Antibacterial activity of hydrophobicity modulated cationic polymers with enzyme and pH-responsiveness

The membrane lipid compositions of prokaryotic and eukaryotic cells are inherently different in many aspects, although some similarities exist in their structure and composition. Therefore, selective targeting of membrane lipids with a compound of therapeutic value, such as an antibacterial copolymer, is often challenging. Hence, developing an ideal copolymer with antibacterial properties demands hydrophobicity/hydrophilicity balance with a high biosafety profile. To integrate hydrophobic/hydrophilic balance and cationic charge in an alternating antibacterial copolymer with enzyme and pH-responsiveness, a lysine appended styrenic monomer was copolymerized with a fatty acid (octanoic acid (OA) or myristic acid (MA)) tethered maleimide monomer via reversible addition-fragmentation chain transfer (RAFT) polymerization. A range of microscopic analyses, including dynamic light scattering (DLS), confirmed the formation of nanoaggregates (size ∼30-40 nm) by these polymers in aqueous solution with positive zeta potential (cationic surface charge). Hydrophobic Nile red (NR) dye was successfully encapsulated in the nanoaggregates, and the in vitro release kinetics of the NR dye were monitored at different pHs and in the presence or absence of esterase/lipase. The in vitro release kinetics of NR revealed ∼85% dye release in the presence of pH 5.5 and lipase, suggesting their suitability for pH/enzyme-triggered therapeutic payload delivery. The standard broth microdilution assay showed significant bactericidal activity against both Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria with an MIC50 value <30 μg mL-1. The effect of polymeric nanoaggregates on bacterial morphology and in vitro survival was further confirmed by field emission scanning electron microscopy (FESEM), agar gel disk diffusion assay, and bacterial live/dead cell count. The significantly low hemolytic activity against red blood cells (RBCs) (HC50 >103 μg mL-1) and nontoxic effect on human intestinal epithelial cells (INT 407) (EC50 >500 μg mL-1) ensure that the polymer nanoaggregates are safe for in vivo use and can serve as a potent antibacterial polymer.

Surface Charge-Switchable Antifouling Block Copolymer with Bacteriostatic Properties

Zwitterionic polymers are an emerging family of effective, low-fouling materials that can withstand unintended interactions with biological systems while exhibiting enhanced activity in bacterial matrix deterioration and biofilm eradication. Herein, we modularly synthesized an amphiphilic block copolymer, ZABCP, featuring potential bacteriostatic properties composed of a charge-switchable polyzwitterionic segment and a redox-sensitive pendant disulfide-labeled polymethacrylate block. The leucine-appended polyzwitterionic segment with alternatively positioned cationic amine and anionic carboxylate functionalities undergoes charge alterations (+ve → 0 → -ve) on pH variation. By introducing appropriate amphiphilicity, ZABCP forms distinct vesicles with redox-sensitive bilayer membranes and zwitterionic shielding coronas, enabling switching of surface charge. ZABCP vesicles exhibit 180 ± 20 nm hydrodynamic diameter, and its charge switching behavior in response to pH was confirmed by the change of zeta potential value from -23 to +36 mV. The binding interaction between ZABCP vesicles with lysozyme and pepsin proteins strengthens when the surface charge shifts from neutral (pH 7.4) to either anionic or cationic. This surface-charge-switchable phenomenon paves the way for implementing cationic ZABCP vesicles for bacterial cell growth inhibition, which is shown by the pronounced transition of cellular morphology, including clustering, aggregation, or elongation as well as membrane disruption for both Bacillus subtilis (Gram-positive) and Escherichia coli (Gram-negative). Such enhanced bacteriostatic activity could be ascribed to a strong electrostatic interaction between cationic vesicles and negatively charged bacterial membranes, leading to cell membrane disruption. Overall, this study provides a tailor-made approach to adopt low-fouling properties and potential bacteriostatic activity using zwitterionic polymers through precise control of pH.

Stimuli-Responsive Aggregation-Induced Emission (AIE)-Active Polymers for Biomedical Applications

At high concentration or in the aggregated state, most of the traditional luminophores suffer from the general aggregation-caused quenching (ACQ) effect, which significantly limits their biomedical applications. On the contrary, a few fluorophores exhibit an aggregation-induced emission (AIE) feature which is just the opposite of ACQ. The luminophores with aggregation-induced emission (AIEgens) have exhibited noteworthy advantages to get tunable emission, excellent photostability, and biocompatibility. Incorporating AIEgens into polymer design has yielded diversified polymer systems with fascinating photophysical characteristics. Again, stimuli-responsive polymers are capable of undergoing chemical and/or physical property changes on receiving signals from single or multiple stimuli. The combination of the AIE property and stimuli responses in a single polymer platform provides a feasible and effective strategy for the development of smart polymers with promising biomedical applications. Herein, the advancements in stimuli-responsive polymers with AIE characteristics for biomedical applications are summarized. AIE-active polymers are first categorized into conventional π-π conjugated and nonconventional fluorophore systems and then subdivided based on various stimuli, such as pH, redox, enzyme, reactive oxygen species (ROS), and temperature. In each section, the design strategies of the smart polymers and their biomedical applications, including bioimaging, cancer theranostics, gene delivery, and antimicrobial examples, are introduced. The current challenges and future perspectives of this field are also stated at the end of this review article.

Styrene-Maleimide/Maleic Anhydride Alternating Copolymers: Recent Advances and Future Perspectives

Alternating sequencing of styrene-maleimide/maleic anhydride (S-MI/MA) in the copolymer chain is known for a long time. But since early 2000, this class of copolymers has been extensively studied using various living/controlled polymerization techniques to design S-MI/MA alternating copolymers with tunable molecular weight, narrow dispersity (Ð), and precise chain-end functionality. The widespread diverse applications of this polymeric backbone are due to its ease of synthesis, cheap starting materials, high precision in alternating sequencing, and facile post-polymerization functionalization with simple organic reactions. Recently, S-MI/MA alternating copolymers have been rediscovered as novel polymers with unprecedented emissive behavior. It outperforms the traditional fluorophores with no aggregation caused quenching (ACQ), aqueous solubility, and greater cell viability. Herein, the origin of alternating sequence, synthesis, and recent (2010-Present) developments in applications of these polymers in different fields are elaborately discussed, including the advantages of the unconventional luminogenic property. This review article also highlights the future research directions of the versatile S-MI/MA copolymers.

AIE-active non-conjugated poly(N-vinylcaprolactam) as a fluorescent thermometer for intracellular temperature imaging

Since temperature is one of the most significant physiological parameters that dictate the cellular status of living organisms, accurate intracellular temperature measurement is crucial and a valuable biomarker for the diagnosis and treatment of diseases. Herein, we introduce the foremost example of a non-conjugated polymer as a next generation fluorescent thermometer which is capable of addressing the key shortcomings including toxicity and thermal-induced fluorescence quenching associated with π-π conjugated system-based thermometers developed so far. We revealed, for the first time, the unique photophysical and aggregation-induced emission (AIE) characteristics of well-known thermoresponsive poly(N-vinylcaprolactam) (PNVCL) devoid of any classical fluorophore entity. PNVCL underwent a coil to globular conformational transition in an aqueous medium and appeared to be fluorescent above its lower critical solution temperature (LCST) near body temperature (38 °C). Eventually, this intriguing aspect enabled higher cellular uptake of PNVCL at the LCST boundary. By virtue of the AIE effect, the thermo-induced aggregation phenomenon has been ingeniously utilized to apply PNVCL as a novel fluorescent thermometer for intracellular temperature determination.

Multifunctional tryptophan-based fluorescent polymeric probes for sensing, bioimaging and removal of Cu2+ and Hg2+ ions

Fluorescent polymeric probes were synthesized by amalgamating tryptophan and pyridine side-chain moieties through an imine bond with the aim of selectively sense and remove both Cu²⁺ and Hg²⁺ ions from aqueous media.

A comprehensive review of the structures and properties of ionic polymeric materials

This review focuses on the mechanistic approach, the structure–property relationship and applications of ionic polymeric materials.

Recent advances in the development and applications of nonconventional luminescent polymers

Recently, nonconventional luminescent polymers (NLPs) have emerged as the most sought-after alternative luminescent materials. This review provides a thorough description of the importance and applications of each class of state-of-the-art NLPs.

AB-alternating copolymers via chain-growth polymerization: synthesis, characterization, self-assembly, and functions

In this review, four topics on alternating copolymers synthesized via chain-growth polymerization are reviewed: (1) how to control the alternating sequence; (2) sequence analysis; (3) self-assembly; and (4) functions.