Sensor coatings

B.D. MacCraith, Enhanced evanescent wave sensors based on sol-gel-derived porous glass coatings, Sensor Actuat. B-Chem., 11(1-3) (1993) 29-34. 

While the amount of electricity that can be conducted by polymer films and wires is limited, on a weight basis the conductivity is comparable with that of copper. These polymeric conductors are lighter, some are more flexible, and they can be laid down in wires that approach being one-atom thick. They are being used as cathodes and solid electrolytes in batteries, and potential uses include in fuel cells, smart windows, nonlinear optical materials, LEDs, conductive coatings, sensors, electronic displays, and in electromagnetic shielding. 

There are also many inorganic polymers such as rocks and ceramics. We also attempt to duplicate these materials in such products as concrete. High-technology inorganic polymers using organometaUic precursors and sol-gel processes are also used to prepare catalysts, coatings, sensors, and linear polymers such as silicones. 

An early example of an MIP-QCM sensor was a glucose monitoring system by Malitesta et al. (1999). A glucose imprinted poly(o-phenylenediamine) polymer was electrosynthesized on the sensor surface. This QCM sensor showed selectivity for glucose over other compounds such as ascorbic acid, paracetamol, cysteine, and fructose at physiologically relevant millimolar concentrations. A unique QCM sensor for detection of yeast was reported by Dickert and coworkers (Dickert et al. 2001 Dickert and Hayden 2002). Yeast cells were imprinted in a sol-gel matrix on the surface of the transducer. The MIP-coated sensor was able to measure yeast cell concentrations in situ and in complex media. A QCM sensor coated with a thin permeable MIP film was developed for the determination of L-menthol in the liquid phase (Percival et al. 2001). The MIP-QCM sensor displayed good selectivity and good sensitivity with a detection limit of 200 ppb (Fig. 15.7). The sensor also displayed excellent enantioselectivity and was able to easily differentiate the l- and D-enantiomers of menthol. 

We focus attention here on titania (Ti02) for the following reasons. The first is that titania is a widely used oxide support for both metal particles and metal oxides, and used in some cases also directly as catalyst (Claus reaction, for example). The second is that it possesses multifunctional properties, such as Lewis and Bronsted sites, redox centres, etc. The third is that it has several applications both as a catalyst and an advanced material for coating, sensors, functional films, etc. The fourth is its high photocatalytic activity which make titania unique materials. 

In this chapter we discuss how solid surfaces can be modified. Surface modification is essential for many applications, for example, to reduce friction and wear, to make implants biocompatible, or to coat sensors [405,406], Solid surfaces can be changed by various means such as adsorption, thin film deposition, chemical reactions, or removal of material. Some of these topics have already been discussed, for example in the chapter on adsorption. Therefore, we focus on the remaining methods. Even then we can only give examples because there are so many different techniques reflecting diverse applications in different communities. 

While conductometric responses of coated and uncoated gold layers on exposure to mercury vapour are similar, a strong difference in then-responses to other compounds is observed (Fig. 12.6). No response of the coated sensors to saturated water vapour was detected (Fig. 12.6a). Also in contrast to the uncoated sensors, practically no effect of either vapour of sulphuric acid (Fig. 12.6b) or butanethiol (Fig. 12.6c) on the coated sensors was observed. The effect of iodine vapour was not changed. 

The sensors display no sensitivity to such typical interferents of mercury chemoresistors based on uncoated gold as volatile thiols and water. Preliminary data demonstrate a possibility to measure Hg(0) even in aqueous solutions. However, the coated sensors still have strong interference with halides and hydrogen halides. 

Polyurethanes have also been employed as outer sensor membranes. Yu et al. evaluated the biocompatibility and analytical performance of a subcutaneous glucose sensor with an epoxy-enhanced polyurethane outer membrane.15 The membrane was mechanically durable and the resulting sensors were functional for up to 56 days when implanted in the subcutaneous tissue of rats. Despite the improved sensor lifetime, all of the polyurethane-coated sensors were surrounded by a fibrous capsule, indicating an enduring inflammatory response that is undesirable due to the aforementioned effects on analytical sensor performance. To date, the clinical success of most passive approaches has been rather limited. It is doubtful that one passive material alone will be capable of imparting long-term (i.e., weeks to months) biocompatibility for in vivo use due to the extremely dynamic nature of the wound environment. 

Fig. 30 Response characteristic of 42-coated sensor to 50,100,150, 200 and 250 ppm of S02 gas at room temperature [48]...

The majority of investigations involving polymer-coated acoustic-wave sensors for vapor detection have employed liquid or rubbery, amorphous, solid polymer coatings and have been restricted to vapor concentrations that are well below saturation. As a result, linear sorption isotherms have been found to prevail. Insofar as a sorption isotherm depends on the distribution of analyte between two phases (ambient and coating), models describing the extent of the distribution process are useful in a-priori prediction of coated-sensor performance. 

The use of polymer-coated acoustic sensors as chromatographic detectors (GIX, HPLC) has also been demonstrated [1,43,218]. In such applications, a lack of selectivity fcH a given analyte is actually beneficial, since the function of the coated sensor is to detect each and every species passing the detector after preseparation by the chromatographic column (see Chapter 6). 

Finally, the rate of response of a coated sensor is temperature dependent. When measurements are made under conditions of equilibrium between free and sorbed analyte, changes in kinetics present no problem unless the response becomes too slow for a chosen application. In some cases, however, the rate of response can be used to identify the species being detected an example is the molecular size-dependent diffusion of organic solvents into some polymer films [35]. In this case, failure to accurately measure and/or control temperature could lead to misidentification of the analyte. 

This simple case will be accurate only to the extent that the responses of each of the coated sensors is linearly proportional to the analyte concentration over the range of interest. 

Figure 1. An SEM photograph of molecular sieve coated sensor.

The detection performance of the molecular sieve coated sensor is examined from the measurement of frequency variation while different concentrations of organic vapor contained air are contacted to the sensor sur e. While organic vapor contained air flows continuously with constant flow rate of 0.4 IVmin., the variation of frequency is monitored and the outcome is converted to the organic concentration. In order to examine the process of adsorption and desorption of the organic vapor on the molecular sieve coated on the sensor surface, fresh air and organic substance contained air are alternately provided. 

In case of ethanol, the sensitivity of the sensor to ethanol-contained air is about 1/S of that of methanol. Because the SA molecular sieve is designed for the separation of the molecular size of methanol, ethane and propane or smaller, the SA molecular sieve coated sensor gives much less sensitivity with ethanol contained air. Unlike other solid particle coated sensor, such as activated carbon sensor, the molecular sieve coated sensor has selectivity to the molecular size of detecting material. The outcome presented in Fig. 3 indicates that the sensor coated with SA molecular sieve satisfactorily discern methanol vapor from bigger molecules, but it does not separate from small molecules. When 4 A molecular sieve coated sensor is implemented to detect methanol, the same result of measuring ethanol with S A molecular sieve sensor is yielded as shown in Fig. 4. In other... 

Figure 5 shows the relation between methanol concentration and frequency shift with 5 A molecular sieve coated sensor. Hus indicates the measured frequency shift gives the concentration of organic vapor. 

Fig. 21.11. Oil-sensitive MIPs sensitivity increases with the layer thickness of the QCM coatings. Sensor effects have been gathered by differential measurements between uncoated and MIP-coated electrodes (difference eliminates viscosity and temperature effects).

FIGURE 10.25 Sn02 sensors modified with zeolitic filters (a) schematic representation of the zeolite-coated sensor and (b) (See color insert following page 588.) front and back view of the as-received Sn02 sensors. 

Figure 3. The increase in sensor performance with decreasing particle size (A) Detection of 1.0 ppm NO in air at 250°C by sol-gel synthesized In203 response denotes relative resistance changes, S=Rjas/Rair, wheiB Rgas and Rair denote the sensor resistance in the presence and in the absence of the NO [6] (B) Detection of 500 ppm CO flame-spray-synthesized (FSP) SnO [7]. With increasing particle diameter the resistance in air, in 500 ppm CO and in 1000 ppm CO decreases. Also the sensor signal for 500 ppm CO decreases with increasing particle size. The measurements have been performed on drop-coated sensors at 350°C (8/3-FSP and commercial powders) and screen-printed sensors at 400°C (5/5-FSP powder) in dry air.

However, Nafion-modified electrodes also exhibited several disadvantages. For example, the response time of the Nafion-coated sensors increases due to a reduced diffusion coefficient value in the film [7], This can pose a serious disadvantage for in vivo work where dopamine and other neurotransmitter releases often occur on a sub-second time scale. In addition, Nafion coatings perform well for applications such as stripping analysis, but their use for direct voltammetric analysis is complicated by slow equilibration of the film with solution species [5 7], Therefore, there is a need for a modification system that can allow for rapid and selective permeation of the ions of interest. 

Remove sensor and rinse with clean PBS. Dispense 0.5mL of antibody solution into a clean 1.2 mL centrifuge tube. Immerse the protein G coated sensor into the tube for 1 h. 

At present, no calibration procedure for black and white standard thermometer is available that includes all stress factors (air temperature, air velocity, and humidity). Today, calibration traceability is guaranteed by a contact thermometric procedure. It would be preferable to measure the temperature at the surface of the coated sensor because this is the temperature of interest. A contactless surface temperature measurement requires knowing the emission ratio of the material and a minimization of the reflected and scattered radiation. For a minor error contact surface temperature measurement, a known method is the multiprobe measurement with extrapolation to the surface temperature. 

---