Much of the interest in noble metal nanoparticles is due to their plasmonic resonance responses and local field enhancement, both of which can be tuned through the size and shape of the particles. However, both properties suffer from the loss of monodispersity that is frequently associated with various morphologies of nanoparticles. Here we show a method to generate diverse and monodisperse anisotropic gold nanoparticle shapes with various tip geometries as well as highly tunable size augmentations through either oxidative etching or seed-mediated growth of purified, monodisperse gold bipyramids. The conditions employed in the etching and growth processes also offer valuable insights into the growth mechanism difficult to realize with other gold nanostructures. The high-index facets and more complicated structure of the bipyramid lead to a wider variety of intriguing regrowth structures than in previously studied nanoparticles. Our results introduce a class of gold bipyramid-based nanoparticles with interesting and potentially useful features to the toolbox of gold nanoparticles.
Nucleic acid amplification techniques have been among the most powerful tools for biological and biomedical research, and the vast majority of the bioassays rely on thermocycling that uses time-consuming and expensive Peltier-block heating. Here, we introduce a plasmonic photothermal method for quantitative real-time PCR, using gold bipyramids and light to achieve ultrafast thermocycling. Moreover, we successfully extend our photothermal system to other biological assays, such as isothermal nucleic acid amplification and restriction enzyme digestion.
Breaking symmetry in colloidal crystals is challenging
due to the
inherent chemical and structural isotropy of many nanoscale building
blocks. If a non-particle component could be used to anisotropically
encode such building blocks with orthogonal recognition properties,
one could expand the scope of structural and compositional possibilities
of colloidal crystals beyond what is thus far possible with purely
particle-based systems. Herein, we report the synthesis and characterization
of novel DNA dendrimers that function as symmetry-breaking synthons,
capable of programming anisotropic and orthogonal interactions within
colloidal crystals. When the DNA dendrimers have identical sticky
ends, they hybridize with DNA-functionalized nanoparticles to yield
three distinct colloidal crystals, dictated by dendrimer size, including
a structure not previously reported in the field of colloidal crystal
engineering, Si2Sr. When used as symmetry-breaking synthons
(when the sticky ends deliberately consist of orthogonal sequences),
the synthesis of binary and ternary colloidal alloys with structures
that can only be realized through directional interactions is possible.
Furthermore, by modulating the extent of shape anisotropy within the
DNA dendrimers, the local distribution of the nanoparticles within
the crystals can be directed.
Although
examples of colloidal crystal analogues to metal alloys
have been reported, general routes for preparing 3D analogues to random
substitutional alloys do not exist. Here, we use the programmability
of DNA (length and sequence) to match nanoparticle component sizes,
define parent lattice symmetry and substitutional order, and achieve
faceted crystal habits. We synthesized substitutional alloy colloidal
crystals with either ordered or random arrangements of two components
(Au and Fe3O4 nanoparticles) within an otherwise
identical parent lattice and crystal habit, confirmed via scanning
electron microscopy and small-angle X-ray scattering. Energy dispersive
X-ray spectroscopy reveals information regarding composition and local
order, while the magnetic properties of Fe3O4 nanoparticles can direct different structural outcomes for different
alloys in an applied magnetic field. This work constitutes a platform
for independently defining substitution within multicomponent colloidal
crystals, a capability that will expand the scope of functional materials
that can be realized through programmable assembly.
Colloidal
crystal engineering with DNA has advanced beyond controlling
the lattice symmetry and parameters of ordered crystals to now tuning
crystal habit and size. However, the predominately used slow-cooling
procedure that enables faceted crystal habits also limits control
over crystal size and uniformity because nucleation and growth cannot
be separated. Here, we explore how DNA sequence design can be used
to deliberately separate nucleation and growth in a given crystallization
process. Specifically, two batches of complementary particles are
created with one batch exhibiting perfectly complementary base pairs
while the other has a strategically introduced mismatch. This design
enables the weaker binding “growth” particles to participate
in heterogeneous growth on the nucleates formed from the stronger
binding “seed” particles, effectively eliminating secondary
nucleation pathways. By eliminating secondary nucleation events, this
approach improves crystal uniformity, as measured by polydispersity
(from PDI = 0.201 to 0.091). By using this approach with two different
particle cores (gold and silver), we show how core–shell colloidal
crystals can be synthesized in a one-pot fashion. This work shows
how tuning DNA interaction strength can profoundly impact crystal
size, uniformity, and structure, parameters central to using such
materials as device components.
In the version of this Article originally published, the diblock copolymer structure in Fig. 2a showed a single bond between the carbon and the oxygen atoms; it should have been a double bond. This has been corrected in all versions of the Article.
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