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 (<i>K</i>), 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 <i>K</i> patterns from semiconducting tubes show preference to certain SWNT chiralities and surfactant-specific cooperativity according to nanotube chirality. Controlling temperature was effective to modulate <i>K</i> 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

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 (<i>K</i>), 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 <i>K</i> patterns from semiconducting tubes show preference to certain SWNT chiralities and surfactant-specific cooperativity according to nanotube chirality. Controlling temperature was effective to modulate <i>K</i> 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

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 (<i>K</i>_a). 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 <i>K</i>_a of SWNTs against an achiral surfactant as a titrant, left- and right-handed SWNTs display two-step PL inflection corresponding to respective <i>K</i>_a 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