A novel 4-(tetrahydro-2-furanmethoxy)-N-octadecyl-1,8-naphthalimide (4-TN1) blue emitting fluorophore was synthesized for solvent polarity probing and protein detection. The effect of various solvents on absorption and fluorescence spectra of 4-TNI was investigated. By comparison with other derivatives with heterocyclic electron-donating groups, 4-TNI had the advantage of higher fluorescence quantum yield. The bathochromic shift observed in absorption and fluorescence spectra of 4-TNI with increasing solvent polarity indicates that the transitions involved are pi --> pi*. The normalized transition energy value E(T)N showed some scattering when plotted versus deltav. According to the quantum mechanics second order perturbation method, the ground and excited state dipole moments of 4-TNI were calculated as 3.91 and 7.12 D, respectively. Density functional calculations were also used to obtain the ground and excited state dipole moments. The result was consistent with the experimental values. Binding of TNI with human serum albumin (HSA) was studied. Fluorescence data revealed that quenching of HSA fluorescence by 4-TNI was due to the formation of a 4-TNI-HSA complex. Hydrogen bonds and hydrophobic interactions are the driving force of complex formation. 4-TNI fluorescence was found to be very sensitive to quenching by HSA. Therefore, a new spectrofluorimetric method for detection of HSA in Tris-HCl buffer solution (pH 7.4) was developed. The linear range of the calibration curve was 0.1-14.2 x 10(-6) M for HSA, with a detection limit (3sigma) of 1.37 x 10(-10) M. The method was applied to determination of total protein in clinical samples of human serum and the results were in good agreement with the data obtained by using Coomassie Brilliant Blue G-250 colorimetry.