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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]

We investigated the sensor behavior of a field effect transistor, the channel of which is made of atomically thin MoS2 layers, focusing on the interaction of the MoS2 channel with the solution containing target molecules. For this purpose, we made a newly designed device in which the mask covered the electrodes of the source and the drain in order to make the solution contact only with the channel. In addition, a micro-fluid tank was fabricated above the channel as a solution reservoir. We examined the FET properties of this device for the sensing of the nicotine molecule for the development of a detection system for this molecule in the human body under in vivo conditions. We detected the sensor behavior both for the drop-cast process and for the condition where the channel contacts with the solution. The drain-current vs. gate-voltage variation of the MoS2-FET with the attachment of the nicotine molecule was clearly observed for both cases. For the latter case, the threshold voltage shifted in the negative gate-voltage direction with the increase of the concentration of the nicotine in the solution. This can be explained by the electron transfer from the molecule to the MoS2 channel, which was further confirmed by analyzing the X-ray photoemission spectroscopy and Raman spectroscopy together with the DFT calculation. The sensor can detect the variation of the nicotine concentration in the IPA solution by detecting the Vth change of the MoS2-FET. [ABSTRACT FROM AUTHOR]