Polymer-based organic light-emitting diodes (OLEDs) with the structure ITO / PEDOT:PSS / MDMO-PPV / Metal were prepared by spin coating. It is known that electroluminescence of these devices is strongly dependent on the material used as cathode and on the deposition parameters of the polymer electroluminescent layer MDMO-PPV. Objective. In this work the effect of i) the frequency of the spin coater (1000-8000 rpm), ii) the concentration of the MDMO-PPV: Toluene solution, and iii) the material used as cathode (Aluminium or Silver) on the electrical response of the devices, was evaluated through current-voltage (I-V) measurements. Materials and methods. PEDOT:PPS and MDMO-PPV organic layers were deposited by spin coating on ITO substrates, and the OLED structure was completed with cathodes of aluminium and silver. The electric response of the devices was evaluated based on the I-V characteristics. Results. Diodes prepared with thinner organic films allow higher currents at lower voltages; this can be achieved either by increasing the frequency of the spin coater or by using concentrations of MDMO-PPV: Toluene lower than 2% weight. A fit of the experimental data showed that the diodes have two contributions to the current. The first one is attributed to parasitic currents between anode and cathode, and the other one is a parallel current through the organic layer, in which the carrier injection mechanism is mediated by thermionic emission. Conclusions. The results fitting and the energy level alignment through the whole structure show that PPV-based OLEDs are unipolar devices, with current mainly attributed to hole transport. Key words: organic semiconductors, OLEDs, electroluminescent polymers, MDMO-PPV, PEDOT:PSS, Spin coating, HOMO, LUMO, carrier injection, thermionic emission.

Here, it is reported an enhancement of the light-extraction efficiency in organic light emitting diodes (OLEDs) by growing a colloidal crystal (CC) matrix inside the luminescent layer of single bottom-emitting heterostructures. The CC matrix is obtained by self-assembly of Silica (SiO ₂ ) spheres during the spin-coating procedure used for the deposition of the luminescent polymer Poly [2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV). The SiO ₂ sphere diameter is estimated assuming a crystalline face-centered-cubic (fcc) structure and performing first principles calculations with the plane wave expansion method. The calculations provided a 250 nm sphere diameter for an adequate decoupling of the waveguide organic mode. From these considerations both, traditional bilayer and nanostructured OLEDs, were elaborated using ITO/PEDOT:PSS/MDMO-PPV/Ag and ITO/PEDOT:PSS/MDMO-PPV+Colloidal crystal/Ag architectures, respectively. The light-extraction efficiencies of both types of OLED architectures are quantified by measuring the wall plug efficiency. The results show a significant increase (near to 30%) on the average efficiency for nanostructured OLEDs. The improvement in the power efficiency is attributed to a reduction in the effective refractive index of the organic layer through the incorporation of a fcc-colloidal crystal into the emission layer (EML) of the OLED, which modifies their reflection properties. This novel method for obtaining CC in a single-step associated to the spin coating process has also the advantage of introducing the scattering medium directly into the EML in order to modify internal coupling modes, instead of focusing on substrate modification.

Studies regarding cytotoxic effects attributed to the use of adhesive bonding agents on pulp tissue are not conclusive. To point out whether these materials are safe for clinical use, in vivo exposure of dental pulp to adhesive bonding agents was simulated using an experimental setup in which Human Dental Pulp Stem Cells (hDPSC) are exposed to the action of two kinds of adhesives: self-etching adhesives and two-step bonding agents through a dentine barrier. Cytotoxic effects on these cells were evaluated by MTT assay protocol and fluorescence microscopy, and their results were contrasted to those obtained through Raman spectra taken on single hDPSCs. Overall, no significant cytotoxic effects were observed by combining all the techniques, and cell viability close to 90% was achieved for a dentine barrier of at least 1 mm thick. Moreover, Raman spectroscopy was able to detect structural DNA damage in some dental pulp cells when exposed to two-step bonding agents, suggesting that this technique could be considered a complementary tool with the potential to evaluate cell toxicity beyond cell viability.

Background: Amelogenesis imperfecta is a hereditary disorder affecting dental enamel. Among its phenotypes, hypocalcified AI is characterized by mineral deficiency, leading to tissue wear and, consequently, dental sensitivity. Excessive fluoride intake (through drinking water, fluoride supplements, toothpaste, or by ingesting products such as pesticides or insecticides) can lead to a condition known as dental fluorosis, which manifests as stains and teeth discoloration affecting their structure. Our recent studies have shown that extracts from Colombian native plants, Ilex guayusa and Piper marginatum, deposit mineral ions such as phosphate and orthophosphate into the dental enamel structure; however, it is unknown whether these extracts produce toxic effects on the dental pulp. Objective: To assess cytotoxicity effects on human dental pulp stem cells (hDPSCs) exposed to extracts isolated from I. guayusa and P. marginatum and, hence, their safety for clinical use. Methods: Raman spectroscopy, fluorescence microscopy, and flow cytometry techniques were employed. For Raman spectroscopy, hDPSCs were seeded onto nanobiochips designed to provide surface-enhanced Raman spectroscopy (SERS effect), which enhances their Raman signal by several orders of magnitude. After eight days in culture, I. guayusa and P. marginatum extracts at different concentrations (10, 50, and 100 ppm) were added. Raman measurements were performed at 0, 12, and 24 h following extract application. Fluorescence microscopy was conducted using an OLIMPUS fv1000 microscope, a live–dead assay was performed using a kit employing a BD FACS Canto TM II flow cytometer, and data analysis was determined using a FlowJo program. Results: The Raman spectroscopy results showed spectra consistent with viable cells. These findings were corroborated using fluorescence microscopy and flow cytometry techniques, confirming high cellular viability. Conclusions: The analyzed extracts exhibited low cytotoxicity, suggesting that they could be safely applied on enamel for remineralization purposes. The use of nanobiochips for SERS effect improved the cell viability assessment.

Traditionally, the vertical convective self-assembly method (VCSA) uses time-invariant temperature conditions to obtain high-quality colloidal crystal (CC) films, leaving unexplored possible effects due to temperature variations during the elaboration process. Here, the quality of colloidal crystal films grown by the VCSA method when applying temperature ramps is investigated. This study is conducted by combining experimental measurements and simulations of concentration profile and meniscus displacement, using the temperature dependent Kynch’s theory. The quality of the elaborated CC films is evaluated by analyzing the resulting optical Bragg diffraction. Colloidal crystal films deposited under controlled parameters showed that high-quality films can be achieved by positioning the meniscus into the uniform zone under temperature ramping, as a definite relation between sedimentation velocity and evaporation rate, in agreement with modelling results.