Artigo:

Poly(N,N-dimethylaminoethyl methacrylate) modified ITO (indium tin oxide) covered with gold nanoparticles electrode was synthesized by grafting to method and stimuli responses to pH and ionic strength (IS) was evaluated in PBS solution. At pHAndlt;5 PDMAEMA chains were found to be stretched generating a great electrochemical signal and at pHAndlt;5 collapsed with a signal decreasing. In IS≤0.001 molL-1 NaCl polymer chains were stretched and IS≥0.001mol L-1 collapsed. An interesting fact was observed when adding 0.001 molL-1 NaCl in a pH 7.40 solution, the signal response was increased comparing to pH 7.40 and 0 molL-1 NaCl, but when adding 0.001 molL-1 NaCl to a pH 3.00 solution, the signal response was decreased. This behavior can be explained in terms of protonation/deprotonation process and polymer brush regime. PDMAEMA brushes response was investigated by localized surface plasmonic resonance measurements (L-SPR), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). 1. INTRODUCTION Stimuli- responsive interfaces, which switch their physical and chemical properties in response to external stimuli, have attract interest of many field of technology such as drug delivery, diagnostics, tissue engineering and ‘smart’ optical systems, as well as biosensors, micro-electromechanical systems, coatings and textiles (Cohen et al., 2010). Polymer brushes interfaces are typical example of such materials, they are polymer chains with one end attached to a surface, and have the property to switch the configuration, they can be stretched away from the surface (swollen), when in a good solvent for example, or they can be collapsed (shanked), in a bad solvent. Poly(acrilyc acid) has been extensively used as sensor pattern, in a previously work (Sempionatto et al., 2014) it was demonstrated the behavior of PAA when pH solution was changed, in a range pH of 3.00 to 5.00 the polymer chains were stretched and at pH ≥ 6 the chains were collapsed. Poly(2,2 DMAEMA) are also largely study, in a recent work (Crulhas et al, 2014) it was also demonstrated the PDMAEMA brush application as a glucose sensor – with glucose oxidase as an enzyme probe, which demonstrated a detection limit of 5.6x10–6 molL-1. Área temática: Engenharia de Materiais e Nanotecnologia 1The use of gold nanoparticles enhances the electrical signal and provide plasmon signal, plasmon is the phenomena that occurs when confined electrons on the boundaries of their materials oscillate within the electron-magnetic waves, generating quantified plasmon (Link and El-Sayed, 1999). In this work PDMAEMA brushes and gold nanoparticles were used to study their behavior when submitted to a pH range from 3.00 to 7.40 with different IS. Adding 0.001 molL-1 NaCl in a pH 7.40 solution, the signal response increased comparing to pH 7.40 and 0 molL-1 NaCl and when adding 0.001 molL-1 NaCl to a pH 3.00 solution, the signal response decreased. 2. EXPERIMENTAL PROCEDURE 2.1 Chemicals and Reagents PDMAEMA (P-9739-DMAEMA, Mw = 3100 g mol−1, Polymer Source), 3-glycidyloxypropyl-trimethoxysilane (GPS, Sigma-Aldrich), trisodium citrate dehydrate, toluene, Potassium hexacyanoferrate(III) [Fe(CN)6]-4 (Sigma-Aldrich), hydrogen tetrachloroaurate(III) (HAuCl4•3H2O). ITO-coated glass (60 Ω/sq surface resistivity, Sigma-Aldrich) served as the working electrode, Pt/Ti Titanium Wire Anode ETO78 was employed as a counter electrode, and the reference electrode was a Ag/AgCl (3.0 mol L−1) for electrochemical measurements. Ultrapure water from a Milli-Q (Millipore Inc.) purification system was used in all the experiments. 2.2 Modification of electrodes The ITO electrodes were chemically modified with PDMAEMA brushes using the "grafting- to" method (Wittemann et al, 2005) according to the following procedure. ITO-coated glass slides were cut into 30 mm × 8 mm strips. They were cleaned with ethanol in an ultrasonic bath for 15 min and dried in the atmosphere. The cleaning step was repeated using 2-butanone as a solvent. The initial cleaning steps were followed by immersing the strips into a cleaning solution composed of NH4OH, H2O2, and H2O in a ratio of 1:1:1 (v/v/v) for 30 min. Subsequently, the glass strips were rinsed with water and then dried under atmosphere. The freshly cleaned ITO strips were reacted with 0.1% v/v GPS in dry toluene overnight. The silanized ITO was rinsed with several aliquots of toluene. Then 60 μL of a 1% wt PDMAEMA solution in toluene was spin coated to the surface of each ITO glass strip at 3000 rpm and left to react in a vacuum oven at 140 °C overnight. The final cleaning step to remove the unbound polymer consisted of soaking the samples in toluene for 10 min. 2.3 Preparation of Gold Nanoparticles The synthesis of gold nanoparticles is described elsewhere (Roiter et al, 2012). Gold nanoparticles were attached on the PDMAEMA polymer brushes from a 1 mmolL-1 solution in water by incubating the samples overnight. The electrode was rinsed by several times with water to removed unbound gold nanoparticles. 2.4 Equipment and Measurement Electrochemical measurements were performed with an ECO Chemie Autolab Microautolab Área temática: Engenharia de Materiais e Nanotecnologia 2III/FRA2 with an electrochemical analyzer and a NOVA 10.0 software package. CV measurements and EIS analysis were performed with a three-electrode system in a standard eDAQ (Australia) ET-073 cell, using the polymer brush-modified ITO as working electrode, a Ag/AgCl/KCl 3 molL-1 as a reference electrode, and a Pt/Ti Titanium Wire anode ETO78 as a counter electrode. CVs were performed from -0.2 to 0.7 V and the EIS analysis were recorded while applying a bias potential of 0.3 V and using a 10 mV alternative voltage in the frequency range of 100 kHz – 100 mHz. All solutions were buffered (0.01 molL-1 phosphate buffer titrated to the pH values specified in the text with the use of NaOH or HCl) and 1.2 mmolL-1 [Fe(CN)6]-4 was used as redox probe. The measurements were carried out at ambient temperature. The pH measurements were performed with a Metrohm 827 pH Lab. TEM images were recording using a CM100, Philips. The AFM experiments were carried out with an Explorer model from Digital Instruments model Multimade. To minimize the surface deformation and material removal, the experiments were performed in intermittent non-contact mode, using a silicon cantilever with a spring constant of 70 Nm-1 at scan rate of 1Hz. All measurements were taken under air at room temperature. As the roughness value depends on the observation scale, all experiments were carried out on a scale of 4 µm. UV−vis spectra were obtained using a Biochrom Libra S11 spectrophotometer from 400-600 nm with 1.0 nm step and speed of 500 nm mim-1. A reference of PBS at specified pH was taken before each scan. 3. RESULTS AND DISCUTION 3.1 Gold Nanoparticles characterization Figure 1 – A) MEV gold nanoparticle, bar scale: 0.1 µm. B) Size distribution histogram, N=400 (ImageJ). C) Absorbance measurement of gold nanoparticles. Gold colloids are composed of an internal core of pure gold that is surrounded by a surface layer of adsorbed AuCl–2 ions. These negatively charged ions confer a negative charge to the colloidal gold and thus, through electrostatic repulsion, prevent particle aggregation. All gold colloids display a single absorption peak in the visible range between 510 and 550 nm (Chaudhuri and Raychaudhuri, 2001), in this work the absorption peak was 530 nm (Figure 1C) which agrees with the average diameter 23.06 nm (Figure 1B). The gold nanoparticles showed a good size distribution and a relatively good dispersion. 3.2 Electrode Characterization