Fraunhofer Institute for Microengineering and Microsystems

The anisotropic etching behavior of single‐crystal silicon and the behavior of and in an ethylenediaminebased solution as well as in aqueous , , and were studied. The crystal planes bounding the etch front and their etch rates were determined as a function of temperature, crystal orientation, and etchant composition. A correlation was found between the etch rates and their activation energies, with slowly etching crystal surfaces exhibiting higher activation energies and vice versa. For highly concentrated solutions, a decrease of the etch rate with the fourth power of the water concentration was observed. Based on these results, an electrochemical model is proposed, describing the anisotropic etching behavior of silicon in all alkaline solutions. In an oxidation step, four hydroxide ions react with one surface silicon atom, leading to the injection of four electrons into the conduction band. These electrons stay localized near the crystal surface due to the presence of a space charge layer. The reaction is accompanied by the breaking of the backbonds, which requires the thermal excitation of the respective surface state electrons into the conduction band. This step is considered to be rate limiting. In a reduction step, the injected electrons react with water molecules to form new hydroxide ions and hydrogen. It is assumed that these hydroxide ions generated at the silicon surface are consumed in the oxidation reaction rather than those from the bulk electrolyte, since the latter are kept away from the crystal by the repellent force of the negative surface charge. According to this model, monosilicic acid is formed as the primary dissolution product in all anisotropic silicon etchants. The anisotropic behavior is due to small differences of the energy levels of the backbond surface states as a function of the crystal orientation.

The application of microstructured reactors in the chemical process industry has gained significant importance in recent years. Companies that offer not only microstructured reactors, but also entire chemical process plants and services relating to them, are already in existence. In addition, many institutes and universities are active within this field, and process-engineering-oriented reviews and a specialized book are available. Microstructured systems can be applied with particular success in the investigation of highly exothermic and fast reactions. Often the presence of temperature-induced side reactions can be significantly reduced through isothermal operations. Although microstructured reaction techniques have been shown to optimize many synthetic procedures, they have not yet received the attention they deserve in organic chemistry. For this reason, this Review aims to address this by providing an overview of the chemistry in microstructured reactors, grouped into liquid-phase, gas-phase, and gas-liquid reactions.

Abstract Driven by the economics of scale, the size of reaction vessels as the major processing apparatus of the chemical industry has became bigger and bigger [1, 2]. Consequently, the efforts for ensuring mixing and heat transfer have also increased, as these are scale dependent. This has brought vessel operation to (partly severe) technical limits, especially when controlling harsh conditions, e.g., due to large heat releases. Accordingly, processing at a very large scale has resulted in taming of the chemistry involved in order to slow it down to a technically controllable level. Therefore, reaction paths that already turned out too aggressive at the laboratory scale are automatically excluded for later scale‐up, which constitutes a common everyday confinement in exploiting chemical transformations. Organic chemists are barely conscious that even the small‐scale laboratory protocols in their textbooks contain many slow, disciplined chemical reactions. Operations such as adding a reactant drop by drop in a large diluted solvent volume have become second nature, but are not intrinsic to the good engineering of chemical reactions. These are intrinsic to the chemical apparatus used in the past. In contrast, today's process intensification [3–12] and the new flow‐chemistry reactors on the micro‐ and milli‐scale [13–39] allow such limitations to be overcome, and thus, enable a complete, ab‐initio type rethinking of the processes themselves. In this way, space‐time yields and the productivity of the reactor can be increased by orders of magnitude and other dramatic performance step changes can be achieved. A hand‐in‐hand design of the reactors and process re‐thinking is required to enable chemistry rather than subduing chemistry around the reactor [40]. This often leads to making use of process conditions far from conventional practice, under harsh environments, a procedure named here as Novel Process Windows.

Recent studies have firmly established that cellular uptake of nanoparticles is strongly affected by the presence and the physicochemical properties of a protein adsorption layer around these nanoparticles. Here, we have modified human serum albumin (HSA), a serum protein often used in model studies of protein adsorption onto nanoparticles, to alter its surface charge distribution and investigated the consequences for protein corona formation around small (radius ∼5 nm), dihydrolipoic acid-coated quantum dots (DHLA-QDs) by using fluorescence correlation spectroscopy. HSA modified by succinic anhydride (HSAsuc) to generate additional carboxyl groups on the protein surface showed a 3-fold decreased binding affinity toward the nanoparticles. A 1000-fold enhanced affinity was observed for HSA modified by ethylenediamine (HSAam) to increase the number of amino functions on the protein surface. Remarkably, HSAsuc formed a much thicker protein adsorption layer (8.1 nm) than native HSA (3.3 nm), indicating that it binds in a distinctly different orientation on the nanoparticle, whereas the HSAam corona (4.6 nm) is only slightly thicker. Notably, protein binding to DHLA-QDs was found to be entirely reversible, independent of the modification. We have also measured the extent and kinetics of internalization of these nanoparticles without and with adsorbed native and modified HSA by HeLa cells. Pronounced variations were observed, indicating that even small physicochemical changes of the protein corona may affect biological responses.

The etching behavior of highly boron doped silicon in aqueous solutions based of ethylenediamine, , , and was studied. For all etchants, a strong reduction of the etch rate for boron concentrations exceeding approximately was observed. This value is in good agreement with published data for the onset of degeneracy of p‐type silicon. The reduction of the etch rate was found to be inversely proportional to the fourth power of the boron concentration. For a given high boron concentration, the etch stop effect was found to be most effective for ethylenediamine‐based solutions and low concentration and least effective for highly concentrated . On the basis of these results, a model is proposed attributing the etch stop phenomenon to electrical effects of holes rather than chemical effects of boron. Due to the high dopant concentration the width of the space charge layer on the silicon surface shrinks drastically. Therefore, electrons injected into the conduction band by an oxidation reaction cannot be confined to the surface and rapidly recombine with holes from the valence band. The lack of these electrons impedes the reduction of water and thereby the formation of new hydroxide ions at the silicon surface. Since the transfer of four electrons is required for the dissolution of one silicon atom the observed fourth power law for the decrease of the etch rate can be explained. The reduction of the etch rate on silicon doped with germanium or phosphorus is much smaller and follows a different mechanism.

The mixing quality of a single mixing unit and mixer arrays having various designs was characterized. A known test reaction for mixing quality had to be optimized, since a much higher sensitivity as for the characterization of macroscopic mixers was needed. This adapted test reaction allowed not only the characterization of the mixing quality but also analysis of the homogeneity of the flow distribution between parallely aligned mixing units. A comparison of the mixing quality to those of macroscopic reference systems (like mixing in stirred and unstirred vessels as well as to laminar and turbulent mixing-tees) is presented. The mixing quality−volume flow dependence revealed a complex behavior, the hydrodynamic origin of which has been analyzed.

Abstract The present status of theoretical and experimental investigations of solitary excitations in one-dimensional magnetic systems is reviewed. A survey of exact solutions to the nonlinear equations of motion for pertinent classical chain systems (sine-Gordon chain and ferromagnetic Heisenberg chains with various anisotropies) is given. Particular emphasis is devoted to the role of solitons in the thermodynamics of such systems. Models corresponding to real quasi-one-dimensional magnets are broadly discussed to demonstrate the properties of their solitary excitations. The experimental significance of such nonlinear excitations in the static and dynamic quantities of such systems is discussed in detail. The models and model substances are the easy-plane ferromagnet (model substances CsNiF3 and (C6H11NH3)CuBr3), the easy-plane antiferromagnet (CH3)4NMnCl3) and the S = ½ Ising chain with transverse interactions (CsCoCl3). The quantum aspects of solitary excitations are treated in some theoretical detail. Finally, open questions and possible future investigations are discussed.

A second generation micro-mixer, being a further optimised version of a first prototype, relying on the consequent utilisation of the split-and-recombine principle is presented. We show that the mixing can be characterized by a positive finite-time Lyapunov exponent although being highly regular and uniform. Using computational fluid dynamics (CFD) we investigate the mixing performance for Reynolds numbers in the range of about 1 to about 100. In particular for low Reynolds numbers (Re < 15) the CFD results predict an almost ideal multi-lamination. Thus, the developed mixer is especially suited for efficient mixing of highly viscous fluids. Furthermore, the numerical results are experimentally validated by investigations of mixing of water-glycerol solutions. The experimental results are found to be in excellent agreement with the numerical data and prove the high mixing efficiency.

Abstract The mixing due to helical flows in curved micro channels is investigated. A new chaotic mixing mechanism is presented relying on alternately switching between different flow patterns exhibiting four Dean vortices. Flow patterns and interfacial stretching factors are numerically computed for various Dean numbers. For experimental studies a prototype of a chaotic mixer with curved channels was fabricated. The experimental evaluation of the mixing performance corroborates the numerical prediction: the mixing performance found for Dean numbers above 140 is qualitatively different from that at lower Dean numbers; the periodic switching between different vortex patterns leads to efficient mixing, manifesting itself in an exponential growth of interfacial area. In addition to the studies on mixing, residence‐time distributions in the mixing channel are computed numerically. These investigations indicate that due to mass‐transfer enhancement originating from the transversal redistribution of matter in the chaotic flow, hydrodynamic dispersion is substantially reduced relative to a straight channel. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2297–2305, 2004

In biological fluids, proteins may associate with nanoparticles (NPs), leading to the formation of a so-called "protein corona" largely defining the biological identity of the particle. Here, we present a novel approach to assess apparent binding affinities for the adsorption/desorption of proteins to silver NPs based on the impact of the corona formation on the agglomeration kinetics of the colloid. Affinities derived from circular dichroism measurements complement these results, simultaneously elucidating structural changes in the adsorbed protein. Employing human serum albumin as a model, apparent affinities in the nanomolar regime resulted from both approaches. Collectively, our findings now allow discrimination between the formation of protein mono- and multilayers on NP surfaces.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSemiconductor Junction Gas SensorsKarin Potje-KamlothView Author Information Institut fuer Mikrotechnik Mainz, Carl-Zeiss-Strasse 18-20, 55129 Mainz, Germany Cite this: Chem. Rev. 2008, 108, 2, 367–399Publication Date (Web):January 30, 2008Publication History Received11 July 2007Published online30 January 2008Published inissue 1 February 2008https://pubs.acs.org/doi/10.1021/cr0681086https://doi.org/10.1021/cr0681086research-articleACS PublicationsCopyright © 2008 American Chemical SocietyRequest reuse permissionsArticle Views5923Altmetric-Citations222LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Diodes,Electrical conductivity,Layers,Metals,Semiconductors Get e-Alerts

Abstract Interdigital‐type glass micromixers with alternating feed channels to periodically create liquid multilamellae, were fabricated for basic investigations of hydrodynamics for liquid mixing of two streams. Three interdigital designs developed, rectangular, triangular, and slit‐type, differed in their flow‐through mixing chamber. These designs are based on simple polygon geometries and their combinations. The flow patterns of an aqueous solution dyed with blue and uncolored water were investigated for different interdigital mixers. Since mixing in a nonfocused device, such as the rectangular mixer, was not completed, focusing techniques were applied. Geometric focusing was used to reduce lamellae width and to speed up mixing. In the special version of the triangular mixer, SuperFocus mixer, liquid mixing times are reduced to about 10 ms, as determined by iron‐rhodanide reaction imaging. In the latter case, the lamellae are compressed by a factor of 40, from a width of 160 to 4 μm. For both the triangular and slit‐type micromixer, flow patterns differed from regular multilamination ones. At high flow rates, lamellae tilted. When the flow cross‐sectional area was expanded, jet and associated eddies formed further improved mixing. Mixing investigations also helped develop imaging techniques. The rhodanide imaging complemented water blue contrasting.

A microfabricated electrophoresis chip with an integrated contactless conductivity detection system is described. The new contactless conductivity microchip detector is based on placing two planar sensing aluminum film electrodes on the outer side of a poly(methyl methacrylate) (PMMA) microchip (without contacting the solution) and measuring the impedance of the solution in the separation channel. The contactless route obviates problems (e.g., fouling, unwanted reactions) associated with the electrode-solution contact, offers isolation of the detection system from high separation fields, does not compromise the separation efficiency, and greatly simplifies the detector fabrication. Relevant experimental variables, such as the frequency and amplitude of the applied ac voltage or the separation voltage, were examined and optimized. The detector performance was illustrated by the separation of potassium, sodium, barium, and lithium cations and the chloride, sulfate, fluoride, acetate, and phosphate anions. The response was linear (over the 20 microM-7 mM range) and reproducible (RSD = 3.4-4.9%; n = 10), with detection limits of 2.8 and 6.4 microM (for potassium and chloride, respectively). The advantages associated with the contactless conductivity detection, along with the low cost of the integrated PMMA chip/detection system, should enhance the power and scope of microfluidic analytical devices.

Abstract Mikrostrukturreaktoren haben in den letzten Jahren in der chemischen Verfahrenstechnik deutlich an Bedeutung gewonnen: Erste Firmen, die Prozessanlagen mit Mikrostrukturreaktoren anbieten, sind am Markt, eine Reihe von Instituten erforscht dieses Gebiet, Universitäten bieten Lehrveranstaltungen an, und einige verfahrenstechnisch orientierte Übersichtsartikel liegen bereits vor. Mikrostrukturierte Systeme sind besonders wertvoll für die Untersuchung stark exothermer schneller Reaktionen. Durch isotherme Betriebsweise lassen sich viele thermisch induzierte Nebenreaktionen zurückdrängen, was sicherheitstechnische Vorteile bei Reaktionen mit gefährlichen Reaktanten bringt. Obwohl viele organische Synthesen mit besseren Ergebnissen als in konventionellen Reaktoren durchgeführt wurden, ist der Mikrostrukturreaktor‐Technologie der große Durchbruch in der präparativen Chemie noch nicht gelungen. Aus diesem Grund unterstreicht dieser Aufsatz die Vorteile von Mikrostrukturreaktoren bei Flüssigphasen‐, Gasphasen‐ und Gas‐Flüssigphasen‐Reaktionen.

Abstract Helical flows are investigated in structured microchannels with regard to micromixing by means of computational fluid dynamics (CFD). In the case of bas‐relief structured channels the numerical results are found to be in good agreement with experimental findings. The magnitude of the transverse flow is computed for various Reynolds numbers and for different geometries including channels with bas‐relief structures on two opposite walls. Transverse flows in structured microchannels are compared to secondary flow patterns in curved square channels. The corresponding helical flows are analyzed for Dean numbers ranging from 1 to 900. Special attention is paid to the occurrence of additional vortices close to the center of the outer channel wall. Based on the results a new type of micromixer is proposed that relies on the transition of the secondary flow pattern from two to four vortices. © 2004 American Institute of Chemical Engineers AIChE J, 50: 771–778, 2004

At present, the aims of the investigations with microchemical processing devices are changing from simply proving feasibility for one chemical reaction towards more in-depth scientific studies and industrial piloting. In this way, large data sets are gathered, providing multifaceted information on the topic. To enable industrial exploitation of the technology, future investigations should aim to complete the economic evaluation of the methodology for plant engineering. Hence, commercially oriented studies have to be undertaken, not with the aim to further broaden the scope of information, but rather to achieve a new system-oriented level of know-how. Since this involves the interaction of many parties with many different skills, it is a bridging function that is needed to bring the vast amount of findings documented to a compact format and to compare it to the state of the art in the chemicals-producing industry. Accordingly, this contribution reviews many chemical reactions carried out in either credit-card-sized microdevices or in larger microflow processing tools for reasons of screening/analysis and organic synthesis/industrial piloting, respectively. Quantities which characterize the process itself, the product on a molecular and supramolecular level, and the downstream processing are compared for both microreactor and conventional processing, benchmarking the performance of microflow devices at minute and large throughput levels.

Direct laser writing and subsequent electroless silver plating is introduced as a high-quality fabrication method for three-dimensional plasmonic nanostructures. With this method, we fabricate the first three-dimensional bichiral plasmonic crystals, which exhibit a large difference in transmittance of left-handed and right-handed circularly polarized light in the mid-infrared spectral region between 3 and 5 μm. Our structure has a high degree of isotropy, showing only a weak dependence of the transmittance on the angle of incidence.

Microflow technology, i.e., the use of microfluidic devices for continuous flow synthesis, represents a highly useful and increasingly popular method in organic chemistry. Recently, also an increasing number of polymer synthesis protocols attain benefit from this technique. In particular, the control of highly exothermic, fast polymerization reactions can be improved due to the excellent heat and mass transfer within the small dimensions of the microreactors. Continuous flow setups with different micromixer geometries and flow patterns are currently used for the preparation of a variety of macromolecular architectures by ionic and (controlled) radical polymerization techniques. This Perspectives reviews recent developments in synthetic strategies and reactor design for the homogeneous synthesis of polymers in microflow systems and emphasizes future challenges and promise for applications. Polymer synthesis by radical, anionic, cationic, and coordinative polymerization is considered as well as different polymer topologies generated (linear, branched, and dendritic architectures).

Abstract Flow patterns and mixing properties of micromixing devices described in Part I are investigated by computational fluid dynamics (CFD) and semianalytical methods. The CFD results provide qualitative information on mixing, but, due to numerical artifacts, no quantitative information can be derived in most cases. The geometrical arrangement of liquid lamellae predicted by the CFD simulations explain transmitted‐light experiments reported in Part I. In contrast, the semianalytical method provides a simple model for the mixing devices presented and allows to assess their mixing properties. For two of the four micromixers, mixing times were in the range of milliseconds, thus lending themselves to fast, mass‐transfer‐limited reactions. Experimental data obtained for a specific micromixer support the semianalytical model. Both the model and experiments suggest that geometric focusing of a large number of liquid streams is a powerful micromixing principle.

A fully disposable microanalytical device based on combination of poly(methylmethacrylate) (PMMA) capillary electrophoresis microchips and thick-film electrochemical detector strips is described. Variables influencing the separation efficiency and amperometric response, including separation voltage or detection potential are assessed and optimized. The versatility, simplicity and low-cost advantages of the new design are coupled to an attractive analytical performance, with good precision (relative standard deviation RSD = 1.68% for n = 10). Applicability for assays of mixtures of hydrazine, phenolic compounds, and catecholamines is demonstrated. Such coupling of low-cost PMMA-based microchips with thick-film electrochemical detectors holds great promise for mass production of single-use micrototal analytical systems.