Gold nanorods as a perspective technology platform for SERS analytics

We discuss the application of gold nanorods for forming SERS substrates for chemical and biological sensing. Two approaches are considered: (1) formation of planar arrays on silicon wafers by using suspensions of gold nanorods; and (2) a new approach based on gold nanorod powders that can be easily dissolved in aqueous media. Both SERS platforms are characterized and their SERS enhancement factors are compared.     

Parallel single-cell analysis microfluidic platform

We report a PDMS microfluidic platform for parallel single-cell analysis (PaSCAl) as a powerful tool to decipher the heterogeneity found in cell populations. Cells are trapped individually in dedicated pockets, and thereafter, a number of invasive or non-invasive analysis schemes are performed. First, we report single-cell trapping in a fast (2-5 min) and reproducible manner with a single-cell capture yield of 85% using two cell lines (P3x63Ag8 and MCF-7), employing a protocol which is scalable and easily amenable to automation. Following this, a mixed population of P3x63Ag8 and MCF-7 cells is stained in situ using the nucleic acid probe (Hoechst) and a phycoerythrin-labeled monoclonal antibody directed at EpCAM present on the surface of the breast cancer cells MCF-7 and absent on the myeloma cells P3x63Ag8 to illustrate the potential of the device to analyze cell population heterogeneity. Next, cells are porated in situ using chemicals in a reversible (digitonin) or irreversible way (lithium dodecyl sulfate). This is visualized by the transportation of fluorescent dyes through the membrane (propidium iodide and calcein). Finally, an electrical protocol is developed for combined cell permeabilization and electroosmotic flow (EOF)-based extraction of the cell content. It is validated here using calcein-loaded cells and visualized through the progressive recovery of calcein in the side channels, indicating successful retrieval of individual cell content.     

SERS as an analytical tool in environmental science: The detection of sulfamethoxazole in the nanomolar range by applying a microfluidic cartridge setup

Sulfamethoxazole (SMX) is a commonly applied antibiotic for treating urinary tract infections; however, allergic reactions and skin eczema are known side effects that are observed for all sulfonamides. Today, this molecule is present in drinking and surface water sources. The allowed concentration in tap water is 2·10⁻⁷ mol L⁻¹. SMX could unintentionally be ingested by healthy people when drinking contaminated tap water, representing unnecessary drug intake. To assess the quality of tap water, fast, specific and sensitive detection methods are required, in which consequence measures for improving the purification of water might be initiated in the short term. Herein, the quantitative detection of SMX down to environmentally and physiologically relevant concentrations in the nanomolar range by employing surface-enhanced Raman spectroscopy (SERS) and a microfluidic cartridge system is presented. By applying surface-water samples as matrices, the detection of SMX down to 2.2·10⁻⁹ mol L⁻¹ is achieved, which illustrates the great potential of our proposed method in environmental science.     

Multi-channel microfluidic chip-mass spectrometry platform for cell analysis

In this review, we highlight the latest development of multi-channel microfluidic chip-mass spectrometry (chip-MS) in cell analysis and metabolite detection. Following a brief introduction about history and development of multi-channel microchip and MS combination, we will elaborate the key issues of constructing chip-MS platform interface. Then exciting progresses made in this field should be reviewed with well exemplified works, including chip-MS technology for cell introduction, pretreatment of cell secretions and cell metabolite analysis. We will also describe the development of integrated total analysis systems proposed by our group. We hope this brief review will inspire interested readers and provide knowledge about chip-MS platform in the bioanalysis field, particularly in cell analysis and metabolite identifying applications.     

Engineering superlyophobic surfaces as the microfluidic platform for droplet manipulation

We propose robust engineering superlyophobic surfaces (SLS) as a universal microfluidic platform for droplet manipulation enabling electric actuation, featured with characteristics of highly nonwetting, low adhesion, and low friction for various liquids including water and oil. To functionalize SLS with embedded electrodes, two configurations with continuous and discrete topologies have been designed and compared. The discrete configuration is found to be superior upon comparison of their fabrication, microstructures and nonwetting performances. We also present new formulation of SLS pressure stability for linear, square and hexagonal pattern layouts, and propose a criterion for three wetting states (the Cassie-Baxter, partial Cassie-Baxter and Wenzel states) by introducing two dimensionless parameters, which are supported by our experimental data. Droplet manipulation experiments including deformation and transport on electrode-embedded SLS were performed, showing that present SLS reduce adhesion and flow resistance of oil droplets respectively by 98% and 73% compared with a smooth hydrophobic surface, and the excellent hydrodynamic performances are applicable for a wide range of droplet velocity. Simulation of an oil droplet electrically actuated on SLS predicts the significantly increased droplet motion for a low solid fraction and a relatively large droplet size.     

Filter-based microfluidic device as a platform for immunofluorescent assay of microbial cells

A filter-based microfluidic device was combined with immunofluorescent labeling as a platform to rapidly detect microbial cells. The coin-sized device consisted of micro-chambers, micro-channels and filter weirs (gap = 1-2 microm), and was demonstrated to effectively trap and concentrate microbial cells (i.e., Cryptosporidium parvum and Giardia lamblia), which were larger in size than the weir gap. After sample injection, a staining solution containing fluorescently-labeled antibodies was continuously provided into the device (flow rate = 20 microl min(-1)) to flush the microbial cells toward the weirs and to accelerate the fluorescent labeling reaction. Using a staining solution that was 10 to 100 times more dilute than the recommended concentration used in a conventional glass method, those target cells with a fluorescent signal-to-noise ratio of 12 could be microscopically observed at single-cell level within 2 to 5 min prior to secondary washing.     

Topological constraints in a microfluidic platform

Microfluidics has evolved as a major technological platform for biotechnology, material science and related fields. In virtually all of the areas of application, the flowing matrix is an isotropic fluid. However, replacing the typically isotropic fluid with an anisotropic liquid crystal opens up avenues beyond the viscous-dominated isotropic microfluidics. Especially, the material anisotropy of the flowing LC matrix and the consequent incorporation of topological constraints within the microfluidic device offer smart capabilities ranging from tunable flow-shaping to flexible micro-cargo concepts. The key to such capabilities lies in exploiting the possible topological constraints offered by the microfluidic confinement. As an example, we shall demonstrate how long-range ordering and consequent anisotropy in liquid crystals (LCs) could be utilised to devise a novel route to guided transport of microscopic cargo on ‘soft rails’, i.e. topological defect lines (disclinations). We create, position and navigate disclination lines within the LC matrix by tuning the coupling between flow and LC orientation. As model cargo elements, we have used isolated or self-assembled chains of colloidal particles, and demonstrated the broader capability of this method by transporting aqueous droplets on the defect lines. Topological constraints in combination with flow-director coupling thus endow LC microfluidics with features distinct from its isotropic counterparts.     

Silver nanocubes monolayers as a SERS substrate for quantitative analysis

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopic tool in quantitative analysis of molecules, where the substrate plays a critical role in de...     

A digital microfluidic platform for primary cell culture and analysis

Digital microfluidics (DMF) is a technology that facilitates electrostatic manipulation of discrete nano- and micro-litre droplets across an array of electrodes, which provides the advantages of single sample addressability, automation, and parallelization. There has been considerable interest in recent years in using DMF for cell culture and analysis, but previous studies have used immortalized cell lines. We report here the first digital microfluidic method for primary cell culture and analysis. A new mode of "upside-down" cell culture was implemented by patterning the top plate of a device using a fluorocarbon liftoff technique. This method was useful for culturing three different primary cell types for up to one week, as well as implementing a fixation, permeabilization, and staining procedure for F-actin and nuclei. A multistep assay for monocyte adhesion to endothelial cells (ECs) was performed to evaluate functionality in DMF-cultured primary cells and to demonstrate co-culture using a DMF platform. Monocytes were observed to adhere in significantly greater numbers to ECs exposed to tumor necrosis factor (TNF)-α than those that were not, confirming that ECs cultured in this format maintain in vivo-like properties. The ability to manipulate, maintain, and assay primary cells demonstrates a useful application for DMF in studies involving precious samples of cells from small animals or human patients.     

SERS as a bioassay platform: fundamentals, design, and applications

Bioanalytical science is experiencing a period of unprecedented growth. Drivers behind this growth include the need to detect markers central to human and veterinary diagnostics at ever-lower levels and greater speeds. A set of parallel arguments applies to pathogens with respect to bioterrorism prevention and food and water safety. This tutorial review outlines our recent explorations on the use of surface enhanced Raman scattering (SERS) for detection of proteins, viruses, and microorganisms in heterogeneous immunoassays. It will detail the design and fabrication of the assay platform, including the capture substrate and nanoparticle-based labels. The latter, which is the cornerstone of our strategy, relies on the construction of gold nanoparticles modified with both an intrinsically strong Raman scatterer and an antibody. This labelling motif, referred to as extrinsic Raman labels (ERLs), takes advantage of the well-established signal enhancement of scatterers when coated on nanometre-sized gold particles, whereas the antibody imparts antigenic specificity. We will also examine the role of plasmon coupling between the ERLs and capture substrate, and challenges related to particle stability, nonspecific adsorption, and assay speed.     

Development of a microfluidic platform for single-cell secretion analysis using a direct photoactive cell-attaching method

A precise understanding of individual cellular processes is essential to meet the expectations of most advanced cell biology. Therefore single-cell analysis is considered to be one of possible approach to overcome any misleading of cell characteristics by averaging large groups of cells in bulk conditions. In the present work, we modified a newly designed microchip for single-cell analysis and regulated the cell-adhesive area inside a cell-chamber of the microfluidic system. By using surface-modification techniques involving a silanization compound, a photo-labile linker and the 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer were covalently bonded on the surface of a microchannel. The MPC polymer was utilized as a non-biofouling compound for inhibiting non-specific binding of the biological samples inside the microchannel, and was selectively removed by a photochemical reaction that controlled the cell attachment. To achieve the desired single-macrophage patterning and culture in the cell-chamber of the microchannel, the cell density and flow rate of the culture medium were optimized. We found that a cell density of 2.0 × 10(6) cells/ml was the appropriate condition to introduce a single cell in each cell chamber. Furthermore, the macrophage was cultured in a small size of the cell chamber in a safe way for 5 h at a flow rate of 0.2 μl/min under the medium condition. This strategy can be a powerful tool for broadening new possibilities in studies of individual cellular processes in a dynamic microfluidic device.     

Mixed thread/paper‐based microfluidic chips as a platform for glucose assays

A novel microfluidic thread/paper‐based analytical device (μTPAD) to detect glucose through a colorimetric assay is described. The μTPAD was fabricated from nylon thread trifurcated into three channels terminating at analysis sites comprised of circular zones of chromatography paper, which have previously been spotted with glucose of different concentrations. A solution of glucose oxidase (GOx), horseradish peroxidase (HRP), and potassium iodide (KI) is transported via capillary action to the analysis sites where a yellow‐brown color is observed indicating oxidation of iodide to iodine. The device was then dried, scanned, and analyzed yielding a correlation between yellow intensity and glucose concentrations. Both a flat platform constructed mainly of tape, and a cone platform constructed from tape and polyvinyl chloride, are described. Studies to quantitate glucose in artificial urine showed good correlation using the μTPAD.     

Differentiation-on-a-chip: a microfluidic platform for long-term cell culture studies

Here we demonstrate a microfluidic perfusion system suitable for a long-term (>2 week) culture of muscle cells spanning the whole process of differentiation from myoblasts to myotubes. Cell-adhesive surface microdomains alternating with a robust cell-repellent coating mimic in vivo spatial cues for muscle cell assembly and allow for confining the fusion of myoblasts into aligned, isolated multinucleated myotubes. The microfluidic system provides accurate control of the perfusion rates and biochemical composition of the environment surrounding the cells. Comparing muscle cell-specific differentiation markers and the timing of fusion, we observed no differences in differentiation between microfluidic and traditional cultures. All differentiation assays were fully microfluidic, i.e. they were performed by sequentially changing the fluids in the micro-channels. By delivering fluorescent markers using heterogeneous laminar flows, it was possible to confine a membrane receptor labeling assay to a region smaller than a myotube. Our method can serve as an improved in vitro model for studying muscle cell differentiation and for characterizing extracellular molecules and mechanisms involved in neuromuscular differentiation.     

Education: a microfluidic platform for university-level analytical chemistry laboratories

We demonstrate continuous flow acid-base titration reactions as an educational microfluidic platform for undergraduate and graduate analytical chemistry courses. A series of equations were developed for controlling and predicting the results of acid-base neutralisation reactions conducted in a microfluidic format, including the combinations of (i) a strong base and a strong acid, (ii) a strong base and a weak acid, and (iii) a strong base and a multiprotic acid. Microfluidic titrations yielded excellent repeatability. The small experimental footprint is advantageous in crowded teaching laboratories, and it offers limited waste and exposure to potentially hazardous acids and bases. This platform will help promote the utilisation of microfluidics at an earlier stage of students' careers.     

A microfluidic platform for pharmaceutical salt screening

We describe a microfluidic platform comprised of 48 wells to screen for pharmaceutical salts. Solutions of pharmaceutical parent compounds (PCs) and salt formers (SFs) are mixed on-chip in a combinatorial fashion in arrays of 87.5-nanolitre wells, which constitutes a drastic reduction of the volume of PC solution needed per condition screened compared to typical high throughput pharmaceutical screening approaches. Nucleation and growth of salt crystals is induced by diffusive and/or convective mixing of solutions containing, respectively, PCs and SFs in a variety of solvents. To enable long term experiments, solvent loss was minimized by reducing the thickness of the absorptive polymeric material, polydimethylsiloxane (PDMS), and by using solvent impermeable top and bottom layers. Additionally, well isolation was enhanced via the incorporation of pneumatic valves that are closed at rest. Brightfield and polarized light microscopy and Raman spectroscopy were used for on-chip analysis and crystal identification. Using a gold-coated glass substrate and minimizing the thickness of the PDMS control layer drastically improved the signal-to-noise ratio for Raman spectra. Two drugs, naproxen (acid) and ephedrine (base), were used for validation of the platform's ability to screen for salts. Each PC was mixed combinatorially with potential SFs in a variety of solvents. Crystals were visualized using brightfield polarized light microscopy. Subsequent on-chip analyses of the crystals with Raman spectroscopy identified four different naproxen salts and five different ephedrine salts.     

Fuel cell-powered microfluidic platform for lab-on-a-chip applications

The achievement of a higher degree of integration of components--especially micropumps and power sources--is a challenge currently being pursued to obtain portable and totally autonomous microfluidic devices. This paper presents the integration of a micro direct methanol fuel cell (μDMFC) in a microfluidic platform as a smart solution to provide both electrical and pumping power to a Lab-on-a-Chip system. In this system the electric power produced by the fuel cell is available to enable most of the functionalites required by the microfluidic chip, while the generated CO(2) from the electrochemical reaction produces a pressure capable of pumping a liquid volume through a microchannel. The control of the fuel cell operating conditions allows regulation of the flow rate of a liquid sample through a microfluidic network. The relation between sample flow rate and the current generated by the fuel cell is practically linear, achieving values in the range of 4-18 μL min(-1) while having an available power between 1-4 mW. This permits adjusting the desired flow rate for a given application by controlling the fuel cell output conditions and foresees a fully autonomous analytical Lab-on-a-Chip in which the same device would provide the electrical power to a detection module and at the same time use the CO(2) pumping action to flow the required analytes through a particular microfluidic design.     

Thread‐based microfluidic chips as a platform to assess acetylcholinesterase activity

In this paper, a microfluidic thread‐based analytical device (μTAD) to assess the activity of acetylcholinesterase (AChE) via colorimetric analylsis is described. Fabrication of the device consists of two platforms, both with a nylon thread trifurcated into three channels terminating at open analysis sites at the end of the thread. 5,5’‐Dithiobis‐(2‐nitrobenzoic acid) (DTNB) was spotted and dried on the analysis sites. Acetylthiocholine iodide (ATC) (or cysteine, Cys) is transported through an inlet channel of the nylon thread by capillary action due to the hydrophilic nature of nylon. AChE is transported through the other inlet channel and mixes with the ATC (or Cys) as they travel up to the analysis sites. As the solution reaches the analysis sites, an intense yellow color change occurs indicating the reaction of the thiol with DTNB to produce the yellow anion TNB²⁻. The sites are then dried, scanned, yielding a linear range of inverse yellow mean intensity versus substrate concentration. An IC₅₀ value (1.74 nM) with a known inhibitor, neostigmine bromide (NB), is obtained on the device. The multiplex design enables triplicate data collection in a device that is easy to use. μTADs have great potential to be employed in a myriad of tests including point‐of‐care diagnostic devices for resource‐challenged settings.     

Polarography as a tool in organic analysis

Summary A polarographic micromethod has been developed for the analysis of mono-, di-, tri- and polynitro compounds, standard titanium(III) sulphate solution being used as a reducing agent. The cathodic part of the Kalousek cell has been modified to serve as a reaction vessel. The results agree favourably with those obtained by visual back-titration of excess of titanium(III) with iron(III), thiocyanate being used as indicator, and the relative error is ± 0.5%. This polarographic finish proved to be of general applicability particularly with nitro compounds that give rise to coloured reduction products.

---

Zusammenfassung Eine polarographische Mikromethode zur Analyse von Mono-, Di-, Tri- und Polynitroverbindungen mit Titan(III)-sulfatlösung als Reduktionsmittel wurde ausgearbeitet. Der Kathodenteil der Kalousek-Zelle wurde als Reaktionsgefäß umgestaltet. Die Analysenresultate stimmen sehr gut mit jenen überein, die man durch visuelle Rücktitration des überschüssigen Titan(III) mit Eisen(III) gegen Thiocyanat als Indikator erhält. Der relative Fehler beträgt ± 0,5%. Dieses polarographische Verfahren dürfte allgemein für Nitroverbindungen anwendbar sein, aus denen sich gefärbte Reduktions-produkte bilden können.