Adam C. Curry

Label-free plasmonic detection of biomolecular binding by a single gold nanorod.

We report the use of individual gold nanorods as plasmonic transducers to detect the binding of streptavidin to individual biotin-conjugated nanorods in real time on a surface. Label-free detection at the single-nanorod level was performed by tracking the wavelength shift of the nanorod-localized surface plasmon resonant scattering spectrum using a dark-field microspectroscopy system. The lowest streptavidin concentration that was experimentally measured was 1 nM, which is a factor of 1000-fold lower than the previously reported detection limit for streptavidin binding by biotinylated single plasmonic nanostructures. We believe that the current optical setup is able to reliably measure wavelength shifts as small as 0.3 nm. Binding of streptavidin at 1 nM concentration induces a mean resonant wavelength shift of 0.59 nm suggesting that we are currently operating at close to the limit of detection of the system.

Rational Selection of Gold Nanorod Geometry for Label-Free Plasmonic Biosensors

We present the development of an analytical model that can be used for the rational design of a biosensor based on shifts in the local surface plasmon resonance (LSPR) of individual gold nanoparticles. The model relates the peak wavelength of light scattered by an individual plasmonic nanoparticle to the number of bound analyte molecules and provides an analytical formulation that predicts relevant figures-of-merit of the sensor such as the molecular detection limit (MDL) and dynamic range as a function of nanoparticle geometry and detection system parameters. The model calculates LSPR shifts for individual molecules bound by a nanorod, so that the MDL is defined as the smallest number of bound molecules that is measurable by the system, and the dynamic range is defined as the maximum number of molecules that can be detected by a single nanorod. This model is useful because it will allow a priori design of an LSPR sensor with figures-of-merit that can be optimized for the target analyte. This model was used to design an LSPR sensor based on biotin-functionalized gold nanorods that offers the lowest MDL for this class of sensors. The model predicts a MDL of 18 streptavidin molecules for this sensor, which is in good agreement with experiments and estimates. Further, we discuss how the model can be utilized to guide the development of future generations of LSPR biosensors.

Substrate effect on refractive index dependence of plasmon resonance for individual silver nanoparticles observed using darkfield microspectroscopy.

We use optical darkfield micro-spectroscopy to characterize the plasmon resonance of individual silver nanoparticles in the presence of a substrate. The optical system permits multiple individual nanoparticles to be identified visually for simultaneous spectroscopic study. For silver particles bound to a silanated glass substrate, we observe changes in the Plasmon resonance due to induced variations in the local refractive index. The shifts in the plasmon resonance are investigated using a simple analytical theory in which the contributions from the substrate and environment are weighted with distance from the nanoparticle. The theory is compared with experimental results to determine a weighting factor which facilitates modeling of environmental refractive index changes using standard Mie code. Use of the optical system for characterizing nanoparticles attached to substrates for biosensing applications is discussed.

Epi-illumination through the microscope objective applied to darkfield imaging and microspectroscopy of nanoparticle interaction with cells in culture

Existing darkfield illumination schemes are incompatible with many types of samples and/or procedures. We present a darkfield epi-illumination scheme which addresses these incompatibilities by providing illumination through the imaging objective. We validate the system performance using silver nanospheres in varying refractive index environments, characterize the intensity distribution of the darkfield illumination, and demonstrate system capabilities through a preliminary study of functionalized gold nanosphere interactions with cancer cells in culture. We observe a broadened scattering spectrum from unconjugated nanoparticles, as compared with anti-EGFR conjugated nanoparticles, upon incubation with cancer cells, and discuss the implications of this observation.

Analysis of total uncertainty in spectral peak measurements for plasmonic nanoparticle-based biosensors

One goal of recent research on plasmonic nanoparticle-based sensors is maximizing nanoparticle sensitivity or shift of resonance peak wavelength per refractive index change. Equally important is a measurement systems peak location uncertainty or shift resolution. We provide systematic analyses and discuss optimization of factors that determine peak location uncertainty, reporting values as low as 0.3 nm for the presented scheme. This type of analysis is important, in part, because it provides a means of evaluating detection thresholds for biosensor applications such as analyte binding. We estimate thresholds of 310 streptavidin molecules for the presented scheme and 20 molecules with system improvements.

Catheter for diagnosis and therapy with infrared evanescent waves

We have developed an optical delivery device (catheter) capable of transmitting broadband infrared light (IR wavelengths from 2 to 10 microm) for both diagnostic and therapeutic applications. The catheter is 1.68 mm in outer diameter and 1 m in length. It consists of two hollow glass waveguides coupled to a high-refractive-index optic tip. The IR light interacts with the tissue at the optic-tissue interface to measure the spectral signatures and perform therapy on the tissue at this interface. Fourier-transform IR spectrophotometer light is used to obtain the spectral signatures, and an IR free-electron laser (FEL) is used to study the therapeutic interaction of evanescent waves with the tissue. We present our catheter design; preliminary IR spectroscopy of aorta, blood, fatty tissue, and muscle; and IR FEL therapy on atheroslerotic aorta.

Molecular Imaging of EGFR Expression in Live Cancer Cells Using Immunotargeted Nanoparticles

Using molecular imaging of immunolabeled plasmonic nanoparticles bound by cell surface receptors, we compare epidermal growth factor receptor expression, an indicator of cancerous activity, of both human epithelial carcinoma and brain tumor cell lines.

Epi-Illumination Darkfield through a Microscope Objective for Imaging and Spectral Analysis of Nanoparticle Interaction with Cells in Culture

Existing darkfield illumination schemes are incompatible with many types of samples and/or procedures. We present a darkfield scheme which addresses these incompatibilities. We then apply the scheme to evaluate nanoparticles binding cancer cells in culture.

Measurement system for the high-throughput characterization of metal nanoparticles for biosensors

We present a system for the rapid characterization of nanoparticles for biosensors, providing concurrent atomic force microscopy and scattering spectra of individual nanoparticles and simultaneous spectra from various nanostructures created by electron beam lithography.