A structure-function relationship for the optical modulation of phenyl boronic acid-grafted, polyethylene glycol-wrapped single-walled carbon nanotubes

Phenyl boronic acids (PBA) are important binding ligands to pendant diols useful for saccharide recognition. The aromatic ring can also function to anchor an otherwise hydrophilic polymer backbone to the surface of hydrophobic graphene or carbon nanotube. In this work, we demonstrate both functions using a homologous series of seven phenyl boronic acids conjugated to a polyethylene glycol, eight-membered, branched polymer (PPEG8) that allows aqueous dispersion of single-walled carbon nanotubes (SWNT) and quenching of the near-infrared fluorescence in response to saccharide binding. We compare the 2-carboxyphenylboronic acid (2CPBA); 3-carboxy- (3CPBA) and 4-carboxy- (4CPBA) phenylboronic acids; N-(4-phenylboronic)succinamic acid (4SCPBA); 5-bromo-3-carboxy- (5B3CPBA), 3-carboxy-5-fluoro- (5F3CPBA), and 3-carboxy-5-nitro- (5N3CPBA) phenylboronic acids, demonstrating a clear link between SWNT photoluminescence quantum yield and boronic acid structure. Surprisingly, quantum yield decreases systematically with both the location of the BA functionality and the inclusion of electron-withdrawing or -donating substituents on the phenyl ring. For three structural isomers (2CPBA, 3CPBA, and 4CPBA), the highest quantum yields were measured for para-substituted PBA (4CPBA), much higher than ortho- (2CPBA) and meta- (3CPBA) substituted PBA, indicating the first such dependence on molecular structure. Electron-withdrawing substituents such as nitro groups on the phenyl ring cause higher quantum yield, while electron-donating groups such as amides and alkyl groups cause a decrease. The solvatochromic shift of up to 10.3 meV was used for each case to estimate polymer surface coverage on an areal basis using a linear dielectric model. Saccharide recognition using the nIR photoluminescence of SWNT is demonstrated, including selectivity toward pentoses such as arabinose, ribose, and xylose to the exclusion of the expected fructose, which has a high selectivity on PBA due to the formation of a tridentate complex between fructose and PBA. This study is the first to conclusively link molecular structure of an adsorbed phase to SWNT optical properties and modulation in a systematic manner.