Surface-enhanced Raman scattering detection and tracking of nanoprobes: enhanced uptake and nuclear targeting in single cells

Influence of Nuclear Localization Sequences on the Intracellular Fate of Gold Nanoparticles

Directing nanoparticles to the nucleus by attachment of nuclear localization sequences (NLS) is an aim in many applications. Gold nanoparticles modified with two different NLS were studied while crossing barriers of intact cells, including uptake, endosomal escape, and nuclear translocation. By imaging of the nanoparticles and by characterization of their molecular interactions with surface-enhanced Raman scattering (SERS), it is shown that nuclear translocation strongly depends on the particular incubation conditions. After an 1 h of incubation followed by a 24 h chase time, 14 nm gold particles carrying an adenoviral NLS are localized in endosomes, in the cytoplasm, and in the nucleus of fibroblast cells. In contrast, the cells display no nanoparticles in the cytoplasm or nucleus when continuously incubated with the nanoparticles for 24 h. The ultrastructural and spectroscopic data indicate different processing of NLS-functionalized particles in endosomes compared to unmodified particles. NLS-functionalized nanoparticles form larger intraendosomal aggregates than unmodified gold nanoparticles. SERS spectra of cells with NLS-functionalized gold nanoparticles contain bands assigned to DNA and were clearly different from those with unmodified gold nanoparticles. The different processing in the presence of an NLS is influenced by a continuous exposure of the cells to nanoparticles and an ongoing nanoparticle uptake. This is supported by mass-spectrometry-based quantification that indicates enhanced uptake of NLS-functionalized nanoparticles compared to unmodified particles under the same conditions. The results contribute to the optimization of nanoparticle analysis in cells in a variety of applications, e.g., in theranostics, biotechnology, and bioanalytics.

Effect of the functionalisation agent on the surface-enhanced Raman scattering (SERS) spectrum: Case study of pyridine derivatives

Nowadays, the use of functionalised surface-enhanced Raman scattering (SERS) substrates has become common. These surface modifying agents notably act as Raman reporters, as sensors of biological processes (pH, redox probes) or to increase the sensitivity and/or the specificity of SERS detections. However, the effects of the functionalisation agents are deeply examined in very few studies, even though they can affect the aggregation behaviour of the SERS substrate. Moreover, depending on their concentration and on the pH, their spectral signature can be modified and they can even degrade if stored inappropriately. In this context, this paper aims at emphasising the importance of the different aspects previously listed in the selection of a functionalisation agent. Pyridine derivatives were picked out to highlight these parameters, as some of these compounds are commonly used to be grafted onto SERS substrates. Two widespread syntheses of nanoparticles were selected as SERS substrates: citrate-reduced gold and silver nanoparticles. The surface of the nanoparticles was functionalised with several pyridine derivatives at different concentrations and in several solvents. It was observed that the molecules under study had a concentration-dependent effect on nanoparticle aggregation. A stability study was furthermore conducted in order to determine the best preservation conditions of the grafting solutions. In conclusion, this paper shines a light on the relevance of the investigation of the too-often neglected behaviour of the surface modifying agents. Before their application in SERS analyses, parameters such as the label concentration should therefore be included in an experimental design to optimise the sample preparation.

Surface-Enhanced Raman Spectroscopy Characterization of Breast Cell Phenotypes: Effect of Nanoparticle Geometry

The use of surface-enhanced Raman spectroscopy (SERS) to delineate between the breast epithelial cell lines MCF10A, SK-BR-3, and MDA-MB-231 is explored utilizing varied morphologies of gold nanoparticles. The nanoparticles studied had spherical, star-like, and quasi-fractal (nanocaltrop) morphologies and possessed varying degrees of surface inhomogeneity and complexity. The efficacy of Raman enhancement of these nanoparticles was a function of their size, their surface morphology, and the associated density of "hot spots," as well as their cellular uptake. The spherical and star-like nanoparticles provided strong signal enhancement that allowed for the discernment among the three cell phenotypes based solely on the acquired Raman spectra. The presence of overlapping Raman band spectral regions, as well as unique spectral bands, suggests that the underlying biological differences between these cells can be accessed without the need for tagging the nanoparticles or for specific cell targeting, demonstrating the potential ubiquity of this technique in imaging any cancer. This work provides clear evidence for the potential application of SERS as a tool for mapping cancerous lesions, possibly during surgery and under histopathological analysis.

Functionalized Gold Nanoparticles as Biosensors for Monitoring Cellular Uptake and Localization in Normal and Tumor Prostatic Cells

In the present contribution the fabrication and characterization of functionalized gold nanospheres of uniform shape and controlled size is reported. These nano-objects are intended to be used as Surface Enhanced Raman Spectroscopy (SERS) sensors for in-vitro cellular uptake and localization. Thiophenol was used as molecular reporter and was bound to the Au surface by a chemisorption process in aqueous solution. The obtained colloidal solution was highly stable and no aggregation of the single nanospheres into larger clusters was observed. The nanoparticles were incubated in human prostatic cells with the aim of developing a robust, SERS-based method to differentiate normal and tumor cell lines. SERS imaging experiments showed that tumor cells uptake considerably larger amounts of nanoparticles in comparison to normal cells (up to 950% more); significant differences were also observed in the uptake kinetics. This largely different behaviour might be exploited in diagnostic and therapeutic applications.

Highly stable SERS pH nanoprobes produced by co-solvent controlled AuNP aggregation

Production of gold nanoparticle (AuNP) surface-enhanced Raman spectroscopy (SERS) nanoprobes requires replicable aggregation to produce multimers with high signal intensity. Herein, we illustrate a novel, yet simple, approach to produce SERS nanoprobes through control of co-solvent composition. AuNP multimers were produced by mixing AuNP monomers in water : ethanol co-solvent for variable periods of time. By varying the water : ethanol ratio and the amount of 4-mercaptobenzoic acid (4-MBA) present, the aggregation rate can be systematically controlled. Thiolated poly(ethylene glycol) was then added to halt the aggregation process and provide steric stability. This approach was used to produce pH nanoprobes with excellent colloidal stability in high ionic strength environments and in complex samples. The pH probe exhibits broad pH sensitivity over the range 6-11 and we calculate that a single AuNP dimer in a 35 fL volume is sufficient to generate a detectable SERS signal. As a proof-of-concept, the probes were used to detect the intracellular pH of human prostate cancer cells (PC-3). The internalized probes exhibit a strong 4-MBA signal without any interfering bands from either the cells or the culture media and produce exceptionally detailed pH maps. pH maps obtained from 19 xy surface scans and 14 yz depth scans exhibit highly consistent intracellular pH in the range of 5 to 7, thus indicating the greater reliability and reproducibility of our pH probes compared with other probes previously reported in the literature. Our water : ethanol co-solvent production process is fast, simple, and efficient. Adjustment of solvent composition may become a powerful way to produce SERS tags or nanoprobes in the future.

Nanoparticles and intracellular applications of surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectrocopy (SERS) offers ultrasensitive vibrational fingerprinting at the nanoscale. Its non-destructive nature affords an ideal tool for interrogation of the intracellular environment, detecting the localisation of biomolecules, delivery and monitoring of therapeutics and for characterisation of complex cellular processes at the molecular level. Innovations in nanotechnology have produced a wide selection of novel, purpose-built plasmonic nanostructures capable of high SERS enhancement for intracellular probing while microfluidic technologies are being utilised to reproducibly synthesise nanoparticle (NP) probes at large scale and in high throughput. Sophisticated multivariate analysis techniques unlock the wealth of previously unattainable biomolecular information contained within large and multidimensional SERS datasets. Thus, with suitable combination of experimental techniques and analytics, SERS boasts enormous potential for cell based assays and to expand our understanding of the intracellular environment. In this review we trace the pathway to utilisation of nanomaterials for intracellular SERS. Thus we review and assess nanoparticle synthesis methods, their toxicity and cell interactions before presenting significant developments in intracellular SERS methodologies and how identified challenges can be addressed.

A real-time clinical endoscopic system for intraluminal, multiplexed imaging of surface-enhanced Raman scattering nanoparticles

The detection of biomarker-targeting surface-enhanced Raman scattering (SERS) nanoparticles (NPs) in the human gastrointestinal tract has the potential to improve early cancer detection; however, a clinically relevant device with rapid Raman-imaging capability has not been described. Here we report the design and in vivo demonstration of a miniature, non-contact, opto-electro-mechanical Raman device as an accessory to clinical endoscopes that can provide multiplexed molecular data via a panel of SERS NPs. This device enables rapid circumferential scanning of topologically complex luminal surfaces of hollow organs (e.g., colon and esophagus) and produces quantitative images of the relative concentrations of SERS NPs that are present. Human and swine studies have demonstrated the speed and simplicity of this technique. This approach also offers unparalleled multiplexing capabilities by simultaneously detecting the unique spectral fingerprints of multiple SERS NPs. Therefore, this new screening strategy has the potential to improve diagnosis and to guide therapy by enabling sensitive quantitative molecular detection of small and otherwise hard-to-detect lesions in the context of white-light endoscopy.

Optical nano antennas: state of the art, scope and challenges as a biosensor along with human exposure to nano-toxicology

The concept of optical antennas in physical optics is still evolving. Like the antennas used in the radio frequency (RF) regime, the aspiration of optical antennas is to localize the free propagating radiation energy, and vice versa. For this purpose, optical antennas utilize the distinctive properties of metal nanostructures, which are strong plasmonic coupling elements at the optical regime. The concept of optical antennas is being advanced technologically and they are projected to be substitute devices for detection in the millimeter, infrared, and visible regimes. At present, their potential benefits in light detection, which include polarization dependency, tunability, and quick response times have been successfully demonstrated. Optical antennas also can be seen as directionally responsive elements for point detectors. This review provides an overview of the historical background of the topic, along with the basic concepts and parameters of optical antennas. One of the major parts of this review covers the use of optical antennas in biosensing, presenting biosensing applications with a broad description using different types of data. We have also mentioned the basic challenges in the path of the universal use of optical biosensors, where we have also discussed some legal matters.

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.

Identification, localization, and quantification of neuronal cell membrane receptors with plasmonic probes: role of protein kinase D1 in their distribution

Detecting, imaging, and being able to localize the distribution of several cell membrane receptors on a single neuron are very important topics in neuroscience research. In the present work, the distribution of metabotropic glutamate receptor 1a (mGluR1a) density on neuron cells on subcellular length scales is determined by evaluating the role played by protein kinase D1 (PKD1) in the trafficking of membrane proteins, comparing the distribution of mGluR1a in experiments performed in endogenous PKD1 expression with those in the presence of kinase-inactive protein kinase D1 (PKD1-kd). The localization, distribution, and density of cell surface mGluR1a were evaluated using 90 nm diameter Au nanoparticle (NP) probes specifically functionalized with a high-affinity and multivalent labeling function, which allows not only imaging NPs where this receptor is present but also quantifying by optical means the NP density. This is so because the NP generates a density (ρ)-dependent SERS response that facilitated a spatial mapping of the mGluR1a density distribution on subcellular length scales (dendrites and axons) in an optical microscope. The measured ρ values were found to be significantly higher on dendrites than on axons for endogenous PKD1, while an increase of ρ on axons was observed when PKD1 is altered. The spatial distribution of the NP immunolabels through scanning electron microscopy (SEM) confirmed the results obtained by fluorescence bright-field analysis and dark-field spectroscopy and provided additional structural details. In addition, it is shown using electrodynamic simulations that SERS spectroscopy could be a very sensitive tool for the spatial mapping of cell membrane receptors on subcellular length scales, as SERS signals are almost linearly dependent on NP density and therefore give indirect information on the distribution of cell membrane proteins. This result is important since the calibration of the ρ-dependent near-field enhancement of the Au immunolabels through correlation of SERS and SEM paves the way toward quantitative immunolabeling studies of cell membrane proteins involved in neuron polarity. From the molecular biology point of view, this study shows that in cultured hippocampal pyramidal cells mGluR1a is predominantly transported to dendrites and excluded from axons. Expression of kinase-inactive protein kinase D1 (PKD1-kd) dramatically and selectively alters the intracellular trafficking and membrane delivery of mGluR1a-containing vesicles.

Universal surface-enhanced Raman tags: individual nanorods for measurements from the visible to the infrared (514-1064 nm)

Surface-enhanced Raman scattering (SERS) is a promising imaging modality for use in a variety of multiplexed tracking and sensing applications in biological environments. However, the uniform production of SERS nanoparticle tags with high yield and brightness still remains a significant challenge. Here, we describe an approach based on the controlled coadsorption of multiple dye species onto gold nanorods to create tags that can be detected across a much wider range of excitation wavelengths (514-1064 nm) compared to conventional approaches that typically focus on a single wavelength. This was achieved without the added complexity of nanoparticle aggregation or growing surrounding metallic shells to further enhance the surface-enhanced resonance Raman scattering (SERRS) signal. Correlated Raman and scanning electron microscopy mapping measurements of individual tags were used to clearly demonstrate that strong and reproducible SERRS signals at high particle yields (>92%) were readily achievable. The polyelectrolyte-wrapped nanorod-dye conjugates were also found to be highly stable as well as noncytotoxic. To demonstrate the use of these universal tags for the multimodal optical imaging of biological specimens, confocal Raman and fluorescence maps of stained immune cells following nanoparticle uptake were acquired at several excitation wavelengths and compared with dark-field images. The ability to colocalize and track individual optically encoded nanoparticles across a wide range of wavelengths simultaneously will enable the use of SERS alongside other imaging techniques for the real-time monitoring of cell-nanoparticle interactions.

Delivering colloidal nanoparticles to mammalian cells: a nano-bio interface perspective

Understanding the behavior of multifunctional colloidal nanoparticles capable of biomolecular targeting remains a fascinating challenge in materials science with dramatic implications in view of a possible clinical translation. In several circumstances, assumptions on structure-activity relationships have failed in determining the expected responses of these complex systems in a biological environment. The present Review depicts the most recent advances about colloidal nanoparticles designed for use as tools for cellular nanobiotechnology, in particular, for the preferential transport through different target compartments, including cell membrane, cytoplasm, mitochondria, and nucleus. Besides the conventional entry mechanisms based on crossing the cellular membrane, an insight into modern physical approaches to quantitatively deliver nanomaterials inside cells, such as microinjection and electro-poration, is provided. Recent hypotheses on how the nanoparticle structure and functionalization may affect the interactions at the nano-bio interface, which in turn mediate the nanoparticle internalization routes, are highlighted. In addition, some hurdles when this small interface faces the physiological environment and how this phenomenon can turn into different unexpected responses, are discussed. Finally, possible future developments oriented to synergistically tailor biological and chemical properties of nanoconjugates to improve the control over nanoparticle transport, which could open new scenarios in the field of nanomedicine, are addressed.

Plasmonic nanoprobes: from chemical sensing to medical diagnostics and therapy

This article provides an overview of the development and applications of plasmonics-active nanoprobes in our laboratory for chemical sensing, medical diagnostics and therapy. Molecular Sentinel nanoprobes provide a unique tool for DNA/RNA biomarker detection both in a homogeneous solution or on a chip platform for medical diagnostics. The possibility of combining spectral selectivity and high sensitivity of the surface-enhanced Raman scattering (SERS) process with the inherent molecular specificity of nanoprobes provides an important multiplex diagnostic modality. Gold nanostars can provide an excellent multi-modality platform, combining two-photon luminescence with photothermal therapy as well as Raman imaging with photodynamic therapy. Several examples of optical detection using SERS and photonics-based treatments are presented to illustrate the usefulness and potential of the plasmonic nanoprobes for theranostics, which seamlessly combines diagnostics and therapy.

Cell-penetrating peptide enhanced intracellular Raman imaging and photodynamic therapy

We present the application of a theranostic system combining Raman imaging and the photodynamic therapy (PDT) effect. The theranostic nanoplatform was created by loading the photosensitizer, protoporphyrin IX, onto a Raman-labeled gold nanostar. A cell-penetrating peptide, TAT, enhanced intracellular accumulation of the nanoparticles in order to improve their delivery and efficacy. The plasmonic gold nanostar platform was designed to increase the Raman signal via the surface-enhanced resonance Raman scattering (SERRS) effect. Theranostic SERS imaging and photodynamic therapy using this construct were demonstrated on BT-549 breast cancer cells. The TAT peptide allowed for effective Raman imaging and photosensitization with the nanoparticle construct after a 1 h incubation period. In the absence of the TAT peptide, nanoparticle accumulation in the cells was not sufficient to be observed by Raman imaging or to produce any photosensitization effect after this short incubation period. There was no cytotoxic effect observed after nanoparticle incubation, prior to light activation of the photosensitizer. This report shows the first application of combined SERS imaging and photosensitization from a theranostic nanoparticle construct.

Gold nanorods for ovarian cancer detection with photoacoustic imaging and resection guidance via Raman imaging in living mice

Improved imaging approaches are needed for ovarian cancer screening, diagnosis, staging, and resection guidance. Here, we propose a combined photoacoustic (PA)/Raman approach using gold nanorods (GNRs) as a passively targeted molecular imaging agent. GNRs with three different aspect ratios were studied. Those with an aspect ratio of 3.5 were selected for their highest ex vivo and in vivo PA signal and used to image subcutaneous xenografts of the 2008, HEY, and SKOV3 ovarian cancer cell lines in living mice. Maximum PA signal was observed within 3 h for all three lines tested and increased signal persisted for at least two days postadministration. There was a linear relationship (R(2) = 0.95) between the PA signal and the concentration of injected molecular imaging agent with a calculated limit of detection of 0.40 nM GNRs in the 2008 cell line. The same molecular imaging agent could be used for clear visualization of the margin between tumor and normal tissue and tumor debulking via surface-enhanced Raman spectroscopy (SERS) imaging. Finally, we validated the imaging findings with biodistribution data and elemental analysis. To the best of our knowledge, this is the first report of in vivo imaging of ovarian cancer tumors with a photoacoustic and Raman imaging agent.

Peptide-guided surface-enhanced Raman scattering probes for localized cell composition analysis

The ability to control the localization of surface-enhanced Raman scattering (SERS) nanoparticle probes in bacterial cells is critical to the development of analytical techniques that can nondestructively determine cell composition and phenotype. Here, selective localization of SERS probes was achieved at the outer bacterial membrane by using silver nanoparticles functionalized with synthetic hydrophobic peptides.

Rational design of plasmonic nanostructures for biomolecular detection: interplay between theory and experiments

In this work, we report a simple strategy to obtain ultrasensitive SERS nanostructures by self-assembly and bioconjugation of Au nanospheres (NSs). Homodimer aggregates with an interparticle gap of around 8 nm are generated in aqueous dispersions by the highly specific molecular recognition of biotinylated Au NSs to streptavidin (STV), while random Au NS aggregates with a gap of 5 nm are formed in the absence of STV due to hydrogen bonding among biotinylated NSs. Both types of aggregates depict SERS analytical enhancement factors (AEF) of around 10(7) and the capability to detect biotin concentrations lower than 1 × 10(-12) M. Quite interesting, the AEF for an external analyte, Rhodamine 6G (RH6G), using the dimer aggregates is 1 order of magnitude greater (10(5)) than using random aggregates (around 10(4)). The dependence on the wavelength and the differences of the AEF for Au random aggregates and dimers are rationalized with rigorous electrodynamic simulations. The dimers obtained afford a new type of an in situ self-calibrated and reliable SERS substrate where biotinylated molecules can selectively be "trapped" by STV and located in the nanogap enhanced plasmonic field. Using this concept, powerful molecular-recognition-based SERS assays can be carried out. The capability of the dimeric structures for analytical applications is demonstrated using SPR spectroscopy to detect biotinylated immunoglobulin G at very low concentrations.

Molecular imaging with SERS-active nanoparticles

Raman spectroscopy has been explored for various biomedical applications (e.g., cancer diagnosis) because it can provide detailed information on the chemical composition of cells and tissues. For imaging applications, several variations of Raman spectroscopy have been developed to enhance its sensitivity. To date, a wide variety of molecular targets and biological events have been investigated using surface-enhanced Raman scattering (SERS)-active nanoparticles. The superb multiplexing capability of SERS-based Raman imaging, already successfully demonstrated in live animals, can be extremely powerful in future research where different agents can be attached to different Raman tags to enable the simultaneous interrogation of multiple biological events. Over the last several years, molecular imaging with SERS-active nanoparticles has advanced significantly and many pivotal proof-of-principle experiments have been successfully carried out. It is expected that SERS-based imaging will continue to be a dynamic research field over the next decade.

Localized surface plasmon resonance: nanostructures, bioassays and biosensing--a review

Localized surface plasmon resonance (LSPR) is an optical phenomena generated by light when it interacts with conductive nanoparticles (NPs) that are smaller than the incident wavelength. As in surface plasmon resonance, the electric field of incident light can be deposited to collectively excite electrons of a conduction band, with the result being coherent localized plasmon oscillations with a resonant frequency that strongly depends on the composition, size, geometry, dielectric environment and separation distance of NPs. This review serves to describe the physical theory of LSPR formation at the surface of nanostructures, and the potential for this optical technology to serve as a basis for the development bioassays and biosensing of high sensitivity. The benefits and challenges associated with various experimental designs of nanoparticles and detection systems, as well as creative approaches that have been developed to improve sensitivity and limits of detection are highlighted using examples from the literature.

Activity of psoralen-functionalized nanoscintillators against cancer cells upon X-ray excitation

We report development of a nanoparticle-based, X-ray-activated anticancer "nanodrug" composed of yttrium oxide (Y(2)O(3)) nanoscintillators, a fragment of the HIV-1 TAT peptide, and psoralen. In this formulation, X-ray radiation is absorbed by the Y(2)O(3) nanoscintillators, which then emit UVA light. Absorption of UVA photons by nanoparticle-tethered psoralen has the potential to cross-link adenine and thymine residues in DNA. UVA-induced cross-linking by free psoralen upon activation with UVA light has previously been shown to cause apoptosis in vitro and an immunogenic response in vivo. Studies using the PC-3 human prostate cancer cell line demonstrate that X-ray excitation of these psoralen-functionalized Y(2)O(3) nanoscintillators yields concentration-dependent reductions in cell number when compared to control cultures containing psoralen-free Y(2)O(3) nanoscintillators.