Biomedical Engineering Reference
In-Depth Information
between an ordinary molecule and a typical nanoparticle? It is a simple question that cannot be
easily answered. First, nature is the same and we are talking about galaxies, trees, and atoms.
Indeed, the same laws are applied for biological systems or artificial devices invented by the human
mind. It means that a computer or a sunflower obey exactly the same restrictions regarding thermo-
dynamics, kinetics, or quantum mechanics. Second, the classification in nanometric or kilometric
scale is just a human way to organize our universe. There are not intrinsic compartments in nature
to define a system as “nanometric” or “macroscopic.” It is just human interpretation.
From the words of Ozin and Arsenault
[1]
, nanoscience is defined being the discipline con-
cerned with making, manipulating, and imaging materials having at least one special dimension in
the size range 1
1000 nm and nanotechnology being a device or machine, product or process,
based upon individual or multiple integrated nanoscale components. However, this definition did
not show the most important question: Why was it so important to us classifying matter in this
way? The answer is properties, not length.
We are accustomed to observe nature as a continuum. All the shapes are possible and all the
sizes are allowed. Although shape and size are the easiest method to classify a system, the most
important property of a system is related to energy. Differently to our common and classical
assumption of nature, energy is a discrete variable and any amount of energy (E) should be propor-
tional to an integral multiple of a specific frequency (
ν
), represented by
Eq. (9.1)
, where n is an
integer and h is the proportionality constant
(the Planck constant)
that has the value of
6.629
3
10
2
24
J/s, which is an extremely small value of power.
E
5
nh
ν
(9.1)
For sake of comparison, a human being has an average power of 100 J/s, which is millions of
billions of billions higher than the Planck constant. While a molecular event involves quanta of
approximately 10
2
18
J, any energetic process occurring in our scale of time and length are infinitely
higher, which makes virtually impossible to us to distinguish the existence of discrete levels
[2]
.
A single atom is fully characterized by very precise energetic transitions that can be easily mea-
sured using spectroscopy techniques. The existence of these energetic levels results from the con-
finement of electrons in movement under the influence of the atomic nucleus and has been well
explained by quantum mechanics
[3]
. When a finite number of atoms, for example one hundred of
them, are put together, there is complex interaction between their electronic densities, which results
in chemical bonds and new energetic transitions that were forbidden in an isolated atom. If more
and more atoms are added to the system, their energetic properties reach macroscopic values that
characterize a substance.
“Silver is silver,” a silver miner can say. However, it is true only when the number of atoms is large
enough to achieve the limit properties of a macroscopic sample. “One gram of silver has the same prop-
erties of 1 ton.” But 1 g of silver has 1.6
10
22
atoms. It is a huge number. For samples with a few
atoms, the properties that depend on the energetic levels will be evidently different. Since each silver
atom has a diameter of 0.165 nm, just seven atoms aligned results in a nanometric system of 1nm.
That is the point. Nanoscience is the manipulation of a sufficiently small number of atoms to
form particles with energetic characteristics that are intermediate between a single atom and a macro-
scopic sample. It is the reason why chemists and materials scientists are always developing new tech-
niques to control the number of atoms in their nanoparticles. If a molecule is a fixed number of
atoms organized in a precise structure, a nanoparticle, on the other hand, can be prepared with
3
