Overproduction of hydrogen peroxide is involved in the pathogenesis of inflammatory diseases such as cancer and arthritis. To image hydrogen peroxide via chemiluminescence resonance energy transfer in the near-infrared wavelength range, we prepared quantum dots functionalized with a luminol derivative.

ConspectusPlasmonic coupling-based electromagnetic field localization and enhancement are becoming increasingly important in chemistry, nanoscience, materials science, physics, and engineering over the past decade, generating a number of new concepts and applications. Among the plasmonically coupled nanostructures, metal nanostructures with nanogaps have been of special interest due to their ultrastrong electromagnetic fields and controllable optical properties that can be useful for a variety of signal enhancements such as surface-enhanced Raman scattering (SERS). The Raman scattering process is highly inefficient, with a very small cross-section, and Raman signals are often poorly reproducible, meaning that very strong, controllable SERS is needed to obtain reliable Raman signals with metallic nanostructures and thus open up new avenues for a variety of Raman-based applications. More specifically, plasmonically coupled metallic nanostructures with ultrasmall (∼1 nm or smaller) nanogaps can generate very strong and tunable electromagnetic fields that can generate strong SERS signals from Raman dyes in the gap, and plasmonic nanogap-enhanced Raman scattering can be defined as Raman signal enhancement from plasmonic nanogap particles with ∼1 nm gaps. However, these promising nanostructures with extraordinarily strong optical signals have shown limited use for practical applications, largely due to the lack of design principles, high-yield synthetic strategies with nanometer-level structural control and reproducibility, and systematic, reliable single-molecule/single-particle-level studies on their optical properties. All these are extremely important challenges because even small changes (<1 nm) in the structure of the coupled plasmonic nanogaps can significantly affect the plasmon mode and signal intensity.In this Account, we examine and summarize recent breakthroughs and advances in plasmonic nanogap-enhanced Raman scattering with metal nanogap particles with respect to the design and synthesis of plasmonic nanogap structures, as well as ultrasensitive and quantitative Raman signal detection using these structures. The applications and prospects of plasmonic nanogap particle-based SERS are also discussed. In particular, reliable synthetic and measurement strategies for plasmonically coupled nanostructures with ∼1 nm gap, in which both the nanogap size and the position of a Raman-active molecule in the gap can be controlled with nanometer/sub-nanometer-level precision, can address important issues regarding the synthesis and optical properties of plasmonic nanostructures, including structural and signal reproducibility. Further, single-molecule/single-particle-level studies on the plasmonic properties of these nanogap structures revealed that these particles can generate ultrastrong, quantifiable Raman signals in a highly reproducible manner.

Lanthanide-doped upconverting nanoparticles (UCNPs) have recently attracted enormous attention in the field of biological imaging owing to their unique optical properties: (1) efficient upconversion photoluminescence, which is intense enough to be detected at the single-particle level with a (nonscanning) wide-field microscope setup equipped with a continuous wave (CW) near-infrared (NIR) laser (980 nm), and (2) resistance to photoblinking and photobleaching. Moreover, the use of NIR excitation minimizes adverse photoinduced effects such as cellular photodamage and the autofluorescence background. Finally, the cytotoxicity of UCNPs is much lower than that of other nanoparticle systems. All these advantages can be exploited simultaneously without any conflicts, which enables the establishment of a novel UCNP-based platform for wide-field two-photon microscopy. UCNPs are also useful for multimodal in vivo imaging because simple variations in the composition of the lattice atoms and dopant ions integrated into the particles can be easily implemented, yielding various distinct biomedical activities relevant to magnetic resonance imaging (MRI), computed tomography (CT), and positron emission tomography (PET). These multiple functions embedded in a single type of UCNPs play a crucial role in precise disease diagnosis. The application of UCNPs is extended to therapeutic fields such as photodynamic and photothermal cancer therapies through advanced surface conjugation schemes.

Here, we quantitatively monitored and analyzed the spectral redistributions of the coupled plasmonic modes of trimeric Au nanostructures with two ∼1 nm interparticle gaps and single-dye-labeled DNA in each gap as a function of varying trimer symmetries. Our precise Mie scattering measurement with the laser-scanning-assisted dark-field microscopy allows for individual visualization of the orientations of the radiation fields of the coupled plasmon modes of the trimers and analyzing the magnitude and direction of the surface-enhanced Raman scattering (SERS) signals from the individual plasmonic trimers. We found that the geometric transition from acute-angled trimer to linear trimer induces the red shift of the longitudinally polarized mode and the blue shift of the axially polarized mode. The finite element method (FEM) calculation results show the distinct “on” and “off” of the plasmonic modes at the two gaps of the trimer. Importantly, the single-molecule-level systematic correlation studies among the near-field, far-field, and surface-enhanced Raman scattering reveal that the SERS signals from the trimers are determined by the largely excited coupled plasmon between the two competing plasmon modes, longitudinal and axial modes. Further, the FEM calculation revealed that even 0.5 nm or smaller discrepancy in the sizes of two gaps of the linear trimer led to >10-fold difference in the SERS signal. Granted that two gap sizes are not likely to be completely the same in actual experiments, one of two gaps plays a more significant role in generating the SERS signal. Overall, this work provides the knowledge and handles for the understanding and systematic control of the magnitude and polarization direction of the both plasmonic response and SERS signal from trimeric nanostructures and sets up the platform for the optical properties and the applications of plasmonically coupled trimers and higher multimeric nanostructures.

We report DNA-mediated simple synthetic methods to obtain anisotropic plasmonic nanostructures with a tailorable intra-nanogap distance ranging from 0.9 to 4.0 nm. Anisotropic half-shell structures with sub-1.0 nm intra-nanogaps showed a wavelength-independent surface-enhanced Raman scattering (SERS) intensity and a highly sensitive SERS response to NIR light. We found that the reaction conditions such as pH and NaCl concentration are responsible for the resulting shell structures and intra-nanogap distances. Three noticeable plasmonic nanostructures [i.e., half-shell with sub-1.0 nm nanogaps, closed-shell with a wide nanogap (2.1 nm) and star-shaped with an irregular nanogap (1.5–4.0 nm)] were synthesized, and solution-based and single particle-based Raman measurements showed a strong relationship between the plasmonic structures and the SERS intensity. An understanding of DNA-mediated control for nanogap-engineered plasmonic nanostructures and studies of SERS-activity relationships using single particle-correlated measurements can provide new insights into the design of new plasmonic nanostructures and SERS-based biosensing applications.

The design, synthesis and control of plasmonic nanostructures, especially with ultrasmall plasmonically coupled nanogap (∼1 nm or smaller), are of significant interest and importance in chemistry, nanoscience, materials science, optics and nanobiotechnology. Here, we studied and established the thiolated DNA-based synthetic principles and methods in forming and controlling Au core-nanogap-Au shell structures [Au-nanobridged nanogap particles (Au-NNPs)] with various interior nanogap and Au shell structures. We found that differences in the binding affinities and modes among four different bases to Au core, DNA sequence, DNA grafting density and chemical reagents alter Au shell growth mechanism and interior nanogap-forming process on thiolated DNA-modified Au core. Importantly, poly A or poly C sequence creates a wider interior nanogap with a smoother Au shell, while poly T sequence results in a narrower interstitial interior gap with rougher Au shell, and on the basis of the electromagnetic field calculation and experimental results, we unraveled the relationships between the width of the interior plasmonic nanogap, Au shell structure, electromagnetic field and surface-enhanced Raman scattering. These principles and findings shown in this paper offer the fundamental basis for the thiolated DNA-based chemistry in forming and controlling metal nanostructures with ∼1 nm plasmonic gap and insight in the optical properties of the plasmonic NNPs, and these plasmonic nanogap structures are useful as strong and controllable optical signal-generating nanoprobes.

We present time-gated femtosecond stimulated Raman spectroscopy (fSRS) under the pre-resonance Raman conditions of diethylthiatricarbocyanine (DTTC) iodide. A ‘pseudo emission-free’ condition is achieved by delivering the probe beam ahead of the pump beam. Regeneratively amplified pulse trains are employed to create an angle-geometry (non-collimated) mixing between the pump and probe beams, leading to highly sensitive measurement of the stimulated Raman gain. Time-integrated spectroscopy allows for a more quantitative distinction between the contributions of stimulated Raman scattering and stimulated emission. We successfully obtain a highly sensitive (signal-to-noise ratio >100) stimulated Raman spectrum under the optimized conditions, which compares favourably to results obtained using two-dimensional correlation spectroscopy (2DCOS). Given the optical pre-resonance of ∼0.1 eV, the background signals mostly originate from the stimulated emission of excited electrons and are significantly reduced by partial overlapping of the pump and probe beams; a genuine fSRS spectral profile is obtained for a temporal delay of ∼0.2 ps between the two beams.

•Depth-dependent spatially offset Raman spectra are analyzed using generalized 2D correlation spectroscopy (2DCOS).•2DCOS analysis gives the resolved spectral components and their sequential order.•Linear shaped illumination is employed to reduce the potential photo-damage.The detection of poisonous chemicals enclosed in daily necessaries is prerequisite essential for homeland security with the increasing threat of terrorism. For the detection of toxic chemicals, we combined a sensitive deep Raman spectroscopic method with 2D correlation analysis. We obtained the Raman spectra from concealed chemicals employing spatially offset Raman spectroscopy in which incident line-shaped light experiences multiple scatterings before being delivered to inner component and yielding deep Raman signal. Furthermore, we restored the pure Raman spectrum of each component using 2D correlation spectroscopic analysis with chemical inspection. Using this method, we could elucidate subsurface component under thick powder and packed contents in a bottle.

Understanding the detailed electromagnetic field distribution inside a plasmonically coupled nanostructure, especially for structures with ∼1 nm plasmonic gap, is the fundamental basis for the control and use of the strong optical properties of plasmonic nanostructures. Using a multistep AFM tip-matching strategy that enables us to gain the optical spectra with the optimal signal-to-noise ratio as well as high reliability in correlation measurement between localized surface plasmon (LSP) and surface-enhanced Raman scattering (SERS), the coupled longitudinal dipolar and high-order multipolar LSPs were detected within a dimeric structure, where a single Raman dye is located via a single-DNA hybridization between two differently sized Au–Ag core–shell particles. On the basis of the characterization of each LSP component, the distinct phase differences, attributed to different quantities of the excited quadrupolar LSPs, between the transverse and longitudinal regimes were observed for the first time. By assessing the relative ratio of dipolar and quadrupolar LSPs, we found that these LSPs of the dimer with ∼1 nm gap were simultaneously excited, and large longitudinal bonding dipolar LSP/longitudinal bonding quadrupolar LSP value is required to generate high SERS signal intensity. Interestingly, a minor population of the examined dimers exhibited strong SERS intensities along not only the dimer axis but also the direction that arises from the interaction between the coupled transverse dipolar and longitudinal bonding quadrupolar LSPs. Overall, our high-precision correlation measurement strategy with a plasmonic heterodimer with ∼1 nm gap allows for the observation of the characteristic spectral features with the optimal signal-to-noise ratio and the subpopulation of plasmonic dimers with a distinct SERS behavior, hidden by a majority of dimer population, and the method and results can be useful in understanding the whole distribution of SERS enhancement factor values and designing plasmonic nanoantenna structures.

We investigated the correlations among the structure, Rayleigh scattering, and single-molecule surface-enhanced Raman scattering (SERS) of DNA-tethered Au–Ag core–shell nanoparticles, especially in dimer and trimer forms. For the optimal correlation measurements, accurate information on the position of the nanoparticle is crucial for locating the nanoparticle at the center of the excitation source for the optical measurements. To achieve this, we developed a multistep correlation strategy that enables us to unambiguously correlate the AFM images with optical images within a few nanometers. We also newly defined the correlation accuracy in this paper as a useful concept for the correlation measurements. With this reliable correlation accuracy, we performed various statistical analyses to thoroughly elucidate the relationships between particle structure, Rayleigh scattering and SERS in terms of the incident polarization and scattering intensity ratio.

This perspective presents an overview of single-molecule surface-enhanced Raman scattering (sm-SERS). Our overview is organized as a brief theoretical background, discussion of the factors that enhance SERS, various experimental preparations for inserting a single molecule in a hot spot, recent sm-SERS experiments, and a perspective. Although, there have been numerous review papers on sm-SERS, we mainly concentrated on the logical development of sm-SERS on the basis of the fundamental concepts and their physical significance, so that readers outside this field can understand the motivation and the underlying physics when describing current sm-SERS measurements. Indeed, understanding such current sm-SERS experiments conducted by representative groups would be very helpful for readers to answer for themselves the fundamental and practical questions surrounding sm-SERS: (1) what information can sm-SERS provide? (2) Which factors based on the SERS mechanism should be considered to significantly amplify the SERS signal? (3) What kinds of related microscopy techniques could be combined with sm-SERS to attain more meaningful results? (4) Which statistical approaches can be used and how they can be applied to properly analyze sm-SERS data? We hope that this review article can help readers answer these questions.

Upconverting nanoparticles (UCNPs) have been studied as novel bioimaging probes owing to the absence of autofluorescence and excellent photostability. For practical applications, biocompatible UCNPs with high upconversion efficiency, bright luminescence, and good colloidal stability are desirable. Herein, we report a quantitative and systematic study on the upconversion luminescence from a set of NaYF4:Yb3+,Er3+-based nanoparticles by varying crystal structures, core/shell structures, and surface ligands. Upconversion luminescent properties in colloidal solution and at the single-particle level were examined. Hexagonal-phase core/shell UCNPs exhibited the most intense luminescence among various structures, while the excellent photostability was observed in all different types of UCNPs. To optimize the biomedical imaging capability of UCNPs, various surface coating strategies were tested. By quantitative spectroscopic measurements of surface-modified UCNPs in water, it was suggested that encapsulation with polyethylene glycol (PEG)-phospholipid was found to be effective in retaining both upconversion luminescence intensity and dispersibility in aqueous environment. Finally, UCNPs with different crystal structures were applied and compared in live cells.

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 × 1013 (average value = 1.8 × 1013) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 1012 and 5.9 × 1013. 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.Keywords: core−shell particle; enhancement factor; nanogap; plasmonic probe; single-molecule detection; surface-enhanced Raman scattering

Well-defined surface-enhanced Raman scattering (SERS) active systems were fabricated by single-crystalline noble metal nanowires. Crossed and parallel nanowire pairs were constructed by using a nanomanipulator to create SERS hot spots in the form of nanowire junction. SERS spectra of brilliant cresyl blue (BCB), p-mercaptoaniline (pMA), and p-mercaptobenzoic acid (pMBA) were observed at the junction of two nanowires. The SERS enhancement and polarization dependence are correlated well with the enhanced electric field intensities calculated by the finite difference time domain (FDTD) method for specific nanowire geometries. These simple and effective SERS active systems have a practical advantage that the hot spots can be readily located and visualized by an optical microscope. These well-defined SERS active systems based on noble metal nanowires can be further developed to find applications in a variety of biological and chemical sensing.

We investigated the correlations among the structure, Rayleigh scattering, and single-molecule surface-enhanced Raman scattering (SERS) of DNA-tethered Au–Ag core–shell nanoparticles, especially in dimer and trimer forms. For the optimal correlation measurements, accurate information on the position of the nanoparticle is crucial for locating the nanoparticle at the center of the excitation source for the optical measurements. To achieve this, we developed a multistep correlation strategy that enables us to unambiguously correlate the AFM images with optical images within a few nanometers. We also newly defined the correlation accuracy in this paper as a useful concept for the correlation measurements. With this reliable correlation accuracy, we performed various statistical analyses to thoroughly elucidate the relationships between particle structure, Rayleigh scattering and SERS in terms of the incident polarization and scattering intensity ratio.

We present time-gated femtosecond stimulated Raman spectroscopy (fSRS) under the pre-resonance Raman conditions of diethylthiatricarbocyanine (DTTC) iodide. A ‘pseudo emission-free’ condition is achieved by delivering the probe beam ahead of the pump beam. Regeneratively amplified pulse trains are employed to create an angle-geometry (non-collimated) mixing between the pump and probe beams, leading to highly sensitive measurement of the stimulated Raman gain. Time-integrated spectroscopy allows for a more quantitative distinction between the contributions of stimulated Raman scattering and stimulated emission. We successfully obtain a highly sensitive (signal-to-noise ratio >100) stimulated Raman spectrum under the optimized conditions, which compares favourably to results obtained using two-dimensional correlation spectroscopy (2DCOS). Given the optical pre-resonance of ∼0.1 eV, the background signals mostly originate from the stimulated emission of excited electrons and are significantly reduced by partial overlapping of the pump and probe beams; a genuine fSRS spectral profile is obtained for a temporal delay of ∼0.2 ps between the two beams.

Overproduction of hydrogen peroxide is involved in the pathogenesis of inflammatory diseases such as cancer and arthritis. To image hydrogen peroxide via chemiluminescence resonance energy transfer in the near-infrared wavelength range, we prepared quantum dots functionalized with a luminol derivative.

Lanthanide-doped upconverting nanoparticles (UCNPs) have recently attracted enormous attention in the field of biological imaging owing to their unique optical properties: (1) efficient upconversion photoluminescence, which is intense enough to be detected at the single-particle level with a (nonscanning) wide-field microscope setup equipped with a continuous wave (CW) near-infrared (NIR) laser (980 nm), and (2) resistance to photoblinking and photobleaching. Moreover, the use of NIR excitation minimizes adverse photoinduced effects such as cellular photodamage and the autofluorescence background. Finally, the cytotoxicity of UCNPs is much lower than that of other nanoparticle systems. All these advantages can be exploited simultaneously without any conflicts, which enables the establishment of a novel UCNP-based platform for wide-field two-photon microscopy. UCNPs are also useful for multimodal in vivo imaging because simple variations in the composition of the lattice atoms and dopant ions integrated into the particles can be easily implemented, yielding various distinct biomedical activities relevant to magnetic resonance imaging (MRI), computed tomography (CT), and positron emission tomography (PET). These multiple functions embedded in a single type of UCNPs play a crucial role in precise disease diagnosis. The application of UCNPs is extended to therapeutic fields such as photodynamic and photothermal cancer therapies through advanced surface conjugation schemes.

This perspective presents an overview of single-molecule surface-enhanced Raman scattering (sm-SERS). Our overview is organized as a brief theoretical background, discussion of the factors that enhance SERS, various experimental preparations for inserting a single molecule in a hot spot, recent sm-SERS experiments, and a perspective. Although, there have been numerous review papers on sm-SERS, we mainly concentrated on the logical development of sm-SERS on the basis of the fundamental concepts and their physical significance, so that readers outside this field can understand the motivation and the underlying physics when describing current sm-SERS measurements. Indeed, understanding such current sm-SERS experiments conducted by representative groups would be very helpful for readers to answer for themselves the fundamental and practical questions surrounding sm-SERS: (1) what information can sm-SERS provide? (2) Which factors based on the SERS mechanism should be considered to significantly amplify the SERS signal? (3) What kinds of related microscopy techniques could be combined with sm-SERS to attain more meaningful results? (4) Which statistical approaches can be used and how they can be applied to properly analyze sm-SERS data? We hope that this review article can help readers answer these questions.

We report DNA-mediated simple synthetic methods to obtain anisotropic plasmonic nanostructures with a tailorable intra-nanogap distance ranging from 0.9 to 4.0 nm. Anisotropic half-shell structures with sub-1.0 nm intra-nanogaps showed a wavelength-independent surface-enhanced Raman scattering (SERS) intensity and a highly sensitive SERS response to NIR light. We found that the reaction conditions such as pH and NaCl concentration are responsible for the resulting shell structures and intra-nanogap distances. Three noticeable plasmonic nanostructures [i.e., half-shell with sub-1.0 nm nanogaps, closed-shell with a wide nanogap (2.1 nm) and star-shaped with an irregular nanogap (1.5–4.0 nm)] were synthesized, and solution-based and single particle-based Raman measurements showed a strong relationship between the plasmonic structures and the SERS intensity. An understanding of DNA-mediated control for nanogap-engineered plasmonic nanostructures and studies of SERS-activity relationships using single particle-correlated measurements can provide new insights into the design of new plasmonic nanostructures and SERS-based biosensing applications.