There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

The widespread use of molecular machines in biology has long suggested that great rewards could come from bridging the gap between synthetic molecular systems and the machines of the macroscopic world. In the last two decades, it has proved possible to design synthetic molecular systems with architectures where triggered large amplitude positional changes of submolecular components occur. Perhaps the best way to appreciate the technological potential of controlled molecular-level motion is to recognize that nanomotors and molecular-level machines lie at the heart of every significant biological process. Over billions of years of evolution, nature has not repeatedly chosen this solution for performing complex tasks without good reason. When mankind learns how to build artificial structures that can control and exploit molecular level motion and interface their effects directly with other molecular-level substructures and the outside world, it will potentially impact on every aspect of functional molecule and materials design. An improved understanding of physics and biology will surely follow.

The first steps on the long path to the invention of artificial molecular machines were arguably taken in 1827 when the Scottish botanist Robert Brown observed the haphazard motion of tiny particles under his microscope.1,2 The explanation for Brownian motion, that it is caused by bombardment of the particles by molecules as a consequence of the kinetic theory of matter, was later provided by Einstein, followed by experimental verification by Perrin.3,4 The random thermal motion of molecules and its implications for the laws of thermodynamics in turn inspired Gedankenexperiments (“thought experiments”) that explored the interplay (and apparent paradoxes) of Brownian motion and the Second Law of Thermodynamics. Richard Feynman’s famous 1959 lecture “There’s plenty of room at the bottom” outlined some of the promise that manmade molecular machines might hold.5,6 However, Feynman’s talk came at a time before chemists had the necessary synthetic and analytical tools to make molecular machines. While interest among synthetic chemists began to grow in the 1970s and 1980s, progress accelerated in the 1990s, particularly with the invention of methods to make mechanically interlocked molecular systems (catenanes and rotaxanes) and control and switch the relative positions of their components.7−24

Here, we review triggered large-amplitude motions in molecular structures and the changes in properties these can produce. We concentrate on conformational and configurational changes in wholly covalently bonded molecules and on catenanes and rotaxanes in which switching is brought about by various stimuli (light, electrochemistry, pH, heat, solvent polarity, cation or anion binding, allosteric effects, temperature, reversible covalent bond formation, etc.). Finally, we discuss the latest generations of sophisticated synthetic molecular machine systems in which the controlled motion of subcomponents is used to perform complex tasks, paving the way to applications and the realization of a new era of “molecular nanotechnology”.

1.1. The Language Used To Describe Molecular Machines
Terminology needs to be properly and appropriately defined and these meanings used consistently to effectively convey scientific concepts. Nowhere is the need for accurate scientific language more apparent than in the field of molecular machines. Much of the terminology used to describe molecular-level machines has its origins in observations made by biologists and physicists, and their findings and descriptions have often been misinterpreted and misunderstood by chemists. In 2007 we formalized definitions of some common terms used in the field (e.g., “machine”, “switch”, “motor”, “ratchet”, etc.) so that chemists could use them in a manner consistent with the meanings understood by biologists and physicists who study molecular-level machines.14

The word “machine” implies a mechanical movement that accomplishes a useful task. This Review concentrates on systems where a stimulus triggers the controlled, relatively large amplitude (or directional) motion of one molecular or submolecular component relative to another that can potentially result in a net task being performed. Molecular machines can be further categorized into various classes such as “motors” and “switches” whose behavior differs significantly.14 For example, in a rotaxane-based “switch”, the change in position of a macrocycle on the thread of the rotaxane influences the system only as a function of state. Returning the components of a molecular switch to their original position undoes any work done, and so a switch cannot be used repetitively and progressively to do work. A “motor”, on the other hand, influences a system as a function of trajectory, meaning that when the components of a molecular motor return to their original positions, for example, after a 360° directional rotation, any work that has been done is not undone unless the motor is subsequently rotated by 360° in the reverse direction. This difference in behavior is significant; no “switch-based” molecular machine can be used to progressively perform work in the way that biological motors can, such as those from the kinesin, myosin, and dynein superfamilies, unless the switch is part of a larger ratchet mechanism.14

Artificial Molecular Machines

Hydrogels for biomedical applications.

https://www.alice.cnptia.embrapa.br/bitstream/doc/579864/1/PA09005.pdf

Nanocomposites for food packaging applications

The paper deals with the correlation between the internal structure and dynamics of polyelectrolyte multilayers on one hand and their functional properties on the other hand. It considers different concepts of multilayer formation like driving forces, adsorption kinetics, mode of growth and stability aspects. A further focus is the control of internal structure and dynamics which is of high impact with respect to the design of stimuli-responsive material.

Internal structure of polyelectrolyte multilayer assemblies

The present review is a topical survey of the disjoining pressure in thin liquid foam and emulsion films from both the experimental and the theoretical points of view. Section 2 deals with the latest research work on experimental techniques with which the disjoining pressure Π in foam, emulsion, and pseudo-emulsion films can be measured. Although a lot of techniques are available, the question of the origin of the charges at the water/air and the water/oil interfaces of films, which are stabilized by non-ionic surfactants, has not yet been answered. We address this question in section 3, reviewing the latest relevant literature. The relevance of structural forces for the disjoining pressure is outlined in section 4, which focuses on films which are stabilized by surfactant/polyelectrolyte mixtures.

https://iopscience.iop.org/article/10.1088/0953-8984/15/27/201/pdf

Disjoining pressure in thin liquid foam and emulsion films—new concepts and perspectives

The influence of ionic strength and polymer charge density on the multilayer formation of strong polyelectrolytes is investigated by X-ray reflectivity. To a first approximation the adsorption behavior as a function of the degree of charge, f, is binary:  For f ≤ 50% the films are one order of magnitude thinner than those for f ≥ 75%. This is due to a threshold of charge overcompensation after each adsorption step which seems to be at f between 50% and 75%. Above the charge reversal limit the thickness and the surface roughness increase with decreasing polymer charge. Below a charge density of 50% the film thickness cannot be changed by salt additive whereas the film thickness increases with cNaCl1/2 above a degree of charge of 75%. The density of all investigated films is quite similar.

Influence of Charge Density and Ionic Strength on the Multilayer Formation of Strong Polyelectrolytes

Influence of the ionic strength on the structure of polyelectrolyte films at the solid/liquid interface

The swelling behavior of layer-by-layer assemblies of poly(styrenesulfonate) sodium salt (PSS) and poly(allylamine hydrochloride) (PAH) with various number of layers were investigated. The data presented in this paper suggest that swelling and deswelling are completely reversible and reproducible. At 99% relative humidity, a pronounced “odd−even effect” in the swollen thickness is observed depending on the type of polyelectrolyte in the outermost layer. Contact angle measurements confirmed that the PSS surface is more hydrophilic than the PAH surface. The “odd−even effect” indicates that water has to be pressed out of the multilayer when PAH is adsorbed onto the PSS surface of the multilayer and that again more water penetrates into the multilayer on adsorption of the next PSS layer. It was also found that the relative amount of swelling with respect to the total film thickness decreases with increasing number of layers. This is an indication that the loosely packed outer layers are more sensitive to envi...

Regine von Klitzing

Papers

Internal structure of polyelectrolyte multilayer assemblies

Disjoining pressure in thin liquid foam and emulsion films—new concepts and perspectives

Influence of Charge Density and Ionic Strength on the Multilayer Formation of Strong Polyelectrolytes

Influence of the ionic strength on the structure of polyelectrolyte films at the solid/liquid interface

Swelling Behavior of Polyelectrolyte Multilayers in Saturated Water Vapor