*2.2.1. G4-wires DNA as nanowire*

Nanowires known as G4-wires [34] (or quadruplexes), consist of stacked guanine (G) tetrads (G4). These one-dimensional polymers act as prospective candidates for bio-molecular electronics because, due to the low ionization potential of guanine (the lowest among nucleicacid bases), they might be suitable to mediate charge transport by hole conduction along the helix, and have even been suggested as nano-mechanical extension-contraction machines [35]. In the presence of appropriate metal cations (especially K<sup>+</sup> and Na<sup>+</sup> ), solutions of homoguanylic strands in water [36, 37] as well as lipophilic guanosine monomers in organic solvents [38], self-assemble in right-handed quadruple helices. Recent investigations has been carried out for the G4-nanowires, but the conduction properties of these nanowires are basically unknown and a direct measurement of electrical properties of G4-wires is still missing.

complex samples. By incorporation of signal reporters such as chromophores, fluorophores, electrochemical tags, and Raman tags, these metal-ion-specific DNA sequences have been transformed into colorimetric, fluorescent, electrochemical, and Raman sensors and imaging agents for a broad range of metal ions with high sensitivity and selectivity [50–55]. DNAzymes that is highly selective to use specific metal ions as cofactors to catalyze reactions can be obtained. In this way, DNAzymes that are dependent on bivalent metals for various chemical and biological reactions have been successfully discovered. One report of DNAzyme sensor was a fluorescent sensor for Pb2+ based on DNAzyme [56–58], which showed much higher specificity to Pb2+ over other metal ions in catalyzing the cleavage of DNA substrates with a

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single RNA linkage (rA) at the cleavage site.

These sensors can be further classified into different parts:

**3.** Surface-immobilized fluorescent sensors [59]

**4.** Label-free fluorescent sensors [60–62]

sensors based on C–Ag<sup>+</sup>

*2.2.3. DNA-electrochemical biosensors*

**11.** Ag<sup>+</sup>

by all.

**12.** Sensors for K<sup>+</sup>

**14.** Portable sensors [71]

**1.** Fluorescent sensors based on metal ion-dependent DNAzymes

**2.** Fluorescent sensors labeled with fluorophores and quenchers [59]

**5.** Colorimetric sensors based on metal ion-dependent DNAzymes

**7.** Colorimetric "dipstick" tests using lateral-flow devices [65, 66]

**8.** Electrochemical and Raman sensors based on metal ion-dependent DNAzymes [67]

–C-containing DNA

**13.** Combination based sensors (DNAzymes and metal-binding DNA structure) [70]

In this category, new technologies have been developed to design sensors based on commercialized devices, compatible with portable devices, which could enable to monitor metal ions

DNA biosensors are the integrated receptor-transducer devices that use DNA as biomolecular recognition element to measure specific binding processes with DNA, by electrical, thermal or optical signal transduction methods. The characteristics of DNA probes with the capacity of direct and label-free electrochemical detection find applications in rapid monitoring of

based on G-quadruplex DNA

**6.** Colorimetric sensors based on gold nanoparticles [63, 64]

**9.** Sensors based on metal binding structures [68, 69] **10.** Hg2+ sensors based on T–Hg2+–T-containing DNA

, Pb2+, Cu2+, and Ag<sup>+</sup>
