Hairpin-Contained i‑Motif Based Fluorescent Ratiometric Probe for High-Resolution and Sensitive Response of Small pH Variations

• Published in: Analytical chemistry (Washington) Vol. 90; no. 3; pp. 1889 - 1896
• Main Authors: Ma, Wenjie, Yan, Lv’an, He, Xiaoxiao, Qing, Taiping, Lei, Yanli, Qiao, Zhenzhen, He, Dinggeng, Huang, Kaihang, Wang, Kemin
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06.02.2018
• Subjects: Cells; Chemistry; Design engineering; Detection; Emissions; Fluorescence; Fluorescent indicators; Hybridization; Hydrogen ions; Intracellular; Measurement; pH effects; Rhodamine
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/acs.analchem.7b03972

Intracellular pH (pHi) is an important parameter associated with cellular behaviors and pathological conditions. Sensing pHi and monitoring its changes are essential but challenging due to the lack of high-sensitive probes. Herein, a ratiometric fluorescent probe with ultra pH-sensitivity is developed based on hairpin-contained i-motif strand (I-strand, labeled with Rhodamine Green and BHQ2 at two termini) and complementary strand (C-strand, labeled with Rhodamine Red at its 5′-end). At neutral pH, both I-strand and C-strand hybridize into a rigid duplex (I–C), which holds the Rhodamine Red and the BHQ2 in close proximity. As a result, the fluorescence emission (F597 nm) of the Rhodamine Red is strongly suppressed, while the Rhodamine Green (F542 nm) is in a “signal on” state. However, the slightly acidic pH enforced the I-strand to form an intramolecular i-motif and initiated the dehybridization of I–C duplex, leading to Rhodamine Red in a “signal on” state and a decreased fluorescence of Rhodamine Green. The ratio (F542 nm/F597 nm) can be used as a signal for pH sensing. Due to the rational internal hairpin design of I–C duplex probe, almost 70-fold change in the ratio was observed in the physiological pH range (6.50–7.40). This probe possesses efficient stability, fast response, and reversible pH measurement capabilities. Furthermore, intracellular application of the ratiometric probe was demonstrated on the example of SMMC-7721 cells. With different recognition elements in engineering of i-motif based platforms, the design might hold great potential to become a versatile strategy for intracellular pH sensing.