###### Abstract  
Metallic nanoparticles are a new class of materials with applications in medicine, pharmaceutical and agriculture. Using biological, chemical and physical approaches, nanoparticles with amazing properties are obtained. Copper is one of the most-found elements and plays an important part in the normal functioning of organisms. Coper nanoparticles have superior antibacterial properties when comparing them to present day antibiotics. Moreover, apart from their antibacterial role, antifungal, antiviral and anticancer properties have been described. Although the mechanism of actions is not completely understood, copper nanoparticles can become a viable alternative in fighting multi-resistant bacteria strains. We hereby review the already existing data on copper nanoparticle synthesis, effects and mechanisms of action as well as toxicity. nanoparticles; copper; physiology; antibacterial activity; action mechanism; toxicity 2021  
M.C. [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON] Copper Nanoparticles: Synthesis and Characterization, Physiology, Toxicity and Antimicrobial Applications. _Appl. Sci._**2022**, 12, 141. [[https://doi.org/10.3390/app12010141](https://doi.org/10.3390/app12010141)]([https://doi.org/10.3390/app12010141](https://doi.org/10.3390/app12010141)) Academic Editor: [PERSON]  
Received: 4 November 2021  
Accepted: 21 December 2021  
Published: 24 December 2021  
Published: **24 December 2021**  
Transmission Electron Microscopy (TEM) transmits or scatters electrons from a thin sample using uniform current electron beams. TEM can provide accurate estimations of nanoparticle homogeneity, but it has difficulties when quantifying large numbers of particles. Another limitation is that is produces unreliable images due to orientation effects [8]. High Resolution Transmission Electron Microscopy became the most used technique for characterizing the internal structure of nanoparticles thanks to its high resolution [8]. A method used to characterize size, shape, concentration, agglomeration state and refractive index is UV-VIS spectroscopy. A reference material is used to measure the intensity of the reflected light and then compare it to the sample resolution [8]. Atomic Force Microscopy (AFM) is a technique used to create three dimensional images of surfaces. It is mainly used to determine the size of nanoparticles and it is cost effective, space saving and can be used to assess the interaction of nanoparticles in real-time [10]. FTIR spectroscopy has increased signal to noise ratio and reduced thermal deterioration when used to characterize nanoparticles. An improved version of this technique, attenuated total reflection (ATR-FTIR), is used for evaluation of chemical features on nanoparticles surface [11; 12]. Particles with high charges tend to repel each other and form stable colloidal solutions. Zeta potential is used to analyze the stability of the formed colloidal solutions [8]. Electrophoretic mobility (EPM) is used to measure the surface charge of nanoparticles. Low values are associated with aggregations of iron oxide nanoparticles, while high values are associated with stable nanoparticles over long periods of time [8]. To measure nanoparticles' size in colloidal suspension, dynamic light scattering (DLS) is used. It is a powerful technique when used for real time observation of the aggregation process. When combined with differential centrifugal sedimentation (DCS), the measurements are accurate. A drawback is that it lacks the necessary resolution when working with small aggregates. It is, however, powerful because it is capable of measuring different sizes of nanoparticles at the same time [8]. Photoluminescence (PL) is light emission from matter after absorbing photons. PL is a good fit to study quantum dots and metal nanoclusters. It is used in optical labeling applications [8]. Methods used to measure magnetic properties are Superconducting quantum interference device magnetometry (SQUID) and vibrating sample magnetometry (VSM).