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ScienceDirect Procedía
Engineering
Procedía Engineering 5 (2010) 1240-1243
www.elsevier.com/locate/procedia
Proc. Eurosensors XXIV, September 5-8, 2010, Linz, Austria PHEMA functionalized gold nanoparticle films for vapor sensing
Jun Tang1, E. Skotadis1, V. Tsouti2, D. Tsoukalas12*
1 Department of Applied Physics, National Technical University of Athens 15780 Zographou, Greece 2 Institute of Microelectronics, NCSR Demokritos", Athens, Agia Paraskevi 15310, Greece
Abstract
The fabrication of a chemical sensor based on the technique of gold nanoparticle ink-jet printing and the subsequent functionalization of the nanoparticle film by Polyhydroxyethylmethacrylate (PHEMA) polymer is presented while its chemi-resistive, chemi-capacitive and chemi-impedance response are investigated. By controlling the Gold nanoparticle density and selecting different interdigitated electrode distances and nanoparticle sizes, chemical sensors of different linearity and sensitivity can be obtained.
© 2010 Published by Elsevier Ltd.
Keywords: Chemical sensor, Gold nanoparticles, ink-jet printing, chemiresistance, chemicapacitance, chemi-impedance, PHEMA
1. Introduction
The use of nanostructured materials as building blocks for chemical/biological sensing has seen much progress over the last years. Devices incorporating metal[1] or semiconducting[2] nanoparticles, carbon nanotubes[3], polymer[4] materials or certain hybrid nanomaterials[5] have already been widely investigated while different sensing techniques taking advantage of the resistive, capacitive or the impedance properties of the devices have been presented exhibiting good performance. In this case GNP of varying size (100nm and 5nm) have been delivered onto silicon oxidized wafers previously patterned with gold interdigitated electrodes. On top of this GNP film a second layer of PHEMA Polymer has been delivered as the active sensing layer of the device. Ink-jet printing has been used in order to deliver the desired quantity of colloidal GNP and PHEMA polymer on the substrates. PHEMA
being in liquid phase_
Nomenclature
GNP Gold Nanoparticles
PHEMA Polyhydroxyethylmethacrylate
* Corresponding author. Tel.: 00302107722929; fax: 00302107723338. E-mail address: dtsouk@central.ntua.gr
1877-7058 © 2010 Published by Elsevier Ltd. doi:10.1016/j.proeng.2010.09.337
flows inside the porous GNP film forming a hybrid polymer/nanoparticle material. By controlling the density and the size of the GNP, sensors of different sensitivity and linearity have been obtained.
2. Experimental
Silicon wafers with 1 p,m thermal SiO2 were used as substrates. Gold electrodes of 50nm in thickness and of two different electrode distances (5p,m and 30p,m) were fabricated by e-gun evaporation with a 7nm Ti adhesion layer. GNP of 5nm and 100nm in diameter stabilized in water and capped with citrate acid (negative charged) have been purchased from BBI int. and have been used in the as purchased concentration. PHEMA polymer was diluted in etlyl-lactete with the concentration of 0.5%. A micro-drop ink-jet dispenser system (microdrop technologies Gmbh) has been used to deliver both polymer and GNP to the substrates. All experiments have been conducted under room temperature. Prior to the delivery of the GNP the SiO2 substrates have been functionalized using a solution of 3-[2-(2-aminoethylamino) ethyl amino] pro - pyltrimethoxysilane (AEEPTMS). The hybrid gold nanoparticle and PHEMA film patterns were characterized by optical measurements and Field Emission Scanning Transmission Microscopy (FESEM). Electrical characterization was conducted using Keithley 2000, HP-4278A and HP-4192 instruments for Resistance, Capacitance and Impedance measurements respectively. The evaluation of the sensor's response was performed in a small volume (4mm3) chamber where relative humidity and temperature are controlled to within 0.1% and 0.1 °C.
Fig. 1 (a) Optical images of 1000 drops of 100nm Au nanoparticles deposited on gold electrodes on SiO2 substrate; (b) schematic of the nanoparticle and PHEMA hybrid material deposited on the gold electrodes; (c) FESEM images of 1000 drops of 100nm gold nanoparticle colloid with 1000 drops of PHEMA; (d) FESEM images of 3000 drops of 100nm gold nanoparticle colloid with 1000 drops of PHEMA
3. Results/Discussion
3.1 Sensor fabrication
After selecting a 5Hz printing frequency 5nm and 100nm GNP have been printed on the gold interdigitated electrode structures. As can be seen in figure 1, a rather opaque GNP ring has been formed in the circumference of the printed drops, after the evaporation of the solvent (water). The formation of this ring is associated with a capillary flow mechanism studied extensively and referred to as the ''coffee ring'' effect[6]. The diameter of this nanoparticle ring can vary and is dependent on the total number of ink-jet printed drops, the delivery frequency and
the surface functionalization. As can be seen in figures 1(c) and 1(d) by increasing the total amount of printed drops from 1000 to 3000 a continuous GNP ring is formed making it possible to control the ring's formation by selecting an appropriate number of printed drops.
During the experiments/measurements we found GNP films printed on AEEPTMS functionalized substrates to be more reproducible and stable due to the attraction of the negatively charged gold nanoparticles to the positively charged amino groups on the AEPTMS monolayer therefore deciding on depositing the GNP only on such functionalized substrates. Following the GNP deposition a PHEMA polymer layer has been ink-jet printed on top of the conductive nanoparticle layer thus forming a hybrid material of GNP embedded in a polymer matrix.
Summarizing it's clear that by controlling the overall amount of printed gold colloid drops, GNP ''rings'' of different thicknesses can be achieved. When this is combined with the selection of different nanoparticle size and electrode distances a variety of different sensing devices can be fabricated.
3.2 Sensing mechanism/results
Resistance, capacitance and impedance changes have been monitored for a sensor fabricated by 1000 printed drops of GNP and the subsequent printing of 1000 drops of PHEMA polymer on top of 5p,m and 30|im interdigitated electrodes. The dynamic response for all three electrical parameters of this sensor, when exposed to vapours of water can be seen in figure 2.
The GNP films exhibit electrical conductivity which is well in agreement with an activated tunneling model for charge transport[7]. Humidity absorption from the polymer (PHEMA) will cause its swelling. This swelling will increase the inter nanoparticle distance 8 therefore increasing the resistance of the GNP film as can be seen in figure 2(a),(d). Also the humidity absorption will increase the permittivity of the film due to its relatively high permittivity constant, increasing the tunneling decay constant thus further increasing the resistance. The sensor's chemi-resistive response reveals that for a 5p,m electrode distance higher sensitivity is obtained especially for low concentration values while for a larger electrode distance (30p,m) sensitivity becomes smaller but with a linear response throughout the entire measured concentration range.
The sensing mechanism for chemi-capacitive response can also be attributed to the polymer's absorption of Humidity and to the subsequent polymer permittivity change by 8s. As can be seen in figure 2(b),(e) the permittivity and thickness increase induced by humidity absorption ultimate in a capacitance increase. In this case using a 5p,m electrode distance, lower sensitivity and linear response is obtained for the entire measurement range while for a sensor of 30|im electrode gap higher sensitivity again under low concentrations is achieved.
Finally the sensor's impedance response has been monitored for exposure to water vapors. From the relative sensor response shown in figure 2(c),(f) it is evident that either using 5p,m or 30|im electrode gaps, linear sensor behavior and similar sensitivity in the range of 2000ppm to 20000ppm is obtained.
Changing to a different nanoparticle size by keeping the GNP quantity constant, has greatly increased both resistance and capacitance while the impedance was greatly reduced. Using a 30 p,m electrode gap, response to water vapors for both capacitance and impedance was clearly observed. The resistance response of the sensor was poor and it is not discussed here. The relative change in the sensor's Capacitance showed that again for a 30p,m electrode gap the sensor's sensitivity is higher compared to the linear response of a 5p,m electrode gap, but this time with a higher sensitivity towards higher concentrations of humidity. In addition the chemi-impedance results of the sensor were similar to the ones of a 100nm sensor.
4. Conclusions
Using the ink-jet printing technique hybrid GNP/polymer chemical sensors have been fabricated. Sensor measurements using humidity vapours as a test gas revealed that by actually tuning several parameters such as the GNP density, electrode distance, GNP size chemical sensors of different sensing characteristics can be fabricated giving us the ability to accordingly design a sensor for optimizing measurements in a specific humidity range.
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-30(Jm Electrode gap distance — 5|im Electrode gap distance
a .,s.
Time (mln)
Fig. 2. 100nm GNP sensor. (a)/(d) relative change in Resistance for electrode distances of 30^m and 5^m, (b)/(e) relative change in Capacitance for electrode distances of 30^m and 5^m, (c)/(f) relative change in Impedance for electrode distances of 30^m and 5^m.
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