Helder M. C. Barbosa

Protein quantification in the presence of poly(ethylene glycol) and dextran using the Bradford method

Some experimental methodologies require the quantification of protein in the presence of polymers like poly(ethylene glycol) (PEG) and dextran (DEX). In the aqueous two-phase system (ATPS) extraction of biomolecules, the interference of these phase-forming polymers on the Bradford quantification assay is commonly recognized. However, how these polymers interfere has not been reported hitherto. In this study we show that while dextran concentrations of 20% (w/w) can be used without error, loss of accuracy occurs for solutions with PEG concentrations >10% (w/w). Above this value a substantial decrease on the assay sensitivity is observed.

Affinity partitioning of plasmid DNA with a zinc finger protein

The affinity isolation of pre-purified plasmid DNA (pDNA) from model buffer solutions using native and poly(ethylene glycol) (PEG) derivatized zinc finger-GST (Glutathione-S-Transferase) fusion protein was examined in PEG-dextran (DEX) aqueous two-phase systems (ATPSs). In the absence of pDNA, partitioning of unbound PEGylated fusion protein into the PEG-rich phase was confirmed with 97.5% of the PEGylated fusion protein being detected in the PEG phase of a PEG 600-DEX 40 ATPS. This represents a 1322-fold increase in the protein partition coefficient in comparison to the non-PEGylated protein (Kc = 0.013). In the presence of pDNA containing a specific oligonucleotide recognition sequence, the zinc finger moiety of the PEGylated fusion protein bound to the plasmid and steered the complex to the PEG-rich phase. An increase in the proportion of pDNA that partitioned to the PEG-rich phase was observed as the concentration of PEGylated fusion protein was increased. Partitioning of the bound complex occurred to such an extent that no DNA was detected by the picogreen assay in the dextran phase. It was also possible to partition pDNA using a non-PEGylated (native) zinc finger-GST fusion protein in a PEG 1000-DEX 500 ATPS. In this case the native ligand accumulated mainly in the PEG phase. These results indicate good prospects for the design of new plasmid DNA purification methods using fusion proteins as affinity ligands.

Theoretical study of the influence of salt doping in the functioning of OLEDs

One of the strategies to improve the efficiency of organic light emitting diodes (OLEDs) is to dope the active organic semiconducting layer with inorganic salts, leading to the development of a hybrid organic/inorganic hetero-structure. However, it is hard to know from the experiments how each one of the electronic processes underlying the functioning of OLEDs is affected by the accumulation of inorganic ions of different sign at both organic/electrode interfaces. In order to assess these effects, we performed computer simulations by using a multi-scale model that combines quantum molecular dynamics calculations at the atomistic scale with Monte Carlo calculations at the mesoscopic scale. We focus our attention on the main differences obtained between doped and pristine organic layers, when bipolar charge injection occurs. Our results show a significant drop on the turn-on applied electric field while maintaining rapid response to the applied field as well as a clear increase in recombination rate and recombination efficiency far from the electrodes for the doped situation, which are responsible for the dramatic improvement of doped OLED performance found in the experiments.

Computational study of the presence of defects in semiconducting polymers on exciton formation

Although semiconducting polymers are very attractive to be used in optoelectronic devices due to their molecular structure, they are not pristine semiconductors. After deposition it is possible to find out several structural and chemical defects, with different origins, that strongly influence exciton dynamics since they create deep energetic sites, where excitons can migrate leading to their quenching or reducing exciton diffusion length. By using a self-consistent quantum molecular dynamics method we performed a computational study to understand the influence of well-known polymer defects on excitons dynamics. Our results show that these defects influences mainly intramolecular exciton localization and exciton energy.

Multi-scale modelling of polymer-based optoelectronic devices

The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic cells requires the understanding how molecular properties and the spatial arrangement of the conjugated strands affect the electronic processes underlying the functioning of these devices. Since some of the important features are determined largely by the individual molecular strands and other features depend strongly on the nanostructure, a multi-scale modelling of materials and device properties is needed. In this work we discuss the atomistic and nanoscale modelling of charge injection, transport and trapping single-carrier diode based on poly(p-phenylene venylene) (PPV), which also applies to other optoelectronic devices.

Computational Study of the Influence of Polymer/Polymer Interface Formation on Bilayer-LED Functioning

The actual interest on polymer light emitting diodes (PLEDs) is based on the fact that they are easy to process, which reduces the cost of fabrication and thus opening a new branch in the electronic market – the low-cost electronics. However, these devices present a limited efficiency compared to their inorganic counterparts mainly due to the unbalanced charge injection, which reduces the fluorescence emission. One of the first strategies to improve PLEDs efficiency was using a bilayer structure composed by two polymers to improve charge injection and transport, and at the same time tune charge recombination zone to reduce the effect of the electrodes on exciton quenching. Although this is a very ingenious device architecture some of these bilayer devices showed a lower efficiency than it was expected. The reason for that is attributed to the dissolution of the first polymer layer by the solvent used for the deposition of the second polymer layer, which do not allow to create a define polymer/polymer interface. Although cross-linking the first polymer layer can solve this problem, there is not a clear understanding why the presence of a graded interface between both polymer layers can lead to a change on PLED efficiency. In order to clarify the effect of a graded polymer/polymer interface as compared to a sharp one on the functioning of a PLED, we performed computer experiments using a mesoscopic model of a bilayer PLED developed by us that considers the morphology of both polymers at nanoscale and their properties at molecular scale. The results present in this work show clearly a significant change on the charge recombination profile within the polymer device depending on the type of interface formed between the two polymers, which can be a plausible explanation for the loss of efficiency in the bilayer 7-CN-PPV/PPV LED.

Theoretical Study of the Influence of Chemical Defects on the Molecular Properties of Semiconducting Polymers

Semiconductor polymers are successfully implemented in a broad range of applications such as light emitting diodes, field effect transistors and photovoltaic devices. Most of the achievements reached in the development of these devices were obtained at experimental level, being difficult to identify individually the influence of each factor that limits and controls these devices efficiency. One of the factors that strongly influence the performance of polymer-based devices is the presence of chemical defects in the polymer strands that change their molecular properties. As a result, these polymer strands can work like traps or deep energetic states for charge transport, leading, for instance, to a decrease on charge mobility. At experimental level it is a difficult task to isolate the influence of each type of chemical defects individually on the molecular properties of the polymer strands. It is in this context that theoretical modelling seems to be the most suitable approach to get a deep understanding of the influence of chemical defects on the molecular properties of semiconductor polymers. By performing quantum molecular dynamics calculations we study the influence of chemical defects on the molecular properties of poly(para-phenylenevinylene) (PPV). Our results show clearly a significant difference on the electronic properties of defective polymer strands (e.g. charge carrier localization, ionization potential, electron affinity and electric-field threshold for charge carrier mobility along the polymer backbone) as compared with defect-free strands.

The influence of chain orientation in the electric behaviour of polymer diodes

Recently some experimental results have showed that the spatial alignment of conjugated polymer chains on nanometre length scales can influence the behaviour of polymer-based electronic devices, such as light-emitting diodes, field effect transistors, and photovoltaic cells. The effects of chain orientation at electrode-polymer interfaces on the charge injection process and charge mobility through the polymer layer are not well understood. In this work we use a generalized dynamical Monte Carlo method to study the influence of different polymer chain orientation relative to the electrodes surface on the electric behaviour of single-layer polymer diode, namely density current and charge density.

Computer Simulation of Hole Distribution in Polymeric Materials

Polymers have been known for their flexibility and easy processing into coatings and films, which made them suitable to be applied in a variety of areas and in particular the growing area of organic electronics. The electronic properties of semiconducting polymers made them a serious rival in areas where until now inorganic materials were the most used, such as light emitting diodes or solar cells. Typical polymers can be seen as a network of molecular strands of varied lengths and orientations, with a random distribution of physical and chemical defects which makes them an anisotropic material. To further increase their performance, a better understanding of all aspects related to charge transport and space charge distribution in polymeric materials is required. The process associated with charge transport depends on the properties of the polymer molecules as well as connectivity and texture, and so we adopt a mesoscopic approach to build polymer structures. Changing the potential barrier for charge injection we can introduce holes in the polymer network and, by using a generalised Monte-Carlo method, we can simulate the transport of the injected charge through the polymer layer caused by imposing a voltage between two planar electrodes. Our results show that the way that holes distribute within polymer layer and charge localization in these materials is quite different from the inorganic ones.