Читать книгу Handbook of Aggregation-Induced Emission, Volume 2 - Группа авторов - Страница 45

As one of the key parameters, pH plays a crucial role in all life forms including external environment as well as cellular functions. Small changes in the pH of the environment may even affect the lives of many plants and animals. In addition, pH is a key factor in pharmaceuticals, food, and drinking water. For intracellular pH, the fluctuation has a significant effect on cell growth, enzyme activity, and ion transport. pH is also one of the important parameters to distinguish cancer cells from normal cells. Therefore, monitoring pH is critical to maintaining our living environment and improving the quality of our life.

The hydroxyl groups of SSB experience deprotonation according to the increase of medium pH; thus, most SSB AIE fluorophores show significant fluorescent wavelength change, usually blue‐shifted as pH increases [49–53]. Therefore, SSB is of unique advantage for designing ratiometric fluorescent pH probes. In particular, fluorescent pH probes with a ratiometric response manner are highly preferred for pH monitoring in complex samples because of their visible fluorescence color change and better resistance to variations of sensor concentration and external environment. Till date, a number of ratiometric fluorescent pH probes have been designed based on SSB and successfully applied in test paper‐based detection [50] and in bioimaging [49, 52].

A representative work is the first SSB ratiometric fluorescent pH probe designed and reported by Tong's group in 2011 and applied in the imaging of pH variation in living cells [49]. As shown in Figure 3.21a and b, 4‐carboxylaniline‐5‐chlorosalicylaldehyde Schiff base (34) changed the fluorescence color from orange to green (λ_em = 559/516 nm) when pH of the solution increased from 3.43 to 9.56. The pK_a1 of 34 is 4.8 for deprotonation of the carboxyl group resulting in a decrease of fluorescent intensity of orange light. The pK_a2 of 34 is 7.4 for deprotonation of the hydroxyl group and caused the enhancement of green fluorescence according to further increase of the pH. The fluorescence intensity ratio I₅₁₆/I₅₅₉ changed dramatically from 5.0 to 7.0, indicating that 34 could be a sensitive pH probe at the range of 5.0–7.0. Figure 3.21c demonstrates the ratiometric fluorescent imaging of H⁺ concentration variation in HepG2 cells by probe 34, showing that the SSB molecule 35 deserves in detecting pH variation in living cells.

A series of works designing SSB‐based ratiometric pH probes were then reported in the following years. Molecular structures of the probes (shown in Figure 3.22a) and their applications as test papers were reported with good contrast (Figure 3.22b). Tang and coworkers [52] ameliorated the molecular structure, endowing the optimal detection range as 6.86–8.01, which covers the pH range of blood and intracellular fluid of healthy individuals and achieved satisfied results in cell imaging (Figure 3.22c, d). Compound 36 colocalized well on mitochondria compared with MitoTracker Deep‐Red FM; Pearson's coefficient was obtained as high as 0.92. The plot of the ratiometric fluorescence intensity of 36 in HeLa cells as a function of pH indicated a satisfactory linearity of R = 0.93, showing that SSB‐based ratiometric pH probes perform satisfactorily in real environmental samples as well as for intracellular pH evaluation.