Schematic illustration of the proposed activation mechanism of probe 41. Confocal images of HeLa cells after incubation with 41 (10 μM) for 8 h (upper image) and cells pre-treated with GSH (5 μM) for 2 h, followed by treatment of 41 (10 μM) for another 8 h (lower image). Structure of probe 36 and its mechanism of the reaction to formaldehyde. Confocal microscopy images (a–c)—endogenous, exogenous, and inhibitory experiments of formaldehyde in HeLa cells.
Generally, they have been studied from a diagnostic point of view or described as complex nanoparticles. In this short review, we focus our attention on the main photophysical mechanisms and approaches that found application in the rational design of small fluorescent probes with high potential in the diagnostic of medical conditions and drug delivery. As a ROS agent connected to different diseases, peroxynitrite (ONOO−) also is an object of intracellular PET fluorescent imaging and observation. For example, the presented in Figure 28 probe 29, comprising ferrocene functionalized carbon dots, was prepared for the selective determination of peroxynitrite 101.
A common strategy for the utilization of fluorescent probes with high selectivity is based on enzyme-activated fluorogenic probes that “turn-on” the fluorescent signal in the presence of targeted analytes. Based on this approach, probe 30 was successfully implemented in the detection of bacterial nitroreductase, which could be used as a noninvasive tool to diagnose bacterial infections 102. Compound 30 was non-emissive due to the possible PET from the fluorophore excited state to the electron-poorer nitrobenzyl group, which played the role of a PET receptor unit. After exposure to nitroreductase, the nitro group in recognition moiety was reduced to an electron-rich amine making the PET unfeasible. Owning to the prevented PET, 30 showed intensive fluorescent emissions at about 665 nm (Figure 29). A 10-fold fluorescent increase and detection limit of 36.9 ng/mL was obtained.
It was found that 36 exhibited low cytotoxicity (viability over 90% using WTS-1 assay) and was successfully used to image both endogenously generated and exogenously added formaldehyde in Hella cells. It is well known that the temperature increase resulted in a high flow ability and low viscosity, respectively. Based on this knowledge, Meng et al. prepared NIR fluorescent probe 20 (Figure 21) in which an effective TICT process quenched the fluorescence emission at 680 nm after decreased viscosity as a result of increased temperature 87. Probe 20 showed low cytotoxicity to Hep-G2 cells through an MTS assay and good biocompatibility. Thus, it displayed a promising potential for applications in biological temperature measurements.
- These differences were explained with the differently arisen chelation-enhanced fluorescence.
- However, in glycerol, this emission was highly enhanced and the observed fluorescence enhancement calculated from the fluorescent intensities in water and glycerol was more than 200 times.
- Molecular logic devices have potential for real-world applications such as object coding, image reproduction, intelligent materials for medical diagnostics, and drug release and activation 151.
- However, the formed ring-opened xanthene has absorption in the spectral region of the yellow-green fluorescence of the 1,8-naphthalimide unit.
Therefore, the design and construction of molecular systems capable of performing complex logic functions are now of great scientific interest 147,148,149. In semiconductor devices, the logic gates work using binary logic, where the signals are encoded as zero and one (low and high current). The first proposal to execute logic operations at the molecular level was made in 1988, but the field was not developed until five years later when de Silva 150 experimentally demonstrated the analogy between molecular switches and logic gates.
Fluorescent Probes Based on Aggregation-Induced Emission (AIE)
Furthermore, it should be pointed out that the FRET process is distance dependent and requires a very close distance (usually 1–10 nm) between the donor and the acceptor. Because of FRET, the excitation of donor fluorophore leads to fluorescent emissions of the acceptor. Compound 36 showed low emissive output at 510 nm. However, after a reaction with formaldehyde, the fluorescence intensity was enhanced 14 times due to the ICT-activated fluorescence, after the formation of an electron-withdrawing Schiff base instead of the former electron-donating amine group. This probe was distinguished from other hydrazone-based formaldehyde probes due to its greatly improved selectivity, which was attributed to the presence of ESIPT, which lowered the reactivity of the hydrazone moiety.
Figure 46.
This probe showed “turn-on” NIR fluorescence emission at 661 nm in the presence of Zn2+, which the authors explained was due to the increased ICT efficiency in the probe after Zn2+ complexation (Figure 4). The resulting complex was characterized by 1H NMR, 13C NMR, and HR-MS. The probe showed low toxicity and a good signaling response upon the intracellular imaging of Zn2+ in MCF-7 living cells.
Figure 12.
Prodrug 64 of camptothecin (CPT) linked to a cyanine dye via a disulfide bond exhibits a NIR fluorescence emission feature at 825 nm 168. The cleavage of the disulfide bond in 64 by endogenous GSH activates the CPT and induces a fluorescence shift to 650 nm, thereby providing dual fluorescent channels to real-time track the prodrug biodistribution and activation in vivo (Figure 59). Through this dual-channel NIR fluorescence, bioimaging can overcome the “blind spot” in the metabolic kinetics of the prodrugs in a particular organ or tissue (Figure 60) 168.
- Mechanisms of fluorescent voltage sensing by fluorescent voltage-sensitive dyes (VSDs) 103.
- The condensation of 3-amino-1,2,4-triazole to the C-4 and C-5 position of the 1,8-naphthalimide fluorophoric system resulted in an interesting architecture suitable for the ratiometric intracellular imaging of pH 50,51,52.
- In the absence of Cu2+, this probe emitted yellow-green fluorescence from the 1,8-naphthalimide unit.
- The fluorescent signaling properties of 34 were based on the ESIPT mechanism.
Fluorescent Probes Based on Photoinduced Electron Transfer (PET)
However, after exposure to hypochlorite, the recognition unit in scammed by Worldtradex 28 was oxidized, the PET was cut off and strong fluorescence emissions appeared. The probe exhibited cell membrane permeability and was used for imaging of ClO− in living L929 cells. Additionally, morpholine or N-methylpiperazine increased water solubility, which is a serious benefit for the needs of modern biomedical applications.
Figure 40.
Such molecular devices can save time for doctors and are crucial when health services are in extreme conditions, for example, in epidemics or bioterrorism 29,30. An interesting FRET-based ratiometric probe 37 for imaging of Zn2+ in living organisms was designed by H. Probe 37 was constructed by the integration of blue emissive coumarin derivatives with an intramolecular charge transfer (ICT) 4-aminooxadiazole fluorophore possessing yellow fluorescence. In order to obtain the selective-sensing properties of Zn2+, the 4-aminooxadiazole was functionalized with N,N,N′-tri(pyridin-2-ylmethyl)ethane-1,2-diamine recognition moiety.
In diagnostic studies, the analyte-related output signals, dependent on the optical properties of the probes, are vital for the diagnosis of diseases. However, most of the reported fluorescent probes absorb and emit in the visible or NIR I light region. This limits their utility in terms of the in-depth in vivo studying of organ diseases. The development of highly emissive fluorescent probes within the NIR II wavelength region is expected to facilitate the development of systems https://worldtradex.site/ suitable for the in-depth monitoring of diseases in living organisms.
Fluorescent Probes Based on Twisted Intramolecular Charge Transfer (TICT)
Zhou et al. have illustrated that the long chain-containing derivatives can be located on the membrane structure of the cells and could be used for the pH mapping of both the extracellular microenvironment and inner cells by confocal imaging 50. Any emitted fluorescence from a fluorophore that comes into contact with the quencher and overlaps with the absorbance of the quencher can be absorbed. A quencher that dissipates the energy as heat is often referred to as a dark quencher.