Emerging Leaders Program

Ellen Williams Distinguished Postdoctoral Fellowship
Research Topic: Time-Domain Nanoscience

Surface-Enhanced Absorption

Advisor(s):

Prof. Amy Mullin

Research Description

A postdoctoral research position is available at the University of Maryland, College Park through the Ellen Williams Distinguished Postdoctoral Fellowship program in the area of time-resolved studies of single molecules using surface-enhanced IR absorption on nanostructured substrates. This position in the laboratories of Prof. Amy Mullin will focus on the development of time-resolved photothermal detection of molecules tethered to various nanostructures and the use of surface enhanced IR absorption for single molecule studies.

Progress Upate:

Researcher - Dr. Carlos Toro, Amy S. Mullin Group

September 2011
In this project, we have set out to develop experimental tools for time-resolved infrared (IR) studies of single molecule dynamics by combining recent advances in surface-enhanced spectroscopy along with time-resolved photothermal detection. We have focused our attention on the fabrication of nano-patterned substrates of noble metals (such as gold or silver) for surface-enhanced infrared absorption (SEIRA) as well as in the design of an improved photothermal detection (PD) scheme to measure small changes of refractive index in a medium due to the presence of a localized heat point. The optimization of these two components of the experiment will allow us to ultimately merge them into a powerful spectroscopic tool capable of reaching single molecule sensitivity. To accomplish these tasks, we have explored a number of synthetic strategies of metal nanoparticles reported in the literature to systematically address the effect of size and shape of such nanoparticles in the observation of SEIRA signals.[1-3] The structural properties of these metal nanoparticles are known to govern their optical properties and, therefore, it is important to have a precise control over them to be able to tune their surface plasmon resonance (SPR) band in the desired spectral region. Additionally, in order to gain a deeper understanding of the mechanisms governing the surface-enhanced infrared absorption process from a molecular point of view, we have established strategies to perform simulations of the infrared absorption spectra of isolated cysteine as well as its complex with silver, gold and nickel atoms [4]. The use of state-of-the-art quantum chemistry software packages such as Gaussian 09 [5] will allow us to perform these calculations within ab initio and density functional theory schemes.

In parallel with the nano-fabrication component, effort has also been focused on developing expertise in IR-based photo-thermal detection. We have optimized the detection of laser absorption based on initiating a spatially confined transient photothermal event. This has been accomplished by combining two tightly focused counter-rotating beams of light to drive molecules in high rotational states in a device called an optical centrifuge.[6] Collisions with the surrounding medium rapidly distributed the rotational energy and induce a thermal lens at the focal point of the excitation beams. The strength of the thermal lens depends on the strength of the excitation beam within the sample cell, the identity of the surrounding medium and the IR probe wavelength. Initial measurements have targeted N2O molecules as IR absorbers. N2O was chosen for these studies because it has a relatively simple IR spectral profile yet absorbs in the same IR region as cysteine, the molecule of interest for SIERA measurements. This work has laid the groundwork for future photothermal studies by establishing the necessary conditions for reliable measurements with good signal-to-noise ratio. One journal publication has resulted from this work to date.[7]

[1] Kundu, J.; Le, F.; Nordlander, P.; Halas, N. J. Surface enhanced infrared absorption (SEIRA) spectroscopy on nanoshell aggregate substrates. Chem. Phys. Lett. 2008, 452, 115-119.
[2] Majzik, A.; Patakfalvi, R.; Hornok, V.; Dekany, I. Growing and stability of gold nanoparticles and their functionalization by cisteine. Gold Bull. 2009, 42, 113-123.
[3] Williams, S. M.; Stafford, A. D.; Rodriguez, K. R.; Rogers, T. M.; Coe, J. V. Accesing surface plasmons with Ni microarrays for enhanced IR absorption by monolayers. J. Phys. Chem. B 2003, 107, 11871-11879.
[4] Perry, D. A.; Cordova, J. S.; Spencer, W. D.; Smith, L. G.; Biris, A. S. SERS, SEIRA, TPD, and DFT study of cyanobenzoic acid isomer film growth on silver nanostructured films and powder. J. Phys. Chem. C 2010, 114, 14953–14961.
[5] Frisch, M. J. et al., Gaussian 09, Gaussian Inc., Wallingford, CT, 2009.
[6] Yuan, L.; Teitelbaum, S. W.; Robinson, A.; Mullin, A. S. Dynamics of molecules in extreme rotational states. Proc. Nat. Acad. Sci. 2011, 108, 6872-6877.
[7] Yuan, L.; Toro, C.; Bell, M.; Mullin, A. S. Spectroscopy of molecules in very high rotational states using an optical centrifuge. Faraday Discuss. 2011, 150, 101-111.