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Page 14 of 33 Mao et al. Chem Synth 2023;3:26 https://dx.doi.org/10.20517/cs.2022.41

been prepared through a cation-exchange process by Wang et al. Consequently, the affected Cu
2+
[106]
regulates the single electron spin, revealing the T -weighted MRI ability sensitive to H O concentration, as
2
1
2
shown in Figure 8I-K.
Positron emission tomography
Positron emission tomography (PET) is a formidable biomedical imaging technique widely used for
diagnosing clinical oncology due to its practicality and accurate susceptivity [115-117] . PET is one of the updated
imaging mechanics that could show the metabolism of biomolecules, receptors, and neurotransmitter
activity in vivo . In addition, it has been widely used in differential diagnosis, efficacy evaluation,
[118]
condition evaluation, new drug development, and organ function research. (1) High sensitivity. As we
know, PET is an imaging technique that reflects molecular metabolism. When the disease occurs in the
early stages of molecular level changes, the morphological and pathological changes in the damaged area
may not yet be visible, and MRI and CT examinations cannot provide a clear diagnosis; however, PET
inspection can identify the damaged area and provide 3D images and quantitative analysis to achieve early
diagnosis. (2) High specificity. It is difficult to distinguish between benign and malignant tumors in organs
by MRI and CT examinations; however, PET can be used as a diagnostic tool according to the high
metabolism characteristics of malignant tumors. (3) Whole-body scan. PET can be used as a one-time
whole-body examination to produce images of all parts of the body. (4) Good safety. The radionuclides
required for PET examinations have a certain level of radioactivity, but the amounts used are negligible.
Additionally, the radionuclides have a short half-life of approximately 12 minutes and a long half-life of
approximately 120 minutes, ensuring a fast metabolism. After physical decay and biological metabolism, the
contrast agent stays in the body for a short period of time. The dose of PET required for a whole-body
examination is much lower than that of conventional CT examinations, so it is safe and reliable. Cu was
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successfully applied in vivo imaging as an efficient PET luminescence. Guo et al. depicted a direct composite
of intrinsic radioactive material by amalgamating Cu directly into CuInS/ZnS nanostructures, using CuCl
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as the synthesis precursor , as shown in Figure 9A. It clearly displayed characteristic whole-body coronal
[119]
2
PET images of U87-tumor-bearing mice in Figure 9B. Zhou et al. developed a patented, chelator-free Cu-
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CuS compound that can be used for PET imaging and as a photothermal sensitizer for PTT in tumor
[120]
ablation, as shown in Figure 9C-D. These Cu-CuS compounds were easy to fabricate, exhibited exceptional
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stability, and allowed for almost noninvasive micro-PET imaging. In summary, the combination of smaller
diameter, stronger absorption of NIR, and comprehensiveness of Cu as a configurational constituent made
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these NPs well-suited for synergetic molecular imaging and therapy.
Although PET technology presents great success in the clinic, especially for early tumor diagnosis and
therapy evacuation, there are still some disadvantages that cannot be ignored and limit its development.
PET technology may not be effective in detecting early adenocarcinomas, especially ground glass nodules, as
these may not exhibit active metabolism and will show negative results. Additionally, false positives can
occur when infections, tuberculosis, sarcomatoid lesions, and other diseases are present, as these can show
positive results. Furthermore, because radioactive isotopes must be injected into the body during the
examination, PET has the disadvantage of exposing patients to a large amount of radiation, and it is
absolutely prohibited for use in screening otherwise healthy individuals.

Fluorescence imaging
Fluorescence imaging (FLI) is particularly significant for qualitative preclinical applications . As we know,
[118]
FLI has lots of advantages, including simple synthesis, fluorescence signal visualization, and multiple
marker sites. It is a consummate implement for imaging-guided angiography. Metal NCs have been vastly
utilized in imaging techniques, including PET, MRI, and FLI [121,122] . They exhibited individual optics and