Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Chrysantenbedrijf noodgedwongen proefkonijn met diffuus glas : Linflowers kiest voor meer licht en diffusiteit
    Dueck, Tom - \ 2016
    greenhouse horticulture - cut flowers - chrysanthemums - diffused glass - light transmission - crop production - optical properties - cultural methods - decision making

    Linflowers is het eerste chrysantenbedrijf met diffuus glas. De keuze was een lastige, want behalve aan praktijkervaring ontbreekt het ook aan onderzoeksresultaten. Eigenlijk hebben de broers Van Tuijl vooral gekozen voor hoge lichtdoorlatendheid en is de diffusiteit een extraatje. De eerste ervaringen zijn positief.

    Ordering properties of oligomeric columnar discotic liquid crystals
    Umesh, C.P. - \ 2015
    Wageningen University. Promotor(en): Han Zuilhof, co-promotor(en): Ton Marcelis. - Wageningen : Wageningen University - ISBN 9789462575066 - 152
    liquid crystals - optical properties - synthesis - analytical methods - vloeibare kristallen - optische eigenschappen - synthese - analytische methoden

    The synthesis and liquid crystalline ordering properties of oligomeric discotic liquid crystals were investigated. The phase behaviour and surface ordering properties are dependent on among others core type, spacer length and fluorination.

    Meer licht en toch energie besparen in ideale winterlichtkas : Effect van condens nog in onderzoek
    Arkesteijn, M. ; Kempkes, F.L.K. ; Swinkels, G.L.A.M. - \ 2015
    Onder Glas 12 (2015)6/7. - p. 18 - 19.
    glastuinbouw - kastechniek - beglazing - optische eigenschappen - lichtdoorlating - absorptiegraad - winter - innovaties - optimalisatiemethoden - landbouwkundig onderzoek - greenhouse horticulture - greenhouse technology - glazing - optical properties - light transmission - absorbance - winter - innovations - optimization methods - agricultural research
    Een consortium ontwerpt samen met Wageningen UR een kas die in de periode van oktober tot en met maart maximaal licht doorlaat. Het streven is om in die maanden hiermee een meerproductie van 10% te halen. De lichtwinst moet komen door een optelsom van verschillende aanpassingen en vernieuwingen. Na de theoretische simulatiemodellen en fysieke schaalmodellen, wordt volgend jaar de eerste winterlichtkas als ‘proof of principle’ gebouwd op het Innovatie- en Demonstratiecentrum in Bleiswijk.
    Besteed zorg aan het wensenlijstje en het objectief doormeten : Silke Hemming over keuze juiste diffuse kasdek
    Staalduinen, J. van; Hemming, S. - \ 2015
    Onder Glas 12 (2015)6/7. - p. 10 - 11.
    glastuinbouw - beglazing - diffuus glas - optische eigenschappen - condenseren - verstrooiing - besluitvorming - bemonsteren - behoeften - greenhouse horticulture - glazing - diffused glass - optical properties - condensation - scattering - decision making - sampling - requirements
    Dat een diffuus kasdek – met of zonder antireflectiecoating – meerwaarde biedt ten opzichte van traditioneel basisglas weten telers inmiddels. Het is echter niet eenvoudig om tot een juiste glaskeuze te komen. Onderzoekster Silke Hemming zet de relevante aspecten op een rij. Een onderschatte factor is het doormeten van glasmonsters op basis van goede steekproeven bij grootschalige nieuwbouwprojecten.
    Coniferous needle-leaves, shots and canopies : a remote sensing approach
    Yanez Rausell, L. - \ 2014
    Wageningen University. Promotor(en): Michael Schaepman, co-promotor(en): Z. Malenovsky; Jan Clevers. - Wageningen : Wageningen University - ISBN 9789461738684 - 154
    remote sensing - naaldbossen - pinopsida - naalden, coniferen - bladeren - optische eigenschappen - modellen - remote sensing - coniferous forests - pinopsida - conifer needles - leaves - optical properties - models

    Coniferous forests are important in the regulation of the Earth’s climate and thus continuous monitoring of these ecosystems is crucial to better understand potential responses to climate change. Optical remote sensing (RS) provides powerful methods for the estimation of essential climate variables and for global forest monitoring. However, coniferous forests represent challenging targets for RS methods, mainly due to structural features specific for coniferous trees (e.g. narrow needle leaves, shoot clumping) whose effects on the RS signal are not yet known or not yet fully understood.

    Recognizing the need for a better adaptation of RS methods to such spatially heterogeneous and structurally complex canopies, this thesis contributes to improving the interpretation of the remotely sensed optical signal reflected from coniferous stands by focusing on specific knowledge gaps identified in the RS methods at different scales of the coniferous canopies. In addition, it explores the application of approaches that simplify the way the structural complexity of such an environment is tackled when using canopy-level radiative transfer approaches. Three main levels based on the identified gaps were defined for the analysis: (needle) leaf level (chapter 2 and 3); shoot level (chapter 4) and canopy level (chapter 5).

    At leaf levelthis thesis contributes to minimizing the uncertainties and errors related to leaf optical measuring methods adapted for needle leaves. Although optical properties of coniferous leaves are extensively used in RS approaches (i.e. as input or as validation data), there is only a limited number of techniques available for measuring coniferous leaves. The first focus of this thesis was to review the shortcomings and uncertainties of such methods in order to identify application limits and potential improvements (chapter 2). A review showed that a more standardized measuring protocol was needed, for which measurement uncertainties and errors had to be identified, quantified and preferably removed or minimized. Thus, an experimental set-up improving the original method of Mesarch et al. (1999) was presented (chapter 3), which focused on analyzing uncertainties caused by the presence of the sample holder and by the multiple scattering triggered by both the shape of the specific needle cross-section, and the distance between the needles composing a sample. Results showed that both the sample holder and the multiple scattering, triggered specially by the shape of the non-flat cross section of the coniferous needle-leaves, had a non-negligible effect on the optical signal when measured using a standard spectroradiometer coupled to a single-beam integrating sphere and following the method suggested by Mesarch. Thus, approaches designed to measure optical properties of non-flat coniferous needle samples more comprehensively should take into account these effects in their current signal correction algorithms.

    Needle clumping into shoots quickly transforms the optical signal making the description of the canopy radiative transfer a complex task and encouraging the search for simplified yet robust approaches. Thus, subsequent steps in this thesis focus on one such simplified approach, known as the recollision probability theory (“p-theory”), applied at two hierarchical levels, i.e., shoots (Chapter 4) and the whole canopy (Chapter 5).At shoot level, an empirical verification of the relationship between the photon recollision probability and a structural parameter called STAR was investigated. The approach allows upscaling needle albedo to shoot albedo and was previously theoretically tested only (chapter 4). For this analysis empirical optical measurements of Scots pine needles and shoots were used. Results showed that the approach works well for the VIS and SWIR spectral regions. However, it was less accurate for the NIR and also for sparse shoots (STAR <0.15) with an uneven distribution of photon–needle interactions and a larger influence of the twig bark.

    Finally, accurate modelling of the reflectance signal at canopy levelfor coniferous canopies requires realistic representations of the forest stands, which in general implies a large number of input parameters and computationally demanding algorithms. Radiative transfer modelling based on the photon recollision probability offers an alternative for a simplified definition of the forest canopy structure. The performance of such approach for estimation of the leaf chlorophyll content from satellite imaging spectroscopy data acquired by the CHRIS-PROBA sensor was investigated. The approach was compared to a computationally more demanding one based on a detailed 3D structural description of a forest (chapter 5). For this purposes two canopy models, PARAS and DART, representing the first and second approach respectively, were used. Top-of-canopy bidirectional reflectance factors (BRF) were simulated for both models and used to calculate two optical indices, ANCB670–720 and ANMB670–720.Subsequently, the empirical relationships established between the optical indices and the needle-leaf chlorophyll content (Cab) were applied to the CHRIS-PROBA image of a Norway spruce forest stand to retrieve a map of Cab estimates. Results showed that for the spatial resolution of CHRIS-PROBA (17 m), the simpler model PARAS can be applied to retrieve plausible needle-leaf Cab estimates from satellite imaging spectroscopy data with less intensive model parameterization and reduced computational powerthan when using a model like DART. The ANMB670–720 optical indexwas more robust andresulted in a linear relationship between the Cab estimated by both models. This relationship showed, however, a systematic offset that is potentially caused by differences in the implementation of woody elements in each model or by a different parameterization of leaf optical properties. Thus, further investigation on the impact of parameterization differences related to the needle optical properties and the implementation of woody elements in such a model is recommended.

    Haze-factor maakt plaats voor F-scatter
    Swinkels, G.L.A.M. - \ 2014
    Kas techniek 2014 (2014)1. - p. 24 - 27.
    glastuinbouw - kassen - diffuus glas - innovaties - technologie - optische eigenschappen - verstrooiing - scatterometrie - greenhouse horticulture - greenhouses - diffused glass - innovations - technology - optical properties - scattering - scatterometry
    Uitgelicht licht & scherming - Wageningen UR Glastuinbouw deed vorig jaar onderzoek naar een methode om de lichtspreiding onder diffuus glas te karakteriseren. dit resulteerde in een nieuwe waarde, de zogenaamde F-scatter voor voorwaartse lichtspreiding. Leveranciers van glas en schermdoekmaterialen zijn enthousiast. Maar waar staat de F-scatter nu exact voor? En hoe wordt deze gemeten?
    Ultra-energiezuinige kassystemen met Supertransparante Micro-V Gestructureerde materialen: Rapportage in het kader van het EOS-LT programma van Agentschap NL (Openbare versie)
    Gieling, T.H. ; Ruijven, J.P.M. van; Swinkels, G.L.A.M. ; Holterman, H.J. ; Mohammadkhani, V. ; Ruijs, M.N.A. ; Rijn, C.J.M. van; Baggerman, J. ; Rath, J.K. ; Jong, M. de; Schropp, R.E.I. - \ 2013
    Wageningen : Wageningen UR Glastuinbouw (Rapporten Wageningen UR Glastuinbouw 1277) - 100
    teelt onder bescherming - glastuinbouw - materialen - innovaties - lichtrelaties - optische eigenschappen - milieufactoren - schermen - kastechniek - protected cultivation - greenhouse horticulture - materials - innovations - light relations - optical properties - environmental factors - blinds - greenhouse technology
    Doel van het in dit rapport beschreven onderzoek is het gebruiken van micro- en nanostructuren uit de PV-industrie om te komen tot een diffuus kasdekmateriaal met een hoge hemisferische transmissie. Hierbij wordt gebruik gemaakt van de kennis die in het verleden is opgedaan in het ontwikkelen van het zigzag kasdekmateriaal. Met de ontwikkelde kennis kan energiebesparing gerealiseerd worden door het toepassen van de structuren op dubbel glas, waardoor materiaal gemaakt kan worden met een hoge hemisferische transmissie, diffuse eigenschappen en een hoge isolatiewaarde. Randvoorwaarde hierin is dat het ontwikkelde kasdekmateriaal toepasbaar is in een glastuinbouwmilieu. Dit betekent dat het materiaal niet mag verouderen, condensatie de transmissie niet mag verminderen en extra vervuiling niet mag optreden. Hierbij wordt de Nederlandse situatie als uitgangspunt genomen, maar interessante structuren voor andere klimaten zullen niet worden uitgesloten.
    Nieuw lichtmodel berekent vooraf jaarrond lichtopbrengst nieuwe kastypen (interview met Gert-Jan Swinkels)
    Arkesteijn, M. ; Swinkels, G.L.A.M. - \ 2013
    Onder Glas 10 (2013)6/7. - p. 46 - 47.
    glastuinbouw - kastechniek - lichtrelaties - belichting - optische eigenschappen - wetenschappelijk onderzoek - meting - kassen - schermen - greenhouse horticulture - greenhouse technology - light relations - illumination - optical properties - scientific research - measurement - greenhouses - blinds
    Licht meten in de kas is lastig. Waar meet je licht in de kas, onder welke weersomstandigheden of in virtuele kassen op de ontwerptafel? Met het door onderzoeker Swinkels ontwikkelde lichtmodel, gebaseerd op ray-tracing, is het mogelijk om de jaarrond lichtopbrengst van een bestaande of nieuwe kas te berekenen. In combinatie met een betrouwbare meting van diffuus kasdekmateriaal betekent dit weer een stapje verder naar het ontwerpen van de ideale kas.
    F-scatterwaarden bieden meer zicht op diffuus licht (interview met Tom Dueck, Jim van Ruijven en Gert-Jan Swinkels)
    Kierkels, T. ; Dueck, T.A. ; Ruijven, J.P.M. van; Swinkels, G.L.A.M. - \ 2012
    Onder Glas 9 (2012)12. - p. 18 - 19.
    glastuinbouw - diffuus glas - materialen - licht - verstrooiing - optische eigenschappen - modellen - landbouwkundig onderzoek - groenten - greenhouse horticulture - diffused glass - materials - light - scattering - optical properties - models - agricultural research - vegetables
    Verstrooiing van licht door diffuus glas of een coating verhoogt de opbrengst van tomaat, komkommer en waarschijnlijk een hele range aan andere gewassen. Hoe hoger de haze (het percentage verstrooiing) hoe hoger de productie, blijkt uit proeven. Maar de hazefactor zegt nog niet genoeg. Het is tijd voor een nieuwe aanduiding van materiaaleigenschappen: F-scatter.
    Praktijkervaringen met de DaglichtKas
    Zwart, H.F. de; Noort, F.R. van - \ 2012
    Wageningen : Wageningen UR Glastuinbouw (rapporten GTB 1157) - 60
    glastuinbouw - kassen - daglicht - optische eigenschappen - potplanten - lichtdoorlating - zonne-energieverwarming - elektriciteit - warmteproductie - greenhouse horticulture - greenhouses - daylight - optical properties - pot plants - light transmission - solar heating - electricity - heat production
    De DaglichtKas is een innovatief ontwerp voor kassen voor schaduwminnende teelten waarbij gebruik gemaakt wordt van lenzen in het dak. Deze lenzen buigen het directe licht af naar een brandlijn waar een collector het omzet in warmte en in elektriciteit. De lens is alleen effectief voor direct licht en daardoor heeft het systeem nauwelijks effect op de diffuse doorlatendheid. Dit blijkt te resulteren in een zeer geschikt lichtklimaat voor de teelt van potplanten.
    Daglichtkas
    Zwart, Feije de - \ 2011
    greenhouse horticulture - greenhouses - double glazing - optical properties - daylight - plant development - agricultural energy production
    Glas voor de tuinbouw
    Hemming, Silke - \ 2011
    greenhouses - greenhouse horticulture - glazing - glass - light transmission - optical properties - diffused glass - plant development - agricultural research
    Energieneutrale sierteelt binnen bereik met DaglichtKas (interview met o.a. Feije de Zwart)
    Bezemer, J. ; Zwart, H.F. de - \ 2011
    Onder Glas 8 (2011)6/7. - p. 56 - 57.
    glastuinbouw - kassen - constructie - beglazing - optische eigenschappen - lichtrelaties - zonnecollectoren - energie - duurzaamheid (sustainability) - sierteelt - potplanten - snijbloemen - greenhouse horticulture - greenhouses - construction - glazing - optical properties - light relations - solar collectors - energy - sustainability - ornamental horticulture - pot plants - cut flowers
    De ontwikkeling van de zogenaamde DaglichtKas is ver gevorderd. Deze kas brengt een energieneutrale teelt binnen bereik, met name voor potplanten- en bloementelers. De splitsing van het zonlicht door een daglichtinstallatie in de kas is het geheim daarachter. Direct zonlicht wordt weggevangen voor de productie van warmte en elektriciteit, terwijl het diffuse licht voor het gewas beschikbaar blijft.
    Veel rood licht geeft compactere planten : plantlengte te sturen door combinatie van SON-T met rode en blauwe LED's
    Ieperen, W. van; Heuvelink, E. ; Kierkels, T. - \ 2011
    Onder Glas 8 (2011)3. - p. 12 - 13.
    glastuinbouw - sierplanten - belichting - led lampen - optische eigenschappen - effecten - plantenontwikkeling - groenten - potplanten - greenhouse horticulture - ornamental plants - illumination - led lamps - optical properties - effects - plant development - vegetables - pot plants
    Lichtkleuren beïnvloeden lengte en vertakking van pot- en perkplanten. Maar sturing daarmee is nog lastig. Wageningen Universiteit heeft onderzoek gedaan om meer zicht te krijgen op de mogelijkheden, met petunia en potchrysant als modelplant.
    Plantweerstand omhoog door belichting met rood licht (interview met Jantineke Hofland)
    Arkesteijn, M. ; Hofland-Zijlstra, J.D. - \ 2010
    Onder Glas 7 (2010)4. - p. 59 - 59.
    kassen - teeltsystemen - plantenziektebestrijding - ziekteresistentie - plaagresistentie - belichting - lampen - optische eigenschappen - endofyten - weerstand - gewasbescherming - glastuinbouw - greenhouses - cropping systems - plant disease control - disease resistance - pest resistance - illumination - lamps - optical properties - endophytes - resistance - plant protection - greenhouse horticulture
    Een nieuwe aanpak is nodig in de strijd tegen ziekten en plagen in de glastuinbouw. Verhoging van de plantweerstand is één van de mogelijkheden. Wageningen UR-onderzoekster Jantineke Hofland-Zijlstra bekijkt twee methoden om de plantweerstand te verhogen: via LED-belichting en via endofytische schimmels.
    Rode bladeren gaan anders met licht om dan groene bladeren (interview met Jan Snel)
    Kierkels, T. ; Snel, J.F.H. - \ 2010
    Onder Glas 7 (2010)4. - p. 52 - 53.
    kassen - belichting - lampen - lichtgevende dioden - optische eigenschappen - tests - rozen - rassen (planten) - glastuinbouw - snijbloemen - led lampen - greenhouses - illumination - lamps - light emitting diodes - optical properties - tests - roses - varieties - greenhouse horticulture - cut flowers - led lamps
    Op voorlichtingsavonden wordt vaak een standaardgrafiek getoond die aangeeft welke lichtkleuren de plant het best benut. Die geldt vooral voor groene bladeren. Roos met rood blad gaat veel minder efficiënt om met blauwgroen tot geel licht. Rood licht wordt juist beter benut. Reden om bij gewassen met afwijkende bladkleuren de efficiëntie per lichtkleur te meten als de teler LED-belichting overweegt. Dat kan met een mobiele opstelling van Wageningen UR Glastuinbouw.
    SON-T trekt in laboratoriumproef voorlopig aan het langste eind
    Bezemer, J. ; Marcelis, L.F.M. ; Meinen, E. - \ 2010
    Onder Glas 7 (2010)4. - p. 48 - 49.
    kassen - belichting - kunstlicht - lichtgevende dioden - effecten - plantenontwikkeling - lampen - optische eigenschappen - proeven op proefstations - glastuinbouw - chrysanten - snijbloemen - led lampen - greenhouses - illumination - artificial light - light emitting diodes - effects - plant development - lamps - optical properties - station tests - greenhouse horticulture - chrysanthemums - cut flowers - led lamps
    Wageningen UR Glastuinbouw deed onderzoek bij chrysant naar het effect van verschillende lamptypen. Anders dan verwacht bleken chrysanten onder SON-T-licht een hogere fotosynthese te realiseren en konden de stelen aanzienlijk vroeger (10 tot 14 dagen) worden geoogst. In hoeverre de gekozen verhouding tussen rood en blauw licht bij de LED-lampen de mindere prestaties onder deze lampen verklaart, vereist verder onderzoek.
    Synthesis and optical properties of monodisperse oligodiacetylenes
    Pilzak, G.S. - \ 2009
    Wageningen University. Promotor(en): Han Zuilhof, co-promotor(en): Ernst Sudhölter. - [S.l. : S.n. - ISBN 9789085853961 - 150
    alkynen - synthese - optische eigenschappen - onverzadigde koolwaterstoffen - alkynes - synthesis - optical properties - unsaturated hydrocarbons
    Synthesis and photophysics of functionalized silicon nanoparticles
    Rosso-Vasic, M. - \ 2008
    Wageningen University. Promotor(en): Han Zuilhof; L. De Cola. - [S.l.] : S.n. - ISBN 9789085852940 - 118
    synthese - optische eigenschappen - silicium - deeltjes - fotochemie - nanotechnologie - synthesis - optical properties - silicon - particles - photochemistry - nanotechnology
    Nanotechnology is an emerging multidisciplinary science that involves the formation, investigation and manipulation of nanoobjects (1 - 100 nm). It has a huge potential to revolutionize diverse fields as engineering and medicine since the basis of many different physical processes can now be controlled up to the nanometer-scale and since the nanoworld yields access to novel processes.
    For almost two decades, research into semiconductor nanoclusters has been focused on the properties of quantum dots (QDs) - semiconductor fragments consisting of hundreds to thousands of atoms.1 Due to their unique size–range, characteristically on the boundary between quantum mechanics and Newtonian physics, the properties of nanoparticles differ from those of the bulk and of single atoms. Quantum dots have exceptional optical and electronic properties such as: size dependent - tunable light emission wavelengths,1-3 intense fluorescence,4-7 resistance against photobleaching8 and simultaneous excitation of multiple fluorescent colors.9-12 All these qualities make QDs in many respects superior over organic dyes and fluorescent proteins that are used for bioimaging purposes so far. QDs can be synthesized from a variety of materials with different sizes, shapes and morphologies. The most studied are complex core/shell/coat (CdS/ZnS/silane) QDs which can be produced in a variety of colors (sizes). However, their high intrinsic toxicity and minimal sizes (10 – 20 nm due to their complex structure) are an issue when considering them as candidates for in vivo bioimaging. Therefore, a huge interest arose in the possible development of smaller, non-toxic, stable and more versatile nanoparticles (NPs).
    On the other side, application of Si was limited only to (micro-)electronics and its photoemission potential has not been realized for years. Due to the indirect band gap in bulk Si, light absorption and emission occur only when the absorption or the emission of a photon and a specific change in a lattice vibration mode occurs simultaneously. Consequently, the photoluminescence of bulk silicon is very weak.
    Nonetheless, by creating Si with nanoscale dimensions (silicon nanoparticles), it can be coaxed to emit visible light with relatively high efficiencies. 1-10 nm Si NPs luminesce intensely over a wide range of wavelengths, from UV (for NPs < 2 nm) to IR (NPs > 8 nm).
    Silicon surfaces can be well passivated, by creating stable Si-C bonds. The methods to tailor silicon surfaces were developed on porous and planar Si and can also be applied for the functionalization of Si NPs surfaces. Such a coating of Si NPs could prevent surface oxidation. A high luminescence, well-developed surface passivation principles, and a low inherent toxicity of Si initiated the enthusiasm for the research in Si NPs.
    The goals of the work described in this thesis are:
    - the development and optimization of methods for the preparation of stable and monodisperse Si NPs,
    - photophysical characterization of such NPs, also in dependence on their functionalization ,
    - the exploration of their possible applications, specifically in the realm of bioimaging.
    Chapter 1 describes general properties of semiconductor quantum dots and Si NPs, in particular. The origin of Si NPs luminescence is described in detail, and an overview of published methods for the synthesis of Si NPs is given, with a discussion of the advantages and drawbacks of each method.
    Experimental work is described in Chapter 2, 3, 4 and 5.
    Chapter 2 describes the first gramscale preparation of highly monodisperse alkyl-terminated Si NPs (diameter of 1.57 ± 0.21 nm) in reverse micelles.13 Both steady-state and time-resolved absorption and emission techniques, as well as FTIR (Fourier Transform InfraRed Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy) measurements are used to study their photophysical properties and chemical composition in detail. For the first time, due to the relatively efficient synthesis, the molar extinction coefficient of alkyl-terminated Si NPs is experimentally determined to be 261= 1.7 x 10-4 M-1cm-1, only a factor 4 lower than that of CdS and CdSe NPs of that size. The measured quantum yields of emission ranged from 0.12 (C10H21 capping) to 0.23 (C16H33 capping). Their UV-Vis absorption and emission spectra display clear vibrational progressions in each sample of alkyl-terminated Si NPs. The vibrational peaks were almost equidistant and appeared after 974 ± 14 cm-1. These vibrational bands evidently resemble bulk SiC phonons. These defined peaks confirm the monodispersity of Si NPs as also observed by TEM. Time-resolved fluorescence anisotropy measurements, displayed a strictly monoexponential decay that can only be indicative of monodisperse, ball-shaped nanoparticles. The thus developed method was further applied to obtain Si NPs with different functionalizations that could be used to develop various applications.
    The preparation of very stable and bright water-soluble amine-terminated Si NPs is described in Chapter 3. The Si NPs were synthesized with different alkyl-chain lengths between the amine group and the surface of the NP: Si-C3H6NH2, Si-C6H12NH2 and Si-C11H22NH2 NPs.14 A highly improved synthesis method is presented, and the photophysical characterization of alkyl-amine coated Si NPs, including steady-state and time-resolved fluorescence, as well as STS (Scanning Tunneling Spectroscopy) measurements is discussed. The topography of amine-terminated Si NPs is examined using transmission electron microcopy (TEM) and scanning tunneling microscopy (STM) techniques. TEM measurements indicate a homogeneous size distribution (1.57 ± 0.24 nm). This size was independent of the alkyl spacer length, which excludes effects of the Si NP core size on their optical properties, and also shows that the method used to make such NPs is very reproducible. Size distribution histograms obtained by TEM and STM display highly similar height distributions. All synthesized amine-terminated Si NPs show a broad continuous absorption between 200 and 380 nm. The absorption spectrum of the 3-amino-propyl-terminated Si NPs has a distinctive broad peak at 300 nm, while for Si NPs with longer alkyl chains between the Si core and the amine group, this peak was not pronounced. There is no appearance of a vibrational structure in the absorption spectra, as was observed for the alkyl-terminated Si NPs. The electronic band gap of propyl-amine-terminated Si NPs is determined by STS measurements of individual NPs; ~ 60% of the observed particles have a band gap of ~4 eV, which exactly corresponds to an absorption peak at 300 nm.
    All NPs under study displayed an intense, well-defined emission in the 350 – 600 nm range. The emission spectra, and specifically the wavelength at which the maximum emission intensity is observed, depend on the length of the alkyl spacer. With a short (-C3H6) spacer the maximum emission is observed in the blue with a maximum centered at 475 nm, when exciting Si NPs with 390 nm. For longer chains the maximum emission gradually shifts to the UV: for the C11H22 spacer excmax = 320 nm, with the maximum emission is centered at 385 nm. Two factors may contribute to this shift: the amine-to-Si core distance, and a higher degree of oxidation with shorter alkyl chains, as longer alkyl chains may provide a better passivation of the Si NPs surface. A Stokes shift of ~1 eV also points out to the formation of trapped states. The excited-state lifetimes are in the ns range and indicate direct band-gap processes.
    The stability of NH2-terminated Si NPs is exceptionally good over a wide pH range (1 - 13!) and high temperatures (120 C), which is of great importance for a variety of possible applications including cell contents of various pH values and sterilization conditions. The prepared amine-terminated Si NPs are shown to be highly suitable for bio-imaging studies. They display a very high mobility in water (D = 3.13•10-10 m2s-1), and are readily taken up by BV2 nerve cells. After the uptake Si NPs are located in the cell cytosol. For the first time, proliferation of BV2 cells stained with Si NPs is observed by confocal microscopy. The newly formed daughter cells are also stained with Si NPs, indicating their minimal toxicity. In this way, amine-terminated Si NPs can be successfully used for staining multiple cell generations by only staining the mother cells.
    The possible use of Si NPs as an active energy donor in energy-transfer processes is described in Chapter 4. To that aim, a ruthenium-containing dye was attached to Si NPs, while the chain length of the alkyl spacer between the possible donor and acceptor was varied. In order to examine if an energy transfer occurs, Si NPs emission intensities and lifetimes were measured with and without a Ru dye attached to Si NPs. In this way, it was shown that Si NPs can act as a very efficient energy donor, with energy transfer efficiencies up to 55%. Also, energy transfer efficiency is shown to be distance dependent. This opens up a venue for numerous applications of Si NPs as efficient energy donors in various systems.
    As the fast and efficient synthesis procedure of amine- and alkyl-terminated Si NPs was established, it became highly convenient to functionalize Si NPs via an easy and versatile route. Therefore, Chapter 5 deals with the development of a method for the production of stable azide-terminated Si NPs and their further functionalization using “click” chemistry. The produced azide-terminated Si NPs were studied by FTIR, steady-state and time-resolved absorption and emission spectroscopy. They absorb light in the UV spectral region with a pronounced vibrational structure, and emit light at ~ 350 nm. Stable azide-terminated Si NPs were further functionalized with a number of functional terminal alkynes: undec-10-yn-1-ol, propargyl amine, undec-10-ynoic acid, undec-10-ynyl lactoside, undec-10-ynyl hepta-O-acetyl lactoside and rhodamine-labeled lactoside. In this way, once synthesized, the azide-terminated Si NPs can be rapidly functionalized with many moieties of interest.
    The cellular uptake of rhodamine-labeled Si NPs was successfully demonstrated by using yeast cells (Rhodotorula glutinis). Yeast cells were allowed to actually grow in the presence of Si NPs. It was shown by confocal microscopy that rhodamine-labeled Si NPs are located in mitochondria, and no effect on the cells proliferation rate was observed. Our findings clearly demonstrate that Si NPs are biocompatible and can be internalized by growing cells. The cellular uptake and internal localization under varying physiological conditions can now be studied in more detail, and with more complex functionalities attached onto the Si NPs. The results of this study open the way for further progress in research on Si NPs as imaging agents and effective delivery vehicles for the transport of biologically active compounds into specific cells of interest.

    NCL Symposium Optical dating applications, Wageningen, The Netherlands, April 24, 2008
    Buis, E. ; Temme, A.J.A.M. ; Wallinga, J. - \ 2008
    Delft : NCL Netherlands Centre for Luminescence dating (NCL symposium series vol. 5) - 19
    geologie - leeftijdsbepaling - optische eigenschappen - luminescentie - datering - optica - geology - age determination - optical properties - luminescence - dating - optics
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