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Special Issue "Sol-Gel Chemistry Applied to Materials Science"

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A special issue of Materials (ISSN 1996-1944).

Deadline for manuscript submissions: closed (15 January 2018) | Viewed by 59571

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Dr. Michelina Catauro

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Department of Industrial and Information Engineering, Second University of Naples, Via Roma 29, 81031 Aversa, Italy
Interests: sol-gel technology; biomaterials; bioglass; organic–inorganic hybrid materials; drug delivery; thin films
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Dear Colleagues,

The sol-gel process has been known since the late 1800s; however, in the last three decades, the process has been extensively developed, enabling the preparation of new and advanced materials that are not easy to obtain using other methods.

The sol-gel technique is a versatile synthesis method used to produce glasses and ceramics at low temperatures. Sol-gel chemistry is based on the hydrolysis of metal alkoxide precursors into a water/alcohol solution, and the polycondensation of the formed oligomers, which leads to the formation of a 3D inorganic network. Therefore, the process involves the transition of a system from a mostly colloidal liquid (‘sol’) into a solid ‘gel’. By drying the obtained wet gel, it is possible to prepare xerogels (by exposure to low temperatures) or aerogels (by solvent extraction under supercritical conditions) or dense ceramic and glass by means of further heat treatment at higher temperatures.

The increasing attention given to this method is due to its versatility, which is derived from the various special shapes obtained directly from the gel state (e.g., monoliths, film, fibers, nanoparticles and monosized powders). Many parameters of the sol-gel process, indeed, can be modulated in order to control the chemical composition, microstructure, and, thus, the properties of the obtained materials. Moreover, the low processing temperatures allow to entrap thermolabile molecules (polymers, drugs, biomulecules, etc.) in the inorganic glassy matrix, making the sol-gel process an ideal technique to prepare organic–inorganic hybrid materials. The leading idea in their development is to take the advantageous properties of each component, which forms the hybrid, by trying to decrease or eliminate its drawbacks, through a synergic effect, which results in the generation of a new material with new properties.

The multidisciplinary approach of sol-gel technology, therefore, has allowed the development of materials for a wide range of applications: To produce optical fibers, optical or anticorrosive coatings, catalysts, wave guides, chemical sensors, biosensor, bioglass, and drug delivery systems.

Now potential applications of sol-gel technology are now continuously emerging into new areas of discovery.

It is my pleasure to invite you to submit a manuscript for this Special Issue. Full papers, communications, and reviews are all welcome.

Prof. Michelina Catauro
Guest Editor

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Keywords

Biomaterials
Drug Delivery
Nanoparticles
Thin Films and Coatings
Organic–Inorganic Hybrid Materials
Catalysts
Sensors and biosensors

Published Papers (14 papers)

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Research

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Open AccessArticle

Theoretical Design of Multilayer Dental Posts Using CAD-Based Approach and Sol-Gel Chemistry

and

Materials 2018, 11(5), 738; https://doi.org/10.3390/ma11050738 - 07 May 2018

Cited by 13 | Viewed by 3094

Abstract

A computer-aided design (CAD)-based approach and sol-gel chemistry were used to design a multilayer dental post with a compositional gradient and a Young’s modulus varying from 12.4 to 2.3 GPa in the coronal-apical direction. Specifically, we propose a theoretical multilayer post design, consisting of titanium dioxide (TiO₂) and TiO₂/poly(ε-caprolactone) (PCL) hybrid materials containing PCL up to 24% by weight obtained using the sol-gel method. The current study aimed to analyze the effect of the designed multilayer dental post in endodontically treated anterior teeth. Stress distribution was investigated along and between the post and the surrounding structures. In comparison to a metal post, the most uniform distributions with lower stress values and no significant stress concentration were found when using the multilayer post. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessArticle

Au Nanoparticle Sub-Monolayers Sandwiched between Sol-Gel Oxide Thin Films

and

Materials 2018, 11(3), 423; https://doi.org/10.3390/ma11030423 - 14 Mar 2018

Cited by 1 | Viewed by 3184

Abstract

Sub-monolayers of monodisperse Au colloids with different surface coverage have been embedded in between two different metal oxide thin films, combining sol-gel depositions and proper substrates functionalization processes. The synthetized films were TiO₂, ZnO, and NiO. X-ray diffraction shows the crystallinity of all the oxides and verifies the nominal surface coverage of Au colloids. The surface plasmon resonance (SPR) of the metal nanoparticles is affected by both bottom and top oxides: in fact, the SPR peak of Au that is sandwiched between two different oxides is centered between the SPR frequencies of Au sub-monolayers covered with only one oxide, suggesting that Au colloids effectively lay in between the two oxide layers. The desired organization of Au nanoparticles and the morphological structure of the prepared multi-layered structures has been confirmed by Rutherford backscattering spectrometry (RBS), Secondary Ion Mass Spectrometry (SIMS), and Scanning Electron Microscopy (SEM) analyses that show a high quality sandwich structure. The multi-layered structures have been also tested as optical gas sensors. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessArticle

Further Theoretical Insight into the Mechanical Properties of Polycaprolactone Loaded with Organic–Inorganic Hybrid Fillers

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Materials 2018, 11(2), 312; https://doi.org/10.3390/ma11020312 - 21 Feb 2018

Cited by 49 | Viewed by 3895

Abstract

Experimental/theoretical analyses have already been performed on poly(ε-caprolactone) (PCL) loaded with organic–inorganic fillers (PCL/TiO₂ and PCL/ZrO₂) to find a correlation between the results from the small punch test and Young’s modulus of the materials. PCL loaded with Ti2 (PCL = 12, TiO₂ = 88 wt %) and Zr2 (PCL = 12, ZrO₂ = 88 wt %) hybrid fillers showed better performances than those obtained for the other particle composition. In this context, the aim of current research is to provide further insight into the mechanical properties of PCL loaded with sol–gel-synthesized organic–inorganic hybrid fillers for bone tissue engineering. For this reason, theoretical analyses were performed by the finite element method. The results from the small punch test and Young’s modulus of the materials were newly correlated. The obtained values of Young’s modulus (193 MPa for PCL, 378 MPa for PCL/Ti2 and 415 MPa for PCL/Zr2) were higher than those obtained from a previous theoretical modelling (144 MPa for PCL, 282 MPa for PCL/Ti2 and 310 MPa for PCL/Zr2). This correlation will be an important step for the evaluation of Young’s modulus, starting from the small punch test data. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessFeature PaperArticle

Thermal Behavior and Structural Study of SiO₂/Poly(ε-caprolactone) Hybrids Synthesized via Sol-Gel Method

Stefano Vecchio Ciprioti

Riccardo Tuffi

Alessandro Dell’Era

Francesco Dal Poggetto

and

Flavia Bollino

Materials 2018, 11(2), 275; https://doi.org/10.3390/ma11020275 - 10 Feb 2018

Cited by 6 | Viewed by 2939

Abstract

SiO₂-based organic-inorganic hybrids (OIHs) are versatile materials whose properties may change significantly because of their thermal treatment. In fact, after their preparation at low temperature by the sol-gel method, they still have reactive silanol groups due to incomplete condensation reactions that can be removed by accelerating these processes upon heating them in controlled experimental conditions. In this study, the thermal behavior of pure SiO₂ and four SiO₂-based OIHs containing increasing amount (6, 12, 24 and 50 wt %) of poly(ε-caprolactone) (PCL) has been studied by simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC). The FTIR analysis of the gas mixture evolved at defined temperatures from the samples submitted to the TG experiments identified the mechanisms of thermally activated processes occurring upon heating. In particular, all samples already release ethanol at low temperature. Moreover, thermal degradation of PCL takes place in the richest-PCL sample, leading to 5-hexenoic acid, H₂O, CO₂, CO and ε-caprolactone. After the samples’ treatment at 450, 600 and 1000 °C, the X-ray diffraction (XRD) spectra revealed that they were still amorphous, while the presence of cristobalite is found in the richest-PCL material. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessArticle

Transparent Glass-Ceramics Produced by Sol-Gel: A Suitable Alternative for Photonic Materials

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Materials 2018, 11(2), 212; https://doi.org/10.3390/ma11020212 - 30 Jan 2018

Cited by 48 | Viewed by 5905

Abstract

Transparent glass-ceramics have shown interesting optical properties for several photonic applications. In particular, compositions based on oxide glass matrices with fluoride crystals embedded inside, known as oxyfluoride glass-ceramics, have gained increasing interest in the last few decades. Melt-quenching is still the most used method to prepare these materials but sol-gel has been indicated as a suitable alternative. Many papers have been published since the end of the 1990s, when these materials were prepared by sol-gel for the first time, thus a review of the achievements obtained so far is necessary. In the first part of this paper, a review of transparent sol-gel glass-ceramics is made focusing mainly on oxyfluoride compositions. Many interesting optical results have been obtained but very little innovation of synthesis and processing is found with respect to pioneering papers published 20 years ago. In the second part we describe the improvements in synthesis and processing obtained by the authors during the last five years. The main achievements are the preparation of oxyfluoride glass-ceramics with a much higher fluoride crystal fraction, at least double that reported up to now, and the first synthesis of NaGdF₄ glass-ceramics. Moreover, a new SiO₂ precursor was introduced in the synthesis, allowing for a reduction in the treatment temperature and favoring hydroxyl group removal. Interesting optical properties demonstrated the incorporation of dopant ions in the fluoride crystals, thus obtaining crystal-like spectra along with higher efficiencies with respect to xerogels, and hence demonstrating that these materials are a suitable alternative for photonic applications. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessFeature PaperArticle

Synthesis, Characterization, and Electrochemical Behavior of LiMn_xFe_(1−x)PO₄ Composites Obtained from Phenylphosphonate-Based Organic-Inorganic Hybrids

Alessandro Dell’Era

Mauro Pasquali

Elvira Maria Bauer

Stefano Vecchio Ciprioti

Francesca A. Scaramuzzo

and

Carla Lupi

Materials 2018, 11(1), 56; https://doi.org/10.3390/ma11010056 - 30 Dec 2017

Cited by 10 | Viewed by 3557

Abstract

The synthesis of organic-inorganic hybrid compounds based on phenylphosphonate and their use as precursors to form LiMn_xFe_(1−x)PO₄ composites containing carbonaceous substances with sub-micrometric morphology are presented. The experimental procedure includes the preliminary synthesis of Fe²⁺ and/or Mn²⁺ phenylphosphonates with the general formula Fe_(1−x)Mn_x[(C₆H₅PO₃)(H₂O)] (with 0 < x < 1), which are then mixed at different molar ratios with lithium carbonate. In this way the carbon, obtained from in situ partial oxidation of the precursor organic part, coats the LiMn_xFe_(1−x)PO₄ particles. After a structural and morphological characterization, the electrochemical behavior of lithium iron manganese phosphates has been compared to the one of pristine LiFePO₄ and LiMnPO₄, in order to evaluate the doping influence on the material. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessArticle

1T1R Nonvolatile Memory with Al/TiO₂/Au and Sol-Gel-Processed Insulator for Barium Zirconate Nickelate Gate in Pentacene Thin Film Transistor

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Materials 2017, 10(12), 1408; https://doi.org/10.3390/ma10121408 - 09 Dec 2017

Cited by 11 | Viewed by 4379

Abstract

A one-transistor and one-resistor (1T1R) architecture with a resistive random access memory (RRAM) cell connected to an organic thin-film transistor (OTFT) device is successfully demonstrated to avoid the cross-talk issues of only one RRAM cell. The OTFT device, which uses barium zirconate nickelate (BZN) as a dielectric layer, exhibits favorable electrical properties, such as a high field-effect mobility of 2.5 cm²/Vs, low threshold voltage of −2.8 V, and low leakage current of 10⁻¹² A, for a driver in the 1T1R operation scheme. The 1T1R architecture with a TiO₂-based RRAM cell connected with a BZN OTFT device indicates a low operation current (10 μA) and reliable data retention (over ten years). This favorable performance of the 1T1R device can be attributed to the additional barrier heights introduced by using Ni (II) acetylacetone as a substitute for acetylacetone, and the relatively low leakage current of a BZN dielectric layer. The proposed 1T1R device with low leakage current OTFT and excellent uniform resistance distribution of RRAM exhibits a good potential for use in practical low-power electronic applications. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessArticle

The Influence of the Polymer Amount on the Biological Properties of PCL/ZrO₂ Hybrid Materials Synthesized via Sol-Gel Technique

Michelina Catauro

Elisabetta Tranquillo

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Materials 2017, 10(10), 1186; https://doi.org/10.3390/ma10101186 - 17 Oct 2017

Cited by 10 | Viewed by 3062

Abstract

Organic/inorganic hybrid materials are attracting considerable attention in the biomedical area. The sol-gel process provides a convenient way to produce many bioactive organic–inorganic hybrids. Among those, poly(e-caprolactone)/zirconia (PCL/ZrO₂) hybrids have proved to be bioactive with no toxic materials. The aim of this study was to investigate the effects of these materials on the cellular response as a function of the PCL content, in order to evaluate their potential use in the biomedical field. For this purpose, PCL/ZrO₂ hybrids containing 6, 12, 24, and 50 wt % of PCL were synthesized by the sol-gel method. The effects of their presence on the NIH-3T3 fibroblast cell line carrying out direct cell number counting, MTT, cell damage assays, flow cytometry-based analysis of cell-cycle progression, and immunoblotting experiments. The results confirm and extend the findings that PCL/ZrO₂ hybrids are free from toxicity. The hybrids containing 12 and 24 wt % PCL, (more than 6 and 50 wt % ones) enhance cell proliferation when compared to pure ZrO₂ by affecting cell cycle progression. The finding that the content of PCL in PCL/ZrO₂ hybrids differently supports cell proliferation suggests that PCL/ZrO₂ hybrids could be useful tools with different potential clinical applications. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessArticle

Wet-Chemical Synthesis of 3D Stacked Thin Film Metal-Oxides for All-Solid-State Li-Ion Batteries

Evert Jonathan Van den Ham

Christophe Detavernier

Peter H. L. Notten

Marlies K. Van Bael

and

An Hardy

Materials 2017, 10(9), 1072; https://doi.org/10.3390/ma10091072 - 12 Sep 2017

Viewed by 5002

Abstract

By ultrasonic spray deposition of precursors, conformal deposition on 3D surfaces of tungsten oxide (WO₃) negative electrode and amorphous lithium lanthanum titanium oxide (LLT) solid-electrolyte has been achieved as well as an all-solid-state half-cell. Electrochemical activity was achieved of the WO₃ layers, annealed at temperatures of 500 °C. Galvanostatic measurements show a volumetric capacity (415 mAh·cm⁻³) of the deposited electrode material. In addition, electrochemical activity was shown for half-cells, created by coating WO₃ with LLT as the solid-state electrolyte. The electron blocking properties of the LLT solid-electrolyte was shown by ferrocene reduction. 3D depositions were done on various micro-sized Si template structures, showing fully covering coatings of both WO₃ and LLT. Finally, the thermal budget required for WO₃ layer deposition was minimized, which enabled attaining active WO₃ on 3D TiN/Si micro-cylinders. A 2.6-fold capacity increase for the 3D-structured WO₃ was shown, with the same current density per coated area. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessFeature PaperArticle

Sol-Gel Derived Active Material for Yb Thin-Disk Lasers

Rui M. Almeida

Tiago Ribeiro

and

Luís F. Santos

Materials 2017, 10(9), 1020; https://doi.org/10.3390/ma10091020 - 02 Sep 2017

Cited by 2 | Viewed by 3820

Abstract

A ytterbium doped active material for thin-disk laser was developed based on aluminosilicate and phosphosilicate glass matrices containing up to 30 mol% YbO_1.5. Thick films and bulk samples were prepared by sol-gel processing. The structural nature of the base material was assessed by X-ray diffraction and Raman spectroscopy and the film morphology was evidenced by scanning electron microscopy. The photoluminescence (PL) properties of different compositions, including emission spectra and lifetimes, were also studied. Er³⁺ was used as an internal reference to compare the intensities of the Yb³⁺ PL peaks at ~ 1020 nm. The Yb³⁺ PL lifetimes were found to vary between 1.0 and 0.5 ms when the Yb concentration increased from 3 to 30 mol%. Based on a figure of merit, the best active material selected was the aluminosilicate glass composition 71 SiO₂-14 AlO_1.5-15 YbO_1.5 (in mol%). An active disk, ~ 36 μm thick, consisting of a Bragg mirror, an aluminosilicate layer doped with 15 mol% Yb and an anti-reflective coating, was fabricated. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessFeature PaperArticle

Hydrophobic Coatings by Thiol-Ene Click Functionalization of Silsesquioxanes with Tunable Architecture

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Materials 2017, 10(8), 913; https://doi.org/10.3390/ma10080913 - 08 Aug 2017

Cited by 3 | Viewed by 3964

Abstract

The hydrolysis-condensation of trialkoxysilanes under strictly controlled conditions allows the production of silsesquioxanes (SSQs) with tunable size and architecture ranging from ladder to cage-like structures. These nano-objects can serve as building blocks for the preparation of hybrid organic/inorganic materials with selected properties. The SSQs growth can be tuned by simply controlling the reaction duration in the in situ water production route (ISWP), where the kinetics of the esterification reaction between carboxylic acids and alcohols rules out the extent of organosilane hydrolysis-condensation. Tunable SSQs with thiol functionalities (SH-NBBs) are suitable for further modification by exploiting the simple thiol-ene click reaction, thus allowing for modifying the wettability properties of derived coatings. In this paper, coatings were prepared from SH-NBBs with different architecture onto cotton fabrics and paper, and further functionalized with long alkyl chains by means of initiator-free UV-induced thiol-ene coupling with 1-decene (C10) and 1-tetradecene (C14). The coatings appeared to homogeneously cover the natural fibers and imparted a multi-scale roughness that was not affected by the click functionalization step. The two-step functionalization of cotton and paper warrants a stable highly hydrophobic character to the surface of natural materials that, in perspective, suggests a possible application in filtration devices for oil-water separation. Furthermore, the purification of SH-NBBs from ISWP by-products was possible during the coating process, and this step allowed for the fast, initiator-free, click-coupling of purified NBBs with C10 and C14 in solution with a nearly quantitative yield. Therefore, this approach is an alternative route to get sol-gel-derived, ladder-like, and cage-like SSQs functionalized with long alkyl chains. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessArticle

Synthesis of Bioactive Chlorogenic Acid-Silica Hybrid Materials via the Sol–Gel Route and Evaluation of Their Biocompatibility

Michelina Catauro

and

Severina Pacifico

Materials 2017, 10(7), 840; https://doi.org/10.3390/ma10070840 - 21 Jul 2017

Cited by 23 | Viewed by 3820

Abstract

Natural phenol compounds are gaining a great deal of attention because of their potential use as prophylactic and therapeutic agents in many diseases, as well as in applied science for their preventing role in oxidation deterioration. With the aim to synthetize new phenol-based materials, the sol–gel method was used to embed different content of the phenolic antioxidant chlorogenic acid (CGA) within silica matrices to obtain organic-inorganic hybrid materials. Fourier transform infrared (FTIR) measurements were used to characterize the prepared materials. The new materials were screened for their bioactivity and antioxidant potential. To this latter purpose, direct DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2,2′-azinobis-(3-ethylbenzothiazolin-6-sulfonic acid) methods were applied: radical scavenging capability appeared strongly dependent on the phenol amount in investigated hybrids, and became pronounced, mainly toward the ABTS radical cation, when materials with CGA content equal to 15 wt% and 20 wt% were analyzed. The in vitro biocompatibility of the synthetized materials was estimated by using the MTT assay towards fibroblast NIH 3T3 cells, human keratinocyte HaCaT cells, and the neuroblastoma SH-SY5Y cell line. As cell viability and morphology of tested cell lines seemed to be unaffected by new materials, the attenuated total reflectance (ATR)-FTIR method was applied to deeply measure the effects of the hybrids in the three different cell lines. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Open AccessFeature PaperArticle

Zirconia/Hydroxyapatite Composites Synthesized Via Sol-Gel: Influence of Hydroxyapatite Content and Heating on Their Biological Properties

Flavia Bollino

Emilia Armenia

and

Elisabetta Tranquillo

Materials 2017, 10(7), 757; https://doi.org/10.3390/ma10070757 - 05 Jul 2017

Cited by 67 | Viewed by 4657

Abstract

Zirconia (ZrO₂) and zirconia-based glasses and ceramics are materials proposed for use in the dental and orthopedic fields. In this work, ZrO₂ glass was modified by adding different amounts of bioactive and biocompatible hydroxyapatite (HAp). ZrO₂/HAp composites were synthesized via the sol-gel method and heated to different temperatures to induce modifications of their chemical structure, as ascertained by Fourier transform infrared spectroscopy (FTIR) analysis. The aim was to investigate the effect of both HAp content and heating on the biological performances of ZrO₂. The materials’ bioactivity was studied by soaking samples in a simulated body fluid (SBF). FTIR and scanning electron microscopy (SEM)) analyses carried out after exposure to SBF showed that all materials are bioactive, i.e., they are able to form a hydroxyapatite layer on their surface. Moreover, the samples were soaked in a solution containing bovine serum albumin (BSA). FTIR analysis proved that the synthesized materials are able to adsorb the blood protein, the first step of cell adhesion. WST-8 ([2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt]) assay showed that no cytotoxicity effects were induced by the materials’ extract. However, the results proved that bioactivity increases with both the HAp content and the temperature used for the thermal treatment, whereas biocompatibility increases with heating but is not affected by the HAp content. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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Review

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Open AccessReview

Synthesis of Hollow Sphere and 1D Structural Materials by Sol-Gel Process

Fa-Liang Li

and

Hai-Jun Zhang

Materials 2017, 10(9), 995; https://doi.org/10.3390/ma10090995 - 25 Aug 2017

Cited by 11 | Viewed by 7311

Abstract

The sol-gel method is a simple and facile wet chemical process for fabricating advanced materials with high homogeneity, high purity, and excellent chemical reactivity at a relatively low temperature. By adjusting the processing parameters, the sol-gel technique can be used to prepare hollow sphere and 1D structural materials that exhibit a wide application in the fields of catalyst, drug or gene carriers, photoactive, sensors and Li-ion batteries. This feature article reviewed the development of the preparation of hollow sphere and 1D structural materials using the sol-gel method. The effects of calcination temperature, soaking time, pH value, surfactant, etc., on the preparation of hollow sphere and 1D structural materials were summarized, and their formation mechanisms were generalized. Finally, possible future research directions of the sol-gel technique were outlined. Full article

(This article belongs to the Special Issue Sol-Gel Chemistry Applied to Materials Science)

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