A novel label-free photoelectrochemical sensor based on N,S-GQDs and CdS co-sensitized hierarchical Zn2SnO4 cube for detection of cardiac troponin I

Membraneless, self-powered immunosensing of a cardiac biomarker by exploiting a PEC platform based on CaBi2Ta2O9 combined with bismuth oxyiodides

This work describes the development of a membraneless, self-powered immunosensor exploiting a photoelectrochemical system based on two photoelectrodes for cardiac troponin I (cTn). An electrode based on CaBi2Ta2O9 combined with bismuth oxyiodides (BiOI/Bi4O5I2/Bi5O7I) was modified with the cTnI antibody (anti-cTnI) and applied in a photoelectrochemical cell as a photoanode. To perform the cTnI detection exploiting a self-powered photoelectrochemical setup, the immunosensor (anti-cTnI/BiOI/Bi4O5I2/Bi5O7I/CaBi2Ta2O9/FTO) was coupled to a photoelectrochemical cell containing a photocathode based on CuBi2O4 (CBO/FTO) for zero-biased photoelectrochemical immunosensing of cardiac troponin I (cTnI) biomarker. For comparison purposes, the photoanode was applied for cTnI detection in a three-electrode electrochemical cell. The spectroscopic, structural, and morphological characteristics of the photoelectrochemical (PEC) materials were evaluated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) measurements were performed in the presence and absence of light to investigate the effects of photons on the charge transfer resistance of the photoanode. The influence of the cTnI biomarker on the photoelectrochemical response of the anti-cTnI antibody-modified photoelectrochemical platform (anti-cTnI/BiOI/Bi4O5I2/Bi5O7I/CaBi2Ta2O9/FTO) was evaluated by measuring the photocurrent of the system. The immunosensor presented a linear response ranging from 1 pg mL-1 to 200 ng mL-1 as well as a mean recovery percentage between 95.7% and 108.0% in real human serum samples for the cTnI biomarker.

Designing advanced S-scheme CdS QDs/La-Bi2WO6 photocatalysts for efficient degradation of RhB

Finding effective strategies to design efficient photocatalysts and decompose refractory organic compounds in wastewater is a challenging problem. Herein, by coupling element doping and constructing heterostructures, S-scheme CdS QDs/La-Bi2WO6 (CS/LBWO) photocatalysts are designed and synthesized by a simple hydrothermal method. As a result, the RhB degradation efficiency of the optimized 5% CS/LBWO reached 99% within 70 min of illumination with excellent stability and recyclability. CS/LBWO shows improvement in the adsorption range of visible light and promotes electron-hole pair generation/migration/separation, attributing the superior degradation performance. The degradation RhB mechanism is proposed by a free radical capture experiment, electron paramagnetic resonance, and high-performance liquid chromatography-mass spectrometry results, indicating that h+ and •O2 - play a significant role during four degradation processes: de-ethylation, chromophore cleavage, ring opening, and mineralization. Based on in situ irradiated X-ray photoelectron spectroscopy, Mulliken electronegativity theory, and the work function results, the S-scheme heterojunction of CS/LBWO promotes the transfer of photogenerated electron-hole pairs and promotes the generation of reactive radicals. This work not only reports that 5% CS/LBWO is a promising photocatalyst for degradation experiments but also provides an approach to design advanced photocatalysts by coupling element doping and constructing heterostructures.

Photoelectrochemical Determination of Cardiac Troponin I as a Biomarker of Myocardial Infarction Using a Bi2S3 Film Electrodeposited on a BiVO4-Coated Fluorine-Doped Tin Oxide Electrode

A sensitive and selective label-free photoelectrochemical (PEC) immunosensor was designed for the detection of cardiac troponin I (cTnI). The platform was based on a fluorine-doped tin oxide (FTO)-coated glass photoelectrode modified with bismuth vanadate (BiVO₄) and sensitized by an electrodeposited bismuth sulfide (Bi₂S₃) film. The PEC response of the Bi₂S₃/BiVO₄/FTO platform for the ascorbic acid (AA) donor molecule was approximately 1.6-fold higher than the response observed in the absence of Bi₂S₃. The cTnI antibodies (anti-cTnI) were immobilized on the Bi₂S₃/BiVO₄/FTO platform surface to produce the anti-cTnI/Bi₂S₃/BiVO₄/FTO immunosensor, which was incubated in cTnI solution to inhibit the AA photocurrent. The photocurrent obtained by the proposed immunosensor presented a linear relationship with the logarithm of the cTnI concentration, ranging from 1 pg mL^-1 to 1000 ng mL^-1. The immunosensor was successfully employed in artificial blood plasma samples for the detection of cTnI, with recovery values ranging from 98.0% to 98.5%.

Tailoring enzymatic loading capacity on CdS nanorods@ZnIn2S4 nanosheets 1D/2D heterojunctions: Toward ultrasensitive photoelectrochemical bioassay of tobramycin

Despite advances in the development of photoelectrochemical (PEC) sensor, modulating the PEC response of assembled heterostructure interface is still a great challenge. Here, an ultrasensitive PEC aptasensor for tobramycin (TOB) assay was conducted based on one-dimensional/two-dimensional CdS nanorods@ZnIn₂S₄ nanosheets (1D/2D CdS NRs@ZnIn₂S₄ NSs) heterojunctions by tailoring enzymatic loading capacity. Firstly, alkaline phosphatase modified TOB aptamer (ALP-Apt) was linked via specific base complementary pairing, and insoluble precipitations were then produced through the ALP-triggered catalytic reaction with the aid of Ag⁺, which prevented the charge transfer and resulted in the decrement of photocurrent. In the presence of TOB, partial ALP-Apt detached from the electrode surface due to the strong affinity between TOB and its aptamer, leading to a reduction in the amount of ALP and insoluble precipitate, in turn the PEC response partially recovered. The photocurrents exhibited a wider linear range towards the TOB concentration of 1.0-5.0 × 10⁴ pg mL^-1, with a low detection limit of 0.96 pg mL^-1. The constructed PEC aptasensor gained satisfactory results for TOB assay in milk samples as well, which also offered significant promise for other pollutants in environmental analysis.

Low-dimensionality carbon-based biosensors: the new era of emerging technologies in bioanalytical chemistry

Since the last decade, carbon nanomaterials have had a notable impact on different fields such as bioimaging, drug delivery, artificial tissue engineering, and biosensors. This is due to their good compatibility toward a wide range of chemical to biological molecules, low toxicity, and tunable properties. Especially for biosensor technology, the characteristic features of each dimensionality of carbon-based materials may influence the performance and viability of their use. Surface area, porous network, hybridization, functionalization, synthesis route, the combination of dimensionalities, purity levels, and the mechanisms underlying carbon nanomaterial interactions influence their applications in bioanalytical chemistry. Efforts are being made to fully understand how nanomaterials can influence biological interactions, to develop commercially viable biosensors, and to gain knowledge on the biomolecular processes associated with carbon. Here, we present a comprehensive review highlighting the characteristic features of the dimensionality of carbon-based materials in biosensing.

Development of a Point-of-Care SPR Sensor for the Diagnosis of Acute Myocardial Infarction

A novel point-of-care surface plasmon resonance (SPR) sensor was developed for the sensitive and real-time detection of cardiac troponin I (cTnI) using epitope-imprinted molecular receptors. The surface coverage of a nano-molecularly imprinted polymer (nanoMIP)-functionalized SPR sensor chip and the size of nanoMIPs (155.7 nm) were characterized using fluorescence microscopy and dynamic light scattering techniques, respectively. Atomic force microscopy, electrochemical impedance spectroscopy, square wave voltammetry and cyclic voltammetry techniques confirmed the successful implementation of each step of the sensor fabrication. The SPR bio-detection assay was initially established by targeting the cTnI peptide template, and the sensor allowed the detection of the peptide in the concentration range of 100-1000 nM with a correlation coefficient (R²) of 0.96 and limit of detection (LOD) of 76.47 nM. The optimum assay conditions for protein recognition were subsequently determined, and the cTnI biomarker could be detected in a wide concentration range (0.78-50 ng mL^-1) with high reproducibility (R² = 0.91) and sensitivity (LOD: 0.52 ng mL^-1). The overall sensor results were subjected to three binding isotherm models, where nanoMIP-cTnI interaction followed the Langmuir binding isotherm with the dissociation constant of 2.99 × 10^-11 M, indicating a very strong affinity between the cTnI biomarker and epitope-imprinted synthetic receptor. Furthermore, the selectivity of the sensor was confirmed through studying with a control nanoMIP that was prepared by imprinting a non-specific peptide template. Based on the cross-reactivity tests with non-specific molecules (i.e., glucose, p53 protein, transferrin and bovine serum albumin), the nanoMIP-SPR sensor is highly specific for the target biomarker. The developed biomimetic sensor, relying on the direct assay strategy, holds great potential not only for the early and point-of-care testing of acute myocardial infarction but also for other life-threatening diseases that can be diagnosed by determining the elevated levels of certain biomarkers.

Ultrathin Covalent Organic Framework Nanosheets/Ti3C2Tx-Based Photoelectrochemical Biosensor for Efficient Detection of Prostate-Specific Antigen

Designable and ultrathin covalent organic framework nanosheets (CONs) with good photoelectric activity are promising candidates for the construction of photoelectrochemical (PEC) biosensors for the detection of low-abundance biological substrates. However, achieving highly sensitive PEC properties by using emerging covalent organic framework nanosheets (CONs) remains a great challenge due to the polymeric nature and poor photoelectric activity of CONs. Herein, we report for the first time the preparation of novel composites and their PEC sensing properties by electrostatic self-assembly of ultrathin CONs (called TTPA-CONs) with Ti₃C₂T_x. The prepared TTPA-CONs/Ti₃C₂T_x composites can be used as photocathodes for PEC detection of prostate-specific antigen (PSA) with high sensitivity, low detection limit, and good stability. This work not only expands the application of CONs but also opens new avenues for the development of efficient PEC sensing platforms.

A sensitive ratiometric biosensor for determination cardiac troponin I of myocardial infarction markers based on N, Zn-GQDs

A sensitive unlabeled ratiometric biosensor was developed to the detection of cardiac troponin I (cTnI). This biosensor was established by using the glassy carbon electrode coated with graphene oxide to form a platform bonded with N, Zn co-doped graphene quantum dots (N, Zn-GQDs). The N, Zn-GQDs was successfully prepared as the raw materials of graphite powder and characterized. Antibodies of cTnI were bonded to the surface of N, Zn-GQDs as the nanoprobe by amide bonds. The signals of electrochemiluminescence (ECL) and differential pulse voltammetry (DPV) were exposed to decrease in the presence of cTnI, which caused the signal substance to move farther away from the electrode. It was found that the immune complex layer attenuated the intensity of ECL and DPV which could be used as the good overall signal for determining concentration of cTnI. The ratiometric biosensor had a good response to cTnI with the detection limit is 4.59 pg L^-1 in the concentration range of 10-10⁶ pg L^-1. The developed method was evaluated for the detection of cTnI in human serum, and the obtained results were consistent compared to the reference values obtained by hospital standard enzyme linked immunoassay (ELISA) with 9.09%-11.1% of RSD. Our findings suggested that this ratiometric biosensor could be used to the detection of cTnI in human serum with lower cost and higher sensitivity, it also might be better potential application prospect based on N, Zn-GQDs to detect other biomarkers.

Liposome-Embedded Cu2-xAgxS Nanoparticle-Mediated Photothermal Immunoassay for Daily Monitoring of cTnI Protein Using a Portable Thermal Imager

Functional photothermal nanomaterials have gained widespread attention in the field of precise cancer therapy and early disease diagnosis due to their unique photothermal conversion properties. However, the relatively narrow temperature response range and the outputable accuracy of commercial thermometers limit the accurate detection of biomarkers. Herein, we designed a liposome-embedded Cu_2-xAg_xS amplification-based photothermal sensor for the accurate determination of cardiac troponin I (cTnI) in health monitoring and point-of-care testing (POCT). The combinable 3D-printing detecting device monitored and visualized target signal changes in the testing system under the excitation of near-infrared (NIR) light, which was recorded and evaluated for possible pathogenicity by a smartphone. Notably, we predicted the potentially efficient thermal conversion efficiency of Cu_2-xAg_xS from the structure and charge density distribution, calculated by the first-principles and density functional theory (DFT), which provided a theoretical basis for the construction of novel photothermal materials, and the experimental results proved the correctness of the theoretical projections. Under optimal conditions, the photothermal immunoassay showed a dynamic linear range of 0.02-10 ng mL^-1 with a detection limit of 11.2 pg mL^-1. This work instructively introduces promising theoretical research and provides new insights for the development of sensitive portable photothermal biosensors.

Electroanalytical point-of-care detection of gold standard and emerging cardiac biomarkers for stratification and monitoring in intensive care medicine - a review

Determination of specific cardiac biomarkers (CBs) during the diagnosis and management of adverse cardiovascular events such as acute myocardial infarction (AMI) has become commonplace in emergency department (ED), cardiology and many other ward settings. Cardiac troponins (cTnT and cTnI) and natriuretic peptides (BNP and NT-pro-BNP) are the preferred biomarkers in clinical practice for the diagnostic workup of AMI, acute coronary syndrome (ACS) and other types of myocardial ischaemia and heart failure (HF), while the roles and possible clinical applications of several other potential biomarkers continue to be evaluated and are the subject of several comprehensive reviews. The requirement for rapid, repeated testing of a small number of CBs in ED and cardiology patients has led to the development of point-of-care (PoC) technology to circumvent the need for remote and lengthy testing procedures in the hospital pathology laboratories. Electroanalytical sensing platforms have the potential to meet these requirements. This review aims firstly to reflect on the potential benefits of rapid CB testing in critically ill patients, a very distinct cohort of patients with deranged baseline levels of CBs. We summarise their source and clinical relevance and are the first to report the required analytical ranges for such technology to be of value in this patient cohort. Secondly, we review the current electrochemical approaches, including its sub-variants such as photoelectrochemical and electrochemiluminescence, for the determination  of important CBs highlighting the various strategies used, namely the use of micro- and nanomaterials, to maximise the sensitivities and selectivities of such approaches. Finally, we consider the challenges that must be overcome to allow for the commercialisation of this technology and transition into intensive care medicine.

Graphene Quantum Dots-Based Electrochemical Biosensing Platform for Early Detection of Acute Myocardial Infarction

Heart failure resulting from acute myocardial infarction (AMI) is an important global health problem. Treatments of heart failure and AMI have improved significantly over the past two decades; however, the available diagnostic tests only give limited insights into these heterogeneous conditions at a reversible stage and are not precise enough to evaluate the status of the tissue at high risk. Innovative diagnostic tools for more accurate, more reliable, and early diagnosis of AMI are urgently needed. A promising solution is the timely identification of prognostic biomarkers, which is crucial for patients with AMI, as myocardial dysfunction and infarction lead to more severe and irreversible changes in the cardiovascular system over time. The currently available biomarkers for AMI detection include cardiac troponin I (cTnI), cardiac troponin T (cTnT), myoglobin, lactate dehydrogenase, C-reactive protein, and creatine kinase and myoglobin. Most recently, electrochemical biosensing technologies coupled with graphene quantum dots (GQDs) have emerged as a promising platform for the identification of troponin and myoglobin. The results suggest that GQDs-integrated electrochemical biosensors can provide useful prognostic information about AMI at an early, reversible, and potentially curable stage. GQDs offer several advantages over other nanomaterials that are used for the electrochemical detection of AMI such as strong interactions between cTnI and GQDs, low biomarker consumption, and reusability of the electrode; graphene-modified electrodes demonstrate excellent electrochemical responses due to the conductive nature of graphene and other features of GQDs (e.g., high specific surface area, π-π interactions with the analyte, facile electron-transfer mechanisms, size-dependent optical features, interplay between bandgap and photoluminescence, electrochemical luminescence emission capability, biocompatibility, and ease of functionalization). Other advantages include the presence of functional groups such as hydroxyl, carboxyl, carbonyl, and epoxide groups, which enhance the solubility and dispersibility of GQDs in a wide variety of solvents and biological media. In this perspective article, we consider the emerging knowledge regarding the early detection of AMI using GQDs-based electrochemical sensors and address the potential role of this sensing technology which might lead to more efficient care of patients with AMI.

A Novel Photoelectrochemical Sensor Based on SiNWs@PDA for Efficient Detection of Myocardial Infarction

Based on the necessity and urgency of Cardiac Troponin I (cTnI) detection for the diagnosis of myocardial infarction, a novel unlabeled photoelectrochemical (PEC) immunosensor was developed to detect cTnI rapidly...

A dual-signal electrochemiluminescence immunosensor for high-sensitivity detection of acute myocardial infarction biomarker

Based on two different types of luminescence systems (Ru﹡(bpy)₃²⁺/TPA and SnO₂ NFs/K₂S₂O₈), a new type of electrochemiluminescence (ECL) immunosensor was prepared, which realized the detection of acute myocardial infarction biomarker cTnI. In this strategy, Ru(bpy)₃²⁺, above all, was immobilized on the NH₂-MIL-125 as a capture probe. Subsequently, cTnI and SnO₂ NFs was bonded to the electrode surface through the interaction between antigen and antibody in turn. During this process, Ru(bpy)₃²⁺ and the co-reactant TPA first showed strong and stable ECL emission. As the concentration of cTnI in the test system increased, the signal of SnO₂ NFs and the co-reactant K₂S₂O₈ gradually enhanced, indicating self-calibrating mechanism of the assay system. Therefore, the "off-on" ECL immunosensor can be detected in the linear range of 10^-5 -10⁴ ng/mL, and the limit of detection (LOD) is 3.39 fg/mL (S/N = 3), respectively. The dual-signal electrochemiluminescence method has the advantages of low cost, simple analysis process, wide detection range and good selectivity, providing a promising analysis protocol for clinical applications.

A Novel Graphene-Based Nanomaterial Modified Electrochemical Sensor for the Detection of Cardiac Troponin I

Acute myocardial infarction has a high clinical mortality rate. The initial exclusion or diagnosis is important for the timely treatment of patients with acute myocardial infarction. As a marker, cardiac troponin I (cTnI) has a high specificity, high sensitivity to myocardial injury and a long diagnostic window. Therefore, its diagnostic value is better than previous markers of myocardial injury. In this work, we propose a novel aptamer electrochemical sensor. This sensor consists of silver nanoparticles/MoS₂/reduced graphene oxide. The combination of these three materials can provide a synergistic effect for the stable immobilization of aptamer. Our proposed aptamer electrochemical sensor can detect cTnl with high sensitivity. After optimizing the parameters, the sensor can provide linear detection of cTnl in the range of 0.3 pg/ml to 0.2 ng/ml. In addition, the sensor is resistant to multiple interferents including urea, glucose, myoglobin, dopamine and hemoglobin.

Enzyme-Free Electrochemical Nano-Immunosensor Based on Graphene Quantum Dots and Gold Nanoparticles for Cardiac Biomarker Determination

An ultrasensitive enzyme-free electrochemical nano-immunosensor based on a screen-printed gold electrode (SPGE) modified with graphene quantum dots (GQDs) and gold nanoparticles (AuNPs) was engineered to detect cardiac troponin-I (cTnI) for the early diagnosis of acute myocardial infarction (AMI). The GQDs and in-house synthesized AuNPs were implanted onto the SPGE and allowed for anti-cTnI immobilization prior to quantifying cTnI. The biomarker could be determined in a wide concentration range using square-wave voltammetry (SWV), cyclic voltammetry (CV), electron impedance spectroscopy (EIS) and amperometry. The analyses were performed in buffer, as well as in human serum, in the investigation ranges of 1–1000 and 10–1000 pg mL⁻¹, respectively. The detection time ranged from 10.5–13 min, depending on the electrochemical method employed. The detection limit was calculated as 0.1 and 0.5 pg mL⁻¹ for buffer and serum, respectively. The sensitivity of the immunosensor was found to be 6.81 µA cm⁻² pg mL⁻¹, whereas the binding affinity was determined to be <0.89 pM. The sensor showed high specificity for cTnI with slight responses for nonspecific biomolecules. Each step of the sensor fabrication was characterized using CV, SWV, EIS and atomic force microscopy (AFM). Moreover, AuNPs, GQDs and their nanocomposites were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). This is the first immunosensor that represents the successful determination of an analyte using four different electrochemical techniques. Such a sensor could demonstrate a promising future for on-site detection of AMI with its sensitivity, cost-effectiveness, rapidity and specificity.

Label-Free and Ultrasensitive Electrochemiluminescent Immunosensor Based on Novel Luminophores of Ce2Sn2O7 Nanocubes

In this research article, a novel and simple label-free electrochemiluminescence (ECL) immunosensor using cerium stannite (Ce₂Sn₂O₇) nanocubes as brand-new ECL emitters has been suggested for the first time. Ce₂Sn₂O₇ nanocubes prepared by a simple hydrothermal method displayed bright ECL emission, promising biocompatibility, low noxiousness, and perfect stability. On comparison of ECL and photoluminescence (PL) spectra, a surface-state mechanism was proposed to be involved in the ECL emission. After aminofunctionalization with 3-aminopropyltriethoxysilane (APTES), Ce₂Sn₂O₇ could be decorated with gold nanoparticles through Au-NH₂ covalent linkage, which yielded Au@Ce₂Sn₂O₇ nanocomposites and further enhanced the ECL emission. To confirm the proposed immunosensor feasibility, carcinoembryonic antigen (CEA) was employed as an exemplary analyte. Based on the abovementioned points, our fabricated immunosensor improved the ECL performance to CEA concentrations in a linear range of 0.001-70 ng/mL with a low limit of detection of 0.53 pg/mL (S/N = 3). With outstanding stability, reproducibility, and specificity, this method is expected to be an innovative one for sensitive analyses of CEA and other biomarkers in real samples.

Biosensors Based on Advanced Sulfur-Containing Nanomaterials

In recent years, sulfur-containing nanomaterials and their derivatives/composites have attracted much attention because of their important role in the field of biosensor, biolabeling, drug delivery and diagnostic imaging technology, which inspires us to compile this review. To focus on the relationships between advanced biomaterials and biosensors, this review describes the applications of various types of sulfur-containing nanomaterials in biosensors. We bring two types of sulfur-containing nanomaterials including metallic sulfide nanomaterials and sulfur-containing quantum dots, to discuss and summarize the possibility and application as biosensors based on the sulfur-containing nanomaterials. Finally, future perspective and challenges of biosensors based on sulfur-containing nanomaterials are briefly rendered.

Cardiac troponin I photoelectrochemical sensor: {Mo368} as electrode donor for Bi2S3 and Au co-sensitized FeOOH composite

A suitable electron donor, which guarantees the stability of the whole system, is considered as the driving force of the PEC sensor. Nowadays, searching appropriate electron donor is still one of the orientations to explorate in the field of sensor. Na₄₈[H₄₉₆Mo₃₆₈O₁₄₆₄S₄₈]·ca.1000H₂O (abbr. {Mo₃₆₈}), as a type of polyoxometalate, has perfect morphology, definite size and unique electronic property. Due to the prominent water solubility, {Mo₃₆₈} usually releases small cations and exists as large anions in the ultrapure water. The interesting property endows {Mo₃₆₈} with excellent reducibility, which provides great feasibility to become an outstanding electron donor. In addition, FeOOH prepared through a simple operation owns high adsorption capacity, which ensures the fastness of other materials. Subsequently, the narrow band-gap of Bi₂S₃ and the unique noble metal properties of Au nanoparticles are utilized to co-sensitize FeOOH to improve the light-harvesting capability and photoelectric conversion efficiency. Combined with the specificity recognition of antigen and antibody, a novel photoelectrochemical sensor is constructed with a wide detection range of 1.00 pg mL^-1 - 100 ng mL^-1 and low detection limit (0.76 pg mL^-1), which achieves the sensitive detection of cardiac troponin I in early diagnosis of cardiovascular disease.

A wavelength-resolved electrochemiluminescence resonance energy transfer ratiometric immunosensor for detection of cardiac troponin I

In this study, a wavelength-resolved electrochemiluminescence resonance energy transfer (ECL-RET) ratiometric immunosensor from Au nanoparticle functionalized graphite-like carbon nitride nanosheets (Au-g-C3N4) to Au nanoclusters (Au NCs) has been constructed for the first time. At a working voltage of 0 to -1.2 V, Au-g-C3N4 showed a strong cathodic ECL emission with a peak at 460 nm, which overlapped well with the absorption spectra of Au NCs thus stimulating the fluorescence emission of Au NCs at 610 nm. Moreover, within this voltage range, the Au NCs showed no ECL signal; therefore, they would not interfere with the detection of the system. We used cardiac troponin I (cTnI) as an analytical model to construct a sandwich immunosensor based on the ECL-RET ratiometric strategy. By measuring the responses of the ECL460 nm/FL610 nm ratio at different cTnI concentrations, the sensitive detection of cTnI with a wide range of 50 fg mL-1 to 50 ng mL-1 and a low detection limit of 9.73 fg mL-1 can be achieved. This work enriches the wavelength-resolved ECL-RET system and provides an innovative reference for the development of more efficient and sensitive ECL-RET ratiometry.

An antibody-aptamer sandwich cathodic photoelectrochemical biosensor for the detection of progesterone

The progesterone (P4) level in body fluids can act as an indicator for early pregnancy diagnosis and offers insight into mammalian somatic function. In this work, we designed an antibody-aptamer based sandwich assay as a cathodic photoelectrochemical (PEC) biosensor for P4 detection. The composites of carbon dots and graphene oxide (CDs-GO) with favorable cathodic photocurrent response were used as photoactive materials on which the antibody (Ab) of P4 was immobilized. Meanwhile, high affinity truncated P4 aptamer was immobilized on Au-CuO-Cu₂O to act as a bioconjugate. When P4 was present, the aptamer-Au-CuO-Cu₂O bioconjugate could amplify the cathodic photocurrent of CDs-GO modified electrode through Ab-P4-aptamer interactions. Under optimum conditions, the cathodic photocurrent of the constructed PEC biosensor was found to increase linearly with P4 in a wide concentration range from 0.5 nM to 180 nM, with a low detection limit (3S/N) of 0.17 nΜ. The proposed cathodic PEC sensing platform demonstrated high selectivity, satisfying reproducibility, good stability. The sensor was successfully applied in the determination of P4 in human serum samples.

Applications of Graphene Quantum Dots in Biomedical Sensors

Due to the proliferative cancer rates, cardiovascular diseases, neurodegenerative disorders, autoimmune diseases and a plethora of infections across the globe, it is essential to introduce strategies that can rapidly and specifically detect the ultralow concentrations of relevant biomarkers, pathogens, toxins and pharmaceuticals in biological matrices. Considering these pathophysiologies, various research works have become necessary to fabricate biosensors for their early diagnosis and treatment, using nanomaterials like quantum dots (QDs). These nanomaterials effectively ameliorate the sensor performance with respect to their reproducibility, selectivity as well as sensitivity. In particular, graphene quantum dots (GQDs), which are ideally graphene fragments of nanometer size, constitute discrete features such as acting as attractive fluorophores and excellent electro-catalysts owing to their photo-stability, water-solubility, biocompatibility, non-toxicity and lucrativeness that make them favorable candidates for a wide range of novel biomedical applications. Herein, we reviewed about 300 biomedical studies reported over the last five years which entail the state of art as well as some pioneering ideas with respect to the prominent role of GQDs, especially in the development of optical, electrochemical and photoelectrochemical biosensors. Additionally, we outline the ideal properties of GQDs, their eclectic methods of synthesis, and the general principle behind several biosensing techniques.

Detection signal amplification strategies at nanomaterial-based photoelectrochemical biosensors

This review focusses on unique material modification and signal amplification strategies reported in developing photoelectrochemical biosensors with utmost sensitivity and selectivity.

Chemiluminescence Immunoassays for Simultaneous Detection of Three Heart Disease Biomarkers Using Magnetic Carbon Composites and Three-Dimensional Microfluidic Paper-Based Device

Chemiluminescence (CL) immunoassays for simultaneous detection of early acute myocardial infarction (AMI) biomarkers, including copeptin, heart-type fatty acid binding protein (h-FABP), and cardiac troponin I (cTnI), were developed by using Co²⁺/N-(aminobutyl)-N-(ethylisoluminol) (ABEI) functionalized magnetic carbon composite (Co²⁺-ABEI-Fe₃O₄@void@C) as an interface and a three-dimensional microfluidic paper-based device (3D μPAD) as a detection system. For CL immunoassays, Co²⁺-ABEI-Fe₃O₄@void@C was assembled with chitosan (CS) and gold nanoparticle-conjugated antibody (Au-Ab) sequentially to form the sensing platform (Co²⁺-ABEI-Fe₃O₄@void@C-CS/Au-Ab). In the presence of antigen (Ag), Ag was captured by the sensing interface to form an immunocomplex, leading to an increase in CL intensity due to the catalysis of -COO^- existing in Ag. A 3D μPAD with three detection zones for simultaneous detection of copeptin, h-FABP, and cTnI was designed and fabricated to obtain time-resolved CL signals. Three kinds of immunocomplexes formed with copeptin, h-FABP, and cTnI were added to three detection zones of 3D μPAD, respectively. After injecting H₂O₂, three time-resolved CL signals were generated in one CL detection run by virtue of time-delayed transport of H₂O₂ to different detection zones. The three time-resolved CL signals were used for the simultaneous determination of copeptin, h-FABP, and cTnI. The detection limit of copeptin, h-FABP, and cTnI was 0.40 pg/mL, 0.32 pg/mL, and 0.50 pg/mL, respectively, which is at least 1 order of magnitude lower than most of the reported immunoassays. The immunoassays could be directly used for the detection of copeptin, h-FABP, and cTnI in human serum samples. The proposed immunoassays are simple, fast, sensitive, and selective, and are of great application potential in early diagnosis and treatment of AMI.

Affinity-Based Detection of Biomolecules Using Photo-Electrochemical Readout

Detection and quantification of biologically-relevant analytes using handheld platforms are important for point-of-care diagnostics, real-time health monitoring, and treatment monitoring. Among the various signal transduction methods used in portable biosensors, photoelectrochemcial (PEC) readout has emerged as a promising approach due to its low limit-of-detection and high sensitivity. For this readout method to be applicable to analyzing native samples, performance requirements beyond sensitivity such as specificity, stability, and ease of operation are critical. These performance requirements are governed by the properties of the photoactive materials and signal transduction mechanisms that are used in PEC biosensing. In this review, we categorize PEC biosensors into five areas based on their signal transduction strategy: (a) introduction of photoactive species, (b) generation of electron/hole donors, (c) use of steric hinderance, (d) in situ induction of light, and (e) resonance energy transfer. We discuss the combination of strengths and weaknesses that these signal transduction systems and their material building blocks offer by reviewing the recent progress in this area. Developing the appropriate PEC biosensor starts with defining the application case followed by choosing the materials and signal transduction strategies that meet the application-based specifications.

DNA nanotetrahedron-assisted electrochemical aptasensor for cardiac troponin I detection based on the co-catalysis of hybrid nanozyme, natural enzyme and artificial DNAzyme

The sensitive and accurate detection of cardiac troponin I (cTnI) is critical for myocardial infarction diagnosis. In this work, a dual-aptamer-based electrochemical (EC) biosensor was designed for cTnI detection based on the DNA nanotetrahedron (NTH) capture probes and multifunctional hybrid nanoprobes. First, the NTH-based Tro4 aptamer probes were anchored on a screen printed gold electrode (SPGE) surface through the Au-S bond, providing an enhanced spatial dimension and accessibility for capturing cTnI. Then, the hybrid nanoprobes were fabricated by using magnetic Fe₃O₄ nanoparticles as nanocarriers to load a large amount of cTnI-specific Tro6 aptamer, natural horseradish peroxidase (HRP), HRP-mimicking Au@Pt nanozymes and G-quadruplex/hemin DNAzyme. This signaling nanoprobes are capable of specifically recognizing the target cTnI based on the Tro6 aptamer and amplifying the signals to improve the detection sensitivity via enzymatic processes. We found the remarkable enhanced effect of EC signal to be attributed to the co-catalysis effect of hybrid nanozymes, HRP and DNAzyme. The target cTnI was sandwiched between the two types of aptamers (Tro4 and Tro6) on the electrode interface. Finally, this EC aptasensing platform exhibited great analytical performance with a wide dynamic range of 0.01-100 ng mL^-1 and a low detection limit of 7.5 pg mL^-1 for cTnI. The high selectivity, sensitivity and reliability of EC aptasensor can provide great potential in the clinic disease diagnostics.

Photoelectrochemical determination of malathion by using CuO modified with a metal-organic framework of type Cu-BTC

A photoelectrochemical (PEC) sensor was constructed for the detection of non-electroactive malathion. It is based on the use of a hierarchical CuO material derived from a Cu-BTC metal-organic framework (where BTC stands for benzene-1,3,5-tricarboxylic acid). The modified CuO was obtained by calcination of Cu-BTC at a high temperature (300 °C) and possesses a high photocurrent conversion efficiency. Under irradiation with visible light and in the presence of malathion, the formation of the CuO-malathion complex on the CuO gave rise to an increase in steric hindrance. This results in a decrease in photocurrent. This novel PEC detection method has a lower detection limit of 8.6 × 10^-11 mol L^-1 and a wide linear range (1.0 × 10^-10 ~ 1.0 × 10^-5 mol L^-1). Graphical abstract Schematic presentation of the Cu-BTC MOF derived photoelectrochemical sensor for non-electroactive malathion detection.

Electrochemical dual-aptamer-based biosensor for nonenzymatic detection of cardiac troponin I by nanohybrid electrocatalysts labeling combined with DNA nanotetrahedron structure

The accurate detection of biomarkers for acute myocardial infarction (AMI) plays an important role in clinical diagnosis and management process. In this work, we developed an electrochemical biosensor by using magnetic metal organic framework (MMOF) nanocatalysts and DNA nanotetrahedron (NTH) based dual-aptamer probes for nonenzymatic detection of cardiac troponin I (cTnI), a gold standard biomarker for the early diagnosis of AMI. Firstly, the NTH-assisted dual-aptamer (Tro4 and Tro6) capture probes were immobilized on the screen-printed gold electrode (SPGE) for the highly enhanced capture the target cTnI with steady support and optimized interface density. Then, the MMOF Fe₃O₄@UiO-66 nanozymes were decorated by bimetallic Cu@Au nanoparticles and two kinds of aptamer. This fabricated nonenzymatic nanoprobe1 (NP1) can be applied for recognizing the cTnI specifically and amplifying the current signal by catalyzing the oxidation of hydroquinone (HQ) to benzoquinone (BQ) with H₂O₂. The target proteins were captured to fabricate a supersandwich-like structure on a SPGE interface. Furthermore, the nanoprobe2 (NP2) of Cu@Au nanozymes labeled with dual-complementary DNA (cDNA) to the dual-aptamer, were anchored on the NP1 through DNA hybridization, leading to the formation of cluster-based nanoprobes for further enhancing detection sensitivity. Finally, this enzyme-free electrochemical aptasensor exhibited great analytical performance with a dynamic range of 0.05-100 ng/mL, a low detection limit of 16 pg/mL, high selectivity and good repeatability. The fabricated aptasensor has great potential development in the field of clinic disease diagnostics for AMI.

An aptasensor for troponin I based on the aggregation-induced electrochemiluminescence of nanoparticles prepared from a cyclometallated iridium(III) complex and poly(4-vinylpyridine-co-styrene) deposited on nitrogen-doped graphene

An ultrasensitive electrochemiluminescence (ECL) disposable aptamer sensor (aptasensor) is presented for detection of myocardial infarction biomarker by quantification of troponin I in blood serum. A screen-printed electrode was modified with (a) aptamer-modified gold nanoparticles, (b) cyclometallated iridium(III)-poly-4-vinylpyridine nanoparticles, and (c) nitrogen-doped graphene in order to increase the loading capacity and conductivity of the aptasensor. If the aptasensor is exposed to troponin I, it will bind to the aptamer and desorb the aptamer from gold nanoparticles and the surface of the electrode. This generates an enhancement in ECL emission depending on troponin I concentration. ECL emission is strongly improved by aggregation-induced phenomenon, which is caused by inhibition of the water and oxygen quenching effect on the iridium complex ECL in aqueous media. Under optimum conditions, the aptasensor has a wide dynamic range that extends from 0.1 pM to 10 nM, with a 20 fM detection limit (S/N = 3) and a relative standard deviation of 3.1%. The ECL aptasensor was successfully applied to 20 individual human serum for the detection of troponin I biomarker. Graphical abstract Schematic presentation of electrochemiluminescence aptamer assay fabrication for detection of Troponin I. Carbon screen printed electrode (CSPE) was modified with nitrogen doped graphene (NG), gold nanoparticles (AuNPs), cyclometallated iridium(III)-polyvinylpyridine polymer nanoparticles, ionic liquid and bovine serum albumin.

Facile fabrication of visible light photoelectrochemical immunosensor for SCCA detection based on BiOBr/Bi2S3 heterostructures via self-sacrificial synthesis method

A novel visible light photoelectrochemical immunosensor based on BiOBr/Bi₂S₃ heterostructures was fabricated to detect squamous cell carcinoma antigen (SCCA). Bi₂S₃ nanoparticles formed on the BiOBr microflowers by the self-sacrificial synthesis method based on the facile reaction between BiOBr and S^2- ions. The BiOBr/Bi₂S₃ composites exhibited excellent visible light photoelectrochemical activity, when ascorbic acid (AA) was employed as a perfect electron donor. The photocurrent intensity of BiOBr/Bi₂S₃ modified ITO electrode arrived at around 20 µA, which was approximately 30 times than that of pure BiOBr. Dopamine formed easily polydopamine film via self-polymerization on the surface of BiOBr/Bi₂S₃ composites to immobilize SCCA antibody. Under the optimal condition, this photoelectrochemical immunosensor realized the ultrasensitive determination of SCCA. With the logarithm of SCCA concentration in the range of 0.001-75 ng mL^-1, the specific binding between SCCA and antibody led to the linearly decrease of photocurrent signal with a low detection limit of 0.3 pg mL^-1 (S/N = 3). This facilely constructed photoelectrochemical immunosensor maybe have promising practical application in photocatalysis, analytical detection and biosensor, etc.

An amplification label of core–shell CdSe@CdS QD sensitized GO for a signal-on photoelectrochemical immunosensor for amyloid β-protein

Core–shell CdSe@CdS QDs conjugated with GO can enhance the photocurrent intensity.

A highly sensitive photoelectrochemical VEGF165 biosensor with a dual signal amplification strategy by using AgVO3 as a photoactive material

We developed a novel “signal-on” photoelectrochemical (PEC) aptasensor with a near-zero background signal by using AgVO₃ as a single photoactive material for the sensitive detection of vascular endothelial growth factor (VEGF₁₆₅).

Dual-Wavelength Ratiometric Electrochemiluminescence Immunosensor for Cardiac Troponin I Detection

Ratiometric electrochemiluminescence (ECL) has attracted special focus in the biological analysis field, because it could eliminate the environmental interference and allow for precise measurement. Herein, a dual-wavelength ratiometric ECL biosensor was designed for the detection of cardiac troponin I (cTnI), where (4,4'-dicarboxylic acid-2,2'-bipyridyl) ruthenium(II) (Ru(dcbpy)₃²⁺) and Au nanoparticle-loaded graphene oxide/polyethylenimine (GPRu-Au) nanomaterial acts as an acceptor, and Au nanoparticle-modified graphitic phase carbon nitride nanosheet composite (Au-CNN) acts as donor. Au-CNN shows a high and steady ECL signal centered at 455 nm, which is well-matched with the adsorption of GPRu-Au; thereby, a highly efficient electrochemiluminescent resonance energy transfer (ECL-RET) sensing platform is designed. AuNPs facilitate the immobilization of antibody on the nanomaterials through a Au-N bond. The high surface area of graphene oxide/polyethylenimine allows a large number of Ru(dcbpy)₃²⁺ to be loaded, immensely amplifying the ECL signal. This sensing platform exhibits outstanding analytical performance toward cTnI with a detection limit of 3.94 fg/mL (S/N = 3). The high reliability, selectivity, and sensitivity of this ratiometric ECL biosensor provides a versatile sensing platform for the bioanalysis.

Point-of-Care Compatibility of Ultra-Sensitive Detection Techniques for the Cardiac Biomarker Troponin I-Challenges and Potential Value

Cardiac biomarkers are frequently measured to provide guidance on the well-being of a patient in relation to cardiac health with many assays having been developed and widely utilised in clinical assessment. Effectively treating and managing cardiovascular disease (CVD) relies on swiftly responding to signs of cardiac symptoms, thus providing a basis for enhanced patient management and an overall better health outcome. Ultra-sensitive cardiac biomarker detection techniques play a pivotal role in improving the diagnostic capacity of an assay and thus enabling a better-informed decision. However, currently, the typical approach taken within healthcare depends on centralised laboratories performing analysis of cardiac biomarkers, thus restricting the roll-out of rapid diagnostics. Point-of-care testing (POCT) involves conducting the diagnostic test in the presence of the patient, with a short turnaround time, requiring small sample volumes without compromising the sensitivity of the assay. This technology is ideal for combatting CVD, thus the formulation of ultra-sensitive assays and the design of biosensors will be critically evaluated, focusing on the feasibility of these techniques for point-of-care (POC) integration. Moreover, there are several key factors, which in combination, contribute to the development of ultra-sensitive techniques, namely the incorporation of nanomaterials for sensitivity enhancement and manipulation of labelling methods. This review will explore the latest developments in cardiac biomarker detection, primarily focusing on the detection of cardiac troponin I (cTnI). Highly sensitive detection of cTnI is of paramount importance regarding the rapid rule-in/rule-out of acute myocardial infarction (AMI). Thus the challenges encountered during cTnI measurements are outlined in detail to assist in demonstrating the drawbacks of current commercial assays and the obstructions to standardisation. Furthermore, the added benefits of introducing multi-biomarker panels are reviewed, several key biomarkers are evaluated and the analytical benefits provided by multimarkers-based methods are highlighted.