Label-free photoelectrochemical immunosensor for amyloid β-protein detection based on SnO2/CdCO3/CdS synthesized by one-pot method

The LOD paradox: When lower isn't always better in biosensor research and development

Biosensor research has long focused on achieving the lowest possible Limits of Detection (LOD), driving significant advances in sensitivity and opening up new possibilities in analysis. However, this intense focus on low LODs may not always meet the practical needs or suit the actual uses of these devices. While technological improvements are impressive, they can sometimes overlook important factors such as detection range, ease of use, and market readiness, which are vital for biosensors to be effective in real-world applications. This review advocates for a balanced approach to biosensor development, emphasizing the need to align technological advancements with practical utility. We delve into various applications, including the detection of cancer biomarkers, pathology-related biomarkers, and illicit drugs, illustrating the critical role of LOD within these contexts. By considering clinical needs and broader design aspects like cost-effectiveness, sustainability, and regulatory compliance, we argue that integrating technical progress with practicality will enhance the impact of biosensors. Such an approach ensures that biosensors are not only technically sound but also widely useable and beneficial in real-world applications. Addressing the diverse analytical parameters alongside user expectations and market demands will likely maximize the real-world impact of biosensors.

Diagnosis and therapy of Alzheimer's disease: Light-driven heterogeneous redox processes

Light-driven heterogeneous processes are promising approaches for diagnosing and treating Alzheimer's disease (AD) by regulating its relevant biomolecules. The molecular understanding of the heterogeneous interface environment and its interaction with target biomolecules is important. This review critically appraises the advances in AD early diagnosis and therapy employing heterogeneous light-driven redox processes, encompassing photoelectrochemical (PEC) biosensing, photodynamic therapy, photothermal therapy, PEC therapy, and photoacoustic therapy. The design strategies for heterogeneous interfaces based on target biomolecules and applications are also compiled. Finally, the remaining challenges and future perspectives are discussed. The present review may promote the fundamental understanding of AD diagnosis and therapy and facilitate interdisciplinary studies at the junction of nanotechnology and bioscience.

Interface engineered cascade-type electronic structure of 2D/0D/2D CdS-CdCO3/SnO2 quantum dots/g-C3N4 nanocomposite for boosting solar-driven photocatalysis

A rational design of heterojunctions with high-quality contacts is essential for efficiently separating photogenerated charge carries and boosting the solar-driven harvesting capability. Herein, we fabricated a novel heterojunction of SnO2 quantum dots-anchored CdS-CdCO3 with g-C3N4 nanosheets as a superior photocatalyst. SnO2 quantum dots (SQDs) with positively charged surfaces were tightly anchored on the negatively charged surface of CdS nanosheets (NSs). The resulting CdS@SnO2 was finally decorated with g-C3N4 NSs, and a new crystalline phase of CdS-CdCO3 was formed during the hydrothermal decoration process, g-C3N4 decorated CdS-CdCO3@SnO2 (CdS-CdCO3@SnO2@g-C3N4). The as-synthesized photocatalysts were evaluated for the degradation of methyl orange dye under solar light conditions. The CdS-CdCO3@SnO2@g-C3N4 exhibited 7.7-fold and 2.3-fold enhancements in photocatalytic activities in comparison to those of the bare CdS and CdS@SnO2 NSs, respectively. The optimal performance of CdS-CdCO3@SnO2@g-C3N4 is primarily attributed to the cascade-type conduction band alignments between 2D/0D/2D heterojunctions, which can harvest maximum solar light and effectively separate photoexcited charge carriers. This work provides a new inspiration for the rational design of 2D/0D/2D heterojunction photocatalyst for green energy generation and environmental remediation applications.

High-performance assaying cardiac troponin I using Bi-doped tin-based heterojunction in photoelectrochemical biosensing with the quencher of yolk-shell nanostructure

Cardiac troponin I (cTnI), a protein regulating myocardial contraction, stands the premier biomarker for diagnosing acute myocardial infarction and stratifying heart disease risk. Photoelectrochemical (PEC) biosensing combines traditional PEC analysis with high bioconjugation specificity, rendering a prospective avenue for disease biomarker analysis. However, the performance of sensors often falls short due to inadequate photoelectric materials. Hence, designing heterojunctions with proper band alignment, effective transport and separation of photogenerated carriers is highly expected for PEC sensors. Meanwhile, doping as a synergistic strategy to tune the energy band edges and improve carrier transport in heterojunctions, can also enhance the sensing performance. In this work, bismuth-doped tin oxide and tin disulfide heterojunction (Bi-SnOS) was prepared via a simple one-step hydrothermal method and utilized as a highly sensitive platform. Integrating copper sulfide-coated nano-gold (Au@CuS), a yolk-shell shaped nanocomposites, as the double quenching probe, an excellent PEC biosensor was fabricated to assay cTnI via sandwich immunorecognition. Under optimal conditions, the proposed biosensor displayed a high-performance for cTnI in the range from 0.1 pg/mL to 5.0 ng/mL with a low detection limit (44.7 fg/mL, 3σ). The strong photocurrent response, high stability and suitable selectivity point out that the synergistic effect between heterojunction and doping provides a promising prospect for the design of new PEC materials.

Dual-mode photoelectrochemical radar based on CdS quantum dot and Ce-MOF for detection of low-abundance disease-associated proteins

Herein, we developed a convenient and versatile dual-mode electrochemiluminescence (ECL) and photoelectrochemistry (PEC) sensing radar for the detection of Prostate-specific antigen (PSA), which has important implications for detection of low-abundance disease-associated proteins. Cerium-based metal-organic framework (Ce-MOFs) were firstly modified on the electrode, showing well ECL and PEC property. In particular, a unique multifunctional Au@CdS quantum dots (QDs) probe loaded numerous QDs and antibody was fabricated, not only displaying strong ECL and PEC signals, but also having specific recognition to PSA. After the signal probe was linked to the electrode by immune reaction, much amplified signals of ECL and PEC were generated for double-mode detection of PSA. Therefore, this work proposed a multifunctional Au@CdS QDs signal probe with excellent ECL and PEC performance, and developed an ultrasensitive photoelectric biosensing platform for dual-mode detection, which provides an effective method for health monitoring of cancer patients.

Visible light-induced hole transfer in single-nanoplate Cu1.81S-CdS heterostructures

The separation and transfer of photogenerated carriers in semiconducting materials are essential processes that determine the efficiency of optoelectronic devices and photocatalysts, and transient absorption spectroscopy provides a powerful support for exploring the diffusion and recombination of photogenerated electrons and holes. Herein, high-quality Cu1.81S nanoplates were synthesized by a hot injection method, and were used as starting templates for the preparation of Cu1.81S-CdS heterojunctions and CdS nanoplates by cation exchange. Their carrier dynamics were investigated by transient absorption spectroscopy, which revealed that photogenerated holes may be transferred from the CdS phase to the Cu1.81S phase under 400 nm excitation. This process is in the opposite direction to the hole transfer induced by near-infrared localized surface plasmon resonance in copper sulfide heterostructures. Moreover, density functional theory calculations were used to further explain the visible light-induced hole transport process. This transfer is a potential way to increase the rate of H2 production and enhance the photostability of the catalyst.

Coreactant-free electrochemiluminescence of polyfluorene nanoparticle coupling double quencher for β-amyloid1-42 detection

β-amyloid_1-42 (Aβ_1-42) is a humoral biomarker for early diagnosis of Alzheimer's disease (AD), and exists at a low level in human body. Its sensitive detection is very valuable. The electrochemiluminescence (ECL) assay of Aβ_1-42 has attracted special attention owing to high sensitivity and simple operation. However, currently reported ECL assays for Aβ_1-42 usually required the introduction of exogenous coreactants to improve the detection sensitivity. Introducing exogenous coreactants will lead to non-negligible repeatability and stability problems. This work exploited poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadazole)] nanoparticles (PFBT NPs) as coreactant-free ECL emitters for detecting Aβ_1-42. The PFBT NPs, first antibody (Ab₁) and antigen Aβ_1-42 were successively assembled on glassy carbon electrode (GCE). Silica nanoparticles served as a carrier to grow polydopamine (PDA) in situ, and further assembled Au nanoparticles (Au NPs) and second antibody (Ab₂), producing the secondary antibody complex (SiO₂@PDA-Au NPs-Ab₂). With its assembly on the biosensor, the ECL signal decreased since both PDA and Au NPs could quench ECL emission from PFBT NPs. The limit of detection (LOD) of 0.55 fg/mL and limit of quantification (LOQ) of 37.45 fg/mL for Aβ_1-42 were obtained. PFBT NPs coupling dual-quencher PDA-Au NPs created an excellent ECL system for bioassays, and constructed a sensitive analytical method for Aβ_1-42.

Electrochemiluminescence resonance energy transfer of MnCO3 for ultrasensitive amyloid-β protein detection

A composite material MnCO₃/poly(diallyl dimethyl ammonium chloride) (PDDA)/Ag with excellent electrochemiluminescence (ECL) performance and high biocompatibility was prepared by adding MnCO₃ and PDDA to silver nanoparticles (AgNPs). MnCO₃/PDDA/Ag and Au@SiO₂NPs were used as ECL donors and acceptors, respectively. Thus, an effective ECL-resonance energy transfer (RET) sensing platform was established. In a potassium persulfate (K₂S₂O₈) medium, MnCO₃ exhibited ECL emission with an ECL band appearing at 500-600 nm. In addition, Au@SiO₂ nanoparticles showed a UV-visible absorption at 450-650 nm. The ECL emission spectra of MnCO₃ overlapped with the absorption spectra of Au@SiO₂NPs. The effective ECL quenching resulted in a good response to the concentration of Aβ₄₂ in serum samples. The linear range was 5 fg ⋅ mL^-1 to 100 ng ⋅ mL^-1, and the detection limit was 2 fg ⋅ mL^-1. The recovery ranged from 97.7% to 104%. The high-efficiency ECL-RET immunosensor has potential application in detecting human serum Aβ₄₂ and other biomarkers, and can be used for the early screening of diseases.

Insights into Photothermally Enhanced Photocatalytic U(VI) Extraction by a Step-Scheme Heterojunction

In this work, a CdS/BiVO₄ step-scheme (S-scheme) heterojunction with self-photothermally enhanced photocatalytic effect was synthesized and applied for efficient U(VI) photoextraction. Characterizations such as transient absorption spectroscopy and Tafel test together confirmed the formation of S-scheme heterojunctions, which allows CdS/BiVO₄ to avoid photocorrosion while retaining the strong reducing capacity of CdS and the oxidizing capacity of BiVO₄. Experimental results such as radical quenching experiments and electron spin resonance show that U(VI) is rapidly oxidized by photoholes/^•OH to insoluble UO₂(OH)₂ after being reduced to U(IV) by photoelectrons/^•O₂⁻, which precisely avoids the depletion of electron sacrificial agents. The rapid recombination of electron-hole pairs triggered by the S-scheme heterojunction is found to release large amounts of heat and accelerate the photocatalysis. This work offers a new enhanced strategy for photocatalytic uranium extraction and presents a direction for the design and development of new photocatalysts.

Antibody-receptor bioengineering and its implications in designing bioelectronic devices

Antibodies play a crucial role in the defense mechanism countering pathogens or foreign antigens in eukaryotes. Its potential as an analytical and diagnostic tool has been exploited for over a century. It forms immunocomplexes with a specific antigen, which is the basis of immunoassays and aids in developing potent biosensors. Antibody-based sensors allow for the quick and accurate detection of various analytes. Though classical antibodies have prolonged been used as bioreceptors in biosensors fabrication due to their increased fragility, they have been engineered into more stable fragments with increased exposure of their antigen-binding sites in the recent era. In biosensing, the formats constructed by antibody engineering can enhance the signal since the resistance offered by a conventional antibody is much more than these fragments. Hence, signal amplification can be observed when antibody fragments are utilized as bioreceptors instead of full-length antibodies. We present the first systematic review on engineered antibodies as bioreceptors with the description of their engineering methods. The detection of various target analytes, including small molecules, macromolecules, and cells using antibody-based biosensors, has been discussed. A comparison of the classical polyclonal, monoclonal, and engineered antibodies as bioreceptors to construct highly accurate, sensitive, and specific sensors is also discussed.

Addressable Label-Free Photoelectric Sensor Array with Self-Calibration for Detection of Neuron Specific Enolase

An addressable label-free photoelectric immunosensor array was designed for detection of neuron specific enolase (NSE) based on TiO₂/CdS as substrate materials. In this work, the hydrothermal synthesized TiO₂ nanorod film is evenly grown on the surface of the fluorine-doped tin oxide (FTO), and then CdS with a narrow band gap is added for sensitization through successive ionic layer adsorption reactions. The obtained TiO₂/CdS composite materials with matched energy band structures promote the rapid electron transfer and effectively reduce the recombination of electron hole pairs, which greatly enhance the visible light absorption and increased photocurrent intensity. In order to construct a suitable sensor array, the sensitized FTO electrode is divided into multiple regions of equal size by insulating stickers, and then the addressable and continuous detection of multiple samples can be achieved. Because multiple detection regions are prepared and tested under the same conditions, the difference effectively reduces, and the sensor can realize self-calibration and obtain more accurate results. Under optimal conditions, this sensor array can detect NSE in the linear range of 0.01-100 ng mL^-1 with a detection limit of 2.49 pg mL^-1 (S/N = 3). The sensor array has good selectivity, stability, and reproducibility, making it a viable approach for real sample detection.

Insights into Photothermally Enhanced Photocatalytic U(VI) Extraction by a Step-Scheme Heterojunction

In this work, a CdS/BiVO 4 step-scheme (S-scheme) heterojunction with self-photothermally enhanced photocatalytic effect was synthesized and applied for efficient U(VI) photoextraction. Characterizations such as transient absorption spectroscopy and Tafel test together confirmed the formation of S-scheme heterojunctions, which allows CdS/BiVO 4 to avoid photocorrosion while retaining the strong reducing capacity of CdS and the oxidizing capacity of BiVO 4 . Experimental results such as radical quenching experiments and electron spin resonance show that U(VI) is rapidly oxidized by photoholes/ • OH to insoluble UO 2 (OH) 2 after being reduced to U(IV) by photoelectrons/ • O 2 - , which precisely avoids the depletion of electron sacrificial agents. The rapid recombination of electron-hole pairs triggered by the S-scheme heterojunction is found to release large amounts of heat and accelerate the photocatalysis. This work offers a new enhanced strategy for photocatalytic uranium extraction and presents a direction for the design and development of new photocatalysts.

Versatile photoelectrochemical and electrochemiluminescence biosensor based on 3D CdSe QDs-DNA nanonetwork-SnO2 nanoflower coupled with DNA walker amplification for HIV detection

A novel 3D CdSe quantum dots (QDs)-DNA nanonetwork was assembled to sensitize the mesoporous SnO₂ photoelectrochemical (PEC) substrate, which was coupled with a biped-DNA walker multiple amplification technique to design a versatile electrochemiluminescence (ECL) and PEC biosensor for dual detection of HIV. Firstly, the photosensitive CdSe QDs and SnO₂ nanoflowers have well-matched band-edge energy level, thus their complex can promote effective transfer of the photogenerated carriers, and show better PEC and ECL property. Then, a novel 3D CdSe QDs-DNA nanonetwork was assembled and loaded with a large amount of QDs, which was used as multifunctional PEC and ECL probes. Moreover, the target-triggered biped DNA walker-cascade amplification method was introduced to generate a large amount of output DNA, which was used to link numerous 3D CdSe QDs-DNA nanonetwork probes to the electrode, generating greatly amplified signals for sensitive assay of HIV. The highly photosensitive 3D CdSe QDs-DNA reticulated nanomaterials have high stability and controllability, and display significantly improved PEC and ECL signals of the biosensor. This method opened a new photoelectric nanocomposite of QDs-sensitized SnO₂ nanoflower, and developed a versatile biosensing strategy using the 3D CdSe QD_S DNA sensitization probes for ultra-sensitive detection of biomolecules, which is important for the early diagnosis of diseases.

A synergistic promotion strategy remarkably accelerated electrochemiluminescence of SnO2 QDs for MicroRNA detection using 3D DNA walker amplification

Developing low-cost and efficient methods to enhance the electrochemiluminescence (ECL) intensity of luminophores is highly desirable and challenging. Herein, we develop a synergistic promotion strategy based on three types of co-reaction accelerators to achieve an efficient SnO2 quantum dots (SnO2 QDs)-based ternary ECL system. Specifically, the MnO2 nanoflowers (MnO2 NFs), Ag nanoparticles (Ag NPs) and hemin/G-quadruplex were rationally selected as co-reaction accelerators. Owing to the synergistic effect, the deft integration of three types of co-reaction accelerators enabled better structural stability, more exposed catalytic active sites, and faster charge transfer, thus more effectively facilitating the reduction of co-reactant (S2O82-) compared with that of the single co-reaction accelerator. To demonstrate the practical utility of this principle, an "on-off-super on" ECL biosensor was constructed in combination with a 3D DNA walker, which showed a superior linear range (10 aM-100 pM) and a low detection limit (2.9 aM) for the highly-sensitive miRNA-21 detection. In general, this work firstly reported that three types of co-reaction accelerators were deftly integrated to remarkably amplify the ECL emission of SnO2 QDs, and provided brand-new perspectives for research on the ingenious design of the structure and component of highly efficient co-reaction accelerators.

Femtomolar sensing of Alzheimer's tau proteins by water oxidation-coupled photoelectrochemical platform

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder. A key pathogenic event of AD is the formation of intracellular neurofibrillary tangles that are mainly composed of tau proteins. Here, we report on ultrasensitive detection of total tau (t-tau) proteins using an artificial electron donor-free, BiVO₄-based photoelectrochemical (PEC) analysis. The platform was constructed by incorporating molybdenum (Mo) dopant and iron oxyhydroxide (FeOOH) ad-layer into the BiVO₄ photoelectrode and employing a signal amplifier formed by horseradish peroxidase (HRP)-triggered oxidation of 3,3'-diaminobenzidine (DAB). Despite the absence of additional electron suppliers, the FeOOH/Mo:BiVO₄ conjugated with the Tau5 antibody produced strong current signals at 0 V (vs. Ag/AgCl, 3 M NaCl) under the illumination of a white light-emitting diode. The Mo extrinsic dopants increased the charge carrier density of BiVO₄-Tau5 by 1.57 times, and the FeOOH co-catalyst promoted the interfacial water oxidation reaction of Mo:BiVO₄-Tau5 by suppressing charge recombination. The introduction of HRP-labeled Tau46 capture antibodies to the FeOOH/Mo:BiVO₄-Tau5 platform produced insoluble precipitation on the transducer by accelerating the oxidation of DAB, which amplified the photocurrent signal of FeOOH/Mo:BiVO₄-Tau5 by 2.07-fold. Consequently, the water oxidation-coupled, FeOOH/Mo:BiVO₄-based PEC sensing platform accurately and selectively recognized t-tau proteins down to femtomolar concentrations; the limit of detection and limit of quantification were determined to be 1.59 fM and 4.11 fM, respectively.

A photoelectrochemical immunosensor based on CdS/CdTe-cosensitized SnO2 as a platform for the ultrasensitive detection of amyloid β-protein

An ultrasensitive label-free photoelectrochemical immunosensor was developed to detect amyloid β-protein based on CdS/CdTe-cosensitized SnO₂ nanoflowers.

Chemical sensing platforms for detecting trace-level Alzheimer's core biomarkers

This review provides an overview of recent advances in optical and electrical detection of Alzheimer's disease biomarkers in clinically relevant fluids.

CdS quantum dots/Au nanoparticles/ZnO nanowire array for self-powered photoelectrochemical detection of Escherichia coli O157:H7

In this paper, the hydrothermally grown ZnO nanowire array (NWs) was modified by Au nanoparticles (NPs) and CdS quantum dots (QDs) to construct a high-performance photoelectrochemical (PEC) electrode. The aligned ZnO NWs, which decorated Au NPs and CdS QDs have the effective light absorption range from UV to visible region. This hybrid structure provided a self-powered PEC electrode with a favorable energy-band configuration for fast charge separation and transportation. Meanwhile, the Au NPs and CdS QDs also made increase of the surface area to improve the immobilization of the analytes. After assembling aptamer as recognition element, this composite nanoarray was further developed as a self-powered PEC biosensor by synergizing above multiple enhancement factors. The PEC aptasensor exhibited a rapid response in a wide linear range of 10-10⁷ CFU/mL with the detection limit as low as 1.125 CFU/mL to Escherichia coli O157:H7 (E. coli O157:H7). This approach would offer an alternative PEC transduction for fast environment monitoring and clinical diagnosis related to pathogenic bacteria.

Photoelectrochemical competitive immunosensor for 17β-estradiol detection based on ZnIn2S4@NH2-MIL-125(Ti) amplified by PDA NS/Mn:ZnCdS

A competitive-type PEC immunosensor for 17β-estradiol (E₂) detection was successfully fabricated using ZnIn₂S₄@NH₂-MIL-125(Ti) composite as matrix. The excellent PEC behavior of ZnIn₂S₄@NH₂-MIL-125(Ti) composite could be attributed to that the Ti⁴⁺-Ti³⁺ intervalence cycles in the titanium oxo-cluster of NH₂-MIL-125(Ti) as well as the matching energy level between ZnIn₂S₄ and NH₂-MIL-125(Ti) promote the migration and separation of photocarrier. Besides, polydopamine (PDA) with abundant amino- and quinone-groups was selected to further improve the PEC signals and capture antibody, which implement through the covalent bonding of PDA and BSA-E₂ or carboxyl-group functionalized Mn:ZnCdS QDs in the competitive-type strategy. Concretely, the quinone functional groups in PDA film was applied to immobilize BSA-E₂ through Michael reactions, and the PDA nanosphere loaded Mn:ZnCdS quantum dot (PDA NS/Mn:ZnCdS QDs) was used as antibodies' labels to amplify PEC signals. After PDA NS/Mn:ZnCdS-anti-E₂ immobilized on the modified electrode, a remarkable increase of photocurrent signal was observed owing to the specific bonding of antigen and antibody. Based on the competitive binding of PDA NS/Mn:ZnCdS-anti-E₂ with either free E₂ or bovine serum albumin (BSA)-E₂ causing the change of the photocurrent signal, the standard sample free E₂ could be accuracy detect. Under optimal conditions, the competitive-type PEC immunosensor exhibited the linear range from 0.0005 ng/mL to 20 ng/mL and a limit detection of 0.3 pg/mL (S/N = 3). Meanwhile, the acceptable stability, selectivity and reproducibility of the proposed PEC immunosensing platform indicating the promising detection of small molecular environmental pollutants.