Browsing by Author "Zheng, Yu"

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    Advanced Optical Studies of Single-Wall Carbon Nanotubes Wrapped with Single-Stranded DNA
    (2019-05-23) Zheng, Yu; Weisman, R. Bruce
    Single-wall carbon nanotubes (SWCNTs) exist in a variety of discrete structures, giving distinct optical and electronic properties. Purification of as-grown SWCNTs and sorting of SWCNT mixtures into pure (n,m) fractions are required for enabling basic research progress and developing advanced applications. A remarkably effective method for SWCNT sorting is based on structure-selective wrapping interactions of single-stranded DNA (ssDNA) with nanotubes. In this thesis, optical spectroscopy has been used to investigate the ssDNA-SWCNT system for aspects of photophysics and photochemistry. A simple way to reveal the wrapping ability of ssDNA sequences with SWCNT structural forms is studying fluorescence spectral kinetics during displacement of ssDNA coating by sodium deoxycholate (SDC). It is found that the displacement rate dramatically decreases with increasing nanotube diameter for (GT)20-SWCNT samples. Moreover, the structure-specific interactions of (ATT)4-(7,5) hybrids show strong enough enantiomeric differences to cause resolvable shifts in fluorescence spectra and dramatic kinetic differences in coating displacement. The quenching of ssDNA-SWCNT fluorescence by dissolved oxygen was observed and the effect was quickly and fully reversed at room temperature by displacing the O2. The extent of quenching depends on structure-selective interactions of ssDNA with SWCNTs, allowing their specific affinities to be uncovered and studied through quick and nondestructive optical measurements. In addition, methylene blue (MB), a common dye, has been found to significantly quench ssDNA-SWCNT fluorescence through a charge-transfer process. The distinct MB quenching for different (n,m) species also shows the specific affinities of ssDNA-SWCNT hybrids. Structure-selective photochemistry of SWCNTs has been achieved using near-infrared (NIR) monochromatic irradiation in the presence of dissolved oxygen. The covalent functionalization of (8,3), (6,5), and (7,6) species in unsorted SWCNT dispersions was induced by irradiation with wavelengths of 955, 985, and 1130 nm, respectively. After the photoinduced reaction, fluorescence spectrum was significantly and selectively quenched and the absorption spectrum was heavily and selectively distorted. So the optical properties of SWCNT samples can be easily tailored by such nanotube structure-selective photochemistry. Furthermore, the spatial pattern of covalent functionalization sites on SWCNT sidewalls and the tunable and smooth modification of SWCNT energy levels have been readily obtained using ssDNA as a template to covalently functionalize SWCNTs. A room temperature chemical reaction between the nanotube surface and guanine nucleotides in the ssDNA coating proceeds quickly on exposure to singlet oxygen (1O2). This reaction preserves the SWCNT characteristic fluorescence but causes spectral red-shifts that depend on the spatial guanine density of the ssDNA oligo. Customized spatial patterns and depths of band gap modulation can thus be inscribed on the nanotube by selecting the templating ssDNA sequence. In addition, previously studied sparse chemical doping and DNA-templated covalent functionalization show distinct treated emission features, allowing flexible strategies for modifying SWCNTs.
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    Creating fluorescent quantum defects in carbon nanotubes using hypochlorite and light
    (Springer Nature, 2019) Lin, Ching-Wei; Bachilo, Sergei M.; Zheng, Yu; Tsedev, Uyanga; Huang, Shengnan; Weisman, R. Bruce; Belcher, Angela M.; Smalley-Curl Institute
    Covalent doping of single-walled carbon nanotubes (SWCNTs) can modify their optical properties, enabling applications as single-photon emitters and bio-imaging agents. We report here a simple, quick, and controllable method for preparing oxygen-doped SWCNTs with desirable emission spectra. Aqueous nanotube dispersions are treated at room temperature with NaClO (bleach) and then UV-irradiated for less than one minute to achieve optimized O-doping. The doping efficiency is controlled by varying surfactant concentration and type, NaClO concentration, and irradiation dose. Photochemical action spectra indicate that doping involves reaction of SWCNT sidewalls with oxygen atoms formed by photolysis of ClO- ions. Variance spectroscopy of products reveals that most individual nanotubes in optimally treated samples show both pristine and doped emission. A continuous flow reactor is described that allows efficient preparation of milligram quantities of O-doped SWCNTs. Finally, we demonstrate a bio-imaging application that gives high contrast short-wavelength infrared fluorescence images of vasculature and lymphatic structures in mice injected with only ~100 ng of the doped nanotubes.
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    Dye Quenching of Carbon Nanotube Fluorescence Reveals Structure-Selective Coating Coverage
    (American Chemical Society, 2020) Zheng, Yu; Alizadehmojarad, Ali A.; Bachilo, Sergei M.; Kolomeisky, Anatoly B.; Weisman, R. Bruce; Smalley-Curl Institute
    Many properties and applications of single-wall carbon nanotubes (SWCNTs) depend strongly on the coatings that allow their suspension in aqueous media. We report that SWCNT fluorescence is quenched by reversible physisorption of dye molecules such as methylene blue, and that measurements of that quenching can be used to infer structure-specific exposures of the nanotube surface to the surrounding solution. SWCNTs suspended in single-stranded DNA oligomers show quenching dependent on the combination of nanotube structure and ssDNA base sequence. Several sequences are found to give notably high or low surface coverages for specific SWCNT species. These effects seem correlated with the selective recognitions used for DNA-based structural sorting of nanotubes. One notable example is that dye quenching of fluorescence from SWCNTs coated with the (ATT)4 base sequence is far stronger for one (7,5) enantiomer than for the other, showing that coating coverage is associated with the coating affinity difference reported previously for this system. Equilibrium modeling of quenching data has been used to extract parameters for comparative complexation constants and accessible surface areas. Further insights are obtained from molecular dynamics simulations, which give estimated contact areas between ssDNA and SWCNTs that correlate with experimentally inferred surface exposures and account for the enantiomeric discrimination of (ATT)4.
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    Enantiomers of Single-Wall Carbon Nanotubes Show Distinct Coating Displacement Kinetics
    (American Chemical Society, 2018) Zheng, Yu; Bachilo, Sergei M.; Weisman, R. Bruce; Smalley-Curl Institute
    It is known that specific oligomers of single-stranded DNA (ssDNA) can show remarkable selectivity when coating different structural species of single-wall carbon nanotubes (SWCNTs). We report that (ATT)4ﾠssDNA coatings strongly distinguish between the two optical isomers of (7,5) SWCNTs. This causes resolvable shifts in their fluorescence spectra and differences of 2 orders of magnitude in the room temperature rates of coating displacement, as monitored through changes in nanotube fluorescence wavelength and intensity on exposure to sodium deoxycholate. During coating displacement, the enantiomer with high affinity for the ssDNA oligomer is deduced to form an intermediate hybrid that is not observed for the low affinity enantiomer. These results reveal that enantiomeric differences in SWCNTs complexed with ssDNA are more diverse and dramatic than previously recognized.