Journal of the American Chemical Society
Article
was purchased from Gelest, Inc. Deuterium oxide (D2O) was
purchased from Cambridge Isotope Laboratories and used as received.
All oligonucleotides used in this work were synthesized on a MM12
synthesizer (Bioautomation) fitted with 5-μmol-scale columns and
purified according to the procedures described previously.1,3,5
Sequences can be found in the Supporting Information. SAXS data
were collected at the DuPont-Northwestern-Dow Collaborative Access
Team (DND-CAT) beamline at Argonne National Laboratory’s
Advanced Photon Source with 10 keV (wavelength λ = 1.24 Å)
collimated X-rays calibrated with a silver behenate standard. TEM
images were collected on a Hitachi HD-2300 scanning transmission
electron microscope. Gas adsorption isotherms were measured
volumetrically on an Autosorb-1 analyzer (Micromeritics). FTIR
experiments were performed on a Bruker Tensor 37 spectrometer. 1H
NMR spectra were collected on a Bruker Avance 400 MHz
spectrometer.
Nanoparticle Assembly and Crystallization. For the experi-
ments detailed in this work, nanoparticle functionalization with DNA
was carried out at an approximately 10-fold larger scale than
conventional DNA assembly work1,3,5 in order to produce amounts
sufficient for catalysis and related experiments. Briefly, approximately
500 nmol of the thiolated sequences (sequences 1 and 2 in the
Supporting Information) was reduced with 100 mM DTT at pH 8 and
purified through a Nap-25 Sephadex size-exclusion column. The
thiolated DNA was added to a solution of citrate-capped gold
nanoparticles stabilized with 0.01% volume of the surfactant Tween 20
at a concentration of approximately 5 nmol of DNA per mL of
particles. Following DNA addition, the particles were placed on a
shaker at room temperature for approximately 30 min before
subsequent salt addition. In order to maximize DNA loading on the
particle surface, NaCl was slowly added over the course of several
hours until the final salt concentration reached 0.5 M NaCl. After
allowing the particle solution to shake overnight at room temperature,
the particles were purified by five rounds of centrifugation (10 min,
4000 rpm) using 10K molecular weight cutoff spin filters (Millipore)
to remove the salt and any unbound DNA. After the final
centrifugation, the supernatant was removed and salt was added to
bring the final salt concentration of the solution to 0.5 M NaCl. DNA
and DNA-functionalized nanoparticle concentrations were calculating
using Beer’s law with the absorbance (at 260 nm for the DNA and at
520 nm for the Au nanoparticles) measured on a Cary 5000 (Agilent)
UV/vis spectrophotometer. To synthesize a bcc superlattice, two
solutions of particles functionalized with strands 1 and 2 were added in
a 1:1 ratio, followed by the addition of linker strands 3 and 4 that
initiate rapid particle aggregation (see Supporting Information for
sequences). The aggregates (disordered at this stage) were
subsequently transferred to PCR tube strips and slow-cooled in a
Veriti temperature cycler (Life Technologies) from 60 to 25 °C at a
rate of 0.1 °C/10 min to produce crystallized nanoparticle superlattices
with bcc packing.
and subsequently purified by at least 10 rounds of centrifugation and
resuspension in Nanopure water to remove all unbound nanoparticles.
The pellet was collected and dried in a CentriVap vacuum
concentrator (Labconco) to yield a pink solid. Samples were calcined
at 350 °C for 2 h prior to catalysis experiments. Gold content was
measured by inductively coupled plasma mass spectrometry. See
Supporting Information for TEM images of the silica-supported, non-
assembled Au nanoparticles.
Calcination of Superlattice Assemblies. Dried, silica-encapsu-
lated superlattices were transferred to 3 mm quartz capillaries (Charles
Supper) and placed inside a Lindberg Blue M tube furnace (Thermo
Scientific). Samples were heated in air at 350 °C for 2 h (not including
the 1 h ramp time to 350 °C).
Thermogravimetric Analysis (TGA). Samples were dried under
reduced pressure at 60 °C for 6 h in preparation for TGA
measurements. TGA was performed on a Discovery TGA (TA
Instruments). The temperature was ramped to 350 °C at a rate of 4
°C/min and held at 350 °C for 10 h.
Fourier Transform Infrared Spectroscopy (FTIR). Superlattices
dissolved in toluene were drop cast on a KBr Real Crystal IR sample
card (International Crystal Laboratories). Spectra (averaged over 16
scans) were collected in transmission mode and corrected for H2O and
CO2 in the OPUS software.
N2 Isotherm Measurements. A liquid nitrogen sample bath (77
K) was used, and the N2 gas used was UHP grade. For measurement
of the apparent surface areas (SBET), the BET method was applied
using the adsorption branches of the N2 isotherms assuming a N2
cross-sectional area of 16.2 Å2/molecule.
Small-Angle X-ray Scattering (SAXS). Exposure times of 0.1 and
1 s were used for solution-phase lattices and silica-embedded samples,
respectively. X-ray scattering data were collected on a CCD area
detector, and one-dimensional patterns were obtained from radial
averaging of the two-dimensional data to generate plots of scattering
intensity I(q) as a function of the scattering vector q (q = 4π sin θ/λ,
where θ and λ are the scattering angle and wavelength of the X-rays
used, respectively). Theoretical SAXS patterns were generated using
PowderCell (available free of charge from the Federal Institute for
that simulates scattering patterns for atomic lattices but which provides
a good approximation of SAXS patterns obtained for the nanoparticle
lattices studied in this work. The nanoparticle nearest-neighbor
distance (d, in nm) was calculated using the position of the first
scattering peak q0 in the SAXS pattern:
1
6π
d =
10 q0
Lattice parameters were calculated from the nearest neighbor
distance using geometric considerations for a bcc unit cell.
Silica Embedding. Superlattices were transferred to the solid state
using the modified silica embedding method as previously
described.3,12 As-synthesized superlattices were divided into 1.5 mL
Eppendorf tubes containing approximately 500 μL sample volume,
followed by the addition of 10 μL of TMSPA and 20 μL of TES into
each tube. The reaction was allowed to shake for 4 days on a
thermomixer (Eppendorf), followed by purification by centrifugation
to remove excess silica and precursor molecules. The purified
superlattices were dried in a CentriVap vacuum concentrator
(Labconco) to yield a dark purple solid.
Preparation of Supported, Non-assembled Gold Nano-
particles. To form the non-assembled, supported nanoparticle
catalyst, pre-formed 5 nm diameter Au nanoparticles (not function-
alized with DNA) were added to a silica growth solution to ensure that
particle sizes would be uniform across all catalysis experiments. Next,
100 mL of citrate-capped Au nanoparticles (5.0 × 1013 particles/mL,
as reported by the manufacturer) was added to a round-bottom flask
equipped with a stir bar. This was followed by the addition of 1 mL
TES (same silica precursor as was used in the silica encapsulation
process described above). The reaction was allowed to stir overnight
Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
Gold content was measured by ICP-MS measurements carried out in
Thermo X series II instrument with an automated sample changer.
Dried superlattices were dispersed homogeneously in 2 mL of D2O.
From this stock solution, 5 μL was added to 95 μL of D2O, and 5 μL
of this solution was digested in 995 μL of aqua regia. The solution
containing superlattices in aqua regia was left at 60 °C overnight to
ensure full digestion of the nanoparticles. Samples were prepared with
a multi-element standard and compared to a standard curve generated
using a gold standard solution.
Catalysis Experiments. In a typical catalysis experiment, the
catalyst (0.215 nmol Au nanoparticles), 4-hydroxybenzyl alcohol (6.25
μmol), and K2CO3 (4.38 μmol) were combined in 2.5 mL of D2O and
added to a 20 mL round-bottom flask containing a magnetic stir bar.
In the case of the supported, non-assembled particles that contained
low nanoparticle loading compared to the superlattice sample (0.07 wt
% compared to 10 wt%), side-by-side experiments using the calcined
superlattices containing an equivalent molar amount of nanoparticles
(0.02 nmol) were performed to compare the catalytic activity. The
flask containing the starting materials was fitted with a rubber septum
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J. Am. Chem. Soc. 2015, 137, 1658−1662