Towards hybrid sol-gel devices for optoelectronic biosensors

I sensori per la rilevazione di sostanze inquinanti in acqua, terra e atmosfera così come i dispositivi biosensori per l’identificazione di proteine ed enzimi, rappresentano un interessantissimo ambito di ricerca con forti applicazioni in campo industriale. In questo contesto, una promettente possibilità è rappresentata dallo sviluppo di sensori basati sull’optoelettronica dato che essi assicurano un’alta sensibilità, una buona stabilità meccanica, la possibilità di miniaturizzare i dispositivi e di produrli su larga scala. In particolare, negli ultimi anni molti sforzi sono stati orientati allo sviluppo di sensori ottici a guida d’onda. Il principio di funzionamento di questo tipo di dispositivi è basato sull’interazione fra la componente del campo evanescente di un’onda guidata e la regione che si vuole analizzare. Lo sviluppo di dispositivi optoelettronici, d’altra parte, richiede la possibilità di sintetizzare materiali dalle qualità ottiche appropriate. In particolare, la possibilità di cambiare opportunamente l’indice di rifrazione rappresenta un aspetto fondamentale per la fabbricazione di dispositivi reali. I materiali ibridi organico-inorganici sintetizzati con la tecnologia sol-gel rappresentano una valida alternativa ai più tradizionali metodi per la fabbricazione di dispositivi ottici integrati come la tecnica dello scambio ionico o la CVD (chemical vapor deposition). Questa tecnica ibrida permette lo sviluppo di materiali dalle caratteristiche nuove in modo semplice ed economico. La presente tesi tratta dello sviluppo, della sintesi e della caratterizzazione di guide d’onda sol-gel ibride per possibili applicazioni come sensori ottici a guida d’onda. La caratterizzazione ottica dei dispositivi è stata effettuata principalmente utilizzando un apparato sperimentale sviluppato ed ottimizzato dal candidato durante il primo periodo del lavori di dottorato. Il setup si basa sia sulla tecnica m-line sia sulla tecnica dell’angolo di Brewster e rappresenta uno strumento semplice e a basso costo per l’analisi dell’indice di rifrazione di film sottili. I risultati mostrano che l’apparato permette la stima dell’indice di rifrazione a diverse lunghezze d’onda nel visibile e nel vicino infrarosso di film aventi spessore da poche decine di nanometri a diversi micron. L’errore nella determinazione dell’indice di rifrazione è compreso nel range ±0.001-0.003 a seconda della lunghezza d’onda e delle proprietà del campione. L’ottima accuratezza e affidabilità del nostro apparato è anche confermata dal confronto con i risultati ottenuti da misure di ellissometria spettroscopica. Molti sforzi sono stati indirizzati alla sintesi e caratterizzazione di diverse guide d’onda sol-gel deposte su substrati sia di vetro che di silicio. I campioni sono stati caratterizzati mediante misure di indice di rifrazione di perdite ottiche usando la tecnica dell’analisi della luce di scattering. I risultati hanno mostrato la possibilità di modulare l’indice di rifrazione fra 1.45 e 1.90 semplicemente modificando la sintesi chimica e i trattamenti post-deposizione. Perdite di propagazione dell’ordine di 3-10 dB/cm sono state misurate sui nostri campioni a seconda della lunghezza d’onda, della polarizzazione del modo selezionato. Questi valori sono piuttosto comuni per guide d’onda planari di tipo sol gel ibrido. Durante questo lavoro sono state inoltre sintetizzate a caratterizzate guide d’onda drogate con molecole luminescenti al fine di mostrare la possibilità di usare le nostre strutture guidanti come dispositivi attivi. Si è investigata anche l’opportunità di modulare l’indice di rifrazione di film ibridi utilizzando le proprietà di molecole fotosensibili esposte a luce ultravioletta. Infine, l’ultimo periodo di tesi è stato dedicato con successo ad investigare la possibilità di utilizzare la guide d’onda sol-gel ibride come strutture-base per lo sviluppo di sensori ottici a fluorescenza. A tal fine sono state condotte misure di fluorescenza eccitata mediante onda evanescente.Environment sensors for the detection of polluting substances in water, earth and atmosphere as well as biosensor devices for the recognition of proteins and enzymes represent a very intriguing topic for both research and industrial applications. In this framework, a very promising alternative is represented by the development of sensors based on optoelectronic technology since they combine high sensitivity, mechanical stability, miniaturization and the possibility of mass-production. In particular, extensive research have been devoted to evanescent-field-based optical waveguide sensors. The operation principle of this kind of devices is based on the interaction between the evanescent field component of a guided optical wave and the monitoring region. The development of optoelectronic devices requires the possibility to design materials with suitable optical properties. In particular, the possibility of changing appropriately the refractive index represents a fundamental step for design and fabrication of real devices. Hybrid organic-inorganic materials synthesized by sol-gel technology seem to be a valid alternative to more traditional methods such as ion exchange or chemical vapor deposition for fabrication of integrated optical devices. Hybrid materials combining organic and inorganic networks allow the design and fabrication of new materials with appropriate features in a simple and economic way. This thesis reported on the design, synthesis and characterization of hybrid sol-gel-based waveguides for possible applications as fluorescence-based optical sensors. Optical characterization has been accomplished using a home-made experimental setup built and optimized by the candidate during the first period of the PhD fellowship. The setup is based on both m-line and Brewster methods and represents a completely non-destructive, low cost and very simple tool for thin film refractive index estimation. Results have shown that the apparatus allows the estimation of refractive index at different wavelengths in the visible and near infrared spectral region for films having thickness from few tens of nanometers to several micrometers. The error in refractive index determination was in the range ±0.001-0.003, depending on wavelength and sample features. Comparison with results obtained by ellipsometric measurements have confirmed the high accuracy and reliability of our setup. Many efforts was dedicated to the synthesis and characterization of different hybrid sol-gel waveguides grown both on silicon and glass substrates. Samples were characterized by refractive index determination and propagation loss measurements using scattered light detection technique. Results have shown the possibility to modulate quite easily the refractive index from 1.45 to about 1.90 playing on the chemical synthesis and on the post-deposition treatments. Propagation loss coefficients in the range 3-10 dB/cm were commonly obtained on our samples, depending on wavelengths, polarization, and mode selected. These values are quite common for planar organic/inorganic sol-gel based waveguides. Waveguides doped with fluorescent molecules were synthesized and characterized showing the possibility to use our structures as active optical devices. The modulation of refractive index of hybrid films using photosensitive molecules was investigated exposing films to different UV light dose. Moreover, first efforts to fabricate channel waveguides exploiting photolithographic techniques were accomplished. Finally, the possibility to use hybrid sol-gel planar waveguides as building blocks for a fluorescence-based optical sensor has been demonstrated performing measurements of fluorescence excited by guided wave evanescent field

Spectroscopic ellipsometry analyses of thin films in different environments: An innovative \u201creverse side\u201d approach allowing multi angle measurements

An innovative ellipsometer sample holder has been designed and tested in order to measure thin films optical properties under different environments and so infer the porosity through effective medium approximation models. Compared to commercial cells that require a fixed angle of incidence or a cell with a cylindrical geometry, we present a simple cell in which the sample is mounted in \u2018\u2018reverse side\u2019\u2019, allowing multiple angle analyses without the need for cell windows. Standard ellipsometry measurements are compared to the \u2018\u2018reverse side\u2019\u2019 approach in order to confirm the feasibility of this new procedure, obtaining the same refractive index dispersion curves in both cases. Then different samples have been tested in \u2018\u2018reverse side\u2019\u2019 under different environments to measure porosity. The multiangle approach has been found useful to improve the fitting of the experimental data by reducing both the fitting error and the correlation between parameters

Effect of titania content on the optical properties of dye-doped hybrid sol-gel coatings

Sol-gel waveguides have been realized using a mixture of zirconium and titanium alkoxides with 3-Glycidoxypropyltrimethoxysilane (Glymo) as organic modifier. The molar ratio metal (Ti,Zr)/Glymo was 70/30 in order to obtain a material with high refractive index. Coumarin30 dye was added into the matrix during the synthesis with a concentration 2 x 10(-2) M. optical properties have been investigated by spectroscopic ellipsometry and UV-visible absorption measurements. Crack free 600 runt thick coatings were obtained with refractive index in the range 1.56-1.66, increasing with the Ti-content. Steady-state and time-resolved fluorescence measurements evidenced an almost linear decrease of the Coumarin30 fluorescence efficiency as Ti-content increases. Such a behaviour can be related to an excited state quenching mechanism. (Zr-0.5-Ti-0.5)(70)/Glymo(30) coating could be considered a fair trade-off between wiveguiding performance (n = 1.61; 12 dB/cm) and Coumarin30 fluorescence efficiency. (C) 2008 Elsevier B.V. All rights reserved

Porous inorganic thin films from bridged silsesquioxane sol-gel precursors

International audienceA sol-gel process was exploited to produce porous inorganic thin films from phenyl-bridged silsesquioxanes. The evolution of both structural and optical properties of the starting hybrid so-gel films were monitored, during synthesis and successive thermal curing steps, by Fourier transform infrared (FT-IR) spectroscopy, differential thermal analysis (DTA), thermogravimetric analysis (TGA) and spectroscopic ellipsometry (SE). Involved chemical species, structural and chemical modifications were identified when thermal treatments at increasing temperatures in the range of 60-800 degrees C were applied to the hybrid films. The progressive formation of a crosslinked silica network, template elimination and film densification were observed, resulting in completely inorganic porous thin films of low refractive index. The distinctiveness of this system directly comes from the extremely controlled and uniform dispersion of the porogen at a molecular level, which is intrinsic to the bridged silsesquioxane precursor choice. A quantitative porosity analysis was performed by environmental ellipsometric porosimetry (EEP), studying inorganic film optical and mechanical properties under different relative humidity conditions. A transmission electron microscopy (TEM) in-situ characterization of porosity size and distribution confirms the presence of a spatially structured organization of pores of a few nanometers in diameter