Nico F. de Rooij

Microfluidic Chips for Point‐of‐Care Immunodiagnostics

We might be at the turning point where research in microfluidics undertaken in academia and industrial research laboratories, and substantially sponsored by public grants, may provide a range of portable and networked diagnostic devices. In this Progress Report, an overview on microfluidic devices that may become the next generation of point-of-care (POC) diagnostics is provided. First, we describe gaps and opportunities in medical diagnostics and how microfluidics can address these gaps using the example of immunodiagnostics. Next, we conceptualize how different technologies are converging into working microfluidic POC diagnostics devices. Technologies are explained from the perspective of sample interaction with components of a device. Specifically, we detail materials, surface treatment, sample processing, microfluidic elements (such as valves, pumps, and mixers), receptors, and analytes in the light of various biosensing concepts. Finally, we discuss the integration of components into accurate and reliable devices.

Sample pretreatment on microfabricated devices

The integration of sample pretreatment into microfluidic devices represents one of the remaining hurdles towards achieving true miniaturized total analysis systems (muTAS). The challenge is made more complex by the enormous variation in samples to be analyzed. Moreover, the pretreatment technique has to be compatible with the analysis device to which it is coupled in terms of time, reagent and power consumption, as well as sample volume. This review provides a thorough overview of the developments in this field to date.

Sensing protein molecules using nanofabricated pores

We report the detection of protein molecules with nanofabricated pores using the resistive pulse sensing method. A 20-nm-thick silicon nitride membrane with a nanofabricated pore measuring about 55nm in diameter separated an electrolyte cell into two compartments. Current spike trains were observed when bovine serum albumin (BSA) was added to the negatively biased compartment. The magnitude of the spikes corresponded to particles 7–9nm in diameter (the size of a BSA molecule) passing through the pore. This suggests that the current spikes were current blockages caused by single BSA molecules. The presented nano-Coulter counting method could be applied to detect single protein molecules in free solution, and to study the translocation of proteins through a pore.

A microchip electrophoresis system with integrated in-plane electrodes for contactless conductivity detection

We present a new approach for contactless conductivity detection for microchip‐based capillary electrophoresis (CE). The detector integrates easily with well‐known microfabrication techniques for glass‐based microfluidic devices. Platinum electrodes are structured in recesses in‐plane with the microchannel network after glass etching, which allows precise positioning and batch fabrication of the electrodes. A thin glass wall of 10–15 νm separates the electrodes and the buffer electrolyte in the separation channel to achieve the electrical insulation necessary for contactless operation. The effective separation length is 34 mm, with a channel width of 50 νm and depth of 12 νm. Microchip CE devices with conductivity detection were characterized in terms of sensitivity and linearity of response, and were tested using samples containing up to three small cations. The limit of detection for K+ (18 νM) is good, though an order of magnitude higher than for comparable capillary‐based systems and one recently reported example of contactless conductivity on chip. However, an integrated field‐amplified stacking step could be employed prior to CE to preconcentrate the sample ions by a factor of four.

Label-Free Detection of Single Protein Molecules and Protein-Protein Interactions Using Synthetic Nanopores

Nanofabricated pores in 20 nm-thick silicon nitride membranes were used to probe various protein analytes as well as to perform an antigen-antibody binding assay. A two-compartment electrochemical cell was separated by a single nanopore, 28 nm in diameter. Adding proteins to one compartment caused current perturbations in the ion current flowing through the pore. These perturbations correlated with both the charge and the size of the protein or of a protein-protein complex. The potential of this nanotechnology for studying protein-protein interactions is highlighted with the sensitive detection of beta-human chorionic gonadotropin, a hormone and clinical biomarker of pregnancy, by monitoring in real time and at a molecular level the formation of a complex between hormones and antibodies in solution. In this form, the assay compared advantageously to immunoassays, with the important difference that labels, immobilization, or amplification steps were no longer needed. In conclusion, we present proof-of-principle that properties of proteins and their interactions can be investigated in solution using synthetic nanopores and that these interactions can be exploited to measure protein concentrations accurately.

Sample preconcentration by field amplification stacking for microchip-based capillary electrophoresis.

A microchip structure for field amplification stacking (FAS) was developed, which allowed the formation of comparatively long, volumetrically defined sample plugs with a minimal electrophoretic bias. Up to 20‐fold signal gains were achieved by injection and separation of 400 μm long plugs in a 7.5 cm long channel. We studied fluidic effects arising when solutions with mismatched ionic strengths are electrokinetically handled on microchips. In particular, the generation of pressure‐driven Poiseuille flow effects in the capillary system due to different electroosmotic flow velocities in adjacent solution zones could clearly be observed by video imaging. The formation of a sample plug, stacking of the analyte and subsequent release into the separation column showed that careful control of electric fields in the side channels of the injection element is essential. To further improve the signal gain, a new chip layout was developed for full‐column stacking with subsequent sample matrix removal by polarity switching. The design features a coupled‐column structure with separate stacking and capillary electrophoresis (CE) channels, showing signal enhancements of up to 65‐fold for a 69 mm long stacking channel.

Polyimide membrane with ZnO piezoelectric thin film pressure transducers as a differential pressure liquid flow sensor

Fabrication and characterization of ZnO thin film piezoelectric sensors on a polyimide membrane is presented in this paper. As a test device a differential pressure liquid flow sensor has been fabricated. We discuss new material combinations for the fabrication of the test devices. The pressure sensor membrane is a thin polyimide sheet bonded to a silicon wafer and the sensing material is sputtered ZnO piezoelectric thin film. The fabricated liquid flow sensor has been tested with a piezoelectric micropump for flow rates from 30 µl h−1 to 300 µl h−1. Stroke volumes of 1 to 10 nl have been measured. The strain in the sensing layer has been modeled and a transverse piezoelectric coefficient of e31,f = −0.294 C m−2 has been extracted.

Chemical sensing using an integrated microfluidic system based on the Berthelot reaction

Abstract The Berthelot reaction is a well-established colorimetric method for the determination of ammonia. It has been investigated with the particular aim of incorporating it into a simple, reliable analytical microfluidic sensing system. Absorbance measurements for the complex formed when this reaction is performed in microfluidic chips compare very well to those obtained in a spectrophotometric system. The very high reproducibility and efficiency of mixing by diffusion in the microfluidic chip make it a useful tool for future studies of other chemical methods where kinetics are a limiting factor for the response time.

The temperature dependence of the surface potential at the Al2O3/electrolyte interface

Electrolyte/insulator/silicon structures can be used to measure the variations of surface potential at the insulator/electrolyte interface. We have used this method to measure the temperature dependence of the surface potential Ψ0 at the γ-Al2O3/electrolyte interface. By applying the surface site dissociation model, this measurement can be interpreted in terms of the enthalpy changes of the ionization reactions of OH surface sites. As this theory predicts, it is found that ∂Ψ0ϖT varies linearly with pH. From the slope and intercept of this experimental line, the following enthalpies can be deduced: ΔHa1 = +34.8 ± 2.0 kJ/mole, for the dissociation of a proton from a SOH2+ site, and ΔH2a = +54.0 ± 2.0 kJ/mole, for the dissociation of a proton from a SOH site. These results imply that proton dissociation on Al2O3 surfaces is endothermic, and are in reasonable agreement with those obtained by Griffiths and Furstenau with a colloidal suspension.

A silicon integrated miniature chemical analysis system

Abstract Piezoelectrically driven micromachined silicon pumps are shown to have excellent characteristics for application in miniaturized chemical analysis systems. A system is demonstrated using two micro pumps and a separate glass flow-through cell with a potassium-sensitive ISFET. The measurement protocol is such that the sample solution enters the detector but does not pass the sensitive pump valves, thus improving the practical applicability of the system. During its operation, the sensor is continuously calibrated with a very low consumption of calibrating solution. With a measurement rate of four samples per minute, the use of calibrant is less than 3 ml/h.