11 search hits
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Generation of metal nanoparticles in spherical polyelectrolyte brushes and their application in heterogeneous catalysis
(2005)
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Geeta Sharma
- This thesis describes synthesis of spherical cationic polyelectrolyte brushes by “grafting from” technique. The spherical polyelectrolyte brushes have been used as “nanoreactors” for the synthesis of metal nanoparticles (Gold, Platinum and Silver). The catalytic activity of the resulting metal/polymer nanocomposites has been tested by heterogeneous hydrogenation of carbonyl groups. The synthesis of the cationic spherical polyelectrolyte brushes is achieved by a three-step procedure. Firstly, cationic polystyrene core particles are synthesized by emulsion polymerization by using a cationic surfactant and a cationic thermal initiator. In the second step, a thin layer of photoinitiator is generated around the particles by addition of the photoinitiator under “starved conditions” . The photoinitiator is a monomer, which is added when the formation of core particles is in the last stage. The photoinitiator polymerizes with the styrene to give a covalently bound photoinitiator. In the third and last Step, the brushes are grafted on the core particles by photoemulsion polymerization, where the initiation is triggered by UV/VIS radiation. The important parameters of the brushes- contour length and grafting density are determined by the cleavage of the chains from the surface by alkaline hydrolysis. The ester functionality within the photoinitiator is hydrolyzed under the harsh conditions of hydrolysis. The cleaved chains are analyzed to determine the molecular weight. The grafting of the charged polymeric chains stabilizes the colloids even under unfavorable conditions like high ionic strength and high pH. The behavior of brushes is investigated at different ionic strength and pH. At different ionic strengths brushes show three regimes- osmotic, salted and neutral regime. At increasing ionic strength, a shrinking in brush thickness is observed due to the screening of ionic charges. The cationic brushes (polyamino ethylmethacrylate hydrochloride and poly vinylbenzylamine hydrochloride) bear protonated amine functionality. The brushes are classified as annealed brushes as they are sensitive towards pH. The brushes lose the protons at high pH, to result in the uncharged brushes, causing them to shrink, which is followed by dynamic light scattering. Polyaminoethylmethyacrylate brushes are used as nanoreactors to synthesize the metal nanoparticles of gold and platinum. The water-soluble metal salts are to introduce metal ions in the brushes. The negatively charged metal ions (AuCl4- and PtCl6-2) interact with the cationic chains of the brushes. Dynamic Light Scattering is used to study the influence of the metal ions on the brushes. It is observed that metal ions induce much more pronounced shrinking as compared to the monovalent ions. The shrinking can be compared with the shrinking caused multivalent ions such as MgSO4. The metal ions once introduced are localized within the brushes due to strong correlation of counterions with the polyelectrolyte chains. The excess ions are cleaned by ultrafiltration. The trapped counterions render high contrast to the brushes and hence visualize brushes in Cryo-TEM. The metal ions can be reduced chemically by NaBH4 to generate nanoparticles. The particles are studied by high-resolution Transmission microscopy and cryogenic TEM. The metal nanoparticles formed are well spaced and crystalline in nature. The particles formed are found to be stable against aggregation. The silver nanoparticles are formed by using the anionic brushes (polyacrylic acid) and AgNO3 as the precursor salt. The catalytic activity of Platinum is tested by the heterogeneous hydrogenation of the carbonyl functionalities with dihydrogen under optimum conditions. The platinum nanoparticles give a 90% conversion of butyraldehyde to 1-butanol. The catalyst is found to be recyclable for a number of runs without losing the efficiency. The time dependent studies are carried out to gain insight in the mechanism and the kinetics of the reaction. The particles are found to be stable after the catalytic cycles. The gold nanoparticles supported on the polystyrene are found to be catalytically active for the same reaction. The gold particles are found to be recyclable for a number of runs without losing the efficiency. The bulk gold is completely inactive, hence the origin of catalytic activity is attributed to the quantum size effects. As the polystyrene particles are inert, the role of support particles in the origin of catalytic activity can be ruled out. The nanoparticles are found to become inactive for catalysis after 6 months of standing with a manifold increase in the particle size as revealed by microscopy.
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Katalysatordesign als Beitrag zu einer nachhaltigeren Chemie
(2007)
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Sebastian Proch
- Aminopyridine bzw. Dipyridylamine können durch Monochlorphosphane, wie z.B. Chlordiphenylphosphan, funktionalisiert werden und es entstehen neutrale P,N-Chelatliganden. Diese P,N-Liganden können Palladiumvorstufen für die Suzukikreuzkupplung stabilisieren. Ein besonderer Vertreter [Bis(2-pyridyl)amino]diphenylphosphan bildet mit Rhodium-1,5-cyclooctadien-chlorid-dimer einen homobimetallischen Komplex, welcher sich als Katalysator für die Direktarylierung nichtaktivierter Arene eignet. In einer homogenkatalytischen Reaktion kann hierbei z.B. 3-Chlorpyridin mit Benzol zu 3-Phenylpyridin gekuppelt werden, jedoch sind die eingesetzten Katalysatoren nicht rezyklierbar. Rezyklierbare Katalysatoren für Kreuzkupplungen können aus Palladiumnanopartikeln aufgebaut werden, um eine Filtrierbarkeit der Nanopartikel zu gewährleisten müssen diese auf einem entsprechenden Trägersystem immobilisiert werden. Dazu werden sphärische Polyelektrolytbürsten (SPB) verwendet, welche ein robustes Trägersystem darstellen. Die Anwendung dieses Verbundsystems in der Heck- und Suzukikreuzkupplung liefert eine exzellente Rezyklierbarkeit und ein Auslaugen des Katalysators von nur 6 ppm in das gebildete Produkt. Die Breite an Metallnanopartikeln auf SPB-Träger kann vergrößert werden, z.B. Gold, Platin, Rhodium; dabei können auch bimetallische Partikel hergestellt werden. Diese neue Breite an Metallen liefert den Zugang zu weiteren Reaktionen, die katalysiert werden können. Dabei ist besonders die Oxidation von Alkoholen zu Aldehyden oder Ketonen bei Raumtemperatur mit Luftsauerstoff in Wasser interessant. Sie stellt eine umweltfreundliche Alternative zur industriell verwendeten Oxidation mit äquimolaren Mengen an Chrom- oder Manganreagenzien dar. Durch die Nanopartikelfunktionalisierung eignen sich SPB-Verbundsysteme auch als Wasserstoffspeichermedien bei Raumtemperatur („spillover“), es können bis zu 2 wt% an Wasserstoff gespeichert werden. Überträgt man den gleichen Ansatz auf mikro- bis mesoporöse Feststoffe (MOF-5, Silica) und funktionalisiert diese mit Palladiumnanopartikeln, so erhält man Speicherkapazitäten von bis zu 6 wt% an Wasserstoff.
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Functional Cylindrical Polymer Brushes and Their Hybrids with Inorganic Nanoparticles
(2008)
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Youyong Xu
- Various cylindrical polymer brushes were synthesized via a grafting-from strategy. Very long poly(2-hydroxylethyl methacrylate) backbones of the brushes were prepared by anionic polymerization (DPn=1500), esterified with an ATRP initiator, and subsequently the side-chains were grafted by atom transfer radical polymerizations (ATRP). Cylindrical brushes with different architectures, such as brushes of single component, double-grafted brushes and core-shell brushes, were built according to the need of applications. A number of functional monomers were involved in the preparations of the brushes, providing possibilities for further functionalizations and uses. Nano-hybrids comprising organic cylindrical brushes and inorganic nanoparticles such as magnetite and polyhedral oligomeric silsesquioxane (POSS) were fabricated through non-covalent inclusion and covalent attachment respectively. Double-grafted poly(lauryl methacrylate) brushes carry side-chains containing dodecyl short grafts. The long alkyl chains provided good solubility in hydro-carbon solvents like n-hexane and paraffin oil. DSC measurements revealed that they undergo side-chain crystallizations. Grafting of N,N-dimethylaminoethyl methacrylate (DMAEMA) to the macro-initiator by ATRP yielded weak polyelectrolyte cylindrical brushes. They showed responsiveness to pH and salinity in solution. Strong cationic polyelectrolyte brushes were obtained by further quaternization of the PDMAEMA brushes. Their responses to counterions of different valencies were investigated. The addition of a sufficient amount of mono-valent salt induced the collapse of these brushes. When di- and tri-valent counterions were added, helical transition morphologies were recorded before the brushes collapsed into sphere-like structures. Special trivalent counterions, which can change valency through photo-aquation reactions, allowed switching the morphologies of the cationic brushes from worms to globules and back to worms. The morphologies of the cationic brushes could also be tuned by forming ionic complexes with the anionic surfactant sodium dedecyl sulfonate (SDS) and supramolecular inclusion complexes between cyclodextrins (CDs) and SDS. The brushes underwent transitions from worms, over pearl-necklace structures to totally collapsed spheres when SDS was added. Introducing alpha- or beta-cyclodextrins could bring the collapsed spheres back to worms. Adamantyl ammonium chloride, a more competitive inclusion agent, deprived SDS of CDs, and re-induced the spherical collapse of the brushes. The morphologies of the cationic brushes could be regulated in a similar way by forming inter-polyelectrolyte complexes (IPECs) with anionic linear poly(sodium styrene sulfonate) (PSS) in highly diluted solutions. Worm-to-sphere switching with helix-like transition states was also observed. A new strategy for the direct preparation of strong anionic polyelectrolyte cylindrical brushes without protection was introduced by forming supramolecular complexes between the monomer potassium sulfopropyl methacrylate (SPMA) and crown ether 18-crown-6 in DMSO using ATRP for the grafting-from processes. Well-defined worm-like morphologies were proven by atomic force microscopy (AFM) and cryogenic transmission microscopy (cryo-TEM). Water soluble double-hydrophilic core-shell cylindrical brushes were prepared and showed pH responsiveness. Magnetic hybrid cylinders were formed by introducing magnetite nanoparticles into the core. They could be aligned on a large scale on the substrates by applying magnetic fields. Finally, single-molecular hybrid cylinders were created by covalently attaching thiol-functionalized polyhedral oligomeric silsequioxane (POSS) to poly(glycidyl methacrylate) brushes. Their pyrolysis in air resulted in porous silica materials.
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Smart organic-inorganic nanohybrids of functionalized silsesquioxane nanoparticles
(2008)
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Manuela Schumacher
- The formation and characterization of smart organic-inorganic nanohybrids was studied. The inorganic part was formed by N,N-di(2,3-di¬hydroxy¬propyl)¬3-amino¬propyl¬functional silsesquioxane nanoparticles being highly functionalized with ca. 14 tertiary amino groups per particles, each amino group bearing four hydroxyl groups. Two different polymer systems were used for the organic side: amphiphilic block copolymer micelles of poly(n-butyl acrylate)-block-poly(acrylic acid) and star-shaped poly(acrylic acid)s, the latter serving as a model system for frozen micelles. In all cases the mixing of aqueous solutions of anionic block copolymer micelles or the anionic stars with solutions of the silsesquioxane nanoparticles led to the easy and straightforward formation of organic-inorganic nanohybrids. The structure of the complex nanohybrids depends on pH and salinity. The amount of incorporated silsesquioxane nanoparticles within the micelles or the stars under varying external stimuli was determined using a large number of methods. Complexation preserved the original size of the micelles - consisting of a PnBA core and a PAA corona - according to dynamic light scattering and static light scattering as well as light scattering titration measurements and asymmetric flow field-flow fractionation experiments. Fourier-transform infrared spectroscopy and dialysis measurements with fluorescently labelled silsesquioxane nanoparticles confirmed the nanohybrid formation over a relatively wide range in pH. Cryogenic transmission electron microscopy micrographs indicated a core-shell structure of the nanohybrids with gradual decreasing density of silsesquioxane nanoparticles. LS titrations gave an insight in the postulated interaction mechanism. Complexation in acidic media is driven by hydrogen-bonding and ionic interaction; in alkaline media nanohybrids are mainly formed due to ionic interaction. Depending on ionic strength, attractive Coulomb interactions may be either sufficient to promote complexation even at high pH, where hydrogen-bonding is absent (low ionic strength), or are screened (high ionic strength), resulting in less favourable interactions between micelles and silsesquioxane nanoparticles. The reason for the size conservation is most probably due to the kinetically frozen micellar core and the compensation of increased steric repulsion due to complexation and attractive interactions between the silsesquioxane nanoparticle and the charged PAA. The maximum of the interaction at 0.1 M NaCl could be deduced to be in the range 3.5 < pH < 7.5 NaCl. At low salinity (0.01 M NaCl) more nanoparticles were incorporated within the micelles. Nanohybrids exist even up to very basic conditions (pH < 9.5). The responsiveness of the system on pH and salinity as external stimuli was demonstrated by LS titration, dialysis and FT-IR measurements, thermogravimetric analysis (TGA) and AFFFF measurements. Quantifying the amount of nanoparticles incorporated in the micelles turns out to be a arduous task. SLS of dialysed and undialysed samples and AFFFF of undialysed samples clearly showed increased molecular weights of the formed nanohybrids. TGA - requiring an exhaustive dialysis procedure prior to the measurements - provided information about the amount of incorporated silsesquioxane nanoparticles within the micelles. Isothermal titration calorimetry (ITC) provided the possibility to investigate the complexation mechanism in greater detail. Small angle neutron scattering (SANS) experiments, conducted at basic conditions, provided information on the inner structure of the nanohybrids. A newly developed fitting model enabled the quantification of the amount of interacting nanoparticles under these conditions. All methods to determine the amount of nanoparticles incorporated within the micelles sustained the formation of the organic-inorganic nanohybrids. The absolute number of nanoparticles per micelle is quite high (in the range from 160 to 4300, depending on the used method and conditions), however, the calculated numbers of nanoparticles per acrylic acid unit are quite low (in the range from 0.002 to 0.053). The stars showed behaviour comparable to that of the micelles. According to DLS and SANS experiments their size was preserved during complexation. SANS and LS titration measurements demonstrated the increased mass of the nanohybrid stars compared to the net stars. Cryo-TEM micrographs confirmed the formation of organic-inorganic nanohybrid stars, indicating a morphology with gradually decreasing density of nanoparticles. An appropriate fitting model for the SANS data for this challenging system was developed that proved the interaction between the silsesquioxane nanoparticles and the PAA and enabled the calculation of the amount of entrapped silsesquioxane nanoparticles within one star. The determined values were comparable to the ones calculated for the micellar nanohybrids.
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Synthesis and Characterization of immobilized Gold Nanoparticles and Binary Gold Nanoalloys on Cationic Spherical Polyelectrolyte Brushes and their Application as a Catalyst
(2008)
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Marc Claudius Schrinner
- First of all the up scaled synthesis for cationic and anionic spherical polyelectrolyte brushes (SPB) was introduced and a reproducible method for the synthesis was established (Chapter 3.1). For a better understanding of anionic SPB the complexation of the anionic polyelectrolyte chains with the cationic surfactant cetyl trimethyl ammonium bromide was studied in detail. The models were proved by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS), if it’s in good agreement with the resulting systems (Chapter 3.2). It was possible to show, that cationic SPB could be used for the generation of gold nanoparticles (Chapter 3.3). The synthesized carrier systems were characterized in detail by transmission electron microscopy (TEM), cryo-TEM and disc centrifuge (DCP). The more detail examination of the Au/SPB system by DLS, TEM/cryo-TEM, showed that a reversible immobilization system for gold nanoparticles was synthesized. The immobilized gold nanoparticles@SPB could be complexed by cyanid ions and oxygen. After the complexation of the gold nanoparticles we get the previous carrier system back. Detailed studies by wide angle X-ray scattering (WAXS) and high resolution transmission electron microscopy (HR-TEM) show an amorphous morphology of the gold nanoparticles in the range of 1 nm. This kind of generation allow us to generate gold nanoparticles in the range of 1.0 and 2.5 nm on the surface of the carrier particle. In the following part of the thesis the concepts described above were used for the synthesis of binary gold nanoalloys (Chapter 3.4 and 3.6). The binary systems Au-Pt, Au-Ru, Au-Rh and Au-Ir were sucessfully generated. In the next step the nanoparticular structure of these binary systems were completely clarified by different HR-TEM methods and WAXS. It was shown, that there is a different behaviour between macroscopic and nanoscopic world. Bulk Au-Pt alloys show miscibility gaps, whereas Au-Pt nanoalloys have no such miscibility gap. This alloy obeys the Vegard’s law. For the generation of facetted Pt nanocrystals (Chapter 3.5), the complexation of gold atoms by cyanid ions and oxygen is used. By this way it’s possible to synthesize well-ordered PtNP structures starting from the Au-Pt nanoalloy. In the second part of the thesis the immobilized gold nanoparticles and the binary gold nanoalloys applied as catalysts for industrial interesting oxidation reactions of alcohols and epoxidation reactions (Chapter 3.4 and 3.6) were studied. All reactions could be conducted at room temperature and in water as reaction media. The catalytic activities have a strong dependency on the composition in the nanoalloy. Cryo-TEM characterization showed us no change of the morphology of the catalyst before and after a catalyst cyclus. Concluding this thesis showed successfully a new route for the synthesis of monodispers and well defined gold nanoparticles, gold nanoalloys and facetted platinum nanocrystals. The particle sizes ranges between 1.0 and 7.0 nm. All systems can be used as green catalysts. This is an important point in the discussion of sustainability. All dispersions are not light and air sensitive, so they can be handled without any problems.
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Selbstorganisation von Nanopartikeln an flüssigen Grenzflächen
(2008)
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Sergey Kutuzov
- Im Rahmen dieser Arbeit wird eine umfangreiche Untersuchung der Selbstorganisation von Nanopartikeln als Folge ihrer spontanen Adsorption an flüssigen Grenzflächen vorgestellt. Die hier vorgelegten experimentellen und theoretischen Studien betreffen nicht nur den kinetischen Ablauf der Adsorption von Nanoteilchen unter Berücksichtigung aller darauf wirkenden Parameter, sondern auch die Organisation von Nanoteilchen an der flüssigen Grenzfläche im Hinblick auf die innere Struktur der gebildeten nanopartikulären Filme und ihrer mechanischen Eigenschaften. Die dadurch gewonnenen Erkenntnisse bilden daher eine wissenschaftliche Basis für vielversprechende praktische Anwendungen der Selbstorganisation (bzw. spontanen Adsorption) von Nanoteilchen in der Industrie. Die hier verwendeten Nanopartikel zeigen einen Kern-Schale-Aufbau, wobei der Kern aus Cadmiumselenid (CdSe) betshet und die Schale aus organischen Molekülen (TOPO) gebildet wird, die zur Strabilisierung der Partikel in Lösung beitragen. Hauptsächlich wurden kugelförmige Nanoteilchen mit drei verschiedenen Kern-Durchmessern verwendet: 2,3 nm, 4,6 nm und 6,0 nm. Außerdem wurde das Selbstorganisationsverhalten von stäbchenförmigen Nanopartikeln sowie die Entstehung von nanopartikulären Filmen untersucht. Die Abmessungen der Nanostäbchen betragen: (7 x 30) nm, (4 x 34) nm und (4 x 25) nm. Zur Untersuchung der Adsorption wurden Toluol/Wasser-, oder Luft/Wasser-Grenzflächen erzeugt. Aus der Arbeit ergeben sich folgende Erkenntnisse: 1) die Anlagerung von Nanopartikeln an der flüssigen Grenzfläche hat eine sofortige Minimierung ihrer Gibbschen freier Energie zufolge. Bei Raumtemperatur erfolgt daher die Adsorption von Nanoteilchen spontan. 2) die allgemeinen Vorstellungen über den zeitlichen Ablauf der Adsorption, ursprünglich vorgeschlagen von A. F. H. Ward und L. Tordai für die Adsorption von amphiphilen Molekülen an flüssigen Grenzflächen, haben sich in dieser Arbeit auch bei der Beschreibung des Adsorptionsverhaltens von Nanoteilchen an flüssigen Grenzflächen als richtig erwiesen. 3) die Adsorption verläuft gemäss dem lokalisierten Modell nach Langmuir 4) im Laufe der Adsorption entsteht eine Monolage eines nanopartikuläreren Films 5) aufgrund der anziehenden interpartikulären Kräfte bilden sich von Beginn der Adsorption Nanopartikel-Aggregate an der flüssigen Grenzfläche. Die Größe und die innere Symmetrie der Aggregate hängen von der Größe und Form der verwendeten Nanopartikel ab. Dicht adsorbierte Nanopartikel weisen klare filmbildende Eigenschaften auf und sind zur Herstellung von ultradünnen nanopartikulären Membranen geeignet.
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Click Chemistry as Efficient Ligation Strategy for Complex Macromolecular Architecture and Surface Engineering
(2009)
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Anja Goldmann
- Click-Chemie wurde als Ligations-Strategie für die Synthese von cyclischen Polymeren und zur Oberflächenmodifizierung von großen Mikrokugeln und magnetischen Eisenoxidpartikeln verwendet. Das breite Spektrum dieses universellen und leistungsstarken Instruments im Bereich der komplexen makromolekularen Architektur und Oberflächenmodifizierung ist hier dargelegt. Cyclisches Polystyrol wurde mittels der Kombination der „Reversiblen Additions-Fragmentierungs-Kettenübertragungs-Polymerisation" (RAFT) und der kupferkatalysierten Huisgen [2+3] Cycloadditons Click-Reaktion synthetisiert. Ein Azido-funktionalisiertes Dithiobenzoat Click-RAFT-Agens wurde als Kettenüberträger in der RAFT Polymerisation von Styrol verwendet, die in niedermolekularen azido-terminierten Polymeren resultierte. Der Austausch der Dithio-Gruppe der Polymerkette wurde mit einem Alkin-modifizierten Initiator durchgeführt und führte zu einem heterotelechelischen linearen Polymerprecursor für die Click-Cyclisierung. Die Eigenschaften des makrocyclischen Polymers im Vergleich zum linearen Gegenstück wurden untersucht. Die Kombination aus mehreren Analytikmethoden konnte die cyclische Struktur beweisen. Aus den Viskositätsmessungen im guten Lösungsmittel THF wurde ein Kontraktionsfaktor g´ = [eta]cyc/[eta]lin = 0.70-0.74 bestimmt. Dieser Wert stimmt mit dem theoretisch bestimmten Wert g´=0.67 für theta-Bedingungen überein. Die Oberflächenmodifizierung von großen Poly(divinylbenzol) Mikrokugeln (pDVB, 1,3 mikrom) wurde mit zwei verschiedenen Strategien durchgeführt, zum einen der Huisgen [2+3] Cycloadditionsreaktion und zum anderen mit der Thiol-en Click-Chemie. Die pDVB Mikrokugeln besitzen eine dünne Oberflächenschicht die aus teilweise vernetztem und quellfähigem Poly(divinylbenzol) besteht und darüber hinaus über Vinylgruppen auf ihren Oberflächen verfügen die für eine Modifizierung zugänglich sind, beispielsweise einer direkter Oberflächenmodifizierung durch Pfropfungstechniken („grafting-to“). Die RAFT-Technik wurde benutzt um SH-funktionalisierte Poly(N-Isopropylacrylamid)-Polymere (pNIPAAm-SH) zu synthetisieren und oberflächenmodifizierte Mikrokugeln über Thiol-en-Reaktion zu generieren. Oberflächensensitive Charakterisierungsmethoden wurden zur Identifizierung der charakteristischen Polymerhülle auf der Außenschale verwendet. Die Visualisierung der Partikel wurde mit der Rasterelektronenmikroskopie (REM) durchgeführt. Suspensionsstudien der Mikrokugeln zeigen einen ansprechenden Gewinn der Hydrophilie nachdem sie mit pNIPAAm45 gepfropft wurden und somit nach der Oberflächenmodifizierung in Wasser suspendiert werden können. Diese Beobachtung wurde durch eine Trübungsstudie unterstützt. In einer alternativen Vorgehensweise wurden multifunktionelle Azido-funktionalisierte Mikrokugeln über die Thiol-En-Reaktion von 1-Azido-undecan-11-thiol mit den verbleibenden Doppelbindungen auf der Oberfläche und anschließender 1,3 Huisgen dipolarer Cycloadditionsreaktion hergestellt. Diese oberflächenmodifizierten Partikel wurden mit Poly(hydroxyethylmethacrylat) (pHEMA) gepfropft. Das Aufpfropfen von hydrophilen Polymeren auf hydrophobe Partikel kann die Suspendierungseigenschaften der Partikel im wässrigen Medium deutlich erhöhen. Schließlich wurden Magnetit-Nanopartikel (Fe3O4) mit der Huisgen [2+3] Cycloadditionsreaktion oberflächenmodifiziert. Dabei wurde ein vielseitiger biomimetischer Anker, Dopamin, verwendet um die Partikel zu stabilisieren und gleichzeitig zu funktionalisieren. Die Synthese eines Alkin-Dopamin-Derivats führt zu multifunktionellen stabilen Fe3O4-Nanopartikeln. Die Oberflächenmodifizierung wurde mit einem Azid-funktionalisierten Polyethylenglykol (PEG) und desweiteren mit einem Azid-modifizierten Rhodamin-Derivat durchgeführt. Diese Eisenoxid-Partikel wurden mit konfokaler Fluoreszenzmikroskopie untersucht. Mit diesem Ansatz können hydrophobe Fe3O4-Nanopartikel in wasserlösliche Partikel umgewandelt und in Wasser redispergiert werden. Außerdem führt die hydrophile PEG-Schicht zu einer biokompatiblen Hülle. Im Allgemeinen zeigen all diese neuen Anwendungen die Vielseitigkeit der Click-Chemie und erweitern die Bandbreite alternativer und einfacher Ansätze für Oberflächenmodifizierungsstrategien und den Zugang zu komplexer makromolekularer Architektur.
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Ordering of Nanoparticles by Wrinkle-Assisted Self-Assembly : Controlling Plasmonic Coupling Effects
(2011)
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Alexandra Schweikart
- Structures of spatial scale between 10Å and 1000Å are known as nanomaterials and have attracted immense interest over the last decades (Nobel Prize in physics in 2010 was awarded for the nanomaterial graphene). Materials within this scale show a large surface-to-volume ratio and amplify surface-related properties. Governing and manipulating material on this almost atomic level is one of the most active fields in modern natural science. Nanoscale technology, such as some of the processes involved in steel production and painting, has been empirically utilized in human society for centuries, however, a scientific investigation of phenomena on this spatial scale only began in 1857 when Michael Faraday reported on the synthesis and colors of gold colloids. In 1959 interest in the nanoscale was stimulated by an American physicist, Richard Feynman, in his famous “There’s plenty of room at the bottom” address, and the term nanotechnology first appeared in 1974 from the Japanese Norio Tanigucho. Since these pioneering works, thousands of publications have been focused on the synthesis, modification, properties and assembly of nanoparticles. Great progress has been attained in the preparation of nanoparticles of any desired size, shape and composition. Metal nanoparticles are particularly attractive due to their spectacular size and shape dependent optical and electronic properties. Color variations of nanoparticle suspension for example arise from changes in the composition, size and shape of nanoparticles, as well as from the proximity of other metal nanoparticles. The average distances of nanoparticles in thin films influence the spectral features because of inter-nanoparticle coupling. These effects are often the result of changes in the so-called surface Plasmon resonance, the frequency at which conduction electrons oscillate in response to the alternating electric field. Provided nanoparticles form ordered arrays, they can additionally have unique and fascinating optical properties because of photonic band gap effects with potential applications such as detectors, circuits, light sources, polymeric opals or meta-materials. The present work deals with the controlled placement of nanoparticles by physical con-straints. Exact placement of nanoparticles allows for the control of the inter-nanoparticle distance and thus determines the coupling effects (here: Plasmon coupling) which arise upon interaction with electromagnetic radiation. Different coupling leads to different distance-dependent signals and such substrates can serve as sensors if, for example, Raman spectroscopy is carried out for detection of the signal. Currently, most templates are created using lithographic techniques. Particularly if structures on the sub-micron scale are desired, electron beam lithography has to be used which involves environmentally harmful etching processes. Within this work we show how controlled wrinkling of a thin rigid film on a soft, elastomeric substrate, can be used as an alternative to fabricate nano-templates without using any lithography. As a substrate, a silicon elastomer poly (dimethylsiloxane) (PDMS) was used. Upon stretching such substrates uniaxially, an enlarged surface was exposed to oxygen plasma and converted to silica by oxidation. After releasing the strain, periodic wrinkles appeared perpendicular to the applied strain. Under defined conditions, such wrinkles have a regular sinusoidal topology featuring a single dominant wavelength and amplitude. The formation process could easily be tuned by tuning the plasma exposure to generate periodically structured templates between few hundreds of nanometers and several microns. In this work, wrinkled templates were tailored such that suitably sized nanoparticles could be arbitrarily assembled into a hierarchical structure by drying colloids out of suspension in a channel-like confinement offered by wrinkles in contact with a flat substrate. Using the same template geometry (same wavelength and amplitude of wrinkles) but different particle concentration of spherical polystyrene beads (r = 55nm) we found parallel particle-structures ranging from single parallel lines at low particle concentration to dense prismatic ridges at high particle concentration. The wavelength of the wrinkled template defined the spacing between the particle lines. Moreover, we performed Monte Carlo (MC) computer simulations in collaboration with the theoretical physics department (Prof. Dr. Matthias Schmidt and Dr. Andrea Fortini) at the Uni-versity of Bayreuth to assess the dominant driving forces during the assembly process. Be using MC, colloidal particle assemblies can be characterized in terms of their equilibrium configuration that minimizes the free energy. Simulations were performed on particles in a box delimited by a flat hard wall and a sinusoidal hard wall according to our experimental system. These simulations precisely predicted the exact assembled geometry in thermal equilibrium. Comparing results of simulation and experiment we found perfect agreement between the equilibrium structures. We discovered the confinement itself to be mainly responsible for the assembled morphology of nanoparticle, which makes the process independent of the detailed chemistry of particles. In addition we obtained very similar structures with the same assembly strategy but using gold nanoparticles (r = 33 nm) instead of polymeric particles. We fabricated lines of gold nanoparticles assembled in a single file and lines two particles wide using similar particle concentration but different sizes of the confinement template. The different morphologies of the lines give rise to different optical signals as collective oscillation of conduction electrons result in different interaction with electromagnetic radiation. Surface Plasmon resonance due to Plasmon coupling between adjacent particles arises. Different morphology-dependent signals of nanoparticles in contact within the lines were detected by surface enhanced Raman spectroscopy (SERS). The electromagnetic field was measured to be randomly distributed along the particle lines with strong enhancements at so-called hot spots located at gaps between neighboring nano-particles. To confirm the measured signal we compared theoretical simulations using the finite-difference time-domain (FDTD) method and experimentally measured dark-field spectroscopy signal along differently shaped lines of particles within a collaboration with Weihai Ni and Dr. Ramón Alvarez-Puebla at the University of Vigo in Spain. Good agreement between theory and experiment indicated that indeed plasmonic coupling of the individual nanoparticles is responsible for the observed SERS effects: Using wrinkle-assisted self-assembly it is possible to control the organization of the colloidal particles on the substrate, with a consequent control over the formation of hot spots and the resulting SERS intensity. Such ordered multiplicities of hot spots give rise to quantitative SERS signals with high sensitivity which has applications as diverse as biological detectors, optical filters and sensors. In addition, this work deals with chemical modification of the wrinkled structure to render it accessible to different solvents as PDMS tends to swell in organic solvents and suffers from poor mechanical stability. Additionally, wrinkles fabricated through a buckling instability of a stiff supported layer under compression are not tension free on the microscopic level and suffer from relaxation on a longer time scale. We introduce in this work two different methods to replicate wrinkles by molding. In micro thermoforming, the wrinkled surface was used as a mold (or caliber) to structure different kinds of polymers (polystyrene and poly (methylmethacrylate)) by pressing the originally wrinkled structure onto a ductile material which preserves the nanostructure after curing. The second methodology was carried out in collaboration with PD. Dr. Kerstin Koch and Michael Bennemann at the Nees institute in Bonn and employed a two-step molding process, where wrinkles were molded against wax and in a second step, the structured wax was cast against epoxy resin. Both methods revealed perfect copies of the wrinkled original with high fidelity even at dimensions as small as a few hundred nanometers and hold no residual stresses because there is only one component. Wrinkles made of tough polymers are now accessible to various solvents which make them potential substrates for microfluidics. In the last part of this work, wrinkles are used as stamps in so-called micro contact printing (µCP). In this technique, a structured elastomeric stamp is used to transfer a surface-active molecule out of solution to a flat substrate by mechanical contact. Patterns of different charge density can be created which have applications in the field of biosensors, diagnostic immunoassays and cell culturing. Traditionally, stamps for µCP are prepared by a two step process where a lithographically fabricated structured silicon master serves as mold. An elastomeric polymer is cast against the caliber and preserves the structure after curing and detaching. As already mentioned lithography is expensive and involves environmentally harmful etching processes. Within this work we introduce the one step wrinkling process to fabricate structured stamps. Even though the diversity of stamp geometries created by wrinkling is limited, the simplicity compared to lithographic techniques is evident. The process of wrinkle formation includes plasma oxidation, which renders the topmost surface hydrophilic. Therefore, charged macromolecules out of aqueous solution were adsorbed onto the surface. The coated relief structure was used as a stamp to transfer the molecules selectively from the elevated parts of the wrinkles to another flat, oppositely charged surface by means of µCP. The topography of the resulting pattern was characterized by Atomic Force Microscopy (AFM) imaging as alternating charged pattern of printed (elevated) and non-printed areas. By varying the geometry of the wrinkled stamp (amplitude and wavelength) we studied the limits in which successful µCP with wrinkles can be carried out. We found the limits for wavelength of the wrinkles below 355nm and amplitudes below 40nm at which the printed structure disappeared because material was transferred from the wrinkles’ hills as well as from the bottom parts. The height of the transferred structure increase with increasing wavelength and amplitude of the wrinkles but tended to a limit of 6-7nm, even though the topology of the stamp increases. The smallest structure found in lateral dimensions was as small as 50nm, appearing as areas where no material was transferred.
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Structure and dynamics of new intelligent copolymer hydrogels and hydrogel nanoparticle hybrids
(2011)
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Yvonne Hertle
- Polymeric gels consist of a chemically or physically crosslinked polymer network swollen with a certain amount of solvent and most of these gels show an ability of reversible swelling or shrinking due to small changes in their environment (as changes in pH, temperature or electric field). In the group of hydrogels, poly(N isopropylacrylamide) (poly(NIPAM)) crosslinked with N,N' methylene-bisacrylamide (BIS) is the most well-known member of the class of thermoresponsive "smart" polymers. This work covers the characterization of thermoresponsive poly(NIPAM) gels an shows different kinds of possibilities tuning their properties. The first part of this thesis presents different methods for the preparation of crosslinker gradient macroscopic gels with the dimensions of some cubic centimetres. The swelling behaviour from different zones of the macrogel with a varying crosslinker content was studied as a function of temperature. Furthermore, the internal dynamics of a poly(NIPAM-co-butenoic acid) copolymer macrogel was investigated by neutron spin echo and compared to the results for a chemically identical microgel. Due to the different preparation conditions of the macro- and microgel, a difference in the collective diffusion of the network was expected. Beside this, copolymer microgel particles based on NIPAM and N tert butylacrylamide (NtBAM) were synthesized. The particle size and the swelling behaviour of the obtained colloidal microgels was characterized with respect to the content of the comonomer using different scattering techniques and electron microscopy. In addition, an attempt was made to describe theoretically the temperature induced deswelling with the Flory-Rehner theory. The latter part of this thesis focuses on hybrid materials based on magnetic nanoparticles and thermoresponsive microgels. First of all, different approaches for the synthesis of cobalt and nickel nanoparticles and their protection against oxidation were made. Furthermore, these magnetic particles were incorporated as an inorganic core in poly(NIPAM) core-shell systems, as well as randomly distributed particles within the gel network.Polymergele sind aus chemisch oder physikalisch vernetzten Polymerketten aufgebaut und weisen dabei, bedingt durch ihre Netzwerkstruktur, ein Quellverhalten auf. Die meisten Vertreter dieser Klasse zeigen zusätzlich durch kleine Veränderungen in ihrer Umgebung, wie eine Änderung des pH-Wertes, der Temperatur oder eines elektrischen Feldes, eine reversible Volumenänderung. Das wohlbekannteste Mitglied der sogenannten thermoresponsiven, "intelligenten" Gele ist ein Hydrogel auf der Basis von N,N' Methylenbisacrylamid-vernetztem Poly(N Isopropylacrylamid) (Poly(NIPAM)). Die vorliegende Arbeit beschäftigt sich mit den charakteristischen Eigenschaften von thermoresponsiven Gelen auf Basis von Poly(NIPAM) und den vielfältigen Möglichkeiten diese Eigenschaften gezielt zu steuern. Im ersten Teil der Arbeit werden unterschiedliche Methoden zur Herstellung von Makrogelen basierend auf Poly(NIPAM) mit einem Quervernetzergradient entwickelt, wobei die hier synthetisierten Gele eine Größe von einigen Kubikzentimetern aufweisen. Für eine anschließende Charakterisierung des Quellverhaltens wurde das Quellverhältnis alpha in Abhängigkeit von der Temperatur für unterschiedliche Bereiche des Makrogels bestimmt. Die so erhaltenen Quellkurven konnten dann anhand des Quellvermögens den jeweiligen Bereichen mit unterschiedlichem Quervernetzergehalt zugeordnet werden. Zusätzlich wurde die interne Dynamik eines Poly(NIPAM-co-Butensäure) Copolymer-Makrogels mit Hilfe von Neutronen Spin-Echo Experimenten analysiert. Die durch die Messungen erhaltenen Ergebnisse konnten mit denen chemisch-identischer Mikrogele verglichen werden. Dabei wurde aufgrund der unterschiedlichen Synthesebedingungen von Makro- und Mikrogelen ein Unterschied in der kollektiven Netzwerkdiffusion erwartet. Der gefundene Unterschied in der Netzwerkdynamik war allerdings geringer als erwartet und liegt im Bereich von etwa 10%. Ein weiterer Teil der vorliegenden Arbeit beschäftigt sich mit der Synthese von Mikrogel-Copolymeren auf Basis von NIPAM und N-tert-Butylacrylamid (NtBAM). Mit Hilfe unterschiedlicher Streumethoden, sowie durch Elektronenmikroskopie, wurde sowohl Partikelgröße, als auch Quellverhalten der erhaltenen kolloidalen Mikrogele charakterisiert. Dabei galt es zu bestimmen, welchen Einfluss der Comonomergehalt auf die Eigenschaften des Mikrogels hat. Weiterhin wurde das thermoresponsive Quellen des Gelnetzwerks mit den Vorhersagen der Flory-Rehner Theorie verglichen. Hybridmaterialien aus magnetischen Nanopartikeln und thermoresponsiven Poly(NIPAM) Mikrogelen werden im letzten Teil der Arbeit vorgestellt. Zuerst wurden unterschiedliche Syntheseansätze für Cobalt- und Nickel-Nanopartikel, sowie deren Schutz vor Oxidation, verfolgt. Des Weiteren wurde ein Kern-Schale System angestrebt, in welchem die magnetischen Partikel als Kern vorliegen. Zum anderen wurde beabsichtigt, eine statistische Verteilung der Nanopartikel im Gelnetzwerk zu erreichen.
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Composites of Spherical Polyelectrolyte Brushes and Nanoparticles – Synthesis, Characterization and Their Use in Catalysis
(2011)
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Frank Polzer
- The main objective of this thesis was the synthesis of colloidal stable managanese oxide nanoparticles (MnOxNP) for applications as a catalyst in aqueous solution. Spherical polyelectrolyte brushes (SPBs) with poly(2-trimethyl ammonium ethyl methacrylate chloride) (pTMAEMC) chains were used as support particles to stabilize the MnOxNP by immobilization. In a first step we established and investigated the method of the in situ generation of the MnOxNP within SPBs. It was found that no reducing agent is needed for the reduction of the permanganate precursors and that they do not react with the cationic polymer chains of the SPBs. By a combination of powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and cryogenic TEM (cryoTEM) it was found that the platelet-like MnOxNP are of layered topology built up from MnO6 octahedra denoted as birnessite. The PXRD patterns revealed a disorder along the stacking direction of the single layers of hexagonal sheets. Furthermore, the structure of the composite material observed by TEM strongly differs compared to that in cryoTEM micrographs. The composite material was furthermore analyzed by high resolution TEM (HRTEM) and X-ray absorption fine structure (XAFS) analysis. The qualitative X-ray absorption near-edge structure (XANES) analysis using reference compounds confirmed the crystallographic similarity of the MnOxNP to a c-disordered birnessite. The local structure of the MnOxNP was investigated by a quantitative extended X-ray absorption fine structure (EXAFS) analysis that revealed that no significant difference between the MnOxNP@SPB in aqueous solution and in the dried state. In general, the hexagonal sheets of edge-share MnO6 octahedra are compressed along the c-direction, that is, along the stacking direction. Additionally, a new kind of composite material composed of star-shaped pTMAEMC homopolymer and MnOxNP was synthesized and characterized To test the MnOxNP@SPB composite material for its catalytic activity, the oxidation of morin by hydrogen peroxide was established as a model reaction. It could be shown by UV/vis measurements that the rate of the decomposition is highly depending on the ratio between morin and the oxidant H2O2. This finding was modeled by a Langmuir-Hinshelwood reaction mechanism. The study proved the potential application of the composite material as a catalyst especially for water-based catalysis. Furthermore, a detailed kinetic analysis of the reduction of 4-nitrophenol by sodium borohydride using gold and platinum nanoparticles immobilized on SPBs has been conducted. In analogy to the work on the oxidative decomposition of morin, a Langmuir-Hinshelwood model was used for the description of the reaction mechanism. Using this model, the adsorption constants for both reactants as well as the rate constant of the surface reaction could be determined for both noble metal nanoparticles. This showed that the higher catalytic activity of Pt is mainly due to the higher rate constant of the surface reaction. Additionally, the induction period of the reduction of 4-nitrophenol could be assigned to a surface reconstructuring of the nanoparticles due to the adsorption of 4-nitrophenol. Finally, the synthesis of a novel zwitterionic SPB could be realized using aqueous atom transfer radical polymerization. These particles show a surprisingly high colloidal stability in aqueous medium though the poly(2-(methacryloyloxy)ethyl dimethyl-(3-sulfopropyl)ammonium hydroxide) (pMEDSAH) chains are not soluble in pure water. The solution behavior in water was furthermore studied by dynamic light scattering, TEM and cryoTEM proving the collapsed state of the brush layer. The zwitterionic shell undergoes an internal phase separation leading to a surface-near layer whereas only a minor part of the chains reaches further out into the solution. The collapse was explained by the formation of aggregates of monomer units by zwitterionic or hydrophobic interactions. It was shown that the zwitterionic shell swells upon the addition of high amounts of salts and/or upon increasing the temperature due to the presence of an upper critical solution temperature. In conclusion, this thesis presented a new method for the generation and stabilization of MnOxNP of layered topology using cationic SPBs. The mechanism of the in situ generation could be elucidated as well as the microscopic structure of the composite material in the aqueous dispersed state. Using state of the art characterization methods like XAFS, the local environment of the MnOxNP around the Mn absorber could be determined. The catalytic activity of the MnOxNP was studied in detail applying a Langmuir-Hinshelwood model to the catalytic degradation of morin. A similar study gave new insights into the reduction of 4-nitrophenol using noble metal nanoparticles applying a similar model. The synthesis and analysis of zwitterionic SPBs gave important information about their solution behavior.
