Carbon Quantum Dot-Incorporated Chitosan Hydrogel for Selective Sensing of Hg2+ Ions: Synthesis, Characterization, and Density Functional Theory Calculation

Review of carbon dot-hydrogel composite material as a future water-environmental regulator

As water pollution and scarcity pose severe threats to the sustainable progress of human society, it is important to develop a method or materials that can accurately and efficiently detect pollutants and purify aquatic environments or exploit marine resources. The compositing of photoluminescent and hydrophilic carbon dots (CDs) with hydrogels bearing three-dimensional networks to form CD-hydrogel composites to protect aquatic environments is a "win-win" strategy. Herein, the feasibility of the aforementioned method has been demonstrated. This paper reviews the recent progress of CD-hydrogel materials used in aquatic environments. First, the synthesis methods for these composites are discussed, and then, the composites are categorized according to different methods of combining the raw materials. Thereafter, the progress in research on CD-hydrogel materials in the field of water quality detection and purification is reviewed in terms of the application of the mechanisms. Finally, the current challenges and prospects of CD-hydrogel materials are described. These results are expected to provide insights into the development of CD-hydrogel composites for researchers in this field.

Emerging trends in CDs@hydrogels composites: from materials to applications

Carbon dots (CDs) are nanoscale carbon materials with unique optical properties and biocompatibility. Their applications are limited by their tendency to aggregate or oxidize in aqueous environments. Turning weakness to strengths, CDs can be incorporated with hydrogels, which are three-dimensional networks of crosslinked polymers that can retain large amounts of water. Hydrogels can provide a stable and tunable matrix for CDs, enhancing their fluorescence, stability, and functionality. CDs@hydrogels, known for their ease of synthesis, strong binding capabilities, and rich surface functional groups, have emerged as promising composite materials. In this review, recent advances in the synthesis and characterization of CDs@hydrogels, composite materials composed of CDs and various types of natural or synthetic hydrogels, are summarized. The potential applications of CDs@hydrogels in fluorescence sensing, adsorption, drug delivery, antibacterial activity, flexible electronics, and energy storage are also highlighted. The current challenges and future prospects of CDs@hydrogels systems for the novel functional materials are discussed.

Recent studies on proteins and polysaccharides-based pH-responsive fluorescent materials

Polymer-based pH-responsive fluorescent materials have the characteristics of fast response, real-time monitoring, visualisation, and easy forming. Consequently, they have attracted widespread attention in wound healing, sweat monitoring, security and anti-counterfeiting, freshness detection of aquatic products, metal-ion sensing and bioimaging. This paper analyses the preparation principles and characteristics of pH-responsive fluorescent materials based on cellulose, chitosan and proteins. It then outlines the fluorescence properties, environmental response mechanisms and applications of various luminescent materials. Next, the research indicates that amines, N-heterocyclic rings, carboxyl groups and amino plasmonic groups on the fluorescent molecule structure and polymer skeleton appear to change the degree of ionisation under acid or alkali stimulation, which affects the light absorption ability of chromophore electrons, thus producing fluorescence changes in fluorescent materials under different pH stimuli. On this basis, the challenges and growth encountered in the development of proteins and polysaccharides-based pH-responsive fluorescent materials were prospected to provide theoretical references and technical support for constructing pH-responsive fluorescent materials with high stability, high sensitivity, long-lasting pH-response and wide detection range.

Fiberglass cloth coated by coffee ground waste-derived carbon quantum dots/titanium dioxide composite for removal of caffeine and other pharmaceuticals from water

Coffee ground waste from the coffee beverage preparation is mainly discarded and consequently ends up in landfill, which cause the contamination of caffeine in various environmental compartments. This study focuses on the upcycling of coffee-ground waste to carbon quantum dots (CQDs) for use as a modifying material to improve the visible light activity of titanium dioxide (TiO₂). The CQD solution was synthesized by hydrothermal method, which has an average size of 2.80 ± 0.63 nm. The CQDs/TiO₂ photocatalysts were prepared by combining CQD solutions at various amounts with sol-gel TiO₂ and then coated on the fiberglass cloths (FGCs). The photocatalytic application mainly focuses on the removal of caffeine from the water. The photocatalytic experiment was preliminary run in a simple batch reactor under visible light. The 5CQDs/TiO₂ coated FGC (5 mL of CQD solution/g of Ti-based on sol-gel) showed the best performance, and it was selected for the removal of caffeine and other pharmaceuticals (i.e., carbamazepine and ibuprofen) in the recirculating reactor. The removals of caffeine, carbamazepine, and ibuprofen after irradiation for 9 h were 82%, 88%, and 84%, respectively. The residual concentrations were significantly lower than the reported toxicity levels based on specific species. The changes in total organic carbon were observed, indicating the mineralization of pharmaceuticals in water. The 5CQDs/TiO₂ coated FGC showed good flexible performance. No obvious loss of activity was observed for five runs. The actual wastewater from the coffee pot cleaning process was also tested. The removal was 80% for caffeine and 86% for color in the unit of the American Dye Manufacturers Institute (ADMI).

Recent advances in fluorescent materials for mercury(ii) ion detection

Invading mercury would cause many serious health hazards such as kidney damage, genetic freak, and nerve injury to human body. Thus, developing highly efficient and convenient mercury detection methods is of great significance for environmental governance and protection of public health. Motivated by this problem, various testing technologies for detecting trace mercury in the environment, food, medicines or daily chemicals have been developed. Among them, the fluorescence sensing technology is a sensitive and efficient detection method for detecting Hg2+ ions due to its simple operation, rapid response and economic value. This review aims to discuss the recent advances in fluorescent materials for Hg2+ ion detection. We reviewed the Hg2+ sensing materials and divided them into seven categories according to the sensing mechanism: static quenching, photoinduced electron transfer, intramolecular charge transfer, aggregation-induced emission, metallophilic interaction, mercury-induced reactions and ligand-to-metal energy transfer. The challenges and prospects of fluorescent Hg2+ ion probes are briefly presented. We hope that this review can provide some new insights and guidance for the design and development of novel fluorescent Hg2+ ion probes to promote their applications.

Selective detection of nitenpyram by silica-supported carbon quantum dots

In this study, a fluorescent sensor of nitrogen-doped carbon quantum dots (N-CQDs) and silica gel hybrid was developed for the quantitative detection of nitenpyram, a toxic neonicotinoid existing in groundwater and/or surface water.The prepared N-CQDs@SiO₂ sensor exhibited remarkable sensing selectivity and sensitivity towards nitenpyram among the four pesticides and six metal ions. A prominent fluorescence quenching of N-CQDs@SiO₂ at 445 nm was observed in the presence of nitenpyram with a linear response range of 0-300.0 mg L^-1 and an estimated limit of detection of 1.53 mg L^-1. The main cause for selective sensing is that nitenpyram absorbs the excitation light of N-CQDs@SiO₂, leading to fluorescence quenching of the sensor through the inner filter effect.

Fluorescent Nanocomposite Hydrogels Based on Conjugated Polymer Nanoparticles as Platforms for Alkaline Phosphatase Detection

This work describes the development and characterization of fluorescent nanocomposite hydrogels, with high swelling and absorption capacity, and prepared using a green protocol. These fluorescent materials are obtained by incorporating, for the first time, polyfluorenes-based nanoparticles with different emission bands-poly[9,9-dioctylfluorenyl-2,7-diyl] (PFO) and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(1,4-benzo-{2,1,3}-thiadiazole)] (F8BT)-into a three-dimensional polymeric network based on polyacrylamide. To this end, two strategies were explored: incorporation of the nanoparticles during the polymerization process (in situ) and embedment after the hydrogel formation (ex situ). The results show that the combination of PFO nanoparticles introduced by the ex situ method provided materials with good storage stability, homogeneity and reproducibility properties, allowing their preservation in the form of xerogel. The fluorescent nanocomposite hydrogels have been tested as a transportable and user-friendly sensing platform. In particular, the ability of these materials to specifically detect the enzyme alkaline phosphatase (ALP) has been evaluated as a proof-of-concept. The sensor was able to quantify the presence of the enzyme in an aqueous sample with a response time of 10 min and LOD of 21 nM. Given these results, we consider that this device shows great potential for quantifying physiological ALP levels as well as enzyme activity in environmental samples.

Natural N-Doped Carbon Quantum Dots Derived from Straw and Adhered onto TiO2 Nanospheres for Enhancing the Removal of Antibiotics and Resistance Genes

Antibiotics and antibiotic resistance genes (ARGs) are emerging environmental contaminants. TiO₂ photocatalytic degradation has been proved an important removal technique, but its photocatalytic ability needs be improved. In our work, natural N-doped carbon quantum dots (N-SCQDs) were extracted from hydrothermal carbonization waste liquid of straw and were attached onto TiO₂ nanospheres for remediating antibiotics [sulfadiazine (SA)] and ARGs (sul1, sul2, and intl1). The maximum SA reduction rates were close to 100%, and the ARG reduction rates were 52.91-83.52%/lg10 (sul1), 32.10-68.23%/lg10 (sul2), and 46.29-76.55%/lg10 (inlt1). The temperature of the straw derivatives would influence their photoelectric properties. N-SCQDs@TiO₂ expands the application range of a novel potential high-efficiency degradation catalyst and offers a new way of hydrothermal carbonization waste liquid of agricultural waste.

Naturally Derived Carbon Dots In Situ Confined Self-Healing and Breathable Hydrogel Monolith for Anomalous Diffusion-Driven Phytomedicine Release

Fluorescent nanocarbons are well-proficient nanomaterials because of their optical properties and surface engineering. Herein, Apium graveolens-derived carbon dots (ACDs) have been synthesized by a one-step hydrothermal process without using any surplus vigorous chemicals or ligands. ACDs were captured via an in situ gelation reaction to form a semi-interpenetrating polymer network system showing mechanical robustness, fluorescent behavior, and natural adhesivity. ACDs-reinforced hydrogels were tested against robust uniaxial stress, repeated mechanical stretching, thixotropy, low creep, and fast strain recovery, confirming their elastomeric sustainability. Moreover, the room-temperature self-healing behavior was observed for the ACDs-reinforced hydrogels, with a healing efficacy of more than 45%. Water imbibition through hydrogel surfaces was digitally monitored via "breathing" and "accelerated breathing" behaviors. The phytomedicine release from the hydrogels was tuned by the ACDs' microstructure regulatory activity, resulting in better control of the diffusion rate compared to conventional chemical hydrogels. Finally, the phytomedicine-loaded hydrogels were found to be excellent bactericidal materials eradicating more than 85% of Gram-positive and -negative bacteria. The delayed network rupturing, superstretchability, fluorescent self-healing, controlled release, and antibacterial behavior could make this material an excellent alternative to soft biomaterials and soft robotics.

Microgels based on 0D-3D carbon materials: Synthetic techniques, properties, applications, and challenges

Microgels are three-dimensional (3D) colloidal hydrogel particles with outstanding features such as biocompatibility, good mechanical properties, tunable sizes from submicrometer to tens of nanometers, and large surface areas. Because of these unique qualities, microgels have been widely used in various applications. Carbon-based materials (CMs) with various dimensions (0-3D) have recently been investigated as promising candidates for the design and fabrication of microgels because of their large surface area, excellent conductivity, unique chemical stability, and low cost. Here, we provide a critical review of the specific characteristics of CMs that are being incorporated into microgels, as well as the state-of-the art applications of CM-microgels in pollutant adsorption and photodegradation, H₂ evoluation, CO₂ capture, soil conditioners, water retention, drug delivery, cell encapsulation, and tissue engineering. Advanced preparation techniques for CM-microgel systems are also summarized and discussed. Finally, challenges related to the low colloidal stability of CM-microgels and development strategies are examined. This review shows that CM-microgels have the potential to be widely used in various practical applications.

Mesoporous Silica Modified with 2-Phenylimidazo[1,2-a] pyridine-3-carbaldehyde as an Effective Adsorbent for Cu(II) from Aqueous Solutions: A Combined Experimental and Theoretical Study

In this study, we will present an efficient and selective adsorbent for the removal of Cu(II) ions from aqueous solutions. The silica-based adsorbent is functionalized by 2-phenylimidazo[1,2-a] pyridine-3-carbaldehyde (SiN-imd-py) and the characterization was carried out by applying various techniques including FT-IR, SEM, TGA and elemental analysis. The SiN-imd-py adsorbent shows a good selectivity and high adsorption capacity towards Cu(II) and reached 100 mg/g at pH = 6 and T = 25 °C. This adsorption capacity is important compared to other similar adsorbents which are currently published. The adsorption mechanism, thermodynamics, reusability and the effect of different experimental conditions, such as contact time, pH and temperature, on the adsorption process, were also investigated. In addition, a theoretical study was carried out to understand the adsorption mechanism and the active sites of the adsorbent, as well as the stability of the complex formed and the nature of the bonds.

Localized Surface Plasmon Resonance Decorated with Carbon Quantum Dots and Triangular Ag Nanoparticles for Chlorophyll Detection

This paper demonstrates carbon quantum dots (CQDs) with triangular silver nanoparticles (AgNPs) as the sensing materials of localized surface plasmon resonance (LSPR) sensors for chlorophyll detection. The CQDs and AgNPs were prepared by a one-step hydrothermal process and a direct chemical reduction process, respectively. FTIR analysis shows that a CQD consists of NH₂, OH, and COOH functional groups. The appearance of C=O and NH₂ at 399.5 eV and 529.6 eV in XPS analysis indicates that functional groups are available for adsorption sites for chlorophyll interaction. A AgNP-CQD composite was coated on the glass slide surface using (3-aminopropyl) triethoxysilane (APTES) as a coupling agent and acted as the active sensing layer for chlorophyll detection. In LSPR sensing, the linear response detection for AgNP-CQD demonstrates R² = 0.9581 and a sensitivity of 0.80 nm ppm^-1, with a detection limit of 4.71 ppm ranging from 0.2 to 10.0 ppm. Meanwhile, a AgNP shows a linear response of R² = 0.1541 and a sensitivity of 0.25 nm ppm^-1, with the detection limit of 52.76 ppm upon exposure to chlorophyll. Based on these results, the AgNP-CQD composite shows a better linearity response and a higher sensitivity than bare AgNPs when exposed to chlorophyll, highlighting the potential of AgNP-CQD as a sensing material in this study.

Preparation of Quantum Dot-Embedded Photonic Crystal Hydrogel and Its Application as Fluorescence Sensor for the Detection of Nitrite

The development of fluorescence sensing platforms with excellent photoluminescence capabilities is of great importance for their further application. In this work, a photonic crystal structure was successfully applied to enhance the luminescence performance of fluorescent hydrogel, and the application of the obtained hydrogel as a fluorescence sensor was explored. A polystyrene photonic crystal template was constructed via vertical deposition self-assembly; then, the precursor solution containing polyethylenimine-capped CdS quantum dots (PEI-CdS QDs) and monomers filled in the gap of the template. After the polymerization process, the desired hydrogel was obtained. PEI-CdS QDs endowed the hydrogel with its fluorescence property, while interestingly, the photonic crystal structure showed a significant enhancement effect on the fluorescence-emission capability. The mechanism of this phenomenon was revealed. Moreover, this hydrogel could be used as a reusable fluorescence sensor for the detection of nitrite in water with good selectivity. The limit of detection was determined to be 0.25 μmol/L, which is much lower than the maximum limit for nitrite in drinking water.