INSIGHTS | PERSPECTIVES
By Nicholas A. Kotov1,2,3
Porous metals, semiconductors, and ceramics are widely used in optoelec- tronics, biological sensing, catalysis, and energy conversion and storage. Materials with pore diameters rang- ing from less than a nanometer to a
few nanometers include zeolites (1), metal-organic frameworks (2), supramolecular
coordination frameworks (3), and DNA lattices (4). Alternatively, colloidal crystals can
be used to create micrometer-scale pores.
However, voids that are several nanometers
to several tens of nanometers in size make
atomic lattices unstable and often collapse;
such materials are therefore difficult to
synthesize. On page 514 of this issue, Udayabhaskararao et al. (5) report a method for
engineering primarily two-dimensional and,
in some instances, three-dimensional materials that have regular nanometer-scale spaces
between 5 and 25 nm and do not collapse.
The authors use inorganic nanoparticles,
which have previously been shown to assemble into frameworks with nanoscale
pores; however, nonuniform particle sizes
led to variable pore sizes (6). Udayabhaskararao et al.’s method involves mixtures of inorganic nanoparticles, one type of which is
dissolved (“sacrificed”) to create pores. Such
a sacrificial strategy has been used in the
past to make singular nanoshells (7, 8), but
not for binary nanoparticle constructs that
make possible a wide variety of atom-like
The authors start from monolayers and
multilayers of two chemically dissimilar
nanoparticles—gold (Au) and magnetite
(Fe3O4)—formed at the interface of diethyl-
ene glycol and air. They used the Langmuir-
Blodgett lift-off technique (9, 10) to create
binary nanoparticle superlattices that could
cover any solid surface (see the figure). Dis-
solution of one nanoparticle type would
produce non–close-packed nanoparticle
arrays of the other type, with nanoscale va-
cancies. However, the nanoparticles are
coated with surfactant molecules, providing
partial protection from etching agents. The
authors deprotected them by gently heating
the nanoparticle films to 70°C; at this tem-
perature, the surfactant layers migrate from
the nanoparticles to the underlying carbon
films. After this step, exposure of the closely
packed superlattices to hydrogen chloride or
hydrogen cyanide successfully etched away
Fe3O4 or Au, respectively.
With this method, as much as half of the
particles in two-dimensional lattices can be
removed without collapse into a disorganized or closely packed solid. However, the
nanoparticles reorganize during this process. For example, Au nanoparticles cluster
together to form tetrahedral quintets when
the supporting Fe3O4 nanoparticles are dissolved. After heat treatment, the nanoparticles coalesce further into supraparticles that
are held together primarily by van der Waals
forces (see the figure). This secondary reassembly opens up more space in the nanoparticle lattice while maintaining long-range
organization. Superlattices with high Au
content show variable coalescence patterns;
one of them leads to the long-range assembly
of a conductive membrane with nearly perfect hexagonal elements.
The computational design of nanoparticle
assemblies with vacancies requires consideration of the collective motions of a nanoparticle in three dimensions. Full description of
the close-range interactions of nanoparticles
with surfactant layers in solution (where
most self-assembly processes take place) is
challenging because nanoscale interactions
are not additive in the traditional sense of
colloidal science (11), they cannot, therefore,
be computed by separation into familiar electrostatic, van der Waals, and hydrophobic
components. The authors were nevertheless
able to successfully model the nanoparticle
reorganization because vacuum conditions
and etching simplify the computations by removing the surfactant and solvent molecules
from the calculations.
The ability to create stable, non–
close-packed particle arrays with nanoscale pores
opens multiple pathways for future scientific inquiries and emerging technologies.
The authors demonstrated, for instance,
the drastic—more than one order of magnitude—improvement in Raman scattering
intensity for superlattices with vacancies
of specific shape and size. Nanoscale pores
are also needed for biomedical applications
such as continuous biomonitoring of cancer
markers, biomimetic catalysis, and drug delivery because nanoscale dimensions are typical for biomacromolecules. The formation
of host-guest complexes (see the figure) in
these arrays may be identified with Raman
scattering, electrical conductance surface
capacitance, magnetic measurements, and
A key challenge for some of these non–
close-packed arrays will be to ensure solvent
stability. Bridges between the nanoparticles
will increase the robustness of the superstructure, although at the expense of pore
size uniformity. The possibility of field-driven subwavelength phenomena in electrically conductive networks is intellectually
stimulating and has far-reaching practical
implications for electro-optics and energy
storage (13). j
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3. Y.Inokuma etal.,Nature 495, 461(2013).
4. E.Auyeung et al., J. Am. Chem. Soc. 137,1658(2015).
5. T. Udayabhaskararao et al., Science 358, 514 (2017).
6. K. Hirai et al. , Angew. Chem. Int. Ed. 54, 896 (2015).
7. B. M. Nolan et al., ACS Nano 10, 5391 (2016).
8. D. S. Koktysh et al. , Adv. Funct. Mater. 12, 255 (2002).
9. N. A. Kotov, F. C. Meldrum, C. Wu, J. H. Fendler, J. Phys.
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10. B. O. Dabbousi, C. B. Murray, M. F. Rubner, M. G. Bawendi,
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11. C. A. Silvera Batista, R. G. Larson, N. A. Kotov, Science 350,
12. C. Hamon, S. Novikov, L. Scarabelli, L. Basabe-Desmonts,
L. M. Liz-Marzán, ACS Nano 8, 10694 (2014).
13. J.-Y.Kim,N.A.Kotov, Chem. Mater.26,134(2014).
The art of empty space
Nanoparticle assemblies can be transformed into regular
and stable nanoporous materials
1Department of Chemical Engineering, Department of
Biomedical Engineering, University of Michigan, Ann Arbor,
MI, USA. 2Department of Materials Science and Engineering,
University of Michigan, Ann Arbor, MI, USA. 3Biointerfaces
Institute, University of Michigan, Ann Arbor, MI, USA.
Potential use as a biosensor
Nano-vacancies by self-assembly
Etching away iron oxide particles leaves regular gold
nanoparticle assemblies. The gaps are of ideal size
for biomolecular sensing and other applications.