RESUMO
The effect of the Au crystalline plane on the adsorption of different thiols and selenols is studied via reductive desorption (RD) and X-ray photoelectron spectroscopy (XPS) measurements. Self-assembled monolayers (SAMs) using aliphatic (ATs) and aromatic thiols (ArTs) on both Au(111) and Au(100) were prepared. The electrochemical stability of these SAMs on both surfaces is evaluated by comparing the position of the RD peaks. The longer the AT chain the more stable the SAM on Au(100) when compared to Au(111). By means of XPS measurements, we determine that the binding energy (BE) of the S 2p signal corresponding to the S atoms at the thiol/Au interface, commonly assigned at 162.0 eV, shifts 0.2 eV from Au(111) to Au(100) for SAMs prepared using thiols with the C* (C atom bonded to S) in sp3 hybridization, such as ATs. However, when the thiol presents the C* with an sp2 hybridization, such as in the case of ArTs, the BE remains at 162.0 eV regardless of the surface plane. Selenol-based SAMs were characterized comparatively on both Au(100) and Au(111). Our results show that selenol SAMs become even more electrochemically stable on Au(100) with respect to Au(111) than the analogue sulfur-based SAM. According to our results, we suggest that the electronic distribution around the Au-S/Se bond could be responsible for the different structural arrangements reported in the literature (gold adatoms, etc.), which should be dependent on the crystalline face (Au(hkl)-S) and the chemical nature of the environment of the adsorbates (sp3-C* vs sp2-C* and Au-SR vs Au-SeR).
RESUMO
We show that homogeneously mixed self-assembled monolayers (SAMs) of mercaptoalkanoic acids of different chain lengths can be used to build up a pH-sensitive supramolecular switch. The acids with short and long alkyl chains interact via the strong hydrogen bond between carboxylic acid groups. The pH acts as a trigger by breaking or restoring the hydrogen bond interaction in basic or acidic solutions, respectively. The corresponding changes in the monolayer structure were determined by ellipsometry, surface-enhanced Raman spectroscopy, and contact angle measurements. Density functional theory (DFT) calculations were performed to elucidate the structures of interacting molecules compatible with the surface coverage obtained from electrochemical reductive desorption experiments. The simplicity of the preparation procedure assures a high reproducibility whereas the stability of the homogeneous mixed SAM guarantees the reversibility of the switching process.
RESUMO
We investigated the mechanism of formation and stability of self-assembled monolayers (SAMs) of methaneselenolate on Au(111) prepared by the immersion method in ethanolic solutions of dimethyl diselenide (DMDSe). The adsorbed species were characterized by electrochemical measurements and high-resolution photoelectron spectroscopy (HR-XPS). The importance of the headgroup on formation mechanism and the stability of the SAMs was addressed by comparatively studying methaneselenolate (MSe) and methanethiolate (MT) monolayers. Density Functional Theory (DFT) calculations were performed to identify the elementary reaction steps in the mechanisms of formation and decomposition of the monolayers. Reductive desorption and HR-XPS measurements indicated that a MSe monolayer is formed at short immersion times by the cleavage of the Se-Se bond of DMDSe. However, the monolayer decomposes at long immersion times at room temperature, as evidenced by the appearance of atomic Se on the surface. The decomposition is more pronounced for MSe than for MT monolayers. The MSe monolayer stability can be greatly improved by two modifications in the preparation method: immersion at low temperatures (-20 °C) and the addition of a reducing agent to the forming solution.
Assuntos
Ouro/química , Compostos Organosselênicos/química , Compostos de Sulfidrila/química , Adsorção , Técnicas Eletroquímicas , Etanol/química , Espectroscopia Fotoeletrônica , Teoria Quântica , Soluções , Propriedades de Superfície , TemperaturaRESUMO
The preparation and thermal stability of benzenethiol and benzeneselenol self-assembled monolayers (SAMs) grown on Au(111) have been investigated by electrochemical experiments and high-resolution photoemission spectroscopy. Both techniques confirm the formation of monolayers with high packing densities (θ = 0.27-0.29 ML) and good degrees of order in both cases. Despite many similarities between the two SAMs, the thermal desorption is distinctly different: whereas the benzenethiol SAM desorbs in a single steplike process, the desorption of the benzeneselenol SAM occurs with a much lower activation energy and involves the cleavage of some Se-C bonds and a change in molecular configuration from standing up to lying down. This behavior is explained by considering the different nature of the bonding of the headgroup with the metal surface and with the phenyl ring. Density functional theory calculations show that the breakage of the Se-C bond has a lower activation energy barrier than the breakage of the S-C bond.
Assuntos
Derivados de Benzeno/química , Técnicas Eletroquímicas , Ouro/química , Compostos Organosselênicos/química , Fenóis/química , Teoria Quântica , Compostos de Sulfidrila/química , Temperatura , Adsorção , Espectroscopia Fotoeletrônica , Propriedades de SuperfícieRESUMO
The local structure of the sulfur atom of methanethiolate and ethanethiolate on the Cu(111) and Cu(100) surfaces was investigated from first principles employing the periodic supercell approach in the framework of density functional theory. On the 111 surface, we investigated the (square root 3 x square root 3)R30 degrees and (2 x 2) structures, whereas on the 100 surface, we investigated the p(2 x 2) and c(2 x 2) structures. The landscape of the potential energy surface on each metal surface presents distinctive features that explain the local adsorption structure of thiolates found experimentally. On the Cu(111) surface, the energy difference between the hollow and bridge sites is only 3 kcal/mol, and consequently, adsorption sites ranging from the hollow to the bridge site were observed for increasing surface coverages. On the Cu(100) surface, there is a large energy difference of 12 kcal/mol between the hollow and bridge sites, and therefore, only the 4-fold coordination was observed. The high stabilization of thiolates on the hollow site of Cu(100) may be the driving force for the pseudosquare reconstruction observed experimentally on Cu(111). Density of states analysis and density difference plots were employed to characterize the bonding on different surface sites. Upon interaction with the metal d bands, the pi* orbital of methanethiolate splits into several peaks. The two most prominent peaks are located on either edge of the metal d band. They correspond to bonding and antibonding S-Cu interactions. In the case of ethanethiolate, all the back-bonds are affected by the surface bonding, leading to alternating regions of depletion and accumulation of charge in the successive bonds.
RESUMO
The structure, the surface bonding, and the energetics of alkanethiols adsorbed on Cu(111), Ag(111), and Au(111) surfaces were studied under low and high coverages. The potential energy surfaces (PES) for the thiol/metal interaction were investigated in the absence and presence of externally applied electric fields in order to simulate the effect of the electrode potential on the surface bonding. The electric field affects the corrugation of the PES which decreases for negative fields and increases for positive fields. In the structural investigation, we considered the relaxation of the adsorbate and the surface. The highest relaxation in a direction perpendicular to the surface was observed for gold atoms, whereas silver atoms presented the highest relaxation in a plane parallel to the surface. The surface relaxation is more important in the low coverage limit. The surface bonding was investigated by means of the total and projected density of states analysis. The highest ionic character was observed on the copper surface whereas the highest covalent character occurs on gold. This leads to a strong dependence of the PES with the tilt angle of the adsorbate on Au(111) whereas this dependence is less pronounced on the other metals. Thus, the adsorbate-relaxation and the metal-relaxation contributions to the binding energy are more important on gold. The adsorption of thiols on gold was investigated on the 111 surface as well as on a surface with gold adatoms in order to elucidate the effect of thiols on the surface diffusion of gold. The CH(3)CH(2)S radical adsorbs ontop of the gold adatom. The diffusional barrier of the CH(3)CH(2)SAu species is lower than that for a bare gold adatom and is also lower than that for the bare thiol radical. The adsorption of the molecular species CH(3)SH and CH(3)CH(2)SH was also investigated on Au(111). They adsorb via the sulfur atom ontop of a gold atom. On the other hand, the adsorption of the alkanethiol radicals on the perfect 111 surfaces occurs on the face centered cubic (fcc)-bridge site in the low coverage limit for all metals and shifts toward the fcc site at high coverage on copper and silver.
Assuntos
Alcanos/química , Cobre/química , Ouro/química , Modelos Químicos , Prata/química , Compostos de Sulfidrila/química , Adsorção , Teoria Quântica , Propriedades de SuperfícieRESUMO
V. cholerae non-O1 non-O139 serogroups isolated from clinical and environmental sources in Córdoba, Argentina, were analyzed for the presence and expression of virulence genes. Most of the strains studied contained the genes toxR and hlyA, but lacked ctxA, zot, ace, tcpA and stn. The culture supernatants were tested for hemolytic and cytotoxic activity. The enterotoxic potential of the strains was studied in a rabbit ileal loop assay and their genetic profiles were compared by PFGE. The environmental strains varied in their virulence phenotype and showed no-clonal relationships. The clinical strains were highly enterotoxic, hemolytic, proteolytic and showed indistinguishable PFGE profiles, although they differed in their cytotoxic activity. This is the first description, using cell culture and “in vivo” studies, of the virulence properties of non-O1 non-O139 V. cholerae from Argentina.
En este trabajo se analizó la presencia y expresión de genes de virulencia en V. cholerae no-O1 no-O139 de origen clínico y ambiental, aislados en Córdoba, Argentina. La mayoría de las cepas estudiadas contiene los genes toxR y hlyA, pero no ctxA, zot, ace, tcpA y stn. Se analizó la actividad hemolítica y citotóxica de estas cepas en los sobrenadantes de cultivo, así como su potencial enterotóxico en ensayos de asa ileal ligada de conejo. Además, los aislamientos fueron comparados por sus perfiles genéticos en PFGE. Las cepas del medio ambiente mostraron variación en su fenotipo de virulencia y no mostraron relación clonal. Las cepas clínicas fueron muy enterotóxicas, hemolíticas, proteolíticas y mostraron perfiles indistinguibles de PFGE, aunque mostraron diferencias en su actividad citotóxica. En este trabajo se describen por primera vez, utilizando ensayos de cultivo celular e “in vivo”, propiedades de virulencia de V. cholerae no-O1 no-O139 aislados en Argentina.
Assuntos
Animais , Humanos , Coelhos , Vibrio cholerae não O1/patogenicidade , Argentina/epidemiologia , Técnicas de Tipagem Bacteriana , Proteínas de Bactérias/genética , Proteínas de Bactérias/isolamento & purificação , Proteínas de Bactérias/fisiologia , Chlorocebus aethiops , Células COS/microbiologia , Toxina da Cólera/genética , DNA Bacteriano/genética , Farmacorresistência Bacteriana , Diarreia/epidemiologia , Diarreia/microbiologia , Eletroforese em Gel de Campo Pulsado , Enterotoxinas/genética , Enterotoxinas/isolamento & purificação , Enterotoxinas/fisiologia , Deleção de Genes , Genes Bacterianos , Proteínas Hemolisinas/genética , Proteínas Hemolisinas/isolamento & purificação , Proteínas Hemolisinas/fisiologia , Metaloendopeptidases/genética , Metaloendopeptidases/isolamento & purificação , Metaloendopeptidases/fisiologia , Filogenia , Vibrioses/epidemiologia , Vibrioses/microbiologia , Vibrio cholerae não O1/efeitos dos fármacos , Vibrio cholerae não O1/genética , Vibrio cholerae não O1/isolamento & purificação , Virulência/genética , Microbiologia da ÁguaRESUMO
V. cholerae non-O1 non-O139 serogroups isolated from clinical and environmental sources in Córdoba, Argentina, were analyzed for the presence and expression of virulence genes. Most of the strains studied contained the genes toxR and hlyA, but lacked ctxA, zot, ace, tcpA and stn. The culture supernatants were tested for hemolytic and cytotoxic activity. The enterotoxic potential of the strains was studied in a rabbit ileal loop assay and their genetic profiles were compared by PFGE. The environmental strains varied in their virulence phenotype and showed no clonal relationships. The clinical strains were highly enterotoxic, hemolytic, proteolytic and showed indistinguishable PFGE profiles, although they differed in their cytotoxic activity. This is the first description, using cell culture and "in vivo" studies, of the virulence properties of non-O1 non-O139 V. cholerae from Argentina.
Assuntos
Vibrio cholerae não O1/patogenicidade , Animais , Argentina/epidemiologia , Proteínas de Bactérias/genética , Proteínas de Bactérias/isolamento & purificação , Proteínas de Bactérias/fisiologia , Técnicas de Tipagem Bacteriana , Células COS/microbiologia , Chlorocebus aethiops , Toxina da Cólera/genética , DNA Bacteriano/genética , Diarreia/epidemiologia , Diarreia/microbiologia , Farmacorresistência Bacteriana , Eletroforese em Gel de Campo Pulsado , Enterotoxinas/genética , Enterotoxinas/isolamento & purificação , Enterotoxinas/fisiologia , Deleção de Genes , Genes Bacterianos , Proteínas Hemolisinas/genética , Proteínas Hemolisinas/isolamento & purificação , Proteínas Hemolisinas/fisiologia , Humanos , Metaloendopeptidases/genética , Metaloendopeptidases/isolamento & purificação , Metaloendopeptidases/fisiologia , Filogenia , Coelhos , Vibrioses/epidemiologia , Vibrioses/microbiologia , Vibrio cholerae não O1/efeitos dos fármacos , Vibrio cholerae não O1/genética , Vibrio cholerae não O1/isolamento & purificação , Virulência/genética , Microbiologia da ÁguaRESUMO
V. cholerae non-O1 non-O139 serogroups isolated from clinical and environmental sources in Córdoba, Argentina, were analyzed for the presence and expression of virulence genes. Most of the strains studied contained the genes toxR and hlyA, but lacked ctxA, zot, ace, tcpA and stn. The culture supernatants were tested for hemolytic and cytotoxic activity. The enterotoxic potential of the strains was studied in a rabbit ileal loop assay and their genetic profiles were compared by PFGE. The environmental strains varied in their virulence phenotype and showed no clonal relationships. The clinical strains were highly enterotoxic, hemolytic, proteolytic and showed indistinguishable PFGE profiles, although they differed in their cytotoxic activity. This is the first description, using cell culture and [quot ]in vivo[quot ] studies, of the virulence properties of non-O1 non-O139 V. cholerae from Argentina.
RESUMO
It was assay by biochemical and immunological tests, strains of V. Cholerae non-01 non-0139 isolated from two patients of the province of Córdoba, Argentina. They showed episodes of acute gastroenteritis. Strains non-01 non-0139 were isolated from water samples ingested by patients. We conclude that strains identified from patients would have the source from contaminated environmental water by V. Cholerae.
Assuntos
Gastroenterite/microbiologia , Vibrio cholerae/isolamento & purificação , Adulto , Feminino , Humanos , Microbiologia da ÁguaRESUMO
The increasing levels of resistance of enteropathogenic bacteria against antimicrobial agents present geographic variations. We have analysed the antimicrobial susceptibility of isolates obtained from 4,364 children under 5 years of age with acute diarrhea, in 7 cities of Argentina. Diarrheagenic E. coli exhibited 74.5% of resistance against ampicillin, 64.2% against sulfametoxazole-trimethoprim, and Shigella spp., 62% and 75.6% respectively. Salmonella sp. showed 35%, 14%, 41.8%, 65.4%, 14.5%, and 13.6% of resistance against ampicillin, chloranfenicol, sulfametoxazole-trimetoprim, sulfadiazin, gentamycin, and fosfomycin respectively. These values are higher than the ones observed in developed countries. Aeromonas showed significantly lower resistance percentage. Important differences in our country were observed, consequently, local trials should be carried out in order to apply corrective measures.
Assuntos
Aeromonas/efeitos dos fármacos , Diarreia/microbiologia , Resistência Microbiana a Medicamentos , Enterobacteriaceae/efeitos dos fármacos , Infecções por Bactérias Gram-Negativas/microbiologia , Doença Aguda , Aeromonas/isolamento & purificação , Argentina/epidemiologia , Pré-Escolar , Diarreia/epidemiologia , Diarreia Infantil/epidemiologia , Diarreia Infantil/microbiologia , Disenteria Bacilar/epidemiologia , Disenteria Bacilar/microbiologia , Enterobacteriaceae/isolamento & purificação , Escherichia coli/efeitos dos fármacos , Escherichia coli/isolamento & purificação , Infecções por Escherichia coli/epidemiologia , Infecções por Escherichia coli/microbiologia , Feminino , Infecções por Bactérias Gram-Negativas/epidemiologia , Humanos , Lactente , Masculino , Testes de Sensibilidade Microbiana , Salmonella/efeitos dos fármacos , Salmonella/isolamento & purificação , Infecções por Salmonella/epidemiologia , Infecções por Salmonella/microbiologia , Shigella/efeitos dos fármacos , Shigella/isolamento & purificação , População UrbanaRESUMO
Se estudiaron, mediante pruebas bioquímicas e imunológicas, cepas de V. Cholerae no 01 no 0139 aisladas de dos pacientes de la provincia de Córdoba, que presentaban cuadros agudos de gastroenteritis. En el agua ingerida por estos pacientes se aislaron cepas de V. Cholerae considerados ambientales. (AU)
Assuntos
Humanos , Feminino , Adulto , Vibrio cholerae/isolamento & purificação , Gastroenterite/microbiologia , Microbiologia da ÁguaRESUMO
Se estudiaron, mediante pruebas bioquímicas e imunológicas, cepas de V. Cholerae no 01 no 0139 aisladas de dos pacientes de la provincia de Córdoba, que presentaban cuadros agudos de gastroenteritis. En el agua ingerida por estos pacientes se aislaron cepas de V. Cholerae considerados ambientales.