One-step synthesis of fluorescent graphene quantum dots as an effective fluorescence probe for vanillin detection

Stability of sp3 Carbons in Hydrogenated Graphene Quantum Dots and Their Electronic and Optical Properties Studied Using Density Functional Theory

Graphene quantum dots (GQDs) are zero-dimensional nanomaterials composed of sp2-hybridized carbon atoms, which are widely researched because of their tunable optical properties. GQDs contain defects, such as sp3-hybridized carbon atoms, which may be introduced during synthesis and can affect these materials' properties. In this study, we use hydrogenated polycyclic aromatic hydrocarbons as models for GQDs containing sp3-hybridized carbon atoms. We analyze the effect of sp3 carbons on the stabilities and electronic and optical properties of GQDs using density functional theory (DFT) and time-dependent DFT calculations. We find that sp3 carbons can form stable arrangements as dimers or continuous chains along the edges of GQDs. Our results reveal that the presence of sp3 carbons can tune the HOMO-LUMO gap, dependent on the position of sp3 carbons within the GQD. Calculated optical absorption spectra show a reduction in intensity and a blue shift of the main absorption peak for most of the investigated sp3-containing structures; additionally, the presence of sp3 carbons can extend the optical absorption of these structures into the red and infrared regions of the solar spectrum (600 to 900 nm), depending on the concentration and arrangement of sp3 carbons. These results provide insight into structural factors responsible for the variation of the electronic and optical properties of GQD nanomaterials and suggest that controlling the amount of residual sp3 carbon atoms introduced during synthesis can be used to tailor the properties of GQDs.

Development of a smartphone integrated 3D-printed point of care platform for sensitive detection of bilirubin

Strategic design and development of nanomaterials-based detection platforms specific to critical biomarkers like bilirubin holds immense promise for revolutionizing early disease detection. Bilirubin (BR) plays a pivotal role as a biomarker for liver function, making accurate and timely detection of BR crucial for diagnosing and monitoring of liver diseases. In this work, we synthesized blue light emitting graphene quantum dots (GQDs) via a single step pyrolysis method, which exhibited excellent photostability and biocompatibility. Under optimal conditions, the fluorescence of GQDs was significantly quenched with the successive addition of BR achieving an ultra-low detection limit (38.96 nM) over a concentration range of 0.18 μM-14.29 μM with high selectivity, and rapid response towards free BR. The sensing mechanism was identified as the inner filter effect after extensive investigations. Thereafter, the sensor system was directly applied to human serum and urine samples and was further compared with the conventional Jendrassik and Grof method, yielding satisfactory recoveries. To demonstrate the sensor system's potential for real world applications, we designed and fabricated a prototype point-of-care device (POC) through 3D printing, incorporating paper microfluidic devices and fluorescence image analysis-based android application through smartphone. The compact 3D-printed POC device achieved a detection limit of 114.66 nM for BR detection, proving to be a promising platform for affordable, efficient and rapid BR detection.

ZnFe2O4/GQDs Nanoparticles as Peroxidase Mimics for Sensitive and Selective Colorimetric Detection of Glucose in Real Samples

Glucose detection is critical in addressing health and medical issues related to irregular blood levels. Colorimetry, a simple, cost-effective, and visually straightforward method, is often employed. Traditional enzymatic detection methods face drawbacks such as high costs, limited stability, and operational challenges. To overcome these, enzyme mimics or artificial nano-enzymes based on inorganic nanomaterials have garnered attention, but their cost and susceptibility to inactivation limit applications. This study presents a ZnFe2O4/GQDs nanocomposite as an innovative enzyme mimic, addressing key requirements like low cost, high stability, biocompatibility, and wide operational range. Synthesized using a simple and inexpensive method, the composite benefits from the synergistic interaction between ZnFe2O4 nanoparticles and graphene quantum dots (GQDs), resulting in excellent magnetic properties, high surface area, and functional versatility. The material demonstrated remarkable sensitivity with a detection limit of 7.0 μM across a range of 5-500 μM and achieved efficient peroxidase-like activity with Km values of 0.072 and 0.068 mM and Vmax of 4.58 × 10⁻8 and 8.29 × 10⁻8 M/s for TMB and H2O2, respectively. The nanocomposite also exhibited robust recyclability, retaining performance over six reuse cycles.

Advances in spin properties of plant leaf-derived graphene quantum dots from materials to applications

Over the past decade, graphene quantum dots (GQDs) have gained an inexhaustible deal of attention due to their unique zero-dimensional (0D) and quantum confinement properties, which boosted their wide research implication and reliable applications. As one of the promising 0D member and rising star of the carbon family, plant leaf-derived GQDs have attracted significant attention from scholars working in different research fields. Owing to its novel photophysical properties including high photo-stability, plant leaf-derived GQDs have been increasingly utilized in the fabrication of optoelectronic devices. Their superior biocompatibility finds their use in biotechnology applications, while their fascinating spin and magnetic properties have maximized their utilization in spin-manipulation devices. In order to promote the applications of plant leaf-derived GQDs in different fields, several studies over the past decade have successfully utilized plant leaf as sustainable precursor and synthesized GQDs with various sizes using different chemical and physical methods. In this review, we summarize the Neem and Fenugreek leaves based methods of synthesis of plant leaf-derived GQDs, discussing their surface characteristics and photophysical properties. We highlight the size and wavelength dependent photoluminescence properties of plant leaf-derived GQDs towards their applications in optoelectronic devices such as white light-emitting diodes and photodetectors, as well as biotechnology applications such asin vivoimaging of apoptotic cells and spin related devices as magnetic storage medium. Finally, we particularly discuss possible ways of fine tuning the spin properties of plant leaf-derived GQD clusters by incorporation with superconducting quantum interference device, followed by utilization of atomic force microscopy and magnetic force microscopy measurements for the construction of future spin-based magnetic storage media and spin manipulation quantum devices so as to provide an outlook on the future spin applications of plant leaf-derived GQDs.

Sensitive Electrochemical Determination of Vanillin Using a Bimetallic Hydroxide and Reduced Graphene Oxide Nanocomposite

Vanillin (VAN) is an organic compound which not only functions as a flavoring and fragrance enhancer in some foods but also has antioxidant, anti-inflammatory, anti-cancer, and anti-depressant effects. However, the excessive use of VAN can be associated with negative side effects on human health. As a result, it is crucial to find a reliable method for the rapid determination of VAN to enhance food safety. Herein, we developed a sensor using Ni and Co bimetallic hydroxide and reduced graphene oxide nanostructure (NiCo(OH)2.rGO). Our prepared material was characterized using various physico-chemical techniques. The electrocatalytic efficiency of the NiCo(OH)2.rGO-modified glassy carbon electrode was investigated using cyclic and square wave voltammetry. The developed sensor showed a limit of detection of 6.1 nM and a linear range of 5-140 nM. The synergistic effect of NiCo(OH)2 and rGO improved the active sites and enhanced its catalytic efficiency. The practical applicability of the prepared sensor was investigated for the determination of VAN in food samples such as biscuits and chocolates, showing promise in practical applications.

Self-Assembled Luminescent Droplets from Graphene Quantum Dots Induced by a Gemini Surfactant for Selective Detection of Mercury(II)

Herein, we report the fabrication of a new class of luminescent coacervate droplets from graphene quantum dots (GQDs) and a gemini surfactant in aqueous medium and utilized them toward detection of mercuric ions (Hg2+). The self-assembly of negatively charged GQDs and positively charged gemini surfactant exists mainly because of their electrostatic interaction, leading to coacervation. Confocal laser scanning microscopy (CLSM) and field-emission scanning electron microscopy (FESEM) were utilized to analyze the luminescent and morphological structures of the self-assembled droplets. CLSM images display droplets that are naturally luminescent. The droplets exhibit luminescence quenching in the presence of Hg2+ ions. Our study demonstrates that Hg2+ ions interact through electrostatic interactions with the free carboxylate groups on the surface of GQDs in the hollow structure of the droplets. For Hg2+ ion sensing, the limit of detection (LOD) using the present system is found to be 30.5 nM, which is substantially lower than that of many of the previously reported similar systems. The sensor demonstrated high sensitivity for Hg2+ ions and exhibited a strong linear correlation within the concentration range of 100-500 nM. The current results indicate that the flexibility of surface ligands and organic nanoparticles in hybrid droplets plays a crucial role in the development of multifunctional materials for diverse applications.

A Ratio Fluorescence Method Based on Dual Emissive Copper Nanoclusters for the Detection of Vanillin

In this study, a novel double-emission fluorescence probe at 340 and 400 nm was synthesized by one-pot method using phenylalanine (Phe) and ascorbic acid (AA) as stabilizing and reducing agents. It was found that the fluorescence intensity of the probe at 400 nm could be controlled by controlling the temperature within a certain range, and the ratio of double-emission fluorescence probe could be further regulated. Under the optimal conditions, the fluorescence intensity at 340 nm decreased significantly, while it only showed a slight decrease at 400 nm, which constituted the ratio fluorescence probe. The synthesized fluorescence probe showed good linearity in the range of 0.2-32 μM, and its detection limit was 63.4 nM. Moreover, the method was successfully employed to determine VA in vanilla drink and perfumes, and corresponding results were consistent with those of HPLC.

Self-ratiometric fluorescence approach based on plant extract-assisted synthesized silver nanoparticles for the determination of vanillin

The current study designed and applied a novel self-ratiometric fluorescent nanosensor composed of green-synthesized silver nanoparticles (Ag-NPs) to determine vanillin in adult and infant foods and human plasma. A straightforward microwave-assisted approach is proposed for synthesizing Ag-NPs in less than 1 min using a reducing agent, tailed pepper seed extract. The synthesized Ag-NPs had a strong fluorescence with an intense emission band at 360 nm and a shoulder peak at 430 nm when excited at 265 nm. Upon interaction with vanillin, the fluorescence peak of Ag-NPs at 360 nm decreases in a concentration-dependent manner while being shifted to a longer wavelength, 385 nm. Meanwhile, the shoulder fluorescence peak at 430 nm is only slightly affected by vanillin addition. Thus, a new Ag-NP self-ratiometric probe was designed and validated for vanillin determination using the peak at 385 nm and the shoulder peak at 430 as two built-in reference peaks. The optimized system accurately measured vanillin with a detection limit of 9.0 ng/mL and a linear range of 0.05-8.0 μg/mL without needing pre-derivatization or high-cost instrumentation. The method successfully measured vanillin in adult and infant milk formula, biscuits, and human plasma samples with high percentage recoveries (95.3-104.6%) and excellent precision (relative SD; ≤3.85%).

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging

Advances in nanotechnology and nanomaterials have attracted considerable interest and play key roles in scientific innovations in diverse fields. In particular, increased attention has been focused on carbon-based nanomaterials exhibiting diverse extended structures and unique properties. Among these materials, zero-dimensional structures, including fullerenes, carbon nano-onions, carbon nanodiamonds, and carbon dots, possess excellent bioaffinities and superior fluorescence properties that make these structures suitable for application to environmental and biological sensing, imaging, and therapeutics. This review provides a systematic overview of the classification and structural properties, design principles and preparation methods, and optical properties and sensing applications of zero-dimensional carbon nanomaterials. Recent interesting breakthroughs in the sensitive and selective sensing and imaging of heavy metal pollutants, hazardous substances, and bioactive molecules as well as applications in information encryption, super-resolution and photoacoustic imaging, and phototherapy and nanomedicine delivery are the main focus of this review. Finally, future challenges and prospects of these materials are highlighted and envisaged. This review presents a comprehensive basis and directions for designing, developing, and applying fascinating fluorescent sensors fabricated based on zero-dimensional carbon nanomaterials for specific requirements in numerous research fields.

Highly defective graphene quantum dots-doped 1T/2H-MoS2 as an efficient composite catalyst for the hydrogen evolution reaction

We present a new composite catalyst system of highly defective graphene quantum dots (HDGQDs)-doped 1T/2H-MoS2 for efficient hydrogen evolution reactions (HER). The high electrocatalytic activity, represented by an overpotential of 136.9 mV and a Tafel slope of 57.1 mV/decade, is due to improved conductivity, a larger number of active sites in 1T-MoS2 compared to that in 2H-MoS2, and additional defects introduced by HDGQDs. High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) were used to characterize both the 1T/2H-MoS2 and GQDs components while Fourier-transform infrared spectroscopy (FTIR) was employed to identify the functional groups on the edge and defect sites in the HDGQDs. The morphology of the composite catalyst was also examined by field emission scanning electron microscopy (FESEM). All experimental data demonstrated that each component contributes unique advantages that synergistically lead to the significantly improved electrocatalytic activity for HER in the composite catalyst system.

GQD@NiFe-LDH Nanosheets for Photocatalytic Activity towards Textile Dye Degradation via Lattice Contraction

The functionalized NiFe-LDH with photosensitized GQDs were synthesized through the hydrothermal route by differing the amount of GQDs solution and studied its efficacy towards the mineralization of textile dyes under visible light. The synthesized samples were characterized by XRD, FESEM, HRTEM, DRUV-Vis, RAMAN, XPS, and BET. The combined effect of the hexagonal carbon lattice in GQD and open layered porous structure of NiFe-LDH nanosheets results in the contraction of the lattice. Different reactive and conventional dyes were taken as representative dyes to evaluate the activity of the as-synthesized photocatalysts. The enhanced electron absorption/donor effect between GQDs and NiFe-LDH, and the growth of oxygen-bridged Ni/Fe-C moieties enable the composite to exhibit better photocatalytic activity. Both photocatalytic activity and characterization results confirmed that the GQD@NiFe-LDH nanocomposite heterostructure synthesized at 160 °C by taking 10 mL of GQDs aqueous solution named GNFLDH10 has a higher degree of crystallinity and has the best photocatalytic efficiency compared to other reported visible light catalysts. Specifically, the above optimized GQD@NiFe-LDH photocatalyst is capable of photo-mineralizing 50 ppm of Reactive Green in 20 min, Reactive Red in 20 min, and Congo Red in 25 min respectively following a direct Z-scheme mechanism with substantial reusability.

Lanthanum nickelate spheres embedded acid functionalized carbon nanofiber composite: An efficient electrocatalyst for electrochemical detection of food additive vanillin

Contemporary food marketing is ruined by flavor enhancers rather than emphasizing the nutritional value of food. Vanillin is an overexploited flavor enhancer added to food items, thereby necessitating its detection. In this study, an electrochemical sensor was designed using a modified electrode made up of La2NiO4 functionalized carbon nanofiber (f-CNF) to effectively detect vanillin in food samples. To confirm the successful formation of La2NiO4/f-CNF, structural and morphological studies were performed using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Further electrochemical analysis was performed using cyclic voltammetry and differential pulse voltammetry techniques, which resulted in high sensitivity (0.2899 µA·μM-1·cm-2) and low limit of detection (LOD) (6 nM). This modified electrode material was tested in food samples, which showed an excellent response with recovery percentage and is a promising electrocatalyst for vanillin detection.

A review on nanomaterial-based electrochemical sensors for determination of vanillin in food samples

Vanillin is an organic compound that not only acts as a flavoring and fragrance enhancer in some foods, but also can have antioxidant, anti-inflammatory, anti-cancer and anti-depressant effects. Nevertheless, its excessive use can be associated with side effects on human health. Consequently, there is a need to achieve a rapid vanillin determination approach to enhance food safety. The diversity and high sensitivity of analytical approaches has led researchers to use more advanced and efficient methods providing quantitative and qualitative outcomes in complex matrices. Among these, prominent attention has been drawn to electrochemical sensors for reasons such as reliability, simplicity, cost-effectiveness, portability, selectivity, and ease of operation, especially for the determination of vanillin. Nanomaterials are a good candidate for sensor construction due to their commendable physicochemical attributes. Some advanced nanostructures with promising platforms for high-sensitivity, highly selective, and long-lasting electrochemical sensors include graphene (Gr) and its derivatives, graphite carbon nitride (g-C₃N₄), carbon nanotubes (CNTs), metal nanoparticles, metal organic frameworks, carbon nanofibers (CNFs) and quantum dots. Study about sizes, dimensions, and morphologies of nanomaterials makes strong candidates for improving sensitivity or selectivity according to electrocatalytic abilities. The low LOD and wide linear range of samples demonstrated an excellent catalytic performance towards the vanillin oxidation. Some investigations have reported the synergistic effects like great conductivity of carbon nanomaterials which improved the electrocatalytic performance of nanocomposites which demonstrated the estimable sensitivity of nanomaterial-supported electrochemical sensors for determination of vanillin concentrations. The sensors which have reported have a commendable response to practical potential and evaluated in biscuit, pudding powder, chocolate, custard specimens and etc. sensitivity, stability, reproducibility and repeatability of suggested sensor were investigated. The present review article scrutinizes recent advances in the fabrication of nanomaterial-based electrochemical sensors to detect vanillin in various food matrices.

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...

Lithium titanate nanoplates embedded with graphene quantum dots as electrode materials for high-rate lithium-ion batteries

Anode materials based on lithium titanate (LTO)/graphene composites are considered as ideal candidates for high-rate lithium-ion batteries (LIBs). Considering the blocking effects of graphene nanosheets in electrodes during ion-transfer processes, construction of LTO/graphene composite structures with enhanced electrical and ionic conductivity via facile and scalable techniques is still challenging for high-rate LIB. In this work, structures of anode materials based on LTO nanoplates embedded with graphene quantum dots (GQDs) are demonstrated for high-rate LIB. The hybrids can be facilely prepared viain situintroduction of GQDs during the process LTO preparation, which enables a uniform dispersion of GQDs within LTO. This method is convenient, rapid, and can be easily scaled-up. The introduction of 0.05 wt.% GQDs can greatly enhance the electrochemical performance of the electrodes. The electrodes with 0.05 wt.% GQDs deliver a specific discharge capacity of 185, 181 and 179 mAh g^-1at 5, 10, and 20 C, respectively. The performance enhancement is suggested to be due to the synergistic interactions between LTO and GQDs. The strategy as well as as-designed structures of LTO/GQDs show potentials for application as high-rate anode materials in LIBs application.