Maria Luz Rodriguez-Mendez

Langmuir-Blodgett film and carbon paste electrodes based on phthalocyanines as sensing units for taste

Abstract In this work, the possibility of using voltammetric electrodes based on phthalocyanine compounds as the sensing units of an electronic tongue has been investigated. Sensors based on monophthalocyanines (including cobalt and copper phthalocyanines) and lanthanide bisphthalocyanines (comprising europium, gadolinium and lutetium bisphthalocyanines and the octa- tert butyl substituted praseodymium bisphthalocyanine) have been prepared using two different methods: the Langmuir–Blodgett (LB) technique and the carbon paste electrode (CPE) technique. The sensors have been used to evaluate the five types of basic tastes (sweet, bitter, salty, acid and umami). Each phthalocyanine shows a characteristic electrochemical response for the solutions investigated. The electrochemical response towards the five basic types of flavours is particularly rich in the case of the bisphthalocyanine compounds. The responses of the sensors prepared using the LB technique has been compared with that of the sensing units prepared by the CPE technique. The response of both types of electrodes is similar, but in the case of the LB films the peaks are better resolved, and the electrodes show a very good repeatability. Nevertheless, their reproducibility is lower than for the CPE electrodes. The electrochemical studies indicate that the charge transport in both types of electrodes is diffusion controlled. A multichannel taste sensor has been constructed using those sensors with the best performances. The array of sensors has been exposed to solutions of the five basic flavours. The principal component analysis (PCA) of the obtained signals has allowed the discrimination of basic tastes.

Array of sensors based on conducting polymers for the quality control of the aroma of the virgin olive oil

Abstract In this paper, a sensor array based on thin films of conducting polymers aiming to discriminate among different virgin olive oils is described. The array has been constructed using eight polymeric sensors. They were deposited electrochemically by using different electrodeposition conditions, different monomers (3-methylthiophene, pyrrole and aniline) and doping agents. Olive oil samples with well-defined organoleptic characteristics have been selected by a panel of experts. They included virgin olive oils of different qualities (extra virgin, virgin and lampante) as well as a refined olive oil. Four types of olive oil samples with well-defined “off odours”, that is with unpleasant aromatic notes, named musty, rancid, fusty and muddy, have also been included in the study. The sensors are stable and show good reproducibility and reversibility when exposed to the headspace of the virgin olive oils. The array of sensors combined with a Principal Component Analysis (PCA) allows the discrimination of different types of olive oils.

Lutetium bisphthalocyanine thin films as sensors for volatile organic components (VOCs) of aromas

Lutetium bisphthalocyanine (LuPc2) Langmuir‐Blodgett (LB) and evaporated films were investigated as sensitive materials for the detection of volatile organic compounds (VOCs) responsible for the aroma of food products such as olive oil or wine. The sensitive layers were deposited on ITO interdigitated electrodes in order to study the changes in their conductivity. The system was operated at room temperature and a variety of organic vapours with different chemical functionalities were tested. The results demonstrate the viability of the phthalocyanine thin films as the active species for systems specifically designed for the monitoring of aromatic components in food.

Nanostructured thin films based on phthalocyanines: electrochromic displays and sensors

The group of the University of Valladolid is a multidisciplinary team formed by chemists, physicists and engineers. The activities of the group are focused to the study of the physicochemical properties of nanostructured Langmuir-Blodgett thin films based on phthalocyanines and their applications. Films of a variety of phthalocyanine molecules including several metallophthalocyanines, lanthanide double decker phthalocyanines and heteroleptic derivatives have been prepared. Their spectroelectrochemical properties have been described in detail and compared with those observed in disordered casted films or microcrystalline evaporated films. The group has dedicated special attention to films based on rare earth double decker compounds due to their unique semiconducting, optical and electrochemical properties. A rich electrochromism has been demonstrated in thin films of this family of compounds. The reversibility is improved in nanostructured Langmuir-Blodgett films. This has permitted development of an electrochromic display that can change its color from blue to green and finally to red. At the present moment, our main objective is the design of sensors able to detect gases and liquids. It has been demonstrated that thin film assemblies based on rare earth bisphthalocyanines modify their conductivity and their optical properties in the presence of electron donor or electron acceptors gases. Changes are also observed when the devices are exposed to Volatile Organic Compounds such as esters, alcohols or aldehydes which are responsible of odors in foods and beverages. Liquid sensors have also been developed. Their working principle is based in the fact that the rich electrochemical properties of phthalocyanine thin films are extremely sensitive to the nature of the electrolytic solution. Arrays of phthalocyanines have been used to construct an electronic nose able to discriminate odors from a variety of foods and beverages. Similarly, phthalocyanines have also been used to construct an electronic tongue based on voltammetric sensors. This is one of the main contributions of the group to the field of sensors.

Spectroscopic and electrochemical properties of thin solid films of yttrium bisphthalocyanine

Abstract Langmuir—Blodgett (LB) and evaporated thin solid films of the yytrium bisphthalocyanine complex (YPc2) have been prepared on various substrates. Cyclic voltammograms of films are discussed and the electrochromic effect on LB films is reported. A detailed spectroscopic characterization of the YPc2 material is given using resonance Raman scattering (RRS), surface-enhanced resonance Raman scattering (SERRS), transmission and reflection absorption FT-IR spectroscopy and UV—vis spectra. The spectroscopic characterization of the chemical and electrochemical oxidations products of YPc2 films and solutions was carried out by in situ UV—vis spectroscopy. Potential applications are discussed.

New insights into sensors based on radical bisphthalocyanines

The unique semiconducting, optical and electrochemical properties of radical lanthanide bisphthalocyanines make them ideal materials for sensing applications. A variety of chemical sensors have been developed using rare-earth bisphthalocyanine thin films. In this paper, the characteristics of sensors based on bisphthalocyanines are reviewed. The advantages of these sensors with respect to sensors developed using other metallophthalocyanines are discussed. Resistive sensors based on bisphthalocyanines change their conductivity when exposed to a variety of pollutant gases and volatile organic compounds. Because bisphthalocyanines are intrinsic semiconductors, the conductivity of their thin films is higher than the conductivity of metallophthalocyanine thin films. This facilitates the electrical measurements and enhances the sensitivity of the sensors. Optical sensors have also been developed based on the rich optical properties shown by bisphthalocyanines. Films characterized by a bright green color change to red or to blue upon oxidation or reduction. The changes also affect the charge-transfer band associated to the free radical that bisphthalocyanines show in the near infrared region. This band coincides with telecommunication wavelengths, making possible the fabrication of fiber optic sensors where a phthalocyanine film is deposited at one of the ends of the fiber. Electrochemical sensors have been developed taking advantage of the unique electrochemical behavior associated to the one-electron oxidation and one-electron reduction of the phthalocyanine ring. These reversible processes are extremely sensitive to the nature of the electrolytic solution. This has made possible the development of voltammetric sensors able to produce particular signals when immersed in different liquids. In the last part of the paper, the fundamentals and performance characteristics of electronic noses and electronic tongues based on bisphthalocyanines are described. Such devices have been successfully exploited in quality control, classification, freshness evaluation and authenticity assessment of a variety of food, mainly wines and olive oils.

Conducting polymer-based array for the discrimination of odours from trim plastic materials used in automobiles

Abstract This paper reports the test of the use of an electronic nose based on chemoresistive polymeric sensors for the discrimination of eight sample materials used in the car manufacture, including parts fabricated in polyurethane (PU). Principal component analysis (PCA) and cluster analysis (CA) have been used in order to classify and identify the odours coming from the parts under study. Good separation among odours of samples of comparable composition has been obtained using the first coefficients of the fast Fourier transform (FFT) of the whole curve as the input variable of PCA. These results show the feasibility of the conducting polymer array of sensors to discriminate odours for the air cabin quality control.

Iron phthalocyanine in non-aqueous medium forming layer-by-layer films: growth mechanism, molecular architecture and applications

The application of organic thin films as transducer elements in electronic devices has been widely exploited, with the electrostatic layer-by-layer (LbL) technique being one of the most powerful tools to produce such films. The conventional LbL method, however, is restricted in many cases to water soluble compounds. Here, an alternative way to produce LbL films containing iron phthalocyanine (FePc) in non-aqueous media (chloroform) is presented. This film fabrication was made possible by exploiting the specific interactions between Fe and NH(2) groups from PAH, poly(allylamine hydrochloride) used as the supporting layer, leading to the formation of bilayers structured as (PAH/FePc)(n). We have also incorporated silver nanoparticles (AgNPs) in LbL films with (PAH/FePc/AgNP)(n) trilayers, making it possible to achieve the surface-enhanced Raman scattering (SERS) phenomenon. The molecular architecture of the LbL films was determined through different techniques. The growth was monitored with UV-Vis absorption spectroscopy, their morphology characterized by optical and scanning electron (SEM) microscopes, and their molecular organization determined using FTIR. The electrochemical properties of the LbL films were successfully applied in detecting dopamine in KCl aqueous solutions at different concentrations using cyclic voltammetry. The results confirmed that the LbL films from FePc in non-aqueous media keep their electroactivity, while showing an interesting electrocatalytic effect. The SERS phenomenon suggested that FePc aggregates might be directly involved in the maintenance of the electroactivity of the LbL films.

Improvement of the synthesis of lutetium bisphthalocyanine using the Taguchi method

In this work, a Taguchi experimental design has been used to identify important factors that influence the procedure of synthesis and purification of the lutetium bisphthalocyanine (LuPc2). The working conditions have been improved using an L8 (27) experimental design. A statistical analysis of the experimental data revealed that the most influential factor in the yield is the amount of the starting material (AR), with a 34.4% contribution, followed by the way of performing two chromatographies (C), sequentially or in parallel (26.8%), the temperature of the reaction T (17.8%), the type of chromatography TC, standard or flash (9.1%), and lastly, the volume of the sample injected in the column (5.4%). The time of the reaction (t) and the length of the column (LC) had little effect on the yield. Noise (N) was found to have a negligible influence on the throughput. Maximum yield (20.0%) was obtained when the synthesis was carried out at 280 °C (T) during 1 h 30 min (t) using 0.5 g of lutetium acetate (AR) and performing a standard chromatography (TC) on neutral alumina, injecting 30 ml (V) of the reaction mixture in two columns performed in parallel (C). This yield represents an increase of 186% in relation to the 7% obtained before the application of the Taguchi method. Under these conditions the yield is not significantly influenced by variability of external factors, which validates the proposed procedure.

Response of a sensor based on ytterbium bisphthalocyanine Langmuir–Blodgett films to selected herbicides

Abstract The response of a sensor based on the changes of the conductivity of ytterbium bisphthalocyanine Langmuir–Blodgett (LB) films when exposed to the vapours of three extended pesticides is studied. The herbicides Imazamethabenz-Me, Terbutryn and Tri-allate were selected on the basis of their different chemical group (imidazolinone, triazine and thiocarbamate, respectively) and physical properties (Henry’s Law constant, water solubility and octanol/water partitioning coefficient, among others). The present results suggest that the chemical composition determines the relative change of the measured properties. On the other hand, the kinetics of the adsorption and desorption processes seem to be closely related to the hydrophobicity of the pesticides. The development of sensors suitable for monitoring the volatilisation of pesticides at laboratory and field scales is discussed in terms of the differences observed in the responses to the tested vapours.