Core–shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core–shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core–shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core–shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long-standing limitations of material and morphology generality encountered in traditional SERS. We then explain the SERS-enhancement mechanism with core–shell nanoparticles, as well as three generations of SERS hotspots for surface analysis of materials. To provide a clear view for readers, we summarize various approaches for the synthesis of core–shell nanoparticles and their applications in SERS, such as electrochemistry, bioanalysis, food safety, environmental safety, cultural heritage, materials, catalysis, and energy storage and conversion. Finally, we exemplify about the future developments in new core–shell nanomaterials with different functionalities for SERS and other surface-enhanced spectroscopies.

•Ag@SiO2 with tuneable thickness shell modify exciton excitation and emission rate.•A general method for enhancing the fluorescence of different quantum dots has been established.•The enhancement of H2 production is due to the effect of boosting excitons instead of “hot” electrons.Optically tunable quantum dots (QDs) have drawn significant attention for optoelectronic applications such as light-emitting diodes, photovoltaics, and photodetectors. However, when QDs are assembled as films, the quantum yield decreases significantly, known as the “self-quenching” effect. In this study, we develop a general method for suppressing this self-quenching effect and enhancing energy conversion efficiency using shell-isolated nanoparticles (SHINs), which efficiently promote spontaneous emission of diverse QD films to picosecond timescale. We discover that Ag SHINs with controllable thickness shells exhibit different enhancement factors due to the competition between radiative and non-radiative decay, and localized surface plasmon resonance (LSPR) in nanocavities enhances the fluorescence of QD monolayer films up to nearly 1000 times by SHINs with 6 nm shell. In addition, acting as nanoantennas and amplifying the local photon density, SHINs also effectively enhance excitons excitation and improve the H2 evolution performance of QD-PEC to nearly 12 µmol/h in neutral solution without “hot” electrons effect.Download high-res image (173KB)Download full-size image

•SHINERS technology is extended to Au(111) surface in nonaqueous solutions.•SPR-induced “hot electrons” catalyze hydrogenation reaction on a roughened Au surface.•Shell-isolated nanoparticles isolate “hot electrons” from Au nanoparticles.•Aprotic acetonitrile has no hydrogen source for catalytic hydrogenation.Surface-enhanced Raman spectroscopy (SERS) studies of electrode/solution interfaces are important for understanding electrochemical processes. However, revealing the nature of reactions at well-defined single crystal electrode surfaces, which are SERS-inactive, remains challenging. In this work, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used for the first time to study electrochemical adsorption and hydrogenation reactions at single crystal surfaces in nonaqueous solvents. A roughened Au surface was also studied for comparison. The experimental results show that the hydrogenation of adsorbed p-ethynylaniline (PEAN) on roughened Au electrode surfaces occurred at very negative potentials in methanol because of the catalytic effect of surface plasmon resonance (SPR). However, because “hot electrons” were blocked by the silica shell of Au@SiO2 nanoparticles and aprotic acetonitrile was an ineffective hydrogen source, surface reactions at Au(111) were inhibited in the systems studied. Density functional theory (DFT) calculations revealed that the PEAN triple bond opened, allowing adsorption in a flat configuration on the Au(111) surface via two carbon atoms. This work provides an advanced understanding of electrochemical interfacial processes at single crystal surfaces in nonaqueous systems.Download high-res image (358KB)Download full-size image

•EC-SHINERS is extended to the in situ study of electrochemical reactions on practical nanocatalysts.•The activity for CO electrooxidation is greatly enhanced on PtFe bimetallic nanocatalysts compared to Pt.•The PtCO stretching bands shift to lower frequencies as the Fe content increases.Combination of spectroscopic techniques with electrochemistry is a promising way to elucidate electrocatalytic mechanisms at a molecular level. Surface-enhanced Raman spectroscopy (SERS) is a non-destructive, ultrasensitive fingerprint technique that can detect metal–adsorbate and metal oxide vibrations. However, it is hard to study nanocatalysts because of the morphology limitation of SERS. In this paper, core-shell-satellite nanostructures have been fabricated by assembling PtFe nanocatalysts on shell-isolated nanoparticles (SHINs). CO electrooxidation on these nanostructures was studied by in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS). The in situ spectroscopic results correlate well with the results obtained using cyclic voltammetry, and show that PtFe bimetallic nanocatalysts have improved CO electrooxidation activity. The in situ SHINERS studies also show that this improvement is the result of weaker CO adsorption on PtFe compared to Pt, as revealed by the red shift of the Raman band of the PtC stretching vibration on PtFe. This work demonstrates that this method is a powerful tool for in situ investigation of the electrocatalytic processes occurring on nanocatalysts and provides a better understanding of the reaction mechanisms.Download high-res image (273KB)Download full-size image

Surface plasmon resonance (SPR) has been utilized in many fields, such as surface-enhanced Raman spectroscopy (SERS) and solar energy conversion. Here we developed an Au@CdS core–shell nanostructure, a bifunctional nanoparticle, used as an efficient catalyst for SPR enhanced photocatalytic degradation, and as a substrate for in situ SERS detection of methylene blue (MB) and p-nitrophenol (pNTP). With integration of an Au nanoparticle into a CdS shell, the degradation process was significantly accelerated under 500 nm long-pass (λ > 500 nm) visible light irradiation, which was caused by the injection of hot electrons. Moreover, a highly uniform, monolayer film of Au@CdS nanoparticles (NPs) has been prepared and used as both a SERS substrate and catalyst. The decomposition of MB molecules and nitrogen coupling reaction of pNTP were observed during the 638 nm laser illumination. We demonstrate that a plasmonic core-semiconductor shell nanocomposite can be a promising material for photocatalysis and in situ SERS study.

Fluorescence spectroscopy with strong emitters is a remarkable tool with ultra-high sensitivity for detection and imaging down to the single-molecule level. Plasmon-enhanced fluorescence (PEF) not only offers enhanced emissions and decreased lifetimes, but also allows an expansion of the field of fluorescence by incorporating weak quantum emitters, avoiding photobleaching and providing the opportunity of imaging with resolutions significantly better than the diffraction limit. It also opens the window to a new class of photostable probes by combining metal nanostructures and quantum emitters. In particular, the shell-isolated nanostructure-enhanced fluorescence, an innovative new mode for plasmon-enhanced surface analysis, is included. These new developments are based on the coupling of the fluorophores in their excited states with localized surface plasmons in nanoparticles, where local field enhancement leads to improved brightness of molecular emission and higher detection sensitivity. Here, we review the recent progress in PEF with an emphasis on the mechanism of plasmon enhancement, substrate preparation, and some advanced applications, including an outlook on PEF with high time- and spatially resolved properties.

Nanostructures with symmetry breaking can allow the coupling between dark and bright plasmon modes to induce strong Fano resonance. However, it is still a daunting challenge to prepare bottom-up self-assembled subwavelength asymmetric nanostructures with appropriate gaps between the nanostructures especially below 5 nm in solution. Here we present a viable self-assembly method to prepare symmetry-breaking nanostructures consisting of Ag nanocubes and Au nanospheres both with tunable size (90–250 nm for Au nanospheres; 100–160 nm for Ag nanocubes) and meanwhile control the nanogaps through ultrathin silica shells of 1–5 nm thickness. The Raman tag of 4-mercaptobenzoic acid (MBA) assists the self-assembly process and endows the subwavelength asymmetric nanostructures with surface-enhanced Raman scattering (SERS) activity. Moreover, thick silica shells (above 50 nm thickness) can be coated on the self-assembled nanostructures in situ to stabilize the whole nanostructures, paving the way toward bioapplications. Single particle scattering spectroscopy with a 360° polarization resolution is performed on individual Ag nanocube and Au nanosphere dimers, correlated with high-resolution TEM characterization. The asymmetric dimers exhibit strong configuration and polarization dependence Fano resonance properties. Overall, the solution-based self-assembly method reported here is opening up new opportunities to prepare diverse multicomponent nanomaterials with optimal performance.

Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is employed to investigate the electrochemical behavior of adenine molecules on smooth Ag electrodes. To attain this goal, pinhole-free shell-isolated Ag nanoparticles (Ag SHINs) have been synthesized and then used as signal “amplifiers” in the gap-mode configuration (Ag SHINs are coupled with a Ag electrode surface). The as-prepared Ag SHINs exhibit remarkable plasmonic performance under 488, 532, and 633 nm excitations as revealed by finite-difference time-domain (FDTD) simulations and SHINERS experiments. Furthermore, wavelength-dependent SHINERS investigation of adenine on Ag electrodes is excellently combined with the electrochemical technique. With outstanding chemical stability and plasmonic property, the Ag SHINs are extraordinarily suitable for fundamental studies at various electrochemical interfaces.

•In-situ electrochemical SHINERS was employed to study adsorption and redox transformation process of viologen at Au(hkl).•Electrochemical behaviors correlated very well with the operando SHINERS data.•Facet-dependence of SERS enhancements were observed on Au(hkl) and well explained by theoretical simulations.•Nonlinear resonant Raman process was identified in our system.In-situ Raman/SERS studies of molecular adsorption/reaction behaviors at well-defined electrochemical interfaces are important for understanding the fundamentals of electrochemical processes. However, it is still a great challenge to perform such studies on model single-crystal surfaces as the smooth surface cannot support surface plasmon resonance (SPR). In this work, shell-isolated nanoparticle-enhanced Raman spectroscopy was combined with an electrochemical method (EC-SHINERS) to study the adsorption and redox transformation of a resonant molecule viologen HS-8V8H at Au(hkl) single-crystal electrodes. Changes in the molecular structure with potential were identified on different single-crystal surfaces, which explained the transformation process of viologen from V2 + state to V+ and then V0. Facet-dependent SERS enhancement was also observed, which results from the different imaginary part of the dielectric function on Au(111), Au(100) and Au(110), and is supported by the FEM simulations. Furthermore, a nonlinear resonant Raman process has been directly observed in our experiments, which is consistent with the simulation results. These findings increase our understanding of the electrochemical behavior of molecules in model systems.

Correction for ‘Shell-isolated nanoparticle-enhanced Raman spectroscopy study of the adsorption behaviour of DNA bases on Au(111) electrode surfaces’ by Bao-Ying Wen et al., Analyst, 2016, DOI: 10.1039/c6an00180g.

For the first time, we used the electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) technique to in situ characterize the adsorption behaviour of four DNA bases (adenine, guanine, thymine, and cytosine) on atomically flat Au(111) electrode surfaces. The spectroscopic results of the various molecules reveal similar features, such as the adsorption-induced reconstruction of the Au(111) surface and the drastic Raman intensity reduction of the ring breathing modes after the lifting reconstruction. As a preliminary study of the photo-induced charge transfer (PICT) mechanism, the in situ spectroscopic results obtained on single crystal surfaces are excellently illustrated with electrochemical data.

•The principles of combination systems of scanning probe microscopy (SPM) and mass spectrometry (MS) have been described.•The applications of SPM–MS have been reviewed.•The strengths and the limitations of SPM–MS have been discussed.•Future development trends of SPM–MS have been predicted.The simultaneous acquisition of morphological and chemical information from solid surfaces with a nanoscale lateral resolution is being increasingly considered in modern materials and biological research. Due to their respective advantages, combining scanning probe microscopy (SPM) and mass spectrometry (MS) with field evaporation, thermal desorption, and near-field enhancement is a perfect approach that satisfies the above-outlined research requirements.This review covers the analytical trends of these combined techniques. Relevant research from recent years will be summarized, the principles of the representative techniques highlighted, applications briefly introduced, and potential problems discussed.

In heterogeneous catalysis, characterization of heterogeneous metal interfaces of bimetallic catalysts is a crucial step to elucidate the catalytic performance and is a key to develop advanced catalysts. However, analytical techniques such as X-ray photoelectron spectroscopy can only work in vacuum conditions and are difficult to use for in situ analysis. Here, we present efficient and convenient core–shell nanoparticle-enhanced Raman spectroscopy to explore the in situ electronic structures of heterogeneous interfaces (Au@Pd and Au@Pt core–shell NPs) by varying the shell thickness. The experimental observations reported here clearly show that Pd donates electrons to Au, while Pt accepts electrons from Au at the heterogeneous interfaces. This conclusion gains further support from ex situ X-ray photoelectron spectroscopy results. The Au core greatly affects the electronic structures of both the Pd and Pt shells as well as catalytic behaviors. Finally, the as-prepared core–shell nanoparticles were used to demonstrate their improved catalytic properties in real electrocatalytic systems such as methanol oxidation and oxygen reduction reactions.

Tip-enhanced Raman spectroscopy (TERS) provides topographic and chemical information simultaneously with a spatial resolution well below the optical diffraction limit; however, the typically used noble-metal nanotips are prone to unwanted chemisorption, and when silver is used, the enhancement quickly fades due to chemical degradation during storage and use. In the present work, we demonstrate a method to produce strongly enhancing tips for TERS, which are protected against undesired chemisorption and (in the case of silver) have a significantly longer lifetime than unprotected tips. This was achieved by chemical coating of an ultrathin (few nanometers) and pinhole-free silica shell on top of the enhancing noble-metal nanostructures. The enhancement achieved with the protected silver tips was comparable with the typical tip enhancement for TERS; that is, no major constraints for TERS experiments resulted from the protective coatings. These shell-isolated tips could widen the scope of tip-enhanced Raman spectroscopy by potentially allowing for in situ studies of, for example, surface catalysis and biological systems.

Surface-enhanced Raman scattering (SERS) is a powerful technique that provides fingerprint vibrational information with ultrahigh sensitivity. However, only a few metals (gold, silver and copper) yield a large SERS effect, and they must be rough at the nanoscale. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was developed to overcome the long-standing materials and morphological limitations of SERS. It has already been applied in a variety of fields such as materials science, electrochemistry, surface science, catalysis, food safety and the life sciences. Here, the principles and applications of SHINERS are highlighted. To provide an understanding of the plasmonics involved, finite-difference time-domain (FDTD) calculations and single nanoparticle SHINERS experiments are reviewed. Next, various shell-isolated nanoparticle (SHIN) types are described. Then a number of applications are discussed. In the first application, SHINERS is used to characterize the adsorption processes of pyridine on Au(hkl) single-crystal electrode surfaces. Then, SHINERS' applicability to food inspection and cultural heritage science is demonstrated by the detection of parathion and fenthion pesticides, and Lauth's violet (thionine dye). Finally, graphene-isolated Au nanoparticles (GIANs) are shown to be effective for multimodal cell imaging, photothermal cancer therapy and photothermally-enhanced chemotherapy. SHINERS is a fast, simple and reliable method, suitable for application to many areas of science and technology. The concept of shell-isolation can also be applied to other surface-enhanced spectroscopies such as fluorescence, infrared absorption and sum frequency generation.

Attainment of spatial resolutions far below diffraction limits by means of optical methods constitutes a challenging task. Here, we design nonlinear nanorulers that are capable of accomplishing approximately 1 nm resolutions by utilizing the mechanism of plasmon-enhanced second-harmonic generation (PESHG). Through introducing Au@SiO2 (core@shell) shell-isolated nanoparticles, we strive to maneuver electric-field-related gap modes such that a reliable relationship between PESHG responses and gap sizes, represented by “PESHG nanoruler equation”, can be obtained. Additionally validated by both experiments and simulations, we have transferred “hot spots” to the film-nanoparticle-gap region, ensuring that retrieved PESHG emissions nearly exclusively originate from this region and are significantly amplified. The PESHG nanoruler can be potentially developed as an ultrasensitive optical method for measuring nanoscale distances with higher spectral accuracies and signal-to-noise ratios.

Silver is an ideal candidate for surface plasmon resonance (SPR)-based applications because of its great optical cross-section in the visible region. However, the uses of Ag in plasmon-enhanced spectroscopies have been limited due to their interference via direct contact with analytes, the poor chemical stability, and the Ag+ release phenomenon. Herein, we report a facile chemical method to prepare shell-isolated Ag nanoparticle/tip. The as-prepared nanostructures exhibit an excellent chemical stability and plasmonic property in plasmon-enhanced spectroscopies for more than one year. It also features an alternative plasmon-mediated photocatalysis pathway by smartly blocking “hot” electrons. Astonishingly, the shell-isolated Ag nanoparticles (Ag SHINs), as “smart plasmonic dusts”, reveal a ∼1000-fold ensemble enhancement of rhodamine isothiocyanate (RITC) on a quartz substrate in surface-enhanced fluorescence. The presented “smart” Ag nanostructures offer a unique way for the promotion of ultrahigh sensitivity and reliability in plasmon-enhanced spectroscopies.

Electrochemical methods are combined with shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) for a comprehensive study of pyridine adsorption on Au(111), Au(100) and Au(110) single crystal electrode surfaces. The effects of crystallographic orientation, pyridine concentration, and applied potential are elucidated, and the formation of a second pyridine adlayer on Au(111) is observed spectroscopically for the first time. Electrochemical and SHINERS results correlate extremely well throughout this study, and we demonstrate the potential of EC-SHINERS for thorough characterization of processes occurring on single crystal surfaces. Our method is expected to open up many new possibilities in surface science, electrochemistry and catalysis. Analytical figures of merit are discussed.

Identifying the intermediate species in an electrocatalytic reaction can provide a great opportunity to understand the reaction mechanism and fabricate a better catalyst. However, the direct observation of intermediate species at a single crystal surface is a daunting challenge for spectroscopic techniques. In this work, electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) is utilized to in situ monitor the electrooxidation processes at atomically flat Au(hkl) single crystal electrode surfaces. We systematically explored the effects of crystallographic orientation, pH value, and anion on electrochemical behavior of intermediate (AuOH/AuO) species. The experimental results are well correlated with our periodic density functional theory calculations and corroborate the long-standing speculation based on theoretical calculations in previous electrochemical studies. The presented in situ electrochemical SHINERS technique offers a unique way for a real-time investigation of an electrocatalytic reaction pathway at various well-defined noble metal surfaces.

•The development history of SHINERS technique is introduced.•The applications of SHINERS for in situ monitoring the electrochemical interface reaction process at single crystal surfaces are discussed in detail.•Practical applications of SHINERS at electrochemical interfaces in aqueous and non-aqueous systems are demonstrated.Electrochemical solid/liquid interface earned huge interest due to its wide range of application in various fundamental research fields. Combining with electrochemical methods, shell-isolated nanoparticles-enhanced Raman spectroscopy (SHINERS) can be employed as a promising technique for better understanding of interface reaction processes. In recent times, it has received great attention by research community as it is universally applicable to many material and morphology surfaces in electrochemical and surface science fields. Based on the recent progress of EC-SHINERS, we first briefly reviewed the development of SHINERS history, and then followed with a detailed introduction of SHINERS application in various electrochemical interfaces studies, including the molecule adsorption, corrosion resistance, catalytic reactions and other applications in different fields from aqueous to non-aqueous systems.

Fluorescence spectroscopy with strong emitters is a remarkable tool with ultra-high sensitivity for detection and imaging down to the single-molecule level. Plasmon-enhanced fluorescence (PEF) not only offers enhanced emissions and decreased lifetimes, but also allows an expansion of the field of fluorescence by incorporating weak quantum emitters, avoiding photobleaching and providing the opportunity of imaging with resolutions significantly better than the diffraction limit. It also opens the window to a new class of photostable probes by combining metal nanostructures and quantum emitters. In particular, the shell-isolated nanostructure-enhanced fluorescence, an innovative new mode for plasmon-enhanced surface analysis, is included. These new developments are based on the coupling of the fluorophores in their excited states with localized surface plasmons in nanoparticles, where local field enhancement leads to improved brightness of molecular emission and higher detection sensitivity. Here, we review the recent progress in PEF with an emphasis on the mechanism of plasmon enhancement, substrate preparation, and some advanced applications, including an outlook on PEF with high time- and spatially resolved properties.

Surface-enhanced Raman scattering (SERS) is a powerful technique that provides fingerprint vibrational information with ultrahigh sensitivity. However, only a few metals (gold, silver and copper) yield a large SERS effect, and they must be rough at the nanoscale. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was developed to overcome the long-standing materials and morphological limitations of SERS. It has already been applied in a variety of fields such as materials science, electrochemistry, surface science, catalysis, food safety and the life sciences. Here, the principles and applications of SHINERS are highlighted. To provide an understanding of the plasmonics involved, finite-difference time-domain (FDTD) calculations and single nanoparticle SHINERS experiments are reviewed. Next, various shell-isolated nanoparticle (SHIN) types are described. Then a number of applications are discussed. In the first application, SHINERS is used to characterize the adsorption processes of pyridine on Au(hkl) single-crystal electrode surfaces. Then, SHINERS' applicability to food inspection and cultural heritage science is demonstrated by the detection of parathion and fenthion pesticides, and Lauth's violet (thionine dye). Finally, graphene-isolated Au nanoparticles (GIANs) are shown to be effective for multimodal cell imaging, photothermal cancer therapy and photothermally-enhanced chemotherapy. SHINERS is a fast, simple and reliable method, suitable for application to many areas of science and technology. The concept of shell-isolation can also be applied to other surface-enhanced spectroscopies such as fluorescence, infrared absorption and sum frequency generation.