Etching Triangular Silver Nanoparticles by Self-generated Hydrogen Peroxide to Initiate the Response of an Electrochemiluminescence Sensing Platform

Patterned Au@Ag nanoarrays with electrically stimulated laccase-mimicking activity for dual-mode detection of epinephrine

Epinephrine (EP) is a crucial neurotransmitter in the central nervous system. However, an abnormal level of EP in biological fluids can lead to various diseases. Therefore, it is essential to rapidly and accurately detect EP content. Herein, electrically stimulated patterned Au@Ag nanoarrays with laccase-mimicking activity were designed for the dual-mode detection of EP concentration. The patterned Au@Ag nanoarrays exhibit excellent electrochemical properties and electrically stimulated laccase-mimicking activity. They provide sensitive electrochemical responses for detecting EP content. Simultaneously, the Au@Ag nanoarrays can catalyze the oxidation of EP, enabling its detection through a colorimetric process. This dual-mode approach achieves the detection of EP content over a wide linear range of 0.5-200 μM, with a low detection limit of 0.152 μM. Furthermore, the utility of these nanoarrays for sensing EP in human serum was evaluated. This work provides a convenient method using patterned nanozyme array for the visible, rapid and accurate detection of EP content. It provides the important implication for the development of portable and reliable on-site analytical instruments.

Low-Triggering-Potential and Narrow-Potential-Window Electrochemiluminescence of Silver Nanoclusters for Gene Assay

A low-triggering potential and a narrow-potential window are anticipated to decrease the electrochemical interference and cross talk of electrochemiluminescence (ECL). Herein, by exploiting the low oxidative potential (0.82 V vs Ag/AgCl) of dihydrolipoic acid-capped sliver nanoclusters (DHLA-AgNCs), a coreactant ECL system of DHLA-AgNCs/hydrazine (N2H4) is proposed to achieve efficient and oxidative-reduction ECL with a low-triggering potential of 0.82 V (vs Ag/AgCl) and a narrow-potential window of 0.22 V. The low-triggering-potential and narrow-potential-window nature of ECL can be primarily preserved upon labeling DHLA-AgNCs to probe DNA and immobilizing DHLA-AgNCs onto the Au surface via sandwiched hybridization, which eventually enables a selective ECL strategy for the gene assay at +0.82 V. This gene assay strategy can sensitively determine the gene of human papillomavirus from 10 to 1000 pM with a low limit of detection of 5 pM (S/N = 3) and would open a way to improve the applied ECL bioassay.

Europium Metal-Organic Framework with a Tetraphenylethylene-Based Ligand: A Dual-Mechanism Quenching Immunosensor for Enhanced Electrochemiluminescence via the Coordination Trigger

The effective applications of electrochemiluminescence (ECL) across various fields necessitate ongoing research into novel luminophores and ECL strategies. In this study, self-luminous flower-like nanocomposites (Eu-tcbpe-MOF) were prepared by coordination self-assembly using the aggregation-induced emission material 1,1,2,2-tetrakis(4-carboxyphenyl)ethylene (H4TCBPE) and Eu(III) ions as the precursors. Compared with the monomers and aggregates of H4TCBPE, Eu-tcbpe-MOF exhibits stronger ECL emission. Such enhanced electrochemiluminescence is due to coordination as the coordination-triggered electrochemiluminescence (CT-ECL) enhancement effect. In this study, a cubic-structured nanocomposite (Co9S8@Au@MoS2) was used as an efficient quencher, and a more sensitive ECL detection platform was achieved by two quenching mechanisms: resonance energy transfer and competitive consumption of coreactants. N,N-Diethylethanolamine (DBAE) was used as a coreactant, and DBAE has a faster electron transfer rate and stronger energy supply efficiency than the traditional anodoluminescent coreactant tripropylamine, which effectively improves the ECL signal intensity of Eu-tcbpe-MOF. Hence, a sandwich-type ECL immunosensor was prepared by employing a dual-quenching mechanism, utilizing Eu-tcbpe-MOF as the detection probe and Co9S8@Au@MoS2 as the quencher, achieving precise detection of carcinoembryonic antigen from 0.1 pg·mL-1 to 100 ng·mL-1 with a detection limit of 35.1 fg·mL-1.

Nano-matrixes propped self-enhanced electrochemiluminescence biosensor for microRNA detection

MicroRNAs (miRNA) are the potential biomarker for breast cancer, a biosensor for detecting miRNA-21 was successfully prepared by covalently linking carbohydrazide (CON4H6) and tris (4,4 '- dicarboxylic acid-2,2' - bipyridyl) ruthenium dichloride (Ru (dcbpy)32+) as a self-enhanced emitter (Ru-CON4H6). The biosensor was prepared by coating the electrode with mesoporous silica encapsulated Ru-CON4H6 as luminophores (RMSNs) to covalently link a couple of DNA strands (Q1-H2). The RMSNs coated electrode exhibited strong ECL emission due to the intramolecular electron transfer between the electrochemically oxidized Ru (dcbpy)32+ and co-reactant CON4H6. In the presence of target miRNA-21 and an assistant hairpin H1, H2 could be released from the surface through a strand displacement reaction (SDR), and the reserved Q1 could form G-quadruplex upon the addition of K+. The formed G-quadruplex then interacted with Q2-Fc in the presence of Mg2+ to form a DNA complex on the biosensor surface, which quenched the nano-matrixes propped self-enhanced ECL emission through the electron exchange between Fc and electrode or oxidized ECL intermediates. Under optimal conditions, the ECL decrease showed a correlation with target concentration, leading to a biosensing method for sensitive detection of miRNA-21. The proposed ECL method demonstrated a detectable concentration range from 0.1 fM to 1 nM along with a detection limit of 0.03 fM, good accuracy, and acceptable reproducibility, showing that the self-enhanced ECL biosensing strategy supported by nano-matrix provided a new way for the ultrasensitive detection of miRNA, and promoted the development of breast cancer diagnosis.

Novel inner filter effect-based near-infrared electrochemiluminescence sensor mediated by well-matched AgBr-nitrogen-doped Ti3C2 MXene and nonmetallic plasmon WO3•H2O

The development of innovative and efficient strategy is of paramount importance for near-infrared (NIR) electrochemiluminescence (ECL) sensing, which can substantially promote ECL detection in a wide range of situations. Herein, the inner filter effect (IFE) strategy was designed to construct an ultrasensitive NIR ECL biosensor based on the well-matched AgBr nanocrystals (NCs) decorated nitrogen-doped Ti3C2 MXene nanocomposites (AgBr-N-Ti3C2) and hydrated defective tungsten oxide nanosheets (dWO3•H2O). Specifically, the AgBr-N-Ti3C2 nanocomposites displayed extremely effective NIR ECL emission because N-doping could accelerate electron transfer and boost the red-shift of the ECL spectrum. The nonmetallic plasmon dWO3•H2O was used as an absorber due to its facile tuning of the spectra overlap and higher molar extinction coefficients. Time-resolved emission decay curves proved that the decreased ECL intensity was ascribed to the IFE-based steady quenching mechanism. With the support of tetracycline (TC) aptamer and the complementary DNA chain, the fabricated NIR ECL-IFE biosensor performed a wide linear range of 100 nM ∼ 10 fM with a low detection limit of 2.2 fM (S/N = 3), and it exhibited excellent stability, sensitivity, and reproducibility, so as to be applied to real samples. This strategy opens a new avenue to constructing an efficient NIR ECL-IFE system and shows excellent practical potential in actual sample analysis.

An ultrasensitive split-type electrochemical immunosensor based on controlled-release strategy for detection of CA19-9

In this study, a novel split-type electrochemical immunosensor based on controlled release strategy was proposed for sensitive analysis and detection of tumor marker carbohydrate antigen 199 (CA19-9). Specifically, glucose (Glu) was encapsulated in carrier mesoporous silica (MSN) with encapsulation technology, and surface functionalized Zinc sulfide (ZnS) caps were used as "gatekeepers". The complex is formed by encapsulating Glu within MSN with ZnS (ZnS@MSN-Glu) as a signal amplifier labeled on the signal antibody (Ab₂). And the Ab₂ can detect the presence of antibodies. To reduce the interference of biological analysis, the immune recognition process of ZnS@MSN-Glu-Ab₂ bioconjugate and antigen was carried out in 96-well microplate, which did not interfere with the electrochemical analysis process. Therefore, the low sensitivity detection caused by biofouling of nanomaterials and immunoreaction on the testing platform is eliminated. Subsequently, the opening and timed release of mesopores were controlled by external stimuli, the disulfide bond cleavage by dithiothreitol (DTT), and glucose was effectively released. Then nickel cobalt layered double hydroxide (NiCo-LDH) were directly hydrothermally grown on carbon cloth (CC) electrodeposited with copper selenide (CuSe) nanosheets to construct three-dimensional (3D) cactus-like NiCo-LDH/CuSe/CC sensing platform. It can realize the catalytic oxidation of released glucose, triggering glucose-mediated signal amplification. The synergistic effect of the 3D cactus structure and active nanomaterials promotes electron conduction. Taking the detection of carbohydrate antigen CA19-9 as an example, the immunosensor shows a wide linear concentration range (0.001-100 U/mL) with the limit of detection of 0.0005 U/mL, realizing highly sensitive detection of CA19-9. This biosensing technique has considerable advantages and provides an innovative approach for trace detection of other biomarkers.

Surface-Defect-Involved and Eye-Visible Electrochemiluminescence of Unary Copper Nanoclusters for Immunoassay

A single-stabilizer-capped strategy is proposed for achieving highly efficient and surface-defect-involved electrochemiluminescence (ECL) from unary copper nanoclusters (NCs) via employing l-cysteine (Cys) as a capping agent of luminophore. The Cys-capped CuNCs (Cys-CuNCs) can be electrochemically injected with valence band (VB) holes and exhibit eye-touchable ECL processes around +0.95 and +1.15 V upon employing TPrA as a coreactant. Both accumulated ECL spectra and spooling ECL spectra demonstrated that the two ECL processes are of the same single waveband and spectrally identical to each other with the same maximum emission wavelength of 640 nm. Promisingly, ECL of the Cys-CuNCs/TPrA system is obviously red-shifted for ∼150 nm to PL of Cys-CuNCs, indicating that the bandgap-engineered routes for ECLs of Cys-CuNCs are completely blocked. The oxidative-reduction ECL process of the Cys-CuNCs/TPrA system is a kind of highly efficient, eye-visible, and single-color emission in surface-defect-involved route. The capping agent of Cys can enable the CuNCs/TPrA system with a stronger ECL than other thiol capping agents, so that Cys-CuNCs are utilized as ECL tags for sensitive and selective immunoassays, which exhibit a wide linear response range from 0.05 pg/mL to 0.5 ng/mL and a limit of detection of 0.01 pg/mL (S/N = 3) with carcinoembryonic antigen as the analyte. Moreover, both the luminophore Cys-CuNCs and conjugates Ab₂-CuNCs can be safely stored in aqueous media without any protector, which is promising for the evolution and clinic application of metal NC ECL in the surface-defect-involved route.

Electrochemiluminescence nanoemitters for immunoassay of protein biomarkers

The family of electrochemiluminescent luminophores has witnessed quick development since the electrochemiluminescence (ECL) phenomenon of silicon nanoparticles was first reported in 2002. Moreover, these developed ECL nanoemitters have extensively been applied in sensitive detection of protein biomarker by combining with immunological recognition. This review firstly summarized the origin and development of various ECL nanoemitters including inorganic and organic nanomaterials, with an emphasis on metal-organic frameworks (MOFs)-based ECL nanoemitters. Several effective strategies to amplify the ECL response of nanoemitters and improve the sensitivity of immunosensing were discussed. The application of ECL nanoemitters in immunoassay of protein biomarkers for diagnosis of cancers and other diseases, especially lung cancer and heart diseases, was comprehensively presented. The recent development of ECL imaging with the nanoemitters as ECL tags for detection of multiplex protein biomarkers on single cell membrane also attracted attention. Finally, the future opportunities and challenges in the ECL biosensing field were highlighted.

Designing Triangular Silver Nanoplates with GSH/GSSG Surface Mixed States as Novel Nanoparticle-based Redox Mediators for Electrochemical Biosensing

Herein, a dual signal-quenched electrochemical (EC) biosensing strategy utilizing surface-engineered trisodium citrate (TSC)-glutathione (GSH)/oxidized glutathione (GSSG)-capped triangular silver nanoplates (Tri-Ag NPs_TSC-GSH/GSSG) as a novel nanoparticle-based redox mediator was explored for biomarker determination. In contrast with conventional redox mediators, Tri-Ag NPs_TSC-GSH/GSSG provided more admirable EC performance along with a lower oxidation potential (∼0.14 V). Taking advantage of the split-type mode, the immune response in a 96-well microplate was independent from EC detection, which could effectively eliminate the biological interference and thereby greatly enhance the sensitivity. As for the surface engineering process of Tri-Ag NPs, it was composed of partial GSH replacement and the formation of the GSH/GSSG surface mixed state. Primarily, the signal response of Ag NPs_TSC-GSH decreased due to the hindrance of GSH on electron transfer. Moreover, varying proportions of GSH/GSSG could further impede the oxidation process of Tri-Ag NPs_TSC-GSH/GSSG and eventually realize efficient dual signal quenching of this system. Notably, the ZIF-67@MIL-88B-GO_x nanocomposite as the label was applied for a cascade reaction system with GSH peroxidase-like activities to form the optimal GSH/GSSG proportion, causing sensitive changes in signal response with a range of different antigen concentrations. On this basis, the fabricated biosensor provided measurable outputs of aflatoxin B1 concentrations in a linear range of 0.0005-50 ng/mL with a low detection limit of 0.61 pg/mL (S/N = 3). All of the results indicated that the novel biosensor could be a promising analytical tool for future biomarker detection.

Self-Photocatalysis Boosted Electrochemiluminescence Signal Amplification via In Situ Generation of the Coreactant

The classic luminol-based electrochemiluminescence (ECL) platform generally suffers from self-decomposition of the coreactant (i.e., H₂O₂) during the reaction process, seriously hampering the luminous signal stability, as well as its practical application. To address this issue, apart from the introduction of complex exogenous species, preoxidation of the luminophore, and electrocatalysis for ECL signal amplification, we proposed a novel ECL model to realize the signal enhancement via in situ self-photocatalytic generation of the coreactant H₂O₂. Interestingly, the luminescence of luminol was simultaneously utilized as the light source to promote the conversation of O₂ to H₂O₂ with the assistance of the photocatalyst resorcinol-formaldehyde resin, which could further improve the luminescence of luminol in turn. In comparison with the traditional case, this new ECL model not only exhibited obvious signal amplification but also efficiently boosted its stability of signal output. To sum up, an exogenous coreactant-free, highly stable ECL platform was obtained via simply integrating the photocatalyst RF and the luminol-based system. This work will not only inspire the design of a new integrated ECL system with a coreactant translator but also provide an ingenious insight for the construction of a new generation of ECL models.

A dual-quenched ECL immunosensor for ultrasensitive detection of retinol binding protein 4 based on luminol@AuPt/ZIF-67 and MnO2@CNTs

Background

Retinol binding protein 4 (RBP4) has been regarded as an important serological biomarker for type 2 diabetes mellitus (T2DM). Hence, the construction of a highly sensitive detection method for RBP4 is the key to early prevention and multidisciplinary intervention of T2DM. In this work, a dual-quenched electrochemiluminescence (ECL) immunosensor has been fabricated for ultrasensitive detection of RBP4 by combining zeolitic imidazolate framework-67/AuPt-supported luminol (luminol@AuPt/ZIF-67) with MnO₂ nanosheets-grown on carbon nanotubes (MnO₂@CNTs).

Results

AuPt/ZIF-67 hybrids with high-efficiency peroxidase-like activity could provide multipoint binding sites for luminol and antibodies and significantly boost the amplified initial signal of the ECL immunosensor. Upon glutathione/H₂O₂ coreactants system, MnO₂@CNTs composites could quench the initial signal by inhibiting mimic peroxidase activity of luminol@AuPt/ZIF-67. Moreover, the absorption spectrum of the MnO₂@CNTs composites completely overlaps with the emission spectrum of luminol, which can further reduce initial signal by ECL resonance energy transfer (ECL-RET).

Conclusions

Benefiting from the above-mentioned properties, the designed immunoassay sensitivity exhibited excellent sensitivity and relative stability for RBP4 detection range from 0.0001 to 100 ng mL⁻¹ with a low detection limit of 43 fg mL⁻¹. Therefore, our ECL immunosensor provides an alternative assaying strategy for early diagnosis of T2DM.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01020-1.

A DNAzyme-Based Dual-Stimuli Responsive Electrochemiluminescence Resonance Energy Transfer Platform for Ultrasensitive Anatoxin-a Detection

An effective and precise electrochemiluminescence resonance energy transfer (ECL-RET), including the efficient regulation over the proximity of a donor and an acceptor and the reliable stimuli responsive as well as the avoidance of undesirable probes leakage, etc., is significant for the development of an accurate and sensitive ECL detection method; yet, the current literature in documentation involves only a limited range of such ECL-RET systems. Herein, we propose an ECL-RET strategy with dually quenched ultralow background signals and a dual-stimuli responsive, accurate signal output for the ultrasensitive and reliable detection of anatoxin-a (ATX-a). The dual quenching is accomplished by an integrated ECL-RET probe of metal organic frameworks (MOFs) encapsulated into Ru(bpy)₃²⁺ (Ru-MOF) (donor) coated with silver nanoparticles (AgNPs) shell (acceptor 1) and close proximity with DNA-ferrocene (Fc) (acceptor 2). Multistimuli responsive DNAzyme facilitated the accurate signal switch by both target ATX-a and hydrogen peroxide (H₂O₂). Because of the specific recognition of the aptamer toward ATX-a, an intricate design of the DNA sequence enabled the exposure of the Ag⁺-dependent DNAzyme sequence and H₂O₂ in situ generated Ag⁺ triggering a catalytic cleavage reaction to freely release the two ECL-RET energy acceptors, thus switching the ECL signal significantly and achieving ultrasensitive detection. It is noteworthy that AgNPs are key in this ECL-RET strategy, serving both as the gate-keepers for avoiding ECL probes leakage and also the ECL energy acceptors, and mostly importantly serving as the redox substrate for the subsequent DNAzyme catalytic signal switch. The proposed ECL-RET aptasensor for ATX-a detection displayed splendid monitoring performance with a quite low detection limit of 0.00034 mg mL^-1. This sensor not only led to the development of a dual-quenching ECL-RET system but also provided meaningful multistimuli responsive ECL biosensing platform construction, which shows a promising application prospect in complicated sample analysis.

Nitrogen-Doped Chiral CuO/CoO Nanofibers: An Enhanced Electrochemiluminescence Sensing Strategy for Detection of 3,4-Dihydroxy-Phenylalanine Enantiomers

l-3,4-Dihydroxy-phenylalanine (l-DOPA) is the most effective drug for the treatment of Parkinson's disease, which plays a very important role in clinical and neurochemistry. However, how to achieve high-sensitivity recognition of l-DOPA still faces challenges. Here, a facile strategy is presented to construct nitrogen-doped chiral CuO/CoO nanofibers (N-CuO/CoO NFs) with nanozyme activity and electrochemiluminescence property, in which CuO/CoO NFs are used as the catalytic activity center and chiral cysteine (Cys) is used as the inducer of chiral recognition, for enantioselective catalysis and sensitive recognition of DOPA enantiomers. Notably, N doping not only enhances the enzyme-mimic activity of CuO/CoO NFs but also amplifies their electrochemiluminescence (ECL) signals in the presence of luminol. More importantly, in the presence of DOPA enantiomers, the d-cysteine (d-Cys)-modified N-CuO/CoO NFs exhibit different ECL performances; thus, d-Cys@N-CuO/CoO NFs could selectively distinguish and sensitively detect l-DOPA through ECL signals, and the detection limit is 0.29 nM for l-DOPA. In addition, it also showed good sensing performance for the determination of l-DOPA in fetal bovine serum. This is the first report on the detection of DOPA enantiomers based on an enhanced ECL strategy, providing a robust pathway for chiral discrimination and detection of chiral molecules.

An intermolecular hydrogen-bond-induced quench-type Ru(dcbpy)32+/TPA electrochemiluminescence system by nitrogen-doped carbon quantum dots

Here, we show that nitrogen-doped carbon quantum dots (NCQDs) strongly inhibits the anodic electrochemiluminescence (ECL) signal of a tris(4,4'-dicarboxylic acid-2,2'-bipyridyl) ruthenium(II) (Ru(dcbpy)₃²⁺)/tripropylamine (TPA) aqueous system. To determine the ECL-quenching mechanism, we used photoluminescence spectroscopy, UV-Visible absorption spectroscopy and dynamic simulation technology. Quenching of the ECL signal of Ru(dcbpy)₃²⁺/TPA by NCQDs was predominantly attributed to the interaction between Ru(dcbpy)₃²⁺ and NCQDs rather than that between TPA and NCQDs. Specifically, when Ru(dcbpy)₃²⁺ and NCQDs were in aqueous solution together, the carboxyl (-COOH) groups of Ru(dcbpy)₃²⁺ were in contact with oxygen- and nitrogen-containing groups on the surface of NCQDs and formed intermolecular hydrogen bonds. This process involved energy transfer from the excited-state Ru(dcbpy)₃²⁺ to the intermolecular hydrogen bonds, thus resulting in a decrease in the Ru(dcbpy)₃²⁺ ECL signal. On this basis, a quenching-type ECL sensor for the quantification of NCQDs was fabricated. The sensor had a wide linear range and an estimated detection limit of 0.0012 mg mL^-1, as well as excellent stability and selectivity. Satisfactory recoveries of 97.0-99.5% were obtained using the ECL sensor to quantify NCQDs in tap water. NCQDs could potentially be used as a quenching probe of Ru(dcbpy)₃²⁺ to construct various biosensors with widespread applications in the sensing field.

An electrochemiluminescence aptasensor for diethylstilbestrol assay based on resonance energy transfer between Ag3PO4-Cu-MOF(II) and silver nanoparticles

In this work, a novel electrochemiluminescence (ECL) aptasensor based on the resonance energy transfer (RET) effect between Ag3PO4-Cu-MOF (ii) and silver nanoparticles (Ag NPs) is proposed. The ECL emission spectra of Ag3PO4-Cu-MOF and the ultraviolet absorption spectra of Ag NPs showed a good spectral overlap. Based on this, we designed an "on-off-on" ECL sensing strategy for the sensitive and specific detection of diethylstilbestrol (DES). Under the optimal conditions, the linear range of the sensor for DES detection was 1.0 × 10-12-1.0 × 10-4 M, with a detection limit of 7.2 × 10-13 M (S/N = 3). The method showed simple and fast operation, high sensitivity and selectivity, a strong anti-interference ability and good stability. More importantly, the developed aptasensor exhibited excellent recognition towards residual DES in actual water samples. The sensor has superior measurement capability and potential application value in the field of environment water quality monitoring.

Split-Type Electrochemical Immunoassay System Triggering Ascorbic Acid-Mediated Signal Magnification Based on a Controlled-Release Strategy

This research put forward a novel split-type electrochemical (EC) immunosensor which integrated the controlled-release strategy with EC detection for application in the field of biosensing. Concretely, ascorbic acid (AA) was packaged in a cadmium sulfide (CdS)-capped spherical mesoporous bioactive glass (SBG) nanocarrier (SBG_CdS) on account of encapsulation technology. To reduce the complexity of the bioanalysis, the detection antibody-labeled SBG_CdS-AA bioconjugate was applied in a 96-well microplate for the immunoreaction process, which is independent of the EC determination procedure. Thus, the immune interference and steric hindrance caused by the accumulation of nanomaterials on the electrode could be minimized. Subsequently, AA was released efficiently via the destruction effect of dithiothreitol on the disulfide bond. In addition, for the as-prepared FcAI/l-Cys/gold nanoparticles (GNPs)/porous BiVO₄ (p-BVO)/ITO EC sensing platform in the detection solution, the synergetic catalysis of Fc and GNPs/p-BVO toward the oxidation of the released AA could be realized, which triggered AA-mediated significant signal magnification throughout this study. In particular, p-BVO with an ordered nanoarray structure could accelerate the electron transfer to assist in sensitivity improvement of this system. This novel biosensor was capable of assaying the neuron-specific enolase (NSE) biomarker sensitively, from which a linear range of 0.001-100 ng/mL was derived along with a low detection limit of 1.08 pg/mL. An innovative way could be paved in the bioanalysis of NSE and other biomarkers.