Three-dimensional gold nanostructures fabricated through a novel convective assembly method are treated thermally to obtain a nanoisland morphology. The new structure is proved to be adequate for the detection of bovine growth hormone, by using an immunoassay method based on the localized surface plasmon resonance band of gold. The nanoisland structures are integrated into a microfluidic device and the spectral measurements are carried out by introducing the device directly in the light beam of a ultraviolet-visible spectrophotometer. The principal motivation for this work is the need for a simple and rapid method of detection of hormone levels in milk and milk products.