Surface Patterning for Selective Bioconjugation of Oxide-Based Biosensors

            The fabrication of on-chip, oxide-based biosensors that exhibit both high sensitivity and specificity is of significant interest to fields like security, food safety evaluation, and environmental monitoring, where rapid and real-time detection of pathogens is necessary.  Surface functionalization strategies, which impart specificity towards a biochemical species of interest through the tethering of a recognition element to the sensor surface, play an important role in the development and final utility of these devices. Typical surface functionalization chemistries, such as the coupling of recognition elements to silicon / silica sensor surfaces via silane coupling agents, often result in the functionalization of the entire platform surface, rather than the selective functionalization of the device.  To overcome this, passivation strategies consisting of combinations of grafting reactions and self-assembly techniques are often used to selectively “pattern” the final device surface.  However, this process can be cumbersome, and detrimental to device performance, especially for on-chip optical biosensors, such as the silica-on-silicon microtoroidal whispering gallery mode resonators (Fig. 1), whose performance can be negatively impacted by wet chemistry techniques.  Here, we present an alternative process for the selective functionalization of silica biosensors via a simple dry etching technique during device fabrication.  This dry etching technique uses XeF₂ vapor, instead of the more typical KOH wet etch, to remove silicon, resulting in silica microtoroids supported by silicon pillars on a silicon substrate.  Using this method, we can selectively tailor the silica surface with recognition elements, such as biotin, which can be used to detect (strept)avidin, without also functionalizing the surrounding silicon platform.   

            To demonstrate the selectivity of this process, we created a variety of samples using the typical fabrication and surface functionalization techniques that have been used to tailor the surface of ultra-high sensitivity microtoroidal whispering gallery mode resonators[1].  The fabrication of silica microtoroids from a 2 micron thermal oxide on (100) silicon wafers consists of three simple steps: (1) patterning circular pads of silica using typical photolithography, (2) undercutting the silica pads using silicon etching techniques to form silica disks supported by silicon pillars, and (3) reflowing the silica disks to form silica microtoroids.  From there, the biosensors are functionalized through a three step functionalization process, that in this example tethers a biotin recognition element to the surface: (1) O₂ plasma treatment, (2) chemical vapor deposition of organosilane coupling agents, and (3) attachment of the biotin recognition element to the coupling agent via grafting.  Here, we examined the impacts of dry vs. wet etching (to remove silicon from the surface) on the resulting ability to selectively functionalize the surface of the silica vs. the surface of the silicon.  After the silicon etch, Atomic Force Microscopy (AFM) measurements were taken to determine surface roughness.  X-ray Photoelectron Spectroscopy (XPS) was taken at various points throughout the fabrication and functionalization process to quantify the surface chemistry.  Lastly, Fluorescence Microscopy (FM) measurements of the functionalized surfaces were performed to determine the relative coverage of the surface functionalization on silica vs. silicon resulting from the different etch types.

As expected, the samples which were etched using KOH did not allow for selective functionalization of the silica surface on a silicon/silica wafer.  Additionally, the XPS results showed that the oxygen plasma treatment was unable to remove residual potassium residue on the surface.  In contrast, the samples etched using XeF₂ were pristine after the oxygen plasma, with no elements from the etching step detected on the XPS, even at trace levels.  Subsequent fluorescent microscopy studies further verified that it is possible to selectively functionalize only the silica surface without additional chemical “patterning”.

            The ability to leverage the inherent chemical properties of the substrate to encourage binding only on the sensor surface will improve the sensor's overall performance and collection efficiency.  This opens the door for the creation of more complex on-chip structures in not only for biosensors, but also for many other multi-material devices.  

Figure 1.  (a) Silica microtoroidal optical resonators on silicon pillars; (b) Biotin-functionalized silica microtoroids labeled with Texas Red-Avidin fluorescent dye.[1]

1.         H. K. Hunt, C. Soteropulos, and A. M. Armani, "Bioconjugation Strategies for Microtoroidal Optical Resonators," Sensors. 10, 9317-9336 (2010).