Universal Click-Chemistry Approach for the DNA Functionalization of Nanoparticles.

Nanotechnology has revolutionized the fabrication of hybrid species with tailored functionalities. A milestone in this field is the deoxyribonucleic acid (DNA) conjugation of nanoparticles, introduced almost 30 years ago, which typically exploits the affinity between thiol groups and metallic surfaces. Over the last decades, developments in colloidal research have enabled the synthesis of an assortment of nonmetallic structures, such as high-index dielectric nanoparticles, with unique properties not previously accessible with traditional metallic nanoparticles. However, to stabilize, integrate, and provide further functionality to nonmetallic nanoparticles, reliable techniques for their functionalization with DNA will be crucial. Here, we combine well-established dibenzylcyclooctyne-azide click-chemistry with a simple freeze-thaw method to achieve the functionalization of silica and silicon nanoparticles, which form exceptionally stable colloids with a high DNA surface density of ∼0.2 molecules/nm2. Furthermore, we demonstrate that these functionalized colloids can be self-assembled into high-index dielectric dimers with a yield of over 50% via the use of DNA origami. Finally, we extend this method to functionalize other important nanomaterials, including oxides, polymers, core-shell, and metal nanostructures. Our results indicate that the method presented herein serves as a crucial complement to conventional thiol functionalization chemistry and thus greatly expands the toolbox of DNA-functionalized nanoparticles currently available.

Accessible hotspots for single-protein SERS in DNA-origami assembled gold nanorod dimers with tip-to-tip alignment.

The label-free identification of individual proteins from liquid samples by surface-enhanced Raman scattering (SERS) spectroscopy is a highly desirable goal in biomedical diagnostics. However, the small Raman scattering cross-section of most (bio-)molecules requires a means to strongly amplify their Raman signal for successful measurement, especially for single molecules. This amplification can be achieved in a plasmonic hotspot that forms between two adjacent gold nanospheres. However, the small (≈1-2 nm) gaps typically required for single-molecule measurements are not accessible for most proteins. A useful strategy would thus involve dimer structures with gaps large enough to accommodate single proteins, whilst providing sufficient field enhancement for single-molecule SERS. Here, we report on using a DNA origami scaffold for tip-to-tip alignment of gold nanorods with an average gap size of 8 nm. The gaps are accessible to streptavidin and thrombin, which are captured at the plasmonic hotspot by specific anchoring sites on the origami template. The field enhancement achieved for the nanorod dimers is sufficient for single-protein SERS spectroscopy with sub-second integration times. This design for SERS probes composed of DNA origami with accessible hotspots promotes future use for single-molecule biodiagnostics in the near-infrared range.

DNA Origami Assembled Nanoantennas for Manipulating Single-Molecule Spectral Emission.

The emission spectrum of a dye is given by the energy of all of the possible radiative transitions weighted by their probability. This spectrum can be altered with optical nanoantennas that are able to manipulate the decay rate of nearby emitters by modifying the local density of photonic states. Here, we make use of DNA origami to precisely place an individual dye at different positions around a gold nanorod and show how this affects the emission spectrum of the dye. In particular, we observe a strong suppression or enhancement of the transitions to different vibrational levels of the excitonic ground state, depending on the spectral overlap with the nanorod resonance. This reshaping can be used to experimentally extract the spectral dependence of the radiative decay rate enhancement. Furthermore, for some cases, we argue that the drastic alteration of the fluorescence spectrum could arise from the violation of Kasha's rule.

Gold Nanorod DNA Origami Antennas for 3 Orders of Magnitude Fluorescence Enhancement in NIR.

DNA origami has taken a leading position in organizing materials at the nanoscale for various applications such as manipulation of light by exploiting plasmonic nanoparticles. We here present the arrangement of gold nanorods in a plasmonic nanoantenna dimer enabling up to 1600-fold fluorescence enhancement of a conventional near-infrared (NIR) dye positioned at the plasmonic hotspot between the nanorods. Transmission electron microscopy, dark-field spectroscopy, and fluorescence analysis together with numerical simulations give us insights on the heterogeneity of the observed enhancement values. The size of our hotspot region is ∼12 nm, granted by using the recently introduced design of NAnoantenna with Cleared HotSpot (NACHOS), which provides enough space for placing of tailored bioassays. Additionally, the possibility to synthesize nanoantennas in solution might allow for production upscaling.

DNA Origami-Enabled Plasmonic Sensing.

The reliable programmability of DNA origami makes it an extremely attractive tool for bottom-up self-assembly of complex nanostructures. Utilizing this property for the tuned arrangement of plasmonic nanoparticles holds great promise particularly in the field of biosensing. Plasmonic particles are beneficial for sensing in multiple ways, from enhancing fluorescence to enabling a visualization of the nanoscale dynamic actuation via chiral rearrangements. In this Perspective, we discuss the recent developments and possible future directions of DNA origami-enabled plasmonic sensing systems. We start by discussing recent advancements in the area of fluorescence-based plasmonic sensing using DNA origami. We then move on to surface-enhanced Raman spectroscopy sensors followed by chiral sensing, both utilizing DNA origami nanostructures. We conclude by providing our own views on the future prospects for plasmonic biosensors enabled using DNA origami.

Improved synthesis of concave cubic gold nanoparticles and their applications for Raman analysis of surfaces.

In this contribution, we present a modification of the procedure for producing concave cubic gold (cc-Au) nanoparticles; this modification significantly increases the homogeneity of the product obtained. The synthesis of cc-Au is carried out by the slow growth of seed nanostructures in a solution containing chloroauric acid, silver nitrate, ascorbic acid and hexadecyltrimethylammonium chloride. We show that, when nanoparticles synthesized in a solution containing both chloroauric acid and copper chloride (with the molar ratio equal to ca. 10 : 1) are used as seeds (instead of seeds formed without the addition of copper), one can observe a significant increase in the homogeneity of the cc-Au nanostructures formed. The resulting cc-Au, and cc-Au@Ag nanoparticles (cc-Au covered by a nanometric layer of silver) as well, have been used as plasmonic cores in nanoresonators dedicated for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). To our knowledge, the SHINERS nanoresonators produced in this work display a homogeneity that is significantly better than that of any anisotropic SHINERS nanostructures previously synthesized without the subsequent complex process of purifying the nanoparticles. Concave cubic nanoparticles were about 5 times more efficient as electromagnetic nanoresonators than spherical nanostructures of a similar size formed from the same material.

Silica-covered star-shaped Au-Ag nanoparticles as new electromagnetic nanoresonators for Raman characterisation of surfaces.

One of the tools used for determining the composition of surfaces of various materials is shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). SHINERS is a modification of "standard" surface-enhanced Raman spectroscopy (SERS), in which, before Raman spectra are recorded, the surfaces analysed are covered with a layer of plasmonic nanoparticles protected by a very thin layer of a transparent dielectric. The plasmonic cores of the core-shell nanoparticles used in SHINERS measurements generate a local enhancement of the electric field of the incident electromagnetic radiation, whereas the transparent coatings prevent the metal cores from coming into direct contact with the material being analysed. In this contribution, we propose a new type of SHINERS nanoresonators that contain spiky, star-shaped metal cores (produced from a gold/silver alloy). These spiky, star-shaped Au-Ag nanoparticles have been covered by a layer of silica. The small radii of the ends of the tips of the spikes of these plasmonic nanostructures make it possible to generate a very large enhancement of the electromagnetic field there, with the result that such SHINERS nanoresonators are significantly more efficient than the standard semi-spherical nanostructures. The Au-Ag alloy nanoparticles were synthesised by the reduction of a solution containing silver nitrate and chloroauric acid by ascorbic acid. The final geometry of the nanostructures thus formed was controlled by changing the ratio between the concentrations of AuCl4- and Ag+ ions. The shape of the synthesised star-shaped Au-Ag nanoparticles does not change significantly during the two standard procedures for depositing a layer of silica (by the decomposition of sodium silicate or the decomposition of tetraethyl orthosilicate).