Localized Surface Plasmon Resonance Wavelength Shift Research Articles

Immobilization of Streptavidin on a Plasmonic Au-TiO2 Thin Film towards an LSPR Biosensing Platform.

Optical biosensors based on localized surface plasmon resonance (LSPR) are the future of label-free detection methods. This work reports the development of plasmonic thin films, containing Au nanoparticles dispersed in a TiO2 matrix, as platforms for LSPR biosensors. Post-deposition treatments were employed, namely annealing at 400 °C, to develop an LSPR band, and Ar plasma, to improve the sensitivity of the Au-TiO2 thin film. Streptavidin and biotin conjugated with horseradish peroxidase (HRP) were chosen as the model receptor–analyte, to prove the efficiency of the immobilization method and to demonstrate the potential of the LSPR-based biosensor. The Au-TiO2 thin films were activated with O2 plasma, to promote the streptavidin immobilization as a biorecognition element, by increasing the surface hydrophilicity (contact angle drop to 7°). The interaction between biotin and the immobilized streptavidin was confirmed by the detection of HRP activity (average absorbance 1.9 ± 0.6), following a protocol based on enzyme-linked immunosorbent assay (ELISA). Furthermore, an LSPR wavelength shift was detectable (0.8 ± 0.1 nm), resulting from a plasmonic thin-film platform with a refractive index sensitivity estimated to be 33 nm/RIU. The detection of the analyte using these two different methods proves that the functionalization protocol was successful and the Au-TiO2 thin films have the potential to be used as an LSPR platform for label-free biosensors.

Design and performance simulation of a highly sensitive nano-biosensor based on a realistic array of plasmonic synthesized nanostructures

In this study, simulation, analysis, and synthesis of silver plasmonic nanostructures were investigated for use in biomolecule detection. At first, silver nanoparticles (SNPs) were synthesized by a chemical reduction method using polyethylenimine as both a reducing and a stabilizing agent. The SNPs were characterized by UV–visible spectroscopy, dynamic light scattering, and field emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy measurement techniques and showed spherical morphology with an average particle size of 38 nm and a localized surface plasmon resonance (LSPR) peak around 400 nm. The plasmonic effects of the silver nanostructures were also investigated for detection of glucose as a model analyte, with a concentration range of 0.0–2.603 mol/L. Wavelength shifts of 0.033 and 12.04 nm were obtained for glucose concentrations of 0.0039 and 2.603 mol/L, respectively; these shifts yielded the sensitivities of 482.75 and 189.02 nm/RIU and figures of merit (FOMs) of 13.5 and 5.24, respectively. The simulation results indicated the ability of SNPs of 30 nm to detect glucose concentrations as low as 0.0039 mol/L with an LSPR wavelength shift of about 0.033 nm. Also, the sensitivity and FOM were calculated for a 10 × 10 array of non-uniform size nanoparticles according to the experimental sizes. The simulation results revealed the significance of both the nanoparticle size and the particle size uniformity of a realistic array on the sensitivity and LSPR peak shift of SNPs. These findings show that it is possible to use an LSPR-based sensor to detect other biological samples by tuning their plasmonic features.

Scanning probe microscopy by localized surface plasmon resonance at fiber taper tips.

Plasmonic antenna probes have been widely investigated for detecting electrical permittivity changes on the nanometer scale by employing high-sensitivity localized surface plasmon resonance (LSPR). Although it is intuitive to integrate such a probe onto an atomic force microscope (AFM) to add one more measurable quantity to the family of scanning probe microscopy techniques, the strong scattering background of the AFM tip overwhelms the LSPR scattering signal. To solve this problem, we combined evanescent coupling, polarization and spatial filtering, confocal spectroscopy, and numerical methods to extract clean LSPR spectra from a gold nanosphere-antenna probe attached to the tip of a fiber taper. By mounting the fiber taper on a custom quartz-tuning-fork SPM, we achieved high-quality nanometer-scale imaging of gold nanospheres on glass slides by mapping the LSPR wavelength shift. In addition, we reported an LSPR wavelength shift enhancement by more complicated probe designs and the consequent promise for higher-sensitivity microscopy. Our optical system and spectral processing method provide an effective solution to the long-standing quest for LSPR scanning microscopy.

Color Changes in Ag Nanoparticle Aggregates Placed in Various Environments: Their Application to Air Monitoring.

Fresh Ag nanoparticles (NPs) dispersed on a transparent SiO2 exhibit an intense optical extinction band originating in localized surface plasmon resonance (LSPR) in the visible range. The intensity of the LSPR band weakened when the Ag NPs was stored in ambient air for two weeks. The rate of the weakening and the LSPR wavelength shift, corresponding to visual chromatic changes, strongly depended on the environment in which Ag NPs were set. The origin of a chromatic change was discussed along with both compositional and morphological changes. In one case, bluish coloring followed by a prompt discoloring was observed for Ag NPs placed near the ventilation fan in our laboratory, resulted from adsorption of large amounts of S and Cl on Ag NP surfaces as well as particle coarsening. Such color changes deduce the presence of significant amounts of S and Cl in the environment. In another case, a remarkable blue-shift of the LSPR band was observed for the Ag NPs stored in the desiccator made of stainless steel, originated in the formation of CN and/or HCN compounds and surface roughening. Their color changed from maroon to reddish, suggesting that such molecules were present inside the desiccator.

Label-free, sensitive colorimetric detection of mercury(II) by target-disturbed in situ seeding growth of gold triangular nanoprisms

Gold nanomaterials have been used extensively in colorimetric detection of mercuric ions (Hg2+) due to their shape- and size-dependent, ultrastrong localized surface plasmon resonance (LSPR). Conventional detection was performed by first synthesizing the nanomaterials, and then applying them to signal-transducing reactions. We herein report a convenient method for detecting Hg2+ based on gold triangular nanoprisms (AuTNPs). During the seeding-growth process, Hg2+ added to the growth solution was co-reduced and deposited on the high-energy facets of the gold seeds, affecting the deposition patterns of the subsequently generated Au0 and ultimately leading to the formation of defective AuTNPs. Morphological changes were reflected by the in-plane dipole LSPR wavelength shift, which was proportionally related to the concentration of Hg2+. To improve the selectivity, the interference from Ag+ was eliminated by a stepwise preparation-selective precipitation approach. Under the optimized conditions, Hg2+ could be selectively detected with 20 min, with a detection limit of 0.12 nM. Finally, the method was successfully applied to detecting trace Hg2+ in fortified drinking, mineral and rain water samples, with recoveries ranging from 95.17% to 110.6%.

Rational Calibration Strategy for Accurate and Sensitive Colorimetric Detection of Iodide and l-Thyroxine Based on Gold Triangular Nanoplates

We developed a sensitive and accurate colorimetric method to detect iodide (I–) and l-thyroxine by etching gold triangular nanoplates (AuTNPs) in the presence of H2O2. The morphological changes of AuTNPs resulted in vivid color variations of the nanoprism dispersion, accompanied by a blue shift of the in-plane localized surface plasmon resonance (LSPR) peak, enabling visual and photometric sensing. To improve the accuracy and the linear range, the overlapping out-of-plane and in-plane LSPR peaks of the AuTNPs were deconvoluted, and a new calibration model was established by plotting the square of the in-plane LSPR wavelength shift against the concentration of the analytes. Under optimum conditions, the limits of detection (LODs) for iodide reached 1 μM and 50 nM by the naked eye and photometry, respectively, and the corresponding LODs for l-thyroxine were 200 and 13.7 nM. Importantly, improved accuracy and linear range were obtained by the new data processing strategy. The method was applied in detecting ...

Al-induced tunable surface plasmon resonance of Ag thin film by laser irradiation

Nd:YAG fiber pulsed laser irradiation was employed to tune the localized surface plasmon resonance (LSPR) properties of a single layer of Ag film and Ag/Al bilayer film in this letter. The LSPR wavelength of both films was realized: the tunability in wavelength shift and intensity by varying the laser scan speeds. Compared with the as-irradiated Ag film with limited LSPR wavelength shift and weak intensity, the Al buffer layer has the effects of enhancing the LSPR intensity, widening both the LSPR peak and wavelength shift, suppressing the interband absorption peak and improving the surface enhanced Raman scattering (SERS) performance of the Ag film.

Whole-Cell Pseudomonas aeruginosa Localized Surface Plasmon Resonance Aptasensor.

The detection of whole-cell Pseudomonas aeruginosa presents an intriguing challenge with direct applications in health care and the prevention of nosocomial infection. To address this problem, a localized surface plasmon resonance (LSPR) based sensing platform was developed to detect whole-cell Pseudomonas aeruginosa strain PAO1 using a surface-confined aptamer as an affinity reagent. Nanosphere lithography (NSL) was used to fabricate a sensor surface containing a hexagonal array of Au nanotriangles. The sensor surface was subsequently modified with biotinylated polyethylene glycol (Bt-PEG) thiol/PEG thiol (1:3), neutravidin, and biotinylated aptamer in a sandwich format. The 1:3 (v/v) ratio of Bt-PEG thiol/PEG thiol was specifically chosen to maximize PAO1 binding while minimizing nonspecific adsorption and steric hindrance. In contrast to prior whole-cell LSPR work, the LSPR wavelength shift was shown to be linearly related to bacterial concentration over the range of 10-103 cfu mL-1. This LSPR sensing platform is rapid (∼3 h for detection), sensitive (down to the level of a single bacterium), selective for detection of Pseudomonas strain PAO1 over other strains, and exhibits a clinically relevant dynamic range and excellent shelf life (≥2 months) when stored at ambient conditions. This versatile LSPR sensing platform should be extendable to a wide range of supermolecular analytes, including both bacteria and viruses, by switching affinity reagents, and it has potential to be used in point-of-care and field-based applications.

Nanoplasmonic monitoring of odorants binding to olfactory proteins from honeybee as biosensor for chemical detection

Odorant binding proteins (OBPs) are key soluble sensitive proteins for odor detections in animal olfactory systems. Here, we present a nanosensor based localized surface plasmon resonance (LSPR) to monitor binding of small odorant molecules to OBPs from honeybees. The binding of odorants molecules to OBPs, can be quantified by nanocup arrays from 10nM to 1mM, with a high refractive index sensing of LSPR wavelength shift in transmission spectra. Linear dose-dependent behavior can be observed in relationship of LSPR wavelength versus analyte concentrations. More than floral odorants, this type of bio-functionalized nanocups also showed potential applications in explosive detections for nitro-compounds such as 2,4,6-trinitrotoluene, 2,4-dinitrotoluene and 3-mononitrotoluene. This study demonstrated a LSPR device to monitor interactions between small molecules and proteins, which provided a great biosensor platform for healthcare diagnosis, environment monitoring and food analysis.

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.

Development of Localized Surface Plasmon Resonance-Based Point-of-Care System

This paper describes our point-of-care system development based on localized surface plasmon resonance (LSPR). Although LSPR has been a hot research area for a few decades, there are several bottlenecks which hampered its application for point-of-care (POC) medical diagnostics. The first is the detection sensitivity shortage when the direct LSPR wavelength shift is used for sensing, the second is the mass fabrication of durable metal nanostructures on the substrate, and the third is the microfluidic chip and the POC system which have to be combined with LSPR chips in a seamless but cost-effective way. To solve the above challenges, several novel technologies are initiated and successfully implemented in this work. To increase the sensitivity of the LSPR detection, we use plasmonic field to excite the fluorescence dyes conjugated to the analyte rather than directly detecting the LSPR wavelength shift upon analyte bonding. This method can enhance the biomarker detection sensitivity 10 to 100 times upon careful design of the metal nanostructures and the location of the fluorescence dyes in the bioassay. To mass fabricate the metal nanostructures, a 4″ nickel mold is fabricated by electroplating and employed for UV nanoimprinting lithography. Our technology achieves high yield on wafer-level mass fabrication of the designed metal nanostructures. In terms of the surface modification of the bioassay, the orientation of the capturing antibody is controlled to enhance the sensitivity of biomarker detections, and an antifouling polymer is synthesized inside the gold nanoholes. To accomplish the cost-effective point-of-care system, a plastic multichamber microfluidic chip is fabricated, which contains the metal nanostructures, microfluidic channels, and trenches for controlling the sample flow. The microfluidic chip is inserted into a point-of-care system which consists of micropumps to control the microfluidic flow, a light source for fluorescence excitation, a camera system for fluorescence detection, and software to automate the POC system and to analyze the results. We believe this highly sensitive LSPR point-of-care system has ample applications on medical diagnostics.

Theoretical Study of the Anchoring Influence on Plasmonic Resonance Tunability of Metallic Nanoparticles Embedded in a Liquid Crystal Cell

In this paper, the localized surface plasmon resonance (LSPR) peak position of an ordered gold nanoparticles array embedded in a nematic liquid crystal (LC) media is investigated using finite-difference time-domain method. The influence of the anchoring effects between nematic LC molecules and glass substrate on the shift of LSPR wavelength is taken into account, and results are compared with the case of a perfect alignment of the LC molecules.

Effect of shell thickness on a Au–Ag core–shell nanorods-based plasmonic nano-sensor

In this paper, Au–Ag core–shell nanorods were synthesized with the wet-chemical method, by which localized surface plasmon resonance (LSPR) wavelengths were tuned from 766 to 544 nm by controlling the thickness of the silver shell. The sensing ability of these core–shell nanorods was experimentally investigated by measuring the extinction spectra, which gives out the shift of LSPR wavelength as the refractive index of the surrounding medium changes. Experimental results show that the refractive index sensitivity (RIS) and figure of merit (FOM) of the core–shell nanorods can be increased at a certain thickness of the silver shell.

Analysis of metallic nanoparticle-DNA assembly formation in bulk solution via localized surface plasmon resonance shift

Metallic nanoparticles such as gold and silver are known to exhibit localized surface plasmon resonance (LSPR). Being able to form well defined nanoassemblies of metallic nanoparticles in solution phase can produce LSPR coupling and shift, which represents a unique plasmon signature and have been used for the detection of nanoassembly formation. While most of the existing works focus on nanoassembly formation on a substrate surface, here, we investigated the formation of DNA-modified gold nanoparticle (nAu-DNA) nanoassemblies in bulk solution. Subsequently the nanoassemblies were allowed to bind on a glass substrate in order to study correlations among the LSPR wavelength shift, the plasmon color change, and the nanoassembly structure. We observed that the hybridization percentage of the complementary 50 nm nAu increased with rising nAu concentration, longer hybridization time, and longer complementary duplex DNA length. In addition, due to lower scattering yield and smaller surface area from 10 nm nAu, the 50 nm/10 nm hetero-size system displayed limited observable LSPR shift compared to 50 nm/20 nm hetero-size system, which in turn was inferior to the 50 nm/50 nm homo-size system. For the hetero-size systems, reducing the surface density of ssDNA on the 20 nm and 10 nm nAu also significantly reduced the hybridization percentages. Overall, this study allows us to understand how different experimental parameters can impact the assembly of nAu-DNA probes, particularly the limitation in using smaller size nAu (10 nm) for LSPR study.

On the linear response and scattering of an interacting molecule-metal system

A many-body Green's function approach to the microscopic theory of plasmon-enhanced spectroscopy is presented within the context of localized surface-plasmon resonance spectroscopy and applied to investigate the coupling between quantum-molecular and classical-plasmonic resonances in monolayer-coated silver nanoparticles. Electronic propagators or Green's functions, accounting for the repeated polarization interaction between a single molecule and its image in a nearby nanoscale metal, are explicitly computed and used to construct the linear-response properties of the combined molecule-metal system to an external electromagnetic perturbation. Shifting and finite lifetime of states appear rigorously and automatically within our approach and reveal an intricate coupling between molecule and metal not fully described by previous theories. Self-consistent incorporation of this quantum-molecular response into the continuum-electromagnetic scattering of the molecule-metal target is exploited to compute the localized surface-plasmon resonance wavelength shift with respect to the bare metal from first principles.

Biological and chemical gold nanosensors based on localized surface plasmon resonance

In this paper, we discuss the performances of gold nanosensors based on Localized Surface Plasmon Resonance (LSPR) designed by Electron Beam Lithography (EBL) in the context of biological and chemical sensing. We demonstrate the sensitivity of our gold nanosensors by studying the influence of the concentration of 11-mercaptoundecanoic acid (MUA) on the shift of LSPR wavelength. Additionally, to study the selectivity of our nanosensors, the system Biotin/Streptavidin was used to detect very weak concentration of biomolecules. These results represent new steps for applications in chemical research and medical diagnostics.