Local modification and laser-welding production of functionalized microfluidic chips

The ability to locally functionalize the surface of glass and polymers allows for myriad biomedical and chemical applications. When we started this research enclosed surfaces such as microfluidic channels were not easily amenable to localized functionalization. Furthermore, pre-functionalization of microfluidic devices was incompatible with the typical high-temperature substrate bonding procedures. The work described in this thesis was performed to tackle these two issues. In Chapter 1, an introduction is provided to concepts that are central to microfluidics and surface functionalization. Focus is given to techniques that allow for localized surface functionalization, specifically on silicon oxide and glass surfaces. Although contact methods are often easy and near-ubiquitously used, non-contact methods, such as photolithography, are preferred for scaled-up production and are compatible with enclosed surfaces. In addition, plastics – preferably so-called inert plastics – are emerging as a consistent and cheap substrate for the fabrication of microtechnologies. Therefore, previous methods for the modification of such plastics, e.g. cyclic olefin co-polymers (COC), are described. Most of these methods rely on harsh conditions, e.g. plasma, to oxidize the plastic surface. However, in solution there are reported methods to selectively oxidize C-H bonds on alkanes, under mild conditions. Solution-based procedures are much more compatible with enclosed structures when compared to methods relying on gases or plasmas, as the latter exhibit several diffusion issues. Finally, we address the emergence of laser-based techniques and other low-temperature glass bonding methods which will allow for the pre-functionalization of microfluidic devices prior to the bonding step of their fabrication. An outline of this thesis is provided at the end of the chapter. In Chapter 2, we present the first example of a photochemical modification of hydrogen-terminated glass (H-glass) with terminal alkenes. Both flat glass surfaces and the inside of glass microchannels were modified with a well-defined, covalently attached organic monolayer using a range of wavelengths, including 302 nm ultraviolet light. Mechanistic studies of the surface modification showed that it proceeds via an anti-Markovnikov substitution. Reacting H-glass with 10-trifluoro-acetamide-1-decene (TFAAD) followed by basic hydrolysis afforded the corresponding primary amine-terminated monolayer, enabling additional functionalization of the substrate (with fluorescent nanoparticles). A microchannel was photochemically patterned with a functional linker allowing for surface-directed liquid flow, and a passive flow stop-valve. H-glass was shown to be modifiable with a functional tailor-made organic monolayer, has highly tunable wetting properties and presents potential for many applications that are explored in the following chapters. In Chapter 3, we studied how to increase the wavelength used in the surface modification approach described in the previous chapter, from 302 to 328 nm. Thus, we show the direct photochemical coupling of a N-hydroxysuccinimide-terminated (NHS) ω-alkene onto hydrogen-terminated silicon oxide, and its subsequent functionalization with a catalytically active DNAzyme. In this chapter, we prepared hydrogen-phenyl-terminated glass (H-Φ-glass) by the reaction of glass with H-SiPhCl2. The presence of a radical-stabilizing substituent on the Si atom enabled the covalent modification of bare glass substrates and of the inside of glass microchannels with a functional organic monolayer that allowed direct reaction with an amine-functionalized bio-molecule. Using these NHS-based active esters on the surface, we performed a direct localized attachment of a horseradish peroxidase (HRP)-mimicking hemin/G-quadruplex (hGQ) DNAzyme complex inside a microfluidic channel. This wall-coated hGQ DNAzyme effectively catalyzed the in-flow oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonate) [ABTS] in the presence of hydrogen peroxide. This proof-of-concept of mild bio-functionalization will find use in myriad bio-relevant applications. In Chapter 4, we explored the modification of plastics, such as cyclic olefin copolymer (COC), which are becoming an increasingly popular material for microfluidics. COC is used, in part, due to its (bio)-chemical resistance. However, its inertness and hydrophobicity can be a major downside for many bio-applications. In this chapter, we showed the first example of a surface-bound selective C-H activation of COC into alcohol C-OH moieties under mild aqueous conditions at room temperature. The nucleophilic COC-OH surface allowed for subsequent covalent attachments, such as of a H-terminated silane. The resulting hybrid material (COC-Si-H) was then modified via a photolithographic hydrosilylation in the presence of ω‑functionalized 1-alkenes to form a new highly-stable, solvent-resistant hybrid surface. The work described in this chapter shows great potential for multiple types of plastics, and further shows that the hydrosilylation chemistry described in the previous chapters can be applied onto a multitude of surfaces that contain nucleophilic groups (such as -OH), and thus be able to react with hydrogen-terminated silanes. In Chapter 5, we explored low-temperature bonding methods of glass substrates. These methods are of great interest in the field of microfluidic-based biosensing. We evaluated how laser welding could be used for this, by studying how the compatibility of this technology with the, prior to bonding, modification of glass channels with temperature-sensitive materials. The effects of the welding process were studied by investigating the thermal degradation of a fluorescent monolayer throughout the bonding procedure. Furthermore, these leakage-free devices, as well as preliminary work using picosecond lasers, shows that there is a great potential for improvement and wide applicability. In Chapter 6, we summarize the results obtained in this thesis. In addition, we discuss some ideas and topics where the research could either be improved or achieved via alternative approaches, including some leads from unfinished work that was performed within this project.