Fluorescent Sensors for Hg²⁺ in Micelles: A New Approach that Transforms an ON–OFF into an OFF–ON Response as a Function of the Lipophilicity of the Receptor

Abstract

A new approach to the use of micelles in the fluorescent sensing of metal cations is proposed and applied to the case of Hg2+. We demonstrate how it is possible to transform a system from an ON–OFF to an OFF–ON sensor by changing the length of the chain used to lipophilise a ligand that resides inside TritonX‐100 micelles together with pyrene as the fluorophore. Three tetrathia‐monoaza macrocyclic ligands have been synthesised with the same ring but functionalised on the nitrogen atom with a methyl (C1‐NS4), an n‐butyl (C4‐NS4) or an n‐dodecyl (C12‐NS4) chain. The three ligands have been fully characterised in water containing TritonX‐100 micelles by means of potentiometric titrations and their apparent protonation and complexation constants with Hg2+ were determined. On the basis of the distribution diagrams obtained, the more lipophilic C12‐NS4 has been developed as an ON–OFF fluorescent sensor for mercury: working at pH<4, in the absence of Hg2+ the ligand is inside the micelles, protonated and non‐quenching, while on addition of mercury the [C12‐NS4Hg]2+ complex forms which remains inside the micelles and is quenching. On the other hand, the ligand of intermediate chain length, C4‐NS4, can be used to obtain an OFF–ON sensor at 7.0<pH<9.5. In the absence of added metal at pH>7.0 the ligand is unprotonated, it stays inside the micelles and is quenching, while addition of Hg2+ in the 7.0–9.5 pH range results in the formation of [C4‐NS4Hg]2+, which is hydrophilic enough to leave the micelles and to be released into the bulk solution where it is no longer capable of quenching pyrene fluorescence. Additional studies on C1‐NS4, C3‐NS4 and C8‐NS4 indicate that the optimal chain length to observe this OFF–ON behaviour is C3–C4.

Amphiphilic metalloaggregates: Catalysis, transport, and sensing

“…Playing with the pK a of the amines and pyridines used it is possible to tune the wide of the window and its position along the pH axes. Following this approach the same group has reported other interesting results in the sensing of metal ions [84] and also in the determination of relevant chemical-physical properties of molecules such as their lipophilicity [85]. All these systems, along with others, are reviewed in a sister review in this special issue and represent and important step forward toward the general applicability of this selfassembling approach.…”

Smoothly shifting fluorescent windows: a tunable “off-on-off” micellar sensor for pH

“…12 In particular, in a non-ionic surfactant such as TX100, the observed pKa of protonated amines and pyridines is shifted to lower values, due to the poor solvation of the protonated forms inside the micellar containers and/or to the lowered H 3 O + local concentration. 13 In our paper we were able to obtain systems with the ''on'' window in the 2-5 pH range. We have also demonstrated that a certain control on the ''on'' window position may be obtained by changing the intrinsic pKa of the protonated moieties (i.e., the pKa in bulk water) by modifying the molecular structure during synthesis.…”

An Off–On–Off Fluorescent Sensor for pH Windows Based on the 13aneN4–Zn2+ System

“…The observed trend likely depends on the different combination of opposite contributions from the solvent and from the 1,8‐naphthalimide‐containing arm. As regards the solvent, the slightly decreased hydrophilicity of the environment with respect to water is expected to depress the protonation constants . On the other hand, the 1,8‐naphthalimide‐containing arm might stabilize some of the protonated amino groups through intramolecular H bonds with the oxygen atoms of the imide moiety.…”