Tailoring Fluorescence-Phosphorescence Emission with a Single Nanocavity.

Realizing the dual emission of fluorescence-phosphorescence in a single system is an extremely important topic in the fields of biological imaging, sensing, and information encryption. However, the phosphorescence process is usually in an inherently "dark state" at room temperature due to the involvement of spin-forbidden transition and the rapid non-radiative decay rate of the triplet state. In this work, we achieved luminescent harvesting of the dark phosphorescence processes by coupling singlet-triplet molecular emitters with a rationally designed plasmonic cavity. The achieved Purcell enhancement effect of over 1000-fold allows for overcoming the triplet forbidden transitions, enabling radiation enhancement with selectable emission wavelengths. Spectral results and theoretical simulations indicate that the fluorescence-phosphorescence peak position can be intelligently tailored in a broad range of wavelengths, from visible to near-infrared. Our study sheds new light on plasmonic tailoring of molecular emission behavior, which is crucial for advancing research on plasmon-tailored fluorescence-phosphorescence spectroscopy in optoelectronics and biomedicine.

Highly sensitive and selective SERS substrates with 3D hot spot buildings for rapid mercury ion detection.

Heavy metal ions, which are over-emitted from industrial production, pose a major threat to the ecological environment and human beings. Among the present detection technologies, achieving rapid and on-site detection of contaminants remains a challenge. Herein, capillaries with three-dimensional (3D) hot spot constructures are fabricated to achieve repaid and ultrasensitive mercury ion (Hg2+) detection in water based on surface-enhanced Raman scattering (SERS). The 4-mercapto pyridine (4-Mpy) serves as the Raman reporter with high selectivity, enabling the detection of Hg2+ by changes in adsorption configuration at the trace level. Under optimized conditions, the SERS response of 4-Mpy for Hg2+ exhibits good linearity, ranging from 1 pM to 0.1 µM in a few minutes, and the detection limit of 0.2 pM is much lower than the maximum Hg2+ concentration of 10 nM allowed in drinking water, as defined by the US Environmental Protection Agency (EPA). Simultaneously, combined with the theoretical simulation and experimental results, the above results indicate that the SERS substrates possess outstanding performances in specificity, recovery rate and stability, which may hold great potential for achieving rapid and on-site environmental pollutant detection using a portable Raman spectrometer.

Advanced plasmonic technologies for multi-scale biomedical imaging.

The use of imaging technologies has been critical in deciphering biological phenomena, structures, and mechanisms across a wide range of spatial scales. The spatial resolution of traditional imaging modalities cannot meet the needs of high-precision research and diagnosis in biomedical fields. Plasmon resonance is the light-matter interaction that allows localizing far-field radiation in the near field with an intense electromagnetic field, enhancing the nanometric ablation, elastic/inelastic scattering of the adsorbate, and photoluminescence of the fluorophore nearby. Further, plasmon resonance scattering of nanoparticles can sensitively indicate the local environmental changes. This is accomplished by combining the spatially resolved capability with molecular spectrometry techniques such as Raman, infrared, fluorescence, etc., leading to a series of excellent imaging techniques to interrogate diverse biological processes from the tissue to subcellular level. In this tutorial review, we first provide the fundamental aspects of plasmonics. Then we give a systematic discussion of the working principle of these plasmon-based imaging techniques with an emphasis on the achievable spatial resolutions: surface-enhanced Raman spectroscopy (micrometre to nanometre), tip-enhanced ablation and ionization mass spectrometry (submicrometre), scattering-type scanning near-field optical microscopy (nanometre), tip-enhanced Raman spectroscopy (nanometre), tip-enhanced fluorescence spectroscopy (nanometre), and plasmon/molecular ruler microscopy (nanometre to angstrom). We also review the recent developments of the bioimaging applications of these techniques and expect that the plasmon-based techniques will not only pave a new way to decipher mysteries in life sciences but also hold great potential to be extended from fundamental research studies to real-life biomedical applications.

Rapid Point-of-Care Assay by SERS Detection of SARS-CoV-2 Virus and Its Variants.

Addressing the spread of coronavirus disease 2019 (COVID-19) has highlighted the need for rapid, accurate, and low-cost diagnostic methods that detect specific antigens for SARS-CoV-2 infection. Tests for COVID-19 are based on reverse transcription PCR (RT-PCR), which requires laboratory services and is time-consuming. Here, by targeting the SARS-CoV-2 spike protein, we present a point-of-care SERS detection platform that specifically detects SARS-CoV-2 antigen in one step by captureing substrates and detection probes based on aptamer-specific recognition. Using the pseudovirus, without any pretreatment, the SARS-CoV-2 virus and its variants were detected by a handheld Raman spectrometer within 5 min. The limit of detection (LoD) for the pseudovirus was 124 TU µL-1 (18 fM spike protein), with a linear range of 250-10,000 TU µL-1. Moreover, this assay can specifically recognize the SARS-CoV-2 antigen without cross reacting with specific antigens of other coronaviruses or influenza A. Therefore, the platform has great potential for application in rapid point-of-care diagnostic assays for SARS-CoV-2.

Shell-Isolated Nanoparticle-Enhanced Electrochemiluminescence.

Enhanced electrochemiluminescence (ECL) aims to promote higher sensitivity and obtain better detection limit. The core-shell nanostructures, owing to unique surface plasmon resonance (SPR) enabling distance-dependent strong localized electromagnetic field, have attracted rising attention in enhanced ECL research and application. However, the present structures usually with porous shell involve electrocatalytic activity from the metal core and adsorption effect from the shell, which interfere with practical SPR enhancement contribution to ECL signal. Herein, to exclude the interference and unveil exact SPR-enhanced effect, shell-isolated nanoparticles (SHINs) whose shell gets thicker and becomes pinhole-free are developed by modifying pH value and particles concentration. Furthermore, allowing for the distribution of hotspots and stronger enhancement, excitation intensity and ECL reaction layer thickness are mainly investigated, and several types of SHINs-enhanced ECL platforms are prepared to fabricate distinct hotspot distribution via electrostatic attraction (submonolayer) and a layer-by-layer deposition method (monolayer). Consequently, the strongest enhancement up to ≈250-fold is achieved by monolayer SHINs with 10 nm shell, and the platform is applied in a "turn-off" mode sensing for dopamine. The platform provides new guidelines to shell preparation, interface engineering and hotspots fabrication for superior ECL enhancement and analytical application with high performance.

Shell-Isolated Nanoparticle-Enhanced Luminescence of Metallic Nanoclusters.

Metallic nanoclusters (NCs) have molecular-like structures and unique physical and chemical properties, making them an interesting new class of luminescent nanomaterials with various applications in chemical sensing, bioimaging, optoelectronics, light-emitting diodes (LEDs), etc. However, weak photoluminescence (PL) limits the practical applications of NCs. Herein, an effective and facile strategy of enhancing the PL of NCs was developed using Ag shell-isolated nanoparticle (Ag SHIN)-enhanced luminescence platforms with tuned SHINs shell thicknesses. 3D-FDTD theoretical calculations along with femtosecond transient absorption and fluorescence decay measurements were performed to elucidate the enhancement mechanisms. Maximum enhancements of up to 231-fold for the [Au7Ag8(C≡CtBu)12]+ cluster and 126-fold for DNA-templated Ag NCs (DNA-Ag NCs) were achieved. We evidenced a novel and versatile method of achieving large PL enhancements with NCs with potential for practical biosensing applications for identifying target DNA in ultrasensitive surface analysis.

Elucidating Molecule-Plasmon Interactions in Nanocavities with 2†nm Spatial Resolution and at the Single-Molecule Level.

The fundamental understanding of the subtle interactions between molecules and plasmons is of great significance for the development of plasmon-enhanced spectroscopy (PES) techniques with ultrahigh sensitivity. However, this information has been elusive due to the complex mechanisms and difficulty in reliably constructing and precisely controlling interactions in well-defined plasmonic systems. Herein, the interactions in plasmonic nanocavities of film-coupled metallic nanocubes (NCs) are investigated. Through engineering the spacer layer, molecule-plasmon interactions were precisely controlled and resolved within 2 nm. Efficient energy exchange interactions between the NCs and the surface within the 1-2 nm range are demonstrated. Additionally, optical dressed molecular excited states with a huge Lamb shift of ≈7 meV at the single-molecule (SM) level were observed. This work provides a basis for understanding the underlying molecule-plasmon interaction, paving the way for fully manipulating light-matter interactions at the nanoscale.

Shell-Isolated Nanoparticle-Enhanced Phosphorescence.

The emerging field of plasmonics has promoted applications of optical technology, especially in plasmon-enhanced spectroscopy (PES). However, in plasmon-enhanced fluorescence (PEF), "metal loss" could significantly quench the fluorescence during the process, which dramatically limits its applications in analysis and high-resolution imaging. In this report, silver core silica shell-isolated nanoparticles (Ag@SiO2 NPs or SHINs) with a tunable thickness of shell are used to investigate the interactions between NPs and emitters by constructing coupling and noncoupling modes. The plasmonic coupling mode between Ag@SiO2 NPs and Ag film reveals an exceeding integrating spectral intensity enhancement of 330 and about 124 times that of the radiative emission rate acceleration for shell-isolated nanoparticle enhanced phosphorescence (SHINEP). The experimental findings are supported by theoretical calculations using the finite-element method (FEM). Hence, the SHINEP may provide a novel approach for understanding the interaction of plasmon and phosphorescence, and it holds great potential in surface detection analysis and singlet-oxygen-based clinical therapy.

Quantitative Analysis of Trace Analytes with Highly Sensitive SERS Tags on Hydrophobic Interface.

Surface-enhanced Raman scattering (SERS) presents a promising avenue for trace matter detection by using plasmonic nanostructures. To tackle the challenges of quantitatively analyzing trace substances in SERS, such as poor enrichment efficiency and signal reproducibility, this study proposes a novel approach using Au@internal standard@Au nanospheres (Au@IS@Au NSs) for realizing the high sensitivity and stability in SERS substrates. To verify the feasibility and stability of the SERS performances, the SERS substrates have exhibited exceptional sensitivity for detecting methyl blue molecules in aqueous solutions within the concentration range from 10-4 M to 10-13 M. Additionally, this strategy also provides a feasible way of quantitative detection of antibiotic in the range of 10-4 M to 10-10 M. Trace antibiotic residue on the surface of shrimp in aquaculture waters was successfully conducted, achieving a remarkably low detection limit of 10-9 M. The innovative approach has great potential for the rapid and quantitative detection of trace substances, which marks a noteworthy step forward in environmental detection and analytical methods by SERS.

Highly sensitive SERS substrates with multi-hot spots for on-site detection of pesticide residues.

Pesticide residues will be a huge threat to food security and ecological environment; therefore, there is an urgent need to achieve rapid and on-site detection of pesticide residues. Herein, a plasmonic substrate with multiple "hot spots" was fabricated by transferring three-dimensional (3D) Au nanoparticles (NPs) onto the polydimethylsiloxane (PDMS) membrane for highly sensitive surface-enhanced Raman scattering (SERS) detection of pesticide residues. In combination with 3D-FDTD simulations, high SERS enhancement (EF = 1.2 × 108) and high detection sensitivity (LOD = 6.3 × 10-10 M) were achieved, mainly due to the enhanced electromagnetic fields around the "hot spots". Additionally, the PDMS-based SERS substrate held good transparency and flexibility, enabling conformal contact with non-planar surfaces and allowing the laser to penetrate the back of the analytes. Combined with a portable Raman spectrometer, the substrates holds great potential for rapid, high-sensitive, and on-site detection of contaminants in food, especially for the analyte on the nonplanar surfaces.

Label-free SERS strategy for rapid detection of capsaicin for identification of waste oils.

Identification of waste oils is challenging in the field of food safety due to the lack of common markers and straightforward analytical methods. Herein, we developed a novel label-free surface-enhanced Raman spectroscopy (SERS) strategy to identify waste oils using Ag nanoparticles solution (Ag NPs sol.) as a SERS substrate to significantly enhance the Raman signal of capsaicin marker molecule usually contained in the waste oils. The enhanced signal was directly detected by a portable Raman spectrometer with the limit of detection (LOD) of 2.9 µg L-1 within 10 min. Concentration-dependent SERS investigation showed the linear relationship between the SERS signal intensity of the characteristic peaks and the concentrations of capsaicin in the range of 10-2500 µg L-1 and the correlation coefficient was 0.9895. Our findings show the sensitivity, accessibility, and reliability of this method for the rapid identification of waste oils and furthermore for the practical applications in the field of food safety.