Near-Infrared Fluorescent Probes with Amine-Incorporated Xanthene Platforms for the Detection of Hypoxia

Deep-Red and Near-Infrared Compact Cyanine Dyes for Sensitive NAD(P)H Sensing in Live Cells and Kidney Disease Tissues

Cyanine dyes constructed for NAD(P)H near-infrared sensing utilize extended π-conjugation but often exhibit delayed fluorescence responses to NAD(P)H due to reduced positive charge density in 3-quinolinium acceptors. This study introduces deep-red and near-infrared compact cyanine dyes represented by probes A and B for mitochondrial NAD(P)H detection in live cells. Probes A and B feature a unique structural design with a double bond connection linking 3-quinolinium to strategically positioned 1-methylquinolinium acceptor units at 2- and 4-positions, correspondingly. Probe A absorbs at 359 and 531 nm, while probe B absorbs at 324 and 370 nm, emitting subtle fluorescence at 587 and 628 nm, respectively, with no NADH present. Upon NADH exposure, probes A and B exhibit significant emission enhancements at 612 and 656 nm, correspondingly, attributed to the efficient reduction of 3-quinolinium units to electron-donative 1-methyl-1,4-dihydroquinoline units. Probe B, chosen for its near-infrared emission and fast response to NAD(P)H, effectively monitored dynamic intracellular NAD(P)H levels throughout diverse experimental conditions. In HeLa cells, minimal basal fluorescence increased upon NADH stimulation. It also identified increased NAD(P)H levels following chemical treatments with acesulfame potassium, cisplatin, carboplatin, and temozolomide, CoCl2-induced hypoxia, and TLR4 activation in macrophages and in disease models of kidney pathology, where diseased tissues exhibited higher fluorescence than normal tissues. In fruit fly larvae under starvation conditions, probe B tracked NAD(P)H increases triggered by exogenous NADH, demonstrating its in vivo applicability for metabolic studies. These findings highlight probe B's utility in elucidating dynamic NAD(P)H fluctuations in diverse biological contexts, offering insights into mitochondrial function and cellular metabolism.

Near-Infrared Ratiometric Hemicyanine Fluorescent Probes for Monitoring Mitochondrial pH Dynamics in Live Cells during Oxidative Stress and Hypoxia

Novel near-infrared ratiometric molecules (probes A and B) produced by linking formyl-functionalized xanthene and methoxybenzene moieties, respectively, onto a xanthene-hemicyanine framework are detailed. Probe A exhibited a primary absorption peak at 780 nm and a shoulder peak at 730 nm and exhibited fluorescence at 740 nm↓ (signifies a downward shift in intensity upon acidification) in a pH 9.3 buffer and 780 nm↑ at pH 2.8 under excitation at 700 nm. Probe B featured absorptions at 618 and 668 nm at pH 3.2 and at 717 nm at pH 8.6, and fluorescence at 693 nm↑ at pH 3.2 and at 739 nm↓ at pH 8.6, in mostly the red to near-IR region. The ratiometric changes in the intensity of the fluorescent absorptions were reversed between A and B upon acidification as indicated by the arrows. Theoretical calculations confirmed that there were slight changes in conformation between probes and the protonated molecules, suggesting that the changes in emission spectra were due mostly to conjugation effects. Calculations at the APFD/6-311+g(d,p) level with a solvent described by the polarizable continuum model resulted in pK a values for A at 6.33 and B at 6.41, in good agreement with the experimentally determined value of 6.97 and an average of 6.40, respectively. The versatilities of the probes were demonstrated in various experimental contexts, including the effective detection of mitochondrial pH fluctuations. Live cell experiments involving exposure to different pH buffers in the presence of H+ ionophores, monitoring mitophagy processes during cell starvation, studying hypoxia induced by CoCl2 treatment, and investigating responses to various oxidative stresses are detailed. Our findings highlight the potential of attaching xanthene and methoxybenzaldehyde groups onto xanthene-hemicyanine structures as versatile tools for monitoring pH changes in a variety of cellular environments and processes.

A near-infrared fluorescence probe with large Stokes shift for selectively monitoring nitroreductase in living cells and mouse tumor models

Hypoxia is commonly regarded as a typical feature of solid tumors, which originates from the insufficient supply of oxygen. Herein, the development of an efficient method for assessing hypoxia levels in tumors is strongly desirable. Nitroreductase (NTR) is an overexpressed reductase in the solid tumors, has been served as a potential biomarker to evaluate the degrees of hypoxia. In this work, we elaborately synthesized a new near-infrared (NIR) fluorescence probe (MR) to monitor NTR activity for assessment of hypoxia levels in living cells and in tumors. Upon exposure of NTR, the nitro-unit of MR could be selectively reduced to amino-moiety with the help of nicotinamide adenine dinucleotide. Moreover, the obtained fluorophore emitted a prominent NIR fluorescence, because it possessed a classical "push-pull" structure. The MR displayed several distinguished characters toward NTR, including intense NIR fluorescent signals, large Stokes shift, high selectivity and low limit of detection (46 ng/mL). Furthermore, cellular confocal fluorescence imaging results validated that the MR had potential of detecting NTR levels in hypoxic cells. Significantly, using the MR, the elevated of NTR levels were successfully visualized in the tumor-bearing mouse models. Therefore, this detecting platform based on this probe may be tactfully constructed for monitoring the variations of NTR and estimating the degrees of hypoxia in tumors.

Near-infrared Absorption and Emission Probes with Optimal Connection Bridges for Live Monitoring of NAD(P)H Dynamics in Living Systems

Two NAD(P)H-biosensing probes consisting of 1,3,3-trimethyl-3H-indolium and 3-quinolinium acceptors, linked by thiophene, A, and 3,4-ethylenedioxythiophene, B, bridges are detailed. We synthesized probes C and D, replacing the thiophene connection in probe A with phenyl and 2,1,3-benzothiadiazole units, respectively. Probe E was prepared by substituting probe A's 3-quinolinium unit with a 1-methylquinoxalin-1-ium unit. Solutions are non-fluorescent but in the presence of NADH, exhibit near-infrared fluorescence at 742.1 nm and 727.2 nm for probes A and B, respectively, and generate absorbance signals at 690.6 nm and 685.9 nm. In contrast, probes C and D displayed pronounced interference from NADH fluorescence at 450 nm, whereas probe E exhibited minimal fluorescence alterations in response to NAD(P)H. Pre-treatment of A549 cells with glucose in the presence of probe A led to a significant increase in fluorescence intensity. Additionally, subjecting probe A to lactate and pyruvate molecules resulted in opposite changes in NAD(P)H levels, with lactate causing a substantial increase in fluorescence intensity, conversely, pyruvate resulted in a sharp decrease. Treatment of A549 cells with varying concentrations of the drugs cisplatin, gemcitabine, and camptothecin (5, 10, and 20 μM) led to a concentration-dependent increase in intracellular fluorescence intensity, signifying a rise in NAD(P)H levels. Finally, fruit fly larvae were treated with different concentrations of NADH and cisplatin illustrating applicability to live organisms. The results demonstrated a direct correlation between fluorescence intensity and the concentration of NADH and cisplatin, respectively, further confirming the efficacy of probe A in sensing changes in NAD(P)H levels within a whole organism.

Turn-on Rhodamine Glycoconjugates Enable Real-Time GLUT Activity Monitoring in Live Cells and In Vivo

The direct relationship between facilitative glucose transporters (GLUTs) and metabolic diseases opens new avenues for sensing metabolic deregulations and drives the development of molecular probes for GLUT-targeted detection of metabolic diseases. Radiotracer-based molecular imaging probes have been effectively utilized in reporting alterations in sugar uptake as an indication of metabolic deregulations, cancer development, or inflammation. Progress in developing fluorophore-based tools facilitated GLUT-specific analyses using more accessible fluorescence-based instrumentation. However, restrictions on the emission range of fluorophores and the requirement for substantial post-treatments to reduce background fluorescence have brought to light the critical directions for improvement of the technology for broader use in screening applications. Here we present turn-on GLUT activity reporters activated upon cells' internalization. We demonstrate a specific delivery of a sizable rhodamine B fluorophore through GLUT5 and showcase a stringent requirement in conjugate structure for maintaining a GLUT-specific uptake. With the turn-on GLUT probes, we demonstrate the feasibility of high-throughput fluorescence microscopy and flow cytometry-based GLUT activity screening in live cells and the probes' applicability for assessing sugar uptake alterations in vivo.

Highly Sensitive Cyanine Dyes for Rapid Sensing of NAD(P)H in Mitochondria and First-Instar Larvae of Drosophila melanogaster

We have developed two highly sensitive cyanine dyes, which we refer to as probes A and B. These dyes are capable of quick and sensitive sensing of NAD(P)H. The dyes were fabricated by connecting benzothiazolium and 2,3-dimethylnaphtho[1,2-d]thiazol-3-ium units to 3-quinolinium through a vinyl bond. In the absence of NAD(P)H, both probes have low fluorescence and absorption peaks at 370 and 400 nm, correspondingly. This is because of their two electron-withdrawing acceptor systems with high charge densities. However, when NAD(P)H reduces the probes' electron-withdrawing 3-quinolinium units to electron-donating 1,4-dihydroquinoline units, the probes absorb at 533 and 535 nm and fluoresce at 572 and 586 nm for A and B correspondingly. This creates well-defined donor-π-acceptor cyanine dyes. We successfully used probe A to monitor NAD(P)H levels in live cells during glycolysis, under hypoxic conditions induced by CoCl2 treatment and after treatment with cancer drugs, including cisplatin, camptothecin, and gemcitabine. Probe A was also employed to visualize NAD(P)H in Drosophila melanogaster first-instar larvae. We observed an increase in NAD(P)H levels in A549 cancer cells both under hypoxic conditions and after treatment with cancer drugs, including cisplatin, camptothecin, and gemcitabine.

Thiophene-based organic dye with large Stokes shift and deep red emission for live cell NAD(P)H detection under varying chemical stimuli

We report a novel method for synthesizing red and deep red cyanine dyes with large Stokes shifts, probes A and B, for live cell NAD(P)H detection. The probes were prepared using thiophene-based organic dyes featuring a π-conjugated bridge of thiophene and 3,4-ethylenedioxythiophene units linking the 1-methylquinolinium acceptor and formyl acceptor, respectively. These probes display weak absorption peaks at 315 nm (A) and 334 nm (B) and negligible fluorescence in the absence of NADH. However, upon the presence of NADH, new absorption and fluorescence peaks appear at 477 nm and 619 nm for probe A and at 486 nm and 576 nm for probe B, respectively. This is due to the NADH-facilitated reduction of the 1-methylquinolinium unit into 1-methyl-1,4-dihydroquinoline, which then acts as the electron donor for the probes, leading to the formation of well-defined electron donor-acceptor dye systems. Probe A has a large Stokes shift of 144 nm, which allows for better separation between the excitation and emission spectra, reducing spectral overlap and improving the accuracy of fluorescence measurements. The probes are highly selective for NAD(P)H, water-soluble, biocompatible, and easily permeable to cells. They are also photostable and were successfully used to monitor changes in NADH concentration in live cells during glycolysis in the presence of glucose, lactate, and pyruvate, treatment of FCCP and cancer drug cisplatin, and under hypoxia triggered by CoCl2. Furthermore, the probes were able to image NAD(P)H in Drosophila melanogaster larvae. Notably, cisplatin treatment increased the NAD(P)H concentration in A459 cells over time. Overall, this work presents a significant advancement in the field of live cell imaging by providing a simple and cost-effective method for detecting changes in NAD(P)H concentration under varying chemical stimuli.

Diagnosis of Bacterial Plant Diseases via a Nitroreductase-Activated Fluorescent Sensor

Plant diseases caused by bacteria have become one of the serious problems that threaten human food security, which led to the remarkable reduction of agricultural yields and economic loss. Nitroreductase (NTR), as an important biomarker, is highly expressed in bacteria, and the level of NTR is closely related to the progression of pathogen infection. Therefore, the design of small-molecule fluorescent sensors targeting NTR is of great significance for the detection and diagnosis of plant pathogenic bacteria. In this study, a new fluorescent sensor targeting NTR was discovered and then successfully applied to the imaging of zebrafish and pathogenic bacteria. Most importantly, the developed sensor achieved the real-time diagnosis of Brassica napus L. infected with bacteria, which provides a promising tool for examining the temporal and spatial infection of plant pathogens in precision agriculture.