**2. Results and Discussion**

OWLS immunosensors were devised both in the direct (immobilised antibody) and competitive (immobilised antigen conjugate)-based formats. Immobilisation of the protein reactants has been carried out by several chemical routes utilising hydroxyl groups on the sensor surface converted into epoxy or amino functionalities, further reacted with appropriate chemical reagents for covalent immobilisation of the protein immunoreagents (Figure 1).

**Figure 1.** Functionalisation of the sensor surface with epoxy, amino, or carboxyl functional groups. GOPS: γ-glycidoxypropyltrimethoxysilane; APTS: (3-aminopropyl) triethoxysilane; GA: glutaraldehyde; SA: succinic anhydride; EDC: 1-ethyl-3-(3 dimethylaminopropyl) carbodiimide; NHS: N-hydroxysuccinimide.
