Dry-reagent microfluidic biosensor for simple detection of NT-proBNP via Ag nanoparticles

The diagnosis of many diseases requires monitoring of biomarker levels over a period of time instead of assessing their concentration only once. For example, in case of heart failure determination, the levels of N-terminal prohormone brain natriuretic peptide (NT-proBNP) in blood vary so strongly amongst individuals, that the current procedure of one-time measurement in combination with clinical examination does not allow for accurate assessment of disease severity and progression. Our microfluidic biosensor addresses key characteristics of desirable home-tests which include low limits of detection, small sample volume (less than 10 μL), simple detection strategies, and ready-to-go all-dried long-term stable reagents. Here, electrochemically superior silver nanoparticles (AgNP) were dried directly within the microfluidic channel in a matrix of trehalose sugar doped with Na₂SO₃ as oxygen scavenger. This successfully prevented AgNP oxidation and enabled dry and ready-to-use storage for at least 18 weeks. Based on this, laser-cut flow chips were developed containing all bioassay reagents needed in a ready-to-go dry format. An oxidation-reduction stripping voltammetry strategy was used for highly sensitive quantification of the AgNPs as electrochemical label. This microfluidic biosensor demonstrated limits of detection for NT-proBNP of 0.57 ng mL^-1 with a mean error of 6% (n ≥ 3) in undiluted human serum, which is below the clinically relevant cut-off of 1 ng mL^-1. This practical approach has the potential to substitute commonly used lateral-flow assays for various biomarkers, as it offers low patient sample volumes hence supporting simple finger-prick strategies well-known also for other electrochemical biosensors, and independence from the notorious variability in fleece fabrication.