We present the use of a microlens array in combination with a conventional optical microscope set-up for the detection of single nanoparticles (NPs) in fluid medium. Optically transparent dielectric microspheres are patterned in a microfabricated well array template and used as microlenses focusing the light originating from a microscope objective into so-called photonic nanojets that expose the medium within the microfluidic channel. When the NPs pass the nanojets, the detection signal is highly enhanced, Au NPs with size down to 50 nm, fluorescent NPs down to 20 nm in size, as well as biomolecule-linked NPs are clearly observed.