# Multifunctional, self-cleaning air filters based on graphene-enhanced ceramic networks

Armin Reimers, Ala Bouhanguel, Erik Greve, Morten Möller, Lena Marie Saure, Yves Andres, Sören Kaps, Lasse Wegner, Ali Shaygan Nia, Xinliang Feng, et al.

<figure><img src="image_1.png" /></figure>

## To cite this version:

Armin Reimers, Ala Bouhanguel, Erik Greve, Morten Möller, Lena Marie Saure, et al., Multifunctional, self-cleaning air filters based on graphene-enhanced ceramic networks. Advanced Device Materials, 2023, 1 (4), pp.100098. 10.1016/j device.2023.100098. hal-04592303

HAL Id: hal-04592303

https://hal.science/hal-04592303

Submitted on 29 May 2024

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<figure><img src="image_2.png" /></figure>
# Multifunctional, self-cleaning air filters based on graphene-enhanced ceramic networks

## Graphical abstract

<figure><img src="image_1.png" /></figure>

## Highlights

* Scalable fabrication of self-cleaning air filters based on graphene

* Filters eliminate captured microorganisms and drastically reduce pollutant buildup

* Graphene enables built-in monitoring capabilities for airflow and load status

## Authors

Armin Reimers, Ala Bouhanguel, Erik Greve, ..., Fabian Schütt, Yves Andres, Rainer Adelung

## Correspondence

fas@tf.uni-kiel.de (F.S.), ra@tf.uni-kiel.de (R.A.)

## In brief

Conventional air filters are unable to inactivate accumulated contaminants and require regular replacement. This work presents a low-cost, scalable approach for the fabrication of multifunctional and self-cleaning air filters based on graphene. The graphene-enhanced filter medium (GeFM) can eliminate captured microorganisms and reduce pollutant buildup by joule heating. In addition, it provides unique capabilities for monitoring the air throughput and load status of the filter. The GeFM paves the way for widespread implementation of more advanced air filters in the future.

<figure><img src="image_2.png" /></figure>
# Multifunctional, self-cleaning air filters based on graphene-enhanced ceramic networks

## Graphical abstract

<figure><img src="image_3.png" /></figure>

## Highlights

* Scalable fabrication of self-cleaning air filters based on graphene

* Filters eliminate captured microorganisms and drastically reduce pollutant buildup

* Graphene enables built-in monitoring capabilities for airflow and load status

## Authors

Armin Reimers, Ala Bouhanguel, Erik Greve, ..., Fabian Schütt, Yves Andres, Rainer Adelung

## Correspondence

fas@tf.uni-kiel.de (F.S.), ra@tf.uni-kiel.de (R.A.)

## In brief

Conventional air filters are unable to inactivate accumulated contaminants and require regular replacement. This work presents a low-cost, scalable approach for the fabrication of multifunctional and self-cleaning air filters based on graphene. The graphene-enhanced filter medium (GeFM) can eliminate captured microorganisms and reduce pollutant buildup by joule heating. In addition, it provides unique capabilities for monitoring the air throughput and load status of the filter. The GeFM paves the way for widespread implementation of more advanced air filters in the future.

<figure><img src="image_4.png" /></figure>
Article

# Multifunctional, self-cleaning air filters based on graphene-enhanced ceramic networks

Armin Reimers,<fnref n="1" /> Ala Bouhanguel,<fnref n="2" /> Erik Greve,<fnref n="1" /> Morten Möller,<fnref n="1" /> Lena Marie Saure,<fnref n="1" /> Sören Kaps,<fnref n="1" /> Lasse Wegner,<fnref n="3" />

Ali Shaygan Nia,<fnref n="4" /> Xinliang Feng,<fnref n="4" /> Fabian Schütt,<fnref n="1" />,* Yves Andres,<fnref n="2" /> and Rainer Adelung<fnref n="1" />,<fnref n="5" />,*

**THE BIGGER PICTURE** Conventional air filters, despite being effective particle removers, typically lack any additional functionalities. They are unable to inactivate accumulated contaminants and require regular maintenance to remain operational. This work presents a scalable approach for the fabrication of multifunctional and self-cleaning air filters by exploiting the properties of graphene on the macroscopic scale. The demonstrated graphene-enhanced filters not only offer improved performance over previously reported self-cleaning filter concepts but also provide unique capabilities for monitoring the air throughput and loading status of the filter. The potential impact of this research is significant: by enabling the low-cost production of graphene-enhanced filters, it paves the way for widespread real-world implementation of more advanced air-filtration devices. Such devices are of utmost importance to face impending challenges of air pollution and pandemics.

## SUMMARY

Particulate air pollution takes an enormous toll on society; it is linked to more than 3 million deaths annually. In addition, airborne microorganisms can spread dangerous diseases, compounding the issue. A widely used method for mitigating the risks of airborne particles is air filtration. While effective at particle removal, conventional air filters typically lack additional functionalities. They cannot inactivate accumulated contaminants, and they require periodic replacement to remain safe. This work presents a multifunctional, self-cleaning air-filtration system using a graphene-enhanced air filter medium (GeFM). The GeFM is capable of capturing >95% of microorganisms and particles >1 µm and can be repeatedly joule heated to >300°C, regenerating the filter. Furthermore, the GeFM provides unique options for monitoring the airflow and loading status of the filter. The GeFM can improve filter lifetime and safety, offering great potential for the development of advanced and likely more sustainable air-filtration solutions in the future.

## INTRODUCTION

The 2019 SARS-CoV-2 pandemic was a grave reminder of our susceptibility to the dangers posed by airborne microorganisms and particulate air pollution. Direct exposure to airborne particulate matter with aerodynamic diameters smaller than 2.5 µm (PM<sub>2.5</sub>) is associated with more than 3 million deaths per year.<fnref n="1" /> In addition, most infectious microorganisms form particles with aerodynamic diameters of 0.5–5 µm.<sup>2</sup> PM<sub>2.5</sub> can therefore promote the spread of dangerous diseases. The most common method to reduce the concentration of harmful PM<sub>2.5</sub> is filtration.

High-performance air filtration is based on two fundamental approaches. Interaction-based filters, like electrostatic precipitators, exploit electrostatic forces to immobilize particles. Structure-based approaches, on the other hand, pass the air through a porous medium with a complex internal structure to capture the particles.<sup>3</sup> State-of-the-art air filter media include hierarchically structured fibrous membranes<sup>4–6</sup> and multimodal nanofiber mats fabricated by humidity-induced electrospinning/netting.<sup>7–9</sup> In the filter medium, particles are filtered through various mechanisms, including size-exclusion sieving, inertial impaction, interception, and diffusion, depending on their size, density, and velocity.<sup>3</sup> Due to interactions between the

<figure><img src="image_3.png" /></figure>

<footnote n="1">¹Functional Nanomaterials, Department of Materials Science, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany</footnote>

<footnote n="2">²IMT-Atlantique GEPEA UMR CNRS 6144, 4 Rue Alfred Castler BP 20722, 44307 Nantes Cedex 3, France</footnote>

<footnote n="3">³Institute for Inorganic Chemistry, Kiel University, Max-Eyth-Straße 2, 24118 Kiel, Germany</footnote>

<footnote n="4">⁴Center for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany</footnote>

<footnote n="5">⁵Lead contact</footnote>

<footnote>*Correspondence: fas@tf.uni-kiel.de (F.S.), ra@tf.uni-kiel.de (R.A.)</footnote>

<footnote>https://doi.org/10.1016/j device.2023.100098</footnote>
## Article

# Multifunctional, self-cleaning air filters based on graphene-enhanced ceramic networks

Armin Reimers,<fnref n="1" /> Ala Bouhanguel,<fnref n="2" /> Erik Greve,<fnref n="1" /> Morten Möller,<fnref n="1" /> Lena Marie Saure,<fnref n="1" /> Sören Kaps,<fnref n="1" /> Lasse Wegner,<fnref n="3" />

Ali Shaygan Nia,<fnref n="4" /> Xinliang Feng,<fnref n="4" /> Fabian Schütt,<fnref n="1" />,* Yves Andres,<fnref n="2" /> and Rainer Adelung<fnref n="1" />,<fnref n="5" />,*

**THE BIGGER PICTURE** Conventional air filters, despite being effective particle removers, typically lack any additional functionalities. They are unable to inactivate accumulated contaminants and require regular maintenance to remain operational. This work presents a scalable approach for the fabrication of multifunctional and self-cleaning air filters by exploiting the properties of graphene on the macroscopic scale. The demonstrated graphene-enhanced filters not only offer improved performance over previously reported self-cleaning filter concepts but also provide unique capabilities for monitoring the air throughput and loading status of the filter. The potential impact of this research is significant: by enabling the low-cost production of graphene-enhanced filters, it paves the way for widespread real-world implementation of more advanced air-filtration devices. Such devices are of utmost importance to face impending challenges of air pollution and pandemics.

## SUMMARY

Particulate air pollution takes an enormous toll on society; it is linked to more than 3 million deaths annually. In addition, airborne microorganisms can spread dangerous diseases, compounding the issue. A widely used method for mitigating the risks of airborne particles is air filtration. While effective at particle removal, conventional air filters typically lack additional functionalities. They cannot inactivate accumulated contaminants, and they require periodic replacement to remain safe. This work presents a multifunctional, self-cleaning air-filtration system using a graphene-enhanced air filter medium (GeFM). The GeFM is capable of capturing >95% of microorganisms and particles >1 µm and can be repeatedly joule heated to >300°C, regenerating the filter. Furthermore, the GeFM provides unique options for monitoring the airflow and loading status of the filter. The GeFM can improve filter lifetime and safety, offering great potential for the development of advanced and likely more sustainable air-filtration solutions in the future.

## INTRODUCTION

The 2019 SARS-CoV-2 pandemic was a grave reminder of our susceptibility to the dangers posed by airborne microorganisms and particulate air pollution. Direct exposure to airborne particulate matter with aerodynamic diameters smaller than 2.5 µm (PM<sub>2.5</sub>) is associated with more than 3 million deaths per year.<fnref n="1" /> In addition, most infectious microorganisms form particles with aerodynamic diameters of 0.5–5 µm.<sup>2</sup> PM<sub>2.5</sub> can therefore promote the spread of dangerous diseases. The most common method to reduce the concentration of harmful PM<sub>2.5</sub> is filtration.

High-performance air filtration is based on two fundamental approaches. Interaction-based filters, like electrostatic precipitators, exploit electrostatic forces to immobilize particles. Structure-based approaches, on the other hand, pass the air through a porous medium with a complex internal structure to capture the particles.<sup>3</sup> State-of-the-art air filter media include hierarchically structured fibrous membranes<sup>4–6</sup> and multimodal nanofiber mats fabricated by humidity-induced electrospinning/netting.<sup>7–9</sup> In the filter medium, particles are filtered through various mechanisms, including size-exclusion sieving, inertial impaction, interception, and diffusion, depending on their size, density, and velocity.<sup>3</sup> Due to interactions between the

<figure><img src="image_5.png" /></figure>

<footnote n="1">¹Functional Nanomaterials, Department of Materials Science, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany</footnote>

<footnote n="2">²IMT-Atlantique GEPEA UMR CNRS 6144, 4 Rue Alfred Castler BP 20722, 44307 Nantes Cedex 3, France</footnote>

<footnote n="3">³Institute for Inorganic Chemistry, Kiel University, Max-Eyth-Straße 2, 24118 Kiel, Germany</footnote>

<footnote n="4">⁴Center for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany</footnote>

<footnote n="5">⁵Lead contact</footnote>

<footnote>*Correspondence: fas@tf.uni-kiel.de (F.S.), ra@tf.uni-kiel.de (R.A.)</footnote>

<footnote>https://doi.org/10.1016/j device.2023.100098</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** The graphene-enhanced filter medium (GeFM)</figcaption></figure>

(A) Photograph of the GeFM with a schematic of the material structure. A highly porous network of tetrapodal ZnO is coated with a few-layer graphene thin film.

(B) Schematic of passive air filtration. The random orientation and high porosity of the GeFM enable reliable capture of various air pollutants at low-pressure drop.

(C) Schematic of active self-cleaning of the GeFM filter. By joule heating of the filter medium itself to 300°C, organic contaminants are decomposed, improving the lifetime and safety of the filter system.

streamline of the fluid and the filter medium, a drop in pressure occurs over the filter.¹⁰ It is desired to have the maximum possible filtration efficiency at the minimum possible pressure drop. High-efficiency particulate air (HEPA) filters can filter >99.99% of even the most penetrating particles (d = ~300 nm) at an initial pressure drop of ~250 Pa.¹¹ However, during operation, pollutants build up on and inside the filter, exponentially increasing its pressure drop.¹⁰ Furthermore, captured microorganisms can survive and proliferate inside the filter medium to later be released back into the airstream, causing downstream contamination. The released microorganisms can include severely harmful pathogenic or toxigenic fungi and bacteria, creating a significant health safety issue.¹²⁻¹⁵ This is especially critical in high-risk scenarios such as hospitals, where airborne infections make up 10%–20% of all nosocomial diseases.¹⁶ Conventional filters therefore need regular maintenance and replacement to ensure safe operation.

To further reduce the risk of microbial penetration and therefore improve safety, air filters can be used in series with air-sterilization systems. The most common technique, UV-C germicidal irradiation, subjects the airstream to high doses of hard ultraviolet light to eliminate microbial contamination.¹⁷ However, if the

microorganisms are embedded in larger particles or shielded in any other way, the lethal dose of radiation might not be met.¹⁸ UV-C and other conventional air-sterilization systems are unable to decompose the various toxins and residues produced by microorganisms upon death. These substances can cause severe health reactions and usually require extensive exposure to dry heat (~250°C) for reliable deactivation.¹⁹ Recently, new concepts for air filters that can disinfect themselves via joule heating have been presented.²⁰⁻²² Particularly interesting approaches utilize the high conductivity and high thermal stability of graphene, a two-dimensional carbon nanomaterial.²⁰,²¹ The most promising study employs sheets of laser-induced graphene as self-sterilizing air filters that can be heated to ~350°C to reliably deactivate captured microbial contaminants and their residues.²⁰ However, the material requires elaborate means of production to fabricate, and even thin layers of filter medium show tremendous pressure drops (~800 Pa) at face velocities typical of large-scale air filtration (0.15 m s⁻¹), making the filter unsuitable for large-scale air filtration. Furthermore, the self-cleaning process is unable to decompose inorganic pollutants with high thermal stability. As the concentration of such pollutants can vary over time and user scenarios, clogging of the laser-induced graphene filter is hard to predict. The filter will therefore still need external monitoring to ensure optimal performance.

While these studies provide extremely useful proof-of-concept information for the design of self-cleaning filter systems in general, further research must be performed to develop more scalable and multifunctional air-filtration systems that can independently operate at maximum efficiency. This work presents a multifunctional, self-cleaning air-filtration system utilizing a graphene-enhanced filter medium (GeFM). The GeFM consist of a highly porous (~94%) backbone of interconnected ZnO tetrapods, coated with a few-layer percolating graphene thin film of ~25 nm thickness, as depicted in Figure 1A. The random orientation of the tetrapod arms allows reliable capture of particulate matter from a passing airstream (Figure 1B), while its high porosity ensures a low-pressure drop. In contrast to conventional filter systems, which offer only passive air filtration, the high conductivity and temperature stability of the graphene thin film enable additional active features such as the pyrolytic self-cleaning visualized in Figure 1C. By joule heating of the filter medium itself to a maximum temperature of ~300°C, captured organic pollutants are effectively decomposed, significantly improving the filter system's safety and lifetime. Furthermore, it is demonstrated that, by analyzing the electrothermal behavior of the GeFM, the air throughput and loading status of the filter can be monitored over time. These unique monitoring features can be used to avoid unnecessary maintenance events, most likely improving the sustainability of the air-filtration system.

## RESULTS AND DISCUSSION

The three-dimensional hybrid networks of the GeFM are prepared as described elsewhere.²³⁻²⁵ Figures 2A–2C illustrate the fabrication of the filter medium. In short, tetrapodal ZnO microparticles are molded and subsequently sintered (5 h at 1,150°C) to obtain high porosity (~94%) networks, e.g., in the form of quadratic plates (68 × 68 × 4 mm) as shown in Figure 2A.
<figure><img src="image_6.png" /><figcaption>**Figure 1.** The graphene-enhanced filter medium (GeFM)</figcaption></figure>

(A) Photograph of the GeFM with a schematic of the material structure. A highly porous network of tetrapodal ZnO is coated with a few-layer graphene thin film.

(B) Schematic of passive air filtration. The random orientation and high porosity of the GeFM enable reliable capture of various air pollutants at low-pressure drop.

(C) Schematic of active self-cleaning of the GeFM filter. By joule heating of the filter medium itself to 300°C, organic contaminants are decomposed, improving the lifetime and safety of the filter system.

streamline of the fluid and the filter medium, a drop in pressure occurs over the filter.¹⁰ It is desired to have the maximum possible filtration efficiency at the minimum possible pressure drop. High-efficiency particulate air (HEPA) filters can filter >99.99% of even the most penetrating particles (d = ~300 nm) at an initial pressure drop of ~250 Pa.¹¹ However, during operation, pollutants build up on and inside the filter, exponentially increasing its pressure drop.¹⁰ Furthermore, captured microorganisms can survive and proliferate inside the filter medium to later be released back into the airstream, causing downstream contamination. The released microorganisms can include severely harmful pathogenic or toxigenic fungi and bacteria, creating a significant health safety issue.¹²⁻¹⁵ This is especially critical in high-risk scenarios such as hospitals, where airborne infections make up 10%–20% of all nosocomial diseases.¹⁶ Conventional filters therefore need regular maintenance and replacement to ensure safe operation.

To further reduce the risk of microbial penetration and therefore improve safety, air filters can be used in series with air-sterilization systems. The most common technique, UV-C germicidal irradiation, subjects the airstream to high doses of hard ultraviolet light to eliminate microbial contamination.¹⁷ However, if the

microorganisms are embedded in larger particles or shielded in any other way, the lethal dose of radiation might not be met.¹⁸ UV-C and other conventional air-sterilization systems are unable to decompose the various toxins and residues produced by microorganisms upon death. These substances can cause severe health reactions and usually require extensive exposure to dry heat (~250°C) for reliable deactivation.¹⁹ Recently, new concepts for air filters that can disinfect themselves via joule heating have been presented.²⁰⁻²² Particularly interesting approaches utilize the high conductivity and high thermal stability of graphene, a two-dimensional carbon nanomaterial.²⁰,²¹ The most promising study employs sheets of laser-induced graphene as self-sterilizing air filters that can be heated to ~350°C to reliably deactivate captured microbial contaminants and their residues.²⁰ However, the material requires elaborate means of production to fabricate, and even thin layers of filter medium show tremendous pressure drops (~800 Pa) at face velocities typical of large-scale air filtration (0.15 m s⁻¹), making the filter unsuitable for large-scale air filtration. Furthermore, the self-cleaning process is unable to decompose inorganic pollutants with high thermal stability. As the concentration of such pollutants can vary over time and user scenarios, clogging of the laser-induced graphene filter is hard to predict. The filter will therefore still need external monitoring to ensure optimal performance.

While these studies provide extremely useful proof-of-concept information for the design of self-cleaning filter systems in general, further research must be performed to develop more scalable and multifunctional air-filtration systems that can independently operate at maximum efficiency. This work presents a multifunctional, self-cleaning air-filtration system utilizing a graphene-enhanced filter medium (GeFM). The GeFM consist of a highly porous (~94%) backbone of interconnected ZnO tetrapods, coated with a few-layer percolating graphene thin film of ~25 nm thickness, as depicted in Figure 1A. The random orientation of the tetrapod arms allows reliable capture of particulate matter from a passing airstream (Figure 1B), while its high porosity ensures a low-pressure drop. In contrast to conventional filter systems, which offer only passive air filtration, the high conductivity and temperature stability of the graphene thin film enable additional active features such as the pyrolytic self-cleaning visualized in Figure 1C. By joule heating of the filter medium itself to a maximum temperature of ~300°C, captured organic pollutants are effectively decomposed, significantly improving the filter system's safety and lifetime. Furthermore, it is demonstrated that, by analyzing the electrothermal behavior of the GeFM, the air throughput and loading status of the filter can be monitored over time. These unique monitoring features can be used to avoid unnecessary maintenance events, most likely improving the sustainability of the air-filtration system.

## RESULTS AND DISCUSSION

The three-dimensional hybrid networks of the GeFM are prepared as described elsewhere.²³⁻²⁵ Figures 2A–2C illustrate the fabrication of the filter medium. In short, tetrapodal ZnO microparticles are molded and subsequently sintered (5 h at 1,150°C) to obtain high porosity (~94%) networks, e.g., in the form of quadratic plates (68 × 68 × 4 mm) as shown in Figure 2A.
<figure><img src="image_5.png" /><figcaption>Figure 2. GeFM fabrication and internal structure</figcaption></figure>

(A) Photograph of 68 × 68 × 4 mm ZnO network for preparation of the filter medium.

(B) Schematic of the drop infiltration process.

(C) Photograph of the finished GeFM.

(D–F) Cross-sectional SEM images: (D) pristine ZnO network; (E) tetrapod arm after the infiltration process, showing the deposited graphene thin film;

(F) network of the GeFM.

The network can be coated thoroughly with a nanomaterial thin film by a simple drop infiltration process as demonstrated in previous works.²⁴,²⁵ The process is schematically shown in Figure 2B. To produce the here-presented GeFM, the free volume of the network is filled with an aqueous dispersion of 1.4 mg mL⁻¹ electrochemically exfoliated graphene. Upon drying of the surplus water, the graphene sheets deposit evenly on the ZnO tetrapods. Repeating the process a total of five times yields a finished GeFM sample like the one shown in Figure 2C. Structural analysis of the material was performed after each fabrication step using scanning electron microscopy (SEM). The SEM image of the ZnO base network presented in Figure 2D shows the randomly oriented interconnected tetrapods. The tetrapod arms are roughly 3 µm in diameter and 50 µm in length. Figure 2E shows a cross section of a tetrapod arm after the infiltration process, demonstrating the homogeneous few-layer thin film formed. An SEM image of a finished GeFM network is shown in Figure 2F. The structure of the ZnO base network is adopted with merely occasional small graphene microstructures extending from the surface.

As mentioned in the introduction, the performance of air filters is determined by their filtration efficiency and their pressure drop. Standard methods for the characterization of filter media measure different pollutants separately, potentially distorting the results. To create a more realistic measurement scenario, an integrated setup was developed to simultaneously measure the filtration efficiency for particulate matter and microorganisms. A combined aerosol of *Bacillus subtilis* and NaCl particles was created by nebulizing a suspension of the microorganisms in an isotonic salt solution. A self-designed nozzle system ensured isokinetic sampling for both pollutants (see Figure S1). The filtration characteristics of the GeFM are shown in Figure 3. The pressure-drop data for filtration velocity relevant in industrial filter systems of GeFM with 3 and 4 mm thickness compared with a 4 mm sample of the pristine base network are shown in Figure 3A. The pressure-drop curves for the pristine ZnO network and GeFM of equal thicknesses are very similar, with the GeFM even having slightly lower pressure drop. This indicates that the graphene film is thin enough not to obstruct the fluid permeability of the network. By fitting the pressure-drop data of the

GeFM with Forchheimer's equation, the initial pressure drop and the fluid permeability coefficients of the GeFM were determined. The initial pressure drop was 50.58 and 74.44 Pa for 3 and 4 mm, respectively. The permeability coefficient $k_1$ for the 3 mm GeFM was calculated as $1.536 \times 10^{-8} \text{ m}^2$ and the non-Darcian permeability $k_2$ as $4.188 \times 10^{-4} \text{ m}$. For the 4 mm GeFM the values for $k_1$ and $k_2$ are $1.358 \times 10^{-8} \text{ m}^2$ and $2.939 \times 10^{-4} \text{ m}$, respectively. More details can be found in the supplemental information (Figure S2). The particle removal efficiency of the pristine tetrapodal zinc oxide (t-ZnO) network compared with a GeFM of equal thickness at a fluid velocity of $0.17 \text{ m s}^{-1}$ is shown in Figure 3B. While the removal efficiencies for particles larger than 1 µm are similarly high (~96%) for both materials, the filtration of sub-micrometer particles is improved by the graphene coating. The mean removal efficiency for particles with diameters between 0.5 and 1 µm increased from ~90% to ~94%. In addition, the GeFM reached its maximum removal efficiency at smaller particle diameters than the uncoated ZnO network, at 0.8 µm instead of 1.1 µm. Filtration of sub-micrometer particles is dominated by the diffusion and interception filtration mechanisms. The increased filtration efficiency in this size regime could therefore be explained by the graphene microstructures that extend from the thin film. The structures provide additional surfaces for the filtration mechanisms to take effect, slightly increasing the filtration efficiency. Overall, it can be concluded that the graphene coating is beneficial to the structure-related air-filtration properties of the underlying network.

Figure 3C shows the fractional particle removal efficiencies of GeFM with 3 and 4 mm thicknesses. For the most penetrating particle size (~0.25 µm), removal efficiencies of 61.2% ± 0.7% and 71.1% ± 0.4% were achieved for 3 and 4 mm, respectively. The removal efficiencies for particles larger than 1 µm are equally high, being >97% for both thicknesses (3 mm, 97.6% ± 0.7%; 4 mm, 97.1% ± 0.7%). As most infectious microorganisms form droplets with sizes ranging from 0.5 to 5 µm,² the GeFM should be able to remove a majority of them. For confirmation, filtration experiments were performed with *Bacillus subtilis* spores, which are ellipsoids ~1.2 µm in length.²⁶ Figures 3D and 3E show the results of the filtration measurements performed with these microorganisms. Both tested GeFM thicknesses were able to reliably capture ~95% of the airborne microorganisms (3 mm, 94.8% ± 0.2%, 4 mm, 96.0% ± 0.6%). As the removal efficiencies for particles larger than 1 µm and *Bacillus subtilis* spores were similar for both tested thicknesses, it is reasonable to assume that filtration
<figure><img src="image_7.png" /><figcaption>Figure 2. GeFM fabrication and internal structure</figcaption></figure>

(A) Photograph of 68 × 68 × 4 mm ZnO network for preparation of the filter medium.

(B) Schematic of the drop infiltration process.

(C) Photograph of the finished GeFM.

(D–F) Cross-sectional SEM images: (D) pristine ZnO network; (E) tetrapod arm after the infiltration process, showing the deposited graphene thin film;

(F) network of the GeFM.

The network can be coated thoroughly with a nanomaterial thin film by a simple drop infiltration process as demonstrated in previous works.²⁴,²⁵ The process is schematically shown in Figure 2B. To produce the here-presented GeFM, the free volume of the network is filled with an aqueous dispersion of 1.4 mg mL⁻¹ electrochemically exfoliated graphene. Upon drying of the surplus water, the graphene sheets deposit evenly on the ZnO tetrapods. Repeating the process a total of five times yields a finished GeFM sample like the one shown in Figure 2C. Structural analysis of the material was performed after each fabrication step using scanning electron microscopy (SEM). The SEM image of the ZnO base network presented in Figure 2D shows the randomly oriented interconnected tetrapods. The tetrapod arms are roughly 3 µm in diameter and 50 µm in length. Figure 2E shows a cross section of a tetrapod arm after the infiltration process, demonstrating the homogeneous few-layer thin film formed. An SEM image of a finished GeFM network is shown in Figure 2F. The structure of the ZnO base network is adopted with merely occasional small graphene microstructures extending from the surface.

As mentioned in the introduction, the performance of air filters is determined by their filtration efficiency and their pressure drop. Standard methods for the characterization of filter media measure different pollutants separately, potentially distorting the results. To create a more realistic measurement scenario, an integrated setup was developed to simultaneously measure the filtration efficiency for particulate matter and microorganisms. A combined aerosol of *Bacillus subtilis* and NaCl particles was created by nebulizing a suspension of the microorganisms in an isotonic salt solution. A self-designed nozzle system ensured isokinetic sampling for both pollutants (see Figure S1). The filtration characteristics of the GeFM are shown in Figure 3. The pressure-drop data for filtration velocity relevant in industrial filter systems of GeFM with 3 and 4 mm thickness compared with a 4 mm sample of the pristine base network are shown in Figure 3A. The pressure-drop curves for the pristine ZnO network and GeFM of equal thicknesses are very similar, with the GeFM even having slightly lower pressure drop. This indicates that the graphene film is thin enough not to obstruct the fluid permeability of the network. By fitting the pressure-drop data of the

GeFM with Forchheimer's equation, the initial pressure drop and the fluid permeability coefficients of the GeFM were determined. The initial pressure drop was 50.58 and 74.44 Pa for 3 and 4 mm, respectively. The permeability coefficient $k_1$ for the 3 mm GeFM was calculated as $1.536 \times 10^{-8} \text{ m}^2$ and the non-Darcian permeability $k_2$ as $4.188 \times 10^{-4} \text{ m}$. For the 4 mm GeFM the values for $k_1$ and $k_2$ are $1.358 \times 10^{-8} \text{ m}^2$ and $2.939 \times 10^{-4} \text{ m}$, respectively. More details can be found in the supplemental information (Figure S2). The particle removal efficiency of the pristine tetrapodal zinc oxide (t-ZnO) network compared with a GeFM of equal thickness at a fluid velocity of $0.17 \text{ m s}^{-1}$ is shown in Figure 3B. While the removal efficiencies for particles larger than 1 µm are similarly high (~96%) for both materials, the filtration of sub-micrometer particles is improved by the graphene coating. The mean removal efficiency for particles with diameters between 0.5 and 1 µm increased from ~90% to ~94%. In addition, the GeFM reached its maximum removal efficiency at smaller particle diameters than the uncoated ZnO network, at 0.8 µm instead of 1.1 µm. Filtration of sub-micrometer particles is dominated by the diffusion and interception filtration mechanisms. The increased filtration efficiency in this size regime could therefore be explained by the graphene microstructures that extend from the thin film. The structures provide additional surfaces for the filtration mechanisms to take effect, slightly increasing the filtration efficiency. Overall, it can be concluded that the graphene coating is beneficial to the structure-related air-filtration properties of the underlying network.

Figure 3C shows the fractional particle removal efficiencies of GeFM with 3 and 4 mm thicknesses. For the most penetrating particle size (~0.25 µm), removal efficiencies of 61.2% ± 0.7% and 71.1% ± 0.4% were achieved for 3 and 4 mm, respectively. The removal efficiencies for particles larger than 1 µm are equally high, being >97% for both thicknesses (3 mm, 97.6% ± 0.7%; 4 mm, 97.1% ± 0.7%). As most infectious microorganisms form droplets with sizes ranging from 0.5 to 5 µm,² the GeFM should be able to remove a majority of them. For confirmation, filtration experiments were performed with *Bacillus subtilis* spores, which are ellipsoids ~1.2 µm in length.²⁶ Figures 3D and 3E show the results of the filtration measurements performed with these microorganisms. Both tested GeFM thicknesses were able to reliably capture ~95% of the airborne microorganisms (3 mm, 94.8% ± 0.2%, 4 mm, 96.0% ± 0.6%). As the removal efficiencies for particles larger than 1 µm and *Bacillus subtilis* spores were similar for both tested thicknesses, it is reasonable to assume that filtration
<figure><img src="image_6.png" /><figcaption>Figure 3. Air-filtration properties of the GeFM</figcaption></figure>

(A) Pressure-drop curves of the 4-mm-thick pristine ZnO network compared with prepared samples of 3 and 4 mm thickness.

(B and C) NaCl particle removal efficiency versus optical particle diameter at 0.17 m s⁻¹ for (B) the base network compared with the finished material and (C) GeFM samples of 3 and 4 mm thickness.

(D) Bacterial filtration measurements for *Bacillus subtilis*.

(E) Calculated removal efficiency for *Bacillus subtilis* by GeFM.

of these pollutants was mostly happening on the surface of the filter. A thinner sheet of the material should therefore also be sufficient to strongly reduce microbial load in an airstream. While the focus of this work is on filtration of PM₂.₅ and microorganisms, the graphene thin film also enhances the filter medium's ability to adsorb and retain non-methane volatile organic compounds (NMVOCs) such as toluene. The GeFM was able to adsorb ~1.7 times the amount of toluene (0.06 wt %) and retain about twice as much (0.03 wt %) compared with the pristine ZnO network. With respect to the active material, the graphene thin film added 121 mg g⁻¹ of toluene adsorption and 75 mg g⁻¹ toluene retention in a nitrogen atmosphere (see Figure S3).

The graphene thin film also provides the GeFM with a high conductivity (~27 S m⁻¹) that leads to low sheet resistances of 12.5 and 9.4 Ω sq⁻¹ for 3- and 4-mm-thick samples, respectively. Combined with the high thermal stability of graphene, the GeFM can be joule heated to high temperatures of >300 °C at relatively low power consumption. Thermographic imaging was performed on the GeFM to determine the temperature distribution and mean temperature of the sample. When heated with 53 W (~1.15 W cm⁻²) of electrical power, the core area of the material heated up rapidly and homogenously. Figure 4A shows a series of infrared (IR) images taken at different times of the heating cycle. The complete IR video is provided as a supplemental item (Video S1). The GeFM reaches a maximum temperature of ~315°C with a mean temperature of ~220°C. The discrepancy between maximum and mean temperature can be attributed to boundary regions of the sample being in contact

with the sample holder. These boundary regions are colder due to conductive heat transfer to the mounting frame, decreasing the mean temperature. The temperature response of the GeFM can be easily tailored by the applied power, as shown in Figure 4B. The maximum temperature increases by 3.5°C W⁻¹ and the mean temperature by ~2.3°C W⁻¹. More details are presented in the supplemental information (Figure S4).

Few-layer graphene demonstrates a negative temperature coefficient of conductivity.²⁷ The sheet resistance of the GeFM therefore drops by ~5% when the maximum temperature is increased from 215°C to 315°C. This behavior lowers the voltage required to reach high temperatures. The low time constant of heating, <10 s, allows the material to reach pyrolytic temperatures, i.e., 250°C, quickly (<20 s), minimizing energy losses when heating up. A histogram of the temperature distribution for the IR image taken after 60 s of heating can be seen in Figure 4C. Approximately 93% of the sample area has a temperature >120°C, with ~70% even exhibiting temperatures of >180°C. These temperatures are the standard temperatures for moist and dry heat sterilization, respectively.²⁸ To prove thermal stability of the material over multiple cycles of heating and cooling, samples were heated to ~300°C for 1 min with a subsequent cooling period of 1 min. The cycle was repeated a total of 30 times. Figure 4D shows a selection of the recorded temperature curves. Except for a small increase in maximum temperature, no changes in the temperature response were observed. The increase is most likely caused by heat built up in the sample holder and contacts. This indicates that no thermal degradation
<figure><img src="image_8.png" /><figcaption>Figure 3. Air-filtration properties of the GeFM</figcaption></figure>

(A) Pressure-drop curves of the 4-mm-thick pristine ZnO network compared with prepared samples of 3 and 4 mm thickness.

(B and C) NaCl particle removal efficiency versus optical particle diameter at 0.17 m s⁻¹ for (B) the base network compared with the finished material and (C) GeFM samples of 3 and 4 mm thickness.

(D) Bacterial filtration measurements for *Bacillus subtilis*.

(E) Calculated removal efficiency for *Bacillus subtilis* by GeFM.

of these pollutants was mostly happening on the surface of the filter. A thinner sheet of the material should therefore also be sufficient to strongly reduce microbial load in an airstream. While the focus of this work is on filtration of PM₂.₅ and microorganisms, the graphene thin film also enhances the filter medium's ability to adsorb and retain non-methane volatile organic compounds (NMVOCs) such as toluene. The GeFM was able to adsorb ~1.7 times the amount of toluene (0.06 wt %) and retain about twice as much (0.03 wt %) compared with the pristine ZnO network. With respect to the active material, the graphene thin film added 121 mg g⁻¹ of toluene adsorption and 75 mg g⁻¹ toluene retention in a nitrogen atmosphere (see Figure S3).

The graphene thin film also provides the GeFM with a high conductivity (~27 S m⁻¹) that leads to low sheet resistances of 12.5 and 9.4 Ω sq⁻¹ for 3- and 4-mm-thick samples, respectively. Combined with the high thermal stability of graphene, the GeFM can be joule heated to high temperatures of >300 °C at relatively low power consumption. Thermographic imaging was performed on the GeFM to determine the temperature distribution and mean temperature of the sample. When heated with 53 W (~1.15 W cm⁻²) of electrical power, the core area of the material heated up rapidly and homogenously. Figure 4A shows a series of infrared (IR) images taken at different times of the heating cycle. The complete IR video is provided as a supplemental item (Video S1). The GeFM reaches a maximum temperature of ~315°C with a mean temperature of ~220°C. The discrepancy between maximum and mean temperature can be attributed to boundary regions of the sample being in contact

with the sample holder. These boundary regions are colder due to conductive heat transfer to the mounting frame, decreasing the mean temperature. The temperature response of the GeFM can be easily tailored by the applied power, as shown in Figure 4B. The maximum temperature increases by 3.5°C W⁻¹ and the mean temperature by ~2.3°C W⁻¹. More details are presented in the supplemental information (Figure S4).

Few-layer graphene demonstrates a negative temperature coefficient of conductivity.²⁷ The sheet resistance of the GeFM therefore drops by ~5% when the maximum temperature is increased from 215°C to 315°C. This behavior lowers the voltage required to reach high temperatures. The low time constant of heating, <10 s, allows the material to reach pyrolytic temperatures, i.e., 250°C, quickly (<20 s), minimizing energy losses when heating up. A histogram of the temperature distribution for the IR image taken after 60 s of heating can be seen in Figure 4C. Approximately 93% of the sample area has a temperature >120°C, with ~70% even exhibiting temperatures of >180°C. These temperatures are the standard temperatures for moist and dry heat sterilization, respectively.²⁸ To prove thermal stability of the material over multiple cycles of heating and cooling, samples were heated to ~300°C for 1 min with a subsequent cooling period of 1 min. The cycle was repeated a total of 30 times. Figure 4D shows a selection of the recorded temperature curves. Except for a small increase in maximum temperature, no changes in the temperature response were observed. The increase is most likely caused by heat built up in the sample holder and contacts. This indicates that no thermal degradation
<figure><img src="image_7.png" /></figure>

of the material took place. To ensure long-term stability of the fil-
ter material, samples were heated to a target temperature of
280°C for 10 h. The maximum sample temperature over the
course of the experiment is plotted in Figure 4E. After a short
period of stabilization to equilibrium temperature, no significant
changes in the maximum temperature were observed. The
GeFM should therefore demonstrate a high cyclic stability and
long-term reliability.

The homogeneous heating behavior of the GeFM can be used
to enable pyrolytic self-cleaning. To determine whether such a
self-cleaning procedure could have an effect on the filter perfor-
mance of the GeFM, the material was subjected to 50 heating cy-
cles of 1 min at 60 W. The particle removal efficiency of the GeFM
was measured before and after the procedure. Figure 5A com-
pares the results of the two measurements. As both curves are
almost identical, it can be concluded that the self-cleaning
procedure should have no detrimental effect on the filter perfor-
mance. To investigate the effectiveness of the self-cleaning pro-
cess, the GeFM was tested in a simulated working environment.
Four-millimeter-thick GeFM was subjected to three simulated
duty cycles of loading for 2 h with a highly concentrated bacterial
suspension of 10⁶ CFU mL⁻¹ in isotonic NaCl solution and sub-
sequent heating at ~60 W for 1 min. Figures 5B and 5D present
the removal efficiency of the tested GeFM before and after the
three duty cycles for particles and *Bacillus subtilis*, respectively.

<caption>Figure 4. Homogeneous joule-heating behavior and long-term stability</caption>

(A) Photograph of 68 × 68 × 4 mm GeFM in aluminum mounting frame with corresponding IR camera images after 20, 40, and 60 s of heating with 1.15 W cm⁻², as well as 20 s after the end of the heating cycle.

(B) Time-temperature curves of the filter when
joule-heated at different powers.

(C) Histogram of temperature distribution of IR
image taken after 60 s of heating.

(D) Time-temperature curves of the first and last
three pulses of 30 cycles of heating to ~300°C.

(E) Time-temperature curve of long-term stability tests at an applied power of 2 W cm⁻². The temperature cutoff at 125°C is due to the measurement range of the high-speed pyrometer.

The results demonstrate that the filtration
efficiency for both pollutants is not
affected by the self-cleaning process.

To determine whether the self-cleaning
process can effectively reduce pollutant
buildup, the pressure drop of the GeFM
was measured after each cycle. As the
network thoroughly joule heats to high
temperatures, heat is transferred directly
to captured pollutants. As heating up is
fast, thermal lack is minimized. Therefore,
even the short heating cycle of 1 min is
already sufficient to significantly reduce
the increase in pressure drop caused
by pollutant buildup (see Figure 5C).

Comparing the estimated lifetime of the reference to the self-
cleaned GeFM shows an improvement by at least a factor of 3
(see Figure S5). To determine the ability of the GeFM to eliminate
captured microorganisms, the amount of viable *Bacillus subtilis*
spores residing in the filter after the three duty cycles was
compared with a reference. The reference was subjected to the
same loading procedure but not heated. As shown in Figure 5D,
the self-cleaning reduced the amount of residual *Bacillus subtilis*
in the filter by at least 3 orders of magnitude. This is supported by
the results of additional structural analysis performed via SEM.
The SEM image presented in Figure 5F shows no evidence of re-
siduals other than NaCl particles. *Bacillus subtilis* is a standard
test germ well known for its high resistance to thermal stress. It
is therefore reasonable to assume that the GeFM is also able to
inactivate other microorganisms with ease. Although the GeFM
operation temperature of ~300°C is already sufficient to eliminate
microorganisms and decompose many organic pollutants, it is
not enough for inorganic temperature-resistant organic particles.
While higher temperatures would surely improve the decomposi-
tion of extremely stable organic compounds further, it could also
result in a slow degradation of the GeFM over time, thereby
limiting its long-term usage. To further increase the operation
temperature of the GeFM, it might be possible in the future to
add a protection layer such as hexagonal boron nitride onto the
graphene.
<figure><img src="image_9.png" /></figure>

of the material took place. To ensure long-term stability of the fil-
ter material, samples were heated to a target temperature of
280°C for 10 h. The maximum sample temperature over the
course of the experiment is plotted in Figure 4E. After a short
period of stabilization to equilibrium temperature, no significant
changes in the maximum temperature were observed. The
GeFM should therefore demonstrate a high cyclic stability and
long-term reliability.

The homogeneous heating behavior of the GeFM can be used
to enable pyrolytic self-cleaning. To determine whether such a
self-cleaning procedure could have an effect on the filter perfor-
mance of the GeFM, the material was subjected to 50 heating cy-
cles of 1 min at 60 W. The particle removal efficiency of the GeFM
was measured before and after the procedure. Figure 5A com-
pares the results of the two measurements. As both curves are
almost identical, it can be concluded that the self-cleaning
procedure should have no detrimental effect on the filter perfor-
mance. To investigate the effectiveness of the self-cleaning pro-
cess, the GeFM was tested in a simulated working environment.
Four-millimeter-thick GeFM was subjected to three simulated
duty cycles of loading for 2 h with a highly concentrated bacterial
suspension of 10⁶ CFU mL⁻¹ in isotonic NaCl solution and sub-
sequent heating at ~60 W for 1 min. Figures 5B and 5D present
the removal efficiency of the tested GeFM before and after the
three duty cycles for particles and *Bacillus subtilis*, respectively.

<caption>Figure 4. Homogeneous joule-heating behavior and long-term stability</caption>

(A) Photograph of 68 × 68 × 4 mm GeFM in aluminum mounting frame with corresponding IR camera images after 20, 40, and 60 s of heating with 1.15 W cm⁻², as well as 20 s after the end of the heating cycle.

(B) Time-temperature curves of the filter when
joule-heated at different powers.

(C) Histogram of temperature distribution of IR
image taken after 60 s of heating.

(D) Time-temperature curves of the first and last three pulses of 30 cycles of heating to ~300°C.

(E) Time-temperature curve of long-term stability tests at an applied power of 2 W cm⁻². The temperature cutoff at 125°C is due to the measurement range of the high-speed pyrometer.

The results demonstrate that the filtration
efficiency for both pollutants is not
affected by the self-cleaning process.

To determine whether the self-cleaning
process can effectively reduce pollutant
buildup, the pressure drop of the GeFM
was measured after each cycle. As the
network thoroughly joule heats to high
temperatures, heat is transferred directly
to captured pollutants. As heating up is
fast, thermal lack is minimized. Therefore,
even the short heating cycle of 1 min is
already sufficient to significantly reduce
the increase in pressure drop caused
by pollutant buildup (see Figure 5C).

Comparing the estimated lifetime of the reference to the self-
cleaned GeFM shows an improvement by at least a factor of 3
(see Figure S5). To determine the ability of the GeFM to eliminate
captured microorganisms, the amount of viable *Bacillus subtilis*
spores residing in the filter after the three duty cycles was
compared with a reference. The reference was subjected to the
same loading procedure but not heated. As shown in Figure 5D,
the self-cleaning reduced the amount of residual *Bacillus subtilis*
in the filter by at least 3 orders of magnitude. This is supported by
the results of additional structural analysis performed via SEM.
The SEM image presented in Figure 5F shows no evidence of re-
siduals other than NaCl particles. *Bacillus subtilis* is a standard
test germ well known for its high resistance to thermal stress. It
is therefore reasonable to assume that the GeFM is also able to
inactivate other microorganisms with ease. Although the GeFM
operation temperature of ~300°C is already sufficient to eliminate
microorganisms and decompose many organic pollutants, it is
not enough for inorganic temperature-resistant organic particles.
While higher temperatures would surely improve the decomposi-
tion of extremely stable organic compounds further, it could also
result in a slow degradation of the GeFM over time, thereby
limiting its long-term usage. To further increase the operation
temperature of the GeFM, it might be possible in the future to
add a protection layer such as hexagonal boron nitride onto the
graphene.
<figure><img src="image_8.png" /><figcaption>Figure 5. Self-cleaning of the GeFM through joule heating</figcaption></figure>

(A) Particle removal efficiency of the 68 × 68 × 4 mm GeFM before and after 50 heating cycles.

(B) Particle removal efficiency after three simulated duty cycles compared with initial measurement.

(C) Pressure drop change at 0.14 cm s⁻¹ fluid velocity for GeFM with and without regular self-cleaning intervals.

(D) Removal efficiency for *Bacillus subtilis* after three simulated duty cycles compared with initial measurement.

(E) Residual microorganisms in the reference GeFM after 6 h of loading compared with self-cleaned sample.

(F) SEM image of GeFM after three duty cycles.

The joule-heating behavior of the GeFM can be used to enable additional active functionalities. Figure 6A shows the temperature response with subsequent cooling of the filter for an electric pulse of 22 V for 1.25 s under varying airflow conditions. With increasing flow, the heat loss effects in the system become more pronounced, leading to a decrease in maximum temperature reached by heat pulses of the GeFM. The results of fitting the cooling curves after the heat pulses (see Figure S6) indicate a combination of at least two heat-loss pathways, one most likely being convective heat transfer to the airflow. These changes in temperature response can therefore be used to enable airflow monitoring. As explained in more detail in the supplemental information (see Figure S6), the relative maximum temperature can be estimated from the fluid velocity by a quadratic relationship. By inverting this relationship, the airflow through the filter can be calculated. Figure 6B shows the determined characteristic flow-monitoring curves for the 3 and 4 mm GeFM. The sensitivity of the airflow monitoring could be further improved by reducing secondary heat losses in the system. Reducing the heat capacity of the electrical contacts, for example, would reduce conductive heat losses. Changes in the maximum peak temperature would then be more directly linked to the airflow. The temperature response does also react to increased dust loading of the filter. Temperature curves of 1 min heating cycles at 60 W were recorded for a 4 mm GeFM sample loaded with different amounts of SiO₂ test dust. Figure 6C shows the initial 20 s of the temperature response. Upon loading, the heat capacity of the test dust

was added to the heat capacity of the filter system. Therefore, the GeFM heats up slightly slower than in its unloaded state. As established in the supplemental information, a linear relationship between the inverse of the initial heating rate (t < 1 s) and the load status of the GeFM can be derived (Figure S7). Although the equation is valid only if heat losses in the system are negligible, the linear relationship still holds for the mean inverse heating rate of the first 10 s of the experiment. Figure 6D shows the corresponding data with a linear fit. However, more precise results could be acquired by measuring the heating rate at the beginning of the joule heating. As the temperature difference between the filter unit and its environment is small at this stage of the heating cycle, distortions from heat losses are smaller as well. Furthermore, the sensitivity of the load monitoring is dictated by the heat capacity of the load compared with the overall heat capacity of the system (Figure S7; Equations 12 and 13). Reducing the heat capacity of the filter unit by, for example, reducing the size of the electrical contacts or removing the underlying ZnO network should improve the resolution of the load monitoring function.

## Conclusion

In conclusion, this study presents a multifunctional, self-cleaning air-filtration system based on a GeFM. The graphene coating improves the removal efficiency for sub-microparticles and the NMVOC adsorption and retention capabilities of the underlying ZnO network without being detrimental to its pressure drop.
<figure><img src="image_10.png" /><figcaption>Figure 5. Self-cleaning of the GeFM through joule heating</figcaption></figure>

(A) Particle removal efficiency of the 68 × 68 × 4 mm GeFM before and after 50 heating cycles.

(B) Particle removal efficiency after three simulated duty cycles compared with initial measurement.

(C) Pressure drop change at 0.14 cm s⁻¹ fluid velocity for GeFM with and without regular self-cleaning intervals.

(D) Removal efficiency for *Bacillus subtilis* after three simulated duty cycles compared with initial measurement.

(E) Residual microorganisms in the reference GeFM after 6 h of loading compared with self-cleaned sample.

(F) SEM image of GeFM after three duty cycles.

The joule-heating behavior of the GeFM can be used to enable additional active functionalities. Figure 6A shows the temperature response with subsequent cooling of the filter for an electric pulse of 22 V for 1.25 s under varying airflow conditions. With increasing flow, the heat loss effects in the system become more pronounced, leading to a decrease in maximum temperature reached by heat pulses of the GeFM. The results of fitting the cooling curves after the heat pulses (see Figure S6) indicate a combination of at least two heat-loss pathways, one most likely being convective heat transfer to the airflow. These changes in temperature response can therefore be used to enable airflow monitoring. As explained in more detail in the supplemental information (see Figure S6), the relative maximum temperature can be estimated from the fluid velocity by a quadratic relationship. By inverting this relationship, the airflow through the filter can be calculated. Figure 6B shows the determined characteristic flow-monitoring curves for the 3 and 4 mm GeFM. The sensitivity of the airflow monitoring could be further improved by reducing secondary heat losses in the system. Reducing the heat capacity of the electrical contacts, for example, would reduce conductive heat losses. Changes in the maximum peak temperature would then be more directly linked to the airflow. The temperature response does also react to increased dust loading of the filter. Temperature curves of 1 min heating cycles at 60 W were recorded for a 4 mm GeFM sample loaded with different amounts of SiO₂ test dust. Figure 6C shows the initial 20 s of the temperature response. Upon loading, the heat capacity of the test dust

was added to the heat capacity of the filter system. Therefore, the GeFM heats up slightly slower than in its unloaded state. As established in the supplemental information, a linear relationship between the inverse of the initial heating rate (t < 1 s) and the load status of the GeFM can be derived (Figure S7). Although the equation is valid only if heat losses in the system are negligible, the linear relationship still holds for the mean inverse heating rate of the first 10 s of the experiment. Figure 6D shows the corresponding data with a linear fit. However, more precise results could be acquired by measuring the heating rate at the beginning of the joule heating. As the temperature difference between the filter unit and its environment is small at this stage of the heating cycle, distortions from heat losses are smaller as well. Furthermore, the sensitivity of the load monitoring is dictated by the heat capacity of the load compared with the overall heat capacity of the system (Figure S7; Equations 12 and 13). Reducing the heat capacity of the filter unit by, for example, reducing the size of the electrical contacts or removing the underlying ZnO network should improve the resolution of the load monitoring function.

## Conclusion

In conclusion, this study presents a multifunctional, self-cleaning air-filtration system based on a GeFM. The graphene coating improves the removal efficiency for sub-microparticles and the NMVOC adsorption and retention capabilities of the underlying ZnO network without being detrimental to its pressure drop.
<figure><img src="image_9.png" /></figure>

The GeFM achieved high removal efficiencies (~95%) for parti-
cles and microorganisms with diameters >1 µm. Beyond air
filtration, the graphene enhancement enables additional active
functionalities. The GeFM can be homogeneously joule heated
to maximum temperatures of ~315°C at low areal power den-
sities (~1.15 W cm⁻²). The material demonstrated high long-
term stability, showing no signs of degradation over at least 30
cycles or after extended periods of heating. Even short (1 min)
heating cycles were demonstrated to effectively decompose
captured organic pollutants, including highly temperature-resis-
tant microorganisms, promising a considerable improvement in
the filter's lifetime and safety. Furthermore, the joule-heating
behavior of the GeFM can be used to monitor the air throughput
and load status of the filter. With these features, the GeFM would
be of great benefit as a primary filter solution in domestic settings
where pollutants mostly comprise organic particles such as hair
and dander, as well as biological pathogens.

In addition, the GeFM could serve as a pre-filter in HEPA filtra-
tion systems for applications where biological pathogens or
downtime due to maintenance are huge issues. These may
include hospitals, public transportation, or airplanes. The
GeFM filters a majority of larger particles responsible for clog-
ging the HEPA filter as well as microorganisms that could prolif-
erate inside of it. With its regular self-cleaning, the GeFM would
be less affected by these issues itself, but would reduce the load
on the HEPA filter to extend its lifetime. The monitoring functions
of GeFM could further be used to ensure optimal performance of
the HVAC system by aiding the airflow regulation. However,
further research will be necessary to make the GeFM a stand-
alone replacement for HEPA filters. The filtration efficiency for
sub-micrometer particles, in particular, needs to be improved.

<caption>Figure 6. Built-in monitoring capabilities for air throughput and load status of the filter</caption>

(A) Temperature-response curves of a 4 mm GeFM sample for various fluid velocities.

(B) Fluid velocity with inverse of quadratic fit
versus corresponding reduction in maximum
temperature reached by heat pulses of equal
energy.

(C) Initial temperature response for a 4 mm GeFM
loaded with different amounts of SiO₂ test dust.

(D) Dust loading versus inverse mean heating rate
in the first 10 s.

This could, for example, be achieved by
optimizing the morphology and size dis-
tribution of the t-ZnO particles used for
the base network. Another approach
could involve electrostatically charging
the conductive graphene thin film of the
GeFM, which could vastly improve the
filtration efficiency for sub-micrometer
particles. Investigating the applicability
of the GeFM concept to other fibrous ma-
terials could also offer potential for the
development of novel multifunctional,
high-performance air filter media. Overall,

the GeFM concept is a promising addition to the field of air-filtration technology, providing many opportunities for the development of advanced, safer, and likely more sustainable filter systems to address the challenges of the future.

EXPERIMENTAL PROCEDURES

Resource availability

Lead contact

The lead contact for this article is Prof. Dr. rer. nat. Rainer Adelung (ra@tf.uni-kiel.de).

Materials availability

The t-ZnO generated in this study will be made available in reasonable amounts on request. There are restrictions to the availability of t-ZnO because of our limited production capability and our need to maintain the stock. We may require a payment and a completed materials transfer agreement if there is potential for commercial application. Large quantities of t-ZnO are available for purchase from PhiStone AG (Mielkendorf, Germany). All 3D files and schematics necessary to reproduce the devices presented in this work will be made available by the corresponding authors upon request. All other materials are available from the respective sources reported in this section.

Data and code availability

All data reported used in this work will be shared by the corresponding authors on request. This work does not report original code. Any additional information and data processing code required to reanalyze the data reported here are also available from the corresponding authors upon request.

Fabrication of filter medium

t-ZnO particles were synthesized using the flame transport synthesis approach described by Mishra et al.²⁹ A crucible with a 2:1 mixture of Mowital B60H (Kuraray Europe, Germany) and zinc dust (Sigma-Aldrich Chemie, Germany) was placed in a furnace at 900°C for 20 min. The ZnO tetrapods were then manually harvested from the crucible.

In this work, material with a base network porosity of 94% was tested. This corresponds to a ZnO network density of 0.3 g cm⁻³. The necessary amount of
<figure><img src="image_11.png" /></figure>

The GeFM achieved high removal efficiencies (~95%) for parti-
cles and microorganisms with diameters >1 µm. Beyond air
filtration, the graphene enhancement enables additional active
functionalities. The GeFM can be homogeneously joule heated
to maximum temperatures of ~315°C at low areal power den-
sities (~1.15 W cm⁻²). The material demonstrated high long-
term stability, showing no signs of degradation over at least 30
cycles or after extended periods of heating. Even short (1 min)
heating cycles were demonstrated to effectively decompose
captured organic pollutants, including highly temperature-resis-
tant microorganisms, promising a considerable improvement in
the filter's lifetime and safety. Furthermore, the joule-heating
behavior of the GeFM can be used to monitor the air throughput
and load status of the filter. With these features, the GeFM would
be of great benefit as a primary filter solution in domestic settings
where pollutants mostly comprise organic particles such as hair
and dander, as well as biological pathogens.

In addition, the GeFM could serve as a pre-filter in HEPA filtra-
tion systems for applications where biological pathogens or
downtime due to maintenance are huge issues. These may
include hospitals, public transportation, or airplanes. The
GeFM filters a majority of larger particles responsible for clog-
ging the HEPA filter as well as microorganisms that could prolif-
erate inside of it. With its regular self-cleaning, the GeFM would
be less affected by these issues itself, but would reduce the load
on the HEPA filter to extend its lifetime. The monitoring functions
of GeFM could further be used to ensure optimal performance of
the HVAC system by aiding the airflow regulation. However,
further research will be necessary to make the GeFM a stand-
alone replacement for HEPA filters. The filtration efficiency for
sub-micrometer particles, in particular, needs to be improved.

<caption>Figure 6. Built-in monitoring capabilities for air throughput and load status of the filter</caption>

(A) Temperature-response curves of a 4 mm GeFM sample for various fluid velocities.

(B) Fluid velocity with inverse of quadratic fit
versus corresponding reduction in maximum
temperature reached by heat pulses of equal
energy.

(C) Initial temperature response for a 4 mm GeFM
loaded with different amounts of SiO₂ test dust.

(D) Dust loading versus inverse mean heating rate
in the first 10 s.

This could, for example, be achieved by
optimizing the morphology and size dis-
tribution of the t-ZnO particles used for
the base network. Another approach
could involve electrostatically charging
the conductive graphene thin film of the
GeFM, which could vastly improve the
filtration efficiency for sub-micrometer
particles. Investigating the applicability
of the GeFM concept to other fibrous ma-
terials could also offer potential for the
development of novel multifunctional,
high-performance air filter media. Overall,

the GeFM concept is a promising addition to the field of air-filtration technology, providing many opportunities for the development of advanced, safer, and likely more sustainable filter systems to address the challenges of the future.

EXPERIMENTAL PROCEDURES

Resource availability

Lead contact

The lead contact for this article is Prof. Dr. rer. nat. Rainer Adelung (ra@tf.uni-kiel.de).

Materials availability

The t-ZnO generated in this study will be made available in reasonable amounts on request. There are restrictions to the availability of t-ZnO because of our limited production capability and our need to maintain the stock. We may require a payment and a completed materials transfer agreement if there is potential for commercial application. Large quantities of t-ZnO are available for purchase from PhiStone AG (Mielkendorf, Germany). All 3D files and schematics necessary to reproduce the devices presented in this work will be made available by the corresponding authors upon request. All other materials are available from the respective sources reported in this section.

Data and code availability

All data reported used in this work will be shared by the corresponding authors on request. This work does not report original code. Any additional information and data processing code required to reanalyze the data reported here are also available from the corresponding authors upon request.

Fabrication of filter medium

t-ZnO particles were synthesized using the flame transport synthesis approach described by Mishra et al.²⁹ A crucible with a 2:1 mixture of Mowital B60H (Kuraray Europe, Germany) and zinc dust (Sigma-Aldrich Chemie, Germany) was placed in a furnace at 900°C for 20 min. The ZnO tetrapods were then manually harvested from the crucible.

In this work, material with a base network porosity of 94% was tested. This corresponds to a ZnO network density of 0.3 g cm⁻³. The necessary amount of
t-ZnO for the respective target geometry was weighed and molded into shape. The samples were then sintered at 1,150°C for 5 h.

The ZnO networks were then infiltrated by a dispersion of graphene in deionized water. A dispersion of electrochemically exfoliated graphene was prepared as described elsewhere.³⁰ The dispersion was kindly provided by Sixonia Tech (Dresden, Germany) and used without further modification. The dispersion was diluted with deionized water to 1.4 mg mL⁻¹ and tip sonicated with 3 s pulses for a total of 10 min and 13.16 kJ using a Sonoplus HD4100 (Bandelin, Germany). The dispersion was then drop infiltrated into the samples until the free volume was filled. The samples were then dried for 4 h at 50°C. The procedure was repeated a total of five times.

The samples were mounted to 3D-printed sample holders and electrically conducted using brass rods and conductive silver paste (ACHESON 1415, Plano, Germany). The array was then sealed using high-temperature-resistant silicone (Pattex Ofen und Kamin Spezial Silikon, Henkel, Germany).

## Preparation of *Bacillus subtilis* spore suspension

*Bacillus subtilis* spores were selected as test germs for their high resistance to thermal stress. *Bacillus subtilis subtilis* (CIP 52.65, CRBIP Institut Pasteur, France) was cultivated in double-strength Schaeffer sporulation medium for 4 days at 37°C. The spores were extracted from the medium by centrifugation at 8,000 rpm, 7,585g. The spores were then washed with deionized water and stored at 80°C for 10 min. Using such prepared spores, a suspension of 10⁵ CFU mL⁻¹ in 9 g L⁻¹ NaCl (Fisher Chemical, Germany) solution was prepared for the air-filtration experiments. An additional suspension of 10⁶ CFU mL⁻¹ was prepared for self-cleaning experiments.

Growth media were prepared using 20 g L⁻¹ nutrient broth (10 g L⁻¹ Tryp-tone, 5 g L⁻¹ meat extract, 5 g L⁻¹ NaCl; Biokar Diagnostics, France) and 12 g L⁻¹ bacteriological agar type E (Biokar Diagnostics, France). The media were autoclaved at 121°C for 15 min, and 20 mL of each medium was poured into Petri dishes and left to cool before being stored for later use. The growth media were used within 3 days of preparation.

## Filtration measurements

The air-filtration properties were measured using an integrated self-designed setup that allows for the simultaneous sampling of pressure drop and particle and bacterial removal efficiency. For these experiments, samples of 68 × 68 × 3 and 68 × 68 × 4 mm were mounted to a self-designed nozzle system enabling isokinetic sampling of both pollutants (Figure S1) and placed in an air duct of 0.297 × 0.297 m. Additional airflow needed to equalize the fluid velocity in the air duct and the nozzle system was supplied by a rotary pump connected to the nozzle outlet. The airflow rate in the duct was set to 60 m³ h⁻¹. All measurements were performed at room temperature and a relative air humidity of 50%. The pressure drop over the sample was measured for various flow rates using an LPX 5481 differential pressure transducer (Druck SA, Leicester, UK) with a range of 0–2,000 Pa. A suspension of 10⁵ CFU mL⁻¹ of *Bacillus subtilis* in an isotonic NaCl solution was nebulized using an AGK 2000 (Palas, Germany) with a complementary pressurized air dryer. The aerosol generation pressure was set to 2.4 bar. For particle sampling the airflow in the reference and sampling nozzle was set to 1.2 and 2.6 m³ h⁻¹, respectively. This equates to a sampling fluid velocity of 0.17 m s⁻¹. Particle counting was performed utilizing a Welas aerosol spectrometer (Palas, Germany). The air was sampled at 5 L min⁻¹ for 1 min. Four particle count measurements upstream were performed alternating with three measurements downstream. The particle separation efficiency *PFE* for each particle size was calculated from the particle counts upstream before (*C*<sub>Up i</sub>) and after (*C*<sub>Up i+1</sub>) a downstream measurement compared with the respective downstream particle count (*C*<sub>Down i</sub>), using Equation 1:

$$PFE = 1 - \frac{2 * C_{\text{Down } i}}{(C_{\text{Up } i} + C_{\text{Up } i+1})} * 100. \quad (\text{Equation 1})$$

The ultimately reported particle separation efficiency and its error are the mean and standard deviation of the three *PFE* values.

Reference measurements on the particle removal efficiency and pressure drop of the ZnO base network were commissioned at the Institute für Luft- und Klimatechnik (Dresden, Germany).

The bacterial filtration efficiency was measured using a six-stage Andersen cascade impactor, TE-10-800 (Tisch Environmental, USA). While 1.3 m³ h⁻¹ of additional airflow was supplied in the sampling nozzle, no additional airflow was needed for the reference nozzle. All six stages of the Andersen impactor were loaded with growth medium prepared as described above. The airstream was sampled upstream and downstream three times each for 1.5 min at 28.3 L min⁻¹. The Petri dishes were then incubated for 16 h at 37°C. The CFU on the dishes were counted manually before being positive-hole corrected using a table published by Macher.³¹ The bacterial separation efficiency *BFE* was calculated from the ratio of the total CFU counts downstream and upstream:

$$BFE = 1 - \frac{\sum CFU_{\text{Down } i}}{\sum CFU_{\text{Up } i}} * 100. \quad (\text{Equation 2})$$

The mean and standard deviation of the three *BFE* values are reported as the bacterial separation efficiency and its error.

## VOC adsorption measurements

Toluene sorption experiments were conducted on cylindrical samples of 6 mm diameter and 3 mm height. The samples were analyzed using a DVS Advantage gravimetric sorption analyzer (Surface Measurement Systems, UK). All data were collected at 25°C with nitrogen as the carrier gas and an overall gas flow rate of 100 sccm min⁻¹.

## Investigation of homogeneous heating

IR images and temperature data were recorded using an IR camera (HD VarioCAM, InfraTec, Germany). An EA PS2042-10B (Elektro-Automatik, Germany) was used as the power supply. A filter unit employing a 68 × 68 × 4 mm sample was primed by joule heating with 26 V (57 W) constantly for 2 min. The unit was then left to cool down to 35°C. Afterward, the unit was joule heated constantly with different powers for 1 min, being left to cool back to 35°C between each experiment. IR images were continuously recorded over the heating period and for the first 30 s of cooling. The temperature response was measured from 21 V (35 W) to 27 V (62 W) in 1 V increments.

Evaluation of the temperature data was performed using the thermography software IRBIS 3.1 (Infratec, Germany). A histogram from the temperature data was generated using a self-written Python program.

## Setup for rapid joule heating

Further measurements on heating of the GeFM were performed using a self-designed rapid heating setup. For temperature measurements, a high-speed Metis H318 pyrometer (SensorTherm, Germany) was used. An EA PS2042-10B was used as power supply. Data recording and setup control were performed by an USB2537 High-Speed DAQ Board (Measurement Computing, USA) in combination with a self-programmed Python software.

## Investigation of long-term stability

To show cycling stability of the material, a sample of 20 × 20 × 4 mm was primed by constantly joule heating it with 9.0 W, 8.3 V, for 2 min. The sample was then left to cool for 1 min. Afterward, the sample was joule heated at ~300°C to 9.4 W, 8.2 V, for 1 min with a subsequent cooling period of 1 min as well. This cycle was repeated 30 times. To prove the long-term temperature stability of the material, a sample of 20 × 20 × 4 mm was joule heated to ~280°C with 7.9 W, 10.90 V, constantly for 10 h.

Samples of 68 × 68 × 4 mm were mounted into the air-handling unit and connected to an electrical power source. The initial particle removal efficiency, bacterial removal efficiency, and pressure drop were measured as described above. The air temperature and relative air humidity were 27°C and 70%, respectively. The sample was then heated with 61 W for 1 min with a subsequent cooling period of 3 min. The cycle was repeated a total of 50 times. After the procedure, particle removal efficiency, bacterial removal efficiency, and pressure drop were measured again.

## Self-cleaning experiments

Samples of 68 × 68 × 4 mm were mounted into the air-handling unit and connected to an electrical power source. The initial particle removal efficiency, bacterial removal efficiency, and pressure drop were measured as described
t-ZnO for the respective target geometry was weighed and molded into shape. The samples were then sintered at 1,150°C for 5 h.

The ZnO networks were then infiltrated by a dispersion of graphene in deionized water. A dispersion of electrochemically exfoliated graphene was prepared as described elsewhere.³⁰ The dispersion was kindly provided by Sixonia Tech (Dresden, Germany) and used without further modification. The dispersion was diluted with deionized water to 1.4 mg mL⁻¹ and tip sonicated with 3 s pulses for a total of 10 min and 13.16 kJ using a Sonoplus HD4100 (Bandelin, Germany). The dispersion was then drop infiltrated into the samples until the free volume was filled. The samples were then dried for 4 h at 50°C. The procedure was repeated a total of five times.

The samples were mounted to 3D-printed sample holders and electrically conducted using brass rods and conductive silver paste (ACHESON 1415, Plano, Germany). The array was then sealed using high-temperature-resistant silicone (Pattex Ofen und Kamin Spezial Silikon, Henkel, Germany).

## Preparation of *Bacillus subtilis* spore suspension

*Bacillus subtilis* spores were selected as test germs for their high resistance to thermal stress. *Bacillus subtilis subtilis* (CIP 52.65, CRBIP Institut Pasteur, France) was cultivated in double-strength Schaeffer sporulation medium for 4 days at 37°C. The spores were extracted from the medium by centrifugation at 8,000 rpm, 7,585g. The spores were then washed with deionized water and stored at 80°C for 10 min. Using such prepared spores, a suspension of 10⁵ CFU mL⁻¹ in 9 g L⁻¹ NaCl (Fisher Chemical, Germany) solution was prepared for the air-filtration experiments. An additional suspension of 10⁶ CFU mL⁻¹ was prepared for self-cleaning experiments.

Growth media were prepared using 20 g L⁻¹ nutrient broth (10 g L⁻¹ Tryp-tone, 5 g L⁻¹ meat extract, 5 g L⁻¹ NaCl; Biokar Diagnostics, France) and 12 g L⁻¹ bacteriological agar type E (Biokar Diagnostics, France). The media were autoclaved at 121°C for 15 min, and 20 mL of each medium was poured into Petri dishes and left to cool before being stored for later use. The growth media were used within 3 days of preparation.

## Filtration measurements

The air-filtration properties were measured using an integrated self-designed setup that allows for the simultaneous sampling of pressure drop and particle and bacterial removal efficiency. For these experiments, samples of 68 × 68 × 3 and 68 × 68 × 4 mm were mounted to a self-designed nozzle system enabling isokinetic sampling of both pollutants (Figure S1) and placed in an air duct of 0.297 × 0.297 m. Additional airflow needed to equalize the fluid velocity in the air duct and the nozzle system was supplied by a rotary pump connected to the nozzle outlet. The airflow rate in the duct was set to 60 m³ h⁻¹. All measurements were performed at room temperature and a relative air humidity of 50%. The pressure drop over the sample was measured for various flow rates using an LPX 5481 differential pressure transducer (Druck SA, Leicester, UK) with a range of 0–2,000 Pa. A suspension of 10⁵ CFU mL⁻¹ of *Bacillus subtilis* in an isotonic NaCl solution was nebulized using an AGK 2000 (Palas, Germany) with a complementary pressurized air dryer. The aerosol generation pressure was set to 2.4 bar. For particle sampling the airflow in the reference and sampling nozzle was set to 1.2 and 2.6 m³ h⁻¹, respectively. This equates to a sampling fluid velocity of 0.17 m s⁻¹. Particle counting was performed utilizing a Welas aerosol spectrometer (Palas, Germany). The air was sampled at 5 L min⁻¹ for 1 min. Four particle count measurements upstream were performed alternating with three measurements downstream. The particle separation efficiency *PFE* for each particle size was calculated from the particle counts upstream before (*C*<sub>Up i</sub>) and after (*C*<sub>Up i+1</sub>) a downstream measurement compared with the respective downstream particle count (*C*<sub>Down i</sub>), using Equation 1:

$$PFE = 1 - \frac{2 * C_{\text{Down } i}}{(C_{\text{Up } i} + C_{\text{Up } i+1})} * 100. \quad (\text{Equation 1})$$

The ultimately reported particle separation efficiency and its error are the mean and standard deviation of the three *PFE* values.

Reference measurements on the particle removal efficiency and pressure drop of the ZnO base network were commissioned at the Institute für Luft- und Klimatechnik (Dresden, Germany).

The bacterial filtration efficiency was measured using a six-stage Andersen cascade impactor, TE-10-800 (Tisch Environmental, USA). While 1.3 m³ h⁻¹ of additional airflow was supplied in the sampling nozzle, no additional airflow was needed for the reference nozzle. All six stages of the Andersen impactor were loaded with growth medium prepared as described above. The airstream was sampled upstream and downstream three times each for 1.5 min at 28.3 L min⁻¹. The Petri dishes were then incubated for 16 h at 37°C. The CFU on the dishes were counted manually before being positive-hole corrected using a table published by Macher.³¹ The bacterial separation efficiency *BFE* was calculated from the ratio of the total CFU counts downstream and upstream:

$$BFE = 1 - \frac{\sum CFU_{\text{Down } i}}{\sum CFU_{\text{Up } i}} * 100. \quad (\text{Equation 2})$$

The mean and standard deviation of the three *BFE* values are reported as the bacterial separation efficiency and its error.

## VOC adsorption measurements

Toluene sorption experiments were conducted on cylindrical samples of 6 mm diameter and 3 mm height. The samples were analyzed using a DVS Advantage gravimetric sorption analyzer (Surface Measurement Systems, UK). All data were collected at 25°C with nitrogen as the carrier gas and an overall gas flow rate of 100 sccm min⁻¹.

## Investigation of homogeneous heating

IR images and temperature data were recorded using an IR camera (HD VarioCAM, InfraTec, Germany). An EA PS2042-10B (Elektro-Automatik, Germany) was used as the power supply. A filter unit employing a 68 × 68 × 4 mm sample was primed by joule heating with 26 V (57 W) constantly for 2 min. The unit was then left to cool down to 35°C. Afterward, the unit was joule heated constantly with different powers for 1 min, being left to cool back to 35°C between each experiment. IR images were continuously recorded over the heating period and for the first 30 s of cooling. The temperature response was measured from 21 V (35 W) to 27 V (62 W) in 1 V increments.

Evaluation of the temperature data was performed using the thermography software IRBIS 3.1 (Infratec, Germany). A histogram from the temperature data was generated using a self-written Python program.

## Setup for rapid joule heating

Further measurements on heating of the GeFM were performed using a self-designed rapid heating setup. For temperature measurements, a high-speed Metis H318 pyrometer (SensorTherm, Germany) was used. An EA PS2042-10B was used as power supply. Data recording and setup control were performed by an USB2537 High-Speed DAQ Board (Measurement Computing, USA) in combination with a self-programmed Python software.

## Investigation of long-term stability

To show cycling stability of the material, a sample of 20 × 20 × 4 mm was primed by constantly joule heating it with 9.0 W, 8.3 V, for 2 min. The sample was then left to cool for 1 min. Afterward, the sample was joule heated at ~300°C to 9.4 W, 8.2 V, for 1 min with a subsequent cooling period of 1 min as well. This cycle was repeated 30 times. To prove the long-term temperature stability of the material, a sample of 20 × 20 × 4 mm was joule heated to ~280°C with 7.9 W, 10.90 V, constantly for 10 h.

Samples of 68 × 68 × 4 mm were mounted into the air-handling unit and connected to an electrical power source. The initial particle removal efficiency, bacterial removal efficiency, and pressure drop were measured as described above. The air temperature and relative air humidity were 27°C and 70%, respectively. The sample was then heated with 61 W for 1 min with a subsequent cooling period of 3 min. The cycle was repeated a total of 50 times. After the procedure, particle removal efficiency, bacterial removal efficiency, and pressure drop were measured again.

## Self-cleaning experiments

Samples of 68 × 68 × 4 mm were mounted into the air-handling unit and connected to an electrical power source. The initial particle removal efficiency, bacterial removal efficiency, and pressure drop were measured as described
above. The nebulized suspension was changed to have a bacterial concentra-
tion of 10⁶ CFU mL⁻¹. The sample was loaded with particle aerosol for 2 h and
subsequently joule heated with 61 W for 1 min. After several minutes of cool-
ing, the pressure drop over the sample was measured. The cycle was repeated
a total of three times. The bacterial suspension was now changed back to hav-
ing 10⁵ CFU mL⁻¹. Particle and bacterial removal efficiency were re-measured
as described previously. For the reference, the heating cycles were skipped.
The samples were dismounted and placed in a drying furnace at 100°C for
10 min to eliminate potential contamination. Afterward, the sample was
removed from its holder. Four grams of the sample was crushed with a spatula
and placed in 20 mL of a solution of 0.01 M MgSO₄ (Acros Organics, Belgium)
and 0.25% Tween 20 (Fisher Chemical, Germany) in deionized water. The sus-
pension was shaken at 150 rpm for 1 h. One hundred microliters of the suspen-
sion was then sampled on growth medium. After 16 h of incubation at 37°C, the
colony-forming units were counted manually.

Demonstrating airflow monitoring

Samples of 20 × 20 × 4 and 20 × 20 × 3 mm were mounted to a sample holder with an aperture of 16 mm diameter. The experiment was performed using the heating setup described above. The setup was integrated into the measurement setup shown in Figure S6. The airflow rate was measured using an SFTE-10U-Q4-B-2.5K gas flow transmitter (Festo, Germany).

To increase the airflow passing through the material, the temperature response to a power pulse of 48 W, 25 V, for 3 mm and 44 W, 22 V, for 4 mm was recorded. The pulse length was set to 1.25 s. The investigated range of fluid velocities was 0–0.27 cm s⁻¹.

Demonstrating load status monitoring

The high-speed heating setup was incorporated into a self-designed dust-
loading setup. Pressure drop over the sample was measured using a PDA
1L differential manometer (PCE Deutschland, Germany). Dust could be
dispersed into the airflow by a self-designed Venturi nozzle (see Figure S9).
Samples of 68 × 68 × 4 mm were weighed and then mounted into the setup.
Airflow through the setup was set to 0.2 m s⁻¹. The samples were then primed at
26 V for 2 min with subsequent cooling before the temperature response to a
pulse of ~50 W, 25 V, for 1 min was measured. Afterward, 2 g of SiO₂ ISO
12103 A2 fine dust (98% SiO₂, 5 µm; DMT, Germany) was dispersed using
the Venturi nozzle operating at 2 bar. The sample was now dismounted and
weighed again. The process of measuring the temperature response, loading,
and weighing was repeated several times.

Electron microscopy

SEM images of the samples were taken using a Zeiss Supra 55VP with an in-
lens detector.

SUPPLEMENTAL INFORMATION

Supplemental information can be found online at https://doi.org/10.1016/j.
device.2023.100098.

ACKNOWLEDGMENTS

We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG)
under contract AD 183/12. This project has received funding from the Euro-
pean Union's Horizon 2020 Research and Innovation Programme under grant
agreement No GrapheneCore3 881603 in the framework of the spearhead
project "AEROGrAFT" - Next-Generation Aerospace Filtration. We would
also like to thank Dr. Martin Lohe from Sixonia Tech GmbH for providing addi-
tional exfoliated graphene dispersion used in this work.

AUTHOR CONTRIBUTIONS

The GeFM concept was envisioned by F.S., A.S.N., and R.A., as well as X.F.
These authors also secured the necessary funding. The required resources
were provided by R.A., Y.A., X.F., and L.W. The undertaking was supervised
by F.S., A.B., R.A., A.S.N., X.F., and S.K. Project administration was handled

by A.R., F.S., M.M., and R.A. The methodology for this work was developed
by A.R., E.G., F.S., M.M., A.B., R.A., Y.A., and A.S.N., as well as X.F. Any
custom software was programmed by A.R., S.K., and E.G. The investigations
were carried out by A.R., E.G., M.M., and L.M.S. The acquired data were
curated by A.R., E.G., and F.S. before being validated by A.R., A.B., E.G.,
L.W., and Y.A. Formal analysis was performed by A.R., E.G., A.B., F.S.,
R.A., and L.W. The results were visualized by A.R., E.G., F.S., and M.M. The
first draft was devised by A.R. in cooperation with F.S. All of the authors
contributed to reviewing and editing the manuscript.

DECLARATION OF INTERESTS

The authors declare no competing interests.

INCLUSION AND DIVERSITY

One or more of the authors of this paper self-identifies as a gender minority in
their field of research.

Received: June 23, 2023

Revised: June 29, 2023

Accepted: September 12, 2023

Published: October 10, 2023

REFERENCES

1. Jerrett, M. (2015). Atmospheric science: The death toll from air-pollution sources. Nature 525, 330-331. https://doi.org/10.1038/525330a.

2. Fernstrom, A., and Goldblatt, M. (2013). Aerobiology and its role in the transmission of infectious diseases. J. Pathog. 2013, 493960. https://doi.org/10.1155/2013/493960.

3. Han, S., Kim, J., and Ko, S.H. (2021). Advances in air filtration technologies: structure-based and interaction-based approaches. Materials Today Advances 9, 100134. https://doi.org/10.1016/j.mtadv.2021.100134.

4. Wang, Z., Pan, Z., Wang, J., and Zhao, R. (2016). A Novel Hierarchical Structured Poly(lactic acid)/Titania Fibrous Membrane with Excellent Antibacterial Activity and Air Filtration Performance. J. Nanomater. 2016, 1-17. https://doi.org/10.1155/2016/6272983.

5. Al-Attabi, R., Morsi, Y., Kujawski, W., Kong, L., Schütz, J.A., and Dumée, L.F. (2019). Wrinkled silica doped electrospun nano-fiber membranes with engineered roughness for advanced aerosol air filtration. Separ. Purif. Technol. 215, 500-507. https://doi.org/10.1016/j.seppur.2019.01.049.

6. Wang, W., Wang, Z., Liu, Z., Zhu, Z., Zhu, F., Sun, X., Lam, T.P., Cheng, J.C.y., and Qiu, Y. (2015). Preparation of hierarchical structured nano-sized/porous poly(lactic acid) composite fibrous membranes for air filtration. Eur. Spine J. 24, 1168-1174. https://doi.org/10.1016/j.apsusc.2015.08.211.

7. Zhang, S., Liu, H., Tang, N., Zhou, S., Yu, J., and Ding, B. (2020). Spider-Web-Inspired PM 0.3 Filters Based on Self-Sustained Electrostatic Nano-structured Networks. Adv. Mater. 32, 2002361. https://doi.org/10.1002/adma.202002361.

8. Mei, Y., Wang, Z., and Li, X. (2013). Improving filtration performance of electrospun nanofiber mats by a bimodal method. J. Appl. Polym. Sci. 128, 1089-1094. https://doi.org/10.1002/app.38296.

9. Liu, H., Zhang, S., Liu, L., Yu, J., and Ding, B. (2019). A Fluffy Dual-Network Structured Nanofiber/Net Filter Enables High-Efficiency Air Filtration. Adv. Funct. Mater. 29, 1904108. https://doi.org/10.1002/adfm.201904108.

10. Thomas, D., Contal, P., Renaudin, V., Penicot, P., Leclerc, D., and Vendel, J. (1999). Modelling pressure drop in hepa filters during dynamic filtration. J. Aerosol Sci. 30, 235-246. https://doi.org/10.1016/S0021-8502(98)00036-6.

11. Da Roza, R.A. (1982). Particle size for greatest penetration of HEPA filters - and their true efficiency. https://doi.org/10.2172/6241348.
above. The nebulized suspension was changed to have a bacterial concentra-
tion of 10⁶ CFU mL⁻¹. The sample was loaded with particle aerosol for 2 h and
subsequently joule heated with 61 W for 1 min. After several minutes of cool-
ing, the pressure drop over the sample was measured. The cycle was repeated
a total of three times. The bacterial suspension was now changed back to hav-
ing 10⁵ CFU mL⁻¹. Particle and bacterial removal efficiency were re-measured
as described previously. For the reference, the heating cycles were skipped.
The samples were dismounted and placed in a drying furnace at 100°C for
10 min to eliminate potential contamination. Afterward, the sample was
removed from its holder. Four grams of the sample was crushed with a spatula
and placed in 20 mL of a solution of 0.01 M MgSO₄ (Acros Organics, Belgium)
and 0.25% Tween 20 (Fisher Chemical, Germany) in deionized water. The sus-
pension was shaken at 150 rpm for 1 h. One hundred microliters of the suspen-
sion was then sampled on growth medium. After 16 h of incubation at 37°C, the
colony-forming units were counted manually.

Demonstrating airflow monitoring

Samples of 20 × 20 × 4 and 20 × 20 × 3 mm were mounted to a sample holder with an aperture of 16 mm diameter. The experiment was performed using the heating setup described above. The setup was integrated into the measurement setup shown in Figure S6. The airflow rate was measured using an SFTE-10U-Q4-B-2.5K gas flow transmitter (Festo, Germany).

To increase the airflow passing through the material, the temperature response to a power pulse of 48 W, 25 V, for 3 mm and 44 W, 22 V, for 4 mm was recorded. The pulse length was set to 1.25 s. The investigated range of fluid velocities was 0–0.27 cm s⁻¹.

Demonstrating load status monitoring

The high-speed heating setup was incorporated into a self-designed dust-
loading setup. Pressure drop over the sample was measured using a PDA
1L differential manometer (PCE Deutschland, Germany). Dust could be
dispersed into the airflow by a self-designed Venturi nozzle (see Figure S9).
Samples of 68 × 68 × 4 mm were weighed and then mounted into the setup.
Airflow through the setup was set to 0.2 m s⁻¹. The samples were then primed at
26 V for 2 min with subsequent cooling before the temperature response to a
pulse of ~50 W, 25 V, for 1 min was measured. Afterward, 2 g of SiO₂ ISO
12103 A2 fine dust (98% SiO₂, 5 µm; DMT, Germany) was dispersed using
the Venturi nozzle operating at 2 bar. The sample was now dismounted and
weighed again. The process of measuring the temperature response, loading,
and weighing was repeated several times.

Electron microscopy

SEM images of the samples were taken using a Zeiss Supra 55VP with an in-
lens detector.

SUPPLEMENTAL INFORMATION

Supplemental information can be found online at https://doi.org/10.1016/j.
device.2023.100098.

ACKNOWLEDGMENTS

We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) under contract AD 183/12. This project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement No GrapheneCore3 881603 in the framework of the spearhead project "AEROGrAFT" - Next-Generation Aerospace Filtration. We would also like to thank Dr. Martin Lohe from Sixonia Tech GmbH for providing additional exfoliated graphene dispersion used in this work.

AUTHOR CONTRIBUTIONS

The GeFM concept was envisioned by F.S., A.S.N., and R.A., as well as X.F.
These authors also secured the necessary funding. The required resources
were provided by R.A., Y.A., X.F., and L.W. The undertaking was supervised
by F.S., A.B., R.A., A.S.N., X.F., and S.K. Project administration was handled

by A.R., F.S., M.M., and R.A. The methodology for this work was developed
by A.R., E.G., F.S., M.M., A.B., R.A., Y.A., and A.S.N., as well as X.F. Any
custom software was programmed by A.R., S.K., and E.G. The investigations
were carried out by A.R., E.G., M.M., and L.M.S. The acquired data were
curated by A.R., E.G., and F.S. before being validated by A.R., A.B., E.G.,
L.W., and Y.A. Formal analysis was performed by A.R., E.G., A.B., F.S.,
R.A., and L.W. The results were visualized by A.R., E.G., F.S., and M.M. The
first draft was devised by A.R. in cooperation with F.S. All of the authors
contributed to reviewing and editing the manuscript.

DECLARATION OF INTERESTS

The authors declare no competing interests.

INCLUSION AND DIVERSITY

One or more of the authors of this paper self-identifies as a gender minority in
their field of research.

Received: June 23, 2023

Revised: June 29, 2023

Accepted: September 12, 2023

Published: October 10, 2023

REFERENCES

1. Jerrett, M. (2015). Atmospheric science: The death toll from air-pollution sources. Nature 525, 330-331. https://doi.org/10.1038/525330a.

2. Fernstrom, A., and Goldblatt, M. (2013). Aerobiology and its role in the transmission of infectious diseases. J. Pathog. 2013, 493960. https://doi.org/10.1155/2013/493960.

3. Han, S., Kim, J., and Ko, S.H. (2021). Advances in air filtration technologies: structure-based and interaction-based approaches. Materials Today Advances 9, 100134. https://doi.org/10.1016/j.mtadv.2021.100134.

4. Wang, Z., Pan, Z., Wang, J., and Zhao, R. (2016). A Novel Hierarchical Structured Poly(lactic acid)/Titania Fibrous Membrane with Excellent Antibacterial Activity and Air Filtration Performance. J. Nanomater. 2016, 1-17. https://doi.org/10.1155/2016/6272983.

5. Al-Attabi, R., Morsi, Y., Kujawski, W., Kong, L., Schütz, J.A., and Dumée, L.F. (2019). Wrinkled silica doped electrospun nano-fiber membranes with engineered roughness for advanced aerosol air filtration. Separ. Purif. Technol. 215, 500-507. https://doi.org/10.1016/j.seppur.2019.01.049.

6. Wang, W., Wang, Z., Liu, Z., Zhu, Z., Zhu, F., Sun, X., Lam, T.P., Cheng, J.C.y., and Qiu, Y. (2015). Preparation of hierarchical structured nano-sized/porous poly(lactic acid) composite fibrous membranes for air filtration. Eur. Spine J. 24, 1168-1174. https://doi.org/10.1016/j.apsusc.2015.08.211.

7. Zhang, S., Liu, H., Tang, N., Zhou, S., Yu, J., and Ding, B. (2020). Spider-Web-Inspired PM 0.3 Filters Based on Self-Sustained Electrostatic Nano-structured Networks. Adv. Mater. 32, 2002361. https://doi.org/10.1002/adma.202002361.

8. Mei, Y., Wang, Z., and Li, X. (2013). Improving filtration performance of electrospun nanofiber mats by a bimodal method. J. Appl. Polym. Sci. 128, 1089-1094. https://doi.org/10.1002/app.38296.

9. Liu, H., Zhang, S., Liu, L., Yu, J., and Ding, B. (2019). A Fluffy Dual-Network Structured Nanofiber/Net Filter Enables High-Efficiency Air Filtration. Adv. Funct. Mater. 29, 1904108. https://doi.org/10.1002/adfm.201904108.

10. Thomas, D., Contal, P., Renaudin, V., Penicot, P., Leclerc, D., and Vendel, J. (1999). Modelling pressure drop in hepa filters during dynamic filtration. J. Aerosol Sci. 30, 235-246. https://doi.org/10.1016/S0021-8502(98)00036-6.

11. Da Roza, R.A. (1982). Particle size for greatest penetration of HEPA filters - and their true efficiency. https://doi.org/10.2172/6241348.
12. Maus, R., Goppelsröder, A., and Umhauer, H. (2001). Survival of bacterial and mold spores in air filter media. Atmos. Environ. 35, 105–113. https://doi.org/10.1016/S1352-2310(00)00280-6.

13. Möritz, M., Peters, H., Nipko, B., and Rüden, H. (2001). Capability of air filters to retain airborne bacteria and molds in heating, ventilating and air-conditioning (HVAC) systems. Int. J. Hyg Environ. Health 203, 401–409. https://doi.org/10.1078/1438-4639-00054.

14. Joe, Y.H., Yoon, K.Y., and Hwang, J. (2014). Methodology for modeling the microbial contamination of air filters. PLoS One 9, e88514. https://doi.org/10.1371/journal.pone.0088514.

15. Bonnevie Perrier, J.C., Le Coq, L., Andrès, Y., and Le Cloirec, P. (2008). SFGP 2007 - Microbial Growth onto Filter Media Used in Air Treatment Devices. Int. J. Chem. React. Eng. 6. https://doi.org/10.2202/1542-6580.1675.

16. Beggs, C.B. (2003). The Airborne Transmission of Infection in Hospital Buildings: Fact or Fiction? Indoor Built Environ. 12, 9–18. https://doi.org/10.1177/1420326X03012001002.

17. Sehulster, L., and Chinn, R.Y.W.CDC; HICPAC (2003). Guidelines for environmental infection control in health-care facilities. Recommendations of CDC and the Healthcare Infection Control Practices Advisory Committee (HICPAC). MMWR. Recommendations and reports : Morbidity and mortality weekly report. MMWR Recomm. Rep. (Morb. Mortal. Wkly. Rep.) 52, 1–42.

18. Kollu, K., and Ormeci, B. (2012). Effect of particles and bioflocculation on ultraviolet disinfection of Escherichia coli. Water Res. 46, 750–760. https://doi.org/10.1016/j.watres.2011.11.046.

19. S. Lerouge and A. Simmons, eds. (2012). Sterilisation of biomaterials and medical devices (Woodhead Publishing).

20. Stanford, M.G., Li, J.T., Chen, Y., McHugh, E.A., Liopo, A., Xiao, H., and Tour, J.M. (2019). Self-Sterilizing Laser-Induced Graphene Bacterial Air Filter. ACS Nano 13, 11912–11920. https://doi.org/10.1021/acsnano.9b05983.

21. Gupta, A., Sharma, C.P., Thamaraiselvan, C., Pisharody, L., Powell, C.D., and Arnusch, C.J. (2021). Low-Voltage Bacterial and Viral Killing Using Laser-Induced Graphene-Coated Non-woven Air Filters. ACS Appl. Mater. Interfaces 13, 59373–59380. https://doi.org/10.1021/acsmaterials.1c20198.

22. Kim, Y.-I., Kim, M.-W., An, S., Yarin, A.L., and Yoon, S.S. (2021). Reusable Filters Augmented with Heating Microfibers for Antibacterial and Antiviral

Sterilization. ACS Appl. Mater. Interfaces 13, 857–867. https://doi.org/10.1021/acsmaterials.0c16471.

23. Mishra, Y.K., Modi, G., Cretu, V., Postica, V., Lupan, O., Reimer, T., Paulowicz, I., Hrkac, V., Benecke, W., Kienle, L., and Adelung, R. (2015). Direct Growth of Freestanding ZnO Tetrapod Networks for Multifunctional Applications in Photocatalysis, UV Photodetection, and Gas Sensing. ACS Appl. Mater. Interfaces 7, 14303–14316. https://doi.org/10.1021/acsmaterials.5b02816.

24. Rasch, F., Schütt, F., Saure, L.M., Kaps, S., Strobel, J., Polonskyi, O., Nia, A.S., Lohe, M.R., Mishra, Y.K., Faupel, F., et al. (2019). Wet-Chemical Assembly of 2D Nanomaterials into Lightweight, Microtube-Shaped, and Macroscopic 3D Networks. ACS Appl. Mater. Interfaces 11, 44652–44663. https://doi.org/10.1021/acsmaterials.9b16565.

25. Schütt, F., Rasch, F., Deka, N., Reimers, A., Saure, L.M., Kaps, S., Rank, J., Carstensen, J., Kumar Mishra, Y., Misseroni, D., et al. (2021). Electrically powered repeatable air explosions using microtubular graphene assemblies. Mater. Today 48, 7–17. https://doi.org/10.1016/j.mattod.2021.03.010.

26. Ricca, E., and Cutting, S.M. (2003). Emerging Applications of Bacterial Spores in Nanobiotechnology. J. Nanobiotechnol. 1, 6. https://doi.org/10.1186/1477-3155-1-6.

27. Fang, X.-Y., Yu, X.-X., Zheng, H.-M., Jin, H.-B., Wang, L., and Cao, M.-S. (2015). Temperature- and thickness-dependent electrical conductivity of few-layer graphene and graphene nanosheets. Phys. Lett. 379, 2245–2251. https://doi.org/10.1016/j.physleta.2015.06.063.

28. HEWLETT, R.T. (1940). A Textbook of Microbiology. Nature 146, 149. https://doi.org/10.1038/146149a0.

29. Mishra, Y.K., Kaps, S., Schuchardt, A., Paulowicz, I., Jin, X., Gedamu, D., Freitag, S., Claus, M., Wille, S., Kovalev, A., et al. (2013). Fabrication of Macroscopically Flexible and Highly Porous 3D Semiconductor Networks from Interpenetrating Nanostructures by a Simple Flame Transport Approach. Part. Part. Syst. Char. 30, 775–783. https://doi.org/10.1002/ppsc.201300197.

30. Vázquez, R.; Adrián Technische Universität Dresden (2022). Scaling up Electrochemical Exfoliation: In-Situ Functionalization and Processing of Graphene Materials.

31. Macher, J.M. (1989). Positive-hole correction of multiple-jet impactors for collecting viable microorganisms. Am. Ind. Hyg. Assoc. J. 50, 561–568. https://doi.org/10.1080/15298668991375164.
12. Maus, R., Goppelsröder, A., and Umhauer, H. (2001). Survival of bacterial and mold spores in air filter media. Atmos. Environ. 35, 105–113. https://doi.org/10.1016/S1352-2310(00)00280-6.

13. Möritz, M., Peters, H., Nipko, B., and Rüden, H. (2001). Capability of air filters to retain airborne bacteria and molds in heating, ventilating and air-conditioning (HVAC) systems. Int. J. Hyg Environ. Health 203, 401–409. https://doi.org/10.1078/1438-4639-00054.

14. Joe, Y.H., Yoon, K.Y., and Hwang, J. (2014). Methodology for modeling the microbial contamination of air filters. PLoS One 9, e88514. https://doi.org/10.1371/journal.pone.0088514.

15. Bonnevie Perrier, J.C., Le Coq, L., Andrès, Y., and Le Cloirec, P. (2008). SFGP 2007 - Microbial Growth onto Filter Media Used in Air Treatment Devices. Int. J. Chem. React. Eng. 6. https://doi.org/10.2202/1542-6580.1675.

16. Beggs, C.B. (2003). The Airborne Transmission of Infection in Hospital Buildings: Fact or Fiction? Indoor Built Environ. 12, 9–18. https://doi.org/10.1177/1420326X03012001002.

17. Sehulster, L., and Chinn, R.Y.W.CDC; HICPAC (2003). Guidelines for environmental infection control in health-care facilities. Recommendations of CDC and the Healthcare Infection Control Practices Advisory Committee (HICPAC). MMWR. Recommendations and reports : Morbidity and mortality weekly report. MMWR Recomm. Rep. (Morb. Mortal. Wkly. Rep.) 52, 1–42.

18. Kollu, K., and Ormeci, B. (2012). Effect of particles and bioflocculation on ultraviolet disinfection of Escherichia coli. Water Res. 46, 750–760. https://doi.org/10.1016/j.watres.2011.11.046.

19. S. Lerouge and A. Simmons, eds. (2012). Sterilisation of biomaterials and medical devices (Woodhead Publishing).

20. Stanford, M.G., Li, J.T., Chen, Y., McHugh, E.A., Liopo, A., Xiao, H., and Tour, J.M. (2019). Self-Sterilizing Laser-Induced Graphene Bacterial Air Filter. ACS Nano 13, 11912–11920. https://doi.org/10.1021/acsnano.9b05983.

21. Gupta, A., Sharma, C.P., Thamaraiselvan, C., Pisharody, L., Powell, C.D., and Arnusch, C.J. (2021). Low-Voltage Bacterial and Viral Killing Using Laser-Induced Graphene-Coated Non-woven Air Filters. ACS Appl. Mater. Interfaces 13, 59373–59380. https://doi.org/10.1021/acsami.1c20198.

22. Kim, Y.-I., Kim, M.-W., An, S., Yarin, A.L., and Yoon, S.S. (2021). Reusable Filters Augmented with Heating Microfibers for Antibacterial and Antiviral

Sterilization. ACS Appl. Mater. Interfaces 13, 857–867. https://doi.org/10.1021/acsami.0c16471.

23. Mishra, Y.K., Modi, G., Cretu, V., Postica, V., Lupan, O., Reimer, T., Paulowicz, I., Hrkac, V., Benecke, W., Kienle, L., and Adelung, R. (2015). Direct Growth of Freestanding ZnO Tetrapod Networks for Multifunctional Applications in Photocatalysis, UV Photodetection, and Gas Sensing. ACS Appl. Mater. Interfaces 7, 14303–14316. https://doi.org/10.1021/acsami.5b02816.

24. Rasch, F., Schütt, F., Saure, L.M., Kaps, S., Strobel, J., Polonskyi, O., Nia, A.S., Lohe, M.R., Mishra, Y.K., Faupel, F., et al. (2019). Wet-Chemical Assembly of 2D Nanomaterials into Lightweight, Microtube-Shaped, and Macroscopic 3D Networks. ACS Appl. Mater. Interfaces 11, 44652–44663. https://doi.org/10.1021/acsami.9b16565.

25. Schütt, F., Rasch, F., Deka, N., Reimers, A., Saure, L.M., Kaps, S., Rank, J., Carstensen, J., Kumar Mishra, Y., Misseroni, D., et al. (2021). Electrically powered repeatable air explosions using microtubular graphene assemblies. Mater. Today 48, 7–17. https://doi.org/10.1016/j.mattod.2021.03.010.

26. Ricca, E., and Cutting, S.M. (2003). Emerging Applications of Bacterial Spores in Nanobiotechnology. J. Nanobiotechnol. 1, 6. https://doi.org/10.1186/1477-3155-1-6.

27. Fang, X.-Y., Yu, X.-X., Zheng, H.-M., Jin, H.-B., Wang, L., and Cao, M.-S. (2015). Temperature- and thickness-dependent electrical conductivity of few-layer graphene and graphene nanosheets. Phys. Lett. 379, 2245–2251. https://doi.org/10.1016/j.physleta.2015.06.063.

28. HEWLETT, R.T. (1940). A Textbook of Microbiology. Nature 146, 149. https://doi.org/10.1038/146149a0.

29. Mishra, Y.K., Kaps, S., Schuchardt, A., Paulowicz, I., Jin, X., Gedamu, D., Freitag, S., Claus, M., Wille, S., Kovalev, A., et al. (2013). Fabrication of Macroscopically Flexible and Highly Porous 3D Semiconductor Networks from Interpenetrating Nanostructures by a Simple Flame Transport Approach. Part. Part. Syst. Char. 30, 775–783. https://doi.org/10.1002/ppsc.201300197.

30. Vázquez, R.; Adrián Technische Universität Dresden (2022). Scaling up Electrochemical Exfoliation: In-Situ Functionalization and Processing of Graphene Materials.

31. Macher, J.M. (1989). Positive-hole correction of multiple-jet impactors for collecting viable microorganisms. Am. Ind. Hyg. Assoc. J. 50, 561–568. https://doi.org/10.1080/15298668991375164.