Artium Khatchatouriants

Near-field optics: from subwavelength illumination to nanometric shadowing

Near-field optics uniquely addresses problems of x, y and z resolution by spatially confining the effect of a light source to nanometric domains. The problems in using far-field optics (conventional optical imaging through a lens) to achieve nanometric spatial resolution are formidable. Near-field optics serves a bridging role in biology between optical imaging and scanned probe microscopy. The integration of near-field and scanned probe imaging with far-field optics thus holds promise for solving the so-called inverse problem of optical imaging.

Second-harmonic generation of biological interfaces: probing the membrane protein bacteriorhodopsin and imaging membrane potential around GFP molecules at specific sites in neuronal cells of C. elegans

Abstract Second-harmonic generation (SHG) is applied to problems of probing membrane proteins and functionally imaging around selective sites and at single molecules in biological membranes. The membrane protein bacteriorhodopsin (bR) has been shown to have large second-harmonic (SH) intensities that are modulated by protein/retinylidene chromophore interactions. The nonlinear optical properties of model compounds, which simulate these protein chromophore interactions in retinal proteins, are studied in this work by surface SHG and by hyper-Rayleigh scattering. Our results indicate that non-conjugated charges and hydrogen bonding effects have a large effect on the molecular hyperpolarizability of the retinal chromophore. However, mbR, the model system studies suggest that polarizable amino acids strongly affect the vertically excited state of the retinylidene chromophore and appear to play the major role in the observed protein enhancement (>50%) of the retinylidene chromophore molecular hyperpolarizability and associated induced dipole. Furthermore, the data provide insights on emulating these interactions for the design of organic nonlinear optical materials. Our studies have also led to the development of dyes with large SH intensities that can be embedded in cell membranes and can functionally image membrane potential. Single molecules of such dyes in selected single molecular regions of a cell membrane have been detected. SHG from green fluorescent protein (GFP) selectively expressed in concert with a specific protein in neuronal cells in a transgenic form of the worm C. elegans is also reported. The membrane potential around the GFP molecules expressed in these cells has been imaged with SHG in live animals.

GFP Is a Selective Non-Linear Optical Sensor of Electrophysiological Processes in Caenorhabditis elegans

Electrophysiology of the nematode Caenorhabditis elegans has the potential to bridge the wealth of information on the molecular biology and anatomy of this organism with the responses of selected cells and cellular neural networks associated with a behavioral response. In this paper we report that the nonlinear optical phenomenon of second harmonic generation (SHG) can be detected using green fluorescent protein (GFP) chimeras expressed in selected cells of living animals. Alterations in the SHG signal as a result of receptor ligand interactions and mechanical stimulation of the mechanosensory cells indicate that this signal is very sensitive to membrane potential. The results suggest that this approach to membrane potential measurements in C. elegans and in other biological systems could effectively couple data on selective locations within specific cells with functional responses that are associated with behavioral and sensory processes.

Controlled fabrication of silver or gold nanoparticle near-field optical atomic force probes: Enhancement of second-harmonic generation

A simple method is described for producing silver and gold nanoparticles at the tip of an atomic force microscope sensor. It is shown that these nanoparticles can enhance the nonlinear optical phenomenon of second-harmonic generation of a molecular system such as a styryl dye.

Near-field optical and atomic force constraints for superresolution 3D deconvolution in far-field optical microscopy

We demonstrate that near-field optical and atomic force microscopy data can be used for super-resolution 3D image restoration in optical sectioning fluorescence microscopy. A crucial feature in this approach is the full integration of such data sets with digital far-field images. The scanned probe data is used to provide modalities for boundary constraints which define surface optical information and spatial domains of optical alterations in a sample with a spatial precision that has been unachievable in the past. A restoration algorithm that can use such a complex of data for 3D image deconvolution is presented. It uses a Tikhonov- Miller regularization scheme and allows for the imposition of different types of constraints to obtain super-resolution deconvolved images. Performance was tested by using simulated 3D imaging. An example is given of the restoration of a 3D wide field optical image of a biological specimen in the presence of atomic force constraints.