**4.2 Organometallic complexes**

Organometallic complexes consist of centrally located metal atoms or ions and completely or partly coordinated organic ligands. The organometallic complexes with transition metals have the advantages of strong redox signal, good chemical

stability, low toxicity, and high structural flexibility. They interact with biomolecules via the Intermolecular interaction force and electrostatic interaction.

Ferrocene (Fc), is a yellow organometallic complexes with transition metal (Fe) and aromatic ligands (cyclopentadiene rings). Because Fc has two freely rotating cyclopentadiene rings, it can be labeled with the biomolecules, such as DNA via hydrophobic interactions. As an electrical signal molecule, in the combination of bio-receptors and target molecules, Fc generates electrical signals mainly by adjusting the distance between the Fc and the electrode surface to realize the change of electrical signal and achieve the purpose of detection.

K3[Fe(CN)6]/K4[Fe(CN)6], is a pair of dyes with bright red and yellow color, respectively. Mainly, they are used as electron transfer agents in amperometric biosensors, to replac the natural electron transfer agents of the enzymes. In the commercial blood glucose meters, the glucose in the blood reacts with glucose oxidase and K3[Fe(CN)6] fixed on the surface of the test strip to produce gluconic acid and K4[Fe(CN)6]. Applying a constant working voltage to the test strip, K4[Fe(CN)6] is oxidized to K3[Fe(CN)6], generating an oxidation current that is proportional to the glucose concentration.

#### **4.3 Nanomaterials**

#### *4.3.1 Quantum dots*

One of the most commonly used electrochemical biosensor is cadmium selenide (CdSe) QDs, which employ as electrical signal molecules for the labeling of nucleic acid strands [86]. The Pb2+ cleavage ribozyme sequence was modified on the surface of the magnetic beads, and designed an electrochemical biosensor for detecting Pb2+ by using rolling circle amplification reaction and a signal probe labeled with CdS QDs [87]. Based on Ni2+ cleavage ribozyme and CdSe QDs, The Ni2+ was detected and the detection limit was 6.67 nmol/L [88]. As electrical signal molecules, QDs have versatility and low background signal, which has great application prospects.

#### *4.3.2 Graphene quantum dots*

Graphene quantum dots (GQDs) are actually sheets of graphene with dimensions less than 100 nm with sp2 hybridized honeycomb structures, and their shapes are mostly circular and elliptical, but square and hexagonal QDs are also available. Basically, GQDs are characterized as graphene-like, consisting of C, O, and H as well as carbonyl, carboxyl, hydroxyl, and epoxy groups. GQDs can bind to ssDNA through π-π interactions, but it has no such effect on double-stranded DNA. Park et al. used GQDs as electrical signal substances to detect the Hg2+ concentration by measuring the current generated during the electrochemical reduction of GQDs [89].

#### *4.3.3 Metal–organic frameworks*

Metal–organic frameworks (MOFs) are crystalline materials with an infinitely regular and infinitely expanding periodic network structure formed by the selfassembly of metal ions and organic ligands through coordination bonds, covalent bonds, and weak intermolecular bonds (π-π stacking, van der Waals forces, hydrogen bonding, and other electrostatic interactions, etc.) [90]. MOFs are nanomaterials with good stability, large porosity, and specific surface area that are of great interest in gas storage, drug delivery, and sensors. Due to the intrinsic peroxidase

**49**

*Dyes as Labels in Biosensing*

**5. Conclusion**

**Acknowledgements**

**Conflict of interest**

*DOI: http://dx.doi.org/10.5772/intechopen.96540*

catalytic activity, MOFs can also be used in electrochemical biosensors. Xu et al. constructed a Pb2+ electrochemical biosensor based on the MOFs prepared based on Fe [91], and AgPt nanoparticles are employed to increase its electrical conductivity and electrocatalytic activity, and the obtained sensitivity approaches 0.032 pmol/L. However, even though MOFs have enzymatic activity to improve sensitivity, their synthesis process is very complicated, and the characterization of the modification

Investigation and evaluation of dyes play a vital role in the process of introduction novel labels and their corresponding sensing methods, which signify opportunities for the development of biosensors. This chapter highlights the utilization of dyes as biosensing labels and some most important sensing mechanisms for biological, biotechnological, and biomedical applications. These designs and applications have been much attracted for in vivo and in vitro analysis due to their high sensitivity and selectivity, fast response, biocompatibility, etc. Further developments in novel synthetic approaches of functional nanomaterials and sensing strategies will accelerate the discovery of unique properties of dyes, which will further improve

The authors are grateful to ÅForsk Foundation (grant number, 20-280), Formas

(grant number, 2019-01583), STINT (grant number, IB2020-8594) and I Bergh scholarship. Qilu young scholar program of Shandong University (grant number,

11500082063141) is also acknowledged for the financial support.

process is also very critical, so it is not suitable for routine use.

their applications towards future biosensing platforms.

The authors declare no conflict of interest.

#### *Dyes as Labels in Biosensing DOI: http://dx.doi.org/10.5772/intechopen.96540*

catalytic activity, MOFs can also be used in electrochemical biosensors. Xu et al. constructed a Pb2+ electrochemical biosensor based on the MOFs prepared based on Fe [91], and AgPt nanoparticles are employed to increase its electrical conductivity and electrocatalytic activity, and the obtained sensitivity approaches 0.032 pmol/L. However, even though MOFs have enzymatic activity to improve sensitivity, their synthesis process is very complicated, and the characterization of the modification process is also very critical, so it is not suitable for routine use.
