Introduction

Abstract

In the search for safe, economical, and efficient means of providing for the health and well-being of mankind, modern science has produced numerous active agents that manipulate the biological environment around and within us. Nevertheless, the use of these active agents is fraught with inefficiencies stemming from an inability to deliver these agents to their targets at the right time and in the right amounts. This results in their loss and in undesirable side effects and leads to a regimen requiring repeated treatment to produce and sustain the desired effect. With drugs, periodic dosage produces peaks and valleys in the concentration of the drug in the blood stream, possibly between harmful and ineffective levels (see Fig. 1.1). Agricultural chemicals such as pesticides, fertilizers, herbicides, and fumigants create the same problems when applied directly. To counter these problems, scientists have conventionally looked to altering the persistency and effectiveness of the reagents through modification of the reagents themselves. However, this approach tends to be difficult, time consuming, and expensive.