School of Chemistry - Research Publications
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ItemSpectroscopic study of L-DOPA and dopamine binding on novel gold nanoparticles towards more efficient drug-delivery system for Parkinson's disease(PERGAMON-ELSEVIER SCIENCE LTD, 2022-03-05)Nano-drug delivery systems may potentially overcome current challenges in the treatment of Parkinson's disease (PD) by enabling targeted delivery and more efficient blood-brain penetration ability. This study investigates novel gold nanoparticles (AuNPs) to be used as delivery systems for L-DOPA and dopamine by considering their binding capabilities in the presence and absence of a model protein, bovine serum albumin (BSA). Four different AuNPs were prepared by surface functionalization with polyethylene glycol (PEG), 1-adamantylamine (Ad), 1-adamantylglycine (AdGly), and peptidoglycan monomer (PGM). Fluorescence and UV-Vis measurements demonstrated the strongest binding affinity and L-DOPA/dopamine loading efficiency for PGM-functionalized AuNPs with negligible impact of the serum protein presence. Thermodynamic analysis revealed a spontaneous binding process between L-DOPA or dopamine and AuNPs that predominantly occurred through van der Waals interactions/hydrogen bonds or electrostatic interactions. These results represent PGM-functionalized AuNPs as the most efficient at L-DOPA and dopamine binding with a potential to become a drug-delivery system for neurodegenerative diseases. Detailed investigation of L-DOPA/dopamine interactions with different AuNPs was described here for the first time. Moreover, this study highlights a cost- and time-effective methodology for evaluating drug binding to nanomaterials.
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ItemSynthesis and Characterisation of Helicate and Mesocate Forms of a Double-Stranded Diruthenium(ii) Complex of a Di(terpyridine) Ligand(CSIRO Publishing, 2019)A diruthenium(II) complex involving the di(terpyridine) ligand 1,2-bis{5-(5″-methyl-2,2′:6′,2″-terpyridinyl)}ethane was synthesised by heating an equimolar ratio of RuCl3 and the ligand under reflux conditions in ethylene glycol for 3 days, realising double-stranded helicate and mesocate forms which were chromatographically separated. The two species were obtained in relatively low yield (each ~7–9 %) from the reaction mixture. X-Ray structural studies revealed differences in the cavity sizes of the two structures, with the helicate structure having a significantly smaller cavity. Furthermore, the helicate and mesocate forms pack with notably different arrangements of the structures with the helicate having large solvent and anion filled pores. 1D/2D NMR studies revealed rigidity in the mesocate structure relative to that of the helicate, such that the –CH2CH2– signal was split in the former and appeared as a singlet in the latter. In a manner analogous to the behaviour of the parent [Ru(tpy)2]2+ coordination moiety (tpy = 2,2′:6′,2″-terpyridine), photophysical studies indicated that both the helicate and mesocate forms were non-emissive at ~610 nm at room temperature, but at 77 K in n-butyronitrile, both isomers showed emission at ~610 nm (λex 472 nm). However, the temporal emission characteristics were very different: time-resolved studies showed the emission of the helicate species decayed with a dominant emission lifetime of ~10 μs (similar to the emissive properties of free [Ru(tpy)2]2+ under the same conditions), whereas for the mesocate the emission lifetime was at least three orders of magnitude lower (~4 ns).
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ItemLow-Bandgap Conjugated Polymer Dots for Near-Infrared Fluorescence Imaging(AMER CHEMICAL SOC, 2018-09)Low-bandgap conjugated polymers attract significant research interests because of their broad light absorption spectra in the red and near-infrared regions, making them desirable materials for solar photovoltaics. To date, low-bandgap conjugated polymers yield some of the best power conversion efficiencies offered by polymer solar cells. In addition to their applications as solar photovoltaic materials, nanoparticles of these polymers may be potentially beneficial for cell imaging because of their red and near-infrared absorption features, which are required for significant light penetration into biological samples. In this work, conjugated polymer dots (CPdots) of PCPDTBT, PSBTBT, PTB7, PCDTBT, and PBDTTPD are prepared in aqueous solution using nanoprecipitation. The maximum fluorescence wavelengths of these CPdots range from 800 to 1000 nm. The CPdots exhibit an average zeta potential of -30 mV, giving rise to colloidal stability of these nanoparticles. Dynamic light scattering results show that the CPdots have a hydrodynamic diameter of approximately 100 nm. Furthermore, analyses of atomic force microscopy images of the low-bandgap donor-acceptor CPdots show an average height of approximately 20 nm. The CPdots are introduced to live THP-1 cells, a human monocytic cell line, and the internalization of CPdots by these cells is observed. Confocal fluorescence microscopy images of cells labeled with the low-bandgap CPdots show the presence of these bright nanoparticles in the cells. In short, we demonstrate the preparation of low-bandgap CPdots as an aqueous dispersion and their applications in cell imaging.
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ItemPhotophysical and Fluorescence Anisotropic Behavior of Polyfluorene β-Conformation Films(AMER CHEMICAL SOC, 2018-01-18)We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (β-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene β-conformational spin-coated films. Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) β-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, β-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to β-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF β-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer.
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ItemNo Preview AvailableA simple and ubiquitous device for picric acid detection in latent fingerprints using carbon dots(Royal Society of Chemistry, 2020-05-07)This work addresses the synthetic optimization of carbon dots (CDs) and their application in sensing picric acid from latent fingerprints by exploiting a smartphone-based RGB tool. The optimization of the synthesis of CDs is investigated towards achieving shorter reaction time, better product yield and fluorescence quantum efficiency. Precursors such as citric acid and thiourea were chosen for the synthesis of CDs. Among the various synthetic methodologies, it is found that the pyrolysis method offers ∼50% product yield within 15 min. The morphology and optical properties of the prepared CDs are characterized using the typical microscopic and spectroscopic techniques, respectively. The synthesized CDs exhibit quasi-spherical shape with an average particle size of 1.7 nm. The excitation dependent emissive properties of CDs are investigated by time resolved fluorescence spectroscopy. Furthermore, the excellent fluorescence properties (ϕ = 11%) of CDs are explored as a fluorescent fingerprint powder for the identification of latent fingerprints on various substrates. In addition, the presence of picric acid in latent fingerprints was detected. Furthermore, this study is extended to perform real time detection of fingerprints and harmful contaminants in fingerprints by utilizing a smartphone-based RGB color analysis tool. Based on these investigations, the prepared CDs could be a prospective fluorescent material in the field of forensics.
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ItemSurface Lattice Resonances in Self-Assembled Gold Nanoparticle Arrays: Impact of Lattice Period, Structural Disorder, and Refractive Index on Resonance Quality(American Chemical Society, 2020-11-17)Surface lattice resonances are optical resonances composed of hybridized plasmonic and diffractive modes. These collective resonances occur in periodic arrays of plasmonic nanoparticles with wavelength-scale interparticle distances. The appearance and strength of surface lattice resonances strongly depend on the single particle localized surface plasmon resonance and its spectral overlap with the diffractive modes of the array. Coupling to in-plane orders of diffraction is also strongly affected by the refractive index environment and its symmetry. In this work, we address the impact of the interparticle distance, the symmetry of the refractive index environment, and structural imperfections in self-assembled colloidal monolayers on the plasmonic–diffractive coupling. For this purpose, we prepared hexagonally ordered, nonclose packed monolayers of gold nanoparticles using a fast and efficient, interface-mediated, colloidal self-assembly approach. By tuning the thickness and deformability of the polymer shells, we were able to prepare monolayers with a broad range of interparticle distances. The optical properties of the samples were studied experimentally by UV–Vis spectroscopy and theoretically by finite difference time domain simulations. The measured and simulated spectra allow a comprehensive analysis of the details of electromagnetic coupling in periodic plasmonic arrays. In particular, we identify relevant criteria required for surface lattice resonances in the visible wavelength range with optimized quality factors in self-assembled monolayers.
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ItemSedimentation of C60 and C70: Testing the Limits of Stokes' Law(AMER CHEMICAL SOC, 2018-11-01)Virtually all dynamic methods for determining particle size on the nanoscale use the Stokes-Einstein-Sutherland (SES) equation to convert the diffusion coefficient into a hydrodynamic radius. The validity of this equation on the nanoscale has not been rigorously validated by experiment. Here we measure the sedimentation rates and diffusion coefficients of C60 and C70 in toluene using analytical ultracentrifugation and compare the results to the SES equation. We find that the SES equation for the drag force (nonslip boundary condition) works down to 1 nm length scales.
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ItemFabrication of a Three-Dimensional Plasmon Ruler Using an Atomic Force Microscope(AMER CHEMICAL SOC, 2019-08-15)We have assembled a three-dimensional (3D) plasmon ruler using an atomic force microscope (AFM) tip to manipulate single gold nanocrystals on top of electron beam lithography fabricated base layers. The 3D structures exhibit several polarization-dependent surface plasmon scattering peaks, including symmetric and asymmetric Fano resonances. We map these resonances as a function of the degree of asymmetry of the structure. We show that the coupled surface plasmon resonances are extremely sensitive to the position of the upper particle and that the resonances can be engineered and tuned using an AFM tip to move the upper nanocrystal just a few Angstroms.
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ItemGrowth of Gold Nanorods: A SAXS Study(AMER CHEMICAL SOC, 2021-09-16)Using simultaneous, in situ optical spectroscopy and time-resolved, small-angle X-ray scattering (SAXS), we have directly monitored the seeded growth of nearly monodisperse gold nanorods using hydroquinone as the reductant. Growth of the rods is much slower than with the ascorbate ion, allowing the rate of growth along both the longitudinal and transverse directions to be independently determined. The thickness of the stabilizing CTAB layer (3.2 ± 0.3 nm) has also been extracted. We find that increasing the hydrogen tetrachloroaurate(III) concentration produces longer rods, while conversely, increasing the hydroquinone concentration reduces the final aspect ratio. The final number of gold rods is smaller than the initial number of seed particles and decreases in the presence of larger concentrations of HAuCl4. The SAXS data reveal an early transition from a spherical morphology to an ellipsoidal one and then to spherically capped cylinders. The growth curve exhibits at least three distinct regimes: an initial phase comprising spherical seed growth, followed by symmetry breaking and slow elongation. A third phase is marked by rapid rod growth and increases in the aspect ratio. This process is temporally well resolved from the initial symmetry breaking but typically occurs when the rods are around 6 nm in diameter using hydroquinone as the reductant. The results provide qualitative support for the “popcorn model” proposed by Edgar et al. [ Formation of Gold Nanorods by a Stochastic “Popcorn” Mechanism. ACS Nano 2012, 6, 1116 1125 ].
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ItemTuning Single Quantum Dot Emission with a Micromirror(AMER CHEMICAL SOC, 2018-02)The photoluminescence of single quantum dots fluctuates between bright (on) and dark (off) states, also termed fluorescence intermittency or blinking. This blinking limits the performance of quantum dot-based devices such as light-emitting diodes and solar cells. However, the origins of the blinking remain unresolved. Here, we use a movable gold micromirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. We observe that the quantum yield of the bright state is close to unity for these single QDs. Furthermore, we also study the effect of a micromirror on blinking, and then evaluate excitation efficiency, biexciton quantum yield, and detection efficiency. The mirror does not modify the off-time statistics, but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened due to an increase in the radiative recombination rate.