Review of Microfluidic Liquid–Liquid Extractors

Reshaping Capillary Electrophoresis With State-of-the-Art Sample Preparation Materials: Exploring New Horizons

Capillary electrophoresis (CE) is a powerful analysis technique with advantages such as high separation efficiency with resolution factors above 1.5, low sample consumption of less than 10 µL, cost-effectiveness, and eco-friendliness such as reduced solvent use and lower operational costs. However, CE also faces limitations, including limited detection sensitivity for low-concentration samples and interference from complex biological matrices. Prior to performing CE, it is common to utilize sample preparation procedures such as solid-phase microextraction (SPME) and liquid-phase microextraction (LPME) in order to improve the sensitivity and selectivity of the analysis. Recently, there have been advancements in the development of novel materials that have the potential to greatly enhance the performance of SPME and LPME. This review examines various materials and their uses in microextraction when combined with CE. These materials include carbon nanotubes, covalent organic frameworks, metal-organic frameworks, graphene and its derivatives, molecularly imprinted polymers, layered double hydroxides, ionic liquids, and deep eutectic solvents. The utilization of these innovative materials in extraction methods is being examined. Analyte recoveries and detection limits attained for a range of sample matrices are used to assess their effects on extraction selectivity, sensitivity, and efficiency. Exploring new materials for use in sample preparation techniques is important as it enables researchers to address current limitations of CE. The development of novel materials has the potential to greatly enhance extraction selectivity, sensitivity, and efficiency, thereby improving CE performance for complex biological analysis.

A Coalescing Filter for Liquid-Liquid Separation and Multistage Extraction in Continuous-Flow Chemistry

Presented here is the design and performance of a coalescing liquid-liquid filter, based on low-cost and readily available meltblown nonwoven substrates for separation of immiscible phases. The performance of the coalescer was determined across three broad classes of fluid mixtures: (i) immiscible organic/aqueous systems, (ii) a surfactant laden organic/aqueous system with modification of the type of emulsion and interfacial surface tension through the addition of sodium chloride, and (iii) a water-acetone/toluene system. The first two classes demonstrated good performance of the equipment in effecting separation, including the separation of a complex emulsion system for which a membrane separator, operating through transport of a preferentially wetting fluid through the membrane, failed entirely. The third system was used to demonstrate the performance of the separator within a multistage liquid-liquid counterflow extraction system. The performance, robust nature, and scalability of coalescing filters should mean that this approach is routinely considered for liquid-liquid separations and extractions within the fine chemical and pharmaceutical industry.

Microfluidic based continuous enzyme immobilization: A comprehensive review

Conventional techniques for enzyme immobilization suffer from suboptimal activity recovery due to insufficient enzyme loading and inadequate stability. Furthermore, these techniques are time-consuming and involve multiple steps which limit the applicability of immobilized enzymes. In contrast, the use of microfluidic devices for enzyme immobilization has garnered significant attention due to its ability to precisely control immobilization parameters, resulting in highly active immobilized enzymes. This approach offers several advantages, including reduced time and energy consumption, enhanced mass-heat transfer, and improved control over the mixing process. It maintains the superior structural configuration in immobilized form which ultimately affects the overall efficiency. The present review article comprehensively explains the design, construction, and various methods employed for enzyme immobilization using microfluidic devices. The immobilized enzymes prepared using these techniques demonstrated excellent catalytic activity, remarkable stability, and outstanding recyclability. Moreover, they have found applications in diverse areas such as biosensors, biotransformation, and bioremediation. The review article also discusses potential future developments and foresees significant challenges associated with enzyme immobilization using microfluidics, along with potential remedies. The development of this advanced technology not only paves the way for novel and innovative approaches to enzyme immobilization but also allows for the straightforward scalability of microfluidic-based techniques from an industrial standpoint.

Combining a hybrid chip and tube microfluidic system with fluorescent molecularly imprinted polymer (MIP) core-shell particles for the derivatisation, extraction, and detection of peptides with N-terminating phosphorylated tyrosine

The reliable identification and quantitation of phosphorylated amino acids, peptides and proteins is one of the key challenges in contemporary bioanalytical research, an area of particular interest when attempting to diagnose and treat diseases at an early stage. We have developed a synthetic probe for targeting phosphorylated amino acids, based on core-shell submicron-sized particles consisting of a silica core, coated with a molecularly imprinted polymer (MIP) shell. The MIP layer contains a fluorescent probe crosslinker which binds selectively to phosphorylated tyrosine (pY) moieties with a significant imprinting factor (IF) and responds with a "light-up" fluorescence signal. The bead-based ratiometric detection scheme has been successfully transferred to a microfluidic chip format and its applicability to rapid assays has been exemplarily shown by discriminating a pY-terminating oligopeptide against its non-phosphorylated counterpart. Such miniaturised devices could lead to an automated pY or pY N-terminated peptide measurement system in the future. The setup combines a modular microfluidic system for amino acid derivatisation, extraction (by micropillar co-flow) and selective adsorption and detection with the fluorescent MIP core-shell particle probes. A miniaturised optical assembly for low-light fluorescence measurements was also developed, based on miniaturised opto-electronic parts and optical fibres. The emission from the MIP particles upon binding of pY or pY N-terminated peptides could be monitored in real-time.

Polyaniline-polycaprolactone electrospun nanofibrous mat: new polymeric support with anion exchange characteristic for immobilizing liquid membrane in efficient on-chip electromembrane extraction of polar acidic drugs

The potential of application of an electrospun nanofiber sheet as new polymeric support for immobilizing the liquid membrane, instead of a common commercial polypropylene sheet, in on-chip electromembrane extraction (EME) of some acidic polar drugs followed by HPLC with ultraviolet detection is presented. The nanofiber sheet was prepared by electrospinning a mixture of polycaprolactone and polyaniline. The successful synthesis of the electrospun nanofiber sheet was confirmed by field emission-scanning electron microscopy, energy-dispersive X-ray spectroscopy, elemental mapping, and atomic force microscopy. Several parameters affecting the efficiency of the microextraction method, including pHs of the donor and acceptor phases, applied voltage, sample flow rate, phosphate content of the acceptor phase, and sample volume, were investigated and optimized. After optimization, the linearity range of 0.5-250.0 µg L^-1 and detection limits of 0.2-1.0 µg L^-1 were obtained for the analytes. The extraction recovery values and preconcentration factors were 10.7-55.3% and 16-83, respectively. The presence of polyaniline in the composition of the nanofibers significantly improved the extraction efficiency of the polar acidic drugs due to providing the possibility of various interactions with the target analytes such as hydrogen bonding, π-stacking, and anion exchange. The obtained results demonstrate the excellent efficiency of the synthesized electrospun nanofibrous mat as a novel support membrane for immobilizing 1-octanol and as an interactive substrate for electromembrane extraction of acidic polar drugs. Eventually, the proposed on-chip EME method exhibits acceptable precision (relative standard deviations less than 9.7% (n = 3)) and good accuracy (86-112%) for determining the target analytes in the plasma samples.

Modular Flow Reactors for Valorization of Kraft Lignin and Low-Voltage Hydrogen Production

Recent studies have found that green hydrogen production and biomass utilization technologies can be combined to efficiently produce both hydrogen and value-added chemicals using biomass as an electron and proton source. However, the majority of them have been limited to proof-of-concept demonstrations based on batch systems. Here the authors report the design of modular flow systems for the continuous depolymerization and valorization of lignin and low-voltage hydrogen production. A redox-active phosphomolybdic acid is used as a catalyst to depolymerize lignin with the production of aromatic compounds and extraction of electrons for hydrogen production. Individual processes for lignin depolymerization, byproduct separation, and hydrogen production with catalyst reactivation are modularized and integrated to perform the entire process in the serial flow. Consequently, this work enabled a one-flow process from biomass conversion to hydrogen gas generation under a cyclic loop. In addition, the unique advantages of the fluidic system (i.e., effective mass and heat transfer) substantially improved the yield and efficiency, leading to hydrogen production at a higher current density (20.5 mA cm-2 ) at a lower voltage (1.5 V) without oxygen evolution. This sustainable eco-chemical platform envisages scalable co-production of valuable chemicals and green hydrogen for industrial purposes in an energy-saving and safe manner.

Flow Physics of Planar Bistable Fluidic Oscillator with Backflow Limbs

Fluidic oscillators (FOs) are used in a variety of applications, including process control and process intensification. Despite the simple design and operation of FOs, the fluid dynamics of FOs exhibit rich complexities. The inherently unstable flow, jet oscillations, and resulting vortices influence mixing and other transport processes. In this work, we computationally investigated the fluid dynamics of a new design of a planar FO with backflow limbs. The design comprised of two symmetric backflow limbs leading to bistable flow. The unsteady flow dynamics, internal recirculation, jet oscillations, secondary flow vortices were computationally studied over a range of inlet Reynolds numbers (2400-12,000). The nature and frequency of the jet oscillations were quantified. The computed jet oscillation frequency was compared with the experimentally measured (using imaging techniques) jet oscillation frequency. The flow model was then used to quantitatively understand mixing, heat transfer, and residence time distribution. The approach and the results presented in this work will provide a basis for designing FO's with desired flow and transport characteristics for various engineering applications.

A microchip device based liquid-liquid-solid microextraction for the determination of permethrin and cypermethrin in water samples

In this work, for the first time, a microchip device integrating liquid-liquid-solid phase microextraction is presented. As a novel approach to microchip systems, liquid-liquid-solid microextraction was performed in a sandwiched microchip device. The microchip device consisted of three poly(methyl methacrylate) layers along with a double "Y"-shaped microchannel. As the stationary phase, polyacrylonitrile-C18 was synthesized and immobilized in the upper channel, while the beneath channel was used as a reservoir for the stagnant volume ratio of sample-to-extraction solvent phase. In this way, analytes were extracted from an aqueous sample through an organic phase into the stationary phase. The analytes were finally desorbed with a minimum amount of acetonitrile as the desorption solvent. Permethrin and cypermethrin were selected as the model analytes for extraction and subsequent analysis by gas chromatography-flame ionization detection. Under optimum conditions (extraction solvent; n-hexane, sample -to-extraction solvent volume ratio; 2:1, extraction time; 20 min, desorption solvent; acetonitrile, desorption volume; 200 μL, and desorption time; 15 min) detection limits were 3.5 and 6.0 ng mL^-1 for permethrin and cypermethrin, respectively. Relative standard deviations for intra- and inter-day reproducibility were below 8.3%. Device-to-device precision was in the range of 8.1-9.6%. The proposed microchip device was successfully applied to determine permethrin and cypermethrin in water samples with recoveries in the range of 73-96%.

Microextraction of lanthanum using a rotating microchannel extractor

This work introduced a novel microchannel extractor. The extraction system was to extract lanthanum nitrate aqueous solution with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA). Different feeding methods and inner rotors were explored first. The results showed that parallel feeding and inner rotors engraved with spiral stripes were more favorable for extraction. Next, the effect of various factors on the extraction was explored, including the pH of the aqueous phase, rotational inner rotor speed (R) and the fluid volumetric flow rate (Q). The results showed that these factors are closely related to the extraction. Finally, the experiment was verified by CFD numerical simulation, the simulation result was consistent with the experiment. In this device, active mixing was introduced into the microchannel extraction, which significantly improved the extraction efficiency. Under certain conditions, the extraction efficiency of this device exceeded stirring extraction equilibrium. Moreover, the extraction in this device is faster than conventional stirring extraction. These advantages provide a possibility for highly efficient extraction and use of rare earth elements.

Multiphase flow in microfluidics: From droplets and bubbles to the encapsulated structures

Microfluidic technologies have a unique ability to control more precisely and effectively on two-phase flow systems in comparison with macro systems. Controlling the size of the droplets and bubbles has led to an ever-increasing expansion of this technology in two-phase systems. Liquid-liquid and gas-liquid two-phase flows because of their numerous applications in different branches such as reactions, synthesis, emulsions, cosmetic, food, drug delivery, etc. have been the most critical two-phase flows in microfluidic systems. This review highlights recent progress in two-phase flows in microfluidic devices. The fundamentals of two-phase flows, including some essential dimensionless numbers, governing equations, and some most well-known numerical methods are firstly introduced, followed by a review of standard methods for producing segmented flows such as emulsions in microfluidic systems. Then various encapsulated structures, a common two-phase flow structure in microfluidic devices, and different methods of their production are reviewed. Finally, applications of two-phase microfluidic flows in drug-delivery, biotechnology, mixing, and microreactors are briefly discussed.

High-throughput screening by droplet microfluidics: perspective into key challenges and future prospects

In two decades of development, impressive strides have been made for automating basic laboratory operations in droplet-based microfluidics, allowing the emergence of a new form of high-throughput screening and experimentation in nanoliter to femtoliter volumes. Despite advancements in droplet storage, manipulation, and analysis, the field has not yet been widely adapted for many high-throughput screening (HTS) applications. Broad adoption and commercial development of these techniques require robust implementation of strategies for the stable storage, chemical containment, generation of libraries, sample tracking, and chemical analysis of these small samples. We discuss these challenges for implementing droplet HTS and highlight key strategies that have begun to address these concerns. Recent advances in the field leave us optimistic about the future prospects of this rapidly developing technology.

Liquid-liquid-liquid three-phase microsystem: hybrid slug flow-laminar flow

Liquid-liquid-liquid three-phase microfluidics provides abundant flow patterns and attractive characteristics that substantially extend the applications of liquid-liquid two-phase microfluidics. Although the manipulation of stable interfaces between adjacent liquid phases is a prerequisite for the successful utilization of three-phase flows, it is challenging. In this study, we develop a novel liquid-liquid-liquid three-phase microsystem that is a hybrid slug flow-laminar flow microchip, in which one aqueous phase makes contact with one organic phase containing slugs of another aqueous phase. The organic phase separates the two aqueous phases, and the three phases co-currently flow. The microchip can simultaneously provide a stable continuous water-oil interface and multiple segregated oil-water interfaces. The design guideline, flow pattern map, and influence rules for the flow rates and the interfacial tension are discussed in detail. Furthermore, a hybrid slug flow-laminar flow microchip is demonstrated for simultaneous extraction/stripping. This three-phase microsystem presents the advantages of slug flow and laminar flow and has potential in various applications such as sample purification, analysis, synthesis, and micro-reactions.

A microfluidic study of synergic liquid–liquid extraction of rare earth elements

A membrane based liquid–liquid extraction microfluidic device coupled with X-ray fluorescence enables the first quantification of free energies of transfer dependence for a complex mixture of rare earth elements and iron using synergic extractants.

On-chip ion pair-based dispersive liquid-liquid extraction for quantitative determination of histamine H2 receptor antagonist drugs in human urine

In the present work, an ion-pair based dispersive liquid-liquid microextraction was performed on a centrifugal chip for the first time. The entire DLLME procedure, including flow direction, desperation, and sedimentation of the extracting phase, can be fulfilled automatically on a solitary chip. The chip was made of Poly(methyl methacrylate) (PMMA) and was of two units for two parallel extractions, each consisting of three chambers (for the sample solution, extracting solvents, and sedimentation). As the chip rotated, fluids flowed within the chip, and the dispersion, mixing, extraction, and sedimentation of the final phase were performed on the chip by simply adjusting the spin speed. Determination of two histamine H₂ receptor antagonist drugs, cimetidine and ranitidine, as the model analytes from the urine samples was done using the developed on-chip ion-pair based DLLME method followed by an HPLC-UV. The effective parameters on the extraction efficiency of the model analytes were investigated and optimized using the one variable at a time method. Under optimized conditions, the calibration curve was linear in the range of 15-2000 μg L^-1 with a coefficient of determination (R²) more than 0.9987. The relative standard deviations (RSD %) for extraction and determination of the analytes were less than 3.7% based on five replicated measurements. LODs less than 10.0 μg L^-1 and preconcentration factors higher than 39-fold were obtained for both of the model analytes. The proposed chip enjoys the advantages of both the DLLME method and miniaturization on a centrifugal chip.

Perspectives on the Use of Liquid Extraction for Radioisotope Purification

The reliable and efficient production of radioisotopes for diagnosis and therapy is becoming an increasingly important capability, due to their demonstrated utility in Nuclear Medicine applications. Starting from the first processes involving the separation of ^99mTc from irradiated materials, several methods and concepts have been developed to selectively extract the radioisotopes of interest. Even though the initial methods were based on liquid-liquid extraction (LLE) approaches, the perceived difficulty in automating such processes has slowly moved the focus towards resin separation methods, whose basic chemical principles are often similar to the LLE ones in terms of chelators and phases. However, the emerging field of flow chemistry allows LLE to be easily automated and operated in a continuous manner, resulting in an even improved efficiency and reliability. In this contribution, we will outline the fundamentals of LLE processes and their translation into flow-based apparatuses; in addition, we will provide examples of radioisotope separations that have been achieved using LLE methods. This article is intended to offer insights about the future potential of LLE to purify medically relevant radioisotopes.

An improved liquid–liquid separator based on an optically monitored porous capillary

We report an automated device for the efficient microscale separation of immiscible liquids.

Microfluidic approaches for accessing thermophysical properties of fluid systems

Thermophysical properties of fluid systems under high pressure and high temperature conditions are highly desirable as they are used in many industrial processes both from a chemical engineering point of view and to push forward the development of modeling approaches.