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 (F1: λ1,ex = 420 nm, λ1,em = 462 nm; F2: λ2,ex = 470 nm, λ2,em = 498 nm) properties for ratiometric (RF1,F2 = F1/F2) normalization of sensor signal. The sensor demonstrated an exponentially decreasing ratiometric response with increasing pH, with a linear correlation (R2 = 0.9936) between 10log(RF1,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 (Itot, 10–700 mM) were more pronounced for the electrode, with a linear (R2 = 0.9976) increase in response (δE/δpH) with increasing Itot. The DHPDS-based fluorosensor, however, retained sensitivity (δ10log(RF1,F2)/δpH = 0.8024 ± 0.0145), though with an overall increase in ratiometric signal with increasing Itot. 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.