Wavelength-Dependent Optical Force Aggregation of Gold Nanorods for SERS in a Microfluidic Chip

Optical printing of metal-nanoparticle-protein complexes in microfluidic chips is of particular interest in view of the potential applications in biomolecular sensing by surface-enhanced Raman spectroscopy (SERS). SERS-active aggregates are formed when the radiation pressure pushes the particle-protein complexes on an inert surface, enabling the ultrasensitive detection of proteins down to pM concentration in short times. However, the role of plasmonic resonances in the aggregation process is still not fully clear. Here, we study the aggregation velocity as a function of excitation wavelength and power. We use a model system consisting of complexes formed of gold nanorods featuring two distinct localized plasmon resonances bound with bovine serum albumin. We show that the aggregation speed is remarkably accelerated by 300 or 30% with respect to the off-resonant case if the nanorods are excited at the long-axis or minor-axis resonance, respectively. Power-dependent experiments evidence a threshold below which no aggregation occurs, followed by a regime with a linear increase in the aggregation speed. At powers exceeding 10 mW, we observe turbulence, bubbling, and a remarkable 1 order of magnitude increase in the aggregation speed. Results in the linear regime are interpreted in terms of a plasmon-enhanced optical force that scales as the extinction cross section and determines the sticking probability of the nanorods. Thermoplasmonic effects are invoked to describe the results at the highest power. Finally, we introduce a method for the fabrication of functional SERS substrates on demand in a microfluidic platform that can serve as the detection part in microfluidic bioassays or lab-on-a-chip devices.