Two distinct fluorescent quantum clusters of gold starting from metallic nanoparticles by pH-dependent ligand etching

Two fluorescent quantum clusters of gold, namely Au25 and Au8, have been synthesized from mercaptosuccinic acid-protected gold nanoparticles of 4-5 nm core diameter by etching with excess glutathione. While etching at pH ~3 yielded Au25, that at pH 7-8 yielded Au8. This is the first report of the synthesis of two quantum clusters starting from a single precursor. This simple method makes it possible to synthesize well-defined clusters in gram quantities. Since these clusters are highly fluorescent and are highly biocompatible due to their low metallic content, they can be used for diagnostic applications

Switching Plasmons: Gold Nanorod–Copper Chalcogenide Core–Shell Nanoparticle Clusters with Selectable Metal/Semiconductor NIR Plasmon Resonances

Exerting control over the near-infrared (NIR) plasmonic response of nanosized metals and semiconductors can facilitate access to unexplored phenomena and applications. Here we combine electrostatic self-assembly and Cd²⁺/Cu⁺ cation exchange to obtain an anisotropic core–shell nanoparticle cluster (NPC) whose optical properties stem from two dissimilar plasmonic materials: a gold nanorod (AuNR) core and a copper selenide (Cu_2–<i>x</i>Se, <i>x</i> ≥ 0) supraparticle shell. The spectral response of the AuNR@Cu₂Se NPCs is governed by the transverse and longitudinal plasmon bands (LPB) of the anisotropic metallic core, since the Cu₂Se shell is nonplasmonic. Under aerobic conditions the shell undergoes vacancy doping (<i>x</i> > 0), leading to the plasmon-rich NIR spectrum of the AuNR@Cu_2–<i>x</i>Se NPCs. For low vacancy doping levels the NIR optical properties of the dually plasmonic NPCs are determined by the LPBs of the semiconductor shell (along its major longitudinal axis) and of the metal core. Conversely, for high vacancy doping levels their NIR optical response is dominated by the two most intense plasmon modes from the shell: the transverse (along the shortest transversal axis) and longitudinal (along the major longitudinal axis) modes. The optical properties of the NPCs can be reversibly switched back to a purely metallic plasmonic character upon reversible conversion of AuNR@Cu_2–<i>x</i>Se into AuNR@Cu₂Se. Such well-defined nanosized colloidal assemblies feature the unique ability of holding an all-metallic, a metallic/semiconductor, or an all-semiconductor plasmonic response in the NIR. Therefore, they can serve as an ideal platform to evaluate the crosstalk between plasmonic metals and plasmonic semiconductors at the nanoscale. Furthermore, their versatility to display plasmon modes in the first, second, or both NIR windows is particularly advantageous for bioapplications, especially considering their strong absorbing and near-field enhancing properties

Growth of In Situ Functionalized Luminescent Silver Nanoclusters by Direct Reduction and Size Focusing

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Luminescent quantum clusters of gold in bulk by albumin-induced core etching of nanoparticles: metal ion sensing, metal-enhanced luminescence, and biolabeling

The synthesis of a luminescent quantum cluster (QC) of gold with a quantum yield of ~4 % is reported. It was synthesized in gram quantities by the core etching of mercaptosuccinic acid protected gold nanoparticles by bovine serum albumin (BSA), abbreviated as AuQC@BSA. The cluster was characterized and a core of Au38 was assigned tentatively from mass spectrometric analysis. Luminescence of the QC is exploited as a "turn-off" sensor for Cu2+ ions and a "turn-on" sensor for glutathione detection. Metal-enhanced luminescence (MEL) of this QC in the presence of silver nanoparticles is demonstrated and a ninefold maximum enhancement is seen. This is the first report of the observation of MEL from QCs. Folic acid conjugated AuQC@BSA was found to be internalized to a significant extent by oral carcinoma KB cells through folic acid mediated endocytosis. The inherent luminescence of the internalized AuQC@BSA was used in cell imaging

Growth of <i>In Situ</i> Functionalized Luminescent Silver Nanoclusters by Direct Reduction and Size Focusing

We have used one phase growth reaction to prepare a series of silver nanoparticles (NPs) and luminescent nanoclusters (NCs) using sodium borohydride (NaBH₄) reduction of silver nitrate in the presence of molecular scale ligands made of polyethylene glycol (PEG) appended with lipoic acid (LA) groups at one end and reactive (−COOH/–NH₂) or inert (−OCH₃) functional groups at the other end. The PEG segment in the ligand promotes solubility in a variety of solvents including water, while LAs provide multidentate coordinating groups that promote Ag–ligand complex formation and strong anchoring onto the NP/NC surface. The particle size and properties were primarily controlled by varying the Ag-to-ligand (Ag:L) molar ratios and the molar amount of NaBH₄ used. We found that while higher Ag:L ratios produced NPs, luminescent NCs were formed at lower ratios. We also found that nonluminescent NPs can be converted into luminescent clusters, <i>via</i> a process referred to as “size focusing”, in the presence of added excess ligands and reducing agent. The nanoclusters emit in the far red region of the optical spectrum with a quantum yield of ∼12%. They can be redispersed in a number of solvents with varying polarity while maintaining their optical and spectroscopic properties. Our synthetic protocol also allowed control over the number and type of reactive functional groups per nanocluster

Bright, NIR-emitting Au<SUB>23</SUB> from Au<SUB>25</SUB>: characterization and applications including biolabeling

A novel interfacial route has been developed for the synthesis of a bright-red-emitting new subnanocluster, Au23, by the core etching of a widely explored and more stable cluster, Au25SG18 (in which SG is glutathione thiolate). A slight modification of this procedure results in the formation of two other known subnanoclusters, Au22 and Au33. Whereas Au22 and Au23 are water soluble and brightly fluorescent with quantum yields of 2.5 and 1.3 %, respectively, Au33 is organic soluble and less fluorescent, with a quantum yield of 0.1 %. Au23 exhibits quenching of fluorescence selectively in the presence of Cu2+ ions and it can therefore be used as a metal-ion sensor. Aqueous- to organic-phase transfer of Au23 has been carried out with fluorescence enhancement. Solvent dependency on the fluorescence of Au23 before and after phase transfer has been studied extensively and the quantum yield of the cluster varies with the solvent used. The temperature response of Au23 emission has been demonstrated. The inherent fluorescence of Au23 was used for imaging human hepatoma cells by employing the avidin-biotin interaction

Tunable and Linker Free Nanogaps in Core–Shell Plasmonic Nanorods for Selective and Quantitative Detection of Circulating Tumor Cells by SERS

Controlling the size, number, and shape of nanogaps in plasmonic nanostructures is of significant importance for the development of novel quantum plasmonic devices and quantitative sensing techniques such as surface-enhanced Raman scattering (SERS). Here, we introduce a new synthetic method based on coordination interactions and galvanic replacement to prepare core–shell plasmonic nanorods with tunable enclosed nanogaps. Decorating Au nanorods with Raman reporters that strongly coordinate Ag⁺ ions (e.g., 4-mercaptopyridine) afforded uniform nucleation sites to form a sacrificial Ag shell. Galvanic replacement of the Ag shell by HAuCl₄ resulted in Au–AgAu core–shell structure with a uniform intra-nanoparticle gap. The size (length and width) and morphology of the core–shell plasmonic nanorods as well as the nanogap size depend on the concentration of the coordination complexes formed between Ag⁺ ions and 4-mercaptopyridine. Moreover, encapsulating Raman reporters within the nanogaps afforded an internal standard for sensitive and quantitative SERS analysis. To test the applicability, core–shell plasmonic nanorods were functionalized with aptamers specific to circulating tumor cells such as MCF-7 (Michigan Cancer Foundation-7, breast cancer cell line). This system could selectively detect as low as 20 MCF-7 cells in a blood mimicking fluid employing SERS. The linking DNA duplex on core–shell plasmonic nanorods can also intercalate hydrophobic drug molecules such as Doxorubicin, thereby increasing the versatility of this sensing platform to include drug delivery. Our synthetic method offers the possibility of developing multifunctional SERS-active materials with a wide range of applications including biosensing, imaging, and therapy

Strong Quantum Confinement Effects and Chiral Excitons in Bio-Inspired ZnO–Amino Acid Cocrystals

Elucidating the underlying principles behind band gap engineering is paramount for the successful implementation of semiconductors in photonic and optoelectronic devices. Recently it has been shown that the band gap of a wide and direct band gap semiconductor, such as ZnO, can be modified upon cocrystallization with amino acids, with the role of the biomolecules remaining unclear. Here, by probing and modeling the light-emitting properties of ZnO–amino acid cocrystals, we identify the amino acids’ role on this band gap modulation and demonstrate their effective chirality transfer to the interband excitations in ZnO. Our 3D quantum model suggests that the strong band edge emission blue-shift in the cocrystals can be explained by a quasi-periodic distribution of amino acid potential barriers within the ZnO crystal lattice. Overall, our findings indicate that biomolecule cocrystallization can be used as a truly bio-inspired means to induce chiral quantum confinement effects in quasi-bulk semiconductors