Ruthenium(II) Complex-Based Luminescent Bifunctional Probe for Ag⁺ and Phosphate Ions: Ag⁺‑Assisted Detection and Imaging of rRNA

A new bis-heteroleptic
Ru­(II) complex (1) of benzimidazole-substituted 1,2,3-triazole
pyridine ligand has been designed and constructed for the photoluminescent
detection of cationic and anionic analytes, Ag+ and phosphate
ions. Compound, 1­[PF6]2 was fully characterized by various spectroscopic
techniques and the solid-state structure was determined via single-crystal
X-ray diffraction. The cation and anion sensing properties in 50%
aqueous buffer (pH 9.2) and pure acetonitrile were carefully examined
in photoluminescence (PL) spectroscopy. The 1­[PF6]2 was found
to be highly selective to pyrophosphate; PPi/HP2O73– and H2PO4– ions in CH3CN. It showed ∼10-fold PL intensity
enhancement at 583 nm in the presence of only 1 and 2 equiv of PPi
and H2PO4– ions, respectively.
The PL titrations of 1­[PF6]2 with PPi and H2PO4– in CH3CN furnished the association
constant (Ka = 3.3 × 103 M–1 and 6.8 × 103 M–1) and the detection limit was as low as 5.73 and 5.19 ppb, respectively.
The 1­[PF6]2 also selectively detected Ag+ over other
competitive cations through the luminescence light up in 50% aqueous
buffer (pH 9.2) media. The PL titration of 1­[PF6]2 with Ag+ showed ∼8-fold luminescence enhancement at 591 nm
and yielded association constant, Ka =
3.5 × 104 M–1 and the detection
limit was determined to be 5.05 ppb. A new cation sensing mechanism
has been established where the Ag+ ion is detected in photoluminescence
spectroscopy through the unique cyclometalated Ag+-triazolide complex formation. The high selectivity of 1­[PF6]2 for phosphates and Ag+ was established by PL in
the presence of various competing ions. Finally, for biological application,
the cytotoxicity study was performed. The probe showed low cytotoxicity
and was suitable for intracellular Ag+ imaging. The cell
imaging and in vitro photoluminescence study revealed
that the probe stained the cell nucleoli and specifically bind with
ribosomal RNA (rRNA) and, therefore, it can also serve as a luminescent
probe for rRNA in the presence of Ag+.