Vladimir V. Vinogradov

Metal–organic frameworks as competitive materials for non-linear optics

The last five years have witnessed a huge breakthrough in the creation and the study of the properties of a new class of compounds - metamaterials. The next stage of this technological revolution will be the development of active, controllable, and non-linear metamaterials, surpassing natural media as platforms for optical data processing and quantum information applications. However, scientists are constantly faced with the need to find new methods that can ensure the formation of quantum and non-linear metamaterials with higher resolution. One such method of producing metamaterials in the future, which will provide scalability and availability, is chemical synthesis. Meanwhile, the chemical synthesis of organized 3D structures with a period of a few nanometers and a size of up to a few millimeters is not an easy task and is yet to be resolved. The most promising avenue seems to be the use of highly porous structures based on metal-organic frameworks that have demonstrated their unique properties in the field of non-linear optics (NLO) over the past three years. Thus, the aim of this review is to examine current progress and the possibilities of using metal-organic frameworks in the field of non-linear optics as chemically obtained metamaterials of the future. The review begins by presenting the theoretical principles of physical phenomena represented by mathematical descriptions for clarity. Major attention is paid to the second harmonic generation (SHG) effect. In this section we compare inorganic single crystals, which are most commonly used to study the effect in question, to organic materials, which also possess the required properties. Based on these data, we present a rationale for the possibility of studying the non-linear optical properties of metal-organic structures as well as describing the use of synthetic approaches and the difficulties associated with them. The second part of the review explicitly acquaints the reader with a new class of materials which successfully combines the positive properties of organic and inorganic materials. Using recently synthesized metal-organic frameworks and coordination polymers in the field of non-linear optics as an example, we consider synthetic approaches used for obtaining materials with desired properties and the factors to be considered in this case. Finally, probable trends towards improving the quality of the synthesized materials with regards to their further use in the field of non-linear optical effects are described.

Inkjet Color Printing by Interference Nanostructures.

Color printing technology is developing rapidly; in less than 40 years, it moved from dot matrix printers with an ink-soaked cloth ribbon to 3D printers used to make three-dimensional color objects. Nevertheless, what remained unchanged over this time is the fact that in each case, dye inks (CMYK or RGB color schemes) were exclusively used for coloring, which inevitably limits the technological possibilities and color reproduction. As a next step in printing color images and storing information, we propose the technology of producing optical nanostructures. In this paper, we report use of inkjet technology to create colored interference layers with high accuracy without the need for high-temperature fixing. This was made possible due to using titania-based colloidal ink yielding monolithic coatings with a high refractive index (2.00 ± 0.08 over the entire visible range) when naturally dried. By controlling the film thickness by using inkjet deposition, we produced images based on controlled interference and implementing color printing with one ink. The lack of dyes in the proposed method has good environmental prospects, because applied systems based on a crystalline anatase sol are nontoxic and biologically inert. The paper explains in detail the principle of producing interference images by the classical inkjet method and shows the advantages of this technique in depositing coatings with uniform thickness, which are required for large-scale interference color imaging even on unprepared polymer films. This article demonstrates the possibility of inkjet printing of nanostructures with a precision in thickness of up to 50 nm, we believe that the proposed approach will be the groundwork for developing interference color printing approach and allow to implement new methods of forming optical nano-objects by widely available techniques.

Exceptional thermal stability of therapeutical enzymes entrapped in alumina sol–gel matrices

Alumina sol-gel matrices emerge in this study as very interesting biocompatible superior materials for entrapment and stabilization of enzymes. Three therapeutic enzymes were selected for this study: acid phosphatase (AcP) which is used for treatment of bone dysplasia; a peroxidase (HRP), used for release of toxic drugs in vivo from their pro-drugs; and asparaginase (ASP) used for starving cancerous leukemia cells. The thermal stability of enzyme@alumina is so high that by heating, for instance, AcP@alumina to 60 °C, the enzymatic activity is not only kept, but actually increases, whereas is solution at that temperature there is total denaturation; and cycles of heating-cooling AcP@alumina at that temperature leave it unaffected. An unprecedented increase of six-orders of magnitude in the Arrhenius pre-factor was observed! This indicates the beneficial role of the microporosity in this type of entrapment. This exceedingly high stability is seen also with the other two enzymes: thus, when HRP or ASP are heated in solution to 75 °C the activity drops by 65% and 72%, respectively, but when entrapped it drops only by 1.2% and 1.9%, respectively. The thermal stability was studied in detail, by the follow-up of kinetics, by differential scanning calorimetry and by circular dichroism. These techniques indicate that the entrapment shifts the temperatures of denaturation higher by 30-50 °C. A special protein-friendly synthetic procedure using ultrasound was developed for the synthesis of these biomaterials. As alumina is already approved for injections (as an adjuvant in vaccinations), these findings highlight the possibility of overcoming the hurdle of the use of sol-gel materials as injectable carriers of therapeutic components.

Low-temperature sol–gel synthesis of crystalline materials

Sol–gel chemistry has opened a new era of modern materials science by enabling the production of ceramic materials at near-room temperature. Thousands of papers have been published since its inception, and new hybrid materials and composites widely used in our everyday life have been obtained. From a chemical point of view, these materials actually have compositions identical to their high-temperature ceramic analogs, but there is a drastic difference in structure and phase composition. In the majority of cases, oxide systems produced using the sol–gel method possess an amorphous structure and huge surface area with narrow micro/mesopore size distribution. At the same time, there are a great variety of oxides and mixed-oxide systems with quite a number of polymorphic modifications and, consequently, certain properties can only be produced by high-temperature treatment. Investigation of the mechanisms and methods of crystallization for such systems in the colloidal state at temperatures less than 100 °C would significantly contribute to the development of new materials obtained by low-temperature sol–gel synthesis. Taking into account the millions of different thermosensitive organic, inorganic, and bio-organic substances that could be used in producing hybrids and composites, the potential of low-temperature sol–gel technology is immense. In fact, it is a ‘second wind’ for developing classical sol–gel technology, with its more than a hundred-year history. The present review describes the fundamental principles of crystallization of oxide sol–gel systems in solution and gives examples of the applications of composites produced by low-temperature sol–gel synthesis.

A universal magnetic ferrofluid: Nanomagnetite stable hydrosol with no added dispersants and at neutral pH.

A facile method to produce highly stable magnetite magnetic fluid at neutral pH without any stabilizing agents, resulting in pure Fe3O4 nanoparticles dispersed in water is described. The hydrosol which consists of only two components - magnetite and water - behaves as a typical ferrofluid, that is, although it responds to a magnetic field, the magnetic particles cannot be phase-separated from the water by that field. No such pure magnetic fluid have been described before, making it a universal carrier which can be easily modified for any application in materials science and chemistry, and in particular for a range of applications where non-corrosivity, low viscosity, and mild conditions are needed, such as in most bioapplications and in nano electro-mechanical systems. Under optimal conditions the hydrosol is stable for at least three months.

Exceptional thermal stability of industrially-important enzymes by entrapment within nano-boehmite derived alumina

We developed an alumina sol–gel matrix based on boehmite nanorods as a superior carrier for enzyme immobilization. Proteinase and xylanase were chosen for this study, as important representatives of industrially applied enzymes. For these two enzymes we observed exceptional thermal stability by entrapment within the alumina (enzyme@alumina). We show – using kinetics, DSC and CD analyses – that alumina holds strongly and thus keeps the native structures of the proteins, preventing unfolding at high temperatures. For instance, the activity of xylanase entrapped within alumina increases with temperature up to 80 °C (!), whereas being in solution or entrapped in silica drops the activities to zero at that temperature; whereas CD clearly shows that proteinase undergoes conformational changes above 30 °C, in the case of the entrapped enzyme, the ellipticity remains constant up to 90 °C. The importance of the nanoporosity of the nanorods derived alumina is shown for this superior stability. The findings open the door to potential new applications of these enzymes for high temperature organic syntheses.

Leach-proof magnetic thrombolytic nanoparticles and coatings of enhanced activity

Despite the fact that magnetic thrombolytic composites is an emerging area, all known so far systems are based on the similar mechanism of action: thrombolytic enzyme releases from the magnetic carrier leaving non-active matrix, thus making the whole system active only for a limited period of time. Such systems often have very complex structure organization and composition, consisting of materials not approved for parenteral injection, making them poor candidates for real clinical trials and implementation. Here we report, for the first time, the production of thrombolytic magnetic composite material with non-releasing behavior and prolonged action. Obtained composite shows good thrombolytic activity, consists of fully biocompatible materials and could be applied as infinitely active thrombolytic coatings or magnetically-targetable thrombolytic agents.

Silica Foams for Fire Prevention and Firefighting

We report the new development of fire-extinguishing agents employing the latest technology of fighting and preventing fires. The in situ technology of fighting fires and explosions involves using large-scale ultrafast-gelated foams, which possess new properties and unique characteristics, in particular, exceptional thermal stability, mechanical durability, and full biocompatibility. We provide a detailed description of the physicochemical processes of silica foam formation at the molecular level and functional comparison with current fire-extinguishing and fire-fighting agents. The new method allows to produce controllable gelation silica hybrid foams in the range from 2 to 30 s up to 100 Pa·s viscosity. Chemical structure and hierarchical morphology obtained by scanning electron microscopy and transmission electron microscopy images develop thermal insulation capabilities of the foams, reaching a specific heat value of more than 2.5 kJ/(kg·°C). The produced foam consists of organized silica nanoparticles as determined by X-ray photoelectron spectroscopy and X-ray diffraction analysis with a narrow particle size distribution of ∼10-20 nm. As a result of fire-extinguishing tests, it is shown that the extinguishing efficiency exhibited by silica-based sol-gel foams is almost 50 times higher than that for ordinary water and 15 times better than that for state-of-the-art firefighting agent aqueous film forming foam. The biodegradation index determined by the time of the induction period was only 3 d, while even for conventional foaming agents this index is several times higher.

Biocomposites for wound-healing based on sol–gel magnetite

Currently, efficient wound-healing materials are booming due to increasing health care costs and world population aging, but also because of a sharp increase in the incidence of diabetes and obesity. Exacting demands are placed upon modern wound-healing materials as these should affect all stages of healing by accelerating them. In this paper, we demonstrate for the first time that drug entrapped magnetite xerogels can be effectively used for this purpose. To prepare a healing biocomposite, we have combined four medicaments in a magnetite matrix: chlorhexidine digluconate as an antimicrobial agent, lidocaine as a painkiller, prednisolone as an anti-inflammatory agent and chymotrypsin as a necrolytic agent. Compared to the control group, the wound healing rate with a biocomposite exhibited a ∼1.5-fold increase (21 and 14 days for complete healing, respectively). Moreover application of a magnetite-based biocomposite provided strong scar size decrease. Characteristics of the magnetite matrix as well as wound-healing composite material are fully described by XRD, XPS, SEM, TEM and N2 physisorption analysis.

A synergistic biocomposite for wound healing and decreasing scar size based on sol–gel alumina

The synthesis of new biocomposites exhibiting a synergistic effect is a promising step in the healing of acute and chronic wounds. In the present study we have combined four materials: chlorhexidine digluconate as a antimicrobial agent, lidocaine as a painkiller, chymotrypsin as a necrolytic agent, and sol–gel processed alumina as a carrier for the sustained delivery of drugs and as an established wound healer. Composites were synthesized and characterized for surface morphology, crystalline structure and in vitro drug release. In vivo wound healing efficacy was assessed using a full thickness excision wound model in Wistar rats. The main result, was that a marked decrease in scar size was observed because of the wound healing composite, in fact the area of the scar in the test group of rats was 2.4 times smaller than that in the control group. Wound closure analysis revealed that complete epithelialization was observed after 15 ± 1 days using the biocomposite, whereas this took 17 ± 1 days and 19 ± 1 days using the healing solution alone or pure alumina gel, respectively. It was concluded that the synergistic combination of healing drugs, with sol–gel alumina as dressing material, provides a highly attractive biomaterial for the treatment of surface wounds, burns and foot ulcers.