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

The cross-correlation among tracers of the underlying large-scale mass distribution in the universe

[eng] The work presented in this Ph.D. thesis is mostly related to the measurement and interpretation of the cross-correlation of different species. This is achieved using spectroscopic data from the BOSS survey of which I am a member. Along the thesis I will be visiting the cross-correlation of different tracers. This tracers include the Mg II absorbers, the Damped Lyα Systems (DLA), the Lyα forest, the quasars and the galaxies. Save the last mentioned, all tracers are measured from quasar spectra from BOSS. The galaxies are taken from the BOSS CMASS catalogue. The first cross-correlation I address is that of Mg II absorbers with CMASS galaxies. I present a new method for measuring the equivalent width of metal absorbers, developed in collaboration with Britt Lundgren and Jordi Miralda- Escud´e, and I apply it to the absorption doublet. Then I shift my attention to the DLAs and the Ly forest. Since the Lyα forest autocorrelation has already been studied (see e.g. Delubac et al. 2015; Blomqvist et al. 2015), the Lyα parameters are well constrained. I use those constraints to provide more accurate measurements of the DLA bias when analysing the cross-correlation of DLAs with the Lyα forest. The study has been made in collaboration with Andreu Font-Ribera, Nicol´as Busca, and Jordi Miralda-Escud´e. In particular, the estimator for the cross-correlation was orig- inally developed by Nicol´as Busca for the analysis of the Lyα autocorrelation and later adapted to the measurement of the cross-correlation. The first two cross-correlations studied and more importantly their interpre- tation, are mostly focused on large scales, where the linear theory is valid. At smaller scales, non-linear effects start to kick in and the linear approximation is no longer valid. I measure the cross-correlation of quasars with the Lyα forest, and focus on the small scales effects. I show that a contamination from metals is clearly present in the cross-correlation. Also, I discuss some of the possible non-linear effects and give a simple theoretical model to explain the effect that quasar radiation can have in the surrounding hydrogen clouds.[cat] El treball fet en aquesta tesi doctoral es basa principalment en la mesura i interpretació de la correlació creuada entre diferents traçadors. Els traçadors emprats en aquesta tesi són les galàxies del catàleg CMASS, els quàsars del catàleg de Baryon Oscillations Spectroscopic Survey (BOSS), els sistemes absorbidors de MgII, el bosc de Lya i els sistemes Lya esmorteïts o DLAs (els tres darrers són sistemes absorbidors detectats als espectres dels quàsars). El projecte BOSS és un dels quatre projectes que han composat la tercera fase de la col·laboració de l’Sloan Digital Sky Survey (SDSS) el principal objectiu del qual ha estat la detecció del senyal de les oscil·lacions acústiques dels barions (BAO, de l’anglès Baryon Acoustic Oscillations) i que ha estat recollint espectres electromagnètics de quàsars i galàxies des de la tardor de 2013 fins a la tardor de 2014. L’estructura d’aquesta tesi es divideix en tres capítols que fan referència a l’estudi de diferents traçadors. En el capítol ì els traçadors emprats són les galàxies de CMASS i els sistemes absorbidors de MgII; en el capítol 3, els DLA i el bosc de Lya, i en el capítol 5, els quàsars i el bosc de Lya. A més la tesi compta amb una introducció en català, una altra en anglès i un compendi de les conclusions obtingudes

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

The Cross-correlation of MgII Absorption and Galaxies in BOSS

We present a measurement of the cross-correlation of MgII absorption and massive galaxies, using the DR11 main galaxy sample of the Baryon Oscillation Spectroscopic Survey of SDSS-III (CMASS galaxies), and the DR7 quasar spectra of SDSS-II. The cross-correlation is measured by stacking quasar absorption spectra shifted to the redshift of galaxies that are within a certain impact parameter bin of the quasar, after dividing by a quasar continuum model. This results in an average MgII equivalent width as a function of impact parameter from a galaxy, ranging from 50 kpc to more than 10 Mpc in proper units, which includes all MgII absorbers. We show that special care needs to be taken to use an unbiased quasar continuum estimator, to avoid systematic errors in the measurement of the mean stacked MgII equivalent width. The measured cross-correlation follows the expected shape of the galaxy correlation function, although measurement errors are large. We use the cross-correlation amplitude to derive the bias factor of MgII absorbers, finding bMgII = 2.33 \pm? 0.19, where the error accounts only for the statistical uncertainty in measuring the mean equivalent width. This bias factor is larger than that obtained in previous studies and may be affected by modeling uncertainties that we discuss, but if correct it suggests that MgII absorbers at redshift z \simeq 0:5 are spatially distributed on large scales similarly to the CMASS galaxies in BOSS. Keywords: galaxies: haloes, galaxies: formation, quasars: absorption lines, large-scale structure of universeComment: Accepted for publication to MNRAS. Accepted 2014 December 12. Received 2014 November 29; in original form 2014 February

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

A Strong Blend in the Morning: Studying the Circumgalactic Medium Before Cosmic Noon with Strong, Blended Lyman-α\alpha Forest Systems

We study of the properties of a new class of circumgalactic medium absorbers identified in the Lyman-$\alpha$ forest: "Strong, Blended Lyman-$\alpha$" (or SBLA) absorption systems. We study SBLAs at $2.4<z<3.1$ in SDSS-IV/eBOSS spectra by their strong extended Lyman-$\alpha$ absorption complexes covering 138 km/s with an integrated $\log (N_{HI}/$cm$^{-2}) =16.04^{+0.05}_{-0.06}$ and Doppler parameter $b=18.1^{+0.7}_{-0.4}$ km/s. Clustering with the Lyman-$\alpha$ forest provides a large-scale structure bias of $b = 2.34\pm0.06$ and halo mass estimate of $M_h \approx 10^{12}{\rm h^{-1}M_{sol}}$ for our SBLA sample. We measure the ensemble mean column densities of 22 metal features in the SBLA composite spectrum and find that no single-population multiphase model for them is viable. We therefore explore the underlying SBLA population by forward modelling the SBLA absorption distribution. Based on covariance measurements and favoured populations we find that $\approx 25$% of our SBLAs have stronger metals. Using silicon only we find that our strong metal SBLAs trace gas with a $\log(n_H /$cm$^{-3}) > -2.45$ for $T=10^{3.5}$K and show gas clumping on $<255$ parsec scales. We fit multiphase models to this strong sub-population and find a low ionization phase with $n_H=1$cm$^{-3}$, $T=10^{3.5}$K and $[X/H]=0.8$, an intermediate ionization phase with $\log(n_H /$cm$^{-3}) = -3.35$, $T=10^{3.5}$K and $[X/H]=-1.1$, and a poorly constrained higher ionization phase. We find that the low ionization phase traces cold, dense super-solar metallicity gas with a clumping scale of just 0.009 parsecs.Comment: 28 pages, submitted to MNRA

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