Structure control of poly(p-phenylene vinylene) in layer-by-layer films by deposition on a charged poly(o-methoxyaniline) cushion

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Structure control of poly(p-phenylene vinylene) in layer-by-layer films by deposition on a charged poly(o-methoxyaniline) cushion Alexandre Marletta, Silésia de Fátima Curcino da Silva, Erick Piovesan, K. R. Campos, Hugo Santos Silva, N. C. de Souza, Maria Letícia Vega, Maria Raposo, Carlos J. L Constantino, Raigna A. Silva, and Osvaldo N. Oliveira Jr. Citation: Journal of Applied Physics 113, 144509 (2013); doi: 10.1063/1.4798937 View online: View Table of Contents: Published by the AIP Publishing [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to ] IP: On: Wed, 05 Feb 2014 15:45:18 JOURNAL OF APPLIED PHYSICS 113, 144509 (2013) Structure control of poly(p-phenylene vinylene) in layer-by-layer films by deposition on a charged poly(o-methoxyaniline) cushion sia de Fa tima Curcino da Silva,1 Erick Piovesan,1 Alexandre Marletta,1,a) Sile K. R. Campos,1 Hugo Santos Silva,1 N. C. de Souza,2 Maria Letıcia Vega,3 Maria Raposo,4 Carlos J. L Constantino,5 Raigna A. Silva,1 and Osvaldo N. Oliveira, Jr.6 1 Instituto de Fısica, Universidade Federal de Uberl^ andia, CP 593, 38400-902 Uberl^ andia-MG, Brazil Grupo de Materiais Nanoestruturados, Campus Universit ario do Araguaia, Universidade Federal de Mato Grosso, CEP: 78698-000, Barra do Garças, Mato Grosso, Brazil 3 Departamento de Fısica, Universidade Federal do Piauı, 64049-690 Teresina, PI, Brazil 4 CEFITEC, Departamento de Fısica, Faculdade de Ci^ encias e Tecnologia, UNL, Campus de Caparica, 2829-516 Caparica, Portugal 5 Faculdade de Ci^ encias e Tecnologia, Departamento de Fısica Quımica e Biologia, UNESP Universidade Estadual Paulista, 19060-900 Presidente Prudente, SP, Brazil 6 Instituto de Fısica de S~ ao Carlos, Universidade de S~ ao Paulo, CP 369, 13560-970 S~ ao Carlos, SP, Brazil 2 (Received 20 February 2013; accepted 13 March 2013; published online 11 April 2013) In this paper, we demonstrate that the intrinsic electric field created by a poly(o-methoxyaniline) (POMA) cushion layer hinders the changes in molecular conformation of poly(p-phenylenevinylene) (PPV) in layer-by-layer with dodecylbenzene sulfonic acid (DBS). This was modeled with density functional theory (DFT) calculations where an energy barrier hampered molecular movements of PPV segments when they were subjected to an electric field comparable to that caused by a charged POMA layer. With restricted changes in molecular conformation, the PPV film exhibited Franck-Condon transitions and the photoexcitation spectra resembled the absorption spectra, in contrast to PPV/DBS films deposited directly on glass, with no POMA cushion. Other effects from the POMA cushion were the reduced number of structural defects, confirmed with Raman spectroscopy, and an enhanced PPV emission at high temperatures (300 K) in comparison with the films on bare glass. The positive effects from the POMA cushion may be exploited for enhanced opto-electronic devices, especially as the intrinsic electric field may assist in separating photoexcited electron-hole pairs in photovoltaic devices. C 2013 American Institute of Physics. [] V I. INTRODUCTION The control of supramolecular architectures is one of the key features exploited in the fabrication of layer-by-layer (LbL) films,1 which may be considered within the realms of the newly coined area of nanoarchitectonics.2 In addition to reaching synergy in the properties of distinct materials in the same film, it is possible to control the characteristics of some layers via direct interaction with the others, as in surface functionalization.2 Taking as an example the LbL films from semiconducting polymers, their optical and electrical properties may be tuned by combining with other materials, including fullerenes,3 and carbon nanotubes,4 in order to obtain enhanced performance in polymer light emitting diodes (PLEDs),5 organic field effect transistors,6 and photovoltaic cells.7 A widely used luminescent polymer in this regard is poly(p-phenylenevinylene) (PPV), the first electroluminescent polymer employed in organic electronics.8 Even though the luminescent properties of PPV are inferior to those of other polymers (see a review on luminescent polymers in Ref. 9), it is still useful in fundamental studies, particularly in proof-of-concept type of experiment as it may be processed using various film-forming techniques such as casting, spin-coating, LbL, and Langmuir-Blodgett (LB) methods. a) Author to whom correspondence should be addressed. Electronic mail: Tel.: þ55 34 32394190. Fax: þ55 34 32394190. 0021-8979/2013/113(14)/144509/7/$30.00 Significantly, the synthesis route to yield PPV using the polymer precursor poly(xilidenotetrahydrothiophenium chloride) (PTHT) already offers opportunities to tune the PPV properties, as the PTHT chorine counter ion may be replaced by dodecylbenzene sulfonic acid (DBS),10 thus allowing for thermal conversion into PPV in considerably less time and at lower temperatures than in typical conversion procedures.11 Furthermore, with such facile conversion, the number of structural defects arising from oxidative chemical groups (e.g., carbonyl groups) is decreased considerably. Among the several ways to control the structuring of materials in LbL films is the deposition of the layers on a cushion, for several purposes. The cushion may be used to control surface roughness, to decrease the electrical charge on the surface, to alter interaction with the substrate, or to obtain a layer compatible (or incompatible) with a given type of molecule. Therefore, in choosing the material for the cushion, one has to take into account its charge in solution in addition to the desired final property for the LbL films. In earlier work,5 we showed that the operating voltage of PLEDs could be decreased fourfold and the thermal stability of the luminescent properties improved if the PPV LbL films were deposited on a poly (o-methoxyaniline) (POMA) cushion. In this paper, we extend the study to investigate possible effects from changing the thickness of the cushion or of the LbL PPV film and explain the origin of the phenomenon leading to the enhanced 113, 144509-1 C 2013 American Institute of Physics V [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to ] IP: On: Wed, 05 Feb 2014 15:45:18 144509-2 Marletta et al. FIG. 1. Chemical formula of poly(o-methoxyaniline) emeraldine salt. performance. The latter was done with the identification of structural defects from Raman spectroscopy data and with ab initio calculations for PPV molecules submitted to an external electric field that controls their molecular conformation. II. METHODOLOGIES Figure 1 shows the chemical structure of POMA in the salt emeraldine form, which is water soluble and used in the preparation of LbL films. Details of the POMA synthesis and deposition processes can be found in Ref. 12. Aqueous solutions were obtained by dissolving 0.0615 g of POMA in acetonitrile and then in ultrapure water (Milli-Q) (acetonitrile/water (1 ml:49 ml)). The solution was kept in an ultrasound bath during 1 h. After 2 h at rest, the solution was filtered with a 25–50 lm porosity filter because POMA is not completely soluble in acetonitrile and water. The concentration of the filtered solution was estimated at 0.6 g l1 by measuring the ultraviolet-visible spectrum of the solution and using the calibration procedure established in Ref. 12. The semiconducting polymer PPV shown in Fig. 2(c) is commonly obtained by thermal conversion of a nonconjugated polymer precursor, PTHT (Fig. 2(a)), through the elimination of the PTHT lateral group via thermal treatment at high temperatures (200  C) for long periods of time (6 h) in vacuum. The thermal conversion introduces structural defects due to thermal oxidation, such as carbonyl groups (C ¼ O), which reduce the effective conjugation length and behave as photoquenching centers, leading to a decreased quantum efficiency for luminescence. This problem can be alleviated by using an alternative method10 where conversion may take place at low temperatures (around 110  C), within short periods of time (3 min) and in air. This route consists in the partial substitution of the Cl counter-ion of PTHT with dodecylbenzene sulfonic acid (DBS) ions, as indicated in Fig. 2(b). The substrates used were BK7glass slides (1  2 cm2) washed initially with water and detergent. To obtain J. Appl. Phys. 113, 144509 (2013) hydrophobic substrates, the BK7 slides were washed using the RCA procedure,13 i.e., with a solution containing hydrogen peroxide (H2O2) and sulfuric acid (H2SO4) in a 3:7 ratio, during 1 h in an ultrasound bath. Then, the substrates were rinsed with ultrapure water and immersed in a solution with ultrapure water (Milli Q), hydrogen peroxide (H2O2), and ammonium hydroxide (NH4OH) in the volume ratio of 5:1:1, for 30 min in an ultrasound bath. Finally, the substrates were rinsed in ultrapure water. The LbL films of POMA-PTHT/DBS were deposited onto BK7 substrates by immersing the substrates for 2 min in a solution of POMA, with concentration of 0.6 g l1, and subsequent drying with N2. This procedure was repeated several times to obtain the number of desired layers of POMA. Note that multiple layers of POMA can be deposited on the same substrate, without alternating with layers of another material, because POMA adsorption may occur via secondary interactions, such as H-bonding, in addition to electrostatic interactions.14 The substrates coated with POMA LbL films were alternately immersed in a 0.3 mg ml1 PTHT solution and a 0.1 mol l1, pH ¼ 5 DBS solution. The immersion time of the substrate in the POMA solution was 1 min, while it was 30 s for PTHT and DBS solutions. This procedure was repeated to get the number of PTHT/DBS bilayers desired. Finally, conversion into PPV was achieved via thermal treatment at high and low temperatures under vacuum (101 atm) for 2 h, thus leading to the architecture BK7/POMA-PPV/DBS LbL films. For the sake of clarity, the samples are referred to by using the label AXXXYYZZ, where XXX is the thermal conversion temperature (TC), i.e., 110  C or 230  C; YY is the number of POMA layers: 00, 02, 05, 10, and 20, and ZZ is the number of PPV/DBS bilayers: 05, 25, and 50. Linear absorption measurements were performed using a Shimatzu UV-Vis spectrometer model UV-160, under room conditions, in the range between 350 and 1000 nm. Photoluminescence (PL) measurements for various sample temperatures (16–300 K) were performed using a He closed cycle cryostat under vacuum (1.3  102 mbar) and the 458 nm line of an Arþ laser with average excitation of 5 nW cm2. The detection system was mounted using a photomultiplier on a 0.5 m monochromator for detection in the lock in mode. The ambient photoluminescence excitation (PLE) was performed in a spectrometer Hitachi F-450. Raman spectroscopy measurements were performed using a micro-Raman Spectrograph RENISHAW model in-Via, using a 785 nm FIG. 2. Thermal conversion of PTHT (a) into PPV (c), through the alternative route (b) using DBS as counter ion. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to ] IP: On: Wed, 05 Feb 2014 15:45:18 144509-3 Marletta et al. laser, grating with 1200 lines/mm and a 50 objective lens with a focal length of ca. 0.3 lm. As for the theoretical calculations, the geometry of the PPV ground state with 3 monomers in the gaseous state was determined by Density Functional Theory (DFT). The ground state formulation was based on functional hybrid B3LYP exchange and correlation, with Gaussian 6–31 G** in the Orca 2.9 package. Starting from the molecule geometry, a super-unit cell was chosen and then the torsion between adjacent monomers was obtained without allowing for structural relaxation. With periodic boundary conditions applied, the dependence of total energy with and without the electric field bias was studied in the package Siesta 3.1, based on DZP and functional exchange and correlation GGA-PBE. III. RESULTS AND DISCUSSION A. Effects from the POMA cushion The formation of PPV/DBS LbL films on a POMA cushion leads to optical absorption features characteristic of a PPV film, with some modifications caused by absorption of POMA. Figure 3 shows the absorption spectra of POMA-PPV/DBS LbL films with various numbers of bilayers and thermally converted at 110 and 230  C, where the spectra were shifted in the y-axis to facilitate visualization. For films converted at low temperature (110  C) in Figure 3(a), the first band at 445 nm is assigned to transitions from delocalized states of PPV and POMA conjugated segments, while the second one is the POMA polaronic band centered at 800 nm.10 No absorption is expected from DBS, which absorbs from 180 to 350 nm.10 The absorption of the first band increased with the number of PPV/DBS layers deposited, as expected. The conversion temperature did not affect the band assigned to PPV in the POMA-PPV/DBS LbL films, with the spectra in Figure 3(b) exhibiting essentially the same features as in Figure 3(a). On the other hand, the POMA polaronic band was red shifted for 25 and 50-bilayer PPV/DBS LbL films converted at the lower temperature (110  C). This is due to the elimination of HCl or DBS species during PPV conversion J. Appl. Phys. 113, 144509 (2013) process, which then diffuse to the POMA-PPV interface and co-dope the POMA layer.5 The polaronic band was less defined and blue shifted for the higher thermal conversion temperature (230  C), as seen in Figure 3(b), because polymer degradation is expected to increase above 150  C.10 Figures 3(c) and 3(d) display the maximum of the absorbance intensity of POMA þ PPV p!p* band (<500 nm) as a function of the number of PPV/DBS bilayers. Since the absorbance spectra between 350 and 500 nm can be described by A ¼ APOMA þ APPV, where APOMA and APPV are the absorbance of POMA and PPV, respectively, for a fixed number of POMA layers the adsorption rate for PTHT/DBS may be dA PPV obtained from the slope dAdN jAPOMA . Figures 3(c) and  dN PPV 3(d) show an almost constant slope, dAdN ffi 103 , regardless of the number of POMA layers. Furthermore, the adsorption rate is preserved after the PPV conversion procedure. The amount of material adsorbed per layer is comparable to that for PPV/DBS bilayers deposited on glass substrates.14,15 A drastic improvement is seen in thermal stability of emission properties when the POMA cushion is used for deposition of PPV/DBS films, regardless of the thermal conversion temperature. Figures 4(a) and 4(b) illustrate this finding with the emission spectra obtained at various temperatures, from 16 to 300 K, for samples A2300225 (with a 2-layer POMA cushion) and A2300025 (no cushion), respectively. Figure 4(c) shows the temperature dependence for the integrated area of the spectra (normalized to unity for 16 K) for all samples, from which one can infer a decrease of 80% in emission when the temperature is increased for the film without POMA cushion. In contrast, the decrease is much less for the PPV/DBS LbL films adsorbed on the POMA cushion. For the latter, the PL line shape and peak positions were essentially the same for all samples, regardless of the POMA or PPV/DBS thickness or conversion temperature. An electronic transition appears at 517 nm with phonon first and second replica at 555 nm and 600 nm, respectively. For the higher thermal conversion temperature, there is a small band broadening and blue shift (5 nm), probably because the film was more structurally disordered due to structural FIG. 3. Linear absorption spectra of POMA-PPV/DBS films converted at (a) 110  C and (b) 230  C for POMA layers YY ¼ 10 and PPV/DBS bilayers ZZ ¼ 05, 25, and 50. Absorbance intensity at 440 nm as a function of the number of layers of PPV/DBS bilayers for films converted at 110  C (c) and 230  C (d). The dashed lines are just to guide the eyes. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to ] IP: On: Wed, 05 Feb 2014 15:45:18 144509-4 Marletta et al. J. Appl. Phys. 113, 144509 (2013) FIG. 4. Temperature dependence of photoluminescence spectra of LbL POMA-PPV/DBS films: (a) A2300225 and (b) A2300025. (c) Temperature dependence of the integrated photoluminescence for samples A1100205, A2300205, A1102025, A2302025, and A2300025. defects, such as carbonyl groups, as the PPV glass transition temperature (Tg) is 200  C.16 For the PPV/DBS LbL films deposited on glass substrates (no cushion), the PL spectra were shifted by 15 nm in addition to the decrease in intensity.6 In summary, the quenching channels activated at high temperatures are eliminated, so that PPV films display an almost temperature-independent PL. This inactivation of quenching channels is related to elimination of carbonyl defects, as will be proven with Raman spectroscopy later on. In subsidiary experiments, we observed that this inactivation does not depend on the thickness of the POMA cushion or on the conversion temperature (see supplementary material).17 The vibrational transition probabilities were not affected by the increase in temperature for the POMA-PPV/DBS LbL films, again in contrast to the PPV/DBS film without cushion. This is shown in Table I which brings the Huang-Rhys factor, S, for all samples. This factor was calculated by fitting the emission spectra with enfocar la importancia de la producción mediática de los niños en su descubrimiento del mundo, sobre todo utilizando el periódico escolar y la imprenta. Asimismo las asociaciones de profesores trabajaron en esta línea e incluso la enseñanza católica se comprometió desde los años sesenta realizando trabajos originales en el marco de la corriente del Lenguaje Total. Páginas 43-48 45 Comunicar, 28, 2007 En el ámbito de los medios, también desde el principio del siglo XX hay ciertas corrientes de conexión. Pero es a lo largo de los años sesenta cuando se constituyeron asociaciones de periodistas apasionados por sus funciones de mediadores, que fomentaron la importancia ciudadana de los medios como algo cercano a los jóvenes, a los profesores y a las familias. Así se crearon la APIJ (Asociación de Prensa Información para la Juventud), la ARPEJ (Asociación Regional de Prensa y Enseñanza para la Juventud), el CIPE (Comité Interprofesional para la Prensa en la Escuela) o la APE (Asociación de Prensa y Enseñanza), todas ellas para la prensa escrita Estas asociaciones fueron precedidas por movimientos surgidos en mayo de 1968, como el CREPAC que, utilizando películas realizadas por periodistas conocidos, aclaraba temas que habían sido manipulados por una televisión demasiado próxima al poder político y realizaba encuentros con grupos de telespectadores. cipio del siglo XX, y nos han legado textos fundadores muy preciados, importantes trabajos de campo y muchos logros educativos y pedagógicos. La educación en medios ha tenido carácter de oficialidad de múltiples maneras, aunque nunca como una enseñanza global. Así la campaña «Operación Joven Telespectador Activo» (JTA), lanzada al final de los años setenta y financiada de manera interministerial para hacer reflexionar sobre las prácticas televisuales de los jóvenes, la creación del CLEMI (Centro de Educación y Medios de Comunicación) en el seno del Ministerio de Educación Nacional en 1983, la creación de la optativa «Cine-audiovisual» en los bachilleratos de humanidades de los institutos en 1984 (primer bachillerato en 1989) y múltiples referencias a la educación de la imagen, de la prensa, de Internet. La forma más visible y rápida de evaluar el lugar de la educación en medios es valorar el lugar que se le ha reservado en los libros de texto del sistema educa- 2. Construir la educación en los medios sin nombrarla El lugar que ocupa la edu- La denominación «educación en medios», que debería cación en los medios es muy ambiguo, aunque las cosas están cambiando recientemente. entenderse como un concepto integrador que reagrupase todos los medios presentes y futuros, es a menudo percibida En principio, en Francia, co- por los «tradicionalistas de la cultura» como una tendencia mo en muchos otros países, la educación en los medios no es hacia la masificación y la pérdida de la calidad. una disciplina escolar a tiempo completo, sino que se ha ido conformado progresivamente a través de experiencias y reflexiones teóricas que han tivo en Francia. Una inmersión sistemática nos permi- permitido implantar interesantes actividades de carác- te constatar que los textos oficiales acogen numerosos ter puntual. Se ha ganado poco a poco el reconoci- ejemplos, citas, sin delimitarla con precisión. miento de la institución educativa y la comunidad es- colar. Podemos decir que ha conquistado un «lugar», 3. ¿Por qué la escuela ha necesitado casi un siglo en el ámbito de la enseñanza transversal entre las dis- para oficilializar lo que cotidianamente se hacía en ciplinas existentes. ella? Sin embargo, la escuela no está sola en esta aspi- Primero, porque las prácticas de educación en me- ración, porque el trabajo en medios es valorado igual- dios han existido antes de ser nombradas así. Recor- mente por el Ministerio de Cultura (campañas de foto- demos que no fue hasta 1973 cuando aparece este grafía, la llamada «Operación Escuelas», presencia de término y que su definición se debe a los expertos del colegios e institutos en el cine ), así como el Minis- Consejo Internacional del Cine y de la Televisión, que terio de la Juventud y Deportes que ha emprendido en el seno de la UNESCO, definen de esta forma: numerosas iniciativas. «Por educación en medios conviene entender el estu- Así, esta presencia de la educación en los medios dio, la enseñanza, el aprendizaje de los medios moder- no ha sido oficial. ¡La educación de los medios no apa- nos de comunicación y de expresión que forman parte rece oficialmente como tal en los textos de la escuela de un dominio específico y autónomo de conocimien- francesa hasta 2006! tos en la teoría y la práctica pedagógicas, a diferencia Este hecho no nos puede dejar de sorprender ya de su utilización como auxiliar para la enseñanza y el que las experiencias se han multiplicado desde el prin- aprendizaje en otros dominios de conocimientos tales Páginas 43-48 46 Comunicar, 28, 2007 como los de matemáticas, ciencias y geografía». A pe- mente en todas las asignaturas. Incluso los nuevos cu- sar de que esta definición ha servido para otorgarle un rrículos de materias científicas en 2006 para los alum- reconocimiento real, los debates sobre lo que abarca y nos de 11 a 18 años hacen referencia a la necesidad no, no están totalmente extinguidos. de trabajar sobre la información científica y técnica y En segundo lugar, porque si bien a la escuela fran- el uso de las imágenes que nacen de ella. cesa le gusta la innovación, después duda mucho en Desde junio de 2006, aparece oficialmente el tér- reflejar y sancionar estas prácticas innovadoras en sus mino «educación en medios» al publicar el Ministerio textos oficiales. Nos encontramos con una tradición de Educación los nuevos contenidos mínimos y las sólidamente fundada sobre una transmisión de conoci- competencias que deben adquirir los jóvenes al salir mientos muy estructurados, organizados en disciplinas del sistema educativo. escolares que se dedican la mayor parte a transmitir Este documento pretende averiguar cuáles son los conocimientos teóricos. La pedagogía es a menudo se- conocimientos y las competencias indispensables que cundaria, aunque los profesores disfrutan de una ver- deben dominar para terminar con éxito su escolaridad, dadera libertad pedagógica en sus clases. El trabajo seguir su formación y construir su futuro personal y crítico sobre los medios que estaba aún en elaboración profesional. Siete competencias diferentes han sido te- necesitaba este empuje para hacerse oficial. nidas en cuenta y en cada una de ellas, el trabajo con Aunque el trabajo de educación en los medios no los medios es reconocido frecuentemente. Para citar esté reconocido como disciplina, no está ausente de un ejemplo, la competencia sobre el dominio de la len- gua francesa definen las capa- cidades para expresarse oral- La metodología elaborada en el marco de la educación en mente que pueden adquirirse con la utilización de la radio e, medios parece incluso permitir la inclinación de la sociedad incluso, se propone fomentar de la información hacia una sociedad del conocimiento, como defiende la UNESCO. En Francia, se necesitaría unir el interés por la lectura a través de la lectura de la prensa. La educación en los medios las fuerzas dispersas en función de los soportes mediáticos y orientarse más hacia la educación en medios que al dominio adquiere pleno derecho y entidad en la sección sexta titulada «competencias sociales y cívi- técnico de los aparatos. cas» que indica que «los alum- nos deberán ser capaces de juz- gar y tendrán espíritu crítico, lo que supone ser educados en los las programaciones oficiales, ya que, a lo largo de un medios y tener conciencia de su lugar y de su influencia estudio de los textos, los documentalistas del CLEMI en la sociedad». han podido señalar más de una centena de referencias a la educación de los medios en el seno de disciplinas 4. Un entorno positivo como el francés, la historia, la geografía, las lenguas, Si nos atenemos a las cifras, el panorama de la las artes plásticas : trabajos sobre las portadas de educación en medios es muy positivo. Una gran ope- prensa, reflexiones sobre temas mediáticos, análisis de ración de visibilidad como la «Semana de la prensa y publicidad, análisis de imágenes desde todos los ángu- de los medios en la escuela», coordinada por el CLE- los, reflexión sobre las noticias en los países europeos, MI, confirma año tras año, después de 17 convocato- información y opinión rias, el atractivo que ejerce sobre los profesores y los Esta presencia se constata desde la escuela mater- alumnos. Concebida como una gran operación de nal (2 a 6 años) donde, por ejemplo, se le pregunta a complementariedad source of circulating FGF-21. The lack of association between circulating and muscle-expressed FGF-21 also suggests that muscle FGF-21 primarily works in a local manner regulating glucose metabolism in the muscle and/or signals to the adipose tissue in close contact to the muscle. Our study has some limitations. The number of subjects is small and some correlations could have been significant with greater statistical power. Another aspect is that protein levels of FGF-21 were not determined in the muscles extracts, consequently we cannot be sure the increase in FGF-21 mRNA is followed by increased protein expression. In conclusion, we show that FGF-21 mRNA is increased in skeletal muscle in HIV patients and that FGF-21 mRNA in muscle correlates to whole-body (primarily reflecting muscle) insulin resistance. These findings add to the evidence that FGF-21 is a myokine and that muscle FGF-21 might primarily work in an autocrine manner. Acknowledgments We thank the subjects for their participation in this study. Ruth Rousing, Hanne Willumsen, Carsten Nielsen and Flemming Jessen are thanked for excellent technical help. The Danish HIV-Cohort is thanked for providing us HIV-related data. 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