Ag⁰ Nanoparticles Synthesized in R-phycoerythrin: Change in Bioconjugate Properties upon Ripening of Nanoparticles

Background: Bioconjugates prepared from photoactive molecules and metal nanoparticles are suitable for the development of new optoelectronic devices and for theranostic applications in medicine. Methods: We produced a bioconjugate of R-phycoerythrin (R-PE), a photosynthetic pigment of red algae, with Ag0 nanoparticles (Ag0 NPs) synthesized in its tunnel cavities by mixing aqueous solutions of AgNO3 and R-PE without exogenous reductant. In freshly prepared bioconjugate (Ag0R-PE), the diameter of Ag0 NPs measured in electron microscopic images was 6.5 ± 0.5 nm, being commensurate with the length of R-PE tunnel cavity, and they were found to emit bright visible fluorescence. Result: The initial aggregation temperature (T0) of freshly prepared Ag0R-PE was decreased, compared to that of native R-PE (33 vs. 44°C). Upon the ripening of Ag0 NPs, their fluorescence was almost entirely quenched, and giant surface-enhanced Raman scattering was recorded. The bioconjugate self-assembled into nanorods 25 ± 5 nm long and could withstand heating to 90°C. Further heating (90°C → 100°C) resulted in the formation of filaments which are 0.5 ± 0.2 μm long and spherical aggregates 60 ± 10 nm in diameter. Conclusion: These data can contribute to the development of practical applications for the Ag0R-PE bioconjugate.