Aerosol particles are ubiquitous in the atmosphere, and currently contribute a large uncertainty to climate models. Part of the endeavour to reduce this uncertainty takes the form of improving our understanding of aerosol at the microphysical level, thus enabling chemical and physical processes to be more accurately represented in larger scale models. In addition to modeling efforts, there is a need to develop new instruments and methodologies to interrogate the physicochemical properties of aerosol. This perspective presents the development, theory, and application of optical trapping, a powerful tool for single particle investigations of aerosol. After providing an overview of the role of aerosol in Earth's atmosphere and the microphysics of these particles, we present a brief history of optical trapping and a more detailed look at its application to aerosol particles. We also compare optical trapping to other single particle techniques. Understanding the interaction of light with single particles is essential for interpreting experimental measurements. In the final part of this perspective, we provide the relevant formalism for understanding both elastic and inelastic light scattering for single particles. The developments discussed here go beyond Mie theory and include both how particle and beam shape affect spectra. Throughout the entirety of this work, we highlight numerous references and examples, mostly from the last decade, of the application of optical trapping to systems that are relevant to the atmospheric aerosol.

We investigate Raman scattering from optically deformed droplets both theoretically and experimentally. Using a dual-beam optical trap, single aqueous aerosol microdroplets are held in an environmentally controlled cell and deformed, while both input and output resonances are simultaneously excited. Our systematic investigation shows that, depending on the scattering angle and whispering gallery mode (WGM) order and polarization, drastically different line shapes can be observed. The inelastic light scattering formalism developed here accounts for the profile of the excitation beam and the nonspherical shape of the optically deformed particle. Model calculations are in good agreement with measured Raman spectra and correctly predict the splitting of WGMs in measured spectra. The experiments and model presented here enable the accurate characterization of nanometer-sized deformations in micrometer-sized particles, which has many potential applications.

Optical trapping has become an important tool in a wide range of fields. While these traps are most commonly realized using optical tweezers, dual-beam optical traps offer specific advantages for certain experiments. It is commonly assumed that a particle will become trapped midway between the focal points of the two beams. However, this is not always the case. We perform a theoretical and experimental investigation of trapping positions of weakly absorbing, spherical particles in a dual-beam optical trap. We evaluate the effect of offsetting the beams in the direction of propagation and identify four regimes with distinct trapping behavior. The effect of an offset perpendicular to the propagation direction and an imbalance in power between the two beams is also considered. Experiments utilize an aqueous aerosol particle whose size can be readily controlled and monitored over hundreds of nanometers. As such, it serves as an excellent probe of the optical trap. We demonstrate that it is possible to fit the evolution of the particle trapping position in order to determine the position of the particle relative to the focal point of each beam. The results presented here provide key insights into the workings of dual-beam optical traps, elucidating more complex behaviors than previously known.

A dual-beam optical trap is used to controllably deform single aerosol particles. Collection of cavity-enhanced Raman spectra allows the deformation to be measured, and enables the calculation of particle surface tension.

Aqueous proton transport plays a key role in acid-base neutralization, and energy transport through biological membranes and hydrogen fuel cells. Extensive experimental and theoretical studies have resulted in a highly detailed elucidation of one of the underlying microscopic mechanisms for aqueous excess proton transport, known as the von Grotthuss mechanism, involving different hydrated proton configurations with associated high fluxional structural dynamics. Hydroxide transport, with approximately two-fold lower bulk diffusion rates than those of excess protons, has received much less attention. We present femtosecond UV/IR pump-probe experiments and ab initio molecular dynamics simulations of different proton transport pathways of the bifunctional photoacid 7-hydroxyquinoline (7HQ) in water-methanol mixtures. For 7HQ solvent-dependent photoacidity, free energy-reactivity correlation behaviour and QM/MM trajectories point to a dominant OH-/CH3O- transport pathway, for all water-methanol mixing ratios investigated. This provides conclusive evidence for the hydrolysis/methanolysis acid-base neutralization pathway formulated by Manfred Eigen half a century ago.

We present an effective oscillator model for the wavelength‐dependent complex refractive index of weakly absorbing aqueous solutions. The model is derived using the causal connection between the real and imaginary parts of the refractive index that is described through the Kramers‐Kronig relation. Using cavity‐enhanced Raman spectroscopy, we measure both the real and imaginary parts of the refractive index of several aqueous solutions whose optical properties are relevant to seawater, aqueous sea‐salt particles, and aqueous organic aerosol. It is demonstrated that the effective oscillator model accurately describes both the real and imaginary parts of the refractive index of aqueous solutions across a wide range of water activities and optical wavelengths. Finally, through a comparison with measurements, we verify that mixing rule calculations utilizing oscillator parameters from solutions containing a single solute and water can be used to predict the optical properties of aqueous solutions containing multiple solutes.

Advancements in designing complex models for atmospheric aerosol science and aerosol-cloud interactions rely vitally on accurately measuring the physicochemical properties of microscopic particles. Optical tweezers are a laboratory-based platform that can provide access to such measurements as they are able to isolate individual particles from an ensemble. The surprising ability of a focused beam of light to trap and hold a single particle can be conceptually understood in the ray optics regime using momentum transfer and Newton's second law. The same radiation pressure that results in stable trapping will also exert a deforming optical stress on the surface of the particle. For micron-sized aqueous droplets held in the air, the deformation will be on the order of a few nanometers or less, clearly not observable through optical microscopy. In this study, we utilize cavity-enhanced Raman scattering and a phenomenon known as thermal locking to measure small deformations in optically trapped droplets. With the aid of light-scattering calculations and a model that balances the hydrostatic pressure, surface tension, and optical pressure across the air-droplet interface, we can accurately determine surface tension from our measurements. Our approach is applied to 2 systems of atmospheric interest: aqueous organic and inorganic aerosol.

Femtosecond UV/IR pump-probe experiments and ab initio molecular dynamics simulations of 7-hydroxyquinoline in water-methanol mixtures demonstrate an unexpectedly dominant OH-/CH3O- transport pathway but consistent with a solvent-dependent photoacidity free energy-reactivity correlation behaviour.

A quantitative understanding of light scattering by small homogeneous particles requires accurate knowledge of particle geometry and complex refractive index, m = n + ik. In weakly absorbing particles, k can be on the order of 10-9, which is well below the detection limit of almost all light scattering based instruments. Here, we describe a dual-beam optical trap that can simultaneously determine n, k, and the radius, s, of weakly absorbing aerosol particles. We utilize cavity-enhanced Raman scattering to determine n and s and electromagnetic heating from the trapping laser itself to determine k. The relationship between particle size, the trapping cell conditions, the parameters of the trapping laser, and electromagnetic heating is thoroughly discussed and it is shown that the proper choice of a light scattering model is necessary to retrieve accurate values of k when fitting measurements. The phenomenon of optical multistability and its connection to thermal locking and thermal jumping is investigated through both modeling and measurements as understanding this behavior is essential when interpreting results from electromagnetic heating experiments. Measurements are made on three different atmospheric aerosol model systems and k as low as 5.91 × 10-9 are found.

Optical trapping allows for high precision studies of many microphysical and chemical processes as it enables measurements on the single-particle level. This has been a tremendous benefit to fundamental aerosol research. In the vast majority of these experiments, trapped particles are characterized using light scattering - most often angular light scattering (phase functions) or Mie resonance spectroscopy. In this report, we compare the radii and refractive indices of best-fit found with these two light scattering methods by trapping single aerosol particles in a relative humidity-controlled cell where we can simultaneously measure both phase functions and Mie resonances, the latter of which are found using cavity-enhanced Raman scattering. Additionally, we compare best-fits found using both one- and two-dimensional phase functions. The application of Mie theory to these light scattering problems is thoroughly reviewed. Both the accuracy and uncertainty of the best-fits that these light scattering techniques produce are investigated using a model aqueous inorganic aerosol particle. (C) 2018 Elsevier Ltd. All rights reserved.

From gene expression to nanotechnology, understanding and controlling DNA requires a detailed knowledge of its higher order structure and dynamics. Here we take advantage of the environment-sensitive photoisomerization of cyanine dyes to probe local and global changes in DNA structure. We report that a covalently attached Cy3 dye undergoes strong enhancement of fluorescence intensity and lifetime when stacked in a nick, gap or overhang region in duplex DNA. This is used to probe hybridization dynamics of a DNA hairpin down to the single-molecule level. We also show that varying the position of a single abasic site up to 20 base pairs away modulates the dye-DNA interaction, indicative of through-backbone allosteric interactions. The phenomenon of stacking-induced fluorescence increase (SIFI) should find widespread use in the study of the structure, dynamics and reactivity of nucleic acids.

A characteristic property of colloidal conductor nanocrystal quantum dots (QDs) is their emission intermittency. Although a unifying theory of QD photo processes remains elusive, the importance of charged states is clear. We now report a new approach to directly study the role of surface charge on QD emission by adding metal ions to individual, core-only QDs immobilized in aqueous solution in an agarose gel. The CdTe QDs shbw very stable emission in the absence of metal ions but a dramatic and reversible increase in blinking due to the resence of trivalent metal ions. Our results support a charge-separation model, in which the major blinking pathway is the surface trapping of electrons; transiently bound metal ions dose to the QD surface enhance this process.

The first single-molecule fluorescence detection of a structurally-defined synthetic carbohydrate is reported: a heparan sulfate (HS) disaccharide fragment labeled with Alexa488. Single molecules have been measured whilst freely diffusing in solution and controlled encapsulation in surface-tethered lipid vesicles has allowed extended observations of carbohydrate molecules down to the single-molecule level. The diverse and dynamic nature of HS-protein interactions means that new tools to investigate pure HS fragments at the molecular level would significantly enhance our understanding of HS. This work is a proof-of-principle demonstration of the feasibility of single-molecule studies of synthetic carbohydrates which offers a new approach to the study of pure glycosaminoglycan (GAG) fragments.