Nanostars shine bright for you

Synthesis of anisotropic gold nanoparticles in binary surfactant mixtures: a review on mechanisms of particle formation

Gold nanoparticles are promising candidates for a wide spectrum of biomedical applications ranging from diagnostics and sensors to therapeutics. Their plasmonic properties are dependent on their size and shape among other factors, which can be controlled by understanding various growth mechanisms. Since the breakthrough of the seed-mediated growth approach reported in 2001, the scientific community has actively engaged in the synthesis of tailored anisotropic gold nanoparticles. Surfactants are known for their shape-controlling abilities and since Nikoobakht and El-Sayed in 2003 used a binary surfactant mixture, this method has been adopted as a common synthesis strategy. A wide range of particle shapes have been produced in binary surfactant mixtures using different synthesis approaches, and different working mechanisms have been proposed. This calls for a thorough and critical evaluation of the synthetic methods with an aim to bridge the link between the use of binary surfactants and the control of morphology of anisotropic gold nanoparticles. This review gives a systematic overview of experimental procedures using binary surfactant mixtures to produce gold nanoparticles with tuned properties. The resulting shapes include gold nanorods, bipyramids, tetrahexahedra, and other anisotropic structures. Different mechanisms proposed based on experimental, simulation and modelling analyses are discussed based on the type of binary surfactant systems. Current challenges that need to be addressed and future prospects that may open up new avenues of applications for anisotropic gold nanoparticles are also discussed.

Two-Step Reshaping of Acicular Gold Nanoparticles

Anisometric plasmonic nanoparticles find applications in various fields, from photocatalysis to biosensing. However, exposure to heat or to specific chemical environments can induce their reshaping, leading to loss of function. Understanding this process is therefore relevant both for the fundamental understanding of such nano-objects and for their practical applications. We followed in real time the spontaneous reshaping of gold nanotetrapods in solution via optical absorbance spectroscopy, revealing a two-step kinetics (fast tip flattening into {110} facets, followed by slow arm shortening) with characteristic times a factor of 6 apart but sharing an activation energy around 1 eV. Synchrotron-based X-ray scattering confirms this time evolution, which is much faster in solution than in the dry state, highlighting the importance of the aqueous medium and supporting a dissolution-redeposition mechanism or facilitated surface diffusion. High-temperature transmission electron microscopy of the dry particles validates the solution kinetics.

Aerogels Based on Chitosan and Collagen Modified with Fe2O3 and Fe3O4 Nanoparticles: Fabrication and Characterization

The necessity to mitigate the intrinsic issues associated with tissue or organ transplants, in order to address the rising prevalence of diseases attributable to increased life expectancy, provides a rationale for the pursuit of innovation in the field of biomaterials. Specifically, biopolymeric aerogels represent a significant advancement in the field of tissue engineering, offering a promising solution for the formation of temporary porous matrices that can replace damaged tissues. However, the functional characteristics of these materials are inadequate, necessitating the implementation of matrix reinforcement methods to enhance their performance. In this study, chemical and green iron oxide nanoparticles, previously synthesized and documented in existing research, were incorporated into hybrid aerogels combining collagen (C) and chitosan (CH). The characterization of these aerogels was conducted through rheological, microstructural, and functional analyses. The results demonstrate that the incorporation of iron oxide nanoparticles has a significant influence on the properties of the aerogels fabricated with them. In particular, the incorporation of these nanoparticles has been observed to modify the mechanical properties, with an increase in strength and porosity that may support cell proliferation.

Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

Mellitic Acid-Supported Synthesis of Anisotropic Nanoparticles Used as SERS Substrate

A method for the synthesis of a new SERS substrate-anisotropic silver nanoparticles using mellitic acid as a new capping agent is presented. The synthesis is free of toxic substances and does not require special temperature or lighting conditions. Moreover, it is fast, easy, and inexpensive. Depending on the concentration of silver ions and nanoparticle seeds, four different colloids were obtained, representing the evolution of nanoparticle growth along different paths from the first common stage. One of the synthesized colloids consists mainly of triangular nanoplates, while the other consists of polyhedral NPs. The analysis of the synthesis process together with the observation of TEM images and UV-vis extinction spectra enabled the proposal of the mechanism of interaction of mellitic acid molecules as the capping agent. The ability of mellitic acid molecules to form a hydrogen bond network, together with a ratio of silver ions to the mellitic acid concentration, turned out to be crucial for determining the shape of the NPs. All obtained colloids strongly enhance the Raman spectra of analyte molecules, thus proving their applicability as efficient new SERS substrates. For the one that enhanced the spectra the most, the detection limit was set at 10-9 M. Using it as a SERS substrate enables the identification of a trace amount of a designer drug, i.e., 4-chloromethcathinone (4-CMC, clephedrone). For the first time, SERS spectra of this substance, illegal in many countries, are presented.

Operation-friendly and accurate naked-eye observation assay for fast zoonotic echinococcosis and pulmonary tuberculosis monitoring in clinics

Echinococcosis and tuberculosis are two common zoonotic diseases that can cause severe pulmonary infections. Early screening and treatment monitoring are of great significance, especially in areas with limited medical resources. Herein, we designed an operation-friendly and rapid magnetic enrichment-silver acetylene chromogenic immunoassay (Me-Sacia) to monitor the antibody. The main components included secondary antibody-modified magnetic nanoparticles (MNP-Ab2) as capture nanoparticles, specific peptide (EG95 or CFP10)-modified silver nanoparticles (AgNP-PTs) as detection nanoparticles, and alkyne-modified gold nanoflowers as chromogenic nanoparticles. Based on the magnetic separation and plasma luminescence techniques, Me-Sacia could completely replace the colorimetric assay of biological enzymes. It reduced the detection time to approximately 1 h and simplified the labor-intensive and equipment-intensive processes associated with conventional ELISA. Meanwhile, the Me-Sacia showed universality for various blood samples and intuitive observation with the naked eye. Compared to conventional ELISA, Me-Sacia lowered the detection limit by approximately 96.8 %, increased the overall speed by approximately 15 times, and improved sensitivity by approximately 7.2 %, with a 100 % specificity and a coefficient of variation (CV) of less than 15 %.

Tecoma stans Floral Extract-Based Biosynthesis for Enhanced Surface Plasmon-Coupled Emission and a Preliminary Study on Fluoroimmunoassay

We demonstrate the synthesis of biogenic supported silver spiked star architectures and their application to increase the electromagnetic field intensity at its tips that enhance plasmon-coupled emission. Tecoma stans floral extract has been used to synthesize silver nanocubes and spiked stars. We observe ∼445-fold and ∼680-fold enhancements in spacer and cavity configurations, respectively, in the SPCE platform. The hotspot intensity and Purcell factor are evaluated by carrying out finite-difference time-domain (FDTD) simulations. Time-based studies are presented to modulate the sharpness of the edges wherein an increase in the tip sharpness with the increase in reaction time up to 5 h is observed. The unique morphology of the silver architectures allowed us to utilize them in biosensing application. A SPCE-based fluoroimmunoassay was performed, achieving a 1.9 pg/mL limit of detection of TNF-α cytokine. This combination of anisotropic architectures, SPCE and immunoassay prove to be a powerful platform for the ultrasensitive detection of biomarkers in surface-bound assays.

Role of Surface Curvature in Gold Nanostar Properties and Applications

Gold nanostars (AuNSs) are nanoparticles with intricate three-dimensional structures and shape-dependent optoelectronic properties. For example, AuNSs uniquely display three distinct surface curvatures, i.e. neutral, positive, and negative, which provide different environments to adsorbed ligands. Hence, these curvatures are used to introduce different surface chemistries in nanoparticles. This review summarizes and discusses the role of surface curvature in AuNS properties and its impact on biomedical and chemical applications, including surface-enhanced Raman spectroscopy, contrast agent performance, and catalysis. We examine the main synthetic approaches to generate AuNSs, control their morphology, and discuss their benefits and drawbacks. We also describe the optical characteristics of AuNSs and discuss how these depend on nanoparticle morphology. Finally, we analyze how AuNS surface curvature endows them with properties distinctly different from those of other nanoparticles, such as strong electromagnetic fields at the tips and increased hydrophilic environments at the indentations, together making AuNSs uniquely useful for biosensing, imaging, and local chemical manipulation.

Plasmonic Dodecahedral-Walled Elongated Nanoframes for Surface-Enhanced Raman Spectroscopy

Here, elongated pseudohollow nanoframes composed of four rectangular plates enclosing the sides and two open-frame ends with four ridges pointing at the tips for near-field focusing are reported. The side facets act as light-collecting domains and transfer the collected light to the sharp tips for near-field focusing. The nanoframes are hollow inside, allowing the gaseous analyte to penetrate through the entire architecture and enabling efficient detection of gaseous analytes when combined with Raman spectroscopy. The resulting nanostructures are named Au dodecahedral-walled nanoframes. Synthesis of the nanoframes involves shape transformation of Au nanorods with round tips to produce Au-elongated dodecahedra, followed by facet-selective Pt growth, etching of the inner Au, and regrowth steps. The close-packed assembly of Au dodecahedral-walled nanoframes exhibits an attomolar limit of detection toward benzenethiol. This significant enhancement in SERS is attributed to the presence of a flat solid terrace for a large surface area, sharp edges and vertices for strong electromagnetic near-field collection, and open frames for effective analyte transport and capture. Moreover, nanoframes are applied to detect chemical warfare agents, specifically mustard gas simulants, and 20 times higher sensitivity is achieved compared to their solid counterparts.

Sensitization Strategies of Lateral Flow Immunochromatography for Gold Modified Nanomaterials in Biosensor Development

Gold nanomaterials have become very attractive nanomaterials for biomedical research due to their unique physical and chemical properties, including size dependent optical, magnetic and catalytic properties, surface plasmon resonance (SPR), biological affinity and structural suitability. The performance of biosensing and biodiagnosis can be significantly improved in sensitivity, specificity, speed, contrast, resolution and so on by utilizing multiple optical properties of different gold nanostructures. Lateral flow immunochromatographic assay (LFIA) based on gold nanoparticles (GNPs) has the advantages of simple, fast operation, stable technology, and low cost, making it one of the most widely used in vitro diagnostics (IVDs). However, the traditional colloidal gold (CG)-based LFIA can only achieve qualitative or semi-quantitative detection, and its low detection sensitivity cannot meet the current detection needs. Due to the strong dependence of the optical properties of gold nanomaterials on their shape and surface properties, gold-based nanomaterial modification has brought new possibilities to the IVDs: people have attempted to change the morphology and size of gold nanomaterials themselves or hybrid with other elements for application in LFIA. In this paper, many well-designed plasmonic gold nanostructures for further improving the sensitivity and signal output stability of LFIA have been summarized. In addition, some opportunities and challenges that gold-based LFIA may encounter at present or in the future are also mentioned in this paper. In summary, this paper will demonstrate some feasible strategies for the manufacture of potential gold-based nanobiosensors of post of care testing (POCT) for faster detection and more accurate disease diagnosis.

Photodynamic toluidine blue-gold nanoconjugates as a novel therapeutic for Staphylococcal biofilms

Staphylococci are among the most frequent bacteria known to cause biofilm-related infections. Pathogenic biofilms represent a global healthcare challenge due to their high tolerance to antimicrobials. In this study, water soluble polyethylene glycol (PEG)-coated gold nanospheres (28 ppm) and nanostars (15 ppm) with electrostatically adsorbed photosensitizer (PS) Toluidine Blue O (TBO) ∼4 μM were successfully synthesized and characterized as PEG-GNPs@TBO and PEG-GNSs@TBO. Both nanoconjugates and the TBO 4 μM solution showed remarkable, if similar, antimicrobial photodynamic inactivation (aPDI) effects at 638 nm, inhibiting the formation of biofilms by two Staphylococcal strains: a clinical methicillin-resistant Staphylococcus aureus (MRSA) isolate and Staphylococcus epidermidis (S. epidermidis) RP62A. Alternatively in biofilm eradication treatments, the aPDI effects of PEG-GNSs@TBO were more effective and yielded a 75% and 50% reduction in viable count of MRSA and S. epidermidis RP62A preformed biofilms, respectively and when compared with untreated samples. This reduction in viable count was even greater than that obtained through aPDI treatment using a 40 μM TBO solution. Confocal laser microscopy (CLSM) and scanning electron microscope (SEM) images of PEG-GNSs@TBO's aPDI treatments revealed significant changes in the integrity and morphology of biofilms, with fewer colony masses. The generation of reactive oxygen species (ROS) upon PEG-GNSs@TBO's aPDI treatment was detected by CLSM using a specific ROS fluorescent probe, demonstrating bright fluorescence red spots across the surfaces of the treated biofilms. Our findings shine a light on the potential synergism between gold nanoparticles (AuNPs) and photosensitizers in developing novel nanoplatforms to target Staphylococcal biofilm related infections.

Recent advances in nanoflowers: compositional and structural diversification for potential applications

In recent years, nanoscience and nanotechnology have emerged as promising fields in materials science. Spectroscopic techniques like scanning tunneling microscopy and atomic force microscopy have revolutionized the characterization, manipulation, and size control of nanomaterials, enabling the creation of diverse materials such as fullerenes, graphene, nanotubes, nanofibers, nanorods, nanowires, nanoparticles, nanocones, and nanosheets. Among these nanomaterials, there has been considerable interest in flower-shaped hierarchical 3D nanostructures, known as nanoflowers. These structures offer advantages like a higher surface-to-volume ratio compared to spherical nanoparticles, cost-effectiveness, and environmentally friendly preparation methods. Researchers have explored various applications of 3D nanostructures with unique morphologies derived from different nanoflowers. The nanoflowers are classified as organic, inorganic and hybrid, and the hybrids are a combination thereof, and most research studies of the nanoflowers have been focused on biomedical applications. Intriguingly, among them, inorganic nanoflowers have been studied extensively in various areas, such as electro, photo, and chemical catalysis, sensors, supercapacitors, and batteries, owing to their high catalytic efficiency and optical characteristics, which arise from their composition, crystal structure, and local surface plasmon resonance (LSPR). Despite the significant interest in inorganic nanoflowers, comprehensive reviews on this topic have been scarce until now. This is the first review focusing on inorganic nanoflowers for applications in electro, photo, and chemical catalysts, sensors, supercapacitors, and batteries. Since the early 2000s, more than 350 papers have been published on this topic with many ongoing research projects. This review categorizes the reported inorganic nanoflowers into four groups based on their composition and structure: metal, metal oxide, alloy, and other nanoflowers, including silica, metal-metal oxide, core-shell, doped, coated, nitride, sulfide, phosphide, selenide, and telluride nanoflowers. The review thoroughly discusses the preparation methods, conditions for morphology and size control, mechanisms, characteristics, and potential applications of these nanoflowers, aiming to facilitate future research and promote highly effective and synergistic applications in various fields.

Role of the Deep Eutectic Solvent Reline in the Synthesis of Gold Nanoparticles

This work presents a mechanistic understanding of the synthesis of small (<3 nm) gold nanoparticles in a nontoxic, eco-friendly, and biodegradable eutectic mixture of choline chloride and urea (reline) without the addition of external reducing or stabilization agents. Reline acts as a reducing agent by releasing ammonia (via urea hydrolysis), forming gold nanoparticles even at trace ammonia concentration levels. Reline also affects the speciation of the gold precursor forming gold chloro-complexes, stabilizing Au⁺ species, leading to an easier reduction and avoiding the otherwise fast disproportionation reaction. Such a capability is however lost in the presence of large amounts of water, where water replaces the chloride ligands in the precursor speciation. In addition, reline acts as a weak stabilizing agent, leading to small particles (<3 nm) and narrow distributions although agglomerates quickly form. Such properties are maintained in the presence of water, indicating that it is linked to the urea stabilization rather than the hydrogen-bonding network. This work has important implications in the field of green synthesis of nanoparticles with small sizes, especially for biomedical and health care applications, due to the nontoxic nature of the components of deep eutectic solvents in contrast to the conventional routes.

Cationic Polymer Coating Increases the Catalytic Activity of Gold Nanoparticles toward Anionic Substrates

Organic coatings on catalytic metal nanoparticles (NPs) typically hinder their activity due to the blocking of active sites. Therefore, considerable effort is made to remove organic ligands when preparing supported NP catalytic materials. Here, cationic polyelectrolyte coatings are shown to increase the catalytic activity of partially embedded gold nanoislands (Au NIs) toward transfer hydrogenation and oxidation reactions with anionic substrates compared to the activity of identical but uncoated Au NIs. Any potential steric hindrance caused by the coating is countered by a decrease in the activation energy of the reaction by half, resulting in overall enhancement. The direct comparison to identical but uncoated NPs isolates the role of the coating and provides conclusive evidence of enhancement. Our findings show that engineering the microenvironment of heterogeneous catalysts, creating hybrid materials that cooperatively interact with the reactants involved, is a viable and exciting path to improving their performance.

Practical Considerations for Simulating the Plasmonic Properties of Metal Nanoparticles

Simulating the plasmonic properties of colloidally derived metal nanoparticles with accuracy to their experimentally observed measurements is challenging due to the many structural and compositional parameters that influence their scattering and absorption properties. Correlation between single nanoparticle scattering measurements and simulated spectra emphasize these strong structural and compositional relationships, providing insight into the design of plasmonic nanoparticles. This Perspective builds from this history to highlight how the structural features of models used in simulation methods such as those based on the Finite-Difference Time-Domain (FDTD) method and Discrete Dipole Approximation (DDA) are of critical consideration for correlation with experiment and ultimately prediction of new nanoparticle properties. High-level characterizations such as electron tomography are discussed as ways to advance the accuracy of models used in such simulations, allowing the plasmonic properties of structurally complex nanoparticles to be better understood. However, we also note that the field is far from bringing experiment and simulation into agreement for plasmonic nanoparticles with complex compositions, reflecting analytical challenges that inhibit accurate model generation. Potential directions for addressing these challenges are also presented.

Challenges and opportunities for SERS in the infrared: materials and methods

In the wake of a global, heightened interest towards biomarker and disease detection prompted by the SARS-CoV-2 pandemic, surface enhanced Raman spectroscopy (SERS) positions itself again at the forefront of biosensing innovation. But is it ready to move from the laboratory to the clinic? This review presents the challenges associated with the application of SERS to the biomedical field, and thus, to the use of excitation sources in the near infrared, where biological windows allow for cell and through-tissue measurements. Two main tackling strategies will be discussed: (1) acting on the design of the enhancing substrate, which includes manipulation of nanoparticle shape, material, and supramolecular architecture, and (2) acting on the spectral collection set-up. A final perspective highlights the upcoming scientific and technological bets that need to be won in order for SERS to stably transition from benchtop to bedside.

Femtosecond laser patterned silicon embedded with gold nanostars as a hybrid SERS substrate for pesticide detection

We have developed simple and cost-effective surface-enhanced Raman scattering (SERS) substrates for the trace detection of pesticide (thiram and thiabendazole) and dye (methylene blue and Nile blue) molecules. Surface patterns (micro/nanostructures) on silicon (Si) substrates were fabricated using the technique of femtosecond (fs) laser ablation in ambient air. Different surface patterns were achieved by tuning the number of laser pulses per unit area (4200, 8400, 42 000, and 84 000 pulses per mm²) on Si. Subsequently, chemically synthesized gold (Au) nanostars were embedded in these laser-patterned areas of Si to achieve a plasmonic active hybrid SERS substrate. Further, the SERS performance of the as-prepared Au nanostar embedded Si substrates were tested with different probe molecules. The as-prepared substrates allowed us to detect a minimum concentration of 0.1 ppm in the case of thiram, 1 ppm in the case of thiabendazole (TBZ), 1.6 ppb in the case of methylene blue (MB), and 1.8 ppb in case of Nile blue (NB). All these were achieved using a simple, field-deployable, portable Raman spectrometer. Additionally, the optimized SERS substrate demonstrated ∼21 times higher SERS enhancement than the Au nanostar embedded plain Si substrate. Furthermore, the optimized SERS platform was utilized to detect a mixture of dyes (MB + NB) and pesticides (thiram + TBZ). The possible reasons for the observed additional enhancement are elucidated.

Gold Nanostar Characterization by Nanoparticle Tracking Analysis

We demonstrate the application of nanoparticle tracking analysis (NTA) for the quantitative characterization of gold nanostars (GNSs). GNSs were synthesized by the seed-mediated growth method using triblock copolymer (TBP) gold nanoparticles (GNPs). These GNPs (≈ 10 nm) were synthesized from Au³⁺ (≈ 1 mM) in aqueous F127 (w/v 5%) containing the co-reductant ascorbic acid (≈ 2 mM). The GNS tip-to-core aspect ratio (AR) decreased when higher concentrations of GNPs were added to the growth solution. The AR dependency of GNSs on Au³⁺/Au(seed) concentration ratio implies that growth is partly under kinetic control. NTA measured GNS sizes, concentrations, and relative scattering intensities. Molar absorption coefficients ∼ 10⁹-10¹⁰ M^-1 cm^-1 (ε_400 nm) for each batch of GNSs were determined using the combination of extinction spectra and NTA concentrations for heterogeneous samples. NTA in combination with UV-vis was used to derive the linear relationships: (1) hydrodynamic size versus localized surface plasmon peak maxima; (2) ε_400 nm versus localized surface plasmon peak maxima; (3) ε_400 nm versus hydrodynamic size. NTA for quantitative characterization of anisotropic nanoparticles could lead to future applications, including heterogeneous colloidal catalysis.

Single-step controlled synthesis of flower-like gold nanoparticles stabilized by chitosan for sensitive detection of heparin using a surface-enhanced Raman scattering method

A novel single-step and template-free procedure, including controlled synthesis of gold flowers (AuNFs), conjugation to a 4-MBA reporter, and stabilization with chitosan, is proposed to develop the SERS tags-based nanoparticles for trace detection of heparin. This SERS detection assay is based on the aggregation/non-aggregation balance of AuNFs-4-MBA@chitosan nanoparticles, which was induced by adding a very low concentration of heparin in the as-synthesized colloidal solutions. SERS-tag colloids are prepared by mixing chitosan with HAuCl4 and 4-mercapto benzoic acid before being reduced with ascorbic acid under appropriate pH conditions. The formed AuNFs-4-MBA@chitosan nanoparticles were positively charged with high stability and well-dispersed in aqueous media. Based on understanding each reaction component's role in the preparation of the SERS tag colloid, we aim to simplify the controlled synthesis and Raman probe conjugation process. The average size of AuNFs is below 90 nm, fine-tuned in shape and effectively conjugated to the Raman reporter molecules 4-MBA. These as-prepared SERS tag-based AuNFs have good biocompatibility and are virtually non-toxic, as studied with fibroblast and MCF-7 cells. Through these SERS-tag colloids, the trace detection of heparin is improved, with a wide detection window (0.01 to 100 ppm), high reproducibility (RSD value of 3.56%), limit of detection (LOD) of 0.054 ppm, and limit of quantification (LOQ) of 0.17 ppm. Comparison experiments show that the SERS-tag colloids possess good selectivity over other ions, and organic and amino acid substances. The results provide the capability and the potential for application under complex biological conditions and future biosensing based on SERS signal amplification.

Size Effect in Hybrid TiO2:Au Nanostars for Photocatalytic Water Remediation Applications

TiO₂:Au-based photocatalysis represents a promising alternative to remove contaminants of emerging concern (CECs) from wastewater under sunlight irradiation. However, spherical Au nanoparticles, generally used to sensitize TiO₂, still limit the photocatalytic spectral band to the 520 nm region, neglecting a high part of sun radiation. Here, a ligand-free synthesis of TiO₂:Au nanostars is reported, substantially expanding the light absorption spectral region. TiO₂:Au nanostars with different Au component sizes and branching were generated and tested in the degradation of the antibiotic ciprofloxacin. Interestingly, nanoparticles with the smallest branching showed the highest photocatalytic degradation, 83% and 89% under UV and visible radiation, together with a threshold in photocatalytic activity in the red region. The applicability of these multicomponent nanoparticles was further explored with their incorporation into a porous matrix based on PVDF-HFP to open the way for a reusable energy cost-effective system in the photodegradation of polluted waters containing CECs.

Surface-Enhanced Raman Spectroscopy Chips Based on Silver Coated Gold Nanostars

Surface-enhanced Raman scattering (SERS) is becoming widely used as an analytical tool, and the search for stable and highly responsive SERS substrates able to give ultralow detection of pollutants is a current challenge. In this paper we boosted the SERS response of Gold nanostars (GNS) demonstrating that their coating with a layer of silver having a proper thickness produces a 7-fold increase in SERS signals. Glass supported monolayers of these GNS@Ag were then prepared using simple alcoxyliane chemistry, yielding efficient and reproducible SERS chips, which were tested for the detection of molecules representative of different classes of pollutants. Among them, norfloxacin was detected down to 3 ppb, which is one of the lowest limits of detection obtained with this technique for the analyte.

Colloidal nanomaterials for water quality improvement and monitoring

Water is the most important resource for all kind forms of live. It is a vital resource distributed unequally across different regions of the globe, with populations already living with water scarcity, a situation that is spreading due to the impact of climate change. The reversal of this tendency and the mitigation of its disastrous consequences is a global challenge posed to Humanity, with the scientific community assuming a major obligation for providing solutions based on scientific knowledge. This article reviews literature concerning the development of nanomaterials for water purification technologies, including collaborative scientific research carried out in our laboratory (nanoLAB@UA) framed by the general activities carried out at the CICECO-Aveiro Institute of Materials. Our research carried out in this specific context has been mainly focused on the synthesis and surface chemical modification of nanomaterials, typically of a colloidal nature, as well as on the evaluation of the relevant properties that arise from the envisaged applications of the materials. As such, the research reviewed here has been guided along three thematic lines: 1) magnetic nanosorbents for water treatment technologies, namely by using biocomposites and graphite-like nanoplatelets; 2) nanocomposites for photocatalysis (e.g., TiO2/Fe3O4 and POM supported graphene oxide photocatalysts; photoactive membranes) and 3) nanostructured substrates for contaminant detection using surface enhanced Raman scattering (SERS), namely polymers loaded with Ag/Au colloids and magneto-plasmonic nanostructures. This research is motivated by the firm believe that these nanomaterials have potential for contributing to the solution of environmental problems and, conversely, will not be part of the problem. Therefore, assessment of the impact of nanoengineered materials on eco-systems is important and research in this area has also been developed by collaborative projects involving experts in nanotoxicity. The above topics are reviewed here by presenting a brief conceptual framework together with illustrative case studies, in some cases with original research results, mainly focusing on the chemistry of the nanomaterials investigated for target applications. Finally, near-future developments in this research area are put in perspective, forecasting realistic solutions for the application of colloidal nanoparticles in water cleaning technologies.

Direct and Water-Mediated Adsorption of Stabilizers on SERS-Active Colloidal Bimetallic Plasmonic Nanomaterials: Insight into Citrate-AuAg Interactions from DFT Calculations

In previous studies, AuAg colloidal nanostar formulations were developed with the two-fold aim of producing optimized surface-enhanced Raman spectroscopy (SERS) substrates and investigating the nature of the capping process itself. Findings demonstrated that the nanoparticle metals are alloyed and neutral, and capping by stabilizers occurs via chemisorption. This study utilizes citrate as the model stabilizer and investigates the mechanistic aspects of its interaction with mono- (Au₂₀) and bimetallic (Au₁₉Ag) surfaces by density functional theory (DFT) calculations. Citrate was modeled according to the colloid's pH and surrounded by a water and sodium first solvation shell. A population of stable cluster-citrate structures was obtained, and energies were refined at the uB3LYP//LANL2TZ(f)/cc-pVTZ level of theory. Solvation was accounted for both explicitly and implicitly by the application of the continuum model SMD. Results indicate that both direct binding and binding by water proxy through the charge-transfer complex formation are thermodynamically favorable. Water participation in citrate adsorption is supported by the adsorption behavior observed experimentally and the comparison between experimental and DFT-simulated IR spectra. Vibrational mode analysis suggests the possible presence of water within a crystal in dried nanostar residues. All ΔG_ads(aq) indicate a weak chemisorptive process, leading to the hypothesis that citrate could be displaced by analytes during SERS measurements.

SERS and Fluorescence-Active Multimodal Tessellated Scaffolds for Three-Dimensional Bioimaging

With the ever-increasing use of 3D cell models toward studying bio-nano interactions and offering alternatives to traditional 2D in vitro and in vivo experiments, methods to image biological tissue in real time and with high spatial resolution have become a must. A suitable technique therefore is surface-enhanced Raman scattering (SERS)-based microscopy, which additionally features reduced photocytotoxicity and improved light penetration. However, optimization of imaging and postprocessing parameters is still required. Herein we present a method to monitor cell proliferation over time in 3D, using multifunctional 3D-printed scaffolds composed of biologically inert poly(lactic-co-glycolic acid) (PLGA) as the base material, in which fluorescent labels and SERS-active gold nanoparticles (AuNPs) can be embedded. The combination of imaging techniques allows optimization of SERS imaging parameters for cell monitoring. The scaffolds provide anchoring points for cell adhesion, so that cell growth can be observed in a suspended 3D matrix, with multiple reference points for confocal fluorescence and SERS imaging. By prelabeling cells with SERS-encoded AuNPs and fluorophores, cell proliferation and migration can be simultaneously monitored through confocal Raman and fluorescence microscopy. These scaffolds provide a simple method to follow cell dynamics in 4D, with minimal disturbance to the tissue model.

Gold nanostar‐based complexes applied for cancer theranostics

Cancer remains a major health problem that plagues human beings, calling widespread attention to develop novel theranostics to achieve sensitive diagnosis and efficient therapy. Multifunctional nanomedicine that can integrate diagnosis with treatment formulations has been emerging as a powerful strategy to overcome the current drawbacks in conventional clinical cancer treatments. Due to the good biocompatibility, easy surface modification, surface‐enhanced Raman spectroscopy (SERS)/computed tomography (CT)/photoacoustic (PA) imaging properties, and exceptional photothermal performance of gold nanostars (AuNSs), various AuNS‐based complexes or nanohybrids including metal compound/AuNSs, SiO2/AuNSs, polymer/AuNSs, and dendrimer/AuNSs, and so forth have been developed, holding great blueprint in cancer theranostics. Herein, we concisely review the recent progresses in the intriguing design of AuNS‐based nanoplatforms, and their applications in bioimaging, therapy and imaging‐guided cancer treatment, and clarify the possible future perspectives for the design of AuNS‐facilitated cancer theranostics.

Laser intensity profile based terahertz field enhancement from a mixture of nano-particles embedded in a gas

Nano-particle embedded system plays an importance in developing of future terahertz (THz) radiation source for real-world applications. The laser interactions with nanoparticle embedded system can produce a wide range of THz radiation due to plasma oscillations excitation. We investigate THz field generation from the laser-beat wave interaction with a mixture of spherical and cylindrical graphite nanoparticles in argon gas. Different laser intensity distributions such as Gaussian, cosh-Gaussian, flat-top and ring shape laser pulses have been studied in this work. The relevant plasmon resonance conditions with appropriate symmetry of spherical nanoparticles and cylindrical nanoparticles are discussed. THz field is enhanced upto the order of 10 2 when the laser intensity redistributes along the polarization direction for a ring shape field envelope.

Surface-Enhanced Raman Spectroscopy for the Investigation of Chromogenic Motion Picture Films: A Preliminary Study

In the present work, surface-enhanced Raman spectroscopy (SERS) is proposed for the identification of dyes in chromogenic films. These substances, which are generated within the film itself during the colour development process, are first studied on reference molecules synthesised for the purpose and, subsequently, on dyes extracted from a chromogenic film. SERS spectroscopy proved to be an efficient technique for their investigation, providing a proof of concept for its applicability for further studies on this complex topic. SERS spectra of the reference molecules were also compared with normal Raman spectra, acquired with a spectrometer based on SSE™ technology. Furthermore, the excellent SERS properties of anisotropic nanomaterials, such as silver nanostars, have been confirmed.

Photothermal and Photodynamic Therapy of Tumors with Plasmonic Nanoparticles: Challenges and Prospects

Cancer remains one of the leading causes of death in the world. For a number of neoplasms, the efficiency of conventional chemo- and radiation therapies is insufficient because of drug resistance and marked toxicity. Plasmonic photothermal therapy (PPT) using local hyperthermia induced by gold nanoparticles (AuNPs) has recently been extensively explored in tumor treatment. However, despite attractive promises, the current PPT status is limited by laboratory experiments, academic papers, and only a few preclinical studies. Unfortunately, most nanoformulations still share a similar fate: great laboratory promises and fair preclinical trials. This review discusses the current challenges and prospects of plasmonic nanomedicine based on PPT and photodynamic therapy (PDT). We start with consideration of the fundamental principles underlying plasmonic properties of AuNPs to tune their plasmon resonance for the desired NIR-I, NIR-2, and SWIR optical windows. The basic principles for simulation of optical cross-sections and plasmonic heating under CW and pulsed irradiation are discussed. Then, we consider the state-of-the-art methods for wet chemical synthesis of the most popular PPPT AuNPs such as silica/gold nanoshells, Au nanostars, nanorods, and nanocages. The photothermal efficiencies of these nanoparticles are compared, and their applications to current nanomedicine are shortly discussed. In a separate section, we discuss the fabrication of gold and other nanoparticles by the pulsed laser ablation in liquid method. The second part of the review is devoted to our recent experimental results on laser-activated interaction of AuNPs with tumor and healthy tissues and current achievements of other research groups in this application area. The unresolved issues of PPT are the significant accumulation of AuNPs in the organs of the mononuclear phagocyte system, causing potential toxic effects of nanoparticles, and the possibility of tumor recurrence due to the presence of survived tumor cells. The prospective ways of solving these problems are discussed, including developing combined antitumor therapy based on combined PPT and PDT. In the conclusion section, we summarize the most urgent needs of current PPT-based nanomedicine.

Surfactant-free Synthesis of Spiky Hollow Ag-Au Nanostars with Chemically Exposed Surfaces for Enhanced Catalysis and Single-Particle SERS

Spiky/hollow metal nanoparticles have applications across a broad range of fields. However, the current bottom-up methods for producing spiky/hollow metal nanoparticles rely heavily on the use of strongly adsorbing surfactant molecules, which is undesirable because these passivate the product particles' surfaces. Here we report a high-yield surfactant-free synthesis of spiky hollow Au-Ag nanostars (SHAANs). Each SHAAN is composed of >50 spikes attached to a hollow ca. 150 nm diameter cubic core, which makes SHAANs highly plasmonically and catalytically active. Moreover, the surfaces of SHAANs are chemically exposed, which gives them significantly enhanced functionality compared with their surfactant-capped counterparts, as demonstrated in surface-enhanced Raman spectroscopy (SERS) and catalysis. The chemical accessibility of the pristine SHAANs also allows the use of hydroxyethyl cellulose as a weakly bound stabilizing agent. This produces colloidal SHAANs that remain stable for >1 month while retaining the functionalities of the pristine particles and allows even single-particle SERS to be realized.

Impurities in polyvinylpyrrolidone: the key factor in the synthesis of gold nanostars

Control over the synthesis of anisotropic nanoparticles is crucial as slight differences in their size, shape, sharpness, or the number of tips in the case of gold nanostars, has an inordinate influence on their properties and functionality for future applications. Herein, we show that the supplier and purity of polyvinylpyrrolidone (PVP) can significantly alter the synthesis of gold nanostars, demonstrating that impurities, not PVP itself, are the main factor responsible for star-like shape formation. We demonstrate that in the presence of pure PVP and N,N-dimethylformamide, the use of hydrazine leads to the formation of branched nanoparticles. This synthetic approach opens the door to solving issues associated with the use of commercial PVP during the synthesis of gold nanostars.

Plasmon-driven photocatalytic properties based on the surface of gold nanostar particles

Surface plasmon resonance (SPR) generated in gold nanoparticles can induce the conversion of p-Aminothiophenol (PATP) molecules into p,p'-dimercaptoazobenzene (DMAB) molecules by coupling reaction under the action of excitation light. Molecular detection of samples by surface enhanced Raman spectroscopy (SERS) techniques allows the study of their plasma-driven photocatalytic reaction processes. In this study, we used gold nanostars (GNS) as the substrate to study its catalytic performance and sensitivity. On this basis, catalytic substrates of gold nanospheres (GNPs) were prepared for comparison. The catalytic reactions of PATP molecules on each of the above two substrates were systematically investigated under 633 nm laser irradiation. The reduction process was subsequently observed by introducing NaBH₄ solution. The results show that photocatalytic reactions can be achieved on both substrates under laser excitation at the same wavelength. However, the catalytic and reduction reaction rates on GNSs as a substrate are much faster than those of GNPs. This phenomenon may be due to the abundant nano-branched microstructures on the surface of GNSs, which will generate more and stronger local surface plasma hot spots under the irradiation of excitation light. In order to test the above hypothesis, the surface electromagnetic field distribution of two nanostructures was numerically simulated using the finite-difference time domain (FDTD) method. It is found that the star-like nanostructures not only have the same inter-particle hot spot system as the spherical nanostructures, but also have a large number of high-intensity single-particle hot spot systems arising from the abundance of branched nanostructures on their own surfaces. Compared with the spherical nanostructures, they are characterized by a dual hot spot system, which accelerates the photocatalytic reaction rate. The above experiments are of some reference significance for the in-depth study of multi-branched nanostructures and surface plasma distribution properties and their applications.

Facile tuning of tip sharpness on gold nanostars by the controlled seed-growth method and coating with a silver shell for detection of thiram using surface enhanced Raman spectroscopy (SERS)

Developing SERS substrates based on individual gold and silver metals, either with rough surfaces or bare nanoparticles, has certain limitations in practical analysis applications. In order to improve the range of applications of the noble metallic substrates, a comprehensive approach has been proposed for preparing non-traditional SERS nano-substrates by combining tip-enhanced gold nanostars and Raman signal amplification of the silver layer. This preparation process is conducted in two steps, including tuning the sharpness and length of tips by a modified seed growth method followed by coating the silver layer on the formed star-shaped nanoparticles. The obtained AuNS-Ag covered with an average size of around 100 nm exhibited interesting properties as a two-component nano-substrate to amplify the activities in SERS for detecting thiram. The controllable and convenient preparation route of gold nanostars is based on the comproportionation reaction of Au seed particles with Au(iii) ions, achieved by governing the stirring times of the mixture of the Au seed and the growth solution. Thus, the citrate-seed particles decreased in size (below 2 nm) and grew into nanostars with sharp tips. The thickness of Ag covering the Au particles' surface also was appropriately controlled and the tips were still exposed to the outside, which is a benefit for matching with the source excitation wavelength to achieve good SERS performance. The Raman signals of thiram can be instantly and remarkably detected with the enhancement of the substrates. Thiram can be determined without any pretreatment. It was found that the limit of detection for thiram is 0.22 ppm, and the limit of quantification is 0.73 ppm. These experimental results shed some light on developing the SERS method for detecting pesticide residue.

Developing SERS substrates based on the star-like morphology of gold nanoparticles covered by a silver layer to overcome limitations in practical analysis application.

Recent Advances in Metallic Nanoparticle Assemblies for Surface-Enhanced Spectroscopy

Robust and versatile strategies for the development of functional nanostructured materials often focus on assemblies of metallic nanoparticles. Research interest in such assemblies arises due to their potential applications in the fields of photonics and sensing. Metallic nanoparticles have received considerable recent attention due to their connection to the widely studied phenomenon of localized surface plasmon resonance. For instance, plasmonic hot spots can be observed within their assemblies. A useful form of spectroscopy is based on surface-enhanced Raman scattering (SERS). This phenomenon is a commonly used in sensing techniques, and it works using the principle that scattered inelastic light can be greatly enhanced at a surface. However, further research is required to enable improvements to the SERS techniques. For example, one question that remains open is how to design uniform, highly reproducible, and efficiently enhancing substrates of metallic nanoparticles with high structural precision. In this review, a general overview on nanoparticle functionalization and the impact on nanoparticle assembly is provided, alongside an examination of their applications in surface-enhanced Raman spectroscopy.

Modification of Aerosol Gold Nanoparticles by Nanosecond Pulsed-Periodic Laser Radiation

This study investigates the processes of interaction of nanosecond pulsed-periodic laser radiation with the flow of aerosol agglomerates of gold nanoparticles synthesized in a spark discharge. Nanoparticles in a gas flow are spatially separated nano-objects whose interaction with each other and with the walls of an experimental cell was insignificant. Therefore, the energy absorbed by nanoparticles was used only for their own heating with further shape and size modification and on heat transfer to the surrounding gas. In the research, we used laser radiation with wavelengths of 527 and 1053 nm at pulse energies up to 900 µJ and pulse repetition rates up to 500 Hz. The dynamics of changes in the nanoparticles size during their sintering process depending on the laser pulses energy is characterized by an S-shaped shrinkage curve. Complete sintering of the initial agglomerates with their transformation into spherical nanoparticles is achieved by a series of impacting laser pulses. The result of nanoparticles’ laser modification is largely determined by the pulse energy and the efficiency of the nanoparticles’ radiation absorption.

Live Cell Poration by Au Nanostars to Probe Intracellular Molecular Composition with SERS

A new type of flat substrate has been used to visualize structures inside living cells by surface-enhanced Raman scattering (SERS) and to study biochemical processes within cells. The SERS substrate is formed by stabilized aggregates of gold nanostars on a glass microscope slide coated with a layer of poly (4-vinyl pyridine) polymer. This type of SERS substrate provides good cell adhesion and viability. Au nanostars' long tips can penetrate the cell membrane, allowing it to receive the SERS signal from biomolecules inside a living cell. The proposed nanostructured surfaces were tested to study, label-free, the distribution of various biomolecules in cell compartments.

Optothermal properties of plasmonic inorganic nanoparticles for photoacoustic applications

Plasmonic systems are becoming a favourable alternative to dye molecules in the generation of photoacoustic signals for spectroscopy and imaging. In particular, inorganic nanoparticles are appealing because of their versatility. In fact, as the shape, size and chemical composition of nanoparticles are directly correlated with their plasmonic properties, the excitation wavelength can be tuned to their plasmon resonance by adjusting such traits. This feature enables an extensive spectral range to be covered. In addition, surface chemical modifications can be performed to provide the nanoparticles with designed functionalities, e.g., selective affinity for specific macromolecules. The efficiency of the conversion of absorbed photon energy into heat, which is the physical basis of the photoacoustic signal, can be accurately determined by photoacoustic methods. This review contrasts studies that evaluate photoconversion in various kinds of nanomaterials by different methods, with the objective of facilitating the researchers' choice of suitable plasmonic nanoparticles for photoacoustic applications.

Surface-enhanced Raman spectroscopy for bioanalysis and diagnosis

In recent years, bioanalytical surface-enhanced Raman spectroscopy (SERS) has blossomed into a fast-growing research area. Owing to its high sensitivity and outstanding multiplexing ability, SERS is an effective analytical technique that has excellent potential in bioanalysis and diagnosis, as demonstrated by its increasing applications in vivo. SERS allows the rapid detection of molecular species based on direct and indirect strategies. Because it benefits from the tunable surface properties of nanostructures, it finds a broad range of applications with clinical relevance, such as biological sensing, drug delivery and live cell imaging assays. Of particular interest are early-stage-cancer detection and the fast detection of pathogens. Here, we present a comprehensive survey of SERS-based assays, from basic considerations to bioanalytical applications. Our main focus is on SERS-based pathogen detection methods as point-of-care solutions for early bacterial infection detection and chronic disease diagnosis. Additionally, various promising in vivo applications of SERS are surveyed. Furthermore, we provide a brief outlook of recent endeavours and we discuss future prospects and limitations for SERS, as a reliable approach for rapid and sensitive bioanalysis and diagnosis.

Bespoke nanostars: synthetic strategies, tactics, and uses of tailored branched gold nanoparticles

Interest in branched colloidal gold nanosystems has gained increased traction due to the structures' outstanding optical and plasmonic properties, resulting in utilization in techniques such as surface-enhanced spectroscopy and bioimaging, as well as plasmon photocatalysis and photothermal therapy. The unique morphologies of nanostars, multipods, urchins, and other highly branched nanomaterials exhibit selective optical and crystallographic features accessible by alterations in the respective wet-chemical syntheses, opening a vast array of useful applications. Examination of discriminatory reaction conditions, such as seeded growth (e.g., single-crystalline vs. multiply twinned seeds), underpotential deposition of Ag(i), galvanic replacement, and the dual use of competing reducing and capping agents, is shown to reveal conditions necessary for the genesis of assorted branched nanoscale gold frameworks. By observing diverse approaches, including template-directed, microwave-mediated, and aggregation-based methods, among others, a schema of synthetic pathways can be constructed to provide a guiding roadmap for obtaining the full range of desired branched gold nanocrystals. This review presents a comprehensive summary of such advances and these nuances of the underlying procedures, as well as offering mechanistic insights into the directed nanoscale growth. We conclude the review by discussing various applications for these fascinating nanomaterials, particularly surface-enhanced Raman spectroscopy, photothermal and photodynamic therapy, catalysis, drug delivery, and biosensing.

Controlled pDNA Release in Gemini Cationic Lipoplexes by Femtosecond Laser Irradiation of Gold Nanostars

The design of nanovectors able to overcome biological barriers is one of the main challenges in biomedicine. Gemini cationic lipids are considered potential candidates for gene therapy due to their high biocompatibility and capacity to condense nucleic acids safely in the form of lipoplexes. However, this approach presents difficulties regarding genetic unpacking and, therefore, control over this process becomes crucial to ensure successful transfection. In this work, gemini cationic lipoplexes were prepared in the presence of plasmonic gold nanostars (AuNSs) to afford a nanovector that efficiently releases plasmid DNA (pDNA) upon irradiation with near-infrared femtosecond laser pulses. A critical AuNSs concentration of 50 pM and optimized laser power density of 400 mW led to successful pDNA release, whose efficiency could be further improved by increasing the irradiation time. Agarose gel electrophoresis was used to confirm pDNA release. UV-Vis-NIR spectroscopy and transmission electron microscopy studies were performed to monitor changes in the morphology of the AuNSs and lipoplexes after irradiation. From a physicochemical point of view, this study demonstrates that the use of AuNSs combined with gemini cationic lipoplexes allows control over pDNA release under ultrafast laser irradiation.

Gold nanomaterials for optical biosensing and bioimaging

Gold nanoparticles (AuNPs) are highly compelling nanomaterials for biomedical studies due to their unique optical properties. By leveraging the versatile optical properties of different gold nanostructures, the performance of biosensing and biomedical imaging can be dramatically improved in terms of their sensitivity, specificity, speed, contrast, resolution and penetration depth. Here we review recent advances of optical biosensing and bioimaging techniques based on three major optical properties of AuNPs: surface plasmon resonance, surface enhanced Raman scattering and luminescence. We summarize the fabrication methods and optical properties of different types of AuNPs, highlight the emerging applications of these AuNPs for novel optical biosensors and biomedical imaging innovations, and discuss the future trends of AuNP-based optical biosensors and bioimaging as well as the challenges of implementing these techniques in preclinical and clinical investigations.

Stable and scalable SERS tags conjugated with neutravidin for the detection of fibroblast activation protein (FAP) in primary fibroblasts

SERS tags are a class of nanoparticles with great potential in advanced imaging experiments. The preparation of SERS tags however is complex, as they suffer from the high variability of the SERS signals observed even at the slightest sign of aggregation. Here, we developed a method for the preparation of SERS tags based on the use of gold nanostars conjugated with neutravidin. The SERS tags here obtained are extremely stable in all biological buffers commonly employed and can be prepared at a relatively large scale in very mild conditions. The obtained SERS tags have been used to monitor the expression of fibroblast activation protein alpha (FAP) on the membrane of primary fibroblasts obtained from patients affected by Crohn's disease. The SERS tags allowed the unambiguous identification of FAP on the surface of cells thus suggesting the feasibility of semi-quantitative analysis of the target protein. Moreover, the use of the neutravidin-biotin system allows to apply the SERS tags for any other marker detection, for example, different cancer cell types, simply by changing the biotinylated antibody chosen in the analysis.

Iodide-Mediated Rapid and Sensitive Surface Etching of Gold Nanostars for Biosensing

Iodide-mediated surface etching can tailor the surface plasmon resonance of gold nanostars through etching of the high-energy facets of the nanoparticle protrusions in a rapid and sensitive way. By exploring the underlying mechanisms of this etching and the key parameters influencing it (such as iodide, oxygen, pH, and temperature), we show its potential in a sensitive biosensing system. Horseradish peroxidase-catalyzed oxidation of iodide enables control of the etching of gold nanostars to spherical gold nanoparticles, where the resulting spectral shift in the surface plasmon resonance yields a distinct color change of the solution. We further develop this enzyme-modulated surface etching of gold nanostars into a versatile platform for plasmonic immunoassays, where a high sensitivity is possible by signal amplification via magnetic beads and click chemistry.

Asymmetries in the Electronic Properties of Spheroidal Metallic Nanoparticles, Revealed by Conduction Electron Spin Resonance and Surface Plasmon Resonance

Using electron spin resonance spectroscopy, we demonstrate that the morphological asymmetries present in small spheroidal metallic nanoparticles give rise to asymmetries in the behavior of electrons held in states near the metal's Fermi energy. We find that the effects of morphological asymmetries for these spheroidal systems are more important than the effects of size distributions when explaining the asymmetry in electronic behavior. This is found to be true for all the particles examined, which were made from Cu, Ag, Pd, Ir, Pt, and Au, bearing dodecanethiolate ligands. In the case of the Ag particles, we also demonstrate that the same model used to account for morphological effects in the electron spin resonance spectra can be used to account for small asymmetries present in the plasmon spectrum. This result demonstrates that the electronic properties of even small particles are tunable via morphological changes.

Facile synthesis of gold-silver/copper sulfide nanoparticles for the selective/sensitive detection of chromium, photochemical and bactericidal application

In this project, bimetallic Au-Agnanoparticles/CuS nanoparticles were prepared via simple hydrothermal methods, which were used as highly efficient material for Cr (III) detection, photocatalytic, and biological process. The Au-Ag/CuS nanoparticles was studied via UV-visible spectroscopy, field-emission scanning electron microscopy, Dynamic light scattering, and X-ray diffraction. The zeta potential and effective size of Au-Ag/CuS nanoparticles was -32.1 mV and 25 nm respectively. The response time of Cr (III) ions interaction was 2 min. The lowest detection of Cr (III) by Au-Ag/CuS nanoparticles was 0.5 nM. The Au-Ag/CuS nano catalyst was applied to decomposition of drug under visible lamp irradiation. The photo degradation response of drug was 100.0% in 30 min irradiation. The particles exhibited excellent antibacterial activities.

Gold nanostar as an ultrasensitive colorimetric probe for picomolar detection of lead ion

The sensitivity for analytes of interest is vital for environment protection and food safety. Here, we propose an extremely sensitive assay toward Pb²⁺ by using gold nanostars (GNSs) as probes based on the catalytic activity of Pb on etching gold atoms after being reduced in the presence of 2-mercaptoethanol (2-ME) and sodium thiosulfate. GNSs were prepared by using 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid as both the reducing and capping agents, enabling high stability and sensitivity for quantitation of Pb²⁺. Upon increasing Pb²⁺ concentration over the range of 0-10 μM, GNS solution color changed from greenish-blue to blue to purple to red, and eventually to colorless. The color change can be distinguished by naked eye at the Pb²⁺ concentration as low as 200 pM. Through monitoring longitudinal localized surface plasmon of GNSs, Pb²⁺ could be detected with a limit of detection of 1.5 pM, and the working range is 2 pM-1 μM. The ultra-high sensitivity of our assay stems from the high catalysis of Pb on etching gold on tips and branches in the presence of 2-ME and sodium thiosulfate, leading to the shape deformation to spherical gold nanoparticle and the corresponding significant changes in their optical properties. The assay provides high selectivity of Pb²⁺ over the tested interfering metal ions like Cu²⁺. With high sensitivity and selectivity, the assay was efficiently validated by analyzing water samples and monitoring the migration of Pb²⁺ from the tested container to water.