Electrochemical synthesis of silver nanoclusters on electrochemiluminescent resonance energy transfer amplification platform for Apo-A1 detection

Challenges and Opportunities of Using Fluorescent Metal Nanocluster-Based Colorimetric Assays in Medicine

Development of rapid colorimetric methods based on novel optical-active metal nanomaterials has provided methods for the detection of ions, biomarkers, cancers, etc. Fluorescent metal nanoclusters (FMNCs) have gained a lot of attention due to their unique physical, chemical, and optical properties providing numerous applications from rapid and sensitive detection to cellular imaging. However, because of very small color changes, their colorimetric applications for developing rapid tests based on the naked eye or simple UV-vis absorption spectrophotometry are still limited. FMNCs with peroxidase-like activity have significant potential in a wide variety of applications, especially for point-of-care diagnostics. In this review, the effect of using various capping agents and metals for the preparation of nanoclusters in their colorimetric sensing properties is explored, and the synthesis and detection mechanisms and the recent advances in their application for ultrasensitive chemical and biological analysis regarding human health are highlighted. Finally, the challenges that remain as well as the future perspectives are briefly discussed. Overcoming these limitations will allow us to expand the nanocluster's application for colorimetric diagnostic purposes in medical practice.

Synergetic-effect-enhanced electrochemiluminescence of zein-protected Au-Ag bimetallic nanoclusters for CA15-3 detection

In this work, a sandwich-type electrochemiluminescence (ECL) system was constructed for the detection of CA15-3. Gold-silver bimetallic nanoclusters (Au-Ag BNCs) with zein as a protective ligand were synthesized, and the excellent ECL performance of this material was demonstrated for the first time. Zein carrying a variety of groups that ligated with Au-Ag BNCs, forming a protective shell of zein, effectively prevented clusters from aggregating or growing into larger nanoparticles. The synergistic effect of the bimetal promotes the ECL emission, making this nanoscale material an ideal ECL probe. GO-PANI, which effectively promoting the production of sulfate radicals of the co-reactant and significantly increasing the ECL strength, was a good sensing platform for antibody immobilization. Consequently, we constructed an ECL sensor with GO-PANI as the sensing platform and Au-Ag BNCs@zein as the ECL probe, with a detection range of 0.001-100 U mL-1 and a detection limit of 0.0003 U mL-1, provided a strong support for the sensor for future CA15-3 detection applications.

An electrochemical aptasensor based on AuNRs/AuNWs for sensitive detection of apolipoprotein A-1 (ApoA1) from human serum

For early detection and diagnosis of cancer, it is essential to develop an electrochemical biosensor that is quick, accurate, and sensitive. Here, we use gold nanorod (AuNR) and gold nanowire (AuNW) nanocomposites (AuNR/AuNW/CS) as electrode modifiers on a glassy carbon electrode (GCE) to construct a sensitive label-free electrochemical aptasensor to detect ApoA1. The thiolated ApoA1-specific aptamers were immobilized onto the modified electrode surface through self-assembled monolayers. Electrochemical techniques, such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV), were used to analyze the fabrication steps. The concentration of ApoA1 was measured with DPV on the aptasensor, with a linear range of 0.1 to 1000 pg mL^-1 and a detection limit of 0.04 pg mL^-1. When compared to results from ELISA tests (which have a detection limit of 80 pg mL^-1), the results achieved here were over 2000 times better. The aptasensor's performance was successfully evaluated using human serum spiked with ApoA1, suggesting that it has great potential for practical application. The electrochemical apatsensor additionally demonstrated outstanding selectivity responses and strong stability toward the target analyte.

A novel sandwich impedimetric immunosensor for detection of apolipoprotein-A1 based on the gold nanoparticle-hybridized mercapto-β-cyclodextrin-Pb(II) metal-organic framework

A novel sandwich electrochemical impedimetric immunosensor was proposed to detect apolipoprotein-A1 (Apo-A1), a common biomarker for bladder cancer. The molybdenum disulfide/graphene quantum dot (MoS₂/GQD) nanocomposites were modified on the surface of a glassy carbon electrode (GCE) and used to immobilize the biotinylated antibody (Ab₁) with the help of chitosan and glutaraldehyde (denoted as BSA/Ab₁/CHIT/MoS₂/GQD/GCE). Pb(II)-thiol-β-cyclodextrin metal-organic framework (denoted as Pb-MOF) was synthesized with lead metal ions and thiol-β-cyclodextrin ligands by a one-pot solvothermal method, and then, gold nanoparticles were modified on the surface of Pb-MOF (Pb-MOF-AuNPs) by Au-S bond, which was used as signal label for the recombinant antibody (Ab₂). When the immunosensor of BSA/Ab₁/CHIT/MoS₂/GQD/GCE reacted with Apo-A1, Pb-MOF-AuNPs-Ab₂/BSA was connected to the electrode when immunoreaction occurred, and an immune sandwich structure was formed, which led to significantly increased charge transfer resistance of electrochemical probe for ferrocyanide (II)/(III) within the frequency range 10^-1 ~ 10⁵ Hz at 5 mV amplitude and the potential of 0.180 V (vs. SCE). Based on this principle, the quantitative detection of Apo-A1 was established. The relative change of electrochemical resistance and the logarithmic value of Apo-A1 concentration showed a linear relationship with a linear coefficient of 0.9989 in the range 1.00 pg mL^-1 and 1.00 μg mL^-1 with the limit of detection of 0.30 pg mL^-1. The selectivity, repeatability, and other performance of the proposed immunosensor were also investigated. The immunosensor was successfully applied to the detection of real serum and urine samples with recovery in the range 96.4 ~ 109.1% (RSD < 3.8%), indicating that it could be helpful for the clinical diagnosis of bladder cancer.

Label-Free Electrochemiluminescence Nano-aptasensor for the Ultrasensitive Detection of ApoA1 in Human Serum

A molybdenum sulfide/zirconium oxide/Nafion (MoS₂/ZrO₂/Naf) based electrochemiluminescence (ECL) aptasensor for the selective and ultrasensitive detection of ApoA1 is proposed, with Ru(bpy)₃ ²⁺ as the luminophore. The chitosan (CS) modification on the nanocomposite layer allowed glutaraldehyde (GLUT) cross-linking, resulting in the immobilization of ApoA1 aptamers. Scanning electron microscopy, tunneling electron microscopy, and energy dispersive X-ray spectroscopy were used to characterize the nanocomposite, while electrochemiluminescence (ECL), cyclic voltammetry, and electrochemical impedance spectroscopy were used to analyze the aptasensor assembly. The nanocomposite was used as an electrode modifier, which increased the intensity of the ECL signal. Due to the anionic environment produced on the sensor surface following the specific interaction of the ApoA1 biomarker with the sensor, more Ru(bpy)₃ ²⁺ were able to be electrostatically attached to the aptamer-ApoA1 complex, resulting in enhanced ECL signal. The ECL aptasensor demonstrated outstanding sensitivity for ApoA1 under optimal experimental conditions, with a detection limit of 53 fg/mL and a wide linear dynamic range of 0.1-1000 pg/mL. The potential practical applicability of this aptasensor was validated by analyzing ApoA1 in human serum samples, with recovery rates of 94-108% (n = 3). The proposed assay was found to be substantially better compared to the commercially available enzyme-linked immunosorbent assay method, as reflected from over 1500 times improvement in the detection limit for ApoA1.

Confined Gold Single Atoms-MXene Heterostructure-Based Electrochemiluminescence Functional Material and Its Sensing Application

Herein, based on electronic metal-support interaction (EMSI), a gold single atom confined MXene (Au_SA/MXene) heterostructure was developed as the highly efficient electrochemiluminescence (ECL) functional material, which greatly improved the electrochemical properties and broadened the sensing application of MXenes. Gold single atoms were confined into the vacancy defects of Ti₃C₂T_x MXene, which could effectively avoid the masking of catalytic active sites. Meanwhile, electron transport could be accelerated with the assistance of titanium dioxide on the MXene nanosheets. Therefore, the Au_SA/MXene heterostructure had high catalytic activity and electrical activity to promote hydrogen peroxide to generate free radicals, which achieved high-efficiency ECL. Eventually, the Au_SA/MXene heterostructure was used to construct a Faraday cage-type ECL sensor with fluid nanoislands to detect miRNA-187 in triple-negative breast cancer tumor tissues.

Recent progress of metal nanoclusters in electrochemiluminescence

Metal nanoclusters (MeNCs), composed of a few to hundreds of metal atoms and appropriate surface ligands, have attracted extensive interest in the electrochemiluminescence (ECL) realm owing to their molecule-like optical, electronic, and physicochemical attributes and are strongly anticipated for discrete energy levels, fascinating electrocatalytic activity, and good biocompatibility. Over the past decade, huge efforts have been devoted to the synthesis, properties, and application research of ECL-related MeNCs, and this field is still a subject of heightened concern. Therefore, this perspective aims to provide a comprehensive overview of the recent advances of MeNCs in the ECL domain, mainly covering the emerged ECL available MeNCs, unique chemical and optical properties, and the general ECL mechanisms. Synthesis strategies for desirable ECL performance are further highlighted, and the resulting ECL sensing applications utilizing MeNCs as luminophores, quenchers, and substrates are discussed systematically. Finally, we anticipate the future prospects and challenges in the development of this area.

Copper nanocluster-Ru(dcbpy)32+ as a cathodic ECL-RET probe combined with 3D bipedal DNA walker amplification for bioanalysis

In this study, an intra-molecular ECL-RET probe combining the 3D bipedal DNA walker amplification strategy was exquisitely designed to monitor platelet-derived growth factor BB (PDGF-BB).

MXene-Derived Quantum Dot@Gold Nanobones Heterostructure-Based Electrochemiluminescence Sensor for Triple-Negative Breast Cancer Diagnosis

MXene material has been gradually studied in recent years due to its fascinating characteristics. This work developed a novel MXene-derived quantum dots (MQDs) @gold nanobones (Au NBs) heterostructure as the electrochemiluminescence (ECL) sensor. First, MXene and MQDs were synthesized via the green preparation process, which avoided the harm of hydrofluoric acid to humans and the environment. There was a strong ECL signal enhancement in the MQD@Au NBs heterostructure. On the one hand, Au NBs with surface plasmon resonance (SPR) effect acted as an "electronic regulator" that can transfer electrons to itself to control over-injection of electrons into the conduction band of MQDs. The luminous signal of MQDs can be efficiently generated and significantly amplified in the ECL sensing process. On the other hand, the work function of MQDs with excellent conductivity was relatively close to that of Au NBs in the heterostructure. So, ECL quenching caused by short-distance electron transfer between luminophore and Au nanomaterial has been effectively suppressed. The MQD@Au NBs heterostructure-based ECL sensing system was applied to determine miRNA-26a in the serum of patients with triple-negative breast cancer. It not only provides ideas for the green synthesis of MXene but also provides a guide for the application of MQD@Au NBs heterostructure in the field of ECL sensing.

Contrary Logic Pair Library, Parity Generator/Checker and Various Concatenated Logic Circuits Engineered by a Label-Free and Immobilization-Free Electrochemiluminescence Resonance Energy Transfer System

Herein, a label-free and immobilization-free electrochemiluminescence resonance energy transfer (ECL-RET) system based on graphitic carbon nitride nanosheets (GCNNs)/Ru(phen)32+ donor/acceptor pair is developed, in which the ECL-RET is regulated by regulating the diffusivity of Ru(phen)32+ molecules toward the negatively charged GCNNs through logically programmed DNA hybridization reactions. The two optical signals of GCNNs (445 nm) and Ru(phen)32+ (593 nm) show completely opposite changes through the same one-time DNA hybridization reaction. Based on this ECL-RET system, a contrary logic pair (CLP) library, a parity generator/checker system for differentiating the erroneous bits during data transmission, the parity checker to identify the even/odd natural numbers from 0 to 9, and a series of concatenated logic circuits including a six-input logic gate capable of implementing of 64 input combinations for meeting the needs of computational complexity are developed. The ECL-RET-based molecular logic system avoids the time-consuming, costly and inefficient labeling procedures and the laborious processes of immobilization, presenting great potential for building more complicated and advanced logic gates, and providing a refreshing inspiration for the construction of combinatorial logic circuits based on ECL method.

Enhanced electrochemiluminescence ratiometric cytosensing based on surface plasmon resonance of Au nanoparticles and nanosucculent films

Ramos cells are human Burkitt's lymphoma cells, which are a kind of cancer cells to facilitate the monitoring of the relevant biological processes of cancers. Sensitive and accurate detection of Ramos cells using emerging ratiometric ECL biosensing technology shows increasing importance, however, the target analytes of current ratiometric ECL biosensors are mainly limited to DNA/RNA or proteins. In this study, we proposed a dual-potential ratiometric sensing strategy for the electrochemiluminescence detection of Ramos cells based on two types of electrochemiluminescence (ECL)-responding molecular. Au nanosucculent films (AuNFs) were electrodeposited on the fluorine doped tin oxide (FTO) electrode to increase the effective area of the electrode for more efficient assembly of DNA and effectively improving the conductivity of the sensing interfaces. In the presence of Ramos cells, aptamers capped with Au@luminol would conjugate with Ramos cells and then remove from AuNFs, accompanying the decrease of ECL signal from Au@luminol. Then, Au-DNA was captured and alternately hybridized with DNA-modified CdS nanocrystals (NCs) on the surface of AuNFs with the formation of a super reticulate structure. The reticulate structure not only raised another identified ECL signal from CdS NCs but also greatly promoted its ECL intensity from the surface plasmon resonance originating from Au NPs. The value of log (ECL_CdS/ECL_luminol) and the logarithm of the number of cells exhibit considerable linear relation ranging from 80 to 8 × 10⁵ cells mL^-1 with a low detection limit of 20 cells mL^-1 (S/N = 3). The selectivity and specificity of this dual-potential ECL sensor showed good performance and indicated considerable promise in avoiding false-positive results in detection.

Rational Fabrication of a Smart Electrochemiluminescent Sensor: Synergistic Effect of a Self-Luminous Faraday Cage and Biomimetic Magnetic Vesicles

A smart electrochemiluminescent (ECL) sensor has been designed in this work. The sensing system consisted of Ag NPs-Ti₃AlC₂ nanosheets (Ag-TACS) as the self-luminous Faraday cage and biomimetic magnetic vesicles as the functional substrate. By engineering the structure and properties of Ti₃AlC₂ nanosheets to induce the Faraday-cage effect, the outer Helmholtz plane (OHP) was extended to contribute to ECL enhancement. Compared with the Faraday cage that further incorporated luminous materials, the self-luminous Faraday cage in the "direct label" model kept all the luminous materials on the OHP. Meanwhile, biomimetic magneticvesicles with highly efficient fluidity were used to improve the sensing efficiency and obtain a perfect Faraday-cage structure to enhance the ECL signals. The highest ECL enhancement (ca. 25 times) has been achieved by the synergistic effect of the Faraday cage and biomimetic magnetic vesicles. This sensing system was used to detect the wild-type K-ras gene in the colorectal tumor tissue. It provides not only an important guide for the novel ECL sensing concept but also a smart modulation system of the electromagnetic field.

Direct Laser Writing of Fluorescent Silver Nanoclusters: A Review of Methods and Applications

Metal nanoclusters (NCs) are nanomaterials of size of less than 2 nm that exhibit a set of unique physical, chemical, optical, and electronic properties. Because of recent interest in NCs, a great deal of effort is being made to develop synthetic routes that allow control over the NC size, shape, geometry, and properties. Direct laser writing is one of the few synthesis methods that allow the generation of photostable NCs with high quantum yield in a highly controlled fashion. A key advantage of laser-written NCs is the ability to create easy-to-use solid-state devices for a range of applications. This review will present necessary background and recent advances in laser writing of silver NCs and their applications in different solid-state matrixes such as glass, zeolites, and polymer substrate. This topic will be of interest to researchers in the fields of materials science, optics and photonics, chemistry, and biomedical sciences.

Fe3O4 NP@ZIF-8/MoS2 QD-based electrochemiluminescence with nanosurface energy transfer strategy for point-of-care determination of ATP

Herein, Fe₃O₄ NP@ZIF-8/MoS₂ QD-based electrochemiluminescence (ECL) biosensor with nanosurface energy transfer strategy was successfully developed for point-of-care determination of ATP. With the porous structure and poor electron transfer ability, Fe₃O₄ NP@ZIF-8 complex was first used as an excellent catalyst in ECL. The complex catalyzed the coreactant for more free radicals and hindered the quenching effect of Fe₃O₄ nanoparticles (NPs) on quantum dots (QDs). In ECL-nanosurface energy transfer (NSET) system, through the specific binding of complementary DNA linked to MoS₂ QDs (QDs-cDNA) and aptamer linked to Au NPs, interaction between the point dipole of MoS₂ QDs and the collective dipoles of Au NPs quenched ECL signal. When ATP was captured by aptamer, the ECL-NSET system was taken apart, which resulted in the recovery of ECL signal. Moreover, changes of the ECL imaging can be captured by a smartphone, which enabled point-of-care determination of ATP from 0.05 nmol L^-1 to 200 nmol L^-1 with LOD of 0.015 nmol L^-1. With superior specificity and stability, the sensing system showed significant potential about the application of catalysts coated with ZIF and NSET in point-of-care ECL determination.

Electrochemical sandwich-type thrombin aptasensor based on dual signal amplification strategy of silver nanowires and hollow Au-CeO2

Signal amplification is crucial in electrochemical biosensor to obtain low detection limits. In this work, a highly sensitive sandwich-type thrombin aptasensor is constructed, based on dual signal amplification of uniform silver nanowires (AgNWs) and hollow Au-CeO₂ nanocomposites. AgNWs are decorated on the ITO surface to immobilize amino functionalized thrombin capture apemeter 1 (Apt1). And Au nanoparticles (AuNPs) grown directly on the surface of hollow CeO₂ microstructure are used to immobilize sulfydryl functionalized thrombin reporter apemeter 2 (Apt2). Thus, sandwich-type apatasensor has been successfully designed, due to the specific recognition between thrombin and the two kinds of aptamers. One of the signal amplifications is from the good conductivity of uniform AgNWs. Moreover, uniform AgNWs together with hollow Au-CeO₂ exhibit the good catalytic performance for the reduction of H₂O₂, further resulting in significant electrochemical signal amplification. Because the electrochemical signal amplification is closely related to the thrombin concentration, differential pulse voltammetry is used to specifically detect thrombin. Under the optimized conditions, the proposed method has a good linear response ranged from 0.5 pM to 30 nM with a low detection limit of 0.25 pM (S/N = 3) for thrombin. The proposed thrombin aptasensor displays good selectivity, reproducibility and stability, providing a good platform for the ultrasensitive detection of thrombin.

Electrochemiluminescence "turn-off" detection of curcumin via energy transfer using luminol-doped silica nanoparticles

A method is presented for electrochemiluminescent (ECL) detection of the food additive curcumin via an energy transfer strategy and by using luminol-doped silica nanoparticles (luminol-NPs). The ECL emission of the luminol-NPs (peaking at 425 nm) is reduced in the presence of curcumin due to spectral overlap. The assay can be performed within 1 min, response is linear in the 0.1 to 100 µM curcumin concentration range, and the limit of detection is 32 nM. The method is selective over many ions, adenosine triphosphate, ascorbic acid, cysteine and folic acid. It was successfully applied to the determination of curcumin in spiked human serum and urine. The average recoveries range from 99.0 to 102.6%. Graphical abstract Electrochemiluminescence (ECL) "turn-off" detection of curcumin at levels as low as 32 nM via energy transfer using luminol-doped silica nanoparticles. No hydrogen peroxide (H₂O₂) is used in ECL detection which makes the luminol-NPs ECL system more stable than the conventional luminol-H₂O₂ ECL system.

Recent advances of ratiometric electrochemiluminescence biosensors

Ratiometric electrochemiluminescence (ECL) assays have been widely applied in biosensing because of eliminated outside interferences and improved reliability in detection.

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.