According to the central dogma of molecular biology, deoxyribonucleic acid (DNA) is the central carrier of genetic information in living organisms. The data encoded within DNA can be transcripted and translated into a variety of functional and structural proteins that are essential for life. The detection of specific DNA sequences provides the fundamental basis for monitoring a wide variety of genetic diseases, viral infections, and infectious diseases. DNA sensors based on nucleic-acid recognition processes are rapidly being developed towards the goal of rapid and inexpensive testing of various diseases. In general, DNA sensors rely on the analyte-induced conformational change of a functional oligonucleotide, sometimes referred to as “structure switching”. The DNA thus plays the role of the "receptor unit", and its conformational change upon analyte binding can be transduced into an electrochemical, colorimetric, or luminescent output. Transition metal complexes are widely used in the field of DNA sensing by virtue of their excellent properties, which depend mainly on the output of optical signals. Transition metal complexes can bind DNA backbone through intercalation, noncovalent groove binding, and electrostatic and some other types of interactions.
The transition metal complexes possess the following excellent properties and can be utilized for the development of DNA-based luminescence sensors.
(1) They have the ability to bind strongly but reversibly to one particular conformation of DNA, and with low affifinity to other nucleic acid species that may be present in the sensing assay.
(2) The photophysical properties of the metal complex change upon binding to the target DNA structure, so that the structure-switching response of the nucleic acid can be conveniently monitored.
(3) They are easy to prepare, less toxic and less costly.
Nowadays, transition metal complexes show potential and have been successfully applied for DNA-based sensing. A few simple examples are given below.
Gupta and co-workers  reported square planar Pt (II) complexes as luminescent biosensors for DNA detection in solution. The sensing is attributed to the aggregation induced bright red photoluminescence (AIPE) of the complexes in the presence of DNA that can be seen with the naked eye using only a 360 nm light source.
Figure 1. Schematic representation of Pt (II) complexes as luminescent biosensors for the detection of DNA
Liu and co-workers  synthesized Iridium (III) complex with carboxyl group and activated with N-hydroxy succinimide, followed by tagging to the amino terminate of single-stranded DNA (ssDNA). The Ir-ssDNA probe was further combined with graphene oxide (GO) nanosheets to develop a GO-based biosensor for target ssDNA detection. Based on the high luminescence quenching efficiency of GO toward iridium (III) complex, the GO-Ir-ssDNA biosensor exhibited minimal background signals, while strong emission was observed when Ir-ssDNA desorbed from GO nanosheets and formed a double helix with the specific target, leading to a high signal-to-background ratio.
Figure 2. Scheme for the mechanism of GO-based biosensor for the DNA detection
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- Gupta, P.; et al. Luminescent terpyridine appended geminal bisazide and bistriazoles: multinuclear Pt (II) complexes and AIPE-based DNA detection with the naked eye. Dalton Trans. 2021, 50: 10225-10236.
- Liu, J.; et al. Luminescent iridium (III) complex labeled DNA for graphene oxide-based biosensors. Anal. Chem. 2016, 88: 1892-1899.