Nanopipette-Based SERS Aptasensor for Subcellular Localization of Cancer Biomarker in Single Cells

Tailoring strategies of SERS tags-based sensors for cellular molecules detection and imaging

As an emerging nanoprobe, surface enhanced Raman scattering (SERS) tags hold significant promise in sensing and bioimaging applications due to their attractive merits of anti-photobleaching ability, high sensitivity and specificity, multiplex, and low background capabilities. Recently, several reviews have proposed the application of SERS tags in different fields, however, the specific sensing strategies of SERS tags-based sensors for cellular molecules have not yet been systematically summarized. To provide beneficial and comprehensive insights into the advanced SERS tags technique at the cellular level, this review systematically elaborated on the latest advances in SERS tags-based sensors for cellular molecules detection and imaging. The general SERS tags-based sensing strategies for biomolecules and ions were first introduced according to molecular classes. Then, aiming at such molecules located in the extracellular, cellular membrane and intracellular regions, the tailored strategies by designing and manipulating SERS tags were summarized and explored through several key examples. Finally, the challenges and perspectives of developing high performance of advanced SERS tags were briefly discussed to provide effective guidance for further development and extended applications.

Development of a SERS aptasensor for the determination of L-theanine using a noble metal nanoparticle-magnetic nanospheres composite

An ultrasensitive surface-enhanced Raman spectroscopy (SERS) aptamer sensor (aptasensor) using a noble metal nanoparticle-magnetic nanospheres composite was developed for L-theanine detection. It makes use of Fe3O4@Au MNPs and Au@Ag NPs embedded with the Raman reporter 4-mercaptobenzoic acid (4MBA). Au@4MBA@Ag NPs modified by aptamer and Fe3O4@Au MNPs modified by cDNA created the aptasensor with the strongest Raman signal of 4MBA through the specific binding of the aptamer. With the preferred binding of L-theanine aptamer to L-theanine, Au@4MBA@Ag NPs were released from Fe3O4@Au MNPs, causing a linear decrease in SERS intensity to achieve the SERS detection of the L-theanine. The SERS peak of 4MBA at 1078 cm-1 was used for quantitative determination. SERS intensity showed a good log-linear relationship within the range 10-10 to 10-6 M of L-theanine. The aptasensor has a high selectivity for L-theanine compared with other twelve tested analytes. Hence, this aptasensor is a promising analytical tool for L-theanine detection. The developed method was applied to the analysis of real samples, demonstrating excellent performance. The comparison with the standard liquid chromatography mass spectrometry method showed an error within 20%.

"Light-On" Fluorescent Nanoprobes for Monitoring Dynamic Distribution of Cellular Nucleolin During Pyroptosis

Nucleolin (NCL) is a multifunctional nuclear protein that plays significant roles in regulating physiological activities of the cells. However, it remains a challenge to monitor the dynamic distribution and expression of nucleolin within living cells during cell stress processes directly. Here, we designed "turn-on" fluorescent nanoprobes composed of specific AS1411 aptamer and nucleus-targeting peptide on gold nanoparticles (AuNPs) to effectively capture and track the NCL distribution and expression during pyroptosis triggered by electrical stimulation (ES). The distribution of nucleolin in the cell membrane and nucleus can be easily observed by simply changing the particle size of the nanoprobes. The present strategy exhibits obvious advantages such as simple operation, low cost, time saving, and suitability for living cell imaging. The ES can induce cancer cell pyroptosis controllably and selectively, with less harm to the viability of normal cells. The palpable cell nuclear stress responses of cancerous cells, including nucleus wrinkling and nucleolus fusion after ES at 1.0 V were obviously observed. Compared with normal cells (MCF-10A), NCL is overexpressed within cancerous cells (MCF-7 cells) using the as-designed nanoprobes, and the ES can effectively inhibit NCL expression within cancerous cells. The developed NCL sensing platform and ES-based methods hold great potential for cellular studies of cancer-related diseases.

Ammonia Toxicity and Associated Protein Oxidation: A Single-Cell Surface Enhanced Raman Spectroscopy Study

Ammonia (NH3) is a commonly used industrial chemical to which exposure at high concentrations can result in severe skin damage. Moreover, high levels of ammonia in the human body can lead to hyperammonemia conditions and enhanced cancer metabolism. In this work, the toxicity mechanism of NH3 has been studied against human dermal fibroblast (HDF) cells using surface-enhanced Raman spectroscopy (SERS). For this purpose, gold nanoparticles of size 50 nm have been prepared and used as probes for Raman signal enhancement, after being internalized inside HDF cells. Following the exposure to ammonia, HDF cells showed a significant variation in the protein ternary structure's signals, demonstrating their denaturation and oxidation process, together with early signs of apoptosis. Meaningful changes were observed especially in the Raman vibrations of sulfur-containing amino acids (cysteine and methionine) together with aromatic residues. Fluorescence microscopy revealed the formation of reactive oxygen and nitrogen species in cells, which confirmed their stressed condition and to whom the causes of protein degradation can be attributed. These findings can provide new insights into the mechanism of ammonia toxicity and protein oxidation at a single-cell level, demonstrating the high potential of the SERS technique in investigating the cellular response to toxic compounds.

Solid-State Nanopores for Biomolecular Analysis and Detection

Advances in nanopore technology and data processing have rendered DNA sequencing highly accessible, unlocking a new realm of biotechnological opportunities. Commercially available nanopores for DNA sequencing are of biological origin and have certain disadvantages such as having specific environmental requirements to retain functionality. Solid-state nanopores have received increased attention as modular systems with controllable characteristics that enable deployment in non-physiological milieu. Thus, we focus our review on summarizing recent innovations in the field of solid-state nanopores to envision the future of this technology for biomolecular analysis and detection. We begin by introducing the physical aspects of nanopore measurements ranging from interfacial interactions at pore and electrode surfaces to mass transport of analytes and data analysis of recorded signals. Then, developments in nanopore fabrication and post-processing techniques with the pros and cons of different methodologies are examined. Subsequently, progress to facilitate DNA sequencing using solid-state nanopores is described to assess how this platform is evolving to tackle the more complex challenge of protein sequencing. Beyond sequencing, we highlight the recent developments in biosensing of nucleic acids, proteins, and sugars and conclude with an outlook on the frontiers of nanopore technologies.

In situ monitoring of ROS secretion from single cells with a dual-nanopore biosensor

We report here a dual-nanopore biosensor based on modulation of surface charge density coupled with a microwell array chip for in situ monitoring of ROS secretion from single MCF-7 cells.

Deciphering the Potential of Multidimensional Carbon Materials for Surface-Enhanced Raman Spectroscopy through Density Functional Theory

Surface-enhanced Raman spectroscopy (SERS) is a potent analytical tool, particularly for molecular identification and structural analysis. Conventional metallic SERS substrates, however, suffer from low reproducibility and compatibility with biological molecules. Recently, metal-free SERS substrates based on chemical enhancement have emerged as a promising alternative with carbon-based materials offering excellent reproducibility and compatibility. Nevertheless, our understanding of carbon materials in SERS remains limited, which hinders their rational design. Here we systematically explore multidimensional carbon materials, including zero-dimensional fullerenes (C60), one-dimensional carbon nanotubes, two-dimensional graphene, and their B-, N-, and O-doped derivatives, for SERS applications. Using density functional theory, we elucidate the nonresonant polarizability-enhanced and resonant charge-transfer-based chemical enhancement mechanisms of these materials by evaluating their static/dynamic polarizability and electron excitation properties. This work provides a critical reference for the future design of carbon-based SERS substrates, opening a new avenue in this field.

Atomic layer deposition (ALD)-constructed TaS2nanoflakes for cancer-related nucleolin detection

Sensitive detection of nucleolin (NCL) is of great significance for the early diagnosis of cancer. In this work, as a new type of two-dimensional (2D) transition metal dichalcogenides (TMDCs), TaS₂nanoflakes (NFs) were precisely constructed by atomic layer deposition (ALD) on carbon fiber paper (CFP) with high specific surface area.In situobservation showed that the nucleation and growth of TaS₂nanoflakes were precisely controlled by the number of ALD cycles, thereby regulating their electrochemical properties. The electrochemical performance of TaS₂NFs was observed in depth, and compared with that of traditional 2D TMDCs. Due to the high surface area and conductivity, anodic/cathodic current of ∼1570μA of TaS₂NFs/CFP can be obtained. Subsequently, an electrochemical biosensor based on ALD-constructed TaS₂NFs/CFP for cancer-related NCL detection was fabricated. Due to the excellent electrochemical performance of TaS₂NFs/CFP, ultrasensitive detection of NCL in the linear range of 0.1 pM-10 nM with a detection limit of 0.034 pM was achieved.

A biodegradable and cofactor self-sufficient aptazyme nanoprobe for amplified imaging of low-abundance protein in living cells

Despite the progress on the analysis of proteins either in vitro or in vivo, detection and imaging of low-abundance proteins in living cells still remains challenging. Herein, a novel biodegradable and cofactor self-sufficient DNAzyme nanoprobe has been deve-loped for catalytic imaging of protein in living cells with signal amplification capacity. This DNAzyme nanoprobe is constructed by assembling a DNAzyme subunit-containing aptamer hairpin (HP), another DNAzyme subunit strand (DS), and the molecular beacon (MB) substrate strand onto pH-sensitive ZnO@polydopamine nanorods (ZnO@PDA NRs) that work as DNAzyme cofactor suppliers. Such a nanoprobe can facilitate cellular uptake of DNA molecules and protection of them from nuclease degradation as well as release of them in cells by lysosomal acid-triggered dissolution of ZnO@PDA NRs into Zn²⁺ as DNAzyme cofactor. Upon recognition and binding with the intracellular protein target, the stem of HP is opened, after which the opened HP hybridizes with DS and generates activated DNAzymes. Each activated DNAzyme can catalyze the cleavage of many MB substrates through true enzymatic multiple turnovers, resulting in the separation of the quenched fluorophore/quencher pair labeled in MB and the generation of significantly amplified fluorescence. Using nucleolin (NCL) as a model protein, this nanoprobe enables the analysis of NCL with a detection limit of 1.8 pM, which are at least two orders of magnitude lower than that of non-catalytic imaging probe. Moreover, it could accurately distinguish tumor cells and normal cells by live cell NCL imaging. And the experimental results are also further verified by flow cytometry assays. The developed nanoprobe can be easily extended to detect other biomolecules by the change of their corresponding aptamer sequences, thus providing a promising tool for highly sensitive imaging of low-abundance biomolecules in living cells.

A Background-Free SERS Strategy for Sensitive Detection of Hydrogen Peroxide

The accurate and sensitive detection of biomolecules by surface-enhanced Raman spectroscopy (SERS) is possible, but remains challenging due to the interference from biomolecules in complex samples. Herein, a new SERS sensor is developed for background-free detection of hydrogen peroxide (H₂O₂) with an ultralow detection limit (1 × 10^-10 mol/L), using a Raman-silent strategy. The Au microparticles (Au-RSMPs) resembling rose-stones are devised as SERS substrates with a high enhancement effect, and 4-mercaptophenylboronic acid (4-MPBA) is selected as an H₂O₂-responsive Raman reporter. Upon the reaction with H₂O₂, the phenylboronic group of 4-MPBA was converted to a phenol group, which subsequently reacted with 4-diazonium-phenylalkyne (4-DP), an alkyne-carrying molecule via the azo reaction. The formed product exhibits an intense and sharp SERS signal in the Raman-silent region, avoiding interference of impurities and biomolecules. As a proof-of-concept demonstration, we show that this SERS sensor possesses significant merits towards the determination of H₂O₂ in terms of broad linear range, low limit of detection, and high selectivity, showing promise for the quantitative analysis of H₂O₂ in complicated biological samples.

Surface potential modulation as a tool for mitigating challenges in SERS-based microneedle sensors

Raman spectroscopic-based biosensing strategies are often complicated by low signal and the presence of multiple chemical species. While surface-enhanced Raman spectroscopy (SERS) nanostructured platforms are able to deliver high quality signals by focusing the electromagnetic field into a tight plasmonic hot-spot, it is not a generally applicable strategy as it often depends on the specific adsorption of the analyte of interest onto the SERS platform. This paper describes a strategy to address this challenge by using surface potential as a physical binding agent in the context of microneedle sensors. We show that the potential-dependent adsorption of different chemical species allows scrutinization of the contributions of different chemical species to the final spectrum, and that the ability to cyclically adsorb and desorb molecules from the surface enables efficient application of multivariate analysis methods. We demonstrate how the strategy can be used to mitigate potentially confounding phenomena, such as surface reactions, competitive adsorption and the presence of molecules with similar structures. In addition, this decomposition helps evaluate criteria to maximize the signal of one molecule with respect to others, offering new opportunities to enhance the measurement of analytes in the presence of interferants.

Prospects of Surface-Enhanced Raman Spectroscopy for Biomarker Monitoring toward Precision Medicine

Future precision medicine will be undoubtedly sustained by the detection of validated biomarkers that enable a precise classification of patients based on their predicted disease risk, prognosis, and response to a specific treatment. Up to now, genomics, transcriptomics, and immunohistochemistry have been the main clinically amenable tools at hand for identifying key diagnostic, prognostic, and predictive biomarkers. However, other molecular strategies, including metabolomics, are still in their infancy and require the development of new biomarker detection technologies, toward routine implementation into clinical diagnosis. In this context, surface-enhanced Raman scattering (SERS) spectroscopy has been recognized as a promising technology for clinical monitoring thanks to its high sensitivity and label-free operation, which should help accelerate the discovery of biomarkers and their corresponding screening in a simpler, faster, and less-expensive manner. Many studies have demonstrated the excellent performance of SERS in biomedical applications. However, such studies have also revealed several variables that should be considered for accurate SERS monitoring, in particular, when the signal is collected from biological sources (tissues, cells or biofluids). This Perspective is aimed at piecing together the puzzle of SERS in biomarker monitoring, with a view on future challenges and implications. We address the most relevant requirements of plasmonic substrates for biomedical applications, as well as the implementation of tools from artificial intelligence or biotechnology to guide the development of highly versatile sensors.

Highly accurate and label-free discrimination of single cancer cell using a plasmonic oxide-based nanoprobe

The detection of cancer cells at the single-cell level enables many novel functionalities such as next-generation cancer prognosis and accurate cellular analysis. While surface-enhanced Raman spectroscopy (SERS) has been widely considered as an effective tool in a low-cost and label-free manner, however, it is challenging to discriminate single cancer cells with an accuracy above 90% mainly due to the poor biocompatibility of the noble-metal-based SERS agents. Here, we report a dual-functional nanoprobe based on dopant-driven plasmonic oxides, demonstrating a maximum accuracy above 90% in distinguishing single THP-1 cell from peripheral blood mononuclear cell (PBMC) and human embryonic kidney (HEK) 293 from human macrophage cell line U937 based on their SERS patterns. Furthermore, this nanoprobe can be triggered by the bio-redox response from individual cells towards stimuli, empowering another complementary colorimetric cell detection, approximately achieving the unity discrimination accuracy at a single-cell level. Our strategy could potentially enable the future accurate and low-cost detection of cancer cells from mixed cell samples.

A multimer-based SERS aptasensor for highly sensitive and homogeneous assay of carcinoembryonic antigens

Carcinoembryonic antigens (CEAs) are known as one of the most common tumor markers. Their facile and affordable detection is critical for early diagnosis of malignant tumors, especially in resource-constrained settings. Here, we report a novel multimer-based surface-enhanced Raman scattering (SERS) aptasensor for a specific CEA assay. The aptasensor is fabricated through aptamer-assisted self-assembly of silver-coated gold nanoparticles (Au@Ag NPs), and the self-assembled multimeric structure possesses abundant hot-spots to provide high SERS response. When CEA is introduced, the specific recognition of CEA by aptamers will lead to the disassembly of Au@Ag multimers due to the lack of a bridging aptamer between Au@Ag NPs. As a result, the number of hot-spots in the multimeric system is decreased, and the intensity at 1585 cm-1 of the SERS reporter (4-mercaptobenzoic acid, 4-MBA) on the surface of NPs will also be decreased. The Raman intensity is proportional to the logarithm of the concentration of CEA. The detection sensitivity can be down to the pg mL-1 level. The analytical method only needs a droplet of 2 μL of sample, and the detection time is less than 20 min. The multimer-based SERS aptasensor can be applied in sensitive and inexpensive detection of CEA in serum samples.

Single-Cell Oxidative Stress Events Revealed by a Renewable SERS Nanotip

A nanotip sensitive to reactive oxygen species (ROS) and NAD⁺/NADH (oxidized/reduced forms of nicotinamide adenine dinucleotide) was designed and prepared to identify the redox events in a single living cell by surface-enhanced Raman scattering (SERS) spectroscopy. The nanotips were prepared by the one-step laser-induced Ag growth and deposition. A redox-reversible Raman reporter, 4-mercaptophenol (4-MP), was employed for the nanotip decoration along with the Ag deposition. 4-MP can be converted to SERS-inactive 4-mercaptocyclohexa-2,5-dienone (4-MC) by Fe³⁺ ions to complete signal rezeroing for multiple oxidative stress event loops. The SERS signal conversion from 4-MC to 4-MP provides a cue for the reduction process that is NADH-dependent. In contrast, by the conversion from 4-MP to 4-MC, the oxidative stress events and the signal transduction mechanism of cells stimulated by drugs (phorbol 12-myristate 13-acetate and H₂O₂) can be explored by SERS. This sensor is easy to fabricate and can be recycled. This tip-typed SERS nanosensor can be extendedly available for tracing other key markers in other NAD⁺/NADH-mediated respiratory chain and glycolysis, e.g., lactic acid, pyruvic acid, adenosine triphosphate, and antioxidants. It will be useful for investigating the diseases of abnormal oxidative stress and mitochondrial metabolism at the single-cell level.

Competitive immunosensor for sensitive and optical anti-interference detection of imidacloprid by surface-enhanced Raman scattering

The sensitive detection of pesticides in complex environment is important but still challenging in presence of organic-rich water sample and food matrix. Herein, we reported a nitrile-mediated SERS immunosensor for sensitive and optical anti-interference determination of imidacloprid. Raman tag contained CN bond could provide a sharp characteristic peak in the Raman-silent spectral window (1800 ~ 2800 cm^-1), which could resist the optical noises from the fingerprint region (<1800 cm^-1). Au_core-Ag_shell bimetallic nanocuboid (AuNR@Ag) connected with antigen and Raman tag was used as Raman probe, while Fe₃O₄ magnetic nanoparticle functionalized with anti-imidacloprid antibody was applied as signal enhancer. Owing to the specific recognition ability between antigen and antibody, the competitive system with imidacloprid was formed. Under the optimal condition, the linear relationship was developed in the range of 10-400 nM. Finally, the SERS immunosensor was successfully applied to determine imidacloprid in real samples with recoveries from 96.8% to 100.5%.

Ultrasensitive plasmon enhanced Raman scattering detection of nucleolin using nanochannels of 3D hybrid plasmonic metamaterial

Detection of cancer biomarker is of great significance in cancer diagnostics. In this work, we propose an ultrasensitive and in situ method for plasmon enhanced Raman scattering (PERS) detection of nucleolin (NCL) using a 3D hybrid plasmonic metamaterial (PM). In this aptasensor, thiolated complementary DNA (cDNA) immobilized on PM can hybridize with Rox-labeled NCL-binding aptamer (AS1411-Rox) to form a rigid double-stranded DNA (dsDNA). When NCL passes through the PM nanochannels under a transmembrane voltage bias, it interacts with AS1411-Rox to form G-quadruplexes (G4-AS1411-Rox), resulting in the release of AS1411-Rox from the nanochannels surface and the decrease in PERS signal of the reporter Rox. This change in PERS signals can be recorded in situ without the interference of external environment. With the help of the enrichment function of nanochannel, the present method is able to achieve fast NCL detection within 10 min with a detection limit as low as 71 pM. Furthermore, our method shows excellent specificity, reversibility, uniformity (relative standard deviation of ~6.86%) and reproducibility (~6.65%), providing a new platform for reliable cancer auxiliary diagnosis and drug screening.

Interfacial interactions of SERS-active noble metal nanostructures with functional ligands for diagnostic analysis of protein cancer markers

Noble metal nanostructures with designed hot spots have been widely investigated as surface-enhanced Raman spectroscopy (SERS)-active substrates, particularly for selective and sensitive detection of protein cancer markers. For specific target recognition and efficient signal amplification, SERS probe design requires a choice of SERS-active nanostructures as well as their controlled functionalization with Raman dyes and target recognition entities such as antibodies. However, the chemical conjugation of antibodies and Raman dyes to SERS substrates has rarely been discussed to date, despite their substantial roles in detection schemes. The interfacial interactions of metal nanostructures with functional ligands during conjugation are known to be strongly influenced by the various chemical and physical properties of the ligands, such as size, molecular weight, surface charge, 3-dimensional structures, and hydrophilicity/hydrophobicity. In this review, we discuss recent developments in the design of SERS probes over the last 4 years, focusing on their conjugation chemistry for functionalization. A strong preference for covalent bonding is observed with Raman dyes having simpler molecular structures, whereas more complicated ones are non-covalently adsorbed. Antibodies are both covalently and non-covalently bonded to nanostructures, depending on their activity in the SERS probes. Considering that ligand conjugation is highly important for chemical stability, biocompatibility, and functionality of SERS probes, this review is expected to expand the understanding of their interfacial design, leading to SERS as one of the most promising spectroscopic analytical tools for the early detection of protein cancer markers.

Highly sensitive detection of DNA damage in living cells by SERS and electrochemical measurements using a flexible gold nanoelectrode

Guanine (G) oxidation products, such as 8-hydroxy-2'-deoxyguanosine (8-OHdG) and 8-oxo-guanine (8-OXOG), have been widely studied as promising biomarkers for DNA oxidative damage. In this work, we develop a new method to detect G oxidative products released from live cells after chromium (vi) ion or hydrogen peroxide treatments by using a glass nanopipette-based flexible gold nanoelectrode (fGNE). Specific response to G oxidative products with high sensitivity can be detected from the fGNE tip through integrated electrochemical measurements and surface-enhanced Raman spectroscopy. The fGNE apex can be positioned very close to the cell membrane noninvasively because of its high flexibility and nanoscale tip size. With the assistance of the electrophoretic force, the fGNEs can effectively collect and detect the G-derived DNA damage products released from individual cells in the cell culture medium with high sensitivity.

Recent progress of surface-enhanced Raman spectroscopy for subcellular compartment analysis

Organelles are involved in many cell life activities, and their metabolic or functional disorders are closely related to apoptosis, neurodegenerative diseases, cardiovascular diseases, and the development and metastasis of cancers. The explorations of subcellular structures, microenvironments, and their abnormal conditions are conducive to a deeper understanding of many pathological mechanisms, which are expected to achieve the early diagnosis and the effective therapy of diseases. Organelles are also the targeted locations of drugs, and they play significant roles in many targeting therapeutic strategies. Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical tool that can provide the molecular fingerprint information of subcellular compartments and the real-time cellular dynamics in a non-invasive and non-destructive way. This review aims to summarize the recent advances of SERS studies on subcellular compartments, including five parts. The introductions of SERS and subcellular compartments are given. SERS is promising in subcellular compartment studies due to its molecular specificity and high sensitivity, and both of which highly match the high demands of cellular/subcellular investigations. Intracellular SERS is mainly cataloged as the labeling and label-free methods. For subcellular targeted detections and therapies, how to internalize plasmonic nanoparticles or nanostructure in the target locations is a key point. The subcellular compartment SERS detections, SERS measurements of isolated organelles, investigations of therapeutic mechanisms from subcellular compartments and microenvironments, and integration of SERS diagnosis and treatment are sequentially presented. A perspective view of the subcellular SERS studies is discussed from six aspects. This review provides a comprehensive overview of SERS applications in subcellular compartment researches, which will be a useful reference for designing the SERS-involved therapeutic systems.

Determination of NADH by Surface Enhanced Raman Scattering Using Au@MB@Ag NPs

Nicotinamide adenine dinucleotide (NADH) is an important coenzyme involved in various metabolic processes of living cells. As an important biomarker, NADH is associated with breast cancer and Alzheimer’s disease. In this paper, silver plated gold core–shell nanoparticles containing Raman signal molecules were synthesised on the basis of bare gold. Using the Raman peak corresponding to the 4-mercaptobenzonitrile (MB) silent region C≡N vibration for quantification, while avoiding competition with the precious metal surface binding site to be measured, it can also be free from the interference of endogenous biomolecules. On the one hand, it can correct the working curve, on the other hand, it can avoid competing with the binding site. Compared with the core–shell structure prepared here, the limit of detection (LOD) for NADH was only 10−5 M for bare gold and the LOD for the core–shell structure prepared on the basis of bare gold was 3.3 × 10−7 M. In terms of correction, with Rhodamine 6G (R6G) as a Raman signalling molecule, the R2 value before SERS detection and correction is only 0.9405, and the R2 value after correction increases to 0.9853. The unique fingerprint peak of SERS was used to realise the quantitative detection of NADH, which realizes the detection of NADH in complex biological samples of serum and provides the possibility for expanding the early diagnosis of breast cancer.

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.

Sensing with Nanopores and Aptamers: A Way Forward

In the 90s, the development of a novel single molecule technique based on nanopore sensing emerged. Preliminary improvements were based on the molecular or biological engineering of protein nanopores along with the use of nanotechnologies developed in the context of microelectronics. Since the last decade, the convergence between those two worlds has allowed for biomimetic approaches. In this respect, the combination of nanopores with aptamers, single-stranded oligonucleotides specifically selected towards molecular or cellular targets from an in vitro method, gained a lot of interest with potential applications for the single molecule detection and recognition in various domains like health, environment or security. The recent developments performed by combining nanopores and aptamers are highlighted in this review and some perspectives are drawn.

Dynamic single-cell intracellular pH sensing using a SERS-active nanopipette

Glass nanopipettes have shown promise for applications in single-cell manipulation, analysis, and imaging. In recent years, plasmonic nanopipettes have been developed to enable surface-enhanced Raman spectroscopy (SERS) measurements for single-cell analysis. In this work, we developed a SERS-active nanopipette that can be used to perform long-term and reliable intracellular analysis of single living cells with minimal damage, which is achieved by optimizing the nanopipette geometry and the surface density of the gold nanoparticle (AuNP) layer at the nanopipette tip. To demonstrate its ability in single-cell analysis, we used the nanopipette for intracellular pH sensing. Intracellular pH (pHi) is vital to cells as it influences cell function and behavior and pathological conditions. The pH sensitivity was realized by simply modifying the AuNP layer with the pH reporter molecule 4-mercaptobenzoic acid. With a response time of less than 5 seconds, the pH sensing range is from 6.0 to 8.0 and the maximum sensitivity is 0.2 pH units. We monitored the pHi change of individual HeLa and fibroblast cells, triggered by the extracellular pH (pHe) change. The HeLa cancer cells can better resist pHe change and adapt to the weak acidic environment. Plasmonic nanopipettes can be further developed to monitor other intracellular biomarkers.

Superparamagnetic Nanostructures for Split-Type and Competitive-Mode Photoelectrochemical Aptasensing

Photoelectrochemical sensing has developed rapidly in the past decade because of its inherent advantages of economic devices and low background noise. However, traditional assembly of photoelectric beacons, probes, and targets on the ITO electrode solid-liquid interface inevitably leads to time-consuming, limited selectivity, poor stability, and nonreproducibility. To overcome these drawbacks, in this work, a unique split-type PEC aptasensor for carcinoembryonic antigen (CEA) was developed in virtue of the sandwich-like structure comprised of magnetic-optical Fe₃O₄@SiO₂@CdS-DNA₁, CEA aptamer, and signal element SiO₂-Au-DNA₂. The sandwich-like structure is easily formed in the liquid phase and can be triggered by competition from low-abundance CEA, resulting in dissociation. By further photocurrent measurement in pure phosphate buffer saline (PBS), coexisting species can be effectively removed from the modified electrode, improving selectivity, stability, and repeatability. These advantages benefit from the preparation of uniform and monodispersed Fe₃O₄@SiO₂@CdS and SiO₂-Au particles, DNAs assembly, and an elegant design. Additionally, the as-designed signal-on PEC aptasensor is highly sensitive, short time-consuming, and economical, enabling the detection of CEA in serum specimens. It not only provides an alternative to CEA immunosensors, but also paves the way for high-performance PEC aptasensors.

Molecular-Imprinting-Based Surface-Enhanced Raman Scattering Sensors

Molecularly imprinted polymers (MIPs) receive extensive interest, owing to their structure predictability, recognition specificity, and application universality as well as robustness, simplicity, and inexpensiveness. Surface-enhanced Raman scattering (SERS) is regarded as an ideal optical detection candidate for its unique features of fingerprint recognition, nondestructive property, high sensitivity, and rapidity. Accordingly, MIP based SERS (MIP-SERS) sensors have attracted significant research interest for versatile applications especially in the field of chemo- and bioanalysis, showing excellent identification and detection performances. Herein, we comprehensively review the recent advances in MIP-SERS sensors construction and applications, including sensing principles and signal enhancement mechanisms, focusing on novel construction strategies and representative applications. First, the basic structure of the MIP-SERS sensors is briefly outlined. Second, novel imprinting strategies are highlighted, mainly including multifunctional monomer imprinting, dummy template imprinting, living/controlled radical polymerization, and stimuli-responsive imprinting. Third, typical application of MIP-SERS sensors in chemo/bioanalysis is summarized from both small and macromolecular aspects. Lastly, the challenges and perspectives of the MIP-SERS sensors are proposed, orienting sensitivity improvement and application expanding.

SERS-based nanostrategy for rapid anemia diagnosis

Iron detection is one of the critical markers to diagnose multiple blood-related disorders that correspond to various biological dysfunctions. The currently available anemia detection approach can be used only for pre-treated blood samples that interfere with the actual iron level in blood. Real-time detection approaches with higher sensitivity and specificity are certainly needed to cope with the commercial level clinical analyses. Herein, we presented a novel strategy to determine the blood iron that can be easily practiced at commercial levels. The blend of well-known iron-cyanide chemistry with nanotechnology is advantageous with ultrahigh sensitivity in whole blood analysis without any pre-treatments. This approach is a combined detection system of the conventional assay (UV-visible spectroscopy) with surface-enhanced Raman scattering (SERS). Organic cyanide modified silver nanoparticles (cAgNPs) can selectively respond to Fe3+ ions and Hb protein with a detection limit of 10 fM and 0.46 μg mL-1, respectively, without being affected by matrix interfering species in the complex biological fluid. We confirmed the clinical potential of our new cAgNPs by assessing iron-status in multiple anemia patients and normal controls. Our SERS-based iron quantitation approach is highly affordable for bulk-samples, cheap, quick, flexible, and useful for real-time clinical assays. Such a method for metal-chelation has extendable features of therapeutics molecular tracking within more complex living systems at cellular levels.

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.

Operando characterization of chemical reactions in single living cells using SERS

Operando probing of chemical reactions for the delivery of gaseous signaling molecules in living cells that is critical for understanding the physiological metabolic processes.

True one cell chemical analysis: a review

The constantly growing field of True One Cell (TOC) analysis has provided important information on the direct chemical composition of various cells and cellular components.

Single-Cell Patterning Based on Immunocapture and a Surface Modified Substrate

Micropatterning technology offers powerful methods for biological analyses at the molecular level, enabling the investigation of cell heterogeneities, as well as high throughput detection. We herein propose an approach for single-cell patterning. The substrate was prepared using micro fabrication and surface modification processes, and the patterning template was prepared using bovine serum albumin and streptavidin, which can be employed for the patterning of any biological molecules containing biotin. Subsequent to photolithography, etching, chemical vapor deposition (CVD), and polyethylene glycol (PEG) treatment, the optimized patterns were obtained with high accuracy, strong contrast, and good repeatability, thus providing good foundations for the subsequent single-cell patterning. The surface passivation method was proven effective, preventing unwanted binding of the antibodies and cells. Based on this streptavidin template, the specific binding between the biotinylated antibodies and the antigens expressed on the surface of the cells was enabled, and we successfully achieved single-cell patterning with a single-cell capture rate of 92%. This single-cell array offers an effective method in the investigation of cell heterogeneity and drug screening. Further, these methods can be used in the final step for the screening and enrichment of certain cells, such as circulating tumor cells.

Photoactivated Specific mRNA Detection in Single Living Cells by Coupling "Signal-on" Fluorescence and "Signal-off" Electrochemical Signals

The spatiotemporal detection of a target mRNA in a single living cell is a major challenge in nanoscience and nanomedicine. We introduce a versatile method to detect mRNA at a single living cell level that uses photocleavable hairpin probes as functional units for the optical (fluorescent) and electrochemical (voltammetric) detection of MnSOD mRNA in single MCF-7 cancer cells. The fluorescent probe is composed of an ortho-nitrophenylphosphate ester functionalized hairpin that includes the FAM fluorophore in a caged configuration quenched by Dabcyl. The fluorescent probe is further modified with the AS1411 aptamer to facilitate the targeting and internalization of the probe into the MCF-7 cells. Under UV irradiation, the hairpin is cleaved, leading to the intracellular mRNA toehold-stimulated displacement of the FAM-functionalized strand resulting in a switched-on fluorescence signal upon the detection of the mRNA in a single cell. In addition, a nanoelectrode functionalized with a methylene blue (MB) redox-active photocleavable hairpin is inserted into the cytoplasm of a single MCF-7 cell. Photocleavage of the hairpin leads to the mRNA-mediated toehold displacement of the redox-active strand associated with the probe, leading to the depletion of the voltammetric response of the probe. The parallel optical and electrochemical detection of the mRNA at a single cell level is demonstrated.

Ultrasensitive Simultaneous Detection of Multiplex Disease-Related Nucleic Acids Using Double-Enhanced Surface-Enhanced Raman Scattering Nanosensors

Developing ultrasensitive probes holds great significance for simultaneous detection of multiplexed cancer-associated nucleic acids. Bimetallic nanoparticles containing silver may be exploited as nanoprobes for disease detection, which can produce stable and strong surface-enhanced Raman scattering (SERS) signals. However, it remains extremely challenging that such SERS nanoprobes are directly synthesized. Herein gold-silver nanosnowmen, grown via a DNA-mediated approach and attached to thiol-containing Raman dyes, are successfully synthesized. Stable SERS-enhanced gold substrates are also prepared and used as the enriching containers, where the capture DNAs are tethered to sense the target genes jointly enhanced by the SERS nanoprobes in a sandwich hybridization assay. This means detection of the target gene can obtain a limit of detection close to 0.839 fM. Such double-enhanced SERS nanosensors are further employed to simultaneously detect the three types of prostate carcinoma-related genes with high sensitivity and specificity, which meanwhile exhibit robust capacity of resisting disturbance in practical samples. Simultaneous and multiplexed detection of cancer-related genes may provide further biomedical applications with new opportunity.

Exploring the margins of SERS in practical domain: An emerging diagnostic modality for modern biomedical applications

Excellent multiplexing capability, molecular specificity, high sensitivity and the potential of resolving complex molecular level biological compositions augmented the diagnostic modality of surface-enhanced Raman scattering (SERS) in biology and medicine. While maintaining all the merits of classical Raman spectroscopy, SERS provides a more sensitive and selective detection and quantification platform. Non-invasive, chemically specific and spatially resolved analysis facilitates the exploration of SERS-based nano probes in diagnostic and theranostic applications with improved clinical outcomes compared to the currently available so called state-of-art technologies. Adequate knowledge on the mechanism and properties of SERS based nano probes are inevitable in utilizing the full potential of this modality for biomedical applications. The safety and efficiency of metal nanoparticles and Raman reporters have to be critically evaluated for the successful translation of SERS in to clinics. In this context, the present review attempts to give a comprehensive overview about the selected medical, biomedical and allied applications of SERS while highlighting recent and relevant outcomes ranging from simple detection platforms to complicated clinical applications.

Signalling probe displacement electrochemical aptasensor for malignant cell surface nucleolin as a breast cancer biomarker based on gold nanoparticle decorated hydroxyapatite nanorods and silver nanoparticle labels

Nucleolin is a multifunctional protein that is markedly overexpressed on the surface of most cancer cells. By taking advantage of the high affinity and specificity of the AS1411 aptamer for nucleolin, a signalling probe displacement electrochemical aptasensor was developed. The thiolated AS1411 aptamer was conjugated to hydroxyapatite nanorods (HApNRs) decorated with gold nanoparticles (AuNPs). To further increase the electrical conductivity of the interface, the ionic liquid 1-ethyl-3-methylimidazolium alanine with its high ion conductivity was placed on the electrode surface. Then, the aptamer was immobilized on the modified electrode and conjugated to signalling c-DNA tagged with AgNPs (c-DNA@AgNPs). In the presence of the MCF7 target cells, the signalling probe is displaced and released from the electrode surface. This leads to a decrease in the current that is proportional to the concentration of cancer cells in the range from 10 to 106 cells mL-1, with a detection limit as low as 8 ± 2 cells mL-1 (n = 3) (based as 3σ/m, where σ is the standard deviation of the blank and m is the slope of the calibration plot). This method presents a promising tool for highly sensitive and selective detection of surface nucleolin on MCF7 cancer cells. Graphical abstract HApNR-AuNP-AS1411 aptamer nanocomposite as an electrochemical sensing interface was immobilized on the gold electrode surface and conjugated to signaling c-DNA tagged with AgNPs for determination of surface nucleolin on MCF7 cancer cells.