A novel nanocatalytic SERS detection of trace human chorionic gonadotropin using labeled-free Vitoria blue 4R as molecular probe

Target-mediated silver deposition-based electrochemical biosensor for highly sensitive detection of human chorionic gonadotropin

As a glycoprotein hormone, human chorionic gonadotropin (hCG) is an established marker for pregnancy test. On the basis of the target-mediated silver deposition (TSD), in this work, we report the development of an amplification-free electrochemical biosensor for the highly sensitive detection of hCG. The detection of hCG involves the use of the affinity peptide-modified electrode for hCG capture (the CGGSSPPLRINRHILTR peptide containing the hCG-binding domain of the PPLRINRHILTR sequence is used as the affinity peptide), the oxidation of the diol sites of the glycan chains on hCG hormones into aldehyde groups by NaIO4, and the deposition of silver nanoparticles (AgNPs) for the solid-state voltammetric stripping analysis. Due to the deposition of multiple AgNPs while the solid-state Ag/AgCl voltammetric process has a high signal-to-noise ratio, the TSD-based electrochemical biosensor can be applied to the highly sensitive detection of hCG without the need for signal amplification. Under optimal conditions, the stripping current increased linearly with an increasing hCG concentration over the range from 1.0 to 25 mIU/mL, with a detection limit of 0.45 mIU/mL. Owing to the high specificity of the hCG-binding peptide PPLRINRHILTR, this electrochemical hCG biosensor exhibits high selectivity. The results of the quantitative assay of hCG in urine samples at the concentrations of 25, 10, and 1.0 mIU/mL are desirable, indicating the good anti-interference capability. As the TSD-based electrochemical biosensor allows the amplification-free detection of low-abundance hCG, it is easy to use and cost-effective, showing great promise in point-of-care assay of hCG for pregnancy test.

Tris(2,2'-bipyridine)ruthenium(II)-silver nanoparticle electrostatic nanoaggregates (AgNPs@[Ru(bpy)3]2+ ENAs) as novel SERS nanotags for rapid, sensitive and selective immunosensing

Tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)3]2+), as a versatile molecule, has been widely applied in various fields, such as photocatalysis, electrochemiluminescence and fluorescence probes, solar cell and LED due to its excellent optical and electrical properties, good water solubility, high chemical stability. In this work, we prepared electrostatic nanoaggregates from [Ru(bpy)3]2+ and silver nanoparticles (AgNPs@[Ru(bpy)3]2+ ENAs) as a new type of SERS nanotags. Each [Ru(bpy)3]2+ ion carries two positive charges with strong affinity to negative surfaces, which enables a strong electrostatic interaction between [Ru(bpy)3]2+ and negatively charged silver nanoparticles (AgNPs) and fast (within 10 min) formation of AgNPs@[Ru(bpy)3]2+ ENAs. The prepared AgNPs@[Ru(bpy)3]2+ ENAs had a very strong and stable SERS activity due to abundant bipyridine molecules in [Ru(bpy)3]2+ and the location of many [Ru(bpy)3]2+ SERS reporters at the electromagnetic "hot spots" (i.e. the junction of two adjacent AgNPs), and thus could act as novel and excellent SERS nanotags. Further conjugated with antibodies, AgNPs@[Ru(bpy)3]2+ nanotags were used to develop new SERS-based immunochromatography test strips (SERS-ICTSs), showing excellent sensing performances. The AgNPs@[Ru(bpy)3]2+ ENAs based SERS-ICTSs not only inherit the merit of fast and visualize quantitative analysis from traditional ICTSs, but also realize much more sensitive biosensing (with detection limit of 25 pg/mL HCG) using the SERS technology.

Molecular Plasmonic Silver Forests for the Photocatalytic-Driven Sensing Platforms

Structural electronics, as well as flexible and wearable devices are applications that are possible by merging polymers with metal nanoparticles. However, using conventional technologies, it is challenging to fabricate plasmonic structures that remain flexible. We developed three-dimensional (3D) plasmonic nanostructures/polymer sensors via single-step laser processing and further functionalization with 4-nitrobenzenethiol (4-NBT) as a molecular probe. These sensors allow ultrasensitive detection with surface-enhanced Raman spectroscopy (SERS). We tracked the 4-NBT plasmonic enhancement and changes in its vibrational spectrum under the chemical environment perturbations. As a model system, we investigated the sensor's performance when exposed to prostate cancer cells' media over 7 days showing the possibility of identifying the cell death reflected in the environment through the effects on the 4-NBT probe. Thus, the fabricated sensor could have an impact on the monitoring of the cancer treatment process. Moreover, the laser-driven nanoparticles/polymer intermixing resulted in a free-form electrically conductive composite that withstands over 1000 bending cycles without losing electrical properties. Our results bridge the gap between plasmonic sensing with SERS and flexible electronics in a scalable, energy-efficient, inexpensive, and environmentally friendly way.

Au/Fe3O4-based nanozymes with peroxidase-like activity integrated in immunochromatographic strips for highly-sensitive biomarker detection

Because of their simplicity, rapidity, and cost-effectiveness, immunochromatographic strips (ICTs) have been widely used as an effective tool in various fields. However, typical strips for the preliminary screening suffer from limited detection sensitivity, particularly in biomarker detection with trace concentration. Herein, to tackle this challenge, we integrated homemade gold-decorated Fe₃O₄ nanoparticles (Au/Fe₃O₄ NPs) with flexible strips, exploring the excellent peroxidase-like activity of this labeled material, and then enhancing the detection sensitivity via signal amplification. The limit of detection (LOD) of the strips is as low as 0.05 mIU mL^-1 when human chorionic gonadotropin (hCG) is as a biomarker model, which is 500 times lower than that of the traditional color-based strip. Overall, our results demonstrated the potential for Au/Fe₃O₄ NP based-ICTs for the rapid detection of the biomarker in an instrument-free and point-of-care testing format.

Sandwich-Type Electrochemiluminescence Immunosensor Based on CDs@dSiO2 Nanoparticles as Nanoprobe and Co-Reactant

In general, co-reactants are essential in highly efficient electrochemiluminescence (ECL) systems. Traditional co-reactants are usually toxic, so it is necessary to develop new environmentally friendly co-reactants. In this work, carbon dots (CDs) were assembled with dendritic silica nanospheres (CDs@dSiO₂ NPs) to form a co-reactant of Ru(bpy)₃²⁺. Subsequently, a sandwich immunosensor for detecting human chorionic gonadotropin (HCG) was constructed based on CDs@dSiO₂ NPs as co-reactants, the nanoprobe loaded with the secondary antibody, and Ru(bpy)₃²⁺ as a luminophore. In addition, compared to directly as a signal probe, the luminophore Ru (bpy)₃²⁺ as a part of the electrolyte solution is simpler in this work. The immunosensor has an extremely low limit of detection of 0.00019 mIU/mL. This work describes the synthesis of low-toxic, efficient, and environmentally friendly CDs, which have become ideal co-reactants of Ru(bpy)₃²⁺, and proposes an ECL immunosensor with excellent stability and selectivity, which has great potential in clinical applications.

A nanosol SERS/RRS aptamer assay of trace cobalt(ii) by covalent organic framework BtPD-loaded nanogold catalytic amplification

The determination of heavy metal ions has always been a hot topic in the field of environmental analysis. In this study, a new covalent organic framework-loaded gold nanoparticle (AuCOF) nanocatalytic amplification signal strategy was developed to determine trace Co²⁺ in water. The COF of BtPD was synthesized from 1,3,5-benzene tricarboxaldehyde and p-phenylenediamine, and a new kind of AuBtPD nanosol was prepared by reduction of HAuCl₄ to AuNPs on the BtPD carrier. It has strong catalysis of the new indicator reaction of sodium formate reducing HAuCl₄ to AuNP sol with strong resonance Rayleigh scattering (RRS) at 370 nm and surface enhanced resonance Raman scattering (SERS) activity at 1614 cm^-1 in the presence of a Victoria blue 4R (VB4R) molecular probe. Combining the nanocatalytic reaction to amplify the dual-scattering signals and specific aptamer (Apt) of cobalt ions, a new, fast, stable, sensitive and specific dual mode method for detecting Co²⁺ was established; the RRS signal I _370nm and SERS signal I _1614cm^-1 show a linear relationship with the concentration of 0.033-1 nmol L^-1 Co²⁺ and with a limit of detection (LOD) of 0.02 nmol L^-1. The two methods have been applied to the determination of Co²⁺ in industrial wastewater, tap water and river water, and the results are satisfactory.

Electrochemiluminescence immunoassay of human chorionic gonadotropin using silver carbon quantum dots and functionalized polymer nanospheres

A composite, reduced graphene oxide (rGO) doped with silver nanoparticles (Ag NPs), was prepared by using binary reductants of sodium citrate and hydrazine hydrate. Carbon quantum dots (CQDs) synthesized by papaya peel combined with silver ions to form a CQDs-loaded silver nanoparticle (AgCQDs) nanocomposite. Polymer nanospheres (PNS) were generated via the infinite coordination polymer of ferrocene dicarboxylic acid and employed as carriers to load AgCQDs. The prepared AgCQDs@PNS-PEI has good biocompatibility and electrical conductivity and can be used as a matrix for the immobilization of a secondary antibody (Ab2). A sandwich-type electrochemiluminescence (ECL) immunosensor using AgCQDs@PNS-PEI nanocomposite as probe has been developed for the detection of human chorionic gonadotropin (HCG). The proposed immunosensor exhibits a linear range from 0.00100 to 500 mIU mL^-1 and the detection limit is 0.33 μIU mL^-1 (S/N = 3) under optimal conditions. The sensor exhibits excellent selectivity, good reproducibility, and high stability. These features demonstrate that the proposed method has promising potential for clinical protein detection and displays a new strategy to fabricate an immunosensor. Graphical abstract.

Recent Progress in Optical Sensors for Biomedical Diagnostics

In recent years, several types of optical sensors have been probed for their aptitude in healthcare biosensing, making their applications in biomedical diagnostics a rapidly evolving subject. Optical sensors show versatility amongst different receptor types and even permit the integration of different detection mechanisms. Such conjugated sensing platforms facilitate the exploitation of their neoteric synergistic characteristics for sensor fabrication. This paper covers nearly 250 research articles since 2016 representing the emerging interest in rapid, reproducible and ultrasensitive assays in clinical analysis. Therefore, we present an elaborate review of biomedical diagnostics with the help of optical sensors working on varied principles such as surface plasmon resonance, localised surface plasmon resonance, evanescent wave fluorescence, bioluminescence and several others. These sensors are capable of investigating toxins, proteins, pathogens, disease biomarkers and whole cells in varied sensing media ranging from water to buffer to more complex environments such as serum, blood or urine. Hence, the recent trends discussed in this review hold enormous potential for the widespread use of optical sensors in early-stage disease prediction and point-of-care testing devices.

Ultrasensitive Fluorescence Detection of Alzheimer's Disease Based on Polyvalent Directed Peptide Polymer Coupled to a Nanoporous ZnO Nanoplatform

Amyloid-beta 42 (Aβ₄₂), the key biomarker of Alzheimer's disease (AD), aggregates to form neurotoxic amyloid plaques. In this work, we modified two fluorescein isothiocyanate-labeled Aβ₄₂-targeting peptides and designed an Aβ₄₂-specific ultrasensitive polyvalent-directed peptide polymer (PDPP) to enhance AD diagnosis sensitivity. The dissociation constant of Aβ₄₂ by PDPP was 10³-fold higher than the single-site-directed peptide. The improved binding was due to the ability of PDPP to detect multiple receptors on the target. The power of the PDPP diagnostic probe was verified in its application to detect Aβ₄₂ in cerebrospinal fluid (CSF), which showed a lower limit of detection (LOD) in the fg mL^-1 range that is more sensitive than detection by antibodies or single peptides. In addition, we present a novel ultrasensitive diagnostic system using an array of nanoporous ZnO nanoparticles, which play a role in fluorescence signal amplification, to further improve AD diagnosis sensitivity. We enhanced the signal on the basis of the properties of nanoporous ZnO nanoparticles and measured and quantified an ultralow concentration (ag mL^-1 range) of Aβ₄₂. This PDPP coupled to the nanoporous ZnO-based system is a novel approach to AD diagnosis that might also be useful for the detection of other target biomarkers and clinical applications.

Machine Learning Approach to Enhance the Performance of MNP-Labeled Lateral Flow Immunoassay

The use of magnetic nanoparticle (MNP)-labeled immunochromatography test strips (ICTSs) is very important for point-of-care testing (POCT). However, common diagnostic methods cannot accurately analyze the weak magnetic signal from ICTSs, limiting the applications of POCT. In this study, an ultrasensitive multiplex biosensor was designed to overcome the limitations of capturing and normalization of the weak magnetic signal from MNPs on ICTSs. A machine learning model for sandwich assays was constructed and used to classify weakly positive and negative samples, which significantly enhanced the specificity and sensitivity. The potential clinical application was evaluated by detecting 50 human chorionic gonadotropin (HCG) samples and 59 myocardial infarction serum samples. The quantitative range for HCG was 1-1000 mIU mL^-1 and the ideal detection limit was 0.014 mIU mL^-1, which was well below the clinical threshold. Quantitative detection results of multiplex cardiac markers showed good linear correlations with standard values. The proposed multiplex assay can be readily adapted for identifying other biomolecules and also be used in other applications such as environmental monitoring, food analysis, and national security.

Analysis of Biomolecules Based on the Surface Enhanced Raman Spectroscopy

Analyzing biomolecules is essential for disease diagnostics, food safety inspection, environmental monitoring and pharmaceutical development. Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for detecting biomolecules due to its high sensitivity, rapidness and specificity in identifying molecular structures. This review focuses on the SERS analysis of biomolecules originated from humans, animals, plants and microorganisms, combined with nanomaterials as SERS substrates and nanotags. Recent advances in SERS detection of target molecules were summarized with different detection strategies including label-free and label-mediated types. This comprehensive and critical summary of SERS analysis of biomolecules might help researchers from different scientific backgrounds spark new ideas and proposals.

Noble metal nanostructures in optical biosensors: Basics, and their introduction to anti-doping detection

Nanotechnology has illustrated significant potentials in biomolecular-sensing applications; particularly its introduction to anti-doping detection is of great importance. Illicit recreational drugs, substances that can be potentially abused, and drugs with dosage limitations according to the prohibited lists announced by the World Antidoping Agency (WADA) are becoming of increasing interest to forensic chemists. In this review, the theoretical principles of optical biosensors based on noble metal nanoparticles, and the transduction mechanism of commonly-applied plasmonic biosensors are covered. We review different classes of recently-developed plasmonic biosensors for analytic determination and quantification of illicit drugs in anti-doping applications. The important classes of illicit drugs include anabolic steroids, opioids, stimulants, and peptide hormones. The main emphasis is on the advantages that noble metal nano-particles bring to optical biosensors for signal enhancement and the development of highly sensitive (label-free) biosensors. In the near future, such optical biosensors may be an invaluable substitute for conventional anti-doping detection methods such as chromatography-based approaches, and may even be commercialized for routine anti-doping tests.

Immunocontrolling Graphene Oxide Catalytic Nanogold Reaction and Its Application to SERS Quantitative Analysis

The gold nanoreaction between HAuCl4 and H2O2 is very slow at 50 °C, and the nanoenzyme of graphene oxide (GO) greatly catalyzes the nanoreaction to form gold nanoparticles (AuNPs) with high SERS activity in the presence of Vitoria blue 4R (VB4r) molecular probes, strong resonance Rayleigh scattering (RRS), and surface plasmon resonance (SPR) absorption effect. With the increase of GO, the SERS, RRS, and SPR absorptions were enhanced linearly due to the formation of more AuNPs. The rabit antibody of human chorionic gonadotropin (RHCG) strongly adsorbed on the GO surface to inhibit its catalysis. Upon addition of human chorionic gonadotropin (HCG), the RHCG is separated from the GO surface due to the formation of HCG-RHCG specific immunocomplexes, which led to the recovery of GO catalysis. Using the new strategy of immunocontrolling GO catalysis, three types of resonance methods including SERS, RRS, and surface plasmon resonance (SPR) absorption have been developed for detection of HCG.

A Label-Free and Ultrasensitive Immunosensor for Detection of Human Chorionic Gonadotrophin Based on Graphene FETs

We report on a label-free immunosensor based on graphene field effect transistors (G-FETs) for the ultrasensitive detection of Human Chorionic Gonadotrophin (hCG), as an indicator of pregnancy and related disorders, such as actopic pregnancy, choriocarcinoma and orchic teratoma. Pyrene based bioactive ester was non-covalently anchored onto the graphene channel in order to retain the sp² lattice. The G-FET transfer characteristics showed repeatable and reliable responses in all surface modifying steps using a direct current (DC) readout system. The hCG concentration gradient showed a detection limit of ~1 pg·mL^-1. The proposed method facilitates the cost-effective and viable production of graphene point-of-care devices for clinical diagnosis.

Nanotechnology-Enhanced No-Wash Biosensors for in Vitro Diagnostics of Cancer

In vitro biosensors have been an integral component for early diagnosis of cancer in the clinic. Among them, no-wash biosensors, which only depend on the simple mixing of the signal generating probes and the sample solution without additional washing and separation steps, have been found to be particularly attractive. The outstanding advantages of facile, convenient, and rapid response of no-wash biosensors are especially suitable for point-of-care testing (POCT). One fast-growing field of no-wash biosensor design involves the usage of nanomaterials as signal amplification carriers or direct signal generating elements. The analytical capacity of no-wash biosensors with respect to sensitivity or limit of detection, specificity, stability, and multiplexing detection capacity is largely improved because of their large surface area, excellent optical, electrical, catalytic, and magnetic properties. This review provides a comprehensive overview of various nanomaterial-enhanced no-wash biosensing technologies and focuses on the analysis of the underlying mechanism of these technologies applied for the early detection of cancer biomarkers ranging from small molecules to proteins, and even whole cancerous cells. Representative examples are selected to demonstrate the proof-of-concept with promising applications for in vitro diagnostics of cancer. Finally, a brief discussion of common unresolved issues and a perspective outlook on the field are provided.

Resonance Rayleigh Scattering and SERS Spectral Detection of Trace Hg(II) Based on the Gold Nanocatalysis

Mercury (Hg) is a heavy metal pollutant, there is an urgent need to develop simple and sensitive methods for Hg(II) in water. In this article, a simple and sensitive resonance Rayleigh scattering (RRS) method was developed for determination of 0.008-1.33 µmol/L Hg, with a detection limit of 0.003 μmol/L, based on the Hg(II) regulation of gold nanoenzyme catalysis on the HAuCl₄-H₂O₂ to form gold nanoparticles (AuNPs) with an RRS peak at 370 nm. Upon addition of molecular probes of Victoria blue B (VBB), the surface-enhanced Raman scattering (SERS) peak linearly decreased at 1612 cm-1 with the Hg(II) concentration increasing in the range of 0.013-0.5 μmol/L. With its good selectivity and good accuracy, the RRS method is expected to be a promising candidate for determining mercury ions in water samples.

SERS spectral study of HAuCl4-cysteine nanocatalytic reaction and its application for detection of heparin sodium with label-free VB4r molecular probe

In the presence of nanocatalyst, L-cysteine reduce HAuCl₄ rapidly to form gold nanoparticles (AuNP), and a quick nanocatalytic preparation procedure was established for Au/AuNP sol with highly active surface enhanced Raman scattering (SERS) effect and good stability. The nanoreaction was also studied by absorption, resonance Rayleigh scattering (RRS), transmission electron microscopy (TEM) and energy spectra. In the selected conditions, the analyte heparin sodium (HS) could react with victoria blue 4 R (VB4r) to form associated complexes which have very weak SERS effect to make the SERS signals decrease. The SERS signals at 1617 cm⁻¹ reduced linearly with HS concentration increasing. Upon addition of FeCl₃, it hydrolyzed to form stable Fe(OH)₃ sol platform that carried SERS active Au/AuNPs to enhance the sensitivity. Accordingly, we established a SERS quantitative analysis method in the sol substrate of Fe(OH)₃-Au/AuNPs, with a linear range of 0.5–75 ng/mL HS and a detection limit of 0.2 ng/mL. HS in real samples was determined, with a relative standard deviation of 2.65–7.63% and a recovery of 99.3–101%.

A novel label-free electrochemical immunosensor based on the composite of LPCs-SnS2 and AuNPs for the detection of human chorionic gonadotropin

LPCs-SnS₂ with a 3D conduction architecture was prepared by a facile hydrothermal process.

Recent advances in biosensor development for the detection of cancer biomarkers

Cancer is the second largest disease throughout the world with an increasing mortality rate over the past few years. The patient's survival rate is uncertain due to the limitations of cancer diagnosis and therapy. Early diagnosis of cancer is decisive for its successful treatment. A biomarker-based cancer diagnosis may significantly improve the early diagnosis and subsequent treatment. Biosensors play a crucial role in the detection of biomarkers as they are easy to use, portable, and can do analysis in real time. This review describes various biosensors designed for detecting nucleic acid and protein-based cancer biomarkers for cancer diagnosis. It mainly lays emphasis on different approaches to use electrochemical, optical, and mass-based transduction systems in cancer biomarker detection. It also highlights the analytical performances of various biosensor designs concerning cancer biomarkers in detail.