Investigation of Micro-volume Viscosity with Janus Microbeads Based on Rotational Brownian Motion.

Viscosity is an important property of liquids. A viscosity change of aqueous substances that deviates from their normal levels usually implies a compromise in quality due to degradation or microorganism proliferation. Monitoring of macro-scale viscosity can be simply realized by various conventional tools, such as rotational viscometers, capillary tubes, falling bodies, and so forth. Nevertheless, today, micro-volume viscosity measurement remains a challenging endeavor, resulting in rare, expensive, or difficult-to-obtain samples not very well studied. For this reason, a novel technique for micro-viscosity based on rotational Brownian motion is presented in this paper. Janus microbeads were made by coating fluorescent polystyrene beads with gold film. Taking advantage of the bead configuration of half gold/half fluorescence, the rotational Brownian signal was expressed in terms of blinking fluorescent intensity. The characteristic correlation time was derived from the blinking intensity of trace amounts of a selected medium over a certain time period, and results were correlated with viscosity. Given a volume of only 2 µL for each measurement, calibration of a series of glycerol⁻water mixtures (100%⁻1% (v/v) water content) yielded good agreement with the expected viscosity predictions over the range of 0.8⁻574.8 cP. Five common oil products, including lubricant oil, baby oil, food oil, olive oil, and motor oil, were further investigated to demonstrate the feasibility and practicability of the proposed technique. Data measured by the rotational Brownian motion-based diffusometer were comparable with those measured by a commercial rotational viscometer. The method also explicitly showed viscosity degradation after the oils were heated at a high temperature of over 100 °C for 10 min. Evaluation proved the proposed Janus microbead-enabled rotational diffusometric technique to be a promising approach for rapid and micro-scale viscosity measurement.

An Electrochemical Sensor Based on Gold Nanodendrite/Surfactant Modified Electrode for Bisphenol A Detection.

In the present work, we reported the simple way to fabricate an electrochemical sensing platform to detect Bisphenol A (BPA) using galvanostatic deposition of Au on a glassy carbon electrode covered by cetyltrimethylammonium bromide (CTAB). This material (CTAB) enhances the sensitivity of electrochemical sensors with respect to the detection of BPA. The electrochemical response of the modified GCE to BPA was investigated by cyclic voltammetry and differential pulse voltammetry. The results displayed a low detection limit (22 nm) and a linear range from 0.025 to 10 µm along side with high reproducibility (RSD = 4.9% for seven independent sensors). Importantly, the prepared sensors were selective enough against interferences with other pollutants in the same electrochemical window. Notably, the presented sensors have already proven their ability in detecting BPA in real plastic water drinking bottle samples with high accuracy (recovery range = 96.60%-102.82%) and it is in good agreement with fluorescence measurements.

An electrochemical sensor based on copper-based metal-organic framework-reduced graphene oxide composites for determination of 2,4-dichlorophenol in water.

In the present work, we reported the fabrication of a novel electrochemical sensing platform to detect 2,4-dichlorophenol (2,4-DCP) by using a copper benzene-1,3,5-tricarboxylate-graphene oxide (Cu-BTC/GO) composite. The sensor was prepared by drop-casting Cu-BTC/GO suspension onto the electrode surface followed by electrochemical reduction, leading to the generation of an electrochemically reduced graphene oxide network (ErGO). By combining the large specific area of the Cu-BTC matrix with the electrical percolation from the graphene network, the number of accessible reaction sites was strongly increased, which consequently improved the detection performance. The electrochemical characteristics of the composite were revealed by cyclic voltammetry and electrochemical impedance spectroscopy. For the detection of 2,4-DCP, differential pulse voltammetry was used to emphasize the faradaic reaction related to the oxidation of the analyte. The results displayed a low detection limit (83 × 10-9 M) and a linear range from 1.5 × 10-6 M to 24 × 10-6 M alongside high reproducibility (RSD = 2.5% for eight independent sensors) and good stability. Importantly, the prepared sensors were sufficiently selective against interference from other pollutants in the same electrochemical window. Notably, the presented sensors have already proven their ability in detecting 2,4-DCP in real field samples with high accuracy (recovery range = 97.17-104.15%).

Phase separation of ternary self-assembled monolayers into hydrophobic 1-dodecanethiol domains and electrostatically stabilized hydrophilic domains composed of 2-aminoethanethiol and 2-mercaptoethanesulfonic acid on Au(111).

Ternary self-assembled monolayers (SAM) composed of 2-aminoethanethiol (AET), 2-mercaptoethanesulfonic acid (MES), and 1-dodecanethiol (DDeT) form two types of domains as if it were a two-component SAM: DDeT-rich hydrophobic domains and electrostatically stabilized hydrophilic domains composed of MES and AET on Au(111). MES and AET behave virtually as a single surface-active species. Two distinct reductive desorption peaks in cyclic voltammograms (CV) and binarized images of scanning tunneling microscopy clearly show nanometer scale, yet macroscopically distinguishable, phase separation over a wide range of the mixing ratio of DDeT and MES-AET in the bathing solution. X-ray photoelectron spectroscopy measurements indicate that the ratio of MES to AET in the hydrophilic domains is unity and that both terminal groups are in the charged states, that is, the sulfonate group and the ammonium group. With decreasing the total concentration of the thiols, the mole fraction of DDeT in the bathing solution at which the surface coverage of MES-AET domains is equal to that of DDeT domains dramatically decreases. This suggests that the adsorption kinetics plays a crucial role in the formation of the domains structure.