Stimuli-responsive SERS nanoparticles: conformational control of plasmonic coupling and surface Raman enhancement

Poly(ionic liquid)s-Based Thermal-Responsive Microgel for Use as SERS Substrates with "ON-OFF" Switchable Effect

A temperature-responsive surface-enhanced Raman scattering (SERS) substrate with "ON-OFF" switching based on poly(ionic liquid)s (PILs) block copolymer microgels have been designed and synthesized. The PIL units act as a joint component to anchor the gold nanoparticles (AuNPs) and analytes onto poly(N-isopropylacrylamide) (PNIPAm). This anchor allows the analytes to be fixed at the formed hot spots under temperature stimulus. Owing to the regulation of the PNIPAm segment, the SERS substrates exhibit excellent thermally responsive SERS activity with a reversible "ON-OFF" effect. Additionally, because of the anion exchange of PILs, microgels can introduce new analytes, which offers more flexibility for the system. The substrate shows excellent reversibility, controllability, and flexibility of SERS activity, which is expected to have a broad application in the field of practical SERS sensors.

Hybrid Polymer Vesicles: Controllable Preparation and Potential Applications

Hybrid polymer vesicles contain functional nanoparticles (NPs) in their walls, interfaces, coronae, or cavities. NPs render the hybrid vesicles with specific physical properties, while polymers endow them with structural stability and may significantly reduce the high toxicity of NPs. Therefore, hybrid vesicles integrate fascinating multifunctions from both NPs and polymeric vesicles, which have gained tremendous attention because of their diverse promising applications. Various types of delicate hybrid polymeric vesicles with size control and tunable localization of NPs in different parts of vesicles have been constructed via in situ and ex situ strategies, respectively. Their potential applications have been widely explored, as well. This review presents the progress of block copolymer (BCP) vesicle systems containing different types of NPs including metal NPs, magnetic NPs, and semiconducting quantum dots (QDs), etc. The strategies for controlling the location of NPs within hybrid vesicles are discussed. Typical potential applications of the elegant hybrid vesicles are also highlighted.

Stimuli-responsive plasmonic core-satellite hybrid nanostructures with tunable nanogaps

Incorporating stimuli-responsive block copolymers to hierarchical metallic nanoparticles (MNPs) is of particular interest due to their tunable plasmonic properties responding to environmental stimuli. We herein report thermo-responsive plasmonic core-satellite hybrid nanostructures with tunable nanogaps as surface-enhanced Raman scattering (SERS) nanotags. Two different diblock copolymers with opposite charges, poly(acrylic acid-b-N-isopropylacrylamide) (p(AAc-b-NIPAM)) and poly(N,N-dimethylaminoethyl methacrylate-b-N-isopropylacrylamide) (p(DMAEMA-b-NIPAM)), were synthesized. The negatively charged p(AAc-b-NIPAM)s were bound to gold nanospheres (GNSs), while the positively charged p(DMAEMA-b-NIPAM)s were conjugated to gold nanorods (GNRs) via gold-sulfur bonds. When p(AAc-b-NIPAM)-GNSs and p(DMAEMA-b-NIPAM)-GNRs were electrostatically complexed, plasmonic hybrid nanostructures consisting of both GNS satellites and a GNR core were formed. Dynamic tuning of electromagnetic coupling of their nanogaps was achieved via a temperature-triggered conformational change of p(NIPAM) blocks. Furthermore, a sandwich-type immunoassay for the detection of immunoglobulin G was performed to demonstrate these core-satellites as potential SERS nanotags. Our results showed that these plasmonic core-satellites with stimuli-responsiveness are promising for SERS-based biosensing applications.

Magnetic-Responsive Surface-Enhanced Raman Scattering Platform with Tunable Hot Spot for Ultrasensitive Virus Nucleic Acid Detection

Surface-enhanced Raman scattering (SERS)-based biosensors are promising tools for virus nucleic acid detection. However, it remains challenging for SERS-based biosensors using a sandwiching strategy to detect long-chain nucleic acids such as nucleocapsid (N) gene of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) because the extension of the coupling distance (CD) between the two tethered metallic nanostructures weakens electric field and SERS signals. Herein, we report a magnetic-responsive substrate consisting of heteoronanostructures that controls the CD for ultrasensitive and highly selective detection of the N gene of SARS-CoV-2. Significantly, our findings show that this platform reversibly shortens the CD and enhances SERS signals with a 10-fold increase in the detection limit from 1 fM to 100 aM, compared to those without magnetic modulation. The optical simulation that emulates the CD shortening process confirms the CD-dependent electric field strength and further supports the experimental results. Our study provides new insights into designing a stimuli-responsive SERS-based platform with tunable hot spots for long-chain nucleic acid detection.

Gap-Dependent Surface-Enhanced Raman Scattering (SERS) Enhancement Model of SERS Substrate-Probe Combination Using a Polyelectrolyte Nanodroplet as a Distance Controller

The development of surface-enhanced Raman scattering (SERS) biosensor platforms based on the sandwich combination of an SERS substrate and Raman reporter coated gold nanoparticle (AuNP) labeled with antibody has been widely performed for highly sensitive detection of biomolecules. The size of biomolecules located between these SERS-active materials dictates the sensitivity enhancement of the sensor. However, no suitable molecular size is provided. In this study, we report the gap-dependent SERS enhancement model using the combination of two SERS-active materials of 2D arrays of gold core-silver shell nanoparticles (Au@Ag core-shell NPs) as SERS-active substrates and mercaptobenzoic acid (MBA)-labeled AuNPs as SERS probes. The distance between these two materials is finely tuned using layer-by-layer assembled polyelectrolyte multilayer films. The morphology of the polyelectrolyte spacer is controlled into a droplet nanostructure, which is assumed to have a comparable shape with globular biomolecules. The well-controlled height or thickness of polyelectrolyte nanodroplet was achieved by changing number of deposition cycles. By increasing the thickness of the polyelectrolyte nanodroplet, MBA SERS intensities gradually decreased until at 40 nm-thick nanodroplet film and maintained afterward. This spacer thickness defined the limit of plasmonic coupling effect from this SERS probe-substrate combination. The SERS enhancement capability of this model was compared to conventional SERS immunoassay using three different antigen-antibody complex sizes of prostate-specific antigen, carcinoembryonic antigen, and carbohydrate antigen 19-9. Good agreement of the limitation of plasmon coupling as a function of the distance between the SERS substrate-probe combination using this developed model and SERS immunoassay was found. The finding provides valuable guidelines for immune-system selection in SERS immunosensors based on SERS substrate-probe combination.

Janus bimetallic nanorod clusters-poly(aniline) nanocomposites with temperature-responsiveness for Raman scattering-based biosensing

Herein, Janus bimetallic nanorod clusters-poly(aniline) nanocomposites (JRCPCs) with gold nanorod clusters (GNRCs) in side-by-side (SBS) or end-to-end (ETE) configuration are synthesized, and applied to surface-enhanced Raman scattering (SERS)-based biosensing of carcinoembryonic antigen (CEA). Taking advantage of their geometrical and chemical anisotropy, GNRCs in both SBS and ETE configurations are prepared by addition of negatively charged citrate anions and poly(acrylic acid)-block-poly(N-isopropylacrylamide) (PAAc-b-PNIPAM), respectively, to electrostatically interact with cationic cetyltrimethylammonium bromide surfactant on the side of the gold nanorods (GNRs). Subsequently, the JRCPCs are prepared by unidirectional growth of polyaniline and additional growth of Ag onto these GNRCs. JRCPCs with GNRCs in either the SBS or the ETE configuration show strong enhancement of electromagnetic field at both GNR aggregates and GNRC core-Ag shell gaps of bimetallic nanorod cluster components. In particular, because temperature-responsive PAAc-b-PNIPAM of JRCPCs is embedded at GNR junctions, interparticle gaps generated in GNRCs in ETE configuration are controlled via temperature-triggered hydration-dehydration of the PAAc-b-PNIPAM chains such that optical properties are largely changed. With distinct surface functionalities from JRCPCs, SERS-based quantitative analysis of CEA is achieved using JRCPCs as SERS nanoprobes. This work presents the great potential of advanced Janus nanocomposites for SERS-based biosensing applications.

Directional self-assembly of anisotropic bimetal-poly(aniline) nanostructures for rheumatoid arthritis diagnosis in multiplexing

Anisotropic organic-inorganic hybrid nanoparticles possessing different functionalities and physicochemical properties from each compartment have attracted significant interest for the development of advanced functional materials. Moreover, their self-assembled structures exhibit unique optical properties for photonics-based biosensing. We report herein the fabrication of anisotropic bimetal-polymer nanoparticles (ABPNs) via combination of oxidative polymerization and additional growth of metallic nanoparticles on Au seeds as well as their directional clustering mediated via noncovalent interactions. Polymerization of anilines for poly (aniline) shell was conducted by reducing silver nitrate onto the Au seed in the presence of a surfactant, giving rise to spatially distinct bimetallic Au core and Ag shell compartment and the poly (aniline) counter-one that comprise the ABPNs. Furthermore, ABPNs were directionally clustered in a controlled manner via hydrophobic interaction, when the bimetallic compartment was selectively modified. These nanoclusters showed highly enhanced optical properties owing to the increased electromagnetic fields while the poly (aniline) being used to offer antibody binding capacity. Taking advantages of those properties of the ABPN nanoclusters, surface-enhanced Raman scattering (SERS) intensity-based quantification of two different biomarkers: autoantibodies against cyclic citrullinated peptide and rheumatoid factor was demonstrated using ABPN nanoclusters as SERS nanoprobes. Conclusively, this work has great potential to satisfy a need for multiplexing in diagnosis of early stage of rheumatoid arthritis.

Responsive Plasmonic Nanomaterials for Advanced Cancer Diagnostics

Plasmonic nanostructures, particularly of noble-metal Au and Ag, have attracted long-lasting research interests because of their intriguing physical and chemical properties. Under light excitation, their conduction electrons can form collective oscillation with the electromagnetic fields at particular wavelength, leading to localized surface plasmon resonance (LSPR). The remarkable characteristic of LSPR is the absorption and scattering of light at the resonant wavelength and greatly enhanced electric fields in localized areas. In response to the chemical and physical changes, these optical properties of plasmonic nanostructures will exhibit drastic color changes and highly sensitive peak shifts, which has been extensively used for biological imaging and disease treatments. In this mini review, we aim to briefly summarize recent progress of preparing responsive plasmonic nanostructures for biodiagnostics, with specific focus on cancer imaging and treatment. We start with typical synthetic approaches to various plasmonic nanostructures and elucidate practical strategies and working mechanism in tuning their LSPR properties. Current achievements in using responsive plasmonic nanostructures for advanced cancer diagnostics will be further discussed. Concise perspectives on existing challenges in developing plasmonic platforms for clinic diagnostics is also provided at the end of this review.

Stimuli-Responsive Plasmonic Assemblies and Their Biomedical Applications

Among the diverse development of stimuli-responsive assemblies, plasmonic nanoparticle (NP) assemblies functionalized with responsive molecules are of a major interest. In this review, we outline a comprehensive and up-to-date overview of recently reported studies on in vitro and in vivo assembly/disassembly and biomedical applications of plasmonic NPs, wherein stimuli such as enzymes, light, pH, redox potential, temperature, metal ions, magnetic or electric field, and/or multi-stimuli were involved. Stimuli-responsive assemblies have been applied in various biomedical fields including biosensors, surfaced-enhanced Raman scattering (SERS), photoacoustic (PA) imaging, multimodal imaging, photo-activated therapy, enhanced X-ray therapy, drug release, stimuli-responsive aggregation-induced cancer therapy, and so on. The perspectives on the use of stimuli-responsive plasmonic assemblies are discussed by addressing future scientific challenges involving assembly/disassembly strategies and applications.

Surface-enhanced Raman scattering-active AuNR array cellulose films for multi-hazard detection

In this study, we report a surface-enhanced Raman scattering (SERS)-active array film, which is based on regenerated cellulose hydrogels and gold nanorods (AuNRs), by combining a silicon rubber mask with a vacuum filtration method. This strategy enables the direct AuNR array formation on hydrogel surface with a precisely controlled number density. Moreover, the control of interparticle nanogap has been realized by the spatial deformation of hydrogels. A decrease in gaps between AuNRs deposited on hydrogels can result in SERS enhancement because 3D porous hydrogel structures turned into 2D closely packed hydrogel films during drying. In our experiments, SERS sensor arrays show excellent SERS activity to detect rhodamine 6 G and thiram down to 10 pM and 100 fM with competitive enhancement factors of 3.9 × 10⁸ and 9.5 × 10⁹, respectively. Importantly, the resultant SERS-active arrays with nine sensor units can efficiently detect nine different analytes on a single substrates at a time. Moreover, we demonstrate that physical bending has little effect on the SERS activity of flexible AuNR array hydrogel films, which indicates the high reproducibility of SERS measurement. This SERS array film has great potential to simultaneously detect multiple hazards for the practical application of SERS analysis.

Flexible Surface-Enhanced Raman Scattering Chip: A Universal Platform for Real-Time Interfacial Molecular Analysis with Femtomolar Sensitivity

We propose and demonstrate a flexible surface-enhanced Raman scattering (SERS) chip as a versatile platform for femtomolar detection and real-time interfacial molecule analysis. The flexible SERS chip is composed of a flexible and transparent membrane and embedded plasmonic dimers with ultrahigh particle density and ultrasmall dimer gap. The chip enables rapid identification for residuals on solid substrates with irregular surfaces or dissolved analytes in aqueous solution. The sensitivity for liquid-state measurement is down to 0.06 molecule per dimers for 10^-14 mol·L^-1 Rhodamine 6G molecule without molecule enrichment. Strong signal fluctuation and blinking are observed at this concentration, indicating that the detection limit is close to the single-molecule level. Meanwhile, the homogeneous liquid environment facilities accurate SERS quantification of analytes with a wide dynamic range. The synergy of flexibility and liquid-state measurement opens up avenues for the real-time study of chemical reactions. The reduction from p-nitrothiophenol (PNTP) to p-aminothiophenol (PATP) in the absence of the chemical reducing agents is observed at liquid interfaces by in situ SERS measurements, and the plasmon-induced hot electron is demonstrated to drive the catalytic reaction. We believe this robust and feasible approach is promising in extending the SERS technique as a general method for identifying interfacial molecular traces, tracking the evolution of heterogeneous reactions, elucidating the reaction mechanisms, and evaluating the environmental effects such as pH value and salty ions in SERS.

Dynamically Tunable Plasmonic Band for Reversible Colorimetric Sensors and Surface-Enhanced Raman Scattering Effect with Good Sensitivity and Stability

A colorimetric sensor based on plasmonic nanoparticles (NPs) is a promising and convenient detection tool, but its reproducibility and adjustability remain a challenge because the NPs are mainly random and uncontrollable. Herein, a colorimetric sensor with good reversibility and reproducibility was prepared by embedding the two-dimensional (2D) Au NP arrays on the surface of the polyacrylamide hydrogel film to form 2D Au NP arrays attached a hydrogel composite. For this composite, with the change of the interspacing distance of Au NPs driven by the swelling-shrinking behavior of the hydrogel carrier, the diffraction peaks faded away and plasmonic coupling peaks appeared, accompanied by a series of obvious color changes (iridescence ↔ violet ↔ golden yellow ↔ red), which can be correlated to the applied water content. Importantly, the composite had good reproducibility as a result of a highly ordered array structure. Additionally, this colorimetric sensor with a dynamically tunable plasmonic band can be used as a high-quality surface-enhanced Raman scattering (SERS) substrate because the gap distance of the Au NPs can be uniformly controlled. We demonstrated that, as the active gap distance decreased, the SERS signals can be significantly intensified. When the water content reached 40%, this SERS substrate exhibited high sensitivity (10^-10 M for 4-aminothiophenol and 10^-9 M for thiram) and good reproducibility (relative standard deviation of <20%) using the excitation laser of 785 nm because of the small gap between two adjacent Au NPs and the highly ordered periodic structure. Such 2D Au NP arrays attached to a hydrogel composite could be a new strategy to obtain a high-quality colorimetric sensor and dynamic SERS substrate.

Magnetic tuning of SERS hot spots in polymer-coated magnetic-plasmonic iron-silver nanoparticles

Plasmonic nanostructures are intensively studied for their ability to create electromagnetic hot spots, where a great variety of optical and spectroscopic processes can be amplified. Understanding how to control the formation of hot spots in a dynamic and reversible way is crucial to further expand the panorama of plasmon enhanced phenomena. In this work, we investigate the ability to modulate the hot spots in magnetic-plasmonic iron-doped silver nanoparticles dispersed in aqueous solution, by applying an external magnetic field. Evidence of magnetic field induction of hot spots was achieved by measuring the amplification of surface enhanced Raman scattering (SERS) from analytes dispersed in the solution containing Ag-Fe NPs. A polymeric shell was introduced around Ag-Fe NPs to confer colloidal stability, and it was found that the length and density of the polymer chains have a significant influence on SERS performance, and therefore on the formation of electromagnetic hot spots, under the action of the external magnetic field. These findings are expected to provide an important contribution to understanding the growing field of tuneable electromagnetic enhancement by external stimuli, such as magnetic fields applied to magnetic-plasmonic nanoparticles.

Nanoscale uniformity in the active tuning of a plasmonic array by polymer gel volume change

Active plasmonic tuning is an attractive but challenging research subject, leading to various promising applications. As one of the approaches, nanostructures are placed in or on soft matter, such as elastomers and gels, and their gap distances are tuned by the mechanical extension or volume change of the supporting matrices. As hydrogels possess various types of stimuli-responsiveness with large volume change and biocompatibility, they are good candidates as supporting materials for active nanostructure tuning. However, it remains unclear how accurately we can control their nanogap distance changes using polymer gels with a low deviation due to major difficulties in the precise observation of nanostructures on the gels. Here, we prepared gold arrays with sub-100 nm dots on silicon substrates by electron beam lithography and transferred them onto the hydrogel surface. Then, their nanopattern was actively tuned by the changes in gel size in water and their structural changes were confirmed by optical microscopy, microspectroscopy, and atomic force microscopy (AFM). Further, we successfully prepared ionic liquid (IL) gels with various degrees of swelling via solvent exchange. Scanning electron microscopy (SEM) observation of the IL gels provided clear pictures at nanoscale resolution. Finally, we calculated the plasmonic spectra using a finite difference time domain (FDTD) simulation based on the SEM images and compared them with the measured spectra. The results in this study totally support the notion that active changes in plasmonic nanodot patterns via volume changes in the hydrogel are quite homogenous on a several nanometer scale, making them ideal for precise active surface plasmon tuning.

Stimuli-Responsive Optical Nanomaterials

Responsive optical nanomaterials that can sense and translate various external stimuli into optical signals, in the forms of observable changes in appearance and variations in spectral line shapes, are among the most active research topics in nanooptics. They are intensively exploited within the regimes of the four classic optical phenomena-diffraction in photonic crystals, absorption of plasmonic nanostructures, as well as color-switching systems, refraction of assembled birefringent nanostructures, and emission of photoluminescent nanomaterials and molecules. Herein, a comprehensive review of these research activities regarding the fundamental principles and practical strategies is provided. Starting with an overview of their substantial developments during the latest three decades, each subtopic discussion is led with fundamental theories that delineate the correlation between nanostructures and optical properties and the delicate research strategies are elaborated with specific attention focused on working principles and optical performances. The unique advantages and inherent limitations of each responsive optical nanoscale platform are summarized, accompanied by empirical criteria that should be met and perspectives on research opportunities where the developments of next-generation responsive optical nanomaterials might be directed.

Split aptamer-based detection of adenosine triphosphate using surface enhanced Raman spectroscopy and two kinds of gold nanoparticles

An ultrasensitive and highly selective method is described for the determination of adenosine triphosphate (ATP) via surface-enhanced Raman scattering (SERS). Two split aptamers are used for specific recognition of ATP. They were attached to two SERS substrates. The first was placed on a nanolayer of gold nanoparticle-decorated graphene oxide (GO/Au3), and the other on gold nanoparticles (Au2). When ATP is introduced, it will interact with the split aptamers on the gold nanostructures to form a sandwich structure that brings the GO/Au3 nanolayer and the Au2 nanoparticle in close proximity. Consequently, the SERS signal, best measured at 1072 cm^-1, is strongly enhanced. The sandwich structure also displays good water solubility and stability. Under optimized conditions, the SERS signal increases in the 10 pM - 10 nM ATP concentration range, and the limit of detection (LOD) is 0.85 pM. The method was applied to the determination of ATP in spiked human serum, and the LODs in serum and buffer are comparable. In our perception, the method has a wide scope in that numerous other aptamers may be used. This may result in a variety of other highly sensitive aptasensors for use in in-vitro diagnostics. Graphical abstract Schematic presentation of a self-assembly sandwich nanostructure as unique SERS assay platform for the sensitive detection of ATP.

External-Stimuli-Assisted Control over Assemblies of Plasmonic Metals

Assembly of plasmonic nanoparticles (NPs) in suspensions is a promising approach for the control of optical and sensing properties that depend on the assembled states of plasmonic NPs. This review focuses on the controlling methods to assemble the NP via external stimuli such as pH, temperature, light, magnetic field, and electric field. External stimuli are introduced as powerful tools to assemble the NPs because of various operational factors, such as the intensity, application time, and frequency, which can be employed. In addition to a summary of recent studies on the controlling methods, a future study on the reversible control over assembled states of the plasmonic NPs via external stimuli is proposed.

Introducing 12 new dyes for use with oligonucleotide functionalised silver nanoparticles for DNA detection with SERS

Oligonucleotide functionalised metallic nanoparticles (MNPs) have been shown to be an effective tool in the detection of disease-specific DNA and have been employed in a number of diagnostic assays. The MNPs are also capable of facilitating surface enhanced Raman scattering (SERS) enabling detection to become highly sensitive. Herein we demonstrate the expansion of the range of specific SERS-active oligonucleotide MNPs through the use of 12 new Raman-active monomethine and trimethine chalcogenopyrylium and benzochalcogenopyrylium derivatives. This has resulted in an increased ability to carry out multiplexed analysis beyond the current small pool of resonant and non-resonant Raman active molecules, that have been used with oligonucleotide functionalised nanoparticles. Each dye examined here contains a variation of sulphur and selenium atoms in the heterocyclic core, together with phenyl, 2-thienyl, or 2-selenophenyl substituents on the 2,2′,6, and 6′ positions of the chalcogenopyrylium dyes and 2 and 2′ positions of the benzochalcogenopyrylium dyes. The intensity of SERS obtained from each dye upon conjugate hybridisation with a complementary single stranded piece of DNA was explored. Differing concentrations of each dye (1000, 3000, 5000 and 7000 equivalents per NP-DNA conjugate) were used to understand the effects of Raman reporter coating on the overall Raman intensity. It was discovered that dye concentration did not affect the target/control ratio, which remained relatively constant throughout and that a lower concentration of Raman reporter was favourable in order to avoid NP instability. A relationship between the dye structure and SERS intensity was discovered, leaving scope for future development of specific dyes containing substituents favourable for discrimination in a multiplex by SERS. Methine dyes containing S and Se in the backbone and at least 2 phenyls as substituents give the highest SERS signal following DNA-induced aggregation. Principal component analysis (PCA) was performed on the data to show differentiation between the dye classes and highlight possible future multiplexing capabilities of the 12 investigated dyes.

12 new Raman active dyes are reported to increase the SERS intensity upon hybridisation of a targeted DNA to oligonucleotide-NP conjugates and can be potentially used together in a multiplex.

Driving Chemical Reactions in Plasmonic Nanogaps with Electrohydrodynamic Flow

Nanoparticles from colloidal solution-with controlled composition, size, and shape-serve as excellent building blocks for plasmonic devices and metasurfaces. However, understanding hierarchical driving forces affecting the geometry of oligomers and interparticle gap spacings is still needed to fabricate high-density architectures over large areas. Here, electrohydrodynamic (EHD) flow is used as a long-range driving force to enable carbodiimide cross-linking between nanospheres and produces oligomers exhibiting sub-nanometer gap spacing over mm² areas. Anhydride linkers between nanospheres are observed via surface-enhanced Raman scattering (SERS) spectroscopy. The anhydride linkers are cleavable via nucleophilic substitution and enable placement of nucleophilic molecules in electromagnetic hotspots. Atomistic simulations elucidate that the transient attractive force provided by EHD flow is needed to provide a sufficient residence time for anhydride cross-linking to overcome slow reaction kinetics. This synergistic analysis shows assembly involves an interplay between long-range driving forces increasing nanoparticle-nanoparticle interactions and probability that ligands are in proximity to overcome activation energy barriers associated with short-range chemical reactions. Absorption spectroscopy and electromagnetic full-wave simulations show that variations in nanogap spacing have a greater influence on optical response than variations in close-packed oligomer geometry. The EHD flow-anhydride cross-linking assembly method enables close-packed oligomers with uniform gap spacings that produce uniform SERS enhancement factors. These results demonstrate the efficacy of colloidal driving forces to selectively enable chemical reactions leading to future assembly platforms for large-area nanodevices.

Nanoscale Structural Switching of Plasmonic Nanograin Layers on Hydrogel Colloidal Monolayers for Highly Sensitive and Dynamic SERS in Water with Areal Signal Reproducibility

Developing substrates that enable both reproducible and highly sensitive Raman detection of trace amounts of molecules in aqueous systems remains a challenge, although these substrates are crucial in biomedicine and environmental sciences. To address this issue, we report spatially uniform plasmonic nanowrinkles formed by intimate contact between plasmonic nanograins on the surface of colloidal crystal monolayers. The Au or Ag nanograin layers coated on hydrogel colloidal crystal monolayers can reversibly wrinkle and unwrinkle according to changes in the water temperature. The reversible switches are directed by surface structural changes in the colloidal crystal monolayers, while the colloids repeat the hydration-dehydration process. The Au and Ag nanowrinkles are obtained upon hydration, thus enabling the highly reproducible detection of Raman probes in water at the nano- and picomolar levels, respectively, throughout the entire substrate area. Additionally, the reversible switching of the nanostructures in the plasmonic nanograin layers causes reversible dynamic changes in the corresponding Raman signals upon varying the water temperature.

Optimizing Melamine Resin Microspheres with Excess Formaldehyde for the SERS Substrate

Influence of the excess monomer within the synthetic reaction solution of melamine resin microspheres (MFMSs) on the surface-enhanced Raman spectroscopy (SERS) enhancement from Rhodamine 6G (R6G) was investigated, where the R6G was adsorbed on the silver nanoparticles (AgNPs) that were loaded on the MFMSs. Surface characteristics of the MFMSs were modified by the excess monomer (i.e., the excessive melamine or formaldehyde) through its terminal overreaction, which can be simply controlled by some of the synthetic reaction conditions, thus further allowing us to optimize the assembly of the loaded AgNPs for the SERS detection. These SERS substrates incorporating the optimized MFMSs with the excess formaldehyde can also be used for tracing analyses of more environmental and food contaminants.

A New Smart Surface-Enhanced Raman Scattering Sensor Based on pH-Responsive Polyacryloyl Hydrazine Capped Ag Nanoparticles

A novel pH-responsive Ag@polyacryloyl hydrazide (Ag@PAH) nanoparticle for the first time as a surface-enhanced Raman scattering (SERS) substrate was prepared without reducing agent and end-capping reagent. Ag@PAH nanoparticles exhibited an excellent tunable detecting performance in the range from pH = 4 to pH = 9. This is explained that the swelling-shrinking behavior of responsive PAH can control the distance between Ag NPs and the target molecules under external pH stimuli, resulting in the tunable LSPR and further controlled SERS. Furthermore, Ag@PAH nanoparticles possessed an ultra-sensitive detecting ability and the detection limit of Rhodamine 6G reduced to 10-12 M. These advantages qualified Ag@PAH NP as a promising smart SERS substrate in the field of trace analysis and sensors.

pH-Responsive Coassembly of Oligo(ethylene glycol)-Coated Gold Nanoparticles with External Anionic Polymers via Hydrogen Bonding

Stimuli-responsive assembly of gold nanoparticles (AuNPs) with precise control of the plasmonic properties, assembly size, and stimuli responsivity has shown potential benefits with regard to biosensing devices and drug-delivery systems. Here we present a new pH-responsive coassembly system of oligo(ethylene glycol) (OEG)-coated AuNPs with anionic polymers as an external mediator via hydrogen bonding in water. Hydrogen-bond-driven coassemblies of OEG-AuNPs with poly(acrylic acid) (PAA) were confirmed by the monitoring of plasmonic peaks and hydrodynamic diameters. In this system, the protonation of anionic polymers on change in pH triggered the formation of hydrogen bond between the OEG-AuNPs and polymers, providing sensitive pH responsivity. The plasmonic properties and assembly size are affected by both the ratio of PAA to AuNPs and the molecular weight of PAAs. In addition, the attachment of hydrophobic groups to the surface ligand or anionic polymer changed the responsive pH range. These results demonstrated that the coassembly with an external mediator via hydrogen bonding provides a stimuli-responsive assembly system with tunable plasmonic properties, assembly size, and stimuli responsivity.

Core-Shell Nanoparticle-Enhanced Raman Spectroscopy

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.

Observing the dynamic “hot spots” on two-dimensional Au nanoparticles monolayer film

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays.

Directed assembly of functionalized nanoparticles with amphiphilic diblock copolymers

We theoretically propose a simple approach to achieve soft nanoparticles with a self-patchiness nature, which are further directed to assemble into a rich variety of highly ordered superstructures.

Biocomputing nanoplatforms as therapeutics and diagnostics

Biocomputing nanoplatforms are designed to detect and integrate single or multiple inputs under defined algorithms, such as Boolean logic gates, and generate functionally useful outputs, such as delivery of therapeutics or release of optically detectable signals. Using sensing modules composed of small molecules, polymers, nucleic acids, or proteins/peptides, nanoplatforms have been programmed to detect and process extrinsic stimuli, such as magnetic fields or light, or intrinsic stimuli, such as nucleic acids, enzymes, or pH. Stimulus detection can be transduced by the nanomaterial via three different mechanisms: system assembly, system disassembly, or system transformation. The increasingly sophisticated suite of biocomputing nanoplatforms may be invaluable for a multitude of applications, including medical diagnostics, biomedical imaging, environmental monitoring, and delivery of therapeutics to target cell populations.

Nanogels: An overview of properties, biomedical applications and obstacles to clinical translation

Nanogels have emerged as a versatile hydrophilic platform for encapsulation of guest molecules with a capability to respond to external stimuli that can be used for a multitude of applications. These are soft materials capable of holding small molecular therapeutics, biomacromolecules, and inorganic nanoparticles within their crosslinked networks, which allows them to find applications for therapy as well as imaging of a variety of disease conditions. Their stimuli-responsive behavior can be easily controlled by selection of constituent polymer and crosslinker components to achieve a desired response at the site of action, which imparts nanogels the ability to participate actively in the intended function of the carrier system rather than being passive carriers of their cargo. These properties not only enhance the functionality of the carrier system but also help in overcoming many of the challenges associated with the delivery of cargo molecules, and this review aims to highlight the distinct and unique capabilities of nanogels as carrier systems for the delivery of an array of cargo molecules over other nanomaterials. Despite their obvious usefulness, nanogels are still not a commonplace occurrence in clinical practice. We have also made an attempt to highlight some of the major challenges that need to be overcome to advance nanogels further in the field of biomedical applications.

Surface-Enhanced Raman Spectroscopy Assisted by Radical Capturer for Tracking of Plasmon-Driven Redox Reaction

The deep understanding about the photocatalytic reaction induced by the surface plasmon resonance (SPR) effect is desirable but remains a considerable challenge due to the ultrafast relaxation of hole-electron exciton from SPR process and a lack of an efficient monitoring system. Here, using the p-aminothiophenol (PATP) oxidation SPR-catalyzed by Ag nanoparticle as a model reaction, a radical-capturer-assisted surface-enhanced Raman spectroscopy (SERS) has been used as an in-situ tracking technique to explore the primary active species determining the reaction path. Hole is revealed to be directly responsible for the oxidation of PATP to p, p′-dimercaptoazobenzene (4, 4′-DMAB) and O₂ functions as an electron capturer to form isolated hole. The oxidation degree of PATP can be further enhanced through a joint utilization of electron capturers of AgNO₃ and atmospheric O₂, producing p-nitrothiophenol (PNTP) within 10 s due to the improved hole-electron separation efficiency.

Fine Golden Rings: Tunable Surface Plasmon Resonance from Assembled Nanorods in Topological Defects of Liquid Crystals

Unprecedented, reversible, and dynamic control over an assembly of gold nanorods dispersed in liquid crystals (LC) is demonstrated. The LC director field is dynamically tuned at the nanoscale using microscale ring confinement through the interplay of elastic energy at different temperatures, thus fine-tuning its core replacement energy to reversibly sequester nanoscale inclusions at the microscale. This leads to shifts of 100 nm or more in the surface plasmon resonance peak, an order of magnitude greater than any previous work with AuNR composites.

Plasmonic Nanogels for Unclonable Optical Tagging

We demonstrate the fabrication of novel functional gel coatings with randomized physical and chemical patterns that enable dual encoding ability to realize unclonable optical tags. This design is based on swelling-mediated massive reconstruction of an ultrathin responsive gelatinous polymer film uniformly adsorbed with plasmonic nanostructures into a randomized network of interacting folds, resulting in bright electromagnetic hotspots within the folds. We reveal a strong correlation between the topology and near-field electromagnetic field enhancement due to the intimate contact between two plasmonic surfaces within the folds, each of them representing a unique combination of local topography and chemical distribution caused by the formation of electromagnetic hotspots. Because of the efficient trapping of the Raman reporters within the uniquely distributed electromagnetic hotspots, the surface enhanced Raman scattering enhancement from the morphed plasmonic gel was found to be nearly 40 times higher compared to that from the pristine plasmonic gel. Harnessing the nondeterministic nature of the folds, the folded plasmonic gel can be employed as a multidimensional (with dual topo-chemical encoding) optical taggant for prospective anticounterfeiting applications. Such novel optical tags based on the spontaneous folding process are virtually impossible to replicate because of the combination of nondeterministic physical patterns and chemical encoding.

Plasmonic nanohalo optical probes for highly sensitive imaging of survivin mRNA in living cells

A novel strategy for sensitive detection of survivin mRNA based on Rayleigh light scattering spectroscopy of AuNP nanohalo probes.

Plasmonic Vesicles of Amphiphilic Nanocrystals: Optically Active Multifunctional Platform for Cancer Diagnosis and Therapy

Vesicular structures with compartmentalized, water-filled cavities, such as liposomes of natural and synthetic amphiphiles, have tremendous potential applications in nanomedicine. When block copolymers self-assemble, the result is polymersomes with tailored structural properties and built-in releasing mechanisms, controlled by stimuli-responsive polymer building blocks. More recently, chemists are becoming interested in multifunctional hybrid vesicles containing inorganic nanocrystals with unique optical, electronic, and magnetic properties. In this Account, we review our recent progress in assembling amphiphilic plasmonic nanostructures to create a new class of multifunctional hybrid vesicles and applying them towards cancer diagnosis and therapy. Localized surface plasmon resonance (LSPR) gives plasmonic nanomaterials a unique set of optical properties that are potentially useful for both biosensing and nanomedicine. For instance, the strong light scattering at their LSPR wavelength opens up the applications of plasmonic nanostructures in single particle plasmonic imaging. Their superior photothermal conversion properties, on the other hand, make them excellent transducers for photothermal ablation and contrast agents for photoacoustic imaging. Of particular note for ultrasensitive detection is that the confined electromagnetic field resulting from excitation of LSPR can give rise to highly efficient surface enhanced Raman scattering (SERS) for molecules in close proximity. We have explored several ways to combine well-defined plasmonic nanocrystals with amphiphilic polymer brushes of diverse chemical functionalities. In multiple systems, we have shown that the polymer grafts impart amphiphilicity-driven self-assembly to the hybrid nanoparticles. This has allowed us to synthesize well-defined vesicles in which we have embedded plasmonic nanocrystals in the shell of collapsed hydrophobic polymers. The hydrophilic brushes extend into external and interior aqueous environment to stabilize the vesicular structure. More importantly, we have demonstrated that strong interparticle coupling greatly enhances the optical properties (scattering, photothermal conversion, and SERS) in plasmonic vesicles. In combination with the loading capacity of the vesicles, this technology can provide unique opportunities for integrated diagnosis and therapy, multimodality combination therapy, and imaging-guided therapy. One key property differentiating the plasmonic vesicles from other vesicular structures containing nanocrystals is that we can tailor the interparticle coupling and disintegration of the plasmonic vesicles by altering structural parameters and conformational changes of the covalently bound polymer brushes. This gives us tremendous flexibility to engineer plasmonic vesicles for ultrasensitive detection and targeted therapy. Through bringing together advances in nanochemistry, polymer chemistry, self-assembly, and nanophotonics, we expect to further expand our capability of tailoring optical and structural characteristics of plasmonic vesicles to address challenges in medical settings.

Facile Preparation of Ultrasmall Void Metallic Nanogap from Self-Assembled Gold-Silica Core-Shell Nanoparticles Monolayer via Kinetic Control

A facile preparation of ultrasmall 1-2 nm void metallic nanogaps on various solid substrates is proposed by utilizing the self-assembly of a uniform gold-silica core-shell nanoparticle monolayer at interfaces and chemical etching. The ultrasmall void metallic nanogap shows key advantages such as a strong near-field enhancement and free diffusion of analytes to the gap, which are useful in molecular sensing and monitoring.