Now showing 1 - 10 of 26
  • Publication
    Photophysical studies of CdTe quantum dots in the presence of a zinc cationic porphyrin
    The photophysical properties of 2.3 nm thioglycolic acid (TGA) coated CdTe quantum dots (QDs) prepared by a reflux method have been studied in the presence of cationic meso-tetrakis(4-N-methylpyridyl) zinc porphyrin (ZnTMPyP4). Addition of the CdTe QDs to the porphyrin in H2O results in a marked red-shift and hypochromism in the porphyrin absorption spectrum, indicative of a non-covalent binding interaction with the QD surface. Only low equivalents of the quantum dot were required for complete quenching of the porphyrin fluorescence revealing that one quantum dot may quench more than one porphyrin. Similarly addition of porphyrin to the quantum dot provided evidence for very efficient quenching of the CdTe photoluminescence, suggesting the formation of CdTe'porphyrin aggregates. Definitive evidence for such aggregates was gathered using small angle X-ray spectroscopy (SAXS). Ultrafast transient absorption data are consistent with very rapid photoinduced electron transfer (1.3 ps) and the resultant formation of a long-lived porphyrin species.
      652Scopus© Citations 26
  • Publication
    Multimodal Microscopy Distinguishes Extracellular Aggregation and Cellular Uptake of Single-Walled Carbon Nanohorns
    The low toxicity, high surface area, and ease of functionalisation of carbon nanohorns (CNH) makes them attractive systems for cellular imaging, diagnostics and therapeutics. However, challenges remain for the biomedical translation of these and other nanomaterials. A significant task is tuning the surface chemistry to achieve optimal cellular interactions. Herein, we combine real-time fluorescent imaging of nanoparticle cellular uptake and real-time differential interference contrast (DIC) imaging of extracellular media to monitor a) nanoparticle/nanoparticle and b) nanoparticle/cell interactions for CNHs covalently modified with an OFF/ON near-IR dye, the fluorescence of which is switched OFF in extracellular environments and triggered upon cellular internalisation. CHN samples modified with different loadings of the hydrophobic dye are taken as a simple model of drug-loaded nanoparticle systems. The punctate fluorescence suggests the CNHs are delivered to lysosomes and other vesicles of the endocytic pathway. DIC imaging highlights the competition that exists for many particle types, between extracellular aggregation and cellular internalization, the efficiency of which would be dependent upon the amount of fluorophore loading. The results of this study illustrate how complementary real-time imaging methods together with physicochemical characterisation can be used to address the challenges involved in optimising nanoparticle/cell interactions for biomedical applications.
      404Scopus© Citations 6
  • Publication
    Excited state behaviour of substituted dipyridophenazine Cr(III) complexes in the presence of nucleic acids
    The photophysics and photochemistry of [Cr(phen)2(dppz)]3+ and its 11,12-substituted derivatives [Cr(phen)2(X2dppz)]3+ {X = Me or F} have been studied in the presence of purine nucleotides or DNA using steady state and time-resolved absorption and luminescence spectroscopy. 5'-Adenosine monophosphate (5'-AMP) shows only a weak interaction with the excited states of each complex. By contrast they are efficiently quenched by 5'-guanosine monophosphate (5'-GMP), consistent with photo-induced electron transfer. Laser flash photolysis spectroscopy in the presence of 5'--GMP suggests that both forward and back electron-transfers are rapid. All complexes also display a strong affinity for DNA and evidence for both static and dynamic quenching mechanisms is provided.
      546Scopus© Citations 13
  • Publication
    Direct observation by time-resolved infrared spectroscopy of the bright and the dark excited states of the [Ru(phen)2(dppz)]2+ light-switch compound in solution and when bound to DNA
    The [Ru(phen)2(dppz)]2+ complex (1) is non-emissive in water but is highly luminescent in organic solvents or when bound to DNA, making it a useful probe for DNA binding. To date, a complete mechanistic explanation for this “light-switch” effect is still lacking. With this in mind we have undertaken an ultrafast time resolved infrared (TRIR) study of 1 and directly observe marker bands between 1280–1450 cm−1, which characterise both the emissive “bright” and the non-emissive “dark” excited states of the complex, in CD3CN and D2O respectively. These characteristic spectral features are present in the [Ru(dppz)3]2+ solvent light-switch complex but absent in [Ru(phen)3]2+, which is luminescent in both solvents. DFT calculations show that the vibrational modes responsible for these characteristic bands are predominantly localised on the dppz ligand. Moreover, they reveal that certain vibrational modes of the “dark” excited state couple with vibrational modes of two coordinating water molecules, and through these to the bulk solvent, thus providing a new insight into the mechanism of the light-switch effect. We also demonstrate that the marker bands for the “bright” state are observed for both Λ- and Δ-enantiomers of 1 when bound to DNA and that photo-excitation of the complex induces perturbation of the guanine and cytosine carbonyl bands. This perturbation is shown to be stronger for the Λ-enantiomer, demonstrating the different binding site properties of the two enantiomers and the ability of this technique to determine the identity and nature of the binding site of such intercalators.
      199Scopus© Citations 51
  • Publication
    Spectro-electrochemical Studies on [Ru(TAP)2 (dppz)]2+ - Insights into the Mechanism of its Photosensitized Oxidation of Oligonucleotides
    (American Chemical Society, 2019-01-07) ; ; ;
    [Ru(TAP) 2 (dppz)] 2+ (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido[3,2-a:2′,3′-c]phenazine) is known to photo-oxidize guanine in DNA. Whether this oxidation proceeds by direct photoelectron transfer or by proton-coupled electron transfer is still unknown. To help distinguish between these mechanisms, spectro-electrochemical experiments have been carried out with [Ru(TAP) 2 (dppz)] 2+ in acetonitrile. The UV-vis and mid-IR spectra obtained for the one-electron reduced product were compared to those obtained by picosecond transient absorption and time-resolved infrared experiments of [Ru(TAP) 2 (dppz)] 2+ bound to guanine-containing DNA. An interesting feature of the singly reduced species is an electronic transition in the near-IR region (with λ max at 1970 and 2820 nm). Density functional and time-dependent density functional theory simulations of the vibrational and electronic spectra of [Ru(TAP) 2 (dppz)] 2+ , the reduced complex [Ru(TAP) 2 (dppz)] + , and four isomers of [Ru(TAP)(TAPH)(dppz)] 2+ (a possible product of proton-coupled electron transfer) were performed. Significantly, these predict absorption bands at λ > 1900 nm (attributed to a ligand-to-metal charge-transfer transition) for [Ru(TAP) 2 (dppz)] + but not for [Ru(TAP)(TAPH)(dppz)] 2+ . Both the UV-vis and mid-IR difference absorption spectra of the electrochemically generated singly reduced species [Ru(TAP) 2 (dppz)] + agree well with the transient absorption and time-resolved infrared spectra previously determined for the transient species formed by photoexcitation of [Ru(TAP) 2 (dppz)] 2+ intercalated in guanine-containing DNA. This suggests that the photochemical process in DNA proceeds by photoelectron transfer and not by a proton-coupled electron transfer process involving formation of [Ru(TAP)(TAPH)(dppz)] 2+ , as is proposed for the reaction with 5′-guanosine monophosphate. Additional infrared spectro-electrochemical measurements and density functional calculations have also been carried out on the free TAP ligand. These show that the TAP radical anion in acetonitrile also exhibits strong broad near-IR electronic absorption (λ max at 1750 and 2360 nm).
      226Scopus© Citations 10
  • Publication
    Monitoring one-electron photo-oxidation of guanine in DNA crystals using ultrafast infrared spectroscopy
    To understand the molecular origins of diseases caused by ultraviolet and visible light, and also to develop photodynamic therapy, it is important to resolve the mechanism of photoinduced DNA damage. Damage to DNA bound to a photosensitizer molecule frequently proceeds by one-electron photo-oxidation of guanine, but the precise dynamics of this process are sensitive to the location and the orientation of the photosensitizer, which are very difficult to define in solution. To overcome this, ultrafast time-resolved infrared (TRIR) spectroscopy was performed on photoexcited ruthenium polypyridyl-DNA crystals, the atomic structure of which was determined by X-ray crystallography. By combining the X-ray and TRIR data we are able to define both the geometry of the reaction site and the rates of individual steps in a reversible photoinduced electron-transfer process. This allows us to propose an individual guanine as the reaction site and, intriguingly, reveals that the dynamics in the crystal state are quite similar to those observed in the solvent medium.
      237Scopus© Citations 61
  • Publication
    Reversal of a Single Base-Pair Step Controls Guanine Photo-Oxidation by an Intercalating Ruthenium(II) Dipyridophenazine Complex
    Small changes in DNA sequence can often have major biological effects. Here the rates and yields of guanine photo-oxidation by Λ-[Ru(TAP)2(dppz)]2+ have been compared in 5′-{CCGGATCCGG}2 and 5′-{CCGGTACCGG}2 using pico/nanosecond transient visible and time-resolved IR (TRIR) spectroscopy. The inefficiency of electron transfer in the TA sequence is consistent with the 5′-TA-3′ versus 5′-AT-3′ binding preference predicted by X-ray crystallography. The TRIR spectra also reveal the differences in binding sites in the two oligonucleotides.
      252Scopus© Citations 30
  • Publication
    Ultrafast IR spectroscopy of polymeric cytosine nucleic acids reveal the long-lived species is due to a localised state
    The decay pathways of UV-excited cytosine polymers are investigated using picosecond time-resolved infrared spectroscopy. Similar yields of a non-emissive (1)nÏ * state are found in the single-stranded dC(30) polymer as in the dCMP monomer, but with a longer lifetime in the polymer (80 ps vs. 39 ps). A longer lifetime is also found in the d(CpC) dinucleotide. No evidence of excimer states is observed, suggesting that localised (1)nÏ * excited states are the most significant intermediates present on the picosecond timescale.
      403Scopus© Citations 13
  • Publication
    Enantiomeric conformation controls rate and yield of photoinduced electron transfer in DNA sensitized by Ru(II) dipyridophenazine complexes
    Photosensitized oxidation of guanine is an important route to DNA damage. Ruthenium polypyridyls are very useful photosensitizers, as their reactivity and DNA-binding properties are readily tunable. Here we show a strong difference in the reactivity of the two enantiomers of [Ru(TAP)2(dppz)]2+, by using time-resolved visible and IR spectroscopy. This reveals that the photosensitized one-electron oxidation of guanine in three oligonucleotide sequences proceeds with similar rates and yields for bound Δ-[Ru(TAP)2(dppz)]2+, whereas those for the λ enantiomer are very sensitive to base sequence. It is proposed that these differences are due to preferences of each enantiomer for different binding sites in the duplex.
      174Scopus© Citations 27
  • Publication
    Carbon nanohorn modified platinum electrodes for improved immobilisation of enzyme in the design of glutamate biosensors
    (The Royal Society of Chemistry, 2019-07-25) ; ; ;
    Electrochemical enzymatic biosensors are the subject of research due to their potential for in vivo monitoring of glutamate, which is a key neurotransmitter whose concentration is related to healthy brain function. This study reports the use of biocompatible oxidised carbon nanohorns (o-CNH) with a high surface area, to enhance the immobilization of glutamate oxidase (GluOx) for improved biosensor performance. Two families of biosensors were designed to interact with the anionic GluOx. Family-1 consists of covalently functionalised o-CNH possessing hydrazide (HYZ) and amine (PEG-NH2) terminated surfaces and Family-2 comprised non-covalently functionalised o-CNH with different loadings of polyethyleneimine (PEI) to form a cationic hybrid. Amperometric detection of H2O2 formed by enzymatic oxidation of glutamate revealed a good performance from all designs with the most improved performance by the PEI hybrid systems. The best response was from a o-CNH:PEI ratio of 1:10 mg mL-1, which yielded a glutamate calibration plateau, JMAX, of 55 ± 9 μA cm-2 and sensitivity of 111 ± 34 μA mM-1 cm-2. The low KM of 0.31 ± 0.05 mM indicated the retention of the enzyme function, and a limit of detection of 0.02 ± 0.004 μM and a response time of 0.88 ± 0.13 s was determined. The results demonstrate the high sensitivity of these biosensors and their potential for future use for the detection of glutamate in vivo.
      352Scopus© Citations 15