Determination of the Localized Surface Plasmon Resonance Alteration of AgNPs via Multiwavelength Evanescent Scattering Microscopy for Pb(II) Detection

We developed multiwavelength evanescent scattering microscopy (MWESM), which can acquire plasmonic nanoparticle images at the particle level using the evanescent field as the incident source and distinguish different LSPR (localized surface plasmon resonance) spectral peaks among four wavelengths. Our microscope could be easily and simply built by modifying a commercial total internal reflection fluorescence microscope (TIRFM) with the substitution of a beamsplitter and the addition of a semicircular stop. The ultrathin depth of illumination and rejection of the reflected incident source together contribute to the high sensitivity and contrast of single nanoparticle imaging. We first validated the capability of our imaging system in distinguishing plasmonic nanoparticles bearing different LSPR spectral peaks, and the results were consistent with the scattering spectra results of hyperspectral imaging. Moreover, we demonstrated high imaging quality from the aspects of the signal/noise ratio and point spread function of the single-particle images. Meaningfully, the system can be utilized in rapidly determining the concentration of toxic lead ions in environmental and biological samples with good linearity and sensitivity, based on single-particle evanescent scattering imaging through the detection of the alteration of the LSPR of silver nanoparticles. This system holds the potential to advance the field of nanoparticle imaging and foster the application of nanomaterials as sensors.

Detection and imaging of Hg(II) in vivo using glutathione-functionalized gold nanoparticles

The optical and biological properties of functionalized gold nanoparticles (GNPs) have been widely used in sensing applications. GNPs have a strong binding ability to thiol groups. Furthermore, thiols are used to bind functional molecules, which can then be used, for example, to detect metal ions in solution. Herein, we describe 13 nm GNPs functionalized by glutathione (GSH) and conjugated with a rhodamine 6G derivative (Rh6G2), which can be used to detect Hg(II) in cells. The detection of Hg²⁺ ions is based on an ion-catalyzed hydrolysis of the spirolactam ring of Rh6G2, leading to a significant change in the fluorescence of GNPs-GSH-Rh6G2 from an "OFF" to an "ON" state. This strategy is an effective tool to detect Hg²⁺ ions. In cytotoxicity experiments, GNPs-GSH-Rh6G2 could penetrate living cells and detect mercury ions through the fluorescent "ON" form.

Dark-field/hyperspectral microscopy for detecting nanoscale particles in environmental nanotoxicology research

Nanoscale contaminants (including engineered nanoparticles and nanoplastics) pose a significant threat to organisms and environment. Rapid and non-destructive detection and identification of nanosized materials in cells, tissues and organisms is still challenging, although a number of conventional methods exist. These approaches for nanoparticles imaging and characterisation both inside the cytoplasm and on the cell or tissue outer surfaces, such as electron or scanning probe microscopies, are unquestionably potent tools, having excellent resolution and supplemented with chemical analysis capabilities. However, imaging and detection of nanomaterials in situ, in wet unfixed and even live samples, such as living isolated cells, microorganisms, protozoans and miniature invertebrates using electron microscopy is practically impossible, because of the elaborate sample preparation requiring chemical fixation, contrast staining, matrix embedding and exposure into vacuum. Atomic force microscopy, in several cases, can be used for imaging and mechanical analysis of live cells and organisms under ambient conditions, however this technique allows for investigation of surfaces. Therefore, a different approach allowing for imaging and differentiation of nanoscale particles in wet samples is required. Dark-field microscopy as an optical microscopy technique has been popular among researchers, mostly for imaging relatively large specimens. In recent years, the so-called "enhanced dark field" microscopy based on using higher numerical aperture light condensers and variable numerical aperture objectives has emegred, which allows for imaging of nanoscale particles (starting from 5 nm nanospheres) using almost conventional optical microscopy methodology. Hyperspectral imaging can turn a dark-field optical microscope into a powerful chemical characterisation tool. As a result, this technique is becoming popular in environmental nanotoxicology studies. In this Review Article we introduce the reader into the methodology of enhanced dark-field and dark-field-based hyperspectral microscopy, covering the most important advances in this rapidly-expanding area of environmental nanotoxicology.

A compact prospective investigation on the colorimetric recognition of Hg2+ ion and photostimulated degradation of discharged toxic organic dyes motivated by H. mutabilis directed silver nanoparticles

A colorimetric sensing method for Hg²⁺ ion was developed using H. mutabilis motivated silver NPs. The calculated detection limit was estimated ∼48 pM. The nanoparticles also work as a good photo catalyst for degradation of TB and Rh-B.

Variable Growth and Characterizations of Monolayer-Protected Gold Nanoparticles Based on Molar Ratio of Gold and Capping Ligands

Controlling the size of nanoscale entities is important because many properties of nanomaterials are directly related to the size of the particles. Gold nanoparticles represent classic materials and are of particular interest due to their potential application in a variety of fields. In this study, hexanethiol-capped gold nanoparticles are synthesized via the Brust-Schiffrin method. Synthesized nanoparticles were characterized by various analytical techniques such as transmission electron microscopy, scanning tunneling microscopy (STM), UV-visible absorption spectroscopy and electrochemical techniques. We have varied the molar ratio of gold to the protecting agent (hexanethiol) to discover the effect of gold-to-hexanethiol ligand ratio on the size of gold particles. The clear correlation between particle size and molar ratio is found that the averaged particle size decreases from 4.28 ± 0.83 to 1.54 ± 0.67 nm as the gold-to-ligand molar ratio changes from 1:1 to 1:9. In contrast to a recent report that thiolated gold nanoparticles are under spontaneous disintegration when they are assembled on a gold substrate, our STM experiments proved that these gold nanoparticles can form a stable monolayer or multiple layers on the platinum electrode without observing disintegration within 72 h. Therefore, our STM experiments demonstrate that the disintegration behavior of gold nanoparticles is related to the type of ligands and the nature of substrate materials. In electrochemical experiments, these gold nanoparticles displayed an electrochemical quantized charging effect, making these nanoparticles useful in the device applications such as electrochemical or biological sensors.

Direct versus ligand-exchange synthesis of [PtAg28(BDT)12(TPP)4]4- nanoclusters: effect of a single-atom dopant on the optoelectronic and chemical properties

Heteroatom doping of atomically precise nanoclusters (NCs) often yields a mixture of doped and undoped products of single-atom difference, whose separation is extremely difficult. To overcome this challenge, novel synthesis methods are required to offer monodisperse doped NCs. For instance, the direct synthesis of PtAg₂₈ NCs produces a mixture of [Ag₂₉(BDT)₁₂(TPP)₄]^3- and [PtAg₂₈(BDT)₁₂(TPP)₄]^4- NCs (TPP: triphenylphosphine; BDT: 1,3-benzenedithiolate). Here, we designed a ligand-exchange (LE) strategy to synthesize single-sized, Pt-doped, superatomic Ag NCs [PtAg₂₈(BDT)₁₂(TPP)₄]^4- by LE of [Pt₂Ag₂₃Cl₇(TPP)₁₀] NCs with BDTH₂ (1,3-benzenedithiol). The doped NCs were thoroughly characterized by optical and photoelectron spectroscopy, mass spectrometry, total electron count, and time-dependent density functional theory (TDDFT). We show that the Pt dopant occupies the center of the PtAg₂₈ cluster, modulates its electronic structure and enhances its photoluminescence intensity and excited-state lifetime, and also enables solvent interactions with the NC surface. Furthermore, doped NCs showed unique reactivity with metal ions - the central Pt atom of PtAg₂₈ could not be replaced by Au, unlike the central Ag of Ag₂₉ NCs. The achieved synthesis of single-sized PtAg₂₈ clusters will facilitate further applications of the LE strategy for the exploration of novel multimetallic NCs.

Luminescent turn-on detection of Hg(II) via the quenching of an iridium(III) complex by Hg(II)-mediated silver nanoparticles

A novel luminescent turn-on detection method for Hg(II) was developed. The method was based on the silver nanoparticle (AgNP)-mediated quenching of Ir(III) complex 1. The addition of Hg(II) ions causes the luminescence of complex 1 to be recovered due to the oxidation of AgNPs by Hg(II) ions to form Ag(I) and Ag/Hg amalgam. The luminescence intensity of 1 increased in accord with an increased Hg(II) concentration ranging from 0 nM to 180 nM, with the detection limit of 5 nM. This approach offers an innovative method for the quantification of Hg(II).

Distinct metal-exchange pathways of doped Ag25 nanoclusters

Atomically precise metal nanoclusters (NCs) containing more than one type of metal atom (i.e., doped or alloyed), due to synergistic effects, open new avenues for engineering the catalytic and optical properties of NCs in a manner that homometal NCs cannot. Unfortunately, it is still a major challenge to controllably introduce multimetallic dopants in NCs, understanding the dopants' positions, mechanism, and synergistic effects. To overcome these challenges, we designed a metal-exchange approach involving NCs as molecular templates and metal ions as the source of the incoming dopant. In particular, two structurally similar monodoped silver-rich NCs, [MAg₂₄(SR)₁₈]^2- (M = Pd/Pt and SR: thiolate), were synthesized as templates to study their mechanistic transformation in response to the introduction of gold atoms. The controllable incorporation of Au atoms into the MAg₂₄ framework facilitated the elucidation of distinct doping pathways through high-resolution mass spectrometry, optical spectroscopy and elemental analysis. Interestingly, gold replaced the central Pd atom of [PdAg₂₄(SR)₁₈]^2- clusters to produce predominantly bimetallic [AuAg₂₄(SR)₁₈]^- clusters along with a minor product of an [Au₂Ag₂₃(SR)₁₈]^- cluster. In contrast, the central Pt atom remained intact in [PtAg₂₄(SR)₁₈]^2- clusters, and gold replaced the non-central Ag atoms to form trimetallic [Au_xPtAg_24-x(SR)₁₈]^2- NCs, where x = 1-2, with a portion of the starting [PtAg₂₄(SR)₁₈]^2- NCs remaining. This study reveals some of the unusual metal-exchange pathways of doped NCs and the important role played by the initial metal dopant in directing the position of a second dopant in the final product.

Real-Time Plasmonic Monitoring of Single Gold Amalgam Nanoalloy Electrochemical Formation and Stripping

Direct electrodeposition of mercury onto gold nanorods on an ITO substrate, without reducing agents, is reported. The growth of single gold amalgam nanoalloy particles and subsequent stripping was monitored in real-time monitoring by plasmonic effects and single-nanoparticle dark-field spectroelectrochemistry techniques. Time-dependent scattering spectral information conferred insight into the growth and stripping mechanism of a single nanoalloy particle. Four critical stages were observed: First, rapid deposition of Hg atoms onto Au nanorods; second, slow diffusion of Hg atoms into Au nanorods; third, prompt stripping of Hg atoms from Au nanorods; fourth, moderate diffusion from the inner core of Au nanorods. Under high Hg(2+) concentrations, homogeneous spherical gold amalgam nanoalloys were obtained. These results demonstrate that the morphology and composition of individual gold amalgam nanoalloys can be precisely regulated electrochemically. Moreover, gold amalgam nanoalloys with intriguing optical properties, such as modulated plasmonic lifetimes and quality factor Q, could be obtained. This may offer opportunities to extend applications in photovoltaic energy conversion and chemical sensing.

Real-time dark-field scattering microscopic monitoring of the in situ growth of single Ag@Hg nanoalloys

A comprehensive understanding of the growth mechanism of nanoalloys is beneficial in designing and synthesizing nanoalloys with precisely tailored properties to extend their applications. Herein, we present the investigation in this aspect by real-time monitoring of the in situ growth of single Ag@Hg nanoalloys, through direct amalgamation of Ag nanoparticles with elemental mercury, by dark-field scattering microscopy. Four typically shaped Ag nanoparticles, such as rods, triangular bipyramids, cubes, and spheres, were used as seeds for studying the growth of Ag@Hg nanoalloys. The scattered light of Ag nanoparticles of different shapes, on exposure to the growth solution, exhibited a noticeable blue-shift followed by a red-shift, suggesting the growth of Ag@Hg nanoalloys. The formation of Ag@Hg nanoalloys was confirmed by scanning electron microscopy, high-resolution transmit electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and elemental mapping and line scanning. Further analysis of the time-dependent spectral data and morphological change of single nanoparticles during the growth led to the visual identification of the growth mechanism of single Ag@Hg nanoalloys. Three important steps were involved: first, rapid adsorption of Hg atoms onto Ag nanoparticles; second, initial diffusion of Hg atoms into Ag nanoparticles, rounding or shortening the particles; third, further diffusion of Hg atoms leading to the formation of spherical Ag@Hg nanoalloys. On the basis of these results, Ag@Hg nanoalloys with given optical properties can be synthesized. Moreover, dark-field scattering microscopy is expected to be a powerful tool used for real-time monitoring of the in situ growth of other metal nanoparticles.

Facile and rapid synthesis of a dithiol-protected Ag7 quantum cluster for selective adsorption of cationic dyes

We report a facile and rapid (less than 15 min) synthesis of atomically precise, dithiol-protected, silver quantum cluster, Ag7(DMSA)4 (DMSA: meso-2,3-dimercaptosuccinic acid), through a modified solid state route. The as-synthesized cluster exhibits molecular optical absorption features with a prominent λmax at ~500 nm. Composition of the cluster was confirmed using various spectroscopic and microscopic techniques such as electrospray ionization mass spectrometry (ESI MS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive analysis of X-rays (EDAX). Clusters supported on neutral alumina have been shown as better adsorbents for selective adsorption of cationic dyes (over anionic dyes) from water. This selectivity for cationic dyes was evaluated by zeta potential (ζ) measurements. The efficiency of clusters for removal of dyes is very high when compared to nanoparticles (NPs) protected with ligands (citrate and mercaptosuccinic acid (MSA)) possessing similar chemical structures as that of DMSA. The higher efficiency of clusters for the removal of dyes is attributed to their smaller size and large surface area compared to the NPs in addition to favorable electrostatic interactions between the clusters and cationic dyes. Adsorption of dyes (cationic and anionic) was enhanced when dye molecules contain hydrogen bond forming functional groups. Supported clusters have been reused up to five cycles without the loss of activity once the adsorbed dye is extracted using suitable solvents.