Halloysite is an alumosilicate tubular clay with a diameter of 50 nm, an inner lumen of 15 nm and a length of 600-900 nm. It is a natural biocompatible nanomaterial available in thousands of tons at low price, which makes it a good candidate for nanoarchitectural composites. The inner lumen of halloysite may be adjusted by etching to 20-30% of the tube volume and loading with functional agents (antioxidants, anticorrosion agents, flame-retardant agents, drugs, or proteins) allowing for formulations with sustained release tuned by the tube end-stoppers for hours and days. Clogging the tube ends in polymeric composites allows further extension of the release time. Thus, antioxidant-loaded halloysite doped into rubber enhances anti-aging properties for at least 12 months. The addition of 3-5 wt% of halloysite increases the strength of polymeric materials, and the possibility of the tube's orientation promises a gradient of properties. Halloysite nanotubes are a promising mesoporous media for catalytic nanoparticles that may be seeded on the tube surface or synthesized exclusively in the lumens, providing enhanced catalytic properties, especially at high temperatures. In vitro and in vivo studies on biological cells and worms indicate the safety of halloysite, and tests for efficient adsorption of mycotoxins in animals' stomachs are also carried out.

Halloysite Clay Nanotubes for Loading and Sustained Release of Functional Compounds.

Functional polymer–clay nanotube composites with sustained release of chemical agents

This chapter highlights the synthetic aspects that generally remain in the shadows in order to provide the scientific community with a user-friendly guide for the production of gold nanorods. The mechanism behind the formation of gold nanorods is still a matter of much interest because a general mechanistic model would allow us to identify specific guidelines for the design of a synthetic pathway for each nanostructure. Gold nanorod synthesis requires both thermodynamic and kinetic control, which significantly increases the number of parameters that should be taken into account. The historical turning point in the development of efficient nanorod wet synthesis methods was the introduction of the so-called seeded growth protocol, where nucleation is performed separately to prepare the seeds, which are subsequently added to the growth solution for nanorod production. The growth of gold nanorods is characterized in general by a slow kinetics, meaning that several hours are needed to complete particle growth.

http://pubs.acs.org/doi/pdf/10.1021/acs.jpclett.5b02123

A "Tips and Tricks" Practical Guide to the Synthesis of Gold Nanorods.

Halloysite clay nanotubes loaded with corrosion inhibitors benzotriazole (BTA), 2-mercaptobenzimidazole (MBI), and 2-mercaptobenzothiazole (MBT) were used as additives in self-healing composite paint coating of copper. These inhibitors form protective films on the metal surface and mitigate corrosion. Mechanisms involved in the film formation have been studied with optical and electron microscopy, UV–vis spectrometry, and adhesivity tests. Efficiency of the halloysite lumen loading ascended in the order of BTA < MBT < MBI; consequently, MBI and MBT halloysite formulations have shown the best protection. Inhibitors were kept in the tubes buried in polymeric paint layer for a long time and release was enhanced in the coating defects exposed to humid media with 20–50 h, sufficient for formation of protective layer. Anticorrosive performance of the halloysite-based composite acrylic and polyurethane coatings have been demonstrated for 110-copper alloy strips exposed to 0.5 M aqueous NaCl for 6 months.

Self-healing coatings based on halloysite clay polymer composites for protection of copper alloys

Halloysite, naturally occurring clay nanotubes, is described as an additive for functional polymer composites. Due to the empty tubular lumen capable of being loaded with chemically active agents, halloysite provides additional functions (drug delivery, antibiotic, flame-retardant, anticorrosion, and crack self-healing) to polymeric composites synergistically combined with enhanced tensile, impact and adhesive strength. Doping loaded clay nanotubes into a polymeric matrix provides a kind of ceramic “skeleton”, and these “skeleton bones” are loaded with functional chemicals like real bones loaded with marrow. Tunable controlled release of active agents through synthesis of artificial nano-caps at the tube endings and halloysite lumen enlargement by selective etching allowed for tubular nanocontainers with chemicals release time from 10 to 200 h and a loading capacity of ca. 30 wt%. Halloysite is well mixable with polymers of high and medium polarities without any surface modification.

Halloysite clay nanotubes as a ceramic “skeleton” for functional biopolymer composites with sustained drug release

The first direct comparison between two common methods for loading halloysite with a small molecule for controlled release is presented. While the methods differ in the degree of simplicity, they provide essentially the same level of loading and release kinetics. A tentative explanation of the "burst" effect often seen in the release of low molecular weight molecules from halloysite is provided. The ability of halloysite to mediate the release rate of a water soluble drug, tetracycline, from solution cast polyvinyl alcohol and polymethyl methacrylate films was evaluated. In some films, montmorillonite was also incorporated. The addition of montmorillonite to solutions used to cast tetracycline containing films significantly reduced the release rate from the dried films. The same overall effect was seen when the drug was loaded into halloysite prior to preparation of the films. In both cases, the release was best fit with the simple Higuchi model. However, when montmorillonite was added to solutions of polyvinyl alcohol and drug loaded halloysite the release profiles were better fit by the Ritgar-Peppas model for anomalous transport. Release from polymethyl methacrylate was reduced by a factor of three by incorporating the drug in halloysite prior to producing the films.

Controlled release of tetracycline-HCl from halloysite-polymer composite films

The effects of the seed reaction conditions on the two-step seed-mediated growth of gold nanorods and the effect of gold and reducing agent content in the growth solution were evaluated. Results indicate that the reaction conditions used to produce the seeds have a significant impact on the aspect ratio of the gold nanorods produced. Increasing the concentration of gold or the reaction temperature in the seed production step results in lower length to diameter (aspect ratio) gold rods. In addition, the amount of prepared seed added to the growth solution impacts the rod aspect ratio, with increasing amounts of seed reducing the aspect ratio. The effects of reducing agent, ascorbic acid (AA), and gold content of the growth solution on the aspect ratio of the produced rods are strongly interrelated. There exists a minimum ascorbic acid to gold concentration below which rods will not form; however, increasing the ratio above this minimum results in shorter rods being formed. Characterization of nanorod growth is performed by UV-vis-NIR spectrophotometry and transmission electron microscopy (TEM).

/pdf/seed-mediated-growth-of-gold-nanorods-limits-of-length-to-3nad2j88pz.pdf

Seed-Mediated growth of gold nanorods: limits of length to diameter ratio control

Halloysite Nanoclay for Controlled Release Applications

In this paper we introduce a new data link control protocol appropriate for a low earth orbit (LEO) satellite network. The principle characteristics of an LEO satellite network are high error rates, long propagation delay, and high mobility. Although this area is traditionally dominated by forward error correction (FEC) we present a reliable protocol suited to this environment and derive the throughput for it given a finite buffer model. In our target network environment, reliability constraints may be relaxed provided that the protocol provides zero packet loss capability. The proposed scheme (called LAMS-DLC) provides a datagram service at the link level based on a Negative Acknowledgement ARQ scheme to accomplish error recovery. In LAMS-DLC the receiver sends commands for the following reasons only: to enable the sender to release buffer space held for previously transferred information frames (I-Frames); and to identify when the sender has failed. Commands are sent by the receiver so long as the link is active. The performance of the protocol is compared with the popular ARQ protocol HDLC. We show that LAMS-DLC performs better than the selective repeat HDLC and offers certain advantages in satellite networks.

The LAMS-DLC ARQ protocol

Students learning computer organization and assembly language programming often have difficulty understanding the relationship between hardware functions and software instructions. This paper describes how a software tool can help students understand the organization and use of the Motorola 68000 processor family. XSIM68K employs a graphical interface to enhance a traditional “black box” simulator so that students can observe first-hand the effect of a running program on registers, memory, and II0 devices. Multiple data representations allow the student to choose the most appropriate format for data entry and display, which reduces confusion and eliminates conversion mistakes.

Christopher J. Ward

Papers

Controlled release of tetracycline-HCl from halloysite-polymer composite films

Seed-Mediated growth of gold nanorods: limits of length to diameter ratio control

Halloysite Nanoclay for Controlled Release Applications

The LAMS-DLC ARQ protocol

Visual execution of assembly language programs