Highly selective and sensitive surface enhanced Raman scattering nanosensors for detection of hydrogen peroxide in living cells

Quantitative Factors Introduced in the Feasibility Analysis of Reactive Oxygen Species (ROS)-Sensitive Triggers

Reactive oxygen species (ROS) are critical for cellular signaling. Various pathophysiological conditions are also associated with elevated levels of ROS. Hence, ROS-sensitive triggers have been extensively used for selective payload delivery. Such applications are predicated on two key functions: (1) a sufficient magnitude of concentration difference for the interested ROS between normal tissue/cells and intended sites and (2) appropriate reaction kinetics to ensure a sufficient level of selectivity for payload release. Further, ROS refers to a group of species with varying reactivity, which should not be viewed as a uniform group. In this review, we critically analyze data on the concentrations of different ROS species under various pathophysiological conditions and examine how reaction kinetics affect the success of ROS-sensitive linker chemistry. Further, we discuss different ROS linker chemistry in the context of their applications in drug delivery and imaging. This review brings new insights into research in ROS-triggered delivery, highlights factors to consider in maximizing the chance for success and discusses pitfalls to avoid.

Application of the Electrospinning Technique in Electrochemical Biosensors: An Overview

Electrospinning is a cost-effective and flexible technology for producing nanofibers with large specific surface areas, functionalized surfaces, and stable structures. In recent years, electrospun nanofibers have attracted more and more attention in electrochemical biosensors due to their excellent morphological and structural properties. This review outlines the principle of electrospinning technology. The strategies of producing nanofibers with different diameters, morphologies, and structures are discussed to understand the regulation rules of nanofiber morphology and structure. The application of electrospun nanofibers in electrochemical biosensors is reviewed in detail. In addition, we look towards the future prospects of electrospinning technology and the challenge of scale production.

Fluorescent sensor based on bismuth metal-organic frameworks (Bi-MOFs) mimic enzyme for H2O2 detection in real samples and distinction of phenylenediamine isomers

Although peroxidase-like nano-enzymes have been widely utilized in biosensors, nano-enzyme based biosensors are seldom used for both quantitative analysis of H2O2 and differentiation of isomers of organic compounds simultaneously. In this study, a dual-functional mimetic enzyme-based fluorescent sensor was constructed using metal-organic frameworks (Bi-MOFs) with exceptional oxidase activity and fluorescence properties. This mimetic enzyme sensor facilitated quantitative analysis of H2O2 and accurate discrimination of phenylenediamine isomers. The sensor exhibited a wide linear range (0.5-400 μM) and low detection limit (0.16 μM) for the detection of H2O2. Moreover, the sensor can also be used for the discrimination of phenylenediamine isomers, in which the presence of o-phenylenediamine (OPD) leads to the appearance of a new fluorescence emission peak at 555 nm, while the presence of p-phenylenediamine (PPD) significantly quenched its fluorescence due to the internal filtration effect. The proposed strategy exhibited a commendable capability in distinguishing phenylenediamine isomers, thereby paving the way for novel applications of MOFs in the field of environmental science.

Efficient Electrochemical Microsensor for the Simultaneous Measurement of Hydrogen Peroxide and Ascorbic Acid in Living Brains

Hydrogen peroxide (H2O2) and ascorbic acid (AA), acting as two significant indicative species, correlate with the oxidative stress status in living brains, which have historically been considered to be involved mainly in neurodegenerative disorders such as Alzheimer's disease, Huntington's disease, and Parkinson's disease (PD). The development of efficient biosensors for the simultaneous measurement of their levels in living brains is vital to understand their roles played in the brain and their interactive relationship in the progress of these diseases. Herein, a robust ratiometric electrochemical microsensor was rationally designed to realize the determination of H2O2 and AA simultaneously. Therefore, a specific probe was designed and synthesized with both recognition units responsible for reacting with H2O2 to produce a detectable signal on the microsensor and linkage units helping the probe modify onto the carbon substrate. A topping ingredient, single-walled carbon nanotubes (SWCNTs) was added on the surface of the electrode, with the purpose of not only facilitating the oxidation of AA but also absorbing methylene blue (MB), prompting to read out the inner reference signal. This proposed electrochemical microsensor exhibited a robust ability to real-time track H2O2 and AA in linear ranges of 0.5-900 and 10-1000 μM with high selectivity and accuracy, respectively. Eventually, the efficient electrochemical microsensor was successfully applied to the simultaneous measurement of H2O2 and AA in the rat brain, followed by microinjection, and in the PD mouse brain.

Fabrication of multifunctional g-C3N4-modified Au/Ag NRs arrays for ultrasensitive and recyclable SERS detection of bisphenol A residues

Monolayer g-C3N4-modified Au/Ag nanorods (g-C3N4/Au/Ag NRs) array is fabricated as a dual-function platform with high surface-enhanced Raman scattering (SERS) response and excellent photocatalytic degradation ability for bisphenol A (BPA) residues. FDTD simulation results of Au/Ag NRs proves that the electromagnetic field intensity is significantly enhanced at the gap of Ag NRs and Au NPs and the protrusion of Au NPs, which endows the arrays with excellent SERS activity. The arrays exhibit high sensitivity for rhodamine 6G (R6G) (LOD = 1.1 × 10-11 mol/L) and high SERS enhancement (EF = 9.2 × 107). In addition, the g-C3N4/Au/Ag NRs could degrade ˃90% of BPA adsorbed on the substrate surface within 140 min under visible light irradiation, and maintains its SERS activity after repeated use for 4 times. The dual-function platform with high SERS response and excellent recycling capability is proved to be reliable and is very promising for monitoring of BPA residues in food.

Smart Ratiometric SERS Nanoprobe for Real-Time Monitoring Hydrogen Peroxide in Living Cells during NADH Treatment Associated with Ferroptosis

Studying the oxidative stress, especially the reactive oxygen species (ROS) response of ferroptosis, is crucial for the diagnosis and treatment of cancer based on ferroptosis. However, reliable quantitative analysis of intracellular ROS in cancer treatment for drug screening is still a challenge. Herein, a superior ratiometric SERS nanoprobe was developed for in situ, real-time, and highly sensitive detection of content variation of H2O2 within living cells. The SERS nanoprobe was prepared by coassembly of the internal standard molecule p-mercaptobenzonitrile and the reporter molecule p-mercaptophenylboronic acid on the surface of gold nanoparticles, used for synergistic calibration and detection of H2O2, which enables reliable detection of the true content of intracellular H2O2 without the interference of other substances in cells. Based on the nanoprobe, we found that the level of intracellular H2O2 of cancer cells was increased after the nicotinamide adenine dinucleotide (NADH) treatment, with a dose-dependence to the concentration of NADH. High doses of NADH (above 20 mM) can induce cell death by means of ferroptosis associated with the level elevation of intracellular lipid hydroperoxides. This study highlights the potential of the SERS nanoprobe for tracking content variation of cellular H2O2 and understanding its roles in screening new anticancer drugs.

Research Progress of SERS Sensors Based on Hydrogen Peroxide and Related Substances

Hydrogen peroxide (H2O2) has an important role in living organisms, and its detection is of great importance in medical, chemical, and food safety applications. This review provides a comparison of different types of Surface-enhanced Raman scattering (SERS) sensors for H2O2 and related substances with respect to their detection limits, which are of interest due to high sensitivity compared to conventional sensors. According to the latest research report, this review focuses on the sensing mechanism of different sensors and summarizes the linear range, detection limits, and cellular applications of new SERS sensors, and discusses the limitations in vivo and future prospects of SERS technology for the detection of H2O2.

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-enhanced Raman scattering: An emerging tool for sensing cellular function

Continuous long-term intracellular imaging and multiplexed monitoring of biomolecular changes associated with key cellular processes remains a challenge for the scientific community. Recently, surface-enhanced Raman scattering (SERS) has been demonstrated as a powerful spectroscopic tool in the field of biology owing to its significant advantages. Some of these include the ability to provide molecule-specific information with exquisite sensitivity, working with small volumes of precious samples, real-time monitoring, and optimal optical contrast. More importantly, the availability of a large number of novel Raman reporters with narrower full width at half maximum (FWHM) of spectral peaks/vibrational modes than conventional fluorophores has created a versatile palette of SERS-based probes that allow targeted multiplex sensing surpassing the detection sensitivity of even fluorescent probes. Due to its nondestructive nature, its applicability has been recognized for biological sensing, molecular imaging, and dynamic monitoring of complex intracellular processes. We critically discuss recent developments in this area with a focus on different applications where SERS has been used for obtaining information that remains elusive for conventional imaging methods. Current reports indicate that SERS has made significant inroads in the field of biology and has the potential to be used for in vivo human applications. This article is categorized under: Diagnostic Tools > In Vitro Nanoparticle-Based Sensing Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > Biosensing Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Cr2O3-TiO2-Modified Filter Paper-Based Portable Nanosensors for Optical and Colorimetric Detection of Hydrogen Peroxide

In the present approach, a Cr₂O₃-TiO₂-modified, portable, and biomimetic nanosensor was designed to meet the requirement of a robust and colorimetric sensing of hydrogen peroxide. Cr₂O₃-TiO₂ nanocomposites prepared via the hydrothermal method were fabricated as a transducer surface on the filter paper using the sol-gel matrix. The color on the filter paper sensor changed from green to blue upon the addition of hydrogen peroxide in the presence of TMB. This change in the color intensity was linear with the concentration of H₂O₂. RGB software was used as a color analyzing model to evaluate the optical signals. This paper-based colorimetric platform provided us with an improved analytical figure of merit with a linear range of 0.005-100 μM with 0.003 μM limit of detection. The real sample analysis and excellent anti-interference potential results proved the good analytical performance of the proposed design, providing a more promising tool for colorimetric H₂O₂ detection. Introducing Cr₂O₃-TiO₂ nanocomposite-based paper sensors, being a novel method for optical and colorimetric detection, can pave the way for the development of other sensing devices for the detection of different analytes.

Acid-responsive endosomolytic polymeric nanoparticles with amplification of intracellular oxidative stress for prodrug delivery and activation

Prodrug strategy especially in the field of chemotherapy of cancers possesses significant advantages reducing the side toxicity of anticancer drugs. However, high-efficiency delivery and in situ activation of prodrugs for tumor growth suppression are still a great challenge. Herein, we report rationally engineered pH-responsive endosomolytic polymeric micelles for the delivery of an oxidation-activable prodrug into the cytoplasm of cancer cells and amplification of intracellular oxidative stress for further prodrug activation. The prepared block copolymers consist of a poly(ethylene glycol) (PEG) block and a segment grafted by endosomolytic moieties and acetal linkage-connected cinnamaldehyde groups. The amphiphilic diblock copolymers can self-assemble to form micelles in water for loading the oxidation-activable phenylboronic pinacol ester-caged camptothecin prodrug (ProCPT). The obtained micelles can release free cinnamaldehyde under acidic conditions in tumor tissues and endo/lysosomes followed by efficient endosomal escape, which further induces enhancement of intracellular reactive oxygen species (ROS) to activate the prodrugs. Simultaneously, intracellular glutathione (GSH) can be reduced by quinone methide that was produced during prodrug activation. The ProCPT-loaded micelles can finally achieve efficient tumor accumulation and retention as well as effective tumor growth inhibition. More importantly, hematological and pathological analysis of toxicity reveals that the ProCPT-loaded micelles do not cause obvious toxic side effects toward important organs of mice. A positive immunomodulatory microenvironment in tumor tissue and serum can be detected after treatment with ProCPT-loaded micelles. Therefore, the endosomolytic ProCPT-loaded micelles exert synergistic therapeutic effects toward tumors through amplification of intracellular oxidative stress and activation of the prodrugs.

Plasma membrane anchored nanosensor for quantifying endogenous production of H2O2 in living cells

Hydrogen peroxide (H₂O₂) is one of the main second messengers involved in signaling pathways controlling cell metabolism. During tumorigenesis H₂O₂ is generated on the extracellular space by membrane-associated NADPH oxidases and superoxide dismutase to stimulate cell proliferation and preservation of the transformed state. Accordingly, a characteristic feature of malignant cells is overproduction of H₂O₂ in the extracellular milieu and the subsequent absorption in the cytosol. Since the most significant gradients of endogenous extracellular H₂O₂ can be observed only in a very shallow region of the fluid in contact with the plasma membrane, we show here the use of a newly designed nanosensor anchored to the outer cell surface and capable of quantifying H₂O₂ at nanometer distance from the membrane proteins responsible for its production. This biosensor is built upon gold nanoparticles functionalized with a H₂O₂-sensitive boronate compound that is probed using surface enhanced Raman spectroscopy (SERS). The highly localized information obtained on the cell surface by SERS analysis is combined with analytical methods of redox biology to estimate the associated levels of intracellular H₂O₂ responsible for cell signaling. The results obtained from A549 lung cancer cell line show localized spots on the cell surface at concentration up to 12 μM, associated to intracellular concentration up to 5.1 nM.

Bioapplications of Nanomaterials

Nanobiotechnology is known as the application of nanoscaled techniques in biology which bridges natural science to living organism for improving the quality of life of humans. Nanotechnology was first issued in 1959 and has been rapidly developed, supplying numerous benefits to basic scientific academy and to clinical application including human healthcare, specifically in cancer therapy. This chapter discusses recent advances and potentials of nanotechnology in pharmaceutics, therapeutics, biosensing, bioimaging, and gene delivery that demonstrate the multifunctionality of nanotechnology.

In Situ Growth of AuNPs on Glass Nanofibers for SERS Sensors

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

Applications of advanced materials in bio-sensing in live cells: Methods and applications

A wide variety of species, such as different ions, reactive oxygen species, and biomolecules play critical roles in many cell functions. These species are responsible for a range of cellular functions such as signaling, and disturbed levels could be involved in many diseases, such as diabetes, cancer, neurodegeneration etc. Thus, sensitive and specific detection methods for these biomarkers could be helpful for early disease detection and mechanistic investigations. New ultrasensitive sensors for detection of markers within living cells are a growing field of research. The present review provides updates in live cell-based biosensing, which have been published within the last decade. These sensors are mainly based on carbon, gold and other metals, and their physicochemical advantages and limitations are discussed. Advanced materials can be incorporated into probes for the detection of various analytes in living cells. The sensitivity is strongly influenced by the intrinsic properties of the nanomaterials as well their shape and size. The mechanisms of action and future challenges in the developments of new methods for live cell based biosensing are discussed.

Luminescent europium(III)-organic framework for visual and on-site detection of hydrogen peroxide via a tablet computer

A luminescent metal-organic framework of type Eu(III)-MOF has been fabricated for visual and on-site fluorometric determination of hydrogen peroxide (H₂O₂) via a tablet computer. The maximum excitation and emission peaks of type Eu(III)-MOF were found at λex = 290 nm and λem = 615 nm, respectively. The average length of Eu-MOF is 1.21 ± 0.07 μm. In the presence of the target H₂O₂, Fe²⁺ is transmitted into Fe³⁺ via Fenton reaction, leading to a fluorescence quenching of Eu-MOF. Therefore, visible color change occurred from bright red into colorless. Interestingly, by means of tablet computer's digital camera and ImageJ software, fluorescent signals were captured and transduced into digital parameters, resulting in a linear relationship between fluorescence intensity and the concentration of H₂O₂. As a result, the determination of H₂O₂ without the aid of complicated instruments is achieved in the range 2.0 μM to 0.2 mM with a detection limit of 1.02 μM. Our approach has been successfully applied to quantify H₂O₂ in serum, urine, and waste water with good recovery and precision (< 2.5% RSD). Besides, our assay has been exploited for visual detection of H₂O₂ released from HepG2 cells with the advantages of portability and accuracy. Moreover, the strategy displays acceptable selectivity and stability. Hence, our assay provides an alternative practical method for on-site determination of H₂O₂ without the need for instruments. Graphical abstract Schematic representation of the synthesis procedure of a luminescent Eu-MOF, which has been successfully applied for on-site detection of H₂O₂ via Fenton reaction and imaging analysis technique. The method exhibits handheld and accuracy for H₂O₂ determination, holding the potential for biochemical and clinical applications in remote regions.

Directed evolution of a transcription factor PbrR to improve lead selectivity and reduce zinc interference through dual selection

Directed evolution has been proven as a powerful tool for developing proteins and strains with novel or enhanced features. In this study, a dual selection system was designed to tune the binding specificity of a transcription factor to a particular ligand with the ampicillin resistance gene amp (ON selection) as the positive selection marker and the levansucrase gene sacB (OFF selection) as the negative selection marker. It was applied to the lead responsive transcription factor PbrR in a whole-cell lead biosensor previously constructed in our lab (Jia et al. in Fems Microbiol Lett 365:fny157, 2018). After multiple rounds of ON–OFF selection, two mutants with higher specificity for lead were selected. Structural analysis revealed that the mutation C134 located on the metal-binding loop at the C-terminal of PbrR is likely associated with the enhanced binding to both lead and cadmium. The double mutations D64A and L68S close to the metal-binding residue C79 may lead to the reduced binding specificity toward zinc ions. This dual selection system can be applied to engineer the specificity of other transcription factors and provide fine-tuned tools to synthetic biology.

A simple enzyme-free SERS sensor for the rapid and sensitive detection of hydrogen peroxide in food

A simple enzyme-free method based on surface-enhanced Raman scattering (SERS) was developed for the first time to detect H₂O₂ in food by etching a self-assembled film of silver nanoparticles (Ag NPs) on a glass substrate.

Intracellular and Cellular Detection by SERS-Active Plasmonic Nanostructures

Surface-enhanced Raman scattering (SERS), with greatly amplified fingerprint spectra, holds great promise in biochemical and biomedical research. In particular, the possibility of exciting a library of SERS probes and differentially detecting them simultaneously has stimulated widespread interest in multiplexed biodetection. Herein, recent progress in developing SERS-active plasmonic nanostructures for cellular and intracellular detection is summarized. The development of nanosensors with tailored plasmonic and multifunctional properties for profiling molecular and pathological processes is highlighted. Future challenges towards the routine use of SERS technology in quantitative bioanalysis and clinical diagnostics are further discussed.

Phenylboronic acid derivative-modified (6,5) single-wall carbon nanotube probes for detecting glucose and hydrogen peroxide

In this paper, we presented a new method for constructing near-infrared fluorescence probes and their applications in detecting glucose and hydrogen peroxide (H₂O₂).

A microfluidic surface-enhanced Raman spectroscopy approach for assessing the particle number effect of AgNPs on cytotoxicity

Silver nanoparticles (Ag NPs) have well-known antibacterial properties and are widely applied in various medical products and general commodities. Although many studies have addressed the toxicity of Ag NPs to mammalian cells, the direct relationship between the number of Ag NPs in living cells and the corresponding cell toxicity has not yet been explicitly demonstrated. In this work, a simple and reusable microfluidic device composed of a quartz cover slip and a glass plate with etched micro-channel and micro-wells was employed for separating and trapping single living cells. The device was silanized to render the surface hydrophobic. For simplicity, HeLa cells as the model cancer cells were used in the study, which were pipette-loaded into an array of micro wells based on dead-end filling. Surface enhanced Raman spectroscopy (SERS) was then employed to examine the living cancer cells and assessed number and distribution of Ag NPs in the cells. Combined with the cell viability assay, we therefore correlated the number of Ag NPs in the cell with the toxicity to the cell directly.

Ultrasensitive detection of thyrotropin-releasing hormone based on azo coupling and surface-enhanced resonance Raman spectroscopy

Surface-enhanced resonance Raman scattering (SERRS) has been used to establish a rapid and quantitative assay based on the diazotization coupling reaction for thyrotropin-releasing hormone (TRH). Ultrahigh sensitivity of this approach originates from two factors: changing TRH to an azo compound and the SERRS effect with the addition of silver nanoparticles (AgNPs) at 532 nm excitation wavelength. The lowest detectable concentration of TRH was found to be as low as 1 pg mL(-1), which is 10-fold lower than the lowest normal reference value in human serum reported in previous literature. The quantitative measurements in human serum based on this method were conducted, and the results showed its feasibility for detection in complex biological samples. In comparison with conventional TRH identification and quantification methodologies, radioimmunoassay (RIA) and subsequent various hyphenated techniques, the main advantages of this study are simplicity, rapidness (2 minutes), time effectiveness, no additional steps required to further characterize the immunogenic material, highest sensitivity (57.1 fg), high selectivity, practicality and reliability. Thus, this work puts forward a research tool that may be applied to the determination of TRH in practical assays.

Flexible, transparent and highly sensitive SERS substrates with cross-nanoporous structures for fast on-site detection

A flexible and transparent film assembled from the cross-nanoporous structures of Au on PET (CNS of Au@PET) is developed as a versatile and effective SERS substrate for rapid, on-site trace analysis with high sensitivity. The fabrication of the CNS of Au can be achieved on a large scale at low cost by employing an etching process with super-aligned carbon nanotubes as a mask, followed by metal deposition. A strongly enhanced Raman signal with good uniformity can be obtained, which is attributed to the excitation of "hot spots" around the metal nanogaps and sharp edges. Using the CNS of Au@PET film as a SERS platform, real-time and on-site SERS detection of the food contaminant crystal violet (CV) is achieved, with a detection limit of CV solution on a tomato skin of 10-7 M. Owing to its ability to efficiently extract trace analytes, the resulting substrate also achieves detection of 4-ATP contaminants and thiram pesticides by swabbing the skin of an apple. A SERS detection signal for 4-ATP has a relative standard deviation of less than 10%, revealing the excellent reproducibility of the substrate. The flexible, transparent and highly sensitive substrates fabricated using this simple and cost-effective strategy are promising for practical application in rapid, on-site SERS-based detection.

Aptamer based SERS detection of Salmonella typhimurium using DNA-assembled gold nanodimers

The authors describe a surface-enhanced Raman scattering (SERS) based aptasensor for Salmonella typhimurium (S. typhimurium). Gold nanoparticles (AuNPs; 35 nm i.d.) were functionalized with the aptamer (ssDNA 1) and used as the capture probe, while smaller (15 nm) AuNPs were modified with a Cy3-labeled complementary sequence (ssDNA 2) and used as the signalling probe. The asymmetric gold nanodimers (AuNDs) were assemblied with the Raman signal probe and the capture probe via hybridization of the complementary ssDNAs. The gap between two nanoparticles is a "hot spot" in which the Raman reporter Cy3 is localized. It experiences a strong enhancement of the electromagnetic field around the particle. After addition of S. typhimurium, it will be bound by the aptamer which therefore is partially dehybridized from its complementary sequence. Hence, Raman intensity drops. Under the optimal experimental conditions, the SERS signal at 1203 cm^-1 increases linearly with the logarithm of the number of colonies in the 10² to 10⁷ cfu·mL^-1 concentration range, and the limit of detection is 35 cfu·mL^-1. The method can be performed within 1 h and was successfully applied to the analysis of spiked milk samples and performed very well and with high specificity. Graphical abstract DNA-assembled asymmetric gold nanodimers (AuNDs) were synthesized and appllied in a SERS-based aptasensor for S. typhimurium. Capture probe was preferentially combined with S. typhimurium and the structure of the AuNDs was destroyed. The "hot spot" vanished partly, this resulting in the decreased Raman intensity of Cy3.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.

Recent progress in surface-enhanced Raman spectroscopy for biological and biomedical applications: from cells to clinics

The application of surface-enhanced Raman spectroscopy (SERS) in biological and biomedical detection schemes is feasible due to its excellent molecular specificity and high sensitivity as well as the capability of SERS to be performed in complex biological compositions. SERS-based investigation of cells, which are the basic structure and functional unit of organisms, represents the starting point of this review. It is demonstrated that SERS provides a deep understanding of living cells as well as their microenvironment which is needed to assess the development of diseases. The clinical relevance of SERS is proved by its application for the detection of cancer cells and tumour margins under in vivo conditions and examples for theranostic approaches are discussed. This review article provides a comprehensive overview of the recent progress within the last 3 years.

Dual fluorescence nanoconjugates for ratiometric detection of reactive oxygen species in inflammatory cells

Reactive oxygen species (ROS) are largely produced under pathological situations. To understand the etiology of disease, it is urgent to develop efficacious probes for detecting ROS. Herein, a novel nanoconjugate detection system constructed from gold clusters (AuNCs) and quantum dots (QDs) for fluorescence ratiometric-sensing ROS was reported. Upon interacting with ROS, the red emission fluorescence (645 nm from QDs) in the detection system gradually decreased, while the green fluorescence (480 nm from AuNCs) changed little. The fluorescence ratio at the 2 wavelengths (I_{480 nm} /I_{645 nm} ) was linearly correlated with the ROS, which could be used for the real-time ratiometric detection of ROS. The developed nanoconjugates could be applied to monitor the ROS in inflammatory cells for its ability of generating abundant ROS and uptaking ability to nanoparticles. The stimulated ROS in inflammatory cells were monitored by AuNC-QD and the results were consistent with the traditional 2', 7'-dichlorofluorescin diacetate method, confirming the reliability of the developed method. Featured with the merits of higher photostability, low background, high accuracy of ratiometric detection, the AuNC-QD conjugate demonstrated its potential to be the probe for real-time ROS detection in inflammatory cells.

In-site encapsulating gold "nanowires" into hemin-coupled protein scaffolds through biomimetic assembly towards the nanocomposites with strong catalysis, electrocatalysis, and fluorescence properties

An efficient and green biomimetic assembly protocol was developed for the fabrication of multifunctional nanocomposites by mimicking the configuration of natural protein enzymes. Bovine serum albumin (BSA) as the protein model was first split to produce the disassembled BSA (dBSA) of linear polymer and then coupled with catalytic Hemin (Hem). The yielded dBSA-Hem scaffolds were utilized to in-site encapsulate gold nanoclusters (AuNCs) through biominerization, yielding the dBSA-Hem-AuNCs. It was discovered that the nanocomposites could display the well-defined composition and spheric morphology. In particular, they could exhibit unexpectedly strong catalysis, electrocatalysis, and fluorescence properties, in which the biominerized AuNCs would act as fluorescence sources and "nanowires" for promoting the electron-transfer of the catalytic nanocomposites. Colorimetric investigations show that the developed enzyme mimics could present peroxidase-like catalysis activities comparable to natural horseradish peroxidase. In addition, they could facilitate the direct electrocatalysis for H₂O₂ at concentrations as low as 0.40 μM. Moreover, strong red fluorescence of AuNCs in nanocomposites could be expected for the fluorimetric analysis of H₂O₂ with linear concentrations ranging from 50 nM to 100 μM. Such a biomimetic assembly route may open a new door toward the preparation of diverse nanocomposites with multifunctional catalysis and fluorescence, thus promising extensive applications of catalysis and detection in the chemical, environmental, and biomedical fields.

Photoelectrochemical Bioanalysis Platform for Cells Monitoring Based on Dual Signal Amplification Using in Situ Generation of Electron Acceptor Coupled with Heterojunction

By using in situ generation of electron acceptor coupled with heterojunction as dual signal amplification, a simple photoelectrochemical (PEC) bioanalysis platform was designed. The synergic effect between the photoelectrochemical (PEC) activities of carbon nitride (C₃N₄) nanosheets and PbS quantum dots (QDs) achieved almost nine-fold photocurrent intensity increment compared with the C₃N₄ alone. After the G-quadruplex/hemin/Pt nanoparticles (NPs) with catalase-like activity toward H₂O₂ were introduced, oxygen was in situ generated and acted as electron donor by improving charge separation efficiency and further enhancing photocurrent response. The dually amplified signal made enough sensitivity for monitoring H₂O₂ released from live cells. The photocathode was prepared by the stepwise assembly of C₃N₄ nanosheets and PbS QDs on indium tin oxide (ITO) electrode, which was characterized by scanning electron microscope. A signal-on protocol was achieved for H₂O₂ detection in vitro due to the relevance of photocurrent on the concentration of H₂O₂. Under the optimized condition, the fabricated PEC bioanalysis platform exhibited a linear range of 10-7000 μM with a detection limit of 1.05 μM at S/N of 3. Besides, the bioanalysis platform displayed good selectivity against other reductive biological species. By using HepG2 cells as a model, a dual signal amplifying PEC bioanalysis platform for monitoring cells was developed. The bioanalysis platform was successfully applied to the detection of H₂O₂ release from live cells, which provided a novel method for cells monitoring and would have prospect in clinical assay.

Small and Sharp Triangular Silver Nanoplates Synthesized Utilizing Tiny Triangular Nuclei and Their Excellent SERS Activity for Selective Detection of Thiram Residue in Soil

The great harm of thiram residue in soil to environment and human health is usually ignored. Due to the complexity of soil compositions, the detection of thiram residue in soil faces considerable difficulties. In this work, a highly sensitive and selective surface-enhanced Raman scattering (SERS) substrate based on the triangular silver nanoplates (TSNPs) with small size and sharp corners is developed and used for the detection of thiram residue in soil for the first time. These TSNPs are synthesized by replacing the conventional seeds in the seed-mediated chemical reduction route with the tiny and uniform triangular silver nuclei (TSN) which can provide more growing space for generating sharp corners during the growth of TSNPs. It is interesting that the TSNPs with the smaller size have the better SERS performance. The possible mechanism behind this phenomenon is explained by the electromagnetic enhancement theory. On the basis of the Raman activity of the smallest TSNPs, a SERS-active substrate is prepared for detecting the thiram residue in soil. The thiram solution detection shows that the limit of detection (LOD) of these smallest TSNPs is lower than other nanoparticles, such as nanospheres, nanocubes, etc. For sensing the thiram residue in soil, the addition of poly(sodium 4-styrenesulfonate) realizes the specific adsorption of thiram by TSNPs. This method exhibits a good linear response from 0.12 to 4.8 μg/g with a low LOD of 90 ng/g, which is better than conventional methods. This work shows the great potential of the small TSNPs as a novel SERS substrate and its broader applications in pesticides detection.

Sensing of hydrogen peroxide and glucose in human serum via quenching fluorescence of biomolecule-stabilized Au nanoclusters assisted by the Fenton reaction

Sensitive detection of H₂O₂ and glucose were realized by Fenton reaction assistant oxidation of Au NCs.

From near-infrared and Raman to surface-enhanced Raman spectroscopy: progress, limitations and perspectives in bioanalysis

Over recent decades, spreading environmental concern entailed the expansion of green chemistry analytical tools. Vibrational spectroscopy, belonging to this class of analytical tool, is particularly interesting taking into account its numerous advantages such as fast data acquisition and no sample preparation. In this context, near-infrared, Raman and mainly surface-enhanced Raman spectroscopy (SERS) have thus gained interest in many fields including bioanalysis. The two former techniques only ensure the analysis of concentrated compounds in simple matrices, whereas the emergence of SERS improved the performances of vibrational spectroscopy to very sensitive and selective analyses. Complex SERS substrates were also developed enabling biomarker measurements, paving the way for SERS immunoassays. Therefore, in this paper, the strengths and weaknesses of these techniques will be highlighted with a focus on recent progress.

PtW/MoS2 hybrid nanocomposite for electrochemical sensing of H2O2 released from living cells

Hydrogen peroxide (H2O2) as an important reactive oxygen species (ROS) is reactive and potentially harmful to cells, causing oxidation of lipids, proteins and DNA. Herein, we report a PtW/MoS2 hybrid nanocomposite with ultrasensitive and highly specific response for the detection of H2O2 released from breast cancer 4T1 cells. Upon exposure to 5 nM of H2O2, the electrochemical response is still visible. This PtW/MoS2 hybrid nanocomposite could be facilely synthesized through in-situ growth of PtW nanocrystals on the surface of MoS2 nanosheets. The incorporation of PtW nanocrystals and MoS2 nanosheets in conjunction with each other to form hybrid nanocomposite improves the selective interaction of H2O2 with sensing material surface, and further increases the sensitivity and selectivity of sensor.

A novel SERS nanoprobe for the ratiometric imaging of hydrogen peroxide in living cells

Via the assembly of 4-mercaptophenylboronic ester on the gold nanorod's surface, a novel ratiometric SERS nanosensor for H₂O₂ imaging in living cells or cancer tissues has been developed in the present study.