In this work, we demonstrate a high-performance Field effect transistor-based biosensor (BioFET) on Al functionalized beta-Bi2O3 nanofibers for highly selective and rapid detection of serotonin. The beta-Bi2O3 nanofibers were synthesized using electrospinning technique and conductivity was tailored by surface functionalization using Al nanoparticles. Interdigitated silver electrodes deposited over the flexible polyimide (PI) substrate acts as contact terminals and dielectric substrate respectively while Al functionalized beta-Bi2O3 nanofibers acts as n-type semiconductor channel and a thermally evaporated silver electrode beneath the PI tape as a back-gate terminal. Detailed characterization reveals the formation of 0-phase monoclinic, high mobility, 1-D Bi2O3 nanofibers, and Al functionalization was validated using elemental composition studies. The fabricated FET exhibited a trans conductance of 5.23 pS and mobility of 64.4 cm(2) V(-1)s(-1) with an Ion/ Ioff ratio similar to 106. The BioFET exhibited an excellent sensitivity of 51.64 pA/nM towards serotonin over a wide linear detection range of 10 nM(-1) pM and a limit of detection of 0.29 nM. This can be ascribed to the excellent change in surface potential upon interaction of serotonin molecules with the electroactive surface functionalized multidimensional nanoparticle-nanofiber network and the back-gate electrode-based control of the Debye screening length. The fabricated BioFET exhibits excellent sensitivity, stability and reproducibility with a rapid response time of 0.8 s. Excellent selectivity towards serotonin in the presence of interfering analytes like ascorbic acid (AA), uric acid (UA), Glucose, Na+, K+, Dopamine (DA) is observed owing to the formation of selective binding complexes between the aromatic amine and beta-Bi2O3. The real-time applicability of the sensor was successfully tested using urine samples as a test matrix thus establishing it as a promising platform for label free detection of biomolecules in point-of care diagnostics.