Avocado seeds derived carbon dots for highly sensitive Cu (II)/Cr (VI) detection and copper (II) removal via flocculation

Highlights

• Green and sustainable synthesis of carbon dots using avocado seeds as carbon source.

• Increase in limit of detection with an increase in carbonization temperature.

• Carbon dots applied as optical sensors with high selectivity towards Cr⁶⁺ and Cu²⁺.

• Real water remediation demonstrated through removal of Cu²⁺ via flocculation.

• Measured limit of detection in river water was 0.04 and 0.09 nM for Cr⁶⁺ and Cu²⁺ respectively.

Abstract

Sustainable, scalable and green carbon dots (CDs) were derived from avocado seeds using carbonization at different temperatures (250, 400, and 600 °C) and were applied as fluorescent nanoprobes for the detection of heavy metal ions. The CDs were found to be highly selective towards Cr⁶⁺ and Cu²⁺ ions and acted as fluorescent turn-off sensors. An increase in the carbonization temperature not only decreased the dimensions of the CDs from 4.6 to 3.2 nm, but also enhanced the crystallinity and sensitivity towards Cu²⁺/Cr⁶⁺. Although the CDs act as accurate optical sensors at low concentration range for Cu²⁺ ions, at higher concentration flocculation (visible to the naked eye) takes place and hence their applicability for possible water treatment is elucidated. The high sensitivity of CDs synthesized at 600 °C is revealed through a low limit of detection (LOD), i.e., 0.04 nM and 0.09 nM for Cr⁶⁺ and Cu²⁺ ions respectively. The efficacy of the proposed particles has been successfully demonstrated in the possible real-life applications via sensitivity tests in river/tap water samples.

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.