Andrea M. Armani

Experimental demonstration of application of ring down measurement approach to microcavities for biosensing

Cavity ring down measurement approach is a promising technique for biosensing as it is insensitive to intensity uctuations of a laser source. This technique in conjunction with ultra high Q microcavities have a great potential for ultra sensitive biosensing. Until now, most work on microcavity biosensors has been based on measurement of the resonant frequency shift induced by binding event on surface of the microcavity. Such measurements suer from the noise due to intensity uctuations of the laser source. However, the binding event will also introduce shift in quality factor of the microcavity, which can be tracked by using cavity ring down spectroscopy. In this work, we report on experimental demonstration of application of ring down measurement approach to microcavities for biosensing by tracking disassociation phase of a biotin-streptavidin reaction. These measurements were performed by using a bioconjugated ultra high Q microtoroidal cavity immersed in a liquid microacquarium. We found that disassociation curves agree with previously reported results on the protein kinetics measurements.

Improving the specificity and stability of label-free optical biosensors

The detection of antigens, bacteria, and viruses for medical diagnostics and environmental monitoring requires the development of highly sensitive and selective biosensors. Conventionally, fluorescent immunoassays have been the primary detection platforms in complex environments, such as serum or whole blood. This technique is particularly successful due to its high specificity to the target molecule and the good signal-to-noise provided by the fluorescent molecule. However, because a dye molecule is required as a read-out mechanism, these approaches are not suitable for real-time, in-line monitoring of biological processes. In contrast, label-free optical sensors, such as those based on optical waveguides and resonant cavity devices, are able to detect the molecule itself, without the use of a secondary fluorescent probe. However, while many of these optical devices are inherently sensitive transducers, detection specificity is equally, if not more, important in many applications. Therefore, it is critical to improve the specificity of detection without degrading the device performance.

Bioconjugation of ultra-high-Q optical microcavities for label-free sensing

The development of label-free biosensors with high sensitivity and specificity is of significant interest for medical diagnostics and environmental monitoring, where rapid and real-time detection of antigens, bacteria, viruses, etc., is necessary. Ultra-high-Q optical microcavities are uniquely suited to sensing applications, but previous research efforts in this area have focused on the development of the sensor itself. While device sensitivity is crucial to sensor development, specificity is an equally important feature. Therefore, it is crucial to develop a high density, covalent surface functionalization process, which also maintains the devices performance. Here, we demonstrate a facile method to impart specificity to optical microcavities, without adversely impacting their optical performance. In this approach, we selectively functionalize the surface of the silica microtoroids with amine-terminated silane coupling agents, and examine the impact of differing reaction schemes on the overall quality of the devices. The chemistries investigated here result in uniform surface coverage, and no microstructural damage, and do not adversely impact the optical performance of the devices, as measured by their quality factors before and after functionalization. This work represents one of the first examples of non-physisorption-based bioconjugation of optical microtoroid resonators, which can be used for the label-free detection of biomolecules.

Optical devices for label-free detection

Innovation in technology routinely leads the way for discovery in chemistry and biology. Most notably, x-ray diffraction data was instrumental in the elucidation of the structure of DNA. To explore the inherent complexity present in biological systems, existing technologies are being pushed to their limits. Once again, scientists are looking to engineers to create innovative solutions to enable their exploration and discovery. Many of the new methods currently being developed focus on increasing the sensitivity of the detection technique by inventing new devices as well as increasing the specificity of the device by engineering synthetic targeting moieties and improved attachment methods. This presentation will focus on new optical technologies with an emphasis on bio/chem-detection applications. Specifically, the experimental and theoretical optical properties of several new optical devices, which were designed for the express purpose of biological and chemical detection, will be discussed. Additionally, biomolecule attachment strategies which can improve both the stability and specificity of the sensors surface functionalization will be presented.

Silica suspended waveguide splitter-based biosensor

Recently, a novel integrated optical waveguide 50/50 splitter was developed. It is fabricated using standard lithographic methods, a pair of etching steps and a laser reflow step. However, unlike other integrated waveguide splitters, the waveguide is elevated off of the silicon substrate, improving its interaction with biomolecules in solution and in a flow field. Additionally, because it is fabricated from silica, it has very low optical loss, resulting in a high signal-to-noise ratio, making it ideal for biosensing. By functionalizing the device using an epoxy-silane method using small samples and confining the protein solutions to the device, we enable highly efficient detection of CREB with only 1 μL of solution. Therefore, the waveguide coupler sensor is representative of the next generation of ultra-sensitive optical biosensors, and, when combined with microfluidic capabilities, it will be an ideal candidate for a more fully-realized lab-on-a-chip device.

Low-loss silica on silicon integrated waveguides

Low-loss waveguides integrated on a silicon substrate are essential components in the design and fabrication of photonic circuits. For this application, a wide operational bandwidth - from visible to infrared wavelengths - is critical. Previous research has yielded waveguides made with various materials and geometries. Several of these devices have achieved low, <0.1dB/cm loss in either the visible or the near-IR. However, to obtain effective confinement of light from the visible through the near-IR, it is necessary to develop waveguides which have near-constant loss and minimal non-linear effects across the entire wavelength range. To overcome this challenge, we have developed novel silica on silicon waveguides fabricated using conventional lithographic techniques and CO2 laser reflow. The entire waveguide is elevated above the higher refractive index silicon substrate, creating an isolated, air-clad waveguide. The cylindrical waveguides loss was determined by coupling light from 658nm, 980nm, and 1550nm lasers into the waveguide using lensed optical fibers. Due to the inherently low material loss of silica and the isolation from the silicon substrate, the device has low optical loss (0.7-0.9dB/cm) and linear behavior across the entire wavelength, polarization, and input power ranges studied. These on-chip waveguides will benefit many applications, including biodetection and integrated photonics.

Reconfigurable visible quantum dot microlasers integrated on a silicon chip

Developing on-chip, dynamically reconfigurable visible lasers that can be integrated with additional optical and electronic components will enable adaptive optical components. In the present work, we demonstrate a reconfigurable quantum dot laser based on an integrated silica ultra high-Q microcavity. By attaching the quantum dot using a reversible, non-destructive bioconjugation process, the ability to remove and replace it with an alternative quantum dot without damaging the underlying microcavity device has been demonstrated. As a result of the absorption/emission characteristics of quantum dots, the same laser source can be used to excite quantum dots with distinct emission wavelengths.

Evanescent field excitation of Cy5-conjugated lipid bilayers using optical microcavities

Whispering gallery mode optical microresonators are devices used for performing ultra-sensitive optical detection. Although the majority of the sensor research has been focused on label-free detection strategies for diagnostics, a whispering gallery mode device is ideally suited to perform fluorescent label-based biodetection as well. However, previous research using optical microcavities to excite fluorescent molecules has focused on cavity quantum electrodynamics applications and fundamental studies of the interactions of large fluorescent nanoparticles with the resonant cavity. In the present work, a method for forming self-assembled lipid bilayers, a mimic for cell membranes, on a spherical microresonator is developed. Solid-supported lipid bilayers, which are approximately 5nm thick, have been shown to accurately model cell membranes, and researchers use lipid bilayers in combination with fluorescent microscopy when developing theoretical models for the transport of molecules across the cell membrane. The bilayernature is verified using both fluorescent resonance energy transfer and fluorescence recovery after photobleaching. The evanescent tail of the microresonator is used to excite a Cy5-conjugated lipid located within the bilayer while the underlying optical device behavior is characterized at 633nm and 980nm. The emission wavelength of the Cy5 dye and the optical performance (Q factor) of the microcavity agree with theoretical predictions.

Bioconjugation strategies for improved optical sensor performance

Measuring the binding kinetics of molecular systems is fundamental in understanding the interaction between biomolecules within a binding pair. One emerging label-free detection method is based on silica optical microcavities. The majority of research to date with microcavity-based sensors has focused on applications in the diagnostics realm. Here, we develop and characterize a covalent surface attachment strategy for microsphere resonators. We also measure the optical performance (quality factor) of the functionalized microcavities and use them to determine the dissociation constant of the biotinstreptavidin pair. The measured value is within acceptable range of previously published dissociation constants for the biotin-streptavidin pair.

Recyclable optical microcavities for label-free sensing

High-sensitivity, label-free biosensors, such as optical microcavities, have shown tremendous potential in medical diagnostics, environmental monitoring, and food safety evaluation, particularly when paired with a biochemical recognition element that grants high specificity towards a target of interest. Their primary limitation is that these systems are single-use, unless the recognition element can be regenerated. Therefore, the ability to selectively functionalize the optical microcavity for a specific target molecule and then recycle the system, without degrading device performance, is extremely important. Here, we present a bioconjugation strategy that not only imparts specificity to optical microcavities, but also allows for biosensor recycling. In this approach, we selectively functionalize the surface of silica microtoroids with a biotin recognition element. We then use a non-destructive O2 plasma treatment to remove the surface chemistry, refresh the recognition element, and recycle the device. The surface chemistry and optical performance of the functionalized and recycled devices are characterized by microcavity analysis, and typical spectroscopic techniques, respectively. The resulting devices can be recycled several times without performance degradation, and show high density surface coverage of biologically active recognition elements. This work represents one of the first examples of a recyclable, bioconjugation strategy for optical microtoroid resonators.