## 4 Antibacterial Nanotheranostics
### Photothermal Therapy (PTT)  
PTT is a key technology for the treatment of bacterial diseases. However, PTT remains in a nascent stage (experimentally and only in vitro studies) compared with PDT [108]. Within the PTT technologies, GNPs receive much interest given their elicited optical properties from their plasmon resonance absorption. Different GNPs (spheres, rods, tubes) contribute to the shifting of the absorption band into the near-infrared (NIR) wavelength range and further functionalization [108]. The NIR wavelength range is long (700 to 1700 nm) and is coined as \"NIR window\" or \"optical window\" [229; 230]. Utilizing this long NIR light-mediated approach has attracted much interest because it allows deep-tissue penetration and minimal photodamage to biological tissue [229; 231; 232]. In contrast, short-wavelength UV light facilitates both low-tissue penetration and DNA damage, making it unsuitable for clinical therapy [231], and visible light also facilitates low-tissue penetration [229]. GNPs also positively promote the localized thermal damaging effects in the nanometer range surrounding them [108]. In PTT studies using GNPs, [PERSON] et al. [233] developed a theranostic PTT method for the selective laser killing of targeted protein A of _S. aureus_ (nanotherapy) by secondary IgG (Immunoglobulin G)-conjugated 40 nm GNPs. A real-time assessment of the nanotherapy was exercised using a photothermal microscope (nanodiagnosis) with higher sensitivity than the transmission technique. Bound _S. aureus_ with conjugated GNPS were irradiated with laser pulses (420 to 570 nm, 12 ns, 100 pulses), killing bacterial cells aptly combined with the bubble-formation phenomenon around the clustered GNPs. This apt killing was achieved as increasing the laser energy formed bubbles (life span of 0.1 and 2 ms and size of 1 to 8 \(\upmu\)m) around the hot clustered GDNPs, penetrating bacterial cells causing local cell-wall damage and subsequent complete bacterial disintegration. [PERSON] et al. [234] prepared iron oxide (Fe3O4)@Au nanoeggs and reported a temperature increase in the nanoegg suspension from 23 to about 55 \({}^{\circ}\)C after NIR irradiation (808 nm) for 3 min. They have further immobilized vancomycin on the surface of nanoeggs. The treated MRSA were entirely covered with the vancomycin-bound nanoeggs and killed (99%) after NIR irradiation (808 nm, 3 min). The magnetic properties of nanoeggs aggregated the bacterial cells and contributed to an ace photothermal effect of the vancomycin-bound nanoeggs. [PERSON] and [PERSON] [235] assembled magnetic NPs (MNPs, around 15 nm) surrounding Au nanorods, forming an Fe3O4-Au100 necklace-like probe. They further functionalized the probe with specific antibodies to _E. coli_ and investigated its simultaneous detection, separation, and photokilling properties to _E. coli_ within a cocktail of pathogens. The authors recorded a strong longitudinal plasmon for the probe at 765 nm, which decreased after 30 min incubation with _E. coli_. This decrease implied the selective binding of several antibody-bound probes to its targeted much larger _E. coli_ cell (around 1 to 3 \(\upmu\)m), even at a concentration as low as 10\({}^{2}\) CFU/mL. Following irradiation with NIR light (785 nm) for 15 min, the probe-bound _E. coli_ cells were magnetically separated and cultured, showing no grown colonies and implying probe absorption to adequate energy after excitation in the NIR region killing the targeted _E. coli_. Similar to GNPs, Fe3O4 MNPs also possess photothermal properties under NIR light illumination, where [PERSON] et al. [236] have found that Fe3O4/alumina core/shell MNPs specifically targeted and inhibited the cell growth of antibiotic-resistant nosocomial bacteria by over 95% within 10 min of illumination. Moreover, the graphene-based PTT was investigated by [PERSON] et al. [237], preparing MNPs (5 to 8 nm) with reduced graphene oxide (sheet-like structure) functionalized with glutaraldehyde (GA, cross-linking or capturing agent), namely MRGOGA.