Identification of Metalloporphyrins with High Sensitivity Using Graphene-Enhanced Resonance Raman Scattering

Graphene-enhanced resonance Raman scattering (GERRS) was performed for the detection of three different metallo-octaethylporphyrins (M-OEPs; M = 2H, FeCl, and Pt) homogeneously thermal vapor deposited on a graphene surface. GERRS of M-OEPs were measured using three different excitation wavelengths, λ_ex = 405, 532, and 633 nm, and characterized detail vibrational bands for the identification of M-OEPs. The GERRS spectra of Pt-OEP at λ_ex = 532 nm showed ∼29 and ∼162 times signal enhancement ratio on graphene and on graphene with Ag nanoclusters, respectively, compared to the spectra from bare SiO₂ substrate. This enhancement ratio, however, was varied with M-OEPs and excitation wavelengths. The characteristic peaks and band shapes of GERRS for each M-OEP were measured with high sensitivity (100 pmol of thermal vapor deposited Pt-OEP), and these facilitate the selectively recognition of molecules. Also, the peaks shift and broadening provide the evidence of the interaction between graphene and M-OEPs through the charge transfer and π-orbital interaction. The increase of graphene layer induced the decrease of signal intensity and GERRS effect was almost not observed on the thick graphite flakes. Further experiments with various substrates demonstrated that the interaction of single layer of graphene with molecule is the origin of the Raman signal enhancement of M-OEPs. In this experiment, we proved the graphene is a good alternative substrate of Raman spectroscopy for the selective detection of various metalloporphyrins with high sensitivity

PSA Detection with Femtomolar Sensitivity and a Broad Dynamic Range Using SERS Nanoprobes and an Area-Scanning Method

Recently, surface-enhanced Raman scattering (SERS)-based immunoassays (SIA) have drawn much attention as diagnostic tools with large multiplex capacity and high sensitivity. However, several challengessuch as a low reproducibility, a time-consuming read-out process, and limited dynamic rangeremain. In this study, we report a reliable and sensitive SIA platform for prostate specific antigen (PSA) detection. Reliability and sensitivity were achieved by two approaches: (1) well-established SERS probes, so-called SERS dots that have high sensitivity (single particle detection) and little particle-to-particle variation in SERS intensity; and (2) a whole area-scanning readout method for rapid and reliable chip analysis rather than point scanning. As a feasibility test, PSA could be detected with high sensitivity (ca. 0.11 pg/mL, 3.4 fM LOD), with a wide dynamic range (0.001–1000 ng/mL). Thus, the developed platform will facilitate development of reliable immunoassays with high sensitivity and a wide dynamic range

Ag Shell–Au Satellite Hetero-Nanostructure for Ultra-Sensitive, Reproducible, and Homogeneous NIR SERS Activity

It is critical to create isotropic hot spots in developing a reproducible, homogeneous, and ultrasensitive SERS probe. Here, an Ag shell–Au satellite (Ag–Au SS) nanostructure composed of an Ag shell and surrounding Au nanoparticles was developed as a near-IR active SERS probe. The heterometallic shell-satellite structure based SERS probe produced intense and uniform SERS signals (SERS enhancement factor ∼1.4 × 10⁶ with 11% relative standard deviation) with high detectability (100% under current measurement condition) by 785 nm photoexcitation. This signal enhancement was independent of the laser polarizations, which reflects the isotropic feature of the SERS activity of Ag–Au SS from the three-dimensional (3D) distribution of SERS hot spots between the shell and the surrounding satellite particles. The Ag–Au SS nanostructure shows a great potential as a reproducible and quantifiable NIR SERS probe for in vivo targets

Subnanomolar Sensitivity of Filter Paper-Based SERS Sensor for Pesticide Detection by Hydrophobicity Change of Paper Surface

As a cost-effective approach for detecting trace amounts of pesticides, filter paper-based SERS sensors have been the subject of intensive research. One of the hurdles to overcome is the difficulty of retaining nanoparticles on the surface of the paper because of the hydrophilic nature of the cellulose fibers in paper. This reduces the sensitivity and reproducibility of paper-based SERS sensors due to the low density of nanoparticles and short retention time of analytes on the paper surface. In this study, filter paper was treated with alkyl ketene dimer (AKD) to modify its property from hydrophilic to hydrophobic. AKD treatment increased the contact angle of the aqueous silver nanoparticle (AgNP) dispersion, which consequently increased the density of AgNPs. The retention time of the analyte was also increased by preventing its rapid absorption into the filter paper. The SERS signal was strongly enhanced by the increased number of SERS hot spots owing to the increased density of AgNPs on a small contact area of the filter surface. The reproducibility and sensitivity of the SERS signal were optimized by controlling the distribution of AgNPs on the surface of the filter paper by adjusting the concentration of the AgNP solution. Using this SERS sensor with a hydrophobicity-modified filter paper, the spot-to-spot variation of the SERS intensity of 25 spots of 4-aminothiophenol was 6.19%, and the limits of detection of thiram and ferbam as test pesticides were measured to be 0.46 nM and 0.49 nM, respectively. These proof-of-concept results indicate that this paper-based SERS sensor can serve for highly sensitive pesticide detection with low cost and easy fabrication

Screening of Pro–Asp Sequences Exposed on Bacteriophage M13 as an Ideal Anchor for Gold Nanocubes

Bacteriophages are thought to be ideal vehicles for linking antibodies to nanoparticles. Here, we define the sequence of peptides exposed as a fusion protein on M13 bacteriophages to yield optimal binding of gold nanocubes and efficient bacteriophage amplification. We generated five helper bacteriophage libraries using AE­(X)₂DP, AE­(X)₃DP, AE­(X)₄DP, AE­(X)₅DP, and AE­(X)₆DP as the exposed portion of pVIII, in which X was a randomized amino acid residue encoded by the nucleotide sequence NNK. Efficient phage amplification was achievable only in the AE­(X)₂DP, AE­(X)₃DP, and AE­(X)₄DP libraries. Through biopanning with gold nanocubes, we enriched the phage clones and selected the clone with the highest fold change after enrichment. This clone displayed Pro–Asp on the surface of the bacteriophage and had amplification yields similar to those of the wild-type helper bacteriophage (VCSM13). The clone displayed even binding of gold nanocubes along its length and minimal aggregation after binding. We conclude that, for efficient amplification, the exposed pVIII amino acid length should be limited to six residues and Ala–Glu–Pro–Asp–Asp–Pro (AEPDDP) is the ideal fusion protein sequence for guaranteeing the optimal formation of a complex with gold nanocubes

Gold Nanoparticle/Graphene Oxide Hybrid Sheets Attached on Mesenchymal Stem Cells for Effective Photothermal Cancer Therapy

Cell-mediated nanoparticle delivery has been proposed for an effective cancer therapy. However, there are limitations in loading nanoparticles within cells as the internalized nanoparticles cause cytotoxicity and leak out of the cells via exocytosis. Here, we introduce hybrid sheets composed of gold nanoparticles (AuNPs) and graphene oxide (GO), which stably adhere to the cell surface and exhibit a remarkable photothermal effect. To form AuNP/GO sheets in which GO is sandwiched between two AuNP monolayers, AuNPs are coated with α-synuclein protein and subsequently adsorbed onto GO sheets. Attaching AuNP/GO sheets to the tumor-tropic mesenchymal stem cell (MSC) surface enhances the loading efficiency of AuNPs in MSCs by avoiding the cytotoxicity and exocytosis issues. Furthermore, the tight packing of AuNPs on microscaled GO sheets enhances the photothermal effect via strong plasmon coupling between AuNPs. The injection of AuNP/GO sheet-attached MSCs into tumor-bearing mice significantly improves the photothermal therapeutic efficacy by delivering larger amounts of AuNPs to the tumor and generating higher heat at the tumor region compared to injection of AuNP-internalized MSCs. The system of attaching AuNP/GO hybrid sheets to the tumor-tropic cell surface may be an effective platform for cancer therapy

Single-Step and Rapid Growth of Silver Nanoshells as SERS-Active Nanostructures for Label-Free Detection of Pesticides

We explored a single-step approach for the rapid growth of Ag nanoshells (Ag NSs) under mild conditions. Without predeposition of seed metals, a uniform and complete layer of Ag shells was rapidly formed on silica core particles within 2 min at 25 °C via single electron transfer from octylamine to Ag⁺ ions. The size and thickness of the Ag NSs were effectively tuned by adjusting the concentration of silica nanoparticles (silica NPs) with optimal concentrations of AgNO₃ and octylamine. This unusually rapid growth of Ag NSs was attributed to a significant increase in the reduction potential of the Ag⁺ ions in ethylene glycol (EG) through the formation of an Ag/EG complex, which in turn led to their facile reduction by octylamine, even at room temperature. A substantial enhancement in the surface-enhanced Raman scattering (SERS) of the prepared Ag NSs was demonstrated. The Ag NSs were also utilized as SERS-active nanostructures for label-free detection of the pesticide thiram. The Ag NS-based SERS approach successfully detected thiram on apple peel down to the level of 38 ng/cm² in a label-free manner, which is very promising with respect to its potential use for the on-site detection of residual pesticides

Physicochemical Determinants of Multiwalled Carbon Nanotubes on Cellular Toxicity: Influence of a Synthetic Method and Post-treatment

Since the discovery of carbon nanotubes (CNTs), scientists have performed extensive studies on nanotubes in the fields of materials science, physics, and electronic engineering. Because multiwalled CNTs (MWCNTs) are not homogeneous materials, and because it is not feasible to test every newly synthesized MWCNT, this study was aimed at investigating the physicochemical properties that primarily determine the cellular toxicity of MWCNTs. This study analyzed the relationship between cell viability and physicochemical characteristics following exposure to eight different MWCNTs. We generated eight different MWCNTs using various synthetic methods and post-treatments. From this analysis, we sought to identify the major physicochemical determinants that could predict the cellular toxicity of MWCNTs, regardless of the synthetic method and post-treatment conditions. Creation of binding sites on the tube walls by breaking C–C bonds played a pivotal role in increasing toxicity and was most clearly demonstrated by a Raman G peak shift and the <i>I</i>_D/<i>I</i>_G ratio. In addition, several factors were found to be strongly related to cellular toxicity: surface charge in the case of MWCNTs created by the chemical vapor deposition method and surface area and EPR intensity in the case of MWCNTs created by the arc discharge based method. The methods developed in this study could be applied to the prediction of the toxicity of newly synthesized MWCNTs

Concave Rhombic Dodecahedral Au Nanocatalyst with Multiple High-Index Facets for CO₂ Reduction

A concave rhombic dodecahedron (RD) gold nanoparticle was synthesized by adding 4-aminothiophenol (4-ATP) during growth from seeds. This shape is enclosed by stabilized facets of various high-indexes, such as (331), (221), and (553). Because it is driven thermodynamically and stabilized by 4-ATP ligands, the concave RD maintains its structure over a few months, even after rigorous electrochemical reactions. We discussed the mechanism of the shape evolution controlled by 4-ATP and found that both the binding energy of Au–S and the aromatic geometry of 4-ATP are major determinants of Au atom deposition during growth. As a possible application, we demonstrated that the concave RD exhibits superior electrocatalytic performance for the selective conversion of CO₂ to CO in aqueous solution

Polymer-Mediated Formation and Assembly of Silver Nanoparticles on Silica Nanospheres for Sensitive Surface-Enhanced Raman Scattering Detection

To impart a desired optical property to metal nanoparticles (NPs) suitable for surface-enhanced Raman scattering (SERS) applications, it is crucial to assemble them in two or three dimensions in addition to controlling their size and shape. Herein, we report a new strategy for the synthesis and direct assembly of Ag NPs on silica nanospheres (AgNPs-SiNS) in the presence of poly­(ethylene glycol) (PEG) derivatives such as PEG-OH, bis­(amino)-PEGs (DA-PEGs), and <i>O</i>,<i>O</i>′-bis­(2-aminopropyl)­PEG (DAP-PEG). They exhibited different effects on the formation of Ag NPs with variable sizes (10–40 nm) and density on the silica surface. As the molecular weight (MW) of DA-PEGs increased, the number of Ag NPs on the silica surface increased. In addition, DAP-PEG (MW of 2000), which has a 2-aminopropyl moiety at both ends, promoted the most effective formation and assembly of uniform-sized Ag NPs on a silica surface, as compared to the other PEG derivatives with the same molecular weight. Finally, we demonstrated that AgNPs-SiNS bearing 4-fluorobenzenethiol on its surface induced the strong SERS signal at the single-particle level, indicating that each hybrid particle has internal hot spots. This shows the potential of AgNPs-SiNS for SERS-based sensitive detection of target molecules