Fabrication of an all-in-one self-enhanced solid-state electrochemiluminescence sensing platform for the selective detection of spermine

The fabrication of an all-in-one solid-state ECL sensing platform is beneficial not only for expediting the miniaturization of sensing devices, but also, more importantly, for enabling point-of-care applications. In the present work, a self-enhanced solid-state ECL sensing platform is fabricated using newly synthesised silica polyethylene nanoparticles (SiO2-PEI NPs) which generate a co-reactant in situ and easily self-assemble with Ru(bpy)32+ and shows selective and sensitive detection of spermine at physiological pH (7.4). Spermine induces the maximum ECL emission intensity compared to other biogenic amines due to the presence of two secondary amines. A possible ECL reaction mechanism has been proposed based on CV and ECL experiments, DFT calculations, and in situ ECL spectrum analysis. The developed solid-state sensor showed a linear increase in ECL intensity with increasing spermine concentration in the range of 10 nM to 100 nM with an LOD of 12.2 nM. Compared to other biogenic amines in previous works, chemically synthesised SiO2-PEI NPs used in the present study act as an effective label- and enzyme-free sensor, and the new method is observed to be simple and cost-effective, to overcome various limitations of solution-phase ECL and to avoid the usage of any noble metals.

A covalently linked nickel(ii) porphyrin–ruthenium(ii) tris(bipyridyl) dyad for efficient photocatalytic water oxidation

Visible-light-induced oxidation of water to dioxygen, catalyzed by a newly synthesized dyad consisting of a ruthenium tris(bipyridyl), [Ru(bpy)3]2+ as a photosensitizer, and a nickel porphyrin, NiP as a cheaper water oxidation catalyst is reported.

Two in One: A Dinuclear Ru(II) Complex for Deep-Red Light-Emitting Electrochemical Cells and as an Electrochemiluminescence Probe for Organophosphorus Pesticides

The emissive properties of two Ru(II) complexes, [Ru(dmbipy)₂L₁][PF₆]₂ (1) and [Ru₂(dmbipy)₄L₂][PF₆]₄ (2), (where L₁ and L₂ are π-extended phenanthroline-based ligands and dmbipy = 4,4'-dimethyl-2,2'-bipyridine) have been explored for dual applications, namely, deep-red light-emitting electrochemical cells (LECs) and electrochemiluminescence (ECL) sensors for the detection of organophosphorus pesticides (OPs) that include chlorpyrifos (CPS). A simple single-layer deep-red LEC device comprising 2 is reported that outperforms both its mononuclear derivative 1 and all previously reported dinuclear LECs, with a maximum brightness of 524 cd/m², an external quantum efficiency of 0.62%, and a turn-on voltage of 3.2 V. Optoelectronic studies reveal that the ECL response of 2 is improved when compared to its mononuclear counterpart 1 and benchmark [Ru(bipy)₃]²⁺ (3). Modified glassy carbon electrodes coated with 2 are highly sensitive deep-red ECL sensors that facilitate the detection of CPS directly from river water and fruit samples without any complex pretreatment steps, operating over a broad logarithmic concentration range, with a low detection limit.

Emission from the working and counter electrodes under co-reactant electrochemiluminescence conditions

We present a new approach to explore the potential-dependent multi-colour co-reactant electrochemiluminescence (ECL) from multiple luminophores. The potentials at both the working and counter electrodes, the current between these electrodes, and the emission over cyclic voltammetric scans were simultaneously measured for the ECL reaction of Ir(ppy)₃ and either [Ru(bpy)₃]²⁺ or [Ir(df-ppy)₂(ptb)]⁺, with tri-n-propylamine as the co-reactant. The counter electrode potential was monitored by adding a differential electrometer module to the potentiostat. Plotting the data against the applied working electrode potential and against time provided complementary depictions of their relationships. Photographs of the ECL at the surface of the two electrodes were taken to confirm the source of the emissions. This provided a new understanding of these multifaceted ECL systems, including the nature of the counter electrode potential and the possibility of eliciting ECL at this electrode, a mechanism-based rationalisation of the interactions of different metal-complex luminophores, and a previously unknown ECL pathway for the Ir(ppy)₃ complex at negative potentials that was observed even in the absence of the co-reactant.

Exploration of potential-dependent, multi-colour co-reactant electrochemiluminescence from multiple luminophores at the working and counter electrodes reveals new pathways to emission.

Photophysics and Cell Uptake of Self-Assembled Ru(II)Polypyridyl Vesicles

Effective delivery of luminescent probes for cell imaging requires both cell membrane permeation and directing to discrete target organelles. Combined, these requirements can present a significant challenge for metal complex luminophores, that have excellent properties as imaging probes but typically show poor membrane permeability. Here, we report on highly luminescent Ruthenium polypyridyl complexes based on the parent; [Ru(dpp)2(x-ATAP)](PF6)2 structure, where dpp is 4,7-diphenyl-1,10-phenanthroline and x-ATAP is 5-amino-1,10-phenanthroline with pendant alkyl-acetylthio chains of varying length; where x is 6; 5-Amido-1,10-phenanthroline-(6-acetylthio-hexanyl). 8; 5-Amido-1,10-phenanthroline-(8-acetylthio-octanyl). 11; 5-Amido-1,10-phenanthroline-(11-acetylthio-undecanyl); and 16; 5-Amido-1,10-phenanthroline-(16-acetylthio-hexadecanyl). Soluble in organic media, the alkyl-acetylthiolated complexes form nanoaggregates of low polydispersity in aqueous solution. From dynamic light scattering the nanoaggregate diameter was measured as 189 nm and 135 nm for 5 × 10-6 M aqueous solutions of [Ru(dpp)2(N∧N)](PF6)2 with the hexadecanoyl and hexanyl tails respectivly. The nanoaggregate exhibited dual exponential emission decays with kinetics that matched closely those of the [Ru(dpp)2(16-ATAP)]2+ incorporated into the membrane of a DPPC liposome. Cell permeability and distribution of [Ru(dpp)2(11-ATAP)]2+ or [Ru(dpp)2(16-ATAP)]2+ were evaluated in detail in live HeLa and CHO cell lines and it was found from aqueous media, that the nanoaggregate complexes spontaneously cross the membrane of mammalian cells. This process seems, on the basis of temperature dependent studies to be activated. Fluorescence imaging of live cells reveal that the complexes localize highly specifically within organelles and that organelle localization changes dramatically in switching the pendent alkyl chains from C16 to C11 as well as on cell line identity. Our data suggests that building metal complexes capable of self-assembling into nano-dimensional vesicles in this way may be a useful means of promoting cell membrane permeability and driving selective targeting that is facile and relatively low cost compared to use of biomolecular vectors.

Near-Infrared Electrochemiluminescence from Bistridentate Ruthenium(II) Di(quinoline-8-yl)pyridine Complexes in Aqueous Media

We report the synthesis, photophysics, electrochemistry and electrochemiluminescence (ECL) of two dqp (dqp=2,6-di(quinoline-8-yl)pyridine) based ruthenium(II) complexes, bearing either a n-butyl ester (1) or the corresponding carboxylic acid functionality (2). The complexes were prepared from [Ru(dqp)(MeCN)₃ ][PF₆ ]₂ by reaction with the dqp precursor using microwave irradiation. In both cases, photoluminescence spectra present strong ³ MLCT-based red/near-infrared (NIR) emissions centred at about 710 nm. The photoluminescence quantum yields were 6.1 % and 1.8 % for 1 and 2 respectively while the excited state lifetimes were 3.60 μs and 2.37 μs. Both complexes are ECL active, although ECL efficiency (Φ_ECL ) of 1 was substantially higher than 2, due to its more favourable electrochemical properties. Importantly, 1 also gave strong ECL in aqueous media, which is rare for near-infrared emitters. The results suggest the possibility of very interesting ECL sensing applications for this class of emitter in biological media.

A conceptual framework for the development of iridium(iii) complex-based electrogenerated chemiluminescence labels

Translation of the highly promising electrogenerated chemiluminescence (ECL) properties of Ir(iii) complexes (with tri-n-propylamine (TPrA) as a co-reactant) into a new generation of ECL labels for ligand binding assays necessitates the introduction of functionality suitable for bioconjugation. Modification of the ligands, however, can affect not only the photophysical and electrochemical properties of the complex, but also the reaction pathways available to generate light. Through a combined theoretical and experimental study, we reveal the limitations of conventional approaches to the design of electrochemiluminophores and introduce a new class of ECL label, [Ir(C^N)2(pt-TOxT-Sq)]+ (where C^N is a range of possible cyclometalating ligands, and pt-TOxT-Sq is a pyridyltriazole ligand with trioxatridecane chain and squarate amide ethyl ester), which outperformed commercial Ir(iii) complex labels in two commonly used assay formats. Predicted limits on the redox potentials and emission wavelengths of Ir(iii) complexes capable of generating ECL via the dominant pathway applicable in microbead supported ECL assays were experimentally verified by measuring the ECL intensities of the parent luminophores at different applied potentials, and comparing the ECL responses for the corresponding labels under assay conditions. This study provides a framework to tailor ECL labels for specific assay conditions and a fundamental understanding of the ECL pathways that will underpin exploration of new luminophores and co-reactants.

Tuning the electrochemiluminescent properties of iridium complexes of N-heterocyclic carbene ligands

A series of five heteroleptic Ir(iii) complexes of the general form Ir(dfppy)2(C^C) have been prepared (where dfppy represents 2-(2,4-difluorophenyl)pyridine and C^C represents a bidentate cyclometalated phenyl substituted imidazolylidene ligand). The cyclometalated phenyl ring of the imidazolylidene ligand was either unsubstituted or substituted with electron donating (OMe and Me) or electron withdrawing (Cl and F) groups in the 2 and 4 positions. The synthesised Ir(iii) complexes have been characterised by elemental analysis, NMR spectroscopy, cyclic voltammetry and electronic absorption and emission spectroscopy. The molecular structures for four Ir(iii) complexes were determined by single crystal X-ray diffraction. Each of the Ir(iii) complexes exhibited intense photoluminescence in acetonitrile solution at room temperature with quantum yields (ΦPL) ranging from 58% to 86%. Cyclic voltammetry experiments revealed one oxidation process (formally ascribed to the metal centre), and two ligand-based reductions for each complex. Complexes 1-5 gave moderate to intense annihilation and co-reactant electrochemiluminescence (ECL). Consideration of the electrochemical, spectroscopic and theoretical investigations provide insights into the electrochemiluminescence behaviour.

Mixed annihilation electrogenerated chemiluminescence of iridium(iii) complexes

Previously reported annihilation ECL of mixtures of metal complexes have generally comprised Ir(ppy)3 or a close analogue as a higher energy donor/emitter (green/blue light) and [Ru(bpy)3]2+ or its derivative as a lower energy acceptor/emitter (red light). In contrast, here we examine Ir(ppy)3 as the lower energy acceptor/emitter, by combining it with a second Ir(iii) complex: [Ir(df-ppy)2(ptb)]+ (where ptb = 1-benzyl-1,2,3-triazol-4-ylpyridine). The application of potentials sufficient to attain the first single-electron oxidation and reduction products can be exploited to detect Ir(ppy)3 at orders of magnitude lower concentration, or enhance its maximum emission intensity at high concentration far beyond that achievable through conventional annihilation ECL of Ir(ppy)3 involving comproportionation. Moreover, under certain conditions, the colour of the emission can be selected through the applied electrochemical potentials. We have also prepared a novel Ir(iii) complex with a sufficiently low reduction potential that the reaction between its reduced form and Ir(ppy)3+ cannot populate the excited state of either luminophore. This enabled, for the first time, the exclusive formation of either excited state through the application of higher cathodic or anodic potentials, but in both cases, the ECL was greatly diminished by parasitic dark reactions.

New perspectives on the annihilation electrogenerated chemiluminescence of mixed metal complexes in solution

Preliminary explorations of the annihilation electrogenerated chemiluminescence (ECL) of mixed metal complexes have revealed opportunities to enhance emission intensities and control the relative intensities from multiple luminophores through the applied potentials. However, the mechanisms of these systems are only poorly understood. Herein, we present a comprehensive characterisation of the annihilation ECL of mixtures of tris(2,2'-bipyridine)ruthenium(ii) hexafluorophosphate ([Ru(bpy)3](PF6)2) and fac-tris(2-phenylpyridine)iridium(iii) ([Ir(ppy)3]). This includes a detailed investigation of the change in emission intensity from each luminophore as a function of both the applied electrochemical potentials and the relative concentrations of the two complexes, and a direct comparison with two mixed (Ru/Ir) ECL systems for which emission from only the ruthenium-complex was previously reported. Concomitant emission from both luminophores was observed in all three systems, but only when: (1) the applied potentials were sufficient to generate the intermediates required to form the electronically excited state of both complexes; and (2) the concentration of the iridium complex (relative to the ruthenium complex) was sufficient to overcome quenching processes. Both enhancement and quenching of the ECL of the ruthenium complex was observed, depending on the experimental conditions. The observations were rationalised through several complementary mechanisms, including resonance energy transfer and various energetically favourable electron-transfer pathways.

Mobile phone based electrochemiluminescence detection in paper-based microfluidic sensors

The development of simple, inexpensive paper-based sensors for medical diagnostics and other applications is now an important emerging area in the field of biosensors; however, the electronic instrument or reader used to interrogate such sensors adds significantly to the cost of the analysis. In this chapter we describe the design and construction of novel, low-cost disposable electrochemiluminescent (ECL) sensors based on screen printed carbon electrodes and paper-based microfluidics. Moreover, a method to interrogate these sensors using only a mobile phone is articulated. This is realized by exploiting the audio output of the device to achieve electrochemical control, while using the camera to detect the resulting light emitted during the ECL reaction. The combination of cell phone technology with low-cost paper microfluidic sensors dramatically reduces the cost of sensing and has the potential to enhance health-care outcomes by exploiting the functionality, connectivity, and close to worldwide penetration of mobile phone technology.

Chemiluminescence detection with water-soluble iridium(III) complexes containing a sulfonate-functionalised ancillary ligand

The chemiluminescence from four cyclometalated iridium(III) complexes containing an ancillary bathophenanthroline-disulfonate ligand exhibited a wide range of emission colours (green to red), and in some cases intensities that are far greater than the commonly employed benchmark reagent, [Ru(bpy)3](2+). A similar complex incorporating a sulfonated triazolylpyridine-based ligand enabled the emission to be shifted into the blue region of the spectrum, but the responses with this complex were relatively poor. DFT calculations of electronic structure and emission spectra support the experimental findings.

Iridium(iii) N-heterocyclic carbene complexes: an experimental and theoretical study of structural, spectroscopic, electrochemical and electrogenerated chemiluminescence properties

Theoretical and experimental studies of a series of iridium N-heterocyclic carbene complexes.

Annihilation electrogenerated chemiluminescence of mixed metal chelates in solution: modulating emission colour by manipulating the energetics

We demonstrate the mixed annihilation electrogenerated chemiluminescence of tris(2,2'-bipyridine)ruthenium(ii) with various cyclometalated iridium(iii) chelates. Compared to mixed ECL systems comprising organic luminophores, the absence of T-route pathways enables effective predictions of the observed ECL based on simple estimations of the exergonicity of the reactions leading to excited state production. Moreover, the multiple, closely spaced reductions and oxidations of the metal chelates provide the ability to finely tune the energetics and therefore the observed emission colour. Distinct emissions from multiple luminophores in the same solution are observed in numerous systems. The relative intensity of these emissions and the overall emission colour are dependent on the particular oxidized and reduced species selected by the applied electrochemical potentials. Finally, these studies offer insights into the importance of electronic factors in the question of whether the reduced or oxidized partner becomes excited in annihilation ECL.

Labeling phospholipid membranes with lipid mimetic luminescent metal complexes

Lipid-mimetic metallosurfactant based luminophores are promising candidates for labeling phospholipid membranes without altering their biophysical characteristics. The metallosurfactants studied exhibit high structural and physicochemical similarity to phospholipid molecules, designed to incorporate into the membrane structure without the need for covalent attachment to a lipid molecule. In this work, two lipid-mimetic phosphorescent metal complexes are described: [Ru(bpy)2(dn-bpy)](2+) and [Ir(ppy)2(dn-bpy)](+) where bpy is 2,2'-bipyridine, dn-bpy is 4,4'-dinonyl-2,2'-bipyridine and ppy is 2-phenylpyridine. Apart from being lipid-mimetic in size, shape and physical properties, both complexes exhibit intense photoluminescence and enhanced photostability compared with conventional organic fluorophores, allowing for prolonged observation. Moreover, the large Stokes shift and long luminescence lifetime associated with these complexes make them more suitable for spectroscopic studies. The complexes are easily incorporated into dimyristoil-phosphatidyl-choline (DMPC) liposomes by mixing in the organic solvent phase. DLS reveals the labeled membranes form liposomes of similar size to that of neat DMPC membrane. Synchrotron Small-Angle X-ray Scattering (SAXS) measurements confirmed that up to 5% of either complex could be incorporated into DMPC membranes without producing any structural changes in the membrane. Fluorescence microscopy reveals that 0.5% label content is sufficient for imaging. Atomic Force Microscopic imaging confirms that liposomes of the labeled bilayers on a mica surface can fuse into a flat lamellar membrane that is morphologically identical to neat lipid membranes. These results demonstrate the potential of such lipid-mimetic luminescent metal complexes as a new class of labels for imaging lipid membranes.

Red-green-blue electrogenerated chemiluminescence utilizing a digital camera as detector

Exploiting the distinct excitation and emission properties of concomitant electrochemiluminophores in conjunction with the inherent color selectivity of a conventional digital camera, we create a new strategy for multiplexed electrogenerated chemiluminescence detection, suitable for the development of low-cost, portable clinical diagnostic devices. Red, green and blue emitters can be efficiently resolved over the three-dimensional space of ECL intensity versus applied potential and emission wavelength. As the relative contribution ratio of each emitter to the photographic RGB channels is constant, the RGB ECL intensity versus applied-potential curves could be effectively isolated to a single emitter at each potential.

Electrochemiluminescent monomers for solid support syntheses of Ru(II)-PNA bioconjugates: multimodal biosensing tools with enhanced duplex stability

The feasibility of devising a solid support mediated approach to multimodal Ru(II)-peptide nucleic acid (PNA) oligomers is explored. Three Ru(II)-PNA-like monomers, [Ru(bpy)(2)(Cpp-L-PNA-OH)](2+) (M1), [Ru(phen)(2)(Cpp-L-PNA-OH)](2+) (M2), and [Ru(dppz)(2)(Cpp-L-PNA-OH)](2+) (M3) (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, dppz = dipyrido[3,2-a:2',3'-c]phenazine, Cpp-L-PNA-OH = [2-(N-9-fluorenylmethoxycarbonyl)aminoethyl]-N-[6-(2-(pyridin-2yl)pyrimidine-4-carboxamido)hexanoyl]-glycine), have been synthesized as building blocks for Ru(II)-PNA oligomers and characterized by IR and (1)H NMR spectroscopy, mass spectrometry, electrochemistry and elemental analysis. As a proof of principle, M1 was incorporated on the solid phase within the PNA sequences H-g-c-a-a-t-a-a-a-a-Lys-NH(2) (PNA1) and H-P-K-K-K-R-K-V-g-c-a-a-t-a-a-a-a-lys-NH(2) (PNA4) to give PNA2 (H-g-c-a-a-t-a-a-a-a-M1-lys-NH(2)) and PNA3 (H-P-K-K-K-R-K-V-g-c-a-a-t-a-a-a-a-M1-lys-NH(2)), respectively. The two Ru(II)-PNA oligomers, PNA2 and PNA3, displayed a metal to ligand charge transfer (MLCT) transition band centered around 445 nm and an emission maximum at about 680 nm following 450 nm excitation in aqueous solutions (10 mM PBS, pH 7.4). The absorption and emission response of the duplexes formed with the cDNA strand (DNA: 5'-T-T-T-T-T-T-T-A-T-T-G-C-T-T-T-3') showed no major variations, suggesting that the electronic properties of the Ru(II) complexes are largely unaffected by hybridization. The thermal stability of the PNA·DNA duplexes, as evaluated from UV melting experiments, is enhanced compared to the corresponding nonmetalated duplexes. The melting temperature (T(m)) was almost 8 °C higher for PNA2·DNA duplex, and 4 °C for PNA3·DNA duplex, with the stabilization attributed to the electrostatic interaction between the cationic residues (Ru(II) unit and positively charged lysine/arginine) and the polyanionic DNA backbone. In presence of tripropylamine (TPA) as co-reactant, PNA2, PNA3, PNA2·DNA and PNA3·DNA displayed strong electrochemiluminescence (ECL) signals even at submicromolar concentrations. Importantly, the combination of spectrochemical, thermal and ECL properties possessed by the Ru(II)-PNA sequences offer an elegant approach for the design of highly sensitive multimodal biosensing tools.

Electrochemiluminescent peptide nucleic acid-like monomers containing Ru(II)-dipyridoquinoxaline and Ru(II)-dipyridophenazine complexes

A series of Ru(II)-peptide nucleic acid (PNA)-like monomers, [Ru(bpy)(2)(dpq-L-PNA-OH)](2+) (M1), [Ru(phen)(2)(dpq-L-PNA-OH)](2+) (M2), [Ru(bpy)(2)(dppz-L-PNA-OH)](2+) (M3), and [Ru(phen)(2)(dppz-L-PNA-OH)](2+) (M4) (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, dpq-L-PNA-OH = 2-(N-(2-(((9H-fluoren-9-yl)methoxy)carbonylamino)ethyl)-6-(dipyrido[3,2-a:2',3'-c]phenazine-11-carboxamido)hexanamido)acetic acid, dppz-L-PNA-OH = 2-(N-(2-(((9H-fluoren-9-yl) methoxy)carbonylamino)ethyl)-6-(dipyrido[3,2-f:2',3'-h]quinoxaline-2-carboxamido)acetic acid) have been synthesized and characterized by IR and (1)H NMR spectroscopy, mass spectrometry, and elemental analysis. As is typical for Ru(II)-tris(diimine) complexes, acetonitrile solutions of these complexes (M1-M4) show MLCT transitions in the 443-455 nm region and emission maxima at 618, 613, 658, and 660 nm, respectively, upon photoexcitation at 450 nm. Changes in the ligand environment around the Ru(II) center are reflected in the luminescence and electrochemical response obtained from these monomers. The emission intensity and quantum yield for M1 and M2 were found to be higher than for M3 and M4. Electrochemical studies in acetonitrile show the Ru(II)-PNA monomers to undergo a one-electron redox process associated with Ru(II) to Ru(III) oxidation. A positive shift was observed in the reversible redox potentials for M1-M4 (962, 951, 936, and 938 mV, respectively, vs Fc(0/+) (Fc = ferrocene)) in comparison with [Ru(bpy)(3)](2+) (888 mV vs Fc(0/+)). The ability of the Ru(II)-PNA monomers to generate electrochemiluminescence (ECL) was assessed in acetonitrile solutions containing tripropylamine (TPA) as a coreactant. Intense ECL signals were observed with emission maxima for M1-M4 at 622, 616, 673, and 675 nm, respectively. At an applied potential sufficiently positive to oxidize the ruthenium center, the integrated intensity for ECL from the PNA monomers was found to vary in the order M1 (62%) > M3 (60%) > M4 (46%) > M2 (44%) with respect to [Ru(bpy)(3)](2+) (100%). These findings indicate that such Ru(II)-PNA bioconjugates could be investigated as multimodal labels for biosensing applications.