**Acknowledgements**

comprised two strands of DNA whereby each of the strand has G-rich sequence forming DNAzyme (1) and (2) and a T-containing site that functions as the recognition site of Hg2+, (3) and (4). As the T-containing sites of (3) and (4) were partially complementary, (1) and (2) could not assemble to form a G-quadruplex. However, in the presence of Hg2+, the T-Hg2+-T complexes formed between the T-containing sites of (3) and (4) and formed a duplex structure. Therefore, this led to the formation of the hemin/G-quad complex between (1) and (2). The hemin/G-quad complex catalyzed the oxidation of luminol in the presence of H2O2, generating chemiluminescence signal. Then, this signal acted as an internal light source that can excite CdSe/ZnS QDs, resulting in CRET to the QDs. Then, this stimulat‐

ed the luminescence signal from QDs [51].

132 Nucleic Acids - From Basic Aspects to Laboratory Tools

**8. Conclusion**

**Figure 8.** Detection of Hg2+ through CRET from luminol that is oxidized by DNAzyme to QD.

from small drug molecules, chemical compounds to large biomolecules.

In comparison with conventional reporter systems, G-quad-based systems provide many advantages in terms of cost, thermostability and ease of synthesis. With all these advantages, it makes them very useful for the development of sensing probes for diagnosis, applicable to DNA, protein and metal detection. The vast application of G-quad structures to generate various forms of readouts ranging from colorimetric to electrochemical-based readouts puts it at the forefront of sensing reporters. G-quad-based assays are flexible as they can be adapted to many different types of diagnostic platforms. This makes G-quad an attractive alternative for the development of sensitive reporter systems for the sensing of various samples ranging

The authors would like to acknowledge funding from the Malaysian Ministry of Education under the Fundamental Research Grant Scheme (Grant No. 203/CIPPM/6711473).
