Luminescent lanthanide reporters: new concepts for use in bioanalytical applications

Triplet-Triplet Annihilation Upconversion-Based Oxygen Sensors to Overcome the Limitation of Autofluorescence

Autofluorescence is one of the many challenges in bioimaging as it can mask the emission from fluorescent probes or markers, a limitation that can be overcome via upconversion. Herein, we have developed a nanosensor that uses triplet-triplet annihilation upconversion to optically report changes in the dissolved oxygen concentration. Using a sensitizer-annihilator dye pairing of platinum(II) octaethylporphyrin and 9,10-diphenylanthracene, we monitored the oxygen consumption (as a proxy for metabolic activity) over time in a biological system─Saccharomyces cerevisiae (brewing yeast). The nanosensor demonstrated good reversibility over multiple cycles and showed good signal and colloidal stability when tested over the course of 7 days, and it was sensitive to dissolved oxygen from 0.00 to 3.17 mg/L O2. Additionally, there was no signal overlap between the nanosensor emission and S. cerevisiae autofluorescence, thus underscoring the utility of upconversion as a facile and economical means of overcoming autofluorescence.

Luminescence nanothermometry using a trivalent lanthanide co-doped perovskite

This study investigates in detail the laser-mediated upconversion emission and temperature-sensing capability of (Ca_0.99-a Yb_0.01Er _a )TiO₃. Samples were prepared at different concentrations to observe the effect of erbium on upconversion while increasing its concentration and keeping all the other parameters constant. Doping is a widespread technological process which involves incorporating an element called a dopant in a lower ratio to the host lattice to derive hybrid materials with desired properties. The (Ca_0.99-a Yb_0.01Er _a )TiO₃ perovskite nanoparticles were synthesized via a sol-gel technique. The frequency upconversion was performed using a 980 nm laser diode excitation source. X-ray diffractometry (XRD) confirmed that the synthesized samples are crystalline in nature and have an orthorhombic structure. The temperature-sensing ability was examined using the fluorescence intensity ratio (FIR) algorithm of two emission bands (²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2) of the Er³⁺ ion. Temperature-dependent upconversion luminescence is observed over a broad temperature range of 298-623 K. The maximum sensor sensitivity obtained is 6.71 × 10^-3 K^-1 at 110°.

Ultrasensitive Point-of-Care Test for Tumor Marker in Human Saliva Based on Luminescence-Amplification Strategy of Lanthanide Nanoprobes

The point-of-care detection of tumor markers in saliva with high sensitivity and specificity remains a daunting challenge in biomedical research and clinical applications. Herein, a facile and ultrasensitive detection of tumor marker in saliva based on luminescence-amplification strategy of lanthanide nanoprobes is proposed. Eu2O3 nanocrystals are employed as bioprobes, which can be easily dissolved in acidic enhancer solution and transform into a large number of highly luminescent Eu3+ micelles. Meanwhile, disposable syringe filter equipped with nitrocellulose membrane is used as bioassay platform, which facilitates the accomplishment of detection process within 10 min. The rational integration of dissolution enhanced luminescent bioassay strategy and miniaturized detection device enables the unique lab-in-syringe assay of tumor marker like carcinoembryonic antigen (CEA, an important tumor marker in clinic diagnosis and prognosis of cancer) with a detection limit down to 1.47 pg mL-1 (7.35 × 10-15 m). Upon illumination with a portable UV flashlight, the photoluminescence intensity change above 0.1 ng mL-1 (0.5 × 10-12 m) of CEA can be visually detected by naked eyes, which allows one to qualitatively evaluate the CEA level. Moreover, we confirm the reliability of using the amplified luminescence of Eu2O3 nanoprobes for direct quantitation of CEA in patient saliva samples, thus validates the practicality of the proposed strategy for both clinical diagnosis and home self-monitoring of tumor marker in human saliva.

Thermoluminescence Enhancement of LiMgPO4 Crystal Host by Tb3+ and Tm3+ Trivalent Rare-Earth Ions Co-doping

We investigated the influence of terbium and thulium trivalent rare-earth (RE) ions co-doping on the luminescent properties enhancement of LiMgPO₄ (LMP) crystal host. The studied crystals were grown from the melt by micro-pulling-down (MPD) technique. Luminescent properties of the obtained crystals were investigated by thermoluminescence (TL) method. The most favorable properties and the highest luminescence enhancement were measured for Tb and Tm double doped crystals. A similar luminescence level can be also obtained for Tm, B co-doped samples. In this case, however, the low-temperature TL components have a significant contribution. The measured luminescent spectra showed a typical emission of Tb³⁺ and Tm³⁺ ions of an opposite trapping nature, namely the holes and electron-trapping sites, respectively. The most prominent transitions of ⁵D₄ → ⁷F₃ (550 nm for Tb³⁺) and ¹D₂ → ³F₄ (450 nm for Tm³⁺) were observed. It was also found that Tb³⁺ and Tm³⁺ emissions show temperature dependence in the case of double doped LMP crystal sample, which was not visible in the case of the samples doped with a single RE dopant. At a low temperature range (up to around 290 °C) Tm³⁺ emission was dominant. At higher temperatures, the electrons occupying Tm³⁺ sites started to be released giving rise to emissions from Tb-related recombination centers, and emissions from Tm³⁺ centers simultaneously decreased. At the highest temperatures, emission took place from Tb³⁺ recombination centers, but only from deeper ⁵D₄ level-related traps which had not been emptied at a lower temperature range.

Unveiling the Effects of Rare-Earth Substitutions on the Structure, Mechanical, Optical, and Imaging Features of ZrO2 for Biomedical Applications

The impact of selective rare-earth (RE) additions in ZrO₂-based ceramics on the resultant crystal structure, mechanical, morphological, optical, magnetic, and imaging contrast features for potential applications in biomedicine is explored. Six different RE, namely, Yb³⁺, Dy³⁺, Tb³⁺, Gd³⁺, Eu³⁺, and Nd³⁺ alongside their variations in the dopant concentrations were selected to accomplish a wide range of combinations. The experimental observations affirmed the roles of size and dopant concentration in determining the crystalline phase behavior of ZrO₂. The significance of tetragonal ZrO₂ (t-ZrO₂) → monoclinic ZrO₂ degradation is evident with 10 mol % of RE substitution, while RE contents in the range of 20 and 40 mol % ensured either t-ZrO₂ or cubic ZrO₂ (c-ZrO₂) stability until 1500 °C. High RE content in the range of 80-100 mol % still confirmed the structural stability of c-ZrO₂ for lower-sized Yb³⁺, Dy³⁺, and Tb³⁺, while the c-ZrO₂ → RE₂Zr₂O₇ phase transition becomes evident for higher-sized Gd³⁺, Eu³⁺, and Nd³⁺. A steady decline in the mechanical properties alongside a quenching effect experienced in the emission phenomena is apparent for high RE concentrations in ZrO₂. On the one hand, the paramagnetic characteristics of Dy³⁺, Tb³⁺, Gd³⁺, and Nd³⁺ fetched excellent contrast features from magnetic resonance imaging analysis. On the other hand, Yb³⁺ and Dy³⁺ added systems exhibited good X-ray absorption coefficient values determined from computed tomography analysis.

Inorganic Complexes and Metal-Based Nanomaterials for Infectious Disease Diagnostics

Infectious diseases claim millions of lives each year. Robust and accurate diagnostics are essential tools for identifying those who are at risk and in need of treatment in low-resource settings. Inorganic complexes and metal-based nanomaterials continue to drive the development of diagnostic platforms and strategies that enable infectious disease detection in low-resource settings. In this review, we highlight works from the past 20 years in which inorganic chemistry and nanotechnology were implemented in each of the core components that make up a diagnostic test. First, we present how inorganic biomarkers and their properties are leveraged for infectious disease detection. In the following section, we detail metal-based technologies that have been employed for sample preparation and biomarker isolation from sample matrices. We then describe how inorganic- and nanomaterial-based probes have been utilized in point-of-care diagnostics for signal generation. The following section discusses instrumentation for signal readout in resource-limited settings. Next, we highlight the detection of nucleic acids at the point of care as an emerging application of inorganic chemistry. Lastly, we consider the challenges that remain for translation of the aforementioned diagnostic platforms to low-resource settings.

Insulin: a review of analytical methods

Insulin is an important polypeptide hormone that regulates carbohydrate metabolism.

Chiral dinuclear Ln(iii) complexes derived from S- and R-2-(6-methoxy-2-naphthyl)propionate. Optical and magnetic properties

Eight dinuclear chiral compounds of the formula [Ln₂(S-L)₆(phen)₂]·3DMF·H₂O or [Ln₂(R-L)₆(phen)₂]·3DMF·H₂O [Ln = Eu, Gd, Tb and Dy], (S)-(+)- or (R)-(−)-2-(6-methoxy-2-naphthyl)propionic acid, S-HL or R-HL, are reported. Luminescence and magnetic studies are also reported.

Slow magnetic relaxation and luminescence properties in lanthanide(iii)/anil complexes

The initial use of anils, i.e. bidentate Schiff bases derived from the condensation of anilines with salicylaldehyde or its derivatives, in 4f-metal chemistry is described. The 1 : 1 reactions between Ln(NO3)3·xH2O (Ln = lanthanide) or Y(NO3)3·6H2O and N-(5-bromosalicylidene)aniline (5BrsalanH) in MeCN has provided access to complexes [Ln(NO3)3(5BrsalanH)2(H2O)]·MeCN (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) and [Y(NO3)3(5BrsalanH)2(H2O)]·MeCN, respectively, in good yields. The structures of the isomorphous complexes with Ln = Pr(1·MeCN), Sm(3·MeCN), Gd(5·MeCN), Dy(7·MeCN) and Er(9·MeCN) have been determined by single-crystal X-ray crystallography. The other complexes were proven to be isostructural with the fully structurally characterized compounds based on elemental analyses, IR spectra, unit cell determinations and powder X-ray patterns. The 9-coordinate LnIII centre in the [Ln(NO3)3(5BrsalanH)2(H2O)] molecules is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms that belong to the organic ligands and one oxygen atom from the aquo ligand. The 5BrsalanH molecules behave as monodentate O-donors; the acidic H atom is clearly located on the imino N atom and thus the formally neutral ligands adopt an extremely rare coordination mode participating in the zwitterionic form. The coordination polyhedra defined by the nine donor atoms around the LnIII centres are best described as spherical capped square antiprisms. Various intermolecular interactions build the crystal structures and Hirshfeld surface analysis was applied to evaluate the magnitude of interactions between the molecules. Solid-state IR and UV/VIS data are discussed in terms of structural features. 1H NMR data prove that the diamagnetic [Y(NO3)3(5BrsalanH)2(H2O)] complex decomposes in DMSO. Combined dc and ac magnetic susceptibility, as well as magnetization data for 7 suggest that this complex shows field-induced slow magnetic relaxation. Two magnetization relaxation processes are evident. The fit to the Arrhenius law has been performed using the 6.5-8.5 K ac data, affording an effective barrier for the magnetization reversal of 27 cm-1. Cole-Cole plot analysis in the temperature range in which the Orbach relaxation process is assumed, reveals a narrow distribution of relaxation times. The solid Dy(iii) complex 7 emits green light at 338 nm, the emission being ligand-centered. The perspectives of the present, first results in the lanthanide(iii)-anil chemistry are critically discussed.

Slow Magnetic Relaxation in a Palladium-Gadolinium Complex Induced by Electron Density Donation from the Palladium Ion

Incorporating palladium in the first coordination sphere of acetato-bridged lanthanoid complexes, [Pd₂ Ln₂ (H₂ O)₂ (AcO)₁₀ ]⋅2 AcOH (Ln=Gd (1), Y (2), Gd_0.4 Y_1.6 (3), Eu (4)), led to significant bonding interactions between the palladium and the lanthanoid ions, which were demonstrated by experimental and theoretical methods. We found that electron density was donated from the d⁸ Pd²⁺ ion to Gd³⁺ ion in 1 and 3, leading to the observed slow magnetic relaxation by using local orbital locator (LOL) and X-ray absorption near-edge structure (XANES) analysis. Field-induced dual slow magnetic relaxation was observed for 1 up to 20 K. Complex 3 and frozen aqueous and acetonitrile solutions of 1 showed only one relaxation peak, which confirms the role of intermolecular dipolar interactions in slowing the magnetic relaxation of 1. The slow magnetic relaxation occurred through a combination of Orbach and Direct processes with the highest pre-exponential factor (τ_o =0.06 s) reported so far for a gadolinium complex exhibiting slow magnetic relaxation. The results revealed that transition metal-lanthanoid (TM-Ln) axial interactions indeed could lead to new physical properties by affecting both the electronic and magnetic states of the compounds.

Application of lanthanide luminescence in probing enzyme activity

Enzymes play critical roles in the regulation of cellular function and are implicated in numerous disease conditions. Reliable and practicable assays are required to study enzyme activity, to facilitate the discovery of inhibitors and activators of enzymes related to disease. In recent years, a variety of enzyme assays have been devised that utilise luminescent lanthanide(iii) complexes, taking advantage of their high detection sensitivities, long luminescence lifetimes, and line-like emission spectra that permit ratiometric and time-resolved analyses. In this Feature article, we focus on recent progress in the development of enzyme activity assays based on lanthanide(iii) luminescence, covering a variety of strategies including Ln(iii)-labelled antibodies and proteins, Ln(iii) ion encapsulation within defined peptide sequences, reactivity-based Ln(iii) probes, and discrete Ln(iii) complexes. Emerging approaches for monitoring enzyme activity are discussed, including the use of anion responsive lanthanide(iii) complexes, capable of molecular recognition and luminescence signalling of polyphosphate anions.

Heteroleptic β-diketonate Ln(iii) complexes decorated with pyridyl substituted pyridazine ligands: synthesis, structure and luminescence properties

A substituted pyridazine acts as a sensitizer in mononuclear heteroleptic Ln(iii) complexes.

Fluorescence Sensing of Circulating Diagnostic Biomarkers Using Molecular Probes and Nanoparticles

The interplay of photonics, nanotechnology, and biochemistry has significantly improved the identification and characterization of multiple types of biomarkers by optical biosensors. Great achievements in fluorescence-based technologies have been realized, for example, by the advancement of multiplexing techniques or the introduction of nanoparticles to biochemical and clinical research. This review presents a concise overview of recent advances in fluorescence sensing techniques for the detection of circulating disease biomarkers. Detection principles of representative approaches, including fluorescence detection using molecular fluorophores, quantum dots, and metallic and silica nanoparticles, are explained and illustrated by pertinent examples from the recent literature. Advanced detection technologies and material development play a major role in modern biosensing and consistently provide significant improvements toward robust, sensitive, and versatile platforms for early detection of circulating diagnostic biomarkers.

Following the stability of amphiphilic nanoaggregates by using intermolecular energy transfer

An intermolecular energy transfer system is developed for studying the stability of nanoaggregate(s) (NAs) in complex solution and cell culture by one- and two-photon fluorescence microscopy and optical imaging. The system allows facile addition of one or more tumor targeting molecules, one of which is exemplified here. NAs functionalized with an MRI and optical probe, with and without folic acid, remain stable in fetal bovine serum for at least 4 hours. HeLa cell cultures showed a clear difference between NAs non-targeted and targeted to folate receptors, with both NAs appearing to be taken up by the cells through different mechanisms. An MRI relaxivity, r₁, of 9 mM^-1 s^-1 at 310 K and 1.4 T was measured associated with the increased rotational correlation time of the NAs. These NAs may have application in the targeted drug delivery of hydrophobic drugs such as doxorubicin (DOX).

Dissolution Mechanism of Upconverting AYF4:Yb,Tm (A = Na or K) Nanoparticles in Aqueous Media

The dissolution of upconverting AYF4:Yb,Tm (A = Na or K) nanoparticles (UCNPs) in aqueous media was systematically studied. UCNPs with a cubic structure and sizes of between 10 and 33 nm were synthesized solvothermally in ethylene glycol at 200 °C. The UCNPs of both compositions showed an upconversion fluorescence emission characteristic of Tm(3+). The effects of the A cation, the particle size, the temperature, the pH, and the composition of the aqueous medium on the dissolution of the UCNPs were evaluated. The degree of dissolution was determined from the fraction of dissolved fluoride (F(-)) using potentiometry. Unexpectedly, the composition of aqueous media had the most significant effect on the dissolution of the UCNPs. The highest degree of dissolution and rate were measured for the phosphate-buffered saline (PBS), which can be explained by the formation of stable lanthanide compounds with phosphates. The degree of dissolution was much lower in water and in the phthalate buffer, which was attributed to the release of F(-) as a result of the hydrolysis of the UCNPs' surfaces.

Future prospects of luminescent nanomaterial based security inks: from synthesis to anti-counterfeiting applications

Counterfeiting of valuable documents, currency and branded products is a challenging problem that has serious economic, security and health ramifications for governments, businesses and consumers all over the world. It is estimated that counterfeiting represents a multi-billion dollar underground economy with counterfeit products being produced on a large scale every year. Counterfeiting is an increasingly high-tech crime and calls for high-tech solutions to prevent and deter the acts of counterfeiting. The present review briefly outlines and addresses the key challenges in this area, including the above mentioned concerns for anti-counterfeiting applications. This article describes a unique combination of all possible kinds of security ink formulations based on lanthanide doped luminescent nanomaterials, quantum dots (semiconductor and carbon based), metal organic frameworks as well as plasmonic nanomaterials for their possible use in anti-counterfeiting applications. Moreover, in this review, we have briefly discussed and described the historical background of luminescent nanomaterials, basic concepts and detailed synthesis methods along with their characterization. Furthermore, we have also discussed the methods adopted for the fabrication and design of luminescent security inks, various security printing techniques and their anti-counterfeiting applications.

Europium(III) tris-dibenzoylmethanate as an efficient chemosensor for detection of ammonia

The effect of ammonia vapor on luminescence of Eu(III) tris-dibenzoylmethanate immobilized in various matrices has been investigated. It has been revealed that interaction of Eu(III) tris-dibenzoylmethanate with analyte vapor results in increase of the intensity of Eu(III) luminescence. The mechanism of the effect of ammonia vapors on intensification of the Eu(III) luminescence has been suggested using the data of IR spectroscopy, X-ray diffraction analysis and quantum chemistry calculations. The mechanism of luminescence sensitization consists in bonding of an analyte molecule with a water molecule into the coordination sphere of Eu(III). As a result, the bond of a water molecule with the luminescence centre weakens and the blockage of the quenching of luminescence on OH-vibrations takes place.

Development of a Rapid Insulin Assay by Homogenous Time-Resolved Fluorescence

Direct measurement of insulin is critical for basic and clinical studies of insulin secretion. However, current methods are expensive and time-consuming. We developed an insulin assay based on homogenous time-resolved fluorescence that is significantly more rapid and cost-effective than current commonly used approaches. This assay was applied effectively to an insulin secreting cell line, INS-1E cells, as well as pancreatic islets, allowing us to validate the assay by elucidating mechanisms by which dopamine regulates insulin release. We found that dopamine functioned as a significant negative modulator of glucose-stimulated insulin secretion. Further, we showed that bromocriptine, a known dopamine D2/D3 receptor agonist and newly approved drug used for treatment of type II diabetes mellitus, also decreased glucose-stimulated insulin secretion in islets to levels comparable to those caused by dopamine treatment.

Sub-5 nm lanthanide-doped lutetium oxyfluoride nanoprobes for ultrasensitive detection of prostate specific antigen

It remains challenging to develop ultrasmall (<5 nm) but highly luminescent bioprobes with a large linear detection range for the early diagnosis and monitoring of prostate cancer (PCa). Benefiting from the high molar density of lanthanide ions in an oxyfluoride matrix and the superior dissolution capability of Lu₆O₅F₈ nanoparticles in the enhancer solution, we demonstrated the successful use of novel sub-5 nm Lu₆O₅F₈:Eu³⁺ nanoprobes for the detection of prostate specific antigen (PSA) in clinical serum samples. The limit of detection for PSA is as low as 0.52 pg mL^-1, which is almost a 200-fold improvement relative to that of a commercial dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA) kit. The PSA levels detected in 23 patient serum samples were consistent with those measured independently by the DELFIA kit, showing the assay's reliability with a correlation coefficient of 97%. A linear range of 4 orders of magnitude ranging from 8.5 × 10^-4 to 5.6 ng mL^-1 for the assay of PSA was achieved, which is highly promising for the early diagnosis of PCa and monitoring of PCa relapse of patients after radical prostatectomy.

Paramagnetic Nanoparticles Leave Their Mark on Nuclear Spins of Transiently Adsorbed Proteins

The successful application of nanomaterials in biosciences necessitates an in-depth understanding of how they interface with biomolecules. Transient associations of proteins with nanoparticles (NPs) are accessible by solution NMR spectroscopy, albeit with some limitations. The incorporation of paramagnetic centers into NPs offers new opportunities to explore bio-nano interfaces. We propose NMR paramagnetic relaxation enhancement as a new tool to detect NP-binding surfaces on proteins with increased sensitivity, also extending the applicability of NMR investigations to heterogeneous biomolecular mixtures. The adsorption of ubiquitin on gadolinium-doped fluoride-based NPs produced residue-specific NMR line-broadening effects mapping to a contiguous area on the surface of the protein. Importantly, an identical paramagnetic fingerprint was observed in the presence of a competing protein-protein association equilibrium, exemplifying possible interactions taking place in crowded biological media. The interaction was further characterized using isothermal titration calorimetry and upconversion emission measurements. The data indicate that the used fluoride-based NPs are not biologically inert but rather are capable of biomolecular recognition.

A Simple and Sensitive Method to Quantify Biodegradable Nanoparticle Biodistribution using Europium Chelates

The biodistribution of biodegradable nanoparticles can be difficult to quantify. We report a method using time resolved fluorescence (TRF) from a lanthanide chelate to minimize background autofluorescence and maximize the signal to noise ratio to detect biodegradable nanoparticle distribution in mice. Specifically, antenna chelates containing europium were entrapped within nanoparticles composed of polylactic acid-polyethylene glycol diblock copolymers. Tissue accumulation of nanoparticles following intravenous injection was quantified in mice. The TRF of the nanoparticles was found to diminish as a second order function in the presence of serum and tissue compositions interfered with the europium signal. Both phenomena were corrected by linearization of the signal function and calculation of tissue-specific interference, respectively. Overall, the method is simple and robust with a detection limit five times greater than standard fluorescent probes.