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Biomedical subjects

Ricardo F Aroca

Publications and source records attributed to Ricardo F Aroca.

14 recordsLinked to original sources
PubMed2006

Chemically selective sensing through layer-by-layer incorporation of biorecognition into thin film substrates for surface-enhanced resonance Raman scattering.

In this work, the fabrication, characterization, and application of avidin/Ag nanoparticle layer-by-layer (LbL) films as chemically selective substrates for surface-enhanced resonance Raman scattering (SERRS) is demonstrated. The biospecific interaction between avidin and the small molecule biotin, one of the strongest known to exist in nature, is exploited to preferentially capture biotinylated species from solution. This highly favored adsorption is shown to yield SERRS concentration enhancements and improved detection sensitivities of ca. 102 for commercially available and in situ prepared biotinylated species over their nontagged counterparts.

Avidin↗
PubMed2006

The role of Azopolymer/Dendrimer layer-by-layer film architecture in photoinduced birefringence and the formation of surface-relief gratings.

The fabrication of nanostructured layer-by-layer (LbL) films strives for molecular control of the film properties directly connected with modifications in the film architecture. In the present report, the photoinduced birefringence and formation of the surface-relief gratings in LbL films obtained with an azopolymer (PS119) are shown to be strongly affected by the generation of the dendrimer employed in the alternating layers. Stronger adsorption of PS119 occurred when polypropylenimine tetrahexacontaamine dendrimer (DAB) of higher generations is used, due to a larger number of sites available to interact with azochromophores in PS119. In contrast, the photoinduced birefringence for LbL films made with the generation 1 dendrimer (DABG1) was higher, which can be explained by weaker interactions between adjacent layers. Strong interactions in LbL films consisting of PS119 and generation 3 or 5 dendrimers restrict the chromophore mobility, leading to a smaller birefringence. The interpretation is supported by the fact that surface-relief gratings with larger amplitudes were obtained for 35-bilayer films of DABG1/PS119 (31 nm) in comparison with films from DABG5/PS119 (5 nm). These gratings were formed with mass transport arising from a light-driven mechanism, as photoinscription was successful only with p-polarized light and not with s-polarized light.

Journal Article↗
PubMed2006

Investigation of the two-photon absorption cross-section in perylene tetracarboxylic derivatives: nonlinear spectra and molecular structure.

We investigated the 2PA absorption spectrum of a family of perylene tetracarboxylic derivatives (PTCDs): bis(benzimidazo)perylene (AzoPTCD), bis(benzimidazo)thioperylene (Monothio BZP), n-pentylimidobenzimidazoperylene (PazoPTCD), and bis(n-butylimido)perylene (BuPTCD). These compounds present extremely high two-photon absorption, which makes them attractive for applications in photonics devices. The two-photon absorption cross-section spectra of perylene derivatives obtained via Z-scan technique were fitted by means of a sum-over-states (SOS) model, which described with accuracy the different regions of the 2PA cross-section spectra. Frontier molecular orbital calculations show that all molecules present similar features, indicating that nonlinear optical properties in PTCDs are mainly determined by the central portion of the molecule, with minimal effect from the lateral side groups. In general, our results pointed out that the differences in the 2PA cross-sections among the compounds are mainly due to the nonlinearity resonance enhancement.

Journal Article↗
PubMed2005

Inherent complexities of trace detection by surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS) are powerful optical scattering techniques used in such frontier areas of research as ultrasensitive chemical analysis, the characterization of nanostructures, and the detection of single molecules. However, measuring and, most importantly, interpreting SERS/SERRS spectra can be incredibly challenging. This is the result of modifications to the measured spectra that are due to of a variety of instabilities and contributions. These interferences and modifications arise from the nature of the enhancement itself, as well as the conditions used to attain SERS spectra. The present report is an attempt to collect in one place the analytical interferences that are most commonly found during the collection of SERS/SERRS spectra.

Journal Article↗
PubMed2005

Surface-enhanced Raman scattering on nanoshells with tunable surface plasmon resonance.

Fabrication, characterization, and optical enhancement applications of bimetallic AgAu nanoparticles and nanoshells are reported. Nanoparticles with tunable surface plasmon resonances are synthesized at room temperature and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and photon correlation spectroscopy. The collective electron oscillation of the nanoparticles shows a controllable tunability in the 400-990 nm spectral range, in agreement with plasmon absorption calculated using Mie theory, providing an optimum substrate for surface plasmon-assisted enhanced spectroscopy. Surface-enhanced Raman scattering experiments show that the average enhancement factor obtained with nanoshells could be higher than those obtained with silver sols.

Journal Article↗
PubMed2005

Surface-enhanced raman scattering on dendrimer/metallic nanoparticle layer-by-layer film substrates.

In this paper, the fabrication, characterization, and application of unique layer-by-layer (LBL) films of dendrimers and metallic nanoparticles is reported. Silver nanoparticles (d = approximately 20 nm) are produced in solution by sodium citrate reduction and incorporated into thin films with generation 1 and 5 DAB-Am dendrimers (polypropylenimine dendrimers with amino surface groups) by the LBL technique. The resulting nanocomposite films are characterized by UV-visible surface plasmon absorption and atomic force microscopy (AFM) measurements, and employed as substrates for surface-enhanced Raman scattering (SERS) of 2-naphthalenethiol. Through variation of the molecular size (dendrimer generation) and concentration of the cross-linker used, as well as the number of layers produced, the optical properties of several different possible architectures are studied. In the films, Ag nanoparticles are shown to be effectively immobilized and stabilized with increased control over their spacing and aggregation. Moreover, the films are shown to be excellent substrates for SERS measurements, demonstrating significant enhancement capability. As expected, large electromagnetic enhancement of Raman scattering signals is found to be strongly dependent on interparticle coupling between neighboring metallic nanoparticles. Finally, the possibility of detecting SERS signals from architectures with intervening layers between the metal nanoparticles and analyte molecules is explored. It is shown that although there are decreases in intensity with increasing number of intervening layers (as is expected from the distance dependence of SERS), electromagnetic enhancement is still able to function at these distances, thus offering the possibility of developing sensors with external layers that are chemically selective for specific analytes.

Journal Article↗
PubMed2005

Role of nanoparticle surface charge in surface-enhanced Raman scattering.

In this work, the role of nanoparticle surface charge in surface-enhanced Raman scattering (SERS) is examined for the common case of measurements made in colloidal solutions of Ag and Au. Average SERS intensities obtained for several analytes (salicylic acid, pyridine, and 2-naphthalenethiol) on Ag and Au colloids are correlated with the pH and zeta potential (zeta) values of the nanoparticle solutions from which they were recorded. The consequence of the electrostatic interaction between the analyte and the metallic nanoparticle is stressed. The zeta potentials of three commonly used colloidal solutions are reported as a function of pH, and a discussion is given on how these influence SERS intensity. Also examined is the importance of nanoparticle aggregation (and colloidal solution collapse) in determining SERS intensities, and how this varies with the pH of the solution. The results show that SERS enhancement is highest at zeta potential values where the colloidal nanoparticle solutions are most stable and where the electrostatic repulsion between the particles and the analyte molecules is minimized. These results suggest some important criteria for consideration in all SERS measurements and also provide important insights into the problem of predicting SERS activities for different molecular systems.

Journal Article↗
PubMed2005

Silver nanowire layer-by-layer films as substrates for surface-enhanced Raman scattering.

In this paper, the fabrication of highly stable, surface-enhanced Raman scattering (SERS) active dendrimer/silver nanowire layer-by-layer (LBL) films is reported. Ag nanowires, approximately 100 nm in diameter, were produced in solution and transferred, using the LBL technique, onto a single fifth-generation DAB-Am dendrimer layer on a glass substrate. The Ag nanowires, and the resulting LBL films were characterized using UV-visible surface plasmon absorbance, while the LBL films were further characterized by atomic force microscopy measurements and surface-enhanced Raman and resonance Raman scattering of several analytes. The dendrimer was found to effectively immobilize the Ag nanowires with increased control over spacing and aggregation of the particles. These films are shown to be excellent substrates for SERS/SERRS measurements, demonstrating significant enhancement, and trace detection capability. Several trial analytes were tested using a variety of excitation energies, and results confirmed effective enhancement of Raman signals throughout the visible range (442-785 nm) with different molecules. Analytes were deposited onto the enhancing Ag nanowire LBL films surface using both casting and Langmuir-Blodgett monolayer transferring techniques.

Dendrimers↗
PubMed2004

Surface-enhanced vibrational microspectroscopy of fulvic acid micelles.

Micro-Raman spectroscopy, infrared absorption microspectroscopy, and AFM images of nano- or microsized micelles formed by fulvic acid (FA) solutions, prepared at different pHs, and cast on glass slides or gold island films, are reported. FA films cast on gold islands are characterized by surface-enhanced infrared absorption (SEIRA), surface-enhanced infrared reflection absorption, and surface-enhanced Raman scattering (SERS). Based on spectral evidence, it is expected that the chemisorption of FA on gold island films takes place through thiol groups, which become more active as pH increases. The SEIRA spectra of these films show increased peak intensity, as well as improved band resolution. Microspectroscopy SERS studies show that, at pH 5, FA form small aggregates on gold surfaces. At pH 8, FA tends to expand due to electrostatic repulsion, giving rise to a fractal surface composed of different domains. SERS studies of these domains reveal that the most polar molecules are located on the external faces. At pH 11, fractal conformations are even more pronounced and give rise to radial patterned structures. At this pH, the position of fulvic acid molecules in the fractal micelles is the same as observed at pH 8. In this way, SERS can be viewed as a powerful tool for the analysis of the composition, apparent contribution of the surface functional groups of FA films, and the FA building blocks (i.e., catechol, gallic, salicylic, or ftalic acids) in the structures of these materials.

Benzopyrans↗
PubMed2004

Gold nanoparticle embedded, self-sustained chitosan films as substrates for surface-enhanced Raman scattering.

In this work, self-sustained, biocompatible, biodegradable films containing gold nanostructures have been fabricated for potential application in nanobioscience and ultrasensitive chemical and biochemical analysis. We report a novel synthesis of gold nanoparticles mediated by the biopolymer chitosan. Self-supporting thin films are formed from the resultant gold-chitosan nanocomposite solutions and characterized by UV-visible surface plasmon absorption, transmission electron microscopy, atomic force microscopy, infrared absorption, and Raman scattering measurements. Results demonstrate control over the size and distribution of the nanoparticles produced, which is promising for several applications, including the development of biosensors. As a proof of principle, we demonstrate that gold-chitosan films can be employed in trace analysis using surface-enhanced Raman scattering.

Biocompatible Materials↗
PubMed2004

Photoinduced phenomena in layer-by-layer films of poly(allylamine hydrochloride) and Brilliant Yellow azodye.

The nanoscale interactions between adjacent layers of layer-by-layer (LBL) films from poly(allylamine hydrochloride) (PAH) and azodye Brilliant Yellow (BY) have been investigated, with the films employed for optical storage and the formation of surface-relief gratings. Using Fourier transform infrared spectroscopy, we identified interactions involving SO3- groups from BY and NH+ groups from PAH. These electrostatic interactions were responsible for the slow kinetics of writing in the optical storage experiments, due to a tendency to hinder photoisomerization and the subsequent reorientation of the azochromophores. The photoinduced birefringence did not saturate after one hour of exposure to the writing laser, whereas in azopolymer films, saturation is normally reached within a few minutes. On the other hand, the presence of such interactions prevented thermal relaxation of the chromophores after the writing laser was switched off, leading to a very stable written pattern. Moreover, the nanoscale interactions promoted mass transport for photoinscription of surface-relief gratings on PAH/BY LBL films, with the azochromophores being able to drag the inert PAH chains when undergoing the trans-cis-trans photoisomerization cycles. A low level of chromophore degradation was involved in the SRG photoinscription, which was confirmed with micro-Raman and fluorescence spectroscopies.

Allylamine↗
PubMed2003

Surface-enhanced Raman scattering in the ultraviolet spectral region: UV-SERS on rhodium and ruthenium electrodes.

We report the first observation of surface-enhanced Raman scattering (SERS) excited with ultraviolet (UV) light from transition metal electrodes. Adsorbed pyridine and SCN- on rough rhodium (Rh) and ruthenium (Ru) electrodes, respectively, have been studied using 325 nm laser excitation. In contrast, the best enhancers in the visible and near infrared, silver and gold, do not produce UV-SERS. The experimental data of UV-SERS are in agreement with our preliminary theoretical calculation based on the electromagnetic enhancement mechanism. The enhancement factor is about 2 orders of magnitude for the Rh and Ru electrodes when they are excited at 325 nm.

Electrodes↗
PubMed2003

Overtones and combinations in single-molecule surface-enhanced resonance Raman scattering spectra.

The observation of overtones and combinations in the SERRS spectra of single molecules dispersed in Langmuir-Blodgett monolayers is confirmed for a family of molecules. The detection of fundamentals, combinations, and overtones in single-molecule spectra of a series of perylenetetracarboxylic diimides (PTCD) is achieved with spatially resolved surface-enhanced resonance Raman scattering (SERRS). The Langmuir-Blodgett technique is used to create monomolecular thick films on metal islands containing on average one probed molecule within the field of view of the Raman microscope. The enhancement needed for single-molecule detection is achieved through the multiplicative effects of electromagnetic enhancement by metal nanostructures and resonance Raman enhancement by excitation into molecular electronic absorption bands. Overtone and combination progressions are well resolved in the average SERRS spectra of all three PTCD molecules.

Journal Article↗