Gregory E. LeCroy

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 “Quantum” Dots for Fluorescence Labeling of Cells

The specifically synthesized and selected carbon dots of relatively high fluorescence quantum yields were evaluated in their fluorescence labeling of cells. For the cancer cell lines, the cellular uptake of the carbon dots was generally efficient, resulting in the labeling of the cells with bright fluorescence emissions for both one- and two-photon excitations from predominantly the cell membrane and cytoplasm. In the exploration on labeling the live stem cells, the cellular uptake of the carbon dots was relatively less efficient, though fluorescence emissions could still be adequately detected in the labeled cells, with the emissions again predominantly from the cell membrane and cytoplasm. This combined with the observed more efficient internalization of the same carbon dots by the fixed stem cells might suggest some significant selectivity of the stem cells toward surface functionalities of the carbon dots. The needs and possible strategies for more systematic and comparative studies on the fluorescence labeling of different cells, including especially live stem cells, by carbon dots as a new class of brightly fluorescent probes are discussed.

Flexible Graphene–Graphene Composites of Superior Thermal and Electrical Transport Properties

Graphene is known for high thermal and electrical conductivities. In the preparation of neat carbon materials based on graphene, a common approach has been the use of well-exfoliated graphene oxides (GOs) as the precursor, followed by conversion to reduced GOs (rGOs). However, rGOs are more suitable for the targeted high electrical conductivity achievable through percolation but considerably less effective in terms of efficient thermal transport dictated by phonon progression. In this work, neat carbon films were fabricated directly from few-layer graphene sheets, avoiding rGOs completely. These essentially graphene-graphene composites were of a metal-like appearance and mechanically flexible, exhibiting superior thermal and electrical transport properties. The observed thermal and electrical conductivities are higher than 220 W/m · K and 85000 S/m, respectively. Some issues in the further development of these mechanically flexible graphene-graphene nanocomposite materials are discussed and so are the associated opportunities.

Polymer/carbon nanocomposites for enhanced thermal transport properties - carbon nanotubes versus graphene sheets as nanoscale fillers

Light-weight composite materials of superior thermal transport properties are important to thermal management and other applications. Carbon nanomaterials with their high thermal conductivities have been widely pursued for such a purpose. Specifically, carbon nanotubes have been shown both theoretically and experimentally to possess extraordinarily high thermal conductivities at the individual nanotube level, and thus are logically considered as ideal fillers for highly thermally conductive polymeric nanocomposites. However, the predicted dramatically enhanced thermal transport in polymers upon the incorporation of carbon nanotubes has not yet materialized. Recently, graphene research has brought new opportunities to the development of polymer/carbon nanocomposites of high thermal conductivities, with already some successful uses of exfoliated graphite sheets as nanoscale fillers. In this work poly(vinyl alcohol) (PVA) was selected as the polymer matrix for the dispersion of single-walled carbon nanotubes (seamlessly with PVA functionalization and solubilization) vs. few-layer graphene sheets as nanoscale carbon fillers for a more direct comparison on the thermal transport performance in the resulting nanocomposites. The effect of aligning the nanotubes embedded in the nanocomposite films via mechanical stretching was also evaluated. Implications of the comparison between the nanotubes and nanosheets with respect to their potentials in thermally conductive polymeric nanocomposites are discussed.

Visible-Light-Activated Bactericidal Functions of Carbon "Quantum" Dots.

Carbon dots, generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged as a new class of quantum-dot-like nanomaterials, with their optical properties and photocatalytic functions resembling those typically found in conventional nanoscale semiconductors. In this work, carbon dots were evaluated for their photoinduced bactericidal functions, with the results suggesting that the dots were highly effective in bacteria-killing with visible-light illumination. In fact, the inhibition effect could be observed even simply under ambient room lighting conditions. Mechanistic implications of the results are discussed and so are opportunities in the further development of carbon dots into a new class of effective visible/natural light-responsible bactericidal agents for a variety of bacteria control applications.

Host-Guest Carbon Dots for Enhanced Optical Properties and Beyond

Carbon dots, generally small carbon nanoparticles with various forms of surface passivation, have achieved the performance level of semiconductor quantum dots in the green spectral region, but their absorption and fluorescence in red/near-IR are relatively weaker. Conceptually similar to endofullerenes, host-guest carbon dots were designed and prepared with red/near-IR dyes encapsulated as guest in the carbon nanoparticle core. Beyond the desired enhancement in optical properties, the host-guest configuration may significantly broaden the field of carbon dots.

Photoexcited state properties of carbon dots from thermally induced functionalization of carbon nanoparticles

Carbon dots are small carbon nanoparticles with various surface passivation schemes, in which more effective has been the deliberate chemical functionalization of the nanoparticles for brighter fluorescence emissions, though the synthesis method is more tedious and subject to some limitations in the selection of functionalization molecules. Another more popular synthesis method has been the carbonization of organic species, with the method being more efficient and versatile, but less controllable in the synthesis and for the desired dot structure and performance. In this work, a hybrid approach combining the advantageous characteristics of the two synthesis methods was applied to the preparation of carbon dots with polyethyleneimine (PEI) for surface passivation, where pre-processed and selected small carbon nanoparticles were functionalized with PEI in microwave-induced thermal reactions. The optical absorption and fluorescence emission properties were evaluated, and the results suggested that the carbon dots thus prepared shared the same photoexcited state characteristics with those from the deliberate chemical functionalization, including comparable fluorescence colors and other properties. A further demonstration on the similarity in photoexcited state properties was based on the same visible light-activated bactericidal functions of the PEI-carbon dots as those found in carbon dots from the deliberate chemical functionalization. The advantages and potential limitations of the hybrid approach for more controllable yet versatile and efficient syntheses of carbon dots are highlighted and discussed.

Host–guest carbon dots as high-performance fluorescence probes

Host–guest carbon dots (G@CDots) represent a new platform in the rapidly advancing and expanding research field of carbon “quantum” dots or carbon dots, enabling the development of novel carbon hybrid nanostructures of unique and/or advantageous properties and capabilities beyond those of conventional carbon dots. In this study, the red/near-IR emissive dye nile blue (NB) was selected as the guest, and NB@CDots were prepared in a microwave-assisted one-pot thermal carbonization synthesis with oligomeric polyethylene glycols (PEGs) as the precursor and also surface passivation species in the resulting host–guest dots. The NB@CDots exhibited unique and/or favorable characteristics, including especially the bright red/near-IR fluorescence emissions, with the observed fluorescence quantum yield due to the guest nile blue species more than an order of magnitude higher than that of free nile blue molecules, all in aqueous solutions. The higher fluorescence intensities were coupled with a longer fluorescence lifetime, and also accompanied by excellent photochemical stability, making the NB@CDots high-performance fluorescence probes for bioimaging-sensing and related applications. Indeed, their potential in this regard was explored and demonstrated in the fluorescence imaging of live stem cells. The NB@CDots were also evaluated in cytotoxicity assays, and the results suggested their being nontoxic. Excellent opportunities in further development of the host–guest carbon dots platform for high-performance yet nontoxic fluorescence probes are discussed.

Carbon nanotube-assisted capturing of bacterial pathogens

This study explored the use of co-polymer poly(propionylethyleneimine-co-ethyleneimine (PPEI-EI) functionalized multi-walled carbon nanotubes (MWNTs) as a coating material on filters for capturing of bacterial pathogens from aqueous solutions. Polycarbonate membranes with pore sizes of 1.2 and 3.0 μm were coated with different PPEI-EI-MWNTs and cross-linked PPEI-EI-MWNTs samples at various coating densities, and then evaluated for capturing of E. coli cells at flow rates of 0.25 and 0.5 mL min−1. With a good combination of PPEI-EI-MWNTs sample, coating density, appropriate filter pore size and flow rate, a capture efficiency of higher than 4 log (up to 6 log or larger) of bacterial cells was achieved. The filters coated with the cross-linked PPEI-EI-MWNTs were unexpectedly less efficient than those with the other PPEI-EI-MWNTs samples, likely due to the poorer dispersibility of the cross-linked sample and consequently the less homogeneous coating on filters. The results of this study demonstrated the feasibility of PPEI-EI-MWNTs as a coating material on filters for highly effective capturing of bacterial pathogens, and also presented both challenges and opportunities for further investigations in controlling the coating material synthesis to improve performance in capturing bacterial cells.

Carbon–TiO2 hybrid dots in different configurations – optical properties, redox characteristics, and mechanistic implications

Carbon-based hybrid nanostructures, especially carbon hybrid dots with metal oxides, are important to both fundamental studies and technological applications. In this work, carbon–TiO2 hybrid dots in two different structural configurations were prepared and investigated comparatively for their optical properties and photoinduced redox characteristics, and the results suggested major differences. In the configuration of small carbon nanoparticles each coated/doped with only a small amount of TiO2, the TiO2 played the role of enhancing the surface passivation effect in conjunction with organic surface functionalization agents for the hybrid dots to exhibit a much improved performance over that of their corresponding neat carbon dots. In the other configuration, however, the hybrid dots of TiO2 nanocrystals each composited with nanoscale carbon domains were apparently similar to dye-sensitized TiO2 nanoparticle systems, with the carbon domains providing the dye function for the harvesting of visible photons. While combining nanoscale carbon and TiO2 for various applications has been a popular topic in the recent literature, the results reported here provide new insights into the different structural configurations or arrangements between the carbon and TiO2 and their associated effects on the optical and photoinduced redox properties of the hybrid nanostructures. Significant implications of the results on an understanding of the mechanistic relationships between the hybrid nano-structures/configurations and optical/redox properties are highlighted, and opportunities in the further exploration of carbon–metal oxide hybrid dots and their applications are discussed.