The stabilization of carbon nanotubes via the filling with inner tubes is demonstrated by probing the optical transitions in double-walled carbon nanotube bundles under hydrostatic pressure with optical spectroscopy. Double-walled carbon nanotube films were prepared from fullerene peapods and characterized by HRTEM and optical spectroscopy. In comparison to single-walled carbon nanotubes, the pressure-induced redshifts of the optical transitions in the outer tubes are significantly smaller below ∼10 GPa, demonstrating the enhanced mechanical stability due to the inner tube already at low pressures. Anomalies at the critical pressure P d ≈12 GPa signal the onset of the pressure-induced deformation of the tubular cross-sections. The value of P d is in very good agreement with theoretical predictions of the pressure-induced structural transitions in double-walled carbon nanotube bundles with similar average diameters.

We present a high‐pressure optical spectroscopy study on double‐walled carbon nanotubes with dinner≈0.80 nm and douter≈1.45 nm using nitrogen, argon, and alcohol‐mixture as pressure transmitting medium (PTM). The pressure‐induced redshift of the optical transitions in the outer tubes is very small below 10 GPa, demonstrating the enhanced mechanical stability due to the inner tube. An anomaly at the critical pressure Pd ≈ 12 GPa signals the onset of the pressure‐induced deformation of the tubular cross‐sections. Using argon as PTM affects the results quantitatively: The Pd value is lower and the absorption bands are broadened considerably. Furthermore, alcohol‐mixture as PTM seems to destroy the tubes completely above 10 GPa due to its solidification. The results are compared with those for single‐walled carbon nanotubes.

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The high-pressure behavior of single-walled carbon nanotubes (SWCNTs) filled with C 60 molecules, socalled C 60 @SWCNT peapods, has been investigated by optical spectroscopy using nitrogen and argon as pressure transmitting media. Peapods with a high filling ratio were prepared by the sublimation method and characterized by high-resolution transmission electron microscopy. Optical transmission measurements under high pressure were conducted on films of SWCNTs and C 60 @SWCNT peapods. As with SWCNTs, the absorption bands observed for peapods exhibit a red-shift under pressure. The relative energy shifts of the optical transitions under pressure are higher than those for empty SWCNTs, demonstrating enhanced hybridization and/or symmetry breaking effects. We find an anomaly in the pressure-induced shifts of the optical transitions at P c1 ≈ 6.5 GPa (P c1 ≈ 5.5 GPa) when using nitrogen (argon) as pressure transmitting medium. The anomaly signals the deformation of the nanotube from circular to an oval shape. The value of P c1 is in good agreement with theoretical predictions of the pressure-induced deformation for highly filled peapods with similar average diameter. A plateau in the pressure-induced shifts with an onset at P c2 = 12−13 GPa indicates a saturation of the pressure-induced effects above this critical pressure.

Ivabradine hydrochloride (IVR) is a medically important drug because of its ability to lower the heart rate. Techniques reported for IVR determination were expensive, laborious, besides being of poor selectivity. In this study, iron oxide @ carbon nanotube (Fe2O3@MWCNTs) nanocomposite and molecularly imprinted polymer (MIP) were synthesized and used in the fabrication of carbon paste electrodes (CPEs) for the potentiometric detection of IVR in biological and pharmaceutical samples. CPEs of the best sensor were formulated from graphite (41 wt%) as a carbon source, MIP (3 wt.%) as an ionophore, Fe2O3@MWCNTs (5 wt%) as a modifier, and nitrophenyl octyl ether (NPOE, 51 wt.%) as a conductive oil so-called plasticizer. The best sensor exhibits a Nernstian slope (response) of 56 mV decade–1 within the IVR concentration range from 1.0 × 10–3 M to 9.8 × 10–8 M with high selectivity against interfering species (ascorbic, maltose, glucose, lactose, dopamine, glycine) over those reported earlier. The use of Fe2O3@MWCNTs together with MIP in the electrode formulation was found to improve the limit of detection (LOD) from 630 to 98 nM along with high reversibility, a short response time of 30 s, and a good lifetime of more than 2 weeks. The sandwich membrane (SMM) method was used to quantify the H-bonding complexing strength of the MIP binding sites for IVR with Log βILn = 11.33. The constructed sensors were successfully applied for the IVR determination in blood serum, urine, and commercial formulations (Savapran®) with high sensitivity.