(627a) Controlling the Sensitivity of Integrated Silica Based Biosensors Through Surface Chemistry

Significant advances have been made in sensor device engineering and optimization, resulting in numerous high sensitivity transducers.  While the limit of detection which is the most frequently reported metric is important, there are numerous other performance characteristics of equal importance, including the device working range.   However, complementary research into methods to effectively control or tune the working range has not kept pace.

Recently, a method for covalently attaching targeting molecules to optical resonant microcavities was developed¹.  Using this approach, it is possible to both maintain the sensing performance of the microcavity and form dense, uniform layers of binding sites.  However, a dense layer is not always desired, as the binding sites can interfere with each other.  Additionally, the number of binding sites on the surface plays a role in determining the overall sensing threshold and detection response time.  Therefore, by developing methods to tune the density of the binding sites, it will be possible to change the device performance.

In the present work, the number of active sites on the surface of integrated silica microcavity sensors is controlled by introducing a secondary “blocker” molecule, which is attached in parallel with the binding site.  By changing the relative concentrations of the binding site and the null site, the overall density of the active sites on the surface can be controlled without introducing vacancies in the functionalization.  In this proof of concept demonstration, two different attachment strategies are explored: one is based on the conventional NHS-ester method¹ and the second is based on the click chemistry (azide-alkyne) approach.  Previous work combining PEG layers and microcavity sensors has demonstrated that PEG is an effective method for blocking non-specific binding².  By using two different attachment methods, it is possible to attach a wide range of probe molecules (eg antibodies, biotin) using commercially available material, enabling the rapid transition of the developed protocols.

Experiments are performed on both control substrates and integrated optical sensors.  The silica sensors are lithographically fabricated in large arrays on silicon substrates.  To verify and optimize the changes in binding site density, both the probe molecules and the PEG chain are fluorescently labeled. The resulting surfaces are characterized using fluorescence microscopy, and the differential fluorescence intensity is measured.  This change indicates the proportion of PEG:binding site.  Ongoing work is exploring the sensor performance (detection limit, response and equilibrium time, working range) as a function of the binding site density using both strategies.

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

2.           C. Soteropulos, K. M. Zurick, M. T. Bernards, H. K. Hunt, “Tailoring the protein adsorption properties of whispering gallery mode optical biosensors,” Langmuir, 28, 15743-15750 (2012).