Optical Probing of Local pH and Temperature in Complex Fluids with Covalently Functionalized, Semiconducting Carbon Nanotubes

We show that local pH can be optically probed through defect photoluminescence from semiconducting carbon nanotubes covalently functionalized with aminoaryl groups. Switching between protonated and deprotonated forms of the amino moiety produces an energy shift in the defect state of the functionalized nanotube by as much as 33 meV in the near-infrared region. This unexpected observation enables a new optical pH sensor that features ultrabright near-infrared II (1.1-1.4 mu m) photoluminescence, a sensitivity for pH changes as small as 0.2 pH units over a wide working window that covers the entire physiologic pH range, and potentially molecular resolution. Independent of pH, this nanoprobe can simultaneously act as a nanothermometer by monitoring temperature-modulated changes in photoluminescence intensity, which follows the vant Hoff equation. This work opens new opportunities for quantitative probing of local pH and temperature changes in complex biological systems.