Pengju G. Luo

Carbon “quantum” dots for optical bioimaging

Carbon dots, generally referring to small carbon nanoparticles with various levels of surface passivation, have emerged as a new class of quantum dot-like fluorescent nanomaterials. Since the original report in 2006, carbon dots have been investigated by many research groups worldwide, with major advances already made in their syntheses, structural and mechanistic understandings, and evaluations for biocompatibilities and potential bio-applications. In this article, representative studies responsible for these advances in the development and understanding of carbon dots are reviewed, and those targeting the use of carbon dots as high-performance yet nontoxic fluorescence agents for optical bioimaging in vitro and in vivo are highlighted and discussed.

Carbon-based quantum dots for fluorescence imaging of cells and tissues

Carbon dots (or carbon quantum dots in some literature reports), generally small carbon nanoparticles with various surface passivation effects, have attracted widespread attention in recent years, with a rapidly increasing number of research publications. The reported studies covered many aspects of carbon dots, from the development of many new synthetic methodologies to an improved mechanistic elucidation and to the exploration of application opportunities, especially for those in the fluorescence imaging of cells and tissues. There have also been significant advances in the establishment of a shared mechanistic framework for carbon dots and other carbon-based quantum dots, graphene quantum dots in particular. In this article, representative recent studies for more efficient syntheses of better-performing carbon dots are highlighted along with results from explorations of their various bioimaging applications in vitro and in vivo. Similar fluorescence properties and potential imaging uses of some graphene quantum dots are also discussed, toward a more consistent and uniform understanding of phenomenologically different carbon-based quantum dots.

Competitive performance of carbon "quantum" dots in optical bioimaging.

Carbon-based “quantum” dots or carbon dots are surface-functionalized small carbon nanoparticles. For bright fluorescence emissions, the carbon nanoparticles may be surface-doped with an inorganic salt and then the same organic functionalization. In this study, carbon dots without and with the ZnS doping were prepared, followed by gel-column fractionation to harvest dots of 40% and 60% in fluorescence quantum yields, respectively. These highly fluorescent carbon dots were evaluated for optical imaging in mice, from which bright fluorescence images were obtained. Of particular interest was the observed competitive performance of the carbon dots in vivo to that of the well-established CdSe/ZnS QDs. The results suggest that carbon dots may be further developed into a new class of high-performance yet nontoxic contrast agents for optical bioimaging.

Carbon dots of different composition and surface functionalization: cytotoxicity issues relevant to fluorescence cell imaging

Nanoscale carbon particles have emerged as versatile precursors for a new class of highly fluorescent nanomaterials that resemble semiconductor quantum dots. The surface-passivated fluorescent carbon nanoparticles, dubbed ‘carbon dots’, were already demonstrated for their potential optical bioimaging applications in vitro and in vivo. In this study, we conducted a systematic cytotoxicity evaluation on the carbon dots prepared by various combinations of precursor carbon nanoparticles and molecules for the particle surface functionalization. The results suggested that the cytotoxicity of carbon dots was dependent on the selection of surface passivation molecules. Those dots showing more significant cytotoxicity at higher concentrations were also evaluated for their effects on the fluorescence imaging of live cells. The implications of the results on the eventual use of carbon dots as cell imaging agents are discussed.

Single-walled carbon nanotube as a unique scaffold for the multivalent display of sugars.

Single-walled carbon nanotube (SWNT) is a pseudo-one-dimensional nanostructure capable of carrying/displaying a large number of bioactive molecules and species in aqueous solution. In this work, a series of dendritic beta-D-galactopyranosides and alpha-D-mannopyranosides with a terminal amino group were synthesized and used for the functionalization of SWNTs, which targeted the defect-derived carboxylic acid moieties on the nanotube surface. The higher-order sugar dendrons were more effective in the solubilization of SWNTs, with the corresponding functionalized nanotube samples of improved aqueous solubility characteristics. Through the functionalization, the nanotube apparently serves as a unique scaffold for displaying multiple copies of the sugar molecules in pairs or quartets. Results on the synthesis and characterization of these sugar-functionalized SWNTs and their biological evaluations in binding assays with pathogenic Escherichia coli and with Bacillus subtilis (a nonvirulent simulant for Bacillus anthracis or anthrax) spores are presented and discussed.

Fullerene-Conjugated Doxorubicin in Cells

The conjugation of fullerene with well-established drug molecules has been a representative strategy to impart fullerene-specific properties for improved formulation. However, conjugates involving fullerenes or other nanomaterials often differ significantly from the free drug molecules in cellular uptake and distributions. For the highly effective anticancer drug doxorubicin (DOX), its strong absorption and fluorescence in the visible spectral region enable the tracking of DOX-containing conjugates by optical techniques. In this work, a stoimetrically and structurally well-defined fullerene-DOX conjugate was studied in terms of fluorescence microscopy, including the fluorescence imaging with two-photon excitation, to examine the uptake and distribution in human breast cancer cells. The results suggested that the conjugate was distributed mostly in the cytoplasm, significantly different from free DOX molecules (predominantly in the cell nucleus, as already reported in the literature). Mechanistic implications of the results are discussed. Also discussed are potentials of conjugated DOX species as self-labeled fluorescent probes in bioimaging and other mechanistic investigations on drug delivery.

Selective interactions of sugar-functionalized single-walled carbon nanotubes with Bacillus spores.

It was reported previously that monosaccharide-functionalized single-walled carbon nanotubes (SWNTs) could interact with Bacillus anthracis (Sterne) spores with the mediation of a divalent cation such as Ca(2+) to result in significant spore aggregation and reduction in colony forming units. In this work a more systematic investigation was performed on interactions of the SWNTs functionalized with individual mannose and galactose moieties and their various dendritic configurations with B. anthracis and B. subtilis spores in the presence and absence of a divalent cation. Significant differences and selectivity between the Bacillus spores and between different sugars and their configurations were observed. The relevant results are presented, and their mechanistic implications are discussed.

Carbon nanoparticles trapped in vivo-similar to carbon nanotubes in time-dependent biodistribution.

Carbon nanoparticles are in all of the carbon nanomaterials that are presently widely pursued for potential bioapplications, but their in vivo biodistribution-related properties are largely unknown. In this work, highly (13)C-enriched carbon nanoparticles were prepared to allow their quantification in biological samples by using isotope-ratio mass spectroscopy. The in vivo biodistribution results are presented and discussed, and also compared with those of the aqueous suspended carbon nanotubes reported previously. The distribution profile and time dependencies are largely similar between the nanoparticles and nanotubes, with results on both suggesting meaningful accumulation in some major organs over an extended period of time. Therefore, the surface modification of carbon nanoparticles, preferably the chemical functionalization of the nanoparticles with biocompatible molecules or species, is desirable or necessary in the pursuit of these nanomaterials for various bioapplications.

Graphene Oxides as Substrate for Enhanced Mammalian Cell Growth

Graphene Oxides as Substrate for Enhanced Mammalian Cell Growth Graphene oxides (GOs), widely used as precursors in the reduction for single-layer graphene materials (rGOs), have found application potentials independent of those for graphene. In particular, the excellent aqueous compatibility of GOs has attracted growing interest in their biological applications. Studies on cellular interactions of GOs have been centered on the cytotoxicity evaluations. Beyond cytotoxicity, however, the cellular interactions of GOs apparently have other consequences. In this work, GOs as coating on solid-state substrate were found to be significantly enhancing mammalian cell growth, despite the fact that GOs in a more concentrated aqueous suspension were somewhat toxic to the same cell lines. Results for a comparison of cell growth on other surfaces based on carbon nanomaterials, including rGOs, carbon nanotubes, and composites of GOs with carbon nanotubes, are presented and discussed.

Photoluminescent Carbon Nanomaterials: Properties and Potential Applications

Carbon nanoparticles and nanotubes upon surface passivation or modification via chemical functionalization exhibit strong photoluminescence in the visible and into the near-IR. In this Chapter, the general features and related optical characteristics of the photoluminescence are highlighted, mechanistic issues discussed, and their potential material and biomedical applications explored. For single-walled carbon nanotubes, the similarities and differences between the defect-derived emission and the band-gap fluorescence (emission from individualized single-walled carbon nanotubes) are also discussed.