Over the last decade, copper-iron selenide semiconductor nanoparticles have attracted significant attention for photothermal cancer therapy applications. [1-4] Owing to the presence of iron-related intermediate band that extends absorption into the infrared region, these nanoparticles efficiently convert infrared light into heat. [5] Conventional high-temperature colloidal synthesis typically yields the CuFeSe₂ phase with a fixed Cu:Fe ratio of 1:1 within the nanocrystals. [1,4,6] Apart from non-uniform Fe³⁺ doping of Cu_2-xSe nanocrystals, compositionally tunable Cu-Fe-Se nanocrystals spanning a broad range of Cu:Fe ratios have not been previously demonstrated. [7] In addition, the incorporation of iron into the nanoparticles introduces magnetic properties that are advantageous for biomedical applications such as magnetic resonance imaging and magnetothermal therapy. [1,4] Copper selenide nanoparticles have also been widely used for photothermal therapy; however, in that case heat generation arises from surface plasmon resonance due to a high concentration of copper vacancies. [8-9] Therefore, varying the Cu and Fe content in Cu-Fe-Se nanocrystals provides an opportunity to directly compare the heat-generation efficiency associated with Cu-based surface plasmon resonance and Fe-based intermediate band.
In the present study, we synthesized novel colloidal Cu-Fe-Se nanoparticles with tunable compositions for the first time via a cation-exchange method, covering a range from Cu-rich (Cu_0.7Fe_0.3Se) to Fe-rich (Cu_0.35Fe_0.65Se) nanocrystals, as determined by ICP-OES analysis. Absorption spectroscopy revealed a gradual transition from the characteristic NIR signature of pure Cu_2-xSe to absorption features attributed to Fe-related intermediate band in Cu-Fe-Se compositions. For the Cu-rich Cu_0.7Fe_0.3Se phase, coexistence of both phenomena was observed.
Following encapsulation in an amphiphilic polymer and dispersion in water, the photothermal properties of the nanoparticles were investigated. Macroscopic measurements were performed by normalizing the optical density at 808 nm (the irradiation wavelength), while microscale heat generation was evaluated using thermal lens spectrometry (TLS). [10] The results indicate that copper-rich Cu-Fe-Se phases exhibit superior photothermal performance compared to other compositions, including Cu_0.55Fe_0.45Se with a Cu:Fe ratio close to 1:1 - the composition normally reported in this material system.