Langmuir 1994,10, 1642-1646
1642
Application of the Quartz Crystal Microbalance To Characterize the Interaction of Solutes with Modified Interfaces John A. Roush, David L. Thacker, and Mark R. Anderson* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virgina 24061 Received January 18,1994. I n Final Form: April 11,1994’ Piezoelectric quartz crystals, modified by the spontaneous adsorption of either 4-aminothiophenol or octadecanethiol,were utilized to investigate the selective interactions that exist between solutes and the surface modification layer. It is found that the exposed amine functionality of the 4-aminothiophenol monolayer increases the sensitivity of the quartz crystal microbalance to phenols relative to the response found at either a crystal modified with an octadecanethiol monolayer or an unmodified quartz crystal. Protonation of the amino group decreases the sensitivityof the 4-aminothiophenol modified crystaltoward the phenol solutes, to a level approximately the same as seen for the octadecanethiolmonolayer and the unmodified quartz crystal. The response of the three quartz interfaces studied to structurally similar aniline molecules ia nearly the same for each interface and is approximately the same as observed for the phenols at both the octadecanethiolmodified interface and the unmodified quartz crystal. These results indicate that a selective interaction between the solute and monolayer leads to increased sensitivityof the quartz crystal microbalance to the solute. For the phenols, the principal mode of interaction at the interface likely involves hydrogen bonding with the basic 4-aminothiophenol. Theoretical fit of the adsorption data to Langmuir isothermsprovides a mechanism to determine the free energy of adsorption for the phenols at the 4-aminothiophenol modified quartz crystal. The free energy of adsorption was found to be -16.2 f 0.5 kJ/mol for 4-tert-butylphenol,-14.4 f 0.6 kJ/mol for 4-isopropylphenol, and -14.8 f 0.8 kJ/mol for 4-ethylphenol when interacting with a 4-aminothiophenol modifed surface.
Introduction
electrode structures or (ii) generating specific molecular interactions with solutes. Crooks and co-workers have published a series of papers that describe their efforts to There has been a great deal of interest recently in trying design interfaces with specific molecular interaction/ to understand, on a molecular level, the interactions that recognition proper tie^.^'^ In these reports, electrochemioccur a t interfaces and to use this information to undercalmeasurements, infraredspectroscopy,ellipsometry,and stand fundamental properties of interfacial phenomena surface acoustic wave (SAW) devices were used to (e.g. adhesion, lubrication, and charge transport).l4 Much characterize the surface interactions that occur with of this research has involved the spontaneous adsorption analytes. Most of the work, however, was conducted with of organic mercaptans onto gold substrates to form, by vapor phase material that was interacting with the self-assembly,a structurally well ordered monolayer. With interface. This paper investigates the utility of the quartz these self-assembled monolayers, the influence of mocrystal microbalance (QCM) for understanding selective lecular structure upon these interfacial processes may be molecular interactions that occur at modified interfaces studied. Although much of the research conducted thus with solutes present in an adjacent condensed phase. far has focused upon the structural importanceof relatively The quartz crystal microbalance is a sensitive device simply alkyl mercaptans, recent interest has involved using capable of measuring mass changes that occur at an the spontaneous adsorption process to place functional interface. It has been estimated that the limit of detection groups at defined positions within the surface monoof a 15-MHz crystal is approximately 10-l2g under ideal layer.”14 These defined structures may then be used for conditions.16 The QCM has been applied many times to specific purposes, such as (i) generating nanoporous the determination of low levels of organic compounds in gases. Extension of this technique to measurements in * To whom correspondence should be sent. the solution phase has been slow because of problems not 0 Abstract published in Advance ACS Abstracts, May 15,1994. (1)Chidsey, C. E.D.Science 1991,261,919, encountered in gas phase applications. Greater energy (2)Swalen,J.D.;Allara,D.L.;Andrade,J.D.;Chandroee,E.A.;Garoff,losses at the solution-crystal interface make stable crystal S.; Israelachivili, J.; McCa~thy,T. J.; Murray, R.; Pease,R. F.; Rabolt, oscillation more difficult to achieve in the presence of J. F.; Wynne, K.J.; Yu, H.Langmuir 1987,3,932. (3)Nuzzo, R. G.;Fueco, F. A.; Allara, D. L. J. Am. Chem. SOC.1987, liquids. The oscillation frequency is thought to be 109,2358. dependent on the density and viscosity of the gas or liquid (4)Kurihara, K.; Kunitake, T. Langmuir 1992,8,2486. in contact with the crystal. Slight density or viscosity (5)Bain,C. D.;Troughton, E. B.;Tao, Y. T.; Evall, J.; Whitesides, G. changes of the solvent or sample, therefore, may cause a M.; Nuzzo, R. G. J. Am. Chem. SOC.1989,111,321. (6)Bilewicz, R.;Majda, M. J. Am. Chem. SOC.1991,113,5464. drift in resonant frequency. Finally, progress has been (7)Sabitani. E.;Rubenebin, I.; Mom, R.; Sagiv. J. J. Electroanal. hampered by the lack of coating procedures which will Chem. 1987,219,365. result in stable and reproducible surfaces that can interact (8)Sagiv, J. J. Am. Chem. SOC.1980,102,92. (9)Sun, L.;Johnson, B.;Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, strongly with solutes.ls 94,8869. In gas-phase applications, the resonant frequency of (10)Sun, L.; Thomas, R. C.; Crooks, R. M. J. Am. Chem. SOC.1991, 113,8550. the crystal is linearly dependent on the mass of the analyte (11)Bryant,M.; Crooks, R. M. Langmuir 1993,9,385. added to the crystal surface, in accordance with the
(12)Sun, L.;Crooks, R. M.; Ricco, A. J. Langmuir l993,9,1775. (13)Chailapakul, 0.; Crooks,R. M. Langmuir 1993,9,884. (14)Kepley, L.J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992,64, 3191.
(15)King,W. H.Anal. Chem. 1964,367,1735. (16)Charlesworth, C. W.Anal. Chem. 1990,62,76.
0743-7463/94/2410-1642$04.50/0 0 1994 American Chemical Society
Langmuir, Vol. 10, No. 6, 1994 1643
Letters
Sauerbrey expression17 Av =
-2 A m n v
t
A/ (S,d,)
where Av is the frequency change, YO is the fundamental oscillating frequency of the quartz wafer, S, is the shear modulus of the quartz wafer, d, is the density of the quartz, and A m is the change in mass on the oscillating surface. Recently, several research groups have developed theoretical explanations for the behavior of uncoated piezoelectric crystals in the presence of 1iquids.ls-" Many of these treatments include some contribution from the density or viscosity of the adjacent solution, in addition to the response due to the specific adsorption of solutes to the surface,to generate the measured frequency change. Despite the disadvantages, the QCM has found some applications for the determination of analytes in solution. In most of these applications, the sensing ability is imparted to the quartz crystal by depositing a material on the surface of the quartz which is capable of a selective interaction with the analyte of interest. Konash and Bastiaans were the first to achieve reliable oscillation of quartz crystals in contact with solutions.2s These workers chemically bonded octadecyltrichlorosilane or docosodimethyl(dimethy1amino)silane to the face of the crystal that was exposed to solvent flow in an effort to simulate liquid chromatographic conditions. Others have followed using predominantly polymer coating to generate the specific interaction.l6*2G2g In this paper we report upon the utility of the quartz crystal microbalance for understanding the chemical and/ or physical interactions that may exist between solutes in a condensed phase and chemically modified interfaces. The applicability of the QCM for the determination of the free energy of adsorption of solutes to homogeneous, chemically modified surfaces is also investigated. Experimental Section Quartz crystals with 10-MHz resonant frequency were purchased from International Crystal Manufacturing Co., Inc. (Oklahoma City, OK). The crystals were lab monitor AT cut, measuring 11.4 mm in diameter, with a thickness of 0.25 mm. The crystals had centrally deposited gold electrodes 5 mm in diameter on each face. The electrical circuit used to measure crystal oscillation frequency was a modified version of the circuit published by Bruckenstein and Shay.20 A flow cell was constructed so that the indicator quartz crystal was held between two O-rings which were partially recessed into the upper and lower halves of the cell. The flow cavity consisted of the space between the upper face of the crystal and the top half of the flow cell and had an estimated volume of 5 pL. The incoming solvent stream was introduced normal to the center of the quartz crystal, flowing radially across the surface to two exit porta which were located at the outer edges of the flow cavity. (17) Sauerbrey, G.Z. Phys. 1959, 155,206. (18) Kanazawa, K. K.; Gordon, J. G.Anal. Chem. 1985,57,1772. (19) Kanazawa, K. K.; Gordon, J. G.Anal. Chim. Acta 1988,175,99. (20) Bruckenstein, S.; Shay, S. Electrochim. Acta 198930, 1295. (21) Hager, H. E. Chem. Eng. Commun.1986,43,25. (22) Muramateu, H.; Tamiya, E.; Karube, 1. Anal. Chem. 1988, 60, 2142. (23) Nomura, T.; Okuhara, M. Anal. Chim. Acta 1982, 142, 281. (24) Yao, S. 2.;Wou, T. A. A d . Chim. Acta 1988,212,61. (25) Konash, P. L.; Bastiaans, G. L. Anal. Chem. 1980,52,1929. (26) Nomura, T.; Okuhara, T.; Hasegawa, T. Anal. Chim.Acta 1986, 183, 261. (27) Nomura, T.; Sakai, M. Anal. Chim.Acta 1986,183, 301. (28) Nomura, R.; Ando, M. Anal. Chim. Acta 1986,172,353. (29)Okahata, T.; Ebatao, H.; Taguchi, K. J. Chem. Soc., Chem. Commun. 1987,1363.
- A 110 -
I
120
loo
-
90-
8070
-
805040-
B
302010
-
0 -
-
C
hh
I I 1 I I I ' Figure 1. Quartz crystal microbalance frequency response to a solution containing lo00 ppm 4-tert-butylphenol. Letter A corresponds to the data obtained with the 4-aminothiophenol modified crystal, B corresponds to the data obtained with the octadecanethiol modified crystal, and C corresponds to the data obtained with the unmodified crystal.
-10
The solvent delivery system consisted of an ISCO Model 314 syringe pump and a RheodyneMode17120 high-pressure injection valve with a 20-pL sample loop. All postinjector tubing was 0.010 in. i.d. Teflon. Data were collected using a Servocorder Model 6252 chart recorder. Octadecanethiol, Cethylphenol, 4-isopropylphenol, Ctertbutylphenol, 4-ethylaniline,Cisopropylaniline, and 4-tert-butylaniline were purchased from Aldrich (Milwaukee, WI)and used without further purification. 4-Aminothiophenol was also purchased from Aldrich, and it was purified by sublimation prior to being used. Experiments were conducted using acetonitrilewatermixtures as solvents. Acetonitrile was nonspectro grade (Burdick & Jackson, Muskegon, MI) and was used without further purification. All water was purified by a Barnsted Nanopure I1 water purification system. Unless otherwise noted, the QCM experiments were conducted using a solvent consisting of 20% acetonitrile and 80% water. Each time the flow cell was assembled, the solvent was pumped through the cell for a minimum of 24 h prior to the performance of an experiment. The solvent flow rate was 40pLlmin. for all experiments. All samples were made using the same solvent batch as was being pumped through the flow cell. The procedure for surface modification followedstandard selfassembly techniques." Briefly,the quartz crystalswere immersed in a warm solution of concentrated sulfuric acid and 30% hydrogen peroxide (3:l volume ratio), so-called piranha solution, in order to remove surface contaminants. Caution should be exercised when wing "piranha" solution as it is potentially erplosive.31 After wage it should be immediately and properly disposed. The crystals were then rinsed with deionized water, 95 5% ethanol, and chloroform. The crystals were then immersed in solutions of the organic thiol (50 mM, in chloroform), and the spontaneous adsorption was allowed to proceed at room temperature for 24 h. After this period of time, the crystals were rinsed with chloroform and 95% ethanol, then blown dry with a rapid stream of nitrogen and immediately placed in the flow cell.
Results and Discussion Figure 1shows the frequency response of the different quartz crystal interfaces to solutions containing loo0ppm 4-tert-butylphenol. The frequency response is found to be substantially higher at the 4-aminothiophenolmodified (30) Gatin, M. R.; Anderson, M. R. Vib. Spectrosc. 1993,5,256. (31) Dobbs, D. A.; Bergman, R. G.; Theopold,K. H. Chem. Eng. News 1990, 68 (17), 2.
1644 Langmuir, Vol. 10, No. 6, 1994 Table 1. Frequency Response (Hz) of the Modified Quartz Crystals to Solutions Containing lo00 ppm of Various Phenol Solutes (The Error Associated with the Frequency Measurement Is t2 Hz) &aminothiophenol octadecanethiol modified modified unmodified quartz quartz quartz solute crystal CryStal crystal 4-tert-butylphenol 135 16 25 4-isopropylphenol 52 16 28 4-ethylphenol 32 12 5
interface than the response observed at a quartz crystal modified with octadecanethiol or at an unmodified quartz crystal. Table 1lists the frequency response data for loo0 ppm solutions of the different phenols at the modified quartz crystal interfaces. The data for the three different phenol solutions are similar in that the crystal modified with 4-aminothiophenol has a larger response to the phenols than do the other quartz crystal surfaces. These data suggest that there is a selective interaction between the 4-aminothiophenol monolayer and the solution dissolved phenol that leads to the higher frequency response than is observed at the other interfaces. Given the relative acidity of the phenol and the basicity of the 4-aminothiophenol monolayer (estimated pKa = 4.6 for 4-aminothiophenolg), it is likely that a hydrogen bonding interaction exists between the monolayer and the phenol solutes. The difference in response between the quartz crystals modified with 4-aminothiophenolor with octadecanethiol illustrates that the presence of the organic monolayer, by itself, does not significantly increase the frequency response to the organicsolutes by a hydrophobic interaction (relative to the response at an unmodified quartz crystal). Rather, there must exist some other selective interaction between the phenol solutes and the 4-aminothiophenol modified interface that increases the quartz crystal microbalance response. The absence of a difference in the frequency response between the unmodified crystal and the quartz crystal modified with octadecanethiol may be attributed to the impermeability of the self-assembled octadecanethiol monolayer to solutes in aqueous solutions.32133 The steric bulk of the phenols also probably contributes to its inability to partition into the tightly packed monolayer. Any lipophillic interaction that may exist between the organic solute and the organicmonolayer, therefore, would likely occur at the monolayer-solution interface rather than by solute permeation into the monolayer. Such an interaction at the surface of the monolayer is not likely to be much different from the interaction that occurs at the unmodified crystal interface (the unmodified quartz crystal is also likely to have some lipophilliccharacter due to the adsorption of adventitious organic contaminants from the laboratory atmosphere, as suggested by the presence of carbon in XPS analysis of an unmodified quartz crystal), possibly explaining the correspondence of the results at these two interfaces. Konash and Bastiaans also reported that quartz crystals modified with octadecylsilanedemonstrated little response to aromatic solutes.2s To demonstrate the importance of hydrogen bonding between the 4-aminothiophenol monolayer and the phenol solutes, the mobile phase for the flow injection experiments was adjusted to a pH of approximately 3 by the addition of 0.001M HC1. At this pH, the exposed amine group on (32) Porter, M. D.; Bright, T. B.;Allara, D. L.; Chidsey,C. E. D. J. Am. Chem. SOC. 1987,109,3559. (33) Wang, J.; Hui, W.; Angnes, L. Anal. Chem. 1993,65, 1893.
Letters Table 2. Frequency Response (Hz) of the 4-Aminophenol Modified Quartz Crystals to the Phenols in the Presence and Absence of 0.001 M HCl in the Mobile Phase (The Error Associated with the Frequency Measurement Is A2 Hz) solute without added HC1 with added HCl 14 4-tert-butylphenol 133 4-isopropylphenol 53 9 4-ethylphenol 33 7 Table 3. Frequency Response (Hz)of the Modified Quartz Crystals to Solution Containing lo00 ppm of Various Aniline Solutes (The Error Associated with the Frequency Measurement Is *2 Hz) 4-aminothiophenol octadecanethiol modified modified unmodified quartz quartz solute CrvStal cryetal CrvStal 4-tert-butylaniline 18 12 17 Cisopropylaniline 14 10 31 4-ethylaniline 8 6 4
the surface confined 4-aminothiophenol is expected to be protonated, eliminating the ability of the amine functionality of the surface layer to interact as a base with the weakly acidic phenol solutes in the adjacent solution.When the lo00 ppm phenol solutions are flowed past the protonated 4-aminothiophenolsurface layer, the frequency response is much lower than the response observed for the same species with the unprotonated surface (Table 2). The frequency response at the protonated surface was also about the same as was observed with both the unmodified crystal and the crystal modified with the octadecanethiol monolayer. These results support the conclusionthat the ability of solute moleculesto selectively interact with the surface layer results in greater QCM sensitivity. Preventing this interaction from occurring by protonation of the surface amine results in a situation where the quartz crystal responds in a manner similar to the mechanism of response by the unmodified quartz crystal or by the crystal modified with octadecanethiol. Without a selective interaction with the surface layer, the frequency response to the solutes is nearly the same regardless of the identity of the surface coating. In these instances, the QCM is likely responding to the changes in the bulk properties in the solution as the solute flows past the quartz crystal. Additional experiments were conducted using the same modified interfaces interacting with 4-tert-butylaniline, 4-isopropylaniline,and 4-ethylaniline. These solutes have nearly the same formula weight and a similar structure to the corresponding phenols investigated. The anilines, however, are basic and should not have a selective interaction with the 4-aminothiophenol monolayer. The frequency response to solutions containing lo00 ppm of the anilines was found to be nearly the same at the crystals modified with 4-aminothiophenol, at the unmodified quartz crystal, and at the crystal modified with octadecanethiol (Table 3). In addition, the frequency response of the anilines at all the surfaces investigated was nearly the same magnitude as the response of the phenols to (i) the unmodified quartz crystal, (ii) the crystal modified with octadecanethiol, and (iii) the protonated 4-aminothiophenol modified quartz crystal. These results support the conclusion that a selective interaction must exist between the acidic phenols and the basic amine group of the 4-aminothiophenol monolayer for the increased frequency response to occur. In the absence of a selective interaction, the QCM responds only to the changing properties of the solution.
Letters 3 20 300
3.20
-
2.60 3.00
200 240 220 280
n
'2
x,
N
a
e
3
2.60
2.40 2.20
1.0 1.40 1.20 1.w 0.60 0.60 -
200-
2.00
100 140 180
120
1.60
-
ii : : A
in
ow040
020
OW
-
I
0
OW1
0002
0003
0004
OW5
OOW
0007
0
Concentration, &J Figure 2. Adsorption data, uncorrected for the frequency response due to changing bulk solution properties, plotted as a function of 4-tert-butylphenolsolution concentration. The line represents a theoretical Langmuir isotherm in which rsequals 3.5 X 1o-B mol/cm2. The frequency changes observed at the modified quartz crystals indicate that the microbalance may respond to both a specific molecular interaction between the solute and the surface and to the changing bulk properties of the adjacent solution. At high solute concentrations, it is likely that the total measured frequency response is a combination of the two contributions. A plot of the bulk concentration of 4-tert-butylphenol versus the amount of material adsorbed to the quartz crystal surface (as calculated with the Sauerbrey equation assuming that the experimental frequency change comes only from specific adsorption, Figure 2) shows that the experimental data begin to diverge from a calculated Langmuir isotherm at a concentration of approximately 0.0017 M 4-tert-butylphenol. Assuming that the flow cell cavity has a volume Of 5 pL,at concentrations of the 4-tert-butylphenol greater than 0.0017 M, the flow cell cavity contains more material than is required to saturate the monolayer. Even if the monolayer is saturated, at these solute concentrations, material would remain in the solution generating a bulk response by the QCM. To conduct a thermodynamic analysis of the specific adsorption, therefore, the contribution from the bulk response must be separated from the response due to the specific interaction. For the theoretical isotherm shown in Figure 2, the mol/ surface capacity, rS,was estimated to be 3.5 X cm2 by measuring the frequency change that occurs when an unmodified quartz crystal is exposed to a solution containing containing 0.050 M 4-aminothiophenol. It is assumed that each exposed amine group on the surface represents one site for potential solute interaction; therefore, the monolayer capacity for the adsorption of the phenols is given by the number of moles of 4-aminothiophenol on the surface. This value for the maximum surface coverage, 3.5 X 10-9 mol/cm2, is higher than coverage estimated to be 1.2 X 10-9 mol/cm2 by Crooks and co-workers.'O Although it in unclear why our measurement is much higher than that reported by Crooks et al., a possible explanation for this discrepancy may be the roughness of the gold electrodes on the quartz crystal utilized. Experiments which utilized a variety of solutes which are thought not to have a specific interaction with the surface show a linear dependence of the frequency response
I
I
,
I
I
,
0.002
0.003
0.004
0.005
0.008
0.007
0.00
0.001
Concentration, &J Figure 3. Adsorption data in which the QCM frequency change has been corrected for the bulk solution response plotted as a function of 4-tert-butylphenolsolution concentration. The line represents a theoretical Langmuir isotherm in which equals 3.5 x 1o-B mol/cm2.
as a function of solute concentration. An estimate of the bulk response, therefore, may be made from the response of the phenol solutes at either the unmodified quartz crystal or at the crystal modified with octadecanethiol. Assuming that the two contributions to the frequency change are additive, the response to the bulk changes of the solution may be subtracted from the total frequency response to leave the contribution from the specific molecular interaction. When this subtraction is conducted for the data shown in Figure 2, it is found that the experimental data points more closely approximate the calculated theoretical isotherm (Figure 3),particularly a t high solute concentration. From the calculated Langmuir isotherm, a value for AGah is determined to be -16.2 f 0.5 kJ/mol for 4-tert-butylphenol, -14.4 f 0.8 kJ/mol for 4-isopropylphenol, and -14.8 f 0.8 kJ/mol for 4-ethylphenol. Importantly, for all of the phenols studied, the calculated values for the free energy of adsorption are closeto each other, indicating that the mode of interaction is the same for each phenol tested. This is the expected result if the phenols are all interacting with the 4-aminothiophenol monolayer by the same type of mechanism. The values of AGa& obtained are in the range that might be expected for a physisorption process, and these are values that might be expected for hydrogen bonding interactions between the confined 4-aminothiophenol and the phenol solutes.34 Summary The interactions that exist between solutes and chemically modified interfaces have been studied using the quartz crystal microbalance. It is found that the ability of phenol solutes to selectively interact with the basic 4-aminothiophenol monolayer resulta in a larger frequency response than is observed for solutes and/or modified interfaces which have no ability to selectivelyinteract with each other. The selective interaction between the phenol solutes and the 4-aminothiophenol monolayer is thought to be due to a hydrogen bonding association. These results correlate well to the results reported by Crooks et al. for (34)Atkim, P.W.Physical Chemistry; W.H.Freeman and Co.: San Francisco, CA, 1978,p 938.
1646 Langnuir, Vol. 10, No. 6,1994
the acid-base interaction between a monolayer containing an exposed carboxyllic acid group and an organic amine in the adjacent gas phase.12 When no selectiveinteraction between the solute and interface occurs, the quartz crystal apparently responds to the changing bulk properties of the solution, as shown by Konash and Bastiaans= and suggested in many theoretical treatmenh2S2' The total frequency response of the QCM appears to be the sum of the bulk response and the response due to the specific interaction, especially at high solute concentration. The frequency response to the changes in the bulk solution properties may be subtracted from the total response to provide a measure of the specific interaction that occurs between the solute and the surface layer. A simple subtraction of an estimated bulk response in the presence of a specific surface interaction, however, may be inadequate at low concentrationswhere the amount of material in the solution is below the surface saturation limit. When
Letters a selective interaction occurs, the frequency response data may be utilized to construct adsorption isotherms and extract thermodynamic information. These results may have application to the design of molecular interfaces for use as in situ chemical sensors or for molecular recognition applications,similar to the gas phase surfaceacousticwave sensor reported by Kepley et a1.I' Research is currently being conducted in which other modified interfaces are prepared to investigate their specific interactions with condensedphase solutes, and which is using in situ infrared spectroscopy to try and identify the specific interactions that may be occurringbetween the solutesand the modified interface.
Acknowledgment. Acknowledgment is made to the Thomas R. and Kate Miller Jeffress Memorial Trust for financial support of this research.
