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. 2016 Jun 1;6:26952. doi: 10.1038/srep26952

Figure 1. LIQUISOR concept.

Figure 1

(a) The LIQUISOR employs gold NRs (35 nm diameter, 90 nm length), capped with Cetyltrimethylammonium Bromide (CTAB) and dispersed in deionized water. NRs are mixed with the target biomolecules dissolved in phosphate saline (b). Upon mixing, the biomolecules + PBS action destabilizes the CTAB and bind to the NRs (c). A tiny aliquot of the biomolecule-NRs solution (75 μL) is pipetted into a hemispheric glass microcell (d), sealed with a coverslip and put under a Raman micro-spectrometer. A laser beam is focused on a micron scale spot in liquid, close to the sidewalls of the microcell. Calculations of the optical forces show that for field polarization parallel to the NRs long axis, the gradient force (e, red line) has a repulsive nature, due to the blue shift of the laser energy with respect to the long axis LSPR. The scattering force (e, blue line) is even more intense. When the NR is oriented along to the propagation direction, k, i.e. field parallel to the short axis, the gradient force (f, red line) shows an attractive, “trapping” character, but also in this case the magnitude scattering force is markedly higher (f, blue line). In both orientations the net optical force acting on the NRs is positive, i.e., tends to expel the rod from the laser focus along the propagation direction. The relative z = 0 position refers to the center of the laser spot. The z axis is parallel to the wavevector k that points “downward” in our experimental configuration. The NRs are, consequently pushed towards the bottom of the cell, inducing the formation of SERS-active aggregates that embed the biomolecules (g). Laser scattering (h) and bright field images (i) show that aggregates can reach micron scale dimensions after several tens of minutes. (l) SEM image of a further larger aggregate. The zoom (m) highlights the presence NRs aggregated (white box) and kneaded with the biomolecules. Sparse individual NRs (blue box) are also visible on some points. (n) Locally NRs aligned in a side-by-side configuration can be found.