Dendritic polyelectrolytes as fluorescent aptasensors for potassium ion

Abstract

A tricationic phenyleneethynylene fluorophore containing three ammonium groups was synthesized from Sonogashira coupling of triiodotriphenylamine with three equivalents of N,N-dimethyl-4-ethynylaniline using Pd/Cu catalyst. Quaternization of the three dimethylaniline groups was accomplished by methylation with methyl iodide. The iodide salt, however, tended to demethylate upon storage in a solid form and became poorly soluble in water. The strongly nucleophilic I- was thus exchanged to a less nuleophilic Cl- using Amberlite IRA-410 anionic ion exchange resin. The fluorophore containing Cl- counter ion was stable and retained its water solubility upon storage. In Tris-HCl buffer, the synthesized compound exhibited maximum absorption and emission at 372 and 386 nm, respectively, with a quantum yield of 6.6%. The polycationic periphery was designed for charge interaction with polyanionic oligonucleotide chain. Three potassium aptameric ODNs i.e. Apt12, Apt15 and Apt21, and one non-aptameric ODN (nApt15) were tested for K+ aptasensing properties. A job’s plot showed the binding ratios of fluorophore/aptamer of 1:2, 1:1 and 1:1 for Apt12, Apt15 and Apt21, respectively. Upon complexation with the aptamer ODNs, the fluorescent signal was enhanced probably by the reduction of geometrical relaxation and aggregation process. In the presence of K+, the fluorescent signal decreases by the formation of the K+-aptamer G-quaduplex structure. Monitoring the fluorescent signal provided a method for determination of K+ concentration in urine samples.