Defect related anomalous mobility of small polarons in dielectric oxides at the example of congruent lithium niobate

Polarons play a major role in the description of optical, electrical and dielectrical properties of several ferroelectric oxides. The motion of those particles occurs by elementary hops among the material lattice sites. In order to compute macroscopic transport parameters such as charge mobility, normal (i.e. Fickian) diffusion laws are generally assumed. In this paper we show that when defect states able to trap the polarons for long times are considered, significant deviations from the normal diffusion behaviour arise. As an example of this behavior, we consider here the case of lithium niobate (LN). This can be considered as a prototypical system, having a rich landscape of interacting polaron types and for which a significant wealth of information is available in literature. Our analysis considers the case of a stoichiometric, defect-free lithium niobate containing a certain concentration of small electron polarons hopping on regular Nb sites, and compares it to the material in congruent composition, which is generally found in real-life applications and which is characterized by a large concentration of antisite NbLi defects. While in the first case the charge carriers are free polarons hopping on a regular Nb sublattice, in the second case a fraction of polarons is trapped on antisite defects. Thus, in the congruent material, a range of different hopping possibilities arises, depending on the type of starting and destination sites. We develop a formalism encompassing all these microscopic processes in the framework of a switching diffusion model which can be well approximated by a mobile-immobile transport model providing explicit expressions for the polaron mobility. Finally, starting from the Marcus-Holstein's model for the polaron hopping frequency we verify by means of a Monte Carlo approach the diffusion/mobility of the different polarons species showing that, while free polarons obey the laws for normal diffusion as expected, bound polarons follow an anomalous diffusion behaviour and that in the case of the congruent crystal where mixed free and bound polaron transport is involved, our expressions indeed provide a satisfactory description.

Genetic Material Manipulation and Modification by Optical Trapping and Nanosurgery-A Perspective

Light can be employed as a tool to alter and manipulate matter in many ways. An example has been the implementation of optical trapping, the so called optical tweezers, in which light can hold and move small objects with 3D control. Of interest for the Life Sciences and Biotechnology is the fact that biological objects in the size range from tens of nanometers to hundreds of microns can be precisely manipulated through this technology. In particular, it has been shown possible to optically trap and move genetic material (DNA and chromatin) using optical tweezers. Also, these biological entities can be severed, rearranged and reconstructed by the combined use of laser scissors and optical tweezers. In this review, the background, current state and future possibilities of optical tweezers and laser scissors to manipulate, rearrange and alter genetic material (DNA, chromatin and chromosomes) will be presented. Sources of undesirable effects by the optical procedure and measures to avoid them will be discussed. In addition, first tentative approaches at cellular-level genetic and organelle surgery, in which genetic material or DNA-carrying organelles are extracted out or introduced into cells, will be presented.

Time evolution of photovoltaic fields generated by arbitrary light patterns in z-cut LiNbO3:Fe: application to optoelectronic nanoparticle manipulation

The bulk photovoltaic effect is a phenomenon that generates high electric fields in certain ferroelectric crystals under illumination, as iron doped lithium niobate (LiNbO₃:Fe). A variety of innovative applications of these electric fields require using of z-cut plates, in which the polar axis is normal to the larger crystal faces. However, the kinetics and distribution of the photovoltaic fields in this configuration have not been investigated in depth. In this work, the photovoltaic charge transport of z-cut configuration is studied through a complete finite element analysis. Light patterns commonly used for particle trapping applications are used to study the temporal evolution of the electric field developed by the crystal. Results show that photovoltaic currents perpendicular to the optical axis play a key role in the development of the final charge distribution. Moreover, there is a relevant, localized charge accumulation inside the crystal which is required to reach the saturation electric field in the whole illuminated volume. The role of crystal thickness and light absorption are analysed. It has been found that they are important to determine the time evolution of the process. The simulations are expected to be a key tool to analyse and improve photovoltaic optoelectronic tweezers.

Advancing Versatile Ferroelectric Materials Toward Biomedical Applications

Ferroelectric materials (FEMs), possessing piezoelectric, pyroelectric, inverse piezoelectric, nonlinear optic, ferroelectric-photovoltaic, and many other properties, are attracting increasing attention in the field of biomedicine in recent years. Because of their versatile ability of interacting with force, heat, electricity, and light to generate electrical, mechanical, and optical signals, FEMs are demonstrating their unique advantages for biosensing, acoustics tweezer, bioimaging, therapeutics, tissue engineering, as well as stimulating biological functions. This review summarizes the current-available FEMs and their state-of-the-art fabrication techniques, as well as provides an overview of FEMs-based applications in the field of biomedicine. Challenges and prospects for future development of FEMs for biomedical applications are also outlined.

Peroxoniobium inhibits leukemia cell growth

A new class of polyoxoniobate complex has been synthesized and characterized as a novel anticancer agent for photodynamic therapy. The complex inhibits the growth of chronic myelogenous leukemia cells with an IC50 value of 30 μM, in the dark. However, upon exposure to light (365 nm) there is a fivefold increase in the cytotoxic activity. Light radiation activate the complex with the formation of radical species capable of interacting with DNA according to our experimental and theoretical data.

Recent Achievements on Photovoltaic Optoelectronic Tweezers Based on Lithium Niobate

This review presents an up-dated summary of the fundamentals and applications of optoelectronic photovoltaic tweezers for trapping and manipulation of nano-objects on the surface of lithium niobate crystals. It extends the contents of previous reviews to cover new topics and developments which have emerged in recent years and are marking the trends for future research. Regarding the theoretical description of photovoltaic tweezers, detailed simulations of the electrophoretic and dielectrophoretic forces acting on different crystal configurations are discussed in relation to the structure of the obtained trapping patterns. As for the experimental work, we will pay attention to the manipulation and patterning of micro-and nanoparticles that has experimented an outstanding progress and relevant applications have been reported. An additional focus is now laid on recent work about micro-droplets, which is a central topic in microfluidics and optofluidics. New developments in biology and biomedicine also constitute a relevant part of the review. Finally, some topics partially related with photovoltaic tweezers and a discussion on future prospects and challenges are included.

Perturbation of physiological systems by nanoparticles

Nanotechnology is having a tremendous impact on our society. However, societal concerns about human safety under nanoparticle exposure may derail the broad application of this promising technology. Nanoparticles may enter the human body via various routes, including respiratory pathways, the digestive tract, skin contact, intravenous injection, and implantation. After absorption, nanoparticles are carried to distal organs by the bloodstream and the lymphatic system. During this process, they interact with biological molecules and perturb physiological systems. Although some ingested or absorbed nanoparticles are eliminated, others remain in the body for a long time. The human body is composed of multiple systems that work together to maintain physiological homeostasis. The unexpected invasion of these systems by nanoparticles disturbs normal cell signaling, impairs cell and organ functions, and may even cause pathological disorders. This review examines the comprehensive health risks of exposure to nanoparticles by discussing how nanoparticles perturb various physiological systems as revealed by animal studies. The potential toxicity of nanoparticles to each physiological system and the implications of disrupting the balance among systems are emphasized.

Photovoltaic versus optical tweezers

The operation of photovoltaic (PV) tweezers, using the evanescent light-induced PV fields to trap and pattern nano- and micro-meter particles on a LiNbO(3) crystal surface, is discussed. The case of a periodic light pattern is addressed in detail, including the role of particle shape and the modulation index of the light pattern. The use of a single Gaussian light beam is also considered. Illustrative experiments for the two situations are presented. The performance of such PV tweezers in comparison to the best established case of optical tweezers, using optical forces, is considered. Differential features between the two trapping approaches are remarked.