**6. Current challenges**

Current LFAs use label materials (in the conjugate pad) that are synthesized in the hydrophobic state, unstable, lack colourimetric fluorescence stability and are incompatible with the protein–particle binding. For example, using enzymes results in detection sensitivity that depends on an enzyme-substrate combination [59]. Colloidal carbon nanoparticles as labels suffer from the non-specificity and the presence of irregular shapes [59]. Although quantum dots seemingly addressed some of these issues. The use of binary QDs (composed of groups I-VI and VII) is of concern as they are toxic by their very nature due to the presence of heavy metals such as Cd and Pb. Moreover, the risk of toxic heavy metals leaking within the LFA is a significant concern, raising toxicity, health, and environmental concerns. Additionally, the synthesis of currently used QDs has been via organic reagent, thus requiring ligand exchange which compromises the optoelectrical properties of the QDs required for ideal LFA development. Another shortcoming of current LFAs is that they: (i) lack the adequate sensitivity that enables color changes that the human eye can view, (ii) loss of conformation and recognition functionality in non-aqueous media by antibody biosensors, and (iii) challenges in the maintenance of colloidal and fluorescence stability in aqueous biological environments, particularly in enzyme-mediated conditions, (iv) challenges of ligand exchange and (v) qualitative results of that suffer from obstruction of pores due to matrix components. To address these challenges, there is a need to develop multi-colored ternary and quaternary QDs-based LFA for the clinical detection of single and multi-disease/virus biomarkers.
