Sensing of Iron(III) Ion via Modulation of Redox Potential on Biliverdin Protected Silver Nanosurface

Silver nanoparticle shows distinctive electrochemical properties, and it has a wide range of applications in most areas of science and technology. In the current work we reported the synthesis of biliverdin protected silver nanosurface (Ag-BV) that could sense Fe­(III) ion via reduction despite of their unfavorable reduction potential in aqueous medium. The addition of Fe­(III) to the Ag-BV suspension resulted an initial red shift in its surface plasmon resonance (SPR) band (420–450 nm) and a color change from straw yellow to deep brown due to the agglomeration of the nanoparticles. Subsequently a redox reaction caused the disappearance of the deep brown color and a significant blue shift occurred in its SPR band (up to 410 nm). The analysis further suggested that the aromatic π system of biliverdin (BV) on the Ag-BV nanosurface could make an electron carrier bridge that favors the transfer of an electron from atomic silver to an empty d orbital of Fe­(III) ion. The reduction of Fe­(III) ion resulted in oxidation of silver nanoparticles and loss of the nanostructure, which were evidenced in transmission electron microscopy analysis. Further investigation revealed that the partial charge on the iron center was ∼+1.16 in the Fe­(II)–biliverdin complex compared to ∼+1.26 in the Fe­(III)–biliverdin complex, suggesting a shift of electron density to the metal ion center. Thus, the biliverdin coated silver nanoparticle could be useful as a specific metal ion detector and a redox modulator for an Fe­(III)/Fe­(II) aqueous system. This might be the first report of its kind as the sensing mechanism involves an exceptional redox type phenomenon instead of mere coagulation of the nanoparticles in the presence of specific ions and produces a different color as an indicator for the ion detection.