Ultrafast sensing of single-particle dynamics with an optofluidic microcavity

Nanoparticles are everywhere, and as they become increasingly relevant in science,
medicine and technology, increasingly sophisticated methods are required to investigate
them. Since most biochemistry occurs in an aqueous environment, techniques to measure
nanoparticles in their natural environment are desired. Many methods for the detection
of nanoparticles dispersed in a liquid rely on fluorescently labelling the samples with
fluorophores which are not only often costly or toxic, but also may alter the behaviour of
the sample itself. As a result, it is of high relevance to study label-free sensing techniques.
A Fabry-Perot microcavity allows the detection of unlabelled particles thanks to the
cavity-enhancement of interactions between the optical field and a nanoparticle. In the
form presented in this work, the microcavity is integrated into a microfluidic platform,
allowing the sample open access to the cavity field. Furthermore, a detection scheme with
an actively stabilised cavity is adopted, offering a high sensitivity and high measurement
bandwidth. Single gold nanospheres down to 3 nm in diameter could thus be detected. ... mehrTo
enable the quantitative analysis of single-particle measurements, an existing theoretical
model of diffusion was used to derive an analytical autocorrelation function which allowed
the direct determination of the diffusion coefficient and diameter of individual nanospheres,
and the results hence obtained were in excellent agreement with measurements using other
methods.
The high-bandwidth measurement capability of the stabilised microcavity was then
exploited for the measurement of the dynamics of diffusing nanorods, which also exhibit
rotational diffusion on the microsecond scale. Simultaneously analysing diffusion
and rotation will allow the determination of both the translational and rotational
diffusion coefficients, and hence the length and diameter of nanorods. Additionally, a
polarisation-split detection scheme was developed to enable the simultaneous measurement
of orthogonal polarisation modes in the cavity, allowing the orientation of individual
diffusing nanorods to be tracked in two dimensions with a temporal resolution of 20 ns.
The sensitivity of this nanosensor already approaches that of single-molecule sensing
techniques, and its relevance to biosensing was demonstrated by sensing ensembles of
∼ 10 protein molecules, as well as single DNA nanostructures. This high sensitivity, fast
measurement speed and possibility of direct quantitative analysis of particle dynamics
make the microcavity a promising technique for the investigation of various nanosystems
and biochemical processes on the nanoscale.