Gold Nanohole Research Articles

Graphene-enhanced plasmonic nanohole arrays for environmental sensing in aqueous samples

The label-free nature of surface plasmon resonance techniques (SPR) enables a fast, specific, and sensitive analysis of molecular interactions. However, detection of highly diluted concentrations and small molecules is still challenging. It is shown here that in contrast to continuous gold films, gold nanohole arrays can significantly improve the performance of SPR devices in angle-dependent measurement mode, as a signal amplification arises from localized surface plasmons at the nanostructures. This leads consequently to an increased sensing capability of molecules bound to the nanohole array surface. Furthermore, a reduced graphene oxide (rGO) sensor surface was layered over the nanohole array. Reduced graphene oxide is a 2D nanomaterial consisting of sp2-hybridized carbon atoms and is an attractive receptor surface for SPR as it omits any bulk phase and therefore allows fast response times. In fact, it was found that nanohole arrays demonstrated a higher shift in the resonance angle of 250–380% compared to a continuous gold film. At the same time the nanohole array structure as characterized by its diameter-to-periodicity ratio had minimal influence on the binding capacity of the sensor surface. As a simple and environmentally highly relevant model, binding of the plasticizer diethyl phthalate (DEP) via π-stacking was monitored on the rGO gold nanohole array realizing a limit of detection of as low as 20 nM. The concentration-dependent signal change was studied with the best performing rGO-modified nanohole arrays. Compared to continuous gold films a diameter-to-periodicity ratio (D/P) of 0.43 lead to a 12-fold signal enhancement. Finally, the effect of environmental waters on the sensor was evaluated using samples from sea, lake and river waters spiked with analytically relevant amounts of DEP during which significant changes in the SPR signal are observed. It is expected that this concept can be successfully transferred to enhance the sensitivity in SPR sensors.

Surface-Enhanced Raman Scattering on Gold Nanohole Arrays in Symmetrical Dielectric Environments Exhibiting Electric Field Extension

The electromagnetic enhancement in surface-enhanced Raman scattering (SERS) caused by localized surface plasmon resonance is a near-field effect, often limiting the practicality of SERS in many applications. However, no attempts have been made to investigate field extension through symmetrical refractive index modulation in a SERS-based system. Here, we report the development and characterization of refractive index-matched SERS substrates supporting electric field extension to realize what is termed “long-range SERS” (LR-SERS). Finite-difference time-domain simulations were employed to tune the plasmonic responses and investigate electric field distributions of gold nanohole arrays (NHAs) as a function of the dielectric environment and geometric parameters. SERS substrates supporting long-range behavior were compared against “conventional” substrates without long-range behavior. SERS intensities from rhodamine 6G (R6G) aqueous solutions of 2.0 × 103 and 1.5 × 102 counts s–1 mW–1 were produced by the LR-S...

Gold Nanohole Arrays Fabricated by Interference Lithography Technique as SERS Probes for Chemical Species Such As Rhodamine 6G and 4,4′-Bipyridine

In the present study, we report the SERS effect of rhodamine 6G (Rh6G) and 4-4′ bipyridine (Bipy) molecules on the surface of gold nanohole (AuNh) arrays fabricated by the interference lithography (IL) technique. It was observed an enhancement of the main bands of Rh6G and 4-4′ Bipy molecules, when deposited over the surface of AuNh, in comparison when that was deposited on the surface of 5-nm flat gold film. Our study indicates that metallic nanostructures fabricated by IL technique have the potential for high impact on biochemical sensing, such as DNA and bacterial detection, real time glucose sensing for diabetes, and in situ identification of reaction products.

Graphene-based plasmonic tweezers

Conventional plasmonic tweezers with the ability to attract and immobilize nearby sub-diffraction limit sized particles can only enhance the trapping efficiency by changing the shape of the metal nanostructures. There are several problems with conventional plasmonic tweezers. First, trapped particles can easily escape from the trap by disturbances coming from the heat absorption of the metallic surfaces. These disturbances prevent prolonged observation of the trapped particles. Second, observation of the particles becomes a challenge because the opaqueness of the metal blocks the illumination pathways. These problems can be solved by using graphene, which has high transmittance and thermal conductivity. The carrier density of the graphene is tuned by externally controlling the Fermi level through the gate voltage. Tuning the carrier density alters the local field enhancement factor far beyond the capabilities provided by other metal-based plasmonic structures. In this paper, we have shown that particles can be trapped by graphene nanoholes with larger forces than gold nanoholes. The trapping forces on gold and graphene nanoholes were compared to illustrate the benefit of graphene nanoholes. Furthermore, various trapping modes of a particle under various geometries and configurations of graphene nanoholes is discussed.

96-Well Plasmonic Sensing with Nanohole Arrays

A multiwell plasmonic reader was designed and validated for higher throughput analysis of biological interactions with a platform of the same size as standard 96-well plates. While the plasmonic sensor can be read with standard 96-well plate readers, a custom 96-well plate reader was designed to analyze nanohole arrays at high incident angles required for higher sensitivity. Gold nanohole arrays were manufactured on a 4 in. glass wafer using a photolithographic process. In comparison to single channel measurements with nanohole arrays fabricated with nanosphere lithography, the nanohole array sensors greatly enhanced the signal-to-noise ratio of the plasmonic signal and precision of the measurements with the multiwell plate system. As proof of concept, the detection of IgG in the low nanomolar range was achieved with the multiwell plate reader. The multiwell plasmonic plate reader was also applied to the screening of several prostate specific (PSA) antibodies for secondary detection of PSA and for the ana...

Probing Graphene χ((2)) Using a Gold Photon Sieve.

Nonlinear second harmonic optical activity of graphene covering a gold photon sieve was determined for different polarizations. The photon sieve consists of a subwavelength gold nanohole array placed on glass. It combines the benefits of efficient light trapping and surface plasmon propagation to unravel different elements of graphene second-order susceptibility χ((2)). Those elements efficiently contribute to second harmonic generation. In fact, the graphene-coated photon sieve produces a second harmonic intensity at least two orders of magnitude higher compared with a bare, flat gold layer and an order of magnitude coming from the plasmonic effect of the photon sieve; the remaining enhancement arises from the graphene layer itself. The measured second harmonic generation yield, supplemented by semianalytical computations, provides an original method to constrain the graphene χ((2)) elements. The values obtained are |d31 + d33| ≤ 8.1 × 10(3) pm(2)/V and |d15| ≤ 1.4 × 10(6) pm(2)/V for a second harmonic signal at 780 nm. This original method can be applied to any kind of 2D materials covering such a plasmonic structure.

Influences of Nanostructure Arrays on Light Absorption in Amorphous Silicon Thin-Film Solar Cells

By means of finite-difference time-domain (FDTD) numerical method, we investigate the possibility to enhance the light absorption in solar cells by employing different nanostructures. The solar cells are made of 100-nm-thick amorphous silicon (α-Si). The impacts of gold nanohole arrays, dielectric nanosphere arrays, and gold nanoparticle arrays on the light absorption are simulated, compared, and analyzed. The results show that gold nanohole arrays functioning as the back reflective layer, dielectric nanosphere arrays, and gold nanoparticle arrays can significantly enhance the light absorption for the solar cells, and the former two can increase the short-circuit current by more than 40 %, showing a great potential to improve the utilization efficiency of solar energy.

Template-Stripped Tunable Plasmonic Devices on Stretchable and Rollable Substrates.

We use template stripping to integrate metallic nanostructures onto flexible, stretchable, and rollable substrates. Using this approach, high-quality patterned metals that are replicated from reusable silicon templates can be directly transferred to polydimethylsiloxane (PDMS) substrates. First we produce stretchable gold nanohole arrays and show that their optical transmission spectra can be modulated by mechanical stretching. Next we fabricate stretchable arrays of gold pyramids and demonstrate a modulation of the wavelength of light resonantly scattered from the tip of the pyramid by stretching the underlying PDMS film. The use of a flexible transfer layer also enables template stripping using a cylindrical roller as a substrate. As an example, we demonstrate roller template stripping of metallic nanoholes, nanodisks, wires, and pyramids onto the cylindrical surface of a glass rod lens. These nonplanar metallic structures produced via template stripping with flexible and stretchable films can facilitate many applications in sensing, display, plasmonics, metasurfaces, and roll-to-roll fabrication.

Improving the performance of gold nanohole array biosensors by controlling the optical collimation conditions.

An experimental investigation on how the bulk and surface sensitivities of gold nanohole arrays fabricated by interference lithography affect the degree of white light beam collimation is presented. The optical transmission response of nanohole arrays has been recorded by focused and collimated beam transmission spectra. The results show that both the bulk and surface sensitivities for the collimated case are much larger than for the focused case. In particular, the shape of the spectra was dependent on the degree of beam collimation. The results showed that improved sensing performance (around 3.5 times) and higher figure of merit (around 4.4 times) can be obtained by simply adjusting the incident/collection experimental conditions in transmission measurements.

Plasmonic Gold Nanohole Arrays for Surface-Enhanced Raman Scattering Biosensing

Gold nanohole arrays can serve as promising substrates for construction of surface-enhanced Raman scattering (SERS) biosensors. Gold nanohole arrays support tunable localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP). However, it is a great challenge to optimize their performance due to coexistence of LSPR and SPP. Here, finite difference time domain (FDTD) simulation is used in combination with measured transmission and reflection to discern which resonance mode leads to the electromagnetic field enhancement. The SERS peak signal intensity has been mapped with the nanohole size, which shows that the “gap” between neighboring nanoholes in the array pattern and the absorption play an important role in SERS enhancement. Optimization of hole size and pitch is necessary for development of nanohole array based SERS biosensors.

Nanoimprinted thrombin aptasensor with picomolar sensitivity based on plasmon excited quantum dots

To achieve sensitive biomolecular detection, a gold nanohole array sensing chip was designed by plasmonic simulation and fabricated by high fidelity nanoimprinting technology. The gold nanohole array on a glass substrate serves as an optical antenna array, to produce localized surface plasmon resonance (LSPR) of 10–100 times stronger than the illumination light for quantum dot (QD) excitation. In this work, a robust sandwich bioassay of primary aptamer/thrombin/secondary aptamer with QD655 was established on the chip, to detect thrombin at a limit of detection (LOD) of 1ng/ml (27pM), with low non-specific binding even at the presence of 1000 times more concentrated bovine serum albumin (BSA). The conformation and QD enhancement mechanism of the sandwich assay were characterized by atom force microscopy (AFM) and dual polarization interferometry (DPI). This is the first time that DNA is surface functionalized on gold nanostructures for protein detection by plasmon enhanced QD emission. Considering the low-cost, easy regeneration and long shelf-life of the capturing aptamer, low non-specific binding and high sensitivity of the bioassay, as well as the inexpensive mass fabrication of the high quality chips, this novel aptasensor platform is particularly useful for a point-of-care system on medical diagnostics.

Gold Nanohole Array with Sub-1nm Roughness by Annealing for Sensitivity Enhancement of Extraordinary Optical Transmission Biosensor.

Nanofabrication technology plays an important role in the performance of surface plasmonic devices such as extraordinary optical transmission (EOT) sensor. In this work, a double liftoff process was developed to fabricate a series of nanohole arrays of a hole diameter between 150 and 235 nm and a period of 500 nm in a 100-nm-thick gold film on a silica substrate. To improve the surface quality of the gold film, thermal annealing was conducted, by which an ultra-smooth gold film with root-mean-square (RMS) roughness of sub-1 nm was achieved, accompanied with a hole diameter shrinkage. The surface sensitivity of the nanohole arrays was measured using a monolayer of 16-mercaptohexadecanoic acid (16-MHA) molecule, and the surface sensitivity was increased by 2.5 to 3 times upon annealing the extraordinary optical transmission (EOT) sensor.

Enhancing Eu(3+) magnetic dipole emission by resonant plasmonic nanostructures.

We demonstrate the enhancement of magnetic dipole spontaneous emission from Eu3+ ions by an engineered plasmonic nanostructure that controls the electromagnetic environment of the emitter. Using an optical microscope setup, an enhancement in the intensity of the Eu3+ magnetic dipole emission was observed for emitters located in close vicinity to a gold nanohole array designed to support plasmonic resonances overlapping with the emission spectrum of the ions.

A gold nanohole array based surface-enhanced Raman scattering biosensor for detection of silver(I) and mercury(II) in human saliva.

A surface-enhanced Raman scattering (SERS) biosensor has been developed by incorporating a gold nanohole array with a SERS probe (a gold nanostar@Raman-reporter@silica sandwich structure) into a single detection platform via DNA hybridization, which circumvents the nanoparticle aggregation and the inefficient Raman scattering issues. Strong plasmonic coupling between the Au nanostar and the Au nanohole array results in a large enhancement of the electromagnetic field, leading to amplification of the SERS signal. The SERS sensor has been used to detect Ag(I) and Hg(II) ions in human saliva because both the metal ions could be released from dental amalgam fillings. The developed SERS sensor can be adapted as a general detection platform for non-invasive measurements of a wide range of analytes such as metal ions, small molecules, DNA and proteins in body fluids.

Tailoring plasmonic properties of gold nanohole arrays for surface-enhanced Raman scattering.

The wide plasmonic tuning range of nanotriangle and nanohole array patterns fabricated by nanosphere lithography makes them promising in surface-enhanced Raman scattering (SERS) sensors. Unfortunately, it is challenging to optimize these patterns for SERS sensing because their optical response is a complex mixture of localized surface plasmon resonance (SPR) and propagating surface plasmon polariton (SPP). In this paper, transmission and reflection measurements are combined with finite difference time domain simulations to identify and separate each plasmonic mode, discerning which resonance leads to the electromagnetic field enhancement. The SERS enhancement is found to be dominated by the absorption, which is shifted from the transmission and reflection dips usually used as tuning points, and by the 'gap' defects formed within the pattern. These effects have different spectral and geometric dependences, forming two optimization curves which can be used to predict the best performance for a given excitation wavelength. The developed model is verified with experimental SERS measurements for several nanohole sizes and periodicities, and then used to give optimal fabrication parameters for a range of measurement conditions. The results will promote the application of two-dimensional plasmonic nanoarrays in SERS sensors.

Electrofocusing-enhanced localized surface plasmon resonance biosensors.

Localized surface plasmon resonance (LSPR) biosensors typically suffer from diffusion limited mass transport and nonspecific adsorption upon detection of biomolecules in real biofluids. We employ here a peptide-modified plasmonic gold nanohole (AuNH) array for real-time detection of human troponin I (cTnI). Applying a negative electric bias on the AuNH sensor chip enables us to attract and concentrate cTnI at the sensor surface, while repelling other proteins thus decreasing interferences due to nonspecific adsorption.

Nearly Perfect Absorption by Bimetallic Surface Plasmonic Crystal and Its Application as Sensor

The author proposes and numerically investigates nearly perfect absorption in a bimetallic surface plasmonic crystal formed by two physically adjoined gold and silver nanohole array layers. The absorption over 99% at 650-nm wavelength is obtained and its high quality factor is preserved for oblique incidence. The structure is examined as a plasmonic sensor to measure the refractive indexes. The author achieves a very high modified figure-of-merit of 335.

Extraordinary mid-infrared transmission of subwavelength holes in gold films.

Gold (Au) nanoholes are fabricated with electron-beam lithography and used for the investigation of extraordinary transmission in mid-infrared regime. Transmission properties of the nanoholes are studied as the dependence on hole-size. Transmittance spectra are characterized by Fourier transform infrared spectroscopy (FTIR) and enhanced transmittance through the subwavelength holes is observed. The transmission spectra exhibit well-defined maximum and minimum of which the position are determined by the lattice of the hole array. The hole-size primarily influence the transmission intensity and bandwidth of the resonance peak. With an increase of hole-size, while keep lattice constant fixed, the intensity of the resonance peak and the bandwidth increases, which are due to the localized surface plasmons. Numerical simulation for the transmission through the subwavelength holes is performed and the simulated results agree with the experimental observations.

Recycling gold nanohole arrays

The authors report the impact of common cleaning methods on the stability of gold nanohole arrays used as extraordinary optical transmission surface plasmon resonance sensors. Their optical sensitivity, physical structure, and surface contamination levels were measured before and after multiple cycles of monolayer adsorption and removal with various wet chemicals (sulfochromic acid, piranha, or ammonium hydroxide: hydrogen peroxide) and dry oxygen plasma etchants. While these oxidative chemical and plasma etches remove organic monolayers and other contaminants, the oxidation and associated heating also damages the gold nanostructures to varying degrees. The authors observed decreases in the arrays' optical sensitivities via changes in the shapes and positions of their surface plasmon resonance peaks. The optimum recycling process was a room temperature, aqueous ammonium hydroxide: hydrogen peroxide treatment (15 min) commonly referred to as Radio Corporation of America Clean 1, followed by immersion in dilute nitric acid (0.1M, 30 min). This method was effective at removing an alkanethiol self-assembled monolayer of 11-mercaptoundecanoic acid; after six recycles, no loss in optical sensitivity was detected with minimal changes in the gold film thickness (−10%), hole area (−10%), and hole circularity (+6%).

Dielectrophoresis-Enhanced Plasmonic Sensing with Gold Nanohole Arrays

We experimentally demonstrate dielectrophoretic concentration of biological analytes on the surface of a gold nanohole array, which concurrently acts as a nanoplasmonic sensor and gradient force generator. The combination of nanohole-enhanced dielectrophoresis, electroosmosis, and extraordinary optical transmission through the periodic gold nanohole array enables real-time label-free detection of analyte molecules in a 5 μL droplet using concentrations as low as 1 pM within a few minutes, which is more than 1000 times faster than purely diffusion-based binding. The nanohole-based optofluidic platform demonstrated here is straightforward to construct, applicable to both charged and neutral molecules, and performs a novel function that cannot be accomplished using conventional surface plasmon resonance sensors.