Continuous microflow synthesis of fluorescent phosphorus and nitrogen co-doped carbon quantum dots

Highlights

• A microflow technique is applied for N/P co-doped carbon quantum dots synthesis.

• The continuous microflow approach is effective for the co-doping of heteroatoms.

• The properties of the co-doped carbon dots can be controlled by process parameters.

• The photoluminescence performance of the carbon dots is further evaluated.

Abstract

Fluorescent carbon quantum dots (CQDs) doped with heteroatoms are highly promising for diverse applications ranging from bioimaging to environmental sensing. However, the ability for doping and functionalizing CQDs in a continuous, scalable, industry-relevant flow chemistry process, remains limited. Here we overcome this limitation by developing a continuous, facile and efficient method for the preparation of nitrogen and phosphorus co-doped carbon quantum dots (NP-CQDs), using microflow reactors combined with localized, energy-efficient heating of ethanolamine and phosphoric acid aqueous solution. The products are characterized by advanced microanalysis including fluorescence spectrophotometry, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) measurements. The results show ultrasmall fluorescent CQDs with narrow size distribution were successfully prepared, where N and P atoms proved to be effectively doped into the CQDs. The reaction conditions for the continuous synthesis of CQDs are investigated, including the influence of residence time and reaction temperature on the obtained CQDs (particle size, distributions, fluorescence intensity, bandgap energies, conductivity, and fluorescence lifetime) are analyzed. This study is expected to provide guidance for the continuous and controllable preparation of fluorescent CQDs with effective doping of heteroatoms.

Introduction

In the past two decades, the development of functional nanomaterials has witnessed rapid acceleration and expansion. Nanomaterials, as suitable biomedical, energy, and environment materials, have attracted strong attention from academy and industry (Ma et al., 2019). Due to their unique physical and chemical properties, they have been actively explored for applications in nanotechnology and chemical engineering. In last decades, fluorescent nanomaterials were found capable of visualizing and imaging the evolution of biological processes at the cellular or subcellular level, making them a versatile tool in bio-applications (Lin et al., 2019). Among them, carbon quantum dots (CQDs) have key benefits of low toxicity, good water solubility and chemical stability, strong fluorescence, as well as low cost. These features, along with excellent biocompatibility have endowed CQDs with many merits that are desirable in biomedical applications, such as drug delivery, disease diagnosis, bio-sensing, and imaging-guided therapy.

The electrochemical and optical properties of CQDs can be dramatically enhanced by doping heteroatoms. For instance, nitrogen doping proved to be an effective strategy to overcome the inherent low quantum yields of conventional CQDs. The introduction of dopant atoms like N, S, O, P, etc. into CQDs can create different emission centers and traps, as well as can modify the electronic structures of CQDs. As a consequence, their emission spectra can be flexibly tuned to a large extent (Ma et al., 2020). Recently, it has been reported that the co-doping CQDs, such as N, S co-doping (NS-CQDs); N, B co-doping (NB-CQDs); N, P co-doping (NP-CQDs), etc., can be used to achieve better optical and electrochemical properties compared with the single-element doping. This is due to the synergistic effect of different heteroatoms (Li et al., 2019). CQDs co-doped with multi-elements also possess abundant functional groups (e.g. −OH, −COOH, −SH, −NH₂, and −PO₃, etc.) on their surface, endowing them with extra functionalities by conjugation with functional molecules or polymers, which can expand their application range (Park et al., 2016). Thus, doping/co-doping of heteroatoms in CQDs becomes a robust and versatile way to further improve the chemical composition and properties of CQDs.

Currently, several methods have been developed to synthesize fluorescent CQDs. Castro et al. (2016) reported a laser ablation method to prepare ultra-small CQDs (1.5−3.0 nm) in ionic liquids media, where CQDs could be stabilized by an electrostatic layer of ionic liquids. As a result, the fluorescent emission of CQDs became much broadened due to the altered electrostatic potential. Ling et al. (2020) developed a magnetic heating method for rapid, single-step synthesis of excitation-dependent fluorescent CQDs, which were further used as ink for inkjet printing to produce fluorescent patterns. In a recent study, N-CQDs of stable fluorescence, excellent cell permeability, and high photothermal conversion efficiency (42.4%) were produced via a hydrothermal method. By converting sunlight to heat, the as-prepared N-CQDs also showed intrinsic anti-cancer and anti-fungal activities. Qu et al. (2020) demonstrated the construction of a colorimetric and ratiometric dual-mode fluorescence probe using N-CQDs, which were successfully applied for highly sensitive and selective detection of formaldehyde in living cells.

Despite the significant progress in the synthesis and application of heteroatom-doped CQDs, several major limitations still exist. Traditional methods rely on labor- and energy-consuming selection of reagents and experimental conditions, so the synthesis of high performance CQDs or CQDs with targeted dopant atoms is often a tedious and time-consuming work. There are also many factors affecting the fluorescence performance of CQDs, such as carbon precursors, reaction time, solvents, dopants, surface functional groups, and particle size. However, there are no exhaustive guiding principles for the synthesis of CQDs with customized properties. The situation becomes even more complex for CQDs with multi-element doping. Thus, the ability for precise CQDs synthesis and engineering, especially in a continuous, scalable, industry-relevant flow chemistry process, remains essentially limited. Furthermore, the emission mechanism of CQDs in many cases still remain unclear. Consequently, the controllable preparation of fluorescent CQDs and the study of the fluorescence mechanism are crucial for practical applications of CQDs (El-Shabasy et al., 2021).

Compared with conventional batch reactors, microreactors have the advantages of high surface area to volume ratio, high efficiency of heat and mass transfer, good reaction reproducibility, as well as feasibility for industrial scale-up, which can benefit chemical, nucleation-based nanomaterial formations (Lin et al., 2021a, Lin et al., 2021b). In addition, microreactors can be used to resolve the process dynamics both in time and position along the microchannel, thus making the study of the nucleation and growth mechanism of nanocrystals feasible (Song et al., 2019). Therefore, microreactor technology provides an effective way to solve the problems of large-scale continuous flow synthesis, process control and safety of CQDs.

In the present study, NP-CQDs were prepared via a microflow method by using ethanolamine and phosphoric acid as precursors, aiming to develop a continuous and rapid method to prepare CQDs with uniform size distribution and multi-element doping. The influence of processing parameters on the structure and fluorescence properties of the products was also investigated. The demonstrated ultra-small luminescent NP-CQDs together with the simple, continuous, and controllable microflow process contribute to the development of new microreactor-based technologies for the scalable production of advanced functional nanomaterials for diverse applications.