Red-shift Of Localized Surface Plasmon Resonance Research Articles

Plasmon extinguishment by bandedge shift identified as a second-order spectroscopic differentiation

Abstract Optical excitation of metallic nanostructures induces strong intraband transitions, leaving transient depletion below the Fermi level, which allows transient interband transition to this depletion band. This is equivalent to the lowering of the threshold for interband transitions and pushes the plasmonic band to the red. As a result, localized surface plasmon resonance (LSPR) is “extinguished” or “quenched” around the bandedge, in contrast, the interband optical absorption becomes enhanced and redshifted. The corresponding transient absorption (TA) signals have equal lifetimes and opposite signs. Moreover, the TA spectrum is found to be a second-order differential of the steady-state optical extinction spectrum over the studied band. This is a commonly existing mechanism for metallic nanostructures and verified with gold in this work. Such a discovery is completely different from the optical-excitation-induced redshift of LSPR through enhanced electronic scattering and is important for understanding the ultrafast spectroscopic response of plasmonic nanostructures with clear photophysical insights, supplying solid basis for exploring optical logic device and optical data processing techniques.

A multicolor colorimetric assay for sensitive detection of sulfide ions based on anti-etching of triangular gold nanoplates

This study presents a viable colorimetric assay for sensitive sensing of sulfide ions (S2−) using triangular gold nanoplates (AuNPLs) as nanoprobes. The sensing mechanism is based on regulating the etching process of AuNPLs upon the addition of different concentration of S2−. In the presence of Cu2+ and I−, the AuNPLs would be etched by triiodide ions (I3−) produced from the redox reaction between Cu2+ and I−, resulting in the transformation of the shape of AuNPLs from triangular into sphere and blue shifted localized surface plasmon resonance (LSPR). However, in the presence of S2−, a preferential reaction between Cu2+ and S2− occurred, and the inhibition of the formation of I3− prevented the AuNPLs from being etched. Correspondingly, a red shift of LSPR can be observed with an increase of the concentration of S2−, accompanying a readily distinguishable color transition that goes from pink to purplish red, purple, bluish purple and blue. The color alterations allow for a visual estimation of the concentrations of S2−, and the lowest visual concentration through naked-eyes was 0.1 μM. A good linear relationship (R2 = 0.997) between the peak shifts and the concentration of S2− was obtained in the range of 0.02–1.5 μM, and the limit of detection (3 s/k) by UV–vis spectroscopy was as low as 16 nM. The as-established assay displayed excellent selectivity, which was successfully applied for analysis of S2− in environmental water samples.

Label-Free, Single-Particle, Colorimetric Detection of Permanganate by GNPs@Ag Core-Shell Nanoparticles with Dark-Field Optical Microscopy.

Rapid quantification of permanganate (MnO4-) in aqueous solution with a convenient and sensitive single-particle-detection (SPD) method is demonstrated by dark-field optical microscopy. The design is based on the selective etching of the Ag shell of a glucose-protected GNPs@Ag nanoparticle by MnO4-. In the presence of MnO4-, a noticeable red-shift of localized surface-plasmon resonance (LSPR, from blue to green) together with a tremendous decrease in the extinction coefficient from individual GNPs@Ag nanoparticles is observed. MnO4- can then be quantified by calculating the ratio between the number of green and blue particles on the cover glass surface after the etching process. A linear dynamic range of 0-6 μM and a limit of detection (LOD) as low as 46 nM were readily achieved, which are much lower than those of spectroscopic measurements in bulk solution. In tap water, a comparable LOD (50 nM) and satisfactory recovery efficiency are demonstrated. As a consequence of these merits, the method demonstrated herein will find promising applications for the ultrasensitive detection of MnO4- under complex milieu in the future.

Gold nanoprobe for inhibition and reactivation of acetylcholinesterase: An application to detection of organophosphorus pesticides

The organophosphorus compound (OP) induced inhibition of acetylcholinesterase (AChE) and their reactivation has been monitored using gold nanoprobe. The method is based on the colour change and red shift of localized surface plasmon resonance (LSPR) absorption band of AuNPs in UV–vis region after the addition of acetylthiocholine in presence of AChE. The aggregation of AuNP is induced by the thiocholine (TCh) through Au-S binding and electrostatic force of interactions. The inhibitory attack of OPs at the active serine site of the AChE leads to the decrease in formation of TCh molecule, which consequently, prevents the aggregation of the AuNPs. Conversely, the red shift of LSPR has been regained upon the reactivation of OP inhibited AChE by oximates (2-PyOx−, 4-C12PyOx−) and salicylhydroxamate (SHA). The inhibition protocols of the AChE have been used for detection of OP pesticides. The linear range for determination of OPs was found in range of 0.30–17.30 ng mL−1 with their limit of detection being 0.13 ng mL−1, 0.37 ng mL−1, 0.42 ng mL−1 and 0.20 ng mL−1 for paraoxon, parathion, fenitrothion and diazinon, respectively. The proposed method was found to be simple, sensitive and cost effective for the determination of OPs in real water samples.

Nanosecond Laser-Assisted Fabrication of Colloidal Gold and Silver Nanoparticles and Their Conjugation with S-Ovalbumin

In the present work, we have investigated the functionalization of protein S-ovalbumin with laser-generated gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) using spectroscopic techniques. Biocompatible gold and silver nanoparticles were fabricated by nanosecond pulsed laser ablation (PLA) of metal target in water and directly conjugated to protein S-ovalbumin. The localized surface plasmon resonance (LSPR) of gold and silver nanoparticles is sensitive to particle size, shape, and surrounding medium refractive index. UV–visible absorption spectrum of both AuNPs and AgNPs showed red shift in LSPR after protein addition. The effect of laser fluence on the size of AuNPs and nanoparticle-protein conjugation in the size range 6–32 nm has been systematically studied. The presence of NPs resulted in broadening of S-ovalbumin absorption peak around 278 nm. Raman spectra of S-ovalbumin-NP assembly demonstrated breaking of disulfide bond and appearance of peak around 290 and 206 cm−1 corresponding to Au–S and Ag–S bond formation, respectively. Further, increase in the ratio of the 1350/1320-cm−1 doublet was observed, which indicates that hydrophobicity for tryptophan residues in the protein-NP assembly has increased. Fluorescence spectroscopy showed quenching in emission intensity of S-ovalbumin in the presence of AuNPs and AgNPs. The number of binding sites between AuNPs and S-ovalbumin has been found to be 1.4 with binding constant 4.5 × 109 M−1. The calculated values of binding constant and number of binding sites for AgNPs and S-ovalbumin were 2.4 × 109 M−1and 0.96, respectively.

Fabrication LSPR sensor chip of Ag NPs and their biosensor application based on interparticle coupling

We introduce a simple method to synthesize localized surface plasmon resonance (LSPR) sensor chip of Ag NPs on the hydrogenated amorphous carbon by co-deposition of RF-Sputtering and RF-PECVD. The X-ray photoelectron spectroscopy revealed the content of Ag and C atoms. X-ray diffraction profile and atomic force microscopy indicate that the Ag NPs have fcc crystal structure and spherical shape and by increasing deposition time, particle sizes do not vary and only Ag NPs aggregation occurs, resulting in LSPR wavelength shift. Firstly, by increasing Ag NPs content, in-plan interparticles coupling is dominant and causes redshift in LSPR. At the early stage of agglomeration, out-plane coupling occurs and in-plane coupling is reduced, resulting a blueshift in the LSPR. By further increasing of Ag NPs content, agglomeration is completed on the substrate and in-plan coupling rises, resulting significant redshift in the LSPR. Results were used to implement biosensor application of chips. Detection of DNA primer at fM concentration was achieved based on breaking interparticles coupling of Ag NPs. A significant wavelength shift sensitivity of 30nm and a short response time of 30min were obtained, where both of these are prerequisite for biosensor applications.

Optimization of ultrathin carbon film coated silver nanoshell for biomedical applications in vivo

The extinction spectroscopy and near-field enhancement of dielectric-silver nanoshell coated by tetrahedral amorphous carbon [ta-C] layer (DSC) have been calculated by using Mie theory. With decreasing the Ag layer thickness, the localized surface plasmon resonance (LSPR) of DSC nanoshell moves from the visible region into the near-infrared region and the corresponding local field factor (LFF) increases first and then decreases. In addition, the increase of ta-C shell thickness leads to red-shift of LSPR and the decrease of LFF in DSC nanoshell. We further find that the increase of the dielectric constant for the outer shell can induce a significant enhancement of near-field. Based on the simulation analysis, we show that the DSC nanoshell can provide strong near-field in near-infrared region and may be suitable for the biomedical applications in vivo.

Localized surface plasmon resonance propertiesof elliptical gold nanotubes

The effects of ellipse shape, polarization direction of incident light, shell thickness, and dielectric constants of core and embedding medium on the localized surface plasmon resonance (LSPR) of elliptical gold nanotube have been investigated by the Finite Difference Time Domain (FDTD) method. When the semimajor axis is fixed, it is found that with the increase of the semiminor axis of the ellipse the extinction peak of the gold nanotube has a red-shift. With the increase of the angle between the incident polarization and the semimajor axis, the extinction peak has a red-shift. With the shell thickness decreasing, the extinction peak of gold nanotube also has a red-shift. Furthermore, we also find that the increase of the dielectric constant for core or embedding medium will induce a red-shift of LSPR in gold nanotube. The change of the extinction peak is ascribed to the plasmon hybridization and the competition between the variations of conduction and oscillation electrons.