Solution-based molecularly-mediated bottom-up assembly of gold nanostars and nanospheres in regiospecific core-satellite nanoarchitectures is reported. The controlled assembly is driven by coupling ...reactions in solution between small, rigid, Raman-active organic molecules bound to the surface of the nanoparticles, and leads to much narrower interparticle gaps than achievable with DNA-based assembly methods. In the described system, gold nanostars with multiple sharp spikes, ideal for electromagnetic field enhancement, are used as the core particle onto which spherical satellites are assembled. Transmission electron micrographs show that the core-satellite structures assemble with <2 nm interparticle gaps and regiospecific binding of only one sphere per spike, and the process can be followed by monitoring changes in the surface enhanced Raman scattering (SERS) spectra of the Raman active linkers. The assembled structures give rise on average to two orders of magnitude SERS signal enhancement per nanoparticle in comparison to their constituents, which can be attributed to the creation of SERS "hot spots" between the nanostar tip and the satellite sphere. Two dimensional finite element electromagnetic models show strongly confined electromagnetic field intensity in the narrow interparticle gaps of core-satellite assemblies, which is significantly enhanced in comparison to the constituent nanoparticles, thus corroborating the experimental findings. Thus, the assemblies reported here can be envisioned as SERS-tags for imaging purposes as well as a model system for SERS-based chemical sensing with improved sensitivity.
The response of living systems to nanoparticles is thought to depend on the protein corona, which forms shortly after exposure to physiological fluids and which is linked to a wide array of ...pathophysiologies. A mechanistic understanding of the dynamic interaction between proteins and nanoparticles and thus the biological fate of nanoparticles and associated proteins is, however, often missing mainly due to the inadequacies in current ensemble experimental approaches. Through the application of a variety of single molecule and single particle spectroscopic techniques in combination with ensemble level characterization tools, we identified different interaction pathways between gold nanorods and bovine serum albumin depending on the protein concentration. Overall, we found that local changes in protein concentration influence everything from cancer cell uptake to nanoparticle stability and even protein secondary structure. We envision that our findings and methods will lead to strategies to control the associated pathophysiology of nanoparticle exposure in vivo.
Ligand exchange on gold nanorods (NRs) is still too often dismissed or not given the importance it should deserve. The many applications of gold NRs, mainly in plasmonics, biological imaging, and ...sensing, are made possible by finely tuning not only the optical properties of the metallic core but also the tethered functional groups. Gold NRs are mainly synthesized by using CTAB as the morphology‐guiding surfactant, and an intimate relationship between the crystallographic facets of the rod and the CTAB bilayer exists. Because of this, it is imperative to fully understand the ligand exchange mechanisms that allow replacing CTAB with functional ligands, including the energetic contributions. Here, the major applications of gold NRs are briefly overviewed, and what is known about ligand exchange mechanisms is summarized, as well as why it is important to achieve complete removal of CTAB, including the techniques that are used to characterize the exchange reaction products. The concept of interface in gold NRs is briefly examined, and explained why the scientific community should focus more on understanding and characterizing it. Starting from the published literature, the reader is guided through the reasons why it is thought that ligand exchange on gold NRs is perhaps the next grand challenge in the nanoparticle field.
Ligand exchange on gold nanorods is examined in this Progress Report. The main applications of gold nanorods are summarized, and the importance of ligand exchange in this context is highlighted. Ligand exchange mechanisms, CTAB toxicity and removal, and the relevant characterization techniques are overviewed. The importance of understanding the interface and devising experiments to characterize it are also described.
Herein, a new class of multifunctional materials combining a clustered nanoparticle-based probe is presented for surface enhanced Raman scattering (SERS)-based microscopy and surface ...functionalization for tissue targeting. Controlled assembly of spherical gold nanoparticles into dimers (DNP-REP) is engineered using a small, rigid Raman-active dithiolated linking reporter (REP) to yield narrow internanoparticle gaps and to strategically generate the "hot spot" while concurrently placing the reporter within the region of highest SERS enhancement. Peptide functionalized DNP-REP materials are highly stable even upon incubation with living cells and show controlled levels of binding and intracellular endocytosis. To demonstrate the functionality of such probes for disease detection, differentially targeted DNP-REPs are incubated over various time points with cultured human glioblastoma cells. Using human glioblastoma cells, the SERS maps of targeted tumor cells show the markedly enhanced signals of the DNP-REP, compared to conventional confocal fluorescence based approaches, especially at low incubation times. Even with as few as 40 internalized DNP-REP, a relatively intense SERS signal is measured, demonstrating the high signal to noise ratio and inherent biocompatibility of the materials. Thus, these Raman reporter-based nanoparticle cluster probes present a promising and versatile optical imaging tool for fast, reliable, selective, and ultrasensitive tissue targeting and disease detection and screening.
Methods combining immunology and surface-enhanced Raman scattering (SERS) have been developed for the simultaneous detection, identification, and localization of proteinaceous binding media found in ...artworks. However, complex surface topographies and heterogeneous compositions of art samples represent significant challenges for the general optimization of this technique. In particular, aggregation of immuno-SERS nanoparticles can lead to non-specific SERS response across the sample surface, resulting in inaccurate identification of binding media or dubious localization maps. This aggregation also diminishes the sample area available for analysis, as excitation of visible nanoparticle aggregates by the Raman laser must be avoided during data collection. In the present work, we synthesize several types of immuno-SERS nanoparticles and investigate their applicability for the detection and localization of ovalbumin-rich (egg-based) binding media in art samples. Dimers of gold nanoparticles (Au NPs) connected by a Raman-active dithiolated linker are conjugated to secondary antibodies through either an amino or a carboxyl functional group (SERS tags). The SERS tags display localized surface plasmon resonance (LSPR) at 532 nm. SERS spectra are acquired at 633 nm (SERS-633) in order to maximize tuning between laser excitation and LSPR, while avoiding sample burning. In an indirect immunoassay applied to replica art samples, carboxy-terminated SERS-633 tags show strong Raman reporter signal, specificity for the target protein, robust response in the presence of various inorganic pigments, and reduced aggregation on sample surfaces compared to amino-terminated or commercial SERS tags. Scanning electron microscopy (SEM) is used to visualize Au NPs bound to egg media in situ, demonstrating that carboxy-terminated SERS-633 tags remain as discrete dimer units throughout the assay.
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