Host–Guest Complexation of a Lipoic Acid Conjugate of Calix[4]arene with Pyridinium Moiety on Gold Nanorods for Mitochondrial Tracking Followed by Cytotoxicity in HeLa Cells under 633 nm Laser Light

A new 1,3-lower rim-lipoic acid appended calix[4]­arene (L) has been synthesized and characterized by different spectral techniques. Lipoic acid functionality helps L to anchor onto a noble metal surface, and in the present case these are the gold nanorods (GNRs). As the conjugate of calix[4]­arene acts as a host due to the presence of its arene cavity, pyridinium containing guests have been explored to study their complexation, since the pyridinium containing molecules will accumulate in mitochondria in cells. The host–guest complexation has been explored in solution by 1H NMR, ESI-MS, UV–vis, and fluorescence spectral titration studies. The loading of host and the guest onto the GNR surface has been analyzed by spectroscopy and microscopy and found that their combination results in nanocomplexation. The presence of pyridinium functionality on the guest leads to targeted delivery to mitochondria of GNR nanocomposites as shown by confocal laser scanning microscopy imaging. The nanocomplex has been studied for cancer cell imaging and laser-induced cell killing with the help of plasmonic gold nanorods. When excited at the longitudinal SPR band of GNRs using laser (λ = 633 nm), cancer cell killing was observed due to laser-induced cell death. The flow cytometry studies supported ∼97% killing when the GNR-nanocomplex administered HeLa cells were irradiated under laser light for 10 min, which is a ∼4 fold increase in their cell as compared to the study carried out in the absence of laser light irradiation, and this has been partly attributed to the greater internalization of these into cells.