Gold layers, thin

Fig. 10. A SPR Detection realized in a BIAcore system. A fan of polarized light passes a prism and is focused at the interface to an aqueous phase under conditions of total reflection. An evanescent wave enters the solvent phase. If the prism is coated with a thin gold layer at the interface the free electrons in the metal absorb energy from the evanescent wave for a distinct angle, depending on the refractive index of the solvent near the interface. B The gold layer can be modified with, e.g., a carboxydextrane matrix, where catcher molecules can be immobilized by standard chemistry. If a ligand is applied with the aqueous phase it may interact with the catcher and accumulate in the matrix, causing a shift in the resonance angle. If no specific binding occurs the refractive index in proximity of the sensor is less affected...

Aluminum Oxide Moisture Sensor. This type of sensor is a capacitor, formed by depositing a layer of porous aluminum oxide onto a conductive substrate, and then coaling the oxide with a thin film of gold The conductive base and the gold layer become the capacitor s electrodes. Water vapor penetrates the gold layer and is absorbed by the porous oxidation layer The number of water molecules absorbed determines the electrical impedance of the capacity, which is. m turn, a measure of water vapor pressure. 

In fact, the chemical etching is isotropic which is unfavorable for the structure we want to obtain. But the gold layer thickness is in the order of some 10 nm which is too thin for the isotropy to occur. 

TRANSDUCERS FOR MERCURY SENSORS BASED ON THIN GOLD LAYERS... 

High affinity of gold to mercury provides a simple way for the development of chemical sensors for mercury vapour. A number of transducers can be used for detection of mercury binding to gold all of them are based on exploitation of thin gold layers with a thickness from about 10 to about 100 nm. 

Fig. 12.5. Resistance change of thin gold layer AR due to increase of its surface resistance Rs, ARs, depends on the ratio Rh/Rs + Rh- where Rh is the bulk resistance of the gold layer (a). However, an increase of Rb by decreasing the thickness of the gold layer is limited by surface roughness (b). Changes in the resistance (AR) of thin gold layers due to exposure to 10 ng/1 of mercury vapour on reciprocal thickness of these layers (1 Id) are shown in (c) (adapted from Ref. [25]). With kind permission of Springer Science and Business Media.

Fig. 12.6. Influence of typical interferents on mercury sensors based on changes of lateral resistance of thin gold layers. Sensors bare gold layers (open symbols), gold layers coated by self-assembled monolayers of 1-hex-adecanethiol (filled symbols). Interferents 100% humidity (a), saturated vapour of sulphuric acid (b), 10 pg/1 of 1-butanethiol (c), 10 pg/1 of iodine vapour (d), Thickness of gold layers 42 nm [25].

Mercury sensors based on thin gold layers require regular calibrations. In ideal cases, the calibration should be performed before each measurement. Therefore, a calibration technique should be compatible with design and concept of the mercury vapour sensor. Several approaches were suggested. 

Figure 3.10 (a) Optical image of gold wires which have been coated on a silicon wafer. The gold layers are themselves covered with a thin polymer film, (b) Chemical image of the polymer overlayer (peak height of the vas CH2, at 2920 cm-1), recorded with a dual aperture of 3 x 3 pm2, 64 scans at 4 cm-1 resolution. 

Plasma polymerized N-vinyl-2-pyrrolidone films were deposited onto a poly(etherurethaneurea). Active sites for the immobilization were obtained via reduction with sodium borohydride followed by activation with l-cyano-4-dimethyl-aminopyridinium tetrafluoroborate. A colorometric activity determination indicated that 2.4 cm2 of modified poly(etherurethaneurea) film had an activity approximately equal to that of 13.4 nM glucose oxidase in 50 mM sodium acetate with a specific activity of 32.0 U/mg at pH 5.1 and room temperature. Using cyclic voltammetry of gold in thin-layer electrochemical cells, the specific activity of 13.4 nM glucose oxidase in 0.2 M aqueous sodium phosphate, pH 5.2, was calculated to be 4.34 U/mg at room temperature. Under the same experimental conditions, qualitative detection of the activity of a modified film was demonstrated by placing it inside the thin-layer cell. 

The impedance hygrometer measures the water content of a sample by means of a probe whose electrical impedance is a function of the vapor pressure of moisture in the fluid. The probe consists of an aluminum strip that is anodized to form a porous layer of aluminum oxide. A thin coat of gold is applied over the aluminum oxide. Water vapor penetrates the gold layer and equilibrates on the aluminum oxide. Leads from the gold and aluminum electrodes of the probes connect the sensing element to the measuring circuitry. The moisture content of solids can also be measured indirectly by detecting the moisture in the atmosphere above or near the process solids because the atmosphere near the solids is in equilibrium with the moisture content of the process materials. 

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