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Microfluidic Devices: Compact Laboratories from Design to Application
Full text linkThis doctoral thesis resumes the most interesting experimental results achieved between 2009 and 2011 by the author, during his stay in the laboratories of the department of chemical sciences of the university of Padova, under the supervision of prof. M. Maggini. The main theme behind this work is microfluidics, an ensemble of techniques for the manipulation of fluids on the micrometer scale. Such manipulation is carried out inside microchannels, confined environments whose geometries can be exploited to optimize unit operations – such as mixing and heat transfer – of profound interest for chemists.
Microfabrication is the first theme treated, and one that has been constantly developed during this internship. The fast prototyping of microfluidic chips is described thoroughly, from the fabrication of chip-external world interfaces, to that of microchannels and electrodes, in order to allow for their easy reproduction. Two techniques widely employed were soft photolithography and embossing. The former makes use of photocurable resins, in particular thiol-ene ones, employing masks to copy a design to the resin bulk in order to make the channels. The latter is based on the use of a cutting plotter to work directly on the materials the microchannels are made of, such as biadhesive tapes and Parafilm M laboratory film.
A commercial thiol-ene resin, NOA, was the object of two experimental works: one aimed at increasing its solvent resistance and the other at achieving wettability control for its surfaces. Solvent resistance is possibly the foremost requirement for a chemical reactor; while NOA displays better chemical compatibility towards organic solvents than other materials commonly used for microfluidic applications, such as poly(dimethylsiloxane), its scarce resistance towards halogenated solvents is a drawback for its employment. The deposition of a silica layer proved to ease this problem, even though it did not solve it, by extending the working life of microchannels made in polymeric material when aggressive solvents were used.
Another interesting characteristic is wettability, which controls the interaction between solvent and the reactor’s inner surfaces, playing an important role in the flow characteristics of the device. To control this feature, NOA was functionalized by silanizing the hydroxide groups on its surface by condensation with trichlorosilane derivatives. A demonstrator device consisting in a passive valve that can discriminate between aqueous and organic liquids was fabricated in a few hours thanks to this method.
Being able to quickly produce microfluidic devices led to the development of various applications, the most interesting of which are reported in the third part of the thesis. As a first example, a very common laboratory material such as Parafilm M was used to fabricate an elegant microreactor capable of producing two diazo dyes in parallel, in a multistep synthesis whose first step relies on the production in confined environment of an intermediate (diazonium salt) which is potentially explosive.
Next, a flow cell interfaced with a spectrophotometer is described, which was used for the study of adsorption kinetics of organic dyes (photosensitizers) on porous titanium dioxide. Such process is of general interest, being a fundamental step in the fabrication of dye sensitized solar cells.
The optical transparency of NOA resins is exploited in the third application presented: a microreactor that was used to carry out photochemical reactions. Light emitting diodes were used as cold, highly efficient light sources to direct the growth phenomena of metal nanostructures, to produce silver nanoparticles with controlled sizes and shapes.
The same masking technology developed for photocurable materials was also exploited to produce gold electrodes. A microfluidic device for electrochemical impedance spectroscopy was then developed and a description of its design, production and electronic characterization is reported at the end of the third section.
Finally, the last part of the thesis report two kinetic studies carried out on important processes for fullerene chemistry: a 1,3-dipolar cycloaddition of azomethyne ylides (the Prato-Maggini reaction) and a cyclopropanation reaction to the fullerene cage with bromomalonate derivatives (the Bingel-Hirsch reaction). The interest behind these studies is two-fold: i) they allow a critical comparison between batch and flow chemistry and ii) notwithstanding the widespread employment of these two reactions in chemical laboratories, few kinetic studies are available on the subject.Questa tesi è un riassunto delle più rappresentative attività sperimentali svolte dall’autore nel triennio 2009-2011 presso i laboratori del dipartimento di scienze chimiche dell’università di Padova, sotto la supervisione del prof. M. Maggini. Il filo conduttore che lega le tematiche toccate dal presente lavoro è la tecnologia microfluidica, intesa come insieme di tecniche per la manipolazione dei fluidi su scala micrometrica. Tale manipolazione avviene all’interno di microcanali, ambienti confinati nello spazio la cui geometria può essere sfruttata per ottimizzare operazioni unitarie – quali il mescolamento e il trasferimento di calore – di profondo interesse dal punto di vista chimico.
Una porzione importante di questo elaborato è dedicata alle tecniche di microfabbricazione per la prototipizzazione rapida di chips microfluidici, un tema sviluppato costantemente durante tutto l’internato di tesi. I protocolli sviluppati per la fabbricazione di interfacce con l’ambiente esterno, di microcanali e di elettrodi, sono riportati punto per punto e commentati in modo da permetterne una fedele riproduzione. Due tecniche largamente sfruttate sono la fotolitografia “soft” e l’embossing. La prima si basa sulla fotopolimerizzazione di resine commerciali (in particolare di tipo tiolenico), utilizzando una maschera disegnata al computer e stampata su acetato per imprimere un disegno sul materiale polimerico e ricavarne quindi dei canali. La seconda prevede l’uso di un plotter da taglio per lavorare materiali quali nastri biadesivi o film di cera dai quali ricavare direttamente le microstrutture desiderate.
La resina tiolenica commerciale NOA è anche al centro di due lavori sperimentali riguardanti gli aspetti più chimici della prototipizzazione: la resistenza ai solventi e la bagnabilità. Fra i requisiti principali di un materiale per la fabbricazione di reattori chimici, c’è senz’altro la resistenza ai solventi utilizzati nelle sintesi per cui il reattore stesso deve essere utilizzato. Nonostante la resina in oggetto presenti caratteristiche di compatibilità interessanti, soprattutto se confrontata con altri materiali tipicamente usati in microfluidica, quali il poli(dimetilsilossano), la sua resistenza verso i solventi alogenati non ne consente un ampio uso in ambito sintetico. La deposizione di uno strato di silice, ad ogni modo, ha permesso di aumentare la sua resistenza, pur non rendendola completamente resistente. Altra caratteristica importante, in quanto controlla l’interazione fra solvente e superficie interna del reattore è la bagnabilità. Nel caso del NOA, si dimostra come essa possa essere controllata derivatizzando la resina grazie alla condensazione dei gruppi idrossili superficiali con derivati triclorosilani.
La possibilità di produrre rapidamente dispositivi microfluidici ha portato allo sviluppo di varie applicazioni, le più interessanti delle quali sono descritte nella terza parte della tesi.
Innanzitutto viene dimostrato come un elegante dispositivo fabbricato con un materiale ceroso molto comune come il Parafilm, possa essere utilizzato per la sintesi parallela di diversi coloranti diazoici attraverso una sintesi multistep che prevede la produzione in ambiente confinato di un intermedio potenzialmente esplosivo come un sale di diazonio.
In seguito viene descritta l’applicazione di una cella a flusso interfacciata a uno spettrofotometro, con cui si possono ricavare interessanti informazioni sulle cinetiche di adsorbimento di sensibilizzatori sulla superficie dell’ossido di titanio. Tale processo è di interesse generale, essendo alla base della produzione delle celle fotovoltaiche a colorante organico.
La trasparenza ottica della resina tiolenica NOA viene poi sfruttata per condurre reazioni fotochimica all’interno di canali microfluidici fabbricati in questo materiale. Luci monocromatiche “fredde” LED vengono qui usate per direzionare il fenomeno di crescita di nanostrutture metalliche, producendo nanoparticelle di forma e dimensioni controllate.
La possibilità di fabbricare elettrodi direttamente all’interno di canali ha avuto come sbocco naturale la costruzione di dispositivi per elettrochimica. Quello presentato in questa tesi è stato progettato per compiere misure di spettroscopia elettrochimica di impedenza e ne viene presentata la fabbricazione e la caratterizzazione elettronica di base.
L’ultima parte della tesi riporta gli studi cinetici relativi a due importanti reazioni nell’ambito della chimica dei fullereni: la cicloaddizione di speci 1,3-dipolari alla gabbia fullerenica (reazione di Prato-Maggini) e la reazione di ciclopropanazione con derivati bromomalonati (reazione di Bingel-Hirsch). L’interesse dietro a questi studi è duplice: da un lato studiare queste reazioni fornisce dati interessanti per un confronto critico fra la chimica condotta in flusso e quella condotta con tecniche tradizionali, d’altra parte studi di tipo cinetico su queste due reazioni non sono presenti in letteratura nonostante l’indiscussa utilità delle reazioni suddette
Shape-selective growth of silver nanoparticles under continuous flow photochemical conditions
No full textA microfluidic setup for the photochemical nucleation and growth of silver nanoparticles with controlled morphologies is described. The combination of microstructured reactors and efficient LED illumination speeds the growth process up and enhances the shape-wise homogeneity of the produced nanostructures
A comparative Electron Paramagnetic Resonance study of expanded graphites and graphene
No full textGraphene, a novel electronic system with unprecedented characteristics, can be obtained using different methods, each producing materials with specific characteristics from the electronic point of view.
Among these procedures, methods based on the expansion of graphite allow to obtain graphene material in rather high quantities. We have, then, conducted a comparative study of graphene materials produced by these methods by using electron paramagnetic resonance (EPR) techniques; single-layer commercial graphene produced using the Hummers method has been used as the reference. EPR techniques enable the study of some magnetic properties of different types of electrons exhibiting paramagnetic nature. We have analysed the EPR spectra to identify the different types of paramagnetic centres contributing to the spectrum. The analysis of the temperature-dependent EPR spectra and the use of pulse techniques allowed us to separate and characterize the contribution of free conduction
electrons from the contributions of localized edge states and molecular-type paramagnetic states
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Functional palladium metal films for plasmonic devices: an experimental proof
No full textPalladium-based, Inverted surface plasmon resonance substrates with functional surfaces are presented and characterized. The advantages of the use of palladium in lieu of other metals (most notably gold) are discussed and assayed experimentally. We demonstrate how the bare metal films can be functionalized by thiol chemistry or by adsorption of functional graphene sheets and how the features of the surfaces affect the performance of the substrate
Tailoring the wetting properties of thiolene microfluidic materials
No full textA post functionalization method for the control of the wettability of thiolene resins of the NOA
family is presented. Treatment of open model surfaces or closed microchannels with chlorosilane
derivatives resulted in dramatic changes in the behaviour of droplets and streams contacting the
surfaces. The experimental findings are confirmed by the fabrication of a Y-junction device that
works as a passive valve for water streams
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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