**1. Introduction**

The need for water remediation systems designed to eliminate contaminants and pollutants ravaging water sources is a global problem and, thus, is part of the goals of the 2030 Agenda for Sustainable Development [1]. Nevertheless, the struggle to remove heavy metal ions, pesticides and other dissolved organic pollutants is a di fficult war and some of the e fforts of researchers to accomplish such a target are directed towards the development of environmentally friendly porous nanomaterials [2,3]. In fact, big bets are being placed on systems derived from naturally occurring polymers, such as polysaccharides [4,5]. Within the portfolio of commended biopolymers, cellulose and its nanoscale forms, namely cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs) and bacterial nanocellulose (BNC), show tremendous potential for environmental and water remediation as recently reviewed [6–8]. The high interest in the latter nanocellulose, *viz*. the exopolysaccharide BNC biosynthesized by some non-pathogenic bacterial strains [9], lies in its 3D structure with an ultrafine network of physically entangled cellulose nanofibers, which is responsible for its *in-situ* moldability, shape retention, inherent biodegradability, high water-holding capacity and porous structure [10,11].

BNC, its derivatives and composites [12] have already been used in the manufacture of water remediation systems for the removal of dyes [13,14], oil [15] and heavy metals [16,17]. To the best of our knowledge, the partnership between BNC and a zwitterionic polymer has never been explored for the fabrication of nanocomposites, aiming at the simultaneous removal of anionic and cationic organic dyes. Under these premises, 2-methacryloyloxyethyl phosphorylcholine (MPC) was selected as a non-toxic polymerizable monomer due to its methacrylic functional group and zwitterionic phosphorylcholine moiety, consisting of a phosphate anion and a trimethylammonium cation [18], which are prone to establish electrostatic interactions with positively and negatively charged molecules. Furthermore, the unique hydration state [18], antimicrobial, bioinert and antifouling properties of MPC polymer [19–21] will be a major asset in reducing or avoiding bacterial growth in contaminated water.

The present study contemplates the fabrication of nanocomposite membranes consisting of cross-linked poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and BNC via the one-pot polymerization of the corresponding non-toxic zwitterionic monomer (*i.e*. MPC) within the BNC three-dimensional porous network. The structure, morphology, thermal stability, mechanical properties, antibacterial activity towards *Staphylococcus aureus* and *Escherichia coli*, water-uptake capacity and removal of cationic and anionic organic dyes of the resulting nanocomposites were assessed.

#### **2. Materials and Methods**

#### *2.1. Chemicals, materials and microorganisms*

2-Methacryloyloxyethyl phosphorylcholine (MPC, 97%), 2,2-azobis(2-methylpropionamidine) dihydrochloride (AAPH, 97%), *<sup>N</sup>*,*N*-methylenebis(acrylamide) (MBA, 99%), methylene blue (MB, dye content ≥ 82%), methyl orange (MO, dye content 85 %) and para ffin oil (puriss., 0.827–0.890 g mL−<sup>1</sup> at 20 ◦C) were purchased from Sigma-Aldrich (Sintra, Portugal) and used as received. Ultrapure water (Type 1, 18.2 MX·cm at 25 ◦C) was obtained from a Simplicity ® Water Purification System (Merck, Darmstadt, Germany). Other chemicals and solvents were of laboratory grade.

Bacterial nanocellulose (BNC) wet membranes were biosynthesized in our laboratory using the *Gluconacetobacter sacchari* bacterial strain [22]. *Staphylococcus aureus* (ATCC 6538) and *Escherichia coli* (ATCC 25922) was provided by DSMZ – Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (German Collection of Microorganisms and Cell Cultures).

#### *2.2. Preparation of PMPC*/*BNC nanocomposites*

Wet BNC membranes (diameter: *ca*. 70 mm) with 40% water content were placed in stoppered glass-reactors and purged with nitrogen. Simultaneously, aqueous solutions of monomer (MPC, 1:3 and 1:5 BNC/MPC weight fraction), cross-linker (MBA, 5.0% w/w relative to monomer), and radical initiator (AAPH, 1.0% w/w relative to monomer) were prepared and transferred to the glass-reactors containing the drained BNC membranes. After the complete incorporation of the corresponding solution into the BNC membrane, during 1 h in ice, the reaction mixtures were heated in an oil bath at 70 ◦C and left to react for 6 h. Afterwards, the nanocomposites were repeatedly washed with water, oven dried at 40 ◦C for 12 h, and kept in a desiccator until further use. All experiments were made in triplicate and samples of cross-linked PMPC were also prepared in the absence of BNC for comparison.

## *2.3. Characterization methods*
