Functional Single-Walled Carbon Nanotubes for Anion Sensing

Abstract

We report an anion-sensing platform wherein conductance changes are triggered by chemical interactions between selectors and anions. The selector design incorporates both a cationic moiety (i.e., pyridinium) and a thiourea-based dual-hydrogen-bond donor. Anion binding by a model selector (2) was studied using H-1 NMR and UV-vis titrations, which reveal a binding strength toward acetate ions (AcO-) followed by Cl- ˃ Br- ˃ NO3-. These studies reveal that selector 2 is deprotonated upon addition of AcO-, whereas it undergoes hydrogen bonding associated with Cl-, Br-, and NO3-. The cationic pyridinium moiety improves anion binding affinity by lowering the pK(a) value of selector 2 and enhancing the hydrogen-bond donor capability as confirmed by spectroscopic titrations and DFT calculations. The selector is covalently attached to poly(4-vinylpyridine) (P4VP), which wraps single-walled carbon nanotubes (SWCNTs) (i.e., P4VP-2-SWCNT) to transduce an electrical signal. As a result, continuous anion sensing was achieved with high sensitivity represented by a normalized resistance change of 101.9 +/- 10.3% toward 16.7 mM AcO-, whereas negligible sensitivity was observed toward Cl-, Br-, and NO3-. The sensitivity transition was attributed to the internal charge transfer of 2 by deprotonation of the thiourea proton upon addition of AcO-.