Antonio Carlos Seabra

A Monolithic Continuous-Flow Microanalyzer with Amperometric Detection Based on the Green Tape Technology

The development of micro total analysis systems (muTAS) has become a growing research field. Devices that include not only the fluidics and the detection system but also the associated electronics are reported scarcely in the literature because of the complexity and the cost involved for their monolithic integration. Frequently, dedicated devices aimed at solving specific analytical problems are needed. In these cases, low-volume production processes are a better alternative to mass production technologies such as silicon and glass. In this work, the design, fabrication, and evaluation of a continuous-flow amperometric microanalyzer based on the green tape technology is presented. The device includes the microfluidics, a complete amperometric detection system, and the associated electronics. The operational lifetime of the working electrode constitutes a major weak point in electrochemical detection systems, especially when it is integrated in monolithic analytical devices. To increase the overall system reliability and its versatility, it was integrated following an exchangeable configuration. Using this approach, working electrodes can be readily exchanged, according to the analyte to be determined or when their surfaces become passivated or poisoned. Furthermore, the electronics of the system allow applying different voltamperometric techniques and provide four operational working ranges (125, 12.5, 1.25, and 0.375 microA) to do precise determinations at different levels of current intensity.

Tailoring magnetic vortices in nanostructures

Tailoring the properties of magnetic vortices through the preparation of structured multilayers is discussed. The dependence of the vortex core radius r on the effective anisotropy is derived within a simple model, which agrees with our simulations. As the perpendicular anisotropy increases, r also increases until a perpendicular magnetization appears in the disk rim. Co/Pt multilayer disks were studied; x-ray microscopy confirms qualitatively the predicted behavior. This is a favorable system for implementing vortex-based spin-transfer nano-oscillator devices, with enhanced rf power resulting both from the increase in the core size and synchronization afforded by the coupling of the Co layers.

Vortex configuration flow cell based on low-temperature cofired ceramics as a compact chemiluminescence microsystem.

The integration of optical detection methods in continuous flow microsystems can highly extend their range of application, as long as some negative effects derived from their scaling down can be minimized. Downsizing affects to a greater extent the sensitivity of systems based on absorbance measurements than the sensitivity of those based on emission ones. However, a careful design of the instrumental setup is needed to maintain the analytical features in both cases. In this work, we present the construction and evaluation of a simple miniaturized optical system, which integrates a novel flow cell configuration to carry out chemiluminescence (CL) measurements using a simple photodiode. It consists of a micromixer based on a vortex structure, which has been constructed by means of the low-temperature cofired ceramics (LTCC) technology. This mixer not only efficiently promotes the CL reaction due to the generated high turbulence but also allows the detection to be carried out in the same area, avoiding intensity signal losses. As a demonstration, a flow injection system has been designed and optimized for the detection of cobalt(II) in water samples. It shows a linear response between 2 and 20 microM with a correlation of r > 0.993, a limit of detection of 1.1 microM, a repeatability of RSD = 12.4%, and an analysis time of 17 s. These results demonstrate the suitability of the proposal to the determination of compounds involved in CL reactions by means of an easily constructed versatile device based on low-cost instrumentation.

Local hysteresis loop measurements by magneto-optical scanning near-field optical microscope

We performed magnetic-field-induced experiments on micron-sized patterned Co70.4Fe4.6Si15B10 square thin-film elements with in-plane magnetic anisotropy by magneto-optical scanning near-field optical microscopy (MO-SNOM) with a spatial resolution better than 200nm. Markedly different local hysteresis loops (LHLs) were measured on selected positions in one element. Some LHLs presented an unusual shape intrinsic of local magnetic-field-induced process. Comparison of the MO-SNOM imaging results with high-resolution far-field Kerr microscopy has confirmed the local character of the MO-SNOM measurements. This has also helped us to understand the unusual LHLs shapes as related to the field-induced rearrangement of the domain structure within the square element during the magnetization process. The magnetic structure in small field is well described by two overlapping four-domain flux-closure configurations that are well modeled by micromagnetic calculations.

Magnetic characterization of microscopic particles by MO-SNOM

This paper reports on the development of a magneto‐optical scanning near‐field optical microscope and the experimental near‐field study of the domain structure for a model magnetic particle of 16 × 16 µm2 of a Co70.4Fe4.6Si15B10 amorphous thin film, deposited on a silicon substrate. We present the topographic, optical and magneto‐optical differential susceptibility (MODS) images of the particle. Imaging by using the local MODS reveals the domain structure. These images are also used for positioning the tip in order to acquire local hysteresis loops, with submicrometre spatial resolution.

Rifampicin Nanoprecipitation using Flow Focusing Microfluidic Device

We present the nanoprecipitation of rifampicin performed in a microfluidic device as a means to reduce the particle size and enhance the dissolution rate. The microfluidic device was microfabricated in glass substrate with a 45° flow-focusing geometry. The dimensions of the central and side channels are 100 µm and 110 µm in width, respectively, and 85 µm in depth. We analyze the influence of different parameters in the rifampicin particles size, such as: rifampicin concentration, the presence of surfactant, the total fluid flow and solvent to anti-solvent flow rate ratio. The processed rifampicin was evaluated not only in terms of size, but also morphology, crystallinity, thermal characteristics and dissolution rate. We produce particle sizes in a controlled manner with sizes ranging from 100 nm to 1.2 µm. The particles present an amorphous profile and enhanced dissolution rate as compared to commercial raw rifampicin. These results are promising and have enabled us to better understand the rifampicin self-assembly process in microfluidic device .

Imaging of domain wall motion in small magnetic particles through near-field microscopy

We report magneto-optical scanning near-field optical microscopy images for 4×4×0.08μm3 and 16×16×0.08μm3 low magnetic anisotropy Co70.4Fe4.6Si15B10 particles. Measuring magneto-optical differential susceptibility we acquired images of the domain wall movement driven by an applied magnetic field with a spatial resolution better than λ∕4. For the 4×4×0.08μm3 sized particle, a sequence of 27 magneto-optical differential susceptibility images reveals the evolution of the magnetic domain structure between positive and negative saturation fields passing through the four-domain flux-closure magnetization structure. On the 16×16×0.08μm3 particle, we studied the role of the oscillating driving field on the susceptibility distribution. Comparing the different magneto-optical differential susceptibility sequences, the role of the shape anisotropy on the field induced domain wall movement is evidenced. Micromagnetic simulations were used to provide a better understanding of the domain wall movement.

Increasing the optical coupling efficiency of planar photodetectors: electron beam writing of an integrated microlens array on top of an MSM device

In this paper we describe the fabrication of an array of integrated cylindrical microlenses on top of a single GaAs MSM photodetector. Experimental data shows an increase of about 65% on the photocurrent of the MSM photodiode as a result of the improved optical coupling efficiency.

Fabrication of Microlenses With a Novolak-type Polymer

In this work, the fabrication of arrays of parabolic convergent and divergent microlenses is presented. The used material is the Novolak-based polymer All-Resist AR P322, which can be used both for optical UV lithography and for electron beam direct write lithography. Gratings of parabolic divergent microlenses with f-number of 0.5 were fabricated using traditional optical lithography, employing the diffraction characteristics of de-focussed light during the photolithography exposure. Direct write electron beam lithography was used to obtain convergent parabolic microlenses, with different diameters and different heights, allowing the control of the focal length of these lenses. The same technique was employed to manufacture gratings of parabolic convergent microlenses with different diameter and focal length, what enables one to control the intensity of the different orders of the diffracted light. These structures have several applications in the fields of pattern recognition, robotic vision and optical sensors.

Development of Sub-half Micrometric Structures with High Aspect Ratio Using a Multi-layer Lithography e-beam Process and Plasma Dry Etching

In this work we are using the AR-P 322 ( All Resist GmbH ) DQN-novolac photoresist, with 3 µm resolution specified by the manufacturer in the development of sub micrometric structures with high aspect ratios (10:1). In order to obtain these structures (sub-half micrometric and nanometric) we are studying the possible application of electron beam lithography and plasma etching. The resolution limit of the photoresist AR-P 322 is increased to 0.25 µm (nanometric resolution), using an electron beam spot size of 50 nm and dry development.