Luís F. Santos

Rare-earth-doped transparent glass ceramics

Abstract Glass ceramics are a known class of polycrystalline ceramic materials, where, depending on the glass matrix and the particular crystalline phases, one can obtain materials with improved mechanical, thermal, electrical or optical properties. The characteristics and applications of optical glass ceramics are reviewed, with particular emphasis on rare-earth-doped transparent glass ceramics for photonics, including the search for new transparent glass ceramic compositions and the development of suitable methods to process such materials into functional devices.

Tissue characterization using high wave number Raman spectroscopy.

Raman spectroscopy is a powerful diagnostic tool, enabling tissue identification and classification. Mostly, the so-called fingerprint (approximately 400-1800 cm(-1)) spectral region is used. In vivo application often requires small flexible fiber-optic probes, and is hindered by the intense Raman signal that is generated in the fused silica core of the fiber. This necessitates filtering of laser light, which is guided to the tissue, and of the scattered light collected from the tissue, leading to complex and expensive designs. Fused silica has no Raman signal in the high wave number region (2400-3800 cm(-1)). This enables the use of a single unfiltered fiber to guide laser light to the tissue and to collect scattered light in this spectral region. We show, by means of a comparison of in vitro Raman microspectroscopic maps of thin tissue sections (brain tumors, bladder), measured both in the high wave number region and in the fingerprint region, that essentially the same diagnostic information is obtained in the two wave number regions. This suggests that for many clinical applications the technological hurdle of designing and constructing suitable fiber-optic probes may be eliminated by using the high wave number region and a simple piece of standard optical fiber.

Vibrational spectra and structure of alkali germanate glasses

Abstract Numerous structural studies have been carried out so far to elucidate the nature of the so-called germanate anomaly, but the correct structural model is still unknown. The objective of the present paper is to shed further light on the structure of alkali germanate glasses, namely those containing sodium and cesium oxides. Two series of glass samples were prepared, having molar compositions (100−x)GeO2·xM2O (M=Na and 0⩽x⩽35, or M=Cs and 0⩽x⩽25). Polarized Raman and infrared absorption spectra were recorded for these glasses, as a function of the alkali content; infrared reflectivity measurements were also carried out and Kramers–Kronig analysis of the near-normal incidence spectra were compared with the polarized Raman spectra of the glasses. An overall structural picture has emerged, which is compatible with the presence of a fraction of higher coordinated Ge atoms, either five-fold or six-fold.

XPS and NEXAFS studies of rare-earth doped amorphous sol–gel films

Abstract Information about the structural environment of rare-earth (RE) ions in different host matrices is necessary in order to select a glass composition with optimized spectral properties for integrated optic devices, such as lasers or amplifiers. The present study is aimed at determining the influence of co-dopant elements (P and Al) on the structural environment around (RE) cations (Nd 3+ and Er 3+ ) in amorphous silica–titania matrices. For this purpose, thin film samples in the SiO 2 –TiO 2 –P 2 O 5 –Nd 2 O 3 and SiO 2 –TiO 2 –Al 2 O 3 –Er 2 O 3 systems were prepared by sol–gel processing and studied by X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS), after being densified at 900°C. These measurements allowed the determination of the concentrations of the different types of oxygen atoms present in the Si–O–Ti, Si–O–P and Si–O–Al bonding sequences and these were found to reach maximum total values for compositions containing ∼10 mol% of the PO 2.5 or AlO 1.5 co-dopants.

Raman spectra and structure of fluoroaluminophosphate glasses

Fluoroaluminophosphate glasses are promising materials for rare earth doping, but their structure is not yet well known. The structural changes which take place in the 39AlF 3 -11NaF-10LiF-(40 - x)(CaF 2 -MgF 2 -SrF 2 -BaF 2 )-xNaPO 3 system, with increasing NaPO 3 content (0-15 mol%), have been studied by polarised Raman and also infrared reflectance spectroscopies. The Raman spectra of the glasses, at low-phosphate content, presented a polarised band peaking near 520 cm -1 , with a shoulder on the high-frequency side, which was the band with the largest amplitude. This band could be deconvoluted into three Gaussian functions near 515, 570 and 620 cm -1 , assigned to octahedral [AlF 6 ], five-coordinated [AlF 5 ] and tetrahedral [AlF 4 ] structural units, respectively. The addition of NaPO 3 appeared to promote the formation of [AlF 5 ] units up to 3 mol% and to increase the conversion of [AlF 4 ] and [AlF 5 ] into [AlF 6 ] octahedra, beyond 3 mol% of NaPO 3 . A picture has emerged for the structure of these ionic glasses, in which, for the low-phosphate content, the largest fraction of structural units are [AlF 4 ] tetrahedra, containing both bridging and non-bridging fluorine, some of which are replaced by [AlF 6 ] octahedra, surrounded mostly by non-bridging fluorine, as the NaPO 3 content increases, bonded to short phosphate chains.

Structure, surface reactivity and physico-chemical degradation of fluoride containing phospho-silicate glasses

We report on the structure, apatite-forming ability and physicochemical degradation of glasses along the fluorapatite [FA; Ca5(PO4)3F]–diopside (Di; CaMgSi2O6) join. A series of glasses with varying FA/Di ratio have been synthesised by melt-quenching technique. The amorphous glasses could be obtained only for compositions up to 40 wt.% of FA. The detailed structural analysis of the glasses has been made by infrared spectroscopy (FTIR), Raman spectroscopy and magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR). Silicon was predominantly present as Q2 (Si) species while phosphorus was found in an orthophosphate type environment in all the investigated glasses. The apatite forming ability of glasses was investigated by immersion of the glass powders in simulated body fluid (SBF) for times varying between 1 h–28 days. An extensive precipitation of calcite (CaCO3) after immersion in SBF was found in all the glasses, which considerably masked the formation of hydroxyapatite [HA; Ca5(PO4)3OH]. The possible mechanism favouring the formation of calcite instead of HA has been explained on the basis of experimental results obtained for the structure of the glasses, leaching profile of glass powders in SBF solution and pH variation in SBF solution. Furthermore, the physico-chemical degradation of the glasses has been studied in accordance with ISO 10993-14 “Biological evaluation of medical devices – Part 14: Identification and quantification of degradation products from ceramics” in Tris HCl and citric acid buffer. All the FA containing glasses exhibited a weight gain (instead of weight loss) after immersion in citric acid buffer due to the formation of different crystalline products.

Comparison of two hydrogel formulations for drug release in ophthalmic lenses.

In the present work two types of polymers were investigated as drug releasing contact lens materials: a poly-hydroxyethylmethacrylate (pHEMA) based hydrogel and a silicone hydrogel. The silicone hydrogel resulted from the addition of TRIS, a hydrophobic monomer containing silicon (3-tris(trimethylsilyloxy)silylpropyl 2-methylprop-2-enoate), to pHEMA. Both hydrogels were loaded with an antibiotic (levofloxacin) and an antiseptic (chlorhexidine) by soaking in the drug solutions. The hydrogel properties were determined to be within the range demanded for lens materials. The release profiles of both drugs from the hydrogels were obtained and eventual drug/polymer interactions were assessed with the help of Raman spectra. A mathematical model, developed to mimic the eye conditions, was applied to the experimental results in order to predict the in vivo efficacy of the studied systems. The release profiles were compared with those resulting from the application of commercial eyedrops. The pHEMA based hydrogel demonstrated to be the best material to achieve a controlled release of levofloxacin. In the case of chlorhexidine, the silicone hydrogel seems to lead to better results. In both cases, our results suggest that these materials are adequate for the preparation of daily disposable therapeutic contact lenses.

Local order around Er3+ ions in SiO2–TiO2–Al2O3 glassy films studied by EXAFS

Abstract The structural environments of Er3+ ions were studied by extended X-ray absorption fine structure (EXAFS) measurements above the Er LIII edge, in SiO2–TiO2–Al2O3 sol–gel derived glassy films with varying Er and co-dopant (Al) contents. The 4 I 13/2 excited state lifetimes of the Er3+ ion were also measured for selected samples containing equal amounts of Er2O3 and Yb2O3, the latter being added as a fluorescence sensitizer. Both the first and the second Er3+ co-ordination shells were analyzed. The first co-ordination shell was composed of ∼6–7 oxygen atoms at distances varying between 0.225 and 0.229 nm, with small Debye–Waller factors, varying between 1.2 and 2.2×10 −4 nm 2 . The second shell was basically composed of tetrahedrally co-ordinated second near neighbor (SNN) atoms, bonded through corner-sharing Er–O–SNN bridges. The SNN species were found to be either Si, or a combination of Si and Al, depending on the matrix composition. Although the measured lifetime values for the Er3+ fluorescence at 1.54 μm varied between ca. 0.5 and 5.0 ms, depending on the Al content and indicated the occurrence of Er3+ concentration quenching above 0.5 at.% Er in films co-doped with 9% Al, no Er–Er interactions were detected by EXAFS in films doped with up to 1.75% Er, within the sensitivity of this technique (∼0.5 nm). This suggests that there was no rare-earth (RE) clustering in the present films, but that certain mechanisms of RE ion–ion interactions at longer distances were active.

Chitosan/alginate based multilayers to control drug release from ophthalmic lens.

In this study we investigated the possibility of using layer-by-layer deposition, based in natural polymers (chitosan and alginate), to control the release of different ophthalmic drugs from three types of lens materials: a silicone-based hydrogel recently proposed by our group as drug releasing soft contact lens (SCL) material and two commercially available materials: CI26Y for intraocular lens (IOLs) and Definitive 50 for SCLs. The optimised coating, consisting in one double layer of (alginate - CaCl2)/(chitosan+glyoxal) topped with a final alginate-CaCl2 layer to avoid chitosan degradation by tear fluid proteins, proved to have excellent features to control the release of the anti-inflammatory, diclofenac, while keeping or improving the physical properties of the lenses. The coating leads to a controlled release of diclofenac from SCL and IOL materials for, at least, one week. Due to its high hydrophilicity (water contact angle≈0) and biocompatibility, it should avoid the use of further surface treatments to enhance the useŕs comfort. However, the barrier effect of this coating is specific for diclofenac, giving evidence to the need of optimizing the chemical composition of the layers in view of the desired drug.

Effect of plasma treatment on the performance of two drug-loaded hydrogel formulations for therapeutic contact lenses

Although the plasma technology has long been applied to treat contact lenses, the effect of this treatment on the performance of drug-loaded contact lenses is still unclear. The objective of this work is to study the effect of nitrogen plasma treatment on two drug-loaded polymeric formulations which previously demonstrated to be suitable for therapeutic contact lenses: a poly-hydroxyethylmethacrylate (pHEMA) based hydrogel loaded with levofloxacin and a silicone-based hydrogel loaded with chlorhexidine. Modifications of the surface and the optical properties, and alterations in the drug release profiles and possible losses of the antimicrobial activities of the drugs induced by the plasma treatment were assessed. The results showed that, depending on the system and on the processing conditions, the plasma treatment may be beneficial for increasing wettability and refractive index, without degrading the lens surface. From the point of view of drug delivery, plasma irradiation at moderate power (200 W) decreased the initial release rate and the amount of released drug, maintaining the drug activity. For lower (100 W) and higher powers (300 W), almost no effect was detected because the treatment was, respectively, too soft and too aggressive for the lens materials.