(308e) 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).