A single DNA aptamer functions as a biosensor for ricin. Electronic supplementary information (ESI) available: Fig. S1: Schematic of ricin B chain aptamer selection viasystemic evolution of ligands by exponential enrichment. An aptamer library (∼1014sequences) was exposed twice to unconjugated tosylactivated beads to remove non-specific candidates. Aptamer candidates in the supernatant were subsequently incubated with ricin B chain bound tosylactivated beads for 8 rounds of SELEX. Upon the eighth round of SELEX, aptamers were cloned, sequenced, and examined for redundancy. See DOI: 10.1039/c1an15352h

The use of microorganisms or toxins as weapons of death and fear is not a novel concept; however, the modes by which these agents of bioterrorism are deployed are increasingly clever and insidious. One mechanism by which biothreats are readily disseminated is through a nation's food supply. Ricin, a toxin derived from the castor bean plant, displays a strong thermostability and remains active at acidic and alkaline pHs. Therefore, the CDC has assigned ricin as a category B reagent since it may be easily amendable as a deliberate food biocontaminate. Current tools for ricin detection utilize enzymatic activity, immunointeractions and presence of castor bean DNA. Many of these tools are confounded by complex food matrices, display a limited dynamic range of detection and/or lack specificity. Aptamers, short RNA and single stranded DNA sequences, have increased affinity to their selected receptors, experience little cross-reactivity to other homologous compounds and are currently being sought after as biosensors for bacterial contaminants in food. This paper describes the selection and characterization of a single, dominant aptamer, designated as SSRA1, against the B-chain of ricin. SSRA1 displays one folding conformation that is stable across 4–63 °C (ΔG= −5.05). SSRA1 is able to concentrate at least 30 ng mL−1of ricin B chain from several liquid food matrices and outcompetes a currently available ELISA kit and ricin aptamer. Furthermore, we show detection of 25 ng mL−1of intact ricin A–B complex using SSRA1 combined with surface enhanced Raman scattering technique. Thus, SSRA1 would serve well as pre-analytical tool for processing of ricin from liquid foods to aid current diagnostics as well as a sensor for direct ricin detection. [ABSTRACT FROM AUTHOR]