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Photo-switching of smart surfaces for integrated biosensors

Final Report Summary - PHOTOSMART (Photo-switching of smart surfaces for integrated biosensors)

The project PhotoSmart aimed to master the first steps towards integrated biosensors with photo-switchable smart surfaces. The idea is to integrate an array of organic light emitting diodes (OLEDs) with a surface of immobilized molecular motor molecules. Such a surface is termed a “smart” surface as the surface properties may be adjusted automatically during operation. In order to demonstrate molecular switching with relatively low intensity OLEDs, losses at the surface need to be minimized and the intensity delivered to the surface needs to be maximized.

We employ azobenzene molecules as molecular motors as they may be switched with light reversibly between the trans and cis isomer. These two isomers differ markedly in their molecular geometries and electronic properties. We implemented and compared three different immobilization methods for azobenzene molecules on dielectric surfaces (click chemistry, crosslinker applications, mussel-inspired adhesion methods with polydopamine). Dielectric surfaces were chosen as they have significantly lower absorption losses than metal surfaces. All three types of functionalization processes were tested on different types of dielectric surfaces (glass, PDMS, SiO2, ZnO, TiO2) as well as on nanostructured surfaces. The polydopamine-based process was found to be most suitable for application as it is a facile process that does not damage an underlying nanostructure.

An on-chip light source for molecular switching was realized employing blue OLEDs based on the fluorescent emitter DPvBi:BCzVBi. Switching from the cis to the trans state within 5 minutes was demonstrated for self-assembled monolayers (SAMs) of azobenzene molecules on a flat glass substrate. Successful switching of an azobenzene-functionalized lotus-leaf with an on-chip OLED light source was demonstrated. Light concentration at the surface was demonstrated using a periodically nanostructured surface (photonic crystal slab). For enhanced excitation in both switching directions of the azobenzene we engineered a compound grating structure with two superimposed grating periods of 180 nm and 220 nm.

Wettability control was achieved for azobenzene-functionalized surfaces as well as for spiropyran-functionalized surfaces. Rough surfaces on the nano- and microscale were implemented for increased switching angles. In the first case, nanoporous hybrid-layer surfaces cast with polydopamine-assisted azobenzene-SiO2-nanoparticles give a change of contact angle of 60°. By combining the azobenzene-SiO2-nanoparticles with hierarchical ZnO-PDMS surfaces a stable surface was demonstrated that allowed for repeated drop placement at the same position with a slightly reduced change of contact angle of 50°. In the second case noncovalent binding of spiropyran to candle-soot-covered surfaces was realized. A reversible switching with UV light and blue or green light was achieved, starting from an initial contact angle of 130°. The highest contact angle difference is 41° and reversibility was shown for several switching cycles.

We developed a compact readout system for imaging readout of biomolecular binding to dielectric surfaces. The overall system response was designed such that a change in the spectral resonance is converted into an intensity change that may be detected with a simple camera. Using this setup we measured the binding kinetics of several biomarkers to locally spotted functionalization sites. This approach has the particular advantage that the complete surface is captured simultaneously allowing for dynamic background subtraction. A first microfluidic test chip was realized. For switching of binding kinetics we designed a thrombin receptor molecule attached to an azobenzene photoswitch. By drop casting surfaces were locally functionalized with the azobenzene-functionalized thrombin-binding aptamer. The remaining surface was passivated. Upon addition of thrombin the local binding was confirmed in temporally-resolved measurements. These measurements were carried out with different blue and UV illumination states. We demonstrated that photo switching of the azoaptamer changes the dissociation rate and may serve sensor regeneration.