Green Nanomaterials. Siddharth Patwardhan

microscopy can be used to visualise targeted quantum dots using confocal microscopy [6]. The density of gold nanoparticles can also be utilised as a probe for TEM imaging in what is commonly referred to as immune gold staining, where gold nanoparticles can be targeted to a specific biological site. TEM analysis will then identify structures decorated with dense gold dots, distinguishing them from the other biological tissue (figure 2.9(Bii)). Recently, a combined approach of both gold and CdSe quantum dots have been used to image a cell, highlighting different aspects (the gold to the nucleus, and the quantum dot to the insulin granules) offering multimodal enhanced imaging (figures 2.9(Biii), (Biv)) [14]. In vitro imaging applications are considered mainly to conduct scientific research and cross over into the area of nanosensing (section 2.4.1.5). In vivo imaging is where the real medical applicability lies. Again, gold nanoparticles have been used in diagnostic imaging, using their SPR properties in optical coherent tomography (OCT) [15]. This non-invasive, high resolution optical technique relies on reflective interference between emitted and backscatter light (akin to an optical ultrasound), so can penetrate biological tissue making it applicable in vivo. Gold nanoparticles of various morphologies (tuned so the SPR lies within the emitted light source) make ideal contrast agents for OCT as the gold nanoparticles absorb this specific light wavelength, enhancing the image at this location. Hollow nanocages have proved to be the best contrast enhancing agent, as they offer the strongest absorption in the near infrared. Figure 2.9(Bv) shows a phantom with and without enhancement with gold 30 nm nanocages [15].

      Figure 2.9. Summary of some inorganic nanoparticle uses in nanomedicine. (A) Schematic of and key attributes of a generic biomedical nanoparticle. (B) Some examples of how nanoparticles have been used for imaging. (i) Light scattering image of malignant epithelial cells labelled with antibody (anti-EGFR) coated gold nanoparticles (reproduced with permission from [12], copyright 2005 American Chemical Society). (ii) TEM image of human deep posterior lingual gland selectively labeled with immunogold nanoparticles. Scale bar 1 μm (reproduced from [13], copyright 2006 with permission of Elsevier). (iii) and (iv) Targeted labelling of the nucleus of a cell with gold nanoparticles (red) and insulin granules with quantum dots (green/blue) (scale bar 2 μm) (reproduced from [14]). (v) Spectroscopic optical coherence tomography (SOCT) images of a phantom (top intensity and bottom spectroscopic image) showing contrast due to nanocages (reproduced with permission from [15], copyright The Optical Society). (vi) MPI of a mouse with a lower right flank tumour 9 h after injection with 5 mg kg⁻¹ of (superparamagnetic iron-oxide nanoparticle) SPION tracer. A CT scan of the skeleton is overlaid (reproduced with permission from [16], copyright 2017 American Chemical Society) and (C) shows some examples of how nanoparticles are being researched for therapeutic use. (i) Schematic showing the formation of the DOX-loaded SPION@HP core@shell system, and the sustained release at a steady rate of DOX by diffusing through the polymeric matrix as the HP shell degraded. Reproduced from [17]. (ii) An in vivo experiment of the treatment of a tumour with magnetosomes and a SPION control. (a) Shows the experimental setup in the alternating field coil, (b) shows the shrinkages of the tumour with treatment of magnetosome chains compared to (c) treatment with SPIONs, and (d) shows the cancer cell with magnetosome chains after 24 h (reproduced from [18] with permission from Taylor & Francis Ltd). (Di) Schematic of a multifunctional combined imaging and therapeutic nanoparticle termed a theragnostic nanoparticle. (ii) Schematic representing the dual function of Fe@Si–DOX–CD–PEG nanoparticles having both MRI contrast agents and for SMART targeted drug release. Fe@Si cores are magnetite nanoparticles coated with nanoporous silica shown in the TEM image on the right. DOX is the anticancer drug doxorubicin located in the silica pores. CD is cyclodextrin which can release the DOX when exposed to glutathione (GSH) which is present in high concentrations in cancer cells (adapted from [19]).

      Magnetic resonance imaging (MRI) is a non-invasive technique which has been used internationally since the 1970s. The technique makes use of the massive number of protons in the body. In basic terms, when a magnetic field is applied to the body (in an MRI scanner) the net magnetisation of protons in the body aligns with the direction of the field. A radio frequency is then applied giving the protons energy. When the radio frequency is removed the protons then relax back, and this relaxation can be measured. Using gradient magnets of varying strength allows the targeting of specific areas for imaging of all three dimensions. Magnetic nanoparticles have great potential as MRI contrast agents, as the interaction of the particle’s local magnetic field with the surrounding protons can significantly enhance contrast in vivo, by increasing relaxation time, resulting in the darkening of this area of the image, without the toxicity of gadolinium-based agents. The size of the MNPs greatly affects the MRI signal, offering tunable contrast modalities [21].

      Magnetic particle imaging (MPI) is a new non-invasive imaging technique that images the magnetic signal from superparamagnetic iron-oxide nanoparticles (SPIONs). Utilising the inherent magnetic properties (from the electrons) results in far greater (×22 million) intensity compared to MRI (which utilises the nuclear moment). SPIONs are targeted to a site in the body then imaged by applying a saturating magnetic field, then rostering an excitation field, which causes the SPIONs to flip, which can be detected at this location in 3D. It has been used to successfully locate and image cancer tumours (figure 2.9(Bvi)) [16] but its sensitivity means it could be used for a range of more subtle diagnostics too.

      2.4.1.3 Therapeutics

      Targeted drug delivery uses a nanoparticle (for this book, specifically an inorganic nanoparticle) as a vehicle to transport a drug to a specific disease cell sites. Targeted drug delivery is a game-changer for global pharmacology. Most cancer drugs’ activity depends on their toxicity to cancer cells, but the obvious issue is that these are invariably toxic to healthy cells too. This means that previous therapies had terrible side effects, as the body had to cope with being poisoned overall in order to poison the cancer tumour. Targeted drug delivery means that the drugs are delivered to just the cancer cells, and thus are not administered to the whole body. This has two advantages: (1) the side effects of the treatment are dramatically reduced as the effect on healthy cells is now negligible; (2) the quantity of drug required is vastly reduced, as it is only used on the much smaller affected area. Targeting can be achieved passively or actively.

      Passive targeting takes advantage of the fact that tumours have more ‘leaky’ vasculature and faulty drainage, so have enhanced permeability and retention (EPR). This means that tumours are susceptible to nanoparticles of between 12–500 nm accumulating preferentially within tumours, without any other ‘active targeting’. However, accumulation quantities can be significantly increased by active targeting also. Active targeting can take the form of specific antibodies, as discussed above, which will target just the specific cancer cells. There are also other physiochemical properties that differentiate cancer cells from healthy cells which can be taken advantage of, such as pH. Cancer cells are more acidic, and as such a pH-responsive nanoparticle coating can be designed to release the drug only at the pH of the cancer cell, targeting the drug release.

      Magnetic nanoparticles can be further exploited for targeting by utilising a magnetic field gradient to move the particles remotely in vivo. Magnetite or maghemite SPIONs have been most widely used due to their biocompatibility and easy availability for iron precursors. A permanent magnetic field near the tumour can enable the field gradient to concentrate and retain the drug carrying magnetic nanoparticles at the tumour