Fluorescent cellulose aerogels containing covalently immobilized (ZnS)x(CuInS2)1−x/ZnS (core/shell) quantum dots

Photoluminiscent (PL) cellulose aerogels of variable shape containing homogeneously dispersed and surface-immobilized alloyed (ZnS)_x(CuInS₂)_1?x/ZnS (core/shell) quantum dots (QD) have been obtained by (1) dissolution of hardwood prehydrolysis kraft pulp in the ionic liquid 1-hexyl-3-methyl-1H-imidazolium chloride, (2) addition of a homogenous dispersion of quantum dots in the same solvent, (3) molding, (4) coagulation of cellulose using ethanol as antisolvent, and (5) scCO₂ drying of the resulting composite aerogels. Both compatibilization with the cellulose solvent and covalent attachment of the quantum dots onto the cellulose surface was achieved through replacement of 1-mercaptododecyl ligands typically used in synthesis of (ZnS)_x(CuInS₂)_1?x/ZnS (core–shell) QDs by 1-mercapto-3-(trimethoxysilyl)-propyl ligands. The obtained cellulose—quantum dot hybrid aerogels have apparent densities of 37.9–57.2 mg cm^?3. Their BET surface areas range from 296 to 686 m² g^?1 comparable with non-luminiscent cellulose aerogels obtained via the NMMO, TBAF/DMSO or Ca(SCN)₂ route. Depending mainly on the ratio of QD core constituents and to a minor extent on the cellulose/QD ratio, the emission wavelength of the novel aerogels can be controlled within a wide range of the visible light spectrum. Whereas higher QD contents lead to bathochromic PL shifts, hypsochromism is observed when increasing the amount of cellulose at constant QD content. Reinforcement of the cellulose aerogels and hence significantly reduced shrinkage during scCO₂ drying is a beneficial side effect when using α-mercapto-ω-(trialkoxysilyl) alkyl ligands for QD capping and covalent QD immobilization onto the cellulose surface.     

Comparative study of ruthenium(II) tris(bipyridine) derivatives for electrochemiluminescence application

Several [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine and its derivatives) complexes were synthesized and compared electrochemically and spectroscopically in the search for better luminophores for electrochemiluminescence (ECL)-based analytical applications. ECL measurement in [Ru(bpy)3]2+/tripropylamine (TPA) aqueous buffer solutions has led to a conclusion that due to the complexity of the ECL generation process, the photoluminescence efficiency cannot be used to predict ECL intensity and there is no obvious relationship between the photoluminescence quantum yield and the ECL intensity. Under the present experimental condition, when compared with the pristine [Ru(bpy)3]2+, the ethoxycarbonyl-substituted derivative, [Ru(bpy-COOEt)3]2+, one of the most efficient luminophores under photoexcitation, did not generate reasonably intense ECL, whereas luminophores with lower photoluminescence quantum yields demonstrated higher ECL. These findings are useful for further efforts in the search for more efficient ECL luminophores.     

Highly efficient quenching of coreactant electrogenerated chemiluminescence by phenolic compounds

We describe the quenching effects of phenolic compounds on the electrogenerated chemiluminescence (ECL) of the Ru(bpy)3(2+) (bpy = 2,2'-bipyridine)/tri-n-propylamine (TPrA) system in aqueous solution. First, the emissions via different ECL routes were examined in the presence of 1,4-benzoquinone. It was found that the interception of the ECL intermediate radicals by the quencher molecules significantly influenced the light emission, especially when the direct coreactant oxidation played a predominant role in producing ECL. The most efficient quenching was observed for the low-oxidation-potential (LOP) ECL at a low concentration of TPrA (<5 mM). The Stern-Volmer constant (K(SV)) of the LOP ECL quenching could be as high as 1.3 x 10(6) M(-1), approximately 700 times larger than that of the photoluminescence quenching. Other phenolic compounds, such as phenol, hydroquinone, catechol, and dopamine, would be oxidized at the potential where the ECL was generated, and the benzoquinone-containing products exhibited ECL quenching effects similar to that of 1,4-benzoquinone. The highly efficient quenching of the LOP ECL by the phenolic compounds may provide a new approach for the determination of these pharmaceutically and environmentally important molecules.     

Fundamentals and bioanalytical applications of functional quantum dots as electrogenerated emitters of chemiluminescence

Since the electrochemiluminescence (ECL) of quantum dots (QDs) of silicon was reported by Science in 2002, lots of QDs (e.g., II-VI, III-V and IV-VI) with different sizes and shapes have been used as ECL emitters for bioanalysis. Especially, QDs functionalized with multitudinous biomolecules offer excellent ECL signal-transduction platforms for designing a new generation of biosensing devices.In this article, we focus on recent advances in the ECL principles of functional QDs, and their bioanalytical applications in DNA analysis, immunoassay, cytosensing and detection of other biological molecules.     

Bio serves nano: biological light-harvesting complex as energy donor for semiconductor quantum dots

Light-harvesting complex (LHCII) of the photosynthetic apparatus in plants is attached to type-II core-shell CdTe/CdSe/ZnS nanocrystals (quantum dots, QD) exhibiting an absorption band at 710 nm and carrying a dihydrolipoic acid coating for water solubility. LHCII stays functional upon binding to the QD surface and enhances the light utilization of the QDs significantly, similar to its light-harvesting function in photosynthesis. Electronic excitation energy transfer of about 50% efficiency is shown by donor (LHCII) fluorescence quenching as well as sensitized acceptor (QD) emission and corroborated by time-resolved fluorescence measurements. The energy transfer efficiency is commensurable with the expected efficiency calculated according to F?rster theory on the basis of the estimated donor-acceptor separation. Light harvesting is particularly efficient in the red spectral domain where QD absorption is relatively low. Excitation over the entire visible spectrum is further improved by complementing the biological pigments in LHCII with a dye attached to the apoprotein; the dye has been chosen to absorb in the "green gap" of the LHCII absorption spectrum and transfers its excitation energy ultimately to QD. This is the first report of a biological light-harvesting complex serving an inorganic semiconductor nanocrystal. Due to the charge separation between the core and the shell in type-II QDs the presented LHCII-QD hybrid complexes are potentially interesting for sensitized charge-transfer and photovoltaic applications.     

Emission of tris(2,2'-bipyridine)ruthenium(II) by coreactant electrogenerated chemiluminescence: from O2-insensitive to highly O2-sensitive

We describe the influence of dissolved oxygen on the emission of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) by electrogenerated chemiluminescence (ECL) with tertiary amine as coreactant in aqueous solutions. The significance of the reactions between molecular oxygen and the ECL intermediate reducing radicals has been demonstrated for the first time. By varying the experimental conditions, the oxygen effect on different ECL routes of the Ru(bpy)3(2+)/tri-n-propylamine (TPrA) system was examined. When coreactant direct oxidation played a predominant role in producing ECL, the maximum emission intensity, especially that of the low-oxidation-potential (LOP) ECL, could change from O2-insensitive to highly O2-sensitive with decreasing TPrA concentration. This behavior can be interpreted as follows: A large excess of intermediate reducing radicals was produced at high [TPrA], and the dissolved oxygen within the ECL reaction layer was completely reduced by these radicals and exerted no quenching effect on the emission. At low [TPrA], however, coreactant oxidation generated a relatively small amount of reducing intermediates, and molecular oxygen acted as an interceptor, destroying the intermediates before they participated in the ECL pathways, which led to the obvious reduction of the emission intensity. In the latter case, the less efficient LOP ECL route was more remarkably affected. When ECL was generated primarily via the catalytic route at high [Ru(bpy)3(2+)], the reactions consuming the intermediate radicals by O2 became insignificant, and he drop of emission intensity in the presence of oxygen could mainly be ascribed to the excited-state quenching. A similar oxygen effect was also observed for the Ru(bpy)3(2+)/triethylamine (TEA) system.     

DC-electrochemiluminescence (ECL with a coreactant)—principle and applications in organic chemistry

This review is focused on the so-called DC-electrochemiluminescence (or ECL with coreactant), its fundamentals and use in basic research in organic and coordination chemistry, photo(electro)chemistry and photophysics, where ECL is not still fully explored. This review is also meant to remind the fellow scientists to the substantial contribution of Prof. Fritz Pragst to this field. An appropriate attention is given to the following two types of research problems where ECL plays a crucial role: (1) selective generation of triplet states enabling investigation of their reactions and estimation of their energies; (2) Use of DC-ECL for mechanistic studies in organic electrochemistry, for interception of short-living intermediates and for visualization of electrode processes. The main aim of this paper is not to rewrite already existing reviews in another form, but to complete the whole picture of ECL research and to remind the reader to several fundamental studies, ideas, and papers hoping that they could be inspirative for organic and molecular electrochemists.     

Polymer Electrochemiluminescence Featuring Thermally Activated Delayed Fluorescence

Electrochemiluminescence (ECL) based on conjugated polymers or oligomers is persistently being pursued owing to its huge application scope ranging from ultra-sensitive bioanalysis to ultra-resolution imaging and spectroscopy. Because of the theoretical limit in radiative exciton generation yield (typically ∼25 %) of those polymers or oligomers, the corresponding ECL efficiency is still limited, which hampers its ECL performance and its related applications. Herein, we report ECL based on a thermally activated delayed fluorescence (TADF) polymer scaffold, which is characteristic of all-exciton harvesting in the ECL process, and thus potentially capable of achieving ∼100 % ECL efficiency. These desired properties of the TADF polymer ECL is attributed to a fast and efficient up-conversion process from non-radiative triplet to radiative singlet states under thermal activation, which is absent in conventional fluorescent polymers/oligomers, such as F8BT. In this study, various ECL modes, including annihilation or co-reactant mode using TPrA or S₂O₈²⁻ as co-reactant, are confirmed for our model TADF polymer ECL system, which was different from fluorescent polymer ECL counterpart. Furthermore, solid-state ECL sensing on L-cysteine (an important marker of disease) is also evaluated by using the model TADF polymer. Ultralow detection limit in combination with high sensitivity and good specificity are achieved for this model system, indicative of a high potential of the TADF polymer scaffold for applications in the broad field of ECL.     

Evaluation of electrogenerated chemiluminescence from a neutral Ir(III) complex for quantitative analysis in flowing streams

Though recently Ir(III) complexes have attracted much interest in electrochemiluminescent (ECL) analysis due to their high emission in various wavelengths, there were a few studies reported on its analytical applications. In this study, we evaluate the ECL from (pq)(2)Ir(acac) (pq = 2-phenylquinolate, acac = acetylacetonate) for the use in flow injection analysis. An aqueous solution of the analyte and (pq)(2)Ir(acac) passes through the reaction/observation cell, and then ECL reaction is generated by electrochemical initiation on the analyte and (pq)(2)Ir(acac). Tri-n-propylamine (TPrA) is used as a representative analyte for evaluation. Additionally, a comparison is made of the relative ECL intensities obtained for a variety of analytes including oxalate, amino acids, aliphatic amines, and NADH. The (pq)(2)Ir(acac) produces efficient ECL upon TPrA exhibiting the limit of detection of 5 nM with a linear range of 3 orders of magnitude in concentration whereas 20 nM is observed in the conventional Ru(bpy)(3)(2+) system. It shows particular sensitivity advantages for oxalate, proline, and tartaric acid. The ECL generation upon various analytes proposes direct applicability of (pq)(2)Ir(acac) as a post-column detection tool.     

Pre‐oxidation of Gold Nanoclusters Results in a 66 % Anodic Electrochemiluminescence Yield and Drives Mechanistic Insights

Gold nanoclusters (AuNCs) are attractive electrochemiluminescence (ECL) emitters because of their excellent stability, near IR emission, and biocompatibility. However, their ECL quantum yield is relatively low, and our limited fundamental understanding has hindered rational improvement of this parameter. Herein, we report drastic enhancement of the ECL of ligand‐stabilized AuNCs by on‐electrode pre‐oxidation with triethylamine (TEA) as a co‐reactant. The l ‐methionine‐stabilized AuNCs resulted in a record high ECL yield of 66 %. This strategy was successfully extended to other AuNCs, and it is more effective for ligand shells that allow more effective electron transfer. In addition, excitation of the pre‐oxidized ECL required a lower potential than conventional methods, and no additional instrument was required. This work opens avenues for solving a challenging problem of AuNC‐based ECL probes and enriches fundamental understanding, greatly broadening their potential applications.     

Theory of transient and steady-state ECL generation at double-hemicylinder assemblies using conformal mapping and simulations

Rigorous analytical expressions for steady-state currents and electrogenerated chemiluminescence (ECL) intensities are derived for two-hemicylinder assemblies operating in the ECL generation mode based on a specific conformal mapping transform. This transform is also used for fast and accurate simulations of transient currents and ECL intensities leading to an excellent agreement at long times with the analytical steady state solutions. These results are compared to those relative to two-band assemblies to investigate the effect of hemicylindrical electrode protuberance on ECL efficiencies. This evidence that electrode protuberance results in a drastic increase of the ECL generation and shortening of commutation times when the gap is not too large as compared to the electrode radii.     

Effect of Ionic Strength on the Electrochemiluminescence Generation by Tris(2,2'-bipyridyl)ruthenium(II)/Tri-n-propylamine

Electrochemiluminescence(ECL) is a powerful transduction technique used in biosensing and in vitro diagnosis, while the mechanism of ECL generation is complicated and affected by various factors. Herein the effect of ionic strength on ECL generation by the classical tris(2,2'-bipyridyl)ruthenium(II)[Ru(bpy)₃²⁺]/tri-n-propylamine(TPrA) system was investigated. It is clear that the ECL intensity decreases significantly with the increase of ionic strength, most likely arising from the reduced deprotonation rate of TPrA^+·. We further combined microtube electrode(MTE) with ECL microscopy to unravel the evolution of ECL layer with the variation of ionic strength. At a low concentration of Ru(bpy)₃²⁺, the thickness of ECL layer(TEL) nearly kept unchanged with the ionic strength, indicating the surface-confined ECL generation is dominated by the oxidative-reduction route. While at a high concentration of Ru(bpy)₃²⁺, ECL generation is dominated by the catalytic route and TEL increases remarkably with the increase of ionic strength, because of the extended diffusion length of Ru(bpy)₃³⁺ at a reduced concentration of TPrA^·.     

Pushing the band gap envelope: mid-infrared emitting colloidal PbSe quantum dots

Efficient mid-infrared sources are of considerable general interest for gas analysis, remote sensing, and atmospheric monitoring, but existing technologies are limited. Here, we report the synthesis of the first colloidal QDs having photoluminescence (PL) in the mid-infrared. We show particle-size-tunable mid-infrared emission for large (10-17 nm), but quantum-confined, colloidal PbSe QDs, with efficient, narrow-bandwidth PL at energies as low as 0.30 eV (4.1 mum). Applying two new synthetic routes, we have achieved fine control of QD size and size distribution, allowing us to provide the first systematic correlation of QD size with PL energy for PbSe QDs emitting at wavelengths longer than 2 mum, results which are compared with a literature model. For the entire spectral range reported, we provide measured quantum yields in emission, showing a marked decrease with increasing QD size, for which we include a possible explanation. Finally, we present very promising preliminary results for overcoating PbSe with CdSe, a wider-gap semiconductor. We show PL enhanced by approximately 6-fold for such core/shell samples.     

量子点电化学发光研究进展及展望

电化学发光因具有低背景、高灵敏度的优势已成为当前最先进的体外诊断技术之一.以三联吡啶钌为代表的分子型电化学发光体系虽然实现了商业化应用,但其光学性质已无法满足电化学发光分析的发展需求.量子点作为新一代的理想发光材料在电化学发光领域表现出巨大的应用前景.然而,由于对量子点电化学发光的过程和机理研究尚不充分,目前量子点电化学发光的各项性能均有待提升.本文聚焦于量子点电化学发光领域的关键科学问题,在总结该领域重要研究进展的基础上,指出光谱学、合成化学及电分析化学等多领域学科交叉是未来量子点电化学发光研究的重要发展方向.     

Scanning electrochemical microscopy study of ion annihilation electrogenerated chemiluminescence of rubrene and [Ru(bpy)3]2+

Scanning electrochemical microscopy (SECM) was used for the study of electrogenerated chemiluminescence (ECL) in the radical annihilation mode. The concurrent steady-state generation of radical ions in the microgap formed between a SECM probe and a transparent microsubstrate provides a distance-dependent ECL signal that can provide information about the kinetics, stability, and mechanism of the light emission process. In the present study, the ECL emission from rubrene and [Ru(bpy)(3)](2+) was used to model the system by carrying out experiments with the SECM and light-detecting apparatus inside an inert atmosphere box. We studied the influence of the distance between the two electrodes, d, and the annihilation kinetics on the ECL light emission profiles under steady-state conditions, as well as the ECL profiles when carrying out cyclic voltammetry (CV) at a fixed d. Experimental results are compared to simulated results obtained through commercial finite element method software. The light produced by annihilation of the ions was a function of d; stronger light was observed at smaller d. The distance dependence of the ECL emission allows the construction of light approach curves in a similar fashion as with the tip currents in the feedback mode of SECM. These ECL approach curves provide an additional channel to describe the reaction kinetics that lead to ECL; good agreement was found between the ECL approach curve emission profile and the simulated results for a fast, diffusion-limited second-order annihilation process (k(ann) > 10(7) M(-1) s(-1)). In the CV mode at fixed distance, the ECL emission of rubrene showed two distinct signals at different potentials when fixing the substrate to generate the radical cation and scanning the tip to generate the radical anion. The first signal (pre-emission) corresponded to an emission well before reaching the generation of the radical anion and was more intense on Au than on Pt. The second ECL signal showed the expected steady-state behavior from the second-order annihilation reaction and agreed well with the simulation. A comparison of the emission obtained with rubrene and [Ru(bpy)(3)](2+) to test the direct formation of lower energy triplets directly at the electrode showed that triplets are not the cause of the pre-emission observed. Wavelength selection experiments for the rubrene system showed that the pre-emission ECL signal also appeared slightly red-shifted with respect to the main luminophore emission; a possible explanation for this phenomenon is inverse photoemission, where the injection of highly energetic holes by the oxidized species into the negatively biased tip electrode causes emission of states in the metal that appear at a different wavelength than the singlet emission from the ECL luminophore.     

Quenching of the electrochemiluminescence of tris(2,2'-bipyridine)ruthenium(II) by ferrocene and its potential application to quantitative DNA detection

Efficient and stable quenching of electrochemiluminescence (ECL) of tris(2,2'-bipyridine)ruthenium(II) by oxidizing ferrocene methanol (FcMeOH) at the electrode is reported. Bimolecular energy or electron transfer between Ru(bpy)(3)(2+*) and ferrocenium (Fc(+)), the oxidized species of Fc, along with suppression of radical reactions is suggested as the mechanism for quenching ECL. Fc shows more efficient quenching of ECL compared with the known quenchers phenol and 1,1-dimethyl-4,4'-bipyridine dication (MV(2+)). The ECL quenching rate constant was 5.6 x 10(10) M(-)(1) s(-)(1). Using Fc as a quencher label on a complementary DNA sequence, an intramolecular ECL quenching in hybridized oligonucleotide strands has been realized. With essentially complete quenching efficiency, this system has the potential for application to sequence-specific DNA detection.     

Copolymerization of luminol on screen-printed cells for single-use electrochemiluminescent sensors

An efficient electrochemiluminescent (ECL) single-use sensor for H(2)O(2) is presented based on an electropolymerized film prepared on screen-printed gold electrode (gold SPE). A study of the copolymerization of luminol in the presence of different monomers was carried out. The polymeric films were grown potentiodynamically with a potential interval between -0.2 and 1.0 V in 0.2 M H(2)SO(4) and were characterized by their electrochemical, electrochemiluminescent, and superficial features. The polymer with the most efficient growth and ECL emission was poly(luminol-3,3',5,5'-tetramethylbenzidine) at 1:5 ratio. These prepared SPE cells present good mechanical and photoemissive properties. A semi-logarithmic linearization shows a noticeable four decade-width concentration range with a limit of detection (LOD) of 2.6 × 10(-9) M and a precision of 10.2% (n = 5; as relative standard deviation, RSD) in the medium range level. The described SPE ECL sensors will be useful for the determination of oxidase substrates in ECL single-use biosensors.     

Electrogenerated chemiluminescence for potentiometric sensors

We report here on a generic approach to read out potentiometric sensors with electrogenerated chemiluminescence (ECL). In a first example, a potassium ion-selective electrode acts as the reference electrode and is placed in contact with the sample solution. The working electrode of the three-electrode cell is responsible for ECL generation and placed in a detection solution containing tris(2,2'-bipyridyl)ruthenium(II) [Ru(bpy)(3)(2+)] and the coreactant 2-(dibutylamino)ethanol (DBAE), physically separated from the sample by a bridge. Changes in the sample potassium concentration directly modulate the potential at the working electrode, and hence the ECL output, when a constant-potential pulse is applied between the two electrodes. A linear response of the ECL intensity to the logarithmic potassium concentration between 10 μm and 10 mM was found.     

Amplified quenching of electrochemiluminescence from CdS sensitized TiO2 nanotubes by CdTe-carbon nanotube composite for detection of prostate protein antigen in serum

This work reports an ECL immunoassay method for ultrasensitive detection of prostate protein antigen (PSA), by remarkably efficient energy-transfer induced electrochemiluminescence (ECL) quenching from the CdS nanoparticles (NPs) sensitized TiO(2) nanotube array (CdS-TiO(2) NTs) to the activated CdTe NPs functionalized multi-wall carbon nanotubes (CdTe-MWNTs) composite. The coupling of TiO(2) and CdS NPs results in a cathodic ECL intensity 14.7 times stronger than that of the pure TiO(2) NTs electrode, which could be efficiently quenched by the CdTe-MWNTs. The enhanced mechanism of TiO(2) NTs ECL by CdS NPs was studied in detail by cyclic voltammetry and ECL spectroscopy. The strong absorption of the CdTe-MWNTs in the wavelength range of 400-800 nm renders them highly efficient for ECL quenching labeled on anti-PSA antibody. Based on a sandwich structure, we developed an ECL immunoassay method for the sensitive and selective detection of PSA. The ECL intensity decrement was logarithmically related to the concentration of the PSA in the range of 1.0 fg mL(-1) to 10 pg mL(-1) with a detection limit of 1 fg mL(-1). Human serum samples were then tested using the proposed immunoassay with excellent correlations, suggesting that the proposed immunoassay method is of great promise in clinical screening of cancer biomarkers.