Usage of coconut coir for sustainable production of high-valued carbon dots with discriminatory sensing aptitude toward metal ions

https://doi.org/10.1016/j.mtchem.2020.100247Get rights and content

Highlights

  • Development of coconut waste-derived fluorescent carbon dots was carried out by a thermal calcination.

  • The biocompatible behavior was evaluated by employing quantitative multi-assay approach on different species.

  • The developed system found highly selective toward cadmium and copper metal ions.

  • The practical utilities of the biocompatible C-dots have been checked over different water resources.

Abstract

A novel, remunerative, green synthetic methodology without adding any kind of extra chemical reagent has been proposed for synthesizing highly fluorescent carbon dots (C-dots) by using waste coconut coir as the C source. The as-prepared C-dots possessed a quantum yield of 48% and displayed a high solubility rate in aqueous media. The fluorescence emission intensity of C-dots is found to be excitation dependent with high stability toward variation in the reaction medium. The bio-congenial performance of C-dots has been checked by employing quantitative multi-assay including fungus, plant, aquatic animals, and at the chromosomal level. It is notable that the antibacterial studies of as-prepared C-dots have presented appealing outcomes which advocated that C-dots prepared using coconut coir are biocompatible toward both Gram-positive and Gram-negative bacteria with no zone of inhibition in much higher concentration ranges. The as-prepared C-dots provided a simple sensing podium with high sensitivity and selectivity with a “turn-on and turn-off” fluorescence response for the detection of dual metal ions i.e. cadmium and copper ions in aqueous media with a detection limit of 0.18 nM and 0.28 nM, respectively. The practical utilities of the biocompatible C-dots have been checked over different water resources. The current work provided a simple, easy, bio-matched, harmless, rapid, and cost-effective sensory probe for wastewater remediation.

Introduction

The environmental depreciation via the toxic impact of heavy metal ions has turned out to be a major concern due to their harsh impact on our bionomics [1,2]. Out of different types of existing metal ions, the lethal impact of cadmium(II) (Cd2+) and copper(II) (Cu2+) ions has produced severe damage to our ecosystem and human health due to their higher rate of stability [3,4]. The overexposure to copper metal ion could produce rigorous harm to our nervous system and lead to various neurodegenerative diseases like Wilson's disease and Alzheimer’s disease [5,6]. In addition, the overexposure to Cd2+ ions leads to diseases like “itai-itai”, failure of liver and kidney, and lethal impact on reproductive, immune, and respiratory systems of living beings [7,8]. Simultaneously, Cd2+ ions are considered to have high inclination to intercommunicate with thiol groups of proteins and form highly stable and poisonous cadmium–metallothionein complexes in living systems [9,10]. Therefore, the detection of the respective ion from different types of water resources with high selectivity and sensitivity is congregating so much consideration among researchers [9,11]. To date, various available analytical techniques such as atomic absorption spectroscopy, potentiometric techniques, and inductively coupled plasmon resonance strategies have been employed for sensing the existence of metal ions. However, the main difficulties associated with these techniques are their hefty procedural cost with preliminary time-consuming preparation processing and requirement of professional expertise for handling sophisticated instruments [[12], [13], [14], [15]].

In contrast to available techniques, the fluorescence spectrophotometry-based methodology has been found to be quite effective for metal ion sensing due to its high selectivity and sensitivity and low procedural expenditure [[16], [17], [18]]. Out of different types of conventional fluorophores, carbon-based nanodots are considered one of the potential sensory advanced materials. The existence of high stability toward various reaction medium, photostability, and biocompatible nature with excellent solubility in aqueous media made C-dots superior contender for the diverse range of applications in environmental remediation. In the literature, Wang et al. have synthesized C-dots by utilizing a chemical oxidation process and liquid fuels as a precursor. The developed C-dots show definitive discrimination toward copper ion and the value of detection limit has been found to be 0.039 μM [19]. Gu et al. adapted lily bulbs as a predecessor material for the synthesis of C-dots by using microwave-assisted analysis. The value of developed C-dots has been reported to be 17.6%. The developed C-dots show exceptional selectivity and sensitivity toward copper metal with 26 nm value of detection limit [20]. Niu et al. prepared carbon quantum dot/AuNC nanohybrids by applying a chemical synthetic method. The developed systems show ratiometric fluorescent sensing toward cadmium ion. The value of detection limit has been reported to be 32.5 nM [21]. Li et al. have also prepared nitrogen doped C-dots–graphene oxide hybrid by applying microwave synthesis. The developed hybrid system was found to be electrochemically sensitive and selective toward cadmium ion. The value of detection limit has been found to be 7.45 μg/ml [22]. The major problems associated with the above-mentioned methods are their time-consuming synthesis along with expensive methods and lower value of detection limit and their approach toward biocompatibility and toxicity has not been explored. The conventional synthetic processes such as laser ablation, hydrothermal, microwave, electrochemical, and pyrolysis methods have been employed for the production of C-dots. However, the multistep processing and expensive procedural cost have displayed a major hitch in the popularity of these methods during the production of C-dots. Therefore, researchers from different fields are now applying great efforts in developing fairly easy and economical methodologies for large-scale production of C-dots [[23], [24], [25], [26]]. In this regard, the application of greener precursors such as seeds of wheat, rice, barley, grasses, banana, and orange juice has fulfilled the quest for producing economical viable C-dots [[27], [28], [29], [30], [31]].

Coconut coir is one of such starting precursors, which can be employed for the fabrication of C-dots. The coconut coir is obtained from the exterior thick mesocarp of the coconut fruit with approximately 75% of fibrous component as its major constituent [32,33]. In addition, the presence of lignin, cellulose, and hemicelluloses in coconut coir has signified its utilization as a starting precursor for the synthesis of C-dots. The usage of such bulk carbon precursor for the preparation of C-dots has overcome the disposal problem of this waste coir to form useful fluorescent probes for the selective and sensitive detection of metal ions. Therefore, the current work has proposed the novel, remunerative, green synthetic methodology without adding any kind of extra chemical reagent for developing highly fluorescent carbon dots (C-dots) by using waste coconut coir as the carbon source. The quantum yield and fluorescence emission intensity have been estimated in aqueous media. The bio-congenial (biocompatibility) performance of as-prepared C-dots has also been analyzed by using quantitative multi-assay approaches via different bacterial and fungus strains. Simultaneously, the different other biocompatibility assessment approaches have been performed for C-dots on algal species, plants (Vigna radiata seeds), and at the chromosomal level (Allium cepa) which shows their enhanced embellishment in the medical field also. The fluorescence emission properties of as-prepared C-dots have further been employed as a simple sensing podium for the detection of metal ions with high sensitivity and selectivity in aqueous media. In view of our observation, this is one of the primary testimonies where application of coconut coir has been utilized as a renewable starting precursor for the fabrication of highly fluorescent C-dots. Additionally, the outstanding PL activities exhibited by biocompatible C-dots in different pH have also been studied and that would endorse the fabrication strategy and usage of such waste biomass-derived C-dots as long-range emissive fluorophoric probes for metal ions sensing. The promising results from various water resources explored their utility in practical application. The current work provided a simple, easy, bio-matched, harmless, rapid, and cost-effective sensory probe for wastewater remediation.

Section snippets

Materials

Coconut coir has been purchased from Sector-15 Chandigarh local market. All the used metal salts of chloride and nitrate (barium, copper, cadmium, zinc, potassium, sodium, silver, calcium, magnesium, iron, lead, ammonium, and mercury) have been purchased from Sigma-Aldrich with 99% purity. Muller Hinton Agar, Agar-Agar type I, malt extract powder, and nutrient broth have been purchased from HiMedia with 98% purity for antimicrobial and antifungal activity. Chlorella algal sp. was procured from

Optical and morphological characterization of C-dots prepared using coconut coir

The excitation wavelength-dependent emission properties of C-dots have been checked by fluorescence spectroscopy (Fig. 2a) as a function of excitation wavelength ranging from 250 to 380 nm. By varying the excitation values, the maximum intensity was observed with λexc = 280 nm with the emission peak at λemi = 530 nm. Therefore, further fluorescence emission studies were carried out by using λexc = 280 nm. The emission intensity of C-dot has been found to be controlled by the greater number of

Conclusion

The current work has presented the easy and affordable hydrothermal-based method for the synthesis of C-dots with size less than 10 nm from waste coconut coir as the starting precursor source. The different characterization techniques have been used for the detailed structural and physicochemical verification of C-dots. The as-prepared C-dots possessed a quantum yield of 48% and displayed a high solubility rate in aqueous media. The fluorescence emission intensity of C-dots is found to be

Author contribution

Pooja Chauhan: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft; Smriti Dogra: Software, Investigation, Data Curation; Savita Chaudhary: Conceptualization, Methodology, Validation, Formal analysis, Resources, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding acquisition; Rajeev Kumar: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Savita Chaudhary is greatly thankful for the Purse Grant II and DST Inspire Faculty Award [IFA-17] for the financial assistance. Authors are highly thankful to Panjab University SAIF CIL for technical support.

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