On-chip integration of sequential ion-sensing system based on intermittent reagent pumping and formation of two-layer flow

Ultrasensitive Ionophore-Based Liquid Sensors for Colorimetric Ion Measurements in Blood

The self-monitoring of electrolytes using a small volume of capillary blood is needed for the management of many chronic diseases. Herein, we report an ionophore-based colorimetric sensor for electrolyte measurements in a few microliters of blood. The sensor is a pipet microtip preloaded with a segment of oil (plasticizer) containing a pH-sensitive chromoionophore, a cation exchanger, and an ionophore. The analyte is extracted from the sample into the oil via a mixing protocol controlled by a stepper motor. The oil with an optimized ratio of sensing chemicals shows an unprecedentedly large color response for electrolytes in a very narrow concentration range that is clinically relevant. This ultrahigh sensitivity is based on an exhaustive response mode with a novel mechanism for defining the lower and higher limits of detection. Compared to previous optodes and molecular probes for ions, the proposed platform is especially suitable for at-home blood electrolyte measurements because (1) the oil sensor is interrogated independent of the sample and therefore works for whole blood without requiring plasma separation; (2) the sensor does not need individual calibration as the consistency between liquid sensors is high compared to solid sensors, such as ion-selective electrodes and optodes; and (3) the sensing system consisting of a disposable oil sensor, a programmed stepper motor, and a smartphone is portable, cost-effective, and user-friendly. The accuracy and precision of Ca2+ sensors are validated in 51 blood samples with varying concentrations of total plasma Ca2+. Oil sensors with an ultrasensitive response can also be obtained for other ions, such as K+.

Nanoliter-Scale Droplet-Droplet Microfluidic Microextraction Coupled with MALDI-TOF Mass Spectrometry for Metabolite Analysis of Cell Droplets

The further miniaturization of liquid-phase microextraction (LPME) systems has important significance and major challenges for microscale sample analysis. Herein, we developed a rapid and flexible droplet-droplet microfluidic microextraction approach to perform nanoliter-scale miniaturized sample pretreatment, by combining droplet-based microfluidics, robotic liquid handling, and LPME techniques. Differing from the previous microextraction methods, both the extractant and sample volumes were decreased from the microliter scale or even milliliter scale to the nanoliter scale. We utilized the ability of a liquid-handling robot to manipulate nanoliter-scale droplets and micrometer-scale positioning to overcome the scaling effect difficulties in performing liquid-liquid extraction of nanoliter-volume samples in microsystems. Two microextraction modes, droplet-in-droplet microfluidic microextraction and droplet-on-droplet microfluidic microextraction, were developed according to the different solubility properties of the extractants. Various factors affecting the microextraction process were investigated, including the extraction time, recovery method of the extractant droplet, static and dynamic extraction mode, and cross-contamination. To demonstrate the validity and adaptability of the pretreatment and analysis of droplet samples with complex matrices, the present microextraction system coupled with MALDI-TOF mass spectrometry (MS) detection was applied to the quantitative determination of 7-ethyl-10-hydroxylcamptothecin (SN-38), an active metabolite of the anticancer drug irinotecan, in 800-nL droplets containing HepG2 cells. A linear relationship (y = 0.0305x + 0.376, R² = 0.984) was obtained in the range of 4-100 ng/mL, with the limits of detection and quantitation being 2.2 and 4.5 ng/mL for SN-38, respectively.

Ionophore-based pH independent detection of ions utilizing aggregation-induced effects

Ionophores have been integrated into various electrochemical and optical sensing platforms for the selective detection of ions. Previous ionophore-based optical sensors rely on a H⁺ chromoionophore as the signal transducer and consequently, suffered from a pH cross-response. pH independent methods were proposed very recently by utilizing the solvatochromic dyes or the exhaustive mode. Here, we report a pH independent sensing principle based on nanospheres containing ionophores. As the ion-exchange occurs, the signal transducer undergoes aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ), leading to a dramatic change in fluorescence intensity. The principle was evaluated on different ionophores including those selective for K⁺, Na⁺, Ca²⁺, and Pb²⁺. The nanospheres were also introduced into microfluidic chips and successfully applied for the determination of sodium and potassium ion concentrations in diluted blood serum and urine samples.

Stable biphasic interfaces for open microfluidic platforms

We present an open microfluidic platform that enables stable flow of an organic solvent over an aqueous solution. The device features apertures connecting a lower aqueous channel to an upper solvent compartment that is open to air, enabling easy removal of the solvent for analysis. We have previously shown that related open biphasic systems enable steroid hormone extraction from human cells in microscale culture and secondary metabolite extraction from microbial culture; here we build on our prior work by determining conditions under which the system can be used with extraction solvents of ranging polarities, a critical feature for applying this extraction platform to diverse classes of metabolites. We developed an analytical model that predicts the limits of stable aqueous-organic interfaces based on analysis of Laplace pressure. With this analytical model and experimental testing, we developed generalized design rules for creating stable open microfluidic biphasic systems with solvents of varying densities, aqueous-organic interfacial tensions, and polarities. The stable biphasic interfaces afforded by this device will enable on-chip extraction of diverse metabolite structures and novel applications in microscale biphasic chemical reactions.

Interfacial Phenomena and Fluid Control in Micro/Nanofluidics

Fundamental aspects of rapidly advancing micro/nanofluidic devices are reviewed from the perspective of liquid interface chemistry and physics, including the influence of capillary pressure in microfluidic two-phase flows and phase transitions related to capillary condensation.

Solvent Extraction of Uranium by Laminar Flow in Microfluidic Chips

The system of uranyl nitrate aqueous solution and tributylphosphate (TBP) dissolved in kerosene is widely used all around the world, no matter in nuclear fuel cycle or analysis of impurities in uranium. The extraction process is mature, but the amount of consumption of organic reagent and generation of radioactive waste solution is huge, that is not environment friendly. Microfluidic is very popular nowadays, and one of its many applications is extraction. Here in this paper, we studied the flow situation of water and 20%TBP-kerosene, find the relationship between width ratio and flow rate ratio. And we used chemical modification to get a good separated outlet, and we observed the stability of OTS modified glass, that is stable in our circumstance. Finally we use the modified channels to achieve the extraction process, and find the extraction ratio is increases by contact time increases, and has no relationship of initial concentration.

Thermal lens detection device

A thermal lens detection device was developed to realize an easy-to-use, portable and sensitive detector for nonfluorescent molecules. Two laser diodes (658 nm for excitation and 785 nm for probe) were made coaxial in an optical unit and were coupled to a single-mode optical fiber. On a microfluidic chip, a small holder for the optical fiber was fixed, and micro-lenses (numerical aperture of 0.2) were also integrated inside the holder. The micro-lenses were designed to realize an adequate chromatic aberration (50 μm), which was essential for sensitive thermal lens detection. Compared with conventional thermal lens detection systems which required very laborious and accurate optical alignment with the microchannel, the new device needed just attachment-detachment of the optical fiber, which was important for practical application. The lower limit of detection was 10 nM for nickel(II) phthalocyaninetetrasulfonic acid tetrasodium salt solutions (model sample), and the absorbance was 9 × 10(-6) AU. The absolute number of molecules detected was less than 200 zmol. The coefficient of variance for 5-time attachment-detachment of the optical probe was as small as 3.6%. The technical development allowed integration of the thermal lens detection devices inside a microsystem (e.g. enzyme-linked immuno-sorbent assay system), and practical microsystems were realized with sensitivities several-orders higher than absorptiometry.

An integrated enzyme-linked immunosorbent assay system with an organic light-emitting diode and a charge-coupled device for fluorescence detection

A fluorescence detection system for a microfluidic device using an organic light-emitting diode (OLED) as the excitation light source and a charge-coupled device (CCD) as the photo detector was developed. The OLED was fabricated on a glass plate by photolithography and a vacuum deposition technique. The OLED produced a green luminescence with a peak emission at 512 nm and a half bandwidth of 55 nm. The maximum external quantum efficiency of the OLED was 7.2%. The emission intensity of the OLED at 10 mA/cm(2) was 13 μW (1.7 mW/cm(2)). The fluorescence detection system consisted of the OLED device, two band-pass filters, a five microchannel poly(dimethylsiloxane) (PDMS) microfluidic device and a linear CCD. The fluorescence detection system was successfully used in a flow-based enzyme-linked immunosorbent assay on a PDMS microfluidic device for the rapid determination of immunoglobulin A (IgA), a marker for human stress. The detection limit (S/N=3) for IgA was 16.5 ng/mL, and the sensitivity was sufficient for evaluating stress. Compared with the conventional 96-well microtiter plate assay, the analysis time and the amounts of reagent and sample solutions could all be reduced.

Microfluidics in inorganic chemistry

The application of microfluidics in chemistry has gained significant importance in the recent years. Miniaturized chemistry platforms provide controlled fluid transport, rapid chemical reactions, and cost-saving advantages over conventional reactors. The advantages of microfluidics have been clearly established in the field of analytical and bioanalytical sciences and in the field of organic synthesis. It is less true in the field of inorganic chemistry and materials science; however in inorganic chemistry it has mostly been used for the separation and selective extraction of metal ions. Microfluidics has been used in materials science mainly for the improvement of nanoparticle synthesis, namely metal, metal oxide, and semiconductor nanoparticles. Microfluidic devices can also be used for the formulation of more advanced and sophisticated inorganic materials or hybrids.

A three-phase microfluidic chip for rapid sample clean-up of alkaloids from plant extracts

A three-phase microchip was developed for the rapid and efficient small-scale purification of alkaloids from plant extracts. As part of the development of such a three-phase microchip, first a two-phase microchip with two channels (3.2 cm and 9.3 cm) was used to study the extraction efficiency of strychnine nitrate and strychnine at various flow rates. Strychnine was extracted from a basic aqueous phase to a chloroform phase (extraction) or strychnine was extracted from a chloroform phase into an acidic aqueous phase (back extraction). Subsequently, the "simultaneous extraction and back extraction" of strychnine was carried out in a three-phase microchip. The experimental extraction rate and yield were compared with model data. At a residence time of 25 sec, 79.5% of strychnine was extracted into the acidic aqueous phase using the three-phase microchip. In general, a good correlation was found between experimental results and model data for both two- and three-phase extractions. Finally, the three-phase microchip was employed in the purification of alkaloids (strychnine and brucine) from Strychnos seed extracts.

A micro-ELISA system for the rapid and sensitive measurement of total and specific immunoglobulin E and clinical application to allergy diagnosis

We have developed a novel, practical micro-ELISA system for sensitive and rapid allergy diagnosis. The enzymatic reactions occurred under stopped-flow conditions, resulting in both high precision and high sensitivity. A BSA-biotin-avidin linker was introduced for the immobilization of water-soluble allergens on polystyrene microbeads, enabling immobilization of allergens in sufficient density to provide high sensitivity. Evaluation of the system's performance showed a good detection limit (2 ng/mL) for total IgE measurement. In addition, a good correlation with a conventional method (CAP method) was demonstrated using human serum samples from 85 allergy patients. Importantly, sample volumes (5 microL) were 10 times smaller and analysis time (12 min) was >20 times faster than the conventional method. All procedures were automatically regulated with our simple microfluidic system, and all the fluidic, optic and electronic components were integrated for portability. We believe that our system has the potential to become a very powerful tool, particularly for point-of-care diagnosis.

Microchip-based cell analysis and clinical diagnosis system

Cell analysis and clinical diagnosis systems are now becoming the largest field of application for microchip-based analytical systems. Technological advantages include: small volume, fast analysis time, highly integrated analytical functions, easy operation and small size. For these purposes, basic methodologies for general micro-integration and basic technologies, including fluidic control and ultrasensitive detection, are required. In this review, we introduce our approach to the general integration of various analytical functions and the application of cell analysis systems with cultured cells in microchannels, as well as practical analytical systems for clinical diagnosis utilizing human serum samples.

Liquid filling method for nanofluidic channels utilizing the high solubility of CO2

We developed a fabrication method and a liquid filling method for a nano chemical reactor that used Y-shaped nanochannels specially designed for mixing and reacting. In order to reduce the pressure loss and to utilize the characteristics of the nanochannel, inlet microchannels were fabricated just beside the nanochannels. We investigated an initial liquid filling method into the nanochannels that ensured there were no air bubbles that could cause a flow stack due to the capillary pressure. In our method, the micro- and nanochannels were filled with carbon dioxide and any remaining air during the initial liquid introduction was dissolved utilizing the high solubility of carbon dioxide. We propose that chemical reactions in nanospaces can be realized by utilizing these fabrication and liquid introduction techniques.

Multiphase flow in microfluidic systems --control and applications of droplets and interfaces

Micro- and nanotechnology can provide us with many tools for the production, study and detection of colloidal and interfacial systems. In multiphase flow in micro- and nanochannels several immiscible fluids will be separated from each other by flexible fluidic interfaces. The multiphase coexistence and the small-volume confinement provide many attractive characteristics. Multiphase flow in microfluidic systems shows a complicated behavior but has many practical uses compared to a single-phase flow. In this paper, we discuss the methods of controlling multiphase flow to generate either micro- or nano-droplets (or bubbles) or stable stratified interfaces between fluidic phases. Furthermore, applications of the droplets and interfaces in microchannels are summarized.

Preparation of the capillary-based microchips for solid phase extraction by using the monolithic frits prepared by UV-initiated polymerization

A microfluidic solid phase extraction (SPE) array for sample enrichment was prepared by a simple method, a hot embossing technique. Five fused-silica capillaries (250 microm i.d., 380 microm o.d.) were partly embedded parallel in a polymethyl methacrylate (PMMA) microchip to serve as the extraction channels. Within each of the channels, a 2-mm-long monolithic porous polymer was prepared by in-situ photoinitiated polymerization. This then acted as the frit for packing of the extraction materials (octadecylsilica beads, ODS). By defining the light-exposure window on the channels, one can easily control the length and location of the polymer frits and the ODS beads can be packed at the desired location. With this method, solid phase extraction channels for microfluidic use can be easily prepared without complex fabrication of microstructures. Several SPE channels can be conveniently made in one microchip since the frits can be prepared in different channels through one polymerization; packing of the different channels can also be performed simultaneously. With the use of dilute ephedrine solutions, the sample loading capacity, linearity, and reproducibility were characterized. Coupled with the fast capillary electrophoresis separation, this microchip SPE array was applied for the detection of ephedrines in human urine.

Recent advances in synthetic micro reaction technology

Although in its infancy, the field of micro reaction technology is growing rapidly, with many research groups investigating the practical advantages associated with reaction miniaturisation. With this in mind, the following Feature Article aims to provide an overview of the progress made in the past decade, paying particular attention to the field of synthetic organic chemistry.

Pressure-driven flow control system for nanofluidic chemical process

We developed a novel flow control system for a nanofluidic chemical process. Generally, flow control in nanochannels is difficult because of its high-pressure loss with very small volume flow rate. In our flow control method, liquid pressure in a microchannel connected to the nanochannels is regulated by utilizing a backpressure regulator. The flow control method was verified by using simple structured microchip, which included parallel nanochannels. We found that the observed flow rate was three times lower than the value expected from Hagen-Poiseuille's equation. That implied a size-dependent viscosity change in the nanochannels. Then, we demonstrated mixing of two different fluorescent solutions in a Y-shaped nanochannel and also a proton exchange reaction in the Y-shaped nanochannel. The flow control method will contribute to further integration of nanochemical systems.

A microfluidic chip based sequential injection system with trapped droplet liquid-liquid extraction and chemiluminescence detection

A microfluidic chip-based sequential injection system with trapped droplet liquid-liquid extraction preconcentration and chemiluminescence detection was developed for achieving high sensitivity with low reagent and sample consumption. The microfabricated glass lab-chip had a 35 mm long extraction channel, with 134 shrunken opening rectangular recesses (L 100 microm x W 50 microm x D 25 microm) arrayed within a 1 mm length on both sides of the middle section of the channel. Ketonic peroxyoxalate ester solution was filled in the recesses forming organic droplets, and keeping the aqueous sample solution flowing continuously in the extraction channel; analytes were transferred from the aqueous phase into the droplets through molecular diffusion. After liquid-liquid extraction preconcentration, catalyst and hydrogen peroxide solutions were introduced into the channel, and mixed with analytes and peroxyoxalate ester to emit chemiluminescence light. The performance of the system was tested using butyl rhodamine B, yielding a precision of 4% RSD (n = 5) and a detection limit of 10(-9) M. Within a 17 min analytical cycle, the consumptions of sample and peroxyoxalate solutions were 2.7 microL and 160 nL, respectively.

Optical and electrochemical detection techniques for cell-based microfluidic systems

The ability to fabricate microfluidic systems with complex structures and with compatible dimensions between the microfluidics and biological cells have attracted significant attention in the development of microchips for analyzing the biophysical and biochemical functions of cells. Just as cell-based microfluidics have become a versatile tool for biosensing, diagnostics, drug screening and biological research, detector modules for cell-based microfluidics have also undergone major development over the past decade. This review focuses on detection methods commonly used in cell-based microfluidic systems, and provides a general survey and an in-depth look at recent developments in optical and electrochemical detection methods for microfluidic applications for biological systems, particularly cell analysis. Selected examples are used to illustrate applications of these detection systems and their advantages and weaknesses.

Microfluidic Polymer Chip Integrated with an ISFET Detector for Cationic Surfactant Assay in Dental Rinses

A cationic surfactant ion-selective field-effect transistor (cationic surfactant-ISFET) has been developed based on the tetraphenylborate derivative known as sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. The cationic surfactant-ISFET shows an almost Nernstian response to tetradecyldimethylbenzylammonium chloride (Zephiramine) over a concentration range between 1.0 x 10(-6) M and 1.0 x 10(-3) M, with a slope of 58.5 +/- 1.7 mV/decade. The cationic surfactant-ISFET can be used over a range of pH values, between pH 3 and 9. The cationic surfactant-ISFET shows excellent selectivity for Zephiramine over small inorganic cations, but shows similar selectivity for other cationic surfactants, such as hexadecyltrimethylammonium and stearyltrimethylammonium ions. A microfluidic polymer chip was integrated with the cationic surfactant-ISFET, and this was fabricated using polystyrene plates and stainless wires as a template for the channel. Cationic surfactant-ISFETs used in a batch system and microchips integrated with cationic surfactant-ISFETs showed very similar performance in terms of low detection limits, slope sensitivity and the stability of the potential response. The microfluidic polymer chip was then applied to the determination of cationic surfactants in dental rinses.

Reflective thermal lens detection device

A reflective thermal lens detection device was developed for realizing a portable and sensitive detector for a microsystem. An aluminum mirror was formed on the main plate of a microchip, and a reflected probe beam was detected with a single pick-up unit. The background signal due to light absorption of the aluminum mirror was 60 times reduced when the microchannel and the mirror were separated with an interval of 600 microm. The tilt angle of the microchip significantly affected the precision of the measurement. Then a quadrant photodiode was used to detect the center of gravity of the reflected probe beam to regulate the tilt angle within +/-0.05 degrees , and this value was enough to achieve 1% CV (coefficient of variance) precision in the measurements. The limit of detection (LOD) was 60 nM for xylene cyanol solution, and the absorbance was 9.4 x 10(-6) AU. About 40 times higher sensitivity was obtained in comparison with a spectrophotometer.

Determination of inorganic ions using microfluidic devices

The separation and detection of inorganic ions on microfluidic devices has received little attention since the 'lab-on-a-chip' concept has revolutionised the field of electrokinetically driven analysis. This review presents a summary and discussion of the published literature on inorganic analysis using microfluidic devices and includes sections on electromigration separation methods, namely isotachophoresis (ITP), capillary electrophoresis (CE), and hyphenated ITP-CE, together with a brief account of flow injection analysis. The review concludes with the authors' perspective on future directions for inorganic analysis on microfluidic devices.

Electroosmotic guiding of sample flows in a laminar flow chamber

The so-called address-flow principle is described: a valveless, electroosmotically driven technology used for controlling the stream profile in a laminar flow chamber. The method is explained, and a theoretical description and experimental verification are presented. Adjustment of the flow of two electroosmotically controlled guiding streams, running parallel to a central sample stream, can be used for positioning the sample stream in the dimension perpendicular to the flow direction. The results presented show that address-flow microfluidics allow easy and accurate control of sample stream position and width. The electroosmotic flow (EOF)-controlled guiding of microfluidic flows described in this paper, is a new unit operation that might aid in separation and collection in microfluidic devices. One possible application of address-flow microfluidics is guiding of capillary electrophoresis-separated components over a multisensor array, in order to perform affinity assays.