The mechanism of polyelectrolyte-assisted retention of TiO2 filler particles during paper formation

Structural and Double Layer Forces between Silica Surfaces in Suspensions of Negatively Charged Nanoparticles

Direct force measurements between negatively charged silica microparticles are carried out in suspensions of like-charged nanoparticles with atomic force microscopy (AFM). In agreement with previous studies, oscillatory force profiles are observed at larger separation distances. At smaller distances, however, soft and strongly repulsive forces are present. These forces are caused by double layer repulsion between the like-charged surfaces and can be quantitatively interpreted with the Poisson-Boltzmann (PB) model. However, the PB model must be adapted to a strongly asymmetric electrolyte to capture the nonexponential nature of these forces. Thereby, the nanoparticles are modeled as highly charged co-ions, while the counter ions are monovalent. This model permits extraction of the effective charge of the nanoparticles, which is well comparable to the one obtained from electrophoresis. The PB model also explains the presence of a particle-free layer close to the interface.

Raman Characterization of Nanoparticle Transport in Microfluidic Paper-Based Analytical Devices (μPADs)

There is great interest in the use of microfluidic paper-based analytical devices (μPADs) for low-cost diagnostics. In this contribution, we illustrate the utility of Raman spectral imaging for both μPAD characterization and for quantification of the transport of applied reagents and analytes within these devices. We evaluated the transport of nanoscale particles within μPADs using a suite of differentially functionalized gold (AuNP) and silver (AgNP) nanoparticles with diameters of 8-64 nm. Nanoparticle transport within the cellulose matrix was characterized by collection of both Raman and surface-enhanced Raman spectroscopy (SERS) spectral maps that enabled differentiation of cellulose fibers and characterization of analyte deposition patterns. The transport of citrate (cit), BSA, PEG, PVP, and DNA functionalized AuNP and AgNP in wax-printed μPADs was primarily affected by nanoparticle surface chemistry rather than particle size or core composition. Sample pH (3-10) influenced the transport of 15 nm BSA-cit-AuNP, but not 15 nm cit-AuNP, because of the effects of solution pH on the charge and conformation of BSA. Derjaguin, Landau, Verwey, and Overbeek theory (DLVO) and extended DLVO (xDLVO) theory are used to explain the collected experimental results.