Breast cancer is the most common cancer globally, accounting for 12% of all new annual cancer cases worldwide, according to the World Health Organization, and early detection is a key issue as survival improves when cancer is detected early. NanoNIR will develop a novel fluorescence-based nanotool for the detection and quantification of miR-99a-5p in liquid biopsy samples from breast cancer patients. The nanotool will consist of upconverting nanoparticles (UCNPs) decorated with aptamers and small gold nanoparticles (AuNPs). UCNPs are inorganic nanocrystals that convert near-infrared (NIR) light into shorter wavelength emissions and exhibit narrow emission bandwidths and large anti-Stokes shifts (λex = 980 nm, λem = 540, 655 nm). These photophysical properties make them excellent candidates for fluorescence biosensing allowing for effective sensing with diminished background noise in a complicated detection system. The AuNPs have a large extinction coefficient and a broad UV–Vis absorption band (500 – 580 nm) making them excellent fluorescence quenching agents. The combination of UCNPs (donors) with AuNPs (acceptors) linked through complementary aptamers will result in a fluorescence emission quenching of UCNPs at 540 nm. Following the detection of miR-99a-5p by its complementary aptamer sequence, AuNPs will be displaced from the UCNPs surface restoring the luminescence of UCNPs at 540 nm with an intensity-dependent to miRNA-9a-5p concentration allowing the detection of miR-99a-5p in real samples. Additionally, the luminescence signal at 655 nm could be used for the ratiometric measurements.