R. Esparza

Surface functionalized halloysite nanotubes decorated with silver nanoparticles for enzyme immobilization and biosensing

Improving enzyme immobilization with high loading capacity and achieving direct electron transfer (DET) between the enzyme and the electrode surface is key to designing highly sensitive enzymatic electrochemical biosensors. Herein, we report a novel approach based on the selective modification of the outer surface of halloysite nanotubes (HNTs) that supports silver nanoparticles (AgNPs) to obtain a hybrid nanocomposite. AgNPs of about 10 nm average size could be uniformly supported on silane-modified HNTs through in situ reduction of Ag+ ions. The resultant nanocomposite shows an excellent support capability for the effective immobilization and electrical wiring of redox enzyme glucose oxidase (GOx). The GOx immobilized HNT/AgNPs were deposited on the glassy carbon electrode (GCE) and utilized for the bioelectrocatalyzed electrochemical detection of glucose. The GOx modified composite electrodes show glucose sensitivity as high as 5.1 μA mM-1 cm-2, which is higher than for the electrodes prepared without surface functionalization.

Chitosan supported silver nanowires as a platform for direct electrochemistry and highly sensitive electrochemical glucose biosensing

The development of low-cost and sensitive glucose biosensors has been the focus of substantial research interest due to their diverse applications in medical diagnosis, healthcare, and environmental monitoring. Herein, we report the successful use of chitosan (CS) supported silver nanowires (AgNWs) based enzyme electrodes for highly sensitive electrochemical glucose biosensing. The glucose oxidase (GOx) enzyme is electrically contacted using highly conductive AgNWs and thereby significant enhancement in the direct electron transfer (DET) between the redox enzymes and the electrode surface. In addition, the CS polymer matrix enables distinct self-assembly of GOx enzymes adjacent to the electrode surface, which further favours DET by increasing the charge transfer. Characterization by Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM) measurements were used to evidence the intermolecular interaction and self-assembly of the GOx on CS polymers. AFM results clearly revealed the self-assembly of the GOx on CS surfaces. The immobilized GOx exhibits a well-defined quasi-reversible redox peak with an electron rate constant (ks) of 6.52 s−1 compared to the bare glassy carbon electrode (GCE). The resultant biosensor demonstrates a high sensitivity of 16.72 μA mM−1 cm−2 with a wide linear range (1–15 mM), good selectivity and long-term stability for glucose detection. Our current approach represents a promising platform for the immobilization and electrical wiring of biomolecules with higher loading efficiency for designing low-cost, high sensitive enzymatic biosensors.

Effect of the surfactant on the growth and oxidation of iron nanoparticles

Fe nanoparticles and branched nanostructures of iron oxide were synthesized by chemical reduction in aqueous phase. The mechanism of formation of iron oxides as a function of the amount of surfactant employed during the synthesis process was studied. Specifically Fe, Fe2O3, and Fe3O4 nanoparticles were obtained. The oxidation of Fe to Fe3O4 and finally to Fe2O3 was carried out by oxidative etching process, decreasing the amount of stabilizer agent. The structures obtained were characterized by high resolution (HRTEM) and scanning/transmission (STEM) electron microcopies, energy dispersive spectroscopy (EDS), and optical spectroscopy (UV-Vis and IR).

Effects of Minor Element Additions to the Nanocrystalline FeAl Intermetallic Alloy Obtained by Mechanical Alloying

Chemical, microstructural, and mechanical properties of nanocrystalline FeAl intermetallic alloys with Li, Ce, and Ni additions have been assessed. Mechanical alloying and sintering procedures were used to produce and consolidate the alloys. The sintering procedure was based on room temperature uniaxial pressing followed by annealing in air of the pressed specimens. The mechanically alloyed powders have a microstructure consisting of micrometer-size particles that contain FeAl intermetallic nanocrystals. The three minor additional elements form a solid solution with the B2 intermetallic structure of the FeAl alloy. Densification greater than 90% has been obtained. The hardness values are higher than those obtained from specimens produced with conventional casting procedures. High resolution transmission electron microscopy images showed clusters of less than 5 nm with well-defined structure corresponding to Fe 3 Al.

Chitosan-Covered Pd@Pt Core–Shell Nanocubes for Direct Electron Transfer in Electrochemical Enzymatic Glucose Biosensor

Development of biosensors with high sensitivity, high spatial resolution, and low cost has received significant attention for their applications in medical diagnosis, diabetes management, and environment-monitoring. However, achieving a direct electrical contact between redox enzymes and electrode surfaces and enhancing the operational stability still remain as challenges. Inorganic metal nanocrystals (NCs) with precisely controlled shape and surface structure engineered with an appropriate organic coating can help overcome the challenges associated with their stability and aggregation for practical biosensor applications. Herein, we describe a facile, room-temperature, seed-mediated solution-phase route to synthesize monodisperse Pd@Pt core–shell nanocubes with subnanometer-thick platinum (Pt) shells. The enzyme electrode consisting of Pd@Pt core–shell NCs was first covered with a chitosan (CS) polymer and then glucose oxidase (GOx) immobilized by a covalent linkage to the CS. This polymer permits covalent immobilization through active amino (−NH) side groups to improve the stability and preserve the biocatalytic functions while the Pd@Pt NCs facilitate enhanced direct electron transfer (DET) in the biosensor. The resultant biosensor promotes DET and exhibits excellent performance for the detection of glucose, with a sensitivity of 6.82 μA cm–2 mM–1 and a wide linear range of 1–6 mM. Our results demonstrate that sensitive electrochemical glucose detection based on Pd@Pt core–shell NCs provides remarkable opportunities for designing low-cost and sensitive biosensors.

Structural analysis and shape-dependent catalytic activity of Au, Pt and Au/Pt nanoparticles

Metallic nanoparticles of Au, Pt and Pt/Au have been synthesized using a chemical reduction approach. The structural characterization of these metallic nanoparticles has been carried out using conventional transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HR-TEM) techniques. HR-TEM images of Pt nanoparticles reveal their fcc-like structures, in the [103] and [011] orientations. However, in case of Au, the main structures are fcc-like and decahedral morphologies with five-fold symmetry axes. The structure of the gold nanoparticles was verified through their theoretically-simulated images, using a multislice approach of the dynamical theory of electron diffraction. These small particles have been used as electrocatalysts in Nafion membranes for fuel cell applications. The electrochemical characterization of the membranes shows that the Pt dispersion gives best performance for fuel cell applications.

Synthesis and Characterization of Bifunctional α-Fe2O3-Ag Nanoparticles

The synthesis of α-Fe2O3-Ag bimetallic nanoparticles using a novel and simplified route is presented in this work. These hybrid nanoparticles were produced using a modification of the chemical reduction method by sodium borohydride (NaBH4). Fe(III) chloride hexahydrate (FeCl3·6H2O) and silver nitrate (AgNO3) as precursors were employed. Particles with semispherical morphology and dumbbell configuration were observed. High-resolution transmission electron microscopy (HRTEM) technique reveals the structure of the dumbbell-like α-Fe2O3-Ag nanoparticles. Some theoretical models further confirm the formation of the α-Fe2O3-Ag structures. Analysis by cyclic voltammetry reveals an interesting catalytic behavior which is associated with the combination of the individual properties of the Ag and α-Fe2O3 nanoparticles.

Synthesis of Ag Nanoparticles-Clinoptilolite Composite by Homogeneous and Heterogeneous Nucleation

In this study, a natural zeolite clinoptilolite-type was impregnated through homogeneous and heterogeneous nucleation with silver nanoparticles. The synthesis of Ag nanoparticles was carried out by chemical reduction of silver nitrate (AgNO3) with sodium borohydride (NaBH4). In the case of homogeneous nucleation, colloidal solution of Ag nanoparticles at concentrations of 1, 2 and 4 parts per million was added and magnetically mixed with the porous material. With respect to heterogeneous nucleation, a solution of clinoptilolite and silver nitrate (0.01 M) was prepared and stirred; subsequently, the reduction of Ag was possible due to the addition of an aqueous solution of sodium borohydride. For the structural characterization, transmission electron microscopy (TEM), X-ray diffraction (XRD) and infrared spectroscopy (IR) techniques were carried out. The results were compared and discussed in both types of nucleation.

High-Energy Ball-Milling of FeAl2 and Fe2Al5 Intermetallic Systems

In this study, FeAl2 and Fe2Al5 intermetallic alloys were prepared by conventional casting technique. In order to study their structural stability the alloys were subjected to high-energy ball milling process for 1, 2.5, 5 and 10 h. The structural and chemical characterizations were conducted by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. After 10 h of milling, the experimental results indicated a phase transformation from FeAl2-triclinic phase to Fe2Al5-ortorrombic structure. This phase transformation is characterized by a change from low to high symmetry systems.

Spectroscopy Study of Silver Nanoparticles Produced by Chemical Reduction

In this study, Ag nanoparticles were synthesized using two chemical reduction agents; ethylene glycol and sodium borohydride. Different particle size distributions were obtained and characterized by transmission electron microscopy. Ag nanoparticles concentrations of 1, 2 and 4 parts per million (gmL-1) were prepared and studied by ultraviolet-visible spectroscopy (UV-Vis) and atomic absorption spectrophotometry (AAS). In the UV-Vis results a characteristic band at 420 nm were observed. However, when the concentration of silver decreased, a change in band intensity was detected. Atomic absorption spectrophotometry measurements from different solutions of Ag nanoparticles showed a linear behavior similar to the silver standard solution in the concentration range 1 to 4 mgL-1. However, up 4 mgL-1 concentrations, the slope of the calibration curve is increases when the concentration of Ag nanoparticles is increased too.