Stability, immunosensor

Because of their chemical stability and past industrial applications, PCBs are ubiquitous contaminants in the environment marine sediment, soil and food are the matrices where the presence of PCBs is more documented. For their characteristics electrochemical immunosensors are valid analytical tools to carry out a fast screening analysis on a large number of potentially polluted samples. 

Konig, B., and Gratzel, M. Long-term stability and improved reusability of a piezoelectric immunosensor for human erythrocytes. Anal. Chim. Acta 1993, 280, 37—41. 

To demonstrate a way of the use of aptamers in design of biomimetic sensors, two examples will be cited from the recent literature. The piezoelectric sensor for protein IgE has been developed with the use of commercially available anti-IgE aptamer oligonucleotide.167 The obtained sensor shows specificity and sensitivity equivalent to these of immunosensor, but for aptamer-based sensor a less decrease of sensitivity after consecutive cycles of analyte binding and regeneration, as well as relative heat resistance and stability over several weeks was shown. A more complex mechanism of sensing was employed in adenosine aptamer-based sensor.168 Detection was based on enzymatic activity measurements by fluorescence polarization with the use of aptameric enzyme subunit, which was a DNA aptamer composed of enzyme-inhibiting aptamer and adenosine-binding aptamer. 

The type of immobilisation dictates the ability to regenerate the sensor surface, or the antibody surface, for reuse as well as the stability and storage conditions needed to extend the lifetime of the sensor. The importance of immobilisation is reflected in the fact that a significant percentage of Pz immunosensor publications deal with the development of novel or the improvement of existing immobilisation methods. 

Combining immobilisation methods seems to be the best option, such as incorporating the stability and reproducibility of covalent methods such as SAMs with the orientated binding of protein A/G. The first example of this was in 1987 by Muramatsu et al. [4]. The authors cross-linked protein A to a layer of APTES, although it is unhkely that it was done to combine the advantages of both immobihsation methods since this was one of the earhest Pz immunosensors and more basic fundamental problems were being overcome at this stage. 

The main disadvantage of Pz immunosensors with a view to commercialisation seems to be their stability. As can be seen from the many previous examples they generally have a working lifetime of only a few days. Even unused sensors only last a number of weeks. This lack of stabihty is unacceptable if commercial success is to be realised. 

Theoretically Pz immunosensors are still very promising. In the short few years since the first immunosensor huge developments have been made. As more fundamental research paves the way for a better understanding of the exact mechanistics, the benefits should be conveyed in improved sensors. To overcome the stability problem a completely new approach may be required, rather than just trying to improve on already existing techniques. 

SPR is a representative physical phenomenon that is widely utilized for label-free characterization of molecules on thin metal films. The basic principle and operation of SPR has been described in more detail in several review articles [77, 78]. The reports on SPR-based immune sensors have steeply increased for detection of analytes with low molecular weights in recent years. SPR detection in microfluidic systems can provide various advantages. Immunoreactions are completed within a short time due to small sample volumes down to the nanolitre scale. Kim et al. developed a simple and versatile miniaturized SPR immunosensor enabling parallel analyses of multiple analytes [79]. Their SPR sensor was claimed to exhibit good stability and reusability for 40 cycles and more than 35 days. Feltis et al. demonstrated a low-cost handheld SPR-based immunosensor for the toxin Ricin [80]. Springer et al. reported a dispersion-free microfluidic system with a four-channel SPR sensor platform, which considerably improved the response time and sensitivity [81]. The sensor was able to detect short sequences of nucleic acids down to a femtomole level for 4 min. Waswa et al. demonstrated the immunological detection of E. coli 0157 H7 in milk, apple juice, and meat juice extracted from... 

The FABS, whose working principle was very similar to that of the AFM biosensor, was a cantilever-based immunosensor [7]. However, its configuration was much simpler than that of the AFM. Rather than using a piezoceramic translator to pull on intermolecular bonds, it used magnetic particles, which eliminated the need to manually position a tip and sample next to each other with picometer precision and stability. The cantilever-beam force transducer was the only element of the AFM retained by the FABS. 

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