Highly-sensitive fluorescent detection of chemical compounds via photonic nanojet excitation

•A novel photonic nanojet-based sensor system has been developed.•This system is an array of Al2O3 microspheres covered with a polymer layer impregnated with a sensitive probe.•When illuminated, the microstructure generates photonic nanojets that efficiently excite the sensitive layer.•Excitation of the sensitive layer via photonic nanojets provides 4-fold better sensing performance.•The demonstrated sensor system concept is customizable and can be modified to cover many non-trivial detection tasks. A photonic nanojet phenomenon that is strong electromagnetic field local enhancement is widely used for various optical tasks like surface-enhanced Raman scattering or surface-enhanced fluorescence. Here we present a novel implementation of this effect for boosting the sensory response of the fluorescent optical probe through its more efficient pumping via photonic nanojets. For this, a sensitive layer consisting of chitosan polymer impregnated with a fluorescent probe designed for the selective determination of Au3+ ions was deposited onto the surface of an array of dielectric alumina microspheres. The photonic nanojet effect was numerically predicted and experimentally observed in the developed system. It was demonstrated that the change in the refractive index of the matrix caused by polymer swelling has a significant effect on the characteristics of the sensor. Under certain conditions, such as the diameter of the microspheres and the contrast of the refractive index a four-fold decrease in the detection limit of the chemosensor was achieved.