The present disclosure is generally related to aerosol generation.
The relationship between aerosol particles and cloud systems is a poorly understood nonlinear process and is the largest uncertainty to accurately predicting climate and extreme weather events.1,2 Aerosol particles serve as nucleation sites for water molecules to condense into droplets that can then form into clouds. Recent work posited that aerosol particles from the exhaust of ships enhanced the intensity and electrification of storms, showing that the density of lightning strikes doubled over shipping lanes.3 Moreover, ultrafine aerosol particles (diameter <50 nm), once thought to be too small to influence cloud formation, have recently been shown to significantly intensify the convective strength of cloud systems,2 indicating that nanoparticle aerosols may also be used for geoengineering applications.4-10
The influence of nanoparticle aerosols on cloud formation is extremely complex and hard to disentangle, and a significant need exists to experimentally model these systems in controlled environments to carefully examine the nanoscale mechanisms governing these macroscale processes. Aerosols composed of micrometer-sized particles have been thoroughly investigated for decades.11 However, the experimental aerosolization and optical detection of nanoparticle aerosols is a longstanding challenge due to factors such as aggregation upon the liquid-gas phase transition, relatively dilute concentrations, or small light-matter coupling.12
Plasmonic nanoparticles are promising candidates for benchtop aerosol studies. They couple strongly to light, leading to the capability to optically detect them in dilute concentrations, and they are also sensitive to changes in their surrounding environment. A simple harmonic oscillator model can be used to describe the behavior of the plasmonic nanoparticles in an optical field.11 From this model, the imaginary electric susceptibility of a plasmonic nanoparticle is χ″=βωp2ω/[(Lωp2−ω2)2+β2ω2], where β is the damping constant, L is the depolarization factor, ωp is the plasma frequency, and ω is the frequency of the incident light. The imaginary susceptibility, and consequently the absorption, is a maximum at resonance, ω=√{square root over (L)}ωp, yielding χ″max=ωp/(β√{square root over (L)}). Therefore, a pragmatic nanoparticle to maximize the absorption is a gold nanorod13-15 due to its large ωp, small L (along the long axis of the nanorod), and mature chemical-based fabrication.
The nanorods will be thermodynamically stable in the gas state when the gravitational force ΔρVg is less than the stabilizing thermal forces kBT/l, where Δρ is the density difference between the nanorod and gas, V=¾πr2l is the volume of the nanorod, l is the length and r is the radius of the nanorod, kB is the Boltzmann constant, and T is the absolute temperature. Accordingly, if the length of the gold nanorods is smaller than 3kBT/4πΔρgr2≈μm, then they will remain suspended in the gas state. Gold is also an inert metal, making it biocompatible and environmentally friendly. Additionally, recent gram-scale, colloidal gold nanorod synthesis breakthroughs have now made these materials accessible in large quantities.16
Disclosed herein is an apparatus comprising: a vessel for containing a suspension comprising a liquid and solid particles suspended therein; a tube having a narrowed portion configured to draw the suspension from the vessel into the tube when a gas flows through the tube; an aerosol generator coupled to the tube for forming an aerosol from the suspension; a dehydrator coupled to the aerosol generator for removing the liquid from the aerosol forming a dried aerosol; a multiple-pass spectroscopic absorption cell coupled to the dehydrator to pass the dried aerosol into the absorption cell; and a Fourier transform spectrometer coupled to the absorption cell to measure an absorption spectrum of the dried aerosol.
A more complete appreciation will be readily obtained by reference to the following Description of the Example Embodiments and the accompanying drawings.
In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that the present subject matter may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the present disclosure with unnecessary detail.
Disclosed herein is a solution to a decades-old problem of simultaneously aerosolizing and measuring the optical response of plasmonic nanoparticles in the gas phase, thereby uniting the fields of plasmonics and aerosols. It is shown that the aerosols are optically homogeneous, thermodynamically stable, with wide wavelength tunability, and extremely high sensitivities to their environment that may be useful in aiding geoengineering challenges. It is anticipated that plasmonic aerosols will open up broad and innovative approaches to understand the underlying physics of inaccessible climatology, astronomy, petroleum, and medical environments. In the context of vacuum microelectronics,17-19 if plasmonic aerosols are encapsulated into micron-sized elements and gated using external electric fields, then the electro-optic properties of the element may be reconfigurable by controlling the orientational order of the nanorods.20,21 These materials may also be useful for nonlinear optics6-8, nanojet printing22, molecular diagnostics23, or nanomedicines24.
The disclosed apparatus is illustrated in
The suspension then flows into an aerosol generator which forms an aerosol from the suspension. This aerosol contains liquid droplets with suspended particles. The droplets may be, for example, up to 1 micron in diameter. The aerosol then flows through a dehydrated that removes all or most of the liquid to make a dried aerosol. The dehydrator may include a desiccant the dries the aerosol by diffusion. The dried aerosol may contain only the particles suspended in the gas, or there may be trace amounts of liquid remaining.
The dried aerosol then flows into a multiple-path spectroscopic absorption cell, such as a Herriott cell. The optical path length of the cell may be, for example, up to 20 meters long. A Fourier transform spectrometer is then used to measure an absorption spectrum of the dried aerosol. The spectrum may include the IR, visible, and/or UV range, including near and/or far IR.
A vacuum pump at the spectrometer end may be used to draw the gas or air through the entire apparatus.
The following examples are given to illustrate specific applications. These specific examples are not intended to limit the scope of the disclosure in this application.
Materials—Hexadecyltrimethylammonium bromide (CTAB, >98%) and silver nitrate (>99.99%) were purchased from GFS Chemicals. Octadecyltrimethylammonium bromide (OTAB, >98%), benzyldimethylhexadecylammonium chloride (BDAC, >95%), and L-ascorbic acid (>99.0%) were purchased from TCI. Gold (III) chloride trihydrate (99.9%), sodium borohydrate (99.99%), hydroquinone (>99%), and trisodium citrate dihydrate were purchased from Sigma-Aldrich. Thiol-terminated polystyrene (Mn=5000 Da) was purchased from Polymer Source, Inc.
Nanorod synthesis—Multiple nanorod synthesis techniques were used based on the desired aspect ratio. High aspect ratio nanorods (30-40) were synthesized according to the procedure described by Kitahata et al.25 A CTAB concentration of 100 mM was used for all high aspect ratio synthesis methods while the OTAB concentration was varied between 30-75 mM to change the nanorod aspect ratio. The temperature was fixed at 20° C. for both the seed and growth solutions. Nanorods with an aspect ratio of approximately 17 were prepared using the seed-mediated procedure described by Zubarev et al.27 Nanorods with an aspect ratio of 5 were synthesized with a CTAB/BDAC surfactant growth solution as described by Park et al.16 The resulting gold nanorod suspensions were centrifuged at either 5,000 rpm (AR 30-40) or 10,000 rpm (AR 5-17) for 15 min. and resuspended in DI water to their initial volumes before another centrifugation step and 10-fold concentration. For redispersion in toluene, nanorods were phase transferred using thiol-terminated polystyrene according to a previously established procedure.20
Aerosolization—The experimental setup is schematically illustrated in
COMSOL simulations—A gold nanorod was constructed in a box with the length of the rod in the z-direction and the center of the nanorod located at x=y=z=0. The refractive index of the surrounding materials was considered to be constant for the wavelengths studied (air, water, or toluene, n=1, 1.33, or 1.475, respectively). The refractive index of gold was taken from Rakić.32 The computational requirements were reduced by symmetry, computing the electric field of only ¼ of the structure. The nanorod and simulation box were cut in half in the xy-plane at z=0 and in the xz-plane at y=0. The xy-plane was specified as a Perfect Electric Conductor (PEC) and the xz-plane was specified as a Perfect Magnetic Conductor (PEC). All other edges were surrounded by Perfectly Matched Layers (PML) to absorb reflections. An illustration of the model can be seen in
A background electric field propagating in the x-direction and polarized in the z-direction was specified in the calculation to excite the plasmon mode, Eb,z=E0exp(−ik0nmedx), where Eb,z is the z-component of the background electric field, E0=1 V/m, k0 is the free space wavevector, and nmed is the refractive index of the medium. Absorption cross sections (σabs) were calculated by integrating the power dissipation, Q, over the volume of the nanorod,
where Pin is the input power, which is calculated as,
where Z0 is the characteristic impedance of vacuum. Scattering cross sections (σscat) were calculated by integrating the Poynting vector, S, over a surface surrounding the simulation domain (the boundary between the surrounding medium and the PML),
Both σabs and σscat were multiplied by a factor of four to adjust for calculating the electric field of only ¼ of the structure.
Results and discussion—
The absorbance spectra of the gold nanorods in the gas phase are shown in
Due to water's prohibitively large absorption bands beyond 1.2 μm, the nanorods were phase transferred from water into toluene suspensions20 to enable the absorbance spectra in the infrared region to be measured in the liquid phase, as seen in
The aspect ratio of the gold nanorods was varied from 1.5, 2.5, 3, and 4.5 in water to 10, 15, and 30 in toluene in
The optical response of gold nanorods in the gas phase with aspect ratios of 5, 15, and 30 are shown in
Three-dimensional finite-element simulations in
To understand the spectroscopic evolution of the evaporating water-nanorod droplets, further simulations were carried out in
Transitioning the nanorods from the liquid to the gas state results in large shifts in the absorbance peak wavelength, as shown in
To validate that the nanorods are dispersed in air, experiments and simulations were carried out measuring the absorbance peak wavelength at various aspect ratios of the gold nanorods: 7.5, 13, 16.8, 33, and 38 in toluene and air (
The simulations show that the absorbance peak wavelength depends linearly on the aspect ratio of the nanorods, λtoluene=0.135(l/d)+0.422 and λair=0.0883 (l/d)+0.359, and the peak redshifts as the refractive index of the host medium increases.28
The experimental data were found to agree well with the simulation data, within experimental uncertainty, and that the absorbance peak is proportional to the aspect ratio. Furthermore, the experimental data for the nanorods in air agree well with the simulation predictions for their peak wavelengths, implying the nanorods are homogeneously dispersed as an aerosol.
To further confirm that the nanorods are uniformly suspended in air, the longitudinal absorption peak wavelength can be related to the refractive index of the host medium nm29 by
where λp is the plasma wavelength of gold, ns is the refractive index of the ligand shell coating the nanorods, and f is the ellipsoidal volume fraction of the inner nanorod to the outer ligand shell. The depolarization factor of the long axis of the nanorod is
L∥=[(1−ϵ2)/ϵ]{[(½ϵ)[ln(1−ϵ)/(1+ϵ)]−1]}
where ϵ=√{square root over (1−(l/d)2)}.
If the nanorods are very long (1/L∥>>1) and there is no ligand shell (ns=nm), then Eq. (1) can be differentiated with respect to nm and then series expanded about L∥ to approximate the sensitivity,
This result implies that if the geometric (L∥) and material (λp) properties of the nanorod are known, the shift in the absorption peak wavelength can be estimated as the host medium surrounding the nanorods is varied.
In
The absorbance peak wavelength shifts are on the order of several microns per refractive index unit (RIU) in
In summary, the aerosolization of gold nanorods from concentrated liquid suspensions, while simultaneously measuring their optical spectra at benchtop scales was demonstrated. The plasmonic aerosol absorption peaks are sharp and well defined with effective quality factors as large as 2.4. It was shown that by controlling the aspect ratio of the nanorods, the aerosol absorption peaks are broadly tunable over 2500 nm from visible to midwave infrared wavelengths. It was found that the sensitivity of the longitudinal absorption peak wavelength to the refractive index of the host medium depends linearly on the nanorod aspect ratio and can be estimated from the geometric and material properties of the nanorod. Utilizing this sensitivity dependence, it was also shown that minute changes of the host refractive index of 10−4 may be detectable, suggesting these materials could be useful for environmental or remote sensing.
Obviously, many modifications and variations are possible in light of the above teachings. It is therefore to be understood that the claimed subject matter may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles “a”, “an”, “the”, or “said” is not construed as limiting the element to the singular.
This application claims the benefit of U.S. Provisional Application No. 62/918,713, filed on Feb. 16, 2019.
Number | Name | Date | Kind |
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20160214075 | Suslick | Jul 2016 | A1 |
Entry |
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Geldmeier et al., “Plasmonic aerosols” Phys. Rev. B 99, 081112(R) (Feb. 13, 2019). |
Number | Date | Country | |
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20200261939 A1 | Aug 2020 | US |
Number | Date | Country | |
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62918713 | Feb 2019 | US |