**3. Conclusions**

*Gold Nanoparticles - Reaching New Heights*

cancer treatment are fairly common practice [37–39]. While these studies are predominantly focused on the biological effects in the vicinity of cancerous tissue, they highlight how photons interact with these high-atomic-number (high-Z) nanoparticles to deposit their energy locally. High energy photons are also expected to cause Frenkel pair displacement damage (i.e., vacancy and interstitial defect pairs), primarily through the energetic electrons that they tend to generate. However, the effects of these displacement effects on the long-term stability of nanoparticle structures, if any, have not been studied to our knowledge. Charged particles are common by-products of radioactive decay and nuclear reactions and are a primary component of cosmic radiation. Accelerators and ion beams are also common ion sources used in both research and industry. For example, ions are frequently used in radiation damage experiments to simulate material microstructures resulting from neutron radiation exposure in nuclear reactor environments primarily because they are much less costly and can achieve similar damaged microstructures in a fraction of time [40]. Accelerators also have a slew of other potential applications ranging from materials analysis (e.g., electron microscopes and Rutherford backscattering) to ion beam modification. Beta (electron or positron) radiation either incident on or produced in gold will primarily lose its energy via ionization and bremsstrahlung radiation. As mentioned in the introduction, bremsstrahlung radiation occurs due to electron acceleration from interaction with an atomic nucleus and results in the production of a photon of energy equivalent to the energy lost by the electron. This interaction is again Z-dependent and is quite common for high-Z materials like gold [41]. While displacement or knock-on damage resulting from electrons is commonly observed in transmission electron microscopy experiments [42], the significant difference in the masses of an electron and a gold nucleus requires electron energies in excess of 1.35 MeV to create a single Frenkel pair [43]. In many cases, the role of the electron beam in altering the nanoparticle stability during these studies is not a result of interaction with the gold atoms itself, but with the organic capping ligands, as a result, the stability of the gold nanoparticles to beta radiation is often dictated by the organic capping agent chosen [44]. Alpha particles are common products of radioactive decay for actinides and other heavy radioactive isotopes, but, along with protons, deuterons, and tritons, they can also result from and induce a number of different nuclear reactions. For example, irradiation of Au-197 with protons with energies of 4.5 MeV or higher can cause a (p, n) reaction resulting in the production of metastable Hg-197 m [45]. However, the threshold energies of these reactions for gold are, in most cases, sufficiently high and reaction cross sections sufficiently low that these types of interactions rarely occur in practical applications. More often, light ions will interact via ionization and through coulombic forces. Similar to the other types of interactions discussed, ionization has the potential to result in the emission of characteristic X-rays and other secondary radiation. Coulombic interactions with other atomic nuclei can tend to cause displacement damage, usually in the form of Frenkel pairs or small,

In general, the response of nanostructured materials to radiation damage is still poorly understood [46]. Despite the limited understanding in the general field, free-standing gold nanoparticles (usually drop-cast onto carbon or SiN TEM grids) have been used as the model system for testing and validation of TEM with in-situ ion irradiation capabilities [47–49]. Expanding on the known enhanced sputtering rate observed in gold thin foils exposed to a range of noble

**6**

isolated cascades.

Gold nanoparticles have diverse properties compared to those of bigger particles, which can be exploited in a broad spectrum of fields such as in renewable energy, nanomedicine, catalysis*,* information technologies, energy production*,* energy storage, sensitive nanosensors*,* nanomaterials, manufacturing, and real-time environmental applications. Taking into account the interesting photo-physical properties of sensitive gold nanoparticles, their easy functionalization, the use of aqueous solutions, and the detection by the naked eye, it can be concluded that the red or blue signal will continue to be very present in the molecular sensing field. It is concluded that selective gold nanoparticles have been extensively used for the design of colorimetric sensors as well as probes due to their interesting photo-physical properties. The use of gold nanoparticles for the preparation of colorimetric sensors is a very active field. The changes in the color of colloidal gold nanoparticles in solution because of the change in the surface plasmon absorption band upon aggregation or disaggregation processes can be easily used to transform the molecular recognition event into a macroscopic measurable signal. This change from red to blue can be perceived by the naked eye allowing in this way cheap and easy detection of the target analytes. The initial studies that have been done to observe the radiation response of gold nanoparticles to charged particle irradiation indicate that a significant enhancement of sputtering yield is present. This enhanced sputtering leads to the rapid disintegration of the original nanoparticle and the formation of unique satellite nanoscale arrangements as well as neurotransmitters, nerve agents, pesticides, and carboxylates of biological interest. The sensitivity and selectivity of the gold nanoparticle–based sensors toward the different bio- or chemical analytes will depend on the detection as well as recognition properties of the selective molecules attached to the surface of the low-dimensional nanoparticles. In this book, a selection of optically as well as biologically active molecules have been considered as analytes in broad interest such as photocatalysis, electrocatalysis, plasma-absorption, neurotransmitters, nerve agents, pesticides, and carboxylates.

*Gold Nanoparticles - Reaching New Heights*
