Optical Trapping-Polarized Raman Microspectroscopy of Single Ethanol Aerosol Microdroplets: Droplet Size Effects on Rotational Relaxation Time and Viscosity

Optical trapping of multiple particles based on a rotationally-symmetric power-exponent-phase vortex beam

In this paper, the optical trapping of multiple particles based on a rotationally-symmetric power-exponent-phase vortex beam (RSPEPVB) was introduced and demonstrated. Based on the theories of tight focusing and optical force, the optical force model of RSPEPVB was established to analyze the optical trapping force of tightly focused RSPEPVB. Then, an experimental setup of optical tweezer, by utilizing the RSPEPVB, was built to demonstrate that the optical tweezer of RSPEPVBs can achieve the optical trapping of multiple particles, and the number of captured particles is equal to the topological charge l of RSPEPVB, which shows that the RSPEPVBs can achieve multi-particles trapping with controllable number. Moreover, compared to vortex beam, the captured particles by RSPEPVB will not rotate around the circular light intensity distribution. The results will provide a new option for optical trapping of multiple particles in biomedicine, laser cooling and so on.

Optical collection of extracellular vesicles in a culture medium enhanced by interactions with gold nanoparticles

Extracellular vesicles (EVs) exist in biological fluids such as blood, urine, and cerebrospinal fluid and are promising cancer biomarkers. Attempts to isolate and analyze trace EVs, however, have been a challenge for researchers studying their functions and secretion mechanisms, which has stymied the options for diagnostic application. This study demonstrated a collection of EVs that was enhanced by gold nanoparticles (AuNPs) via the use of optical force. The collection system consists of an inverted microscope equipped with a CCD camera, a square capillary connected with a PTFE tube, and an Nd:YAG laser that generates optical force. The laser beam was focused on a capillary wall in which a cell culture medium containing EVs flowed continuously. Control of the surface charges on both the capillary wall and the AuNPs achieved the collection and retention of EVs on the capillary wall. The positively charged capillary wall retained EVs even after the laser irradiation was halted due to the negative charges inherent on the surface of EVs. Conversely, positively charged AuNPs had a strong electrostatic interaction with EVs and enhanced the optical force acting on them, which made collecting them a much more efficient process.

In Situ Detection of Chemical Compositions at Nanodroplet Surfaces and In-Nanodroplet Phases

Small-volume nanodroplets play an increasingly common role in chemistry and biology. Such nanodroplets are believed to have unique chemical and physical properties at the interface between a droplet and its surrounding medium, however, they are underexamined. In this study, we present the novel technique of vibrational sum frequency scattering (VSFS) spectroscopy as an interface-specific, high-performance method for the in situ investigation of nanodroplets with sub-micron radii; as well as the droplet bulk through simultaneous hyper-Raman scattering (HRS) spectroscopy. We use laboratory-generated nanodroplets from aqueous alcohol solutions to demonstrate this technique's ability to separate the vibrational phenomena which take place at droplet surfaces from the underlying bulk phase. In addition, we systemically examine interfacial spectra of nanodroplets containing methanol, ethanol, 1-propanol, and 1-butanol through VSFS. Furthermore, we demonstrate interfacial differences between such nanodroplets and their analogous planar surfaces. The sensitivity of this technique to probe droplet surfaces with few-particle density at standard conditions validates VSFS as an analytical technique for the in situ investigation of small nanodroplets, providing breakthrough information about these species of ever-increasing relevance.