Ligand Controlled Morphology Evolution of Active Intermediates for the Syntheses of Gold Nanostars

Gold nanostars and nanourchins for enhanced photothermal therapy, bioimaging, and theranostics

Photothermal therapy (PTT), a recently emerging method for eradicating tumors, utilizes hyperthermia induced by photo-absorbing materials to generate heat within cancer cells. Gold nanoparticles (AuNPs) have gained reliability for in vitro and in vivo applications in PTT due to their strong light absorbance, stability, and biocompatibility. Yet, their potential is limited by their spherical shape, impacting their size capabilities, electromagnetic enhancement effects, and localized surface plasmon resonance (LSPR). Anisotropic shapes have been tested and implemented in this treatment to overcome the limitations of spherical AuNPs. Nanostars (AuNSs) and nanourchins (AuNUs) offer unique properties, such as increased local electron density, improved catalytic activity, and an enhanced electromagnetic field, which have proven to be effective in PTT. Additionally, these shapes can easily reach the NIR-I and NIR-II window while exhibiting improved biological properties, including low cytotoxicity and high cellular uptake. This work covers the critical characteristics of AuNS and AuNUs, highlighting rough surface photothermal conversion enhancement, significantly impacting recent PTT and its synergy with other treatments. Additionally, the bioimaging and theranostic applications of these nanomaterials are discussed, highlighting their multifaceted utility in advanced cancer therapies.

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.

Controlling morphology evolution of titanium oxide-gold nanourchin for photocatalytic degradation of dyes and photoinactivation of bacteria in the infected wound

We report a one-pot, room-temperature, morphology-controlled synthesis of titanium oxide (TiO_x)-gold nanocomposites (TiO_x-Au NCs) using HAuCl₄ and TiCl₃ as precursors, and catechin as reducing agent. TiO_x-Au NCs have a range of morphologies from star-like to urchin-like shape depending on the concentration of TiCl₃ in the reaction mixture. The urchin-shaped TiO_x-Au NCs exhibited excellent photocatalytic activity toward dye degradation due to strong light absorption, plasmon-induced excitation, high conductivity of the gold, and reduced hole-electron pair recombination. TiO_x-Au NCs have the advantage of a wide range of light absorption and surface plasmon absorption-mediated excitation due to their abundant gold spikes, which enabled the degradation of dyes over 97% in 60 min, using a xenon lamp as a light source. In addition, TiO_x-Au NCs are highly efficient for the photoinactivation of Escherichia coli and methicillin-resistant Staphylococcus aureus (MRSA), and Candida albicans through the photodynamic generation of reactive oxygen species (ROS) and damage to the bacterial membrane. The catechin derivatives on the NCs effectively promoted curing MRSA infected wounds in rats through inducing collagen synthesis, migration of keratinocytes, and neovascularization.

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.

Greater SERS Activity of Ligand-Stabilized Gold Nanostars with Sharp Branches

Sharp branches of gold nanostars are critical in tuning the plasmonic properties of these nanostars and maximizing the activities in surface-enhanced Raman scattering (SERS). The interaction between the capping ligands and nanostars plays an essential role in determining the morphology of the branches on the gold nanostars. In this Article, we show that 4-mercapto benzoic acid can effectively control the morphology of branched gold nanostars, and these gold nanostars can be used for the colloidal SERS detection of probe molecules at a nanomolar concentration. We also find that the sharp branches on gold nanostars will provide extra SERS activities as compared to the ones with a rough surface. Using the method of principal component analysis, we can easily distinguish the addition of 4-mercapto pyridine molecules at a concentration of 2 nM. Our work indicated the promising applications of these gold nanostars in colloidal SERS studies for various ultrasensitive chemical analyses.

Insight into growth of Au-Pt bimetallic nanoparticles: an in situ XAS study

Au-Pt bimetallic nanoparticles have been synthesized through a one-pot synthesis route from their respective chloride precursors using block copolymer as a stabilizer. Growth of the nanoparticles has been studied by simultaneous in situ measurement of X-ray absorption spectroscopy (XAS) and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at Indus-2 SRS at RRCAT, Indore, India. In situ XAS spectra, comprising both X-ray near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) parts, have been measured simultaneously at the Au and Pt L₃-edges. While the XANES spectra of the precursors provide real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed in the intermediate stages of growth. This insight into the formation process throws light on how the difference in the reduction potential of the two precursors could be used to obtain the core-shell-type configuration of a bimetallic alloy in a one-pot synthesis method. The core-shell-type structure of the nanoparticles has also been confirmed by ex situ energy-dispersive spectroscopy line-scan and X-ray photoelectron spectroscopy measurements with in situ ion etching on fully formed nanoparticles.

A closer look at the physical and optical properties of gold nanostars: an experimental and computational study

A combined experimental and computational study was carried out to design a semi-empirical method to determine the volume, surface area, and extinction coefficients of gold nanostars. The values obtained were confirmed by reconstructing the nanostar 3D topography through high-tilt TEM tomography and introducing the finite elements in COMSOL Multiphysics through which we have also calculated the morphology-dependent extinction coefficient. Doing so, we have, for the first time, modeled the heat losses of a real, experimentally synthesized nanostar, and found the plasmon resonances to be in excellent agreement with those obtained experimentally. We believe that our approach could substantially improve the applicability of this remarkable nanomaterial.