The general strategies regarding fluorescence probes as molecular logic devices and fluorescence–drug conjugates Worldtradex for theranostic and drug delivery systems are discussed. This work could be of help for researchers working in the field of fluorescence sensing compounds, molecular logic gates, and drug delivery. The probe was designed as an ICT naphthalimide fluorophore, conjugated with a 3-hydroxybenzyl unit at the C-4 position as a receptor for selective tyrosinase reaction.
- The nerve signals were changed using rapid potential difference that was set up in a cell membrane by an imbalance of cations such as Ca2+, Na+, and K+ across a lipid bilayer.
- Moreover, all of the three probes showed low cytotoxicity against Hella cells with a calculated viability higher than 80% at 15 μM concentration, and they were used for imaging the fluctuations of the mitochondrial viscosity in Hella cells.
- However, due to the different acidity of both receptors, the 5 and 6 were suitable for pH determination at different pH windows.
- Moreover, it was found that the probe was biocompatible, organism-permeable, and was used for the detection of intracellular Hg2+ Cy-PT cells and zebrafish.
Figure 6.
However, in sunflower oil, due to the high viscosity of the oil, the molecular motion of these probes was limited, which reinforce AIE fluorescence. Furthermore, they exhibited a large Stokes shift, impressive photophysical properties, great biocompatibility, and excellent photostability. It was found that they showed the great cellular-imaging ability of lipid droplets and were used for the visualization of lipid droplets in Hella cells and mice atherosclerosis models. The obtained results illustrated the great potential of 43 and 44 to provide an option for the diagnosis of atherosclerosis. Compound 42 is an AIE probe that exhibited a 112 nm red shifting of its pH-dependent emission at 503 nm to 615 nm, which is due to the enhanced intramolecular charge transfer after protonation of the pyridine moiety (Figure 43) 133. Based on both the changes, probe 42 showed ratiometric measurements with highly selective and reversible pH responses.
- The cytotoxicity test revealed more than 80% cell viability, suggesting that the probe can be used safely in living cell imaging.
- This process occurs by the nonradioactive, resonant transfer of energy from an excited donor fluorophore to a closely placed acceptor molecule in the ground state.
- This observation has several advantages for the monitoring of living organisms and it was successfully applied for real-time detection of Hg2+ in living cells.
- This probe has a bright emission at 730 nm and contains an electron-donating amino group as a metal recognition unit that takes place in a strong ICT process during excitation.
- Hence, through fluorescent dye screening and rational design strategy, chemists can cleverly construct functional fluorescent probes with great photostability and chemical stability.
After desulfurization in the presence of Hg2+, electron-accepting thiocarbonyl in 8 was replaced with a strong electron-donating hydroxyl group; thus, an ICT interaction was raised, and a NIR fluorescent emission at 708 nm was observed. Moreover, it was found that the probe was biocompatible, organism-permeable, and was used for the detection of intracellular Hg2+ Cy-PT cells and zebrafish. The recent advances in the design and synthesis of fluorescent probes for diagnostic purposes and monitoring in drug delivery systems are discussed in several reviews 31,32,33,34,35.
Figure 34.
The observed fluorescence properties are environmentally dependent; thus, the TICT fluorophores are the ideal platform for the construction of fluorescent probes. Notably, the TICT excited state is strongly solvent-viscosity dependable due to the prevented bond rotation in more viscous liquids and makes this phenomenon especially valuable for imaging microenvironmental viscosity in biological objects 79,80,81. Co-workers have developed disulfide-based fluorescent-drug conjugate 62 by conjugating on NIR fluorescent molecule DCM-NH2, glutathione (GSH)-activatable disulfide linker, and Combretastatin A-4 166. This is due to the loss of the electron-donating ability of the amine group of DCM-NH2 when attached to the linker. The higher molecular weight and lower water solubility were the main disadvantages of the FRET-based multi-chromophoric systems, which seriously reduced their biomedical applications.
The cytotoxicity of conjugate 67 against MCF-7 cells under white-light irradiation is significantly higher than that of light-irradiated free DOX at the same concentrations, proving the excellent synergistic chemo-/PDT therapeutic effect of 67. The ESIPT-based probe could be a useful tool for the monitoring of intracellular ROS units. Jiang et al. designed probe 35 as an efficient fluorescent reporter for ClO− in living cells 113. For the detection of ClO−, diacylhydrazine moiety was introduced into rhodamine, which selectively was oxidized into diimide by ClO− and, further, underwent a decomposition in water.
Figure 38.
The excited-state intramolecular proton transfer (ESIPT) due to increased acidity/basicity in an excited state is a typical phenomenon in molecules, which have a proton (hydrogen)-donating group placed closely to accepting groups 105. For instance, the hydroxyl hydrogen in salicylic acid decreased its electron density (generated a positive charge) in the excited state while the neighbor carboxylic oxygen increased its electron density (generated a negative charge). This caused a proton translocation from the hydroxyl to the carboxylic group.
Design of Fluorescent Probes
Synthetic oligonucleotides with covalently-attached CDPI3 have enhanced DNA affinity and have improved the hybridization properties of sequence-specific DNA probes. Glen Research offers a diverse selection of fluorophores and fluorescence quenchers that span most of the visible spectrum. Phosphoramidite versions and support-bound reagents are available, allowing for incorporating these products at the 5′- or 3′-terminus or internally within the sequence during oligonucleotide synthesis.
Typical examples of non-cleavable fluorescent-drug delivery systems are heptamethine cyanine dye (HMCD)-drug conjugates 159. As we mentioned above, the FRET process is strongly distance dependable. This makes the cleavage reaction another important strategy for the https://worldtradex.bid/ design of FRET-based fluorescence probes. Compound 41 is a typical example of a cleavage reactive probe for the detection of glutathione (GSH), which operates on FRET communication 126. The monitoring of glutathione in living organisms is very attractive from a diagnostic point of view because its concentration levels in cancer cells are much higher than in normal cells 127. Probe 41 was designed as a FRET bichromophoric system consisting of three modules, a BODIPY-based energy donating fluorophore, an N,N′-dimethylamino-based BODIPY energy acceptor, and a bio-reducible disulfide linker (Figure 42).
Principle of the ONOO− nanoprobe 29 based on PET. Confocal fluorescence imaging of MCF-7 cells incubated with the nanoprobe 29 (50 μg/mL) and then treated with LPS (1 µg/mL) and IFN-γ (100 ng/mL). The fluorescence emission is collected from 555 to 780 nm.
Fluorescence signaling allows noninvasive and harmless real-time imaging with great spectral resolution in living objects, which is extremely useful for modern biomedical applications. This review presents the basic photophysical principles and strategies for the rational design of fluorescent probes as visualization agents in medical diagnosis and drug delivery systems. The presented examples are focused on the visualization of pH, biologically important cations and anions, reactive oxygen species (ROS), viscosity, biomolecules, and enzymes that find application for diagnostic purposes.
Due to the effective spectral overlap between the emission of coumarin and 4-aminooxadiazole absorption, a FRET process occurred in 37, which showed typical energy-acceptor yellow emissions. However, the Zn2+ binding decreased the electron-donating ability of the 4-amino group, which weakened the ICT effect in oxadiazole and blue shifted its absorption. Thus, the former spectral overlap was strongly reduced and the FRET was interrupted. As a result, the blue fluorescence of the donor coumarin was observed (Figure 38). Using the fluorescence changes at 480 nm and 560 nm, a ratiometric analysis for the quantitative determination of Zn2+ was applied.
This encouraged Sidhu et al. to obtain the 1,8-naphthalimide 14 as an efficient fluorescent ratiometric probe for intracellular imaging of tyrosinase (Figure 14) 76. At slightly alkaline media, this amine 7 was deprotonated and formed an anion that strongly increased the electron density of the substituent at the C-4 position. As a result, the ICT efficiency in the probe was enhanced and the former fluorescence at 440 nm was red-shifted to 480 nm.
Moreover, the resulting fluorescence changes at 728 nm and 663 nm in 1 allow a ratiometric signaling output. In ratiometric methods, the presence of an analyte is quantified by the ratio of fluorescent intensities of two different fluorescent bands, which provides a self-calibration effect and built-in correction for environmental effects including biomolecules. Additionally, probe 1 showed good cell permeability and low toxicity. The discriminative detection of two or more metal cations at the same time could play an essential role for better diagnostics in medicine and biology.
The NHS ester versions enable post-deprotection fluorophore labeling. Schematic diagram of pH-responsive drug release and the mechanism of synergistic chemo-/PDT antitumor therapy. Schematic presentation of drug release in the presence of nitroreductase (NRD) 171. Chemical system 49 performs AND logic operation under the action of two chemical inputs (H+ and Na+) and the corresponding truth table 150.
Amyloid β-peptide (Aβ) aggregation is a primary biomarker for Alzheimer’s disease; hence, the preparation of a fluorescent probe for its detection plays an important diagnostic role. It was found that the N,N-diethylaniline recognition groups were selectively binding Aβ oligomers and were not responsive to other biomolecular small molecules. The resulting complex was strongly sensitive to the Aβ aggregation process due to the environmentally sensing ICT nature of 15 and showed off–on emissions during the agglomeration of Aβ. Probe 15 showed low cytotoxicity toward HaCat, HeLa, and bEnd.3 cells, and it was used for a brain-imaging Alzheimer’s disease model mice. The observed results illustrated the great potential of 15 for early in vivo diagnosis of Alzheimer’s disease. Because of ESIPT attractiveness, many fluorescent probes for different analytes have been synthesized including physiological pH, biological important cations and anions, ROS agents, and biomolecules 106.
