Miniaturization of biosensors

As an outlook, the optimization of the efficiency of microorganisms with methods of genetic engineering will result in an increased sensitivity, selectivity, and stability, in connection with the further miniaturization of biosensor systems, especially the development of portable biosensor measuring devices, represents a promising feature for environmental monitoring by microbial... 

Miniaturization of biosensors is important for a variety of reasons 1) potential use in neurological and physiological studies. 2) improved biocompatibility. 3) measurement in very small volumes. 

A considerable body of research work is directed at the miniaturization of biosensors and the creation of multifunctional sensors by the use of small-scale electronic devices. Semiconductor biosensors have been developed by the biochemical modification of gas-sensitive or ion-sensi-... 

Future directions in biosensors will require the miniaturization of individual sensors for in-vivo use, and for use in microsensor arrays (7). Ideally, these biosensors should be capable of direct, rather than differential measurement, and should have reasonable lifetimes. 

Biosensors are being increasingly used as detectors in FIA systems [284,285, 322, 379, 476]. The drawbacks of biosensors as direct in situ sensors, namely their low dynamic range, their lack of ability to survive sterilization, their limited lifetime, etc. are no longer valid ex situ because the analyzer interfaces the biosensor which can be changed at any time and FIA can provide samples in optimal dilution. The need for chemicals and reagents can be drastically reduced when employing biosensors, specifically when the entire system is miniaturized [48]. 

Based on the high specificity of enzymatic reactions enzymatic fuel cells can be constructed compartmentless, i.e., without a physical separation of the anodic and the cathodic compartments. This allows miniaturization of the devices, e.g., for biomedical (implantable) devices and -> biosensors [iii]. 

Semiconductor fabrication techniques have also been successfully applied to the construction of conventional transducers sensitive to hydrogen peroxide, oxygen, and carbon dioxide, A hydrogen peroxide-sensitive silicon chip was made by using metal deposition techniques (28,29). The combination of the hydrogen peroxide-sensitive transducer and enzyme-immobilized membranes gave a miniaturized and multifunctional biosensor. Similarly, an oxygen- and a carbon dioxide-sensitive device was made cmd applied to the construction of biosensors (25, 30, 31). 

All the above reported examples utilized electrochemical biosensors because these kinds of biosensors have numerous and still unsurpassed advantages in terms of versatility, reliability, low cost plus offering the possibility of fabrication in different materials and shapes and of miniaturization (also of the related instrumentation). 

An excellent overview and in-depth description of miniaturization techniques is given in Ref. [2]. Here, only a few examples of how miniaturized biosensors can he fabricated are described. A clear distinction can be made between the miniaturization of the biosensor fluid flow system and the use of a miniaturized transducer. Both result in different advantages for the biosensor assay, as it will be discussed below. In the case of biosensor fluid flow system, microfabrication tools used are by far fewer than those applied to the fabrication of micro/nanotransducers, which can span all of the micro/ nanofabiication techniques developed. 

Related to the volume processed, the detection limit of biosensors has to be considered. Take, for example, a hypothetical biosensor for HIV detection that has a detection limit of one virus per analysis. If only 100 nl of sample are analyzed, then the detection limit of biosensor cannot be below 10,000 virus particles per milliliter of sample, which is neither a stunning nor an acceptable limit of detection for rapid detection early in the disease. Thus, in this case, a sample preparation step that reduces a 1 ml blood sample to 100 nl is of imminent importance (i.e., now, reduced reagent costs make the miniaturized biosensor advantageous over a macroscopic counter part). The same problem is encountered in most environmental and food sample analyses. Thus, the incorporation of sample pretreatment steps into the bioanalysis is very important, and for miniaturized biosensors even more than for their macro cotmterparts. 

Low detection limits of 10/xgl were established. Similar approaches for the miniaturization of SPR can be found in literature, however, similar to the miniaturized cantilever biosensor, any surface-active interfering compound in samples will cause significant analytical challenges. Borchers and coworkers used a microchip evanescent waveguide for the detection of realtime DNA hybridization events. A lower detection limit of 0.21 nmol 1 was demonstrated. The authors also showed multi-analyte detection capabilities of their system and suggested that this strategy can be utilized in real-time DNA array format with analysis times as short as 2 min. 

Miniaturized biosensors combined with miniaturized sample pretreatment steps will offer one of the true advantages of biosensors over other analytical and bioanalytical methods portability. These future bioanalytical microsystems will have the size of a calculator, and will be much smaller and lighter than a laptop computer. Thus, research and development are needed not only for the miniaturized biosensor part of these bioanalytical microsystems but also for miniaturized sample pretreatment steps. The concentration of sample volumes (especially for environmental and food samples), the purification and extraction of the analyte from a complex sample are as important as the detection of the analyte itself. Finally, integrated pump systems that are independent from the sample components (such as pH and ionic strength), that require little energy and are stable and rugged are still needed. 

A miniaturized thermal flow-injection analysis biosensor was coupled with a microdialysis probe for continuous subcutaneous monitoring of glucose [34]. The system (Scheme 1) consisted of a miniaturized thermal biosensor with a small column containing co-immobilized glucose oxidase and catalase. The analysis buffer passed through the column at a flow rate of 60 pl/min via a 1-pl sample loop connected to a microdialysis probe (Fig. 11). 

The feasibility of miniaturizing thermal biosensors in different constructions, sizes and materials,by employing conventional machining and micromachining technologies could be further exploited in several directions. The miniaturiza-... 

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