Determination of the Absolute Enantiomeric Excess of the Carbon Nanotube Ensemble by Symmetry Breaking Using the Optical Titration Method.

Symmetry breaking of single-walled carbon nanotubes (SWNTs) has profound effects on their optoelectronic properties that are essential for fundamental study and applications. Here, we show that isomeric SWNTs that exhibit identical photoluminescence (PL) undergo symmetry breaking by flavin mononucleotide (FMN) and exhibit dual PLs and different binding affinities (Ka). Increasing the FMN concentration leads to systematic PL shifts of SWNTs according to structural modality and handedness due to symmetry breaking. Density gradient ultracentrifugation using a FMN-SWNT dispersion displays PL shifts and different densities according to SWNT handedness. Using the optical titration method to determine the PL-based Ka of SWNTs against an achiral surfactant as a titrant, left- and right-handed SWNTs display two-step PL inflection corresponding to respective Ka values with FMN, which leads to the determination of the enantiomeric excess (ee) of the SWNT ensemble that was confirmed by circular dichroism measurement. Decreasing the FMN concentration for the SWNT dispersion leads to enantiomeric selection of SWNTs. The titration-based ee determination of the widely used sodium cholate-based SWNT dispersion was also demonstrated by using FMN as a cosurfactant.

The effects of dendrimer size and central metal ions on photosensitizing properties of dendrimer porphyrins.

A series of dendrimer porphyrins (G(n)DP(M); n = generation of dendrimer, n = 1-3; M = coordination metal, M = freebase, Zn, Pt) were prepared and their photosensitizing properties were compared. All G(n)DP(M) exhibited sharp absorption in organic solvents. However, the Soret absorptions of G(n)DP(M)(CO(2)H) in 10 mM phosphate buffer solution (pH = 7.4) are broader than those of G(n)DP(M) in organic solvents, indicating inhomogeneous microenvironments of the focal porphyrin derivatives. All G(3)DP(M)(CO(2)H) successfully formed globular polyion complex micelles that were uniform in size. Under dark conditions, all G(n)DP(M)(CO(2)H) showed negligible cytotoxicity. However, all samples exhibited concentration-dependent photocytotoxicity under light irradiation. In vitro photocytotoxicity as well as singlet oxygen generation revealed that G(3)DP(Zn)(CO(2)H) is the best dendritic PS of the three different dendrimer porphyrin species.

Effect of tight flavin mononucleotide wrapping and its binding affinity on carbon nanotube covalent reactivities.

The controlled functionalization of single-walled carbon nanotubes (SWNTs) is a key to using them in high-end applications. We show that nanotube reactivity after covalent diazonium modification is governed by a chirality-specific surfactant binding affinity to SWNTs. Both metallic and semiconducting SWNTs tightly organized by a helical flavin mononucleotide (FMN) assembly exhibit two hundred times slower reactivity toward 4-methoxy benzenediazonium (4-MBD) than those wrapped by sodium dodecyl sulfate and this reactivity enables chirality- and metallicity-specific behaviours to be probed, as confirmed by absorption, Raman, and photoluminescence (PL) spectroscopy. Each reaction kinetic of the two-step SWNT PL decays originating from electron transfer and the covalent reaction of 4-MBD, respectively, is inversely proportional to the binding affinity (Ka) between FMN and the SWNTs. The observed marginally higher reaction rate of the metallic nanotube compared to the semiconducting one results from the weaker Ka value of the metallic nanotubes with FMN. An enrichment demonstration of a few nanotube chiralities using selective and slow covalent diazonium chemistry demonstrates the importance of the binding affinity between the surfactant and the SWNTs. The study provides a handle on chirality-specific covalent chemistry via surfactant-SWNT binding affinity and impacts on future-sensing schemes.

Binding affinities and thermodynamics of noncovalent functionalization of carbon nanotubes with surfactants.

Binding affinity and thermodynamic understanding between a surfactant and carbon nanotube is essential to develop various carbon nanotube applications. Flavin mononucleotide-wrapped carbon nanotubes showing a large redshift in optical signature were utilized to determine the binding affinity and related thermodynamic parameters of 12 different nanotube chiralities upon exchange with other surfactants. Determined from the midpoint of sigmoidal transition, the equilibrium constant (K), which is inversely proportional to the binding affinity of the initial surfactant-carbon nanotube, provided quantitative binding strengths of surfactants as SDBS > SC ≈ FMN > SDS, irrespective of electronic types of SWNTs. Binding affinity of metallic tubes is weaker than that of semiconducting tubes. The complex K patterns from semiconducting tubes show preference to certain SWNT chiralities and surfactant-specific cooperativity according to nanotube chirality. Controlling temperature was effective to modulate K values by 30% and enables us to probe thermodynamic parameters. Equally signed enthalpy and entropy changes produce Gibbs energy changes with a magnitude of a few kJ/mol. A greater negative Gibbs energy upon exchange of surfactant produces an enhanced nanotube photoluminescence, implying the importance of understanding thermodynamics for designing nanotube separation and supramolecular assembly of surfactant.