Dejan P. Kepić

In vitro comparison of the photothermal anticancer activity of graphene nanoparticles and carbon nanotubes

The present study compared the photothermal anticancer activity of near-infrared (NIR)-excited graphene nanoparticles and carbon nanotubes (CNT). Despite lower NIR-absorbing capacity, suspension of polyvinylpyrrolidone-coated graphene sheets exposed to NIR radiation (808 nm, 2 W/cm(2)) generated more heat than DNA or sodium dodecylbenzenesulfonate-solubilized single-wall CNT under the same conditions. Accordingly, graphene nanoparticles performed significantly better than CNT in inducing photothermal death of U251 human glioma cells in vitro. The superior photothermal sensitivity of graphene sheets could be largely explained by their better dispersivity, which has been supported by a simple calculation taking into account thermodynamic, optical and geometrical properties of the two type of carbon nanoparticles. The mechanisms of graphene-mediated photothermal killing of cancer cells apparently involved oxidative stress and mitochondrial membrane depolarization resulting in mixed apoptotic and necrotic cell death characterized by caspase activation/DNA fragmentation and cell membrane damage, respectively.

Graphene quantum dots as autophagy-inducing photodynamic agents

The excellent photoluminescent properties of graphene quantum dots (GQD) makes them suitable candidates for biomedical applications, but their cytotoxicity has not been extensively studied. Here we show that electrochemically produced GQD irradiated with blue light (470 nm, 1W) generate reactive oxygen species, including singlet oxygen, and kill U251 human glioma cells by causing oxidative stress. The cell death induced by photoexcited GQD displayed morphological and/or biochemical characteristics of both apoptosis (phosphatidylserine externalization, caspase activation, DNA fragmentation) and autophagy (formation of autophagic vesicles, LC3-I/LC3-II conversion, degradation of autophagic target p62). Moreover, a genetic inactivation of autophagy-essential LC3B protein partly abrogated the photodynamic cytotoxicity of GQD. These data indicate potential usefulness of GQD in photodynamic therapy, but also raise concerns about their possible toxicity.

Modification of Structural and Luminescence Properties of Graphene Quantum Dots by Gamma Irradiation and Their Application in a Photodynamic Therapy

Herein, the ability of gamma irradiation to enhance the photoluminescence properties of graphene quantum dots (GQDs) was investigated. Different doses of γ-irradiation were used on GQDs to examine the way in which their structure and optical properties can be affected. The photoluminescence quantum yield was increased six times for the GQDs irradiated with high doses compared to the nonirradiated material. Both photoluminescence lifetime and values of optical band gap were increased with the dose of applied gamma irradiation. In addition, the exploitation of the gamma-irradiated GQDs as photosensitizers was examined by monitoring the production of singlet oxygen under UV illumination. The main outcome was that the GQDs irradiated at lower doses act as better photoproducers than the ones irradiated at higher doses. These results corroborate that the structural changes caused by gamma irradiation have a direct impact on GQD ability to produce singlet oxygen and their photostability under prolonged UV illumination. This makes low-dose irradiated GQDs promising candidates for photodynamic therapy.

Nanosecond laser-assisted nitrogen doping of graphene oxide dispersions.

N-doped reduced graphene oxide (RGO) has been prepared in bulk form by laser irradiation of graphene oxide (GO) dispersed in an aqueous solution of ammonia. A pulsed Nd:YAG laser with emission wavelengths in the infrared (IR) 1064 nm, visible (Vis) 532 nm, and ultraviolet (UV) 266 nm spectral regions was employed for the preparation of the N-doped RGO samples. Regardless of the laser energy employed, the resulting material presents a higher fraction of pyrrolic nitrogen compared to nitrogen atoms in pyridinic and graphitic coordination. Noticeably, whereas increasing the laser fluence of UV and Vis wavelengths results in an increase in the total amount of nitrogen, up to 4.9 at. % (UV wavelength at 60 mJ cm-2 fluence), the opposite trend is observed when the GO is irradiated in ammonia solution through IR processing. The proposed laser-based methodology allows the bulk synthesis of N-doped reduced graphene oxide in a simple, fast, and cost efficient manner.

Gamma ray-assisted irradiation of few-layer graphene films: a Raman spectroscopy study

This paper represents results of a Raman spectroscopy study of gamma-irradiated few-layer graphene thin films at three different doses: 25, 50 and 110 kGy. Graphene thin films were deposited by the vacuum filtration method and then transferred onto glass substrate. Raman spectroscopy and atomic force microscopy analysis have shown that the average in-plane crystallite size La of graphene thin films varies slightly when an irradiation dose is applied. Raman spectroscopy revealed that gamma irradiation of graphene thin films resulted in slight p-doping of the graphene thin film surface. It was found that during gamma irradiation at a dose of 110 kGy, the graphene sheets merged. As a result, the number of incorporated defects in the graphene structure was reduced (the ID/IG ratio decreased with the increase in the applied dose).

Semi-transparent, conductive thin films of electrochemical exfoliated graphene

The electrochemical exfoliation of graphite to give one-atom-thick graphene with desirable properties is a green, cost-effective method for high-yield graphene production. This paper presents the results of electrochemical exfoliation of two different graphite precursors under an applied direct current voltage of +12 V. The used characterization techniques (elemental analysis, Fourier transform infrared spectroscopy, X-ray diffraction, X-photoelectron spectroscopy, Raman spectroscopy, field emission scanning electron microscopy and atomic force microscopy) showed that the exfoliated powder is highly functionalized with a low carbon/oxygen content that is similar to graphene oxide. The exfoliated graphene sheets dispersed in N,N′-dimethylformamide were deposited on ano-discs by vacuum filtration and transferred to glass ceramic substrates. The thermal annealing of the as-deposited films at 600 °C for 30 minutes resulted in an increase in the carbon/oxygen ratio by more than 3 fold and a decrease in the sheet resistance by 25%. The lowest values for the sheet resistance of the annealed graphene thin films were in the range of 0.32 ± 0.04 to 0.84 ± 0.1 kohm sq−1 depending on the graphite source that was used.

Novel method for graphene functionalization

In this paper we present a novel method to obtain a stable dispersion of graphene in water using carbon quantum dots as surface active agents. In this way it is possible to achieve graphene concentrations in dispersion up to 2.7 mg ml−1. Fourier transform infrared spectroscopy and UV–Vis measurements confirmed the presence of oxygen-containing functional groups in the graphene–carbon quantum dot (gCQD) structure, responsible for its good solubility in water. The stability of the gCQD dispersion is due to π–π interactions formed between graphene and graphene-like sites of carbon quantum dots. According to Raman spectroscopy, as well as transmission electron microscopy and atomic force microscopy analysis, graphene sheets consist of several layers.

Ambient light induced antibacterial action of curcumin/graphene nanomesh hybrids

Curcumin and its derivates are well-known for their different biological activities including antibacterial. On the other hand there are controversial reports concerning the antibacterial potential of graphene and, in particular, graphene oxide. In this study we have reported for the first time the antibacterial activity of curcumin/graphene nanomesh hybrids under ambient light conditions. The graphene nanomesh was synthesized by electrochemical exfoliation of highly oriented pyrolytic graphite in 1 M solution of ammonium persulfate and further functionalized by curcumin. Identical values of minimum inhibitory concentration (1 mg mL−1) were determined for pure curcumin and curcumin/graphene nanomesh hybrids toward Staphylococcus aureus. All tested samples had more pronounced antibacterial activity against Gram positive bacteria, Staphylococcus aureus compared to Escherichia coli as a representative of Gram negative strains. The poor antibacterial potential of exfoliated graphene improves significantly by the functionalization with curcumin, which allows for its usage as a antibacterial coating.

Selective Laser-Assisted Synthesis of Tubular van der Waals Heterostructures of Single-Layered PbI2 within Carbon Nanotubes Exhibiting Carrier Photogeneration

The electronic and optical properties of two-dimensional layered materials allow the miniaturization of nanoelectronic and optoelectronic devices in a competitive manner. Even larger opportunities arise when two or more layers of different materials are combined. Here, we report on an ultrafast energy efficient strategy, using laser irradiation, which allows bulk synthesis of crystalline single-layered lead iodide in the cavities of carbon nanotubes by forming cylindrical van der Waals heterostructures. In contrast to the filling of van der Waals solids into carbon nanotubes by conventional thermal annealing, which favors the formation of inorganic nanowires, the present strategy is highly selective toward the growth of monolayers forming lead iodide nanotubes. The irradiated bulk material bearing the nanotubes reveals a decrease of the resistivity as well as a significant increase in the current flow upon illumination. Both effects are attributed to the presence of single-walled lead iodide nanotubes in the cavities of carbon nanotubes, which dominate the properties of the whole matrix. The present study brings in a simple, ultrafast and energy efficient strategy for the tailored synthesis of rolled-up single-layers of lead iodide (i.e., single-walled PbI2 nanotubes), which we believe could be expanded to other two-dimensional (2D) van der Waals solids. In fact, initial tests with ZnI2 already reveal the formation of single-walled ZnI2 nanotubes, thus proving the versatility of the approach.

Surface modification of single-wall carbon nanotube thin films irradiated by microwaves: a Raman spectroscopy study

In this work, we present the results of a Raman spectroscopy study of single-wall carbon nanotube (SWCNT) thin films treated with microwave irradiation. SWCNT thin films were deposited by the vacuum filtration method and transferred onto alumina substrate. These thin films were exposed to microwave irradiation of 25 and 250 W at 2.45 GHz. All samples were characterized by Raman spectroscopy and atomic force microscopy. Raman spectroscopy analysis showed that there was neither selective destruction of metallic nor semiconducting nanotubes. It was noticed that samples were heated only during microwave irradiation at 250 W. The major effect of microwave irradiation on SWCNTs was their debundling.