Nitrogen-doped carbon dots originating from unripe peach for fluorescent bioimaging and electrocatalytic oxygen reduction reaction

Abstract

This paper reports the robust hydrothermal synthesis of nitrogen doped carbon dots (N-CDs) using the unripe fruit of Prunus persica (peach) as the carbon precursor and aqueous ammonia as the nitrogen source. The optical properties of synthesized N-CDs were characterized by ultraviolet visible (UV–Vis) and fluorescence spectroscopy techniques. The synthesized N-CDs were emitted blue light when excitated with a portable UV lamp. The materials with the optical properties were characterized further by high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), Raman, Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). The mean size of the N-CDs was approximately 8 nm, as calculated from the HRTEM image. The d-spacing of N-CDs, calculated using Bragg law, was approximately 0.21 nm, which was consistent with the interlayer distance calculated from the HRTEM image. FT-IR spectroscopy and XPS revealed the presence of the phytoconstituents functionalities of peach fruit over the N-CDs surface and a high level of nitrogen doping on carbon dots (CDs) was confirmed by XPS studies. These results suggest that the unripe fruit extract of peach is an ideal candidate for the preparation of N-CDs. The resulting N-CDs showed excellent optical properties in water. The synthesized N-CDs exhibited a high fluorescence quantum yield and low cytotoxicity, and can be used as fluorescence imaging probes. In addition, the N-CDs were catalytically activite towards the oxygen reduction reaction (ORR). The N-CDs exhibited good catalytic activity in an alkaline medium (0.1 M KOH) with a remarkable ORR of approximately 0.72 V vs reversible hydrogen electrode (RHE), and O₂ reduction follows mainly a 2 electron pathway by being reduced to hydrogen peroxide. The 2-electron reduction pathway is used in industry for H₂O₂ production.

Introduction

Recently, photoluminescent nanomaterials have attracted considerable attentions owing to their attractive properties, such as optical and electronic properties. Semiconductor quantum dots (SQDs) and organic dyes have attracted much attention for their potential applications in many fields such as photoluminescence, sensors, energy and bio-imaging [1], [2], [3], [4]. On other hand, the heavy metal containing SQDs which may lead to potential cytotoxicity in biological systems and the photostability of organic dyes is poor. This drawback can be overcome by a new fluorescent nanomaterials, photoluminescent carbon dots (CDs) as a new class of nanoparticles, which have attracted considerable attention due to exceptional advantages, such as low toxicity, high chemical stability, good biocompatibility, excellent optical performance, and low photo bleaching. Recently, nitrogen-doped carbon dots (N-CDs) have great attention due to their fascinating properties and applications, one of which is the highly enhanced photoluminescence [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].

The most common methods for the synthesis of the CDs, include the chemical oxidation method, ultrasonic method, hydrothermal synthesis, solvothermal method, microwave method, and laser ablation method. However, it is still a problem for progress in controlling the morphology, particle size, and surface chemistry of the resultant products with high quantum yields. The green chemical routes of CDs from plant materials are simple and eco-friendly. Authors already reported the synthesis of florescent CDs using Prunus mume and were applied as a straining probe for bio-imaging application [15]. In the present study, Prunus persica (peach) was chosen as a carbon source for the synthesizing CDs due to it is easily available and affordable, and were consumed as a common food and medicinal materials. Peach belongs to family rosaceae is known for its nutritional value and therapeutic properties. The major constituents of peach fruit are carbohydrates, organic acids, minerals, and dietary fibre, which contribute to the nutritional quality of both the fresh fruits and the juice [16], [17]. Peach being a potential source of bioactive compounds has been demonstrated to possess medicinal properties. Phenolic acids, flavonoids, and anthocyanin are the major sources of potential antioxidants in peach fruit. The nanomaterial synthesis and activity of them depends on the phytoconstituents present in the particular extract. The sucrose and glucose were dominant sugars, and citric acid was dominant organic acids in unripe peach fruits, this might be helpful to formation of carbon materials. The unripe fruits possess more acidic constituents (citric acid) compared to ripen fruits [18]. The glucose content nearly same for ripen as well as unripe fruits. Based on the above reasons, the unripe peach has been chosen as a carbon source for the synthesis of CDs. Recently, hydrothermal-carbonization has proved to be an eco-friendly, easy and soft chemical route for the synthesis of CDs and N-CDs in aqueous media, which produces highly efficient fluorescent probes for cell imaging in biomedical application [19], [20], [21], [22]. The synthesized N-CDs have also been used as an electrocatalyst for the oxygen reduction reaction (ORR) which is an important reaction in energy converting systems, such as fuel cells [23], [24], [25], [26]. ORR in aqueous solutions occurs mainly by two pathways: the direct 4-electron reduction from O₂ to H₂O, and the 2-electron reduction from O₂ to hydrogen peroxide (H₂O₂)·H₂O₂ has been one of the most important chemicals in the world and the 2-electron reduction pathway is used in industry for H₂O₂ production [27], [28], [29].

Herein, highly photoluminescent N-CDs with narrow size distribution were synthesized for the first time by simple hydrothermal process using the extract of unripe peach fruit and aqueous ammonia as the carbon precursor and nitrogen source, respectively. Aqueous ammonia was also used as a nitrogen dopant of the peach fruit extract. The optical properties of obtained nanoparticles were examined by using UV–Vis and fluorescence spectroscopy in water. In Addition, the fluorescent N-CDs exhibited hypotoxicity and can be a biocompatible candidate for bio-imaging in living cells. The synthesized N-CDs were used as catalyst towards ORR. To the best of our knowledge, the unripe peach is first time used for the synthesis of N-CDs and as a staining probe for fluorescence cell imaging and electrocatalyst for ORR.