Promising Anodic Electrochemiluminescence of Nontoxic Core/Shell CuInS2/ZnS Nanocrystals in Aqueous Medium and Its Biosensing Potential

Multifunctional tunable Cu2O and CuInS2 quantum dots on TiO2 nanotubes for efficient chemical oxidation of cholesterol and ibuprofen

In this study, a CuInS2/Cu2O/TiO2 nanotube (TNT) heterojunction-based hybrid material is reported for the selective detection of cholesterol and ibuprofen. Anodic TNTs were co-decorated with Cu2O and CuInS2 quantum dots (QDs) using a modified chemical bath deposition (CBD) method. QDs help trigger the chemical oxidation of cholesterol by cathodically generating hydroxyl radicals (˙OH). The small size of QDs can be used to tune the energy levels of electrode materials to the effective redox potential of redox species, resulting in highly improved sensing characteristics. Under optimal conditions, CuInS2/Cu2O/TNTs show the highest sensitivity (∼12 530 μA mM-1 cm-2, i.e. up to 11-fold increase compared to pristine TNTs) for cholesterol detection with a low detection limit (0.013 μM) and a fast response time (1.3 s). The proposed biosensor was successfully employed for the detection of cholesterol in real blood samples. In addition, fast (4 s) and reliable detection of ibuprofen (with a sensitivity of ∼1293 μA mM-1 cm-2) as a water contaminant was achieved using CuInS2/Cu2O/TNTs. The long-term stability and favourable reproducibility of CuInS2/Cu2O/TNTs illustrate a unique concept for the rational design of a stable and high-performance multi-purpose electrochemical sensor.

Potential-selective electrochemiluminescence of AgInS2/ZnS nanocrystals and its immunoassay application

Potential-selective electrochemiluminescence (ECL) with tunable maximum-emission-potential ranging from 0.95 to 0.30 V is achieved using AgInS2/ZnS nanocrystals, which is promising in the design of multiplexed bioassay on commercialized ECL setups. The model system AgInS2/ZnS/N2H4 exhibits efficient ECL around 0.30 V and can be exploited for sensitive immunoassays with less electrochemical interference and crosstalk.

Wavelength-dependent photoelectrochemical response demonstrated by the determination of acetaminophen and rutin in differential molecularly imprinted polymers strategy

Herein, the excitation wavelength-dependent responses of the molecularly imprinted polymer (MIP) photoelectrochemical (PEC) sensors were investigated, using acetaminophen (AP), rutin (RT) and perfluorooctanoate (PFOA) as the model templates, pyrrole as functional monomer, CuInS2@ZnS/TiO2 NTs as the basic photoelectrode. With wavelength λ > 240 nm, the photocurrent of MIPPFOA enhanced at higher concentrations of PFOA. With increasing AP concentration, the photocurrents of MIPAP could decline with λ < 271 nm, not change at λ = 270 nm, or increase with λ > 270 nm. As RT concentration increased, the photocurrents of MIPRT could decrease (λ < 431 nm), not change (λ = 431 nm) or increase (λ > 431 nm). The PEC responses depend on the comprehensive interaction of two contrary mechanisms from the template molecules within the MIP membrane. The photocurrent is enhanced by the role of the electron donor for photo-generated holes but attenuated due to the steric hindrance effect and the excitation light intensity loss via absorption or scattering. The apparent molar absorption coefficient of AP and RT within MIP membranes are 9.1-19.4 folds of those measured from dilute solutions. By using a routine UV lamp as the light source, the photocurrents of MIPRT at 254 nm and MIPAP at 365 nm were used to determine RT and AP, with the detection limits of 5.3 and 16 nM, respectively. The interference from the non-specific adsorption of interferents on the surfaces of MIPAP and MIPRT was reduced by one order of magnitude via a differential strategy.

A novel differential ratiometric molecularly imprinted electrochemical sensor for determination of sulfadiazine in food samples

Herein, we report a novel differential ratiometric molecularly imprinted polymer (MIP) electrochemical sensor for sulfadiazine (SDZ). An MIP membrane with double templates, SDZ and propyl gallate (PG), was fabricated on glassy carbon electrode (GCE) modified by CuInS2/ZnS nanocomposites. After adding PG in the samples as the reference, the current differences between MIP@CuInS2/ZnS/GCE and non-imprinted polymer@CuInS2/ZnS/GCE at the potentials of 0.18 V (ΔIPG) and 0.92 V (ΔISDZ) were measured. The ratio of ΔISDZ/ΔIPG was used for SDZ determination in the differential and ratiometric dual-mode. The influence of the variations in electrode modification and sample enrichment conditions on the determination of SDZ can be suppressed by 2.8 ∼ 13.2-fold, enhancing the reproducibility and stability of the MIP sensor. The interference level was reduced by one order of magnitude compared with the normal MIP mode. The proposed sensors were used to determine SDZ in food samples, with the detection limit of 2.1 nM.

Review: Synthesis of metal organic framework-based composites for application as immunosensors in food safety

Ensuring food safety continues to be one of the major global challenges. For effective food safety monitoring, fast, sensitive, portable, and efficient food safety detection strategies must be devised. Metal organic frameworks (MOFs) are porous crystalline materials that have attracted attention for use in high-performance sensors for food safety detection owing to their advantages such as high porosity, large specific surface area, adjustable structure, and easy surface functional modification. Immunoassay strategies based on antigen-antibody specific binding are one of the important means for accurate and rapid detection of trace contaminants in food. Emerging MOFs and their composites with excellent properties are being synthesized, providing new ideas for immunoassays. This article summarizes the synthesis strategies of MOFs and MOF-based composites and their applications in the immunoassays of food contaminants. The challenges and prospects of the preparation and immunoassay applications of MOF-based composites are also presented. The findings of this study will contribute to the development and application of novel MOF-based composites with excellent properties and provide insights into advanced and efficient strategies for developing immunoassays.

Integration of CuS/ZnIn2S4 flower-like heterojunctions and (MnCo)Fe2O4 nanozyme for signal amplification and their application to ultrasensitive PEC aptasensing of cancer biomarker

Vascular endothelial growth factor 165 (VEGF₁₆₅) is a crucial regulator of angiogenesis and works as a major protein biomarker of cancer metastasis. Therefore, its quantitative detection is pivotal in clinic. In this work, CuS/ZnIn₂S₄ flower-like heterojunctions had strong and stable photocurrents, which behaved as photoactive material to construct a photoelectrochemical (PEC) aptasensor for detecting VEGF₁₆₅, combined by home-prepared (MnCo)Fe₂O₄ nanozyme-mediated signal amplification. The interfacial photo-induced electron transfer mechanism was chiefly discussed by UV-vis diffuse reflectance spectroscopy in details. Specifically, the (MnCo)Fe₂O₄ modified VEGF₁₆₅ aptamer was released from the PEC aptasensing platform for its highly specific affinity to target VEGF₁₆₅, which terminated the color precipitation reaction, ultimately recovering the PEC signals. The developed sensor displayed a wider linear range from 1 × 10^-2 to 1 × 10⁴ pg mL^-1 with a smaller limit of detection (LOD) of 0.1 fg mL^-1. This study provides some valuable insights for building other ultrasensitive aptasensors for clinical assays of cancer biomarkers in practice.

Multifunctional Photoelectrochemical Biosensor Based on ZnIn2S4/ZnS QDs@Au-Ag-Reversed Photocurrent of Cu-Metal-Organic Framework Coupled with CRISPR/Cas-12a-Shearing for Assay of Dual Targets

False positives and negatives in bioanalytical assays remain a persistent problem. Herein, a multifunctional photoelectrochemical (PEC) biosensor based on ZnIn₂S₄ (ZIS)/ZnS quantum dots (QDs)@Au-Ag-reversed photocurrent of Cu-metal-organic framework (MOF) coupled with CRISPR/Cas-12a-shearing was innovatively developed for assay of dual targets. First, Cu-MOF as a good PEC material shows cathodic photocurrent. Then, numerous ZIS/ZnS QDs were assembled to the Au-Ag nanoparticles (NPs) to prepare a stable and highly amplified signal probe, which can just match the energy level of Cu-MOFs and realized the polarity-reversed photocurrent of Cu-MOF for the first time. As the empty-core nanostructure of Au-Ag NPs has a high specific surface area and low material density, the bimetallic nanocrystal can much increase the reaction rate and improve the redox efficiency. When target CEA-produced cDNA opened the hairpin DNA (HP1 DNA) on the electrode, the ZIS/ZnS QDs@Au-Ag signal probe was conjugated to the electrode via DNA hybridization, achieving a significantly reversed PEC current for CEA detection. Moreover, the specific binding of kanamycin/aptamer generated the acDNA (activator), which can activate the trans-cleavage activity of the CRISPR-CAS12a system on ssDNA, so the signal probe was sheared and caused the obvious decrease of PEC signal for kanamycin detection. The newly developed ZIS/ZnS QDs@Au-Ag NPs displayed excellent PEC properties and reversed photocurrent to MOF and were combined with the unique CRISPR-Cas12a system to achieve sensitive detection of dual targets, which can open a new polarity-reversed PEC sensing platform for rapid and accurate analysis of multiple targets and can effectively avoid false positives results in clinical testing.

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.

Surface Defect-Involved and Single-Color Electrochemiluminescence of Gold Nanoclusters for Immunoassay

Single-color electrochemiluminescence (ECL) of nanoparticles is normally achieved in a bandgap engineered route via passivating the nanoparticle surface. Herein, when linear mercaptoalkanoic acids are employed as the thiol-capping agent of unary Au nanoclusters (NCs), a single-stabilizer-capped strategy is proposed to achieve surface defect-involved and single-color ECL from the AuNCs with hydrazine (N2H4) as the coreactant. The carbon skeleton of the linear mercaptoalkanoic acids exhibits important effects on the ECL of the AuNCs, and efficient oxidative-reductive ECL is achieved with 8-mercaptooctanoic acid (MOA), 11-mercaptoundecanoic acid (MUA), and 12-mercaptododecanoic acid (MDA) capped AuNCs, respectively. The ECL of these AuNCs not only exhibits similar ECL intensity-potential profiles with the same maximum emission potential of ∼1.20 V (vs Ag/AgCl), but also demonstrates almost identical spectral ECL profiles of the same maximum emission wavelength around 713 nm as well as the same fwhm of 64 nm. The ECL of AuNCs/N2H4 is obviously red-shifted to the photoluminescence of AuNCs, which not only provides unambiguous evidence that bandgap-engineered ECL of these AuNCs is quenched but also manifests that the capping agent of linear mercaptoalkanoic acid is promising for the achievement of surface defect-involved and single-color ECL from AuNCs. The MUA capped AuNCs can be utilized as an ECL tag for a sensitive and selective immunoassay, which exhibits a broad linear range from 0.5 mU/mL to 1 U/mL with a low limit of detection of 0.1 mU/mL (S/N = 3) with CA125 as the model analyte. This work provides a promising alternative to the traditional surface-passivating strategy for the achievement of single-color ECL from nanoparticle luminophores.

Coreactant-free and Near-Infrared Electrochemiluminescence Immunoassay with n-Type Au Nanocrystals as Luminophores

The electrochemiluminescence (ECL) bioassay is prominently carried out with the involvement of the coreactant. To remove the detrimental effects of the coreactant on the ECL of luminophores, herein, a promising ECL immunoassay strategy with biocompatible nanoparticles as the luminophore is proposed, which involves directly and electrochemically oxidizing the luminophore methionine-capped Au (Met@Au) nanocrystals (NCs) without the participation of any coreactant. Met@Au NCs are a kind of n-type nanoparticles, and they can be electrochemically injected with valence band (VB) holes around +0.80 and +1.10 V (vs Ag/AgCl). The electrochemically injected exogenous VB hole can recombine with the endogenous conduction band electron of Met@Au NCs and eventually bring out two coreactant-free and near-infrared ECL processes around 0.80 V (ECL-1) and 1.10 V (ECL-2). The intensity of coreactant-free ECL is primarily determined by the electrochemical oxidation-induced hole-injection process. ECL-2 is considerably stronger than ECL-1 and can be exploited for determining the carcinoembryonic antigen (CEA) in a sandwich immunoassay procedure with a linear range from 0.1 to 50 pg/mL as well as a limit of detection of 0.03 pg/mL (S/N = 3).

Low-Triggering-Potential Single-Color Electrochemiluminescence from Bovine Serum Albumin-Stabilized Unary Au Nanocrystals for Immunoassays

Herein, low-triggering-potential (LTP) electrochemiluminescence (ECL) with an onset around 0.0 V (vs Ag/AgCl) is proposed with bovine serum albumin (BSA)-stabilized Au nanocrystals (BSA-AuNCs) as a luminophore and hydrazine hydrate (N₂H₄) as a coreactant. The BSA-AuNCs/N₂H₄ system can exhibit efficient LTP-ECL around 0.37 V with the luminophore of both monodispersed and surface-confined states. The LTP-ECL of BSA-AuNCs/N₂H₄ is a kind of single-color emission with a maximum emission wavelength around 740 nm, which is obviously red-shifted for 80 nm from that of BSA-AuNCs PL, and indicates that the ECL is generated in a surface-defect-involved route instead of the band-gap-engineered route. Importantly, BSA-AuNCs can be utilized as ECL tags to perform sandwich-type immunoassays with acceptable sensitivity and selectivity, which exhibits a wide linear response for determining CA125 from 0.5 to 1000 mU/mL and a limit of detection of 0.05 mU/mL (S/N = 3).

A One-Step Electrochemical Aptasensor Based on Signal Amplification of Metallo Nanoenzyme Particles for Vascular Endothelial Growth Factor

In this study, a one-step electrochemical aptasensor was developed to detect the biomarker vascular endothelial growth factor (VEGF), an important protein in the pathogenesis of many retinal diseases, including age-related macular degeneration, diabetic retinopathy, retinopathy of prematurity, and retinal vein occlusion. The aptamer has a good affinity and can rapidly identify and capture VEGF based on its unique structure. We designed a VEGF aptasensor based on the aptamer recognition and complex metallo nanoenzyme particles as an electron exchange center and bridge between capture DNA and electrode. The aptamers maintained the hairpin structure to avoid nonspecific surface adsorption and expose the capture sequence outwards when the target was inexistent. Conversely, the aptamers opened the hairpin structure to release space to accomplish binding between VEGF and DNA, resulting in increased impedance. The performance of the electrochemical aptasensor is detected by electrochemical impedance spectroscopy (EIS). The limit of detection by EIS was as low as 8.2 pg ml-1, and the linear range was 10 pg ml-1-1 μg ml-1. The electrochemical aptasensor also showed high specificity and reproducibility.

Glow and Flash Adjustable Chemiluminescence with Tunable Waveband from the Same CuInS2@ZnS Nanocrystal Luminophore

All commercial chemiluminescence (CL) assays are conducted with either glow or flash CL of eye-visible waveband from chemical luminophores. Herein, glow and flash, as well as waveband adjustable CL from the same nanoparticle luminophore of thiol-capped CuInS₂@ZnS nanocrystals (CIS@ZnS-Thiol), are proposed via extensively exploiting the differed redox nature of CL triggering reagents. Taking thiosalicylic acid (TSA) as the model thiol-capping agent, the electron-injection-initiated charge transfer between CIS@ZnS-TSA and reductant can bring out efficient glow CL while the hole-injection-initiated charge transfer between CIS@ZnS-TSA and oxidant can give off obvious flash CL under optimum conditions. The maximum emission wavelength for CL of CIS@ZnS-TSA is adjustable from 730 nm to 823 nm via employing different triggering agents. Promisingly, the coexistent reductant of N₂H₄·H₂O and oxidant of H₂O₂ can be employed as dual triggering reagents to trigger eye-visible and highly efficient flash CL from CIS@ZnS-TSA. The maximum emission intensity for flash CL of CIS@ZnS-TSA/N₂H₄-H₂O₂ is 101-fold greater than the glow CL of CIS@ZnS-TSA/N₂H₄ and 22-fold greater than the flash CL of CIS@ZnS-TSA/H₂O₂, respectively. The flash CL from CIS@ZnS-TSA/N₂H₄-H₂O₂ is qualified for highly sensitive and selective CL immunoassay in a commercialized typical procedure with the entire operating process manually terminated within 35 min.

Cysteine Modification of Glutathione-Stabilized Au Nanoclusters to Red-Shift and Enhance the Electrochemiluminescence for Sensitive Bioanalysis

Screening new electrochemiluminescence (ECL) emitters for the design of sensitive detection strategies with even long emission wavelength is intensively anticipated in ECL evolution. Herein, a promising modification strategy for improving the ECL performance of Au nanoclusters (AuNCs) as a water-soluble luminophore was proposed. Upon the introduction of l-cysteine (l-Cys) onto the surface of glutathione (GSH)-stabilized AuNCs (GSH-AuNCs), the dual-thiol bond between l-Cys and GSH was formed to limit the intramolecular motion and nonradiative relaxation of the excited state from the capping agents, which resulted in the enhancement of monochromatic ECL emission of GSH-AuNCs with a red-shifted wavelength. By utilizing triethylamine as a coreactant, the ECL of l-Cys/GSH-AuNCs was about 1.5-fold stronger than that of GSH-AuNCs, and the emission wavelength red-shifted from 660 to 780 nm at a relatively low potential, which could decrease the interference in bioassay and the photochemical damage in nondestructive detection. As a proof of application, a sandwich-type immunosensing method for CYFRA 21-1 was proposed with l-Cys/GSH-AuNCs as the signal tag, which displayed a wide linear ranging from 0.2 fg/mL to 2 ng/mL and a limit of detection down to 0.067 fg/mL at 3S/N. This work provides a wonderful strategy for promoting the performance of ECL emitters.

PEGylation Improved Electrochemiluminescence Supramolecular Assembly of Iridium(III) Complexes in Apoferritin for Immunoassays Using 2D/2D MXene/TiO2 Hybrids as Signal Amplifiers

Dynamic self-assembly of iridium complexes in water-soluble nanocontainers is an important bottom-up process for fabricating electrochemiluminescence (ECL) bioprobes. PEGylated apoferritin (PEG-apoHSF) as the host offers a confined space to alter and modify the self-assembly of trans-bis(2-phenylpyridine)(acetylacetonate)iridium(III) [Ir(ppy)₂(acac)] based on a pH-dependent depolymerization/reassembly pathway, allowing the formation of ECL-active iridium cores in PEG-apoHSF cavities (Ir@PEG-apoHSF). With an improved encapsulation ratio in PEG-apoHSF, the coreactant ECL behavior of the fabricated Ir@PEG-apoHSF nanodots with tri-n-propylamine (TPrA) was further demonstrated, exhibiting maximum ECL emission at 530 nm that was theoretically dominated by the band gap transition. The application of Ir@PEG-apoHSF as a bioprobe in a "signal-on" ECL immunosensing system was developed based on electroactive Ti₃C₂T_x MXenes/TiO₂ nanosheet (Ti₃C₂T_x/TiO₂) hybrids. Combining with the efficiently catalyzed electro-oxidation of TPrA and Ir(ppy)₂(acac) by Ti₃C₂T_x/TiO₂ hybrids, the developed immunosensor showed dramatically amplified ECL responses toward the target analyte of neuron-specific enolase (NSE). Under experimental conditions, linear quantification of NSE from 100 fg/mL to 50 ng/mL was well established by this assay, achieving a limit of detection (LOD) of 35 fg/mL. The results showcased the capability of PEGylated apoHSF to host and stabilize water-insoluble iridium complexes as ECL emitters for aqueous biosensing and immunoassays.

Low-Triggering-Potential Electrochemiluminescence from Surface-Confined CuInS2@ZnS Nanocrystals and their Biosensing Applications

Electrochemiluminescence (ECL) of low triggering potential is strongly anticipated for ECL assays with less inherent electrochemical interference and improved long-term stability of the working electrode. Herein, effects of the thiol capping agents and the states of luminophores, i.e., the thiol-capped CuInS₂@ZnS nanocrystals (CuInS₂@ZnS-Thiol), on the ECL triggering potential of CuInS₂@ZnS-Thiol/N₂H₄·H₂O were explored on the Au working electrode. The thiol capping agent of glutathione (GSH) not only enabled CuInS₂@ZnS-Thiol/N₂H₄·H₂O with the stronger oxidative-reduction ECL than other thiol capping agents but also demonstrated the largest shift for the ECL triggering potential of CuInS₂@ZnS-Thiol/N₂H₄·H₂O upon changing the luminophores from the monodispersed state to the surface-confined state. CuInS₂@ZnS-GSH/N₂H₄·H₂O exhibited an efficient oxidative-reduction ECL around 0.78 V (vs Ag/AgCl) with CuInS₂@ZnS-GSH of the monodispersed state. Upon employing CuInS₂@ZnS-GSH as the ECL tag and immobilizing them onto the Au working electrode, the oxidative-reduction ECL of CuInS₂@ZnS-GSH/N₂H₄·H₂O was lowered to 0.32 V (vs Ag/AgCl), which was about 0.88 V lower than that of traditional Ru(bpy)₃²⁺/TPrA (typically ∼1.2 V, vs Ag/AgCl). The ECL of the CuInS₂@ZnS-GSH/N₂H₄·H₂O system with the luminophore of both monodispersed and surface-confined states was spectrally identical to each other, indicating that this surface-confining strategy exhibited negligible effect on the excited state for the ECL of CuInS₂@ZnS-GSH. A surface-confined ECL sensor around 0.32 V was fabricated with CuInS₂@ZnS-GSH as a luminophore, which could sensitively and selectively determine the K-RAS gene from 1 to 500 pM with a limit of detection at 0.5 pmol L^-1 (S/N = 3).

Ultrasensitive detection of cyclin D1 by a self-enhanced ECL immunosensor based on Bi2S3 quantum dots

Bismuth sulfide quantum dots (Bi₂S₃ QDs), which have excellent optical and thermoelectric properties, represent a green and non-toxic semiconductor material that has been widely used in catalysis and photoelectric conversion devices. At present, research on this material has gradually expanded into the biological field. Herein, the biomineralization method mediated by bovine serum albumin (BSA) was utilized to synthesize Bi₂S₃ QDs with monodispersity, excellent colloidal stability, and good biocompatibility. This is the first study on the electrochemiluminescence (ECL) characteristics of Bi₂S₃ QDs and related ECL mechanisms in detail. In addition, on the basis of Bi₂S₃ QDs, an ECL immunosensor was used for the ultrasensitive measurement of cyclin D1 (CCND1). The composite material, namely Au@Cu-Bi₂S₃ QDs was used as a high-sensitivity ECL probe, in which AuNPs were connected with Bi₂S₃ QDs through a copper(ii) ion bridge. PDA-AgNPs made of dopamine (DA) and silver nanoparticles (AgNPs) were utilized as a carrier for fixing the primary antibody (Ab₁), ultimately presenting a relatively wide detection range of 10 fg mL^-1-1 μg mL^-1. Moreover, quite a low detection limit (6.34 fg mL^-1) was also obtained for an assay of CCND1. Results indicated that the immunosensor can provide a potential platform with fine stability and creditable reproducibility for clinical diagnosis.

Electrochemiluminescence ultrasensitive immunoassay for carbohydrate antigen 125 based on AgInS2/ZnS nanocrystals

We developed a near-infrared (NIR) electrochemiluminescence (ECL) immunosensor for sensitively and selectively determining carbohydrate antigen 125 (CA125) with toxic-element-free and environmental-friendly AgInS₂/ZnS nanocrystals (NCs) as tags. The core/shell-structured AgInS₂/ZnS NCs not only can be conveniently prepared via an aqueous synthetic procedure, but also has high photoluminescence quantum yield (PLQY) of up to 61.7%, highly monodispersed, water-soluble, and desired biological compatibility. As AgInS₂/ZnS NCs can be oxidized via electrochemically injecting holes into their valence band at + 0.84 V, both the monodispersed AgInS₂/ZnS NCs in solution and the surface-confined AgInS₂/ZnS NCs immobilized in sandwich-typed immuno-complexes with CA125 as analyte can exhibit efficient oxidative-reduction ECL around 695 nm under physiological conditions with the presence of tri-n-propylamine (TPrA). The ECL intensity from the AgInS₂/ZnS NCs immobilized in sandwich-typed immuno-complexes increases linearly and selectively with an increased concentration of CA125 from 5 × 10^-6 to 5 × 10^-3 U/mL, and limit of detection (LOD) was 1 × 10^-6 U/mL (S/N = 3). This reliable platform can provide an effective detection method in the early diagnosis and treatment of ovarian cancer.

Enhanced Near-Infrared Electrochemiluminescence from Trinary Ag-In-S to Multinary Ag-Ga-In-S Nanocrystals via Doping-in-Growth and Its Immunosensing Applications

Screening toxic-element-free and biocompatible electrochemiluminophores was crucial for electrochemiluminescence (ECL) evolution. Herein, l-glutathione (GSH)-capped Ag-Ga-In-S (AGIS) nanocrystals (NCs) were prepared by doping Ag-In-S (AIS) NCs in a doping-in-growth way and utilized as a model for both ECL modulating and developing multinary NC-based electrochemiluminophores with enhanced ECL performance than trinary NCs. AGIS NCs not only primarily preserved the morphology, size, phase structure, and water monodisperse characteristics of AIS NCs with broadened band gap but also demonstrated obviously enhanced oxidative-reduction ECL than AIS NCs. Importantly, ECL of AGIS NCs was located at the near-infrared region with a maximum emission wavelength of 744 nm and could be utilized for an ECL immunoassay with human prostate-specific antigen (PSA) as a model, which exhibited a linearity range from 0.05 pg/mL to 1.0 ng/mL and a low limit of detection of 0.01 pg/mL (S/N = 3). This work provided a promising alternative to the traditional binary NCs for developing toxic-element-free and biocompatible electrochemiluminophores with efficient near-infrared ECL.

Formation of a Photoelectrochemical Z-Scheme Structure with Inorganic/Organic Hybrid Materials for Evaluation of Receptor Protein Expression on the Membrane of Cancer Cells

Quantitative analysis of receptor protein expression is essential to give new insights into tumor-related research. Benefitting from their high sensitivity and low background, photoelectrochemical (PEC) platforms are considered as powerful tools for evaluating the expression of receptor proteins. Herein, to reduce the cytotoxicity and facilitate the subsequent assembly, l-cysteine-modified Ag-ZnIn₂S₄ quantum dots (l-Cys AZIS QDs) are prepared and PEC responses under the irradiation of long wavelength light are obtained. To further improve the PEC behavior, iron phthalocyanine (FePc) is employed to form a Z-scheme structure with l-Cys AZIS QDs. The Z-scheme structure based on l-Cys AZIS QDs/FePc hybrid materials exhibits high photo-to-electric conversion efficiency and can be excited with near-infrared range light. Because hyaluronic acid linked to photoactive materials can recognize CD44 expressed on the membrane of cancer cells, cancer cells are immobilized onto l-Cys AZIS QDs/FePc hybrid materials, inducing a decrease of the photocurrent intensity. Consequently, a PEC cytosensor is constructed to quantify cancer cells expressing CD44. The PEC analytical platform is able to determine A549 cells in the range of 2 × 10² to 4.5 × 10⁶ cells/mL, and a detection limit of 15 cells/mL is realized in the case of S/N = 3. In addition, the expression of CD44 in A549 and other five cancer cells is measured with this PEC method. Depending on our data, the expression of CD44 in different cancer cells is distinct, indicating great potential of this method in receptor protein-related studies.

Electrochemically Lighting Up Luminophores at Similar Low Triggering Potentials with Mechanistic Insights

Electrochemiluminescence (ECL) with high electrode compatibility and less electrochemical interference has conventionally been envisioned by lowering the oxidative potential of luminophores and/or screening luminophores with a low oxidative potential. Herein, an alternative was developed by employing the environmental-friendly carbohydrazide as a coreactant, which enabled serial luminophores with oxidative-reduction ECL at one similar low triggering potential around 0.55 V versus Ag/AgCl, including Ru(bpy)₃²⁺ as well as CdTe, CdSe, CuInS₂/ZnS, and Au nanocrystals. Because the eight-electron releasing process of carbohydrazide was electrochemically triggered at ∼0.25 V versus Ag/AgCl, the radicals generated via electrochemical oxidation of carbohydrazide could reduce the luminophores at a much lower potential than those of traditional coreactants. All the luminophore/carbohydrazide systems exhibited one ECL process around 0.55 V, which was about 0.65 V lower than that of a traditional Ru(bpy)₃²⁺/tri-n-propylamine system (typically around +1.2 V), and even lower than the oxidative potential of some luminophores. The ECL of the luminophore/carbohydrazide system was spectrally close to that of the corresponding luminophore/tri-n-propylamine system; the maximum emission wavelength of the low triggering potential ECL could shift from 540 to 783 nm via the selection of luminophores in this case. The coreactant screening strategy would be a favorable addition to the expected luminophore screening strategy for achieving enhanced ECL performance. This work created an avenue toward a deeper understanding of the ECL mechanism.

Shell-Free Copper Indium Sulfide Quantum Dots Induce Toxicity in Vitro and in Vivo

Semiconductor quantum dots (QDs) are attractive fluorescent contrast agents for in vivo imaging due to their superior photophysical properties, but traditional QDs comprise toxic materials such as cadmium or lead. Copper indium sulfide (CuInS₂, CIS) QDs have been posited as a nontoxic and potentially clinically translatable alternative; however, previous in vivo studies utilized particles with a passivating zinc sulfide (ZnS) shell, limiting direct evidence of the biocompatibility of the underlying CIS. For the first time, we assess the biodistribution and toxicity of unshelled CIS and partially zinc-alloyed CISZ QDs in a murine model. We show that bare CIS QDs breakdown quickly, inducing significant toxicity as seen in organ weight, blood chemistry, and histology. CISZ demonstrates significant, but lower, toxicity compared to bare CIS, while our measurements of core/shell CIS/ZnS are consistent with literature reports of general biocompatibility. In vitro cytotoxicity is dose-dependent on the amount of metal released due to particle degradation, linking degradation to toxicity. These results challenge the assumption that removing heavy metals necessarily reduces toxicity: indeed, we find comparable in vitro cytotoxicity between CIS and CdSe QDs, while CIS caused severe toxicity in vivo compared to CdSe. In addition to highlighting the complexity of nanotoxicity and the differences between the in vitro and in vivo outcomes, these unexpected results serve as a reminder of the importance of assessing the biocompatibility of core QDs absent the protective ZnS shell when making specific claims of compositional biocompatibility.

Mechanistic investigations into synergistically enhanced radiative-charge-transfer in Au–Ag bimetallic nanoclusters

Synergetic effects in Au–Ag bimetallic nanoclusters more favorably enhanced electrochemical redox induced electrochemiluminescence over photoexcitation induced photoluminescence.

Synthesis of europium(iii)-doped copper nanoclusters for electrochemiluminescence bioanalysis

We reported an electrochemiluminescence biosensing platform based on europium(iii)-doped copper nanoclusters that exhibited excellent analytical performances of high stability and enhanced intensity.

Promising Mercaptobenzoic Acid-Bridged Charge Transfer for Electrochemiluminescence from CuInS2@ZnS Nanocrystals via Internal Cu+/Cu2+ Couple Cycling

Screening novel electrochemiluminescence (ECL) systems with less inherent interference is strongly anticipated for ECL evolution. Herein, near-infrared ECL (∼730 nm) with an ultralow triggering potential of 0.45 V (vs Ag/AgCl) is achieved under physiological conditions with 4-mercaptobenzoic acid (MBA) and citrate capped CuInS₂@ZnS (CIS@ZnS) nanocrystals (NCs), which is promising for less autofluorescence and electrochemical interference. Cu⁺ species within the CIS@ZnS NCs can be electrochemically oxidized at 0.45 V to form internal Cu²⁺ defects, while the capping agent MBA can bridge a direct charge transfer between the oxidized NCs and the traditional coreactant tripropylamine (TPrA) for weak ECL at 0.45 V. When hydrazine hydrate is adopted as coreactant, CIS@ZnS NCs/hydrazine hydrate exhibits 8k-fold enhanced oxidative-reduction ECL via the internal Cu⁺/Cu²⁺ couple cycling at 0.45 V in comparison to CIS@ZnS NCs/TPrA. This work opens a way to enhance the radiative charge transfer of NCs.

Polyelectrolyte-Mediated Nontoxic AgGa xIn1- xS2 QDs/Low-Density Lipoprotein Nanoprobe for Selective 3D Fluorescence Imaging of Cancer Stem Cells

Cancer stem cells, which are a population of cancer cells sharing common properties with normal stem cells, have strong self-renewal ability and multi-lineage differentiation potential to trigger tumor proliferation, metastases, and recurrence. From this, targeted therapy for cancer stem cells may be one of the most promising strategies for comprehensive treatment of tumors in the future. We design a facile approach to establish the colon cancer stem cells-selective fluorescent probe based on the low-density lipoprotein (LDL) and the novel AgGa _xIn_{(1- x)}S₂ quantum dots (AGIS QDs). The AGIS QDs with a high crystallinity are obtained for the first time via cation-exchange protocol of Ga³⁺ to In³⁺ starting from parent AgInS₂ QDs. Photoluminescence peak of AGIS QDs can be turned from 502 to 719 nm by regulating the reaction conditions, with the highest quantum yield up to 37%. Subsequently, AGIS QDs-conjugated LDL nanocomposites (NCs) are fabricated, in which a cationic polyelectrolyte was used as a coupling reagent to guarantee the electrostatic self-assembly. The structural integrity and physicochemical properties of the LDL-QDs NCs are found to be maintained in vitro, and the NCs exhibit remarkable biocompatibility. The LDL-QDs can be selectively delivered into cancer stem cells that overexpress LDL receptor, and three-dimensional imaging of cancer stem cells is realized. The results of this study not only demonstrate the versatility of nature-derived lipoprotein nanoparticles, but also confirm the feasibility of electrostatic conjugation using cationic polyelectrolyte, allowing reseachers to design nanoarchitectures for targeted diagnosis and treatment of cancer.

Electrochemical-Signal-Amplification Strategy for an Electrochemiluminescence Immunoassay with g-C3N4 as Tags

Signal amplification for electrochemiluminescence (ECL) has conventionally been achieved by employing effective matrixes that can accelerate the electrochemical redox processes or carry more electrochemiluminophores. Herein, a convenient signal-amplification strategy was proposed for an ECL immunoassay with carboxylated g-C₃N₄ nanosheets (NSs) as tags and carcinoembryonic antigen (CEA) as the model target via electrochemically pretreating the substrate: a glassy-carbon electrode (GCE) modified with a polymerized 2-aminoterephthalic acid (ATA) film (GCE/ATA). Bioconjugates of g-C₃N₄ NSs and the signal CEA antibody (Ab₂) (i.e., g-C₃N₄ NS-Ab₂) were immobilized on GCE/ATA via a sandwich immunoreaction to form GCE/ATA-Ab₁-Ag-Ab₂-NSs. Electrochemical-impedance spectroscopy and potential-resolved ECL characterization proved that GCE/ATA plays an important role in the electron-transfer resistance ( R_et) of the GCE/ATA-Ab₁-Ag-Ab₂-NSs for ECL and that successively scanning GCE/ATA-Ab₁-Ag-Ab₂-NSs from 0 to -1.6 V in K₂S₂O₈- and H₂O₂-containing medium could reduce the R_et and bring out 3.3-times-enhanced ECL at the 10th scan cycle compared with that of the 1st scan cycle, which was about 10.2 times the ECL of the GCE/ATA-Ab₁-Ag-Ab₂-NSs in medium containing merely K₂S₂O₈. Inspired by this, direct and successive scanning of GCE/ATA in K₂S₂O₈- and H₂O₂-containing medium was employed during fabrication, which dramatically reduced the R_et of GCE/ATA-Ab₁-Ag-Ab₂-NSs and brought out obviously enhanced ECL responses for selectively determining CEA from 0.1 pg/mL to 1 ng/mL, with a detection limit of 3 fg/mL.