| The biological application of optical and spectroscopic probes is the science of making visible the invisible. Most biochemistry occurs on a scale too small to be seen, and optically active nanoparticles are presently an intensely interesting way to shed light on biomolecules and their interactions. This research describes two classes of ultra-small particles and their application to bioanalytical problems: noble metal nanoclusters and semiconductor nanocrystals (quantum dots or QDs). Each of these materials has desirable optical properties that can be exploited in analytical applications, molecular and cell biology, and medical diagnostics.; Gold and silver particles exhibit two distinct scattering phenomena: Surface Enhanced Raman Scatting (SERS) and plasmon resonant Rayleigh scattering. To use SERS as the detection modality, a metal nanoparticle is simultaneously loaded with a biomolecule and a reporter molecule. The reporter molecule, typically an organic dye, provides a spectroscopically distinct "fingerprint" that can be associated with the coadsorbed biomolecule. The particle serves as both scaffold and SERS substrate. For example, these enhancing particles are used to rapidly and accurately identify cancerous tissues in a microarray format. Larger (40--200 nm) particles are used can be visualized directly under a microscope as plasmon resonant scatterers. Such particles exhibit size tunable coloration, and are extremely bright, with photon fluxes exceeding those of single fluorescein by a million fold. Microscopic detection of single particles enables visualization of DNA and protein interactions.; QDs, here cadmium chalcogenides smaller than the exiton (3--10 nm), have broad excitation profiles, narrow spectral line widths, size tunable emission, and long term photostability. These intensely fluorescent particles can be efficiently excited by a single source, and are ideal for multiplex applications, such a flow cytometry. Covalent coupling of DNA, proteins or peptides renders these particles useful in a wide array of biological applications. This research addresses both the in vitro cytotoxicity and the bio-distribution and accumulation of QD bioconjugates, and demonstrates the successful use of QD bioconjugates for retrograde neural tracing. Further, QDs are used to label and encode paramagnetic microspheres, which have enormous potential as multi-purpose probes with an integral means of separation. |
