Manuela Adami

Nano-assembly of glucose oxidase on the in situ self-assembled films of polypyrrole and its optical, surface and electrochemical characterizations

An in situ self-assembly technique with nanometre control over thicknesses and multilayered structures was used to manufacture the ultrathin films of polypyrrole (PPY). The PPY film was deposited on a polyanion, poly(styrene sulfonate) (PSS) modified surface as a function of time, previously PSS had been deposited on various substrates (glass, mica, indium-tin-oxide coated glass plates). Later, alternate PPY and PSS films were fabricated on such substrates using a layer-by-layer (LBL) technique. The glucose oxidase (GOD) molecules were also deposited on such self-assembled PPY films by the LBL technique. The PSS/PPY/GOD, PPY/PPY/GOD/poly(ethylene imine) (PEI)/GOD etc, film configurations were fabricated, and functionally as well as structurally characterized by UV-visible, electrochemical and scanning probe microscopy techniques, respectively. The results of a scanning probe microscopic study on the films were analysed to understand the molecular orientation of GOD on the PPY surface, and the dependence of the enzyme concentration for the depositing solutions. Moreover, the electrochemical studies performed provided information with respect to the electron transfer processes on the spatial arrangement of GOD molecules on the PPY surfaces. The immobilization of GOD on a conductive PPY represented a crucial and important step, and allowed us to construct the glucose-responsive biosensors.

Minority carrier diffusion length effects on light-addressable potentiometric sensor (LAPS) devices

Abstract Alternating photocurrent measurements with light-addressable potentiometric sensors (LAPSs) have been used to monitor pH, redox potential, and ionic concentrations at discrete locations in an electrolyte in contact with LAPS devices. We report here the results of AC photocurrent measurements with insulated semiconductor LAPS devices where the semiconductor either is illuminated through the insulator (frontside) or alternatively from the opposite side (backside). Such comparative AC photocurrent measurements were made with semiconductors of varied thickness, at varied frequency of light intensity modulation, and at several different photoexcitation wavelengths. The results are fit to a theoretical expression which predicts the dependence of photocurrent on modulation frequency, wafer thickness, bulk minority carrier lifetime, and surface recombination velocity. The results are useful to optimize the design of LAPS devices with regard to these parameters. The results also predict optimal conditions for minimal lateral spacing of adjacent sensing areas in LAPS devices.

Comparison between a LAPS and an FET-based sensor for cell-metabolism detection

Several silicon-based biosensors have been developed for various applications, such as enzymatic and immunological activity determination and cell-metabolism detection. This work describes an electrochemical characterization of ion-sensitive field-effect transistor (ISFET) and light-addressable potentiometric sensor (LAPS) transducers and a test of such devices as detectors of 3T6 (Swiss albino mouse embryo, fibroblasts) metabolism. In particular, our investigation is devoted to some fundamental parameters of these transducers (e.g., pH sensitivity, drift, temperature dependence of the output signal, speed of response) for the purpose of comparing their performance related to cell-metabolism evaluation. The final goal is therefore to analyze the capabilities of these silicon-based transducers for use in a biosensing system for cell-metabolism detection.

Investigation of carrier transport through silicon wafers by photocurrent measurements

Measurement of the ac photocurrent in metal/insulator/semiconductor capacitors can be used as a tool to measure minority‐carrier diffusion and lifetime. The amplitude of the ac photocurrent generated at a silicon surface biased into inversion depends on the number of excess minority carriers present at that surface. By comparing this amplitude when intensity‐modulated light is directed to each side of the same device, minority‐carrier diffusion from the back to the front of the device can be characterized. An analytical model of this transport process predicts the dependence of the ac photocurrent on frequency and wafer thickness, and allows the determination of a value of the bulk lifetime free of the influence of surface recombination. Measurements under low‐light intensity levels are presented on n‐type silicon wafers with lifetimes in the 10–100 μs range. Lifetimes are found about a factor of 2 lower than those measured with noncontact photoconductive decay, at high‐light intensity levels. This is exp...

Comparative studies on Langmuir-Schaefer films of polyanilines

Abstract Langmuir isotherms of polyaniline (PANI), poly( o -toluidine) (POT), poly( o -anisidine) (POAS) and poly( o -ethoxy aniline) (PEOA) were investigated at aqueous subphase of pH 1, where doping during monolayer formation appeared as an essential step for high quality of the film. The effect of substituent groups in polyanilines plays a prominent role for the formation of Langmuir films. The area per unit repeat molecule was shown to increase by an increment of the substituent groups in polyanilines. Ultra-thin films of PANI, POT, POAS and PEOA were engineered by Langmuir–Schaefer (LS) technique. The uniformity of the deposited polyanilines LS films was verified by atomic force microscopy (AFM). The electrochemical properties of polyanilines LS films were investigated by cyclic voltammetry and current transient measurements, and the electrical characteristics were investigated by depositing the films on interdigitated electrodes. The electrochromic switching response time and diffusion coefficient of such LS films were also estimated by electrochemical surveyings.

Prototypes of Newly Conceived Inorganic and Biological Sensors for Health and Environmental Applications

This paper describes the optimal implementation of three newly conceived sensors for both health and environmental applications, utilizing a wide range of detection methods and complex nanocomposites. The first one is inorganic and based on matrices of calcium oxide, the second is based on protein arrays and a third one is based on Langmuir-Blodgett laccase multi-layers. Special attention was paid to detecting substances significant to the environment (such as carbon dioxide) and medicine (drug administration, cancer diagnosis and prognosis) by means of amperometric, quartz crystal microbalance with frequency (QCM_F) and quartz crystal microbalance with dissipation monitoring (QCM_D) technologies. The resulting three implemented nanosensors are described here along with proofs of principle and their corresponding applications.

Monitoring of enzymatic activity and quantitative measurements of substrates by means of a newly designed silicon-based potentiometric sensor

Abstract By utilizing a modified light addressable potentiometric sensor (LAPS) technology, we designed a device for investigation of enzymatic reactions. The sensing principle is the detection of pH variations due to the catalyzed reaction. The system is divided into two main parts, a reaction chamber and a measuring chamber which contains the silicon transducer. By monitoring the H+ production, we obtain information either about the substrate concentration (by using a fixed amount of enzyme) or about the enzyme concentration and activity (if the initial substrate concentrations are constant). Theoretical analysis and modeling were carried out in order to optimize the performance of the system and to predict and explain the experimental curves with alcohol dehydrogenase. By using this set-up it was possible to detect enzyme concentrations down to the 10-10 M range.

Biosensors: a step to bioelectronics

Biosensors are analytical devices which combine a biologically sensitive element with a physical or chemical transducer to selectively and quantitatively detect the presence of specific compounds in a given external environment. The interaction of the compound (the analyte) and the biological element generates a signal (electrical, optical or mechanical) which is measured by an optical or electronic system, usually following amplification. Increasingly, biosensors are exploiting state-of-the-art technologies such as protein engineering and advanced microelectronics to offer easy-to-use, widely applicable devices which are compact as well as cheap.

High-sensitivity biosensor based on LB technology and on nanogravimetry

Abstract Quartz-resonator nanobalances are utilized here as effective transducers capable of determining with high sensitivity the mass changes due to specific protein-protein, antigen-antibody and ligand-receptor binding or self-assembly of functional complexes. The resulting highly sensitive biosensor is based on two quartz resonators (one active and the second used as a reference) and on Langmuir-Blodgett (LB) protein monolayers. The electronics are composed of two separate blocks, one designed to acquire by a personal computer the data coming from the other card, a 24-bit digital counter directly connected to the two oscillators. In the case of immunosensor application, the active and reference oscillators are covered respectively by antibodies specific to a given antigen and by antibodies non-specific to the antigen, in order to disciminate the physical adsorption effects. Deposition of antibody monolayers is performed by the LB technique in a surface pressure range of 20–35 mN m−1 onto gluteraldehyde pre-treated quartz resonators. A thermal treatment of the antibody layer up to 150 °C results in the reorganization of the film, and significantly improves the sensitivity and the properties of the immunosensor.

New measuring principle for LAPS devices

Abstract The light-addressable potentiometric sensor (LAPS) has good measuring features, including high stability and linearity, and the capability to address and test different sample areas at the same time. Usual data acquisition and processing steps can be speeded up by the simultaneous utilization of an n- and a p-type silicon wafer in contact with the same electrolyte. By combining the two signals, a new characteristic curve (measured signal versus bias potential) is created, whose minimum value is directly related to the ionic concentration of the solution. Therefore, the latter can be easily monitored by a few hardware circuits. A LAPS-based biosensing device is presented here; it allows faster pH detection and permits the construction of measuring (flow) chambers of lower volumes, at the same time preserving the possibility to address selected areas. The experimental features of the device appear to be in good agreement with the theoretical predictions of an ad hoc physical model.