The fabrication of excitation-dependent fluorescence boron/nitrogen co-doped carbon quantum dots and their employment in bioimaging

Abstract

Synthesis of carbon quantum dots (CQDs) with high quantum yield (QY) and excitation-dependent fluorescence emission is of great importance and provides unquestionable opportunities in biomedical applications. In this study, we have proposed a novel strategy for the fabrication of boron (B) and nitrogen (N)-co-doped CQDs (B/N-CQDs) by a simple yet facile hydrothermal method. For this, for the first time, dopamine and sodium borohydride were employed as C/N and B sources, respectively. CQDs were synthesized with excitation-dependent emission nature and high QY (25 %, the highest). The B/N-CQDs exhibited low toxicity and good biocompatibility through the cell viability tests by using MRC5 and A549 cell lines with IC50 values of 981 and 583 ppm, respectively. Lastly, the incubation of B/N-CQDs with an A549 cell line led to the emergence of blue and green colors under an excitation wavelength of 405 and 488 nm, respectively, indicating the excitation-dependent nature.

Introduction

In the last two decades, carbon quantum dots (CQDs) have arisen as a novel carbon-based nanomaterial and attracted significant attention worldwide [1]. CQDs are zero-dimensional nanomaterials with very small sizes and significantly high fluorescence activity [2], [3]. In addition, these carbon nanosystems exhibit highly desirable and tunable features such as extraordinary optical properties, high water solubility, high stability, low toxicity, excellent biocompatibility, and eco-friendly nature [4]. With these unique properties, CQDs have become promising and robust candidates for wide applications including bioimaging, biosensing, chemical sensing, nanomedicine, photocatalysis, electrocatalysis, and solar cells [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Furthermore, CQDs have superior features in comparison to traditional QDs in terms of optical properties, chemical inertness, photochemical stability, low toxicity, and, high biocompatibility [16]. These unique features make them more suitable, especially in biomedical applications [17]. For the fabrication of CQDs, both the top-down and bottom-up strategies are utilized by using different precursors [1], [4], [18]. In the top-down approach, the relatively larger carbon materials such as graphite, graphene, etc. are broken down into small pieces through various processes including arc discharge, laser ablation, and electrochemical oxidation [19], [20], [21], [22], [23], [24]. The requirement of high energy and complicated device, poor control over the process, and resultant CQDs are the main drawbacks of this strategy that has to be considered. However, in the bottom-up approach, small organic molecules are used as precursors and CQDs are synthesized through carbonization-condensation via thermal treatment [25]. Pyrolysis, microwave irradiation, combustion, solvothermal and hydrothermal treatments are commonly used methods as bottom-up strategies [25], [26]. The bottom-up strategies provide noticeable advantages in terms of simple and flexible synthesis procedures, low cost, easily accessible precursors, and short process time. Of these strategies, the hydrothermal method is the most preferred one since it is easy, cheap, and has experimental parameters such as time, temperature, and pressure that can be manipulated easily. These properties enable to tune the optical properties of CQDs through adjustable synthesis conditions [27], [28].

Despite the excellent characteristics of CQDs and recent progress in synthesis procedures, limited fluorescence activity with low quantum yield (QY, less than 10 %) and optical stability are major drawbacks that must be considered [29], [30]. Surface passivation or functionalization and element doping are the most commonly used methods to overcome these difficulties [31], [32], [33], [34]. The surface passivation or functionalization modification strategies require an additional and time-consuming procedure, produce CQDs with larger sizes, and are not feasible on a large scale [19], [35]. The doping with heteroatoms such as boron, nitrogen, phosphorus, sulfur, etc., is the most effective and easiest method to eliminate such limitations by introducing new energy levels and functional groups [36]. The emergence of new surface groups creates trap states with different energy levels resulting in the excitation-dependent fluorescence emission [19], [33], [37]. In heteroatom doping, boron (B) and nitrogen (N) are mostly utilized elements due to their noticeable advantages. Firstly, B and N are the adjacent neighbors of the C in the periodic table. Secondly, both B and N have an atomic size similar to C. Lastly, the substitution of C by B or N dramatically changes the local chemical reactivity, which enhances the optical performance [38], [39], [40], [41], [42]. Although there have been some reports on the fabrication of B/N co-doped CQDs (B/N-CQDs), further studies are still required to clarify the phenomena behind these nanosystems. Also, these CQDs with high QY and an excitation-dependent emission nature would provide unquestionable benefits in biomedical applications.

In this study, we proposed a simple yet facile hydrothermal method for the synthesis of B/N-CQDs. For this, dopamine and sodium borohydride were utilized as C/N and B source, respectively. To the best of our knowledge, this is the first time dopamine and sodium borohydride have been employed in the synthesis of CQDs. We systematically investigated the effect of synthesis parameters such as synthesis temperature, reaction time, and the ratio of precursors on UV–visible absorption and fluorescence spectra of resultant B/N-CQDs. For all runs, CQDs were fabricated with high QY and excitation-dependent emission characteristics. The B/N-CQDs obtained in optimized conditions were furtherly characterized through electron microscopy (TEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The stability of B/N-CQDs was examined by collecting UV and fluorescence spectra at certain times from the samples stored under daylight and dark conditions at room temperature. Also, the cytotoxic effect of the as-prepared CQDs was determined by using human lung carcinoma (A549) and human healthy lung cell line (MRC5). A549 cells were incubated with a certain amount of B/N-CQDs for different times for bioimaging with a confocal microscope at different excitation wavelengths. The cytotoxicity test and confocal imaging indicated the low toxicity, high biocompatibility, and suitability of the proposed nanosystem for biomedical applications.