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

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.

Arginine-modified black phosphorus quantum dots with dual excited states for enhanced electrochemiluminescence in bioanalysis

The electrochemiluminescence (ECL) is generally emitted via radiative transition of singlet or triplet excited state (S₁ or T₁). Herein, an ECL mechanism with the transitions of both S₁ and T₁ of black phosphorus quantum dots (BPQDs) is found, and an arginine (Arg) modification strategy is proposed to passivate the surface oxidation defects of BPQDs, which could modulate the excited states for enhancing the ECL efficiency of BPQDs. The Arg modification leads to greater spatial overlap of highest and lowest occupied molecular orbitals, and spectral shift of radiative transitions, and improves the stability of anion radical of BPQDs. To verify the application of the proposed mechanism, it is used to construct a sensitive method for conveniently evaluating the inhibiting efficiency of cyclo-arginine-glycine-aspartic acid-d-tyrosine-lysine to cell surface integrin by using Arg containing peptide modified BPQDs as signal tag. The dual excited states mediated ECL emitters provide a paradigm for adjustable ECL generation and extend the application of ECL analysis.

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.

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).

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.

Coreactant-Free and Direct Electrochemiluminescence from Dual-Stabilizer-Capped InP/ZnS Nanocrystals: A New Route Involving n-Type Luminophore

Electrochemiluminescence (ECL) is conventionally generated in either an annihilation or a coreactant route, and the overwhelming majority of ECL research is conducted in the coreactant route via oxidizing or reducing the coexisting coreactant and luminophore. The coreacant-free ECL generated via merely oxidizing the luminophore would break through the ceiling of coreactant ECL via excluding the detrimental effects of exogenous coreactant and dissolved oxygen. Herein, by exploiting the rich-electron nature of n-type nanocrystals (NCs), coreacant-free ECL is achieved via merely oxidizing 3-mercaptopropionic acid (MPA) and mercaptosuccinic acid (MSA) capped InP/ZnS NCs, i.e., InP/ZnS_MPA-MSA. The electron-rich InP/ZnS_MPA-MSA can be electrochemically injected with holes via two oxidative processes at around +0.75 and +1.37 V (vs Ag/AgCl), respectively, and the exogenous hole can directly combine the conduction band (CB) electron of InP/ZnS_MPA-MSA, resulting in two coreactant-free ECL processes without employing any exogenous coreactant. The deprotonation process for the carboxyl group of the capping agents can provide a negatively charged surface to InP/ZnS_MPA-MSA and enhance the coreactant-free ECL. The hole-injecting process at +1.37 is much stronger than that at +0.75 V and eventually enables an ∼2000-fold enhanced ECL at +1.37 V than that at +0.75 V. The ECL at +1.37 V can be utilized for coreactant-free ECL immunoassay with prostate-specific antigen (PSA) as analyte, which exhibits an acceptable linear response from 5 pg·mL^-1 to 1 ng·mL^-1 with a limit of detection of 0.3 pg·mL^-1. The coreactant-free ECL route would provide an alternative to both annihilation and coreactant routes, simplify the ECL assay procedure and deepening the ECL mechanism investigations.

Graphite-like Carbon Nitride Nanotube for Electrochemiluminescence Featuring High Efficiency, High Stability, and Ultrasensitive Ion Detection Capability

Herein, for the first time, we introduced a novel electrochemiluminescence (ECL) luminophore based on a one-dimensional g-C₃N₄ nanotube using K₂S₂O₈ as the coreactant. The g-C₃N₄ nanotube/K₂S₂O₈ couple displayed very satisfactory ECL performance, i.e., an ECL efficiency (Φ_ECL) of 437% (vs 100% for the Ru(bpy)₃²⁺/K₂S₂O₈ reference) and excellent ECL stability (the relative standard deviation (RSD) = 0.78%). By contrast, Φ_ECL and RSD of the control g-C₃N₄ nanosheet/K₂S₂O₈ couple were merely 196% and 45.34%, respectively. The mechanism study revealed that the g-C₃N₄ nanotube features a large surface area and much lower interfacial impedance in the porous microstructure, which are beneficial for accelerating the charge transfer rate and stabilizing charge/excitons for ECL. Moreover, using the g-C₃N₄ nanotube/K₂S₂O₈ system as a sensing platform, excellent Cu²⁺ detection capability was also achieved. Our work thus triggers a promising g-C₃N₄ nanomaterial system toward ECL application.

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).

Hydrazine Hydrate and Dissolved Oxygen-Triggered Near-Infrared Chemiluminescence from CuInS2@ZnS Nanocrystals for Bioassays

The overwhelming majority of commercially available chemiluminescence (CL) assays are conducted in the eye-visible region. Herein, a near-infrared (NIR) aqueous CL strategy was proposed with CuInS₂@ZnS nanocrystals (CIS@ZnS NCs) as emitters. Hydrazine hydrate (N₂H₄·H₂O) could inject electrons into the conduction band of the CIS@ZnS NCs and simultaneously transformed to the intermediate radical N₂H₃^•. N₂H₃^• reduced dissolved oxygen (O₂) to O₂^-•, while the O₂^-• could inject holes into the valence band of the CIS@ZnS NCs. The recombination of electrons and holes at Cu⁺ defects in CIS@ZnS NCs eventually yielded efficient NIR CL at around 824.1 nm, which is the longest waveband for NCs CL to the best of our knowledge. The NIR CL could be conveniently performed in the neutral aqueous medium (pH 7.0) with a quantum yield of 0.0155 Einstein/mol and was successfully employed for constructing a signal-off CL biosensor with ascorbic acid as the analyte as well as a signal-on CL biosensor for determining ascorbate oxidase, which indicates that this NIR CL system has a promising potential for bioassays in diverse ways.

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.

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.