Highly sensitive and uniform surface-enhanced Raman spectroscopy from grating-integrated plasmonic nanograss

Grating-Integrated Gold Nanograsses Encapsulated with ZIF-8: A Quantitative and Ultrasensitive Surface-Enhanced Raman Scattering Substrate

We proposed and demonstrated highly sensitive hybrid surface-enhanced Raman scattering (SERS) substrates, which are grating-integrated gold nanograsses (GIGN) that are tip-selectively encapsulated by ZIF-8 nanospheres (GIGN/tip-ZIF). This unique structure is realized through the tip-selective modification of GIGN by polyvinylpyrrolidone (PVP), and then, the tips of the GIGN were encapsulated by ZIF-8 nanospheres. The ZIF-8 nanospheres can adsorb analyte molecules, resulting in the spatial overlap between the analyte molecules and the "hotspots" on the tips of GIGN. Such a unique GIGN/tip-ZIF hybrid SERS substrate exhibits high sensitivity and quantitative detection ability. The detection limits can reach as low as 10-11 M, and the relative standard deviation is 5.59% for 4-aminothiophenol (4-ATP). In a wide range of concentrations from 10-5 to 10-11 M, the SERS intensity and concentration relationship can be fitted as a sigmoidal curve with R2 = 0.988. These indicate that the GIGN/tip-ZIF hybrid SERS substrates have broad applications in detecting toxic and harmful substances in food safety, disease diagnosis, and environmental monitoring.

Plasma amplifiers: multiscale light-enhanced uniform SERS composite substrates for breaking through resonance limitations

A phenomenon known as plasmon resonance constitutes a unique optical effect that can induce an enhancement in localized electromagnetic fields, resulting in a substantial increase in the electromagnetic field intensity surrounding metallic nanostructures. In this work, the coupling effect of excitation of surface plasmon polaritons and local surface plasmons in nanoparticles is deeply studied under the background of nanoparticles/one-dimension grating composite structures through grating matching. By employing finite-difference time-domain simulations as our methodological approach, we discern gratings with a periodicity of 1.5 μm support surface plasmon bound states between the gratings. Furthermore, the modulation of SPs along the vertical sidewalls of the grating due to standing wave effects exhibits oscillatory behavior with varying grating heights. Experimental results obtained from the nanoparticle/grating composite SERS substrate validate theoretical predictions, demonstrating higher enhanced Raman signals at 633 nm compared to 532 nm. Remarkably, this structure exhibits good performance, with R6G detection sensitivity down to concentrations as low as 10-10 M and mapping achieving a relative standard deviation of 7.79%, underscoring its uniformity and capability of electromagnetic field enhancement.

Double-sided plasmonic metasurface for simultaneous biomolecular separation and SERS detection

Porous membrane-based nanofiltration separation of small biomolecules is a widely used biotechnology for which size-based selectivity is a critical parameter of technological relevance. Efficient determination of size selectivity calls for an advanced detection method capable of performing sensitive, rapid, and on-membrane examination. Surface-enhanced Raman spectroscopy (SERS) is such a detection method that has been widely recognized as an ultrasensitive technique for trace-level detection with sensitivity down to the single-molecule level. In this work, we for the first time develop a double-sided hierarchical porous membrane-like plasmonic metasurface to realize high-selectivity bimolecular separation and simultaneous ultrasensitive SERS detection. This highly flexible device, consisting of subwavelength nanocone pairs surrounded by randomly orientated sub-5 nm nanogrooves, was prepared by combining customized "top-down" fabrication of conical nanopores in an ion-track registered polycarbonate membrane and self-assembly of nanogrooves on the membrane surface through physical vapor deposition. The unique tip-to-tip oriented conical nanopores in the device enables excellent size-based molecular selectivity; the hierarchical groove-pore structure supports a peculiar cascaded electromagnetic near-field enhancement mechanism, endowing the device with SERS-based molecular detection of ultrahigh sensitivity, uniformity, repeatability, and polarization independence. With such dual structural merits and performance enhancement, we demonstrate effective nanofiltration separation of small-sized adenine from big-sized ss-DNA and synergistic SERS determination of their species. We experimentally demonstrate an ultrasensitive detection of 4-mercaptopyridine down to 10 pM. Together with its unparalleled mechanical flexibility, this double-side-responsive plasmonic metasurface membrane can find great potential in real-world molecular filtration and detection under extremely complex working conditions.

Particle-in-Molybdenum Disulfide-Coated Cavity Structure with a Raman Internal Standard for Sensitive Raman Detection of Water Contaminants from Ions to <300 nm Nanoplastics

To develop a universal and precise detection strategy that can be applied to water contaminants of various sizes, we designed a particle-in-MoS₂ coated cavity structure of AAO/MoS₂/Ag with a Raman internal standard. This modified particle-in-cavity structure not only successfully integrates both "surface hot spots" and "volume hot spots" via dressing and manipulating the cascaded optical-field mode inside the cavity but also introduces the chemical enhancement and internal standard attribute of MoS₂. Because of its unique three-dimensional structure, AAO/MoS₂/Ag accurately detects water contaminants of various sizes from ions to nanoplastics (<300 nm) for the first time. This work proposes a novel and universal surface-enhanced Raman scattering strategy for detecting multiple-size water contaminants and demonstrates the potential to build a security line in early warning systems for the prevention of water pollution.

Coupling enhancement mechanisms, materials, and strategies for surface-enhanced Raman scattering devices

Surface-enhanced Raman scattering (SERS) has become one of the most sensitive analytical techniques for identifying the chemical components, molecular structures, molecular conformations, and the interactions between molecules. However, great challenges still need to be addressed until it can be widely accepted by the absolute quantification of analytes. Recently, many efforts have been devoted to addressing these issues via various electromagnetic (EM), chemical (CM), and EM-CM hybrid coupling enhancement strategies. In comparison with uncoupled SERS devices, they offer key advantages in terms of sensitivity, reproducibility, uniformity, stability, controllability and reliability. This review provides an in-depth analysis of coupled SERS devices, including coupling enhancement mechanisms, materials and approaches. Finally, we also discuss the remaining bottlenecks and possible strategies for the development of coupling-enhanced SERS devices in the future.

Octahedral silver oxide nanoparticles enabling remarkable SERS activity for detecting circulating tumor cells

The detection of circulating tumor cells (CTCs) is a crucial tool for early cancer diagnosis, prognosis, and postoperative evaluation. However, detection sensitivity remains a major challenge because CTCs are extremely rare in peripheral blood. To effectively detect CTCs, octahedral Ag₂O nanoparticles (NPs) with high dispersibility, good biocompatibility, remarkable surface-enhanced Raman scattering (SERS) enhancement, and obvious enhancement selectivity are designed as an SERS platform. Ag₂O NPs with many oxygen vacancy defects are successfully synthesized, which exhibit an ultra-high SERS enhancement factor (1.98×10⁶) for 4-mercaptopyridine molecules. The remarkable SERS activity of octahedral Ag₂O NPs is derived from the synergistic effect of the surface defect-promoted photo-induced charge transfer (PICT) process and strong vibration coupling resonance in the Ag₂O-molecule SERS complex, greatly amplifying the molecular Raman scattering cross-section. The promoted PICT process is confirmed using ultraviolet-visible (UV-Vis) absorption spectroscopy, demonstrating that obvious PICT resonance occurs in Ag₂O SERS system under visible light. An additional growth step of SERS bioprobe is proposed by modifying the Raman signal molecules and functional biological molecules on Ag₂O NPs for CTC detection. The Ag₂O-based SERS bioprobe exhibits excellent detection specificity for different cancer cells in rabbit blood. Importantly, the high-sensitivity Ag₂O-based SERS bioprobe satisfies the requirement for rare CTC detection in the peripheral blood of cancer patients, and the detection limit can reach 1 cell per mL. To our knowledge, this study is the first time that a semiconductor SERS substrate has been successfully utilized in CTC detection. This work provides new insights into CTC detection and the development of novel semiconductor-based SERS platforms for cancer diagnosis.

Hierarchical growth and morphological control of ordered Cu–Au alloy arrays with high surface enhanced Raman scattering activity

Low-cost Cu–Au alloy hierarchical structures are fabricated by coelectrodeposition, and the highest SERS activity is obtained when the atom ratio of Cu and Au is about 88 : 12.

Surface-enhanced Raman scattering induced by the coupling of the guided mode with localized surface plasmon resonances

Surface-enhanced Raman spectroscopy (SERS) is considered to be a powerful analysis tool for the detection of molecules due to its ultra-high sensitivity and non-destructive nature. Here, we introduce a new type of hybrid SERS substrate, where gold nanorods are assembled on a structured support containing a top dielectric grating, dielectric spacer and gold mirror. Compared with the conventional metal nanoparticle assemblies on a flat support, our hybrid substrate shows an approximately 30-fold enhancement in the SERS signal. Numerical simulations show that such a substantial boost arises from the amplification of the absorption cross sections of the gold nanorods and the heating of the "hot spots" around the gold nanorods by the coupling between the guided mode in the structured support and the localized surface plasmon resonances. This mode coupling can be easily tuned by changing the thickness of the spacer. In addition, this substrate also presents uniform spot-to-spot and sample-to-sample SERS signals of the analyte molecules (relative standard deviations down to 7.4% and 6.1%, respectively). Moreover, the performance of this substrate has been demonstrated with the detection of melamine and cytosine, suggesting its great potential in food safety regulation and bioassays. This grating-mirror-enhanced strategy is available to any other SERS-active nanoparticles synthesized by chemical methods, which might offer new opportunities for improving the performance of the chemically prepared nanoparticles in realistic SERS-related applications.

Layer-by-Layer Assembly of Three-Dimensional Optical Functional Nanostructures

Nanotransfer printing (nTP) technology can generate highly functional three-dimensional (3D) nanostructures in a low-cost and high-throughput fashion. Nevertheless, the fabrication yield and quality of the transferred nanostructures are often limited by the merging of the surface patterns of replica stamps during transfer printing. Here, an nTP technology was developed to fabricate large-area and crack-free 3D multilayer nanostructures. Instead of directly depositing materials on the patterned flexible stamp in conventional nTPs, we transferred the nanostructures straightforwardly onto an attached polydimethylsiloxane slab by removing a sacrificial water-soluble poly(acrylic acid) film, which can avoid the cracking of metal film and the failures of printing nanostructures onto target substrates. Based on this approach, subwavelength-thick polarization rotators working at infrared wavelengths were fabricated. Excellent performance of linear polarization rotation over a broadband was realized. This nTP approach could complement existing fabrication techniques and benefit the development of various functional nanostructures with complex multilayer hierarchies.

Plasmonic heating induced by Au nanoparticles for quasi-ballistic thermal transport in multi-walled carbon nanotubes

The plasmon resonances of nanostructures enable wide applications from highly sensitive sensing to high-resolution imaging, through the improvement of photogeneration rate stimulated by the local field enhancement. However, quantitative experimental studies on the localized heating and the thermal transport process in the vicinity of plasmonics are still lacking because of the diffraction limit in conventional optothermal methodologies. In this work, we demonstrate an approach based on Raman thermometry to probe the near-field heating caused by plasmonics. An array of Au nanoparticles (AuNPs) fabricated by the template-assisted method is used to generate the near field effect. Multi-walled carbon nanotubes (MWCNTs) dispersed on the AuNPs are employed to quantify the near-field heating from their Raman peak shifts. Results show that the temperature rise in MWCNTs on AuNPs is much higher than that in a control group under the same laser irradiation. Further analysis indicates that the enhanced photon absorption of MWCNTs attributed to plasmon resonances is partially responsible for the different heating effect. The nonuniform thermal hot spots at the nanoscale can result in the quasi-ballistic thermal transport of phonons in MWCNTs, which is another reason for the temperature rise. Our results can be used to understand plasmonic heating effects as well as to explore quasi-ballistic thermal transport in carbon-based low-dimensional materials by tailoring the geometry or size of plasmonic nanostructures.

Reproducible Plasmonic Nanopyramid Array of Various Metals for Highly Sensitive Refractometric and Surface-Enhanced Raman Biosensing

Localized surface plasmon resonance (LSPR) biosensors show great potential for practical/commercial use in clinical diagnosis, home healthcare, environmental analysis, and public healthcare. However, two main issues, that is, low refractometric sensitivity and low reproducibility (large-area uniformity and batch-to-batch consistency), hinder the extensive applications of LSPR biosensors. Therefore, plasmonic nanostructures with high sensitivity and excellent reproducibility are desirable for preparing reliable LSPR sensors. Herein, we have fabricated plasmonic nanopyramid arrays (NPAs) for several batches with reproducible morphology and optical properties by elastic soft lithography and metal thermal evaporation. NPAs of various metals (i.e., Al, Au, and Ag) were also prepared by thermal evaporation with the according metals. The transmission spectra of these NPAs showed several narrow LSPR peaks in the visible-infrared wavelength region. The refractometric sensitivities of the LSPR peaks were systematically studied, and high refractometric sensitivities of 774.0, 472.8, and 421.0 nm/RIU were achieved on Al, Au, and Ag NPAs, respectively. To demonstrate the potential of the NPAs for multiplex applications, we first applied this highly sensitive Al NPA biosensor to monitoring the process of proliferation of HeLa cancer cells, in situ and in real time. Then, we demonstrated that the Au NPA was able to identify the absorbed analytes on its surface through the surface-enhanced Raman scattering spectrum. In addition, the finite difference time domain simulations were performed to reveal the electromagnetic field enhancement on NPAs. Because of the properties of high sensitivity and excellent reproducibility of the metal NPA LSPR substrates, as well as the simplicity and cost efficiency of the fabrication method, our proposed work will accelerate the practical use of LSPR sensors.

Cluster Assemblies Produced by Aggregation of Preformed Ag Clusters in Ionic Liquids

Room-temperature ionic liquids (RTILs) can be used as electrosterical stabilizers for nanoparticles without adding stabilizing agents. However, the nanoparticle stability and its mechanisms are still in discussion. We deposited preformed 2 nm ±0.6 nm silver clusters into the ionic liquid C₄MIM PF₆ using in situ UV/vis absorption to monitor the deposition process. The time- and temperature-dependent cluster aggregation process was studied with ex situ UV/vis absorption spectroscopy analyzed with electrodynamic calculations using generalized Mie theory. On an atomistic level, the sample structure was investigated using EXAFS and a neural network based analysis of XANES. The combination of all methods shows that an aggregation of the original 2 nm clusters without coalescence takes place, which can be controlled or stopped by choosing an appropriate sample temperature. This approach allows the controlled production of chainlike cluster aggregates in RTIL, promising for a number of applications.

Self-Assembly of Au@Ag Nanoparticles on Mussel Shell To Form Large-Scale 3D Supercrystals as Natural SERS Substrates for the Detection of Pathogenic Bacteria

Herein, we developed a natural surface-enhanced Raman scattering (SERS) substrate based on size-tunable Au@Ag nanoparticle-coated mussel shell to form large-scale three-dimensional (3D) supercrystals (up to 10 cm2) that exhibit surface-laminated structures and crossed nanoplates and nanochannels. The high content of CaCO3 in the mussel shell results in superior hydrophobicity for analyte enrichment, and the crossed nanoplates and nanochannels provided rich SERS hot spots, which together lead to high sensitivity. Finite-difference time-domain simulations showed that nanoparticles in the channels exhibit apparently a higher electromagnetic field enhancement than nanoparticles on the platelets. Thus, under optimized conditions (using Au@AgNPs with 5 nm shell thickness), highly sensitive SERS detection with a detection limit as low as 10-9 M for rhodamine 6G was obtained. Moreover, the maximum electromagnetic field enhancement of different types of 3D supercrystals shows no apparent difference, and Au@AgNPs were uniformly distributed such that reproducible SERS measurements with a 6.5% variation (613 cm-1 peak) over 20 spectra were achieved. More importantly, the as-prepared SERS substrates can be utilized for the fast discrimination of Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa by discriminant analysis. This novel Au@Ag self-assembled mussel shell template holds considerable promise as low-cost, durable, sensitive, and reproducible substrates for future SERS-based biosensors.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Highly robust, uniform and ultra-sensitive surface-enhanced Raman scattering substrates for microRNA detection fabricated by using silver nanostructures grown in gold nanobowls

Highly sensitive and reproducible surface enhanced Raman spectroscopy (SERS) requires not only a nanometer-level structural control, but also superb uniformity across the SERS substrate for practical imaging and sensing applications. However, in the past, increased reproducibility of the SERS signal was incompatible with increased SERS sensitivity. This work presents multiple silver nanocrystals inside periodically arrayed gold nanobowls (SGBs) via an electrochemical reaction at an overpotential of -3.0 V (vs. Ag/AgCl). The gaps between the silver nanocrystals serve as hot spots for SERS enhancement, and the evenly distributed gold nanobowls lead to a high device-to-device signal uniformity. The SGBs on the large sample surface exhibit an excellent SERS enhancement factor of up to 4.80 × 10⁹, with excellent signal uniformity (RSD < 8.0 ± 2.5%). Furthermore, the SGBs can detect specific microRNA (miR-34a), which plays a widely acknowledged role as biomarkers in diagnosis and treatment of diseases. Although the small size and low abundance of miR-34a in total RNA samples hinder their detection, by utilizing the advantages of SGBs in SERS sensing, reliable and direct detection of human gastric cancer cells has been successfully accomplished.

Attomolar detection of extracellular microRNAs released from living prostate cancer cells by a plasmonic nanowire interstice sensor

Prostate cancer (PC) is the second leading cause of cancer death for men worldwide. The serum prostate-specific antigen level test has been widely used to screen for PC. This method, however, exhibits a high false-positive rate, leading to over-diagnosis and over-treatment of PC patients. Extracellular microRNAs (miRNAs) recently provided valuable information including the site and the status of the cancers and thus emerged as new biomarkers for several cancers. Among them, miR141 and miR375 are the most pronounced biomarkers for the diagnosis of high-risk PC. Herein, we report an attomolar detection of miR141 and miR375 released from living PC cells by using a plasmonic nanowire interstice (PNI) sensor. This sensor showed a very low detection limit of 100 aM as well as a wide dynamic range from 100 aM to 100 pM for all target miRNAs. In addition, the PNI sensor could discriminate perfectly the diverse single-base mismatches in the miRNAs. More importantly, the PNI sensor successfully detected the extracellular miR141 and miR375 released from living PC cell lines (LNCaP and PC-3), proving the diagnostic ability of the sensor for PC. We anticipate that the present PNI sensor can hold great promise for the precise diagnosis and prognosis of various cancer patients as well as PC patients.

Hybrid magnetite-gold nanoparticles as bifunctional magnetic-plasmonic systems: three representative cases

Hybrid systems based on magnetite and gold nanoparticles have been extensively used as bifunctional materials for bio- and nano-technology. The properties of these composites are assumed to be closely related to the magnetite to gold mass ratio and to the geometry of the resulting hetero-structures. To illustrate this, we compare and analyze the optical and magnetic properties of core-shell, dumbbell-like dimers and chemical cross-linked pairs of magnetite and gold nanoparticles in detail. We explore how the combination of gold with magnetite can lead to an improvement of the optical properties of these systems, such as tunability, light scattering enhancement or an increase of the local electric field at the interface between magnetic and plasmonic constituents. We also show that although the presence of gold might affect the magnetic response of these hybrid systems, they still show good performance for magnetic applications; indeed the resulting magnetic properties are more dependent on the NP size dispersion. Finally, we identify technological constraints and discuss prospective routes for the development of further magnetic-plasmonic materials.