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Polyelectrolyte and molecular host ion self-assembly to multilayer thin films: An approach to thin film chemical sensors
Sensors and Actuators B 45 (1997) 87 – 92
Polyelectrolyte and molecular host ion self-assembly to multilayer thin films: An approach to thin film chemical sensors X. Yang, S. Johnson, J. Shi, T. Holesinger, B. Swanson * Chemical Science and Technology, Los Alamos National Laboratory, CST-1, MS J565, Los Alamos, NM 87545, USA Received 31 January 1997; received in revised form 12 August 1997; accepted 18 August 1997
Abstract Multilayer molecular films of polyelectrolyte/calixarene and polyelectrolyte/cyclodextrin hosts were fabricated by alternating adsorption of charged species in aqueous solutions onto a substrate (quartz or silicon wafer). This layer-by-layer molecular deposition approach has been successfully used to integrate molecular recognition reagents into polymer films as chemically selective layers for surface acoustic wave (SAW) chemical sensing applications. The resulting sensors have high sensitivity and selectivity to the organic vapors studied. The films were characterized with SEM, infrared and UV-vis spectroscopy, and monitored by SAW devices. These measurements revealed that the deposition process is highly reproducible and the resulting films are uniform and stable. © 1997 Elsevier Science S.A. Keywords: Polyelectrolyte; Calixarene; Self-assembly; Thin-film; Sensor
1. Introduction The use of polymer-coated surface acoustic wave (SAW) chemical sensors for directly detecting organic vapors has attracted increasing attention [1 – 5]. Polymer coatings not only increase the sensitivity but also the selectivity of SAW sensors towards organic vapors at very low concentrations. However, the current techniques (dip-coating, spray, solution casting and brushing) of fabricating polymer thin films have minimum control of film properties such as film thickness, uniformity and stability. Moreover, there are problems associated with the method currently used to control the thickness of the polymer by observing a total frequency shift, since the frequency shifts observed are not only the function of mass loading but also that of polymer elasticity and temperature. Polymers with various functional groups such as CF3, CN, pyridyl, OH, and perfluoro units have been used to promote selective vapor absorption for SAW chemical sensors [4]. However, selectivity towards structurally similar hydrocarbons is still low. In general, the polymers used are not commercially available and the polar groups in* Corresponding author. Tel.: +1 505 6674686; fax: + 1 505 6654631. 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 7 4 - 8
corporated result in polymers with high glass transition points and low permeability [4]. Problems associated with film dewetting (island formation) and poor film adhesion were also reported. An alternative approach in developing selective sensor coatings relies on the use of molecular recognition reagents such as cyclodextrins [6] calixarenes [7–9], cavitands [10], metalloporphyrins[11]. Although it has been demonstrated that selectivity of the sensing layer is improved, it is difficult to fabricate ordered multilayer molecular films incorporating these organic host species. Solution casting of molecular species results in films with low coverage [10], poor uniformity [11], low stability and low reproducibility. Decher et al. recently developed a technique for polymer thin film growth using a polyelectrolyte self-assembly approach [12–14]. Alternating adsorption of anionic and cationic electrolytes on a charged substrate by sequential dipping of the substrate into aqueous polyelectrolyte solutions yields a uniform thin film that is both durable and reproducible in terms of film thickness. The driving force for film formation is multi-point electrostatic interactions between polyions. The resulting thin film has a well organized bilayer structure, high thermal stability and high uniformity. Multilayer thin films with precisely controlled thickness and molecular
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Scheme 1.
architectures can therefore be successfully fabricated. The layer-by-layer assembly of bilayer films has been applied to fabricating conductive polymer films [15,19], electo-optical films [16], biomimic membranes [17], protein films [18] and amphiphile organic films [20]. In this paper, we present our work on constructing organic sensing layers on SAW devices via polyelectrolyte and molecular host ion self-assembly. Replacing polyanions, highly negatively charged molecular species (Scheme 1) were used for film fabrication. These molecular reagents capable of binding organic species were deposited as functional components into thin films. This unique approach incorporates polymer and molecular elements into the sensing film and thus results in films with polymer’s physical properties and molecular film’s selectivity. By incorporating molecular recognition functions into polymer film, rugged, sensitive and selective SAW chemical sensors were fabricated. The SAW sensor responses to VOCs will be reported and discussed.
2. Experimental
2.1. Chemicals Poly(sodium 4-styrenesulfonate) (PSS) (Mw =70 000, d= 0.801), poly(diallyldimethylammonium chloride)
(PDDA) (20 wt.% in water, d= 1.040), poly(allylamine hydrochloride) (PAH) (Mw = 50 000), nickel(II) phthalocyaninetetrasulfonic acid, tetrasodium salt and tbutyl-calix[6]arene were purchased from Aldrich and used without further purification. (2) and (3) were prepared according to the literature procedures [21]. (4) was from Cerestar USA, and was used without further purification.
2.2. Film fabrication and SAW measurements A 250 MHz SAW device was rinsed with chloroform and plasma-cleaned for 20 min. The device was first treated with aminopropyltrimethoxysilane in chloroform, followed with deposition of PSS and then PDDA polyelectrolytes by dipping the device into the aqueous solutions of the polyelectrolytes, respectively. After this, alternating depositions of negatively charged molecular host species (Scheme 1) and PDDA were carried out until the desired number of bilayers was reached. Between each deposition, the device was thoroughly rinsed with deionized water. For film characterizations, silicon wafers or quartz substrates were used for film depositions. The polyelectrolyte and molecular ion assembly was monitored by UV-vis absorption spectroscopy and mass loading was measured with SAW devices. Quartz substrates were used for optical measurements. Silicon
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wafers were used for surface ATR-FTIR study. ATRFTIR spectra were recorded on a Bio-Rad Spectrophotometer. ST-cut quartz 250 MHz SAW devices were purchased from Microsensor Systems. The SAW frequencies of the device were measured with a HP 5350B Microwave Frequency Counter with a resolution of 1 Hz. A vapor generator (VG-400, Microsensor Systems) was used for the vapor generation and dilution. Vapor temperature was set at 15.0°C and controlled with a thermostat. SEM study was performed with a JOEL 6300FXV field emission SEM at Materials Science Laboratory of Los Alamos National Laboratory.
3. Results and discussion SEM image of 5-bilayer film of PDDA/(3) with (3) on the surface layer is shown in Fig. 1. The film is amorphous and uniform with some aggregates at ca. 50 nm diameter. No cracks or islands were observed on the surface. As mentioned above, SAW device has been used to determine the polymer mass loading on the surface of SAW devices by monitoring the change in frequency of the SAW device. Recently, Kunitake and his co-worker studied protein depositions using quartz crystal microbalance (QCM) technique monitoring the polyelectrolyte and protein deposition in in situ [18]. Similarly, we used SAW device to monitor the mass loading of each PDDA/(3) bilayer deposition. Starting from the first bilayer of PDDA/(3), each bilayer deposition results in a frequency shifts of about 24 kHz (Fig. 2). We can estimate the film thickness from Eq. (1), where l is the film thickness (cm) and d is the film density (g cm − 3) and F0 is the fundamental frequency of the SAW device.
Fig. 2. SAW device frequency shifts after each PDDA and calixarene (3) bilayer deposition, along with SAW device frequency shifts after each PDDA and PSS bilayer deposition.
Df = − 1.26×10 − 6F 20ld
(1)
If unit density is assumed for the polymer and (3), a 24 kHz shift caused by each bilayer deposition corresponds to a bilayer film thickness of ca. 3 nm, which is in the range of the reported thickness of each polyelectrolyte bilayers [12]. After a total of five bilayers, film thickness is 17 nm, including the silane and PSS layer. Similarly, we used SAW devices to study polyelectrolyte deposition of PSS and PDDA. A linear frequency drop of the SAW device due to mass loading of the polyelectrolytes was also observed (Fig. 2). A 8 kHz shift due to each polyelectrolyte bilayer deposition was one-third of that of PDDA/(3) bilayer deposition. This leads us to believe that the mass loading of (3) is more significant than that of the polyelectrolyte, since the mass per charge of (3) is much larger than those of PDDA or PSS. ATR-FTIR was used to characterize the PDDA/(3) films and it was found that the peaks appear at 1186 and 1070 cm − 1 corresponding to the sulfonate groups on (3). Polyelectrolyte (PDDA or PAH) assembly with NiPc(1) was followed with UV-vis spectroscopy (Fig. 3). The film was deposited on a quartz substrate and the absorbance of (1) at 624 nm was found to be a linear function of the layer number of (1) up to 20 layers studied. This is a clear indication that about the same amount of (1) was deposited on the film at each adsorption step and (1) deposited on the film was not stripped off during the deposition process.
3.1. SAW response Fig. 1. SEM photograph of a self-assembled PDDA and calixarene (3) bilayer film with (3) on the surface layer.
SAW sensors coated with (1–4) were exposed to a variety of organic solvent vapors. It was found that
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Fig. 3. UV-vis measurements on PDDA/(1) and PAH/(1) bilayer films.
sensor coated with (3) showed increased sensitivity with the increasing number of calixarene layers deposited on the SAW device (Fig. 4). However, the sensitivity to carbon tetrachloride and 1,1,1-trichloroethane increases only slightly. The sensor responses to perchloroethylene (PCE) increase with increasing number of calixarene (3) layers. Similar results were observed for sensors coated with (1) or (2), though the sensitivity increase is not significant. The result indicates that, in addition to the surface layer, the vapor sorption also involves inner layer species. It is worth mentioning that a SAW device coated with PDDA/PSS didn’t show increased sensitivity upon increasing thickness of the film. Real time sensor (coated with (3)) responses upon exposure to TCE, PCE and chloroform are shown in Fig. 5. The sensor is completely reversible with response and recovery time in the order of seconds. The sensor can detect halogenated hydrocarbons in tens of ppm
Fig. 5. SAW responses of a calixarene (3) coated device: (A) 13.8 ppm perchloroethylene (v/v); (B) 55 ppm trichloroethylene (v/v); (C) 164 ppm chloroform (v/v).
range without difficulty. The sensor is stable and retains its sensitivity and responses towards halogenated hydrocarbons after 3 months when it was retested. For a comparison study, a polyelectrolyte 5-bilayer film containing PDDA and PSS was coated on a 250 MHz device with PSS at the surface layer. The sensor responses to a group of selected organic vapors are shown in Fig. 6, along with the sensor responses of a 5-bilayer PDDA and calix[6]arene (3), 20 bilayer PDDA/(1), and 5-bilayer PDDA/(4), respectively. The sensor with (3) coating is much more sensitive (5–6 times larger) and has higher selectivity towards organic vapors. It is obvious that the presence of calixarene cavities and long alkyl chains in the film provide hydrophobic sites for organic vapors adsorption. The high sensitivity toward toluene is expected, since toluene, in addition to van der Waals interactions, can have p–p interactions with the calixarene aromatic rings. A solid state structure of toluene and p-t-butylcalix[4]arene complex has been reported [22]. On the other hand, PDDA/(4), PDDA/(1) and PDDA/PSS films showed less sensitivity and almost no selectivity towards halogenated hydrocarbons and toluene. These highly charged films are expected to be highly hydrophilic and permeability of organic vapors in these films are expected to be minimum. It is also expected that these films might have appreciable responses to humidity in the air. We are now testing these films for sensing polar organic species such as alcohols and amines.
4. Conclusion
Fig. 4. SAW responses to CTC (carbon tetrachloride), PCE (perchloroethylene), and TCA (1,1,1-trichloroethane) vs. numbers of PDDA/(3) bilayers.
An alternating electrostatic adsorption approach has been successfully utilized to incorporate molecular recognition elements into polymer films for chemical sensing applications. The process is highly efficient and
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Fig. 6. SAW sensor responses to organic vapors with sensors coated with PDDA/PSS (five bilayer), PDDA/(1) (20 bilayer), PDDA/(3) (five bilayer), and PDDA/(4) (five bilayer), respectively.
reproducible for fabricating multilayer molecular films with high uniformity and stability. SAW devices coated with (3) multilayer films displayed high sensitivity and selectivity to organic vapors studied. We are currently expanding our studies to include other molecular host species and the result will be reported.
Acknowledgements This work was supported by Laboratory Directed Research and Development (LDRD) funds and the Advanced Technologies for Proliferation Detection program of the Nuclear Nonproliferation Office of the Department of Energy.
References [1] H. Wohltjen, R.E. Dessy, Surface acoustic-wave probe for chemical-analysis. 1. Introduction and instrument description, Anal. Chem. 51 (1979) 1458–1464. [2] H. Wohltjen, N.L. Jarvis, J.R. Lint, Surface acoustic-wave chemical microsensors and sensor arrays for industrial-process control and pollution prevention, ACS Symp. Ser. 508 (1992) 86 – 102. [3] H. Wohltjen, D.S. Ballantine Jr., N.L. Jarvis, R.W. Murray, R.E. Dessy, Vapor detection with surface acoustic-wave microsensors, ACS Symp. Ser. 403 (1989) 157–175. [4] R.A. McGill, M.H. Abraham, J.W. Grate, Choosing polymercoatings for chemical sensors, Chemtech. 24 (1994) 27–37. [5] K. Bodenho¨fer, A. Hierlemann, G. Noetzel, U. Weimar, W. Go¨pel, Performances of mass-sensitive devices for gas-sensing: thickness-shear mode and surface-acoustic-wave transducers, Anal. Chem. 68 (1996) 2210–2218. [6] C.S.I. Lai, G.J. Moody, J.D.R. Thomas, Piezoelectric quartz crystal detection of benzene vapor using chemically modified cyclodextrins, J. Chem. Soc. Perkin Trans. (1988) 319–324.
[7] K.D. Schierbaum, Application of organic supramolecular and polymeric compounds for chemical sensors, Sensors and Actuators B 18 (1994) 71 – 76. [8] K.D. Schierbaum, T. Weiss, E.U. Thoden, T. Velzen, J.F.J. Engbersen, D.N. Reinhoudt, W. Go¨pel, Molecular recognition by self-assembled monolayers of cavitand receptors, Science 265 (1994) 1413 – 1415. [9] J. Rickert, T. Weiss, W. Go¨pel, Self-assembled monolayers for chemical sensors: molecular recognition by immobilized supramolecular structures, Sensors and Actuators B 31 (1996) 45 – 50. [10] P. Nelli, E. Dalcanale, G. Faglia, G. Sberveglieri, P. Soncini, Cavitands as selective materials for QMB sensors for nitrobenzene and other aromatic vapors, Sensors and Actuators B 13 (1993) 302 – 304. [11] J.A.J. Brunink, C.D. Natale, F. Bungaro, F.A.M. Davide, A.A.D. Amico, R. Paolesse, T. Boschi, M. Faccio, G. Ferri, The application of metalloporphyrins as coating material for quartz microbalance-based chemical sensors, Anal. Chem. Acta 325 (1996) 53 – 64. [12] G. Decher, J.D. Hong, Buildup of ultrathin multilayer films by a self-assembly process. 2. Consecutive adsorption of anionic and cationic bipolar amphiphiles and polyelectrolytes on charged surfaces, Bear. Bunsenges. Phys. Chem. 95 (1991) 1430–1434. [13] G. Decher, J.D. Hong, J. Schmitt, Buildup of ultrathin multilayer films by a self-assembly process. 3. Consecutively alternating adsorption of anionic and cationic polyelectrolytes on charged surfaces, Thin Solid Films 210/211 (1992) 831 –835. [14] Y. Lvov, G. Decher, H. Mo¨hwald, Assembly, structural characterization, and thermal-behavior of layer-by-layer deposited ultrathin films of poly(vinyl sulfate) and poly(allylamine), Langmuir 9 (1993) 481 – 486. [15] J.H. Cheung, A.F. Fou, M.F. Rubner, Molecular self-assembly of conducting polymers, Thin Solid Films 244 (1994) 985–989. [16] M. Ferreira, J.H. Cheung, M.F. Rubner, Molecular self-assembly of conjugated polyions: a new process for fabricating multilayer thin-film heterostructures, Thin Solid Films 244 (1994) 806 – 809. [17] P.J. Lukes, G.R. Ivanov, M.C. Petty, J. Yarwood, M.H. Greenhall, Y. Lvov, Deposition and structural-properties of langmuirblodgett-films of a fluorescently labeled phospholipid, Langmuir 10 (1994) 1877 – 1881.
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[18] Y. Lvov, K. Ariga, I. Ichinose, T. Kunitake, Assembly of multicomponent protein films by means of electrostatic layer-bylayer adsorption, J. Am. Chem. Soc. 117 (1995) 6117–6123. [19] A.C. Fou, M.F. Rubner, Molecular-level processing of conjugated polymers. 2. Layer-by-layer manipulation in-situ polymerized p-type doped conducting polymers, Macromolecules 28 (1995) 7115 – 7120. [20] I. Ichinose, K. Fujioshi, S. Mizuki, Y. Lvov, T. Kunitake, Layer-by-layer assembly of aqueous bilayer-membranes on charged surfaces, Chem. Lett. (1996) 257–258. [21] S. Shinka, S. Mori, H. Koreishi, T. Tsubaki, O. Manabe, Hexasulfonated calix[6]arene derivatives: a new class of catalysts, surfactants, and host molecules, J. Am. Chem. Soc. 108 (1986) 2409 – 2416. [22] S.G. Bott, A.W. Coleman, J.L. Atwood, Inclusion of both cation and neutral molecule by a calixarene: structure of the para-tertbutylmethoxycalix[4]arene-sodium-toluene, J. Am. Chem. Soc. 108 (1986) 1709 – 1710.
Biographies Xiaoguang Yang is a technical staff member at Los Alamos National Laboratory. He received his B.Sc. degree in chemistry from Fuzhou University, Fuzhou, China, his M.Sc. degree from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, and his Ph.D. in inorganic chemistry from University of Illinois at Urbana-Champaign in 1990. He has broad experience and interest in metal chalcogenide chemistry, molecular nonlinear materials, molecular self-assembly, thin-film materials, host-guest chemistry, carborane chemistry, chemical and biosensors. He has co-authored over 30 research articles. Sabina Rene Johnson obtained her B.Sc. from Washington State University in 1991 and her Ph.D. in veterinary medicine from Oregon and Washington State Universities in 1993. From there she has had a varied career including research work at the Wyoming State Veterinary lab and Washington State Immunology section studying antigen antibody interactions. She is currently working as a technician at Los Alamos National Laboratory, interested in thin films, molecular recognition, molecular interactions and alternative detection techniques for a variety of different environmental toxins.
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Jing-Xuan Shi received her B.Sc. in chemistry from Beijing University, Beijing, China, and M.Sc. in analytical materials chemistry from Penn. State University in 1992. After working 2 years as an analytical lab technician at the University of Florida, she became a graduate research assistant at Los Alamos. Her primary research interest is organic thin films for the development of thin film microsensors. Terry Holesinger is a technical staff member at Los Alamos National Laboratory (LANL). He received his B.A. (Summa Cum Laude, 1986) from Central College and Ph.D. (1993) from Iowa State University in applied physics. While at Iowa State, he worked with a joint appointment at Argonne National Laboratory on the materials properties of the high-temperature superconductor, Bi2Sr2CaCu2Oy with Scott Chumbley and Douglas Finnemore (ISU) and Dean Miller (Argonne). In 1993, he joined Los Alamos National Laboratory as a post-doctoral researcher and then staff scientist in 1996. Dr Holesinger is (co)author of over 30 research articles and two patents. His current research interests are in the structural and materials properties of superconducting and magnetoresistive perovskites. Basil I. Swanson is a technical staff member and Laboratory Fellow at Los Alamos National Laboratory. He received his B.A. in chemistry in 1966 from Colorado School of Mines and Ph.D. in inorganic chemistry from Northwestern University in 1970. Professional experience: postdoctoral fellow Los Alamos National Laboratory, 70–71; asisstant professor New York University 71–73; asisstant professor University Texas at Austin, 73–79; Los Alamos National Lab 80–present, and Faculty Assoc Physics, Colorado State University, 93–present. His research include structure and dynamics of condensed matter systems; structural phase transformations; high pressure phenomena; low-dimensional electronic materials; chemical and biochemical sensors; self assembly techniques and molecular level control of materials. Dr Swanson has co-authored over 200 articles and has four patents.
Polyelectrolyte and molecular host ion self-assembly to multilayer thin films: An approach to thin film chemical sensors X. Yang, S. Johnson, J. Shi, T. Holesinger, B. Swanson * Chemical Science and Technology, Los Alamos National Laboratory, CST-1, MS J565, Los Alamos, NM 87545, USA Received 31 January 1997; received in revised form 12 August 1997; accepted 18 August 1997
Abstract Multilayer molecular films of polyelectrolyte/calixarene and polyelectrolyte/cyclodextrin hosts were fabricated by alternating adsorption of charged species in aqueous solutions onto a substrate (quartz or silicon wafer). This layer-by-layer molecular deposition approach has been successfully used to integrate molecular recognition reagents into polymer films as chemically selective layers for surface acoustic wave (SAW) chemical sensing applications. The resulting sensors have high sensitivity and selectivity to the organic vapors studied. The films were characterized with SEM, infrared and UV-vis spectroscopy, and monitored by SAW devices. These measurements revealed that the deposition process is highly reproducible and the resulting films are uniform and stable. © 1997 Elsevier Science S.A. Keywords: Polyelectrolyte; Calixarene; Self-assembly; Thin-film; Sensor
1. Introduction The use of polymer-coated surface acoustic wave (SAW) chemical sensors for directly detecting organic vapors has attracted increasing attention [1 – 5]. Polymer coatings not only increase the sensitivity but also the selectivity of SAW sensors towards organic vapors at very low concentrations. However, the current techniques (dip-coating, spray, solution casting and brushing) of fabricating polymer thin films have minimum control of film properties such as film thickness, uniformity and stability. Moreover, there are problems associated with the method currently used to control the thickness of the polymer by observing a total frequency shift, since the frequency shifts observed are not only the function of mass loading but also that of polymer elasticity and temperature. Polymers with various functional groups such as CF3, CN, pyridyl, OH, and perfluoro units have been used to promote selective vapor absorption for SAW chemical sensors [4]. However, selectivity towards structurally similar hydrocarbons is still low. In general, the polymers used are not commercially available and the polar groups in* Corresponding author. Tel.: +1 505 6674686; fax: + 1 505 6654631. 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 7 4 - 8
corporated result in polymers with high glass transition points and low permeability [4]. Problems associated with film dewetting (island formation) and poor film adhesion were also reported. An alternative approach in developing selective sensor coatings relies on the use of molecular recognition reagents such as cyclodextrins [6] calixarenes [7–9], cavitands [10], metalloporphyrins[11]. Although it has been demonstrated that selectivity of the sensing layer is improved, it is difficult to fabricate ordered multilayer molecular films incorporating these organic host species. Solution casting of molecular species results in films with low coverage [10], poor uniformity [11], low stability and low reproducibility. Decher et al. recently developed a technique for polymer thin film growth using a polyelectrolyte self-assembly approach [12–14]. Alternating adsorption of anionic and cationic electrolytes on a charged substrate by sequential dipping of the substrate into aqueous polyelectrolyte solutions yields a uniform thin film that is both durable and reproducible in terms of film thickness. The driving force for film formation is multi-point electrostatic interactions between polyions. The resulting thin film has a well organized bilayer structure, high thermal stability and high uniformity. Multilayer thin films with precisely controlled thickness and molecular
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Scheme 1.
architectures can therefore be successfully fabricated. The layer-by-layer assembly of bilayer films has been applied to fabricating conductive polymer films [15,19], electo-optical films [16], biomimic membranes [17], protein films [18] and amphiphile organic films [20]. In this paper, we present our work on constructing organic sensing layers on SAW devices via polyelectrolyte and molecular host ion self-assembly. Replacing polyanions, highly negatively charged molecular species (Scheme 1) were used for film fabrication. These molecular reagents capable of binding organic species were deposited as functional components into thin films. This unique approach incorporates polymer and molecular elements into the sensing film and thus results in films with polymer’s physical properties and molecular film’s selectivity. By incorporating molecular recognition functions into polymer film, rugged, sensitive and selective SAW chemical sensors were fabricated. The SAW sensor responses to VOCs will be reported and discussed.
2. Experimental
2.1. Chemicals Poly(sodium 4-styrenesulfonate) (PSS) (Mw =70 000, d= 0.801), poly(diallyldimethylammonium chloride)
(PDDA) (20 wt.% in water, d= 1.040), poly(allylamine hydrochloride) (PAH) (Mw = 50 000), nickel(II) phthalocyaninetetrasulfonic acid, tetrasodium salt and tbutyl-calix[6]arene were purchased from Aldrich and used without further purification. (2) and (3) were prepared according to the literature procedures [21]. (4) was from Cerestar USA, and was used without further purification.
2.2. Film fabrication and SAW measurements A 250 MHz SAW device was rinsed with chloroform and plasma-cleaned for 20 min. The device was first treated with aminopropyltrimethoxysilane in chloroform, followed with deposition of PSS and then PDDA polyelectrolytes by dipping the device into the aqueous solutions of the polyelectrolytes, respectively. After this, alternating depositions of negatively charged molecular host species (Scheme 1) and PDDA were carried out until the desired number of bilayers was reached. Between each deposition, the device was thoroughly rinsed with deionized water. For film characterizations, silicon wafers or quartz substrates were used for film depositions. The polyelectrolyte and molecular ion assembly was monitored by UV-vis absorption spectroscopy and mass loading was measured with SAW devices. Quartz substrates were used for optical measurements. Silicon
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wafers were used for surface ATR-FTIR study. ATRFTIR spectra were recorded on a Bio-Rad Spectrophotometer. ST-cut quartz 250 MHz SAW devices were purchased from Microsensor Systems. The SAW frequencies of the device were measured with a HP 5350B Microwave Frequency Counter with a resolution of 1 Hz. A vapor generator (VG-400, Microsensor Systems) was used for the vapor generation and dilution. Vapor temperature was set at 15.0°C and controlled with a thermostat. SEM study was performed with a JOEL 6300FXV field emission SEM at Materials Science Laboratory of Los Alamos National Laboratory.
3. Results and discussion SEM image of 5-bilayer film of PDDA/(3) with (3) on the surface layer is shown in Fig. 1. The film is amorphous and uniform with some aggregates at ca. 50 nm diameter. No cracks or islands were observed on the surface. As mentioned above, SAW device has been used to determine the polymer mass loading on the surface of SAW devices by monitoring the change in frequency of the SAW device. Recently, Kunitake and his co-worker studied protein depositions using quartz crystal microbalance (QCM) technique monitoring the polyelectrolyte and protein deposition in in situ [18]. Similarly, we used SAW device to monitor the mass loading of each PDDA/(3) bilayer deposition. Starting from the first bilayer of PDDA/(3), each bilayer deposition results in a frequency shifts of about 24 kHz (Fig. 2). We can estimate the film thickness from Eq. (1), where l is the film thickness (cm) and d is the film density (g cm − 3) and F0 is the fundamental frequency of the SAW device.
Fig. 2. SAW device frequency shifts after each PDDA and calixarene (3) bilayer deposition, along with SAW device frequency shifts after each PDDA and PSS bilayer deposition.
Df = − 1.26×10 − 6F 20ld
(1)
If unit density is assumed for the polymer and (3), a 24 kHz shift caused by each bilayer deposition corresponds to a bilayer film thickness of ca. 3 nm, which is in the range of the reported thickness of each polyelectrolyte bilayers [12]. After a total of five bilayers, film thickness is 17 nm, including the silane and PSS layer. Similarly, we used SAW devices to study polyelectrolyte deposition of PSS and PDDA. A linear frequency drop of the SAW device due to mass loading of the polyelectrolytes was also observed (Fig. 2). A 8 kHz shift due to each polyelectrolyte bilayer deposition was one-third of that of PDDA/(3) bilayer deposition. This leads us to believe that the mass loading of (3) is more significant than that of the polyelectrolyte, since the mass per charge of (3) is much larger than those of PDDA or PSS. ATR-FTIR was used to characterize the PDDA/(3) films and it was found that the peaks appear at 1186 and 1070 cm − 1 corresponding to the sulfonate groups on (3). Polyelectrolyte (PDDA or PAH) assembly with NiPc(1) was followed with UV-vis spectroscopy (Fig. 3). The film was deposited on a quartz substrate and the absorbance of (1) at 624 nm was found to be a linear function of the layer number of (1) up to 20 layers studied. This is a clear indication that about the same amount of (1) was deposited on the film at each adsorption step and (1) deposited on the film was not stripped off during the deposition process.
3.1. SAW response Fig. 1. SEM photograph of a self-assembled PDDA and calixarene (3) bilayer film with (3) on the surface layer.
SAW sensors coated with (1–4) were exposed to a variety of organic solvent vapors. It was found that
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Fig. 3. UV-vis measurements on PDDA/(1) and PAH/(1) bilayer films.
sensor coated with (3) showed increased sensitivity with the increasing number of calixarene layers deposited on the SAW device (Fig. 4). However, the sensitivity to carbon tetrachloride and 1,1,1-trichloroethane increases only slightly. The sensor responses to perchloroethylene (PCE) increase with increasing number of calixarene (3) layers. Similar results were observed for sensors coated with (1) or (2), though the sensitivity increase is not significant. The result indicates that, in addition to the surface layer, the vapor sorption also involves inner layer species. It is worth mentioning that a SAW device coated with PDDA/PSS didn’t show increased sensitivity upon increasing thickness of the film. Real time sensor (coated with (3)) responses upon exposure to TCE, PCE and chloroform are shown in Fig. 5. The sensor is completely reversible with response and recovery time in the order of seconds. The sensor can detect halogenated hydrocarbons in tens of ppm
Fig. 5. SAW responses of a calixarene (3) coated device: (A) 13.8 ppm perchloroethylene (v/v); (B) 55 ppm trichloroethylene (v/v); (C) 164 ppm chloroform (v/v).
range without difficulty. The sensor is stable and retains its sensitivity and responses towards halogenated hydrocarbons after 3 months when it was retested. For a comparison study, a polyelectrolyte 5-bilayer film containing PDDA and PSS was coated on a 250 MHz device with PSS at the surface layer. The sensor responses to a group of selected organic vapors are shown in Fig. 6, along with the sensor responses of a 5-bilayer PDDA and calix[6]arene (3), 20 bilayer PDDA/(1), and 5-bilayer PDDA/(4), respectively. The sensor with (3) coating is much more sensitive (5–6 times larger) and has higher selectivity towards organic vapors. It is obvious that the presence of calixarene cavities and long alkyl chains in the film provide hydrophobic sites for organic vapors adsorption. The high sensitivity toward toluene is expected, since toluene, in addition to van der Waals interactions, can have p–p interactions with the calixarene aromatic rings. A solid state structure of toluene and p-t-butylcalix[4]arene complex has been reported [22]. On the other hand, PDDA/(4), PDDA/(1) and PDDA/PSS films showed less sensitivity and almost no selectivity towards halogenated hydrocarbons and toluene. These highly charged films are expected to be highly hydrophilic and permeability of organic vapors in these films are expected to be minimum. It is also expected that these films might have appreciable responses to humidity in the air. We are now testing these films for sensing polar organic species such as alcohols and amines.
4. Conclusion
Fig. 4. SAW responses to CTC (carbon tetrachloride), PCE (perchloroethylene), and TCA (1,1,1-trichloroethane) vs. numbers of PDDA/(3) bilayers.
An alternating electrostatic adsorption approach has been successfully utilized to incorporate molecular recognition elements into polymer films for chemical sensing applications. The process is highly efficient and
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Fig. 6. SAW sensor responses to organic vapors with sensors coated with PDDA/PSS (five bilayer), PDDA/(1) (20 bilayer), PDDA/(3) (five bilayer), and PDDA/(4) (five bilayer), respectively.
reproducible for fabricating multilayer molecular films with high uniformity and stability. SAW devices coated with (3) multilayer films displayed high sensitivity and selectivity to organic vapors studied. We are currently expanding our studies to include other molecular host species and the result will be reported.
Acknowledgements This work was supported by Laboratory Directed Research and Development (LDRD) funds and the Advanced Technologies for Proliferation Detection program of the Nuclear Nonproliferation Office of the Department of Energy.
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Biographies Xiaoguang Yang is a technical staff member at Los Alamos National Laboratory. He received his B.Sc. degree in chemistry from Fuzhou University, Fuzhou, China, his M.Sc. degree from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, and his Ph.D. in inorganic chemistry from University of Illinois at Urbana-Champaign in 1990. He has broad experience and interest in metal chalcogenide chemistry, molecular nonlinear materials, molecular self-assembly, thin-film materials, host-guest chemistry, carborane chemistry, chemical and biosensors. He has co-authored over 30 research articles. Sabina Rene Johnson obtained her B.Sc. from Washington State University in 1991 and her Ph.D. in veterinary medicine from Oregon and Washington State Universities in 1993. From there she has had a varied career including research work at the Wyoming State Veterinary lab and Washington State Immunology section studying antigen antibody interactions. She is currently working as a technician at Los Alamos National Laboratory, interested in thin films, molecular recognition, molecular interactions and alternative detection techniques for a variety of different environmental toxins.
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Jing-Xuan Shi received her B.Sc. in chemistry from Beijing University, Beijing, China, and M.Sc. in analytical materials chemistry from Penn. State University in 1992. After working 2 years as an analytical lab technician at the University of Florida, she became a graduate research assistant at Los Alamos. Her primary research interest is organic thin films for the development of thin film microsensors. Terry Holesinger is a technical staff member at Los Alamos National Laboratory (LANL). He received his B.A. (Summa Cum Laude, 1986) from Central College and Ph.D. (1993) from Iowa State University in applied physics. While at Iowa State, he worked with a joint appointment at Argonne National Laboratory on the materials properties of the high-temperature superconductor, Bi2Sr2CaCu2Oy with Scott Chumbley and Douglas Finnemore (ISU) and Dean Miller (Argonne). In 1993, he joined Los Alamos National Laboratory as a post-doctoral researcher and then staff scientist in 1996. Dr Holesinger is (co)author of over 30 research articles and two patents. His current research interests are in the structural and materials properties of superconducting and magnetoresistive perovskites. Basil I. Swanson is a technical staff member and Laboratory Fellow at Los Alamos National Laboratory. He received his B.A. in chemistry in 1966 from Colorado School of Mines and Ph.D. in inorganic chemistry from Northwestern University in 1970. Professional experience: postdoctoral fellow Los Alamos National Laboratory, 70–71; asisstant professor New York University 71–73; asisstant professor University Texas at Austin, 73–79; Los Alamos National Lab 80–present, and Faculty Assoc Physics, Colorado State University, 93–present. His research include structure and dynamics of condensed matter systems; structural phase transformations; high pressure phenomena; low-dimensional electronic materials; chemical and biochemical sensors; self assembly techniques and molecular level control of materials. Dr Swanson has co-authored over 200 articles and has four patents.