A simple method for the assessment of electrophoretic mobility in porous media.

Developing paper-based electrophoretic methods involve dealing with significant uncertainty levels when compared to their capillary counterparts. Critical information for developing these kinds of methods are the electrophoretic mobility of background electrolytes and samples. This work presents the design and characterization of a device for measuring the electrophoretic mobilities of dyes in porous media. The device was developed with the aim of validating a previously presented model and also proposing a protocol for the straightforward determination of electrophoretic mobilities in porous media when open-channel values are already known. Whatman #1 paper was used as a model substrate as far as it is the most common porous medium substrate for paper-based electrophoresis. The device was designed using a numerical simulation-assisted approach, utilizing OpenFOAM® and specific solvers for capillary transport and electromigration, namely porousMicroTransport and electroMicroTransport, respectively. The electrophoretic mobilities of five dyes were analyzed experimentally with the proposed device. To establish appropriate comparative values at different pHs, experiments in fused silica capillaries were also performed. An effective parameter model for describing the electrophoretic behavior of dyes in porous media, that is, the constriction factor, was found consistent with previous reports for the Whatman #1 paper. This consistency was found after considering (via direct measurements) the chromatographic effect of the medium over each dye. Consequently, the recorded values hold significant worth due to their potential for direct application in designing new experiments or devices in Whatman #1 paper. With the validation of the model through the experiments with the proposed device, those researchers interested on developing electrophoretic methods in porous substrates can make use of the open-channel electrophoretic mobilities reported in the literature, or in the well-known software databases, and correct them for the media of interest just by performing two simple characterization steps.

Field-deployable aptasensor with automated analysis of stain patterns for the detection of chlorpyrifos in water.

Organophosphorus compounds such as chlorpyrifos (CPS) are causing serious environmental problems worldwide. Efficient monitoring of the CPS levels in water resources demands portable devices for on-field testing. Here we report the development of a CPS sensor coupled with smartphones for automated reading and data analysis. The sensing mechanism makes use of gold nanoparticles stabilized by a CPS-specific aptamer, where colloidal destabilization occurs in presence of competing CPS molecules. In particular, an innovative readout is proposed: quantitative analysis of the stain patterns left by evaporating drops of the test solutions. We have found that the CPS-induced destabilization suppresses the typical coffee-ring stain of the colloidal gold, and then exploited the phenomenon to quantitatively determine the CPS concentration in water samples. A strong correlation between CPS level in samples and the alteration of the stain patterns was established for a wide range of CPS concentrations (0.048 µM-482 µM). The limit of detection of the sensor was 0.2 µM. The assay was implemented on Whatman filter paper cards that were specially patterned by wax-printing. A smartphone-based Python program was written for fully automated image capture and processing. Notably, as we analyze the spatial configuration of the stains, the reading system is independent of external illumination. The system also enables data management and traceability, which are highly desirable attributes for environmental monitoring.

A free customizable tool for easy integration of microfluidics and smartphones.

The integration of smartphones and microfluidics is nowadays the best possible route to achieve effective point-of-need testing (PONT), a concept increasingly demanded in the fields of human health, agriculture, food safety, and environmental monitoring. Nevertheless, efforts are still required to integrally seize all the advantages of smartphones, as well as to share the developments in easily adoptable formats. For this purpose, here we present the free platform appuente that was designed for the easy integration of microfluidic chips, smartphones, and the cloud. It includes a mobile app for end users, which provides chip identification and tracking, guidance and control, processing, smart-imaging, result reporting and cloud and Internet of Things (IoT) integration. The platform also includes a web app for PONT developers, to easily customize their mobile apps and manage the data of administered tests. Three application examples were used to validate appuente: a dummy grayscale detector that mimics quantitative colorimetric tests, a root elongation assay for pesticide toxicity assessment, and a lateral flow immunoassay for leptospirosis detection. The platform openly offers fast prototyping of smartphone apps to the wide community of lab-on-a-chip developers, and also serves as a friendly framework for new techniques, IoT integration and further capabilities. Exploiting these advantages will certainly help to enlarge the use of PONT with real-time connectivity in the near future.

Numerical simulations of paper-based electrophoretic separations with open-source tools.

A new tool for the solution of electromigrative separations in paper-based microfluidics devices is presented. The implementation is based on a recently published complete mathematical model for describing these types of separations, and was developed on top of the open-source toolbox electroMicroTransport, based on OpenFOAM® , inheriting all its features as native 3D problem handling, support for parallel computation, and a GNU GPL license. The presented tool includes full support for paper-based electromigrative separations (including EOF and the novel mechanical and electrical dispersion effects), compatibility with a well-recognized electrolyte database, and a novel algorithm for computing and controlling the electric current in arbitrary geometries. Additionally, the installation on any operating system is available due to its novel installation option in the form of a Docker image. A validation example with data from literature is included, and two extra application examples are provided, including a 2D free-flow IEF problem, which demonstrates the capabilities of the toolbox for dealing with computational and physicochemical modeling challenges simultaneously. This tool will enable efficient and reliable numerical prototypes of paper-based electrophoretic devices to accompany the contemporary fast growth in paper-based microfluidics.

Generalized model of the linear theory of electromigration and its application to electrokinetic chromatography: Capillary zone electrophoretic systems with complex-forming equilibria.

We discuss several possible phenomena in electrophoretic systems with complexing agents present in the background electrolyte. In our previous work, we extended the linear theory of electromigration with the first-order nonlinear term, which originally applied to acid-base equilibria only, by generalizing it to any fast chemical equilibria. This extension provides us with a fresh insight into the well-established technique of elecktrokinetic chromatography (EKC). We combine mathematical analysis of the generalized model with its solution by means of the new version of our software PeakMaster 6, and experimental data. We re-examine the fundamental equations by Wren and Rowe and Tiselius in the frame of the generalized linear theory of electromigration. Besides, we show that selector concentration can increase inside the interacting-analyte zone due to its complexation with the analyte, which contradicts the generally accepted idea of a consumption of a portion of the selector inside the zone. Next, we focus our discussion on interacting buffers (i.e., buffer constituents that form a complex with the selector). We demonstrate how such side-interaction of the selector with another buffer constituent can influence measuring analyte-selector interactions. Finally, we describe occurrence and mobilities of system peaks in these EKC systems. We investigate systems with fully charged analytes and neutral cyclodextrins as selectors. Although the theory is not limited in terms of the charge and/or the degree of (de)protonation of any constituent, this setup allows us to find analytical solutions to generalized model under approximate, yet realistic, conditions and to demonstrate all important phenomena that may occur in EKC systems. An occurrence of system peaks in a system with fully charged selector is also investigated.

Generalized model of the linear theory of electromigration and its application to electrokinetic chromatography: Theory and software PeakMaster 6-Next Generation.

The linear theory of electromigration, including the first-order nonlinear approximation, is generalized to systems with any equilibria fast enough to be considered instantaneous in comparison with the timescale of peak movement. For example, this theory is practically applied in the electrokinetic chromatography (EKC) mode of the CZE. The model enables the calculation of positions and shapes of analyte and system peaks without restricting the number of selectors, the complexation stoichiometry, or simultaneous acid-base equilibria. The latest version of our PeakMaster software, PeakMaster 6-Next Generation, implements the theory in a user-friendly way. It is a free and open-source software that performs all calculations and shows the properties of the background electrolyte and the expected electropherogram within a few seconds. In this paper, we mathematically derive the model, discuss its applicability to EKC systems, and introduce the PeakMaster 6 software.