Real-time dark-field light scattering imaging to monitor the coupling reaction with gold nanorods as an optical probe

Intrinsic Chirality Modulation and Biosensing Application of Helical Gold Nanorods by Anisotropic Etching

The investigation of plasmonic chirality is a profound and intriguing topic, and the distinctive morphology of intrinsically chiral nanoparticles has prompted significant interest in the structure-activity relationship between particle morphology and chirality. In this work, the anisotropic etching of chiral helical gold nanorods (HGNRs) by a cetyltrimethylammonium bromide (CTAB)-HAuCl4 complex was observed with an interesting bidirectional variation of intrinsic chirality that initially enhanced and subsequently weakened, which was related with the diversity in CTAB distribution. In addition, an ultrasensitive and convenient sensing platform for acetylcholinesterase was developed based on the circular dichroism signal recovery of HGNRs caused by the dual inhibition of HGNR etching. The distinctive etching process and mechanism of chiral nanoparticles offer new insights into understanding the structural features and biochemical applications of the plasmonic intrinsic chirality, which could be applied to the acquisition of chiral nanoparticles and sensitive detection platform based on chiral signal changes.

High-Throughput All-Optical Determination of Nanorod Size and Orientation

As a single-particle characterization technique, optical microscopy has transformed our understanding of structure-function relationships of plasmonic nanoparticles, but the need for ex-situ-correlated electron microscopy to obtain structural information handicaps an otherwise exceptional high-throughput technique. Here, we present an all-optical alternative to electron microscopy to accurately and quickly extract structural information about single gold nanorods (Au NRs) using calcite-assisted localization and kinetics (CLocK) microscopy. Color CLocK images of single Au NRs allow scattering from the longitudinal and transverse plasmon modes to be imaged simultaneously, encoding spectral data in CLocK images that can then be extracted to obtain Au NR size and orientation. Moreover, through the use of convolutional neural networks, Au NR length, width, and aspect ratio can be predicted directly from color CLocK images within ∼10% of the true value measured by electron microscopy.

Automated Five-Dimensional Single Particle Tracking by Bifocal Parallax Dark-Field Microscopy with Electronic Tunable Lens

We present a novel method for the precise tracking of plasmonic gold nanorods (AuNRs) in live cells, enabling a comprehensive understanding of the nanocargo's cellular dynamics. Traditional single particle tracking (SPT) struggles with accurately determining all five spatial parameters (x, y, z, ϕ, and θ) in live cells due to various challenges. Our innovation combines electronic tunable lens (ETL) technology with bifocal parallax dark-field (DF) microscopy, allowing continuous adjustment of the imaging focal plane for automatic tracking of both translational and rotational movements of AuNRs. This 5D single-particle orientation and rotational tracking (5D SPORT) method achieves remarkable precision, with 3D localization precisions of 9 (x), 10 (y), and 15 nm (z) and angular resolutions below 2°. To showcase its applicability, we investigated intracellular transport of nanocargos using transferrin-modified AuNRs as the imaging probe. Differentiated transport stages, such as active transport and pause period, were clearly unveiled from the observed dynamics in 5D. This advancement in single particle tracking holds promise for a wide range of applications in biomedical research, particularly when combined with other imaging modalities, such as light sheet fluorescence microscopy.

Rapid and sensitive determination of ascorbic acid based on label-free silver triangular nanoplates

In this study, a new method for the detection of ascorbic acid (AA) was proposed. It was based on the protective effect of AA on silver triangular nanoplates (Ag TNPs) against Cl- induced etching reactions. Cl- can attack the corners of Ag TNPs and etch them, causing a morphological shift from triangular nanoplates to nanodiscs. As a result, the solution changes color from blue to yellow. However, in the presence of AA, the corners of Ag TNPs can be protected from Cl- etching, and the blue color of the solution remains unchanged. Using this effect, a selective sensor was designed to detect AA in the range of 0-40.00 μM with a detection limit of 2.17 μM. As the concentration of AA varies in this range, color changes from yellow to blue can be easily observed, so the designed sensor can be used for colorimetric detection. This method can be used to analyze fruit juice samples.

The enhancement in the performance of ultra-small core-shell Au@AuPt nanoparticles toward HER and ORR by surface engineering

In this work, ultra-small core-shell (USCS) Au_38.4@Au_4.1Pt_57.5 nanoparticles (NPs) with an optimal Pt-to-Au ratio were successfully prepared by the optimal etching treatment of USCS Au@AuPt NPs by Fe(III) ions to remove some exposed Au atoms on their outermost surfaces. The as-prepared USCS Au_38.4@Au_4.1Pt_57.5 NPs with Fe(III)-etching treatment for 2 h loaded on carbon black as catalysts (USCS^2h Au_38.4@Au_4.1Pt_57.5-NP/C catalysts) exhibit superior electrocatalytic activity and durability for both the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in acidic media. For instance, the overpotential of USCS^2h Au_38.4@Au_4.1Pt_57.5-NP/C catalysts toward the HER is 13 mV at a current density of -10 mA cm^-2 (η₁₀ = 13 mV), which is much better than that of commercial Pt/C catalysts (η₁₀ = 31 mV). Moreover, their mass activity (63.8 A mg_Pt^-1) is about 16.4 times larger than that of commercial Pt/C catalysts (3.9 A mg_Pt^-1). In addition, they also present better long-term stability. Furthermore, they also show an improved activity toward the ORR in terms of the half-wave potential (E_1/2) (0.89 V vs. RHE), which is more positive by about 38 mV than commercial Pt/C catalysts (0.852 V). In addition, they also show a higher kinetic current density (14.22 mA cm^-2 at 0.85 V) and better long-term durability. This etching-treatment strategy can be extended to further improve the catalytic performance of ultra-small Au-based bimetallic or multi-metallic NPs by surface engineering.

Two-Dimensional Analysis Method for Highly Sensitive Detection of Dual MicroRNAs in Breast Cancer Cells

Multiple biomarker detection is crucial for early clinical diagnosis, and it is significant to achieve the simultaneous detection of multiple biomarkers with the same nanomaterial. In this work, the hairpin DNA strands were selectively modified on the surface of gold nanorods (AuNRs) to construct two kinds of nanoprobes by rational design. When in the presence of dual microRNAs, AuNRs were assembled to form end-to-end (ETE) and side-by-side (SBS) dimers. Compared with a single AuNR, the dark-field scattering intensity and red color percentage variation of dimers were extremely distinguished, which could be developed for dual microRNA detection by combining the red color percentage and scattering intensity with the data processing method of principal component analysis to construct a two-dimensional analysis method. Especially, the fraction of AuNR dimers presented a linear relationship with the amount of microRNAs. Based on this, microRNA-21 and microRNA Let-7a in breast cancer cells were detected with the detection limits of 1.72 and 0.53 fM, respectively. This method not only achieved the sensitive detection of dual microRNAs in human serum but also realized the high-resolution intracellular imaging, which developed a new way for the oriented assembly of nanomaterials and biological detection in living cells.

The Accurate Imaging of Collective Gold Nanorods with a Polarization-Dependent Dark-Field Light Scattering Microscope

Anisotropic nanomaterials, such as gold nanorods (AuNRs), could be employed as an orientation platform due to their polarization-dependent surface plasmon resonance. However, a variety of factors would affect the dark-field light scattering imaging of anisotropic nanomaterials, resulting in an unstable signal, which is not advantageous to its further application. In this work, the localized surface plasmon resonance properties of a few AuNRs at different angles were excited by polarization with a conventional dark-field microscope, in which it was found that the ratio of AuNRs' light scattering intensity at different polarization angles (I) to that without a polarizer (I₀) reflected the orientation information of AuNRs. Furthermore, the light scattering signal ratio between the parallel polarization (I_p) and that without a polarizer (I₀) was closely related with the aspect ratio of AuNRs, which could not be affected by external conditions. To verify this concept, a highly sensitive and selective assay of the alkaline phosphatase activity in human serum was successfully developed based on the chemical etching of AuNRs, resulting in a lower aspect ratio and a lesser I_p/I₀. This result holds great promise for polarization-dependent colorimetric nanomaterials and single-particle tracers in living cells.

Visual and Ultrasensitive Detection of a Coronavirus Using a Gold Nanorod Probe under Dark Field

Porcine epidemic diarrhea virus (PEDV), a coronavirus that causes highly infectious intestinal diarrhea in piglets, has led to severe economic losses worldwide. Rapid diagnosis and timely supervision are significant in the prophylaxis of PEDV. Herein, we proposed a gold-nanorod (GNR) probe-assisted counting method using dark field microscopy (DFM). The antibody-functionalized silicon chips were prepared to capture PEDV to form sandwich structures with GNR probes for imaging under DFM. Results show that our DFM-based assay for PEDV has a sensitivity of 23.80 copies/μL for simulated real samples, which is very close to that of qPCR in this study. This method of GNR probes combined with DFM for quantitative detection of PEDV not only has strong specificity, good repeatability, and a low detection limit, but it also can be implemented for rapid on-site detection of the pathogens.

In Situ Direct Monitoring of the Morphological Transformation of Single Au Nanostars Induced by Iodide through Dual-Laser Dark-Field Microscopy: Unexpected Mechanism and Sensing Applications

Single nanoparticle imaging is a significant technique to help reveal the reaction mechanism and provides insight into the nanoparticle transformation. Here, we monitor the in situ morphological transformation of Au nanostars (GNSs) induced by iodide (I⁻) in real time using dark-field microscopy (DFM) with 638 nm red (R) and 534 nm green (G) laser coillumination. The two lasers are selected because the longitudinal localized surface plasmon resonance of GNSs is located at 638 nm and that for GNSs after transformation is at 534 nm. Interestingly, I⁻ can interact with GNSs directly without the engagement of other reagents, and upon increasing I⁻ concentrations, GNSs undergo color changes from red to orange, yellow, and green under DFM. Accordingly, green/red channel intensities (G/R ratios) are extracted by obtaining red and green channel intensities of single nanoparticles to weigh the morphological changes and quantify I⁻. A single nanoparticle sensor is constructed for I⁻ detection with a detection limit of 6.9 nM. Finally, a novel mechanism is proposed to elucidate this shape transformation. I⁻ absorbed onto the surface of GNSs binds with Au atoms to form AuI⁻, lowering the energy of its bond with other Au atoms, which facilitates the diffusion of this atom across the nanoparticle surface to low-energy sites at the concaves, thus deforming to spherical Au nanoparticles.

The restructure of Au@Ag nanorods for cell imaging with dark-field microscope

The facile and noninjurious image of cells with high resolution and low toxicity is essential since imaging can offer rich and direct information and insights into metabolic activities, clinical diagnosis, drug delivery and cancer therapy. In this contribution, a smart imaging probe was employed as a contrast agent for dark-field cell imaging. Au core/Ag shell nanorods (Au@Ag NRs) that characterized by X-ray diffraction and X-ray photoelectron spectroscopy, formed Au@Ag@AgI NRs when exposed to iodine, which greatly enhanced the light scattering of nanorods. Herein, the silver shell acted as the response element for iodine as well as the protective agent for Au core. When conjugated with folate, the nanorods can be used to image human cervical cancer cells (HeLa cells) under a dark-field microscope. Nanorods were demonstrated with excellent tumor cellular uptake ability without obvious cytotoxicity, making them ideal candidates in biosensing and bioimaging applications.

Artificial intelligence-assisted enumeration of ultra-small viruses with dual dark-field plasmon resonance probes

Direct visual enumeration of viruses under dark-field microscope (DFM) using plasmon resonance probes (PRPs) is fast and convenient; however, it is greatly limited in the assay of real samples because of its inability to accurately identify false positives owing to non-specific adsorption. In this study, we propose an artificial intelligence (AI)-assisted DFM enumeration strategy for the accurate assay of Enterovirus A71 (an ultra-small human virus) using two PRPs; a 40 nm silver nanoparticle probe (SNP) that appears bright blue under DFM, and a 120 nm gold nanorod probe (GNP) that appears red under DFM. The capture chip was prepared by immobilizing the SNPs with antibodies on the glass to capture the target virus and to form dichromatic sandwich structures with the GNPs, followed by imaging under a dark field (DF). Subsequently, the DF images of the capture chip were subjected to a two-step screening: first, using image processing, and thereafter using the AI algorithm screening to eliminate false positive results and background noise. The results revealed that the data from the AI-assisted dual PRPs assay were highly consistent with those of quantitative PCR (qPCR), and that the sensitivity with a minimum detectable concentration of 3 copies/μL was 5 times higher than that of qPCR. The entire analysis was completed within 45 min. Therefore, our AI-assisted virus enumeration strategy with two DF PRPs holds great potential for ultra-sensitive and accurate quantification of viruses in real samples.

Determination of the Mono and Dibromo Derivatives Ratio Resulting from Semiconductor Bromination Using Ultraviolet-visible Absorption Spectroscopy and Gaussian Peak Fitting

The chemical-industrial production of organic semiconductors urgently needs a cheap and fast approach to determine the components' proportion of the reaction system. In the present work, the Gaussian peak fitting method was applied to process monobromo and dibromo-substituted perylene diimide mixed solutions' ultraviolet-visible absorption curves. The functional relationship formula between the peak-intensity ratio and the component ratio is then concluded. Finally, field experiments of the perylene imide brominating reaction can be used to confirm that such a formula is able to accurately calculate the proportion of ingredients in the synthesis reaction solution system.

Size-modulated optical property of gold nanorods for sensitive and colorimetric detection of thiourea in fruit juice

As an important sulfur compound, thiourea (TU) has caused great concern because of its wide application as well as its serious toxicity and hazard to the environment. Thus, it is necessary to develop a sensitive and selective method for TU analysis. In this work, gold nanorods (AuNRs) acted as an optical probe to realize the sensitive and colorimetric detection of TU. In HCl medium, Fe³⁺ at low concentration was difficult to oxide Au⁰ to form Au⁺ because of the high redox potential or the positive Gibbs free energy change. However, this process was possible when TU was present since the association constant between Au⁺ and TU is great enough to bind with TU to form a stable complex to further promote the etching of AuNRs, resulting in the lower aspect ratio of AuNRs with the blue shift and intensity decrease in extinction spectra, accompanied by the divisive colors of AuNRs solution or colorful dark-field light scattering imaging of single AuNR. The blue-shift of AuNRs longitudinal plasmon resonance absorption (LPRA) band was proportional to the concentration of TU in the range of 1-250 nM and the limit of detection (3σ/k) was as low as 0.4 nM. In addition, the colorimetric method was proven with high selectivity in the presence of potential interfering compounds, which was successfully applied to the detection of TU in fruit juice samples. This proposed colorimetric method provides a simple, sensitive yet selective measurement tool for TU sensing, which may offer new opportunities in the development of colorimetric sensors for food safety in the future.

Nanoscale optical imaging in chemistry

Single-molecule-level measurements are bringing about a revolution in our understanding of chemical and biochemical processes. Conventional measurements are performed on large ensembles of molecules. Such ensemble-averaged measurements mask molecular-level dynamics and static and dynamic fluctuations in reactivity, which are vital to a holistic understanding of chemical reactions. Watching reactions on the single-molecule level provides access to this otherwise hidden information. Sub-diffraction-limited spatial resolution fluorescence imaging methods, which have been successful in the field of biophysics, have been applied to study chemical processes on single-nanoparticle and single-molecule levels, bringing us new mechanistic insights into physiochemical processes. However, the scope of chemical processes that can be studied using fluorescence imaging is considerably limited; the chemical reaction has to be designed such that it involves fluorophores or fluorogenic probes. In this article, we review optical imaging modalities alternative to fluorescence imaging, which expand greatly the range of chemical processes that can be probed with nanoscale or even single-molecule resolution. First, we show that the luminosity, wavelength, and intermittency of solid-state photoluminescence (PL) can be used to probe chemical transformations on the single-nanoparticle-level. Next, we highlight case studies where localized surface plasmon resonance (LSPR) scattering is used for tracking solid-state, interfacial, and near-field-driven chemical reactions occurring in individual nanoscale locations. Third, we explore the utility of surface- and tip-enhanced Raman scattering to monitor individual bond-dissociation and bond-formation events occurring locally in chemical reactions on surfaces. Each example has yielded some new understanding about molecular mechanisms or location-to-location heterogeneity in chemical activity. The review finishes with new and complementary tools that are expected to further enhance the scope of knowledge attainable through nanometer-scale resolution chemical imaging.

Toward Real-Time Monitoring and Control of Single Nanoparticle Properties with a Microbubble Resonator Spectrometer

Optical microresonators have widespread application at the frontiers of nanophotonic technology, driven by their ability to confine light to the nanoscale and enhance light-matter interactions. Microresonators form the heart of a recently developed method for single-particle photothermal absorption spectroscopy, whereby the microresonators act as microscale thermometers to detect the heat dissipated by optically pumped, nonluminescent nanoscopic targets. However, translation of this technology to chemically dynamic systems requires a platform that is mechanically stable, solution compatible, and visibly transparent. We report microbubble absorption spectrometers as a versatile platform that meets these requirements. Microbubbles integrate a two-port microfluidic device within a whispering gallery mode microresonator, allowing for the facile exchange of chemical reagents within the resonator's interior while maintaining a solution-free environment on its exterior. We first leverage these qualities to investigate the photoactivated etching of single gold nanorods by ferric chloride, providing a method for rapid acquisition of spatial and morphological information about nanoparticles as they undergo chemical reactions. We then demonstrate the ability to control nanorod orientation within a microbubble through optically exerted torque, a promising route toward the construction of hybrid photonic-plasmonic systems. Critically, the reported platform advances microresonator spectrometer technology by permitting room-temperature, aqueous experimental conditions, which may be used for time-resolved single-particle experiments on non-emissive, nanoscale analytes engaged in catalytically and biologically relevant chemical dynamics.

Highly monodisperse Au@Ag nanospheres: synthesis by controlled etching route and size-dependent SERS performance of their surperlattices

For accurate experimental and theoretical research, the preparation of nanocrystrals with regular morphology is of significant importance. In this work, we investigated a facile and effective route for generating highly spherical Au@Ag nanospheres (NSs) with tuneable size and uniform morphology at room temperature. The aqueous synthesis mainly involved seed-mediated growth method together with oxidation etching employing sodium hypochlorite (NaClO) as etching agent. The termination of the etching reaction with NaClO as etching agent was simply related to the amount of NaClO and had no connection with time. Thus Au@Ag NSs with controllable diameters in range from 24 to 87 nm were prepared only by varying the amount of NaClO added into the solutions of Au@Ag nanocubes. Additionally, combined with interface self-assembly technique, Au@Ag NSs were assembled into densely arranged two-dimensional (2D) monolayer film. Moreover, the SERS performance of these monolayers were evaluated by calculating the analytical enhancement factor using crystal violet as probe molecule. The AEF increased obviously as the diameter of Au@Ag NSs went up, and the maximum AEF could reach to 0.94 × 10⁷ at the laser excitation wavelength of 785 nm. Besides, the electromagnetic field distribution for Au@Ag NSs array were also confirmed by Mie theory and FDTD solutions software and the results revealed the similar trend with the experimental results. In general, this 2D assemblies in term of high quality Au@Ag NSs have broad prospects to act as promising candidates for SERS analytical sensor and other applications.

Label-free gold nanorods sensor array for colorimetric detection and discrimination of biothiols in human urine samples

Biothiols play important roles in regulating redox balance in biological systems, but their discrimination is challengeable. In this work, a colorimetric nanosensing array for biothiols was established, which was composed of gold nanorods (AuNRs) and metal ions (Hg²⁺, Pb²⁺, Cu²⁺, Ag⁺). By employing label-free AuNRs as the colorimetric probe, and the color and spectral changes of AuNRs as the output signal, principal component analysis (PCA) was applied to processing the signal and generating a clustering map. Due to the different binding affinity between biothiols and metal ions, AuNRs exhibited a unique pattern to form a fingerprint-like colorimetric array, which was able to discriminate five biothiols by the naked eyes. This strategy combines PCA and sensor array to achieve rapid and accurate discrimination and detection of biothiols. In addition, the method shows the great potential in analysis of biothiols in human urine samples.

Time-resolved visual detection of heparin by accelerated etching of gold nanorods

Plasmonic gold nanorods are promising and sensitive light scattering probes, which can reach the single particle level. Herein, we present the light scattering properties of gold nanorods for time-resolved visual detection of heparin based on the rapid etching of gold nanorods under dark-field microscopy.

Matter, energy and information network of a graphene-peptide-based fluorescent sensing system for molecular logic computing, detection and imaging of cancer stem cell marker CD133 in cells and tumor tissues

A graphene-peptide-based fluorescent sensing system for molecular logic operations, sensing and imaging of CD133.

The localized surface plasmon resonance induced edge effect of gold regular hexagonal nanoplates for reaction progress monitoring

The LSPR induced edge effect of Au regular nanoplates gradually disappears accompanied with the change of doughnut-shaped far-field scattering patterns (DNSPs) into scattering solid spots with the reaction proceeding with ferric iron in a thiourea (TU) solution.

Aptamer-modified selenium nanoparticles for dark-field microscopy imaging of nucleolin

Selenium nanoparticles with good water solubility and excellent biocompatibility are used for the first time as a light-scattering nanoprobe with aptamer modification to image nucleolin-overexpressing cancer cells through dark-field microscopy.