Redox-Mediated Indirect Fluorescence Immunoassay for the Detection of Disease Biomarkers Using Dopamine-Functionalized Quantum Dots

An ultrasensitive electrochemical immunosensor based on meso-PdN NCs and Au NPs/N-CNTs for quantitative cTnI detection

Electrochemical immunosensors have gained considerable attention in detecting human disease markers due to their excellent specificity, high sensitivity, and facile operation. Herein, a rational-designed sandwich-type electrochemical immunosensor is constructed for the sensitive detection of cardiac troponin I (cTnI) using nitrogen-doped carbon nanotubes loaded with gold nanoparticles (Au NPs/N-CNTs) as substrate and highly active mesoporous palladium-nitrogen nanocubes (meso-PdN NCs) as secondary antibody markers. Benefitting from its large specific surface area (638.04 m2 g-1) and high nitrogen content, novel polydopamine (PDA)/ halloysite nanotubes (HNTs) hybrid derived one-dimensional (1D) N-CNTs can provide more binding sites for the in-situ growth of Au NPs to connect Ab1. Furthermore, as an ideal substrate material, Au NPs/N-CNTs exhibit finely tuned mesoporous structures and outstanding conductivity, which facilitate the mass and electron transfer during the electrocatalysis process. Besides, highly concave surfaces and crystalline mesopores of meso-PdN NCs expose more surfaces and crevices, providing abundant reactive sites for H2O2 reduction. Remarkably, the as-obtained immunosensor presented a wide linear range (from 10 fg mL-1 to 100 ng mL-1) and an excellent low detection limit (9.85 fg mL-1). This study may offer new insights into the precise fabrication of efficient electrochemical immunosensors for various clinical diagnosis applications.

Visual monitoring of biocatalytic processes using small molecular fluorescent probes: strategies-mechanisms-applications

Real-time monitoring of biocatalytic-based processes is significantly improved and simplified when they can be visualized. Visual monitoring can be achieved by integrating a fluorescent unit with the biocatalyst. Herein, we outline the design strategies of fluorescent probes for monitoring biocatalysis: (1) probes for monitoring biocatalytic transfer: γ-glutamine is linked to the fluorophore as both a recognition group and for intramolecular charge transfer (ICT) inhibition; the probe is initially in an off state and is activated via the transfer of the γ-glutamine group and the release of the free amino group, which results in restoration of the "Donor-π-Acceptor" (D-π-A) system and fluorescence recovery. (2) Probes for monitoring biocatalytic oxidation: a propylamine is connected to the fluorophore as a recognition group, which cages the hydroxyl group, leading to the inhibition of ICT; propylamine is oxidized and subsequently β-elimination occurs, resulting in exposure of the hydroxyl group and fluorescence recovery. (3) Probes for monitoring biocatalytic reduction: a nitro group attached to a fluorophore as a fluorescence quenching group, this is converted to an amino group by catalytic reduction, resulting in fluorescence recovery. (4) Probes for monitoring biocatalytic hydrolysis: β-D-galactopyranoside or phosphate acts as a recognition group attached to hydroxyl groups of the fluorophore; the subsequent biocatalytic hydrolysis reaction releases the hydroxyl group resulting in fluorescence recovery. Following these 4 mechanisms, fluorophores including cyanine, coumarin, rhodamine, and Nile-red, have been used to develop systems for monitoring biocatalytic reactions. We anticipate that these strategies will result in systems able to rapidly diagnose and facilitate the treatment of serious diseases.

Oxygen Vacancies-Induced Antifouling Photoelectrochemical Aptasensor for Highly Sensitive and Selective Determination of α-Fetoprotein

Accurate measurement of cancer markers in urine is a convenient method for tumor monitoring. However, the concentration of cancer markers in urine is so low that it is difficult to achieve their measurement. Photoelectrochemical (PEC) sensors are a promising technology to realize the detection of trace cancer markers due to their high sensitivity. Currently, the interference of nonspecific biomolecules in urine is the main reason affecting the high sensitivity and selectivity of PEC sensors in detecting cancer markers. In this work, a strategy of oxygen vacancy (OV) modulation is proposed to construct a fouling-resistant PEC aptamer sensing platform for the detection of α-fetoprotein (AFP), a liver cancer marker. The introduction of OVs induces the formation of intermediate localized states in the photoelectric material, which not only facilitates the separation of photogenerated carriers but also leads to the redshift of the light absorption edge. More importantly, OVs with positive electrical properties can be employed to modify the antifouling layer (C-PEG) with negatively charged groups through an electrostatic interaction. The synergistic effect of OVs, antifouling layer, and aptamer resulted in a TiO2/OVs/C-PEG-based PEC sensor achieves a wide linear range from 1 pg/mL to 100 ng/mL and a low detection limit of 0.3 pg/mL for AFP. In addition, the sensor successfully realized the determination of AFP in urine samples and accurately differentiated between normal people and liver cancer patients in the early and advanced stages. This project is of great significance in advancing the application of photoelectrochemical bioanalytical technology to achieve the detection of cancer markers in urine by investigating the construction of an OVs-regulated fouling-resistant sensing interface.

A review of nanomaterials for biosensing applications

A biosensor is a device that reacts with the analyte to be analyzed, detects its concentration, and generates readable information, which plays an important role in medical diagnosis, detection of physiological indicators, and disease prevention. Nanomaterials have received increasing attention in the fabrication and improvement of biosensors due to their unique physicochemical and optical properties. In this paper, the properties of nanomaterials such as the size effect, optical and electrical properties, and their advantages in the field of biosensing are briefly summarized, and the application of nanomaterials can effectively improve the sensitivity and reduce the detection limit of biosensors. The advantages of commonly used nanomaterials such as gold nanoparticles (AuNPs), carbon nanotubes (CNTs), quantum dots (QDs), graphene, and magnetic nanobeads for biosensor applications are also reviewed. Besides, the two main types of biosensors using nanomaterials involved in their construction and their working principles are described, and the toxicity and biocompatibility of nanomaterials and the future direction of nanomaterial biosensors are discussed.

A Fluorescence Biosensor for Tyrosinase Activity Analysis Based on Silicon-Doped Carbon Quantum Dots

Tyrosinase (TYR) plays a pivotal role in the biosynthesis of melanin, and its activity level holds critical implications for vitiligo, melanoma cancer, and food nutritional value. The sensitive determination of TYR activity is of great significance for both fundamental research and clinical investigations. In this work, we successfully synthesized silicon-doped carbon quantum dots (Si-CQDs) through a one-pot hydrothermal method with trans-aconitic acid as carbon source and N-[3-(trimethoxysilyl)propyl]ethylenediamine as the dopant, exhibiting remarkable fluorescence quantum yield (QY) and photostability. Correspondingly, Si-CQDs were used as a probe to construct a sensitive, rapid, and user-friendly fluorescence method for TYR detection. The method relied on the oxidation of isoprenaline (ISO) by TYR, where Si-CQDs were employed as a highly efficient probe. The testing mechanism was the internal filtering effect (IFE) observed between Si-CQDs and the oxidative system of ISO and TYR. Under the optimized conditions, the fluorescence strategy exhibited a detection range of 0.05-2.0 U/mL for TYR with a limit of detection (LOD) of 0.041 U/mL. Furthermore, we successfully demonstrated the accurate determination of TYR levels in human serum, showcasing the promising potential of this method in various practical scenarios.

Application of Nanoparticles in the Diagnosis of Gastrointestinal Diseases: A Complete Future Perspective

The diagnosis of gastrointestinal (GI) diseases currently relies primarily on invasive procedures like digestive endoscopy. However, these procedures can cause discomfort, respiratory issues, and bacterial infections in patients, both during and after the examination. In recent years, nanomedicine has emerged as a promising field, providing significant advancements in diagnostic techniques. Nanoprobes, in particular, offer distinct advantages, such as high specificity and sensitivity in detecting GI diseases. Integration of nanoprobes with advanced imaging techniques, such as nuclear magnetic resonance, optical fluorescence imaging, tomography, and optical correlation tomography, has significantly enhanced the detection capabilities for GI tumors and inflammatory bowel disease (IBD). This synergy enables early diagnosis and precise staging of GI disorders. Among the nanoparticles investigated for clinical applications, superparamagnetic iron oxide, quantum dots, single carbon nanotubes, and nanocages have emerged as extensively studied and utilized agents. This review aimed to provide insights into the potential applications of nanoparticles in modern imaging techniques, with a specific focus on their role in facilitating early and specific diagnosis of a range of GI disorders, including IBD and colorectal cancer (CRC). Additionally, we discussed the challenges associated with the implementation of nanotechnology-based GI diagnostics and explored future prospects for translation in this promising field.

Rapid and ultrasensitive detection of acute kidney injury biomarkers CH3L1 and L-FABP using surface-enhanced Raman spectroscopy

Chitinase 3-like 1 (CH3L1) and liver fatty acid binding protein (L-FABP) are promising biomarkers for the early diagnosis of acute kidney injury (AKI). Here, a highly sensitive method for the quantitative detection of CH3L1 and L-FABP by surface-enhanced Raman spectroscopy (SERS) based on graphene oxide/gold and silver core-shell nanoparticles (GO/Au@Ag NPs) was proposed. The results showed that such GO/Au@Ag substrate can achieve rapid sensing of CH3L1 and L-FABP with a wide response range (2 × 10^-1 to 2 × 10^-8 mg/mL and 1.2 × 10^-1 to 1.2 × 10^-8 mg/mL, respectively) and high sensitivity. The detection limits of CH3L1 and L-FABP were 1.21 × 10^-8 mg/mL and 0.62 × 10^-8 mg/mL, respectively. In addition, the simultaneous detection of the two biomarkers in serum was demonstrated, showing the feasibility of this method in the complex biological environment. The detection of CH3L1 and L-FABP will greatly improve the early diagnosis and intervention of AKI.

Biogenic carbon dot-based fluorescence-mediated immunosensor for the detection of disease biomarker

The dynamic interplay of nanotechnology and immunoassay has unlocked an arena for developing next-generation techniques to contribute to detecting disease biomarkers. Herein, the work establishes the strategic fabrication of an immunosensor by sandwich technique based on the fluorescence phenomenon of carbon dots for the detection of the disease biomarker VEGF (Vascular Endothelial Growth Factor). VEGF, a biomarker for angiogenesis, is considered cancerous if found in elevated levels in the blood, and so is paramount for early detection of disease. Carbon dots derived from a biogenic source were synthesized employing a green microwave-assisted method followed by conjugating with a detection antibody, human immunoglobulin G (IgG), via EDC-NHS amidation reaction. On the other hand, the VEGF biomarker was immobilized onto the capture antibody. The detection antibody tagged with the fluorescent probe is employed as a bridge to connect with the VEGF biomarkers bound to the capture antibody. The response to different concentrations of VEGF biomarkers was recorded in terms of the fluorescence intensity of the carbon dots. The fluorescence immunosensor could exhibit a wide linear range of 0.1 fg/mL to 10 pg/mL with a low detection limit of 5.65 pg/mL towards VEGF. The potentiality of this designed immunosensor was qualitatively assessed with human blood plasma samples, showing promising results, thereby upholding the applicability of carbon dots as fluorescent labels in immunoassay techniques.

Acidic pH and thiol-driven homogeneous cathodic electrochemiluminescence strategy for determining the residue of organophosphorus pesticide in Chinese cabbage

Electrochemiluminescent (ECL) sensors for organophosphorus pesticides (OPs) have received considerable attention, whereas complicated electrode's immobilization, response to single hydrolysate and anodic emission correlated with ECL assays restrict their potential utilization. Herein, we developed a homogeneous dual-response cathodic ECL system for highly sensitive and reliable analysis of OP using CdTe QDs as emitters. CdTe QDs, emitting red light, were fabricated through a hydrothermal reaction and generated anodic and cathodic ECL emission upon stimulation of tripropyl amine and K₂S₂O₈, respectively. Notably, CdTe QDs-K₂S₂O₈ showed a simultaneous response to thiol and acidic pH, and were regarded as a ECL sensor for methidathion with limit of detection of 0.016 ng/mL based on hydrolysis of acetylthiocholine into thiocholine and CH₃COOH by acetylcholinesterase (AChE) and OPs' inhibition on AChE activity. This sensor also exhibited good practicability to detect methidathion in Chinese cabbage. Overall, the sensor will supply more useful information for ensuring OPs-related food safety.

Synthesis of Metal–Organic Frameworks Quantum Dots Composites as Sensors for Endocrine-Disrupting Chemicals

Hazardous chemical compounds such as endocrine-disrupting chemicals (EDCs) are widespread and part of the materials we use daily. Among these compounds, bisphenol A (BPA) is the most common endocrine-disrupting chemical and is prevalent due to the chemical raw materials used to manufacture thermoplastic polymers, rigid foams, and industrial coatings. General exposure to endocrine-disrupting chemicals constitutes a serious health hazard, especially to reproductive systems, and can lead to transgenerational diseases in adults due to exposure to these chemicals over several years. Thus, it is necessary to develop sensors for early detection of endocrine-disrupting chemicals. In recent years, the use of metal–organic frameworks (MOFs) as sensors for EDCs has been explored due to their distinctive characteristics, such as wide surface area, outstanding chemical fastness, structural tuneability, gas storage, molecular separation, proton conductivity, and catalyst activity, among others which can be modified to sense hazardous environmental pollutants such as EDCs. In order to improve the versatility of MOFs as sensors, semiconductor quantum dots have been introduced into the MOF pores to form metal–organic frameworks/quantum dots composites. These composites possess a large optical absorption coefficient, low toxicity, direct bandgap, formidable sensing capacity, high resistance to change under light and tunable visual qualities by varying the size and compositions, which make them useful for applications as sensors for probing of dangerous and risky environmental contaminants such as EDCs and more. In this review, we explore various synthetic strategies of (MOFs), quantum dots (QDs), and metal–organic framework quantum dots composites (MOFs@QDs) as efficient compounds for the sensing of ecological pollutants, contaminants, and toxicants such as EDCs. We also summarize various compounds or materials used in the detection of BPA as well as the sensing ability and capability of MOFs, QDs, and MOFs@QDs composites that can be used as sensors for EDCs and BPA.

Quantum Chemical Characterization and Design of Quantum Dots for Sensing Applications

The ability to tune the optoelectronic properties of quantum dots (QDs) makes them ideally suited for the use as fluorescence sensing probes. The vast structural diversity in terms of the composition and size of QDs can make designing a QD for a specific sensing application a challenging process. Quantum chemical calculations have the potential to aid this process through the characterization of the properties of QDs, leading to their in silico design. This is explored in the context of QDs for the fluorescence sensing of dopamine based upon density functional theory and time-dependent density functional theory (TDDFT) calculations. The excited states of hydrogenated carbon, silicon, and germanium QDs are characterized through TDDFT calculations. Analysis of the molecular orbital diagrams for the isolated molecules and calculations of the excited states of the dopamine-functionalized quantum dots establish the possibility of a photoinduced electron-transfer process by determining the relative energies of the electronic states formed from a local excitation on the QD and the lowest QD → dopamine electron-transfer state. The results suggest that the Si₁₆₅H₁₀₀ and Ge₈₄H₆₄ QDs have the potential to act as fluorescent markers that could distinguish between the oxidized and reduced forms of dopamine, where the fluorescence would be quenched for the oxidized form. The work contributes to a better understanding of the optical and electronic behavior of QD-based sensors and illustrates how quantum chemical calculations can be used to inform the design of QDs for specific fluorescent sensing applications.

Surface-enhanced Raman scattering from an electromagnetic induced transparency substrate for the determination of hepatocellular carcinoma

Surface-enhanced Raman scattering (SERS) is a powerful analytical method that is especially suitable for the detection of protein molecules. Detection sensitivity of SERS is directly related to the enhancement factor of the substrate, which is dependent on the strength of a local surface electric field generated by surface plasmonic resonance from substrate. In this study, an electromagnetic induced transparency like (EIT-like) metamaterial was used as the SERS substrate. The corresponding plasmonic resonance structure not only produces stronger optical near field but also reduces the spectral line broadening due to radiation damping. This is very beneficial for SERS process, which is strongly dependent on electric field intensity, to improve the sensitivity of SERS detection. Compared with the single resonance mode substrate, the enhancement factor for SERS with the double-mode substrate was increased by an order of magnitude. The obtained EIT-like substrate was used as a SERS-active substrate to detect Lens culinaris agglutinin (LCA)-reactive fraction of AFP (AFP-L3), a hepatocellular carcinoma (HCC)-specific maker. Experimental results are in good agreement with the clinical diagnosis, which demonstrates the potential application of metamaterials in the SERS-based diagnosis and biosensing.

Phenolic Tyrosinase Substrate with a Formal Potential Lower than That of Phenol to Obtain a Sensitive Electrochemical Immunosensor

The high and selective catalytic activities of tyrosinase (Tyr) have frequently led to its application in sensitive biosensors. However, in affinity-based biosensors, the use of Tyr as a catalytic label is less common compared to horseradish peroxidase and alkaline phosphatase owing to the fact that phenolic Tyr substrates have yet to be investigated in detail. Herein, four phenolic compounds that have lower formal potentials than phenol were examined for their applicability as Tyr substrates, and three reducing agents were examined as potential strong reducing agents for electrochemical-chemical (EC) redox cycling involving an electrode, a Tyr product, and a reducing agent. The combination of 4-methoxyphenol (MP) and ammonia-borane (AB) allows for (i) a high electrochemical signal level owing to rapid EC redox cycling and (ii) a low electrochemical background level owing to the slow oxidation of AB at a low applied potential and no reaction between MP and AB. When this combination was applied to an electrochemical immunosensor for parathyroid hormone (PTH) detection, a detection limit of 2 pg/mL was obtained. This detection limit is significantly lower than that obtained when a combination of phenol and AB was employed (300 pg/mL). It was also found that the developed immunosensor works well in PTH detection in clinical serum samples. This new phenolic substrate could therefore pave the way for Tyr to be more commonly used as a catalytic label in affinity-based biosensors.

Self-assembled multiprotein nanostructures with enhanced stability and signal amplification capability for sensitive fluorogenic immunoassays

Fundamentally improving the sensing sensitivity of immunoassay remains a huge challenge, which limited further critical applications. Herein we designed a new immunoprobe by integrating biometric unit (antibody) and signal amplification element (enzyme) to form urease-antibody-CaHPO₄ hybrid nanoflower (UAhNF) via the biomineralization process. The dual-functional UAhNF enhances the stability of urease in NaCl (10 mmol L^-1) and high temperature (60 °C), and also maintains the ability of antibody recognition, fitting greatly well with the need for immunosensor. Using imidacloprid as a model target, the fixed coating antigens are competed with imidacloprid to capture primary antibodies, and the secondary antibody of UAhNF was linked to construct the competitive-type fluorogenic immunoassays. An in-situ etching process of copper nanoparticles initiated by urease is integrated with UAhNF-based immune response for further improving the detection sensitivity. The proposed immunosensor possessed a 50% inhibition concentration value of 0.72 ng mL^-1, which is 30-fold lower than conventional enzyme-linked immunosorbent assay. This presented approach provided a versatile sensing tool by varying building blocks, making it practically functional for a variety of bioassay applications.

A colorimetric and SERS dual-readout sensor for sensitive detection of tyrosinase activity based on 4-mercaptophenyl boronic acid modified AuNPs

Tyrosinase (TYR) is as a well-known polyphenol oxidase and important biomarker of melanocytic lesions. Thus, developing powerful methods to determine TYR activity is of great value in the early diagnosis of skin disease. Direct surface-enhanced Raman scattering (SERS) detection of biomolecules is usually affected by non-specific interference and complicate structure of the analytes. It is a challenge to develop Raman-active molecules with specific recognition to analytes in complex media. Here, we report a novel colorimetric and surface-enhanced Raman scattering (SERS) dual-readout assay for the determination of TYR using commercially available and economical 4-mercaptophenyl boronic acid (4-MPBA) as a Raman-active and recognition molecule. 4-MPBA provides a unique interactive boronic acid group to the diol group of TYR substrate and exhibits good SERS signal. Also, the introduction of magnetic beads could promptly improve the anti-interference ability of dual-mode sensor. The TYR-incubated tyramine-modified magnetic beads could obviously change the concentration of 4-MPBA-AuNPs in the presence of O₂ and ascorbic acid, where the ultraviolet visible (UV-vis) absorption and SERS intensity were directly related to the concentration of TYR added. The dual-mode sensor had a rapid response to TYR within 1 min under optimized conditions and had high selectivity for TYR with a limit of detection at 0.001 U/mL. In addition, the dual-mode strategy showed promising prospects in the determination of TYR activity in serum samples and could be used to screen TYR inhibitors.

Spectrophotometric Assays for Sensing Tyrosinase Activity and Their Applications

Tyrosinase (TYR, E.C. 1.14.18.1), a critical enzyme participating in melanogenesis, catalyzes the first two steps in melanin biosynthesis including the ortho-hydroxylation of L-tyrosine and the oxidation of L-DOPA. Previous pharmacological investigations have revealed that an abnormal level of TYR is tightly associated with various dermatoses, including albinism, age spots, and malignant melanoma. TYR inhibitors can partially block the formation of pigment, which are always used for improving skin tone and treating dermatoses. The practical and reliable assays for monitoring TYR activity levels are very useful for both disease diagnosis and drug discovery. This review comprehensively summarizes structural and enzymatic characteristics, catalytic mechanism and substrate preference of TYR, as well as the recent advances in biochemical assays for sensing TYR activity and their biomedical applications. The design strategies of various TYR substrates, alongside with several lists of all reported biochemical assays for sensing TYR including analytical conditions and kinetic parameters, are presented for the first time. Additionally, the biomedical applications and future perspectives of these optical assays are also highlighted. The information and knowledge presented in this review offer a group of practical and reliable assays and imaging tools for sensing TYR activities in complex biological systems, which strongly facilitates high-throughput screening TYR inhibitors and further investigations on the relevance of TYR to human diseases.

Proximity hybridization-triggered DNA assembly for label-free surface-enhanced Raman spectroscopic bioanalysis

We have developed a versatile label-free surface-enhanced Raman spectroscopic platform for detecting various biotargets via proximity hybridization-triggered DNA assembly based on the 736 cm^-1 Raman peak of adenine breathing mode. We initially immobilized the first probe to AuNPs and modified the second with poly adenine. Presence of target DNA or protein molecules assembled a sandwich complex that brought the poly adenine close to the AuNPs surface, generating Raman signals, that were proportional to target molecule concentration. These approach exhibits high sensitivity, with a detection limit of 5.4 pM, 47 fM, and 0.51 pg/mL for target DNA, thrombin and CEA, respectively. Owing to a one step proximity dependent complex formation, this technique is simple and can be completed within 40 min, making it a promising candidate for point-of-care testing applications.

Stimuli-responsive azobenzene-quantum dots for multi-sensing of dithionite, hypochlorite, and azoreductase

A new fluorescence turn-on sensing platform has been developed applicable for sensitive profiling of multiple chemical and biological analytes, using azobenzene-quantum dot as a new stimuli-responsive optical nanoprobe. An azobenzene-carrying compound bis [4, 4'-(dithiophenyl azo)-1, 3-benzenediamine] (DTPABDA) is for the first time reported to be used for conjugation with CdSe/ZnS core/shell quantum dots (QDs) via the ligand exchange reaction. Due to the photo-induced electron-transfer (PET) effect, the electron-withdrawing azobenzene groups of DTPABDA can significantly cause the photoluminescence (PL) of QDs quenched. The QDs' PL can be subsequently reignited by the removal of azo moiety cleavable through three types of specific reactions: the dithionite reduction, hypochlorite oxidation, and azoreductase enzymatic catalysis, respectively. By monitoring of reaction-induced recovery of FL signals at 560 nm with an excitation of 450 nm, such azobenzene-QDs conjugates served as a new nanoprobe enabling the fluorescence turn-on sensing of dithionite, hypochlorite, and azoreductase with high sensitivity, broad linear range, and good selectivity. The successful detection of target analytes in real samples reveals the potential of our method in practical applications, such as biosensing, environmental and industrial monitoring. Graphical abstract A new stimuli-responsive fluorescence probe is reported for the sensitive detection of sodium dithionite, hypochlorite, and azoreductase. The probe consists of QDs with an azobenzene-carrying compound as a ligand. The fluorescence of QDs could be quenched by the azo group and subsequently recovered via the removal of azo group by these three compounds, resulting in the "turn-on" sensing of these compounds with high sensitivity, broad linear range, and good selectivity. The successful detection of azoreductase in serum samples reveals the practical use of this method.

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.

3-Aminophenyl Boronic Acid Functionalized Quantum-Dot-Based Ratiometric Fluorescence Sensor for the Highly Sensitive Detection of Tyrosinase Activity

Using the commercially available and economical 6-hydroxycoumarin (6-HC) as the substrate, a dual-emission ratiometric fluorescence sensor was developed to detect tyrosinase (TYR) activity based on 3-aminophenyl boronic acid functionalized quantum dots (APBA-QDs). TYR can catalyze 6-HC, a monohydroxy compound, to form a fluorescence-enhancing o-hydroxy compound, 6,7-dihydroxycoumarin. Owing to the special covalent binding between the o-hydroxyl and boric acid groups, APBA-QDs react with 6,7-dihydroxycoumarin to form a five-membered ring ester dual-emission fluorescence probe for TYR. With an increase in TYR activity, the fluorescence at 675 nm originating from the QDs is gradually quenched, whereas that at 465 nm owing to 6,7-dihydroxycoumarin increases. Referencing the decreasing signal of the dual-emission probe at 675 nm to measure the increasing signal at 465 nm, a ratiometric fluorescence method was established to detect the TYR activity with high sensitivity and selectivity. Under the conditions optimized via response surface methodology, a linear range of 0-0.05 U/mL was obtained for the TYR activity. The detection limit was as low as 0.003 U/mL. This sensing strategy can also be adopted for the rapid screening of the TYR inhibitors.

Equipment-free and visual detection of multiple biomarkers via an aggregation induced emission luminogen-based paper biosensor

Early and accurate disease diagnosis is of great appeal for saving patients' life, but requires biomarkers to be sensitively detected with simplicity, convenience, and low cost. Exploring the development of a high-performance fluorescence biosensor for biomarkers solid, equipment-free and visual biosensing is highly urgent but faces enormous challenges. Herein, we proposed a brand-new fluorescence system by integrating a typical aggregation induced emission dye (TPE-BTD) with dopamine for multiple biomarkers sensitive detection based on target-induced catalyzing oxidation. The system comprising TPE-BTD and dopamine emits strong fluorescence; with horseradish peroxidase (HRP) or HRP-mimicking DNAzyme and H₂O₂ being added, significant oxidation on dopamine occurs to generate dopachrome, which actuated the inner filter effect (IFE) due to the overlap of its absorbtion curve and emission spectrum of TPE-BTD, subsequently decreasing fluorescence emission and displaying a rapid and sensitive response to H₂O₂ and G-quadruplex DNA. We further apply TPE-BTD/dopamine system in analysis of glucose and DNA adenine methylation methyltransferase (Dam MTase) based on target-initiated signal transduction. Finally, TPE-BTD was employed as emitters in fabrication of paper biosensors, which can achieve solid, equipment-free and visual detection of multiple biomarkers based on the high emission performance of TPE-BTD, opening up a new pathway to development of biosensors for practical application. We expect this sensing conception will be helpful in development of practical biosensors, and this sensor will find more applications in disease diagnosis.

Enzyme-free amplified SERS immunoassay for the ultrasensitive detection of disease biomarkers

We developed an enzyme-free, amplified SERS immunoassay by combining AgNP-linked immunoreaction and SERS transduction for the ultrasensitive detection of disease biomarkers.

A novel colorimetric immunoassay based on enzyme-regulated instant generation of Turnbull's blue for the sensitive determination of ochratoxin A

The aim of this study was to develop a novel colorimetric sensing method based on enzyme-regulated instant generation of Turnbull's blue, serving as a chromogenic agent, for a sensitive immunoassay for the determination of ochratoxin A (OTA).

Nanomaterials for Healthcare Biosensing Applications

In recent years, an increasing number of nanomaterials have been explored for their applications in biomedical diagnostics, making their applications in healthcare biosensing a rapidly evolving field. Nanomaterials introduce versatility to the sensing platforms and may even allow mobility between different detection mechanisms. The prospect of a combination of different nanomaterials allows an exploitation of their synergistic additive and novel properties for sensor development. This paper covers more than 290 research works since 2015, elaborating the diverse roles played by various nanomaterials in the biosensing field. Hence, we provide a comprehensive review of the healthcare sensing applications of nanomaterials, covering carbon allotrope-based, inorganic, and organic nanomaterials. These sensing systems are able to detect a wide variety of clinically relevant molecules, like nucleic acids, viruses, bacteria, cancer antigens, pharmaceuticals and narcotic drugs, toxins, contaminants, as well as entire cells in various sensing media, ranging from buffers to more complex environments such as urine, blood or sputum. Thus, the latest advancements reviewed in this paper hold tremendous potential for the application of nanomaterials in the early screening of diseases and point-of-care testing.

Nanohoneycomb Surface-Enhanced Raman Spectroscopy-Active Chip for the Determination of Biomarkers of Hepatocellular Carcinoma

Overexpression of the Lens culinaris agglutinin-reactive fraction of alpha-fetoprotein (AFP-L3) is an essential biomarker for early diagnosis of hepatocellular carcinoma (HCC). In this study, we designed a new surface-enhanced Raman spectroscopy active chip for the detection of AFP with high sensitivity and excellent repeatability. This chip was composed of a honeycomb gold nanostructure array with strong electromagnetic field coupling due to the special cavity geometric characteristics of the honeycomb structure. The honeycomb structure exhibited extraordinary performance for the specific detection of AFP in the range of 0.003-3 ng/mL and also determined the proportion of AFP-L3 with a high degree of accuracy, which has shown great potential for application in the clinical diagnosis of HCC.

Quantum dots in biomedical applications

Semiconducting nanoparticles, more commonly known as quantum dots, possess unique size and shape dependent optoelectronic properties. In recent years, these unique properties have attracted much attention in the biomedical field to enable real-time tissue imaging (bioimaging), diagnostics, single molecule probes, and drug delivery, among many other areas. The optical properties of quantum dots can be tuned by size and composition, and their high brightness, resistance to photobleaching, multiplexing capacity, and high surface-to-volume ratio make them excellent candidates for intracellular tracking, diagnostics, in vivo imaging, and therapeutic delivery. We discuss recent advances and challenges in the molecular design of quantum dots are discussed, along with applications of quantum dots as drug delivery vehicles, theranostic agents, single molecule probes, and real-time in vivo deep tissue imaging agents. We present a detailed discussion of the biodistribution and toxicity of quantum dots, and highlight recent advances to improve long-term stability in biological buffers, increase quantum yield following bioconjugation, and improve clearance from the body. Last, we present an outlook on future challenges and strategies to further advance translation to clinical application. STATEMENT OF SIGNIFICANCE: Semiconducting nanoparticles, commonly known as quantum dots, possess unique size and shape dependent electrical and optical properties. In recent years, they have attracted much attention in biomedical imaging to enable diagnostics, single molecule probes, and real-time imaging of tumors. This review discusses recent advances and challenges in the design of quantum dots, and highlights how these strategies can further advance translation to clinical applications.

Ratiometric fluorescent immunoassay for the cardiac troponin-I using carbon dots and palladium-iridium nanocubes with peroxidase-mimicking activity

A nanozyme-linked immunosorbent assay is described for cardiac troponin I which is a biomarker for myocardial infarction. The method is based on the use of Pd-Ir nanocubes with excellent peroxidase-like activity. The nanocubes catalyze the oxidization of nonfluorescent o-phenylenediamine (OPD) by H₂O₂ to form a yellow fluorescent product (oxOPD) with excitation/emission maxima at 400/570 nm. Carbon dots are added as a reference fluorophore. Under the same excitation wavelength, they display blue fluorescence (450 nm). The ELISA uses the Pd-Ir nanocubes as a label for the secondary antibody and OPD as substrate. The ratio of fluorescence intensities at 570 and 450 nm increases in the 1 pg·mL^-1 to 1 ng·mL^-1 cardiac troponin I concentration range, and the detection limit is 0.31 pg·mL^-1. The method was applied to analyze spiked serum samples, and the results compared well with those obtained by a commercial chemiluminescence assay. Graphical abstract Schematic presentation of the ratiometric fluorescence immunoassay for cardiac troponin-I. Pd-Ir nanocubes were employed to fabricate nanozyme-based signal labels for its excellent peroxidase-mimicking activity.

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.

An fluorescent aptasensor for sensitive detection of tumor marker based on the FRET of a sandwich structured QDs-AFP-AuNPs

The detection of alpha-fetoprotein (AFP) is of great importance for hepatocellular carcinoma (HCC) diagnosis, but it needs to be further improved because of poor sensitivity and complicated operating steps. In this paper, a simple and sensitive homogeneous apatasensor for AFP has been developed based on Förster resonance energy transfer (FRET) where the AFP aptamer labeled luminescent CdTe quantum dots (QDs) as a donor and anti-AFP antibody functional gold nanoparticles (AuNPs) as an acceptor. In the presence of AFP, the bio-affinity between aptamer, target, and antibody made the QDs and AuNPs close enough, thus the fluorescence of CdTe QDs quenched though the FRET between QD and AuNP. The fluorescent aptasensor for AFP showed a concentration-dependent decrease of fluorescence intensity in the low nanomolar range and a detecting linear range of 0.5-45 ng mL^-1, with a detection limit of 400 pg mL^-1. Moreover, this homogeneous aptasensor is simple and reliable, and obtained satisfying results for the detection of AFP in human serum samples. With more and more aptamers for biomarkers have been selected gradually, this approach could be easily extended to detection of a wide range of biomarkers. The proposed aptasensor has great potential for carcinoma screening in point-of-care testing and even in field use.

An ultrasensitive fluorescent aptasensor for detection of cancer marker proteins based on graphene oxide-ssDNA

A novel biosensing platform was developed by integrating a new ssDNA aptamer and graphene oxide (GO) for highly sensitive and selective detection of liver cancer biomarkers (alpha-fetoprotein, AFP). The key concept of this biosensing platform is that the fluorescence of dye-modified ssDNA can be effectively quenched by GO after forming the hybrid structure of graphene oxide-ssDNA (GO-ssDNA). The AFP can selectively react with GO-ssDNA and lead to the decomposition of GO-ssDNA, which results in the recovery of fluorescence, and an increase in fluorescence intensity with the increasing concentration of AFP in the range of 0 to 300 pg mL-1. The linear range was obtained from 1 to 150 pg mL-1 and the detection limit was 0.909 pg mL-1. Moreover, this biosensing platform can be applied to serum and cell imaging for the detection of AFP. The results show that the proposed biosensor has great potential application in AFP-related clinical diagnosis and research.

An ultrasensitive flow cytometric immunoassay based on bead surface-initiated template-free DNA extension

An ultrasensitive flow cytometric immunoassay (TdT-FCI) is developed based on bead surface-initiated template-free DNA extension.

Proteins lack the duplication mechanism like nucleic acids, so the connection of immunoassays with effective nucleic acid amplification techniques has become a powerful way for the detection of trace protein biomarkers in biological fluids. However, such immunoassays generally suffer from rather stringent DNA sequence design and complicated operations. Herein, we propose a simple but highly sensitive flow cytometric immunoassay (FCI) by employing on-bead terminal deoxynucleotidyl transferase (TdT)-initiated template-free DNA extension as an effective signal amplification pathway (TdT-FCI), and gold nanoparticles (AuNPs) co-functionalized with both the detection antibody and a 3′-OH oligonucleotide (ODN) as the transducer to bridge the immunoassay and subsequent TdT-mediated DNA amplification. The target antigen can sandwich with the capture antibody immobilized on the magnetic beads (MBs) and the detection antibody on the AuNPs to bring a lot of ODNs onto the surface of MBs. Each ODN on the MBs can be effectively elongated by TdT in a template-free manner to produce a long poly(T) tail, which will then bind to many 6-carboxyfluorescein (FAM)-labeled poly(A)25. Since each AuNP can carry multiple ODNs and each extended ODN can ultimately capture numerous FAM-poly(A)25, efficiently amplified fluorophore accumulation on the MBs can be achieved. The fluorescent MBs can be individually interrogated with a flow cytometer and thus quantitative analysis of the target antigen can be realized. Coupled with the powerful flow cytometry analysis, the simple but efficient TdT-based signal amplification mechanism has pushed the detection limit of prostate specific antigen (PSA) down to a low level of 0.5 pg mL^–1. Furthermore, based on an elegant bead size-encoding principle, we have further advanced the TdT-FCI for multiplexed antigen detection in a single reaction. Sharing the unique merits of simple design and operation, efficient signal amplification, powerful signal readout and the capability for multiplexed analysis, this TdT-FCI provides a versatile tool for detecting trace antigen biomarkers towards clinical diagnosis as well as prognosis.

Nanosensors to Detect Protease Activity In Vivo for Noninvasive Diagnostics

Proteases are multi-functional enzymes that specialize in the hydrolysis of peptide-bonds and control broad biological processes including homeostasis and allostasis. Moreover, dysregulated protease activity drives pathogenesis and is a functional biomarker of diseases such as cancer; therefore, the ability to detect protease activity in vivo may provide clinically relevant information for biomedical diagnostics. The goal of this protocol is to create nanosensors that probe for protease activity in vivo by producing a quantifiable signal in urine. These protease nanosensors consist of two components: a nanoparticle and substrate. The nanoparticle functions to increase circulation half-life and substrate delivery to target disease sites. The substrate is a short peptide sequence (6-8 AA), which is designed to be specific to a target protease or group of proteases. The substrate is conjugated to the surface of the nanoparticle and is terminated by a reporter, such as a fluorescent marker, for detection. As dysregulated proteases cleave the peptide substrate, the reporter is filtered into urine for quantification as a biomarker of protease activity. Herein we describe construction of a nanosensor for matrix metalloproteinase 9 (MMP9), which is associated with tumor progression and metastasis, for detection of colorectal cancer in a mouse model.

Dual-mode fluorescent and colorimetric immunoassay for the ultrasensitive detection of alpha-fetoprotein in serum samples

We present a novel dual-mode fluorescent and colorimetric immunosensor based on conventional immunoassay platforms by utilizing a gold nanoflower (AuNF)-loaded fluorescein molecule (AuNF@Fluorescein) as signal output. The AuNFs were modified with thiolated carboxyl ligand, which consisted of a hydrophobic alkane chain as hydrophobic wallet for fluorescein encapsulation, a tetra (ethylene glycol) unit for biocompatibility and solubility, and a functional carboxyl group for the conjugation of biorecognition molecules for biosensing. The resultant AuNFs showed a high loading capacity of 3.74 × 10⁶ fluorescein molecules per AuNF because of its flower-like shape with many complex branches. By adjusting the solution pH to 8.0, the fluorescein molecules can almost entirely be released from the hydrophobic wallet of AuNF@Fluorescein, which led to strong fluorescent-signal amplification. Under the optimal detection conditions, the proposed immunoassay based on fluorescent signal exhibited ultrahigh sensitivity for alpha-fetoprotein (AFP) detection, with a limit of detection (LOD) of 29 fg/mL. This value is approximately 9.3 × 10³-fold lower than that of corresponding horseradish peroxidase (HRP)-based immunoassay (LOD = 270 pg/mL). The fluorescein molecule also had intrinsic peroxidase-like activity to catalyze 3,3',5,5'-tetramethylbenzidine oxidation with hydrogen peroxide for colorimetric signal. The proposed method with colorimetric mode further exhibited a sensitivity with a LOD of 17.7 pg/mL, which is about 15-fold lower than that of conventional HRP-based immunoassay. The recoveries of the proposed dual-mode immunoassay for AFP spiked serum samples ranged within 89.85%-100.0%, with the coefficient of variations ranging from 0.5% to 2.4%, indicating acceptable accuracy and precision for AFP quantitative detection. The reliability of the developed dual-mode immunoassay was further compared with a commercial chemiluminescence immunoassay kit by analyzing 20 clinical serum samples, showing that the two methods well agreed with each other, with high correlation coefficients of 0.997 and 0.986 based on recorded fluorescence and colorimetric signals, respectively. In summary, the proposed method was highly suitable for the ultrasensitive analysis of biomarkers or infectious diseases by fluorescence mode and can be used for routine clinical diagnosis by colorimetric mode.