One- and two-photon brightness of proteins interacting with gold. A closer look at gold-insulin conjugates

Red luminophores displaying large Stokes shift and high-quantum yields are obtained when gold salts are reacted with proteins under strongly alkaline conditions. Although bovine serum albumin (BSA) has mainly been used as a protein template, other attempts to prepare red luminophores have been proposed using other proteins. Here, we report on the structural characterization and nonlinear optical properties of insulin-gold conjugates. Such conjugates display strong luminescence at ∼670 nm with quantum yields that reach 5.4%. They also display long luminescence lifetimes allowing efficient reactive oxygen species generation, with a quantum yield of ¹O₂ generation reaching 13%. In addition, they exhibit remarkable nonlinear optical properties and in particular a strong two-photon excited fluorescence (TPEF) cross section in the range of 800-1100 nm. By combining experimental studies and time-dependent density functional theory simulations (TD-DFT), we show the formation of insulin-Au(III) conjugates. The interaction of Au(III) ions with the aromatic rings of tyrosine induces charge transfer-like excitation in the visible range. Experimental investigations, together with molecular dynamics simulations of insulin and calculations of electronic properties in a model system, are performed to explore the origin of optical features and the structure-optical property relationship, leading the way to new concepts for nonlinear optics using protein-Au(III) conjugates.