Hydrothermal synthesis of N-doped carbon quantum dots and their application in ion-detection and cell-imaging

Highlights

• N-CQDs were prepared by a facile hydrothermal of glucose and m-phenylenediamine.

• N-CQDs emit stable blue emission under UV irradiation in different conditions.

• N-CQDs are excellent fluorescent probes of Fe³⁺ and CrO₄²⁻.

• The static quenching mechanism is based on inner filter effect.

• N-CQDs are superior cell-imaging reagents for Hela cells.

Abstract

Carbon quantum dots (CQDs), owing to their characteristic luminescent properties, have become a new favorite in the field of luminescence. They have been widely used in light emitting diode, ion detection, cell-imaging, ect. Herein a facile synthesis method of nitrogen-doped carbon quantum dots (N-CQDs) has been developed via a one-step hydrothermal of glucose and m-phenylenediamine. The chemical composition, surface functional groups, and crystal structure of so prepared N-CQDs were systematically characterized. The characterizations indicate that nitrogen has been chemically doped in the CQDs and the N-CQDs crystallize in a graphene structure. Photoluminescence (PL) measurements show that the N-CQDs emit strong blue emission under the irradiation of ultraviolet. The emission is excitation-dependent, is resistant to photo bleaching and high ionic strength, and slightly decreases with the increase of temperature. The quantum yield of them is about 17.5%. The PL intensity of N-CQDs quenches linearly with the increase of the concentrations of Fe³⁺(0.5–1.0 mM) and CrO₄²⁻(0.3–0.6 mM), which are a kind of excellent fluorescent probe for the detection of Fe³⁺ and CrO₄²⁻. The quenching mechanism of Fe³⁺ and CrO₄²⁻is verified to be a static quenching mechanism based on inner filter effect. The N-CQDs are also found to be a good cell-imaging reagent of Hela cells.

Introduction

In recent years, carbon-based nanomaterials, such as carbon nanotubes, graphene, and fullerene, have attracted the attention of chemists and material scientists [1], [2], [3], [4], [5], [6]. These materials have been widely used in catalysis, sensor, solar cell, ect. Nevertheless, they have no standing in the field of luminescent materials due to their conductive property [7], [8], [9], [10], [11]. In 2004, carbon quantum dots (CQDs) were discovered in the purification of single-walled carbon nanotubes, and the fluorescent properties of which were first detected, even though their fluorescence intensity was relatively weak [12]. This discovery triggered intensive researches on light emitting CQDs and their potential applications [13], [14], [15], [16], [17], [18], [19], [20], [21]. In the past few years, it has been verified that a chemical doping of heteroatoms, such as borium, silicon, nitrogen, sulphur, or phosphorus, into CQDs can effectively improve their fluorescence quantum efficiency [22], [23], [24], [25], [26], [27]. Since nitrogen is adjacent to carbon in the periodic table, and can fit well at the lattice position of carbon-based materials, nitrogen doped carbon quantum dots (N-CQDs) have been greatly valued over other element doped CQDs [28], [29], [30], [31].

The synthetic strategies of N-CQDs are based on either nitrogenization of CQDs or fusion of carbon precursor and nitrogenization agent [32], [33], [34], [35]. The former approach, always a top-down approach, involves oxidative cutting, reductive cutting, physical grinding, or supersonic cutting of large graphitized carbon materials followed by nitrogenization [36], [37], [38]. The latter approach, usually a bottom-up method, includes a reaction of carbon precursor and nitrogenization agent followed by dehydration, aggregation (or polymerization) [39], [40], [41], [42]. In comparison, the top-down method always utilizes low-cost raw materials, but it often requires long reaction time, has low production yield. The bottom-up method, however, possesses a mild reaction condition and higher production yield, more importantly, it is convenient to introduce heteroatoms into CQDs [43], [44]. So, the bottom-up method is greatly favored in the synthesis of N-CQDs. Hydrothermal carbonization is traditionally a way to prepare carbon-based materials, hence It is certainly an approach to synthesize CQDs. It is greatly valued due to its convenience, low-cost, and mild reaction condition over other bottom-up methods.

A great many precursors have been used to synthesize CQDs, among which carbohydrate is greatly favored for its low cost, free availability, and environmental friendliness. CQDs prepared from carbohydrate, however, are always sp³ carbon-based CQDs [45], [46], [47]. Carbonyl is the basic functional group of glucose, which can react with amino to form CN group. Dehydrating such a nitrogenized glucose compound maybe is a rational approach to synthesize N-CQDs. In 2015, Lin group prepared high quality nitrogen-doped graphene quantum dots using phenylenediamine as precursor [48]. The amino group in phenylenediamine can probably introduce nitrogen into CQDs by reacted with carbonyl group, and induce the formation of nitrogen-doped graphene structure. Herein, a one-step hydrothermal of glucose and phenylenediamine was developed to synthesize N-CQDs without any passivation, and the hydrothermal of glucose and m-phenylenediamine is verified to be a feasible approach to synthesize grapheme structured N-CQDs. The structure, fluorescent properties of so prepared N-CQDs was systematically characterized. The synthesized N-CQDs exhibit low toxicity, good water solubility, considerable quantum yields, and good photostability. Moreover, some applications of the N-CQDs in ion-detection (ferric and chromate ions) and cell-imaging have been committed.