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Glutathione (GSH) levels are closely related to the homeostasis of redox state which directly affects human disease occurrence by regulating cell apoptosis. Hence, real-time monitoring of dynamic changes in intracellular GSH levels is urgently needed for disease early diagnosis and evaluation of therapy efficiency. In this study, an endogenous cysteine (Cys)-assisted detection system based on GSH@AgNCs and reduced graphene oxide (rGO) with high sensitivity and specificity was developed for GSH detection. Compared with GSH, GSH@AgNCs with weaker affinity and bonding force was quite easier to extrude from the rGO surface when competing against GSH, leading to the obvious change in fluorescence signal. This phenomenon was termed as “a crowding out effect”. Furthermore, the presence of Cys can improve GSH assay sensitivity by enhancing the quenching efficiency of rGO on the GSH@AgNCs. In vitro assay indicated that the efficiency of fluorescence recovery was positively related with GSH concentration in the range from 0 to 10 mM. In addition, the method was employed for real-time monitoring of the dynamic changes in GSH levels regulated by natural drugs. The imaging results showed that the natural compound 3(C3) can downregulate GSH levels in HepG2 cells, which was accompanied by reactive oxygen species (ROS) release and apoptosis induction. Finally, the method was used to monitor the change of GSH levels in serum samples with chronic hepatitis B (CHB) infection. The results demonstrated that the occurrence and development of CHB may be positively correlated with GSH levels to some extent. Overall, the above results demonstrate the potential application of this new nanosystem in anticancer natural drug screening and clinical assay regarding GSH levels.

As a highly conserved damage repair protein, RNase H can specifically hydrolyze RNA in DNA–RNA chimeric strands. DNAzyme, a synthetic single-stranded DNA consisting of binding and catalytic sites, can cleave RNA in the presence of cofactors. In this study, we establish a highly sensitive RNase H assay assisted with DNAzyme’s cleavage property. A DNA–RNA chimeric strand, which contains DNAzyme sequences, is used as the hydrolysis substrate of RNase H. The RNase H hydrolysis of the chimeric substrate results in the release of DNAzyme. Subsegment DNAzyme digest, a molecular beacon, causes a “turn-on” fluorescence signal by disrupting its hairpin structure. Furthermore, the fluorescence signal is amplified by cyclic digestion of DNAzyme to the substrate of molecular beacon. Under the optimal conditions, the detection limit of RNase H is 0.01 U/mL, which is superior to those of several alternative approaches. Additionally, the method was further used for RNase H detection in heterogeneous biological samples as well as to investigate the effects of natural compounds on this enzyme. In summary, these results show that the method not only provides a universal platform for monitoring RNase H activity but also shows great potential in biomedical studies and drug screening.

In addition to being an important object in theoretical and experimental studies in enzymology, RNase A also plays an important role in the development of many kinds of diseases by regulating various physiological or pathological processes, including cell growth, proliferation, differentiation, and invasion. Thus, it can be used as a useful biomarker for disease theranostics. Here, a simple, sensitive, and low-cost assay for RNase A was constructed by combining a fluorogenic substrate with reduced graphene oxide (rGO). The method with detection limit of 0.05 ng/mL was first applied for RNase A targeted drug screening, and 14 natural compounds were identified as activators of this enzyme. Then, it was applied to detect the effect of drug treatment and Hepatitis B virus (HBV) infection on RNase A activity. The results indicated that RNase A level in tumor cells was upregulated by G-10 and Chikusetsusaponin V in a concentration-dependent manner, while the average level of RNase A in the HBV infection group was significantly inhibited compared with that in the control group. Furthermore, the concentration-dependent inhibitory effect of heavy metal ions on RNase A was observed using the method and the results indicated that Ba2+, Co2+, Pb2+, As3+, and Cu2+inhibited RNase A activity with IC50values of 93.7 μM (Ba2+), 90.9 μM (Co2+), 110.6 μM (Pb2+), 171.5 μM (As3+), and 165.1 μM (Cu2+), respectively. In summary, considering the benefits of rapidity and high sensitivity, the method is practicable for RNase A assay in biosamples and natural compounds screening in vitro and in vivo.