Domingo I. Garcia-Gutierrez

Encapsulation and immobilization of papain in electrospun nanofibrous membranes of PVA cross-linked with glutaraldehyde vapor.

In this paper, papain enzyme (E.C. 3.4.22.2, 1.6 U/mg) was successfully immobilized in poly(vinyl alcohol) (PVA) nanofibers prepared by electrospinning. The morphology of the electrospun nanofibers was characterized by scanning electron microscopy (SEM) and the diameter distribution was in the range of 80 to 170 nm. The presence of the enzyme within the PVA nanofibers was confirmed by infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDXS) analyses. The maximum catalytic activity was reached when the enzyme loading was 13%. The immobilization of papain in the nanofiber membrane was achieved by chemical crosslinking with a glutaraldehyde vapor treatment (GAvt). The catalytic activity of the immobilized papain was 88% with respect to the free enzyme. The crosslinking time by GAvt to immobilize the enzyme onto the nanofiber mat was 24h, and the enzyme retained its catalytic activity after six cycles. The crosslinked samples maintained 40% of their initial activity after being stored for 14 days. PVA electrospun nanofibers are excellent matrices for the immobilization of enzymes due to their high surface area and their nanoporous structure.

Cobalt ferrite nanowhiskers as T2 MRI contrast agent

A novel, one-step synthesis of one-dimensional cobalt ferrite nanowhiskers (CfW) is reported. SQUID studies confirmed that the monodispersed surfactant-free CfW exhibit superparamagnetic behaviour. This characteristic makes CfW an ideally suited contrast agent for T2-weighted MRI, thus proving to be very effective for biomedical applications.

Enhancement of electrochemical properties on TiO2/carbon nanofibers by electrospinning process

In the present work, we report the preparation of TiO2–carbon/carbon dual nanofibers using an electrospinning technique. The dual nanofibers were synthesized using a modified side-by-side spinneret, which allowed the fabrication of the desired dual nanofiber architecture in a one-step process. A subsequent heat treatment permitted the control on the crystal structure of the synthesized dual nanofibers. Scanning electron microscopy and transmission electron microscopy results confirmed the continuity and duality of the obtained nanofibers. The difference in composition between the fibers composing the dual fibers was clearly observed by energy dispersive X-ray spectroscopy. The effect of heat treatment on crystallinity was evident on the results obtained from the X-ray diffraction and selected area electron diffraction studies; where, depending on the heat treatment conditions, clear signals for anatase and rutile phases of TiO2 were observed. Electrochemical studies suggest an improvement on the conduction properties of TiO2–carbon/carbon dual nanofibers compared to single TiO2–carbon nanofibers, attributed to the carbon nanofiber contribution attached to the TiO2 nanofibers. Based on the morphological and structural features of this novel nanostructured material, and to the electrochemical performance observed, it has a wide range of potential applications.

Ultrasonic irradiation-assisted synthesis of Bi2S3 nanoparticles in aqueous ionic liquid at ambient condition

In this work, an easy, fast and environmentally friendly method to obtain Bi2S3 nanostructures with sphere-like morphology is introduced. The promising material was successfully synthesized by a sonochemical route in 20% 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][EtSO4] ionic liquid solution (IL). Morphological studies by electron microscopy (SEM and TEM) show that the use of IL in the synthesis of Bi2S3 favors the formation of nanocrystals non-agglomerated. Micro Raman and energy dispersive X-ray spectroscopy (EDXS) were used to determine the composition and purity of the synthesized material. X-ray powder diffraction (XRD) and selective area electron diffraction (SAED) revealed that ultrasonic radiation accelerated the crystallization of Bi2S3 into orthorhombic bismuthinite structure. The band gap calculated from the diffuse reflectance spectra (DRS) was found to be 1.5eV.

Metal-Induced Production of a Novel Bioadsorbent Exopolysaccharide in a Native Rhodotorula mucilaginosa from the Mexican Northeastern Region

There is a current need to develop low-cost strategies to degrade and eliminate industrially used colorants discharged into the environment. Colorants discharged into natural water streams pose various threats, including: toxicity, degradation of aesthetics and inhibiting sunlight penetration into aquatic ecosystems. Dyes and colorants usually have complex aromatic molecular structures, which make them very stable and difficult to degrade and eliminate by conventional water treatment systems. The results in this work demonstrated that heavy metal-resistant Rhodotorula mucilaginosa strain UANL-001L isolated from the northeast region of Mexico produce an exopolysaccharide (EPS), during growth, which has colorant adsorption potential. The EPS produced was purified by precipitation and dialysis and was then physically and chemically characterized by Scanning Electron Microscopy, Fourier Transform Infrared Spectroscopy, and chemical elemental analysis. Here, the ability of the purified EPS produced to adsorb methylene blue (MB), which served as a model colorant, is studied. MB adsorption by the EPS is found to follow Langmuir Adsorption Isotherm kinetics at 25°C. Further, by calculating the Langmuir constant the adsorption capabilities of the EPS produced by the Rhodotorula mucilaginosa strain UANL-001L is compared to that of other adsorbents, both, microbially produced and from agroindustrial waste. The total adsorption capacity of the EPS, from the Rhodotorula mucilaginosa strain UANL-001L, was found to be two-fold greater than the best bioadsorbents reported in the literature. Finally, apart from determining which heavy metals stimulated EPS production in the strain, the optimal conditions of pH, heavy metal concentration, and rate of agitation of the growing culture for EPS production, was determined. The EPS reported here has the potential of aiding in the efficient removal of colorants both in water treatment plants and in situ in natural water streams.

Mesoporous titania nanofibers by solution blow spinning

Fast and large-scale production of mesoporous titania nanofibers was achieved by solution blow spinning. The blow spinning setup provides a method to prepare titania nanofibers in a safe and scalable way without using a high-voltage electric field. Titania microstructure and porosity can be modified by adding a suitable template, such as pluronic polymers. The blow spun titania nanofibers had a good performance on the photocatalytic degradation of tetracycline and could be easily removed from the tetracycline aqueous solution due to their large aspect ratio. Solution blow spinning method has a great potential for the large-scale production of titania nanofibers with good photocatalytic properties.Graphical Abstract

Transmission electron microscope calibration methods for critical dimension standards

Abstract. One of the key challenges in critical dimension (CD) metrology is finding suitable dimensional calibration standards. The transmission electron microscope (TEM), which produces lattice-resolved images having scale traceability to the SI (International System of Units) definition of length through an atomic lattice constant, has gained wide usage in different areas of CD calibration. One such area is critical dimension atomic force microscope (CD-AFM) tip width calibration. To properly calibrate CD-AFM tip widths, errors in the calibration process must be quantified. Although the use of TEM for CD-AFM tip width calibration has been around for about a decade, there is still confusion on what should be considered in the uncertainty analysis. We characterized CD-AFM tip-width samples using high-resolution TEM and high angle annular dark field scanning TEM and two CD-AFMs that are implemented as reference measurement systems. The results are used to outline how to develop a rigorous uncertainty estimate for TEM/CD-AFM calibration, and to compare how information from the two electron microscopy modes are applied to practical CD-AFM measurements. The results also represent a separate validation of previous TEM/CD-AFM calibration. Excellent agreement was observed.

Influence of the Capping Ligand on the Band Gap and Electronic Levels of PbS Nanoparticles through Surface Atomistic Arrangement Determination

Lead sulfide (PbS) nanoparticles were synthesized by chemical methods with different sizes and different capping ligands (oleic acid, myristic acid, and hexanoic acid), avoiding ligand exchange procedures, to study the effect of characteristics of the capping ligands on their energy levels and band gap values. Experimental results (UV–vis–NIR, Fourier transform infrared, and Raman spectroscopies, cyclic voltammetry, transmission electron microscopy, and electron energy loss spectroscopy) showed a marked influence of the capping ligand nature on the electro-optical properties of PbS nanoparticles with a very similar size. Differences were observed in the atomistic arrangement on the nanoparticle surface and phonon vibrations with the different capping ligands. These observations suggest that the electro-optical properties of PbS nanoparticles are not only determined by their size, through quantum confinement effects, but also strongly affected by the atomistic arrangement on the nanoparticle surface, which is determined by the capping ligand nature.

Enhancement of the Optoelectronic Properties of PEDOT:PSS–PbS Nanoparticles Composite Thin Films Through Nanoparticles’ Capping Ligand Exchange

The influence of the capping ligand on nanoparticles’ optical and electronic properties is a topic of great interest currently being investigated by several research groups in different countries. In the present study, PbS nanoparticles originally synthesized with oleic acid, myristic acid and hexanoic acid underwent a ligand exchange process to replace the original carboxylic acid for l-cysteine as the capping layer, and were thoroughly characterized by means of transmission electron microscopy and its related techniques, such as energy dispersive x-ray spectroscopy and scanning-transmission electron microscopy, and Fourier transform infrared, Raman and x-ray photoelectron spectroscopy. Afterwards, these PbS nanoparticles were dispersed into a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) matrix to fabricate a composite thin film which displayed the optical absorption properties of the PbS nanoparticles and the electrical transport properties of the PEDOT:PSS matrix, in order to evaluate the impact of the nanoparticles’ capping ligand on the optoelectronic properties of the fabricated composite thin films. Composite thin films with PbS nanoparticles showing l-cysteine as the capping layer displayed clear photoresponse and a threefold increment in their conductivities compared to pristine PEDOT:PSS. The properties of PEDOT:PSS, known as a hole transport layer in most organic photovoltaic devices, were enhanced by adding PbS nanoparticles with different capping ligands, producing a promising composite material for optoelectronic applications by proper selection of the nanoparticles’ capping layer.

Electron diffraction and crystal orientation phase mapping under scanning transmission electron microscopy

Electron diffraction pattern acquisition in scanning transmission electron microscopy (STEM) mode is a very attractive technique for the study of the crystallographic characteristics of nanostructured materials. One of the most important aspects of this technique is to ensure an illumination on the sample as parallel as possible, which translates into reducing the convergence angle of the electron beam as much as possible. Different parameters of electron microscopes have a direct impact on the convergence angle of the electron beam; once these parameters are identified, and their effect on the convergence angle is studied, optimum conditions for the acquisition of electron diffraction patterns while in STEM mode (D-STEM) can be identified. In the present study, several of these parameters were identified and assessed; among these parameters we can mention the condenser aperture 2 size, the excitation of the condenser minilens, and the spot size used, among others. The results obtained allowed to identify the optimum conditions to produce a convergence angle smaller than 1 mrad, with an electron probe size smaller than 3 nm. When combined with precession electron diffraction (PED), this D-STEM technique allows obtaining crystal orientation phase maps with a spatial resolution determined mainly by the electron probe size. Several examples of these combined techniques applied to different nanostructured systems, like lead chalcogenide nanoparticles, Au clusters, GaN nanofilms, Co nanowires, and Au decahedral nanoparticles, are presented.