Wastewater surveillance for SARS-CoV-2 has been demonstrated to be a valuable tool in monitoring community-level virus circulation and assessing new outbreaks. It may become a useful tool in the early detection and response to future pandemics, enabling public health authorities to implement timely interventions and mitigate the spread of infectious diseases with the fecal excretion of their agents. It also offers a chance for cost-effective surveillance. Reverse transcription-quantitative polymerase chain reaction (RTqPCR) is the most commonly used method for viral RNA detection in wastewater due to its sensitivity, reliability, and widespread availability. However, recent studies have indicated that reverse transcription droplet digital PCR (RTddPCR) has the potential to offer improved sensitivity and accuracy for quantifying SARS-CoV-2 RNA in wastewater samples. In this study, we compared the performance of RTqPCR and RTddPCR approaches for SARS-CoV-2 detection and quantification on wastewater samples collected during the third epidemic wave in Saxony, Germany, characterized by low-incidence infection periods. The determined limits of detection (LOD) and quantification (LOQ) were within the same order of magnitude, and no significant differences were observed between the PCR approaches with respect to the number of positive or quantifiable samples. Our results indicate that both RTqPCR and RTddPCR are highly sensitive methods for detecting SARS-CoV-2. Consequently, the actual gain in sensitivity associated with ddPCR lags behind theoretical expectations. Hence, the choice between the two PCR methods in further environmental surveillance programs is rather a matter of available resources and throughput requirements.

GaAs-based nanowires are attractive building blocks for the development of future (opto)electronic devices owing to their excellent intrinsic material properties, such as the direct band gap and high electron mobility. A pre-requisite for the implementation of novel functionalities on a single Si chip is the monolithic integration of the nanowires on the well-established Si complementary-metal-oxide-semiconductor (CMOS) platform with precise control of the nanowire growth process. The self-catalyzed (Ga-assisted) growth of GaAs nanowires on Si(111) substrates using molecular beam epitaxy has offered the possibility to obtain vertical nanowires with predominant zinc blende structure, while potential contamination by external catalysts like Au is eliminated. Although the growth mechanism is fairly well understood, control of the nucleation stage, the nanowire number density and the crystal structure has been proven rather challenging. Moreover, conventional growth processes are typically performed at relatively high substrate temperatures in the range of 560-630 °C, which limit their application to the industrial Si platform. This thesis provides two original methods in order to tackle the aforementioned challenges in the conventional growth processes. In the first part of this thesis, a simple surface modification procedure (SMP) for the in situ preparation of native-SiOx/Si(111) substrates has been developed. Using a pre-growth treatment of the substrates with Ga droplets and two annealing cycles, the SMP enables highly synchronized nucleation of all nanowires on their substrate and thus, the growth of exceptionally uniform GaAs nanowire ensembles with sub-Poissonian length distributions. Moreover, the nanowire number density can be tuned within three orders of magnitude and independent of the nanowire dimensions without prior ex situ patterning of the substrate. This work delivers a fundamental understanding of the nucleation kinetics of Ga droplets on native-SiOx and their interaction with SiOx, and confirms theoretical predictions about the so-called nucleation antibunching, the temporal anti-correlation of consecutive nucleation events. In the second part of this thesis, an alternative method called droplet-confined alternate-pulsed epitaxy (DCAPE) for the self-catalyzed growth of GaAs nanowires and GaAs/AlxGa1-xAs axial nanowire heterostructures has been developed. DCAPE enables nanowire growth at unconventional, low temperatures in the range of 450-550 °C and is compatible with the standard Si-CMOS platform. The novel growth approach allows one to precisely control the crystal structure of the nanowires and, thus, to produce defect-free pure zinc blende GaAs-based nanowires. The strength of DCAPE is further highlighted by the controlled growth of GaAs/AlxGa1-xAs axial quantum well nanowires with abrupt interfaces and tunable thickness and Al-content of the AlxGa1-xAs sections. The GaAs/AlxGa1-xAs axial nanowire heterostructures are interesting for applications as single photon emitters with tunable emission wavelength, when they are overgrown with thick lattice-mismatched InxAl1-xAs layers in a core-shell fashion. All results presented in this thesis contribute to paving the way for a successful monolithic integration of highly uniform GaAs-based nanowires with controlled number density, dimensions and crystal structure on the mature Si platform. / GaAs-basierte Nanodrähte sind attraktive Bausteine für die Entwicklung von zukünftigen (opto)elektronischen Bauelementen dank ihrer exzellenten intrinsischen Materialeigenschaften wie zum Beispiel die direkte Bandlücke und die hohe Elektronenbeweglichkeit. Eine Voraussetzung für die Realisierung neuer Funktionalitäten auf einem einzelnen Si Chip ist die monolithische Integration der Nanodrähte auf der etablierten Si-Metall-Oxid-Halbleiter-Plattform (CMOS) mit präziser Kontrolle des Wachstumsprozesses der Nanodrähte. Das selbstkatalytische (Ga-unterstützte) Wachstum von GaAs Nanodrähten auf Si(111)-Substrat mittels Molekularstrahlepitaxie bietet die Möglichkeit vertikale Nanodrähte mit vorwiegend Zinkblende-Struktur herzustellen, während die potentielle Verunreinigung der Nanodrähte und des Substrats durch externe Katalysatoren wie Au vermieden wird. Obwohl der Wachstumsmechanismus gut verstanden ist, erweist sich die Kontrolle der Nukleationsphase, Anzahldichte und Kristallstruktur der Nanodrähte als sehr schwierig. Darüber hinaus sind relativ hohe Temperaturen im Bereich von 560-630 °C in konventionellen Wachstumsprozessen notwendig, die deren Anwendung auf der industriellen Si Plattform begrenzen. Die vorliegende Arbeit liefert zwei originelle Methoden um die bestehenden Herausforderungen in konventionellen Wachstumsprozessen zu bewältigen. Im ersten Teil dieser Arbeit wurde eine einfache Prozedur, bezeichnet als surface modification procedure (SMP), für die in situ Vorbehandlung von nativem-SiOx/Si(111)-Substrat entwickelt. Die Substratvorbehandlung mit Ga-Tröpfchen und zwei Hochtemperaturschritten vor dem Wachstumsprozess ermöglicht eine synchronisierte Nukleation aller Nanodrähte auf ihrem Substrat und folglich das Wachstum von sehr gleichförmigen GaAs Nanodraht-Ensembles mit einer sub-Poisson Verteilung der Nanodrahtlängen. Des Weiteren kann die Anzahldichte der Nanodrähte unabhängig von deren Abmessungen und ohne ex situ Vorstrukturierung des Substrats über drei Größenordnungen eingestellt werden. Diese Arbeit liefert außerdem ein grundlegendes Verständnis zur Nukleationskinetik von Ga-Tröpfchen auf nativem-SiOx und deren Wechselwirkung mit SiOx und bestätigt theoretische Voraussagen zum sogenannten Nukleations-Antibunching, dem Auftreten einer zeitlichen Anti-Korrelation aufeinanderfolgender Nukleationsereignisse. Im zweiten Teil dieser Arbeit wurde eine alternative Methode, bezeichnet als droplet-confined alternate-pulsed epitaxy (DCAPE), für das selbstkatalytische Wachstum von GaAs Nanodrähten und GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen entwickelt. DCAPE ermöglicht das Nanodrahtwachstum bei unkonventionell geringeren Temperaturen im Bereich von 450-550 °C und ist vollständig kompatibel mit der Standard-Si-CMOS-Plattform. Der neue Wachstumsansatz erlaubt eine präzise Kontrolle der Kristallstruktur der Nanodrähte und folglich das Wachstum von defektfreien Nanodrähten mit phasenreiner Zinkblende-Struktur. Die Stärke der DCAPE Methode wird des Weiteren durch das kontrollierte Wachstum von GaAs/AlxGa1-xAs axialen Quantentopf-Nanodrähten mit abrupten Grenzflächen und einstellbarer Dicke und Al-Anteil der AlxGa1-xAs-Segmente aufgezeigt. Die GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen sind interessant für den Einsatz als Einzelphotonen-Emitter mit einstellbarer Emissionswellenlänge, wenn diese mit gitterfehlangepassten InxAl1-xAs-Schichten in einer Kern-Hülle-Konfiguration überwachsen werden. Alle Ergebnisse dieser Arbeit tragen dazu bei, den Weg für eine erfolgreiche monolithische Integration von sehr gleichförmigen GaAs-basierten Nanodrähten mit kontrollierbarer Anzahldichte, Abmessungen und Kristallstruktur auf der industriell etablierten Si-Plattform zu ebnen.

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Confronted with the global demographic changes and the increasing pressure on modern healthcare system, there has been a surge of developing new technology platforms in the past decades. Droplet microfluidics is a prominent example of such technology platforms, which offers an efficient format for massively parallelized screening of a large number of samples and holds great promise to boost the throughput and reduce the costs of modern biomedical activities. Despite recent achievements, the realization of a compact and generic screening system which is suited for resource-limited settings and point-of-care applications remains elusive. To address the above challenges, the dissertation focuses on the development of a compact multifunctional droplet micro-magnetofluidic system by exploring the advantages of magnetic in-flow detection principles. The methodologies behind a novel technique for biomedical applications, namely, magnetic in-flow cytometry have been put forth, which encompass magnetic indexing schemes, quantitative multiparametric analytics and magnetically-activated sorting. A magnetic indexing scheme is introduced and intrinsic to the magnetofluidic system. Two parameters characteristic of the magnetic signal when detecting magnetically functionalized objects, i.e. signal amplitude and peak width, providing information which is necessary to perform quantitative analysis in the spirit of optical cytometry has been proposed and realized. Magnetically-activated sorting is demonstrated to actively select individual droplets or to purify a population of droplets of interest. Together with the magnetic indexing scheme and multiparametric analytic technique, this functionality synergistically enables controlled synthesis, quality administration and screening of encoded magnetic microcarriers, which is crucial for the practical realization of magnetic suspension arrays technologies. Furthermore, to satisfy the needs of cost-efficient fabrication and high-volume delivery, an approach to fabricate magnetofluidic devices on flexible foils is demonstrated. The resultant device retains high performance of its rigid counterpart and exhibits excellent mechanical properties, which promises long-term stability in practical applications.

Confronted with the global demographic changes and the increasing pressure on modern healthcare system, there has been a surge of developing new technology platforms in the past decades. Droplet microfluidics is a prominent example of such technology platforms, which offers an efficient format for massively parallelized screening of a large number of samples and holds great promise to boost the throughput and reduce the costs of modern biomedical activities. Despite recent achievements, the realization of a compact and generic screening system which is suited for resource-limited settings and point-of-care applications remains elusive. To address the above challenges, the dissertation focuses on the development of a compact multifunctional droplet micro-magnetofluidic system by exploring the advantages of magnetic in-flow detection principles. The methodologies behind a novel technique for biomedical applications, namely, magnetic in-flow cytometry have been put forth, which encompass magnetic indexing schemes, quantitative multiparametric analytics and magnetically-activated sorting. A magnetic indexing scheme is introduced and intrinsic to the magnetofluidic system. Two parameters characteristic of the magnetic signal when detecting magnetically functionalized objects, i.e. signal amplitude and peak width, providing information which is necessary to perform quantitative analysis in the spirit of optical cytometry has been proposed and realized. Magnetically-activated sorting is demonstrated to actively select individual droplets or to purify a population of droplets of interest. Together with the magnetic indexing scheme and multiparametric analytic technique, this functionality synergistically enables controlled synthesis, quality administration and screening of encoded magnetic microcarriers, which is crucial for the practical realization of magnetic suspension arrays technologies. Furthermore, to satisfy the needs of cost-efficient fabrication and high-volume delivery, an approach to fabricate magnetofluidic devices on flexible foils is demonstrated. The resultant device retains high performance of its rigid counterpart and exhibits excellent mechanical properties, which promises long-term stability in practical applications.