Mn(II)-coordinated Fluorescent Carbon Dots: Preparation and Discrimination of Organic Solvents

doi:10.1016/j.optmat.2018.02.026

Optical Materials

Volume 78, April 2018, Pages 118-125

https://doi.org/10.1016/j.optmat.2018.02.026 Get rights and content

Highlights

•
Fluorescence/magnetic resonance bimodal carbon dots were prepared by Mn(II) coordination.
•
The fluorescence quantum efficiency of the carbon dots increased by Mn(II) coordination.
•
White light emitting carbon dots were obtained to discriminate organic solvents.

Abstract

Herein, we prepared a Mn(II)-coordinated carbon dots (CDs) with fluorescence and MRI (magnetic resonance imaging) bimodal properties by a one-pot solvothermal method and separated via silica column chromatography. The quantum yield of the CDs increased greatly from 2.27% to 6.75% with increase of Mn(II) doping, meanwhile the CDs exhibited a higher MR activity (7.28 mM⁻¹s⁻¹) than that of commercial Gd-DTPA (4.63 mM⁻¹s⁻¹). In addition, white light emitting CDs were obtained by mixing the different types of CDs. Notably, these CDs exhibited different fluorescence emissions in different organic solvents and could be used to discriminate organic solvents based on the polarity and protonation of the solvents.

Introduction

Carbon dots (CDs) have drawn much attention due to their various fascinating advantages, which include tunable fluorescence, excellent photo- and chemical-stability, low toxicity, and favorable biocompatibility [[1], [2], [3], [4], [5]], which have been applied in biological sensing and imaging [[6], [7], [8], [9], [10], [11]]. However, their usage remains challenging due to few methods available for fluorescence enhancement and functionalization [12].

Metal-enhanced fluorescence has emerged as an efficient way to improve the fluorescence properties of CDs. Zhang et al. found that the fluorescence of CDs could be enhanced via a metal-enhanced fluorescence process using silver island films [13]. CDs with excellent fluorescence can also be obtained by doping with ZnO or ZnS, as shown by Sun et al. [14]. Wang et al. prepared Mn(II)-coordinated CDs that exhibited yellow emission and could be used for the discrimination of volatile organic compounds [15]. Thus, metal-enhanced fluorescence has been shown to be an efficient way of improving the optical properties of CDs [16,17] and the use of Mn(II) ions as a doping source is regarded as an excellent archetype for the investigation of doping effects [18,19].

Further, magnetic resonance imaging (MRI) has been widely used and exhibited an excellent collaboration ability with fluorescence imaging [20]. Many kinds of fluorescence/MR bimodal CDs have been prepared using gadolinium as MR unit. However, gadolinium ions are highly toxic because of calcium channels inhibition [21,22]. Mn(II) ions exhibit low toxicity and have been reported as an efficient contrast agent for MRI [23], which would be a good choice to CDs for building new fluorescence/MR bimodal probes [24]. Therefore, the design of a CDs coordinated with Mn(II) ions for fluorescence enhancement and obtaining MR activity is highly desired.

Solvothermal method as a novel and simple method has become a mature technology for preparing nanomaterials. For example, Zhang et al. prepared PS@ZnO nanoparticles by two steps of fabrication of polystyrene (PS) spheres and fabrication of PS@ZnO in ethanol [25], and they also demonstrate a double-phase method for synthesizing the blue emitting ZnO nanoparticles that are stable in aqueous solutions [26]. Not only for semiconductor nanomaterials, solvothermal method is mostly preferred because of its simplicity, controlled reaction conditions, and cost-effectiveness for the preparation of CDs. Herein, we prepare Mn(II)-coordinated CDs (Mn-CDs) with fluorescence and MR bimodal properties via the one-pot solvothermal reaction of a mixture of p-phenylenediamine, ethanol, and MnCl₂·4H₂O at 180 °C for 4 h. We found that the fluorescence intensity and quantum yeild of the resulting CDs (Mn-CDs) were enhanced, and the quantum yield of the CDs increased greatly from 2.27% to 6.75%, meanwhile the CDs exhibited a higher MR activity (7.28 mM⁻¹s⁻¹) than that of commercial Gd-DTPA (4.63 mM⁻¹s⁻¹). The product could be separated by silica gel column chromatography into four fractions that emitted different colors from red to blue, which were denoted as CDs-1, CDs-2, CDs-3, and CDs-4, respectively. We found that the fluorescence emission of the CDs blue-shifted with the increase of particle size. After the characterization of X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP), Mn(II) was found to be centered on the yellow emitting CDs-2, which had high fluorescence and excellent magnetic resonance properties. White light-emitting CDs obtained by mixing CDs, had CIE coordinates of (0.37, 0.35). Notably, the white light emitting CDs showed solvent-dependent fluorescence emission, which can be used for the discrimination of organic solvents. Therefore, these Mn(II)-coordinated CDs can be used for rapid identification of organic solvents. This work not only offers a simple, cost-effective method of synthesizing fluorescence/MRI bimodal CDs with excellent fluorescence properties for the discrimination of organic solvents, but also promotes the development of metal-coordinated CDs in general (Scheme 1).

Section snippets

Materials

P-phenylenediamine was purchased from J&K Co., Beijing, China. Ethanol, methanol, n-propanol, n-butanol, petroleum ether, dichloromethane, tetrahydrofuran, N, N-dimethylformamide, petroleum ether and MnCl₂·4H₂O were obtained from Tianjin Chemical Co. Ltd. All of the reagents were used without any further purification.

Synthesis of Mn-CDs

Mn-CDs were prepared according to a previously CDs method with fine modification [27]. Briefly, 0.2 mmol of p-phenylenediamine and 0.4 mmol MnCl₂·4H₂O were added into 10 mL ethanol

Preparation and characterization of Mn(II)-coordinated CDs

To date, a range of works have been reported that metals can influence the fluorescence properties of CDs [13,14]. We attempted to synthesize Mn(II)-coordinated CDs to investigate the influence of Mn coordination on fluorescence. Mn(II)-coordinated CDs were prepared by solvothermal treatment of p-phenylenediamine and MnCl₂·4H₂O according to a previously reported method [27]. The as-prepared products were separated via silicon column chromatography using a mixture of petroleum ether and ethyl

Conclusions

We reported a simple one-pot solvothermal treatment of a mixture of p-phenylenediamine, ethanol, and MnCl₂·4H₂O to prepare Mn(II)-coordinated CDs with full-color emission that is independent of the excitation wavelength. Four types of CDs could be separated via column chromatography after the hydrothermal treatment, which was also beneficial for exploring the fluorescence mechanisms of the CDs. After being subjected to the same separation and purification, the products can be dispersed again in

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Nos. 21605036, 21771050 and 21271060), the Natural Science Foundation of Hebei Province (Nos. B2016202147, B2016202149, B2017202048 and B2017202068) and the Educational Committee of Hebei Province (No. QN2016172).

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Mn(II)-coordinated Fluorescent Carbon Dots: Preparation and Discrimination of Organic Solvents

Highlights

Abstract

Introduction

Section snippets

Materials

Synthesis of Mn-CDs

Preparation and characterization of Mn(II)-coordinated CDs

Conclusions

Acknowledgments

Biomaterials

Carbon

Carbon

Coord. Chem. Rev.

Brain Res.

Luminescent carbon nanodots: emergent nanolights

Angew. Chem. Int. Ed.

Carbon nanodots: synthesis, properties and applications

J. Mater. Chem.

Carbon quantum dots and their applications

Chem. Soc. Rev.

Green preparation of carbon dots with papaya as carbon source for effective fluorescent sensing of Iron (III) and Escherichia coli

Biosens. Bioelectron.

Functional surface engineering of C-dots for fluorescent biosensing and in vivo bioimaging

Acc. Chem. Res.

Highly biocompatible carbon nanodots for simultaneous bioimaging and targeted photodynamic therapy in vitro and in vivo

Adv. Funct. Mater.

Graphene oxide nanoparticles as a nonbleaching optical probe for two-photon luminescence imaging and cell therapy

Angew. Chem. Int. Ed.

Carbon “quantum” dots for optical bioimaging

J. Mater. Chem. B

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Chem. Commun.

Doped carbon nanoparticles as a new platform for highly photoluminescent dots

J. Phys. Chem. C

Intraparticle FRET of Mn(ii)-doped carbon dots and its application in discrimination of volatile organic compounds

Nanoscale

Heavily doped semiconductor nanocrystal quantum dots

Science