Green and cost-effective fluorescent carbon nanoparticles for the selective and sensitive detection of iron (III) ions in aqueous solution: Mechanistic insights and cell line imaging studies
Graphical abstract
Introduction
The design and synthesis of carbon nanoparticles (CNPs) based sensors for sensing of ionic species at extremely low concentrations, relevant environment and biology. In particular sensing transition metal ions have been of interest [1], [2], [3]. Transition metals are naturally occurring materials and it can be extracted from mines, industrial wastes, vehicle emissions, fertilizers, paints and treated woods. Transition metals enter inside plant through the uptake of water and inside animal and human tissues via air inhalation, diet and manual handling [4]. Because of the transition metals importance in human life and plant, numerous reports have been focused on selective and sensitive detection of these metal ions (Cu2+, Pb2+, Hg2+, Cr6+, and Ag+) using fluorescent nanoparticles as a sensing probe [5], [6], [7], [8], [9]. However, baring a few scattered reports, the CNPs have not been used widely as a fluorescent sensor for Fe3+ ion.
The sensing of iron is important owing to the fact that iron is one of the most abundant transition metal ion (including Cu2+ and Zn2+) in human body and its trivalent form (Fe3+) is essential in human being [10]. Fe3+ is involved in various biological processes such as oxygen-carrying function, electron transfer processes in DNA and RNA and cofactor in many enzymatic reactions [11], [12]. Both the deficiency of iron (hypoferremia) and iron overload (hyperferremia) are harmful for human beings but lack of iron is even more harmful than efficiency of iron under certain condition [13]. The recommended amount of Fe3+ range in normal adults is 8–18 mg per day [14].
Although significant efforts have been directed at the sensing of Fe3+ in mixed solvent (aqueous/nonaqueous solvent) using fluorescent dye [13], [15], [16], [17], [18], [19], polymer [20], [21] and nanoparticles [22], [23], [24] as a probe, still there exists some limitations; of sensitivity and selectivity. Thus, the development of selective and sensitive fluorescence sensor for the detection of Fe3+ ion in aqueous medium is still a challenge. Recent years have seen an upsurge in the use of nanoparticles as fluorescence sensors due to their outstanding photo stability, green synthesis, favorable biocompatibility, good water solubility [25], [26], [27], [28]. Till date a vast number of methods have been reported for preparation of CNPs from different sources via (1) microwave irradiation of sucrose [29], polyethylene glycol (PEG) [30], (2) hydrothermal treatment of chitosan [25], pomelo peel [31], (3) dispersion of lamp soot prepared from mustard oil [32], nitric acid oxidation of carbon soot prepared from candle [33], castor oil soot by acid treatment [34], (4) laser irradiation on graphite powder [26], (5) by ultrasonic acid and alkali treatment of glucose [35], carbonization of glucose with different dehydrating agent [36] and (6) by electrochemical synthesis [28]. Recently, Yang et al. [37] have synthesized CNPs from glucose by light induced method for Fe3+ sensing. Although the CNPs synthesized from glucose by light induced method have been efficiently utilized in the sensing of Fe3+, the detection limit is restricted to ppm level [37]. Thus, selective and sensitive detection of Fe3+ is highly desirous at present. (Table 1).
In the present work, we report green and cost-effective CNPs obtained from acid assisted carbonization of glucose based on iron sensing in aqueous medium with ppb level detection limit.
Section snippets
Materials
d-glucose and concentrated hydrochloric acid were purchased from Merck for the synthesis of CNPs. PbCl2, Mn(CH3COO)2·4H2O, BaCl2·2H2O, CdCl2, CoCl2·6H2O, CrCl3·6H2O, FeCl3, FeCl2, CuSO4, NiCl2·6H2O, CH3COOAg, HgCl2, ZnCl2 and KCl, KBr, KI, KSCN, KH2PO4, KNO3, KSO4, NaF, CH3COOK were purchased from Sigma–Aldrich for ion sensing.
Instruments
UV–vis electronic absorption spectra measurements were carried out in a Jasco V-650 UV–vis Spectrophotometer with a scan rate of 1000 nm s−1. Fluorescence emission spectra
Morphological characterization
The core size of CNPs, was determined by transmission electron microscopy (TEM). Fig. 2A shows the TEM image of the aqueous monodispersed CNPs. Size of CNPs was obtained as less than 5 nm. The spherical particles are completely soluble in water. Average size distribution of CNPs was also calculated from dynamic light scattering (DLS) experiment which exhibits larger sizes of CNPs (220 nm diameter) due to aggregation in solution (Fig. S1 in the Supplementary information). The surface compositions
Fluorescence quenching study
All the experiments were performed at room temperature in 10 mM Tris–HCl buffer solution at pH 6.75 for the detection of Fe3+. Series of solution of Fe3+ (2–1000 μM) were added to the CNPs, and fluorescence emission spectra were recorded. It was found that the fluorescence intensity of CNPs gets quenched by Fe3+ ion significantly by increasing the concentration of Fe3+, (Fig. 4A). The quenching of CNPs fluorescence by Fe3+ was due to adsorption of Fe3+ on the surface of CNPs. The sensing
Conclusions
In summary, we have used water soluble fluorescent CNPs as a green and cheap sensor for the sensing of Fe3+ in aqueous medium. The CNPs were synthesized from d-glucose using green and cost effective way and even without the need of extra immobilization steps for CNPs. The present CNPs system in aqueous medium provides selective recognition of Fe3+ via static quenching in ppb level. Importantly, this strategy reduces the need to use organic dyes, quantum dots and organic solvents, which are not
Acknowledgements
We thank CSIR, New Delhi for financial support for this work. V.S. is thankful to university grant commission (UGC) New Delhi for a research fellowship. We thank Prof. R. S.Verma, Department of Biotechnology IIT Madras and Prof. E. Prasad, Department of Chemistry for fluorescence microscopy and DLS facilities respectively.
Vikram Singh has completed his B. Sc. (2008) from Ewing Christian College, Allahabad (University of Allahabad, India), and M.Sc. (2010) from Lucknow Christian College, Lucknow (University of Lucknow, India). Currently he is a PhD student at Department of Chemistry, Indian Institute of Technology Madras Chennai, India. His research interests are on designing green and cost-effective novel fluorescent sensors and white light emitting materials.
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Vikram Singh has completed his B. Sc. (2008) from Ewing Christian College, Allahabad (University of Allahabad, India), and M.Sc. (2010) from Lucknow Christian College, Lucknow (University of Lucknow, India). Currently he is a PhD student at Department of Chemistry, Indian Institute of Technology Madras Chennai, India. His research interests are on designing green and cost-effective novel fluorescent sensors and white light emitting materials.
Ashok Kumar Mishra is a professor in Department of Chemistry, Indian Institute of Technology Madras Chennai India. He received doctor degree from Indian Institute of Technology Kanpur, India in 1985. His interest is in the general area of physical photochemistry and fluorescence spectroscopy.