Localized Surface Plasmon Resonance (LSPR) Biosensor for the Protein Detection

In this paper, we investigate the ability of the gold nanorods (GNRs) to detect some proteins and demonstrate their potential to be used as plasmonic nanobiosensors. The GNRs were synthesized by a two-step seed-mediated growth procedure at room temperature. Firstly, a seed solution of gold nanoparticles was synthesized in the presence of cetyltrimethylammonium bromide surfactant and, subsequently, incorporated with appropriate amount of silver nitrate and tetrachloroauric acid solutions to grow GNRs with average length of 50 nm and diameter of 14 nm. We study the interaction of GNRs with proteins whose molecular weight varies from 6.5 up to 75 kDa. We investigate the resulting solutions by means of UV–vis absorption spectroscopy to determine the effect of the proteins characteristics on the shift of the localized surface plasmon resonance (LSPR). We show that for the case when proteins are in large excess compared to the GNRs concentration, whatever the protein is, the LSPR shift is constant and does not depend on the protein molecular weight. Moreover, we have been able to demonstrate that the sensitivity of such LSPR sensor is around 10^–9 M/nm on a concentration range from 10^–10 to 10^–8 M. Some comparison with finite-difference time-domain simulations have also shown that the number of proteins adsorbed at the GNRs surface is around 40.     

Nonlinear Optical Properties of Large-Sized Gold Nanorods

The nonlinear optical properties of single gold nanorods (GNRs) with a large diameter of ～200 nm and a long length of ～800 nm were investigated by using a focused femtosecond (fs) laser light with tunable wavelength. While the linear and nonlinear optical properties of small-sized GNRs have been extensively studied, the nonlinear optical properties of large-sized GNRs and the effects of high-order surface plasmon resonances remain unexplored. Second harmonic generation (SHG) or/and two-photon-induced luminescence (TPL) were observed in the nonlinear response spectra, and their dependences on excitation wavelength and polarization were examined. The scattering and absorption spectra of the small- and large-sized GNRs were compared by using the discrete dipole approximation method. It was found that the extinction of large-sized GNRs is dominated by scattering rather than absorption, which is dominant in small-sized GNRs. In addition, it was revealed that the excitation wavelength-dependent SHG of a GNR is governed by the linear scattering of the GNR and the maximum SHG is achieved at the valley of the scattering spectrum. In comparison, the excitation wavelength dependence of TPL is determined by the absorption spectrum of the GNR. The polarization-dependent SHG of a GNR exhibits a strong dependence on the dimension of the GNR, and it may appear as bipolar distributions parallel or perpendicular to the long axis of the GNR or multipole distributions.     

Tissue Distribution and Efficacy of Gold Nanorods Coupled with Laser Induced Photoplasmonic Therapy in Ehrlich Carcinoma Solid Tumor Model

Gold nanorods (GNR) within tumor microregions are characterized by their ability to absorb near IR light and emit heat in what is called photoplasmonic effect. Yet, the efficacy of nanoparticles is limited due to intratumoral tissue distribution reasons. In addition, distribution of GNRs to normal tissue might result in non specific toxicity. In the current study, we are assessing the intratumoral and tissue distribution of PEGylated GNRs on the top of its antitumor characteristics when given intravenously or intratumoral to solid tumor bearing mice and coupled with laser photoplasmonic sessions. PEGylated GNRs with a longitudinal size of less than 100 nm were prepared with aspect ratio of 4.6 showing strong surface plasmon absorption at wavelength 800 nm. Pharmacokinetics of GNR after single I.V. administration (0.1 mg/kg) showed very short systemic circulating time (less than 3 h). On the other hand, tissue distribution of I.V. GNR (0.1 mg/kg) to normal animals showed preferential deposition in spleen tissue. Repeated administration of I.V. GNR resulted in preferential accumulation in both liver and spleen tissues. In addition, I.V. administration of GNR to Ehrlich carcinoma tumor bearing mice resulted in similar tissue distribution; tumor accumulation and anti-tumor effect compared to intratumoral administration. In conclusion, the concentration of GNR achieved within tumors microregions after I.V. administration was comparable to I.T. administration and sufficient to elicit tumoral growth arrest when coupled with laser-aided photoplasmonic treatment.     

Enhancing LSPR Sensitivity of Au Gratings through Graphene Coupling to Au Film

A particular interesting plasmonic system is that of metallic nanostructures interacting with metal films. As the localized surface plasmon resonance (LSPR) behavior of gold nanostructures (Au NPs) on the top of a gold thin film is exquisitely sensitive to the spacer distance of the film-Au NPs, we investigate in the present work the influence of a few-layered graphene spacer on the LSPR behavior of the NPs. The idea is to evidence the role of few-layered graphene as one of the thinnest possible spacer. We first show that the coupling to the Au film induces a strong lowering at around 507 nm and sharpening of the main LSPR of the Au NPs. Moreover, a blue shift in the main LSP resonance of about 13 nm is observed in the presence of a few-layered graphene spacer when compared to the case where gold nanostructures are directly linked to a gold thin film. Numerical simulations suggest that this LSP mode is dipolar and that the hot spots of the electric field are pushed to the top corners of the NPs, which makes it very sensitive to surrounding medium optical index changes and thus appealing for sensing applications. A figure of merit of such a system (gold/graphene/Au NPs) is 2.8, as compared to 2.1 for gold/Au NPs. This represents a 33 % gain in sensitivity and opens-up new sensing strategies.     

Investigation of the Plasmonic Interaction of Gold Nanoparticles Toward Plasmonic Photothermal Therapeutics

Plasmonic interaction of nanoparticles located in close proximity, embedded in breast tissue, is simulated for estimating the optical characteristics like optical absorption cross-section, plasmonic wavelength as well as full-width half maxima (FWHM). The computations are done for the monomers, homodimers, and heterodimers of spherical and rod-shaped gold nanoparticles considering various interparticle spacings for gold nanospheres and the interparticle spacing as well as the orientation for gold nanorods (GNRs). The results indicate that for the spherical dimer, with the change in interparticle spacing from 1 to 20 nm, the peak absorption cross-section decreases by 43%. Whereas for the GNRs, the absorption cross-section increases/decreases, within 9–18%, depending on the homodimer or heterodimer configuration. Furthermore, secondary peaks for the absorption cross-section are obtained within wavelengths of 630–940 nm due to antibonding modes for GNR heterodimers. For GNR heterodimer located end-to-end, this secondary peak for the absorption cross-section appears at 780 nm irrespective of interparticle spacing within 1–5 nm. The absorption coefficient is considerably dependent on the configuration and proximity of GNRs located within the tissue. While FWHM is not significantly influenced by GNRs configuration and interparticle spacing. For interparticle spacing from 1 to 20 nm, the plasmonic wavelength shifts by 38 nm for the spherical dimer and by 35–86 nm for various GNR dimers. The findings of this study are useful for plasmonic photothermal therapeutics as the heat generation is governed by the resulting absorption cross-section due to plasmonic coupling of the closely spaced and different orientations of the nanoparticles.

     

In Vitro Identification of Gold Nanorods through Hyperspectral Imaging

Due to their unique plasmonic and optical properties, gold nanorods (GNR) have shown tremendous potential for nano-based applications extending into a variety of fields including bioimaging, sensor development, electronics, and cancer therapy. These distinctive, shape-specific properties are strongly dependent upon the GNR aspect ratio, thus producing the ability to be targeted for an application by fine-tuning their physical parameters. It is owing to their characteristic spectral signature, which is vastly different from that of a cellular setting, that GNRs are emerging as an ideal candidate for nano-based imaging applications. However, one challenge that has emerged in the field of bioimaging is the need to account for the observed plasmon coupling effect that arises from GNR agglomeration in a physiological environment. In this study, GNRs with aspect ratios of 2.5 and 6.0 were actively identified in an in vitro setting through a hyperspectral imaging (HSI) analysis; which successfully recognized and separated the light scattering pattern of these particles from that of the surrounding cells. Through inclusion of agglomerated GNR spectral patterns in the HSI spectral library, this imaging technique was able to overcome the complication of plasmon coupling, though to varying degrees. These results demonstrate the tremendous potential of GNRs coupled with HSI analysis to advance the field of nano-based sensing and imaging mechanisms.     

Dynamic change in optical properties of a nanoparticle embedded tumor phantom for plasmonic photothermal cancer therapeutics

In this study, the temporal dynamic changes in optical properties of gold nanorods (GNR) embedded tumor phantom, during photothermal interaction, are reported for plasmonic photothermal therapeutics. Tumor mimicking bilayer phantoms were prepared by using 1% agarose incorporated with 0.1% coffee powder, 0.3% intralipid solution as epidermis layer; 3% intralipid solution and 0.3% human hemoglobin (Hb) powder as dermis layer. On incorporating GNRs of concentrations 10, 20, and 40 μg/ml within the phantom, the absorption coefficients increases 4–8 times, while there is minimal change in the reduced scattering coefficients. Further the absorption coefficient increased by ~8% with the incorporation of GNRs of concentration 40 μg/ml, while no considerable dynamic change in the optical properties is observed for the phantom embedded with GNRs of concentrations 10, and 20 μg/ml. The discussed results are useful for the selection of GNRs dose for pre-treatment planning of plasmonic photothermal cancer therapeutics.     

Tailoring LSPR-Based Absorption and Scattering Efficiencies of Semiconductor-Coated Au nanoshells

Absorption and scattering efficiencies of semiconductor-coated Au nanoshell have been studied by the extended Mie theory for their possible solar cell, optical imaging, and photothermal applications, etc. The effect of Au shell layer thickness, core size, and surrounding medium on the absorption and scattering efficiencies at the localized surface plasmon resonance (LSPR) wavelengths has been reported. It has been found that both the absorption and scattering efficiencies get blue-shifted with an increase in Au shell layer thickness from 2 to 10 nm and with an increase in surrounding refractive index whereas the corresponding LSPR peaks shift towards red. It has also been found that the spectra are red-shifted with an increase in the core radius from 20 to 40 nm while keeping the shell thickness same. The effect of shell thickness on the absorption peak position and absorption linewidth has also been studied. Hence, the optical response of both CdSe- and CdTe-coated Au nanoshells can be tuned and controlled from the visible to the near-infrared (NIR) region of the electromagnetic (EM) spectrum. Finally, the CdSe-coated Au nanoshell exhibits high scattering and absorption efficiencies in comparison to the CdTe-coated nanoshell.     

金纳米棒-聚苯胺核壳纳米材料的光学相干层析成像能力研究

对比试剂的使用能够显著提升光学相干层析(OCT)的成像效果。聚苯胺(PANI)是一种有机导电聚合物,在近红外(NIR)区有着很强的光吸收。本文采用PANI对常见的OCT成像对比试剂--金纳米棒(GNRs)进行修饰,合成了PANI/GNRs核壳粒子,并对其OCT成像对比能力进行了研究。PANI/GNRs展现出良好的NIR光吸收特性;同时,PANI对GNRs的包裹也显著提升了金纳米结构的稳定性、降低了GNRs原有的毒性。选用离体猪肝组织作为检测样本,发现纳米材料使用能够显著提升OCT的成像效果。与未修饰的GNRs及PANI粒子相比,PANI/GNRs的OCT成像对比效果明显更好。因此,PANI包裹的GNRs核壳纳米材料有望成为一种低毒性且效果良好的OCT对比试剂用于生物组织成像。     

Modulation of Visible and Near-Infrared Surface Plasmon Resonance of Au Nanoparticles Based on Highly Doped Graphene

We study an active modulation of surface plasmon resonance (SPR) of Au nanoparticles based on highly doped graphene in visible and near-infrared regions. We find that compared to the traditional metal SPR, the SPR of Au nanoparticles based on graphene causes a remarkable blue shift. The field intensity in the gap is redistributed to standing wave. The field intensity of standing wave is about one order of magnitude higher than the traditional model. Moreover, the SPR of Au nanoparticles can be actively modulated by varying the graphene Fermi energy. We find the maximum modulation of field intensity of absorption spectra is more than 21.6 % at λ?=?822?nm and the amount of blue shift is 17.4 nm, which is about 2.14 % of the initial wavelength λ ₀?=?813.4?nm, with increasing monolayer graphene Fermi energy from 1.0 to 1.5 ev. We find that the SPR sensitivity to the refractive index n of the environment is about 642 nm per refractive index unit (RIU). The SPR wavelengths have a big blue shift, which is about 33 nm, with increasing number of graphene layers from 1 to 3, and some shoulders on the absorption spectra are observed in the models with multilayer graphene. Finally, we study the Au nanorod array based on monolayer graphene. We find that the blue shift caused by the graphene increases from 14 to 24 nm, with increasing gap g _y from 10 to 20 nm. Then, it decreases from 24 to 14 nm, with increasing gap g _y from 20 to 50 nm. This study provides a new way for actively modulating the optical and optoelectronic devices.     

In vivo pump‐probe optical coherence tomography imaging in Xenopus laevis

Currently, optical coherence tomography (OCT), is not capable of obtaining molecular information often crucial for identification of disease. To enable molecular imaging with OCT, we have further developed a technique that harnesses transient changes in light absorption in the sample to garner molecular information. A Fourier‐domain Pump‐Probe OCT (PPOCT) system utilizing a 532 nm pump and 830 nm probe has been developed for imaging hemoglobin. Methylene blue, a biological dye with well‐know photophysics, was used to characterize the system before investigating the origin of the hemoglobin PPOCT signal. The first in vivo PPOCT images were recorded of the vasculature in Xenopus laevis. The technique was shown to work equally well in flowing and nonflowing vessels. Furthermore, PPOCT was compared with other OCT extensions which require flow, such as Doppler OCT and phase‐variance OCT. PPOCT was shown to better delineate tortuous vessels, where nodes often restrict Doppler and phase‐variance reconstruction. (© 2013 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Cancer cell‐specific protein delivery by optoporation with laser‐irradiated gold nanorods

The delivery of macromolecules into living cells is challenging since in most cases molecules are endocytosed and remain in the endo‐lysosomal pathway where they are degraded before reaching their target. Here, a method is presented to selectively improve cell membrane permeability by nanosecond laser irradiation of gold nanorods (GNRs) with visible or near‐infrared irradiation in order to deliver proteins across the plasma membrane, avoiding the endo lysosomal pathway. GNRs were labeled with the anti‐EGFR (epidermal growth factor receptor) antibody Erbitux to target human ovarian carcinoma cells OVCAR‐3. Irradiation with nanosecond laser pulses at wavelengths of 532 nm or 730 nm is used for transient permeabilization of the cell membranes. As a result of the irradiation, the uptake of an anti‐Ki‐67 antibody was observed in about 50 % of the cells. The results of fluorescence lifetime imaging show that the GNR detached from the membrane after irradiation.     

Plasmonically Tunable Blue-Shifted Emission from Coumarin 153 in Ag Nanostructure Random Media: A Demonstration of Fast Dynamic Surface-Enhanced Fluorescence

Enhancement of intensity and wavelength tunability of emission are desirable features for light-emitting device applications. We report on the large and tunable blue shift (60 nm) in emission from an environment-sensitive fluorophore (Coumarin153) embedded in Ag plasmonic random media. Coumarin 153 having emission at 555 nm, show a systematic blue shift (to 542, 503 and 495 nm) upon infiltration into random media fabricated by Ag nanowires of different aspect ratio (hence, surface plasmon resonances at 426, 445 and 464 nm). The blue shift is due to the fast dynamic surface-enhanced fluorescence mechanism and can be tuned by controlling the surface plasmon resonance and hotspot density in random media. Enhanced emission at desired wavelength is achieved by using nanostructures having higher extinction coefficient but same-surface plasmon resonance. Ag nanostructures of different aspect ratio used for fabricating the random media are synthesized by chemical route.     

A Localized Surface Plasmon Resonance Biosensor Based on Integrated Controllable Au2S/AuAgS-Coated Gold Nanorods Composite

In this work, we have demonstrated that the exquisite optical properties based on localized surface plasmon resonance (LSPR) of Au₂S/AuAgS-coated gold nanorods (Au₂S/AuAgS-coated GNRs) can be utilized to develop a simple and sensitive biosensor, and goat anti-human IgG can be detected by the human IgG probe as low as 0.2 nM. Moreover, we introduce an integrated LSPR biosensor constructed by integrating Au₂S/AuAgS-coated GNRs immobilized on glass slide and isolated Au₂S/AuAgS-coated GNRs in the form of liquid. The detection of target binding was performed via direct spectral changes induced by changes of refractive index in the vicinity of individual particles. The integrated LSPR optical biosensor is label-free, cost-effective, and easy to fabricate and requires only a visible/near-infrared spectrometer for detection purposes. Additionally, the investigation on the mutual influence of the two types of nanorods in the integrated LSPR biosensor was performed. The results of separate experiments indicate that the nanorods in the form of isolate or in integrated exhibit a similar behavior.     

Intercoupling surface plasmon resonance and diffusion reflection measurements for real‐time cancer detection

Spatial diffusion reflection (DR) measurements of gold nanorods (GNR) were recently suggested as a simple and highly sensitive non‐invasive and non‐ionizing method for real‐time cancer detection. In this paper we demonstrate that wavelength dependent DR measurements enable the spectral red‐shift observation of highly concentrated GNR. By conjugating targeting moieties to the GNR, large density of GNR can specifically home onto cancer cells. The inter‐particle plasmon resonance pattern of the highly concentrated GNR leads to an extension and a red‐shift (Δλ) in the absorption spectrum of the concentrated GNR. Dark‐field microscopy was used in order to measure the expected Δλ in different GNR concentrations in vitro. Double‐wavelength DR measurements of tissue‐like phantoms and tumor bearing mice containing different GNR concentrations are presented. We show that the DR profile of the highly concentrated GNR directly correlate with the spectral extension and red‐shift. This presented work suggests that wavelength dependent DR method can serve as a promising tool for real‐time superficial tumor detection. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Effect of Aspect Ratio of Gold Nanorods on Cell Imaging with Two-Photon Excitation

To make the gold nanorod (AuNR) a better photoluminescence (PL) probe for cell imaging under two-photon excitation (TPE), the effect of the aspect ratio of AuNRs was studied. The AuNRs with the aspect ratios of 2.7, 3.2, 4.1, and 4.5 and correlated longitudinal surface plasmon resonance (LSPR) bands of 710, 760, 820, and 870 nm were compared. The approach of two-photon excited PL was used to measure the two-photon absorption cross section (TPACS) of these AuNRs in aqueous solutions. Under TPE of an 800-nm femtosecond laser, the TPACS of AuNRs with an aspect ratio of 3.2 was found to be the highest (about 3?×?10⁹ GM), and that of AuNRs (aspect ratio of 2.7) was only 1.5?×?10⁹ GM. The probe function of these two AuNRs was further compared in cell imaging studies using the human liver cancer cell (QGY) as the cell model. Both TPE PL image and confocal reflectance image of AuNR-loaded cells were acquired comparatively in measurements. The brightness and contrast of confocal reflectance images for these two AuNRs in cells are similar. In contrast, the PL images of cellular AuNRs (2.7) under TPE of 800 nm are weak but that of cellular AuNRs (3.2) is much better. These results show that when the LSPR band of AuNRs is coincided with the excitation wavelength, the TPACS of these AuNRs will be enhanced ensuring a good quality of cell imaging under TPE. The LSPR band is correlated to the aspect ratio of AuNRs. Therefore, in cell imaging studies with TPE, the aspect ratio effect of AuNRs should be taken into consideration.     

Large Scale Fabrication of Gold Nano-Structured Substrates Via High Temperature Annealing and Their Direct Use for the LSPR Detection of Atrazine

The present work is reporting on the fabrication of localized surface plasmonic resonant (LSPR) gold nano-structures on glass substrate by using different high annealing temperatures (500 °C, 550 °C, 600 °C) of initially created semi-continue gold films (2 nm and 5 nm) by the electron beam evaporation technique. Interestingly, well-defined gold nano-structures were also obtained from continuous 8 nm evaporated gold film - known as the value above gold percolated thickness - once exposed to high temperatures. The surface morphology and plasmonic spectroscopy of “annealed” nano-structures were controlled by key experimental parameters such as evaporated film thickness and annealing temperature. By using scanning electron microscopy (SEM) characterization of annealed surface it was noticed that the size and inter-particle distance between nano-structures were highly dependent on the evaporated thin film thickness, while the nanoparticle shape evolution was mainly affected by the employed annealing temperature. Due to the well-controlled morphology of gold nano-particles, prominent and stable LSPR spectra were observed with good plasmon resonance tunability from 546 nm to 780 nm that recommend the developed protocol as a robust alternative to fabricate large scale LSPR surface. An example of a LSPR-immunosensor is reported. Thus, the monoclonal anti-atrazine antibodies immobilizion on the “annealed” gold nano-structures, as well as the specific antigen (atrazine) recognition were monitored as variations of the resonance wavelength shifts and optical density changes in the extinction measurements.     

Improvement of Figure of Merit for Gold Nanobar Array Plasmonic Sensors

We report a strategy to improve two types of the figure of merit (FOM and FOM*) of the refractive index sensitivity of a gold nanobar array localized surface plasmon resonance (LSPR) biosensor by simply placing it close to a thin gold film with a dielectric spacer. The thickness of the dielectric spacer determines the plasmon coupling strength between the gold nanobars and the gold film and consequently the FOM and FOM* of the biosensor. From our calculations, when the spacer thickness is 20 nm, the FOM and FOM* reach maximal (4.68 and 310, respectively) and the sensitivity remains at a high value of 600 nm per refractive index unit. This biosensor scheme is practically realizable, and this strategy is also potentially applicable to the LSPR biosensors with other geometries.     

Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n ₂?=?2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ?=?2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.     

Plasmonic Biosensor Based on Triangular Au/Ag and Au/Ag/Au Core/Shell Nanoprisms onto Indium Tin Oxide Glass

Gold–silver core–shell triangular nanoprisms (Au/AgTNPs) were grown onto transparent indium tin oxide (ITO) thin film-coated glass substrate through a seed-mediated growth method without using peculiar binder molecules. The resulting Au/AgTNPs were characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, UV–vis spectroscopy, and cyclic voltammograms. The peak of dipolar plasmonic resonance was located at near infrared region of ～700 nm, which showed the refractive index (RI) sensitivity of 248 nm/RIU. Moreover, thin gold shells were electrodeposited onto the surface of Au/AgTNPs in order to stabilize nanoparticles. Compared with the Au/AgTNPs, this peak of localized surface plasmon resonance (LSPR) was a little red-shift and decreased slightly in intensity. The refractive index sensitivity was estimated to be 287 nm/RIU, which showed high sensitivity as a LSPR sensing platform. Those triangular nanoprisms deposited on the ITO substrate could be further functionalized to fabricate LSPR biosensors. Results of this research show a possibility of improving LSPR sensor by using core–shell nanostructures.