To control indoor air quality, a novel freon gas sensor of piezoelectric microcantilever coated with zeolite has been developed in this paper. Excited by an ac voltage, the microcantilever is employed to detect the concentration of sample freon-12 gas ranged from 0 to 100 ppm by the effect of the specific MFI zeolite modification. High selectivity and sensitivity combined with excellent repeatable and reversible performances are shown. The relationship between the frequency shift in percent and the concentration of freon gas is linear. The minimum mass changing of 3.5×10−9 g and the sensitivity of −0.0024%/ppm are determined from the experimental results.
A micro-total analysis system (μ-TAS) or a “Lab-On-Chip” for integrated chemical and biochemical analysis has grown dramatically in the past decade. The concept extends the scope since its introduction by Manz at Transducers’89 and now encompasses analysis and synthesis for applications ranging from chemistry through to biology. By using a high degree of parallelism in the designs it has become clear that automation of high sample throughput is possible. Meanwhile, an enormous amount of researches have been devoted to the development to miniaturize chemical and biochemical sensors, which has great effect on expanding the application fields, such as quality and process control, disposable diagnostic biosensor for medical analysis, fragrance design, oenology, and as sensing devices for gaseous analytes. However, the miniaturization of chemical sensor is of great complexity, functionality and compactness, and the sensitivity of the device and consequently the analytical power should be enhanced. Therefore, it is important to find a sensor with a high sensitivity and easy to be miniaturized and mass-produced.
A microcantilever in resonating mode has attracted intensive interest these years on account of its much higher sensitivity than the classical methods [1], [2] and [3]. Cantilevers used as nanoscale sensors for atomic force microscope (AFM) have recently been extended beyond those of a surface-imaging tool. The masses of such sensors are typically in nanogram range, thus enabling short response time (milliseconds) and high sensitivity well beyond what is achievable with standard techniques. Such micromechanical sensors have been configured to be used as calorimeters and surface stress sensors. A piezoelectric microcantilever in resonating mode, which shifts its resonance frequency due to mass loading, has shown very high sensitivity for sensing chemicals and has broad and potential application areas related to environmental control [4], artificial nose [5], drug discovery [6], etc. Air quality is one aspect of environmental control. With the extensive use of air conditioner, problems with freon gas emission have increased in recent years. Rapidly and accurately determining the concentration of freon gas plays an important role on air quality control and assay, especially on indoor air quality control.
Although cantilever presents high sensitivity, in order to recognise different specific chemicals, improving the sensor’s selectivity becomes one of the critical factors. A great many researches of sensing materials have been done to increase selectivity, including polymer [7] and [8], metal [3], oxide [4] and so on. Zeolites are the subject of intense interest as chemical sensors and as advanced materials. The nano-sized channel system of the zeolites provides a size and shape-selective matrix for absorbed molecules while maintaining a high surface-to-mass ratio. Microcomposite coatings with molecular sieving properties have been deposited on surface acoustic wave devices [9], quartz microbalances [10], and zeolite composites.
For the first time, a novel piezoelectric microcantilever sensor with a layer of microporous material zeolite has been fabricated and used to determine the freon-12 gas in this paper. Because of the effect of zeolite, high selectivity has been obtained simultaneously in addition to intrinsic high sensitivity of the piezoelectric microcantilever in resonance. A linear relationship between the frequency shift of the microsensor and the concentration of freon-12 gas is also obtained. The sensitivity of microsensor to the response of freon gas is −0.0024%/ppm and a minimum detectable mass loading of 3.5×10−9 g is exhibited when the specific zeolite MFI sensitive to freon-12 gas is applied. Very good repeatability and reversibility of the microcantilever sensor are also presented after the deposition of zeolite, which represents the very effective combination from another point of view.
Moreover, this paper demonstrates the potential applications for the combination of zeolite and microcantilever not only in the gas sensor field, but in all chemical and biological sensor filed if only appropriate zeolite available.
