Extended Data Fig. 5: Lateral flow and FND benchmarking.

a, Measurement of flow rate of lateral flow strips. During wetting, the flow follows the Washburn equation, where \(V\approx {t}^{\frac{1}{2}}\) (inset), and during fully-wetted flow, Darcy’s law for capillary flow is followed (V ≈ t), with a constant flow rate of 6.9 μl min⁻¹. b, Using a one-to-one receptor–ligand binding approximation, the binding of biotinylated FNDs to streptavidin was modelled kinetically, indicating that all the FNDs bind with a residency time of more than about 10⁻³ s. Here, the residency time is measured as 4 s, using the flow rate from a, so all the FNDs should bind. c, An example of the measurement of FND concentration. FND fluorescence is unaffected by surface chemistry, so is used to quantify concentration. A serial dilution of FND suspensions from a known stock concentration was performed (dots showing means with error bars showing s.d., n = 6 measurement replicates). This was then fitted with a linear regression (lines) to find a relationship between fluorescence intensity and concentration. After each FND functionalization, the fluorescence intensities of the final suspensions were measured, and the linear fit was used to estimate concentration (crosses). d, Fundamental LODs for different sized FNDs on LFAs, using a model biotin–avidin interaction. Suspensions (55 μl) of BSA–biotin-functionalized FNDs were run at different concentrations on poly-streptavidin strips. Concentrations were chosen to span the dynamic range of the camera, limited by over-exposure, as seen with the top concentration of 200 and 600 nm FNDs. Dots show means and error bars show s.d. (n = 3 technical replicates, n = 3 measurement replicates). Each series is fitted to a simple linear regression, shown as the solid line, with 95% confidence intervals shown shaded. LODs for 120, 200 and 600 nm diameter FNDs are 200 aM, 46 aM, and 820 zM respectively, defined by the intersection of the lower 95% confidence intervals of the linear fit with the upper 95% confidence intervals of the blanks for each particle size.