Fig. 1. The workflow of cell-free biosensors.
Fig. 2. Recognition-response mechanisms of cell-free biosensor
systems. (A) After the ligand is identified, reporter genes can be
activated by binding activated transcription factors or releasing
inhibitory transcription factors. Unknown ligands can be converted into
detectable ligands by using metabolic enzymes before the identification.
(B) Cell-free biosensors use NASBA-CRISPR/Cas9 cleavage system to
identify different strains of virus. (C) Nucleic acid ligands are used
to identify thrombin. When thrombin is absent, the reporter protein is
successfully expressed, but the translation is blocked in the presence
of thrombin. (D) When samples containing amino acids are added,
incomplete CFPS is activated, leading to successful expression of the
reporter protein.
Fig. 3. Signal output of cell-free biosensor systems. (A) The
reporter gene gfp is expressed in the CFPS system and output in
the form of optical signal. (B) The reporter gene luc is
expressed in the CFPS system and output in the form of optical signal.
(C) The reporter gene lacZ is expressed in a paper-based
cell-free system, and the color change on the measured paper can be
easily detected by the naked eye. (D) The biosensor based on the
cell-free genetic circuit captures the analyte through the
nanostructured microelectrode to generate the electrochemical signal
output.