Suspended, micron-scale corner cube retroreflectors as ultra-bright optical labels

Flexible corner cube retroreflector array for temperature and strain sensing

Optical sensors for detecting temperature and strain play a crucial role in the analysis of environmental conditions and real-time remote sensing. However, the development of a single optical device that can sense temperature and strain simultaneously remains a challenge. Here, a flexible corner cube retroreflector (CCR) array based on passive dual optical sensing (temperature and strain) is demonstrated. A mechanical embossing process was utilised to replicate a three-dimensional (3D) CCR array in a soft flexible polymer film. The fabricated flexible CCR array samples were experimentally characterised through reflection measurements followed by computational modelling. As fabricated samples were illuminated with a monochromatic laser beam (635, 532, and 450 nm), a triangular shape reflection was obtained at the far-field. The fabricated flexible CCR array samples tuned retroreflected light based on external stimuli (temperature and strain as an applied force). For strain and temperature sensing, an applied force and temperature, in the form of weight suspension, and heat flow was applied to alter the replicated CCR surface structure, which in turn changed its optical response. Directional reflection from the heated flexible CCR array surface was also measured with tilt angle variation (max. up to 10°). Soft polymer CCRs may have potential in remote sensing applications, including measuring the temperature in space and in nuclear power stations.

A flexible corner cube retroreflector (CCR) array based passive dual sensing is demonstrated to measure external stimuli (temperature/mechanical force as weight suspension).

An embedded microretroreflector-based microfluidic immunoassay platform

We present a microfluidic immunoassay platform based on the use of linear microretroreflectors embedded in a transparent polymer layer as an optical sensing surface, and micron-sized magnetic particles as light-blocking labels. Retroreflectors return light directly to its source and are highly detectable using inexpensive optics. The analyte is immuno-magnetically pre-concentrated from a sample and then captured on an antibody-modified microfluidic substrate comprised of embedded microretroreflectors, thereby blocking reflected light. Fluidic force discrimination is used to increase specificity of the assay, following which a difference imaging algorithm that can see single 3 μm magnetic particles without optical calibration is used to detect and quantify signal intensity from each sub-array of retroreflectors. We demonstrate the utility of embedded microretroreflectors as a new sensing modality through a proof-of-concept immunoassay for a small, obligate intracellular bacterial pathogen, Rickettsia conorii, the causative agent of Mediterranean Spotted Fever. The combination of large sensing area, optimized surface chemistry and microfluidic protocols, automated image capture and analysis, and high sensitivity of the difference imaging results in a sensitive immunoassay with a limit of detection of roughly 4000 R. conorii per mL.

Microretroreflector-sedimentation immunoassays for pathogen detection

Point-of-care detection of pathogens is medically valuable but poses challenging trade-offs between instrument complexity and clinical and analytical sensitivity. Here we introduce a diagnostic platform utilizing lithographically fabricated micron-scale forms of cubic retroreflectors, arguably one of the most optically detectable human artifacts, as reporter labels for use in sensitive immunoassays. We demonstrate the applicability of this novel optical label in a simple assay format in which retroreflector cubes are first mixed with the sample. The cubes are then allowed to settle onto an immuno-capture surface, followed by inversion for gravity-driven removal of nonspecifically bound cubes. Cubes bridged to the capture surface by the analyte are detected using inexpensive, low-numerical aperture optics. For model bacterial and viral pathogens, sensitivity in 10% human serum was found to be 10⁴ bacterial cells/mL and 10⁴ virus particles/mL, consistent with clinical utility.

Retroreflective imaging system for optical labeling and detection of microorganisms

A retroreflective imaging system for imaging microscopic targets over macroscopic sampling areas is introduced. Detection of microorganism-bound retroreflector (RR) targets across millimeter-scale samples is implemented according to retroreflection directionality, collimation, and contrast design characteristics. Retroreflection directionality is considered for corner-cube (CC) and spherical geometries. Spherical-RRs improve directionality and reliability. Retroreflection collimation is considered for spherical-RRs. Retroreflective images for micro-CC-RRs and micro-spherical-RRs with varying refractive indices show optimal results for high refractive index BaTiO3 micro-spherical-RRs. A differential imaging technique improves retroreflection contrast by 35 dB. High refractive index micro-spherical-RRs and differential imaging, together, can detect microscopic RR targets across macroscopic areas.

Detection and Monitoring of Microparticles Under Skin by Optical Coherence Tomography as an Approach to Continuous Glucose Sensing Using Implanted Retroreflectors

We demonstrate the feasibility of using optical coherence tomography (OCT) to image and detect 2.8 μm diameter microparticles (stationary and moving) on a highly-reflective gold surface both in clear media and under skin in vitro. The OCT intensity signal can clearly report the microparticle count, and the OCT response to the number of microparticles shows a good linearity. The detect ability of the intensity change (2.9% ± 0.5%) caused by an individual microparticle shows the high sensitivity of monitoring multiple particles using OCT. An optical sensing method based on this feasibility study is described for continuously measuring blood sugar levels in the subcutaneous tissue, and a molecular recognition unit is designed using competitive binding to modulate the number of bound microparticles as a function of glucose concentration. With further development, an ultra-small, implantable sensor might provide high specificity and sensitivity for long-term continuous monitoring of blood glucose concentration.

High-throughput top-down fabrication of uniform magnetic particles

Ion Beam Aperture Array Lithography was applied to top-down fabrication of large dense (10(8)-10(9) particles/cm(2)) arrays of uniform micron-scale particles at rates hundreds of times faster than electron beam lithography. In this process, a large array of helium ion beamlets is formed when a stencil mask containing an array of circular openings is illuminated by a broad beam of energetic (5-8 keV) ions, and is used to write arrays of specific repetitive patterns. A commercial 5-micrometer metal mesh was used as a stencil mask; the mesh size was adjusted by shrinking the stencil openings using conformal sputter-deposition of copper. Thermal evaporation from multiple sources was utilized to form magnetic particles of varied size and thickness, including alternating layers of gold and permalloy. Evaporation of permalloy layers in the presence of a magnetic field allowed creation of particles with uniform magnetic properties and pre-determined magnetization direction. The magnetic properties of the resulting particles were characterized by Vibrating Sample Magnetometry. Since the orientation of the particles on the substrate before release into suspension is known, the orientation-dependent magnetic properties of the particles could be determined.