Fatemeh Eftekhari

Nanoholes as nanochannels: flow-through plasmonic sensing.

We combine nanofluidics and nanoplasmonics for surface-plasmon resonance (SPR) sensing using flow-through nanohole arrays. The role of surface plasmons on resonant transmission motivates the application of nanohole arrays as surface-based biosensors. Research to date, however, has focused on dead-ended holes, and therefore failed to harness the benefits of nanoconfined transport combined with SPR sensing. The flow-through format enables rapid transport of reactants to the active surface inside the nanoholes, with potential for significantly improved time of analysis and biomarker yield through nanohole sieving. We apply the flow-through method to monitor the formation of a monolayer and the immobilization of an ovarian cancer biomarker specific antibody on the sensing surface in real-time. The flow-through method resulted in a 6-fold improvement in response time as compared to the established flow-over method.

Polarization-dependent sensing of a self-assembled monolayer using biaxial nanohole arrays

We demonstrate surface plasmon resonance (SPR) sensing based on the polarization-dependent extraordinary optical transmission through a biaxial nanohole array. The biaxial array has two periodicities in a single array that can be individually probed by varying polarization. Here, the SPR polarization-spectral dependence is demonstrated for the detection of a self-assembled monolayer for four sets of biaxial array periodicities. By monitoring the polarization dependence of transmission through the nanohole arrays with biaxial periodicity, surface-sensitive refractive index induced intensity variations may be separated from other effects such as absorption, scattering, and intensity fluctuations, while using a single wavelength source. Biaxial sensing is useful for ongoing microfluidic integration of nanohole SPR, where the light source is transmitted through a microfluidic channel.

Enhanced Second Harmonic Generation From Noncentrosymmetric Nanohole Arrays in a Gold Film

A subwavelength aperture array in a gold film is fabricated for the purpose of second harmonic generation (SHG). The sunglass aperture shape is chosen to have an apex to increase the local field; however, unlike past research on double-hole structures, the inversion symmetry of the structure is intentionally broken to enhance the SHG. A systematic study of the influence of array periodicity and aperture shape is provided. The conditions for maximum SHG are discussed in terms of local field enhancement, excitation of the localized surface plasmon resonances, and the influence of array periodicity. Fully vectorial finite-difference electromagnetic calculations provide the effective index of the aperture modes, as well as the surface electric field to quantify the discussion. Both the sunglass and double-hole structures are promising for future implementation with less-expensive optical lithography since it is mainly composed of two holes; yet they produce sharp apexes for enhanced local field and thereby enhanced nonlinear light-matter interaction.

Development of plasmonic substrates for biosensing

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.

Development of portable SPR sensor devices based on integrated periodic arrays of nanoholes

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.

Microfluidic and nanofluidic integration of plasmonic substrates for biosensing

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.

Biaxial nanohole array sensing and optofluidic integration

Polarization dependent SPR-based sensing is demonstrated using a biaxial nanohole array. This enables increased spectral diversity and sensitivity with single line excitation. Towards further optofluidic integration, a flow- through array detection scheme is evaluated computationally.

Real-time monitoring of self-assembled monolayer using biaxial nanohole arrays

We demonstrate surface plasmon resonance (SPR) sensing based on the polarization dependence of extraordinary optical transmission through a biaxial nanohole array. This structure provides the benefit of having two nearly-degenerate periodicities in a single array that can be individually probed by varying polarization. This enables spectral diversity, even when using a single wavelength laser source; for a suitable wavelength, one polarization has increased transmission and the orthogonal polarization has reduced transmission. By monitoring both polarizations, surface-sensitive refractive index induced intensity variations can be separated from other effects, such as absorption, scattering and detected intensity fluctuations. This is particular useful to the on-going microfluidic integration of nanohole SPR, where the light source must be transmitted through the microfluidic channel and it is cost-effective to have an efficient semiconductor laser source and intensity detector.

Apex-enhanced second harmonic generation from asymmetric nanoscale arrays in a gold film

The second harmonic generation from a non-centrosymmetric nanostructure in gold film is studied comprehensively. The sunglass aperture shape is chosen to have an apex to increase the local field.

NANOHOLE ARRAYS AS OPTICAL AND FLUIDIC ELEMENTS FOR SENSING

Arrays of nanoholes in metal films present several opportunities as surface based sensors in lab-on-chip systems. Metallic nanohole arrays support surface electromagnetic waves that enable enhanced transmission through the holes and have been harnessed for chemical and biological sensing. Nanohole array based sensing performed to date has involved nanoholes that end shortly beyond the metallic film layer on a substrate such as glass. Such dead-ended holes fail to harness the potential of through-hole nanohole arrays including enhanced transport of reactants to the active area and a solution sieving action that is unique among surface-based sensing methods. In this work we investigate the potential of a flow-through-array sensing format.Copyright