Optical metasurfaces are inscribed in surface relief on azobenzene molecular glass thin films following double and triple sequential exposures to laser interference patterns with very close periodicities, resulting in two-fold and three-fold hierarchical Moiré gratings respectively. These metasurfaces formed due to the unique photomechanical effect in azobenzene materials, in which molecules migrate from zones of high to low laser irradiance. The laser interference patterns were obtained using a Lloyd mirror interferometer and a continuous wave laser having a wavelength of 532 nm and an irradiance of 200 mW/cm2. The resulting optical metasurfaces, which resembled surface features of a Peruvian lily flower petal, were characterized using atomic force microscopy, optical microscopy and surface profilometry techniques. It was found that the highly-customizable surface characteristics of the resulting metasurfaces can significantly alter their hydrophobicity.
Chirped-pitch crossed surface relief gratings (CP-CSRGs) were fabricated on photoactive azobenzene thin films using a simple two-step procedure. The resulting gratings had a constant pitch in one direction and a varying (chirped) pitch in the orthogonal direction. They were coated with silver and tested for their ability to excite surface plasmon resonance (SPR). It was observed that incident light can be transmitted or blocked on different locations of the CP-CSRG device only as a function of the light’s wavelength. These SPR-based sensors were used to detect changes in the refractive index of aqueous sucrose solutions and a maximum sensitivity of 778.6 nm/RIU was obtained.
Mobile communications have massively populated the consumer electronics market over the past few years and it is now ubiquitous, providing a timeless opportunity for the development of smartphone-based technologies as point-of-care (POC) diagnosis tools1 . The expectation for a fully integrated smartphone-based sensor that enables applications such as environmental monitoring, explosive detection and biomedical analysis has increased among the scientific community in the past few years2,3. The commercialization forecast for smartphone-based sensing technologies is very promising, but reliable, miniature and cost-effective sensing platforms that can adapt to portable electronics in still under development. In this work, we present an integrated sensing platform based on flow-through metallic nanohole arrays. The nanohole arrays are 260 nm in diameter and 520 nm in pitch, fabricated using Focused Ion Beam (FIB) lithography. A white LED resembling a smartphone flash LED serves as light source to excite surface plasmons and the signal is recorded via a Complementary Metal-Oxide-Semiconductor (CMOS) module. The sensing abilities of the integrated sensing platform is demonstrated for the detection of (i) changes in bulk refractive index (RI), (ii) real-time monitoring of surface modification by receptor-analyte system of streptavidin-biotin.
Metallic nanohole arrays support surface electromagnetic waves that enable enhanced optical transmission and may be
exploited for sensing. Our group has been active in the application of enhanced optical transmission to chemical and
biological sensing, and in the optofluidic integration nanohole arrays. Our recent work in this area is described here. Our
research on the combined photonic and fluidic characteristics of flow-through nanohole arrays and their application to
sensing is presented. Flow-through nanohole arrays provide a biomarker sieving capacity that is unique among
plasmonic sensors as well as rapid transport of reactants to the sensing surface. Our experiments indicate a order of
magnitude improvement in sensor response time for flow-through operation as compared to current flow-over sensing
methods. Transport analysis results indicate that more than a 20-fold improvement may be expected for small
biomolecules with rapid reaction kinetics.
Extraordinary optical transmission through nanohole arrays in metal films shows enhanced performance in surface
plasmon resonance sensing, and efforts to develop this technology have been undertaken by many research groups
worldwide. The challenge is to integrate a nanohole array sensor into a handheld design that is compact, cost effective,
and capable of multiplexing. A number of implementations have been suggested, using components such as lasers and
spectrometers, but these designs are often bulky, expensive and unacceptably noisy. We have developed an approach
that is simple, inexpensive and reliable: an integrated handheld SPR imaging sensing platform using the nanohole array
chip as the sensing element, a two-color LED source for spectral diversity, and a CCD module for multiplexed detection.
A PDMS microfluidic chip made by conventional photolithographic techniques is assembled with the nanohole arrays
and incorporated into the integrated module in order to transport the testing solutions, which offers the flexibility for
future multiplexing. Results of preliminary tests show surface binding detection and have been promising.
Periodic arrays of nanoholes are being developed by several groups for integrated and portable real-time sensing
based on surface plasmon resonance (SPR). Recent advances have allowed for nanohole sensitivity comparable to
ATR SPR. Here, we will present our new advances in developing integrated and multiplexed SPR sensors using
nanohole arrays. For the first time, we will present our dual-wavelength approaches that remove the need for a
spectrometer, thus greatly reducing cost and size. We will also present our recent achievements in (1) in-hole
sensing, demonstrating attomolar detection, and (2) flow-through sensing, where the detection time is greatly
reduced due to the rapid diffusion inside the nanoholes themselves.
Metallic nanohole arrays support surface electromagnetic waves that enable enhanced optical transmission and may be
exploited for sensing. Our group has been active in the application of enhanced optical transmission to chemical and
biological sensing, and in the optofluidic integration nanohole arrays. Recent work in this area is described here. Recent
work using a blocking layer to limit the exposed metal surface to the in-hole region resulted in effective sensing in a
much smaller, nanoconfined volume. This result motivates the use of through nanoholes, (i.e. nanoholes as
nanochannels) to directly address the sensing area. A flow-through nanohole array based sensing format is presented that
leads to enhanced transport of reactants to the active area and a solution sieving action that is unique among surfacebased
sensing methods. The pertinent fluid and solid mechanics aspects of the flow-through nanohole array sensing are
discussed and recent flow-through sensing results are presented. The application of dielectrophoresis to influence
particle transport in flow-through nanohole arrays is also discussed. Specifically, simulations indicate that equivalent
dielectrophoretic forces are compatible with drag forces for flow rates in the range already defined in the context of
biomarker transport and membrane strength considerations. Importantly, these results indicate that dielectrophoretic
trapping is viable in these systems. The confinement of particles in the nanoholes opens opportunities for analyte
concentration and surface enhanced Raman scattering in flow-through nanohole array based fluidic systems.
The transmission of normally incident light through arrays of subwavelength holes (nanoholes) in gold thin films is enhanced at the wavelengths that satisfy the surface plasmon resonance (SPR) condition. Our group has been active on the implementation of schemes for the application of this phenomenon for chemical sensing. For instance, we have shown that the interaction between adsorbates with nanoholes modified the SP resonance conditions, leading to a shift in the wavelength of maximum transmission. The output sensitivity of this substrate was found to be 400 nm RIU-1 (refractive index units), which is comparable to other grating-based surface plasmon resonance devices. The array of nanoholes was also integrated into a microfluidic system and the characteristics of the solution flow and detection systems were evaluated. In this work, we will concentrate on improving the efficiency of the nanohole arrays for applications in chemical in chemical sensing. Attempts to improve the sensitivity of the device will be discussed. In-hole sensing is suggested as an alternative to decrease the number of probe molecules, and enhance sensitivity. A biaxial sensing scheme will also be introduced. The biaxial scheme allows for polarization-modulation detection that can account for background fluctuations. A flow-through approach should lead to an optimized transport situation of the analytes to the immobilized species at the surface, which should significantly improve the time and sensitivity of the analysis. Finally, we will discuss the implementation of multiplexing detection using these arrays. Multiplexing detection in zero-order transmission is simpler to implement than the common multiplexing imaging from angle-resolved SPR.
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