Patent Publication Number: US-11650144-B2

Title: Interdigitated capacitive sensor for real-time monitoring of sub-micron and nanoscale particulate matters

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to and claims priority under 35 U.S.C. § 119(e) from U.S. Patent Application No. 62/972,988, filed Feb. 11, 2020 entitled “Interdigitated Capacitive Sensor for Real-time Monitoring of Sub-micron and Nanoscale Particulate Matters,” the entire contents of which is incorporated herein by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present invention generally relate to systems and methods for monitoring of environmental conditions, and more particularly, to systems and methods for real-time monitoring of sub-micron and/or nanoscale particle particulate matters (PM) in an environment. 
     BACKGROUND 
     Particulate matter (PM) is a mixture of particles and droplets in air consisting of various compounds. PM exists everywhere in various sizes, although some of the particles may be toxic and respirable to human. Some work environments have more dangerous respirable particles than others and can cause harmful effects to human through inhalation. For example, miners in a mining environment are often exposed to higher level of hazardous particles such as coal, silica (SiO2) and diesel exhaust. Continuous exposure to such particles can cause severe damage to the human respiratory system. For instance, exposure to mine dust can cause coal workers&#39; pneumoconiosis (CWP), also known as black lung disease, which is very common in coal miners. Workers mining minerals are often at a high risk for silicosis as they are exposed to mine dust containing high amount of silica. Accordingly, the National Institute for Occupational Safety and Health (NIOSH) recommends that workers&#39; exposure to respirable coal mine dust should be limited to 1 mg/m 3  and crystalline silica should be limited to 0.05 mg/m 3  (up to 10 hours per day over a 40-hour work week). Recently, Mine Safety and Health Administration (MSHA) lowered the concentration limit of respirable coal mine dust from 1 mg/m 3  to 0.5 mg/m 3  for underground and surface coal mines. 
     Devices have been commercially developed to monitor air quality or collect airborne particles in the mining environments. In one approach known as gravimetric sampling, a personal sampler including a cyclone, filter holder, and a small pump is worn by workers to obtain samples of the environment. The concentration of dust is calculated by the average mass gain over the sampling time which is then analyzed via electron microscopy and x-ray diffraction spectroscopy (XPS) to examine the accurate concentration and components of the collected particles, such as silica. However, the gravimetric sampling approach typically requires several hours to collect the particles and send them out for analysis. Other approaches include direct reading of particle concentrations through various monitoring techniques. For example, a light scattering method to measure size distribution of PM in real time has been developed to monitor PM concentrations by translating the sampler&#39;s light scattering into the corresponding concentration. In another example, some monitors utilize a tapered-element oscillation microbalance (TEOM) approach to monitor the coal-dust concentration in the mining environment. In general, TEOM devices include a replaceable filter cartridge mounted to the tip of the tapered element which oscillates like a tuning fork during operation. The oscillation frequency changes in real-time with respect to the mass collected on the filter and the integrated particle mass can be analyzed by gravimetric method after measurement. 
     Further, recent research has demonstrated that nanoparticles (NPs) have stronger and unique adverse health effects compared to micrometer-sized particles of the same material. For example, particle deposition efficiency in the human respiratory track has been measured to vary with particle diameter; while the highest efficiency (˜90%) is at particle diameter of 10 μm and reduces to 15% with decreased diameter, the efficiency starts increasing again when the diameter is 0.2 μm. The efficiency reaches almost 80% with particle diameter of ˜0.01 μm. Moreover, such particles can penetrate deep into the lung or other organs by circulating through the body. To prevent such adverse health effects, accurate characterization of NP exposure in an environment is needed. However, due to the smaller volume of particles as well as their smaller mass, detecting sub-micron and nanoparticles using current methods is challenging since it requires orders of magnitude higher sensitivity compared to detecting microscale particles. In parallel, there is an effort to change the mass-based regulation to number of NPs for accurate evaluations. While a number of government agencies and private entities have established mass-based occupational exposure limits (OELs) for carbon nanotubes (CNTs), one type of nanomaterial, some agencies have started to rely on number of concentrations. 
     Commercially available PM sensors are not yet developed to detect ultra-fine particles from noisy environment. For instance, the use of some monitors is inhibited in gassy underground mines as it is impacted by moisture in the mine air and calibration using gravimetric measurement is necessary. Therefore, it is not recommended for environments where accuracy is the topmost concern. In addition, TEOM monitors may not be suitable for monitoring nanomaterials in the mining environment as mine dust contains a portion of respirable particles in nanometer sizes as well as in microscale. However, current devices are mainly affected by larger particles while the response from smaller particles are masked by the response of the larger particles. 
     It is with these observations in mind, among others, that aspects of the present disclosure were conceived. 
     SUMMARY 
     One aspect of the present disclosure relates to a particulate matters sensing device comprising a sensor cartridge and a readout circuit. The sensor cartridge may include an interdigitated capacitance sensor comprising a plurality of interdigitated electrodes, each of the plurality of interdigitated electrodes separated from another of the plurality of interdigitated electrodes by a spacing, wherein sub-micron or nanoscale particular matters (PMs) of an environment are deposited within the spacing. The readout circuit may include a processor and a tangible storage medium encoded with instructions that are executed by the processor to perform operations of a method. The operations may include receiving a measurement signal corresponding to a capacitance of the interdigitated capacitance sensor, correlating the capacitance of the interdigitated capacitance sensor to a concentration of the deposited sub-micron or nanoscale PMs of the environment, and displaying an indication of the concentration of the deposited sub-micron or nanoscale PMs of the environment. 
     In some instances, the sensor cartridge may further include a micro-heater circuit generating heat for the sensor cartridge and the readout circuit may further include a display device such that the method may also include displaying the indication of the concentration of the deposited sub-micron or nanoscale PMs of the environment on the display device. The readout circuit may further including a wireless communication unit receiving the indication of the concentration of the deposited sub-micron or nanoscale PMs of the environment and transmitting the indication via the wireless transmitter. 
     In other instances, a width of at least one of the plurality of electrodes of the particulate matters sensing device may be between 10 nm to 3 μm and a width of the spacing may be between 10 nm to 3 μm. The sensor cartridge may further comprise a flexible, printed circuit board comprising a first conductive path electrically connected to a first portion of the plurality of interdigitated electrodes and a second conductive path electrically connected to a second portion of the plurality of interdigitated electrodes. The sensor cartridge may also include a resistor connected in series with the interdigitated capacitance sensor, the resistor and interdigitated capacitance sensor comprising a resistor-capacitor (RC) circuit. In such instances, the method may include the operations of transmitting a monitoring signal to the RC circuit, the RC circuit providing an output signal comprising a delay of the monitoring signal, the delay corresponding to the capacitance of the interdigitated capacitance sensor of the RC circuit and comparing a delay of the measurement signal to the monitoring signal to determine the capacitance of the interdigitated capacitance sensor. 
     In still other instances, the indication of the concentration of the deposited sub-micron or nanoscale PMs of the environment comprises at least one of an auditory alarm, a tactile alarm, or a visual alarm. 
     Another aspect of the present disclosure relates to a method for monitoring particulate matters of an environment. The method may include the operations of locating a sensor cartridge in a sampling cassette, the sensor cartridge comprising an interdigitated capacitance sensor comprising a plurality of interdigitated electrodes, each of the plurality of interdigitated electrodes separated from another of the plurality of interdigitated electrodes by a spacing, determining, at a monitoring circuit, a capacitance of the interdigitated capacitance sensor, the capacitance corresponding to a concentration of deposited sub-micron or nanoscale PMs of the environment on the spacing of the interdigitated capacitance sensor, and displaying, on a display device, an indication of the concentration of the deposited sub-micron or nanoscale PMs of the environment on the spacing of the interdigitated capacitance sensor. 
     The method may also include the operations of receiving a measurement signal comprising a delay of the monitor signal, the delay corresponding to the capacitance of the interdigitated capacitance sensor, determining a number of output pulses of the measurement signal with a duration equal to or more than a minimum duration value, and correlating the number of output pulses of the measurement signal to the concentration of deposited sub-micron or nanoscale PMs of the environment on the spacing of the interdigitated capacitance sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of necessary fee. 
       The foregoing and other objects, features, and advantages of the present disclosure set forth herein should be apparent from the following description of particular embodiments of those inventive concepts, as illustrated in the accompanying drawings. The drawings depict only typical embodiments of the present disclosure and, therefore, are not to be considered limiting in scope. 
         FIG.  1    is a block diagram of a particulate matters sensor for real-time monitoring of sub-micron and nanoscale particulate matters in accordance with one embodiment. 
         FIG.  2    illustrates an example interdigitated capacitance sensor  110  for use with a particular matters sensor  102  to monitoring for sub-micron and nanoscale particular matters in an environment in accordance with one embodiment. 
         FIG.  3    is a graph illustrating the response of the interdigitated capacitive sensor  110  with different spacing of the electrodes in accordance with one embodiment. 
         FIG.  4    illustrates an overhead view of an interface board for interconnecting the interdigitated capacitance sensor and/or micro-heater with a readout circuit in accordance with one embodiment. 
         FIG.  5 A  is a circuit diagram of a readout circuit in communication with a capacitance sensor for measuring sub-micron and nanoscale particulate matters in an environment in accordance with one embodiment. 
         FIG.  5 B  illustrates voltage signals of nodes of the readout circuit of  FIG.  5 A  in accordance with one embodiment. 
         FIG.  6    is a flowchart of a method for measuring a relative capacitance of the interdigitated capacitance sensor  110  for real-time monitoring of sub-micron and nanoscale particulate matters in accordance with one embodiment. 
         FIG.  7 A  illustrates a graph of a time-response of a particulate matters sensor during a road-dust test and an airflow test in accordance with one embodiment. 
         FIG.  7 B  illustrates a graph of measured capacitance versus frequency of the particulate matters sensor before and after the road dust illustrated in  FIG.  7 A  in accordance with one embodiment. 
         FIG.  8    is an illustration of radial distribution of particles within a sampling cassette about a PM sensor in accordance with one embodiment. 
         FIG.  9    is a diagram illustrating an example of a computing system which may be used in implementing embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure involve systems, methods, and the like, for a fabrication of a particulate matter (PM) sensor that utilizes a capacitance sensor to detect sub-micrometer and nanoparticles in the respirable range of an environment. In one implementation, the capacitance sensor may comprise interdigitated electrodes between which a capacitance may be measured. PM deposited on the sensor may cause the capacitance between the electrodes to be altered and such a change in capacitance may be measured by the PM sensor. This measurement of the change in capacitance of the interdigitated capacitance sensor may therefore be correlated to the presence of sub-micrometer and nanoparticles in an environment. 
     In one particular implementation, the PM sensor may further include a micro-heater circuit, a readout circuit, and an interface connecting the readout circuit to the micro-heater/capacitance sensor of the PM sensor. The interdigitated capacitance sensor may have a detection capability of sub-micron and nanoscale particles in 1 mm×1.5 mm sensing area. This miniaturized sensor enables an easy integration with standard sampling cassettes minimizing the interference of air flow for particle collection. The readout circuit may utilize, in one implementation, a resistance-capacitance (RC) delay time constant to monitor capacitance shift due to particle deposition in real-time and may be is separately designed for re-use. The capacitance sensor may mounted on a personal sampler and located away from center to increase the probability of accepting sub-micron particles while rejecting larger particles. The PM sensor is tested and provides a clear response with respect to particle deposition; and the positive capacitance shift is consistent with the increased sensor counting. The micro-heater allows the sensor temperature to be maintained at constant temperature above dew point for stable sensor reading. In this manner, a sensor comprising an interdigitated capacitor may be used to detect sub-micrometer and nanoparticles in the respirable range of an environment as a continuous particle monitoring device. 
     Turning first to  FIG.  1   , a block diagram of a particulate matters (PM) sensor  102  for real-time monitoring of sub-micron and nanoscale PMs in accordance with one embodiment in shown. In some instances, the capacitance sensor  102  may include a readout circuit  104 , an interface  106 , a micro-heater  108 , and an interdigitated capacitance sensor  110 . Although shown as including the above-noted components, more or fewer components or circuits may be included with or in communication with the PM sensor  102 . For example, the PM sensor  102  may be in communication with a warning system to alert a wearer of the sensor of a particular level of sensed sub-micron and nanoscale PMs. In another implementation, one or more components of the PM sensor  102  may be in communication with a wired or wireless transmitter configured to transmit a measurement of PMs determined by the PM sensor  102 . Such measurements may be transmitted to a mobile device, a computing device, or a network device to further analysis and processing by the receiving device. Further, although illustrated in  FIG.  1    as interconnected in a particular construction, each of the components of the PM sensor  102  may be connected to or otherwise communicate with any other component of the sensor. A more detailed description of the components, circuits, systems, and/or portions of the PM sensor  102  is discussed in more detail below. 
     As shown in  FIG.  1   , the PM sensor  102  may include an interdigitated capacitance sensor  110  for detecting a change in capacitance. In general, an interdigitated capacitance sensor  110  may include a multi-fingered, micro-strip electrodes interdigitated on a substrate board, such as that illustrated in  FIG.  2   . In particular,  FIG.  2    illustrates an example interdigitated capacitance sensor  110  for use with a PM sensor  102  to monitoring for sub-micron and nanoscale PMs in an environment, in accordance with one embodiment. The capacitance sensor  110  includes a first conductor plate  202  in electrical communication with a first electrical lead  204 . A plurality of first electrodes  206  extend from the first conductor plate  202  with a space between each of the plurality of first electrodes. A second conductor plate  208  is located opposite the first conductor plate  202  and is connected to or otherwise in electrical communication with a second electrical lead  210 . A plurality of second electrodes  212  extend from the second conductor plate  208  with a space between each of the plurality of second electrodes. The plurality of first electrodes  206  and the plurality of second electrodes  212  are interdigitated such that one of the first electrodes is located within the space between two of the second electrodes, and vice versa. In one implementation, the leads, conductor plates, and electrodes of the capacitance sensor  110  may be constructed from copper or other conducting metal. During use, a capacitance may occur across the narrow gap between the electrodes  206 ,  212  (shown in  FIG.  1    as gap  214 ) which may be detected by readout circuit  104  via leads  204 ,  210 . As explained in more detail below, the measured capacitance may be related to sub-micron and/or nanoscale PMs in an environment such that a measurement or estimation of concentration of sub-micron and/or nanoscale PMs may be obtained from the detected capacitance of the interdigitated capacitive sensor  110 . 
     In one particular implementation, the capacitive sensor  110  may be constructed to be integrated with a standard air sampling cassettes which use 25-37.5 mm filters. The sampling cassette allows the selection of sub-micron particles while rejecting larger particles with a mass median aerodynamic diameter (MMAD) of 3.8 μm. Further, the sensor strip may have a rectangular shape with dimensions of 2 mm×12 mm to minimize the air flow interference when integrated with the sampling cassette and to facilitate easy connection with the readout board. The interdigitated patterns may provide maximum sensitivity in a given area. In one particular implementation, the sensing area of the interdigitated capacitance sensor  110  may be a 1 mm (shown by arrow  216 ) by 1.5 mm (shown by arrow  218 ) sampling area. In addition, the width  220  of each of the interdigitated electrodes  206 ,  212  may, in one implementation, be 2 μm with spacing  214  between the electrodes being nominally between 2 μm to 3 μm. In general, width  220  of the electrodes  206 ,  212  and/or the spacing between the electrodes may be selected to capture particularly-sized particulates. Thus, to capture the presence of nanoparticles, the width  220  of the electrodes  206 ,  212  and/or the spacing between the electrodes may be as small as 10 nm and may be larger than 3 μm. Electrode widths and spacings between electrodes may thus be any length as desired. In one example, the spacing  214  between the electrodes  206 ,  212  may be chosen based on projected fabrication yield. 
     In general, the capacitance of interdigitated patterns may be calculated as sum of interior capacitances (CI) and exterior capacitances (CE), with multiplication of the number of electrodes where CI and CE are capacitances with respect to a ground plane in the halfway between two electrodes. In order to understand both the capacitance of the interdigitated sensor  110  (without particles) and the effects of particle loading, simulations may be performed with different electrode thickness and spacing and the deposited particle layer is represented by a uniform-thickness layer of dielectric. The result may then be normalized with respect to the entire sensor area. 
       FIG.  3    is a graph  300  illustrating the response of the interdigitated capacitive sensor  110  with different spacing between the electrodes in accordance with one implementation. In particular, the graph  300  illustrates measured capacitance  302  of the capacitive sensor  110  versus a thickness of the electrodes  304 , ranging from 0 nm to 2000 nm. The plots for the different spacing illustrated include a width of 3 μm  312 , 2 μm  310 , 1 μm  308 , and 0.5 μm  306 . As shown, when there is no particle layer, the calculated capacitance may be approximately 8.37 pF for 2 μm spacing and 7.23 pF for 3 μm respectively. The capacitance may linearly increase with an increased particle layer thickness, with saturation beginning as the layer thickness exceeds 400 nm. The slope in the linear region represents the sensitivity which increases as the spacing is reduced. The results indicate that ΔC due to dielectric material is higher when particles are deposited in between electrodes. In general, there is a trade-off between electrode spacing and sensitivity; a capacitive sensor with narrower spacing between the electrodes may be most useful for smaller particle detection, but such a design may be inefficient for particles larger than the spacing as they will not be collected in between electrodes. Also, taller electrodes may be beneficial for extending the maximum capacity of sensor with a constant response, but inefficient particle collection may occur as taller electrodes may disturb airflow inside the sampler, which may disturb particle collection in between the sensor electrodes. With these constraints in mind, one particular implementation of the interdigitated capacitive sensor  110  may include an electrode width  220  of 2 μm and a spacing  214  between the electrodes of 2-3 μm, although other dimensions are contemplated for different types and sizes of PMs. 
     Returning to  FIG.  1   , the PM sensor  102  may also include a resistance-based micro-heater circuit  108  for maintaining the sensor at an elevated temperature. The micro-heater circuit  108  may mitigate the effects of condensation/water droplets on the PM sensor  102  and enhances the stability of the capacitance of the interdigitated capacitive sensor  110 . In addition, direct integration of the micro-heater  108  on the sensor reduces the complexity of installing a heater inside a sampler. The serpentine pattern  222  is used in order to maximize the resistance (minimize the current) in the limited area of the sensor strip, with four electrodes enabling accurate temperature measurement. Further, while silicon substrate provides solid and high fabrication yields, such a substrate provides a high parasitic capacitance, potentially reducing the effect of capacitance change from the sensor  110 . Thus, in one implementation, a polyimide material may be used as it has lower relative dielectric constant in comparison to silicon to minimize the parasitic capacitance from the substrate and improve the accuracy of the capacitive sensor  110 . Batches of flexible sensor/heater strips may be fabricated through scalable microfabrication approaches for use in the real-time monitoring of PMs in an environment. 
     To facilitate integration between the interdigitated capacitance sensor/micro-heater strip with the readout board  104 , the PM sensor  102  may include an interface  106 . In one implementation, the interface  106  may be a custom-made printed circuit board (PCB) that includes electrical connections between readout circuit  104  and the sensor/heater strip. The interface PCB  106  may enable robust electrical and mechanical connections to the readout board, via one or more pin header connections included in the interface.  FIG.  4    illustrates an overhead view of an interface board  106  for interconnecting the interdigitated capacitance sensor  110  and/or micro-heater  108  with a readout circuit  104 . In particular, the interface  106  may electrically connect a strip  402  including the electrical leads  204 ,  210  of the interdigitated capacitance sensor  110  and/or portions of the micro-heater  108  to a readout circuit  104  once the strip is mounted on the interface. One or more interface ports  404  may be located on the interface  106  for transmission of signals, measurements, data, etc. from the strip  402  to the readout circuit  104 . In some instances, signals, data, and the like from the readout circuit  104  may be transmitted to the capacitive sensor  110  and/or the micro-heater  108  via the interface ports  404 . The sensor  110  and micro-heater  108  may, in one implementation, be wire-bonded to the interface  106  using gold wires. Similar to the batch-fabricated sensor strip  402 , the interface  106  can be disposed, as needed. 
       FIG.  5 A  is a circuit diagram of a readout circuit in communication with a capacitance sensor for measuring sub-micron and nanoparticles in an environment in accordance with one embodiment. In general, the readout circuit  104  includes a microcontroller  502  in electrical communication and providing a clock signal to a first conductor of a resistor  506  and a first conductor of an exclusive OR gate  510 . A second conductor of the resistor  506  is in electrical communication with the capacitance sensor  110  discussed above and an input to an inverter  508 . The output of the inverter is connected to a second conductor of the exclusive OR gate  510 , the output of which is provided back to the microcontroller  502 . In general, the readout circuit  104  operates to measure the time constant of the capacitance sensor  110  and an external resistor  506 . In particular,  FIG.  5 B  illustrates voltage signals of nodes of the readout circuit of  FIG.  5 A  in accordance with one embodiment. In particular,  FIG.  5 B  illustrates the clock signal (CLK)  520 , a voltage signal  522  at node V 1  (the input to the inverter  508 ), a voltage signal  524  at node V 2  (the output of the inverter  508 ), and a voltage signal  526  at node V 3  (the output of the exclusive OR (XOR) gate  510 ). 
       FIG.  6    is a flowchart of an example method for measuring a relative capacitance of the interdigitated capacitance sensor  110  for real-time monitoring of sub-micron and nanoscale particulate matters in accordance with one embodiment. The operations of the method  600  may be performed or executed by the readout circuit  104  or any other computing or electrical component, such as the computing device described in greater detail below, of or associated with the PM sensor  102 . The operations may be performed by one or more hardware components of the PM sensor  102  or other computing device, one or more programs executed by a hardware processor, or a combination of both hardware and software components. The operations are described herein as performed or executed at least partially by the microcontroller  502 . 
     Beginning in operation  602 , the microcontroller  502  may transmit a repeating monitor signal to a resistor-capacitance (RC) circuit in which the capacitance portion of the RC circuit is the interdigitated capacitance sensor  110 . In one implementation, the monitor signal may be the clock signal (CLK)  520  discussed above. Thus, using the circuit of  FIG.  5 A  as an example, the microcontroller  502  may transmit the clock signal  520  to resistor  506 . In general, the clock signal  520  may include multiple, repeating pulses, as illustrated in the signal graph of  FIG.  5 B . One example of such a clock signal  520  may be a 100 Hz square wave signal with a 50% duty cycle, although other repeating signals may be used to monitor the capacitance sensor  110 . The clock signal  520  is transmitted to the series-connected resistor  506  and capacitance sensor  512 . This RC circuit acts on the clock signal  520  to generate voltage signal V 1    522 . In general, the slope of the V 1  signal  522  is dependent on the values of the resistance value of the resistor  506  and the capacitance of the capacitor sensor  512 . The V 1  wave form  522  may be transmitted as an input to inverter  508  and transformed into a square wave signal (signal V 2    522 ) after passing through the inverter. In particular, the inverter  508  may be associated with a reference voltage (V REF ), which may be provided to the inverter as an input. In one particular implementation, the V REF  may have a value of 0.5 V, although any reference voltage value may be selected or otherwise provided to the inverter  508 . The inverter  508  operates to output a high value when the input signal (V 1    522 ) reaches or is less than the V REF  value and to output a low value when the input signal is greater than the V REF  value. The output signal V 2  of the inverter  508  is illustrated as signal  524  of  FIG.  5 B . The value of V REF  is also illustrated in waveform  522  for reference. 
     The output signal  524  V 2  from the inverter  508  is transmitted to the XOR gate  510  for comparison with the initial clock signal  520  at the XOR gate. The XOR gate  510  operates to output a high value if either the input signal V 2    524  or the clock signal  520  is high and to output a low if both the input signal V 2  and the clock signal are low, as illustrated in voltage signal V 3    526 . The output of the XOR  510  (signal V 3    526 ) is fed back to the microcontroller  502  for comparison to the clock signal  520 , as explained in more detail below. 
     In operation  604 , the microcontroller  502  receives the voltage signal V 3    526  as the measurement signal. In general, as the capacitance of the interdigitated capacitive sensor  110  increases due to particle deposition, a delay in the time constant delay of the RC circuit occurs (as shown in the red curve in signal V 1    522 ). In other words, as the capacitance of the capacitor sensor  512  increases due to deposition of PMs, a delay in voltage signal V 1    522  occurs. This delay propagates through the inverter  508  (illustrated as the red signal in voltage signal V 2    524 ) and the XOR gate  510  (illustrated as the red signal in voltage signal V 3    526 ). Further increase in the capacitance of the sensor  512  may cause a longer delay in the signal which is subsequently propagated through the circuit into measurement signal V 3    526 . This signal is then received at the microcontroller  502  for analysis. 
     In particular, the microcontroller  502  may, in operation  606 , count a number of output pulses within the measurement signal based on a minimum pulse width value. In particular, the microcontroller  502  counts a number of output pulses of signal V 3    526  following a rising edge of the clock signal and that last at least a minimum detectable pulse width duration of D. For example, as shown in voltage waveform  526 , the microcontroller  502  may count the pulses indicated by the duration D in the voltage signal as these pulses follow the rising edge of a corresponding pulse of the clock signal  520  and have a high value for at least the duration D. In general, a counting interval may be set by the microcontroller  502 , such as a counting interval of 1 microseconds (μS) or any other timeframe. Although in the illustrated example, only the “rise” pulses for the V 3  signal  526  may be chosen for counting, any trigger in the measurement signal  526  and/or the clock signal  520  may be selected. The microcontroller  502  may count the number of such pulses of the measurement signal V 3    526  that occur in the counting interval. Further, in operation  608 , the microcontroller  502  may average the number of counted pulses that are detected for a given number of pulses of the monitor signal, such as the clock signal  520 . For example, the microcontroller  502  may count the number of qualifying output pulses of the measurement signal V 3    526  that occur over 500 pulses of the clock signal  520 . 
     In operation  610 , the microcontroller  502  may then convert the averaged count of output pulses of the measurement signal V 3    526  into a measurement of sub-micron and nanoscale particulate matters in an environment. In particular, as the deposition of sub-micron and nanoscale PMs on the capacitance sensor  512  increases, the delay (represented by the red line in the voltage signal graphs of  FIG.  5 B ) may increase such that one or more output pulses of measurement signal V 3    526  no longer has a duration above the minimum detectable pulse width D. As the delay in the measurement signal increases due to the build-up of PMs, fewer output pulses may be counted by the microcontroller  502  such that the calculated average may drop. The microcontroller  502  may correlate the determined average number of output pulses over the measurement period to an estimated concentration of PMs in the monitored environment. For example, the microcontroller  502  may store or have access to a database of correlated determined averages of output pulses over the measurement period to a concentration of sub-micron and nanoscale PMs in an environment, such as a look-up table maintaining such correlations. In another example, the microcontroller  502  may generate the estimated concentration of PMs in the monitored environment as a relative value to a baseline average number of output pulses over the measurement period. Regardless of the mechanism through which the estimated concentration of PMs in the monitored environment, an indication of such estimation may be displayed on a display device in operation  612 . In one particular implementation, the readout circuit  104  may include a display, such as a LED display, that is controllable to display the estimated concentration of PMs. In some instances, the indication of the estimated PMs may include an auditory, visual, or tactile alert or alarm. 
     The effectiveness of the PM sensor  102  described herein has been verified through simulation and testing. In one particular example, a PM sensor  102  with an interdigitated capacitance sensor  110  with a sensing area of 1 mm by 1.5 mm, including electrodes  206 ,  212  with a width of 2 μm and a spacing  214  between the electrodes being nominally between 2 μm to 3 μm is tested. Initially, such a fabricated PM sensor  102  is calibrated. During calibration of PM sensors  102  with 2 μm-spacing between electrodes  206 ,  212 , calibrated measured capacitances ranged from 11 to 12 pF and for PM sensors  102  with 3 μm-spacing between electrodes  206 ,  212 , calibrated measured capacitances ranged from 7 to 8 pF. Comparing with the calculated nominal capacitances of 2 μm and 3 μm-spacing sensors, which are 8.37 pF and 7.23 pF respectively, the higher measured values should originate from the parasitic capacitance of the interface  106  as well as fabrication nonidealities. The PM sensor  102  sensitivity may be estimated by calculating the RC time constant (i) as a result of capacitance shift. That is
 
 V=V   0 (1− e   −t/τ )  (1)
 
where V 0  is an initial voltage and t represents time. During simulation of the PM sensor  102  circuit, about 15 femtofarad (fF) of capacitance shift is required to delay 1 μs of rising time. As the readout circuit  104  itself has a fixed resistance and parasitic capacitance, a non-zero number of counts has been observed for zero sample capacitance. The results indicate that counting increases linearly with increasing capacitance. After calibration with fixed capacitors, fabricated PM sensors  102  were compared and the counting of two sensor chips (8.03 pF and 11.54 pF) matched well with a linearly extrapolated curve.
 
     The use of the interdigitated capacitance sensor  110  to detect sub-micron and nanoscale PMs has been demonstrated with test dust, with the PM sensor  102  mounted in a sampler device. The outlet of the sampler may be connected to an air pump for constant air flow (0.3 L/min), and road dust may be sprayed periodically through the top opening of a test chamber to test the PM sensor  102 . Since the dust consists primarily of silica, it was assumed that the dielectric constant of the test dust is same as that of silica, which is about 3.9. 
       FIG.  7 A  illustrates a graph  702  of a time-response of a PM sensor  102  during two tests: one using road-dust and the other with the same airflow but without particles. In particular, an average of counted output pulses is graphed for the designated test time. At the beginning of each test, the stability of the PM sensor  102  was monitored via measurement during 5 minutes without airflow and 15 minutes of airflow without particles. After 60 minutes of tests, an additional 10 minutes of measurement without air flow was performed. While the PM sensor  102  was stable during dry test (no particles generated), a clear differential response was observed when particles are generated showing a counting shift of 3.24 counts. This indicates that the duration of output pulse changed by 3.24 μs as a result of capacitance change. The sub-integers of counts are the results of averaging as described above, as well as 2 minutes of window averaging from the recorded data. 
       FIG.  7 B  illustrates a graph of a frequency-response of the PM sensor  102  before and after the above test for comparison. The graph  720  illustrates an increase in sensor capacitance after testing, in agreement with positive shifts in counting from the readout circuit  104 . The frequency dependence of capacitance is observed in the graph  720  from the sensor before and after test; the frequency-dependent dielectric constant of polyimide substrate could reduce the capacitance with increased frequency. The readout calibration using fixed capacitors estimated a resolution of ˜15 fF/count, however, the calculated resolution from the test results using road dust appeared to be ˜42 fF/count. One possibility is that the actual capacitance shifts due to particles could be masked under continuous air flow which causes dried condition, resulting the reduced capacitance shifts during tests. 
     The PM sensor  102  tested in the manner disclosed above was inspected with a scanning-electron microscope (SEM) for post-analysis. Through this analysis, it was determined that about 77% of particles on the PM sensor  102  are smaller than 1 μm while 23% are larger than 1 μm. Whereas most particles had sub-micrometer diameters, a few larger particles were observed; each of these is counted as a single particle due to a limited resolution. In order to estimate total volume of particles within each size range, particles are assumed as spheres with an average diameter (D eff ) corresponding to each bin. By summing the estimated volumes for all size ranges, the total volume of particles on the sensor is calculated to be 1.15×10 4  μm 3 . Since the test dust mostly consists of silica, the density of silica (2.65 g/cm 3 ) is used for the calculation of the effective mass (m eff ). The calculated total m eff  on the tested PM sensor  102  is 3×10 −8  g. The calculation indicates that the positive capacitive sensor response corresponds to the volume of particles collected on sensor. Although a large fraction of the overall particle count may come from sub-micron particles, the total volume of particulate material is dominated by the particles larger than 1 micron. The volume fraction of each particle size range may thus correlate to the relative contribution to capacitance response. It is noted that a nucleation of particles after landing on sensor could result such agglomerations. 
     To understand the sensor sensitivity with volume, the calculated volume may be compared with simulation results. The total volume may be converted into effective thickness (T eff ), i.e. the thickness of a uniform thin film containing the same volume of material. Assuming the material is uniformly deposited over the sensing area of the tested PM sensor  102  (1 mm×1.5 mm), the T eff  is estimated to be ˜8 nm. This T eff  may then compared with a simulation result. Since the effective thickness of the tested sensor is about 8 nm, sensor response is in the regime where the dielectric change is linearly proportional to the volume of each particle. While the fractional change in capacitance (ΔC/C) from experiment is about 1.7%, that from simulation becomes 0.6% with 8 nm of increase in T eff . As such, the comparison using ΔC/C still shows that the PM sensor  102  response is in the linear regime. 
     A simulation study using a computational fluid dynamic program indicates that particle distribution inside a sampling cassette depends on the particle size; particles smaller than 3 gm are uniformly distributed over the filter area, while particles larger than 3 gm are concentrated in the center area.  FIG.  8    is an illustration of radial distribution of particles within a sampling cassette about a PM sensor in accordance with one embodiment. The illustration  800  provides simulation results with three representative particles; while particles of 11.5 nm-diameter (red) is uniformly distributed, both 5.8 um-diameter (blue) and 9.0 um-diameter (green) are collected preferably in the center. The PM sensor  102  in this simulation is located about 2.5 mm away from the center. According to the simulation result, the PM sensor  102  rejects most of particles larger than 9 gm while some of 5 gm-diameter particles may land on the sensor. Therefore, placing the sensor away from the center increases the probability of accepting sub-micron particles, rather than microparticles. 
     With all given information, we can convert our results into the standard particle concentration expression; g/m 3 . Using chain rule, 
                       mass   (   g   )       Volume   (     m   3     )       =       Count   min     ⁢     X   ⁡   (         mass   (   g   )     Count     ⁢   X   ⁢     1     Flow   ⁢           rate   (     L   /   min     )         ⁢   X   ⁢   C     )               (   2   )               
where C is the mass calibration factor. The C represents the ratio of particle mass on filter to particle mass on sensor, which are obtained by gravimetric method and post-analysis, respectively. While the terms in the parenthesis are known, only the rate of change in count will vary with respect to the environment. For instance, the rate of change may be about 1 count/10 minutes for the first 30 minutes. Therefore,
 
     
       
         
           
             
               
                 
                   
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     The calculation result shows that under the given test conditions using road dust, the particle concentration is −4 mg/m 3  for 10 minutes of sampling. On the other hand, the rate decreases after 30 minutes of particle sampling with the ratio of 0.33 count/10 minutes. That is, 
                     1.36     mg     m   3         =         0.33         Count       10   ⁢         min       ⁢     X   ⁡   (         3   .04   ×     10     -   8       ⁢     (   g   )         3.24         Count       ⁢   X   ⁢     1     0.3     (     L   /   min     )         ⁢   X   ⁢       1   ⁢     0   3     ⁢   L       m   3       ⁢   X   ⁢       1316     )               (   4   )               
By monitoring the rate of change in counts, the airborne particle concentration can be calculated at intervals on the order of 10-20 minutes.
 
     Therefore, an interdigitated capacitive sensor  110  is made for sub-micron and nanoscale particulate matters detection in an environment. The batch-fabricated sensor strip may be designed for the integration with personal sampling cassettes and a readout circuit  104  may enable continuous monitoring of capacitance shifts due to particles. Sensor response with respect to test dust showed differential behavior from tests without dust and good agreement with positive capacitance shift. Most of the collected particles appeared to be sub-micrometer sized particles with diameter below 1 μm, and micrometer sized particles are agglomerates of smaller particles. A simulation study showed that the PM sensor  102  response is linearly proportional to the volume of collected particles. The particle mass on sensor  102  with the consideration of radial dependence of particle deposition and sensor location can estimate the total mass concentration of deposited particles. An incorporated heater improved stable capacitance sensor reading by mitigating variations from surroundings such as air flow and relative humidity. Finally, the sensor response is converted into a standard airborne particle concentration (g/m 3 ) demonstrating an example of continuous particle monitoring. This disposable and real-time particle sensing device  102  could be integrated with standard personal sampling cassettes and utilized for workers in the mining environmental and other diverse workplaces who are exposed to hazardous sub-micrometer and nanometer-sized particles. 
       FIG.  9    is a block diagram illustrating an example of a computing device or computer system  900  which may be used in implementing the embodiments of the components of the network disclosed above. For example, the computing system  900  of  FIG.  9    may perform one or more operations of the method  600  of  FIG.  6    discussed above. The computer system (system) includes one or more processors  902 - 906 . Processors  902 - 906  may include one or more internal levels of cache (not shown) and a bus controller or bus interface unit to direct interaction with the processor bus  912 . Processor bus  912 , also known as the host bus or the front side bus, may be used to couple the processors  902 - 906  with the system interface  914 . System interface  914  may be connected to the processor bus  912  to interface other components of the system  900  with the processor bus  912 . For example, system interface  914  may include a memory controller  914  for interfacing a main memory  916  with the processor bus  912 . The main memory  916  typically includes one or more memory cards and a control circuit (not shown). System interface  914  may also include an input/output (I/O) interface  920  to interface one or more I/O bridges or I/O devices with the processor bus  912 . One or more I/O controllers and/or I/O devices may be connected with the I/O bus  926 , such as I/O controller  928  and I/O device  930 , as illustrated. 
     I/O device  930  may also include an input device (not shown), such as an alphanumeric input device, including alphanumeric and other keys for communicating information and/or command selections to the processors  902 - 906 . Another type of user input device includes cursor control, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to the processors  902 - 906  and for controlling cursor movement on the display device. 
     System  900  may include a dynamic storage device, referred to as main memory  916 , or a random access memory (RAM) or other computer-readable devices coupled to the processor bus  912  for storing information and instructions to be executed by the processors  902 - 906 . Main memory  916  also may be used for storing temporary variables or other intermediate information during execution of instructions by the processors  902 - 906 . System  900  may include a read only memory (ROM) and/or other static storage device coupled to the processor bus  912  for storing static information and instructions for the processors  902 - 906 . The system set forth in  FIG.  9    is but one possible example of a computer system that may employ or be configured in accordance with aspects of the present disclosure. 
     According to one embodiment, the above techniques may be performed by computer system  900  in response to processor  904  executing one or more sequences of one or more instructions contained in main memory  916 . These instructions may be read into main memory  916  from another machine-readable medium, such as a storage device. Execution of the sequences of instructions contained in main memory  916  may cause processors  902 - 906  to perform the process steps described herein. In alternative embodiments, circuitry may be used in place of or in combination with the software instructions. Thus, embodiments of the present disclosure may include both hardware and software components. 
     A machine readable medium includes any mechanism for storing or transmitting information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). Such media may take the form of, but is not limited to, non-volatile media and volatile media and may include removable data storage media, non-removable data storage media, and/or external storage devices made available via a wired or wireless network architecture with such computer program products, including one or more database management products, web server products, application server products, and/or other additional software components. Examples of removable data storage media include Compact Disc Read-Only Memory (CD-ROM), Digital Versatile Disc Read-Only Memory (DVD-ROM), magneto-optical disks, flash drives, and the like. Examples of non-removable data storage media include internal magnetic hard disks, SSDs, and the like. The one or more memory devices  606  may include volatile memory (e.g., dynamic random access memory (DRAM), static random access memory (SRAM), etc.) and/or non-volatile memory (e.g., read-only memory (ROM), flash memory, etc.). 
     Computer program products containing mechanisms to effectuate the systems and methods in accordance with the presently described technology may reside in main memory  916 , which may be referred to as machine-readable media. It will be appreciated that machine-readable media may include any tangible non-transitory medium that is capable of storing or encoding instructions to perform any one or more of the operations of the present disclosure for execution by a machine or that is capable of storing or encoding data structures and/or modules utilized by or associated with such instructions. Machine-readable media may include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more executable instructions or data structures. 
     Embodiments of the present disclosure include various steps, which are described in this specification. The steps may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a general-purpose or special-purpose processor programmed with the instructions to perform the steps. Alternatively, the steps may be performed by a combination of hardware, software and/or firmware. 
     Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations together with all equivalents thereof. 
     While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the disclosure. Thus, the following description and drawings are illustrative and are not to be construed as limiting. Numerous specific details are described to provide a thorough understanding of the disclosure. However, in certain instances, well-known or conventional details are not described in order to avoid obscuring the description. References to one or an embodiment in the present disclosure can be references to the same embodiment or any embodiment; and, such references mean at least one of the embodiments. 
     Reference to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. 
     The terms used in this specification generally have their ordinary meanings in the art, within the context of the disclosure, and in the specific context where each term is used. Alternative language and synonyms may be used for any one or more of the terms discussed herein, and no special significance should be placed upon whether or not a term is elaborated or discussed herein. In some cases, synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and is not intended to further limit the scope and meaning of the disclosure or of any example term. Likewise, the disclosure is not limited to various embodiments given in this specification. 
     Without intent to limit the scope of the disclosure, examples of instruments, apparatus, methods and their related results according to the embodiments of the present disclosure are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the disclosure. Unless otherwise defined, technical and scientific terms used herein have the meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. In the case of conflict, the present document, including definitions will control. 
     Additional features and advantages of the disclosure will be set forth in the description which follows, and in part will be obvious from the description, or can be learned by practice of the herein disclosed principles. The features and advantages of the disclosure can be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the disclosure will become more fully apparent from the following description and appended claims, or can be learned by the practice of the principles set forth herein.