Biosynthesized Quantum Dot for Facile and Ultrasensitive Electrochemical and Electrochemiluminescence Immunoassay

Classification and applications of nanomaterials in vitro diagnosis

With the rapid development of clinical diagnosis and treatment, many traditional and conventional in vitro diagnosis technologies are unable to meet the demands of clinical medicine development. In this situation, nanomaterials are rapidly developing and widely used in the field of in vitro diagnosis. Nanomaterials have distinct size-dependent physical or chemical properties, and their optical, magnetic, electrical, thermal, and biological properties can be modulated at the nanoscale by changing their size, shape, chemical composition, and surface functional groups, particularly because they have a larger specific surface area than macromaterials. They provide an amount of space to modify different molecules on their surface, allowing them to detect small substances, nucleic acids, proteins, and microorganisms. Combining nanomaterials with in vitro diagnosis is expected to result in lower detection limits, higher sensitivity, and stronger selectivity. In this review, we will discuss the classfication and properties of some common nanomaterials, as well as their applications in protein, nucleic acids, and other aspect detection and analysis for in vitro diagnosis, especially on aging-related nanodiagnostics. Finally, it is summarized with guidelines for in vitro diagnosis.

An electrochemiluminescence sensor based on lanthanide bimetallic MOFs with a "cascade sensitization mechanism" for the sensitive detection of CA242

Lanthanide metal-organic frameworks (Ln-MOFs) broaden the optical sensing applications of lanthanide ions due to the antenna effect between organic ligands and metals. However, the sensitization ability of the ligand to metal ions is limited, and maximizing the sensitization of the electrochemiluminescence behavior of Eu3+ is still a challenge for the application of Ln-MOFs. Therefore, under the guidance of the "cascade sensitization mechanism" based on the antenna effect sensitizing the electrochemiluminescence of bimetallic Ln-MOFs, we proposed Eu/Tb-MOFs with high luminescence intensity as a signal probe. According to the antenna effect, the conjugated structure and high extinction coefficient of the benzene ring of 2-amino terephthalic acid (NH2-BDC) can enhance the ECL luminescence intensity of Eu/Tb-MOFs. Tb3+ can act as an energy bridge between NH2-BDC and Eu3+, buffering the energy gap. The bimetallic sensitization is formed between Tb3+ and Eu3+, which can inhibit the reverse internal flow of energy and ensure the high luminous efficiency of Eu3+. In addition, the nanosphere mixed valence Fe3O4 as a co-reactant accelerator promotes the formation of transient free radical SO4•- through the valence change of Fe2+/Fe3+. The ECL immunosensor constructed by luminophores Eu/Tb-MOFs and nanosphere Fe3O4 provided a new explanation for the ECL self-luminous of Eu/Tb-MOFs.

Nucleic acid aptamer-based electrochemical sensor for the detection of serum P-tau231 and the instant screening test of Alzheimer's disease

The instant screening of patients with a tendency towards developing Alzheimer's disease (AD) is significant for providing preventive measures and treatment. However, the current imaging-based technology cannot meet the requirements in the early stage. Developing biosensor-based liquid biopsy technology could be overcoming this bottleneck problem. Herein, we developed a simple, low-cost, and sensitive electrochemical aptamer biosensor for detecting phosphorylated tau protein threonine 231 (P-tau231), the earliest and one of the most efficacious abnormally elevated biomarkers of AD. Gold nanoparticles (AuNPs) were electrochemically synthesized on a glassy carbon electrode as the transducer, exhibiting excellent conductivity, and were applied to amplify the electrochemical signal. A nucleic acid aptamer was designed as the receptor to capture the P-tau231 protein, specifically through the formation of an aptamer-antigen complex. The proposed biosensor showed excellent sensitivity in detecting P-tau 231, with a broad linear detection range from 10 to 107 pg/mL and a limit of detection (LOD) of 2.31 pg/mL. The recoveries of the biosensor in human serum ranged from 97.59 to 103.26%, demonstrating that the biosensor could be used in complex practical samples. In addition, the results showed that the developed biosensor has good repeatability, reproducibility, and stability, which provides a novel method for the early screening of AD.

Nonmetallic SERS-based biosensor for ultrasensitive and reproducible immunoassay of ferritin mediated by magnetic molybdenum disulfide nanoflowers and black phosphorus nanosheets

To improve the curability of cancer patients, it is essential to propose an early diagnosis technology with ultra-high sensitivity and reliable biocompatibility. Herein, a sophisticated nonmetallic SERS-based immunosensor, comprised by a MoS₂ @Fe₃O₄ nanoflower-based immunoprobe with magnetism and a black phosphorus (BP) nanosheet-based immunosubstrate, was proposed for the specific in-situ monitoring of ferritin (FER). The sandwich immunosensor was endowed with an excellent SERS performance mainly ascribed to a synergistic chemical enhancement as well as an additional electrostatic adsorption effect, achieving a limit of detection down to 7.3 × 10^-5 μg/mL. Particularly, all the Raman label, target FER, and anti-FER could be completely degraded within 70 min under visible light irradiation owing to the favorable photocatalytic activities of MoS₂ and BP which could be then effectively separated and collected with the assistance of an external magnet. Such a recyclable nonmetallic immunosensor holds great potential and practicality in the clinical screening of cancer.

Self-corrected dual-optical immunosensors using carbon dots@SiO2@MnO2 improving diethyl phthalate detection accuracy

The traditional immunoassay is widely used for pollutant detection and bioanalysis, but there are still some challenges in ensuring its sensitivity and reliable accuracy. Dual-optical measurement can prove mutual evidence to effectively improve the accuracy of the method by self-correction, which will overcome this problem. In this study, we developed a "visualization and sensing" dual-modal immunoassay based on blue carbon dots@SiO₂@MnO₂ (B-CDs@SiO₂@MnO₂) as "color and fluorescence" immunosensors. Here, MnO₂ nanosheets have the activity of simulating oxidase. 3,3', 5,5'-Tetramethylbenzidine (TMB) can be oxidized to TMB²⁺ under acidic conditions and the color of the solution from colorless to yellow. On the other hand, the MnO₂ nanosheets can quench the fluorescence of B-CDs@SiO₂. After adding ascorbic acid (AA), MnO₂ nanosheets were reduced to Mn²⁺, thereby the fluorescence of B-CDs@SiO₂ was restored. Under the optimum conditions, as the concentration of target substance (diethyl phthalate) increased from 0.05 to 100 ng/mL, the method showed a good linear relationship. The fluorescence measurement signal and the color change signal of the solution visualization support each other and give the information of the corresponding material content. The results of the dual-optical immunoassay maintain good consistency, which proves the accuracy of the developed dual-optical immunoassay for detection of diethyl phthalate is reliable. Additionally, it is demonstrated that the dual-modal method exhibits high accuracy and stability in the assays, pointing to a broad range of application prospects in pollutant analysis.

Engineered Escherichia coli as a Controlled-Release Biocarrier for Electrochemical Immunoassay

Micro/nanocarriers hold great potential in bioanalysis for molecular recognition and signal amplification but are frequently hampered by harsh synthesis conditions and time-consuming labeling processes. Herein, we demonstrate that Escherichia coli (Ec) can be engineered as an efficient biocarrier for electrochemical immunoassay, which can load ultrahigh amounts of redox indicators and simultaneously be decorated with detection antibodies via a facile polydopamine (PDA)-mediated coating approach. Compared with conventional carrier materials, the entire preparation of the Ec biocarrier is simple, highly sustainable, and reproducible. Moreover, immune recognition and electrochemical transduction are performed independently, which eliminates the accumulation of biological interference on the electrode and simplifies electrode fabrication. Using human epidermal growth factor receptor 2 (HER2) as the model target, the proposed immunosensor exhibits excellent analytical performance with a low detection limit of 35 pg/mL. The successful design and deployment of Ec biocarrier may provide new guidance for developing biohybrids in biosensing applications.

Electrochemiluminescence immunoassay strategies based on a hexagonal Ru-MOF and MoS2@GO nanosheets: detection of 5-fluorouracil in serum samples

Herein, a competitive-type electrochemiluminescence immunosensor for ultrasensitive detection of 5-fluorouracil (5-FU) was fabricated. Ruthenium(II)-metal-organic framework (Ru-MOF) nanosheets were selected to act a promising ECL luminophore using tris(4,4'-dicarboxylic acid-2,2'-bipyridyl) ruthenium(II) dichloride (Ru(dcbpy)₃²⁺) as the organic ligand. The two-dimensional (2D) Ru-MOF nanosheets achieved an increased loading of Ru(dcbpy)₃²⁺ and effectively prevented leakage of the ECL emitter during application, which exhibited satisfactory ECL performance. Thin two-dimensional MoS₂@GO was used to modify the electrode as the sensing platform for improving the electron transfer rate and loading more 5-FU coating antigens due to its large specific surface area and piezoelectric catalytic efficiency. Under the optimized conditions, the proposed immunosensor presented high sensitivity, a wide detection range (0.0001 ng-100 ng mL^-1), a low limit of detection (0.031 pg mL^-1, S/N = 3), good specificity and stability. Furthermore, the immunosensor was successfully applied for the detection of 5-FU in human serum samples with satisfactory results, proving this strategy has potential applications in bioanalysis and clinical diagnosis.

Novel electrochemical and electrochemiluminescence dual-modality sensing platform for sensitive determination of antimicrobial peptides based on probe encapsulated liposome and nanochannel array electrode

With the increasing application of antimicrobial peptides (AMPs) to replace antibiotics in medicine, food and agriculture, it is highly desired to develop a fast, reliable, and convenient strategy for sensitive detection of AMPs. Herein, a novel electrochemical (EC) and electrochemiluminescence (ECL) dual-modality sensing platform was developed based on probe encapsulated liposomes and nanochannel array modified electrodes, which enables sensitive determination of nisin in food samples. The bifunctional probe with both EC and ECL signals, tris(2,2-bipyridyl) dichlororuthenium (II) (Ru(bpy)₃²⁺), was chosen to be easily encapsulated in liposomes (Ru(bpy)₃²⁺@liposome). Based on the unique sterilization mechanism that AMPs can disrupt cell membranes, Ru(bpy)₃²⁺@liposome can be destroyed by nisin and release a large number of Ru(bpy)₃²⁺ probes. Vertically-ordered mesoporous silica-nanochannel film (VMSF) modified ITO electrodes (VMSF/ITO) prepared by electrochemically assisted self-assembly (EASA) method were applied as the sensing electrode. Due to the efficient enrichment of Ru(bpy)₃²⁺ by the negatively charged nanochannel arrays, VMSF/ITO enables detection of the EC/ECL signals of the released Ru(bpy)₃²⁺ probes with ultrahigh sensitivity. In consequence, sensitive dual-modality detection of nisin was achieved by the combination of Ru(bpy)₃²⁺@liposome and VMSF/ITO. The developed sensing system can realize sensitive determination of nisin in ECL mode in the concentration range of 10 ng/ml to 50 μg/ml with a limit of detection (LOD) of 9.3 ng/ml, or in EC mode from 800 ng/ml to 100 μg/ml with a LOD of 70 ng/ml. Combined with the excellent anti-fouling and anti-interference performance of VMSF, rapid and sensitive detection of nisin in milk or egg white was also achieved by the sensor.

Hemin-graphene oxide-gold nanoflower-assisted enhanced electrochemiluminescence immunosensor for determination of prostate-specific antigen

An ultrasensitive luminol electrochemiluminescence (ECL) immunosensor was constructed for the detection of prostate specific antigen (PSA) using glucose oxidase-decorated hemin-graphene oxide-gold nanoflowers ternary nanocomposites as probes. Graphene oxide was first modified with hemin and then with gold nanoflowers through an in situ growth method, which has significantly boosted the catalytic activity of this graphene oxide-based peroxidase mimetics. The biocatalytical activity of this ECL immunosensor was thoroughly investigated to achieve selective recognition of the analyte molecules (PSA) by specific binding between antigens and antibodies. The limit of detection was calculated to be 0.32 pg mL^-1 with a signal-to-noise ratio of 3. A broad linear range from 7.5 × 10^-4 to 2.5 ng mL^-1 was obtained. Such step-by-step assembled biosensor showed controlled nanostructure and exhibited promising application in analysis of human serum samples with a recovery range of 90.6-111.8% and a RSD range of 3.9-5.5%.

Quantum dots for electrochemiluminescence bioanalysis - A review

Electrochemiluminescence (ECL) bioanalysis has become increasingly important in various fields from bioanalysis to clinical diagnosis due to its outstanding merits, including low background signal, high sensitivity, and simple instrumentation. Quantum dots (QDs) are a significant theme in ECL bioanalysis since their excellent optical, electrochemical properties, and ease of functionalization endow them with versatile roles and new mechanisms of signal transduction in ECL. Herein, this review details recent advances of QDs-based ECL bioanalysis by using QDs as ECL emitters, coreactants, or ECL resonance energy transfer donors/acceptors, mainly focused on their optical and electrochemical properties and ECL reaction mechanism. In the end, we will discuss the current limitations and future developments in QDs ECL bioanalysis to address the requirement about selectivity, sensitivity, toxicity, and emerging applications.

Biomimetic-mineralized bifunctional nanoflowers for enzyme-free and colorimetric immunological detection of protein biomarker

Detection of protein biomarkers relies largely on the development of modern immunological methods. Herein, a new enzyme-free immunological method is proposed to detect protein biomarkers. Employment of antibody-Cu₃(PO₄)₂ hybrid nanoflowers, which are prepared through a facile and mild biomimetic-mineralizing process, is the core concept of the method. These nanoflowers can perform functions: one is to bind to target protein biomarkers with high specificity; the other is to release large amounts of Cu²⁺ upon acid treatment, which can interact with creatinine and exert peroxidase-mimicking enzyme activity, therefore producing a distinctly amplified signal. Using osteocalcin, a well-known circulating protein biomarker for bone formation, as a model, the method affords a linear range from 0.1 to 50 ng/mL with a detection limit of 0.042 ng/mL, which is superior to reported methods. Moreover, the method shows considerable specificity, desirable performance in serum samples and eliminates the use of enzymes, so a great potential for this method is expected to meet the need of the clinical diagnosis.

Bio-inspired dynamic biomolecule assembling for fine regulation of protein activity

A versatile approach for the fine control of DNA-based hierarchical assembly via dual stimuli and two assembly strategies is developed. Moreover, with a reasonable design of functional thrombin aptamer structures on the formed DNA nanoassembly, it can achieve precise regulation of thrombin activity.

Dual-Mode Sensing Platform for Electrochemiluminescence and Colorimetry Detection Based on a Closed Bipolar Electrode

Development of sensors uniting different sensing principles is in line with the concept of reliable, comprehensive, and diversified equipment construction. However, the current exploration in this field is obstructed by compromise of reaction conditions and inevitable mutual interference arising from different sensing modes. This work reported a closed bipolar electrode (c-BPE) strategy for dual-modality detection or dual-target detection. To this end, a c-BPE sensing platform installed in physically separated anode and cathode compartments was well designed and carefully optimized. If luminol was present in the anode section and Prussian blue (PB) was at the cathode part, single stimulation could realize electrochemiluminescence (ECL) from luminol at the anode and conversion of PB to Prussian white (PW) at the cathode. The latter reaction helped elevate the ECL signal and also prepared for colorimetric detection as color change from PW to PB under the trigger of oxidant (like H₂O₂) was used to track the content of the oxidant. Thus, dual signals were obtained for dual-modality detection of single target or the detection of different targets was realized at different poles. Detection of glucose was carried out to validate the application for dual-modality detection, while VLDL/AChE and NADH/H₂O₂ assays illustrated the potential of dual-target detection. The proposed platform possesses outstanding sensing performance including selectivity, repeatability, long-term stability, accuracy, and so forth. This work implements a breakthrough in designing dual-mode sensors and is expected to present a rational basis for development of a diversified sensing platform.

Multifunctional bacteria-derived tags for advancing immunoassay analytical performance with dual-channel switching and antibodies bioactivity sustaining

Constructing multifunctional immunochromatographic assays (ICA) carriers with multiple signals and retaining bioactivity of monoclonal antibodies (mAbs) are conducive to the sensitive and accurate point-of-care testing (POCT). To fulfill this pressing need, a microorganism-based microsphere mediated dual-modal ICA (DICA) was developed for sensitive and reliable detection of zearalenone (ZEN). As the key to the biosensor, a superb biotag with an intact coccus morphology was designed based on Staphylococcus aureus biosynthesized quantum dots incorporating Ru(bpy)₃²⁺ (SAQDsRu), in which SA offered a specific recognition capacity for Fc region of mAbs, QDs endowed a naked-eye discernible colorimetric signal on the SA, and robust fluorescence signal that remedied for the insufficient brightness of QDs was derived from Ru(bpy)₃²⁺. The characterization of SAQDsRu-labeled mAb (SAQDsRu-mAb) probe demonstrated strong luminescence, excellent stability and high affinity with ZEN (affinity constant was approximately 1.723 × 10⁹ M^-1), which can significantly improve the detection sensitivity. Impressively, a portable sensing system was developed by the integration of SAQDsRu-DICA with a smartphone-based readout. After optimization, this DICA indicated a limit of detection reaching down to 0.008 ng/mL (colorimetric mode) and 0.0058 ng/mL (fluorescent mode), which were much lower than that of conventional gold nanoparticles-based ICA (0.1029 ng/mL), possessing favorable specificity and repeatability (relative standard deviation (RSD) < 10%). Moreover, the feasibility of the immunoassay was further assessed by measuring ZEN in real samples with satisfactory recoveries, and the results are good consistent with liquid chromatography tandem mass spectrometry (LC-MS/MS).

Tumor-targeting inorganic nanomaterials synthesized by living cells

Inorganic nanomaterials (NMs) have shown potential application in tumor-targeting theranostics, owing to their unique physicochemical properties. Some living cells in nature can absorb surrounding ions in the environment and then convert them into nanomaterials after a series of intracellular/extracellular biochemical reactions. Inspired by that, a variety of living cells have been used as biofactories to produce metallic/metallic alloy NMs, metalloid NMs, oxide NMs and chalcogenide NMs, which are usually automatically capped with biomolecules originating from the living cells, benefitting their tumor-targeting applications. In this review, we summarize the biosynthesis of inorganic nanomaterials in different types of living cells including bacteria, fungi, plant cells and animal cells, accompanied by their application in tumor-targeting theranostics. The mechanisms involving inorganic-ion bioreduction and detoxification as well as biomineralization are emphasized. Based on the mechanisms, we describe the size and morphology control of the products via the modulation of precursor ion concentration, pH, temperature, and incubation time, as well as cell metabolism by a genetic engineering strategy. The strengths and weaknesses of these biosynthetic processes are compared in terms of the controllability, scalability and cooperativity during applications. Future research in this area will add to the diversity of available inorganic nanomaterials as well as their quality and biosafety.

Diverse Dyes-Embedded Staphylococcus aureus as Potential Biocarriers for Enhancing Sensitivity in Biosensing

Nanomaterials-based immunochromatographic assays (ICAs) have gained great commercial success in real-life point-of-care testing (POCT). Exploring novel carriers of ICAs with improved signaling and sustained activity favors the development of sensitive POCT. Herein a potent signal biotag, colored Staphylococcus aureus (SA), was created for ICA carriers through a mild self-assembly strategy, providing high luminance and abundant specific binding sites for immobilization of monoclonal antibodies (mAbs). The biocompatible SA-dyes (SADs) retained both an intact surface structure for mAbs labeling (Fc portion) and the superior bioactivity of immobilized mAbs (affinity constant was about 10⁹ M^-1), thus waiving the intrinsic limitations of traditional nanomaterials and endowing high sensitivity. Proof-of-concept was demonstrated by employing Congo red- or/and fluorescein isothiocyanate-embedded SA (SACR, SAFITC, and SACR-SAFITC) as ICA carriers to detect zearalenone (ZEN) through colorimetric or/and fluorimetric signals. Furthermore, the ICAs satisfied the clinical requirement perfectly, including limit of detection (0.013 ng/mL, which was at least an 85-fold improvement over that of traditional gold nanoparticles-based ICA), linearity (R² > 0.98), reproducibility (RSD < 8%), selectivity, and stability. Importantly, the proposed biosensors could be well-applied in four real samples for ZEN monitoring with satisfactory recoveries, correlating well with the results from liquid chromatography-tandem mass spectrometry (LC-MS). This work also proved a universal design for tailoring coloration bands for SAD-ICA detection of multiple analytes.

Electrochemiluminescence and Photoluminescence of Carbon Quantum Dots Controlled by Aggregation-Induced Emission, Aggregation-Caused Quenching, and Interfacial Reactions

Carbon quantum dots (CQDs) show promise in optoelectronics as a light emitter due to simple synthesis, biocompatibility and strong tunable light emissions. However, CQDs commonly suffer from aggregation caused quenching (ACQ), inhibiting the full potential of these light emitters. Studies into different ideal light emitters have shown enhancements when converting common ACQ effects to aggregation induced emission (AIE) effects. We report CQD synthesis using citric acid and high/low thiourea concentrations, or sample 2/1. These two CQDs exhibited AIE and ACQ PL effects, respectively. CQD characterizations and photoluminescence interrogations of CQD films and solutions revealed that these unique emission mechanisms likely arose from different S incorporations into the CQDs. Furthermore, it was discovered that sample 2 emitted electrochemiluminescence (ECL) more intensely than sample 1 in a homogenous solution with S₂O₈^2- as a coreactant, due to aggregation and interactions of CQD species in solution. Very interestingly, sample 1's CQD film|S₂O₈^2- system achieved an ECL efficiency of 26% and emitted roughly 26 times more efficiently than sample 2 in the same conditions. Predominant interfacial reactions and surface state emission produced intense white light with a correlated color temperature of 2000 K. Spooling ECL spectroscopy was utilized to investigate emission mechanisms. Sample 2's CQD film|TPrA system had four times higher ECL intensity than that of sample 1, most likely due to π-cation interactions leading to a strong CQD^•+ stability, thereby, enhancing ECL. It is anticipated that ECL enhancement of CQD films or solutions by means of AIE will lead to wide CQD optoelectronic applications.

Dual-Modular Aptasensor for Detection of Cardiac Troponin I Based on Mesoporous Silica Films by Electrochemiluminescence/Electrochemical Impedance Spectroscopy

A simple, dual-modular aptasensor for accurate determination of cardiac troponin I (cTnI), a sensitive biomarker of acute myocardial infarction, is reported. It has the parallel output of electrochemiluminescence (ECL) and electrochemical impedance spectroscopy (EIS) based on target-gated transportation of signal probes (luminol/H₂O₂ or Fe(CN)₆^3-/4-). The sensing capacity is originated from the amino-functionalized mouth margin of the nanochannels in a vertically oriented mesoporous silica film, which was in situ-grown on indium tin oxide-coated glass. With the linkage of glutaraldehyde to couple the aptamer as a trapper, it brings in the high specific target-gated response toward cTnI as decreased ECL or increased EIS. The concentration of cTnI is measurable by the ECL response within a wide linear range from 0.05 pg mL^-1 to 10 ng mL^-1, as well as the EIS response for a linear range between 0.05 pg mL^-1 and 1 ng mL^-1. Significantly, the self-verification of these two data from ECL and EIS validated each other with a satisfactory linear correlation (R² = 0.999), thereby realizing the more reliable and accurate quantification to avoid false results. The designed strategy is an effective method for detection of cTnI, which is of great potential to apply in clinical detection.

Electrogenerated Chemiluminescence and Electroluminescence of N-Doped Graphene Quantum Dots Fabricated from an Electrochemical Exfoliation Process in Nitrogen-Containing Electrolytes

Artificial lighting sources are one of the most important technological developments for our modern lives; the search for cost-effective and efficient luminophores is therefore crucial to a sustainable future. Graphene quantum dots (GQDs) are carbon-based nanomaterials that exhibit exceptional optical and electronic properties, making them a prime candidate for a luminophore in a light-emitting device. Nitrogen-doped GQDs fabricated from a facile top-down electrochemical exfoliation process with a nitrogen-containing electrolyte in this report showed strong photoluminescent emission at 450 nm, and electrogenerated chemiluminescence at 660 nm in the presence of benzoyl peroxide as a coreactant. When introduced into solid-state light-emitting electrochemical cells, for the first time, the GQDs displayed a broad white emission centered at 610 nm, corresponding to Commision Internationale de l'eclairage (CIE) colour coordinates of (0.38, 0.36).