We report a new sensing technique of proteins using the Vroman effect in a microfluidic device. The sensor relies on the competitive nature of protein adsorption onto a surface, directly depending upon protein's adsorption strength. The sensor uses SPR (surface plasmon resonance) for highly sensitive biomolecular interactions detection and the Vroman effect for highly selective detection. A target protein displaces a pre-adsorbed weak-affinity protein; however a pre-adsorbed strong-affinity protein is not displaced by the target protein. In a microfluidic device, we engineer two gold surfaces covered by two known proteins. The sensor allows selective protein detection by being displaced by a target protein on only one of the surfaces. The SPR sensorgrams show that four different human serum proteins, albumin (Alb), immunoglobulin G (IgG), fibrinogen (Fib), and thyroglobulin (Tg) have different adsorption strengths to the surface and the competitive adsorption of individuals controls the exchange sequence. Based on the exchange reaction, we demonstrate that the sensor has a high selectivity for Tg which is a thyroid cancer biomarker. By using the technique, we bypass having to rely on bio-receptors and their attachment to transducers, a process known to be complex and time-consuming.