Devanand K. Shenoy

Supramolecular surface plasmon resonance (SPR) sensors for organophosphorus vapor detection

We synthesized cavitands containing COOH moieties at the upper rim of the cavity and evaluated the interaction between these cavitands and the sarin nerve gas simulant, dimethylmethylphosphonate (DMMP) using surface plasmon resonance (SPR) spectroscopy. The carboxylic acid group on the cavitand is expected to form a hydrogen bond with the PO group of the organophosphorus vapors. Films of these cavitands produced a rapid and reversible SPR response to low concentrations of DMMP. We observed concentration dependent sorption of the DMMP molecule into the COOH containing layer in the ppb to ppm range. Spin-cast films and Langmuir–Blodgett (LB) depositions of bilayer thick cavitand films produced identical SPR shifts upon exposure to DMMP. The sensitivity of the sensor was enhanced via LB deposition of multiple bilayers. Eight-layer-thick films of the COOH cavitand showed sensitivity to DMMP concentrations as low as 16 ppb. The orientation of the COOH group into or out of the cavity did not affect DMMP binding, but strongly influenced the water uptake. In both cases the molecular recognition event responsible for the DMMP uptake has been elucidated via crystal structure analyses of the complexes of COOH in/out cavitands with DMMP. Furthermore, we demonstrated that the COOH containing cavitand had an SPR sensitivity to DMMP higher than the standard fluoropolyol sensing layer, and that the cavitand layer was less prone to water vapor and alcohol interferences. Hence, cavitand layers containing a COOH moiety are promising for use as sensitive and specific sensors for nerve gas agents.

Photo-dimerized monolayer (PDML) versus rubbed polyimide (RPI): a comparison of electro-optic properties

Electro-Optic response characteristics as well as polar anchoring energies of liquid crystal cells prepared using a new photo-alignment technique i.e., photo-dimerized monolayer, have been measured. These data are compared with those for cells that use a rubbed polyimide alignment layer. The two alignment methods yield comparable electro-optic properties showing thereby that the new photo-alignment technique is a viable alternative to the rubbed alignment technique for display applications.

Tuning polar anchoring energy through chemical modification of photodimerized surfaces

A detailed study of the dependence of the polar anchoring energy of liquid crystals on the chemical structure of photoaligning layers is presented. The monolayer alignment layers consist of different chemical groups at the terminus so that the interaction of the alignment layer with the liquid-crystal molecules is systematically varied. The results demonstrate the ability to tune the polar anchoring energy by chemical modification of the alignment layers.

Molecular structure and pretilt control of photodimerized-monolayers (PDML)

We have studied the alignment of nematic liquid crystals on photo-sensitive chemisorbed monolayers. Surface modification and a single UV exposure at normal incidence resulted in photo-dimerized monolayers. A uniform, planar alignment of liquid crystals is realized on these surfaces. Chemical modification of the photo-sensitive chromophores of the monolayer allow fine-tuning of the pretilt. For a given alignment layer, there is a good correlation between the value of the pretilt and the polar properties of the liquid crystal used. Furthermore, the value of the pretilt depends on the chemical functionality at the outermost portion of the photo-alignment layer.

Influence of Annealing on the Dielectric Properties of a Ferroelectric Liquid Crystal

The noise level in a pyroelectric device and therefore the sensitivity is dictated mainly by the dielectric losses in pyroelectric materials. An annealing procedure has been developed that reduces the dielectric loss significantly. The procedure consists of subjecting the ferroelectric liquid crystal sample to an electric field in the isotropic phase and cooling it into the smectic phase at a certain rate. After measurement of the dielectric parameters, the sample is reheated into the isotropic phase and then cooled again at a slower rate into the smectic phase. The entire process is repeated for successively slower cooling rates. We present the results of this annealing treatment and explain the origin of this phenomenon.

Device properties and polar anchoring of nematic molecules at photodimerized surfaces

A photo-dimerized monolayer (PDML) has been recently developed by us for liquid crystal alignment. Electro-optic properties of a single pixel prototype device with the PDML alignment layer have been measured. Data on the contrast, threshold voltage and slope of the response curve are presented which demonstrate the efficacy of the PDML layer for LCD applications. In addition, the polar anchoring energy was measured for PDML layers differing in the chemical composition. The ability to tune this anchoring energy through chemical modification is demonstrated. It is shown that the anchoring strength is comparable to that obtained for liquid crystal cells using the mechanically rubbed polyimide alignment layer.

A surface plasmon resonance investigation of the selective interaction of organic vapors with cavitands

A class of supramolecules, called cavitands, that have been shown to exhibit discotic phases depending on the structure, are shown to have potential for sensing applications. Certain cavitands (macrocyclic compounds based on resorcinarenes) display selectivity of interactions with organic vapors. We use Surface Plasmon Resonance (SPR) to demonstrate this principle. The two cavitands chosen for this study, had both a different size and shape of the pre-organized cavity and were exposed to a variety of aromatic and chlorinated hydrocarbons. QxCav-1 (cavitand) was found to have a marked preference for the aromatic compounds; with the sequence of selectivity, determined by SPR to be nitrobenzene>toluene>benzene. MeCav-2 (cavitand) on the other hand, showed higher selectivity to dichloromethane with respect to aromatic vapors at room temperature. Cavitands therefore represent attractive sensing materials, with potential for application in devices using optical transduction schemes based on a refractive index change.

Effect of annealing on the dielectric parameters of ferroelectric TSiKN65

The ionic conductivity of ferroelectric liquid crystals used for pyroelectric detection is a measure of the noise contribution to the output of the device. This is obtained from a measurement of the dielectric loss of the liquid crystal material. An annealing procedure has been developed that reduces the ionic conductivity and hence the dielectric loss significantly. The procedure consists of subjecting the ferroelectric liquid crystal sample to an electric field in the isotropic phase and cooling it into the smectic phase at a certain rate. After measurement of the dielectric parameters, the sample is reheated into the isotropic phase and then cooled again at a slower rate into the smectic phase. The dielectric measurements are repeated again. The entire process is repeated for successively slower cooling rates. We present the results of the effect of this annealing treatment on the dielectric parameters and explain the origin of this phenomenon.

Photodimerized monolayers for liquid crystal alignment

A non-rubbing process suitable for active matrix LCDs based on a photo-dimerized monolayer (PDML) has been developed. The PDML yields defect free alignment of nematic liquid crystal molecules. The electro-optic characteristics of LCDs fabricated using the photo-dimerized layers are shown to be comparable to those of a rubbed polyimide alignment layer.