  Detection of biomolecules is important in proteomics and clinical diagnosis
and treatment of diseases. Here, we apply functionalized, macromolecular,
single walled carbon nanotubes SWNTs as multi-color Raman labels to protein
arrays for highly sensitive, multiplexed protein detection. Raman detection
utilizes the sharp peaks of SWNTs with minimal background interference,
affording a high signal to noise ratio needed for ultra-sensitive detection.
Surface-enhanced Raman scattering SERS combined with the strong resonance Raman
intensity of SWNTs, affords detection sensitivity down to 1 fM, a three order
of magnitude improvement over most of reported fluorescence-based protein
detections. We show that human autoantibodies to Proteinase 3 aPR3, a biomarker
for the autoimmune disease Wegeners granulomatosis, is detected by Raman in
human serum up to a 107 dilution. Moreover, SWNT Raman tags are stable against
photobleaching and quenching, and by conjugating different antibodies to pure
12C and 13C SWNT isotopes, we demonstrate two-color SWNT Raman-based protein
detection in a multiplexed fashion.
