Polydopamine functionalized hydrogel beads as magnetically separable antibacterial materials.

In the present study, magnetically separable hydrogel beads of ionically cross-linked alginate were functionalized with polydopamine (PDA). The rationale behind this was to enhance the structural stability and antibacterial profile of PDA/Alg/Fe 3 O 4 beads (K3). Incorporation of superparamagnetic magnetite (Fe 3 O 4 ) nanoparticles endowed the hydrogel beads with magnetism. X-ray diffraction (XRD) analysis revealed the successful formation of pure Alg/Fe 3 O 4 nanoparticles having an inverse spinel structure. Vibrating sample magnetometry (VSM) confirmed their superparamagnetic behaviour with M s values of 36.18 and 30.46 emu g -1 at 5 and 300 K, respectively. High resolution-transmission electron microscopy (HR-TEM) images showed alginate capping and the size of the Alg/Fe 3 O 4 nanoparticles (∼8 nm). The successful deposition of PDA granules on the K3 bead surface was verified by field emission-scanning electron microscopy (FE-SEM). The PDA functionalization was further justified by VSM, XRD and Fourier-transform infrared spectroscopy (FT-IR). During swelling experiments, K3 beads displayed appreciable structural stability compared to bare/non-functionalized beads. Wettability studies revealed K3 beads to be hydrophilic with a contact angle of ∼55°. Rheological parameters including storage modulus (G') and shear viscosity of K3 increased upon PDA functionalization. During antibacterial tests, K3 strongly inhibited E. coli , S. typhi , S. aureus and L. monocytogenes in a concentration and time dependent manner. Fluorescence staining experiments showed that K3 could greatly alter the bacterial membrane integrity. Reusability experiments with K3 beads substantiated their effective broad-spectrum antibacterial performance for three consecutive cycles.