Plasmonic nanostructures have demonstrated unique capabilities for label-free biosensing through the excitation of localized surface plasmon resonance (LSPR) or the propagation surface plasmon polaritons (SPPs). Carefully designed nanoplasmonic biosensors convert small changes in the local refractive index caused by surface bio-molecular binding into spectral shifts. Here, we investigate a class of plasmonic interferometric biosensors that consist of arrays of circular aperture-groove nanostructures patterned on a gold film for phase-sensitive biomolecular detection. When the whole structure is illuminated by a collimated white light beam, the nanogrooves excite and focus the SPPs to the central aperture, where the SPPs interfere with the light that is directly transmitted through the aperture and modulate the far-field transmission. These biosensors achieve superior performance within a microscale footprint by combining SPR interactions with sensitive interferometric techniques. The phase and amplitude of interfering SPPs in the proposed device can be effectively engineered by structural tuning, providing a flexible and efficient control over the plasmon line shape observed through SPP interference. By careful structural tuning, spectral fringes with high contrast, narrow linewidth, and large amplitude have been experimentally measured and permit sensitive detection of protein surface coverage as low as 0.4 pg/mm2. This sensor resolution compares favorably with commercial prism-based surface plasmon resonance systems (0.1 pg/mm2), but is achieved here using a significantly simpler collinear transmission geometry, a miniaturized sensor footprint (150-150-m2), and a low-cost compact spectrometer, showing great promise to develop fast, inexpensive, compact biomedical devices for personal healthcare. The circular plasmonic interferometric biosensors were also operated in the intensity interrogation mode for high-throughput sensing applications, achieving a record high sensing figure-of-merit (FOM*) of 146 in the visible region, surpassing previous plasmonic biosensors and facilitating ultrasensitive high-throughput detection. Efforts were also made to differentiate surface analyte binding events in complex solutions from bulk refractive index variations due to changes in temperature and concentration of non-specific components in multi-component solutions.