On the Chemical Stability of DNA-Stabilized Silver Nanoclusters.

DNA-stabilized silver nanoclusters (DNA-AgNCs) are atomically precise emitters, whose chemical stability is commonly defined by their stable optical response over time. Here, two isotopically pure versions (Ag and Ag) of DNA-[AgCl] were mixed, and we demonstrate for the first time for DNA-AgNCs that silver atoms are continuously exchanged between individual clusters. This atom exchange was monitored by time-resolved mass spectrometry.

Fluorescence Screening of DNA-AgNCs with Pulsed White Light Excitation.

DNA-stabilized silver nanoclusters (DNA-AgNCs) are a class of fluorophores with interesting photophysical properties dominated by the choice of DNA sequence. Screening methods with ultraviolet excitation and steady state well plate readers have previously been used for deepening the understanding between DNA sequence and emission color of the resulting DNA-AgNCs. Here, we present a new method for screening DNA-AgNCs by using pulsed white light excitation (λ ≈ 490-900 nm).

Analytical method for the determination of the absorption coefficient of DNA-stabilized silver nanoclusters.

DNA-stabilized silver nanoclusters (DNA-AgNCs) are biocompatible emitters formed by silver atoms and cations encapsulated in DNA oligomers. Here, we present an analytical approach to calculate the molar absorption coefficient () of these systems, which consists of combining UV-Vis spectroscopy, electrospray ionization-mass spectrometry (ESI-MS), and inductively coupled plasma-optical emission spectrometry (ICP-OES). ESI-MS enables the determination of the number of silvers bound to the DNA strands, whereas ICP-OES allows measurement of the total amount of silver in solution.

Time gated Fourier transform spectroscopy with burst excitation for time-resolved spectral maps from the nano- to millisecond range.

We demonstrate burst-mode Time Gated Fourier Transform Spectroscopy (bmTG-FTS), a technique for simultaneously capturing and disentangling emission signals from short- (ns) and long-lived (μs-ms) states. We showcase the possibilities of the technique by preparing time gated temporal-spectral maps from a dual-emissive DNA-stabilized silver nanocluster (DNA-AgNC).

A DNA-Stabilized Ag Cluster with Excitation-Intensity-Dependent Dual Emission.

DNA-stabilized silver nanoclusters (DNA-AgNCs) are easily tunable emitters with intriguing photophysical properties. Here, a DNA-AgNC with dual emission in the red and near-infrared (NIR) regions is presented. Mass spectrometry data showed that two DNA strands stabilize 18 silver atoms with a nanocluster charge of 12+.

Bioconjugation of a Near-Infrared DNA-Stabilized Silver Nanocluster to Peptides and Human Insulin by Copper-Free Click Chemistry.

DNA-stabilized silver nanoclusters (DNA-AgNCs) are biocompatible emitters with intriguing properties. However, they have not been extensively used for bioimaging applications due to the lack of structural information and hence predictable conjugation strategies. Here, a copper-free click chemistry method for linking a well-characterized DNA-AgNC to molecules of interest is presented.

DNA-AgNC Loaded Liposomes for Measuring Cerebral Blood Flow Using Two-Photon Fluorescence Correlation Spectroscopy.

Unraveling the transport of drugs and nanocarriers in cerebrovascular networks is important for pharmacokinetic and hemodynamic studies but is challenging due to the complexity of sensing individual particles within the circulatory system of a live animal. Here, we demonstrate that a DNA-stabilized silver nanocluster (DNA-AgNC) that emits in the first near-infrared window upon two-photon excitation in the second NIR window can be used for multiphoton fluorescence correlation spectroscopy for the measurement of cerebral blood flow rates in live mice with high spatial and temporal resolution. To ensure bright and stable emission during experiments, we loaded DNA-AgNCs into liposomes, which served the dual purposes of concentrating the fluorescent label and protecting it from degradation.

Chloride Ligands on DNA-Stabilized Silver Nanoclusters.

DNA-stabilized silver nanoclusters (Ag-DNAs) are known to have one or two DNA oligomer ligands per nanocluster. Here, we present the first evidence that Ag-DNA species can possess additional chloride ligands that lead to increased stability in biologically relevant concentrations of chloride. Mass spectrometry of five chromatographically isolated near-infrared (NIR)-emissive Ag-DNA species with previously reported X-ray crystal structures determines their molecular formulas to be (DNA)[AgCl].

Excited-State Dynamics in a DNA-Stabilized Ag Cluster with Near-Infrared Emission.

Due to desirable optical properties, such as efficient luminescence and large Stokes shift, DNA-templated silver nanoclusters (DNA-AgNCs) have received significant attention over the past decade. Nevertheless, the excited-state dynamics of these systems are poorly understood, as studies of the processes ultimately leading to a fluorescent state are scarce. Here we investigate the early time relaxation dynamics of a 16-atom silver cluster (DNA-AgNC) featuring NIR emission in combination with an unusually large Stokes shift of over 5000 cm.

Experimental assignment of long-range magnetic communication through Pd & Pt metallophilic contacts.

Record-breaking magnetic exchange interactions have previously been reported for 3d-metal dimers of the form [M(Pt(SAc))(pyNO)] (M = Ni or Co) that are linked in the solid state metallophilic Pt⋯Pt bridges. This contrasts the terminally capped monomers [M(Pt(SAc))(py)], for which neither metallophilic bridges nor magnetic exchange interactions are found. Computational modeling has shown that the magnetic exchange interaction is facilitated by the pseudo-closed shell d⋯d metallophilic interaction between the filled Pt 5d orbitals.

The effect of inosine on the spectroscopic properties and crystal structure of a NIR-emitting DNA-stabilized silver nanocluster.

The effect of replacing guanosines with inosines in the two stabilizing strands (5'-CACCTAGCGA-3') of the NIR emissive DNA-AgNC was investigated. The spectroscopic behavior of the inosine mutants is position-dependent: when the guanosine in position 7 was exchanged, the nanosecond fluorescence decay time shortened, while having the inosine in position 9 made the decay time longer. Thanks to structural information gained from single crystal X-ray diffraction measurements, it was possible to propose a mechanistic origin for the observed changes.

DNA Stabilizes Eight-Electron Superatom Silver Nanoclusters with Broadband Downconversion and Microsecond-Lived Luminescence.

DNA oligomers are known to serve as stabilizing ligands for silver nanoclusters (Ag-DNAs) with rod-like nanocluster geometries and nanosecond-lived fluorescence. Here, we report two Ag-DNAs that possess distinctly different structural properties and are the first to exhibit only microsecond-lived luminescence. These emitters are characterized by significant broadband downconversion from the ultraviolet/visible to the near-infrared region.

Probing emission of a DNA-stabilized silver nanocluster from the sub-nanosecond to millisecond timescale in a single measurement.

A method for measuring emission over a range of sub-nanosecond to millisecond timescales is presented and demonstrated for a DNA-stabilized silver nanocluster (DNA-AgNC) displaying dual emission. This approach allows one to disentangle the temporal evolution of the two spectrally overlapping signals and to determine both the nano- and microsecond decay times of the two emission components, together with the time they take to reach the steady-state equilibrium. Addition of a second near-infrared laser, synchronized with a fixed delay, enables simultaneous characterization of optically activated delayed fluorescence (OADF).

The effect of deuterium on the photophysical properties of DNA-stabilized silver nanoclusters.

We investigated the effect of using DO HO as solvent on the spectroscopic properties of two NIR emissive DNA-stabilized silver nanoclusters (DNA-AgNCs). The two DNA-AgNCs were chosen because they emit in the same energy range as the third overtone of the O-H stretch. Opposite effects on the ns-lived decay were observed for the two DNA-AgNCs.

Frequency Encoding of Upconversion Nanoparticle Emission for Multiplexed Imaging of Spectrally and Spatially Overlapping Lanthanide Ions.

We present frequency encoded upconversion (FE-UPCON) widefield microscopy, an imaging approach that allows for multiplexed signal recovery based on frequency encoding of selected upconverted lanthanide ion emission rather than separation based on energy or time. FE-UPCON allows for the separation of luminescence from spectrally and spatially overlapping trivalent lanthanide ions (Ln) in upconversion nanoparticles (UCNPs). Utilizing the numerous electronic energy levels of Ln, one can generate a frequency encoded signal by periodic coexcitation with a secondary light source (modulated at a chosen frequency) that, for a particular wavelength, enhances the luminescence of the Ln of interest.

Observation of microsecond luminescence while studying two DNA-stabilized silver nanoclusters emitting in the 800-900 nm range.

We investigated two DNA-stabilized silver nanoclusters (DNA-AgNCs) that show multiple absorption features in the visible region, and emit around 811 nm (DNA811-AgNC) and 841 nm (DNA841-AgNC). Both DNA-AgNCs have large Stokes shifts and can be efficiently excited with red light. A comparison with the commercially available Atto740 yielded fluorescence quantum yields in the same order of magnitude, but a higher photon output above 800 nm since both DNA-AgNCs are more red-shifted.

Insights from Studies on the Early Stages of Platinum Nanoparticle Formation.

Understanding the formation of nanomaterials down to the atomic level is key to rational design of advanced materials. Despite their widespread use and intensive study over the years, the detailed formation mechanism of platinum (Pt) nanoparticles remains challenging to explore and rationalize. Here, various characterization techniques, and in particular X-ray total scattering with pair distribution function (PDF) analysis, are used to follow the structural and chemical changes taking place during a surfactant-free synthesis of Pt nanoparticles in alkaline methanol.

Single-Molecule Detection of DNA-Stabilized Silver Nanoclusters Emitting at the NIR I/II Border.

The near-infrared (NIR) I and II regions are known for having good light transparency of tissue and less scatter compared to the visible region of the electromagnetic spectrum. However, the number of bright fluorophores in these regions is limited. Here we present a detailed spectroscopic characterization of a DNA-stabilized silver nanocluster (DNA-AgNC) that emits at around 960 nm in solution.

Structure and luminescence of DNA-templated silver clusters.

DNA serves as a versatile template for few-atom silver clusters and their organized self-assembly. These clusters possess unique structural and photophysical properties that are programmed into the DNA template sequence, resulting in a rich palette of fluorophores which hold promise as chemical and biomolecular sensors, biolabels, and nanophotonic elements. Here, we review recent advances in the fundamental understanding of DNA-templated silver clusters (Ag -DNAs), including the role played by the silver-mediated DNA complexes which are synthetic precursors to Ag -DNAs, structure-property relations of Ag -DNAs, and the excited state dynamics leading to fluorescence in these clusters.

Rational Design of Bright Long Fluorescence Lifetime Dyad Fluorophores for Single Molecule Imaging and Detection.

Increasing demand for detecting single molecules in challenging environments has raised the bar for the fluorophores used. To achieve better resolution and/or contrast in fluorescence microscopy, it is now essential to use bright and stable dyes with tailored photophysical properties. While long fluorescence lifetime fluorophores offer many advantages in time-resolved imaging, their inherently lower molar absorption coefficient has limited applications in single molecule imaging.

Solvent-Dependent Growth and Stabilization Mechanisms of Surfactant-Free Colloidal Pt Nanoparticles.

Understanding the formation of nanoparticles (NPs) is key to develop materials by sustainable routes. The Co4Cat process is a new synthesis of precious metal NPs in alkaline mono-alcohols well-suited to develop active nanocatalysts. The synthesis is 'facile', surfactant-free and performed under mild conditions like low temperature.

Unusually large fluorescence quantum yield for a near-infrared emitting DNA-stabilized silver nanocluster.

A near-infrared emitting DNA-stabilized silver nanocluster (DNA-AgNC) with an unusually high fluorescence quantum yield is presented. The steady-state and time-resolved fluorescence properties of the DNA-AgNC were characterized, together with its ability to generate optically activated delayed fluorescence (OADF) and upconversion fluorescence (UCF).

UV-induced syntheses of surfactant-free precious metal nanoparticles in alkaline methanol and ethanol.

Surfactant-free UV-induced syntheses of Pt and Ir nanoparticles in alkaline methanol and ethanol are presented. Small size nanoparticles 2 nm in diameter are obtained without surfactants in a wide range of base concentration.

Intrinsic anti-Stokes emission in living HeLa cells.

Intrinsic fluorescence of biological material, also called auto-fluorescence, is a well-known phenomenon and has in recent years been used for imaging, diagnostics and cell viability studies. Here we show that in addition to commonly observed auto-fluorescence, intrinsic anti-Stokes emission can also be observed under 560 nm or 633 nm excitation. The anti-Stokes emission is shown to be spatially located on/in the mitochondria.