Label-free DNA biosensor based on localized surface plasmon resonance coupled with interferometry.

In this report, we developed a new optical biosensor in connection with a gold-deposited porous anodic alumina (PAA) layer chip. In our sensor, we observed that the gold deposition onto the chip surface formed a "caplike" layer on the top of the oxide nanostructures in an orderly fashion, so we called this new surface formation a "gold-capped oxide nanostructure". As a result of its interferometric and localized surface plasmon resonance properties, the relative reflected intensity (RRI) at surface of the chip resulted in an optical pattern that was highly sensitive to the changes in the effective thickness of the biomolecular layer. We demonstrated the method on the detection of picomolar quantities of untagged oligonucleotides and the hybridization with synthetic and PCR-amplified DNA samples. The detection limit of our PAA layer chip was determined as 10 pM synthetic target DNA. The capability of observing both RRI increment and wavelength shift upon biomolecular interactions promises to make our chip widely applicable in various analytical tests.