Ellagic acid-functionalized fluorescent carbon dots for ultrasensitive and selective detection of mercuric ions via quenching
Graphical abstract
Introduction
Mercury and its compounds are known to be extremely toxic and ubiquitously distributed in environment [1]. Hg2+ is highly water-soluble and the most stable inorganic metal ion. Through microbial-mediated methylation, Hg2+ ions get converted to methyl mercury; a well-known neurotoxin [2], [3], [4]. The continuous exposure of mercury through drinking water leads to its accumulation in muscles and liver of humans and also causes serious damages to DNA and central nervous system [5], [6], [7]. The exposure limit for inorganic mercury (Hg2+) in drinking water is set at 2 ppb (2 µg/L) by the United States Environmental Protection Agency (US-EPA) [8]. In the past decades there have been intense researches dedicated to the development of selective and sensitive mercury detection methods [9], [10]. Even though traditional and well established analytical methods like Atomic Absorption Spectroscopy (AAS), Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) and Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and electrochemical techniques have good sensitivity (down to sub-µg/L or ng/L) [9], [10]. However, these methods have various drawbacks such as high-cost, lack of mobility and/or the requirement of significant sample preparation protocols. Hg2+ ions have no optical-spectroscopic signature because of its closed-shell d10 configuration which limits a direct spectroscopic detection of the pollutant [11]. Thus, probes based on changes in the fluorescence (intensity or wavelength) induced by the presence of mercuric ions are particularly attractive due to its simplicity of use and cost-effectiveness [10], [12]. Even though variety of remarkable probes based on small organic molecules, semiconductor quantum dots and metal nanoparticles have been used for the detection of Hg2+ in drinking water, but the wide application of these probes are limited due to their complex synthesis and/or toxicity [12], [13].
Fluorescent carbon quantum dots (CDs), are sub-10 nm carbon nanoparticles, with sp2-hybridised graphite core and surface-functionalized with several groups. Thanks to the unique features of CDs such as non-toxicity, excellent aqueous dispersity, lack of photo-blinking, chemical inertness and remarkable emission quantum yield, they have been used in wide range of applications such as photovoltaics, photocatalysis, energy conversion/storage, optoelectronics, bioimaging/biosensing and chemical sensing [14], [15], [16], [17]. Interesting optoelectronic features of CDs like UV-absorption, luminescence in blue-green region and excitation-wavelength dependent/independent emission from the size-quantization in the graphitic nanodomains in the core, in conjunction with the surface functionality. The UV-absorption by the CDs may be assigned to the interband electronic excitations resulting from the confined sp2 conjugation in the carbon core. Whereas the photoluminescence, may be due to the de-excitation of the photo-generated carriers in the emissive sites formed by the hybridization of the carbon network and the functional groups [18]. The fabrication strategies of CDs are generally classified into top-down and bottom-up approaches. Top-down synthesis relies on the fragmentation of the bulk graphitic materials until the nanoparticles become luminescent. But in bottom-up route, molecular precursors are carbonized at suitable conditions [16], [17]. In the latter method, during the formation of CDs, the surface-functional groups of the precursors and/or the reactants are intrinsically transferred to the unsatisfied valences of the surface carbon atoms. Hence, not only the internal characteristics of the carbon nanoparticles (such as particle-size, crystallinity of the graphitic core, elemental composition, dispersity in solvents), the chemical and optoelectronic attributes also are strongly dependent on the synthesis strategies and the choice of precursors. In order to realize CDs with specific and custom-tailored properties; the steps of nanoparticle preparation as well as the surface-functionalization of the CDs are integrated to a single process known as Synthesis-modification integration [19], [20], [21], [22], [23], [24], [25]. This technique relies on identifying a precursor which can act as the carbon source and with suitable functional groups. Because the functional groups on the precursor will decorate the surface of the nanoparticles during the synthesis.
In this work we report a one-step fabrication of UV-photoluminescent, ellagic acid-functionalized carbon quantum dots through synthesis-modification integration strategy using tannic acid (TA) as the carbon source. The precursor, TA when mixed with sulphuric acid (without any external heat), undergo acid-catalyzed hydrolysis and further dehydration to form carbon dots. The resulting CDs exhibit excitation-wavelength independent emission at 340 nm, upon irradiation with UV light (excitation wavelengths in the range 210–350 nm). TA is selected as the precursor because of two reasons: (i) TA is a plant-polyphenol with carbohydrate (glucose) core and with ten gallic acid (GA) units [21]; primarily, the pyranose glucose ring is a good carbon source and (ii) crucially, gallic acid/digallic units can be functionalized on the surface of carbon dots during the synthesis. It have already been reported that two gallic acid units may couple to form ellagic acid (EA) derivatives in the presence of oxidizing agents [26]. The representative chemical structures of TA, GA and EA are shown in Fig. S1.
Herein this work, H2SO4 plays the role of dehydrating agent (converting the glucose core of TA to nanographitic clusters); as well as oxidizing agent (in the formation of ellagic acid derivatives, through the coupling of GA units). The physico-chemical characterizations of the synthesized nanoparticles indicate that they are surface-functionalized with ellagic acid derivatives. The CDs reported in this work were utilized for highly selective and sensitive detection of Hg2+ ions.
Section snippets
Materials
Tannic acid and sulphuric acid were acquired from Sigma Aldrich. Ethyl acetate was purchased from Merck chemicals. Dialysis tubing with molecular weight cut-off 1 kDa was purchased from Spectrum Laboratories, USA. The deionized water from Labostar TWF water purification system (18.2 MΩ cm, Siemens Ultrapure Water Systems) was used for all experiments.
Synthesis of carbon dots
In a typical experiment, 0.125 g of tannic acid was added to 10 mL conc. H2SO4 and was kept under stirring at ambient conditions for 5 min. After
Physico-chemical characterization
Fig. 1a represents typical TEM image of the CDs. The nanoparticles are well separated and apparently spherical; and have a size distribution with an average size of about 3 nm as represented in Fig. 1b. The nanoparticles were further examined by HRTEM. The inset of Fig. 1b provides the HRTEM image of a typical nanoparticle, which reveals a graphitic-like layered structure with an inter-layer separation of about 0.2 nm. Selected Area Electron Diffraction (SAED) pattern of the nanoparticles
Conclusions
In summary, we have used a rapid and single-pot, synthesis-modification strategy to prepare ultra-compact photoluminescent CDs by acid-catalyzed ester hydrolysis of tannic acid – a gallic ester of D-glucose, without the need for external heat. The prepared nanoparticles have an average size of about 3 nm, and exhibited excellent aqueous dispersion and strong photoluminescence. No special equipment is necessary for the synthesis procedure, and the chemicals used are readily available and
Acknowledgements
The authors thank Dr. Lakshmi C. (Department of Chemistry, NITC) for providing facilities for fluorescence measurements and Dr. Parameswaran P. (Department of Chemistry, NITC), for fruitful and intellectual discussions. We are grateful to Dr. Harish C. Barshilia, CSIR-National Aerospace Laboratories (CSIR-NAL), Bangalore for the XPS survey data of the nanoparticles.
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