**3.8 Carbon nanodots**

*Novel Nanomaterials*

**3.7 DNAzyme**

of most known DNAzymes [88].

tions of metal ions difficult to distinguish [89].

but also an excellent biological functional material [93].

future cellular and portable detection technologies.

rates of amplification and molecular weights [84].

Compared to HCR, this reaction consists of more complex components, including a trigger sequence, two double stranded substrates with bridging loops in the middle, and two helper sequences [83]. Thus, non-linear HCR can achieve higher

Expanding the application of DNA nanomaterials to bioassays in the future may

DNAzymes are single-stranded (ss) DNA sequences are able to catalyze a number of reactions, including cleavage of the phosphodiester backbone at a ribonucleotide or deoxyribonucleotide site [87]. It has been shown that metal ions play an important role in the catalytic process and are essential for the catalytic activity

The ability to select a DNAzyme with metal ion specific activity without previous chemical knowledge of the DNAzyme structure, and then to subsequently modify DNAzyme binding arms and other insignificant nucleotides with minimal to no effect on sensitivity and selectivity has made DNAzymes ideal metal-selective components for new metal ion sensing technologies. RNA-cleaving DNAzyme is a very useful biomaterial for the determination of metal ions, but some parts of DNAzymes can be cleaved by several metal ions, which makes different concentra-

In the last two decades, the rapid development of nanomaterials and biomaterials [90] offers more opportunities to improve electrochemical sensor performance. For the determination of Cu (II) and Hg (II), many highly sensitive sensors are manufactured using small molecules, peptides, proteins and antibodies at low cost. The ligand sites of proteases composed of nitrogen, oxygen or sulfur can combine with heavy metal ions to form a stable complex [91]. Cu(II) is a small ion that has to be chelated first and then bind to the antibody recognition [92]. Both antibody and enzyme work best under physiological conditions that limit application in real environment. DNA is not only the genetic material of most living organisms,

Metal ions can be specifically bound with a single-stranded DNA to form a stable metal-mediated DNA, and this mechanism is applied to detect metal ions [94, 95]. Therefore, numerous studies have focused on the newly discovered biosensor using different DNA-based aptamers functionalized with nanomaterials to increase sensitivity. DNAzymes that break down RNA as DNA-based catalysts are obtained through in vitro selection, which turned out to be a very useful platform for the identification of metal ions. After binding with heavy metal ions, many biochemical and biophysical studies have been conducted on DNAzymes due to their high metal ion selectivity and high catalytic efficiency [96]. Therefore, DNAzymes have been applied in various biosensors (colorimetric, electrochemical and fluorescent) that realize the detection of various metal ions such as Mg(II) [97], Ag(I) [98], Pb(II) [99], Zn(II) [100], Hg(II) [101],

The field of DNAzyme-based metal ion sensing is continuing to develop for

To join non-linear HCR and Y-DNA nanostructures, the Y-DNA's terminals were designed as triggers that could initiate the amplification reaction. As a result, the new biosensing method can provide high-precision and selective detection of biological molecules. An unlabeled DNA nanostructured electrochemical biosensor was designed to detect miRNA-25, which is reported to be a potential molecular

biomarker for non-small cell lung cancer and heart failure [85, 86].

enable early and effective detection of various diseases.

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UO2(II) [102].

Carbon dots (CDs) are nanomaterials less than 10 nm in size and became the new potential material for the electrode modifier [103]. Formerly, CDs have been applied in electrochemical sensing platforms, mainly focusing on their electrocatalytic properties toward analytes of interest [104, 105] rather than electrode modifiers. Thus, the studies on carbon dots owing a noticeable potential to be used as electrode modifiers in electrochemical techniques to increase the sensitivity of the electrochemical sensor has been exploited.

Recently, a new member of CDs, have gained attention because of their water solubility, fine properties, high luminescence, low cytotoxicity and good conductivity [106]. Depending on the precursors employed in their synthesis, CNDs are surrounded by different functional groups including, among others, hydroxyl, amide groups and carboxyl which facilitate the immobilization of biomolecules. Hence, due to their ability to be modified with a wide variety of biomolecules, and in conjunction with the excellent properties mentioned above, CNDs have been employed in many biological applications such as solar cell development and photocatalysis [107, 108]. Concerning the employment of CNDs for electrochemical biosensors, it should be highlighted that despite the previously mentioned advantages, very few attempts to incorporate CNDs into electrodes are reported. Reporting the application of CNDs in electrochemical sensors are focused on the electrocatalytic properties of this nanomaterial toward oxygen reduction [109], biomedical application [110], exploited for glucose biosensing [111] and DNA sensing [112].

Transmission electron microscopy (TEM) of carbon nanodots in different scale from 20 nm to 2 nm are illustrate in **Figure 7** [110].

**Figure 7.** *High resolution transmission electron microscopic images of fish scale derived carbon nanodots (a-c).*
