Presence of photoluminescent carbon dots in Nescafe^® original instant coffee: Applications to bioimaging

Highlights

• The presence of photoluminescent carbon dots was observed in commercial Nescafe instant coffee.

• The multicolored carbon dots possess upconversion properties which are strongly photoluminescent, pH stable and biocompatible to live cells.

• The direct use of these carbon dots in live cells and guppy fish imaging revealed that such nanomaterials derived from human drinks may have more potential biological applications.

Abstract

The presence of the carbon dots (C-dots) in food is a hotly debated topic and our knowledge about the presence and the use of carbon dots (C-dots) in food is still in its infancy. We report the finding of the presence of photoluminescent (PL) C-dots in commercial Nescafe instant coffee. TEM analysis reveals that the extracted C-dots have an average size of 4.4 nm. They were well-dispersed in water and strongly photoluminescent under the excitation of ultra-violet light with a quantum yield (QY) about 5.5%, which were also found to possess clear upconversion PL properties. X-ray photoelectron spectroscopy characterization demonstrates that the C-dots contain C, O and N three elements with the relative contents ca. 30.1, 62.2 and 7.8%. The X-ray diffraction (XRD) analysis indicates that the C-dots are amorphous. Fourier-transform infrared (FTIR) spectra were employed to characterize the surface groups of the C-dots. The C-dots show a pH independent behavior by varying the pH value from 2 to 11. The cytotoxicity study revealed that the C-dots did not cause any toxicity to cells at a concentration as high as 20 mg/mL. The C-dots have been directly applied in cells and fish imaging, which suggested that the C-dots present in commercial coffee may have more potential biological applications.

Introduction

In recent years carbon nanomaterials, such as fullerenes, carbon nanotubes, graphene and carbon dots(C-dots), have attracted significant attention because of their unique electronic, mechanical, thermal, optical and chemical properties [1], [2]. Among them, C-dots are a new class of quantum dot (QD)-like photoluminescent nanomaterials which are chemically stable, environmentally and biologically compatible in contrast to the traditional semiconductor QDs [3]. The wavelength-tuneable emission property has made them promising candidates as new ‛nanolights’ for biological applications. Unlike the semiconductor QDs essentially contain heavy metals which have known toxicity and are environmentally hazardous; the C-dots are more safe and non-toxic both in cell and animal level, suggesting their good biocompatibility for biomedical applications [3], [4]. The syntheses, fluorescent properties and evaluations for sensing, imaging and photochemical catalysis of carbon dots have been intensively investigated worldwide since the original report in 2006 [5], [6], [7], [8]. The photoluminescent C-dots are considered very useful as high-performance yet nontoxic fluorescence agents for optical bio-imaging in biomedical field [9], [10]

Various synthetic methods have been developed for preparation of C-dots, such as the top-down methods of laser ablation [5] and electrochemical oxidation, [11] bottom-up methods of microwave-assisted synthesis, [12], [13] hydrothermal synthetic route [14] and combustion thermal oxidation [15]. However, all the synthetic methods suffer to some degree from drawbacks such as harsh reaction conditions, tedious processes or toxicity starting materials. In this regard, the idea of searching for C-dots within regular food can overcome the above mentioned drawbacks. This prompted us to search for C-dots in food items that can potentially be used for biological applications. Some foods have been consumed by human for centuries, that are hardly considered as toxic. From this point, the concern of the origin and toxicity of the C-dots can be easily dispelled when they are used for biological applications.

Our knowledge about the presence and the use of carbon dots in food is still in its infancy. The presence of the C-dots within food is a hotly debated topic [16]. The discovery of the C-dots in food and evaluation of their potential biological applications is a hardly mentioned area. We have come across the photoluminescence (PL) of Nescafe Original instant coffee, in which we assumed the presence of the C-dots. This motivated us to analyze the components of the commercial coffee for the discovery of photoluminescent C-dots. In this paper, the presence of photoluminescent C-dots in instant coffee was reported and their photoluminescent properties were characterized originally. Further, the direct use of C-dots was evaluated for live cells and in vivo guppy fish imaging. Nevertheless, it is hard to find such photoluminescent nanomaterials in human drinks that intrinsically satisfy the criteria for bio-imaging with strong PL and good biocompatibility, simultaneously. To the best of our knowledge, only one type of such C-dots has been reported in carbohydrate based food caramels, viz. bread, jaggery, sugar caramel, corn flakes and biscuits [16]. Thanks to their presence in human food, the C-dots in coffee have been consumed by human beings with no known toxicity and thus we believe that they may be preferred as fluorescent probes for bio-medical applications.