To use porous silicon as an optical interferometric biosensor, the pores must be sufficiently large to allow easy ingress of reagents and the layer must also display Fabry‐Perot optical cavity modes. Here the detection antibody is rabbit IgG and the analyte is α‐rabbit IgG conjugated to horseradish peroxidase (HRP). For this model system, the pores should be >50 nm in diameter. Such diameters have been obtained in 0.05 Ω cm n‐type silicon using anodisation followed by chemical etching in ethanolic KOH and also by anodising 0.005 Ω cm p‐type material. The latter also displays optical cavity modes. The silicon surface is oxidised in ozone, silanised using aminopropylmethoxysilanes with one, two or three methoxy groups, and cross linked to IgG using glutaraldehyde. High specific binding is found for mono‐, di‐ and tri‐methoxy silanes, but the lowest non‐specific binding is found for silanisation with the tri‐methoxy silane.