Elsevier

Sensors and Actuators B: Chemical

Volume 282, 1 March 2019, Pages 972-983
Sensors and Actuators B: Chemical

Fuel waste to fluorescent carbon dots and its multifarious applications

https://doi.org/10.1016/j.snb.2018.11.144Get rights and content

Highlights

  • Herein, we report the conversion of fuel (kerosene) soot into fluorescent carbon dots.

  • KCDs show an excitation wavelength-independent emissive property.

  • The detection limit of both explosives and metal ions are in the order of nanogram level.

  • The unreacted soot used as an affordable filter bed for waste water treatment.

Abstract

Herein, we report the fluorescent carbon dots (KCDs) from kerosene fuel soot via simple one pot oxidative acid treatment. The obtained KCDs were thoroughly characterized by pivotal techniques. The prepared KCDs are 5 nm sized quasi-spherical particles and exhibited an outstanding stability against pH, NaCl and light irradiation. Intriguingly, the KCDs display an excitation wavelength independent emissive property. Further, the KCDs were successfully utilized for multifarious applications such as photoinduced electron transfer (PET) and sensing studies. For PET studies, charged porphyrins (tetra pyridyl and tetra sulphonato) were selected and the fluorescence of associated porphyrin was quenched and the quenching is due electron transfer. The lower recombination rate obtained from transient absorption spectral studies suggests that KCDs can be used as electron shuttling material in a cascade system of photovoltaic device. On the other hand, the fluorescent nature of KCDs was used for the selective and sensitive detection of explosives and metal ions. The detection limit for picric acid, Cu2+ and Fe3+ is found to be 86 ng/ml, 15.3 ng/ml and 0.36 μg/ml respectively. Moreover, the unreacted soot during the acid treatment has been calcined and used as an affordable filter bed for waste water purification.

Introduction

The majority of energy needs of humans is met by burning fossil fuels, which leads two major threats to the environment such as green house effect and ozone layer depletion causing global warming, acid rain, etc [1,2]. Regulatory authorities in most countries are seriously concerned with the emission of soot to the atmosphere by various activities. Inhalation of such soot causes many adverse health effects such as lung inflammation, cardiovascular problems and defects in brain activity, etc [3,4]. Therefore conversion of waste soot into value added product is one of the recent scenario among the research community. In other area of contemporary research is the design and synthesis of nanomaterials for diverse fields. Nanomaterials [5] are of great interest owing to their distinctive optical and other functional characteristics which direct them for potential applications in electronic as well as biological fields [6,7]. Among the numerous types of nanomaterials reported, carbon dots (CDs) are emerged as potential competitor to other conventional semiconducting nanomaterials [8]. CDs are characteristically nanomaterials consist of ill-defined nanocrystalline cores with sp2 hybridised carbon sheets (graphite/graphene/graphene oxide) merged by three dimensional sp3 carbon insertions [9]. They were revealed by Xu et al. in 2004 while purifying carbon nanotubes [10]. Generally both top-down [[10], [11], [12]] and bottom-up [[13], [14], [15], [16]] methodologies were known to produce CDs. Notably, a green synthetic approach, under bottom-up method, was introduced by Li et al. in which ethanol was electrochemically treated in NaOH medium for the synthesis of CDs [17]. Thereafter, numerous reports for the usage of inexpensive precursors for the production of CDs followed by various protocols were developed [[18], [19], [20], [21], [22], [23], [24]]. In this juncture, we have also derived the CDs from green resources such as tamarind and centella asiatica via one pot hydrothermal method [25,26]. However, few drawbacks like diversity in particle size, costly processing, small scale production and poor chemical stability were faced while using green resources as precursor [[27], [28], [29]].

Conversion of waste biomass or pollutant into CDs is a contemporary in research which has been kicked off by Lee and his research group [30]. They have introduced bulk synthesis of CDs from food waste via ultrasonication method. Similarly, a few reports were presented for the conversion of soot collected by burning candle, natural gas etc into CDs for biological applications [31,32]. Based on this context, our ultimate aim is to convert a waste material such as kerosene fuel soot into value added product like nanostructured CDs for the effective utilization in the field of photovoltaic as well as sensing fields.

The usage of carbon based materials in photovoltaic and sensing applications are growing interest [[33], [34], [35], [36], [37]]. For instance, in dye-sensitized solar cells (DSCs) integration of graphene sheets into TiO2 photoanodes gave five times higher power conversion efficiencies than those obtained from TiO2 photoanodes without graphene sheets [36]. Imahori et al. reported the efficient hierarchical electron transfer cascade system on a semiconducting electrode by using rGO sheet to anchoring porphyrins/ZnO hybrid materials and that cascade system exhibited remarkably high photocurrent generation with an incident-photon-to-current efficiency (IPCE) of up to 70% compared with the reference device without rGO sheets [30]. Kamat et al. reported the CdSe–graphene composites in quantum dot sensitized solar cells display improved photocurrent response (∼150%) over those prepared without GO [37]. Therefore, it is significant to study the interactions between light harvesting materials and carbon nanomaterials. For interaction studies, porphyrin was selected due to its higher efficiency in dye sensitized solar cells over than other dyes [[38], [39], [40]]. The selected porphyrins were meso-tetrakis (1-methyl-4-pyridinio) porphyrin [TMPyP] and meso-tetrakis(4-sulfonatophenyl) porphyrin [TSPP] shown in Scheme S1.

On the other hand, CDs were highly utilized due to their fluorescence property and water soluble in nature. For instance, Wu et al. developed simple amine-capped C-dots for the selective detection of picric acid (PA) in water medium and explored that there is a strong electrostatic interaction between the amine capped CDs and PA [41]. Liu and co workers were reported, the high fluorescence quantum yield of CDs capped with BPEI using bamboo leaves as carbon source. The prepared CDs were utilized for the sensitive and selective detection of Cu2+ in real water sample [42]. For the sensitivity of Fe3+, the blue emitting CDs were developed from citric acid and ethylene diamine via a hydrothermal method. The mechanism of sensing was due to complex formation between Fe3+ ion and phenolic hydroxyl groups (−OH) on the CDs surface leads electron transfer from excited state CDs to unfilled d orbital of Fe3+ ion [43]. These milieu are tempted us to utilize the KCDs in PET and sensing studies.

In the present work, we have successfully converted the waste soot collected from the kerosene fuel burning into value added product namely carbon dots (KCDs). Since it is reported that CDs have wide range of applications, herein we have utilized the prepared KCDs in multi-target applications such as PET studies with porphyrins in view of usage of KCDs as electron shuttling mediator in cascade system of photovoltaic device and the fluorescent nature of KCDs is effectively used for the selective and sensitive detection of explosives and heavy metal ions. The sensing studies have also been tested in real water sample in order to prove the practical applicability of the analysis. On the other hand, the unreacted soot has also been applied as an affordable filter bed for waste water purification. Therefore, the present manuscript unambiguously gives the knowledge of effective usage of the prepared nanomaterial (Waste to Nano) for multifarious applications.

Section snippets

Materials

Kerosene was purchased from a local provisional store. Porphyrins, [meso-tetrakis (1-methyl-4-pyridinio) porphyrin and meso-tetrakis (4-sulfonatophenyl) porphyrin] were procured from Sigma–Aldrich. Analytical grade sodium carbonate and nitric acid were acquired from LOBA Chemicals (India). Picric acid and other nitro aromatic compounds were purchased from Merck. ZnCl2, NiCl2.6H2O, CuCl2.2H2O, MgCl2.6H2O, CoCl2.6H2O, BaCl2.2H2O, PbCl2, FeCl2, NaCl, KCl, MnCl2.4H2O and FeCl3 were purchased from

Results and discussion

The ultimate aim of this work is the effective utilization of carbon dots from a waste material for multifarious applications in view of sustainable development. From the oxidative acid treatment of kerosene soot, the obtained brown solid product (Scheme 1) was highly water soluble in nature and refrigerated at 4 °C which exhibited the storage stability of more than three months without precipitation. Fig. 1a shows the TEM image of KCDs measured in 20 nm scale which indicates that most of the

Conclusion

In conclusion, the kerosene fuel soot was successfully converted into nanostructured carbon dots via a one-step oxidative acid treatment. The KCDs particles were quasi-spherical in shape and average diameter of 5 nm which consist of various oxygen enclosed surface functionalities imparting excitation independent fluorescence behavior. The intermolecular interactions between the porphyrin and KCDs in aqueous medium were investigated and the possibility of electron injection from porphyrin to

Acknowledgments

V.S. thanks to the Department of Chemistry and BSACIST for the Junior Research Fellowship. M.A.J. thanks the Department of Science and Technology (DST), New Delhi, India for the DST-SERB Project (Ref. No. SB/FT/CS-125/2013, Dt. 30/06/2014). MAJ also thanks to SAIF, IIT Madras for assistance with NMR, HR-SEM and TCSPC facilities. MAJ also acknowledge to Ms. S. Thulasi, project fellow and Ms. G. Loganayagi, PG student for their assistance in soot collections and benchmark experiments. A.K. thanks

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