Sungmoon Choi

Oligonucleotide-stabilized Ag nanocluster fluorophores.

Single-stranded oligonucleotides stabilize highly fluorescent Ag nanoclusters, with emission colors tunable via DNA sequence. We utilized DNA microarrays to optimize these scaffold sequences for creating nearly spectrally pure Ag nanocluster fluorophores that are highly photostable and exhibit great buffer stability. Five different nanocluster emitters have been created with tunable emission from the blue to the near-IR and excellent photophysical properties. Ensemble and single molecule fluorescence studies show that oligonucleotide encapsulated Ag nanoclusters exhibit significantly greater photostability and higher emission rates than commonly used cyanine dyes.

Shuttle-based fluorogenic silver cluster biolabels

Most molecular/cellular labeling utilizes organic dyes conjugated to bioactive molecules through general organic/inorganic chemistry. Though shortcomings of organic dyes such as poor photostability[1] and low brightness[2] limit observable copy numbers, lack of selectivity in labeling often more seriously limits their application in live cell imaging and single molecule studies[3]. Specificity can be addressed through antibodies and other strong affinity pairs such as avidin-biotin[4, 5], but direct covalent technologies are crucial at sub-pM concentrations, as this is beyond the binding limits of antibody-based affinities. Genetically encoded fluorescent proteins are an excellent solution for specific fluorescent labeling in live cells, but disadvantageous organic dye photophysical instabilities (blinking and bleaching) remain, coupled with the potential perturbation due to large label size[6]. Attempts to overcome photoinstabilities of organic labels have produced much brighter quantum dot labels[7], but, while providing excellent signals, these emitters introduce additional problems such as large physical size, aggregation, toxicity, polyvalency, and strong fluorescence intermittency[8–10]. Simultaneously addressing concerns of brightness, photostability, monovalency, size, and fluorescence intermittency, newly emerging silver cluster-based labels offer excellent potential as molecular labeling agents[11–15]. Spectrally-pure emitters have been produced ranging from the blue to the near IR, with fluorescence quantum yields (ΦF) up to 40% and hydrodynamic radii of the fully assembled ssDNA-encapsulated SCs of ~2.5 nm[16]. Moreover, at bulk and single molecule levels, SCs also show both excellent brightness and photostability[17].

Live Cell Surface Labeling with Fluorescent Ag Nanocluster Conjugates

DNA‐encapsulated silver clusters are readily conjugated to proteins and serve as alternatives to organic dyes and semiconductor quantum dots. Stable and bright on the bulk and single molecule levels, Ag nanocluster fluorescence is readily observed when staining live cell surfaces. Being significantly brighter and more photostable than organics and much smaller than quantum dots with a single point of attachment, these nanomaterials offer promising new approaches for bulk and single molecule biolabeling.

Autofluorescence generation and elimination: a lesson from glutaraldehyde

Glutaraldehyde causes especially high autofluorescence. It reacted with proteins and peptides to generate visible to near-IR emitters. A model indicated that ethylenediamine and a secondary amine in the molecule were key components for the formation of emissive species. The mechanism enables us to control the generation and elimination of autofluorescence.

DNA-encapsulated silver nanodots as ratiometric luminescent probes for hypochlorite detection

DNA-encapsulated silver nanodots are noteworthy candidates for bio-imaging probes, thanks to their excellent photophysical properties. The spectral shift of silver nanodot emitters from red to blue shows excellent correlations with the concentration of reactive oxygen species, which makes it possible to develop new types of probes for reactive oxygen species (ROS), such as hypochlorous acid (HOCl), given the outstanding stability of the blue in oxidizing environments. HOCl plays a role as a microbicide in immune systems but, on the other hand, is regarded as a disease contributor. Moreover, it is a common ingredient in household cleaners. There are still great demands to detect HOCl fluxes and their physiological pathways. We introduce a new ratiometric luminescence imaging method based on silver nanodots to sensitively detect hypochlorite. The factors that influence the accuracy of the detection are investigated. Its availability has also been demonstrated by detecting the active component in cleaners.PACS82; 82.30.Nr; 82.50.-m

Recent development in deciphering the structure of luminescent silver nanodots

Matrix-stabilized silver clusters and stable luminescent few-atom silver clusters, referred to as silver nanodots, show notable difference in their photophysical properties. We present recent research on deciphering the nature of silver clusters and nanodots and understanding the factors that lead to variations in luminescent mechanisms. Due to their relatively simple structure, the matrix-stabilized clusters have been well studied. However, the single-stranded DNA (ssDNA)-stabilized silver nanodots that show the most diverse emission wavelengths and the best photophysical properties remain mysterious species. It is clear that their photophysical properties highly depend on their protection scaffolds. Analyses from combinations of high-performance liquid chromatography, inductively coupled plasma-atomic emission spectroscopy, electrophoresis, and mass spectrometry indicate that about 10 to 20 silver atoms form emissive complexes with ssDNA. However, it is possible that not all of the silver atoms in the c...

Understanding Interactions between Cellular Matrices and Metal Complexes: Methods To Improve Silver Nanodot-Specific Staining

Metal complexes are frequently used for biological applications due to their special photophysical and chemical characteristics. Due to strong interactions between metals and biomacromolecules, a random staining of cytoplasm or nucleoplasm by the complexes results in a low signal-to-background ratio. In this study, we used luminescent silver nanodots as a model to investigate the major driving force for non-specific staining in cellular matrices. Even though some silver nanodot emitters exhibited excellent specific staining of nucleoli, labeling with nanodots was problematic owing to severe non-specific staining. Binding between silver and sulfhydryl group of proteins appeared to be the major factor that enforced the silver staining. The oxidation of thiol groups in cells with hexacyanoferrate(III) dramatically weakened the silver-cell interaction and consequently significantly improved the efficiency of targeted staining.

Selective self-assembly of adenine-silver nanoparticles forms rings resembling the size of cells

Self-assembly has played critical roles in the construction of functional nanomaterials. However, the structure of the macroscale multicomponent materials built by the self-assembly of nanoscale building blocks is hard to predict due to multiple intermolecular interactions of great complexity. Evaporation of solvents is usually an important approach to induce kinetically stable assemblies of building blocks with a large-scale specific arrangement. During such a deweting process, we tried to monitor the possible interactions between silver nanoparticles and nucleobases at a larger scale by epifluorescence microscopy, thanks to the doping of silver nanoparticles with luminescent silver nanodots. ssDNA oligomer-stabilized silver nanoparticles and adenine self-assemble to form ring-like compartments similar to the size of modern cells. However, the silver ions only dismantle the self-assembly of adenine. The rings are thermodynamically stable as the drying process only enrich the nanoparticles-nucleobase mixture to a concentration that activates the self-assembly. The permeable membrane-like edge of the ring is composed of adenine filaments glued together by silver nanoparticles. Interestingly, chemicals are partially confined and accumulated inside the ring, suggesting that this might be used as a microreactor to speed up chemical reactions during a dewetting process.

Tuning the hydride reductions catalyzed on metal nanoparticle surfaces

Electrostatic interactions between the nanoparticle surface and reactants in solution have a more predominant impact on the catalytic activity on the nanoparticle surface than the size of the nanoparticle, which consequently influences the competitive transfer hydrogenation of water and nitro compounds from the activated hydride on nanoparticles. The higher surface charge density of a smaller nanoparticle led to a stronger repulsion from the nanoparticle surface, resulting in a slower approach of reactants, such as nitrophenol, to the nanoparticles. The generation of hydrogen overtook the nitrophenol reduction. However, larger nanoparticles presented a more heterogeneous surface coating than a smaller one even though both had a similar average surface charge, which provided some unprotected metal surfaces for nitrophenol reduction to proceed. Consequently, we observed an inverse dependence of the catalytic reaction on the surface area, i.e., the smaller the nanoparticles, the larger the surface area per unit mass, the slower the catalytic reaction of nitrophenol reduction. However, when the electrostatic repulsion was not obvious, decreased particle size had no effect on the reaction rates of the same reaction. As a competing reaction to the nitrophenol reduction, hydrogen generation depended strongly on the surface area of nanoparticles.

Silica nanoparticle stability in biological media revisited

The stability of silica nanostructure in the core-silica shell nanomaterials is critical to understanding the activity of these nanomaterials since the exposure of core materials due to the poor stability of silica may cause misinterpretation of experiments, but unfortunately reports on the stability of silica have been inconsistent. Here, we show that luminescent silver nanodots (AgNDs) can be used to monitor the stability of silica nanostructures. Though relatively stable in water and phosphate buffered saline, silica nanoparticles are eroded by biological media, leading to the exposure of AgNDs from AgND@SiO2 nanoparticles and the quenching of nanodot luminescence. Our results reveal that a synergistic effect of organic compounds, particularly the amino groups, accelerates the erosion. Our work indicates that silica nanostructures are vulnerable to cellular medium and it may be possible to tune the release of drug molecules from silica-based drug delivery vehicles through controlled erosion.