**7. Multiwalled carbon nanotubes**

A number of chemically modified QDs have been developed using multiwalled carbon nanotubes (MWCNTs) because of their unique properties, such as high electrical conductivity, high mechanical strength, high thermal conductivity, and large surface area to volume. In analytical sensing, multiwalled carbon nanotubes (MWCNTs) have been shown to reduce detection limits, increase sensitivity, and prevent surface fouling.

As part of their study, Baslak et al. functionalized MWCNT surfaces with silver nanoparticles and encapsulated the silver nanoparticles with poly(glycidyl methacrylate) (pGMA). An initiated chemical vapor deposition (CVD) method was used to coat MWCNT surfaces with pGMA. The chemical vapor deposition (CVD) method produces thin films in a dry environment. By initiating chemical vapor deposition (iCVD), complex geometries can be coated uniformly without solvent-related damages, which are typically observed in conventional wet coating. During the iCVD process, the substrate is cooled so that the species can adsorb and grow on the surface. This prevents the substrate from being damaged by high temperatures, plasma, or light sources, which can alter the substrate's properties physically or chemically. As a result, iCVD was selected as an ideal method for functionalizing and encapsulating MWCNT surfaces [57].

The authors of Vinoth et al. describe a method for integrating ZnO QDs into MWCNTs using ultrasonication to decorate the QDs on the surface of the nanotubes (MWCNT/ZnO QDs), using both electromechanical detection of glucose and photocurrents simultaneously. A chronoamperometry measurement, a differential pulse voltammetry measurement, and a cyclic voltammetry measurement were used to evaluate the performance of the enzyme-free glucose sensor. Ascorbic acid, uric acid, and dopamine all interfere with the glucose sensor based on MWCNT/ZnO QDs, while sucrose exhibits good anti-interference properties. A nanocomposites heterostructure made from MWCNT/ZnO QDs shows excellent photocurrent activity toward visible detection [58].

### **8. Polymer**

Multifunctional polymer layers can enhance QDs' interaction with target analytes by incorporating a variety of functionalities. QDs are usually synthesized with polymers that contain organic cyclic chains such as calixarene and cyclodextrin. The use of polyethylene glycol (PEG) derivatives in the synthesis of QDs has also become increasingly popular in recent years. PEG derivatives are readily available, require simple encapsulation processes, and are readily available.

According to Yi et al., a fluorescence nanoprobe detecting parathion-methyl (MP) through host-guest recognition is the first enzyme-free fluorescent nanoprobe. It is possible to form distinct molecular recognition function on MoS2 QD surfaces by introducing this molecule on the surface of MoS2 QDs, as shown in **Figure 4**.

With the PET process, p-NP molecules from MP hydrolysis under alkaline conditions can enter into small cavities of molecular beads and further quench their fluorescent properties, thereby providing a fluorescence sensing platform for MP. Based on the optimization of various experimental conditions, the results show that the molecular probe is excellent in terms of selectivity and sensitivity, has a wide linear range, and has been able to detect MP with a low detection limit [59].

Gupta et al. studied interactions between chitosan, the most abundant biopolymer, and luminescent CdSe QDs synthesized by cyclic voltammetry and capped with MPA. MPA-CdSe QDs were found to react dynamically with chitosan (crystal size 2.3 × 0.5 nm, zeta potential 47 × 6 mV). Based on our assessments, the number of chitosan molecules bound and the binding constant Ka by MPA-CdSe QDs are ∼1.z 3 and 7.332 × 1015 L Mol-1 at 298 K, respectively. Chitosan's strong affinity for MPA-CdSe QDs is confirmed by the high Ka. Using the above calculations, chitosan-MPA CdSe QDs were found to be stable over a long period of time and to disperse in water homogeneously. Biomolecule interaction can be strongly influenced by the size, shape, and surface chemistry of QDs [60].

**Figure 4.** *Schematic illustration of parathion-methyl detection using β-CD-functionalized MoS2 QDs [59].*

A ratiometric fluorescence probe developed by Yu et al. detects H2O2 and glucose selectively using cationic conjugated polymer (CCP) and cationic transition metal dichalcogenides (CdTe/CdS QDs). H2O2 causes fluorescence quenching of QDs due to FRET between CCP and QDs. In order to create new H2O2 and glucose enzymatic assays, these phenomena can be exploited. This method can be applied to other oxidases to quantify substrates such as choline, xanthine, cholesterol, and lacate, since many oxidases generate H2O2.

According to the proposed design, it is able to provide two advantages and advantages over conventional systems: (I) we use the signal transformation process to design a dual-emission nanoprobe with excellent properties such as high selectivity, fast response time, and resistance to outside interferences, and (II) because the QDs exhibit excellent NIR properties, we were able to directly measure glucose in whole blood [61].

#### **9. Conclusion**

A comprehensive summary of recent advances in surface modification QDs for biological applications is presented in this chapter. Surface ligands are the most important factor in obtaining useful materials from quantum dots for biological applications. The surface of quantum dots can be easily modified with, metal ions doped, metal-organic frameworks, molecularly imprinted polymer, aptamers, multiwalled carbon nanotubes, graphene and carbon quantum dots, silanization, polymer, and transition metal oxide, which further expands the application of quantum dots in the sensor field.

The surface modification QDs have shown that design of QDs to detect, with high sensitivity and selectivity, various analytes was an effective strategy. Traditional QDs have challenges such as being environmentally friendly, biocompatible, having a broad excitation spectrum, and having poor photostability, wavelength tenability dependent on size, and low quantum yield. Modification QDs help overcome these issues by being extremely energy-efficient and highly quantum yielding.

*Recent Advances in Quantum Dots-Based Biosensors DOI: http://dx.doi.org/10.5772/intechopen.108205*
