Progress in the biosensing techniques for trace-level heavy metals. 
Diverse classes of sensors have been developed over the past few decades for on-site detections of heavy metals.  Most of these sensor systems have exploited optical, electrochemical, piezoelectric, ion-selective (electrode), and electrochemical measurement techniques.  As such, numerous efforts have been made to explore the role of biosensors in the detection of heavy metals based on well-known interactions between heavy metals and biomolecules (e.g. proteins, peptides, enzymes, antibodies, whole cells, and nucleic acids).  In this review, we cover the recent progress made on different types of biosensors for the detection of heavy metals.  Our major focus was examining the use of biomolecules for constructing these biosensors.  The discussion is extended further to cover the biosensors' performance along with challenges and opportunities for practical utilization.