Highly fluorescent carbon dots derived from Mangifera indica leaves for selective detection of metal ions
Graphical abstract
Introduction
Over the past three decades, there have been numerous efforts to apply fluorescent materials to optoelectronic, sensing, and bio-imaging devices, due to their high fluorescent quantum yield, interference free signal, etc. (Hirano et al., 2000; Li et al., 2015; Rasheed et al., 2019, Rasheed et al., 2018a, Rasheed et al., 2018b; Soroka et al., 1987; Tang et al., 2017; Wang et al., 2016; Yoon et al., 2018). Among different fluorescent materials, the use of semiconductor quantum dots (QDs) has been prevalent due to their high photostability, absolute photoluminescence quantum yields, and size-tunable absorption and emission (Hardman, 2006; Sun et al., 2006; Wu et al., 2003). Typical fluorescent materials in the form of QDs have been prepared from compounds of gold, silver, lead, silicon, and cadmium (Branham et al., 2006; Hutter and Maysinger, 2011; Jin and Gao, 2009). However, these typical QDs are disadvantageous because of their potential toxicity, posing adverse environmental health hazards (Manshian et al., 2017). Therefore, there has been a demand to develop a sustainable method to prepare stable and high intensity fluorescent QDs with high photo-stability, biocompatibility, and low toxicity.
In this respect, fluorescent carbon quantum dots (CQDs) are a new class of carbon family that are recognized as promising alternative to replace conventional semiconductor quantum dots (Lim et al., 2015). Note that conventional QDs comprises of heavy metals which are extremely toxic even at comparatively low concentration. In this respect, CQDs may take a number of advantages such as low toxicity and biocompatibility along with common properties (e.g., high photoluminescence, low photodegradation, and high-water solubility). All of these properties are desired for effective sensing of metal ions and bio-imaging applications (Ju et al., 2018; Kalytchuk et al., 2017; Mehta et al., 2018; Sharma et al., 2018; Yu et al., 2017).
During the electrophoretic analysis of single-walled carbon nanotube, fluorescent CQDs were serendipitously discovered (Xu et al., 2004). In their study, they observed that this carbon nanotube fragment can be segmented into a number of moieties with size-dependent fluorescent properties. This prompted the creation of CQDs as a new viable class of fluorescent nanomaterials to replace existing QDs. For the first time, fluorescent carbon nanoparticles were recognized as “carbon quantum dots” (Sun et al., 2006). These authors proposed a new route for the synthesis of CQDs through surface modification that improved fluorescence emissions considerably (Sun et al., 2006). These studies reported that CQDs contain a high amount of diamond-type sp3 hybridization of carbon atoms along with graphite-type sp2 hybridization (Ray et al., 2009; Sun et al., 2006). Consequently, they can exhibit irregular π-electronic conjugation in the presence of hydroxyl and carbonyl groups (Yang et al., 2011). Therefore, CQDs may contain a number of elementary constituents such as graphene, graphene oxide, and diamond. These exceptional structural properties of CODs indicate the existence of numerous fluorophores.
To date, the synthesis of CQDs can be carried out based on either top-down or bottom-up approaches. In the former route, CQDs are obtained by cutting or evaporating the bulk source of carbon into nanoparticles through physical processes, such as electrochemical methods (Liu et al., 2007), laser-ablation (Thongpool et al., 2012), plasma induction (Li et al., 2013), and arc evaporation (Dey et al., 2014). In contrast, in the bottom-up approach, CQDs are obtained by nucleation of individual atoms. This includes thermal methods (Peng and Travas-Sejdic, 2009), solvothermal/hydrothermal methods (Liu et al., 2011; Wu et al., 2013), microwave treatment (Zhu et al., 2009), and ultrasonic-assisted synthetic methods (Li et al., 2011; Lu et al., 2016). However, they have many disadvantages, such as harsh synthesis conditions (e.g., high temperature, reactive chemical reactions), expensive precursors and apparatus, and long synthesis times. In addition, these methods involve toxic chemicals, such as strong acids and bases, that generate wastewater (Lim et al., 2015; Namdari et al., 2017; Wang and Hu, 2014). Therefore, there have been numerous efforts to develop alternate natural, non-toxic, and inexpensive materials as precursors for application toward a sustainable synthesis procedure for CQDs. For example, the synthesis of CQDs has been tested using grass (Liu et al., 2012), egg white (Cai et al., 2011), honey (Mandani et al., 2017), dairy waste (Devi et al., 2017), sweet potato (Wen et al., 2014), bamboo leaves (Liu et al., 2014), pomelo peel (Lu et al., 2012), orange juice (Sahu et al., 2012), tulsi, neem leaves (Meena et al., 2019), banana juice (Chaudhry et al., 2020), lemon juice (Hoan et al., 2019), and tea extract (Chen et al., 2019).
Iron is an indispensable micronutrient in our diets, with the human body requiring about 30 mg per day (Eid et al., 2017; He et al., 2008). It plays a momentous role in biological systems, such as in oxygen transport, binding with regulatory proteins, and enzyme activity. For example, it acts as a binding site to attract oxygen from breathed air to produce hemoglobin (Karami et al., 2010). However, an excess amount of iron ions may lead to serious health problems, such as cytotoxicity, Parkinson's disease, need for blood transfusions, and metabolic disorders. Further, free or excess iron ions can also result in damage of cellular systems or produce reactive oxygen species due to their tendency to accept or gain electrons from surroundings (Ananthanarayanan et al., 2014; Britton et al., 2002; Eid et al., 2017; Puntarulo, 2005). Therefore, detection of iron ions in biological systems and the environment is of great concern.
This study attempted to prepare highly fluorescent CQDs using the renewable carbon source Mangifera indica: M. indica (mango) leaves. M. indica leaves were chosen as the carbon source of CQDs because of their abundance (mango is one of the most common fruits cultivated in India) (Mukherjee, 1953). The synthesized CQDs were employed for selective detection of Fe2+ ions in water as well as using a Livogen tablet (as a real sample to validate the efficacy of the study) over a wide range of pH values. To the best of the authors' knowledge, this is the first study on the synthesis of CQDs using mango leaves, which have high fluorescent and efficient sensing sensitivity toward Fe2+ ions. Moreover, the use of M. indica leaves extract for the synthesis of CQDs offers an extra benefit in terms of the excellent binding capability with Fe2+ ions (e.g., due to carboxylic/amine groups in such extract) (Masibo and Qian, 2008). This study should help pave the new road for the development of sustainable synthesis of a highly efficient sensing platform toward Fe2+ ions in water.
Section snippets
Materials
Mangifera indica (M. indica) leaves were collected from Fatehgarh Sahib, Punjab, India, in July 2017. Hydrogen peroxide (H2O2), ferrous chloride (FeCl2), buffer solutions (pH from 1 to 11), and deionized (DI) water were purchased from Fisher Scientific, Emsure, Merck, and Sigma-Aldrich, respectively.
Synthesis of M. indica leaf-derived CQDs
The M. indica leaf-derived CQDs were synthesized as follows. (1) M. indica leaves were cleaned by washing with water and then chopped; (2) the chopped leaves were heated at 300 °C in a muffle
Physical and chemical properties of the CQDs derived from M. indica leaves
An optical absorption spectrum of the CQDs was recorded from 200 nm to 800 nm as shown in Fig. 1a. It showed a broad peak around 213 nm, which likely reflects the p–p* transition in the conjugated CC domain of the carbon-core state. According to the PL spectrum of the CQDs in Fig. 1b, a high-intensity emission peak around 525 nm was clearly visible. This massive blue light emission was likely due to differences in particle size distribution and light radiative state present on the surface of
Conclusions
In this study, synthesis of photoluminescent CQDs was carried out using a renewable carbon source (M. indica leaves) via pyrolysis without pre-/post-chemical treatments. Various spectroscopic and electron microscopic techniques confirmed the spherical shape and amphiphilic nature of the CQDs in a size range of 1–5 nm. The synthesized CQDs exhibited effective sensing potential for Fe2+ ions in water and in a Livogen tablet (as a real sample), with a quantum yield of ~18.2%. The method introduced
CRediT authorship contribution statement
Jagpreet Singh:Methodology, Writing - original draft, Writing - review & editing.Sukhmeen Kaur:Methodology, Writing - original draft, Writing - review & editing.Jechan Lee:Formal analysis.Akansha Mehta:Data curation, Investigation.Sanjeev Kumar:Data curation, Investigation.Ki-Hyun Kim:Conceptualization, Supervision.Soumen Basu:Data curation, Investigation.Mohit Rawat:Conceptualization, Supervision.
Declaration of competing interest
This experimental research work is original and was carried out at the authors' institute. This work possesses no intellectual or financial conflict of interests.
Acknowledgements
This work was supported by financial assistance from the Shromani Gurdwara Prabhandak Committee (SGPC), Amritsar. The authors are thankful to the Vice-Chancellor of SGGSW and Thapar University for providing laboratory facilities.
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These authors contributed equally to this work.