforementioned issues, in the present work, we synthesized CDs with unique excitation and concentration-dependent luminescence via facile one-pot pyrolysis of citric acid (CA) and thiourea (TU) (Scheme 1). The emission of CDs shows a red shift by 154 nm by adjusting the concentration of CDs. In order to investigate the origin of multi-emissions in CDs, we prepared Zr-doped CDs (ZrCDs) in a similar way as a control. Surprisingly, no multicolored emissions in ZrCDs were observed, even the concentration was increased from 0.002 mg mL−1 to 1 mg mL−1, meaning that zirconium complexes on the surface of ZrCDs suppresses the concentration-dependent luminescence of CDs. The morphologies and sizes of CDs and ZrCDs determined by transmission electron microscopy (TEM) demonstrate that CDs aggregate gradually with increasing the concentration of CDs, while, ZrCDs do not. The zeta-potential of CDs at low concentration (0.002 mg mL−1) is −10.59 mV, as the concentration went up, it changed to nearly 0 mV, implying that CDs at high concentrations are unstable and tend to aggregate into larger particles. On the contrary, the zeta-potential of ZrCDs gradually decreased from −4.75 mV to −17.83 mV with the concentration increased, indicating that ZrCDs are in a highly dispersed state. The band gaps of CDs deduced from plots of (αhv)1/2 versus hv lower from 2.18 eV (0.002 mg mL−1) to 1.56 eV (1.0 mg mL−1), the interactions between closely adjacent CDs become stronger as the concentration increases, resulting in a decrease in the energy gap. However, the existence of zirconium complexes prevents ZrCDs from aggregation and maintains the band gap of ZrCDs at about 2.21 eV. Based on the experimental results, we propose that aggregation favors energy transfer between CDs, decreases band gaps and induces the multicolor emission of CDs.
Section snippets
Chemicals
Citric acid (≥99.5%), thiourea (99%) and ZrCl4 (≥99.9%) were purchased from Aladdin Reagent Corporation. The ultrapure water was prepared from a Milli-Q system (Millipore, USA).
Characterization
Ultraviolet–visible (UV–vis) absorption spectra were obtained by using a Shimadzu UV-2450 PC UV–vis spectrophotometer. Fluorescence intensity tests were performed on a PerkinElmer LS-55 spectrofluorophotometer. The Zeta potentials of the CDs and ZrCDs were determined by a Malvern Zeta-sizer Nano. The morphology of the
Preparation and characterization of CDs
CDs were prepared by pyrolysis of CA and TU as described in the Experimental Section. Different reaction time and CA/TU molar ratios were screened to optimize the reaction conditions. When the molar ratio of CA to TU was 1:3, the reaction time was varied from 0.5 h to 1 h and 2 h, respectively. The obtained CDs was characterized by photoluminescence (PL) spectra and shown in Figs. S1–S3. The fluorescence intensity of CDs (1 mg mL−1) prepared at 1 h is the highest at the same condition. So 1 h was
Conclusion
In summary, Carbon dots (CDs) with remarkable concentration-dependent luminescence were synthesized via a simple one-step pyrolysis method. By contrast, Zr-doped CDs (ZrCDs) with concentration-independent emission were prepared in a similar way. As the concentration increases, the aggregation of CDs results in a continuous fluorescence redshift. On the contrary, the existence of zirconium carboxylate impedes ZrCDs from aggregation and renders ZrCDs with a good dispersion, which helps ZrCDs be
CRediT authorship contribution statement
Ya Su: Methodology, Investigation, Software, Writing - original draft. Zhigang Xie: Resources, Supervision, Writing - review & editing. Min Zheng: Conceptualization, Resources, Supervision, Writing - review & editing, Data curation.
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.
Acknowledgements
The financial support from the National Natural Science Foundation of China (No. 51873023 and 51522307), Talent Development Fund of Jilin Province and the Research Project of Science and Technology of the Education Department of Jilin Province (No. JJKH20200648KJ). The authors acknowledge the help of Prof. Siyu Lu.
References (55)
- et al.
Red fluorescent carbon dots with phenylboronic acid tags for quick detection of Fe(III) in PC12 cells
J. Colloid Interface Sci.
(2018) - et al.
One-step synthesis of photoluminescent carbon dots with excitation-independent emission for selective bioimaging and gene delivery
J. Colloid Interface Sci.
(2017) - et al.
Fluorescent carbon dots with tunable negative charges for bio-imaging in bacterial viability assessment
Carbon
(2017) - et al.
Facile approach to synthesize highly fluorescent multicolor emissive carbon dots via surface functionalization for cellular imaging
J. Colloid Interface Sci.
(2018) - et al.
Carbogenic nanodots derived from organo-templated zeolites with modulated full-color luminescence
Chem. Sci.
(2016) - et al.
Luminescent carbon dots with concentration-dependent emission in solution and yellow emission in solid state
J. Colloid Interface Sci.
(2020) - et al.
Concentration-dependent color tunability of nitrogen-doped carbon dots and their application for iron(III) detection and multicolor bioimaging
J. Colloid Interface Sci.
(2018) - et al.
Exploring of multicolor emissive carbon dots with novel double emission mechanism
Sens. Actuators
(2018) - et al.
Effective and selective adsorption of organoarsenic acids from water over a Zr-based metal-organic framework
Chem. Eng. J.
(2019) - et al.
Highly selective recovery of Au(III) from wastewater by thioctic acid modified Zr-MOF: Experiment and DFT calculation
Chem. Eng. J.
(2020)
Nitrogen-doped carbon dots modified dibismuth tetraoxide microrods: A direct Z-scheme photocatalyst with excellent visible-light photocatalytic performance
J. Colloid Interface Sci.
Highly fluorescent nitrogen-doped carbon dots derived from Phyllanthus acidus utilized as a fluorescent probe for label-free selective detection of Fe(3+) ions, live cell imaging and fluorescent ink
Biosens. Bioelectron.
Reusable sensor based on functionalized carbon dots for the detection of silver nanoparticles in cosmetics via inner filter effect
Anal. Chim. Acta
Investigation of phosphorous doping effects on polymeric carbon dots: Fluorescence, photostability, and environmental impact
Carbon
Enhanced photoluminescence properties of a carbon dot system through surface interaction with polymeric nanoparticles
J. Colloid Interface Sci.
53% Efficient red emissive carbon quantum dots for high color rendering and stable warm white-light-emitting diodes
Adv. Mater.
Near-infrared excitation/emission and multiphoton-induced fluorescence of carbon dots
Adv. Mater.
In vivo theranostics with near-infrared-emitting carbon dots-highly efficient photothermal therapy based on passive targeting after intravenous administration
Light Sci. Appl.
Carbon dots for in vivo bioimaging and theranostics
Small
Near-infrared light-mediated photodynamic/photothermal therapy nanoplatform by the assembly of Fe3O4 carbon dots with graphitic black phosphorus quantum dots
Int. J. Nanomedicine
Tunable photoluminescence across the entire visible spectrum from carbon dots excited by white light
Angew. Chem. Int. Ed. Engl.
Full-colour carbon dots: integration of multiple emission centres into single particles
Nanoscale
Full-color light-emitting carbon dots with a surface-state-controlled luminescence mechanism
ACS Nano
A building brick principle to create transparent composite films with multicolor emission and self-healing function
Small
Full color carbon dots through surface engineering for constructing white light-emitting diodes
J. Mater. Chem. C
Blue, green, and red full-color ultralong afterglow in nitrogen-doped carbon dots
Nanoscale
A postmodification strategy to modulate the photoluminescence of carbon dots from blue to green and red: synthesis and applications
J. Mater. Chem. B
Cited by (62)
Biomass-derived carbon dots as emerging visual platforms for fluorescent sensing
2024, Environmental ResearchRecent advances in solid-state fluorescent of red carbon dots: A comprehensive review
2024, Journal of Alloys and CompoundsSynthesis of oil-soluble carbon dots via pyrolysis and their diverse applications in doxycycline detection, fluorescent ink and film
2024, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy