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- Catalyst discovery through megalibraries of nanomaterials.
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One high priority for the company has been increasing the speed of integration when a new customer signs up for Catalyst. Chiu told me that the startup often starts with onboarding a customer success team, only to later find that C-suite and other team leads have also joined and are also interacting together on the platform. To explore the ability of spray coating to generate such combinatorial libraries, a dual spray-coating experiment was performed in which the two airbrushes were loaded with the same polymer—fluorophore inks as previously used but were aimed above two adjacent corners of the pen array rather than along a line that passes through the center of the array Fig.
To characterize this massive array of features, large-scale fluorescence images were acquired, which clearly showed a macroscopic gradient in fluorescence across the patterned surface Fig. Indeed, the average fluorescence intensity is well described by a linear gradient across the entire patterned surface horizontally Fig. Also, fluorescence intensity and atomic force microscopy AFM data show a nonlinear size gradient of features ranging from 1. Large-scale nanopatterned gradients of composition and size. A Stitched confocal fluorescence image of compositional and size gradients of two fluorophores in polymeric domes.
Scale bar: 2 mm. B Average fluorescence intensity of each fluorophore across the array and C average total fluorescence intensity blue curve in the vertical axis plotted with the average volume of individual features as measured by AFM red dots; error bars represent SD. D High-resolution dark-field micrograph of a region showing the patterns written by four pens. E Schematic of the airbrush position during spraying. F AFM of the largest patterned features 1.
While prior experiments focused on patterning gradients with easy to detect fluorophores to visualize and characterize the gradients, it is important to validate that combinatorial libraries may be generated using substances where the function can, in principle, dramatically change across the array.
When combined with SPBCL, catalytically active vide infra multimetallic nanoparticles with gradients of composition can be readily synthesized.
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To determine if combining SPBCL and dual spray coating can be used to generate compositional gradients, we explored the patterning of metal ion-loaded block copolymers that can be compositionally quantified using X-ray fluorescence XRF To explore the patterning of these inks, a pen array was dual spray coated using aqueous solutions of PEO- b -P2VP: one with auric acid and the other with sodium tetrachloropalladate.
After spray coating, this pen array was used to pattern features on an HMDS-treated silicon wafer. The resulting features were heat treated to form nanoparticles and were characterized using XRF, which allowed for the calculation of the local atomic ratio of Au to Pd in sections across the array SI Appendix , Figs. S6 and S7. Pd was used for this analysis as opposed to Cu, which is studied later, due to the larger signal separation from Au in the XRF spectrum.
Equally important to synthesizing these centimeter-scale continuous compositional gradients is developing methods to measure the properties of the resulting nanoparticles. Using a rotational stage and a three-axis piezo actuator to align the pen array with the substrate Fig.
Following a three-step heat treatment precursor aggregation, particle formation, and calcination , catalytically active particles were synthesized on top of the 3, micropillars, allowing for high-throughput SWNT growth and in situ characterization. Patterning compositional nanoparticle gradients onto ARES micropillars.
B Dark-field optical image. Inset Four pillars with the nanodome polymer features in a square array on top of the micropillars. Magnification: 2. While transition metal catalysts, like Fe and Co, are traditionally used for SWNT growth, recent breakthroughs using bimetallic catalysts, such as W-Co 38 , prompted us to study an alloy system for this application, Au-Cu, which is known to form fully miscible phases at the nanoscale over all compositions 28 , For this initial proof-of-concept catalyst screen, we chose to only study particle composition effects on SWNT growth.
Due to the size restriction 1—3 nm of catalytically active particles for SWNT growth, a size gradient was not included in this screen. The 5-mm width of the ARES substrate requires the 1. We know that this is accurate, because the tight correlation between ink composition and resulting particle composition has been studied in detail 27 , All three substrates were heat treated to form catalytically active particles and placed into the ARES reaction chamber.
Each pillar was individually heated to the growth temperature under a total pressure of 20 torr, consisting of C 2 H 4 partial pressure SWNT growth occurred over 1 min. After each growth, the laser power was turned down, and a low-power s Raman scan was collected at room temperature. The integrated intensity of the G band, normalized to the Si peak, from this room temperature scan G max is proportional to the yield of the SWNTs on the micropillar. Ten growths were performed for each nanoparticle composition five in each temperature range , amounting to a total of growth experiments.
The average G max values are shown as a function of nanoparticle composition in Fig.
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While this composition was not previously known for its ability to catalyze SWNT growth, it has been shown to have enhanced catalytic activity in electrochemistry, with Au 3 Cu exhibiting a peak activity for the reduction of CO 2 greater than any other composition in the Au-Cu system Moreover, we observed higher overall SWNT yield in the lower-temperature range. Taking melting point depression into account, these growth temperatures are near the melting points of Au-Cu nanoalloys, with particles containing a higher gold content having higher melting temperatures In addition to the in situ data, diameter-dependent radial breathing modes were collected using ex situ Raman spectroscopy, which indicate increasing SWNT diameters with Au fraction SI Appendix , Fig.
Compositional breakdown of catalytic activity. Error bars represent SEM. We independently confirmed the ARES results with more traditional tube furnace-based thermal CVD growth experiments, where the appropriate amounts of Au and Cu were used in the form of salts auric acid and cupric nitrate dissolved in ethanol followed by dip coating of SiO 2 substrates. While the products from these growths exhibited a higher D-band intensity due to an excess of ethylene being present the tube furnace growths were performed at atmospheric pressure, while ARES growths were carried out at 20 torr; excess ethylene leads to more disordered C , the G-band trends for the growths performed with the SPBCL-synthesized particles were mirrored using this more conventional particle synthesis and growth method SI Appendix , Fig.
In conclusion, we have reported a method for rapidly synthesizing and screening massively complex combinatorial megalibraries of nanostructures. These greater than 1-million particle libraries are the first of their kind and lay the foundation for a type of inorganic combinatorial science that focuses on using such libraries to identify properties associated with nanostructures of interest.
These libraries contain approximately as many new inorganic materials as scientists have cumulatively made to date.
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The ability to generate megalibraries of polyelemental nanoparticles will allow researchers to ask and answer key questions in fields beyond catalysis, including photonics, plasmonics, magnetism, solar light harvesting, biology, and nanomedicine. Moreover, this synthetic capability will challenge researchers in these fields to develop rapid ways of probing discrete locations of the libraries to screen for properties of interest, which may dramatically vary from system to system. Polydimethylsiloxane was made from 3. Glass slides were obtained from VWR. Metal compounds, auric acid, sodium tetrachloropalladate, and cupric nitrate were purchased from Sigma-Aldrich and used without additional purification.
PPL arrays were made as previously reported 25 , Solutions with fluorophore compounds were wrapped in foil and kept on a shaker for 1 h. Solutions containing metal precursors were brought to pH values of three to four and shaken for 12—24 h. All pen arrays were plasma cleaned for 2 min under oxygen at 60 W before spray inking. The two airbrushes were sprayed in unison.
After spraying, the pen arrays were placed with the pens facing up and covered for 15 min while the solution dried. To convert polymer features into nanoparticles, the substrates were put into a tube furnace and thermally annealed. The heating conditions were as follows. The metal precursors aggregate during the first step under Ar followed by nanoparticle formation and reduction during the second step under H 2.
For SWNT catalyst particle preparation, an additional calcination step was performed in air. Images were processed in Nanoscope Analysis 1.
Images were processed with ImageJ and Zen Blue. Maximum intensity projections of confocal stacks were generated for each tile. Tiles were stitched with vignette corrections. Intensity plots were created from pixel moving averages.
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Fluorescence spectra were collected at an incident energy of A horizontal slit size of 3 mm was used, and the sample was translated to probe the Au-Pd gradient at various points on the sample. The vertical slit size was adjusted according to footprint calculation, such that the entire sample length would be measured, to both maximize fluorescence counts from the elements of interest and achieve a global representation of the Au-Pd gradient.
Polymer nanoreactors containing Au and Cu precursor ions were deposited on nm silicon nitride membranes using dip pen nanolithography and thermally treated under the same conditions as described earlier. The high-angle annular dark-field images were collected with a collection angle of 68— mrad, and annular bright-field images were collected with a collection angle of 8—34 mrad. To write the particles onto the micropillar substrates used for ARES, a rotational stage was integrated into the patterning stage to allow for the alignment of pens with pillars.