Nanoscale wells have been fabricated in a chip to construct a photonic crystal that is used for enhanced immunoassays of a common food-borne toxin, Staphylococcal enterotoxin B (SEB). The nanostructure of the photonic crystal (PC) in the array enhanced the fluorescent signal due to a guided mode resonance. Nanoparticles were used as the solid substrate for attachment of capture antibodies; the particles were then isolated in individual wells of the chip by using an electrophoretic particle entrapment system (EPES). The standard curve generated from the chip consisted of two log-linear regions: the first region with a greater sensitivity, limited by the K_d of the antibody, resembling the 96-well plate ELISA and the other that shows greater than six orders of linearity extending to attomolar concentrations, which is unique to the device we have developed. SEB dissolved in phosphate buffered saline was resolved to levels as low as 35 aM with 10~6-fold better limit of detection than a conventional 96-well-ELISA. Different concentrations of SEB spiked into milk were tested to assess the reliability of the device and the efficacy of the extended log-linear regime in a "real" food matrix. The presence of the milk did not significantly alter the limit of detection. With very low amounts of sample (less than 10 μL) and fast read-out time, the PC-based system shows great promise for the detection of a wide range of target molecules with close to a single molecule level of sensitivity.
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