Label-free, real-time monitoring of membrane binding events at zeptomolar concentrations using frequency-locked optical microresonators.

Binding events to elements of the cell membrane act as receptors which regulate cellular function and communication and are the targets of many small molecule drug discovery efforts for agonists and antagonists. Conventional techniques to probe these interactions generally require labels and large amounts of receptor to achieve satisfactory sensitivity. Whispering gallery mode microtoroid optical resonators have demonstrated sensitivity to detect single-molecule binding events. Here, we demonstrate the use of frequency-locked optical microtoroids for characterization of membrane interactions in vitro at zeptomolar concentrations using a supported biomimetic membrane. Arrays of microtoroids were produced using photolithography and subsequently modified with a biomimetic membrane, providing high quality (Q) factors (>106) in aqueous environments. Fluorescent recovery after photobleaching (FRAP) experiments confirmed the retained fluidity of the microtoroid supported-lipid membrane with a diffusion coefficient of 3.38±0.26 µm2⋅s-1. Utilizing this frequency-locked membrane-on-a-chip model combined with auto-balanced detection and non-linear post-processing techniques, we demonstrate zeptomolar detection levels The binding of Cholera Toxin B- monosialotetrahexosyl ganglioside (GM1) was monitored in real-time, with an apparent equilibrium dissociation constant kd=1.53 nM. The measured affiny of the agonist dynorphin A 1-13 to the κ-opioid receptor revealed a kd=3.1 nM using the same approach. Radioligand binding competition with dynorphin A 1-13 revealed a kd in agreement (1.1 nM) with the unlabeled method. The biosensing platform reported herein provides a highly sensitive real-time characterization of membrane embedded protein binding kinetics, that is rapid and label-free, for toxin screening and drug discovery, among other applications.

Lensfree Air-Quality Monitoring of Fine and Ultrafine Particulate Matter Using Vapor-Condensed Nanolenses.

Current commercial air-quality monitoring devices lack a large dynamic range, especially at the small, ultrafine size scale. Furthermore, there is a low density of air-quality monitoring stations, reducing the precision with which local particulate matter hazards can be tracked. Here, we show a low-cost, lensfree, and portable air-quality monitoring device (LPAQD) that can detect and measure micron-sized particles down to 100 nm-sized particles, with the capability to track and measure particles in real time throughout a day and the ability to accurately measure particulate matter densities as low as 3 µg m-3. A vapor-condensed film is deposited onto the coverslip used to collect particles before the LPAQD is deployed at outdoor monitoring sites. The vapor-condensed film increases the scattering cross section of particles smaller than the pixel size, enabling the sub-pixel and sub-diffraction-limit-sized particles to be detected. The high dynamic range, low cost, and portability of this device can enable citizens to monitor their own air quality to hopefully impact user decisions that reduce the risk for particulate matter-related diseases.

Assembly of multicomponent structures from hundreds of micron-scale building blocks using optical tweezers.

The fabrication of three-dimensional (3D) microscale structures is critical for many applications, including strong and lightweight material development, medical device fabrication, microrobotics, and photonic applications. While 3D microfabrication has seen progress over the past decades, complex multicomponent integration with small or hierarchical feature sizes is still a challenge. In this study, an optical positioning and linking (OPAL) platform based on optical tweezers is used to precisely fabricate 3D microstructures from two types of micron-scale building blocks linked by biochemical interactions. A computer-controlled interface with rapid on-the-fly automated recalibration routines maintains accuracy even after placing many building blocks. OPAL achieves a 60-nm positional accuracy by optimizing the molecular functionalization and laser power. A two-component structure consisting of 448 1-µm building blocks is assembled, representing the largest number of building blocks used to date in 3D optical tweezer microassembly. Although optical tweezers have previously been used for microfabrication, those results were generally restricted to single-material structures composed of a relatively small number of larger-sized building blocks, with little discussion of critical process parameters. It is anticipated that OPAL will enable the assembly, augmentation, and repair of microstructures composed of specialty micro/nanomaterial building blocks to be used in new photonic, microfluidic, and biomedical devices.

Optimized sensing of sparse and small targets using lens-free holographic microscopy.

Lens-free holographic microscopy offers sub-micron resolution over an ultra-large field-of-view >20 mm2, making it suitable for bio-sensing applications that require the detection of small targets at low concentrations. Various pixel super-resolution techniques have been shown to enhance resolution and boost signal-to-noise ratio (SNR) by combining multiple partially-redundant low-resolution frames. However, it has been unclear which technique performs best for small-target sensing. Here, we quantitatively compare SNR and resolution in experiments using no regularization, cardinal-neighbor regularization, and a novel implementation of sparsity-promoting regularization that uses analytically-calculated gradients from Bayer-pattern image sensors. We find that sparsity-promoting regularization enhances the SNR by ~8 dB compared to the other methods when imaging micron-scale beads with surface coverages up to ~4%.

Fundamental Limits of Optical Tweezer Nanoparticle Manipulation Speeds.

Optical tweezers are a noncontact method of 3D positioning applicable to the fields of micro- and nanomanipulation and assembly, among others. In these applications, the ability to manipulate particles over relatively long distances at high speed is essential in determining overall process efficiency and throughput. In order to maximize manipulation speeds, it is necessary to increase the trapping laser power, which is often accompanied by undesirable heating effects due to material absorption. As such, the majority of previous studies focus primarily on trapping large dielectric microspheres using slow movement speeds at low laser powers, over relatively short translation distances. In contrast, we push nanoparticle manipulation beyond the region in which maximum lateral movement speed is linearly proportional to laser power, and investigate the fundamental limits imposed by material absorption, thus quantifying maximum possible speeds attainable with optical tweezers. We find that gold and silver nanospheres of diameter 100 nm are limited to manipulation speeds of ∼0.15 mm/s, while polystyrene spheres of diameter 160 nm can reach speeds up to ∼0.17 mm/s, over distances ranging from 0.1 to 1 mm. When the laser power is increased beyond the values used for these maximum manipulation speeds, the nanoparticles are no longer stably trapped in 3D due to weak confinement as a result of material absorption, heating, microbubble formation, and enhanced Brownian motion. We compared this result to our theoretical model, incorporating optical forces in the Rayleigh regime, Stokes' drag, and absorption effects, and found good agreement. These results show that optical tweezers can be fast enough to compete with other common, serial rapid prototyping and nanofabrication approaches.

Radiotherapy fiber dosimeter probes based on silver-only coated hollow glass waveguides.

Manifestation of Cerenkov radiation as a contaminating signal is a significant issue in radiation therapy dose measurement by fiber-coupled scintillator dosimeters. To enhance the scintillation signal transmission while minimizing Cerenkov radiation contamination, we designed a fiber probe using a silver-only coated hollow waveguide (HWG). The HWG with scintillator inserted in its tip, embedded in tissue-mimicking phantoms, was irradiated with clinical electron and photon beams generated by a medical linear accelerator. Optical spectra of the irradiated tip were taken using a fiber spectrometer, and the signal was deconvolved with a linear fitting algorithm. The resultant decomposed spectra of the scintillator with and without Cerenkov correction were in agreement with measurements performed by a standard electron diode and ion chamber for electron and photon beam dosimetry, respectively, indicating the minimal effect of Cerenkov contamination in the HWG-based dosimeter. Furthermore, compared with a silver/dielectric-coated HWG fiber dosimeter design, we observed higher signal transmission in the design based on the use of silver-only HWG.

Silver/cyclic olefin copolymer hollow glass waveguides for infrared laser delivery.

Metal/dielectric-coated hollow glass waveguides (HGWs) have been studied extensively for the efficient transmission of radiation over a broad spectral range. In this study, a low-absorption optical polymer, cyclic olefin copolymer (COC), is investigated as a dielectric material for HGWs designed for the delivery of various IR lasers. Using established silver (Ag) plating techniques and a newly optimized polymer-coating procedure, Ag/COC HGWs with low attenuation coefficients are fabricated for operation at the following three wavelengths: 808 nm, 1.064 µm, and 2.94 µm. The spectral responses of the HGW designs are used to develop a film thickness dependency of the COC layer formation based on the concentration of the solution used in the polymer deposition procedure. Further, the attenuation coefficients of the HGWs are measured using the cutback method at the three wavelengths as a function of the curvature of the waveguide. In order of increasing operation, the attenuation coefficients are measured to be 0.549, 0.095, and 0.298 dB/m for the HGWs in the straight configuration. These experimental values for the straight attenuation coefficients are compared to theoretical values calculated using a ray transfer matrix approach and are found to be in good agreement.

Theory and practical considerations of multilayer dielectric thin-film stacks in Ag-coated hollow waveguides.

This analysis explores the theory and design of dielectric multilayer reflection-enhancing thin film stacks based on high and low refractive index alternating layers of cadmium sulfide (CdS) and lead sulfide (PbS) on silver (Ag)-coated hollow glass waveguides (HGWs) for low loss transmission at midinfrared wavelengths. The fundamentals for determining propagation losses in such multilayer thin-film-coated Ag hollow waveguides is thoroughly discussed, and forms the basis for further theoretical analysis presented in this study. The effects on propagation loss resulting from several key parameters of these multilayer thin film stacks is further explored in order to bridge the gap between results predicted through calculation under ideal conditions and deviations from such ideal models that often arise in practice. In particular, the effects on loss due to the number of dielectric thin film layers deposited, deviation from ideal individual layer thicknesses, and surface roughness related scattering losses are presented and thoroughly investigated. Through such extensive theoretical analysis the level of understanding of the underlying loss mechanisms of multilayer thin-film Ag-coated HGWs is greatly advanced, considerably increasing the potential practical development of next-generation ultralow-loss mid-IR Ag/multilayer dielectric-coated HGWs.

Terahertz wave transmission in flexible polystyrene-lined hollow metallic waveguides for the 2.5-5 THz band.

A low-loss and low-dispersive optical-fiber-like hybrid HE11 mode is developed within a wide band in metallic hollow waveguides if their inner walls are coated with a thin dielectric layer. We investigate terahertz (THz) transmission losses from 0.5 to 5.5 THz and bending losses at 2.85 THz in a polystyrene-lined silver waveguides with core diameters small enough (1 mm) to minimize the number of undesired modes and to make the waveguide flexible, while keeping the transmission loss of the HE11 mode low. The experimentally measured loss is below 10 dB/m for 2 < ν < 2.85 THz (~4-4.5 dB/m at 2.85 THz) and it is estimated to be below 3 dB/m for 3 < ν < 5 THz according to the numerical calculations. At ~1.25 THz, the waveguide shows an absorption peak of ~75 dB/m related to the transition between the TM11-like mode and the HE11 mode. Numerical modeling reproduces the measured absorption spectrum but underestimates the losses at the absorption peak, suggesting imperfections in the waveguide walls and that the losses can be reduced further.

Fabrication and characterization of improved Ag/PS hollow-glass waveguides for THz transmission.

This study involves the fabrication and characterization of improved quality silver (Ag)/polystyrene (PS) thin-film-coated hollow-glass waveguides for the low-loss transmission of terahertz radiation via modified dynamic liquid phase deposition techniques. High-quality PS thin films were deposited from aqueous PS solutions, and the spectral response of fabricated samples was measured from λ=1-100 µm. Fabricated samples exhibited highly defined spectral responses throughout this entire range indicative of PS films of excellent quality. The spectra of experimental samples were compared to the theoretical and bulk PS spectra in the near-IR and far-IR regions. The thickness of deposited PS thin films was found to depend on total sample length and to vary from approximately 10-16 µm for sample lengths ranging from 115 to 140 cm. Such PS film thicknesses are adequate for low-loss delivery from approximately 2-4 THz. Furthermore, film thickness was found to vary minimally along the waveguide length regardless of total sample length.

Theoretical and experimental investigation of infrared properties of tapered silver/silver halide-coated hollow waveguides.

Silver/silver halide-coated hollow-glass waveguides (HGWs) are capable of low-loss, broadband transmission at infrared wavelengths with the advantage of optical response tunability through alteration of a number of key design parameters. Generally, the design of circular HGWs has primarily involved optimization of the waveguide bore size and deposited film structure in order to obtain the desired optical response, with the waveguide bore size being held constant as a function of length. In this study, the effects of HGW structures consisting of linearly tapered inner diameters on the optical response at infrared wavelengths are theoretically and experimentally investigated. Theoretical analysis involving numerical ray optics methods accounting for the dynamic nature of bore size, and consequently light propagation, along the waveguide length is presented and compared to experimental results in order to gain a deeper understanding of these atypical HGW structures.