Silica-Based Materials Containing Inorganic Red/NIR Emitters and Their Application in Biomedicine

The low absorption of biological substances and living tissues in the red/near-infrared region (therapeutic window) makes luminophores emitting in the range of ~650-1350 nm favorable for in vitro and in vivo imaging. In contrast to commonly used organic dyes, inorganic red/NIR emitters, including ruthenium complexes, quantum dots, lanthanide compounds, and octahedral cluster complexes of molybdenum and tungsten, not only exhibit excellent emission in the desired region but also possess additional functional properties, such as photosensitization of the singlet oxygen generation process, upconversion luminescence, photoactivated effects, and so on. However, despite their outstanding functional applicability, they share the same drawback-instability in aqueous media under physiological conditions, especially without additional modifications. One of the most effective and thus widely used types of modification is incorporation into silica, which is (1) easy to obtain, (2) biocompatible, and (3) non-toxic. In addition, the variety of morphological characteristics, along with simple surface modification, provides room for creativity in the development of various multifunctional diagnostic/therapeutic platforms. In this review, we have highlighted biomedical applications of silica-based materials containing red/NIR-emitting compounds.

Dye-sensitized lanthanide containing nanoparticles for luminescence based applications

Due to their exceptional luminescent properties, lanthanide (Ln) complexes represent a unique palette of probes in the spectroscopic toolkit. Their extremely weak brightness due to forbidden Ln electronic transitions can be overcome by indirect dye-sensitization from the antenna effect brought by organic ligands. Despite the improvement brought by the antenna effect, (bio)analytical applications with discrete Ln complexes as luminescent markers still suffers from low sensitivity as they are limited by the complex brightness. Thus, there is a need to develop nano-objects that cumulate the spectroscopic properties of multiple Ln ions. This review firstly gives a brief introduction of the spectral properties of lanthanides both in complexes and in nanoparticles (NPs). Then, the research progress of the design of Ln-doped inorganic NPs with capping antennas, Ln-complex encapsulated NPs and Ln-complex surface functionalized NPs is presented along with a summary of the various photosensitizing ligands and of the spectroscopic properties (excited-state lifetime, brightness, quantum yield). The review also emphasizes the problems and limitations encountered over the years and the solutions provided to address them. Finally, a comparison of the advantages and drawbacks of the three types of NP is provided as well as a conclusion about the remaining challenges both in the design of brighter NPs and in the luminescence based applications.

Recent Progress in Time-Resolved Biosensing and Bioimaging Based on Lanthanide-Doped Nanoparticles

Luminescent nanomaterials have attracted great attention in luminescence-based bioanalysis due to their abundant optical and tunable surface physicochemical properties. However, luminescent nanomaterials often suffer from serious autofluorescence and light scattering interference when applied to complex biological samples. Time-resolved luminescence methodology can efficiently eliminate autofluorescence and light scattering interference by collecting the luminescence signal of a long-lived probe after the background signals decays completely. Lanthanides have a unique [Xe]4f^N electronic configuration and ladder-like energy states, which endow lanthanide-doped nanoparticles with many desirable optical properties, such as long luminescence lifetimes, large Stokes/anti-Stokes shifts, and sharp emission bands. Due to their long luminescence lifetimes, lanthanide-doped nanoparticles are widely used for high-sensitive biosensing and high-contrast bioimaging via time-resolved luminescence methodology. In this review, recent progress in the development of lanthanide-doped nanoparticles and their application in time-resolved biosensing and bioimaging are summarized. At the end of this review, the current challenges and perspectives of lanthanide-doped nanoparticles for time-resolved bioapplications are discussed.

A substituentpara-to-orthopositioning effect drives the photoreactivity of a dibenzothiophene-based oxalate series used as LED-excitable free radical photoinitiators

Dibenzothiophene-based oxalate derivatives were synthesized as type I photoinitiators, and their photoinitiation properties depend on the substituentpara-to-orthopositioning effect of the oxalates.

Effect of the Composition of Lanthanide Complexes on Their Luminescence Enhancement by Ag@SiO₂ Core-Shell Nanoparticles

Metal-enhanced luminescence of lanthanide complexes by noble metal nanoparticles has attracted much attention because of its high efficiency in improving the luminescent properties of lanthanide ions. Herein, nine kinds of europium and terbium complexes—RE(TPTZ)(ampca)₃·3H₂O, RE(TPTZ)(BA)₃·3H₂O, RE(phen)(ampca)₃·3H₂O, RE(phen)(PTA)_1.5·3H₂O (RE = Eu, Tb) and Eu(phen)(BA)₃·3H₂O (TPTZ = 2,4,6-tri(2-pyridyl)-s-triazine, ampca = 3-aminopyrazine-2-carboxylic acid, BA = benzoic acid, phen = 1,10-phenanthroline, PTA = phthalic acid)—have been synthesized. Meanwhile, seven kinds of core-shell Ag@SiO₂ nanoparticles of two different core sizes (80–100 nm and 40–60 nm) and varied shell thicknesses (5, 12, 20, 30 and 40 nm) have been prepared. The combination of these nine types of lanthanide complexes and seven kinds of Ag@SiO₂ nanoparticles provides an opportunity for a thorough investigation of the metal-enhanced luminescence effect. Luminescence spectra analysis showed that the luminescence enhancement factor not only depends on the size of the Ag@SiO₂ nanoparticles, but also strongly relates to the composition of the lanthanide complexes. Terbium complexes typically possess higher enhancement factors than their corresponding europium complexes with the same ligands, which may result from better spectral overlap between the emission bands of Tb complexes and surface plasmon resonance (SPR) absorption bands of Ag@SiO₂. For the complexes with the same lanthanide ion but varied ligands, the complexes with high enhancement factors are typically those with excitation wavelengths located nearby the SPR absorption bands of Ag@SiO₂ nanoparticles. These findings suggest a combinatorial chemistry strategy is necessary to obtain an optimal metal-enhanced luminescence effect for lanthanide complexes.

Encapsulation-Stabilized, Europium Containing Nanoparticle as a Probe for Time-Resolved luminescence Detection of Cardiac Troponin I

The use of a robust optical signaling probe with a high signal-to-noise ratio is important in the development of immunoassays. Lanthanide chelates are a promising material for this purpose, which provide time-resolved luminescence (TRL) due to their large Stokes shift and long luminescence lifetime. From this, they have attracted considerable interest in the in vitro diagnostics field. However, the direct use of lanthanide chelates is limited because their luminescent signal can be easily affected by various quenchers. To overcome this drawback, strategies that rely on the entrapment of lanthanide chelates inside nanoparticles, thereby enabling the protection of the lanthanide chelate from water, have been reported. However, the poor stability of the lanthanide-entrapped nanoparticles results in a significant fluctuation in TRL signal intensity, and this still remains a challenging issue. To address this, we have developed a Lanthanide chelate-Encapsulated Silica Nano Particle (LESNP) as a new immunosensing probe. In this approach, the lanthanide chelate is covalently crosslinked within the silane monomer during the silica nanoparticle formation. The resulting LESNP is physically stable and retains TRL properties of the parent lanthanide chelate. Using the probe, a highly sensitive, sandwich-based TRL immunoassay for the cardiac troponin I was conducted, exhibiting a limit of detection of 48 pg/mL. On the basis of the features of the LESNP such as TRL signaling capability, stability, and the ease of biofunctionalization, we expect that the LESNP can be widely applied in the development of TRL-based immunosensing.

Rare Earth Doped Silica Nanoparticles via Thermolysis of a Single Source Metallasilsesquioxane Precursor

Rare earth metal doped silica nanoparticles have significant advantages over traditional organic dyes and quantum dots. Silsesquioxanes are promising precursors in the production of silica nanoparticles by thermolysis, due to their structural similarities with silica materials. This manuscript describes the production of a new Eu3+-based metallasilsesquioxane species and its use as a single source precursor in the thermolytic production of luminescent rare earth metal doped silica nanoparticles with characteristic emission in the visible region of the spectrum.

A visible-light-excited Eu3+complex-based luminescent probe for highly sensitive time-gated luminescence imaging detection of intracellular peroxynitrite

A visible-light-excited europium complex was developed for highly sensitive imaging of intracellular peroxynitrite with time-gated luminescence mode.

A lysosome targetable luminescent bioprobe based on a europium β-diketonate complex for cellular imaging applications

A unique bright luminescent europium coordination compound with excellent biocompatibility has been developed that serves as a selective bioprobe for particular organelles within the cells.

Preparation, characterization, and properties of PMMA-doped polymer film materials: a study on the effect of terbium ions on luminescence and lifetime enhancement

Poly(methylmethacrylate) (PMMA) doped with Tb-based imidazole derivative coordination polymer {[Tb(3)(L)(μ(3)-OH)(7)]·H(2)O}(n) (1) (L = N,N'-bis(acetoxy)biimidazole) was synthesized and its photophysical properties were studied. The L'(L' = N,N'-bis(ethylacetate)biimidazole) ligand was synthesized by an N-alkylation reaction process followed by ester hydrolysis to produce ligand L. Polymer 1 and ligand L' have been characterized by (1)H NMR and IR spectroscopy, elemental analysis, PXRD and X-ray single-crystal diffraction. Coordination polymer 1 is the first observation of a CdCl(2) structure constructed with hydroxy groups and decorated by ligand L in lanthanide N-heterocyclic coordination polymers. In the 2D layered structure of 1, each Tb3 metal center is connected with three Tb1 and three Tb2 metal centers by seven hydroxyl groups in different directions, resulting in a six-membered ring. After doping, not only the luminescence intensity and lifetime enhanced, but also their thermal stability was increased in comparison with 1. When 1 was doped into poly(methylmethacrylate) (1@PMMA), polymer film materials were formed with the PMMA polymer matrix (w/w = 2.5%-12.5%) acting as a co-sensitizer for Tb(3+) ions. The luminescence intensity of the Tb(3+) emission at 544 nm increases when the content of Tb(3+) was 10%. The lifetime of 1@PMMA (914.88 μs) is more than four times longer than that of 1 (196.24 μs). All τ values for the doped polymer systems are higher than coordination polymer 1, indicating that radiative processes are operative in all the doped polymer films. This is because PMMA coupling with the O-H oscillators from {[Tb(3)(L)(μ(3)-OH)(7)]·H(2)O}(n) can suppress multiphonon relaxation. According to the variable-temperature luminescence (VT-luminescence) investigation, 1@PMMA was confirmed to be a stable green luminescent polymer film material.

A new fluorescent chemosensor for highly selective and sensitive detection of inorganic phosphate (Pi) in aqueous solution and living cells

A new fluorescein-based chemosensor, FP-Fe³⁺, was developed for the detection of inorganic phosphate (Pi) in aqueous solution and living cells.

Tuning of the excitation wavelength in Eu3+-aminophenyl based polyfluorinated β-diketonate complexes: a red-emitting Eu3+-complex encapsulated in a silica/polymer hybrid material excited by blue light

A series of new antenna complexes of Eu³⁺ based on aminophenyl β-diketonate ligands was designed and their photophysical properties were evaluated.

Practical implementation, characterization and applications of a multi-colour time-gated luminescence microscope

Time-gated luminescence microscopy using long-lifetime molecular probes can effectively eliminate autofluorescence to enable high contrast imaging. Here we investigate a new strategy of time-gated imaging for simultaneous visualisation of multiple species of microorganisms stained with long-lived complexes under low-background conditions. This is realized by imaging two pathogenic organisms (Giardia lamblia stained with a red europium probe and Cryptosporidium parvum with a green terbium probe) at UV wavelengths (320-400 nm) through synchronization of a flash lamp with high repetition rate (1 kHz) to a robust time-gating detection unit. This approach provides four times enhancement in signal-to-background ratio over non-time-gated imaging, while the average signal intensity also increases six-fold compared with that under UV LED excitation. The high sensitivity is further confirmed by imaging the single europium-doped Y₂O₂S nanocrystals (150 nm). We report technical details regarding the time-gating detection unit and demonstrate its compatibility with commercial epi-fluorescence microscopes, providing a valuable and convenient addition to standard laboratory equipment.

Preparation and functionalization of a visible-light-excited europium complex-modified luminescent protein for cell imaging applications

Lanthanide complex-based luminescent bioprobes have shown great utility in a variety of time-resolved luminescence bioassays, but these bioprobes often require UV excitation and suffer from problems related to bioaffinity and biocompatibility for in vivo applications. In this work, a new visible-light-excited europium(III) complex with the maximum excitation wavelength over 400 nm, BHHBB-Eu(3+)-BPT {BHHBB: 1,2-bis[4'-(1'',1'',1'',2'',2'',3'',3''-heptafluoro-4'',6''-hexanedion-6''-yl)-benzyl]-benzene; BPT: 2-(N,N-diethylanilin-4-yl)-4,6-bis(pyrazol-1-yl)-1,3,5-triazine}, has been synthesized for the preparation of an artificial luminescent protein that can be used as a visible-light-excited luminescent bioprobe for cell imaging. By encapsulating BHHBB-Eu(3+)-BPT into apoferritin with a simple dissociation-reassembly method, the luminescent protein, Eu@AFt, with a maximum excitation peak at 420 nm and a long luminescence lifetime of 365 μs was fabricated and successfully used for visible-light-excited time-resolved luminescence cell imaging. Moreover, by conjugating a mitochondria-targeting molecule, (5-N-succinimidoxy-5-oxopentyl)-triphenylphosphonium bromide (SPTPP), onto the surface of Eu@AFt, a mitochondria-specifically-tracking luminescent probe, Eu@AFt-SPTPP, was further prepared and used for visible-light-excited confocal luminescence microscopy imaging to visualize the mitochondria of living cells.

How to build a time-gated luminescence microscope

The sensitivity of filter-based fluorescence microscopy techniques is limited by autofluorescence background. Time-gated detection is a practical way to suppress autofluorescence, enabling higher contrast and improved sensitivity. In the past few years, three groups of authors have demonstrated independent approaches to build robust versions of time-gated luminescence microscopes. Three detailed, step-by-step protocols are provided here for modifying standard fluorescent microscopes to permit imaging time-gated luminescence.

Organosilylated complex [Eu(TTA)3(Bpy-Si)]: a bifunctional moiety for the engeneering of luminescent silica-based nanoparticles for bioimaging

A new highly luminescent europium complex with the formula [Eu(TTA)3(Bpy-Si)], where TTA stands for the thenoyltrifluoroacetone, (C4H3S)COCH2COCF3, chelating ligand and Bpy-Si, Bpy-CH2NH(CH2)3Si(OEt)3, is an organosilyldipyridine ligand displaying a triethoxysilyl group as a grafting function has been synthesized and fully characterized. This bifunctional complex has been grafted onto the surface of dense silica nanoparticles (NPs) and on mesoporous silica microparticles as well. The covalent bonding of [Eu(TTA)3(Bpy-Si)] inside uniform Stöber silica nanoparticles was also achieved. The general methodology proposed could be applied to any silica matrix, allowed high grafting ratios that overcome chelate release and the tendency to agglomerate. Luminescent silica-based nanoparticles SiO2-[Eu(TTA)3(Bpy-Si)], with a diameter of 28 ± 2 nm, were successfully tested as a luminescent labels for the imaging of Pseudomonas aeruginosa biofilms. They were also functionalized by a specific monoclonal antibody and subsequently employed for the selective imaging of Escherichia coli bacteria.

Determination of fluoroquinolone antibiotics through the fluorescent response of Eu(III) based nanoparticles fabricated by layer-by-layer technique

The present work introduces the determination of fluoroquinolone antibiotics (FQs) in aqueous solutions through the fluorescent response of Eu(TTA)3 and [Eu(TTA)(3)1] (TTA(-) and 1 are thenoyltrifluoroacetonate and phosphine oxide derivative) complexes encapsulated into the polyelectrolyte capsules fabricated through layer-by-layer deposition of poly(sodium 4-styrenesulfonate) (PSS) and polyethyleneimine (PEI). The variation of luminescent core, polyelectrolyte deposition and concentration conditions reveals two modes of fluorescent response on FQs of diverse structure namely the sensitization and quenching of Eu(III) centered luminescence. The obtained regularities reveal the ternary complex formation and the ligand exchange occurring at the interface of polyelectrolyte coated [Eu(TTA)(3)1] based colloids as the reasons of the diverse fluorescent response of Eu(III) centered luminescence on FQs. The factors affecting the fluorescent response have been revealed, which are: the content of luminescent core, the mode of polyelectrolyte deposition, concentration and structure of FQs. The discrimination of moxifloxacin and lomefloxacin from levofloxacin, ofloxacin, difloxacin, perfloxacin through the quenching of Eu(III) luminescence in PSS-[Eu(TTA)(3)1] colloids has been revealed.

Cadmium-free quantum dots as time-gated bioimaging probes in highly-autofluorescent human breast cancer cells

We report cadmium-free, biocompatible (Zn)CuInS(2) quantum dots with long fluorescence lifetimes as superior bioimaging probes using time-gated detection to suppress cell autofluorescence and improve the signal : background ratio by an order of magnitude. These results will be important for developing non-toxic fluorescence imaging probes for ultrasensitive biomedical diagnostics.

Enhanced emission of ultra-small-sized LaF3:RE3+ (RE = Eu, Tb) nanoparticles through 1,2,4,5-benzenetetracarboxylic acid sensitization

Uniform, ultra-small-sized and well-water-dispersible LaF(3) nanoparticles doped with trivalent rare earth (RE) ions (Eu(3+) or Tb(3+)) have been synthesized by a simple, low temperature synthesis route. The nanoparticles, with sizes of about 3.2 nm (for those doped with Eu(3+)) and 3.0 nm (for those doped with Tb(3+)), are roughly spherical and monodisperse. 1,2,4,5-Benzenetetracarboxylic acid (labeled as BA) as sensitizer has been bonded to the surface of the nanoparticles, which can sensitize the emission of RE(3+) in the LaF(3) nanoparticles. The BA-LaF(3):RE(3+) (RE = Eu or Tb) nanoparticles have a broad absorption band in the UV domain, and show enhanced luminescence of RE(3+) based on an energy transfer from BA ligands to RE(3+) ions (i.e. the so-called "antenna effect"). Due to the dual protection of organic ligands (BA) and inorganic matrices (LaF(3)), BA-LaF(3):RE(3+) (RE = Eu or Tb) nanoparticles have longer excited state lifetimes than LaF(3):RE(3+) (RE = Eu or Tb) nanoparticles as well as lanthanide coordination polymers of BA.

Preparation and time-gated luminescence bioimaging applications of long wavelength-excited silica-encapsulated europium nanoparticles

Silica-encapsulated luminescent lanthanide nanoparticles have shown great potential as biolabels for various time-gated luminescence bio-detections in recent years. The main problem of these nano-biolabels is their short excitation wavelengths within the UV region. In this work, a new type of silica-encapsulated luminescent europium nanoparticle, with a wide excitation range from UV to visible light in aqueous solutions, has been prepared using a conjugate of (3-isocyanatopropyl)triethoxysilane bound to a visible light-excited Eu(3+) complex, 2,6-bis(1',1',1',2',2',3',3'-heptafluoro-4',6'-hexanedion-6'-yl)-dibenzothiophene-Eu(3+)-2-(N,N-diethylanilin-4-yl)-4,6-bis(pyrazol-1-yl)-1,3,5-triazine (IPTES-BHHD-Eu(3+)-BPT conjugate), as a functionalized precursor. The nanoparticles, which are prepared by the copolymerization of the IPTES-BHHD-Eu(3+)-BPT conjugate, tetraethyl orthosilicate and (3-aminopropyl)triethoxysilane in a water-in-oil reverse microemulsion consisting of Triton X-100, n-octanol, cyclohexane and water in the presence of aqueous ammonia, are monodisperse, spherical and uniform in size. Their diameter is 42 ± 3 nm and they are strongly luminescent with a wide excitation range from UV to ∼475 nm and a long luminescence lifetime of 346 μs. The nanoparticles were successfully used for streptavidin labeling and the time-gated luminescence imaging detection of two environmental pathogens, cryptosporidium muris and cryptosporidium parvium, in water samples. The results demonstrated the practical utility of the new nanoparticles as visible light-excited biolabels for time-gated luminescence bioassay applications.

Luminescent nanoparticles and their use for in vitro and in vivo diagnostics

Fluorescence spectroscopy has been shown to be a useful tool for a broad variety of biological and medical applications. Many of the analytical methods, as used for tumor marker and gene mutation detection, recognition of pathogens or monitoring of cell-related processes, are based on the labeling of the investigating object with luminescent nanoparticles. Owing to their size, which is comparable to that of biomolecules, and to their extraordinary optical properties, luminescent nanoparticles could well improve the sensitivity and flexibility of current detection techniques. This article provides a general overview of the synthesis, properties and application of luminescent semiconductor, metal and inorganic nanoparticles for in vitro and in vivo diagnostics, also reflecting the aspect of biocompatibility.

Design and synthesis of a terbium(III) complex-based luminescence probe for time-gated luminescence detection of mercury(II) ions

Time-gated luminescence detection technique using lanthanide complexes as luminescent probes is a useful and highly sensitive method. However, the effective application of this technique is limited by the lack of the target-responsive luminescent lanthanide complexes that can specifically recognize various analytes in aqueous solutions. In this work, a dual-functional ligand that can form a stable complex with Tb(3+) and specifically recognize Hg(2+) ions in aqueous solutions, N,N,N(1),N(1)-{[2,6-bis(3'-aminomethyl-1'-pyrazolyl)-4-[N,N-bis(3″,6″-dithiaoctyl)-aminomethyl]- pyridine]} tetrakis(acetic acid) (BBAPTA), has been designed and synthesized. The luminescence of its Tb(3+) complex is weak, but can be effectively enhanced upon reaction with Hg(2+) ions in aqueous solutions. The luminescence response investigations of BBAPTA-Tb(3+) to various metal ions indicate that the complex has a good luminescence sensing selectivity for Hg(2+) ions, but not for other metal ions. Thus a highly sensitive time-gated luminescence detection method for Hg(2+) ions was developed by using BBAPTA-Tb(3+) as a luminescent probe. The dose-dependent luminescence enhancement of the probe shows a good linearity with a detection limit of 17 nM for Hg(2+) ions. These results demonstrated the efficacy and advantages of the new Tb(3+) complex-based luminescence probe for the sensitive and selective detection of Hg(2+) ions.

Biocompatible hydroxyapatite nanoparticles as a redox luminescence switch

A redox luminescence switch was prepared by doping hydroxyapatite nanoparticles with CePO(4):Tb. The resulting multifunctional material exhibits good biocompatibility, biological affinity, and potential drug-carrying capability. The luminescent hydroxyapatite nanoparticles may find important applications in biomedical diagnostics, drug delivery, and biological sensors.