Facile green synthesis of fingernails derived carbon quantum dots for Cu²⁺ sensing and photodegradation of 2,4-dichlorophenol

Highlights

• FN-CQDs were produced via hydrothermal treatment of human fingernails.

• Produced FN-CQDs were sensitive to detect Cu²⁺ ions as low as 1 nM concentration.

• FN-CQD/g-C₃N₄(50) composite successfully removed 100 % of 2,4-DCP in 75 min.

• FN-CQDs enable photosensitizing effect and extend lifespan of charge carriers.

Abstract

The present study reports a facile method to prepare carbon quantum dots (CQDs) via hydrothermal treatment using human fingernails as a green precursor. Fingernails derived CQDs (FN-CQDs) could selectively detect copper ions (Cu²⁺) at the concentration as low as 1 nM. Different weightage of FN-CQDs (30 wt%, 40 wt% and 50 wt%) were coupled with pure graphitic carbon nitride (g-C₃N₄) to remove 2,4-dicholorophenol (2,4-DCP) under sunlight irradiation. The composite loaded with 50 wt% of FN-CQD removed 100 % of 2,4-DCP in 75 min, which was almost 2 times higher than g-C₃N₄. The photocatalytic performance was in agreement with ultraviolet–visible diffuse reflectance spectra (UV–vis DRS) in which the photosensitizing effect was significantly exerted by 50 wt% of CQDs. In the presence of FN-CQDs, g-C₃N₄ was sensitized by sunlight with an average light intensity of ∼ 937 × 100 lx to donate more electrons for the generation of oxidizing radicals. Excessive loading of FN-CQDs (up to 50 wt%) created trap state that decreased the charge carrier transport in FN-CQDs/g-C₃N₄(50). Such drawbacks did not affect the overall performance of FN-CQDs/g-C₃N₄(50). The higher loading of FN-CQDs exerted stronger photosensitizing effects to overcome the limitation of high recombination rate of charge carriers and lower surface area. FN-CQDs could act as photosensitizer to increase the light absorption range to generate more electron and holes. It also served as electron reservoir for the reduction of oxygen molecule to produce superoxide anion radical (O₂⁻). Scavenging tests identified that O₂⁻ was the most active radical in the photodegradation of 2,4-DCP.

Introduction

Copper (Cu²⁺) ions are one of the most hazardous and recalcitrant heavy metal ions pollutants in untreated industrial effluents. The imbalanced concentration of Cu²⁺ ions in the human body upon exposure to pollutants could result in Menkes disease, Parkinson’s disease, Alzheimer’s disease and Wilson’s disease [1]. Therefore, an effective analytical method for sensitive and selective Cu²⁺ ions detection is important for pollutant identification. Fluorescence detection tools can be miniaturized at a high sensitivity of parts per billion or trillion with inexpensive and easy procedures. Carbon quantum dots (CQDs) are designed in fluorescence detection tools as they can be excited with a single excitation source for multiplexed detection compared to traditional dye techniques [2]. CQDs also possess other distinctive properties such as low toxicity, low cost, biocompatible, unique optical and electronic properties, and chemically inert with broad and tunable excitation spectrum due to its quantum sized [[3], [4], [5]]. Hence, CQDs were used as fluorophores to detect heavy metal ions in untreated industrial effluents. Jiao et al. utilized the mango peel derived CQDs for the detection of Fe²⁺ in ferrous succinate tablets with the linearity range of 4 μM–16 μM [6]. Liu et al. reported CQDs modified with lysine and bovine serum albumin, which selectively detect Cu²⁺ in tap water [7]. Pramanik et al. developed CQDs by hydrothermal carbonization of Aegle Marmelos leave powder which could be applied as a fluorescence Fe³⁺ ion sensor [8]. In this study, human fingernails were used as the green precursor for the hydrothermal synthesis of water-soluble and fluorescent CQDs using a top-down approach [9]. The fingernails derived CQDs (FN-CQDs) served as an effective fluorescent probe for sensitive and selective detection of Cu²⁺.

Additionally, the endocrine disrupting chemicals (EDCs) present in untreated industrial effluents such as 2,4-dichlorophenol (2,4-DCP) could induce faint, itch, anemia and phospholipid bilayer disruption in the human cell by altering endocrine system function [10,11]. The 2,4-DCP was also reported to destroy the soil microbiota due to its toxicity through organic matter and clay dissociation [12]. However, their removal was difficult as they were stable in the molecular structure. Hence, the photocatalytic process was suggested to remove 2,4-DCP since the photocatalytic process utilized synergistic light and catalyst effect to degrade recalcitrant contaminants into harmless products. Among the photocatalysts, graphitic carbon nitride (g-C₃N₄) shines as a popular metal-free polymeric semiconducting compound due to its low band gap energy (2.7 eV), high thermal stability, non-toxicity and low cost [13]. Yet, the photocatalytic performance of g-C₃N₄ is limited by its short light absorption range, slow charge mobility and fast recombination of electron and hole pairs [14]. The shortcomings of g-C₃N₄ could be reduced by coupling with various semiconductors (TiO₂ and BiVO₄) [15,16] and coupling with metals (gold and silver) [17,18]. Jiang et al. reported a combination of N self-doping and thermal exfoliation process, in which the porous nitrogen self-doped g-C₃N₄ demonstrated 81.72 % of tetracycline degradation in 60 min [19]. Wang et al. (2019) combined g-C₃N₄ with 3D TiO₂ microflowers to form direct Z-scheme heterojunction in the interface, leading to excellent charge separation and transportation [20]. Dang et al. loaded gold nanoparticles onto g-C₃N₄ nanosheet, facilitating the charge carrier separation and eventually improving the photoactivity for H₂ evolution by 5.3 times higher than pure g-C₃N₄ [17]. He et al. prepared Bi₂O₃/g-C₃N₄ direct Z-scheme photocatalyst via the combination of photoreduction and subsequent oxidation by air. The composite showed improved photodegradation of phenol than pure Bi₂O₃ and g-C₃N₄ due to the Z-scheme charge transfer [21]. Despite recording positive properties and results, chemical-based doping agents are harmful to our environment and health whereas the noble metal doping is relatively more costly. As an alternative, carbon-based material such as CQDs is derived from a cheaper and greener precursor. Despite extensive research, few discernments are yet to be discovered to improve past studies’ data. This includes the application of CQDs derived from human fingernails in Cu²⁺ sensing and photocatalysis. Many works mainly focused on the applications of CQDs/g-C₃N₄ for photocatalytic water splitting and photodegradation of organic dye. The outcome is positive but there has been a little attempt on the photodegradation of EDCs using CQDs/g-C₃N₄. In this study, our group coupled g-C₃N₄ with FN-CQDs to enhance the photocatalytic performance. We further explored the metal sensing application in terms of selectivity and sensitivity of the prepared FN-CQDs towards various heavy metals.