1. Technical Field
The present disclosure relates generally to surface acoustic wave (SAW) sensors employed in the identification of chemicals, and more particularly to coatings applied to SAW sensors to reduce surface contamination due to chemical adsorption.
2. Related Art
Surface acoustic wave (SAW) devices are widely used in chemical detection sensor applications. A SAW sensor is generally defined by an input transducer, which converts a known input electronic signal into an acoustic/mechanical wave, and an output transducer that converts that wave back into an electronic signal for further processing. The transducers, which are interdigitated, are disposed on a piezoelectric substrate such as quartz. There are generally two types of SAW sensor configurations based upon different electrode patterns. One of these configurations is referred to as a resonator, where a first set of reflector electrodes and a second set of reflector electrodes surrounds a set of interdigitated electrodes. The other configuration is referred to as a delay line, where the input interdigitated transducer electrodes are spaced apart from the output interdigitated transducer electrodes.
The frequency of the output electronic signal may vary due to mass loading on the SAW surface. When the surface is clean and free from additional mass loading, the output electronic signal has known characteristics, that is, known frequency. With additional mass loading, the oscillation frequency is reduced, and there is understood to be a linear relation between the additional mass and the reduction in oscillation frequency. This behavior may be utilized for detecting and identifying various chemical compounds.
A SAW detector may be comprised of multiple SAW sensors that are coated with selected polymers and define a sensor array. The particular polymer used for the coating is understood to enhance the adsorption of certain chemical compounds, or groups of chemical compounds, and selectivity of the polymers is understood to be based upon the function groups thereof. The adsorption is understood to increase the mass loading on the sensor surface, resulting in the reduction of the oscillation frequency. Each sensor in the array is understood to respond different to specific target chemical compounds, target groups of chemical compounds, and target mixtures of different chemical compounds. Response patterns for each of the foregoing may be established. The coating polymers are thus selected based upon the targets to be detected, and the selective adsorption capabilities of the coating surface are enhanced therefor. Thus, selectivity and sensitivity of the sensors may be optimized to detect the target chemical compounds.
With such types of sensors, the chemical compound samples interact directly with the polymer rather than the SAW surface. The adsorption of the chemical molecules by the polymer coating increases the mass loading on the SAW surface, which in turn reduces the oscillation frequency of the SAW as discussed above. There is understood to be no interaction between the chemical compound sample and the SAW device surface.
Alternatively, uncoated SAW devices may be used as sensors, specifically for gas chromatography applications and detectors therefor. Initially, the chemical compounds are separated in a gas chromatography column. When the vapor of a chemical compound elutes from the gas chromatography column, such vapor contact the uncoated SAW sensor. The chemical compound is understood to condense onto the surface of the sensor when the SAW temperature thereof is below the dew point of the vapor. The condensed chemical compound, in turn, affects the oscillation frequency of the SAW device. Detection and identification of the chemical compound sample is based upon the alteration to the oscillation frequency, and the retention time in the gas chromatography column.
With these sensors, chemical compounds are understood to interact directly with the surface of the SAW device without any barrier against the condensate, with the mass loading of the chemical by condensation being the basis for the sensor response. When the concentration in the eluting vapor is below the saturation level at the sample vapor-SAW interface, each chemical compound evaporates from the SAW surface without an increase in the temperature of the SAW sensor.
There are generally three contributors to the mass loading on the surface of uncoated SAW devices. First, there is condensation due to the saturation of chemical compounds in the vapor in contact with the SAW sensor. Chemical compounds loaded on the surface via condensation evaporate when the concentration in the contacted vapor is below the saturation level at the sample vapor-SAW surface interface. It is not necessary for the SAW sensor to be heated to release the condensed chemical compounds, and the release is understood to be complete without any residues remaining on the surface of the SAW sensor. If the chemical compounds load on the SAW sensor surface only by this process, short and long term operational stability is possible.
Second, there is physical adsorption due to weak Van der Waals forces between the substrate surface and the molecules in the vapor. Such adsorption may require external energy to reverse, and completely release the adsorbed chemical compounds. As the bond is based upon weak Van der Waals forces, only a slight increase in temperature of the SAW sensor may be needed to break the bond. It is understood that residues do not remain on the surface, and therefore raising the temperature of the sensor by a proscribed degree after each analysis cycle is deemed sufficient to clean the chemical compounds physically adsorbed on the SAW sensor surface.
Third, there is chemical adsorption due to the formation of chemical bonds between the active sites of the substrate/quartz surface and the interdigitate electrodes, and the molecules in the vapor. For example, although the surface of the quartz substrate, which is one of the most used piezoelectric materials for a SAW device, is generally regarded as inert, they are slightly acidic and highly adsorptive due to the presence of hydroxyl groups (—OH). Such reactive groups are understood to interact with different function groups such as amine (—NH), carboxylic acid (—COOH), hydroxyl (—OH), or thiol (—SH) via hydrogen bonding. The reaction between the SAW surface and the function groups results in the chemical adsorption of the compounds that contain such function groups into untreated surfaces of the SAW sensor. Chemical compounds in a sample being analyzed which contain these function groups are understood to bond to the surface of the SAW sensor, and cannot be removed without significant external energy. Furthermore, the chemical compounds that remain on the surface of the SAW sensor may create different active sites thereon and react with other chemical compounds, thereby increasing its chemical adsorption capability for those chemical compounds in contact with the surface of the SAW sensor. Over time and multiple analysis cycles, there may be an accumulation of residual compounds that, in turn, affect the oscillation frequency, resulting in instrument drift. When residual chemical compounds have accumulated beyond a certain point, oscillation may be reduced to such an extent that there will be no further response to additional mass. This is understood to affect both long term stability and repeatability.
To minimize accumulation of residual compounds on the SAW sensor, presently, the sensor is briefly heated to a higher temperature at the end of each analysis cycle than during analysis. This is understood to break the chemical bond between the surface of the SAW sensor and the chemically adsorbed chemical compounds, and release the adsorbed molecules following each analysis cycle. Under some circumstances, it may not be possible to completely reverse the chemical adsorption, even with high temperatures. The aforementioned chemical reactions between the SAW sensor and the compounds, and the attendant chemical bonds formed as a consequence, may prevent full removal. Thus, the surface of the SAW sensor may not be entirely clean, or as clean as its initial state, if a high amount of chemical compounds have been chemically adsorbed. Heavily contaminated sensors require heating for longer durations and at higher temperatures. Where heating the SAW sensor is insufficient, conventionally they may be washed with solvents such as acetone. To the extent a solvent wash is also insufficient, the SAW sensor may require replacement.
Accordingly, there is a need in the art for an improved, uncoated SAW sensor that is not susceptible to chemical adsorption of the analyzed chemical compound sample. There is also a need in the art for such SAW sensors with both short term and long term stability.