Sergio Paddeu

Cholesterol biosensors prepared by layer-by-layer technique.

The analysis of formation, deposition and characterization of cholesterol oxidase (COX) layer-by-layer films were performed. Initially, a layer of polyanion, poly(styrene sulfonate) (PSS) was adsorbed followed by a layer of polycation, poly(ethylene imine) (PEI) on each solid substrate from aqueous solutions. The alternating layers were formed by consecutive adsorption of polycations (PEI) and negatively charged proteins (COX) and cholesterol esterase (CE). A strong interaction between protein and polyelectrolyte improves the stability of the alternating multilayer; however, it can change a native protein conformation and impair the protein activity. The PSS/PEI/COX, PSS/PEI/COX/PEI/CE, PSS/PEI/COX-CE/PEI etc. layered structures were prepared on the surface of a platinum electrode, ITO coated glass plate, quartz crystal microbalance, quartz plates, mica and silicon substrates. Optical and gravimetric measurements based on an ultraviolet-visible absorption spectroscopy and a quartz crystal microbalance revealed that the enzyme multilayers thus prepared consist of molecular layered of the proteins. The surface morphology of such bilayer films was investigated by using atomic force microscopy. The electrochemical redox processes of the enzyme-layered films deposited either on platinum or ITO coated glass plate were investigated. The response current of cholesterol oxidase electrode with concentration of cholesterol was investigated at length.

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.

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.

Optical, structural and fluorescence microscopic studies on reduced form of polyaniline : The leucoemeraldine base

Electrochemically deposited thin films of emeraldine salt (ES) on indium-tin-oxide (ITO) glass plate and [111] silicon crystal were treated chemically with a phenyl hydrazine (PNH) solution in order to obtain a leucoemeraldine base (LB) form of polyaniline (PANI). The NMP-treated PANI films were washed using different organic solvents. ES, LB and air-oxidized LB films were examined using UV-visible, optical microscopy, X-ray diffraction and fluorescence microscopic techniques. The obtained results showed a direct correlation in the optical, structural and fluorescence behaviour of the LB form of PANI. An X-ray diffraction study revealed the partial crystallinity in the ES, whereas the LB and the air-oxidized LB showed amorphous structures. The first evidence of high fluorescence microscopic behaviour in the LB form of PANI has been observed. The gradual decrease in fluorescence signal as a function of time for the LB film has been interpreted as being due to the diffusion phenomenon in relation to the contact of the complex of the phenyl ring with O2 molecules.

Nanoassemblies of sulfonated polyaniline multilayers

A self-assembly layer-by-layer (LBL) technique was used for the sequential adsorption of polycation, poly(diallyldimethylammonium chloride) (PDDA) and polyanion, sulfonated polyaniline (SPANI) on glass, indium-tin-oxide (ITO) coated glass plates, polystyrene sulfonate (PSS)/glass and PSS/ITO surfaces, respectively. The building up of such multilayers was characterized by the increment of the adsorbed amount through UV-visible spectroscopy and cyclic voltammetry. The atomic force microscopic study showed the granular surface topology of such self-assembled films of sulfonated polyaniline. PDDA/SPANI LBL films were electrically active and detailed electrochemical parameters were investigated.

A physical insight into the gas-sensing properties of copper (II) tetra-(tert-butyl)-5,10,15,20-tetraazaporphyrin Langmuir–Blodgett films

Abstract The Langmuir isotherm of copper (II) tetra-( tert -butyl)-5,10,15,20-tetraazaporphyrin [CuPaz( t -Bu) 4 ] has been investigated at the air–water interface. The Langmuir monolayer displays two phase regions, which have a phase transition at 7 mN/m. Langmuir–Blodgett (LB) films have been fabricated onto glass slides, interdigitated electrodes and quartz crystal microbalance electrodes, and characterized by UV-VIS spectroscopy, electrical and nanogravimetric measurements, respectively. The CuPaz( t -Bu) 4 LB films were used to quantity some of the volatile gases, i.e. vapors of hexane, benzene and toluene. UV-VIS spectroscopic measurements reveal that there is an interaction between CuPaz( t -Bu) 4 molecules and the above organic solvent molecules, which is manifested by a red shift and broadening of Soret and Q bands. The response and recovery behaviors, sensitivity and selectivity of the LB films to different organic vapors have been investigated by means of nanogravimetric measurements. Such results offer a potential application of such a porphyrin derivative for the detection of aromatic compounds, even at room temperature.

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.

LB-based PAB immunosystem : activity of an immobilized urease monolayer

Abstract A new configuration of the biosensing system named potentiomeric alternating biosensor (PAB) is analyzed. The system is based on an Si/SiO 2 /Si 3 N 4 transducer and allows the monitoring of several biochemical events, such as immunoenzymatic reactions and cell metabolism. the utilization of PAB a immunosensor implies the possibility of using a modified Langmuir-Blodgett (LB) technique for the production of an antibody monolayer. In the first stage the activity of an immobilized LB monolayer of an enzyme (urease) is monitored in order to simulate the final step of the immunoenzymatic assay; subsequently, the enzymatic activity of an immobilized monolayer of antigen (MAb) after incubation with antibodies labelled with urease is monitored. Vacuum silanization with glycidoxypropyltrimethoxysilane (GOPTS) is utilized to ensure covalent binding of the protein LB monolayer to a glass substrate. The analysis with PAB of films thus prepared is preceded by characterization of the layer by means of nanogravimetric and spectrophotometric techniques, which yield useful information on the surface density and activity, respectively. Furthermore, the operative lifetime of the enzyme layer within a period comparable to the standard time of an immunoassay has been studied.

Langmuir-Schaefer films of a poly(o-anisidine) conducting polymer for sensors and displays

Langmuir-Schaefer (LS) films of poly(o-anisidine) (POAS) were fabricated and characterized by means of Brewster-angle microscopy, ellipsometry and electrochemical techniques. The studied optical, cyclic voltammetric and ellipsometric properties of films underlined a regular deposition up to at least 40 monolayers of POAS conducting polymer. The development of surface irregularities beyond 40 monolayers in LS films showed an electrochemical kinetic similar to electrodeposited films. More importantly, the electrochemical kinetic in a small number of monolayers was indicative of the fast transfer process of the electrons. The nature of anions caused meaningful changes in the redox properties of POAS LS films. The electrochromic switching response time and diffusion coefficient of the LS films were estimated through electrochemical surveying. Later, POAS LS films were used as a sensing element for a survey of 0.1 ppm of acid in water through conductimetric measurement.

Effect of annealing on physical properties of conducting poly(ortho-anisidine) Langmuir—Blodgett films

Langmuir—Blodgett (LB) films of poly(ortho-anisidine) (POAS) have been fabricated at pH 1 of the subphase, where the doping during the monolayer formation is essential for high quality of the ultrathin films. POAS LB films have been systematically characterized using low-angle X-ray diffraction, UV—Vis and FTIR spectroscopic, current—voltage and capacitance—frequency measurements, respectively. The effect of annealing on the physical properties of POAS LB films has been studied as a function of temperature (from 60 to 150 °C).It has been shown that annealing causes changes in structural, optical and electrical characteristics of POAS LB films. The decrease in thickness (from 24 to 18.1 A) of each monolayer of POAS film has been observed for annealing at 150 °C. The value of conductivity of POAS LB film has been shown to be varying from 0.4 to 10−8 S cm−1 as a function of annealing temperature. The continuous shift of the UV absorption bands from 840 to 590 nm with annealing temperature is arising due to the simultaneous change of charge defects in POAS LB films. Differential scanning calorimetry studies show a complete evaporation of dopant ions occurring at 180 °C in the powder form of POAS, whereas complete removal of dopants ions for LB films have been observed for annealing at 150 °C. The Cole—Cole plot obtained at 80 °C for POAS LB film shows a mark deviation from a semicircle which indicates the operation of complex behaviour in the POAS LB films. It also shows the typical behaviour of interfacial polarization caused by the contact effect of POAS LB films with chromium metal of interdigited electrodes for annealing at 150 °C.