A Dual-Mode Near-Infrared Optical and Photoacoustic Imaging Agent Based on a Low Energy Absorbing Ytterbium Complex.

Near-infrared (NIR) luminescence and photoacoustic (PA) imaging have attracted increasing attention for the real-time monitoring of biological samples due to high sensitivity, resolution, and pronounced signal detection depth, respectively. For improved contrast, both techniques require imaging agents possessing high absorption in the red-NIR range. Herein, we took advantage of a ternary complex formed with the anionic ytterbium(III) tetrakis(2-thenoyltrifluoroacetonate) ([Yb(tta)4]-) and the cationic NIR-absorbing chromophore, 1,1'-diethyl-2,2'-dicarbocyanine (Cy+), to evaluate its potential to act as a dual-mode NIR luminescence and PA imaging agent. We demonstrated that, upon excitation with red-NIR light, Cy[Yb(tta)4] encapsulated into polystyrene nanoparticles is able to generate both NIR Yb3+ emission and a PA signal in an imaging experiment performed in a tissue-mimicking phantom.

Lanthanide-Based Probes for Imaging Detection of Enzyme Activities by NIR Luminescence, T1- and ParaCEST MRI.

Applying a single molecular probe to monitor enzymatic activities in multiple, complementary imaging modalities is highly desirable to ascertain detection and to avoid the complexity associated with the use of agents of different chemical entities. We demonstrate here the versatility of lanthanide (Ln3+) complexes with respect to their optical and magnetic properties and their potential for enzymatic detection in NIR luminescence, CEST and T1 MR imaging, controlled by the nature of the Ln3+ ion, while using a unique chelator. Based on X-ray structural, photophysical, and solution NMR investigations of a family of Ln3+ DO3A-pyridine model complexes, we could rationalize the luminescence (Eu3+, Yb3+), CEST (Yb3+) and relaxation (Gd3+) properties and their variations between carbamate and amine derivatives. This allowed the design of L n L G a l 5 ${{{\bf L n L}}_{{\bf G a l}}^{5}}$ probes which undergo enzyme-mediated changes detectable in NIR luminescence, CEST and T1-weighted MRI, respectively governed by variations in their absorption energy, in their exchanging proton pool and in their size, thus relaxation efficacy. We demonstrate that these properties can be exploited for the visualization of ß-galactosidase activity in phantom samples by different imaging modalities: NIR optical imaging, CEST and T1-weighted MRI.

Lower Energy Excitation of Water Soluble Near-Infrared Emitting Mixed-Ligand Metallacrowns.

By combining advantages of two series of lanthanide(III)/zinc(II) metallacrowns (MCs) assembled using pyrazine- (pyzHA2- ) and quinoxaline- (quinoHA2- ) hydroximate building blocks ligands, we created here water-soluble mixed-ligand MCs with extended absorption to the visible range. The YbIII analogue demonstrated improved photophysical properties in the near-infrared (NIR) range in cell culture media, facilitating its application for NIR optical imaging in living HeLa cells.

Tunable Optical Molecular Thermometers Based on Metallacrowns.

The effect of ligands' energy levels on thermal dependence of lanthanide emission was examined to create new molecular nanothermometers. A series of Ln2Ga8L8'L8″ metallacrowns (shorthand Ln2L8'), where Ln = Gd3+, Tb3+, or Sm3+ (H3L' = salicylhydroxamic acid (H3shi), 5-methylsalicylhydroxamic acid (H3mshi), 5-methoxysalicylhydroxamic acid (H3moshi), and 3-hydroxy-2-naphthohydroxamic acid (H3nha)) and H2L″ = isophthalic acid (H2iph), was synthesized and characterized. Within the series, ligand-centered singlet state (S1) energy levels ranged from 23,300 to 27,800 cm-1, while triplet (T1) energy levels ranged from 18,150 to 21,980 cm-1. We demonstrated that the difference between T1 levels and relevant energies of the excited 4G5/2 level of Sm3+ (17,800 cm-1) and 5D4 level of Tb3+ (20,400 cm-1) is the major parameter controlling thermal dependence of the emission intensity via the back energy transfer mechanism. However, when the energy difference between S1 and T1 levels is small (below 3760 cm-1), the S1 → T1 intersystem crossing (and its reverse, S1 ← T1) mechanism contributes to the thermal behavior of metallacrowns. Both mechanisms affect Ln3+-centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for Tb2L8' and from 0.0267(7)% to 2.27(5)% for Sm2L8'. The maximal thermal dependence varies over a wide thermal range (ca. 150-350 K) based on energy gaps between relevant ligand-based and lanthanide-based electronic states. By mixing Tb2moshi8' with Sm2moshi8' in a 1:1 ratio, an optical thermometer with a relative thermal sensitivity larger than 3%/K at 225 K was created. Other temperature ranges are also accessible with this approach.

Near-Infrared Emitting Poly(amidoamine) Dendrimers with an Anthraquinone Core toward Versatile Non-Invasive Biological Imaging.

Today, there is a very strong demand for versatile near-infrared (NIR) imaging agents suitable for non-invasive optical imaging in living organisms (in vivo imaging). Here, we created a family of NIR-emitting macromolecules that take advantage of the unique structure of dendrimers. In contrast to existing fluorescent dendrimers bearing fluorophores at their periphery or in their cavities, a NIR fluorescent structure is incorporated into the core of the dendrimer. Using the poly(amidoamine) dendrimer structure, we want to promote the biocompatibility of the NIR-emissive system and to have functional groups available at the periphery to obtain specific biological functionalities such as the ability to deliver drugs or for targeting a biological location. We report here the divergent synthesis and characterization by NMR and mass spectrometries of poly(amidoamine) dendrimers derived from the fluorescent NIR-emitting anthraquinone core (AQ-PAMAF). AQ-PAMAFs ranging from the generation -0.5 up to 3 were synthesized with a good level of control resulting in homogeneous and complete dendrimers. Absorption, excitation, and emission spectra, as well as quantum yields, of AQ-PAMAFs have been determined in aqueous solutions and compared with the corresponding properties of the AQ-core. It has been demonstrated that the absorption bands of AQ-PAMAFs range from UV to 750 nm while emission is observed in the range of 650-950 nm. Fluorescence macroscopy experiments confirmed that the NIR signal of AQ-PAMAFs can be detected with a satisfactory signal-to-noise ratio in aqueous solution, in blood, and through 1 mm thick tissue-mimicking phantom. The results show that our approach is highly promising for the design of an unprecedented generation of versatile NIR-emitting agents.

Near-Infrared Lanthanide-Based Emission from Fused Bis[Ln(III)/Zn(II) 14-metallacrown-5] Coordination Compounds.

A set of (Ln[14-MCZn(II)N(quinHA)-5])2Ln2Zn2(quinHA)2(ph)2(Hph)4(OH)8(H2O)4 metallacrowns (Ln-1, Ln = Tb, Gd, or Yb; H2quinHA = quinaldic hydroxamic acid, H2ph = phthalic acid) have been synthesized via solution-state self-assembly. The metallacrowns possess an uncommon topology within the metallacrown family where two rarely seen 14-metallacrown-5 moieties are fused by a Yb2Zn2(quinHA)2 bridge. Moreover, Yb-1 analyzed in the solid state exhibits a characteristic near-infrared luminescence signal arising from Yb3+ 2F5/2→2F7/2 transition despite the proximity of high energy O-H oscillators.

Pyridazino-1,3a,6a-Triazapentalenes as Versatile Fluorescent Probes: Impact of Their Post-Functionalization and Application for Cellular Imaging.

Pyridazino-1,3a,6a-triazapentalenes (PyTAP) are compact fused 6/5/5 tricyclic scaffolds which exhibit promising fluorescent properties. Chemically stable, they can be post-functionalized using standard Pd-catalyzed cross-coupling chemistry. Several original PyTAP bearing additional unsaturated substituents in positions 2 and 8 were synthetized and their spectroscopic properties analyzed. They have been successfully tested as fluorescent probes for cellular imaging.

Doxorubicin-Sensitized Luminescence of NIR-Emitting Ytterbium Liposomes: Towards Direct Monitoring of Drug Release.

Drug-loaded liposomes are typical examples of nanomedicines. We show here that doxorubicin, the anti-cancer agent in the liposomal drug Doxil, can sensitize Ytterbium (Yb3+ ) and generate its near-infrared (NIR) emission. When doxorubicin and amphiphilic Yb3+ chelates are incorporated into liposomes, the sensitized emission of Yb3+ is dependent on the integrity of the particles, which can be used to monitor drug release. We also established the first demonstration that the NIR Yb3+ emission signal is observable in living mice following intratumoral injection of the Yb3+ -doxorubicin-liposomes, using a commercial macroscopic setup equipped with a NIR camera.

Dy3+ White Light Emission Can Be Finely Controlled by Tuning the First Coordination Sphere of Ga3+/Dy3+ Metallacrown Complexes.

Single lanthanide(III) ion white light emission is in high demand since it provides the advantage of requiring only one chromophore for the control of the color. Herein, a series of Ga3+/Dy3+ metallacrowns (MCs) is presented, demonstrating outstanding white light colorimetric properties with CIE chromaticity coordinates of (0.309, 0.334) and correlated color temperature (CCT) equal to 6670 K for the MC emitting the closest to the standard white color. Experimental data reveal that the CIE coordinates within the studied series of MCs are controlled mainly by the Dy3+-centered emission rather than by the ligand-centered bands, implying that Dy3+ can be tuned as a single ionic white light emitter by a simple modification of the coordination environment.

Ship-in-a-Bottle Preparation of Long Wavelength Molecular Antennae in Lanthanide Metal-Organic Frameworks for Biological Imaging.

While metal-organic frameworks (MOFs) have been identified as promising materials for sensitizing near-infrared emitting lanthanide ions (Ln3+) for biological imaging, long-wavelength excitation of such materials requires large, highly delocalized organic linkers or guest-chromophores. Incorporation of such species generally coincides with fewer Ln3+ emitters per unit volume. Herein, the excitation bands of ytterbium-based MOFs are extended to 800 nm via the postsynthetic coupling of acetylene units to form a high density of conjugated π-systems throughout MOF pores. The resulting long wavelength excitation/absorption bands are a synergistic property of the composite material as they are not observed in the individual organic components after disassociation of the MOFs, thus circumventing the need for large organic chromophores. We demonstrate that the long wavelength excitation and emission properties of these modified MOFs are maintained in the biological conditions of cell culture (aqueous environment, salts, heating), pointing toward their promising use for biological imaging applications.

A Six-Armed Phenhomazine Ligand with a Potential "Turn-Off" Copper(II) Sensing Capability through Terbium(III) Luminescence Quenching.

Herein, the design, synthesis, and characterization of a phenhomazine ligand are described. The ligand has six pendant acetate arms designed for the combined coordination of copper(II) and lanthanide(III) ions, with the perspective of developing a "turn-off" copper sensor. The key step for the ligand preparation was the one-step endomethylene bridge fission of a diamino Tröger's base with a concomitant alkylation. Fluorescence and absorption spectroscopies as well as nuclear magnetic resonance (NMR) experiments were performed to analyze and understand the coordination properties of the ligand. Transition metal coordination was driven by the synergistic effect of the free nitrogen atoms of the diazocinic core and the two central acetate arms attached to those nitrogen atoms, whereas lanthanide coordination is performed by the external acetate arms, presumably forming a self-assembled 2:2 metallosupramolecular structure. The terbium complex shows the typical green emission with narrow bands and long luminescence lifetimes. The luminescence quenching produced by the presence of copper(II) ions was analyzed. This work sets, therefore, a starting point for the development of a phenhomazine-based "turn-off" copper(II) sensor.

Innovative Multipodal Ligands Derived from Tröger's Bases for the Sensitization of Lanthanide(III) Luminescence.

Herein, the synthesis and characterization of the first family of multipodal ligands with a Tröger's base framework designed for the preparation of luminescent lanthanide(III) complexes are reported. Eight ligands were designed and synthesized using different strategies, including alkylation reactions, amide couplings, and Ugi multicomponent reactions. All the ligands bear carboxylate groups for the coordination of the lanthanide(III) ions, with the lanthanide(III)-sensitizing units consisting of the Tröger's base framework itself or attached benzamides. Upon irradiation of the chromophoric ligands, green terbium(III) emission was efficiently generated, whereas europium(III) emission was negligible. The geometry and substitution pattern of the ligands allow control of the stoichiometry of the species formed and the TbIII luminescence sensitization efficiency, showing that para-substitution patterns are more efficient than meta substitution for the formation of coordination compounds with lower TbIII /ligand ratio. We propose that the species formed are self-assembled 2:2 or 2:4 metallosupramolecular structures.

Using Native Chemical Ligation for Site-Specific Synthesis of Hetero-bis-lanthanide Peptide Conjugates: Application to Ratiometric Visible or Near-Infrared Detection of Zn2.

The interest in ratiometric luminescent probes that detect and quantify a specific analyte is growing. Owing to their special luminescence properties, lanthanide(III) cations offer attractive opportunities for the design of dual-color ratiometric probes. Here, the design principle of hetero-bis-lanthanide peptide conjugates by using native chemical ligation is described for perfect control of the localization of each lanthanide cation within the molecule. Two zinc-responsive probes, r-LZF1Tb|Cs124|Eu and r-LZF1Eu|Cs124|Tb are described on the basis of a zinc finger peptide and two DOTA (DOTA=1,4,7,10-tetraaza-cyclododecane-1,4,7,10-tetraacetic acid) complexes of terbium and europium. Both display dual-color ratiometric emission in response to the presence of Zn2+ . By using a screening approach, anthracene was identified for the sensitization of the luminescence of two near-infrared-emitting lanthanides, Yb3+ and Nd3+ . Thus, two novel zinc-responsive hetero-bis-lanthanide probes, r-LZF3Yb|Anthra|Nd and r-LZF3Nd|Anthra|Yb were assembled, the former offering a neat ratiometric response to Zn2+ with emission in the near-infrared around 1000 nm, which is unprecedented.

[Ga3+ 8 Sm3+ 2 , Ga3+ 8 Tb3+ 2 ] Metallacrowns are Highly Promising Ratiometric Luminescent Molecular Nanothermometers Operating at Physiologically Relevant Temperatures.

Nanothermometry is the study of temperature at the submicron scale with a broad range of potential applications, such as cellular studies or electronics. Molecular luminescent-based nanothermometers offer a non-contact means to record these temperatures with high spatial resolution and thermal sensitivity. A luminescent-based molecular thermometer comprised of visible-emitting Ga3+ /Tb3+ and Ga3+ /Sm3+ metallacrowns (MCs) achieved remarkable relative thermal sensitivity associated with very low temperature uncertainty of Sr =1.9 % K-1 and δT<0.045 K, respectively, at 328 K, as an aqueous suspension of polystyrene nanobeads loaded with the corresponding MCs. To date, they are the ratiometric molecular nanothermometers offering the highest level of sensitivity in the physiologically relevant temperature range.

Iodinated Metallacrowns: Toward Combined Bimodal Near-Infrared and X-Ray Contrast Imaging Agents.

Multimodal probes capable of combining imaging modalities within a single molecule are in high demand today as they can provide information at both molecular and anatomical levels. Herein, a study was conducted on a series of gallium(III)/lanthanide(III) bis(12-MC-4) metallacrowns (MCs) with the general composition {Ln[12-MCGa III N(shi) -4]}2 (iph)4 (Ln-Ix , x=0, 4, 8, 12), where shi and iph are salicylhydroximate and isophthalate ligands, respectively, or their iodinated derivatives. For Yb-Ix , the attenuation in X-ray computed tomography (XCT) imaging and near-infrared (NIR) luminescence properties can be finely tuned by controlled structural modifications based on iodo groups. Solutions of Yb-Ix appear to be 22-40 times more efficient as XCT agents in comparison to the commercially available iobitridol, while providing an intense emission signal in the NIR range with total quantum yields up to 8.6 %, which are among the highest values reported so far. Therefore, these molecules are promising potential bimodal agents for combined NIR luminescence and XCT imaging.

Visible, Near-Infrared, and Dual-Range Luminescence Spanning the 4f Series Sensitized by a Gallium(III)/Lanthanide(III) Metallacrown Structure.

Lanthanide(III) ions (Ln3+) in coordination compounds exhibit unique luminescence properties with narrow and characteristic f-f transitions throughout the visible and near-infrared (NIR) ranges. In addition, some Ln3+ such as Pr3+, Sm3+, Dy3+, Ho3+, Er3+, and Tm3+ possess an exceptional ability, although less explored, to exhibit dual-range emissions. Such remarkable features allow highly specific use in materials science and biology, for example, for the creation of sophisticated barcode modules or for the next generation of optical imaging applications. Herein, a series of Ga3+/Ln3+ metallacrowns (MCs) with the general composition [LnGa8(shi)8(OH)4]Na·xCH3OH·yH2O (Ln-1, Ln = Pr3+, Nd3+, Sm3+-Yb3+ and analogue Y3+; H3shi = salicylhydroxamic acid) is presented. Ln-1 were obtained by reacting Ga3+ and Ln3+ nitrate salts with the H3shi ligand. X-ray single crystal unit cell analysis confirmed that all MCs are isostructural. The crystal structure was solved for the Nd3+ analogue and revealed that Nd3+ is centered between two [12-MCGaIIIN(shi)-4] MC rings and bound to eight hydroximate oxygen ions (four from each ring) in a pseudosquare antiprismatic fashion adopting a pseudo-D4h symmetry. Pulsed gradient spin echo diffusion ordered 1H NMR spectroscopy and electrospray ionization mass spectrometry confirmed that the structure of Ln-1 remains intact in methanol solutions while mass spectrometry suggests that four OH- bridges are exchanged with CH3O-/CD3O-. An exceptional ability of this series of MCs to sensitize the characteristic emission of Ln3+ was confirmed with the observation of bright red and green emission signals of Eu-1 and Tb-1, NIR emissions of Yb-1 and Nd-1, and dual-range emissions of Pr-1, Sm-1, Dy-1, Ho-1, Er-1, and Tm-1 in the solid state upon excitation into ligand-centered bands at 340 nm. The luminescence properties of Ln-1 (Ln = Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, and Yb3+) were also investigated in CH3OH and CD3OD solutions. For Eu-1 and Yb-1 MCs, more extensive analyses of the photophysical properties were performed, which included the determination of radiative lifetimes, intrinsic quantum yields, and sensitization efficiencies. The absolute quantum yields (QLnL) of Ln-1 in the visible and NIR ranges have been determined. In the case of Sm-1, the values of QLnL in CH3OH and CD3OD solutions are exceptionally high, that is, 10.1(5) and 83(1) %. Values obtained for Yb-1, that is, 0.78(4) % in CH3OH and 8.4(1)% in CD3OD, are among the highest ones reported today for Yb3+ complexes formed with nondeuterated and nonhalogenated ligands.

Galactofuranose-Related Enzymes: Challenges and Hopes.

Galactofuranose is a rare form of the well-known galactose sugar, and its occurrence in numerous pathogenic micro-organisms makes the enzymes responsible for its biosynthesis interesting targets. Herein, we review the role of these carbohydrate-related proteins with a special emphasis on the galactofuranosidases we recently characterized as an efficient recombinant biocatalyst.

Toward MRI and Optical Detection of Zwitterionic Neurotransmitters: Near-Infrared Luminescent and Magnetic Properties of Macrocyclic Lanthanide(III) Complexes Appended with a Crown Ether and a Benzophenone Chromophore.

Thanks to their versatile magnetic and luminescence features, lanthanide complexes have gained a central position in biomedical imaging as magnetic resonance imaging (MRI) contrast agents and optical imaging probes. In addition, appropriate chemical design allows modification of the magnetic relaxation properties of GdIII complexes and the optical properties of visible- or near-infrared (NIR)-emitting lanthanide chelates upon interaction with various biomarkers, which makes them ideal candidates for the creation of responsive agents. In this Forum Article, we demonstrate such design principles as well as the difficulties encountered in the context of neurotransmitter (NT) detection. Lanthanide(III) complexes of a macrocyclic ligand incorporating a benzophenone chromophore and a monoazacrown ether (LnL3) have been synthesized as responsive probes to monitor amino acid NTs either in MRI (Ln = Gd) or in NIR optical detection (Ln = Nd or Yb). The parameters characterizing the water exchange and rotational dynamics of the gadolinium(III) complex were assessed by 17O NMR and 1H NMRD. In the presence of zwitterionic NTs, the inner-sphere water molecule is replaced by the carboxylate function of the NTs in the gadolinium(III) complex, leading to a decrease of the longitudinal relaxivity from 6.7 to 2-2.5 mM-1 s-1 (300 MHz and 37 °C). The apparent affinity constants range from Ka = 35 for γ-aminobutyric acid (GABA) to 80 M-1 for glycine and glutamate, and there is no selectivity with respect to hydrogen carbonate (Ka = 232; pH 7.4). The gadolinium(III) complex interacts with human serum albumin (HSA), resulting in a 60% increase in the relaxivity (20 MHz, 37 °C) in the absence of NTs. The HSA-bound complex, however, was revealed to be less responsive to NTs because of displacement of the GdIII-bound water by HSA, which was confirmed by the hydration number calculated from luminescence lifetimes of the HSA-bound europium(III) complex. The creation of an imaging agent suitable for NIR detection of NTs for an enhanced sensitivity in biological systems using the benzophenone (BP) moiety as the sensitizer of lanthanide luminescence was also attempted. Upon excitation at 300 nm of the BP chromophore in aqueous solutions of NdL3 and YbL3, characteristic NIR emissions of NdIII and YbIII were observed because of 4F3/2 → 4IJ (J = 9/2-13/2) and 2F5/2 → 2F7/2 transitions, respectively, indicating that this chromophore is a suitable antenna. Despite these promising results, luminescence titrations of NdIII and YbIII complexes with NTs were not conclusive because of chemical conversion of the ligand triggered by light, preventing quantitative analysis. The observed photochemical reaction of the ligand is strongly dependent on the nature of the lanthanide chelated; it is considerably slowed down in the presence of NdIII and EuIII.

On-Demand Degradation of Metal-Organic Framework Based on Photocleavable Dianthracene-Based Ligand.

We have designed a rigid photocleavable dianthracene-based ligand that reacts with ytterbium as coordination metal ion for the creation of a class of tridimensional light-degradable metal-organic framework (MOF). We demonstrated that we can obtain a high level of control on the disassembly of the MOF formed with this ligand which can be triggered either through light irradiation or temperature increase. The reversible 4π-4π photodimerization is the intrinsic chemical mechanism ruling the ligand and MOF cleavage. In the fields of biology and medicine, MOFs have sparked a strong interest as highly porous vehicles for drug release but have only been explored so far through the passive leakage of their payloads. The designed light-degradable MOFs can potentially overcome this limitation and serve as prototypes for drug delivery and corresponding therapeutic applications.

A Unique LnIII {[3.3.1]GaIII Metallacryptate} Series That Possesses Properties of Slow Magnetic Relaxation and Visible/Near-Infrared Luminescence.

A new family of [3.3.1] metallacryptates with the general composition [LnGa6 (H2 shi)(Hshi)(shi)7 (C5 H5 N)] (Ln-1; shi3- =salicylhydroximate; Ln = Pr, Nd, Sm-Yb) has been synthesized and characterized. Ln-1 display both interesting magnetic and luminescent properties. Sm-1 has sharp emission bands in the visible and the near-infrared (NIR) regions with quantum yield values (QSmL ) of 1.64(9) and 5.5(2). 10-2 %, respectively. Tb-1 exhibits a weak green emission (QTbL =1.89(3). 10-1 %) while Pr-1, Nd-1, Ho-1, Er-1, and Yb-1 possess emission bands in the NIR range with QPrL =3.7(2). 10-3 %, QNdL =1.71(5). 10-1 %, QHoL =1.1(2). 10-3 %, QErL =7.1(2). 10-3 % and QYbL =0.65(3) %. Nd-1, Dy-1, and Yb-1 display slow magnetization relaxation in an applied field, where only Dy-1 has been observed to follow an Orbach process (Ueff =12.7 K). The combination of NIR emission with magnetic properties makes Nd-1 and Yb-1 attractive candidates as smart materials addressable in two manners.