Nano-objects (8) with feature size smaller than the classical Abbe's diffraction limit can be optically detected via a microlens (4) array configured in a miniaturized microfluidic device. A hydrophilic microwell (3) array in a hydrophobic layer (2) is firstly fabricated by standard microfabrication processes. Dielectric microspheres (4) with high refractive index are used as microlenses and patterned inside the hydrophilic microwells (3) resulting in a microlens array format with high uniformity over a large surface area. These microlenses (4) focus the illuminating light into a region that is referred to as a photonic nanojet (6), which is characterized by a high optical intensity and sub-diffraction transverse dimension. The microlens array is integrated with a microfluidic channel (9), where a fluid medium (7) with nano-objects (8) is present. When the nano-objects (8) pass through the photonic nanojet (6), the retroreflected light from the microsphere-nano-object ensemble is highly enhanced and measured by an optical signal detection component (11) (conventional optical microscope, etc.), and the enhancement of the retroreflected light is very sensitive to the size of the nano-objects (8). Therefore, not only the existence of the nano-objects (8) can be qualitatively monitored, but also the size of these nano-objects can be quantitatively determined by the proposed technique.