Microbial colonization of indwelling devices remains a major concern in modern healthcare. Developing approaches to prevent biomaterial-associated infections (BAI) is, therefore, in great demand. This study aimed to immobilize two antimicrobial peptides (polymyxins B and E) onto polydimethylsiloxane (PDMS) using two polydopamine (pDA)-based approaches: the conventional two-step method involving the deposition of a pDA layer to which biomolecules are immobilized, and a one-step method where peptides are dissolved together with dopamine before its polymerization. Surface characterization confirms the immobilization of polymyxins onto PDMS at a non-toxic concentration. Immobilization of polymyxins using a one-step pDA-based approach is able to prevent Pseudomonas aeruginosa adhesion and kill a significant fraction of the adherent ones. Living cells adhered to these modified surfaces exhibit the same susceptibility pattern as cells adhered to unmodified surfaces, highlighting no resistance development. Results suggest that polymyxins immobilization holds a great potential as an additional antimicrobial functionality in the design of biomaterials.

Iron oxide nanoparticles (IONPs) modified with polydopamine were covalently immobilized with nisin to inhibit the growth of Alicyclobacillus acidoterrestris in apple juice. The minimum bactericidal concentration (MBC) of IONPs @ pDA-nisin composites against Alicyclobacillus acidoterrestris cells and spores in three kinds of apple juice were 1.25 mg/mL and 2.5 mg/mL, respectively. The concentration of cells and spores decreased from 10 to 10 CFU/mL in apple juice after pretreatment with the composites at the concentration of 20 mg/mL for 10 min. Meanwhile, the investigation of the content of polyphenols, organic acids, volatile compounds and sugars indicated that there was no significant influence of the composites on the apple juices. The results of cytotoxicity (HepG2, Caco 2, SH-SY5Y, and BV2 cells) and acute toxicity test demonstrated the biosecurity and nontoxicity of the composites, which suggested that it was promising to apply the proposed composites to apple juice industry for the control of Alicyclobacillus acidoterrestris pollution.

Contact lens is a major risk factor for microbial keratitis among contact lens wearers. Chemical strategies that can prevent microbial adhesion and biofilm formation are required to improve a wearer's hygiene and safety. Taking advantage of the material-independent properties of a polydopamine (pDA) coating, we investigated the role of covalent/noncovalent interactions of the antimicrobials and pDA in conferring long-term antimicrobial activities. The developed antimicrobial contact lenses not only retain their antibacterial efficiency against different bacterial strains for 2 weeks but also inhibit microbial adhesion and biofilm formation on the lens surfaces. The designed antimicrobial coatings were found to be safe for ocular cell lines. Moreover, the antimicrobial coatings did not affect the functional and surface properties of coated contact lenses. This methodology can be used to protect the contact lenses from microbial contamination for prolonged periods and has the potential to be extended for designing antimicrobial coatings for other medical devices as well.

Bacterial infection commonly occurs in clinical settings when the procedure involves a medical implant. Thus, the fabrication of antimicrobial medical materials has attracted much attention in recent years. To improve the antibacterial properties of titanium (Ti)-based biomedical materials, surface microporous structures, with antimicrobial peptide coatings, were employed in this study. Native Ti substrates were endowed with a certain level of antibacterial activity after treatment with the micro-arc oxidation (MAO). A multilayer consisting of polydopamine, cationic antimicrobial peptides LL-37, and phospholipid (POPC) was coated onto MAO substrates, leading to antibacterial activity against both Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria. The combination of polydopamine-LL-37-POPC was found to alleviate the burst release of LL-37 in the initial phase. This multilayer coated onto microporous Ti substrates also showed favorable cytocompatibility to both mesenchymal stem cells (MSCs) and osteoblasts. These findings illustrate a novel strategy for the development of antibacterial Ti-based implants.

Catheter-associated urinary tract infections (CAUTIs) are the most common hospital-acquired infections worldwide, aggravating the problem of antimicrobial resistance and patient morbidity. There is a need for a potent and robust antimicrobial coating for catheters to prevent these infections. An ideal coating agent should possess high antimicrobial efficacy and be easily and economically conjugated to the catheter surface. In this study, we report a simple yet effective immobilization strategy to tether a potent synthetic antimicrobial peptide, CWR11, onto catheter-relevant surfaces. Polydopamine (PD) was deposited as a thin adherent film onto a polydimethylsiloxane (PDMS) surface to facilitate attachment of CWR11 onto the PD-functionalized polymer. Surface characterization of the CWR11-tethered surfaces confirmed the successful immobilization of peptides onto the PD-coated PDMS. The CWR11-immobilized PDMS slides displayed excellent antimicrobial (significant inhibition of 5×10(4) colony-forming units of CAUTI-relevant microbes) and antibiofilm (∼92% enhanced antibacterial adherence) properties. To assess its clinical relevance, the PD-based immobilization platform was translated onto commercial silicone-coated Foley catheters. The CWR11-impregnated catheter displayed potent bactericidal properties against both Gram-positive and Gram-negative bacteria, and retained its antimicrobial functionality for at least 21days, showing negligible cytotoxicity against human erythrocyte and uroepithelial cells. The outcome of this study demonstrates the proof-of-concept potential of a polydopamine-CWR11-functionalized catheter to combat CAUTIs.

This work presents a systematic evaluation of 2-amino, 5-amino and 2,5-diamino substituted benzimidazo[1,2-a]quinoline-6-carbonitriles as novel pH probes with a potential application in pH sensing materials or as H fluoroionophores in bulk optode membranes. The study was carried out by varying the length, type and position of amino substituents in ten fluorescent dyes with the same benzimidazo[1,2-a]quinoline-6-carbonitrile core. The photophysical and acid-base properties of the dyes were investigated by the UV/Vis absorption and fluorescence spectroscopies, and interpreted by the electronic structure DFT calculations. pH sensing mechanisms and structure-property relations affecting the fluorescence response were discussed through a detailed analysis of the piperidine substituted derivatives 1-4. Push-pull donor-acceptor interactions stimulate strong fluorescence in the visible spectral range (up to Φ = 0.65 for 7) and induce significant changes in the photophysical properties associated with the acid-base equilibria (up to a 50-fold increase in the fluorescence intensity). pK values in aqueous and mixed solutions (v/v HO:EtOH 99:1, 50:50), appear suitable for monitoring acidic pH in solution. The most promising candidates were immobilised in thin polymer matrices by the spin coating technique to form fluorescent sensing materials - optodes, and examined as novel pH-sensitive fluoroionophores. In the liquid membrane environment, dyes exhibited significant increase of the apparent pKs by almost 4 units. Bright and blue emissive thin films showed pH response and dynamic range around pK = 5, making them suitable for a wide range of optical sensing applications.

A versatile folate-receptor-mediated targeting tumor theranostics has been designed for pH/reduction dual-responsive controlled anticancer drug release and pH-modulated fluorescent tumor imaging via facile ionic (pH sensitive) and covalent (reduction responsive) double-cross-linking (DCL) of the folic acid (FA) and Rhodamine 6G modified hyaluronic acid (HA) (FA-HA-Rh 6G). After optimizing the morphology and diameter of the resultant nanoparticles (DCL FA-HA-Rh 6G NPs) via modulating the concentration of the ionic and covalent cross-linking agents, the one with Ca and S contents of 1.70 and 2.84 wt % and an average hydrodynamic diameter of 154 nm was chosen as the desired drug delivery system (DDS) for DOX. They not only had high drug loading capacity and drug encapsulation efficiency (716 ± 34 mg/g and 71.6 ± 3.4%) but also possessed perfect triggered release and strong fluorescence intensity in the stimulated tumor microenvironment. The MTT assay and CLSM analysis revealed that the proposed double-cross-linked HA-based DDS had favorable cytocompatibility and folate-receptor-mediated targeting functionality to the HeLa cells and could obviously enhance the anticancer efficiency of DOX. The integration of the pH and reduction dual-responsiveness, folate-receptor-mediated targeting functionality, and pH-dependent fluorescence intensity into the biodegradable and biocompatible HA nanoparticles make the DCL FA-HA-Rh 6G NPs significant potential for future visualized chemotherapy of cancers.

Stimuli-responsive pigments are very interesting for several technological applications, such as food and cosmetic colorants, color-based sensors and fluorescence probes, among others. In this work, the synthesis of a new xanthylium-derived pigment was developed and the chemical and photophysical properties in hydroalcoholic solution at different pH values were investigated by UV-Vis and fluorescence spectroscopy. The UV-Vis titration of 3,6,8-trihydroxy-11-methylpyranoxanthylium has shown four different colored chemical species (AH, A, A and A) in hydroalcoholic solution in a pH range between 1 and 12 with the thermodynamic acidic constants of pK = 4.80 ± 0.03, pK = 6.51 ± 0.05 and pK = 8.64 ± 0.01. Regarding fluorescence properties, this dye revealed an interesting pH-dependent emission behavior. In fact, the anionic quinoidal base A predominant at pH range between 5 and 9 should be mainly responsible for the pronounced fluorescence intensity observed at λ 467 nm/λ 510 nm pair (maximum at pH 7.5). This set of new insights make this dye useful as a potential "off-on-off" pH-responsive fluorescent probe for biological applications. A pyranoxanthylium dye was developed and revealed a selective fluorescence emission between 5 < pH < 9, being maximum at pH 7.5, which make it very interesting as a pH-responsive "off-on-off" fluorescent probe for biomedical applications.

This review summarizes state of the art synthesis and applications of carbon dots (CDs) with pH-responsive fluorescence. Following an introduction, the first section covers methods for the preparation of pH-responsive CDs, with subsections on general methods for preparing CDs (by hydrothermal, solvothermal, electrochemical, microwave, laser ablation, pyrolysis or chemical oxidation polymerization methods), and on precursors for synthesis. This is followed by a section on the mechanisms of pH-responsivity (by creating new functional groups, change of energy levels, protonation and deprotonation, aggregation, or by introduction shells). Several Tables are presented that give an overview of the wealth of methods and materials. A final section covers applications of carbon dots (CDs) with pH-responsive fluorescence for sensing, drug delivery, and imaging. The conclusion summarizes the current status, addresses challenges, and gives an outlook on potential future trends. Graphical abstract The synthesis and biological applications of carbon dots(CDs) with pH-responsive fluorescence are summarized. Precursors and methods for preparation of pH-responsive CDs, mechanisms of pH-responsivity, and biological applications of CDs with pH-responsive fluorescence for sensing, drug delivery, and imaging are discussed.