Detection with NO Modification: (N═O)-Au Interactions for Instantaneous Label-Free Detection of N-Nitrosodiphenylamine.

Increasing concerns have been raised about dangerous, yet nearly undetectable levels of nitrosamines in foods, medications, and drinking water. Their ubiquitous presence and carcinogenicity necessitates a method of sensitive and selective detection of these potent toxins. While the detection of two major nitrosamines─N-nitrosodimethylamine and N-nitrosodiethylamine─has seen success, low detection limits are scarcer for the other members of this class. One member, N-nitrosodiphenylamine (NDPhA), has had little progress not only in its detection in low quantities but also in its detection at all. NDPhA has unique difficulty in its identification due to its aromaticity, making it far more problematic to distinguish in the common GC-MS or LC-MS/MS methods used for nitrosamine sensing. Despite this detection barrier, it has been listed among the top 6 carcinogenic nitrosamines by the Food and Drug Administration as of 2023. Due to its evasive nature, a unique methodology must be applied to facilitate its sensitive identification. Herein, we describe the use of surface-enhanced Raman spectroscopy for the first account of liquid-phase detection of NDPhA using cysteamine-functionalized gold nanostars and a portable Raman spectrometer. Our methodology requires no chemical modification to the nitrosated structure as well as the usage of two well-understood biocompatible materials: cysteamine and gold nanoparticles.

Synthesis and Characterization of Magnetoplasmonic Air-Stable Au@FeCo.

The synthesis of FeCo alloys as highly magnetic nanoparticles has been valuable, as far as applications for magnetic nanoparticles are concerned. However, recently, a field of magnetoplasmonics in which magnetic nanoparticles such as the FeCo alloys doped with plasmonic materials such as Au and Ag to create a hybrid nanostructure with both properties has emerged. These magnetoplasmonic metamaterials have greatly enhanced the limit of detection of analytes in spectroscopic methods, as well as providing a more widely applicable nanoparticle to broaden the use of FeCo alloys even further. Herein, we discuss the synthesis of high-yield and fairly monodisperse spherical FeCo and Au-doped FeCo (Au@FeCo) with varying compositions of Au synthesized via the thermal decomposition of iron pentacarbonyl (Fe(CO)5) and dicobalt octacarbonyl (Co2(CO)8), followed by the addition of Au atoms using triphenylphosphine gold(I) chloride ((Ph3P)AuCl) via both coprecipitation and by delayed addition methods. The products were separated using a hand-held magnet, and then characterized via ultraviolet-visible light (UV-vis), scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), flame atomic absorption spectrometry (F-AAS), and magnetization measurements. Optical studies revealed a plasmonic peak at 550 nm in the Au@FeCo nanoparticles that had a gold content (%Au) of >2% (by weight), determined using F-AAS. Colocation of the Fe, Co, and Au were demonstrated through EDX analysis. Location of the Au atoms in the core were seen through high-resolution bright-field imaging. To understand the use of these nanoparticles for potential application in therapeutics and/or electronics, resistance measurements were performed to assess power loss as a function of frequency. We also achieved magnetization values as high as 150 emu/g and as low as 50 emu/g for gold-loaded samples based on %Au by weight. This paves the way to continue to develop magneto-plasmonic structures chemically using these synthesis strategies.

Gold nanoparticles loaded with cullin-5 DNA increase sensitivity to 17-AAG in cullin-5 deficient breast cancer cells.

HSP90 inhibitors have the potential to treat many types of cancer due to the dependence of tumor cells on HSP90 for cell growth and proliferation. The Cullin-5 (Cul5) E3 ubiquitin ligase is required for HSP90 inhibitors to induce client protein degradation and subsequent cell death. Cul5 is expressed at low levels in breast cancer cells compared to patient matched controls. This observed low Cul5 expression may play a role in the reported decreased efficacy of 17-AAG and related HSP90 inhibitors as a monotherapy. We have developed a method for delivery of 17-AAG plus Cul5 DNA to cells via gold nanoparticles (AuNPs). Delivery of AuNPs containing Cul5 DNA increases the sensitivity of Cul5 deficient AU565 cells to 17-AAG. Characterization of AuNPs by UV-vis spectrum, TEM, gel electrophoresis assay and 1H NMR indicate attachment of both 17-AAG and DNA payload as well as AuNP stability. Studies in Cul5 deficient AU565 cells reveal that delivery of Cul5 and 17-AAG together increase cytotoxicity. Our results provide evidence that delivery of DNA with drug may serve as a method to sensitize drug resistant tumor cells.

Spatial modulation spectroscopy imaging of nano-objects of different sizes and shapes.

Spatial modulation spectroscopy (SMS) is a powerful method for interrogating single nanoparticles. In these experiments optical extinction is measured by moving the particle in and out of a tightly focused laser beam. SMS is typically used for particles that are much smaller than the laser spot size. In this paper, we extend the analysis of the SMS signal to particles with sizes comparable to or larger than the laser spot, where the shape of the particle matters. These results are important for the analysis of polydisperse samples that have a wide range of sizes. The presented example images and analysis of a carbon microparticle sample show the utility of the derived expressions. In particular, we show that SMS can be used to generate extinction cross-section information about micrometer-sized particles with complex shapes.

Role of Resonances in the Transmission of Surface Plasmon Polaritons between Nanostructures.

Understanding how surface plasmon polaritons (SPPs) propagate in metal nanostructures is important for the development of plasmonic devices. In this paper, we study the transmission of SPPs between single-crystal gold nanobars on a glass substrate using transient absorption microscopy. The coupled structures were produced by creating gaps in single nanobars by focused ion beam milling. SPPs were launched by focusing the pump laser at the end of the nanobar, and the transmission across the gaps was imaged by scanning the probe laser over the nanostructure. The results show larger losses at small gap sizes. Finite element method calculations were used to investigate this effect. The calculations show two main modes for nanobars on a glass surface: a leaky mode localized at the air-gold interface, and a bound mode localized at the glass-gold interface. At specific gap sizes (approximately 50 nm for our system), these SPP modes can excite localized surface plasmon modes associated with the gap, which dissipate energy. This increases the energy losses at small gap sizes. Experiments and simulations were also performed for the nanobars in microscope immersion oil, which creates a more homogeneous optical environment, and consistent results were observed.

Absorption Spectroscopy of Single Optically Trapped Gold Nanorods.

Extinction spectra of single gold nanorods optically trapped in water were measured by spatial modulation spectroscopy. Comparison of the extinction cross sections and resonance frequencies to finite element calculations allows us to determine the dimensions of the nanorod and estimate the contribution of radiation damping to the LSPR line width. Subtracting the radiation damping and bulk contributions from the measured line widths yields the electron-surface scattering contribution. The results show that the surfactant coating for the nanorods causes large electron-surface scattering effects with significant particle-to-particle variations. These effects are more pronounced than those seen for substrate-supported particles in previous single particle studies. Indeed, the measured line widths are only slightly narrower than that of the ensemble spectrum. These results show the importance of removing surfactant for sensing applications of these materials.

Imaging nano-objects by linear and nonlinear optical absorption microscopies.

Absorption based microscopy measurements are emerging as important tools for studying nanomaterials. This review discusses the three most common techniques for performing these experiments: transient absorption microscopy, photothermal heterodyne imaging, and spatial modulation spectroscopy. The focus is on the application of these techniques to imaging and detection, using examples taken from the authors' laboratory. The advantages and disadvantages of the three methods are discussed, with an emphasis on the unique information that can be obtained from these experiments, in comparison to conventional emission or scattering based microscopy experiments.

Spatial modulation spectroscopy for imaging and quantitative analysis of single dye-doped organic nanoparticles inside cells.

Imaging of non-fluorescent nanoparticles in complex biological environments, such as the cell cytosol, is a challenging problem. For metal nanoparticles, Rayleigh scattering methods can be used, but for organic nanoparticles, such as dye-doped polymer beads or lipid nanoparticles, light scattering does not provide good contrast. In this paper, spatial modulation spectroscopy (SMS) is used to image single organic nanoparticles doped with non-fluorescent, near-IR croconaine dye. SMS is a quantitative imaging technique that yields the absolute extinction cross-section of the nanoparticles, which can be used to determine the number of dye molecules per particle. SMS images were recorded for particles within EMT-6 breast cancer cells. The measurements allowed mapping of the nanoparticle location and the amount of dye in a single cell. The results demonstrate how SMS can facilitate efforts to optimize dye-doped nanoparticles for effective photothermal therapy of cancer.

Compressible Viscoelastic Liquid Effects Generated by the Breathing Modes of Isolated Metal Nanowires.

Transient absorption microscopy is used to examine the breathing modes of single gold nanowires in highly viscous liquids. By performing measurements on the same wire in air and liquid, the damping contribution from the liquid can be separated from the intrinsic damping of the nanowire. The results show that viscous liquids strongly reduce the vibrational lifetimes but not to the extent predicted by standard models for nanomaterial-liquid interactions. To explain these results a general theory for compressible viscoelastic fluid-structure interactions is developed. The theory results are in good agreement with experiment, which confirms that compressible non-Newtonian flow phenomena are important for vibrating nanostructures. This is the first theoretical study and experimental measurement of the compressible viscoelastic properties of simple liquids.

Effect of substrate discontinuities on the propagating surface plasmon polariton modes in gold nanobars.

The surface plasmon polariton (SPP) modes of gold nanobars (nanowires with rectangular dimensions) have been investigated by scanning pump-probe microscopy. In these experiments the nanobars were suspended over trenches cut in glass coverslips, and propagating SPP modes were launched in the supported portion of the nanobar by focusing a near-IR pump laser beam at the end of the nanobar. Transient absorption images were then collected by scanning the probe laser over the nanobar using a galvo-mirror system. The images show that the trench has a large effect on the SPP modes, specifically, for approximately half the nanowires the propagation length is significantly reduced after the trench. Finite element calculations were performed to understand this effect. The calculations show that the pump laser excites bound and leaky modes (modes that have their fields localized at the nanobar/glass or nanobar/air interfaces, respectively) in the supported portions of the nanobars. These modes propagate along the nanobar. When they meet the trench their field distributions are altered. The modes that derive from the bound mode are strongly damped over the trench. Thus, the bound mode is not reconstituted on the opposite side of the trench, and only the leaky mode contributes to the signal. Because the bound and leaky modes can have different propagation lengths, the propagation lengths measured in our experiments can change from one side of the trench to the other. The results show how the substrate can be engineered to control the SPP modes in metal nanostructures.

Imaging and Analysis of Single Optically Trapped Gold Nanoparticles Using Spatial Modulation Spectroscopy.

The extinction cross sections and spectra of single nanoparticles can be directly measured by moving the particle in and out of a tightly focused laser beam. This technique, known as spatial modulation spectroscopy, yields detailed information about the size, shape, and environment of the particles. These experiments are typically done on particles immobilized on a substrate. Here we demonstrate for the first time the use of spatial modulation spectroscopy to interrogate single, optically trapped nanoparticles in solution. Gold nanoparticles as small as 15 nm were trapped and imaged. The experiments were performed by modulating the position of the probe laser beam while scanning it over the trapped particle with a galvo-scanning mirror system. This technique opens up the possibility of precisely measuring the optical properties of single nanoparticles in liquid environments, free from the influence of a surface.

Detection of single gold nanoparticles using spatial modulation spectroscopy implemented with a galvo-scanning mirror system.

The optical extinction of single nanoparticles can be sensitively detected by spatial modulation spectroscopy (SMS), where the particle is moved in and out of a tightly focused laser beam with a piezo-device. Here we show that high sensitivity can be obtained by modulating the beam with a galvo-mirror system, rather than by moving the sample. This work demonstrates an inexpensive method for making a SMS microscope, and shows how an existing laser scanning microscope can be adapted for SMS measurements. The galvo-mirror technique also allows SMS measurements to be performed in a liquid, which is difficult to do with piezo-modulation.

Optical detection of single nano-objects by transient absorption microscopy.

In recent years there has been considerable effort in developing ultra-sensitive imaging techniques based on absorption. This mini-review describes recent results from our laboratory on detecting single nano-objects using transient absorption microscopy. This technique is extremely flexible, allowing the detection of single semiconductor and metal nanostructures with high sensitivity. The goal of this review is to illustrate key points in implementing transient absorption microscopy for ultra-sensitive detection, as well as to discuss the advantages and disadvantages of this technique compared to other optical absorption based methods.