Daniéll Malsch

Towards a quantitative SERS approach--online monitoring of analytes in a microfluidic system with isotope-edited internal standards.

In this contribution a new approach for quantitative measurements using surface-enhanced Raman spectroscopy (SERS) is presented. Combining the application of isotope-edited internal standard with the advantages of the liquid-liquid segmented-flow-based approach for flow-through SERS detection seems to be a promising means for quantitative SERS analysis. For the investigations discussed here a newly designed flow cell, tested for ideal mixing efficiency on the basis of grayscale-value measurements, is implemented. Measurements with the heteroaromatics nicotine and pyridine using their respective deuterated isotopomers as internal standards show that the integration of an isotopically labeled internal standard in the used liquid-liquid two-phase segmented flow leads to reproducible and comparable SERS spectra independent from the used colloid. With the implementation of an internal standard into the microfluidic device the influence of the properties of the colloid on the SERS activity can be compensated. Thus, the problem of a poor batch-to-batch reproducibility of the needed nanoparticle solutions is solved. To the best of our knowledge these are the first measurements combining the above mentioned concepts in order to correct for differences in the enhancement behaviour of the respective colloid.

Nanoparticle Layer Deposition for Plasmonic Tuning of Microstructured Optical Fibers

Plasmonic nanoparticles with spectral properties in the UV-to-near-IR range have a large potential for the development of innovative optical devices. Similarly, microstructured optical fibers (MOFs) represent a promising platform technology for fully integrated, next-generation plasmonic devices; therefore, the combination of MOFs and plasmonic nanoparticles would open the way for novel applications, especially in sensing applications. In this Full Paper, a cost-effective, innovative nanoparticle layer deposition (NLD) technique is demonstrated for the preparation of well-defined plasmonic layers of selected particles inside the channels of MOFs. This dynamic chemical deposition method utilizes a combination of microfluidics and self-assembled monolayer (SAM) techniques, leading to a longitudinal homogeneous particle density as long as several meters. By using particles with predefined plasmonic properties, such as the resonance wavelength, fibers with particle-adequate spectral characteristics can be prepared. The application of such fibers for refractive-index sensing yields a sensitivity of about 78 nm per refractive index unit (RIU). These novel, plasmonically tuned optical fibers with freely selected, application-tailored optical properties present extensive possibilities for applications in localized surface plasmon resonance (LSPR) sensing.

Quantitative CARS Microscopic Detection of Analytes and Their Isotopomers in a Two‐Channel Microfluidic Chip

Raman microspectroscopy provides label-free vibrational contrast at submicron spatial resolution without the need for sample preparation, and has therefore become an indispensable characterization method in various disciplines, including analytical, life, and materials sciences. The technique is particularly useful for spatially resolved quantification of the concentrations of chemical constituents in a sample, and in situations where labeling of low-molecular-weight compounds by fluorescent labels is not possible or not desired. Coherent anti-Stokes Raman scattering (CARS) microscopy benefits from significantly faster acquisition rates than conventional Raman microspectroscopy. The combination of this nonlinear Raman technique withmicrofluidics for reactionmonitoring and cytometry has been introduced recently. Unfortunately, CARS is not background-free: the signal generation leads to both a coherent excitation of molecular vibrations (nuclear motions) and an intrinsic non-Ramanresonant background (electronic response). This chemically nonspecific background constitutes a severe limitation for CARS detection and the quantification of analytes at low concentrations. Multiplex CARS microspectroscopy with subsequent CARS band-shape analysis allows extraction of

Functionalization of Microstructured Optical Fibers by Internal Nanoparticle Mono-Layers for Plasmonic Biosensor Applications

For fully integrated next-generation plasmonic devices, microstructured optical fibers (MOFs) represent a promising platform technology. This paper describes the use of a dynamic technique to demonstrate the wet chemical deposition of gold and silver nanoparticles (NPs) within MOFs. The plasmonic structures were realized on the internal capillary walls of a three-hole suspended core fiber. Electron micrographs, taken of the inside of the fiber holes, confirm the even distribution of the NP in the MOF over a length of up to 6 m. Accordingly, this procedure is highly productive and makes the resulting MOF-based sensors potentially (very) cost efficient. In proof-of-principle experiments with liquids of different refractive indices, the dependence of the localized surface plasmon resonance (LSPR) on the surroundings was confirmed. Comparing Raman spectra of MOFs with and without NP layers, each one filled with crystal violet, a significant signal enhancement demonstrates the usability of such functionalized MOFs for surface-enhanced Raman spectroscopy (SERS) experiments.

Effects of Fluid and Interface Interaction on Droplet Internal Flow in All-Glass Micro Channels

Segmented flow of aqueous droplets transported by an immiscible separation medium in all-glass micro channels is characterized by an interplay of interface interaction and phase internal flow field formation. As a result, pressure drop, local shear rates and mixing performance are determined by this interplay. Yet, these effects are strongly influenced by the properties of the employed fluids, wettability of the channel walls and experimental conditions like flow rate and fluid phase ratio. We utilize micro Particle Imaging Velocimetry (μPIV) aside direct pressure measurements as a tool to measure and visualize these effects and show their dependency on fluidic properties and experimental conditions. All-glass micro channels have been prepared by wet etching of glass half channels and anodic bonding of two glass substrates. Micro channels have coplanar faces at top and bottom and sidewalls with a half circular shape. Wetting properties of the internal micro channel surfaces are adjusted by treatment with octadecyl-trichlorosilane. Sample droplets that fully seal the channel are embedded in a separation fluid that completely wets the channel. A permanent thin film of separation fluid prevents direct contact between sample fluid and micro channel walls. In contrast to micro channels with rectangular cross-section [1,2], we observe alternating flow circulation in separation medium and droplets under these conditions [3]. As a consequence, the total pressure drop is almost independent of droplet viscosity.Copyright

Microstructured optical fiber with homogeneous monolayer of plasmonic nanoparticles for bioanalysis

Microstructured optical fibers (MOFs) represent a promising platform technology for fully integrated, next generation plasmonic devices. This paper details the use of a dynamic chemical deposition technique to demonstrate the wet chemical deposition of gold and silver nanoparticles (NP) within MOFs with longitudinal, homogenously-distributed particle densities. The plasmonic structures were realized on the internal capillary walls of a three-hole suspended core fiber. The population density of the NP on the surface, which directly influences the usable / necessary sensor length, can be tailored via the controlled pre-treatment of the fiber. With the proposed procedure we can coat several meters of fiber and, afterwards, cut the fiber into the desired lengths. Accordingly, this procedure is highly productive and makes the resulting MOF-based sensors potentially very cheap. Electron microscope micrographs, taken of the inside of the fiber holes, confirm the even distribution of the NP. A transversal through-light setup was used for the non-destructive layer characterization. In proof-of-principle experiments with liquids of different refractive indices, the LSPR dependence on the surroundings was confirmed and compared with Mie-theory based calculations.

Two Channel Microfluidic CARS for Quantifying Pure Vibrational Contrast of Model Analytes

We discuss the combination of a CARS-imaging system with microfluidics. Such system is a versatile tool to quantify the relative contributions of resonant and non-resonant scattering at the CARS frequency. We will show that the twochannel microfluidic chip employed in combination with deuterated isotopomers as an internal standard allows for fast and quantitative detection of organic molecules by CARS microscopy. The experimental design enables the simultaneous measurement of both the chemically relevant Raman-resonant signal and the non-Raman-resonant background.

Application of Self-Control in Droplet-Based Microfluidics

Droplet-based microfluidics provide a powerful platform for high-throughput operations applied in micro analytics, micro reaction technology and live sciences. Todays research interests focus on the development of highly integrated fluidic networks for sample processing according to a microchemical or microanalytical protocol. Normally, fluidic networks with integrated fluidic loops and bypasses are very complicated systems that require a huge effort for external control and integration of actor components. In contrast, in droplet-based microfluidics interface generated forces can be used to temporarily seal bypasses or to generate well defined pressure gradients at strictures. This potential can be used to implement self-control and self-synchronization at functional nodes in order to minimize the effort for external control and actors integration. Here we report on progress in development of functional nodes for self-synchronized 1:1 coalescence of two independently generated droplet sequences at a Y-shaped junction and on approaches for droplet aliquotation at a Y-shaped bifurcation. The droplet connector automatically balances the time delay between two droplets arriving at the junction. On this account, strictures are integrated into the Y-junction and an additional bypass connects the arriving channels. The first arriving droplet stops at the stricture until its fusion partner arrives. The droplet splitter performs an 1:1 aliqoutation of all elements of a droplet sequence. The main challenges are the balancing of pressure differences at the outlets and the correct aliquotation of droplets independent of their volume at a wide range of flow rates. The splitter design is based on the rule that forces required for splitting are always lower than the forces required for complete droplet inflow into only one of the outlet channels without splitting.Copyright

Homogenous metallic nanoparticle monolayer inside a microstructured optical fiber

Microstructured optical fibers (MOFs) represent a promising platform technology for new biosensing devices. Using MOFs with adapted cavity diameters of about 20 to 30 μm, they can be used to carry the biofluids of analytical interest. Such cavities with their walls coated by transducer material form in combination with adequate microfluidic chips a platform for fully integrated next generation plasmonic devices. This paper describes the use of a dynamic chemical nanoparticle layer deposition (NLD) technique to demonstrate the wet chemical deposition of gold and silver nanoparticles (NP) within MOFs with longitudinal, homogenously-distributed particle densities. The plasmonic structures were realized on the internal capillary walls of a three-hole suspended core fiber. Electron micrographs, taken of the inside of the fiber holes, confirm the even distribution of the NP. With the proposed procedure fiber lengths of several meters can be coated and afterwards cut up into small pieces of desired lengths. Accordingly, this procedure is highly productive and makes the resulting MOF-based sensors potentially cost efficient. In proof-of-principle experiments with liquids of different refractive indices, the dependence of the localized surface plasmon resonance (LSPR) on the surroundings was confirmed. Comparing Raman spectra of NP coated and uncoated MOFs, each filled with crystal violet, a significant signal enhancement demonstrates the usability of such functionalized MOFs for surfaceenhanced Raman spectroscopy (SERS) experiments.