Sensing of Iron(III) Ion via Modulation of Redox Potential on Biliverdin Protected Silver Nanosurface Mritunjoy Maity, †,∥ Kaushik Bera, ∥ Uttam Pal, ‡ Krishnendu Khamaru, and Nakul C. Maiti* Structural Biology and Bioinformatics Division, CSIRIndian Institute of Chemical Biology, 4- Raja S. C. Mullick Road, Kolkata 700032, India * S Supporting Information ABSTRACT: Silver nanoparticle shows distinctive electro- chemical 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 nanosur- face (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. KEYWORDS: silver nanoparticle, biliverdin, heavy metal, sensing, surface plasmon resonance, reduction potential, iron ■ INTRODUCTION Nanoparticles are of great scientific interest as they bridge the gap between bulk materials and atomic or molecular structures. 1 Among various metal nanoparticles, silver nanoparticle (AgNP) is one of the most promising one due to its wide range of applications in optoelectronics, renewable energy, and many other active fields of science and technology. 2−4 Therefore, serious efforts are made to make silver nanosurfaces of different shapes and sizes. In addition, surface modification of nano- particles are often carried out to modulate both the physical and chemical aspects of silver atoms on the naosurfaces. One of the most important characteristics of AgNPs is their localized surface plasmon resonance (SPR). The SPR band of silver nanoparticles is caused by the collective oscillations of conduction electrons excited by light and is manifested by an absorption band around 400 nm. 5,6 The absorption wavelengths depend on the size, shape, and refractive index of the surrounding environment of the dispersed nanoparticles. 7−10 The shift of the SPR band also depends on the pH of the nanosuspension and the presence of different types of heavy metal ions. 11 Therefore, any changes in their surface structure and aggregation, the medium’s refractive index, and the presence of other ions may affect the absorption band position and color of the nanoparticle dispersion. 8 Based on these plasmon resonance properties, several colorimetric sensor methods are developed. 12−14 The SPR is also used as an important descriptor to realize the chemistry within and around the metal nanosurfaces. 12,15,16 The colorimetric sensors provide a plat- form through the immediate color change in the presence of analyte and are highly demanding due to their simplicity, rapidity, high sensitivity, and ease of measurement. There are several methods for the detection of Fe(III), Hg(II), and Pb(II) based on chromophores or fluorophores, organic compounds, and polymers. 17−20 Shyamal et al. synthesized an anthracene based fluorescent probe, which exhibited Hg(II)-selective “on− off” type fluorescence switching via ground state complex- ation. 21 Zhao et al. prepared a thiocarbazone derivative based selective fluorescent sensor which underwent desulfurization reaction in the presence of Hg(II) ions. 22 Madhu et al. developed a benzimidazole substituted BODIPY sensor for the Received: July 27, 2018 Accepted: October 4, 2018 Published: October 4, 2018 Article www.acsanm.org Cite This: ACS Appl. Nano Mater. 2018, 1, 6099-6111 © 2018 American Chemical Society 6099 DOI: 10.1021/acsanm.8b01311 ACS Appl. Nano Mater. 2018, 1, 6099−6111 Downloaded via CSIR-INDIAN INST CHEMICAL BIOLOGY on April 16, 2019 at 07:45:04 (UTC). 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