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Lee et al. Microstructures 2023;3:2023021 https://dx.doi.org/10.20517/microstructures.2023.08 Page 7 of 19

[55]
particularly challenging until Yaghi’s team reported the first COFs in 2005 . To obtain well-ordered
crystalline three-dimensional organic structures held together by strong covalent bonds between each block,
it is crucial to carefully design the covalent and noncovalent interaction sites in the primary structure. In the
absence of a proper guiding system, the organic chains may grow randomly on a two-dimensional plane,
resulting in either an amorphous structure or a packed one-dimensional (linear) crystalline structure . To
[57]
address this issue, researchers have been studying different types of linkers, organic blocks, and fabrication
conditions to tailor the COFs with diverse structures, as depicted in Figure 3 . These explorations opened
[58]
new opportunities for COFs to be considered as porous host materials due to their high specific surface
area, tailorable pores and structure, and low density.

Mn-ZnS QDs are considered as one of the promising materials for chemical contamination fluorescent
detectors as ZnS has low toxicity and a wide band gap of 3.7 eV . Furthermore, it has been broadly studied
[59]
for metal ions, small molecules, and biopolymers detections. However, pure Mn-ZnS QDs have the
limitation of low stability in a complex medium as the selectivity and sensitivity of the material reduce
significantly over time . To overcome these issues, molecularly imprinted polymers (MIPs) methods have
[60]
been investigated. MIP is a procedure to generate specifically tailored cavities in polymers that works like
antibody-antigen systems in our bodies to select precisely aimed molecules in the matrix . This hybrid
[61]
material showed great selectivity from shaped polymer and high sensitivity from QD material. However,
achieving a uniform distribution and increasing the active material volume density remain challenging
issues to address. Zhang et al. reported a new hybrid composite consisting of Mn-ZnS QDs embedded in
TpPa-1 COFs with molecularly imprinted polymer (MIP) to improve the detection limit and stability of the
composite in water . This technique was further developed to fabricate fluorescent probes for ferulic acid
[62]
[63]
with high selectivity and sensitivity, as demonstrated in Figure 4 . The fabricated composite demonstrated
improved stability at room temperature, as well as a high photoluminescence quantum yield (PLQY) of
37%. In these applications, the COF serves not only as a passive protective framework but also as a platform
for achieving even distribution and minimising aggregation of QDs.

Carbon quantum dots (CQDs) have been studied widely as a new type of fluorescent nanomaterials
primarily due to their favourable characteristics such as environmental friendliness, functionalisability,
excellent stability, biocompatibility, solubility, and low toxicity. These properties have rendered CQDs an
ideal candidate for various applications such as sensing cellular copper, glucose, nucleic acids, and cancer
detection and treatment. As a result, the investigation and exploration of CQDs continue to grow
significantly [64,65] . Despite significant advantages, the limited size control, uniformity, and low quantum yield
of carbon quantum dots present major challenges. To address these issues, researchers have attempted to
improve synthesis methods using various techniques such as solvent engineering, electrochemical
fabrication, and laser ablation, as well as fabricating hybrid composites that include encapsulating CQDs
with polymers and embedding them within porous matrices. Recent progress in encapsulating carbon
quantum dots (CQDs) involves surface functionalization by coating them with amphiphilic monomers or
polymers. Amphiphilic molecules contain both hydrophilic and hydrophobic components, which allows
them to interact with both water and the CQDs surface. This method can provide a protective layer around
CQDs which can improve their stability, reduce aggregation, and provide other desirable properties such as
solubility in different solvents [66-68] . Encapsulating CQDs within micro to mesoporous matrices such as
mesoporous silica is a common approach to improve their stability and efficacy. Mesoporous silica, in
particular, has attracted significant interest due to its large specific surface area, low toxicity, and good
biocompatibility.