Publications by authors named "Niels R Tas"

20 Publications

  • Page 1 of 1

Evaporation-driven colloidal cluster assembly using droplets on superhydrophobic fractal-like structures.

Soft Matter 2021 Jan 24;17(3):506-515. Epub 2020 Nov 24.

Physics of Fluids Group, Faculty of Science and Technology, Mesa+ Institute, University of Twente, 7500 AE Enschede, The Netherlands.

Microparticles can be considered building units for functional systems, but their assembly into larger structures typically involves complex methods. In this work, we show that a large variety of macro-agglomerate clusters ("supra-particles") can be obtained, by systematically varying the initial particle concentration in an evaporating droplet, spanning more than 3 decades. The key is the use of robust superhydrophobic substrates: in this study we make use of a recently discovered kind of patterned surface with fractal-like microstructures which dramatically reduce the contact of the droplet with the solid substrate. Our results show a clear transition from quasi-2D to 3D clusters as a function of the initial particle concentration, and a clear transition from unstable to stable 3D spheroids as a function of the evaporation rate. The origin of such shape transitions can respectively be found in the dynamic wetting of the fractal-like structure, but also in the enhanced mechanical stability of the particle agglomerate as its particle packing fraction increases.
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http://dx.doi.org/10.1039/d0sm01346cDOI Listing
January 2021

Attomolar SERS detection of organophosphorous pesticides using silver mirror-like micro-pyramids as active substrate.

Mikrochim Acta 2020 03 26;187(4):247. Epub 2020 Mar 26.

Departamento de Ingeniería Química y Tecnologías del Medio Ambiente, Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, 50009, Zaragoza, Spain.

Surface-enhanced Raman spectroscopy (SERS) is gaining importance as an ultrasensitive analytical tool for routine high-throughput analysis of a variety of molecular compounds. One of the main challenges is the development of robust, reproducible and cost-effective SERS substrates. In this work, we study the SERS activity of 3D silver mirror-like micro-pyramid structures extended in the z-direction up to 3.7 μm (G0 type substrate) or 7.7 μm (G1 type substrate), prepared by Si-based microfabrication technologies, for trace detection of organophosphorous pesticides, using paraoxon-methyl as probe molecule. The average relative standard deviation (RSD) for the SERS intensity of the peak displayed at 1338 cm recorded over a centimetre scale area of the substrate is below 13% for pesticide concentrations in the range 10 to 10 mol L. This data underlies the spatial uniformity of the SERS response provided by the microfabrication approach. According to finite-difference time-domain (FDTD) simulations, such remarkable feature is mainly due to the contribution on electromagnetic field enhancement of edge plasmon polaritons (EPPs), propagating along the pyramid edges where the pesticide molecules are preferentially adsorbed. Graphical abstract.
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http://dx.doi.org/10.1007/s00604-020-4216-9DOI Listing
March 2020

Wafer-scale 3D shaping of high aspect ratio structures by multistep plasma etching and corner lithography.

Microsyst Nanoeng 2020 23;6:25. Epub 2020 Mar 23.

Mesoscale Chemical System Group, MESA+ Institute, University of Twente, 7522 NB Enschede, The Netherlands.

The current progress of system miniaturization relies extensively on the development of 3D machining techniques to increase the areal structure density. In this work, a wafer-scale out-of-plane 3D silicon (Si) shaping technology is reported, which combines a multistep plasma etching process with corner lithography. The multistep plasma etching procedure results in high aspect ratio structures with stacked semicircles etched deep into the sidewall and thereby introduces corners with a proper geometry for the subsequent corner lithography. Due to the geometrical contrast between the gaps and sidewall, residues are left only inside the gaps and form an inversion mask inside the semicircles. Using this mask, octahedra and donuts can be etched in a repeated manner into Si over the full wafer area, which demonstrates the potential of this technology for constructing high-density 3D structures with good dimensional control in the bulk of Si wafers.
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http://dx.doi.org/10.1038/s41378-020-0134-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8433478PMC
March 2020

Wafer-scale fabrication of high-quality tunable gold nanogap arrays for surface-enhanced Raman scattering.

Nanoscale 2019 Jul 13;11(25):12152-12160. Epub 2019 Jun 13.

BIOS Lab-on-a-Chip Group, MESA+ Institute, Max Planck Center for Complex Fluid Dynamics, University of Twente, 7522 NB Enschede, The Netherlands.

We report a robust and high-yield fabrication method for wafer-scale patterning of high-quality arrays of dense gold nanogaps, combining displacement Talbot lithography based shrink-etching with dry etching, wet etching, and thin film deposition techniques. By using the self-sharpening of <111>-oriented silicon crystal planes during the wet etching process, silicon structures with extremely smooth nanogaps are obtained. Subsequent conformal deposition of a silicon nitride layer and a gold layer results in dense arrays of narrow gold nanogaps. Using this method, we successfully fabricate high-quality Au nanogaps down to 10 nm over full wafer areas. Moreover, the gap spacing can be tuned by changing the thickness of deposited Au layers. Since the roughness of the template is minimized by the crystallographic etching of silicon, the roughness of the gold nanogaps depends almost exclusively on the roughness of the sputtered gold layers. Additionally, our fabricated Au nanogaps show a significant enhancement of surface-enhanced Raman scattering (SERS) signals of benzenethiol molecules chemisorbed on the structure surface, at an average enhancement factor up to 1.5 × 10.
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http://dx.doi.org/10.1039/c9nr02215eDOI Listing
July 2019

Large-scale fabrication of highly ordered sub-20 nm noble metal nanoparticles on silica substrates without metallic adhesion layers.

Microsyst Nanoeng 2018 23;4. Epub 2018 Apr 23.

1BIOS Lab-on-a-Chip Group, MESA+ Institute for Nanotechnology, MIRA Institute for Biomedical Technology and Technical Medicine, Max Planck Center for Complex Fluid Dynamics, University of Twente, Enschede, 7522 NB The Netherlands.

Periodic noble metal nanoparticles offer a wide spectrum of applications including chemical and biological sensors, optical devices, and model catalysts due to their extraordinary properties. For sensing purposes and catalytic studies, substrates made of glass or fused-silica are normally required as supports, without the use of metallic adhesion layers. However, precise patterning of such uniform arrays of silica-supported noble metal nanoparticles, especially at sub-100 nm in diameter, is challenging without adhesion layers. In this paper, we report a robust method to large-scale fabricate highly ordered sub-20 nm noble metal nanoparticles, i.e., gold and platinum, supported on silica substrates without adhesion layers, combining displacement Talbot lithography (DTL) with dry-etching techniques. Periodic photoresist nanocolumns at diameters of ~110 nm are patterned on metal-coated oxidized silicon wafers using DTL, and subsequently transferred at a 1:1 ratio into anti-reflection layer coating (BARC) nanocolumns with the formation of nano-sharp tips, using nitrogen plasma etching. These BARC nanocolumns are then used as a mask for etching the deposited metal layer using inclined argon ion-beam etching. We find that increasing the etching time results in cone-shaped silica features with metal nanoparticles on the tips at diameters ranging from 100 nm to sub-30 nm, over large areas of 3×3 cm. Moreover, subsequent annealing these sub-30 nm metal nanoparticle arrays at high-temperature results in sub-20 nm metal nanoparticle arrays with ~10 uniform particles.
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http://dx.doi.org/10.1038/s41378-017-0001-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6161447PMC
April 2018

3D Fractals as SERS Active Platforms: Preparation and Evaluation for Gas Phase Detection of G-Nerve Agents.

Micromachines (Basel) 2018 Jan 31;9(2). Epub 2018 Jan 31.

Nanoscience Institute of Aragon, Department of Chemical & Environmental Engineering, University of Zaragoza, Edif I+D+i, Campus Río Ebro, C/Mariano Esquillor, s/n, 50018 Zaragoza, Spain.

One of the main limitations of the technique surface-enhanced Raman scattering (SERS) for chemical detection relies on the homogeneity, reproducibility and reusability of the substrates. In this work, SERS active platforms based on 3D-fractal microstructures is developed by combining corner lithography and anisotropic wet etching of silicon, to extend the SERS-active area into 3D, with electrostatically driven [email protected] nanoparticles (NPs) assembly, to ensure homogeneous coating of SERS active NPs over the entire microstructured platforms. Strong SERS intensities are achieved using 3D-fractal structures compared to 2D-planar structures; leading to SERS enhancement factors for R6G superior than those merely predicted by the enlarged area effect. The SERS performance of Au monolayer-over-mirror configuration is demonstrated for the label-free real-time gas phase detection of 1.2 ppmV of dimethyl methylphosphonate (DMMP), a common surrogate of G-nerve agents. Thanks to the hot spot accumulation on the corners and tips of the 3D-fractal microstructures, the main vibrational modes of DMMP are clearly identified underlying the spectral selectivity of the SERS technique. The Raman acquisition conditions for SERS detection in gas phase have to be carefully chosen to avoid photo-thermal effects on the irradiated area.
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http://dx.doi.org/10.3390/mi9020060DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6187359PMC
January 2018

Three-Dimensional Fractal Geometry for Gas Permeation in Microchannels.

Micromachines (Basel) 2018 Jan 27;9(2). Epub 2018 Jan 27.

Mesoscale Chemical Systems, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

The novel concept of a microfluidic chip with an integrated three-dimensional fractal geometry with nanopores, acting as a gas transport membrane, is presented. The method of engineering the 3D fractal structure is based on a combination of anisotropic etching of silicon and corner lithography. The permeation of oxygen and carbon dioxide through the fractal membrane is measured and validated theoretically. The results show high permeation flux due to low resistance to mass transfer because of the hierarchical branched structure of the fractals, and the high number of the apertures. This approach offers an advantage of high surface to volume ratio and pores in the range of nanometers. The obtained results show that the gas permeation through the nanonozzles in the form of fractal geometry is remarkably enhanced in comparison to the commonly-used polydimethylsiloxane (PDMS) dense membrane. The developed chip is envisioned as an interesting alternative for gas-liquid contactors that require harsh conditions, such as microreactors or microdevices, for energy applications.
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http://dx.doi.org/10.3390/mi9020045DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6187368PMC
January 2018

Scalable 3D Nanoparticle Trap for Electron Microscopy Analysis.

Small 2018 11 15;14(48):e1803283. Epub 2018 Oct 15.

Mesoscale Chemical Systems, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500, AE, Enschede, The Netherlands.

Arrays of nanoscale pyramidal cages embedded in a silicon nitride membrane are fabricated with an order of magnitude miniaturization in the size of the cages compared to previous work. This becomes possible by combining the previously published wafer-scale corner lithography process with displacement Talbot lithography, including an additional resist etching step that allows the creation of masking dots with a size down to 50 nm, using a conventional 365 nm UV source. The resulting pyramidal cages have different entrance and exit openings, which allows trapping of nanoparticles within a predefined size range. The cages are arranged in a well-defined array, which guarantees traceability of individual particles during post-trapping analysis. Gold nanoparticles with a size of 25, 150, and 200 nm are used to demonstrate the trapping capability of the fabricated devices. The traceability of individual particles is demonstrated by transferring the transmission electron microscopy (TEM) transparent devices between scanning electron microscopy and TEM instruments and relocating a desired collection of particles.
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http://dx.doi.org/10.1002/smll.201803283DOI Listing
November 2018

Let's twist again: elasto-capillary assembly of parallel ribbons.

Soft Matter 2016 Sep 8;12(34):7186-94. Epub 2016 Aug 8.

MESA + Institute for Nanotechnology, University of Twente, The Netherlands.

We show the self-assembly through twisting and bending of side by side ribbons under the action of capillary forces. Micro-ribbons made of silicon nitride are batch assembled at the wafer scale. We study their assembly as a function of their dimensions and separating distance. Model experiments are carried out at the macroscopic scale where the tension in ribbons can easily be tuned. The process is modeled considering the competition between capillary, elastic and tension forces. Theory shows a good agreement for macroscale assemblies, while the accuracy is within 30% at the micrometer scale. This simple self-assembly technique yields highly symmetric and controllable structures which could be used for batch fabrication of functional 3D micro-structures.
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http://dx.doi.org/10.1039/c6sm00910gDOI Listing
September 2016

Elasto-Capillary Folding Using Stop-Programmable Hinges Fabricated by 3D Micro-Machining.

PLoS One 2015 19;10(5):e0125891. Epub 2015 May 19.

MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands; KIST Europe, Saarbrücken, Germany.

We show elasto-capillary folding of silicon nitride objects with accurate folding angles between flaps of (70.6 ± 0.1)° and demonstrate the feasibility of such accurate micro-assembly with a final folding angle of 90°. The folding angle is defined by stop-programmable hinges that are fabricated starting from silicon molds employing accurate three-dimensional corner lithography. This nano-patterning method exploits the conformal deposition and the subsequent timed isotropic etching of a thin film in a 3D shaped silicon template. The technique leaves a residue of the thin film in sharp concave corners which can be used as an inversion mask in subsequent steps. Hinges designed to stop the folding at 70.6° were fabricated batchwise by machining the V-grooves obtained by KOH etching in (110) silicon wafers; 90° stop-programmable hinges were obtained starting from silicon molds obtained by dry etching on (100) wafers. The presented technique has potential to achieve any folding angle and opens a new route towards creating structures with increased complexity, which will ultimately lead to a novel method for device fabrication.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125891PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4437908PMC
April 2016

3D nanofabrication of fluidic components by corner lithography.

Small 2012 Dec 21;8(24):3823-31. Epub 2012 Aug 21.

Transducers Science and Technology Group, MESA+ Institute for Nanotechnology, University of Twente, P. O. Box 217, 7500 AE Enschede, The Netherlands.

A reproducible wafer-scale method to obtain 3D nanostructures is investigated. This method, called corner lithography, explores the conformal deposition and the subsequent timed isotropic etching of a thin film in a 3D shaped silicon template. The technique leaves a residue of the thin film in sharp concave corners which can be used as structural material or as an inversion mask in subsequent steps. The potential of corner lithography is studied by fabrication of functional 3D microfluidic components, in particular i) novel tips containing nano-apertures at or near the apex for AFM-based liquid deposition devices, and ii) a novel particle or cell trapping device using an array of nanowire frames. The use of these arrays of nanowire cages for capturing single primary bovine chondrocytes by a droplet seeding method is successfully demonstrated, and changes in phenotype are observed over time, while retaining them in a well-defined pattern and 3D microenvironment in a flat array.
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http://dx.doi.org/10.1002/smll.201201446DOI Listing
December 2012

Limits of miniaturization: assessing ITP performance in sub-micron and nanochannels.

Lab Chip 2012 Aug 12;12(16):2888-93. Epub 2012 Jun 12.

Department of Analytical Biosciences, Leiden/Amsterdam Centre for Drug Research (LACDR), Leiden University, P.O. Box 9502, 2300 RA, Leiden, The Netherlands.

The feasibility of isotachophoresis in channels of sub micrometer and nanometer dimension is investigated. A sample injection volume of 0.4 pL is focused and separated in a 330 nm deep channel. The sample consists of a biomatrix containing the fluorescently-labeled amino acids glutamate and phenylalanine, 20 attomoles of each. Isotachophoretic focusing is successfully demonstrated in a 50 nm deep channel. Separation of the two amino acids in the 50 nm deep channel however, could not be performed as the maximum applicable voltage was insufficient. This limit is imposed by bubble formation that we contribute to cavitation as a result of the mismatch in electro-osmotic flow, so called electrocavitation. This represents an unexpected limit on the miniaturization of ITP. Nonetheless, we report the smallest isotachophoretic separation and focusing experiment to date, both in terms of controlled sample injection volume and channel height.
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http://dx.doi.org/10.1039/c2lc21011hDOI Listing
August 2012

Capillarity at the nanoscale.

Chem Soc Rev 2010 Mar 2;39(3):1096-114. Epub 2010 Feb 2.

Transducers Science and Technology Group, MESA+ Institute for Nanotechnology and IMPACT Institute of Mechanics, Processes and Control, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

In this critical review we treat the phenomenon of capillarity in nanoscopic confinement, based on application of the Young-Laplace equation. In classical capillarity the curvature of the meniscus is determined by the confining geometry and the macroscopic contact angle. We show that in narrow confinement the influence of the disjoining pressure and the related wetting films have to be considered as they may significantly change the meniscus curvature. Nanochannel based static and dynamic capillarity experiments are reviewed. A typical effect of nanoscale confinement is the appearance of capillarity induced negative pressure. Special attention is paid to elasto-capillarity and electro-capillarity. The presence of electric fields leads to an extra stress term to be added in the Young-Laplace equation. A typical example is the formation of the Taylor cone, essential in the theory of electrospray. Measurements of the filling kinetics of nanochannels with water and aqueous salt solutions are discussed. These experiments can be used to characterize viscosity and apparent viscosity effects of water in nanoscopic confinement. In the final section we show four examples of appearances of capillarity in engineering and in nature (112 references).
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http://dx.doi.org/10.1039/b909101gDOI Listing
March 2010

Capillary negative pressure measured by nanochannel collapse.

Langmuir 2010 Feb;26(3):1473-6

Transducers Science and Technology Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

A new method is presented to measure capillarity-induced negative pressure. Negative pressures of several bars have been measured for five different liquids (ethanol, acetone, cyclohexane, aniline, and water) over a range of surface tension. Capillary negative pressure was measured in 79 +/- 3 nm silica nanochannels on the basis of the determination of the critical channel width for elastocapillary collapse of the flexible plate covering the channels. The results are consistent with the Young-Laplace equation.
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http://dx.doi.org/10.1021/la903649nDOI Listing
February 2010

Solution titration by wall deprotonation during capillary filling of silicon oxide nanochannels.

Anal Chem 2008 Nov 1;80(21):8095-101. Epub 2008 Oct 1.

Department of Analytical Biosciences, University of Leiden, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

This paper describes a fundamental challenge when using silicon oxide nanochannels for analytical systems, namely the occurrence of a strong proton release or proton uptake from the walls in any transient situation such as channel filling. Experimentally, when fluorescein solutions were introduced into silicon oxide nanochannels through capillary pressure, a distinct bisection of the fluorescence was observed, the zone of the fluid near the entrance fluoresced, while the zone near the meniscus, was dark. The ratio between the zones was found to be constant in time and to depend on ionic strength, pH, and the presence of a buffer and its characteristics. Theoretically, using the Gouy-Chapman-Stern model of the electrochemical double layer, we demonstrate that this phenomenon can be effectively modeled as a titration of the solution by protons released from silanol groups on the walls, as a function of the pH and ionic strength of the introduced solution. The results demonstrate the dominant influence of the surface on the fluid composition in nanofluidic experiments, in transient situations such as filling, and changes in solvent properties such as the pH or ionic strength. The implications of these fundamental properties of silicon oxide nanochannels are important for analytical strategies and in particular the analysis of complex biological samples.
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http://dx.doi.org/10.1021/ac800603mDOI Listing
November 2008

Micromachined Fabry-Pérot interferometer with embedded nanochannels for nanoscale fluid dynamics.

Nano Lett 2007 Feb;7(2):345-50

MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

We describe a microfabricated Fabry-Pérot interferometer with nanochannels of various heights between 6 and 20 nm embedded in its cavity. By multiple beam interferometry, the device enables the study of liquid behavior in the nanochannels without using fluorescent substances. During filling studies of ethanol and water, an intriguing filling mode for partially wetting water was observed, tentatively attributed to the entrapment of a large amount of gas inside the channels.
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http://dx.doi.org/10.1021/nl062447xDOI Listing
February 2007

A MALDI-chip integrated system with a monitoring window.

Lab Chip 2005 Apr 16;5(4):378-81. Epub 2005 Feb 16.

Laboratory of Supramolecular Chemistry and Technology, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

The integration of a monitoring port along the microfluidic path of a MALDI-chip integrated device is described. Optimization of the microreactor design allows longer reaction and measuring times. The Schiff base reaction between 4-tert-butylaniline (1) and 4-tert-butylbenzaldehyde (2) in ethanol was carried out on-chip in the MALDI ionization chamber and the formed imine 3 was detected in real time, demonstrating the feasibility of the "monitoring window" approach. This preliminary result opens the way to on-chip kinetic studies by MALDI-MS, by opening multiple monitoring windows along the microchannel.
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http://dx.doi.org/10.1039/b418986hDOI Listing
April 2005

Electrokinetic pumping and detection of low-volume flows in nanochannels.

Electrophoresis 2004 Nov;25(21-22):3687-93

BIOS, MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands.

Electrokinetic pumping of low-volume rates was performed on-chip in channels of small cross sectional area and height in the sub-microm range. The flow was detected with the current monitoring technique by monitoring the change in resistance of the fluid in the channel upon the electroosmosis-driven displacement of an electrolyte solution by a second electrolyte solution. Flow rates in the order of 0.1 pL/s were successfully generated and detected. The channels were fabricated with the sacrificial layer technology.
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http://dx.doi.org/10.1002/elps.200406083DOI Listing
November 2004

1-D nanochannels fabricated in polyimide.

Lab Chip 2004 Jun 24;4(3):161-3. Epub 2004 Mar 24.

MESA+ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

A simple method using spin-deposition and sacrificial layer etching is used to fabricate all-polyimide nanochannels (100 and 500 nm channel height). Channels are characterized using spontaneous capillary filling with water, ethanol and isopropanol, and with electroosmotic flow. The channels can be produced with simple cleanroom equipment, namely spinning and metal deposition facilities. Polyimide is an excellent material for micro- and nanofluidic channels due to its favourable electrical and mechanical properties and its biocompatibility.
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http://dx.doi.org/10.1039/b315859dDOI Listing
June 2004

Integrated microfluidic system enabling (bio)chemical reactions with on-line MALDI-TOF mass spectrometry.

Anal Chem 2002 Aug;74(16):3972-6

Laboratory of Supramolecular Chemistry and Technology, MESA+ Research Institute, University of Twente, Enschede, The Netherlands.

A continuous flow micro total analysis system (micro-TAS) consisting of an on-chip microfluidic device connected to a matrix assisted laser desorption ionization [MALDI] time-of-flight [TOF] mass spectrometer (MS) as an analytical screening system is presented. Reaction microchannels and inlet/outlet reservoirs were fabricated by powderblasting on glass wafers that were then bonded to silicon substrates. The novel lab-on-a-chip was realized by integrating the microdevice with a MALDI-TOFMS standard sample plate used as carrier to get the microfluidic device in the MALDI instrument. A novel pressure-driven pumping mechanism using the vacuum of the instrument as a driving force induces flow in the reaction microchannel in a self-activating way. Organic syntheses as well as biochemical reactions are carried out entirely inside the MALDI-MS ionization vacuum chamber and analyzed on-line by MALDI-TOFMS in real time. The effectiveness of the micro-TAS system has been successfully demonstrated with several examples of (bio)chemical reactions.
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http://dx.doi.org/10.1021/ac020185nDOI Listing
August 2002
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