Nonspecific sensor arrays ("electronic tongue") for chemical analysis of liquids (IUPAC Technical Report)

The history of the development of potentiometric sensors over the past century demonstrates progress in constructing single, discrete (i.e., separate, to distinguish from sensor arrays) ion sensors, which have been made as selective as possible. Only a few types reveal high selectivity. However, easy measurement procedure, with low cost and availability, give rise to the search for new ways for their successful application. The present document describes a new concept for application of potentiometric multisensor systems, viz., sensor arrays for solution analysis, and the performance of this new analytical tool - the "electronic tongue". The electronic tongue is a multisensor system, which consists of a number of low-selective sensors and uses advanced mathematical procedures for signal processing based on the pattern recognition (PARC) and/or multivariate analysis [artificial neural networks (ANNs), principal component analysis (PCA), etc.]. Definitions of the multisensor systems and their parameters are suggested. Results from the application of the electronic tongue, both for quantitative and qualitative analysis of different mineral water and wine samples, are presented and discussed.

Electronic tongues and their analytical application

Electronic tongues for liquid analysis, based on the organizational principles of biological sensory systems, developed rapidly during the last decade. A brief historical overview of the research and development in the field of electronic tongue systems is presented. Current achievements of scientific groups working in this field are outlined and critically reviewed. The performance of electronic tongues in quantitative analysis and in classification of multicomponent media is considered. The exciting possibility of establishing a correlation between the output from an electronic tongue and human sensory assessment of food flavour, thereby enabling quantification of taste and flavour, is described. Application areas of electronic tongue systems including foodstuffs, clinical, industrial, and environmental analysis are discussed in depth. Prospective research and development in the field of electronic tongues is discussed.

Electronic tongue for pharmaceutical analytics: quantification of tastes and masking effects

The organoleptic aspects of pharmaceutical formulations affect their acceptability to the patient and hence can have an important effect on concordance with treatment. Objective evaluation of these aspects, particularly the taste of the formulation and the drug substance it contains, is difficult. Whilst volunteer taste panels can be used to good effect their utility is limited, particularly during very early stage development when the toxicological profile of the active pharmaceutical ingredient (API) is yet to be established in detail. A potentiometric "electronic tongue" has been applied to analyse a variety of 41 individual substances and mixtures of particular interest for pharmaceutical research and development. The electronic tongue (ET) was capable of discriminating between substances with different taste modalities and could also distinguish different substances eliciting the same basic taste; the ET is promising in terms of quantifying the content of each substance and has an ability to detect nuances of the basic taste (e.g. lingering or short-lived). After calibration the electronic tongue was successfully applied to predicting bitterness strength of binary mixtures with a sweetener in terms of "apparent" or "perceived" quinine content. In order to render a formulation palatable it is often necessary to mask the (usually bitter) taste of the API by the addition of masking agents such as sweeteners and flavours. The ET proved capable of distinguishing between formulations with different levels of sweetener and/or flavour in a manner that was consistent with their masking efficiency as perceived by a small human taste panel. A suitably calibrated ET could have the benefit of providing the pharmaceutical formulator with reliable data concerning the taste of the product quickly and with a reduced need to ask volunteers to taste active pharmaceutical samples. Early development activities could be facilitated when human tasting is usually not possible in the absence of the required toxicological data.

Monitoring batch fermentations with an electronic tongue

An electronic tongue comprising 21 potentiometric chemical sensors with pattern recognition tools was used for the rapid off-line monitoring of batch Escherichia coli fermentations. The electronic tongue was capable of monitoring the changes in the media composition as the fermentation progressed, and could correlate this with an increase in biomass. The electronic tongue was also able to monitor the increase in organic acids, especially acetic acid, throughout the fermentation. This technique clearly shows promise as a rapid tool for fermentation monitoring.

1,10-Phenanthroline-2,9-dicarboxamides as ligands for separation and sensing of hazardous metals

1,10-Phenanthroline-2,9-dicarboxamides of various structures were synthesized and studied as ligands for separation and sensing of d- and f-metals.

Evaluation of a novel chemical sensor system to detect clinical mastitis in bovine milk

Automatic detection of clinical mastitis is an essential part of high performance and robotic milking. Currently available technology (conductivity monitoring) is unable to achieve acceptable specificity or sensitivity of detection of clinical mastitis or other clinical diseases. Arrays of sensors with high cross-sensitivity have been successfully applied for recognition and quantitative analysis of other multicomponent liquids. An experiment was conducted to determine whether a multisensor system ("electronic tongue") based on an array of chemical sensors and suitable data processing could be used to discriminate between milk secretions from infected and healthy glands. Measurements were made with a multisensor system of milk samples from two different farms in two experiments. A total of 67 samples of milk from both mastitic and healthy glands were in two sets. It was demonstrated that the multisensor system could distinguish between control and clinically mastitic milk samples (p=0.05). The sensitivity and specificity of the sensor system (93 and 96% correspondingly) showed an improvement over conductivity (56 and 82% correspondingly). The multisensor system offers a novel method of improving mastitis detection.

Real-Time Water Quality Monitoring with Chemical Sensors

Water quality is one of the most critical indicators of environmental pollution and it affects all of us. Water contamination can be accidental or intentional and the consequences are drastic unless the appropriate measures are adopted on the spot. This review provides a critical assessment of the applicability of various technologies for real-time water quality monitoring, focusing on those that have been reportedly tested in real-life scenarios. Specifically, the performance of sensors based on molecularly imprinted polymers is evaluated in detail, also giving insights into their principle of operation, stability in real on-site applications and mass production options. Such characteristics as sensing range and limit of detection are given for the most promising systems, that were verified outside of laboratory conditions. Then, novel trends of using microwave spectroscopy and chemical materials integration for achieving a higher sensitivity to and selectivity of pollutants in water are described.

Fermentation monitoring using multisensor systems: feasibility study of the electronic tongue

The electronic tongue based on an array of 30 non-specific potentiometric chemical sensors has been applied to qualitative and quantitative monitoring of a batch fermentation process of starting culture for light cheese production. Process control charts were built by using PLS regression and data from fermentations run under "normal" operating conditions. Control charts allow discrimination of samples from fermentation batches run under "abnormal" operating conditions from "normal" ones at as early as 30-50% of fully evolved fermentations. The capability of the electronic tongue to quantify concentrations of important organic acids (citric, lactic and orotic) in the present type of fermentation media was demonstrated. Average prediction errors were assessed in the range 5-13% based on test set validation. Correlation between peptide profiles determined using HPLC and the electronic tongue output was also established. The electronic tongue was demonstrated to be a promising tool for fermentation process monitoring and quantitative analysis of growth media.

Differentiation of four Aspergillus species and one Zygosaccharomyces with two electronic tongues based on different measurement techniques

Two electronic tongues based on different measurement techniques were applied to the discrimination of four molds and one yeast. Chosen microorganisms were different species of Aspergillus and yeast specie Zygosaccharomyces bailii, which are known as food contaminants. The electronic tongue developed in Linköping University was based on voltammetry. Four working electrodes made of noble metals were used in a standard three-electrode configuration in this case. The St. Petersburg electronic tongue consisted of 27 potentiometric chemical sensors with enhanced cross-sensitivity. Sensors with chalcogenide glass and plasticized PVC membranes were used. Two sets of samples were measured using both electronic tongues. Firstly, broths were measured in which either one of the molds or the yeast grew until late logarithmic phase or border of the stationary phase. Broths inoculated by either one of molds or the yeast was measured at five different times during microorganism growth. Data were evaluated using principal component analysis (PCA), partial least square regression (PLS) and linear discriminant analysis (LDA). It was found that both measurement techniques could differentiate between fungi species. Merged data from both electronic tongues improved differentiation of the samples in selected cases.

Potentiometric and theoretical studies of the carbonate sensors based on 3-bromo-4-hexyl-5-nitrotrifluoroacetophenone

Novel carbonate ionophore, trifluoroacetophenone derivative (TFA) substituted by two acceptor substituents in the phenyl ring (3-bromo-4-hexyl-5-nitrotrifluoroacetophenone), was synthesized. Solvent polymeric membrane sensors based on this ionophore exhibited heightened selectivity to carbonate ions in the presence of the most important interfering anions. A wide range of potentiometric properties were studied and compared with those of sensors based on mono-substituted ionophores. Special attention was paid to pH dependence of sensor responses and to elaboration of appropriate conditions for carbonate analysis. A segmented-sandwich membrane method was applied for determination of the stoichiometry of ionophore-carbonate complexes, which was determined to be 1:3, and apparent complex formation constants which were 14.4 and 13.6 for DOS- and NPOE-plasticized membranes, respectively. Theoretical studies on TFA derivatives by semi-empirical (AM1 and PM3) and ab initio(6-31+G*) methods were performed, considering different types of possible ionophore-ion interactions. The formation of hydrogen bonds between carbonate and hydrated TFA was proved to be much more favourable in terms of energy compared to tetrahedral nucleophilic adducts that earlier were postulated to being formed in the membrane phase. The final conclusion on the mechanism of carbonate sensing by TFA-based solvent polymeric membrane sensors was made on the basis of computational data and detailed analysis of the literature.