Open Access
Add to BibSonomy
Abstract
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.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The rapid detection and characterization of bioaerosol particles has attracted great attention for more than two decades, and it continues to motivate relevant fundamental research and drive the development of corresponding technologies [1,2]. Although there have been quite a few commercial instruments available for real-time bioaerosol detection and study (e.g. BAWS, WIBS, UVAPS), the most sensitive instruments are still not only costly but also bulky in size. Researchers have been working on the development of miniaturized biosensors, with the goal of making them as small and inexpensive as a smoke detector at low false alarm rate. At the present time, the developed technologies for rapid bioaerosol detection are mainly based on the measurement of optical signatures of single flowing through aerosol particles. Among them, elastic light scattering (ELS) has the largest scattering cross section, suitable for instrumentation that does not need a dispersing device, which is essential for the spectroscopic methods such as fluorescence, Raman, and mass spectroscopy [1–3]. Therefore, ELS has the high potential to be developed into small-size and inexpensive biosensors.
Although ELS signals contain very rich information about the scattering particles, it is still far from being able to retrieve enough parameters for the identification of bioaerosols. Therefore, ELS is generally used as a supplemental technology in bioaerosol detection, providing information on particle’s size, shape, symmetricity, refractive index, and/or depolarization, but with limited information on specific chemical compositions [4–11]. However, Mueller matrix elements can supply more information on the property of the scattering particle, in particular, the polarization containing elements such as S12, S34, and S14 [12–16]. The normalized Mueller matrix element -S14/S11, i.e. the circular intensity differential scattering (CIDS) [14], was reported to be able to detect the helical structures of DNA molecules in biological systems in the 1970s-1980s, and a few experiments were carried out using a polarization modulator, a lock-in amplifier, and a rotation goniometer for recording the CIDS phase function from a group of particles, e.g. cells and PSL microspheres, in suspension [e.g. 13,14,17–23]. Although this technology has been advanced into CIDS microscopic imaging for extracting information on chiral DNA molecules inside cell nuclei or obtaining molecular conformation within complex biopolymers without using specific fluorescent labels [24–26], there has been little improvement for aerosol characterization, especially no report on rapid CIDS (or S14) measurement from single micron-sized aerosol particles. In this paper, we report an advanced method of measuring CIDS phase functions from single aerosol particles. In this method, no moving parts, no polarization modulator, and no lock-in amplifier were required, and the scattering phase function from a single particle could be obtained in tens of μs instead of tens of minutes as in the traditional method. The CIDS signals obtained from a few types of test particles demonstrated that this method can be used for fast discrimination of bioaerosol from non-bioaerosol particles.
2. Circular intensity differential scattering (CIDS)
In general, the polarization state of a light beam can be described by Stokes vector using four parameters as [15],
Given an input beam Sin, the output beam Sout after interacting with a scattering object can be determined by operating a 4×4 Mueller matrix [14,16] describing the optical system as M, that is,
In general, ELS characteristics contained in M is determined by the size, shape and chemistry of the scattering particle. It also depends on particle orientation relative to the illuminating light and light polarization state. The chemical properties are contained in the complex refractive index of the particle n = nr+ini, where nr and ni are the real and imaginary parts. Most studies assume the refractive index is homogeneous, although this approximation is not always right for heterogeneous particles [28]. The more complicated part is that, in some cases, different particle can produce the same Sout, especially for the complex particle systems [29]. Therefore, it is still very challenging to retrieve the complete properties or parameters of a scattering particle from its ELS measurements, even with all matrix elements resolved. Fortunately, the element S14 or CIDS was reported to be able to distinguish chiral structures in particles, such as the helical structures of DNA molecules in biological systems.
For the right and left circular polarization (RCP and LCP) light in Eq. (2), the S vector incident on the system is $\left[ {\begin{array}{l} 1\\ 0\\ 0\\ 1 \end{array}} \right]\textrm{and} \left[ {\begin{array}{c} 1\\ 0\\ 0\\ { - 1} \end{array}} \right]$. Then, the output intensity after illumination of the circularly polarized light is Iout,R = ${S_{11}}$+${S_{14}}$ and Iout,L = ${S_{11}}$ − ${S_{14}}$, respectively. Then,
The measurement of element S14 or CIDS has been used to study helical structures for more than 40 years [e.g. 15,19], such as those from suspensions of sperm cells of Eledone cirrhosa [21,23,30]. These experimental results are in a good agreement with theoretical simulations based on the first Born approximation [23], or coupled-dipole approximation [31]. However, there has been limited advancement and application since these initial studies. The key barriers are the requirements for precise optical components/devices with highly accurate alignments in the experiment, as well as in being able to measure very weak signals, which are on the order of 10−3 to 10−6 for most biological systems. Therefore, the sum of polarization anisotropy caused by the light source, optical components, and any artificial effects produced during the light and signal propagating processes including the detector itself must be controlled to be well below 1% [23]. Past measurements of CIDS or S14 phase functions were carried out using the polarization modulation technique, a lock-in amplifier and a rotation goniometer [15,21,23,30]. It takes at least 10s of minutes or longer to perform such a measurement, and it is impossible to obtain CIDS from single flowing through aerosol particles in such experimental arrangements.
3. Experimental arrangement for CIDS measurement from single aerosol particles
Figure 1 shows the experimental arrangement for measuring CIDS phase function from single aerosol particles. As shown in Fig. 1(a), a horizontally linearly polarized beam from a 532 nm continuous wave (CW) laser (Coherent Verdi-V6, 1 W) was cleaned and formed into a collimated beam using a 100 μm pinhole and a pair of focusing lenses. This beam was divided into two beams by a 50:50 beam splitter, one of the beams was turned into a vertically polarized beam by a half wave plate, then the two beams were recombined (i.e. overlapping each other) by a Glan-laser polarizer (Lambda Research Optics, extinction ratio 5×105:1). The linearity of the polarizations for the two beams was further purified by the Glan-laser polarizer. The two parts within the combined beam were turned into a RCP & a LCP beams, by a quarter wave plate adjusted at 45°. The intensities of the two parts were adjusted to be equal by finely rotating the half wave plate, which was placed into the beam with a slightly higher intensity after the beam splitter. Individually flowing through single aerosol particle is illuminated by each of the two parts: RCP and LCP alternately, when the particle is located at a well-defined scattering volume that is in the depth field of an elliptical reflector. Here, the half wave plate, the Glan-laser polarizer, and the quarter wave plate were combined to produce the RCP and LCP beams instead of using an expensive electro-optic polarization modulator that requires a lock-in amplifier for signal detection. Traditionally, a rotation goniometer and a photodetector were used for measuring CIDS phase function, it takes at least minutes to complete a measurement from different scattering angles [32]. In our experiments, the custom-designed elliptical reflector (Fig. 1(b)) was arranged to project the angle-dependent scattering light onto an ICCD camera (Andor iStar), and the two-dimensional angular optical scattering (TAOS) patterns of aerosol particles were obtained from the forward hemisphere and the backward hemisphere simultaneously [33,34]. A cross-beam trigger scheme [35], which consisted of two orthogonally aligned diode lasers (638 nm and 685 nm) and two photomultiplier tubes (PMT), was used to detect the presence of aerosol particles at the focal point, then the logical gate AND signal from the two PMTs was used to trigger the ICCD for signal recording with a 1 μs accumulation.
Fig. 1. (a) and (b) Schematics of the experimental arrangement for measuring CIDS phase function from single flowing through particles; (c) ELS patterns from a single PSL microsphere of 3 μm in diameter; (d) and (e) corresponding scattering polar angle θ and azimuthal angle ϕ; and (f) an example of scattering phase functions along θ at ϕ=180ο by reading the intensity distribution from the red box area of (c).
The system was calibrated routinely using NIST-traceable polystyrene latex (PSL) microspheres (ThermoFisher Scientific). The microspheres were aerosolized by an ink-jet aerosol generator (IJAG) [36] and fed into the inlet of a nozzle that focused the aerosol particles and delivered them across the focal point of the reflector. The concentration of PSL in the IJAG cartridge was diluted to be less than one microsphere in every three droplets produced by the IJAG, to make sure there was only one microsphere (or none) contained in each dried particle (from each droplet) and being delivered to the scattering volume. For the other test samples, the certain concentrations were prepared so that the 50 micron-sized droplets, produced by the IJAG, dried into particles at the designed sizes, approximately 3 μm in diameter. Figure 1(c) shows one of the typical scattering TAOS patterns from a single PSL microsphere at 3 μm in diameter (Fig. 1(b)). The TAOS patterns captured from individual single particles were projected onto spherical coordinates (θ, φ), then separated into forward and backward scattering hemispheres by ray tracing [37] and, finally, compared with the Lorenz–Mie theoretical calculations. Figure 1(d) and (e) show the corresponding scattering polar angle θ and azimuthal angle ϕ, respectively, in the spherical coordinates (θ, φ) for the TAOS pattern in Fig. 1(c). Therefore, scattering phase functions [Fig. 1(f)] could be obtained by reading the scattering intensities along θ from 0ο to 180ο at ϕ=180ο [red box area in Fig. 1(c)], except for the missing data around θ = 0ο, 90ο, and 180ο because of the three holes in the reflector. In this study, we recorded the scattering phase functions via RCP & LCP illuminations separately, and their subsequent subtraction yielded CIDS phase functions.
4. Experimental results and discussion
We recorded hundreds of TAOS patterns by RCP and LCP 532 nm laser illuminations from four types of aerosol particles at 3 μm in diameter: polystyrene latex (PSL) microspheres, DNA-tagged PSL microspheres, tryptophan particles, and aggregates of Bacillus subtilis spores. The results demonstrated that the TAOS patterns illuminated by RCP and LCP beams have almost identical features, without noticeable difference. Here, the four samples were selected to test how the helical structures within the particles are reflected in the CIDS phase functions. Figure 2 shows three typical RCP illuminating TAOS patterns from three different testing aerosol particles (PSL microsphere, tryptophan, and B. subtilis aggregate) associated with their SEM images. The different false colors in these images are used to present patterns from different types of particles. The dark part (low intensity) in the upper portion of each pattern was from the shadow of the nozzle tip, and the vertical black line in the middle was from the shadow of a post that held a 3 mm mirror for blocking the 638 nm diode laser beam. The three black spots at the middle height were from the three holes drilled on the reflector to provide the paths for the laser beams (from left to right: 532 nm laser exiting, 638 nm laser entering, and 532 nm laser entering, respectively (see Fig. 1(b)).
Fig. 2. Typical SEM images (left column) and TAOS patterns (right three columns) from single PSL microspheres, tryptophan particles, and B. subtilis aggregates using RCP 532 nm laser illumination. Different false colors in the images are used to present patterns from different types of particles.
As we were only interested in the scattering phase function [intensities at θ from 0° to 180°, see Fig. 1(d)] around φ=180° [see Fig. 1(e)], we recorded the TAOS patterns only within the range between φ=(90°, 270°) as shown in Fig. 2, which covered many fewer angles compared to that in Fig. 1(c). For a microsphere, the TAOS patterns consist of concentric rings in both forward and backward directions. As the particle shape drifts away from a perfect sphere, or its homogeneity in composition or in surface roughness changes to be heterogeneous, as shown in the SEM images in the left column of Fig. 2, these rings become broken and distorted, and the back scattering region is more sensitive to these changes [38]. For the polystyrene microspheres, there is little intensity variation along φ in the concentric rings. For the particles composed of tryptophan molecules and B. subtilis spores, their overall shapes were formed to be near sphere by the surface tension when they dried from the droplets generated by IJAG. Small surface roughness was produced in tryptophan particles due to the uneven vapor evaporation and heat diffusion in the drying process, and larger roughness and inhomogeneous density distribution were generated for the aggregates of B. subtilis spores due to their natural shapes of spores [2,35]. Therefore, the forward scattering patterns still retained good concentric ring structures (left side), but they became gradually distorted from tryptophan to B. subtilis. There were increasingly significant intensity variations in the backward direction (right side), and they eventually turned into speckle islands for the less spherical and rough B. subtilis aggregates (bottom panel).
Figure 3 shows the averaged scattering phase functions from 50 particles using both RCP and LCP illuminations. The phase functions were similar between RCP and LCP illuminations with all four types of tested aerosol particles, but showed noticeable differences for the DNA-tagged PSL microspheres and aggregates of B. subtilis spores.
Fig. 3. Averaged scattering phase functions by the RCP and LCP illuminations from 50 particles at 3-μm in diameter for (a) polystyrene latex (PSL) microsphere, (b) tryptophan particle, (c) DNA-tagged PSL microsphere, and (d) aggregate of Bacillus subtilis spores.
Figure 4 summarizes the averaged CIDS phase functions from the four types of particles. They are the normalized subtraction results from the intensity phase functions illuminated by the RCP and LCP beams from Fig. 3. From these results, we learn that there are relatively weak signals (peak |CIDS| ≤ 0.025) for PSL microspheres, which should have a zero value since they do not contain any molecules with a helical structure. There is also a weak signal for tryptophan particles with peak |CIDS| ≤ 0.025, while the DNA-tagged PSL microsphere and B. subtilis aggregate demonstrated relatively strong signals (peak |CIDS| ≥ 0.065), which are about 3 times stronger than those from the PSL microspheres and tryptophan particles. This result demonstrated the potential discrimination between biological and non-biological particles using the CIDS measurements from single airborne particles.
Fig. 4. CIDS phase functions from PSL, DNA-tagged PSL microspheres, tryptophan, and Bacillus subtilis particles.
In order to obtain the two intensity scattering phase functions from exactly the same single particle as it is within the depth field around the focal point of the reflector while illuminated by the RCP and LCP beams, the displacement between the two positions for a particle being illuminated during data acquisition should be less than 50 μm. This is essential to avoid the pattern distortion caused by the particle drifting away from the focal point [39]. To achieve this, we need to turn-on and –off the RCP and LCP beams within 5 μs, since the particle was moving at a speed of ∼10 m/sec, and the data acquisition frame rate of the ICCD needs to be higher than 2 MHz. Unfortunately, the maximum frame rate for an ICCD is only around 1 kHz, far below the needed 2 MHz, even when it is binned into the one-dimension (spectral) mode. Further, the two laser switches (shutter 1 and 2) used here have a response time on the order of ms, therefore, the data illuminated by the RCP and LCP beams presented in this study were recorded from different particles. Although the data shown in Figs. 3 and 4 are from the average over the measurements of 50 particles, they are still very noisy up to the order of 10−2 (Fig. 4). In the future, a multiple-anode (e.g. 32-anode) photomultiplier tube (PMT) [40] will replace the ICCD for faster data acquisition. Meanwhile, a fast light deflector, such as an acousto-optic modulators (AOM) or electro-optic deflector, can direct the laser beam into either the horizontal or the vertical polarization path on the order of ns; or a polarization rotator, such as a Pockels cell, can turn the beam into linearly horizontal or vertical polarization to produce RCP and LCP at a few ns. Such an arrangement can no doubt measure the CIDS phase function from the exactly same single particles, but it will also adversely impact the size and cost of the whole system.
The CIDS signal was estimated to be on the order of 10−3 to 10−6 for biological aerosols, and the sum of polarization anisotropy must be controlled to be well below 1% for a good CIDS measurement [23]. However, the smallest anisotropy or ellipticity [(Imax-Imin)/ (Imax+Imin)]×100% of the RCP and LCP beams we could achieve was 1.2%, where Imax and Imin are the maximum and minimum light intensity measured by a linearly polarization analyzer and a power meter around all the angles. The polarization anisotropy from the other optical components was higher than 1% with different magnitudes at different scattering angles. This could be one of the key reasons for the high noise of CIDS signals.
Although the phase functions were designed to be recorded by the ICCD (1 μs integration window) only when the particles were moving through the focal point of the elliptical reflector, there were still some distorted data recorded when the particles were interrogated at a position that was slightly drifted away from the focal point. This could be another key reason for the high noise of CIDS signals recorded here.
Tryptophan is the major fluorescence molecule in bioaerosol particles, and it is the main indicator for fluorescence-based biosensors [1,2]. Particles containing tryptophan do not mean they are viable bioaerosols or living organisms that have microbiological activity and the potential to multiply themselves, so to detect the DNA and chromosomes could be a better indicator for viable bioaerosols [e.g. 41]. Therefore, tryptophan was chosen for the CIDS test and check if it can be distinguished from the particles containing DNA molecules. As expected, it appeared that the tryptophan itself did not contribute strong CIDS signals compared to DNA-tagged PSL microspheres and B. subtilis spores. In proteins, the polypeptide chain forms a number of right- and left-handed helices and superhelices with tryptophan incorporated into the structure, but not tryptophan itself, so its CIDS gave the similar intensity level to the noise from PSL microsphere. This result demonstrated that CIDS can be used to discriminate bioaerosol particles containing DNA molecules from that just with tryptophan only, and has a better ability toward bioaerosol identification than fluoresce-based biosensors.
It was expected that the DNA-tagged PSL microsphere and B. subtilis aggregate would have relatively strong CIDS signals. Interestingly, there was only positive peaks from the DNA-tagged PSL microsphere, while B. subtilis had positive peaks in small scattering angle and negative peaks at large scattering angles. Coincidentally, the overall CIDS profile from B. subtilis is very similar to the observed CIDS features from sperm cells of Eledone cirrhosa [23]. Certainly, the intensity and profile of a CIDS phase function depends on the packing density, orientation, and level of chiral structures [22,42,43]. Therefore, theoretical calculations for particles with different size and helical structures are under studying, while experimental CIDS measurements from single particles that is optically trapped at the exact focal point of the elliptical reflector are underway. Both of these results will help understanding the observed phenomena and determining the best path toward developing a single particle CIDS measurement.
5. Summary
We reported a new method that is able to measure the phase function of circular intensity differential scattering (CIDS), i.e. the normalized Mueller matrix element -S14/S11 along the scattering angles, from single aerosol particles. Additionally, with this method, a phase function measurement can be completed in tens of μs instead of tens of minutes through the implementation of a reflector to collect the angle-dependent scattering signals. The system has no moving parts, modulator, or lock-in amplifier involved, which were generally required in the traditional experimental setup. This new configuration presents significant advantages in potential instrumentation using the aforementioned system. The results from this exploratory study showed that the configuration used for the CIDS measurement here is able to discriminate biological from non-biological aerosol particles. Leveraging this approach may enable the development of a small and low cost biosensor based on CIDS measurements.
Funding
CCDC-ARL (Mission fund); Defense Threat Reduction Agency (CB10745).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Row data presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request after public release approved.
References
1. J. A. Huffman, A. E. Perring, N. J. Savage, B. Clot, B. Crouzy, F. Tummon, O. Shoshanim, B. Damit, J. Schneider, V. Sivaprakasam, M. A. Zawadowicz, I. Crawford, M. Gallagher, D. Topping, D. C. Doughty, S. C. Hill, and Y. L. Pan, “Real-time sensing of bioaerosols: Review and current perspectives,” Aerosol Sci. Technol. 54(5), 465–495 (2020). [CrossRef]
2. Y. L. Pan, J. D. Eversole, P. H. Kaye, V. Foot, R. G. Pinnick, S. C. Hill, M. W. Mayo, J. R. Bottiger, A. Huston, V. Sivaprakasarn, and R. K. Chang, “Bio-aerosol fluorescence - Detecting and characterising bio-aerosols via UV light-induced fluorescence spectroscopy,” Nato Sci Ser Ii-Math 238, 63–164 (2007). [CrossRef]
3. Y. L. Pan, K. B. Aptowicz, R. K. Chang, M. Hart, and J. D. Eversole, “Characterizing and monitoring respiratory aerosols by light scattering,” Opt. Lett. 28(8), 589–591 (2003). [CrossRef]
4. K. B. Aptowicz, Y. L. Pan, R. K. Chang, R. G. Pinnick, S. C. Hill, R. L. Tober, A. Goyal, T. Leys, and B. V. Bronk, “Two-dimensional angular optical scattering patterns of microdroplets in the mid infrared with strong and weak absorption,” Opt. Lett. 29(17), 1965–1967 (2004). [CrossRef]
5. K. B. Aptowicz, R. G. Pinnick, S. C. Hill, Y. L. Pan, and R. K. Chang, “Optical scattering patterns from single urban aerosol particles at Adelphi, Maryland, USA: A classification relating to particle morphologies,” J. Geophys. Res. 111(D12), 6774 (2006). [CrossRef]
6. P. H. Kaye, K. AlexanderBuckley, E. Hirst, S. Saunders, and J. M. Clark, “A real-time monitoring system for airborne particle shape and size analysis,” J. Geophys. Res. 101(D14), 19215–19221 (1996). [CrossRef]
7. P. H. Kaye, J. E. Barton, E. Hirst, and J. M. Clark, “Simultaneous light scattering and intrinsic fluorescence measurement for the classification of airborne particles,” Appl. Opt. 39(21), 3738–3745 (2000). [CrossRef]
8. M. Kerker, “Light scattering instrumentation for aerosol studies: An historical overview,” Aerosol Sci. Technol. 27(4), 522–540 (1997). [CrossRef]
9. D. Li, F. Chen, N. Zeng, Z. G. Qiu, H. H. He, Y. H. He, and H. Ma, “Study on polarization scattering applied in aerosol recognition in the air,” Opt. Express 27(12), A581–A595 (2019). [CrossRef]
10. Y. L. Pan, S. C. Hill, R. G. Pinnick, H. Huang, J. R. Bottiger, and R. K. Chang, “Fluorescence spectra of atmospheric aerosol particles measured using one or two excitation wavelengths: Comparison of classification schemes employing different emission and scattering results,” Opt. Express 18(12), 12436–12457 (2010). [CrossRef]
11. P. J. Wyatt, “Differential Light Scattering - a Physical Method for Identifying Living Bacterial Cells,” Appl. Opt. 7(10), 1879 (1968). [CrossRef]
12. W. S. Bickel and G. Videen, “Stokes Vectors, Mueller Matrices and Polarized Scattered-Light - Experimental Applications to Optical-Surfaces and All Other Scatterers,” P Soc Photo-Opt Ins 1530, 7–14 (1991). [CrossRef]
13. C. Bustamante, M. F. Maestre, and I. Tinoco, “Circular Intensity Differential Scattering of Light by Helical Structures .1. Theory,” J Chem Phys 73(9), 4273–4281 (1980). [CrossRef]
14. A. Diaspro, M. Bertolotto, L. Vergani, and C. Nicolini, “Polarized-Light Scattering of Nucleosomes and Polynucleosomes - Insitu and Invitro Studies,” IEEE Trans. Biomed. Eng. 38(7), 670–678 (1991). [CrossRef]
15. A. J. Hunt and D. R. Huffman, “New Polarization-Modulated Light-Scattering Instrument,” Rev Sci Instrum 44(12), 1753–1762 (1973). [CrossRef]
16. R. J. Perry, A. J. Hunt, and D. R. Huffman, “Experimental Determinations of Mueller Scattering Matrices for Nonspherical Particles,” Appl. Opt. 17(17), 2700–2710 (1978). [CrossRef]
17. C. Bustamante, M. F. Maestre, and I. Tinoco, “Circular Intensity Differential Scattering of Light by Helical Structures .2. Applications,” J Chem Phys 73(12), 6046–6055 (1980). [CrossRef]
18. C. Bustamante, I. Tinoco, and M. F. Maestre, “Circular Intensity Differential Scattering of Light by Helical Structures .3. A General Polarizability Tensor and Anomalous Scattering,” J Chem Phys 74(9), 4839–4850 (1981). [CrossRef]
19. B. P. Dorman and M. F. Maestre, “Experimental Differential Light-Scattering Correction to Circular-Dichroism of Bacteriophage T2,” P Natl Acad Sci USA 70(1), 255–259 (1973). [CrossRef]
20. C. T. Gross, H. Salamon, A. J. Hunt, R. I. Macey, F. Orme, and A. T. Quintanilha, “Hemoglobin Polymerization in Sickle Cells Studied by Circular Polarized-Light Scattering,” Biochim Biophys Acta 1079(2), 152–160 (1991). [CrossRef]
21. M. F. Maestre, C. Bustamante, T. L. Hayes, J. A. Subirana, and I. Tinoco, “Differential Scattering of Circularly Polarized-Light by the Helical Sperm Head from the Octopus Eledone-Cirrhosa,” Nature 298(5876), 773–774 (1982). [CrossRef]
22. C. W. Patterson, S. B. Singham, G. C. Salzman, and C. Bustamante, “Circular Intensity Differential Scattering of Light by Hierarchical Molecular-Structures,” J Chem Phys 84(3), 1916–1921 (1986). [CrossRef]
23. K. S. Wells, D. A. Beach, D. Keller, and C. Bustamante, “An Analysis of Circular Intensity Differential Scattering Measurements - Studies on the Sperm Cell of Eledone-Cirrhosa,” Biopolymers 25(11), 2043–2064 (1986). [CrossRef]
24. A. L. Gratiet, L. Pesce, M. Oneto, R. Marongiu, G. Zanini, P. Bianchini, and A. Diaspro, “Circular intensity differential scattering (CIDS) scanning microscopy to image chromatin-DNA nuclear organization,” OSA Continuum 1(3), 1068–1078 (2018). [CrossRef]
25. A. L. Gratiet, R. Marongiu, and A. Diaspro, “Circular intensity differential scattering for label-free chromatin characterization: a review for optical microscopy,” Polymers 12(10), 2428 (2020). [CrossRef]
26. R. Marongiu, A. L. Gratiet, L. Pesce, P. Bianchini, and A. Diaspro, “ExCIDS: a combined approach coupling Expansion Microscopy (ExM) and Circular Intensity Differential Scattering (CIDS) for chromatin-DNA imaging,” OSA Continuum 3(7), 1770–1780 (2020). [CrossRef]
27. W. S. Bickel and W. M. Bailey, “Stokes Vectors, Mueller Matrices, and Polarized Scattered-Light,” Am J Phys 53(5), 468–478 (1985). [CrossRef]
28. P. Chylek and G. Videen, “Scattering by a composite sphere and effective medium approximations,” Opt. Commun 146(1-6), 15–20 (1998). [CrossRef]
29. E. Zubko, K. Muinonen, O. Munoz, T. Nousiainen, Y. Shkuratov, W. B. Sun, and G. Videen, “Light scattering by feldspar particles: Comparison of model agglomerate debris particles with laboratory samples,” Journal of Quantitative Spectroscopy and Radiative Transfer 131, 175–187 (2013). [CrossRef]
30. D. B. Shapiro, P. G. Hull, Y. Shi, M. S. Quinbyhunt, M. F. Maestre, J. E. Hearst, and A. J. Hunt, “Toward a Working Theory of Polarized-Light Scattering from Helices,” J Chem Phys 100(1), 146–157 (1994). [CrossRef]
31. D. B. Shapiro, M. F. Mestre, W. M. Mcclain, P. G. Hull, Y. Shi, M. S. Quinbyhunt, J. E. Hearst, and A. J. Hunt, “Determination of the Average Orientation of DNA in the Octopus Sperm Eledone-Cirrhossa through Polarized-Light Scattering,” Appl. Opt. 33(24), 5733–5744 (1994). [CrossRef]
32. S. Nothelfer, F. Foschum, and A. Kienle, “Goniometer for determination of the spectrally resolved scattering phase function of suspended particles,” Rev Sci Instrum 90(8), 083110 (2019). [CrossRef]
33. M. Bartholdi, G. C. Salzman, R. D. Hiebert, and M. Kerker, “Differential Light-Scattering Photometer for Rapid Analysis of Single Particles in Flow,” Appl. Opt. 19(10), 1573–1581 (1980). [CrossRef]
34. G. E. Fernandes, Y. L. Pan, R. K. Chang, K. Aptowicz, and R. G. Pinnick, “Simultaneous forward- and backward-hemisphere elastic-light-scattering patterns of respirable-size aerosols,” Opt. Lett. 31(20), 3034–3036 (2006). [CrossRef]
35. Y. L. Pan, S. Holler, R. K. Chang, S. C. Hill, R. G. Pinnick, S. Niles, and J. R. Bottiger, “Single-shot fluorescence spectra of individual micrometer-sized bioaerosols illuminated by a 351- or a 266-nm ultraviolet laser,” Opt. Lett. 24(2), 116–118 (1999). [CrossRef]
36. J. Bottiger, P. Deluca, E. Stuebing, and D. R. Vanreenen, “An ink jet aerosol generator,” Journal of Aerosol Science - J AEROSOL SCI 29 (1998).
37. K. B. Aptowicz, Y. L. Pan, S. D. Martin, E. Fernandez, R. K. Chang, and R. G. Pinnick, “Decomposition of atmospheric aerosol phase function by particle size and asphericity from measurements of single particle optical scattering patterns,” Journal of Quantitative Spectroscopy and Radiative Transfer 131, 13–23 (2013). [CrossRef]
38. R. Fu, C. J. Wang, O. Munoz, G. Videen, J. L. Santarpia, and Y. L. Pan, “Elastic back-scattering patterns via particle surface roughness and orientation from single trapped airborne aerosol particles,” Journal of Quantitative Spectroscopy and Radiative Transfer 187, 224–231 (2017). [CrossRef]
39. K. B. Aptowicz, “Angularly-resolved elastic light scattering of micro-particles,” Ph.D. Dissertation (Yale University, 2005).
40. Y. L. Pan, P. Cobler, S. Rhodes, A. Potter, T. Chou, S. Holler, R. K. Chang, R. G. Pinnick, and J. P. Wolf, “High-speed, high-sensitivity aerosol fluorescence spectrum detection using a 32-anode photomultiplier tube detector,” Rev. Sci. Instrum. 72(3), 1831–1836 (2001). [CrossRef]
41. J. Narula, A. Kuchina, D.-Y. D. Lee, M. Fujita, G. M. Suel, and O. A. Igoshin, “Chromosomal Arrangement of Phosphorelay Genes Couples Sporulation and DNA Replication,” Cell 162(2), 328–337 (2015). [CrossRef]
42. D. Guirado, J. W. Hovenier, and F. Moreno, “Circular polarization of light scattered by asymmetrical particles,” Journal of Quantitative Spectroscopy and Radiative Transfer 106(1-3), 63–73 (2007). [CrossRef]
43. M. W. Ashraf, A. L. Gratiet, and A. Diaspro, “Computational Modeling of Chromatin Fiber to Characterize Its Organization Using Angle-Resolved Scattering of Circularly Polarized Light,” Polymers 13(19), 3422 (2021). [CrossRef]
