Luminescent materials have found a wide variety of applications as phosphors for fluorescent lighting, display devices, X-ray monitoring and imaging, scintillators, and in biomedical imaging. The research on nanostructured materials resulted in the development of novel synthetic methods to control their structure, morphology, and doping. When the size of crystalline powders is tailored down to the nanoscale, several advantages are achieved, like the reduction of the emitted light scattering when fabricating optical nanocomposites. Nanoscale dimensions are also necessary in biotech applications where the material is required to travel in blood vessels and penetrate into cells. Finally, the realization of high density optical ceramics by nanoparticles (NPs) compaction can be pursued, especially with materials that possess cubic crystalline structure, leading to the bottom-up fabrication of a new class of luminescent materials. Hafnium oxide (HfO2) has gained interest in the last years as an attractive nanophosphor because of its excellent physical and chemical properties. In this work, the luminescence and scintillation properties of pure and rare-earth (RE) doped HfO2 NPs with a diameter < 5 nm have been investigated, obtained through a purposely designed synthetic strategy. This work was aimed at controlling the structural properties of NPs while optimizing their optical features. A particular attention has been paid to the role of doping with europium and lutetium ions through the non-aqueous sol-gel method. Structure and morphology characterization by XRD, TEM/SEM, elemental analyses, and Raman/IR vibrational spectroscopies have confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations larger than 5% mol for trivalent Lu3+ and Eu3+ ions. Optical properties have been investigated by radio- and photo-luminescence spectroscopy. Besides the relevance in application related issues, the results here reported represent an important dataset for a better comprehension of the structure-property relationship in materials confined into nanoscale dimensions. We also demonstrated the possibility of tuning the emission spectrum by multiple RE doping, while deputing the NP cubic structural stabilization to optically inert Lu3+ ions. Given the importance of HfO2 as host material for RE, its intrinsic optical response is also worth of investigation. Undoped HfO2 NPs were studied considering the effect of the size and of the crystal phase. A broad composite emission was observed in the visible range, potentially correlated both to intrinsic surface defects and to impurities. Its intensity can be varied by thermal treatments leading to surface modifications as well as to variations of particle dimensions. Its efficiency has been found to be comparable to that of standard commercial materials, evidencing the potential of pure HfO2 NPs as efficient phosphors. In parallel, we also investigated the use of emitting NPs for biological applications. Novel approaches for high contrast, deep tissue, in vivo fluorescence biomedical imaging are based on infrared-emitting NPs working in the so-called second biological window (1000 -1400 nm), where the partial transparency of tissues allows for the acquisition of high resolution, deep tissue images. In addition, the infrared excitation also leads to a reduction of auto-fluorescence generated by tissues, intra-body components, and specimen's diet. In my work, I exploited how the 1.3m emission band of Nd3+ ions embedded in SrF2 nanoparticles can be used to produce auto-fluorescence free, high contrast fluorescence images and bio-distribution studies. The strong brightness, the chemical and physical stability as well as high biocompatibility make Nd:SrF2 nanocrystals very promising infrared nanoprobes for in vivo imaging experiments in the second biological window.
I materiali luminescenti nanostrutturati sono largamente studiati per applicazioni in lampade e display, come scintillatori e nell’imaging biomedico. Pertanto, la ricerca nei nanomateriali prevede lo sviluppo di metodi di sintesi all’avanguardia per il controllo della loro struttura, morfologia e drogaggio. L’utilizzo di polveri nanocristalline per la fabbricazione di nanocompositi permette di ridurre l’incorrere di diverse problematiche come la diffusione della luce emessa; inoltre la dimensione nanometrica dei materiali è un requisito fondamentale per le applicazioni in biotecnologia, per la loro veicolazione attraverso il sangue e la penetrazione nelle cellule. Infine, la realizzazione di nanoparticelle (NP) aventi fase cristallina cubica permetterebbe la progettazione di ceramiche ottiche ad alta densità e quindi di una nuova classe di materiali luminescenti. L’ossido di afnio (HfO2) è stato considerato come fosforo di grande interesse grazie alle sue eccellenti proprietà chimiche e fisiche. In questo lavoro si sono investigate le proprietà di luminescenza e scintillazione di NP di HfO2 di diametro < 5 nm. Le NP pure e drogate con ioni di terre rare (TR) sono state fabbricate attraverso un processo di sintesi appositamente elaborato e ottimizzato. Il lavoro condotto ha permesso di controllare simultaneamente le proprietà strutturali e di luminescenza nelle NP. Particolare attenzione è stata rivolta al ruolo del drogaggio con ioni europio e lutezio tramite sintesi sol-gel non acquosa. L’analisi elementale, la caratterizzazione strutturale e morfologica con XRD, TEM/SEM, insieme alla spettroscopia vibrazionale Raman/IR, hanno confermato la trasformazione della fase cristallina da monoclina a cubica per concentrazioni > 5% mol di ioni Lu3+e Eu3+. Le proprietà ottiche sono state studiate attraverso tecniche di radio- e foto-luminescenza. I risultati ottenuti rappresentano un importante traguardo sia per una migliore comprensione della relazione struttura-proprietà di materiali di dimensione nanometrica, che per l’analisi della applicabilità di questi ultimi in campo tecnologico. In questo lavoro è stata dimostrata la possibilità di modificare lo spettro di emissione delle NP drogando simultaneamente con diverse TR e stabilizzando la fase cubica con l’incorporazione di ioni di Lu3+ otticamente inerte. L’HfO2 è un promettente materiale sia come matrice ospite per le TR che per la sua luminescenza intrinseca. NP non drogate sono state studiate considerando l’effetto della dimensione e della fase cristallina sulla luminescenza. Si è individuata la presenza di una banda di emissione composita nell’intervallo di lunghezze d’onda visibili, possibilmente correlata a difetti di superficie intrinseci o a impurezze del materiale. La sua intensità varia in funzione di trattamenti termici che portano alla modifica della superficie e del diametro delle NP, ed è confrontabile all’efficienza di materiali luminescenti commerciali usati come standard. In parallelo, sono state studiate le proprietà di NP luminescenti per applicazioni biologiche. Le nuove tecniche diagnostiche per immagini in vivo a fluorescenza con alta risoluzione e profondità di penetrazione si basano sulla luminescenza di NP nella finestra di trasparenza del tessuto biologico (1000-1400 nm). Inoltre, l’eccitazione a basse energie porta alla riduzione dell’autofluorescenza generata dai tessuti, componenti intra corporee e molecole organiche della dieta degli animali trattati con le NP. In questa ricerca, è stato dimostrato che l’utilizzo della banda a 1.3 m di ioni di Nd3+ in SrF2 permette di effettuare analisi di biodistribuzione e ottenere immagini in assenza di autofluorescenza e ad alto contrasto. La luminosità, la stabilità chimica e fisica così come l’elevata biocompatibilità rendono le NP di SrF2 promettenti per applicazioni biotecniche, bioimmagini a fluorescenza e future strategie diagnostiche.
(2015). Structural and morphological tuning of inorganic luminescent nanophosphors - towards applications in sensing and lighting.. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2015).
Structural and morphological tuning of inorganic luminescent nanophosphors - towards applications in sensing and lighting.
VILLA, IRENE
2015
Abstract
Luminescent materials have found a wide variety of applications as phosphors for fluorescent lighting, display devices, X-ray monitoring and imaging, scintillators, and in biomedical imaging. The research on nanostructured materials resulted in the development of novel synthetic methods to control their structure, morphology, and doping. When the size of crystalline powders is tailored down to the nanoscale, several advantages are achieved, like the reduction of the emitted light scattering when fabricating optical nanocomposites. Nanoscale dimensions are also necessary in biotech applications where the material is required to travel in blood vessels and penetrate into cells. Finally, the realization of high density optical ceramics by nanoparticles (NPs) compaction can be pursued, especially with materials that possess cubic crystalline structure, leading to the bottom-up fabrication of a new class of luminescent materials. Hafnium oxide (HfO2) has gained interest in the last years as an attractive nanophosphor because of its excellent physical and chemical properties. In this work, the luminescence and scintillation properties of pure and rare-earth (RE) doped HfO2 NPs with a diameter < 5 nm have been investigated, obtained through a purposely designed synthetic strategy. This work was aimed at controlling the structural properties of NPs while optimizing their optical features. A particular attention has been paid to the role of doping with europium and lutetium ions through the non-aqueous sol-gel method. Structure and morphology characterization by XRD, TEM/SEM, elemental analyses, and Raman/IR vibrational spectroscopies have confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations larger than 5% mol for trivalent Lu3+ and Eu3+ ions. Optical properties have been investigated by radio- and photo-luminescence spectroscopy. Besides the relevance in application related issues, the results here reported represent an important dataset for a better comprehension of the structure-property relationship in materials confined into nanoscale dimensions. We also demonstrated the possibility of tuning the emission spectrum by multiple RE doping, while deputing the NP cubic structural stabilization to optically inert Lu3+ ions. Given the importance of HfO2 as host material for RE, its intrinsic optical response is also worth of investigation. Undoped HfO2 NPs were studied considering the effect of the size and of the crystal phase. A broad composite emission was observed in the visible range, potentially correlated both to intrinsic surface defects and to impurities. Its intensity can be varied by thermal treatments leading to surface modifications as well as to variations of particle dimensions. Its efficiency has been found to be comparable to that of standard commercial materials, evidencing the potential of pure HfO2 NPs as efficient phosphors. In parallel, we also investigated the use of emitting NPs for biological applications. Novel approaches for high contrast, deep tissue, in vivo fluorescence biomedical imaging are based on infrared-emitting NPs working in the so-called second biological window (1000 -1400 nm), where the partial transparency of tissues allows for the acquisition of high resolution, deep tissue images. In addition, the infrared excitation also leads to a reduction of auto-fluorescence generated by tissues, intra-body components, and specimen's diet. In my work, I exploited how the 1.3m emission band of Nd3+ ions embedded in SrF2 nanoparticles can be used to produce auto-fluorescence free, high contrast fluorescence images and bio-distribution studies. The strong brightness, the chemical and physical stability as well as high biocompatibility make Nd:SrF2 nanocrystals very promising infrared nanoprobes for in vivo imaging experiments in the second biological window.File | Dimensione | Formato | |
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PhD_unimib_760820.pdf
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Descrizione: Tesi dottorato
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Doctoral thesis
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