Maryanne M. Collinson

Exciting new directions in the intersection of functionalized sol–gel materials with electrochemistry

The implications of organically-modified silica-based materials in electrochemical science is reviewed along with some selected recent trends in the field of functionalized and sol–gel silica electrochemistry. These recent trends include the electro-assisted generation of organosilica films on solid electrode surfaces, the preparation and applications of sol–gel derived composite (carbon, gold, nanotubes) electrodes, the electrochemical characterisation of mass transfer reactions in porous functionalized silicas, solid-state electrochemistry and gas sensors involving sol–gel materials, imprinted functionalized silica, and the electrochemical characterisation and applications of ordered mesoporous organosilicas.

Recent trends in analytical applications of organically modified silicate materials

Organically modified silicate (ORMOSIL) materials prepared by the sol-gel process have become an attractive field of study due to the versatility and flexibility associated with this method of preparation. In this review, the development of analytical applications involving ORMOSILs for the specific case where the individual precursors are covalently bound is described. A brief explanation of sol-gel processing is given followed by applications in the areas of chemical sensing, ion-exchange coatings, chromatography, and templated materials. A specific focus of this article will be on material published in the last 5 years.

Analytical applications of organically modified silicates

Inorganic-organic hybrid materials prepared by the sol-gel process have become an attractive field of study due to the immense versatility associated with this method of composite material preparation. The blending of inorganic precursors with organosilicon reagents enables unique materials to be fabricated with the desired chemical and physical characteristics. The ability to control the interfacial polarity, the degree of porosity, and the chemical functionality in the matrix has been shown to be a powerful tool in the design of materials for sensor, optical, chromatographic, and catalytic applications. In this review, the preparation of the organically modified silicates (ORMOSILs) where the individual precursors are covalently bound to each other is discussed and selected examples of their potential usefulness in analytical applications is presented.

Electrochemical Properties of Nanostructured Porous Gold Electrodes in Biofouling Solutions

The effect of electrode porosity on the electrochemical response of redox active molecules (potassium ferricyanide, ruthenium(III) hexammine, and ferrocene methanol) in the presence of bovine serum albumin or fibrinogen was studied at macroporous (pore diameter: 1200 nm), hierarchical (1200/60 nm), and nanoporous (<50 nm) gold. These electrodes were prepared using standard templating or dealloying techniques, and cyclic voltammetry (CV) was utilized to evaluate the effect of protein adsorption on the electron transfer of the diffusing redox probes. Following exposure to albumin (or fibrinogen) under near neutral pH conditions, planar gold electrodes showed an immediate reduction in Faradaic peak current and increase in peak splitting for potassium ferricyanide. The rate at which the CV curves changed was highly dependent on the morphology of the electrode. For example, the time required for the Faradaic current to drop to one-half of its original value was 3, 12, and 38 min for planar gold, macroporous gold, and hierarchical gold, respectively. Remarkably, for nanoporous gold, only a few percent drop in the peak Faradaic current was observed after an hour in solution. A similar suppression in the voltammetry at planar gold was also noted for ruthenium hexammine at pH 3 after exposure to albumin for several hours. At nanoporous gold, no significant loss in response was observed. The order of performance of the electrodes as judged by their ability to efficiently transfer electrons in the presence of biofouling agents tracked porosity with the electrode having the smallest pore size and largest surface area, providing near ideal results. Nanoporous gold electrodes when immersed in serum or heparinized blood containing potassium ferricyanide showed ideal voltammetry while significant fouling was evident in the electrochemical response at planar gold. The small nanopores in this 3D open framework are believed to restrict the transport of large biomolecules, thus minimizing passivation of the inner surfaces while permitting access to small redox probes to efficiently exchange electrons.

Permselectivities and ion-exchange properties of organically modified sol-gel electrodes

Abstract Sol—gel films prepared from organosilanes containing acidic/basic sites have been investigated as permselective and ion-exchange coatings for electroanalytical and bioanalytical investigations. When a glassy carbon electrode was modified with a sol—gel film fabricated from 3-aminopropyl-methyl-diethoxysilane (silane—NH 2 ), excellent permselectivity and anion-exchange properties were obtained. For a pH 7.4, 1 mM potassium ferricyanide solution, an eight-fold increase in current was observed after the electrode was immersed in solution for 10min whereas complete suppression of the electrochemical response for ruthenium hexaamine and methyl viologen was observed. Sol—gel films fabricated from trimethoxysilylpropyl ethylenediamine triacetic acid (silane—(COOH) 3 ) exhibited nearly complementary behavior. An approximately 10-fold increase in current was observed for pH 7.4, 1 mM ruthenium hexaammine solutions and complete suppression of the potassium ferricyanide response was observed. These ion-exchange properties can be attributed to the strong electrostatic interactions between the acid/basic functional group in the matrix and the highly charged analyte molecules. When glassy carbon electrodes were modified with diethyl-(triethoxysilypropyl) malonate (silane—(COOEt) 2 ), the films did not exhibit distinct ion-exchange properties but rather suppressed the reduction of potassium ferricyanide. The observed permselectivity results from the electron dense carbonyl group and/or hydrolyzed ester functionalities in the film.

Nanoporous Gold Electrodes and Their Applications in Analytical Chemistry

Nanoporous gold prepared by dealloying Au:Ag alloys has recently become an attractive material in the field of analytical chemistry. This conductive material has an open, 3D porous framework consisting of nanosized pores and ligaments with surface areas that are 10s to 100s of times larger than planar gold of an equivalent geometric area. The high surface area coupled with an open pore network makes nanoporous gold an ideal support for the development of chemical sensors. Important attributes include conductivity, high surface area, ease of preparation and modification, tunable pore size, and a bicontinuous open pore network. In this paper, the fabrication, characterization, and applications of nanoporous gold in chemical sensing are reviewed specifically as they relate to the development of immunosensors, enzyme-based biosensors, DNA sensors, Raman sensors, and small molecule sensors.

Conducting polymer-silk biocomposites for flexible and biodegradable electrochemical sensors.

UNLABELLED Approaches to form flexible biosensors require strategies to tune materials for various biomedical applications. We report a facile approach using photolithography to fabricate poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) ( PEDOT PSS) sensors on a fully biodegradable and flexible silk protein fibroin support. A benchtop photolithographic setup is used to fabricate high fidelity and high resolution PEDOT PSS microstructures over a large (cm) area using only water as the solvent. Using the conductive micropatterns as working electrodes, we demonstrate biosensors with excellent electrochemical activity and stability over a number of days. The fabricated biosensors display excellent nonspecific detection of dopamine and ascorbic acid with high sensitivity. These devices are mechanically flexible, optically transparent, electroactive, cytocompatible and biodegradable. The benign fabrication protocol allows the conducting ink to function as a matrix for enzymes as shown by a highly sensitive detection of glucose. These sensors can retain their properties under repeated mechanical deformations, but are completely degradable under enzymatic action. The reported technique is scalable and can be used to develop sensitive, robust, and inexpensive biosensors with controllable biodegradability, leading to applications in transient or implantable bioelectronics and optoelectronics.

Diffusion Coefficients of Redox Probes Encapsulated within Sol−Gel Derived Silica Monoliths Measured with Ultramicroelectrodes

The diffusional mobilities of potassium ferricyanide (Fe(CN)63-) and ferrocene methanol (FcCH2OH) encapsulated within silica monoliths have been investigated using cyclic voltammetry and chronoamperometry at an ultramicroelectrode. In this work, a 13-μm-radius Pt microdisk working electrode and a silver chloride coated silver wire (r = 0.5 mm) reference electrode were inserted into doped silica sols prepared by the sol−gel process. The resultant gels were aged and slowly dried under a relative humidity atmosphere of 60−70% to minimize gel cracking. Fast-scan voltammetry (up to 100 V/s) confirmed that the gel−electrode interface remained intact throughout the duration of the drying period (typically 20−50 days). During this time frame, an ca. 30% loss in mass and ca. 50% reduction in volume of the gels were observed. The diffusion coefficients of gel-encapsulated Fe(CN)63- and FcCH2OH were measured without prior knowledge of dopant concentration via normalization of the chronoamperometric response with the...

The effects of drying time and relative humidity on the stability of sol-gel derived silicate films in solution

The stability of thin silicate films in solution has been evaluated as a function of drying time and drying conditions using scanning probe microscopy and an electrochemical probe technique. In these experiments, the silicate films were spin coated on various substrates using a sol formed by the acid catalyzed hydrolysis and condensation of tetramethoxysilane. The silicate films were then dried under a relative humidity of 25, 50, or 75% for 3, 12, 24, or 48 h. After drying, the films were immediately placed in a solution of potassium nitrate or a solution of a redox probe, specifically ferrocene methanol, ruthenium hexaammine, or cobalt phenanthroline. Cyclic voltammetry in conjunction with atomic force microscopy was used to monitor the disruption in gel structure as the films sat in aqueous solution. The stability of each film was found to be essentially independent of the substrate it was coated on but a strong function of the length of the drying time and the relative humidity under which it was dried. Films that were dried at 25% humidity for 48 h stayed on the electrode for several days. In contrast, films that were dried at 75% humidity for 3 h fell off the electrode within 24 h.

Electrodeposited silicate films: importance of supporting electrolyte.

Silica and hybrid organic-inorganic films, ca. 100-200 nm thick, can be grown on glassy carbon electrodes through reactions initiated by electrogenerated hydroxide or hydronium ions in water under reductive and oxidative conditions, respectively. A variety of different alkoxysilanes (tetramethoxysilane and organoalkoxysilanes) and supporting electrolytes were used to evaluate whether film formation takes place on glassy carbon electrodes. The results of the study indicate that the acid-base properties of the supporting electrolyte are an important factor in determining whether film formation will take place. For cathodic electrodeposition, thin films can be formed using supporting electrolytes that are close to neutral, such as KCl, KNO3, and NaClO4. For anodic electrodeposition, thin films can be formed using supporting electrolytes that are acidic, such as, KH2PO4, HNO3, H2SO4, etc. The acidity/basicity effects of the electrolytes arise in part from the strong dependence of the hydrolysis and condensation rates of the silicon alkoxide precursors on pH.