Biological and catalytic applications of green synthesized fluorescent N-doped carbon dots using Hylocereus undatus

Highlights

• The highly fluorescent N-CDs was synthesized from H. undatus extract.

• The simple hydrothermal carbonization method was employed for the synthesis of N-CDs.

• The hydrothermal carbonization method is cost effective, safe route and eco-friendly.

• The synthesized N-CDs used as an excellent catalyst for the dye reduction.

• The N-CDs, exhibits excellent cytotoxicity on L-929 and MCF-7 cells.

Abstract

In this work, a simple hydrothermal route for the synthesis of fluorescent nitrogen doped carbon dots (N-CDs) is reported. The Hylocereus undatus (H. undatus) extract and aqueous ammonia are used as carbon and nitrogen source, respectively. The optical properties of synthesized N-CDs are analyzed using UV–Visible (UV–Vis) and fluorescence spectroscopy. The surface morphology, elemental composition, crystallinity and functional groups present in the N-CDs are examined using high resolution transmission electron microscopy (HR-TEM) with energy dispersive spectroscopy (EDS), selected area electron diffraction (SAED), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy, respectively. The synthesized N-CDs emit strong blue fluorescence at 400 nm under the excitation of 320 nm. Further, the excitation dependent emission properties are also observed from the fluorescence of synthesized N-CDs. The HR-TEM results reveal that synthesized N-CDs are in spherical shape with average diameter of 2.5 nm. The XRD pattern exhibits, the graphitic nature of synthesized N-CDs. The doping of nitrogen is confirmed from the EDS and FT-IR studies. The cytotoxicity and biocompatibility of N-CDs are evaluated through MTT assay on L-929 (Lymphoblastoid-929) and MCF-7 (Michigan Cancer Foundation-7) cells. The results indicate that the fluorescent N-CDs show less cytotoxicity and good biocompatibility on both L-929 and MCF-7 cells. Moreover, the N-CDs show excellent catalytic activity towards the reduction of methylene blue by sodium borohydride.

Introduction

The carbon dots (CDs) are new class of carbon materials with size ranging below 10 nm and they are accidently discovered during the purification of single wall nanotubes (SWNT's) in 2004 [1], [2], [3]. The CDs attract many researchers due to their fascinating properties such as excellent water solubility, high photo stability, unique excitation-dependent fluorescence, low toxicity and superior biocompatibility [4], [5], [6]. Owing to the unique properties, CDs are applied in the fields of bioimaging, sensing, catalysis, and light emitting devices [7], [8], [9], [10]. Several physical, chemical and electrochemical methods have been employed for the synthesis of CDs such as laser ablation of graphite, hydrothermal and solvo-thermal carbonization of organic compounds, microwave pyrolysis and ultra-sonication [11], [12]. Among these, hydrothermal carbonization method is simple, eco-friendly, safe and cost effective for the synthesis of CDs. Many carbon precursors including carbohydrates, ascorbic acid, citric acid, tannic acid and organic sources have been utilized for the synthesis of CDs through the hydrothermal method. The synthesis of CDs mainly depends on acidic, basic and neutral constituents present in the plant extracts. The most favorable condition for preparing high yield CDs is that, the plant extracts must have acidic (ascorbic acid, citric acid and tartaric acid) and neutral (carbohydrate) constituents. The fluorescent properties of CDs are purely based on size, dopants, pH and solvents [13], [14], [15]. Doping of heteroatoms such as nitrogen, boron and sulphur on CDs, enhance the fluorescence and catalytic properties. Hence, the heteroatom doped CDs are mainly used as fluorescent probe in biological, catalytic and sensing applications [16], [17], [18], [19].

Hylocereus undatus (H. undatus) is belonging to the Cactaceae family and native to South and Central America. H. undatus is a well-known plant for its fruit, and is commonly known as “dragon fruit”. It is fully covered by red skin with edible white flesh and commercially cultivated in Vietnam, Thailand, Malaysia, Israel as well as India and Sri Lanka [20], [21], [22]. The phytochemical screening of the H. undatus showed the presence of triterpenoids, alkaloids, flavonoids, saponins, vitamin C, polyunsaturated fatty acids and lipids [23], [24], [25]. In the view of above, the fruit extract has been chosen as a carbon source for the preparation of CDs.

Methylene blue (MB) is a carcinogenic thiazine dye and it creates severe health problems such as mutations, cancer, skin irritations, allergic dermatitis and eye burns to both human and animals [26]. MB is widely used in surgery, microbiology, diagnostic fields and also in the aquaculture industry [27], [28], [29]. Since, MB is commonly found as pollutant in the effluents of these industries and hence the reduction of MB is environmentally and biologically important [30], [31].

This work reports, the synthesis of nitrogen doped CDs (N-CDs) using H. undatus fruit extract and aqueous ammonia as a carbon and nitrogen precursor, respectively. The synthesized N-CDs are characterized by UV–Visible (UV–Vis), fluorescence spectroscopy, high resolution transmission electron microscopy (HR-TEM) with energy dispersive spectroscopy (EDS), selected area electron diffraction (SAED), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy techniques. Further, the cytotoxicity and biocompatibility of N-CDs are tested on the L-929 (Lymphoblastoid-929) and MCF-7 (Michigan Cancer Foundation-7) cells. The catalytic activity of synthesized N-CDs are also analyzed towards the reduction of MB using sodium borohydride as reducing agent.