Carbon Dots
Carbon dots (CDs) were discovered at the beginning of the 21st century as an alternative to the toxic quantum dots. However, many biological tissues and organs have autofluorescence, which interferes with the generally short emission of CDs and results in the difficulty and inaccuracy in the observation of CDs in vivo. Therefore, our research group currently is interested in the development of long-emissive CDs especially in the red-NIR region, which will be systematically characterized through well-established techniques such as UV/vis, fluorescence, Fourier-transform IR, Raman, X-ray diffraction, X-ray photoelectron spectroscopies, atomic force, transmission electron microscopies, thermogravimetric analysis, NMR, mass spectrometry, and zeta potential.
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Figure 1. (a) Graphical illustration of the structure of CDs; (b) TEM image of CDs.
Drug Delivery across the Blood-Brain Barrier
The blood-brain barrier is hard to overcome. It protects the central nervous system but also forms an obstacle for drug delivery. Due to all the favored properties of CDs, our research group is focusing on the drug delivery across the blood-brain barrier using CDs as drug nanocarrier with zebrafish, mouse and rat as models. The main aim is to deliver drugs to the brain to treat central nervous system (CNS) diseases.
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Figure 2. (a) Graphical illustration of the structure of the blood-brain barrier; (b) Image of transgenic zebrafish.
Alzheimer’s Research
Neurodegeneration is a progressive and irreversible process of neuronal function loss in the brain, which is often associated with deposition of protein aggregates, such as amyloid and Lewy body. Among the neurodegenerative diseases, Alzheimer’s disease (AD) is the 6th leading cause of death in the US and there are more than 5 million Americans are living with AD based on the Alzheimer’s Disease Fact Sheet from National Institutes of Health. Unfortunately, disease-modifying drugs which are able to stop or cure this disease are not yet available, leaving with only symptom-alleviating medications in hand. Therefore, our research group focuses on developing CDs or CD conjugates as drugs and drug carriers to treat the AD. Several CD species or CD conjugates have shown the capability to cross the BBB with the capacity to inhibit the aggregation of amyloid-beta (Aβ) and hyperphosphorylated tau proteins, which are two main hallmarks of AD.
Figure 3. In vitro efficacy of Y-CDs. The Chinese hamster ovary (CHO) cells that are stably overexpressing APP were incubated with 0, 0.1, 1 and 10 µM Y-CDs aqueous dispersion for 24 hrs. (A) Y-CDs significantly reduced APP production. Representative micrographs of CHO cells treated with vehicle and 10 µM of Y-CDs showing APP (red) and nuclei (DAPI), scale - 20 µm. The CHO cells overexpressing human APP751 were treated with Y-CDs and were fixed at 24 hrs and permeabilized cells were detected using APP antibody and stained with streptavidin-Alexa647. Robust APP staining was observed in vehicle treated cells compared to cells treated with 10 µM of Y-CDs. (B) Quantification of APP mean fluorescence intensity (MFI) per cell following Y-CDs treatments. The number of cells and the MFI per cell was quantified using NIH Image J software with more than 300 cells counted per treatment. (C) Quantification of the secreted β-Amyloid (Aβ) monomers in cell culture media following Y-CDs treatments. Data expressed as mean±SEM of two independent experiments. * P<0.05; ** P<0.01; ***P<0.001 based on the ANOVA (analysis of variance) and Tukey’s post hoc test compared to vehicle (0 µM) control.
Chemotherapy in the Treatment of Tumor
Pediatric Brain Tumor
One of the main research focuses of Leblanc’s groups is on the treatment of pediatric brain tumor. Brain tumors are now the leading cause of cancer-related deaths in children. Additionally, adverse consequences of the current treatment procedures are severe including neurological deficits and increased risk of additional cancers. For these diseases we are interested in using CDs as a platform to selectively target tumor cells for fluorescent-guided surgery and to deliver anti-cancer drugs. Thanks to their exceptional properties, CDs are a promising new approach for real-time fluorescence imaging during surgery and as a drug delivery system. We have successfully managed to conjugate three ligands namely, transferrin (targeting ligand) and two anti-cancer drugs, epirubicin and temozolomide, to build the triple conjugated system. In this study, we showed that triple conjugated CDs were strongly cytotoxic to brain tumor cell lines. In addition, our triple conjugated system displayed a synergistic effect compared to dual conjugated systems. Additionally, we have synthesized carbon nitride dots as a selective bioimaging nanomaterial and a drug delivery vehicle for targeted cancer therapy. These CDs were capable of selectively entering the cytoplasm of SJGBM2 tumor cells and emitting bright photoluminescence (PL) in the red region. This could be an effective labeling technique in tumor identification as well as fluorescence-guided brain surgery.
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Figure 4. (a) Schematic illustration of a triple conjugated system composed of transferrin, epirubicin and temozolomide on the carboxylic acid functionalized C-dots (CDs). The drawings are not according to the exact scale and the ratio. Only a 1 : 1 : 1 : 1 ratio of -COOH : transferrin : epirubicin : temozolomide is shown for the sake of clarity; (b) Cytotoxicity profiles of the three transferrin-conjugated samples on each cell line compared to the non-treated (NT) sample.
Lymphoma
A novel approach to treat diffuse large B-cell lymphoma (DLBCL) was presented by our group in collaboration with Johnathan Schatz’ Lab (University of Miami). It consists of direct delivery of doxorubicin (Dox) to the tumor through the use of a chemotherapeutic-nanocarrier system, composed of carbon nitride dots (CND), Dox and transferrin (TF). According to the gain and loss of function studies, the TF is the key of tumor targeting. This system has shown 10-100 times more effectiveness against DLBCL cell lines than Dox alone. In vivo studies also presented significant results in mice survivability due to the reduced toxicity and tumor reduction caused by the direct delivery of drug. In order to improve the novel drug delivery system, studies are redirected to the increase of drug loading of Dox on CND and analyses of biomechanisms of drug release inside the cell.
Figure 5. Chemical structure of the CND-Dox-TF conjugate.
Bone Infection Treatment
Our research group observed CDs prepared from carbon nanopowder could specifically target bones using zebrafish as a model. Since CDs have small size, high PL, water dispersity, nontoxicity, biocompatibility and abundant surface functional groups, CDs are promising drug nanocarrier to load drugs and deliver drugs in vivo, which can be tracked by their excellent PL. Therefore, we are trying to conjugate CDs with bone-related antibiotics and use zebrafish larvae as a model to examine whether the antibiotics could be efficiently delivered to the bones of zebrafish.
Figure 6. Bone-specific targeting and bioimaging with CDs synthesized from carbon nanopowder.
Photodynamic Therapy
The lowest excited state of oxygen, singlet oxygen, has been shown to have promising results in medicinal purposes for many years. Typically, singlet oxygen is produced through irradiation of a photosensitizer, which are generally porphyrins, chlorophylls or dyes. Singlet oxygen possesses high reactivity towards a wide range of molecules, due to its strong oxidizing nature. This has led many to use it as a source of cancer treatment through selectively irradiating a tumorous area in the presence of a specific photosensitizer. Our goal is to use this approach externally, for treatments of Methicillin-resistant Staphylococcus aureus (MRSA) and fungal infections found on the eye. Treatment outside the body allows for a wider range of possible photosensitizers to be used, as the irradiation source does not need to penetrate many layers of skin or flesh to reach the infected area.
Photocatalytic Degradation of Environmental Contaminants
A microwave-assisted approach was adopted to create tunable sized CDs that showed unique PL behavior, structural properties, and photocatalytic activity and stability. However, there was not a trend between size of CDs and the PL color (wavelength). This shows the PL mechanism of CDs cannot be explained by quantum confinement. The diffuse reflectance spectroscopy analysis indicated semiconductor (fraction 3) properties with a narrow band gap of 2.04 eV. The size of the CDs demonstrated significant effect on their photocatalytic activity in the degradation of organic dyes. The 2-nm CDs showed substantial degradation of RhB and MB under simulated sunlight irradiation. The mechanism of the dye degradation was analyzed using sacrificial scavengers and argon protection, separately. We observed the photocatalytic dye degradation catalyzed by the 2-nm CDs is mainly attributed to the oxidation of the dye by the holes and superoxide radical anions. Fraction 3 was applied to degrade other organic pollutants such as p-nitrophenol and it showed 70% degradation upon irradiation by full spectrum. Also, fraction 3 demonstrated good stability as a photocatalyst for multiple cycles of dye degradation. The study, for the first time, reports a size-dependent photocatalytic dye degradation of CDs, showing a comparable result to those of many metal-containing photocatalysts. These results show the potential of bare CDs with smaller sizes as excellent photocatalysts for pollutant degradation, important for future environmental protection.
Figure 7. Graphical representation of the size-dependent photocatalytic activity of CDs.
Printing and Cosmetics
Due to the excellent PL, CDs have been applied in our group to make security, 3D printings and fingerprint. 3D printing was achieved through embedding CDs into a superabsorbent polymer. In addition, many photoluminescent cosmetic products have been developed such as photoluminescent nail polish, sun cream, and permanent hair colorant.
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Figure 8. Graphical illustration of CDs applied in (a) security printing and (b) 3D printing.
Thermoelectric Effect of Carbon Dots
Cu2Se is a promising thermoelectric (TE) system due to its earth-abundant constituents and high TE figure of merit for mid-temperature range applications. This study aimed to further enhance the TE conversion efficiency of these materials by doping with CD nanomaterials. Gel-like CD (G-CDs) were fabricated using a facile and rapid solvothermal method. Cu2Se powders were doped with these CDs and spark plasma sintered to oved fabricate hybrid TE systems. The hybrid systems showed superior TE figure of merit compared to undoped Cu2Se marking the maximum figure of merit of 2.1 at 880 oK with a CD dopant ratio of 2 wt%. To the best of our knowledge, this is the highest figure of merit at this temperature achieved for these material systems in literature. The structural analysis conducted on the samples suggested high levels of purity, which is a significant factor that contributed towards the high ZT achieved. Other factors that accounted for this achievement are the synergetic presence of quasi-spherical CD nanoparticles, intensive grain boundaries and high density of sintered Cu2Se matrix which amplified phonon scattering and electrical conductivity. Therefore, this study establishes the novel methodologies for fabricating G-CD-doped Cu2Se material systems where the scaled-up nano dopant preparation procedures and enhanced TE conversion efficiencies are reported. It is expected that, the outcomes of this study will help wider adoption of Cu2Se for energy harvesting applications in near future.
Hybrid Rocket Fuel with Carbon Dots
In this research, the combustion characteristics of a novel hybrid rocket fuel composed of acrylonitrile butadiene styrene (ABS) and paraffin were studied in order to understand the effects of the introduction of a novel nanomaterial, namely G-CDs, into the paraffin matrix. Hollow ABS cylinders with straight port were 3D printed and used as molds into which neat and 1 wt% CD-loaded paraffin matrices were casted separately. For control, pure ABS fuels were also printed. All fuel grains were exposed to ballistic tests using the lab-scale test setup where gaseous oxygen (GOX) was employed as oxidizer. As per the ballistic test results, ABS/CD-loaded paraffin fuel grains exhibited a maximum combustion efficiency and regression rate of 88% and 1.13 mm/s, which marked enhancements of about 7% and 11% compared to ABS/pure paraffin fuel grains, respectively. The enhancements in combustion behavior of CD-loaded samples were attributed to the higher particle entrainment of paraffin which ensued as a result of reduction in viscosity and increase in thermal conductivity, as justified by rheological measurements, thermal conductivity characterization and scanning electron microscopy (SEM).
Figure 9. The multi-step experimental strategy followed in this research.
