Ratiometric optical sensor for high-resolution imaging of pH with low cross-talk.

We synthesize lipophilic, highly efficient, and pH-insensitive oleic acid-modified quantum dots (QDs) with maximum emission at a wavelength of 628 nm. The pH sensing film is fabricated by encapsulating 5-hexadecanoylamino-fluorescein and QDs as the reference in D4-hydromed and plasticized polystyrene. Using a light-emitting diode with a central wavelength of 410 nm as an excitation source, it is shown that the emission wavelengths of the pH sensitive indicator and reference dye have no spectral overlap and match respectively the channels of a 3CCD (RGB) camera with low cross-talk. A series of validation experiments shows that this ratiometric pH optode has good properties of high sensitivity, long-term stability, and photostability. It had a fast response time of <20  s when going from pH 6.3 to pH 8.0. The pH images suggest that the proposed ratiometric pH-sensing approach has great advantage and promise for field applications.