Anton Naumov

Quasi-Molecular Fluorescence from Graphene Oxide

Aqueous dispersions of graphene oxide (GO) have been found to emit a structured, strongly pH-dependent visible fluorescence. Based on experimental results and model computations, this is proposed to arise from quasi-molecular fluorophores, similar to polycyclic aromatic compounds, formed by the electronic coupling of carboxylic acid groups with nearby carbon atoms of graphene. Sharp and structured emission and excitation features resembling the spectra of molecular fluorophores are present near 500 nm in basic conditions. The GO emission reversibly broadens and red-shifts to ca. 680 nm in acidic conditions, while the excitation spectra remain very similar in shape and position, consistent with excited state protonation of the emitting species in acidic media. The sharp and structured emission and excitation features suggest that the effective fluorophore size in the GO samples is remarkably well defined.

Analyzing Absorption Backgrounds in Single-Walled Carbon Nanotube Spectra

The sources of broad backgrounds in visible-near-IR absorption spectra of single-walled carbon nanotube (SWCNT) dispersions are studied through a series of controlled experiments. Chemical functionalization of nanotube sidewalls generates background absorption while broadening and red-shifting the resonant transitions. Extensive ultrasonic agitation induces a similar background component that may reflect unintended chemical changes to the SWCNTs. No major differences are found between spectral backgrounds in sample fractions with average lengths between 120 and 650 nm. Broad background absorption from amorphous carbon is observed and quantified. Overlapping resonant absorption bands lead to elevated backgrounds from spectral congestion in samples containing many SWCNT structural species. A spectral modeling method is described for separating the background contributions from spectral congestion and other sources. Nanotube aggregation increases congestion backgrounds by broadening the resonant peaks. Essentially no background is seen in sorted pristine samples enriched in a single semiconducting (n,m) species. By contrast, samples enriched in mixed metallic SWCNTs show broad intrinsic absorption backgrounds far from the resonant transitions. The shape of this metallic background component and its absorptivity coefficient are quantitatively assessed. The results obtained here suggest procedures for preparing SWCNT dispersions with minimal extrinsic background absorptions and for quantifying the remaining intrinsic components. These findings should allow improved characterization of SWCNT samples by absorption spectroscopy.

Quantifying the semiconducting fraction in single-walled carbon nanotube samples through comparative atomic force and photoluminescence microscopies.

A new method was used to measure the fraction of semiconducting nanotubes in various as-grown or processed single-walled carbon nanotube (SWCNT) samples. SWCNT number densities were compared in images from near-IR photoluminescence (semiconducting species) and AFM (all species) to compute the semiconducting fraction. The results show large variations among growth methods and effective sorting by density gradient ultracentrifugation. This counting-based method provides important information about SWCNT sample compositions that can guide controlled growth methods and help calibrate bulk characterization techniques.

Gate-Variable Light Absorption and Emission in a Semiconducting Carbon Nanotube

We investigate the gate field dependence of light absorption and emission of an individual, suspended semiconducting carbon nanotube using Raman and photoluminescence spectroscopies. We find a strong reduction in the absorption strength and a red shift of the E(33) state of the nanotube with increasing gate field. The photoluminescence from the E(11) state is quenched even stronger. We explain these observations in terms of field-doping and its effects on both the radiative and nonradiative decay rates of the excitons. Thus, gate field-induced doping constitutes an effective means of controlling the optical properties of carbon nanotube devices.

Graphene Oxide: A One- versus Two-Component Material

The structure of graphene oxide (GO) is a matter of discussion. While established GO models are based on functional groups attached to the carbon framework, another frequently used model claims that GO consists of two components, a slightly oxidized graphene core and highly oxidized molecular species, oxidative debris (OD), adsorbed on it. Those adsorbents are claimed to be the origin for optical properties of GO. Here, we examine this model by preparing GO with a low degree of functionalization, combining it with OD and studying the optical properties of both components and their combination in an artificial two-component system. The analyses of absorption and emission spectra as well as lifetime measurements reveal that properties of the combined system are distinctly different from those of GO. That confirms structural models of GO as a separate oxygenated hexagonal carbon framework with optical properties governed by its internal structure rather than the presence of OD. Understanding the structure of GO allows further reliable interpretation of its optical and electronic properties and enables controlled processing of GO.

Carbon Nanotubes: Solution for the Therapeutic Delivery of siRNA?

Carbon nanotubes have many unique physical and chemical properties that are being widely explored for potential applications in biomedicine especially as transporters of drugs, proteins, DNA and RNA into cells. Specifically, single-walled carbon nanotubes (SWCNT) have been shown to deliver siRNA to tumors in vivo. The low toxicity, the excellent membrane penetration ability, the protection afforded against blood breakdown of the siRNA payload and the good biological activity seen in vivo suggests that SWCNT may become universal transfection vehicles for siRNA and other RNAs for therapeutic applications. This paper will introduce a short review of a number of therapeutic applications for carbon nanotubes and provide recent data suggesting SWCNT are an excellent option for the delivery of siRNA clinically.

Carbon nanotube photo- and electroluminescence in longitudinal electric fields.

The photoluminescence of a partially suspended, semiconducting carbon nanotube that forms the active channel of a field-effect transistor is quenched and red-shifted upon application of a longitudinal electrical (source-drain) field. The quenching can be explained by a loss of oscillator strength and an increased Auger-like nonradiative decay of the E(11) exciton. The spectral shifts are due to drain-field-induced doping that leads to enhanced dielectric screening. Electroluminescence due to electron impact excitation of E(11) excitons is red-shifted and broadened with respect to the zero-field photoluminescence. A combination of screening and heating of the carbon nanotube can explain both spectral shift and broadening of the electrically induced light emission.

Modifying optical properties of reduced/graphene oxide with controlled ozone and thermal treatment in aqueous suspensions

Graphene possesses a number of advantageous properties, however, does not exhibit optical emission, which limits its use in optoelectronics. Unlike graphene, its functional derivative, graphene oxide (GO) exhibits fluorescence emission throughout the visible. Here, we focus on controlled methods for tuning the optical properties of GO. We introduce ozone treatment of reduced graphene oxide (RGO) in order to controllably transform it from non-emissive graphene-like material into GO with a specific fluorescence emission response. Solution-based treatment of RGO for 5-45 min with ∼1.2 g l-1 ozone/oxygen gas mixture yields a drastic color change, bleaching of the absorption in the visible and the stepwise increase in fluorescence intensity and lifetime. This is attributed to the introduction of oxygen-containing functional groups to RGO graphitic platform as detected by the infrared spectroscopy. A reverse process: controllable quenching of this fluorescence is achieved by the thermal treatment of GO in aqueous suspension up to 90 °C. This methodology allows for the wide range alteration of GO optical properties starting from the dark-colored non-emissive RGO material up to nearly transparent highly ozone-oxidized GO showing substantial fluorescence emission. The size of the GO flakes is concomitantly altered by oxidation-induced scission. Semi-empirical PM3 theoretical calculations on HyperChem models are utilized to explore the origins of optical response from GO. Two models are considered, attributing the induced emission either to the localized states produced by oxygen-containing addends or the islands of graphitic carbon enclosed by such addends. Band gap values calculated from the models are in the agreement with experimentally observed transition peak maxima. The controllable variation of GO optical properties in aqueous suspension by ozone and thermal treatments shown in this work provides a route to tune its optical response for particular optoelectronics or biomedical applications.

Abstract C137: Single-walled carbon nanotubes provide a safe and effective means for delivery of siRNA.

Carbon nanotubes have unique physical and chemical properties that are being widely explored for applications in biomedicine, especially as transporters of drugs, proteins, DNA and RNA. We have shown that siRNA/SWCNT complexes can be delivered safely to animals at high doses over an extended period with no weight loss or changes in blood hematology or chemistry, and produce excellent biological activity including target knockdown and antitumor activity. The current study aimed to prepare stable, optimized SWCNT solutions with consistent siRNA compositions and physical properties and explore cellular uptake. Atomic force microscopy revealed siRNA covers the SWCNT on a mass ratio basis and provided length distributions of the samples. Quantitation of siRNA payload on the SWCNT was achieved using electrophoretic separation of the siRNA from the SWCNT. The stability of siRNA/SWCNT was explored in ribonuclease at 37°C over time and found that siRNA in the complex was stable in 37°C with 80% remaining after 1 hr and 40% after 6 hr versus free siRNA where none remained in solution at 1 hr. Labeled Cy-3-siRNA whose fluorescence is quenched when complexed with SWCNT was used to determine whether the siRNA payload is released intracellularly from the SWCNT following delivery into cells. MiaPaCa pancreatic carcinoma and H2122 non-small cell lung cancer cells were exposed to Cy-3 siRNA/SWCNT for 30 min to 6 hrs. The cells were examined microscopically using near infrared fluorescence detection to observe intracellular SWCNT and visible fluorescence detection for intracellular Cy-3-siRNA. The siRNA/SWCNT complexes readily enter cells within 1 hr and the amount of intracellular SWCNT increase over time to 6 hr. Cy-3-siRNA was released within 1 hr and the siRNA is widely dispersed throughout the cytoplasm. Control cells showed no fluorescence under either condition and intact Cy-3-siRNA/SWCNT complex was not fluorescent due to quenching. We also used Cy-3 labeled siRNA to compare the transfection efficiency of siRNA/SWCNT to that of liposomal delivery. Using equivalent amounts of siRNA, SWCNT/siRNA complexes delivered and released siRNA intracellularly more quickly and distributed the siRNA more evenly within a 1 hr time frame as compared to liposomal delivery. Finally, in vivo delivery of Cy-3-siRNA/SWCNT appears to demonstrate a more prolonged and pronounced distribution of siRNA than that of free Cy-3 siRNA. The studies reported here provide data showing the stability of the SWCNT/siRNA complexes in vitro and in vivo and illustrate their cell penetrating ability with release of the payload intracellularly. We believe the low toxicity, the excellent membrane penetration ability, the protection afforded against blood breakdown of the siRNA payload and the good biological activity seen in vivo will allow SWCNTs to become universal transfection vehicles for siRNA and other RNAs for therapeutic applications. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C137.