Molecular sensing is performed using either optical, electrochemical, electrical, thermal, magnetic, or piezo-electric sensors. Optical sensors are preferred and represent the most powerful approach because of their speed, safety, sensitivity, and robustness. In addition, they allow in situ sensing and real-time, nondestructive measurements. Further advantages include their suitability for component miniaturization and their remote and multi-analyte sensing capabilities.1
Although optical sensors come in many different forms, evanescent-wave-based sensors—in which surface-plasmon and/or waveguide mode(s) can be exited to detect molecular changes in the evanescent-wave field on the sensor's surface—represent an innovative approach. Very small variations in the molecular layer (see Figure 1) may alter the spatial and spectral characteristics of the incident light (such as its intensity, polarization, phase, or frequency). Although measurable outputs of an optical sensor include fluorescence/phosphorescence, biochemical luminescence, reflectance, or Raman scattering,2 label-free measurements can generally be performed. Likely sensor outputs may be intrinsic—such as total internal reflection (TIR)—or extrinsic, such as scattering and Raman scattering. Most evanescent-wave sensors are characterized by intrinsic output. This is easy to detect and not affected by external factors. However, in some cases, understanding complex molecular events is limited—due to the characteristics of the reflected light—when available data is based solely on angular or spectral TIR measurements. Scattering measurements must be accompanied by reflection measurements to determine the concentration and organization of molecules on a sensor's surface.
Figure 1. Layout and measurable optical output of a nanostructured sensor. TIR: Total internal reflection. tm, tw: Molecular-layer, waveguide thicknesses. L, h, d: Length, average height, average diameter of the corrugations. t = h+tw.
Surface nanostructuring is one way to provide and control the dual (TIR and scattering) sensor output. Figure 1 shows the schematic operation of a nanostructured evanescent-wave sensor. In addition to tunable dual output, nanostructuring provides further advantages over the conventional evanescent-wave-sensor approach, including a more complete understanding of the molecular organization, unprecedented sensitivity for a label-free optical sensor (which will allow analysis of information down to sub-pmol/cm2 levels because of its enhanced electromagnetic field), and the ability to control the molecular-binding characteristics of the surface. Therefore, current needs for better sensors render exploration of novel approaches for fabrication of nanostructures timely and relevant.
Techniques are available for fabrication of micro- and nanostructures on a substrate that are either top-down or bottom-up. Top-down, advanced lithographic approaches provide most control over fabrication of nanostructures of uniform size, shape, and spacing. Nanostructures have been fabricated using electron-beam lithography (EBL),3 x-ray lithography,4 ion-beam lithography,5 phase-shifting photolithography,6nanoimprint lithography,7 and soft-interference lithography.6 Even though EBL is probably the best established, most versatile tool, achieving inexpensive fabrication of nanostructures is currently a major challenge.3
We are using hydrothermal-process-driven fabrication as a straightforward and more economic method for nanostructuring. Surface nanopillars with a controllable height distribution and a given concentration can be fabricated by adjusting process parameters such as temperature and duration. First, a thin film of alumina (Al2O3) is produced by atomic-layer deposition.9Subsequently, a hydrothermal process is applied to the alumina film to form nanopillars. Light is coupled into the nanostructured waveguide through a grating or prism and travels along a 34mm waveguide. Some of the light, tunneled into the molecular layer as radiating component, decreases exponentially and the rest travels along the waveguide and outcouples from the grating or prism.
With this arrangement, we can achieve dual output and tuning of the output ratio while controlling the height of the nanopillars. By simply adjusting water temperature and exposure time, we managed to control8 their average heights to reach between 20 and 75nm with sensor-output ratios (scattering/TIR) between 0.09 and 1.0. Figure 2 shows nanopillars formed on an alumina waveguide that was 307nm thick after the atomic-layer deposition. Using this innovative fabrication process, we are working on developing novel sensors for biological and chemical applications ranging from drug discovery to gas detection.
Typical nanopillars formed on the alumina waveguide (scanning-electron-microscopy image of the cross section).8
Support for this four-year study was provided by the European Union's Seventh Framework Programme's Marie Curie action (FP7-PEOPLE-RG, grant PIRG07-GA-2010-268144).
Mustafa M. Aslan
Materials Institute TÜBITAK Marmara Research Center
Mustafa Aslan received his PhD from the Engineering Science Department of Pennsylvania State University in July 2000. He has since gained more research experience in the fields of experimental optics and photonics at the University of Kentucky, followed by an appointment at the Photonics Laboratory at the University of Louisville. He is currently a senior research scientist.
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