Photophysics of chitosan-iron oxide nanoparticle composites

Abstract

In this study, we aim to elucidate the photophysical behavior of chitosan-iron oxide (Ch:Fe₃O₄) composite nanoparticles using steady-state fluorescence and phosphorescence investigations. The XRD analysis of Ch:Fe₃O₄ composite revealed (220), (311), (400), (511) and (440) difraction planes that agree well with JCPDS file, PDF No. 65-3107. The position and relative intensity of all the diffraction peaks of the Ch:Fe₃O₄ were consistent with the crystalline pattern of the Fe3O4 phase. The involvement of chitosan did not result in a phase change but only broadened difraction peaks. Fluorescence spectra of Ch:Fe3O4 (λexc. = 375 nm) exhibits maxima at 427 nm, 449 nm, 508 nm, and 569 nm. The band positioned at 427 nm correspond to a slightly shifted π–π* transition like that of pure chitosan, suggesting electronic interactions between the nanoparticles and the functional groups of chitosan, while the emission band at 449 nm accord with modified n–π* transitions induced by enhanced spin–orbit coupling introduced by Fe₃O4. The glycosidic bonds of chitosan influenced by the presence of Fe₃O₄ nanoparticles exhibit a band situated at 505 nm. The band situated at 566 nm implies a charge-transfer transition between Fe₃O₄ and chitosan, in which electrons are transferred from the chitosan to the Fe₃O₄ core, leading to lower-energy ligand-to-metal charge transfer transitions. The time-correlated single photon counting (TCSPC) measurements was used to estimate the excited state lifetime of the Ch:Fe₃O₄ composite. The fluorescence decay lifetime curve fit bi-exponential function with two lifetimes and is characterized by values of 1.18 ns and 4.44 ns. An enhanced of value of quantum yield of 0.24% was reached for Ch:Fe3O4, compared with a lower quantum yield of 0.07% for chitosan. Phosphorescence analysis of pure chitosan and the Ch:Fe₃O₄ composite show similar spectral profiles, except that the Ch:Fe₃O₄ maxima are shifted to shorter wavelengths. The deconvolution of the phosphorescence spectra for pure chitosan discloses emission maxima at 402, 425, 508, 547, and 607 nm, while for Ch:Fe₃O₄, the 607 nm peak shifts to 638 nm, going into the red spectral region.The red shift from 425 nm to 442 nm and the presence of the 638 nm band suggest that Fe₃O₄ nanoparticles stabilize the triplet states, introducing additional low-energy states that facilitate phosphorescent emission. Phosphorescence lifetime measurements of the Ch:Fe₃O₄ composite show triplet lifetimes of 1.09 μs and 8.83 μs, while pure Fe₃O₄ displays lifetimes of 1.10 μs and 8.46 μs. These enhancement in the photophysical parameters of Ch:Fe₃O₄, such as singlet and triplet lifetimes, along with the values of fluorescence and phosphorescence quantum yields are expected to support significant advancements in medical and bioscience applications.