Tunable Surface Assembly of Gold Nanorods for Biosensor Applications Ferhan Abdul Rahim Division of Bioengineering School of Chemical and Biomedical Engineering 20 September 2010

Outline Introduction to Gold Nanorods Biological Applications Gold Nanorod Surface Assemblies Tuning of Assemblies Conclusion

Introduction to Gold Nanorods Gold nanorods have shape and size-dependent optical properties originating from anisotropic shape and tunable aspect ratio. Nikoobakht et al. Chem Mater. 2003, 15, 1957-1962. Aspect Ratio (AR) = L/W L W

Introduction to Gold Nanorods Under electromagnetic field of light, the conduction band electrons undergo a collective coherent oscillation in resonance with the frequency of the incident light. This is known as the localized surface plasmon resonance (LSPR). Due to their anisotropic shape, 2 extinction peaks can be observed from gold nanorods. Huang et al. Adv Mater. 2009, 21, 4880-4910.

Introduction to Gold Nanorods They are commonly synthesized via the seed-mediated method in the presence of cetyltrimethylammonium bromide (CTAB) surfactant. In most cases, surfactant needs to be removed or exchanged prior to utilization of gold nanorods. Murphy et al. Adv Mater. 2002, 14, 80-82.

Biological Applications Nanorod surface can be incorporated with biomolecules such as antibodies and oligonucleotides. This allows site-specific targeting in vivo and biosensing of biomolecules of interest.

Biological Applications Biosensing Specific antibody-antigen binding or sequence-specific hybridization events can be detected via spectral extinction peak shifts.

Gold Nanorod Surface Assemblies To develop a portable and reusable diagnostic device, it is necessary to assemble nanorods on surfaces of appropriate substrates. Tuning the density of assembled nanorods ensures optimum accessibility of a variety of target proteins. A full-immersion method based on electrostatic interactions was adopted. Glass substrates coated with APTS-PSS were immersed in gold nanorod solution for 1 h at room temperature. Ferhan et al. Langmuir. 2010, 26, 12433-12442.

Tuning of Assemblies Ferhan et al. Langmuir. 2010, 26, 12433-12442. Correlation between UV-Vis absorbance and ionic strength. Corresponding UV-Vis spectra (inset). Variation in [NaCl] lead to different UV-Vis absorbance peak intensities originating from assembled gold nanorods.

No NaCl 40 mM 120 mM 160 mM 80 mM Tuning of Assemblies SEM characterization confirmed surface density variation as a function of NaCl concentration. SEM images of gold nanorod assembly on silicon surface from a solution with moderate CTAB concentration. Ferhan et al. Langmuir. 2010, 26, 12433-12442.

Tuning of Assemblies Correlation between UV-Vis absorbance and ionic strength. Ferhan et al. Langmuir. 2010, 26, 12433-12442. Variation in both [NaCl] and [CTAB] lead to different UV-Vis absorbance peak intensities originating from assembled gold nanorods.

Tuning of Assemblies Zeta-potential measurements show variation of surface potential around gold nanorods with increasing ionic strength. Ferhan et al. Langmuir. 2010, 26, 12433-12442.

Tuning of Assemblies Mechanism A two-tiered shielding effect exists in nanorod solution at slightly higher CTAB concentration. Such shielding effect regulates inter-nanorod and nanorod-substrate interactions. Ferhan et al. Langmuir. 2010, 26, 12433-12442.

Conclusion Gold nanorods were electrostatically assembled onto modified glass surfaces via full-immersion. We have achieved gold nanorod assembly without complete removal or exchange of surfactant. Such assembly can be tunable through variation of both ionic strength and surfactant concentration. There exists a two-tiered shielding effect which regulates inter-nanorod and nanorod-substrate interactions.

References Murphy et al. ‘Controlling the aspect ratio of inorganic nanorods and nanowires’, Adv Mater. 2002, 14, 80-82. Huang et al. ‘Gold nanorods: From synthesis and properties to biological and biomedical applications’, Adv Mater. 2009, 21, 4880-4910. Nikoobakht et al. ‘Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method’, Chem Mater. 2003, 15, 1957-1962. Marinakos et al. ‘Plasmonic detection of a model analyte in serum by a gold nanorod sensor’, Anal Chem. 2007, 79, 5279-5283. Nusz et al. ‘Rational selection of gold nanorod geometry for label-free plasmonic biosensors’, ACS Nano. 2009, 3, 795-806. Ferhan et al. ‘Influence of ionic strength and surfactant concentration on electrostatic surfacial assembly of cetyltrimethylammonium bromide-capped gold nanorods on fully-immersed glass’, Langmuir. 2010, 26, 12433-12442. 15

Thank You 16