A high-performance fluorosensor for pH measurements between 6 and 9.

This study presents a high-performance ratiometric pH optode based on the fluorophore 6,8-dihydroxypyrene-1,3-disulfonic acid (DHPDS). The two pH-sensitive terminal hydroxy groups of DHPDS facilitated dual excitation/dual emission (F(1): lambda(1,ex)=420 nm, lambda(1,em)=462 nm; F(2): lambda(2,ex)=470 nm, lambda(2,em)=498 nm) properties for ratiometric (R(F1,F2)=F(1)/F(2)) normalization of sensor signal. The sensor demonstrated an exponentially decreasing ratiometric response with increasing pH, with a linear correlation (R(2)=0.9936) between (10)log(R(F1,F2)) and pH within the pH interval 6-9. Precision determined as the IUPAC pooled standard deviation for the pH values 6.00, 7.01 and 9.01, was 0.0057 pH units for the fluorosensor and 0.0054 for a commercially available pH electrode used for comparison. Between the end-points of calibration at pH 7.01, the precision of the sensor was 0.0037 pH units. Effects from changes in ionic strength (I(tot), 10-700 mM) were more pronounced for the electrode, with a linear (R(2)=0.9976) increase in response (deltaE/deltapH) with increasing I(tot). The DHPDS-based fluorosensor, however, retained sensitivity (delta(10)log(R(F1,F2))/deltapH=0.8024+/-0.0145), though with an overall increase in ratiometric signal with increasing I(tot). The preserved sensitivity despite changes in ionic strength was possibly a consequence from the dual photo-acidic properties of DHPDS. Analytical characteristics of immobilized DHPDS therefore not only facilitated high-performance measurements over a wide pH range, but also opened for straightforward simultaneous measurements of pH and ionic strength.