2D SFG Spectroscopy

Biofunctionalized inorganic interfaces play a role in a wide range of emerging materials and analytical technologies. Peptides tethered to gold surfaces have recently been used as the basis for a variety of sensors, rangin from the detection of enzymes and cell metabolites to metal ions. As for all proteins, the sequence and structure of the protein is critical for the function of the interface. Methods capable of assessing both structure and dynamics are highly desireable.

Sum Frequency Generation (SFG) is a well-established surface/interface specific non-linear spectroscopy. The laser pulse sequence of SFG Vibrational Spectroscopy (SFG-VS) consists of an on-resonance IR pulse, followed by a non-resonance visible pulse. A typical SFG pulse sequence is shown in Figure 1B. Since the inherent symmetry of the non-linear response, the SFG signal can only come from non-central symmetric parts of the system. Taking advantage of this property, SFG has been used to investigate many surface/interfacial systems, such as water orientation at the air/water interface.

Figure 1: Pulse sequence and the corresponding spectra of SFG, 2D IR, and 2D SFG.

2D SFG is a perfect match between 2D IR and SFG. In 2D IR, we can learn about the coupling and dynamics of a system. However, the signal of 2D IR is not surface specific, as the isgnal coming from surface molecular layers can easily be buried in bulk background signal (Figure 1A). In order to study this thin molecular layer, we need to incorporate the surface selectivity of SFG into 2D IR. In our previous work, we have demonstrated how the central part of 2D IR, the pulse shaper, can be integrated with SFG. Here, we integrate 2D IR with SFG by adding a non-resonant up-converting visible pulse (Figure 1F).

With 2D SFG, we have already studied two systems: carbon monoxide adsorption on a platinum surface and an alpha helical peptide on a gold surface. In the CO/Pt system, we found that the inhomogeneity of the CO vibration is actually larger than previously thought. This can be caused by surface roughness and/or water hydrogen bonding to CO. This implies that the environment of CO on a Pt surface is more complicated than the simple "atop" configuration assumed in previous studies.

In our study of a surface-bound peptide, we tried to understand how the gold surface affects the second order structure of a peptide. We designed an Alpha Helical Peptide (AHP), which is functionalized with a thio group (-SH) at the end of the peptide to bind to the gold surface. With the help of a heterodyne detection technique, we demonstrate how a 2D SFG spectrum can be directly compared to a 2D IR spectrum (Figure 2). In the paper, we concluded that the AHP still retains most of its helicity, standing upright on the gold surface.

Figure 2: Comparison between 2D IR from a bulk peptide solution and 2D SFG from a surface tethered peptide.

In a waiting time experiment (Figure 3), we also observed a previously predicted, interesting cross peak between the amide I mode on both the ordered helical and disordered random coil tail. The ability to measure the SFG inactive mode (amide I from the random coil) by measuring its cross peak with an SFG active mode shows another possibility of 2D SFG spectroscopy.

Figure 3: Waiting time experiments of both 2D IR and 2D SFG of a peptide in solution and on gold, respectively.

In order to extract the structural information from a 2D SFG spectrum, we also derived the analytical expression of the 2D SFG response functionand developed a two-exciton Hamiltonian model for spectral simulation. Figure 4 shows how different the 2D SFG spectra can be for different orientations between two coupled vibrational modes.

Figure 4: Example of 2D SFG simulation for different molecular orientations.

With a more in-depth understanding of 2D SFG, we can extract both structural and dynamical information of surface/interfacial systems. There are still many more interesting systems to study and we are confident that 2D SFG can provide more insight into surface chemistry.