We use optical tweezers to position heterogeneous nanoscale building blocks in complex 3D geometries for nanophotonic metamaterials and devices. Designs are efficiently optimized using the discrete dipole approximation. © 2025 The Author(s)

ABSTRACT 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 (>10 6 ) 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 μm 2 ·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 (k d ) = 1.53 nM. The measured affiny of the agonist dynorphin A 1-13 to the κ-opioid receptor revealed a k d = 3.1 nM using the same approach. Radioligand binding competition with dynorphin A 1-13 revealed a K d 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.

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.

We use optical positioning and linking (OPAL) based on optical tweezers to fabricate arrays of hundreds of building blocks in 3D shapes. Automation is provided through feedback from a quadrant photodiode. Applications include nanophotonic sensors.

Our optical positioning and linking (OPAL) platform enables the assembly of complex 3D microstructures using optical tweezers and a biochemical linking mechanism. Advantages to OPAL include incorporating building blocks of different sizes and materials and augmentation of existing devices. Updates to system components and processes have improved the speed and accuracy of assembly. The collection of the backscattered signal using a quadrant photodiode permits classification of the number of trapped particles, which is used for automated assembly. The fully automated assembly will increase the efficiency of fabrication and is beneficial for rapid prototyping and producing larger, more complex structures.

Microtoroid resonators are one of the most sensitive chemical sensing technologies. However, coupling light into microtoroids typically requires equipment such as vibration-isolation tables and piezoelectric nanopositioning stages. Translating microtoroids to platforms with small size, weight, power, and cost for chemical vapor sensing remains a challenge. We demonstrate an approach to position photonic nanostructures on the surface of microtoroids to facilitate free-space coupling via inexpensive optics. We have designed the nanophotonic coupler using finite element simulations with novel boundary conditions to accommodate a large simulation domain. The nanophotonic coupler is assembled using a custom manufacturing platform based on automated optical tweezers.

Optical trapping provides a non-contact method for the three-dimensional (3D) manipulation of diverse objects. The optical positioning and linking (OPAL) platform used in this study uses an optical trap for object manipulation and a biochemical linking mechanism for building up extended 3D structures. Here we demonstrate the feasibility of the OPAL platform for the fabrication of several large-scale 3D assemblies consisting of hundreds of building blocks. We develop an efficient computer-controlled platform and explore the optimal parameters for the biochemical linking mechanism. Finally, we discuss future applications of the OPAL platform and its viability for nanostructure fabrication.

The ability to fabricate three-dimensional structures with microscale resolution is critical to many emerging applications. We use an optical tweezers platform with biochemical linking mechanism to assemble 3D structures consisting of hundreds of individual building blocks.

Many air-quality monitoring devices can detect different sizes of particles but cannot quantify individual particle sizes across the range 0.1-10 μm. Here we show such a device that is furthermore cost-effective and portable.

Three-dimensional (3D) structures with nanoscale building blocks often benefit from light-matter interaction at the nanometer size scale. The most popular existing technologies for the 3D fabrication of these small structures (e.g. direct laser writing) typically encounter significant restrictions. Here we demonstrate our optical positioning and linking (OPAL) platform for the assembly of multi-material 3D devices consisting of nanoscale components. OPAL integrates an optical trapping setup with a computer-controlled motion platform, microfluidic building block delivery apparatus, and imaging feedback system, overcoming the limitations of many traditional nanofabrication approaches, and establishing a versatile tool for the fabrication of complex 3D nanophotonic structures.

The optical tweezers platform provides an attractive approach for the fabrication of three-dimensional structures. Existing studies on optical tweezers assembly have mostly focused on the formation of small-scale structures consisting of a single type of dielectric building block. In this work, we demonstrate the potential of our automated optical positioning and linking (OPAL) platform that uses optical trapping for object manipulation and biochemical binding as a linking mechanism. We show a comprehensive analysis of system parameters and fabricate the largest microstructure built to date using this approach. Furthermore, we assemble multicomponent structures consisting of nanoscale building blocks.

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.

Optical tweezers provide a non-contact three-dimensional (3D) method for nanoscale positioning. We assemble a 3D light guiding structure comprising a chain of gold-silica nanoshells and show structures assembled from hundreds of chemically-linked building blocks.

For many micro- and nano-photonic applications, current 3D prototyping approaches are unable to provide the necessary resolution or material integration. Optical tweezers (OT) are a potentially attractive solution due to their ability to manipulate various small objects with high precision. Here we show a custom-built automated OT 3D assembly platform that operates with manipulation speeds up to 0.22 mm/s and positioning accuracy better than 50 nm. Furthermore, to the best of our knowledge, we assemble the largest 3D structure to date using an OT platform, consisting of several hundred objects of multiple compositions.

Optical tweezers are a powerful platform for nano- and micro-assembly, as they provide a non-contact and biologically friendly method for the three-dimensional manipulation of objects over a range of sizes and of varying material properties. Three-dimensional micro- and nano-scale structures that are composed of multiple materials often achieve improved performance over single-material designs. In the case of optical devices, the inclusion of both metallic and dielectric media allows for the possibility of achieving functionality which is otherwise inaccessible. Although there are many methods for fabricating small-scale three-dimensional optical devices, the majority of these approaches only deal with a single material or type of material. Thus, in order to create structures that consist of multiple materials, it is typically necessary to use a combination of methods over the course of several steps. Here we show that optical tweezers are a promising technology for the assembly of heterogeneous optical structures in a single process. We demonstrate our approach by fabricating structures using core-shell nanoparticles with metallic shells and dielectric cores as building blocks. To the best of our knowledge, these structures represent the first nanoscale, multi-material devices built using the optical tweezer platform. Furthermore, we discuss several relevant metrics regarding the assembly process such as object translation speeds, placement accuracy, and overall rates of fabrication. Currently, we have achieved lateral speeds up to 0.2 mm/s and placement repeatability down to 50 nm. We suggest future applications of this fabrication method and discuss the next steps in its evolution.

Improved 3D nanofabrication approaches could enable higher performance photonic devices. Existing approaches are limited in resolution, material compatibility, and/or the ability to fabricate complex geometries. Optical tweezers are attractive due to their ability to trap and position nanoscale objects of various materials with high precision. However, it is challenging to develop an automated platform that is robust and high-speed enough for rapid prototyping. Here we present such a system, discuss its positioning accuracy and speed, and, as a proof-of-concept, use it to assemble a light-guiding chain composed of biochemically functionalized 110 nm gold-silica nanoshells connected to a fluorescent nanoparticle.

New approaches are needed to prototype heterogeneous 3D photonic materials and devices with 100 nm or smaller feature sizes. We demonstrate that optical tweezers can provide the necessary speed and positioning accuracy for rapid prototyping.

Complex 3D nanophotonic devices demand robust methods for the design and rapid prototyping of heterogeneous material structures with sub-100 nm resolution. Here we present an automated optical tweezer platform for assembling nanoparticles of multiple materials. In pursuit of higher throughput, we have demonstrated record nanoparticle manipulation speeds >150 um/s. To design structures, we use a coupled dipole model that is orders of magnitude faster than commercial tools. In developing this method, we have found that a longstanding practice of basing a metallic nanoparticle’s dipole moment on its skin depth is significantly less accurate than calculations based on full particle volume.

Three-dimensional structure fabrication using discrete building blocks provides a versatile pathway for the creation of complex nanophotonic devices. The processing of individual components can generally support high-resolution, multiple-material, and variegated structures that are not achievable in a single step using top-down or hybrid methods. In addition, these methods are additive in nature, using minimal reagent quantities and producing little to no material waste. In this article, we review the most promising technologies that build structures using the placement of discrete components, focusing on laser-induced transfer, light-directed assembly, and inkjet printing. We discuss the underlying principles and most recent advances for each technique, as well as existing and future applications. These methods serve as adaptable platforms for the next generation of functional three-dimensional nanophotonic structures.

The optical trapping of nanoparticles is important in the assembly of nanostructured materials and in fundamental studies of plasmonics and coupled light-matter interactions. These applications demand an accurate model of the trapping force and the ability to manipulate nanoparticles at high speeds over long distances. The trapping force is most simply modeled using a dipole approximation for particles much smaller than the wavelength, resulting in a force proportional to particle volume. For metallic nanoparticles, it was previously thought to be more accurate to replace the full particle volume with an effective volume based on a spherical shell with thickness equal to the metallic skin depth. The resulting optical trapping force is then proportional to surface area rather than volume. However, experimental studies have generally failed to find forces that scale with surface area, with qualitative explanations such as enhanced radiation pressure displacing particles from the beam focus or the presence of spherical aberration. Here we show through comparison to rigorous Mie theory that the complex permittivity of the metal fully accounts for the skin effect in metallic nanoparticles, and it is more accurate to use the full volume with a radiation reaction correction rather than an effective volume based on the skin depth. We compare these predictions to experiments, where we also show particularly high-speed (>0.1 mm/s) and long-distance (1 mm) manipulation of gold, silver, and polystyrene nanoparticles using a high-powered laser and low-aberration optical tweezer. We hope that that these results will help to enable high-speed nano-assembly.