Collecting single molecules with conventional optical tweezers

The size of particles that can be trapped in optical tweezers ranges from tens of nanometers to tens of micrometers. This size regime also includes large single molecules. Here we present experiments demonstrating that optical tweezers can be used to collect polyethylene oxide molecules suspended in water. The molecules that accumulate in the focal volume do not aggregate and therefore represent a region of increased molecule concentration, which can be controlled by the trapping potential. We also present a model that relates the change in concentration to the trapping potential. Since many protein molecules have molecular weights for which this method is applicable the effect may be useful in assisting nucleation of protein crystals.

Figure 1

Schematic diagram of the experimental setup. The ytterbium fiber laser was used to create the trapping potential while the local increase in the molecule concentration was monitored by collecting the scattered light of the collinear $\mathrm{He}\text{−}\mathrm{Ne}$ laser. A beam block in the detection beam path increased the sensitivity of the detection system.

Figure 2

Typical time series of the scattered $\mathrm{He}\text{−}\mathrm{Ne}$ light intensity (in arbitrary units) as a result of the concentration increase due to a trapping potential created by the trapping laser (switched on at $t=0$). The solid line shows a fitted curve using the function $S=S_0(1-e^{-t∕\tau })$.

Figure 3

(a) Scattered $\mathrm{He}\text{−}\mathrm{Ne}$ light intensity (in arbitrary units) for a stepwise changing power of the trapping laser (dashed lines). Graph is for PEO with molecular weight $m_w=300\mathrm{kDa}$ and an initial PEO concentration of $0.027\mathrm{wt}\%$, the trapping laser powers in the different sections (I–VII, left to right) were 0.58, 0.10, 0.60, 0.10, 0.64, 0.10, and $-0.7\mathrm{W}$. (b) Summary of collection rates $R$ for PEO with molecular weights $m_w=300$ $(*)$ and $900\mathrm{kDa}$ $(▿)$. The data points at $0.1\mathrm{W}$ give the reciprocal of the time constants for the diffusion-driven decrease extracted from (d). (c) Increase of $\mathrm{He}\text{−}\mathrm{Ne}$ light intensity for different laser powers [taken from (a)], fitted with $S=S_0(1-e^{-t∕\tau })$. (d) For laser powers below the threshold the $\mathrm{He}\text{−}\mathrm{Ne}$ light intensities decreased exponentially with time.

Figure 4

Calculated surface area of the equipotential contour of the trap for a potential depth of $kT$ for different molecular weight PEO molecules. The dash-dotted line gives the surface area of the event horizon for PEO molecules with molecular weight of $900\mathrm{kDa}$, the dotted line for $300\mathrm{kDa}$, and the solid line for $100\mathrm{kDa}$.