Renato S. Lima

Capacitively coupled contactless conductivity detection on microfluidic systems—ten years of development

The use of capacitively coupled contactless conductivity detection (C4D) on miniaturized systems has increased considerably over the last few years. Since the first report, 10 years ago, several advances on the detection cell geometry, strategies for increasing the sensitivity and a wide range of applications have been reported. This review intends to cover the main features related to the instrumental setup of this detection method for analytical and bioanalytical assays on microfluidic chips.

Contactless conductivity biosensor in microchip containing folic acid as bioreceptor

We report a glass/PDMS-based microfluidic biosensor that integrates contactless conductivity transduction and folic acid, a target for tumor biomarker, as a bioreceptor. The device presents relevant advantages such as direct determination--dismiss the use of redox mediators as in faradaic electrochemical techniques--and the absence of the known drawbacks related to the electrode-solution interface. Characterizations of the functionalization processes and chemical sensor are described in this communication.

Glass/PDMS hybrid microfluidic device integrating vertically aligned SWCNTs to ultrasensitive electrochemical determinations

This communication reports a promising platform for rapid, simple, direct, and ultrasensitive determination of serotonin. The method is related to integration of vertically aligned single-walled carbon nanotubes (SWCNTs) in electrochemical microfluidic devices. The required microfabrication protocol is simple and fast. In addition, the nanomaterial influenced remarkably the obtained limit-of-detection (LOD) values. Our system achieved a LOD of 0.2 nmol L(-1) for serotonin, to the best of our knowledge one of the lowest values reported in the literature.

Sacrificial adhesive bonding: a powerful method for fabrication of glass microchips

A new protocol for fabrication of glass microchips is addressed in this research paper. Initially, the method involves the use of an uncured SU-8 intermediate to seal two glass slides irreversibly as in conventional adhesive bonding-based approaches. Subsequently, an additional step removes the adhesive layer from the channels. This step relies on a selective development to remove the SU-8 only inside the microchannel, generating glass-like surface properties as demonstrated by specific tests. Named sacrificial adhesive layer (SAB), the protocol meets the requirements of an ideal microfabrication technique such as throughput, relatively low cost, feasibility for ultra large-scale integration (ULSI), and high adhesion strength, supporting pressures on the order of 5 MPa. Furthermore, SAB eliminates the use of high temperature, pressure, or potential, enabling the deposition of thin films for electrical or electrochemical experiments. Finally, the SAB protocol is an improvement on SU-8-based bondings described in the literature. Aspects such as substrate/resist adherence, formation of bubbles, and thermal stress were effectively solved by using simple and inexpensive alternatives.

Self-regenerating and hybrid irreversible/reversible PDMS microfluidic devices

This paper outlines a straightforward, fast, and low-cost method to fabricate polydimethylsiloxane (PDMS) chips. Termed sandwich bonding (SWB), this method requires only a laboratory oven. Initially, SWB relies on the reversible bonding of a coverslip over PDMS channels. The coverslip is smaller than the substrate, leaving a border around the substrate exposed. Subsequently, a liquid composed of PDMS monomers and a curing agent is poured onto the structure. Finally, the cover is cured. We focused on PDMS/glass chips because of their key advantages in microfluidics. Despite its simplicity, this method created high-performance microfluidic channels. Such structures featured self-regeneration after leakages and hybrid irreversible/reversible behavior. The reversible nature was achieved by removing the cover of PDMS with acetone. Thus, the PDMS substrate and glass coverslip could be detached for reuse. These abilities are essential in the stages of research and development. Additionally, SWB avoids the use of surface oxidation, half-cured PDMS as an adhesive, and surface chemical modification. As a consequence, SWB allows surface modifications before the bonding, a long time for alignment, the enclosure of sub-micron channels, and the prototyping of hybrid devices. Here, the technique was successfully applied to bond PDMS to Au and Al.

Determination of glyphosate and AMPA on polyester-toner electrophoresis microchip with contactless conductivity detection

This paper reports a method for rapid, simple, direct, and reproducible determination of glyphosate and its major metabolite aminomethylphosphonic acid (AMPA). The platform described herein uses polyester‐toner microchips incorporating capacitively coupled contactless conductivity detection and electrophoresis separation of the analytes. The polyester‐toner microchip presented 150 μm‐wide and 12 μm‐deep microchannels, with injection and separation lengths of 10 and 40 mm long, respectively. The best results were obtained with 320 kHz frequency, 4.5 Vpp excitation voltage, 80 mmol/L CHES/Tris buffer at pH 8.8, injection in −1.0 kV for 7 s, and separation in −1.5 kV. RSD values related to the peak areas for glyphosate and AMPA were 1.5 and 3.3% and 10.1 and 8.6% for intra‐ and interchip assays, respectively. The detection limits were 45.1 and 70.5 μmol/L, respectively, without any attempt of preconcentration of the analytes. Finally, the method was applied to river water samples in which glyphosate and AMPA (1.0 mmol/L each) were added. The recovery results were 87.4 and 83.7% for glyphosate and AMPA, respectively. The recovery percentages and LOD values obtained here were similar to others reported in the literature.

Glass/SU‐8 microchip for electrokinetic applications

In this communication, we describe the fabrication and electric characterization of a hybrid glass/SU‐8 microchannels for high‐performance electrokinetic applications. The bonding process employed SU‐8 film as intermediate layer with reduced baking times; all the procedure took less than 50 min (only about 10 min disregarding the cleaning and dehydration steps). Additionally, further steps to improve the adhesion of the substrate to the SU‐8 were not needed. The developed configuration aggregates the advantages of both substrates, including (i) simple fabrication techniques; (ii) high compatibility for integration of microelectromechanical, optical, and electrochemical components (SU‐8); (iii) high and stable electroosmotic mobility (μEO); and (iv) satisfactory heat dissipation capacity (glass). Electroosmotic mobilities were measured as a function of the pH using the current monitoring method, whereas the heat dissipation capacity was investigated through Ohms law plots for both glass and glass/SU‐8 microchips. The measured μEO values were similar for both microdevices, with mobilities of the order of 4.0–4.5 × 10−4 cm2 V−1 cm−1 at 4–12 pH range using phosphate buffer (10 and 20 mmol/L). The heat dissipation assays were carried out in microchannels filled with 20 mmol/L phosphate buffer. A considerable Joule heating was observed only at electric field strengths greater than 580 V cm−1 in hybrid glass/SU‐8 microdevices, representing a substantial increase of 48% when compared to all SU‐8 microdevices.

An integrated platform for gas-diffusion separation and electrochemical determination of ethanol on fermentation broths

An integrated platform was developed for point-of-use determination of ethanol in sugar cane fermentation broths. Such analysis is important because ethanol reduces its fuel production efficiency by altering the alcoholic fermentation step when in excess. The custom-designed platform integrates gas diffusion separation with voltammetric detection in a single analysis module. The detector relied on a Ni(OH)2-modified electrode. It was stabilized by uniformly depositing cobalt and cadmium hydroxides as shown by XPS measurements. Such tests were in accordance with the hypothesis related to stabilization of the Ni(OH)2 structure by insertion of Co(2+) and Cd(2+) ions in this structure. The separation step, in turn, was based on a hydrophobic PTFE membrane, which separates the sample from receptor solution (electrolyte) where the electrodes were placed. Parameters of limit of detection and analytical sensitivity were estimated to be 0.2% v/v and 2.90 μA % (v/v)(-1), respectively. Samples of fermentation broth were analyzed by both standard addition method and direct interpolation in saline medium based-analytical curve. In this case, the saline solution exhibited ionic strength similar to those of the samples intended to surpass the tonometry colligative effect of the samples over analyte concentration data by attributing the reduction in quantity of diffused ethanol vapor majorly to the electrolyte. The approach of analytical curve provided rapid, simple and accurate analysis, thus contributing for deployment of point-of-use technologies. All of the results were accurate with respect to those obtained by FTIR method at 95% confidence level.

Microemulsification: An Approach for Analytical Determinations

We address a novel method for analytical determinations that combines simplicity, rapidity, low consumption of chemicals, and portability with high analytical performance taking into account parameters such as precision, linearity, robustness, and accuracy. This approach relies on the effect of the analyte content over the Gibbs free energy of dispersions, affecting the thermodynamic stabilization of emulsions or Winsor systems to form microemulsions (MEs). Such phenomenon was expressed by the minimum volume fraction of amphiphile required to form microemulsion (Φ(ME)), which was the analytical signal of the method. Thus, the measurements can be taken by visually monitoring the transition of the dispersions from cloudy to transparent during the microemulsification, like a titration. It bypasses the employment of electric energy. The performed studies were: phase behavior, droplet dimension by dynamic light scattering, analytical curve, and robustness tests. The reliability of the method was evaluated by determining water in ethanol fuels and monoethylene glycol in complex samples of liquefied natural gas. The dispersions were composed of water-chlorobenzene (water analysis) and water-oleic acid (monoethylene glycol analysis) with ethanol as the hydrotrope phase. The mean hydrodynamic diameter values for the nanostructures in the droplet-based water-chlorobenzene MEs were in the range of 1 to 11 nm. The procedures of microemulsification were conducted by adding ethanol to water-oleic acid (W-O) mixtures with the aid of micropipette and shaking. The Φ(ME) measurements were performed in a thermostatic water bath at 23 °C by direct observation that is based on the visual analyses of the media. The experiments to determine water demonstrated that the analytical performance depends on the composition of ME. It shows flexibility in the developed method. The linear range was fairly broad with limits of linearity up to 70.00% water in ethanol. For monoethylene glycol in water, in turn, the linear range was observed throughout the volume fraction of analyte. The best limits of detection were 0.32% v/v water to ethanol and 0.30% v/v monoethylene glycol to water. Furthermore, the accuracy was highly satisfactory. The natural gas samples provided by the Petrobras exhibited color, particulate material, high ionic strength, and diverse compounds as metals, carboxylic acids, and anions. These samples had a conductivity of up to 2630 μS cm(-1); the conductivity of pure monoethylene glycol was only 0.30 μS cm(-1). Despite such downsides, the method allowed accurate measures bypassing steps such as extraction, preconcentration, and dilution of the sample. In addition, the levels of robustness were promising. This parameter was evaluated by investigating the effect of (i) deviations in volumetric preparation of the dispersions and (ii) changes in temperature over the analyte contents recorded by the method.

Simple, rapid and, cost‐effective fabrication of PDMS electrophoresis microchips using poly(vinyl acetate) as photoresist master

This study describes a simple, rapid, and cost‐effective fabrication of PDMS electrophoresis microchips using poly(vinyl acetate) (PVAc) emulsion as photoresist master. High‐relief microfluidic structures were defined on poly(vinyl acetate) previously deposited on printed circuit boards surfaces without cleanroom facilities and sophisticated instrumentation. After a UV exposure, channels with heights ranging from 30 to 140 μm were obtained by controlling the emulsion mass deposited on the master surface. The developing stage was performed using water rather than the organic solvents that are applied for conventional masks. The surface morphology was characterized by optical imaging, profilometry, and SEM. Based on the achieved results, the proposed method offers suitable reproducibility for the prototyping of electrophoresis microchips in PDMS. The feasibility of the resulting PDMS electrophoresis chips was successfully demonstrated with the separation of major inorganic cations within 100 s using a contactless conductivity detection system. The separation efficiencies ranged from ca. 67 900 to 125 600 plates/m. Due to the satisfactory performance and simplified instrumentation, we believe this fabrication protocol presents potential to be implemented in any chemical, biochemical, or biological laboratory.