Page 90 - Nanotechnology an introduction

macroscopic mixers (cf. Figure 2.4), one very important benefit of which is much more predictable selection of reaction products, wherever several
are possible (in consequence, yields can be raised to 100%), and (in principle) great ease of scale up, simply by having many micromixers in
parallel (although this does not yet seem to have been convincingly demonstrated for any industrial-scale production facility). It is, however, by no
means clear that even greater success will attend further miniaturization down to the nanoscale. On the contrary, performance may be degraded.
This needs further investigation.

7.9.2. Chemical and Biochemical Sensors

These devices are included under fluidics since the sample to be analyzed is almost invariably a fluid (cf. Section 11.5). The Holy Grail of clinical
sensing is continuous, noninvasive monitoring (cf. Section 4.2). Currently, most tests require a sample of the relevant biofluid (e.g., blood) to be
drawn from the patient. For most people this is a somewhat unpleasant procedure, hence the tests are carried out infrequently. It is, however,
recognized that much more insight into a patient's pathological state could be obtained by frequent, ideally continuous, monitoring. At present, this
is only possible in intensive care stations, where the patient is immobilized, and even there continuous invasive monitoring does not take place (the
oxygen content of the blood is monitored noninvasively by analyzing the optical reflectance spectrum of the skin covering blood vessels). It seems to
be a very difficult technical problem to extend such noninvasive analysis to the plethora of biomarkers currently under intensive study as
symptomatic of disease or incipient disease. An alternative approach is to develop sensors so tiny that they can be semipermanently implanted
inside the body, where they can continuously monitor their surroundings.

Because of the large and growing number of afflicted people, diabetes has received overwhelmingly the most attention. The sensing requirement is
for glucose in the blood. The glucose sensor follows classic biosensing design: a recognition element to capture the analyte (glucose) mounted on
a transducer that converts the presence of captured analyte into an electrical signal (Figure 7.23). The recognition element is typically a biological
molecule, the enzyme glucose oxidase, hence (if small enough) this device can be categorized as both nanobiotechnology and bionanotechnology.

Figure 7.23 A prototypical biosensor. The capture layer concentrates the analyte in the vicinity of the transducer, which reports the concentration of analyte in the capture layer, which is directly related to the concentration in the sample.
Both components of the biosensor are excellent candidates for the application of nanotechnology. Molecular recognition depends on a certain
geometrical and chemical arrangement of atoms in some sense complementary to the analyte molecules, together with cooperative motions to
enhance affinity. Atom-by-atom assembly therefore represents the perfect way to artificially fabricate recognition elements. The ultimate goal of the
transducer is to detect a single captured analyte molecule, hence the smaller it can be made the better, provided issues related to noise and
detection efficiency (Section 10.8) can be overcome. The goal is to exploit the phenomenon that a minute change in an atomically scaled device
(e.g., the displacement of a single atom) can significantly change its properties, including functionality.

7.9.3. Energy Conversion Devices

Originally inspired by the photosynthetic machinery in blue-green algae and plants, in order to overcome the problem that the output of a
photovoltaic or thermal solar energy converter (i.e., electricity or heat, respectively) must be subsequently stored, possibly via further conversion
and inevitably involving losses, attempts have been made to create devices that convert the solar energy directly into chemical form. The basic
principle is that the absorption of light by a semiconductor creates highly reducing and highly oxidizing species (respectively, the conduction band
(CB) electrons and the valence band (VB) holes). Instead of separating them in a space charge as in the conventional p–n junction device and
collecting the current, if the semiconductor is a nano-object with its characteristic dimension comparable to the Bohr radii of the charge carriers
(Section 2.5), they may react with chemical species adsorbed on the object's surface before they have time to recombine (Figure 7.24).

Figure 7.24 Radiant energy harvesting using semiconductor nanoparticles. The absorption of a photon raises an electron from the valence band (VB) to the conduction band (CB), leaving a positive hole in the former. The CB electron ⊖ is
a strong reducing agent and the VB hole ⊕ a strong oxidizing agent, possibly capable of, respectively, reducing and oxidizing water to hydrogen and oxygen. At present, this concept of nanoparticle photoelectrochemistry is used to
sacrificially destroy persistent organic pollutants allowed to adsorb on the particles. This is also the basis of self-cleaning windows and compact water purification devices.