Journal articles, book contributions, reviews

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Visaveliya, Nikunjkumar R.; Mazétyté-Stasinskiené, Raminta; Köhler, Michael;
Stationary, continuous, and sequential surface-enhanced raman scattering sensing based on the nanoscale and microscale polymer-metal composite sensor particles through microfluidics: a review. - In: Advanced optical materials, ISSN 2195-1071, Bd. 10 (2022), 7, 2102757, S. 1-25

Surface-enhanced Raman scattering (SERS) is a label-free and accurate analytical technique for the detection of a broad range of various analytes such as, biomolecules, pesticides, petrochemicals, as well as, cellular and other biological systems. A key component for the SERS analysis is the substrate which is required to be equipped with plasmonic features of metal nanostructures that directly interact with light and targeted analytes. Either metal nanoparticles can be deposited on the solid support (glass or silicon) which is suitable for stationary SERS analysis or dispersed in the solution (freely moving nanoparticles). Besides these routinely utilizing SERS substrates, polymer-metal composite particles are promising for sustained SERS analysis where metal nanoparticles act as plasmon-active (hence SERS-active) components and polymer particles act as support to the metal nanoparticles. Composite sensor particles provide 3D interaction possibilities for analytes, suitable for stationary, continuous, and sequential analysis, and they are reusable/regenerated. Therefore, this review is focused on the experimental procedures for the development of multiscale, uniform, and reproducible composite sensor particles together with their application for SERS analysis. The microfluidic reaction technique is highly versatile in the production of uniform and size-tunable composite particles, as well as, for conducting SERS analysis.
Visaveliya, Nikunjkumar R.; Mazétyté-Stasinskiené, Raminta; Köhler, Michael;
General background of SERS sensing and perspectives on polymer-supported plasmon-active multiscale and hierarchical sensor particles. - In: Advanced optical materials, ISSN 2195-1071, Bd. 10 (2022), 4, 2102001, S. 1-27

Surface-enhanced Raman scattering (SERS) is one of the most powerful analytical techniques for the identification of molecules. The substrate, on which SERS is dependent, contains regions of nanoscale gaps (hotspots) that hold the ability to concentrate incident electromagnetic fields and effectively amplify vibrational scattering signals of adsorbed analytes. While surface plasmon resonance from metal nanostructures is a central focus for the SERS effect, the support of polymers can be significantly advantageous to provide larger exposure of structured metal surfaces for efficient interactions with analytes. Characteristics of the polymer particles such as softness, flexibility, swellability, porosity, optical transparency, metal-loading ability, and high surface area can allow diffusion of analytes and penetrating light deeply that can enormously amplify sensing outcomes. As polymer-supported plasmon-active sensor particles can emerge as versatile SERS substrates, the microfluidic platform is promising for the generation of sensor particles as well as for performing sequential SERS analysis of multiple analytes. Therefore, in this perspective article, the development of multifunctional polymer-metal composite particles, and their applications as potential sensors for SERS sensing through microfluidics are presented. A detailed background from the beginning of the SERS field and perspectives for the multifunctional sensor particles for efficient SERS sensing are provided.
Köhler, Michael;
Vaccination, immunity and breakthrough: quantitative effects in individual immune responses illustrated by a simple kinetic model. - In: Applied Sciences, ISSN 2076-3417, Bd. 12 (2022), 1, 31, insges. 15 S.

The personal risks of infection, as well as the conditions for achieving herd immunity, are strongly dependent on an individual’s response to the infective agents on the one hand, and the individual’s reactions to vaccination on the other hand. The main goal of this work is to illustrate the importance of quantitative individual effects for disease risk in a simple way. The applied model was able to illustrate the quantitative effects, in the cases of different individual reactions, after exposition to viruses or bacteria and vaccines. The model was based on simple kinetic equations for stimulation of antibody production using different concentrations of the infective agent, vaccine and antibodies. It gave a qualitative explanation for the individual differences in breakthrough risks and different requirements concerning a second, third or further vaccinations, reconsidering different efficiencies of the stimulation of an immune reaction.
Stolle, Heike Lisa Kerstin Stephanie; Kluitmann, Jonas; Csáki, Andrea; Köhler, Michael; Fritzsche, Wolfgang;
Shape-dependent catalytic activity of gold and bimetallic nanoparticles in the reduction of methylene blue by sodium borohydride. - In: Catalysts, ISSN 2073-4344, Bd. 11 (2021), 12, 1442, S. 1-20

In this study the catalytic activity of different gold and bimetallic nanoparticle solutions towards the reduction of methylene blue by sodium borohydride as a model reaction is investigated. By utilizing differently shaped gold nanoparticles, i.e., spheres, cubes, prisms and rods as well as bimetallic gold–palladium and gold-platinum core-shell nanorods, we evaluate the effect of the catalyst surface area as available gold surface area, the shape of the nanoparticles and the impact of added secondary metals in case of bimetallic nanorods. We track the reaction by UV/Vis measurements in the range of 190-850 nm every 60 s. It is assumed that the gold nanoparticles do not only act as a unit transferring electrons from sodium borohydride towards methylene blue but can promote the electron transfer upon plasmonic excitation. By testing different particle shapes, we could indeed demonstrate an effect of the particle shape by excluding the impact of surface area and/or surface ligands. All nanoparticle solutions showed a higher methylene blue turnover than their reference, whereby gold nanoprisms exhibited 100% turnover as no further methylene blue absorption peak was detected. The reaction rate constant k was also determined and revealed overall quicker reactions when gold or bimetallic nanoparticles were added as a catalyst, and again these were highest for nanoprisms. Furthermore, when comparing gold and bimetallic nanorods, it could be shown that through the addition of the catalytically active second metal platinum or palladium, the dye turnover was accelerated and degradation rate constants were higher compared to those of pure gold nanorods. The results explore the catalytic activity of nanoparticles, and assist in exploring further catalytic applications.
Kronfeld, Klaus-Peter; Mazétyté-Stasinskiené, Raminta; Zheng, Xuejiao; Köhler, Michael;
Textured and hierarchically constructed polymer micro- and nanoparticles. - In: Applied Sciences, ISSN 2076-3417, Bd. 11 (2021), 21, 10421, insges. 17 S.

Microfluidic techniques allow for the tailored construction of specific microparticles, which are becoming increasingly interesting and relevant. Here, using a microfluidic hole-plate-device and thermal-initiated free radical polymerization, submicrometer polymer particles with a highly textured surface were synthesized. Two types of monomers were applied: (1) methylmethacrylate (MMA) combined with crosslinkers and (2) divinylbenzene (DVB). Surface texture and morphology can be influenced by a series of parameters such as the monomer-crosslinker-solvent composition, surfactants, and additives. Generally, the most structured surfaces with the simultaneously most uniform particles were obtained in the DVB-toluene-nonionic-tensides system. In a second approach, poly-MMA (PMMA) particles were used to build aggregates with bigger polymer particles. For this purpose, tripropyleneglycolediacrylate (TPGDA) particles were synthesized in a microfluidic co-flow arrangement and polymerized by light- irradiation. Then, PMMA particles were assembled at their surface. In a third step, these composites were dispersed in an aqueous acrylamide-methylenebisacrylamide solution, which again was run through a co-flow-device and photopolymerized. As such, entities consisting of particles of three different size ranges - typically 0.7/30/600 [my]m - were obtained. The particles synthesized by both approaches are potentially suitable for loading with or incorporation of analytic probes or catalysts such as dyes or metals.
Kronfeld, Klaus-Peter; Ellinger, Thomas; Köhler, Michael;
Micro flow photochemical synthesis of Ca-sensitive fluorescent sensor particles. - In: Engineering in life sciences, ISSN 1618-2863, Bd. 21 (2021), 8/9, S. 518-526

Fluorescence probes have widely been used for detecting and imaging Ca2+-enriched parts of cells but more rarely for quantitative determination of concentrations. In this study we show how this can be achieved by a novel approach using hydrogel particles. In a microfluidic co-flow arrangement spherical droplets were generated from an aqueous solution of acrylamide, N,N'-methylenebisacrylamide crosslinker and photoinitiator and subsequently photo-cured in situ yielding gel particles in a sub millimeter range. These particles were separated, dried under reduced pressure and re-swollen in water containing Rhod-5N tri potassium salt as calcium ion selective fluorescence probe. After that the particles were dried again and stored for further investigations. Upon exposure of dried particles to calcium chloride solutions they swell and take up Ca2+-ions forming a strong fluorescing complex with Rhod-5N. Thus, fluorescence intensity increases with calcium ion concentration. Up to ca. 0.50 mM the enhancement effect is strong and then becomes considerably weaker. The intensity-concentration-dependence is well described by an equation derived from the equilibrium of the formation of a 1:1 Ca2+:Rhod-5N complex. The particles allow for a fast optical determination of Ca2+-concentrations up to 0.50 mM in analyte volumes down to below 10 [my]L.
Köhler, Michael; Kluitmann, Jonas; Günther, Mike;
Metal nanoparticles as free-floating electrodes. - In: Encyclopedia, ISSN 2673-8392, Bd. 1 (2021), 3, S. 551-565

Colloidal metal nanoparticles in an electrolyte environment are not only electrically charged but also electrochemically active objects. They have the typical character of metal electrodes with ongoing charge transfer processes on the metal/liquid interface. This picture is valid for the equilibrium state and also during the formation, growth, aggregation or dissolution of nanoparticles. This behavior can be understood in analogy to macroscopic mixed-electrode systems with a free-floating potential, which is determined by the competition between anodic and cathodic partial processes. In contrast to macroscopic electrodes, the small size of nanoparticles is responsible for significant effects of low numbers of elementary charges and for self-polarization effects as they are known from molecular systems, for example. The electrical properties of nanoparticles can be estimated by basic electrochemical equations. Reconsidering these fundamentals, the assembly behavior, the formation of nonspherical assemblies of nanoparticles and the growth and the corrosion behavior of metal nanoparticles, as well as the formation of core/shell particles, branched structures and particle networks, can be understood. The consequences of electrochemical behavior, charging and self-polarization for particle growth, shape formation and particle/particle interaction are discussed.
Köhler, Michael;
Challenges for nanotechnology. - In: Encyclopedia, ISSN 2673-8392, Bd. 1 (2021), 3, S. 618-631

The term "Nanotechnology" describes a large field of scientific and technical activities dealing with objects and technical components with small dimensions. Typically, bodies that are in-at least-two dimensions smaller than 0.1 [my]m are regarded as "nanobjects". By this definition, a lot of advanced materials, as well as the advanced electronic devices, are objects of nanotechnology. In addition, many aspects of molecular biotechnology as well as macromolecular and supermolecular chemistry and nanoparticle techniques are summarized under "nanotechnology". Despite this size-oriented definition, nanotechnology is dealing with physics and chemistry as well as with the realization of technical functions in the area between very small bodies and single particles and molecules. This includes the shift from classical physics into the quantum world of small molecules and low numbers or single elementary particles. Besides the already established fields of nanotechnology, there is a big expectation about technical progress and solution to essential economic, medical, and ecological problems by means of nanotechnology. Nanotechnology can only meet these expectations if fundamental progress behind the recent state of the art can be achieved. Therefore, very important challenges for nanotechnology are discussed here.
Cao, Jialan; Chande, Charmi; Kalensee, Franziska; Schüler, Tim; Köhler, Michael;
Microfluidically supported characterization of responses of Rhodococcus erythropolis strains isolated from different soils on Cu-, Ni-, and Co-stress. - In: Brazilian journal of microbiology, ISSN 1678-4405, Bd. 52 (2021), 3, S. 1405-1415

We present a new methodological approach for the assessment of the susceptibility of Rhodococcus erythropolis strains from specific sampling sites in response to increasing heavy metal concentration (Cu2+, Ni2+, and Co2+) using the droplet-based microfluid technique. All isolates belong to the species R. erythropolis identified by Sanger sequencing of the 16S rRNA. The tiny step-wise variation of metal concentrations from zero to the lower mM range in 500 nL droplets not only provided accurate data for critical metal ion concentrations but also resulted in a detailed visualization of the concentration-dependent response of bacterial growth and autofluorescence activity. As a result, some of the isolates showed similar characteristics in heavy metal tolerance against Cu2+, Ni2+, and Co2+. However, significantly different heavy metal tolerances were found for other strains. Surprisingly, samples from the surface soil of ancient copper mining areas supplied mostly strains with a moderate sensitivity to Cu2+, Ni2+, and Co2+, but in contrast, a soil sample from an excavation site of a medieval city that had been covered for about eight centuries showed an extremely high tolerance against cobalt ion (up to 36 mM). The differences among the strains not only may be regarded as results of adaptation to the different environmental conditions faced by the strains in nature but also seem to be related to ancient human activities and temporal partial decoupling of soil elements from the surface. This investigation confirmed that microfluidic screening offers empirical characterization of properties from same species which has been isolated from sites known to have different human activities in the past.
Kluitmann, Jonas; Zheng, Xuejiao; Köhler, Michael;
Tuning the morphology of bimetallic gold-platinum nanorods in a microflow synthesis. - In: Colloids and surfaces, Bd. 626 (2021), 127085

An automated microfluidic system with computer-controlled syringe pumps was applied for screening a three-dimensional concentration space for the formation of binary gold-platinum metal nanorods. Leveraging the micro segmented flow technique, precise residence and reactant addition timings as well as concentration spaces were addressed. The density and thickness of quasi-isotropic platinum shells on gold nanorod cores were tuned from isolated spots to a dense arrangement of high-aspect-ratio columns. The changing optical properties of the particles in the platinum deposition were used for monitoring the reaction progress and the products by the means of a fiber based micro flow-through spectrophotometer allowing to optimize process times. From our data, we propose an electrochemical model, postulating a diode-like effect and limitations for the formation of Pt nuclei on the gold surface and the formation of nano local elements. This point of view is supported by the observed decoration effects of gold facets and to the formation of columnar structures of the platinum shell.