The advance of nanotechnology has boosted the development of ultra-sensitive biosensors for
biomedical applications. Most recently, optical detection based biosensors have been
demonstrated in medical imaging and diagnosis employing nanocrystals such as fluorescent
quantum dots (QDs) and plasmon resonant metal nanoparticles to achieve femto-molar detection.
An intriguing but far less explored approach for biological diagnostics relies on an emerging
ultrasensitive technology -- surface enhanced Raman scattering (SERS) spectroscopy. We have
developed a stable SERS nano-tag by grafting hydrophilic polymer to gold nanoparticle-dye
molecule complexes to preserve the spectral signature and fully control the aggregation states.
The light-emitting power and scattered light of both QDs and SERS nano-tags have been recorded
under the same experimental conditions using dark field microscope, fluorometer, and Raman
instrument. A comparison in brightness, sensitivity level, and quantum efficiency between SERS
nano-tags and near infrared (NIR) QDs has been assessed on both bulk colloidal solution and
single particle measurements. Well-designed SERS nano-tags exhibit excellent advantages over