Simultaneous Force and Darkfield Measurements Reveal Solvent- Dependent Axial Control of Optically Trapped Gold Nanoparticles

Single molecule force spectroscopy using optical tweezers (OT) has enabled nanoresolved measurements of dynamic biological processes but not of 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.

Automated summary

Single molecule force spectroscopy using optical tweezers has been successful in measuring dynamic biological processes, but not in synthetic molecular mechanisms. In this study, we demonstrate the trapping of gold nanoparticles in both aqueous and organic conditions using a custom optical tweezers and darkfield instrument, allowing simultaneous measurement of the force and scattering spectra of single gold nanoparticles. Our findings reveal that standard trapping models developed for aqueous conditions cannot explain the observed trends in different media. We also determine that higher pushing forces can mitigate the trapping force increase in higher index organic solvents and lead to axial displacement of the particle, which can be controlled through trap intensity. This work establishes a new model framework incorporating axial forces for understanding nanoparticle dynamics in an optical trap, and proves the effectiveness of the combined darkfield optical tweezers with gold nanoparticles as a probe for single molecule and single particle spectroscopy experiments, with nanoscale control over particle location.