Bacteria-induced aggregation of bioorthogonal gold nanoparticles for SERS imaging and enhanced photothermal ablation of Gram-positive bacteria

Synergizing Nanomaterials and Artificial Intelligence in Advanced Optical Biosensors for Precision Antimicrobial Resistance Diagnosis

Antimicrobial resistance (AMR) poses a critical global One Health concern, ensuing from unintentional and continuous exposure to antibiotics, as well as challenges in accurate contagion diagnostics. Addressing AMR requires a strategic approach that emphasizes early stage prevention through screening in clinical, environmental, farming, and livestock settings to identify nonvulnerable antimicrobial agents and the associated genes. Conventional AMR diagnostics, like antibiotic susceptibility testing, possess drawbacks, including high costs, time-consuming processes, and significant manpower requirements, underscoring the need for intelligent, prompt, and on-site diagnostic techniques. Nanoenabled artificial intelligence (AI)-supported smart optical biosensors present a potential solution by facilitating rapid point-of-care AMR detection with real-time, sensitive, and portable capabilities. This Review comprehensively explores various types of optical nanobiosensors, such as surface plasmon resonance sensors, whispering-gallery mode sensors, optical coherence tomography, interference reflection imaging sensors, surface-enhanced Raman spectroscopy, fluorescence spectroscopy, microring resonance sensors, and optical tweezer biosensors, for AMR diagnostics. By harnessing the unique advantages of these nanoenabled smart biosensors, a revolutionary paradigm shift in AMR diagnostics can be achieved, characterized by rapid results, high sensitivity, portability, and integration with Internet-of-Things (IoT) technologies. Moreover, nanoenabled optical biosensors enable personalized monitoring and on-site detection, significantly reducing turnaround time and eliminating the human resources needed for sample preservation and transportation. Their potential for holistic environmental surveillance further enhances monitoring capabilities in diverse settings, leading to improved modern-age healthcare practices and more effective management of antimicrobial treatments. Embracing these advanced diagnostic tools promises to bolster global healthcare capacity to combat AMR and safeguard One Health.

Phytic Acid-Promoted Deposition of Gold Nanoparticles with Grafted Cationic Polymer Brushes for the Construction of Synergistic Contact-Killing and Photothermal Bactericidal Coatings

Medical implants are constantly facing the risk of bacterial infections, especially infections caused by multidrug resistant bacteria. To mitigate this problem, gold nanoparticles with alkyl bromide moieties (Au NPs-Br) on the surfaces were prepared. Xenon light irradiation triggered the plasmon effect of Au NPs-Br to induce free radical graft polymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA), leading to the formation of poly(DMAEMA) brush-grafted Au NPs (Au NPs-g-PDM). The Au NPs-g-PDM nanocomposites were conjugated with phytic acid (PA) via electrostatic interaction and van der Waals interaction. The as-formed aggregates were deposited on the titanium (Ti) substrates to form the PA/Au NPs-g-PDM (PAP) hybrid coatings through surface adherence of PA and the gravitational effect. Synergistic bactericidal effects of contact-killing caused by the cationic PDM brushes, and local heating generated by the Au NPs under near-infrared irradiation, conferred strong antibacterial effects on the PAP-deposited Ti (Ti-PAP) substrates. The synergistic bactericidal effects reduced the threshold temperature required for the photothermal sterilization, which in turn minimized the secondary damage to the implant site. The Ti-PAP substrates exhibited 97.34% and 99.97% antibacterial and antiadhesive efficacy, respectively, against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli), compared to the control under in vitro antimicrobial assays. Furthermore, the as-constructed Ti-PAP surface exhibited a 99.42% reduction in the inoculated S. aureus under in vivo assays. In addition, the PAP coatings exhibited good biocompatibility in the hemolysis and cytotoxicity assays as well as in the subcutaneous implantation of rats.

Bioorthogonal Chemistry in Translational Research: Advances and Opportunities

Bioorthogonal chemistry is a rapidly expanding field of research that involves the use of small molecules that can react selectively with biomolecules in living cells and organisms, without causing any harm or interference with native biochemical processes. It has made significant contributions to the field of biology and medicine by enabling selective labeling, imaging, drug targeting, and manipulation of bio-macromolecules in living systems. This approach offers numerous advantages over traditional chemistry-based methods, including high specificity, compatibility with biological systems, and minimal interference with biological processes. In this review, we provide an overview of the recent advancements in bioorthogonal chemistry and their current and potential applications in translational research. We present an update on this innovative chemical approach that has been utilized in cells and living systems during the last five years for biomedical applications. We also highlight the nucleic acid-templated synthesis of small molecules by using bioorthogonal chemistry. Overall, bioorthogonal chemistry provides a powerful toolset for studying and manipulating complex biological systems, and holds great potential for advancing translational research.

Smart chitosan-based microgels for enhanced photothermal-assisted antibacterial activity

During recent years, antibiotic-resistant bacteria have rapidly emerged owing to the irrational use of antibiotics, rendering a global problem. Currently, few studies introduce customized antibacterial nanoplatforms to overcome antibiotic-resistance according to specific characteristic of bacteria, rather than abuse of antibiotic. Herein, with regard to personalized antibacterial nanoplatform, we design a novel antibiotic delivery nanocarrier composed of polyaniline-grafted-chitosan, presenting pH-responsive, conductive, photothermal, and biodegradable properties. After treatment with divalent anion (SO42-), the negatively charged nanocarriers are obtained for improving the loading efficacy of cationic vancomycin. Meanwhile, the controlled vancomycin release is achieved by lysozyme-triggered degradation of the nanocarrier. With the assistance of photothermal effect, the photothermal-assisted antibacterial effect of the nanocarriers have been effectively enhanced rather than that of a single antibacterial effect of vancomycin. Owing to the low heat resistance of Escherichia coli, photothermal effect can break the antibiotic-resistant bacteria membrane to render the convenient antibiotic entry, leading to the improved antibacterial efficacy. Therefore, the customization of a photothermal-assisted antibacterial on account of the characteristic of specific bacteria can definitely expand our arsenal for enhancing the antibacterial effect against antibiotic-resistant bacteria.

Limits of antibacterial activity of triangular silver nanoplates and photothermal enhancement thereof for Bacillus subtilis

Currently, nanoparticles are being actively explored for antimicrobial applications involving variety of pathogens. Bacillus subtilis is a major concern considering its sporulation and biofilm formation capability which involves high bacteria loadings. Also, there is natural ability of B subtilis to adapt and develop resistance to the silver nanoparticles alone. So, this study reports the limits of antibacterial activity of triangular silver nanoplates (∆AgNPs) and further photothermal enhancement for B. subtilis ATCC 6051 for considerably high bacterial load of 2.5 × 107 to 5 × 108 CFU/ml. Triangular silver nanoplates were synthesized using one pot synthesis method and showed significant photothermal response i.e., ∼36 °C temperature rise on near infrared irradiation as well as photothermal stability. Triangular silver nanoplates alone showed absolute destruction for 2.5 × 107 CFU/ml initial B. subtilis load in 5 min. Whereas, for further higher bacterial loads, the antibacterial efficacy of ∆AgNPs is observed to be insignificant. For higher initial bacterial loads of 5 × 107 CFU/ml and 5 × 108 CFU/ml, photothermally enhanced triangular silver nanoplates resulted in complete destruction of bacteria in about 5 and 10 min, respectively. Antibacterial efficacy and mechanism of the destruction assessed via scanning electron microscopy and LIVE/DEAD assay confirmed morphological deformities. Further the generation of higher levels of reactive oxygen species is also confirmed due to photothermal activation of ∆AgNPs. The study concludes that ∆AgNPs alone are effective only up to bacterial load of 2.5 × 107 CFU/ml. Whereas, for higher bacterial loads of B. subtilis, photothermally activated ∆AgNPs lead to irreversible damage due to multiple targeting mechanisms leading to absolute elimination in short span of 5-10 min for the chosen irradiation conditions. Ultimately, this study demonstrates photothermally enhanced silver nanoplates as a potential antimicrobial agent for considerably high bacterial loads of B. subtilis. Overall, the broader window of considered high bacterial loadings and its irradiation by this technique shows the full-proof nature of photothermal applications for scenarios involving high cell density such as biofilms and wound infections etc. Further, the concept may be useful for sterilization or decontamination of samples, devices, etc. because B. subtilis and its spores are the challenges during sterilization.

Vaginal Drug Delivery Systems to Control Microbe-Associated Infections

The vagina has been regarded as a crucial route for drug delivery. Despite the wide range of available vaginal dosage forms for vaginal infection control, poor drug absorptivity remains a significant challenge due to various biological barriers in the vagina, such as mucus, epithelium, immune systems, and others. To overcome these barriers, different types of vaginal drug delivery systems (VDDSs), with outstanding mucoadhesive, mucus-penetrating properties, have been designed to enhance the absorptivity of vagina-administered agents in the past decades. In this Review, we introduce a general understanding of vaginal administration, its biological barriers, the commonly used VDDSs, such as nanoparticles and hydrogels, and their applications in controlling microbe-associated vaginal infections. Additionally, further challenges and concerns regarding the design of VDDSs will be discussed.

Photothermal/Photoacoustic Therapy Combined with Metal-Based Nanomaterials for the Treatment of Microbial Infections

The increased spread and persistence of bacterial drug-resistant phenotypes remains a public health concern and has contributed significantly to the challenge of combating antibiotic resistance. Nanotechnology is considered an encouraging strategy in the fight against antibiotic-resistant bacterial infections; this new strategy should improve therapeutic efficacy and minimize side effects. Evidence has shown that various nanomaterials with antibacterial performance, such as metal-based nanoparticles (i.e., silver, gold, copper, and zinc oxide) have intrinsic antibacterial properties. These antibacterial agents, such as those made of metal oxides, carbon nanomaterials, and polymers, have been used not only to improve antibacterial efficacy but also to reduce bacterial drug resistance due to their interaction with bacteria and their photophysical properties. These nanostructures have been used as effective agents for photothermal therapy (PTT) and photodynamic therapy (PDT) to kill bacteria locally by heating or the controlled production of reactive oxygen species. Additionally, PTT or PDT therapies have also been combined with photoacoustic (PA) imaging to simultaneously improve treatment efficacy, safety, and accuracy. In this present review, we present, on the one hand, a summary of research highlighting the use of PTT-sensitive metallic nanomaterials for the treatment of bacterial and fungal infections, and, on the other hand, an overview of studies showing the PA-mediated theranostic functionality of metal-based nanomaterials.

Highly efficient photothermal branched Au-Ag nanoparticles containing procyanidins for synergistic antibacterial and anti-inflammatory immunotherapy

Periodontitis is an inflammatory disease caused by bacterial infection. Excessive immune response and high levels of reactive oxygen species (ROS) further lead to the irreversible destruction of surrounding tissues. Developing new antimicrobial materials that regulate the immune system to resist inflammation can effectively treat periodontal inflammation. A nanoplatform integrating Ag⁺, photothermal therapy (PTT), and procyanidins (PC) for precision antibacterial and synergistic immunotherapy for periodontitis was proposed. This work loaded PC into AuAg nanoparticles, and the resulting nanocomposite was named AuAg-PC. PTT can effectively remove pathogenic bacteria, but high temperatures can cause tissue damage. Ag⁺ contributes to the preparation of a nanoparticle branched structure that improves the photothermal efficiency and helps PTT achieve an excellent antibacterial effect and avoid periodontal tissue damage. PC regulates host immunity by eliminating intracellular ROS, inhibiting inflammatory factors, and regulating macrophage polarisation in periodontal disease sites. It enhances the host's resistance to bacterial inflammation. AuAg-PC exerted an excellent anti-inflammatory effect and promoted tissue repair in animal periodontal inflammation models. Hence, AuAg-PC significantly combats periodontal pathogens and shows great application potential in the photothermal-assisted immunotherapy of periodontitis. This design provided a new controllable and efficient treatment platform for controlling persistent inflammation infection and regulating immunity.

Peptidoglycan-Directed Chemical Ligation for Selective Inhibition on Gram-Positive Bacteria

Microbicides with distinct antibacterial mechanisms show potential to combat multi-drug resistance bacteria. We herein report peptidoglycan-directed chemical ligation (PGCL) between alkyne-bearing vancomycin and an azide-tagged cationic polymer. The former binds peptidoglycan and inhibits peptidoglycan crosslinking, while the latter interferes the integrity of the bacterial membrane. PGCL results in enhanced bactericidal activity against Gram-positive Staphylococcus aureus (S. aureus) over Gram-negative Escherichia coli (E. coli). These data indicate the potential of PGCL to selectively and synergistically inhibit Gram-positive pathogens via dual modality antibacterial mechanisms of peptidoglycan-inhibiting antibiotics and bacterial membrane-disrupting polycations.

Recent advances in developing active targeting and multi-functional drug delivery systems via bioorthogonal chemistry

Bioorthogonal chemistry reactions occur in physiological conditions without interfering with normal physiological processes. Through metabolic engineering, bioorthogonal groups can be tagged onto cell membranes, which selectively attach to cargos with paired groups via bioorthogonal reactions. Due to its simplicity, high efficiency, and specificity, bioorthogonal chemistry has demonstrated great application potential in drug delivery. On the one hand, bioorthogonal reactions improve therapeutic agent delivery to target sites, overcoming off-target distribution. On the other hand, nanoparticles and biomolecules can be linked to cell membranes by bioorthogonal reactions, providing approaches to developing multi-functional drug delivery systems (DDSs). In this review, we first describe the principle of labeling cells or pathogenic microorganisms with bioorthogonal groups. We then highlight recent breakthroughs in developing active targeting DDSs to tumors, immune systems, or bacteria by bioorthogonal chemistry, as well as applications of bioorthogonal chemistry in developing functional bio-inspired DDSs (biomimetic DDSs, cell-based DDSs, bacteria-based and phage-based DDSs) and hydrogels. Finally, we discuss the difficulties and prospective direction of bioorthogonal chemistry in drug delivery. We expect this review will help us understand the latest advances in the development of active targeting and multi-functional DDSs using bioorthogonal chemistry and inspire innovative applications of bioorthogonal chemistry in developing smart DDSs for disease treatment.

Multifunctional Au@Pt@Ag NPs with color-photothermal-Raman properties for multimodal lateral flow immunoassay

Multimodal lateral flow immunoassay (LFIA) has displayed its potential to improve practicability and elasticity of point-of-care testing. Herein, multifunctional core-shell-shell Au@Pt@Ag NPs loaded with dual-layer Raman reporter molecules of 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) with a characteristic combination of color-photothermal-Raman performance were constructed for colorimetric LFIA (CM-LFIA), photothermal LFIA (PT-LFIA) and surface-enhanced Raman scattering-based LFIA (SERS-LFIA), respectively. The highly specific nanoprobes, being obtained through the combination of the resulted dual-layer DTNB modified Au@Pt@Ag NPs with the antibody, were triumphantly utilized in exploring multimodal LFIA with one visual qualitative and two optional quantitative modes with excellent sensing sensitivity. Under optimal conditions, the limit of detection (LOD) for the model hazardous analyte dehydroepiandrosterone (DHEA) were 1.0 ng mL^-1 for CM-LFIA, 0.42 ng mL^-1 for PT-LFIA, and 0.013 ng mL^-1 for SERS-LFIA, three of which were over 100-fold, 200-fold and 7 000-fold more sensitive than conventional visual AuNPs-based LFIA, respectively. In addition, the quantitative PT-LFIA and SERS-LFIA sensors worked well in spiked real samples with acceptable recoveries of 96.2 - 106.7% and 98.2 - 105.2%, respectively. This assay demonstrated that the developed multimodal LFIA had a great potential to be a powerful tool for accurate tracing hazardous analytes in complex samples.

Optimization of laser deposited silver nanoparticle substrates for surface-enhanced raman spectroscopy

Creating sensitive and reproducible substrates for surface-enhanced Raman spectroscopy (SERS) has been a challenge in recent years. While SERS offers significant benefits over traditional Raman spectroscopy, certain hindrances have limited their commercial use, especially in settings where low limits of detection are necessary. We studied a variety of laser-deposited silver microstructured SERS substrates with different morphology as a means to optimize analyte detection. We found that using a 405 nm laser to deposit lines of silver nanoparticles (AgNPS) from a 2 mM silver nitrate and sodium citrate solution offered not only the best enhancement, but also the most consistent and reproducible substrates. We also found that the probability of deposition by laser was wavelength dependent and that longer wavelengths were less likely to deposit than shorter wavelengths. This work offers a better understanding of the laser deposition process as well as how substrate shape and structure effect SERS signals.

Nanoglycocluster based diagnostic platform for colorimetric detection of bacteria; A comparative study analysing the effect of AuNPs size, linker length, and glycan diversity

Nanoglycoclusters, an upcoming class of functional nanomaterial are known to drive various processes like detection, imaging, targeting proteins, cells, and bacteria. Nanoglycoclusters are a type of nanomaterial functionalized with various glycans. The array of glycan in multiple copies enhances binding affinity with proteins. Selective and sensitive bacteria/lectin interactions using nanomaterials are an emerging area of research. The measurement of different ligand receptor interactions require sophisticated analytical tools that limit the application in biosensor domain. Recently, colorimetric biosensors gained importance in the field of the biosensor for the detection of bacteria/lectins. Herein we have demonstrated that different size of gold nanoparticles (AuNPs) along with various polyethylene glycol (PEG) linkers, functionalized with synthesized monopod and tripod of mannose and galactose that have different bacteria/lectins specificity. The newly synthesized nanoglycoclusters were able to discriminate between different lectins and bacteria. The aggregation of specific nanoglycocluster upon interaction with specific bacteria/lectins revealed that mannose monopod (MM) and mannose tripod (MT) are specific to Escherichia coli and concanavalin A (ConA) lectin, while galactose monopod (GM) and galactose tripod (GT) are specific to Pseudomonas aeruginosa and Peanut agglutinin (PNA) lectin. Further, the binding events depict the affinity of tripod glycans is more with respect to its corresponding monopod glycans. Our findings explored the potential of colorimetric sensing depending upon the size of AuNPs, linker length, specificity, along with glycans density to develop user friendly diagnostic system for the detection of bacteria.

Infection microenvironment-related antibacterial nanotherapeutic strategies

The emergence and spread of antibiotic resistance is one of the biggest challenges in public health. There is an urgent need to discover novel agents against the occurrence of multidrug-resistant bacteria, such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci. The drug-resistant pathogens are able to grow and persist in infected sites, including biofilms, phagosomes, or phagolysosomes, which are more difficult to eradicate than planktonic ones and also foster the development of drug resistance. For years, various nano-antibacterial agents have been developed in the forms of antibiotic nanocarriers. Inorganic nanoparticles with intrinsic antibacterial activity and inert nanoparticles assisted by external stimuli, including heat, photon, magnetism, or sound, have also been discovered. Many of these strategies are designed to target the unique microenvironment of bacterial infections, which have shown potent antibacterial effects in vitro and in vivo. This review summarizes ongoing efforts on antibacterial nanotherapeutic strategies related to bacterial infection microenvironments, including targeted antibacterial therapy and responsive antibiotic delivery systems. Several grand challenges and future directions for the development and translation of effective nano-antibacterial agents are also discussed. The development of innovative nano-antibacterial agents could provide powerful weapons against drug-resistant bacteria in systemic or local bacterial infections in the foreseeable future.

A biodegradable and near-infrared light-activatable photothermal nanoconvertor for bacterial inactivation

The development of biodegradable nanomaterials for near-infrared photothermal antibacterial is of great significance to improve the biosafety of nano-antibacterial strategies in clinical application. In this study, a new nano-antibacterial strategy was developed, in which a biodegradable charge-transfer nanocomplex acted as a high-efficiency near-infrared light-activatable photothermal nanoconvertor. The charge-transfer nanocomplex was synthesized through oxidation-induced self-assembly of 3,3',5,5'-tetramethylbenzidine molecules. This nanocomplex can efficiently convert light energy around 900 nm into heat energy, with a photothermal conversion efficiency of up to 30%. More importantly, the nanocomplex can spontaneously degrade under physiological conditions within 12 hours. Utilizing the photothermal effect of this nanocomplex, both Gram-positive bacteria and Gram-negative bacteria can be inactivated within 2 minutes. In addition, the inactivation mechanism was systematically discussed and the results indicated that the photothermal effect induced bacterial cell membrane damage was probably responsible for the antibacterial effect.

Prospects and applications of synergistic noble metal nanoparticle-bacterial hybrid systems

Hybrid systems composed of living cells and nanomaterials have been attracting great interest in various fields of research ranging from materials science to biomedicine. In particular, the interfacing of noble metal nanoparticles and bacterial cells in a single architecture aims to generate hybrid systems that combine the unique physicochemical properties of the metals and biological attributes of the microbial cells. While the bacterial cells provide effector and scaffolding functions, the metallic component endows the hybrid system with multifunctional capabilities. This synergistic effort seeks to fabricate living materials with improved functions and new properties that surpass their individual components. Herein, we provide an overview of this research field and the strategies for obtaining hybrid systems, and we summarize recent biological applications, challenges and current prospects in this exciting new arena.

Comparative study of an antimicrobial peptide and a neuropeptide conjugated with gold nanorods for the targeted photothermal killing of bacteria

There are certain disadvantages in treating bacterial infections through conventional methods. For this reason, the current study does focus on combating bacterial wound infections by photothermal therapy assisted by gold nanorod-peptide conjugates (GNR-peptide conjugates). Two peptides, the cationic antimicrobial peptide LL-37 and neuropeptide ANGIOPEP-2 both with specificity for targeted bacterial binding, were conjugated with GNR surface through electrostatic interactions. The GNR-peptide conjugates showed good biocompatibility, sufficient stability, enhanced targeting, potential photothermal killing of bacteria, and possible acceleration of wound healing. The photo-biomodulation properties of NIR improved the wound closure rates through enhanced cell migration. The multifunctional LL37-conjugated GNRs significantly enhanced photothermal therapeutic outcomes based on bacterial targeting with promising wound healing properties.

The recent progress in photothermal-triggered bacterial eradication

Increasing evidence suggested that bacterial infection diseases posed a great threat to human health and became the leading cause of mortality. However, the abuse of antibiotics and their residues in the environment result in the emergence and prevalence of drug-resistant bacteria. Photothermal therapy (PTT) has received considerable attention owing to its noninvasiveness, and proved to be promising in preventing bacterial infection diseases. In this review, we first surveyed the recent progress of PTT-based responsive targeting strategies for bacterial killing. We then highlighted the PTT-based smart designs of bio-films, hydrogels and synergistic methods for treating bacterial infections. Existing challenges and perspectives are also discussed to inspire the future development of a PTT-based platform for the efficient therapy of bacterial infections.

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.

Gold Nanoparticles: Can They Be the Next Magic Bullet for Multidrug-Resistant Bacteria?

In 2017 the World Health Organization (WHO) announced a list of the 12 multidrug-resistant (MDR) families of bacteria that pose the greatest threat to human health, and recommended that new measures should be taken to promote the development of new therapies against these superbugs. Few antibiotics have been developed in the last two decades. Part of this slow progression can be attributed to the surge in the resistance acquired by bacteria, which is holding back pharma companies from taking the risk to invest in new antibiotic entities. With limited antibiotic options and an escalating bacterial resistance there is an urgent need to explore alternative ways of meeting this global challenge. The field of medical nanotechnology has emerged as an innovative and a powerful tool for treating some of the most complicated health conditions. Different inorganic nanomaterials including gold, silver, and others have showed potential antibacterial efficacies. Interestingly, gold nanoparticles (AuNPs) have gained specific attention, due to their biocompatibility, ease of surface functionalization, and their optical properties. In this review, we will focus on the latest research, done in the field of antibacterial gold nanoparticles; by discussing the mechanisms of action, antibacterial efficacies, and future implementations of these innovative antibacterial systems.

Metabolic Labeling Mediated Targeting and Thermal Killing of Gram-Positive Bacteria by Self-Reporting Janus Magnetic Nanoparticles

Nanoparticles have been widely used in detection and killing of bacteria; however, targeting bacteria is still challenging. Delicate design of nanoparticles is required for simultaneous targeting, detection, and therapeutic functions. Here the use of Au/MnFe₂ O₄ (Au/MFO) Janus nanoparticles to target Gram-positive bacteria via metabolic labeling is reported and realize integrated self-reporting and thermal killing of bacteria. In these nanoparticles, the Au component is functionalized with tetrazine to target trans-cyclooctene group anchored on bacterial cell wall by metabolic incorporation of d-amino acids, and the MFO part exhibits peroxidase activity, enabling self-reporting of bacteria before treatment. The spatial separation of targeting and reporting functions avoids the deterioration of catalytic activity after surface modification. Also important is that MFO facilitates magnetic separation and magnetic heating, leading to easy enrichment and magnetic thermal therapy of labeled bacteria. This method demonstrates that metabolic labeling with d-amino acids is a promising strategy to specifically target and kill Gram-positive bacteria.

Gold-silver nanoshells promote wound healing from drug-resistant bacteria infection and enable monitoring via surface-enhanced Raman scattering imaging

Chronic infections, caused by multidrug-resistant (MDR) bacteria, constitute a serious problem yet often underappreciated in clinical practice. The in situ monitoring of the bacteria-infected disease is also necessary to track and verify the therapeutic effect. Herein we present a facile approach to overcome the above challenges through a Raman tag 3,3'-diethylthiatricarbocyanine iodide (DTTC)-conjugated gold-silver nanoshells (AuAgNSs). With a strong responsive of the near-infrared laser due to surface plasmon resonance (SPR) from hybrid metallic nanoshell structure, AuAgNSs exhibits an efficient photothermal effect, and it simultaneously releases silver ions during laser irradiation to bacterial eradicate. Herein, two MDR bacteria strain, methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum β-lactamase Escherichia coli, are chosen as models and studied both in vitro and in vivo. As a result, the AuAgNSs-DTTC substrates enable surface-enhanced Raman scattering imaging to provide a non-invasive and extremely high sensitive detection (down to 300 CFU mL^-1 for MRSA) and prolonged tracking (at least 8 days) of residual bacteria. In a chronic MRSA-infected wound mouse model, the AuAgNSs gel-mediated photothermal therapy/silver-release leads to a synergistic would healing with negligible toxicity or collateral damage to vital organs. These results suggest that AuAgNSs-DTTC is a promising anti-bacterial tool for clinical translation.

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.

Development and Application of Aptamer-Based Surface-Enhanced Raman Spectroscopy Sensors in Quantitative Analysis and Biotherapy

Surface-enhanced Raman scattering (SERS) is one of the most special and important Raman techniques. An apparent Raman signal can be observed when the target molecules are absorbed onto the surface of the SERS substrates, especially on the "hot spots" of the substrates. Early research focused on exploring the highly active SERS substrates and their detection applications in label-free SERS technology. However, it is a great challenge to use these label-free SERS sensors for detecting hydrophobic or non-polar molecules, especially in complex systems or at low concentrations. Therefore, antibodies, aptamers, and antimicrobial peptides have been used to effectively improve the target selectivity and meet the analysis requirements. Among these selective elements, aptamers are easy to use for synthesis and modifications, and their stability, affinity and specificity are extremely good; they have been successfully used in a variety of testing areas. The combination of SERS detection technology and aptamer recognition ability not only improved the selection accuracy of target molecules, but also improved the sensitivity of the analysis. Variations of aptamer-based SERS sensors have been developed and have achieved satisfactory results in the analysis of small molecules, pathogenic microorganism, mycotoxins, tumor marker and other functional molecules, as well as in successful photothermal therapy of tumors. Herein, we present the latest advances of the aptamer-based SERS sensors, as well as the assembling sensing platforms and the strategies for signal amplification. Furthermore, the existing problems and potential trends of the aptamer-based SERS sensors are discussed.