Maria Mercedes Velázquez

Universidad de Salamanca · Department of Physical Chemistry

In the last years N-doped carbon nanoparticles have been shown to have improved antibacterial activity over the undoped nanomaterial, but it is difficult to find correlations between the structure of the nanoparticle and its antibacterial activity. This prevents us from proposing a clear antibacterial mechanism and makes it difficult to select mate...

Graphene-based nanomaterials have emerged as promising materials for a wide range of applications.. Therefore, in the last years carbon nanoparticles have attracted great attention due to their low toxicity, high biocompatibility and easy preparation. Recently, N-doped carbon nanoparticles have been shown to have improved antibacterial activity ove...

The reduction of CO2 emissions and its elimination from the atmosphere has become one of the major problems worldwide, since CO2 is the main cause of the greenhouse effect and climate change. In recent years, a great number of carbonaceous materials that can be used as CO2 adsorbents have been synthesized. The strategy is usually to synthesize the...

Carbon nanoparticles (CNPs) have received a great attention because of their unique optical and electronic properties. To modulate their photoluminescence properties, nitrogen doping has been previously reported, but it was difficult to find correlations between the photoluminescence properties and the nanoparticle structure. For this purpose, we a...

The Langmuir–Blodgett (LB) technique is a very convenient tool to build thin films, with sizes that vary from one to a few molecular layers. In general, the technique was applied to amphiphilic molecules that are capable of generating stable Langmuir monolayers at the water/air interface, which can then be transferred to a given solid substrate; ho...

Graphene oxide is a derivate of graphene obtained by oxidation of graphite and other carbonaceous materials. The more accepted structure consists in carbonyl and carboxyl groups located at the edge of the graphene network and hydroxyl and epoxy groups attached to the basal plane. The percentage of O-groups depends on the synthesis route and the mat...

The Raman spectra of the topological crystalline insulator Pb0.77Sn0.23Se flakes of thickness below 150 nm show the presence of plasma enhanced phonons related to topologically protected surface electronic states. The Fröhlich interaction between the topologically protected surface Dirac electrons and the phonons activates the forbidden phonons nea...

Journal of COLLOID AND INTERFACE SCIENCE The unique luminescence of upconverting nanoparticles, combined with stunning Graphene oxide quenchers, allowed a new generation of optical sensors. The underlying action mechanisms were revealed. Contribution of resonance energy transfer to the luminescence quenching of upconversion nanoparticles with graph...

Raman spectroscopy is a technique widely used to detect defects in semiconductors because it provides information of structural or chemical defects produced in its structure. In the case of graphene monolayer, the Raman spectrum presents two bands centered at 1582 cm−1 (G band) and 2700 cm−1 (2D band). However, when the periodic lattice of graphene...

We address here the role of oxidation impurities on the structure of graphene oxide films at the air-water interface by specular neutron scattering (SNR). We study films of purified (PGO) and non-purified graphene oxide (GO) in the close-packed state. Non-purified graphene oxide is constituted by GO layers with oxidation impurities adsorbed on the...

Upconversion nanoparticles (UCNP) are increasingly used due to their advantages over conventional fluorophores, and their use as resonance energy transfer (RET) donors has permitted their application as biosensors when they are combined with appropriate RET acceptors such as graphene oxide (GO). However, there is a lack of knowledge about the desig...

Carbon nanomaterials are widespread in the atmospheric aerosol as a result of the combustion processes and their extensive industrial use. This has raised many question about the potential toxicity associated with the inhalation of such nanoparticles, and its incorporation into the lung surfactant layer. In order to shed light on the main physical...

New nanocomposites based on graphene oxide with polyaniline (PANI) or Fe3O4 nanoparticles (MG) were synthesized for CO2 capture. To study the effect of graphene oxide structure on the CO2 retention capacity, CO2/N2 selectivity, adsorbent regeneration and adsorption kinetics, two different graphene oxides were synthesized by oxidation of graphite fl...

Raman spectra of graphene oxides (GOs) with different chemical composition and synthesized by oxidation of distinct starting materials were analyzed in order to relate spectral features with the structural properties. The chemical composition of different graphene oxides was determined by X-ray photoelectron spectroscopy (XPS) and nanoplatelets wer...

The Langmuir‐Blodgett (LB) methodology is based on the transfer process of a monolayer adsorbed at the water interface, Langmuir film, from the air‐water interface onto solids by vertical dipping of the substrate immersed on the subphase. The technique allows the continuous variation of material density, packing, and arrangement by compressing or e...

A pharmaceutical vehicle based on the encapsulation of liposomes with unmodified albumin has been designed, formulated, and in vitro characterized. Microscopy was used to investigate particle morphology and dynamic light scattering to determine the size and zeta potential. Vancomycin was selected as a model drug for water-soluble and moderately alb...

Silver nanowires 90 nm in diameter and 9 �m in length have been synthesized using different capping agents: polyvinyl pyrrolidone (PVP) and alkyl thiol of different chain lengths. The nanowire structure is not influenced by the displacement of PVP by alkyl thiols, although alkyl thiols modify the lateral aggregation of nanowires. We examined the ef...

Raman spectra of graphene oxide and thermal reduced graphene oxide were analyzed in order to relate spectral parameters with the structural properties. The chemical composition of different graphene oxides was determined by Organic Elemental Analysis and the microstructure of nanocrystals was analyzed by X-ray diffraction. We find five reported ban...

We synthesized graphene oxide sheets of different functionalization by oxidation of two different starting materials, graphite and GANF® nanofibers, followed by purification based on alkaline washing. The chemical structure of graphene oxide materials was determined by XPS and the nanoplatelets were characterized by zeta potential and Dynamic Light...

We used Quartz Crystal Microbalance with Dissipation to study the mechanical properties, kinetics of adsorption and the amount of CdSe QDs adsorbed onto SiO2 sensor, referred as bare sensor, onto the sensor modified with a film of the polymer poly (maleic anhydride-alt-1-octadecene), PMAO, or with a film of the Gemini surfactant ethyl-bis (dimethyl...

We propose to use the self-assembly ability of a block copolymer combined with compression-expansion cycles to obtain CdSe quantum dots (QDs) structures of different morphology. The methodology proposed consists in transferring onto mica mixed Langmuir monolayers of QDs and the polymer poly (styrene-co-maleic anhydride) partial 2 buthoxy ethyl este...

We study the effect of oxidative impurities on the properties of graphene oxide and on the graphene oxide Langmuir-Blodgett films (LB). The starting material was grupo Antolín nanofibers (GANF) and the oxidation process was a modified Hummers method to obtain highly oxidized graphene oxide. The purification procedure reported in this work eliminate...

An industrial process to obtain few layer sheets of graphene oxide (GRAnPH®) by using GANF® carbon nanofibers as starting material is presented. GRAnPH® sheets are suitable for the preparation of highly exfoliated and stable suspensions in different polar solvents, avoiding the use of additional stabilizers. These suspensions have been deposited ov...

Thin films of colloidal semiconductor CdSe quantum-dots (QDs) and a styrene/maleic anhydride copolymer were prepared by the Langmuir–Blodgett technique and their photoluminescence was characterized by confocal fluorescence lifetime microscopy. In the films, the photoluminescence dynamics are strongly affected by excitation energy migration between...

We studied the ability of poly(octadecene-co-maleic anhydride) (PMAO) and a Gemini surfactant [C18H37 (CH3)2N+Br−–(CH2)2–N+Br−(CH3)2 C18H37] (18-2-18) to assist in the self-assembly process of CdSe quantum dots (QDs) at the air–water interface. Results show that, while QD agglomeration is generally inhibited by the addition of these components to t...

Different LB films of poly(octadecene-co-maleic anhydride), PMAO, poly(styrene-co-maleic anhydride) partial 2 butoxy ethyl ester cumene terminated, PS-MA-BEE, and Gemini surfactant ethyl-bis(dimethyl octadecylammonium bromide), 18-2-18, have been used to study the effect of the substrate coating on the surface self-assembly of CdSe quantum dots (QD...

We propose to use the self-assembly ability of a block copolymer to obtain CdSe quantum dots (QDs) structures of different morphology. The methodology proposed consist in transferring mixed Langmuir monolayers of QDs and the polymer poly (styrene-co-maleic anhydride) partial 2 buthoxy ethyl ester cumene terminated, PS-MA-BEE onto mica by the Langmu...

Films of a few layers in thickness of reduced graphite oxide (RGO) sheets functionalized by the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDPS) are obtained by using the Langmuir-Blodgett method. The quality of the RGO sheets is checked by analyzing the degrees of reduction and defect repair by means of X-ray phot...

Fluorescence probing and ζ-potential measurements have been used to characterize the interactions in bulk between the zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethyl-ammonium]-1-propanesulphonate, CHAPS and the polymer polyethylene imine, PEI. Results show that the strongest attractive interactions are produced between the surfactant and th...

Vesicles are widely used in technological applications including cosmetic products, in microencapsulation for drug delivery, as anticancer agents and in the technology of adhesives, paints and inks. The vesicle size and the surface charge are very important properties from a technological point of view. Thus, the challenge in formulation is to find...

We report the study of the equilibrium and dynamic properties of Langmuir monolayers of poly(styrene-co-maleic anhydride) partial 2-buthoxyethyl ester cumene terminated polymer and the effect of the Mg(NO(3))(2) addition in the water subphase on the film properties. Results show that the polymer monolayer becomes more expanded when the electrolyte...

Fluorescence probing, and ζ-potential measurements have been used to study the interactions between the zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethyl-ammonium]-1-propanesulphonate, CHAPS and two polyelectrolytes with different electric charge. The polyelectrolytes chosen are poly (diallyl-dimethyl-ammonium chloride), PDADMAC, and poly (so...

The glass transition temperature of poly (maleic anhydride-alt-1-octadecen) and poly (styrene-co-maleic anhydride) cumene-terminated thin films has been measured by mechanical relaxation of Langmuir films of these polymers. The dynamical properties show glass-like features (non-Arrhenius relaxation times and non-Debye mechanical response) interpret...

We have studied the effect of the addition of sodium poly(styrene sulfonate) polymers on the properties of monolayers formed by the adsorption of two carboxybetaines with different number of separation methylenes between their charged groups. Fluorescence and surface tension measurements indicate that above the critical aggregation concentration a...

Dynamic light scattering and Cryo-TEM measurements have allowed us to obtain the size and structure of spontaneous aggregates formed by mixtures of Aerosol OT, AOT, and ethylene glycol polymers of different molecular mass. The results presented in this work show that small unilamellar vesicles predominate in pure Aerosol OT solutions and in dilute...

In this laboratory experiment students analyze the role of attractive interactions between the hydrocarbon tails and repulsive interactions between the surfactant head groups on the micellar association process by determining the CMC of two surfactant classes. Using electrical conductivity measurements and pyrene fluorescence emission, the CMCs of...

The dynamic surface tension of micellar surfactant solutions was measured using the maximum bubble pressure method. The chosen surfactants were carboxy- and sulfo-betaines. At the beginning, surface tension decay curves are consistent with a diffusion-controlled adsorption. However, at long times these curves were analyzed by using the Fainerman mo...

To study the influence of the head group in the properties of the mixed monolayers adsorbed at the air-water interface, the surface tension and surface potential of binary mixtures of surfactant have been determined as a function of the surfactant composition. Experiments were carried out with anionic-zwitterionic sodium dodecyl sulfate and dodecyl...

In order to study the effect of charge on the adsorption of surfactants at the air–water interface, two carboxybetaines have been synthesized with different number of separation methylenes between their charged groups. After purification and structure confirmation, the equilibrium and dynamic surface tensions were measured as a function of surfacta...

We have studied the effect of the hydrocarbon length, electrical charge and the addition of inert electrolytes on the mixed aggregation process of alkyl ammonium propane sulfonate and the ionic surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTAB). Using conductivity and fluorescence probing, we determine the cmc o...

We report the formation of Langmuir monolayers of pure zwitterionic hexadecyl 1-N-L-tryptophan glycerol ether (C(16)-TGE) surfactant and mixed monolayers of cationic-zwitterionic surfactant obtained modifying the pH of the subphase. The pressure-area and surface potential-area isotherms and fluorescence microscopy measurements have been used to cha...

The properties of the interface of vesicles of pure sodium bis-(2-ethyl-hexyl) sulfosuccinate (AOT) and binary mixtures composed of AOT with poly(ethylene) glycol (PEG), poly(sodium 4-styrensulfonate) (PSS) and sodium chloride were investigated using absorption and steady-state fluorescence of nabumetone and electrophoretic mobility measurements. R...

Dynamic surface tension of aqueous alkaline solutions of 1-N-l-tryptophan–glycerol-ether surfactants, Cn-TEG, below its critical micelle concentration has been measured as a function of the surfactant concentration and over a temperature range of 19.7–31.5°C. The static tension of the alkyl tryptophan–glycerol-ether surfactants with n=9–11 is analy...

Vesicles of different morphologies were found to form in aqueous solutions of the surfactant sodium bis(2-ethylhexyl)sulfosuccinate, AOT, and in binary mixtures composed of AOT with poly(ethylene glycol) and poly(sodium 4-styrenesulfonate). Using electrical conductivity and fluorescence probing, two critical vesicle concentrations, cvc and cac, wer...

Using equilibrium and dynamic surface tension measurements, we have studied the effect of the addition of poly(sodium 4-styrenesulfonate) (PSS), sodium chloride, and 1,4,7,10,13,16-hexaoxyacyclooctadecane (18-crown-6) on the surface properties of sodium bis(2-ethyl hexyl sulfosuccinate) (AOT). The addition of PSS or NaCl weakly increases the maximu...

By using electrical conductivity and static and dynamic light scattering measurements, the behavior of water-oil (w/o) microemulsions formed by Aerosol OT dissolved in toluene has been examined. To study the effect of the droplet volume fraction on the microemulsion behavior, the water and the surfactant concentrations were systematically modified...

Vesicles were found to form easily in diluted aqueous mixtures of two cationic amphiphiles, the double-tailed, bilayer-forming didodecyldimethylammonium bromide (DDAB) and the single-tailed, micelle-forming dodecyltrimethylammonium chloride (DTAC), and therefore are believed to be spontaneous. A combination of equilibrium tensiometry, conductivity,...

The inclusion of the anti-inflammatory drug, Nabumetone, in -, - and hydroxypropyl--cyclodextrin (CDs) is studied using UV-VIS absorption and steady-state fluorescence emission. Binding constants and thermodynamic parameters of complex formation are determined by spectrofluorimetry. The inclusion phenomena of Nabumetone with the three cyclodextrins...

Using surface tension measurements the interaction between 3-(dimethyldodecylammonio)propanesulfonate, DPS, and poly(vinylpyrrolidone), PVP, poly(acrylic acid sodium salt), PAC, and poly(sodium 4-styrenesulfonate), PSS, has been studied. No interaction between PVP and PAC and the surfactant was observed, while a significant interaction with PSS was...

Using IR spectroscopy and the UV–Vis absorption spectra of solubilized methyl orange (MO) as a probe, the properties of the water pool of some bis(2-ethylhexyl) sodium sulfosuccinate, (Aerosol OT) w/o microemulsions were investigated. The solvents used to prepare the micromulsions were cyclohexane, isooctane, and toluene. From IR water spectra, fou...

In this lab experiment we describe an investigation of the thermodynamics of the formation of cyclodextrin inclusion complexes by using fluorescence spectroscopy. A naphthalene derivative, 4-(6-methoxy-2-naphthyl)-2 butanone (nabumetone), was used as a guest and two different cyclodextrins, β-cyclodextrin (β-CD) and hydroxypropyl β-cyclodextrin, (H...

The interaction between cetyl pyridinium chloride and different-molecular-weight poly(vinylpyrrolidone) (PVP) and poly(ethylene glycol) (PEG) polymers has been investigated by means of electrical conductivity and fluorescence probing. No interaction between the low-molecular-weight PVP polymers and the surfactant was detected, whereas a significant...

Using FTIR, dynamic light scattering, and electrical conductivity measurements, the effect of the addition of two different poly(vinylpyrrolidone) polymers, PVP K15 (MW) 10000 g/mol) and PVP K25 (MW) 24 000 g/mol), on the structure and size of Aerosol OT/isooctane water-in-oil microemulsions has been examined. Results show that the addition of thes...

Using Fourier transform infrared (FT-IR) spectroscopy and electrical conductivity measurements, the study of water-in-oil (w/o) microemulsions of Aerosol OT dissolved in toluene with different water contents, in the absence and in the presence of poly(vinylpyrrolidone) (MW 10 000), PVP K15, and poly(sodium 4-styrenesulfonate) (MW 75 000), PSS, has...

UV-absorption spectroscopy technique enables the determination of the binding constants of some non-steroidal anti-inflammatory drugs to α-, β-, and γ-cyclodextrins. In all cases the stoichiometry of the complex is 1:1 and the drugs have a higher affinity for β-cyclodextrin than for α-cyclodextrin or γ-cyclodextrin. The dielectric constant of the c...

Photophysical properties of a non-steroidal anti-inflammatory drug, Naproxen (6-methoxy alpha-methyl-2-naphthalene acetic acid sodium salt), were investigated in solvents of different polarity, hydrogen donor ability and also in cyclodextrins. The results indicate that in all cases the emitting state is the 1L(b) singlet. In alcoholic solvents, an...

Fluorescence data-emission intensities and decay profiles of benzyl 1-pyrenoate in the presence of dimethylaniline in anionic homomicelles and in mixed micelles have been analyzed with two different models: a kinetic model of diffusion-controlled reactions in micelles with a reflecting boundary surface and a fractal reaction kinetic model. A consta...

The binding of dibenzepin to α-,β-, and γ-cyclodextrins, and to the anionic surfactants: sodium octyl, decyl, dodecyl and tetradecyl sulfates, in aqueous solution, has been studied by a UV spectrophotometric technique. A particular dependence on cyclodextrin cavity size and on surfactant hydrocarbon chain length is evident from the obtained results...

A kinetic study of the acid hydrolysis of aqueous diazepam, bromazepam, and flunitrazepam was carried out at 25 degrees C using a spectrophotometric method. For diazepam and flunitrazepam, the experimental pseudo first-order rate constant decreased as the acid concentration was increased. The contrary behavior was found in the case of bromazepam. A...

The distribution of zinc porphyrin (ZnTPP) and derivatives in reversed micelles of benzyldimethyl n-hexadecyl ammonium chloride (BHDC) depends on the water content and size of the water pool.Steady-state and transient state fluorescence methods enable the establishment of two spectroscopically distinct molecular species.Analysis of fluorescence dec...

A new method for the determination of critical micelle concentrations, cmc, of ionic surfactants from conductivity measurements has been developed. The method is based on deconvolution into Gaussians of the second derivative of the conductivity/concentration data, followed by two consecutive numerical integrations. In testing this procedure, aqueou...

The characterization and study of the polarity of mixed micelles of sodium decyl and sodium dodecyl sulfate and sodium cholate by fluorescence probing have been carried out. Pyrene and Pyrene-1-carboxaldehyde were used. The obtained results allow the discussion of the changes of micelle structure with composition.

A study of the fluorescence quenching of pyrene by N-hexadecyl pyridinium chloride in mixed micelles of sodium decyl and dodecyl sulphate, using a transient-state time-resolved fluorescence technique, enabled the determination of aggregation numbers and micellar volumes. The former vary between 49 to 65 and the latter between 16.6 and 20 dm3 mol–1...

The solubility of diazepam and prazepam in aqueous polyoxyethylen-10-dodecanol, polyoxyethylen-23-dodecanol and polyoxyethylen-20-hexadecanol, has been determined at 25.0 degrees C. Diazepam seems to achieve a higher micellar penetration than prazepam, in spite of an expected smaller hydrophobic character. Thermodynamic interpretation of the micell...

The rate of the perchloric acid hydrolysis of aqueous ethyl and butyl vinyl ethers at 25.0°C, in the presence of micellar aggregates [anionic, sodium dodecyl sulfate (SDS); cationic, cetyl trymethyl ammonium bromide (CTAB); and nonionic, polyoxyethylen23dodecanol, (Brij 35)], has been studied. Negligible effects were observed in the cases of cation...

The comparison of the kinetic behaviors presented by these experiments may be of fundamental importance in clarifying the catalyzing and inhibiting effects in microheterogeneous catalytic processes. The experiments can be carried out in four to five 3-hr laboratory periods since each kinetic run may take only from 10-15 min.

Realizado en la Facultad de Químicas de Salamanca, por 3 profesores del centro para la asignatura Experimentación en Química Física de cuarto curso de la Licenciatura en Ciencias Químicas. El objetivo era el montaje y puesta a punto de una práctica de Espectroscopia Molecular para el laboratorio de Química Física. La metodología para la construcció...