Vibration-assisted optical injection of a single fluorescent sensor into a target cell

In this paper, we propose the selective adhesion and rapid injection of a fluorescent sensor into a target cell via the optical control of zeta potential and local vibration stimulus using optical tweezers. A multi-fluorescent sensor, which can respond to both temperature and pH, was encapsulated in anionic lipid layers containing a photochromic material (spiropyran) via the layer-by-layer method. The zeta potential of the lipid layers containing spiropyran was adjusted from negative to positive by photo-isomerization of spiropyran using UV illumination. A single sensor was manipulated by optical tweezers and transferred to a cell surface, thereafter adhering selectively to the cell surface under UV illumination without excess sensor adhesion. We then drove the focal point of the optical tweezers to move up and down circularly near the sensor, mimicking a vibration on the sensor or rapid injection. The surface zeta potential of the liposome layers was measured using a zeta potential analyzer. The fluorescence resonance energy transfer (FRET) method was used to observe the changes in contact area between the adhered sensor and cell membrane before and after vibration. Holographic optical tweezers (HOT) and laser confocal microscopy were used to manipulate the single sensor and to capture fluorescent images. The results showed that the vibration applied on the sensor could push down the sensor, inducing a downward displacement. This displacement caused a corresponding deformation of the cell membrane, which increased the contact area between the sensor and the cell membrane. Without vibration, the sensor was injected into the cytoplasm in 5 h at an injection rate of 40%. By applying the vibration stimulus, we succeeded in the rapid injection of the sensor in 30 min at an injection rate of 80%.