The type of aggregation in conjugated polymers determines their use in electronic devices. H-type... more The type of aggregation in conjugated polymers determines their use in electronic devices. H-type aggregates are most suitable for solar cell applications, while J-type aggregates are recommended for light-emitting diodes. In this work, we used three methods to determine the type of aggregates in a benzodithiophene–isoindigo-based (PBDTI-DT) copolymer, namely, Huang–Rhys factor evolution with temperature, Franck–Condon analysis, and relative quantum yield (QY) calculation. All three methods indicate that both aggregation types are present, and the QY calculation clearly indicates that H-aggregates are more dominant. Time-dependent density functional theory was used to identify the two absorption bands of PBDTI-DT as local π − π* and intramolecular charge-transfer transitions.
Real-time feedback-driven single-particle tracking (RT-FD-SPT) is a class of techniques in the fi... more Real-time feedback-driven single-particle tracking (RT-FD-SPT) is a class of techniques in the field of single-particle tracking that uses feedback control to keep a particle of interest in a detection volume. These methods provide high spatiotemporal resolution on particle dynamics and allow for concurrent spectroscopic measurements. This review article begins with a survey of existing techniques and of applications where RT-FD-SPT has played an important role. We then systematically discuss each of the core components of RT-FD-SPT in order to develop an understanding of the trade-offs that must be made in algorithm design and to create a clear picture of the important differences, advantages, and drawbacks of existing approaches. These components are feedback tracking and control, ranging from simple proportional-integral-derivative control to advanced nonlinear techniques, estimation to determine particle location from the measured data, including both online and offline algorithms, and techniques for calibrating and characterizing different RT-FD-SPT methods. We then introduce a collection of metrics for RT-FD-SPT to help guide experimentalists in selecting a method for their particular application and to help reveal where there are gaps in the techniques that represent opportunities for further development. Finally, we conclude with a discussion on future perspectives in the field.
Biochimica Et Biophysica Acta - Bioenergetics, Apr 1, 2019
General rights Copyright and moral rights for the publications made accessible in the public port... more General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Journal of Physical Chemistry Letters, Mar 5, 2018
and it is a condition of accessing publications that users recognise and abide by the legal requi... more and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have ... more In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have an important function to quench chlorophyll (Chl) triplet states and therefore avoid the production of harmful singlet oxygen. The resulting Car triplet states lead to a non-linear self-quenching mechanism called singlet-triplet (S-T) annihilation that strongly depends on the excitation density. In this work we investigated the fluorescence decay kinetics of single immobilized LHCIIs at room temperature and found a two-exponential decay with a slow (3.5 ns) and a fast (35 ps) component. The relative amplitude fraction of the fast component increases with increasing excitation intensity, and the resulting decrease in the fluorescence quantum yield suggests annihilation effects. Modulation of the excitation pattern by means of an acousto-optic modulator (AOM) furthermore allowed us to resolve the time-dependent accumulation and decay rate (B7 ms) of the quenching species. Inspired by singlet-singlet (S-S) annihilation studies, we developed a stochastic model and then successfully applied it to describe and explain all the experimentally observed steady-state and time-dependent kinetics. That allowed us to distinctively identify the quenching mechanism as S-T annihilation. Quantitative fitting resulted in a conclusive set of parameters validating our interpretation of the experimental results. The obtained stochastic model can be generalized to describe S-T annihilation in small molecular aggregates where the equilibration time of excitations is much faster than the annihilation-free singlet excited state lifetime.
Light-harvesting pigment-protein complexes of photosystem II of plants have a dual function: they... more Light-harvesting pigment-protein complexes of photosystem II of plants have a dual function: they efficiently use absorbed energy for photosynthesis at limiting sunlight intensity and dissipate the excess energy at saturating intensity for photoprotection. Recent single-molecule spectroscopy studies on the trimeric LHCII complex showed that environmental control of the intrinsic protein disorder could in principle explain the switch between their light-harvesting and photoprotective conformations in vivo. However, the validity of this proposal depends strongly on the specificity of the protein dynamics. Here, a similar study has been performed on the minor monomeric antenna complexes of photosystem II (CP29, CP26, and CP24). Despite their high structural homology, similar pigment content and organization compared to LHCII trimers, the environmental response of these proteins was found to be rather distinct. A much larger proportion of the minor antenna complexes were present in permanently weakly fluorescent states under most conditions used; however, unlike LHCII trimers the distribution of the single-molecule population between the strongly and weakly fluorescent states showed no significant sensitivity to low pH, zeaxanthin, or low detergent conditions. The results support a unique role for LHCII trimers in the regulation of light harvesting by controlled fluorescence blinking and suggest that any contribution of the minor antenna complexes to photoprotection would probably involve a distinct mechanism.
Plants harvest sunlight by converting light energy to electron flow through the primary events in... more Plants harvest sunlight by converting light energy to electron flow through the primary events in photosynthesis. One important question is how the light harvesting machinery adapts to fluctuating sunlight intensity. As a result of various regulatory processes, efficient light harvesting and photoprotection are balanced. Some of the biological steps in the photoprotective processes have been extensively studied and physiological regulatory factors have been identified. For example, the effect of lumen pH in changing carotenoid composition has been explored. However, the importance of photophysical dynamics in the initial light-harvesting steps and its relation to photoprotection remain poorly understood. Conformational and excited-state dynamics of multi-chromophore pigment-protein complexes are often difficult to study and limited information can be extracted from ensemble-averaged measurements. To address the problem, we use the Anti-Brownian ELectrokinetic (ABEL) trap to investigate the fluorescence from individual copies of light-harvesting complex II (LHCII), the primary antenna protein in higher plants, in a solution-phase environment. Perturbative surface immobilization or encapsulation schemes are avoided, and therefore the intrinsic dynamics and heterogeneity in the fluorescence of individual proteins are revealed. We perform simultaneous measurements of fluorescence intensity (brightness), excited-state lifetime, and emission spectrum of single trapped proteins. By analyzing the correlated changes between these observables, we identify forms of LHCII with different fluorescence intensities and excited-state lifetimes. The distinct forms may be associated with different energy dissipation mechanisms in the energy transfer chain. Changes of relative populations in response to pH and carotenoid composition are observed, which may extend our understanding of the molecular mechanisms of photoprotection.
Plants can safely dissipate excess excitation energy during light harvesting to prevent the forma... more Plants can safely dissipate excess excitation energy during light harvesting to prevent the formation of triplet chlorophyll, which can generate deleterious singlet oxygen. With this regulation, known as non-photochemical quenching
We report on the properties of hematite thin films prepared by spray pyrolysis on fluorine-doped ... more We report on the properties of hematite thin films prepared by spray pyrolysis on fluorine-doped tin oxide (FTO)-coated glass substrates and investigated the effect of the spray volume, tetraethoxysilicate treatment of the hematite, and post-annealing at 500 °C for 2 h with 10 °C/min ramping. Raman spectroscopy confirmed the characteristic Raman spectrum of all the films, while high-resolution confocal Raman microscopy showed a uniform intensity, suggesting a homogeneous coating of the hematite films on the FTO substrates. Ultrafast transient absorption spectroscopy indicates that all three experimental parameters-a larger spray volume, tetraethoxysilicate treatment, and annealing-slowed down electron-hole recombination. Global analysis of the difference absorption data resolved the spectra and associated decay lifetimes of three distinct processes, operating on the ultrafast, tens of picoseconds, and hundreds of picoseconds timescales.
Journal of the Optical Society of America, May 2, 2018
The concept of orthonormal vector circle polynomials is revisited by deriving a set from the Cart... more The concept of orthonormal vector circle polynomials is revisited by deriving a set from the Cartesian gradient of Zernike polynomials in a unit circle using a matrix-based approach. The heart of this model is a closed-form matrix equation of the gradient of Zernike circle polynomials expressed as a linear combination of lower-order Zernike circle polynomials related through a gradient matrix. This is a sparse matrix whose elements are twodimensional standard basis transverse Euclidean vectors. Using the outer product form of the Cholesky decomposition, the gradient matrix is used to calculate a new matrix, which we used to express the Cartesian gradient of the Zernike circle polynomials as a linear combination of orthonormal vector circle polynomials. Since this new matrix is singular, the orthonormal vector polynomials are recovered by reducing the matrix to its row echelon form using the Gauss-Jordan elimination method. We extend the model to derive orthonormal vector general polynomials, which are orthonormal in a general pupil by performing a similarity transformation on the gradient matrix to give its equivalent in the general pupil. The outer form of the Gram-Schmidt procedure and the Gauss-Jordan elimination method are then applied to the general pupil to generate the orthonormal vector general polynomials from the gradient of the orthonormal Zernike-based polynomials. The performance of the model is demonstrated with a simulated wavefront in a square pupil inscribed in a unit circle.
Theoretical comparison of real-time feedback-driven single-particle tracking techniques Real-time... more Theoretical comparison of real-time feedback-driven single-particle tracking techniques Real-time feedback-driven single-particle tracking is a technique that uses feedback control to enable single-molecule spectroscopy of freely diffusing particles in native or near-native environments. A number of different RT-FD-SPT approaches exist, and comparisons between methods based on experimental results are of limited use due to differences in samples and setups. In this study, we used statistical calculations and dynamical simulations to directly compare the performance of different methods. The methods considered were the orbital method, the Knight's Tour (grid scan) method and MINFLUX, and we considered both fluorescence-based and interferometric scattering (iSCAT) approaches. There is a fundamental trade-off between precision and speed, with the Knight's Tour method being able to track the fastest diffusion but with low precision, and MINFLUX being the most precise but only tracking slow diffusion. To compare iSCAT and fluorescence, different biological samples were considered, including labeled and intrinsically fluorescent samples. The success of iSCAT as compared to fluorescence is strongly dependent on the particle size and the density and photophysical properties of the fluorescent particles. Using a wavelength for iSCAT that is negligibly absorbed by the tracked particle allows an increased illumination intensity, which results in iSCAT providing better tracking for most samples. This work highlights the fundamental aspects of performance in RT-FD-SPT and should assist with the selection of an appropriate method for a particular application. The approach used can easily be extended to other RT-FD-SPT methods.
Abstract Hematite nanoparticles were synthesized using chemical spray pyrolysis. Precursor concen... more Abstract Hematite nanoparticles were synthesized using chemical spray pyrolysis. Precursor concentrations of 30, 40, and 50 mM, deposition temperatures of 250, 340, and 400 °C were used. X-ray diffraction measurements confirmed the purity of hematite with no other phases of iron oxide. The (104) and (110) diffraction peaks associated with corundum structure of hematite were revealed in all the prepared thin films. The rhombohedral unit cell with space group R 3 ¯ C and average lattice constants, a = 5.042 A and c = 13.673 A were achieved. Scanning electron microscopy measurements obtained surface morphology of different grain sizes that ranged from 7 to 33 nm. From the UV–vis measurements, bandgaps that ranged from 2.10 to 1.92 eV for all films prepared at different precursor concentrations and temperatures were obtained. Photocurrent densities that ranged from 0.78 to 89 μA cm−2 were obtained for different precursor concentrations and deposition temperatures thin films. Further, all the samples had a donor density of 1018 cm−3 with a positive flat band potential in the range of 0.045 and 0.213 V.
Journal of The Optical Society of America B-optical Physics, Sep 14, 2020
Plasmon coupling between the dipolar localized surface plasmons of a nanoegg and the longitudinal... more Plasmon coupling between the dipolar localized surface plasmons of a nanoegg and the longitudinal dipolar localized surface plasmons of a nearby gold nanorod is investigated within a dipolar-quasistatic limit. This was achieved by varying the core-offset of the nanoegg for different nanorod sizes at a fixed coupling distance. With respect to the plasmon peaks of the isolated nanoegg, we studied blue shifted, resonant, and red shifted nanorods. We show that besides plasmon-induced resonance shifts, which occurred in all three cases studied, transparency dips are induced in both the absorption and scattering spectra of the nanoegg–nanorod dimer. The latter effect depends on the plasmon detuning frequency and the nanorod absorption cross section. In comparison to a nanoegg–nanosphere dimer, the optical properties of the nanoegg–nanorod dimer are more enhanced.
SummaryIn hyper‐arid soil environments, photosynthetic microorganisms are largely restricted to h... more SummaryIn hyper‐arid soil environments, photosynthetic microorganisms are largely restricted to hypolithic (sub‐lithic) habitats: i.e., on the ventral surfaces of translucent pebbles in desert pavements. Here, we combined fluorometric, spectroscopic, biochemical and metagenomic approaches to investigate in situ the light transmission properties of quartz stones in the Namib Desert, and assess the photosynthetic activity of the underlying hypolithic cyanobacterial biofilms. Quartz pebbles greatly reduced the total photon flux to the ventral surface biofilms and filtered out primarily the short wavelength portion of the solar spectrum. Chlorophylls d and f were not detected in biofilm pigment extracts; however, hypolithic cyanobacterial communities showed some evidence of adaptation to sub‐lithic conditions, including the prevalence of genes encoding Helical Carotenoid Proteins, which are associated with desiccation stress. Under water‐saturated conditions, hypolithic communities showed no evidence of light stress, even when the quartz stones were exposed to full midday sunlight. This initial study creates a foundation for future in‐situ and laboratory exploration of various adaptation mechanisms employed by photosynthetic organisms forming hypolithic microbial communities.
The concept of orthonormal polynomials is revisited by developing a Zernike-based orthonormal set... more The concept of orthonormal polynomials is revisited by developing a Zernike-based orthonormal set for a non-circular pupil that is transmitting an aberrated, non-uniform field. We refer to this pupil as a general pupil. The process is achieved by using the QR form of the Gram Schmidt procedure on Zernike circle polynomials and is interpreted as a process of balancing each Zernike circle polynomial by adding those of lower order in the general pupil, a procedure which was previously performed using classical aberrations. We numerically demonstrate this concept by comparing the representation of phase in a square-Gaussian pupil using the Zernike-Gauss square and Zernike-circle polynomials. As expected, using the Strehl ratio, we show that only specific lower-order aberrations can be used to balance specific aberrations, for example, tilt cannot be used to balance spherical aberration. In the process, we present a possible definition of the Maréchal criterion for the analysis of the tolerance of systems with apodized pupils.
The conversion of solar radiation to chemical energy in plants and green algae takes place in the... more The conversion of solar radiation to chemical energy in plants and green algae takes place in the thylakoid membrane. This amphiphilic environment hosts a complex arrangement of light-harvesting pigment-protein complexes that absorb light and transfer the excitation energy to photochemically active reaction centers. This efficient light-harvesting capacity is moreover tightly regulated by a photoprotective mechanism called nonphotochemical quenching to avoid the stress-induced destruction of the catalytic reaction center. In this review we provide an overview of single-molecule fluorescence measurements on plant light-harvesting complexes (LHCs) of varying sizes with the aim of bridging the gap between the smallest isolated complexes, which have been well-characterized, and the native photosystem. The smallest complexes contain only a small number (10-20) of interacting chlorophylls, while the native photosystem contains dozens of protein subunits and many hundreds of connected pigments. We discuss the functional significance of conformational dynamics, the lipid environment, and the structural arrangement of this fascinating nanomachinery. The described experimental results can be utilized to build mathematical-physical models in a bottom-up approach, which can then be tested on larger in vivo systems. The results also clearly showcase the general property of biological systems to utilize the same system properties for different purposes. In this case it is the regulated conformational flexibility that allows LHCs to switch between efficient light-harvesting and a photoprotective function.
Photosynthesis operates at the bottom of the food chain to convert the energy of light into carbo... more Photosynthesis operates at the bottom of the food chain to convert the energy of light into carbohydrates at a remarkable global rate of about 130 TW. Nonetheless, the overall photosynthetic process has a conversion efficiency of a few percent at best, significantly less than bottom-up photovoltaic cells. The primary photosynthetic steps, consisting of light harvesting and charge separation, are often presented as having near-unity quantum efficiency but this holds only true under ideal conditions. In this Review, we discuss the importance of energy loss processes to establish robustness in photosynthetic light harvesting. Thermal energy dissipation of light-harvesting complexes in different environments is investigated and the relationships and contrasts between concentration quenching of high pigment concentrations, photoprotection (non-photochemical quenching), quenching due to protein aggregation, and fluorescence blinking are discussed. The role of charge-transfer states in light harvesting and energy dissipation is highlighted and the importance of controlled protein structural disorder to switch the light-harvesting antennae between effective light harvesters and efficient energy quenchers, is underscored. The main light-harvesting complex of plants, LHCII, is used as prime example.
The type of aggregation in conjugated polymers determines their use in electronic devices. H-type... more The type of aggregation in conjugated polymers determines their use in electronic devices. H-type aggregates are most suitable for solar cell applications, while J-type aggregates are recommended for light-emitting diodes. In this work, we used three methods to determine the type of aggregates in a benzodithiophene–isoindigo-based (PBDTI-DT) copolymer, namely, Huang–Rhys factor evolution with temperature, Franck–Condon analysis, and relative quantum yield (QY) calculation. All three methods indicate that both aggregation types are present, and the QY calculation clearly indicates that H-aggregates are more dominant. Time-dependent density functional theory was used to identify the two absorption bands of PBDTI-DT as local π − π* and intramolecular charge-transfer transitions.
Real-time feedback-driven single-particle tracking (RT-FD-SPT) is a class of techniques in the fi... more Real-time feedback-driven single-particle tracking (RT-FD-SPT) is a class of techniques in the field of single-particle tracking that uses feedback control to keep a particle of interest in a detection volume. These methods provide high spatiotemporal resolution on particle dynamics and allow for concurrent spectroscopic measurements. This review article begins with a survey of existing techniques and of applications where RT-FD-SPT has played an important role. We then systematically discuss each of the core components of RT-FD-SPT in order to develop an understanding of the trade-offs that must be made in algorithm design and to create a clear picture of the important differences, advantages, and drawbacks of existing approaches. These components are feedback tracking and control, ranging from simple proportional-integral-derivative control to advanced nonlinear techniques, estimation to determine particle location from the measured data, including both online and offline algorithms, and techniques for calibrating and characterizing different RT-FD-SPT methods. We then introduce a collection of metrics for RT-FD-SPT to help guide experimentalists in selecting a method for their particular application and to help reveal where there are gaps in the techniques that represent opportunities for further development. Finally, we conclude with a discussion on future perspectives in the field.
Biochimica Et Biophysica Acta - Bioenergetics, Apr 1, 2019
General rights Copyright and moral rights for the publications made accessible in the public port... more General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Journal of Physical Chemistry Letters, Mar 5, 2018
and it is a condition of accessing publications that users recognise and abide by the legal requi... more and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have ... more In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have an important function to quench chlorophyll (Chl) triplet states and therefore avoid the production of harmful singlet oxygen. The resulting Car triplet states lead to a non-linear self-quenching mechanism called singlet-triplet (S-T) annihilation that strongly depends on the excitation density. In this work we investigated the fluorescence decay kinetics of single immobilized LHCIIs at room temperature and found a two-exponential decay with a slow (3.5 ns) and a fast (35 ps) component. The relative amplitude fraction of the fast component increases with increasing excitation intensity, and the resulting decrease in the fluorescence quantum yield suggests annihilation effects. Modulation of the excitation pattern by means of an acousto-optic modulator (AOM) furthermore allowed us to resolve the time-dependent accumulation and decay rate (B7 ms) of the quenching species. Inspired by singlet-singlet (S-S) annihilation studies, we developed a stochastic model and then successfully applied it to describe and explain all the experimentally observed steady-state and time-dependent kinetics. That allowed us to distinctively identify the quenching mechanism as S-T annihilation. Quantitative fitting resulted in a conclusive set of parameters validating our interpretation of the experimental results. The obtained stochastic model can be generalized to describe S-T annihilation in small molecular aggregates where the equilibration time of excitations is much faster than the annihilation-free singlet excited state lifetime.
Light-harvesting pigment-protein complexes of photosystem II of plants have a dual function: they... more Light-harvesting pigment-protein complexes of photosystem II of plants have a dual function: they efficiently use absorbed energy for photosynthesis at limiting sunlight intensity and dissipate the excess energy at saturating intensity for photoprotection. Recent single-molecule spectroscopy studies on the trimeric LHCII complex showed that environmental control of the intrinsic protein disorder could in principle explain the switch between their light-harvesting and photoprotective conformations in vivo. However, the validity of this proposal depends strongly on the specificity of the protein dynamics. Here, a similar study has been performed on the minor monomeric antenna complexes of photosystem II (CP29, CP26, and CP24). Despite their high structural homology, similar pigment content and organization compared to LHCII trimers, the environmental response of these proteins was found to be rather distinct. A much larger proportion of the minor antenna complexes were present in permanently weakly fluorescent states under most conditions used; however, unlike LHCII trimers the distribution of the single-molecule population between the strongly and weakly fluorescent states showed no significant sensitivity to low pH, zeaxanthin, or low detergent conditions. The results support a unique role for LHCII trimers in the regulation of light harvesting by controlled fluorescence blinking and suggest that any contribution of the minor antenna complexes to photoprotection would probably involve a distinct mechanism.
Plants harvest sunlight by converting light energy to electron flow through the primary events in... more Plants harvest sunlight by converting light energy to electron flow through the primary events in photosynthesis. One important question is how the light harvesting machinery adapts to fluctuating sunlight intensity. As a result of various regulatory processes, efficient light harvesting and photoprotection are balanced. Some of the biological steps in the photoprotective processes have been extensively studied and physiological regulatory factors have been identified. For example, the effect of lumen pH in changing carotenoid composition has been explored. However, the importance of photophysical dynamics in the initial light-harvesting steps and its relation to photoprotection remain poorly understood. Conformational and excited-state dynamics of multi-chromophore pigment-protein complexes are often difficult to study and limited information can be extracted from ensemble-averaged measurements. To address the problem, we use the Anti-Brownian ELectrokinetic (ABEL) trap to investigate the fluorescence from individual copies of light-harvesting complex II (LHCII), the primary antenna protein in higher plants, in a solution-phase environment. Perturbative surface immobilization or encapsulation schemes are avoided, and therefore the intrinsic dynamics and heterogeneity in the fluorescence of individual proteins are revealed. We perform simultaneous measurements of fluorescence intensity (brightness), excited-state lifetime, and emission spectrum of single trapped proteins. By analyzing the correlated changes between these observables, we identify forms of LHCII with different fluorescence intensities and excited-state lifetimes. The distinct forms may be associated with different energy dissipation mechanisms in the energy transfer chain. Changes of relative populations in response to pH and carotenoid composition are observed, which may extend our understanding of the molecular mechanisms of photoprotection.
Plants can safely dissipate excess excitation energy during light harvesting to prevent the forma... more Plants can safely dissipate excess excitation energy during light harvesting to prevent the formation of triplet chlorophyll, which can generate deleterious singlet oxygen. With this regulation, known as non-photochemical quenching
We report on the properties of hematite thin films prepared by spray pyrolysis on fluorine-doped ... more We report on the properties of hematite thin films prepared by spray pyrolysis on fluorine-doped tin oxide (FTO)-coated glass substrates and investigated the effect of the spray volume, tetraethoxysilicate treatment of the hematite, and post-annealing at 500 °C for 2 h with 10 °C/min ramping. Raman spectroscopy confirmed the characteristic Raman spectrum of all the films, while high-resolution confocal Raman microscopy showed a uniform intensity, suggesting a homogeneous coating of the hematite films on the FTO substrates. Ultrafast transient absorption spectroscopy indicates that all three experimental parameters-a larger spray volume, tetraethoxysilicate treatment, and annealing-slowed down electron-hole recombination. Global analysis of the difference absorption data resolved the spectra and associated decay lifetimes of three distinct processes, operating on the ultrafast, tens of picoseconds, and hundreds of picoseconds timescales.
Journal of the Optical Society of America, May 2, 2018
The concept of orthonormal vector circle polynomials is revisited by deriving a set from the Cart... more The concept of orthonormal vector circle polynomials is revisited by deriving a set from the Cartesian gradient of Zernike polynomials in a unit circle using a matrix-based approach. The heart of this model is a closed-form matrix equation of the gradient of Zernike circle polynomials expressed as a linear combination of lower-order Zernike circle polynomials related through a gradient matrix. This is a sparse matrix whose elements are twodimensional standard basis transverse Euclidean vectors. Using the outer product form of the Cholesky decomposition, the gradient matrix is used to calculate a new matrix, which we used to express the Cartesian gradient of the Zernike circle polynomials as a linear combination of orthonormal vector circle polynomials. Since this new matrix is singular, the orthonormal vector polynomials are recovered by reducing the matrix to its row echelon form using the Gauss-Jordan elimination method. We extend the model to derive orthonormal vector general polynomials, which are orthonormal in a general pupil by performing a similarity transformation on the gradient matrix to give its equivalent in the general pupil. The outer form of the Gram-Schmidt procedure and the Gauss-Jordan elimination method are then applied to the general pupil to generate the orthonormal vector general polynomials from the gradient of the orthonormal Zernike-based polynomials. The performance of the model is demonstrated with a simulated wavefront in a square pupil inscribed in a unit circle.
Theoretical comparison of real-time feedback-driven single-particle tracking techniques Real-time... more Theoretical comparison of real-time feedback-driven single-particle tracking techniques Real-time feedback-driven single-particle tracking is a technique that uses feedback control to enable single-molecule spectroscopy of freely diffusing particles in native or near-native environments. A number of different RT-FD-SPT approaches exist, and comparisons between methods based on experimental results are of limited use due to differences in samples and setups. In this study, we used statistical calculations and dynamical simulations to directly compare the performance of different methods. The methods considered were the orbital method, the Knight's Tour (grid scan) method and MINFLUX, and we considered both fluorescence-based and interferometric scattering (iSCAT) approaches. There is a fundamental trade-off between precision and speed, with the Knight's Tour method being able to track the fastest diffusion but with low precision, and MINFLUX being the most precise but only tracking slow diffusion. To compare iSCAT and fluorescence, different biological samples were considered, including labeled and intrinsically fluorescent samples. The success of iSCAT as compared to fluorescence is strongly dependent on the particle size and the density and photophysical properties of the fluorescent particles. Using a wavelength for iSCAT that is negligibly absorbed by the tracked particle allows an increased illumination intensity, which results in iSCAT providing better tracking for most samples. This work highlights the fundamental aspects of performance in RT-FD-SPT and should assist with the selection of an appropriate method for a particular application. The approach used can easily be extended to other RT-FD-SPT methods.
Abstract Hematite nanoparticles were synthesized using chemical spray pyrolysis. Precursor concen... more Abstract Hematite nanoparticles were synthesized using chemical spray pyrolysis. Precursor concentrations of 30, 40, and 50 mM, deposition temperatures of 250, 340, and 400 °C were used. X-ray diffraction measurements confirmed the purity of hematite with no other phases of iron oxide. The (104) and (110) diffraction peaks associated with corundum structure of hematite were revealed in all the prepared thin films. The rhombohedral unit cell with space group R 3 ¯ C and average lattice constants, a = 5.042 A and c = 13.673 A were achieved. Scanning electron microscopy measurements obtained surface morphology of different grain sizes that ranged from 7 to 33 nm. From the UV–vis measurements, bandgaps that ranged from 2.10 to 1.92 eV for all films prepared at different precursor concentrations and temperatures were obtained. Photocurrent densities that ranged from 0.78 to 89 μA cm−2 were obtained for different precursor concentrations and deposition temperatures thin films. Further, all the samples had a donor density of 1018 cm−3 with a positive flat band potential in the range of 0.045 and 0.213 V.
Journal of The Optical Society of America B-optical Physics, Sep 14, 2020
Plasmon coupling between the dipolar localized surface plasmons of a nanoegg and the longitudinal... more Plasmon coupling between the dipolar localized surface plasmons of a nanoegg and the longitudinal dipolar localized surface plasmons of a nearby gold nanorod is investigated within a dipolar-quasistatic limit. This was achieved by varying the core-offset of the nanoegg for different nanorod sizes at a fixed coupling distance. With respect to the plasmon peaks of the isolated nanoegg, we studied blue shifted, resonant, and red shifted nanorods. We show that besides plasmon-induced resonance shifts, which occurred in all three cases studied, transparency dips are induced in both the absorption and scattering spectra of the nanoegg–nanorod dimer. The latter effect depends on the plasmon detuning frequency and the nanorod absorption cross section. In comparison to a nanoegg–nanosphere dimer, the optical properties of the nanoegg–nanorod dimer are more enhanced.
SummaryIn hyper‐arid soil environments, photosynthetic microorganisms are largely restricted to h... more SummaryIn hyper‐arid soil environments, photosynthetic microorganisms are largely restricted to hypolithic (sub‐lithic) habitats: i.e., on the ventral surfaces of translucent pebbles in desert pavements. Here, we combined fluorometric, spectroscopic, biochemical and metagenomic approaches to investigate in situ the light transmission properties of quartz stones in the Namib Desert, and assess the photosynthetic activity of the underlying hypolithic cyanobacterial biofilms. Quartz pebbles greatly reduced the total photon flux to the ventral surface biofilms and filtered out primarily the short wavelength portion of the solar spectrum. Chlorophylls d and f were not detected in biofilm pigment extracts; however, hypolithic cyanobacterial communities showed some evidence of adaptation to sub‐lithic conditions, including the prevalence of genes encoding Helical Carotenoid Proteins, which are associated with desiccation stress. Under water‐saturated conditions, hypolithic communities showed no evidence of light stress, even when the quartz stones were exposed to full midday sunlight. This initial study creates a foundation for future in‐situ and laboratory exploration of various adaptation mechanisms employed by photosynthetic organisms forming hypolithic microbial communities.
The concept of orthonormal polynomials is revisited by developing a Zernike-based orthonormal set... more The concept of orthonormal polynomials is revisited by developing a Zernike-based orthonormal set for a non-circular pupil that is transmitting an aberrated, non-uniform field. We refer to this pupil as a general pupil. The process is achieved by using the QR form of the Gram Schmidt procedure on Zernike circle polynomials and is interpreted as a process of balancing each Zernike circle polynomial by adding those of lower order in the general pupil, a procedure which was previously performed using classical aberrations. We numerically demonstrate this concept by comparing the representation of phase in a square-Gaussian pupil using the Zernike-Gauss square and Zernike-circle polynomials. As expected, using the Strehl ratio, we show that only specific lower-order aberrations can be used to balance specific aberrations, for example, tilt cannot be used to balance spherical aberration. In the process, we present a possible definition of the Maréchal criterion for the analysis of the tolerance of systems with apodized pupils.
The conversion of solar radiation to chemical energy in plants and green algae takes place in the... more The conversion of solar radiation to chemical energy in plants and green algae takes place in the thylakoid membrane. This amphiphilic environment hosts a complex arrangement of light-harvesting pigment-protein complexes that absorb light and transfer the excitation energy to photochemically active reaction centers. This efficient light-harvesting capacity is moreover tightly regulated by a photoprotective mechanism called nonphotochemical quenching to avoid the stress-induced destruction of the catalytic reaction center. In this review we provide an overview of single-molecule fluorescence measurements on plant light-harvesting complexes (LHCs) of varying sizes with the aim of bridging the gap between the smallest isolated complexes, which have been well-characterized, and the native photosystem. The smallest complexes contain only a small number (10-20) of interacting chlorophylls, while the native photosystem contains dozens of protein subunits and many hundreds of connected pigments. We discuss the functional significance of conformational dynamics, the lipid environment, and the structural arrangement of this fascinating nanomachinery. The described experimental results can be utilized to build mathematical-physical models in a bottom-up approach, which can then be tested on larger in vivo systems. The results also clearly showcase the general property of biological systems to utilize the same system properties for different purposes. In this case it is the regulated conformational flexibility that allows LHCs to switch between efficient light-harvesting and a photoprotective function.
Photosynthesis operates at the bottom of the food chain to convert the energy of light into carbo... more Photosynthesis operates at the bottom of the food chain to convert the energy of light into carbohydrates at a remarkable global rate of about 130 TW. Nonetheless, the overall photosynthetic process has a conversion efficiency of a few percent at best, significantly less than bottom-up photovoltaic cells. The primary photosynthetic steps, consisting of light harvesting and charge separation, are often presented as having near-unity quantum efficiency but this holds only true under ideal conditions. In this Review, we discuss the importance of energy loss processes to establish robustness in photosynthetic light harvesting. Thermal energy dissipation of light-harvesting complexes in different environments is investigated and the relationships and contrasts between concentration quenching of high pigment concentrations, photoprotection (non-photochemical quenching), quenching due to protein aggregation, and fluorescence blinking are discussed. The role of charge-transfer states in light harvesting and energy dissipation is highlighted and the importance of controlled protein structural disorder to switch the light-harvesting antennae between effective light harvesters and efficient energy quenchers, is underscored. The main light-harvesting complex of plants, LHCII, is used as prime example.
World Health Organization - International EMF Project, 2021
The international electromagnetic field (EMF) project of the World Health Organization (WHO) is a... more The international electromagnetic field (EMF) project of the World Health Organization (WHO) is a collaborative effort of the WHO teams for Environment, Climate Change and Health (ECCH) and Radiation and Health (ionizing and non-ionizing). The purpose of the effort is twofold. Firstly it seeks to create and house a database of legislation pertaining to EMF exposure. Secondly, it seeks to promote dialogue on the risks related to EMF exposure.
The establishment of the EMF Project is motivated by the fact that EMF of a very broad range of frequencies represents one of the most common and fastest growing environmental influences on human health. EMF may have notable positive or negative effects on health, depending on the exposure context. EMF has been around since the birth of the universe, with light being its most familiar form. Electric and magnetic fields are part of the spectrum of electromagnetic radiation, which extends from static electric and magnetic fields, through radiofrequency, ultraviolet, and infrared radiation, to X-rays.
Various Organs of State have reviewed the research endeavors of the NRF in this emerging field and found that the research is innovative and strongly aligned with South Africa’s national interests. The EMF project is a multidisciplinary research endeavor integrating efforts from multiple sectors. Of particular import to South Africa is research into EMF exposure risks, EMF shielding modifications, and bio-adaptation mitigation options related to climate change. The EMF Project is endorsed by the Surgeon General, and the Nation’s doctor is provided with the best available scientific information on how to improve health outcomes and reduce the risk of illness and injury. The mission of the EMF Project is to protect, promote, and advance the health of our Nation.
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Papers by Tjaart Krüger
The establishment of the EMF Project is motivated by the fact that EMF of a very broad range of frequencies represents one of the most common and fastest growing environmental influences on human health. EMF may have notable positive or negative effects on health, depending on the exposure context. EMF has been around since the birth of the universe, with light being its most familiar form. Electric and magnetic fields are part of the spectrum of electromagnetic radiation, which extends from static electric and magnetic fields, through radiofrequency, ultraviolet, and infrared radiation, to X-rays.
Various Organs of State have reviewed the research endeavors of the NRF in this emerging field and found that the research is innovative and strongly aligned with South Africa’s national interests. The EMF project is a multidisciplinary research endeavor integrating efforts from multiple sectors. Of particular import to South Africa is research into EMF exposure risks, EMF shielding modifications, and bio-adaptation mitigation options related to climate change. The EMF Project is endorsed by the Surgeon General, and the Nation’s doctor is provided with the best available scientific information on how to improve health outcomes and reduce the risk of illness and injury. The mission of the EMF Project is to protect, promote, and advance the health of our Nation.