  Label-free biosensors are important tools for clinical diagnostics and for
studying biology at the single molecule level. The development of optical
label-free sensors has allowed extreme sensitivity, but can expose the
biological sample to photodamage. Moreover, the fragility and complexity of
these sensors can be prohibitive to applications. To overcome these problems,
we develop a quantum noise limited exposed-core fiber sensor providing robust
platform for label-free biosensing with a natural path toward microfluidic
integration. We demonstrate the detection of single nanoparticles down to 25 nm
in radius with optical intensities beneath known biophysical damage thresholds.
