Wendell Karlos Tomazelli Coltro

Toner and paper‐based fabrication techniques for microfluidic applications

The interest in low‐cost microfluidic platforms as well as emerging microfabrication techniques has increased considerably over the last years. Toner‐ and paper‐based techniques have appeared as two of the most promising platforms for the production of disposable devices for on‐chip applications. This review focuses on recent advances in the fabrication techniques and in the analytical/bioanalytical applications of toner and paper‐based devices. The discussion is divided in two parts dealing with (i) toner and (ii) paper devices. Examples of miniaturized devices fabricated by using direct‐printing or toner transfer masking in polyester‐toner, glass, PDMS as well as conductive platforms as recordable compact disks and printed circuit board are presented. The construction and the use of paper‐based devices for off‐site diagnosis and bioassays are also described to cover this emerging platform for low‐cost diagnostics.

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.

Recent advances in low-cost microfluidic platforms for diagnostic applications

The use of inexpensive materials and cost‐effective manufacturing processes for mass production of microfluidic devices is very attractive and has spurred a variety of approaches. Such devices are particularly suited for diagnostic applications in limited resource settings. This review describes the recent and remarkable advances in the use of low‐cost substrates for the development of microfluidic devices for diagnostics and clinical assays. Thus, a plethora of new and improved fabrication methods, designs, capabilities, detections, and applications of microfluidic devices fabricated with paper, plastic, and threads are covered.

A handheld stamping process to fabricate microfluidic paper-based analytical devices with chemically modified surface for clinical assays

This paper describes the development and use of a handheld and lightweight stamp for the production of microfluidic paper-based analytical devices (μPADs). We also chemically modified the paper surface for improved colorimetric measurements. The design of the microfluidic structure has been patterned in a stamp, machined in stainless steel. Prior to stamping, the paper surface was oxidized to promote the conversion of hydroxyl into aldehyde groups, which were then chemically activated for covalent coupling of enzymes. Then, a filter paper sheet was impregnated with paraffin and sandwiched with a native paper (n-paper) sheet, previously oxidized. The metal stamp was preheated at 150 °C and then brought in contact with the paraffined paper (p-paper) to enable the thermal transfer of the paraffin to the n-paper, thus forming the hydrophobic barriers under the application of a pressure of ca. 0.1 MPa for 2 s. The channel and barrier widths measured in 50 independent μPADs exhibited values of 2.6 ± 0.1 and 1.4 ± 0.1 mm, respectively. The chemical modification for covalent coupling of enzymes on the paper surface also led to improvements in the colour uniformity generated inside the sensing area, a known bottleneck in this technology. The relative standard deviation (RSD) values for glucose and uric acid (UA) assays decreased from 40 to 10% and from 20 to 8%, respectively. Bioassays related to the detection of glucose, UA, bovine serum albumin (BSA), and nitrite were successfully performed in concentration ranges useful for clinical assays. The semi-quantitative analysis of all four analytes in artificial urine samples revealed an error smaller than 4%. The disposability of μPADs, the low instrumental requirements of the stamp-based fabrication, and the improved colour uniformity enable the use of the proposed devices for the point-of-care diagnostics or in limited resources settlements.

Rational selection of substrates to improve color intensity and uniformity on microfluidic paper-based analytical devices

A systematic investigation was conducted to study the effect of paper type on the analytical performance of a series of microfluidic paper-based analytical devices (μPADs) fabricated using a CO2 laser engraver. Samples included three different grades of Whatman chromatography paper, and three grades of Whatman filter paper. According to the data collected and the characterization performed, different papers offer a wide range of flow rate, thickness, and pore size. After optimizing the channel widths on the μPAD, the focus of this study was directed towards the color intensity and color uniformity formed during a colorimetric enzymatic reaction. According to the results herein described, the type of paper and the volume of reagents dispensed in each detection zone can determine the color intensity and uniformity. Therefore, the objective of this communication is to provide rational guidelines for the selection of paper substrates for the fabrication of μPADs.

Comparison of the analytical performance of electrophoresis microchannels fabricated in PDMS, glass, and polyester-toner

This paper compares the analytical performance of microchannels fabricated in PDMS, glass, and polyester‐toner for electrophoretic separations. Glass and PDMS chips were fabricated using well‐established photolithographic and replica‐molding procedures, respectively. PDMS channels were sealed against three different types of materials: native PDMS, plasma‐oxidized PDMS, and glass. Polyester‐toner chips were micromachined by a direct‐printing process using an office laser printer. All microchannels were fabricated with similar dimensions according to the limitations of the direct‐printing process (width/depth 150 μm/12 μm). LIF was employed for detection to rule out any losses in separation efficiency due to the detector configuration. Two fluorescent dyes, coumarin and fluorescein, were used as model analytes. Devices were evaluated for the following parameters related to electrophoretic separations: EOF, heat dissipation, injection reproducibility, separation efficiency, and adsorption to channel wall.

A toner-mediated lithographic technology for rapid prototyping of glass microchannels

A simple, fast, and inexpensive masking technology without any photolithographic step to produce glass microchannels is proposed in this work. This innovative process is based on the use of toner layers as mask for wet chemical etching. The layouts were projected in graphic software and printed on wax paper using a laser printer. The toner layer was thermally transferred from the paper to cleaned glass surfaces (microscope slides) at 130 degrees C for 2 min. After thermal transference, the glass channel was etched using 25% (v/v) hydrofluoric acid (HF) solution. The toner mask was then removed by cotton soaked in acetonitrile. The etching rate was approximately 7.1 +/- 0.6 microm min(-1). This process is economically more attractive than conventional methods because it does not require any sophisticated instrumentation and it can be implemented in any chemical/biochemical laboratory. The glass channel was thermally bonded against a flat glass cover and its analytical feasibility was investigated using capacitively coupled contactless conductivity detection (C(4)D) and laser-induced fluorescence (LIF) detection.

Modification of microfluidic paper-based devices with silica nanoparticles

This paper describes a silica nanoparticle-modified microfluidic paper-based analytical device (μPAD) with improved color intensity and uniformity for three different enzymatic reactions with clinical relevance (lactate, glucose, and glutamate). The μPADs were produced on a Whatman grade 1 filter paper and using a CO2 laser engraver. Silica nanoparticles modified with 3-aminopropyltriethoxysilane were then added to the paper devices to facilitate the adsorption of selected enzymes and prevent the washing away effect that creates color gradients in the colorimetric measurements. According to the results herein described, the addition of silica nanoparticles yielded significant improvements in color intensity and uniformity. The resulting μPADs allowed for the detection of the three analytes in clinically relevant concentration ranges with limits of detection (LODs) of 0.63 mM, 0.50 mM, and 0.25 mM for lactate, glucose, and glutamate, respectively. An example of an analytical application has been demonstrated for the semi-quantitative detection of all three analytes in artificial urine. The results demonstrate the potential of silica nanoparticles to avoid the washing away effect and improve the color uniformity and intensity in colorimetric bioassays performed on μPADs.

Colorimetric determination of nitrite in clinical, food and environmental samples using microfluidic devices stamped in paper platforms

This study reports the use of microfluidic paper-based analytical devices (μPADs) associated with colorimetric detection for the determination of nitrite in clinical, food and environmental samples. μPADs were fabricated by a simple and fast stamping process in a geometry containing eight circular detection zones and one central zone to sample inlet interconnected by microfluidic channels. The colorimetric determination of nitrite was performed through the modified Griess reaction. Detection zones were spotted with a 0.75 μL aliquot of a solution containing 50 mM sulfanilamide, 1.2 M hydrochloric acid and 4 mM N-(1-naphthyl)ethylenediamine. The monitoring of the background colorimetric response revealed good stability over 12 h for devices stored in the absence of light. After the addition of standard or real samples, the resulting images were captured with a scanner, converted to a color scale and analyzed in the magenta channel. The analytical sensitivity and the limit of detection achieved after a preconcentration stage were 0.56 (AU μM−1) and 5.6 μM, respectively. The preconcentration provided an enrichment factor of ca. 3.2 times. The concentration levels of nitrite were successfully determined in saliva, preservative water, ham, sausage and river water samples. The concentration levels attained for each sample using μPADs were compared to the values found by spectrophotometry and there was no significant difference from one another at a confidence level of 95%.

Fabrication and integration of planar electrodes for contactless conductivity detection on polyester‐toner electrophoresis microchips

In this report, we describe the microfabrication and integration of planar electrodes for contactless conductivity detection on polyester‐toner (PT) electrophoresis microchips using toner masks. Planar electrodes were fabricated by three simple steps: (i) drawing and laser‐printing the electrode geometry on polyester films, (ii) sputtering deposition onto substrates, and (iii) removal of toner layer by a lift‐off process. The polyester film with anchored electrodes was integrated to PT electrophoresis microchannels by lamination at 120°C in less than 1 min. The electrodes were designed in an antiparallel configuration with 750 μm width and 750 μm gap between them. The best results were recorded with a frequency of 400 kHz and 10 Vpp using a sinusoidal wave. The analytical performance of the proposed microchip was evaluated by electrophoretic separation of potassium, sodium and lithium in 150 µm wide×6 µm deep microchannels. Under an electric field of 250 V/cm the analytes were successfully separated in less than 90 s with efficiencies ranging from 7000 to 13 000 plates. The detection limits (S/N = 3) found for K+, Na+, and Li+ were 3.1, 4.3, and 7.2 μmol/L, respectively. Besides the low‐cost and instrumental simplicity, the integrated PT chip eliminates the problem of manual alignment and gluing of the electrodes, permitting more robustness and better reproducibility, therefore, more suitable for mass production of electrophoresis microchips.