Surface-Coverage Dependence of Surface-Enhanced Raman Scattering from Gold Nanocubes on Self-Assembled Monolayers of Analyte

Multiwavelength SERS of Magneto-Plasmonic Nanoparticles Obtained by Combined Laser Ablation and Solvothermal Methods

The present study introduces a novel method for the synthesis of magneto-plasmonic nanoparticles (MPNPs) with enhanced functionality for surface-enhanced Raman scattering (SERS) applications. By employing pulsed laser ablation in liquid (PLAL) to synthesize plasmonic nanoparticles and wet chemistry to synthesize magnetic nanoparticles, we successfully fabricated chemically pure hybrid Fe3O4@Au and Fe3O4@Ag nanoparticles. We demonstrated a straightforward approach of an electrostatic attachment of the plasmonic and magnetic parts using positively charged polyethylenimine. The MPNPs displayed high SERS sensitivity and reproducibility, and the magnetic part allowed for the controlled separation of the nanoparticles from the reaction mixture, their subsequent concentration, and their precise deposition onto a specified surface area. Additionally, we fabricated alloy based MPNPs from AgxAu100-x (x = 50 and 80 wt %) targets with distinct localized surface plasmon resonance (LSPR) wavelengths. The compositions, morphologies, and optical properties of the nanoparticles were characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-vis spectroscopy, and multiwavelength Raman spectroscopy. A standard SERS marker, 4-mercaptobenzoic acid (4-MBA), validated the enhancement properties of the MPNPs and found an enhancement factor of 2 × 108 for the Fe3O4@Ag nanoparticles at 633 nm excitation. Lastly, we applied MPNP-enhanced Raman spectroscopy for the analysis of the biologically relevant molecule adenine and found a limit of detection of 10-7 M at 785 nm excitation. The integration of PLAL and wet chemical methods enabled the relatively fast and cost-effective production of MPNPs characterized by high SERS sensitivity and signal reproducibility that are required in various fields, including biomedicine, food safety, materials science, security, and defense.

Exploring Intensity Distributions and Sampling in SERS-Based Immunoassays

Surface-enhanced Raman scattering (SERS) is a sensitive, widely used spectroscopic technique. However, SERS is perceived as poorly reproducible and insufficiently robust for standard applications in analytical chemistry. Here, we demonstrated that reliable SERS immunoassay quantification at low concentrations (pM range) can be achieved by careful experimental design and appropriate data analysis statistics. A SERS-based immunoassay for IgG in human serum (3.1-50.0 ng mL^-1 or 20.6-333 pM) was developed as a proof of concept. Calibration curves were created using the population median of the band area, centered at 592 cm^-1, of a SERS reporter (Nile Blue A). Histograms of 7200 SERS spectra show lognormal distributions. SEM images of the sensor platform confirm a correlation between the number of SERS probes (ERLs) at the surface and the SERS intensity response. The IgG immunosensor reported here presented a limit of detection of 1.11 ng mL^-1 or 7.39 pM and a limit of quantification of 9.04 ng mL^-1 or 60.30 pM, within a 95% confidence level. The % error of the predicted versus the actual response of a quality control (QC) sample was 0.13%. The percent error of the QC sample decreases exponentially with the number of measurements. Randomly selected spatially separated measurements provided lower QC % error than a larger number of measurements that were closely spaced. We propose that it is necessary to describe the measured populations using an appropriate sample size for good statistics and consider the interrogation of sufficiently large and well-separated areas of the sensor surface to achieve a reliable sampling.

Plasmonic Au nanocube enhanced SERS biosensor based on heated electrode and strand displacement amplification for highly sensitive detection of Dam methyltransferase activity

In this work, a novel "turn-on" mode Au nanocubes (AuNCs) enhanced surface-enhanced Raman scattering (SERS) biosensing platform coupled with heated Au electrode (HAuE) and strand displacement amplification (SDA) strategy was proposed for highly sensitive detection of DNA adenine methylation (Dam) Methyltransferase (MTase) activity. The Dam MTase and DpnI enzyme activities were significantly increased by elevating the HAuE surface temperature, resulting in the rapid production of template DNA for later SDA. During the SDA process, the released single-stranded DNA (ssDNA) could be amplified exponentially, and its concentration was positively related to the Dam MTase activity. The plasmonic AuNCs in SERS tags could provide significant SERS enhancement due to their "lightning rod" effect resulting from the sharp feature of the edges and corners of AuNCs. Because of these factors, the proposed biosensors exhibited high sensitivity in detecting the Dam MTase activity. The limit of detection was estimated to be 8.65 × 10^-5 U mL^-1, which was lower than that in most of the sensors for detection of Dam MTase activity in the literature. This SERS biosensor could also be used to screen inhibitors of Dam MTase and had the potential for detecting Dam MTase activity in real biological samples.

Thermoresponsive chiral plasmonic nanoparticles

Chiral metallic nanoparticles can exhibit novel plasmonic circular dichroism (PCD) in the ultraviolet and visible range of the electromagnetic spectrum. Here, we investigate how thermoresponsive dielectric nanoenvironments will influence such PCD responses through poly(N-isopropylacrylamide) (PNIPAM) modified chiral gold nanorods (AuNRs). We observed the temperature-dependent chiral plasmonic responses distinctly from unmodified counterparts. As for the modified systems, the PCD peaks for both L-AuNRs and D-AuNRs at 50 °C red shifted simultaneously with enhanced intensities compared to the results at 20 °C. In contrast, the unmodified L-AuNRs and D-AuNRs exhibited no peak shift with reduced intensities. Subsequent simulation and experimental studies demonstrated that the enhanced PCD was attributed to PNIPAM chain collapse causing the increase of the refractive index by expelling minute water out of the corona surrounding chiral plasmonic AuNRs. Notably, such thermoresponsive chiral plasmonic responses are reversible, general, and extendable to other types of chiral plasmonic nanoparticles.

Magnetic nanoprobes for rapid detection of copper ion in aqueous environment by surface-enhanced Raman spectroscopy

Excessive copper ions in drinking water could cause serious health issues, such as gastrointestinal disorders and cirrhosis, and they are associated with Alzheimer's disease. ICP-OES, ICP-MS, and AAS are the most common methods of copper ion determination. However, the high cost of sample preparation and labor limit the possibility of on-site detection. In this study, rapid monitoring of copper ion through the SERS technique was evaluated. Fe₃O₄@SiO₂–Ag–4MBA nanoparticles were investigated as SERS-activated magnetic nanoprobes. These magnetic nanoprobes underwent superparamagnetism for rapid aggregation in seconds and provided selectivity in sensing copper ions. According to the dose–response curve of the SERS spectra, the limit of detection (LOD) was 0.421 ppm and the dynamic range was from 0.5 to 20 ppm in the presence of other metal ions. Copper ion detection through SERS was highly correlated with ICP-OES (R² = 0.95, slope = 0.974). These results demonstrate that magnetic nanoprobes may ultimately be used in a platform for on-site detection.

Magnetic SERS probes can rapidly detect copper ions within high precision and accuracy.

Effects of macrophage polarization on gold nanoparticle-assisted plasmonic photothermal therapy

Tumor associated macrophages (TAM) are key pathogenic factors in neoplastic diseases. They are known to have plasticity and can polarize into two opposing phenotypes, including the tumoricidal M1 and the protumoral M2 phenotypes with high prevalence of M2-phentoypes in patients with poor prognosis. Strategies for targeting M2-TAM may consequently increase the efficacy of therapeutic strategies for cancer treatment. Gold nanorod-assisted plasmonic photothermal therapy (PPTT) has emerged as a promising treatment for cancer but the effects of macrophage polarization parameters in the performance of this new treatment modality is still unknown. Herein, human monocytic THP-1 cells were polarized into two opposite phenotypic macrophages (M1-TAM and M2-TAM) and their response to PPTT was examined. M2-TAM exhibits a three-fold increase in AuNP uptake compared to M1-TAM. Laser irradiation results in selective killing of pro-tumoral M2-TAM after treatment with AuNPs with limited effects on anti-tumoral M1-TAM. A positive correlation between the expression of CD206 marker and the AuNP uptake may indicate the role of CD206 in facilitating AuNP uptake. Our findings also suggest that the differences in AuNP avidity and uptake between the M1-TAM and M2-TAM phenotypes may be the rationale behind the effectiveness of PPTT in the treatment of solid tumors.

A preferential uptake of gold nanoparticles by macrophages with a protumoral M2 phenotype result in efficient killing upon laser irradiation while keeping M1 phenotypes relatively undamaged.

Live-Cell Surface-Enhanced Raman Spectroscopy Imaging of Intracellular pH: From Two Dimensions to Three Dimensions

Visualization of intracellular pH (i-pH) using surface-enhanced Raman spectroscopy (SERS) plays an important role toward understanding of cellular processes including their interactions with nanoparticles. However, conventional two-dimensional SERS imaging often fails to take into consideration changes occurring in the whole-cell volume. We therefore aimed at obtaining a comprehensive i-pH profile of living cells by means of three-dimensional (3D) SERS imaging, thereby visualizing dynamic i-pH distribution changes in a single cell. We devised here a biocompatible and highly stable SERS pH probe, comprising plasmonic gold nanostars functionalized with a pH-sensitive Raman reporter tag-4-mercaptobenzoic acid-and protected by a cationic biocompatible polymer, poly-l-arginine hydrochloride (PA). The positively charged PA coating plays a double role in enhancing cell uptake and providing chemical and colloidal stability in cellular environments. The SERS-active pH probe allowed visualization of local changes in i-pH, such as acidification during nanoparticle (NP) endocytosis. We provide evidence of i-pH changes during NP endocytosis via high-resolution 3D SERS imaging, thereby opening new avenues toward the application of SERS to intracellular studies.

Study on the Assembly Structure Variation of Cetyltrimethylammonium Bromide on the Surface of Gold Nanoparticles

In this work, the self-assembly behavior of cetyltrimethylammonium bromide (CTAB) on the surface of citrate-capped gold nanoparticles (AuNPs) in solution has been studied by UV-vis absorption spectroscopy, fluorescence probe techniques, ζ potentiometric methods, transmission electron microscopy, etc. The UV-vis spectra show that the color with the increase of CTAB for the mixture containing CTAB and a given amount of AuNPs changes from red to blue and then to red. The absolute value of ζ potential corresponding to this color change decreases initially and then increases. Specially, the reversible color change, from red to blue and then to red, could be observed only in the case of a gradual addition of a AuNP solution to a CTAB solution; however, this reversible change is not suitable for the mixture formed in a reverse order of mixing. The results from pyrene used as the fluorescence probe indicate that the features in the fluorescence spectrum (including fluorescence quenching, I ₁/I ₃, and the excimer) well correspond to those from the UV-vis spectrum mentioned above. Based on the experimental results, the mechanism of the assembly structure variation of CTAB on the surface of negatively charged AuNPs was proposed. For a given amount of AuNPs, the assembly structure of CTAB on the surface of AuNPs undergoes the transformation from a monolayer to a bilayer with the increase of CTAB. In the case of the concentration of CTAB far beyond its critical micelle concentration (CMC) and the higher ratio of CTAB and AuNPs, there is a possibility of the formation of an extra micellar structure only after the formation of a double-layer structure.

In Situ Self-Assembly of Ultrastable Gold Nanoparticles on Polyvinyl Alcohol Nanofibrous Mats for Use as Highly Reusable Catalysts

Designing highly stable and reusable catalytic systems based on Au nanoparticles (NPs) is a significant challenge in nanocatalysis research. Here, we have fabricated polyvinyl alcohol (PVA) nanofibrous mat/Au NP composite catalysts with NPs in uniform size and good distribution by use of a developed in situ growth approach. In this method, Au seeds were first adsorbed on PVA nanofibrous mat surfaces rather than on relatively large Au NPs and then used to grow NPs in Au seed solution; thus, the steric hindrance effect was alleviated and a high loading was used for Au NPs up to 11 wt %. Strong interfacial interactions between the Au NPs and the PVA nanofibrous mats due to introducing a large number of hydrogen bonds provide high thermal stability for the PVA side chains, long-term catalytic stability, and excellent reusability. Consequently, the proposed in situ grown PVA/Au NP nanofibrous mats produce high catalytic activity for at least 15 cycles over a 30 d period. This work provides a potential approach for fabricating highly stable and reusable metal NPs on polymer nanofibrous mats to facilitate a wide variety of applications.

A General Approach to Free-Standing Nanoassemblies via Acoustic Levitation Self-Assembly

Droplets suspended by acoustic levitation provide genuine substrate-free environments for understanding unconventional fluid dynamics, evaporation kinetics, and chemical reactions by circumventing solid surface and boundary effects. Using a fully levitated air-water interface by acoustic levitation in conjunction with drying-mediated nanoparticle self-assembly, here, we demonstrate a general approach to fabricating free-standing nanoassemblies, which can totally avoid solid surface effects during the entire process. This strategy has no limitation for the sizes or shapes of constituent metallic nanoparticle building blocks and can also be applied to fabricate free-standing bilayered and trilayered nanoassemblies or even three-dimensional hollow nanoassemblies. We believe that our strategy may be further extended to quantum dots, magnetic particles, colloids, etc. Hence, it may lead to a myriad of homogeneous or heterogeneous free-standing nanoassemblies with programmable functionalities.

Monitoring the dynamics of hemeoxygenase-1 activation in head and neck cancer cells in real-time using plasmonically enhanced Raman spectroscopy

Real-time monitoring of the dynamics of pharmacologically generated HO-1 in mammalian cells by using plasmonically enhanced Raman spectroscopy (PERS).

We report for the first time the usage of plasmonically enhanced Raman spectroscopy (PERS) to directly monitor the dynamics of pharmacologically generated hemeoxygenase-1 (HO-1) by evaluating the kinetics of formation of carbon monoxide (CO), one of the metabolites of HO-1 activation, in live cells during cisplatin treatment. Being an endogenous signaling molecule, CO plays an important role in cancer regression. Many aspects of HO-1's and CO's functions in biology are still unclear largely due to the lack of technological tools for the real-time monitoring of their dynamics in live cells and tissues. In this study, we found that, together with nuclear region-targeted gold nanocubes (AuNCs), cisplatin treatment can dramatically trigger the activation of HO-1 and thereby the rate and production of CO in mammalian cells in a dose-dependent manner. Though quantitative molecular data revealed that a lower concentration of cisplatin up-regulates HO-1 expression in cancer cells, PERS data suggest that it poorly facilitates the activation of HO-1 and thereby the production of CO. However, at a higher dose, cisplatin along with AuNCs could significantly enhance the activation of HO-1 in cancer cells, which could be probed in real-time by monitoring the CO generation by using PERS. Under the same conditions, the rate of formation of CO in healthy cells was relatively higher in comparison to the cancer cells. Additionally, molecular data revealed that AuNCs have the potential to suppress the up-regulation of HO-1 in cancer cells during cisplatin treatment at a lower concentration. As up-regulation of HO-1 has a significant role in cell adaptation to oxidative stress in cancer cells, the ability of AuNCs in suppressing the HO-1 overexpression will have a remarkable impact in the development of nanoformulations for combination cancer therapy. This exploratory study demonstrates the unique possibilities of PERS in the real-time monitoring of endogenously generated CO and thereby the dynamics of HO-1 in live cells, which could expedite our understanding of the signaling action of CO and HO-1 in cancer progression.

Operando Investigation into Dynamic Evolution of Cathode-Electrolyte Interfaces in a Li-Ion Battery

While Li-ion battery cathode-electrolyte interfaces (CEIs) have been extensively investigated in recent decades, accurately identifying the chemical nature and tracking the dynamics of the CEIs during electrochemical cycling still remain a grand challenge. Here we report our findings in the investigation into the dynamic evolution of the interface between a LiNi_0.33Co_0.33Mn_0.33O₂ (LNMC) cathode and an ethylene carbonate/dimethyl carbonate (EC/DMC)-based electrolyte using surface-enhanced Raman spectroscopy (SERS) performed on a model cell under typical battery operating conditions. In particular, the strong SERS activity provided by a monolayer of Au nanocubes deposited on a model LNMC electrode (additive-free) enables quasi-quantitative assessment of the CEI evolution during cycling, proving information vital to revealing the dynamics of the species adsorbed on the LNMC surface as a function of cell potential. Furthermore, our theoretical calculation, which is based on the interaction between a model interface-bound molecule and a model LNMC surface, agrees with our experimental observation. The carefully designed operando SERS platform has demonstrated high sensitivity, good surface specificity, and excellent compatibility with extensive electrochemical measurements; it is also applicable to fundamental studies of dynamic interfaces in other electrochemical energy storage and conversion systems.

Plasmonic Nanocomposites Based on Silver Nanocube-Polymer Blends Displaying Nearly Perfect Absorption in the UV Region

Plasmonic nanocomposites based on well-dispersed silver nanocubes in poly(vinylpyrrolidone) are presented that are solution-processed into layers of varying volume fractions of nanocubes. We show that the high-energy modes of the nanocubes are almost insensitive to plasmonic coupling within the nanocube assemblies, leading to a linear increase in light absorption in the UV region with the nanocube densities. Concerning the main dipolar resonance mode at 450 nm, it is strongly affected by the formation of these assemblies, leading to an increased absorption in the UV region as well as a large absorption band in the visible region. Simulations of the optical response of the nanocube assemblies as a function of nanocube spacing and electric field polarization reveal that optical features in the visible region are due to intercube couplings at short intercube distances and parallel electric field orientation. In contrast, the additional plasmonic band in the UV region has its origin in residual dipolar oscillations of the nanocubes in combination with weak dipolar coupling for both parallel and transversal field polarizations. The combination of these effects leads to an enlarged absorption band in the UV region with nearly perfect light absorption of 98.8% at a high silver volume fraction of 8% that is accompanied by a very weak specular reflection of only 0.28%. Although such perfect absorption is usually observed only when nanocubes are assembled on a gold surface, nearly perfect absorption herein is achieved on a large palette of substrates including glass, plastic, and cheap metals such as aluminum, making it a promising approach for solution-processed robust and cheap quasi-perfect absorption coatings.

Seed-Mediated Growth of Colloidal Metal Nanocrystals

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

Overgrowth of Silver Nanodisks on a Substrate into Vertically Aligned Nanopillars for Chromatic Light Polarization

Vertically aligned and well-separated 1D silver nanopillars (AgNPLs) are prepared on a large-area quartz surface using a robust colloidal chemical technique. Silver nanodisk (AgND) monolayers were first deposited on quartz using the Langmuir-Blodgett technique, and the presence of the substrate induced asymmetric chemical overgrowth of the AgNDs into AgNPLs. The height and diameter of the prepared AgNPLs were controlled by changing the rate of the overgrowth reaction. Chloride ions were used during overgrowth to etch the silver atoms that formed sharp features on the sides of the AgNDs and to limit growth in the lateral direction. The grown AgNPLs displayed two surface plasmon resonance modes corresponding to the transverse and longitudinal electron oscillations. The intensity of the longitudinal mode increased by a factor of 9 while the intensity of the transverse mode decreased by a factor of 2.5 upon increasing the angle of incidence of the exciting light from 0° to 60°. This interesting property makes these AgNPL arrays on quartz useful as chromatic light polarizers.

In vitro biomechanical properties, fluorescence imaging, surface-enhanced Raman spectroscopy, and photothermal therapy evaluation of luminescent functionalized CaMoO4:Eu@Au hybrid nanorods on human lung adenocarcinoma epithelial cells

Highly dispersible Eu3+-doped CaMoO4@Au-nanorod hybrid nanoparticles (HNPs) exhibit optical properties, such as plasmon resonances in the near-infrared region at 790 nm and luminescence at 615 nm, offering multimodal capabilities: fluorescence imaging, surface-enhanced Raman spectroscopy (SERS) detection and photothermal therapy (PTT). HNPs were conjugated with a Raman reporter (4-mercaptobenzoic acid), showing a desired SERS signal (enhancement factor 5.0 × 105). The HNPs have a heat conversion efficiency of 25.6%, and a hyperthermia temperature of 42°C could be achieved by adjusting either concentration of HNPs, or laser power, or irradiation time. HNPs were modified with antibody specific to cancer biomarker epidermal growth factor receptor, then applied to human lung cancer (A549) and mouse hepatocyte cells (AML12), and in vitro PTT effect was studied. In addition, the biomechanical properties of A549 cells were quantified using atomic force microscopy. This study shows the potential applications of these HNPs in fluorescence imaging, SERS detection, and PTT with good photostability and biocompatibility.

Influence of Surfactant Bilayers on the Refractive Index Sensitivity and Catalytic Properties of Anisotropic Gold Nanoparticles

Shape-controlled synthesis of gold nanoparticles generally involves the use of surfactants, typically cetyltrimethylammonium (CTAX, X = Cl(-) , Br(-)), to regulate the nucleation growth process and to obtain colloidally stable nanoparticles. The surfactants adsorb on the nanoparticle surface making further functionalization difficult and therefore limit their use in many applications. Herein, the influence of CTAX on nanoparticle sensitivity to local dielectric environment changes is reported. It is shown, both experimentally and theoretically, that the CTAX bilayer significantly reduces the refractive index (RI) sensitivity of anisotropic gold nanoparticles such as nanocubes and concave nanocubes, nanorods, and nanoprisms. The RI sensitivity can be increased by up to 40% by removing the surfactant layer from nanoparticles immobilized on a solid substrate using oxygen plasma treatment. This increase compensates for the otherwise problematic decrease in RI sensitivity caused by the substrate effect. Moreover, the removal of the surfactants both facilitates nanoparticle biofunctionalization and significantly improves their catalytic properties. The strategy presented herein is a simple yet effective universal method for enhancing the RI sensitivity of CTAX-stabilized gold nanoparticles and increasing their potential as transducers in nanoplasmonic sensors, as well as in catalytic and biomedical applications.

Controlled graphene oxide assembly on silver nanocube monolayers for SERS detection: dependence on nanocube packing procedure

Hybrid graphene oxide/silver nanocubes (GO/AgNCs) arrays for surface-enhanced Raman spectroscopy (SERS) applications were prepared by means of two procedures differing for the method used in the assembly of the silver nanocubes onto the surface: Langmuir-Blodgett (LB) transfer and direct sequential physisorption of silver nanocubes (AgNCs). Adsorption of graphene oxide (GO) flakes on the AgNC assemblies obtained with both procedures was monitored by quartz crystal microbalance (QCM) technique as a function of GO bulk concentration. The experiment provided values of the adsorbed GO mass on the AgNC array and the GO saturation limit as well as the thickness and the viscoelastic properties of the GO film. Atomic force microscopy (AFM) measurements of the resulting samples revealed that a similar surface coverage was achieved with both procedures but with a different distribution of silver nanoparticles. In the GO covered LB film, the AgNC distribution is characterized by densely packed regions alternating with empty surface areas. On the other hand, AgNCs are more homogeneously dispersed over the entire sensor surface when the nanocubes spontaneously adsorb from solution. In this case, the assembly results in less-packed silver nanostructures with higher inter-cube distance. For the two assembled substrates, AFM of silver nanocubes layers fully covered with GO revealed the presence of a homogeneous, flexible and smooth GO sheet folding over the silver nanocubes and extending onto the bare surface. Preliminary SERS experiments on adenine showed a higher SERS enhancement factor for GO on Langmuir-Blodgett films of AgNCs with respect to bare AgNC systems. Conversely, poor SERS enhancement for adenine resulted for GO-covered AgNCs obtained by spontaneous adsorption. This indicated that the assembly and packing of AgNCs obtained in this way, although more homogeneous over the substrate surface, is not as effective for SERS analysis.

Self-assembled structures of polyhedral gold nanocrystals: shape-directive arrangement and structure-dependent plasmonic enhanced characteristics

Self-assembly structures of different types of polyhedral nanocrystals through drop casting method and their plasmonic enhancement characteristics and SERS performances due to the nano-antenna effect.

Elucidation of ultraviolet radiation-induced cell responses and intracellular biomolecular dynamics in mammalian cells using surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy has been used to elucidate biomolecular dynamics on the response of mammalian cells towards UV light irradiation.

Fingerprinting biochemical changes associated with cellular responses to external stimuli can provide vital information on the dynamics of biological processes and their defense mechanisms. In this study, surface-enhanced Raman spectroscopy (SERS) has been used to elucidate biomolecular dynamics on the response of healthy and cancerous cells towards ultraviolet (UV) light irradiation at the cellular level in real-time. We have identified a number of physiochemical damages to proteins, especially to the chemical structure of the sulfur and aromatic amino acid containing moieties, as well as changes in secondary structure. Furthermore, we found that continuous exposure of short wave UV-C light (254 nm) to living cells can photolytically damage intracellular proteins and can completely arrest nanoparticle transport and trigger apoptosis. However, under similar conditions, this was not observed when the cells were exposed to long wave UV-A light (365 nm). These biomolecular events were probed in real-time using SERS and dark-field (DF) imaging. Specifically, this technique has been utilized for the real-time evaluation of a unique cellular defense mechanism in cancer cells towards UV exposure. Our technique provides a powerful approach to understand the mechanisms of UV light-triggered cell death, protein dynamics, and enhanced cell repair and defense machinery within cancer cells through actively monitoring molecular vibrations.

High conversion of HAuCl4 into gold nanorods: A re-seeding approach

Gold nanorods with varying aspect ratios have been utilized in recent years for a wide range of applications including vaccines, surface enhanced Raman spectroscopy (SERS) substrates, and as medicinal therapeutic agents. The surfactant-directed seed mediated approach is an aqueous based protocol that produces monodisperse nanorods with controlled aspect ratios. However, an inherent problem with this approach is poor efficiency of gold conversion from HAuCl4 into nanorods. In fact only ∼15% of gold is converted, motivating the need for alternate synthetic protocols in order to make the process more scalable and efficient as gold nanorods progress toward commercial applications. In the current study, we have significantly improved this conversion by growing rods in several iterations of supernatant solutions that were previously discarded as waste. Inductively coupled plasma mass spectrometry (ICP-MS) data indicates ∼14% gold conversion per nanorod solution with a total recovery of ∼75%. Gold nanorods prepared in consecutive supernatant solutions generally have slightly increased aspect ratios and maintain stability and monodispersity as measured by UV-vis and TEM. The increased nanorod yield minimizes gold waste and results in a greener synthetic approach.

Super-radiant plasmon mode is more efficient for SERS than the sub-radiant mode in highly packed 2D gold nanocube arrays

The field coupling in highly packed plasmonic nanoparticle arrays is not localized due to the energy transport via the sub-radiant plasmon modes, which is formed in addition to the regular super-radiant plasmon mode. Unlike the sub-radiant mode, the plasmon field of the super-radiant mode cannot extend over long distances since it decays radiatively with a shorter lifetime. The coupling of the plasmon fields of gold nanocubes (AuNCs) when organized into highly packed 2D arrays was examined experimentally. Multiple plasmon resonance optical peaks are observed for the AuNC arrays and are compared to those calculated using the discrete dipole approximation. The calculated electromagnetic plasmon fields of the arrays displayed high field intensity for the nanocubes located in the center of the arrays for the lower energy super-radiant mode, while the higher energy sub-radiant plasmon mode displayed high field intensity at the edges of the arrays. The Raman signal enhancement by the super-radiant plasmon mode was found to be one hundred fold greater than that by sub-radiant plasmon mode because the super-radiant mode has higher scattering and stronger plasmon field intensity relative to the sub-radiant mode.

On-column enrichment and surface-enhanced Raman scattering detection in nanoparticles functionalized porous capillary monolith

A monolithic column functionalized with gold nanoparticles (GNPs) was designed to provide ultrasensitive detection with surface-enhanced Raman scattering (SERS). The monolithic column based on poly (glycidyl methacrylate-co-ethylene dimethacrylate) (GMA-co-EDMA) was served as the enrichment sorbent to concentrate GNPs and targets. We found that 60 nm GNPs-functionalized monolithic columns demonstrated the best SERS enhancement, and the lowest detectable concentration for PATP and CV could be achieved at 10(-7) and 10(-11) M, respectively. Moreover, the columns exhibit a good reproducibility in both spot-to-spot (∼10%) and batch-to-batch (∼15%). The SERS monolithic column with a high sensitivity and reproducibility has a great potential in the field-based rapid detection of targets in complex real-world samples.

Concave gold nanocube assemblies as nanotraps for surface-enhanced Raman scattering-based detection of proteins

SERS detection of proteins is typically performed by using labeling agents with stable and high Raman scattering cross sections. This is a valuable approach for trace detection and quantification of a target protein but is unsuitable for inspecting its inherent structural and functional properties. On the other hand, direct SERS of proteins has been mainly devoted to the study of short peptides and aminoacid sequences or of prosthetic groups with intense Raman signals, which is of scarce interest for a thorough characterization of most proteins. Here we try to overcome these limitations by setting-up an effective platform for the structural SERS analysis of proteins. The platform consists of an extended bidimensional array of gold concave nanocubes (CNCs) supported on a PDMS film. CNCs are closely-packed through face-face and face-corner interactions generating a monolayered arrangement featuring well distributed nanoholes. Here the protein homogeneously experiences an E-field enhancement outward from the metal surfaces surrounding it, which causes a large number of vibrations to be contemporarily amplified. The proposed platform provides stable and detailed SERS spectra and confers rapidity and reproducibility to the analysis.

Using gold nanoparticles as probe for detection of salmeterol xinafoate by resonance Rayleigh light scattering

The paper explores the method of determination of salmeterol xinafoate at nanogram level with gold nanoparticles (AuNPs) probe, to measure the intensity of resonance Rayleigh light scattering (RLS) by a common spectrofluorometer. The RLS intensity of salmeterol xinafoate was greatly enhanced by AuNPs, with the maximum scattering peak at 357 nm. The salmeterol xinafoate was determined basing on the binding of salmeterol xinafoate to AuNPs by electrostatic adsorption. Under the optimum conditions, the enhanced RLS intensity was directly proportional to the concentration of salmeterol xinafoate in the range of 0.054-6.038 μg mL(-1) with a good linear relationship (r=0.9928). The limit of detection (LOD) was 9.48 ng mL(-1). The interference tests were performed carefully. With the proposed method, the synthetic samples were analyzed satisfactorily, the recovery and RSD were 102.5-103.0% and 0.67-1.0% respectively.

Plasmonic nanorattles with intrinsic electromagnetic hot-spots for surface enhanced Raman scattering

The synthesis of plasmonic nanorattles with accessible electromagnetic hotspots that facilitate highly sensitive detection of chemical analytes using surface enhanced Raman scattering (SERS) is demonstrated. Raman spectra obtained from individual nanorattles demonstrate the significantly higher SERS activity compared to solid plasmonic nanostructures.

Unraveling the biomolecular snapshots of mitosis in healthy and cancer cells using plasmonically-enhanced Raman spectroscopy

Owing to the dynamic and complex nature of mitosis, precise and timely executions of biomolecular events are critical for high fidelity cell division. In this context, visualization of such complex events at the molecular level can provide vital information on the biomolecular processes in abnormal cells. Here, we explored the plasmonically enhanced light scattering properties of functionalized gold nanocubes (AuNCs) together with surface-enhanced Raman spectroscopy (SERS) to unravel the complex and dynamic biological processes involved in mitosis of healthy and cancerous cells from its molecular perspectives. By monitoring various stages of mitosis using SERS, we noticed that relatively high rate of conversion of mitotic proteins from their α-helix structure to β-sheet conformation is likely in the cancer cells during meta-, ana-, and telophases. Unique biochemical modifications to the lipid and amino acid moieties, associated with the observed protein conformational modifications, were also identified. However, in healthy cells, the existence of proteins in their β conformation was momentary and was largely in the α-helix form. The role of abnormal conformational modifications of mitotic proteins on the development of anomalous mitotic activities was further confirmed by looking at plasmonic nanoparticle-induced cytokinesis failure in cancer cells. Our findings illustrate the vast possibilities of SERS in real-time tracking of complex, subtle, and momentary modifications of biomolecules in live cells, which could provide new insights to the role of protein conformation dynamics during mitosis on the development of cancer and many other diseases.

Gold nanostar substrates for SERS-based chemical sensing in the femtomolar regime

We report a novel approach for fabricating gold nanostar-functionalized substrates for highly sensitive surface enhanced Raman spectroscopy (SERS)-based chemical sensing. Gold nanostars immobilized on a gold substrate via a Raman silent organic tether serve as the SERS substrate, and facilitate the chemical sensing of analytes that can either be chemisorbed or physisorbed on the nanostars. Our SERS substrates are capable of detecting chemisorbed 4-mercaptobenzoic acid at a concentration as low as 10 fM with a reproducible SERS enhancement factor of 10(9), and enable the semi-quantitative multiplexed identification of analytes from mixtures in which they have been dissolved in variable stoichiometry. Most importantly, they afford the detection of physisorbed analytes, such as crystal violet, with an excellent signal-to-noise ratio, hence serving as a versatile platform for the chemical identification of in principle any molecular analyte. These characteristics make a strong case for the use of our nanostar-based SERS substrate in practical chemical sensing applications.

Plasmon Resonance Hybridization of Gold Nanospheres and Palladium Nanoshells Combined in a Rattle Structure

Gold and palladium nanoparticles are characterized by their localized surface plasmon resonance (LSPR). In contrast with the sharp LSPR spectrum of gold nanoparticles, palladium nanoparticles had a broad LSPR spectrum. Palladium-gold nanorattles (PdAuNRT) are an ideal system with optical properties that are a hybrid of gold and palladium nanoparticles. The PdAuNRTs consisted of small gold nanospheres (AuNSs) located inside hollow palladium nanospheres (PdHNSs) of larger sizes without touching each other. PdAuNRTs of various sizes were synthesized by systematic variation of the experimental parameters. Interestingly, for the PdAuNRTs, where PdHNSs and AuNSs are separated by a distance, it was found that the broad plasmon resonance band of the PdHNSs hybridizes with the sharp plasmon resonance of the AuNSs located in its center. This was further confirmed experimentally by optical absorption measurements and theoretically using discrete dipole approximation technique. The plasmon resonance hybridization resulted in broadening of the LSPR spectrum of the PdAuNRTs and the appearance of a dip due to a Fano resonance.

Silica-coated dimers of silver nanospheres as surface-enhanced Raman scattering tags for imaging cancer cells

Surface-enhanced Raman scattering (SERS) tags have been actively explored as a multiplexing platform for sensitive detection of biomolecules. Here, we report a new type of SERS tags that was fabricated by sequentially functionalizing dimers made of 50 nm Ag nanospheres with 4-mercaptobenzoic acid as the Raman reporter molecule, silica coating as a protective shell and antibody as a targeting ligand. These dimer-based tags give highly enhanced and reproducible Raman signals owing to the presence of a well-defined SERS hot spot at the junction between two Ag nanospheres in the dimer. The SERS enhancement factor (EF) of an individual dimer tag supported on a glass slide can reach a level as high as 4.3 × 10(6). In comparison, the EFs dropped to 2.8 × 10(5) and 8.7 × 10(5), respectively, when Ag nanospheres and nanocubes with sizes similar to the spheres in the dimer were used to fabricate the tags using similar procedures. The SERS signals from aqueous suspensions of the dimer-based tags also showed high intensity and good stability. Potential use of the dimer-based tags was demonstrated by imaging cancer cells overexpressing HER2 receptors with good specificity and high sensitivity.

Probing distance-dependent plasmon-enhanced near-infrared fluorescence using polyelectrolyte multilayers as dielectric spacers

Owing to their applications in biodetection and molecular bioimaging, near-infrared (NIR) fluorescent dyes are being extensively investigated. Most of the existing NIR dyes exhibit poor quantum yield, which hinders their translation to preclinical and clinical settings. Plasmonic nanostructures are known to act as tiny antennae for efficiently focusing the electromagnetic field into nanoscale volumes. The fluorescence emission from NIR dyes can be enhanced by more than thousand times by precisely placing them in proximity to gold nanorods. We have employed polyelectrolyte multilayers fabricated using layer-by-layer assembly as dielectric spacers for precisely tuning the distance between gold nanorods and NIR dyes. The aspect ratio of the gold nanorods was tuned to match the longitudinal localized surface plasmon resonance wavelength with the absorption maximum of the NIR dye to maximize the plasmonically enhanced fluorescence. The design criteria derived from this study lays the groundwork for ultrabright fluorescence bullets for in vitro and in vivo molecular bioimaging.

Plasmonic cavities derived from silver nanoparticles atop a massed silver surface for surface enhancement Raman scattering

Various plasmonic cavities (PC) are formed by positioning silver nanocubes or nanospheres on a massed silver surface, being magnificently useful for surface enhancement Raman scattering (SERS) application.

Copper-templated synthesis of gold microcages for sensitive surface-enhanced Raman scattering activity

Nanoparticle-aggregated Au microcages have been successfully synthesized from sacrificial hollow Cu microstructures, and exhibit remarkable surface-enhanced Raman scattering activity for 4-mercaptobenzoic acid.

Ni and p-Cu2O nanocubes with a small size distribution by templated electrodeposition and their characterization by photocurrent measurement

A method for the reproducible formation of Ni and Cu2O nanocubes with dimensions of 200-500 nm and a small size distribution is introduced. For this, the well-known templated electrodeposition technique was extended to cubic PMMA templates made by nanoimprint lithography. When making cubic templates in larger quantities, this method has the potential to become simple and cost-effective. This method was successfully used for the formation of Ni and p-Cu2O nanocubes as well as for the formation of segmented nanobars containing both phases. The lateral dimensions of the nanocubes exactly resembled the dimensions of the template, and the height could be varied by adjusting the deposition time. Nanocubes formed via this method can remain attached to the substrate or can be dispersed in solution. p-Cu2O is considered to be one of the most promising photocathode materials for solar water splitting. It is demonstrated that the activity of the p-Cu2O nanocubes for photocatalytic water splitting can be measured, and it was found that the nanocube morphology enhances the photocatalytic activity compared to thin films.

Figures of merit of a SERS method for Sudan I determination at traces levels

A substrate for Surface-Enhanced Raman Scattering spectroscopy (SERS), electropolished Al, is proposed as a tool for a rapid and low cost determination of Sudan I. This dye has been used as an additive in some foodstuffs but it is now banned because of the health risk associated with its carcinogenic and mutagenic properties. Despite the presence of fluorescence, Raman spectra of Sudan I can be obtained using excitation lasers at 633 and 785 nm. To get rid of the spectral noise and fluorescence background, Savitzky-Golay smoothing and polynomial corrections were applied, respectively. The Raman signal was proved to be enhanced. A linear dependence was found between the logarithmic intensity at 1598 cm(-1) peak versus the logarithmic concentration. The figures of merit were studied obtaining high sensitivity and low detection limits (10(-7) M). A multivariate exploratory analysis (PCA) was used to study the ability of SERS to distinguish Sudan I from other similar compounds. Therefore, results show that SERS is a potential tool to determine Sudan I quickly and effectively.

Solution-dispersible Au nanocube dimers with greatly enhanced two-photon luminescence and SERS

We report the synthesis of 43-nm diameter Au nanocube dimers by using Ag(+) ions as competitive ligands to freeze L-cysteine-induced assembly process of the nanocubes to a desirable stage. Ascribed to the resonant interparticle coupling with an newly arising plasmon band at 710 nm and local field enhancement, the two-photon luminescence intensity of the Au nanocube dimers in solution was over 20 times stronger than that of the monomers in the wavelength range 555-620 nm. Furthermore, by coupling Raman tags onto the nanocube surface, a solution-based surface-enhanced Raman scattering (SERS) of the nanocube dimers had an enhancement factor of over 10 times compared to the isolated nanocubes. To sum up, with high stability in solution and attractive optical properties, the Au nanocube dimers have potential applications in in vivo bio-imaging and solution-based SERS.

Plasmonic planet-satellite analogues: hierarchical self-assembly of gold nanostructures

In the past few years, a remarkable progress has been made in unveiling novel and unique optical properties of strongly coupled plasmonic nanostructures, known as plasmonic molecules. However, realization of such plasmonic molecules using nonlithographic approaches remains challenging largely due to the lack of facile and robust assembly methods. Previous attempts to achieve plasmonic nanoassemblies using molecular ligands were limited to dipolar assembly of nanostructures, which typically results in polydisperse linear and branched chains. Here, we demonstrate that core-satellite structures comprised of shape-controlled plasmonic nanostructures can be achieved through self-assembly using simple molecular cross-linkers. Prevention of self-conjugation and promotion of cross-conjugation among cores and satellites plays a key role in the formation of core-satellite heteroassemblies. The in-built electromagnetic hot-spots and Raman reporters of core-satellite structures make them excellent candidates for surface-enhanced Raman scattering probes.