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

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.

Modular Drug-Loaded Nanocapsules with Metal Dome Layers as a Platform for Obtaining Synergistic Therapeutic Biological Activities

Multifunctional drug-loaded polymer-metal nanocapsules have attracted increasing attention in drug delivery due to their multifunctional potential endowed by drug activity and response to physicochemical stimuli. Current chemical synthesis methods of polymer/metal capsules require specific optimization of the different components to produce particles with precise properties, being particularly complex for Janus structures combining polymers and ferromagnetic and highly reactive metals. With the aim to generate tunable synergistic nanotherapeutic actuation with enhanced drug effects, here we demonstrate a versatile hybrid chemical/physical fabrication strategy to incorporate different functional metals with tailored magnetic, optical, or chemical properties on solid drug-loaded polymer nanoparticles. As archetypical examples, we present poly(lactic-co-glycolic acid) (PLGA) nanoparticles (diameters 100-150 nm) loaded with paclitaxel, indocyanine green, or erythromycin that are half-capped by either Fe, Au, or Cu layers, respectively, with application in three biomedical models. The Fe coating on paclitaxel-loaded nanocapsules permitted efficient magnetic enhancement of the cancer spheroid assembly, with 40% reduction of the cross-section area after 24 h, as well as a higher paclitaxel effect. In addition, the Fe-PLGA nanocapsules enabled external contactless manipulation of multicellular cancer spheroids with a speed of 150 μm/s. The Au-coated and indocyanine green-loaded nanocapsules demonstrated theranostic potential and enhanced anticancer activity in vitro and in vivo due to noninvasive fluorescence imaging with long penetration near-infrared (NIR) light and simultaneous photothermal-photodynamic actuation, showing a 3.5-fold reduction in the tumor volume growth with only 5 min of NIR illumination. Finally, the Cu-coated erythromycin-loaded nanocapsules exhibited enhanced antibacterial activity with a 2.5-fold reduction in the MIC50 concentration with respect to the free or encapsulated drug. Altogether, this technology can extend a nearly unlimited combination of metals, polymers, and drugs, thus enabling the integration of magnetic, optical, and electrochemical properties in drug-loaded nanoparticles to externally control and improve a wide range of biomedical applications.

Crystal structure of bulky-ligand-protected Au24(S-C4H9)16

Atomically precise thiolate-protected gold nanomolecules have attracted interest due to their distinct electronic and chemical properties. The structure of these nanomolecules is important for understanding their peculiar properties. Here, we report the X-ray crystal structure of a 24-atom gold nanomolecule protected by 16 tert-butylthiolate ligands. The composition of Au24(S-C4H9)16 {poly[hexadecakis(μ-tert-butylthiolato)tetracosagold]} was confirmed by X-ray crystallography and electrospray ionization mass spectrometry (ESI–MS). The nanomolecule was synthesized in a one-phase synthesis and crystallized from a hexane–ethanol layered solution. The X-ray structure confirms the 16-atom core protected by two monomeric and two trimeric staples with four bridging ligands. The Au24(S-C4H9)16 cluster follows the shell-closing magic number of 8.

Multimodal Potentials of Gold Nanoparticles for Bone Tissue Engineering and Regenerative Medicine: Avenues and Prospects

Osseous tissue repair has advanced due to the introduction of tissue engineering. The key elements required while engineering new tissues involve scaffolds, cells, and bioactive cues. The macrostructural to the nanostructural framework of such complex tissue has engrossed the intervention of nanotechnology for efficient neo-bone formation. Gold nanoparticles (GNPs) have recently gained interest in bone tissue regeneration due to their multimodal functionality. They are proven to modulate the properties of scaffolds and the osteogenic cells significantly. GNPs also influence different metabolic functions within the body, which directly or indirectly govern the mechanism of bone regeneration. Therefore, this review highlights nanoparticle-mediated osteogenic development, focusing on different aspects of GNPs ranging from scaffold modulation to cellular stimulation. The toxic aspects of gold nanoparticles studied so far are critically explicated, while further insight into the advancements and prospects of these nanoparticles in bone regeneration is also highlighted.

Dimension switchable auto-fluorescent peptide-based 1D and 2D nano-assemblies and their self-influence on intracellular fate and drug delivery

The production of dynamic, environment-responsive shape-tunable biomaterials marks a significant step forward in the construction of synthetic materials that can easily rival their natural counterparts. Significant progress has been made in the self-assembly of bio-materials. However, the self-assembly of a peptide into morphologically distinct auto-fluorescent nanostructures, without the incorporation of any external moiety is still in its infancy. Hence, in this study, we have developed peptide-based self-assembled auto-fluorescent nanostructures that can shuttle between 1D and 2D morphologies. Different morphological nanostructures are well known to have varied cellular internalization efficiencies. Taking advantage of our morphologically different particles emanating from the same peptide monomer, we further explored the intracellular fate of our nanostructures. We observed that the nanostructures' cellular internalization is a complex process that gets influenced by particle morphology and this might further affect their intracellular drug delivery potential. Overall, this study provides initial cues for the preparation of environment-responsive shape-shifting peptide-nano assemblies. Efforts have also been made to understand their shape driven cellular uptake behaviour, along with establishing them as nanocarriers for the cellular delivery of therapeutic molecules.

Biodegradation of metal-based ultra-small nanoparticles: A combined approach using TDA-ICP-MS and CE-ICP-MS

The knowledge of the fate of metal-containing nanoparticles in biological media in aqueous media is of utmost importance for the future use of these promising theranostic agents for clinical applications. A methodology based on the combination of TDA-ICP-MS and CE-ICP-MS was applied to study the degradation pathway of AGuIX, a phase 2 clinical ultrasmall gadolinium-containing nanoparticle. Nanoparticle size measurements and gadolinium speciation performed in different media (phosphate buffer, urine and serum) demonstrated an accelerated dissolution of AGuIX in serum, without any release of free gadolinium for each medium.

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.