Plasmon-Enhanced Electrochemiluminescence of Silver Nanoclusters for microRNA Detection

Nanoconfined Silver Nanoclusters Combined with X-Shaped DNA Recognizer-Triggered Cascade Amplification for Electrochemiluminescence Detection of APE1

Apurinic/apyrimidinic endonuclease 1 (APE1), as a vital base excision repair enzyme, is essential for maintaining genomic integrity and stability, and its abnormal expression is closely associated with malignant tumors. Herein, we constructed an electrochemiluminescence (ECL) biosensor for detecting APE1 activity by combining nanoconfined ECL silver nanoclusters (Ag NCs) with X-shaped DNA recognizer-triggered cascade amplification. Specifically, the Ag NCs were prepared and confined in the glutaraldehyde-cross-linked chitosan hydrogel network using the one-pot method, resulting in a strong ECL response and exceptional stability in comparison with discrete Ag NCs. Furthermore, the self-assembled X-shaped DNA recognizers were designed for APE1 detection, which not only improved reaction kinetics due to the ordered arrangement of recognition sites but also achieved high sensitivity by utilizing the recognizer-triggered cascade amplification of strand displacement amplification (SDA) and DNAzyme catalysis. As expected, this biosensor achieved sensitive ECL detection of APE1 in the range of 1.0 × 10-3 U·μL-1 to 1.0 × 10-10 U·μL-1 with the detection limit of 2.21 × 10-11 U·μL-1, rendering it a desirable approach for biomarker detection.

A Novel Rhodamine-Phenolphthalein Architecture for Selective Mercury Ion Detection in Aqueous Media

In this study, the primary objective is to synthesize a novel fluorescent Rh-PP-Rh compound and explore its extensive range of photochemical behaviors. Initially, the synthesis of the novel Rh-PP-Rh was carried out for this purpose. Subsequently, UV-Vis and fluorescence spectroscopy were employed to investigate the interactions between Rh-PP-Rh and a diverse array of ions in aqueous solvent systems. Through fluorescence and UV-Vis studies, it was observed that Rh-PP-Rh demonstrated turn-on sensor properties in the presence of Hg2+ ions. Furthermore, the limits of detection (LOD) and association constant (Ka) values for Rh-PP-Rh/Hg2+ were determined as 334 nM and 9.13×1011 M-2, respectively. Additionally, the reversible studies demonstrated a switchable on/off response upon alternate addition of HgCl2 and [Bu4N]F to Rh-PP-Rh. These findings suggest that the probe Rh-PP-Rh also possesses specific sensor properties for F- ions in the presence of mercury. In addition, the investigation encompassed an assessment of the visual analysis of the color alterations of Rh-PP-Rh both on filter paper and in an EtOH/H2O solution. The findings demonstrated that Rh-PP-Rh can be successfully utilized in solutions containing mercury, as it generates significant color transformations.

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.

Cationic fluorescent carbon dots with solution ultra-stability and its rapid/on-site sensing application for HClO

Carbon dots (CDs) as a remarkable fluorescent nanomaterial have the advantages of easy preparation, good photostability and high sensitivity. However, the poor aqueous solution stability of carbon dots largely limited their practical application due to the characteristic of easily forming precipitation for long time storage. Here, a kind of cationic fluorescent carbon dots CDs-P(Ph)3 was designed by introducing a cationic compound, (4-carboxybutyl) triphenyl phosphonium bromide, to construct an electrostatic shell outside the dots. Such electrostatic shell could highly improve carbon dots stability in an aqueous solution to make CDs-P(Ph)3 stable for long-term storage with negligible aggregation. Meanwhile, the sensitivity of CDs-P(Ph)3 for hypochlorous acid (HClO) was also enhanced on the basis of the electron-withdrawing effect of cationic substituents on the surface of carbon dots. The limit of detection of CDs-P(Ph)3 for HClO was as low as ∼0.32 μM. Additionally, the fluorescence of CDs-P(Ph)3 could be rapid quenched by HClO with a quenching efficiency of more than 80% within 30 s. The excellent stability of CDs-P(Ph)3 in an aqueous solution made it suitable for on-site detecting HClO in real samples, such as tap, well and lake water. Such designed fluorescent nanomaterial would provide a practical application pathway for optical sensing detection in environmental samples.

Green Synthesis of Silver Nanoparticles Using Acacia ehrenbergiana Plant Cortex Extract for Efficient Removal of Rhodamine B Cationic Dye from Wastewater and the Evaluation of Antimicrobial Activity

Silver nanoparticles (Ag-NPs) exhibit vast potential in numerous applications, such as wastewater treatment and catalysis. In this study, we report the green synthesis of Ag-NPs using Acacia ehrenbergiana plant cortex extract to reduce cationic Rhodamine B (RhB) dye and for antibacterial and antifungal applications. The green synthesis of Ag-NPs involves three main phases: activation, growth, and termination. The shape and morphologies of the prepared Ag-NPs were studied through different analytical techniques. The results confirmed the successful preparation of Ag-NPs with a particle size distribution ranging from 1 to 40 nm. The Ag-NPs were used as a heterogeneous catalyst to reduce RhB dye from aqueous solutions in the presence of sodium borohydride (NaBH₄). The results showed that 96% of catalytic reduction can be accomplished within 32 min using 20 μL of 0.05% Ag-NPs aqueous suspension in 100 μL of 1 mM RhB solution, 2 mL of deionized water, and 1 mL of 10 mM NaBH₄ solution. The results followed a zero-order chemical kinetic (R² = 0.98) with reaction rate constant k as 0.059 mol L^-1 s^-1. Furthermore, the Ag-NPs were used as antibacterial and antifungal agents against 16 Gram-positive and Gram-negative bacteria as well as 1 fungus. The green synthesis of Ag-NPs is environmentally friendly and inexpensive, as well as yields highly stabilized nanoparticles by phytochemicals. The substantial results of catalytic reductions and antimicrobial activity reflect the novelty of the prepared Ag-NPs. These nanoparticles entrench the dye and effectively remove the microorganisms from polluted water.

The synthesis of gold nanoclusters with high stability and their application in fluorometric detection for Hg2+ and cell imaging

Sensing Hg²⁺ is significant to protecting human health and environmental ecosystems, for its toxicity and genotoxicity. Here, highly stable fluorescent folic acid (FA)-protected Au nanoclusters (FA-AuNCs) were synthesized by optimizing the reactive parameters with high quantum yield of 34.7%. Main components of Au₄L were confirmed by MALDI-TOF, and the electron-rich residues of FA shell enabled FA-AuNCs excellent photostability. FA-AuNCs exhibited sensitive response behavior to Hg²⁺ with a minimum detectability of 1.3 nM, and presented extreme effect to the detection of Hg²⁺ in real water. Notably, the cellular imaging and in-situ detection of Hg²⁺ in cells can be achieved visually. The high selectivity was attributed to the chemical bond formed between Au⁺ (4f¹⁴5d¹⁰) and Hg²⁺ (4f¹⁴5d¹⁰). And the internal filter effect and static quenching effect were proved triggering the quenching of FA-AuNCs. The ultra-stable FA-AuNCs provide a potential promising opportunity for the in-situ tracing Hg²⁺ from environmental and biological samples.

Cytotoxicity Study of Gold Nanoparticle Synthesis Using Aloe vera, Honey, and Gymnema sylvestre Leaf Extract

Gold nanoparticles (AuNPs) have gained importance in the field of biomedical research and diagnostics due to their unique physicochemical properties. This study aimed to synthesize AuNPs using Aloe vera extract, honey, and Gymnema sylvestre leaf extract. Physicochemical parameters for the optimal synthesis of AuNPs were determined using 0.5, 1, 2, and 3 mM of gold salt at varying temperatures from 20 to 50 °C. X-ray diffraction was used to evaluate the crystal structure of AuNPs, which came out to be a face-centered cubic structure. Scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis confirmed the size and shape of AuNPs between 20 and 50 nm from the Aloe vera, honey, and Gymnema sylvestre, as well as large-sized nanocubes in the case of honey, with 21-34 wt % of gold content. Furthermore, Fourier transform infrared spectroscopy confirmed the presence of a broadband of amine (N-H) and alcohol groups (O-H) on the surface of the synthesized AuNPs that prevents them from agglomeration and provides stability. Broad and weak bands of aliphatic ether (C-O), alkane (C-H), and other functional groups were also found on these AuNPs. DPPH antioxidant activity assay showed a high free radical scavenging potential. The most suited source was selected for further conjugation with three anticancer drugs including 4-hydroxy Tamoxifen, HIF1 alpha inhibitor, and the soluble Guanylyl Cyclase Inhibitor 1 H-[1,2,4] oxadiazolo [4,3-alpha]quinoxalin-1-one (ODQ). Evidence of the pegylated drug conjugation with AuNPs was reinforced by ultraviolet/visible spectroscopy. These drug-conjugated nanoparticles were further checked on MCF7 and MDA-MB-231 cells for their cytotoxicity. These AuNP-conjugated drugs can be a good candidate for breast cancer treatment that will lead toward safe, economical, biocompatible, and targeted drug delivery systems.

Towards site-specific emission enhancement of gold nanoclusters using plasmonic systems: advantages and limitations

Photoluminescent gold nanoclusters are widely seen as a promising candidate for applications in biosensing and bioimaging. Although they have many of the required properties, such as biocompatibility and photostability, the luminescence of near infrared emitting gold nanoclusters is still relatively weak compared to the best available fluorophores. This study contributes to the ongoing debate on the possibilities and limitations of improving the performance of gold nanoclusters by combining them with plasmonic nanostructures. We focus on a detailed description of the emission enhancement and compare it with the excitation enhancement obtained in recent works. We prepared a well-defined series of gold nanoclusters attached to gold nanorods whose plasmonic band is tuned to the emission band of gold nanoclusters. In the resultant single-element hybrid nanostructure, the gold nanorods control the luminescence of gold nanoclusters in terms of its spectral position, polarization and lifetime. We identified a range of parameters which determine the mutual interaction of both particles including the inter-particle distance, plasmon-emission spectral overlap, dimension of gold nanorods and even the specific position of gold nanoclusters attached on their surface. We critically assess the practical and theoretical photoluminescence enhancements achievable using the above strategy. Although the emission enhancement was generally low, the observations and methodology presented in this study can provide a valuable insight into the plasmonic enhancement in general and into the photophysics of gold nanoclusters. We believe that our approach can be largely generalized for other relevant studies on plasmon enhanced luminescence.

A fluorescent aptasensor for ATP based on functional DNAzyme/walker and terminal deoxynucleotidyl transferase-assisted formation of DNA-AgNCs

The development of sensitive adenosine triphosphate (ATP) sensors is imperative due to the tight relationship between the physiological conditions and ATP levels in vivo. Herein, a fluorescent aptasensor for ATP is presented, which adopts a strategy that combines a split aptamer and a DNAzyme/walker with terminal deoxynucleotidyl transferase (TDT)-assisted formation of DNA-AgNCs to realize fluorescence detection of ATP. A multifunctional oligonucleotide sequence is rationally designed, which integrates a split aptamer, a DNAzyme and a DNA walker. Both multifunctional oligonucleotide and its substrate strand are connected to the surface of Fe₃O₄@Au nanoparticles via Au-S bonds. The existence of ATP can induce the formation of the complete aptamer, and then activate the DNAzyme to circularly cleave the substrate strand, leaving 2',3'-cyclophosphate at the 3'end of the strand. This blocks the polymerization of dCTP to form poly(C) even in the presence of TDT and dCTP, due to the lack of free 3'-OH. In contrast, when ATP is absent, the DNAzyme/walker cannot work and then TDT catalyzes the formation of poly(C) at the free 3'-OH of the substrate strand, which is subsequently utilized as the template to prepare DNA-AgNCs. The fluorescence response derived from AgNCs thus reflects the ATP concentration. Under the optimum conditions, the aptasensor shows a linear response range from 5 nM to 10 000 nM, with a detection limit of 0.27 nM. The level of ATP in human serum can be effectively measured by the aptasensor with good recovery, indicating its application potential in medical samples.

An electrochemiluminescence aptasensor based on highly luminescent silver-based MOF and biotin-streptavidin system for mercury ion detection

In this study, for the first time, a silver-based metal-organic framework (Ag-MOF) was synthesized and used as the electrochemiluminescence (ECL) emitter for building an ECL sensor. After modification with chitosan (CS) and gold nanoparticles (Au NPs), the ECL stability of Ag-MOF was improved. To detect mercury ions, a biosensor was constructed using the mercury ion aptamer and steric effect of streptavidin. First, the capture strand (cDNA) with terminal-modified sulfhydryl group was attached to the electrode surface by the Au-S bond. Then, the mercury-ion aptamer (Apt-Hg) modified with biotin was anchored to the electrode by complementary pairing with cDNA. Streptavidin (SA) could be fixed on the electrode by linking with biotin, thereby reducing the ECL signal. However, in the presence of mercury ions, the aptamer was removed and streptavidin could not be immobilized on the electrode. Hence, the ECL signal of the sensor increased with the concentration of mercury ions, which was linear in the range from 1 μM to 300 fM. The detection limit could reach 66 fM (S/N = 3). The sensor provided a new method for the detection of mercury ions.

Monitoring of organophosphorus pesticide residues in plant and vegetable tissues by a novel silver nanocluster probe

More and more attention has been paid to the problem of pesticide residues, especially in plants and vegetables, due to their close relationship with human health and food safety. Compared to conventional detecting techniques, the fluorescence sensing method has achieved good results for pesticides detection. However, most of the reported fluorescent probes needed two or more steps to achieve the detection of pesticide residues, which greatly limited their application for in vivo imaging and in situ analysis of agricultural residues in plants and vegetables. In this paper, an Ag nanoclusters (AgNCs) based fluorescent probe is developed for one-step detection of pesticide residues. The novel nanoprobe displayed impressive advantages, such as a selective response to glyphosate pesticide, rapid response within 1 min and an ultralow detection limit of 21 nM. The sensing mechanism is attributed to the unique coordination interaction between AgNCs and glyphosate, which not only increased the size of AgNCs to form big Ag particles, but also caused the fluorescence quenching of AgNCs system. Due to its favorable properties, the probe has been successfully applied to imaging the organophosphorus pesticide of glyphosate in the leave tissues of Arabidopsis thaliana and the root tip cells of lettuce for the first time.

Recent Progress in Plasmonic based Electrochemiluminescence Biosensors: A Review

Electrochemiluminescence (ECL) analysis has become a powerful tool in recent biomarker detection and clinic diagnosis due to its high sensitivity and broad linear range. To improve the analytical performance of ECL biosensors, various advanced nanomaterials have been introduced to regulate the ECL signal such as graphene, gold nanomaterials, and quantum dots. Among these nanomaterials, some plasmonic nanostructures play important roles in the fabrication of ECL biosensors. The plasmon effect for the ECL signal includes ECL quenching by resonant energy transfer, ECL enhancement by surface plasmon resonance enhancement, and a change in the polarized angle of ECL emission. The influence can be regulated by the distance between ECL emitters and plasmonic materials, and the characteristics of polarization angle-dependent surface plasmon coupling. This paper outlines the recent advances of plasmonic based ECL biosensors involving various plasmonic materials including noble metals and semiconductor nanomaterials. The detection targets in these biosensors range from small molecules, proteins, nucleic acids, and cells thanks to the plasmonic effect. In addition to ECL biosensors, ECL microscopy analysis with plasmonic materials is also highlighted because of the enhanced ECL image quality by the plasmonic effect. Finally, the future opportunities and challenges are discussed if more plasmonic effects are introduced into the ECL realm.

Strategies for Enhancing the Sensitivity of Electrochemiluminescence Biosensors

Electrochemiluminescence (ECL) has received considerable attention as a powerful analytical technique for the sensitive and accurate detection of biological analytes owing to its high sensitivity and selectivity and wide dynamic range. To satisfy the growing demand for ultrasensitive analysis techniques with high efficiency and accuracy in complex real sample matrices, considerable efforts have been dedicated to developing ECL strategies to improve the sensitivity of bioanalysis. As one of the most effective approaches, diverse signal amplification strategies have been integrated with ECL biosensors to achieve desirable analytical performance. This review summarizes the recent advances in ECL biosensing based on various signal amplification strategies, including DNA-assisted amplification strategies, efficient ECL luminophores, surface-enhanced electrochemiluminescence, and ratiometric strategies. Sensitivity-enhancing strategies and bio-related applications are discussed in detail. Moreover, the future trends and challenges of ECL biosensors are discussed.

Dual-Mechanism Quenching of Electrochemiluminescence Immunosensor Based on a Novel ECL Emitter Polyoxomolybdate-Zirconia for 17β-Estradiol Detection

The exploration of novel electrochemiluminescence (ECL) reagents has been a breakthrough work in ECL immunoassay. In this work, the ECL properties of polyoxomolybdate-zirconia (POM-ZrO₂) were discovered for the first time and their luminescence mechanism was initially explored. Virgulate POM-ZrO₂ was synthesized from phosphomolybdic acid hydrate and zirconium oxychloride by solvothermal method, which achieved intense and stabilized cathode ECL emission at a negative potential. Polyaniline@Au nanocrystals (PANI@AuNPs) as the executor of the dual-mechanism quenching strategy were used to reduce the output signal. The quenching efficiency was significantly enhanced by the dual mechanisms of ECL energy transfer and electron transfer. Specifically, PANI@AuNPs can serve as an energy receptor to absorb the energy emitted by POM-ZrO₂ (energy donor), while the appropriate energy level can be regarded as the condition for electron transfer to quench the ECL intensity of POM-ZrO₂. Herein, the proposed dual-mechanism quenching strategy was applied to the immunoassay of 17β-estradiol by constructing a competitive immunosensor. As expected, the immunosensor demonstrated favorable analytical performance and a wide sensing range from 0.01 pg/mL to 200 ng/mL. Hence, it provides a novel method for the sensitive analysis of other biomolecules, such as disease markers and environmental estrogens.

Enhanced cathodic electrochemiluminescent microcystin-LR aptasensor based on surface plasmon resonance of Bi nanoparticles

Sensitive testing for Microcystins-LR (MC-LR) is needed because of its serious environmental and human health hazards. In this work, a new type of surface plasmon resonance (SPR) enhanced cathodic electrochemiluminescence (ECL) aptasensing platform was designed in which boron and nitrogen co-doped graphene quantum dots (BN-GQDs) were used as the luminary and bismuth nanoparticles (Bi NPs) were used as the SPR source. SPR effect of non-precious metal Bi NPs can induce and enhance ECL signal of BN-GQDs because the fluorescence spectrum of BN-GQDs overlaps well with the ultraviolet-visible absorption spectrum of Bi NPs. On this basis, a sensitive sensing system based on the Bi NPs and BN-GQDs was established for MC-LR detection. The results showed that the ECL sensing signal obtained was linear with the negative logarithm of the target MC-LR concentration in the range of 0.01-5000 pM, and the detection limit was 0.003 pM. In addition, the sensor had high stability and good reproducibility, which can be applied to the detection of MC-LR in actual samples. The method had good specificity and can not be disturbed by its homolog, which can be used for sensitive and reliable detection of complex samples.

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.

Gold nanoparticle-based signal amplified electrochemiluminescence for biosensing applications

Since the content levels of biomarkers at the early stage of many diseases are generally lower than the detection threshold concentration, achieving ultrasensitive and accurate detection of these biomarkers is still one of the major goals in bio-analysis. To achieve ultrasensitive and reliable bioassay, it requires developing highly sensitive biosensors. Among all kinds of biosensors, electrogenerated chemiluminescence (ECL) based biosensors have attracted enormous attention due to their excellent properties. In order to improve the performance of ECL biosensors, gold nanoparticles (Au NPs) have been widely utilized as signal amplification tags. The introduction of Au NPs could dramatically enhance the performance of the constructed ECL biosensors via diverse ways such as electrode modification material, efficient energy acceptor in ECL resonant energy transfer (ECL-RET), reaction catalyst, surface plasmon resonance (SPR) enhancer, and as nanocarrier. Herein, we summarize recent developments and progress of ECL biosensors based on Au NPs signal amplification strategies. We will cover ECL applications of Au NPs as a signal amplification tag in the detection of proteins, metal ions, nucleic acids, small molecules, living cells, exosomes, and cell imaging. Finally, brief summary and future outlooks of this field will be presented.

A Novel Fluorescence Aptasensor Based on Magnetic Beads/Gold Nanoparticles/DNA-Stabilized Silver Nanoclusters for Detection of Salmonella Typhimurium

Salmonella Typhimurium (S. Typhimurium) is a globally distributed foodborne pathogen, which can lead to outbreaks of foodborne infectious diseases. It is essential to guarantee food safety by timely and correct detection of S. Typhimurium. In this investigation, an original fluorescence aptasensor was constructed to detect S. Typhimurium rapidly and sensitively. Through the coupling of magnetic beads, aptamer, and gold nanoparticles (AuNPs), a fluorescence quenching system with a "sandwich structure" was established. The aptamer acted as a link, and its specific binding to S. Typhimurium could release AuNPs from the system. Meanwhile, fluorescent DNA-stabilized silver nanoclusters (DNA-AgNCs) were synthesized. The fluorescence intensity changes caused by the fluorescence resonance energy transfer between DNA-AgNCs and AuNPs were utilized to detect S. Typhimurium. The purposed aptasensor exhibited high selectivity and sensitivity with a linear response to S. Typhimurium, ranging from 3.7 × 10² to 3.7 × 10⁵ cfu/mL. The limit of detection (LOD) was estimated to be 98 cfu/mL within 2 h 10 min. In addition, this method showed excellent application for detection of S. Typhimurium in artificially contaminated milk, with LOD reaching 3.4 × 10² cfu/mL. Therefore, the developed fluorescence aptasensor has great potential to identify S. Typhimurium in foodstuffs.

Novel pathway for the sonochemical synthesis of silver nanoparticles with near-spherical shape and high stability in aqueous media

The present study shows the development of a novel sonochemical synthesis pathway of sub-15 nm silver nanoparticles (AgNPs) with quasi-spherical shape and high stability in aqueous suspension. Different analytical techniques such as on-line UV-Vis spectroscopy, Atomic Force Microscopy (AFM), and Transmission Electron Microscopy (TEM) were complementarily used to characterize the evolution of the properties of AgNPs synthesized with this new route. Furthermore, different centrifugation conditions were studied to establish a practical, simple and straightforward purification method. Particle size was determined by TEM employing two different deposition methods, showing that purified AgNPs have a size of 8.1 nm ± 2.4 nm with a narrow dispersion of the size distribution (95% coverage interval from 3.4 to 13 nm). Critical information of the shape and crystalline structure of these sub-15 nm AgNPs, provided by shape descriptors (circularity and roundness) using TEM and high resolution (HR)-TEM measurements, confirmed the generation of AgNPs with quasi-spherical shapes with certain twin-fault particles promoted by the high energy of the ultrasonic treatment. Elemental analysis by TEM-EDS confirmed the high purity of the sub-15 nm AgNPs, consisting solely of Ag. At the optical level, these AgNPs showed a bandgap energy of (2.795 ± 0.002) eV. Finally, the evaluation of the effects of ultraviolet radiation (UVC: 254 nm and UVA: 365 nm) and storage temperature on the spectral stability revealed high stability of the optical properties and subsequently dimensional properties of sub-15 nm AgNPs in the short-term (600 min) and long-term (24 weeks).

A turn-on fluorescence sensor based on Cu2+ modulated DNA-templated silver nanoclusters for glyphosate detection and mechanism investigation

The abuse application of glyphosate can result in a potential hazard for environment and human, however its ultrasensitive detection remains challenging. Herein, a Cu2+ modulated DNA-templated silver nanoclusters (DNA-AgNCs) sensor was constructed to sensitively determine glyphosate based on the turn-on fluorescence strategy. The fluorescence quenching of DNA-AgNCs occurred with the existence of Cu2+. Upon the presence of glyphosate, the functional groups on the surface of glyphosate could chelate with Cu2+, following the fluorescence recovery of DNA-AgNCs. Through the stoichiometric methods, we unveil that Cu2+-trigged fluorescence quenching mode is a combination of static and dynamic quenching with the static mode being predominant. In DNA-AgNCs/Cu2+ system, the carboxylate, amine, and phosphonate groups of glyphosate interact with Cu2+ through chelation, in which the carboxylate oxygen, the phosphonate oxygen atoms, and the monoprotonated secondary amine nitrogen atom and Cu2+ form chelate rings. This fluorescence sensor showed a desired linearity of glyphosate analysis under the optimum conditions, ranging from 15 to 100 μg/L with a low detection down to 5 μg/L. Moreover, the proposed sensor was successfully utilized to measure glyphosate in real samples, indicating a promising application in pesticide residues detection.

Plasmon-Enhanced Electrochemiluminescence of PTP-Decorated Eu MOF-Based Pt-Tipped Au Bimetallic Nanorods for the Lincomycin Assay

Plasmonic bimetal nanostructures can be employed to amplify electrochemiluminescence (ECL) signals. In this work, a high-performance ECL platform was constructed using a europium metal-organic framework (MOF) as a luminophore and Au-Pt bimetallic nanorods (NRs) as a plasma source. Due to the SPR effect of Au-Pt NRs, the aptasensor exhibits 2.6-fold ECL intensity compared to that of pure polyaniline (PANI)-decorated perylene tetracarboxylic dianhydride (PTCA)/Eu MOF. Moreover, decoration with PTP greatly enhances the conductivity and stability of Eu MOF, resulting in sizeable plasmon-enhanced electrochemical luminescence. The as-designed plasmon-enhanced ECL aptasensor displayed highly sensitive detection for lincomycin (Lin). The as-proposed aptasensor could quantify Lin from 0.1 mg/mL to 0.1 ng/mL with a limit of detection (LOD) of 0.026 ng/mL.

Bifunctional monomer oligomers-based composite molecularly imprinted membranes for the electrochemical monitoring of Sudan I

P(L)ICO and PASCO-independent bifunctional monomer oligomers were utilized to construct a molecularly imprinted electrochemical sensor for monitoring Sudan I.

Plasmon-Enhanced Nitrogen Vacancy-Rich Carbon Nitride Electrochemiluminescence Aptasensor for Highly Sensitive Detection of miRNA

The development of biosensors for biologically important substances with ultralow content such as microRNA is of great significance. Herein, a novel surface plasmon-enhanced electrogenerated chemiluminescence-based aptasensor was developed for ultrasensitive sensing of microRNA by using nitrogen vacancy-rich carbon nitride nanosheets as effective luminophores and gold nanoparticles as plasmonic sources. The introduction of nitrogen vacancies improved the electrochemiluminescence behavior due to improved conductance and electrogenerated chemiluminescence activity. The introduction of plasmonic gold nanoparticles increased the electrochemiluminescence signal intensity by more than eightfold. The developed surface plasmon-enhanced electrogenerated chemiluminescence aptasensor exhibited good selectivity, ultrasensitivity, excellent stability, and reproducibility for the determination of microRNA-133a, with a dynamic linear range of 1 aM to 100 pM and a limit of detection about 0.87 aM. Moreover, the surface plasmon-enhanced electrogenerated chemiluminescence sensor obtained a good recovery when detecting the content of microRNA in actual serum.

Surface Plasmon Coupling Electrochemiluminescence Immunosensor Based on Polymer Dots and AuNPs for Ultrasensitive Detection of Pancreatic Cancer Exosomes

Polymer dots (Pdots) have become attractive electrochemiluminescence (ECL) luminophores due to their facile synthesis, easy modification, and stable electrochemical and optical properties. However, their ECL efficiency is not high enough for practical applications. In this work, we proposed an ECL immunosensor based on localized surface plasmon resonance (LSPR) between AuNPs and Pdots for the determination of pancreatic cancer exosomes. Based on the finite-difference time-domain simulations and the band energy of Pdots and AuNPs, we proposed the possible LSPR mechanism. The hot electrons of plasmonic AuNPs were photoexcited to surface plasmon states by ECL emission of Pdots, and then the excited hot electrons were transferred to the conduction band of Pdots, which significantly improved the ECL efficiency of Pdots. The ECL immunosensor displayed a wide calibration range of 1.0 × 10³ to 1.0 × 10⁶ particles/mL with a detection limit of 400 particles/mL. Cancer-related protein profiling revealed high selectivity toward different expressions of exosomal surface proteins from PANC-01, HeLa, MCF-7, and HPDE6-C7 cell lines. The proposed ECL system exhibits a promising prospect for protein biomarker profiling and early cancer-related diagnosis.

Recent advances of functional nucleic acids-based electrochemiluminescent sensing

Electroluminescence (ECL) has been used in extensive applications ranging from bioanalysis to clinical diagnosis owing to its simple device requirement, low background, high sensitivity, and wide dynamic range. Nucleic acid is a significant theme in ECL bioanalysis. The inherent versatile selective molecular recognition of nucleic acids and their programmable self-assembly make it desirable for the robust construction of nanostructures. Benefiting from their unique structures and physiochemical properties, ECL biosensing based on nucleic acids has experienced rapid growth. This review focuses on recent applications of nucleic acids in ECL sensing systems, particularly concerning the employment of nucleic acids as molecular recognition elements, signal amplification units, and sensing interface schemes. In the end, an outlook of nucleic acid-based ECL biosensing will be provided for future developments and directions. We envision that nucleic acids, which act as an essential component for both bioanalysis and clinical diagnosis, will provide a new thinking model and driving force for developing next-generation sensing systems.

Au nanoparticle plasmon-enhanced electrochemiluminescence aptasensor based on the 1D/2D PTCA/CoP for diclofenac assay

The combination of localized surface plasmon resonance (LSPR) and electrochemiluminescence (ECL) can be an effective way to amplify the signal intensity. In this work, an ECL aptasensor with 3,4,9,10-perylenetetracarboxylic acid-decorated cobalt phosphate (denoted as PTCA/CoP) as the ECL emitter and Au nanoparticles (NPs) as plasma was proposed for diclofenac assay. The prepared PTCA/CoP with special 1D/2D structure exhibited good ability and excellent ECL performance. The diclofenac aptamer acted as a bridge to link the PTCA/CoP and Au NPs; thus, the ECL performance of PTCA/CoP was greatly improved due to the plasma effect of Au NPs. Besides, it was found that the ECL signal of the aptasensor was obviously quenched by the introduction of diclofenac, which might be due to the transformation from the LSPR process to the resonance energy transform (RET) process. Under optimal conditions, the difference of ECL intensity was negatively correlated with the concentration of diclofenac in the range 0.1 pM to 10 μM with a low detection limit of 0.072 pM at the potential of -1.8 V vs. Ag/AgCl (S/N = 3). The aptasensor was proved to be suitable for the detection of diclofenac in real samples, suggesting its great practicability.

Recent advances in templated synthesis of metal nanoclusters and their applications in biosensing, bioimaging and theranostics

As a kind of promising nanomaterials, metal nanoclusters (MNCs) generally composed of several to hundreds of metal atoms have received increasing interest owing to their unique properties, such as ultrasmall size (<2 nm), fascinating physical and chemical properties, and so on. Recently, template-assisted synthesis of MNCs (e.g., Au, Ag, Cu, Pt and Cd) has attracted extensive attention in biological fields. Up to now, various templates (e.g., dendrimers, polymers, DNAs, proteins and peptides) with different configurations and spaces have been applied to prepare MNCs with the advantages of facile preparation, controllable size, good water-solubility and biocompatibility. Herein, we focus on the recent advances in the template-assisted synthesis of MNCs, including the templates used to synthesize MNCs, and their applications in biosensing, bioimaging, and disease theranostics. Finally, the challenges and future perspectives of template-assisted synthesized MNCs are highlighted. We believe that this review could not only arouse more interest in MNCs but also promote their further development and applications by presenting the recent advances in this area to researchers from various fields, such as chemistry, material science, physiology, biomedicine, and so on.

Recent Advances in Electrochemiluminescence and Chemiluminescence of Metal Nanoclusters

Metal nanoclusters (NCs), including Au, Ag, Cu, Pt, Ni and alloy NCs, have become more and more popular sensor probes with good solubility, biocompatibility, size-dependent luminescence and catalysis. The development of electrochemiluminescent (ECL) and chemiluminescent (CL) analytical methods based on various metal NCs have become research hotspots. To improve ECL and CL performances, many strategies are proposed, from metal core to ligand, from intermolecular electron transfer to intramolecular electron transfer. Combined with a variety of amplification technology, i.e., nanostructure-based enhancement and biological signal amplification, highly sensitive ECL and CL analytical methods are developed. We have summarized the research progresses since 2016. Also, we discuss the current challenges and perspectives on the development of this area.

Label-Free Analysis of H5N1 Virus Based on Three-Segment Branched DNA-Templated Fluorescent Silver Nanoclusters

Since H5N1 virus is a highly infectious pathogen that causes outbreaks of avian influenza, developing a sensitive and rapid diagnostic platform to sense it becomes significant. Here, a novel label-free fluorescence sensing platform based on DNA-templated silver nanoclusters (DNA-Ag NCs) is developed to detect the H5N1 gene sequence representing H5N1 virus. The three-segment-branched DNA structure with closed cytosine-rich loop is designed as an effective template to produce fluorescent Ag NCs, which is different with the previous design of cytosine-rich loop formed by hairpin-like single-stranded DNA or double-stranded DNA. The proposed fluorescence detection approach gives a wide linear range (500 pM-2 μM) and a low detection limit (500 pM) to sense H5N1 gene sequence. Furthermore, selective analysis of target DNA shows that our constructed analytical strategy has a high selectivity to H5N1 gene sequence. It is regarded as a promising method for highly sensitive and selective sensing of H5N1 virus.

Plasmon-enhanced quantum dots electrochemiluminescence aptasensor for selective and sensitive detection of cardiac troponin I

The development of highly sensitive electrochemiluminescence (ECL) immunosensors by using functional nanoparticles as signal amplifiers is a solution towards sensitive determination of many low concentration disease biomarkers. Herein, a sensitive aptamer-based, sandwich-type surface plasmon enhanced electrochemiluminescence (SPEECL) immunosensor was demonstrated for the detection of cardiac troponin I (cTnI), by means of aptamer conjugated CdS QDs and AuNPs as ECL luminophores and plasmon sources, respectively, in which Tro4 aptamer was used as a capture probe for cTnI and Tro6 aptamer as a detecting probe. The signal of the developed SPEECL system showed ~ 5-fold increment as compared to that of without AuNPs. Using this ECL platform for the detection of cTnI, a linear range and the limit of detection (LOD) were found to be 1 fg/mL - 10 ng/mL and 0.75 fg/mL, respectively.

Current and Perspective Diagnostic Techniques for COVID-19

Since late December 2019, the coronavirus pandemic (COVID-19; previously known as 2019-nCoV) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been surging rapidly around the world. With more than 1,700,000 confirmed cases, the world faces an unprecedented economic, social, and health impact. The early, rapid, sensitive, and accurate diagnosis of viral infection provides rapid responses for public health surveillance, prevention, and control of contagious diffusion. More than 30% of the confirmed cases are asymptomatic, and the high false-negative rate (FNR) of a single assay requires the development of novel diagnostic techniques, combinative approaches, sampling from different locations, and consecutive detection. The recurrence of discharged patients indicates the need for long-term monitoring and tracking. Diagnostic and therapeutic methods are evolving with a deeper understanding of virus pathology and the potential for relapse. In this Review, a comprehensive summary and comparison of different SARS-CoV-2 diagnostic methods are provided for researchers and clinicians to develop appropriate strategies for the timely and effective detection of SARS-CoV-2. The survey of current biosensors and diagnostic devices for viral nucleic acids, proteins, and particles and chest tomography will provide insight into the development of novel perspective techniques for the diagnosis of COVID-19.

Construction of a Novel Biosensor Based on the Self-assembly of Dual-Enzyme Cascade Amplification-Induced Copper Nanoparticles for Ultrasensitive Detection of MicroRNA153

MicroRNAs (miRNAs) have received extensive attention because of their potential as biomarkers for cancer diagnosis and monitoring, and their effective detection is very significant. Here, a specific, one-pot, rapid, femtomolar sensitive miRNAs detection biosensor was developed based on the target-triggered three-way junction (3-WJ) and terminal deoxynucleotide transferase (TDT)/Nt.BspQI in combination with activated copper nanoparticles (CuNPs) self-assembly. To this end, a 3-WJ hairpin probe and helper probe were designed to selectively identify the target miRNA, so as to form a stable 3-WJ structure that further triggered the double-enzyme cycling to produce poly T to activate the self-assembly of CuNPs. Based on the simplicity of CuNPs generation, the poly T template fluorescence CuNPs can detect the minimum detection limit of 1 fm within 1.75 h. In addition, the applicability of this method in complex samples was demonstrated by analyzing the whole-blood RNA extraction from Parkinson patients, consisting of the results of commercial miRNA kits. The developed strategy performs powerful implications for miRNA detection, which may be beneficial for the effective diagnostic assays and biological research of Parkinson's disease.

Electrochemiluminescence Biosensor for Nucleolin Imaging in a Single Tumor Cell Combined with Synergetic Therapy of Tumor

Nucleolin, a nuclear biological multifunctional protein, plays significant roles in modulating the proliferation, survival, and apoptosis of tumor cells. Different from the traditional electrochemiluminescence (ECL) method, a new ECL biosensor was built to perform ECL imaging of nucleolin in a single HeLa cell with high sensitivity and throughput. Briefly, mesoporous silica nanoparticles (MSN) loaded with doxorubicin (DOX) and phorbol 12-myristate 13-acetate (PMA) were used as drug carriers and could be specifically opened by nucleolin in a HeLa cell. PMA then induced the HeLa cell to produce reactive oxygen species (ROS) and realized ECL imaging of nucleolin. After that, ROS could damage DNA and proteins of the tumor cell and DOX could induce the apoptosis of HeLa cells by inhibiting genetic material, nucleic acid, synthesis. HeLa cells were then efficiently killed by DOX and ROS in a synergetic pathway. Herein, a new ECL biosensor for ECL imaging of nucleolin in a single HeLa cell and synergetic tumor therapy was built.

Targeted DNA-driven catalytic assembly light-up ratiometric fluorescence of biemissive silver nanoclusters for amplified biosensing

We report a ratiometric fluorescence strategy using biemissive silver nanoclusters that are harbored in a functional hairpin beacon for rapid, specific and sensitive detection of specific HIV-related DNA as a model.

Label-free probes using DNA-templated silver nanoclusters as versatile reporters

DNA-templated silver nanoclusters (DNA-AgNCs) have demonstrated pervasive applications in analytical chemistry recently. As a way of signal output in DNA-based detection methods, DNA-AgNCs have prominent advantages: first, the recognition and synthesizing sequences are naturally integrated in one DNA probe without any chemical modification or connection; second, the emissive wavelength of DNA-AgNCs can be adjusted in a wide range by employing different sequences; third, DNA-AgNCs can be utilized for producing not only fluorescence, also electrochemiluminescence and electrochemical signals. Besides, they also show potential applications for cell imaging, and are considered to be one of the most ideal nanomaterials for in-vivo imaging due to their ultra-small particle size. In this review, a brief and comprehensive introduction of DNA-AgNCs is firstly given, then label-free probes using DNA-AgNCs are classified and summarized, lastly concluding perspectives are provided on the defects and application potentials.

Distance-dependent plasmon-enhanced electrochemiluminescence biosensor based on MoS2 nanosheets

Nonmetallic plasmonic MoS₂ nanosheets were synthesized by hydrothermal top-down method. MoS₂ nanosheets had shown strong surface plasmon coupling (SPC) light absorption in the visible and near-infrared region. Herein, the nonmetallic plasmonic MoS₂ nanosheets were employed to enhance the electrochemiluminescence (ECL) signal of sulfur doped boron nitrogen QDs (S-BN QDs) in this work. It is important to regulate the distance between ECL luminophore and plasmonic nanoparticles. On one hand, too closed distance can cause energy or electron transfer, which could quench the ECL intensity of nano-luminophore. On the other hand, plasmonic nanostructure cannot significantly affect the luminescence in the far distance. Therefore, we discussed the distance-dependent plasmon-enhanced ECL in detail with different length DNA chains. Furthermore, we constructed a hybridization chain reaction (HCR) amplification ECL sensing mode with the SPC-ECL strategy. The proposed DNA sensor can quantify hepatitis C virus (HCV) gene from 0.5 pmoL/L to 1 nmoL/L with a limit of detection (LOD) of 0.17 pmoL/L.