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.

Corner cube retroreflectors are objects with three mutually perpendicular reflective surfaces that return light directly to its source and are therefore extremely bright and easy to detect. In this work, we have fabricated suspended corner cube retroreflectors, 5 microns in size, consisting of a transparent epoxy core and three surfaces coated with gold as ultra-bright labels for use in a rapid, low-labor diagnostic platform. The authors have demonstrated that individual cubes are easily imaged using low-cost, low numerical aperture objectives in suspension and that they remain suspended over long periods of time. Moreover, we have demonstrated that the gold outer surfaces can be decorated with proteins, and that individual cubes can be bound to magnetic sample preparation particles bearing antibodies which recognize these proteins. The bound cubes can be imaged and tracked as they move through solution in response to an external magnetic field, and we have, as such, demonstrated the principle of the new biosensing approach.

Ion beam aperture array lithography was developed to rapidly form large arrays of periodic patterns whose shape is defined at the printing step. This is accomplished with a massive array of ion beamlets formed by exposing a stencil mask with a dense array of openings to a broad ion beam; these can be moved across the wafer surface to write the shape of the unit element with each beamlet. In this work, the authors demonstrate that the uniformity of the aperture array mask image can be significantly improved by averaging the exposure with images formed by the neighboring apertures. This reduces the tolerances for patterns in the stencil mask and allows for the correction of random defects that may occur during the mask fabrication process. To achieve this, each aperture image is formed as the sum of the images of itself and its neighbors. They demonstrate, through modeling and experiment, that this process can form images where the standard deviation of the openings has been reduced by a factor of 3 when averaging the exposure over nine neighboring apertures. They show that the size distribution scales inversely with the square root of the number of near neighbor exposures.

In this work, a collimated helium beam was used to activate a thiol-poly(ethylene glycol) (SH-PEG) monolayer on gold to selectively capture proteins in the exposed regions. Protein patterns were formed at high throughput by exposing a stencil mask placed in proximity to the PEG-coated surface to a broad beam of helium particles, followed by incubation in a protein solution. Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (ATR–FTIR) spectra showed that SH-PEG molecules remain attached to gold after exposure to beam doses of 1.5–60 µC/cm2 and incubation in PBS buffer for one hour, as evidenced by the presence of characteristic ether and methoxy peaks at 1120 cm−1 and 2870 cm−1, respectively. X-ray Photoelectron Spectroscopy (XPS) spectra showed that increasing beam doses destroy ether (C–O) bonds in PEG molecules as evidenced by the decrease in carbon C1s peak at 286.6 eV and increased alkyl (C–C) signal at 284.6 eV. XPS spectra also demonstrated protein capture on beam-exposed PEG regions through the appearance of a nitrogen N1s peak at 400 eV and carbon C1s peak at 288 eV binding energies, while the unexposed PEG areas remained protein-free. The characteristic activities of avidin and horseradish peroxidase were preserved after attachment on beam-exposed regions. Protein patterns created using a 35 µm mesh mask were visualized by localized formation of insoluble diformazan precipitates by alkaline phosphatase conversion of its substrate bromochloroindoyl phosphate-nitroblue tetrazolium (BCIP-NBT) and by avidin binding of biotinylated antibodies conjugated on 100 nm gold nanoparticles (AuNP). Patterns created using a mask with smaller 300 nm openings were detected by specific binding of 40 nm AuNP probes and by localized HRP-mediated deposition of silver nanoparticles. Corresponding BSA-passivated negative controls showed very few bound AuNP probes and little to no enzymatic formation of diformazan precipitates or silver nanoparticles.

A new approach to in vivo biosensor design is introduced, based on the use of an implantable micron-sized retroreflector-based platform and non-invasive imaging of its surface reflectivity by Optical Coherence Tomography (OCT). The possibility of using OCT for the depth-resolved imaging and detection of micro-retroreflectors in highly turbid media, including tissue, is demonstrated. The maximum imaging depth for the detection of the micro-retroreflector-based platform within the surrounding media was found to be 0.91 mm for porcine tissue and 1.65 mm for whole milk. With further development, it may be possible to utilize OCT and micro-retroreflectors as a tool for continuous monitoring of analytes in the subcutaneous tissue.