(Invited) Ligand-Functionalised Conducting Polymer Films for Analysis of Metal Ions in Solution
Authors: Hillman A. Robert , Mohammed Mohammed 
Published: July 2021

Innumerable technological devices and applications utilise metals and metal ions that ultimately find their way into the environment. While many metals are required at trace level for good health, their presence at higher levels is generally detrimental to human health and to the environment at large. For example, higher levels of copper are associated with liver and kidney disease; of nickel with cancer, cardiovascular disease, neurological conditions and infant developmental disorders; and of cobalt with respiratory conditions. This motivates the need to detect and determine these species at trace level; a related goal (not addressed directly here) is their extraction for the purposes of remediation. Although numerous analytical techniques exist for the determination of metal ions, electrochemically-based devices have a number of advantages, including portability for remote application. This work explores underpinning fundamental electrochemistry and interfacial characterisation associated with the fabrication and performance of carboxylate ligand-functionalised conducting polymer films as metal ion sensing moieties immobilised on electrode surfaces.

Typical chelating ligands for metal ions are relatively bulky compared to the electropolymerizable monomer units, for example based on pyrrole or thiophene, commonly used to generate electroactive polymer films. As a consequence, electropolymerization of ligand-functionalised monomers is not generally effective: steric hindrance simply impedes coupling of the monomer units. We therefore adopt the alternative approach of electropolymerizing monomers functionalised with relatively small labile groups, followed by hydrolysis of the labile groups and re-functionalisation with the desired ligand. Here we explore this strategy for pyrrole-, thiophene- and aniline-based monomers functionalised with 9-fluorenylmethoxycarbonyl (fmoc) or pentafluorophenyl groups via ester linkages. Following electrochemically controlled polymerization, the resulting polymer films were treated with base to remove the labile groups, then with Nα,Nα-bis-(carboxymethyl)-L-lysine to couple N-nitrilotriacetic acid functionalities via amide formation.

The polymerization process and the subsequent electroactivity of each of the parent films were studied using voltammetric and nanogravimetric (QCM) techniques. The ultimate effectiveness of the surface synthetic sequence, effected by post-deposition functionalisation with the ligand species, was validated using FTIR spectroscopy. This was accomplished by monitoring the bands associated with the carbonyl functionality: this is initially part of an ester, then an acid and finally an amide, each of which has a characteristic frequency. In the case of initially pentafluorophenyl-functionalised monomers, the hydrolysis of this group is associated with the loss of the C-F functionality. Measurements as a function of time permit the dynamics of these processes to be followed. Additionally, nanogravimetric measurements allow the time course of as-deposited polymer hydrolysis and subsequent functionalisation to be followed and quantitatively compared with the reaction stoichiometry.

The effectiveness of these ligand-functionalised films was explored in terms of their ability to take up nickel, cobalt and copper ions from aqueous solution. Coulometric and gravimetric (QCM) data at various points in the process and as a function of solution metal ion concentration permit the acquisition of isotherms for metal ion uptake. At high metal ion concentration, the ligand sites are saturated; this permits metal:ligand stoichiometry to be evaluated. The relative merits of Langmuir, Frumkin, Temkin and Freundlich isotherms are discussed and the viability of the films for metal ion sensing evaluated. Unsurprisingly, the Langmuir isotherm provides a relatively poor description of the variation of metal ion uptake with concentration; this is attributed to the rather simplistic assumptions of this model. Of the remaining models, the Frumkin isotherm provides the best description of metal ion uptake for all systems considered. The binding constants evaluated from the polymer isotherms are significantly smaller than for the same ligand in solution; this is presumably a consequence of steric effects in the surface-confined polymer environment. Of relevance to potential applications, the films can be regenerated, i.e. restored to metal-free state, by exposure to EDTA solution.