1st Place Global Finalist, SARC 2026

1. Abstract:

The escalation of heavy metal pollution in aquatic environments poses a significant and immediate threat to human health and ecological stability. Effective environmental management requires the development of rapid, accurate, and accessible ion monitoring technologies. To overcome the limitations of conventional analytical methods—such as high equipment costs and the need for specialized personnel—biomass-derived carbon quantum dots (CQDs) have emerged as highly promising materials for simultaneous fluorescent sensing and metal ion adsorption. In this study, we propose a sustainable synthesis of CQDs through a onestep green hydrothermal treatment of spent coffee grounds (SCGs) and chitosan, a natural nitrogen dopant. These CQDs are integrated into a polymer matrix to fabricate freestanding nanofiber composite membranes via electrospinning. The resulting membranes exhibit a strong blue fluorescence response, enabling the sensitive detection and evaluation of ferric ions (Fe3+) in lake water through fluorescence quenching.

 

2. Introduction
The direct discharge of heavy metal pollutants into aquatic ecosystems, such as rivers and lakes, has escalated into a critical environmental crisis that threatens biodiversity and public health. These metallic ions are characterized by high toxicity and a propensity for bioaccumulation, often acting as harmful agents within biological systems. Consequently, chronic human exposure leads to irreversible physiological damage to vital organ systems. In response to these challenges, various functional materials have been explored for heavy metal detection and removal (Yusufoğlu et al., 2024). Among these, biomass-derived CQDs have gained significant attention as promising materials for fluorescent sensing systems due to their inherent nontoxicity, excellent biocompatibility, and rich surface functional groups (Usman et al., 2024; Kang et al., 2020). SCGs represent an exceptionally versatile and cost-effective carbon precursor that is currently underutilized and primarily discarded as waste (Jeong et al., 2023). Therefore, this study focuses on the fabrication of freestanding nanofiber composite membranes via an electrospinning process to promote resource circularity and practical scalability. By integrating CQDs into a polymer matrix, these membranes enable Fe3+ detection through fluorescence quenching and allow efficient metal ion removal via chelation-driven adsorption.

 

3. Literature Review

A variety of functional materials, including metal-organic frameworks (MOFs), polymers, and nanoparticles, have demonstrated significant efficiency in the detection and removal of heavy metal ions (Li et al., 2023). Notably, CQDs and carbon-based fluorescent probes exhibit dual functionality, as they can act both as effective adsorbents and sensitive optical sensors (Zhong et al., 2024). Compared to other fluorescent nanomaterials, CQDs offer distinct advantages, including straightforward experimental protocols, high water solubility, and tunable selectivity (Meng et al., 2019). In recent years, the green synthesis of CQDs from biomass waste has created new opportunities for their use in sensing, bioimaging, and catalysis (Kang et al., 2020). With approximately 6 million tons of SCGs are generated globally each year (Franca et al., 2022), upcycling this waste into high-performance fluorescent sensors offers a meaningful approach to resource recycling (Nazar et al., 2024). Furthermore, the integration of chitosan provides a sustainable source of nitrogen doping, which can enhance both the photoluminescence (PL) and Fe3+ selectivity of CQDs. However, a persistent technical challenge in the field is the difficulty of recovering nano-sized CQDs from water after use. To address this limitation and ensure widespread practical utility, the development of freestanding membranes is essential.

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4. Methodology
4.1. Green Synthesis of N-doped CQDs via Chitosan Doping: For the synthesis of nitrogendoped CQDs (N-CQDs), a sustainable one-step hydrothermal method (Rateb et al., 2023) is employed, using SCGs as the main carbon source and chitosan as a natural nitrogen dopant. Initially, dried SCG powder and chitosan are mixed in deionized water at a specific weight ratio and transferred to a Teflon-lined stainless steel autoclave. The mixture is then subjected to highpressure hydrothermal treatment at a controlled temperature of 180°C to 200°C for several hours, which allows both the carbonization of SCGs and the incorporation of nitrogen from chitosan into the CQD structure. Following the reaction, the solution is filtered through a 0.22 μm membrane filter to remove large carbon particles. The resulting filtrate undergoes dialysis against deionized water for 24 to 48 hours, allowing the collection of purified, size-controlled CQDs. Finally, the CQDs are concentrated into a powder form using a rotary evaporator.

 

4.2. Fabrication of Nanofiber Composite Membranes: The synthesized CQDs are integrated into a polymer matrix through an electrospinning process to create freestanding nanofiber composite membrane sheets. To prepare the matrix, two distinct types of polymer solutions are used: a 10 wt% aqueous polyvinyl alcohol (PVA) solution and a PVA/polyacrylic acid (PVA:PAA) blend solution. Approximately 5-10 wt% of the CQDs are dispersed into each polymer solution to ensure a uniform distribution of the fluorescent probes. These composite solutions are then electrospun under optimized parameters to produce uniform nanofiber sheets (Abdulhussain et al., 2023). To ensure the structural integrity and water resistance of the membranes for the use in lake water applications, the sheets undergo a crosslinking process: PVA-based sheets are chemically crosslinked using glutaraldehyde (Tian et al., 2019), while the PVA:PAA blend sheets are stabilized through thermal crosslinking (Zhu et al., 2018).

 

4.3. Structural and Morphological Characterization: The physicochemical and structural properties of both the CQDs and the fabricated membranes are characterized using a range of analytical techniques. Transmission electron microscopy (TEM) is performed to investigate the particle size distribution and crystalline lattice structure of the CQDs, while X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FT-IR) spectroscopy are used to identify the elemental composition and surface functional groups, thereby confirming successful nitrogen doping from the chitosan precursor. Furthermore, scanning electron microscopy (SEM) is carried out to examine the surface morphology and fiber diameters of the electrospun composite sheets.

 

4.4. Fluorescence Quenching and Sensing Performance: The sensing and adsorption capabilities of the composite membranes toward Fe 3+ are evaluated through a series of fluorescence response tests. The membrane sheets are exposed to varying concentrations of Fe3+ ions, ranging from nanomolar to millimolar levels, to evaluate their detection sensitivity. Using PL spectroscopy, the output emission signal is measured under UV light to determine the extent of fluorescence quenching caused by the presence of the target ions. Additionally, the response kinetics are monitored through time-dependent absorption measurements taken at 10-min intervals for up to 1.5 h.

 

4.5. Data Analysis and Evaluation: The final stage of the methodology involves analyzing the experimental results to determine the Limit of Detection (LOD) and overall sensitivity of the membrane sensors. Moreover, the selectivity of the membranes is evaluated by comparing the fluorescence quenching response of Fe3+ with that of other non-target metal ions typically found in aquatic ecosystems, ensuring the specificity of the sensor for practical lake water monitoring.

 

5. Conclusions

This research demonstrates the development of a sustainable nanofiber composite membrane incorporating CQDs synthesized through a green hydrothermal process using spent coffee grounds and chitosan. The synergistic effect of these bio-derived materials allows for dual functionality, enabling high-sensitivity detection through fluorescence quenching and efficient Fe3+ removal through chelation with surface functional groups. By offering a freestanding and practical format that overcomes the recovery challenges of traditional powder-form sensors, this study provides an accessible and effective solution for monitoring and reducing heavy metal contamination in lake water.

 

References:

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