Monitoring of particulate matter using spatially-curved thin film

Abstract

A portable air-quality-monitoring apparatus configured to detect and measure micron- and sub-micron-sized particles (down to 100 nm in dimension) in real time. The apparatus includes a film of fluid deposited onto the coverslip that is used to collect particles before the apparatus is deployed at outdoor monitoring sites, and can be used both in the environment devoid of a conventional optical lens element and in the environment employing such optical lens element. The presence of the film increases the scattering cross-section of particles smaller than the pixel size, thereby enabling the sub-pixel and sub-diffraction-limit sized particles to be detected.

Description

MONITORING OF PARTICULATE MATTER USING SPATIALLY-CURVED THIN FILMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority from and benefit of the US Provisional PatentApplication No. 63 / 586,612 filed on September 29, 2023, the disclosure of which is incorporated herein by reference.STATEMENT REGARDING FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Grants Numbers NNX12AL47G and NNX15AJ22G awarded by NASA and Grant Number 1531243 awarder by the National Science Foundation. Hie U.S. government has certain rights in the invention.RELATED ART

[0003] Air quality monitoring is of growing concern as studies continue to confirm correlation between inhaled particulate matter and respiratory diseases and cognitive decline in human subjects. Current commercial devices configured to detect and measure particulate matter suspended in are do not posses sufficiently large dynamic range (as far as sizes of particles are concerned), and the number of operational air quality monitoring stations is not that significant, which together inevitably reduces the precision with which local particulate matter (PM) hazards can be tracked. These practical gaps in current technology are especially challenging to fix for ultrafine (-100 nm) particulate matter.

[0004] The shortcomings of currently installed and operated air quality monitoring devices (such as high-cost, low-portability, and low-density - that is. the number of air quality devices per number of people) have fueled investigation and research into alternative portable low-cost air quality monitoring devices to enable people to monitor their personal exposure levels based on their unique habits and locations. Many of these low-cost personal devices integrate smartphone technology and machine learning to image collections of PM. How ever, these devices are not able to perform long-term monitoring of particles - and, in particular, of ultrafine particles - in a fixed location. Another possible alternative w ith comparable resolution is a lensfree holographic device. A lensfree holographic devices utilizes computational imaging to post-process images of diffraction patterns collected on the sensor of the device. With no conventional lens element present and a short sample-to-sensor distance, atypical lensfree holographic device operated at a unit magnification configuration while enabling imaging over the whole sensor. This results in images with resolution comparable to lens-based systems over tire whole ficld-of-vicw of tire sensor. Previously, lensfreeholographic devices have been used for air quality monitoring (see, for example. Wu, Y.-C. et al. Air quality monitoring using mobile microscopy and machine learning; Light Set Appl 6, el7046-e 17046, 2017) and volatile aerosol characterization of e-cigarette smoke (see Luo, Y. etal. Dynamic Imaging and Characterization of Volatile Aerosols in E-Cigarette Emissions Using Deep Learning-Based Holographic Microscopy, ACS Sens. 6, 2403-2410; 2021). However, currently both tire smartphone-based and lensfree holographic devices are operationally limited in that they provide spatial resolution on the order of 1 pm, or in some specific cases no less than 500 nm (which, understandably, prevents the existing devices from detection of PM with smaller dimensions).

[0005] Other optical-fiber based systems measure change in transmission caused by PM and could detect particles smaller than 1 pm in size. However, these devices require the use of laser systems that are often neither low-cost nor easily portable. Other non-imaging based low-cost portable devices were reported that are able to detect particles smaller than 500 nm by measuring charged particle mobility. However, these devices cannot be as easily deployed cn masse or replicated by citizen scientists as lensfree holographic systems can, since lensfree holographic systems often only require a commercial sensor, a light source such as an LED, and a 3D printed housing for which files can be made easily accessible.SUMMARY OF THE INVENTION

[0006] Embodiments of tire invention provide a method that includes several steps: step (a) of depositing particular matter to be imaged (a set of particles) onto a film of liquid that is carried by an optically-transparent substrate (notably, each of the optically transparent substrate and the film of liquid is configured such as to not contain - that is, to be devoid of- such particular matter); step (b) of illuminating such set of particles located on a film of liquid with a first lightwave having a substantially spherical wavefront; step (c) of increasing a light-scattering cross-section of a particle of the set of particles by propagating light of the first lightwave through the film of liquid; step (d) of receiving, at an optical detection system separated from the set of particles by the film of liquid, the light of tire first lightwave that has interacted with the set of particles to fonn an optical image or recording of the set of particles without the use of an optical lens element between the film of liquid and tire optical detection system. The method also includes step (e) of transforming the optical image or recording into a resolved image of the set of particles with the use of programmable electronic circuitry, and step (f) of repeating steps (a) through (e) - at a moment of time that comes later than a moment of time corresponding to said illuminating - to form an auxiliary resolved image of the set of particles in which a number of particles has increased. Finally, tire method includes an additional step of determining change of concentration of particulate matter in said air by particlesize with time based on comparison of the resolved image with the auxiliary resolved image (with the use of a computer processor). In one specific implementation, the method may additionally include delivering air that contains particulate matter to the film of fluid along a path that is not within a field-of-view of the optical sensor, and / or the method may satisfy tire condition under which the process of transforming includes reconstructing an optical filed over an entirety of a field-of-view of an optical sensor of the optical detection system. Alternatively or in addition, and at least in one specific implementation, tire method may include a process of time-resolved and real time imaging of the set of particles in which a number of particles increases with time (such process is configured to form an optical image of a particle - of the set of particles - that has a dimension smaller than a diffraction limit associated with the method and smaller than a size of a pixel of an optical sensor of the optical detection system). Alternatively or in addition, and substantially in every implementation of the method, the step of increasing a light-scattering cross-section of a particle of the set of particles may include changing a degree of convergence or divergence of an optical field, emanating from tire particle, upon the propagating light of the first lightwave through the film of liquid.

[0007] Embodiments of the invention additionally provide an optical imaging apparatus that includes an optical detection system (which contains an optical sensor or detector positioned to intersect a first axis and receive light propagating along the first axis), an optically-transparcnt substrate positioned across the first axis to intersect the light propagating along the first axis towards tire optical sensor, and a film of liquid deposited on (and, therefore, carried by) the optically-transparent substrate. Notably, neither the optically transparent substrate nor tire film of liquid contains any particulate matter to be imaged with the optical imaging apparatus (that is, each of the substate and tire film of liquid is devoid of such particulate matter). Such apparatus is configured to be operably mated with a source of light structured to generate the light and direct this light along the first axis (such has a substantially spherical wavefront). In at least one implementation, the apparatus is devoid of (that is, does not contain) an optical lens between the film of liquid and the optical sensor. In at least one of the above-mentioned implementations, the apparatus may include a dedicated housing dimensioned to enclose at least the optical sensor, the optically-transparent substrate, and the film of liquid, and containing an inlet structured to receive a flow of gas (for example - air) delivered from an outside of the housing. When present, such housing is judiciously dimensioned to direct the flow towards tire cover slip glass along a path that is outside of a field-of-view of tire optical detection system. Optionally, an embodiment of the apparatus may be configured to satisfy at least one of the following conditions: to incorporate tire source of light alluded to above: to include a bandpass optical filter betw een the source of light and the optically-transparent substrate, (while the bandpass optical filter is configured to transmit light having partial temporal coherence); and to have the source of light include apinhole separated from the optical sensor by the optically-transparent substrate. A method of using substantially every embodiment of the apparatus discussed above includes: (a) illuminating a set of particles located on (not within) the fdm of liquid with a first lightwave having a substantially spherical wavefront; (b) increasing a light-scattering cross-section of a particle of the set of particles by irradiating such particle with light of the first lightwave and propagating said light through tire film of liquid; (c) receiving, at the optical detection system separated from the set of particles by the film of liquid, the light that has interacted with the set of particles to form an optical image of the set of particles without the use of an optical lens placed betw een the film of liquid and the optical detection system; (d) with programmable electronic circuitry, transforming tire optical image into a resolved image of the set of particles; and (e) at a moment of time that comes later than a moment of time corresponding to tire illuminating, initiating a repetition of steps (a) through (d) to form an auxiliary resolved image of the set of particles in which a number of particles has increased. Alternatively or in addition, and substantially in every implementation of the method, the method may include collecting at least one particle of the set of particles from air at the film of liquid prior to step (a) and / or the step of illuminating may include illuminating the set of particles by transmitting light through an optical lens element and / or the step of increasing a light-scattering cross-section of a particle may include changing a degree of convergence or divergence of an optical field (emanating from the particle) upon the propagating light of the first lightwave through the film of liquid and / or tire method may include a process of time-resolved and real-time imaging of the set of particles in which a number of particles increases with time, said process being configured to form an optical image of a particle (of the set of particles) that has a dimension smaller than a diffraction limit associated with the method and that is smaller than a size of a pixel of the optical detector and / or the method may additionally contain a step of vapor-depositing the film of liquid onto tire optically-transparent substrate and positioning the optically-transparent substrate carrying the film of liquid onto the optical sensor of the optical detection system prior to step (a). Alternatively or in addition, and substantially in every implementation, the method may include one or more of the following: (i) configuring the first lightwave as a lightwave having a substantially spherical wavefront and (ii) the first lightwave as a partially coherent Lightwave. Furthermore, a step of illuminating may include illuminating the set of particles through an optical pinhole and / or at least one of the following conditions may be satisfied: (1) the step of transforming includes reconstructing an optical field over an entirety of a ficld-of-vicw of the optical sensor; and (2) the method includes an additional step of delivering air to the set of particles supported by the film of fluid along a path that is not within the field-of-view of the optical sensor.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention will be more fully understood by referring to tire following Detailed Description of Specific Embodiments in conjunction with the Drawings, of which:

[0009] FIG. 1 schematically illustrates a conventional embodiment of a lensfree microscopic arrangement of related art.

[0010] FIG. 2 provides a schematic diagram illustrating an embodiment of the invention.

[0011] FIGs. 3 A, 3B, 3C are illustrations of portions of physical hardware employed in an embodiment of tire invention: FIG. 3 A: Main device with air pump. FIG. 3B: Main device and pump in bottom portion of housing. FIG. 3C: Main device and pump completely enclosed in housing. The LCD display on the device shows particulate matter as deposited onto the PEG film in real time.

[0012] FIG. 4 presents a reconstructed hologram of a diffraction pattern generated by 200 nm spherical nanoparticles deposited onto PEG film using the device. Arrows show some of the 200 nm particles.

[0013] FIG. 5 illustrates a device used to condense PEG onto coverslip before being placed in the embodiment of the LPAQD.

[0014] FIG. 6 illustrates experimental data pertaining to evaporation-based formation of PEG 600 films with different thicknesses of PEG 600 films. Dotted lines show exponential fits to data.

[0015] FIGs. 7A, 7B, and 7C provide illustrations to tire process of confimiation of operability of tire embodiment of the invention as to detection of spherical particles of sizes between about 1 micron and 100 nm. FIG. 7A: A small region of interest of a reconstructed hologram from the diffraction pattern of 1 micron-sized particles aerosolized into the LPAQD and deposited on the PEG film. FIG. 7B Confirmation of location of 1 micron-sized particles with an image from a fluorescence microscope of the same sample. FIG. 7C Reconstructed hologram from tire diffraction pattern from 100 mn spherical nanoparticles aerosolized in the device and deposited on PEG film. Insets show images of the same particles in an SEM to confinn the locations of tire 100 nm particles. The scale bars in the SEM images are 100 mn.

[0016] FIG. 8 depicts experimental data representing peak phases of particles in holographic reconstructions as the PEG material evaporated.

[0017] FIGs. 9A, 9B, 9C, and 9D provide demonstration of spatial curving of tire PEG film deposited on the supporting substrate and receiving the particles there on, leading to the nano-lensing effect. FIG. 9A: Coverslip with no PEG and 500 nm aerosolized particles. The lines in the image are scratches used as registration markers. FIG. 9B: Image of the same sample as that imaged in FIG. 9A but using a fluorescence microscope, demonstrating that there are 500 nm particles present. FIG. 9C: Coverslip withPEG and 500 nm aerosolized particles clearly observable. FIG. 9D Image of the same sample as that imaged in FIG> 9C but using a fluorescence microscope, validating the locations of the 500 nm particles located in the holographic reconstruction.

[0018] FIG. 10 illustrates correlation of new particles counted with tire use of an embodiment ofLPAQD each hour with the PM25 hourly concentrations reported by the user.

[0019] FIG. 11 is a bar diagram representing concentration of particulate matter (PM concentration) measured with the embodiment of the LPAQD by particle size for each hour.

[0020] Generally, like elements or components in different Drawings may be referenced by like numerals or labels and / or tire sizes and relative scales of elements in Drawings may be set to be different from actual ones to appropriately facilitate simplicity, clarity, and understanding of tire Drawings. For the same reason, not all elements present in one Drawing may necessarily be shown in another.DETAILED DESCRIPTION

[0021] In accordance with embodiments of the present invention, methods and apparatus are disclosed for quantification of airborne pollution due to particulate matter at spatial scale of several tens of nanometers (for simplicity, of about 100 nm) and greater that can be employed with or without conventional microscopic apparatus. Particulate matter of 100 nm and smaller in size is the so-called "ultrafine" particulate matter, and is particularly hazardous to human health. Despite this fact, the ultrafine particular matter is not being widely monitored - unlike the so-called “fine” particulate matter (which is about or less than 2.5 microns in size).

[0022] Imaging systems configured as lensfree microscopes have advantages over conventional microscopes due to their cost-effective hardware, compact and light-weight form factor, and high spacebandwidth product, which is conventionally defined as the ratio of the field-of-view to the area of the smallest resolvable spot. (A typical lensfree microscope is knowm and recognized to not include conventional optical imaging elements but to employ a semicoherent light source used to illuminate an object that is positioned directly atop an image sensor. See FIG. 1, in which the object 110, positioned betw een the light source 104 and the optical sensor 114. is shown in the form of an extended plate; the distance between the object and the sensor is typically on the order of 1 mm). The recorded at the optical detector (not show n) distribution of optical irradiance is the appropriately reconstructed into images of the object. A high space-bandwidth product allow s a large number of objects to be imaged simultaneously within a large field of view.

[0023] The discussed below embodiment of air quality monitoring device (LPAQD) is judiciously configured to detect and measure a set of generally micron-sized and nano-sized particles (from about 1 micron down to particles of about 100 nm in size) with real -time tracking capability. Tire proposed approach involves the use of a supporting optically-transparent substrate (coverslip) that carries a liquid polymer film, judiciously deposited on the substrate before a flow of air is delivered to volume containing the substrate using a custom-built prototype device. The liquid film receives and captures particles and also generates a large optical scattering signature when particles are captured (due to surface tension locally deforming the film in the vicinity of the nanoparticles that landed on the film). The vapor- condensed film operates by increasing the scattering cross-section of particles that are smaller than a pixel of the used optical detector, thereby enabling the sub-pixel and sub-diffraction-limit sized particles to be detected. Hie high dynamic range, low -cost, and portability of this device can enable an individual / user to monitor the quality of air surrounding this particular individual to impact user decisions that reduce risk for particulate matter related diseases. The scattering strength of the scattering signature is then used to measure the size of each particle as it lands onto the film, in real time and as a function of time - since nothing prevents more nanoparticles carries by the air flow to fall / land on the film after the first one docs. Tire process of monitoring of the scattering strength as a function of time allows the user to track the size of and time at which each particle is captured, thereby enabling time-resolved measurements of air pollution levels.

[0024] FIG. 1 provides a schematic of a conventional lensfree microscopic arrangement configured as optical imaging system. Here, the optical sensor 114 of the optical detection system, separated from the object 110 (that is irradiated with light from the light source 104) by a distance correspondence to a far-field of light produced by the source 104, registers the optical field propagated from the object 110.

[0025] FIG. 2 shows a schematic and principle of operation of an embodiment of the invention, which is interchangeably referred to herein as a lensfree, portable, air quality monitoring device (LPAQD). The LPAQD 200 necessarily includes an optical sensor 204 such as the camera sensor (for example, Arducam MT9 JOO 1 with a pixel size of 1.67 tyn) and the optically transparent coverslip or cover substrate 208 with a vapor-condensed film 212 on the top surface of the cover substrate 208. (The substrate 208 may be placed directly on tire sensor with the bottom surface of the substrate 208 facing tire sensor 204.) Notably, the sensor 204 may still have the manufacturer's protective cover glass attached, so the distance between the sample and tire sensor is approximately 1 mm. The real-time tracking is enabled by an appropriatelyprogrammed computer processor (in one implementation, a Raspberry Pi 3 single board computer was used) and an LCD screen (ELECROW 5” Raspberry Pi Screen).

[0026] For the purposes of this disclosure and accompanying claims, a real-time perfonnance of a system is understood as perfonnance which is subject to operational deadlines from a given event to a system’s response to that event. For example, a real-time optical imaging of a particle with the use of the discussed optical imaging system may be one triggered by the user or by the electronic circuitry and executed simultaneously with and without intermption of a process of collection of particle from the air or gas that is in contact with the fdm 212 and during which such optical images are being recorded. In a related example, a real-time extraction of optical information (such as spatial distribution of irradiance or intensity) from an optical detector may be one triggered and / or executed simultaneously with and without intermption of the process of collection of optical data during which such optical information has been recorded. The term ’‘set” is defined as a collection of elements containing one or more of such elements.

[0027] The embodiment also includes a source of light with partial temporal coherence. 216, configured such as to deliver a substantially spherical optical wavefront to the surface of the film 212. (As a skilled artisan knows, the temporal coherence is defined as follows: Considering a fixed point on the electromagnetic wave front, if at any time tire phase difference at this point between time t and time t+dt remains the same - where dt is the time delay period - tire electromagnetic wave is considered to have temporal coherence over a time dt. If dt can be any value, tire electromagnetic wave is considered to have substantially perfect temporal coherence. If the phase difference at hand is maintained the same only within a time range 0<dt< t0, the electromagnetic wave is considered to have partial temporal coherence, with a coherence time equal to tO.)

[0028] In one example, the substantially spherical optical wavefront can be achieved with the use of a pinhole, as indicated. In at least one implementation - and, in particular, when the optical sensor 204 was configured to be a monochrome optical sensor (such as. for example, Arducam MT9I00 Iwith a pixel size of 1.67 / tm) - an additional bandpass filter could be employed. Constituent components of the embodiment 200 were optionally assembled in tire housing (which in one case may be a 3D-printed housing; not shown). In one example, light generated by the LED (such as Golden DRAGON, A = 528 + 15 nm; not shown in FIG. 2) was spatially filtered by a pinhole 220 (of about 250 / tm diameter in one example) thereby forming lightwave with substantially spherical optical wavefront propagating towards and then filtered by an530.5.. .533.5 nm bandpass filter 224 (Thorlabs FL532-3) to achieve a partially coherent plane wave at the plane of the sensor 204.

[0029] Referring again to FIG. 2 and in further reference to FIGs. 3A, 3B, and 3C, air was pulled into / delivered inside the housing of tire LPQAD 200 using a chosen pump - for example, an aquarium air pump (AIRPO) with a flow rate of 4.5 L / min. The embodiment 200 does not use an impactor, and the nozzle of the air pump is placed in a hole at the top of the device to direct the air towards the coverslip while staying out of the field-of-view of the sensor 204.In one implementation, to monitor the particulate matter collected over time, the optical detection system (and tire sensor 204 as part of it) was controlled using a Python code run by a Raspberry Pi 3 and programmed to take multiple (20. in one specific case) images, which were later averaged, every 10 minutes. (For the purposes of this disclosure and accompanying claims, the term image refers to an ordered representation of detector signals corresponding to spatial positions. For example, an image may be an array of values within an electronic memory, or, alternatively, a visual image may be formed on a display device such as a video screen or printer.) Hie collected images then were displayed on a portable LCD screen as seen on the top of the device. FIG. 4 presents a reconstructed image (a hologram) of a diffraction pattern generated by 200 nm spherical nanoparticles deposited onto PEG film 212 using the device 200. Arrows in FIG. 4 point to some of the 200 nm particles.

[0030] Hie combination of the coverslip 208 with the liquid film 212 on it was prepared by the process of Polyethylene Glycol (PEG) Deposition, hi particular, PEG 600 was vapor-condensed onto a plasma-treated coverslip 208 (Globe Scientific) using the system shown in FIG. 5. Hie coverslip 208 was placed at the opening on top of the system and a room temperature aluminum plate was placed on top of the coverslip 212 to ensure uniform condensation. The PEG 600 was heated to about 140°C in a beaker using a U-shaped heater. The temperature of the heater was maintained by a proportional -integral-derivative (PID) temperature controller driven by a Lab VIEW program, which also monitored the temperature of the PEG from a temperature probe to ensure proper performance of the PID loop. PEG was deposited for 5 minutes, resulting in a thickness of several hundreds of nm (for example, ~ 110 nm; FIG. 6) . After the PEG was deposited, the film thickness was measured using an ellipsometer (M-2000 J. A. Woollam).

[0031] An embodiment of the proposed apparatus (such as LPAQ 200) was initially tested by aerosolizing particles of known size into the device using an aerosol generation system (Brechtel Manufacturing Inc. 9200). Solutions with particles of known sizes (1 [lm, 500 nm, 300 nm, 200 nm, and 100 nm) were diluted and placed into the aerosolizer, which blew the particles into the LPAQ 200, where theywere captured on the PEG film 212. Fluorescent particles were used when it was needed to confirm the particle locations with images taken on a benchtop fluorescence microscope. Upon arriving to the film 212, the rested on (in contradistinction to being within) tire film 212 particles spatially curved the film (which, in its virgin deposited form - that is, before the introduction of particles - had substantially perfectly planar upper surface and tire lower surface that w as confirming to tire substantially planar upper surface of the coverslip 208). thereby causing a local effect of changing a degree of convergence and / or divergence of light propagating through the film (and therefore enabling an increase in a light-scattering cross-section of a given particle that led to a change of a degree of convergence or divergence of an optical field, emanating from such particle that has been irradiated with light from tire LED source of light.) The particles were optically detected, sized, and counted using a previously-published image processing approach, written in MATLAB (see, for example, McLeod, E. el al. High-Throughput and Label-Free Single Nanoparticle Sizing Based on Time-Resolved On-Chip Microscopy. ACS Nano 9, 3265-3273; 2015). The code sizes and counts the particles in the image reconstructed using the angular spectrum method (ASM) based on the phase of the identified particles. The peak phase of particles of known size were measured over days and used to determine particle size in the image reconstractions.

[0032] Images were reconstructed using the ASM by decomposing an optical field into a superposition of plane waves that are traveling at different angles as described by Eq. 1.

[0033] The spatial frequency components along x and y are described by kxand ky. k is the wavenumber, z is the propagation distance, and T and fF-1describe the Fourier and inverse Fourier transformations.

[0034] Considering the lack of conventional optical lens components, scalar fields were assumed, and so the vector field E became the scalar field E. To perform the image reconstruction, light detected at the sensor plane. E (x, y, 0). w as back-propagated to the object plane, E (%, y, z). to reconstruct the object field, where z was the negative of the object-to-sensor distance. The reconstraction was done over the whole field of view of the sensor, making high-resolution, large field-of-view reconstructions achievable in a single step.Results and Discussion.

[0035] To determine the optimum time of particle deposition onto and thickness of the film 212 that changes its spatial curvature when a particle with a size of about 1 micron and another particle with a size of about 100 nm are received at the fdm simultaneously, the thickness of vapor-condensed PEG films was measured for various deposition times using an ellipsometer. Since PEG evaporates, tire thickness was measured over a week to determine the “shelf-stability” of the PEG film (FIG. 6).

[0036] It was determined that a 5 -minute deposition was substantially optimal for fonning film- caused nano-lenses around particles based on nano-lens formation theory (McLeod, E. et al. Tunable Vapor- Condensed Nanolenses. ACS Nano 8, 7340-7349; 2014). Hie film thickness was optimized for imaging of tire 100 nm particles (and not the 1 micron sized particles) since peak phase detection was more demanding for smaller particles (as bigger particles can be more easily detected even without any film) and it does not take much for small particles to become buried inside / within the film. It was determined that the optimal film thickness was approximately equal to the radius of the particle. Extending this to 100 nm particles, an approximately 50 nm film thickness was considered to be preferred for performing the desired particle monitoring.

[0037] It was also determined that, once the PEG was deposited for 5 minutes, the resulting PEG film 212 maintained a thickness of around 50 nm for a day or two, after which the thickness was decayed due to continued gradual evaporation.

[0038] Furthermore, particles of know n sizes were aerosolized in the device and onto the PEG- coated coverslip 208 to detennine the smallest particle size that could be detected. Fluorescing 1 pm. 500 nm, 300 nm, and 200 nm particles were used so that particle locations in the holographic reconstructions could be confirmed against images from a fluorescence microscope. A scanning electron microscope (SEM) was used to determine the particle locations of identified 100 nm particles in the holographic reconstructions. FIGs. 7A, 7B, 7C show the reconstruction and validation images of the largest and smallest particles we study here, corresponding to 1 pm and 100 nm particles.

[0039] For particles of known sizes, the optical recordings or images were taken over a 10 day period to measure the peak phase of each particle as the PEG evaporated (FIG. 8). The phase measured at time 0 was used in the MATLAB script for sizing and counting particles of unknowm size. After one day, the200 nm particles were no longer visible in tire reconstructions and after 6 days the 500 nm particles were no longer visible in the reconstructions, indicating that the chosen methodology for measuring such a wide range of particle sizes is stable for a relatively short time. For the first three days, the peak phase of tire 1 |im and 500 nm particles decreased as expected. However, on day four, the peak phase increased. This could be related to film instability, rupture, and reorganization, which was observed in FIG. 6 for the PEG deposited for 9 minutes. The 1 |tm and 500 nm particles may serve as nucleating seeds for the film reorganization after rupture, creating a larger meniscus and enhancing tire nano-lensing effect produced by the film of liquid 208 that was locally spatially curved due to the presence of particle on it.

[0040] FIGs. 9A, 9B, 9C, and 9D show the local effect produced by the spatially-curve (under the500 nm particles) film 212 of the PEG on the convergence / divergence of light propagating through the film. For comparison, without the presence of the PEG film 212 on the coverslip 208 tire particles were undetectable in the obtained optical recordings or image(s) even though their presence was convincingly confirmed by tire image obtained with the fluorescence microscope.

[0041] Field testing of the proposed LPAQD methodology was carried out at tire Children’s ParkPima County testing site (Tucson, AZ) by outdoor air quality monitoring. There, images taken at each hour were processed to obtain the number of particles counted over the previous hour and determine the corresponding size distributions. The Pima County air quality monitoring sites report the PM concentration in |tg / m3each hour. FIG. 10 shows the correlation between the number of new particles detected each hour in the LPAQD and the PM25 hourly concentrations published by tire count}'. Hie error bars in x- show the error due to rounding since tire county reported the hourly PM25 concentration as whole numbers. The vertical error bars show tire uncertainty in particle counts from tire particle detection and counting code. The range of uncertainty was determined by minimizing and maximizing the value of the noise threshold in the code: the value above which a pixel in the holographic reconstruction was considered to be from a detected particle rather than noise.

[0042] The correlation demonstrated that LPAQD 200 could detect a change in number of particles each hour (which correlated with the measured change in PM25 concentration from a commercial device). The number of particles and particle sizes determined from the data collected by LPAQD 200 can be converted into a PM concentration using Eq. (2)

[0043] where r is the particle’s radius. Nris tire particle count of particles of radius r, Vris the particle volume of particles with radius r. p is the particle density, which is assumed to be the density of polystyrene, Q is the volumetric air flow rate, t is the collection time, and rj is the collection efficiency of LPAQD which is the ratio of particles that land on the PEG to the number of particles drawn into the device. For LPAQD 200, J] was determined to be is 0.0119.

[0044] The proposed LPAQD-based methodology facilitated the detection of particles with sizes betw een about 100 nm and about 1 pm, as shown in tire plot of FIG. 11. FIG. 11 show s the measured change in PM concentration by particle size every hour. Commercially available devices are not configured to provide such information, which is critical for understanding PMi and PMo.i levels and for assessing air that could be harmful. Quantify ing the PM concentration by particle size per hour also enables understanding of sources of PM based on particle size and timing of polluting events. Notably, in stark contradistinction with methodology discussed in US Patent No. 10,088,663 (where the very methodology was turning on the required for the operation of the imaging apparatus and necessary for preparation of the particulate matter to- be-imaged incorporation / enclosure of the target particulate matter inside or w ithin the film of liquid or by encapsulating of the target particulate matter between the supporting substrate and the film of liquid) ), the set of particles to be imaged with tire use of the proposed methodology is necessarily not covered by or incorporated within the liquid but, to the contrary, rests on the surface of the film of liquid. As the skilled person w ill readily appreciate, the operational disadvantage of the approach discussion in the ‘663 patent necessarily prevents real-time monitoring of the process of deposition of the particles on the supporting surface (since all particles have to be captured / pre-deposited before tire very first occurrence of the optical detection). As a result, the approach of ‘663 patent does not allow one to configure the process of optical monitoring in time while changing a number and / or content of particulate matter to be imaged.

[0045] References throughout this specification to "one embodiment," "an embodiment," "a related embodiment," or similar language mean that a particular feature, structure, or characteristic described in connection with the referred to "embodiment" is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment, “ "in an embodiment, “ and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. It is to beunderstood that no portion of disclosure, taken on its own and in possible connection with a figure, is intended to provide a complete description of all features of the invention.

[0046] In this specification, embodiments have been described in a way that enables a clear and concise specification to bet written, but it is intended and will be appreciated that embodiments may be variously combined or separated without parting from the scope of the invention. In particular, it will be appreciated that all features described herein at applicable to all aspects of the invention.

[0047] For the purposes of this disclosure and the appended claims, the use of the terms"substantially", "approximately", "about" and similar terms in reference to a descriptor of a value, element, property' or characteristic at hand is intended to emphasize that the value, element, property, or characteristic referred to, while not necessarily being exactly as stated, w ould nevertheless be considered, for practical purposes, as stated by a person of skill in the art. These terms, as applied to a specified characteristic or quality descriptor means "mostly", "mainly", "considerably", "by and large", "essentially", "to great or significant extent", "largely but not necessarily wholly tire same" such as to reasonably denote language of approximation and describe the specified characteristic or descriptor so that its scope would be understood by a person of ordinary skill in the art. In one specific case, the terms "approximately", "substantially", and "about", when used in reference to a numerical value, represent a range of plus or minus 20% with respect to the specified value, more preferably plus or minus 10%. even more preferably plus or minus 5%, most preferably plus or minus 2% with respect to the specified value. As a non-limiting example, two values being "substantially equal" to one another implies that the difference between the two values may be within the range of + / - 20% of the value itself, preferably within the + / - 10% range of tire value itself, more preferably within the range of + / - 5% of the value itself, and even more preferably within the range of + / - 2% or less of the value itself.

[0048] The use of these temis in describing a chosen characteristic or concept neither implies nor provides any basis for indefiniteness and for adding a numerical limitation to the specified characteristic or descriptor. As understood by a skilled artisan, the practical deviation of the exact value or characteristic of such value, element, or property from that stated falls and may vary within a numerical range defined by an experimental measurement error that is typical when using a measurement method accepted in the art for such purposes.

[0049] For example, a reference to an identified vector or line or plane being substantially parallel to a referenced line or plane is to be construed as such a vector or line or plane that is the same as or very close to that of the referenced line or plane (with angular deviations from the referenced line or plane that are considered to be practically typical in related art, for example between zero and fifteen degrees, preferablybetween zero and ten degrees, more preferably between zero and 5 degrees, even more preferably between zero and 2 degrees, and most preferably between zero and 1 degree). In another example, a reference to an identified vector or line or plane being substantially perpendicular to a referenced line or plane is to be construed as such a vector or line or plane tire normal to tire surface of which lies at or very close to tire referenced line or plane (with angular deviations from the referenced line or plane that are considered to be practically typical in related art, for example between zero and fifteen degrees, preferably between zero and ten degrees, more preferably between zero and 5 degrees, even more preferably between zero and 2 degrees, and most preferably between zero and 1 degree). The use of the term "substantially flat" or “substantially planar’’ in reference to the specified surface implies that such surface may possess a degree of non-flatness and / or roughness that is sized and expressed as commonly understood by a skilled artisan in the specific situation at hand. Other specific examples of the meaning of the terms "substantially", "about", and / or "approximately" as applied to different practical situations may have been provided elsewhere in this disclosure.

[0050] The term “A and / or B” is defined to be equivalent to the term “at least one of A and B”.

[0051] While at least one implementation of the idea of the invention may be embodied in software, the functions necessary to implement tire invention may optionally or alternatively be embodied in part or in whole using finnware and / or hardw are components (such as combinatorial logic, Application Specific Integrated Circuits or ASICs, Field-Programmable Gate Arrays or FPGAs, or other hardware or some combination of hardware, software and / or firmware components), and may include an specific electronic circuitry or a processor controlled by instructions stored in a tangible, non-transient memory medium. The computer-usable or computer-readable medium may be, for example, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. For example, computer-usable or computer-readable medium may include a tangible non-transitory storage medium, such as. without limitation, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only mcmon (EPROM or Flash memory), a compact disc read-only memory (CDROM), and / or an optical storage mcmon medium, or any other memory, or combination thereof, suitable for storing control software or other instructions and data. The computer-usable or computer- readable medium may comprise and / or be complemented with an apparatus that contains, stores, communicates, propagates, or transports program code for use by or in connection with tire instruction execution system, apparatus, or device. Tire computer program product may comprise program code stored in a computer readable medium that, when executed on a computing device, causes the computing device toperform and / or govern one or more of the processes described herein. The computer program product can be written in any conventional programming language (such as, in one example, C++) or the like.

[0052] The invention should not be viewed as being limited to the disclosed embodiment(s).

Claims

CLAIMSWhat is claimed is:

1. An optical imaging apparatus comprising: an optical detection system including an optical sensor positioned to intersect a first axis and receive light propagating along the first axis; an optically-transparent substrate positioned across the first axis to intersect tire light propagating along the first axis towards the optical sensor; and a film of liquid carried by the optically-transparent substrate; wherein each of the substrate and the film of liquid is devoid of particulate matter to be imaged by the apparatus; wherein the apparatus in configured to be operably mated with a source of light that is configured to generate the light and direct said light along the first axis, wherein the light has a substantially spherical wavefront.

2. An apparatus according to claim 1, devoid of an optical lens between the film of liquid and the optical sensor.

3. An apparatus according to claim 1, comprising a housing enclosing at least said optical sensor, the optically-transparent substrate, and the film of liquid and containing an inlet configured to receive a flow of gas delivered from an outside of tire housing, said housing configured to direct the flow towards tire cover slip glass along a path that is outside of a field-of-view of the optical detection system.

4. An apparatus according to claim 3, wherein at least one of the following conditions is satisfied:(4 A) the apparatus further comprises said source of light;(4B) the apparatus further comprises a bandpass optical filter between the source of light and the optically- transparent substrate, wherein the bandpass optical filter is configured to transmit light having partial temporal coherence; and(4C) the source of light includes a pinhole separated from the optical sensor by the optically-transparent substrate.

5. An apparatus according to claim 2, comprising a housing that encloses at least said optical sensor, the optically-transparent substrate, and the film of liquid and that contains an inlet configured to receive a flow of gas with tire particulate matter delivered from an outside of the housing, said housing configured to direct the flow towards the cover slip glass along a path that is outside of a field-of-view of the optical detection system.

6. An apparatus according to claim 5. wherein at least one of the following conditions is satisfied:(6 A) the apparatus further comprises said source of light;(6B) the apparatus further comprises a bandpass optical filter between the source of light and the optically- transparent substrate, wherein the bandpass optical filter is configured to transmit light having partial temporal coherence; and(6C) the source of light includes a pinhole separated from tire optical sensor by the optically-transparent substrate.

7. A method comprising: using the apparatus of claim 1 by at least:Step a): collecting a set of particles on the film liquid carried by tire optical substrate;Step b): illuminating the set of particles located on or within the film of liquid with a first lightwave having a substantially spherical wavefront;Step c): increasing a light-scattering cross-section of a particle of the set of particles by irradiating said particle with light of the first lightwave and propagating said light through the film of liquid;Step d): receiving, at the optical detection system separated from the set of particles by said film of liquid, said light that has interacted with the set of particles to form an optical recording of the set of particles without the use of an optical lens placed between the film of liquid and the optical detection system;Step e):with programmable electronic circuitry, transforming the optical recording into a resolved image of the set of particles; andStep f): at a moment of time that comes later than a moment of time corresponding to said illuminating, initiating a repetition of steps a) through e) to form an auxiliary resolved image of tire set of particles in which a number of particles has increased.

8. A method according to claim 7, further comprising collecting at least one particle of the set of particles from air at tire film of liquid prior to step a).

9. A method according to claim 7, wherein said illuminating includes illuminating the set of particles by transmitting light through an optical lens element.

10. A method according to claim 7, wherein said increasing a light-scattering cross-section of a particle includes changing a degree of convergence or divergence of an optical field, emanating from tire particle, upon tire propagating light of the first lightwave through the film of liquid.

11. A method according to claim 7, comprising a process of time-resolved and real-time imaging of the set of particles in which a number of particles increases with time, said process being configured to form anoptical image of a particle, of the set of particles, that has a dimension smaller than a diffraction limit associated with the method and that is smaller than a size of a pixel of the optical detector.

12. A method according to claim 7, further comprising: vapor-depositing tire film of liquid onto tire optically-transparent substrate and positioning the optically-transparent substrate carrying the film of liquid onto the optical sensor of the optical detection system prior to step a), wherein each of the optically-transparent substrate and the film of liquid is devoid of a particle of the set of particles.

13. A method according to claim 12, wherein the vapor-depositing includes depositing tire film of liquid with thickness between about 1 nm and about 1000 nm on a glass substrate.

14. A method according to claim 7, including:(14A) configuring said first lightwave as a lightwave having a substantially spherical wavefront; and / or(14B) configuring said first lightwave as a partially coherent lightwave.

15. A method according to claim 7, wherein said illuminating includes illuminating the set of particles through an optical pinhole.

16. A method according to claim 7, wherein at least one of the following conditions is satisfied:(16A) wherein the transforming includes reconstructing an optical field over an entirety of a field- of-view of the optical sensor; and(16B) the method further comprises delivering air to the set of particles supported by the film of fluid along a path that is not within the field-of-view of the optical sensor.

17. A method comprising:Step a): depositing a set of particles to be imaged onto a film of liquid that is carried by an optically-transparent substrate, wherein each of the film and the substrate does not contain a particle from the set of particles;Step b): illuminating the set of particles located on the film of liquid with a first lightwave having a substantially spherical wavefront;Step c): increasing a light-scattering cross-section of a particle of the set of particles by propagating light of the first lightwave through the film of liquid;Step d): receiving, at an optical detection system separated from the set of particles by said film of liquid, the light of the first lightwave that has interacted with the set of particles to fonn an optical image of the set of particles without the use of an optical lens element between the film of liquid and the optical detection system;Step e):with programmable electronic circuitry, transforming the optical image into a resolved image of the set of particles;Step f) : at a moment of time that comes later than a moment of time corresponding to said illuminating, initiating a repetition of steps a) through e) to form an auxiliary resolved image of tire set of particles in which a number of particles has increased; andStep g): with a computer processor, detennining change of concentration of particulate matter in said air by particle size with time based on comparison of the resolved image with the auxiliary resolved image.

18. A method according to claim 17, wherein at least one of the following conditions is satisfied:(18A) the method further comprises delivering air containing said set of particles to the film of fluid along a path that is not within a field-of-view of the optical sensor; and( 18B) wherein the transforming includes reconstructing an optical filed over an entirety of a field-of-view of an optical sensor of the optical detection system.

19. A method according to claim 17, comprising a process of time-resolved and real time imaging of the set of particles in which a number of particles increases with time, said process configured to form an optical image of a particle of the set of particles, wherein said particle has a dimension smaller than a diffraction limit associated with said method and smaller than a size of a pixel of an optical sensor of the optical detection system.

20. A method according to claim 17, wherein said increasing a light-scattering cross-section of a particle of the set of particles includes changing a degree of convergence or divergence of an optical field, emanating from tire particle, upon said propagating light of tire first lightwave through the film of liquid.

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