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. Author manuscript; available in PMC: 2020 Nov 18.
Published in final edited form as: ACS Nano. 2020 Jul 14;14(7):9136–9144. doi: 10.1021/acsnano.0c04513

Real-Time Measurement of the Hygroscopic Growth Dynamics of Single Aerosol Nanoparticles with Bloch Surface Wave Microscopy

Yan Kuai 1,, Zhibo Xie 2,, Junxue Chen 3, Huaqiao Gui 4, Liang Xu 5, Cuifang Kuang 5, Pei Wang 6, Xu Liu 7, Jianguo Liu 8, Joseph R Lakowicz 9, Douguo Zhang 10
PMCID: PMC7673255  NIHMSID: NIHMS1645576  PMID: 32649174

Abstract

The growth in aerosol particles caused by water uptake during increasing ambient relative humidity alters the physical and chemical properties of aerosols, which then affects public health, atmospheric chemistry, and the Earth’s climate. The temporal resolution and sensitivity of current techniques are not sufficient to measure the growth dynamics of single aerosol nanoparticles. Additionally, the specific time required for phase transition from solid to aqueous has not been measured. Here, we describe a label-free photonic microscope that uses the Bloch surface waves as the illumination source for imaging and sensing to provide real-time measurements of the hygroscopic growth dynamics of a single aerosol (diameter <100 nm) containing the main components of air pollution. This specific time can be measured for both pure and mixed aerosols, showing that organics will delay the phase transition. This photonic microscope can be extended to investigate physicochemical reactions of various aerosols, and then knowing this specific time will be favorable for understanding the reaction kinetics among single aerosols and the surrounding medium.

Keywords: single aerosol nanoparticles, hygroscopic growth dynamics, Bloch surface waves, photonic microscopy, imaging and sensing

Graphical Abstract

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Atmospheric aerosols are solid and liquid particles that are suspended in the air. The water uptake (hygroscopic growth) of these atmospheric aerosols alters each aerosol’s chemical reactivity, aging, atmospheric composition, and atmospheric visibility.1,2 Furthermore, the interaction between water vapor and the aerosol particles is central to the climate effects of these atmospheric aerosols, including their direct effects (e.g., radiation scattering) and their indirect effects (e.g., modification of cloud properties).3,4 Aerosols have also been linked with human health effects; the ability of the aerosols to interact with the pulmonary function is linked to their penetration depth within the lungs.5 There are many experimental techniques for use in aerosol hygroscopicity studies.6 For example, the hygroscopicity tandem differential mobility analyzer (HTDMA) is often used to characterize the water uptake of an ensemble of aerosol particles (but not that of a single particle) by measuring the light scattering enhancement factor due to their water uptake, that is, the growth factor of the particle due to the increasing relative humidity (RH) of the environment.7 Individual particle analysis using methods such as environmental scanning electron microscopy (ESEM) and environmental transmission electron microscopy (ETEM) can resolve the differences in the sizes, chemical compositions, and states of mixtures of individual aerosol particles.8,9 However, the environment in the chamber used to perform the electron microscopy is completely different from the real atmospheric environment. The high energy electronic beams used in ESEM and ETEM methods always damage the atmospheric aerosols, especially those of very small diameter.

Additionally, both HTDMA and electron microscopy methods cannot deliver sufficient temporal resolution; for example, they cannot provide measurements of the growth dynamics of an atmospheric particle related to its water uptake that occurs when the ambient RH reaches the deliquescent point. Measuring the critical RH at which the solid particles become liquid upon hydration (deliquescence relative humidity, DRH) can give valuable insights into the phase state of aerosols at atmospheric conditions. Nonetheless, such critical RH measurements do not provide information on the dynamics of the particle deliquescence or growth. Recent observations suggest that both processes can be more complex than previously thought and occur in multiple steps with intermediate states.10 Therefore, it is necessary to develop methods to measure the hygroscopic growth dynamics of aerosols with sufficient temporal resolution.11

As demonstrated in published works, the hygroscopic growth versus time during the deliquescence can be measured with sufficient temporal resolution using temporal light intensity fluctuations and image processing. In these measurements, a single aerosol particle was trapped using an existing technique, electrodynamic balance (EDB) method, and the aerosol should be charged or labeled with a charge. The size of the single aerosol is also very large, such as 5 μm in diameter, which is larger than the optical diffraction limit.12 The single aerosol particles also can be trapped with another existing technique, a counter-propagating optical tweezer method, and then two-dimensional angular optical scattering and broadband light scattering spectroscopy can be used to real-time monitor hygroscopic growth dynamics.11 Due to the use of the free-space optical tweezer, the size of the aerosol particles is still larger than the diffraction limit. For both optical tweezers and EDB, the trapped microscale aerosol is unsupported and suspended in the atmosphere.

In this work, a label-free wide-field photonic microscope was developed for real-time observations of the hygroscopic growth dynamics of single aerosol nanoparticles with diameters of less than 100 nm. It will be of great importance to investigate the properties of small aerosols because the smaller aerosol penetrates into the respiratory organs deeper than do the larger ones. Also, the distance of aerosol dissemination will be increased if the size of the aerosol is decreased, then a small aerosol will increase the spreading of haze. In our experiment, the aerosol nanoparticle will be located on the top surface of the substrate, which does not need to be charged or labeled for the measurements. The sample chamber for this optical microscope can have the same conditions as those in the atmosphere.

RESULTS AND DISCUSSION

Experimental Setup and the Bloch Surface Wave (BSW) Multilayer Platform.

A schematic diagram of the photonic microscopy setup is shown in Figure 1A, and the details of the RH controller are shown in Figure S1A of the Supporting Information. A polarizer and a half-wave plate are used to control the polarization orientation of the 635 nm incident laser beam. Using a pair of scanning galvanometers and a focusing lens, the expanded and collimated laser beam can be focused on any point in the back focal plane (BFP) of the oil immersion objective (100×, numerical aperture (NA) of 1.49). This oil immersion objective was used to generate the BSWs because of phase matching restrictions for excitation of the BSWs. The oil immersion system acts as a high-refractive-index prism that permits coupling of free-space light to BSWs.

Figure 1.

Figure 1.

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Schematics of the experimental setup and the samples. (A) Bloch surface wave microscope. The incident angle of the beam was controlled by two scanning galvanometers to excite the BSWs. Polairzer-2 was used to reject the reflected beam from the substrate and allowed only scattering light from the nanoparticle to reach the detector (sCMOS). (B) Dielectric multilayer operating as the substrate for the photonic microscope. The aerosol was placed on this multilayer, and the thickness of each layer is labeled on the right. The incident angle was set as θ = 45° so that BSWs can be excited. The curve of intensity versus Z illustrates that the electric field of BSWs was confined at the top surface of the dielectric multilayer. The red-dashed lines in (A,B) represent the scattering light from the aerosol that was illuminated by the BSWs. (C) Plane intensity distribution of the scattering light from a hypothetical aerosol nanoparticle, which was calculated with finite difference time domain method. The diameter (D) of the nanoparticle changes from 90 to 120 nm, and the effective refractive index (n) changes from 1.5 to 1.4 due to the uptake of water. The intensity of the scattered light changes when the aerosol undergoes the uptake of water from the air gas, as shown in (C), due to the sensitivity of the BSWs.

In the imaging experiments, because of the scanning of a pair of galvanometers, this focal point will rotate and form a ring at the center of the BFP of the objective. The expanded and collimated beam will then exit the objective and strike the substrate at a given angle of incidence (θ) from all azimuthal directions (0–360°). This illumination mode is called azimuthal rotation illumination and can improve the spatial resolution of this type of surface-imaging microscope when compared to the single direction illumination.13,14

The substrate used to support the single aerosol nanoparticle was a Bloch surface wave multilayer dielectric (Figure 1B) that allows BSWs to be excited on their top surface when the angle of incidence (here, θ = 45°) and the incident polarization (as controlled by polarizer-1 in Figure 1A) match the excitation conditions for BSWs, which is similar to the excitation of surface plasmons on a metallic film with an oil immersion objective.15 To ensure that the particles can be recrystallized after deliquescence, we added a layer of silicon nitride (thickness: 3 nm) on the top of the multilayer dielectric film to enhance the hydrophobicity of the substrate. The BSWs were used as the illumination source for imaging of the atmospheric aerosols rather than free-space light used in a conventional microscope. The photonic microscope was thus called a Bloch surface wave microscope (BSWM).13

BSWs are electromagnetic surface waves that are excited at the interface between a truncated periodic dielectric multilayer with a photonic band gap and the surrounding medium.16,17 These evanescent waves are localized in the near-field region (e.g., 100–200 nm or less) of the top dielectric multilayer (as illustrated in Figure 1B, the curve of intensity vs Z). Then, only aerosol nanoparticles inside this region can be illuminated and imaged. On the contrary, if the free space light was used, all of the objects along the propagation path will be illuminated and then the scattering light from these objects will form a background and decrease the contrast of the image. This function of BSWM is the same as the total internal reflection fluorescence microscopy (TIRFM),18,19 but a BSW electric field is stronger than that of the evanescent waves used in TIRFM.20

The thicknesses of each of the dielectric layer and the total number of layers (19 layers) are shown in detail in Figure 1B. It can sustain transverse magnetic (TM)-polarized BSWs (verified by the BFP image on Figure S1B of the Supporting Information).21 The beam reflected from the multilayer platform and the scattered BSW signals from the aerosol are collected via the same objective and imaged on a scientific complementary metal oxide semiconductor (Neo sCMOS detector from Andor-Oxford Instruments (United Kingdom) with a pixel size of 6.5 μm) detector using a tube lens. To improve both the imaging contrast and the signal-to-noise ratio, another polarizer (polarizer-2 in Figure 1A) was placed before the sCMOS to reject the reflected laser beam that did not contribute to the excitation of the BSWs, hence only the scattering light from the aerosol nanoparticle can be collected by the oil immersion objective at all angles and reach the sCMOS to form the image of this aerosol.

It was reported that BSWs are more sensitive to the environmental changes on surfaces (such as the changes of aerosols on the surface) than the evanescent waves that are used in TIRFM.20 BSWs have been used to develop high-precision optical biosensors, where a high-refractive-index prism was used for the BSWs’ excitation.22,23 This bulky prism-based biosensor is not favorable for the investigation of single nanoparticles. In contrast, the objective-based setup, as shown in Figure 1A, has the ability of both imaging and sensing any selected single nanoparticles simultaneously, due to the use of a high numerical aperture oil immersion objective.

In our experiment, an objective with a NA of 1.49 was used to excite the BSWs on a finite area containing a single aerosol. A tiny change inside the aerosol (such as refractive index change and diameter change of the ammonium sulfate (AS) aerosol due to the uptake of water) will induce intensity changes of the propagating BSWs below this aerosol and the scattered light from this aerosol. This phenomenon can be simply demonstrated or verified by the numerical simulations based on the finite difference time domain (FDTD) method, as shown in Figure 1C. The structural parameters of the dielectric multilayer are the same as those shown in Figure 1, and an aerosol nanoparticle is placed on this multilayer. As an example, assume the uptake of water can change the diameter (D) of an aerosol nanoparticle from 90 to 120 nm, and its refractive index (n) is decreased from 1.5 to 1.4 (the refractive index of aerosols is always larger than that of water). The intensity of the far-field scattering light from this aerosol can be calculated using the FDTD method, as shown in Figure 1C, which clearly shows the change of the scattering intensity with particle size and refractive index. By recording the intensity variations of the scattered light with a sCMOS array imager, the changes in the aerosol particle can be observed in real-time. This working principle is similar to that of surface plasmon resonance microscopy (SPRM), which has found numerous applications in the study of biological targets including DNA,24 viruses,25 the local electrochemical reactions of heterogeneous surfaces,26 and of single polymeric and protein nanoparticles.27 The difference between SPRM and BSWM is that BSWM uses an all-dielectric multilayer as the sample substrate; this substrate is more stable and robust than the thin metallic film that is used in SPRM. Details of the experimental setup for BSWM can be found in a previous work.13

Hygroscopic Growth of Single Aerosol Nanoparticles Containing Pure Ammonium Sulfate.

AS was selected as a representative inorganic salt because it is a dominant component of the submicron aerosol mass present in the atmosphere and is an efficient scatterer of solar radiation.28 This substance was used as the reference material to evaluate the BSWM setup to monitor hygroscopic growth of AS with a well-known deliquescence point. The diameter of the AS nanoparticle is approximately 90 nm, as shown in the scanning electron microscopy (SEM) image in Figure S1C of the Supporting Information. An in-house-built chamber is placed on the sample holder, as shown in Figure 1A, and allows the RH of the environment for this AS particle to be controlled dynamically from 60 to 90% with an uncertainty of 1% RH. Figure 2A shows focal plane optical images of this AS nanoparticle that were captured via BSWM for a series of ambient RHs. To allow the aerosol to equilibrate at each selected RH, the residence time for each aerosol measurement is 30 min. The sCMOS integration time for the capture of each image is kept constant at 0.01 s; then, after another 0.01 s, a second image can be captured, and 50 images can be captured in 1 s. The rotation speed of the focused spot on the BFP can be as fast as approximately 1 ms per circle, meaning that an BSWM image can be acquired in only 1 ms (this image acquisition speed is also dependent on the sCMOS response speed, and the full frame rate of the sCMOS used here is 100 fps). In our experiment, the rotation speed is set at 10 ms.

Figure 2.

Figure 2.

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Hygroscopic growth of AS aerosol nanoparticle measured via BSWM. (A) BSWM images of AS nanoparticles at four ambient RH points (60, 75, 84, and 90%). The dashed box represents the square area (13 × 13 pixels). (B) Hygroscopic growth factor (left Y-axis) characteristics of AS as a function of the RH (X-axis), predicted using extended aerosol inorganic model and represented by a solid line. The intensity (right Y-axis) of the AS nanoparticle in the BSWM image is represented by the asterisks (*); this intensity varies with the RH. (C) Curve of scattering intensity vs time when the ambient is set around the DRH point, from which the specific time for solid-to-aqueous phase transition can be measured (60 ms). (D) Images before deliquescence and (E) images after deliquescence. The top panels of (D,E) are bright-field focal plane optical images that show little difference in the particle intensity before and after deliquescence. The bottom panels of (D,E) are BSWM focal plane images that show obvious differences in intensity and thus demonstrate the high sensitivity of the BSWM technique to the changes in the particle due to the water uptake. Error bars in (B) indicate the standard deviation.

As shown in Figure 2A, we present BSWM images acquired at four RH set-points, which show that the intensity of the AS nanoparticle increases obviously when the RH changes from 75 to 84%. The single AS nanoparticle looks like a dipole on the BSWM images because of the unique polarization of the BSWs, azimuthal rotation illumination, and two polarizers used in the optical path (Figure 1A).13 This is the same behavior that would be observed when a polarizer is used after either a radially or an azimuthally polarized beam, where the cross section of the doughnut-shaped beam would also be split into two spots (like a dipole).29 This dipole-like pattern of the single AS nanoparticle can be regarded as the point spread function of the BSWM setup in Figure 1.30

To quantitatively describe the hygroscopic growth of the AS nanoparticle, the pixel intensity within a square area (13 × 13 pixels, as shown in Figure 2A) with this AS nanoparticle at its center was summed, and the total intensity represents the growth of the AS nanoparticle due to its uptake of water. The curve of this total intensity versus the ambient RH is plotted in Figure 2B, as indicated by the asterisks (*). When the RH is below 80%, the total intensity changes only slightly, meaning that the AS nanoparticle does not change, and the water uptake activity is not observed at these RH points. When the RH reaches 80%, the total intensity changes suddenly display a stepwise increase due to water uptake, as seen from the brighter images at this RH point (Figure 2A). It is well-known that a threshold RH exists for pure AS particles at which a solid-to-aqueous phase transition occurs.6,28,31 This process is called deliquescence, and the RH at which it occurs is called the deliquescence RH (which is 80% for the AS used here). When it becomes aqueous, the aerosol nanoparticle will continue to grow by taking up more water as the RH increases (from 81 to 90%, as shown in Figure 2B). For each RH condition, five single particles were studied and the error bars are provided, as shown in Figure 2B.

The hygroscopic growth factor of the AS particle with an increasing ambient RH can be predicted using the extended aerosol inorganic model (E-AIM) (Figure 2B, solid curve) [http://www.aim.env.uea.ac.uk/aim/aim.php]. The E-AIM in the predicted growth factor versus RH is similar to that in the experimental results (indicated by asterisks (*)), which also shows very little change before the DRH, followed by a jump growth at the DRH and then continuous change after the DRH. These experimental observations are also consistent with the results measured previously using the HTDMA technique.31 The experimental results presented in Figure 2A,B clearly demonstrate that BSWM can be used for real-time imaging of the hygroscopic growth of individual AS nanoparticles.

When using the HTDMA technique and E-AIM predictions, the hygroscopic growth factor (the ratio of the volume of the aerosol after water uptake to that of the dry aerosol) can be obtained precisely. For our BSWM technique, the hygroscopic growth was represented by the intensity change of the aerosol in the BSWM image. In principle, this intensity change can be used to derive the diameter change in the aerosol if a standard nanoparticle with known diameter and the same refractive index as that of AS was used to calibrate the relationship between the intensity and particle diameter.25 However, one of the main aims to investigate the hygroscopic growth-induced change in the size or mass of the aerosol is to predict the changes in the light scattering and absorption properties of the aerosol. This can be realized directly from the BSWM image as a result of the use of a photonic microscopy method, where the total intensity change of the aerosol in the image can directly represent the changes in the light scattering and absorption properties of this aerosol that are caused by the water uptake. In future work, the working laser beam will be replaced with a broadband light source to allow measurement of the light scattering enhancement factor (f(RH, λ)), which is a crucial parameter for description of the aerosol’s hygroscopic growth properties.6

The difference between BSWM and the current techniques used to measure the hygroscopic growth of aerosol particles (e.g., HTDMA or ESEM/ETEM) is its favorable temporal resolution. Because of its label-free, highly sensitive, and wide-field imaging capabilities, BSWM can record the rapid hygroscopic growth dynamics of the AS nanoparticle at the DRH point, as shown in Figure 2C. When the ambient RH reaches the DRH, the pure AS nanoparticles deliquesce rapidly and their size, mass, and refractive index change in a very short time period, due to the phase transition from solid to aqueous. Our experimental results demonstrate that this process happens within a time span of approximately 60 ms (Figure 2C), which can be defined as the specific time for solid-to-aqueous phase transition of a single aerosol. To the best of our knowledge, this growth dynamic around the DRH of an individual AS nanoparticle has not been measured previously.

This deliquesce process cannot be imaged using common bright-field transmission optical microscopy, as demonstrated by comparisons between panels D and E of Figure 2. The top panels of both Figure 2D,E are the bright-field optical images, which show little difference in the intensity of the AS particle before and after deliquescence. This is because the AS nanoparticle is smaller in size than the diffraction limit32 and the change in its intensity due to deliquescence is not easily detected. This change cannot be resolved clearly when using a common optical microscope. However, the BSWM images (bottom panels of Figure 2D,E) show distinct intensity differences for the AS nanoparticle. It is evident that the AS nanoparticle in Figure 2E appears brighter than that shown in Figure 2D, and detecting the variations in AS nanoparticle size is more easily observed using the BSWM.

Hygroscopic Growth of Single Mixed Aerosol Nanoparticles.

In addition to the airborne ultrafine particles containing pure AS (inorganic compound of the aerosol) which has a DRH, nondeliquescent aerosol particles such as ammonium nitrate (AN) aerosols are one of the key components of fine urban particles. The nitrate concentration keeps increasing in polluted air in China, and organic glucose particles have been found to absorb and desorb water continuously with changes in the ambient RH.33 In this work, the hygroscopic growth of nondeliquescent aerosol particles (i.e., a glucose nanoparticle and an AN nanoparticle) was measured by BSWM, with results shown in Figure 3. The diameters of these laboratory-generated nanoparticles are also around 90 nm. The experimental results measured by BSWM (asterisks in Figure 3A,C) are in good agreement with the predictions obtained using E-AIM (solid lines, Figure 3A,C) and the results of previously reported work33 and show continuous water uptake by both the glucose nanoparticle and the AN nanoparticle from low to high RH. Additionally, no deliquescence phase transition was observed.

Figure 3.

Figure 3.

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Hygroscopic growth of a glucose nanoparticle, AN nanoparticle, glucose + AS mixed nanoparticle, and AN + AS mixed nanoparticle. Hygroscopic growth factors (left Y-axis) of glucose (A), AN (B), glucose + AS (C), and AN + AS (D) predicted using the E-AIM are represented by the solid lines, whereas those predicted using the Zdanovski-Stokes-Robinson relation are represented by dashed lines (C,D). The total intensities (right Y-axis) of the AS nanoparticle (A), the AN nanoparticle (B), the glucose + AS mixed nanoparticle (C), and the AN + AS mixed nanoparticle (D) are represented by asterisks (*), which varied with the ambient RH. Error bars in (A–D) indicate the standard deviation.

It is well-known that real aerosols always contain multiple constituents. For example, the particles in biomass burning smoke are enriched with hygroscopic organic and inorganic constituents, which are believed to act as efficient cloud condensation nuclei. Therefore, it is important to investigate the hygroscopicity of both organic-inorganic and inorganic-inorganic mixed aerosols.34,35 We studied the effects of the organic surrogate constituent (e.g., the glucose) and the inorganic constituent (AN) on the water uptake behavior of the mixed aerosols containing AS. The dry mass ratio of the nanoparticle mixture was set at 1:4 (glucose/AS = 1:5 for Figure 3B, AN/AS = 1:5 for Figure 3D). Figure 3B,D (asterisks, experimental results) shows that a DRH still exists for this mixed nanoparticle but with a slightly lower DRH point (about 78%). When compared with the results for the pure AS nanoparticle (Figure 2B), a smooth hygroscopic growth process (Figure 3B,D) was observed for AS because of the use of the mixture with the nondeliquescent constituents. This smoothing was also present in the numerical predictions obtained using E-AIM (solid lines in Figure 3B,D).

For the mixed aerosol particles, another approximate method based on the Zdanovski-Stokes-Robinson (ZSR) relationship was used to simulate the hygroscopic growth factors,36 with results shown in Figure 3B,D (dashed lines). The results for the two numerical models clearly show that the largest deviations between the calculations and the experimental results occur around the DRH points. At the other RH points, the calculated ZSR results are in good agreement with the experimental values. These deviations around the DRH points have also been observed in other reported works34 and can be attributed to the inherent limits of the numerical models for these mixed aerosol particles. The model does not account precisely for chemical reactions that occur among the compounds around the DRH, where the phase transition from solid to liquid happens rapidly, and it is also difficult to know the exact mixed states of the particles.

Apart from the deviations between the experimental results and the numerical simulations around the DRH, Figure 3 clearly demonstrates that the organic (glucose) and inorganic (AN) nondeliquescent constituents both affect the water uptake of the AS nanoparticle, as indicated by the smooth hygroscopic growth curves. However, the curves shown in Figure 3B,D are similar, thus indicating that the effect enabled by AN is not very different from that enabled by the glucose. These curves are taken from steady-state measurements and do not show the growth dynamics of the mixed aerosols at the DRH point.

Dynamics of Phase Transition of Single Mixed Aerosols with Various Dry Mass Ratios.

We measured growth dynamics around the DRH of single mixed aerosol particles. The differences from the characteristics of the pure particles (Figure 2C) and the mixed, as shown in Figure 4A,B, are obvious. The curve of the scattering intensity from the AS + glucose nanoparticle versus time is much smoother than the corresponding curve for the AS + AN mixed nanoparticle. For the mixed glucose + AS (dry mass ratio of 1:5) nanoparticle, the specific time for solid-to-aqueous phase transition is approximately 4 s; however, for a mixed AN + AS nanoparticle with the same dry mass ratio, the specific time is on the same order as that for the pure AS nanoparticle (Figure 2C), which is approximately 220 ms.

Figure 4.

Figure 4.

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Specific time for solid-to-aqueous phase transition of single mixed aerosols with various dry mass ratios. (A,B) Curve of scattering intensity vs time when the ambient is set around the DRH point. The dry mass ratio of glucose to AS is 1:5 (A). The dry mass ratio of AN to AS is 1:5 (B). (C) Specific time for solid-to-aqueous phase transition of the mixed aerosol as a function of the dry mass ratios of the two compounds (glucose/AS or AN/AS). Error bars indicate the standard deviation.

In our experiments, the dry mass ratio was varied for the two types of mixed nanoparticles, and the specific times for solid-to-aqueous phase transition of these mixed aerosol nanoparticles were measured by the BSWM technique, as shown in Figure 4C. As the glucose mass ratio increases, the transition time of the glucose + AS nanoparticle is increased (the original curves of scattering intensity versus time for the various dry mass ratios are presented in Figure S2 in the Supporting Information). In contrast, the specific time is mostly constant as the AN mass ratio increases (double-pointed triangles in Figure 4C; the corresponding curves for the various dry mass ratios are presented in Figure S3 of the Supporting Information). These phenomena clearly demonstrate that the organic constituent (glucose) has a more significant effect on the specific time for solid-to-aqueous phase transition of the AS than the inorganic constituent (AN), and it delays the phase transition. To the best of our knowledge, these phenomena have not been observed previously and will be helpful in understanding the properties of atmospheric aerosols and the processes that occur in them. The reason for the differences between an organic mixture and pure AS/mixture of AS and AN can be analyzed as follows. An organic component (such as the glucose used here) can have a kinetic effect on aerosol hygroscopicity. This effect provides a mass transfer barrier to retard the transport rates of the water molecules into the particles.37 This phenomenon was also consistent with the observations that a significant delay happened in the deliquescence of the organic coating particles and that the aerosol particles of organic coatings may require a longer equilibrium time at each ambient RH than in their uncoated pure form.38

It is well-recognized that AS and AN are key components of fine urban particles. In this paper, we only measured the laboratory-generated aerosols, but the method proposed here is also applicable for measuring the hygroscopic growth of ambient aerosol particles (such as fine urban particles) that are collected from the atmosphere. Because we use an optical microscopy for label-free measurements, there is no need for the high voltages that are required in scanning/transmission electronic microscopy, and because the aerosol particles do not need to be charged as required in the electrodynamic balance method, it can be anticipated that our setup has the ability for field studies or outdoor test laboratories.

In our experiments, the aerosol nanoparticles were illuminated with the BSWs that are localized at the near-field region of the substrate. Based on previous work,13 particles with a radius as small as about 50 nm can be imaged with this setup. On the other hand, the decay depth of the BSWs into the air space is less half of the wavelength (about 200 nm). When the aerosol particle is larger than 200 nm, such as a particle with a diameter of several or tens of microns, this particle cannot be fully illuminated or imaged with BSWM. In this case, we can change the illumination mode to the bright-field transmission illumination, then our setup can work as a normal optical microscope. A normal optical microscope can measure the hygroscopic growth of aerosol particles on the micrometer size scale,39 so the size range of atmospheric particles that can be used for this setup is from 50 nm to tens of micrometers.

CONCLUSIONS

In summary, we have proposed a method for real-time measurement of the hygroscopic growth dynamics (such as the solid-to-aqueous phase transition) of single air ultrafine particles (for both pure and mixed aerosols) with diameters of less than 100 nm, which has not been reported previously. Using this time-resolved single nanoparticle measurement, the effect of an organic compound such as glucose on the water uptake behavior of the AS aerosol was found to be more significant than that of an inorganic compound (AN), which is useful for understanding and analyzing the reaction kinetics between aerosols and their surrounding mediums. By taking advantage of the wide-field imaging ability of the photonic microscope, multiple aerosol nanoparticles composed of different compounds can be measured simultaneously for real-time comparisons. The constituents of these mixed aerosols could be determined by Raman spectroscopy when a Raman spectrometer is installed on the experimental setup and the electric field enhancement of the BSWs could enhance the Raman scattering signals.20,37

Because of the universality of photonic microscopy, the time-resolved measurement also can be used to investigate the dynamic process of physicochemical or biochemical reactions, such as to measure the mass transfer rate to and through various kinds of coating layers on a single AS aerosol nanoparticle.38,40 The advantage of this photonic microscopy is that the BSWs can respond quickly and sensitively to environmental changes. These changes can be the mass, morphology, or composition of the single aerosols that are illuminated by the BSWs, which always happens with varying chemicals. For example, both field and model studies showed that heterogeneous reactions of SO2, NO2, and NH3 on wet aerosols accelerated the haze formation in northern China due to the changes of the size, mass, and refractive index of the aerosols. These heterogeneous reactions can be monitored in real-time with high temporal resolution and high sensitivity, which are useful for research in atmospheric chemistry and physics.38

METHODS

Bloch Surface Wave Multilayer Platform.

The dielectric multilayer platforms for BSWs were fabricated by plasma-enhanced chemical vapor deposition (PECVD; Oxford System 100) of SiO2 and Si3N4 layers on a standard microscope cover glass (thickness: 0.17 mm) under a vacuum of <0.0133 Pa at a temperature of 300 °C. Prior to PECVD of the dielectric multilayers, the cover glass was cleaned with a piranha solution followed by nanopure deionized water and was dried using a N2 stream. SiO2 is the low (L) refractive index dielectric, and Si3N4 is the high (H) refractive index dielectric. Their layer thicknesses are 110 and 80 nm, respectively. There are nine pairs of SiO2 + Si3N4 layers. The SiO2 layer thickness for the last pair was approximately 350 nm. The hydrophilicity of the substrate will affect the morphology of the aerosol particles after the solid-liquid phase transition. For example, the substrate hydrophobility might affect the formation and spreading/growth after the solid-to-aqueous phase transition, so in our experiment, another thin Si3N4 layer (thickness: 3 nm) was then coated on the top SiO2 layer to enhance the hydrophobicity of the substrate. The aerosol to be imaged was deposited on this Si3N4 layer because of the hydrophobic property of Si3N4. There are a total of 19 layers, and the exact thicknesses of each of the layers are shown in Figure 1B.

Preparation of the Aerosol Nanoparticle at the Required Ambient RH.

In the deposition channel, an atomizer (Metone255, Beckman Coulter, USA), a diffusion drying tube (3062, TSI, USA), and a long-differential mobility analyzer (3081, TSI, USA) were used to produce aerosol particles with diameters of approximately 90 nm. The analytical pure AS, AN, and glucose were purchased from Sinopharm Chemical Reagent Corporation (China) and were all dissolved in ultrapure water. The concentrations of the three solutions were all the same at 2 g/L. These particles were then deposited on the dielectric multilayer substrate based on the principle of plate impact. The humidification channel was mainly composed of a gas drying pipe, a gas humidification pipe, and flow control devices. The two pipes, which are based on Nafion technology, are both produced by Perma Pure Incorporation (USA). To control the RH, we tuned a proportion-integration-differentiation parameter feedback-based three-way solenoid valve based on the required RH and the real-time RH, as measured using a hygrometer, and subsequently adjusted the volume ratio between the dry gas and the wet gas in the mixing chamber. Aerosol particle deposition and humidification occurred in the same sample chamber, which was placed on the same Bloch surface wave multilayer platform, and the two channels could be selected using a three-way valve. Full details of this setup are shown in Figure S1A of the Supporting Information.

Numerical Simulations of the Far-Field Scattering Intensity from Single Aerosol Nanoparticles That Are Illuminated by the BSWs.

In the simulations, the refractive indices of SiO2, Si3N4, and a glass substrate at a wavelength 635 nm are chosen as 1.48, 2.65, and 1.515, respectively. The FDTD method41 was used to simulate the near-field distributions of the electromagnetic field scattered from the nanoparticle that are illuminated by the BSWs. Based on the data of the near-field distributions, angular-dependent far-field intensity of the nanoparticle’s scattering was obtained using the RETOP software package,42 as shown in Figure 1C.

Supplementary Material

Supplementary figures

ACKNOWLEDGMENTS

This work was supported by National Nature Science Foundation of China (91544218, 61427818, 11774330), Ministry of Science and Technology of China (2016YFA0200601), Anhui Initiative in Quantum Information Technologies, Anhui Provincial Science and Technology Major Projects (18030901005), the foundation of Key Laboratory of Environmental Optics and Technology of Chinese Academy of Sciences (Grant No. 2005DP173065-2019-XX). J.R.L. is supported by grants from the National Institutes of Health (R01 GM125976, R21 GM129561, and S10OD19975). The work was partially carried out at the University of Science and Technology of China’s Center for Micro and Nanoscale Research and Fabrication. D.Z. is supported by USTC Tang Scholarship and Advanced Laser Technology Laboratory of Anhui Province (Grant No. 20192301) and acknowledges valuable discussions with Dr. Yunfeng Xiao (Peking University), Dr. Weijun Li (Zhejiang University), and Dr. Mingjin Tang (Guangzhou Institute of Geochemistry).

Footnotes

Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.0c04513.

Experimental setup with details of the RH controller, back focal plane image of light reflected from the dielectric multilayer and scanning electron microscope image of the single AS aerosol nanoparticles; curve of scattering intensity vs time when the ambient is set around the DRH point (glucose + AS); curve of scattering intensity vs time when the ambient is set around the DRH point (AN + AS) (PDF)

The authors declare no competing financial interest.

Contributor Information

Yan Kuai, Advanced Laser Technology Laboratory of Anhui Province and Institute of Photonics, Department of Optics and Optical Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China.

Zhibo Xie, Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, China.

Junxue Chen, School of Science, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China.

Huaqiao Gui, Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, China.

Pei Wang, Advanced Laser Technology Laboratory of Anhui Province and Institute of Photonics, Department of Optics and Optical Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China.

Xu Liu, State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China.

Jianguo Liu, Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, China.

Joseph. R. Lakowicz, Center for Fluorescence Spectroscopy, Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland 21201, United States

Douguo Zhang, Advanced Laser Technology Laboratory of Anhui Province and Institute of Photonics, Department of Optics and Optical Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China;.

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Supplementary Materials

Supplementary figures
NIHMS1645576-supplement-Supplementary_figures.pdf (584KB, pdf)

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