Click Chemistry on the Surface of Ultrasmall Gold Nanoparticles (2 nm) for Covalent Ligand Attachment Followed by NMR Spectroscopy

Increased Cytotoxicity of Bimetallic Ultrasmall Silver-Platinum Nanoparticles (2 nm) on Cells and Bacteria in Comparison to Silver Nanoparticles of the Same Size

Ultrasmall nanoparticles (diameter 2 nm) of silver, platinum, and bimetallic nanoparticles (molar ratio of Ag:Pt 0:100; 20:80; 50:50; 70:30; 100:0), stabilized by the thiolated ligand glutathione, were prepared and characterized by transmission electron microscopy, differential centrifugal sedimentation, X-ray photoelectron spectroscopy, small-angle X-ray scattering, X-ray powder diffraction, and NMR spectroscopy in aqueous dispersion. Gold nanoparticles of the same size were prepared as control. The particles were fluorescently labeled by conjugation of the dye AlexaFluor-647 via copper-catalyzed azide-alkyne cycloaddition after converting amine groups of glutathione into azide groups. All nanoparticles were well taken up by HeLa cells. The cytotoxicity was assessed with an MTT test on HeLa cells and minimal inhibitory concentration (MIC) tests on the bacteria Escherichia coli and Staphylococcus xylosus. Notably, bimetallic AgPt nanoparticles had a higher cytotoxicity against cells and bacteria than monometallic silver nanoparticles or a physical mixture of silver and platinum nanoparticles. However, the measured release of silver ions from monometallic and bimetallic silver nanoparticles in water was very low despite the ultrasmall size and the associated high specific surface area. This is probably due to the surface protection by a dense layer of thiolated ligand glutathione. Thus, the enhanced cytotoxicity of bimetallic AgPt nanoparticles is caused by the biological environment in cell culture media, together with a polarization of silver by platinum.

The Application of Ultrasmall Gold Nanoparticles (2 nm) Functionalized with Doxorubicin in Three-Dimensional Normal and Glioblastoma Organoid Models of the Blood-Brain Barrier

Among brain tumors, glioblastoma (GBM) is very challenging to treat as chemotherapeutic drugs can only penetrate the brain to a limited extent due to the blood-brain barrier (BBB). Nanoparticles can be an attractive solution for the treatment of GBM as they can transport drugs across the BBB into the tumor. In this study, normal and GBM organoids comprising six brain cell types were developed and applied to study the uptake, BBB penetration, distribution, and efficacy of fluorescent, ultrasmall gold nanoparticles (AuTio-Dox-AF647s) conjugated with doxorubicin (Dox) and AlexaFluor-647-cadaverine (AF647) by confocal laser scanning microscopy (CLSM), using a mixture of dissolved doxorubicin and fluorescent AF647 molecules as a control. It was shown that the nanoparticles could easily penetrate the BBB and were found in normal and GBM organoids, while the dissolved Dox and AF647 molecules alone were unable to penetrate the BBB. Flow cytometry showed a reduction in glioblastoma cells after treatment with AuTio-Dox nanoparticles, as well as a higher uptake of these nanoparticles by GBM cells in the GBM model compared to astrocytes in the normal cell organoids. In summary, our results show that ultrasmall gold nanoparticles can serve as suitable carriers for the delivery of drugs into organoids to study BBB function.

The Molecular Footprint of Peptides on the Surface of Ultrasmall Gold Nanoparticles (2 nm) Is Governed by Steric Demand

Ultrasmall gold nanoparticles were functionalized with peptides of two to seven amino acids that contained one cysteine molecule as anchor via a thiol-gold bond and a number of alanine residues as nonbinding amino acid. The cysteine was located either in the center of the molecule or at the end (C-terminus). For comparison, gold nanoparticles were also functionalized with cysteine alone. The particles were characterized by UV spectroscopy, differential centrifugal sedimentation (DCS), high-resolution transmission electron microscopy (HRTEM), and small-angle X-ray scattering (SAXS). This confirmed the uniform metal core (2 nm diameter). The hydrodynamic diameter was probed by 1H-DOSY NMR spectroscopy and showed an increase in thickness of the hydrated peptide layer with increasing peptide size (up to 1.4 nm for heptapeptides; 0.20 nm per amino acid in the peptide). 1H NMR spectroscopy of water-dispersed nanoparticles showed the integrity of the peptides and the effect of the metal core on the peptide. Notably, the NMR signals were very broad near the metal surface and became increasingly narrow in a distance. In particular, the methyl groups of alanine can be used as probe for the resolution of the NMR spectra. The number of peptide ligands on each nanoparticle was determined using quantitative 1H NMR spectroscopy. It decreased with increasing peptide length from about 100 for a dipeptide to about 12 for a heptapeptide, resulting in an increase of the molecular footprint from about 0.1 to 1.1 nm2.

Structural Insights into Antibacterial Payload Release from Gold Nanoparticles Bound to E. coli Peptide Deformylase

The lack of new antibiotics and the rapidly rising number of pathogens resistant to antibiotics pose a serious problem to mankind. In bacteria, the cell membrane provides the first line of defence to antibiotics by preventing them from reaching their molecular target. To overcome this entrance barrier, it has been suggested[1] that small Gold-Nanoparticles (AuNP) could possibly function as drug delivery systems for antibiotic ligands. Using actinonin-based ligands, we provide here proof-of-principle of AuNP functionalisation, the capability to bind and inhibit the target protein of the ligand, and the possibility to selectively release the antimicrobial payload. To this end, we successfully synthesised AuNP coated with thio-functionalised actinonin and a derivative. Interactions between 15N-enriched His-peptide deformylase 1-147 from E. coli (His-ecPDF 1-147) and compound-coated AuNP were investigated via 2D 1H-15N-HSQC NMR spectra proving the direct binding to His-ecPDF 1-147. More importantly by adding dithiothreitol (DTT), we show that the derivative is successfully released from AuNPs while still bound to His-ecPDF 1-147. Our findings indicate that AuNP-conjugated ligands can address and bind intracellular target proteins. The system introduced here presents a new delivery platform for antibiotics and allows for the easy optimisation of ligand coated AuNPs.

Clickable bisreactive small gold nanoclusters for preparing multifunctionalized nanomaterials: application to photouncaging of an anticancer molecule

In this study, we successfully synthesized a small-sized gold nanocluster (2 nm) coated with homogeneous tripeptides bearing azido and amino groups that enable facile multifunctionalizations. Using sodium phenoxide to reduce tetrachloroauric(iii) acid in the presence of the cysteine-containing tripeptide, we efficiently prepared the gold nanoclusters without damaging the azido group. We then utilized this clickable bisreactive nanocluster as a versatile platform for synthesizing multifunctionalized gold nanomaterials. The resulting nanoclusters were conjugated with an anticancer compound connected to an indolizine moiety for photoinduced uncaging, a photodynamic therapy agent acting as a photosensitizer for uncaging, and a cyclic RGD peptide. The cytotoxicity of the multifunctionalized gold nanoclusters was demonstrated through red light irradiation of human lung cancer-derived A549 cells treated with the synthesized nanomaterials. The significant cytotoxicity exhibited by the cells underscores the potential utility of this method in advanced cancer therapies.

Applications of Gold Nanoparticles in Plasmonic and Nanophotonic Biosensing

The unique properties of plasmonic nanoparticles and nanostructures have enabled a broad range of applications in a diverse set of fields, ranging from biological sensing, cancer therapy, to catalysis. They have been some of the most studied nanomaterials due in part to their chemical stability and biocompatibility as well as supporting theoretical efforts. The synthesis and fabrication of plasmonic nanoparticles and nanostructures have now reached high precision and sophistication. We review here their fundamental optical properties, discuss their tailoring for biological environments, and then detail examples on how they have been used to innovate in the biological and biomedical fields.

The Why and How of Ultrasmall Nanoparticles

ConspectusIn this Account, we describe our research into ultrasmall nanoparticles, including their unique properties, and outline some of the new opportunities they offer. We will summarize our perspective on the current state of the field and highlight what we see as key questions that remain to be solved. First, there are several nanostructure size-scale regimes, with qualitatively distinct functional biological attributes. Broadly generalized, larger particles (e.g., larger than 300 nm) tend to be more efficiently swept away by the first line of the immune system (for example macrophages). In the "middle-sized" regime (20-300 nm), nanoparticle surfaces and shapes can be recognized by energy-dependent cellular reorganizations, then organized locally in a spatial and temporally coherent way. That energy is gated and made available by specific cellular recognition processes. The relationship between particle surface design, endogenously derived nonspecific biomolecular corona, and architectural features recognized by the cell is complex and only purposefully and very precisely designed nanoparticle architectures are able to navigate to specific targets. At sufficiently small sizes (<10 nm including the ligand shell, associated with a core diameter of a few nm at most) we enter the "quasi-molecular regime" in which the endogenous biomolecular environment exchanges so rapidly with the ultrasmall particle surface that larger scale cellular and immune recognition events are often greatly simplified. As an example, ultrasmall particles can penetrate cellular and biological barriers within tissue architectures via passive diffusion, in much the same way as small molecule drugs do. An intriguing question arises: what happens at the interface of cellular recognition and ultrasmall quasi-molecular size regimes? Succinctly put, ultrasmall conjugates can evade defense mechanisms driven by larger scale cellular nanoscale recognition, enabling them to flexibly exploit molecular interaction motifs to interact with specific targets. Numerous advances in control of architecture that take advantage of these phenomena have taken place or are underway. For instance, syntheses can now be sufficiently controlled that it is possible to make nanoparticles of a few hundreds of atoms or metalloid clusters of several tens of atoms that can be characterized by single crystal X-ray structure analysis. While the synthesis of atomically precise clusters in organic solvents presents challenges, water-based syntheses of ultrasmall nanoparticles can be upscaled and lead to well-defined particle populations. The surface of ultrasmall nanoparticles can be covalently modified with a wide variety of ligands to control the interactions of these particles with biosystems, as well as drugs and fluorophores. And, in contrast to larger particles, many advanced molecular analytical and separation tools can be applied to understand their structure. For example, NMR spectroscopy allows us to obtain a detailed image of the particle surface and the attached ligands. These are considerable advantages that allow further elaboration of the level of architectural control and characterization of the ultrasmall structures required to access novel functional regimes and outcomes. The ultrasmall nanoparticle regime has a unique status and provides a potentially very interesting direction for development.

Ultrastructure and Surface Composition of Glutathione-Terminated Ultrasmall Silver, Gold, Platinum, and Alloyed Silver-Platinum Nanoparticles (2 nm)

Alloyed ultrasmall silver-platinum nanoparticles (molar ratio Ag:Pt = 50:50) were prepared and compared to pure silver, platinum, and gold nanoparticles, all with a metallic core diameter of 2 nm. They were surface-stabilized by a layer of glutathione (GSH). A comprehensive characterization by high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), differential centrifugal sedimentation (DCS), and UV spectroscopy showed their size both in the dry and in the water-dispersed state (hydrodynamic diameter). Solution NMR spectroscopy (1H, 13C, COSY, HSQC, HMBC, and DOSY) showed the nature of the glutathione shell including the number of GSH ligands on each nanoparticle (about 200 with a molecular footprint of 0.063 nm2 each). It furthermore showed that there are at least two different positions for the GSH ligand on the gold nanoparticle surface. Platinum strongly reduced the resolution of the NMR spectra compared to silver and gold, also in the alloyed nanoparticles. X-ray photoelectron spectroscopy (XPS) showed that silver, platinum, and silver-platinum particles were at least partially oxidized to Ag(+I) and Pt(+II), whereas the gold nanoparticles showed no sign of oxidation. Platinum and gold nanoparticles were well crystalline but twinned (fcc lattice) despite the small particle size. Silver was crystalline in electron diffraction but not in X-ray diffraction. Alloyed silver-platinum nanoparticles were almost fully amorphous by both methods, indicating a considerable internal disorder.

Specific Instantaneous Detection of Klebsiella pneumoniae for UTI Diagnosis with a Plasmonic Gold Nanoparticle Conjugated Aptasensor

Urinary tract infection (UTI), which can be caused by various pathogens, if not detected at an early stage can be fatal. It is essential to identify the specific pathogen responsible for UTI for appropriate treatment. This study describes a generic approach to the fabrication of a prototype for the noninvasive detection of a specific pathogen using a tailor-made plasmonic aptamer-gold nanoparticle (AuNP) assay. The assay is advantageous because the adsorbed specific aptamers passivate the nanoparticle surfaces and reduce and/or eliminate false-positive responses to nontarget analytes. Based on the localized surface plasmon resonance (LSPR) phenomena of AuNP, a point-of-care aptasensor was designed that shows specific changes in the absorbance in the visible spectra in the presence of a target pathogen for robust and fast screening of UTI samples. In this study, we demonstrate the specific detection of Klebsiella pneumoniae bacteria with LoD as low as 3.4 × 103 CFU/mL.

Potentiating Tweezer Affinity to a Protein Interface with Sequence-Defined Macromolecules on Nanoparticles

Survivin, a well-known member of the inhibitor of apoptosis protein family, is upregulated in many cancer cells, which is associated with resistance to chemotherapy. To circumvent this, inhibitors are currently being developed to interfere with the nuclear export of survivin by targeting its protein-protein interaction (PPI) with the export receptor CRM1. Here, we combine for the first time a supramolecular tweezer motif, sequence-defined macromolecular scaffolds, and ultrasmall Au nanoparticles (us-AuNPs) to tailor a high avidity inhibitor targeting the survivin-CRM1 interaction. A series of biophysical and biochemical experiments, including surface plasmon resonance measurements and their multivalent evaluation by EVILFIT, reveal that for divalent macromolecular constructs with increasing linker distance, the longest linkers show superior affinity, slower dissociation, as well as more efficient PPI inhibition. As a drawback, these macromolecular tweezer conjugates do not enter cells, a critical feature for potential applications. The problem is solved by immobilizing the tweezer conjugates onto us-AuNPs, which enables efficient transport into HeLa cells. On the nanoparticles, the tweezer valency rises from 2 to 16 and produces a 100-fold avidity increase. The hierarchical combination of different scaffolds and controlled multivalent presentation of supramolecular binders was the key to the development of highly efficient survivin-CRM1 competitors. This concept may also be useful for other PPIs.

Tuning Nanobodies' Bioactivity: Coupling to Ultrasmall Gold Nanoparticles Allows the Intracellular Interference with Survivin

Nanobodies are highly affine binders, often used to track disease-relevant proteins inside cells. However, they often fail to interfere with pathobiological functions, required for their clinical exploitation. Here, a nanobody targeting the disease-relevant apoptosis inhibitor and mitosis regulator Survivin (SuN) is utilized. Survivin's multifaceted functions are regulated by an interplay of dynamic cellular localization, dimerization, and protein-protein interactions. However, as Survivin harbors no classical "druggable" binding pocket, one must aim at blocking extended protein surface areas. Comprehensive experimental evidence demonstrates that intracellular expression of SuN allows to track Survivin at low nanomolar concentrations but failed to inhibit its biological functions. Small angle X-ray scattering of the Survivin-SuN complex locates the proposed interaction interface between the C-terminus and the globular domain, as such not blocking any pivotal interaction. By clicking multiple SuN to ultrasmall (2 nm) gold nanoparticles (SuN-N), not only intracellular uptake is enabled, but additionally, Survivin crosslinking and interference with mitotic progression in living cells are also enabled. In sum, it is demonstrated that coupling of nanobodies to nanosized scaffolds can be universally applicable to improve their function and therapeutic applicability.

Amidoxime-derived rice husk as biocatalyst and scavenger for organophosphate neutralization and removal

Organophosphates are a worldwide threat because of their presence in agrochemicals and chemical warfare. Situations of misuse, apprehensions of prohibited chemicals (e.g. pesticides), undesired stockpiles and chemical attacks require effective measures for neutralization and removal. Herein, a green approach is shown by functionalizing the agricultural waste rice husk with amidoximes leading to heterogeneous catalysts that were applied in the degradation/scavenging of toxic organophosphates. In aqueous medium, the waste-derived catalyst was efficient in the catalytic neutralization of a phosphotriester (increments up to 1 × 10⁴-fold), while allying important features: selective, recyclable and lead to less toxic products. Curiously, the amidoximated rice husk behaved as a scavenger in the aprotic polar solvents MeCN and acetone by covalently bonding to the phosphoryl moiety. Upon addition of water, this bond is broken and the phosphoryl liberated (hydrolyzed) to the aqueous medium. Thus, the scavenging process is reversible and can be used to remove toxic organophosphates. ³¹P nuclear magnetic resonance spectroscopy was crucial for confirming the overall mechanisms involved. In summary, a sustainable material was synthetized from a waste source and employed as catalyst and scavenger for eliminating threatening organophosphates. This is promising for assuring chemical security such as in chemical emergencies.

An Electrolyte-Gated Graphene Field-Effect Transistor for Detection of Gadolinium(III) in Aqueous Media

In this work, an electrolyte-gated graphene field-effect transistor is developed for Gd³⁺ ion detection in water. The source and drain electrodes of the transistor are fabricated by photolithography on polyimide, while the graphene channel is obtained by inkjet-printing a graphene oxide ink subsequently electro-reduced to give reduced graphene oxide. The Gd³⁺-selective ligand DOTA is functionalized by an alkyne linker to be grafted by click chemistry on a gold electrode without losing its affinity for Gd³⁺. The synthesis route is fully described, and the ligand, the linker and the functionalized surface are characterized by electrochemical analysis and spectroscopy. The as functionalized electrode is used as gate in the graphene transistor so to modulate the source-drain current as a function of its potential, which is itself modulated by the concentration of Gd³⁺captured on the gate surface. The obtained sensor is able to quantify Gd³⁺ even in a sample containing several other potentially interfering ions such as Ni²⁺, Ca²⁺, Na⁺ and In³⁺. The quantification range is from 1 pM to 10 mM, with a sensitivity of 20 mV dec^-1 expected for a trivalent ion. This paves the way for Gd³⁺ quantification in hospital or industrial wastewater.

Fluorescent Gold Nanoparticles in Suspension as an Efficient Theranostic Agent for Highly Radio-Resistant Cancer Cells

Gold nanoparticles are a promising candidate for developing new strategies of therapy against cancer. Due to their high atomic number and relative biocompatibility, they are commonly investigated as radiosensitizers to locally increase the dose of radiotherapy. In order to optimize this radiosensitizing effect, it is necessary to control the positioning of the nanoparticles in the cells. The purpose of this study is to investigate, by means of fluorescent gold nanoparticles in suspension, the dose enhancement on highly radio-resistant cancer cells. These nanoparticles were successfully produced using modern click-chemistry methods, first by attaching a chelating agent Diethylenetriamine pentaacetate benzylamine to L-cysteine, bonding the resulting ligand to a gold core, grafting propargylamine and then utilizing copper-catalyzed azide-alkyne cycloaddition (CuAAC) to fuse AlexaFluor 647 to the ligands. The results of this study prove the success of the reactions to produce a minimally cytotoxic and highly stable nanoparticle suspension that increases the radiosensitivity of gliosarcoma 9L tumor cells, with a 35% increase in cell death using 5 Gy kilovoltage radiation. Their fluorescent functionalization allowed for their simple localization within living cells and detection in vivo post-mortem.

CuAAC ensembled 1,2,3-triazole linked nanogels for targeted drug delivery: a review

Copper(i) catalyzed alkyne azide cycloaddition (CuAAC), the quintessential example of 'click chemistry', provides an adaptable and adequate platform for the synthesis of nanogels for sustained drug release at targeted sites because of their better biocompatibility. The coupling of drugs, carried out via various synthetic routes including CuAAC, into long-chain polymeric forms like nanogels has exhibited considerable assurance in therapeutic advancements and intracellular drug delivery due to the progression of water solubility, evacuation of precocious drug release, and improved upthrust of the pharmacokinetics of the nanogels, thereby rendering them as better and efficient drug carriers. The inefficiency of drug transmission to the target areas due to the resistance of complex biological barriers in vivo is a major hurdle that impedes the therapeutic translation of nanogels. This review compiles the data of nanogels synthesized specifically via CuAAC 'click' methodology, as scaffolds for targeted drug delivery and their assimilation into nanomedicine. In addition, it elaborates the ability of CuAAC to graft specific moieties and conjugating biomolecules like proteins and growth factors, onto orthogonally functionalized polymer chains with various chemical groups resulting in nanogels that are not only more appealing but also more effective at delivering drugs, thereby enhancing their site-specific target approach and initiating selective therapies.

Silencing of proinflammatory NF-κB and inhibition of herpes simplex virus (HSV) replication by ultrasmall gold nanoparticles (2 nm) conjugated with small-interfering RNA

Azide-terminated ultrasmall gold nanoparticles (2 nm gold core) were covalently functionalized with alkyne-terminated small-interfering siRNA duplexes by copper-catalyzed azide-alkyne cycloaddition (CuAAC; click chemistry). The nanoparticle core was visualized by transmission electron microscopy. The number of attached siRNA molecules per nanoparticle was determined by a combination of atomic absorption spectroscopy (AAS; for gold) and UV-Vis spectroscopy (for siRNA). Each nanoparticle carried between 6 and 10 siRNA duplex molecules which corresponds to a weight ratio of siRNA to gold of about 2.2 : 1. Different kinds of siRNA were conjugated to the nanoparticles, depending on the gene to be silenced. In general, the nanoparticles were readily taken up by cells and highly efficient in gene silencing, in contrast to free siRNA. This was demonstrated in HeLa-eGFP cells (silencing of eGFP) and in LPS-stimulated macrophages (silencing of NF-κB). Furthermore, we demonstrated that nanoparticles carrying antiviral siRNA potently inhibited the replication of Herpes simplex virus 2 (HSV-2) in vitro. This highlights the strong potential of siRNA-functionalized ultrasmall gold nanoparticles in a broad spectrum of applications, including gene silencing and treatment of viral infections, combined with a minimal dose of gold.

Uptake of Functional Ultrasmall Gold Nanoparticles in 3D Gut Cell Models

Ultrasmall gold nanoparticles (2 nm) easily penetrate the membranes of intestinal murine epithelial cells (MODE-K) and colorectal cancer cells (CT-26). They are also taken up by 3D spheroids (400 µm) of these cell types and primary gut organoids (500 µm). In contrast, dissolved dyes are not taken up by any of these cells or 3D structures. The distribution of fluorescent ultrasmall gold nanoparticles inside cells, spheroids, and gut organoids is examined by confocal laser scanning microscopy. Nanoparticles conjugated with the cytostatic drug doxorubicin and a fluorescent dye exhibit significantly greater cytotoxicity toward CT-26 tumor spheroids than equally concentrated dissolved doxorubicin, probably because they enter the interior of a spheroid much more easily than dissolved doxorubicin. Comprehensive analyses show that the cellular uptake of ultrasmall gold nanoparticles occurs by different endocytosis pathways.

Water-Based Synthesis of Ultrasmall Nanoparticles of Platinum Group Metal Oxides (1.8 nm)

Ultrasmall nanoparticles of platinum group metal oxides (core diameter of about 1.8 nm) were prepared by alkaline hydrolysis of metal precursors in the presence of NaBH₄ and by colloidal stabilization with tripeptide glutathione. We obtained water-dispersed nanoparticles of Rh₂O₃, PdO, RuO₂, IrO₂, Os/OsO₂, and Pt/PtO. Their size was probed using high-resolution transmission electron microscopy, differential centrifugal sedimentation, small-angle X-ray scattering, and diffusion-ordered ¹H NMR spectroscopy (¹H DOSY). Their oxidation state was clearly determined using X-ray photoelectron spectroscopy, X-ray powder diffraction, and electron diffraction. The chemical composition of the nanoparticles, that is, the ratio of the metal oxide core and glutathione capping agent, was quantitatively determined by a combination of these methods.

Covalent Attachment of Aggregation-Induced Emission Molecules to the Surface of Ultrasmall Gold Nanoparticles to Enhance Cell Penetration

Three different alkyne-terminated aggregation-induced emission molecules based on a para-substituted di-thioether were attached to the surface of ultrasmall gold nanoparticles (2 nm) by copper-catalyzed azide–alkyne cycloaddition (click chemistry). They showed a strong fluorescence and were well water-dispersible, in contrast to the dissolved AIE molecules. The AIE-loaded nanoparticles were not cytotoxic and easily penetrated the membrane of HeLa cells, paving the way for an intracellular application of AIE molecules, e.g., for imaging.

27 Al NMR diffusometry of Al13 Keggin nanoclusters

Although nanometer-sized aluminum hydroxide clusters (i.e., ϵ-Al₁₃ , [Al₁₃ O₄ (OH)₂₄ (H₂ O)₁₂ ]⁷⁺ ) command a central role in aluminum ion speciation and transformations between minerals, measurement of their translational diffusion is often limited to indirect methods. Here, ²⁷ Al pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy has been applied to the AlO₄ core of the ϵ-Al₁₃ cluster with complementary theoretical simulations of the diffusion coefficient and corresponding hydrodynamic radii from a boundary element-based calculation. The tetrahedral AlO₄ center of the ϵ-Al₁₃ cluster is symmetric and exhibits only weak quadrupolar coupling, which results in favorable T₁ and T₂ ²⁷ Al NMR relaxation coefficients for ²⁷ Al PFGSTE NMR studies. Stokes-Einstein relationship was used to relate the ²⁷ Al diffusion coefficient of the ϵ-Al₁₃ cluster to the hydrodynamic radius for comparison with theoretical simulations, dynamic light scattering from literature, and previously published ¹ H PFGSTE NMR studies of chelated Keggin clusters. This first-of-its-kind observation proves that ²⁷ Al PFGSTE NMR diffusometry can probe symmetric Al environments in polynuclear clusters of greater molecular weight than previously considered.

Pathways for Oral and Rectal Delivery of Gold Nanoparticles (1.7 nm) and Gold Nanoclusters into the Colon: Enteric-Coated Capsules and Suppositories

Two ways to deliver ultrasmall gold nanoparticles and gold-bovine serum albumin (BSA) nanoclusters to the colon were developed. First, oral administration is possible by incorporation into gelatin capsules that were coated with an enteric polymer. These permit the transfer across the stomach whose acidic environment damages many drugs. The enteric coating dissolves due to the neutral pH of the colon and releases the capsule's cargo. Second, rectal administration is possible by incorporation into hard-fat suppositories that melt in the colon and then release the nanocarriers. The feasibility of the two concepts was demonstrated by in-vitro release studies and cell culture studies that showed the easy redispersibility after dissolution of the respective transport system. This clears a pathway for therapeutic applications of drug-loaded nanoparticles to address colon diseases, such as chronic inflammation and cancer.

Metal-Ligand Interface and Internal Structure of Ultrasmall Silver Nanoparticles (2 nm)

Ultrasmall silver nanoparticles were prepared by reduction with NaBH₄ and surface-terminated with glutathione (GSH). The particles had a solid core diameter of 2 nm as shown by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). NMR-DOSY gave a hydrodynamic diameter of 2 to 2.8 nm. X-ray photoelectron spectroscopy (XPS) showed that silver is bound to the thiol group of the central cysteine in glutathione under partial oxidation to silver(+I). In turn, the thiol group is deprotonated to thiolate. X-ray powder diffraction (XRD) together with Rietveld refinement confirmed a twinned (polycrystalline) fcc structure of ultrasmall silver nanoparticles with a lattice compression of about 0.9% compared to bulk silver metal. By NMR spectroscopy, the interaction between the glutathione ligand and the silver surface was analyzed, also with ¹³C-labeled glutathione. The adsorbed glutathione is fully intact and binds to the silver surface via cysteine. In situ ¹H NMR spectroscopy up to 85 °C in dispersion showed that the glutathione ligand did not detach from the surface of the silver nanoparticle, i.e. the silver-sulfur bond is remarkably strong. The ultrasmall nanoparticles had a higher cytotoxicity than bigger particles in in vitro cell culture with HeLa cells with a cytotoxic concentration of about 1 μg mL^-1 after 24 h incubation. The overall stoichiometry of the nanoparticles was about Ag_∼250GSH_∼155.

Targeting the Surface of the Protein 14-3-3 by Ultrasmall (1.5 nm) Gold Nanoparticles Carrying the Specific Peptide CRaf

The surface of ultrasmall gold nanoparticles with an average diameter of 1.55 nm was conjugated with a 14-3-3 protein-binding peptide derived from CRaf. Each particle carries 18 CRaf peptides, leading to an overall stoichiometry of Au(115)Craf(18). The binding to the protein 14-3-3 was probed by isothermal titration calorimetry (ITC) and fluorescence polarization spectroscopy (FP). The dissociation constant (K_D ) was measured as 5.0 μM by ITC and 0.9 μM by FP, which was close to the affinity of dissolved CRaf to 14-3-3σ. In contrast to dissolved CRaf, which alone did not enter HeLa cells, CRAF-conjugated gold nanoparticles were well taken up by HeLa cells, opening the opportunity to target the protein inside a cell.

Controlling the Surface Functionalization of Ultrasmall Gold Nanoparticles by Sequence-Defined Macromolecules

Ultrasmall gold nanoparticles (diameter about 2 nm) were surface-functionalized with cysteine-carrying precision macromolecules. These consisted of sequence-defined oligo(amidoamine)s (OAAs) with either two or six cysteine molecules for binding to the gold surface and either with or without a PEG chain (3400 Da). They were characterized by 1 H NMR spectroscopy, 1 H NMR diffusion-ordered spectroscopy (DOSY), small-angle X-ray scattering (SAXS), and high-resolution transmission electron microscopy. The number of precision macromolecules per nanoparticle was determined after fluorescent labeling by UV spectroscopy and also by quantitative 1 H NMR spectroscopy. Each nanoparticle carried between 40 and 100 OAA ligands, depending on the number of cysteine units per OAA. The footprint of each ligand was about 0.074 nm2 per cysteine molecule. OAAs are well suited to stabilize ultrasmall gold nanoparticles by selective surface conjugation and can be used to selectively cover their surface. The presence of the PEG chain considerably increased the hydrodynamic diameter of both dissolved macromolecules and macromolecule-conjugated gold nanoparticles.

Ultrasmall gold and silver/gold nanoparticles (2 nm) as autofluorescent labels for poly(D,L-lactide-co-glycolide) nanoparticles (140 nm)

Ultrasmall metallic nanoparticles show an efficient autofluorescence after excitation in the UV region, combined with a low degree of fluorescent bleaching. Thus, they can be used as fluorescent labels for polymer nanoparticles which are frequently used for drug delivery. A versatile water-in-oil-in-water emulsion-evaporation method was developed to load poly(D,L-lactide-co-glycolide) (PLGA) nanoparticles with autofluorescent ultrasmall gold and silver/gold nanoparticles (diameter 2 nm). The metallic nanoparticles were prepared by reduction of tetrachloroauric acid with sodium borohydride and colloidally stabilised with 11-mercaptoundecanoic acid. They were characterised by UV-Vis and fluorescence spectroscopy, showing a large Stokes shift of about 370 nm with excitation maxima at 250/270 nm and emission maxima at 620/640 nm for gold and silver/gold nanoparticles, respectively. The labelled PLGA nanoparticles (140 nm) were characterised by dynamic light scattering (DLS), scanning electron microscopy (SEM), and UV-Vis and fluorescence spectroscopy. Their uptake by HeLa cells was followed by confocal laser scanning microscopy. The metallic nanoparticles remained inside the PLGA particle after cellular uptake, demonstrating the efficient encapsulation and the applicability to label the polymer nanoparticle. In terms of fluorescence, the metallic nanoparticles were comparable to fluorescein isothiocyanate (FITC).

Transport of ultrasmall gold nanoparticles (2 nm) across the blood-brain barrier in a six-cell brain spheroid model

The blood–brain barrier (BBB) is an efficient barrier for molecules and drugs. Multicellular 3D spheroids display reproducible BBB features and functions. The spheroids used here were composed of six brain cell types: Astrocytes, pericytes, endothelial cells, microglia cells, oligodendrocytes, and neurons. They form an in vitro BBB that regulates the transport of compounds into the spheroid. The penetration of fluorescent ultrasmall gold nanoparticles (core diameter 2 nm; hydrodynamic diameter 3–4 nm) across the BBB was studied as a function of time by confocal laser scanning microscopy, with the dissolved fluorescent dye (FAM-alkyne) as a control. The nanoparticles readily entered the interior of the spheroid, whereas the dissolved dye alone did not penetrate the BBB. We present a model that is based on a time-dependent opening of the BBB for nanoparticles, followed by a rapid diffusion into the center of the spheroid. After the spheroids underwent hypoxia (0.1% O₂; 24 h), the BBB was more permeable, permitting the uptake of more nanoparticles and also of dissolved dye molecules. Together with our previous observations that such nanoparticles can easily enter cells and even the cell nucleus, these data provide evidence that ultrasmall nanoparticle can cross the blood brain barrier.

Ultrasmall gold nanoparticles (2 nm) can penetrate and enter cell nuclei in an in vitro 3D brain spheroid model

The neurovascular unit (NVU) is a complex functional and anatomical structure composed of endothelial cells and their blood-brain barrier (BBB) forming tight junctions. It represents an efficient barrier for molecules and drugs. However, it also prevents a targeted transport for the treatment of cerebral diseases. The uptake of ultrasmall nanoparticles as potential drug delivery agents was studied in a three-dimensional co-culture cell model (3D spheroid) composed of primary human cells (astrocytes, pericytes, endothelial cells). Multicellular 3D spheroids show reproducible NVU features and functions. The spheroid core is composed mainly of astrocytes, covered with pericytes, while brain endothelial cells form the surface layer, establishing the NVU that regulates the transport of molecules. After 120 h cultivation, the cells self-assemble into a 350 µm spheroid as shown by confocal laser scanning microscopy. The passage of different types of fluorescent ultrasmall gold nanoparticles (core diameter 2 nm) both into the spheroid and into three constituting cell types was studied by confocal laser scanning microscopy. Three kinds of covalently fluorophore-conjugated gold nanoparticles were used: One with fluorescein (FAM), one with Cy3, and one with the peptide CGGpTPAAK-5,6-FAM-NH₂. In 2D cell co-culture experiments, it was found that all three kinds of nanoparticles readily entered all three cell types. FAM- and Cy3-labelled nanoparticles were able to enter the cell nucleus as well. The three dissolved dyes alone were not taken up by any cell type. A similar situation evolved with 3D spheroids: The three kinds of nanoparticles entered the spheroid, but the dissolved dyes did not. The presence of a functional blood-brain barrier was demonstrated by adding histamine to the spheroids. In that case, the blood-brain barrier opened, and dissolved dyes like a FITC-labelled antibody and FITC alone entered the spheroid. In summary, our results qualify ultrasmall gold nanoparticles as suitable carriers for imaging or drug delivery into brain cells (sometimes including the nucleus), brain cell spheroids, and probably also into the brain. STATEMENT OF SIGNIFICANCE: 3D brain spheroid model and its permeability by ultrasmall gold nanoparticles. We demonstrate that ultrasmall gold nanoparticles can easily penetrate the constituting cells and sometimes even enter the cell nucleus. They can also enter the interior of the blood-brain barrier model. In contrast, small molecules like fluorescing dyes are not able to do that. Thus, ultrasmall gold nanoparticles can serve as carriers of drugs or for imaging inside the brain.