A nanopipette biosensor capable of detecting a small concentration of target molecules within a sample solution using optical detection methods. The biosensor includes a nanopipette that connects a nanocolloid reservoir containing a nanocolloid solution and a sample reservoir containing a sample solution, where the nanopipette is tapered at the end connected to the sample reservoir. The nanocolloid solution includes nanoparticles functionalized with probes specific to miRNA of the target molecules and reporters. During the detection process, the nanocolloids nanoparticles aggregate such that plasmonic hotspots are formed. These hotspots magnify the reporter signals produced when the probes hybridize with target molecules.