Graphene quantum dots as optical-electrochemical dual mode-biosensors for the detection of dopamine and serotonin biomarkers of depression

Graphene Quantum Dots (GQDs) and Carbon Quantum Dots (CQDs) are nanomaterials with rising popularity as an alternative to traditional semiconductor quantum dots and organic dyes.1 In addition to simple fabrication methods and low production costs, GQDs and CQDs exhibit a good chemical stability and solubility, unique photophysical properties, photochemical stability, and biocompatibility. These are remarkable properties for applications in fields such as catalysis, photovoltaic devices, bioimaging or medical diagnosis.2 With the objective of constructing versatile and functional ensembles for nanoelectronics and optoelectronics, we lately embarked in the synthesis of these carbon nanomaterials and their covalent or supramolecular modification with ligands that modify their fundamental properties. For instance, adding a chiroptical response to the semiconductor properties of GQDs will provide the extra value of their potential application in photonics. In this sense, we recently proof the concept that GQDs are able to become chiral and that this property can be transferred to a supramolecular structure built with pyrene molecules, where the chiral-GQDs/pyrene ensembles show a characteristic chiroptical response depending on the configuration of the introduced organic ligands.3 We have also combined GQDs and CQDs materials with p-quinonoid π-extended tetrathiafulvalenes (exTTFs)4 in the search for new electron donor-acceptor systems (Figure). The electronic interactions between the CQDs and exTTF have been investigated in the ground and excited states. The characterization of the obtained GQDs and CQDs nanomaterials by a combination of analytical, microscopic and spectroscopic techniques will be presented and discussed, along with the photophysical properties of some of the aggregates formed.

Graphene quantum dots (GQDs) have garnered significant attention across numerous fields due to their ultrasmall size and exceptional properties. However, their extensive applications may lead to environmental exposure and subsequent uptake by humans. Yet, conflicting reports exist regarding the potential toxicity of GQDs based on experimental investigations. Therefore, a comprehensive understanding of GQD biosafety requires further microscopic and molecular-level investigations. In this study, we employed molecular dynamics (MD) simulations to explore the interactions between GQDs and graphene oxide quantum dots (GOQDs) with a protein model, the human intestinal fatty acid binding protein (HIFABP), that plays a crucial role in mediating the carrier of fatty acids in the intestine. Our MD simulation results reveal that GQDs can be adsorbed on the opening of HIFABP, which serves as an entrance for the fatty acid molecules into the protein’s interior cavity. This adsorption has the potential to obstruct the opening of HIFABP, leading to the loss of its normal biological function and ultimately resulting in toxicity. The adsorption of GQDs is driven by a combination of van der Waals (vdW), π-π stacking, cation-π, and hydrophobic interactions. Similarly, GOQDs also exhibit the ability to block the opening of HIFABP, thereby potentially causing toxicity. The blockage of GOQDs to HIFABP is guided by a combination of vdW, Coulomb, π-π stacking, and hydrophobic interactions. These findings not only highlight the potential harmful effects of GQDs on HIFABP but also elucidate the underlying molecular mechanism, which provides crucial insights into GQD toxicology.

Graphene quantum dots (GQDs) and carbon quantum dots (CQDs) are promising nanomaterials with tunable electronic and optical properties influenced by quantum confinement effects and structural morphology. In this study, the band structures of GQDs and nitrogen-doped CQDs (N-CQDs) were compared to elucidate how particle size, structure, and synthesis methods affect their electronic properties. GQDs were synthesized via electrochemical exfoliation, allowing size control through current density adjustments, while N-CQDs were synthesized hydrothermally with citric acid and urea as precursors to ensure compositional similarity. Transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, UV-Vis spectroscopy, and synchrotron-based X-ray photoelectron spectroscopy (XPS) were employed to characterize particle sizes, band structures, and semiconductor behavior. Results indicate that GQDs exhibit stronger quantum confinement than N-CQDs, attributed to their sp²-hybridized, two-dimensional structures, which supports lateral electron mobility and increased axial quantum confinement. Furthermore, GQDs exhibited n-type semiconductor behavior, while N-CQDs displayed p-type characteristics, underscoring a fundamental difference in charge transport mechanisms. These findings highlight the critical role of structural and morphological factors in tuning the electronic properties of quantum dots, offering insights into the design of nanomaterials for optoelectronic applications.

Graphene quantum dots (GQDs), as a new graphene material, possess advantageous chemical-physical properties. Owing to their distinctive photoluminescence (PL) property and low biological toxicity, GQDs have been shown to be good candidates for applications in bioimaging and medical analysis. However, the prepared GQDs have their own slew of problems, including low luminescent efficiency, uncertain luminescent mechanism and lack of effective methods to tuning the luminescence property. Heteroatom doping is a good strategy, which could partially address these above problems. Because the doping atoms will be located in the internal structure of carbon nanomaterials thus change their local electronic configuration, polarizability, defect degree and band structure, etc., the physical and chemical properties of GQDs could be well tuned. At present, GQDs doped with Cl, F, N, S, B, P, etc. have been prepared. These obtained GQDs with heteroatom doping not only exhibited tunable optical properties, but also showed enormous potential applications in the field of photocatalysis. Herein, in order to further improve the PL performance of GQDs and extend their application, we prepared pure GQDs and single elemental-doped GQDs with Cl, N, P and S by using the electrochemical method and hydrothermal method. Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy, (XPS) and Raman measurements were used to characterize their elemental composition, surface element state and structural defect. Based on the experimental results, the position and bonding state of heteroatoms in doped GQDs were analyzed. The doping amounts in these doped GQDs are different, i.e., 1.35% of Cl-GQDs, 7.95% of N-GQDs, 10.08% of P-GQDs and 3.25% of S-GQDs, respectively. The degree of defect state is decreased in the order as follows: P-GQDs>S-GQDs>GQDs>Cl-GQDs>N-GQDs. Meanwhile, the PL performance was tested, and the fluorescent quantum efficiencies were calculated to be 8.2% for Cl-GQDs, 5.3% for N-GQDs, 4.0% for GQDs, 2.8% for S-GQDs, and 0.037% for P-GQDs, respectively. It can be concluded that the diverse doping atoms play different roles for the improvement of PL performance. The doping of Cl and N can form the luminescent center, which improves the fluorescent intensity of GQDs. Especially for the Cl doping, the fluorescent intensity of Cl-GQDs is increased twice compared to the pure GQDs, due to larger atomic radius and more outer shell electron of Cl than that of the others doped elementals. In the S-GQDs, the doped S could become small quenching centers, which decreased the fluorescent intensity slightly compared to that of pure GQDs. However, different from the above doped GQDs, P-GQDs showed the negligible fluorescence. Because the P atoms mainly are present in the surface functional groups of P-GQDs, these P-atoms induced defects may become the large fluorescent quenching center and greatly decrease their PL intensity. Besides the measurement and analysis of luminescence intensity, the emission peak positions and corresponding excitation wavelengths of these doped GQDs were also well studied. The excitation wavelength of pure GQDs is located at 340 nm, while the doped GQDs are all located at 360 nm. The strongest emission peaks of Cl-GQDs and N-GQDs are around 450 nm, while the strongest emission peaks of S-GQDs and pure GQDs are around 430 nm. These results indicate that the doping of heteroatoms could change the band gap of GQDs to some extent.

In this study, we aimed to develop simple, efficient, economic and sustainable approach for synthesis of carbon based quantum dots from agricultural waste. Sugarcane bagasse is one of the major solid agricultural waste and through its utility a novel strategy is evolved which makes a good sense both environmentally and economically. Herein, graphene quantum dots have been synthesized by chemical cutting from graphene oxide, while chemical oxidation followed by exfoliation method was used to form carbon quantum dots. The manufactured carbon based quantum dots were structurally characterised by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). The quantum dots were optically characterised by UV–vis spectroscopy, Fluorescence microscopy and Photoluminescence spectroscopy. SEM and TEM analysis indicated size, shape and monodispersed nature of carbon based quantum dots along with structural information about graphene oxide and sugarcane bagasse. The presence of large amount of oxygenous functionality was confirmed by FT-IR spectra and UV–vis spectroscopy. The optical properties reveal the bright blue luminescent nature of carbon quantum dots and graphene quantum dots with a produced yield of 25.7% and 12.54% respectively.

Resonant transport and negative differential resistance in the graphene and graphyne quantum dots

Currently, graphene has attracted much attention in the fields of bioimaging, biolabeling and drug delivery. Theoretical and experimental studies have shown that the graphene quantum dots (GQDs) are expected to show good optical properties due to their quantum confinement and edge effect. In this report, using the electrochemical assay the fluorescent GQDs with a diameter between 5 and 10 nm could be obtained via electrolysing graphite in alkaline condition and with hydrazine hydrate as a reducing agent at room temperature. The structure of the GQDs was confimed by means of transmission electron microscope (TEM) and atomic force microscope (AFM). The finding showed that the GQDs have an uniform size, and most of them are separate graphene. The GQDs mainly consist of single layer with less than 1 nm. Their features and properties were characterized by fourier transform infrared spectroscopy (FTIR), photoluminescence spectra (PL), UV-visible spectroscopy (UV-vis) and X-ray diffraction (XRD). The results indicated that the GQDs have bright yellow luminescence with a 14 % quantum yield, which is higher than that of traditional carbon quantum dots reported previously. When they were excited by different excitation wavelengths, the intensity of photoluminescence increased to the maximum, and then decreased gradually. The fluorescent emission peak of the GQDs remained unshifted, suggesting a novel kind of quantum dots different from those of graphene oxide quantum dots depending excitation wavelengths. The luminescence of GQDs arises from the graphene modified with the phthalhydrazide-like groups and hydrazide groups at the edge. The highly fluorescent GQDs have high water solubility, good photostability and biocompatibility, indicating that the GQDs can easily enter the cells. By incorporating the GQDs with A549 (lung cancer) and MCF-7 (breast cancer) cells through MTT assay, the newly obtained GQDs exhibited low cytotoxicity with an advantage of strong photoluminescence in the cells, and thus the GQDs might be used as a bioimaging marker in tumor cell imaging.

Graphene is a key material for nanoelectronics. Nevertheless, its zero gap makes it unsuitable for applications needing semiconductors with sizeable energy gaps. One way to open a gap in graphene is to use size reduction effects. The reduction of one dimension leads to carbon nanotubes and graphene nanoribbons that are 1D carbon nanostructures. Reducing one more dimension leads to 0D graphene quantum dots. The optical properties of carbon nanotubes have been investigated for approximately 20 years, while the study of graphene quantum dots and nanoribbons is at its infancy.Among potential application of these sp2 carbon nanostructures, the use of graphene nanoribbons and quantum dots as light emitters attracts a lot of attention. Here, I will present our recent results on the optical properties of graphene quantum dots synthesized by bottom-up chemistry [1].

Because of their unique and tunable photoluminescence properties, exceptional physicochemical properties, high photostability, biocompatibility and small size, Graphene quantum dots (GQDs) have received a lot of attention. However, insufficient investigations have been carried out on GQD fluids. In this paper, the properties of a prepared GQD fluid are studied experimentally, involving the physical stability, rheology, thermal conductivity, optical properties and corrosion characteristics. It is found that a highly physically stable GQD fluid could be easily achieved because the selected GQDs are well dispersed. It is also found that the addition of GQDs had a slight effect on the base fluid viscosity, but it could significantly increase the thermal conductivity of the fluid. In addition, the investigation of the optical properties shows that the GQD fluid exhibited high absorption to sunlight. The transmittance of ultraviolet and near-infrared light is close to zero. In contrast, the transmittance of GQDs to visible light is high at low weight concentrations, but significantly decreases with the increase of the proportion of GQDs. The corrosion characteristics of the copper and carbon steel samples in the selected GQD fluid or deionized water were experimentally investigated. It is found that the selected GQD fluid can greatly accelerate the corrosion of copper. However, nearly the same corrosion rate is observed for carbon steel in the GQD fluid as that in deionized water. The high stability, low viscosity, enhanced thermal conductivity and unique optical and corrosion properties allowed the GQD fluid to have excellent potential for applications in the energy sector.

The rapid outbreak of the deadly and contagious SARS-CoV-2 virus in 2019 that caused COVID-19 disease has demanded the development of novel antiviral materials. Since medical treatment and drug evaluation and approval by health authorities takes a longer time, nanomaterials can play a significant role in combating deadly disease. Carbon-based materials, such as carbon nanotubes (CNTs), graphene, fullerene, and carbon quantum dots (CQDs), have been widely reported in the literature as contributing to fighting the viral disease. Among them, CQDs have received significant attention as bionanomaterials recently, particularly in the biomedical field to treat various viral infections. Therefore, this mini-review discusses the recent achievements of CQDs and graphene quantum dots (GQDs) as bionanomaterials in fighting viral disease, specifically COVID-19 and other COVID-19-related works such as sensing and treatment, as well as virus inhibition.

Due to Klein's tunneling the electronic states of a quantum dot in graphene have finite widths and an electron in quantum dot has a finite trapping time. This property introduces a special type of interdot coupling in a system of many quantum dots in graphene. The interdot coupling is realized not as a direct tunneling between quantum dots but as coupling through the continuum states of graphene. As a result the interdot coupling modifies both the positions and the widths of the energy levels of the quantum dot system. We study the system of quantum dots in graphene theoretically by analyzing the complex energy spectra of the quantum dot system. We show that in a double-dot system some energy levels become strongly localized with an infinite trapping time. Such strongly localized states are achieved only at one value of the interdot separation. We also study a periodic array of quantum dots in graphene within a tight-binding mode for a quantum dot system. The values of the hopping integrals in the tight-binding model are found from the expression for the energy spectra of the double quantum dot system. In the array of quantum dots the states with infinitely large trapping time are realized at all values of interdot separation smaller than some critical value. Such states have nonzero wave vectors.

Graphene quantum dots (GQDs) serve as remarkable agents for drug and gene therapeutic delivery targeted treatment of complex diseases including cancer, sensing of biomolecules, cancer-generated genes and environmental conditions including the temperature and pH. GQDs design to emit fluorescence in the near-infrared (NIR) due to its high penetration depth are now utilized in biological imaging within subcellular and intracellular environments as well as in biological tissues and live animal models. Given their promising potential in image-guided delivery, the outlets of NIR-emissive GQDs in live targets need to be explored to further focus and enhance their applications in vivo. In the present work we explore GQD in vivo toxicity (describing maximum amounts that could be used for delivery and imaging), biodistribution (showing the tissues that can be targeted by the GQDs and the regions within particular animal tissues that acquire maximum GQD accumulation) and pharmacokinetics (showing GQD circulation times in blood that could help identify relevant clinical targets). In this study GQD content is analyzed vi a NIR fluorescence imaging allowing for semi-quantitative assessment through the layers of biological tissue. When injected intravenously (IV) or intraperitonially (IP) into live BALB/c mice GQDs distribute into multiple murine organs with maximum fluorescence intensity in liver, kidneys spleen, heart, spleen, thymus, brain and joints with most of the organs experiencing maximum accumulations at 6 – 24 h time points. During first pass metabolism we expect GRDs to partially clear through liver and kidneys. Kidneys expectedly experience faster (~1 h) clearance of the GQDs than liver (6 – 12 h) with the prolonged residence time noted for spleen (12 – 24 h). While the bulk fluorescence signal provides semi-quantitative accumulation information, the location of the GQDs within the tissue in the cells is further studied in animal organ slice microscopy that explores the possibility of GQDs entering such complex targets as brain. While accumulation in the majority of the aforementioned organ targets is expected from metabolic pathways of the GQDs, their high and prominent in mouse joints with over 48 h retention times is unforeseen and can serve toward advancement of joint therapy delivery and joint imaging applications.While being amphiphilic but only nanometer-sized particles, GQDs experience appreciable blood circulation times of 3.8 h allowing for a variety of therapeutic delivery approaches including even non-targeted cancer therapeutics. Their early-stage clearance is attributed to the renal pathway, while at later time points it is expected to be dominated by the reticuloendothelial system. Toxicologically GQDs appear to be superior to a plethora of their nanomaterial counterparts with concentration up to 16.5 mg/mL showing no toxic response in mice in terms of weight and behavior for the period of 3 weeks post administration. These concentrations are surpass the ones used for imaging 10-fold. Conformational tissue changes are further analyzed using H&E-stained organ slice microscopy. This work demonstrates that GQDs have a promising potential to serve as a viable platform for therapeutic delivery and imaging in multiple organs at ultra-high non-toxic concentrations with their circulation times allowing for anticancer therapeutic delivery applications.

Because of the fabulous fluorescence characteristics, semiconductor quantum dots have attracted a lot attention since the first reported. Meanwhile, the applications of quantum dots have been limited by its own toxicity. The appearance of graphene quantum dots (GQDs) breaks the awkward situation. GQDs can also realize from ultraviolet to near infrared full visible light broad band emitting. The lower cost, better biocompatibility and stability than semiconductor quantum dots make it possible to replace inorganic quantum dots for chemical sensing, biomarkers, and optoelectronic devices. In this paper, the synthesis of doped GQDs is mainly studied. The nitrogen-chlorine doped GQDs were synthesized by hydrothermal method. The as prepared GQDs were characterized by TEM, XPS, FTIR, PL and other characterization methods. In this study, by utilizing common industrialized technology to fabricate GQDs white LED could achieve large-scale production. The rendering index of as prepared white GQDs LED was 85.4. It is much higher than YAG phosphor white LED. And the chromaticity coordinates of GQDs LED located at (0.34, 0.36). The correlated color temperature of 5075K was very consistent with indoor lighting scene. That means this GQDs white LED has great application prospects in the field of high-end lighting.

Graphene oxide (GO) and graphene quantum dots (GQDs) are two well-known graphene-related materials derived from a single layer of C-atoms which are arranged in a honeycomb structure. Due to their unique physicochemical properties such as high specific surface area, excellent electrical conductivity, good mechanical strength, and intrinsic biocompatibility, GO and GQDs have drawn great attention in versatile scientific domains, especially in biomedical applications. Recently, they have been recognised as potential sources for new therapeutic approaches, particularly in influencing inflammatory responses and cancer therapy. The drug and imaging agent loading and delivery ability, ease of transport and release, targeting ability, and imaging canny have met them to be attractive carriers for next generation nanomedicine. Particularly their involvement in selective drug delivery systems represents an opportunity to improve so to speak the therapeutic efficacy, by confining the exposure to the therapeutic such to reach the desired tissue/tumor further abating the treatment of the healthy cells. In this review, we discuss the recent advances in the development and application of GO- or GQD-based delivery systems and put emphasis on strategies utilized to improve the targeting capability, biocompatibility, and therapeutic efficacy for cancer treatment.

A facile bottom-up method for the synthesis of highly fluorescent graphene quantum dots (GQDs) has been developed using a one-step pyrolysis of a natural amino acid, L-glutamic acid, with the assistance of a simple heating mantle device. The developed GQDs showed strong blue, green and red luminescence under the irradiation of ultra-violet, blue and green light, respectively. Moreover, the GQDs emitted near-infrared (NIR) fluorescence in the range of 800-850 nm with the excitation-dependent manner. This NIR fluorescence has a large Stokes shift of 455 nm, providing significant advantage for sensitive determination and imaging of biological targets. The fluorescence properties of the GQDs, such as quantum yields, fluorescence life time, and photostability, were measured and the fluorescence quantum yield was as high as 54.5 %. The morphology and composites of the GQDs were characterized using TEM, SEM, EDS, and FT-IR. The feasibility of using the GQDs as a fluorescent biomarker was investigated through in vitro and in vivo fluorescence imaging. The results showed that the GQDs could be a promising candidate for bioimaging. Most importantly, compared to the traditional quantum dots (QDs), the GQDs is chemically inert. Thus, the potential toxicity of the intrinsic heavy metal in the traditional QDs would not be a concern for GQDs. In addition, the GQDs possessed an intrinsic peroxidase-like catalytic activity that was similar to the graphene sheets and carbon nanotubes. Coupled with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), the GQDs can be used for the sensitive detection of hydrogen peroxide with a limit of detection of 20 μM.