Lateral Flow Tests: How Gold Nanoparticles Make a Visible Line
Lateral flow tests are cheap, equipment-free rapid diagnostics that wick a sample along a membrane past antibody-coated gold nanoparticles. The particles' red plasmonic color forms the visible test line, as in pregnancy and COVID-19 tests.
A lateral flow test (lateral flow immunoassay) is a simple paper-based diagnostic that delivers a result in roughly 5 to 30 minutes without lab equipment. A liquid sample is dropped on a sample pad, which acts as a sponge, and capillary action wicks it along a membrane. It first passes through a conjugate pad holding freeze-dried detection particles bound to antibodies. The most common detection label is gold nanoparticles (colloidal gold), whose red color comes from localized surface plasmon resonance; latex beads (blue) are an alternative. See Gold Nanoparticles and Localized Surface Plasmon Resonance. If the target molecule is present, it binds the antibody-gold conjugate; the complex flows onward and is captured at a test line striped with immobilized antibodies, where accumulated gold particles concentrate into a visible colored band. A second control line captures conjugate regardless of the target, confirming the sample flowed correctly and the reagents are active — which is why a valid negative still shows one line. Familiar examples include home pregnancy tests (detecting human chorionic gonadotropin in urine), COVID-19 rapid antigen tests, and rapid tests for malaria and HIV. The format's advantages are decisive for point-of-care and home use: low cost, little or no sample preparation, room-temperature stability, naked-eye readout, and no instrument required. Fluorescent or magnetic labels offer higher sensitivity but need an electronic reader, sacrificing the equipment-free appeal.