Plasmonic nanoprobes: from chemical sensing to medical diagnostics and therapy

Stabilization and Encapsulation of Gold Nanostars Mediated by Dithiols

Surface chemistry plays a pivotal role in regulating the morphology of nanoparticles, maintaining colloidal stability, and mediating the interaction with target analytes toward practical applications such as surface-enhanced Raman scattering (SERS)-based sensing and imaging. The use of a binary ligand mixture composed of 1,4-benzenedithiol (BDT) and hexadecyltrimethylammonium chloride (CTAC) to provide gold nanostars with long-term stability is reported. This is despite BDT being a bifunctional ligand, which usually leads to bridging and loss of colloidal stability. It is found however that neither BDT nor CTAC alone are able to provide sufficient colloidal and chemical stability. BDT-coated Au nanostars are additionally used as seeds to direct the encapsulation with a gold outer shell, leading to the formation of unusual nanostructures including semishell-coated gold nanostars, which are characterized by high-resolution electron microscopy and electron tomography. Finally, BDT is exploited as a probe to reveal the enhanced local electric fields in the different nanostructures, showing that the semishell configuration provides significantly high SERS signals as compared to other core-shell configurations obtained during seeded growth, including full shells.

Hollow Au/Ag nanostars displaying broad plasmonic resonance and high surface-enhanced Raman sensitivity

Bimetallic Au/Ag hollow nanostar (HNS) nanoparticles with different morphologies were prepared in this work. These nanoplatforms were obtained by changing the experimental conditions (concentration of silver and chemical reductors, hydroxylamine and citrate) and by using Ag nanostars as template nanoparticles (NPs) through galvanic replacement. The goal of this research was to create bimetallic Au/Ag star-shaped nanoparticles with advanced properties displaying a broader plasmonic resonance, a cleaner exposed surface, and a high concentration of electromagnetic hot spots on the surface provided by the special morphology of nanostars. The size, shape, and composition of Ag as well as their optical properties were studied by extinction spectroscopy, hyperspectral dark field microscopy, transmission and scanning electron microscopy (TEM and SEM), and energy dispersive X-ray spectroscopy (EDX). Finally, the surface-enhanced Raman scattering (SERS) activity of these HNS was investigated by using thioflavin T, a biomarker of the β-amyloid fibril formation, responsible for Alzheimer's disease. Lucigenin, a molecule displaying different SERS activities on Au and Ag, was also used to explore the presence of these metals on the NP surface. Thus, a relationship between the morphology, plasmon resonance and SERS activity of these new NPs was made.

Multifunctional gold nanostars for molecular imaging and cancer therapy

Plasmonics-active gold nanoparticles offer excellent potential in molecular imaging and cancer therapy. Among them, gold nanostars (AuNS) exhibit cross-platform flexibility as multimodal contrast agents for macroscopic X-ray computer tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), as well as nanoprobes for photoacoustic tomography (PAT), two-photon photoluminescence (TPL), and surface-enhanced Raman spectroscopy (SERS). Their surfactant-free surface enables versatile functionalization to enhance cancer targeting, and allow triggered drug release. AuNS can also be used as an efficient platform for drug carrying, photothermal therapy, and photodynamic therapy (PDT). This review paper presents the latest progress regarding AuNS as a promising nanoplatform for cancer nanotheranostics. Future research directions with AuNS for biomedical applications will also be discussed.

Multiplexed TEM Specimen Preparation and Analysis of Plasmonic Nanoparticles

We describe a system for rapidly screening hundreds of nanoparticle samples using transmission electron microscopy (TEM). The system uses a liquid handling robot to place up to 96 individual samples onto a single standard TEM grid at separate locations. The grid is then transferred into the TEM and automated software is used to acquire multiscale images of each sample. The images are then analyzed to extract metrics on the size, shape, and morphology of the nanoparticles. The system has been used to characterize plasmonically active nanomaterials.

Nanostar Clustering Improves the Sensitivity of Plasmonic Assays

Star-shaped Au nanoparticles (Au nanostars, AuNS) have been developed to improve the plasmonic sensitivity, but their application has largely been limited to single-particle probes. We herein describe a AuNS clustering assay based on nanoscale self-assembly of multiple AuNS and which further increases detection sensitivity. We show that each cluster contains multiple nanogaps to concentrate electric fields, thereby amplifying the signal via plasmon coupling. Numerical simulation indicated that AuNS clusters assume up to 460-fold higher field density than Au nanosphere clusters of similar mass. The results were validated in model assays of protein biomarker detection. The AuNS clustering assay showed higher sensitivity than Au nanosphere. Minimizing the size of affinity ligand was found important to tightly confine electric fields and improve the sensitivity. The resulting assay is simple and fast and can be readily applied to point-of-care molecular detection schemes.

Surface-enhanced Raman scattering (SERS)-active gold nanochains for multiplex detection and photodynamic therapy of cancer

Multifunctional nanomedicine holds considerable promise as the next generation of medicine that will enable early detection of diseases, as well as simultaneous monitoring and therapy with minimal toxicity. In particular, surface-enhanced Raman scattering (SERS) technology with high sensitivity and multiplexing capabilities is emerging as a powerful alternative for identifying specific biological targets in live cells. In this paper, we present the synthesis of SERS-active gold nanochains (AuNCs) as a potential theranostic system for multiplex detection and photodynamic therapy (PDT) of cancer. AuNCs were prepared by a simple physical mixing method to assemble citrate-stabilized gold nanoparticles into nanochains using hyaluronic acid and hydrocaffeic acid (HA-HCA) conjugates as templates. In addition, Raman reporters and photosensitizers (PSs) were conjugated onto the surface of the AuNCs for multiplex detection and PDT action. After mixing with HA-HCA conjugates, citrate-stabilized gold nanoparticles formed the AuNC structure, and AuNC length was controlled by the HCA conjugation ratio in the HA-HCA conjugates. AuNCs exhibited maximal absorption in the near-infrared (NIR) spectral region and effective SERS property. Confocal microscopy, flow cytometry, Raman spectroscopy and Bio-TEM measurements were used to determine cellular uptake of the Raman reporter, PS and AuNCs in HeLa cells. AuNCs conjugated with Raman reporter and PS (HA-HCAn-Au-Pheo-NPT) showed more than 99% cellular uptake and exhibited excellent phototoxicity even at low PS concentrations compared with free PS after laser irradiation. This SERS-active AuNC (HA-HCAn-Au-Pheo-NPT) shows promise for applications in theranostics, integrating SERS imaging and PDT.

A Plasmonic Gold Nanostar Theranostic Probe for In Vivo Tumor Imaging and Photothermal Therapy

Nanomedicine has attracted increasing attention in recent years, because it offers great promise to provide personalized diagnostics and therapy with improved treatment efficacy and specificity. In this study, we developed a gold nanostar (GNS) probe for multi-modality theranostics including surface-enhanced Raman scattering (SERS) detection, x-ray computed tomography (CT), two-photon luminescence (TPL) imaging, and photothermal therapy (PTT). We performed radiolabeling, as well as CT and optical imaging, to investigate the GNS probe's biodistribution and intratumoral uptake at both macroscopic and microscopic scales. We also characterized the performance of the GNS nanoprobe for in vitro photothermal heating and in vivo photothermal ablation of primary sarcomas in mice. The results showed that 30-nm GNS have higher tumor uptake, as well as deeper penetration into tumor interstitial space compared to 60-nm GNS. In addition, we found that a higher injection dose of GNS can increase the percentage of tumor uptake. We also demonstrated the GNS probe's superior photothermal conversion efficiency with a highly concentrated heating effect due to a tip-enhanced plasmonic effect. In vivo photothermal therapy with a near-infrared (NIR) laser under the maximum permissible exposure (MPE) led to ablation of aggressive tumors containing GNS, but had no effect in the absence of GNS. This multifunctional GNS probe has the potential to be used for in vivo biosensing, preoperative CT imaging, intraoperative detection with optical methods (SERS and TPL), as well as image-guided photothermal therapy.

Interactions of skin with gold nanoparticles of different surface charge, shape, and functionality

The interactions between skin and colloidal gold nanoparticles of different physicochemical characteristics are investigated. By systematically varying the charge, shape, and functionality of gold nanoparticles, the nanoparticle penetration through the different skin layers is assessed. The penetration is evaluated both qualitatively and quantitatively using a variety of complementary techniques. Inductively coupled plasma optical emission spectrometry (ICP-OES) is used to quantify the total number of particles which penetrate the skin structure. Transmission electron microscopy (TEM) and two photon photoluminescence microscopy (TPPL) on skin cross sections provide a direct visualization of nanoparticle migration within the different skin substructures. These studies reveal that gold nanoparticles functionalized with cell penetrating peptides (CPPs) TAT and R7 are found in the skin in larger quantities than polyethylene glycol-functionalized nanoparticles, and are able to enter deep into the skin structure. The systematic studies presented in this work may be of strong interest for developments in transdermal administration of drugs and therapy.

Plasmonic gold nanostars for multi-modality sensing and diagnostics

Gold nanostars (AuNSs) are unique systems that can provide a novel multifunctional nanoplatform for molecular sensing and diagnostics. The plasmonic absorption band of AuNSs can be tuned to the near infrared spectral range, often referred to as the "tissue optical window", where light exhibits minimal absorption and deep penetration in tissue. AuNSs have been applied for detecting disease biomarkers and for biomedical imaging using multi-modality methods including surface-enhanced Raman scattering (SERS), two-photon photoluminescence (TPL), magnetic resonance imaging (MRI), positron emission tomography (PET), and X-ray computer tomography (CT) imaging. In this paper, we provide an overview of the recent development of plasmonic AuNSs in our laboratory for biomedical applications and highlight their potential for future translational medicine as a multifunctional nanoplatform.

Au nanostructure arrays for plasmonic applications: annealed island films versus nanoimprint lithography

This paper attempts to compare the main features of random and highly ordered gold nanostructure arrays (NSA) prepared by thermally annealed island film and nanoimprint lithography (NIL) techniques, respectively. Each substrate possesses different morphology in terms of plasmonic enhancement. Both methods allow such important features as spectral tuning of plasmon resonance position depending on size and shape of nanostructures; however, the time and cost is quite different. The respective comparison was performed experimentally and theoretically for a number of samples with different geometrical parameters. Spectral characteristics of fabricated NSA exhibited an expressed plasmon peak in the range from 576 to 809 nm for thermally annealed samples and from 606 to 783 nm for samples prepared by NIL. Modelling of the optical response for nanostructures with typical shapes associated with these techniques (parallelepiped for NIL and semi-ellipsoid for annealed island films) was performed using finite-difference time-domain calculations. Mathematical simulations have indicated the dependence of electric field enhancement on the shape and size of the nanoparticles. As an important point, the distribution of electric field at so-called 'hot spots' was considered. Parallelepiped-shaped nanoparticles were shown to yield maximal enhancement values by an order of magnitude greater than their semi-ellipsoid-shaped counterparts; however, both nanoparticle shapes have demonstrated comparable effective electrical field enhancement values. Optimized Au nanostructures with equivalent diameters ranging from 85 to 143 nm and height equal to 35 nm were obtained for both techniques, resulting in the largest electrical field enhancement. The application of island film thermal annealing method for nanochips fabrication can be considered as a possible cost-effective platform for various surface-enhanced spectroscopies; while the NIL-fabricated NSA looks like more effective for sensing of small-size objects.

SERS nanosensors and nanoreporters: golden opportunities in biomedical applications

This article provides an overview of recent developments and applications of surface-enhanced Raman scattering (SERS) nanosensors and nanoreporters in our laboratory for use in biochemical monitoring, medical diagnostics, and therapy. The design and fabrication of different types of plasmonics-active nanostructures are discussed. The SERS nanosensors can be used in various applications including pH sensing, protein detection, and gene diagnostics. For DNA detection the 'Molecular Sentinel' nanoprobe can be used as a homogenous bioassay in solution or on a chip platform. Gold nanostars provide an excellent multi-modality theranostic platform, combining Raman and SERS with two-photon luminescence (TPL) imaging as well as photodynamic therapy (PDT), and photothermal therapy (PTT). Plasmonics-enhanced and optically modulated delivery of nanostars into brain tumor in live animals was demonstrated; photothermal treatment of tumor vasculature may induce inflammasome activation, thus increasing the permeability of the blood brain-tumor barrier. The imaging method using TPL of gold nanostars provides an unprecedented spatial selectivity for enhanced targeted nanostar delivery to cortical tumor tissue. A quintuple-modality nanoreporter based on gold nanostars for SERS, TPL, magnetic resonance imaging (MRI), computed tomography (CT), and PTT has recently been developed. The possibility of combining spectral selectivity and high sensitivity of the SERS process with the inherent molecular specificity of bioreceptor-based nanoprobes provides a unique multiplex and selective diagnostic modality. Several examples of optical detection using SERS in combination with other detection and treatment modalities are discussed to illustrate the usefulness and potential of SERS nanosensors and nanoreporters for medical applications.

Fano resonance properties of gold nanocrescent arrays

The Fano resonance induced by symmetry breaking could improve the sensitivity of localized surface plasmon resonance sensors. In this work, the spectra of gold nanocrescent arrays are measured and confirmed by simulation results through the finite element method (FEM). The Fano resonance presented in the spectra could be modulated by the symmetry breaking with different waist widths, which are understood through plasmonic hybridization theory with the help of surface charge distribution. Our results indicate the Fano lineshape is generated by the coherent coupling of the quadrupole plasmon mode Q_H of nanohole and the antibonding plasmon mode D(AB) of nanocrescent. Finally, the high figure of merit (FoM=1.6-3.5) of the Q mode in the visible region illustrates this nanocrescent Fano sensor is of great value in the biological and chemical scientific fields.

Au nanorod quartets and Raman signal enhancement: towards the design of plasmonic platforms

Quartets of Au nanorods were designed by combining the methodologies of lateral and longitudinal assemblies. A high electric field prevailing at the quartet junctions results in large enhancement in the Raman signals of molecules. FDTD simulations showed that the displacement of the lateral dimers in quartets expands the scope of hot spot distribution.

In vivo near-infrared photothermal therapy and computed tomography imaging of cancer cells using novel tungsten-based theranostic probe

Photothermal therapy, as a physical therapeutic technique to kill cancer, has generated a great deal of interest. Photothermal agents hence play a critical role in this modern therapy. We report the use of transition metal oxides as photothermal agents based on PEGylated WO3-x nanoparticles. The well-prepared nanoparticles presented effective results during photothermal therapy both in vitro and in vivo by using near-IR laser irradiation (980 nm, 0.5 W cm(-2)). The tumor cells were effectively damaged using low power density during a short irradiation time without destroying healthy tissues. In vitro results of photothermal therapy with PEGylated WO3-x nanoparticles proved to be effective on 4T1 murine breast cancer cells via a confocal microscopy method and MTT assay. In vivo results were further confirmed by hematoxylin and eosin (H & E) histological staining. Additionally, PEGylated WO3-x nanoparticles were shown to be effective as a CT imaging contrast agent on a tumor-bearing mouse model. Our results suggest that this generation of PEGylated WO3-x nanoparticles can potentially be used in oncological CT imaging and photothermal therapy.

Plasmonics-enhanced and optically modulated delivery of gold nanostars into brain tumor

Plasmonics-active gold nanostars exhibiting strong imaging contrast and efficient photothermal transduction were synthesized for a novel pulsed laser-modulated plasmonics-enhanced brain tumor microvascular permeabilization. We demonstrate a selective, optically modulated delivery of nanoprobes into the tumor parenchyma with minimal off-target distribution.

DNA bioassay-on-chip using SERS detection for dengue diagnosis

A signal-on DNA bioassay-on-chip using SERS detection and a single incubation step without any washing was developed for dengue diagnosis.

Branching and size of CTAB-coated gold nanostars control the colorimetric detection of bacteria

The size and degree of branching of gold nanostars control the colorimetric response when detecting Staphylococcus aureus.