Ashwini Gopal

Robust flow measurement with multi-exposure speckle imaging

Laser Speckle Contrast Imaging (LSCI) is a minimally invasive full field optical technique used to generate blood flow maps with high spatial and temporal resolution. The lack of quantitative accuracy and the inability to predict flows in the presence of static scatterers such as an intact or thinned skull have been the primary limitation of LSCI. We present a new Multi-Exposure Speckle Imaging (MESI) instrument that has potential to obtain quantitative baseline flow measures. We show that the MESI instrument extends the range over which relative flow measurements are linear. We also present a new speckle model which can discriminate flows in the presence of static scatters. We show that in the presence of static scatterers the new model used along with the new MESI instrument can predict correlation times of flow consistently to within 10% of the value without static scatterers compared to an average deviation of more than 100% from the value without static scatterers using traditional LSCI. We also show that the new speckle model used with the MESI instrument can maintain the linearity of relative flow measurements in the presence of static scatterers.

Multi-color colloidal quantum dot based light emitting diodes micropatterned on silicon hole transporting layers.

We present a colloidal quantum dot based light emitting diode (QD-LED) which utilizes the p-type silicon substrate as the hole transporting layer. A microcontact printing technique was introduced to pattern self-assembled CdSe/ZnS QD films, which allowed creation of an LED with well-defined geometry suitable for monolithic integration on silicon substrates. Our QD-LED consists of multi-layers of inorganic materials: a combination of Au (thickness: 5 nm) and Ag (12 nm) as the cathode, a ZnO:SnO(2) mixture (ratio 3:1, 40 nm) as the electron transporting layer, CdSe/ZnS QDs as the light emission layer, 1 nm SiO(2) as an energy barrier layer, and p-type silicon as the hole transporting layer. These printed QD-LEDs are capable of multi-color emission peaked at wavelengths of 576 nm, 598 nm, and 622 nm, corresponding to sizes of the embedded QDs with the diameters of 8.4 nm, 9.0 nm, and 9.8 nm respectively. The optimal thickness of the quantum dot layers needed for light emission is characterized using atomic force microscopy: for 8.4 nm QDs, the value is 33 nm (+/- 5 nm) or approximately 4 ML (monolayers). Larger turn on voltages were measured (2, 4 and 5 V) for the smaller average particle diameters (9.8 nm, 9.0 nm and 8.4 nm, respectively). The mixture ratio of Zn and Sn was optimized (40% Zn and 25% Sn) to maintain proper hole-electron recombination at the QD layer and avoid the yellowish-white emission from ZnO/SnO(2).

Experimental validation of the effects of microvasculature pigment packaging on in vivo diffuse reflectance spectroscopy.

Diffuse reflectance spectroscopy (DRS) uses the steady‐state diffuse reflectance measured from the tissue surface to determine absorption and scattering properties of sampled tissue. Many inverse models used to determine absorber properties have assumed a homogeneous distribution of blood. However, blood in tissue is confined to blood vessels that occupy a small fraction of the overall volume. This simplified assumption can lead to large errors when measuring optical properties. The objective of this study was to examine the effect of confining absorbers to small volumes, such as the microvasculature, on in vivo DRS.

Enhanced microcontact printing of proteins on nanoporous silica surface

We demonstrate porous silica surface modification, combined with microcontact printing, as an effective method for enhanced protein patterning and adsorption on arbitrary surfaces. Compared to conventional chemical treatments, this approach offers scalability and long-term device stability without requiring complex chemical activation. Two chemical surface treatments using functionalization with the commonly used 3-aminopropyltriethoxysilane (APTES) and glutaraldehyde (GA) were compared with the nanoporous silica surface on the basis of protein adsorption. The deposited thickness and uniformity of porous silica films were evaluated for fluorescein isothiocyanate (FITC)-labeled rabbit immunoglobulin G (R-IgG) protein printed onto the substrates via patterned polydimethlysiloxane (PDMS) stamps. A more complete transfer of proteins was observed on porous silica substrates compared to chemically functionalized substrates. A comparison of different pore sizes (4-6 nm) and porous silica thicknesses (96-200 nm) indicates that porous silica with 4 nm diameter, 57% porosity and a thickness of 96 nm provided a suitable environment for complete transfer of R-IgG proteins. Both fluorescence microscopy and atomic force microscopy (AFM) were used for protein layer characterizations. A porous silica layer is biocompatible, providing a favorable transfer medium with minimal damage to the proteins. A patterned immunoassay microchip was developed to demonstrate the retained protein function after printing on nanoporous surfaces, which enables printable and robust immunoassay detection for point-of-care applications.

Nanoscale fluorescence imaging with quantum dot near-field electroluminescence

We demonstrate near-field fluorescence excitation and imaging with a quantum dot (QD) light emitting diode (QDLED) integrated at the tip of a scanning probe. The tip-embedded QDLED is employed in a near-field scanning optical microscopy setup to directly excite a secondary colloidal QD sample. Electrically pumped QDs enable multi-color, self-illuminating probes with no conventional optics needed for light coupling. Monolayer QDs stamped at the very tip of a micromachined silicon probe facilitates precise position control of the ultra-thin (10–15 nm) light source. Sensitivity of fluorescence intensity to the QDLED–QD sample distance was measured down to 50 nm order, demonstrating spatially resolved imaging.

Photolithographic patterning of subwavelength top emitting colloidal quantum dot based inorganic light emitting diodes on silicon

The combination of lithographic patterning and nanostamping methods makes it possible to accurately define diffraction-limited multicolor (wavelengths 560–620 nm) light sources on a silicon substrate. We demonstrate a postprocessing technique that utilizes standard photolithography process to pattern the cathode of top emitting diode. Correlation of electroluminescence, photoluminescence, and atomic force microscopy topography showed that the emission region is well defined through the robust multiscale patterning techniques, with the fineness of the emitting area mainly limited by the point spread function of the observing microscope.

Microfluidic enrichment of small proteins from complex biological mixture on nanoporous silica chip

The growing field of miniaturized diagnostics is hindered by a lack of pre-analysis treatments that are capable of processing small sample volumes for the detection of low concentration analytes in a high-throughput manner. This letter presents a novel, highly efficient method for the extraction of low-molecular weight (LMW) proteins from biological fluids, represented by a mixture of standard proteins, using integrated microfluidic systems. We bound a polydimethylsiloxane layer patterned with a microfluidic channel onto a well-defined nanoporous silica substrate. Using rapid, pressure-driven fractionation steps, this system utilizes the size-exclusion properties of the silica nanopores to remove high molecular weight proteins while simultaneously isolating and enriching LMW proteins present in the biological sample. The introduction of the microfluidic component offers important advantages such as high reproducibility, a simple user interface, controlled environment, the ability to process small sample volumes, and precise quantification. This solution streamlines high-throughput proteomics research on many fronts and may find broad acceptance and application in clinical diagnostics and point of care detection.

Near-Field Scanning Nanophotonic Microscopy—Breaking the Diffraction Limit Using Integrated Nano Light-Emitting Probe Tip

We introduce a novel scanning ldquonanophotonicrdquo microscope through monolithic integration of a nanoscale LED (Nano-LED) on a silicon cantilever. We review two recent trends of incorporating miniature light sources on the scanning probes for near-field scanning optical microscopy: one is to attach fluorephores at the tip to define a small light source, while the other is to integrate an LED and a nanometer aperture into scanning probes, based on silicon microfabrication techniques. The creation of Nano-LED combines the advantages of previous two approaches: no external sources are required and the reduction of the light source size directly leads to resolution improvement. Two types of Nano-LEDs have been successfully demonstrated utilizing nanofabrication and microelectromechanical systems technologies: 1) formation of thin silicon dioxide light-emitting layer between heavily doped p + and n+ silicon layers created by a focused ion beam and 2) electrostatic trapping and excitation of CdSe/ZnS core-shell nanoparticles in a nanogap. We employed these probes into a standard near-field scanning and excitation setup. The probe successfully measured optical as well as topographic images of chromium test patterns with imaging resolutions of 400 and 50 nm, respectively. In addition, the directional resolution dependence of the acquired images suggests the size and shape of the light source. To our knowledge, these results are probably the first successful near-field images directly measured by such tip-embedded light sources. With the potential emission capability from near UV to IR and additional mass producibility, the nanophotonic microscope presents exciting opportunities in near-field optics, integrated circuit technology, nanomanufacturing and molecular imaging, and sensing in biomedicine.

Micro-patterned drug delivery device for light-activated drug release.

The primary goal of this study was the fabrication, long‐term stability, and measured release of a marker dye from a micro‐patterned drug delivery device using (i) mechanical puncture and (ii) photodisruption with an ophthalmic Nd:YAG laser.

MEMS scanner enabled real-time depth sensitive hyperspectral imaging

We demonstrate a hyperspectral and depth sensitive optical scattering diffusion imaging microsystem, where fast scanning is provided by a CMOS compatible 2-axis MEMS mirror. By using lissajous scanning patterns, large field-of-view (FOV) of 1.2cm × 1.2cm images with lateral resolution of 100µm can be taken at 1.3 frame-per-second (fps). Hyperspectral and depth-sensitive images were acquired on phantom samples consisting of quantum dots (QDs) patterned at various depths in PDMS, showing 6 nm spectral resolution and 0.43 wavelength per second acquisition speed. Images were also acquired on biological sample of porcine epitheliu m with QDs placed underneath the surface.