Calorimetric studies of small-molecule adsorption to carbon nanotubes

Abstract

Isothermal titration calorimetry (ITC) was developed as a technique for qualitatively comparing the heat of absorption of small molecules to single-walled carbon nanotubes (SWCNTs). In agreement with other studies, it was shown that polyaromatic hydrocarbons (PAHs) that can achieve a greater degree of n-orbital overlap with the curved surface of carbon nanotubes (CNTs) adsorb more strongly. ITC studies also indicated that adsorption of a in-basic metalloaromatic gold pyrazolate trimer, [Au(3,5- Et2Pz)] 3, to SWCNTs in DMF is accompanied by a large negative enthalpy change, while the structurally-related it-acidic macrocycle, [Hg(C 6F4)]3, does not have a large enthalpy of adsorption. This result agrees with previous studies indicating that CNT binding energy depends upon the polarizability of the ligand. Molecules with large CNT binding affinity have potential for use in CNT-based sensing applications, serving to anchor analyte receptors to the surface of CNTs. In this context, a phosphoresorcin[4]arene decorated with four pendant pyrene moieties was investigated as a bifunctional SWCNT adsorbent and receptor for sensing biogenic amines. Though the cooperative binding effect of the pendant pyrene moietites effectively tethers the cavitand receptor to CNTs, primary ammonium salts do not bind to the cavitand more strongly than to the SWCNT surface. Highly selective receptors with large binding affiniites for both CNTs and analyte are necessary to optimize CNT-based sensing technologies. ITC can be useful tool for identifying effective receptor molecules.