13 research outputs found
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Design and assembly of metal oxide nanocrystal gels via depletion attractions
Achieving and implementing macroscopic materials capable of displaying the unique properties inherent to inorganic nanocrystals requires bridging the nanoscale and the length scales of larger orders of magnitude in a systematic, controllable, and scalable way. Over the last 15 years, nanocrystal gels have been developed and investigated as potential materials to tackle this need. However, available gelation methods rely on chemical reactions and interactions specific to the stabilizing molecules on the nanocrystal surface, and are therefore not readily adaptable across broad types of materials. Specifically, most studies have focused on gelation methods for metal chalcogenides and noble metals, whereas progress on metal oxide nanocrystal gels has lagged behind.
This dissertation investigates and demonstrates an alternative gelation method based on entropic depletion attractions that are not dependent on specific surface chemistries and have not been explored in nanoscale colloidal gels. In the first study, a proof of concept system is developed, where depletion attractions induce the gelation of tin-doped indium oxide nanocrystals in the presence of a polymer depletant and achieve a macroscopic material with optical properties reflective of both the microstructure and the nanostructured building blocks. The mechanism of gelation is assessed by comparing the observed phase behavior to theoretical predictions and the microstructure is characterized by small-angle X-ray scattering (SAXS). Next, the universal applicability of depletion attractions is demonstrated by varying the composition and shape of the building blocks while fixing size and nanocrystal volume fraction. The gelation of spherical nanocrystals occurs at the same depletant concentration and this phase transition threshold does not depend on the specific composition of the metal oxide nanocrystal. Consistent with theoretical phase boundary calculations, cubic nanocrystals form gels at a lower depletant concentration than spherical nanocrystals due to the ability to pack face-to-face and therefore increase the overlap excluded volume during assembly. Finally, a method to polymer-wrap tin-doped indium oxide nanocrystals in a controllable way while maintaining colloidal stability is investigated in an effort to tune the physicochemical properties of the metal oxide building blocks available for gelation.Chemical Engineerin
Intrinsic Optical and Electronic Properties from Quantitative Analysis of Plasmonic Semiconductor Nanocrystal Ensemble Optical Extinction
The optical extinction spectra arising from localized surface plasmon
resonance in doped semiconductor nanocrystals (NCs) have intensities and
lineshapes determined by free charge carrier concentrations and the various
mechanisms for damping the oscillation of those free carriers. However, these
intrinsic properties are convoluted by heterogeneous broadening when measuring
spectra of ensembles. We reveal that the traditional Drude approximation is not
equipped to fit spectra from a heterogeneous ensemble of doped semiconductor
NCs and produces fit results that violate Mie scattering theory. The
heterogeneous ensemble Drude approximation (HEDA) model rectifies this issue by
accounting for ensemble heterogeneity and near-surface depletion. The HEDA
model is applied to tin-doped indium oxide NCs for a range of sizes and doping
levels but we expect it can be employed for any isotropic plasmonic particles
in the quasistatic regime. It captures individual NC optical properties and
their contributions to the ensemble spectra thereby enabling the analysis of
intrinsic NC properties from an ensemble measurement. Quality factors for the
average NC in each ensemble are quantified and found to be notably higher than
those of the ensemble. Carrier mobility and conductivity derived from HEDA fits
matches that measured in the bulk thin film literature
Assembly of colloidal nanocrystals into open networks
Inorganic nanocrystals exhibit a wide variety of optical, electronic, chemical, and electrochemical functionality that is synthetically tunable based on their size and composition. Their properties and emerging methods for functionalizing their surfaces with specific chemical agents pose the exciting prospect to program the assembly of nanostructured materials whose properties depend intimately on both the characteristics of the building
blocks and the mesoscale organization of these in the assembly. In this presentation, I describe novel strategies for assembling optically active nanocrystals into organized gel networks. In particular, theoretical frameworks predict open gel architectures when the extent of inter-particle bonding (i.e. valence) is constrained.[1] To achieve a chemically tunable valence, we functionalized semiconductor nanocrystals with highly charged chalcogenidometallates clusters that induce long range repulsive interactions.[2] The addition of controlled amounts of a cationic crosslinking agent determines the assembly of the nanocrystals into a low volume fraction gel. In another assembly strategy, short range attractive forces are induced between charge-stabilized nanocrystal colloids by the introduction of oligomeric polyethylene glycol (PEG). At low PEG concentrations, it can crosslink nanocrystals into a gel. At higher concentrations, PEG effectively passivates the nanocrystal surfaces, yet excess PEG can induce gel network assembly through the depletion attraction. The organization of the gel networks is characterized by small angle X-ray scattering, from which the fractal dimension that describes the gel topology is determined. We present an integrated approach leveraging theory, synthesis, characterization, and simulation to predict, realize, and analyze the formation of low volume fraction gels from colloidal nanocrystals with unusual optical properties in the visible and infrared spectral ranges.
References:
[1] BA Lindquist, RB Jadrich, DJ Milliron, TM Truskett, “On the Formation of Equilibrium Gels via a Macroscopic Bond Limitation,” J. Chem. Phys. 145 (2016), 074906.
[2] A Singh, BA Lindquist, GK Ong, RB Jadrich, A Singh, H Ha, CJ Ellison, TM Truskett, DJ Milliron, “Linking Semiconductor Nanocrystals into Gel Networks through All-Inorganic Bridges,” Angew. Chem. Int. Ed. 54 (2015), 14840-14844
Gelation of Plasmonic Metal Oxide Nanocrystals by Polymer-Induced Depletion-Attractions
Gelation of colloidal nanocrystals (NCs) emerged as a strategy to preserve
inherent nanoscale properties in multiscale architectures. Yet available
gelation methods still struggle to reliably control nanoscale optical phenomena
such as photoluminescence and localized surface plasmon resonance (LSPR) across
NC systems due to processing variability. Here, we report on an alternative
gelation method based on physical inter-NC interactions: short-range
depletion-attractions balanced by long-range electrostatic repulsions. The
latter are established by removing the native organic ligands that passivate
tin-doped indium oxide (ITO) NCs while the former are introduced by mixing with
small polyethylene glycol (PEG) chains. As we incorporate increasing
concentrations of PEG, we observe a reentrant phase behavior featuring two
favorable gelation windows; the first arises from bridging effects while the
second is attributed to depletion-attractions according to phase behavior
predicted by our unified theoretical model. The NCs remain discrete within the
gel network, based on X-ray scattering and high-resolution transmission
electron microscopy. The infrared optical response of the gel is reflective of
both the NC building blocks and the network architecture, being characteristic
of ITO NC LSPR with coupling interactions between neighboring NCs
Universal Gelation of Metal Oxide Nanocrystals via Depletion Attractions
Nanocrystal gelation provides a powerful framework to translate nanoscale
properties into bulk materials and to engineer emergent properties through the
assembled microstructure. However, many established gelation strategies rely on
chemical reactions and specific interactions, e.g., stabilizing ligands or ions
on the surface of the nanocrystals, and are therefore not easily transferrable.
Here, we report a general gelation strategy via non-specific and purely
entropic depletion attractions applied to three types of metal oxide
nanocrystals. The gelation thresholds of two compositionally distinct spherical
nanocrystals agree quantitatively, demonstrating the adaptability of the
approach for different chemistries. Consistent with theoretical phase behavior
predictions, nanocrystal cubes form gels at a lower polymer concentration than
nanocrystal spheres, allowing shape to serve as a handle to control gelation.
These results suggest that the fundamental underpinnings of depletion-driven
assembly, traditionally associated with larger colloidal particles, are also
applicable at the nanoscale
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In Situ Optical Quantification of Extracellular Electron Transfer Using Plasmonic Metal Oxide Nanocrystals**
Extracellular electron transfer (EET) is a critical form of microbial metabolism that enables respiration on a variety of inorganic substrates, including metal oxides. However, quantifying current generated by electroactive bacteria has been predominately limited to biofilms formed on electrodes. To address this, we developed a platform for quantifying EET flux from cell suspensions using aqueous dispersions of infrared plasmonic tin-doped indium oxide nanocrystals. Tracking the change in optical extinction during electron transfer enabled quantification of current generated by planktonic Shewanella oneidensis cultures. Using this method, we differentiated between starved and actively respiring cells, cells of varying genotype, and cells engineered to differentially express a key EET gene using an inducible genetic circuit. Overall, our results validate the utility of colloidally stable plasmonic metal oxide nanocrystals as quantitative biosensors in aqueous environments and contribute to a fundamental understanding of planktonic S. oneidensis electrophysiology using simple in situ spectroscopy.All data needed to evaluate the conclusions in the paper are
present in the paper and/or the Supporting Information. S. onei-
densis ΔmtrCΔomcAΔmtrF was a generous gift from Prof. Jeffrey
Gralnick (U. Minnesota), and E. coli MG1655 and S. oneidensis
S2933 were generous gifts from Prof. Lydia Contreras (UT Aus-
tin). Christopher Dundas is thanked for his contribution of the
engineered S. oneidensis strain. A. J. G. and S. L. G. were sup-
ported through National Science Foundation Graduate Research
Fellowships (Program Award No. DGE-1610403). Portions of this
work, including polymer synthesis and characterization by
Swagat Sahu and Brett A. Helms, were carried out as a user
project at the Molecular Foundry, which is supported by the
Office of Science, Office of Basic Energy Sciences of the U. S.
Department of Energy under contract no. DE-AC02-05CH11231.
This research was primarily supported by the National Science
Foundation through the Center for Dynamics and Control of
Materials: an NSF Materials Research Science and Engineering
Center under DMR-1720595. Additional research support was
provided by the Welch Foundation (Grants F-1929 and F-1848),
the National Institute of General Medical Sciences of the
National Institutes of Health under Award Number
R35 GM133640 (B. K. K.), and an NSF CAREER Award (1944334,
B. K. K.). The authors acknowledge use of shared research
facilities supported in part by the Texas Materials Institute
including the SAXS instrument acquired under an NSF MRI
grant (CBET-1624659), the Center for Dynamics and Control of
Materials: an NSF MRSEC (DMR-1720595), and the NSF National
Nanotechnology Coordinated Infrastructure (ECCS-1542159).Center for Dynamics and Control of Material
Aqueous Processing of Polymer-Coated Tin-Doped Indium Oxide (ITO) Nanocrystals as Electrochromic Thin Films
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Gelation of Plasmonic Metal Oxide Nanocrystals by Polymer-Induced Depletion-Attractions
Gelation of colloidal nanocrystals (NCs) emerged as a strategy to preserve inherent nanoscale properties in multiscale architectures. Yet available gelation methods still struggle to reliably control nanoscale optical phenomena such as photoluminescence and localized surface plasmon resonance (LSPR) across NC systems due to processing variability. Here, we report on an alternative gelation method based on physical inter-NC interactions: short-range depletion-attractions balanced by long-range electrostatic repulsions. The latter are established by removing the native organic ligands that passivate tin-doped indium oxide (ITO) NCs while the former are introduced by mixing with small polyethylene glycol (PEG) chains. As we incorporate increasing concentrations of PEG, we observe a reentrant phase behavior featuring two favorable gelation windows; the first arises from bridging effects while the second is attributed to depletion-attractions according to phase behavior predicted by our unified theoretical model. The NCs remain discrete within the gel network, based on X-ray scattering and high-resolution transmission electron microscopy. The infrared optical response of the gel is reflective of both the NC building blocks and the network architecture, being characteristic of ITO NC LSPR with coupling interactions between neighboring NCs.This research was supported by the National Science Foundation (NSF) through the Center for
Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agreement DMR-
1720595. SAXS data were collected at the Advanced Light Source’s beamline 7.3.3 at the
Lawrence Berkeley National Laboratory, a user facility supported by the U.S. Department of
Energy (DOE) Office of Science under contract no. DE-AC02CH11231. G.K.O was supported in
part by a NSF Graduate Research Fellowship under grant number DGE-1106400. Further support
is acknowledged from the Welch Foundation (F-1848 and F-1696 and the NSF (CHE-1609656
and CBET-1247945).Center for Dynamics and Control of Material