Unary Au Nanocrystal with Prestored Electrons and Intrinsic Low Hole-Injected Potential for Low-Triggering Potential Electrochemiluminescence

Screening a novel electrochemiluminescence (ECL) system and lowering the ECL triggering potential are essential to ECL evolution. Herein, the near-infrared (NIR) ECL system with low-triggering potential ECL was constructed with weakly reductive tert-butylamine borane as coreactant and mercaptosuccinic acid/citrate (MSA/Cit)-capped Au (MSA/Cit@AuNCs) as luminophores. Toxic-element-free and dual-ligand MSA/Cit@AuNCs were prepared via ligand exchange and utilized as a model for developing unary metal NCs-based luminophores with more enhanced ECL performance than monoligand Au nanocrystals (AuNCs), which exhibited a two hole-injected process at around 0.48 and 0.80 V, respectively. Beneficial to the intrinsic low hole-injected potential of AuNCs, MSA/Cit@AuNCs exhibited similar low-triggering ECL potential at around 0.57 V with the participation of series coreactants or not, originating from the recombination of an internal prestored electron within the conduction band (CB) and electroinjected holes at around 0.25 V. Furthermore, the enhanced low-triggering potential around 0.57 V and NIR ECL around 835 nm of MSA/Cit@AuNCs was eventually obtained with the reductive tert-butylamine borane or N2H4·H2O containing a -C-N single-bond structure merely as coreactant. The low-triggering potential ECL of MSA/Cit@AuNCs/tert-butylamine borane system at 0.57 V can be harnessed to selectively determine a carcinoembryonic antigen (CEA) with one linear range spanning from 2 to 20000 fg/mL and a limit of detection of 0.33 fg/mL (S/N = 3). This study will contribute to a more comprehensive understanding of the ECL mechanism in terms of both regulating NCs and selecting coreactants.

Electrochemiluminescence of Semiconductor Quantum Dots and Its Biosensing Applications: A Comprehensive Review

Electrochemiluminescence (ECL) is the chemiluminescence triggered by electrochemical reactions. Due to the unique excitation mode and inherent low background, ECL has been a powerful analytical technique to be widely used in biosensing and imaging. As an emerging ECL luminophore, semiconductor quantum dots (QDs) have apparent advantages over traditional molecular luminophores in terms of luminescence efficiency and signal modulation ability. Therefore, the development of an efficient ECL system with QDs as luminophores is of great significance to improve the sensitivity and detection flux of ECL biosensors. In this review, we give a comprehensive summary of recent advances in ECL using semiconductor QDs as luminophores. The luminescence process and ECL mechanism of semiconductor QDs with various coreactants are discussed first. Specifically, the influence of surface defects on ECL performance of semiconductor QDs is emphasized and several typical ECL enhancement strategies are summarized. Then, the applications of semiconductor QDs in ECL biosensing are overviewed, including immunoassay, nucleic acid analysis and the detection of small molecules. Finally, the challenges and prospects of semiconductor QDs as ECL luminophores in biosensing are featured.

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).

Electrochemiluminescence with semiconductor (nano)materials

Electrochemiluminescence (ECL) is the light production triggered by reactions at the electrode surface. Its intrinsic features based on a dual electrochemical/photophysical nature have made it an attractive and powerful method across diverse fields in applied and fundamental research. Herein, we review the combination of ECL with semiconductor (SC) materials presenting various typical dimensions and structures, which has opened new uses of ECL and offered exciting opportunities for (bio)sensing and imaging. In particular, we highlight this particularly rich domain at the interface between photoelectrochemistry, SC material chemistry and analytical chemistry. After an introduction to the ECL and SC fundamentals, we gather the recent advances with representative examples of new strategies to generate ECL in original configurations. Indeed, bulk SC can be used as electrode materials with unusual ECL properties or light-addressable systems. At the nanoscale, the SC nanocrystals or quantum dots (QDs) constitute excellent bright ECL nano-emitters with tuneable emission wavelengths and remarkable stability. Finally, the challenges and future prospects are discussed for the design of new detection strategies in (bio)analytical chemistry, light-addressable systems, imaging or infrared devices.

Quantum Dots with Highly Efficient, Stable, and Multicolor Electrochemiluminescence

Outstanding photoluminescence (PL) and electroluminescence properties of quantum dots (QDs) promise possibilities for them to meet challenging expectations of electrochemiluminescence (ECL), which at present relies on inefficient and spectral-irresolvable emitters based on transition-metal complexes (such as Ru(bpy)3 2+). However, ECL is reported to be extremely sensitive to the surface traps on the QDs likely because of the spatially and temporally separated electrochemical charge injections. Results here reveal that, by engineering the interior inorganic structure (CdSe/CdS/ZnS core/shell/shell structure) and inorganic-organic interface using new synthetic methods, the trap-insensitive QDs with near-unity PL quantum yield and monoexponential PL decay dynamics in water generated narrow band-edge ECL with efficiencies about six orders of magnitude higher than that of the standard Ru(bpy)3 2+. The band-edge and spectrally resolved ECL from CdSe/CdS/ZnS core/shell/shell QDs demonstrated a new readout scheme using electrochemical potential. Excellent ECL performance of QDs uncovered here offer opportunities to realize the full potential of ECL for biomedical detection and diagnosis.

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.

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.

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.

Tunable Electron-Injection Channels of Heterostructured ZnSe@CdTe Nanocrystals for Surface-Chemistry-Involved Electrochemiluminescence

Tunable charge transfer in and out of nanocrystals (NCs) is crucial to their profound light-emitting applications. Herein a convenient strategy toward tunable electron-injection channels of NCs was achieved by partially coating highly passivated CdTe NCs with unperfected ZnSe shell. Potential- and spectrum-resolved electrochemiluminescence (ECL) characterizations proved that radiative charge recombination for ECL of the heterostructured ZnSe@CdTe NCs only occurred within CdTe core, whereas configurational ions in ECL solution could electrostatically or chemically change the surface states of both the ZnSe shell and the uncoated CdTe core, resulting in tunable electron-injection channels for ECL of ZnSe@CdTe NCs. S^2- anion postponed the electron-injection channel for ECL of ZnSe@CdTe NCs from -1.44 to -1.58 V, Zn²⁺ cation presented two electron-injection channels for ECL at -1.53 and -1.22 V, respectively, whereas Cd²⁺ cation enabled three electron-injection channels for ECL at -1.53, -1.18, and -0.95 V, respectively. The "valve"-like role of configurational ions on the electron-injection channels of ZnSe@CdTe NCs is promising to design novel electrochemiluminophores.

Spectrum-Resolved Triplex-Color Electrochemiluminescence Multiplexing Immunoassay with Highly-Passivated Nanocrystals as Tags

Nanocrystals (NCs) were extensively employed in optical-multiplexing with their size-dependent and narrow waveband photoluminescence (PL). Herein, to achieve NCs-based novel optical-multiplexing strategy with improved sensitivity, a spectrum-resolved triplex-color electrochemiluminescence (ECL) multiplexing immunoassay (MIA) was proposed for the first time by extensively exploring the color-different monochromatic ECL from dual-stabilizers-capped CdTe and CdSe NCs and the inherent lower background of ECL than PL. As a proof of concept, carcinoembryonic antigen (CEA), prostate specific antigen (PSA), and alpha fetoprotein (AFP) were adopted as model analytes and formed three antigen-NCs pairs at the glassy carbon electrode surface via a one-pot immune-reaction with CdSe(550), CdTe(650), and CdTe(776) NCs as tags, respectively. The spectral ECL responses of three antigen-NCs pairs were simultaneously measured via one-pot ECL assay, and the maximum ECL intensity of each tag was directly utilized to determine corresponding antigen without any assistance from either optical filters or signal deconvolution. This ECL-MIA displayed negligible cross-reactivity, high color-selectivity, and excellent sensitivity with limits of detection down to 1, 10, and 0.01 pg/mL for CEA, PSA, and AFP, respectively, which might provide a promising alternative to NCs PL multiplexing.

Signal enhancement and low oxidation potentials for miniaturized ECL biosensors via N-butyldiethanolamine

We present studies on ruthenium-based electrochemiluminescence (ECL) focusing on conditions supporting signal enhancement and low oxidation potentials. Low oxidation potentials (LOPs) are especially attractive for miniaturized ECL biosensors, as microfabricated electrodes tend to detach from their support when used with high currents and operated at high potentials. Furthermore, high potentials or current densities can lead to damage of typical biosensor surface coatings and biological probes. The possibility of generating LOP ECL signals at a potential below 900 mV was therefore studied for Ru(bpy)₃²⁺ with two typical coreactants, i.e. 2-(dibutylamino)ethanol (DBAE) and tripropylamine (TPA), as well as with the tertiary amine N-butyldiethanolamine (NBEA). Furthermore, the effect of buffer components and pH values on ECL signal generation was investigated. We could show a significant LOP ECL signal for NBEA. We found that Tris buffer, with its ability to form complexes with transition metal ions, has a positive influence on this ECL signal in terms of signal strength and LOP capabilities. Specifically, at basic pH values significant increases in ECL signals were observed at 900 mV and at 1.2 V. In fact, the ECL signal at 1.2 V was three times higher than the signal observed in phosphate buffer at a pH of 7, and it was thirty times higher than the ECL signal for TPA under these conditions. The LOP signal for NBEA in Tris buffer at pH 8.5 was similar to the signal obtained for TPA in phosphate buffer at pH 8.5 but three times higher than for TPA at pH 7.0. Interestingly, the coreactant DBAE was neither significantly influenced by the buffer system or pH nor did it present a valuable LOP ECL signal. Finally, it was found that high peak currents in cyclic voltammograms are not the indicators for high ECL signals, which should be obvious because the ECL mechanism requires more complex electron transfers. Overall, the standard TPA ECL at 1.2 V in phosphate buffer at pH 7.0 can successfully be replaced by NBEA ECL at 900 mV in Tris at pH 8.5 providing significantly higher signals accompanied by more gentle electrochemical conditions.

Turn-on near-infrared electrochemiluminescence sensing of thrombin based on resonance energy transfer between CdTe/CdS coresmall/shellthick quantum dots and gold nanorods

Here we designed a near-infrared electrochemiluminescence (NECL) aptasensor for turn-on ultrasensitive determination of thrombin. It was based on the ECL resonance energy transfer (ECL-RET) of CdTe/CdS coresmall/shellthick quantum dots (QDs) to gold nanorods (AuNRs). AuNRs which functioned as ECL acceptors were assembled onto CdTe/CdS film by DNA hybridization between aptamers and their complementary oligonucleotides. In the absence of thrombin, the NECL of QDs was quenched as a result of the ECL-RET of QDs to AuNRs. In the presence of thrombin, the NECL of the system was "turned on" because thrombin can replace the AuNRs onto the QDs film, owing to the specific aptamer-protein affinity interactions. In this way, the increment of ECL intensity and the concentration of thrombin showed a logarithmic linear correlation in the range of 100 aM to 10 fM with a detection limit of 31 aM (S/N=3). Importantly, the developed aptasensor was successfully applied to thrombin sensing in real serum samples.

Anchoring AgBr nanoparticles on nitrogen-doped graphene for enhancement of electrochemiluminescence and radical stability

AgBr nanoparticles anchored nitrogen-doped graphene nanocomposites were designed to obtain enhanced electrochemiluminescence intensity and better stability.

Electrochemiluminescence of blue-luminescent graphene quantum dots and its application in ultrasensitive aptasensor for adenosine triphosphate detection

A simple approach based on exfoliating and disintegrating treatments for graphite oxide, followed by hydrothermal synthesis, was developed to prepare water-soluble graphene quantum dots (GQDs). The as-prepared GQDs exhibited bright blue emission under ultraviolet irradiation (∼365nm), and showed an excitation-independent photoluminescence feature. More importantly, a newly anodic electrochemiluminescence (ECL) was observed from the water-soluble GQDs with H2O2 as coreactant for the first time, and the ECL induced a strong light emission at a low potential (ca. 0.4V vs. Ag/AgCl). The ECL mechanism is investigated in detail. Employing SiO2 nanospheres as signal carrier, a novel SiO2/GQDs ECL signal amplification labels were synthesized based on which a ultrasensitive ECL aptamer sensor was proposed. Under the optimized experimental conditions, the proposed ECL aptamer sensor exhibited excellent analytical performance for adenosine triphosphate (ATP) determination, ranging from 5.0×10(-12) to 5.0×10(-9)molL(-1) with the detection limit of 1.5×10(-12)molL(-1). Due to the low cytotoxicity and excellent biocompatibility, GQDs are demonstrated to be an eco-friendly material as well as excellent ECL labeling agents for biosensor.

A novel aptasensor for the detection of adenosine in cancer cells by electrochemiluminescence of nitrogen doped TiO2 nanotubes

The highly enhanced electrochemiluminescence from a nitrogen doped TiO(2) nanotube array has been reported for the first time, and successfully applied to develop an ultrasensitive ECL aptasensor for the detection of adenosine in cancer cells.