Multivariable Response of Semiconductor Nanocrystal-Dye Sensors: The Case of pH

Emissive semiconductor nanocrystals can be efficient FRET donors to chemically responsive organic fluorophores; conjugating the two creates a ratiometric (or self-calibrating) sensor. Given that the organic component engenders chemical and/or biological sensitivity, it is natural to assume that the NC-dye response is reflective of the photochemical properties of the neat dye. In the case of a pH sensor like fluorescein, the dye emission can be analytically described by the Henderson-Hasselbalch equation. Our research demonstrates that the ratiometric sensing response of CdS/ZnS-fluorescein pH sensors, however, does not directly reflect the Henderson-Hasselbalch relationship and is difficult to predict. The reasons for this complexity are the alternations of the dye microenvironment within the nanoscale architecture and the nature of energy transfer from the NC donor to the acceptor dye.