A combination of surface plasmon field enhanced fluorescence spectroscopy (SPFS) and isotachophoresis (ITP) technologies for detecting biomolecules is disclosed. It uses ITP to preconcentrate the reactants and accelerate the reaction, and then delivers the reacted sample to an SPFS sensor for detection. A microfluidic device with a T-junction is provided, which has two reservoirs respectively containing a low-mobility trailing electrolyte buffer and a high-mobility leading electrolyte buffer, and a main fluid channel between the two reservoirs, where the SPFS sensor is located on a side channel joined to the main channel. A two-step technique is employed, including a step of sample loading and ITP extraction, and a step of delivery of concentrated sample to the detector chamber by pressure-driven flow. In another embodiment, the SPFS sensor is located on the main fluid channel between the two reservoirs. In a particular example, the technique is used in a DNAzyme assay.