Spectroelectrochemistry with Ultrathin lon-Selective Membranes

Sammanfattning: Rapid and decentralized chemical sensing strategies are highly demanded in contemporary society to monitor ongoing processes relevant to the environment, food analysis, healthcare, sports performance, etc. In this context, ion-selective electrodes (ISEs) based on polymeric membranes have emerged as a promising analytical technique owing to their features of low cost, portability, versatility, and energy efficiency. Despite their undeniable success in potentiometric sensing, the interrogation of such electrodes under dynamic electrochemical techniques opens new horizons. An interesting example is voltammetric ISEs comprising the tandem ultrathin membranes and poly(3-octylthiophene) (POT) as the redox mediator. In these ISEs, ion transfer (IT) processes at the sample-membrane interface can be modulated by the electron transfer (ET) at the underlaying POT film, presenting an interconnected IT-ET system that is promising for various analytical purposes.Conveniently, the UV-Vis absorption spectra of a POT film present clear differences between its oxidized and reduced forms. Accordingly, spectroelectrochemical properties of POT can be dynamically monitored to track the ET in the film, and ultimately the associated IT processes, in voltammetric ISEs. Effectively, the obtained information is valuable from both a theoretical and analytical application point of views. In this regard, this doctoral project focuses on the spectroelectrochemical study of voltammetric ISEs based on ultrathin ion-selective membranes interconnected with POT films.The first chapter provides a general introduction to the concepts involved in this thesis, emphasizing the all-solid-state ISEs, the development of ultrathin membranes working under voltammetric mode, and UV-Vis spectroelectrochemistry. The second chapter describes the experimental details of the thesis. The third chapter, which is composed of four sections, presents the main results of this thesis and the corresponding discussions.More in detail, the first section reports the utilization of spectroelectrochemistry to characterize the correlation between IT and ET processes in voltammetric ISEs. The dynamic absorbance readout unequivocally corresponds to the ET process resulting along the gradual oxidation of POT. The electrochemical signal, whichinvolves dynamically integrated charge, describes in turn the IT process at the membrane-solution interface. The two processes are proved to be totally interconnected, both exhibiting sigmoidal-shaped features that can be described with the mathematical Boltzmann-Sigmoidal model.The second section presents the visualization of ionophore-assisted and nonassisted IT processes along the same voltammetric scan of a voltammetric ISE, using a series of ultrathin membranes rationalized to produce assisted and nonassisted ITs at different degrees. Essentially, the modification of the ionophore/ion exchanger molar ratio in the membrane tunes the nature of the IT peaks (for the analyte binding with ionophore or not). This derives into the easy calculation of important thermodynamic parameters (e.g., selectivity coefficients and binding constants) based on a semi-empirical approach.The third section investigates the analytical application of the interconnected IT-ET processes based on spectroelectrochemistry. Advantageously, by changing the initial accumulation protocol of the analyte into the membrane, three distinct working ranges at millimolar, micromolar, and nanomolar concentration levels can be realized by the same ISE using absorbance-based readout. Notably, this is a very unique performance for an analytical technique.The fourth section explores the possibility of calibration-free sensing based on theinterconnected IT-ET mechanism. Effectively, introducing a thin-layer sample makes it feasible to achieve complete transport of a cation (e.g., potassium) from the sample to the membrane and vice versa through a cyclic voltammetryinterrogation. Importantly, the charge under the IT peak can be directly utilized for calculating the concentration in the sample, precisely knowing the volume in the developed microfluidic cell. This investigation has demonstrated the huge potential of the developed ISEs (integrated in a microfluidic format) towards the realization of calibration-free analytical determinations.