The invention relates to a sensor device for detecting CO.sub.2 wherein the said device has (a) a support base, (b) an acid-base indicator composition that stably changes colour upon exposure to CO.sub.2; and (c) a viewer cover, and wherein least (a) and (c) is CO.sub.2 permeable and wherein (b) includes at least one acid-base indicator that has a pK.sub.ln of or above 9.5 and (b) is disposed between (a) and (c) such that the colour change of (b) can be observed through (c). The sensor device of the present invention may be utilized to monitor the ripening process of fresh produces such as fruits, the fermentation or spoilage of food, and/or activity of micro-organisms such as soil and root nodule bacteria in soil.