Room temperature phosphorescence pH optosensor based on energy transfer Original Research Article

In this paper, the development of a room temperature phosphorescence (RTP) optosensor for pH monitoring in aqueous media based on the effect of the pH on the energy transfer from a phosphor molecule (acting as a donor) to an adequate mixture of pH-sensitive dyes (acceptors) is presented. The proposed optosensor, to our knowledge the first RTP optical sensor for pH developed so far, is based on the immobilization in a solid support of a pH-insensitive phosphor molecule along with a mixture of three pH-sensitive dyes. 6-Bromo-2-naphthyl sulfate (BNS) and α-bromonaphthalene (BrN) are compared as donor molecules, because both can produce significant RTP emission in aqueous media (without deoxygenation) when retained in a polymeric resin or encapsulated in a rigid sol–gel matrix. Absorption spectra of the mixture of phenol red (PhR), bromocresol purple (BCP) and bromophenol blue (BPB) in a concentration ratio of 10−5:5×10−6:10−5 mol/l, respectively, possesses a desirable spectral overlap with the emission spectra of the donors (BNS and BrN) in a pH range around 3.5–9.2. Consequently, nonradiative energy transfer occurs from the phosphor molecules to the mixture of pH indicator dyes. An increase in the pH of the solution causes an absorption increase of the mixture of the pH indicators (acceptors) and, therefore, an increase in the amount of energy transfer occurs which brings about a decrease of the RTP emission. Thus, any changes in the absorption of the mixture of the three pH indicators are detected as measurable changes in the RTP emission of the sensing material. The proposed RTP optosensor has been evaluated for pH monitoring of different water samples.