John-David R. Rocha

Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes by Single-Molecule Reactions

Single-molecule chemical reactions with individual single-walled carbon nanotubes were observed through near-infrared photoluminescence microscopy. The emission intensity within distinct submicrometer segments of single nanotubes changed in discrete steps after exposure to acid, base, or diazonium reactants. The steps were uncorrelated in space and time and reflected the quenching of mobile excitons at localized sites of reversible or irreversible chemical attack. Analysis of step amplitudes revealed an exciton diffusional range of about 90 nanometers, independent of nanotube structure. Each exciton visited about 10,000 atomic sites during its lifetime, providing highly efficient sensing of local chemical and physical perturbations.

Structure-dependent fluorescence efficiencies of individual single-walled carbon nanotubes.

Single-nanotube photometry was used to measure the product of absorption cross section and fluorescence quantum yield for 12 (n,m) structural species of semiconducting single-walled carbon nanotubes in aqueous SDBS suspension. These products ranged from 1.7 to 4.5 x 10(-19) cm(2)/C atom, generally increasing with optical band gap as described by the energy gap law. The findings suggest fluorescent quantum yields of approximately 8% for the brightest, (10,2) species and introduce the empirical calibration factors needed to deduce quantitative (n,m) distributions from bulk fluorimetric intensities.

Structure-Dependent Reactivity of Semiconducting Single-Walled Carbon Nanotubes with Benzenediazonium Salts

The addition of diazonium salts to single-walled carbon nanotubes (SWCNTs) in aqueous surfactant suspensions quenches the intrinsic near-infrared fluorescence of semiconducting SWCNTs through sidewall chemical reactions. Spectrally resolved fluorescence spectroscopy of mixed SWCNT samples has been used to measure structure-dependent relative reactivities in the initial stages of these reactions. For several 4-substituted benzenediazonium salts, Ar-R (Ar = N 2 (+)-C 6H 4 and R = Cl, NO 2, OMe), reactivities at pH 10 were found to be greatest for SWCNTs having the largest band gaps. The magnitude of this band gap dependence varies according to the R-group of the salt, with R = OMe showing the strongest variation. For R = OH, acidification of the sample to pH 5.5 results in reversal of the structural trend, as smaller band gap SWCNTs show slightly greater reactivities. The derivatization reactions observed here proceed concurrently, although at different rates, for semiconducting and metallic SWCNT species. These results therefore provide insight into the difficulties of separating metallic and semiconducting SWCNTs through selective reaction schemes and underscore the need for fluorescence spectroscopy to be used in assessing semiconducting SWCNT reactions.

Peptides that non-covalently functionalize single-walled carbon nanotubes to give controlled solubility characteristics

Methods which solubilize single-walled carbon nanotubes (SWNTs) in water as individuals, not bundles, while retaining their unique electronic, photonic and mechanical properties are highly desirable. Furthermore, functionalization with a diverse array of selectable chemical moieties would allow the range of useful applications to be significantly extended and may permit the designed assembly of SWNT networks. This paper presents a series of peptides that non-covalently solubilize carbon nanotubes in water using a design motif that combines a combinatorial library sequence to bind to nanotubes with a rationally designed section to create environmentally tuned solubility characteristics. The ability of the peptides to individually disperse carbon nanotubes without altering their electronic structure is shown by vis-NIR absorbance, fluorescence, and regular and vitreous ice cryo-TEM. Identification of the species composition of each sample by NIR fluorescence reveals that the peptides exhibit some diameter selectivity. Additionally, one of the rationally designed modifications addresses the poor stability of non-covalently solubilized SWNT suspensions by including cysteine residues for covalent crosslinking between adjacent peptides.

Self-Assembling Peptide Coatings Designed for Highly Luminescent Suspension of Single-Walled Carbon Nanotubes

A series of self-assembling multidomain peptides have been designed, synthesized, and tested for their ability to individually suspend single-walled carbon nanotubes (SWCNTs) in water while preserving strong near-IR nanotube luminescence. Photometric and spectral measurements on individual SWCNTs revealed that emission in the common biocompatible coating agents Pluronic F127, ss-DNA, and BSA is approximately an order of magnitude weaker than in the bioincompatible ionic surfactant SDBS. By contrast, one of the engineered peptides gave SWCNT emission approximately 40% as intense as in SDBS. A strong inverse correlation was also found between the spectral line widths of coated SWCNTs and the efficiency of their emission. Peptides with rationally designed self-assembly properties appear to be promising coatings that may enable SWCNT optical sensing applications in biological environments.

High-Performance Hydrogen Production and Oxidation Electrodes with Hydrogenase Supported on Metallic Single-Wall CarbonNanotube Networks

We studied the electrocatalytic activity of an [FeFe]-hydrogenase from Clostridium acetobutylicum (CaH2ase) immobilized on single-wall carbon nanotube (SWNT) networks. SWNT networks were prepared on carbon cloth by ultrasonic spraying of suspensions with predetermined ratios of metallic and semiconducting nanotubes. Current densities for both proton reduction and hydrogen oxidation electrocatalytic activities were at least 1 order of magnitude higher when hydrogenase was immobilized onto SWNT networks with high metallic tube (m-SWNT) content in comparison to hydrogenase supported on networks with low metallic tube content or when SWNTs were absent. We conclude that the increase in electrocatalytic activities in the presence of SWNTs was mainly due to the m-SWNT fraction and can be attributed to (i) substantial increases in the active electrode surface area, and (ii) improved electronic coupling between CaH2ase redox-active sites and the electrode surface.

Computational studies of the potential energy surface for O(3P)+H2S: Characterization of transition states and the enthalpy of formation of HSO and HOS

Structures and vibrational frequencies for minima and 11 transition states on the O(3P)+H2S potential energy surface have been characterized at the MP2=FULL/6‐31G(d) level. GAUSSIAN‐2 theory was employed to calculate ΔHf,298 for HSO and HOS of −19.9 and −5.5 kJ mol−1, respectively. The kinetics of HSO=HOS isomerization are analyzed by Rice–Ramsperger–Kassel–Marcus theory. Transition state theory analysis for O+H2S suggests OH+HS is the dominant product channel, with a rate constant given by 1.24×10−16 (T/K)1.746 exp(−1457 K/T) cm3 molecule−1 s−1. Kinetic isotope effects and the branching ratio for H+HSO production are also analyzed. The other possible products H2+SO and H2O+S do not appear to be formed in single elementary steps, but low‐barrier pathways to these species via secondary reactions are identified. No bound adducts of O+H2S were found, but results for weakly bound triplet HOSH are presented. The likely kinetics for the reactions OH+SH→S(3P)+H2O, OH+SH→cis and trans 3HOSH, cis 3HOSH→HOS+H, and ...

Efficient spectrofluorimetric analysis of single-walled carbon nanotube samples.

A new method and instrumentation are described for rapid compositional analysis of single-walled carbon nanotube (SWCNT) samples. The customized optical system uses multiple fixed-wavelength lasers to excite NIR fluorescence from SWCNTs individualized in aqueous suspensions. The emission spectra are efficiently captured by a NIR spectrometer with InGaAs multichannel detector and then analyzed by a computer program that consults a database of SWCNT spectral parameters. The identities and relative abundances of semiconducting SWCNTs species are quickly deduced and displayed in graphs and tables. Results are found to be consistent with those based on manual interpretation of full excitation-emission scans from a conventional spectrofluorometer. The new instrument also measures absorption spectra using a broadband lamp and multichannel spectrometers, allowing samples to be automatically characterized by their emission efficiencies. The system provides rapid data acquisition and is sensitive enough to detect the fluorescence of a few picograms of SWCNTs in ~50 μL sample volumes.

Theoretical studies of the RSOO, ROSO, RSO2 and HOOS (R=H, CH3) radicals

Abstract The geometries of the radicals HSOO, HOSO, HSO 2 and HOOS have been optimized at the MP2=FULL/6-31G * level, and energies obtained with Gaussian-2 theory. Internal rotations and vibrational frequencies are analyzed. The results yield values of Δ H f,298 for the four doublet radicals of 111.5, −241.4, −141.4 and 58.9 kJ mol −1 , respectively. Implications for reactions of interest in combustion and atmospheric chemistry are discussed. The results are employed to derive Δ H f,0 for CH 3 SOO, CH 3 OSO and CH 3 SO 2 of 91.8, −222.6 and −199.4 kJ mol −1 , respectively. The calculated CH 3 SOO bond strength is in excellent accord with a recent measurement.

Computational studies of the potential energy surface for O(1D)+H2S: Characterization of pathways involving H2SO, HOSH, and H2OS

Structures and vibrational frequencies for minima and transition states on the O(1D)+H2S potential energy surface have been characterized at the unrestricted second‐order Mo/ller–Plesset (UMP2)=full/6‐31G(d) level. The results for the thioperoxide HOSH agree with experimental IR spectra. Gaussian‐2 theory was employed to calculate ΔHf,298 for HOSH of −119.3 kJ mol−1, −47.1 kJ mol−1 for the sulfoxide H2SO, and 47.0 kJ mol−1 for the thiooxonium ylide H2OS. We also derived ΔHf,0 for HOS and HSO of −2.7 and −17.0 kJ mol−1, respectively. Comparisons with ΔHf for known asymptotes on the potential energy surface gave good agreement, except in the case of HSO. Rice–Ramsperger–Kassel–Marcus (RRKM) analysis suggests that in most environments, except at low pressures and temperatures, H2OS will be short lived, and rate constants for isomerization of the three bound adducts under thermally equilibrated conditions are derived. The potential energy surface is discussed in the context of single‐collision experiments, an...