Near-Infrared Photoluminescence\nResponses of Single-Walled\nCarbon Nanotubes Induced by Biomolecules Detected on a Microbead Surface

We demonstrate near-infrared (NIR) photoluminescence\n(PL) microscopy\nof single-walled carbon nanotubes (SWNTs) attached to individual micrometer-sized\nbead surfaces. Monodispersed SWNTs wrapped with sodium carboxymethyl\ncellulose (CMC-SWNT hybrids) were attached to the microbead surfaces,\nand the functionalized beads were immobilized on a coverslip. It is\nknown that NIR PL spectra of the SWNTs were quenched upon exposure\nto potassium permanganate (KMnO<sub>4</sub>) and recovered by the\naddition of papain molecules by macroscopic PL spectroscopy. In this\nstudy, the PL responses were successfully detected on a microbead\nsurface by NIR microscopy. In particular, PL responses of (6, 5) chirality\nSWNTs were selectively detected by using two bandpass filters. When\nKMnO<sub>4</sub> (final concentration: 1 μM) was dropped onto\nthe coverslip surface, the PL intensity of the SWNTs decreased to\napproximately 20% of the initial intensity without KMnO<sub>4</sub>. The average PL intensity of each bead was estimated from the microscopy\nimages. When a papain solution (final concentration: 5 mg/mL) was\nadded to the same sample, the PL intensity recovered to 60–70%\nof the initial intensity. A similar recovery was observed with papain\nsolutions preheated at 60 and 100 °C. The microscopic observation\nof the beads was performed sequentially. Although the PL responses\ninduced by papain molecules have been reported in previous studies\nusing macroscopic PL spectroscopy, this is the first report to demonstrate\nresponses on a single-bead surface. Our results hold significance\nfor investigating enzyme reactions using SWNT PL responses at the\nnano and micro levels.