An update on recent advances in fluorescent materials for fluorescence molecular imaging: a review

Abstract

Fluorescence molecular imaging (FMI) is a powerful imaging technique used primarily in biomedical research and clinical applications to visualize molecular and cellular processes of tumors and other diseases. FMI involves the use of fluorescent molecules (fluorophores) that absorb light at one wavelength and emit it at a longer wavelength. These fluorophores can be attached to specific molecules and markers (such as proteins, nucleic acids, or small molecules) in a biological sample. FMI typically offers non-radioactive and safe, real-time and higher spatial resolution compared to positron emission tomography (PET) for superficial tumors. Additionally, sensitivity and specificity of FMI for superficial tumors in better than PET is some cases. However, FMI and the materials used in molecular imaging (MI) have revolutionized biomedical research, diagnostics, and therapeutic monitoring. In contrast, despite their significant contributions, several challenges remain to be solved to improve the effective application of fluorescence-based techniques. These challenges are related to poor tissue penetration depth, background autofluorescence, photobleaching of fluorophores, low signal-to-noise ratio in deep tissues and the necessity for biocompatible and photostable probes. Hence, ongoing improvements in probe development, imaging technologies and analytical methods are required to overcome current challenges. Future advancements in fluorescence materials and imaging techniques hold promise for making MI more accurate, efficient and applicable for clinical and research scenarios. This review gives an overview of recent advances in the materials used in MI and findings of FMI. Finally, limitations of FMI are highlighted and recommendations for future research directions are proposed.

Fig. 1. Principal of FI using luminescence agent such as green fluorescence probe (GFP) and confocal microscopy. (A), a schematic of principles of FMI, (B), fluorescence confocal microscopy and various stages of FI and recording image from sample.

Fig. 2. Fluorescence microscopic images of BODIPY-based viscosity probe (BTV) in HeLa cells with stained with MitoTracker. DeepRed. Images were taken with a 40× objective. Scale bar represents 20 μm. (a) Phase contrast; (b) MitoTracker DeepRed; (c) BTV; (d) merged of MitoTracker DeepRed and BTV. Adapted from ref. (under the terms of the Creative Commons CC BY 4.0 license (e) chemical structure of BODIPY dye.

Fig. 3. Fluorescence microscopy image of neuronal cells (ND7/23) incubated with Rhodamine B. Adapted from ref. , under the terms of the Creative Commons CC BY 4.0 license.

Fig. 4. Fluorescence microscopy imaging of A549 cells. Cells were treated with TGA-QDs (green) for 2 h at 37 °C, (A) overlap of two images, control nuclei staining with DAPI (blue). Cells were treated with TGA-QDs for 2 h at 4 °C, (B) overlap of two images, control nuclei staining with DAPI. Anti-HER2 TGA-QDs showed a high binding affinity for A549 cells, which resulted in strong fluorescent signals. Reproduced from ref. with permission from Elsevier, copyright 2014.

Fig. 5. Fluorescence microscopy images of NIH-3T3. (A) Cells were treated with TGA-QDs/anti-HER2 for 2 h at 37 °C, (B) overlap of two images, control nuclei staining with DAPI. Anti-HER2 TGA-QDs showed poor binding affinity for NIH-3T3 cells, as demonstrated by low fluorescence signals compared to those in A549 cells. Reproduced from ref. with permission from Elsevier, copyright 2014.

Fig. 6. Intraoperative ICG fluorescence imaging of a hepatoblastoma nodule not visualised on preoperative CT scan. (A) Preoperative CT scan showing no sign of the nodule (B) the same nodule clearly visualized under ICG/NIR fluorescence during surgery (C) gross pathological specimen of the resected nodule, confirming hepatoblastoma adapted from under the terms of the Creative Commons CC BY 4.0 license (D) chemical structure of ICG.