Synthesis of hybrid silica-organic materials for the development of electrochemical biosensing applications

Resumen

INTRODUCTION In recent years, the importance of monitoring and controlling many different parameters in fields such as clinical diagnoses, the food industry, the environment, forensic analysis or drug development has been increasing. A sensor can be defined, in a very general way, as a device that responds to a signal or stimulus. A stimulus can be understood as any quantity, property or condition. For the most part, modern sensors are not independent devices. They are part of larger systems that can incorporate other detectors, conditioners and signal processors, memory devices, data recorders and transducers. Consequently, the sensors must provide a signal that is readable by all other elements of the system in which they are incorporated. Therefore, in most artificial systems, information about the stimulus, first transmitted by the sensor response, is processed and transmitted. as an electrical signal: a voltage, a current or a load. This electrical signal can be described in greater detail in terms of amplitude, frequency and / or phase [1,2]. A chemical sensor is an autonomous analytical device that can provide information about the chemical composition of its environment, that is, a liquid or gas phase [3]. The information is provided in the form of a measurable physical signal that correlates with the concentration of a certain chemical species (called analyte). THEORETICAL DEVELOPMENT An electrochemical sensor is a device that transforms electrochemical information into a useful analytical signal. An electrochemical sensor is normally composed of a working electrode, a reference electrode and a counter electrode connected to a potentiostat / electroplating. The working electrode acts as a receiver and is also a component of the transducer. Electrochemical detection is a promising analytical field and has found its applicability in the fields of energy, health, the environment, the food and pharmaceutical industry [4]. Electrochemical detection leads to the development of chemical sensors and biosensors. The chemical sensor provides analytical information about a particular amount of certain chemical species in the surrounding environment. A biosensor is an integrated device that provides quantitative and semi-quantitative analytical information through the use of a biological recognition element that is in direct spatial contact with a transduction element [5]. Electrochemical sensors have certain advantages that include their profitability, applicability to a wide range of chemicals, ease of use and functionalization, robust nature, high sensitivity and selectivity. In this thesis a study of the modification of electrodes by thin films of silica using the sol-gel method is carried out. Sol-gel chemistry offers a flexible approach to obtain a wide range of materials. It allows to achieve different chemicals and offers the ability to produce a wide range of nano / microstructures [6]. Silica exists in a wide variety of forms, with crystalline and amorphous structure. It is a material that has been thoroughly examined [7], Its physical and chemical properties are well known [8], which gives it an extraordinary number of applications, such as its use in chromatography, thermal insulation, catalysis and as a support for catalysts, It is used in the reinforcement of polymers and as a support to immobilize enzymes, among others. Silica can be modified with a wide variety of functional groups, which leads to considerable enrichment and control of its surface properties. For example, these properties have been exploited largely in chromatography, designing new stationary phases. In addition, the large surface area combined with the chemical properties of the surface makes silica an excellent material to use as a catalyst support. Despite all these attractive properties, the use of silica in electrochemical methods was not widespread. Silica gels prepared by sol-gel chemistry are interesting materials for electrode modification, and since they are not electroactive, they can be used as support for electroactive species, either by adsorption or entrapment during their formation, thus improving their amperometric detection [9–14]; The most attractive properties of silica materials are that they have a great capacity to accumulate several analytes by adsorption, silica can be modified with organic groups in a simple way and these modifications involve the development of a wide range of new materials. In addition, these materials serve as support for the immobilization of enzymes in the manufacture of biosensors. But the fundamental reason for the use of sol-gel materials in electrochemistry is the existence of multiple ways to combine the properties of inorganic materials with a wide variety of organic compounds through ORMOSIL. Electrochemical synthesis is a powerful technique for the preparation of various compounds due to their ability to control chemical and electrical parameters that affect the general reaction [15]. Electrochemical methods can contribute significantly to the protection of the environment through minimization of the production of waste and toxic materials. Therefore, electrochemistry is an ecological method for synthetic applications [16]. Thin silica films are traditionally formed through rotation coating, immersion coating or spray coating of a sun on a flat surface [8,17,18]. In 1999, Mandler and his collaborators demonstrated that it is possible to use the electrodeposit to create thin films of silica based on sol-gel on a conductive surface [19]. In this case, the film was prepared from a prehydrolyzed methyltrimethoxysilane sol. Hydroxide ions were formed by applying a sufficiently negative potential to form methylated films that vary in thickness from nanometers to micrometers. In 2003, Collinson and his colleagues used electrodeposition to create thin silica films on glassy carbon electrodes from soles prepared solely from tetramethoxysilane [20]. CONCLUSION AND RESULTS -Poly (3,4-ethylenedioxythiophene) doped with poly-(styrenesulfonate), PEDOT–PSS, films were synthesized by electrochemical methods from aqueous solution. The polymer thickness can be controlled by the potentiodynamic synthesis. Films synthesised deposited over a carbon support present smooth morphology due to the favorable interaction with the polymer. -PEDOT-PSS thin films present a styrenesulfonate-to-EDOT ratio around 4, indicating that the overall charge of the film is negative. As polymer becomes thicker a progressive depletion of polyelectrolyte is observed. -The electrocatalytic properties of PEDOT-PSS were studied against ferrocene redox probe. The electrochemical response of ferrocene can be clearly observed in thin polymer films. The heterogeneous constant for this transfer ranges between 2 and 6×10-3 cms-1. The electrons transfer to the ferrocene seems progressively hindered as thicker the PEDOT-PSS films. -Conventional Silica and organically modified silica (ORMOSIL) monoliths were synthesized by sol-gel methodology, making use of tetraethylorthosilicate and alkyltriethoxysilane precursors. -Silica monoliths were characterized by thermogravimetric analyses (TG), in all cases we detect in silica gel the presence of ethanol molecules that remained in the pores of silica coming from the precursor solution. -TG and FTIR analyses allow the quantification of the organic group incorporated in ORMOSIL monoliths. In general terms as higher the concentration of alkylsilicate precursor higher the concentration of organic group in the silica monolith, except in the case of methyl groups. -The electrochemical behaviour of p-aminophenol encapsulated in conventional silica and ORMOSIL monoliths were performed. The electrochemical response of p-aminophenol encapsulated in conventional silica is more irreversible than in solution, and the diffusion coefficient of p-aminophenol encapsulate in silica is near four times higher than in solution. -The electrochemical behaviour of ferro/ferricyanide encapsulated in conventional silica and ORMOSIL monoliths were performed. The electrochemical response of ferro/ferricyanide encapsulated in conventional silica is more reversible than in solution, the diffusion coefficient for silica of ferro/ferricyanide is slightly smaller than the value in solution. -Alkaline phosphatase was encapsulated in conventional silica monolith to develop a prototype of biosensor. 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