the sol-gel causes the apparent pKa* to shift to lower pH. TX-100 is non-ionic and therefore, any pKa* change is not due to
ionic interactions between resorufin and the surfactant, but instead must be due to molecular reorientation of the resorufin
into more compatible domains within the matrix. The small decrease in pKa* (~0.4 pH units) is probably due to R
− anions
being located in more hydrophobic domains created within the S3 sol-gel by the non-ionic TX-100. As such, more acidic
conditions are required for protonation and the pH response curve was shifted to lower pH values. The S3 system has a
slightly larger change in
τ
over the 3 – 9 range (Fig. 4A), but there is only a small ~0.5 ns increase in the range pH 6 - 8
which makes it unsuitable for biological applications. .
0
20
40
60
80
100
3 4 5 6 7 8 9
B
pH
% Amplitude
0.5
1.0
1.5
2.0
2.5
3 4 5 6 7 8 9
pH
Average
τ
0.5
1.0
1.5
2.0
2.5
3 4 5 6 7 8 9
D
A
pH
Average
τ
0.5
1.0
1.5
2.0
2.5
3 4 5 6 7 8 9
C
pH
Average
τ
Figure 4: (A&C&D) Average fluorescence lifetime (
τ
) variation with pH for resorufin doped sol-gels: S2 (о), S3 (□) and S4 (∇), at
emission wavelengths of (A) 600 nm, (C) 650 nm, and (D) 550 nm. (B) Percentage amplitudes of τ 1 and τ 2 at emission wavelength 600
nm: (о) τ 1 in S2, (□) τ 2 in S2, (●) τ 1 in S3 and (■) τ 2 in S3. All measurements made using 460 nm excitation in 0.1M phosphate buffers.