ABSTRACT
We propose a laser speckle based scheme that allows the analysis of local scattering properties of light diffusely reflected from turbid media. This turbid medium can be a soft material such as a colloidal or polymeric material but can also be biological tissue. The method provides a 2D map of the scattering properties of a complex, multiple scattering medium by recording a single image. We demonstrate that the measured speckle contrast can be directly related to the local transport mean free path l* or the reduced scattering coefficient µt = 1/l* of the medium. In comparison to some other approaches, the method does not require scanning (of a laser beam, detector or the sample itself) in order to generate a spatial map. It can conveniently be applied in a reflection geometry and provides a single characteristic value at any given position with an intrinsic resolution typically on the order of 5-50 µm. The actual resolution is however limited by the transport mean free path itself and can thus range from microns to millimeters.
Subject(s)
Algorithms , Lasers , Nephelometry and Turbidimetry/methods , Solutions/chemistry , DiffusionABSTRACT
Accurate characterization using static light scattering (SLS) and dynamic light scattering (DLS) methods mandates the measurement and analysis of singly scattered light. In turbid samples, the suppression of multiple scattering is therefore required to obtain meaningful results. One powerful technique for achieving this, known as 3D cross-correlation, uses two simultaneous light scattering experiments performed at the same scattering vector on the same sample volume in order to extract only the single scattering information common to both. Here we present a significant improvement to this method in which the two scattering experiments are temporally separated by modulating the incident laser beams and gating the detector outputs at frequencies exceeding the timescale of the system dynamics. This robust modulation scheme eliminates cross-talk between the two beam-detector pairs and leads to a fourfold improvement in the cross-correlation intercept. We measure the dynamic and angular-dependent scattering intensity of turbid colloidal suspensions and exploit the improved signal quality of the modulated 3D cross-correlation DLS and SLS techniques.
Subject(s)
Light , Optical Devices , Scattering, Radiation , LasersABSTRACT
A theory is derived to describe the relationship between photonic crystal (PC) label-free imaging resolution and PC resonance spectral linewidth and location. PCs are fabricated and patterned with a resolution standard photomask in order to verify this relationship experimentally. Two distinct linear resolutions of <1 microm and 3.5 microm are demonstrated in orthogonal directions on a single device, where the former is limited by the imaging system optics and the latter is constrained by finite resonant mode propagation. In order to illustrate the utility of improved design control, the spectral response of a PC is optimized for label-free imaging of immobilized DNA capture spots on a microarray.
Subject(s)
Crystallization/methods , DNA/ultrastructure , Image Enhancement/methods , Image Interpretation, Computer-Assisted/methods , Microscopy, Fluorescence/methods , Oligonucleotide Array Sequence Analysis/methods , Refractometry/methods , Photons , Staining and LabelingABSTRACT
We report on the design and demonstration of an optical imaging system capable of exciting surface-bound fluorophores within the resonant evanescent electric field of a photonic crystal surface and gathering fluorescence emission that is directed toward the imaging objective by the photonic crystal. The system also has the ability to quantify shifts in the local resonance angle induced by the adsorption of biomolecules on the photonic crystal surface for label-free biomolecular imaging. With these two capabilities combined within a single detection system, we demonstrate label-free images self-registered to enhanced fluorescence images with 328x more sensitive fluorescence detection relative to a glass surface. This technique is applied to a DNA microarray where label-free quantification of immobilized capture DNA enables improved quality control and subsequent enhanced fluorescence detection of dye-tagged hybridized DNA yields 3x more genes to be detected versus commercially available microarray substrates.
Subject(s)
Biosensing Techniques/instrumentation , Microscopy, Fluorescence/methods , Optics and Photonics , Adsorption , Animals , Crystallization , Glass , Humans , Lasers , Metals/chemistry , Nucleic Acid Hybridization , Oligonucleotide Array Sequence Analysis , Photons , Quality Control , Surface Plasmon Resonance/methodsABSTRACT
Efficient recovery of light emitted by fluorescent molecules by employing photonic structures can result in high signal-to-noise ratio detection for biological applications including DNA microarrays, fluorescence microscopy and single molecule detection. By employing a model system comprised of colloidal quantum dots, we consider the physical basis of the extraction effect as provided by photonic crystals. Devices with different lattice symmetry are fabricated ensuring spectral and spatial coupling of quantum dot emission with leaky eigenmodes and the emission characteristics are studied using angle-resolved and angle-integrated measurements. Comparison with numerical calculations and lifetime measurements reveals that the enhancement occurs via resonant redirection of the emitted radiation. Comparison of various lattices reveals differences in the enhancement factor with a maximum enhancement factor approaching 220. We also demonstrate the first enhanced extraction biosensor that allows for over 20-fold enhancement of the fluorescence signal in detection of the cytokine TNF-alpha by a fluorescence sandwich immunoassay.
