Surface Pretreatment Boosts the Performance of Supramolecular

Article pubs.acs.org/ac

Surface Pretreatment Boosts the Performance of Supramolecular Affinity Materials on Quartz Crystal Microbalances for Sensor Applications Malte Brutschy,† Daniel Lubczyk,† Klaus Müllen,‡ and Siegfried R. Waldvogel*,† †

Institute for Organic Chemistry, University of Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany

‡

S Supporting Information *

ABSTRACT: A Teflon-like coating is the key for the boost in sensitivity of quartz microbalances for the tracing of airborne analytes. Since the undesired signals for the interfering compounds are suppressed and the ones for the targeted compounds (e.g., peroxide explosives) are enhanced, the PCA output is improved.

Q

electrode.12 This is caused by a depth diffusion of the analyte through the affinity layer (Figure 1).13

uartz crystal microbalances (QCM) have been in constant focus of research on chemical sensors over the years.1 This is caused due to their low cost, their robust nature toward different chemical environments,2 and the possibility of real-time analysis of adsorption processes onto the surface of the electrode or the deposited material.3 In particular, when the quartz is cut in the AT direction, the sensors show marginal dependence upon temperature fluctuations.2 Consequently, they are superior to other gravimetric sensors based on metal oxide4 or surface acoustic wave technology when5 being operated at room temperature. Especially for gas sensing applications, high fundamental frequency (HFF) QCMs with a resonance frequency of 195 MHz have been proven to be a very useful technology, since their theoretical detection limit is 10−13 g.6 The high frequency is achieved at the expense of the oscillation stability. Therefore, the operation of such devices in a damping environment like liquids is not possible.7 By coating the QCMs with suitable affinity materials, a high sensitivity and selectivity are achieved. Typical candidates are supramolecular assemblies,8 molecularly imprinted materials,9 or conjugated polymers.10 The adsorption of an analyte to the affinity layer creates a signal based on a shift in the resonance frequency of the QCM. As found by Sauerbrey et al., this alteration of frequency is directly proportional to the mass of the adsorbed material on the surface of the QCM.11 In general, the specific interaction of compounds with the affinity material is complicated by an unselective physisorption on the surface of the QCM © XXXX American Chemical Society

Figure 1. Quartz crystal microbalance with affinity layer and undesired depth diffusion of analytes (yellow and blue).

Since aluminum oxide is used in various sensor applications for humidity or volatile organic compounds (VOCs) detection as an adsorbant,14 the discrimination of a particular analyte from humidity and interfering and ubiquitous VOCs, which are continuously released by different sources in the environment like petroleum, humans, or cosmetics, is essential. Under the assumption that the depth diffusion of small molecules cannot be prevented by the affinity material itself without high synthetic work, a suitable method for the protection of the QCM electrode surface is highly desired. In this work, we Received: August 14, 2013 Accepted: September 30, 2013

A

dx.doi.org/10.1021/ac4025818 | Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry

Article

report on a monolayer-like modification which significantly reduces the unspecific adsorption of analytes while not interfering with the vibrational characteristics of the QCM. The modification should be chemically inert, hydro-, and lipophobic. These requirements are best fulfilled by fluorous surface modifications. Fluorous surfaces are known for their low free surface energies and their inertness.15 Fluorous chains equipped with an ethylene spacer between the fluorous chain and the functional group (Figure 2) provide similar surface

Figure 3. Response of modified (green) and nontreated (red) QCMs to water and cyclohexane.

Figure 2. Employed modifiers for HFF-QCMs.

chemistry like their completely fluorinated congener while being easier to synthesize and process.16 In order to seal the surface with minimum impact on the oscillator properties,1f,8 a covalently bound, dense modification is required.11 Therefore, self-assembled monolayers (SAMs) are envisioned. SAMs in general have been widely used in molecular sensing,17 corrosion prevention,18 microfluidics, and nanolubricants.19 With dependence on the surface, a wide range of reagents are known to achieve a formation of SAMs.20

respectively. In order to achieve a selective affinity, QCMs modified with 3 were loaded with a layer of specific affinity material. For this investigation, we decided to use the affinity materials 5−9 we identified in 2010 as the ideal candidates for the identification of triacetone triperoxide (TATP) and its discrimination from ubiquitous water since they had proven outstanding performance.6 The coating of the genuine as well as the pretreated QCMs was accomplished by an electrospray procedure.23 These compounds comprise derivatives of polyphenylene dendrimers,24 triphenylene ketals,25 cyclodextrins,8,26 and sodium cholate (Figure 4). The individual affinities were determined by recording the sensor response at different analyte concentrations and subsequent fitting of these signals against concentration of the analyte with the Langmuir equation (Supporting Information).8 The slope of the linear part of the Langmuir isotherm is regarded as a measure for the affinity to the analytes and used for comparison. Table 1 summarizes the relative alteration of affinities upon deposition of the materials 5−9 on pretreated and genuine QCMs, respectively. Additionally, one QCM without affinity material is given. The investigated analytes are of relevance for the TATP identification process.6,12,27 Table 1 clearly reveals the superior characteristics of the pretreated QCMs when compared to the genuine QCMs. Most probably the modifier seals the polar surface and nanoporosity of the alumina surface (Supporting Information). The lipophobic nature of this pretreatment will lead to an ameliorated exposure of affinity materials for the incorporation of the desired analytes. In particular, the supramolecular action of molecular voids is supported. The affinity materials 5, 6, and 8 exhibit shape-persistent cavities, which are not locked up by collapsing or aggregation. The affinity material 7 representing a lower generation of dendrimer 5 shows a completely different behavior. This might be attributed to aggregational phenomena.28 While almost all of the undesired analytes are suppressed in their affinity, TATP is boosted for all surfaces except for the QCM without any affinity material. In order to compare the quality of this new array with its predecessor,6 the array responses were aggregated using a principal component analysis (PCA) for both systems (for details, see the Supporting Information). Figures 5 and 6 display the mean coefficients of the two “strongest” principal components required to describe the measurement results as well as their standard deviations.

■

RESULTS AND DISCUSSION Since the HFF-QCMs possess aluminum electrodes, we focused on partially fluorinated phosphonic acids as well as the corresponding alkyl phosphonic acids as modifiers. Figure 2 provides a brief overview over the phosphonic acids which were used in this study. We also investigated the modification by treatment with alkyltriethoxysilanes (Supporting Information) via a vapor deposition, but the results were far inferior compared to the properties obtained with the compounds displayed in Figure 2. After modification, according to standard conditions,21 the surface hydrophobicity was evaluated by contact angle measurements with water. Since the geometry of a fully equipped, ready-to-use HFF-QCM device does not allow an exact measurement, a planar quartz disk coated with an aluminum electrode of the same characteristics as the electrodes of the HFF-QCM was employed. The contact angles for the electrode surfaces modified with fluorinated alkyl chains are about 15% higher than those of the nontreated surfaces (Figure S2 of the Supporting Information). There is no clear dependence upon chain length. However, since 3 provides the best results in terms of contact angles, is better to process, and represents a low-cost agent, we focused on this phosphonic derivative. In a practical sensing scenario, humidity and hydrocarbons are ubiquitous, interfering compounds.22 Therefore, we evaluated the effect of the fluorous surface by determination of affinities toward water and cyclohexane (Figure 3). The sensor response was recorded for different concentrations of these analytes in the gas phase (Supporting Information). The signal for both, polar and apolar analytes, was dramatically reduced upon modification of the QCM. At these concentrations, the undesired sensor responses for water and cyclohexane are decreased by 55% and by 74%, B



Article

Figure 5. PCA output of the sensor array without pretreatment.

Figure 6. PCA output of the sensor array with pretreatment.

and H2O2 is possible but not that pronounced since the affinity materials of the array were not optimized for their identification (Figure 6). No more overlapping clouds can be observed, and large separating areas for the organic analytes occur in this representation. Consequently, a much more secure and faster identification of TATP becomes possible. In the near future, flourous modified supramolecular systems30 will be applied on such QCMs to facilitate the coating process.

Figure 4. Affinity materials used for the TATP sensor array.

Table 1. Relative Change in the Individual Affinities for Interfering Analytes and for a Peroxide Explosivea H2O acetone tBuOOtBu H2O2 TATP a

5 (%)

6 (%)

7 (%)

8 (%)

9 (%)

− (%)

−4 −30 −12 12 102

−32 −29 −37 −18 22

191 91 17 41 187

−49 −2 −32 −14 19

−66 −3 −5 12 −5

−87 −32 −58 −47 −55

■

CONCLUSIONS In summary, the introduction of a fluorous interlayer prior to the coating with supramolecular affinity materials tremendously improves both, the selectivity and affinity of sensor array toward the desired analytes. In particular, supramolecular hosts with shape-persistent voids experience a boost in both parameters. The fluorous layer inhibits the diffusion to the alumina surface, causing unspecific physisorption and facilitates an appropriate orientation on the pretreated quartz crystal microbalance. Both effects enhance the desired interactions which can be exploited for the tracing volatile organic compounds, like the peroxide explosive TATP. The improved sensor data allow an improved identification of the target analyte. This fluorous effect might be more general and, therefore, successfully applied in several other fields or other surface-based sensor technologies,31 whereby the interaction to a polar support is causing unspecific signals.

The quotient of the affinity on pretreated/genuine QCMs is given.

The smaller the resulting ellipsoids are and the better their separation is, the easier and more reliable is the identification/ classification process.29 While in the original sensor array, a discrimination of TATP from other organic compounds with a similar shape, like tBuOOtBu, was not possible without the help of the third principal component (Figure 5); the fluorous interlayer already allows a clear discrimination by only two dimensions. In addition, the “clouds” wherein the PCAs of the signals of the analytes of interest are located, are substantially focused. For instance, the separation of the TATP “area” from the water signals and the separation from hydrogen peroxide is highly improved (Figures 5 and 6). The differentiation of H2O C



■

Article

2011, 21, 1907−1912. (c) Park, S.-H.; Ryu, J.-Y.; Choi, H.-H.; Kwon, T.-H. Sens. Actuators, B 1998, 46, 75−79. (d) Al-Abadleh, H. A.; Grassian, V. H. Langmuir 2003, 19, 341−347. (e) Long, Z.; Hill, K.; Sepaniak, M. J. Anal. Chem. 2010, 82, 4114−4121. (15) Grainger, D. W.; Stewart, C. W. Fluorinated Coatings and Films: Motivation and Significance. In Fluorinated Surfaces, Coatings, and Films; Castner, D. C., Grainger, D. W., Eds.; ACS Symposium Series 787; American Chemical Society: Washington, DC, 2001; pp 1−14. (16) Barriet, D.; Lee, T. R. Curr. Opin. Colloid Interface Sci. 2003, 8, 236−242. (17) (a) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 2000, 12, 1315−1328. (b) Samanta, D.; Sarkar, A. Chem. Soc. Rev. 2011, 40, 2567−2592. (18) Jennings, G. K.; Munro, J. C.; Yong, T.-H.; Laibinis, P. E. Langmuir 1998, 14, 6130−6139. (19) (a) Deng, K.; Collins, R. J.; Mehregany, M.; Sukenik, C. N. J. Electrochem. Soc. 1995, 142, 1278−1285. (b) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960−10965. (20) (a) Pellerite, M. J.; Dunbar, T. D.; Boardman, L. D.; Wood, E. J. J. Phys. Chem. B 2003, 107, 11726−11736. (b) Maxisch, M.; Thissen, P.; Giza, M.; Grundmeier, G. Langmuir 2011, 27, 6042−6048. (c) Giza, M.; Thissen, P.; Grundmeier, G. Langmuir 2008, 24, 8688− 8694. (d) Franquet, A.; De Laet, J.; Schram, T.; Terryn, H.; Subremanian, V.; van Ooij, W. J.; Vereecken, J. Thin Solid Films 2001, 384, 37−45. (e) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370−378. (f) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103−1169. (21) (a) Hoque, E.; DeRose, J. A.; Hoffmann, P.; Bhushan, B.; Mathieu, H. J. J. Phys. Chem. C 2007, 111, 3956−3962. (b) Franquet, A.; De Laet, J.; Schram, T.; Terryn, H.; Subramanian, V.; van Ooij, W. J.; Vereecken, J. Thin Solid Films 2001, 384, 37−45. (22) (a) Jia, C.; Batterman, S. A.; Reyea, G. E. Environ. Pollut. 2012, 169, 152−159. (b) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Science 1997, 276, 1045−1052. (23) (a) Fenn, J. B. Angew. Chem. 2003, 115, 3999−4024. Angew. Chem., Int. Ed. 2003, 42, 3871−3894. (b) Rapp, M.; Bender, F.; Bargon, J.; Lubert, K.-H.; Voigt, A.; Wächter, L.; Klesper, G.; Klesper, H.; Fusshöller, G. Vorrichtung zum Aufbringen von Elektrospraybeschichtungen auf elektrisch nicht leitfähigen Oberflächen. DE Patent 10344135 A1, 2003. (24) (a) Bauer, R. E.; Clark, C. G.; Müllen, K. New J. Chem. 2007, 31, 1275−1282. (b) Minard-Basquin, C.; Weil, T.; Hohner, A.; Radler, J. O.; Müllen, K. J. Am. Chem. Soc. 2003, 125, 5832−5838. (c) Shifrina, Z. B.; Rajadurai, M. S.; Firsova, N. V.; Bronstein, L. M.; Huang, X. L.; Rusano, A. L.; Muellen, K. Macromolecules 2005, 38, 9920−9932. (d) Weil, T.; Wiesler, U. M.; Herrmann, A.; Bauer, R.; Hofkens, J.; De Schryver, F. C.; Müllen, K. J. Am. Chem. Soc. 2001, 123, 8101−8108. (e) Wiesler, U. M.; Berresheim, A. J.; Morgenroth, F.; Lieser, G.; Müllen, K. Macromolecules 2001, 34, 187−199. (f) Wiesler, U. M.; Müllen, K. Chem. Commun. 1999, 2293−2294. (g) Wiesler, U. M.; Weil, T.; Müllen, K. Nanosized Polyphenylene Dendrimers. In Dendrimers III; Vögtle, F., Ed.; Topics in Current Chemistry 212; Springer-Verlag: Berlin, 2001; pp 1−40. (25) (a) Waldvogel, S. R.; Fröhlich, R.; Schalley, C. A. Angew. Chem. 2000, 112, 2580−2583. Angew. Chem., Int. Ed. 2000, 39, 2472−2475. (b) Waldvogel, S. R. Angew. Chem. 2003, 115, 624−625. Angew. Chem., Int. Ed. 2003, 42, 604−605. (c) Siering, C.; Kerschbaumer, H.; Nieger, M.; Waldvogel, S. R. Org. Lett. 2006, 8, 1471−1474. (d) Siering, C.; Beermann, B.; Waldvogel, S. R. Supramol. Chem. 2006, 18, 23−27. (26) (a) Girschikofsky, M.; Rosenberger, M.; Belle, S.; Brutschy, M.; Waldvogel, S. R.; Hellmann, R. Sensors 2012, 12, 2018−2025. (b) Girschikofsky, M.; Rosenberger, M.; Belle, S.; Brutschy, M.; Waldvogel, S. R.; Hellmann, R. Sens. Actuators, B 2012, 171−172, 338−342. (c) Girschikofsky, M.; Rosenberger, M.; Belle, S.; Brutschy,

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental methods and additional data. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 (0) 6131 39 26067. Fax: +49 (0) 6131 39 26069. Notes

The authors declare no competing financial interests.

■ ■

ACKNOWLEDGMENTS We would like to thank the state Northrhine-Westfalia (ENQUETE consortium) for generous financial support. REFERENCES

(1) (a) Schlupp, M.; Weil, T.; Berresheim, A. J.; Wiesler, U. M.; Bargon, J.; Müllen, K. Angew. Chem. 2001, 113, 4124−4129. Angew. Chem., Int. Ed. 2001, 40, 4011−4015. (b) Haas, T.; Schulze Lammers, P.; Diekmann, B.; Horner, G.; Boeker, P. Sens. Actuators, B 2008, 132, 545−550. (c) Kleefisch, G.; Kreutz, C.; Bargon, J.; Silva, G.; Schalley, C. A. Sensors 2004, 4, 136−146. (d) Loergen, J.; Kreutz, C.; Bargon, J.; Krattinger, P.; Wenners, H. Sens. Actuators, B 2005, 107, 366−371. (e) Baldini, L.; Melegari, M.; Bagnacani, V.; Casnati, A.; Dalcanale, E.; Sansone, F.; Ungaro, R. J. Org. Chem. 2011, 76, 3720−3732. (f) Janshoff, A.; Galla, H.-J.; Steinem, C. Angew. Chem. 2000, 112, 4164−4195. Angew. Chem., Int. Ed. 2000, 39, 4004−4032. (2) Dickert, F. L.; Lieberzeit, P. A. Imprinted Polymers in Chemical Recognition for Mass-Sensitive Devices. In Piezoelectric Sensors; Steinem, C. Janshoff, A., Eds.; Springer Series on Chemical Sensors and Biosensors 5; Springer-Verlag: Berlin, 2007; pp 181−183. (3) Fung, Y. S.; Wong, Y. Y. Anal. Chem. 2001, 73, 5302−5309. (4) Nato, H.; Stetter, J. R. Introduction to Chemosensors. In Handbook of Machine Olfaction. Electronic Nose Technology; Pearce, T. C., Schiffman, S. S., Nagle, H. T., Gardner, J. W., Eds.; Wiley-VCH: Weinheim, Germany, 2003; pp 79−104. (5) Neubig, B.; Briese, W. Das große Quartzkochbuch; Franzis: Feldkirchen, Germany, 1997. (6) Lubczyk, D.; Siering, C.; Lörgen, J.; Shifrina, Z. B.; Müllen, K.; Waldvogel, S. R. Sens. Actuators, B 2010, 143, 561−566. (7) Urbakh, M.; Tsionsky, V.; Gileadi, E.; Daikhin, L. Probing the Solid/Liquid Interface with Quartz Crystal Microbalances. In Piezoelectric Sensors; Steinem, C., Janshoff, A., Eds.; Springer Series on Chemical Sensors and Biosensors 5; Springer-Verlag: Berlin, Heidelberg, 2007, 111−149. (8) (a) Brutschy, M.; Schneider, M. W.; Mastalerz, M.; Waldvogel, S. R. Adv. Mater. 2012, 48, 6049−6052. (b) Brutschy, M.; Schneider, M. W.; Mastalerz, M.; Waldvogel, S. R. Chem. Commun. (Cambridge, U. K.) 2013, 49, 8398−8400. (c) Dickert, F. L.; Haunschild, A. Adv. Mater. 1993, 5, 887−895. (9) (a) Latif, U.; Rohrer, A.; Lieberzeit, P. A.; Dickert, F. L. Anal. Bioanal. Chem. 2011, 400, 2457−2462. (b) Dickert, F. L.; Lieberzeit, P.; Miarecka, S. G.; Mann, K. J.; Hayden, O.; Palfinger, C. Bionsens. Bioelectron. 2003, 20, 1040−1044. (10) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339−1386. (11) Sauerbrey, G. Z. Phys. 1959, 155, 206−222. (12) Lubczyk, D.; Grill, M.; Baumgarten, M.; Waldvogel, S. R.; Müllen, K. ChemPlusChem 2012, 77, 102−105. (13) (a) Graewe, B.; Rang, A.; Schalley, C. A.; Haubrich, J.; Bargon, J. Sens. Actuators, B 2006, 119, 302−307. (b) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864−11873. (14) (a) Nahar, R. K.; Khanna, V. K. Sens. Actuators, B 1998, 46, 35− 41. (b) Cheng, B.; Tian, B.; Xie, C.; Xiao, Y.; Lei, S. J. Mater. Chem. D



Article

M.; Waldvogel, S. R.; Hellmann, R. Anal. Chim. Acta 2013, 791, 51− 59. (27) Lin, H.; Suslick, K. S. J. Am. Chem. Soc. 2010, 132, 15519− 15521. (28) (a) Bauer, R.; Liu, D.; Ver Heyen, A.; De Schryver, F.; De Feyter, S.; Müllen, K. Macromolecules 2007, 40, 4753−4761. (b) Türp, D.; Nguyen, T.-T.-T.; Baumgarten, M.; Müllen, K. New J. Chem. 2012, 36, 282−298. (29) Ehret, B.; Safenreiter, K.; Lorenz, F.; Biermann, J. Sens. Actuators, B 2011, 158, 75−88. (30) (a) Chu, Q.; O’Neal, K.; Osipov, M.; Ngwendson, J. N.; Geib, S. J.; Weber, S. G.; Curran, D. P. New J. Chem. 2010, 34, 2732−2734. (b) Rezanka, M.; Eignerová, B.; Jindrich, J.; Kotora, M. Eur. J. Org. Chem. 2010, 6256−6262. (31) (a) Dickert, F. L.; Forth, P.; Bulst, W.-E.; Fischerauer, G.; Knauer, U. Sens. Actuators, B 1998, 46, 120−125. (b) Grate, J. W. Chem. Rev. 2000, 100, 2627−2648.

E


Surface Pretreatment Boosts the Performance of Supramolecular

Recommend Documents