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.
Surface plasmon resonance sensors are a popular technology for the optical detection of surface adsorption, for
applications ranging from drug-development to pathogen detection. Here, we will discuss the integration of nano-hole
arrays to provide high-sensitivity detection, with a lower detection limit, speed and cost. Calculations will be presented
that suggest that in-hole detection is more sensitive than detecting binding from the surface around the nano-holes. In-hole
detection also has the benefit of increased speed due to rapid diffusive transport, yet it provides the challenge of
microfluidic/nanofluidic integration. We will outline our recent efforts to produce nano-hole arrays with through-hole
detection.
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 (SP) resonance condition. The enhanced transmission is accompanied by strong field localization and has potential for applications in several fields, ranging from quantum information processing to nanolithography. In this work, arrays of nanoholes were used as chemical sensors to monitor the binding of organic and biological molecules to metallic surfaces. In a first approach, the interaction between the adsorbate with the metallic nanostructure modified the SP resonance conditions, leading to a shift in the wavelength of maximum transmission. The 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. The second approach to sensor development using this class of substrate involved the observation of enhanced spectroscopic signal from species located within the SP field. Surface-enhanced Raman scattering and surface- enhanced fluorescence spectroscopy were observed from adsorbed dyes. The enhanced spectroscopic signal was dependent on the fabrication parameters of the array. The largest enhancement was observed when the periodicity of the nanoholes matched the energy of the laser excitation. Among the main advantages of this substrate for chemical sensing is the collinear optical geometry. This simplifies the alignment with respect to the traditional reflection arrangement used in SPR sensing.
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