shell saturated dendrimers behaved like noble gas atomic elements and did not
exhibit this autoreactivity. In fact this nanoscale atom mimicry constituted a
primary hypothesis upon which a new nano-periodic system for unifying
nanoscience was proposed [137]. More specifically, it provided a fundamental
paradigm for explaining why many well-defined nanoscale building blocks
(i.e., both soft and hard nano-elements) were observed to combine in well-defined
stoichiometries. These soft and hard nano-elements have been observed to produce
extensive libraries of literature-documented chemically bonded nanocompounds
and supramolecularly derived nano-assemblies, as will be described later.
These superatoms or atom mimics appear to fulfill a pivotal role as nanoscale
building blocks, much as elemental atoms function at the pico- or subnanoscale
level. As such, these poly(atomic) structures or entities have been classified and
referred to as “nano-element categories” [137, 138]. Furthermore, these nanoelement categories have been shown to form stoichiometric nanocompounds or
assemblies that exhibit well-defined intrinsic nano-periodic property patterns in
much the same way as atomic elements and their compounds.
In the context of this perspective and using “traditional chemistry first principles” initiated by Lavoisier, Dalton, Mendeleev and others, a new systematic
framework for unifying and defining nanoscience was proposed. Just as the
nineteenth century first principles led to a central paradigm and a periodic system
for traditional elemental atom and small molecule chemistry, it was proposed that a
similar nano-periodic system might be defined for discrete, well-defined
nanomodules at the nanolevel (Fig. 17).
The initial nano-periodic framework of nano-elemental categories should be
viewed as a “works in progress”. This framework is expected to be expanded and
better articulated with time, just as Dalton’s original list of atomic elements has
grown from 23 in 1808 to now over 117 known atomic elements [150]. The current
system is based on 12 nano-element categories, which are differentiated equally
into two main groups consisting of six categories each: (1) hard nano-element
categories (i.e., inorganic modules) and (2) soft nano-element categories
(i.e., organic modules). The inorganic-like, hard nano-element categories are arbitrarily designated as [H-1] metal nanoclusters, [H-2] metal chalcogenide
nanocrystals, [H-3] metal oxide nanocrystals, [H-4] silica nanoparticles, [H-5]
fullerenes, and [H-6] carbon nanotubes. The organic-like, soft nano-element categories include [S-1] dendrons/dendrimers, [S-2] nano-latexes, [S-3] polymeric
micelles, [S-4] proteins, [S-5] viral capsids, and [S-6] RNA/DNA (Fig. 18). Single
units of these various elements (i.e., chemically bonded or supramolecularly
assembled modules) are 1–100 nm in at least one dimension, contain between
10
3 and 10
9 atoms with masses of 10
4 –10
10 Da. In order to be included as a
nano-element category, each type of nanomaterial had to exhibit:
1. Discrete, well-defined monodispersity (i.e., >90% monodisperse as a function of
size or mass)
2. Exist as well-defined nanostructures, assemblies, or collections of units that
mimic or behave like atoms
354
D.A. Tomalia
exhibit this autoreactivity. In fact this nanoscale atom mimicry constituted a
primary hypothesis upon which a new nano-periodic system for unifying
nanoscience was proposed [137]. More specifically, it provided a fundamental
paradigm for explaining why many well-defined nanoscale building blocks
(i.e., both soft and hard nano-elements) were observed to combine in well-defined
stoichiometries. These soft and hard nano-elements have been observed to produce
extensive libraries of literature-documented chemically bonded nanocompounds
and supramolecularly derived nano-assemblies, as will be described later.
These superatoms or atom mimics appear to fulfill a pivotal role as nanoscale
building blocks, much as elemental atoms function at the pico- or subnanoscale
level. As such, these poly(atomic) structures or entities have been classified and
referred to as “nano-element categories” [137, 138]. Furthermore, these nanoelement categories have been shown to form stoichiometric nanocompounds or
assemblies that exhibit well-defined intrinsic nano-periodic property patterns in
much the same way as atomic elements and their compounds.
In the context of this perspective and using “traditional chemistry first principles” initiated by Lavoisier, Dalton, Mendeleev and others, a new systematic
framework for unifying and defining nanoscience was proposed. Just as the
nineteenth century first principles led to a central paradigm and a periodic system
for traditional elemental atom and small molecule chemistry, it was proposed that a
similar nano-periodic system might be defined for discrete, well-defined
nanomodules at the nanolevel (Fig. 17).
The initial nano-periodic framework of nano-elemental categories should be
viewed as a “works in progress”. This framework is expected to be expanded and
better articulated with time, just as Dalton’s original list of atomic elements has
grown from 23 in 1808 to now over 117 known atomic elements [150]. The current
system is based on 12 nano-element categories, which are differentiated equally
into two main groups consisting of six categories each: (1) hard nano-element
categories (i.e., inorganic modules) and (2) soft nano-element categories
(i.e., organic modules). The inorganic-like, hard nano-element categories are arbitrarily designated as [H-1] metal nanoclusters, [H-2] metal chalcogenide
nanocrystals, [H-3] metal oxide nanocrystals, [H-4] silica nanoparticles, [H-5]
fullerenes, and [H-6] carbon nanotubes. The organic-like, soft nano-element categories include [S-1] dendrons/dendrimers, [S-2] nano-latexes, [S-3] polymeric
micelles, [S-4] proteins, [S-5] viral capsids, and [S-6] RNA/DNA (Fig. 18). Single
units of these various elements (i.e., chemically bonded or supramolecularly
assembled modules) are 1–100 nm in at least one dimension, contain between
10
3 and 10
9 atoms with masses of 10
4 –10
10 Da. In order to be included as a
nano-element category, each type of nanomaterial had to exhibit:
1. Discrete, well-defined monodispersity (i.e., >90% monodisperse as a function of
size or mass)
2. Exist as well-defined nanostructures, assemblies, or collections of units that
mimic or behave like atoms
354
D.A. Tomalia