Carbon dots with concentration-modulated fluorescence: Aggregation-induced multicolor emission

https://doi.org/10.1016/j.jcis.2020.04.004Get rights and content

Highlights

  • CDs with concentration-dependent emission are successfully synthesized.

  • ZrCDs with no concentration-dependent emission are successfully synthesized.

  • With the increase of concentration, CDs will aggregate and ZrCDs no aggregate.

Abstract

Carbon dots (CDs) with concentration-modulated luminescence were obtained by pyrolysis of citric acid (CA) and thiourea (TU). CDs exhibit multi-colored emission by varying their concentrations. As a contrast, Zr-doped CDs (ZrCDs) were prepared from CA, TU and ZrCl4 in a similar way. To our surprise, no concentration-dependent emission were observed in ZrCDs. Structural and optical characterizations prove that the concentration-tunable fluorescence is attributed to the aggregation between CDs. As the concentration increases, the aggregation of CDs results in a gradual decrease in the band gaps (from 2.18 eV to 1.56 eV), which ultimately causes the fluorescence to redshift continuously. However, ZrCDs are always in a highly dispersed state no matter how the concentration goes up, the zirconium complexes on the surface of ZrCDs prevents them from aggregation and maintains the band gap of ZrCDs at about 2.21 eV. Therefore, we proposed an aggregation-induced multicolor emission mechanism for CDs, which has a certain reference significance for exploiting the potential applications of CDs.

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.

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