Carlos D. Garcia

Recent Applications of Carbon-Based Nanomaterials in Analytical Chemistry: Critical Review

The objective of this review is to provide a broad overview of the advantages and limitations of carbon-based nanomaterials with respect to analytical chemistry. Aiming to illustrate the impact of nanomaterials on the development of novel analytical applications, developments reported in the 2005-2010 period have been included and divided into sample preparation, separation, and detection. Within each section, fullerenes, carbon nanotubes, graphene, and composite materials will be addressed specifically. Although only briefly discussed, included is a section highlighting nanomaterials with interesting catalytic properties that can be used in the design of future devices for analytical chemistry.

Direct determination of carbohydrates, amino acids and antibiotics by microchip electrophoresis with pulsed amperometric detection

The separation and detection of underivatized carbohydrates, amino acids, and sulfur-containing antibiotics in an electrophoretic microchip with pulsed amperometric detection (PAD) is described. This report also describes the development of a new chip configuration for microchip electrophoresis with PAD. The configuration consists of a layer of poly(dimethylsiloxane) that contains the microfluidic channels, reservoirs, and a gold microwire, sealed to a second layer of poly(dimethylsiloxane). Example separations of carbohydrates, amino acids, and sulfur-containing antibiotics are shown. The effect of the separation and injection potentials, buffer pH and composition, injection time, and PAD parameters were studied in an effort to optimize separations and detection. Detection limits ranging from 6 fmol (5 microM) for penicillin and ampicillin to 455 fmol (350 microM) for histidine were obtained.

Protein adsorption onto nanomaterials for the development of biosensors and analytical devices: a review.

An important consideration for the development of biosensors is the adsorption of the biorecognition element to the surface of a substrate. As the first step in the immobilization process, adsorption affects most immobilization routes and much attention is given into the research of this process to maximize the overall activity of the biosensor. The use of nanomaterials, specifically nanoparticles and nanostructured films, offers advantageous properties that can be fine-tuned to maximize interactions with specific proteins to maximize activity, minimize structural changes, and enhance the catalytic step. In the biosensor field, protein-nanomaterial interactions are an emerging trend that span across many disciplines. This review addresses recent publications about the proteins most frequently used, their most relevant characteristics, and the conditions required to adsorb them to nanomaterials. When relevant and available, subsequent analytical figures of merits are discussed for selected biosensors. The general trend amongst the research papers allows concluding that the use of nanomaterials has already provided significant improvements in the analytical performance of many biosensors and that this research field will continue to grow.

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.

Determination of nitrite in saliva using microfluidic paper-based analytical devices.

Point-of-care platforms can provide fast responses, decrease the overall cost of the treatment, allow for in-home determinations with or without a trained specialist, and improve the success of the treatment. This is especially true for microfluidic paper-based analytical devices (μPAD), which can enable the development of highly efficient and versatile analytical tools with applications in a variety of biomedical fields. The objective of this work was the development of μPADs to identify and quantify levels of nitrite in saliva, which has been proposed as a potential marker of periodontitis. The devices were fabricated by wax printing and allowed the detection of nitrite by a colorimetric reaction based on a modified version of the Griess reaction. The presented modifications, along with the implementation of a paper-based platform, address many of the common drawbacks (color development, stability, etc.) associated with the Griess reaction and are supported by results related to the design, characterization, and application of the proposed devices. Under the optimized conditions, the proposed devices enable the determination of nitrite in the 10-1000 μmol L(-1) range with a limit of detection of 10 μmol L(-1) and a sensitivity of 47.5 AU [log (μmol L(-1))](-1). In order to demonstrate the potential impact of this technology in the healthcare industry, the devices were applied to the analysis of a series of real samples, covering the relevant clinical range.

Recent Developments in Instrumentation for Capillary Electrophoresis and Microchip-Capillary Electrophoresis

Over the last years, there has been an explosion in the number of developments and applications of CE and microchip‐CE. In part, this growth has been the direct consequence of recent developments in instrumentation associated with CE. This review, which is focused on the contributions published in the last 5 years, is intended to complement the articles presented in this special issue dedicated to instrumentation and to provide an overview of the general trends and some of the most remarkable developments published in the areas of high‐voltage power supplies, detectors, auxiliary components, and compact systems. It also includes a few examples of alternative uses of and modifications to traditional CE instruments.

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.

Versatile 3-channel high-voltage power supply for microchip capillary electrophoresis

The fabrication of a battery operated 3-channel high voltage power supply for microchip capillary electrophoresis is described. The power supply consists of two positive and one negative DC-DC converters, a microprocessor controlled timer, a battery and a transformer to recharge the battery and feed the high voltage relays. This arrangement allows the possibility to control the potentials applied in the 0 to +/-4000 V range to a variety of microchip setups. It can also be easily adapted to perform either gated or pinched injection. The inclusion of a rechargeable battery was adopted to feed the DC-DC converters to reduce noise levels and achieve portability.

Interaction of D-Amino Acid Oxidase with Carbon Nanotubes: Implications in the Design of Biosensors

We have investigated the interaction of d-amino acid oxidase (DAAO) with single-walled carbon nanotubes (CNT) by spectroscopic ellipsometry. Dynamic adsorption experiments were performed at different experimental conditions. In addition, the activity of the enzyme adsorbed at different conditions was studied. Our results indicate that DAAO can be adsorbed to CNT at different pH values and concentrations by a combination of hydrophobic and electrostatic interactions. Considering that the highest enzymatic activity was obtained by adsorbing the protein at pH 5.7 and 0.1 mg x mL(-1), our results indicate that DAAO can adopt multiple orientations on the surface, which are ultimately responsible for significant differences in catalytic activity.

Pulsed amperometric detection with poly(dimethylsiloxane)-fabricated capillary electrophoresis microchips for the determination of EPA priority pollutants

A miniaturized analytical system for separation and detection of three EPA priority phenolic pollutants, based on a poly(dimethylsiloxane)-fabricated capillary electrophoresis microchip and pulsed amperometric detection is described. The approach offers a rapid (less than 2 min), simultaneous measurement of three phenolic pollutants: phenol, 4,6-dinitro-o-cresol and pentachlorophenol. The highly stable response (RSD = 6.1%) observed for repetitive injections (n > 100) reflects the effectiveness of Au working electrode cleaned by pulsed amperometric detection. The effect of solution conditions, separation potential and detection waveform were optimized for both the separation and detection of phenols. Under the optimum conditions (5.0 mM phosphate buffer pH = 12.4, detection potential: 0.7 V, separation potential: 1200 V, injection time: 10 s) the baseline separation of the three selected compounds was achieved. Limits of detection of 2.2 microM (2.8 fmol), 0.9 microM (1.1 fmol), and 1.3 microM (1.6 fmol) were achieved for phenol, 4,6-dinitro-o-cresol and pentachlorophenol, respectively. A local city water sample and two over-the-counter sore-throat medicines were analyzed in order to demonstrate the capabilities of the proposed technique to face real applications.