Chemiresistive Properties of a Novel Composite Comprised of ITO-Nanoparticles and 1 , 8-Diaminooctane †

Indium-Tin-Oxide (ITO) nanoparticles have been cross-linked with organic 1,8-Diaminooctane (C8A) molecules. The network has been prepared by layer-by-layer self-assembly on substrates with interdigitated electrodes and their sensing properties were studied with vapors of toluene, 1-propanol, water, and 4-methyl-2-pentanone. We demonstrated that the network based chemiresistor shows sensing properties towards volatile organic compounds (VOC).


Introduction
Over the past few years, one effort of materials research was the development of composite materials which can be tailor-made for certain applications on the molecular level.Especially nanocomposites based on metal nanoparticles and organic molecules show attractive possibilities in chemiresistive sensing [1].Nearly all composites use gold nanoparticles stabilized or linked with thiol based molecules.A replacement of the expensive noble metal nanoparticles with nanoparticles comprised of conductive oxides [2] or semiconductors [3] could reduce the cost of the sensor, enable transparent sensitive films, allow larger material variations for sensor arrays and may prevent long-term materials degradation.

Materials and Methods
ITO nanoparticles were synthesized based on a previously reported method with an In:Sn ratio of 95:5 and dispersed in n-hexane [4].1,8-Diaminooctane was dissolved in 2-propanol with a concentration of 5 mmol/L.As substrates, a 4.95 MHz quartz crystal microbalance (QCM) crystal with gold electrodes (MicroVacuum, Budapest, Hungary) were used, while oxidized silicon wafers equipped with gold interdigitated electrodes were used as chemiresistive sensor substrates.The QSM substrates were cleaned by a short ozone treatment, while the silicon dies were cleaned by an oxygen plasma followed by a gas phase silanisation procedure with 3-aminopropyldimethylethoxysilane.As shown in Figure 1, the preparation of the sensitive material has been performed in an automated layer-by-layer self-assembly procedure using two microfluidic cells [5] where solutions of ITO nanoparticles and 1,8-Diaminooctane were pumped through, alternating with pure solvents.While coating of the sensor substrate took place in the custom-made cell, the coating progress was monitored in the QCM cell.With this preparation method nanoparticle composite multilayer can be achieved.

Layer-by-Layer Self-Assembly
The frequency change and change of dissipation of the QCM during the layer-by-layer self-assembly process is shown in Figure 2. The frequency change curves are dominated by the effects of the solvent changes from 2-propanol to n-hexane due to different viscoelastic properties of the solvents.The curves have a distinct shape reflecting the stepwise layer-by-layer deposition scheme with large frequency changes during the ITO deposition step and an almost constant frequency during the linker steps.This observation can be explained by the much higher mass of the ITO nanoparticles compared to the light organic linker molecules.
It can be observed that in the first 6 cycles the shape of the frequency change curves is slightly different.The reason may be that in the first cycles the assembly takes place on the transducer surface while later on, it is on the composite material itself and thus the surface chemistry changes.
This can be also seen in the linear decrease of the curve, where the slope is lower in the first 6 cycles and larger after the 7th.
The dissipation change curve is also dominated by the change in the solvents.Neglecting these effects, a dissipation change about 80 × 10 6 is reached.Thus, in accordance with the literature [6], a rigid film behavior (and thus validity of the Sauerbrey equation) is assumed.

Chemiresistive Sensing
The sensing properties towards 100-5000 ppm of vapors of toluene, 4-methyl-2-pentanone, 1-propanol, and water measured at room temperature are shown in Figure 3.It can be seen that the chemiresistor responds very fast to all volatile organic compounds in the entire concentration regime.For toluene, the sensitivity increases with concentration up to 800 ppm and then comes close to saturation.For propanol and water, the sensor responses are independent of the concentration regime and obviously are saturated above a concentration of 100 ppm.In contrast to the other analytes, the sensor response of 4-methyl-2-pentanone (4M2P) is not rectangular way but changes with concentration.The time-dependent chemiresistance exhibits at least two components: a very fast one leading to a decrease of the resistance and a slower one that leads to an increase of the resistance.Also, the recovery at higher concentration is much slower in the case of the 4M2P.Interestingly, the sensors respond mainly with a decrease of the resistance towards the vapors.Thus, the ITO nanoparticles may play a different role than metal nanoparticles in comparable, known composites, or the sensing mechanism is different.
We postulate that in the ITO/Diaminooctane composite sorption sites exist which lead to a decrease in resistance.These sites can be accessed by all analytes, and they saturate faster with propanol and water than with toluene or 4M2P.For known metal nanoparticle composites such a process is attributed to filling of pores in the material.However, the dielectric constant of toluene is smaller than that of the organics in the composite and thus would lead to an increase in the resistance.The increase in resistance of the material during sorption with 4M2P can be explained by analyte induced swelling of the composite.However, it is not clear why this is only possible for 4M2P and not for the other analytes.

Conclusions and Outlook
We demonstrated that we were able to form a multilayer composite by an automated layer-by-layer self-assembly procedure comprising of ITO nanoparticles and 1,8-Diaminooctane.The composite showed chemiresistive behavior to volatile organic compounds.The sensing mechanism seems to differ from comparable known composites based on metal nanoparticles or the ITO nanoparticles expose different sorption sites.
To get a further insight into this class of materials, the assembly parameters and the resulting film structure have to be optimized.Additionally, other diamines or organic molecules with different binding groups should be investigated to get a detailed insight in the sensing mechanism and to enhance sensitivity.Furthermore, variations in the ITO nanoparticles concerning the Sn content as well as the particle size are interesting to improve the signal-to-noise ratio and tune possible ITO induced sorption sites.Finally, the longtime robustness as well as drift and the temperature dependency of the measurements should be investigated.

Figure 2 .
Figure 2. (a) Frequency change and change of dissipation during the automated layer-by-layer self-assembly process indicating multilayer formation; (b) detailed information of the 6th and 7th cycle (color code of the flowing solutions is the same as in Figure 1).

Figure 3 .
Figure 3. Chemiresistive sensing properties of ITO nanoparticles interlinked with 1,8-Diaminooctane towards the indicated concentrations of the indicated vapors.The arrows mark the delivery of the VOC to the sensor.