The preparation and characterization of different functional headgroup containing alkylthiol stabilized gold nanoparticles and investigations for possible applications in the sensorics Nikoletta Molnár-Vörös 1 *, Ilona Felhősi 1, Judit Telegdi 1, Imre Dékány 2 1
Chemical Research Center of the Hungarian Academy of Sciences, Department of Surface Modification and Nanostructures, 1025 Budapest, Pusztaszeri street, 59-67, Hungary 2 Department of Colloid Chemistry and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, University of Szeged 6720 Szeged, Aradi vértanuk tere 1., Hungary *e-mail: niki@chemres.hu
There are already some works in the literature, dealing with gold nanoparticles coated sensors for sensing different organic vapours [1,2]. The problem with them is that they are not selective yet. Our work is aimed towards development of special interdigitated electrodes coated with with different funcional group containing gold nanoparticles. The main target with these sensors is to be able to distinguish polar, apolar and organic vapours. Gold nanoparticles were prepared in an inverse micellar system [3]. Hexanethiol, octanethiol, 8-mercapto-1-octanol, and 2-phenylethanethiol surface functionalized 2-10 nm diameter gold nanoparticles were prepared. The as prepared nanosols were characterized by TEM, UV-VIS, TG-DSC, FT-IR. The surface modified nanoparticles are forming stable dispersions in aromatic (toluene) or polar organic solvents (ethanol). 1,4
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AuPhEthThiol Synthetized Spectrum
In the spectra of the 1-octanethiol and 2-phenylethanethiol the vibration of the free –SH group can be detected, which disappears in the spectra of the surface modified Au nanosol. Which shows, that the thiol molecule had bound to the Au surface. 10
Coating the interdigital electrodes with thiol- functionalized Au nanoparticles
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Sensors were manufactured using interdigital microelectrodes (Auter Kft, Budapest), made of 100 µm thick copper coated with 10 µm gold film. The gold nanosol toluenic dispersions were applied on the electrode surface using a fine brush, to form a uniform film.The sensors were connected to a Keithley 2400 multimeter (UK) and the current intensity was detected at a voltage of 0.5 V as a function of time. After detection in air, the sensor was placed in organic vapour (above toluene or ethanol) and current intensity was repeatedly determined.
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In ethanol vapors the highest sensitivity is achieved with the Au-8-mercapto-1-octanol coated sensor. This can be explained by the formation of hydrogen bonds between the – OH groups in ethanol and the 8-mercapto-1octanol. The sensitivity of the Au-1octanethiol coated sensor is lower, because the ethanol molecules are bound to the surface by weak dispersion forces (alkyl chain in ethanol and alkyl chain in octanethiol). The lowest sensitivity has the Au-2-Phenylethanthiol coated sensor, because the molecule is a relative small one, and has no affinity to the ethanol molecule.
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The sensitivity of the Au-2phenylethanethiol coated sensor should be highest for toluene vapours (due to π-π interactions), but the measurements show the contrary, this phenomena could be explained,by the fact, that the “spacer” molecules (-CH2CH2-) are too short and the phenyl molecule is too voluminous, thus they do not allow the toluene molecules to diffuse between the 2phenylethanethiol chains.
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The most interesting results are, that the selectivity of the sensors depend significantly on the structure of the layers that stabilize the gold nanoparticles, additionally the sensor signal depends significantly on the vapour concentration.
Previous measurements have already demonstrated, that the alkyl or aryl thiol surface modified Au based sensors have high sensitivity to toluene vapours. Due to the fact, that the toluene molecules are diffusing into the layer between the alkyl chains, thus the distance between the nanoparticles increases, the result of this process is a decrease in the tunneling current and an increase in the sensor resistance. According to the results, there is a linear variation between the toluene vapour concentration and impedance.
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Parameter Value Error ------------------------------------------A 7.40278 0.72098 B 2.59366 0.05073 ------------------------------------------
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Linear Regression: Y = A + B * X
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y = y0 + A1*(1 - exp(-x/t1)) Chi^2/DoF = 932.46517 R^2 = 0.9951
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R SD N P -----------------------------------------0.99943 1.26597 5 <0.0001 ------------------------------------------
Nikoletta Molnár-Vörös wishes to thank to Judith Mihály and Csaba Németh for the FT-IR measurements.
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Acknowledgement:
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[1] N.L. Jarvis, R.R. Smardzewsk, A.W. Snow H. Wohltjen: Alkylthiol Stabilized Gold Nanoclusters for CB Detection [2] M.Brust, M.Walker, D. Bethell, D.J.Schiffrin, R.Whyman, J. Chem. Soc. Chem. Commun, 1994, 801-802 [3] N.Molnár, R.Patakfalvi, I.Dékány, Coll. Surf. A, 2008, 329, 205.
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In ethanol vapours the character of the impedance spectra is time dependent. At the beginning of the measurement a transient can be observed, which stabilize during a period of 20-30 minutes. This transient can be observed in the spectra measured at 1 kHz too. In the presence of ethanol vapours the impedance variation is instantaneous, which is a favorable feature for the detection of ethanol. The response of the sensor is very fast, concentration dependent and regeneration is instantaneous.
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