Multilevel microfluidics via single-exposure photolithography

This communication reports a new method to form multilevel features in a single layer of SU-8 photoresist to facilitate the generation of 3D microfluidic chips. The method utilizes the spatial dependence of diffracted light intensity to selectively overexpose masked regions of photoresist and requires only a UV light source and a single transparency mask. 3D structures are formed within microfluidic channels using this selective overexposure method, with feature sizes being determined by the exposure dose and mask feature sizes. The dimensions of the internal features and the microfluidic channels can be varied independently according to these parameters, and any number of different heights can be obtained in a single exposure step. The method provides a simple means of forming 3D microfluidic structures with integrated features, including mixing structures, flow stabilization ridges, and separation weirs to increase the capabilities of microfluidic chips in a variety of microchemical applications.     

Simple methods for the direct assembly, functionalization, and patterning of acid-terminated monolayers on Si111

This article describes mild methods to directly assemble, functionalize, and pattern monolayers of undecylenic acid on hydrogen-terminated Si(111). These monolayers were assembled under very mild conditions from a neat solution of undecylenic acid containing 0.1 mol % 4-(decanoate)-2,2,6,6-tetramethylpiperidinooxy at room temperature without the need for UV light. Because of these mild conditions, monolayers exposing carboxylic acids could be assembled in one step without the need to protect the acid prior to its assembly. The monolayers were extensively characterized by horizontal attenuated total reflection infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle goniometry. The monolayers bonded to the silicon surface preferentially through the olefin with no detectable bonds between the carboxylic acids and silicon. The crystallinity of the monolayer was studied by infrared spectroscopy through the antisymmetric--v(a)(CH(2))--and symmetric--v(s)(CH(2))--stretches for methylene. Because it is important for future applications to assemble functional surfaces, methods to react the acid-terminated monolayers with trifluoroacetic anhydride and triethylamine to yield a symmetric anhydride on the monolayer were studied. These anhydrides were reacted with a variety of milligram-quantity amines to yield amide-terminated surfaces. This method was general, and a variety of amines could be bonded to the monolayer. The stabilities of these monolayers upon exposure to ambient conditions and under a variety of solvents were described. Because patterned monolayers have found wide applications, we have developed methods to pattern 1-octadecylamine and poly(ethylenimine) on the micrometer scale using soft lithography. In addition, polymer brushes of polynorbornene with thicknesses from 32 to 150 nm were grown from monolayers patterned with the Grubbs' catalyst. The patterned surfaces were imaged by scanning electron microscopy, scanning probe microscopy, and ellipsometry to determine the thicknesses of the patterns and the fidelity of the method.     

Active control of the depletion boundary layers in microfluidic electrochemical reactors

In this paper, we describe three methods to improve the performance of pressure-driven laminar flow-based microreactors by manipulating reaction-depletion boundary layers to overcome mass transfer limitations at reactive surfaces on the walls, such as electrodes. The transport rate of the reactants to the reactive surfaces is enhanced by (i) removing the depleted zone through multiple periodically-placed outlets; (ii) adding fresh reactants through multiple periodically-placed inlets along the reactive surface; or (iii) producing a spiraling, transverse flow through the integration of herringbone ridges along the channel walls. For approaches (i) and (ii), the network of microfluidic channels needs to be designed such that under the operating conditions used the right amount of boundary layer at each outlet or inlet is removed or replenished, respectively. Here, we report a set of design rules, derived with the help of a fluidic resistance circuit model, to aid in the design of appropriate microfluidic networks. Also, the actual enhancement of the performance of the electrochemical microreactor, i.e. chemical conversion efficiency, using multiple inlets, multiple outlets, or herringbone ridges is reported.     

Air-breathing laminar flow-based microfluidic fuel cell

This communication reports the design and characterization of an air-breathing laminar flow-based microfluidic fuel cell (LFFC). The performance of previous LFFC designs was cathode-limited due to the poor solubility and slow transport of oxygen in aqueous media. Introduction of an air-breathing gas diffusion electrode as the cathode addresses these mass transfer issues. With this design change, the cathode is exposed to a higher oxygen concentration, and more importantly, the rate of oxygen replenishment in the depletion boundary layer on the cathode is greatly enhanced as a result of the 4 orders of magnitude higher diffusion coefficient of oxygen in air as opposed to that in aqueous media. The power densities of the present air-breathing LFFCs are 5 times higher (26 mW/cm2) than those for LFFCs operated using formic acid solutions as the fuel stream and an oxygen-saturated aqueous stream at the cathode ( approximately 5 mW/cm2). With the performance-limiting issues at the cathode mitigated, these air-breathing LFFCs can now be further developed to fully exploit their advantages of direct control over fuel crossover and the ability to individually tailor the chemical composition of the cathode and anode media to enhance electrode performance and fuel utilization, thus increasing the potential of laminar flow-based fuel cells.     

Metastable States of small-molecule solutions

Metastable states such as gels and glasses that are commonly seen in nanoparticle suspensions have found application in a wide range of products including toothpaste, hand cream, paints, and car tires. The equilibrium and metastable state behavior of nanoparticle suspensions are often described by simple fluid models where particles are treated as having hard cores and interacting with short-range attractions. Here we explore similar models to describe the presence of metastable states of small-molecule solutions. We have recently shown that the equilibrium solubilities of small hydrogen-bonding molecules and nanoparticles fall onto a corresponding-states solubility curve suggesting that with similar average strengths of attraction these molecules have similar solubilities. This observation implies that metastable states in small-molecule solutions may be found under conditions similar to those where metastable states are observed in nanoparticle and colloidal suspensions. Here we seek confirmation of this concept by exploring the existence of metastable states in solutions of small molecules.     

Methanol dehydrogenation and oxidation on Pt(111) in alkaline solutions

Adsorption, dehydrogenation, and oxidation of methanol on Pt(111) in alkaline solutions has been examined from a fundamental mechanistic perspective, focusing on the role of adsorbate-adsorbate interactions and the effect of defects on reactivity. CO has been confirmed as the main poisoning species, affecting the rate of methanol dehydrogenation primarily through repulsive interactions with methanol dehydrogenation intermediates. At direct methanol fuel cell (DMFC)-relevant potentials, methanol oxidation occurs almost entirely through a CO intermediate, and the rate of CO oxidation is the main limiting factor in methanol oxidation. Small Pt island defects greatly enhance CO oxidation, though they are effective only when the CO coverage is 0.20 ML or higher. Large Pt islands enhance CO oxidation as well, but unlike small Pt islands, they also promote methanol dehydrogenation. Perturbations in electronic structure are responsible for the CO oxidation effect of defects, but the role of large Pt islands in promoting methanol dehydrogenation is primarily explained by surface geometric structure.     

Laminar flow-based electrochemical microreactor for efficient regeneration of nicotinamide cofactors for biocatalysis

One of the long-standing challenges in biocatalysis is the search for methods to continuously regenerate essential cofactors such as NADH that would enable a wide range of enzymes to be used in the more environmentally friendly synthesis of chiral fine chemicals including pharmaceuticals, cosmetics, and food additives. This communication reports a microreactor-based cofactor regeneration method that exploits the microfluidic phenomenon of laminar flow: a reactant stream and a buffer stream are introduced in a microchannel and continue to flow side by side without turbulent mixing between two electrodes that cover opposing channel walls. Adjustment of the flow rate ratio of the two streams in laminar flow enables focusing of the reactant stream close to the cathode, thereby reversing a normally unfavorable reaction equilibrium essential for cofactor regeneration. The absence of a bulk phase in these microreactors prevents the undesired reverse reaction to take place, which has prevented the use of electrochemical cofactor regeneration in macroscale processes. Here, we demonstrate the regeneration of NADH with conversion efficiencies as high as 31%. We also show the subsequent in situ conversion of an achiral substrate, pyruvate, into a chiral product, l-lactate, within this microreactor.     

Cross metathesis on olefin-terminated monolayers on Si(111) using the Grubbs' catalyst

This paper reports the functionalization and patterning of olefin-terminated monolayers on Si(111) through cross metathesis. A simple, one-step synthesis of a diolefin--CH2=CH(CH2)9O(CH2)9CH=CH2--was developed from commercially available starting materials. Mixed partially olefin-terminated monolayers of this novel diolefin and 1-octadecene on hydrogen-terminated Si(111) were obtained. The olefins are raised above the rest of the monolayer and thus sterically accessible for further functionalization. Olefin-terminated monolayers were reacted with the Grubbs' first generation catalyst and olefins in solution that were terminated with fluorines, carboxylic acids, alcohols, aldehydes, and alkyl bromides. Characterization of these monolayers using X-ray photoelectron spectroscopy and horizontal attenuated total reflection infrared spectroscopy demonstrated that olefins on the surface had reacted via cross metathesis to expose fluorines, carboxylic acids, aldehydes, alcohols, and bromides. Through calibration experiments, we demonstrated a simple 1:1 correspondence between the ratio of olefins in solution used in the assembly and the final composition of the mixed monolayers. Finally, these monolayers on silicon were patterned on the micrometer-size scale by soft lithography using microfluidic channels patterned into poly(dimethylsiloxane) (PDMS) stamps. Micrometer-wide lines of polymer brushes were synthesized on these monolayers and characterized by scanning electron microscopy. In addition, olefin-terminated monolayers were patterned into micrometer-sized lines exposing carboxylic acids by cross metathesis with olefins in solution. This method of patterning is broadly applicable and can find applications in a variety of fields including the development of biosensors and nanoelectronics.