**11. Future Prospects and Conclusions**

The current review discussed the biological synthesis methods in depth with emphasis on the lichen-mediated synthesis of NPs. The biological synthesis of NPs has recently become an increasingly active area of research. Through exhibiting several advantageous qualities and numerous potential applications, biological synthesis methods of NPs have proven to be superior to the traditional chemical and physical methods. These qualities include being cost effective, eco-friendly, and vastly applicable in biomedical fields due to biocompatibility. Lichen species are seldom considered as biomachinery for the biofabrication of NPs. In this review, we have highlighted the potential of these organisms as natural biofactories for NP formation. The symbiosis between fungi and cyanobacteria or algae or sometimes plants makes lichen a promising alternative biomachinery for NPs fabrication. Due to the variation in their biomolecule contents and structures, which are responsible for reducing metal ions into NPs. Devasena et al. reported that lichen-mediated synthesis NPs are distinguished from other alternative biological methods by being lesser toxic and needing low-processing conditions [161]. Lichen species have a reducible activity to fabricate different types of NPs, including gold (Au)-NPs, silver (Ag)-NPs, metal oxide-NPs such as iron oxide- and zinc oxide-NPs, and other nanomaterials such as bimetallic alloys (Au-Ag NPs) and nanocomposites such as ZnO@TiO2@SiO<sup>2</sup> and Fe3O4@SiO2. These biogenic NPs have significant antimicrobial activities against both Gram-positive and Gram-negative bacteria, and fungi, and they also display mosquitocidal activity. Additionally, these NPs

act as potential catalytic materials, bioremediatory agents for heavy metals, antidiabetics, antioxidants, and neuroprotection agents against neurotoxin.

Extending the utilization of lichen-mediated green synthesis methods and exploring the optimum conditions of these processes to fabricate applicable, bioactive, scalable, and biocompatible nanoproducts may lead to the development of novel green NPs with unique physicochemical and biological features that can be applied in different sectors, including agriculture, industry, medicine, biotechnology, and pharmaceutics. Moreover, there remain many barriers against the biological synthesis process, including toxicity and agglomeration, polydispersity, stability, and the nonuniform size of NPs. These issues can be solved by increasing the optimization studies for green synthesis of NPs to obtain the desirable NPs. Additionally, exploring the synthesis mechanism of NPs using natural sources will facilitate the development and launch of nanodrugs in different fields.

**Author Contributions:** Conceptualization, R.S.H.; methodology, R.S.H., L.B. and M.A.A.; software, R.S.H. and M.A.A.; investigation, R.S.H. and L.B.; data curation, R.S.H., L.B. and M.A.A.; writing original draft preparation, R.S.H. and L.B.; writing—review and editing, R.S.H. and L.B.; visualization, R.S.H.; supervision, N.E.A., M.M.B.-M. and M.I.A.-Z.; project administration, R.S.H.; funding acquisition, M.M.B.-M. and M.I.A.-Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data supporting this article are shown in Figures 1–7 and one table. The data sets analyzed in the present study are available from the corresponding author upon reasonable request.

**Acknowledgments:** This research was funded by the Deanship of Scientific Research at Princess Nourah bint Abdulrahman University through the Fast-track Research Funding Program.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

