Applications of cellulose and chitin/chitosan derivatives and composites as antibacterial materials: current state and perspectives.

The bacterial infections have always a serious problem to public health. Scientists are developing new antibacterial materials to overcome this problem. Polysaccharides are promising biopolymers due to their diverse biological functions, low toxicity, and high biodegradability. Chitin and chitosan have antibacterial properties due to their cationic nature, while cellulose/bacterial cellulose does not possess any antibacterial activity. Moreover, the insolubility of chitin in common solvents, the poor solubility of chitosan in water, and the low mechanical properties of chitosan have restricted their biomedical applications. In order to solve these problems, chemical modifications such as quaternization, carboxymethylation, cationization, or surface modification of these polymers with different antimicrobial agents, including metal and metal oxide nanoparticles, are carried out to obtain new materials with improved physiochemical and biological properties. This mini review describes the recent progress in such derivatives and composites with potential antibacterial applications.

Synthesis and fluorescent property of carboxymethyl chitosan with different degrees of carboxymethylation and its application for fluorescence turn-on detection of Cd(II) ion.

Recently, carboxymethyl chitosan (CMCS) has proved to be an intrinsically fluorescent material with aggregation-induced emission characteristic. In order to elucidate the influence of CMCS's chemical structure and solution behavior on its fluorescent intensity, two series of CMCS with different degrees of carboxymethylation (DCM) are synthesized by adjusting the mass ratio of monochloroacetic acid and sodium hydroxide to chitosan at various reaction temperature and time, and then characterized using Fourier-transform infrared spectroscopy and nuclear magnetic resonance. Their solution behaviors at different pH values are studied via zeta potential and ultraviolet-visible measurements. The data reveal that the isoelectronic point (IEP) of a CMCS decreases with increasing DCM, and all the CMCSs have good water solubility at pH range below their IEPs. Fluorescence spectra indicate that a CMCS shows the highest fluorescent intensity in a slightly acidic environment next to its IEP, and the photoluminescence of a CMCS solution increases significantly after heat treatment above T = 70 °C. Furthermore, we compare for the first time the use of heated CMCS solutions for the fluorescence turn-on detection of cadmium ion in pure and tap water, respectively. The limit of detection is found to be ca. 1 µM for the Cd2+ in pure water.

Ultrafast transient absorption microscopy studies of carrier dynamics in epitaxial graphene.

Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate.

Facile production of three-dimensional chitosan fiber embedded with zinc oxide as recoverable photocatalyst for organic dye degradation.

Herein we report on a facile and green strategy for continuous production of chitosan-zinc oxide fibers and then compare their photodegradation performance against three organic dyes (i.e., methylene blue (MB), methyl orange (MO) and Rhodamine B, respectively) under different lights. Chitosan-zinc hydrogel fibers (CS/Zn) with different zinc loadings are obtained by direct mixing of chitosan and zinc acetate solutions using a double-syringe injection device. The as-prepared CS/Zn fibers are then immersed into glutaraldehyde (GA) and sodium hydroxide solutions, respectively, and dried at T = 50 °C. The resultant CS/ZnO/GA fibers of ca. 617 µm in diameter are characterized using X-ray diffraction (XRD), thermogravimetric analysis and field emission scanning electron microscope (FE-SEM). XRD and FE-SEM data confirm that the CS/ZnO/GA fibers consist of a large amount of hexagonal wurtzite ZnO nanorods up to 550 nm in length, and exhibit three-dimensional interconnected macroporous architecture. Photodegradation results clearly show that the CS/ZnO/GA fibers are effective for the removal of organic dyes upon UV irradiation and can be easily recovered and reused for at least 6 consecutive cycles. Unlike most reported CS/ZnO nanocomposites, the current CS/ZnO/GA fiber shows a higher adsorption of cationic MB rather than anionic MO, the mechanism of which is proposed.

Gelation process of carboxymethyl chitosan-zinc supramolecular hydrogel studied with fluorescence imaging and mathematical modelling.

Herein we report on a detailed study about the gelation kinetics of carboxymethyl chitosan-zinc (CMCh-Zn) supramolecular hydrogel by taking advantage of its intrinsic fluorescence property. A specific gelation device is designed and the gel front can be directly visualized under 365 nm UV light. The results show that when increasing Zn2+ concentration from 0.1 M to 1.0 M, the apparent diffusion coefficient increases gradually from 2.72 × 10-6 cm2/s to 4.50 × 10-6 cm2/s. The gelation kinetics then is described with a "zero order" mathematical model, proving that the gel thickness is related to the square root of the gelation time and the diffusion step is the controlling step of the gelation process. Later a more advanced model, developed in 1D geometry and solved numerically, is used to describe and predict experimental results, proving its reliability and the correct description of all the phenomena involved in the gelation process of CMCh-Zn hydrogel.

Spatial correlation of confocal Raman scattering and secondary ion mass spectrometric molecular images of lignocellulosic materials.

A detailed chemical and structural understanding of pre-enzymatic processing of lignocellulosic materials (LCMs) is a key objective in the development of renewable energy. Efficient rendering of biomass components into fermentable substrates for conversion into biofuel feedstocks would benefit greatly from the development of new technologies to provide high-quality, spatially resolved chemical information about LCMs during the various processing states. In an effort to realize this important goal, spatially correlated confocal Raman and mass spectrometric images allow the extraction of three-dimensional information from the perennial grass, Miscanthus x giganteus. An optical microscopy-based landmark registry scheme was developed that allows samples to be transferred between laboratories at different institutions, while retaining the capability to access the same physical regions of the samples. Subsequent to higher resolution imaging via confocal Raman microscopy and secondary ion mass spectrometry (SIMS), laser desorption-ionization mass spectrometry was used to place these regions within the overall sample architecture. Excellent sample registry was evident in the highly correlated Raman and SIMS images. In addition, the correlation of vibrational Raman scattering with mass spectra from specific spatial locations allowed confirmation of the assignment of intracellular globular structures to hemicellulose-rich lignin complexes, an assignment which could only be made tentatively from either image alone.

Enhanced fluorescence of carboxymethyl chitosan via metal ion complexation in both solution and hydrogel states.

Recently, biopolymer-based non-traditional luminogens had attracted a great deal of interest because of their potential applications in biomedical field. Herein, we report for the first time that carboxymethyl chitosan (CMCh) can exhibit strong blue fluorescence at λ = 436.8 nm when brought in contact with zinc ion (Zn2+) in both solution and hydrogel states. The resultant CMCh-Zn sample exhibits a typical fluorescence lifetime of 3.68 ns and a quantum yield of 6.8%. The fluorescence behaviors of CMCh-Zn samples at different excitation wavelengths, CMCh concentrations, temperature, and pH values, are also investigated. The results clearly indicate clustering-triggered emission characteristic of the CMCh-Zn. In order to further elucidate the chemical nature of this new fluorescence system, a series of CMCh-Zn samples are characterized by using ultraviolet-visible spectrometer, Fourier-transform infrared spectrometer and X-ray diffractometer. The data suggest that the metal-ligand complexation of CMCh with Zn2+ account for the generation of such an enhanced fluorescence.

Continuous production of antibacterial carboxymethyl chitosan-zinc supramolecular hydrogel fiber using a double-syringe injection device.

Large-scale production of an antibacterial hydrogel is of critical importance for its practical application in biomedical field. In this regard, herein we report on the construction of a double-syringe injection device by using all the commercial parts and its use for continuous production of carboxymethyl chitosan-zinc (CMCh-Zn) supramolecular hydrogel fiber. The resultant CMCh-Zn hydrogel fibers exhibit good stretchability and knittability. The Zn loading into the hydrogel fibers can be easily controlled by adjusting the concentration of Zn2+ solution. Scanning electron microscope measurements indicate that the CMCh-Zn hydrogel fibers have a relatively smooth and thin skin layer, as well as, a 3-dimensional interconnected microporous interior architecture. Antibacterial activities of the CMCh-Zn hydrogel fibers against both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli are also investigated. The results show that the intrinsic blue fluorescence of the as-prepared CMCh-Zn hydrogel fibers can be employed as optical indicator of their antibacterial effectiveness.

Development and antibacterial activities of bacterial cellulose/graphene oxide-CuO nanocomposite films.

The absence of antibacterial activity of bacterial cellulose (BC) restricts its applications in the biomedical field. To introduce antimicrobial properties into BC, we studied the synthesis, structure, and antimicrobial properties of a novel nanocomposite film comprising BC, graphene oxide (GO), and copper-oxide (CuO) nanosheets. The nanocomposite film was synthesized by incorporating GO-CuO nanohybrids into BC matrix through homogenized blending. The CuO nanosheets, with a length range of 50 nm-200 nm and width range of 20 nm-50 nm, which were uniformly grown on the GO along with even distribution of GO-CuO nanohybrids on the surface of the cellulose fibers. The nanocomposites displayed better antibacterial activity against gram-positive than gram-negative bacteria. BC/GO-CuO nanocomposites showed higher antibacterial activity than BC/CuO. We also elucidated the mechanism of antibacterial activity of the nanocomposites. Further, the nanocomposites exhibited biocompatibility towards mice fibroblast cells. The nanocomposites might serve as an excellent source for development of antibacterial materials.

Polyadenine-mediated Immobilization of Aptamers on a Gold Substrate for the Direct Detection of Bacterial Pathogens.

Nucleic acid aptamers have been widely used as synthetic probes for bioanalytical applications. Herein, we carried out a detailed study on the immobilization of a series of aptamers ranging from 37 to 88 bases, which are specific to either Escherichia coli (E. coli) or Staphylococcus aureus (S. aureus), on a planar gold substrate via a polyadenine-mediated immobilization method. The resultant surfaces were characterized by both surface plasmon resonance spectroscopy (SPR) and X-ray photoelectron spectroscopy. The results clearly show that the aptamer solution at a lower ionic strength gives rise to a higher lateral density of the aptamer when compared to that at a higher ionic strength. The SPR aptasensors are then employed for detecting their corresponding bacteria (i.e., E. coli and S. aureus, respectively). The data indicate that the SPR aptasensor with a higher density of aptamer exhibits a better capture of target bacteria.

Direct immobilization of sugar probes on bovine serum albumin-coated gold substrate for the development of glycan biosensors.

Glycan biosensors based on surface plasmon resonance (SPR) spectroscopy have attracted a great deal of interest due to their potential applications in numerous biological and biomedical fields. Controlled immobilization of sugar probes on a gold substrate is believed to be critical for the performance of these SPR biosensors. In this regard, herein the authors report a direct coupling of mannose probes with bovine serum albumin (BSA) layer on the gold substrate via a squaric acid-mediated reaction under mild conditions, in which the BSA layer provides not only reactive amine groups but also a nonfouling surface property. SPR measurements show that the resultant biosensor with an appropriate amount of mannose probes exhibits high affinity to its corresponding lectin (i.e., concanavalin A) and at the same time could resist nonspecific adsorption of other lectins. The limit of detection of the current SPR biosensor is 1.9 nM. Thus, the squaric acid-mediated immobilization strategy appears to be effective and useful for the fabrication of bioanalytical devices.

Preparation and antibacterial activity of silver-loaded poly(oligo(ethylene glycol) methacrylate) brush.

Herein, we report on a robust approach to fabricate antibacterial nanocomposite coating simply by immersing poly(oligo(ethylene glycol) methacrylate) (POEGMA) brush into a silver perchlorate solution without using any external reducing agents. The POEGMA brush of 48.3 nm in thickness is prepared via surface-initiated atom transfer radical polymerization method. Field-emission scanning electron microscope and Raman measurements indicate that silver nanoparticles of 14 ∼ 25 nm in diameter are successfully embedded into the POEGMA brush. Antibacterial activities of the resultant silver-loaded POEGMA brushes against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus are measured by zone of inhibition and colony-counting methods, respectively. The results show that the silver-loaded POEGMA coatings exhibit enhanced antibacterial efficiency compared to bare POEGMA brush. In order to elucidate their antibacterial mechanism, silver release behaviors of these silver-loaded POEGMA brushes are monitored via inductively coupled plasma mass spectrometry.

A facile construction of bacterial cellulose/ZnO nanocomposite films and their photocatalytic and antibacterial properties.

Bacterial cellulose (BC) has numerous excellent properties but the absence of antibacterial activity restricts its applications in biomedical field. Therefore, in order to introduce the antibacterial characteristics into BC; herein, a facile method for incorporation of ZnO nanoparticles (ZnO-NPs) is presented. BC films were first immersed in zinc nitrate solution, followed by treating with NaOH solution, the BC loaded ZnO nanocomposite films were dried by a sheet former instrument at 80 °C for 20 min. The obtained BC/ZnO nanocomposites were characterized by different techniques. XRD results showed the hexagonal wurtzite structure of ZnO-NPs while FE-SEM results displayed the particle size of ZnO-NPs was ranging from 70 to 100 nm. Thermogravimetric study revealed the thermal stability of nanocomposite films. The nanocomposite exhibited photocatalytic activity and revealed 91% degradation of methyl orange (MO) under UV-irradiation within 2 h. Moreover, the nanocomposites demonstrated significant UV-blocking properties and showed antibacterial activities against tested Gram-positive and Gram-negative bacterial strains. This work provides a simple and novel method for the synthesis of BC/ZnO nanocomposite as a functional biomaterial.

Development of bacterial cellulose/chitosan based semi-interpenetrating hydrogels with improved mechanical and antibacterial properties.

In the present work, novel bacterial cellulose (BC) and chitosan (CS) semi-interpenetrating network (semi-IPN) hydrogels were prepared via blending the slurry of BC with CS solution followed by cross-linking with glutaraldehyde. The structure and properties of BC-CS hydrogels were characterized by different techniques including; FTIR, XRD, FE-SEM, TGA and rotational rheometry. The results indicated cross-linking of chitosan chains by glutaraldehyde while BC was physically connected to network forming semi-IPN hydrogels. Microscopic study of cross-sectional freeze-dried hydrogels showed microporous openings. BC-CS hydrogels exhibited higher thermal stability than pure BC film or CS hydrogel alone. The rheological results presented significant mechanical properties of semi-IPN hydrogels. Moreover, the hydrogels showed antibacterial properties against tested Gram-positive and Gram-negative bacteria. The antibacterial properties were dependent on the ratio of BC to CS. Hydrogels with 20% BC to CS reduced the viable colonies by ~88%. The development of this new class of BC-CS antibacterial, mechanically strong and stable soft-material could be a promising candidate for antibacterial applications.

Reusable ternary PVA films containing bacterial cellulose fibers and ε-polylysine with improved mechanical and antibacterial properties.

The combination of high mechanical properties, antibacterial activity and a green synthesis of the polyvinyl alcohol (PVA) based films remains challenging. This study presents a ternary system of PVA films containing bacterial cellulose (BC) and epsilon-polylysine (ε-PL) by a green solution casting method. The prepared composite films showed more than 99% antibacterial properties against both Staphylococcus aureus and Escherichia coli bacteria. Moreover, the films were collected after a single use and were reused twice, which still exhibited strong antibacterial activity. The films showed thermal stability and higher mechanical properties as compared to pure PVA films. In addition, the cytotoxicity of the films was evaluated by MTT assay against NIH 3T3 mouse fibroblast cells. The results showed no toxicity of the films towards tested cells. We believe that these antibacterial films may find applications in active food packaging and biomedical fields.

Plasma polymerized epoxide functional surfaces for DNA probe immobilization.

The development of functional surfaces for the immobilization of DNA probe is crucial for a successful design of a DNA sensor. In this report, epoxide functional thin films were achieved simply by pulsed plasma polymerization (PP) of glycidyl methacrylate (GMA) at low duty cycle. The presence of epoxide groups in the resulting ppGMA films was confirmed by Fourier transform infrared spectroscopy. The ppGMA coatings were found to be resistant to the non-specific adsorption of DNA strands, while the epoxide groups obtained could react with amine-modified DNA probes in a mild basic environment without any activation steps. A DNA sensor was made, and was successfully employed to distinguish different DNA sequences with one base pair mismatch as seen by surface plasmon enhanced fluorescence spectroscopy (SPFS). The regeneration of the present DNA sensor was also discussed. This result suggests that surface modification with ppGMA films is very promising for the fabrication of various DNA sensors.

Injectable self-healing carboxymethyl chitosan-zinc supramolecular hydrogels and their antibacterial activity.

Injectable and self-healing hydrogels have found numerous applications in drug delivery, tissue engineering and 3D cell culture. Herein, we report an injectable self-healing carboxymethyl chitosan (CMCh) supramolecular hydrogels cross-linked by zinc ions (Zn2+). Supramolecular hydrogels were obtained by simple addition of metal ions solution to CMCh solution at an appropriate pH value. The mechanical properties of these hydrogels were adjustable by the concentration of Zn2+. For example, the hydrogel with the highest concentration of Zn2+ (CMCh-Zn4) showed strongest mechanical properties (storage modulus~11,000Pa) while hydrogel with the lowest concentration of Zn2+ (CMCh-Zn1) showed weakest mechanical properties (storage modulus~220Pa). As observed visually and confirmed rheologically, the CMCh-Zn1 hydrogel with the lowest Zn2+ concentration showed thixotropic property. CMCh-Zn1 hydrogel also presented injectable property. Moreover, the antibacterial properties of the prepared supramolecular hydrogels were studied against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) by agar well diffusion method. The results revealed Zn2+ dependent antibacterial properties against both kinds of strains. The inhibition zones were ranging from ~11-24mm and ~10-22mm against S. aureus and E. coli, respectively. We believe that the prepared supramolecular hydrogels could be used as a potential candidate in biomedical fields.

BSA-Sugar Conjugates as Ideal Building Blocks for SPR-Based Glycan Biosensors.

Controlled immobilization of sugar probes is of key importance for the development of glycan biosensors. To this end, a series of BSA-sugar conjugates with different numbers of mannose units are prepared via the squaric acid-mediated coupling reaction. The conjugates can absorb directly on gold substrate without any derivation reactions, thus providing a simple and effective method for the construction of SPR-based glycan biosensors. SPR measurements show that the BSA-mannose conjugate with 11 mannoses exhibit the highest affinity to the lectin concanavalin A with a limit of detection of ca. 1.8 nM. Regeneration and specificity of the obtained glycan biosensors are also investigated.

Facile fabrication of moldable antibacterial carboxymethyl chitosan supramolecular hydrogels cross-linked by metal ions complexation.

Herein, carboxymethyl chitosan (CMCh) supramolecular hydrogels cross-linked by metal ions (Ag+, Cu2+ and Zn2+) are reported. The hydrogels were formed within a few seconds by simple mixing of CMCh solution with metallic salt solutions at an appropriate pH. The prepared hydrogels were characterized by using FT-IR, XRD, SEM and rotational rheometery. FT-IR measurements suggested that the facile complexation of metal ions with carboxylic, amino and hydroxyl groups of CMCh chains promoted the rapid hydrogelation. SEM images revealed a cross-linked structure of hydrogels, while microstructural openings were observed by cross-sectional studies of the freeze-dried hydrogels. Moreover, the hydrogels showed a remarkable moldability to form free standing objects. The antibacterial activities of the hydrogels were also studied against Escherichia coli and Staphylococcus aureus by agar well diffusion method. The results demonstrated an excellent antibacterial activity of the supramolecular hydrogels. Therefore, the developed CMCh supramolecular hydrogels might be used effectively in biomedical field.

Recent Advances in Antimicrobial Hydrogels Containing Metal Ions and Metals/Metal Oxide Nanoparticles.

Recently, the rapid emergence of antibiotic-resistant pathogens has caused a serious health problem. Scientists respond to the threat by developing new antimicrobial materials to prevent or control infections caused by these pathogens. Polymer-based nanocomposite hydrogels are versatile materials as an alternative to conventional antimicrobial agents. Cross-linking of polymeric materials by metal ions or the combination of polymeric hydrogels with nanoparticles (metals and metal oxide) is a simple and effective approach for obtaining a multicomponent system with diverse functionalities. Several metals and metal oxides such as silver (Ag), gold (Au), zinc oxide (ZnO), copper oxide (CuO), titanium dioxide (TiO2) and magnesium oxide (MgO) have been loaded into hydrogels for antimicrobial applications. The incorporation of metals and metal oxide nanoparticles into hydrogels not only enhances the antimicrobial activity of hydrogels, but also improve their mechanical characteristics. Herein, we summarize recent advances in hydrogels containing metal ions, metals and metal oxide nanoparticles with potential antimicrobial properties.