# Nanostructured Biosilica of Diatoms: From Water World to Biomedical Applications
## 2 Chemical Modification of the Frustule Surface
[45] described an innovative colon-focused drug delivery system (CFDD) based on the functionalization of diatom frustules (DEM) with vitamin B\({}_{12}\) (DEM-B\({}_{12}\)), which binds the transcobalamin (II) receptor (TC II) overexpressed on cancer cells. Additionally, the DEM-B\({}_{12}\) surface was modified with either porphyrin as photosensitizer elements or the 2,2'-bipyridine-3,4-dicarboxylic acid manganese tricarbonyl compound as a photo-triggered CO-releasing molecule (Figure 2). Through the multi-step functionalization process, the authors developed a photo-therapeutic platform able to produce highly toxic ROS or carbon monoxide (CO) and deliver rhenium(I)-tricarbonyl complexes with anticancer effects. ROS and CO produced by light-activation are known to exhibit antiproliferative effects and induce tumors cells apoptosis, potentiating the cytotoxic effect of the delivered drug. The so assembled photo-therapeutic platform showed enhanced adherence to colorectal cancer (CRC) cells via TC (II) receptors and a gradual drug release for a few days. Moreover, cytotoxicity tests on the human colorectal cell line (HCT-116) showed a 2 fold increase of cytotoxicity upon light-activation of both modified frustules, lowering the drug dose needed for equal therapy effectiveness. Also, MTT assay carried out on HCT-116 cell lines under dark/light conditions confirmed the efficacy of light-activable compounds on the surface. Hence, the synergistic chemo-photodynamic therapy could be adopted for lowering the chemotherapeutic needed dose and its side effects for effective cancer therapy. The ability of surface-engineered systems to identify and target cancer cells was exploited by [PERSON] et al. [46], who reported a novel magnetic antibody-labeled biosilica structure with either biosensing and targeted drug delivery potentialities. In this work, the frustules of _Chaetocros_ sp. Figure 2: Left: Concept showing the bio-inspired hybrid drug delivery system described by [PERSON] et al. [45]. Diatom frustule surface was functionalized with photoactivable molecules (orange spheres) linked to vitamin B-12 (red sphere) acting as a tumor-targeting tag. The system can be loaded with chemotherapeutic drugs (light blue spheres), which can be selectively delivered to colorectal cancer cells. Besides, diatomite microparticles can be photoactivated to generate carbon monoxide or free radicals inducing tumor cell apoptosis. Right: Photo-induced chemical reactions at the surface DEM. Reproduced from [45]. diatoms were magnetized through iron oxide nanoparticles and labeled with the antiHER-2 monoclonal antibody Trastuzumab. Paramagnetic antibody-labeled frustules were able to discriminate between HER-2 positive or negative cells through to the specific antibody-antigen interactions. This specific binding ability was detected by fluorescence microscopy thanks to the optical properties of the silica frustule. Moreover, the ability of magnetic particles to move in the presence of a neodymium magnet was outlined to show that the paramagnetic antibody-labeled system can be useful to discriminate between targeted cells from untargeted ones. [PERSON] et al. [47] also investigated a surface bioengineering approach to diatomite nanoparticles (DNPs) based on polyethylene glycol (PEG) and cell-penetrating peptide (CPP) conjugation to improve nanoparticles physicochemical and biological properties. In this study, DNPs obtained by mechanical crushing, sonication, filtering, and purification of natural diatomaceous earth (DE) were amino-modified by APTES and chemically conjugated to a heterobifunctional PEG polymer ending up with CPP at the N-terminal extremity. The biocompatible platform was engineered for delivering the poorly-water soluble drug Sorafenib to cancer cells. Cellular uptake analysis evaluated by confocal fluorescence microscopy indicated that CPP-modified DNPs were internalized into MCF-7 cells with a homogeneous distribution in the cytoplasm and close to the nucleus, whereas no cellular uptake was observed for the unmodified-DNPs.