Turning an aptamer into a light-switch probe with a single bioconjugation

Inorganic-organic hybrid nanoparticles with carbonate-triggered emission-colour-shift

(Eu3+4[PTC]4-3)0.78(Eu3+[TREN-1,2-HOPO]3-)0.22 inorganic-organic hybrid nanoparticles (IOH-NPs) contain Eu3+, tris[(1-hydroxy-2-oxo-1,2-dihydropyridine-6-carboxamido)ethyl]amine (TREN-1,2-HOPO) and perylene-3,4,9,10-tetracarboxylate (PTC). The IOH-NPs are prepared in water and exhibit a rod-type shape, with a length of 60 nm and a diameter of 5 nm. Particle size and chemical composition are examined by different methods (SEM, DLS, FT-IR, TG, C/H/N analysis). With TREN-1,2-HOPO as antenna, the IOH-NPs show Eu3+-based red emission, whereas the PTC emission is totally quenched due to π-stacking in the solid nanoparticles. After addition of carbonate, PTC is released from the IOH-NPs into solution, resulting in an increasing green emission of free PTC. The resulting carbonate-driven shift of the emission colour from red to green surprisingly allows to determine the carbonate concentration qualitatively and quantitatively in a concentration range of 1 μM to 2 mM and was tested for tap water as a specific example.

Smart Applications of Lanthanide Chelates-based Luminescent Probes in Bio-imaging

Luminescent Lanthanide (III) (Ln (III)) bioprobes (LLBs) have been extensively used in the last two decades as intracellular molecular probes in bio-imaging for the efficient revelation of analytes, to signal intracellular events (enzymes/protein activity, antigen-antibody interaction), target specific organelles, and determine parameters of particular biophysical interest, to gain important insights on pathologies or diseases. The choice of using a luminescent Ln (III) coordination compound with respect to a common organic fluorophore is intimately connected to how their photophysical sensitization (antenna effect) can be finely tuned and especially triggered to respond (even quantitatively) to a certain biophysical event, condition or analyte. While there are other reviews focused on how to design chromophoric ligands for an efficient sensitization of Ln (III) ions, both in the visible and NIR region, this mini-review is application-driven: it is a small collection of particularly interesting examples where the LLB's emissive information is acquired by imaging the emission intensity and/or the fluorescence lifetime (fluorescence lifetime imaging microscopy, FLIM).

Luminescent Lanthanide Probes for Inorganic and Organic Phosphates

Inorganic and organic phosphates-including orthophosphate, nucleotides, and DNA-are some of the most fundamental anions in cellular biology, regulating numerous processes of both medical and environmental significance. The characteristic long lifetimes of emitting lanthanides, including the brighter europium(III) and terbium(III), make them ideally suited for the development of molecular probes for the detection of phosphates directly in complex aqueous media. Moreover, given their high oxophilicity and the exquisite sensitivity of their quantum yields to their hydration number, those luminescent lanthanides are perfect for the detection of phosphates. Herein we discuss the principles that have guided the recent developments of molecular probes selective for inorganic or organic phosphates and how these lanthanide complexes facilitate the study of numerous biological processes.

Probing the Mechanism of Structure-Switching Aptamer Assembly by Super-Resolution Localization of Individual DNA Molecules

Oligonucleotide aptamers can be converted into structure-switching biosensors by incorporating a short, typically labeled oligonucleotide that is complementary to the analyte-binding region. Binding of a target analyte can disrupt the hybridization equilibrium between the aptamer and the labeled-complementary oligo producing a concentration-dependent signal for target-analyte sensing. Despite its importance in the performance of a biosensor, the mechanism of analyte-response of most structure-switching aptamers is not well understood. In this work, we employ single-molecule fluorescence imaging to investigate the competitive kinetics of association of a labeled complementary oligonucleotide and a target analyte, l-tyrosinamide (L-Tym), interacting with an L-Tym-binding aptamer. The complementary readout strand is fluorescently labeled, allowing us to measure its hybridization kinetics with individual aptamers immobilized on a surface and located with super-resolution techniques; the small-molecule L-Tym analyte is not labeled in order to avoid having an attached dye molecule impact its interactions with the aptamer. We measure the association kinetics of unlabeled L-Tym by detecting its influence on the hybridization of the labeled complementary strand. We find that L-Tym slows the association rate of the complementary strand with the aptamer but does not impact its dissociation rate, suggesting an S_N1-like mechanism where the complementary strand must dissociate before L-Tym can bind. The competitive model revealed a slow association rate between L-Tym and the aptamer, producing a long-lived L-Tym-aptamer complex that blocks hybridization with the labeled complementary strand. These results provide insight about the kinetics and mechanism of analyte recognition in this structure-switching aptamer, and the methodology provides a general means of measuring the rates of unlabeled-analyte binding kinetics in aptamer-based biosensors.

Forced Intercalation (FIT)-Aptamers

Aptamers are oligonucleotide sequences that can be evolved to bind to various analytes of interest. Here, we present a general design strategy that transduces an aptamer-target binding event into a fluorescence readout via the use of a viscosity-sensitive dye. Target binding to the aptamer leads to forced intercalation (FIT) of the dye between oligonucleotide base pairs, increasing its fluorescence by up to 20-fold. Specifically, we demonstrate that FIT-aptamers can report target presence through intramolecular conformational changes, sandwich assays, and target-templated reassociation of split-aptamers, showing that the most common aptamer-target binding modes can be coupled to a FIT-based readout. This strategy also can be used to detect the formation of a metallo-base pair within a duplexed strand and is therefore attractive for screening for metal-mediated base pairing events. Importantly, FIT-aptamers reduce false-positive signals typically associated with fluorophore-quencher based systems, quantitatively outperform FRET-based probes by providing up to 15-fold higher signal to background ratios, and allow rapid and highly sensitive target detection (nanomolar range) in complex media such as human serum. Taken together, FIT-aptamers are a new class of signaling aptamers which contain a single modification, yet can be used to detect a broad range of targets.

First synthesis of orthogonally 1,7-diprotected cyclens

Six novel orthogonally 1,7-heterodiprotected cyclen derivatives have been prepared through an efficient and chromatography-free procedure.

Achieving selectivity for copper over zinc with luminescent terbium probes bearing phenanthridine antennas

A family of terbium probes was synthesized and evaluated for the luminescence detection of copper and zinc in water at neutral pH. Each probe incorporates a terbium ion chelated by a macrocyclic polyaminocarboxylate and conjugated to either one, two, or three phenanthridine antennas via a diamine linker. All three probes, Tb-1Phen, Tb-2Phen, and Tb-3Phen, exhibit similar responses toward copper and zinc. In each case, the terbium-centered time-gated phosphorescence decreases upon binding either Cu^I or Cu^II but not upon addition of Zn^II. The phosphorescence of Tb-2Phen is also not significantly affected by other metal ions including Mg^II, Ca^II, Mn^II, Fe^II, Ni^II, Cd^II, and Hg^II. Tb-1Phen, on the other hand, responds weakly to Mn^II, Fe^II and Ni^II. The lack of affinity of each probe for Zn^II was further confirmed by competition experiments with Cu^I and Cu^II. Notably, whereas the terbium-centered emission of each probe is quenched upon copper coordination, the phenanthridine-centered luminescence emission is not. As such, each probe functions as a ratiometric probe for the selective detection of copper over zinc. Theoretical calculations further demonstrate that the turn off response of the probe is due to an increase in the distance separating the lanthanide ion from its phenanthridine antennas upon coordination of copper, which in turn decreases the efficiency of terbium sensitization by the phenanthridines.

A Schiff base/quaternary ammonium salt bifunctional graphene oxide as an efficient adsorbent for removal of Th(IV)/U(VI)

A novel approach for facile covalent functionalization of graphene oxide (GO) was proposed in the present study in order to effectively avoid necessary anhydrous conditions and the usage of harsh reagents during the chemical functionalization of GO. Herein, a GO derivative that was functionalized with a primary amine derivative bearing a positively charged quaternary ammonium group, GO-S, was synthesized through a Schiff base condensation reaction between the amine groups of the primary amine derivative and the aldehyde groups of GO. The introduction of the quaternary ammonium groups can prevent GO from stacking and improve the dispersibility of GO after modification. The formation of imine bonds (NCH) between the primary amine and GO has been confirmed by Fourier transform infrared and X-ray photoelectron spectroscopy. The GO-S demonstrated good dispersion stability in aqueous medium and also exhibited better adsorption performance than GO for Th(IV) and U(VI), with a maximum thorium adsorption capacity of 2.22mmol/g and a maximum uranium adsorption capacity of 0.83mmol/g, suggesting a great potential for the application of graphene oxide-based materials for facilitating the removal of Th(IV) and U(VI) from nuclear waste solutions.

Responsive, Water-Soluble Europium(III) Luminescent Probes

The design principles, mechanism of action and performance of europium(III) complexes that serve as strongly emissive and responsive molecular probes in water are critically discussed. Examples of systems designed to assess pH, selected metal ions and anions, including chiral species, as well as selected small molecules and biopolymers are considered, and prospects evaluated for improved performance in more complex biological media such as in bio-fluids and within living cells. Modulation of the emission spectral form, lifetime and degree of circular polarisation can be used to quantify the spectral response and permit calibration.