Differential light scattering photometer for rapid analysis of single particles in flow

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.

Miniature Optical Particle Counter and Analyzer Involving a Fluidic-Optronic CMOS Chip Coupled with a Millimeter-Sized Glass Optical System

Our latest advances in the field of miniaturized optical PM sensors are presented. This sensor combines a hybrid fluidic-optronic CMOS (holed retina) that is able to record a specific irradiance pattern scattered by an illuminated particle (scattering signature), while enabling the circulation of particles toward the sensing area. The holed retina is optically coupled with a monolithic, millimeter-sized, refracto-reflective optical system. The latter notably performs an optical pre-processing of signatures, with a very wide field of view of scattering angles. This improves the sensitivity of the sensors, and simplifies image processing. We report the precise design methodology for such a sensor, as well as its fabrication and characterization using calibrated polystyrene beads. Finally, we discuss its ability to characterize particles and its potential for further miniaturization and integration.

Monolithically-integrated cytometer for measuring particle diameter in the extracellular vesicle size range using multi-angle scattering

Size measurement of extracellular vesicles is hampered by the high cost and measurement uncertainty of conventional flow cytometers which is mainly due to the use of non-specialised free space optics. Integrated cytometry, where the optics and fluidics are embedded in a monolithic chip shows promise for the production of low cost, micro-flow cytometers dedicated for extracellular vesicle (EV) analysis with improved size measurement accuracy and precision. This research demonstrates a unique integrated cytometer for sub-micron particle size measurement using multi-angle scattering analysis. A combination of three technologies is used: (i) Dean-based hydrodynamic focussing to deliver a tight sample core stream to the analysis region, (ii) integrated waveguides with multimode interference devices to focus a narrow excitation beam onto the sample stream, and (iii) an angular array of collection waveguides to measure particle scattering distribution and calculate diameter. Low index 200 nm liposomes could be detected and polystyrene size standards as small as 400 nm diameter could be measured with an uncertainty of ±21 nm (1/2 IQR) demonstrating a first step on the path to high performance integrated cytometry of EVs.

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.

Scattering pulse of label free fine structure cells to determine the size scale of scattering structures

Scattering pulse is sensitive to the morphology and components of each single label-free cell. The most direct detection result, label free cell's scattering pulse is studied in this paper as a novel trait to recognize large malignant cells from small normal cells. A set of intrinsic scattering pulse calculation method is figured out, which combines both hydraulic focusing theory and small particle's scattering principle. Based on the scattering detection angle ranges of widely used flow cytometry, the scattering pulses formed by cell scattering energy in forward scattering angle 2°-5° and side scattering angle 80°-110° are discussed. Combining the analysis of cell's illuminating light energy, the peak, area, and full width at half maximum (FWHM) of label free cells' scattering pulses for fine structure cells with diameter 1-20 μm are studied to extract the interrelations of scattering pulse's features and cell's morphology. The theoretical and experimental results show that cell's diameter and FWHM of its scattering pulse agree with approximate linear distribution; the peak and area of scattering pulse do not always increase with cell's diameter becoming larger, but when cell's diameter is less than about 16 μm the monotone increasing relation of scattering pulse peak or area with cell's diameter can be obtained. This relationship between the features of scattering pulse and cell's size is potentially a useful but very simple criterion to distinguishing malignant and normal cells by their sizes and morphologies in label free cells clinical examinations.

Polarization-resolved near-backscattering of airborne aggregates composed of different primary particles

We measured the polarization-resolved angular elastic scattering intensity distribution of aggregates composed of primary particles with different shapes and packing densities in the near-backward directions (155°-180°). Specifically, we compare aggregates composed of spherical polystyrene latex spheres, cylinder-like Bacillus subtilis particles, and Arizona road dust, as well as tryptophan particles. We observe clearly differentiable polarization aspect ratios and find that the negative polarization dip is more pronounced in more densely packed aggregates or particles. This work indicates that the polarization aspect ratio in the near-backward direction may be used as a fingerprint to discriminate between aggregates with the same size and overall shape by differences in their constituent particles.

Automated bacterial identification by angle resolved dark-field imaging

We propose and demonstrate a dark-field imaging technique capable of automated identification of individual bacteria. An 87-channel multispectral system capable of angular and spectral resolution was used to measure the scattering spectrum of various bacteria in culture smears. Spectra were compared between various species and between various preparations of the same species. A 15-channel system was then used to prove the viability of bacterial identification with a relatively simple microscope system. A simple classifier was able to identify four of six bacterial species with greater than 90% accuracy in bacteria-by-bacteria testing.

Effect of the size and shape of a red blood cell on elastic light scattering properties at the single-cell level

We demonstrate the use of a double-beam optical tweezers system to stabilize red blood cell (RBC) orientation in the optical tweezers during measurements of elastic light scattering from the trapped cells in an angle range of 5-30 degrees. Another laser (He-Ne) was used to illuminate the cell and elastic light scattering distribution from the single cell was measured with a goniometer and a photomultiplier tube. Moreover, CCD camera images of RBCs with and without laser illumination are presented as complementary information. Light scattering from a RBC was measured in different fixed orientations. Light scattering from cells was also measured when the length of the cell was changed in two different orientations. Light scattering measurements from spherical and crenate RBCs are described and the results are compared with other cell orientations. Analysis shows that the measured elastic light scattering distributions reveal changes in the RBC's orientation and shape. The effect of stretching on the changes in scattering is larger in the case of face-on incidence of He-Ne laser light than in rim-on incidence. The scattering patterns from RBCs in different orientations as well as from a spherical RBC were compared with numerical results found in literature. Good correlation was found.

Microscope-based label-free microfluidic cytometry

A microscope-based label-free microfluidic cytometer capable of acquiring two dimensional light scatter patterns from single cells, pattern analysis of which determines cellular information such as cell size, orientation and inner nanostructure, was developed. Finite-difference time-domain numerical simulations compared favorably with experimental scatter patterns from micrometer-sized beads and cells. The device was capable of obtaining light scattering patterns from the smallest mature blood cells (platelets) and cord blood hematopoietic stem/progenitor cells 
(CD34 + cells) and myeloid precursor cells. The potential for evaluation of cells using this label-free microfluidic cytometric technique was discussed.

Retrieval of size and refractive index of spherical particles by multiangle light scattering: neural network method application

A method to retrieve the radius and the relative refractive index of spherical homogeneous nonabsorbing particles by multiangle scattering is proposed. It is based on the formation of noise-resistant functionals of the scattered intensity, which are invariant with respect to the linear homogeneous transformations of an intensity-based signal and approximation of the retrieved parameters' dependence on the functionals by a feed-forward neural network. The neural network was trained by minimization of the mean squared relative error in the range of particle radii from 0.6 mkm up to 13.6 mkm and relative refractive index from 1.015 up to 1.28. In comparison with training on a minimum of the mean squared error, this method enables one to increase the accuracy of the radius retrieval in the range of radii from 0.6 to 2 microm and refractive index in the range from 1.015 to 1.1. The values of intensity of light scattered in the interval of angles 10 degrees-60 degrees are used as input data. If the measurement error is 20%, the mean errors of the radius and relative refractive index are 0.8% and 7%, respectively. The results obtained by the proposed method and by the trial and error method with published experimental data (measured with a scanning flow cytometer) are compared. The maximal difference in the retrieval results of radius and the relative refractive index of particles obtained by both methods is under 5%.

White-light optical particle spectrometer for in situ measurements of condensational growth of aerosol particles

A new optical particle counter was developed to provide fast in situ sizing of cloud droplets in the Leipzig Aerosol and Cloud Interaction Simulator (LACIS). The new instrument features white light for the illumination of the sampling volume: two off-axis elliptical mirrors, providing a wide angle of collection for light scattered by particles; and an optically defined sampling volume. The smooth unambiguous response characteristic for water droplets allows direct conversion of the measured signal amplitudes into droplet diameters. Preliminary response measurements for dry polystyrol microspheres and water droplets, grown in the LACIS on NaCl particles, have shown good agreement with the corresponding calculated response curves.

Elastic light scattering from single cells: orientational dynamics in optical trap

Light-scattering diagrams (phase functions) from single living cells and beads suspended in an optical trap were recorded with 30-ms time resolution. The intensity of the scattered light was recorded over an angular range of 0.5-179.5 degrees using an optical setup based on an elliptical mirror and rotating aperture. Experiments revealed that light-scattering diagrams from biological cells exhibit significant and complex time dependence. We have attributed this dependence to the cell's orientational dynamics within the trap. We have also used experimentally measured phase function information to calculate the time dependence of the optical radiation pressure force on the trapped particle and show how it changes depending on the orientation of the particle. Relevance of these experiments to potential improvement in the sensitivity of label-free flow cytometry is discussed.

Single-particle sizing from light scattering by spectral decomposition

A Fourier transform was applied to size an individual spherical particle from an angular light-scattering pattern. The position of the peak in the amplitude spectrum has a strong correlation with the particle size. A linear equation retrieved from regression analysis of theoretically simulated patterns provides a relation between the particle size and the location of the amplitude spectrum's peak. The equation can be successfully applied to characterize particles of size parameters that range from 8 to 180 (corresponding to particle sizes that range from 1.2 to 27.2 microm at a wavelength of 0.633 microm). The precision of particle sizing depends on the refractive index and reaches a value of 60 nm within refractive-index region from 1.35 to 1.70. We have analyzed four samples of polystyrene microspheres with mean diameters of 1.9, 2.6, 3.0, and 4.2 microm and a sample of isovolumetrically sphered erythrocytes with a scanning flow cytometer to compare the accuracy of our new method with that of others.

Characterizing and monitoring respiratory aerosols by light scattering

The elastic-scattering intensity pattern from a single particle as a function of spherical coordinate angles theta and phi provides detailed information on the pattern's morphology. By use of an ellipsoidal reflector and a CCD camera, a single-laser-shot intensity pattern from a large angular range (theta from 90 degrees to 168 degrees and phi from 0 degrees to 360 degrees) was detected from a single aerosol (e.g., a Bacillus subtilisspore, a 1-microm-diameter polystyrene latex sphere, or a cluster of either of these) flowing through the reflectors focal volume at 5 m/s. Noticeable difference in the large-angle-range two-dimensional angular optical scattering (LATAOS) suggest that the LATAOS pattern could be useful in differentiating and classifying life-threatening aerosols from normal background aerosols.

Particle sizing with a simple differential light-scattering photometer: homogeneous spherical particles

A simple light-scattering photometer has been designed to measure the angular distribution of the intensity of polarized laser light scattered by micrometer and submicrometer samples. The photometer uses an ellipsoidal reflector and simple optical components to collect the He-Ne laser-scattered light and to focus it onto a 512-element photodiode array. Experimental data have been obtained for several monodisperse aqueous solutions of latex spheres of different sizes. The results have been satisfactorily interpreted on the basis of the electromagnetic scattering theory. In particular the Mie formulation has been properly reformulated to take into account the corrections to the scattered intensity determined by the apparatus design making it possible to perform particle sizing.

High-resolution size measurement of single spherical particles with a fast Fourier transform of the angular scattering intensity

A technique is described and demonstrated to measure the size of spherical particles of known index of refraction by laser light scattering with an accuracy of better than 1%. This technique entails imaging the angular scattering intensity onto a photodiode array and applying a fast Fourier transform to the array output to obtain a frequency and phase corresponding to the number and angular position of the scattering lobes. Errors associated with particle trajectory effects and changes in the index of refraction are also considered. Results are not affected by the former, whereas variations of the refractive index by 2%, as may be typical, for example, of the transient heat up of a liquid hydrocarbon droplet, cause a deterioration of sizing accuracy to approximately 3%. The technique can in principle be applied in real time at data rates as high as 20-30 kHz with a modest equipment investment. Therefore, the measurement of droplet evaporation rates in dilute sprays with unprecedented accuracy appears to be feasible.

Elastic light-scattering measurements of single biological cells in an optical trap

We have developed an instrument for determination of the angular light scattering of beads and biological cells. The instrument uses radiation pressure for levitation of particles inside a cuvette. The setup consists of two 780-nm diode lasers in a vertical double-beam trapping configuration. In the horizontal direction a weakly focused 633-nm probe beam is used to illuminate the trapped particle. One can detect scattered light over the range of from - 150 to 150 deg with an angular resolution of 0.9 deg using an avalanche photodiode. With this setup light scattering from polystyrene beads was measured, and the obtained scattering patterns were compared with theoretical scattering patterns from Lorenz-Mie theory. The results show that the setup is stable, gives reproducible patterns, and qualitatively agrees with the calculations. Trapping of biological cells is more difficult than trapping of beads, because smaller forces result from smaller refractive indices. We present an angular scattering pattern measured from a human lymphocyte measured from 20 to 60 deg.

Measurement of scattering properties of individual particles with a scanning flow cytometer

A hydrofocusing head with an optical cuvette has been developed for the flow cytometer to generate complete scatter patterns of single particles at scattering angles ranging from 10° to 120°. The scatter signal has been measured as a function of the angle (a flying indicatrix) by the use of particle motion within a scanning system of the flow cytometer by the use of a single photomultiplier. Scattering data measured with the flow cytometer have been compared with those calculated from Mie theory for latex particles. A calculation algorithm has been used to estimate the size and the refractive index of spherical particles from the scattering data measured.

Light scattering from nonspherical airborne particles: experimental and theoretical comparisons

A laser light-scattering instrument has been designed to permit an investigation of the spatial intensity distribution of light scattered by individual airborne particles constrained within a laminar flow, with a view to providing a means of classifying the particles in terms of their shape and size. Ultimately, a means of detecting small concentrations of potentially hazardous particles, such as asbestos fiber, is sought. The instrument captures data relating to the spatial distribution of light scattered from individual particles in flow. As part of an investigation to optimize orientation control over particles within the sample airstream, the instrument has been challenged with nonspherical particles of defined shape and size, and a simple theoretical treatment based on the Rayleigh-Gans formalism has been used to model the spatial intensity distribution of light scattered from these particle types and hence derive particle orientation data. Both experimental and theoretical scattering data arepresented, showing good agreement for all particle types examined.

Light scattering by laser levitated particles

A new light scattering facility utilizes a laser particle levitation technique to measure the angular distribution of light scattered by solid particles of size range 10-100 microm, over a- range of scattering angles between 16 and 167 degrees . The performance of the facility is illustrated by an excellent match between the observed scattering from a 33 microm-diam sphere and Mie theory prediction.

Scattering matrix elements of biological particles measured in a flow through system: theory and practice

Light scattering techniques (including depolarization experiments) applied to biological cells provide a fast nondestructive probe that is very sensitive to small morphological differences. Until now quantitative measurement of these scatter phenomena were only described for particles in suspension. In this paper we discuss the symmetry conditions applicable to the scattering matrices of monodisperse biological cells in a flow cytometer and provide evidence that quantitative measurement of the elements of these scattering matrices is possible in flow through systems. Two fundamental extensions to the theoretical description of conventional scattering experiments are introduced: large cone integration of scattering signals and simultaneous implementation of the localization principle to account for scattering by a sharply focused laser beam. In addition, a specific calibration technique is proposed to account for depolarization effects of the highly specialized optics applied in flow through equipment.

Elastic and inelastic light scattering in flow cytometry

A review of the fundamental aspects of elastic light scattering suggests that information about the shape and internal structure may best be obtained from signals measured in the backscattering directions. Size information can be most readily extracted from forward scattering signals. Spectral analysis of scattered signals with incident white light is a subject that merits further study. Signals from cells that have been stained with fluorescent dyes are proportional to dye content in the usual flow-cytometric configuration except in the case of dense, strongly anisometric structures such as sperm cells. The recent discovery that Raman signals from molecules adsorbed on small silver particles are strongly enhanced suggests the possibility of utilizing this effect for identification of molecular species inside biological cells.