Circular RNAs (circRNAs) represent single-stranded RNA species that contain covalently closed 3′ and 5′ ends that provide them more stability than linear RNA, which has free ends. Emerging evidence indicates that circRNAs perform essential functions in many DNA viruses, including coronaviruses, Epstein–Barr viruses, cytomegalovirus, and Kaposi sarcoma viruses. Recent studies have confirmed that circRNAs are present in viruses, including DNA and RNA viruses, and play various important functions such as evading host immune response, disease pathogenesis, protein translation, miRNA sponges, regulating cell proliferation, and virus replication. Studies have confirmed that circRNAs can be biological signatures or pathological markers for autoimmune diseases, neurological diseases, and cancers. However, our understanding of circRNAs in DNA and RNA viruses is still limited, and functional evaluation of viral and host circRNAs is essential to completely understand their biological functions. In the present review, we describe the metabolism and cellular roles of circRNA, including its roles in various diseases and viral and cellular circRNA functions. Circular RNAs are found to interact with RNA, proteins, and DNA, and thus can modulate cellular processes, including translation, transcription, splicing, and other functions. Circular RNAs interfere with various signaling pathways and take part in vital functions in various biological, physiological, cellular, and pathophysiological processes. We also summarize recent evidence demonstrating cellular and viral circRNA’s roles in DNA and RNA viruses in this growing field of research.

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An easy and effective strategy for synthesizing a ratiometric fluorescent nanosensor has been demonstrated in this work. Novel fluorescent BSA-AuNPs@Tb-AMP (BSA, bovine serum albumin; AMP, adenosine 5'-monophosphate; AuNPs, Au nanoparticles) metal-organic framework (MOF) nanostructures were synthesized by encapsulating BSA-AuNPs into Tb-AMP MOFs for the detection of 2,6-pyridinedicarboxylic acid (DPA) and Hg . DPA could strongly co-ordinate with Tb to replace water molecules from the Tb center and accordingly enhanced the fluorescence of Tb-AMP MOFs. The fluorescence of BSA-AuNPs at 405 nm remained constant. While the fluorescence of BSA-AuNPs at 635 nm was quenched after Hg was added, the fluorescence of Tb-AMP MOFs remained constant. Accordingly, a ratiometric fluorescence nanosensor was constructed for detection of DPA and Hg . The ratiometric nanosensor exhibited good selectivity to DPA over other substances. The F /F linearly increased with increase of DPA concentration in the range of 50 nM to 10 μM with a detection limit as low as 17.4 nM. F /F increased linearly with increase of Hg concentration ranging from 50 nM to 1 μM with a detection limit as low as 20.9 nM. Additionally, the nanosensor could be successfully applied for the determination of DPA and Hg in running water.

A ratiometric fluorescence sensor for Cu 2+ detection was developed by employing carbon dots (CDs) as the reference fluorophore, and bovine serum albumin (BSA)-Au nanoclusters (AuNCs) both as specific recognition element and as response signal.The CDs were prepared by heating an acetic acid solution of chitosan at 180°C. The proposed nanoprobe was prepared by simply mixing the CDs and BSA-AuNCs. Here, the CDs were associated with BSA-AuNCs to form CDs/BSA-AuNCs nanoprobes based on electrostatic interaction and hydrogen bonds. In the hybrid fluorescent CDs/BSA-AuNCs nanoprobe, the emission of BSA-AuNCs at 640 nm was quenched by Cu 2+ , whereas the emission of CDs at 424 nm was maintained. The fluorescence changes of I 640 /I 424 had good linearity with Cu 2+ concentration in the range 1.5-600 nM, and the detection limit of the nanoprobe was 0.5 nM for Cu 2+ detection, almost 4 × 10 4 times lower than the maximum allowable amount of Cu 2+ in drinking water. The proposed method for detecting Cu 2+ also had high selectivity and feasibility.

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