Other PH Probes

In chemistry, pH is a scale used to specify the acidity or alkalinity of a water-based solution. Acidic solutions have lower pH values, while alkaline solutions have higher pH values. At room temperature (25°C or 77°F), purified water is neither acidic nor alkaline, with a pH of 7. The pH is logarithmic and instead indicates the concentration of hydrogen ions in the solution (a lower pH value indicates a higher hydrogen ion concentration). This is because the formula used to calculate pH approximates the negative base-10 logarithm of the hydrogen ion molar concentration in the solution. More precisely, pH is the negative base-10 logarithm of hydrogen ion activity. At 25°C, solutions with a pH value of less than 7 are acidic, while solutions with a pH value greater than 7 are alkaline. The neutral value of pH depends on the temperature. If the temperature rises, the pH value is lower than 7. For very strong acids, the pH can be less than 0, and for very strong bases, the pH can be greater than 14. The pH can be traced back to a set of standard solutions, the pH of which is determined according to international agreements. Determine the original pH standard value by measuring the potential difference between the hydrogen electrode and a standard electrode (such as a silver chloride electrode) using a transferable concentration cell. The pH of the aqueous solution can be measured with a glass electrode and a pH meter or a discoloration indicator. pH measurement is important in chemistry, agronomy, medicine, water treatment and many other applications.

Figure 1. Relation between p[OH] and p[H] (red = acidic region, blue = basic region)

Lipophilic pH probe

The measurement of pH near the surface of the membrane is often complicated by the electrostatic charge and solvation of pKa of the surface-bound indicator. The pKa of the membrane-inserted fluorescein DHPE is ~ 6.2, which is very close to the pKa of free fluorescein. Researchers have used the pH-dependent fluorescence of fluorescein DHPE to measure lateral proton conduction along the lipid monolayer. This fluorescein-labeled phospholipid has also been used to track the displacement of protons from compartments inside the phospholipid vesicles. For more acidic environments, Oregon Green 488 DHPE has similar potential applications. Other related lipophilic fluorescein derivatives include 5-dodecylaminofluorescein and 5-hexadecamidofluorescein (H110).

Amine-reactive pHrodo pH probes

pHrodo dye is an amino rhodamine pH indicator that increases in fluorescence as the surrounding pH becomes more acidic. The amine-reactive succinimide ester form of the dye provides access to a variety of user-defined bioconjugates by following the detailed protocol provided in the accompanying product information sheet. pHrodo dyes are extremely sensitive to their local environment. Therefore, the pH response of each bioconjugate must be determined individually. pHrodo succinimidyl esters have been used to label dexamethasone-treated thymocytes, flow cytometry analysis of spleen or peritoneal macrophages on phagocytosis, and antigen transfer from human B lymphocytes to macrophages. Confocal imaging of live cells.

Other amine reactive dyes

To generate pH-sensitive tracers, probes useful at near neutral pH (and many of the pH indicators described in Probes useful at acidic pH can be bound to biomolecules. The resulting conjugates can be used in subsequent endocytosis, phagocytosis, organelle transport, and other processes as described in probes for binding and phagocytosis. For example, the pH sensitivity of fluorescein-labeled transferrin is often used to detect pH changes associated with endocytosis of this important iron-transporting glycoprotein. For these types of applications, fluorescein conjugates are not optimal because they are barely sensitive in the pH range below pH 5.5 The response range can be extended using the conjugate of our Oregon Green 488 dye, which has a much lower pK a than fluorescein, but the spectra are essentially the same.

Reference:

Lim, Kieran F. Negative pH Does Exist. Journal of Chemical Education. 2006, 83 (10): 1465.

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