A comparative study of tubular halloysite and platy kaolinite as carriers for the loading and release of the herbicide amitrole

Over recent years, drug release/dissolution from solid pharmaceutical dosage forms has been the subject of intense and profitable scientific developments. Whenever a new solid dosage form is developed or produced, it is necessary to ensure that drug dissolution occurs in an appropriate manner. The pharmaceutical industry and the registration authorities do focus, nowadays, on drug dissolution studies. The quantitative analysis of the values obtained in dissolution/release tests is easier when mathematical formulas that express the dissolution results as a function of some of the dosage forms characteristics are used. In some cases, these mathematic models are derived from the theoretical analysis of the occurring process. In most of the cases the theoretical concept does not exist and some empirical equations have proved to be more appropriate. Drug dissolution from solid dosage forms has been described by kinetic models in which the dissolved amount of drug (Q) is a function of the test time, t or Q=f(t). Some analytical definitions of the Q(t) function are commonly used, such as zero order, first order, Hixson-Crowell, Weibull, Higuchi, Baker-Lonsdale, Korsmeyer-Peppas and Hopfenberg models. Other release parameters, such as dissolution time (tx%), assay time (tx min), dissolution efficacy (ED), difference factor (f1), similarity factor (f2) and Rescigno index (xi1 and xi2) can be used to characterize drug dissolution/release profiles.

Halloysite aluminosilicate nanotubes with a 15 nm lumen, 50 nm external diameter, and length of 800 +/- 300 nm have been developed as an entrapment system for loading, storage, and controlled release of anticorrosion agents and biocides. Fundamental research to enable the control of release rates from hours to months is being undertaken. By variation of internal fluidic properties, the formation of nanoshells over the nanotubes and by creation of smart caps at the tube ends it is possible to develop further means of controlling the rate of release. Anticorrosive halloysite coatings are in development and a self-healing approach has been developed for repair mechanisms through response activation to external impacts. In this Perspective, applications of halloysite as nanometer-scale containers are discussed, including the use of halloysite tubes as drug releasing agents, as biomimetic reaction vessels, and as additives in biocide and protective coatings. Halloysite nanotubes are available in thousands of tons, and remain sophisticated and novel natural nanomaterials which can be used for the loading of agents for metal and plastic anticorrosion and biocide protection.

Chemical pesticides have been a boon to equatorial, developing nations in their efforts to eradicate insect-borne, endemic diseases, to produce adequate food and to protect forests, plantations and fibre (wood, cotton, clothing, etc.). Controversy exists over the global dependence on such agents, given their excessive use/misuse, their volatility, long-distance transport and eventual environmental contamination in colder climates. Many developing countries are in transitional phases with migration of the agricultural workforce to urban centres in search of better-paying jobs, leaving fewer people responsible for raising traditional foods for themselves and for the new, industrialized workforce. Capable of growing two or three crops per year, these same countries are becoming "breadbaskets" for the world, exporting nontraditional agricultural produce to regions having colder climates and shorter growing seasons, thereby earning much needed international trade credits. To attain these goals, there has been increased reliance on chemical pesticides. Many older, nonpatented, more toxic, environmentally persistent and inexpensive chemicals are used extensively in developing nations, creating serious acute health problems and local and global environmental contamination. There is growing public concern in these countries that no one is aware of the extent of pesticide residue contamination on local, fresh produce purchased daily or of potential, long-term, adverse health effects on consumers. Few developing nations have a clearly expressed "philosophy" concerning pesticides. There is a lack of rigorous legislation and regulations to control pesticides as well as training programs for personnel to inspect and monitor use and to initiate training programs for pesticide consumers.