Abstract
<title>Abstract</title> <p>The analytical performance of gold-nanoparticle (AuNP) colorimetric aptasensors is decided upstream, at the point where the recognition aptamer is coupled to the particle surface. For thiolated DNA on citrate-capped AuNPs the classical salt-aging route is slow (1–2 days), aggregation-prone and difficult to reproduce, a decisive obstacle in resource-limited laboratories. A compact, reagent-frugal protocol is reported for conjugating a thiolated anti-HER2 DNA aptamer to citrate AuNPs, coupling three unit operations: dithiothreitol reduction of the 5′-disulfide followed by ethanol precipitation, which removes the reductant completely and fixes the aptamer concentration; reagentless freezing-directed conjugation, a single freeze–thaw with no added salt; and an equipment-free salt-challenge quality gate applied before any washing. The dominant early failure mode is not synthesis quality — the citrate AuNPs were reproducible, with a localised surface-plasmon resonance maximum λmax = 519.8 ± 1.3 nm across ten batches — but inadequate aptamer surface density compounded by premature exposure to high-salt buffer: standard 1× phosphate-buffered saline (~ 137 mM NaCl) exceeds the critical coagulation concentration of the bare colloid, bracketed at ~ 46–97 mM NaCl, and triggers irreversible aggregation before the DNA layer can protect the surface. Strict buffer discipline (water or low-salt media upstream, phosphate-buffered saline only after conjugation), an aptamer-to-particle feed ratio raised from ~ 216:1 to ~ 500:1, and an unstructured rather than pre-folded aptamer presentation together yield conjugates that survive a 500 mM NaCl challenge. The protocol requires only a − 20°C freezer, a microcentrifuge and a UV–visible spectrophotometer, and every step is diagnosable by eye.</p>