Magnetic Bead-based Biosensor

(1) Food Borne Pathogen ImmunoMagnetic Bead Based Biosensor

Bacterial and viral contamination of food has always been a serious problem for consumers and food producers. Food borne illness caused by bacteria such as E. coli, Listeria, and Salmonella result in hundreds of thousands of hospitalizations each year. For food producers the problem is to be able to detect pathogen presence as rapidly as possible and at the minimum possible concentration. Classical methods for the detection of food borne bacteria or other bacteria still depends on culturing and plating, requiring from 24 to 48 hours to confirm the analysis only under the circumstances that minimally 10,000 colony forming units per milliliter (cfu/ml) have been cultured. PCR (polymerase chain reaction), a DNA based method, requires at least 36 hours which is also time consuming.

A new process for detecting biomaterial has been developed using immunomagneticbeads that amplify the sensitivity and specificity of the biosensor. The magnetic beads are treated beforehand so that antibodies can be immobilized on their surface. When the antibody reacts with the antigen presented by the pathogen, the sensor measures a shift in frequency on the gold electrodes. The Immunomagnetic Bead Biosensor can detect 100 cells of E. coli in 2 hours, without pre-enrichment or incubation. The two hours is the total detection time from sample preparation to result display. The sensor is portable and can be used onsite, and the same technology will work to detect nonbiological materials as well as biological pathogens.

(2) Magnetic bead-based DNA detection

Of all the recent technical advances in molecular biology, the PCR has been by far proven to be the most useful and powerful especially for nucleic acid amplification. The detection techniques have been employed for the detection of Escherichia coli O157:H7, including PCR and real-time PCR assays. However, there are several drawbacks, such as complexity, time consuming procedures, and narrowing the target. Recently, other detection methods were also developed, such as quartz crystal microbalance DNA magnetic biosensor. The detection limit range was between 102 and 105 colony-forming units (CFU)/ml. Please see the sachematic above.

(3) Antibody-Antigen interaction

Investigations with antigen antibody binding were also performed, with initial experiments addressing the development of an assay for prostate-specific antigen (PSA) detection. PSA is a possible biomarker that allows for the early detection of prostate cancer, which is an increasingly vital health issue in the western countries . The magnetic-bead-based biosensor has been evaluated as an immunosensor towards PSA detection taking advantage of antigen-antibody affinity. One of the key elements in this evaluation process is the development of an assay to detect PSA. In this indirect method, a sandwich assay being formed, the direct detection of the specific analyte (i.e. PSA) is followed by one or more binding events resulting in a higher sensor response. One of the molecules in this binding cascade is labeled with magnetic beads. Top picture illustrates a typical sandwich assay for PSA. A specific conventional antibody against PSA is immobilized covalently onto a mixed SAM of thiols on the gold surface of the transducer. Next, the PSA analyte molecules are affinity-bound (direct detection) onto the immobilized surface and a biotinylated secondary antibody is introduced. Finally, streptavidin-coated magnetic beads are introduced and interact strongly with the biotinylated secondary antibody. Bottom picture shows the initial results for the specific recognition of PSA versus non-specific detection. The bottom picture shows the capturing of the magnetic beads on the specific antibody ¨C antigen surface.

(4) Magnetic Beads Based Microsystem For Biological Analysis

• Paramagnetic labels attach to solution DNA and the biochip surface is functionalized with probe DNA
• Hybridization with immobilized complementary target DNA
• An external magnetic field is applied
• The sensor detects the external magnetic field minus the particle field

• Modular and portable system
• Small electronics card for sensor driving and reading
• Capable of communicating with a handheld device

• Digital signal processor (up to 30 MIPS), 1 Mbit SRAM
• Biochip interface, switches and current generator
• Acquisition, maximum gain 1000, 24 bit@3.5kHz