Functionalised nanostructured polyaniline? A new substrate for building adaptive sensing surfaces

A new method for covalently binding side-chains to the surface of solution based conducting polymer nanostructures is introduced in this paper. Modification of the structures is achieved by convenient reflux in the presence of a nucleophile, and post-functionalization purification is subsequently carried out by centrifugation. The entire process is easily scalable and hence suitable for bulk production of functionalized nanomaterials. In particular we focus on the modification of polyaniline nanofibres which can be synthesized by interfacial polymerization. Mercaptoundecanoic acid side-chains are attached to the polymer nanostructures, with the intrinsic nano-morphology of the material being maintained during the process. The modified PAni nanofibres provide a template for the attachment of other specific functional groups which could be used to target a particular species

Towards the development of adaptive nanostructured platforms

Since their discovery in 1977, intrinsically conducting polymers have been studies for applications such as electronic devices, sensors and actuators[1-3]. Polyaniline (PAni) is an example of a stable conducting polymer and can be classified as an ‘adaptive material’ in that it can be switched between two or more forms (each with their own distinct characteristics) using an external stimulus. In contrast to a classical metallic conductor or a polymeric insulator, PAni can be switched reversibly between an insulating emeraldine base form and a conducting emeraldine salt. More recently, interest has developed in the area of nanostructured polyaniline[4-6]. These one-dimensional objects combine the advantages of an organic conductor and a high surface area material, thus making them suitable for a diverse range of applications such as chemical sensors, flash memory and electro-optic devices[7-9]. Here we present how polyaniline nanofibres can be successfully functionalised with both amine and carboxylate groups. The modified nanofibres maintain their ability to switch between diffferent forms displaying distinctly different optical properties (as shown by Raman and UV-vis spectroscopy), thus making them suitable for adaptive sensing applications. The attachment of functional groups to polyaniline nanofibres provides a route for manipulating the surface chemistry of nanofibres. While interesting materials in themselves, these functionalised nanofibres are also attractive as molecular scaffolds for building yet more innovative derivatives that nonetheless retain the basic underlying nanostructure and intrinsic characteristics of PAni. That we have demonstrated the ability to regulate the extent of side-chain attachment to one-dimensional objects, in a safe and simple manner, represents a step forward in the area of adaptive nano-structured materials. Functionalisation can be controlled using a simple, scalable and inexpensive technique[10-11]. [1] C. O. Baker, B. Shedd, P. C. Innis, P. G. Whitten, G. M. Spinks, G. G. Wallace, R. B. Kaner, Adv Mater 20 (2008) 155-+. [2] W. R. Small, F. Masdarolomoor, G. G. Wallace, M. Panhuis, J Mater Chem 17 (2007) 4359-4361. [3] J. G. Roh, H. R. Hwang, J. B. Yu, J. O. Lim, J. S. Huh, Journal of Macromolecular Science-Pure and Applied Chemistry A39 (2002) 1095-1105. [4] J. X. Huang, R. B. Kaner, Chemical Communications (2006) 367-376. [5] N. R. Chiou, C. M. Lui, J. J. Guan, L. J. Lee, A. J. Epstein, Nat. Nanotechnol. 2 (2007) 354-357. [6] F. Masdarolomoor, P. C. Innis, S. Ashraf, R. B. Kaner, G. G. Wallace, Macromol. Rapid Commun. 27 (2006) 1995-2000. [7] S. Virji, J. X. Huang, R. B. Kaner, B. H. Weiller, Nano Lett. 4 (2004) 491-496. [8] S. Virji, R. B. Kaner, B. H. Weiller, J. Phys. Chem. B 110 (2006) 22266-22270. [9] S. Virji, R. B. Kaner, B. H. Weiller, Chemistry of Materials 17 (2005) 1256-1260. [10] E. Lahiff, T. Woods, W. Blau, G.G. Wallace, D. Diamond, Synth. Metals, accepted. [11] E. Lahiff, S. Bell, D. Diamond, Mat. Res. Soc. Symp. Proc., Vol. 1054, FF-05-05, 200

Functionalised polyanaline nanofibers

Polyaniline (PAni) is a conducting polymer which switches between distinct states exhibiting dramatically different properties. The colour, conductivity and redox state of PAni all depend on the local chemical environment of the material. Consequently PAni has great potential for sensing applications. The nanostructured form of PAni is particularly interesting as it provides a very large surface-to-volume ratio that can lead to dramatic enhancement of sensor sensitivity and response time. In this work, we focus on derivatising polyaniline nanofibres. Using the technique described, carboxylate terminated side-chains can be covalently bound to solution based fibres

Modified polyaniline nanofibres for ascorbic acid detection

Polyaniline nanofibres (PAni) can be surface modified to improve electroactivity over a broader pH range. The technique we describe here can be used to attach carboxylic acid terminated substituents. Modified nanofibres maintain their high surface area, and ability to switch between different redox states. These properties make the material suitable for sensing applications. Unlike unmodified PAni, the functionalised material is self-doping and hence more stable in higher pH solutions. Here we demonstrate how modified PAni fibres can be used for the detection of ascorbic acid

Sensing the flow: Adaptive coatings based on polyaniline for direcct observation of mixing processes in micro-fluidic systems

In this abstract, we report the specific mixing and fluidic behavior of two reacting solutions of HCl and NaOH in a glass/PDMS microchip, using adaptive coatings, covalently attached to the microchannel walls, based on the conductive polymer, polyaniline (PAni). Lab-on-a-chip technology is attracting great interest as the miniaturisation of reaction systems offers practical advantages over classical bench-top chemical synthesis. In particular, rapid mixing of the fluids flowing through a micro-channel is very important for various applications of micro-fluidic systems [1]. In addition, on-chip detection techniques are essential for the continuous monitoring of the mixing behavior of confluent streams. For this purpose many spectroscopic detection methods have been employed: laser-induced fluorescence, confocal fluorescence microscopy, ultraviolet absorption, chemiluminescence. These spectroscopic techniques provide good opportunities for the detection of chemical species and are suitable for studying mixing in micro-fluidic devices. However, these techniques typically require the addition of a dye or pretreatment of a solute species with florescent tags to allow on-chip detection. Consequently, in these approaches one follows the bulk behaviour of an added solute, rather than the solvent/liquid itself. In this paper we demonstrate the possibility of quantitatively evaluate the mixing process in label and solute free conditions, using chromo-responsive coatings based on polyaniline. Polyaniline is an example of conductive polymer whose optical proprieties change in response to changes in the local environment. Thus PAni has huge potential for sensing applications and has been used extensively as material for optical pH sensors due to its strong pH sensitivity [2]. We were interested in investigating whether coatings based on PAni could be used to study pH gradients in micro-fluidic devices. The functionalisation of the inner walls of the micro-fluidic channel with PAni nanofibres was achieved using the "grafting from" approach. In this way, homogeneous PAni coatings were obtained on the microchannel surface while maintaining the nanomorphology of PAni. These PAni coatings respond very well to changes in pH as shown by the absorption measurements of the channel coating. To study mixing in this device, colorless hydrochloric acid (10-2M, pH=2) and sodium hydroxide (10-3M, pH=11) solutions were pumped into the two arms of a Y-shaped microchannel, 1000x100μm and 30mm long. The two liquid streams meet at the Y-junction, and have an interaction time defined by the flow rate, which was varied between 0.5-3μl/min. A plot of the mixing point (i.e. the point at which the blue colour disappears relative to the meeting point at the Y- junction) against flow rate presents good linearity showing the utility of this approach for investigating diffusion and mixing processes of solutions in micro-channels and also for obtaining useful results for the optimal design of micro-reactors for chemical synthesis applications. Moreover these coatings can also be employed as indicators in the case of non-reacting fluids offering a new method of studying proton diffusion with and without a chemical reaction [3]. REFERENCES: 1. “Chaotic mixer for microchannels,” A. D. Stroock, S. K. W. Dertinger, A. Ajdari, I. Mezic, H. A. Stone, G. M. Whiteside, Science (Washington, D.C.) 295, 647 (2002). 2. “Optical sensing of pH using thin films of substituted polyanilines,” E. Pringsheim, E. Terpetschnig, O.S. Wolfbeis, Analytica Chimica Acta, 357, 247 (1997). 3. “Rapid proton diffusion in microfluidic devices by means of micro-LIF technique,” K. Shinohara, Y. Sugii, A. Hibara, M. Tokeshi, T. Kitamori, K. Okamoto; Experiments in Fluids 38, 117 (2005)

Adaptive coatings based on polyaniline for dynamic pH sensing in micro-fluidic devices

Polymer brushes based on the conducting polymer polyaniline were synthesized on the interior of micro-capillaries and micro-fluidic channels to study the influence of these brushes on solvent flow through a confined space. The polyaniline brushes are formed using

Self-doping polyaniline nanofibres

Polyaniline (PAni) is an example of a conducting polymer whose properties (optical/electrical) change in response to the immediate environment of the material. PAni thus has huge potential for both sensing and electrostatic discharge applications. By focusing on PAni nanofibres we can increase the surface area of the material [1]. Our focus is to explore the self doping behavior of acid functionalised PAni nanofibres. Functionalisation is achieved by a quick and scalable reflux process [2,3]. Carboxylic acid side-groups can be attached and these groups act to self-dope the polymer nanofibres. Using this technique the stability of the material improves, whilst simultaneously maintaining its ability to switch between different forms displaying distinctly different properties. The resulting material is characterised using electron microscopy, nuclear magnetic resonance and a range of spectroscopic techniques. The techniques used both confirm the covalent attachment of functional side-groups and also reveal the material to be self-doping. While interesting materials themselves these functionalised PAni nanofibres are also attractive as molecular scaffolds for building new more innovative derivatives that retain the nanostructure and characteristics of Pani while improving its selectivity. [1] J.X Huang, S, J.Am.Chem.Soc.125 (2003), 314-315. [2] E.Lahiff, Synth. Met. 159 (2009), 741-748. [3] E. Lahiff, D. Diamond, International Journal of Nanomanufacturing, Vol. 5, (2010)

Dynamic pH sensing in micro-fluidic devices using adaptive coatings based on polyaniline

In this abstract, we present a micro-fluidic device that has integrated pH optical sensing capabilities based on polyaniline (PAni) suitable for pH detection in continuous flow. The polyaniline coating is covalently attached to the inner wall of the micro-channel using the “grafting from” approach. The optical proprieties of these polyaniline coatings change in response to the pH of the solution that is flushed inside the micro-channel. These unique properties offer the possibility of monitoring pH in continuous flow over a wide pH range over the entire channel length. In the last few years conducting polymers have been used to prepare optical pH sensors. Through oxidative polymerisation of an appropriate monomer, conducting polymer films with suitable optical properties for pH sensing can be obtained. This technique eliminates the need of using organic dyes as in conventional optical sensors. Among these conducting polymers, polyaniline has received significant attention because of its suitability over a wide pH range [2-4]. By focusing on PAni nanofibres we can dramatically increase the surface area of the material, which manifests in improved response times and sensitivity. Therefore our approach presents a new, simple, and fast photometric method to measure pH using PAni based coatings in micro-channels. The pH measurement can be in continuous flow mode using fiber-optic light guides, or along the entire micro-fluidic system using digital imaging. The functionalisation of the inner walls of the micro-fluidic channel with PAni nanofibres was achieved. Briefly, immediately after exposure of the PDMS chips to oxygen plasma and sealing to the glass slide/PDMS layer, the activated channels (1000x100μm) were flushed with a 20%wt solution of N-[3-(Trimethoxylsilyl) propyl]aniline in ethanol for 60 min at a flow rate of 0.5 μl/min. The channel was then washed with ethanol and filled with 1M HCl solution containing an equimolar concentration of oxidant (ammonium peroxydisulfate) and aniline. After polymerisation, the channels were washed extensively with water to remove any unattached polyaniline nanofibres. Using this technique, homogeneous PAni coatings were obtained, covalently attached to the internal walls of micro-channels made of PDMS/PDMS or PDMS/glass. The transformation of PAni between its Emeraldine Salt form (ES) to the Emeraldine Base (EB) in response to basic solution passing through the channel is accompanied by significant changes in colour. The UV-VIS spectra of the polyaniline coating shows the potential of these adaptive coatings for pH sensing in micro-fluidic devices. A plot of the absorbance at 580 nm vs. pH shows the excellent performance for detection over the range pH 2-10. Moreover, the rapid dynamics of the coating response across the entire micro- fluidic system implies that it can be used to monitor spatial effects and diffusion processes in channels. REFERENCES: 1.”Polymer brushes: surface-immobilized macromolecules,” B. Zhao, WJ. Brittain, Progress in Polymer Science, 25, 677 (2000). 2.”Fiberoptic pH sensor-based on evanescent-wave absorption-spectroscopy,” Z. Ge, C.W. Brown, L. Sun, S.C. Yang, Analytical Chemistry, 65, 2335 (1993). 3.”Optical sensing of pH using thin films of substituted polyanilines,” E. Pringsheim, E. Terpetschnig, O.S. Wolfbeis, Analytica Chimica Acta, 357, 247 (1997). 4.”Polyaniline based optical pH sensor,” U.-W. Grummt, A. Pron, M. Zagorska, S. Lefrant, Analytical Chimica Acta, 357, 253 (1997)

Polyaniline nanofibres as templates for the covalent immobilisation of biomolecules

The attachment of antibodies onto polyaniline nanofibres using covalent chemistry was investigated for the first time. Polyaniline nanofibres were functionalised post-polymerisation to attach either amide or carboxylic acid side-groups. These templates could then be further modified to attach antibodies, specifically in this instance mouse immunoglobulin G (IgG). The resultant conjugates were characterised using a variety of techniques including infrared, UV–vis and Raman spectroscopy. Conjugates were then used to detect secondary antibodies (anti-IgG). Results from enzyme-linked immunoassay studies indi- cate successful binding of the antibody to the polyaniline nanofibres. Carboxyl functionalised polyaniline nanofibres are shown in particular to decrease non-specific binding in the immunoassay. Direct electri- cal communication between polyaniline nanofibres covalently linked to peroxidase-labelled antibodies was observed during cyclic voltammetry, which demonstrates their potential for further development as nano-dimensional immunosensors