Advances in amplification-free detection of nucleic acid: CRISPR/Cas system as a powerful tool

Amplification technologies such as polymerase chain reaction (PCR) play an important role in nucleic acid detection. However, they require bulky and sophisticated thermal cycling instrument, as well as are prone to get false-positive results due to amplicon contamination. Currently, CRISPR/Cas system has become an increasingly popular diagnostic tool for nucleic acid with the discovery of its trans-cleavage activity which can degrade single-stranded DNA or RNA at a very high turnover rate. This inherent signal amplification capability allows CRISPR/Cas system to detect unamplified nucleic acids. Here, we reviewed the recent advances of CRISPR-based amplification-free methods for nucleic acid detection. With the assistance of various signal enhancement strategies, the detection sensitivity could be comparable to that of amplification-based methods. We then presented the pros and cons of these methods. And the subsistent challenges including sample preparation, off-target effect, sequences limit, quantitative and multiplex detection were further discussed in this review. It is probable for CRISPR-powered detection methods to pave the road for rapid, cheap, highly sensitive and specific on-site detection without amplification.