Desenvolupament de sensors i llengües electròniques voltamperomètriques per a la determinació d’ions metàl·lics en mostres d’interès ambientals

[cat] La problemàtica mediambiental associada a la contaminació per ions metàl·lics és mundialment coneguda. En les últimes dècades hi ha hagut un enduriment de la legislació associada a aquests contaminants que ha permès disminuir les seves emissions al medi ambient però, degut a la seva elevada persistència i al seu caràcter no biodegradable, les concentracions dels ions metàl·lics en mostres mediambientals són encara preocupants. Actualment, tot i que existeixen diverses tècniques analítiques per a la determinació d’ions metàl·lics, encara és necessari el desenvolupament de noves metodologies analítiques que permetin determinar-los on-site i a nivell traça. En aquest sentit, les tècniques voltamperomètriques de redissolució són una bona opció ja que són tècniques sensibles i reproduïbles que proporcionen anàlisis ràpides sense requerir instrumentació voluminosa o excessivament costosa. A més, el caràcter portàtil d’aquestes tècniques s’ha vist afavorit amb la introducció dels elèctrodes serigrafiats, que són dispositius compactes, miniaturitzats, versàtils i que es poden produir en massa. Un dels avantatges que proporcionen els elèctrodes serigrafiats és la possibilitat de modificar el seu elèctrode de treball per tal de millorar la seva sensibilitat i selectivitat o permetre treballar a diferents condicions experimentals. En aquest sentit, una primera part d’aquesta tesi doctoral s’ha centrat en el desenvolupament de nous elèctrodes serigrafiats voltamperomètrics per a la determinació d’ions metàl·lics a nivell traça en mostres mediambientals. Concretament, s’han estudiat diferents estratègies de modificació com la modificació química, la formació de pel·lícules metàl·liques i l’ús de diversos nanomaterials (nanopartícules metàl·liques, nanoal·lòtrops de carboni i silici porós). Per a cada estratègia s’han estudiat diferents aspectes del procés de modificació i s’ha avaluat l’aplicabilitat del sensor que proporciona millors resultats en mostres reals. Sovint però, en l’anàlisi de mostres reals trobem mescles complexes on els ions metàl·lics s’interfereixen entre si o donen lloc a pics solapats. En aquests casos l’ús d’un sol sensor no permet determinar correctament la concentració dels diferents ions i cal recórrer a altres estratègies d’anàlisi més sofisticades. Aquest és el cas de les llengües voltamperomètriques, on es combinen diversos sensors no específics amb un tractament de dades multivariant. Així, la segona part d’aquesta tesi doctoral s’ha centrat en el desenvolupament de llengües voltamperomètriques per a l’anàlisi de mostres complexes d’ions metàl·lics. Concretament s’han estudiat el sistema Tl(I) i In(III) i el sistema Cd(II), Pb(II), Tl(I) i Bi(III) en presència de Zn(II) i In(III), que es caracteritzen, respectivament, per la presència de dos metalls que donen lloc a senyals fortament solapats i per la presència d’un elevat nombre de metalls. Concretament, s’ha estudiat la selecció dels sensors que componen la llengua voltamperomètrica, la seva disposició i el disseny experimental. Un altre aspecte crucial en el desenvolupament de les llengües voltamperomètriques és el tractament de dades. Així, en la segona part d’aquesta tesi doctoral també s’ha fet èmfasi en aquest tema, on s’han estudiat diversos pretractaments i s’han construït diversos models quimiomètrics basats en la regressió per mínims quadrats parcials (PLS). A més, s’han estudiat també dos sistemes de calibratge multivariant, el calibratge multivariant extern i l’addició estàndard multivariant, per a la qual s’ha desenvolupat una nova estratègia basada en l’ús de PLS i la simulació del blanc de la mostra a partir de l’omissió de l’etapa de preconcentració de la mesura voltamperomètrica. Aquest segon mètode de calibratge proporciona prediccions més acurades per a mostres amb un fort efecte matriu. Els resultats obtinguts a partir dels estudis realitzats al llarg d’aquesta tesi doctoral han donat lloc a 12 articles, 9 d’ells referents a la part de desenvolupament de sensors voltamperomètrics i 3 referents al desenvolupament de llengües voltamperomètriques.[eng] On-site monitoring of metal ions at trace levels is crucial for safety and environmental reasons. In this sense, stripping voltammetry is a good option due to its high sensitivity and reproducibility and the fact that it provides fast analysis with relatively low cost and portable equipment. These last features are further enhanced by the coupling of stripping voltammetry with screen-printed electrodes, which are compact, miniaturized and versatile devices that can be mass-produced in a reproducible manner. A first part of this PhD thesis has focused on the development of new screen-printed voltammetric sensors for the determination of metal ions at trace levels in environmental samples. Particularly, several modification strategies (chemical modification, metal films and nanomaterials) have been studied. For each strategy, some aspects of the modification process were evaluated and their applicability to real samples was tested for the sensor that provided better results. Frequently though, the analysis of real samples involves complex mixtures where metal ions interact with each other or give rise to highly overlapped peaks. In these scenarios, it is necessary to resort to more sophisticated analytical strategies like voltammetric electronic tongues. Thus, the second part of this PhD thesis has focused on the development of voltammetric tongues for the analysis of complex mixtures of metal ions. In particular, two systems have been studied: Tl(I) and In(III), two metals that give rise to highly overlapped peaks, and Cd(II), Pb(II), Tl(I) and Bi(III) in the presence of Zn(II) and In(III). In both cases, several studies involving the selection of sensors, their arrangement and the experimental design were performed. The second part of this PhD thesis has also focused on data treatment, a key aspect in the development of voltammetric tongues. Two multivariate calibration systems were studied, multivariate external calibration and multivariate standard addition, for which a new strategy based on PLS and the simulation of a blank signal by skipping the deposition step of stripping voltammetry was developed. The results achieved throughout this PhD thesis have resulted in 12 articles, 9 related to the development of voltammetric sensors and 3 related to the development of voltammetric electronic tongues

Electrochemical detection of trace silver

Increasing utilization of silver and silver nanoparticles (AgNPs) in daily processes and products has led to a significant growth in scientific interest in methods for monitoring silver. In particular, the amount of silver ions (Ag+) released to the environment is known to have a detrimental effect on aquatic ecology, and thus some control actions have been implemented in recent years; for example, the manufacturing industry is now required to control and certify the quantity of AgNPs present in products. Electrochemical sensors are well suited to the task of silver monitoring due to several beneficial properties, including low costs, fast measurements, and facile adaptation to miniaturized, portable instrumentation. The predominant method for electrochemical silver determination involves potentiometric ion selective electrodes (ISEs) and voltammetric measurements. Reviewing the literature of the last ten years reveals significant improvements in the analytical performance of electrochemical sensors, mainly related to the development of new protocols, selective receptors, and electrode materials. Remarkably, ISEs with limits of detection (LOD) in the nanomolar range have been reported, employing careful control of ion fluxes across the membrane interfaces. What's more, sub-nanomolar LODs are attainable by stripping voltammetry using either ligand-based deposition strategies or thin layer membranes coupled to conducting polymers. Selectivity has also been optimized through the membrane composition of ISEs, with special focus on Ag+ ionophores. Furthermore, novel voltammetric methods allow for discrimination between Ag+ and AgNPs. However, there is still a dearth of studies applying such electrochemical sensors to on-site water analysis, and hence, further research is needed in order to translate these laboratory scale achievements to real-world contexts. Overall, this review describes the state-of-the-art in electrochemical silver detection, and provides a comprehensive description of those aspects contributing to the further development and improvement of analytical performance

A hybrid sensing system combining simultaneous optical and electrochemical measurements: application to beer discriminations

A hybrid sensing system, which combines simultaneous cyclic voltammetric (CV) and UV-vis absorbance measurements using a commercial carbon screen-printed electrode and a set of optical fibres in disposable cuvettes, is proposed. The hybrid system approach was applied to 27 samples of recognized beer brands, improving the classification power as compared to only voltammetric or only spectrophotometric measurements. The developed partial least squares discriminant analysis (PLS-DA) model was able to discriminate between five types of beer (lager, marzen, black/stout, alcohol-free and white/ale). The model was also successfully applied to 28 beer samples of white-label brands sold in local supermarkets, demonstrating their similarity to recognized brand beers

Authentication of soothing herbs by UV-vis spectroscopic and chromatographic data fusion strategy

A data fusion approach combining chromatographic and spectroscopic profiles is proposed for the discrimination and classification of soothing herbs in different types of herbal preparations. Particularly, chamomile, lavender, passionflower, and valerian were considered. The proposed data fusion approach revealed a higher clusterization ability than each analytical technique in a separate way, which was assessed through an exploratory analysis based on Principal Component Analysis (PCA) coupled to Silhouette analysis: percentage of samples with a negative Silhouette width were 19, 15 and 10 for chromatography, spectroscopy and data fusion, respectively. Furthermore, a Partial Least Squares - Discriminant Analysis (PLS-DA) model developed based on data fusion was able to perfectly discriminate samples of chamomile, passionflower, and valerian in a set of 20 samples, overcoming the difficulties related to dealing with different types of herbal preparations including pure herbs, infusions, tablets, capsules and herbal drops

Microneedle based electrochemical (bio)sensing: towards decentralized and continuous health status monitoring

Microneedle (MN) based electrochemical (bio)sensing has become a growing field within the discipline of analytical chemistry as a result of its unique capacity for continuous, decentralized health status monitoring. There are two significant advantages to this exclusive feature: i) the ability to directly analyze interstitial fluid (ISF), a body fluid with a similar enough composition to plasma (and blood) to be considered a plentiful source of information related to biologically relevant molecules and biomarkers; and ii) the capacity to overcome some of the major limitations of blood analysis including painful extraction, high interferant concentrations, and incompatibility with diagnosis of infants (and especially newborns). Recent publications have demonstrated important advancements in electrochemical MN sensor technology, among which are included new MN fabrication methods and various modification strategies, providing different architectures and allowing for the integration of electronics. This versatility highlights the undeniable need for interdisciplinary efforts towards tangible progress in the field. In a context evidently dominated by glucose sensing, which is slowly being expanded towards other analytes, the following crucial questions arise: to what extent are electrochemical MN (bio)sensors a reliable analytical tool for continuous ISF monitoring? Which is the best calibration protocol to be followed for in vivo assays? Which strategies can be employed to protect the sensing element during skin penetration? Is there an appropriate validation methodology to assess the accuracy of electrochemical MN (bio)sensors? How significant is the distinction between successful achievements in the laboratory and the real commercial feasibility of products? This paper aims to reflect on those previous questions while reviewing the progress of electrochemical MN (bio)sensors in the last decade with a focus on the analytical aspects. Overall, we describe the current state of electrochemical MN (bio)sensors, the benefits and challenges associated to ISF monitoring, as well as key features (and bottlenecks) regarding its implementation for in vivo assays

Why not glycine electrochemical biosensors?

Glycine monitoring is gaining importance as a biomarker in clinical analysis due to its involvement in multiple physiological functions, which results in glycine being one of the most analyzed biomolecules for diagnostics. This growing demand requires faster and more reliable, while affordable, analytical methods that can replace the current gold standard for glycine detection, which is based on sample extraction with subsequent use of liquid chromatography or fluorometric kits for its quantification in centralized laboratories. This work discusses electrochemical sensors and biosensors as an alternative option, focusing on their potential application for glycine determination in blood, urine, and cerebrospinal fluid, the three most widely used matrices for glycine analysis with clinical meaning. For electrochemical sensors, voltammetry/amperometry is the preferred readout (10 of the 13 papers collected in this review) and metal-based redox mediator modification is the predominant approach for electrode fabrication (11 of the 13 papers). However, none of the reported electrochemical sensors fulfill the requirements for direct analysis of biological fluids, most of them lacking appropriate selectivity, linear range of response, and/or capability of measuring at physiological conditions. Enhanced selectivity has been recently reported using biosensors (with an enzyme element in the electrode design), although this is still a very incipient approach. Currently, despite the benefits of electrochemistry, only optical biosensors have been successfully reported for glycine detection and, from all the inspected works, it is clear that bioengineering efforts will play a key role in the embellishment of selectivity and storage stability of the sensing element in the sensor

Toward in vivo transdermal pH sensing with a validated microneedle membrane electrode

We present herein the most complete characterization of microneedle (MN) potentiometric sensors for pH transdermal measurements for the time being. Initial in vitro assessment demonstrated suitable analytical performances (e.g., Nernstian slope, linear range of response from 8.5 to 5.0, and fast response time) in both buffer media and artificial interstitial fluid (ISF). Excellent repeatability and reproducibility together with adequate selectivity and resiliency facilitate the appropriateness of the new pH MN sensor for transdermal ISF analysis in healthcare. The ability to resist skin insertions was evaluated in several ex vivo setups using three different animal skins (i.e., chicken, pork, and rat). The developed pH MN sensor was able to withstand from 5 to 10 repetitive insertions in all the skins considered with a minimal change in the calibration graph (<3% variation in both slope and intercept after the insertions). Ex vivo pH measurements were validated by determining the pH with the MN sensor and a commercial pH electrode in chicken skin portions previously conditioned at several pH values, obtaining excellent results with an accuracy of <1% and a precision of <2% in all cases. Finally, pH MN sensors were applied for the very first time to transdermal measurements in rats together with two innovative validation procedures: (i) measuring subcutaneous pH directly with a commercial pH microelectrode and (ii) collecting ISF using hollow MNs and then the pH measurement of the sample with the pH microelectrode. The pH values obtained with pH MN sensors were statistically more similar to subcutaneous measurements, as inferred by a paired sample t-test at 95% of confidence level. Conveniently, the validation approaches could be translated to other analytes that are transdermally measured with MN sensors

Lactate biosensing for reliable on-body sweat analysis

Wearable lactate sensors for sweat analysis are highly appealing for both the sports and healthcare fields. Electrochemical biosensing is the approach most widely used for lactate determination, and this technology generally demonstrates a linear range of response far below the expected lactate levels in sweat together with a high influence of pH and temperature. In this work, we present a novel analytical strategy based on the restriction of the lactate flux that reaches the enzyme lactate oxidase, which is immobilized in the biosensor core. This is accomplished by means of an outer plasticized polymeric layer containing the quaternary salt tetradodecylammonium tetrakis(4-chlorophenyl) borate (traditionally known as ETH500). Also, this layer prevents the enzyme from being in direct contact with the sample, and hence, any influence with the pH and temperature is dramatically reduced. An expanded limit of detection in the millimolar range (from 1 to 50 mM) is demonstrated with this new biosensor, in addition to an acceptable response time; appropriate repeatability, reproducibility, and reversibility (variations lower than 5% for the sensitivity); good resiliency; excellent selectivity; low drift; negligible influence of the flow rate; and extraordinary correlation (Pearson coefficient of 0.97) with a standardized method for lactate detection such as ion chromatography (through analysis of 22 sweat samples collected from 6 different subjects performing cycling or running). The developed lactate biosensor is suitable for on-body sweat lactate monitoring via a microfluidic epidermal patch additionally containing pH and temperature sensors. This applicability was demonstrated in three different body locations (forehead, thigh, and back) in a total of five on-body tests while cycling, achieving appropriate performance and validation. Moreover, the epidermal patch for lactate sensing is convenient for the analysis of sweat stimulated by iontophoresis in the subjects' arm, which is of great potential toward healthcare applications

Equations for the Correlation and Prediction of Partition Coefcients of Neutral Molecules and Ionic Species in the Water-Isopropanol Solvent System

We use literature data on solubilities of 46 compounds in the water-isopropanol (IPA) system to obtain the corresponding partition coefficients, P, for transfer from water to water- IPA mixtures. We have then used our previously constructed linear free energy equation to obtain equations that correlate log10 P at water-IPA intervals across the entire water-IPA system. These equations can then be used to predict partition coefficients and solubilities of further compounds in the water-IPA systems at 298 K. The coefficients in our linear free energy equation encode information on the physicochemical properties of the water- IPA mixtures. We show that the hydrogen bond basicity of the water-IPA mixtures only increases slightly from water to IPA, but that the hydrogen bond acidity of the mixtures decreases markedly from water to IPA in a smooth continuous manner. We have also used data on ions and on ionic species to set out equations for the estimation of their partition coefficients from water to water-IPA mixtures. We find that for partition from water to IPA itself, log10 P = − 1.81 for H+

Simultaneous determination of iron and copper using screen-printed carbon electrodes by adsorptive stripping voltammetry with o-phenanthroline.

The simultaneous voltammetric determination of iron and copper was accomplished by an unmodified screen-printed carbon electrode based on the complexation with o-phenanthroline assisted by the reducing agent ferrocyanide. The detection limits were 3.74 and 0.34 µg L−1 for iron and copper, respectively, and a linear response was observed from 12.5 to 400 µg L−1 for iron and from 1.14 to 400 µg L−1 for copper. This method successfully avoids interferences between iron and copper while allowing the simultaneous determination of both low and high metal concentrations usually found in the environment. Finally, a certified wastewater reference material was successfully analyzed, confirming the precision and accuracy of the proposed method