An enzyme-induced Au@Ag core-shell nanoStructure used for an ultrasensitive surface-enhanced Raman scattering immunoassay of cancer biomarkers

Aerobic oxidation catalyst vanadyl acetylacetonate boosts luminol chemiluminescence for sensitive detection of alkaline phosphatase and ascorbic acid

Vanadyl acetylacetonate, VO(acac)2, is a well-known environmentally friendly catalyst for aerobic oxidations. It has been exploited as an efficient co-reactant of luminol chemiluminescence (CL) in aqueous media for the first time. The luminol-VO(acac)2 system generates an intense CL and ascorbic acid can quench its CL remarkably. Based on the strong quenching effect of ascorbic acid, the luminol-VO(acac)2 CL was used in the quantification of clinically important ascorbic acid and alkaline phosphatase (ALP) by enzymatic conversion of L-ascorbic acid 2-phosphate into ascorbic acid. The linear ranges for detecting ascorbic acid and alkaline phosphatase are 1 to 100 μM and 0.005 to 200 U/L, respectively. The limits of detection of ascorbic acid and alkaline phosphatase are 0.5 μM and 0.005 U/L, respectively. The proposed CL method has shown high selectivity for the ALP among the tested biomolecules and metal ions. It also shows good recoveries in human serum samples.

Immunoassays: Analytical and Clinical Performance, Challenges, and Perspectives of SERS Detection in Comparison with Fluorescent Spectroscopic Detection

Immunoassays (IAs) with fluorescence-based detection are already well-established commercialized biosensing methods, such as enzyme-linked immunosorbent assay (ELISA) and lateral flow immunoassay (LFIA). Immunoassays with surface-enhanced Raman spectroscopy (SERS) detection have received significant attention from the research community for at least two decades, but so far they still lack a wide clinical commercial application. This review, unlike any other review that we have seen, performs a three-dimensional performance comparison of SERS IAs vs. fluorescence IAs. First, we compared the limit of detection (LOD) as a key performance parameter for 30 fluorescence and 30 SERS-based immunoassays reported in the literature. We also compared the clinical performances of a smaller number of available reports for SERS vs. fluorescence immunoassays (FIAs). We found that the median and geometric average LODs are about 1.5-2 orders of magnitude lower for SERS-based immunoassays in comparison to fluorescence-based immunoassays. For instance, the median LOD for SERS IA is 4.3 × 10-13 M, whereas for FIA, it is 1.5 × 10-11 M. However, there is no significant difference in average relative standard deviation (RSD)-both are about 5-6%. The analysis of sensitivity, selectivity, and accuracy reported for a limited number of the published clinical studies with SERS IA and FIA demonstrates an advantage of SERS IA over FIA, at least in terms of the median value for all three of those parameters. We discussed common and specific challenges to the performances of both SERS IA and FIA, while proposing some solutions to mitigate those challenges for both techniques. These challenges include non-specific protein binding, non-specific interactions in the immunoassays, sometimes insufficient reproducibility, relatively long assay times, photobleaching, etc. Overall, this review may be useful for a large number of researchers who would like to use immunoassays, but particularly for those who would like to make improvements and move forward in both SERS-based IAs and fluorescence-based IAs.

Smart Nanoplatform for Visualizing Hydrogen Sulfide and Amplifying Oxidative Stress to Tumor Apoptosis

Oxidative stress is involved in various signaling pathways and serves a key role in inducing cell apoptosis. Therefore, it is significant to monitor oxidative stress upon drug release for the assessment of therapeutic effects in cancer cells. Herein, a glutathione (GSH)-responsive surface-enhanced Raman scattering (SERS) nanoplatform is proposed for ultra-sensitively monitoring the substance related with oxidative stress (hydrogen sulfide, H2S), depleting reactive sulfur species and releasing anticancer drugs to amplify oxidative stress for tumor apoptosis. The Au@Raman reporter@Ag (Au@M@Ag) nanoparticles, where a 4-mercaptobenzonitrile molecule as a Raman reporter was embedded between layers of gold and silver to obtain sensitive SERS response, were coated with a covalent organic framework (COF) shell to form a core-shell structure (Au@M@Ag@COFs) as the SERS nanoplatform. The COF shell loading doxorubicin (DOX) of Au@M@Ag@COFs exhibited the GSH-responsive degradation capacity to release DOX, and its Ag layer as the sensing agent was oxidized to Ag2S by H2S to result in its prominent changes in SERS signals with a low detection limit of 0.33 nM. Moreover, the releasing DOX can inhibit the generation of H2S to promote the production of reactive oxygen species, and the depletion of reactive sulfur species (GSH and H2S) in cancer cells can further enhance the oxidative stress to induce tumor apoptosis. Overall, the SERS strategy could provide a powerful tool to monitor the dynamic changes of oxidative stress during therapeutic processes in a tumor microenvironment.

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

Preparation of Janus fluorescent probe based on an asymmetrical silica and its application in glucose and alpha-fetoprotein detection

Janus particles have been considered suitable for biomedicine owing to their asymmetric structure and unique properties. Although Janus particles have been applied in biosensing for dual-mode sensing, there are almost no reports for the detection of multiple indicators. In fact, many patients require different diagnoses, such as the examination of hepatogenic diseases in diabetics. Here, a Janus particle based on SiO₂ was synthesized using a Pickering emulsion method. A novel strategy for detecting glucose and alpha-fetoprotein (AFP) based on different principles using this Janus particle was then constructed as a detection platform. Composed of adjustable dendritic silica loaded with gold nanoclusters (Au NCs) and glucose oxidase (GOx) and spherical SiO₂ coupled with AFP antibody, this Janus fluorescent probe achieved the double detection of glucose and AFP. With the protection of dendritic silica, the enzyme temperature stability was enhanced. Moreover, the low limit of detection for glucose (0.5 μM in PBS and 2.5 μM in serum) and AFP (0.5 ng mL^-1) illustrated the feasibility of the application of the Janus material in integrated detection. This work not only supported the use of a Janus fluorescent probe as a detection platform toward glucose and AFP but also showed the potential of Janus particles in integrated detection in the future.

SERS immuno- and apta-assays in biosensing/bio-detection: Performance comparison, clinical applications, challenges

Surface Enhanced Raman Spectroscopy is increasingly used as a sensitive bioanalytical tool for detection of variety of analytes ranging from viruses and bacteria to cancer biomarkers and toxins, etc. This comprehensive review describes principles of operation and compares the performance of immunoassays and aptamer assays with Surface Enhanced Raman scattering (SERS) detection to each other and to some other bioassay methods, including ELISA and fluorescence assays. Both immuno- and aptamer-based assays are categorized into assay on solid substrates, assays with magnetic nanoparticles and assays in laminar flow or/and strip assays. The best performing and recent examples of assays in each category are described in the text and illustrated in the figures. The average performance, particularly, limit of detection (LOD) for each of those methods reflected in 9 tables of the manuscript and average LODs are calculated and compared. We found out that, on average, there is some advantage in terms of LOD for SERS immunoassays (0.5 pM median LOD of 88 papers) vs SERS aptamer-based assays (1.7 pM median LOD of 51 papers). We also tabulated and analyzed the clinical performance of SERS immune and aptamer assays, where selectivity, specificity, and accuracy are reported, we summarized the best examples. We also reviewed challenges to SERS bioassay performance and real-life application, including non-specific protein binding, nanoparticle aggregation, limited nanotag stability, sometimes, relatively long time to results, etc. The proposed solutions to those challenges are also discussed in the review. Overall, this review may be interesting not only to bioanalytical chemist, but to medical and life science researchers who are interested in improvement of bioanalyte detection and diagnostics.

Heterometallic nanomaterials: activity modulation, sensing, imaging and therapy

Heterometallic nanomaterials (HMNMs) display superior physicochemical properties and stability to monometallic counterparts, accompanied by wider applications in the fields of catalysis, sensing, imaging, and therapy due to synergistic effects between multi-metals in HMNMs. So far, most reviews have mainly concentrated on introduction of their preparation approaches, morphology control and applications in catalysis, assay of heavy metal ions, and antimicrobial activity. Therefore, it is very important to summarize the latest investigations of activity modulation of HMNMs and their recent applications in sensing, imaging and therapy. Taking the above into consideration, we briefly underline appealing chemical/physical properties of HMNMs chiefly tailored through the sizes, shapes, compositions, structures and surface modification. Then, we particularly emphasize their widespread applications in sensing of targets (e.g. metal ions, small molecules, proteins, nucleic acids, and cancer cells), imaging (frequently involving photoluminescence, fluorescence, Raman, electrochemiluminescence, magnetic resonance, X-ray computed tomography, photoacoustic imaging, etc.), and therapy (e.g. radiotherapy, chemotherapy, photothermal therapy, photodynamic therapy, and chemodynamic therapy). Finally, we present an outlook on their forthcoming directions. This timely review would be of great significance for attracting researchers from different disciplines in developing novel HMNMs.

Recent Developments in Metallic Nanomaterials for Cancer Therapy, Diagnosing and Imaging Applications

Cancer continues to represent a global health concern, imposing an ongoing need to research for better treatment alternatives. In this context, nanomedicine seems to be the solution to existing problems, bringing unprecedented results in various biomedical applications, including cancer therapy, diagnosing, and imaging. As numerous studies have uncovered the advantageous properties of various nanoscale metals, this review aims to present metal-based nanoparticles that are most frequently employed for cancer applications. This paper follows the description of relevant nanoparticles made of metals, metal derivatives, hybrids, and alloys, further discussing in more detail their potential applications in cancer management, ranging from the delivery of chemotherapeutics, vaccines, and genes to ablative hyperthermia therapies and theranostic platforms.

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 enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.

Bimetallic Nanocrystals: Structure, Controllable Synthesis and Applications in Catalysis, Energy and Sensing

In recent years, bimetallic nanocrystals have attracted great interest from many researchers. Bimetallic nanocrystals are expected to exhibit improved physical and chemical properties due to the synergistic effect between the two metals, not just a combination of two monometallic properties. More importantly, the properties of bimetallic nanocrystals are significantly affected by their morphology, structure, and atomic arrangement. Reasonable regulation of these parameters of nanocrystals can effectively control their properties and enhance their practicality in a given application. This review summarizes some recent research progress in the controlled synthesis of shape, composition and structure, as well as some important applications of bimetallic nanocrystals. We first give a brief introduction to the development of bimetals, followed by the architectural diversity of bimetallic nanocrystals. The most commonly used and typical synthesis methods are also summarized, and the possible morphologies under different conditions are also discussed. Finally, we discuss the composition-dependent and shape-dependent properties of bimetals in terms of highlighting applications such as catalysis, energy conversion, gas sensing and bio-detection applications.

Alkaline Phosphatase-Triggered Etching of Au@FeOOH Nanoparticles for Enzyme Level Assay under Dark-Field Microscopy

In clinical diagnosis, the level of biological enzymes in serum has been generally regarded as markers of human diseases. In this work, a kind of simple and sensitive plasmonic probe (indicated as Au@FeOOH) has been synthesized with the guidance of plasmonic imaging and subsequently developed for the alkaline phosphatase (ALP) level detection under dark-field microscopy (DFM). As a kind of hydrolysis enzyme, ALP can promote the hydrolysis of l-ascorbic acid 2-phosphate to ascorbic acid (AA). AA further acts as a strong reduction reagent for the decomposition of the FeOOH shell, which results in a blue shift of localized surface plasmon resonance spectra and an obvious color change under DFM. RGB analyses show that using a ΔR/G value instead of scattering wavelength or R/G value as the analytical signal, the deviation attributed to the size distribution of the initial Au NPs is greatly suppressed, and a linear range from 0.2 to 6.0 U/L (R² = 0.99) and a limit of detection of 0.06 U/L are acquired with various concentrations of ALP during the detection. Besides, this approach exhibits excellent selectivity in complex biological serum samples, which is expected to be applied for the early diagnosis of clinical diseases by monitoring various biomarkers in the future.

Dendritic Forest-Like Ag Nanostructures Prepared Using Fluoride-Assisted Galvanic Replacement Reaction for SERS Applications

Dendritic forest-like Ag nanostructures were deposited on a silicon wafer through fluoride-assisted galvanic replacement reaction (FAGRR) in aqueous AgNO₃ and buffered oxide etchant. The prepared nanostructures were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma–optical emission spectroscopy, a surface profiler (alpha step), and X-ray diffraction. Additionally, the dendritic forest-like Ag nanostructures were characterized using surface-enhanced Raman scattering (SERS) when a 4-mercaptobenzoic acid (4-MBA) monolayer was adsorbed on the Ag surface. The Ag nanostructures exhibited intense SERS signal from 4-MBA because of their rough surface, and this intense signal led to an intense local electromagnetic field upon electromagnetic excitation. The enhancement factor for 4-MBA molecules adsorbed on the Ag nanostructures was calculated to be 9.18 × 10⁸. Furthermore, common Raman reporters such as rhodamine 6G, 4-aminothiolphenol, 5,5′-dithiobis-2-nitrobenzoic acid, and carboxyfluorescein (FAM) were characterized on these dendritic forest-like Ag nanostructures, leading to the development of an ultrasensitive SERS-based DNA sensor with a limit of detection of 33.5 nM of 15-mer oligonucleotide.

Au/Ag composite-based SERS nanoprobe of Cr3

Quantitative characterization of Cr³⁺, an important element revealing human metabolism and biological environmental variation, is still difficult to achieve by conventional biochemical methods due to the lack of high-sensitivity, real-time techniques with rapid response detection. Using surface-enhanced Raman scattering (SERS), we construct an Au/Ag composite-based SERS nanoprobe for the quantitative characterization of Cr³⁺ content in solution, in which DL-mercaptosuccinic acid (DL-MSA) is employed for Raman signal enhancement, and 4-mercaptobenzoic acid (4-MBA) is chosen as the Raman reporter. The achieved result demonstrates obvious advantages of the synthesized Au/Ag composite-based SERS nanoprobe in sensitivity and response speed. Importantly, this Au/Ag composite-based SERS nanoprobe might provide a new strategy for dynamic monitoring of Cr³⁺ content in human metabolism.

Metal Nanoparticles/MoS2 Surface-Enhanced Raman Scattering-Based Sandwich Immunoassay for α-Fetoprotein Detection

The detection of cancer biomarkers at an early stage of tumor development is vital for effective diagnosis and treatment of cancer. Current diagnostic tools can often detect cancer only when the biomarker levels are already too high, so that the tumors have spread and treatments are less effective. It is urgent therefore to develop highly sensitive assays for the detection of such biomarkers at the lowest possible concentration. In this context, we developed a sandwich immunoassay based on surface-enhanced Raman scattering (SERS) for the ultrasensitive detection of α-fetoprotein (AFP), which is typically present in human serum as a biomarker indicative of early stages of hepatocellular carcinoma. In the immunoassay design, molybdenum disulfide (MoS2) modified with a monoclonal antibody was used as a capture probe for AFP. A secondary antibody linked to an SERS-encoded nanoparticle was employed as the Raman signal reporter, that is, the transducer for AFP detection. The sandwich immunocomplex "capture probe/target/SERS tag" was deposited on a silicon wafer and decorated with silver-coated gold nanocubes to increase the density of "hot spots" on the surface of the immunosensor. The developed SERS immunosensor exhibits a wide linear detection range (1 pg mL-1 to 10 ng mL-1) with a limit of detection as low as 0.03 pg mL-1 toward AFP with good reproducibility (RSD < 6%) and stability. These parameters demonstrate that the proposed immunosensor has the potential to be used as an analytical platform for the detection of early-stage cancer biomarkers in clinical applications.

Highly sensitive detection of cytochrome c in the NSCLC serum using a hydrophobic paper based-gold nanourchin substrate

Cytochrome c (Cyt c) is a biomarker of early apoptosis that plays a critical role in the diagnosis and therapy of non-small cell lung cancer (NSCLC). In this work, we proposed a novel surface-enhanced Raman scattering (SERS)-based biosensor to implement the ultrasensitive detection of Cyt c in the serum of NSCLC patients. The SERS-supporting substrates based on hydrophobic filter paper were composed of gold nanourchins (GNUs) surface-functionalized with the Cyt c aptamer and the cyanine 5-labeled complementary DNA. In the existence of Cyt c, it could specifically bind to its aptamer, which leads to the detachment of complementary strands modified with Cy5 and the great weakness of SERS signal. The finite-difference time domain (FDTD) simulation showed that the excellent SERS performance of GNUs aggregation was strongly dependent on a large number of "hot spots" at the tips and between the nanogaps of aggregated GNUs. Alkyl ketene dimer (AKD) was used to make the filter paper modify its property from hydrophilic to hydrophobic, which consequently increased the density of GNUs and extended the retention time of the analyte. SERS biosensors based on hydrophobic paper exhibited prominent reproducibility and selectivity. The detection limit of Cyt c in PBS was 1.148 pg/mL, while the detection limit in human serum was 1.79 pg/mL. Moreover, the analysis of the serum samples of healthy subjects and NSCLC patients confirmed the feasibility of its clinical application. The results were consistent with enzyme-linked immunosorbent assay results. This method can be a powerful strategy for quantitative detection of extracellular Cyt c, and it is expected that the SERS-based biosensors could be applied in the practical clinical diagnoses of NSCLC.

Applications of surface-enhanced Raman spectroscopy in detection fields

Surface-enhanced Raman spectroscopy (SERS) is a Raman spectroscopy technique that has been widely used in food safety, environmental monitoring, medical diagnosis and treatment and drug monitoring because of its high selectivity, sensitivity, rapidness, simplicity and specificity in identifying molecular structures. This review introduces the detection mechanism of SERS and summarizes the most recent progress concerning the use of SERS for the detection and characterization of molecules, providing references for the later research of SERS in detection fields.

SERS Immunosensor of Array Units Surrounded by Particles: A Platform for Auxiliary Diagnosis of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the diseases with high mortality worldwide, so its early diagnosis and treatment have attracted much attention. Due to the advantages of the high sensitivity of surface-enhanced Raman scattering (SERS) detection, SERS has excellent application value in the diagnosis of HCC. In this paper, silver nanoparticles (AgNPs) are modified by magnetron sputtering on the surface of polystyrene (PS) templates with spheres of two different diameters. The array of units surrounded by particles is successfully prepared and the SERS performance is characterized. The effect of the gap between AgNPs on plasmon coupling and hot spot distribution is discussed. Finite-difference time domain (FDTD) simulation is used to verify the electric fields and hot spot distribution of the array. The differences in the concentrations of HCC markers are analyzed by using the change of SERS signal intensity of the array. The whole process proves that the preparation of structures with a strong local electric field to provide highly sensitive SERS signals is a key link in the detection of HCC markers, which is conducive to the diagnosis of HCC and has potential application value in clinical diagnosis.

Nonenzyme Cascaded Amplification Biosensor Based on Effective Aggregation Luminescence Caused by Disintegration of Silver Nanoparticles

Sensitive and portable quantification of biomarkers has particular significance in the monitoring and treatment of clinical diseases. Conventional immunoassays were accustomed to introducing or incorporating enzymes for signal amplification, which commonly suffered from poor stability and inferior tolerance. Herein, we constructed a novel nonenzyme amplification methodology based on fluorogenic Ag⁺-tetrazolate aggregation coupled with silver corrosion sensitization for biomarker determination. A significant cascade enhancement strategy was achieved by the valid aggregation luminescence caused by the potent disintegration of silver nanoparticles. Furthermore, efficient magnetic separation was also combined and performed for the rapidity and simplicity of operation. As the target, the detection limit of prostate-specific antigen was 15.66 pg/mL in our designed biosensor. Besides, a good linear relationship was obtained. The designed biosensor demonstrated good specificity and was successfully applied to clinical serum sample detection.

Synthesis of water soluble CuGaS2/ZnS quantum dots for ultrasensitive fluorescent detection of alkaline phosphatase based on inner filter effect

Developing monitoring technique for alkaline phosphatase (ALP) is crucial due to the important role it plays in living cells. Here, a kind of biocompatible glutathione-modified CuGaS₂/ZnS quantum dots (GSH-CGS/ZnS QDs) was used as a fluorescent substance and then fabricated "turn-off" fluorescent biosensor for detection of ALP by help of inner filter effect (IFE). Firstly, we prepared CuGaS₂/ZnS (CGS/ZnS) QDs using solvothermal method and explored the efficient ligand (GSH) exchanges strategy for transferring oil-soluble CGS/ZnS QDs to aqueous phase. More importantly, we also explored the potential biological applications of the nanohybrid QDs. The obtained GSH-CGS/ZnS QDs emitted strong yellow fluorescence with the maximum excitation (400 nm) and emission (601 nm). Then, GSH-CGS/ZnS QDs were mixed with p-nitrophenylphosphate (PNPP) and ALP. PNPP could be hydrolyzed to p-nitrophenol (PNP) by help of catalysis of ALP, and the excitation spectrum of the GSH-CGS/ZnS QDs overlapped well with the absorption spectrum of PNP, so the fluorescence of GSH-CGS/ZnS QDs was initially quenched via the so-called "IFE". Finally, a novel "turn-off" biosensor for sensitive detection of ALP in the range of 0.05-10 U L ^-1(R² = 0.98) with a detection limit of 0.01 U L^-1 was successfully obtained. Results indicated that I-III-VI₂ nanocrystals have great potential for their promising biomedical application.

A portable photoacoustic device for facile and sensitive detection of serum alkaline phosphatase activity

It is still a high challenge to develop a simple, sensitive and portable approach for bioassay in strong scattering medium. Herein, a photoacoustic (PA) device is developed for the detection of alkaline phosphatase (ALP) in serum with silver nanoparticles (AgNPs) as signal probe, without any requirements for expensive equipment, professional operation and pre-processing of real samples. ALP as an important disease marker could catalyze the breakdown of sodium L-ascorbyl-2-phosphate (AAP) into ascorbic acid (AA), thereby reducing Ag⁺ to AgNPs. AgNPs could generate strong PA signal under the irradiation of modulated 638-nm laser due to their localized plasmon resonance, and detected by the self-made portable PA device. Under the optimized experimental conditions, the present PA device exhibits excellent photostability and reproducibility with the relative standard deviation (RSD) of 2.2% at the concentration of 25 U L^-1 ALP. Linear calibration graph is obtained within 5-70 U L^-1 for ALP, along with a detection limit of 1.1 U L^-1. This portable PA device is applied to detect ALP in serum samples, providing satisfactory spiking recoveries and competitive analytical performances with the current techniques. The PA-based analytical strategy obviously opens up a new avenue to the detection of disease-correlated biomarker in practice.

DNA Nanofirecrackers Assembled through Hybridization Chain Reaction for Ultrasensitive SERS Immunoassay of Prostate Specific Antigen

Isothermal nucleic acid amplification technology has been widely adopted for analytical chemistry with the purpose of sensitivity improvement. Herein we present an ultrasensitive concatenated hybridization chain reaction (C-HCR) based surface-enhanced Raman scattering (SERS) immunoassay by forming antibody-antigen-aptamer heterosandwich structures with the model analyte of total prostate specific antigens (tPSA). In the C-HCR, two HCRs, one proceeds with two hairpins and the other with four biotin-modified hairpins, are coupled, making the formation of DNA nanofirecrackers with the lengths longer than 200 nm and more than four hundred million binding sites of streptavidin modified enzymes. These types of DNA nanofirecrackers through the aptamer encoded linker strand to form heterosandwich structures could provide a general signal application platform such as enzyme catalysis with high amplification efficiency. As a proof of concept, the Au@Ag core-shell nanostructure based SERS immunoassay with excellent signal amplification has been developed by employing the streptavidin modified alkaline phosphatase (SA-ALP) through its catalysis of 2-phospho-l-ascorbic acid trisodium salt (AAP) to form Au@Ag core-shell nanostructures via the formation of ascorbic acid (AA) to reduce AgNO₃ and deposition of silver element on gold nanorods (AuNRs). The newly developed method has a detection limit as low as 0.94 fg/mL and has successfully achieved the detection of serum samples from clinical patients, which was consistent with the clinical test results, showing that this C-HCR strategy to form DNA nanofirecrackers has great potential in clinical applications.

A magnetic SERS immunosensor for highly sensitive and selective detection of human carboxylesterase 1 in human serum samples

Hepatocellular carcinoma (HCC) is a common and lethal cancer. New serum markers for detecting HCC are urgently needed. Human carboxylesterase 1 (hCE1) is an important member of the serine hydrolase superfamily and is closely related to the occurrence of HCC. It can be used as a good serum marker for early diagnosis of HCC. Here, we developed a surface enhanced Raman scattering (SERS)- based magnetic immunosensor that specifically recognizes and detects trace amounts of hCE1 in human serum via a sandwich structure consisting of a SERS tags, magnetic supporting substrates, and target antigen (hCE1). The SERS tags are 4-mercaptobenzoic acid (4-MBA)-labeled AgNPs, and the SERS supporting substrates are composed of a raspberry-like morphology of Fe₃O₄@SiO₂@AgNPs magnetic nanocomposites surface-functionalized with a hCE1 antibody. The prepared SERS magnetic immunosensor exhibits excellent selectivity and extremely high sensitivity for hCE1 detection. The SERS signal and logarithm of hCE1 concentration presented a wide linear response range of 0.1 ng mL^-1 to 1.0 mg mL^-1, and the detection limit of hCE1 was 0.1 ng mL^-1. The results indicate that the immunosensor can be used for the rapid determination of hCE1 in human serum without a complicated sample pre-treatment. Furthermore, the immunosensor has good reproducibility and stability, and has a promising prospect for the quantitative detection of other tumor markers in early clinical diagnosis.

Ratiometric detection of alkaline phosphatase based on aggregation-induced emission enhancement

Alkaline phosphatase (ALP) is an important enzyme that is associated with many human diseases, so the quantitative detection of ALP is vital from a clinical perspective. Nevertheless, most fluorescent assays for monitoring ALP depend on aggregation-induced quenching (ACQ), single-signal modulation, or a "signal off" mode, which suffer from poor sensitivity, a "false positive" problem, and low signal output. In this work, we utilized the electrostatically driven self-assembly of glutathione-capped gold nanoclusters (GSH-AuNCs, which show aggregation-induced emission, AIE) and amino-modified silicon nanoparticles (SiNPs) to create a hybrid probe (SiNPs@GSH-AuNCs). This nanohybrid probe showed emission from the SiNPs at around 470 nm as well as aggregation-induced emission enhancement (AIEE) of the GSH-AuNCs at 580 nm. The AIEE of the GSH-AuNCs was quenched in the presence of KMnO₄, but the AIEE was recovered by adding ascorbic acid as an oxidation-reduction reaction occurred between KMnO₄ and the ascorbic acid. The fluorescence of the SiNPs remained constant whether the AIEE was quenched or not, meaning that the fluorescence of the SiNPs could be used as an internal reference. In a typical enzymatic reaction, ascorbic acid 2-phosphate is hydrolyzed by ALP to produce ascorbic acid. Therefore, the hybrid probe was shown to allow the ratiometric detection of ALP, with a linear range of 0.5-10 U L^-1 and a limit of detection (LOD) of 0.23 U L^-1. Finally, the proposed analytical strategy was successfully applied to detect ALP in human serum samples and to determine the concentration of an ALP inhibitor. Graphical Abstract.

Overview of Optical and Electrochemical Alkaline Phosphatase (ALP) Biosensors: Recent Approaches in Cells Culture Techniques

Alkaline phosphatase (ALP), which catalyzes the dephosphorylation process of proteins, nucleic acids, and small molecules, can be found in a variety of tissues (intestine, liver, bone, kidney, and placenta) of almost all living organisms. This enzyme has been extensively used as a biomarker in enzyme immunoassays and molecular biology. ALP is also one of the most commonly assayed enzymes in routine clinical practice. Due to its close relation to a variety of pathological processes, ALP's abnormal level is an important diagnostic biomarker of many human diseases, such as liver dysfunction, bone diseases, kidney acute injury, and cancer. Therefore, the development of convenient and reliable assay methods for monitoring ALP activity/level is extremely important and valuable, not only for clinical diagnoses but also in the area of biomedical research. This paper comprehensively reviews the strategies of optical and electrochemical detection of ALP and discusses the electrochemical techniques that have been addressed to make them suitable for ALP analysis in cell culture.

Hierarchical bioresponsive nanocarriers for codelivery of curcumin and doxorubicin

Hierarchical responsive nanocarriers have received much attention for targeted delivery of chemotherapeutics. In this study, we designed pH and redox dual-stage responsive nanocarriers in the different delivery stages for co-delivery phosphorylated curcumin (p-Cur) with doxorubicin (Dox). The MSNs nanocarriers were functionalized via specific cleavable PEGylation and hydrogel coating crosslinked by disulfide bonds: MSNs as core load Dox; p-Cur encapsulated in hydrogel coating. In blood circulation, PEGylation endow the nanocarriers with long time during blood circulation; while in tumor tissue, PEG shells could be cleaved due to the pH-sensitive bond and expose the cationic hydrogel coating to improve cell uptake; while inside tumor cells, hydrogel coating could be cleaved due to the GSH and release the drugs. The results showed that the dual-responsive shells endowed the nanocarriers with tumor extracellular pH-triggered cell uptake and specific cancer cell target release. The synergistic effects of the p-Cur and Dox enhanced cellular apoptosis in Hela cells.

Photothermal Soft Nanoballs Developed by Loading Plasmonic Cu2- xSe Nanocrystals into Liposomes for Photothermal Immunoassay of Aflatoxin B1

Photothermal effects (PTEs) have been greatly concerned with the fast development of new photothermal nanomaterials. Herein we propose a photothermal immunoassay (PTIA) by taking mycotoxins (AFB1) as an example based on the PTEs of plasmonic Cu2- xSe nanocrystals (NCs). By loading plasmonic Cu2- xSe NCs into liposomes to form photothermal soft nanoballs (ptSNBs), on which aptamer of AFB1 previously assembled, a sandwich structure of AFB1 could be formed with the aptamer on ptSNBs and capture antibody. The heat released from the ptSNBs under NIR irradiation, owing to the plasmonic photothermal light-to-heat conversion through photon-electron-phonon coupling, makes the temperature of substrate solution increased, and the increased temperature has a linear relationship with the AFB1 content. Owing to the large amounts of plasmonic Cu2- xSe NCs in the ptSNBs, the PTEs get amplified, making AFB1 higher than 1 ng/mL detectable in food even if with a rough homemade immunothermometer. The proposal of PTIA opens a new field of immunoassay including developing photothermal nanostructures, new thermometers, PTIA theory, and so on.

Ag@Au Core⁻Shell Porous Nanocages with Outstanding SERS Activity for Highly Sensitive SERS Immunoassay

A highly sensitive immunoassay of biomarkers has been achieved using 4-mercaptobenzoic acid-labeled Ag@Au core⁻shell porous nanocage tags and α-fetoprotein immuno-sensing chips. The Ag@Au porous nanocages were uniquely synthesized by using an Ag core as a self-sacrificial template and reducing agent, where the slow reaction process led to the formation of a porous Au layer. The size of the remaining Ag core and surface roughness of the Au shell were controlled by adjusting the chloroauric acid concentration. The porous cage exhibited excellent surface-enhanced Raman spectroscopy (SERS) activity, presumably due to a synergetic interaction between newly generated hot spots in the rough Au shell and the retained SERS activity of the Ag core. Using α-fetoprotein as a model analyte for immunoassay, the SERS signal had a wide linear range of 0.20 ng mL-1 to 500.0 ng mL-1 with a detection limit of 0.12 ng mL-1. Without the need of further signal amplification, the as-prepared Ag@Au bimetallic nanocages can be directly used for highly sensitive SERS assays of other biomarkers in biomedical research, diagnostics, etc.

Label-Free, Single-Particle, Colorimetric Detection of Permanganate by GNPs@Ag Core-Shell Nanoparticles with Dark-Field Optical Microscopy

Rapid quantification of permanganate (MnO₄^-) in aqueous solution with a convenient and sensitive single-particle-detection (SPD) method is demonstrated by dark-field optical microscopy. The design is based on the selective etching of the Ag shell of a glucose-protected GNPs@Ag nanoparticle by MnO₄^-. In the presence of MnO₄^-, a noticeable red-shift of localized surface-plasmon resonance (LSPR, from blue to green) together with a tremendous decrease in the extinction coefficient from individual GNPs@Ag nanoparticles is observed. MnO₄^- can then be quantified by calculating the ratio between the number of green and blue particles on the cover glass surface after the etching process. A linear dynamic range of 0-6 μM and a limit of detection (LOD) as low as 46 nM were readily achieved, which are much lower than those of spectroscopic measurements in bulk solution. In tap water, a comparable LOD (50 nM) and satisfactory recovery efficiency are demonstrated. As a consequence of these merits, the method demonstrated herein will find promising applications for the ultrasensitive detection of MnO₄^- under complex milieu in the future.

Silver nanoparticles deposited on graphene oxide for ultrasensitive surface-enhanced Raman scattering immunoassay of cancer biomarker

Graphene oxide (GO) exhibits distinctive Raman scattering features for its high frequency D (disordered) and tangential modes (G-band), which are characteristically sharp at 1580 cm-1 and 1350 cm-1, respectively, but are too weak for sensitive quantitation purposes. By depositing silver nanoparticles on the surface of GO in this contribution, both D and G bands of GO become enhanced. The enzyme label of this method controls the dissolution of silver nanoparticles on the surface of GO through hydrogen peroxide which is produced by the oxidation of the enzyme substrate. With the dissolution of the silver nanoparticles a greatly decreased SERS signal of GO was obtained. This strategy involves dual signal amplification of the enzyme and nanocomposites to improve the detection sensitivity. As a proof of concept, prostate specific antigen (PSA), a biomarker for prostate cancer, is successfully detected as a target by forming a sandwich structure in immunoassay. The SERS immunoassay possesses excellent analytical performance in the range 0.5 pg mL-1 to 500 pg mL-1 with a limit of detection of 0.23 pg mL-1, making the detection of PSA serum samples from prostate cancer patients satisfactory, demonstrating that the sensitive enzyme-assisted dissolved AgNPs SERS immunoassay of PSA has potential applications in clinical diagnosis.

Fluorescence assay for alkaline phosphatase activity based on energy transfer from terbium to europium in lanthanide coordination polymer nanoparticles

An effective fluorescence assay alkaline phosphatase (ALP) method was developed by using bimetallic lanthanide coordination polymer nanoparticles (Tb-GMP-Eu CPNs).

Quantitative and multiplex dot-immunoassay using gap-enhanced Raman tags

A highly specific, quantitative, and multiplex dot immunoassay has been developed. The immunoassay utilizes functionalized plasmonic gap-enhanced Raman tags (GERTs) as labels and nitrocellulose membrane as a substrate.