Herein, a library of gold nanorods (GNR) decorated with polyethylene glycol-thiol (PEG-SH) containing different functionalities were synthesized and characterized by optical absorption spectroscopy, zeta potential, dynamic light scattering (DLS), transmission electron microscope (TEM) and proton nuclear magnetic resonance ( 1 H-NMR). The colloidal stability of GNR when exposed to skin, and their preferential accumulation into excised human skin layers were investigated. Confocal laser scanning microscopy, transmission electron microscope (TEM) and inductively coupled plasma-optical emission spectroscopy (ICP-OES) were utilized to track the penetration of GNR into different skin layers. The results demonstrated that cholesterol-PEG coated GNR were preferentially loaded up in the upper layers of skin (stratum corneum), while phospholipid-PEG coated counterparts were drastically deposited in skin dermis. Neutral methoxy-PEG-coated GNR were distributed in both SC and dermis skin layers, while charged GNR (anionic-carboxylic acid-PEG-GNR and cationic-amine-PEG-GNR) revealed a minimal accumulation into skin. DSPE-PEG-GNR and Chol-PEG-GNR demonstrated antibacterial activities against Staphylococcus aureus ( S aureus ) at MIC values of 0.011 nM and 0.75 nM, respectively. Photothermal treatment for S. aureus at sub-MIC concentrations resulted in a significant bactericidal effect when using Chol-PEG-GNR but not DSPE-PEG-GNR. Gold-based nanoscale systems have great value as a promising platform for skin diseases therapy.

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In this study, the photothermal-induced bactericidal activity of phospholipid-decorated gold nanorods (DSPE-AuNR) suspension against Pseudomonas aeruginosa planktonic and biofilm cultures was investigated. We found that the treatment of planktonic culture of Pseudomonas aeruginosa with DSPE-AuNR suspension (0.25–0.03 nM) followed by a continuous laser beam exposure resulted in ~6 log cycle reduction of the bacterial viable count in comparison to the control. The percentage reduction of Pseudomonas aeruginosa biofilm viable count was ~2.5–6.0 log cycle upon laser excitation with different concentrations of DSPE-AuNR as compared to the control. The photothermal ablation activity of DSPE-AuNR (0.125 nM) loaded into poloxamer 407 hydrogel against Pseudomonas aeruginosa biofilm resulted in ~4.5–5 log cycle reduction in the biofilm viable count compared to the control. Moreover, transmission electron microscope (TEM) images of the photothermally-treated bacteria revealed a significant change in the bacterial shape and lysis of the bacterial cell membrane in comparison to the untreated bacteria. Furthermore, the results revealed that continuous and pulse laser beam modes effected a comparable photothermal-induced bactericidal activity. Therefore, it can be concluded that phospholipid-coated gold nanorods present a promising nanoplatform to eradicate Pseudomonas aeruginosa biofilm responsible for common skin diseases.

The potential photothermal bactericidal activity of hydrophilic functionalized poly ethylene glycol (PEG)-gold nanorods (GNR) and hydrophobic functionalized polystyrene (PS)-GNR was evaluated towards strains of Staphylococcus aureus (S. aureus) and Propionibacterium acnes (P. acnes) by measuring the percentage reduction of bacterial viable count upon GNR excitation with a near infra-red (NIR) laser beam. Our results suggest that functionalized GNR had a minimal bactericidal activity against S. aureus and P. acnes (≤85%, i.e. ≤1 log10 cycle reduction of bacterial viable count). However, the local heat generated upon exciting the functionalized GNR with NIR laser beam has a significant photothermal ablation effect (≥99.99%, i.e. ≥4 log10 cycles reduction of bacterial viable count). Such photothermolysis effect could potentiate the antibacterial activity of GNR, which may call for minimum concentration and side effects of these nanotherapeutics.

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As an emerging field, nanotoxicology is gaining significant interest from scientists as well as from international regulatory firms in an attempt to build accumulated knowledge on this topic, which will be the basis for regulatory codes and safer nanotechnology. However, conflicting results and findings are abundant in the literature calling for more careful experimental design, result interpretation, and detailed reporting. In this perspective, we focus on misinterpretation in nanotoxicology and highlight the importance of proper experimental practice to avoid artifacts by discussing various examples from the literature.

The antibacterial activity of gold nanorod (GNR) suspensions of different surface functionalities was investigated against standard strains of Staphylococcus aureus and Propionibacterium acnes , taking into consideration two commonly “overlooked” factors: the colloidal stability of GNR suspensions upon mixing with bacterial growth media and the possible contribution of “impurities/molecules” in GNR suspensions to the observed antibacterial activity. The results demonstrated that cationic polyallylamine hydrochloride (PAH)-GNR were severely aggregated when exposed to bacterial growth media compared to other GNR suspensions. In addition, the free cetyltrimethylammonium bromide (CTAB) present in GNR suspensions is most likely the origin of the observed antibacterial activity. However, the antibacterial activity of GNR themselves could not be excluded. Probing these two critical control studies prevents misinterpretations and artifacts of the antibacterial activity of nanoparticles. Unfortunately, these practices are usually ignored in the published studies and may explain the significant conflicting results. In addition, this study indicates that GNR could be a promising candidate for the treatment of skin follicular diseases such as acne vulgaris.

Gold nanorods demonstrate a recognized role in the treatment of breast cancer cell lines as an efficient nanocarrier for chemotherapeutic drug delivery.