In Vivo Platelet Detection Using a Glycoprotein IIb/IIIa-Targeted Near-Infrared Fluorescence Imaging Probe

Platelets play a prominent role in multiple diseases, in particular arterial and venous thrombosis and also in atherosclerosis and cancer. To advance the in vivo study of the biological activity of this cell type from a basic experimental focus to a clinical focus, new translatable platelet-specific molecular imaging agents are required. Herein, we report the development of a near-infrared fluorescence probe based upon tirofiban, a clinically approved small-molecule glycoprotein IIb/IIIa inhibitor (GPIIb/IIIa). Through in vitro experiments with human platelets and in vivo ones in a murine model of deep-vein thrombosis, we demonstrate the avidity of the generated probe for activated platelets, with the added benefit of a short blood half-life, thereby enabling rapid in vivo visualization within the vasculature.

Blood Accessibility to Fibrin in Venous Thrombosis is Thrombus Age-Dependent and Predicts Fibrinolytic Efficacy: An In Vivo Fibrin Molecular Imaging Study

Fibrinolytic therapy of venous thromboembolism (VTE) is increasingly utilized, yet limited knowledge is available regarding in vivo mechanisms that govern fibrinolytic efficacy. In particular, it is unknown how age-dependent thrombus organization limits direct blood contact with fibrin, the target of blood-based fibrinolytic agents. Utilizing high-resolution in vivo optical molecular imaging with FTP11, a near-infrared fluorescence (NIRF) fibrin-specific reporter, here we investigated the in vivo interrelationships of blood accessibility to fibrin, thrombus age, thrombus neoendothelialization, and fibrinolysis in murine venous thrombosis (VT). In both stasis VT and non-stasis VT, NIRF microscopy showed that FTP11 fibrin binding was thrombus age-dependent. FTP11 localized to the luminal surface of early-stage VT, but only minimally to subacute VT (p<0.001). Transmission electron microscopy of early stage VT revealed direct blood cell contact with luminal fibrin-rich surfaces. In contrast, subacute VT exhibited an encasing CD31+ neoendothelial layer that limited blood cell contact with thrombus fibrin in both VT models. Next we developed a theranostic strategy to predict fibrinolytic efficacy based on the in vivo fibrin accessibility to blood NIRF signal. Mice with variably aged VT underwent FTP11 injection and intravital microscopy (IVM), followed by tissue plasminogen activator infusion to induce VT fibrinolysis. Fibrin molecular IVM revealed that early stage VT, but not subacute VT, bound FTP11 (p<0.05), and experienced higher rates of fibrinolysis and total fibrinolysis (p<0.05 vs. subacute VT). Before fibrinolysis, the baseline FTP11 NIRF signal predicted the net fibrinolysis at 60 minutes (p<0.001). Taken together, these data provide novel insights into the temporal evolution of VT and its susceptibility to therapeutic fibrinolysis. Fibrin molecular imaging may provide a theranostic strategy to identify venous thrombi amenable to fibrinolytic therapies.

Site-specific labeling of proteins and peptides with trans-cyclooctene containing handles capable of tetrazine ligation

There is a growing library of functionalized non-natural substrates for the enzyme protein farnesyltransferase (PFTase). PFTase covalently attaches these functionalized non-natural substrates to proteins ending in the sequence CAAX, where C is a cysteine that becomes alkylated, A represents an aliphatic amino acid, and X is Ser, Met, Ala, or Gln. Reported substrates include a variety of functionalities that allow modified proteins to undergo subsequent bioconjugation reactions. To date the most common strategy used in this approach has been copper catalyzed azide-alkyne cycloaddition (CuAAC). While being fast and bioorthogonal CuAAC has limited use in live cell experiments due to copper's toxicity.(1) Here, we report the synthesis of trans-cyclooctene geranyl diphosphate. This substrate can be synthesized from geraniol in six steps and be enzymatically transferred to peptides and proteins that end in a CAAX sequence. Proteins and peptides site-specially modified with trans-cyclooctene geranyl diphosphate were subsequently targeted for further modification via tetrazine ligation. As tetrazine ligation is bioorthogonal, fast, and is contingent on ring strain rather than the addition of a copper catalyst, this labeling strategy should prove useful for labeling proteins where the presence of copper may hinder solubility or biological reactivity.

Upconverting organic dye doped core-shell nano-composites for dual-modality NIR imaging and photo-thermal therapy

Nanotechnology approaches offer the potential for creating new optical imaging agents with unique properties that enable uses such as combined molecular imaging and photo-thermal therapy. Ideal preparations should fluoresce in the near-infrared (NIR) region to ensure maximal tissue penetration depth along with minimal scattering and light absorption. Due to their unique photophysical properties, upconverting ceramics such as NaYF₄:Er³⁺,Yb³⁺ nanoparticles have become promising optical materials for biological imaging. In this work, the design and synthesis of NaYF₄:Er³⁺,Yb³⁺@SiO₂ core-shell nano-composites, which contain highly absorbing NIR carbocyanine dyes in their outer silica shell, are described. These materials combine optical emission (from the upconverting core nanoparticle) with strong NIR absorption (from the carbocyanine dyes incorporated into the shell) to enable both optical imaging and photo-thermal treatment, respectively. Ultimately, this hybrid composite nanomaterial approach imparts the ability to both visualize, via upconversion imaging, and treat, via photo-thermal heating, using two distinct optical channels. Proof-of-principle in vitro experiments are presented to demonstrate the combined imaging and photo-thermal properties of this new functional nano-composite.

Bioorthogonal reaction pairs enable simultaneous, selective, multi-target imaging

Mutually orthogonal tetrazine–transcyclooctene and azide–cyclooctyne cycloaddition reactions were used simultaneously for the bioorthogonal labeling of two different live cell populations in the same culture. These small-molecule probes show good chemical reactivity and can be readily incorporated into biological systems.

Synthesis and evaluation of a series of 1,2,4,5-tetrazines for bioorthogonal conjugation

1,2,4,5-Tetrazines have been established as effective dienes for inverse electron demand [4 + 2] Diels-Alder cycloaddition reactions with strained alkenes for over 50 years. Recently, this reaction pair combination has been applied to bioorthogonal labeling and cell detection applications; however, to date, there has been no detailed examination and optimization of tetrazines for use in biological experiments. Here, we report the synthesis and characterization of 12 conjugatable tetrazines. The tetrazines were all synthesized in a similar fashion and were screened in parallel to identify candidates most ideally suited for biological studies. In depth follow-up studies revealed compounds with varying degrees of stability and reactivity that could each be useful in different bioorthogonal applications. One promising, highly stable, and water-soluble derivative was used in pretargeted cancer cell labeling studies, confirming its utility as a bioorthogonal moiety.

Modular strategy for the construction of radiometalated antibodies for positron emission tomography based on inverse electron demand Diels-Alder click chemistry

A modular system for the construction of radiometalated antibodies was developed based on the bioorthogonal cycloaddition reaction between 3-(4-benzylamino)-1,2,4,5-tetrazine and the strained dienophile norbornene. The well-characterized, HER2-specific antibody trastuzumab and the positron emitting radioisotopes ⁶⁴Cu and ⁸⁹Zr were employed as a model system. The antibody was first covalently coupled to norbornene, and this stock of norbornene-modified antibody was then reacted with tetrazines bearing the chelators 1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetraacetic acid (DOTA) or desferrioxamine (DFO) and subsequently radiometalated with ⁶⁴Cu and ⁸⁹Zr, respectively. The modification strategy is simple and robust, and the resultant radiometalated constructs were obtained in high specific activity (2.7–5.3 mCi/mg). For a given initial stoichiometric ratio of norbornene to antibody, the ⁶⁴Cu-DOTA- and ⁸⁹Zr-DFO-based probes were shown to be nearly identical in terms of stability, the number of chelates per antibody, and immunoreactivity (>93% in all cases). In vivo PET imaging and acute biodistribution experiments revealed significant, specific uptake of the ⁶⁴Cu- and ⁸⁹Zr-trastuzumab bioconjugates in HER2-positive BT-474 xenografts, with little background uptake in HER2-negative MDA-MB-468 xenografts or other tissues. This modular system—one in which the divergent point is a single covalently modified antibody stock that can be reacted selectively with various chelators—will allow for both greater versatility and more facile cross-comparisons in the development of antibody-based radiopharmaceuticals.

Facile Synthesis of Monofunctional Pentamethine Carbocyanine Fluorophores

A high yield route to symmetric, conjugatable pentamethine carbocyanine dyes with far-red/near infrared (NIR) emission between 650 and 700 nm is reported. The dyes are prepared via condensation of indolium or benz[e]indolium inner salts with an alkyl carboxylic acid derivatized malonaldehyde dianil or alternatively in a one-pot reaction without isolation of the malonaldehyde intermediate. The fluorophores are water-soluble, have bright fluorescence emission, are easily prepared in good yield, and are promising candidates for use in a variety of biochemical and in vivo imaging applications.

Development of a bioorthogonal and highly efficient conjugation method for quantum dots using tetrazine-norbornene cycloaddition

We present a bioorthogonal and modular conjugation method for efficient coupling of organic dyes and biomolecules to quantum dots (QDs) using a norbornene-tetrazine cycloaddition. The use of noncoordinating functional groups combined with the rapid rate of the cycloaddition leads to highly efficient conjugation. We have applied this method to the in situ targeting of norbornene-coated QDs to live cancer cells labeled with tetrazine-modified proteins.

Bioorthogonal chemistry amplifies nanoparticle binding and enhances the sensitivity of cell detection

Nanoparticles have emerged as key materials for biomedical applications because of their unique and tunable physical properties, multivalent targeting capability, and high cargo capacity. Motivated by these properties and by current clinical needs, numerous diagnostic and therapeutic nanomaterials have recently emerged. Here we describe a novel nanoparticle targeting platform that uses a rapid, catalyst-free cycloaddition as the coupling mechanism. Antibodies against biomarkers of interest were modified with trans-cyclooctene and used as scaffolds to couple tetrazine-modified nanoparticles onto live cells. We show that the technique is fast, chemoselective, adaptable to metal nanomaterials, and scalable for biomedical use. This method also supports amplification of biomarker signals, making it superior to alternative targeting techniques including avidin/biotin.

Divergent oriented synthesis for the design of reagents for protein conjugation

Instead of using diversity oriented syntheses (DOS) to obtain compounds with biological activities, we employed the DOS method to efficiently obtain multifunctional single attachment point (MSAP) reagents for the conjugation to proteins. Acid insensitive functional groups (chelators, fluorochromes) were attached to Lys-Cys-NH(2) or Lys-Lys-betaAla-Cys-NH(2) peptide scaffolds. After cleavage from solid supports, the modified peptide intermediates were split and further modified by two solution phase, chemoselective reactions employing the single amine and single thiol presented on the intermediates. MSAP-based fluorochrome-chelates were obtained, some possessing a third functional group like a polyethylene glycol (PEG) polymer or "click chemistry" reactive alkynes and azides. The DOS of MSAP reagents permitted the efficient generation of panels of MSAP reagents that can be used to obtain multifunctional proteins with a single modified amino acid (a single attachment point).

Oxazine conjugated nanoparticle detects in vivo hypochlorous acid and peroxynitrite generation

The current lack of suitable probes has limited the in vivo imaging of reactive oxygen/nitrogen species (ROS/RNS). ROS/RNS are often generated by ischemia-induced inflammation; defining the extent of tissue involvement or ROS/RNS-related damage would have a significant clinical impact. We present the preparation and demonstration of a fluorogenic sensor for monitoring peroxynitrite (ONOO(-)) and myeloperoxidase (MPO) mediated hypochlorous acid (HOCl/OCl(-)) production. The sensor consists of a long circulating biocompatible nanoparticle that targets phagocytic cells in vivo and is coated with approximately 400 quenched oxazine fluorophores that are released by reaction with HOCl or ONOO(-) but are stable toward oxidants such as hydroxyl radical, hydrogen peroxide, and superoxide. MPO-dependent probe activation is chloride ion dependent and is negated in flow cytometry studies of MPO inhibitor treated neutrophils. Fluorescence reflectance imaging and microscopic fluorescence imaging in mouse hearts after myocardial infarction showed probe release into neutrophil-rich ischemic areas, making this ROS/RNS sensor a novel prognostic indicator.

Labels and probes for live cell imaging: overview and selection guide

Fluorescence imaging is an important tool for molecular biology research. There is a wide array of fluorescent labels and activatable probes available for investigation of biochemical processes at a molecular level in living cells. Given the large number of potential imaging agents and numerous variables that can impact the utility of these fluorescent materials for imaging, selection of the appropriate probes can be a difficult task. In this report an overview of fluorescent imaging agents and details on their optical and physical properties that can impact their function are presented.

Transillumination fluorescence imaging in mice using biocompatible upconverting nanoparticles

We report on a systematic study of upconverting fluorescence signal generation within turbid phantoms and real tissues. An accurate three-point Green's function, describing the forward model of photon propagation, is established and experimentally validated. We further demonstrate, for the first time to our knowledge, autofluorescence-free transillumination imaging of mice that have received biocompatible upconverting nanoparticles. The method holds great promise for artifact-free whole-body visualization of optical molecular probes.

Upconverting luminescent nanomaterials: application to in vivo bioimaging

In this report, the development of multi-channel anti-Stokes luminescent Y2O3 nanoparticles for application to in vivo upconversion imaging is detailed.

A DNA-binding Gd chelate for the detection of cell death by MRI

GadoTO, a MR contrast agent for the detection of cell death, consists of a nucleic acid-binding fluorophore attached to a gadolinium chelate.

Monofunctional carbocyanine dyes for bio- and bioorthogonal conjugation

A facile synthetic route to prepare monofunctional carbocyanine dyes for biological application is developed. Three pentamethine carbocyanine dyes have been successfully modified with a variety of functional groups such as: carboxylic acids, azides, or alkynes. The new dyes are characterized by strong NIR fluorescence emission, high extinction coefficients and good quantum yields. The azide and alkyne dyes have potential utility as components in bioorthogonal labeling schemes via [2 + 3] dipolar cycloaddition "click" reactions. The application of one derivative, CyAM-5 alkyne, for bioorthogonal labeling is demonstrated. Fluorescence microscopy shows coupling of CyAM-5 alkyne to Chinese hamster ovary (CHO) cells preincubated with azide modified glycans.

Tetrazine-based cycloadditions: application to pretargeted live cell imaging

Bioorthogonal tetrazine cycloadditions have been applied to live cell labeling. Tetrazines react irreversibly with the strained dienophile norbornene forming dihydropyrazine products and dinitrogen. The reaction is high yielding, selective, and fast in aqueous media. Her2/neu receptors on live human breast cancer cells were targeted with a monoclonal antibody modified with a norbornene. Tetrazines conjugated to a near-infrared fluorochrome selectively and rapidly label the pretargeted antibody in the presence of serum. These findings indicate that this chemistry is suitable for in vitro labeling experiments, and suggests that it may prove a useful strategy for in vivo pretargeted imaging under numerous modalities.

Near infrared fluorescence-based bacteriophage particles for ratiometric pH imaging

Fluorogenic imaging agents emitting in the near-infrared are becoming important research tools for disease investigation in vivo. Often pathophysiological states such as cancer and cystic fibrosis are associated with disruptions in acid/base homeostasis. The development of optical sensors for pH imaging would facilitate the investigation of these diseased conditions. In this report, the design and synthesis of a ratiometric near-infrared emitting probe for pH quantification is detailed. The pH-responsive probe is prepared by covalent attachment of pH-sensitive and pH-insensitive fluorophores to a bacteriophage particle scaffold. The pH-responsive cyanine dye, HCyC-646, used to construct the probe, has a fluorogenic pKa of 6.2, which is optimized for visualization of acidic pH often associated with tumor hypoxia and other diseased states. Incorporation of pH-insensitive reference dyes enables the ratiometric determination of pH independent of the probe concentration. With the pH-responsive construct, measurement of intracellular pH and accurate determination of pH through optically diffuse biological tissue is demonstrated.

A fluorescent probe for the detection of myeloperoxidase activity in atherosclerosis-associated macrophages

The myeloperoxidase (MPO)-derived oxidant hypochlorous acid (HOCl/OCl(-)) is implicated in the pathogenesis of atherosclerosis and other inflammatory states. We have synthesized an imaging probe, sulfonaphthoaminophenyl fluorescein (SNAPF), that selectively reacts with HOCl. SNAPF detects HOCl produced by stimulated MPO-expressing cells cultured from human whole blood, as well as HOCl from bone marrow (BM)-derived macrophages isolated from transgenic mice that express human MPO. Two lines of evidence indicate that SNAPF permits the in vivo imaging of HOCl production. First, we used this approach to demonstrate HOCl production by neutrophils in experimental murine peritonitis. Second, we detected HOCl production by MPO expressing cells in human atherosclerotic arteries. Thus, fluorescence reflectance imaging by SNAPF may provide a valuable noninvasive molecular imaging tool for implicating HOCl and MPO in the damage of inflamed tissues.

A highly selective fluorescent probe for thiol bioimaging

A new fluorescent turn-on probe (3) for the selective sensing and bioimaging of thiols is reported. In aqueous buffer solutions at physiological pH, thiols cleave the 2,4-dinitrobenzenesulfonyl group to release the red-emissive donor-acceptor fluorophore (4). The probe displays excellent immunity to interference from nitrogen and oxygen nucleophiles and the imaging of thiols in living cells is demonstrated.

Optimized pH-responsive cyanine fluorochromes for detection of acidic environments

Modulation of pH-responsive cyanine dye pK(a) values via heteroatom substitution allows for design of fluorescent reporters that are tuned for potential imaging of biologically relevant acidic environments.

Sugar sensing based on induced pH changes

A sensory assembly consisting of a pH sensitive NIR dye and an arylboronic acid shows ratiometric absorption changes with increased fluorescence intensity upon addition of sugar in aqueous media; this demonstrates a new signal transduction mechanism for the detection of sugar based on pH changes induced in the microenvironment of the sensory assembly.

Enzyme-targeted fluorescent imaging probes on a multiple antigenic peptide core

Peptide dendrimers have a variety of applications in biology such as the vehicles for drug and gene delivery, molecular inhibitors, protein mimics, and synthetic vaccines. The multiple antigenic peptide (MAP) system is a well-known example of a discrete, dendrimeric scaffold. We explored a novel application of the MAP-based scaffold by designing molecular probes that fluoresce only after enzymatic treatment. The probes, which were synthesized on solid support, incorporate a cathepsin S dipeptide substrate (Leu-Arg), and a poly(ethylene glycol) (PEG) chain in their dendritic arms. The fluorescence emission of the near-infrared fluorochromes attached to the N-termini of the dendritic arms was quenched. Mechanistic studies revealed formation of H-type dye aggregates within the tetravalent MAP system. By varying the length of the PEG chain, three probes were synthesized, CyPEG-1, CyPEG-2, and CyPEG-3 with 4, 8, and 12 ethylene oxide units, respectively. CyPEG-2 showed optimum aqueous solubility and quenching efficiency for imaging applications. Upon proteolytic activation with cathepsin S (EC 3.4.22.27), CyPEG-2 showed greater than 70-fold increase and more than 95% recovery in fluorescence emission.

Monofunctional Near-Infrared Fluorochromes for Imaging Applications

In this report, the development of a new class of monocarboxylate functionalized cyanine derivatives using improved synthetic procedures is detailed. The employed synthetic strategy relies on efficient nucleophilic attack of alkyl-thiols on cyanine dyes bearing chloro-substituted polymethinic linkers. Monocarboxylate derivatized fluorochromes (CyTE dyes) can be prepared in one step in greater than 90% yield without the need for additional purification. Several of the fluorochromes synthesized by this route show no tendency to aggregate in aqueous solution and have excitation and emission maxima greater than 800 nm. The potential utility of the CyTE fluorochromes was demonstrated through direct labeling of phage displaying a vascular cellular adhesion molecule-1 (VCAM-1) targeting peptide. Endothelial cell internalization of the VCAM-1 targeted phage was monitored via near-infrared fluorescence microscopy.

Nitric oxide reactivity of fluorophore coordinated carboxylate-bridged diiron(II) and dicobalt(II) complexes

The synthesis, structural characterization, and NO reactivity of carboxylate-bridged dimetallic complexes were investigated. The diiron(II) complex [Fe(2)(mu-O(2)CAr(Tol))(4)(Ds-pip)(2)] (1), where O(2)CAr(Tol) = 2,6-di(p-tolyl)benzoate and Ds-pip = dansyl-piperazine, was prepared and determined by X-ray crystallography to have a paddlewheel geometry. This complex reacts with NO within 1 min with a concomitant 4-fold increase in fluorescence emission intensity ascribed to displacement of Ds-pip. Although the diiron complex reacts with NO, as revealed by infrared spectroscopic studies, its sensitivity to dioxygen renders it unsuitable as an atmospheric NO sensor. The air-stable dicobalt(II) analogue was also synthesized and its reactivity investigated. In solution, the dicobalt(II) complex exists as an equilibrium between paddlewheel [Co(2)(mu-O(2)CAr(Tol))(4)(Ds-pip)(2)] (2) and windmill [Co(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(Ds-pip)(2)] (3) geometric isomers. Conditions for crystallizing pure samples of each of these isomers are described. Reaction of 2 with excess NO proceeds by reductive nitrosylation giving [Co(mu-O(2)CAr(Tol))(2)(NO)(4)] (5), which is accompanied by release of the Ds-pip fluorophore that is N-nitrosated in the process. This reaction affords an overall 9.6-fold increase in fluorescence emission intensity, further demonstrating the potential utility of ligand dissociation as a strategy for designing fluorescence-based sensors to detect nitric oxide in a variety of contexts.

Cobalt Chemistry with Mixed Aminotroponiminate Salicylaldiminate Ligands: Synthesis, Characterization, and Nitric Oxide Reactivity

A new class of mixed aminotroponimine salicylaldimine ligands and their corresponding cobalt(II) complexes are reported. This work expands the family of cobalt(II) aminotroponiminato complexes to include salicylaldiminate and derivatized fluorescein moieties. The H2iPrSATI-n (n = 3, 4) ligands 3 and 4, respectively, contain an aminotroponimine moiety and a salicylaldimine fragment connected with an alkyl linker. In the H2iPrFATI-n (n = 3, 4) ligands 5 and 6, a derivatized fluorescein replaces the salicylaldimine fragment. The cobalt(II) complexes [Co(iPrSATI-3)] (7) and [Co2(iPrSATI-4)(2)] (9) were prepared and structurally characterized. The reaction of NO with both complexes ultimately results in the formation of a dinitrogen-containing species. The mononitrosyl, [Co(iPrSATI-3)(NO)] (8), was isolated and characterized. The reactivity of [Co(iPrFATI-3)] (10) and [Co(iPrFATI-4)] (11) with NO mimics that observed for the salicylaldimine derivatives, as monitored by solution IR spectroscopy. When followed by fluorescence spectroscopy, reaction of 11 with NO evoked a 3-fold increase in emission intensity after 22 h.

Synthesis and characterization of zinc sensors based on a monosubstituted fluorescein platform

The synthesis of a new fluorescein carboxaldehyde asymmetrically substituted on the xanthene (top) ring is reported. This molecule is a key precursor for two of three monofunctionally derivatized fluorescein-based Zn(II) sensors presented in this work. Detailed preparative routes to, and photophysical characterization of, these sensors are described. The sensors are based on the previously reported ZP4 motif (Burdette, S. C.; Frederickson, C. J.; Bu, W.; Lippard, S. J. J. Am. Chem. Soc. 2003, 125, 1778-1787) and incorporate a di(2-picolyl)amine-containing aniline-derivatized ligand framework. By varying the nature of the substituent (X) para to the aniline nitrogen atom, which is responsible for PET quenching of the unbound ZP dye, we investigated the extent to which such electronic tuning might improve the fluorescent properties of asymmetrical ZP sensors. Although a comparison of probes with X = H, F, Cl, OMe reveals that the photophysical behavior of these dyes is not readily predictable, our methodology illustrates the ease with which aniline-based ligands may be linked to fluorescein dyes.

Dirhodium Tetracarboxylate Scaffolds as Reversible Fluorescence-Based Nitric Oxide Sensors

We report the synthesis and characterization of dirhodium tetracarboxylate complexes [Rh(2)(mu-O(2)CR)(4)(L)(2)], with R = Me and L = dansyl-imidazole (Ds-im) or dansyl-piperazine (Ds-pip). The fluorophores coordinate to the axial sites of the dirhodium core through the imidazole or piperazine N-atom and emit only weakly when excited at 365 or 345 nm for the Ds-im and Ds-pip complexes, respectively. These fluorophore-containing complexes were investigated for their ability to elicit a fluorescence response in the presence of NO. An immediate increase in fluorescence emission of greater than 15-fold occurs when NO is admitted to solutions containing [Rh(2)(mu-O(2)CMe)(4)] and Ds-pip or Ds-im. In both systems, the fluorescence response, which arises by NO-induced displacement of the axially coordinated fluorophore, is reversible with a sensitivity of approximately 4 microM. The related dinitrosyl complexes [Rh(2)(mu-O(2)CR)(4)(NO)(2)], where R = Me, Et, or n-Pr, were prepared, structurally characterized, and found to be air-stable, losing NO upon standing in solution. Sequestration of a methylene chloride solution of the Ds-pip complex from aqueous media by a NO-permeable membrane allows for fluorescence detection of NO for potential applications in biological fluids.