Protein-Based Self-Healing Cu(II)-Metallohydrogels: Efficient Third-Order Nonlinear Optical Materials in Terms of an Intensity-Dependent Refractive Index and Two-Photon Absorption

Two protein-based self-healing Cu(II)-metallohydrogels named BSA-CuA and BSA-CuCl have been synthesized by mixing acetate and chloride salts of Cu(II) distinctly with the protein bovine serum albumin (BSA) in water medium. Experimentally investigated rheological parameters of both synthesized metallohydrogels not only expose the viscoelastic semi solid nature and mechanical toughness but also reveal the self-healing properties of both metallohydrogel materials. Counteranion-directed morphological variations of these metallohydrogels are visualized through field-emission scanning electron microscopic images. The third-order optical nonlinear susceptibility x(3) of these synthesized metallohydrogels has been studied using the Z-scan technique at a wavelength of 550 nm under the femtosecond regime in the excitation intensity range from 66 to 283 GW/cm2. BSA-CuA and BSA-CuCl metallohydrogels exhibit a high value of the positive nonlinear refractive index n2I and two-photon absorption coefficient βeff, which are very important for all-optical switching, optical limiting, and other photonic applications. The polarity possibly associated with the self-healing property of these two synthesized metallohydrogels has been justified through the experimentally measured high value of optical nonlinearity. At 88 GW/cm2 intensity of the excitation beam, the n2I values for BSA-CuA and BSA-CuCl are (14.40 ± 0.16) × 10–7 cm2/GW and (10.99 ± 0.15) × 10–7 cm2/GW, respectively, and at 283 GW/cm2 intensity, the βeff values are (0.0662 ± 0.0002) cm/GW and (0.0540 ± 0.0001) cm/GW, respectively. The (x(3)) of BSA@CuA and BSA@CuCl at 283 GW/cm2 is (1.757 ± 0.018) (esu ×10–14) and (1.560 ± 0.017) (esu ×10–14), respectively.