Target-Triggered Catalytic Hairpin Assembly-Induced Core–Satellite Nanostructures for High-Sensitive “Off-to-On” SERS Detection of Intracellular MicroRNA

Advances, Challenges, and Opportunities in Plasmonic Nanogap-Enhanced Raman Scattering with Nanoparticles

Surface-enhanced Raman scattering has been widely used for molecular/material characterization and chemical and biological sensing and imaging applications. In particular, plasmonic nanogap-enhanced Raman scattering (NERS) is based on the highly localized electric field formed within the nanogap between closely spaced metallic surfaces to more strongly amplify Raman signals than the cases with molecules on metal surfaces. Nanoparticle-based NERS offers extraordinarily strong Raman signals and a plethora of opportunities in sensing, imaging and many different types of biomedical applications. Despite its potential, several challenges still remain for NERS to be widely useful in real-world applications. This Perspective introduces various plasmonic nanogap configurations with nanoparticles, discusses key advances and critical challenges while addressing possible misunderstandings in this field, and provides future directions for NERS to generate stronger, more uniform, and stable signals over a large number of structures for practical applications.

Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

An advanced 3D DNA nanoplatform for spatiotemporally confined enhanced dual-mode biosensing MicroRNA in cancer cell

Dual-mode signal output platforms have demonstrated considerable promise due to their improved anti-interference capability and inherent signal self-correction. Nevertheless, traditional discrete-distributed signal probes often encounter significant drawbacks, including limited mass transfer efficiency, diminished signal strength, and instability in intricate biochemical environments. In response to these challenges, a scalable and hyper-compacted 3D DNA nanoplatform resembling "periodic focusing heliostat" has been developed for synergistically enhanced fluorescence (FL) and surface-enhanced Raman spectroscopy (SERS) biosensing of miRNA in cancer cells. Our approach utilized a distinctive assembly strategy integrating gold nanostars (GNS) as fundamental "heliostat units" linked by palindromic DNA sequences to facilitate each other hand-in-hand cascade alignment and condensed into large scale nanostructures. This configuration was further augmented by the incorporation of gold nanoparticles (GNP) via strong Au-S bonds, resulting in a sturdy framework for improved signal transduction. The initiation of this assembly process was mediated by the hybridization of dsDNA to miRNA-21, which served as a primer for polymerization and nicking reactions, thus generating a multifunctional T2 probe. This probe is intricately designed with three distinct parts: a 3'-palindromic end for structural integrity, a central region for capturing SERS-active probes (Cy3-P2), and a 5'-segment for attaching fluorescence reporters. Upon integration T2 into the GNS-based heliostat unit, it promotes palindromic arm-induced aggregation and plasma exciton coupling between plasma nanoparticles and signal transduction tags. This clustered arrangement creates a high-density "hot spot" array that maximizes the local electromagnetic fields necessary for enhanced SERS and FL response. This superstructure supports enhanced aggregation-induced signal amplification for both SERS and FL, offering exceptional sensitivity with LOD as low as 0.0306 pM and 0.409 pM. The efficacy of this method was demonstrated in the evaluation of miRNA-21 in various cancer cell lines.

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.

EXPAR for biosensing: recent developments and applications

Emerging as a promising novel amplification technique, the exponential amplification reaction (EXPAR) offers significant advantages due to its potent exponential amplification capability, straightforward reaction design, rapid reaction kinetics, and isothermal operation. The past few years have witnessed swift advancements and refinements in EXPAR-based technologies, with numerous high-performance biosensing systems documented. A deeper understanding of the EXPAR mechanism has facilitated the proposal of novel strategies to overcome limitations inherent to traditional EXPAR. Furthermore, the synergistic integration of EXPAR with diverse amplification methodologies, including the use of a CRISPR/Cas system, metal nanoparticles, aptamers, alternative isothermal amplification techniques, and enzymes, has significantly bolstered analytical efficacy, aiming to enhance specificity, sensitivity, and amplification efficiency. This comprehensive review presents a detailed exposition of the EXPAR mechanism and analyzes its primary challenges. Additionally, we summarize the latest research advancements in the biomedical field concerning the integration of EXPAR with diverse amplification technologies for sensing strategies. Finally, we discuss the challenges and future prospects of EXPAR technology in the realms of biosensing and clinical applications.

Face-to-face Assembly Strategy of Au Nanocubes: Induced Generation of Broad Hotspot Regions for SERS-Fluorescence Dual-Signal Detection of Intracellular miRNAs

While designing anisotropic noble metal nanoparticles (NPs) can enhance the signal intensity of Raman dyes, more sensitive surface-enhanced Raman scattering (SERS) probes can be designed by oriented self-assembly of noble metal nanomaterials into dimers or higher-order nanoclusters. In this study, we engineered a self-assembly strategy in living cells for real-time fluorescence and SERS dual-channel detection of intracellular microRNAs (miRNAs), using Mg2+-dependent 8-17E DNAzyme sequences as the driving motors, gold nanocubes (AuNCs) as the driver components, and three-branched double-stranded DNA as the linking tool. The assembly selects adenine in DNA as a reporter molecule, simplifying the labeling process of Raman reporter molecules and reducing the synthesis process. In addition, adenine is stably distributed between the faces of AuNCs and the wide hotspot region gives good reproducibility of the adenine SERS signal. In this strategy, the SERS channel was consistently stable and more sensitive compared to the fluorescence channel. Among them, the detection limit of the SERS channel was 2.1 pM and the coefficient of variation was 1.26% in the in vitro liquid phase and 1.49% in MCF-7 cells. The strategy successfully achieved accurate tracking and quantification of miRNA-21 in cancer cells, showing good reproducibility in complex samples as well as cells. The reported strategy provides ideas for exploring intracellular specific triggering of nanoparticles for precise control of self-assembly.

Smart Stimuli-Responsive Spherical Nucleic Acids: Cutting-Edge Platforms for Biosensing, Bioimaging, and Therapeutics

Spherical nucleic acids (SNAs) with exceptional colloidal stability, multiple modularity, and programmability are excellent candidates to address common molecular delivery-related issues. Based on this, the higher targeting accuracy and enhanced controllability of stimuli-responsive SNAs render them precise nanoplatforms with inestimable prospects for diverse biomedical applications. Therefore, tailored diagnosis and treatment with stimuli-responsive SNAs may be a robust strategy to break through the bottlenecks associated with traditional nanocarriers. Various stimuli-responsive SNAs are engineered through the incorporation of multifunctional modifications to meet biomedical demands with the development of nucleic acid functionalization. This review provides a comprehensive overview of prominent research in this area and recent advancements in the utilization of stimuli-responsive SNAs in biosensing, bioimaging, and therapeutics. For each aspect, SNA nanoplatforms that exhibit responsive behavior to both internal stimuli (including sequence, enzyme, redox reactions, and pH) and external stimuli (such as light and temperature) are highlighted. This review is expected to offer inspiration and guidance strategies for the rational design and development of stimuli-responsive SNAs in the field of biomedicine.

DNA-directed formation of plasmonic core-satellite nanostructures for quantification of hepatitis C viral RNA

Hepatitis C virus (HCV) continues to be a significant public health challenge, affecting an estimated 71 million people globally and posing risks of severe liver diseases. Despite advancements in treatments, diagnostic limitations hinder the global elimination efforts targeted by 2030. This study introduces an innovative diagnostic approach, integrating catalytic hairpin assembly (CHA) with plasmonic core-satellite gold nanoparticle (AuNP) assemblies, to enable sensitive and specific detection of HCV RNA. We optimized the stoichiometry of DNA hairpins to form highly stable three-way junctions (3WJs), minimizing non-specific reactions in an enzyme-free, isothermal amplification process. The resulting dual-transduction biosensor combines colorimetric and surface-enhanced Raman spectroscopy (SERS) techniques, utilizing the Raman reporter malachite green isothiocyanate (MGITC) for signal generation. Our system targets a conserved 23-nucleotide sequence within the HCV 5'-UTR, essential for RNA replication, facilitating pan-genotypic HCV detection that complements direct-acting antiviral strategies. We evaluated the biosensor's efficacy using fluorescence spectroscopy, native PAGE, AFM, and TEM. Findings indicate that the 60 nm core AuNPs surrounded by 20 nm satellite AuNPs achieved a ten-fold increase in sensitivity over the 10 nm satellites, detecting HCV RNA concentrations as low as 1.706 fM. This sensitivity is crucial, given the extremely low viral loads present during early infection stages. Our research demonstrates the promise of enzyme-free molecular biosensors for HCV, with the potential to provide cost-efficient, rapid, point-of-care testing, although further sensitivity enhancements are needed to address the challenges of early-stage detection.

Recent advances in SERS-based bioanalytical applications: live cell imaging

Raman scattering can provide information on molecular fingerprints, which have been widely applied in various fields of material science and nanobiotechnology. Notably, low interference with water molecules in obtaining the Raman spectra between 500 and 2000 cm-1 made it a powerful spectroscopic tool in biology, such as imaging and signaling for a living cell. To be a robust tool for cell biology, the performance of obtaining molecular-specific information with high sensitivity, high resolution in real time, and without inducing cell damage is strongly required. The conventional fluorescence-based method has been suffered from the rapid photobleaching of organic fluorophores and the lack of molecular information. In contrast, Raman scattering is a promising spectroscopic tool to acquire cellular information, and the extremely low signal intensity of Raman scattering could be amplified by incorporating the plasmonic nanomaterials. Along with the fundamental research focus on surface-enhanced Raman scattering (SERS), the practical approaches of SERS for cellular imaging as a new tool for drug screening and monitoring cellular signals have been extensively explored based on new optical setups and new designing strategies for the nanostructures. Diverse nanostructure and surface chemistry for targeting or sensing have been played pivotal roles in acquiring cellular information and high resolution cell imaging. In this regard, this review focused on the recent advances of SERS-based technologies for a live cell imaging investigated such as potential drug screening, signaling for chemicals or biomolecules in cell, in situ sensing, and high spatiotemporal resolution.

Smart Tumor Cell-Derived DNA Nano-Tree Assembly for On-Demand Macrophages Reprogramming

Without coordinated strategies to balance the population and activity of tumor cells and polarized macrophages, antitumor immunotherapy generally offers limited clinical benefits. Inspired by the "eat me" signal, a smart tumor cell-derived proximity anchored non-linear hybridization chain reaction (Panel-HCR) strategy is established for on-demand regulation of tumor-associated macrophages (TAMs). The Panel-HCR is composed of a recognition-then-assembly module and a release-then-regulation module. Upon recognizing tumor cells, a DNA nano-tree is assembled on the tumor cell surface and byproduct strands loaded with CpG oligodeoxynucleotides (CpG-ODNs) are released depending on the tumor cell concentration. The on-demand release of CpG-ODNs can achieve efficient regulation of M2 TAMs into the M1 phenotype. Throughout the recognition-then-assembly process, tumor cell-targeted bioimaging is implemented in single cells, fixed tissues, and living mice. Afterward, the on-demand release of CpG-ODNs regulate the transformation of M2 TAMs into the M1 phenotype by stimulating toll-like receptor 9 to activate the NF-κB pathway and increasing inflammatory cytokines. This release-then-regulation process is verified to induce strong antitumor immune responses both in vitro and in vivo. Altogether, this proposed strategy holds tremendous promise for on-demand antitumor immunotherapy.

Exclusive Core-Janus Satellite Assembly Based on Au-Ag Janus Self-Aligned Distributions with Abundant Hotspots for Ultrasensitive Detection of CA19-9

The development of surface-enhanced Raman scattering (SERS) probes with high sensitivity and stability is imminent to improve the accuracy of cancer diagnosis. Here, an exclusive core-Janus satellite (CJS) assembly was constructed by a hierarchical assembly strategy in which the Au-Ag Janus satellite is vertically self-aligned on the core surface. In the process, a silica shell template was ingeniously employed to asymmetrically mask the presatellites for the in situ formation of the Janus structure, and a series of Janus satellites with different morphologies were developed by regulating the encapsulated area of the presatellites. The ordered-oriented arrangement of Au-Ag Janus and unique heterojunction morphology permit CJS assemblies, featuring two types of plasmonic nanogaps, including intrananocrevices for individual Janus and internanogaps between neighboring Janus, thereby multiplying the "hotspots" compared to conventional core-monotonous satellites, which contributes to superior SERS activity. As anticipated, the enhancement factor of CJS assemblies was as high as 3.8 × 108. Moreover, it is intriguing that the directional distribution and head physically immobilized by Janus provided uniform and stable SERS signals. The SERS probe based on the CJS assembly for the detection of carbohydrate antigen 19-9 resulted in an ultrahigh sensitivity with a limit of detection of 3.7 × 10-5 IU·mL-1, which is nearly 10 times lower than other SERS probes, and a wide detection range of 3 × 10-5 to 1 × 104 IU·mL-1. The CJS assembly with excellent SERS performance is promising to advance further development of the early diagnosis of pancreatic cancer.

SERS-based methods for the detection of genomic biomarkers of cancer

Genomic biomarkers of cancer are based on changes in nucleic acids, which include abnormal expression levels of some miRNAs, point mutations in DNA sequences, and altered levels of DNA methylation. The presence of tumor-related nucleic acids in body fluids (blood, saliva, or urine) makes it possible to achieve a non-invasive early-stage cancer diagnosis. Currently existing techniques for the discovery of nucleic acids require complex, time-consuming, costly assays and have limited multiplexing abilities. Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that is able to provide molecular specificity combined with trace sensitivity. SERS has gained research attention as a tool for the detection of nucleic acids because of its promising potential: label-free SERS can decrease the complexity of assays currently used with fluorescence-based detection due to the absence of the label, while labeled SERS may outperform the gold standard in terms of the multiplexing ability. The first papers about SERS-based methods for the measurement of genomic biomarkers were written in 2008, and since then, more than 150 papers have been published. The aim of this paper is to review and evaluate the proposed SERS-based methods in terms of their level of development and their potential for liquid biopsy application, as well as to contribute to their further evolution by attracting research attention to the field. This goal will be reached by grouping, on the basis of their experimental protocol, all the published manuscripts on the topic and evaluating each group in terms of its limit of detection and applicability to real body fluids. Thus, the methods are classified according to their working principles into five main groups, including capture-based, displacement-based, sandwich-based, enzyme-assisted, and specialized protocols.

Ultrasound-Induced Enrichment of Ultra-Trace miRNA Biosensing in Nanoliter Samples

The rapid and accurate analysis of micro-samples is a crucial foundation for precision medicine, particularly for early screening and monitoring of cancer, where it holds significant importance. Ultrasound-based multifunctional biocompatible manipulation techniques have been extensively applied in a variety of biomedical fields, providing insights for the development of rapid, cost-effective, and accurate biomarker detection strategies. In this chapter, we combine ultrasound-based gradient pressure fields with functionalized microsphere enrichment to develop a biosensing method for ultra-trace miRNA enrichment in nanoliter samples without PCR. This system relies on inexpensive capillaries, enabling simultaneous visual imaging and trace sample detection.

Ultrasound-enhanced catalytic hairpin assembly capable of ultrasensitive microRNA biosensing for the early screening of Alzheimer's disease

Catalytic hairpin assembly (CHA) is a promising enzyme-free, isothermal signal amplification strategy, but the relatively time-consuming strand replacement limits its application scenarios. Here, we developed an ultrasound-enhanced catalytic hairpin assembly (UECHA) biosensing platform for early screening of Alzheimer's disease by introducing a portable acoustic-drive platform with functionalized microspheres for effective biomarkers enrichment and fluorescence enhancement. By constructing a gradient ultrasonic field in a microcavity, the platform concentrates the functionalized microspheres in a central position, accompanied by an enhanced fluorescence signal with a specific release. In addition, the programmable frequency modulation can also modify the acoustic potential well and effectively promote non-equilibrium chemical reactions such as CHA (25 min). Compared with the conventional catalytic hairpin assembly (CHA), UECHA allows for direct and quantitative measurement of AD miRNAs down to 3.55 × 10-15 M in 1 μL samples. This visual analysis of ultra-trace biomarkers based on acoustic enrichment and promotion provides a new perspective for the rapid and highly sensitive clinical detection of Alzheimer's disease.

Cascade signal amplification strategy by coupling chemical redox-cycling and Fenton-like reaction: Toward an ultrasensitive split-type fluorescent immunoassay

An ultrasensitive split-type fluorescent immunobiosensor has been reported based on a cascade signal amplification strategy by coupling chemical redox-cycling and Fenton-like reaction. In this strategy, Cu2+ could oxidize chemically o-phenylenediamine (OPD) to generate photosensitive 2, 3-diaminophenazine (DAP) and Cu+/Cu0. On one hand, the generated Cu0 in turn catalyzed the oxidation of OPD. On the other hand, the introduced H2O2 reacted with Cu + ion to produce hydroxyl radicals (·OH) and Cu2+ ion through a Cu + -mediated Fenton-like reaction. The produced ·OH and recycled Cu2+ ion could take turns oxidizing OPD to generate more photoactive DAP, which triggering a self-sustaining chemical redox-cycling reaction and leading to a remarkable fluorescent improvement. It was worth mentioning that the cascade reaction did not stop until OPD molecules were completely consumed. Based on the H2O2-triggered cascade signal amplification, the strategy was exploited for the construction of split-type fluorescent immunoassay by taking interleukin-6 (IL-6) as the model target. It was realized for the ultrasensitive determination of IL-6 in a linear ranging from 20 fg/mL to 10 pg/mL with a limit of detection of 5 fg/mL. The study validated the practicability of the cascade signal amplification on the fluorescent bioanalysis and the superior performance in fluorescent immunoassay. It is expected that the strategy would offer new opportunities to develop ultrasensitive fluorescent methods for biosensor and bioanalysis.

Engineering of catalytic hairpin-rigidified Y-shaped DNA-functionalized nanomachine to rapidly detect mRNA

The catalytic hairpin-rigidified Y-shaped DNA through layer-by-layer assembly has been fixed on the surface of copper sulfide nanoparticles for the detection of survivin mRNA. The distance between the CHA probes fixed on the Y-shaped DNA is significantly shortened. The results show that the fluorescence of this nanomachine reached the maximum value in 50 min (excitation wavelength at 488 nm and emission wavelength 526 nm), and its reaction rate is more than 5-fold faster than that of the free-CHA control system. In addition, the nanomachine showed high sensitivity (LOD of 3.5 pM) and high specificity for the survivin mRNA detection. Given its fast response time and excellent detection performance, we envision that the catalytic hairpin-rigidified Y-shaped DNA-functionalized nanomachine will offer potential applications in disease diagnostics and clinical applications.

Nucleic Acid Probes in Bio-Imaging and Diagnostics: Recent Advances in ODN-Based Fluorescent and Surface-Enhanced Raman Scattering Nanoparticle and Nanostructured Systems

Raman nanoparticle probes are a potent class of optical labels for the interrogation of pathological and physiological processes in cells, bioassays, and tissues. Herein, we review the recent advancements in fluorescent and Raman imaging using oligodeoxyribonucleotide (ODN)-based nanoparticles and nanostructures, which show promise as effective tools for live-cell analysis. These nanodevices can be used to investigate a vast number of biological processes occurring at various levels, starting from those involving organelles, cells, tissues, and whole living organisms. ODN-based fluorescent and Raman probes have contributed to the achievement of significant advancements in the comprehension of the role played by specific analytes in pathological processes and have inaugurated new possibilities for diagnosing health conditions. The technological implications that have emerged from the studies herein described could open new avenues for innovative diagnostics aimed at identifying socially relevant diseases like cancer through the utilization of intracellular markers and/or guide surgical procedures based on fluorescent or Raman imaging. Particularly complex probe structures have been developed within the past five years, creating a versatile toolbox for live-cell analysis, with each tool possessing its own strengths and limitations for specific studies. Analyzing the literature reports in the field, we predict that the development of ODN-based fluorescent and Raman probes will continue in the near future, disclosing novel ideas on their application in therapeutic and diagnostic strategies.

Electrochemiluminescence detection of miRNA-21 based on dual signal amplification strategies: Duplex-specific nuclease -mediated target recycle and nicking endonuclease-driven 3D DNA nanomachine

DNA nanomachines have shown potential application in the construction of various biosensors. Here, an electrochemiluminescence biosensor for the sensitive detection of miRNA-21 were reported based on three-dimensional (3D) DNA nanomachine and duplex-specific nuclease (DSN)-mediated target recycle amplification strategy. First, the bipedal DNA walkers were obtained by DSN-mediated digestion reaction initiated by target miRNA-21.3D DNA tracks were prepared by modifying Fe₃O₄ magnetic beads (MBs) with ferrocene-labeled DNA (Fc-DNA). The produced DNA walkers autonomously moved along 3D DNA tracks powered by nicking endonuclease. During the movement, ferrocene-labeled DNA was cleaved, resulting in large amounts of Fc-labeled DNA fragments away from the MBs surface. Finally, the liberated Fc-labeled DNA fragments were dropped on the C-g-C₃N₄ modified electrode surface, leading to the quenching of C-g-C₃N₄ electrochemiluminescence (ECL). Benefiting from the dual amplification strategy of 3D DNA nanomachine and DSN-mediated target recycling, the developed ECL biosensor exhibited an excellent performance for miRNA-21 detection with a wide linear range of 10 fM to 10 nM and a low detection limit of 1.0 fM. This work offers a new thought for the application of DNA walkers in the construction of various biosensors.

Active Enrichment of Nanoparticles for Ultra-Trace Point-of-Care COVID-19 Detection

Active enrichment can detect nucleic acid at ultra-low concentrations without relatively time-consuming polymerase chain reaction (PCR), which is an important development direction for future rapid nucleic acid detection. Here, we reported an integrated active enrichment platform for direct hand-held detection of nucleic acid of COVID-19 in nanoliter samples without PCR. The platform consists of a capillary-assisted liquid-carrying system for sampling, integrated circuit system for ultrasound output, and cell-phone-based surface-enhanced Raman scattering (SERS) system. Considering the acoustic responsiveness and SERS-enhanced performance, gold nanorods were selected for biomedical applications. Functionalized gold nanorods can effectively capture and enrich biomarkers under ultrasonic aggregation. Such approaches can actively assemble gold nanorods in 1-2 s and achieved highly sensitive (6.15 × 10-13 M) SERS detection of COVID-19 biomarkers in nanoliter (10-7 L) samples within 5 min. We further demonstrated the high stability, repeatability, and selectivity of the platform, and validated its potential for the detection of throat swab samples. This simple, portable, and ultra-trace integrated active enrichment detection platform is a promising diagnostic tool for the direct and rapid detection of COVID-19.

An Ultrasensitive miRNA-Based Genosensor for Detection of MicroRNA 21 in Gastric Cancer Cells Based on Functional Signal Amplifier and Synthesized Perovskite-Graphene Oxide and AuNPs

In the present research work, the state-of-art label-free electrochemical genosensing platform was developed based on the hybridization process in the presence of [Fe(CN)₆]^3-/4- as an efficient redox probe for sensitive recognition of the miRNA-21 in human gastric cell lines samples. To attain this aim, perovskite nanosheets were initially synthesized. Afterward, the obtained compound was combined with the graphene oxide resulting in an effective electrochemical modifier, which was dropped on the surface of the Au electrode. Then, AuNPs (Gold Nano Particles) have been electrochemically-immobilized on perovskite-graphene oxide/Au-modified electrode surface through the chronoamperometry (CA) technique. Finally, a self-assembling monolayer reaction of ss-capture RNA ensued by the thiol group at the end of the probe with AuNPs on the modified electrode surface. miRNA-21 has been cast on the Au electrode surface to apply the hybridization process. To find out the effectiveness of the synthesized modifier agent, the electrochemical behavior of the modified electrode has been analyzed through DPV (differential pulse voltammetry) and CV (cyclic voltammetry) techniques. The prepared biomarker-detection bioassay offers high sensitivity and specificity, good performance, and appropriate precision and accuracy for the highly-sensitive determination of miRNA-21. Different characterization methods have been used, such as XRD, Raman, EDS, and FE-SEM, for morphological characterization and investigation of particle size. Based on optimal conditions, the limit of detection and quantification have been acquired at 2.94 fM and 8.75 fM, respectively. Furthermore, it was possible to achieve a wide linear range which is between 10^-14 and 10^-7 for miRNA-21. Moreover, the selectivity of the proposed biosensing assay was investigated through its potential in the detection of one, two, and three-base mismatched sequences. Moreover, it was possible to investigate the repeatability and reproducibility of the related bio-assay. To evaluate the hybridization process, it is important that the planned biomarker detection bio-assay could be directly re-used and re-generated.

Universal linker-free assembly of core-satellite hetero-superstructures

Colloidal superstructures comprising hetero-building blocks often show unanticipated physical and chemical properties. Here, we present a universal assembly methodology to prepare hetero-superstructures. This straightforward methodology allows the assembly of building block materials varying from inorganic nanoparticles to living cells to form superstructures. No molecular linker is required to bind the building blocks together and thus the products do not contain any unwanted adscititious material. The Fourier transform infrared spectra, high resolution transmission electron microscopic images and nanoparticle adhesion force measurement results reveal that the key to self-organization is stripping surface ligands by adding non-polar solvents or neutralizing surface charge by adding salts, which allow us to tune the balance between van der Waals attraction and electrostatic repulsion in the colloid so as to trigger the assembling process. As a proof-of-concept, the superior photocatalytic activity and single-particle surface-enhanced Raman scattering of the corresponding superstructures are demonstrated. Our methodology greatly extends the scope of building blocks for superstructure assembly and enables scalable construction of colloidal multifunctional materials.

Simultaneous Visualization of Dual Intercellular Signal Transductions via SERS Imaging of Membrane Proteins Dimerization on Single Cells

The visualization of protein dimerization on live cells is an urgent need and of vital importance for facile monitoring the signal transduction during intercellular communication. Herein, a highly sensitive and specific SERS strategy for simultaneously imaging dual homodimerizations of membrane proteins on single live cells was proposed by networking of AuNPs-based dual-recognition probes (dual-RPs) and SERS tags via proximity ligation-assisted catalytic hairpin assembly (CHA). The dual-RPs were prepared by comodifying hairpin-structured ssDNAs H1-Met and H1-TβRII on 50 nm AuNPs and two SERS tags for membrane proteins Met and TβRII were prepared respectively by labeling their corresponding Raman molecules and hairpin-structured single-stranded DNAs H2-Met or H2-TβRII on 15 nm AuNPs. The membrane proteins were ligated proximally by specific aptamers, and the dimerizations of proteins resulted in the proximity ligation-assisted CHA-based networking of dual-RPs and SERS tags to form 15Au-50Au network nanostructures with significantly enhanced SERS effect. The SERS strategy for visualizing the membrane protein dimerization was established and the good performance on simultaneously SERS imaging dual dimerizations of membrane proteins (i.e., Met-Met and TβRII-TβRII) was confirmed. Furthermore, the membrane protein dimerization-based signaling pathways between cancer cells and stromal cells or stem cells were observed by SERS, which indicates that the proposed SERS strategy is a good method for high-sensitivity monitoring of membrane proteins dimerizations-based multiple intercellular signal transductions in a natural and complex cellular microenvironment.

Recent advances of catalytic hairpin assembly and its application in bioimaging and biomedicine

Catalytic hairpin assembly (CHA) appears to be a particularly appealing nucleic acid circuit because of its powerful amplification capability, simple protocols, and enzyme-free and isothermal conditions, and can combine with various signal output modes for the biosensing of various analytes. Especially in the last five years, vast CHA related studies have sprung up. With the deep exploration of the CHA mechanism, some novel and excellent CHA strategies have been proposed; meanwhile the CHA cascade strategies with various amplification techniques further improve the analysis performance. Furthermore, diverse CHA based biosensors have been tactfully engineered and extensively employed in imaging applications in living cells and in vivo ascribed to its gentle reaction, efficient amplification and universality. Hence, we present a comprehensive and systematic summary of the progress in CHA and its application in bioimaging and biomedicine to date. At first, we introduced the mechanism and diversification of CHA in detail, including the newly developed CHA and its ingenious combination with a variety of other technologies. Concurrently, we summarized the latest application progress of different CHA strategies in bioimaging and biomedicine, highlighting the merits and drawbacks of representative approaches. Finally, we put forward some views on the challenges and prospects of CHA in bioimaging and biomedicine in the future.

Multistage nucleic acid amplification induced nano-aggregation for 3D hotspots-improved SERS detection of circulating miRNAs

Circulating miRNAs in the blood can regulate disease development and thus indicate disease states via their various expression levels. For these reasons, circulating miRNAs constitute useful biomarkers, and an approach to the accurate detection of circulating miRNAs is attractive in the diagnosis and treatment of diseases. However, methods for clinical detecting of circulating miRNA that take both sensitivity and practicality into account are still needed. Therefore, we aimed herein to solve some inherent problems in the actual detection using a robust surface-enhanced Raman scattering (SERS) platform with integrated nucleic acid amplification and nanoparticle aggregation to construct 3D hotspots for improving performance of analyzing circulating miRNAs. After target recognition and initial signal amplification by DNAzyme, we observed that release triggered an open hairpin DNA on gold nanoparticles (AuNPs), which then promote AuNP aggregation, causing the accumulation of a large number of hotspots in three-dimention. The SERS biosensor achieved a better performance than the sandwich-type separation detection, with a low detection limit (0.37 fM) and a broad linear range (1 fM–10 nM) in liquids. This SERS platform can be used as a powerful tool for the detection of circulating miRNAs, and it can be used to improve the sensitivity and accuracy of various clinical-disease diagnoses.

Sense and Validate: Fluorophore/Mass Dual-Encoded Nanoprobes for Fluorescence Imaging and MS Quantification of Intracellular Multiple MicroRNAs

Simultaneously monitoring and quantifying intracellular multiple microRNAs (miRNAs) is highly essential to clinical diagnosis and pathological research. However, revealing the intracellular distribution of multiple miRNAs while determining their content in a multiplex and quantitative format remains challenging. Considering the respective technical merit of fluorescence imaging and mass spectrometry (MS) in in situ detection and multiplex assaying, we herein propose fluorophore/mass dual-encoded nanoprobes (FMNPs) that can execute target-triggered hairpin self-assembly to enable in situ amplified imaging and follow-up MS quantification of intracellular multiple miRNAs. The FMNPs responsive to the target miRNA were constructed by codecorating gold nanoparticles (AuNPs) with locked hairpin DNA probes (LH1) and corresponding mass tags (MTs) for fluorescent and mass spectrometric dual-modal readout. Cellular miRNAs can separately trigger recycled hairpin self-assembly, leading to the continuous liberation of fluorophore-labeled bolt DNA (bDNA) for fluorescence imaging in cells. Moreover, the postreaction FMNPs afford an extra chance to validate the fluorescence output of miRNA-21 and miRNA-141 by accurate MS quantification relying on the ion signal of the barcoded MTs. Fluorescence imaging and MS quantification of miRNA-21 and miRNA-141 have also been successfully accomplished in different cell lines, highlighting its potential in cell subtyping. This "sense-and-validate" strategy creates a new modality for assaying multiple intracellular miRNAs and holds great promise in unveiling multicomponent-involved events in cellular processes and determining multiple biomarkers in accurate clinical diagnosis.

Shedding Light on DNA-Based Nanoprobes for Live-Cell MicroRNA Imaging

DNA-based nanoprobes integrated with various imaging signals have been employed for fabricating versatile biosensor platforms for the study of intracellular biological process and biomarker detection. The nanoprobes developments also provide opportunities for endogenous microRNA (miRNA) in situ analysis. In this review, the authors are primarily interested in various DNA-based nanoprobes for miRNA biosensors and declare strategies to reveal how to customize the desired nanoplatforms. Initially, various delivery vehicles for nanoprobe architectures transmembrane transport are delineated, and their biosecurity and ability for resisting the complex cellular environment are evaluated. Then, the novel strategies for designing DNA sequences as target miRNA specific recognition and signal amplification modules for miRNA detection are presented. Afterward, recent advances in imaging technologies to accurately respond and produce significant signal output are summarized. Finally, the challenges and future directions in the field are discussed.

Recent Advances in Engineered Noble Metal Nanomaterials as a Surface-Enhanced Raman Scattering Active Platform for Cancer Diagnostics

Recently, noble metal nanomaterials have been extensively studied in the fields of biosensing, environmental catalysis, and cancer diagnosis and treatment, due to their excellent electrical conductivity, high surface area, and individual physical and optical properties. Early research on the surface-enhanced Raman scattering (SERS) effect was focused on the cognition of the SERS phenomenon and enhancing its sensitivity for single-molecule detection. With the development of nanomaterials and nanotechnology, the advances and applications based on SERS substrates have been accelerated. Among them, noble metal nanomaterials are mainly used as SERS-active substrates to enhance SERS signals owing to their compelling surface plasmon resonance (SPR) properties. This review provides recent advances, perspectives, and challenges in SERS assays based on engineered noble metal nanomaterials for early cancer diagnosis.

Recent advances in catalytic hairpin assembly signal amplification-based sensing strategies for microRNA detection

Accumulative evidences have indicated that abnormal expression of microRNAs (miRNAs) is closely associated with many health disorders, making them be regarded as potentialbiomarkers for early clinical diagnosis. Therefore, it is extremely necessary to develop a highly sensitive, specific and reliable approach for miRNA analysis. Catalytic hairpin assembly (CHA) signal amplification is an enzyme-free toehold-mediated strand displacement method, exhibiting significant potential in improving the sensitivity of miRNA detection strategies. In this review, we first describe the potential of miRNAs as disease biomarkers and therapeutics, and summarize the latest advances in CHA signal amplification-based sensing strategies for miRNA monitoring. We describe the characteristics and mechanism of CHA signal amplification and classify the CHA-based miRNA sensing strategies into several categories based on the "signal conversion substance", including fluorophores, enzymes, nanomaterials, and nucleotide sequences. Sensing performance, limit of detection, merits and disadvantages of these miRNA sensing strategies are discussed. Moreover, the current challenges and prospects are also presented.

Toward Sensitive and Reliable Surface-Enhanced Raman Scattering Imaging: From Rational Design to Biomedical Applications

Early specific detection through indicative biomarkers and precise visualization of lesion sites are urgent requirements for clinical disease diagnosis. However, current detection and optical imaging methods are insufficient for these demands. Molecular imaging technologies are being intensely studied for reliable medical diagnosis. In the past several decades, molecular imaging with surface-enhanced Raman scattering (SERS) has significant advances from analytical chemistry to medical science. SERS is the inelastic scattering generated from the interaction between photons and substances, presenting molecular structure information. The outstanding SERS virtues of high sensitivity, high specificity, and resistance to biointerference are highly advantageous for biomarker detection in a complex biological matrix. In this work, we review recent progress on the applications of SERS imaging in clinical diagnostics. With the assistance of SERS imaging, the detection of disease-related proteins, nucleic acids, small molecules, and pH of the cellular microenvironment can be implemented for adjuvant medical diagnosis. Moreover, multimodal imaging integrates the high penetration and high speed of other imaging modalities and imaging precision of SERS imaging, resulting in final complete and accurate imaging outcomes and exhibiting robust potential in the discrimination of pathological tissues and surgical navigation. As a promising molecular imaging technology, SERS imaging has achieved remarkable performance in clinical diagnostics and the biomedical realm. It is expected that this review will provide insights for further development of SERS imaging and promote the rapid progress and successful translation of advanced molecular imaging with clinical diagnostics.

Multiplexed Liquid Biopsy and Tumor Imaging Using Surface-Enhanced Raman Scattering

The recent improvements in diagnosis enabled by advances in liquid biopsy and oncological imaging significantly better cancer care. Both these complementary approaches, which are used for early tumor detection, characterization, and monitoring, can benefit from applying techniques based on surface-enhanced Raman scattering (SERS). With a detection sensitivity at the single-molecule level, SERS spectroscopy is widely used in cell and molecular biology, and its capability for the in vitro detection of several types of cancer biomarkers is well established. In the last few years, several intriguing SERS applications have emerged, including in vivo imaging for tumor targeting and the monitoring of drug release. In this paper, selected recent developments and trends in SERS applications in the field of liquid biopsy and tumor imaging are critically reviewed, with a special emphasis on results that demonstrate the clinical utility of SERS.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Dual-Modal FexCuySe and Upconversion Nanoparticle Assemblies for Intracellular MicroRNA-21 Detection

In situ quantification and imaging of low-level intracellular microRNAs (miRs) are important areas in biosensor research. Herein, DNA-driven Fe_xCu_ySe@upconversion nanoparticle (UCNP) core@satellite nanostructures were developed to probe microRNA-21 (miR-21). Fe_xCu_ySe@UCNP probes displayed dual signals: upconversion luminescence (UCL) and magnetic resonance imaging (MRI). In the presence of miR-21, the luminescence signal was restored and the T₂ value was significantly increased because of dissociation of UCNPs from the assemblies. There was a good linear relationship between the dual signals and the expression levels of miR-21 in the range of 0.035-31.824 amol/ng_RNA. The limit of detection (LOD) was 0.0058 amol/ng_RNA for the luminescence intensity and 0.0182 amol/ng_RNA for the MRI signal. This method opens a new avenue for intracellular miR-21 detection with high sensitivity and specificity.

Ultrasensitive and Simultaneous SERS Detection of Multiplex MicroRNA Using Fractal Gold Nanotags for Early Diagnosis and Prognosis of Hepatocellular Carcinoma

Sensitive and simultaneous detection of multiple cancer-related biomarkers in serum is essential for diagnosis, therapy, prognosis, and staging of cancer. Herein, we proposed a magnetically assisted sandwich-type surface-enhanced Raman scattering (SERS)-based biosensor for ultrasensitive and multiplex detection of three hepatocellular carcinoma-related microRNA (miRNA) biomarkers. The biosensor consists of an SERS tag (probe DNA-conjugated DNA-engineered fractal gold nanoparticles, F-AuNPs) and a magnetic capture substrate (capture DNA-conjugated Ag-coated magnetic nanoparticles, AgMNPs). The proposed strategy achieved simultaneous and sensitive detection of three miRNAs (miRNA-122, miRNA-223, and miRNA-21), and the limits of detection of the three miRNAs in human serum are 349 aM for miRNA-122, 374 aM for miRNA-223, and 311 aM for miRNA-21. High selectivity and accuracy of the SERS biosensor were proved by practical analysis in human serum. Moreover, the biosensor exhibited good practicability in multiplex detection of three miRNAs in 92 clinical sera from AFP-negative patients, patients before and after hepatectomy, recurred and relapse-free patients after hepatectomy, and hepatocellular carcinoma patients at distinct Barcelona clinic liver cancer stages. The experiment results demonstrate that our SERS-based assay is a promising candidate in clinical application and exhibited potential for the prediction, diagnosis, monitoring, and staging of cancers.

Recent progress of SERS optical nanosensors for miRNA analysis

This review focuses on emerging applications of surface-enhanced Raman spectroscopy (SERS) optical nanosensors for miRNA analysis, in which the key enhancement factors of the SERS signal, i.e. SERS-active substrates, SERS nanoprobes and nano-assembly strategy, are emphasized. This article includes many nanomaterials for miRNA analysis by the SERS technique. We summarize these reported nanomaterials mainly according to their function in the miRNA assay biosensor. We also briefly summarize the research progress of these nanomaterials in SERS detection of intracellular miRNA. Finally, we discussed the prospect and limitations of SERS nanosensors for analyzing miRNA.

Improving the Sensitivity of the miRNA Assay Coupled with the Mismatched Catalytic Hairpin Assembly Reaction by Optimization of Hairpin Annealing Conditions

The mismatched catalytic hairpin assembly (mCHA), a programmable oligonucleotide circuit, is one of the promising isothermal amplification methods used in nucleic acid detection. Its limitations are related to a high background noise observed due to the target-independent hybridization of the reacting hairpins (HPs). In this work, it was shown that the introduction of salts such as NaCl and MgCl₂ to HP1/HP2 annealing solutions sharply reduces the background in mCHA and simultaneously increases the signal-to-background (S/B) ratio. A comparison of the salts demonstrated the higher activity of MgCl₂ as compared to NaCl. A similar effect of reducing the background was observed with a decrease in the concentration of H1/H2 probes in annealing solutions. Using the favorable annealing conditions allowed the development of an ultrasensitive chemiluminescence assay coupled with mCHA for miRNA quantitation. Except mCHA, the use of a streptavidin-polyHRP conjugate and an enhanced chemiluminescence reaction additionally increased the assay sensitivity. Notably, the optimization of the HP annealing diminished the detection limit of the assay by 2 orders of magnitude and increased the sensitivity and precision of miRNA-141 determination. The discovered fact of reducing the background by the variation of HP annealing conditions may be valuable not only for the mCHA performance but also likely for other HP-based biochemical methods.

Ultrasensitive Nucleic Acid Assay Based on AIE-Active Polymer Dots with Excellent Electrochemiluminescence Stability

Aggregation-induced emission (AIE) active Pdots are attractive nanomaterials applied in electrochemiluminescence (ECL) fields, while the irreversible redox reaction of these Pdots is a prevailing problem, resulting in instability of ECL emission. Herein, we first designed and synthesized an AIE-active Pdot with reversible redox property, which contains a tetraphenylethene derivate and benzothiadiazole (BT) to achieve stable ECL emission. BT has a good rigid structure with excellent electrochemical behaviors, which is beneficial for avoiding the destruction of the conjugated structure as much as possible during the preparation of Pdots, thus maintaining good redox property. The tetraphenylethene derivate, as a typical AIE-active moiety, provides a channel for highly efficient luminescence in the aggregated states. The Pdots exhibited reversible and quasi-reversible electrochemical behaviors during cathodic and anodic scanning, respectively. The stable annihilation, reductive-oxidative, and oxidative-reductive ECL signals were observed. Subsequently, we constructed an ultrasensitive ECL biosensor based on the oxidative-reductive ECL mode for the detection of miRNA-21 with a detection limit of 32 aM. This work provides some inspiration for the future design of ECL materials featuring AIE-active property and stable ECL emission.

Dual-Mode SERS and Electrochemical Detection of miRNA Based on Popcorn-like Gold Nanofilms and Toehold-Mediated Strand Displacement Amplification Reaction

MicroRNA (miRNA) has emerged as one of the ideal target biomarker analytes for cancer detection because its abnormal expression is closely related to the occurrence of many cancers. In this work, we combined three-dimensional (3D) popcorn-like gold nanofilms as novel surface-enhanced Raman scattering (SERS)-electrochemistry active substrates with toehold-mediated strand displacement reactions (TSDRs) to construct a DNA molecular machine for SERS-electrochemistry dual-mode detection of miRNA. 3D popcorn-like spatial structures generated more active "hot spots" and thus enhanced the sensitivity of SERS and electrochemical signals. Besides, the TSDRs showed high sequence-dependence and high specificity. The addition of target miRNA will trigger the molecular machine to perform two TSDRs in the presence of signal DNA strands modified by R6G (R6G-DNA), thus achieving an enzyme-free amplification detection of miRNA with a low limit of detection of 0.12 fM (for the SERS method) and 2.2 fM (for the electrochemical method). This biosensor can also serve as a universally amplified and sensitive detection platform for monitoring different biomarkers, such as cancer-related DNA, messenger RNA, or miRNA molecules, with high selectivity by changing the corresponding probe sequence.

Algorithm-Assisted Detection and Imaging of microRNAs in Living Cancer Cells via the Disassembly of Plasmonic Core-Satellite Probes Coupled with Strand Displacement Amplification

Acute detection and high-resolution imaging of microRNAs (miRNAs) in living cancer cells have attracted great attention in clinical diagnosis and therapy. However, current methods suffer from low detection sensitivity or heavy dependence on expensive and sophisticated spectrometers. Herein, a novel algorithm-assisted system of detecting and imaging miRNAs in living cancer cells was developed via the disassembly of plasmonic core-satellite probes coupled with strand displacement amplification (SDA). The target miRNAs in the system could trigger the disassembly of plasmonic core-satellite probes, leading to the color change in the scattering light of the probes, which could be captured by dark-field microscopy (DFM). The concentration of the target miRNAs was obtained by analyzing the dark-field image based on the proposed algorithm with a detection limit of 2 pM for miRNA-21. Thus, the performance in terms of simplicity and sensitivity of the system compared with one of the conventional spectrophotometers was well presented, which could inspire more clinical applications of inexpensive, intelligent, and rapid screening of cancer cells. The application software based on the proposed algorithm running on the Android platform was also developed, demonstrating the potential of remote diagnosis.

Photonic technologies for liquid biopsies: recent advances and open research challenges

The recent development of sophisticated techniques capable of detecting extremely low concentrations of circulating tumor biomarkers in accessible body fluids, such as blood or urine, could contribute to a paradigm shift in cancer diagnosis and treatment. By applying such techniques, clinicians can carry out liquid biopsies, providing information on tumor presence, evolution, and response to therapy. The implementation of biosensing platforms for liquid biopsies is particularly complex because this application domain demands high selectivity/specificity and challenging limit-of-detection (LoD) values. The interest in photonics as an enabling technology for liquid biopsies is growing owing to the well-known advantages of photonic biosensors over competing technologies in terms of compactness, immunity to external disturbance, and ultra-high spatial resolution. Some encouraging experimental results in the field of photonic devices and systems for liquid biopsy have already been achieved by using fluorescent labels and label-free techniques and by exploiting super-resolution microscopy, surface plasmon resonance, surface-enhanced Raman scattering, and whispering gallery mode resonators. This paper critically reviews the current state-of-the-art, starting from the requirements imposed by the detection of the most common circulating biomarkers. Open research challenges are considered together with competing technologies, and the most promising paths of improvement are discussed for future applications.