Advances in optical trapping beyond biophysics: combining force and optical spectroscopies under diverse chemical conditions
Date
2024
DOI
Authors
Version
OA Version
Citation
Abstract
Optical tweezers (OT) have revolutionized the study of molecular biology as recognized in 2018 by the Nobel prize in Physics to Arthur Ashkin, the inventor of the technique. OT allow selective single particle manipulation and control in solution at the focus of an optical microscope and in combination with other optical spectroscopies. These techniques have been used to apply forces to single molecules in order to measure dynamics and energetics of protein folding, motor protein translocation and RNA structural transitions, to name just a few. Yet, the capacity of OT for molecular mechanistic studies for chemistry and materials applications is vastly underexplored. The subject of this thesis is to develop approaches for expanding the capability of OT outside of the biological domain. In Chapter 2 we discuss address one of the main obstacles to applying OT to study synthetic molecular mechanisms. Standard OT probes made from silica or polystyrene are incompatible with trapping in organic solvents for solution phase chemistry or with force detected absorption spectroscopies. Here, we demonstrate optical trapping of gold nanoparticles in both aqueous and organic conditions using a custom OT and darkfield instrument which can uniquely measure force and scattering spectra of single gold nanoparticles (Au NPs) simultaneously. Our work reveals that standard models of trapping developed for aqueous conditions cannot account for the trends observed in different media here. We determine that higher pushing forces mitigate the increase in trapping force in higher index organic solvents and lead to axial displacement of the particle which can be controlled through trap intensity. This work develops a new model framework incorporating axial forces for understanding nanoparticle dynamics in an optical trap. These results establish the combined darkfield OT with Au NPs as an effective OT probe for single molecule and single particle spectroscopy experiments, with three-dimensional nanoscale control over NP location. Chapter 3 demonstrates an all-optical method using an optical tweezer to perform chemistry on a single particle in solution. Specifically, we controllably and selectively grow high quality zeolitic imidazolate framework (ZIF) nanoshells on the surface of a single gold nanoparticle (AuNPs) and monitor the growth via darkfield spectroscopy. Our single particle approach allows us to localize an individual NP within a microscope slide chamber containing ZIF precursors at the focus of an optical microscope and initiate growth through localized heating without affecting the bulk system. Darkfield spectroscopy is used to characterize changes to the localized surface plasmon resonance (LSPR) of the AuNP resulting from refractive index changes as the ZIF crystal grows on the surface. We show that the procedure can be generalized to grow various types of ZIF crystals, such as ZIF-8, ZIF-11, and a previously undocumented ZIF variety. Utilizing both computational models and experimental methods, we identify the thickness of ZIF layers to be self-limiting to ∼50 nm or less, depending on the trapping laser power. Critically, the refractive index of the shells here was fou nd to be above 1.6, indicating the formation of high-density crystals, previously accessible only through slow atomic layer deposition and not through a bulk heating process. The single particle method developed here opens the door for bottom-up controllable growth of custom nanostructures with tunable optical properties. Chapter 4 of the thesis introduces an approach to studying and controlling gold nanoparticle (AuNP) dimers suspended in solution using optical trapping. The primary objective is to control inter-particle separation in AuNP dimers using OT and to leverage the plasmon scattering resonance signature to measure it in situ. This functionality is crucial for applications in nanophotonics, nanoelectronics, and biosensing, where accurate distance control between nanoparticles can lead to the development of highly sensitive sensors and devices. We use the custom optical trapping instrument described in Chapter 1 that combines an inverted optical microscope with darkfield (DF) illumination, allowing for the manipulation and imaging of metallic nanoparticles. We show that the dimer long axis aligns with the trapping laser polarization, allowing for control of dimers in solution. When the long axis of the dimer is parallel to the excitation polarization, the dimer scattering resonance is maximized, enabling more precise spectroscopic analysis. The shifts in dimer wavelengths with changing inter-particle separation are modeled computationally, leading to the development of a plasmon ruler equation for our system, which allows for conversion between optical wavelength shifts and distance. We form dimers using van der Waals sticking between polymer coated AuNPs. We find that dimers formed with distinct molecular tethers differing distributions of inter-particle distances which depend on molecule length and structure. Furthermore, by tuning the power of the OT laser, we can modulate the temperature at the dimer surface, causing the release of inter-molecular interactions and a gradual separation of the dimer particles. This experiment is the first demonstration of control over dimer geometry in solution. By tracking the evolution of the plasmon spectra during heating and separation, we are able to distinguish between the classical capacitive coupling regime captured by the plasmon ruler relationship and the charge transfer plasmon (CTP) which arises from quantum tunneling in the dimer gap. The methodology established here provides unprecedented control over dimer geometry which can be leveraged for applications in plasmon-driven catalysis, surface-enhanced spectroscopy and charge transfer studies.
Description
2024
License
Attribution 4.0 International