We experimentally investigated remotely excited Raman optical activity (ROA) using propagating surface plasmons in chiral Ag nanowires. Using chiral fmoc-glycyl-glycine-OH (FGGO) molecules, we first studied the local surface plasmon-enhanced ROA. We found that the Raman intensity can be excited by left-and right-circularly polarized lights and that the circular intensity difference (CID) can be significantly enhanced. Second, by selecting vibrational modes with large Raman and ROA intensities that are not influenced by chemical enhancements, we studied remotely excited ROA imaging and the CID of FGGO molecules by propagating a plasmonic waveguide using Ag chiral nanostructures. When laser light was radiated on one of the Ag terminals, the measured CID of the FGG at the other terminal showed little change compared to the local excited CID. Meanwhile, when the laser light was radiated on the Ag nanowires (not on the terminals) and was coupled to the nearby nanoantenna, the CID of the ROA could be manipulated by altering the coupling angle between the Ag nanowires. To directly demonstrate the propagation of ROA along the nanowire and its remote detection, we also measured the remotely excited ROA spectra. Our experimental method has the potential to remotely determine the chirality of molecular structures and the absolute configuration or conformation of a chiral live cell.

No abstract

No abstract

In this paper, we propose a solution for the ultrasensitive optical selection of plasmonic nanoparticles using Fano interference-induced scattering forces. Under a Gaussian beam excitation, the scattering of a plasmonic nanoparticle at its Fano resonance becomes strongly asymmetric in the lateral direction and consequently results in a net transverse scattering force, that is, Fano interference-induced force. The magnitude of this transverse scattering force is comparable with the gradient force in conventional optical manipulation experiments. More interestingly, the Fano scattering force is ultrasensitive to the particle size and excitation frequency due to the phase sensitivity of the interference between adjacent plasmon modes in the particle. Utilizing this distinct feature, we show the possibility of size-selective sorting of silver and gold nanoparticles with an accuracy of about ±10 nm and silica-gold core-shell nanoparticles with shell thickness down to several nanometers. These results would add to the toolbox of optical manipulation and fabrication.

A method is developed to synthesize surface-enhanced Raman scattering (SERS) materials capable of single-molecule detection, integrated with a microfluidic system. Using a focused laser, silver nanoparticle aggregates as SERS monitors are fabricated in a microfluidic channel through photochemical reduction. After washing out the monitor, the aggregates are irradiated again by the same laser. This key step leads to full reduction of the residual reactants, which generates numerous small silver nanoparticles on the former nanoaggregates. Consequently, the enhancement ability of the SERS monitor is greatly boosted due to the emergence of new "hot spots." At the same time, the influence of the notorious "memory effect" in microfluidics is substantially suppressed due to the depletion of surface residues. Taking these advantages, two-step photoreduced SERS materials are able to detect different types of molecules with the concentration down to 10 m. Based on a well-accepted bianalyte approach, it is proved that the detection limit reaches the single-molecule level. From a practical point of view, the detection reproducibility at different probing concentrations is also investigated. It is found that the effective single-molecule SERS measurements can be raised up to ≈50%. This microfluidic SERS with high reproducibility and ultrasensitivity will find promising applications in on-chip single-molecule spectroscopy.