El SERS como sensor de pH en entornos nanométricos

La Espectroscopia SERS (Surface-Enhanced Raman Spectroscopy) se caracteriza por la enorme intensificación de la señal Raman de una molécula cuando se encuentra en las proximidades de una superficie metálica rugosa a escala nanométrica. Se trata, por tanto, de una técnica de alta sensibilidad que permite el registro de muestras a muy baja concentración y además, de alta selectividad, ya que únicamente aquellas moléculas que presenten un centro capaz de interaccionar con el metal, como un par de electrones libres o un grupo funcional con carga negativa, son las que producen intenso efecto SERS. Actualmente, es una técnica que cada vez se utiliza más como herramienta analítica para la detección e identificación molecular a nivel de trazas, del orden de ppb, o incluso de moléculas individuales, y con aplicación en diferentes áreas de interés como son el medio ambiente, la medicina o la bioquímica, para el control de contaminantes ambientales2, glucosa en sangre3 o secuenciación de péptidos4, respectivamente. Sin embargo, una de las aplicaciones más novedosas del SERS es como medidor óptico de pH a escala nanométrica.5 Esto es posible en aquellas moléculas que presentan, además de un centro de interacción con el metal, un grupo funcional que se vea afectado por el pH del medio y que no interviene en el proceso de adsorción superficial como es el caso de la molécula de carbendazim. Carbendazim (MBC) es un derivado del bencimidazol donde el hidrógeno imidazólico es sustituido por un grupo carbamato, el cual presenta un enlace NH sensible al pH del medio, mientras que la interacción con la superficie metálica se produce, principalmente, a través del nitrógeno aromático del anillo imidazólico. Experimentalmente se han registrado los espectros SERS de MBC en coloide de plata en un rango de pH entre 2-12. Las nanopartículas de plata se han obtenido a partir de una disolución de AgNO3 utilizando el hidrocloruro de hidroxilamina como agente reductor.2 Con objeto de conocer la asignación vibracional de las bandas del espectro, se han calculado las frecuencias vibracionales de cada una de las especies iónicas y de la forma neutra de MBC tanto aisladas como unidas a un átomo de plata mediante métodologías DFT. De esta forma, las dos bandas registradas en la región de 1200 cm-1, 1230 y 1270 cm-1, cuyas intensidades son sensibles al pH del medio, se han asignado a la deformación en el plano del enlace NH amídico e imidazólico, respectivamente. La relación de intensidad entre estas dos bandas frente al pH da como resultado una dependencia lineal. Por tanto, podría utilizarse este dispositivo como un nanosensor óptico capaz de medir el pH local de forma continua en un rango de 6-14, en aquellos ambientes donde es difícil medir el pH de forma convencional como son en células animales o vegetales y en tejidos dañados como control de lesiones tumorales.6 Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. 2 López-Tocón, I.; Otero, J.C.; Arenas, J.F.; Garcia-Ramos, J.V.; Sanchez-Cortes, S. Anal. Chem. 2011, 83, 2518. 3 Kong, K.V.; Ho, C.J.H.; Gong, T.X.; Olivo, M. Biosensor & Bioelectronics 2014, 56, 186. 4 Garrell, R.L. Anal. Chem. 1989, 61, 401. 5 Bishnoi, S.; Rozell, C.; Levin, C.; Hallas, N. NanoLett. 2006, 6, 1687. 6 Vaupel, P.; Kallinowski, F.; Okunieff, P. Cancer Res. 1989, 49, 6449.Universidad de Málaga. Campus e Excelencia Internacional Andalucía Tech

Las TIC como elemento innovador en el proceso de enseñanza-aprendizaje de la asignatura de Química Física

Se ha desarrollado un entorno virtual para la asignatura de Química Física General que se imparte en el primer curso del Grado en Química de la UMA, haciendo uso de la plataforma educativa Moodle. Se han creado diversos recursos digitales entre los que destacan: los documentos en formato pdf de temas monográficos, en especial de aquellos que son difíciles de asimilar, y los cuestionarios on-line correspondientes a cada tema del programa teórico de la asignatura que sirven como autoevaluación para el propio alumno. Mediante la incorporación de las nuevas tecnologías de la información y comunicación (TIC) al proceso de enseñanza-aprendizaje de esta asignatura se pretende modernizar la metodología docente para la materia de Química Física, hasta ahora impartida exclusivamente en clases magistrales, además de crear una dinámica pedagógica más activa, participativa e interactiva. En definitiva, se trata de adaptar esta asignatura al nuevo modelo de Espacio Europeo de Enseñanza Superior (EEES).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Análisis psicométrico de un cuestionario Moodle propuesto como examen online en la asignatura de Química Física General

[EN] A virtual questionnaire is proposed, within the Moodle learning platform, as an online exam for the subject “General Physical Chemistry” of the first course in the degree of Chemistry. It is a questionnaire with 20 items of multiple choice type that have more than three distractors, with a single or multiple correct responses, a duration of 90 minutes, a sequential navigation and a delayed feedback. In order to check the effectiveness and validity of the questionnaire, the statistical and psychometric results, calculated on the basis of a classical theory of tests, are analyzed for the questionnaire as a whole and for each item. The psychometric parameters or indicators such as, the facility index, the discrimination index and the internal consistency coefficient are analyzed. These indicators will make possible to detect the weaknesses of this type of test as a virtual assessment tool.[ES] Se propone un cuestionario virtual, dentro de la plataforma de aprendizaje Moodle, como examen online, a distancia, para la asignatura Química Física General de primero de Grado en Química. Se trata de un cuestionario con 20 ítems del tipo opción múltiple con una única o varias respuestas correctas, con una duración de hora y media, una navegación secuencial y una retroalimentación diferida. Para conocer la efectividad y fiabilidad del cuestionario se analizan los resultados estadísticos y psicométricos, calculados en base a la teoría clásica de los tests, del cuestionario en conjunto y de cada uno de los ítems que lo integran. Como parámetros o indicadores psicométricos se analizan el índice de facilidad, el índice de discriminación y el coeficiente de consistencia interna, entre otros. Estos indicadores van a permitir detectar las debilidades de este tipo de prueba como herramienta de evaluación virtual.López-Tocón, I. (2021). Análisis psicométrico de un cuestionario Moodle propuesto como examen online en la asignatura de Química Física General. En IN-RED 2021: VII Congreso de Innovación Edicativa y Docencia en Red. Editorial Universitat Politècnica de València. 690-700. https://doi.org/10.4995/INRED2021.2021.13465OCS69070

Elaboración de material multimedia y uso de un entorno virtual en el proceso de enseñanza-aprendizaje de las prácticas de laboratorio en Química Física

[EN] Using new technologies, such as the Moodle e-learning platform, a new and more active didactic strategy has been developed in the training of lab practices in subjects assigned to the Department of Physical Chemistry, specifically in the subject Basic Operations of Laboratory, which is taught in the first year of the Degree in Chemistry. A new multimedia and audiovisual teaching material has been designed and elaborated on the most basic procedures that are carried out in a chemical lab. This educational material serves as a support for the teacher in the lab classes, besides supposing a complement to the practices’ script for the students. In this way, we try to facilitate the autonomous learning of the student, to optimize the working time in the lab and to increase the availability of the teacher. A question bank has also been developed for each lab practices in order to carry out on-line tests of knowledge once the practice is finished.[ES] Haciendo uso de las nuevas tecnologías, como es la plataforma educativa Moodle, se ha desarrollado una nueva estrategia didáctica más activa y dinámica en la impartición de las prácticas de laboratorio en asignaturas adscritas al Departamento de Química Física, y en concreto en la asignatura de Operaciones Básicas de Laboratorio, que se imparte en el primer curso del Grado en Química. Se ha diseñado y elaborado un nuevo material didáctico multimedia y audiovisual, sobre los procedimientos más básicos que se realizan en un laboratorio químico, y que sirve como apoyo para el docente en las clases presenciales de laboratorio, y como complemento al guión de prácticas que recibe el alumno. De esta forma se intenta facilitar el aprendizaje autónomo del estudiante, optimizar el tiempo de trabajo en el laboratorio y aumentar la disponibilidad del profesor. También se ha elaborado un banco de preguntas para cada una de las prácticas con objeto de realizar pruebas de conocimiento on-line, una vez finalizada la práctica en el laboratorio.Este trabajo se enmarca dentro del Proyecto Docente PIE15-027, correspondiente a la Convocatoria de Proyectos de Innovación Educativa 2015-2017 de la Universidad de Málaga.López Tocón, I. (2017). Elaboración de material multimedia y uso de un entorno virtual en el proceso de enseñanza-aprendizaje de las prácticas de laboratorio en Química Física. En In-Red 2017. III Congreso Nacional de innovación educativa y de docencia en red. Editorial Universitat Politècnica de València. 34-42. https://doi.org/10.4995/INRED2017.2017.6733OCS344

Pseudocomplemented Semilattices, Boolean Algebras, and Compatible Products

AbstractPseudocomplemented semilattices are studied here from an algebraic point of view, stressing the pivotal role played by the pseudocomplements and the relationship between pseudocomplemented semilattices and Boolean algebras. Following the pattern of semiprime ring theory, a notion of Goldie dimension is introduced for complete pseudocomplemented lattices and calculated in terms of maximal uniform elements if they exist in abundance. Products in lattices with 0-element are studied and questions about the existence and uniqueness of compatible products in pseudocomplemented lattices, as well as about the abundance of prime elements in lattices with a compatible product, are discussed. Finally, a Yood decomposition theorem for topological rings is extended to complete pseudocomplemented lattices

The role of the electrolyte on the SERS spectra of pyridine

SERS spectra of pyridine has been recorded on a silver electrode in a potential range from 0.0 to -1.20 V with a saline solution, pyridine / KCl (0.1M / 0.1M), by using the 514.5 nm exciting line by us [1]. Under these experimental conditions, the maximum intensity of the enhanced 8a and 9a bands is reached at -0.75 V and -1.20 V, respectively, being the 9a band what dominates the spectrum at negative electrode potential. Although this behavior has been explained under a resonant charge transfer mechanism, the nature of the electronic resonance processes involved in the enhancement of each band is different. The 8a band is enhanced due to an electronic excitation between the ground and excited charge transfer electronic state of the metal-adsorbate surface complex, while the activity of the 9a band is due to a plasmon-like excitation taking into account an overall electronic structure of small metal clusters [2] which is able to selectively modify the relative intensities of specific SERS band. We intend now to record pyridine SERS spectra under the same experimental conditions but varying only the type of electrolyte in order to check how it affects the relative intensities and vibrational wavenumbers of the bands as well as the electrode potential to which the enhanced bands reach the maximum intensity. Different electrolytes like KCl, NaCl, KBr, NaBr and Na2SO4, have been selected in such way that allows us to compare SERS spectra in which changes only the cation or the anion of the electrolyte. From the analysis of all these SERS spectra, it can be concluded that no significant wavenumbers shifts have been detected, while the relative intensities of the bands and the electrode potential to which the maximum intensity is reached are slightly modified. NaBr electrolyte requieres more negative electrode potential in order to enhance the 8a and 9a bands and to resolve the 12 and 1 pair. Acknowledgements We are grateful to the Spanish MINECO (CTQ2012-31846) and Junta de Andalucía (FQM-5156 and 6778), for financial support. References [1] J.F. Arenas, I. López Tocón, J.C. Otero, and J.I. Marcos, J. Phys. Chem. 100, 9254-9261 (1996). [2] J. Román-Pérez, I. López-Tocón, J.L. Castro, J. F. Arenas, J. Soto and J.C. Otero, Phys. Chem. Chem. Phys.17, 2326-2329 (2015).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Electrochemical SERS spectra of isonicotinic acid analyzed under a photoinduced charge-transfer mechanism

Isonicotinic (IN) acid is one of the three monocarboxilic derivatives of pyridine in which the acid group is located in para-position of the heterocyclic ring. It is a weak acid (pK2=4.86) and therefore, it is not completely ionized in neutral aqueous solutions, being the zwitterion and the anion the majority chemical species at neutral pH. In acidic solutions (pK1=1.84) the pyridinic nitrogen atom can be protonated yielding a third chemical species with positive charge [1]. In addition, IN acid shows two functional centres that can interact with the silver metallic surface such as the carboxilate group and the aromatic nitrogen atom. Therefore, the analysis of the SERS spectra of IN has been focused on identifying the chemical species adsorbed on the silver surface and its centre of interaction by considering the participation of a photoinduced charge-transfer (CT) mechanism in each particular SERS record as we have previously detected in the SERS of pyridine derivatives [2]. SERS spectra of the IN acid (5x10-3 M) have been recorded on silver at electrode potentials ranging from 0.00 up to -1.00 V and at different pH by using 0.1 M Na2SO4 aqueous solution as electrolyte. The figure shows the SERS recorded at basic pH. The experimental set up is described elsewhere and the excitation line of 514.5 nm wavelength was used. [2]. The detection of the presence of CT processes, which are similar to resonance Raman, requires to carry out quantum mechanical calculations [2].Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Frequency shift on the potential-dependent surface-enhanced Raman scattering of pyridine: simplified models for metal and solvent effects

The electronic structure of adsorbates is altered when it interacts with a surface, modifying the properties of both entities and giving rise to interesting phenomena related to heterogeneous catalysis or molecular electronics. If such surface is a metallic substrate, the electrode potential can be used to tune this interaction. Potential-dependent Surface-Enhanced Raman Scattering (SERS) is a particularly useful technique to study the induced effects on the molecule when the metal-adsorbate surface complex is formed, as the observed frequency shifts of the vibrational modes can provide information about it. However, from the computational point of view, these systems are difficult to model, because the macroscopic metal cannot be modelled easily using quantum mechanics. As an approach, we propose a simple model using silver atomic wires with different size and charge bonded to the molecule (AgnPyq, n = 2,3,5,7 and q = 0 and ±1 for n even and odd, respectively) which has been developed by the group and provides a good description of the effect of the electrode potential on the chemical enhancement mechanism of SERS.1-3 Electronic calculations were performed using Density Functional Theory (DFT). In order to study the frequency shifts, solvent effects have been taken into account by using the Polarizable Continuum Model (PCM). We have used three different functionals (B3LYP, PW91 and M06HF) and two basis sets (LANL2DZ for all atoms and LANL2DZ for Ag and 6-31G(d) for C,N,H) and, in all cases, a good agreement is achieved in terms of amplitude and trend of the frequency shift for most of the vibrational modes, especially when solvent interactions are included. The method was extended to other metals and solvents giving results in agreement with the available experimental data.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Charge Transfer mechanism in the Surface Enhanced Raman Scattering of 2,2'-bipyridine recorded on a silver electrode

Nowadays, Surface Enhanced Raman Spectroscopy (SERS) has become a powerful technique to investigate the electronic structure of surface-molecule hybrid systems due to the huge enhancement of the Raman signal. It is established that the origin of this enhancement has two main contributions; the electromagnetic (EM), related to surface plasmons, and the chemical mechanism, due to resonant charge transfer (CT) processes between the adsorbate and the metal. With the aim to investigate the SERS-CT of bipyridine and to identify charge transfer process, the spectra were recorded on silver electrode by using three different wavelengths (473, 532 and 785 nm) in a range from 0.0 up to -1.4 V electrode potential. The electrode potential was modelled in the calculations with atomic silver wires of different size and charge attached to the BPy molecule (AgnBPyq, with q = 0 for n = 2 and q = ±1 for n = 3, 5, 7) and were computed with Density Functional Theory (DFT). Although BPy shows a trans conformation in solution, a cis conformation was chosen for its chelating properties. The results indicate that the intensification of the ~1550 cm-1 band at negative potentials is due the Franck-Condon factors related to the resonant CT process from the metal to the BPy molecule.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

An MS-CASPT2 Study of the Photodecomposition of 4- Methoxyphenyl Azide. Role of Internal Conversion and Intersystem Crossing

Aryl azides photochemistry is strongly dependent on the substituent relative position, as has been studied by time resolved resonant Raman (TR3) spectroscopy for 4-methoxyphenyl azide and its isomer 3-methoxyphenyl azide. When irradiated at 266 nm, the former results in 4,4’-dimethoxyazobenzene whereas the latter forms 1,2-didehydroazepine. It is proposed that the key step of the reactions is the formation of a nitrene derivative. Recently, it has been proposed by us that nitrenes might have a relevant role in the Surface-Enhanced Raman Scattering (SERS) of p-aminothiophenol, however, the molecular mechanism is not well known in neither of these cases. Therefore, we studied the photodecomposition of 4-methoxyphenyl azide using multiconfigurational self-consistent field methods (MC-SCF) with the CAS-SCF and MS-CASPT2 approximations and calculated the resonant Raman spectra of the relevant species using a multi-state version of Albrecht’s vibronic theory. The results propose that the reaction follows a two steps sequence after irradiation at 266 nm: an intersystem crossing 21A’/23A’’ which decays through a 21A’/21A’’ conical intersection producing molecular nitrogen and triplet 4-methoxyphenyl nitrene in its ground state.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech