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Superparamagnetic nanoparticles (SPMNPs) have attracted interest for various biomedical applications due to their unique magnetic behavior, excellent biocompatibility, easy surface modification, and low cost. Their unique magnetic properties, superparamagnetism, and magnetophoretic mobility have led to their inclusion in immunoassays to enhance biosensor sensitivity and allow for rapid detection of various analytes. In this review, we describe SPMNP characteristics valuable for incorporation into biosensors, including the use of SPMNPs to increase detection capabilities of surface plasmon resonance and giant magneto-resistive biosensors. The current status of SPMNP-based immunoassays to improve the sensitivity of rapid diagnostic tests is reviewed, and suggested strategies for the successful adoption of SPMNPs for immunoassays are presented.

The stabilization of carbon dioxide-in-water (C–W) foams with nanoparticles (NPs) becomes highly challenging as the temperature and salinity increase, particularly for divalent ions, as the nanoparticles often aggregate in the brine phase. For silica nanoparticles with a medium coverage (MC) and high coverage (HC) of organic ligands, the hydrophilic–CO2-philic balance (HCB) was found to be in the appropriate range to produce a large reduction in the C–W interfacial tension (IFT). Furthermore, the nanoparticles were colloidally stable in concentrated brine (15% total dissolved solids, TDS) up to 80 °C. With these interfacially active nanoparticles, C–W foams were stabilized with apparent foam viscosities up to 35 cP and foam textures with bubble sizes on the order of 40 μm at various gas fractional flows (foam qualities) in beadpack experiments. At the foam quality where the apparent viscosity was a maximum (transition quality) in the beadpack, we also produced CO2 foams in Boise and Berea cores versus temperature with apparent viscosities up to 26 cP at 70 °C and 15% TDS and hysteresis in the apparent viscosity versus the interstitial velocity. The reductions in the IFT and foam strength at elevated temperature were modestly larger for the HC nanoparticles than for the MC nanoparticles but were low for the low-coverage case. Given that the interfacial adsorption increased with salinity up to 15% TDS, the screening of the charge helped drive the particles from the brine phase to the interface, which was necessary to stabilize the foams.

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The use of foam in gas enhanced oil recovery (EOR) processes has the potential to improve oil recovery by reducing gas mobility. Nanoparticles are a promising alternative to surfactants in creating foam in the harsh environments found in many oil fields. We conducted several CO2-in-brine foam generation experiments in Boise sandstones with surface-treated silica nanoparticle in high-salinity conditions. All the experiments were conducted at the fixed CO2 volume fraction (g = 0.75) and fixed flow rate which changed in steps. We started at low flow rates, increased to a medium flow rates followed by decreasing and then increasing into high flow rates. The steady-state foam apparent viscosity was measured as a function of injection velocity. The foam flowing through the cores showed higher foam generation and consequently higher apparent viscosity as the flow rate increased from low to medium and high velocities. At very high velocities, once foam bubbles were finely textured, the foam apparent viscosity was governed by foam shear-thinning rheology rather than foam creation. A noticeable "hysteresis" occurred when the flow velocity was initially increased and then decreased, implying multiple (coarse and strong) foam states at the same superficial velocity. A normalized generation function was combined with CMG-STARS foam model to cover the full spectrum of foam flow behavior observed during the experiments. The new foam model successfully captures foam generation and hysteresis trends observed in presented experiments in this study and other foam generation experiments at different operational conditions (e.g. fixed pressure drop at fixed foam quality, and fixed pressure drop at fixed water velocity) from the literature. The results indicate once foam is generated in porous media, it is possible to maintain strong foam at low injection rates. This makes foam more feasible in field applications where foam generation is limited by high injection rates (or high pressure gradient) that may only exist near the injection well. Therefore, understanding of foam generation, and foam hysteresis in porous media and accurate modeling of the process are necessary steps for efficient foam design in field.