Lateral flow (LF) immunoassays are one of the most prevalent point-of-care (POC) diagnostics due to their simplicity, low cost, and robust operation. A common criticism of LF tests is that they have poor detection limits compared to analytical techniques, like ELISA, which confines their application as a diagnostic tool. The low detection limit of LF assays and associated long equilibration times is due to kinetically limited surface reactions that result from low target concentrations. Here we use isotachophoresis (ITP), a powerful electrokinetic preconcentration and separation technique, to focus target analytes into a thin band and transport them to the LF capture line resulting is a dramatic increase in the surface reaction rate and equilibrium binding. We show that ITP is able to improve limit of detection (LOD) of LF assays by 400-fold for 90 second assay time and by 160-fold for a longer 5 minutes time scale. ITP-enhanced LF (ITP-LF) also shows up to 30% target extraction from 100 µL of the sample, while conventional LF captures less than 1% of the target. ITP improves LF assay LOD to the level of some lab based immunoassays, such as ELISA, and may provide sufficient analytical sensitivity for application to a broader range of analytes and diseases that require higher sensitivity and lower detection limits.

Lipid bilayers are biomembranes common to cellular life and constitute a continuous barrier between cells and their environment. Understanding the interaction of engineered nanomaterials (ENMs) with lipid bilayers is an important step toward predicting subsequent biological effects. In this study, we assess the effect of varying the surface functionality and concentration of 10 nm-diameter gold (Au) and titanium dioxide (TiO2) ENMs on the disruption of negatively charged lipid bilayer vesicles (liposomes) using a dye leakage assay. Our findings show that Au ENMs having both positive and negative surface charge induce leakage that reaches a steady state after several hours. Positively charged particles with identical surface functionality and different core composition show similar leakage effects and result in faster and greater leakage than negatively charged particles, which suggests that surface functionality, not particle core composition, is a critical factor in determining the interaction between ENMs and lipid bilayers. The results suggest that particles permanently adsorb to bilayers and that only one positively charged particle is required to disrupt a liposome and trigger leakage of its entire contents in contrast to mellitin molecules, the most widely studied membrane lytic peptide, which requires hundred of molecules to generate leakage.

Paper substrates have been widely used to construct point-of-care lateral flow immunoassay (LFIA) diagnostic devices. Paper based microfluidic devices are robust and relatively simple to operate, compared to channel microfluidic devices, which is perhaps their greatest advantage and the reason they have reached a high level of commercial success. However, paper devices may not be well suited for integrated sample preparation, such as sample extraction and preconcentration, which is required in complex samples with low analyte concentrations. In this study, we investigate integration of isotachophoresis (ITP), an electrokinetic preconcentration and extraction technique, onto nitrocellulose-based paper microfluidic devices with the goal to improve the limit of detection of LFIA. ITP has been largely used in traditional capillary based microfluidic devices as a pretreatment method to preconcentrate and separate a variety of ionic compounds. Our findings show that ITP on nitrocellulose is capable of up to a 900 fold increase in initial sample concentration and up to 60% extraction from 100 μL samples and more than 80% extraction from smaller sample volumes. Paper based ITP is challenged by Joule heating and evaporation because it is open to the environment. We achieved high preconcentration by mitigating evaporation induced dispersion using novel cross-shaped device structures that keep the paper hydrated. We show that ITP on the nitrocellulose membrane can be powered and run several times by a small button battery suggesting that it could be integrated to a portable point-of-care diagnostic device. These results highlight the potential of ITP to increase the sensitivity of paper based LFIA under conditions where small analyte concentrations are present in complex biological samples.

Lipid bilayers are biomembranes common to cellular life and constitute a continuous barrier between cells and their environment. Understanding the interaction of nanoparticles with lipid bilayers is an important step toward predicting subsequent biological effects. In this study, we assessed the affinity of functionalized gold nanoparticles (Au NPs) with sizes from 5 to 100 nm to lipid bilayers by determining the Au NP distribution between aqueous electrolytes and lipid bilayers. The Au NP distribution to lipid bilayers reached an apparent steady state in 24 h with smaller Au NPs distributing onto lipid bilayers more rapidly than larger ones. Au NPs distributed to lipid bilayers to a larger extent at lower pH. Tannic acid-functionalized Au NPs exhibited greater distribution to lipid bilayers than polyvinylpyrrolidone-functionalized Au NPs of the same size. Across the various Au NP sizes, we measure the lipid bilayer-water distribution coefficient (K(lipw) = C(lip)/C(w)) as 450 L/kg lipid, which is independent of dosimetric units. This work suggests that the nanoparticle-cell membrane interaction is dependent on solution chemistry and nanoparticle surface functionality. The K(lipw) value may be used to predict the affinity of spherical Au NPs across a certain size range toward lipid membranes.

Biological membranes are one of the important interfaces between cells and pollutants. Many polar and hydrophobic chemicals can accumulate within these membranes. For this reason, artificial biological membranes are appealing surrogates to complex organisms for assessing the bioaccumulation potential of engineered nanomaterials (ENMs). To our knowledge, this work presents the first quantitative study on the distribution of fullerene ENMs between lipid bilayers, used as model biological membranes, and water. We evaluated the lipid bilayer-water association coefficients (K(lipw)) of aqueous fullerene aggregates (nC(60)) and fullerol (C(60)(ONa)(x)(OH)(y), x + y = 24). Kinetic studies indicated that fullerol reached apparent equilibrium more rapidly than nC(60) (2 h versus >9 h). Nonlinear isotherms can describe the distribution behavior of nC(60) and fullerol. The lipid bilayer-water distributions of both nC(60) and fullerol were pH-dependent with the accumulation in lipid bilayers increasing systematically as the pH decreased from 8.6 (natural water pH) to 3 (the low end of physiologically relevant pH). This pH dependency varies with the zeta potentials of the ENMs and leads to patterns similar to those previously observed for the lipid bilayer-water distribution behavior of ionizable organic pollutants. The K(lipw) value for nC(60) was larger than that of fullerol at a given pH, indicating a greater propensity for nC(60) to interact with lipid bilayers. For example, at pH 7.4 and an aqueous concentration of 10 mg/L, K(lipw) was 3.5 times greater for nC(60) (log K(lipw) = 2.99) relative to fullerol (log K(lipw) = 2.45). Comparisons with existing aquatic organism bioaccumulation studies suggested that the lipid bilayer-water distribution is a potential method for assessing the bioaccumulation potentials of ENMs.