4.1 Environmental detection
Many chemical pollutants exist in the environment, which pose some serious threats because they are invisible or hard to spot. The most common harmful substances are heavy metals, such as arsenic and mercury[50], as well as toxic small molecules and antibiotics [51, 52]. The environmental protection agency (EPA) stipulates that some harmful pollutants cannot exceed a certain level in the environment, so it is necessary to test the present level of these substances in the environment. A number of cell-free biosensors have been developed and designed to detect the presence or content of hazardous substances in environmental samples.
When inorganic heavy metals enter the body from the environment, they accumulate in certain organs of the body, cause chronic poisoning, and harm health. Researchers have designed a number of cell-free biosensors for detecting heavy metals. For example, a paper-based mercury biosensor was designed by the iGEM team Peking 2010 [53]. By adjusting the plasmid concentrations and adding new translation enhancement sequences to optimize the cell-free mercury sensor, it increased the yield of the CFPS reaction and enabled accurate quantification of the analytes. It can detect 6 µg/L of mercury concentration. The paper-based cell-free biosensor has a low cost and is convenient for detection. In addition, the team built a 3D-printed box that can be connected to a phone for real-time readings, extending the detection platform to real-time applications.
There are also some cell-free biosensors designed to detect toxic organic molecules. For instance, Khalidet al . [53] developed a cell-free biosensor platform, a ligand-induced activated RNA output sensor (ROSALIND), which uses a variety of TFs to detect harmful pollutants in water samples, such as toxic PPCP compounds (salicylate, benzalkonium chloride and uric acid). However, when testing non-laboratory samples (e.g., drinking water and lake water), the sensitivity of the sensor may be reduced due to the interference of the molecular components in the samples. Therefore, the matrix effect of these test samples needs to be further explored in future work.
The presence of antibiotics in the environment is harmful to the ecological balance and human health. Therefore, its presence in the environment needs to be monitored in a timely manner. Duyen et al . [54] have developed a simple biological sensor based on color paper, inhibition of bacteria used in the detection of protein synthesis of antibiotics. The detection limits of paramycin, tetracycline, erythromycin, and chloramphenicol were further estimated to be 0.5, 2.1, 0.8, and 6.1 μg/mL. Pellinen et al . [55] designed a novel cell-free biosensor for the detection of tetracycline hydrochloride (Tc-HCl). Tc-HCl has been detected at concentrations below 10 ng/ml. The maximum residue limit for Tc-HCl in EU milk samples is 100 ng/ml, and the limit for Tc-HCl in commercial microbial inhibition tests is 30 to 500 ng/ml. It can be seen that the detection limit of cell-free biosensors can reach the ppt level, which has reached the environmental standard level for the detection of some pollutants. The test results can be used to judge whether the samples under test meet the standards or not. Although the sensitivity of current cell-free biosensors can reach a certain level, some substances with lower detection limits still exist or will appear in the future, which requires us to continuously improve and optimize the cell-free biosensors.
These examples demonstrate that cell-free biosensors have been successfully applied to the detection of toxic substances in the environment. Due to its low detection limit and fast and convenient characteristics, cell-free biosensors can quickly detect pollutants, determine the safety limit of analysis samples, and timely monitor the environment for timely prevention or repair. Through the continuous optimization of cell-free biosensors, it has a broad prospect in environmental monitoring.