**5. Use of electro sensors based on QD in the diagnostic area**

Immunodiagnosis refers to techniques capable of detecting immunological analytes based on antibodies used in conjunction with other structures. The union of the high sensitivity and specificity of the antibodies makes the immunological tests widely used and, depending on the test, with relatively low cost, making these methodologies popular in a clinical environment [71].

As with any other platform, the selection of reagents, methodology, antigens, and antibodies used in immunological tests are extremely important, generating different limits of detection, sensitivity, reproducibility, and other parameters capable of affecting the performance of the test based on these precepts, several forms of detection have emerged over time for specific types of analytes [71].

For a long time, immunosensory platforms based on electrochemical signals have been used, bringing advantages such as speed, precision, and sensitivity to the tests [71, 72].

An indirect photoelectrochemical sensor employing enzymes was used to detect acetylcholine by monitoring variation in the photocurrent generated in relation to its concentration in the solution. The system consisted of the enzyme acetylcholinesterase covalently linked to CdS quantum dots connected to the electrode; the enzyme catalyzes the hydrolysis of acetylcholine, which releases acetate and thiocholine, the latter acting as an electron donor to the system, contributing to increasing the photocurrent peak amplitude [73].

In another work, using enzymes associated with QDs, sarcosine oxidase was used with electrodes containing CdSe/ZnS Quantum dots to determine the amount of free sarcosine in the solution. In this case, the catalysis of oxidation generated formaldehyde and glycine. However, oxygen was consumed during light excitation, causing changes in the photocurrent concerning the amount of analyte in the sample [74].

Several sensors use QDs on gold surfaces. In this case, their use for detecting the cytochrome C protein was first done with the binding of QDs CdSe/ZnS to the surface of the electrode with the help of specific linkers. During the tests, it was possible to observe the changes in the photocurrent with the oxidation and reduction states of the protein, resulting in different anode and cathode currents during the experiment [75].

Still on photoelectrochemical assays, an immunosensor capable of detecting the concentration of type G immunoglobulins was developed through the multilayer deposition of QD on the surface of Indium tin oxide electrodes. The photocurrent

variation was generated due to steric hindrances events by the formation of the immune complex [76].

Apart from traditional single-analysis methodologies, a novel approach was emoployed to develop a multiplex immunosensor capable of simultaneously detecting multiple biomarkers. This innovative strategy involved the utilization of PbS, ZnS, and CdS QDs. By employing square-wave anodic stripping voltammetry, it became feasible to detect and differentiate three different antibodies in the same sample [77].

Using the technique of dissolution of QDs in acid solutions, it was possible to develop an electro sensor based on QDs of CdSe/ZnS as a marker of phosphorylated bovine serum albumin. The nanoparticles were used for amplification of the electrical signal when conjugated to a specific secondary antibody, after acid dissolution, an electrochemical stripping analysis was performed making it possible to correlate between QD and antibody concentration [78].

Several types of QDs are useful in the construction of electro sensors to develop a Carcinoembryonic Antigen (CEA). One group used poly(5-formyl indole)/electrochemically-reduced graphene oxide nanocomposite (P5FIn/erGO) and Au nanoparticles (AuNPs) to facilitate the transport of ions during redox events. Moreover, the modification allows the increase of the primary antibody immobilization area, and finally, the gold nanoparticles amplify the system signal [79].

Assays of the "label-free" type have several advantages associated with nanoparticles. Therefore, an electrochemical immunosensor used graphene QDs with gold nanoparticles to detect and quantify Human Chorionic Gonadotropin (HCG) in human serum. The modification made the more sensitive assay increase the electronic flow, allowing the coupling of a greater number of proteins on its surface and greater reproducibility of the test [80].

In addition to electro sensors based on enzymes or antibodies, those based on DNA/RNA are of great use in the diagnostic area. A study involved the simultaneous detection of several proteins from aptamers immobilized on the gold surface of the electrode, and after incubation, different QDs of lead sulfide and cadmium telluride allowed selective detection and differentiation based on other voltammetric characteristics [36].

Still, magnetic nanoparticles based on ZnFe2O4/GQDs were used on DNA-based detection systems to mimic peroxidase-type activities. In this case, the capture DNA immobilized on the electrode surface served to capture the analyte. Susequently, in a sandwich system, the nanoparticles allowed this quantification through the reduction of electrical peaks [81].

In another example, a work proposed a glucose sensor without using enzymes. For this, it used CdTe QD-functionalized ZnO nanosheets with excellent photocatalytic activity, and the labeled capture DNA was immobilized on the associated modified electrode. The detection made by electrocatalysis of Pt/TiO2 NTA on glucose at low potential allows detection with very low interference [82].

A simple "label free" system was developed for the detection of hepatitis B virus through DNA using graphene QDs on the electrode, the method used an immobilized complementary DNA strand for the capture of HBV-DNA that, in the absence of this analyte, caused a low voltammetric value, referring to the low oxidation of potassium ferricyanide, but in the presence of complementary DNA, these values increased drastically, being able to diagnose the presence of the virus [83].

In diagnosing pathogenic bacteria, using electro sensors also proves to be a great alternative to conventional methods. Using graphene QDs on gold electrodes, a "labelfree" type sensor was developed where specific antibodies were immobilized with

*Electro Sensors Based on Quantum Dots and Their Applications in Diagnostic Medicine DOI: http://dx.doi.org/10.5772/intechopen.111920*

bovine serum albumin and ethanolamine. The quantification of the bacteria was done through amperometry, where after the addition of H2O2, the immunocomplex varied the oxidation levels with the growth of the concentration of *Yersinia enter*ocolitis [84].

In addition to common diagnoses, Living-cell detection has received attention associated with toxicity analysis and medical applications. One study used the photoelectrochemistry of graphene-CdS nanocomposites as a mechanism for amplifying the electronic transport process and increasing photocurrent, in addition to ensuring system stability and linear response with a low detection limit [85].
