The present study was designed to develop facile, reproducible and economically viable bacterial sensing assays using functionalized metal/metal oxide nanoparticles/nanoclusters. For this purpose, iron oxide and mixed ferrite nanoparticles functionalized with dopamine were produced and evaluated for their peroxidase mimicking ability. Dopamine capped iron oxide (DOPA) nanoparticles were then utilized to develop antibody-free as well as enzymefree colorimetric assay for the sensing of Escherichia coli (E.coli) and Bacillus subtilis (B.sub) with detection limit as 104 cfu/mL by naked eye and 102 cfu/mL by spectroscopic measurements respectively. The 2-(3-(3,4-dihydroxyphenyl)propanamide)-N,N,Ntrimethylethanaminium chloride capped MnFe2O4 nanoparticles (DPTMA- MnFe2O4) were integrated into the electrochemical sensor as both transducer and recognition element to develop bio-label as well as bio-recognition element free sensing methodology with differentiation limit of bacteria down to 102 cfu/mL. In yet another effort, blue fluorescent polyethylenimine coated silver nanoclusters (PEI-AgNCs) were evaluated for the detection of bacteria in solution. The fluorescent intensity of nanoclusters was gradually decreased in the presence of bacteria resulting in differentiation of bacteria up to limit of 104 cfu/mL.