The Surface Charge Density Influence on the Electrokinetic Properties of Model Colloids: Solvent Composition Effect

Simultaneous Sensing of Multiplex Volatile Organic Compounds by Adsorption and Plasmon Dual-Induced Raman Enhancement Technique

Developing highly efficient gas sensors with excellent performance for rapid and sensitive detection of volatile organic compounds (VOCs) is of critical importance for the protection of human health, ecological environment, and other factors. Here, a robust gas sensor based on Raman technology was constructed by an in situ grown 2D covalent organic framework (COF) on Au nanoparticles' surface in the microchannel. Dual enhancement effects are included for the as-prepared microfluidic sensor. First, acting as a gas confinement chamber, the 2D COF could effectively capture gas molecules with high adsorption capacity and fast adsorption kinetics, resulting in VOCs' preconcentration at a high level in the COF layer. At the same time, after being stacked in the microchannel, abundant hot spots were generated among the nanogaps of Au@COF NPs. The local surface plasmon resonance effect could effectively enhance the Raman intensity. Both factors contribute to the improved detection sensitivity of VOCs. As a demonstration, several representative VOCs with different functional groups were tested. The resultant Raman spectra were subjected to the statistical principal component analysis. Varied VOCs can be successfully detected with a detection limit as low as ppb level and distinguished with 95% confidence interval. The present microfluidic platform provides a simple, sensitive, and fast method for VOCs' sensing and distinguishing, which is expected to hold potential applications in the fields of health, agricultural, and environmental research.

Anion amphiphilic random copolymers and their performance as stabilizers for O/W nanoemulsions

A series of anionic amphiphilic random copolymers with sodium p-styrene sulfonate and dodecyl methacrylate side chains were synthesized via free radical polymerization and their properties in the formation and stabilization of nano-emulsions were investigated. Using poly(sodium p-styrene sulfonate)-ran-poly(dodecyl methacrylate) and Brij 30 as a stabilizer to prepare nanoemulsions, we obtained small droplet size and unimodal distribution nanoemulsions by a low-energy phase inversion composition (PIC) method. The p(SSS)-ran-p(LMA)-Brij 30 co-stabilized nanoemulsions show extraordinary long-term stability and heat resistance, there were almost no variations of droplet size after storing for 35 days and no phase inversion occurred when heating the temperature up to 90 °C. The influence of salinity on the properties of the nanoemulsions was also discussed.

Comparative study of modified/non-modified aluminum and silica aerogels for anionic dye adsorption performance

Developing effective and low-cost adsorbents is of great significance for controlling water contamination.

Colloidal and bacterial deposition: role of gravity

The role of gravitational force on the deposition of 0.5, 1.1, and 1.8 mum carboxylate-modified polystyrene latex (CML) microspheres and bacterium Burkholderia cepacia G4g has been evaluated using a parallel plate flow chamber system. This experimental system utilized an inverted and an upright optical microscope attached with image-capturing devices to directly observe and determine the deposition kinetics onto glass surfaces located at the top and bottom of the flow chamber. Deposition kinetics was quantified at 10 mM KCl under electrostatically unfavorable and favorable attachment conditions and at two flow rates (0.06 and 3 mL/min), simulating the range of flow velocities from groundwater to rapid granular filtration. Comparing the particle deposition kinetics on the top and bottom surfaces under identical flowing exposure time, fluid chemistries, and hydrodynamic conditions, results showed that significant differences were observed between the two surfaces, suggesting that gravity was a significant driving force for the initial stages of deposition of particles that were larger than 1 mum size. Simulation results utilizing a particle trajectory model confirmed these experimental observations. This was further supported by additional deposition experiments with 1.1 mum microspheres suspended in a deuterium oxide (D(2)O)/water mixture (heavy water) where the density of colloid and the suspending heavy water were effectively the same. Under this condition, deposition rates were observed to be identical between the top and bottom surfaces. Results from normal and heavy water solutions indicated that the greater deposition of colloidal particles larger than 1 mum on the bottom in normal water solutions is due to gravity. Finally, the experimental results were compared with deposition studies using smaller 0.5 mum colloids as well as some theoretical calculations of expected rates of particle deposition.

Electrokinetic parameters of colloidal model systems: analysis and comparison between dilute and concentrated dispersions

In the last decades, the interest of many scientists has been focused on the atypical electrokinetic behavior of charged colloidal systems since several studies have shown, in most cases; it is not so ideal as expected. Particularly, two interesting phenomena have not been clearly explained yet. First, the zeta potential magnitude does not decrease monotonically with increasing ionic strength, as expected according to the Gouy-Chapmann model predicts. Second, the zeta potential obtained from different techniques shows discrepancies. More specifically, the zeta potential obtained from streaming potential is lower (in absolute value) than that measured through electrophoretic mobility. However, a recent work has pointed out that these discrepancies seem to disappear if certain conditions (related with the surface charge density) are satisfied. This work also revealed that unexpected results are found when the electric conductivity was used. Spherical polystyrene particles of appropriate particle size and charge density are employed as polymeric colloidal model in the present work. Common and adequate models are used to make clear and easy our theoretical analysis and its interpretation. To test the surface conductance and ionic mobility effects at the solid-liquid interface, both water medium and alcohol-water mixtures are used.

An Experimental Test of the Ion Condensation Theory for Spherical Colloidal Particles

This paper deals with the notion of ion condensation for spherical colloids and, more specifically, with a recent model developed to predict effective charges (Y. Levin, M. C. Barbosa, and M. N. Tamashiro, Europhys. Lett. 41, 123, 1998). Electrophoretic mobility measurements (carried out for a set of well-characterized latexes) were used to find out to what extent this theory is able to accounts for: (i) the insensitivity of mobility to surface charge, and (ii) the small values of electrokinetic charge found at low ionic strength. As the Levin theory was developed assuming no added salt, a previous discussion about the effect of additional electrolyte was needed. Contrary to what other authors have reported, our results do not support the above-mentioned theory. Copyright 2001 Academic Press.