Construction of a Rational-Designed Multifunctional Platform Based on a Fluorescence Resonance Energy Transfer Process for Simultaneous Detection of pH and Endogenous Peroxynitrite

A mitochondria-targeted fluorescent sensor for imaging endogenous peroxynitrite changes in acute lung injury

Acute lung injury (ALI) is a serious pulmonary inflammatory disease resulting from excessive reactive oxygen species (ROS) which could cause the damage of the alveolar epithelial cells and capillary endothelial cells. Peroxynitrite, as one of short-lived reactive oxygen species, is closely related to the process of ALI. Thus, it is important to monitor the fluctuation of peroxynitrite in living system for understanding the process of ALI. Herein, the novel mitochondria-targeted fluorescent probe BHMT was designed to respond to peroxynitrite and pH with distinct fluorescence properties respectively. The absorption spectrum of the probe BHMT exhibited a notable red shift as the pH value declined from 8.8 to 2.6. Upon reaction with peroxynitrite, BHMT had a significant increase of fluorescence intensity (63-fold) with maintaining a detection limit of only 43.7 nM. Furthermore, BHMT could detect the levels of endogenous peroxynitrite and image the intracellular pH in ratiometric channels utilizing cell imaging. In addition, BHMT was successfully applied to revealing the relationship between the peroxynitrite and the extent of ALI. Thus, these results indicated the probe BHMT could be a potential tool for diagnosing the early stage of ALI and revealed the peroxynitrite was likely to be a crucial therapeutic target in ALI treatment.

Single-Cell Sensitive Colorimetric pH Detection Based on Microscope Ratiometric Grayscale

Rapid and accurate identification of the intracellular pH is critical in the field of biomedicine. In this work, we effectively identified and quantified the intracellular pH and its distribution at the single-cell level using an image sensor based on an ordinary bright-field optical microscope that divided the cell staining images into their red (R) and blue (B) channels. The grayscale of the R and B channels was subjected to a ratiometric operation to generate ratiometric grayscale cell images of the microscope. A standard curve of pH against ratiometric grayscale curve was then obtained by incubating HeLa cells at pH 6.00-7.60 in a high concentration K+ ion buffer solution containing nigericin for obtaining certain intracellular pH values. A good correlation was evidenced between pH and the ratiometric grayscale of the R and B channels in the pH range of 6.00-7.60. Subsequently, the intracellular pH value of the A549 cells under the experimental conditions was measured to be 7.22 ± 0.01 by the method. Furthermore, the changes in the intracellular pH of HeLa cells stimulated with hydrogen peroxide were sensitively monitored, which demonstrated the applicability of the method. Due to its ease of use, the developed colorimetric microscopy pH detection and monitoring method provide prospects for pH-related single-cell studies.

Recent advances in fluorescent probes for dual-detecting ONOO- and analytes

Although peroxynitrite (ONOO-) plays an essential role in cellular redox homeostasis, its excess ONOO- will affect the normal physiological function of cells. Therefore, real-time monitoring of changes in local ONOO- will contribute to further revealing the biological functions. Reliable and accurate detection of biogenic ONOO- will definitely benefit for disentangling its complex functions in living systems. In the past few years, more fluorescent probes have been developed to help understand and reveal cellular ONOO- changes. However, there has been no comprehensive and critical review of multifunctional fluorescent probes for cellular ONOO- and other analytes. To highlight the recent advances, this review first summarized the recent progress of multifunctional fluorescent probes since 2018, focusing on molecular structures, response mechanisms, optical properties, and biological imaging in the detection and imaging of cellular ONOO- and analytes. We classified and discussed in detail the limitations of existing multifunctional probes, and proposed new ideas to overcome these limitations. Finally, the challenges and future development trends of ONOO- fluorescence probes were discussed. We hoped this review will provide new research directions for developing of multifunctional fluorescent probes and contribute to the early diagnosis and treatment of diseases.

A novel turn-on fluorescent probe for detection of pH in extremely acidic environment and its application

A water-soluble turn-on fluorescent probe PNAP for pH has been designed and synthesized. PNAP was consist of pyrene as fluorophore and morpholine as receptor. Owing to the photoinduced electron transfer (PET) effect, the fluorescence of PNAP was quenched, while PNAP exhibited a remarkable "turn-on" fluorescence with the increase of acidity. Notably for its pKa of 2.15, PNAP was one of the pH fluorescent probes used in extremely acidic environments. Furthermore, PNAP also displayed good repeatability, strong anti-ion interference ability, high sensitivity and selectivity toward pH. In addition, PNAP has been successfully applied to the test strips and monitor the pH of environment water samples and realistic samples, showing its good promising prospect.

Exploring the microscopic changes of lipid droplets and mitochondria in alcoholic liver disease via fluorescent probes with high polarity specificity

Alcoholic liver disease (ALD) has received extensive attention because of the increasing alcohol consumption globally as well as its high morbidity. It is reported that absorbed alcohol can cause lipid metabolism disorder and mitochondria dysfunction, so here in this work, we planned to study the microscopic changes of the two organelles, lipid droplets (LDs) and mitochondria in hepatocyte, under the stimulation of alcohol, hoping to present some meaningful information for the theranostics of ALD by the technique of fluorescence imaging. Guided by theoretical calculation, two fluorescent probes, named CBu and CBuT, were rationally designed. Although constructed by the same chromophore scaffold, they stained different organelles efficiently and emitted distinctively. CBu with high lipophilicity, ascribed to the two butyl groups, can selectively localize in LDs with green fluorescence, while CBuT bearing a triphenylphosphine unit can specifically target mitochondria due to electrostatic interactions with near-infrared (NIR) fluorescence. Both probes displayed remarkable selectivity and sensitivity to polarity, free from the environmental interferences including viscosity, pH and other bio-species. With these two probes, the accumulation of LDs and polarity decrease in mitochondria were clearly monitored at the green and red channels, respectively, in the ALD cell model. CBuT was further applied to image the mice with ALD in vivo. In short, we have confirmed the valuable organelles, LDs and mitochondria, for ALD study and provided two potent molecular tools to visualize their changes through fluorescence imaging, which would be favorable for the further development of theranostics for ALD.

New "Destruction Seek to Survive" Strategy Based on a Serum Albumin Assembly with a Squaraine Molecule for the Detection of Peroxynitrite

Peroxynitrite (ONOO^-), a kind of active nitrogen species, plays an important role in biological systems. Overproduction of ONOO^- is closely related to the pathogenesis of many diseases. Therefore, it is necessary to quantify intracellular ONOO^- for differentiating health and disease states. Fluorescent probes with near-infrared (NIR) fluorescence can detect ONOO^- with high sensitivity and selectivity. However, there is an inevitable problem that many NIR fluorophores are easily oxidized by ONOO^- to give a false-negative result. To avoid this problem, herein, we ingeniously propose a "destruction to seek to survive" strategy to detect ONOO^-. Two NIR squaraine (SQ) dyes were connected together to form a fluorescent probe (SQDC). This method utilizes the destructive effect of peroxynitrite on one of the SQ moieties of SQDC to eliminate the steric hindrance, enabling the other "survived" SQ segment to enter the hydrophobic cavity of bovine serum albumin (BSA) via the well-known host-guest interactions. The encapsulation of albumin protects the "survived" SQ from further attack of ONOO^-. As a result, a NIR fluorescence turn-on response coming from the host-guest interaction between BSA and the "survived" SQ escaped from SQDC was found, which can be used for the detection of ONOO^-. The assembly of SQDC mixed with BSA can be located in mitochondria to detect endogenous and exogenous ONOO^- sensitively in living cells. As a proof-of-concept method, it is envisioned that this novel detection strategy with a simple assembly would become a powerful means for the detection of ONOO^- when employing NIR fluorophores.

A highly selective red-emitting fluorescent probe and its micro-nano-assembly for imaging endogenous peroxynitrite (ONOO-) in living cells

Endogenous peroxynitrite plays a very important role in the regulation of life activities. However, validated tools for ONOO^- tests are currently insufficient. We designed a fluorescent probe TPA-F-NO₂ with a low fluorescence background in water based on the D-π-A structure for the imaging of endogenous ONOO^- in living cells. TPA-F-NO₂ can realize the naked eye detection of ONOO^- due to the obvious color change. TPA-F-NO₂ has the advantages of large stokes shift, high signal-to-noise ratio, high selectivity and sensitivity. The quantitative detection can be achieved in the range of 0-14 μM ONOO^-. Due to its solvatochromic characteristics, TPA-F-NO₂ has the potential to be used in OLEDs and other fields. In addition, 4-methylumbelliferone has a wide range of anticancer effects as an inhibitor of hyaluronic acid. We prepared TPA-MU-NPs by assembling TPA-F-NO₂ and 4-methylumbelliferone. It also endows TPA-MU-NPs with ONOO^- imaging function and anti-proliferation effect on breast cancer cells and other cells. This 'probe-drug' assembly strategy provides ideas for the design and optimization of dual-functional probes.

pH-Switched Near-Infrared Fluorescent Strategy for Ratiometric Detection of ONOO- in Lysosomes and Precise Imaging of Oxidative Stress in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is well-known as a kind of autoimmune disease, which brings unbearable pain to the patients by multiple organ complications besides arthritis. To date, RA can be hardly cured, but early diagnosis and standard treatment can relieve symptoms and pain. Therefore, an effective tool to assist the early diagnosis of RA deserves considerable attention. On account of the overexpressed ONOO^- during the early stage of RA, a near-infrared (NIR) receptor, Lyso-Cy, is proposed in this work by linker chemistry to expand the conjugated rhodamine framework by cyanine groups. Contributed by the pH-sensitive spiral ring in rhodamine, receptor Lyso-Cy has been found to be workable in lysosomes specifically, which was confirmed by the pH-dependent spectra with a narrow responding region and a well-calculated pK_a value of 5.81. We presented an excellent ratiometric sensing protocol for ONOO^- in an acidic environment, which was also available for targeting ONOO^- in lysosomes selectively. This innovative dual-targeting responsive design is expected to be promising for assisting RA diagnosis at an early stage with respect to the joint inflammatory model established in this work at the organism level.

Enhancing probe's sensitivity for peroxynitrite through alkoxy modification of dicyanovinylchromene

An intramolecular charge transfer (ICT)-based fluorescent probe P-ONOO^- was synthesized to detect ONOO^-. After responding to peroxynitrite, the dicyano-vinyl group of P-ONOO^- generates the aldehyde group, emitting strong green fluorescence accompanied by quenching of the yellow fluorescence. According to the calculated Fukui function, the modification of the alkoxy group can enhance the f⁺ of P-ONOO^-, which can enhance the probe's nucleophilic addition reactivity with ONOO^-. It has been experimentally verified that P-ONOO^- shows fast response (within 30 s), excellent sensitivity (the detection limit = 10.4 nM), and good selectivity towards ONOO^-. Additionally, the probe P-ONOO^- has high membrane permeability and good biocompatibility, which can image endogenous ONOO^- and exogenous ONOO^- in HeLa cells.

A bifunctional fluorescence probe for dual-channel detecting of mitochondrial viscosity and endogenous/exogenous peroxynitrite

The irregular viscosity in the mitochondrial can induce mitochondrial dysfunction. The content of peroxynitrite (ONOO^-) is related to various physiological and pathological processes. However, many mitochondrial probes only realized the detection of viscosity or ONOO^- in single channel, thus it is necessary to explore single fluorescence probe for dual-detecting mitochondrial viscosity and ONOO^-. In this work, we designed and synthesized a novel fluorescence probe (PV) for dual-detecting viscosity and ONOO^-, which was composed by intergrating a ONOO^-- responsive arlyboronate with a twisting intramolecular charge transfer (TICT) mechanism and possessed the mitochondria-targeting ability due to its pyridinium cation. PV exhibited a significant increase in viscosity with red emission at 582 nm and high sensitivity to ONOO^- levels with yellow emission at 507 nm. PV was also applied to living systems (including living cells and zebrafish) for viscosity and ONOO^- detection using two different channels. Moreover, the ability of PV to track mitophagy may make ONOO^- a powerful tool for its role in mitophagy.