Vibrational Sum-Frequency Generation:
Shedding Light on Interface Molecular Properties

Suche




The surface chemistry of interfaces is of critical importance for fundamental issues such as self-assembly, foam stability, catalysis and current topics in nanotechnology. Studies of the interfacial molecular structure under technically relevant conditions are, however, extremely challenging. We use vibrational sum-frequency generation as a powerful optical tool to reveal the molecular composition of interfaces. SFG is broadly applicable to interfaces which are accessible by light. In fact, we are currently investigating not only particle interfaces, but also self-assembled monolayers for printable electronics and water/air interfaces. The latter is part of a project that aims to understand the foam stability from the molecular to the macroscopic scale. Here, proteins, ions or surfactants at water surfaces are of particular interest for life sciences and food industry where e.g. formation, composition and stability of foams are of great importance. In a SFG experiment for surface vibrational spectroscopy a frequency fixed visible and a tunable infrared laser beam are combined at the interface and the intensity of the resulting sum-frequency signal is measured (Figure 1). In the likely case that the IR frequency matches molecular vibration of interfacial molecules, a resonant enhancement of the SFG intensity is observed and can be used to identify molecular fingerprints of surface adsorbed species.

Shematic SHG setup
Fig. 1: Scheme of the SFG spectrometer

Solid/gas-interface: Molecular order of complex self-assembled monolayers Tuning the molecular order and self-assembly of Fluorinated and C60-functionalized phosphonic acids (Figure 2) on alumina substrates is important for organic electronics based on ultra-thin functional layers. SFG spectroscopy offers a fast and nondestructive way to address CF and C60 vibrations and to deduce molecular level information [1-2].

Shematic SHG setup
Fig. 2: Fluorinated and C60-functionalized phosphonic acids (left) and pictographs of pure and mixed SAMs with their corresponding SFG spectra (right)

Liquid/gas-interface: The adsorption of charged proteins at the air/water interface leads to pH dependend charging of the interface (Figure 3). Vibrational SFG spectra reveal electric field induced polar ordering of the adjacent water layer and enable us to determine the isoelectric point of proteins at aqueous interfaces and to deduce the interfacial structure.

Shematic SHG setup
Fig. 3: SFG spectra of Beta-Lactoglobulin at the air/water interface at different pH values. Above the isoelectric point of the protein (pH 5) the interface is negatively charged and vice versa. Consequently the net orientation of water molecules at the interface changes. In the SFG spectra this becomes evident in the inverted interference between water and protein resonances.

Publications:

[1]

M. Novak, C. M. Jäger, A. Rumpel, H. Kropp, W. Peukert,T. Clark, M. Halik, The morphology of integrated self-assembled monolayers and their impact on devices – A computational and experimental approach, Organic Electronics 11 (2010) 1476–1482

[2]

A. Rumpel, M. Novak, J. Walter, B. Braunschweig, M. Halik, and W. Peukert, Tuning the Molecular Order of C60 Functionalized Phosphonic Acid Monolayers, Langmuir (2011), 27, 15016-15023

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