An ideal surface-enhanced Raman scattering (SERS) nanostructure for sensing and imaging applications should induce a high signal enhancement, generate a reproducible and uniform response, and should be easy to synthesize. Many SERS-active nanostructures have been investigated, but they suffer from poor reproducibility of the SERS-active sites, and the wide distribution of their enhancement factor values results in an unquantifiable SERS signal. Here, we show that DNA on gold nanoparticles facilitates the formation of well-defined gold nanobridged nanogap particles (Au-NNP) that generate a highly stable and reproducible SERS signal. The uniform and hollow gap (∼1 nm) between the gold core and gold shell can be precisely loaded with a quantifiable amount of Raman dyes. SERS signals generated by Au-NNPs showed a linear dependence on probe concentration (R(2) > 0.98) and were sensitive down to 10 fM concentrations. Single-particle nano-Raman mapping analysis revealed that >90% of Au-NNPs had enhancement factors greater than 1.0 × 10(8), which is sufficient for single-molecule detection, and the values were narrowly distributed between 1.0 × 10(8) and 5.0 × 10(9).

Surface-enhanced Raman scattering (SERS)-based signal amplification and detection methods using plasmonic nanostructures have been widely investigated for imaging and sensing applications. However, SERS-based molecule detection strategies have not been practically useful because there is no straightforward method to synthesize and characterize highly sensitive SERS-active nanostructures with sufficiently high yield and efficiency, which results in an extremely low cross-section area in Raman sensing. Here, we report a high-yield synthetic method for SERS-active gold-silver core-shell nanodumbbells, where the gap between two nanoparticles and the Raman-dye position and environment can be engineered on the nanoscale. Atomic-force-microscope-correlated nano-Raman measurements of individual dumbbell structures demonstrate that Raman signals can be repeatedly detected from single-DNA-tethered nanodumbbells. These programmed nanostructure fabrication and single-DNA detection strategies open avenues for the high-yield synthesis of optically active smart nanoparticles and structurally reproducible nanostructure-based single-molecule detection and bioassays.

Core/shell upconverting nanoparticles (UCNPs) of NaGdF4:Er3+,Yb3+/NaGdF4 (see figure) are shown to serve as a multimodal imaging probe that works for both background‐free optical imaging and magnetic resonance imaging (MRI). The nonblinking and nonbleaching properties of UCNPs can contribute to minimization of possible artifacts in long‐term imaging experiments. Owing to Gd3+ ions in the host matrix, contrast is enhanced in T1‐weighted MRI.

We extensively study the relationships between single-molecule surface-enhanced Raman scattering (SMSERS) intensity, enhancement factor (EF) distribution over many particles, interparticle distance, particle size/shape/composition and excitation laser wavelength using the single-particle AFM-correlated Raman measurement method and theoretical calculations. Two different single-DNA-tethered Au-Ag core-shell nanodumbbell (GSND) designs with an engineerable nanogap were used in this study: the GSND-I with various interparticle nanogaps from ∼4.8 nm to <1 nm or with no gap and the GSND-II with the fixed interparticle gap size and varying particle size from a 23-30 nm pair to a 50-60 nm pair. From the GSND-I, we learned that synthesizing a <1 nm gap is a key to obtain strong SMSERS signals with a narrow EF value distribution. Importantly, in the case of the GSND-I with <1 nm interparticle gap, an EF value of as high as 5.9 × 10(13) (average value = 1.8 × 10(13)) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 10(12) and 5.9 × 10(13). In the case of the GSND-II probes, a combination of >50 nm Au cores and 514.5 nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50 nm Au cores with 514.5 nm laser or the GSND-II structures with 632.8 nm laser. Our results show the usefulness and flexibility of these GSND structures in studying and obtaining SMSERS structures with a narrow distribution of high EF values and that the GSNDs with < 1 nm are promising SERS probes with highly sensitive and quantitative detection capability when optimally designed.

New spherical nanostructures of titania (TiO2) have been synthesized through formation of liposome−TiO2 nanocomposites by using egg lecithin lipid as a template, and their optical properties have been investigated with regard to the dynamics of surface charge carriers and photocatalytic activities by using UV−vis and photoluminescence (PL) spectroscopic techniques. On the basis of the measurements of X-ray diffraction, transmission electron microscopy, and atomic force microscopy, the spherical titania nanostructures are identified to be anatase crystalline nanodisks with an average diameter of 9 nm and height of 0.5 nm. The nanodisks have a large Brunauer−Emmett−Teller specific surface area of 227 m2/g. The FT-IR and X-ray photoemission spectra of the nanodisks confirm that the skeleton structure of the titania nanodisk is formed through H-bonding of the −Ti−O−Ti− network through tetrahedrally coordinated vacancies designated 4Ti4+−OH. Analysis of the UV−vis and PL spectra reveals that the band-gap energy is red-shifted to 3.02 eV from that of TiO2 nanoparticle dots and its transition nature is exclusively indirect. The PL emission spectrum of the titania nanodisks exhibits a strong structural emission band around 420 nm with shoulders around 470 and 550 nm which is attributed to the transition from three different exciton-trapped surface states. In addition, another surface emission originating from the coordinatively unsaturated ions (Ti3+) is observed at 618 nm. These results suggest that coupling of the surface charge carriers with the lattice phonon of the nanostructures is so strong that the dominant route to charge recombination in titania nanodisks is nonradiative. Supporting the steady-state spectral observations, the decay profiles of the surface emission measured by using a femtosecond laser time-resolved PL system fit into a triexponential function with relatively longer lifetimes (20−30 ps, 1.1−1.5 ns, and 4.5−6.0 ns) as compared to those of simple nanoparticle dots, indicating that recombination of the charge carriers on the nanodisk surface is very prolonged. Being consistent with this, the photocatalytic efficiency for the reduction of methyl orange is much higher in the presence of the titania nanodisks than that observed in the presence of Degussa P-25.