MUC1 Aptamer Targeted SERS Nanoprobes

Recently, surface‐enhanced Raman scattering (SERS) nanoprobes (NPs) have shown promise in the field of cancer imaging due to their unparalleled signal specificity and high sensitivity. This study reports the development of a DNA aptamer targeted SERS NP. Recently, aptamers are being investigated as a viable alternative to more traditional antibody targeting due to their low immunogenicity and low cost of production. A strategy is developed to functionalize SERS NPs with DNA aptamers, which target Mucin1 (MUC1) in human breast cancer (BC). Thorough in vitro characterization studies demonstrate excellent serum stability and specific binding of the targeted NPs to MUC1. In order to test their in vivo targeting capability, MUC1‐targeted SERS NPs are coinjected with nontargeted or blocked MUC1‐targeted SERS NPs in BC xenograft mouse models. A two‐tumor mouse model with differential expression of MUC1 (MDA‐MB‐468 and MDA‐MB‐453) is used to control for active versus passive targeting in the same animals. The results show that the targeted SERS NPs home to the tumors via active targeting of MUC1, with low levels of passive targeting. This strategy is expected to be an advantageous alternative to antibody‐based targeting and useful for targeted imaging of tumor extent, progression, and therapeutic response.     

Single‐Layer MoS2 Nanosheets with Amplified Photoacoustic Effect for Highly Sensitive Photoacoustic Imaging of Orthotopic Brain Tumors

Photoacoustic (PA) imaging, as a fast growing technology that combines the high contrast of light and large penetration depth of ultrasound, has demonstrated great potential for molecular imaging of cancer. However, PA molecular imaging of orthotopic brain tumors is still challenging, partially due to the limited options and insufficient sensitivity of available PA molecular probes. Here, the direct formation of single‐layer (S‐MoS₂), few‐layer (F‐MoS₂), and multi‐layer (M‐MoS₂) nanosheets by the albumin‐assisted exfoliation without further surface modifications is reported. It is demonstrated that the PA effect of the MoS₂ nanosheets is highly dependent on their layered nanostructures. Decreasing the number of nanosheet layers from M‐MoS₂ to S‐MoS₂ can both significantly enhance the near‐infrared light absorption and improve the elastic properties of the nanomaterial, resulting in greatly amplified PA effect. The in vitro experiments demonstrate that the prepared S‐MoS₂ with excellent biocompatibility can be efficiently internalized into U87 glioma cells, producing strong PA signals for highly sensitive detection of brain tumor cells, with a detection limit of ≈100 cells. Intravenous administration of S‐MoS₂ to both U87 subcutaneous and orthotopic tumor‐bearing mice shows highly efficient tumor retention and significantly enhanced PA contrast. Tumor tissue ≈1.5 mm below the skull can still be clearly visualized in vivo. Previous studies suggest that the fabricated S‐MoS₂ with amplified PA effect have high potential to serve as an efficient nanoplatform for sensitive PA molecular imaging and hold promising prospect for translational medicine.     

Gold Nanoframeworks with Mesopores for Raman–Photoacoustic Imaging and Photo‐Chemo Tumor Therapy in the Second Near‐Infrared Biowindow

Gold‐based nanostructures with tunable wavelength of localized surface plasmon resonance (LSPR) in the second near‐infrared (NIR‐II) biowindow receive increasing attention in phototheranostics. In view of limited progress on NIR‐II gold nanostructures, a particular liposome template‐guided route is explored to synthesize novel gold nanoframeworks (AuNFs) with large mesopores (≈40 nm) for multimodal imaging along with therapeutic robustness. The synthesized AuNFs exhibit strong absorbance in NIR‐II region, affording their capacity of NIR‐II photothermal therapy (PTT) and photoacoustic (PA) imaging for deep tumors. Functionalization of AuNFs with hyaluronic acid (HA) endows the targeting capacity for CD44‐overexpressed tumor cells while gatekeeping doxorubicin (DOX) loaded into mesopores. Conjugation of Raman reporter 4‐aminothiophenol (4‐ATP) onto AuNFs yields a surface‐enhanced Raman scattering (SERS) fingerprint for Raman spectroscopy/imaging. In vivo evaluation of HA‐4‐ATP‐AuNFs‐DOX on tumor‐bearing xenografts demonstrates its high efficacy in eradication of solid tumors in NIR‐II under PA–Raman dual image‐guided photo‐chemotherapy. Thus, current AuNFs offer versatile capabilities for phototheranostics.     

Exploring How to Increase the Brightness of Surface‐Enhanced Raman Spectroscopy Nanolabels: The Effect of the Raman‐Active Molecules and of the Label Size

Due to the surface‐enhanced Raman scattering (SERS) effect, SERS labels based on noble‐metal nanoparticles loaded with Raman‐active molecules are good candidates for ultrasensitive multiplexed assays and in vitro/in vivo imaging. However, understanding how to maximize the brightness of such labels is of paramount importance for their widespread application. The effective differential Raman scattering cross‐section (dσ_R/dΩ) of SERS labels made of pegylated gold nanoparticles loaded with various Raman active molecules (Raman reporters) is studied. It is found that proper choice of the Raman reporter and of nanoparticle size can enhance the dσ_R/dΩ by several orders of magnitude. The experimental results are understood by considering the molecular cross‐section for resonant Raman scattering and the local electromagnetic enhancement factor (G_SERS) in the nearby of gold nanoparticles. These results are useful to guide the design of SERS labels with improved performances and to provide a reference for the comparison of the absolute value of the dσ_R/dΩ of SERS labels based on metal nanoparticles.     

Near‐Infrared SERS Nanoprobes with Plasmonic Au/Ag Hollow‐Shell Assemblies for In Vivo Multiplex Detection

For the effective application of surface‐enhanced Raman scattering (SERS) nanoprobes for in vivo targeting, the tissue transparency of the probe signals should be as high as it can be in order to increase detection sensitivity and signal reproducibility. Here, near‐infrared (NIR)‐sensitive SERS nanoprobes (NIR SERS dots) are demonstrated for in vivo multiplex detection. The NIR SERS dots consist of plasmonic Au/Ag hollow‐shell (HS) assemblies on the surface of silica nanospheres and simple aromatic Raman labels. The diameter of the HS interior is adjusted from 3 to 11 nm by varying the amount of Au³⁺ added, which results in a red‐shift of the plasmonic extinction of the Au/Ag nanoparticles toward the NIR (700–900 nm). The red‐shifted plasmonic extinction of NIR SERS dots causes enhanced SERS signals in the NIR optical window where endogenous tissue absorption coefficients are more than two orders of magnitude lower than those for ultraviolet and visible light. The signals from NIR SERS dots are detectable from 8‐mm deep in animal tissues. Three kinds of NIR SERS dots, which are injected into live animal tissues, produce strong SERS signals from deep tissues without spectral overlap, demonstrating their potential for in vivo multiplex detection of specific target molecules.     

Self‐Assembly of Semiconducting Polymer Amphiphiles for In Vivo Photoacoustic Imaging

Despite the advantages of semiconducting polymer nanoparticles (SPNs) over other inorganic nanoparticles for photoacoustic (PA) imaging, their synthetic method is generally limited to nanoprecipitation, which is likely to cause the issue of nanoparticle dissociation. The synthesis of near‐infrared (NIR) absorbing semiconducting polymer amphiphiles (SPAs) that can spontaneously self‐assemble into homogeneous nanoparticles for in vivo PA imaging is reported. As compared with their counterpart nanoparticles (SPN1) prepared through nanoprecipitation, SPAs generally have higher fluorescence quantum yields but similar size and PA brightness, making them superior over SPN1. Optical and simulation studies reveal that the poly(ethylene glycol) (PEG) grafting density plays a critical role in determining the packing of SP segments inside the core of nanoparticles, consequently affecting the optical properties. The small size and structurally stable nanostructure, in conjunction with a dense PEG shell, allow SPAs to passively target tumors of living mice after systemic administration, permitting both PA and fluorescence imaging of the tumors at signals that are ≈1.5‐fold higher than that of liver. This study thus not only provides the first generation of amphiphilic optically active polymers for PA imaging, but also highlights the molecular guidelines for the development of organic NIR imaging nanomaterials.     

Triplex Au–Ag–C Core–Shell Nanoparticles as a Novel Raman Label

Monodispersed, readily‐grafted, and biocompatible surface‐enhanced Raman spectroscopic (SERS) tagging materials are developed; they are composed of bimetallic Au@Ag nanoparticles (NPs) for optical enhancement, a reporter molecule for spectroscopic signature, and a carbon shell for protection and bioconjugation. A controllable and convenient hydrothermal synthetic route is presented to synthesize the layer‐by‐layer triplex Au–Ag–C core–shell NPs, which can incorporate the Raman‐active label 4‐mercapto benzoic acid (4‐MBA). The obtained gold seed–silver coated particles can be coated further with a thickness‐controlled carbon shell to form colloidal carbon‐encapsulated Au_core/Ag_shell spheres with a monodisperse size distribution. Furthermore, these SERS‐active spheres demonstrated interesting properties as a novel Raman tag for quantitative immunoassays. The results suggest such SERS tags can be used for multiplex and ultrasensitive detection of biomolecules as well as nontoxic, in vivo molecular imaging of animal or plant tissues.     

Understanding the In Vivo Fate of Advanced Materials by Imaging

Engineered materials are ubiquitous in biomedical applications ranging from systemic drug delivery systems to orthopedic implants, and their actions unfold across multiple time‐ and length‐scales. The efficacy and safety of biologics, nanomaterials, and macroscopic implants are all dictated by the same general principles of pharmacology as applied to small molecule drugs, comprising how the body affects materials (pharmacokinetics, PK) and conversely how materials affect the body (pharmacodynamics, PD). Imaging technologies play an increasingly insightful role in monitoring both of these processes, often simultaneously: translational macroscopic imaging modalities such as magnetic resonance imaging and positron emission tomography/computed tomography offer whole‐body quantitation of biodistribution and structural or molecular response, while ex vivo approaches and optical imaging via in vivo (intravital) microscopy reveal behaviors at subcellular resolution. In this review, the authors survey developments in imaging the in situ behavior of systemically and locally administered materials, with a particular focus on using microscopy to understand transport, target engagement, and downstream host responses at a single‐cell level. The themes of microenvironmental influence, controlled drug release, on‐target molecular action, and immune response, especially as mediated by macrophages and other myeloid cells, are examined. Finally, the future directions of how new imaging technologies may propel efficient clinical translation of next‐generation therapeutics and medical devices are proposed.     

Complementary Surface-Enhanced Raman Scattering (SERS) and IR Absorption Spectroscopy (SEIRAS) with Nanorods-on-a-Mirror

The surface-enhanced counterparts of Raman scattering (SERS) and infrared (IR) absorption (SEIRAS) are commonly used to probe and identify nanoscale matter and small populations of molecules. The contrasting selection rules offer complementary vibrational information of bulk solids or solutions. In this study, a complementary surface-enhanced vibrational spectroscopy approach is presented to probe the vibrational signature of metal-bound molecular monolayers. Nanocavities are designed and produced with sharp and tunable visible (VIS) and mid-IR gap resonances by placing nanorods on a mirror that is coated with a thin dielectric spacer. Their VIS resonances are tuned to match a 1.61 eV (770 nm) resonant excitation for SERS, while their mid-IR resonances span the 1500–2800 cm⁻¹ range (6.5–3.5 µm) in high resolution for SEIRAS, targeting CN bond vibrations at 2220 cm⁻¹. Both the VIS and mid-IR gap modes support spatially overlapping and highly enhanced near-fields ensuring strong SERS and SEIRAS signals from the same monolayer molecular population. The differences in the vibrational information obtained with the two surface-enhanced spectroscopies when probing coupled molecular vibrations are highlighted and the advantages of using such a platform for investigating cavity-modified chemical reactions are discussed.     

All‐in‐One Phototheranostics: Single Laser Triggers NIR‐II Fluorescence/Photoacoustic Imaging Guided Photothermal/Photodynamic/Chemo Combination Therapy

Development of single near‐infrared (NIR) laser triggered phototheranostics for multimodal imaging guided combination therapy is highly desirable but is still a big challenge. Herein, a novel small‐molecule dye DPP‐BT is designed and synthesized, which shows strong absorption in the first NIR window (NIR‐I) and fluorescence emission in the second NIR region (NIR‐II). Such a dye not only acts as a dual‐modal contrast agent for NIR‐II fluorescence and photoacoustic (PA) imaging, but also serves as a combined therapeutic agent for photothermal therapy (PTT) and photodynamic therapy (PDT). The single NIR laser triggered all‐in‐one phototheranostic nanoparticles are constructed by encapsulating the dye DPP‐BT, chemotherapy drug DOX, and natural phase‐change materials with a folic acid functionalized amphiphile. Notably, under NIR laser irradiation, DOX can effectively release from such nanoparticles via NIR‐induced hyperthermia of DPP‐BT. By intravenous injection of such nanoparticles into Hela tumor‐bearing mice, the tumor size and location can be accurately observed via NIR‐II fluorescence/PA dual‐modal imaging. From in vitro and in vivo therapy results, such nanoparticles simultaneously present remarkable antitumor efficacy by PTT/PDT/chemo combination therapy, which is triggered by a single NIR laser. Overall, this work provides an innovative strategy to design and construct all‐in‐one nanoplatforms for clinical phototheranostics.     

pH‐Activated Near‐Infrared Fluorescence Nanoprobe Imaging Tumors by Sensing the Acidic Microenvironment

Imaging tumors in their early stages is crucial to increase the surviving rate of cancer patients. Currently most fluorescence probes visualize the neoplasia by targeting the tumor‐associated receptor over‐expressed on the cancer cell membrane. However, the expression level of these receptors in vivo is hard to predict, which limits their clinical translation. Furthermore, the signal output of these receptor‐targeting probes usually stays at a high level, which leads to a strong background signal in normal tissue due to non‐specific binding. In contrast to receptors, characteristics of the tumor microenvironment – such as acidosis – are pervasive in almost all solid tumors and can be easily accessed. In this work, a novel biodegradable nanoprobe InNP1 that demonstrates pH‐activated near‐infrared (NIR) fluorescence in both human glioblastoma U87MG cancer cells in vitro and the subcutaneous U87MG tumor xenografts in vivo is developed. Bio‐distribution, in vivo optical imaging, and autoradiography studies demonstrate that the pH‐activated NIR fluorescence is the dominant factor responsible for the high tumor/normal tissue (T/N) ratio of InNP1 in vivo. Overall, the work provides a nanoprobe prototype to visualize the solid tumor in vivo with high sensitivity and minimal systemic toxicity by sensing the tumor acidic microenvironment.     

Interactions Between Tumor Biology and Targeted Nanoplatforms for Imaging Applications

Although considerable efforts have been conducted to diagnose, improve, and treat cancer in the past few decades, existing therapeutic options are insufficient, as mortality and morbidity rates remain high. Perhaps the best hope for substantial improvement lies in early detection. Recent advances in nanotechnology are expected to increase the current understanding of tumor biology, and will allow nanomaterials to be used for targeting and imaging both in vitro and in vivo experimental models. Owing to their intrinsic physicochemical characteristics, nanostructures (NSs) are valuable tools that have received much attention in nanoimaging. Consequently, rationally designed NSs have been successfully employed in cancer imaging for targeting cancer‐specific or cancer‐associated molecules and pathways. This review categorizes imaging and targeting approaches according to cancer type, and also highlights some new safe approaches involving membrane‐coated nanoparticles, tumor cell‐derived extracellular vesicles, circulating tumor cells, cell‐free DNAs, and cancer stem cells in the hope of developing more precise targeting and multifunctional nanotechnology‐based imaging probes in the future.     

Magnetic Targeting Enhanced Theranostic Strategy Based on Multimodal Imaging for Selective Ablation of Cancer

The booming development of nanomedicine offers great opportunities for cancer diagnostics and therapeutics. Herein, a magnetic targeting‐enhanced cancer theranostic strategy using a multifunctional magnetic‐plasmonic nano‐agent is developed, and a highly effective in vivo tumor photothermal therapy, which is carefully planed based on magnetic resonance (MR)/photoacoustic (PA) multimodal imaging, is realized. By applying an external magnetic field (MF) focused on the targeted tumor, a magnetic targeting mediated enhanced permeability and retention (MT‐EPR) effect is observed. While MR scanning provides tumor localization and reveals time‐dependent tumor homing of nanoparticles for therapeutic planning, photoacoustic imaging with higher spatial resolution allows noninvasive fine tumor margin delineation and vivid visualization of three dimensional distributions of theranostic nanoparticles inside the tumor. Utilizing the near‐infrared (NIR) plasmonic absorbance of those nanoparticles, selective photothermal tumor ablation, whose efficacy is predicted by real‐time infrared thermal imaging intra‐therapeutically, is carried out and then monitored by MR imaging for post‐treatment prognosis. Overall, this study illustrates the concept of imaging‐guided MF‐targeted photothermal therapy based on a multifunctional nano‐agent, aiming at optimizing therapeutic planning to achieve the most efficient cancer therapy.     

Folate Receptor-Targeted NIR-II Dual-Model Nanoprobes for Multiscale Visualization of Macrophages in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by joint inflammation. Macrophages accelerate the pathological process of RA through the continuous cell accumulation and secretion of cytokines. Therefore, multiscale visualization of macrophages is of great significance for the early diagnosis of RA. In this work, folate receptor-targeted Cptnc-4F nanoprobes, named FA-CF-NPs, are developed for dual-model molecular imaging of macrophages in RA mouse models. FA-CF-NPs exhibit complementary imaging advantages with high fluorescence sensitivity and resolution in the second near-infrared window (NIR-II) and promising photoacoustic (PA) imaging contrast with deep tissue penetration. The high affinity of the folate ligand on the surface of FA-CF-NPs to macrophages with overexpressed folate receptors ensures the specific targeting function of the nanoprobes for imaging in vivo. FA-CF-NPs are employed to achieve NIR-II fluorescence imaging to accurately localize macrophages in the joint, obtaining a high signal-to-background ratio (SBR) of 15.5. Meanwhile, PA imaging is also performed to observe macrophage distribution in the longitudinal section of the whole joint. Moreover, macrophage infiltration is detected in the joint of RA mice with the absence of RA pathologies such as synovitis and cartilage loss. The results provide an opportunity for in vivo macrophage tracking and early diagnosis of RA.     

Ultrasmall Quantum Dots with Broad‐Spectrum Metal Doping Ability for Trimodal Molecular Imaging

Development of efficient targeting nanomaterials is extremely challenging due to the nonspecific accumulation in immune tissues, such as the liver and the spleen. Ultrasmall nanoparticles (USNPs) could possess small molecule‐like in vivo pharmacokinetic profiles, coupled with integrated functions capacity, improving the molecular imaging efficiency, particularly in oncology. For nuclear imaging, radiometals are often incorporated into the structures of USNPs using chelator and chelator‐free strategies. However, the incorporated chelator may change the surface properties and in vivo behavior of UNSPs, while chelator‐free labeling strategies either involve complicated resynthesis or rely on the active properties of the metal ions. Herein, a novel chelator‐free and postsynthetic strategy for broad‐spectrum metal ion attachment is reported. The ultrasmall Ag₂Se quantum dots (QDs) are developed with an active oxygen layer on the surface, allowing for facile incorporation of both active and inert metals with high labeling efficiency. The particles enable fluorescence, magnetic resonance imaging, and positron emission tomography (PET) trimodality imaging. After conjugation with targeting peptide, the probe yields a high tumor‐to‐muscle ratio of nine in PET imaging. Importantly, the QDs are predominantly excreted from body through the renal route within 12 h. This chelator‐free strategy opens an avenue for exploring broad‐spectrum radiometal isotope labeling and USNP‐based renal‐excreting imaging probes.     

Selective,Quantitative, and Multiplexed Surface‐Enhanced Raman Spectroscopy Using Aptamer‐Functionalized Monolithic Plasmonic Nanogrids Derived from Cross‐Point Nano‐Welding

Recent advances in surface‐enhanced Raman spectroscopy (SERS) have resulted in multiplexing with unprecedented levels of sensitivity and selectivity in trace‐amount detection. However, quantification of multiple trace‐amount molecules with ng‐level accuracy has yet to be demonstrated due to nonuniform distribution of SERS enhancement and random adsorption of molecules at low concentrations. While Raman reporter‐free SERS is favorable for quantification in that the unique fingerprint spectra of molecules enable specific molecular identification, it has yet to be demonstrated due to poor reproducibility and insufficient SERS enhancement. Raman reporter‐free multiplex SERS with highly accurate quantification is successfully realized by versatile aptamer‐functionalized plasmonic Au nanogrids with uniform SERS enhancement. By cross‐point nano‐welding, monolithic Au nanogrids with excellent uniformity and high stability in aqueous media are produced. Raman reporter‐free multiplex detection and highly accurate quantification of concentration and composition is realized at picomolar levels. As a demonstration, Au nanogrids functionalized with bisphenol A‐specific aptamers successfully detect and quantify trace‐amounts of bisphenol A (8.49 ng) from thermal receipt paper. Moreover, principal component analysis is applied to multiplex SERS spectra to establish a ternary composition map, which can potentially serve as a practical reference for future Raman reporter‐free SERS.     

Multiplex SERS Detection of Metabolic Alterations in Tumor Extracellular Media

The composition and intercellular interactions of tumor cells in the tissues dictate the biochemical and metabolic properties of the tumor microenvironment. The metabolic rewiring has a profound impact on the properties of the microenvironment, to an extent that monitoring such perturbations could harbor diagnostic and therapeutic relevance. A growing interest in these phenomena has inspired the development of novel technologies with sufficient sensitivity and resolution to monitor metabolic alterations in the tumor microenvironment. In this context, surface‐enhanced Raman scattering (SERS) can be used for the label‐free detection and imaging of diverse molecules of interest among extracellular components. Herein, the application of nanostructured plasmonic substrates comprising Au nanoparticles, self‐assembled as ordered superlattices, to the precise SERS detection of selected tumor metabolites, is presented. The potential of this technology is first demonstrated through the analysis of kynurenine, a secreted immunomodulatory derivative of the tumor metabolism and the related molecules tryptophan and purine derivatives. SERS facilitates the unambiguous identification of trace metabolites and allows the multiplex detection of their characteristic fingerprints under different conditions. Finally, the effective plasmonic SERS substrate is combined with a hydrogel‐based three‐dimensional cancer model, which recreates the tumor microenvironment, for the real‐time imaging of metabolite alterations and cytotoxic effects on tumor cells.     

Hierarchical Nanohybrids of Gold Nanorods and PGMA‐Based Polycations for Multifunctional Theranostics

Organic/inorganic nanohybrids hold great importance in fabricating multifunctional theranostics to integrate therapeutic functions with real‐time imaging. Although Au nanorods (NRs) have been employed for theranostics, complicated design of materials limits their practical applications. In this work, new multifunctional theranostic agents are designed and synthesized employing Au NRs with desirable near‐infrared absorbance as the cores. A facile “grafting‐onto” approach is put forward to prepare the series of hierarchical nanohybrids (Au‐PGEA and Au‐PGED) of Au NRs and poly(glycidyl methacrylate)‐based polycations. The resultant nanohybrids can be utilized as gene carriers with high gene transfection performances. The structural effect of polycations on gene transfection is investigated in detail, and Au‐PGEA with abundant hydroxyl groups on the surface exhibits superior performance. Au‐PGEA nanohybrids are further validated to possess remarkable capability of combined photothermal therapy (PTT) and gene therapy (GT) for complementary tumor treatment. Moreover, significantly enhanced computed tomography (CT)/photoacoustic (PA) signals are detected both in vitro and in vivo, verifying the potential of Au‐PGEA for dual‐modal imaging with precise and accurate information. Therefore, these multifunctional nanohybrids fabricated from a simple and straightforward strategy are promising for in vivo dual‐modal CT/PA imaging guided GT/PTT therapy with high antitumor efficacy.     

Novel Au–SiO2–WO3 Core–Shell Composite Nanoparticles for Surface‐Enhanced Raman Spectroscopy with Potential Application in Cancer Cell Imaging

With the rapid development of nanotechnology during the last decades, the ability to detect and control individual objects at the nanoscale has enabled us to deal with complex biomedical challenges. In cancer imaging, novel nanoparticles (NPs) offer promising potential to identify single cancer cells and precisely label larger areas of cancer tissues. Herein, a new class of size tunable core–shell composite (Au–SiO₂–WO₃) nanoparticles is reported. These nanoparticles display an easily improvable ≈10³ surface‐enhanced Raman scattering (SERS) enhancement factor with a double Au shell for dried samples over Si wafers and several orders of magnitude for liquid samples. WO₃ core nanoparticles measuring 20–50 nm in diameter are sheathed by an intermediate 10–60 nm silica layer, produced by following the Stöber‐based process and Turkevich method, followed by a 5–20 nm thick Au outer shell. By attaching 4‐mercaptobenzoic acid (4‐MBA) molecules as Raman reporters to the Au, high‐resolution Raman maps that pinpoint the nanoparticles' location are obtained. The preliminary results confirm their advantageous SERS properties for single‐molecule detection, significant cell viability after 24 h and in vitro cell imaging using coherent anti‐stokes Raman scattering. The long‐term objective is to measure SERS nanoparticles in vivo using near‐infrared light.     

Fabrication of a Macroporous Microwell Array for Surface‐Enhanced Raman Scattering

Here, a colloidal templating procedure for generating high‐density arrays of gold macroporous microwells, which act as discrete sites for surface‐enhanced Raman scattering (SERS), is reported. Development of such a novel array with discrete macroporous sites requires multiple fabrication steps. First, selective wet‐chemical etching of the distal face of a coherent optical fiber bundle produces a microwell array. The microwells are then selectively filled with a macroporous structure by electroless template synthesis using self‐assembled nanospheres. The fabricated arrays are structured at both the micrometer and nanometer scale on etched imaging bundles. Confocal Raman microscopy is used to detect a benzenethiol monolayer adsorbed on the macroporous gold and to map the spatial distribution of the SERS signal. The Raman enhancement factor of the modified wells is investigated and an average enhancement factor of 4 × 10⁴ is measured. This demonstrates that such nanostructured wells can enhance the local electromagnetic field and lead to a platform of ordered SERS‐active micrometer‐sized spots defined by the initial shape of the etched optical fibers. Since the fabrication steps keep the initial architecture of the optical fiber bundle, such ordered SERS‐active platforms fabricated onto an imaging waveguide open new applications in remote SERS imaging, plasmonic devices, and integrated electro‐optical sensor arrays.