Immunoassays in a porous silicon interferometric biosensor combined with sensitive signal processing

Orthogonal subspace signal processing algorithms (OSPA) have been developed to extract the optical thickness of a porous silicon layer to within one part in 10 5 from its reflectivity spectrum. This is equivalent to a limit of detection (LOD) of -40 pm change in optical thickness for a 3 μm thick layer, or an LOD of 1/2000 of a monolayer coverage with antibodies, of molecular weight 160 k Daltons, within a layer with pores of 100 nm diameter. A large molecule {horseradish peroxidase (HRP), MWt 40 kDa} has been detected at a concentration of 1 μg/ml by measuring its direct binding to anti-HRP antibodies immobilised within a porous silicon layer. A competitive assay has been demonstrated for the detection of a small molecule {2, 4, 6 trinitrotoluene (TNT), MWt 227 Da} at 10 μg/ml. The projected LODs for HRP and TNT by these assays are 50 ng/ml and 1 μg/ml respectively.