Surface Functionalisation of Sensor Chips Toward Localised Immobilization of Anti-IL-6 Antibodies

Interleukin-6 (IL-6) is an important biomarker for inflammation, sepsis, autoimmune disorders, and cancer. Silicon photonic biosensors based on TriPleX waveguide technology offer a promising platform for future label-free IL-6 detection, but their performance strongly depends on the surface chemistry used to immobilize biomolecules. This thesis develops and characterises an immersion-based surface functionalisation strategy for silicon sensor surfaces with the long-term objective of enabling future covalent immobilisation of anti-IL-6 antibodies. The workflow combines sequential solvent cleaning, oxygen-plasma activation, APTES silanisation, and coupling of carboxyl-terminated polyethylene glycol (HOOC–PEG–COOH) through EDC/NHS chemistry to produce a reactive antifouling interface. Fluorescein amine was employed as a model molecule to qualitatively evaluate surface activation, coupling efficiency, droplet behaviour, and fluorescence localisation. The protocol was optimised on silicon test chips, transferred to four-inch silicon wafers, and subsequently applied to previously used LioniX TriPleX photonic chips. Surface modifications were evaluated through water contact-angle measurements, fluorescence microscopy, and optical intensity monitoring using the Delta measurement setup. Method-2 APTES silanisation (freshly prepared APTES) produced more reproducible and homogeneous surfaces than the pre-hydrolysed protocol. The optimised APTES/PEG–COOH architecture demonstrated improved wettability stability and successful transferability to wafer-scale substrates and photonic chips. The results establish a practical and transferable surface-functionalisation strategy that provides the physicochemical basis for future anti-IL-6 antibody immobilisation and label-free silicon photonic biosensing.