In this paper, a detailed study on dielectric, magnetic, thermal, and optical properties of citric acid modified ultrafine superparamagnetic magnetite (Fe3O4) nanoparticles (Cit-USPMNs) was conducted. Water dispersible Cit-USPMNs were synthesized by modified co-precipitation technique. Hydrazine (N2H4) and sodium hydroxide were used as precipitating agents. The size of Cit-USPMNs was tuned by changing the volume of added N2H4. Lemon juice; a source of citric acid was used as a stabilizing agent for this ferrofluid. Phase purity was obtained by annealing at 150 and 250 degrees C for 4 h. The X-ray diffraction (XRD) pattern was used to confirm the formation of pure magnetite nanoparticles (NPs). The size of Cit-USPMNs lies in the range of 11-15 nm, as calculated by Scherrer's formula which was confirmed by TEM images. The prepared Cit-USPMNs were studied for magnetic and dielectric properties by vibrating sample magnetometer (VSM) and four probe LCR meter respectively. The MH-loop shows that the magnetic NPs are superparamagnetic in nature. The dielectric behavior and tangent losses as a function of required frequency range were also studied. Optical properties were studied with the help of UV Vis spectrophotometer. Direct and indirect energy band gaps for Cit-USPMNs were calculated from the UV Vis spectrum. Size, shape, and surface morphology were tested by TEM image. Surface functionality of Cit-USPMNs and formation of the Fe-O bond was verified by FTIR spectroscopy. Thermal properties were studied by thermogravimetric analysis (TGA). (C) 2017 Elsevier B.V. All rights reserved.