**3. Engineering quantum dots for lateral flow immunoassays**

Quantum dots are semiconductor materials with shape and size-dependent optical properties. They have found increased application in the diagnosis and prognosis of

infectious diseases [23]. QDs conjugated to biomolecules have improved the binding efficiency within the conjugate pad. QDs bonded to bio-recognition antibodies offer a viable system to enhance specificity and sensitivity in the detection zone. Optimizing multiple parameters for adequate flow, release, and stability is necessary for optimal LFA performance.

Most importantly, the label material must be able to detect antibodies to capture and release the test analyte sufficiently. Antibodies are used as bio-recognition components in LFA's detection zone (test and control lines), and they co-join the target analyte via immunochemical exchange [6, 24]. It is, therefore, essential that the QDs used are also able to bind with the conjugate pad material. The conjugate pad plays an essential role in running the LFA. It serves as the location for chemicals and ensures the constant transfer of detector reagents and analytes to the detector zone. It is, therefore, necessary to ensure QDs exhibit non-specific binding to conjugate pad material to ensure sufficient target analytes reach the detection zones. The conjugate pad also houses the detector particles and ensures they are kept functionally stable until the test is used [8, 9, 25]. In instances where the QDs as fluorescent probes are not well coated or unstable, thus losing their properties over time may result in insufficient target analytes reaching the test line, reducing LFA sensitivity and signal intensity. Furthermore, delays in QDs bound to an analyte of interest due to extended interaction with conjugate pad material may cause inconsistent flow. This may result in the test and control lines appearing as fluorescent streaks due to inconsistent flow [9, 25].

To improve the binding efficiency of QDs, conjugation of QDs via carboxylreactive crosslinker reactive groups is commonly used for the labelling/crosslinking of QDs. Water-soluble 1-ethyl-3-(−3-dimethyl aminopropyl) carbodiimide hydrochloride (EDC) is used for aqueous crosslinking in conjunction with *N*-hydroxy succinimide (NHS). NHS is commonly added to EDC coupling to improve its efficiency and stability. For non-aqueous organic synthetic methods, N′, N′-Cyclohexyl carbodiimide (DCC) crosslinker is widely used. Carbodiimide crosslinkers function by activating the -COOH functional group of the QDs, allowing for direct conjugation to primary amines (–NH2) via amide bonds [26]. The functionalization of QDs in LFA enables increased sensitivity and specificity for a 2-part recognition scheme whereby the label and capture agent become specific to the target analyte [21].

QDs are often functionalized with different capping ligands, which provide colloid solubility (i.e. convert hydrophobic material to hydrophilic), inherent stability and assist in chemical binding for biomolecules (i.e. DNA, proteins, peptides, and enzymes) [27, 28]. **Figure 2** shows various generic QDs solubilization and biofunctionalization often performed to condition QDs. QDs synthesized using an organic medium are insoluble and require phase transfer via ligand exchange (**Figure 2a**, **b**) to improve their water solubility and enable their bio-application. Ligand exchange, therefore, converts the hydrophobic ligands with bifunctional (viz., drugs, antibodies, or proteins) moieties enabling direct bio-application of the QDs [28, 29]. **Figure 2c** shows the use of polymers to insulate the hydrophilic surface of the QDs. In contrast, **Figure 2d-f** and **g** depicts the aqueous dispersion and derivatization of the QDs surface and its interaction with amphilic 'diblock' and 'triblock' copolymers and phospholipids. **Figure 2h** shows the attachment of QDs with amino acids (protein or peptides) as a bio-functionalization technique, therefore solubilizing and converting the surface of the QDs.

*Application of Quantum Dots in Lateral Flow Immunoassays: Non-Communicable… DOI: http://dx.doi.org/10.5772/intechopen.107947*

#### **Figure 2.**

*Schematic of generic QDs solubilization and biofunctionalization (a–h). Reprinted by permission from springer nature customer service Centre GmbH: Nature, nature materials IGor L. Medintz, H. Tetsuo Uyeda, Ellen R. Goldman and Hedi MattoussI), [COPYRIGHT] (2005) [28].*
