Formation of cashew gum thin films onto silicon wafers or amino-terminated surfaces and the immobilization of Concanavalin A on them

Abstract

The adsorption behavior of cashew gum (CG), a galactose-rich polysaccharide, onto silicon wafers and amino-terminated surface was investigated by means of ellipsometry, atomic force microscopy (AFM) and contact angle measurements. The pH has a strong influence on the adsorption behavior. At pH values lower than 4 amino-terminated surfaces are positively charged and CG behaves as a polyanion, due to the glucuronic acid units along its chain. Under acidic conditions, adsorption was driven by electrostatic forces, following the screening-reduced adsorption regime. The adsorption constant ( K ads) of CG onto amino-terminated surfaces was determined as 2.5 × 10 5 L mol −1. At pH 6 the adsorption of CG onto bare Si wafers was less favored, indicating that silanol groups (Si–OH) and many negative charges (Si–O −) on the substrate repel the negatively charged CG segments. On the other hand, hydrogen bonding between silanol groups and sugar hydroxyl groups might drive the adhesion of CG onto Si wafers. CG layers deposited onto amino-terminated substrates or Si wafers are hydrophilic and smooth. The adsorption of Concanavalin A (Con A), a mannose and glucose specific lectin, onto CG-covered amino-terminated surfaces increased with Con A concentration, indicating cooperativity effect. On the contrary, the adsorption of Con A onto Si wafers could be fitted with the Langmuir adsorption model, yielding K ads (2.0 ± 0.2) × 10 6 L mol −1. This result evidenced that although CG is predominantly composed of galactose, the presence of glucose (14%) as side-chain favors the interaction with Con A.