**1. Introduction**

Non-communicable diseases (NCDs) (commonly known as chronic diseases) are diseases that result from environmental, behavioral and genetic reasons. As a result, they tend to linger for extended periods. Some NCDs like cancer, diabetes, and heart disease are preventable through lifestyle and behavior changes [1]. In comparison, communicable diseases (CDs) are diseases that can spread from one person to another (i.e. via bodily fluids and blood, inhaling airborne virus) [2]. Early detection and rapid diagnosis of NCDs and CDs are essential for disease screening and treatment. Several diagnostic tools are available for the detection and prognosis of NCDs. These include: biopsy procedures (i.e. endoscopic, pelvic examination, bronchoscopy, pap test and lumbar puncture.) [1], blood tests (i.e. complete blood count, blood protein testing etc.) [3] and diagnostic tests (i.e. computerized tomography (CT) scan and

magnetic resonance imaging (MRI) scans) [4]. Nonetheless, these medical tools are costly, laborious, and require clinical sample preparation and sophisticated instruments with trained operators.

Moreover, they are unsuitable for first-line diagnosis. This has resulted in a need to develop cost-effective, rapid, and reliable methods for clinical diagnoses and prognoses of NCDs and CDs. Point-of-care testing (POCT) tools are medical diagnostic tools used at the time and place of patient care [5]. It employs simple equipment and reduces the time needed to generate clinical results, permitting patients and clinicians to make on-the-spot clinical decisions. POCTs have some advantages over traditional diagnostic tools, i.e. shorter detection time, less costly, limited to no sample processing, basic instrumentation, with minimal operation requirements [6]. Therefore, they permit early detection of diseases, especially in resource-limited areas, thereby improving the time of initial treatment. Nonetheless, the unavailability of clinical POCTs for NCDs such as cancer and the emergency of the COVID-19 pandemic has highlighted the need for developing POCT tools to allow for improved tracking and response time.

Lateral flow assay (LFA) (also known as immunochromatography tests) is one type of POC testing tool that has addressed the challenges of traditional testing streams like the polymerase chain reaction (PCR) and reverse transcription-polymerase Chain Reaction (RT-PCR). The most common LFA example is the pregnancy and human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS) test. LFA is a simple paper or membrane-based device that uses a liquid sample to detect the presence or absence of the test analyte (or disease biomarker). LFA has several advantages as an attractive sensing tool because it is portable, low cost, and enables on-site detection (**Table 1**) [7]. In LFA, various materials have been employed to detect the presence/absence of a test analyte in a liquid sample. The use of nanomaterials in the development of LFA has enabled qualitative naked-eye detection, broadened the type of labels used, eased antibody/antigen conjugation, and improved fluorescent and dual colourimetric signal detection [8]. Several nanoparticles used in LFA include magnetic nanoparticles [9] and colored nanoparticles [10], with fluorescent quantum dots serving as promising fluorescent probes. Quantum dots (QDs) are ideal fluorescent labels for immunoassay application. Their unique optical properties, molecular extinction coefficients, and excellent resistance to chemical and photo-degradation have advantaged them significantly over fluoro-dyes [11]. In LFA application, QDs have been used in antigen-antibody reactions to detect a variety of


#### **Table 1.** *Advantages of lateral flow assay (LFA).*

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

analytes, i.e. tumor/biomarkers [12, 13], bacteria [14], and viruses [15]. Bock et al., 2021 [16] developed a QDs-embedded with silica nanoparticles lateral flow immunoassay to detect prostate-specific antigen (PSA). The study used QDs as a probe for their red-emitting properties. The group coated the test line with anti-PSA antibody and the control line with goat anti-mouse IgG antibody. Bovine serum albumin (BSA), sucrose, Poly(ethylene glycol) (PEG), Tween 20 and QDs-PSA were used to condition the conjugate pad of the LFA. The LFA results were visible within 15 min, with an LOD of 0.138 ng/mL and no false-negative results on the clinical samples.

Moreover, the fluorescence intensity of the strip exhibited no significant decrease for 10 days. Detecting low levels of antibodies at the early stages of diseases has been a significant challenge in clinical diagnosis. Currently used early-stage diagnosis methods are still expensive with long turn-around times. Kim et al., 2022 [17] reported on the highly sensitive LFA kit based on brush-type ligand-coated quantum beads (B-type QBs) with the potential to diagnose several diseases by changing the antibody pair in the LFA kit. The B-type QBs were obtained via the self-assembly of polystyrene-co-poly(acrylic acid) (PS-PAA)and the QDs. The B-type QBs enabled good dispersion and stability and had a high-binding capacity towards antibodies. Controlling the orientation of the antibody created an ultra-high sensitive LFA. This study highlights the effect of antibody orientation and ligand type on LFA performance. Moreover, the human serum spiked hCG detection confirmed the LFA potential application in human body fluid.

This chapter addresses the application of lateral flow assays as a diagnostic tool for communicable and non-communicable diseases. The chapter discusses the basic principle and mechanisms of lateral flow assays. This is followed by discussions on the tailoring and application of quantum dots (QDs) in LFA to detect NCDs and CDs. The chapter then focuses on applying QDs in the LFA detection of various non-communicable and communicable diseases. The chapter further addresses some of the challenges within LFA application and concludes with future perspectives on QDs application in LFA.

### **2. Principle of lateral flow immunoassays**

Lateral flow assays (LFA) are fabricated on a series of capillary beds made of a specific material. The material used (i.e. glass fiber or nitrocellulose membrane) contributes to the LFA performance. It ensures that the samples migrate towards the detector bed, which hosts the test analyte. The central beds of the LFA are (i) sample application bed, (ii) conjugate pad, (iii) substrate membrane, and (iv) absorbent bed (**Figure 1**).

#### **2.1 Mechanism of lateral flow assay**

The sample is presented at the sample bed (made from cellulose fiber) and migrates to the conjugate pad, which consists of material labeled with analyte-specific antibodies. The conjugate antibodies bind and move up with the sample towards the test line. The test and control lines (also known as the detection compromise of nitrocellulose membrane immobilized with biological moieties (primarily antibodies or antigens). When flowing into the detection zone, the conjugated sample reacts with the analyte bound to a specific antibody. Upon detection, visual changes appear as a line (for a positive test result), while the control line appears whether the target analyte is present or not [6, 7]. The conjugation of the label materials (i.e. quantum dots) to biomolecules such as antibodies allows for the improvement in the capture

**Figure 1.**

*Schematic illustration of the lateral flow assay (LFA) . Reproduced from Huo C, Li D, Hu Z, Li G, Hu Y and Sun H (2021) a novel lateral flow assay for rapid and sensitive nucleic acid detection of Avibacterium paragallinarum. Front. Vet. Sci. 8:738558 under license https://creativecommons.org/licenses/by/4.0/ [18].*

and release of the target analyte in the LFA beds (majorly in the conjugate, test, and control beds). This improves the detection of disease biomarkers.

Although LFAs enjoy the above advantages, traditional lateral flow immunoassays do suffer from limitations which include but are not limited to:


v.Qualitative results that suffer from obstruction of pores due to matrix components.

To address these challenges, optimization of LFA using various experimental approaches has been reported. Optimizing multiple parameters on the bed is necessary for optimal LFA performance enabling adequate flow, release, and stability. **Table 2** lists the components (i.e. buffers, stabilizers, detergents/surfactants/wetting agents and blocking agents) commonly used to optimize the efficiency, reproducibility, sensitivity, and minimize non-specific binding of the LFA. To further address some of the above LFA shortcomings, research has shifted towards applying QDs as signal-generating material (SGM) [17] (Kim et al., 2022). For example, Liang et al., 2020 [19] reported on a QDs-based LFA used in conjunction with a portable fluorescence immunoassay chip for detecting exact IgE for mite allergens obtained from patients with allergic rhinitis. Clinical samples were analyzed with good reproducibility, fluorescent test, and control line.

The brightness of QDs has therefore improved LFA sensitivity and *in-situ* monitoring [20]. Moreover, QDs have become essential in LFA due to their ease of bio-functionalization, which enables specific and stable recognition of antibodies, *Application of Quantum Dots in Lateral Flow Immunoassays: Non-Communicable… DOI: http://dx.doi.org/10.5772/intechopen.107947*


#### **Table 2.**

*Optimization parameters.*

addressing the lack of commonly reported conformation. QDs as fluorescent probes have thus become useful for the immune recognition of target analytes [21] (Wilkins et al., 2018), and allow for quantitative detection of biomarkers. In a study by Rong et al., 2021 [22], red-colored quantum dot nanobeads (i.e. silica or polymer nanobeads embedded with hundreds of quantum dots) were used for the multiplex and simultaneous detection of four infectious disease biomarkers. The group were able to detect human immunodeficiency virus (HIV), *Treponema pallidum* (TP), hepatitis C virus (HCV), and hepatitis B virus (HBV) at LOD levels of 0.11 NCU/mL, 0.62 IU/L, 0.14 NCU/mL and 0.22 IU/mL respectively within 20 min. The direct aqueous synthesis of QDs has further eliminated the challenges of colloidal and fluorescence stability of LFA when biomolecule (i.e. blood, serum, plasma etc.) samples are analyzed. While the small sizes and good size distribution of QDs prevents the obstruction/blocking of membrane pores in the sample pad and detection zone.
