Avocado seeds derived carbon dots for highly sensitive Cu (II)/Cr (VI) detection and copper (II) removal via flocculation
Introduction
In the field of materials science, carbon dots (CDs) are among the nanomaterials that exhibit numerous highly attractive properties, such as fluorescence, water-solubility, excellent thermo/photo-stability, and non– or low toxicity, as well as large-scale production viability [1], [2]. The quasi-spherical nanosized (<10 nM) CDs [3] with graphitic, crystalline, or amorphous sp2 hybridized carbon core and an oxidized carbon surface have been successfully synthesized by “top-down” as well as “bottom-up” approach [4]. Specifically, top-down techniques consist of fragmentation of carbon matter into carbon nanoparticles by means of strategies such as arc discharge, laser ablation, electrochemistry, and wet oxidation, etc. On the other hand, the bottom-up approach, means conversion of small molecules with carbon precursors (such as citric acid and urea [5], glucose [6]) to CDs of required size, using hydrothermal, ultrasonic, thermal decomposition, pyrolysis, carbonization, microwave synthesis, and solvothermal method [7]. Among the bottom-up approaches, pyrolysis is widely used, where the organic material from the carbon source is converted into CDs by heating, dehydration, degradation, and carbonization under high temperatures in either vacuum or inert atmospheres [8]. Such CDs are categorized under sustainable nanotechnology due to the popular usage of innumerable plants, fruits, or even biowaste as carbon precursors [9], [10] and have been applied in the fields of bioimaging, cancer therapy, drug delivery, optoelectronic devices (solar cells/light-emitting devices), catalysis, supercapacitors, agriculture, and optical sensors for pollutants and heavy metals [11], [12], [13].
On the other hand, pollution by heavy metals is a great concern due to enormously increasing negative implications to the environment and human health. Hexavalent chromium ions Cr6+ are broadly used in the textile industry, leather tanning, wood preservation, among others [14]. Being carcinogenic and mutagenic, Cr6+ is extremely hazardous for human health [15]. The US Environmental Protection Agency (EPA) water regulation for Cr6+ in drinking water is 0.1 mg/l [16]. Although copper ions (Cu2+) are abundant and indispensable in living cells, due to its extreme toxicity after a certain permissible limit, it is also recognized as an important pollutant by the EPA [17]. According to the EPA, the allowable level of Cu2+ in drinking water is 30 µM [18].
Most of the current techniques to detect heavy metals imply either complex procedures or specialized and expensive equipment [19]. Recently, fluorometric approaches based on toxic [20], [21] and non-toxic [22], [23], [24], [25] chemical approaches have been investigated and considered as easy, fast, cost-effective, and reliable sensing techniques. CDs have been widely applied for the detection of heavy metal ions due to their ease in binding to the carboxyl and amine groups present on the surface of CDs, by coordination type bonding/electrostatic interaction or by free radical reaction [26]. Although detection of Cr (VI) from CDs derived from different other green precursors, such as lemon peel [27], tulsi leaves [28], pineapple juice [29], natural kelp [30], sophora flavescens [31], weissella sp.K1 [32], alkaloid-soluble poria cocos [33], and panax notoginseng [34] has been reported, the formed CDs demonstrate high quantifiable limits (i.e., low sensitivity) for possible practical viability. Similarly, several green sources such as saga waste [35], peanut shells [36], lemon juice [37], lignocellulosic waste [38], bamboo leaves [39], eleusine coracana [40], and spirulina algae [41] have been used for the detection of Cu 2+ ions, nevertheless, its flocculation using CDs has been scarcely reported [42].
In this work, CDs synthesized using avocado seeds (green waste) as carbon source have been demonstrated as fluorometric detectors/sensors for Cr6+ and Cu2+. Apart from low quantifiable/detection limit for Cr6+ and Cu2+, in a certain concentration range, the proposed CDs have been shown to cause flocculation/sedimentation in presence of Cu2+ ions (via complex formation) which makes them a highly attractive and versatile green option towards the water purification/treatment from the Cu2+ contamination.
Section snippets
Materials and methods
Avocado seeds were collected from a local organically grown tree. All the salts, such as AgNO3 (99%), FeSO4 (99%), HgCl2 (99.5%), NiSO4 (98%), CuCl2 (97%) or K2Cr2O7 (99%) were obtained from Sigma Aldrich, NiSO4, Cd(NO3)2, ZnCl, and Mg(NO3) were obtained from Fermont and Rhodamine 6G from Hycel company. The deionized water was employed in all general experiments.
Synthesis of CDs
In the present work, CDs were prepared using carbonization [43], [44], [45] of avocado seeds. Thermal treatment of dried and finely grounded avocado seed powder was performed using muffle furnace at 3 different temperatures (250, 400, and 600 °C) for 2 h each. Carbonization led to a dark black product at 250 °C (CD-250) and 400 °C (CD-400) as compared to dark gray at 600 °C (CD-600), which was further grounded and dispersed in deionized water, for subsequent sonication (4 h). The black color
Structural and optical characterization of CDs
The morphology and microstructure of the CDs, analyzed through HRTEM (Fig. 1) images, revealed quasi-spherical and well dispersed CDs with an average size decreasing from 4.6 nm to 3.2 nm, with an increase in carbonization temperature from 250 °C to 600 °C respectively. In particular, the particle sizes varied from 2.8 −7.1 nm, 2.2 nm to 6.0 nm and 1.9 nm to 4.5 nm for the particles carbonized at 250 °C (Fig. 1(a)), 400 °C (Fig. 1(b)) and 600 °C (Fig. 1(c)) respectively. Similar to the
Conclusions
In this study, CDs from avocado seeds were satisfactorily obtained by a simple one-step carbonization technique. The extensive characterization was performed using high resolution transmission electron microscope, XPS, XRD and FTIR spectroscopy. The synthesis temperature tunable optical properties, morphology, and crystallinity not only disseminate through particle size reduction (from 4.6 to 3.2 nm) but also makes them more selective towards Cr6+ and Cu2+. Using absorbance spectroscopy and
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.
Acknowledgment
VA acknowledges the financial grant from CONACyT Basic Sciences project A1-S-30393. Authors acknowledge useful discussions with Dr. Naveen Kumar Reddy Bogireddy.
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Contributed equally to the work.