Lignin-based fluorescence-switchable graphene quantum dots for Fe3+ and ascorbic acid detection

Effective pretreatment of tea stem via poly-deep eutectic solvent for promoting platform molecule production and obtaining fluorescent lignin

In this study, polyethylene glycol 200 (PEG200) was employed as hydrogen bond acceptor, while organic acids served as hydrogen bond donors, to formulate poly-deep eutectic solvents (PDESs), which were utilized to pretreat tea stem. Specially, combining PEG200 and oxalic acid (OA) exhibited a notably high cellulose retention (82.03 %) and most efficient hemicellulose (97.02 %) and lignin removal (70.89 %). The removal of amorphous lignin enhanced the crystallinity of the residues and improved the conversion efficiency of cellulose into levulinic acid. Additionally, the structural alterations in lignin samples were analyzed in comparison to milled wood lignin (MWL). The PEG200-OA system facilitated the cleavage of β-O-4 and β-5 linkages and resulted in the degradation of S-type lignin, accompanied by increased condensation of G units. The resulting lignin displayed a reduced molecular weight (Mw of 1283 g/mol, Mn value 531 g/mol) and nanoscale particle size (D50 212 nm). Furthermore, fluorescent lignin was synthesized through simple oxidation and was used to detect metal ions. Density functional theory (DFT) calculations supported that both PEG200 and OA played a significant role in lignin dissolution, with the weak interactions between DES and lignin primarily driven by hydrogen bonding (characterized as weak, closed-shell, and electrostatic) and van der Waals forces.

State-of-the-art of lignin-derived carbon nanodots: Preparation, properties, and applications

Lignin-derived carbon nanodots (LCNs) are nanometer-scale carbon spheres fabricated from naturally abundant lignin. Owing to rich and highly heritable graphene like π-π conjugated structure of lignin, to fabricate LCNs from it not only endows LCNs with on-demand tunable size and optical features, but also further broadens the green and chemical engineering of carbon nanodots. Recently, they have become increasingly popular in sensing, bioimaging, catalysis, anti-counterfeiting, energy storage/conversion, and others. Despite the enormous research efforts put into the ongoing development of lignin value-added utilization, few commercial LCNs are available. To have a deeper understanding of this issue, critical impacts on the preparation, properties, and applications of state-of-the-art LCNs are carefully reviewed and discussed. A concise analysis of their unique advantages, limitations for specific applications, and current challenges and outlook is conducted. We hope that this review will stimulate further advances in the functional material-oriented production of lignin.

Transforming bio-waste lignin into amine functionalized carbon quantum dots for selective detection of trace Cu2+ in aqueous system

Developing carbon quantum dots (CQDs) from bio-waste lignin for effectively detecting Cu2+ is of great significance for promoting the value-added utilization of lignin resources. However, the limited amount of surface-active groups and low quantum yield of lignin-based CQDs hinder their application in this regard. Herein, bio-waste lignin was converted into value-added amine functionalized CQDs using a facile two-step hydrothermal approach. The as-synthesized CQDs modified with amino groups exhibit bright green fluorescence, abundant surface functional groups, high water solubility and uniform particle size (3.9 nm). Systematic analysis demonstrates that the rich NH2 groups (~12.3 %) on the CQDs backbone improve their fluorescence properties (quantum yield increased from 3.4 % to 21.1 %) and specific detection ability for Cu2+. The developed NH2-CQDs serve as an efficient fluorescent probe, displaying high sensitivity and selectivity towards Cu2+ in aqueous system, with a detection limit of 2.42 μmol/L, which is lower than the maximum permitted amount of Cu2+ in drinking water (20 μmol/L). The detection mechanism of NH2-CQDs for Cu2+ is attributed to the synergy of static quenching and photo-induced electron transfer. This study provides a valuable reference for the synthesis of high-quality fluorescent CQDs from lignin resources and the effective detection of trace Cu2+ in aquatic environments.

Hydrothermal synthesis of modified lignin-based carbon dots derived from biomass waste for fluorescence determination of valsartan

Recently, carbon dots (CDs) have been extensively investigated as potential tools for numerous applications. Modified lignin-based CDs have been synthesized and used in the field of drug detection. They were found to be highly selective and sensitive to valsartan (VAL). Using a simple hydrothermal method, phosphorus and chlorine co-doped CDs were synthesized using lignin extracted from date seeds. The fluorescence properties of the synthesized CDs are influenced by several factors, which were investigated in detail. The optimal synthesis conditions were 1.50 g of lignin, 18 mL of 2 M NaOH, 1 mM H3PO4, 3 mM HCl and the mixture was heated at 220 °C for 16 hours. The synthesized lignin-based CDs have excellent FL properties and are well soluble in water with reasonable stability. Characterization of the prepared CDs revealed that they have various functional groups with a graphene oxide-like structure. The developed CDs show a good quantum yield of 37.7%. The FL of the CDs is quenched by VAL at λ em 313 nm after λ ex at 275 nm by a combination of static and dynamic quenching mechanisms. The response of VAL was linear in the range of 4.0-100.0 μg mL-1. The detection and quantification limits of VAL were 1.23 and 3.71 μg mL-1, respectively. The nanoprobe was successfully used to analyze VAL in drug samples and provided satisfactory results.

Recent developments in lignin-based fluorescent materials

Biomass-based fluorescent materials are an alternative to plastic-based materials for their multifunctional applications. Lignin, an inexpensive and easily available raw material, demonstrates outstanding environment-responsive properties such as pH, metal ions, dyes sensing, bioimaging and so on. To date, only a little work has been reported on the synthesis of lignin-based fluorescent materials. In this review report, synthetic approaches and light-responsive applications of lignin-based fluorescent carbon dots and other materials are summarized. The results reveal that lignin-based fluorescent carbon dots are prepared by hydrothermal method, exhibit small size <10 nm, reveal significant quantum yield, biocompatibility, non-toxicity, photostability and display substantial tunable emission and can be efficiently employed for sensing, bioimaging and energy storage applications. Finally, the forthcoming challenges, investigations, and options open for the chemical and/or physical modification of lignin into fluorescent materials for future applications are well-addressed. To our knowledge, this is the first comprehensive review report on lignin-based fluorescent materials and their light-responsive applications. In addition, this review will attract remarkable consideration and thrust for the researchers and biochemical technologists working with the preparation of lignin-based fluorescent materials for broad applications.

Sustainable synthesis of lignin-derived carbon dots with visible pH response for Fe3+ detection and bioimaging

Synthesis of lignin-based carbon dots (LCDs) with high quantum yield (QY), stable fluorescence properties and biocompatibility has been a challenge. Here, we propose an improved two-step strategy for producing high-quality LCDs from enzymatic hydrolysis lignin (EHL). The p-aminobenzenesulfonic acid used in the strategy not only provides nitrogen and sulfur elements, but also tailors the disordered three-dimensional structure of EHL. The successful co-doping of N and S elements favors the reduction of the optical energy bandgap (Eg), resulting in a high QY of 45.05% for LCDs. The LCDs exhibited superior selectivity and sensitivity for Fe3+ with a limit of detection (LOD) of 0.15 μM when Fe3+ concentration was 50-500 μM. In addition, LCDs demonstrated significant fluorescence in HepG2 cells and HepG2 cells loaded with LCDs at a concentration of 80 μg/mL showed good viability, suggesting that they are suitable for in vivo applications. The luminescent centers of LCDs change during pH regulation and thus show a special visual response to pH changes, making them have great potential for detecting metabolism in living cells. This work provides a novel and low-cost method for fabricating sustainable fluorescent probes for chemical sensing and bioimaging.

Lignocellulosic Biomass-Based Carbon Dots: Synthesis Processes, Properties, and Applications

Carbon dots (CDs), a new type of carbon-based fluorescent nanomaterial, have attracted widespread attention because of their numerous excellent properties. Lignocellulosic biomass is the most abundant renewable natural resource and possesses broad potential to manufacture different composite and smart materials. Numerous studies have explored the potential of using the components (such as cellulose, hemicellulose, and lignin) in lignocellulosic biomass to produce CDs. There are few papers systemically aiming in the review of the state-of-the-art works related to lignocellulosic biomass-derived CDs. In this review, the significant advances in synthesis processes, formation mechanisms, structural characteristics, optical properties, and applications of lignocellulosic biomass-based CDs such as cellulose-based CDs, hemicellulose-based CDs and lignin-based CDs in latest research are reviewed. In addition, future research directions on the improvement of the synthesis technology of CDs using lignocellulosic biomass as raw materials to enhance the properties of CDs are proposed. This review will serve as a road map for scientists engaged in research and exploring more applications of CDs in different science fields to achieve the highest material performance goals of CDs.

Fluorescence Sensing Mechanisms of Versatile Graphene Quantum Dots toward Commonly Encountered Heavy Metal Ions

Graphene quantum dots (GQDs) have received tremendous attention as fluorescent probes for detection of diverse heavy metal ions (HMIs). Nevertheless, the fluorescence sensing mechanisms of versatile GQDs with respect to different HMIs remain elusive. Herein, the fluorescence sensing behaviors and mechanisms of GQDs with amino and carboxyl groups toward commonly encountered Cr6+, Fe3+, Cu2+, Cr3+, Mn2+, Co2+, Ni2+, Zn2+, Cd2+, and Hg2+ under different pH conditions are systemically explored. The results show that the fluorescence of GQDs can be enhanced by Zn2+/Cd2+ and quenched by other HMIs at pH 5.8, while it can be enhanced by HMIs except Cr6+/Fe3+/Cu2+ at pH 2.0. Systematic studies verify that the fluorescence quenching/enhancing is mediated by the synergistic effect of the inner filter effect (IFE) and the photoinduced electron transfer (PET) or metal orbital-controlled chelation-quenched/enhanced fluorescence (CHQF/CHEF) effect. The strong and weak IFEs of Cr6+/Fe3+ and Cr3+/Cu2+, respectively, are one of the reasons for the fluorescence quenching, while other HMIs have no IFE. Moreover, the PET effect caused by the interaction of GQDs with Hg2+ at pH 5.8 and the CHQF/CHEF effect caused by the interaction of GQDs with other HMIs are also crucial for fluorescence quenching/enhancing. The findings suggest that the pH condition, the existing forms of functional groups on GQDs, and the complexation states of HMIs in aqueous systems dominate the PET and CHQF/CHEF effects. The elucidating of the fluorescence sensing mechanisms of GQDs toward different HMIs paves the way for developing versatile sensing platforms for monitoring of HMI contamination.

Glutathione-modified graphene quantum dots as fluorescent probes for detecting organophosphorus pesticide residues in Radix Angelica Sinensis

A novel fluorescent sensor was developed in this study based on glutathione-functionalized graphene quantum dots (GQDs@GSH) to detect organophosphorus pesticide residues in Radix Angelica Sinensis. GQDs@GSH was synthesized by a one-step pyrolysis method with a fluorescence quantum yield as high as 33.9% and its structure was characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. GQDs@GSH exhibited excellent fluorescence property showing strong blue fluorescence under UV irradiation. The fluorescence of GQDs@GSH could be quenched by Fe³⁺ by electron transfer and the quenched fluorescence could be recovered due to the strong chelating and reducing ability of phytic acid (PA). Under the catalyzation of acetylcholinesterase (AChE) and choline oxidase (ChOx), acetylcholine (ACh) could be decomposed to H₂O₂, which could further oxidize Fe²⁺ to Fe³⁺ thus quenching the fluorescence of GQDs@GSH once again. Coumaphos, a kind of organophosphorus pesticide, could inhibit AChE activity, thus making the quenched fluorescence turn on again. Several parameters influencing the fluorescence response such as Fe³⁺, PA, ACh and coumaphos concentration, pH value and reaction time were optimized. Based on such a fluorescence "off-on-off-on" ngkmechanism, GQDs@GSH was successfully applied to the detection of coumaphos in Radix Angelica Sinensis. A good linear relationship between the fluorescence intensity and coumaphos concentration was obtained in the range of 0.1-10.0 μmol·L^-1. By a standard addition method, the recoveries were measured to be 101.44-117.90% with RSDs lower than 1.98%. The biosensor system is simple, sensitive and accurate. It has a good application prospect in the detection of organophosphorus pesticide residues in traditional Chinese medicine and agricultural products, and also expanded the application scope for glutathione as a highly selective biological molecule.

Recent advances in lignin-based carbon materials and their applications: A review

As the most abundant natural aromatic polymer, tens of million of tons of lignin produced in paper-making or biorefinery industry are used as fuel annually, which is a low-value utilization. Moreover, burning lignin results in large amounts of carbon dioxide and pollutants in the air. The potential of lignin is far from being fully exploited and the search for high value-added application of lignin is highly pursued. Because of the high carbon content of lignin, converting lignin into advanced carbon-based structural or functional materials is regarded as one of the most promising solutions for both environmental protection and utilization of renewable resources. Significant progresses in lignin-based carbon materials (LCMs) including porous carbon, activated carbon, carbon fiber, carbon aerogel, nanostructured carbon, etc., for various valued applications have been witnessed in recent years. Here, this review summarized the recent advances in LCMs from the perspectives of preparation, structure, and applications. In particular, this review attempts to figure out the intrinsic relationship between the structure and functionalities of LCMs from their recent applications. Hopefully, some thoughts and discussions on the structure-property relationship of LCMs can inspire researchers to stride over the present barriers in the preparation and applications of LCMs.

A biomass-derived Schiff base material composited with polylactic acid nanofiber membrane as selective fluorescent 'turn off/on' platform for Pb2+ quantitative detection and characterization

Herein, a biomass-derived compound Z1 is synthesized via 'one pot' method for detection Pb²⁺ using fluorescence and visual dual-mode in aqueous solution. Z1 shows good response to Pb²⁺ with a limit of detection (LOD) of 13.4 nM. Importantly, the coordination mode of Z1 with Pb²⁺ is further evaluated by UV-vis and NMR spectroscopy and a 1:1 stoichiometry is identified. Furthermore, Z1 can be applied to detection Pb²⁺ in practical samples with satisfactory recoveries in range of 96.0 %-112.0 % in real samples. Besides, Z1 is added into polylactic acid (PLA) solution and made as portable fluorescence nanofiber membrane for Pb²⁺ detection. Further, Z1 responds to Pb²⁺ with high selectivity and sensitivity and has been applied for tracking Pb²⁺ changes in soil samples, zebrafish, and plant tissues. These results indicated that Z1 had great application potential in accurate detection Pb²⁺.

Research progress in synthesis and biological application of quantum dots

Quantum dots are an excellent choice for biomedical applications due to their special optical properties and quantum confinement effects. This paper reviews the research and application progress of several quantum...