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Fluorescence polarization (FP) assays are widely used to quantify biomolecules, and their combination with microfluidic devices has the potential for application in onsite analysis. However, the hydrophobic surface of polydimethylsiloxane (PDMS)-based microfluidic devices and the amphiphilicity of the blocking agents can cause the nonspecific adsorption of biomolecules, which in turn reduces the sensitivity of the FP assay. To address this, we demonstrated an FP assay with improved sensitivity in microfluidic devices using a polyethylene glycol-based surface modification to avoid the use of blocking agents. We evaluated the effectiveness of the modification in inhibiting nonspecific protein adsorption and demonstrated the improved sensitivity of the FP immunoassay (FPIA). Our study addressed the lack of sensitivity of FP assays in microfluidic devices, particularly for the quantification of low-abundance analytes. [ABSTRACT FROM AUTHOR]

There is a demand for biosensors working under in vivo conditions, which requires significant device size and endurance miniaturization in solution environments. We demonstrated the detection of uric acid (UA) molecules, a marker of diseases like gout, whose continuous monitoring is required in medical diagnosis. We used a field effect transistor (FET) composed of an atomically thin transition metal dichalcogenide (TMD) channel. The sensor detection was carried out in a solution environment, for which we protected the electrodes of the source and drain from the solution. A microfluidic channel controls the solution flow that can realize evaporation-free conditions and provide an accurate concentration and precise measurement. We detected a systematic change of the drain current with the concentration of the UA in isopropyl alcohol (IPA) solvent with a detection limit of 60 nM. The sensor behavior is reversible, and the drain current returns to its original value when the channel is washed with pure solvent. The results demonstrate the feasibility of applying the MoS2-FET device to UA detection in solution, suggesting its possible use in the solution environment. [ABSTRACT FROM AUTHOR]

A microfluid-assisted solution field-effect transistor (FET) with nanoscale channels of atomically thin MoS2 layers was constructed. The source–drain current (Id) vs gate voltage (Vg) characteristics (Id–Vg) were examined with a focus on the threshold voltage (Vth) at the onset of the Id–Vg curve. Id–Vg changed when the channel contacted the tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) solutions in isopropyl alcohol (IPA), acetonitrile (ACN), and dimethyl sulfoxide (DMSO). The shift in Vth from the pure solvent condition (ΔVth) increased monotonically with the concentration, which was successfully simulated using Langmuir-type adsorption kinetics. We conclude that the TCNQ and F4-TCNQ solutes were partially solvated by the solvent and adsorbed on the MoS2 channel. Simultaneously, the saturated ΔVth value revealed a significant difference between the TCNQ and F4-TCNQ solutes. The ratio of saturated ΔVth of F4-TCNQ compared to that of TCNQ showed a decrease of 4.2, 1.7, and 1.3 for IPA, ACN, and DMSO, respectively. These results coincided with the order of the dielectric constants of these solvents (18.0, 36.0, and 46.6, respectively). The solutes produced the Id–Vg curve by both charge transfer and the gating effect, the latter of which was screened by the presence of a solvent. This study demonstrates that a solution FET can be employed in solid–solution interface chemistry. [ABSTRACT FROM AUTHOR]

A liquid chromatography system, comprising a separation column with a width and depth of a few hundred nanometers, was fabricated on a glass microchip (femto liquid chromatography, fLC). The size of this system was approximately [10.sup.11] times smaller than that of a conventional LC system, the flow rate was subpicoliter/ minute, and the injection volume was a few hundred attoliters. The fLC system did not require packing stationary phase and was capable of separating solutes with different molecular charges (fluorescein and sulforhodamine B) that could not be separated on a conventional LC column whose surface was covered with the same functional group as that of the column of the fLC system. The fLC system represented herein overcomes limitations of conventional chromatography separation, namely, heterogeneity of the stationary phases and eddy diffusion. Scale-down of the chromatography system brought advantages not only in reduction of sample volume but also in separation efficiency. The fLC system can analyze a very small amount of sample with high efficiency and will be useful in analyzing small samples, such as single cells and synaptic clefts, fLC greatly influences and benefits various fields such as life sciences, medicine, environmental science, and manufacturing by the improvement of separation technology. 10.1021/ac9017605

An interracial pressure balance model was proposed and verified for the elucidation of the physical mechanism of micro countercurrent flow in a hydrophilic-hydrophobic selective-modification microchannel. We considered the conditions of the microflow phase separation, where the phase separation entails a single phase flow in each output of the microchannel. In this pressure balance model, the pressure difference between the two phases due to pressure loss in each phase is balanced by the Laplace pressure generated by the interracial tension at the liquid-liquid interface between the separated phases. When the pressure difference between the two phases is sufficiently low, the contact line between the two phases is pinned at the boundary between the hydrophilic and the hydrophobic surfaces. Since the contact angle is restricted to values between the advancing and receding contact angles, the Laplace pressure has a limit. When the pressure difference between the two phases exceeds the limiting Laplace pressure, one of the phases leaks into the output channel of the other phase, and the phase separation fails. In order to experimentally verify this physical picture, a microchip with an asymmetric cross section, whose hydraulic diameters were 19 and 102 [micro]m, was used. In the microchip, a phase separation of a water-toluene micro countercurrent flow was achieved under pressure differences between an upper limit of 6.9 kPa and a lower limit of -9.3 kPa. The upper limit agreed well with the proposed model. The model is also applicable to cocurrent flows, so that it is useful for general multiphase microflows in continuous-flow chemical processing.

An ultrasensitive and nonlabeled detection method of nonfluorescent molecules on a microchip was developed by realizing a thermal lens microscope (TLM) with a 266-nm UV pulsed laser as an excitation light source (UVTLM). Pulsed laser sources have advantages over continuous-wave laser sources in more compact size and better wavelength tuning which are important for microchip-based analytical systems. Their disadvantage is difficulty in applying a lock-in amplifier due to the high (> [10.sup.4]) duty ratio of pulse oscillation. To overcome this problem, we realized a quasi-continuous-wave excitation by modulating the pulse trains at ~1 kHz and detecting the synchronous signal with a lock-in amplifier. The optimum pulse repetition frequency was obtained at 80 kHz, which was reasonable considering thermal equilibrium time. Furthermore, a permissible flow velocity in the range of 6.6-19.8 mm/s was found to avoid sensitivity decrease due to photochemical reactions and thermal energy dissipation. Under these conditions, we detected adenine aqueous solutions on a fused-silica microchip without labeling and obtained a sensitivity that was 350 times higher than that in a spectrophotometric method. The sensitivity was enough for detection on a microchip with an optical path length that was 2-3 orders shorter than that in conventional cuvettes. Finally, the UV-TLM method was applied to liquid chromatography detection. Fluorene and pyrene were separated in a microcolumn and detected in a capillary (50-[micro]m inner diameter) with 150 times higher sensitivity than a spectrophotometric method. Our method provides highly sensitive and widely applicable detections for various analytical procedures and chemical syntheses on microchips.