Symmetry Breaking during Seeded Growth of Nanocrystals

Currently, most of the reported noble-metal nanocrystals are limited to a high level of symmetry, as constrained by the inherent, face-centered cubic (fcc) lattice of these metals. In this paper, we report, for the first time, a facile and versatile approach (backed up by a clear mechanistic understanding) for breaking the symmetry of an fcc lattice and thus obtaining nanocrystals with highly unsymmetrical shapes. The key strategy is to induce and direct the growth of nanocrystal seeds into unsymmetrical modes by manipulating the reduction kinetics. With silver as an example, we demonstrated that the diversity of possible shapes taken by noble-metal nanocrystals could be greatly expanded by incorporating a series of new shapes drastically deviated from the fcc lattice. This work provides a new method to investigate shape-controlled synthesis of metal nanocrystal

Catalytic Gold-Iridium Nanoparticles as Labels for Sensitive Colorimetric Lateral Flow Assay

The colorimetric lateral flow assay (CLFA, also known as test strip) is a widely used point-of-care diagnostic technology. It has been a challenge to significantly improve the detection sensitivity of CLFA without involving additional equipment and/or compromising its simplicity. In this work, we break through the detection limit barrier of CLFA by developing a type of catalytic nanoparticles (NPs) used as labels. Specifically, the NPs were engineered by coating conventional gold NPs (AuNPs) with iridium (Ir) to form an Au-Ir core–shell structure. Such Au-Ir NPs possess ultrahigh peroxidase-like catalytic activities. A single Au-Ir NP can generate up to 107 colored molecules per second by catalyzing peroxidase substrates. The strong color signal from the catalysis ensures a high sensitivity of associated CLFA. The Au-Ir NP-based CLFA was successfully applied to the detection of two different cancer biomarkers that achieved limits of detection at the low picogram per milliliter level, hundreds of times lower than those of conventional AuNP-based CLFA

Quantitative Analysis of the Role Played by Poly(vinylpyrrolidone) in Seed-Mediated Growth of Ag Nanocrystals

This article presents a quantitative analysis of the role played by poly­(vinylpyrrolidone) (PVP) in seed-mediated growth of Ag nanocrystals. Starting from Ag nanocubes encased by {100} facets as the seeds, the resultant nanocrystals could take different shapes depending on the concentration of PVP in the solution. If the concentration was above a critical value, the seeds simply grew into larger cubes still enclosed by {100} facets. When the concentration fell below a critical value, the seeds would evolve into cuboctahedrons enclosed by a mix of {100} and {111} facets and eventually octahedrons completely covered by {111} facets. We derived the coverage density of PVP on Ag(100) surface by combining the results from two measurements: (i) cubic seeds were followed to grow at a fixed initial concentration of PVP to find out when {111} facets started to appear on the surface, and (ii) cubic seeds were allowed to grow at reduced initial concentrations of PVP to see at which concentration {111} facets started to appear from the very beginning. We could calculate the coverage density of PVP from the differences in PVP concentration and the total surface area of Ag nanocubes between these two samples. The coverage density was found to be 140 and 30 repeating units per nm² for PVP of 55 000 and 10 000 g/mol in molecular weight, respectively, for cubic seeds of 40 nm in edge length. These values dropped slightly to 100 and 20 repeating units per nm², respectively, when 100 nm Ag cubes were used as the seeds

Quantitative Analysis of the Coverage Density of Br^– Ions on Pd{100} Facets and Its Role in Controlling the Shape of Pd Nanocrystals

We report an approach based on a combination of inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy for quantitative analysis of the role played by Br^– ions in the synthesis of Pd nanocrystals. The Br^– ions were found to adsorb onto Pd{100} facets selectively with a coverage density of ca. 0.8 ion per surface Pd atom. The chemisorbed Br^– ions could be removed via desorption at an elevated temperature under reductive conditions. They could also be gradually released from the surface when Pd cubic seeds grew into cuboctahedrons and then octahedrons. On the basis of the coverage density information, we were able to estimate the minimum concentration of Br^– ions needed for the formation of Pd nanocubes with a specific size. If the concentration of Br^– ions was below this minimum value, not all of the {100} facets could be stabilized by the capping agent, leading to the formation of nanocubes with truncated corners. The quantitative analysis developed in this study is potentially extendable to other systems involving chemisorbed capping agents

Quantifying the Coverage Density of Poly(ethylene glycol) Chains on the Surface of Gold Nanostructures

The coverage density of poly(ethylene glycol) (PEG) is a key parameter in determining the efficiency of PEGylation, a process pivotal to <i>in vivo</i> delivery and targeting of nanomaterials. Here we report four complementary methods for quantifying the coverage density of PEG chains on various types of Au nanostructures by using a model system based on HS–PEG–NH₂ with different molecular weights. Specifically, the methods involve reactions with fluorescamine and ninhydrin, as well as labeling with fluorescein isothiocyanate (FITC) and Cu²⁺ ions. The first two methods use conventional amine assays to measure the number of unreacted HS–PEG–NH₂ molecules left behind in the solution after incubation with the Au nanostructures. The other two methods involve coupling between the terminal −NH₂ groups of adsorbed −S–PEG–NH₂ chains and FITC or a ligand for Cu²⁺ ion, and thus pertain to the “active” −NH₂ groups on the surface of a Au nanostructure. We found that the coverage density decreased as the length of PEG chains increased. A stronger binding affinity of the initial capping ligand to the Au surface tended to reduce the PEGylation efficiency by slowing down the ligand exchange process. For the Au nanostructures and capping ligands we have tested, the PEGylation efficiency decreased in the order of citrate-capped nanoparticles > PVP-capped nanocages ≈ CTAC-capped nanoparticles ≫ CTAB-capped nanorods, where PVP, CTAC, and CTAB stand for poly(vinyl pyrrolidone), cetyltrimethylammonium chloride, and cetyltrimethylammonium bromide, respectively

Facile Colorimetric Detection of Silver Ions with Picomolar Sensitivity

Although various colorimetric methods have been actively developed for the detection of Ag⁺ ions because of their simplicity and reliability, the limits of detection of these methods are confined to the nanomolar (nM) level. Here, we demonstrate a novel strategy for colorimetric Ag⁺ detection with picomolar (pM) sensitivity. This strategy involves the use of poly­(vinylpyrrolidone)- (PVP-) capped Pt nanocubes as artificial peroxidases that can effectively generate a colored signal by catalyzing the oxidation of peroxidase substrates. In the presence of Ag⁺ ions, the colored signal generated by these Pt cubes is greatly diminished because of the specific and efficient inhibition of Ag⁺ toward the peroxidase-like activity of the Pt cubes. This colorimetric method can achieve an ultralow detection limit of 80 pM and a wide dynamic range of 10^–2–10⁴ nM. To the best of our knowledge, the method presented in this work shows the highest sensitivity for Ag⁺ detection among all reported colorimetric methods. Moreover, this method also features simplicity and rapidness as it can be conducted at room temperature, in aqueous solution, and requires only ∼6 min

Fluorescent Probe-Based Lateral Flow Assay for Multiplex Nucleic Acid Detection

Here we report a rapid, low cost, and disposable dipstick-type DNA biosensor that enables multiplex detection in a single assay. The fluorescent probes labeled with different fluorophores were introduced into the lateral flow nucleic acid testing system. In combination with multiple immobilized probes arranged in an array formant on the membrane, a dual-color fluorescent lateral flow DNA biosensor was developed using a portable fluorescence reader. Up to 13 human papillomavirus types could be detected simultaneously by a single-step operation in less than 30 min after linear-after-the-exponential (LATE)-PCR. The sensitivity was determined to be 10–10² copies plasmid DNA/μL. The specificity study showed no cross-reactivity among the 31 different common HPV types. In the clinical validation, 95.3% overall agreement showed very good potential for this method in the clinical application when compared to a commercial kit

Facile Synthesis of Iridium Nanocrystals with Well-Controlled Facets Using Seed-Mediated Growth

Iridium nanoparticles have only been reported with roughly spherical shapes and sizes of 1–5 nm, making it impossible to investigate their facet-dependent catalytic properties. Here we report for the first time a simple method based on seed-mediated growth for the facile synthesis of Ir nanocrystals with well-controlled facets. The essence of this approach is to coat an ultrathin conformal shell of Ir on a Pd seed with a well-defined shape at a relatively high temperature to ensure fast surface diffusion. In this way, the facets on the initial Pd seed are faithfully replicated in the resultant Pd@Ir core–shell nanocrystal. With 6 nm Pd cubes and octahedra encased by {100} and {111} facets, respectively, as the seeds, we have successfully generated Pd@Ir cubes and octahedra covered by Ir{100} and Ir{111} facets. The Pd@Ir cubes showed higher H₂ selectivity (31.8% vs 8.9%) toward the decomposition of hydrazine compared with Pd@Ir octahedra with roughly the same size

Evaluating the Pharmacokinetics and <i>In Vivo</i> Cancer Targeting Capability of Au Nanocages by Positron Emission Tomography Imaging

Gold nanocages have recently emerged as a novel class of photothermal transducers and drug carriers for cancer treatment. However, their pharmacokinetics and tumor targeting capability remain largely unexplored due to the lack of an imaging modality for quick and reliable mapping of their distributions <i>in vivo</i>. Herein, Au nanocages were prepared with controlled physicochemical properties and radiolabeled with ⁶⁴Cu in high specific activities for <i>in vivo</i> evaluation using positron emission tomography (PET). Our pharmacokinetic studies with femtomolar administrations suggest that 30 nm nanocages had a greatly improved biodistribution profile than 55 nm nanocages, together with higher blood retention and lower hepatic and splenic uptakes. In a murine EMT-6 breast cancer model, the small cages also showed a significantly higher level of tumor uptake and a greater tumor-to-muscle ratio than the large cages. Quantitative PET imaging confirmed rapid accumulation and retention of Au nanocages inside the tumors. The ability to directly and quickly image the distribution of Au nanocages <i>in vivo</i> allows us to further optimize their physicochemical properties for a range of theranostic applications

An Enzyme-Free Signal Amplification Technique for Ultrasensitive Colorimetric Assay of Disease Biomarkers

Enzyme-based colorimetric assays have been widely used in research laboratories and clinical diagnosis for decades. Nevertheless, as constrained by the performance of enzymes, their detection sensitivity has not been substantially improved in recent years, which inhibits many critical applications such as early detection of cancers. In this work, we demonstrate an enzyme-free signal amplification technique, based on gold vesicles encapsulated with Pd–Ir nanoparticles as peroxidase mimics, for colorimetric assay of disease biomarkers with significantly enhanced sensitivity. This technique overcomes the intrinsic limitations of enzymes, thanks to the superior catalytic efficiency of peroxidase mimics and the efficient loading and release of these mimics. Using human prostate surface antigen as a model biomarker, we demonstrated that the enzyme-free assay could reach a limit of detection at the femtogram/mL level, which is over 10³-fold lower than that of conventional enzyme-based assay when the same antibodies and similar procedure were used