Sensitive detection of trace level Cd (II) triggered by chelation enhanced fluorescence (CHEF) 'turn on': Nitrogen-doped graphene quantum dots (N-GQDs) as fluorometric paper-based sensor

Cadmium ion (Cd (II)) is a highly toxic heavy metal usually found in natural water. Exposure to Cd (II) can produce serious effects in human organs such as Itai-Itai disease. Therefore, the maximum allowance levels of Cd (II) in drinking water and herbal medicines imposed by the World Health Organization (WHO) are 3 μg L