Laser trapping of Ag nanoparticles to enhance Raman spectroscopy in aqueous media

Raman spectroscopy of optical-trapped single particle using bull's eye nanostructure

Surface-enhanced Raman spectroscopy (SERS) has enabled single nanoparticle Raman sensing with abundant applications in analytical chemistry, biomaterials, and environmental monitoring. Genuine single particle Raman sensing requires a cumbersome technique, such as atomic force microscopy (AFM) based tip-enhanced Raman spectroscopy; SERS-based single particle Raman sensing still collects an ensemble signal that samples, in principle, a number of particles. Here, we develop in situ Raman-coupled optical tweezers, based on a hybrid nanostructure consisting of a single bowtie aperture surrounded by bull's eye rings, to trap and excite a rhodamine-6G-dye-doped polystyrene sphere. We simulated a platform to ensure sufficient enhancement capability for both optical trapping and SERS of a single nanoparticle. Experiments with well-designed controls clearly attribute the Raman signal origin to a single 15-nm particle trapped at the center of a nanohole, and they also clarified the trapping and Raman enhancement role of the bull's eye rings. We claim Raman sensing of a smallest optically trapped particle.

Scanning Super-Resolution Imaging in Enclosed Environment by Laser Tweezer Controlled Superlens

Super-resolution imaging using microspheres has attracted tremendous scientific attention recently because it has managed to overcome the diffraction limit and allowed direct optical imaging of structures below 100 nm without the aid of fluorescent microscopy. To allow imaging of specific areas on the surface of samples, the migration of the microspheres to specific locations on two-dimensional planes should be controlled to be as precise as possible. The common approach involves the attachment of microspheres on the tip of a probe. However, this technology requires additional space for the probe and could not work in an enclosed environment, e.g., in a microfluidic enclosure, thereby reducing the range of potential applications for microlens-based super-resolution imaging. Herein, we explore the use of laser trapping to manipulate microspheres to achieve super-resolution imaging in an enclosed microfluidic environment. We have demonstrated that polystyrene microsphere lenses could be manipulated to move along designated routes to image features that are smaller than the optical diffraction limit. For example, a silver nanowire with a diameter of 90 nm could be identified and imaged. In addition, a mosaic image could be constructed by fusing a sequence of images of a sample in an enclosed environment. Moreover, we have shown that it is possible to image Escherichia coli bacteria attached on the surface of an enclosed microfluidic device with this method. This technology is expected to provide additional super-resolution imaging opportunities in enclosed environments, including microfluidic, lab-on-a-chip, and organ-on-a-chip devices.