Analog and digital worlds: Part 2. Fourier analysis in signals and data treatment

The most direct scope of Fourier Transform (FT) is to give an alternative representation of a signal: from the original domain to the corresponding frequency domain. The original domain can be time, space or any other independent variable that can be used as the domain of the function. This subject has been treated in Part 1 [1]. In particular, the FT of a signal, also referred to as the frequency spectrum of a signal, has been used to calculate the lowest sampling frequency that provides a correct representation of the signal itself. At the beginning of this contribution, it is illustrated how to implement the so-called windowing process to periodic sequences. Then, the meaning of the operations denominated convolution and deconvolution is discussed. It is shown how FT provides a very effective path to the execution of these operations in the alternative domain by employing the convolution theorem. Finally, the application of convolution and deconvolution operations to experimental signals associated with the 'spontaneous' convolution of two concurrent events is analysed by different examples

Simulation of an experimental database of infrared spectra of complex gaseous mixtures for detecting specific substances. The case of drug precursors

This work is motivated by the need to develop suitable databases in absence of real experimental data, for instance when spectra measured with a newly developed instrumentation on real samples are not available yet. This notwithstanding, in fact, the realization of the physical project should be addressed by a starting database, also invaluable in order to test its effectiveness. In this article we face the issue of simulating gas mixtures spectra for the development of a new sensor for External Cavity-Quantum Cascade Laser Photoacoustic Spectroscopy (EC-QCLPAS) starting from literature FT-IR spectra of pure components: a dataset is realized suitable to realistically represent the ensemble of spectra of the gas mixtures of interest. The informative data deriving from the literature spectra were combined with the stochastic component extracted from a sample spectrum recorded with a prototype instrument, allowing us to build a matrix containing thousands of simulated spectra of gaseous mixtures, accounting for the presence of different components at different concentrations. Signal processing and experimental design techniques were used along the whole path leading to the dataset of simulated spectra. In particular, the goal of the construction of the database lies in the development of a final system to detect drug precursors in the vapour phase. The comparison of some EC-QCLPAS spectra with the corresponding simulated signals confirms the validity of the proposed approach

Electropolymerisation of 3,4-ethylenedioxythiophene in aqueous solutions

The potentiostatic electrosynthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) in aqueous media without addition to the solution of any kind of surfactant has been studied by electrochemical quartz crystal microbalance (EQCM) and by spectroelectrochemistry. These tandem techniques have given valuable new information about the electropolymerisation process, allowing us to relate absorbance-charge and frequency-charge relationships to: (i) oligomers generation and chain propagation, as far as the length leading to precipitation is reached; (ii) growing of the polymer deposit and concomitant p-doping, and even (iii) overoxidation of the polymer film. An analysis of the whole of the data, in fact, shows that the charge spent is not necessarily totally involved in thepolymer deposit formation, growth, and p-doping, so that it is necessary to be particularly careful in the fitting of the experimental data to linear models

Bidimensional chronoabsorptometric study of electropolymerisation of 4,4 '-bis(2-methylbutylthio)-2,2 '-bithiophene

Bidimensional chronoabsorptometry is a novel spectroelectrochemical technique that monitors simultaneously three different signals: current and absorbance both normal to the electrode plane and parallel to this plane during a time in which a fixed potential is imposed. This technique is applied in the visible range to the study of the electropolymerisation of 4,4'-bis(2-methylbutylthio)2,2'-bithiophene (MBTBT). Experiments are performed in a spectroelectrochemical cell under finite diffusion conditions (thin layer cell) with the aim of interpreting the processes taking place both at the electrode surface and in the adjacent solution during the potentiostatic electrogeneration and deposition of the polymer. Correlations are drawn out among the trends of the oligomers concentration in solution, the polymer electrodeposition and charging, and the current flow, on the time scales of the different steps of the process. (C) 2002 Elsevier Science B.V. All rights reserved

In situ atomic force microscopy in the study of electrogeneration of polybithiophene on Pt electrode

Electrochemical AFM technique has been used for the in situ study of the electrogeneration-deposition process of polybithiophene at varying the polymerisation conditions, such as supporting electrolyte, i.e., LiClO4 or tetrabutylammonium hexafluorophosphate, and polymerisation procedure, i.e., either potentiostatic or potentiodynamic method. In order to better follow the evolution of the morphology of the deposit, particularly during the early stages of the polymer film growth, a suitable home-made electrochemical cell has been used

Electrochemical preparation and characterisation of bilayer films composed by Prussian Blue and conducting polymer

Preparation and electrochemical behaviour of bilayer films consisting of iron(Ill) hexacyanoferrate, well known as Prussian Blue, and of poly[4,4´-bis(butylsulphanyl)-2,2´-bithiophene], on a platinum electrode, are reported. The electrochemical features of the Prussian Blue/conducting polymer bilayer system are examined in aqueous and acetonitrile solutions. Cyclic voltammetric studies show that, in acetonitrile solvent, the inner layer Prussian Blue is electroactive to some extent, though the electrochemical response of the system is mainly accounted for by poly[4,4´-bis(butylsulphanyl)-2,2´-bithiophene] outer layer. On the other hand, in aqueous solution Prussian Blue exhibits good electroactivity. Under specific experimental conditions, the individual redox behaviour of each constituent of the bilayer is evidenced in the two solvents separately, i.e., that of PB and that of poly[4,4´-bis(butylsulphanyl)-2,2´bithiophene] in aqueous and in organic solvent, respectively. However, interesting reciprocal influences are evident in the current/potential curves recorded under conditions which are discussed

Development of Quantitative Structure-Property Relationships (QSPR) using calculated descriptors for the prediction of the physico-chemical properties (nD, r, bp, e and h) of a series of organic solvents.

Quantitative structure-property relationship (QSPR) models were derived for predicting boiling point (at 760 mmHg), density (at 25 \ub0C), viscosity (at 25 \ub0C), static dielectric constant (at 25 \ub0C), and refractive index (at 20 \ub0C) of a series of pure organic solvents of structural formula X-CH2CH2-Y. A very large number of calculated molecular descriptors were derived by quantum chemical methods, molecular topology, and molecular geometry by using the CODESSA software package. A comparative analysis of the multiple linear regression techniques (heuristic and best multilinear regression) implemented in CODESSA, with the multivariate PLS/GOLPE method, has been carried out. The performance of the different regression models has been evaluated by the standard deviation of prediction errors, calculated for the compounds of both the training set (internal validation) and the test set (external validation). Satisfactory QSPR models, from both predictive and interpretative point of views, have been obtained for all the studied properties

A feature selection strategy for the analysis of spectra from a photoacoustic sensing system

In the frame of the EU project CUSTOM, a new sensor system for the detection of drug precursors in gaseous samples is being developed, which also includes an External Cavity-Quantum Cascade Laser Photo Acoustic Sensor (ECQCLPAS). In order to define the characteristics of the laser source, the optimal wavenumbers within the most effective 200 cm -1 range in the mid-infrared region must be identified, in order to lead to optimal detection of the drug precursor molecules in presence of interfering species and of variable composition of the surrounding atmosphere. To this aim, based on simulations made with FT-IR spectra taken from literature, a complex multivariate analysis strategy has been developed to select the optimal wavenumbers. Firstly, the synergistic use of Experimental Design and of Signal Processing techniques led to a dataset of 5000 simulated spectra of mixtures of 33 different gases (including the 4 target molecules). After a preselection, devoted to disregard noisy regions due to small interfering molecules, the simulated mixtures were then used to select the optimal wavenumber range, by maximizing the classification efficiency, as estimated by Partial Least Squares - Discriminant Analysis. A moving window 200 cm -1 wide was used for this purpose. Finally, the optimal wavenumber values were identified within the selected range, using a feature selection approach based on Genetic Algorithms and on resampling. The work made will be relatively easily turned to the spectra actually recorded with the newly developed EC-QCLPAS instrument. Furthermore, the proposed approach allows progressive adaptation of the spectral dataset to real situations, even accounting for specific, different environments

Development of a redox polymer based on poly(2-hydroxyethyl methacrylate)for disposable amperometric sensors

Membranes based on poly(2-hydroxyethyl methacrylate), chemically modified with ferrocene functionalities, are proposed as novel coatings for the development of disposable amperometric sensors. They are mass-produced on an inert support through an UV photo-induced polymerization, forming self-standing films that can be transferred on an electrode surface. Thanks to the characteristic of the functionalized methacrylate matrix to rapidly and reversibly swell by incorporation of large amounts of water, the ferrocene moieties are in intimate contact with the electrolytic solution. They can activate effective electrocatalytic processes that can be exploited in the field of amperometric sensing. The performance of the methacrylate coating with respect to the determination of hydroquinone benchmark analyte has been tested both in a static solution and in a flux cell

Systematic study of the correlation between surface chemistry, conductivity and electrocatalytic properties of graphene oxide nanosheets

A main advantage of graphene oxide (GO) over other materials is the high tunability of its surface functional groups and of its electric conductivity. However, the complex chemical composition of GO renders difficult to unravel the correlation between structural and electric properties. Here, we use a combination of electron spectroscopy and electrochemistry to correlate the surface chemistry of GO to its electrical conductivity and electrocatalytic properties with respect to two molecules of high biological interest: β-nicotinamide adenine dinucleotide (NADH) and vitamin C. We demonstrate that the electrocatalytic properties of the material are due to hydroxyl, carbonyl and carboxyl groups residues that, even if already present on pristine GO, become electroactive only upon GO reduction. The results of this study demonstrate the advantages in the use of GO in amperometric biosensing and in enzymatic biofuel cells: it allows the oxidation of the target molecules at low potential values, with a sensitivity >15 times higher with respect to standard, carbon-based electrode materials. Finally, we demonstrate that the right amount of chemical groups to achieve such high performance can be obtained also by direct electrochemical exfoliation of bulk graphite, without passing through GO production, thus rendering this approach suitable for cheap, large-scale applications