A microfluidic approach for the detection of uric acid through electrical measurement using an atomically thin MoS2 field-effect transistor.

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.

Phytochemical-Assisted Synthesis of Fe3O4 Nanoparticles and Evaluation of Their Catalytic Activity.

In this study, magnetite nanoparticles (Fe3O4 NPs) were synthesized using Baccaurea ramiflora leaf extracts and characterized by visual observation, UV-Vis, FTIR, XRD, FESEM, and EDS. The UV-Vis spectrum showed continuous absorption at 300-500 nm, confirming the formation of Fe3O4 NPs. FTIR revealed that compounds containing the O-H group act as reducing agents during Fe3O4 NPs formation. Agglomerated spherical NPs were observed in the FESEM image. The prominent peak at ~6.4 keV in the EDS spectrum ascertained the existence of Fe, while the sharp peak at ~0.53 keV confirmed the presence of elemental oxygen. XRD patterns affirmed the crystalline nature. The size of as-synthesized NPs was observed to be 8.83 nm. The catalytic activity of Fe3O4 NPs for the reduction of methylene blue (MB) dye was monitored by UV-Vis. The maximum absorption peak of MB dye at 664 nm was almost diminished within 20 min, which revealed Fe3O4 NPs could be an excellent catalyst for wastewater treatment.

Synthesis, characterization and application of a novel polyazo dye as a universal acid-base indicator.

A novel organic polyazo dye is synthesized by the diazotization of aromatic aniline, followed by coupling it with sulfanilic acid and N,N-dimethylaniline. Characterization was done by 1H-NMR, 13C-NMR, and FTIR spectroscopy. Differential scanning calorimetry (DSC) reveals that phase transition for this molecule is exothermic. The optical band gap is estimated from the absorption cutoff point using UV-Visible spectroscopy. Thermal gravimetric analysis (TGA) addresses the thermal stability of the molecule and is found to be at ∼250 °C. The structure of the synthesized molecule is analogous to that of methyl orange and contains three azo groups. These three azo groups help accept more than two protons and provide two pK a values when diprotic acid or a mixture of acids is used in different titrations. Specifically, when a polybasic acid is in strong base titration, the pK a values were found to be 3.5 and 9.1. Moreover, for strong base and (strong + weak) acid mixture titration, the pK a values are found to be 9.2 and 3.3. Furthermore, the pK a values are found to be 8.6 and 2.8 for (strong and weak) base mixture and (strong and weak) acid mixture titration, respectively. Owing to its increased proton accepting capacity, it can be found in the two pH ranges of 2.1-3.8 for orange color and 8.2-9.8 for yellow color, thus indicating a unique property as a universal indicator for acid-base titration. The dissociation constant of this dye is found to be 3.4 × 10-6, determined in a mixed aqueous solution of 10 wt% ethanol, and a linear relationship between pK a and pH is observed in this solvent system.

Chemistry of the photoisomerization and thermal reset of nitro-spiropyran and merocyanine molecules on the channel of the MoS2 field effect transistor.

We have explored the chemical reaction of the photoisomerization and thermal reaction of the photochromic spiropyran (SP) 1',3'-Dihydro-1',3',3' trimethyl-6-nitrospiro[2H-1 benzopyran-2,2'-(2H)-indole] molecule deposited on the atomic thin channel of a MoS2 field-effect transistor (FET) through the analysis of the FET property. With four monolayers of SP molecules on the channel, we observed a clear shift of the threshold voltage in the drain-current vs gate-voltage plot with UV-light injection on the molecule, which was due to the change of the SP molecule to merocyanine (MC). A complete reset from MC to SP molecule was achieved by thermal annealing, while the injection of green light could revert the FET property to the original condition. In the process of change from MC to SP, two types of decay rates were confirmed. The quick- and slow-decay components corresponded to the molecules attached directly to the substrate and those in the upper layer, respectively. The activation energies for the conversion of MC to SP molecules were estimated as 71 kJ/mol and 90 kJ/mol for the former and latter, respectively. Combined with DFT calculations, we concluded that the Id-Vg shift with photoisomerization from SP to MC is due to the upper layer molecules and the dipole moment in the surface normal direction. Based on the estimated activation energy of 90 kJ/mol for the reset process, we calculated the conversion rate in a controllable temperature range. From these values, we consider that the chemical state of MC can be maintained and switched in a designated time period, which demonstrates the possibility of this system in logical operation applications.

In situ study of sensor behavior of MoS2 field effect transistor for methyl orange molecule in ultra high vacuum condition.

We investigate the sensor behavior of the MoS2 field effect transistor (FET) device with the deposition of methyl orange (MO) molecule which is widely used as a chemical probe. The channel of the FET is made of the single layer of MoS2 which makes it highly sensitive to the molecule adsorption, but at the same time the behavior depends much on the surface conditions of the MoS2 channel. In order to make the channel-surface conditions more defined, we prepare an in situ experimental system in which the molecule deposition and the surface- and electrical-characterization of the MoS2 FET are executed in a single ultra-high vacuum chamber. This system makes it possible to examine the change of the FET properties with precise control of the molecule coverage in the sub-monolayer region without the effect of the atmosphere. We detected the shift of the I d-V g curve of the MoS2-FET device with the increase of the molecule coverage (θ) of the MO molecule, which is quantitatively analyzed by plotting the threshold voltage (V th) of the I d-V g curve as a function of θ. The V th shifts towards the negative direction and the initial change with θ can be expressed with an exponential function of θ, which can be accounted for with the Langmuir type adsorption of the molecule for the first layer and the charge transfer from the molecule to the substrate. The V th versus θ curve shows a kink at a certain θ, which is conserved as the starting of the second layer growth. We detected the adsorption of MO far less than monolayer and the phase change from the first layer to the second layer growth, which is realized by the benefit of the in situ UHV experimental condition.

Microfluidic tank assisted nicotine sensing property of field effect transistor composed of an atomically thin MoS2 channel.

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.