Nanofiber Near-Field Light–Matter Interactions for Enhanced Detection of Molecular Level Displacements and Dynamics

Abstract

We experimentally demonstrate that plasmonic nanoparticles embedded in the evanescent field of subwavelength optical waveguides (WGs) are highly sensitive to distances normal to the propagation of light, showing an ~10× increase in spatial resolution compared to the optical field decay of the WG. The scattering cross-section of the Au nanoparticle is increased by the plasmon-dielectric coupling interaction when the nanoparticle is placed near the dielectric surface of the WG, and the decay of the scattering signal is enhanced, showing angstrom level distance sensitivity within 10 nm from the WG. Numerical studies with the finite-difference time-domain (FDTD) method correlate well with the experimental results. To demonstrate real-time monitoring of a single molecule stretching in the evanescent field, we linked individual single-stranded DNA molecules between the WG and plasmonic nanoparticles and pushed on the nanoparticles with fluidic forces. The simple design and ease of obtaining optical feedback on molecular displacements makes our approach ideal for new in situ force sensing devices, imaging technologies, and high-throughput molecular analysis.

Quantitative mechanical analysis of thin compressible polymer monolayers on oxide surfaces

“…It is not only important to control the thickness and mechanical properties of thin polymer lms for understanding molecular interactions with target materials in vitro and in vivo, but compressible lms can also be used for mechanical feedback in novel nanoscale sensor designs for biological applications. 9,10 There are several methods to study the mechanical properties of ultrathin polymer lms in the dry state; [11][12][13][14] however, there are currently few reports on the stiffness of thin, uniform polymer monolayers deposited on oxide nanoparticles and/or oxide nanober structures in the liquid state, which is likely rooted in the challenges associated with synthesizing conformal, uniform monolayer lms and the difficulties in accurately quantifying the mechanical properties at the nanometer scale at the same time. [15][16][17][18] Many theories have been proposed to explain the interaction forces involved in nanometer thickness polymer deformations.…”

Gap controlled plasmon-dielectric coupling effects investigated with single nanoparticle-terminated atomic force microscope probes

“…These experiments resulted in slightly different scattering decays compared to those our group measured using polymer spacers to incrementally separate the NPs from the WG surface. 13,14 The simulations and experimental data in this work suggest that the AFM tip and NP position does not interfere with the scattering experiments, so this is an unlikely cause for the different decay constants. Besides the strategy for controllably separating the NP from the waveguide, the collection geometry for this work is also different.…”

“…In the case of a plasmonic nanoparticle (NP) and a dielectric surface, one surface has to be moved relative to the other and either held fixed while an optical measurement is taken or the system has to allow realtime optical feedback while separation distances are modulated. Of the many methods to control gap separation between a metal nanoparticle and dielectric surface which include 3 polyelectrolytes multilayers, [11][12][13][14] self-assembled monolayers, 15,16 and thin oxide layers grown on the NP, 7 the direct manipulation of a single NP using scanning probe methods is preferred as it provides the most straightforward and clean method to reproducibly probe interactions close to a surface. In fact, atomic force microscope (AFM) tips modified with metal NPs have been used for force indentation and tapping-mode imaging as well as to discover the unique optical phenomena such as single molecule fluorescence and second-harmonic generation of dyes.…”

“…13 The NP-WG separation distance in this system is monitored by simply measuring the far-field scattering intensity. The large enhancement in the distance-dependent scattering is due to a combination of effects including the strong plasmon-dielectric coupling between the NP and WG surfaces as well as the exponentially decaying evanescent field of the WG.…”

Nanofibre optic force transducers with sub-piconewton resolution via near-field plasmon–dielectric interactions

“…17). Since the position of the plasmonic nanoparticles can be optically tracked in the far-field with ångström-level spatial resolution 18 , forces on the nanoparticles can be extracted by monitoring the scattering intensity, assuming the mechanical properties of the polymer cladding are well characterized. The force sensitivity of the NP sensor can be calculated by multiplying the distance sensitivity ( D sensitivity ) and the spring constant ( k PEG ) of the PEG film.…”