Ultrasensitive Nanoporous Interferornetric Sensor for Label-Free Detection of Gold(III) Ions

In this study, we present an ultrasensitive sensor based on nanoporous anodic alumina (NAA) for detection of gold(III) ions (Au3+ ions) using reflectometric interference spectroscopy (RIfS). Nanoporous anodic alumina, prepared by two-step electrochemical anodization, was fiinctionalized with 3-mercaptopropyl-tirethoxysilane (MPTES) in order to selectively detect Au3+ ions. Thus prepared, MPTES-NAA sensors were exposed to different concentrations of Au3+ ions ranging from 0.1 to 750 mu M and the changes in the effective optical thickness (Delta EOT) were monitored in real-time. The linear range of these Au3+ sensors was from 0.1 to 80 mu M, with a lower detection limit of 0.1 mu M of Au3+ ions. Furthermore, the specificity of these MPTES-NAA sensors was validated by sequential exposure to 40 mu M solutions of Fe-3+,Fe- Mg2+, Co2+, Cu2+, Ni2+, Ag+, and Pb2+, resulting in negligible changes in EOT as compared to the same concentration of Au3+ ions. Detection of Au3+ ions in complex and environmentally and biologically relevant solvents such as tap water and phosphate buffer solution (PBS) was also successfully carried out in order to demonstrate the real-life application of these sensors. Finally, the binding isotherm for Au3+ ions and thiol (SH) group of MPTES-NAA system was determined by fitting the changes in EOT to Freundlich and Langmuir isotherm models.