Rogerio Furlan

Synthesis and characterization of tin oxide microfibres electrospun from a simple precursor solution

Tin oxide (SnO2) microfibres in the rutile structure were synthesized using electrospinning and metallorganic decomposition techniques. Fibres were electrospun from a precursor solution containing 20 mg poly(ethylene oxide) (molecular weight 900 000), 2 ml chloroform and 1 ml dimethyldineodecanoate tin, and sintered in the air for 2 h at 400, 600 and 800 °C, respectively. Scanning electron microscopy, x-ray diffraction and Raman microspectrometry were used to characterize the sintered fibres. The results showed that the synthesized fibres are composed of SnO2.

Characterization of an electrospinning process using different PAN/DMF concentrations

We performed an extensive characterization of an electrospinning process to evaluate how the process parameters and precursor solution characteristics affect the fibers morphology. The work was conducted using precursor solutions with different concentrations of polyacrylonitrile (PAN) diluted in a fixed amount of N,N/dimethylformamide (DMF). Fibers obtained with this process can find important applications in the field of nanosensors. The characteristics of the electrospun fibers were analyzed as a function of the solution viscosity, applied voltage and distance between the needle tip (positive electrode) and the collector plate (grounded electrode). The electrical current was monitored during the deposition process and its behavior was correlated with the characteristics of the fibers obtained. Our results demonstrate that the diameter of the fibers increases with increasing viscosity and applied voltage. The number of deposited fibers also increases with the applied voltage. Also, viscosity and applied voltage strongly affect the shape, length and morphology of the fibers. Of particular interest, we demonstrated that by monitoring the electrical current it is possible to control the fibers morphology and bead concentration. The distance between tip and collector plate determines the way the fibers arrive on the collector plate. A main contribution of this study was the definition of conditions to controllably obtain fibers that are smooth and that present diameters in the range between 140 and 300 nm.

Electrostatic deposition of nanofibers for sensor application

This work addresses the formation of nanofibers (with hundred of nanometers) by using electrospinning (electrostatic deposition) aiming at applications as sensors. Different quantities of a colloidal dispersion of graphite particles were blended with polyacrylonitrile (PAN) and N,N dimethylformamide (DMF), resulting in a series of solutions with carbon concentrations ranging from 0 to 25%. Precipitation was observed depending on the concentration of carbon added to the precursor blend. As a consequence, the relative viscosity decreases, due to PAN molecules removal from the solution by carbon particles adsorption, forming precipitates. The resulting fibers show an irregular shape, as observed by SEM and the diameters decrease with the increase of the carbon concentration in the precursor blend. The incorporation of carbon particles in the fibers was confirmed by FTIRS and Raman spectroscopy.

Electronic transport properties of incipient graphitic domains formation in PAN derived carbon nanofibers

The carbon nanofibers used in this work were derived from a polyacrylonitrile (PAN)/N, N-dimethyl formamide (DMF) precursor solution using electrospinning and vacuum pyrolysis techniques. Their conductivity, /spl sigma/, was measured at temperatures between 1.9 and 300 K and transverse magnetic field between -9 and 9 T. Zero magnetic field conductivity /spl sigma/(0,T) was found to increase monotonically with the temperature with a convex /spl sigma/(0,T) versus T curve. Conductivity increases with the external transverse magnetic field, revealing a negative magnetoresistance at temperatures between 1.9 and 10 K, with a maximum magnetoresistance of -75 % at 1.9 K and 9 T. The magnetic field dependence of the conductivity and the temperature dependence of the zero-field conductivity are best described using the two-dimensional weak localization effect.

Electrospun Tin Oxide Nanofibers

Ultrafine tin oxide (SnO2) fibers in the rutile structure, with diameters ranging from 60nm to several microns, were synthesized using electrospinning and metallorganic decomposition techniques. In this work we use a precursor solution which is a mixture of a pure SnO2 sol made from SnCl4 : H2O : C3H7OH : 2-C3H7OH at a molar ratio of 1:9:9:6, and a viscous solution made from poly(ethylene oxide) (PEO) (molecular weight 900,000) and chloroform CHCl3 at a ratio of 200mg PEO/10mL CHCl3. This solution allows obtaining an appropriate viscosity for the electrospinning process. The as deposited fibers were sintered at 400, 500, 600, 700 and 800°C in air for two hours. Previous results using this method and characterizing the fibers with scanning electron microscopy (SEM), x-ray diffraction (XRD), Raman microspectrometry and x-ray photoelectron spectroscopy (XPS) showed that up to the sintering temperature of 700°C, the synthesized fibers are composed of SnO2. Further analysis using SEM, Profilometry, Atomic Force Microscopy (SPM), Auger Spectroscopy and I/V analysis is presented in this paper. The results show that the fibers are composed of tin oxide and that smooth and continuous fibers in different shapes (straight, curved, ribbon-like, and spring-like) can be obtained using this method. The change in resistivity as a function of the annealing temperature can be attributed to the thermally activated formation of a nearly stoichoimetric solid.

Micromechanical structure development for chemical analysis: study of porous silicon as an adsorbent

The aim of this work is to investigate the use of microchannels to concentrate pollutants present in the air. Devices with a length of 30 cm, a width of 100 micrometers and a depth of 30 micrometers , sealed by anodically bonded glass, were manufactured. Tests of adsorption characteristics were made using n-hexane. To reliably insert N2 contaminated with 1000 ppm of n-hexane in the microstructure, a simple setup was manufactured. This setup allows to insert the reactant, remove the amount of reactant adsorbed and to detect it. An amount of 20 mg was inserted in the microstructure. In order to improve the adsorption characteristics, PS layers were manufactured in the microchannels using silicon nitride as mask. The sealing of the microstructure with anodic bonded glass showed to be feasible either if the surface present PS or silicon nitride. Samples of PS layers covered by a plasma polymerized film, produced using HMDS, were analyzed by Raman microscopy. It was noticed that the nanocrystals are completely fulfilled by the deposited material, indicating the high reactivity of the PS layer.

Development of a silicon microprobe for NO detection

An array of electrochemical sensors -- amperometric detection -- was developed using silicon planar technology. Our main purpose is to detect the activity of free radicals such as nitric oxide (NO) at cellular dimensions. In this paper we focused on the process sequence used to produce silicon microprobes, based on plasma etching. Typical dimensions of the structures are: a length of 10 mm, a width of 1 mm, a tip width of 60 micrometers, and a thickness of 30 micrometers. Four different probe designs were adopted in order to test mechanical integrity. The defined process using plasma etching revealed to be feasible, although the lateral walls of the obtained probes resulted very rough. Preliminary mechanical tests were performed using probes with a thickness of 300 micrometers. Probes with wider shapes seem to have a better combination of higher fracture force and possibility to place more electrodes close to the tip.

Retention of copper(II) metal ions in a silicon-glass microfluidic device

This work describes the construction of a silicon microchip for retention of copper(II) metal ions. Conventional photolithographic process was applied to transfer the generated pattern to silicon wafers. Using Reactive Ion Etching (RIE), SF6 based, channels 50 µm wide and 10 µm deep were produced. The channels were sealed with borosilicate glass using anodic bonding process. The surface of the channels were modified with N-(b-aminoethyl)-g-aminopropyltrimetoxysilane through a silanization reaction to promote the adsorption of copper(II) ions. An amperometric detector was placed at the microchip outlet and copper(II) ions were detected by a gold electrode at 0 V (against Ag/AgCl(KCl sat.) reference electrode). Copper(II) ions were retained and eluted with HCl 50 µmol L-1 in a micro-flow system at a flow rate about 100 µL min-1. Reproducibility in the peak area and height were about 4.6 % and 10 %, respectively, for three consecutive injections of 600 µL of 10 µmol L-1 copper(II) sample.

Electronic transport properties of incipient graphitic domain formation in PAN-derived carbon nanofibers

We have measured the electronic transport properties of PAN based nano-fibers obtained by electrostatic deposition from 1.9 K to room temperature and carefully fitted the temperature and magnetic field dependence of these measurements to pertinent theoretical models It is noteworthy that the anomalous temperature and magnetic field dependence of conductivity have been found in carbon fibers with diameter larger than 10 microns, and mostly, carbonized at heat treatment temperature (HTT) higher than 1000°C. It is interesting to evaluate the scaling of such effects that is, if similar effects exist after the diameter is reduced into the nano scale. This paper reports such an attempt after the authors obtained carbon nano-fibers by electro-spinning and measured their electronic transport properties. Single carbon nano-fibers were deposited on silicon oxide coated silicon wafer, and with a lithographed gold contact pattern array. The length and cross-section area of the fibers was measured using an optical microscope and a scanning probe microscope (SPM) operated in tapping mode. Four-probe resistance measurement was conducted continuously 300K down to 1.9K, without any applied magnetic field. Resistance was also measured at 1.9, 3, 5 and 10K when the applied magnetic field, perpendicular to the fiber, increasing and decreasing continuously between -9 and 9 Tesla twice. To suppress the possible heating effect, the total measuring power was limited to 5nW. At all the four investigated temperatures, MR is negative. Its magnitude increase with B and decrease with T. It is noteworthy that MR=-0.75 at T=1.9K and B=9T, the highest MR for such system as far as the authors knowledge.

Silicon microstructure for preconcentration of metallic ions

Publisher Summary This chapter describes the fabrication and preliminary tests of a pre-concentrator implemented in silicon using reactive ion etching, SF6 based, to define the channel microstructure and anodic bonding between silicon and glass to seal up the micro-channels. The proposed innovation is the direct coupling of this pre-concentrator with a glass capillary that will perform the electrophoresis analysis. The immobilization of N-2-aminoethyl-3-aminopropylsilane and pre-concentration of copper (II) was verified by means of potentiometric stripping essays, performed with potential and deposition time of 0 V and 120 s, respectively. Pre-concentration of metallic ions aims at environmental applications. The implemented preconcentrator has a pattern, which is similar to that firstly proposed by Regnier and collaborators for electrochromatography applications. The chapter presents the design and implementation of a preconcentrator fabricated on a silicon substrate using silicon micro-channels defined with RIE and sealed with anodic bonding. The fluid injection is achieved by capillarity effects and the surface of the micro-channels is easily modified with defined ligand.