Integrating Colloidal Quantum Dots with Porous Silicon for High Sensitivity Biosensing
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Integrating Colloidal Quantum Dots with Porous Silicon for High Sensitivity Biosensing Girija Gaur1, Dmitry Koktysh2 and Sharon M. Weiss1 Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, U.S.A. 2 Department of Chemistry, Vanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University, Nashville, TN 37235, USA
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ABSTRACT We aim to utilize the high surface area of a porous silicon (PSi) matrix coupled with semiconductor quantum dot (QD) amplifiers for ultrasensitive optical detection of small biomolecules using a dual-mode detection scheme. In our system, QDs attached to the target biomolecule serve as signal amplifiers by providing an additional refractive index increase beyond that of the smaller target molecules. The strong photoluminescence (PL) from the QDs serves as a secondary indication of target molecule attachment in the pores. A resulting increase in optical thickness of ~190 nm and detection sensitivity of ~700 nm/RIU have been demonstrated for attachment of glutathione capped CdTe QDs in the porous silicon matrix. Reflectance and PL measurements, combined with simulations, have been used to characterize the surface area coverage of the QDs within the porous framework, which is estimated at 10% for glutathione capped CdTe QDs. INTRODUCTION PSi has emerged as a popular platform of choice for implementing high sensitivity biosensing [1, 2]. The programmability of pore sizes, morphology and thickness enables fabrication of a finely tuned porous matrix optimized for targeted biomolecule capture and detection [3]. In addition, the ease of fabricating simple PSi nanostructures and the large surface area to volume ratio of PSi make it a suitable platform for high sensitivity biosensing. Monitoring changes in the effective refractive index and photoluminescence in the PSi matrix are techniques independent of surface-analyte distance. This ensures that there is no loss of signal due to longer linker molecules or multiple binding events at the pore walls [2]. Unlike surface plasmon resonance based detection, PSi offers the ability to utilize the entire porous framework as an active sensing region for monitoring real-time infiltration and binding events with high sensitivity. Additionally, most optical sensors rely on a single-mode detection scheme, monitoring changes in either refractive index [2, 4], photoluminescence [1], or exciting a surface plasmon resonance [5]. In order to implement a highly sensitive dual-mode sensing platform that monitors changes in effective refractive index and photoluminescence, we implemented labeled biomolecule detection using QDs as signal amplifiers. The QDs cause a significant increase in the overall refractive index of PSi and enable PL based labeled biomolecule detection that is specific to a particular target molecule. A multiplexed detection scheme can be implemented using QDs with different peak emission wavelengths tagged to different biomolecules, and the intensity of QD fluorescence can be calibrated t
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