UC San Diego

A new distance and force detection platform has been realized based on nanoparticle-nanofiber optic platform. The detection of distance is based on recording the scattering intensity of nanoparticle which is directly related with its distance away from the nanofiber. If there is a well characterized compressible polymer layer between nanoparticle and nanofiber, the force applied on the nanoparticle can be known based with the scattering intensity.

In order to fulfill the force sensing platform, two relationships have been established beforehand: one is uniform and stable polymer film on the nanofiber; another is an accurate calibration of the relationship between distance and scattering intensity. Firstly, different molecular weights of polyethylene glycol have been covalently grafted on the tin oxide nanofiber based on silane chemistry. Force-indentation under different ionic condition and tip modification show the steric force dominate the repulsion between nanofiber and AFM tip within the height of the polymer brush. Secondly, an easy method has been developed to link a single nanoparticle on the end of the AFM tip. A laser excitation system has been integrated with a bioscope AFM platform to directly record the relationship with distance with scattering intensity. Comparison with the reference nanoparticle on the nanofiber nearby show there is no influence of AFM tip on the scattering intensity of plasmonic nanoparticles in the evanescent field of the nanofiber.

With these two baselines, PEG layer has been sandwiched between nanoparticle and nanofiber and provide mechanical feedback for the force applied on the nanoparticle. With laser excitation and bioscope AFM system, a detail calibration of distance and force with the scattering intensity has been demonstrated. The distance sensitivity is measured to be down to angstrom-level; the force sensitivity is multiplied with the spring constant to be down to 160 fN. The new force sensing platform has been used to detect the miroflow force applied on the nanoparticles with active bacteria, the average stress is compared with AFM cantilever system and show higher force sensitivity due to its smaller size and lower damping coefficient. Acoustical frequency signatures from AFM tip modulation and cardiomyocytes beating have also been identified using the force sensing platform with an acoustic sensitivity of -30 dB. With the ability to tune mechanical response of the compressible fiber cladding, detect forces from multiple nanoscale sites on a single fiber, and a geometry that can be inserted into small volumes, the new force sensing platform will become a valuable tool for biomechanical and intracellular studies.