Food analysis on microchip electrophoresis: an updated review

Recent advances in microchip electrophoresis for analysis of pathogenic bacteria and viruses

Life-threatening diseases, such as hepatitis B, pneumonia, tuberculosis, and COVID-19, are widespread due to pathogenic bacteria and viruses. Therefore, the development of highly sensitive, rapid, portable, cost-effective, and selective methods for the analysis of such microorganisms is a great challenge. Microchip electrophoresis (ME) has been widely used in recent years for the analysis of bacterial and viral pathogens in biological and environmental samples owing to its portability, simplicity, cost-effectiveness, and rapid analysis. However, microbial enrichment and purification are critical steps for accurate and sensitive analysis of pathogenic bacteria and viruses in complex matrices. Therefore, we first discussed the advances in the sample preparation technologies associated with the accurate analysis of such microorganisms, especially the on-chip microfluidic-based sample preparations such as dielectrophoresis and microfluidic membrane filtration. Thereafter, we focused on the recent advances in the lab-on-a-chip electrophoretic analysis of pathogenic bacteria and viruses in different complex matrices. As the microbial analysis is mainly based on the analysis of nucleic acid of the microorganism, the integration of nucleic acid-based amplification techniques such as polymerase chain reaction (PCR), quantitative PCR, and multiplex PCR with ME will result in an accurate and sensitive analysis of microbial pathogens. Such analyses are very important for the point-of-care diagnosis of various infectious diseases.

Chip-based separation of organic and inorganic anions and multivariate analysis of wines according to grape varieties

This report describes the use of electrophoresis microchips integrated with contactless conductivity detection for the determination of organic acids and inorganic anions in wine samples and the subsequent classification based on the grape varieties. The best separation was achieved using a buffer composed of 30 mmol L^-1 2-(N-morpholino)ethanesulfonic acid, 15 mmol L^-1l-histidine and 0.05 mmol L^-1 cetyltrimethylammonium bromide (pH 5.8), allowing the determination of chloride, nitrate, sulfate, oxalate, tartrate, maleate, succinate, citrate, acetate, lactate, pyroglutamate and phosphate within ca. 100 s. The relative standard deviations obtained for the migration times were lower than 2%, while the obtained values for peak areas ranged from 2.5 to 8.4%. The limits of detection achieved for all compounds ranged between 3.0 and 12.6 μmol L^-1. A total of 18 wines from Brazil and Chile were successfully investigated, including red, white and rosé, and the anionic species were quantified with recovery values between 92 and 117%. A statistical difference has not been observed between the data obtained by using electrophoresis microchips integrated with contactless conductivity detection (ME-C⁴D) and capillary electrophoresis with ultra-violet detection (CE-UV) and thus the results from newly developed method is validated. Finally, similarities among the anionic profile of wines were investigated by using a multivariate approach, and it was possible to discriminate samples mainly by grapes varieties. Furthermore, the proposed methodology has provided instrumental simplicity and good analytical performance, demonstrating to be useful for routine quality control of wines.

Reduction in sample injection bias using pressure gradients generated on chip

Sample injection in microchip-based capillary zone electrophoresis (CZE) frequently rely on the use of electric fields which can introduce differences in the injected volume for the various analytes depending on their electrophoretic mobilities and molecular diffusivities. While such injection biases may be minimized by employing hydrodynamic flows during the injection process, this approach typically requires excellent dynamic control over the pressure gradients applied within a microfluidic network. The current article describes a microchip device that offers this needed control by generating pressure gradients on-chip via electrokinetic means to minimize the dead volume in the system. In order to realize the desired pressure-generation capability, an electric field was applied across two channel segments of different depths to produce a mismatch in the electroosmotic flow rate at their junction. The resulting pressure-driven flow was then utilized to introduce sample zones into a CZE channel with minimal injection bias. The reported injection strategy allowed the introduction of narrow sample plugs with spatial standard deviations down to about 45 μm. This injection technique was later integrated to a capillary zone electrophoresis process for analyzing amino acid samples yielding separation resolutions of about 4-6 for the analyte peaks in a 3 cm long analysis channel.

Evaluation of dual electrode configurations for microchip electrophoresis used for voltammetric characterization of electroactive species

Microchip electrophoresis coupled with amperometric detection is more popular than voltammetric detection due to the lower limits of detection that can be achieved. However, voltammetry provides additional information about the redox properties of the analyte that can be used for peak identification. In this paper, two dual electrode configurations for microchip electrophoresis are described and evaluated for obtaining voltammetric information using amperometry. The dual-series electrode configuration was first evaluated to generate current ratios in a single run by applying two different potentials to the working electrodes placed perpendicular to the separation channel. However, it was found that it is difficult to obtain realistic current ratios with this configuration, primarily due to the relative placement of electrodes with respect to the channel end of the simple-t microchip. Correction factors were needed to obtain current ratios similar to those that would be obtained for sequential injections at two different potentials using a single electrode. A second approach using a dual-channel chip with two parallel electrodes was then developed and evaluated for obtaining voltammetric identification. The newly developed microchip permitted the injection of same amount of sample into two unique separation channels, each with an electrode at a different detection potential. Migration times and current ratios for several biologically important molecules and potential interferences including nitrite, tyrosine, hydrogen peroxide, and azide were obtained and compared to the responses obtained for analytes found in macrophage cell lysates.

Pre-Concentration and Analysis of Mycotoxins in Food Samples by Capillary Electrophoresis

Mycotoxins are considered one of the most dangerous agricultural and food contaminants. They are toxic and the development of rapid and sensitive analytical methods to detect and quantify them is a very important issue in the context of food safety and animal/human health. The need to detect mycotoxins at trace levels and to simultaneously analyze many different mycotoxin types became mandatory to protect public health. In fact, European Commission regulations specified both their limits in foodstuffs and official sample preparation protocols in addition to analytical methods to verify their presence. Capillary Electrophoresis (CE) includes different separation modes, allowing many versatile applications in food analysis and safety. In the context of mycotoxins, recent advances to improve CE sensitivity, particularly pre-concentration techniques or miniaturized systems, deserve remarkable attention, as they provide an interesting approach in the analysis of such contaminants in complex food matrices. This review summarizes the applications of CE combined with different pre-concentration approaches, which have been proposed in the literature (mainly) in the last ten years. A section is also dedicated to recent microchip–CE devices since they represent the most promising CE mode for this application.

Microchip electrophoresis based aptasensor for multiplexed detection of antibiotics in foods via a stir-bar assisted multi-arm junctions recycling for signal amplification

Microchip electrophoresis (MCE) was a good available method for high-throughput and rapid detecting chemical pollutants in food samples. However, many of the reported MCE assays involve complex design of microchip, laborious operation and poor universality which limited its promotion in multiple antibiotics' detection. Herein, a multiplexed aptasensor was developed based on a universal double-T type microchip to one-step and simultaneously detect several antibiotics within 3 min using chloramphenicol (CAP) and kanamycin (Kana) as representatives. Besides, a novel stir-bar assisted DNA multi-arm junctions recycling (MAJR) strategy was designed for transducing and amplifying the signal. The brief detection mechanism was as following: the added CAP and Kana can specifically react with their aptamer probes on the stir-bar and produce different single-stranded DNA primer, respectively. Afterwards, the primers can trigger MAJR to form a lot of three- and four-arm DNA junctions corresponding to different targets. The DNA multi-arm junctions can be easily separated and detected by MCE for quantification. Moreover, the stir-bar can facilitate phase separation and obviously eliminate matrix interference in food. The assay was successfully applied in milk and fish samples, showing excellent selectivity and sensitivity with a detection limits of 0.52 pg mL^-1 CAP and 0.41 pg mL^-1 Kana (S/N = 3). Thus, the assay holds a great potential application for screening of antibiotics in food.

Sheathless coupling of microchip electrophoresis to ESI-MS utilising an integrated photo polymerised membrane for electric contacting

In this article, we present a novel approach for the sheathless coupling of microchip electrophoresis (MCE) with electrospray mass spectrometry (ESI-MS). The key element is an ion-conductive hydrogel membrane, placed between the separation channel and an adjacent microfluidic supporting channel, contacted via platinum electrodes. This solves the persistent challenge in hyphenation of mass spectrometry to chip electrophoresis, to ensure a reliable electrical connection at the end of the electrophoresis channel without sacrificing separation performance and sensitivity. Stable electric contacting is achieved via a Y-shaped supporting channel structure, separated from the main channel by a photo polymerised, ion permeable hydrogel membrane. Thus, the potential gradient required for performing electrophoretic separations can be generated while simultaneously preventing gas formation due to electrolysis. In contrast to conventional make-up or sheathflow approaches, sample dilution is also avoided. Rapid prototyping allowed the study of different chip-based approaches, i.e. sheathless, open sheathflow and electrode support channel designs, for coupling MCE to ESI-MS. The performance was evaluated with fluorescence microscopy and mass spectrometric detection. The obtained results revealed that the detection sensitivity obtained in such Y-channel chips with integrated hydrogel membranes was superior because sample dilution or loss was prevented. Furthermore, band broadening is reduced compared to similar open structures without a membrane.

Total polyphenols content in white wines on a microfluidic flow injection analyzer with embedded optical fibers

Absorbance detection in food microdevices has not been thoroughly used due to low levels of sensitivity in measurements. Thus, it is necessary to develop microfluidic methods for improving photometric detection. For this purpose, a simple coupled-optical-fiber-polydimethylsiloxane (PDMS) microdevice was developed, to quantify polyphenols content in white wine employing the Folin-Ciocalteu reaction method. A 6V and 10W halogen lamp with an optical path length of 7mm between optical fibers, which were placed into the microchip, using guides at the outlet of the flow, increased the level of sensitivity during detection. The linear range was from 0.03mmol/L to 0.18mmol/L. Thus, the corresponding equation was: Abs=4.00(±0.16) [tannic acid]+0.17(±0.017). Intra-laboratory repeatability and reproducibility percentages were 2.95% and 6.84%, respectively. Such results were compared to those obtained from applying the conventional flow-injection analysis method, based on the same type of reaction. The relative error between methods was less than 13%.

Fast and versatile fabrication of PMMA microchip electrophoretic devices by laser engraving

This paper describes the effects of different modes and engraving parameters on the dimensions of microfluidic structures produced in PMMA using laser engraving. The engraving modes included raster and vector, while the explored engraving parameters included power, speed, frequency, resolution, line-width, and number of passes. Under the optimum conditions, the technique was applied to produce channels suitable for CE separations. Taking advantage of the possibility to cut-through the substrates, the laser was also used to define solution reservoirs (buffer, sample, and waste) and a PDMS-based decoupler. The final device was used to perform the analysis of a model mixture of phenolic compounds within 200 s with baseline resolution.

Capillary and microchip electrophoretic analysis of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants which can reach the environment and food in different ways. Because of their high toxicity, two international regulatory institutions, the US Environmental Protection Agency and the European Food Safety Authority, have classified PAHs as priority pollutants, generating an important demand for the detection and identification of PAHs. Thus, sensitive, fast, and cheap methods for the analysis of PAHs in environmental and food samples are urgently needed. Within this context, electrophoresis, in capillary or microchip format, displays attractive features. This review presents and critically discusses the published literature on the different approaches to capillary and microchip electrophoresis analysis of PAHs.

Evaluation of in-channel amperometric detection using a dual-channel microchip electrophoresis device and a two-electrode potentiostat for reverse polarity separations

In-channel amperometric detection combined with dual-channel microchip electrophoresis is evaluated using a two-electrode isolated potentiostat for reverse polarity separations. The device consists of two separate channels with the working and reference electrodes placed at identical positions relative to the end of the channel, enabling noise subtraction. In previous reports of this configuration, normal polarity and a three-electrode detection system were used. In the two-electrode detection system described here, the electrode in the reference channel acts as both the counter and reference. The effect of electrode placement in the channels on noise and detector response was investigated using nitrite, tyrosine, and hydrogen peroxide as model compounds. The effects of electrode material and size and type of reference electrode on noise and the potential shift of hydrodynamic voltammograms for the model compounds were determined. In addition, the performance of two- and three-electrode configurations using Pt and Ag/AgCl reference electrodes was compared. Although the signal was attenuated with the Pt reference, the noise was also significantly reduced. It was found that lower LOD were obtained for all three compounds with the dual-channel configuration compared to single-channel, in-channel detection. The dual-channel method was then used for the detection of nitrite in a dermal microdialysis sample obtained from a sheep following nitroglycerin administration.

Lights and shadows on food microfluidics

These insights attempt to share with the community the lights and shadows of one emerging and exciting topic, Food Microfluidics, defined as microfluidic technology for food analysis and diagnosis in important areas such as food safety and quality. The reader is invited to question non-easy interrogations such as why Food Microfluidics, what is the next step and what could we do with the available technology. This article invites food analysts to be seduced by this technology and then to take an interesting trip departing from the main gained achievements, having a look at the crossing bridges over Food Microfluidic challenges or having a look at available technology to start. Finally, this trip arrives at a privileged place to gaze the horizons. A wonderful landscape--full of inspiration--for Food Microfluidics is anticipated. These insights have also been written wishing to give improved conceptual and realistic solutions for food analysis, with the additional hope to attract the community with exciting technology, in order to get novel and unexpected achievements in this field.

Getting started with open-hardware: development and control of microfluidic devices

Understanding basic concepts of electronics and computer programming allows researchers to get the most out of the equipment found in their laboratories. Although a number of platforms have been specifically designed for the general public and are supported by a vast array of on-line tutorials, this subject is not normally included in university chemistry curricula. Aiming to provide the basic concepts of hardware and software, this article is focused on the design and use of a simple module to control a series of PDMS-based valves. The module is based on a low-cost microprocessor (Teensy) and open-source software (Arduino). The microvalves were fabricated using thin sheets of PDMS and patterned using CO2 laser engraving, providing a simple and efficient way to fabricate devices without the traditional photolithographic process or facilities. Synchronization of valve control enabled the development of two simple devices to perform injection (1.6 ± 0.4 μL/stroke) and mixing of different solutions. Furthermore, a practical demonstration of the utility of this system for microscale chemical sample handling and analysis was achieved performing an on-chip acid-base titration, followed by conductivity detection with an open-source low-cost detection system. Overall, the system provided a very reproducible (98%) platform to perform fluid delivery at the microfluidic scale.

Microchip electrophoresis-copper nanowires for fast and reliable determination of monossacharides in honey samples

Microchip electrophoresis (ME) with electrochemical detection has been demonstrated to be a powerful tool in food analysis. However, the coupling of ME with electrochemical detection and nanotechnologies is still in its infancy, knowing that nanomaterials can significantly improve the ME analytical performance. This work reports the coupling between ME and copper nanowires (CuNWs) for the selective analysis of monosaccharides in honey samples. Also, in terms of real applicability, the study of analytical reliability of ME is an issue of paramount importance. To this end, a representative group of nine honey samples were analyzed and the results were compared with those previously obtained by HPLC-refractive index. ME-CuNWs approach allowed the separation of glucose and fructose in <250 s under optimized separation (20 mM NaOH + 10 mM H3 BO3 , pH 12; separation voltage + 1000 V) and detection (E = +0.70 V in 20 mM NaOH + 10 mM H3 BO3 , pH 12) conditions. An excellent stability of EOF during sample analysis was achieved with RSDs for migration times <2% and for amperometric currents <9%. The quantitative contents for individual glucose and fructose obtained using ME-CuNWs in comparison with those obtained by HPLC-refractive index were highly in agreement with errors <10% indicating the reliability of the approach. The excellent analytical performance obtained confirms the analytical potency of ME-CuNWs approach, enhancing the maturity of the microchip technology and opening new avenues for future implementation of applications in the field of food analysis.

Development of an SDS-gel electrophoresis method on SU-8 microchips for protein separation with LIF detection: Application to the analysis of whey proteins

This work describes the development of an SDS-gel electrophoresis method for the analysis of major whey proteins (α-lactalbumin, β-lactoglobulin, and BSA) carried out in SU-8 microchips. The method uses a low-viscosity solution of dextran as a sieving polymer. A commercial coating agent (EOTrol LN) was added to the separation buffer to control the EOF of the chips. The potential of this coating agent to prevent protein adsorption on the walls of the SU-8 channels was also evaluated. Additionally, the fluorescence background of the SU-8 material was studied to improve the sensitivity of the method. By selecting an excitation wavelength of 532 nm at which the background fluorescence remains low and by replacing the mercury arc lamp by a laser in the detection system, an LOD in the nanomolar range was achieved for proteins derivatized with the fluorogenic reagent Chromeo P540. Finally, the method was applied to the analysis of milk samples, demonstrating the potential of SU-8 microchips for the analysis of proteins in complex food samples.