6-Color/1-Target Immuno-SERS Microscopy on the Same Single Cancer Cell

Reaction-Based SERS Probes for the Detection of Raman-Inactive Species

Surface-enhanced Raman spectroscopy (SERS) has the advantages of high sensitivity, low water interference, narrow spectral peaks for multicomponent analysis, and rich molecular fingerprint information, presenting great potential to be a robust analytical technology. However, a key issue is the unavailability in directly detecting Raman-inactive species with a small Raman scattering cross-section. Current research has addressed this issue by using specific chemical reactions to induce significant characteristic changes in SERS signals, enabling the sensitive and selective detection of Raman-inactive species. This reaction-activated SERS sensing strategy provides a clever approach to the precise determination of Raman-inactive species. In this review, we have first summarized the design principles and types of reaction-based SERS probes. Furthermore, we have examined the enormous potential of reaction-based SERS probes in the detection of bioactive species, environmental pollutants, and food contaminants. Finally, we have discussed in depth the challenges and prospects of reaction-based SERS probes on stability, reliability, and intelligence. The review is aimed to inspire a more advanced design of reaction-based SERS probes, thus further facilitating their extensive applications in SERS analysis.

Designing SERS nanotags for profiling overexpressed surface markers on single cancer cells: A review

This review focuses on the chemical design and the use of Surface-Enhanced Raman Scattering (SERS)-active nanotags for measuring surface markers that can be overexpressed at the surface of single cancer cells. Indeed, providing analytical tools with true single-cell measurements capabilities is capital, especially since cancer research is increasingly leaning toward single-cell analysis, either to guide treatment decisions or to understand complex tumor behaviour including the single-cell heterogeneity and the appearance of treatment resistance. Over the past two decades, SERS nanotags have triggered significant interest in the scientific community owing their advantages over fluorescent tags, mainly because SERS nanotags resist photobleaching and exhibit sharper signal bands, which reduces possible spectral overlap and enables the discrimination between the SERS signals and the autofluorescence background from the sample itself. The extensive efforts invested in harnessing SERS nanotags for biomedical purposes, particularly in cancer research, highlight their potential as the next generation of optical labels for single-cell studies. The review unfolds in two main parts. The first part focuses on the structure of SERS nanotags, detailing their chemical composition and the role of each building block of the tags. The second part explores applications in measuring overexpressed surface markers on single-cells. The latter encompasses studies using single nanotags, multiplexed measurements, quantitative information extraction, monitoring treatment responses, and integrating phenotype measurements with SERS nanotags on single cells isolated from complex biological matrices. This comprehensive review anticipates SERS nanotags to persist as a pivotal technology in advancing single-cell analytical methods, particularly in the context of cancer research and personalized medicine.

iSERS: from nanotag design to protein assays and ex vivo imaging

Proteins are an eminently important class of ubiquitous biomacromolecules with diverse biological functions, and numerous techniques for their detection, quantification, and localisation have been developed. Many of these methods exploit the selectivity arising from molecular recognition of proteins/antigens by immunoglobulins. The combination of surface-enhanced Raman scattering (SERS) with such "immuno"-techniques to immuno-SERS (iSERS) is the central topic of this review, which is focused on colloidal SERS nanotags, i.e., molecularly functionalised noble metal nanoparticles conjugated to antibodies, for their use in protein assays and ex vivo imaging. After contrasting the fundamental differences between label-free SERS and iSERS, including a balanced description of the advantages and drawbacks of the latter, we describe the usual workflow of iSERS experiments. Milestones in the development of the iSERS technology are summarised from a historical perspective. By highlighting selected examples from the literature, we illustrate the conceptual progress that has been achieved in the fields of iSERS-based protein assays and ex vivo imaging. Finally, we attempt to predict what is necessary to fully exploit the transformative potential of the iSERS technology by stimulating the transition from research in academic labs into applications for the benefit of our society.

SERS-based microdevices for use as in vitro diagnostic biosensors

Advances in surface-enhanced Raman scattering (SERS) detection have helped to overcome the limitations of traditional in vitro diagnostic methods, such as fluorescence and chemiluminescence, owing to its high sensitivity and multiplex detection capability. However, for the implementation of SERS detection technology in disease diagnosis, a SERS-based assay platform capable of analyzing clinical samples is essential. Moreover, infectious diseases like COVID-19 require the development of point-of-care (POC) diagnostic technologies that can rapidly and accurately determine infection status. As an effective assay platform, SERS-based bioassays utilize SERS nanotags labeled with protein or DNA receptors on Au or Ag nanoparticles, serving as highly sensitive optical probes. Additionally, a microdevice is necessary as an interface between the target biomolecules and SERS nanotags. This review aims to introduce various microdevices developed for SERS detection, available for POC diagnostics, including LFA strips, microfluidic chips, and microarray chips. Furthermore, the article presents research findings reported in the last 20 years for the SERS-based bioassay of various diseases, such as cancer, cardiovascular diseases, and infectious diseases. Finally, the prospects of SERS bioassays are discussed concerning the integration of SERS-based microdevices and portable Raman readers into POC systems, along with the utilization of artificial intelligence technology.

Convergence of Surface-Enhanced Raman Scattering with Molecular Diagnostics: A Perspective on Future Directions

Molecular diagnostics (MD) is widely employed in multiple scientific disciplines, such as oncology, pathogen detection, forensic investigations, and the pharmaceutical industry. Techniques such as polymerase chain reaction (PCR) revolutionized the rapid and accurate identification of nucleic acids (DNA, RNA). More recently, CRISPR and its CRISPR-associated protein (Cas) have been a ground-breaking discovery that is the latest revolution in molecular biology, including MD. Surface-enhanced Raman scattering (SERS) is a very attractive alternative to fluorescence as the currently most widely used optical readout in MD. In this Perspective, milestones in the development of MD, SERS-PCR, and next-generation approaches to MD, such as Specific High-Sensitivity Enzymatic Reporter UnLOCKing (SHERLOCK) and DNA Endonuclease-Targeted CRISPR Trans Reporter (DETECTR), are briefly summarized. Our perspective on the future convergence of SERS with MD is focused on SERS-based CRISPR/Cas (SERS-CRISPR) since we anticipate many promising applications in this rapidly emerging field. We predict that major future developments will exploit the advantages of real-time monitoring with the superior brightness, photostability, and spectral multiplexing potential of SERS nanotags in an automated workflow for rapid assays under isothermal, amplification-free conditions.

Multiplexed Surface Protein Detection and Cancer Classification Using Gap-Enhanced Magnetic-Plasmonic Core-Shell Raman Nanotags and Machine Learning Algorithm

Cancer is the second leading cause of death attributed to disease worldwide. Current standard detection methods often rely on a single cancer marker, which can lead to inaccurate results, including false negatives, and an inability to detect multiple cancers simultaneously. Here, we developed a multiplex method that can effectively detect and classify surface proteins associated with three distinct types of breast cancer by utilizing gap-enhanced Raman scattering nanotags and machine learning algorithm. We synthesized anisotropic magnetic core-gold shell gap-enhanced Raman nanotags incorporating three different Raman reporters. These multicolor Raman nanotags were employed to distinguish specific surface protein markers in breast cancer cells. The acquired signals were deconvoluted and analyzed using classical least-squares regression to generate a surface protein profile and characterize the breast cancer cells. Furthermore, computational data obtained via finite-difference time-domain and discrete dipole approximation showed the amplification of the electric fields within the gap region due to plasmonic coupling between the two gold layers. Finally, a random forest classifier achieved an impressive classification and profiling accuracy of 93.9%, enabling effective distinguishing between the three different types of breast cancer cell lines in a mixed solution. With the combination of immunomagnetic multiplex target specificity and separation, gap-enhancement Raman nanotags, and machine learning, our method provides an accurate and integrated platform to profile and classify different cancer cells, giving implications for identification of the origin of circulating tumor cells in the blood system.

Four-Color SERS Monitoring of Size-dependent Nanoparticle Delivery in the Same Tumor

The physicochemical properties of nanoparticles (NPs) significantly influence their deposition at the disease site, ultimately impacting the overall therapeutic efficacy; however, precisely assessing the effects of various factors on NP accumulation within a single cell/tumor tissue is challenging due to the lack of appropriate labeling techniques. Surface-enhanced Raman spectroscopy (SERS) tag is a powerful encoding method that has recently been intensively employed for immunodetection of biomarkers. Herein, we introduce a multiplexed SERS tracking approach for systematic investigation of size-dependent accumulation and distribution of NPs within the same tumor. Four-sized (34, 60, 108, and 147 nm) NPs encoded with different SERS "colors" were fabricated, mixed, and incubated with monolayer tumor cells, multicellular tumor spheroids, or injected into mouse models bearing xenograft solid tumors in a single dose. Multicolor SERS detection of the specimens revealed that NP accumulation in tumor cells, tumor spheroids, and solid tumors was in the order of 34 nm > 60 nm > 108 nm > 147 nm, 60 nm > 34 nm > 108 nm > 147 nm, and 34 nm > 147 nm > 108 nm > 60 nm, respectively. Inductively coupled plasma mass spectroscopy determination performed in parallel samples were in alignment with the four-color SERS probing results, demonstrating the effectiveness of this multiplexed evaluation assay. Furthermore, in combination with fluorescence labeling of specific biomolecules, this method can be applied for the colocalization of different NPs in various pathological structures and provide additional information for analysis of the possible mechanisms.

Capillary Tube Surface-Enhanced Raman Scattering Substrate and High-Sensitivity Molecule Detection

Surface-enhanced Raman scattering (SERS) greatly improves molecule sensitivity compared with ordinary Raman spectroscopy. To excite and detect SERS efficiently, we fabricated glass-made microcapillary tubes decorated with silver nanoparticles inside them. The capillary tubes work as sample containers, where the required sample volume is in the order of a few nanoliters. The capillary tubes also play the role of optical waveguides. The tubes guide the excitation laser light through them so that the light illuminates whole silver nanoparticles inside the tubes at once. The tubes guide the SERS light to the tube end efficiently. The decoration of silver nanoparticles inside the tubes was performed by the silver mirror reaction. By making the tubes thinner and longer, highly sensitive SERS spectroscopy can be achieved. Our method would be a powerful tool for high-sensitivity molecule detection where the sample volume and concentration are extremely low.

An efficient and simple SERS approach for trace analysis of tetrahydrocannabinol and cannabinol and multi-cannabinoid detection

Many countries have legalized cannabis and its derived products for multiple purposes. Consequently, it has become necessary to develop a rapid, effective, and reliable tool for detecting delta-9-tetrahydrocannabinol (THC) and cannabinol (CBN), which are important biologically active compounds in cannabis. Herein, we have fabricated SERS chips by using glancing angle deposition and tuned dimensions of silver nanorods (AgNRs) for detecting THC and CBN at low concentrations. Experimental and computational results showed that the AgNR substrate with film thickness (or nanorod length) of 150 nm, corresponding to nanorod diameter of 79 nm and gap between nanorods of 23 nm, can effectively sense trace THC and CBN with good reproducibility and sensitivity. Due to limited spectral studies of the cannabinoids in previous reports, this work also explored towards identifying characteristic Raman lines of THC and CBN. This information is critical to further reliable data analysis and interpretation. Moreover, multianalyte detection of THC and CBN in a mixture was successfully demonstrated by applying an open-source independent component analysis (ICA) model. The overall method is fast, sensitive, and reliable for sensing trace THC and CBN. The SERS chip-based method and spectral results here are useful for a variety of cannabis testing applications, such as product screening and forensic investigation.

SERS nanotags for folate receptor α detection at the single cell level: discrimination of overexpressing cells and potential for live cell applications

Development of nanotags based on Surface-Enhanced Raman Scattering (SERS) for the discrimination of cancer cells overexpressing folate receptor α. Nanotags are also applicable for live cell measurements.

Current and Future Advancements of Raman Spectroscopy Techniques in Cancer Nanomedicine

Raman scattering is one of the most used spectroscopy and imaging techniques in cancer nanomedicine due to its high spatial resolution, high chemical specificity, and multiplexity modalities. The flexibility of Raman techniques has led, in the past few years, to the rapid development of Raman spectroscopy and imaging for nanodiagnostics, nanotherapy, and nanotheranostics. This review focuses on the applications of spontaneous Raman spectroscopy and bioimaging to cancer nanotheranostics and their coupling to a variety of diagnostic/therapy methods to create nanoparticle-free theranostic systems for cancer diagnostics and therapy. Recent implementations of confocal Raman spectroscopy that led to the development of platforms for monitoring the therapeutic effects of anticancer drugs in vitro and in vivo are also reviewed. Another Raman technique that is largely employed in cancer nanomedicine, due to its ability to enhance the Raman signal, is surface-enhanced Raman spectroscopy (SERS). This review also explores the applications of the different types of SERS, such as SERRS and SORS, to cancer diagnosis through SERS nanoprobes and the detection of small-size biomarkers, such as exosomes. SERS cancer immunotherapy and immuno-SERS (iSERS) microscopy are reviewed.

Selecting Surface-Enhanced Raman Spectroscopy Flavors for Multiplexed Imaging Applications: Beyond the Experiment

Multiplexing capabilities and sensitivity of surface-enhanced Raman spectroscopy (SERS) nanoparticles (NPs) are strongly dependent on the selected Raman reporter. These Raman-active molecules are responsible for giving each batch of SERS NPs its unique spectral fingerprint. Herein, we studied four types of SERS NPs, namely, AuNPs labeled with trans-1,2-bis(4-pyridyl)ethylene (BPE), 4,4'-bis(mercaptomethyl)biphenyl (BMMBP), 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (PODT), and 5-(4-pyridyl)-1H-1,2,4-triazole-3-thiol (PTT), and demonstrated that the best level of theory could be chosen based on inner products of DFT-calculated and experimental Raman spectra. We also calculated the theoretical spectra of these Raman reporters bound to Au₂₀ clusters to interrogate how SERS enhancement would affect their spectral fingerprint. Importantly, we found a correlation between B3LYP-D3 calculated and experimental enhancement factors, which opens up an avenue toward predicting which Raman reporters could offer improved sensitivity. We observed 0.5 and 3 fM limits of detection for BMMBP- and PTT-labeled 60 nm AuNPs, respectively.

Size-Dependent Penetration of Gold Nanoprobes into Fixed Cells

Nanoprobes have been increasingly applied in the biomedical field due to their superior optical, electronic, or magnetic properties. Among the many aspects involved in the interaction between nanoprobes and biospecimens, size plays an essential role. Although the influence of size on their internalization behavior and distribution in live cells has been extensively studied, how does the size affect penetration of nanoprobes into fixed cells remains unknown. We investigate here the influence of size on the penetration behavior of gold nanoprobes into fixed mammalian cells by dark-field microscopy and surface-enhanced Raman scattering (SERS) microspectroscopy. We show that 14, 20, and 29 nm nanoprobes can readily enter into methanol-fixed MCF-7 cells, while 42 and 55 nm nanoprobes cannot cross the cell membrane. For 4% paraformaldehyde-fixed cells, even 14 nm nanoprobes can hardly get into the cells, but after treatment with permeabilization reagents, 14 and 20 nm nanoprobes are permitted to enter the cells. These findings provide important implications in future design of nanoprobes for cellular immunostaining.

Multiplex micro-SERS imaging of cancer-related markers in cells and tissues using poly(allylamine)-coated Au@Ag nanoprobes

Surface-enhanced Raman scattering (SERS) nanoprobes based on Au@Ag core@shell nanoparticles coated with poly(allylamine) were functionalized with small targeting molecules to evaluate simultaneously the level of expression of two cancer-related markers, both in cells and in tissues. The Au@Ag nanoparticles provide a high SERS signal enhancement in the visible range when combined with resonant Raman-active molecules. The poly(allylamine) coating plays a dual key role in (i) protecting the metal surface against the complex biological medium, leading to a stable signal of the Raman-active molecules, and (ii) enabling specific biofunctionalization through its amine functions. Using small targeting molecules linked to the polymer coating, two different nanoprobes (duplex approach) were designed. Each was able to specifically target a particular cancer-related marker: folate receptors (FRs) and sialic acid (SA). We demonstrate that the level of expression of these targeted markers can be evaluated following the SERS signal of the probes incubated on cells or tissues. The potential overexpression of folate receptors and of sialic acid was evaluated and measured in breast and ovarian cancerous tissue sections. In addition, FR and/or SA overexpression in the tumor region can be visualized with high contrast with respect to the healthy region and with high spatial accuracy consistent with histology by SERS imaging of the nanoprobe signal. Owing to the unique spectral signature of the designed nanoprobes, this approach offers an efficient tool for the spatially resolved, in situ measurement of the expression level of several cancer-related markers in tumors at the same time.Graphical abstract.