One-step green approach to synthesize highly fluorescent carbon quantum dots from banana juice for selective detection of copper ions

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Abstract

Green synthesis of carbon quantum dots (CQDs) from natural sources always remain an attraction due to their excellent optical properties. In present work, highly fluorescent N, S co-doped CQDs (NS-CQDs) were synthesized from banana (musa acuminata) juice using a simple one-step hydrothermal process, which not required any surface passivation, oxidizing agent or inorganic salt. Transmission electron microscopic images and X-ray diffraction analysis revealed spherical shaped NS-CQDs of an average size of 1.27 nm with poor crystallinity. The proposed method offers higher quantum yield (32 %) at an excitation wavelength of 330 nm as compared to other reported natural sources-derived CQD. These synthesized NS-CQDs utilized as a fluorescent probe for the detection of Cu (II) in the water sample. The fabricated fluorescent sensor displayed the detection of copper ions in the concentration range of 1−800 μg/mL with the LOD of 0.3 μg mL−1.

Introduction

In the past few years, incredible endeavors have been devoted to the detection and quantification of heavy metals in ecological and biological frameworks due to their conceivable lethal impact on the human being. Among the various heavy metals, Cu is recognized as the third most abundant heavy metal in the human body, which plays a crucial job in physiological procedures, dioxygen transport, activation, signal transduction, and energy generation [1]. The excess amount of Cu2+ in the human body can result in serious diseases such as Menkes and Wilson diseases, liver or kidney damage, hereditary aceruloplasminemia, gastrointestinal disturbance, and Alzheimer [2]. Cu2+ can enter the human body through water contaminated by industrial and consumer waste or by heavy metals released from soil breaking due to acidic rain. On considering potential impacts, the Environmental Protection Agency recognizes Cu2+ as an essential trace pollutant and sets a permissible amount of Cu in drinking water as 1.3 ppm or 20 μM [3]. Whereas, in human blood, its value exists in the range of 100–150 μg/dL (15.7–23.6 μM) [4,5].

Presently, there is incredible enthusiasm for growing new nanomaterials and minimal effort manufactured plans for developing reliable and sensitive analytical methods for the detection of Cu2+ in the ecosystem. There are various conventional analytical techniques available for Cu2+ detection, including potentiometric sensors, atomic fluorescence spectrometry, atomic absorption spectrometry (AAS), and coupled plasma mass spectrometry [[6], [7], [8], [9], [10]]. These analytical techniques are sensitive for Cu2+ detection but require relatively high-cost instruments, consume more time, toxic/hazards chemicals, and complex sample preparation. Now-days, fluorescent assay based sensors have been considered as the most suitable tool for the identification of heavy metals with the objectives of high sensitivity, selectivity, less interferences, cost-efficiency, and rapid response [11,12]. The fluorescent assay technique typically used fluorescent nanomaterials such as semiconductor quantum dots and organic dyes, which involved highly toxic precursors, multiple steps of synthesis, purification, and expensive chemicals for surface modification [13,14]. Therefore, it is crucial to develop a novel, facile, rapid, and cost-effective approach for developing biocompatible fluorescent nanomaterials.

In this scenario, fluorescent carbon quantum dots (CQDs), a smart and attractive family member of carbon nanomaterials, has replaced the semiconductor quantum dots due to its easy and rapid synthesis process, excellent water solubility, less toxicity, rich functional groups, high biocompatibility, and their outstanding physicochemical properties [[15], [16], [17], [18], [19]]. Synthesis of high-quality graphene QDs (GQDs) using simple, cost-effective, and environment-friendly method always remain an interest for the researchers. Commonly, synthesis of CQDs involves the carbonization of carbon sources and or quantum confinement of carbon materials such as graphene and carbon nanotubes. As compared to the confinement method, synthesis of CQDs from natural carbon sources (green synthesis) has great importance because it is straightforward, repeatable, environment-friendly, and financially savvy. The abundant availability of natural sources of carbon is also a reason for enormous attention for the green synthesis of CQDs [20]. So far, fluorescent CQDs were synthesized from a range of natural carbon sources including fruits, fruit juices, fruit peels, beverages, animal derivatives, bakery products, human derivatives, vegetables, spices, and plant leaves through hydrothermal, oxidation, microwave irradiation, heating and pyrolysis methods [12,[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]]. Moreira et al. [36] reported the synthesis of cuprizone functionalized fluorescent CQDs (C-CQDs) from sugarcane juice through the hydrothermal method, and applied for selective quantification of Cu2+. The prepared C-CQDs showed detection of Cu2+ in the range from 5.12 to 100 μmol/L with LOD of 0.76 μmol/L. In another study, Liu et al. [37] used hydrothermal method to synthesize CQDs from pear juice as carbon source and utilized for detecting a range of Cu2+ concentrations from 0.1 mg/L to 50.0 mg/L. Besides these, Tan et al. [38] synthesized CQDs from the carbonization of sago industrial waste using thermal pyrolysis method. They demonstrated linear detection of Cu2+ with LOD of 7.78 μM using standard Stern–Volmer quenching model. Among the aforementioned green synthesis methods, the hydrothermal method has been shown as a straightforward, cost-effective, environment-friendly, one-step, and efficient way for the synthesis of CQDs in an aqueous medium. The hydrothermal route also offers high yield and produces exceedingly proficient fluorescent probes for perceiving different compound species such as heavy metals in water. Thus, keeping given above banana juice is proposed as a natural source for synthesis CQDs using hydrothermal process. The banana juice contains a high amount of carbohydrates like glucose, fructose, sucrose and ascorbic acid as the carbon precursors [39,40].

The present work reports highly fluorescent N, S co-doped CQDs (NS-CQDs) derived from banana juice through the simple, cost-effective, one-step hydrothermal method. The structural and optical properties of synthesized CQDs were investigated using various techniques and finally applied for the detection of heavy metal (Cu) from water. The spherical shape NS-CQDs with the average size of 1.27 nm has revealed high solubility in aqueous media, blue emission, specific detection of Cu ions in the detection range of 1 to 800 μg/mL.

Section snippets

Materials

Banana (Musa acuminata) was purchased from the local market of Delhi, India. Absolute ethanol and quinine sulfate were procured from Sigma-Aldrich. Whereas, copper acetate, borate buffer, boric acid, sodium hydroxide, and other chemicals of reagent grade obtained from SRL Pvt. Ltd. India. All the chemicals were used without any further purification.

Synthesis of NS-CQDs using banana

Fluorescent NS-CQDs from banana juice was derived through a simple one-step hydrothermal method. In a typical experiment, 80 gm of banana was cut

Quantum yield and stability of prepared NS-CQDs

The NS-CQDs production yield was calculated to be 6.8 %, as 1.097 gm of the product was obtained from 16 gm of banana. Therefore, the above synthesis method can be proposed for large-scale synthesis of CQD. Moreover, the quantum yield of prepared NS-CQDs in aqueous medium was estimated using quinine sulfate as a reference at an excitation wavelength of 330 nm and found as 32 %. It can be easily seen that hydrothermal synthesized of NS-CQDs from banana juice has the highest quantum yield of 32 %

Detection of Cu2+ ions

The feasibility of prepared NS-CQDs to detect Cu2+ was explored in terms of a decrease in fluorescence intensity (quenching) of NS-CQDs with Cu2+ concentrations (1–800 μg/mL). The change in fluorescence intensity was further observed by adding Cu2+ concentrations in the NS-CQDs aqueous solution, separately [Fig. 7 (a)]. It can be easily seen that Cu2+ concentrations efficiently quench the fluorescence of NS-CQDs. Moreover, an apparent red shift in emission peak detected with the addition of Cu2+

Conclusions

In this investigation, an easy and large-scale green synthesis of water dispersed fluorescent NS-CQDs from an inexpensive and promptly accessible natural precursor has been proposed. The green-synthesized NS-CQDs with the average size of 1.27 nm have shown blue fluorescence, and the highest quantum yield (32 %) as compared to other reported green synthesized NS-CQDs. Moreover, NS-CQDs have shown high selectivity (fluorescence quenching) for Cu2+ with respect to other heavy metals. The

Author contribution

Mr. Navneet Chaudhary has synthesized the NS-CQDs and characterization work was equally shared by Mr. Navneet Chaudhary and Dr. Pramod Gupta. Cu ions detection idea by conceive by Dr. Pratima R. Solanki and Prof. Sergei Eremin. The complete manuscript was prepared by Mr. Navneet Chaudhary and Dr. Pramod Gupta and revised by Dr. Pratima R. Solanki.

The corresponding author is responsible for ensuring that the descriptions are accurate and agreed by all authors.

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.

Acknowledgments

Authors are thankful to Advanced Instrument Research Facility (AIRF) JNU for facilitating the instrumentation facility. “This investigation was financially supported by project entitled “Rapid detection of bacterial resistance to antibiotics based on changing optical properties of nanosized labels” funded by Department of Biotechnology, India (DBT/IC-2/Indo-Russia/2017-19/02) and Ministry of Education and Science of the Russian Federation in the frame of the State Targeted Program “Research and

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