Fig. 1. Application of infection process to ricin detection. Ricin recognizes cell-surface oligosaccharides having β-Gal or β-GalNAc at the non-reducing terminal and internalizes into the host cell. The recognition is species-specific and thus may be applicable to detection. The different sugar-based nano-materials were designed for the present study, i.e., sugar chips for SPR detection and sugar-coated Au nanoparticles for visual detection (A). Ricin adheres specifically to the sugar chip and responds to an SPR system (B and C). The Au nanoparticles coated with sugar are dispersed in water as a colloidal solution, and the colloid color may change in the presence of the toxin (D).

Scheme 1. Structures of synthetic sugar-probes 1–4. Compounds 1, 3, and 4 have a ceramide moiety mimicking natural glycolipids, while compound 2 has an aromatic spacer at the reducing end.

Fig. 2. SPR responses of ricin at different concentrations (0 and 10 pg/mL) to a sugar chip derived from 1. One resonance unit (1 RU) was calculated as being equivalent to ca. 1 pg of the toxin protein binding on a 1 mm² area of a glyco-chip surface.

Fig. 3. Discriminative analyses of ricin and different proteins using SPR. Ricin is specific to only the lactosyl chip and the agglutinin has an affinity for the lactosyl- and galactosyl chips, while other proteins respond little to the sugar chips.

Fig. 4. Color change of the colloidal suspension in the presence of ricin after (a) 1 min, (b) 5 min, (c) 10 min and a schematic illustration of the hetero-aggregation of the nanoparticles due to cross linking. Three hundred microliters of ricin (10 μg/mL) was added to 600 μL of 1-coated Au nanoparticle suspension in this study. In this illustration, the A-subunit in ricin is neglected for facile understanding of the cross-linking interaction which led to aggregation. The average sizes of the aggregates derived from Au nanoparticles and RCA₁₂₀ were estimated by dynamic light scattering (DLS). The data supported the fact that the agglutinins and the sugar-coated Au nanoparticles cross-linked with each other. This should hold true also in the case of the ricin made of the same B-subunits carrying two carbohydrate-binding sites.

Fig. 5. Facile and discriminative detection of ricin with sugar-coated Au nanoparticles derived from 1, 3, and 4. Hundred microliters of ricin (10 μg/mL) was added to 200 μL of the sugar-coated Au suspension in micro-plate plastic wells.