We present a simple and facile method to synthesize nanoplatelets of 2D Ruddlesden-Popper (RP) perovskites of the type (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 where n = 2. The 2D RP nanoplatelets are synthesized from bulk 2D RP crystals via a reflux pre-treatment mediated-ultrasonication method. The as-synthesized 2D RP nanoplatelets are highly air-stable even after two months of storage under an ambient atmosphere. The bulk 2D RP crystals and 2D RP nanoplatelets are characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Kelvin probe force microscopy, UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence, time correlated single photon counting measurement, etc. A significant blue shift in the ultraviolet-visible absorption spectrum, high photoluminescence in the UV region, and the modified work function of the nanoplatelets indicate a strong quantum confinement effect. The quantum confinement in the nanoplatelets is further confirmed using XPS. A photodetector fabricated using these 2D RP nanoplatelets exhibits a high photodetectivity of 3.09 × 1010 Jones.