Head to your local colleges to work with professors in the proper disciplines to create a study and have students do their thesis on the parts you need. Then you'll have the designs good to go.
Either that or try research on the web.
I want to create a system which will be monitoring the environmental air condition. i want to monitor Carbon monoxide(Co), dust and Sulphur dioxide(SO2). What are the key components which I should have? Anyone with sample circuits pls!.