Branched Au Nanoparticles on Nanofibers for Surface-Enhanced Raman Scattering Sensing of Intracellular pH and Extracellular pH Gradients

Prediction of cardiac differentiation in human induced pluripotent stem cell-derived cardiomyocyte supernatant using surface-enhanced Raman spectroscopy and machine learning

The efficient manufacturing of cardiomyocytes from human-induced pluripotent stem cells (hiPSCs) is essential for advancing regenerative therapies for myocardial injuries. However, ensuring cell quality during production is challenging since traditional methods are invasive, destructive, and time-consuming. In this study, we monitored cardiomyocyte differentiation of WTC11 hiPSCs by analyzing conditioned media collected at various stages using Raman spectroscopy, multivariate analysis, and machine learning. Differentiation efficiency was confirmed via flow cytometry and immunostaining. Raman spectra were processed using standard normal variate and second derivative transformations before performing a principal component analysis (PCA) and machine learning (Random Forest, K-Nearest Neighbors, and Deep Neural Networks [DNN]). Results show that PCA was unable to distinguish cells based on differentiation stages, while machine learning could reliably predict cell differentiation early in the cardiac cell manufacturing process. DNN models achieved accuracies exceeding 82 % in predicting differentiation, highlighting their potential as quality control tools. These findings underscore the potential of Raman spectroscopy coupled with machine learning as a tool for real-time monitoring of cardiomyocyte production.

Mixed atomistic-implicit quantum/classical approach to molecular nanoplasmonics

A multiscale quantum mechanical (QM)/classical approach is presented that is able to model the optical properties of complex nanostructures composed of a molecular system adsorbed on metal nanoparticles. The latter is described by a combined atomistic-continuum model, where the core is described using the implicit boundary element method (BEM) and the surface retains a fully atomistic picture and is treated employing the frequency-dependent fluctuating charge and fluctuating dipole (ωFQFμ) approach. The integrated QM/ωFQFμ-BEM model is numerically compared with state-of-the-art fully atomistic approaches, and the quality of the continuum/core partition is evaluated. The method is then extended to compute surface-enhanced Raman scattering within a time-dependent density functional theory framework.

An Analysis of the Radiosensitiser Applications in the Biomedical Field

BACKGROUND

Various types of radiosensitisers have been introduced from the past until the present day for applications in the biomedical field. However, there is a lack of understanding and comparison between the various parameters introduced in addition to a lack of consensus among researchers on the optimal radiosensitiser for applications in the biomedical field.

OBJECTIVE

This review aimed to investigate the usage of radiosensitisers in the biomedical field, determine their important parameters, and suggest radiosensitisers with potential among the analysed radiosensitisers.

RESULTS AND CONCLUSION

This review has discussed several parameters for radiosensitisers, including median lethal dose, cell survival, tumour size, cell viability, Dose Enhancement Factor (DEF), Reactive Oxygen Species (ROS) concentration, radiosensitiser production complexity, radiosensitiser administration technique, and radiosensitiser toxicity. General trends regarding the development of radiosensitisers, including the types, effectiveness, and their production complexity, have also been discussed within this review article.

Targeted gold nanoparticles for ovarian cancer (Review)

Among all malignant gynecological tumors, ovarian cancer (OC) has the highest mortality rate. OC is often diagnosed at advanced and incurable stages; however, early diagnosis can enable the use of optimized and personalized treatments. Intensive research into the synthesis and characterization of gold nanoparticles (AuNPs) has been performed with the aim of developing innovative materials for use in biological and photothermal therapies for OC. AuNPs can be chemically modified and functionalized by binding to a variety of organic compounds and biomolecules, such as peptides, antibodies and therapeutic agents, via simple synthetic processes. They are particularly suitable for use as carriers for drug delivery. In the present review, the synthesis and characteristics of AuNPs are summarized, and their potential in OC therapy are discussed.

Surface enhanced Raman scattering microtips for microenvironment pH determination of semi-solid preparations

Semi-solid preparations such as ointments, creams, gels, and pastes are common topical dosage forms with complex compositions and microstructures. pH is a critical quality attribute for semi-solid preparations, affecting properties such as particle size distribution, drug dissociation state, and rheological behavior. Currently, traditional pH electrode methods only provide an "average" pH for large-volume matrices, with low spatial resolution and accuracy. Microenvironment pH detection is crucial for accurately assessing semi-solid preparations. Herein, we developed pH-sensitive surface enhanced Raman scattering (SERS) microtips to achieve localized pH detection in semi-solid preparations. SERS microtips were prepared from glass needles with a tip size of around 1 μm and gold nanoparticles (Au NPs) grown in situ on glass surfaces for SERS enhancement. 4-Mercaptopyridine was selected as a pH sensitive Raman reporter and immobilized on the Au NPs, exhibiting characteristic Raman peak shifts within the pH range of 3-10. The SERS microtips were employed to conduct highly sensitive pH measurements in localized areas of 15 commercial ointments, 8 gels, and 1 laboratory-made ointment, providing higher spatial resolution and microenvironment differentiation compared to pH meters. The SERS microtips were used to monitor pH changes over time in ointment applied to localized wounds on live mice. This work introduces a new tool for pH detection in semi-solid preparations, offering a new method to enhance the prescription process and quality assessment of complex preparations like topical semi-solid preparations.

Long-range enhancement for fluorescence and Raman spectroscopy using Ag nanoislands protected with column-structured silica overlayer

We demonstrate long-range enhancement of fluorescence and Raman scattering using a dense random array of Ag nanoislands (AgNIs) coated with column-structured silica (CSS) overlayer of over 100 nm thickness, namely, remote plasmonic-like enhancement (RPE). The CSS layer provides physical and chemical protection, reducing the impact between analyte molecules and metal nanostructures. RPE plates are fabricated with high productivity using sputtering and chemical immersion in gold(I)/halide solution. The RPE plate significantly enhances Raman scattering and fluorescence, even without proximity between analyte molecules and metal nanostructures. The maximum enhancement factors are 107-fold for Raman scattering and 102-fold for fluorescence. RPE is successfully applied to enhance fluorescence biosensing of intracellular signalling dynamics in HeLa cells and Raman histological imaging of oesophagus tissues. Our findings present an interesting deviation from the conventional near-field enhancement theory, as they cannot be readily explained within its framework. However, based on the phenomenological aspects we have demonstrated, the observed enhancement is likely associated with the remote resonant coupling between the localised surface plasmon of AgNIs and the molecular transition dipole of the analyte, facilitated through the CSS structure. Although further investigation is warranted to fully understand the underlying mechanisms, the RPE plate offers practical advantages, such as high productivity and biocompatibility, making it a valuable tool for biosensing and biomolecular analysis in chemistry, biology, and medicine. We anticipate that RPE will advance as a versatile analytical tool for enhanced biosensing using Raman and fluorescence analysis in various biological contexts.

Machine Learning-Driven SERS Nanoendoscopy and Optophysiology

A frontier of analytical sciences is centered on the continuous measurement of molecules in or near cells, tissues, or organs, within the biological context in situ, where the molecular-level information is indicative of health status, therapeutic efficacy, and fundamental biochemical function of the host. Following the completion of the Human Genome Project, current research aims to link genes to functions of an organism and investigate how the environment modulates functional properties of organisms. New analytical methods have been developed to detect chemical changes with high spatial and temporal resolution, including minimally invasive surface-enhanced Raman scattering (SERS) nanofibers using the principles of endoscopy (SERS nanoendoscopy) or optical physiology (SERS optophysiology). Given the large spectral data sets generated from these experiments, SERS nanoendoscopy and optophysiology benefit from advances in data science and machine learning to extract chemical information from complex vibrational spectra measured by SERS. This review highlights new opportunities for intracellular, extracellular, and in vivo chemical measurements arising from the combination of SERS nanosensing and machine learning.

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.

A targeted fluorescent nanosensor for ratiometric pH sensing at the cell surface

The correlation between altered extracellular pH and various pathological conditions, including cancer, inflammation and metabolic disorders, is well known. Bulk pH measurements cannot report the extracellular pH value at the cell surface. However, there is a limited number of suitable tools for measuring the extracellular pH of cells with high spatial resolution, and none of them are commonly used in laboratories around the world. In this study, a versatile ratiometric nanosensor for the measurement of extracellular pH was developed. The nanosensor consists of biocompatible polystyrene nanoparticles loaded with the pH-inert reference dye Nile red and is surface functionalized with a pH-responsive fluorescein dye. Equipped with a targeting moiety, the nanosensor can adhere to cell membranes, allowing direct measurement of extracellular pH at the cell surface. The nanosensor exhibits a sensitive ratiometric pH response within the range of 5.5-9.0, with a calculated pKa of 7.47. This range optimally covers the extracellular pH (pHe) of most healthy cells and cells in which the pHe is abnormal, such as cancer cells. In combination with the nanosensors ability to target cell membranes, its high robustness, reversibility and its biocompatibility, the pHe nanosensor proves to be well suited for in-situ measurement of extracellular pH, even over extended time periods. This pH nanosensor has the potential to advance biomedical research by improving our understanding of cellular microenvironments, where extracellular pH plays an important role.

Hydrogel-Coated SERS Microneedles for Drug Monitoring in Dermal Interstitial Fluid

In vivo drug monitoring is crucial for evaluating the effectiveness and safety of drug treatment. Blood sampling and analysis is the current gold standard but needs professional skills and cannot meet the requirements of point-of-care testing. Dermal interstitial fluid (ISF) showed great potential to replace blood for in vivo drug monitoring; however, the detection was challenging, and the drug distribution behavior in ISF was still unclear until now. In this study, we proposed surface-enhanced Raman spectroscopy (SERS) microneedles (MNs) for the painless and real-time analysis of drugs in ISF after intravenous injection. Using methylene blue (MB) and mitoxantrone (MTO) as model drugs, the innovative core-satellite structured Au@Ag SERS substrate, hydrogel coating over the MNs, rendered sensitive and quantitative drug detection in ISF of mice within 10 min. Based on this technique, the pharmacokinetics of the two drugs in ISF was investigated and compared with those in blood, where the drugs were analyzed via liquid chromatography-mass spectrometry. It was found that the MB concentration in ISF and blood was comparable, whereas the concentration of MTO in ISF was 2-3 orders of magnitude lower than in blood. This work proposed an efficient tool for ISF drug monitoring. More importantly, it experimentally proved that the penetration ratio of blood to ISF was drug-dependent, providing insightful information into the potential of ISF as a blood alternative for in vivo drug detection.

Block copolymer-templated surface-enhanced Raman scattering-active nanofibers for hydrogen sulfide detection

Metabolic disorders involving endogenous H2S have been linked to a variety of serious human diseases, particularly cancer. In this study, we employed nanofibers with surface-enhanced Raman scattering (SERS) activity for the detection of H2S within live cells. These nanofibers were chosen for their minimal invasiveness, high spatial resolution, and enhanced SERS sensitivity. To improve the performance of SERS, highly sensitive core-shell multibranched-Au NPs (MBAuNP)@Ag NPs were decorated on the nanofibers as SERS tags for H2S detection. A SERS probe named MBN, embedded between the Au core and Ag shell, was utilized for quantitative detection. These nanofibers exhibited excellent reproducibility (relative standard deviation (RSD) within 5.7 %) and demonstrated a strong linear relationship with sulfide concentrations ranging from 50 nM to 1 μM, with an estimated detection limit of 0.12 nM. As a proof of concept, the aforementioned nanofibers were successfully applied to detect endogenous H2S in living cells, offering a potential analytical method in the related research of detection.

Surface-enhanced Raman scattering sensing for detection and mapping of key cellular biomarkers

Cellular biomarkers mainly contain proteins, nucleic acids, glycans and many small molecules including small biomolecule metabolites, reactive oxygen species and other cellular chemical entities. The detection and mapping of the key cellular biomarkers can effectively help us to understand important cellular mechanisms associated with physiological and pathological processes, which greatly promote the development of clinical diagnosis and disease treatment. Surface-enhanced Raman scattering (SERS) possesses high sensitivity and is free from the influence of strong self-fluorescence in living systems as well as the photobleaching of the dyes. It exhibits rich and narrow chemical fingerprint spectra for multiplexed detection, and has become a powerful tool to detect and map cellular biomarkers. In this review, we present an overview of recent advances in the detection and mapping of different classes of cellular biomarkers based on SERS sensing. These advances fully confirm that the SERS-based sensors and sensing methods have great potential for the exploration of biological mechanisms and clinical applications. Additionally, we also discuss the limitations of present research and the future developments of the SERS technology in this field.

Recent advances in the design of intracellular pH sensing nanoprobes based on organic and inorganic materials

In the biological system, the intracellular pH (pHi) plays an important role in regulating diverse physiological activities, including enzymatic action, ion transport, cell proliferation, metabolism, and programmed cell death. The monitoring of pH inside living cells is also crucial for studying cellular events such as phagocytosis, endocytosis, and receptor-ligand internalization. Furthermore, some organelles, viz., endosomes and lysosomes, have intracompartmental pH, which is critical for maintaining the stability of protein structure and function. The dysfunction and abnormal pH regulation can result in terminal diseases such as cancer, Alzheimer, and so forth. Therefore, the accuracy of intracellular pH measurement is always the top priority and demands cutting-edge research and analysis. Such techniques, such as Raman spectroscopy and fluorescence imaging, preferably use nanotechnology due to their remarkable advantages, such as a non-invasive approach and providing accuracy, repeatability, and reproducibility. In the past decades, there have been numerous attempts to design and construct non-invasive organic and inorganic materials-based nanoprobes for pHi sensing. For Raman-based techniques, metal nanostructures such as Au/Ag/Cu nanoparticles are utilized to enhance the signal intensity. As for the fluorescence-based studies, the organic-based small molecules, such as dyes, show higher sensitivity toward pH. However, they possess several drawbacks, including high photobleaching rate, and autofluorescence background signals. To this end, there are alternative nanomaterials proposed, including semiconductor quantum dots (QDs), carbon QDs, upconversion nanoparticles, and so forth. Moreover, the fluorescence technique allows for ratiometric measurement of pHi, which as a result, offers a reliable calibration curve. This timely review will critically examine the current progression in the existing nanoprobes. In addition, based on our knowledge and available research findings, we provide a brief future outlook that may advance the state-of-the-art methodologies for pHi sensing.

Recent Development of Polymer Nanofibers in the Field of Optical Sensing

In recent years, owing to the continuous development of polymer nanofiber manufacturing technology, various nanofibers with different structural characteristics have emerged, allowing their application in the field of sensing to continually expand. Integrating polymer nanofibers with optical sensors takes advantage of the high sensitivity, fast response, and strong immunity to electromagnetic interference of optical sensors, enabling widespread use in biomedical science, environmental monitoring, food safety, and other fields. This paper summarizes the research progress of polymer nanofibers in optical sensors, classifies and analyzes polymer nanofiber optical sensors according to different functions (fluorescence, Raman, polarization, surface plasmon resonance, and photoelectrochemistry), and introduces the principles, structures, and properties of each type of sensor and application examples in different fields. This paper also looks forward to the future development directions and challenges of polymer nanofiber optical sensors, and provides a reference for in-depth research of sensors and industrial applications of polymer nanofibers.

A 3D-printed SERS bionic taster for dynamic tumor metabolites detection

The variation of tumor-associated metabolites in extracellular microenvironment timely reflects the development, the progression and the treatment of cancers. Conventional methods for metabolite detection lack the efficiency to grasp the dynamic metabolic alterations. Herein, we developed a SERS bionic taster which enabled real-time analysis of extracellular metabolites. The instant information of cell metabolism was provided by the responsive Raman reporters, which experienced SERS spectral changes upon metabolite activation. Such a SERS sensor was integrated into a 3D-printed fixture which fits the commercial-standard cell culture dishes, allowing in-situ acquisition of the vibrational spectrum. The SERS taster can not only accomplish simultaneous and quantitative analysis of multiple tumor-associated metabolites, but also fulfill the dynamic monitoring of cellular metabolic reprogramming, which is expected to become a promising tool for investigating cancer biology and therapeutics.

Dual-functional gold nanorods micro pattern guiding cell alignment and cellular microenvironment monitoring

Surface topography has become a powerful tool to control cell behaviors, however, it's still difficult to monitor cellular microenvironment changes during topography-induced cell responses. Here, a dual-functional platform integrating cell alignment with extracellular pH (pHe) measurement is proposed. The platform is fabricated by assembling gold nanorods (AuNRs) into micro pattern via wettability difference interface method, which provides topographical cues and surface-enhanced Raman scattering (SERS) effect for cell alignment and biochemical detection respectively. Results demonstrate that contact guidance and cell morphology changes are achieved by the AuNRs micro pattern, and pHe are also obtained by the changes of SERS spectra during cell alignment, where the pHe near cytoplasm is lower than nucleus, revealing the heterogeneity of extracellular microenvironment. Moreover, a correlation between lower extracellular pH and higher cell migration ability is revealed, and AuNRs micro pattern can differentiate cells with different migration ability, which may be an inheritable character during cell division. Furthermore, mesenchymal stem cells response dramatically to AuNRs micro pattern, showing different morphology and increased pHe level, offering the potential of impacting stem cell differentiation. This approach provides a new idea for the research of cell regulation and response mechanism.

Surface-Roughened SERS-Active Single Silver Nanowire for Simultaneous Detection of Intracellular and Extracellular pHs

The simultaneous and accurate detection of intracellular pH (pH_i) and extracellular pH (pH_e) is essential for studying the complex physiological activities of cancer cells and exploring pH-related therapeutic mechanisms. Here, we developed a super-long silver nanowire-based surface-enhanced Raman scattering (SERS) detection strategy for simultaneous sensing of pH_i and pH_e. A surface-roughened silver nanowire (AgNW) with a high aspect ratio is prepared at a nanoelectrode tip using a Cu-mediated oxidation process, which is then modified by pH-sensitive 4-mercaptobenzoic acid (4-MBA) to form 4-MBA@AgNW as a pH sensing probe. With the assistance of a 4D microcontroller, 4-MBA@AgNW is efficient in simultaneously detecting pH_i and pH_e in both 2D and 3D culture cancer cells by SERS, with minimal invasiveness, high sensitivity, and spatial resolution. Further investigation proves that the surface-roughened single AgNW can also be used in monitoring the dynamic variation of pH_i and pH_e of cancer cells upon stimulation with anticancer drugs or under a hypoxic environment.

Microelectromechanical Microsystems-Supported Photothermal Immunoassay for Point-of-Care Testing of Aflatoxin B1 in Foodstuff

Accurate identification of acutely toxic and low-fatality mycotoxins on a large scale in a quick and cheap manner is critical for reducing population mortality. Herein, a portable photothermal immunosensing platform supported by a microelectromechanical microsystem (MEMS) without enzyme involvement was reported for point-of-care testing of mycotoxins (in the case of aflatoxin B₁, AFB₁) in food based on the precise satellite structure of Au nanoparticles. The synthesized Au nanoparticles with a well-defined, graded satellite structure exhibited a significantly enhanced photothermal response and were coupled by AFB₁ antibodies to form signal conversion probes by physisorption for further target-promoted competitive responses in microplates. In addition, a coin-sized miniature NIR camera device was constructed for temperature acquisition during target testing based on advanced MEMS fabrication technology to address the limitation of expensive signal acquisition components of current photothermal sensors. The proposed MEMS readout-based microphotothermal test method provides excellent AFB₁ response in the range of 0.5-500 ng g^-1 with detection limits as low as 0.27 ng g^-1. In addition, the main reasons for the efficient photothermal transduction efficiency of Au with different graded structures were analyzed by finite element simulations, providing theoretical guidance for the development of new Au-based photothermal agents. In conclusion, the proposed portable micro-photothermal test system offers great potential for point-of-care diagnostics for residents, which will continue to facilitate immediate food safety identification in resource-limited regions.

Skin Interstitial Fluid-Based SERS Tags Labeled Microneedles for Tracking of Peritonitis Progression and Treatment Effect

Skin interstitial fluid (ISF)-based microneedle (MN) sensing has recently exhibited wide promise for the minimally invasive and painless diagnosis of diseases. However, it is still a great challenge to diagnose more disease types due to the limited in situ sensing techniques and insufficient ISF biomarker sources. Herein, ISF is employed to pioneer the tracking of acute peritonitis progression via surface-enhanced Raman scattering (SERS) tags labeled MNs patch technique. Densely deposited core-satellite gold nanoparticles and 3-mercaptophenylboronic acid as a Raman reporter enable the developed MNs patch with high sensitivity and selectivity in the determination of H₂O₂, an indicator of peritonitis development. Importantly, the MNs patch not only reliably tracks the different states of peritonitis but also evaluates the efficacy of drugs in the treatment of peritonitis, as evidenced by the altered SERS signal consistent with plasma pro-inflammatory factor (TNF-α) and peritoneum pathological manifestations. Interestingly, the major source of H₂O₂ in ISF of acute peritonitis investigated may not be through conventional blood capillary filtration pathway. This work provides a new route and technique for the early diagnosis of acute peritonitis and the evaluation of drug therapy effects. The developed MNs patch is promising to serve as a universal sensing tool to greatly enrich the variety and prospect of ISF-based disease diagnosis.

A Dual-Mode NADH Biosensor Based on Gold Nanostars Decorated CoFe2 Metal-Organic Frameworks to Reveal Dynamics of Cell Metabolism

Nicotinamide adenine dinucleotide (NADH) is central to metabolism and implicated in various diseases. Herein, nanohybrids of gold nanostars and metal-organic frameworks are devised and demonstrated as a dual-mode NADH sensor, for which colorimetric detection is enabled by its peroxidase-like nanozyme property and Raman detection is realized by its surface-enhanced Raman scattering property with the detection limit as low as 28 pM. More importantly, this probe enables real-time SERS monitoring in living cells, providing a unique tool to investigate dynamic cellular processes involving NADH. Our experiments reveal that metabolism dynamics is accelerated by glucose and is much higher in cancerous cells. The SERS results can also be verified by the colorimetric detection. This sensor provides a new potential to detect biomarkers and their dynamics in situ.

Optical Sensing Strategies for Probing Single-Cell Secretion

Measuring cell secretion events is crucial to understand the fundamental cell biology that underlies cell-cell communication, migration, proliferation, and differentiation. Although strategies targeting cell populations have provided significant information about live cell secretion, they yield ensemble profiles that obscure intrinsic cell-to-cell variations. Innovation in single-cell analysis has made breakthroughs allowing accurate sensing of a wide variety of secretions and their release dynamics with high spatiotemporal resolution. This perspective focuses on the power of single-cell protocols to revolutionize cell-secretion analysis by allowing real-time and real-space measurements on single live cell resolution. We begin by discussing recent progress on single-cell bioanalytical techniques, specifically optical sensing strategies such as fluorescence-, surface plasmon resonance-, and surface-enhanced Raman scattering-based strategies, capable of in situ real-time monitoring of single-cell released ions, metabolites, proteins, and vesicles. Single-cell sensing platforms which allow for high-throughput high-resolution analysis with enough accuracy are highlighted. Furthermore, we discuss remaining challenges that should be addressed to get a more comprehensive understanding of secretion biology. Finally, future opportunities and potential breakthroughs in secretome analysis that will arise as a result of further development of single-cell sensing approaches are discussed.

Surface-enhanced Raman probes based on gold nanomaterials for in vivo diagnosis and imaging

Surface-enhanced Raman scattering (SERS) has received considerable attention from researchers due to its high molecular specificity, high sensitivity, non-invasive and multiplexing. Recently, various metal substrates have been exploited for SERS analysis and imaging. Among them, gold nanomaterials are important SERS substrates with outstanding surface plasmon resonance effects, structural adjustability and good biocompatibility, making them widely used in biomedical diagnosis and clinical fields. In this minireview, we discuss the latest progress about the application of gold-based nanomaterials as SERS probes in biomedical research, primarily for in vivo disease diagnosis and imaging. This review mainly includes the basic shapes and morphologies of gold based SERS probes, such as gold nanoparticles (AuNPs), gold nanorods (AuNRs), gold nanostars (AuNSs), as well as other gold nanostructures. Finally, a brief outlook for the future development of SERS technique in the context of efficient diagnostics and therapy guidance is provided. We hope that this minireview will facilitate the design and future development of Surface-enhanced Raman probes based on gold nanomaterials.

Human metabolite detection by surface-enhanced Raman spectroscopy

Metabolites are important biomarkers in human body fluids, conveying direct information of cellular activities and physical conditions. Metabolite detection has long been a research hotspot in the field of biology and medicine. Surface-enhanced Raman spectroscopy (SERS), based on the molecular “fingerprint” of Raman spectrum and the enormous signal enhancement (down to a single-molecule level) by plasmonic nanomaterials, has proven to be a novel and powerful tool for metabolite detection. SERS provides favorable properties such as ultra-sensitive, label-free, rapid, specific, and non-destructive detection processes. In this review, we summarized the progress in recent 10 years on SERS-based sensing of endogenous metabolites at the cellular level, in tissues, and in biofluids, as well as drug metabolites in biofluids. We made detailed discussions on the challenges and optimization methods of SERS technique in metabolite detection. The combination of SERS with modern biomedical technology were also anticipated.

Au Polyhedron Array with Tunable Crystal Facets by PVP-Assisted Thermodynamic Control and Its Sharp Shape As Well As High-Energy Exposed Planes Co-Boosted SERS Activity

A route is developed for directly growing 2D Au polyhedron arrays with controllable exposed facets of polyhedron by utilizing the substrate-supported 2D Au quasi-spherical nanoparticle arrays as the Au seed arrays, which cannot be realized by traditional lithography. In the reaction system, polyvinyl pyrrolidone (PVP) plays a vital role in guiding the reduced Au atoms and stabilizing the substrate-supported Au seeds. More importantly, by thermodynamic control, PVP as a capping agent can further direct the formation of {111} facets. The key to guarantee the integrity and periodicity of array is a proper reduction of Au ions and low growth rate of crystal. Benefiting from the higher electric field intensity near the sharp vertexes and edges of Au polyhedra and the exposed {110} facets with high energy, the Au polyhedron array with {110} facets encasing polyhedron exhibits good, stable surface enhanced Raman scattering activity toward 4-aminothiophenol among the involved arrays. The proposed fabrication approach tremendously enriches the structural diversity of Au nanoarrays on substrates and greatly overcomes the shortcoming of traditional lithography.

A photochemical approach to anchor Au NPs on MXene as a prominent SERS substrate for ultrasensitive detection of chlorpromazine

As a novel two-dimensional (2D) material, metal carbide (MXene) has been identified as a hotspot research topic in the field of surface-enhanced Raman spectroscopy (SERS). Herein, we report the increment of SERS activity of titanium carbide (TiC) by incorporation of gold nanoparticles (Au NPs) by a facile photoreduction process for the detection of antipsychotic drug. TiC anchored with Au NPs produce a remarkable SERS enhancement by the synergistic action of chemical and electromagnetic mechanisms. The hotspots are formed in the nanometer-scale gaps between Au NPs on the TiC surface for the effective interaction with probe molecules. The proposed TiC/Au-NPs SERS substrate was employed for the detection of chlorpromazine (CPZ) with the wide linear range of 10^-1-10^-10 M and the ultra-low limit of detection of 3.92 × 10^-11 M. Besides, the SERS effect of the optimized TiC/Au-NPs for the 532 nm excitation exhibits the enhancement factor in the order of 10⁹ with the relative standard deviation of < 13% for the uniformity and < 8.80% for the reproducibility. To ensure the practical feasibility of the proposed TiC/Au-NPs SERS substrate, the spike and recovery method was used for the detection of CPZ in human biological fluids like urine and saliva. This work can open up a new approach to improve the SERS activity of MXene-based SERS substrate for practical applications, especially the determination of antipsychotic drugs in environmental pollution management.

Potentiometric pH Nanosensor for Intracellular Measurements: Real-Time and Continuous Assessment of Local Gradients

We present a pH nanosensor conceived for single intracellular measurements. The sensing architecture consisted of a two-electrode system evaluated in the potentiometric mode. We used solid-contact carbon nanopipette electrodes tailored to produce both the indicator (pH nanosensor) and reference electrodes. The indicator electrode was a membrane-based ion-selective electrode containing a receptor for hydrogen ions that provided a favorable selectivity for intracellular measurements. The analytical features of the pH nanosensor revealed a Nernstian response (slope of −59.5 mV/pH unit) with appropriate repeatability and reproducibility (variation coefficients of <2% for the calibration parameters), a fast response time (<5 s), adequate medium-term drift (0.7 mV h^–1), and a linear range of response including physiological and abnormal cell pH levels (6.0–8.5). In addition, the position and configuration of the reference electrode were investigated in cell-based experiments to provide unbiased pH measurements, in which both the indicator and reference electrodes were located inside the same cell, each of them inside two neighboring cells, or the indicator electrode inside the cell and the reference electrode outside of (but nearby) the studied cell. Finally, the pH nanosensor was applied to two cases: (i) the tracing of the pH gradient from extra-to intracellular media over insertion into a single PC12 cell and (ii) the monitoring of variations in intracellular pH in response to exogenous administration of pharmaceuticals. It is anticipated that the developed pH nanosensor, which is a label-free analytical tool, has high potential to aid in the investigation of pathological states that manifest in cell pH misregulation, with no restriction in the type of targeted cells.

Recent Progress of SERS Nanoprobe for pH Detecting and Its Application in Biological Imaging

As pH value almost affects the function of cells and organisms in all aspects, in biology, biochemical and many other research fields, it is necessary to apply simple, intuitive, sensitive, stable detection of pH and base characteristics inside and outside the cell. Therefore, many research groups have explored the design and application of pH probes based on surface enhanced Raman scattering (SERS). In this review article, we discussed the basic theoretical background of explaining the working mechanism of pH SERS sensors, and also briefly described the significance of cell pH measurement, and simply classified and summarized the factors that affected the performance of pH SERS probes. Some applications of pH probes based on surface enhanced Raman scattering in intracellular and extracellular pH imaging and the combination of other analytical detection techniques are described. Finally, the development prospect of this field is presented.

Hybridizing Carbon-Based Dot-Capped Manganese Dioxide Nanosheets and Gold Nanoparticles as a Highly Sensitive Surface-Enhanced Raman Scattering Substrate

Surface-enhanced Raman Scattering (SERS) is a sensitive and nondestructive technique that provides fingerprint structural information of molecules. Designing and constructing sensitive and stable SERS substrates is of great significance for the application of the technique. In this study, single-layer carbon-based dots (CDs) are used as capping agents to synthesize gold nanoparticles (AuNPs/CDs) and manganese dioxide nanosheets (MnO₂/CDs), which are then hybridized through a simple cocentrifugation method. After the hybridization, the monodispersive AuNPs/CDs aggregate obviously into some clusters exhibiting strong SERS activity due to the electromagnetic "hot spots" effect, and the MnO₂/CDs also show outstanding SERS activity due to the charge-transfer resonance effect. The obtained nanohybrids (MnO₂/CDs/AuNPs) with robust chemical stability combine well with the electromagnetic enhancement of AuNPs/CDs and chemical enhancement of MnO₂/CDs, leading to an ultrahigh enhancement factor of 3.9 × 10⁸. Based on the novel SERS substrate, a sensitive and rapid sensing system for the detection of malachite green is developed, with a low detection limit of 1 × 10^-9 M. This work provides a valuable model for designing and fabricating high-performance SERS substrates.

Electrospun polymeric nanofiber decorated with sea urchin-like gold nanoparticles as an efficient and stable SERS platform

Surface enhanced Raman scattering (SERS)-based nanoprobes have been used as well-established analytical tools enabling single-molecule detection. In this work, we report a facile method to decorate sea urchin-like gold nanoparticles (SUGNPs) on the surface of PMMA/P4VP nanofibers. Firstly, PMMA/P4VP nanofibers within the submicrometer size range were prepared by applying the electrospinning technique. Then, the incorporation of SUGNPs on the surface of PMMA/P4VP nanofiber was achieved by immersing PMMA/P4VP nanofiber into freshly prepared SUGNP aqueous solution through the specific Au-N interactions. The as-fabricated SUGNP-coated PMMA/P4VP nanofibers exhibited good sensitivity and reproducibility in SERS measurements with the relative standard deviation down to 6.6%, by employing 4-mercaptobenzoic acid as a probe molecule with 30 min of soaking time. Hence, we envisage that the SUGNP-coated PMMA/P4VP nanofibers can act as efficient and stable SERS substrates for potential applications in molecular detection as well as chemical and biological analysis.

Surface-Enhanced Raman Scattering Optophysiology Nanofibers for the Detection of Heavy Metals in Single Breast Cancer Cells

Mercury(II) ions (Hg²⁺) and silver ions (Ag⁺) are two of the most hazardous pollutants causing serious damage to human health. Here, we constructed surface-enhanced Raman scattering (SERS)-active nanofibers covered with 4-mercaptopyridine (4-Mpy)-modified gold nanoparticles to detect Hg²⁺ and Ag⁺. Experimental evidence suggests that the observed spectral changes originate from the combined effect of (i) the coordination between the nitrogen on 4-Mpy and the metal ions and (ii) the 4-Mpy molecular orientation (from flatter to more perpendicular with respect to the metal surface). The relative intensity of a pair of characteristic Raman peaks (at ∼428 and ∼708 cm^-1) was used to quantify the metal ion concentration, greatly increasing the reproducibility of the measurement compared to signal-on or signal-off detection based on a single SERS peak. The detection limit of this method for Hg²⁺ is lower than that for the Ag⁺ (5 vs 100 nM), which can be explained by the stronger interaction energy between Hg²⁺ and N compared to Ag⁺ and N, as demonstrated by density functional theory calculations. The Hg²⁺ and Ag⁺ ions can be masked by adding ethylenediaminetetraacetate and Cl^-, respectively, to the Hg²⁺ and Ag⁺ samples. The good sensitivity, high reproducibility, and excellent selectivity of these nanosensors were also demonstrated. Furthermore, detection of Hg²⁺ in living breast cancer cells at the subcellular level is possible, thanks to the nanometric size of the herein described SERS nanosensors, allowing high spatial resolution and minimal cell damage.

Recent Advances in the Fabrication and Functionalization of Flexible Optical Biosensors: Toward Smart Life-Sciences Applications

Over the last 30 years, optical biosensors based on nanostructured materials have obtained increasing interest since they allow the screening of a wide variety of biomolecules with high specificity, low limits of detection, and great sensitivity. Among them, flexible optical platforms have the advantage of adapting to non-planar surfaces, suitable for in vivo and real-time monitoring of diseases and assessment of food safety. In this review, we summarize the newest and most advanced platforms coupling optically active materials (noble metal nanoparticles) and flexible substrates giving rise to hybrid nanomaterials and/or nanocomposites, whose performances are comparable to the ones obtained with hard substrates (e.g., glass and semiconductors). We focus on localized surface plasmon resonance (LSPR)-based and surface-enhanced Raman spectroscopy (SERS)-based biosensors. We show that large-scale, cost-effective plasmonic platforms can be realized with the currently available techniques and we emphasize the open issues associated with this topic.

Multiplexed SERS Detection of Microcystins with Aptamer-Driven Core-Satellite Assemblies

We describe surface-enhanced Raman spectroscopy (SERS) aptasensors that can indirectly detect MC-LR and MC-RR, individually or simultaneously, in natural water and in algal culture. The sensor is constructed from nanoparticles composed of successive layers of Au core-SERS label-silver shell-gold shell (Au@label@Ag@Au NPs), functionalized on the outer Au surface by MC-LR and/or MC-RR aptamers. These NPs are immobilized on asymmetric Au nanoflowers (AuNFs) dispersed on planar silicon substrates through DNA hybridization of the aptamers and capture DNA sequences with which the AuNFs are functionalized, thereby forming core-satellite nanostructures on the substrates. This construction led to greater electromagnetic (EM) field enhancement of the Raman label-modified region, as supported by finite-difference time-domain (FDTD) simulations of the core-satellite assembly. In the presence of MC-LR and/or MC-RR, the aptamer-functionalized NPs dissociate from the AuNFs because of the stronger affinity of the aptamers with the MCs, which decreases the SERS signal, thus allowing indirect detection of the MCs. The improved SERS sensitivity significantly decreased the limit of detection (LOD) for separate MC-LR detection (0.8 pM) and for multiplex detection (1.5 pM for MC-LR and 1.3 pM for MC-RR), compared with other recently reported SERS-based methods for MC-LR detection. The aptasensors show excellent selectivity to MC-LR/MC-RR and excellent recoveries (96-105%). The use of these SERS aptasensors to monitor MC-LR production over 1 week in a culture medium of M. aeruginosa cells demonstrates the applicability of the sensors in a realistic environment.

Intracellular optical probing with gold nanostars

Gold nanostars are important nanoscopic tools in biophotonics and theranostics. To understand the fate of such nanostructures in the endolysosomal system of living cells as an important processing route in biotechnological approaches, un-labelled, non-targeted gold nanostars synthesized using HEPES buffer were studied in two cell lines. The uptake of the gold nanostructures leads to cell line-dependent intra-endolysosomal agglomeration, which results in a greater enhancement of the local optical fields than those around individual nanostars and near aggregates of spherical gold nanoparticles of the same size. As demonstrated by non-resonant surface-enhanced Raman scattering (SERS) spectra in the presence and absence of aggregation, the spectroscopic signals of molecules are of very similar strength over a wide range of concentrations, which is ideal for label-free vibrational characterization of cells and other complex environments. In 3T3 and HCT-116 cells, SERS data were analyzed together with the properties of the intracellular nanostar agglomerates. Vibrational spectra indicate that the processing of nanostars by cells and their interaction with the surrounding endolysosomal compartment is connected to their morphological properties through differences in the structure and interactions in their intracellular protein corona. Specifically, different intracellular processing was found to result from a different extent of hydrophobic interactions at the pristine gold surface, which varies for nanostars of different spike lengths. The sensitive optical monitoring of surroundings of nanostars and their intracellular processing makes them a very useful tool for optical bionanosensing and therapy.

Target-Triggered Assembly in a Nanopipette for Electrochemical Single-Cell Analysis

Engineered nanopipette tools have recently emerged as a powerful approach for electrochemical nanosensing, which has major implications in both fundamental biological research and biomedical applications. Herein, we describe a generic method of target-triggered assembly of aptamers in a nanopipette for nanosensing, which is exemplified by sensitive and rapid electrochemical single-cell analysis of adenosine triphosphate (ATP), a ubiquitous energy source in life and important signaling molecules in many physiological processes. Specifically, a layer of thiolated aptamers is immobilized onto a Au-coated interior wall of a nanopipette tip. With backfilled pairing aptamers, the engineered nanopipette is then used for probing intracellular ATP via the ATP-dependent linkage of the split aptamers. Due to the higher surface charge density from the aptamer assembly, the nanosensor would exhibit an enhanced rectification signal. Besides, this ATP-responsive nanopipette tool possesses excellent selectivity and stability as well as high recyclability. This work provides a practical single-cell nanosensor capable of intracellular ATP analysis. More generally, integrated with other split recognition elements, the proposed mechanism could serve as a viable basis for addressing many other important biological species.

In Situ Growth of AuNPs on Glass Nanofibers for SERS Sensors

It is challenging to fabricate plasmonic nanosensors on high-curvature surfaces with high sensitivity and reproducibility at low cost. Here, we report a facile and straightforward strategy, based on an in situ growth technique, for fabricating glass nanofibers covered by asymmetric gold nanoparticles (AuNPs) with tunable morphologies and adjustable spacings, leading to much improved surface-enhanced Raman scattering (SERS) sensitivity because of hotspots generated by the AuNP surface irregularities and adjacent AuNP coupling. First, nanosensors covered with uniform and well-dispersed citrate-capped spherical AuNPs were constructed using a polystyrene-b-poly(4-vinylpyridine) (PS-P4VP, with 33 mol % P4VP content and 61 kg/mol total molecular weight) block copolymer brush-layer templating method, and then, the deposited AuNPs were grown to asymmetric AuNPs. AuNP morphologies and hence the optical characteristics of AuNP-covered glass nanofibers were easily controlled by the choice of experimental parameters, such as the growth time and growth solution composition. In particular, tunable AuNP average diameters between about 40 and 80 nm with AuNP spacings between about 50 and 1 nm were achieved within 15 min of growth. The SERS sensitivity of branched AuNP-covered nanofibers (3 min growth time) was demonstrated to be more than threefold more intense than that of the original spherical AuNP-covered nanofibers using a 633 nm laser. Finite-difference time-domain simulations were performed, showing that the electric field enhancement is highest for intermediate AuNP diameters. Furthermore, SERS applications of these nanosensors for H₂O₂ detection and pH sensing were demonstrated, offering appealing and promising candidates for real-time monitoring of extra/intracellular species in vitro and in vivo.

A blueprint for performing SERS measurements in tissue with plasmonic nanofibers

Plasmonic nanostructures have found increasing utility due to the increased popularity that surface-enhanced Raman scattering (SERS) has achieved in recent years. SERS has been incorporated into an ever-growing list of applications, with bioanalytical and physiological analyses having emerged as two of the most popular. Thus far, the transition from SERS studies of cultured cells to SERS studies involving tissue has been gradual and limited. In most cases, SERS measurements in more intact tissue have involved nanoparticles distributed throughout the tissue or localized to specific regions via external functionalization. Performing highly localized measurements without the need for global nanoparticle uptake or specialized surface modifications would be advantageous to the expansion of SERS measurements in tissue. To this end, this work provides critical insight with supporting experimental evidence into performing SERS measurements with nanosensors inserted in tissues. We address two critical steps that are otherwise underappreciated when other approaches to performing SERS measurements in tissue are used. Specifically, we demonstrate two mechanical routes for controlled positioning and inserting the nanosensors into the tissue, and we discuss two means of focusing on the nanosensors both before and after they are inserted into the tissue. By examining the various combinations of these steps, we provide a blueprint for performing SERS measurements with nanosensors inserted in tissue. This blueprint could prove useful for the general development of SERS as a tool for bioanalytical and physiological studies and for more specialized techniques such as SERS-optophysiology.

Design of Surface Enhanced Raman Scattering (SERS) Nanosensor Array

An advanced Surface-Enhanced Raman Scattering (SERS) Nanosensor Array, dedicated to serve in the future as a pH imager for the real-time detection of chemical reaction, is presented. The full flow of elementary steps—architecture, design, simulations, fabrication, and preliminary experimental results of structural characterization (Focused Ion Beam (FIB), TEM and SEM)—show an advanced SERS pixel array that is capable of providing spatially resolved measurements of chemical pH in a fluid target that became more than desirable in this period. Ultimately, the goal will be to provide real-time monitoring of a chemical reaction. The pixels consist of a nanostructured substrate composed of an array of projections or cavities. The shape of the nanostructures and the thickness of the metallic (Ag or Au) layer can be tuned to give maximal enhancement at the desired wavelength. The number and arrangement of nanostructures is optimized to obtain maximal responsivity.

From single cells to complex tissues in applications of surface-enhanced Raman scattering

This tutorial review explores how three of the most common methods for introducing nanoparticles to single cells for surface-enhanced Raman scattering measurements can be adapted for experiments with complex tissues.