Note that the coating varies in thickness and there are not any CNT agglomerate formations,probably because the protein intercrossing is disordered. The function of coating is to prevent the degradation of the sorbent by adsorping proteins from the sample on its CNTs, during the extraction steps. At the same time, it allows the Pb2þ ions' permeation to the active sites of the oxidized CNTs. The RACNTs, saturated with Pb2þ ions, were analyzed by high resolution transmission electron microscopy (HRTEM) (Fig. 2c and d) using a Gatan GIF Tridiem 2kx2k CCD camera. Fig. 2c shows the image of the RACNTs and Fig. 2d its scanning transmission electron microscopy (STEM), using a high angle annular dark field. Lead atoms are heavier than carbon nanotubes. The dark points in the HRTEM image (Fig. 2c) and the bright contrast in the STEM image (Fig. 2d) can be interpreted as a heavy element (lead) incorporated in the CNTs. Therefore, the images confirm that the adsorption of Pb2þ ions occurs preferably within the RACNTs. In order to confirm the Pb2þ ions’ incorporation, the RACNTs (saturated with Pb2þ) were analyzed by energy dispersive spectroscopy (EDS) using a Thermo
Noran detector. Fig. 3a and b shows the STEM image and its lead map obtained by EDS, respectively. A clear correlation between the bright contrasts in the STEM image with the lead map obtained by EDS can be observed. Therefore, the Pb2þ ions are preferably incorporated within the RACNTs (in the nanotubes). TEM and EDS images collaborate with data on the adsorption and exclusion mechanisms of the restricted access materials for organic compounds and inorganic ions . In this mechanism, the exclusion of proteins and adsorption of analytes are possible when a biological fluid is percolated through the restricted access material in a pH larger than the isoelectric point of the proteins. Inthis case, both proteins from the sample and from the BSA layer are negatively charged, promoting an electrostatic repulsion, responsible for the exclusion of the sample proteins. The analytes can penetrate through the peptide chains of the BSA, being retained
in the core of the RACNTs. Despite their positive charge, Pb2þ ions do not bind efficiently to the negatively charged BSA layer, as it can be seen in Figs. 2 and 3. A hypothesis is that the negative charge distribution on the BSA layer is not sufficiently dense to cause a significant retention of Pb2þ due to its reduced atomic mass (207 Da) in comparison to the molecular weight of albumin (ca. 66 kDa). Thus, Pb2þ ions can penetrate through the peptide chains of this layer with a low probability of approaching the BSA negative groups