Green facile synthesis of Nitrogen-doped, fluorescent carbon dots as multifunctional nanotracker for biomedical applications

Abstract

Carbon dots (CDs) are biocompatible quantum dots with unique, tuneable optoelectronic properties, such as high photoluminescence, small bandgap, long-term photostability, size dependent emission, excellent aqueous solubility with rich surface chemistry. In the recent times, CDs have emerged as promising eco-friendly, next-generation alternative for semiconductor quantum dots and traditional organic dyes, which can bypass the environmental hazards and health concerns posed by heavy metals and other toxic chemicals [1-2]. The strong photoluminescence of CDs facilitates their extensive use as a probe for bioimaging and as a nanotracker in gene/ drug delivery, serving dual role for theranostics [3]. Laser ablation, hydro/solvothermal treatment, electrochemical/ chemical oxidation, pyrolysis and microwave-assisted synthesis are some well-known methods for synthesis of CDs synthesis using different precursors [4]. The performance and properties of CDs can be modulated through surface modifications and doping [5]. Heteroatom doping of boron, phosphorus, nitrogen and sulphur can tune the emission from ultraviolet to visible to even infrared, thus, providing more active sites for surface functionalization and enhancement of quantum yields (QY) [5-8]. Nitrogen is the most effective dopant than others because atomic size of nitrogen and carbon is similar [5]. Nitrogen is highly electronegative with five valence electrons for chemical bonds and one lone pair of electrons available for transfer to the π-orbitals of sp² carbon [5]. Doping of CDs with nitrogen changes the electronic structure and photophysical properties of N-CDs (nitrogen doped CDs), wherein N-doping increases the photoluminescence (PL), results in shift of PL and hence, changes light absorption and emission [5]. In addition, N-doping can effectively lead to diverse composition and structures of CDs and hence enhance their PL emission and quantum yield [7].

In this study, N-CDs were synthesized using citric acid and ammonia as precursor and nitrogen dopant, respectively via microwave pyrolysis method [7]. The schematic representation for synthesis of N-CDs indicates the formation of N-CDs, which was evident from the change in colour of transparent solution to dark yellowish-brown (Fig. 1A). The as-synthesized N-CDs were characterized using UV-Vis and Fluorescence spectroscopy, Transmission Electron Microscopy (TEM), Zeta potential, Energy Dispersive X-ray Spectrometer (EDS) and Fourier Transform Infrared (FTIR) spectroscopy. UV-vis spectrum exhibited two absorption peaks, one at 240 nm and the other at 340 nm (Fig. 1B), depicting the typical absorption of an aromatic  system. Moreover, aqueous dispersion of N-CDs emitted strong blue fluorescence upon excitation at 365 nm (inset of Fig. 1B). These N-CDs displayed a strong emission peak at 450 nm (Fig. 1B) with excitation wavelength-dependent PL behaviour. N-CDs revealed band gap of 3.4 eV (Fig. 1C), The synthesized N-CDs were mono-dispersed and stable with an average size of 2±1 nm (Fig. 1D) and a mean zeta potential value of 15 ± 2 mV (Fig. 1E). EDS spectra revealed that N-CDs were composed of carbon (36.77 %), nitrogen (16.49 %) and oxygen (46.74 %) elements only (Fig 1F). In vitro toxicity of the N-CDs was evaluated against the mammalian HeLa cell lines and fungal pathogen, Candida albicans. The N-CDs efficiently internalized into HeLa cells and fungal cells and exhibited no toxicity. The internalized N-CDs are visible as blue fluorescence inside the fungal cells (Fig. 1G). This study demonstrated that synthesized N-CDs were non-toxic, biocompatible with long term PL stability and suitable as nanotracker for multifunctional biomedical applications.