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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Detection of single molecules in microspheres

Michael D. Barnes, William Barclay Whitten, J. Michael Ramsey, Kim Chew Ng, S. Arnold

Published May 18, 1993• 3 citations
Publisher

Abstract

We have investigated the use of micron-sized liquid droplets as sample medium to detect single fluorescent molecules in solution. The use of microdroplets (5 - 15 micrometers diameter) offers several powerful advantages over single-molecule detection schemes involving measurements on bulk liquids where the probe volume is defined by the laser beam. In addition, cavity-quantum electrodynamical (QED) effects have been observed which influence both spontaneous emission rates and fluorescence yields of dye molecules in these microspheres.

DOI
10.1117/12.144732

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We have investigated the use of micron-sized liquid droplets as sample medium to detect single fluorescent molecules in solution. The use of microdroplets (5 - 15 micrometers diameter) offers several powerful advantages over single-molecule detection schemes involving measurements on bulk liquids where the probe volume is defined by the laser beam. In addition, cavity-quantum electrodynamical (QED) effects have been observed which influence both spontaneous emission rates and fluorescence yields of dye molecules in these microspheres.
A significant limitation in the detection and spectroscopy of individual dye molecules in liquid room temperature samples is the loss of fluorescence signal through chromophore photobleaching. Although often considered a fundamental restriction on sensitivity, it has been shown that the probability per excitation cycle of photobleaching may be reduced by enhancing the spontaneous emission rate of the chromophore located inside a microdroplet, which acts as a optical cavity. Studies of glycerol microdroplets containing R6G dye molecules, and levitated in a three-dimensional electrodynamic trap, have demonstrated a significant enhancement of both the fluorescence decay rate [1] and the total fluorescence yield [2] of the chromophore. Modification of the emission rate is greatest for molecules located near the surface of small (< 10 μm) microdroplets, where the influence of the droplet cavity modes of the droplet is most significant. To exploit this surface sensitivity, surfactant forms of rhodamine dyes were recently studied [3]. Fluorescence emission of the dye molecules, constrained to the droplet surface and with fixed transition moments perpendicular to the surface normal, were shown to be significantly enhanced at small droplet sizes. Additionally, at larger droplet sizes corresponding to higher droplet Q’s, preferential emission into the cavity modes was observed, however the detection of fluorescence photons was delayed by the storage time of the cavity, which may be on the same order as the fluorescence lifetime. These measurements, performed on relatively concentrated dye solutions, suggest that microcavity effects may be exploited to greatly enhance single molecule detection sensitivity in glycerol microdroplets. Furthermore, the large cavity storage time of the high-Q droplets opens the possibility for re-excitation of the molecule following emission into the droplet resonance, and subsequent stimulated emission, that is, single-molecule lasing.
We describe recent results of experiments probing single molecules in the surface of spherical microcavities. Effects of spatial photoselection have been recently observed through changes in the single-molecule fluorescence photon statistics. In addition to being a sensitive probe of the highly interesting boundary region between classical and quantum mechanics, cavity-modified spontaneous emission and the possibility of single-molecule stimulated emission hold exciting promise as a means of exceeding sensitivity limitations such as photobleaching and absorption saturation in single molecule fluorescence detection. Ongoing efforts such as fluorescence imaging of single molecules on the surface of microspheres and attempts to observe single- molecule stimulated emission will also be described.
Significant cavity QED enhancement have been observed of both spontaneous and stimulated emission in 10- to 14-micrometers diameter liquid microdroplets containing fluorophors. Direct emission lifetime reduction was observed as well as greatly enhanced lasing gain. Use of droplets provides an excellent test of cavity QED theory for spherical geometries. The mode number and order of the spherical cavity resonances responsible for the observed emission peaks were identified and their Q values calculated from Lorenz-Mie theory. An experimental procedure, based on equating stimulated gain to cavity losses, is used to estimate the magnitude of the enhancement. Spherical cavity Qs of only 2600 were found to lead to cavity enhancements of over 100 in the emission cross-section of Rhodamine 6G in agreement with theory.
Measurements of the integrated fluorescence yield of Rhodamine 6G (R6G) in levitated microdroplets (4 to 16 {mu}m diameter) display a size dependence which is attributed to a decreased probability per excitation cycle of photochemical bleaching as a result of cavity-enhanced spontaneous emission rates. The average number of fluorescence photons detected per molecule in 4 {mu}m droplets (where emission rate enhancement has been previously demonstrated) is shown to be approximately a factor of 2 larger than the yield measured for larger droplets where emission rate enhancement does not occur. Within some simple approximations, these results suggest that essentially no emission rate inhibition occurs in this system. A mechanism based on spectral diffusion is postulated for the apparent absence of cavity-inhibited emission and is illustrated by Monte Carlo calculations using time-dependent lineshape functions.
Micrometer-sized transparent droplets act as natural high-Q cavities to visible light, and this paper shows them to exhibit strongly enhanced dye emission owing to cavity quantum-electrodynamic effects. A vibrating-orifice aerosol generator1 was used to produce a linear stream of virtually identical (less than 2 parts in 105 diameter variation) falling droplets 14 μm diameter, 10-5 M solution of Rhodamine 6G in ethanol; the droplets were excited by a cw argon-ion laser. Droplet emission was examined with a double 1 m Spex spectrograph and a photomultiplier. Recent development of elastic-scattering-size spectroscopy1,2 allowed identification of the various cavity modes contributing to the emission. This was necessary because of the different Q values and spatial properties of the resonant modes. Q values were calculated from Lorenz-Mie theory and were found to be consistent with observed fluorescence linewidths for the experimentally resolvable fourth-order modes. It was experimentally determined that Q's of only 103 were sufficient to observe sizable enhancements in the A coefficient. Lasing was observed to occur at reduced thresholds, e.g., 400 W/cm2 for fourth order modes having Q's of 2 × 103. This corresponds to a cavity induced enhancement in the B coefficient of 103.