Anne Habermehl

Superresolution optical fluctuation imaging (SOFI) aided nanomanipulation of quantum dots using AFM for novel artificial arrangements of chemically functionalized colloidal quantum dots and plasmonic structures

For single photon experiments or research on novel hybrid structures consisting of several colloidal quantum dots (Qdots) and plasmonic nanoparticles both the precise localization and the optical behavior of the emitters need to be correlated. Therefore, the gap between the high spatial resolution topography information that provides detailed localization of single Qdots and the diffraction limited fluorescence image needs to be overcome. In this paper, we demonstrate the combination of atomic force microscopy (AFM) with wide-field fluorescence microscopy improved by superresolution optical fluctuation imaging (SOFI). With this approach the topography and the superresolution image can be overlaid with sub-diffraction precision. Consequently, we discriminate between single Qdots that are optically active and dark ones. Additionally, the optical time-dependent behavior of molecular emitters can be selectively investigated. This method is, furthermore, useful for an advanced manipulation and characterization toolbox of Qdots in general. In summary, our findings represent an easily adaptable, highly reproducible and comparatively cheap subdiffraction limit imaging method and they facilitate the efficient selection of bright Qdots in a standard lab environment for proof-of-principle nanostructures containing Qdots and for nanomanipulation experiments.

Lab-on-Chip, Surface-Enhanced Raman Analysis by Aerosol Jet Printing and Roll-to-Roll Hot Embossing

Surface-enhanced Raman spectroscopy (SERS) combines the high specificity of Raman scattering with high sensitivity due to an enhancement of the electromagnetic field by metallic nanostructures. However, the tyical fabrication methods of SERS substrates suffer from low throughput and therefore high costs. Furthermore, point-of-care applications require the investigation of liquid solutions and thus the integration of the SERS substrate in a microfluidic chip. We present a roll-to-roll fabrication approach for microfluidics with integrated, highly efficient, surface-enhanced Raman scattering structures. Microfluidic channels are formed using roll-to-roll hot embossing in polystyrene foil. Aerosol jet printing of a gold nanoparticle ink is utilized to manufacture highly efficient, homogeneous, and reproducible SERS structures. The modified channels are sealed with a solvent-free, roll-to-roll, thermal bonding process. In continuous flow measurements, these chips overcome time-consuming incubation protocols and the poor reproducibility of SERS experiments often caused by inhomogeneous drying of the analyte. In the present study, we explore the influence of the printing process on the homogeneity and the enhancement of the SERS structures. The feasibility of aerosol-jet-modified microfluidic channels for highly sensitive SERS detection is demonstrated by using solutions with different concentrations of Rhodamine 6G and adenosine. The printed areas provide homogeneous enhancement factors of ~4 × 106. Our work shows a way towards the low-cost production of tailor-made, SERS-enabled, label-free, lab-on- chip systems for bioanalysis.

Roll-to-roll production of a microfluidic platform and its functionalization by means of digital printing technologies for gas and fluid sensors (Conference Presentation)

The individualized functionalization of mass-produced microstructures is still challenging for the process technology. Here, a rroll-to-roll based process hot embossing is presented for the production of microfluidic structures by means of hot embossing is presented. The resulting microfluidic channels are functionalized modified with different materials. Thereby, digital printing technologies such as aAerosoljet or inkjet are used. This approach allows for mass production of microfluidic channels and their the individualized individual functionalizationfunctionalization of mass produced microfluidic channels. The encapsulation of the channels also takes placeis realized in an R2R-based thermal bonding process without adding any solvent or adhesive. Taking account ofUsing this approach, several sensor systems for gas and / or fluid detection could be demonstrated. Surface -eEnhanced Raman Scattering scattering (SERS) with amplification enhancement factors of up to 107 [1] is demonstrated by printing gold nanoparticles into the microfluidic channel. We evaluate the printed SERS structures using solutions of rhodamine 6G and adenosine as exemplary analytes. Furthermore, these channels could be functionalized with different fluorescent organic semiconductors. Their fluorescence intensity is quenched in the presence of a nitroaromatic compounds. By using different materials simultaneously, we are able to measure a fingerprint like pattern of different analytes, which we evaluated by means ofusing pattern recognition algorithms. This method can be used both in the gas phase (electronic nose) and in fluids (electronic tongue) for the detection of nitroaromatic compounds [2,3]. With the opto-electronic nose, we were able to reach detections limits below 1ppb. [1] A. Habermehl et al, Sensors 17, 2401 (2017). [2] N. Bolse et al, Flexible and Printed Electronics 2, 024001 (2017) [3] N. Bolse et al, ACS Omega 2 (10), 6500-6505 (2017)

Comparing roll-to-roll and laser-assisted hot embossing for micro- and nanofabrication

We demonstrate the suitability of two cost efficient technologies, namely roll-to-roll hot embossing and laser-assisted hot embossing, to fabricate arrays of structures in the microscale down to the sub-100 nm range. We therefore employ polymers with a relatively moderate glass transition temperature, e.g., cyclic olefin copolymer (COC) and polystyrene (PS). We compare the two replication processes regarding their precision and cost using different 1D and 2D nanostructure gratings and microfluidic channels. All nickel shims used for the replication are fabricated in combination of electron beam or UV lithography and nickel electroforming. The replicated structures are used in different applications. The nanopillar arrays are coated with gold and integrated in the hot embossed microfluidic channels for lab-on-a-chip (LoC) surface-enhanced Raman analysis. We evaluate the as-fabricated 2D nanopillar arrays for surface-enhanced Raman spectroscopy (SERS) using solutions of rhodamine 6G as exemplary analytes. The influence of the geometrical parameters like diameter and pitch of the polymer structures as well as the influence of the gold layer thickness are discussed. 1D-gratings will be used as resonators for organic distributed feedback (DFB) lasers. Both elements, the SERS chips and the organic DFB lasers as tunable excitation source can be combined in the future to form one Raman-on-Chip optofluidic platform for sensitive detection of low-concentrated analytes in water.

A low-cost versatile fluorescence quenching detection system for liquid- and vapor-phase sensing

We report on the design and evaluation of a low-cost versatile optical sensor system to detect explosive traces by fluorescence quenching. In comparison to common detection systems, it allows to rapidly sample sensor arrays in various analyte carriers. Moreover, our work enables system development towards low-cost analysis and sensor testing solutions due to the simplicity of the approach. Therefore, we demonstrate the detection of nitroaromatic analytes in toluene, in water and in air by the application of transducing polymers. The results of the system demonstration indicate that the transduction strongly depends on the transducer state and on the analyte carrier.

Discrimination of trace nitroaromatics using linear discriminant analysis on aerosol jet printed fluorescent sensor arrays

In this work, we report on fluorescent sensor arrays fabricated by aerosol jet printing on glass substrates to detect explosives-related nitroaromatic species. The printed sensor arrays consist of six different fluorescent polymers responding to nitroaromatic vapors through a photo-induced electron transfer. This results in a quenched fluorescence proportional to the vapor concentration. Distinct fluorescence quenching patterns are detected for nitroaromatic species including nitrobenzene, 1,3-dinitrobenzene and 2,4-dinitrotoluene. The detected fingerprints are evaluated at low concentrations of only 1, 3 and 10 parts-per-billion in air. Linear discriminant analysis is used to train each sensor array enabling the discrimination of the target analyte vapors. To investigate the reproducibility of multiple sensor arrays on a single substrate, the measured fluorescence quenching patterns are used to benchmark the linear discriminant models. For this purpose, the target analytes and vapor concentrations are predicted for each sensor array. On average, we report low and reproducible misclassification rates of about 4 % indicating excellent discriminatory abilities at low concentrations close to the detection limits. We conclude that digital printing of fluorescent polymers offers the potential to realize low-cost sensor arrays for a reliable detection of trace explosives.

Nitroaromatic explosive vapor detection using a digitally printed sensor array (Conference Presentation)

We report on a fluorescent optoelectronic nose for the trace detection of nitroaromatic explosive vapors. The sensor arrays, fabricated by aerosol-jet printing, consist of six different polymers as transducers. We demonstrate the nose’s ability to discriminate between several nitroaromatics including nitrobenzene, 1,3-dinitrobenzene and 2,4-dinitrotoluene at three different concentrations using linear discriminant analysis (LDA). We assess the within-batch reproducibility of the printing process and we report that the sensor polymers show efficient fluorescence quenching capabilities with detection limits of a few parts-per-billion in air. Our approach enables the realization of highly integrated optical sensor arrays in optoelectronic noses for the sensitive and selective detection of nitroaromatic explosive trace vapors using a potentially low-cost digital printing technique suitable for high-volume fabrication. An important challenge is temperature-dependence which is often neglected even though organic emitters are strongly affected by temperature. For some materials, even small changes of a few Kelvin can lead to large changes in the emission intensity making a temperature-control for sensing applications inevitable. Therefore, the temperature-dependence of these sensors is investigated via a heated transparent thin film on the back of such sensors allowing the active layer to be temperature controlled. All of these led to the development of a portable system.

Fabrication of SERS Substrates by Roll-to-Roll Hot Embossing

Surface-enhanced Raman spectroscopy (SERS) combines the high specificity of Raman scattering with a high sensitivity due to an enhancement of the electromagnetic field by metallic nanostructures. Multiple micro- and nanostructuring methods have been used to fabricate SERS-structures [1–3]. We present an approach to create enhancing arrays of nanopillars [4] by roll-to-roll (R2R) hot-embossing and to functionalize them with a gold layer. The R2R hot embossing process is shown in principle in Fig. 55.1 on the left. The setup mainly consists in two rotating cylinders (one micro- or nanostrcutured) that can be pressed against each other and additionally heated up to exceed the glass transition temperature of the polymer foil in between. The embossing parameters mainly determine the results [5]. We have investigated the influence of the embossing temperature and pressure. With the identified parameter setting we can show the uniform transfer of the nanopillars like shown in Fig. 55.1 on the right. The setup and the result of a Raman measurement of the analyte rhodamine 6G (Rh6G) on a SERS substrate and an unstructured reference substrate is shown in Fig. 55.2. In a following step this SERS substrate can be integrated into a microfluidic chip by manufacturing one master for the microstructure and the nanostructures. Our work show a way to a low cost fabrication of tailor-made sensor chips for SERS analyse.

Electrical and optical properties of reduced graphene oxide thin film deposited onto polyethylene terephthalate by spin coating technique

We present the reduction of solution processed graphene oxide films by hydrogen iodide vapor. The films were studied by Raman spectroscopy and Fourier-transform infrared spectroscopy and its optoelectronic properties characterized. We obtained reduced graphene oxide films on polyethylene terephthalate flexible substrates with good electrical properties, 3.74×10-6  Ω·m, and high optical transmittance of 70% in the visible range. The fabricated layers contain graphene sheets with sizes up to ∼10  μm long and ∼6  μm wide. The presented solution, with highly concentrated processed graphene oxide, could be used as printing ink for manufacturing transparent and conductive electrodes on plastic substrates without the requirement of elevated temperatures.

Transient response of a microfibrous web array for the discrimination of nitroaromatic explosive vapors

We report on the transient response of a microfiber array for the detection of nitroaromatic trace explosives through fluorescence quenching by 1,3-dinitrobenzene, 2,4-dinitrotoluene and nitrobenzene. The sensor array was fabricated from polymer-coated microfibers. An in-house developed portable system was used to study the sensor array response when exposed to vapor concentrations between 1–10 parts-per-billion in air. We show that linear discriminant analysis can be used to discriminate explosives-related compounds close to the detection limits.