Functional nucleic acids (FNAs) have attracted a lot of attention for the rapid detection of metal ions. Cr3+ is one of the major heavy metal ions in natural waters. Due to the slow ligand exchange rate of Cr3+, the FNA-based Cr3+ sensors require long assay times, limiting the on-site applications. In this study, we report that the good\'s buffers containing amino and polyhydroxy groups greatly increase the ligand exchange rate of Cr3+. Using EDTA as a model coordinate ligand, the Tris buffer (100 mM, pH 7.0) showed the best acceleration effect among the eight buffers. It improved the rate constant ∼20-fold, shorten the half-time 19-fold, and lowered the activation energy ∼70% at 40 °C. The Tris buffer was then applied for sensor based on the Cr3+-binding induced fluorescence quenching of fluorescein (FAM)-labeled and single-stranded DNA (ssDNA), which shortened the assay time from 1 h to 1 min. The Tris buffer also ∼100% enhanced the fluorescence intensity of FAM, achieving the 11.4-fold lower limit of detection (LOD = 6.97 nM, S/N = 3). By the combination use of the Tris buffer and ascorbic acid, the strong interference from Cu2+, Pb2+, and Fe3+ suffered in many previous reported Cr3+ sensors was avoided. The practical application of the sensor for the detection of Cr3+ spiked in the real water samples were demonstrated with high recovery percentages. The Tris buffer could be applied for other metal ions with slow ligand exchange rate (such as V2+, Co3+ and Fe2+) to solve diverse issues such as long assay time and low synthesis yield of metal complexes, without the need of heating treatment.

Ruthenium (Ru) complexes are known for their promising anticancer activity presumably due to octahedral coordination geometry, slow ligand exchange rate, the range of different oxidation states and target specificity. This review article summarizes the physicochemical processes which are responsible for the selectivity of Ru complexes toward cancer cells over the normal cells. Emphasis has been given on the activation mechanism of Ru(III) complex administered as a prodrug and then the release of active species in an acidic environment of cancer cell through normal or photo induced hydrolysis or ligand oxidation. This article also elaborates how active Ru complex can be designed by their rate of hydrolysis, kinetics of ligand exchange, pKa of the aquated species. The article further articulates how Ru complexes inhibit tumor growth via multiple events such as transferrin/albumin binding, ROS generation, inhibition of glutathione-S-transferases and kinases and DNA intercalation. Based on the above understanding, examples of various Ru complexes with their in-vitro cell based cytotoxicity and mechanism of action have been described to make this review comprehensive for future Ru based anticancer drug development. In the end, comments have been made on some emerging concepts regarding lack of innertness of Ru(III) complexes vis-à-vis Ru(II) species.