Abstract

We prepared single-walled carbon nanotube (SWNT) suspensions in phosphate buffer solutions containing 1% of a coconut-based natural detergent (COCO) or 1% of sodium dodecyl sulfate (SDS). The suspensions exhibited strong photoluminescence (PL) in the near-infrared region, suggesting that the SWNTs, such as those with (9, 4) and (7, 6) chiralities, were monodispersed. Upon diluting the suspensions with a detergent-free phosphate buffer solution, the PL intensity of the SDS-containing SWNT suspension was significantly lower than that of the COCO-containing SWNT suspension. The COCO-containing SWNT suspension was more stable than the SDS-containing SWNT suspension. The SWNT concentration of the suspensions prepared via bath-type sonication was lower than that of the suspensions prepared via probe-type sonication. However, near-infrared (NIR) PL intensity of the SWNT suspensions prepared via bath-type sonication was much higher than that of the SWNT suspensions prepared via probe-type sonication regardless of the detergent. This suggested that the fraction of monodispersed SWNTs of the suspensions prepared via bath-type sonication was larger than that of the suspensions prepared via probe-type sonication, although the SWNT concentration was low. Our results indicated that COCO favored the fabrication of SWNT suspensions with stable and strong NIR PL, which are useful for various biological applications.

In this study, two biomolecule solutions were distinguished using the capacity difference in the near-infrared photoluminescence (PL) of single-walled carbon nanotubes (SWNTs). Biosensing techniques using sensitive responses of SWNTs have been intensively studied. When a small amount of an oxidant or reductant solution was injected into the SWNT suspensions, the PL intensity of the SWNTs is significantly changed. However, distinguishing between different molecules remains challenging. In this study, we comparably injected saponin and banana solutions, which are known antioxidant chemicals, into an SWNT suspension. The SWNTs were solubilized by wrapping them with DNA molecules. The results show that 69.1 and 155.2% increases of PL intensities of SWNTs were observed after injection of 20 and 59 μg/mL saponin solutions, respectively. Subsequently, the increase in PL was saturated. With the banana solution, 18.1 and 175.4% increases in PL intensities were observed with 20 and 59 μg/mL banana solutions, respectively. Based on these results, the two antioxidant molecules could be distinguished based on the different PL responses of the SWNTs. In addition, the much higher saturated PL intensities observed with the banana solution suggests that the banana solution increased the capacity of the PL increase for the same SWNT suspension. These results provide helpful information for establishing biosensing applications of SWNTs, particularly for distinguishing chemicals.

We fabricated a micron-sized biodevice based on the near-infrared photoluminescence (PL) response of single-walled carbon nanotubes (SWNTs). Various biosensors using the unique optical responses of SWNTs have been proposed by many research groups. Most of these employed either colloidal suspensions of dispersed SWNTs or SWNT films on flat surfaces, such as electrodes. In this study, we attached DNA-wrapped SWNTs (DNA-SWNTs) to frustule (micron-sized nanoporous biosilica) surfaces, which were purified from cultured isolated diatoms. After the injection of an oxidant and a reductant, the SWNTs on the frustules showed prominent PL responses. This suggests that the biodevice functions as a micron-sized redox sensor. Frustules can be easily suspended in aqueous solutions because of their porous structures and can easily be collected as pellets by low-speed centrifugation. Thus, the removal of unbound SWNTs and the recovery of the fabricated DNA-SWNT frustules for reuse were achieved by gentle centrifugation. Our proposal for micron-sized SWNT biodevices would be helpful for various biological applications.

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