Alexander S. Quick

Preparation of Reactive Three-Dimensional Microstructures via Direct Laser Writing and Thiol-ene Chemistry

Three-dimensional microstructures are fabricated employing the direct laser writing process and radical thiol-ene polymerization. The resin system consists of a two-photon photoinitiator and multifunctional thiols and olefins. Woodpile photonic crystals with 22 layers and a rod distance of 2 μm are fabricated. The structures are characterized via scanning electron microscopy and focused ion beam milling. The thiol-ene polymerization during fabrication is verified via infrared spectroscopy. The structures are grafted in a subsequent thiol-Michael addition reaction with different functional maleimides. The success of the grafting reaction is evaluated via laser scanning microscopy and X-ray photoelectron spectroscopy. The grafting density is calculated to be close to 200 molecules μm(-2) .

Wavelength selective polymer network formation of end-functional star polymers.

A wavelength selective technique for light-induced network formation based on two photo-active moieties, namely ortho-methylbenzaldehyde and tetrazole is introduced. The network forming species are photo-reactive star polymers generated via reversible activation fragmentation chain transfer (RAFT) polymerization, allowing the network to be based on almost any vinylic monomer. Direct laser writing (DLW) allows to form any complex three-dimensional structure based on the photo-reactive star polymers.

Cleaving Direct-Laser-Written Microstructures on Demand

Using an advanced functional photoresist we introduce direct-laser-written (DLW) 3D microstructures capable of complete degradation on demand. The networks consist exclusively of reversible bonds, formed by irradiation of a phenacyl sulfide linker, giving disulfide bonds in a radical-free step-growth polymerization via a reactive thioaldehyde. The bond formation was verified in solution by ESI-MS. To induce cleavage, dithiothreitol causes a thiol-disulfide exchange, erasing the written structure. The mild cleavage of the disulfide network is highly orthogonal to other, for example, acrylate-based DLW structures. To emphasize this aspect, DLW structures were prepared incorporating reversible structural elements into a non-reversible acrylate-based standard scaffold, confirming subsequent selective cleavage. The high lateral resolution achievable was verified by the preparation of well-defined line gratings with line separations of down to 300 nm.

Design of broadband SERS substrates by the laser-induced aggregation of gold nanoparticles

Surface-enhanced Raman scattering (SERS) has already demonstrated its significant potential in analytical science. Thus, current efforts are focused on the development of affordable and reproducible SERS substrates, which exhibit high enhancement factors and uniform responses. A large number of strategies were adopted to produce effective SERS substrates; however, most of them are tuned for the use of single excitation wavelength and consequently can only be applied for a limited number of analytes. Hence, SERS substrates that demonstrate broadband plasmonic properties represent a more flexible analytical tool for multi-wavelength or tunable light sources, especially for biological applications. In the current study, we demonstrate that direct laser writing (DLW), which activates a photoreactive moiety and immobilizes functionalized gold nanoparticles on chemically modified glass substrates, can be used to produce SERS substrates of various sizes and geometries. We show that by tuning the DLW parameters a broad plasmonic response is obtained, enabling the use of these substrates for multi-wavelength SERS analysis. Two Raman reporters, a small synthetic benzotriazole azo organic dye and a larger biological molecule, hemin, are tested at three fixed excitation wavelengths in the visible range (473 nm, 532 nm and 660 nm). SERS enhancement factors show a weak dependence on the wavelength used and the molecules investigated; moreover, the possibility of creating arbitrary shaped and uniform structures is demonstrated. The reported results show that DLW is an excellent technique to engineer microstructured and broadband SERS substrates.

Molecular Switch for Sub-Diffraction Laser Lithography by Photoenol Intermediate-State Cis–Trans Isomerization

Recent developments in stimulated-emission depletion (STED) microscopy have led to a step change in the achievable resolution and allowed breaking the diffraction limit by large factors. The core principle is based on a reversible molecular switch, allowing for light-triggered activation and deactivation in combination with a laser focus that incorporates a point or line of zero intensity. In the past years, the concept has been transferred from microscopy to maskless laser lithography, namely direct laser writing (DLW), in order to overcome the diffraction limit for optical lithography. Herein, we propose and experimentally introduce a system that realizes such a molecular switch for lithography. Specifically, the population of intermediate-state photoenol isomers of α-methyl benzaldehydes generated by two-photon absorption at 700 nm fundamental wavelength can be reversibly depleted by simultaneous irradiation at 440 nm, suppressing the subsequent Diels-Alder cycloaddition reaction which constitutes the chemical core of the writing process. We demonstrate the potential of the proposed mechanism for STED-inspired DLW by covalently functionalizing the surface of glass substrates via the photoenol-driven STED-inspired process exploiting reversible photoenol activation with a polymerization initiator. Subsequently, macromolecules are grown from the functionalized areas and the spatially coded glass slides are characterized by atomic-force microscopy. Our approach allows lines with a full-width-at-half-maximum of down to 60 nm and line gratings with a lateral resolution of 100 nm to be written, both surpassing the diffraction limit.

Photo-Induced Click Chemistry for DNA Surface Structuring by Direct Laser Writing

Oligonucleotides containing photo-caged dienes were prepared and shown to react quantitatively in a light-induced Diels-Alder cycloaddition with functional maleimides in aqueous solution within minutes. Due to its high yield and fast rate, the reaction was exploited for DNA surface patterning with sub-micrometer resolution employing direct laser writing (DLW). Functional DNA arrays were written by direct laser writing (DLW) in variable patterns, which were further encoded with fluorophores and proteins through DNA directed immobilization. This mild and efficient light-driven platform technology holds promise for the fabrication of complex bioarrays with sub-micron resolution.