Prasad V. A. Pamidi

Screen-Printable Sol−Gel Enzyme-Containing Carbon Inks

Enzymes usually cannot withstand the high-temperature curing associated with the thick-film fabrication process and require a separate immobilization step in connection with the production of single-use biosensors. We report on the development of sol-gel-derived enzyme-containing carbon inks that display compatibility with the screen-printing process. Such coupling of sol-gel and thick-film technologies offers a one-step fabrication of disposable enzyme electrodes, as it obviates the need for thermal curing. The enzyme-containing sol-gel carbon ink, prepared by dispersing the biocatalyst, along with the graphite powder and a binder, within the sol-gel precursors, is cured very rapidly (10 min) at low temperature (4 °C). The influence of the ink preparation conditions is explored, and the sensor performance is evaluated in connection with the incorporation of glucose oxidase or horseradish peroxidase. The resulting strips are stable for at least 3 months. Such sol-gel-derived carbon inks should serve as hosts for other heat-sensitive biomaterials in connection with the microfabrication of various thick-film biosensors.

Sol−Gel-Derived Gold Composite Electrodes

The preparation, characterization, and analytical utility of sol-gel-derived gold composite electrodes is described. The new metal-ceramic electrodes are comprised of gold powder homogeneously dispersed in a modified silica matrix. They couple the favorable electron-transfer kinetics common to gold surfaces with the regeneration, bulk modification, and versatility features of sol-gel-derived composite materials. The voltammetric characteristics of the composite gold-silica electrodes are explored and compared with conventional gold electrodes. Sol-gel-derived gold biosensors have been prepared by incorporating an oxidase enzyme within the sol-gel gold solution. Analogous thick-film enzyme strips, based on a new screen-printable gold biogel ink, have also been fabricated. To our knowledge, the above represent the first examples of metal-ceramic sensing electrodes and of bulk modification of metallic working electrodes.

Tailoring the macroporosity and performance of sol-gel derived carbon composite glucose sensors

Control of the preparation conditions is used to enhance the performance of sol-gel derived carbon composite enzyme electrodes. In particular, changes in the water content during the acid-catalyzed hydrolysis strongly affect the macroporosity of the resulting biogel network. Hence, while for the low-water process the response is controlled by the enzyme kinetics, high-water preparations lead to mass-transport restrictions. Such diffusional limitations within the electrode interior result in an extended linear dynamic range (without the need for external membranes). Scanning electron microscopy provides images of biogel microstructures, accrued from the use of different water contents. The ceramic-carbon biosensors offer improved retention of artificial electron acceptors compared to conventional carbon composite bioelectrodes. The silica-containing surface also displays an intrinsic electrocatalytic hydrogen peroxide response, and hence offers a low-potential monitoring of the glucose substrate.

Sol-gel carbon composite electrode as an amperometric detector for liquid chromatography.

The performance characteristics of an electrochemical detector for liquid chromatography based on a sol-gel carbon composite working electrode in a wall-jet configuration are described. The new detector combines the versatility of sol-gel processes with several favorable characteristics, including fast electron-transfer kinetics, mechanical rigidity and renewability. Factors influencing the amperometric response are explored and optimized. Detection limits of 58-170 pg are reported for various neurotransmitters. Repetitive injections yield peak heights with relative standard deviations of 2.6-3.7%. The prospects of using sol-gel derived electrochemical detectors are discussed.

Overoxidized poly{pyrrole-co-[3-(pyrrol-1-yl)-propanesulfonate]}coated platinum electrodes for selective detection of catecholamine neurotransmitters

The cation-exchange and anion-exclusion properties of the poly¿pyrrole-co-[3-(pyrrol-1-yl)propanesulfonate]¿-(PPy-PS) copolymer are exploited for imparting higher selectivity to measurements of primary neurotransmitters in the presence of ascorbic acid. Such incorporation of ionizable sulfonated groups in the pyrrole ring prior to its electropolymerization leads to effective rejection of the anionic ascorbate species and preferential collection of the cationic dopamine and norepinephrine. Overoxidized PPy-PS films thus offer better discrimination against ascorbic acid than Nafion or overoxidized polypyrrole coatings. Experimental variables influencing the permselective behavior of the PPy-PS layer, including the electropolymerization time and solution pH, were explored. The selectivity and sensitivity improvements associated with the increased electrostatic character of overoxidized polypyrrole films hold promise for neurochemical electrochemical studies.

Metal-dispersed porous carbon films as electrocatalytic sensors

The attractive features of ultrathin porous carbon films have been coupled with the efficient catalytic action of dispersed metal particles. In particular, loading of these submicrometer foams with ruthenium or platinum centers offers a dramatic increase in the electron transfer rates of important redox systems, such as NADH, uric acid, ascorbic acid, acetaminophen, hydrazine or hydrogen peroxide. Characterization of the electrocatalytic behavior (with respect to the pH, scan rate or metal loading) and the attractive low-potential analytical (sensing) performance are reported. Scanning electron microscopy sheds useful insights into the distribution of the metals within the porous electrode matrix.

β-Cyclodextrin cation exchange polymer membrane for improved second-generation glucose biosensors

Abstract Both condensation β-cyclodextrin polymer (β-CDP) and condensation carboxymethylated β -cyclodextrin polymer (β-CDPA) were used for preparation of membranes for amperometric glucose biosensors. Glucose oxidase (GOD) was covalently immobilized in the membranes and the tetrathiafulvalenium/tetrathiafulvalene (TTF + /TTF) mediating couple was retained in the β-CDP membrane due to supramolecular complex formation while in the β-CDPA one due to supramolecular complex formation as well as ion exchange (by the pending carboxymethyl groups). In the latter case, retention of the mediator was greatly improved, leading to a superior biosensor performance. This performance was tested in phosphate buffer pH 7.4 with respect to the optimum GOD and TTF loadings as well as the membrane thickness. Under the optimum conditions, i.e., at the 90 units GOD and 0.1 mg TTF loadings and ca. 45 μm membrane thickness, the electrode detectability, sensitivity and response time towards glucose were 0.2 mM, 2.54 μA ml −1 and 25.5 s, respectively. The (β-CDPA)-GOD-TTF biosensor displays excellent selectivity towards glucose in the presence of commonly interfering substances, such as ascorbic acid, uric acid and acetaminophen. The (β-CDPA)-GOD-TTF preparation strategy was employed for fabrication of glucose biosensors based on a disposable screen-printed Ag-carbon strip two-electrode transducer.

Electrocatalysis and amperometric detection of aliphatic aldehydes at platinum-palladium alloy coated glassy carbon electrode

Abstract Aliphatic aldehydes undergo an electrocatalytic oxidation at a platinum-palladium (Pt-Pd) alloy coated glassy carbon electrode. The alloy modification offers a highly sensitive and stable, constant (+0.35 V) potential detection of simple aldehydes. The drastically enhanced catalytic response of the alloy deposit, compared to the single component metals, is attributed to synergistic effect associated with changes in the adsorption features of the surface. The influence of the alloy deposition conditions upon the amperometric response is assessed. Scanning electron microscopy, energy dispersive X-ray analysis and elemental distribution mapping offer useful insights into the microstructure and composition of the alloy deposit. Formaldehyde, acetaldehyde and propionaldehyde were detected in flow-injection analysis at levels as low as 0.9, 6.4 and 6.5 ng (30, 160 and 120 pmol), respectively. Such operation offers lower operating potentials and detection limits compared to the recently developed mixed-valent ruthenium coated detector for aldehydes [2].

Disposable screen-printed electrodes for monitoring hydrazines

Disposable amperometric sensors for hydrazines have been fabricated by a judicious tailoring of the surface of screen-printed electrodes. Strong electrocatalytic action towards the oxidation of hydrazines is achieved by incorporating cobalt phthalocyanine within the carbon inks or by covering the printed surface with a mixed valent ruthenium cyanide coating. The electrocatalytic behavior, sensor optimization and analytical performance are reported. The new sensor strips should facilitate on-site environmental and industrial monitoring of hydrazine compounds.

Microelectrode ensembles based on overlaying ultrathin porous polyacrylonitrile on dense carbon films

A new procedure for preparing microelectrode ensembles, based on casting an ultrathin microcellular polyacrylonitrile (PAN) insulating foam onto a dense carbonized PAN film, is described. The resulting partially blocked surfaces display the behavioral characteristics of ensembles of recessed microelectrodes (of small depth), with steady-state currents at moderate and long timescales. Cyclic voltammetry and chronoamperometry were used to elucidate the electrochemical behavior, while scanning electron microscopy sheds useful light on the surface microstructure. Square-wave stripping voltammetry is demonstrated for the quantification of nanomolar concentrations of lead using short deposition periods and unstirred solutions. These and other PAN-derived composite electrodes present new opportunities for various electrochemical and analytical applications.