Dr. Ofer Kedem
Non-equilibrium Energy Research Center (NERC)
Department of Chemistry
Evanston, IL 60208
E-mail: kedem.ofer [at] northwestern.edu
My research focuses on the interaction between plasmonic nanoparticles and both dielectric and optically active materials (such as fluorescent species). As a model system, I use a system previously developed in our group, of thermally evaporated and annealed gold nano island arrays; this procedure produces random arrays of quite polydisperse islands, though the average island size can be tuned by changing the amount of gold evaporated. Though this is a non-ordered system, the ease of fabrication allows me to easily produce dozens of essentially identical transducers, making possible systematic research. For these reasons, this system is central to work in our group, though other systems are in use (see the pages of the other group members for examples).
During my M.Sc., I studied the response of gold nanoparticle films to adsorption of dielectric layers of varying thickness, with immediate implication for their use in sensing, an activity pursued by others in our group, and elsewhere. See details below, and in my 2011 ACS Nano paper.
Later, I examined the differences between two methods of measurement – transmission and reflection spectra. I found that when dealing with bulk changes in refractive index (changing the solvent the plasmonic substrate is immersed in), measuring in the reflection regime affords higher sensitivity, compared to the transmission regime. This is so for thick films as well (40 nm thick adsorbate layer), but not necessarily so for thinner layers. See the bulk changes work in my 2011 J. Phys. Chem. Lett. paper.
Currently, I am working on the interaction between these plasmonic nanoparticle arrays and fluorescent molecules.
Localized surface plasmon resonance for sensing:
Nanostructured metal (e.g., gold) surfaces support localized surface plasmon resonance (LSPR) excitation, exhibited as an optical extinction band in the visible range. The LSPR band (intensity and wavelength) is sensitive to changes in the refractive index near the metal nanostructures, resulting in changes in the extinction spectrum upon molecular binding. In biosensing, the need for a recognition interface comprising bulky receptors raises the issue of the decay length of the plasmon evanescent field. The farther away the layer of analyte is, the weaker the response will be. The decay length needs to be tailored to the application, considering the size of the interface and analyte layers.
With this motivation, I set out to determine the decay length of metal nanoisland systems using a layer-by-layer (LbL) scheme of alternate adsorption of oppositely-charged polyelectrolyte layers. The method provides regular film growth and control over the thickness on the nanometer scale. I used this approach to determining the decay length of various Au island systems (differing in size and morphology), in order to gain understanding of factors affecting the decay length and to optimize the response of localized plasmon transducers.
Often in designing LSPR-based sensors, there is excess focus on a value called the “refractive index sensitivity”, which quantifies the change in LSPR spectrum for changes of bulk refractive index. This parameters is only part of the picture, as biosensors employ thin layers, not bulk changes, and so the distance dependent decay of the plasmon evanescent field must be taken into account. This work highlights the importance of this parameter – we found that the both the decay length and the refractive index sensitivity increase with particle size. Although large particles have high refractive index sensitivities, they also have large sensing volumes; when an analyte is adsorbed, it only changes the refractive index in a small portion of the sensing volume, leading to a relatively weak signal. Our study demonstrated that for thin layers (few nm), it pays to use small particles - thanks to their small sensing volumes, they actually produce larger responses.
- Curriculum Vitae
B.Sc. in Chemistry (summa cum laude) from Tel Aviv University (2007)
M.Sc. in Chemistry, Weizmann Institute of Science (2009), advisor: Prof. Israel Rubinstein, title of thesis: “Distance Sensitivity in Localized Surface Plasmon Resonance (LSPR) Systems”
Ph.D., Weizmann Institute of Science (2013), advisor: Prof. Israel Rubinstein, title of thesis: “Interaction of Localized Surface Plasmon Resonance Transducers with Dielectric and Fluorescent Materials”
* Kedem, O., Tesler, A. B., Vaskevich, A., Rubinstein, I. "Sensitivity and Optimization of Localized Surface Plasmon Resonance Transducers", ACS Nano, 2011, 5, 748-760.
* Kedem, O., Vaskevich, A., Rubinstein, I. "Improved Sensitivity of Localized Surface Plasmon Resonance Transducers Using Reflection Measurements", J. Phys. Chem. Lett., 2011, 2, 1223–1226.
* Kedem, O., Vaskevich, A., Rubinstein, I. "Comparative Assessment of the Sensitivity of Localized Surface Plasmon Resonance Transducers and Interference-Based Fabry-Pérot Transducers", invited contribution to Special Issue: “Plasmonic Sensors”, Ann. Phys. (Berlin), 2012, 524, 713-722.
* Kedem, O., Sannomiya, T., Vaskevich, A., Rubinstein, I. "Oscillatory Behavior of the Long-Range Response of Localized Surface Plasmon Resonance Transducers", J. Phys. Chem. C., 2012, 116, 26865-26873.
* Chaikin, Y., Kedem, O., Raz, J., Vaskevich, A., Rubinstein I. "Stabilization of Metal Nanoparticle Films on Glass Surfaces Using Ultrathin Silica Coating", Anal. Chem. ASAP
Au nanoislands of different sizes can be
obtained by evaporating different thicknesses
Photographs of (from left) - as-evaporated slide,
slide after annealing, and a slide coated with
21 nm of polyelectrolyte multilayer
Scheme of polyelectrolyte adsorption scheme
HRSEM image of 100 nm diameter gold island film
with 42 nm thick polyelectrolyte coating