Josu Martinez-Perdiguero

Bent-core liquid crystals in a route to efficient organic nonlinear optical materials

The design, synthesis and nonlinear optical (NLO) response of some bent compounds, at both the molecular and macroscopic level, are reported. Based on a bent-core NLO-phore with a μβ0 in the range of 390 × 10−48 esu (1907 nm), the second order susceptibility tensor for second harmonic generation is evaluated at excitation wavelengths of 1064 and 1600 nm in a mesogenic material with a SmCP mesophase. The NLO coefficients are in the range 10–100 pm V−1. Thus it is deduced that a molecular design leading to bent-core liquid crystals with large β values is a correct strategy to achieve NLO-materials for bulk second order effects.

Surface plasmon resonance immunoassay for the detection of the TNFα biomarker in human serum

A simple method for the detection of TNF-alpha protein biomarker in human serum with great sensitivity has been developed using a surface plasmon resonance biosensor. Signal amplification based on a sandwich immunoassay including gold nanoparticles was used. Detection in serum proved to be challenging due to high undesirable non-specific binding to the sensor surface stemming from the matrix nature of the sample. After optimization of the assay parameters and, in the case of serum, of a sample dilution buffer to minimize the non-specific binding, very low limits of detection were achieved: 11.6 pg/mL (211 fM) and 54.4 pg/mL (989 fM) for spiked buffer and human serum respectively. The amplification steps with high affinity biotinylated antibodies and streptavidin-fuctionalized nanoparticles greatly enhanced the signal with the advantage of additional specificity. Due to its simplicity and sensitivity, the immunoassay has proved feasible to be used for detection of low concentration biomarkers in real samples.

Laterally Azo-Bridged H-Shaped Ferroelectric Dimesogens for Second-Order Nonlinear Optics: Ferroelectricity and Second Harmonic Generation

Two classes of laterally azo-bridged H-shaped ferroelectric liquid crystals (FLCs), incorporating azobenzene and disperse red 1 (DR-1) chromophores along the FLC polar axes, were synthesized and characterized by polarized light microscopy, differential scanning calorimetry, 2D X-ray diffraction analysis, and electro-optical investigations. They represent the first H-shaped FLC materials exhibiting the ground-state, thermodynamically stable enantiotropic SmC* phase, i.e., ground-state ferroelectricity. Second harmonic generation measurements of one compound incorporating a DR-1 chromophore at the incident wavelength of 1064 nm give a nonlinear coefficient of d(22) = 17 pm/V, the largest nonlinear optics coefficient reported to date for calamitic FLCs. This value enables viable applications of FLCs in nonlinear optics.

Amperometric magnetoimmunoassay for the direct detection of tumor necrosis factor alpha biomarker in human serum

An amperometric immunoassay for the determination of tumor necrosis factor alpha (TNFα) protein biomarker in human serum based on the use of magnetic microbeads (MBs) and disposable screen-printed carbon electrodes (SPCEs) has been developed. The specifically modified microbeads were magnetically captured on the working electrode surface and the amperometric responses were measured at -0.20V (vs. Ag pseudo-reference electrode), upon addition of hydroquinone (HQ) as electron transfer mediator and H2O2 as the enzyme substrate. After a thorough optimization of the assay, extremely low limits of detection were achieved: 2.0pg mL(-1) (36fM) and 5.8pg mL(-1) (105fM) for standard solutions and spiked human serum, respectively. The simplicity, robustness and this clinically interesting LOD proved the developed TNFα immunoassay as a good contender for real clinical application.

The SmA phase of a bent-core V-shaped compound: structure and electric-field response

The smectic A (SmA) phase of a V-shaped mesogen with a molecular bending angle of 60° was studied. According to X-ray measurements, the molecules are arranged with the ‘bow-string’ direction parallel to the smectic layer. This point was confirmed by birefringence analysis and the result is compatible with a non-polar structure. A detailed study of the (001) smectic layer reflection versus temperature was also carried out to determine the smectic translational order parameter. The results indicate that the material presents a high degree of translational order. In addition, the electric field response of the material was studied by means of second harmonic generation. This technique turned out to be highly sensitive to small structural distortions due to the V-shape of the constituent molecules.

Second harmonic generation in laterally azo-bridged H-shaped ferroelectric dimesogens

This contribution describes the analysis of experimental results on second harmonic generation (SHG) in a novel laterally azo-bridged H-shaped ferroelectric liquid crystal (FLC) 2, incorporating a disperse red 1 (DR-1) chromophore along the FLC polar axis, and its two mixtures with a laterally azo-bridged H-shaped FLC host 1, incorporating an azobenzene chromophore. The largest dij coefficient at the fundamental wavelength of λ = 1.6 μm is d22 = 4.5 pm/V, the coefficient along the polar axis that is optimized from the viewpoint of the chemical design of NLO FLC molecules. A linear dependence of d22 on the mole fractions of 2 in mixtures has been found, indicating that i) both 1 and 2 have a similar degree of polar order, and ii) dipole–dipole interaction in the DR-1 chromophore of 2 is not strong due to low chromophore density. This work highlights the importance of synthetic tailorability of FLC materials with extra functionality to second-order nonlinear optics applications.

Real-Time Label-Free Surface Plasmon Resonance Biosensing with Gold Nanohole Arrays Fabricated by Nanoimprint Lithography

In this work we present a surface plasmon resonance sensor based on enhanced optical transmission through sub-wavelength nanohole arrays. This technique is extremely sensitive to changes in the refractive index of the surrounding medium which result in a modulation of the transmitted light. The periodic gold nanohole array sensors were fabricated by high-throughput thermal nanoimprint lithography. Square periodic arrays with sub-wavelength hole diameters were obtained and characterized. Using solutions with known refractive index, the array sensitivities were obtained. Finally, protein absorption was monitored in real-time demonstrating the label-free biosensing capabilities of the fabricated devices.

Local structure of the B4 phase studied by second harmonic generation and X-ray diffraction measurements

Texture observations, second harmonic generation (SHG) and X-ray diffraction measurements have been carried out in the B4 phase of two classical bent-core liquid crystals. Several sample geometries have been used for the different techniques, including a well-aligned cell for the optical studies. An indexation of the whole diffraction diagram was performed on the basis of a three-dimensional lattice. A quantitative analysis of the X-ray data is also reported. The results are compatible with the well-known twist grain boundary (TGB) structure for the B4 phase. However, within the smectic blocks the structure is monoclinic P2, implying an antiferroelectric in-layer arrangement of the molecules instead of the usually accepted ferroelectric. This antiferroelectric packing also explains the low efficiency of the B4 phase for SHG.

Experimental Test of Curvature-Driven Dynamics in the Phase Ordering of a Two Dimensional Liquid Crystal

We study electric field driven deracemization in an achiral liquid crystal through the formation and coarsening of chiral domains. It is proposed that deracemization in this system is a curvature-driven process. We test this prediction using the recently obtained exact result for the distribution of hull-enclosed areas in two-dimensional coarsening with nonconserved scalar order parameter dynamics [J. J. Arenzon et al., Phys. Rev. Lett. 98, 145701 (2007)]. The experimental data are in very good agreement with the theory. We thus demonstrate that deracemization in such bent-core liquid crystals belongs to the Allen-Cahn universality class, and that the exact formula, which gives us the statistics of domain sizes during coarsening, can also be used as a strict test for this dynamic universality class.

Characteristics of a liquid crystal display using optically isotropic phases of bent‐core mesogens

A liquid crystal display (LCD) mode based on non‐oriented samples of bent‐core molecules is presented. The display uses an optically isotropic phase, which is relatively common in bent‐core compounds. The LCD switches between the isotropic phase (off‐state), and a conventional SmCP phase (on‐state) that gives rise to the bright state. The device exhibits good performance: fast response (∼0.1 ms), quite large contrast ratio (>100:1), continuous grey levels and wide viewing angle. These characteristics have been observed down to 80°C. Other remarkable advantages are the robustness of the cells and the simplicity of their preparation, since the material need not be aligned.