Sensitive layer, selectivity

Abstract Makrolon , a commercially available polycarbonate with a glassy ultramicroporous structure (mean pore-volume 0.1 nm3), was often employed as sensitive layer for optical sensors in recent years. Due to the definite pore volume-distribution, it can be used as a size-selective sensitive layer. The interaction behaviour of Makrolon of different layer-thicknesses under the influence of methanol, ethanol and 1-propanol is characterized by Spectral El-lipsometry (SE), Surface Plasmon Resonance Spectroscopy (SPR)... 

A chemical sensor is a device that transforms chemical information into an analytically useful signal. Chemical sensors contain two basic functional units a receptor part and a transducer part. The receptor part is usually a sensitive layer, therefore a well founded knowledge about the mechanism of interaction of the analytes of interest and the selected sensitive layer has to be achieved. Various optical methods have been exploited in chemical sensors to transform the spectral information into useful signals which can be interpreted as chemical information about the analytes [1]. These are either reflectometric or refractometric methods. Optical sensors based on reflectometry are reflectometric interference spectroscopy (RIfS) [2] and ellipsometry [3,4], Evanescent field techniques, which are sensitive to changes in the refractive index, open a wide variety of optical detection principles [5] such as surface plasmon resonance spectroscopy (SPR) [6—8], Mach-Zehnder interferometer [9], Young interferometer [10], grating coupler [11] or resonant mirror [12] devices. All these optical... 

After discussing various sensitive layers later, both principles will be applied to monitor effects in biomolecular or chemical sensitive layers. The applications will demonstrate the feasibility of the methods as well as their advantages and their disadvantages. Therefore various applications are given, sometimes even for the same analyte, in order to demonstrate the normal approach in sensor development to select the best transduction principle for a specific application. 

For the quality of a sensor, not only the transduction is essential but also the sensitive layer. In addition, it determines selectivity sensitivity, stability... 

Accordingly, in Figure 8 sensitive layers can demonstrate either a higher reversibility or a higher selectivity. In combination with these properties, polymers usually show a high reversibility since they have poor selectivity and since they are relatively stable in contrast to biomolecular receptor layers where the selectivity and the sensitivity are high, but where the stability is rather poor as is reversibility. 

During the last years, so-called microhotplates (pHP) have been developed in order to shrink the overall dimensions and to reduce the thermal mass of metal-oxide gas sensors [7,9,15]. Microhotplates consist of a thermally isolated stage with a heater structure, a temperature sensor and a set of contact electrodes for the sensitive layer. By using such microstructures, high operation temperatures can be reached at comparably low power consumption (< 100 mW). Moreover, small time constants on the order of 10 ms enable applying temperature modulation techniques with the aim to improve sensor selectivity and sensitivity. 

Layer doping with catalytic metals can be done either during the powder preparation or later in the deposition and annealing process. Doping is an important procedure in tuning the gas-sensor characteristics (selectivity pattern of the sensitive layers) [67,68]. 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

It is well established, that the poor selectivity of tin-oxide sensors can partly be overcome by adding catalysts to the sensitive layer. Most common additives are noble metals like gold (Au), platinum (Pt) or palladium (Pd). They can be mixed with the tin oxide during paste formation before deposition. The influence of dopants on the gas sensor response is still subject to debates. The two most established mechanisms are the spill-over and the Fermi-level mechanism [82]. 

A typical CC-measurement procedure (for a pH-sensitive LAPS structure) is depicted in Fig. 6.3. From the raw data material of the CC-mode measurement of all measurement spots under the pH-sensitive layer, a calibration plot can be derived. For example, for the above example, an average pH sensitivity of 54.2 mV/pH with a standard deviation of 0.5 mV/pH between the different measurement spots can be calculated. This initial calibration measurement allows furthermore the determination of different measurement parameters, e.g., the hysterisis, overall drift, stability, selectivity and the influence of external disturbances such as light and temperature. These parameters are important to evaluate the performance of the complete LAPS-based measurement system. 

Taha et al. [32] developed a sensitive and selective thin-layer chromatographic method for the determination of lornoxicam and other oxi-cams in the presence of their alkaline degradation products. The method is based on the thin-layer chromatographic separation of the drugs from their alkaline degradation products, followed by densitometric measurement of the intact drug spots for lornoxicam at 380 nm. The developing systems used for separation are ethyl acetate-methanol-26% ammonia... 

Thin-layer chromatography has been used as a selective, sensitive, reliable and simple separation method, and it has been proven a very useful method for optimisation of displacement chromatography 122), and in the determination of lipophilicity 123-25). 

Nanostructured materials can be synthesized from the so-called top down or bottom-up approach. In the first approach, features at the micron (or submicron) length scale are created on a substrate by masking and exposing selected regions of a radiation sensitive layer (typically a polymeric photoresist) to a UV source. This exposure is followed by various chemical treatments and mechanical steps to obtain the desired spatial pattern on a substrate. However, the feature sizes that can be obtained with this approach are limited to the length scale of the wavelength of the radiation employed. If features at the nanometer scale are desired, one must start from the bottom (i.e., use individual molecules or clusters) and assemble templates that will impart the nanostructure to the desired material. 

Chemical sensors are small devices for the detection and quantification of gaseous or solvated species. This is an active research area based on the need to obtain increasing amounts of data in chemical and food process streams as well as environmental monitoring. Most sensors consist of an appropriate transduction principle such as the quartz-crystal-microbalancc (QCM) and a chemically sensitive layer that imparts the desired chemical response behaviour. Most often a chemically selective response is desirable. Zeolite molecular sieves offer size- and shape-selective adsorption behaviour that can be combined with appropriate transduction concepts in order to construct chemically selective sensor devices. 

Sensors based on adsorption of species onto or into lattice structures have been reported for molecules besides water. For example, devices based on the detection of carbon dioxide adsorption onto semiconductor materials have been developed [10]. In other cases, dielectric materials that have some degree of chemical specificity have been used for making chemically-sensitive layers. One such application is the use of the highly porous zeolite lattice to detect adsorbed hydrocarbons [11]. The specific dimensions and shape of the zeolite pores allows for size and chemical selectivity in the lattice. As in the case of the humidity devices, the adsorbed molecules dipoles cause a local change in the electric fields that can be detected through a capacitive effect. 

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