In-situ nanospectroscopic pH monitoring by plasmon resonance energy transfer (PRET).

Real-time detection of pH value in a living cell is in central importance for research about cells and diseases. In spite of the great advances in science and technology, pH measurement in a living cell is still limited in spatial resolution, in-situ detection, and intracellular monitoring. Here, we designed a nanoscale pH meter by Plasmon Resonance Energy Transfer (PRET). In order to highly sensitively measure pH with nanoscale spatial resolution, we choose 80 nm spherical gold nanoparticle (GNP) and phenol red which is commonly used in cell media for pH determination. The resonance energy of GNP is transferred to phenol red because the scattering intensity of GNP is overlapped with the second absorption intensity of phenol red at near 560 nm. Meanwhile, the absorption intensity of phenol red molecules is changing with pH value of the solution. For that reason, the intensity of PRET from GNP to phenol red molecules also changes by the acidity of phenol red solution. Then we can detect pH values with nanoscale spatial resolution through the Rayleigh scattering intensity of GNP. As we changed pH value from 6.0 to 9.0, the scattering intensity of GNP is decreased because the absorbance of phenol red at 560 nm wavelength is increased with increasing pH value. The Gaussian peak of a difference in Rayleigh scattering spectra of GNP between pH 6.0 and pH 9.0 indicates exactly the same as UV-vis spectral difference between basic and acidic phenol red solution. We expect that this pH measuring technique has a significant impact on the pH detection of living cells with nanoscale, and it can make possibility to image the cell structure by pH variation.