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.

A Collection of Molecular Fingerprints of Single Aerosol Particles in Air for Potential Identification and Detection Using Optical Trapping-Raman Spectroscopy.

Characterization, identification, and detection of aerosol particles in their native atmospheric states remain a challenge. Recently, optical trapping-Raman spectroscopy (OT-RS) has been developed and demonstrated for characterization of single, airborne particles. Such particles in different chemical groups have been characterized by OT-RS in recent years and many more are being studied. In this work, we collected single-particle Raman spectra measured using the OT-RS technique and began construction of a library of OT-RS fingerprints that may be used as a reference for potential detection and identification of aerosol particles in the atmosphere. We collected OT-RS fingerprints of aerosol particles from eight different categories including carbons, bioaerosols (pollens, fungi, vitamins, spores), dusts, biological warfare agent surrogates, etc. Among the eight categories, spectral fingerprints of six groups of aerosol particles have been published previously and two other groups are new. We also discussed challenges, limitations, and advantages of using single-particle optical trapping-Raman spectroscopy for aerosol-particle characterization, identification, and detection.

Combined Experimental and Computational Kinetics Studies for the Atmospherically Important BrHg Radical Reacting with NO and O2.

We report on the first experimental determination of the absolute rate constant of the reaction of BrHg + NO in N2 bath gas using a laser photolysis-laser-induced fluorescence (LP-LIF) system. The rate constant of the reaction of BrHg + NO is determined to be 7.0-0.9+1.3 × 10-12 cm3 molecule-1 s-1 over 50-700 Torr and 315-353 K. The absence of a pressure or temperature dependence suggests that this reaction leads mainly to mercury reduction (Hg + BrNO) rather than mercury oxidation (BrHgNO). Our theoretical calculations using NEVPT2 energies on density functional theory (DFT) geometries are consistent with a barrierless reaction to form Hg + BrNO. The equilibrium constants and the rate constants of the reaction BrHg + O2 ⇌ BrHgOO are computed theoretically because they are too low to be measured in the LP-LIF system. Molecular oxygen quenches the LIF signal of BrHg with a large rate constant of (1.7 ± 0.1) × 10-10 cm3 molecule-1 s-1. Thus, different techniques that capture the absolute [BrHg(X̃)] would be advantageous for kinetics studies of BrHg reactions in the presence of O2. The computed equilibrium constant suggests a non-negligible upper limit of the fraction of BrHg stored as BrHgOO (up to 0.5) at low-temperature conditions, e.g., in the upper troposphere and in polar winters at ground level. Preliminary results indicate that BrHgOO behaves like HOO or organic peroxy radicals in reactions with atmospheric radicals.

Measurement of circular intensity differential scattering (CIDS) from single airborne aerosol particles for bioaerosol detection and identification.

The circular intensity differential scattering (CIDS), i.e. the normalized Mueller matrix element -S14/S11, can be used to detect the helical structures of DNA molecules in biological systems, however, no CIDS measurement from single particles has been reported to date. We report an innovative method for measuring CIDS phase functions from single particles individually flowing through a scattering laser beam. CIDS signals were obtained from polystyrene latex (PSL) microspheres with or without coating of DNA molecules, tryptophan particles, and aggregates of B. subtilis spores, at the size of 3 µm in diameter. Preliminary results show that this method is able to measure CIDS phase function in tens of microseconds from single particles, and has the ability to identify particles containing biological molecules.

Active, controlled circular, and spin-rotational movement of optically trapped airborne micro-particles.

We present a novel method for actively controlling circular and/or spin-rotational motion of an optically trapped airborne micro-particle. A 532-nm Gaussian laser beam is shaped into an elliptical ring by a pair of axicons and a cylindrical lens. The shaped beam is then focused into an elliptic cone that produces an optical trap. As the cylindrical lens is rotated, a torque is exerted on the trapped particle, resulting in circular or spin-rotational motion. We show examples of the circular-rotational movement as a function of laser power and the rotation rate of the cylindrical lens.

Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury.

We present a new chemical mechanism for Hg0/HgI/HgII atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg0 by Br and OH, subsequent oxidation of HgI by ozone and radicals, respeciation of HgII in aerosols and cloud droplets, and speciated HgII photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg0 concentrations and HgII wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg0 to HgII. Ozone is the principal HgI oxidant, enabling the efficient oxidation of Hg0 to HgII by OH. BrHgIIOH and HgII(OH)2, the initial HgII products of Hg0 oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of HgII to Hg0 takes place largely through photolysis of aqueous HgII-organic complexes. 71% of model HgII deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of HgI, and speciation and photoreduction of HgII in aerosols and clouds.

Optical-Trapping Laser Techniques for Characterizing Airborne Aerosol Particles and Its Application in Chemical Aerosol Study.

We present a broad assessment on the studies of optically-trapped single airborne aerosol particles, particularly chemical aerosol particles, using laser technologies. To date, extensive works have been conducted on ensembles of aerosols as well as on their analogous bulk samples, and a decent general description of airborne particles has been drawn and accepted. However, substantial discrepancies between observed and expected aerosols behavior have been reported. To fill this gap, single-particle investigation has proved to be a unique intersection leading to a clear representation of microproperties and size-dependent comportment affecting the overall aerosol behavior, under various environmental conditions. In order to achieve this objective, optical-trapping technologies allow holding and manipulating a single aerosol particle, while offering significant advantages such as contactless handling, free from sample collection and preparation, prevention of contamination, versatility to any type of aerosol, and flexibility to accommodation of various analytical systems. We review spectroscopic methods that are based on the light-particle interaction, including elastic light scattering, light absorption (cavity ring-down and photoacoustic spectroscopies), inelastic light scattering and emission (Raman, laser-induced breakdown, and laser-induced fluorescence spectroscopies), and digital holography. Laser technologies offer several benefits such as high speed, high selectivity, high accuracy, and the ability to perform in real-time, in situ. This review, in particular, discusses each method, highlights the advantages and limitations, early breakthroughs, and recent progresses that have contributed to a better understanding of single particles and particle ensembles in general.

Cavity ringdown spectroscopy of nitric oxide in the ultraviolet region for human breath test.

We report the spectrum of nitric oxide (NO) in the ultraviolet (UV) (225.4-227.0 nm) region based on cavity ringdown spectroscopy (CRDS). A cavity ringdown system, which consisted of a tunable UV laser source and a vacuum-pumped ringdown cavity, was constructed to measure NO at room temperature and atmospheric or reduced pressure. The measured spectra were validated using LIFBase simulations. The absorption cross-section of NO at the strongest absorption peak at 226.255 nm was measured to be 7.64 × 10-18 cm2 molecule-1. Using the measured mirror reflectivity of 99.55% at 226.255 nm, the detection limit of NO was determined to be 7.4 ppb (parts per billion) based on the standard 3-σ criteria. The stability and reproducibility of this CRDS system were also tested. Furthermore, exhaled gas samples from 203 human subjects (105 healthy people and 98 lung cancer patients) were measured using the system. Results demonstrated that the cavity ringdown spectroscopy in the deep-UV region has potential for breath NO test.

Optical-trapping of particles in air using parabolic reflectors and a hollow laser beam.

We present an advanced optical-trapping method that is capable of trapping arbitrary shapes of transparent and absorbing particles in air. Two parabolic reflectors were used to reflect the inner and outer parts of a single hollow laser beam, respectively, to form two counter-propagating conical beams and bring them into a focal point for trapping. This novel design demonstrated high trapping efficiency and strong trapping robustness with a simple optical configuration. Instead of using expensive microscope objectives, the parabolic reflectors can not only achieved large numerical aperture (N.A.) focusing, but were also able to focus the beam far away from optical surfaces to minimize optics contamination. This design also offered a large free space for flexible integration with other measuring techniques, such as optical-trapping Raman spectroscopy, for on-line single particle characterization.

Online Characterization of Single Airborne Carbon Nanotube Particles Using Optical Trapping Raman Spectroscopy.

Carbon nanotubes (CNTs) have become recognized as a potential environmental and health hazard as their applications are broadening and manufacturing costs are reducing. Fundamental information of CNTs in air is of significant importance to our understanding of their environmental fate as well as to further applications. Extensive efforts have been made over decades on characterizing CNTs; however, a majority of the studies are of bulk or CNTs dispersed on substrates. In the present study, we characterize single CNT particles in air using optical trapping Raman spectroscopy (OT-RS). Different types of CNT particles, as well as glassy carbon spheres, were optically trapped in air. Their physical properties were viewed by microscopic bright field images and scattering images; their chemical properties and structural information can be inferred from characteristic Raman bands. The system can also spatially resolve the morphology and chemical distribution of optically trapped CNT particles in air. The OT-RS technique combines single-particle morphological and chemical information and offers an online method to characterize the physicochemical properties of single CNT particles at their native states in air.

Liquid-liquid phase separation and evaporation of a laser-trapped organic-organic airborne droplet using temporal spatial-resolved Raman spectroscopy.

Chemical reactions in aerosol particles can occur between the reactive components of the particle or between the particle and its surrounding media. The fate of atmospheric aerosols depends on the environment, the composition and the distribution of components within a particle. It could be very interesting to see how a liquid aerosol particle behaves in ambient air if the particle is composed of mixed chemicals. Do the chemical components remain homogeneously mixed within a particle or separate as the mixed liquid is aerosolized? How do the chemicals within a droplet separate and interact with the air? In this paper, a single microdroplet formed from an organic-organic mixture of diethyl phthalate (DEPh) and glycerol was investigated using laser-trapped position-resolved temporal Raman spectroscopy. For the first time, we were able to directly observe the gradient distributions of the two chemicals at different positions within such an airborne droplet, their time-resolved processes of liquid-liquid phase-separation, and changes of the physical microstructure and chemical micro-composition in the droplet. The results revealed that DEPh migrated to the surface and formed an outer layer and glycerol was more concentrated in the interior of the droplet, DEPh evaporated faster than glycerol, and both organic chemicals within the mixed droplet evaporated faster than either of them within their pure droplets. This technique also provides a new method for studying the fine structure and chemical reactions of different molecules taking place inside a particle and at the interface of a particle with the surrounding microenvironment.

Optical trapping-Raman spectroscopy (OT-RS) with embedded microscopy imaging for concurrent characterization and monitoring of physical and chemical properties of single particles.

The study of physical and chemical properties of a microscopic object, such as a single particle, is made possible using optical trapping (OT) technology combined with other measuring techniques. Here we show a universal optical trap combined with Raman spectroscopy (RS) and microscopy imaging for single-particle studies. The universal optical trap is constructed using two counter-propagating hollow beams and is able to stably levitate single particles of a wide range of properties, such as transparent or absorbing materials, organic (polymers, bioaerosols, etc.) or inorganic constituents (carbon, silica, glass, etc.), and spherical or irregularly shaped morphologies. Both physical and chemical properties and their temporal evolution of the trapped particle can be characterized simultaneously using the integrated OT-RS and imaging system. We created three sample cases to demonstrate the analytical merits of the system: (I) a single particle with no change, (II) partially degraded over the measuring period, and (III) one part from the fragmentized single particle. The particles' chemical compositions, crystalline states, etc. are inferred from their Raman spectra, while their physical properties (sizes, shapes, morphologies, etc.) are revealed by images. This integrated OT-RS system provides a new approach to concurrently characterize and monitor physical and chemical properties of single micrometer-sized objects optically trapped in air.

Position-resolved Raman spectra from a laser-trapped single airborne chemical droplet.

It could be very useful to detect and monitor the molecules and molecular reactions located at different positions within a microsized particle as they respond to various micro-local environments. In this Letter, a particular optical trap using two focusing counterpropagating hollow beams was able to stably trap both absorbing and nonabsorbing particles in air for lengthy observation. A technique that can measure the Raman spectra from different submicrometer positions of a laser-trapped single airborne particle was developed. Spontaneous and stimulated Raman scattering spectra originating from different positions of a diethyl phthalate droplet were recorded, and the strong Raman scattering signals are the result of cavity-enhanced effects and the localized strong light illumination.

Characterization of single airborne particle extinction using the tunable optical trap-cavity ringdown spectroscopy (OT-CRDS) in the UV.

We integrated a rigid optical trap into a tunable pulsed cavity ringdown spectroscopy (OT-CRDS) system to characterize the extinction of single airborne particles in the UV spectral region (306-315 nm). Single solid particles from a multi-walled carbon nanotube (MWCNT), Bermuda grass smut spore, carbon microsphere, and blackened polyethylene microsphere were trapped in air based on the photophoretic force. The improved OT-CRDS system was highly sensitive and able to resolve extinctions of single particles from different materials and sizes at a given wavelength. Further, we successfully manipulated the number of particles, e.g., 1, 2 or more particles, in the trap and measured their distinguishable extinctions using the OT-CRDS. We also show that the particle size and extinction have a good linear correlation from the measurements of 24 single MWCNT particles. Material- and wavelength-dependent extinctions of the four types of airborne particles were also characterized. Results reveal that single airborne particles regardless of their differences in material and size, due to their heterogeneous morphology, have individual-particle dependent extinctions and that dependence can be resolved and characterized using the OT-CRDS technique.

Measurement of back-scattering patterns from single laser trapped aerosol particles in air.

We demonstrate a method for measuring elastic back-scattering patterns from single laser trapped micron-sized particles, spanning the scattering angle range of θ=167.7°-180° and φ=0°-360° in spherical coordinates. We calibrated the apparatus by capturing light-scattering patterns of 10 µm diameter borosilicate glass microspheres and comparing their scattered intensities with Lorenz-Mie theory. Back-scattering patterns are also presented from a single trapped Johnson grass spore, two attached Johnson grass spores, and a cluster of Johnson grass spores. The method has potential use in characterizing airborne aerosol particles, and may be used to provide back-scattering data for lidar applications.

Optical configurations for photophoretic trap of single particles in air.

Since Ashkin's pioneering work in the 1970's, optical trapping (OT) and manipulation have become an indispensable tool in diverse research fields. Today, there are multiple optical trapping schemes in use. In this article, we explore six different optical trapping schemes based on the photophoretic force (PPF). Within these schemes we explore 21 variants differing in such details as laser source, power, beam shape, and focusing optics. We evaluate and rate the trapping quality and performance of the six trapping schemes in terms of four key aspects: simplicity, robustness, flexibility, and efficiency. One of the schemes is novel: we introduce a simple, high quality scheme using a confocal design in which one trapping beam is effectively converted to two counter-propagating beams. The versatility of this new trapping scheme is demonstrated via application of the scheme to cavity ringdown spectroscopy. We hope this exploration of the diversity of PPF trapping schemes will extend applications of OT by providing researchers with information to assist in the selection of specific optical trapping schemes from the first-of-its-kind list of 21 configurations presented herein.

Fiber loop ringdown humidity sensor.

An optical fiber relative humidity (RH) sensor based on the evanescent field-fiber loop ringdown (EF-FLRD) technique is demonstrated. The sensor was placed inside a chamber that provides a humidity reference and is monitored by a humidity meter. The presence of moisture in the chamber changes the refractive index of the medium; thus the ringdown time changes due to a change in the EF scattering loss induced in the sensor head. The sensor demonstrated a fast response (∼1 s), high sensitivity, and excellent reproducibility and reversibly. The EF-FLRD sensor can measure RH in a wide dynamic range of 4% to 100% at a constant temperature of 20±1°C.

A Portable Real-Time Ringdown Breath Acetone Analyzer: Toward Potential Diabetic Screening and Management.

Breath analysis has been considered a suitable tool to evaluate diseases of the respiratory system and those that involve metabolic changes, such as diabetes. Breath acetone has long been known as a biomarker for diabetes. However, the results from published data by far have been inconclusive regarding whether breath acetone is a reliable index of diabetic screening. Large variations exist among the results of different studies because there has been no "best-practice method" for breath-acetone measurements as a result of technical problems of sampling and analysis. In this mini-review, we update the current status of our development of a laser-based breath acetone analyzer toward real-time, one-line diabetic screening and a point-of-care instrument for diabetic management. An integrated standalone breath acetone analyzer based on the cavity ringdown spectroscopy technique has been developed. The instrument was validated by using the certificated gas chromatography-mass spectrometry. The linear fittings suggest that the obtained acetone concentrations via both methods are consistent. Breath samples from each individual subject under various conditions in total, 1257 breath samples were taken from 22 Type 1 diabetic (T1D) patients, 312 Type 2 diabetic (T2D) patients, which is one of the largest numbers of T2D subjects ever used in a single study, and 52 non-diabetic healthy subjects. Simultaneous blood glucose (BG) levels were also tested using a standard diabetic management BG meter. The mean breath acetone concentrations were determined to be 4.9 ± 16 ppm (22 T1D), and 1.5 ± 1.3 ppm (312 T2D), which are about 4.5 and 1.4 times of the one in the 42 non-diabetic healthy subjects, 1.1 ± 0.5 ppm, respectively. A preliminary quantitative correlation (R = 0.56, p < 0.05) between the mean individual breath acetone concentration and the mean individual BG levels does exist in 20 T1D subjects with no ketoacidosis. No direct correlation is observed in T1D subjects, T2D subjects, and healthy subjects. The results from a relatively large number of subjects tested indicate that an elevated mean breath acetone concentration exists in diabetic patients in general. Although many physiological parameters affect breath acetone, under a specifically controlled condition fast (<1 min) and portable breath acetone measurement can be used for screening abnormal metabolic status including diabetes, for point-of-care monitoring status of ketone bodies which have the signature smell of breath acetone, and for breath acetone related clinical studies requiring a large number of tests.

Detection of Melanoma Cancer Biomarker Dimethyl Disulfide Using Cavity Ringdown Spectroscopy at 266 nm.

Skin cells emit volatile organic compounds (VOCs), and some of them can be used as biomarkers for screening specific diseases. Dimethyl disulfide (DMDS) has been recently reported as a biomarker of melanoma skin cancer (Kwak et al. "Volatile Biomarkers from Human Melanoma Cells". J. Chromatogr. B. 2013. 931: 90-96.). With the motivation of diagnosing melanoma using DMDS as its biomarker, we explore the potential of measuring DMDS using an advanced laser spectroscopic technique as an alternative method. We report on the first DMDS measurements using an experimental system based on cavity ringdown spectroscopy (CRDS). The test samples were mixtures of DMDS vapor and nitrogen in different concentrations. Two sampling methods were investigated to dilute the DMDS sample to low concentrations for ringdown measurements. The results showed that the ringdown system responded to various DMDS concentrations linearly and a theoretical detection limit of sub-ppb (parts per billion) could be achieved at the absorption wavelength of 266 nm. This ringdown system exhibited a high dynamic range for DMDS measurements, from ppm (parts per million) to ppt (parts per trillion) levels, given different laser wavelengths used. The feasibility of developing a portable melanoma screening sensor using the CRDS technique was also demonstrated in this study.

A fully integrated standalone portable cavity ringdown breath acetone analyzer.

Breath analysis is a promising new technique for nonintrusive disease diagnosis and metabolic status monitoring. One challenging issue in using a breath biomarker for potential particular disease screening is to find a quantitative relationship between the concentration of the breath biomarker and clinical diagnostic parameters of the specific disease. In order to address this issue, we need a new instrument that is capable of conducting real-time, online breath analysis with high data throughput, so that a large scale of clinical test (more subjects) can be achieved in a short period of time. In this work, we report a fully integrated, standalone, portable analyzer based on the cavity ringdown spectroscopy technique for near-real time, online breath acetone measurements. The performance of the portable analyzer in measurements of breath acetone was interrogated and validated by using the certificated gas chromatography-mass spectrometry. The results show that this new analyzer is useful for reliable online (online introduction of a breath sample without pre-treatment) breath acetone analysis with high sensitivity (57 ppb) and high data throughput (one data per second). Subsequently, the validated breath analyzer was employed for acetone measurements in 119 human subjects under various situations. The instrument design, packaging, specifications, and future improvements were also described. From an optical ringdown cavity operated by the lab-set electronics reported previously to this fully integrated standalone new instrument, we have enabled a new scientific tool suited for large scales of breath acetone analysis and created an instrument platform that can even be adopted for study of other breath biomarkers by using different lasers and ringdown mirrors covering corresponding spectral fingerprints.