Sharp Localized Surface Plasmon Resonance Research Articles

Experimental and Theoretical Screening of Core Gold Nanoparticles and Their Binding Mechanism to an Anticancer Drug, 2-Thiouracil.

This study demonstrated the capability of two readily available optical spectroscopy tools, namely UV-Vis absorption spectrophotometry and Raman/surface-enhanced Raman spectroscopy, to select in a rapid and noninvasive manner the most homogenous gold nanoparticle (AuNP) models and to identify their chemical binding mechanism to 2-thiouracil (2-TU). 2-TU is an anticancer drug of great promise in the antiproliferative and photothermal therapies of cancer. The citrate-capped AuNPs emerged as the most stable as well as time- and cost-effective AuNP model out of the three widely used colloidal nanocores (citrate-, borohydride-citrate-, and sodium dodecyl sulfate (SDS)-capped AuNPs) that were examined. 2-TU chemically attached to the relatively monodispersed AuNPs via a chemisorption mechanism. The 2-TU-AuNPs complex formed through the covalent bonding of the S atom of 2-TU to the nanosurface in a vertical orientation. The spectroscopic results were then confirmed with the help of density functional theory (DFT) calculations and other physicochemical characterization tools for nanomaterials such as transmission electron microscopy (TEM), dynamic light scattering (DLS), and zeta potential. Overall, the purified 2-TU-AuNPs were found to be spherical, had an average diameter of 25 ± 2 nm, a narrow size distribution (1-30 nm), a sharp localized surface plasmon resonance (LSPR) peak at 525 nm, and a negative surface charge (-14 mV).

SPR-based assay kit for rapid determination of Pb2+

A recyclable optical nanosensor was developed by immobilizing l-tyrosine functionalized silver nanoparticles (AgNPs) on the polyethylene terephthalate (PET) substrate for rapid determination of Pb2+ ions. At first, the l-tyrosine functionalized AgNPs were assessed in the solution phase; the response time was lower than 15 s, and a limit of detection lower than 9 nM was obtained in the dynamic range of 1–1000 nM. For fabrication of the optical assay kit, the design of experiment (DOE) was used to optimize the immobilization efficiency of the nanoparticles on PET films by studying AgNO3 concentration and pH as two crucial parameters. The assay kit in optimal conditions showed a sharp localized surface plasmon resonance band suitable for sensitive determination of Pb2+. The fabricated sensor showed promising results for rapid determination of lead ions with the limit of detection value as low as 1 nM (S/N = 3). The sensor reproduced the obtained results even after three consecutive runs, which proved the recyclability of the optical assay kit. The recoveries of the spiked concentration in real samples were in the range of 95%–103%, which confirmed the applicability of the sensor in practical applications.

Synthesis of silver nanoplates with a narrow LSPR band for chemical sensing through a plasmon-mediated process using photochemical seeds

The plasmon-mediated process is one of the most common and well-studied methods to synthesize silver nanoplates (AgNPts). In a typical plasmon-mediated process, silver seeds are generated by chemical reduction using NaBH4 (abbreviated as CR seeds) before the plasmon-mediated process. However, a broad localized surface plasmon resonance (LSPR) band of AgNPts (usually broader than 150 nm) synthesized using the typical plasmon-mediated process would possibly limit their further applications. In this study, silver seeds are generated using a photochemical reduction method (abbreviated as PCR seeds). These PCR seeds then convert to AgNPts with irradiation by green LEDs or by a sodium lamp through the plasmon-mediated process. Furthermore, a very narrow LSPR bandwidth (approximately 67 nm) can be obtained when these AgNPts are further irradiated with red LEDs. Due to high refractive index sensitivity and sharp LSPR bandwidth, the as-prepared AgNPts have a high figure of merit (FOM) and can be used for spectroscopic chemical sensing applications. The silver seeds generated by this photochemical method can provide another choice for the plasmon-mediated process to synthesize AgNPts with high optical quality.

Synthesis of Silver Nanoplates With a Narrow LSPR Band for Chemical Sensing Through a Plasmon-Mediated Process Using Photochemical Seeds

The plasmon-mediated process is one of the most common and well-studied methods to synthesize silver nanoplates (AgNPts). In a typical plasmon-mediated process, silver seeds are generated by chemical reduction using NaBH4 (abbreviated as CR seeds) before the plasmon-mediated process. However, a broad localized surface plasmon resonance (LSPR) band of AgNPts (usually broader than 150 nm) synthesized using the typical plasmon-mediated process would possibly limit their further applications. In this study, silver seeds are generated using a photochemical reduction method (abbreviated as PCR seeds). These PCR seeds then convert to AgNPts with irradiation by green LEDs or by a sodium lamp through the plasmon-mediated process. Furthermore, a very narrow LSPR bandwidth (approximately 67 nm) can be obtained when these AgNPts are further irradiated with red LEDs. Due to high refractive index sensitivity and sharp LSPR bandwidth, the as-prepared AgNPts have a high figure of merit (FOM) and can be used for spectroscopic chemical sensing applications. The silver seeds generated by this photochemical method can provide another choice for the plasmon-mediated process to synthesize AgNPts with high optical quality.

Suitable Polymeric Coatings to Avoid Localized Surface Plasmon Resonance Hybridization in Printed Patterns of Photothermally Responsive Gold Nanoinks.

When using gold nanoparticle (AuNP) inks for writing photothermal readable secure information, it is of utmost importance to obtain a sharp and stable shape of the localized surface plasmon resonance (LSPR) absorption bands in the prints. The T increase at a given irradiation wavelength (ΔTλ) is the retrieved information when printed patterns are interrogated with a laser source. As ΔTλ is proportional to the absorbance at the wavelength λ, any enlargement or change of the absorbance peak shape in a printed pattern would lead to wrong or unreliable reading. With the aim of preparing AuNP inks suitable for inkjet printing of patterns with stable and reliable photothermal reading, we prepared liquid solutions of spherical AuNP coated with a series of different polymers and with or without additional dispersant. The optical stability of the inks and of the printed patterns were checked by monitoring the shape changes of the sharp LSPR absorption band of AuNP in the visible (λmax 519 nm) along weeks of ageing. AuNP coated with neutral polyethylenglycol thiols (HS-PEG) of mw 2000–20000 showed a strong tendency to rapidly agglomerate in the dry prints. The close contact between agglomerated AuNP resulted in the loss of the pristine shape of the LSPR band, that flattened and enlarged with the further appearance of a second maximum in the Near IR, due to plasmon hybridization. The tendency to agglomerate was found directly proportional to the PEG mw. Addition of the ethylcellulose (EC) dispersant to inks resulted in an even stronger and faster tendency to LSPR peak shape deformation in the prints due to EC hydrophobicity, that induced AuNP segregation and promoted agglomeration. The introduction of a charge on the AuNP coating revelead to be the correct way to avoid agglomeration and obtain printed patterns with a sharp LSPR absorption band, stable with ageing. While the use of a simple PEG thiol with a terminal negative charge, HS-PEGCOO(−) (mw 3000), was not sufficient, overcoating with the positively charged polyallylamine hydrochloride (PAH) and further overcoating with the negatively charged polystyrene sulfonate (PSS) yielded AuNP@HS-PEGCOO(−)/PAH(+) and AuNP@HS-PEGCOO(−)/PAH(+)/PSS(−), both giving stable prints. With these inks we have shown that it is possible to write photothermally readable secure information. In particular, the generation of reliable three-wavelength photothemal barcodes has been demonstrated.

Plasmonic sensors relying on nanoparticle arrays created by a template-directed dewetting process

Label-free detection has promising applications in biomedical and analytical fields. Localized surface plasmon resonance (LSPR) of gold (Au) nanoparticles is sensitive to the dielectric constant of the surrounding environment. Therefore, the LSPR-based (or plasmonic) sensor is one of the most promising label-free sensing techniques. Here, we demonstrated that ordered arrays of Au nanoparticles fabricated by a template-directed dewetting process of the patterned Au film could be harnessed to construct high performance LSPR sensors. The size of the Au nanoparticles is determined by and can be simply adjusted through varying the thickness of the thermally evaporated Au film, and in turn the location of their LSPR peaks. The Au nanoparticles in the ordered array have a narrow size distribution, giving rise to sharp LSPR peaks needed for constructing high performance LSPR sensors. The LSPR sensors showed high sensitivity with a figure-of-merit value of 6.6 before they got saturated. Sensitive detection of DNA was used as an example to demonstrate the application potential of the LSPR-based sensors in biomedical fields.

Optimizing the Sensing Performance of SiO2-Au Nanoshells

The localized surface plasmon resonance (LSPR) extinction spectrum and its dependence with permittivity of the surrounding medium have been broadly explored for molecular sensing applications. The sensing performance is governed by the bulk sensitivity and the linewidth of the LSPR peak. Therefore, a sensor needs to attend both requisites: a high bulk sensitivity and a sharp LSPR spectrum, related by the ηb ×FoM parameter. Complex plasmonic structures are proposed for sensing applications due to their high sensitivity values without considering the broad nature of the LSPR peak, that decrease the detection limit of the plasmonic sensor. In this article, we report the optical properties of SiO2 core/Au nanoshell (Au-NS) particles and the feasibility of exploring it as a sensor platform. On tuning the geometric parameters of the Au-NS, we have obtained the optimized the ηb ×FoM parameter with a value of 2057.4 nm/RIU2, which is the highest reported in the literature with spherical shapes. Moreover, for molecular adsorption sensing, Campbell’s model was exploited on a size-dependence approach. Finally, we have studied the influence of a dielectric substrate on the Au-NS sensing performance. Our results provide new bases for the development of high-performance nanostructures for bulk and molecular LSPR sensing.

Gold Nanodisks Plasmonic Array for Hydrogen Sensing at Low Temperature.

We present a novel plasmonic hydrogen sensor consisting of an array of gold nanodisks produced by lithography. The size, height, and spacing of the disks were optimized using finite element simulation to generate a sharp localized surface plasmon resonance peak in the near-infrared wavelength region. The reported results show the possibility of developing an optical gas sensors-based bare Au nanostructures operating at a low temperature.

(Invited) Acceleration of Electrocatalytic Reaction By Photoexciting Localized Surface Plasmon of Octahedral Au@Pt Core-Shell Nanoparticles

Introduction Localized surface plasmon resonance (LSPR) at the surface of noble metal nanoparticles (NPs), such as, copper, silver and gold, has been intensively investigated in the past decade because they demonstrated enhancement of photochemical reactions and highly sensitive spectroscopies. Recently, the LSPR-induced acceleration of electrocatalytic ethanol oxidation reaction was reported by Wang and co-workers using Pd-Au hetero-nanoparticles, in which photogenerated holes in Au NPs could effectively oxidize ethanol in a solution.1 Platinum is one of the most active catalysts for various reactions in a practical application. However, since Pt NPs hardly exhibit LSPR peak in visible-light region, their application to LSPR-induced catalytic reaction has been little investigated. In this study, we try to enhance the catalytic activity of Pt with LSPR of octahedral Au NPs. An electrochemical technique is used for preparing Pt shell layer on octahedral Au NPs, resulting in the formation of Au@Pt core-shell NPs. The photoexcitation of LSPR peaks of octahedral Au core in Au@Pt NPs can modify the catalytic activity of Pt shell layer for oxygen reduction reaction (ORR) as a model reaction. Experimental Octahedral Au NPs were prepared by reduction of HAuCl4 in diethylene glycol containing poly(diallyldimethylammonium chloride) at 230 °C for 1 h. The underpotential deposition (UPD) of Cu monolayer onto octahedral Au NPs immobilized on an HOPG electrode was carried out with constant potential application at +0.3 V vs. RHE in an aqueous solution containing 1.0 mmol dm-3 CuSO4 and 0.1 mol dm-3 H2SO4. Thus-obtained Cu-deposited Au NPs on HOPG substrate immersed in aqueous solution containing 0.5 mmol dm-3 K2PtCl4 for 10 min to replace Cu monolayer by Pt, resulting in the core-shell structure of octahedral Au@Pt NPs. Results and discussions Prepared Au NPs (> 90%) had an octahedral shape with sharp edges and triangle flat surface of {111} facets. The average edge length was determined to be 65 ± 8 nm by SEM measurement. In the extinction spectrum, a sharp LSPR peak appeared at ca. 600 nm, the peak width of which was relatively narrow (FWHM > 90 nm) probably due to the uniformity of shape and size of octahedral Au NPs. The Pt shell were prepared by the galvanic replacement of Cu UPD layer on Au surface with Pt. The electrochemical surface areas of Pt shell and Au NPs were determined with cyclic voltammetry in a 0.1 mol dm-3 H2SO4 aqueous solution. The deposition of Pt layer on Au NPs decreased a peak current assignable to the reduction of Au oxide layer (ca. 1.15 V vs. RHE), while the peaks originating from reduction of Pt oxide layer and H2 desorption on Pt surface were newly observed for Au@Pt NPs at 0.65 and 0.05 V vs. RHE, respectively. These results indicated that the surface of octahedral Au NPs was partially coated with the Pt shell layer. The Pt surface coverage was successfully estimated to be enlarged from 85 to 94% with an increase in the Pt deposition cycles from 1 to 3. TEM measurement revealed that the average edge length of octahedral NPs (original: 65 nm) increased to 66.5 nm and 69 nm by the Pt deposition cycles of 1 and 3, respectively, indicating that the thin Pt layer of ca. 1-2 nm in thickness was formed on Au surface. Hydrodynamic voltammograms for ORR were measured with or without visible-light irradiation, using a ring-disk-rotating electrode loaded with the octahedral Au@Pt NPs (1 cycle of Pt deposition). A cathodic current for ORR was observed at more negative potential than ca. +0.95 V vs. RHE. The current density at +0.5 V vs. RHE was enhanced from -2.6 mA cm-2 to -3.1 mA cm-2 by the visible-light irradiation. The photocurrent action spectrum of the Au@Pt NPs agreed well with their extinction spectra. These results suggested that the LSPR excitation of the Au core in Au@Pt NPs could enhance the electrocatalytic activity of the Pt shell layer for ORR. Reference 1. Wnag, Q., et al., Power Sou., 2016, 316, 29.

Unusually Sharp Localized Surface Plasmon Resonance in Supported Silver Nanocrystals with a Thin Dielectric Coating.

An unusually sharp localized surface plasmon resonance (sLSPR) is observed for a monolayer of glass-supported silver nanocubes coated with a thin, 5-20 nm, Al2O3 film. The resonance becomes significantly narrower and stronger while losing optical anisotropy and sensitivity to the surroundings with increasing overlayer thickness. Surface-enhanced Raman scattering excitation profiles indicate an additional enhancement to the electric field brought in by the sLSPR. The resonance is thought to originate from a Fano-like constructive interference between the quadrupolar and dipolar LSPR modes in supported silver nanocubes leading to enhanced light extinction. This phenomenon is of significance for plasmon-induced charge-transfer processes in photovoltaics and catalysis.

Synthesis of Copper–Silica Core–Shell Nanostructures with Sharp and Stable Localized Surface Plasmon Resonance

Copper nanoparticles exhibit intense and sharp localized surface plasmon resonance (LSPR) in the visible region; however, the LSPR peaks become weak and broad when exposed to air due to the oxidation of Cu. In this work, the Cu nanoparticles are successfully encapsulated in SiO2 by employing trioctyl-n-phosphine (TOP)-capped Cu nanoparticles for the sol–gel reaction, yielding an aqueous Cu–SiO2 core–shell suspension with stable and well-preserved LSPR properties of the Cu cores. With the TOP capping, the oxidation of the Cu cores in the microemulsion was significantly reduced, thus allowing the Cu cores to sustain the sol–gel process used for coating the SiO2 protection layer. It was found that the self-assembled TOP-capped Cu nanoparticles were spontaneously disassembled during the sol–gel reaction, thus recovering the LSPR of individual particles. During the disassembling progress, the extinction spectrum of the nanocube agglomerates evolved from a broad extinction profile to a narrow and sharp peak. Fo...

Tailored Aggregate-Free Au Nanoparticle Decorations with Sharp Plasmonic Peaks on a U-Type Optical Fiber Sensor by Nanosecond Laser Irradiation

A novel experimental methodology is presented for fabricating U-shaped optical fiber probes decorated with aggregate-free Au nanoparticles exhibiting sharp localized surface plasmon resonance (LSPR) spectra. The U-type tip is coated with gold nanoparticles (AuNPs) using a simple and time-efficient dip-coating procedure, without initially taking any care to prevent the formation of nanoparticle aggregates in the coated area. In a second step, the coating was irradiated with a few tens of laser pulses of 5-ns duration at 532 nm with intensities in the range of 2–14 MW/cm2, leading to the formation of aggregate-free LSPR optical fiber probes. The process was monitored and controlled in real time through the changes induced into the fiber’s extinction spectra by the laser irradiation, and the coated fibers were characterized by electron microscopy. The proposed methodology resulted into the fabrication of U-type optical fiber probes coated with AuNPs exhibiting a sharp plasmon peak, which is a perquisite for their application as sensing devices.

Effective Optoelectrical Switching by Using Pseudo-Single Crystal of Monolayer Array of 2D Polymer–Plasmonic Nanoparticles System

The Langmuir–Blodgett (LB) technique is used to assemble molecular compounds and colloidal nanoparticles into a monolayer on the surface of a substrate. Although the LB technique was used successfully in the assembly of the nanoparticles and molecules into monolayer, ordering of the nanoparticles or the molecules inside the LB film was not highly controlled. Functionalization of long chain polymers, which are able to semicrystallize into a monolayer, with the surface of colloidal nanoparticles can lead to highly ordered 2D arrays upon LB assembly. Poly(ethylene glycol) (PEG) with an average molecular weight of 30 000 g/mol is able to organize both isotropic 40 nm gold nanocubes (AuNCs) and anisotropic 40 nm gold nanorods into highly ordered 2D arrays. The separation distances between the nanoparticles in such LB assemblies are comparable, and the distances decreased upon increasing LB surface pressure. A strong sharp localized surface plasmon resonance (LSPR) spectrum of 2D AuNC arrays of full width at half-maximum (fwhm) of 46 nm is obtained when they are organized into highly ordered, well-separated 2D arrays on the surface of a substrate. The high efficiency of the plasmonic AuNC 2D arrays is exploited for optoelectrical switching applications. The LSPR peak of the AuNC 2D arrays coated with a thin film of poly(3-hexylthiophene), an electrochromic polymer that changes its optical properties upon electrochemical oxidation, reversibly blue-shifts by 9 nm upon the electrochemical oxidation of the polymer and shifts back after electrochemical reduction. The narrow LSPR spectrum of the fabricated 2D arrays decreases the value of the figure of merit and thus improves their sensing efficiency.

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.

Preparation of graphene oxide-based surface plasmon resonance biosensor with Au bipyramid nanoparticles as sensitivity enhancer

A sensitive and selective wavelength-modulation surface plasmon resonance (SPR) biosensor based on graphene oxide (GO) and Au bipyramids (AuBPs) is reported for determination of bovine IgM. GO sheets with lengths of 100–300nm are synthesized and assembled on amine-modified Au film. The large surface area and abundant functional groups of GO allow the efficient immobilization of antibody. AuBPs are nanoparticles with a penta-twinned crystal structure, which have a sharp localized surface plasmon resonance (LSPR) band because of their high monodispersity. In the optimal conditions, the GO-based biosensor with AuBPs as sensitivity enhancers shows a satisfactory response to bovine IgM in the concentration range of 0.03–32μgmL−1. For contrast, traditional biosensor, GO-based biosensor and GO-based biosensor with Au nanorods (AuNRs) as sensitivity enhancers for antigen detection were also investigated. Consequently, the as-prepared GO sheets function as promising support for antibody and GO-based SPR biosensor using AuBPs as enhancers has the highest sensitivity among the four types of biosensors.

Improved Localized Surface Plasmon Resonance Immunoassay with Gold Bipyramid Substrates

Gold nanoparticles bound to substrates exhibit localized surface plasmon resonance (LSPR) in their optical extinction spectra at visible and near-infrared wavelengths. The LSPR wavelength is sensitive to the surrounding refractive index, enabling a simple, label-free immunoassay when capture antibodies are bound to the nanoparticles. Gold bipyramids are nanoparticles with a penta-twinned crystal structure, which have a sharp LSPR because of their high monodispersity. Bipyramid substrates were found to have a refractive index sensitivity ranging from 288 to 381 nm/RIU (-0.62 to -0.68 eV/RIU), increasing with the nanoparticle size and aspect ratio. In an immunoassay, the bipyramid substrates yielded higher sensitivity than nanorods and nanospheres. An immunoassay sensitivity constant which depends on both the optical properties of the nanoparticle and conjugation chemistry was found to be K(LSPR) = 0.01 nm x microm(2) for gold bipyramids.