# Quantum Leap from Gold and Silver to Aluminum Nanoplasmonics for Enhanced Biomedical Applications
## 3 Applications
### Plasmonic Medicine
#### 3.2.2 Plasmonic Antibacterial Treatment
AgNO\({}_{3}\) used as an antibacterial treatment of eye infection in infants was shown to have potential toxicity in corneal damage [205]. Al plasmonics may provide an alternative biofilm sensing and treatment approach that could be less expensive, more scalable, less toxic, and more efficient under certain conditions. There has been much less research on Al plasmonic applications to antibacterial treatment. Here, we review the benefits and drawbacks of using Al NPs and plasmonic substrates for biomedical applications. Monitoring and treatment of biofilms plays an important role in biomedical and industrial applications. Methods for monitoring biofilm formation, its mechanical stability, and early detection provide many benefits. For example, biofilm monitoring techniques based on a combination of SPR, leaky mode spectroscopy, and attenuated total internal reflection spectroscopy (ATR) on Al films were used to monitor the daily growth of _Cytophaga_ biofilm in various environments (Figure 5) [206]. Chemical stability of Al compared to Ag in aqueous environments provides an advantage of Al plasmonics in biomedical applications, in addition to the low cost. Moreover, the metal toxicity also needs to be considered for a specific application [206]. Biofilm formation monitoring via SPR is based on measuring the refractive index of the suspension of microorganisms and nutrients that interact with the Al plasmonic substrate, which requires proper control measurements and calibrations. Monitoring the SPR shift of Al plasmonic substrates in an aqueous organic environment containing _Pseudomonas aeruginosa_ allowed for bacterial biofilm sensing via extracellular polymeric substance (EPS) formation, and to evaluate the Al corrosion over time [207]. Low corrosion rates of Al in the presence of the biofilm show the advantage of Al plasmonic sensing compared to that of Ag, which is less stable in the aqueous solution and may more easily detach from the plasmonic sensing device. Due to the challenges of the Al plasmonic nanostructure fabrication, it is desirable to explore the alternative approaches. Previous work used a commercial plasmonic Al-covered digital versatile disk (DVD) to inactivate _E. coli_ and _Geobacillus stearothermophilus_ via 785 nm laser-induced heating, showing promising effective hyperthermal sterilization with inexpensive, scalable substrates (Figure 6) [208]. The antibacterial effect was based on the thermal inactivation mechanism using NIR excitation. It can be further enhanced by combining it with the UV excitation with ROS inactivation mechanisms. The control experiment on an Al-covered flat Si wafer showed a low sterilization efficiency, confirming the SPR-enabled Al plasmonic heating effect (Figure 6C). Bacterial viability culture tests were performed by scraping the treated bacteria off the DVD and placing the carbenicillin-resistant _E. coli_ on LB agar to eliminate contamination. A similar procedure was repeated for thermally-resistant _G. stearothermophilus_ spores cultured at 55 \({}^{\circ}\)C without carbenicillin. The effect of the thermally resistant _G. stearothermophilus_ treatment on Al-covered DVD confirms the high temperature effect. This approach may provide an effective, inexpensive, and scalable strategy for improved medical device sterilization and food safety. It is desirable to combine the plasmonic bacterial treatment with simultaneous monitoring of the treatment efficiency. Recent studies of inactivation and bacterial viability detection on noble metal Au and Ag NPs reported promising results [209]. However, the nature of the enhancement (electromagnetic vs. chemical) remained unclear and the controllability of inactivation was limited. Figure 5: (**A**) Optical setup for monitoring biofilm formation through Al SPR. (**B**) SPR shifts of Al film in contact with a biofilm. (Adapted from [PERSON] et al. [206]).