Colorimetric response of peptide-functionalized gold nanoparticles to metal ions.

The design of nanostructures with controlled surface chemistry for sensing, catalytic, and electronic applications is an important research challenge. Sensing platforms based on the optical properties of gold nanoparticles in combination with the molecular recognition of ligands, such as alkyl thiols, antibodies, nucleic acids, and proteins, are active areas of research. Detection of targets by functionalized gold particles has been performed by using surface-enhanced raman spectroscopy (SERS), quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR) spectroscopy, electrochemical and potentiometric detection, and colorimetric assays. The gold-nanaoparticle-based colorimetric sensors provide simplicity and excellent detection capability encompassing a variety of targets including metal ions, DNA, bacterial toxins, protein conformations, and enzyme activity. For example, Pb2þ was detected by a color change upon the dispersion of gold nanoparticles functionalized with DNAzyme. Recently, DNA-modified gold nanoparticles were used as a colorimetric sensor in the detection of Hg2þ.[14] The aggregation of the ligand-functionalized gold nanoparticles upon binding its target results in a colorimetric response caused by broadening and shifting of the plasmon resonance peak. This shift in the plasmon resonance frequency is employed in sensing strategies. To date, the majority of the colorimetric gold nanoparticle sensing strategies have used nucleic acids as the sensing element. Here, we demonstrate the potential of peptide-functionalized gold nanoparticles (PFNs) as a colorimetric sensor for metal ions. The PFNs were synthesized in a HEPES buffer using the Flg-A3 peptide (-Asp-Tyr-Lys-Asp-Asp-Asp-Asp-LysPro-Ala-Tyr-Ser-Ser-Gly-Pro-Ala-Pro-Pro-Met-Pro-Pro-Phe-). The synergistic contributions of both the Flg-A3 peptide and HEPES buffer result in the formation of peptidefunctionalized suspension of gold nanoparticles. The overall negative charge of the peptide (pI1⁄4 3.9) prevents aggregation of the particles by repulsive forces. The surface of the gold nanoparticles contains amino acid functional groups that can interact with metal ions. Charged, aromatic, and hydroxyl-