A near-field optical method for probing liquid–liquid interfaces

Interfacial Synthesis: Morphology, Structure, and Properties of Interfacial Formations in Liquid–Liquid Systems

Abstract

The results of studies in the field of interfacial synthesis and interfacial formations in liquid–liquid systems are summarized. The mechanisms of the processes of interfacial synthesis are considered. Data on the self-assembly of nanoparticles, films, and 3D materials are given. The properties of materials of interfacial formations in systems with rare-earth elements and di(2-ethylhexyl)phosphoric acid, obtained both in the presence and absence of local vibrations, are described. It was established that materials obtained in the presence of local vibrations in the interfacial layer have higher density, melting point, and magnetic susceptibility and lower electric conductivity. The effect of force field parameters on the properties of interfacial formations is considered. Practical applications and prospects for research in the field of interfacial formations are discussed.

Excited-state dynamics of organic dyes at liquid/liquid interfaces

Liquid/liquid interfaces play a crucial role in numerous areas of science. However, direct spectroscopic access to this thin (~1 nm) region is not possible with conventional optical methods. After a brief review of the most used techniques to perform interfacial optical spectroscopy, we will focus on time-resolved surface second harmonic generation, which allows the measurement of the excited-state dynamics of probe molecules at interfaces. By comparing these dynamics with those measured in bulk solutions, precious information on the properties of the interfacial region can be obtained. To illustrate this, several studies performed in our group will be presented.

Cantilever tip near-field surface-enhanced Raman imaging of tris(bipyridine)ruthenium(II) on silver nanoparticles-coated substrates

The near-field surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) images of tris(bipyridine)ruthenium(II) adsorbed on a silver nanoparticles-coated substrate were obtained with a scanning near-field optical microscope (SNOM, or near-field scanning optical microscopy, NSOM) using a cantilever tip. In comparison with the most widely used fiber tip for SNOM, the cantilever tip has higher optical throughput and better thermal stability, making it more suitable for detecting the extremely low Raman signal in the near-field spectroscopic investigations. Our preliminary results show that the near-field SERS with the higher spatial resolution can provide richer fingerprint information than the far-field SERS. A comparison of the two types of images shows that there are more SERS than SEF hot spots, and the two types of hot spots do not overlap. More surprisingly, the near-field SERS spectra differ from the far-field SERS spectra obtained on the same sample in the band frequency and relative intensities of some major Raman bands, and some IR-active bands were observed with the near-field mode. These results are explained mainly by the electric field gradient effect and heterogeneous polarization character that operate only in the near-field SERS.

Application of Transient Evanescent Grating Techniques to the Study of Liquid/Liquid Interfaces

Transient grating experiments performed with evanescent fields resulting from total internal reflection at an interface between a polar absorbing solution and an apolar transparent solvent are described. The time evolution of the diffracted intensity was monitored from picosecond to millisecond time scales. The diffracted signal originates essentially from two density phase gratings: one in the absorbing phase induced by thermal expansion and one in the transparent solvent due to electrostriction. A few nanoseconds after excitation, the latter grating is replaced by a thermal grating due to thermal diffusion from the absorbing phase. The speed of sound and the acoustic attenuation measured near the interface are found to be essentially the same as in the bulk solutions. However, after addition of a surfactant in the polar phase, the speed of sound near the interface differs substantially from that in the bulk with the same surfactant concentration. This effect is interpreted in terms of adsorption at the liquid/liquid interface. Other phenomena, which are not observed in bulk experiments, such as acoustic echoes and a fast oscillation of the signal intensity, are also described.

New dimension in nano-imaging: breaking through the diffraction limit with scanning near-field optical microscopy

In recent years scanning near-field optical microscopy (SNOM) has developed into a powerful surface analytical technique for observing specimens with lateral resolution equal to or even better than 100 nm. A large number of applications, from material science to biology, have been reported. In this paper, two different kinds of near-field optical microscopy, aperture and scattering-type SNOM, are reviewed together with recent studies in surface analysis and biology. Here, near-field optical techniques are discussed in comparison with related methods, such as scanning probe and standard optical microscopy, with respect to their specific advantages and fields of application.