We have developed a nanoparticle/droplet platform to study aqueous interfaces, taking into account the possibility to measure simultaneously multiple length scales, charge and surface structure. In addition to having the possibility of performing a simultaneous charge and structure measurement, the use of a nanoparticle or droplet dispersion prepared by mixing small volumes of chemicals significantly reduces the challenges posed by impurities that are inherently present in every chemical.
Femtosecond time and Angstrom interfacial length scale information can be obtained with vibrational sum frequency scattering. Information on a larger (~nm) length scale can be obtained with non-resonant second harmonic scattering.
Sum Frequency Scattering
Left: Energy scheme for sum frequency generation. Middle: Schematic sketch of a sum frequency scattering geometry. An IR femtosecond and visible femtosecond or picosecond pulse are overlapped in a sample containing a dispersion of particles or droplets in a liquid or solid phase. From the interface a SF is scattered according to a particular anglular distribution (indicated by the formula). Right: the optical setup.
We developed vibrational sum frequency scattering spectroscopy through several smaller steps:
- We demonstrated the experiment and improved the optical layout by using particles in infrared transparent non-aqueous liquids [Proc. Nat. Acad. Sci 2006, 103, 13310-13314], and more brightly emitting crystallites in solids [Phys. Rev. Lett. 2009, 102 095502, J. Phys. Chem. B (2013),117, 8906].
- By designing a high power sum frequency spectroscopy system with an extended infrared frequency range from 4000 cm-1 down to 500 cm-1 (2.6 – 20 microns) we can probe ionic head groups of amphiphiles at interfaces [App. Phys. B. 2008, 91, 315 PDF, and 3D structures of surface macromolecules, J. Phys. Chem. C (2008), 112, 7531].
Plasma from a focus of the high power amplifier needed to generate broadly tunable femtosecond infrared pulses.
- Theoretical models were developed to interpret the scattering data (see page: Theory).
- We characterized all optical parameters for performing nonlinear light scattering experiments in aqueous solutions [Chem. Phys. Lett. (2011), 512, 76,Phys. Chem. Chem. Phys. (2012), 14, 6826]. Dynamic nonlinear light scattering was also developed [J. Chem. Phys. (2009), 130, 214710-1], which has the potential to probe the water contained within the hydrodynamic radius of a droplet (nm length scale). The detection limit of our instrument is 1 molecule per 27 nm2 of droplet surface of ~ 100 nm particles [Angew. Chem. Int. Ed. (2012), 51, 12938].
Second Harmonic Scattering
Sketch of the second harmonic process and scattering instrument
To obtain information about the molecular directionality of water molecules in the interfacial region (on a nanometer length scale), we have designed a second harmonic scattering instrument and implemented a new optical layout, light source and detection method [Opt. Express, 21, 815, 2013], which allows us to probe interfacial changes on the millisecond time scale; a significant improvement over existing methods.
Calculated scattering pattern for a 200 nm particle containing both chiral (blue) and non-chiral (red) chemical groups.
Simulation of nonlinear light scattering patterns from water droplets in air using the Rayleigh Gans Debye approximation (top) and exact Mie theory (bottom).