Atomically precise pieces of matter of nanometer dimensions composed of noble metals are new categories of materials with many unusual properties. Over 100 molecules of this kind with formulas such as Au25(SR)18, Au38(SR)24, and Au102(SR)44 as well as Ag25(SR)18, Ag29(S2R)12, and Ag44(SR)30 (often with a few counterions to compensate charges) are known now. They can be made reproducibly with robust synthetic protocols, resulting in colored solutions, yielding powders or diffractable crystals. They are distinctly different from nanoparticles in their spectroscopic properties such as optical absorption and emission, showing well-defined features, just like molecules. They show isotopically resolved molecular ion peaks in mass spectra and provide diverse information when examined through multiple instrumental methods. Most important of these properties is luminescence, often in the visible–near-infrared window, useful in biological applications. Luminescence in the visible region, especially by clusters prot...

Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles

Ultra-small fluorescent metal nanoclusters: Synthesis and biological applications

Chemistries that Facilitate Nanotechnology Kim E. Sapsford,† W. Russ Algar, Lorenzo Berti, Kelly Boeneman Gemmill,‡ Brendan J. Casey,† Eunkeu Oh, Michael H. Stewart, and Igor L. Medintz*,‡ †Division of Biology, Department of Chemistry and Materials Science, Office of Science and Engineering Laboratories, U.S. Food and Drug Administration, Silver Spring, Maryland 20993, United States ‡Center for Bio/Molecular Science and Engineering Code 6900 and Division of Optical Sciences Code 5611, U.S. Naval Research Laboratory, Washington, D.C. 20375, United States College of Science, George Mason University, 4400 University Drive, Fairfax, Virginia 22030, United States Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Sacramento, California 95817, United States Sotera Defense Solutions, Crofton, Maryland 21114, United States

Functionalizing nanoparticles with biological molecules: developing chemistries that facilitate nanotechnology.

Metal nanoclusters, composed of several to a few hundred metal atoms, have received worldwide attention due to their extraordinary physical and chemical characteristics. Recently, great efforts have been devoted to the exploration of the potential diagnostic and therapeutic applications of metal nanoclusters. Here we focus on the recent advances and new horizons in this area, and introduce the rising progress on the use of metal nanoclusters for biological analysis, biological imaging, therapeutic applications, DNA assembly and logic gate construction, enzyme mimic catalysis, as well as thermometers and pH meters. Furthermore, the future challenges in the construction of biofunctional metal nanoclusters for diagnostic and therapeutic applications are also discussed. We expect that the rapidly growing interest in metal nanocluster-based theranostic applications will certainly not only fuel the excitement and stimulate research in this highly active field, but also inspire broader concerns across various disciplines.

Metal nanoclusters: novel probes for diagnostic and therapeutic applications

Nonporous Silica Nanoparticles for Nanomedicine Application

Fluorescent noble metal (Au, Ag) nanoclusters have been biolabeled to bovine serum albumin (BSA) by wet chemistry. Spectroscopic and fluorescence investigations relate the role of the pH and the nature of the reducing agent to the size and the oxidation state of metal clusters. Blue-emitting (λ = 450 nm) small gold nanoclusters (eight atoms) prepared at pH 8 weakly bonded to BSA grow at higher pH to form red-emitting (λ = 690 nm) bigger clusters (25 atoms) covalently bonded to BSA via the sulfur group. X-ray photoelectron spectroscopy (XPS) measurements indicate the presence of Au(I) only for the big clusters. Small silver nanoclusters labeled to the protein with a fluorescence emission in the red region are synthesized in the presence of a strong reducing agent and present only Ag(0). Steady-state and lifetime measurements confirm the crucial impact of the size and the oxidation state of Au(I) on the stabilization of the metal core inside the protein and on the presence of a long lifetime component (τ > ...

Formation of Fluorescent Metal (Au, Ag) Nanoclusters Capped in Bovine Serum Albumin Followed by Fluorescence and Spectroscopy

In this paper we describe the synthesis and the optimisation of a new family of fluorescent core–shell nanoparticle using protein-stabilised gold nanoclusters. Fluorescent gold nanoclusters (<2 nm) entrapped in bovine serum albumin (BSA) protein were loaded in a 100 nm-silica nanoparticle with an optimal concentration of 3% (w/w). These nanoparticles kept the fluorescence properties of the metal clusters with a high Stokes shift and an emission in the near infrared region (λ = 670 nm). They were fully characterized and showed a high monodispersity and stability over more than 5 months. Steady-state fluorescence and lifetime measurements indicate the role of silica as a protective host to improve the photostability and the chemical stability of the fluorescent compound. This new label was taken up in tumor lung cells and tracked by a confocal microscope to show the great potential of this biolabel for sensing and imaging.

NIR-emitting fluorescent gold nanoclusters doped in silica nanoparticles

A principal objective in life sciences is the visualization of biochemical processes. Fluorescence-based techniques are widely used to demonstrate transport of relevant substances across cellular membranes. In this paper we report a novel noninvasive, real-time fluorescence lifetime imaging microscopy method for visualizing uptake and release of divalent copper ions (Cu(2+) ) in vivo. For this purpose, we employed a green fluorescent protein (GFP) form able to change its fluorescence lifetime upon Cu(2+) binding. We demonstrate that this technique is selective for Cu(2+) . We show the reversible decrease of the fluorescence lifetime of GFP from 2.2 to 1.6 ns in Escherichia coli and from 1.8 to 1.3 ns in root cells of Arabidopsis after the addition of Cu(2+) . Cu(2+) uptake of epidermal tobacco cells leads to a drop of the GFP lifetime from 2.5 to 2.2 ns. In summary, the spatially resolved visualization of Cu(2+) distribution in vivo is demonstrated in prokaryote and eukaryote cells.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1742-4658.2011.08434.x

Visualization of Cu2+ uptake and release in plant cells by fluorescence lifetime imaging microscopy

The understanding of cellular processes and functions and the elucidation of their physiological mechanisms is an important aim in the life sciences. One important aspect is the uptake and the release of essential substances as well as their interactions with the cellular environment. As green fluorescent protein (GFP) can be genetically encoded in cells it can be used as an internal sensor giving a deeper insight into biochemical pathways. Here we report that the presence of copper(II) ions leads to a decrease of the fluorescence lifetime (τ
 fl) of GFP and provide evidence for Forster resonance energy transfer (FRET) as the responsible quenching mechanism. We identify the His6-tag as the responsible binding site for Cu2 +  with a dissociation constant K
 d
  = 9 ±2 μM and a Forster radius R
 0 = 2.1 ±0.1 nm. The extent of the lifetime quenching depends on [Cu2 + ] which is comprehended by a mathematical titration model. We envision that Cu2 +  can be quantified noninvasively and in real-time by measuring τ
 fl of GFP.

Determination of Copper(II) Ion Concentration by Lifetime Measurements of Green Fluorescent Protein

Copper ions play a fundamental role in plant metabolism where its uptake and distribution within the organism is highly regulated, allowing the cells to sustain an adequate concentration. Shortage or excess of Cu can cause severe damage to the organisms endangering their survival. We recently reported a non-invasive method to follow the intracellular uptake of bivalent copper ion concentration by fluorescence lifetime microscopy of green fluorescent protein within plant cells. Measuring the fluorescence lifetime has the advantage of being independent on the fluorophore concentration and the excitation intensity. The use of GFP is beneficial because the protein can be introduced nondestructively. Here, we discuss the benefits of this approach as well as the possibility of applying this concept for the investigation of Cu redistribution and storage at the subcellular level. The fluorescence lifetime-encoded microscopic images are envisioned to map the copper distribution within plant cells not only qualitat...

Benjamin Hötzer

Papers

Formation of Fluorescent Metal (Au, Ag) Nanoclusters Capped in Bovine Serum Albumin Followed by Fluorescence and Spectroscopy

NIR-emitting fluorescent gold nanoclusters doped in silica nanoparticles

Visualization of Cu2+ uptake and release in plant cells by fluorescence lifetime imaging microscopy

Determination of Copper(II) Ion Concentration by Lifetime Measurements of Green Fluorescent Protein

Investigation of copper homeostasis in plant cells by fluorescence lifetime imaging microscopy.