Table sugar derived Carbon dot – a naked eye sensor for toxic Pb2+ ions
Graphical abstract
Introduction
Carbon dots (CDs), one of the latest entries of carbon nano world, are tiny particles of carbon (less than 10 nm in size) with some sort of surface passivation. These particles were first discovered quite accidentally during the purification of single-walled carbon nanotubes [1]. The structure and components of CDs determine their diverse properties, which in turn decide their applications. Carbon Dots derived from different precursors exhibit different structures and find applications in various fields [2,3]. On account of the consummate features discovered in recent period, they are considered as amazing candidates among the nano carbon family. Many of the outstanding features of carbon dots were exploited in the fields of energy devices, environmental applications, bio-medical fields as well as in the fabrication of composite materials. The ability for electron storage and transport has been utilized in the designing of carbon nanodots based solar cells [[4], [5], [6]], organic LEDs [7,8], photo catalysts [9] and photo detectors [10]. Recently, they are also being explored for their applications as super capacitors [11].The most studied photoluminescence (PL) character outweighs all the other features, and moulds it as a promising green alternative to luminescent organic dyes and inorganic quantum dots in optics.
Carbon dots are conventionally synthesised from carbonaceous materials like graphite and carbon nanotubes by physical methods like laser ablation [12], vapour deposition of soot [13] microwave synthesis [14,15] ultrasonic treatment [16,17] plasma treatment, arc discharge [1], electrochemical oxidation, thermal oxidation and wet chemical/electrochemical methods [[18], [19], [20]].Green chemistry assisted fabrication of CDs has received recent attention, which eases the labour, reduces both the cost and time, and results in the formation of environmentally benign material with high yield. Recently, several groups have succeeded in the green synthesis of carbon dots by hydrothermal treatment from many abundantly available natural materials like banana juice [19] orange peel [20] sugar cane bagasse [21], sugar cane juice [22], trapabispinosa peel [23] etc.
Present work demonstrates an extremely facile and environment friendly fabrication of highly luminescent carbon nanodots from commercially available Table sugar (TS-CDs). Chandra et al. and Mehta et al. have proven that sucrose containing materials can serve as excellent precursors for highly luminescent carbon nanoparticles. The former group has attained luminescent carbon nano particles from sucrose [24]. However, the usage of ortho-phosphoric acid and stringent separation procedures make the synthesis route very complicated and un-welcoming. Mehta and co-workers have reported the fabrication of water dispersible carbon nanodots from Saccharum officinarum (sugar cane) juice which is excellent for bio-imaging application [25]. But, their synthetic strategy was quite tedious and it demanded the use of dichloromethane (DCM) during the purification stage. Time consuming post-chemical treatments including ultra centrifugation also can be raised as the demerit of this procedure.
Present work demonstrates a synthetic route which is entirely devoid of toxic materials as well as tedious post-chemical treatments such as ultra filtration, dialysis, centrifugation etc. The prepared carbon dots are characterised using Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX) analysis, Atomic Force Microscopy (AFM), X-ray Diffraction analysis (XRD), Fourier Transmission Infra-Red (FTIR) spectroscopy, Raman spectroscopy, Ultra-Violet/Visible absorbance spectral analysis and photoluminescence spectroscopy. An interesting application of this system is the visual detection of Pb (II) ions in parts per billion levels, which is achieved through the aggregation of carbon nanoparticles. Scattering of light by the aggregated carbon dots allowed quantification of lead ions by simple turbidimetry method. Reports on carbon dot systems as visual Pb2+ sensor through turbidimetric route are nil in literature. This happens to be a highly significant observation when the lethal character of Pb (II) ions, a highly probable candidate in drinking water, is considered. Lead ions may enter drinking water when service pipes and other related accessories containing lead corrode in presence of acidic water/water with low mineral content [26]. According to the established standards, the maximum permissible amount of lead in drinking water is 15 μ g/L (15 parts per billion), as prescribed by U.S Environmental Protection Agency (EPA) [26]. The limit of detection (LOD) of lead ions achieved by the present system through turbidimetric method is 14 ppb, which is highly remarkable in the light of the allowed concentration specified above.
Section snippets
Microwave assisted synthesis of carbon dots from table sugar (TS-CDs)
Table sugar was collected from the local market. Ammonia solution was obtained from Merck. Carbon dots were synthesised by the following procedure. Table sugar was heated at 150 °C for 3 min, which resulted in the formation of pale yellow slurry enriched with water molecules. Dilute ammonia solution was added, and the solution was subjected to microwave (M.W) irradiation for about 3 min at 120 °C. The solution was then cooled, filtered and subjected to freeze-drying to yield carbon dots.
Sample characterization
HR-TEM
Plausible mechanism of formation of TS-CDs
Possible mechanistic route to TS-CD formation can be obtained as follows. Caramelization of table sugar results in the decomposition of saccharose structure of table sugar into the mixture of component moieties, glucose and fructose [27,28,29]. The reducing sugar thus formed undergoes further reactions. It was observed that ammonia solution enhances the rate of this reaction [30]. The author also cited that “Ammonia caramel is formed in a Maillard-type reaction where carbonyl compounds react
Visible detection of Pb2+ ion in parts per billion (ppb) levels
TS-CDs serve as an excellent tool for the detection of one of the noxious heavy metal ions, Pb2+. It was quite interesting to see that the carbon dot solution turns turbid in presence of ppb level concentrations of lead ions, which can be monitored by naked eye (Fig. 3). The reason behind this phenomenon is supposed to be the aggregation of dots in presence of Pb (II) ions. As indicated in Fig. 4(a), no other ions could evoke such a response when in contact with CD solution at this much
Conclusion
Simple microwave irradiation of ammoniated table sugar slurry was carried out for the fabrication of luminescent carbon dots. Even though the literature is rich with a good quantum of carbon dot based Pb (II) sensors with commendable features, visual sensing of this lethal ion via facile tactics is never reported. Naked eye detection of a toxic metal ion in ppb levels with extreme selectivity is considered to be a laudable feature for a sensing gadget. It is noteworthy that a simple
Disclosure of interest
The authors declare that they have no conflicts of interest concerning this article.
Acknowledgements
Ansi V.A gratefully acknowledges the financial assistance received from the Government of Kerala, and the Department of Chemistry, University of Calicut for the facilities provided.
V.A. Ansi had her Post Graduation and M.Phil. Degree from University of Calicut, Kerala, India. Presently she is pursuing research for Ph.D. in the Department of Chemistry of University of Calicut under the corresponding author. Her research area is Carbon nanodots.
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V.A. Ansi had her Post Graduation and M.Phil. Degree from University of Calicut, Kerala, India. Presently she is pursuing research for Ph.D. in the Department of Chemistry of University of Calicut under the corresponding author. Her research area is Carbon nanodots.
N.K. Renuka is presently Associate Professor in the Department of Chemistry of University of Calicut, Kerala, India. She had her Post Graduation from University of Calicut, and M. Phil. and Ph.D. Degrees from Cochin University of Science and Technology, Kerala. Her current research interests are Cabon nano materials and sensors.