SERS Tags: Novel Optical Nanoprobes for Bioanalysis

Free radical-quenched SERS probes for detecting H2O2 and glucose

For developing a free radical-quenched surface-enhanced Raman scattering (SERS) probe, starch, a linear molecule, was used as a protective layer to coat gold nanoshells (GNSs) as enhancement substrates and then, methylene blue (MB) was absorbed on the starch-coated GNSs as a free radical-responsive element.

Gold nanostars as SERS-active substrates for FT-Raman spectroscopy

Surfactant-free and CTAB-stabilized gold nanostars were synthesized. Their FT-SERS activity was tested. A nanomolar limit of detection and a Raman enhancement factor of more than 10⁵ were found.

Target induced aggregation of modified Au@Ag nanoparticles for surface enhanced Raman scattering and its ultrasensitive detection of arsenic(iii) in aqueous solution

A highly sensitive and selective detection of As(iii) was reported by target induced aggregation of nanoparticles enhanced Raman spectroscopic technique.

SERS investigation of ciprofloxacin drug molecules on TiO2 nanoparticles

TiO₂ nanoparticles with different crystallinity were synthesized and served as SERS-active substrates for SERS detection of ciprofloxacin (CIP) drug molecules.

Graphene oxide wrapped individual silver nanocomposites with improved stability for surface-enhanced Raman scattering

GO coating on Ag nanoparticles enhances SERS signals and improves the long term stability of Ag.

Improved Raman and photoluminescence sensitivity achieved using bifunctional Ag@SiO2 nanocubes

Surface-enhanced Raman scattering (SERS) and metal-enhanced photoluminescence (MEPL) responses can be greatly improved by introducing a thin coating of silica (SiO₂) on silver nanocubes.

Nanostructured and spiky gold in biomolecule detection: improving binding efficiencies and enhancing optical signals

Nanostructured gold can improve the ability to detect biomolecules.

Selective and sensitive SERS sensor for detection of Hg2+ in environmental water base on rhodamine-bonded and amino group functionalized SiO2-coated Au–Ag core–shell nanorods

A novel SERS sensor for trace detection of Hg²⁺ using R6G-derive Schiff base bonded Au@Ag@SiO₂–NH₂ NRs was designed. The LOD is 0.33 pmol L⁻¹.

Synthesis of novel gold mesoflowers as SERS tags for immunoassay with improved sensitivity

A new class of flowerlike gold mesostructure in high yield is successfully synthesized through a facile one-step route using ascorbic acid as a reducing agent of gold salt with cetyltrimethylammonium chloride (CTAC) as surfactant. The as-prepared Au particles have spherical profiles with an averaged diameter of 770 ± 50 nm, but showing a highly rough surface consisting of many irregular and randomly arranged protrusions. The Au mesoflowers exhibit strong surface-enhanced effects and near-infrared absorption which were utilized in the design of efficient surface-enhanced Raman scattering (SERS) tags as immunosensors for immunoassay with improved sensitivity. The experimental results indicate that a good linear relationship is found between the peak intensity at 1071 cm(-1) and the logarithm of H-IgG concentration in the range between 1 ng/mL and 1 fg/mL, and the limit of detection (LOD) is 1 fg/mL.

Design of functional nanoparticles and assemblies for theranostic applications

Nanostructured materials have found increasing applications in medical therapies and diagnostics (theranostics). The main challenge is the ability to impart the nanomaterials with structurally tailored functional properties which can effectively target biomolecules but also provide signatures for effective detection. The harnessing of functional nanoparticles and assemblies serves as a powerful strategy for the creation of the structurally tailored multifunctional properties. This article highlights some of the important design strategies in recent investigation of metals (especially gold and silver), and magnetically functionalized nanoparticles, and molecularly assembled or biomolecularly conjugated nanoparticles with tunable optical, spectroscopic, magnetic, and electrical properties for applications in several areas of potential theranostic interests. Examples include colorimetric detection of amino acids and small peptides, surface-enhanced Raman scattering detection of biomolecular recognition of proteins and DNAs, delivery in cell transfection and bacteria inactivation, and chemiresistive detection of breath biomarkers. A major emphasis is placed on understanding how the control of the nanostructures and the molecular and biomolecular interactions impact these biofunctional properties, which has important implications for bottom-up designs of theranostic materials.

Shedding light on the extinction-enhancement duality in gold nanostar-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) has evolved from an esoteric physical phenomenon to a robust and effective analytical method recently. The need of addressing both the field enhancement and the extinction of nanoparticle suspensions, however, has been underappreciated despite its substantive impact on the sensing performance. A systematic experimental investigation of SERS enhancement and attenuation is performed in suspensions of gold nanostars, which exhibit a markedly different behavior in relation to conventional nanoparticles. The relationship is elucidated between the SERS enhancement and the localized surface plasmon resonance band, and the effect of the concentration of the gold nanostars on the signal propagation is investigated. It is shown that an optimal concentration of gold nanostars exists to maximize the enhancement factor (EF), and the maximum EF occurs when the LSPR band is blue-shifted from the excitation wavelength rather than at the on-resonance position.

Green synthesis of asymmetrically textured silver meso-flowers (AgMFs) as highly sensitive SERS substrates

Highly asymmetrical "flower-like" micron silver particles, so-called hierarchical silver meso-flowers (AgMFs), were facilely synthesized using ascorbic acid at room temperature in the presence of chitosan biopolymer. The time-evolution of TEM images and XRD analysis confirmed the anisotropic growth of AgMFs with single crystalline phase of which the formation mechanism was described in detail. The morphology and size of as-prepared AgMFs were tunable simply by changing the concentration of chitosan biopolymer and/or AgNO3 precursor under otherwise identical conditions. The asymmetrically textured AgMFs dramatically enhanced Raman signals of probe molecules (2-chlorothiophenol, 4-aminothiophenol) even at a single particle level because of their surface morphologies consisting of numerous nanoedges and crevices.

Gold and silver nanoparticle monomers are non-SERS-active: a negative experimental study with silica-encapsulated Raman-reporter-coated metal colloids

Noble metal nanoparticles (NPs) are the most commonly employed plasmonic substrates in surface-enhanced Raman scattering (SERS) experiments. Computer simulations show that monomers of Ag and Au nanocrystals ("spherical" NPs) do not exhibit a notable plasmonic enhancement, i.e., they are essentially non-SERS-active. However, in experiments, SERS enhanced by spherical NP colloids has been frequently reported. This implies that the monomers do not have strong SERS activity, but detectable enhancement should more or less be there. Because of the gap between theory and practice, it is important to demonstrate experimentally how SERS-active the metal colloid actually is and, in case a SERS signal is observed, where it originates from. In particular the aggregation of the colloid, induced by high centrifugal forces in washing steps or due to a harsh ionic environment of the suspension medium, should be controlled since it is the very high SERS activity of NP clusters which dominates the overall SERS signal of the colloid. We report here the experimental evaluation of the SERS activity of 80 nm Au and Ag NP monomers. Instead of showing fancy nanostructures and super SERS enhancement, we present the method on how to obtain negative experimental data. In this approach, no SERS signal was obtained from the colloid with a Raman reporter on the metal surface when the NPs were encapsulated carefully within a thick silica shell. Without silica encapsulation, if a very low centrifugation speed is used for the washing steps, only a negligible SERS signal can be detected even at very high NP concentrations. In contrast, strong SERS signals can be detected when the NPs are suspended in acidic solutions. These results indicate that Au and Ag NP monomers essentially exhibit no SERS activity of practical relevance.

Synchrotron-based X-ray-sensitive nanoprobes for cellular imaging

It is one of the ultimate goals in cell biology to understand the complex spatio-temporal interplay of biomolecules in the cellular context. To this end, there have been great efforts on the development of various probes to detect and localize specific biomolecules in cells with a variety of microscopic imaging techniques. In this Research News, we first summarize several types of microscopy for visualizing specific biomolecular targets. Then we focus on recent advances in the design of X-ray sensitive nanoprobes for applications in synchrotron-based cellular imaging. With the availability of advanced synchrotron techniques, there has been rapid progress toward high-resolution and multi-color X-ray imaging in cells with various types of functional nanoprobes.

Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes for Monitoring and Understanding Heterogeneous Catalysis

ABSTRACT

Surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) were until recently limited in their applicability to the majority of heterogeneous catalytic reactions. Recent developments begin to resolve the conflicting experimental requirements for SERS and TERS on the one hand, and heterogeneous catalysis on the other hand. This article discusses the development and use of SERS and TERS to study heterogeneous catalytic reactions, and the exciting possibilities that may now be within reach thanks to the latest technical developments. This will be illustrated with showcase examples from photo- and electrocatalysis.

GRAPHICAL ABSTRACT

Dawn of advanced molecular medicine: nanotechnological advancements in cancer imaging and therapy

Nanotechnology plays an increasingly important role not only in our everyday life (with all its benefits and dangers) but also in medicine. Nanoparticles are to date the most intriguing option to deliver high concentrations of agents specifically and directly to cancer cells; therefore, a wide variety of these nanomaterials has been developed and explored. These span the range from simple nanoagents to sophisticated smart devices for drug delivery or imaging. Nanomaterials usually provide a large surface area, allowing for decoration with a large amount of moieties on the surface for either additional functionalities or targeting. Besides using particles solely for imaging purposes, they can also carry as a payload a therapeutic agent. If both are combined within the same particle, a theranostic agent is created. The sophistication of highly developed nanotechnology targeting approaches provides a promising means for many clinical implementations and can provide improved applications for otherwise suboptimal formulations. In this review we will explore nanotechnology both for imaging and therapy to provide a general overview of the field and its impact on cancer imaging and therapy.

Assessing telomere length using surface enhanced Raman scattering

Telomere length can provide valuable insight into telomeres and telomerase related diseases, including cancer. Here, we present a brand-new optical telomere length measurement protocol using surface enhanced Raman scattering (SERS). In this protocol, two single strand DNA are used as SERS probes. They are labeled with two different Raman molecules and can specifically hybridize with telomeres and centromere, respectively. First, genome DNA is extracted from cells. Then the telomere and centromere SERS probes are added into the genome DNA. After hybridization with genome DNA, excess SERS probes are removed by magnetic capturing nanoparticles. Finally, the genome DNA with SERS probes attached is dropped onto a SERS substrate and subjected to SERS measurement. Longer telomeres result in more attached telomere probes, thus a stronger SERS signal. Consequently, SERS signal can be used as an indicator of telomere length. Centromere is used as the inner control. By calibrating the SERS intensity of telomere probe with that of the centromere probe, SERS based telomere measurement is realized. This protocol does not require polymerase chain reaction (PCR) or electrophoresis procedures, which greatly simplifies the detection process. We anticipate that this easy-operation and cost-effective protocol is a fine alternative for the assessment of telomere length.

Mesoporous titania based yolk-shell nanoparticles as multifunctional theranostic platforms for SERS imaging and chemo-photothermal treatment

Recently surface-enhanced Raman scattering (SERS) imaging guided theranostic nanoplatforms have attracted considerable attention. Herein, we developed novel yolk-shell gold nanorod@void@mesoporous titania nanoparticles (AuNR@void@mTiO₂ NPs) for simultaneous SERS imaging and chemo-photothermal therapy. Our work showed three highlighted features: first, we proposed a facile and versatile "up to down" SERS labeling strategy for the drug delivery system, in which "empty carriers" were pre-synthesized, followed by co-loading of Raman reporters on AuNR and anti-cancer drug doxorubicin (DOX) in mTiO₂ in sequence. The acquired SERS signal was strong enough for tracking NPs at both living cells and mice levels. Second, we selected mTiO₂ as a novel drug loading material instead of the widely used mesoporous silica (mSiO₂). The mTiO₂ shared satisfactory drug loading and release behavior as mSiO₂ but it was chemically inert. This property not only provided a facile way to form a yolk-shell structure but also rendered it with superior structural stability in a biological system. Third, the near infrared (NIR) light absorbing property of the AuNR SERS substrate was also explored for drug release regulation and photothermal treatment. Significantly greater MCF-7 cell killing was observed when treated together with DOX-loaded NPs and NIR laser irradiation, attributable to the synergistic chemo-thermal therapeutic effect. Our results indicated the established SERS labeled yolk-shell NP as a promising theranostic platform and suggested its potential in vivo applications.

Silicon nanohybrid-based surface-enhanced Raman scattering sensors

Nanomaterial-based surface-enhanced Raman scattering (SERS) sensors are highly promising analytical tools, capable of ultrasensitive, multiplex, and nondestructive detection of chemical and biological species. Extensive efforts have been made to design various silicon nanohybrid-based SERS substrates such as gold/silver nanoparticle (NP)-decorated silicon nanowires, Au/Ag NP-decorated silicon wafers (AuNP@Si), and so forth. In comparison to free AuNP- and AgNP-based SERS sensors, the silicon nanohybrid-based SERS sensors feature higher enhancement factors (EFs) and excellent reproducibility, since SERS hot spots are efficiently coupled and stabilized through interconnection to the semiconducting silicon substrates. Consequently, in the past decade, giant advancements in the development of silicon nanohybrid-based SERS sensors have been witnessed for myriad sensing applications. In this review, the representative achievements related to the design of high-performance silicon nanohybrid-based SERS sensors and their use for chemical and biological analysis are reviewed in a detailed way. Furthermore, the major opportunities and challenges in this field are discussed from a broad perspective and possible future directions.

On-line SERS detection of single bacterium using novel SERS nanoprobes and a microfluidic dielectrophoresis device

The integration of novel surface-enhanced Raman scattering (SERS) nanoprobes and a microfluidic dielectrophoresis (DEP) device is developed for rapid on-line SERS detection of Salmonella enterica serotype Choleraesuis and Neisseria lactamica. The SERS nanoprobes are prepared by immobilization of specific antibody onto the surface of nanoaggregate-embedded beads (NAEBs), which are silica-coated, dye-induced aggregates of a small number of gold nanoparticles (AuNPs). Each NAEB gives highly enhanced Raman signals owing to the presence of well-defined plasmonic hot spots at junctions between AuNPs. Herein, the on-line SERS detection and accurate identification of suspended bacteria with a detection capability down to a single bacterium has been realized by the NAEB-DEP-Raman spectroscopy biosensing strategy. The practical detection limit with a measurement time of 10 min is estimated to be 70 CFU mL(-1) . In comparison with whole-cell enzyme-linked immunosorbent assay (ELISA), the SERS-nanoprobe-based biosensing method provides advantages of higher sensitivity and requiring lower amount of antibody in the assay (100-fold less). The total assay time including sample pretreatment is less than 2 h. Hence, this sensing strategy is promising for faster and effective on-line multiplex detection of single pathogenic bacterium by using different bioconjugated SERS nanoprobes.

Quantitative multiplexing with nano-self-assemblies in SERS

Multiplexed or simultaneous detection of multiple analytes is a valuable tool in many analytical applications. However, complications caused by the presence of interfering compounds in a sample form a major drawback in existing molecular sensor technologies, particularly in multi-analyte systems. Although separating analytes through extraction or chromatography can partially address the problem of interferents, there remains a need for developing direct observational tools capable of multiplexing that can be applied in situ. Surface-enhanced Raman Spectroscopy (SERS) is an optical molecular finger-printing technique that has the ability to resolve analytes from within mixtures. SERS has attracted much attention for its potential in multiplexed sensing but it has been limited in its quantitative abilities. Here, we report a facile supramolecular SERS-based method for quantitative multiplex analysis of small organic molecules in aqueous environments such as human urine.

A "schizophotonic" all-in-one nanoparticle coating for multiplexed SE(R)RS biomedical imaging

SERS nanoprobes for in vivo biomedical applications require high quantum yield, long circulation times, and maximum colloidal stability. Traditional synthetic routes require high metal-dye affinities and are challenged by unfavorable electrostatic interactions and limited scalability. We report the synthesis of a new near-IR active poly(N-(2-hydroxypropyl) methacrylamide) (pHPMA). The integration of various SERS reporters into a biocompatible polymeric surface coating allows for controlled dye incorporation, high colloidal stability, and optimized in vivo circulation times. This technique allows the synthesis of very small (<20 nm) SERS probes, which is crucial for the design of excretable and thus highly translatable imaging agents. Depending on their size, the "schizophotonic" nanoparticles can emit both SERS and fluorescence. We demonstrate the capability of this all-in-one gold surface coating and SERS reporter for multiplexed lymph-node imaging.

SERS-fluorescence monitored drug release of a redox-responsive nanocarrier based on graphene oxide in tumor cells

A redox-responsive drug carrier based on nanoscale graphene oxide (NGO) loaded with Ag nanoparticles, whose intracellular release behavior can be investigated by SERS-fluorescence combined spectroscopy, is presented. In this demonstrated drug carrier, to make the carrier integrated with the redox responsive property, we utilized disulfide linkages to load drug molecules to the surfaces of NGO directly, which can be cleaved by glutathione (GSH). Covalent drug loading and GSH-responsive release strategy can reduce the influence of the surface diffusion barriers introduced by multifunctionalization. Interestingly, the intracellular real-time drug release dynamics can be monitored by the combined SERS-fluorescence signals of the drugs, while the distribution of the drug carrier can simultaneously be tracked by the intrinsic SERS signals of NGO in the whole process. Our results show that upon the internalization of doxorubicin (DOX)-loaded nanocarriers into living cells, DOX was efficiently released under a GSH regulated reducing environment. Because tumor cells generally exhibit a higher concentration of GSH than normal ones, this drug carrier should have potential in the field of tumor therapy.

Mesoporous silica-coated plasmonic nanostructures for surface-enhanced Raman scattering detection and photothermal therapy

The design and fabrication of core-shell and yolk-shell nanostructures with surface plasmon resonance (SPR)-active center protected by permeable mesoporous channels can raise the new vitality into the catalysis and biological applications. Hybrid plasmonic-mesoporous silica nanocarriers consisting of Ag and Au-Ag alloy nanoparticles are fabricated through spatially confined galvanic replacement approach. The plasmonic absorption peaks can be finely controlled to the near-infrared (NIR) region (500-790 nm) that is beneficial for tissue transmittance. The mesoporous silica shell facilitates also protection of Au-Ag cores and affords the channels between the exterior and interior capsule environments, thereby endowing the multiple applications. In the present work, it is successfully demonstrated that mesoporous silica-coated Au-Ag alloy core-shell and yolk-shell nanocarriers can serve as good substrates for surface-enhanced Raman scattering (SERS) detection. The SERS signal intensities of nanocarriers are highly dependent on the SPR peaks and the contents of gold. Simultaneously, the synthesized Au-Ag@mSiO2 nanocarriers with SPR peak at ≈790 nm can be applied in NIR-sensitive SERS detection and photothermal therapy.

Comprehensive spectral endoscopy of topically applied SERS nanoparticles in the rat esophagus

The early detection and biological investigation of esophageal cancer would benefit from the development of advanced imaging techniques to screen for the molecular changes that precede and accompany the onset of cancer. Surface-enhanced Raman scattering (SERS) nanoparticles (NPs) have the potential to improve cancer detection and the investigation of cancer progression through the sensitive and multiplexed phenotyping of cell-surface biomarkers. Here, a miniature endoscope featuring rotational scanning and axial pull back has been developed for 2D spectral imaging of SERS NPs topically applied on the lumenal surface of the rat esophagus. Raman signals from low-pM concentrations of SERS NP mixtures are demultiplexed in real time to accurately calculate the concentration and ratio of the NPs. Ex vivo and in vivo experiments demonstrate the feasibility of topical application and imaging of multiplexed SERS NPs along the entire length of the rat esophagus.

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.

Virus templated gold nanocube chain for SERS nanoprobe

A M13 virus based SERS nanoprobe is presented. Gold nanocubes closely aligned into chains along the length of the virus intensify Raman signals of various reporter molecules serving as specific labels. An antibody is expressed at one end to detect the analyte. This new SERS nanoprobe holds promise for infinitesimal and multiplexed detection of any antigen.

Bifunctional Au@Pt core-shell nanostructures for in situ monitoring of catalytic reactions by surface-enhanced Raman scattering spectroscopy

Optical probes of heterogeneous catalytic reactions are of great importance for in situ determination of the catalytic activity and monitoring of the reaction process. Surface-enhanced Raman scattering (SERS) spectroscopy could be used as a sensitive optical probe for this purpose provided that plasmonic metal nanoparticles for Raman enhancement are properly integrated with catalytic metals to form a single entity. Herein we present a facile approach for synthesizing Au@Pt core-shell nanostructures with a controllable surface density of sub-5 nm Pt nanoparticles on the surface of Au nanorods. Systematic investigations on both SERS and catalytic activities of the hybrid nanostructures reveal an optimized surface coverage of Pt. More importantly, we demonstrate that, due to their dual functionalities, the hybrid nanostructures are able to track the Pt-catalysed reaction in real time by measuring the SERS signals of the reactant, intermediate and final products. This SERS-based synergy technique provides a novel approach for quantitatively studying catalytic chemical reaction processes and is suitable for many applications such as reduction and oxidation reactions in fuel cells and catalytic water splitting.

Wide-field multiplexed imaging of EGFR-targeted cancers using topical application of NIR SERS nanoprobes

AIM

As the possibilities of molecular imaging in personalized medicine evolve rapidly, the optical advantages of extremely narrow and intense spectral bands makes surface-enhanced Raman scattering (SERS) an appealing candidate for multiplexed recognition of targeted biomarkers over other optical imaging modalities.

MATERIALS & METHODS

In this proof-of-concept study, we report wide-field Raman detection of lung cancer using multimodal SERS nanoprobes specific to the EGF receptor family, both in vitro and in vivo.

RESULTS

For the first time, we demonstrate wide-field multiplexed Raman imaging for cancer detection in vivo after topical application of a 'cocktail' of SERS nanoprobes.

CONCLUSION

This advancement represents a key step towards sensitive wide-field Raman endoscopic imaging of multiple biomarkers for early and accurate diagnosis of EGF receptor-expressing tumors of different internal organs.