Nanomaterials for Photothermal Antimicrobial Surfaces

Microbial infection diseases are a major threat to human health and have become one of the main causes of mortality. The search for novel antimicrobial strategies is an important challenge for the scientific community, considering also the constant increase of antimicrobial resistance and the rise of new diseases. Among the new strategies to combat microbial infections, the photothermal effect seems to be one of the most promising. Hyperthermia is an effective and broad spectrum strategy for the removal of microbial infections. Among all of the strategies to reduce the diffusion of microbial infections, the preparation of antimicrobial surfaces seems of primary importance. In many cases, in fact, an infection can be diffused through surfaces just by touching them, or by inoculating microbes through an internalizable device, such as an implant, a prosthesis, or a catheter. In this review, we will summarize the recent advances in the preparation of photothermal antibacterial surfaces.

Nano-Bio Interactions: Biofilm-Targeted Antibacterial Nanomaterials

Biofilm is a spatially organized community formed by the accumulation of both microorganisms and their secretions, leading to persistent and chronic infections because of high resistance toward conventional antibiotics. In view of the tunable physicochemical properties and the related unique biological behavior (e.g., size-, shape-, and surface charge-dependent penetration, protein corona endowed targeting, catalytic- and electronic-related oxidative stress, optical- and magnetic-associated hyperthermia, etc.), nanomaterials-based therapeutics are widely used for the treatment of biofilm-associated infections. In this review, the biological characteristics of biofilm are introduced. And the nanomaterials-based antibacterial strategies are further discussed via biofilm targeting, including preventing biofilm formation, enhancing biofilm penetration, disrupting the mature biofilm, and acting as drug delivery systems. In which, the interactions between biofilm and nanomaterials include mechanical disruption, electron transfer, enzymatic degradation, oxidative stress, and hyperthermia. Additionally, the current advances of nanomaterials for antibacterial nanomaterials by biofilm targeting are summarized. This review aims to present a complete vision of antibacterial nanomaterials-biofilm (nano-bio) interactions, paving the way for the future development and clinical translation of effective antibacterial nanomedicines.

Gold nanoparticle adsorption alters the cell stiffness and cell wall bio-chemical landscape of Candida albicans fungal cells

HYPOTHESIS

Nanomaterials have been extensively investigated for a wide range of biomedical applications, including as antimicrobial agents, drug delivery vehicles, and diagnostic devices. The commonality between these biomedical applications is the necessity for the nanoparticle to interact with or pass through the cellular wall and membrane. Cell-nanomaterial interactions/uptake can occur in various ways, including adhering to the cell wall, forming aggregates on the surface, becoming absorbed within the cell wall itself, or transversing into the cell cytoplasm. These interactions are common to mammalian cells, bacteria, and yeast cells. This variety of interactions can cause changes to the integrity of the cell wall and the cell overall, but the precise mechanisms underpinning such interactions remain poorly understood. Here, we investigate the interaction between commonly investigated gold nanoparticles (AuNPs) and the cell wall/membrane of a model fungal cell to explore the general effects of interaction and uptake.

EXPERIMENTS

The interactions between 100 nm citrate-capped AuNPs and the cell wall of Candida albicans fungal cells were studied using a range of advanced microscopy techniques, including atomic force microscopy, confocal laser scanning microscopy, scanning electron microscopy, transmission electron microscopy, and synchrotron-FTIR micro-spectroscopy.

FINDINGS

In most cases, particles adhered on the cell surface, although instances of particles being up-taken into the cell cytoplasm and localised within the cell wall and membrane were also observed. There was a measurable increase in the stiffness of the fungal cell after AuNPs were introduced. Analysis of the synchrotron-FTIR data showed significant changes in spectral features associated with phospholipids and proteins after exposure to AuNPs.

Research progress of stimulus-responsive antibacterial materials for bone infection

Infection is one of the most serious complications harmful to human health, which brings a huge burden to human health. Bone infection is one of the most common and serious complications of fracture and orthopaedic surgery. Antibacterial treatment is the premise of bone defect healing. Among all the antibacterial strategies, irritant antibacterial materials have unique advantages and the ability of targeted therapy. In this review, we focus on the research progress of irritating materials, the development of antibacterial materials and their advantages and disadvantages potential applications in bone infection.

Interaction between Engineered Pluronic Silica Nanoparticles and Bacterial Biofilms: Elucidating the Role of Nanoparticle Surface Chemistry and EPS Matrix

Nanoparticles (NPs) are considered a promising tool in the context of biofilm control. Many studies have shown that different types of NPs can interfere with the bacterial metabolism and cellular membranes, thus making them potential antibacterial agents; however, fundamental understanding is still lacking on the exact mechanisms involved in these actions. The development of NP-based approaches for effective biofilm control also requires a thorough understanding of how the chosen nanoparticles will interact with the biofilm itself, and in particular with the biofilm self-produced extracellular polymeric matrix (EPS). This work aims to provide advances in the understanding of the interaction between engineered fluorescent pluronic silica (PluS) nanoparticles and bacterial biofilms, with a main focus on the role of the EPS matrix in the accumulation and diffusion of the particles in the biofilm. It is demonstrated that particle surface chemistry has a key role in the different lateral distribution and specific affinity to the biofilm matrix components. The results presented in this study contribute to our understanding of biofilm-NP interactions and promote the principle of the rational design of smart nanoparticles as an important tool for antibiofilm technology.

Functionalized Self-Assembled Monolayers: Versatile Strategies to Combat Bacterial Biofilm Formation

Bacterial infections due to biofilms account for up to 80% of bacterial infections in humans. With the increased use of antibiotic treatments, indwelling medical devices, disinfectants, and longer hospital stays, antibiotic resistant infections are sharply increasing. Annual deaths are predicted to outpace cancer and diabetes combined by 2050. In the past two decades, both chemical and physical strategies have arisen to combat biofilm formation on surfaces. One such promising chemical strategy is the formation of a self-assembled monolayer (SAM), due to its small layer thickness, strong covalent bonds, typically facile synthesis, and versatility. With the goal of combating biofilm formation, the SAM could be used to tether an antibacterial agent such as a small-molecule antibiotic, nanoparticle, peptide, or polymer to the surface, and limit the agent’s release into its environment. This review focuses on the use of SAMs to inhibit biofilm formation, both on their own and by covalent grafting of a biocidal agent, with the potential to be used in indwelling medical devices. We conclude with our perspectives on ongoing challenges and future directions for this field.

Gold Nanostars Embedded in PDMS Films: A Photothermal Material for Antibacterial Applications

Bacteria infections and related biofilms growth on surfaces of medical devices are a serious threat to human health. Controlled hyperthermia caused by photothermal effects can be used to kill bacteria and counteract biofilms formation. Embedding of plasmonic nano-objects like gold nanostars (GNS), able to give an intense photothermal effect when irradiated in the NIR, can be a smart way to functionalize a transparent and biocompatible material like polydimethylsiloxane (PDMS). This process enables bacteria destruction on surfaces of PDMS-made medical surfaces, an action which, in principle, can also be exploited in subcutaneous devices. We prepared stable and reproducible thin PDMS films containing controllable quantities of GNS, enabling a temperature increase that can reach more than 40 degrees. The hyperthermia exerted by this hybrid material generates an effective thermal microbicidal effect, killing bacteria with a near infrared (NIR) laser source with irradiance values that are safe for skin.

Harvesting Light To Produce Heat: Photothermal Nanoparticles for Technological Applications and Biomedical Devices

The photothermal properties of nanoparticles (NPs), that is, their ability to convert absorbed light into heat, have been studied since the end of the last century, mainly on gold NPs. In the new millennium, these studies have developed into a burst of research dedicated to the photothermal ablation of tumors. However, beside this strictly medical theme, research has also flourished in the connected areas of photothermal antibacterial surface coatings, gels and polymers, of photothermal surfaces for cell stimulation, as well as in purely technological areas that do not involve medical biotechnology. These include the direct conversion of solar light into heat, a more efficient sun-powered generation of steam and the use of inkjet-printed patterns of photothermal NPs for anticounterfeit printing based on temperature reading, to cite but a few. After an analysis of the photothermal effect (PTE) and its mechanism, this minireview briefly considers the antitumor-therapy theme and takes an in-depth look at all the other technological and biomedical applications of the PTE, paying particular attention to photothermal materials whose NPs have joined those based on Au.

Membrane-disruptive peptides/peptidomimetics-based therapeutics: Promising systems to combat bacteria and cancer in the drug-resistant era

Membrane-disruptive peptides/peptidomimetics (MDPs) are antimicrobials or anticarcinogens that present a general killing mechanism through the physical disruption of cell membranes, in contrast to conventional chemotherapeutic drugs, which act on precise targets such as DNA or specific enzymes. Owing to their rapid action, broad-spectrum activity, and mechanisms of action that potentially hinder the development of resistance, MDPs have been increasingly considered as future therapeutics in the drug-resistant era. Recently, growing experimental evidence has demonstrated that MDPs can also be utilized as adjuvants to enhance the therapeutic effects of other agents. In this review, we evaluate the literature around the broad-spectrum antimicrobial properties and anticancer activity of MDPs, and summarize the current development and mechanisms of MDPs alone or in combination with other agents. Notably, this review highlights recent advances in the design of various MDP-based drug delivery systems that can improve the therapeutic effect of MDPs, minimize side effects, and promote the co-delivery of multiple chemotherapeutics, for more efficient antimicrobial and anticancer therapy.

Highly efficient photothermal nanoparticles for the rapid eradication of bacterial biofilms

Biofilm-related infections, such as dental plaque, chronic sinusitis, native valve endocarditis, and chronic airway infections in cystic fibrosis have brought serious suffering to patients and financial burden to society. Materials that can eliminate mature biofilms without developing drug resistance are promising tools to treat biofilm-related infections, and thus they are in urgent demand. Herein, we designed and readily prepared organic nanoparticles (NPs) with highly efficient photothermal conversion by harvesting energy via excited-state intramolecular motions and enlarging molar absorptivity. The photothermal NPs can sufficiently eliminate mature bacterial biofilms upon low-power near-infrared laser irradiation. NPs hold great promise for the rapid eradication of bacterial biofilms by photothermal therapy.

Photo-enhanced antibacterial activity of polydopamine-curcumin nanocomposites with excellent photodynamic and photothermal abilities

Background and objective Photodynamic therapy (PDT) and photothermal therapy (PTT) have gradually become options for select anti-tumor and antibacterial treatment . The combination of PDT and PTT show great research value, which may greatly improve the curative effect. The aim of the present study was to prepare a compound system of polydopamine and curcumin (PDA-Cur nanocomposites) with excellent antibacterial effect towards Gram-positive and Gram-negative bacteria. Methods Dopamine hydrochloride was oxidized and self polymerized in alkaline condition to form PDA-Cur nanocomposites. The structure and morphology of PDA-Cur were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), laser scattering microscopy (LSM), ultraviolet spectrophotometer (UV-vis), infrared spectroscopy (IR) and fluorescence emission spectrometer. Using 1,1-diphenyl-2-picrylhydrazyl radical (DPPH), 1,3-diphenylbenzofuran (DPBF) and 2',7'-Dichlorodihydrofluorescein diacetate (DCFH-DA) were used to detect the production of reactive oxygen species (ROS). The thermal stability of PDA-Cur nanocomposites was investigated by temperature rising test. The antibacterial effect of PDA-Cur was determined by plate counting technique using Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) as models. In addition, the stability and antibacterial mechanism of PDA-Cur were investigated. Finally, the biocompatibility was evaluated by cytotoxicity and hemolysis tests. Results The compound system of polydopamine and curcumin was successfully prepared, which showed improved stability compared with Cur. The consumption of DPBF by the singlet oxygen produced by PDA-Cur was as high as 80%. In the heating test, the highest temperature increased to 59 °C, which contributed to the photodynamic and photothermal inactivation of bacteria. PDA-Cur nanocomposites showed good antibacterial activity against S. aureus and E. coli. Under 405 nm light, the bactericidal rate of PDA-Cur against S. aureus can reach 100% at a low concentration of 10^-4 nM, and that against E. coli was 100% at 1 nM. Under 405 + 808 nm light, the bactericidal rate of PDA-Cur against E. coli enhanced to 100% at 0.1 nM. In addition, PDA-Cur had low cytotoxicity and negligible hemolytic activity, showing good biocompatibility. Conclusion PDA-Cur nanocomposites had good photodynamic effect, photo thermal conversion ability and biocompatibility. Compared with free Cur, the antibacterial activity of PDA-Cur was significantly improved, and the antibacterial effect with combined light was stronger than that of free Cur. Therefore, the construction of PDA-Cur nanocomposites have confirmed that the combination of PDT and PTT can greatly improve the antibacterial effect and reach bactericidal effect at low concentration, which provides a strategy for the design of next generation antimicrobial agents.

PVA Films with Mixed Silver Nanoparticles and Gold Nanostars for Intrinsic and Photothermal Antibacterial Action

PVA films with embedded either silver nanoparticles (AgNP), NIR-absorbing photothermal gold nanostars (GNS), or mixed AgNP+GNS were prepared in this research. The optimal conditions to obtain stable AgNP+GNS films with intact, long lasting photothermal GNS were obtained. These require coating of GNS with a thiolated polyethylene glycol (PEG) terminated with a carboxylic acid function, acting as reticulant in the film formation. In the mixed AgNP+GNS films, the total noble metal content is <0.15% w/w and in the Ag films < 0.025% w/w. The slow but prolonged Ag⁺ release from film-embedded AgNP (8–11% of total Ag released after 24 h, in the mixed films) results in a very strong microbicidal effect against planktonic Escherichia coli and Staphylococcus aureus bacterial strains (the release of Au from films is instead negligible). Beside this intrinsic effect, the mixed films also exert an on-demand, fast hyperthermal bactericidal action, switched on by NIR laser irradiation (800 nm, i.e., inside the biotransparent window) of the localized surface plasmon resonance (LSPR) absorption bands of GNS. Temperature increases of 30 °C are obtained using irradiances as low as 0.27 W/cm². Moreover, 80–90% death on both strains was observed in bacteria in contact with the GNS-containing films, after 30 min of irradiation. Finally, the biocompatibility of all films was verified on human fibroblasts, finding negligible viability decrease in all cases.

A mechanistic perspective on targeting bacterial drug resistance with nanoparticles

Bacterial infections are an important cause of mortality worldwide owing to the prevalence of drug resistant bacteria. Bacteria develop resistance against antimicrobial drugs by several mechanisms such as enzyme inactivation, reduced cell permeability, modifying target site or enzyme, enhanced efflux because of high expression of efflux pumps, biofilm formation or drug-resistance gene expression. New and alternative ways such as nanoparticle (NP) applications are being established to overcome the growing multidrug-resistance in bacteria. NPs have unique antimicrobial characteristics that make them appropriate for medical application to overcome antibiotic resistance. The proposed antibacterial mechanisms of NPs are cell membrane damage, changing cell wall penetration, reactive oxygen species (ROS) production, effect on DNA and proteins, and impact on biofilm formation. The present review mainly focuses on discussing various mechanisms of bacterial drug resistance and the applications of NPs as alternative antibacterial systems. Combination therapy of NPs and antibiotics as a novel approach in medicine towards antimicrobial resistance is also discussed.

Plasmonically Modulated Gold Nanostructures for Photothermal Ablation of Bacteria

With the wide utilization of antibiotics, antibiotic-resistant bacteria have been often developed more frequently to cause potential global catastrophic consequences. Emerging photothermal ablation has been attracting extensive research interest for quick/effective eradication of pathogenic bacteria from contaminated surroundings and infected body. In this field, anisotropic gold nanostructures with tunable size/morphologies have been demonstrated to exhibit their outstanding photothermal performance through strong plasmonic absorption of near-infrared (NIR) light, efficient light to heat conversion, and easy surface modification for targeting bacteria. To this end, this review first introduces thermal treatment of infectious diseases followed by photothermal therapy via heat generation on NIR-absorbing gold nanostructures. Then, the usual synthesis and spectral features of diversified gold nanostructures and composites are systematically overviewed with the emphasis on the importance of size, shape, and composition to achieve strong plasmonic absorption in NIR region. Further, the innovated photothermal applications of gold nanostructures are comprehensively demonstrated to combat against bacterial infections, and some constructive suggestions are also discussed to improve photothermal technologies for practical applications.

Intracellular optical probing with gold nanostars

Gold nanostars are important nanoscopic tools in biophotonics and theranostics. To understand the fate of such nanostructures in the endolysosomal system of living cells as an important processing route in biotechnological approaches, un-labelled, non-targeted gold nanostars synthesized using HEPES buffer were studied in two cell lines. The uptake of the gold nanostructures leads to cell line-dependent intra-endolysosomal agglomeration, which results in a greater enhancement of the local optical fields than those around individual nanostars and near aggregates of spherical gold nanoparticles of the same size. As demonstrated by non-resonant surface-enhanced Raman scattering (SERS) spectra in the presence and absence of aggregation, the spectroscopic signals of molecules are of very similar strength over a wide range of concentrations, which is ideal for label-free vibrational characterization of cells and other complex environments. In 3T3 and HCT-116 cells, SERS data were analyzed together with the properties of the intracellular nanostar agglomerates. Vibrational spectra indicate that the processing of nanostars by cells and their interaction with the surrounding endolysosomal compartment is connected to their morphological properties through differences in the structure and interactions in their intracellular protein corona. Specifically, different intracellular processing was found to result from a different extent of hydrophobic interactions at the pristine gold surface, which varies for nanostars of different spike lengths. The sensitive optical monitoring of surroundings of nanostars and their intracellular processing makes them a very useful tool for optical bionanosensing and therapy.

Photothermal bactericidal surfaces: killing bacteria using light instead of biocides

Recent developments of photothermal bactericidal surfaces based on immobilized photothermal agents to kill bacteria through hyperthermia effects are reviewed.

In Situ Shape Control of Thermoplasmonic Gold Nanostars on Oxide Substrates for Hyperthermia-Mediated Cell Detachment

Gold nanostars (AuNSTs) are biocompatible, have large surface areas, and are characterized by high near-infrared extinction, making them ideal for integration with technologies targeting biological applications. We have developed a robust and simple microfluidic method for the direct growth of anisotropic AuNSTs on oxide substrates including indium tin oxide and glass. The synthesis was optimized to yield AuNSTs with high anisotropy, branching, uniformity, and density in batch and microfluidic systems for optimal light-to-heat conversion upon laser irradiation. Surface-enhanced Raman scattering spectra and mesoscale temperature measurements were combined with spatially correlated scanning electron microscopy to monitor nanostar and ligand stability and microbubble formation at different laser fluences. The capability of the platform for generating controlled localized heating was used to explore hyperthermia-assisted detachment of adherent glioblastoma cells (U87-GFP) grafted to the capillary walls. Both flow and laser fluence can be tuned to induce different biological responses, such as ablation, cell deformation, release of intracellular components, and the removal of intact cells. Ultimately, this platform has potential applications in biological and chemical sensing, hyperthermia-mediated drug delivery, and microfluidic soft-release of grafted cells with single-cell specificity.

Thermo-resistance of ESKAPE-panel pathogens, eradication and growth prevention of an infectious biofilm by photothermal, polydopamine-nanoparticles in vitro

Nanotechnology offers many novel infection-control strategies that may help prevent and treat antimicrobial-resistant bacterial infections. Here, we synthesized polydopamine, photothermal-nanoparticles (PDA-NPs) without further surface-functionalization to evaluate their potential with respect to biofilm-control. Most ESKAPE-panel pathogens in suspension with photothermal-nanoparticles showed three- to four-log-unit reductions upon Near-Infra-Red (NIR)-irradiation, but for enterococci only less than two-log unit reduction was observed. Exposure of existing Staphylococcus aureus biofilms to photothermal-nanoparticles followed by NIR-irradiation did not significantly kill biofilm-inhabitants. This indicates that the biofilm mode of growth poses a barrier to penetration of photothermal-nanoparticles, yielding dissipation of heat to the biofilm-surrounding rather than in its interior. Staphylococcal biofilm-growth in the presence of photothermal-nanoparticles could be significantly prevented after NIR-irradiation because PDA-NPs were incorporated in the biofilm and heat dissipated inside it. Thus, unmodified photothermal nanoparticles have potential for prophylactic infection-control, but data also constitute a warning for possible development of thermo-resistance in infectious pathogens.

Phototherapy-based combination strategies for bacterial infection treatment

The development of nanomedicine is expected to provide an innovative direction for addressing challenges associated with multidrug-resistant (MDR) bacteria. In the past decades, although nanotechnology-based phototherapy has been developed for antimicrobial treatment since it rarely causes bacterial resistance, the clinical application of single-mode phototherapy has been limited due to poor tissue penetration of light sources. Therefore, combinatorial strategies are being developed. In this review, we first summarized the current phototherapy agents, which were classified into two functional categories: organic phototherapy agents (e.g., small molecule photosensitizers, small molecule photosensitizer-loaded nanoparticles and polymer-based photosensitizers) and inorganic phototherapy agents (e.g., carbo-based nanomaterials, metal-based nanomaterials, composite nanomaterials and quantum dots). Then the development of emerging phototherapy-based combinatorial strategies, including combination with chemotherapy, combination with chemodynamic therapy, combination with gas therapy, and multiple combination therapy, are presented and future directions are further discussed. The purpose of this review is to highlight the potential of phototherapy to deal with bacterial infections and to propose that the combination therapy strategy is an effective way to solve the challenges of single-mode phototherapy.

Self-Propelled Active Photothermal Nanoswimmer for Deep-Layered Elimination of Biofilm In Vivo

Increasing penetration of antibacterial agents into biofilm is a promising strategy for improvement of therapeutic effect and slowdown of the progression of antibiotic resistance. Herein, we design a near-infrared (NIR) light-driven nanoswimmer (HSMV). Under NIR light irradiation, HSMV performs efficient self-propulsion and penetrates into the biofilm within 5 min due to photothermal conversion of asymmetrically distributed AuNPs. The localized thermal (∼45 °C) and thermal-triggered release of vancomycin (Van) leads to an efficient combination of photothermal therapy and chemotherapy in one system. The active motion of HSMV increases the effective distance of photothermal therapy (PTT) and also improves the therapeutic index of the antibiotic, resulting in superior biofilm removal rate (>90%) in vitro. Notably, HSMV can eliminate S. aureus biofilms grown in vivo under 10 min of laser irradiation without damage to healthy tissues. This work may shed light on therapeutic strategies for in vivo treatment of biofilm-associated infections.

Eradicating Infecting Bacteria while Maintaining Tissue Integration on Photothermal Nanoparticle-Coated Titanium Surfaces

Photothermal nanoparticles locally release heat when irradiated by near-infrared (NIR). Clinical applications initially involved tumor treatment, but currently extend toward bacterial infection control. Applications toward much smaller, micrometer-sized bacterial infections, however, bear the risk of collateral damage by dissipating heat into tissues surrounding an infection site. This can become a complication when photothermal nanoparticle coatings are clinically applied on biomaterial surfaces requiring tissue integration, such as titanium-made, bone-anchored dental implants. Dental implants can fail due to infection in the pocket formed between the implant screw and the surrounding soft tissue ("peri-implantitis"). We address the hitherto neglected potential complication of collateral tissue damage by evaluating photothermal, polydopamine nanoparticle (PDA-NP) coatings on titanium surfaces in different coculture models. NIR irradiation of PDA-NP-coated (200 μg/cm2) titanium surfaces with adhering Staphylococcus aureus killed staphylococci within an irradiation time window of around 3 min. Alternatively, when covered with human gingival fibroblasts, this irradiation time window maintained surface coverage by fibroblasts. Contaminating staphylococci on PDA-NP-coated titanium surfaces, as can be per-operatively introduced, reduced surface coverage by fibroblasts, and this could be prevented by NIR irradiation for 5 min or longer prior to allowing fibroblasts to adhere and grow. Negative impacts of early postoperative staphylococcal challenges to an existing fibroblast layer covering a coated surface were maximally prevented by 3 min NIR irradiation. Longer irradiation times caused collateral fibroblast damage. Late postoperative staphylococcal challenges to a protective keratinocyte layer covering a fibroblast layer required 10 min NIR irradiation for adverting a staphylococcal challenge. This is longer than foreseen from monoculture studies because of additional heat uptake by the keratinocyte layer. Summarizing, photothermal treatment of biomaterial-associated infection requires precise timing of NIR irradiation to prevent collateral damage to tissues surrounding the infection site.

Prussian blue nanoparticles: synthesis, surface modification, and biomedical applications

Prussian blue nanoparticles (PBNPs) are a nanomaterial that presents unique properties and an excellent biocompatibility. They can be synthesized in mild conditions and can be derivatized with polymers and/or biomolecules. PBNPs are used in biomedicine as therapy and diagnostic agents. In biomedical imaging, PBNPs constitute contrast agents in photoacoustic and magnetic resonance imaging (MRI). They are a good adsorbent to be used as antidotes for poisoning with cesium and/or thallium ions. Moreover, the ability to convert energy into heat makes them useful photothermal agents (PAs) in photothermal therapy (PTT) or as nonantibiotic substances with antibacterial properties. Finally, PBNPs can be both reduced to Prussian white and oxidized to Prussian green. A large window of redox potential exists between reduction and oxidation, which result in the enzyme-like characteristics of these NPs.

Rapid and effective photodynamic treatment of biofilm infections using low doses of amoxicillin-coated gold nanoparticles

Bacterial biofilm are complex microbial communities covered by a matrix of extracellular polymeric substances, which develops when a community of microorganisms irreversibly adheres to a living or inert surface. This structure is considered an important virulence factor because it is difficult to eradicate and often responsible for treatment failures. This adherent community represents one of the greatest problems in public health due to the continued emergence of conventional antibiotic-therapy resistance. Photodynamic Antimicrobial Therapy (PACT) is a therapeutic alternative and promises to be an effective treatment against multiresistant bacteria biofilm, demonstrating a broad spectrum of action. This work demonstrates the reduction in biofilms of relevant clinical isolates (as Pseudomonas aeruginosa and Staphylococcus aureus) treated with PACT using low concentrations of amoxicillin-coated gold nanoparticles (amoxi@AuNP) as a photosensitizer. Moreover, the viability reduction of 60% in S. aureus biofilms and 70% in P. aeruginosa biofilms were obtained after three hours of irradiation with white light and amoxi@AuNP. Scanning electron microscopy analysis revealed that amoxi@AuNP could penetrate and cause damage to the biofilm matrix, and interact with bacteria cells. A strong biofilm production in P. aeruginosa was observed by confocal laser scanning microscopy using acridine orange as a probe, and a markedly decrease in live bacteria was appreciated when PACT was applied. The use of amoxi@AuNP for PACT allows the viability reduction of clinical Gram positive and Gram negative biofilms. This novel strategy needs shorter irradiation times and lower concentrations of nanoparticles than other reports described. This could be attributed to two major innovations: the selectivity for the bacterial wall given by the amoxicillin and the polydispersity of size and shapes with seems to contribute to the photo-antibacterial capacity.

Antimicrobial Metal Nanomaterials: From Passive to Stimuli-Activated Applications

The development of antimicrobial drug resistance among pathogenic bacteria and fungi is one of the most significant health issues of the 21st century. Recently, advances in nanotechnology have led to the development of nanomaterials, particularly metals that exhibit antimicrobial properties. These metal nanomaterials have emerged as promising alternatives to traditional antimicrobial therapies. In this review, a broad overview of metal nanomaterials, their synthesis, properties, and interactions with pathogenic micro-organisms is first provided. Secondly, the range of nanomaterials that demonstrate passive antimicrobial properties are outlined and in-depth analysis and comparison of stimuli-responsive antimicrobial nanomaterials are provided, which represent the next generation of microbiocidal nanomaterials. The stimulus applied to activate such nanomaterials includes light (including photocatalytic and photothermal) and magnetic fields, which can induce magnetic hyperthermia and kinetically driven magnetic activation. Broadly, this review aims to summarize the currently available research and provide future scope for the development of metal nanomaterial-based antimicrobial technologies, particularly those that can be activated through externally applied stimuli.

Plasmonic photothermal heating of gold nanostars in a real-size container: multiscale modelling and experimental study

The ability of noble metal nanoparticles (NPs) to convert light into heat has triggered a lot of scientific interest due to the numerous potential applications, including, e.g. photothermal therapy or laser-based nanopatterning. In order for such applications to be practically implemented, the heating behaviour of NPs embedded in their surrounding medium has to be thoroughly understood, and theoretical models capable of predicting this behaviour must be developed. Here we propose a multiscale approach for modelling the photothermal response of a large ensemble of nanoparticles contained within a cm-scale, real-size container. Electromagnetic field, ray tracing and heat transfer simulations are combined in order to model the response of nanostars and nanospheres suspensions contained within a common Eppendorf tube. To validate the model, gold nanostars are then synthesised and characterized by electron microscopy and optical spectroscopy. Laser-induced heating experiments are conducted by irradiating colloid-filled Eppendorf tubes with a 785 nm continuous wave laser and monitoring by a thermographic camera. The experimental results confirm that the proposed model has potential for predicting and analysing the heating efficiency and temperature dynamics upon laser irradiation of plasmonic nanoparticle suspensions in real-scale containers, at cm³ volumes.

Gold nanostars-diagnosis, bioimaging and biomedical applications

Gold Nanostars (GNS) have attracted tremendous attention toward themselves owing to their multi-branched structure and unique properties. These state of the art metallic nanoparticles possess intrinsic features like remarkable optical properties and exceptional physiochemical activities. These star-shaped gold nanoparticles can predominantly be utilized in biosensing, photothermal therapy, imaging, surface-enhanced Raman spectroscopy and target drug delivery applications due to their low toxicity and extraordinary optical features. In the current review, recent approaches in the matter of GNS in case of diagnosis, bioimaging and biomedical applications were summarized and reported. In this regard, first an overview about the structure and general properties of GNS were reported and thence detailed information regarding the diagnostic, bioimaging, photothermal therapy, and drug delivery applications of such novel nanomaterials were presented in detail. Summarized information clearly highlighting the superior capability of GNS as potential multi-functional materials for biomedical applications.

Photothermally active nanoparticles as a promising tool for eliminating bacteria and biofilms

Bacterial contamination is a severe issue that affects medical devices, hospital tools and surfaces. When microorganisms adhere to a surface (e.g., medical devices or implants) they can develop into a biofilm, thereby becoming more resistant to conventional biocides and disinfectants. Nanoparticles can be used as an antibacterial agent in medical instruments or as a protective coating in implantable devices. In particular, attention is being drawn to photothermally active nanoparticles that are capable of converting absorbed light into heat. These nanoparticles can efficiently eradicate bacteria and biofilms upon light activation (predominantly near the infrared to near-infrared spectral region) due a rapid and pronounced local temperature increase. By using this approach new, protective, antibacterial surfaces and materials can be developed that can be remotely activated on demand. In this review, we summarize the state-of-the art regarding the application of various photothermally active nanoparticles and their corresponding nanocomposites for the light-triggered eradication of bacteria and biofilms.

Nanomaterials as Delivery Vehicles and Components of New Strategies to Combat Bacterial Infections: Advantages and Limitations

Life-threatening bacterial infections have been well-controlled by antibiotic therapies and this approach has greatly improved the health and lifespan of human beings. However, the rapid and worldwide emergence of multidrug resistant (MDR) bacteria has forced researchers to find alternative treatments for MDR infections as MDR bacteria can sometimes resist all the present day antibiotic therapies. In this respect, nanomaterials have emerged as innovative antimicrobial agents that can be a potential solution against MDR bacteria. The present review discusses the advantages of nanomaterials as potential medical means and carriers of antibacterial activity, the types of nanomaterials used for antibacterial agents, strategies to tackle toxicity of nanomaterials for clinical applications, and limitations which need extensive studies to overcome. The current progress of using different types of nanomaterials, including new emerging strategies for the single purpose of combating bacterial infections, is also discussed in detail.

Antibody-Functionalized MoS2 Nanosheets for Targeted Photothermal Therapy of Staphylococcus aureus Focal Infection

Bacterial biofilm-related diseases cause serious hazard to public health and bring great challenge to the traditional antibiotic treatment. Photothermal therapy (PTT) has been recognized as a promising alternative solution. However, the therapeutic efficacy of PTT is often compromised by the collateral damage to normal tissues due to the lack of bacteria-targeting capability. Here, a Staphylococcus aureus (S. aureus)-targeted PTT nanoagent is prepared based on antibody (anti-protein A IgG), polydopamine (PDA), and PEG-SH (thiolated poly (ethylene glycol)) functionalized MoS2 nanosheets (MoS2@PDA-PEG/IgG NSs, MPPI NSs). The PDA was used as bio-nano interface to facilitate the covalent conjugation of antibody and PEG-SH onto the surface of MoS2 NSs via facile catechol chemistry. Targeted PTT of MPPI NSs shows excellent inactivation efficiency of larger than 4 log (>99.99%) to S. aureus both in biofilms (in vitro) and in infected tissues (in vivo) without causing damage to normal mammalian cells. By contrast, non-targeted PTT of MoS2@PDA-PEG NSs (MPP NSs) only kills S. aureus by <90% in vitro and <50% in vivo. As a result, S. aureus focal infection in mice healed much faster after PTT of MPPI NSs than that of MPP NSs. The superiority of targeted PTT may originate from the efficient accumulation and close binding of PTT agents to bacterial cells. Therefore, MPPI NSs with bacteria-targeting capability are promising photothermal agents for effective treatment of S. aureus focal infection.

High Stability Thiol-Coated Gold Nanostars Monolayers with Photo-Thermal Antibacterial Activity and Wettability Control

The adhesion and proliferation of bacteria on abiotic surfaces pose challenges in both health care and industrial applications. Gold nanostars (GNSs) monolayers grafted on glass have demonstrated to exert antibacterial action due to their photo-thermal features. Here, these GNS layers were further functionalized using thiols monolayers, in order to impart different wettability to the surfaces and thus adding a feature that could help to fight bacterial proliferation. Thiol that has different functional groups was used and the thiol-protected surfaces were characterized by means of UV-vis spectroscopy, contact angles, SEM and surface enhanced Raman spectroscopy (SERS). We verified that (i) coating with the proper thiol allows us to impart high hydrophilicity or hydrophobicity to the surfaces (with contact angle values ranging from 10 to 120°); (ii) GNS monolayers are strongly stabilized by functionalization with thiols, with shelf stability increasing from a few weeks to more than three months and (iii) photo-thermal features and subsequent antibacterial effects caused by hyperthermia are not changed by thiols layers, allowing us to kill at least 99.99% of representative bacterial strains.

Photothermal-Induced Antibacterial Activity of Gold Nanorods Loaded into Polymeric Hydrogel against Pseudomonas aeruginosa Biofilm

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.

Novel photo-thermally active polyvinyl alcohol-Prussian blue nanoparticles hydrogel films capable of eradicating bacteria and mitigating biofilms

Antibacterial treatment is an essential issue in many diverse fields, from medical device treatments (for example prostheses coating) to food preservation. However, there is a need of novel and light-weight materials with high antibacterial efficiency (preferably due to the physical activation). Utilization of photo-thermally active nanoparticles can lead to novel and re-usable materials that can be remotely activated on-demand to thermally eradicate bacteria and mitigate biofilm formation, therefore meeting the above challenge. In this study polyvinyl alcohol (PVA) hydrogel films containing non-toxic and highly photo-thermally active Prussian blue (PB) nanoparticles were fabricated. The confocal microscopy studies indicated a uniform nanoparticle distribution and a low degree of aggregation. Upon near-infrared (NIR; 700 and 800 nm) light irradiation of PVA-PB films, the local temperature increases rapidly and reaches a plateau (up to ΔT ≅ 78 °C), within ≈6-10 s under relatively low laser intensities, I ≅ 0.3 W cm^-2. The high and localized increase of temperature on the fabricated films resulted in an efficient antibacterial effect on Pseudomonas aeruginosa (P. aeruginosa) bacteria. In addition, the localized photo-thermal effect was also sufficient to substantially mitigate biofilms growth.

Engineered Gold-Based Nanomaterials: Morphologies and Functionalities in Biomedical Applications. A Mini Review

In the last decade, several engineered gold-based nanomaterials, such as spheres, rods, stars, cubes, hollow particles, and nanocapsules have been widely explored in biomedical fields, in particular in therapy and diagnostics. As well as different shapes and dimensions, these materials may, on their surfaces, have specific functionalizations to improve their capability as sensors or in drug loading and controlled release, and/or particular cell receptors ligands, in order to get a definite targeting. In this review, the up-to-date progress will be illustrated regarding morphologies, sizes and functionalizations, mostly used to obtain an improved performance of nanomaterials in biomedicine. Many suggestions are presented to organize and compare the numerous and heterogeneous experimental data, such as the most important chemical-physical parameters, which guide and control the interaction between the gold surface and biological environment. The purpose of all this is to offer the readers an overview of the most noteworthy progress and challenges in this research field.

Nanomaterials with a photothermal effect for antibacterial activities: an overview

Nanomaterials and nanotechnologies have been expected to provide innovative platforms for addressing antibacterial challenges, with potential to even deal with bacterial infections involving drug-resistance. The current review summarizes recent progress over the last 3 years in the field of antibacterial nanomaterials with a photothermal conversion effect. We classify these photothermal nanomaterials into four functional categories: carbon-based nanoconjugates of graphene derivatives or carbon nanotubes, noble metal nanomaterials mainly from gold and silver, metallic compound nanocomposites such as copper sulfide and molybdenum sulfide, and polymeric as well as other nanostructures. Different categories can be assembled with each other to enhance the photothermal effects and the antibacterial activities. The review describes their fabrication processes, unique properties, antibacterial modes, and potential healthcare applications.

Special Issue on Nano-Systems for Antimicrobial Therapy

Antibacterial materials and surfaces designed and built using the toolbox of nanotechnology are becoming the object of an increasingly boosting interest, responding to the pan-drug resistant bacteria emergency [...]

Supramolecular Antibacterial Materials for Combatting Antibiotic Resistance

Antibiotic-resistant bacteria have emerged as a severe threat to human health. As effective antibacterial therapies, supramolecular materials display unprecedented advantages because of the flexible and tunable nature of their noncovalent interactions with biomolecules and the ability to incorporate various active agents in their platforms. Herein, supramolecular antibacterial materials are discussed using a format that focuses on their fundamental active elements and on recent advances including material selection, fabrication methods, structural characterization, and activity performance.

Responsive and Synergistic Antibacterial Coatings: Fighting against Bacteria in a Smart and Effective Way

Antibacterial coatings that eliminate initial bacterial attachment and prevent subsequent biofilm formation are essential in a number of applications, especially implanted medical devices. Although various approaches, including bacteria-repelling and bacteria-killing mechanisms, have been developed, none of them have been entirely successful due to their inherent drawbacks. In recent years, antibacterial coatings that are responsive to the bacterial microenvironment, that possess two or more killing mechanisms, or that have triggered-cleaning capability have emerged as promising solutions for bacterial infection and contamination problems. This review focuses on recent progress on three types of such responsive and synergistic antibacterial coatings, including i) self-defensive antibacterial coatings, which can "turn on" biocidal activity in response to a bacteria-containing microenvironment; ii) synergistic antibacterial coatings, which possess two or more killing mechanisms that interact synergistically to reinforce each other; and iii) smart "kill-and-release" antibacterial coatings, which can switch functionality between bacteria killing and bacteria releasing under a proper stimulus. The design principles and potential applications of these coatings are discussed and a brief perspective on remaining challenges and future research directions is presented.

Robust, reproducible, recyclable SERS substrates: monolayers of gold nanostars grafted on glass and coated with a thin silica layer

We prepared and characterized recyclable surface enhanced Raman spectroscopy (SERS) active glass chips. Gold nanostars were grafted on properly functionalized glasses by means of electrostatic interactions and then they were coated with a silica layer of controllable thickness in the nanometer range. The SERS activity of the obtained substrates were tested in terms of reproducibility and homogeneity intra-samples and inter-samples from different batches using the Raman reporter as the model compound rhodamine 6G. The uncoated substrates were used as reference to evaluate the effect of silica spacers on SERS enhancement factors (EFs). The chemical route to obtain silica-coated SERS chips is described in detail, and the morphology and the optical response of substrates have been characterized. We demonstrate that SERS substrates coated with 1 nm silica conserve a good EF, and that the coating confers to the SERS platform an extreme robustness leading to reusability of the substrates.

Nanoplasmonics for Real-Time and Label-Free Monitoring of Microbial Biofilm Formation

Microbial biofilms possess intrinsic resistance against conventional antibiotics and cleaning procedures; thus, a better understanding of their complex biological structures is crucial in both medical and industrial applications. Existing laboratory methodologies have focused on macroscopic and mostly indirect characterization of mechanical and microbiological properties of biofilms adhered on a given substrate. However, the kinetics underlying the biofilm formation is not well understood, while such information is critical to understanding how drugs and chemicals influence the biofilm formation. Herein, we report the use of localized surface plasmon resonance (LSPR) for real-time, label-free monitoring of E. coli biofilm assembly on a nanoplasmonic substrate consisting of gold mushroom-like structures. Our LSPR sensor is able to capture the signatures of biofilm formation in real-time by measuring the wavelength shift in the LSPR resonance peak with high temporal resolution. We employ this sensor feature to elucidate how biofilm formation is affected by different drugs, including conventional antibiotics (kanamycin and ampicillin) as well as rifapentine, a molecule preventing cell adhesion yet barely affecting bacterial viability and vitality. Due to its flexibility and simplicity, our LSPR based platform can be used on a wide variety of clinically relevant bacteria, thus representing a valuable tool in biofilm characterization and drug screening.

Decorating gold nanostars with multiwalled carbon nanotubes for photothermal therapy

Gold nanoparticles and carbon nanotubes have attracted substantial attention in recent years for their potential applications in photothermal therapy (PTT) as an emerging breakthrough in cancer treatment. Herein, a hybrid nanomaterial of gold nanostars/multiwalled carbon nanotubes (MWCNTs) was synthesized by two-step reduction via the control of several synthetic conditions such as the reducing agent, pH value, concentration and ratio of reagents. The material shows good biocompatibility and high photothermal conversion efficiency, demonstrating its applicability in PTT. The lack of surfactant in the synthesis process made the hybrid nanomaterial cell-friendly, with no effects on viability in vitro. The MWCNT/gold nanostars hybrid nanomaterial presented 12.4% higher photothermal efficiency than gold nanostars alone and showed a 2.4-fold increase over gold nanospheres based on a heating test under 808 nm laser irradiation. Moreover, the MWCNTs/gold nanostars at low concentration (0.32 nM) exhibited remarkably improved photothermal cancer cell-killing efficacy, which may be attributed to the surface plasmon resonance absorption of the gold nanostars and the combined effects of enhanced coupling between the MWCNTs and gold nanostars. Collectively, these results demonstrate that the MWCNTs/gold nanostars developed herein show prominent photothermal value, and thus may serve as a novel photothermal agent for cancer therapy.

Nano-Strategies to Fight Multidrug Resistant Bacteria-"A Battle of the Titans"

Infectious diseases remain one of the leading causes of morbidity and mortality worldwide. The WHO and CDC have expressed serious concern regarding the continued increase in the development of multidrug resistance among bacteria. Therefore, the antibiotic resistance crisis is one of the most pressing issues in global public health. Associated with the rise in antibiotic resistance is the lack of new antimicrobials. This has triggered initiatives worldwide to develop novel and more effective antimicrobial compounds as well as to develop novel delivery and targeting strategies. Bacteria have developed many ways by which they become resistant to antimicrobials. Among those are enzyme inactivation, decreased cell permeability, target protection, target overproduction, altered target site/enzyme, increased efflux due to over-expression of efflux pumps, among others. Other more complex phenotypes, such as biofilm formation and quorum sensing do not appear as a result of the exposure of bacteria to antibiotics although, it is known that biofilm formation can be induced by antibiotics. These phenotypes are related to tolerance to antibiotics in bacteria. Different strategies, such as the use of nanostructured materials, are being developed to overcome these and other types of resistance. Nanostructured materials can be used to convey antimicrobials, to assist in the delivery of novel drugs or ultimately, possess antimicrobial activity by themselves. Additionally, nanoparticles (e.g., metallic, organic, carbon nanotubes, etc.) may circumvent drug resistance mechanisms in bacteria and, associated with their antimicrobial potential, inhibit biofilm formation or other important processes. Other strategies, including the combined use of plant-based antimicrobials and nanoparticles to overcome toxicity issues, are also being investigated. Coupling nanoparticles and natural-based antimicrobials (or other repurposed compounds) to inhibit the activity of bacterial efflux pumps; formation of biofilms; interference of quorum sensing; and possibly plasmid curing, are just some of the strategies to combat multidrug resistant bacteria. However, the use of nanoparticles still presents a challenge to therapy and much more research is needed in order to overcome this. In this review, we will summarize the current research on nanoparticles and other nanomaterials and how these are or can be applied in the future to fight multidrug resistant bacteria.

Prussian Blue Nanoparticles as a Versatile Photothermal Tool

Prussian blue (PB) is a coordination polymer studied since the early 18th century, historically known as a pigment. PB can be prepared in colloidal form with a straightforward synthesis. It has a strong charge-transfer absorption centered at ~700 nm, with a large tail in the Near-IR range. Irradiation of this band results in thermal relaxation and can be exploited to generate a local hyperthermia by irradiating in the so-called bio-transparent Near-IR window. PB nanoparticles are fully biocompatible (PB has already been approved by FDA) and biodegradable, this making them ideal candidates for in vivo use. While papers based on the imaging, drug-delivery and absorbing properties of PB nanoparticles have appeared and have been reviewed in the past decades, a very recent interest is flourishing with the use of PB nanoparticles as photothermal agents in biomedical applications. This review summarizes the syntheses and the optical features of PB nanoparticles in relation to their photothermal use and describes the state of the art of PB nanoparticles as photothermal agents, also in combination with diagnostic techniques.

Thermo-plasmonic gold nanofilms for simple and mass-producible photothermal neural interfaces

In recent years, photothermal stimulation methods using plasmonic metal nanoparticles have emerged as non-genetic optical techniques in neuromodulation. Although nanoparticle-based photothermal stimulation shows great potential in the excitation and the inhibition of neural activity, the complex synthesis processes of the nanoparticles and the lack of large-area deposition methods can be limiting factors for the development of photothermal neural devices. In this paper, we propose a plasmonic gold nanofilm, fabricated by a standard thermal evaporation process, as a simple and mass-producible photothermal neural interface layer for microelectrode array (MEA) chips. The absorption of the gold nanofilm at near infrared wavelengths is optimized to maximize the photothermal effect by varying the thickness and microstructure of the gold nanofilm. With the optimized conditions, a significantly strong photothermal effect is applied on MEAs without affecting the neural signal recording capability. Finally, primary rat hippocampal neuronal cultures are used to show that the photothermal neural inhibition using the gold nanofilm is as effective as that using the plasmonic nanoparticles. Due to the greater simplicity and versatility of the fabrication process, the plasmonic gold nanofilm can provide a promising solution for the mass production of photothermal platforms.

One particle, two targets: A combined action of functionalised gold nanoparticles, against Pseudomonas fluorescens biofilms

Attempts to deal with the problem of detrimental biofilms using nanoparticle technologies have generally focussed on exploiting biocidal approaches. However, it is now recognised that biofilm matrix-components may be targets for the disruption or dispersion of biofilms. Here, we show that the functionalization of gold nanoparticles with the enzyme, proteinase-K (PK) led to both biocidal and matrix disruption effects within Pseudomonas fluorescens biofilms and released cells. This study highlights the potential mechanisms underpinning the properties of Proteinase-K functionalized gold nanoparticles. With the emergence of biocide-resistant biofilm-forming organisms, novel nanoparticle strategies may provide the ideal solution for disrupting and inactivating biofilm cells, thereby minimising the use of biocides or antibiotics.