The Optical Properties of MTPyP Film for HCl Gas Sensor

本研究主要是提出一種在玻璃基材上簡單製備meso-tetra(4-pyridyl)porphyrin (MTPyP or H2TPyP)薄膜的方法，稱之為浸沾式乾燥塗佈法(dip-dry coating)。此MTPyP紫質分子的感測機制為當暴露於酸性氣體時，其大環中心的兩個氮原子與周圍的四個取代基可吸附氫離子H+，會造成紫外可見光吸收光譜改變，因此可應用於感測酸性氣體。而在本研究中，選擇以鹽酸液作為酸性氣體的來源。首先，以不同比例的甲苯與酒精混合液，1%、10%、30%、50%的甲苯溶劑來配置MTPyP溶液，經光譜儀量測發現當以濃度為30%甲苯溶液所配製的MTPyP溶液，在427nm(即Soret band)的吸收光度值最高，並確立以此濃度比例的甲苯溶液來製備MTPyP薄膜。接著重複同樣的製膜方法，以逐層累積的方式完成沉積六次的MTPyP薄膜製備，並與沉積單次的MTPyP薄膜比較其對鹽酸氣體的響應程度。 本研究發現，沉積六次的MTPyP膜對於HCl氣體的感測效果相較於沉積單次的MTPyP薄膜較佳，之後便選擇沉積六次的MTPyP薄膜作為HCl氣體的感測薄膜。接著將之用於0.064、0.64、3.2、4.8ppm等不同濃度的HCl氣體感測實驗中，結果發現吸收光譜在到達穩態時491nm的吸收值與未暴露在HCl氣體時之491nm的吸收值間差值(ΔA491)之於HCl氣體濃度的關係，呈現近乎線性的關係，且對於HCl氣體可偵測極限達0.064ppm，可見其應用於HCl氣體高靈敏感測的應用價值。 此外，本研究亦對此MTPyP薄膜進行其他常見的高揮發性液體感測實驗，觀察此MTPyP薄膜暴露於非鹽酸氣體時的吸收光譜變化，目前進行了丙酮、酒精及甲苯等三種氣體的測詴，氣體濃度分別為163ppm、186ppm及99ppm，當MTPyP薄膜暴露於以上三種氣體時，其ΔA491值約為0.001，相較於MTPyP薄膜暴露於0.064ppm這樣低濃度的HCl氣體，其ΔA491值達0.004。由此可知，MTPyP薄膜用於感測丙酮、酒精及甲苯等三種氣體時，展現極低的靈敏度。此外本研究亦將薄膜用於感測其他常見之工業用酸，如硫酸及磷酸，但由於兩者在室溫下皆屬於不易揮發之液體，所以在吸收光譜的量測結果上，並無顯著的改變。The work presented a simple way to prepare a meso-tetra (4-pyridyl) porphyrin (MTPyP) film for optical gas sensor called dip-dry coating method. The MTPyP molecule of inner nitrogen core of porphyrin ring and the four substituents at the meso positions could conjugate hydrogen ions leading to spectral changes. Therefore, MTPyP molecule could be applied to detect the acid gases. In our study, hydrogen chloride (HCl) gas was chosen to test response of the optical sensor. The MTPyP powder was dissolved in 1%, 10%, 30%, and 50% toluene solutions those were a mixture of ethanol, respectively. The MTPyP solution in 30% toluene solvent shows a stronger absorption band at 427nm than the other MTPyP solutions. Hence, we decided to use MTPyP solution in 30% toluene solution to prepare MTPyP films in our research. The MTPyP multilayer thin films were made by repeating the dip-dry coating method six times which was used to detect HCl gas compare to the one cycle MTPyP film. From the results, the six cycles films shows the better ability to detect HCl gas. Therefore, the six cycles films were chosen to detect the different concentration of 0.064, 0.64, 3.2, 4.8 ppm HCl gas in our study. A linear response was observed when the concentration of HCl was plotted against a difference in absorbance(ΔA491). And the detection limit of the solid state sensor was found to be 0.064ppm. The MTPyP thin film shows good sensitivity to HCl gas. In addition, the MTPyP thin films were used to detect other gases which were 163ppm toluene, 186 ppm acetone, and 99ppm ethanol gases. The values of ΔA491 were approximately 0.001. However, the values ofΔA491 were up to approximately 0.004, when the films exposure to 0.064ppm HCl gas whose the concentration is very low compare to toluene, acetone, and ethanol gases. According to the results, the v MTPyP film shows low sensitivity to high concentrations of toluene, acetone, and ethanol gases. In our study, the MTPyP films were also used to detect other common acid gases, such as H2SO4 and H3PO4. But there are no significant changes in the absorption spectra of MTPyP thin films. The reasons to cause these results are H2SO4 and H3PO4 with low values of vapor pressure that are not easy vaporizing to atmosphere