Effect of solvent refractive index on the surface plasmon resonance nanoparticle optical absorption

Optical properties of plasmon coupled silver and gold nanoparticles were studied as a function of the refractive index of the surrounding medium. Our studies confirmed that the effect of changes in the refractive index of the surrounding medium was more difficult to demonstrate from an experimental point of view, because of the very high susceptibility of nanoparticles to aggregate in aqueous and organic solvents. Whereas the position of the absorption bands of triiodide in these solvents shows a clear dependence on medium's refractive index, the surface plasmon band position of silver and gold nanoparticles do not exhibit the same dependence. This is attributed to a non-negligible interaction of these solvents with nanoparticle surfaces.     

Theoretical limit of localized surface plasmon resonance sensitivity to local refractive index change and its comparison to conventional surface plasmon resonance sensor

In this paper, the theoretical sensitivity limit of the localized surface plasmon resonance (LSPR) to the surrounding dielectric environment is discussed. The presented theoretical analysis of the LSPR phenomenon is based on perturbation theory. Derived results can be further simplified assuming quasistatic limit. The developed theory shows that LSPR has a detection capability limit independent of the particle shape or arrangement. For a given structure, sensitivity is directly proportional to the resonance wavelength and depends on the fraction of the electromagnetic energy confined within the sensing volume. This fraction is always less than unity; therefore, one should not expect to find an optimized nanofeature geometry with a dramatic increase in sensitivity at a given wavelength. All theoretical results are supported by finite-difference time-domain calculations for gold nanoparticles of different geometries (rings, split rings, paired rings, and ring sandwiches). Numerical sensitivity calculations based on the shift of the extinction peak are in good agreement with values estimated by perturbation theory. Numerical analysis shows that, for thin (≤10 nm) analyte layers, sensitivity of the LSPR is comparable with a traditional surface plasmon resonance sensor and LSPR has the potential to be significantly less sensitive to temperature fluctuations.     

Transparent high refractive index nanocomposite thin films

This work reports the preparation of acetic acid-modified TiO₂ nanoparticles by sol–gel synthesis method. The nanoparticles can be incorporated directly into the polymer matrix to form transparent high refractive index nanocomposite thin films. The result shows that increasing the titania content in the hybrid nanocomposite thin films can significantly increase the refractive index. Hybrid nanocomposite thin film with refractive index value of 2.38 had been prepared. All prepared films also exhibit excellent optical transparency in the visible region.     

Dynamically modulating the surface plasmon resonance of doped semiconductor nanocrystals

Localized surface plasmon absorption features arise at high doping levels in semiconductor nanocrystals, appearing in the near-infrared range. Here we show that the surface plasmons of tin-doped indium oxide nanocrystal films can be dynamically and reversibly tuned by postsynthetic electrochemical modulation of the electron concentration. Without ion intercalation and the associated material degradation, we induce a > 1200 nm shift in the plasmon wavelength and a factor of nearly three change in the carrier density.     

Generation of infrared surface plasmon resonances with high refractive index sensitivity utilizing tilted fiber Bragg gratings

We demonstrate the use of tilted fiber gratings to assist the generation of localized infrared surface plasmons with short propagation lengths and a sensitivity of dlambda/dn = 3,365 nm in the aqueous index regime. It was also found that the resonances could be spectrally tuned over 1,000 nm at the same spatial region with high coupling efficiency (in excess of 25 dB) by altering the polarization of the light illuminating the device.     

Simulation on wavelength-dependent complex refractive index of liquids obtained by phase retrieval from reflectance dip due to surface plasmon resonance

A method for the calculation of the wavelength-dependent complex refractive index of absorbing liquid from reflectance in the vicinity of surface plasmon resonance (SPR) is presented. The calculation is based on the maximum entropy method (MEM). As an example, phase retrieval from a simulated SPR reflectance of a red colored liquid solution is carried out. It is proposed that MEM can be applied to wavelength-dependent complex refractive index assessment from reflectance of absorbing liquids in SPR measurement in wavelength scanning mode.     

Characterization of swellable molecularly imprinted polymer particles by surface plasmon resonance spectroscopy

Surface plasmon resonance (SPR) has been used to investigate template binding at sites in theophylline-imprinted poly-[N-(N-propyl) acrylamide] particles. At concentrations as low as 10(-6) M theophylline, particle swelling is detected as a shift in the angle of minimum reflectance. The binding constant of theophylline estimated from the inflection point of the theophylline calibration curve is approximately 10,000. The imprinted polymer particles do not respond to caffeine or theobromine (which differs from theophylline by a single methyl group) at concentrations as high as 10(-2) M. Full-scale response of the imprinted polymer particles to theophylline (template) occurs in less than 15 minutes, and swelling is reversible. The immobilized imprinted polymer particles can undergo approximately 20 to 25 swelling and shrinking cycles before there is significant loss in functionality. A unique aspect of these imprinted polymer particles is that template binding causes the angle of minimum reflectance to decrease, not increase, in magnitude. Adsorption, which causes an increase in the angle of minimum reflectance, can be readily discriminated from template binding.     

Ultrafast pump-probe surface plasmon resonance spectroscopy of thin gold films

A time-resolved reflection pump-probe method was combined with a surface plasmon resonance technique in Kretschmann geometry for the investigation of ultrafast light-induced processes in thin films. Transient changes in the gold layer's reflectivity were observed when the layer was excited by 3 ps duration pulses with photon energy exceeding the interband transition and by probing with photon energy close to the interband transition. Comparison of the experimental and modeling results has shown that the imaginary part of the dielectric function of gold increases linearly during excitation, whereas the real part remains unchanged. The decay of the light-induced changes has two components. The first component is faster than the pulse duration, and the second is much longer than 1.5 ns; they are related to cooling of the electron plasma and lattice, respectively.     

Retrieval of the effective complex refractive index of Bruggeman nanoliquids from surface plasmon resonance reflectance using the maximum entropy method

We show that surface plasmon resonance reflectance measurement can be used to extract the effective complex refractive index of a Bruggeman nanoliquid, i.e. a solution consisting of spherical nanoparticles embedded into a host liquid such as water, using the maximum entropy method. This retrieval can be immediately performed once the optical properties of the host liquid are known. In this paper we investigate the upper limit of the volume fraction of spherical nanoparticles in a solution that still yields a reliable result in the retrieval based on the maximum entropy method. Finally, we show that by squeezing the spectrum we can significantly reduce the errors in the analysis and retrieve the effective complex refractive index for a high volume fraction of nanoparticles.     

Application of surface plasmon resonance sensing to studying elastohydrodynamic lubricant films

We present a new technique based on the spectral characteristics associated with the surface plasmon resonance (SPR) effect for studying lubricants in elastohydrodynamic (EHD) dimples. The pressure inside the EHD dimple causes a localized change of the refractive index (RI) of the entrapped lubricant. This also results in a shift in the spectral SPR absorption dip. By monitoring the color changes within the SPR image, one can obtain a direct measurement of the RI of the entrapped lubricant, from which the pressure distribution within the elastohydrodynamic lubrication (EHL) dimple can be deduced by means of a predetermined relation of pressure and RI of the tested lubricant. Dimples formed with the lubricants PB 2400 and H 1900 were studied in our experiments. Because SPR is sensitive only to the RI variation within a thin region (approximately one wavelength) close to the sensor's surface, the new technique does not require any measurement of the absolute film thickness of the lubricant. This is much simpler than the existing two-beam interferometric technique for measuring the RI of lubricants in EHD dimples, which requires simultaneous measurements of optical film thickness by use of two beams of different angles of incidence. In light of this advantage we anticipate that the new technique can be applied to pressure field mapping in highly loaded rolling and sliding EHL contacts.     

High-throughput immunophenotyping by surface plasmon resonance imaging

Cell binding assays on antibody arrays permit the rapid immunophenotyping of living cells. The throughput of the analysis, however, is still limited due to our inability to perform parallel and quantitative detection of cells captured on the array. To address this limitation, we employed here an imaging technique based on surface plasmon resonance (SPR). SPR has been frequently used to monitor capture of proteins on antibody microarrays, while few cases were reported for capture of cells. Antibody arrays were prepared through the photopatterning of an alkanethiol monolayer on a gold-evaporated glass plate and the subsequent immobilization of various antibodies onto 4-9 separate spots created by photopatterning. A glass slip was mounted onto the array with a thin spacer to construct a parallel-plate chamber. Leukemia cells were injected into the chamber to conduct a binding assay, while refractive index changes at the vicinity of the array surface were monitored by SPR imaging. We observed that SPR signals were intensified on specific antibody spots but not on nonspecific spots. Confocal laser scanning microscopy revealed that the observed SPR signals were attributed to cell deformations caused by multivalent interactions with immobilized antibody, which effectively elevated the refractive index of a medium phase within an evanescent field. This effect could be suitably utilized to monitor quantitatively cell binding to multiple spots from a heterogeneous cell population.     

Controlled nanostructuring of Ag films by ion beam sputter deposition for surface plasmon resonance applications

The growth of Ag nanostrucutres on borosilicate glass substrates by ion beam sputter deposition in the Ar ion energy range from 150 to 600 eV is demonstrated. Rates of deposition as low as 0.01 nm/s are achieved at an Ar ion energy of 150 eV. This leads to the formation of a random array of nearly spherical Ag particles with a mean size of approximately 100 nm, separated by distances of similar order of magnitude. The particles organize themselves into arrays over lengths of at least 10 microm. As the thickness is increased from 3 to 18 nm there is a transition in morphology from an array to linear chains and finally a dense continuous film. There is a similar microstructural evolution as a function of increasing ion energy. The plasmon resonances can be tuned depending on shape, size and interparticle distances. As the thickness of the films increase, the main plasmon peaks can be tuned from 380 to 680 nm. The spheroidal shape of the particles induces additional peaks (localized surface plasmons) centered around 430 +/- 10 nm. Detailed simulations have been carried out based on Maxwell Garnett theory to distinguish the effects of shape and size on plasmon resonances. It is demonstrated that shape rather than the size of the particles has a stronger influence on the shift in plasmon resonances.     

A transducer based on enzyme-induced degradation of thin polymer films monitored by surface plasmon resonance

A novel transducer based on the dissolution of biodegradable polymer films as a direct result of enzymatic reaction has been developed. Three polymers were investigated for use in the transducer: a poly(ester amide), which is degraded by the proteolytic enzyme alpha-chymotrypsin; a dextran hydrogel, which is degraded by dextranase; and poly(trimethylene) succinate, which is degraded by a lipase. Degradation of the polymer films was monitored by surface plasmon resonance (SPR) and impedance measurements. SPR was shown to be suitable for a greater variety of materials, since it does not require the polymer film to be electrically insulating. Rate of degradation was shown to be directly related to enzyme concentration for each polymer/enzyme couple. The poly(ester amide)/alpha-chymotrypsin couple proved to be the most sensitive. Degradation of the films was complete in less than 20 min for enzyme concentrations greater than 9 x 10(-9) mol dm-3. Enzyme concentrations as low as 4 x 10(-11) mol dm-3 were detected in less than 30 min. The transducer has great potential for the detection of enzyme concentrations as well as for use in immunosensing where the enzyme degrading the polymer would be the enzyme label.     

Differential-phase surface plasmon resonance biosensor

In this paper, a novel differential-phase-sensitive surface plasmon resonance biosensor (DP-SPRB) is proposed and developed, in which a two-frequency laser is integrated with a differential amplifier in order to analytically convert the phase modulation into amplitude modulation. With the use of the conventional envelope detection technique, the differential phase is precisely decoded in real time in terms of the demodulated amplitude. In order to verify high detection sensitivity of the DP-SPRB, a sucrose-water solution and glycerin-water solution at low concentrations were both tested, and the experimental results confirm that the detection sensitivity on wt % concentration of the sucrose solution is 0.00001%. Moreover, the real-time monitoring mouse IgG/antimouse IgG interaction shows the minimum concentration of mouse IgG to be at 10 fg/mL. To our knowledge, this is the highest sensitivity ever measured by a surface plasmon resonance biosensor. However, because of the limited dynamic range of DP-SPRB, it can only apply to biomolecule interactions at extremely low concentration.     

Wavelength-scanning surface plasmon resonance imaging

A new surface plasmon resonance (SPR) imaging technique was proposed. After measurements were conducted at varying wavelengths, the wavelength affording the minimum brightness (SPR wavelength) was determined at each pixel of the image. A two-dimensional map of the SPR wavelength could be converted to a thickness profile by use of a nonlinear calibration curve, which was obtained by Fresnel calculation. An array of protein thin layers on a gold film was evaluated in air to present the layers' surface structure in nanometer scale.     

Inverse surface plasmon resonance based effective hydrogen sensing using nanoscale palladium films

We demonstrate a nanoscale palladium (Pd) based inverse surface plasmon resonance (ISPR) setup for sensing highly inflammable hydrogen (H₂) gas. The ISPR setup was employed in Kretschmann configuration to assess the sensitivity of the Pd-films when subjected to H₂ gas exposure. With an adequate broadening of the SPR peak maxima, the SPR angle was found to shift from a value of 46.57° to 50.97°, when the concentration of H₂ was varied between 0% and 0.9%. The shifting can be attributed to the transient development of isolated PdH_x (x < 1) clusters within Pd lattices, resulting in an appreciable change of refractive index locally. The dynamical behaviour of switching on/off states exhibited by a ∼20 nm Pd-film and exposed to 0.1% H₂ gas was monitored over several cycles repetitively. The ISPR based H₂ sensor, as demonstrated in ambient environment, would find scope to detect low level H₂ in industrial and other hazardous areas.     

Determination of ribonuclease H surface enzyme kinetics by surface plasmon resonance imaging and surface plasmon fluorescence spectroscopy

The kinetics of the ribonuclease H (RNase H) surface hydrolysis of RNA-DNA heteroduplexes formed on DNA microarrays was studied using a combination of real-time surface plasmon resonance imaging (SPRI) and surface plasmon fluorescence spectroscopy (SPFS). Time-dependent SPRI and SPFS data at various enzyme concentrations were quantitatively analyzed using a simple model that couples diffusion, enzyme adsorption, and surface enzyme kinetics. This model is characterized by a set of three rate constants, enzyme adsorption (k(a)), enzyme desorption (k(d)), enzyme catalysis (k(cat)), and one dimensionless diffusion parameter (beta). Values of k(a) = 3.15 (+/-0.20) x 10(6) M(-1).s(-1), k(d) = 0.10 (+/-0.05) s(-1), and k(cat) = 0.95 (+/-0.10) s(-1) were determined from fitting all of the SPRI and SPFS data sets. One of the most interesting kinetic parameters is the surface RNase H hydrolysis reaction rate constant (k(cat)), which was found to be approximately 10 times slower than that observed in solution, but approximately 100 times faster than that recently observed for the exonuclease III surface hydrolysis of double-stranded DNA microarrays (k(cat) = 0.009 s(-1)). Moreover, the surface coverage of the intermediate enzyme-substrate complex (ES) was found to be extremely small during the course of the reaction because k(cat) is much larger than the product of k(a) and the bulk enzyme concentration.     

Thermal-induced surface plasmon band shift of gold nanoparticle monolayer: morphology and refractive index sensitivity

In this paper, thermal-induced behaviors of a gold nanoparticle monolayer on glass slides are investigated. First, through horizontal lifting, gold nanoparticle monolayers are transferred from a water/hexane interface to glass slides. Then thermal treatment is carried out in air, after which an apparent color change of the obtained samples is noticed, depending on the annealing temperature, reflecting a shift of the surface plasmon band (SPB). Depending on the trend of SPB shift, the overall thermal process is divided into three stages. In the first stage, SPB shows a redshift trend with concomitant band broadening. Further increase of the annealing temperature in the second stage results in an increase of interparticle distance. Thus an apparent decrease in absorbance takes place with SPB shift to shorter wavelengths. In the third stage, the SPB redshifts again. Bulk refractive index sensitivity (RIS) measurements are taken by immersing the obtained samples in solutions of various refractive indices and a linear dependence of RIS(λ) and RIS(ext) on refractive index is concluded. In particular, the influences of parameters such as particle sizes, location of SPB, substrate effect and morphology effect on RIS are discussed in detail. The corresponding performance of each sample as a localized surface plasmon resonance-based sensor is evaluated by a figure of merit (FOM) represented as FOM(λ) and FOM(ext). It is found that the optimum annealing temperature is 500?°C. In terms of nanoparticle sizes, samples with a 35 nm gold nanoparticle monolayer perform better than those with 15 nm. The current strategy is simple and facile to achieve fine control of the SPB, in which large-size precision instruments or complex chemosynthesis are unnecessary. Therefore, this method has not only significance for theory but also usefulness in practical applications.     

Silicon-based surface plasmon resonance sensing with two surface plasmon polariton modes

Surface plasmon resonance (SPR) sensing on a silicon-based platform is considered. We have studied properties of SPR in a combined silicon-dielectric layer-gold film-sample medium structure and established conditions of the simultaneous excitation of two plasmon polariton modes that provide narrow and well-separated minima of the reflected intensity. It has been shown that the external mode over the gold-sample medium interface demonstrates a highly sensitive response to a change in the refractive index of the sample medium, whereas the internal mode over the dielectric-gold interface is almost insensitive to medium parameters. We propose that the internal mode can be used as an effective reference zero point for miniature and portable SPR-based systems designed for field and multichannel sensing.     

Influence of the surface roughness on the properties of Au films measured by surface plasmon resonance and X-ray reflectometry

Thickness and refractive index of Au films thermally evaporated onto glass substrates and with an underlayer of Cr are determined from surface plasmon resonance. The results for the thickness are found to agree very well with those from X-ray reflectivity when a simple model of layers with flat interfaces is used. Plasmon propagation along thin films is influenced by radiative damping due to scattering from surface roughness. To study this influence the surface roughness of the glass substrate, Cr an Au layers are measured by X-ray reflectometry and the results used to introduce three intermediate layers with effective refractive indices and thicknesses corresponding to the roughness. Then Fresnel's equations are used to fit the reflectivity and to deduce the layer properties. It is found that the roughness affects to a great extent the optical parameters of the Au films even when it is smaller than 1 nm. In particular, the absolute value of real part of the dielectric constant decreases while its imaginary part increases when those effects are not taken into account.