ExpertiseBio & Chemical Sensors

Bio & Chemical Sensors

We develop sensor systems for applications in medical diagnostics, health care, the biotechnological process industry, as well as food-quality and environmental monitoring.

Optimization of antibody binding to different surfaces

We have been studying ways to optimize antibody binding to surfaces as well as the ability of different chemicals to protect the substrate from adsorption of unwanted biomolecules. The characterization of antibody binding to gold surfaces is done by using both intact IgG antibodies and modified antibodies made in-house. The kinetics of antibody binding to gold has been monitored with QCM-D (Quartz Crystal Microbalance with Dissipation) equipment from Q-Sense, which we have in our laboratory. We have also compared the efficiency of different molecules such as bovine serum albumin, casein and polyethylene glycol in blocking the surface against non-specific (unwanted) binding when the substrate surface is exposed to serum or plasma.
We have gained understanding of how the antibodies bind to surfaces and how we can increase the specific binding and decrease the nonspecific one. This knowledge is essential when designing sensor systems for medical diagnostics as well as for the detection of microorganisms and cells.

Functionalization and characterization of nanoparticles

We often use nanoparticles in suspension as substrates. Using nanoparticles in sensing requires profound expertise in their functionalization and subsequent separation from excess reagents. For example, to enhance the fluorescent signal one can use fluorescent nanoparticles covered with antibodies, the so-called detecting (secondary) antibodies, instead of discrete fluorophores. In addition, nanoparticles have to be blocked against non-specific adsorption of other proteins in the solution and then one has to get rid of the excess of detecting antibody and of the blocking molecules.

We have worked with commercially available nanoparticles that have embedded molecular fluorescent probes (FluoSpheres) as well as with quantum dots. The emission spectra of the latter particles are much narrower than the emission from the FluoSpheres. Moreover, quantum dots with different emission spectra can be excited with the same laser. This makes quantum dots suitable for an assay detecting several antigens at the same time by using different fluorescent markers for different antigens. Mastering the functionalization of quantum dots is a necessary prerequisite for constructing sensor systems for the simultaneous detection of several disease markers and for improving diagnostic precision.

Nanoparticles can be used for more than substrates. A chemical reaction, such as functionalization, can change their properties sufficiently to enable detection. This change can be used to monitor the chemical reaction quantitatively in time. For example, using magnetic nanoparticles, one can track reaction-induced changes of their sizes by monitoring the dynamic magnetic susceptibility.

Therefore, we are developing functionalizations of magnetic nanoparticles with different biomolecules such as Protein G and L, Streptavidin and NiNTA or covered by DNA strands for different applications. These functionalized nanoparticles can be used in assays where antibodies or other proteins are quantified using a proprietary detection scheme.

A dedicated portable instrument has been built for rapid analysis of the size changes of magnetic nanoparticles with different functionalizations. We have developed a software package for the quantification of these changes. The technique allows one to follow and quantify every step in an assay without the need for additional labeling. For example, Ni-NTA nanoparticles allow one to control the process of protein production by monitoring concentration of histydine-tagged proteins.
In our laboratory, we have instruments to characterize the properties of magnetic nanoparticles of importance for as substrates for sensing chemical reaction. The magnetic properties of the nanoparticles are determined with instruments that quantify the static and dynamic magnetization. The particle-size distribution can be deduced from the magnetic properties as well as from light-scattering measurements. The latter measurements can be performed on magnetic as well as on non-magnetic particle suspensions with Malvern Instruments, Zeta Sizer Nanoseries, which we have at our laboratory.

Another important factor when working with nanoparticles is the stability of suspensions towards agglutination. This is strongly related to particle charges, which may change during functionalization. The Zeta Sizer enables us to measure the Z-potential of the particles after every functionalization step. The Z-potential is directly related to particle charge.

Non-specific adsorption

It is very important to suppress all non-specific reactions that may occur between the surface or the surface-bound molecules and the proteins (or DNA strand). Only the molecules of interest, the target molecules, should bind to the sensor substrate. To block all non-specific reactions is a problem that is far from trivial. Usually the abundance of a protein of interest in a solution is very low, often well below ppb, which implies that the surface is bombarded predominantly by parasitic molecules and that the incidence of the molecules of interest is very rare.

Bad blocking may lead to adsorption of fluorescently labeled molecules to sites that do not signal the existence of the target molecule. This leads to increased background fluorescence and thus decreased overall assay sensitivity or, in the worst case, to a false positive signal. We are putting much effort into minimizing non-specific adsorption in order to optimize the immunoassay performance.

Wireless pH sensing using pH-sensitive materials

Wireless sensing and particularly the multifunctional RFID probes that include a sensing possibility, has been attracting considerable industrial and scientific attention. We have gained expertise in the development of wireless pH sensors. As sensing materials we use pH-sensitive polymer films and pH-sensitive surface-bound peptides. The transduction is either magnetic, using the magnetoelastic resonance technique, or electrochemical by means of measurements of the pH-induced changes of the impedance of peptide films.

Such pH sensors complement already commercially available temperature RFIDs in many applications. For example, one can monitor the status of environmentally sensitive parcels such as foodstuff or stem-cell cartilages. They may also be used to sense pH changes in vivo, both in animals and in humans. Together with Swedish Veterinary University, we are developing low-cost probes that will be able to continuously monitor the pH of the cud of cows. This will enable farmers to monitor the pH of each cow and so tailor the food composition for each animal.

Molecularly imprinted polymers

Molecularly Imprinted Polymers (MIPs) are presently used to purify, catch or enrich molecules below about 5kDa. We extend their application range to sensing. MIPs enable one to sense specifically either chosen target molecules or families of chemically similar molecules. They are able to detect, with high sensitivity, toxins, narcotics, drugs and pharmaceutical waste, to mention just a few examples. They work only in liquid environments, but the target can be collected from the air and injected into the MIP-containing suspension. A single measurement using only an imprinted polymer usually gives only qualitative measure of target concentration. To obtain quantitative results, one needs to measure the non-specific adsorption. This is done by monitoring the response of both the imprinted polymer and of the identical polymer without imprinted sites.

The detection part of the sensor system is quite generic, so similar detection techniques can be used to sense a variety of different compounds. Our strategy is to evaluate and optimize detection techniques using model imprint systems. At present, we have gained expertise primarily in two detection techniques: optical detection using fluorescence correlation spectroscopy of nanoparticles that consist of MIP polymer shells surrounding quantum dots (patent pending), and electrical transduction of the target-binding event by means of measurements of the changes of complex impedance at different frequencies. We are developing two systems to demonstrate the detection capabilities of each transduction method in field-like conditions.

Electrochemistry

We have developed a sensor chip that monitors surface reactions by measuring the conductivity and capacitance (complex impedance) as a function of the frequency of thin film layers coated on the chip. Depending on the properties of the coating, the chip can be used for a variety of applications. For example, we have used it to monitor pH changes of pH-sensitive polymers, to detect target molecules from complex solutions using molecularly imprinted polymers, and to monitor changes of impedance of tap water containing different urea concentrations. The latter application is of importance for automotive applications since urea is needed for the catalysis of NOx exhausts from diesel engines.
We use a commercial Electrochemical Impedance Analyzer and the Potentiostat / Galvanostat from Solartron Instruments for these measurements. These instruments allow us to measure not only AC impedance but also to tailor and monitor chemical reactions. For example, we have used the equipment both to electrochemically prepare molecularly imprinted polymers against sorbitol and to evaluate its performance. The latter is done by impedance measurements while the former is done using the cyclic voltammetry. When performing cyclic voltammetry measurements, a DC ramp voltage is applied to a sample electrode, and the current resulting from surface reactions on the electrode is measured. This has been done in a miniature electrochemical cell we have developed.

Peptides on surfaces

Although great progress has been made in the development of artificial replacements for body parts, many problems still remain, one of which is rejection by the human immune system. A step towards the development of materials more bio-compatible than those used today can be to tailor implant surfaces for the attachment of particular cells by modifying the surfaces using cell-adhesion promoting peptides.

The goal of one of the research for master´s theses performed at Imego was to investigate if it was possible to tailor one of these cell-adhesion promoting peptides´ architecture on a gold surface. Each end of the peptide was anchored to the gold surface using thiol chemistry, which caused to form a bent structure on the surface. The bent structure of this particular peptide is especially prone to cell adhesion.

The experiments were performed using a Quartz Crystal Microbalance (QCM) with dissipation monitoring. This is a surface-sensitive technique that allows one to monitor quantitatively the time evolution of chemical reactions. It is used in many of ourhe bio-projects to evaluate surface reactions.

People

Sara Bogren
Senior Scientist

sara.bogren [at] ri.se

Kristina Fogel
Senior Scientist/Project Manager
+46 (0)70 915 27 10
kristina.fogel [at] ri.se

Anatol Krozer
Senior Expert
+46 (0)70 915 18 06
anatol.krozer [at] ri.se

Ingemar Petermann
Senior Scientist
+46 (0)70 768 77 27
ingemar.petermann [at] ri.se

Cristina Rusu
Senior Expert, Micro System technology
+46 (0)70 915 18 26
cristina.rusu [at] ri.se

Björn Samel
Department Manager
+46 (0)70 475 00 81
bjorn.samel [at] ri.se

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