Vladimir Bystrov

Temperature-driven phase transformation in self-assembled diphenylalanine peptide nanotubes

Diphenylalanine (FF) peptide nanotubes (PNTs) represent a unique class of self-assembled functional biomaterials owing to a wide range of useful properties including nanostructural variability, mechanical rigidity and chemical stability. In addition, strong piezoelectric activity has recently been observed paving the way to their use as nanoscale sensors and actuators. In this work, we fabricated both horizontal and vertical FF PNTs and examined their optical second harmonic generation and local piezoresponse as a function of temperature. The measurements show a gradual decrease in polarization with increasing temperature accompanied by an irreversible phase transition into another crystalline phase at about 140‐150 ◦ C. The results are corroborated by the molecular dynamic simulations predicting an order‐disorder phase transition into a centrosymmetric (possibly, orthorhombic) phase with antiparallel polarization orientation in neighbouring FF rings. Partial piezoresponse hysteresis indicates incomplete polarization switching due to the high coercive field in FF PNTs. S Online supplementary data available from stacks.iop.org/JPhysD/43/462001/mmedia (Some figures in this article are in colour only in the electronic version)

Theoretical models of conformational transitions and ion conduction in voltage-dependent ion channels: Bioferroelectricity and superionic conduction

Theoretical studies of the structure and function of ion channels are reviewed, with emphasis on the underlying physical phenomena. Nerve and muscle membranes exhibit behavior interpretable as ferroelectric: Studies show that conformational transitions in voltage-dependent ion channels can be understood in terms of transitions from a ferroelectric state to a superionically conducting state. The ferroelectric-superionic transition hypothesis is supported by observations of voltage-gated ion conduction, surface charge, hysteresis, pyroelectricity, piezoelectricity, transition temperatures and Curie-Weiss behavior in these channels. Macroscopically, the opening of the sodium (Na+) channel appears to involve a moving phase boundary traveling from the outer to the inner surface of the axonal membrane. Molecular biology and electrophysiology have provided a partial picture of the microscopic structure of the Na+ channel, demonstrating a pattern of charged residues that suggests a “voltage sensor” role for the f...

Molecular modeling of the piezoelectric effect in the ferroelectric polymer poly(vinylidene fluoride) (PVDF)

In this work, computational molecular modeling and exploration was applied to study the nature of the negative piezoelectric effect in the ferroelectric polymer polyvinylidene fluoride (PVDF), and the results confirmed by actual nanoscale measurements. First principle calculations were employed, using various quantum-chemical methods (QM), including semi-empirical (PM3) and various density functional theory (DFT) approaches, and in addition combined with molecular mechanics (MM) methods in complex joint approaches (QM/MM). Both PVDF molecular chains and a unit cell of crystalline β-phase PVDF were modeled. This computational molecular exploration clearly shows that the nature of the so-called negative piezo-electric effect in the ferroelectric PVDF polymer has a self-consistent quantum nature, and is related to the redistribution of the electron molecular orbitals (wave functions), leading to the shifting of atomic nuclei and reorganization of all total charges to the new, energetically optimal positions, under an applied electrical field. Molecular modeling and first principles calculations show that the piezoelectric coefficient d33 has a negative sign, and its average values lies in the range of d33 ~ −16.6 to −19.2 pC/N (or pm/V) (for dielectric permittivity ε = 5) and in the range of d33 ~ −33.5 to −38.5 pC/N (or pm/V) (for ε = 10), corresponding to known data, and allowing us to explain the reasons for the negative sign of the piezo-response. We found that when a field is applied perpendicular to the PVDF chain length, as polarization increases the chain also stretches, increasing its length and reducing its height. For computed value of ε ~ 5 we obtained a value of d31 ~ +15.5 pC/N with a positive sign. This computational study is corroborated by measured nanoscale data obtained by atomic force and piezo-response force microscopy (AFM/PFM). This study could be useful as a basis for further insights into other organic and molecular ferroelectrics.

Evidence of ferroelectricity and phase transition in pressed diphenylalanine peptide nanotubes

Self-assembled peptide nanotubes (PNT) are unique nanoscale objects having a great potential for a multitude of applications. Strong piezoactivity and polar properties in aromatic dipeptides were recently observed in stand-alone nanotubes using piezoresponse force microscopy and 2nd harmonic generation. In this work, we report macroscopic dielectric and polarization vs. field measurements on pressed PNTs before and after annealing at 150 °C. The results corroborate nanoscale study and present a clear evidence of ferroelectric-like behaviour and phase transition in this technologically important material. The dielectric constant of PNT pellets obeys apparent Curie-Weiss (CW) law with the CW constant C ≈ 230 °C and transition temperature at T ≈ 142 °C.

First principle calculations of molecular polarization switching in P(VDF–TrFE) ferroelectric thin Langmuir–Blodgett films

This paper reports first principle calculations and analysis of the molecular mechanism of the polarization switching in polyvinylidene fluoride and its copolymer with trifluoroethylene (P(VDF–TrFE)) using semi-empirical and ab initio quantum chemical methods based on the HyperChem 7.5 and Gaussian98 programs. The simulations were performed for different copolymer contents in P(VDF–TrFE)—(70:30), (60:40) and pure PVDF. The calculated values of the dipole moment and average polarization of the molecular chains show a clear hysteresis under varying electric field with polarization saturated at ~0.1–0.14 C m−2. The calculated coercive fields (corresponding to the rotation of molecular chains to opposite orientation) are consistent (within an order of magnitude) with experimental data obtained for thin films (Ec = 5–18 MV cm−1). In the absence of external electric fields, the interactions between several molecular chains lead to the orientation of all dipole moments along one direction parallel to the chain plane. This model corresponds to the PVDF layer on the dielectric surface. For the electric field in the perpendicular direction, all chains are rotated along this direction corresponding to the model of conductive substrate.

Nanoscale polarization patterning of ferroelectric Langmuir–Blodgett P(VDF-TrFE) films

This paper reports nanoscale piezoelectric measurements on ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) films prepared by the Langmuir–Blodgett (LB) technique. Polarization patterning, piezoelectric hysteresis and relaxation after poling are studied in this work by piezoresponse force microscopy. High quality P(VDF-TrFE) films with a copolymer content of 30% were fabricated using a Schaefer monolayer transfer setup permitting precise control of the film microstructure. The thickness of the films deposited with 100 transfers was ~64 nm. Local switching resulted in written polarization lines with the lateral size in the range 70–300 nm depending on polarization time. Local hysteresis loop (at a fixed tip position) demonstrated clear ferroelectric switching with the coercive voltage ≈8–10 V that corresponds to a macroscopic switching field (~1.5 MV cm−1) at a ~10 nm depth below the tip. Relatively slow ageing after poling was observed with the characteristic relaxation time of about 1500–2000 s depending on the polarization direction. The obtained results demonstrate that the stable polarization patterns can be created in LB P(VDF-TrFE) films and attest them as suitable candidates for memory and nanotemplate applications.

Computational and experimental studies of size and shape related physical properties of hydroxyapatite nanoparticles

In this work, the properties of hydroxyapatite (HAP) nanoparticles (NPs) have been studied both theoretically and experimentally focusing on computational analysis. HAP is widely used to fabricate implants, for drug delivery, etc. The physical properties of the nanosized HAP particles play an important role in the interaction with cells in the human body and are of great interest. Computer simulation was employed to understand the properties of HAP clusters (Ca(5)(PO(4))(3)OH) including formation energies, dipole moments and polarization (surface charges) by molecular mechanics (MM + , OPLS) and mostly by quantum semi-empirical Hartree-Fock (PM3) methods. The size of the simulated cluster is found to affect its dipole moment, polarization, and, finally, the electron work function- ϕ. These parameters depend on the concentration of hydrogen atoms H (or protons) at the surface. Values of ϕ were experimentally estimated via photoelectron emission measurements. The magnitude of ϕ was demonstrated to have a positive correlation on sizes. The NPs demonstrated a capability to be gathered within conglomerates. This property is confirmed by the calculated data for various sizes. Their sizes have a positive correlation on ϕ by the native particles. The main results show that the distributions of dipole moments have very different space orientations (along the OX, OY and OZ axes, the OZ axis is oriented along the OH column) and change with the addition of hydrogen atoms, which saturate the broken hydrogen bonds. This electrical property of NP leads to different behaviors and motions with consequent aggregation: (1) for the case of NPs having dipole moment oriented preferably perpendicular to the OZ axis (with more hydrogen bonds saturated by added H)-the HAP NP aggregates with hexagonal orientation and forms a wider and more spherical shape (sphere-like or bundle-like); (2) for the case of NPs having dipole moment oriented along the OZ axis (as is the case in the absence of added protons or non-saturated hydrogen bonds)-the NPs firstly rotated and oriented along this axis to form the most elongated cylindrical shape (rod-like).

Piezoelectricity and Ferroelectricity in Biomaterials: From Proteins to Self-assembled Peptide Nanotubes

Piezoelectricity is one of the common ferroelectric material properties, along with pyroelectricity, optical birefringence phenomena, etc. There has been widespread observation of piezoelectric and ferroelectric phenomena in many biological systems and molecules, and these are referred to as biopiezoelectricity and bioferroelectricity. Investigations have been made of these properties in biological and organic macromolecular systems on the nanoscale, by techniques such as atomic force microscopy (AFM) and piezoresponse force microscopy (PFM). This chapter presents a short overview of the main issues of piezoelectricity and ferroelectricity, and their manifestation in organic and biological objects, materials and molecular systems. As a showcase of novel biopiezomaterials, the investigation of diphenylalanine (FF) peptide nanotubes (PNTs) is described in more detail. FF PNTs present a unique class of self-assembled functional biomaterials, owing to a wide range of useful properties, including nanostructural variability, mechanical rigidity and chemical stability. The discovery of strong piezoactivity and polarization in aromatic dipeptides [ACS Nano 4, 610, 2010] opened up a new perspective for their use as nanoactuators, nanomotors and molecular machines as well for possible biomedical applications.

Polarization switching and patterning in self-assembled peptide tubular structures

Self-assembled peptide nanotubes are unique nanoscale objects that have great potential for a multitude of applications, including biosensors, nanotemplates, tissue engineering, biosurfactants, etc. The discovery of strong piezoactivity and polar properties in aromatic dipeptides [A. Kholkin, N. Amdursky, I. Bdikin, E. Gazit, and G. Rosenman, ACS Nano 4, 610 (2010)] opened up a new perspective for their use as biocompatible nanoactuators, nanomotors, and molecular machines. Another, as yet unexplored functional property is the ability to switch polarization and create artificial polarization patterns useful in various electronic and optical applications. In this work, we demonstrate that diphenylalanine peptide nanotubes are indeed electrically switchable if annealed at a temperature of about 150 °C. The new orthorhombic antipolar structure that appears after annealing allows for the existence of a radial polarization component, which is directly probed by piezoresponse force microscopy (PFM) measurements...

Filling carbon nanotubes with magnetic particles

Magnetic carbon nanotube composites were obtained by filling carbon nanotubes with paramagnetic iron oxide particles. Measurements indicate that these functionalized nanotubes are superparamagnetic at room temperature. Details about the production and characterization of these materials are described along with the experimental procedures employed. These magnetic carbon nanotubes have the potential to be used in a wide range of applications, in particular, the production of nanofluids, which can be controlled by appropriate magnetic fields.