Temperature Measurement of Trapped, Thermally Sensitive Single Particles in an Optical Trap Using Raman Spectroscopy

Single particles trapped in an optical trap may experience temperature elevation, yet direct measurement of temperature and its distribution inside the optical trap of several to hundreds of microns in size remains a big challenge. We introduce a method that can measure the temperature inside a universal optical trap (UOT) using Raman spectroscopy of single trapped particles of high thermal conductivity. We measured temperature and temperature distributions inside the UOT using Raman shifts of single-walled carbon nanotubes (SWCNTs) and micron-sized diamonds (MSDs), which are heated by trapping laser beams directly or indirectly, depending on the location of the particle in the trap. We show that the temperature at the center of the UOT is much lower than the temperature along the hollow beams that form a hollow, cage-shaped UOT. In the range of the trapping laser power of 200-2950 mW, the surface temperature of particles trapped at the center of a UOT changes from 322 K to 830 K, correspondingly. This result gives a heating rate as a high thermal-absorbing particle trapped in the center of the UOT with 18.3 ± 0.4 °C/100 mW. In addition, the temperature gradient outside the UOT was also characterized by trapping SWCNT particles outside the UOT. Results show that when a light-absorbing particle is trapped for the study of material property, phase transitions, surface equilibrium process, chemical reactions, etc., this method can be used to measure temperature distribution and its variations in the trap and its surroundings.

Mie scattering from optically levitated mixed sulfuric acid-silica core-shell aerosols: observation of core-shell morphology for atmospheric science

Sulfuric acid is shown to form a core-shell particle on a micron-sized, optically-trapped spherical silica bead. The refractive indices of the silica and sulfuric acid, along with the shell thickness and bead radius were determined by reproducing Mie scattered optical white light as a function of wavelength in Mie spectroscopy. Micron-sized silica aerosols (silica beads were used as a proxy for atmospheric silica minerals) were levitated in a mist of sulfuric acid particles; continuous collection of Mie spectra throughout the collision of sulfuric acid aerosols with the optically trapped silica aerosol demonstrated that the resulting aerosol particle had a core-shell morphology. Contrastingly, the collision of aqueous sulfuric acid aerosols with optically trapped polystyrene aerosol resulted in a partially coated system. The light scattering from the optically levitated aerosols was successfully modelled to determine the diameter of the core aerosol (±0.003 μm), the shell thickness (±0.0003 μm) and the refractive index (±0.007). The experiment demonstrated that the presence of a thin film rapidly changed the light scattering of the original aerosol. When a 1.964 μm diameter silica aerosol was covered with a film of sulfuric acid 0.287 μm thick, the wavelength dependent Mie peak positions resembled sulfuric acid. Thus mineral aerosol advected into the stratosphere would likely be coated with sulfuric acid, with a core-shell morphology, and its light scattering properties would be effectively indistinguishable from a homogenous sulfuric acid aerosol if the film thickness was greater than a few 100 s of nm for UV-visible wavelengths.

Ionic-strength and pH dependent reactivities of ascorbic acid toward ozone in aqueous micro-droplets studied using aerosol optical tweezers

The heterogeneous oxidation reaction of single aqueous ascorbic acid (AH2) aerosol particles with gas-phase ozone was investigated in this study utilizing aerosol optical tweezers with Raman spectroscopy. The measured liquid-phase bimolecular rate coefficients of the AH2 + O3 reaction exhibit a significant pH dependence, and the corresponding values at ionic strength 0.2 M are (3.1 ± 2.0) × 105 M-1 s-1 and (1.2 ± 0.6) × 107 M-1 s-1 for pH ≈ 2 and 6, respectively. These results measured in micron-sized droplets are in agreement with those from previous bulk measurements, indicating that the observed aerosol reaction kinetics can be solely explained by liquid phase diffusion and AH2 + O3 reaction. Furthermore, the results indicate that high ionic strengths could enhance the liquid-phase rate coefficients of the AH2 + O3 reaction. The results also exhibit a negative ozone pressure dependence that can be rationalized in terms of a Langmuir-Hinshelwood type mechanism for the heterogeneous oxidation of AH2 aerosol particles by gas-phase ozone. The results of the present work imply that in acidified airway-lining fluids the antioxidant ability of AH2 against atmospheric ozone will be significantly suppressed.

Optical Trapping-Polarized Raman Microspectroscopy of Single Ethanol Aerosol Microdroplets: Droplet Size Effects on Rotational Relaxation Time and Viscosity

Optical trapping-polarized Raman microspectroscopy of single ethanol (EtOH) microdroplets with a diameter (d) of 6.1-16.5 μm levitated in an EtOH vapor-saturated air/N₂ gas atmosphere has been explored to elucidate the vibrational and rotational motions of EtOH in the droplets at 22.0 °C. The Raman spectral bandwidth of the C-C stretching vibrational mode observed for an aerosol EtOH microdroplet was narrower than that of bulk EtOH, suggesting that the vibrational/rotational motions of EtOH in the aerosol system were restricted compared to those in the bulk system. In practice, polarized Raman microspectroscopy demonstrated that the rotational relaxation time (τ_rot) of EtOH in an aerosol microdroplet with d = 16. 5 μm was slower (2.33 ps) than that in a bulk EtOH (1.65 ps), while the vibrational relaxation times (τ_vib) in the aerosol and bulk EtOH systems were almost comparable with one another: 0.86-0.98 ps. Furthermore, although the τ_vib value of an aerosol EtOH microdroplet was almost unchanged irrespective of d as described above, the τ_rot value increased from 2.33 to 3.57 ps with a decrease in d from 16.5 to 6.1 μm, which corresponded to the increase in EtOH viscosity (η) from 1.33 to 2.04 cP with the decrease in d. The droplet size dependences of τ_rot and η in an aerosol EtOH microdroplet were discussed in terms of the gas/droplet interfacial molecular arrangements of EtOH and Laplace pressure experienced by a spherical EtOH microdroplet in the gas phase.

The reaction of oleic acid monolayers with gas-phase ozone at the air water interface: the effect of sub-phase viscosity, and inert secondary components

Organic films that form on atmospheric particulate matter change the optical and cloud condensation nucleation properties of the particulate matter and consequently have implications for modern climate and climate models. The organic films are subject to attack from gas-phase oxidants present in ambient air. Here we revisit in greater detail the oxidation of a monolayer of oleic acid by gas-phase ozone at the air-water interface as this provides a model system for the oxidation reactions that occur at the air-water interface of aqueous atmospheric aerosol. Experiments were performed on monolayers of oleic acid at the air-liquid interface at atmospherically relevant ozone concentrations to investigate if the viscosity of the sub-phase influences the rate of the reaction and to determine the effect of the presence of a second component within the monolayer, stearic acid, which is generally considered to be non-reactive towards ozone, on the reaction kinetics as determined by neutron reflectometry measurements. Atmospheric aerosol can be extremely viscous. The kinetics of the reaction were found to be independent of the viscosity of the sub-phase below the monolayer over a range of moderate viscosities, , demonstrating no involvement of aqueous sub-phase oxidants in the rate determining step. The kinetics of oxidation of monolayers of pure oleic acid were found to depend on the surface coverage with different behaviour observed above and below a surface coverage of oleic acid of ∼1 × 1018 molecule m-2. Atmospheric aerosol are typically complex mixtures, and the presence of an additional compound in the monolayer that is inert to direct ozone oxidation, stearic acid, did not significantly change the reaction kinetics. It is demonstrated that oleic acid monolayers at the air-water interface do not leave any detectable material at the air-water interface, contradicting the previous work published in this journal which the authors now believe to be erroneous. The combined results presented here indicate that the kinetics, and thus the atmospheric chemical lifetime for unsaturated surface active materials at the air-water interface to loss by reaction with gas-phase ozone, can be considered to be independent of other materials present at either the air-water interface or in the aqueous sub-phase.

Measurement of the Raman spectra and hygroscopicity of four pharmaceutical aerosols as they travel from pressurised metered dose inhalers (pMDI) to a model lung

Particle inhalation is an effective and rapid delivery method for a variety of pharmaceuticals, particularly bronchodilation drugs used for treating asthma and COPD. Conditions of relative humidity and temperature inside the lungs are generally very different from the outside ambient air, with the lung typically being warmer and more humid. Changes in humidity, from inhaler to lung, can cause hygroscopic phase transitions and particle growth. Increasing particle size and mass can negatively affect particle deposition within the lung leading to inefficient treatment, while deliquescence prior to impaction is liable to accelerate drug uptake. To better understand the hygroscopic properties of four pharmaceutical aerosol particles; pharmaceutical particles from four commercially available pressurised metered dose inhalers (pMDIs) were stably captured in an optical trap, and their composition was examined online via Raman spectroscopy. Micron-sized particles of salbutamol sulfate, salmeterol xinafoate, fluticasone propionate and ciclesonide were levitated and examined over a range of relative humidity values inside a chamber designed to mimic conditions within the respiratory tract. The effect of temperature upon hygroscopicity was also investigated for salbutamol sulfate particles. Salbutamol sulfate was found to have significant hygroscopicity, salmeterol xinafoate showed some hygroscopic interactions, whilst fluticasone propionate and ciclesonide revealed no observable hygroscopicity. Thermodynamic and structural modelling is used to explain the observed experimental results.

Fluorescence lifetime imaging of optically levitated aerosol: a technique to quantitatively map the viscosity of suspended aerosol particles

A technique to measure the viscosity of stably levitated single micron-sized aerosol particles.

Heterogeneous oxidation of nitrite anion by gas-phase ozone in an aqueous droplet levitated by laser tweezers (optical trap): is there any evidence for enhanced surface reaction?

Optical trapping of micron-sized droplet morphology and heterogeneous kinetics with gas-phase ozone with nitrite in a wall-less apparatus.

Optical trapping and Raman spectroscopy of solid particles

Stable levitation and spectroscopic interrogation of solid particles is achieved, over extended time periods, using a new optical trap design.

Determining the unique refractive index properties of solid polystyrene aerosol using broadband Mie scattering from optically trapped beads

A method is described to measure the refractive index dispersion with wavelength of optically trapped solid particles in air. Knowledge of the refraction properties of solid particles is critical for the study of aerosol; both in the laboratory and in the atmosphere for climate studies. Single micron-sized polystyrene beads were optically trapped in air using a vertically aligned counter-propagating configuration of focussed laser beams. Each bead was illuminated using white light from a broadband light emitting diode (LED) and elastic scattering within the bead was collected onto a spectrograph. The resulting Mie spectra were analysed to accurately determine polystyrene bead radii to ±0.4 nm and values of the refractive index to ±0.0005 over a wavelength range of 480-700 nm. We demonstrate that optical trapping combined with elastic scattering can be used to both accurately size polystyrene beads suspended in air and determine their wavelength dependent refractive index. The refractive index dispersions are in close agreement with reported values for polystyrene beads in aqueous dispersion. Our results also demonstrate a variation in the refractive index of polystyrene, from bead to bead, in a commercial sample. The measured variation highlights that care must be taken when using polystyrene beads as a calibration aerosol.