Colorimetric detection ofRicinus communis Agglutinin 120 using optimally presented carbohydrate-stabilised gold nanoparticles

Abstract

Ricin is a toxic lectin which presents a potential security threat. Its rapid detection is highly desirable. Here we present a colorimetric bioassay based on the aggregation of carbohydrate-stabilised gold nanoparticles which has been used to detect Ricinus communis Agglutinin 120 (RCA₁₂₀) – a ricin surrogate. To achieve a stable and robust sensing system the anchor chain length and the density of the assembled carbohydrates on the gold particle surface has been examined to determine the optimal coverage for maximal aggregation with both RCA₁₂₀ and Concanavalin A (Con A) lectins. Gold nanoparticles were stabilised with either a thiolated galactose derivative (9-mercapto-3,6-diaoxaoctyl-β-D-galactoside) or a thiolated mannose derivative (9-merapto-3,6-dioxaoctyl-α-D-mannoside), for RCA₁₂₀ and Con A respectively, diluted in each instance with varying ratios of a thiolated triethylene glycol derivative. Aggregation was induced with the respective cognate lectin with the reaction monitored by UV-visible spectrophotometry. The results obtained show that a particle surface with at least 7.5% galactose is required for aggregation with RCA₁₂₀ and 6% mannose coverage is required for aggregation with Con A. For each lectin the sensitivity of the assay could be controlled by adjustment of the carbohydrate density on the gold nanoparticles, but with differing results. Maximal aggregation with Con A was achieved with a monolayer consisting of 100% mannose, whereas for RCA₁₂₀ maximal aggregation occurred with 70% coverage of galactose. The limit of detection for RCA₁₂₀ using the optimally presented galactose-stabilised nanoparticles was 9 nM.