Cationically modified membranes using covalent layer-by-layer assembly for antiviral applications in drinking water
Graphical abstract
Introduction
Approximately 20% of the global mortality rate can be attributed to infectious diseases, where one of the leading causes of such diseases are viruses such as adenovirus and norovirus, they accounting for about one-third of these deaths [1]. The growing demand for safe drinking water is of paramount concern, and therefore investigating alternate approaches for the production of high-quality water is of great importance. Here, especially waterborne viruses are difficult to remove due to their small size (20–100 nm) nm and stable nature [2]. Many approaches are used for the disinfection of contaminated water, each method having its own difficulties and drawbacks [3], [4]. Membrane technology, for instance, can remove viruses almost completely (using ultrafiltration (UF)) or significantly (using microfiltration (MF)) under appropriate conditions. For the removal of polioviruses suspensions from contaminated sources of water using hydrophobic MF membrane, retentions of greater than 2 log10-units reduction (≥99%) were achieved [5], [6]. Moreover, membranes with large pores (MF) facilitate high fluxes at low pressures, and therefore, could be utilized in gravity-driven household water treatment and safe storage (HWTS) systems. Simple and reliable HWTS systems will be essential in the coming decades to produce safe drinking water in remote and low-income settings.
The removal of viruses depends on membrane characteristics including surface properties [7] and long and short-range interactions with viral particles [8]. An appropriate membrane structure that is thick with a complex porous structure can improve the capacity of the membrane to trap viruses [9]. Interactions between viruses and the membrane surfaces can improve the removal efficiency, these interactions include electrostatic repulsion [10] and hydrophobic interactions [11]. Higher virus retentions were, for example, observed with a hydrophobic membrane in comparison to a hydrophilic one [11]. Pre-conditioning of the viral suspension, for example by inducing aggregation, can further improve the virus removal. Such aggregation can be induced by changing the pH to a value around the isoelectric point of a virus or by adding salt [11].
Manipulating the operating condition like flow [12], or the membrane structure, by for example introducing metallic nanoparticles (NPs) [13], enhances viral removal and reduction from contaminated water sources. Antimicrobial NPs of silver and copper have been used for centuries taking advantage of their antimicrobial properties especially for water storage and purification [14]. For example, silver-impregnated polysulfone membranes showed a significant improvement in viral removal. Additionally, this type of modification leads to reduced biofouling [15]. Also copper nanoparticles (CuNPs), can be incorporated into or on substrates to act as long lasting reservoirs for copper ions for enhanced antimicrobial activity [16], [17]. Though metallic NPs can be incorporated in many applications, they have received only little attention for their use in the removal of waterborne viruses from drinking water. It is important to mention that there are ethical issues concerning the use of silver nanoparticles (AgNPs), as their toxicity to human cells is not completely known or fully understood [13].
MF membranes may be combined with other processes to improve viral removal to the same extent as UF membranes [18]. It has also been hypothesised that virus removal by membrane filtration can be improved by inducing repulsive virus-membranes forces to prevent the viruses from approaching the membrane surface using zwitterionic hydrogels [19]. Though this may be true, the authors of this work are convinced that larger pore sizes are essential to reduce energy dependence and to allow gravity-driven HWTS systems. Therefore, an alternative approach is proposed, in this study, to utilise electrostatic attraction between the membrane and viruses, to improve the reduction of pathogenic viral contaminants in drinking water by an initial mechanism of adsorption followed by inactivation.
Cationic polymers have in the past demonstrated antiviral activity against a number of viruses and model viruses and are well suited for the modification of porous polymeric membranes [20], [21]. Amid numerous antimicrobial agents, polyethyleneimine (PEI) comprising of polycationic moieties has been widely used to modify various substrates due to (1) long-term antimicrobial activity with no resistance development, (2) the possibility for regeneration upon loss of activity, (3) minimal cytotoxicity to mammalian cells and (4) biocidal and virucidal activity against a broad variety of pathogens in short contact times [22], [23]. Cationic polymers could provide a simple electrostatic adsorption mechanism that would remove negatively charged viruses from any permeating water. Such an approach has in the past been used for virus concentration [24], [25], but could also potentially lead to MF membranes with the ability to remove waterborne viruses from drinking water.
For HWTS systems, it would be a real breakthrough if MF membranes could be used to remove both bacteria and viruses. It would allow the removal of pathogens from surface water at substantial water production using just gravity as a driving force. Without the need for an additional driving force, the HWTS systems could be produced very cheaply. Still, there are also downsides to expect from these systems if based purely on cationically modified membranes. The membranes could foul more quickly due to adsorption of negatively charged moieties in the water. Moreover the cationic layer might not be stable and lose its function over time.
In our previous work [26], it was demonstrated that the adsorption of a cationic polymer, PEI could increase the viral reduction of MF membrane filtration systems. PEI exerts an attractive electrostatic interaction towards the negatively charged virus and over time also inactivates the virus by causing damage to the virion [27]. Unfortunately, the applied PEI coating was not sufficiently stable, as PEI leached from the membrane. Moreover, while a promising ≥3 log10-units removal was observed, this is not seen as sufficient to produce drinking water free from health risks. Coatings that capture PEI by air drying or electrostatic attraction, easily lose their antiviral ability when the virucidal substance gets released into the surrounding solution or environment [28], [29]. In contrast, surfaces with covalently attached PEI can retain virucidal activity through a contact mechanism [30], [31]. As the functional groups are covalently attached, the surface retains its antiviral properties even after multiple uses.
In this work, we adopt a chemical-crosslinking methodology to fabricate ultrathin PEI multilayers, first on model surfaces (glass slides) and then on MF membranes. This approach was further optimised by incorporating antiviral metallic silver (Ag) and CuNPs. A detailed study was carried out, both on model surfaces and membranes, to determine the viral reduction (removal and/or inactivation) of MS2 bacteriophages. MS2 is a commonly used surrogate for human pathogenic viruses such as hepatitis E. The study futher establishes whether the removal/reduction of the phages is due to adsorption or a combination of adsorption and inactivation (facilitated by the presence of metallic NPs). This was achieved by using quantitative real-time polymerase chain reaction (qRT-PCR). This work was designed to contribute to the development of new membranes targeting enhanced virus reduction for the production of safer drinking water, at low costs, and under simple operating conditions. Indeed, the study will show here that simple modifications of MF membranes can offer high viral reductions under simple gravity-driven filtration conditions.
Section snippets
Materials
Branched Polyethyleneimine (Mw ~25 kDa), sulphuric acid (H2SO4, ACS reagent, 95–98%), hydrogen peroxide (H2O2, contains inhibitor, 30 wt% in water ACS reagent) and terephthalaldehyde (TA), sodium borohydride (NaBH4) and silver nitrate (AgNO3) were all obtained from Sigma-Aldrich (The Netherlands). All chemicals were used without any purification, and unless specified, all solutions were prepared in Milli-Q water (Milli-Q, Millipore Billerica, MA).
Model surface modification
Microscope slides (75 × 25 × 1 mm,
Covalent layer-by-layer (LBL) assembly of PEI layers on model surfaces
The full modification route to obtain TA crosslinked PEI multilayers on model surfaces is depicted in Fig. 1. Step 1 shows PEI being attached to the model surface through electrostatic attraction between the positively charged polycation and the negatively charged glass surface. Next a TA crosslinking step is performed, after which a second layer of PEI is added. As described in detail in the materials and methods section, aldehydes are suitable crosslinkers for amine groups by Schiff base
Conclusions
In this work, a general strategy was developed to fabricate antiviral MF membranes with coatings from chemically–crosslinked PEI multilayers which used a crosslinker terepthalaldehyde to improve their overall performance and stability thus giving them the ability to be used in gravity-driven filtration. These coatings were first studied on model surfaces where the pH controlled growth was monitored by ellipsometry. Surface characterisation techniques, AFM, FTIR, zeta potential and contact angle
Acknowledgements
This work was performed in the TTIW-cooperation framework of Wetsus, European Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslâ̂n, and the City of Leeuwarden and the Northern Netherlands Provinces. We thank the participants of the research theme “Virus Control” for their financial support and helpful discussions.
Author contributions
Terica R. Sinclair, W.M. de Vos and H.D.W. Roesink conceived and designed membrane and glass slide experiments; Terica R. Sinclair and Akshay Patil performed the experiments with advice from Dennis Reurink; Terica R. Sinclair, Sanne K. van den Hengel, Saskia A Rutjes and Ana Maria de Roda Husman analysed virology data, Terica R. Sinclair and Brahzil G. Raza performed the experiments; Terica R. Sinclair and W. de Vos examined membrane and glass slide fabrication and characterization data; Terica
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