key: cord-0824854-0619nahq
authors: Singh, Seema; Shauloff, Nitzan; Prakash Sharma, Chetan; Shimoni, Ran; Arnusch, Christopher J.; Jelinek, Raz
title: Carbon Dot-Polymer Nanoporous Membrane for Recyclable Sunlight-Sterilized Facemasks
date: 2021-02-26
journal: J Colloid Interface Sci
DOI: 10.1016/j.jcis.2021.02.049
sha: 93ab60e8d3d81ab0fe7e0c8ce1a37696c781666e
doc_id: 824854
cord_uid: 0619nahq

Facemasks are considered the most effective means for preventing infection and spread of viral particles. In particular, the coronavirus (COVID-19) pandemic underscores the urgent need for developing recyclable facemasks due to the considerable environmental damage and health risks imposed by disposable masks and respirators. We demonstrate synthesis of nanoporous membranes comprising carbon dots (C-dots) and poly(vinylidene fluoride) (PVDF), and demonstrate their potential use for recyclable, self-sterilized facemasks. Notably, the composite C-dot-PVDF films exhibit hydrophobic surface which prevents moisture accumulation and a compact nanopore network which allows both breathability as well as effective filtration of particles above 100 nm in diameter. Particularly important, self-sterilization occurs upon short solar irradiation of the membrane, as the embedded C-dots efficiently absorb visible light, concurrently giving rise to elevated temperatures through heat dissipation.

The COVID-19 pandemic started in December 2019 and rapidly spread globally. The virus is believed to be transmitted by respiratory droplets through sneezing, coughing and talking. [1] , [2] Those droplets vary in size and the smaller aerosol can float in air for extended time periods. [3] Facial masks combine aerosol filters and/or fabrics, designed to block the droplets and associated viral particles from entering the human body thus preventing infection. [4] , [5] However, fabrication of enormous quantities of facemasks which are, by and large, non-reusable, has resulted in enormous hazardous waste and adverse environmental impact. [6] To address this, chemical disinfectants and incineration are being used for eliminating facemask waste, which in turn induce further environmental harm through, for example, toxic gas release. [7] In an attempt to overcome the hazards associated with non-reusable masks, the World Health Organization (WHO) recommended the use of cloth masks. [8] However, research has shown limited effectiveness of cloth-made facemasks for blocking the spread of infectious diseases. [9] , [10] In particular, cloth materials offer limited protection against nanometer-scale viral particles which can easily pass through. [11] , [12] Furthermore, viral particles might adhere to cloth fibers thereby increasing risk of infection. [10] As such, development of reusable, environmentally friendly facemasks which can effectively block viral transmission is considered a major goal.

Sunlight-mediated sterilization of facemasks has gained interest as a promising avenue for recyclability. [13] , [14] Photothermal materials used in potential facemask applications include graphene coated on surgical masks 13 and silver nanoparticles deposited upon N95 masks 14 . These materials, however, are difficult to fabricate and cumbersome for practical use. Similarly, various antiviral and antibacterial masks have been developed by coating conventional facemask materials with chemical agents. [15] , [16] However, use of such disinfectants in facial mask is undesirable due to possible physiological toxicity and environmental pollution. [16] In this work, we employed for the first time carbon dots (C-dots) as photothermal agents in nanoporous polymer-based sunlight recyclable viral-blocking matrix. C-dots are a unique class of carbon nanoparticles, synthesized from readily available carbonaceous building blocks, exhibiting remarkable optical properties, biocompatibility, and diverse biological and chemical applications. [17] [18] [19] In particular, the excellent photothermal properties of C-dots have been recently employed in solar-enabled water remediation [20] , optical-switching [21] and oil-spill cleanup applications [22] . Here, we report construction of a composite nanoporous membrane comprising C-dots and polyvinylidene fluoride (PVDF) via a simple mixed solvent phase separation scheme. PVDF has been extensively used in varied applications due to its stability and resilience, mechanical strength and thermal stability. [23] PVDF membranes have been utilized in water treatment [24] , gas separation [25] , lithium ion batteries [26] , pollutant removal [27] and oil/water separation [28] . The nanoporous C-dot-PVDF membrane displays hydrophobicity, air permeability, effective nanoparticle filtration, and, particularly important, solar-induced sterilization through sunlight absorbance and concomitant heat dissipation afforded by the embedded C-dots. The new C-dot-PVDF membrane constitutes an inexpensive, readily prepared, environmentally friendly, reusable and self-sterilizing platform which may be employed in viral-blocking facemasks and respirators.

Citric acid and urea, ACS reagents ≥ 99.5%, were purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO), purity ≥ 99.98%, was obtained from Glentham Life Sciences. N, N Dimethylformamide (DMF), AR ≥ 99.5% was purchased from Bio-Lab ltd. Israel. n-Octane (purity = 98%), polyvinylidene fluoride (PVDF) and ethanol (ACS reagent 96%) were supplied by Alfa Aesar.

C-dots were synthesized by hydrothermal heating of citric acid and urea as precursors according to a previously reported method. [29] Citric acid (2 g) and urea (6 g) were added and dissolved in 30 mL of DMSO (DMSO furnishes a lower energy level due to S-doping, thus reducing the optical bandgap and contributing to broad absorbance range in both visible and near IR regions [29] . The transparent solution obtained was then transferred to a Teflon-lined autoclave and heated at 160 o C for 4 h followed by cooling to room temperature. The black-brown C-dot suspension was mixed with excess of ethanol and centrifuged at 10,000 rpm for 15 min. On centrifugation, the precipitate of C-dots was obtained and dried.

The C-dot-PVDF membrane was synthesized through modification of a mixed solvent phase separation (MSPS) method. [30] PVDF (0.75 g), C-dots (0.0375 g), DMF (2.91 mL) and n-Octane (2.1 mL) were mixed and heated to 85 ᴼC for 24 h to get a homogenous casting solution. A thin film of C-dot-PVDF was casted on a preheated glass plate at 85 ᴼC by a doctor blade method. [31] The film was covered with a glass petri dish and left at room temperature for 90 s. This C-dot-PVDF on glass plate was immersed in room-temperature distilled water to detach the membrane from the glass substrate. The membrane was then kept in fresh distilled water for 12 h and subsequently in ethanol for another 12 h to remove solvent traces and unreacted loosely bound moieties. Finally, the membrane was dried in ambient conditions. Similarly, control PVDF membrane was prepared without addition of C-dots.

Single bacterial colonies Escherichia coli (E. Coli) extracted from Luria-Bertani (LB) agar plates were inoculated in 10 mL of LB broth and kept at 37 C o for 12 h in a shaking incubator (220 rpm).

The concentration of bacteria in the medium was obtained by measuring the optical density at 600 nm (OD 600). When the OD 600 reached 0.5, 50µl from the bacterial culture was placed on a the Cdot-PVDF membrane (1 cm diameter). This was followed by light irradiation at different durations (0, 5, 10 or 20 min). Each of the membranes was then immersed for 5 min in 250 µl LB solution.

Turbidity assays were carried out to evaluate bacterial growth after irradiation by adding the (PVDF) at a 0.05 weight ratio (C-dot : PVDF) in dimethylformamide (DMF) and n-octane. Nanoporous composite free-standing C-dot-PVDF films were formed via a mixed solvent phase separation (MSPS) methodology in which the solvent mixture was heated to 85ᴼC, cooled at room temperature leading to solvent separation, and subsequently deposited on a solid substrate. 20, 21 The resultant C-dot-PVDF matrix was stabilized through hydrogen bonding between -OH and -COOH units on the C-dots' surface and -CF 2 residues of PVDF. [22, 30, 31] The as-prepared C-dot-PVDF membrane could be readily attached to a commercially available cotton cloth providing effective thermal insulation. Importantly, as illustrated in Fig. 1 , heat dissipated through sunlight harvesting by the embedded C-dots can lead to destruction of viral particles, without the need to external sterilization procedures.

Covid-19 facemask. The C-dots and PVDF are initially dispersed in a DMF/n-octane mixture. A free-standing nanoporous film is formed through mixed solvent phase separation. Sunlight is absorbed by the film-embedded C-dots, resulting in heat dissipation which can be utilized for concomitant destruction of viral particles.

Physico-chemical characterization of the nanoporous C-dot-PVDF film is presented in Fig. 2 . The experiments outlined in Fig. 2 were specifically designed to investigate incorporation of the C-dots within the PVDF matrix and their impact upon the structural and functional properties of the polymer. (Detailed characterization of the C-dots is presented in Fig. S3 ). Fourier transform infrared (FTIR) spectra of the C-dots in Fig. 2A,i show the characteristic C-dots peaks corresponding to amine and hydroxyl residues in between 3200-3400 cm -1 and carbon-bonded oxygen and nitrogen between 1300-1600 cm -1 . [29] The FTIR spectrum of PVDF in Fig. 2A ,ii features major signals in 1190 cm -1 and 1230 cm -1 assigned to CF 2 residues and a broad peak around 1400 cm -1 corresponding to CH 2 units. [32] Importantly, the FTIR spectrum of the C-dot-PVDF composite in Fig. 2A ,iii reveals the distinctive peaks of the C-dots (see inset in Fig. 2A,iii; the arrows indicate the C-dot-specific signals), confirming incorporation of the C-dots within the PVDF host matrix. Water contact angle (WCA) measurements in Fig. 2B furnish evidence for modulation of PVDF properties through C-dot incorporation. The WCA of bare PVDF membrane (not containing C-dots) was 101 ± 1 ᴼ (Fig. 2B, i) accounting for the hydrophobic nature of the polymer. [23, 33] In the case of the composite C-dot-PVDF, however, the WCA was reduced to 94 ± 0.4 ᴼ (Fig. 2B,ii) corresponding to the presence of the more hydrophilic C-dots within the polymer matrix. Importantly, the WCA determined for C-dot-PVDF indicates that the composite membrane still retained hydrophobic surface properties, allowing for repulsion of water droplets which is essential for effective facemask materials. The scanning electron microscopy (SEM) image of the C-dot-PVDF film in Fig. 2C displays ubiquitous nanopores. The dense pore network is important for allowing efficient air flow through the membrane. Notably, the SEM analysis also reveals that pore diameters were below 100 nm (Fig. 2C , inset; size distribution determined from the SEM experimentation is presented in Fig. S4 ); this pore diameter threshold is crucial for effectively blocking transmission of COVID-19 viral particles (which are around 125 nm [34] ). Further, the pore size distribution of the membrane and total porosity were analyzed through Brunauer-Emmet-Teller (BET) and gravimetric method respectively. The average pore diameter of C-dot-PVDF membrane obtained through BET was found to be ~49 nm (Fig. S5 ) and the total porosity was estimated to be 72±2.4%. The stability of C-dots inside the membrane was also verified through immersing the membrane in water and spectroscopic analysis confirmed the negligible leaching of C-dots in water from C-dot-PVDF membrane (Fig. S6) .

A core functionality of the new C-dot-PVDF membrane is the feasibility of self-sterilization via light absorbance and heat dissipation by the C-dots. Fig. 3 examines the photothermal properties of the membranes, in particularly the critical role of the embedded C-dots in solar-mediated heating. The infrared (IR) thermal images in Fig. 3B ,i and corresponding temperature variation graph in Figure 3B ,ii demonstrate the significant heat dissipation and concurrent temperature increase in the C-dot-PVDF membrane. In the experiments presented in Fig. 3B , we illuminated bare PVDF and Cdot-PVDF membranes using a solar-simulator and recorded the temperature using an infrared (IR) thermograph camera. The thermal images in Fig. 3B ,i and corresponding recorded temperatures ( Fig. 3B ,ii) illustrate the dramatic temperature increase in the case of C-dot-PVDF membrane (from 28ᴼC to 66ᴼC after 20 min irradiation) while the temperature of the bare PVDF membrane rose from 27ᴼC to just 41ᴼC after the same irradiation time. The significant photothermal effect observed in the C-dot-PVDF membrane underscores the feasibility of self-sterilization since inactivation of Coronavirus has been reported at around 60ºC. [35, 36] . The on-off light irradiation experiment depicted in Figure 3C undercores the excellent recyclability of the photothermal effects. considered the recommended air flow rates for respirators. [37] The pressure drop across all film samples was found to increase in higher air flow rates. [38] Importantly, the pressure drop across the C-dot-PVDF was slightly higher than a surgical mask, and significantly lower than the standard N-95 respirator. The breathability of C-dot-PVDF membrane is ascribed to the highly porous Fig. 4 : Air permeability and nanoparticle filtration. A. Air permeability determined for the Cdot-PVDF membrane in comparison with commercial facemasks. Triplicates of each membrane sample were recorded B. Scanning electron microscopy (SEM) images of glass slides exposed to aerosolized silica nanoparticles, without the C-dot-PVDF membrane (control) and with the membrane blocking the aerosol (scheme showing the experimental setup is depicted in Fig. S2 ). i) control glass substrate without the membrane; the inset shows a magnified region highlighting the nanoparticles on the slide. ii) glass substrate for which the C-dot-PVDF membrane (thickness of 120 µm) was placed 10 mm above the surface. The bar diagram presents the percentage of nanoparticles per unit area on the slide, quantified from the SEM images. The results reflect calculations of 10 images from three glass slides after passing the aerosol nanoparticles with triplicate C-dot-PVDF membranes, and 10 images from the control glass slide without the membrane, defined as 100%. Fig. 4B ,i shows ubiquitous nanoparticles on the surface of the control slide (without placement of the C-dot-PVDF membrane). However, hardly any SiO 2 nanoparticles were observed in the SEM image of the glass slide for which a C-dot-PVDF membrane was placed on the path of the aerosolized nanoparticles (Fig. 4B,ii) . The bar diagram in Fig. 4B provides a quantitative analysis of the filtration performance, demonstrating around 97% blockage of the aerosolized nanoparticles by the C-dot-PVDF membrane, which is far better than most mask technologies. [37] We further examined the feasibility of the solar-mediated self-sterilization strategy ( COVID-19 viral particles while attaining good breathability. Importantly, solar-induced selfsterilization could be accomplished due to the highly effective sunlight absorbance by the embedded C-dots and concomitant heat dissipation.

The C-dot-PVDF membrane technology exhibits important advantages in comparison to existing or proposed recyclable facemask systems. In particular, while published studies reported construction of films that could block viral particles [10] , the adhesion of viable viruses onto the filtration surfaces poses significant health risk. Furthermore, reports on self-sterilized membrane systems [12, 15] have generally described the use of chemical systems that are complex and might be harmful by themselves to the human body. Indeed, the uniqueness of the C-dot-PVDF system is due to integrating the distinct properties of the individual constituents -the nanoporosity and hydrophobicity of the polymer framework and photothermal properties of the C-dots. We envision integration of the C-dot-PVDF system with cotton cloths, furnishing commercially available recyclable anti-COVID-19 facemasks. Future expansion of the technology towards microorganism, viral, and nanoparticle filtration systems would be also feasible. # Seema Singh and Nitzan Shauloff contributed equally

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Graphical Abstract A nanoporous self-sterilized membrane employed for recyclable antiviral facemasks was prepared from poly(vinylidene fluoride) (PVDF) and carbon dots. The embedded carbon dots facilitate sunlight absorbance and sterilization through heat dissipation

Conceptualization, Methodology, Investigation, Data curation, Visualization, Writing -Original Draft. Nitzan Shauloff: Methodology, Investigation, Validation, Visualization, Writing -Original Draft, Review and Editing. Chetan Prakash Sharma: Investigation, Validation. Ran Shimoni: Investigation, Validation. Christopher J. Arnusch: Supervision, Review, Editing. Raz Jelinek: Conceptualization, Methodology, Validation