SPR sensor systems

Fig. 8 Portable SPR sensor system developed at the Institute of Radio Engineering and Electronics, Prague with SPR optical platform, fluidic unit, temperature stabilization, and supporting electronic hardware...

In recent years, first prototypes of SPR biosensor systems integrating these elements have been reported and their application for detection of biological analytes in the field have been demonstrated. Undoubtedly, advances in the development of the key elements - sample preparation technology, microfluidics, biomolecular recognition elements, SPR optical platform - will further stimulate this effort and eventually lead to in-field SPR sensor systems becoming a commercial reality. 

As already mentioned, an important application of SPR is in the field of chemical sensors. Such a SPR sensor system should be simple, compact, and relatively inexpensive, while retaining the angular sensitivity. Some representative SPR sensor systems will now be discussed. 

DDT), organophosphorus (chloipyrifos), and carbamate (carbaryl) pesticides. The lowest limit of detection (LOD) was obtained for DDT with 20 ng 1 , while 50 ng 1 and 0.9 pg 1 were achieved for chloipyrifos and carbaryl, respectively. Matrix effects were evaluated for the carbaryl immunoassay in different water t3tpes with detection limits within the range of carbaryl standards in distilled water (0.9-1.4 pg 1 ). This portable SPR-sensor system is already a product on the market, commercialized by the company SENSIA, SL (Spain). The size and the electronic configuration of the device allow its portabihty and utilization at contaminated locations. ... 

As well as fluorescence-based assays, artificial membranes on the surface of biosensors offered new tools for the study of lipopeptides. In a commercial BIA-core system [231] a hydrophobic SPR sensor with an alkane thiol surface was incubated with vesicles of defined size distribution generating a hybrid membrane by fusion of the lipid vesicles with the alkane thiol layer [232]. If the vesicles contain biotinylated lipopeptides their membrane anchoring can be analyzed by incubation with streptavidine. Accordingly, experiments with lipopeptides representing the C-terminal sequence of N-Ras show clear differences between single and double hydrophobic modified peptides in their ability to persist in the lipid layer [233]. 

Figure 3. Portable SPR sensor prototype system including sensor, optics, electronics and flow delivery system.

Sensor systems based on SPR now are commercially available (Nagata and Handa 2000). 

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 noise of sensor output originates in the optical system and readout electronics of an SPR sensor instriunent. Dominant sources of noise are fluctuations in the hght intensity emitted by the light source, statistical properties... 

Figure 15 suggests that SPR sensors with large area detectors such as PDA or 2D array CCD can potentially achieve a better resolution compared to systems using linear CCD detectors. This comparison is related to the fact that more light is measured with large area detectors in the same period of time, which results in the reduction of the shot noise. 

SPR sensors directly measure refractive index. In conjunction with appropriate biorecognition elements, they can be used as affinity biosensors allowing detection of the capture of analyte molecules by biorecognition elements immobibzed on the sensor surface. The abihty of SPR sensors to perform measurements is described by the performance characteristics, of which the most important are the sensitivity, resolution, accuracy, reproducibihty, and hmit of detection. The sensitivity and resolution are primarily determined by the properties of the optical system of the SPR sensor and can be linked to specific design parameters. 

An SPR sensor instrument consists of an optical system, supporting electronics, and a sensor data acquisition and processing system. In the optical system, surface plasmons are optically excited and the output light wave with an encoded SPR signal is detected. The signal from the detector is processed to yield a sensor output. SPR biosensors also incorporate a biorecognition... 

In the optical system of an SPR sensor, surface plasmons are excited by a light wave. The excitation of surface plasmons in the SPR sensor results in a change in one of the characteristics of the light wave. Based on which characteristic of the light wave is modulated and used as a sensor output, SPR sensors can be classified as SPR with (i) angular, (ii) wavelength, (iii) intensity, (iv) phase, or (v) polarization modulation. The first three types of modulation (Fig. 2) are used most frequently in today s SPR sensors. 

In 1988 Matsubara et al. reported an SPR sensor based on prism coupler and angular modulation [18]. The optical system of their sensor is shown in Fig. 5. 

SPR biosensors have been used to detect a wide range of food-related analytes. Table 1 provides a siunmary of the studies listed in this review. SPR sensors are at the forefront of sensing technologies capable of real-time, quantitative detections. This makes them quite suitable as on-hne monitoring systems in food processing plants. Trends in the technology are pushing... 

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