Page 13 - Nanotechnology an introduction

1.3.3. Nanoparticles

If we define nanotechnology ostensively, we have to concede a very long history: there is evidence that PbS nanocrystals were the goal of a
procedure used since Greco-Roman times to color hair black [164]. Nanoparticulate gold has a long history, not least in medicine (see Section
4.2). The Flemish glassmaker John Utynam was granted a patent in 1449 in England for making stained glass incorporating nanoparticulate gold;
and the Swiss medical doctor and chemist von Hohenheim (Paracelsus) prepared and administered gold nanoparticles to patients suffering from
certain ailments in the early 16th century; a modern equivalent is perhaps the magnetic nanoparticles proposed for therapeutic purposes. The
secret of the extraordinarily advanced metallurgical features of Damascus swords made more than 400 years ago has recently been found to be
carbon nanotubes embedded in the blades [146].

With such a long history, it is perhaps hardly surprising that at present they represent almost the only part of nanotechnology with commercial
significance. The fabrication of different kinds of nanoparticles by chemical means seems to have been well established by the middle of the 19th
century (e.g., Thomas Graham's method for making ferric hydroxide nanoparticles [63]). Wolfgang Ostwald lectured extensively on the topic in the
early 20th century in the USA, and wrote up the lectures in what became a hugely successful book, Die Welt der vernachlässigten Dimensionen.
Many universities had departments of colloid science (sometimes considered as physics, sometimes as chemistry), at least up to the middle of the
20th century, then slowly the subject seemed to fall out of fashion, until its recent revival as part of nanotechnology. The field somewhat overlapped
that of heterogeneous catalysis, in which it was well known, indeed almost obvious, that specific activity (i.e., per unit mass) tended to increase with
increasing fineness of division.
1.4. Biology as Paradigm
When Feynman delivered his famous lecture [56] it was already well known that the smallest viable unit of life was the cell, which could be less than
a micrometer in size. It was surmised that cells contained a great deal of machinery at smaller scales, which has since been abundantly evidenced
through the work of molecular biologists. Examples of these machines are molecule carriers (e.g., hemoglobin), enzymes (heterogeneous
catalysts), rotary motors (e.g., those powering bacterial flagella), linear motors (e.g., muscle), pumps (e.g., transmembrane ion channels), and multi-
enzyme complexes carrying out more complicated functions then simple reactions (e.g., the proteasome for degrading proteins, or the ribosome
for translating information encoded as a nucleic acid sequence into a polypeptide sequence). When Drexler developed his explicit schema of
nanoscale assemblers, allusion to nanoscale biological machinery was explicitly made as a “living proof-of-principle” demonstrating the feasibility
of artificial devices constructed at a similar scale [43].

At present, probably the most practically useful manifestation of the biological nanoparadigm is self-assembly. In simple form, self-assembly is well
known in the nonliving world (for example, crystallization). This process does not, however, have the potential to become a feasible industrial
technology for the general-purpose construction of nanoscale objects, because size limitation is not intrinsic to it. Only when highly regular
structures need to be produced (e.g., a nanoporous membrane, or a collection of monosized nanoparticles) can the process parameters be set to
generate an outcome of a prespecified size. Nature, however, has devised a more sophisticated process, known to engineers as programmable
self-assembly, in which every detail of the final structure can be specified in advance by using components that not only interlock in highly specific
ways but are also capable of changing their structure upon binding to an assembly partner in order to block or facilitate, respectively, previously
possible or impossible interlocking actions. Inspiration for harnessing programmable self-assembly arose from the work of virologists who noticed
that pre-assembled components (head, neck, legs) of bacteriophage viruses would further assemble spontaneously into a functional virus merely
upon mixing and shaking in a test-tube. This “shake and bake” approach appeared to offer a manufacturing route to nanodevices obviating: (1) the
many difficulties involved in making Drexlerian assemblers, which would appear to preclude their realization in the near future; and (2) the great
expense of the ultrahigh precision “top–down” approach, whether via UPMT or semiconductor processing. Even if assemblers are ultimately
realized, it might be most advantageous to use them to assemble sophisticated “nanoblocks”, designed to self-assemble into final macroscopic
objects (see Section 8.3.2). In other words, self-assembly's greatest potential utility will probably arise as a means to bridge the size gap between
the nanoscopic products of assemblers and the macroscopic artifacts of practical use for humans. Self-assembly is covered in detail in Chapter 8.
1.5. Why Nanotechnology?
Nanotechnology is associated with at least three distinct advantages:
1. It offers the possibility of creating materials with novel combinations of properties.
2. Devices in the nanoscale need less material to make them, use less energy and other consumables, their function may be enhanced by
reducing the characteristic dimensions, and they may have an extended range of accessibility.
3. It offers a universal fabrication technology, the apotheosis of which is the personal nanofactory.
The burgeoning worldwide activity in nanotechnology cannot be explained purely as a rational attempt to exploit “room at the bottom”, however. Two
other important human motivations are doubtless also playing a role. One is simply “it hasn't been done before”—the motivation of the mountaineer
ascending a peak previously untrodden. The other is the perennial desire to “conquer nature”. Opportunities for doing so at the familiar
macroscopic scale have become very limited, partly because so much has already been done—in Europe, for example, there are hardly any
marshes left to drain or rivers left to dam, historically two of the most typical arenas for “conquering nature”—and partly because the deleterious
effects of such “conquest” are now far more widely recognized, and the few remaining undrained marshes and undammed rivers are likely
nowadays to be legally protected nature reserves. But the world at the bottom, as Feynman picturesquely called it, is uncontrolled and largely
unexplored. On a more prosaic note, nanotechnology may already offer immediate benefit for existing products through substitution or incremental
improvement (Figure 1.3). The space industry has a constant and heavily pressing requirement for making payloads as small and lightweight as
possible. Nanotechnology is ideally suited to this end user—provided the nanomaterials, devices and systems can be made sufficiently reliable
(see Chapter 10).