In this paper, we report on experimental and theoretical studies that investigate how the structural properties of plasmonic nanodome array devices determine their optical properties and sensing performance. We examined the effect of the interdome gap spacing within the plasmonic array structures on the performance for detection of change in local refractive index environment for label-free capture affinity biosensing applications. Optical sensing properties were characterized for nanodome array devices with interdome spacings of 14 nm, 40 nm, and 79 nm, as well as for a device where adjacent domes are in contact. For each interdome spacing, the extinction spectrum was measured using a broadband reflection instrumentation, and finite-difference-time-domain (FDTD) simulation was used to model the local electric field distribution associated with the resonances. Based on these studies, we predict that nanodome array devices with gap between 14 nm to 20 nm provide optimal label-free capture affinity biosensing performances, where the dipole resonance mode exhibits the highest overall surface sensitivity, as well as the lowest limit of detection.
© 2013 Optical Society of America
Sensors based on plasmonic nanostructures are being extensively studied due to their unique optical properties that are capable of supporting localized surface plasmon resonance (LSPR) [1, 2]. LSPR is a phenomenon associated with free electrons of a material collectively oscillating in resonance, driven by an incident electromagnetic field or light. The effect leads to a significant enhancement of the electromagnetic field with a high degree of spatial confinement around the nanostructure, in dimensions well below the optical diffraction limit . Recent advancements in nanofabrication technology, together with the availability of computer simulation tools that enable detailed investigation of the interactions of metal/dielectric nanostructures with electromagnetic fields, has enabled development of a wide variety of nanoparticle shapes and structures (circular, trianglar, and rod-shaped metal nanoparticle or hole arrays) for use as highly sensitive LSPR biosensors for detecting a broad range of biological analytes that include proteins, DNA/RNA, viruses, and bacteria [4–9]. Despite the considerable potential of LSPR for sensitive biomolecular detection, its wide-spread practical utility has not been realized, primarily due to fabrication issues. The majority of nanoparticle structures for the plasmonic devices have been produced via electron-beam lithography or focused ion beam milling [10, 11]. However, the time (and associated cost) of fabricating such structures over surface areas greater than a few square millimeters generally precludes the use of such devices for commercial applications. Practical biosensor applications demand an inexpensive fabrication method viable for large surface areas, and this requirement is driving intense research efforts on the development of suitable processing methods.
Nanoreplica molding has been demonstrated as a low-cost method for manufacturing a variety of devices comprised of nanostructured surfaces. This method employs a low-force imprinting at room temperature to readily form nanometer-scale structures with high uniformity over large surface areas using a patterned silicon wafer as a reusable molding template. In addition, subsequent resist processing, use of chemical solvents, glancing angle deposition, etching, or lift-off, are not required for this method, thereby further reducing the associated fabrication cost. Using this technique, a variety of nanostructured devices have been manufactured, including photonic crystals for label-free biosensing and enhanced fluorescence, distributed-feedback laser biosensors, as well as tunable optical filters [12–15]. Recently, a plasmonic nanodome array fabricated by nanoreplica molding process has been demonstrated as a surface-enhanced Raman scattering surface [16–18]. In those studies, it was observed that the SERS enhancement factor was highly sensitive to interdome spacings, with a smaller spacing producing a significantly higher enhancement factor concentrated near the surface.
Plasmonic devices that are fabricated using a low-cost, large-area method, hold great potential as a highly sensitive, commercially-viable detection platform for a wide range of biomolecular measurements. In order to design and optimize plasmonic devices tailored for specific biosensing measurements, one needs to understand how the material and structural properties of the device affect its performance. In this paper, we report on systematic experimental and theoretical studies focused at investigating the effect of interdome spacing on the optical properties of plasmonic nanodome array devices for label-free capture affinity biosensing applications. We have characterized the optical sensing properties for nanodome array devices fabricated by nanoreplica molding process that have interdome spacings (gap between edges of adjacent nanodomes) of 14 nm, 40 nm, and 79 nm, as well as a device where adjacent domes are in contact. For each interdome spacing, the extinction spectrum was measured using a broadband reflection instrumentation, and finite-difference-time-domain (FDTD) simulation was used to model the local electric field enhancement associated with the resonances. Key aspects of the plasmonic nanodome arrays for label-free biosensing applications were investigated by characterizing their sensitivities to bulk refractive index changes and surface deposition of charged polyelectrolyte layers. Then, we examined the “surface” sensitivity (sensitivity to changes of refractive index for the detection region within 30 nm near the sensor surface) as a metric to predict the optimal interdome spacing for capture affinity-based biosensing measurements.
2. Materials and methods
Certain commercial equipment, instruments, or materials are identified in this document. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor intended to imply that the products identified are necessarily the best available for the purpose
2.1 Plasmonic nanodome array fabrication
Nanoimprint lithography (Molecular Imprints) was used to pattern an 8 inch (200 mm) diameter silicon wafer with a two-dimensional array of 300 nm diameter wells (period = 400 nm, depth = 130 nm), in (8 × 8) mm2 dies to produce a mold template with overall feature dimensions of (120 × 120) mm2. The completed silicon mold template was subsequently silanated by immersion in dimethyl dichlorosilane (GE Healthcare) solution for 5 min followed by ethanol and DI water rinses to promote clean release of the replica. A liquid UV-curable, acrylate-modified silicone polymer (Gelest Inc.) was distributed between the silicon template wafer and a 250 µm thick polyethylene terephthalate (PET) sheet using a Teflon roller. Then the liquid polymer, which conformed to the shape of the features on the wafer, was cured to a solid state by scanning the entire surface with UV light for 90 s using a linear motion stage to translate the wafer. A high-intensity, pulsed UV curing system (Xenon Inc.) with spectral cutoff at 240 nm, peak power density of 405 W cm−2, pulse repetition rate of 120 Hz, and pulse width of 25 μs was used. Varying thicknesses of SiO2 (0 nm to 120 nm) was then deposited over the polymer replica, followed by 20 nm of Ti and 200 nm of Ag, each deposited by electron-beam evaporation (Infinity 22, Denton), to complete the device. A schematic representation of the plasmonic nanodome array structure used in this study is shown in Fig. 1(a). Note that the 20 nm of Ti, used as an adhesion layer for the Ag deposition, is not included in the schematic. Figure 1(b) shows the scanning electron microscope (SEM) image of the fabricated nanodome array with an interdome spacing of 14 nm in a tilted view, and an atomic force microscope (AFM) image across (1.5 × 1.5) µm2 area on the surface is shown in Fig. 1(c). The images of the nanodome surfaces were obtained with a scanning electron microscope (Ultra-60 FESEM, Zeiss) at an acceleration voltage of 15 kV, and with an atomic force microscope (Dimension 3100, Digital Instruments) operated in tapping mode.
The extinction spectra of the nanodome sensors for all experiments were obtained using an optical hardware setup consisting of a tungsten-halogen light source (Ocean Optics) and a spectrometer (HR4000, Ocean Optics) coupled to each arm of a bifurcated optical fiber (400 µm diameter, Ocean Optics). A collimation lens (NA = 0.22) is attached at the common end of the fiber, which was placed above the sensor in a vertical configuration for illumination/collection normal to the surface of the nanodome array. The nanodome sensors were mounted onto a manual x-y translation stage (Newport). From the extinction spectra, the peak wavelength value (PWV) for each resonance mode was determined by polynomial fitting.
The electromagnetic field distributions in the vicinity of the nanodome array were modeled using a finite-difference time domain (FDTD) simulation package (FDTD Solutions, Lumerical). The schematic shown in Fig. 1(a) represents the nanodome array structure used in the model. It was not necessary to include the Ti layer in the simulation model because the thickness of the Ag layer deposited (200 nm) was large compared to its skin depth. In contrast to the extinction spectral measurement where unpolarized light source was used, the nanodome structure in the model was illuminated with a plane wave propagating in the −z direction (towards the nanodome surface), with the electric field polarized along the x-axis, in order to obtain the electric field intensity profile associated with each of the resonance modes. The simulation region was set to one unit volume of the periodic nanodome structure along with periodic boundary conditions imposed on the sidewalls of the simulation boundary. Perfectly matched layers were imposed at boundaries along the z direction and monitors were placed to calculate the reflected, transmitted, and absorbed power as a function of wavelength. The optical properties of Ag and SiO2 were taken from Palik et al .
For the stacked polyelectrolyte layers experiment in this work, chamber gaskets (Life Technologies) were attached to the nanodome substrate to create wells so that analyte reagents could be pipetted onto the sensor surface. Poly(ethylenimine) (PEI, Mw = 60 kDa, Sigma-Aldrich), poly(sodium 4-styrenesulfonate) (PSS, Mw = 60 kDa, Sigma-Aldrich), and poly(allylamine hydrochloride) (PAH, Mw = 70 kDa, Sigma-Aldrich) were dissolved in a 0.9 mol L−1 NaCl solution to a concentration of 5 mg mL−1.
3. Results and discussion
3.1 Nanodome extinction spectra and electric field intensity profile
Figure 2 shows the resonance extinction spectrum and the associated 2-D spatial electric field intensity profiles for plasmonic nanodome array devices with interdome spacings of 14 nm, 40 nm, 79 nm, and also for a device with adjacent dome structures in contact. For each sub-figure at a particular interdome spacing, the plot on the left side includes the resonance extinction spectrum obtained using a broad-band reflection measurement system (solid blue curve), as well as the extinction spectrum obtained from the FDTD simulation (dashed red curve). To the right of the extinction spectrum are the associated top and cross-sectional 2-D views of the spatial electric field intensity profiles obtained from the FDTD modeling for one unit volume of the nanodome array (each side of the plot corresponds to the period of the array, or a length of 400 nm) at each peak resonance wavelength. The scale bar for each electric field intensity profile represents the resonant electric field intensity (E2) enhancement levels, normalized with respect to the incident electric field intensity (Einc2) on a logarithmic scale. In each FDTD model, the incident optical field was set as a plane wave propagating along the –z direction towards the nanodome surface at normal incidence with the polarization (direction of the electric field) set along the x direction.
Figure 2(a) shows the extinction spectrum and the electric field intensity profile for the nanodome array device with an interdome gap of 79 nm. A peak can be observed at 400 nm, which is the grating diffraction mode from the geometric resonance that arises due to the coherent interaction of multiple scattering by periodically-spaced nanodome structures [20, 21]. A broad dipole feature with low magnitude was observed around λ = 700 nm, however the near-field coupling was not strong enough to produce a distinct resonance peak. For the nanodome array structure with an interdome spacing of 40 nm which is shown in Fig. 2(b), in addition to the grating diffraction mode centered at 404 nm, the near-field coupling between adjacent dome structures generated a clear dipole LSPR mode at λ = 723 nm, and the development of a higher order multipole LSPR mode centered at λ = 433 nm was also observed; these features were observed experimentally, as well as in the FDTD modeling. The result for nanodome array device with an interdome spacing of 14 nm shown in Fig. 2(c) indicates that a smaller gap leads to stronger near-field coupling, generating higher levels of resonance extinction peaks, as well as electric field intensity enhancement. As a result, all three resonance modes (grating diffraction, multipole, and dipole) were clearly observed at wavelengths of 400 nm, 450 nm and 710 nm, respectively. For devices in which nanodomes come into contact, the dipole and multipole modes formed in the gap from interdome coupling disappeared, and instead, a cavity mode was generated at the sharp folds that are created between adjacent domes. Figure 2(d) shows the extinction spectrum and 2-D spatial electric field intensity profile for the nanodome array with a 50 nm overlap along the axis through the center of the domes in contact. The cross sectional profile was plotted along the y-z plane to show the electric field enhancement along the groove formed between adjacent domes. A single LSPR resonance mode centered at λ = 533 nm was generated along with the resonance mode from grating diffraction at 400 nm.
For all of the extinction spectral data shown in Fig. 2, the location of the resonances obtained from experimental measurements match well with the results from the FDTD simulation. The discrepancies in the extinction magnitudes between simulation and experiment can be attributed to the structural aspects not taken into account in the calculation such as surface roughness, structural variations from fabrication, etc. Comparing the characteristics of the electric field intensity profile for each mode, the electric field for the grating diffraction mode extends further away from the surface of the nanodomes, whereas multipole and dipole resonances are localized LSPR modes, confined closer to the surface. For devices with nanodome structures touching, the electric field enhancement achieved in sharp creases is also a localized resonance mode, where the electric fields are confined close to the surface.
3.2 Bulk refractive index sensitivity characterization
The sensitivity of the nanodome array devices to changes in the bulk refractive index (n) was measured by sequentially exposing the sensor surface of different interdome gaps to air (n = 1), deionized (DI) water (n = 1.333), acetone (n = 1.359) and isopropyl alcohol (n = 1.377). The bulk refractive index sensitivity of the sensor structure was calculated by performing a linear curve fit to the data on the peak wavelength value (PWV) shift as a function of refractive indices of the fluid media contacting the nanodome sensor surfaces. Figure 3 shows the experimentally measured bulk refractive index sensitivity (Sb = ΔPWV/Δn) as a function of gap spacing between adjacent nanodome structures for each of its associated resonance modes. In this plot, positive spacing values correspond to the interdome gap for devices with nanodome structures that are separated, whereas negative values represent the amount of overlap along the axis through the centers of adjacent domes in contact. The grating diffraction, multipole, dipole, and cavity modes are represented by red circles, green triangles, blue diamonds, and violet squares, respectively, with the error bars (not visible on all data points due to small magnitudes of the deviations) representing ± 1 standard deviation for five different sensing regions within the nanodome sensor area. Also plotted in Fig. 3 are the bulk refractive index sensitivity results obtained from the FDTD simulation where grating diffraction, multipole, dipole, and cavity modes are represented by red hollow circles, green hollow triangles, blue hollow diamonds, and violet hollow squares, respectively, with each set of data points connected by corresponding lines for visual guidance.
In the FDTD modeling, no distinct dipole mode was observed for devices with interdome spacing larger than 60 nm. The cut-off condition for the multipole resonance mode was tighter, with no distinct multipole mode being observed for devices with interdome spacing larger than 40 nm due to the higher order mode being more sensitive to loss from surface roughness and structural deviation. This result was consistent with the previous observation, where stronger interdome near-field coupling achieved with smaller interdome spacing, led to larger and more distinct resonance peaks in the extinction spectra. As with the extinction spectrum measurements, the bulk refractive index sensitivity values obtained from experiments matched well with the results from the FDTD modeling. The slight discrepancy for the grating diffraction mode can be attributed to the resonance peak broadening due to feature size variations and surface roughness associated with the actual structures, compared to the simulation where the structures within the nanodome array were modeled as being ideal (consistent dimensions and perfect smoothness). Since the grating diffraction mode is a geometric resonance that is generated from the interaction of multiple light scattering events by the periodic array structure, its resonance peak in the model, especially in solution media, became extremely narrow. As a result, the sensor response in solution media for the grating diffraction mode obtained from the simulation was higher than the experimental measurements, leading to consistently larger simulated bulk refractive index sensitivity values.
The highest bulk refractive index sensitivity was obtained from the dipole modes, followed by the cavity modes present in devices with nanodome structures in contact. The multipole modes exhibited bulk refractive index sensitivity values similar to those from the grating diffractions modes. The bulk sensitivity values for grating diffraction modes remained relatively constant particularly for devices with separation between the domes, since the resonance condition arises from periodic nanodome structures, rather than near-field interdome coupling effects. For both the dipole and multipole LSPR modes, their bulk refractive index sensitivities rapidly increased as the gap between the domes decreased until the point of contact (smaller interdome spacing leading to stronger interdome near-field coupling).
3.3 Surface sensitivity characterization
The increase of bulk refractive index sensitivity associated with stronger inter-particle coupling for the LSPR particle array system with smaller spacing have been consistently observed in literature and suggested as an optimized configuration for optical sensing of analyte adsorption [22, 23]. For bioassay experiments using capture affinity-based sensors however, most of the biomolecular binding interactions occur at or near the surface of the sensor, so the surface sensitivity, or the response of a sensor to changes in refractive index or to a molecular binding event at or near the surface, provides a more accurate metric of the potential performance for biosensing than the bulk sensitivity. The response of the nanodome sensors were therefore also measured as a function of distance from the surface using stacked, alternating layers of charged polyelectrolyte deposited via a method described previously . The polyelectrolytes used in this experiment were cationic poly(ethylenimine) (PEI), anionic poly(sodium 4-styrenesulfonate) (PSS), and cationic poly(allylamine hydrochloride) (PAH) dissolved in NaCl solution. The polyelectrolyte layer coatings self-limit to a single monolayer with thickness of ≈5 nm . To build up the polymer stack, NaCl buffer was pipetted onto the sensor surface to establish a baseline, and it was subsequently replaced by PEI solution. After a 10 min incubation, the PEI solution was removed and the sensor surface was washed with NaCl buffer. Then, seven alternating layers of PSS-PAH were deposited in sequence, with a NaCl buffer rinse after every PSS or PAH incubation. Figure 4 shows the results of the surface polyelectrolyte stack measurements, displaying the end-point peak wavelength value (PWV) shift obtained from the 14 nm gap nanodome array device as a function of deposited polyelectrolyte thickness for PEI and seven PSS-PAH depositions. The nanodome array device with 14 nm interdome spacing was chosen because strong interdome near-field coupling provided three optical resonance peaks with high intensities and signal-to-noise ratios. The measured sensor response values for the grating diffraction, multipole, and dipole modes are represented by red circles, green triangles, and blue diamonds, respectively. The error bars, representing ± 1 standard deviation for five sensor response values used to calculate the end-point PWV shift, are not visible on all data points due to small magnitudes of the deviations. Also plotted as open symbols in Fig. 4 are the sensor response obtained from the FDTD modeling.
For the surface polyelectrolyte measurement, the dipole mode exhibited the greatest sensitivity, followed by the multipole and grating diffraction modes. In both the experimental measurement and simulation modeling, the response of the dipole resonance mode started to saturate at ≈60 nm of total polyelectrolyte layer stack thickness. The saturation of the signal for the dipole mode is consistent with the cross-sectional spatial electric field intensity plot in Fig. 2(c), where the electric field is confined to within ≈60 nm from the nanodome surface. On the other hand, the signal from the grating diffraction mode provided the largest detection range in terms of the distance from the surface, where saturation has not been reached after 70 nm of total polyelectrolyte thickness, again consistent with the cross-section electric field intensity profile plotted in Fig. 2(c). As with the previous measurements, the results obtained from experiments and theoretical modeling matched well, particularly for the dipole and multipole modes, further validating the FDTD model for predicting the nanodome array sensor response to surface binding events or changes in refractive index near the surface.
In order to investigate the effect of interdome spacing on the surface sensitivity of nanodome array devices, a series of FDTD simulations were performed to obtain the resonance PWV shift response from the polyelectrolyte layers on the surface of the nanodomes as a function of nanodome gaps. Larger PWV shifts in response to deposition of polyelectrolyte layers within the detection region would correspond to a higher surface sensitivity, and therefore offer characteristics for a better capture affinity biosensor. Depending on the specific sensing applications, the size of the biomolecular detection zone for surface interaction assays varies from ≈5 nm to 25 nm for antigen-antibody interaction assays or molecular binding on phospholipid membranes [26, 27]. Here, we considered surface-based biomolecular sensing for which we defined the detection zone as the volume lying within 30 nm from all exposed surfaces of the device. Figure 5 shows the surface sensitivity values (end-point PWV shift response from 30 nm of total polyelectrolyte thickness) obtained from the FDTD modeling for different optical resonance modes, as a function of interdome separation distance. Comparison of the surface sensitivity values among the optical resonance modes shows similar behavior to that observed for the bulk sensitivity measurements, with the dipole modes exhibiting the highest sensitivity, followed by the cavity modes, and grating diffraction modes exhibiting similar sensitivity levels as the multipole modes. However, an important difference resulting from this surface sensitivity analysis is that the smallest interdome gap did not lead to optimal sensor response. As indicated in Fig. 5, the surface PWV shift response of the dipole mode actually decreased when the interdome spacing of the nanodome array device was reduced from 14 nm to 10 nm. Based on these results, there exists a trade-off relationship for the plasmonic nanodome array devices between electric field enhancement vs. space available for analyte biomolecules to bind. As mentioned throughout this paper, a smaller interdome spacing leads to stronger near-field coupling, with higher electric field enhancement and sensitivity to changes near the surface of the nanodomes. However, a reduction of the gap spacing between the nanodomes beyond a certain point, despite the higher electric field enhancement achieved, leads to a reduction in the sensor response because less space is available for analyte molecules to bind within the enhanced electric field or “hot spot” region. Similar results were observed with near-field coupled Au dimers, where an exponential increase in the sensor response for bovine serum albumin (BSA) protein binding was obtained with decreasing dimer separation until ≈30 nm, below which the response dropped sharply . For plasmonic nanodome array structures with a period of 400 nm, devices with interdome gaps in the range between 14 nm to 20 nm provided optical properties that are optimal for label-free capture affinity-based biosensor applications. In comparing the performance among different resonance modes, the noise or standard deviation associated with the signal for each mode needs to be considered in addition to its sensitivity. In order to obtain the standard deviation values for the sensor signal, 60 consecutive resonance spectral data were collected for 1 min (at a rate of 1 Hz) from the 14 nm gap nanodome array device that was exposed to air. The standard deviation values measured for the grating diffraction, multipole, and dipole modes were 0.0177 nm, 0.0361 nm and 0.0154 nm, respectively. Despite the wider resonance peak width associated with the dipole mode, the signal standard deviation was comparable to the values for grating diffraction and multipole modes.
In this work, the effect of near-field interdome coupling strength on the optical properties and sensing performance of plasmonic nanodome arrays were investigated by observing the resonance spectra, bulk refractive index sensitivity and surface sensitivity values of devices with various interdome spacings. From the FDTD modeling and experimental measurements, smaller interdome spacing resulted in higher electromagnetic field enhancement and bulk refractive index sensitivity. However, interdome spacing smaller than 14 nm was not optimal for capture-affinity biosensing applications, as a reduction in the sensor dipole mode response to 30 nm of surface analyte thickness was observed. Although stronger near-field interdome coupling from smaller interdome spacing increases the electromagnetic field intensity of the resonance modes, there is less space for analyte molecules to bind, leading to lower surface sensitivity. Based on the presented experimental and modeling results, we predict that the dipole resonance mode of the nanodome array devices with gaps between 14 nm to 20 nm provide optimal surface sensitivity and limit of detection for label-free capture affinity biosensing applications. At this spacing, three optical resonance modes with high intensities and signal-to-noise ratios were supported, enabling the nanodome array surface to be applicable to a wide range of other capture affinity biosensing applications where either one mode, or combinations of multiple resonant modes could be used for different target analytes to attain optimal sensing performance for each bioassay case. Nanodome array structures were used as a model system in this work, but we expect the results to be applicable to any type of plasmonic nanoparticle array system (metal nanoparticles arranged on a planar substrate) operated in the near-field coupling regime.
The authors thank Kurt Benkstein, Rebecca Zangmeister, Chris Michaels, Pawel Jaruga, and Dean Ripple for valuable comments and assistance relevant to this work. C. J. C. is supported by a National Institute of Standards and Technology (NIST) Postdoctoral Research Associateship Award administered through the National Research Council.
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