Sabrina Jahns

Handheld imaging photonic crystal biosensor for multiplexed, label-free protein detection.

We present a handheld biosensor system for the label-free and specific multiplexed detection of several biomarkers employing a spectrometer-free imaging measurement system. A photonic crystal surface functionalized with multiple specific ligands forms the optical transducer. The photonic crystal slab is fabricated on a glass substrate by replicating a periodic grating master stamp with a period of 370 nm into a photoresist via nanoimprint lithography and deposition of a 70-nm titanium dioxide layer. Capture molecules are coupled covalently and drop-wise to the photonic crystal surface. With a simple camera and imaging optics the surface-normal transmission is detected. In the transmission spectrum guided-mode resonances are observed that shift due to protein binding. This shift is observed as an intensity change in the green color channel of the camera. Non-functionalized image sections are used for continuous elimination of background drift. In a first experiment we demonstrate the specific and time-resolved detection of 90.0 nm CD40 ligand antibody, 90.0 nM EGF antibody, and 500 nM streptavidin in parallel on one sensor chip. In a second experiment, aptamers with two different spacer lengths are used as receptor. The binding kinetics with association and dissociation of 250 nM thrombin and regeneration of the sensor surface with acidic tris-HCl-buffer (pH 5.0) is presented for two measurement cycles.

Optical waveguides with compound multiperiodic grating nanostructures for refractive index sensing

The spectral characteristics and refractive index sensitivity of compound multiperiodic grating waveguides are investigated in theory and experiment. Compound gratings are formed by superposition of two or more monoperiodic gratings. Compared to monoperiodic photonic crystal waveguides, compound grating waveguides offer more degrees of design freedom by choice of component grating periods and duty cycles. Refractive index sensing is achieved by evaluating the wavelength or intensity of guided mode resonances in the reflection spectrum. We designed, fabricated, and characterized 24 different compound multiperiodic nanostructured waveguides for refractive index sensing. Simulations are carried out with the Rigorous Coupled Wave Algorithm (RCWA). The resulting spectra, resonance sensitivities, and quality factors are compared to monoperiodic as well as to three selected aperiodic nanostructures (Rudin-Shapiro, Fibonacci, and Thue-Morse). The refractive index sensitivity of the TE resonances is similar for all types of investigated nanostructures. For the TM resonances the compound multiperiodic nanostructures exhibit higher sensitivity values compared to the monoperiodic nanostructure and similar values as the aperiodic nanostructures. No significant influence of the compound grating duty cycles on the sensitivity is observed.

Simulation methods for multiperiodic and aperiodic nanostructured dielectric waveguides

Nanostructured dielectric waveguides are of high interest for biosensing applications, light emitting devices as well as solar cells. Multiperiodic and aperiodic nanostructures allow for custom-designed spectral properties as well as near-field characteristics with localized modes. Here, a comparison of experimental results and simulation results obtained with three different simulation methods is presented. We fabricated and characterized multiperiodic nanostructured dielectric waveguides with two and three compound periods as well as deterministic aperiodic nanostructured waveguides based on Rudin–Shapiro, Fibonacci, and Thue–Morse binary sequences. The near-field and far-field properties are computed employing the finite-element method (FEM), the finite-difference time-domain (FDTD) method as well as a rigorous coupled wave algorithm (RCWA). The results show that all three methods are suitable for the simulation of the above mentioned structures. Only small computational differences are obtained in the near fields and transmission characteristics. For the compound multiperiodic structures the simulations correctly predict the general shape of the experimental transmission spectra with number and magnitude of transmission dips. For the aperiodic nanostructures the agreement between simulations and measurements decreases, which we attribute to imperfect fabrication at smaller feature sizes.

Imaging label-free biosensor with microfluidic system

We present a microfluidic system suitable for parallel label-free detection of several biomarkers utilizing a compact imaging measurement system. The microfluidic system contains a filter unit to separate the plasma from human blood and a functionalized, photonic crystal slab sensor chip. The nanostructure of the photonic crystal slab sensor chip is fabricated by nanoimprint lithography of a period grating surface into a photoresist and subsequent deposition of a TiO2 layer. Photonic crystal slabs are slab waveguides supporting quasi-guided modes coupling to far-field radiation, which are sensitive to refractive index changes due to biomarker binding on the functionalized surface. In our imaging read-out system the resulting resonance shift of the quasi-guided mode in the transmission spectrum is converted into an intensity change detectable with a simple camera. By continuously taking photographs of the sensor surface local intensity changes are observed revealing the binding kinetics of the biomarker to its specific target. Data from two distinct measurement fields are used for evaluation. For testing the sensor chip, 1 μM biotin as well as 1 μM recombinant human CD40 ligand were immobilized in spotsvia amin coupling to the sensor surface. Each binding experiment was performed with 250 nM streptavidin and 90 nM CD40 ligand antibody dissolved in phosphate buffered saline. In the next test series, a functionalized sensor chip was bonded onto a 15 mm x 15 mm opening of the 75 mm x 25 mm x 2 mm microfluidic system. We demonstrate the functionality of the microfluidic system for filtering human blood such that only blood plasma was transported to the sensor chip. The results of first binding experiments in buffer with this test chip will be presented.

On the effect of broadband, multi-angular excitation and detection in guided-mode resonance biosensors

Guided mode resonance biosensors are of emerging interest as they allow integration on chip with fabrication on mass scale. The guided mode resonances (GMRs), observed in the transmission or reflection spectrum, are sensitive to refractive index changes in the vicinity of the photonic crystal (PhC) surface. Standard measurement setups utilize a collecting lens, focusing the extracted light intensity onto a single-point photo detector. In order to achieve highly miniaturized devices, we consider the integration of planar emitting and detector structures, such as organic light emitting diodes (OLEDs) and organic photo detectors (OPDs), together with the PhC based biosensors, on a single chip. This approach, however, consequently leads to a broadband, multi-angular light excitation as well as to a broadband and multi-angular contribution to the OPD photon count. While GMR effects in PhC slabs with directional light sources have been widely studied, this lens-less scenario requires a deep understanding regarding the broadband and the angular influence of both incident and reflected or transmitted light. We performed finite-difference time-domain (FDTD) calculations for GMR effects in two-dimensional (2D) PhC slabs. We study the effects for broadband emission in the visible spectrum, together with an angular incident beam divergence of up to 80°. We verified the simulated results by performing angle-resolved spectral measurements with a light emitting diode (LED) in a macroscopic, lens-less setup. We further utilize this numerical setup to provide a deeper understanding of the modal behaviour of our proposed OLED and OPD-based integrated biosensor concept.

Photometric aptasensor using biofunctionalized photonic crystal slabs

We present a biosensor using a photonic crystal slab (PCS) surface-functionalized with aptamers (thus aptasensor) and a label-free photometric detection scheme. Resonances in the optical transmission spectrum of the PCS are highly sensitive to mass changes on the surface of the PCS. We align the resonances spectral position to the falling edge of a light emitting diodes (LED) spectrum and use two crossed polarization filters before and after the PCS. Thus, we may detect a molecular binding process on the surface of the PCS with a simple photometric intensity measurement. Using a CMOS camera, we determine the signal and reference intensity at different positions on the PCS. The functionality of this aptasensor is demonstrated by applying a 125 nM thrombin-solution to the PCS bio-functionalized with anti-thrombin aptamers. A significant signal was obtained and the reaction kinetic was observed.

Sensitivity enhancement of an intensity-based photonic crystal biosensor using a laser excitation source

Photonic Crystal Slabs (PCSs) are nanostructured, dielectric waveguides that feature resonances with small bandwidths and very high reflectivity in the visible spectrum. As refractive index-sensitive transducers, PCSs have been studied for biosensing applications in the past decades [1]. Incident light can couple to and from quasi-guided modes at resonance wavelengths. Due to the evanescent field of the guided mode, the resonance wavelength is sensitive to changes of refractive index in the surrounding media. Intensity-based refractive index sensing offers the opportunity for miniaturized and inexpensive readout systems for point-of-care applications. A system of the size of a microtiter plate for the readout of label-free cellular essays has recently been published and shows the high potential of this approach [2].

Plattformtechnologie für die mobile, markerfreie Proteindetektion

Zusammenfassung Wir präsentieren eine Plattformtechnologie zur gleichzeitigen, mobilen, markerfreien Detektion von mehreren Proteinen. Ein photonischer Kristall, der mit Liganden lokal funktionalisiert ist, dient hier als Sensor. Über ein kompaktes, Kamera-basiertes Messsystem wird die Proteinanbindung an die Sensoroberfläche in ein Intensitätssignal umgewandelt, über dessen Amplitude die Proteinkonzentration bestimmt werden kann. Um das Detektionslimit dieser Technologie weiter zu verbessern, werden hier photonische Kristalle mit einer multiperiodischen und aperiodischen Gitterstruktur simulativ und experimentell untersucht. Dafür werden die Gesamtempfindlichkeit und die Resonanzgüte jeder Struktur bestimmt und mit der bisher verwendeten monoperiodischen Struktur verglichen. Es konnte festgestellt werde, dass sich die Resonanzgüte von mono- über multi- bis aperiodisch deutlich verbessert. Eine Steigerung der Gesamtempfindlichkeit konnte nicht festgestellt werden. Jedoch konnte anhand von Analysen der elektrischen Feldverteilung innerhalb der verschiedenen Strukturen beobachtet werden, dass die Modenausbreitungen in den aperiodischen Strukturen stark lokalisiert wird und die elektrische Feldintensität in diesen „Hot-Spots“ deutlich über der mittleren Feldintensität, die eine flächige Funktionalisierung repräsentiert, liegt. Diese lokale Resonanzausbildung konnte zudem bereits in ersten experimentellen Untersuchungen bestätigt werden.

Imaging label-free biosensor for multiplexed protein detection

The specific, time-resolved, and parallel detection of different proteins with a handheld system is demonstrated. Locally-functionalized photonic crystal surfaces are employed as the transducer for an intensity-based, spectrometer-free measurement. The biosensor is fabricated by first forming the grating structure with ultraviolet nanoimprint lithography, depositing an 80-nm thick titanium dioxide layer on top, and immobilising different ligands drop-wise with covalent coupling to the sensor surface. We demonstrate the specific and time-resolved detection of 90.0 nM CD40 ligand antibody and 90.0 nM EGF antibody in parallel on one sensor chip. In a second experiment the detected protein concentration is reduced down to 9.0 nM for CD40 ligand antibody. Additionally, a microfluidic test chip for human blood filtration and imaging label-free detection of multiple biomarkers is designed. The performance is evaluated and the association of 500 nM biotin in buffer on two ligand positions is observed in an imaging intensity measurement.

Simulation of photonic waveguides with deterministic aperiodic nanostructures for biosensing

Photonic waveguides with deterministic aperiodic corrugations offer rich spectral characteristics under surface-normal illumination. The finite-element method (FEM), the finite-difference time-domain (FDTD) method and a rigorous coupled wave algorithm (RCWA) are compared for computing the near-field and far-field properties of structures based on a Rudin-Shapiro binary sequence. All simulation methods predict multiple resonances with sensitivities in the range of 30 nm/RIU to 60 nm/RIU for bulk refractive index measurements. Local functionalization is estimated to improve the sensitivity for biomolecular binding by a factor of 2.97.