Anal. Chem. 1989, 67, 1001-1010
samples with lower fluorescence quantum yields or for nonfluorescing samples, detection sensitivity is expected to be better, since the D4WM signal becomes stronger with more heating due to more nonradiative relaxation. Hence, this nonlinear technique not only complements the fluorescence methods but also is applicable to fluorescing molecules as demonstrated here.
LITERATURE CITED Yariv, A. I€€€ J . Quantum Nectron. 1978, QE- 14, 650-660. Optlcal Phase Conjugation;Fisher, R. A., Ed.; Academic Press: New York, 1983. Heiiwarth, R. W. J . Opt. SOC.Am. 1977, 6 7 , 1-3. Levenson, M. D.; Johnson, K. M.; Hanchett, V. C.; Chaing, K. J . Opt. SOC. Am. 1981, 7 1 , 737-743. White, J. 0.; Yariv, A. Appl. Phys. Lett. 1980, 3 7 , 5-7. Tan-no, N.; Kawauvhi, K.; Yokoto, K. J . Opt. SOC. Am. 6 : Opt. Phvs . 1986. 3 . 60-64. Liio-, P. F.; ‘Bioom, D. M.; Economou, N. P. Appl. Phys. Lett. 1978, 32. 813-015. Tocho, J. 0.; Sibbett, W.; Bradley, D. J . Opt. Commun. 1980, 3 7 , 67-71. Silberberg, Y.; BarJoseph, I. Opt. Commun. 1981, 39, 265-268. Fujiwara, H.; Nakagawa, K. Opt.Commun. 1985, 55, 386-390. Tong, W. G.; Chen, D. A. A.m 1987, 4 1 , 586-590 and . / . Spectrosc. . references therein. Tong, W. G.; Andrews, J. M.; Wu, Zhiqiang. Anal. Chem. 1987, 59, 896-899. Chen, D. A,; Tong, W. G. J . Anal. At. Spectrom. 1988, 3. 531-535.
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(14) Andrews, J. M.; Tong, W.G. Spectrochim. Acta, 8 : A t . Spectrosc., in press. (15) Scouten, W. H.; et ai. Eur. J . Biochem. 1980, 172, 9-16. (16) Petkovic. M. Arch. Pharm. (Belgrade) 1983, 13, 1-4. (17) Emery, Arthur J., Jr.; Hazen, Frances Knapp; Stotz, Elmer. Stain Techno/. 1950, 25, 201-208. (18) Mizunoya, Yasuhisa; Omori, Mitsuaki. Jpn. Anal. 1971, 2 0 , 1171-1 177. (19) Jork, H.; Lehmann. G.; Recktenwaid, U. J . Chromatogr. 1975, 107, 173-1 79. (20) Fujiwara, H.; Nakagawa, K. J . Opt. SOC.Am. 6 : Opt. Phys. 1987, 4 , 121-128. (21) Silberberg, Yaron; BarJoseph, Israel. I€€€ J . Quantum ,Electron. 1981, OE-17,1967-1970. (22) Martin, G.; Heiiwarth, R. W. Appl. Phys. Lett. 1979, 3 4 , 371-373. (23) Hoffman, H. J. I€€€ J . Quantum Nectron. 1988, QE-22, 552-562. (24) Hoffman, H. J.; Perkins, P. E. I€€€ J . Quantum Nectron. 1988, Q€22, 563-568.
RECEIVED for review October 26, 1988. Accepted February 2, 1989. Part of this report was presented a t the 41st ACS Annual Summer Symposium on Analytical Chemistry, “Lasers in Analytical Chemistry”, at Stanford University, June 1988. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. We also gratefully acknowledge support by the National Institutes of HealthGeneral Medical Sciences (GM-41032).
In Situ Fluorescence Studies of Aluminum Ion Complexation by 8-Hydroxyquinoline Covalently Bound to Silica M. R. Weaver and J. M. Harris* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Investigation of Interfacial effects on an immobilized reagent response was undertaken by using in situ fluorescence spectroscopy and flow Injection methods. A procedure to immobilize 8-hydroxyquinoline by an alkyl slloxane linkage to silica gel was used to produce a chemically stable material, capable of producing fluorescent complexes with a number of metal ions. Reactions of this reagent with aluminum ion were studied by using flow injection methods to control the solution conditions and the exposure of the reagent to metal ion samples, providing some insight into the chemical lnteractions that affect the equilibrium behavior. Surface equiiibrium constants were calculated and compared to the free solution values for the aluminum qulnolate complex. Dlfferences in the behavior of the free solution and immobilized reagent under flow injection and steady-state conditions could be related to changes In the surface potential of the silica gel. The pH and Ionic strength dependence of the reaction and saturation of the immobilized ligand response further point out the important role that the double-layer potentlal plays in the reactivity of surface bound reagents for lonlc species.
Much interest has been generated by the development of immobilized chemical reagents for preconcentration of metal ions and optically based chemical sensing. Devices related to the latter application, called “optrodes,”have potential uses in remote sensing in hazardous or inaccessible environments, such as monitoring groundwater contaminants, industrial chemical processes, or in vivo biological systems. The amount
of activity in the area is reflected by a number of recent reviews on the subject in the literature (1-5). While much effort has been directed toward the practical development of immobilized chemical reagents, comparatively little work has been accomplished in characterizing the chemistries involved in the sensing and preconcentration applications. Saari and Seitz (6) have reported p H effects on a surface immobilized morin-A13+ complex that coincide with free solution theory for a 1:lmetal complex. In the same study, interferences due to other ions were also investigated. Ditzler et al. (7) have investigated effects of the immobilization procedure on selectivity. Dill and Leyden have studied matrix effects on aminosilanes chemically bonded on silica gel (8). Problems with interpreting the p H response of immobilized reagents, in the context of thermodynamics, have been discussed by Janata (9). The pH response of 8-hydroxyquinoline immobilized by an azo linkage to silica and controlled pore glass (CPG) has been studied by Marshall and Motolla (10) and by Kolstad et al. (11). Recently, the calcium binding equilibria of this immobilized ligand were explained by Chow and Cantwell (12)in terms of a site-binding model, which included the effects of the surface potential. The purpose of the present work is to apply in situ fluorescence spectroscopy and flow injection methods to examine the reactivity of an immobilized chemical reagent, which could be used for preconcentrating or detecting solution-phase metal ions. The goal here is to elucidate the interfacial phenomena that affect the chemistry of such reagents in order to yield information that will optimize the design of practical sensors and preconcentration materials, as well as increase our understanding of such systems. In selection of a chemical
0003-2700/89/0361-1001$01.50/0 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
system to develop these methods, the reagents, solid support, and immobilization chemistry should be compatible with spectroscopic observation of surface reactions with a variety of analytes to serve as a paradigm for preconcentration and sensing applications. With these goals in mind, 8-hydroxyquinolone bound to silica gel by an alkyl linkage was chosen for study. Porous silica was selected as the solid support, since it is physically and chemically stable compared to other substrates such as synthetic polymer resins and cellulose, especially in chemically harsh environments. In addition, silica gel is optically transparent from the ultraviolet through the near-IR, making it compatible with a variety of in situ spectroscopies. The large surface area of porous silica (100-500 m2/g) allows immobilization of reagents in relatively high concentrations. Finally, the surface chemistry of silica can be modified by well established silane derivatization procedures. &Hydroxyquinoline or oxine was chosen as a ligand because of the literature available on both the photophysics of its fluorescent metal complexes (13-15) and its metal ion complexation behavior in free solution (16). While several metal ions complexed with 8-hydroxyquinoline exhibit fluorescence, AI3+was selected for this study because it forms an extremely stable oxinate complex of high fluorescence quantum yield, which has been used reliably in several analytical procedures for A13+quantitation. Immobilization was achieved by an alkyl siloxane linkage to silica, yielding a chemically stable surface bond which does not appreciably affect the quantum yield of fluorescence. In addition, the alkyl linkage does not participate in acid-base reactions as do linkages involving nitrogen such as amines and azo groups. This has the advantage of reducing the complexity of interpreting the system's p H response. In addition, the experiment was designed around flow injection analysis (FIA) methods allowing control of reaction conditions and analyte exposure, while improving mass transport efficiency by a t least a factor 6 compared t o that of simple stirring (17). This method of sample handling also allows controllable chemical integration of analyte on the reactive surface.
EXPERIMENTAL SECTION Synthesis of Surface Immobilized 8-Hydroxyquinoline. Chemicals used for the synthesis of immobilized 8-hydroxyquinoline were purchased from Aldrich, including 8-hydroxyquinoline, allyl bromide, hexamethyldisilazane (HMDS), chlorotrimethylsilane, chlorodimethylsilane, and hydrogen hexachloroplatinate(1V) (chloroplatinic acid). Allyl bromide was distilled under a N2 purge and stored over molecular sieves; the other reagents were used without further purification. Dimethylaniline was purchased from Mallincrodt and freshly distilled under reduced pressure prior to use. Silica gel having a specific surface area of 450 m2/g as determined by the Nz Brumauer-Emmett-Teller method and a mean pore diameter of 60 A was purchased from Baker and sieved to a particle size range of 250-125 wm. The silica gel was washed with distilled HzO, filtered, air dried, dried in an oven overnight at 145 "C, and then evacuated to 10 mTorr while still hot to remove as much surface adsorbed H 2 0 as possible prior to derivatization. Treating silica gel with hot 1 M HCl and repeating the above washing process yielded a product with increased background luminescence and therefore was not used. The synthesis of silica-immobilized 8-hydroxyquinoline was adapted from a procedure outlined by Plueddemann (1419) and is presented in Figure 1. The first step involved combining 8.15 g (56 mmol) of 8-hydroxyquinoline, 5.9 mL (68 mmol) of allyl bromide, 3.62 g (64 mmol) of KOH pellets, and 150 mL of spectrophotometric grade acetone into a magnetically stirred 500-mL three-necked round-bottom (RB) flask fitted with a water-cooled condenser. The mixture was refluxed under N2 for 12 h, producing the allyl quinoline ether in a red-tea-colored reaction solution with precipitated KBr. After cooling, the mixture was washed with 2 X 100 mL of 10% aqueous NaOH and 2 X 100
Synthesis of surface immobilized 8 H Q
II/b
OH
I
00
+z:
P
- i;i
P
- Sr I
Figure 1. Synthesis scheme for generating silica-immobilized 8-
hydroxyquinoline. mL of H20 and dried over anhydrous Na2S04. After the solvent was removed by using a rotary evaporator, the product was distilled over at 90 "C and 20 mTorr with a Kugelrohr apparatus, and the structure was verified by proton NMR. The allyl quinoline ether is a yellow-green, syrupy liquid. The second step converts the allyl quinoline ether into 7-allyl-8-hydroxyquinolineby a Claissen rearrangement. The ether was added to 3 times i k volume of dimethylanilinein a N2-purged three-necked RB flask fitted with a water-cooled condenser, thermometer,and a magnetic stirring bar. The mixture was stirred and rapidly heated to 180 "C where the mixture turns dark, and its temperature quickly rises to 195-200 "C with vigorous refluxing. The temperature was reduced to 190 "C, and the mixture was refluxed overnight. After cooling, the product was crudely separated from the dimethylaniline by distilling in a Kugelrohr apparatus. The product was further purified by recrystallization in ethanol-H20 and again distilled, the main-product coming over at 105 "C and 20 mTorr. The white solid product, 7-allyl-8hydroxyquinoline,was verified by proton NMR and by its melting point of 46 "C (20). The third step involves blocking the hydroxyl functionality so reduction of the olefin in the next step will be successful. The 7-allyl-8-hydroxyquinoline was dissolved in a 3-fold excess of a 2:l mixture of HMDS and chlorotrimethylsilane under a N2atmosphere in a magnetically stirred RB flask fitted with a water-cooled condenser. A trace of HzS04was added to catalyze the reaction, and the mixture was gently heated and stirred overnight. After cooling, the reaction mixture was taken up in 50 mL of methylene chloride and washed with 2 X 100 mL of HzO, dried over Na2S04,and distilled, isolating 7-allyl-8-(trimethylsily1oxy)quinoline. The product was verified by proton NMR. In some cases, this third step had to be repeated in order to obtain a quantitative yield. The fourth step of the synthesis involved reduction of the allyl group with chlorodimethylsilane. In a condenser-fitted three-neck 250-mL Nz-purged RB flask, 9.2 g (32 mmol) of 7-allyl-S-(trimethylsilyloxy)quinoline, 12 mL (96 mmol) of chlorodimethylsilane, and 30 mL of dry benzene were combined. The mixture was magnetically stirred and slowly heated to 40-50 "C, at which time 3-5 drops of a 5% solution of chloroplatinic acid in dry
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
propanol was added. The mixture immediately foams up and turns bright yellow. The reaction mixture was then heated to reflux for 12 h. At this point the mixture was a red-tea-colored solution. The benzene was then removed by using a rotary evaporator with care being taken not to expose the sample to H20 and the atmosphere. The product, 7-[3-(chlorodimethylsilyl)propyl]-8-(trimethylsilyloxy)quinoline,was separated by distillation with the fraction of interest distilling over at 115 "C and 20 mTorr. The product was verified by proton NMR. A fraction of the product was dissolved in dry toluene to form a 5% solution of derivatizing reagent. Stored in this form, it was stable for months, although crystals of reagent precipitated over time, suggesting a lower percent reagent in toluene be used for storage. For the surface immobilization step, 2.26 g of Baker silica gel were prepared as described above and refluxed for 12 h with 20 mL of the reagent solution and 25 mL of dry toluene in an atmosphere of Nz. After cooling, the silica was filtered with a glass sintered funnel and rinsed with toluene, acetone, distilled HzO, and ethanol several times. A 10 mM NaOH solution was then added to the silica in the funnel and stirred while more NaOH was added to keep the silica immersed for a few minutes. This hydrolyzes the trimethylsilyl blocking group and restores the chelate functionality. While the silica at this point is suitable for chelation experiments, it was found that treatment with H2S04 yielded a pure surface as determined by the C:N ratio from elemental analysis. The surface was treated by adding concentrated HzS04 at room temperature and stirring for 1min, filtering, and rinsing with HzO and then dilute Na2HC03solution, followed by several rinses of distilled H 2 0and acetone. The silica was then air-dried and the solvent further removed by vacuum at 20 mTorr and 70 "C; the product was stored in a desiccator protected from light. The derivatized silica in this form is stable for more than a year. Surface coverages were determined by carbon and nitrogen elemental analysis performed by MHW Laboratories of Phoenix, AZ. Elemental analysis of derivatized silica prior to treatment with H2SO4 yielded 6.91% C and 0.36% N by weight with a C:N mole ratio of 17.52; the acid-treated product had 2.52% C and 0.21% N, yielding a C:N mole ratio of 12.3. These were compared to a theoretical C:N ratio of 12.0 corresponding to bound 8hydroxyquinoline product with no carbon or nitrogen impurities. Since the goal of this work is to investigate the chemical behavior of surface bound reagents, the apparently purer, acid-treated product was subsequently used for this study. The surface concentration of the reagent for this silica corresponds to 0.33 pmol/m2. For comparison purposes, an azo-linked immobilized 8hydroxyquinoline was also synthesized and bound to silica according to the method of Marshall and Motolla (10). The reagent was found not to fluoresce upon complexation with metal cations which normally yield fluorescent products with 8-hydroxyquinoline. For studies involving comparisons between a surface bound and free solution form of the immobilized reagent, 7propyl-8-hydroxyquinoline was prepared from 7-allyl-8hydroxyquinoline by using Wilkinson's catalyst according to a published procedure (21). This compound was found to be insufficiently soluble, however, in the aqueous buffers used in the prepared complexation experiments. 7-Allyl-8-hydroxyquinoline, in the second synthetic step above, had adequate solubility in aqueous solution and was therefore used as the free solution analogue to the surface immobilized reagent. Preparation of Buffers. Chemicals used for preparing pH buffers were purchased from Aldrich Chemical Co. Acetic acid (99.7+%), chloroacetic acid (99+%), and sodium acetate (99.999%) were Aldrich Gold Label and were not further purified. The major impurity in the acetic acid was water; aliquots of the acid were therefore titrated with standardized KOH to determine the acid concentration prior to preparing solutions. Anhydrous sodium perchlorate (99+ % ) was weighed and dissolved in distilled HzO and diluted to volume. The solution was then filtered to remove particulates and twice passed over Amberlite ion-exchange resin (IR-120 H C.P.) previously exchanged with Na+ ions from a concentrated NaCl solution. Aluminum perchlorate nonahydrate (99%) and Mg(C104)2-2Hz0(99%) were used without further purification. AU volumetric glassware was cleaned in chromic acid, washed with Alconox, leached with 30% HC1 for 2-7 days, washed
1003
with Alconox, rinsed at least five times with distilled HzO, leached with distilled HzO, and finally rinsed with distilled HzO prior to use. For trace level work, the glassware was silanized with chlorotrimethylsilane. The buffer system for this work needed to have spectroscopic transparency, negligible fluorescence quenching, a known, constant ionic strength, a linear buffering capacity, and minimal complexation with metal ions in the pH = 2-5 range. For these purposes, the buffer was based on chloroacetic acid (pK, = 2.88 at 25 "C) and acetic acid (pK, = 4.76 at 25 "C) titrated with NaOH, with NaC104 added to keep the ionic strength constant. The buffers were constructed by using (22)
[H+] = K,
~ 1 b-
b
- [H+] + [OH-] + [ B]
+ [H+] - [OH-] - [B]
(1)
where c2Kz (2) [H+l + Kz and where b, cl, and cz are the analytical concentrations of the strong base and the two weak acids, respectively; K1 and K2 are the respective dissociation constants of the weak acids corrected for ionic strength (23)where K1 > Kz. Despite ita higher charge, the A13+concentration was sufficiently low that it did not affect the ionic strength of any buffered solution. After preparation of buffer solutions, the pH predicted by eq 1 was verified at 23 "C with a pH meter. Fluorescence Measurements. A Perkin-Elmer Model 204-A spectrofluorometer was modified by replacing the sample compartment cap with a no. 7, single-hole,rubber stopper. Threading the flow analysis tubing through the stopper allowed access into the fluorometer sample compartment without exposing the compartment interior to the outside environment. The flow cell is a brass structure described elsewhere ( 2 4 , s )that holds a quartz tube containing a weighed amount of derivatized silica. Two modifications to the cell were implemented; due to the large particle size of the silica packing, stainless steel frit could be replaced with glass wool, and the low-pressure, perfluorinated plastic tubing could be press-fit into the quartz tube, avoiding the need for a high-pressure ferrule. A FIAstar flow injection analysis manifold (26) was used either in a sample injection mode or a constant analyte introduction mode depending on the type of experiment. A Rainen six-way injection valve with a 280-pL sample loop was used for injection. The flow rate was monitored by measuring the eluent volume from the flow cell with a 10-mL buret. For the flow analysis experiments, the excitation wavelength was centered at 365 nm and emission was monitored at 520 nm, using 10-nm entrance and exit slits on both monochromators. A Corning 3-71 filter was also used at the entrance of the emission monochromator to reduce scatter. The intensity output signal from the fluorometer was recorded on a Fisher Model 5000 strip chart; in addition, the signal was amplified and then digitized by using a Metrabyte DASH-8 a/d board interfaced to an IBM-PC-XT running LABTECH NOTEBOOK software from Laboratory Technologies Corp. To ensure the reproducibility of results, an aliquot of silica reagent was equilibrated in the flow cell for 20 min with the carrier buffer at a flow rate of 1.00 & 0.05 mL/min prior to metal ion complexation. A13+ion in buffered solution was either injected into the carrier stream or introduced by flow from a reservoir. To remove complexed metal ion from the surface, 1M HC104was used as an eluent. The same silica sample was used for an entire study, and at least two runs at a given set of conditions were performed. To minimize fluorescence quenching and possible photochemical oxidation of the reagent, the buffers were purged with Nz for 5-10 min prior to use. Even when this procedure was used, some decomposition, indicated by a slight loss of fluorescence response, was observed in the longer saturation studies, which typically exposed the surface to 12 h or more of excitation radiation. The most reproducible results were obtained when a series of experiments were performed in the following systematic way. The first trial for each condition was carried out with the sample that gave the largest intensity signal. Following measurement of all samples, the samples were remeasured in reverse order, and the average of the two trials was determined. In this way, an [BI =
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989 1
0.8
0.9 0.7
O.* 0.3
.
HQH+
i
/.
-/
j /
/ /
/
+/
HQ
+ H+
(5) Since the relative amounts of protonated and neutral ligand forms change with pH, it becomes useful to express the total amount of each form of free ligand in terms of a single quantity, Q, where Q = [HQ] + [HQH+]. The neutral and protonated forms of the ligand are related by the apparent acid dissociation quotient .G
Using eq 6 to express [HQH+]in terms of the other quantities and substituted into the above expression for Q with rearrangement gives PH
Flgure 2. pH dependence of aluminum ion complexation with silicaimmobilized 8-hydroxyquinoline: ionic strength, I = 0.05; [AI3'] = 4.51 X M. Squares indicate the steady-state response, the plus
(7)
symbols represent flow injection of 0.30-mL samples, and diamonds are free solution data. The data in all cases is fit by eq 8 with n = 1 and K x , KO = 0.8.
Equation 7 is now substituted back into eq 4; in this form, the mass action expression describes complex formation in terms of the equilibria of both eq 3 and 5. Since fluorescence is only observed from ligands that are complexed with metal ion, the relative intensity should be proportional to the fraction of ligands that are bound, F, which depends on the ratio of bound to free forms: F = r / ( l r). Substituting eq 4 and 7 into this definition of F and rearranging yields the following general expression for the fraction of ligand bound to metal ion:
estimate of the degree of reagent decomposition could be realized, and its systematiceffect on the reported results could be averaged out.
RESULTS AND DISCUSSION Stoichiometry of Complexation Reaction. The surface immobilized 8-hydroxyquinoline forms a reversible fluorescent complex with A13+ ion having an emission maximum a t 520 nm; the band position and emission spectra are similar to that observed from the free solution complex, which has been previously assigned to a ligand excited state (14). In order to determine the equilibrium behavior of aluminum ion complexation with the immobilized ligand, the pH dependence of the reaction was determined, and the results are plotted in Figure 2. Three experiments are presented in the figure; the first data set is the fluorescence response to a continuous flow of sample solution containing A13+at a concentration of 4.51 X lo4 M passed over the reactive silica surface. Equilibration a t each pH condition is carried out for 20 min, and a steady-state signal is reached. As a practical means of carrying out more rapid measurements under controlled conditions, a flow injection strategy was adopted whereby the immobilized ligand was exposed to constant volume injections of aluminum ion into a buffered carrier stream. The second set of data represents flow injection results, where the fluorescence arising from 0.3-mL injections of 4.51 X lo4 M A13+for I = 0.05 is plotted versus pH in Figure 2. The final set of data is the free solution behavior for complexation with excess A13+ also at I = 0.05. Some insight into the nature of the reactive ligand on the silica surface, such as the stoichiometry of its reaction with aluminum ion, can be accurately determined from the pH dependence of the complexation equilibrium since the reaction liberates hydrogen ion (27). Given an overall complexation reaction in which n neutral ligands react with a metal ion and produce n hydrogen ions in solution the observed equilibrium constant, KO, is defined by
K" = r(HY/[Mm+][HQ]n-l
(4)
where r is the ratio of ligands that are complexed to those that are in the free form, with r = n[MQ,]/[HQ]; the square brackets denote molar concentrations and the braces denote activities. This treatment has assumed that the surface ligand is in a neutral form. In free solution, however, it is known that the ligand is protonated below a pH of 5.0 (16)and thus has its own pH dependence as well:
+
F, =
nK"KA"[Mm+]Q"-'
nKOKA"[Mm+]Q"-l
+ (KA"{H+)"+ {H')'")
(8)
This equation has two interesting limiting forms. In the case of KA >> (H+],KA{H') + {H+)2= KA (H+)and the form of F for n = 1 is described by
which is independent of the KA of the ligand. In the case of {H+)>> KA, the amount of complex formed depends on KA and a similar approximation gives the form of F for n = 1 when both equilibria of eq 3 and 5 contribute protons:
In this limiting case, F has a quadratic dependence on proton activity over the rising portion of the pH dependence curve. Thus, it is possible to determine the interfacial acid-base behavior of the immobilized ligand from a knowledge of the pH dependence of the complexation equilibria. This treatment is valid for equilibrium behavior in free solution. In order to apply the same treatment to a surface immobilized ligand, the total free energy change (electrochemical potential) brought about by "transferring" the complexation reaction from the bulk solution to a charged interface should be considered. Since the immobilized ligand readily complexes metal ion and thus is solvated to some degree in the same aqueous medium as the free ligand in bulk solution, the chemical potential of the interfacial solution phase can be assumed to be approximately the same as that of the bulk phase for similar ligand densities. A possible contribution to the chemical potential change is due to adsorption of protons onto the silica surface, resulting in an elevated proton activity near the ligand. However, free energy terms that account for this phenomenon cancel in the final equilibrium expression, and so are omitted to simplify the derivation. Evidence of similar chemical potentials of bound ligand can be found in the study by Marshall and Motolla (IO)of a phenyl azo derivative of 8-hydroxyquinoline immobilized on silica and CPG.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
Over a pH range where the surface charge density of silica is small, the ligand exhibited small average shifts in the pKA of +0.2 for CPG and +0.4 for silica relative to the free solution. While the change in the chemical potential for the immobilized ligand is small, it is possible to have a significant electrical potential contribution to the total free energy change due to the surface potential, as recently shown for surface bound ligands by Chow and Cantwell (12). The surface potential may affect the reactivity of the surface ligand in two ways. First, the ion concentrations near the silica surface are altered from their respective bulk solution values due to the surface potential. If it is assumed that the immobilized ligand extends beyond the Stern layer of adsorbed ions into the diffuse layer of solution-phase ions, the ion densities present can be modeled by Gouy-Chapman theory (28). The Boltzmann relationship for ion density gives the equilibrium interfacial concentrations used in this type of model: (9) In this expression [C,], is the concentration of Ci a t a distance x from the outer Helmholtz plane (OHP) of adsorbed ions; [Ci]b is the bulk concentration of Ci having charge zi; J/, is the potential a t a distance x from the OHP; e is the electronic charge; k is the Boltzmann constant; and T is the absolute temperature (296 K in these experiments). The Boltzmann relationship is only strictly true for ion densities expressed as activities; however, the relationship is routinely applied to concentrations (see ref 28, for example). I t can be assumed here that the activity coefficients cancel in calculating ion densities due to the similar solution-phase composition in the vicinity of the interface, as discussed above. The second manner in which the surface potential may affect the ligand involves a potential-dependent modification of the reaction barrier for the individual rate constants that make up the equilibrium constant. The surface potential will tend to repel like-charge ions or attract different-charge ions as they proceed along the reaction coordinant. For example, in a reaction at a positively charged surface where cations from the bulk approach the interface, the forward rate of reaction is decreased and the reverse rate is increased due to surface-cation repulsions. This effect is analogous to the surface potential-free energy of activation relationship described by the Butlel-Volmer equation for electrode reactions (29),except for two points. First, protons or cations are involved in the charge transfer instead of electrons, and in the latter case, the rate-determining step can have a net charge transfer of greater than 1. Secondly, the surface ligand equilibria between complexed and uncomplexed forms, or protonated or unprotonated forms, can be identified as charge-transfer couples in analogy with redox couples in an electrode reaction. Thus, even though the surface bound ligand has no bulk activity dependency, its free energy changes with the surface potetial according to a Boltzmann relation as a result of stabilizing or destabilizing forces between charged surface and charged immobilized ligands. The apparent equilibrium constant adjacent to a charged interface is related to the apparent interfacial equilibrium constant at zero charge, KO,, by the surface potential and the net change in charge, Az', of the immobilized ligand as reactants proceed to products:
K, = KO, exp(-Az'e$,/kT)
= ICQ"W+t exp(-Az'eJ/,/kT) { HQ)[C(z+l)+]x
(10)
For the protonation equilibrium of eq 5, Ad = -1, while for the complexation equilibrium of eq 3, Ad = +2 for n = 1. The apparent equilibrium constants are corrected by using eq 9, so that the equilibrium of eq 3 for n = 1 yields a surface
1005
equilibrium constant expression
The final term in eq 11is similar to the apparent equilibrium constant KO in free solution, different only in that the ligand species cannot partition into the bulk solution. Because only concentrations can be measured optically, the behavior of K , is indistinguishable from KO. Similarly, an expression can be written for the protonation equilibrium of eq 3:
I t can be seen that substitution of the modified interfacial concentrations (eq 9) and the charge-transfer terms (eq 10) into the total equilibrium expression of eq 8 results in a cancellation of the Boltzmann factors in K,. However, a Boltzmann factor remains in eq 12, and K h has a dependence on the surface potential. This effect can be written in logarithmic form (30, 31):
where PKAo, is the intrinsic surface dissociation constant for the uncharged surface, which, for the reasons described above, is assumed to be equal to the bulk acid dissociation constant, PKAo. Thus, in this treatment, any difference in the observed pH dependence of eq 8 for the surface reaction compared to that for the free solution reaction a t a given ionic strength is due to a potential-dependent shift in pKA, given by eq 13, and the surface equilibrium constant K, should resemble its free solution counterpart KO. It is helpful to use the free solution behavior as a reference to discuss the response of the immobilized ligand. Coordination of metal ions with excess 8-hydroxyquinoline in free solution occurs with a stoichiometry that is generally the same as the charge on metal ion (16,27);A13+,for example, forms a ternary 8-hydroxyquinolate complex in free solution. It will be seen that the surface immobilized ligand forms a 1:l complex; for comparison, the free solution measurements were carried out under equilibrium conditions for 1:l free solution complexation using both 8-hydroxyquinoline and 7-allyl-8hydroxyquinoline, the former being included in the plot. Reactions in free solution were run with a 6-fold concentration excess of A13+over ligand to ensure that the 1:l complex was formed. There was no significant difference in the pH dependence of the two free solution reagents, indicating that the alkyl chain is not responsible for differences in the complexation chemistry of the bound 8-hydroxyquinoline. The data for the surface bound ligand in Figure 2 and for all of the surface bound reagent experiments in this study were fit by eq 8 for n = 1,indicating a 1:l aluminum-quinolate complex is formed on the surface. The low complexation ratio suggests that the surface bound ligands are spaced too far apart on the silica to form 21 or 3:l complexes with aluminum ion. Since the ligand surface density determined by elemental microanalysis of the silica is 0.33 pmol/m2, the average surface area per ligand (neglecting curvature of the surface) is 500 A2, corresponding to an area weighted average distance between ligands of 22 A. A surface density calculation was also performed with a fractal dimension, d = 2.8 (32),that was determined for the system (33) and an estimated ligand cross section of 100 A. The average surface area per ligand in this case is 220 A2 or a 15-%1separation between ligands. The
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
Table I
free solution 8HQ allyl 8HQ 8HQ 8HQ surface 8HQ steady
0.05 0.05
5.00
2.00
0.8 0.8 0.8 0.8
5.13 5.32
0.05
0.8
5.00
0.05
0.8 0.8 20b
0.50
5.00
2.62
140
flow
8HQ FIA 8HQ FIA 8HQ FIA
" From ref
0.50 2.00
5.00 4.15 5.13 4.37 5.32 5.15*
55 40
70
35
lob
12. *Estimated values.
distance from the surface bond to the chelation site for the ligand, calculated from bond lengths, is a maximum 7.9 A, and the effective distance is probably shorter since coordination of two oxinate ligands around A13+is not planar due to steric effects (34). The monomeric complexation of aluminum ion is, therefore, consistent with the rather low surface coverage of the 8-hydroxyquinoline reagent. S u r f a c e Potential Effects on Interfacial Equilibria. Fitting of both free solution and surface immobilized data was achieved by use of eq 8 with n = 1; the results are recorded in Table I. The value of the KAo for a given ionic strength is taken from the literature (16);however, the surface data cannot be fit without invoking eq 13 to describe the Kh. For a given ionic strength, the surface K, was assumed to be equal to the free solution by reasoning discussed above. This assumption was found to be valid in all of the data except for the I = 2.00 FIA experiment. As can be seen in Figure 2, the pH dependence of surface complexation as described by eq 8 exhibits differences in both the shape and the displacement of the curves relative to the free solution behavior for a given ionic strength. Qualitatively, both the surface curves are shifted to a lower pH relative to that of the free solution, and the steady-state flow data are further displaced relative to that of the FIA experiment. While similar shifting of pKh values of surface acid-base indicators is commonly observed (30, 31, 35, 36), the direction of the shift is not predicted from what is known about silica surface chemistry. The silica/electrolyte interface has a net negative (pH dependent) surface potential due to the deprotonation of surface silanols (37);for example q0 has been measured and reported to increase with pH from -5 to -40 mV over the pH range of Figure 2 (38). Such an effect should increase the surface pKk, of the immobilized ligand by eq 13, and such an increase has already been noted for an azo-linked, silica-immobilized 8-hydroxyquinoline (10). The shift observed in Figure 2 suggests that the interface has a net positive charge for the surface experiments. Similar shifts to lower pH are observed for protonated indicators on positively charged micelles (30). The shift of the steady-state experiment relative to the FIA response is due in part to the dispersion of the injected sample zone under FIA conditions, which decreases the metal ion concentration by a factor of 1.6. However, this factor is not enough to account for the total relative displacement of 0.44 pH unit, and the correspondingly larger surface Kh that must be used in eq 8 to fit the data. This implies that a greater positive surface potential (see eq 13) must exist under the steady flow conditions compared t o the FIA experiment; the result is a qualitative decrease in the pKA in the order PKAO (solution) > pKh(FIA) > pK,(steady-state). The shape of the pH dependence for the free solution behavior indicates that two protons per ligand are liberated during reaction; this is consistent with the literature, which
0.1
-
028
32
3.6
4
4.4
40
52
PH
Figure 3. Effect of ionic strength on the formation of aluminum quinolate. Both FIA surface and free solution pH dependence data are fitted according to eq 8 for n = 1. For the surface data (I = 2.00 are the triangles; I = 0.50 are the diamonds), [AI3+] = 4.51 X lo-' M and K, = 20 and 0.8, respectively. Free solution data (I = 0.05 are the square symbols; I = 2.00 are the plus symbols) are for equilibrium measurements with a &fold excess of cation, [AP'] = 3.88 X See text for discussion.
reports PKA's of around 5.0 for I = 0.05 (16). The steady-state surface data indicate that a single proton per ligand is liberated during reaction, producing a curve that resembles the pH dependence given by eq 8a. The FIA data indicate that two protons per ligand are liberated even though the pK, is shifted from that of free solution, producing a curve that resembles the form of eq 8b. By use of the positive surface potential to account for the shift in p K h , the shape of the pH dependence of all surface reactions is well described. Hence a change in a single parameter, surface potential, is adequate to explain differences in the shift and shape of the pH dependence. Similar results are observed a t different ionic strengths. Figure 3 presents both free solution and FIA surface data for I = 0.5 and 2.00. For all but the I = 2.00 surface results, the apparent equilibrium constant, IC'or K,, is determined to be approximately 0.8 at each ionic strength. This is because the reduction in proton activity as a result of increasing the ionic strength is essentially offset by a decrease in the bulk KAo. As with the I = 0.05 data, the surface results are shifted to lower pH relative to the free solution data for a given ionic strength and exhibit a quadratic dependence on proton activity. The free solution curves are displaced to higher pH with increasing ionic strength, reflecting the increase in the pKA of 8-hydroxyquinolinewith increasing ionic strength. The surface curves show the same general trend toward higher pH with increased ionic strength until the I = 2.00 case, where the curve is displaced to a lower pH than any of the other curves. The data can only be fit when a much larger value of K, is used; i.e. a change in K h due to surface potential is not enough to account for the shift. This interesting behavior is discussed below in the section describing kinetics. I t appears from the data that a positive surface potential is present in the chemical environment of the bound ligand, which is greatest under steady-state sampling conditions and at low ionic strengths. Insight into a possible mechanism can be found by comparing the steady-state and FIA surface experiments, which differ only in the A13+ concentration and total moles of A13+ to which the surface is exposed. Although the initial concentration of aluminum ion is similar for both experiments, dispersion of the sample zone in the FIA experiment reduces the A13+concentration by a factor of 1.6 by the time it passes over the reactive surface. This factor has already been taken into account in the fit of the FIA data to eq 8. The steady-state experiment, however, exposes the silica surface to more total moles of aluminum ion over a greater
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989 1.1
Comparing this curve with that for an injection of dye used to estimate the transport profile of metal ion, one can ascertain that the reverse rate of the reaction in eq 5 is slow on the time scale of this experiment. The rate of change of metal complex concentration is given by the difference between the rates of formation and decay:
0.9
0.7
eo
0.6
0.2 01 0
1007
. - J8" i
0
20
40
50
50
100
time, 9ec
Figure 4. Flow injection response of silica-immobilized 8-hydroxyquinoline to a 0.30-mL injection of 4.51 X lo-' M Ai3+ at pH = 4.06 (solid plateau curve). Also shown is the solid peaked curve obtained by injection of quinine bisulfate. The fits of the data to an integral of the dye injection and to an equilibrium expression (eq 17) are given as the plus and X symbols, respectively. The dye injection and the fled curves have been shifted by 7 s to account for a delay in initiating data acquisition.
period of time (60 s vs 20 min). At low ionic strengths, this could allow greater adsorption of aluminum ion onto the surface and a subsequent reorganization of the double layer on the time scale of the experiment. It is known that aluminum ion has an anomalously large adsorption affinity for silica and has been observed to be strong enough to neutralize and even reverse the surface potential of colloidal silica (37). The resulting positive surface potential from this process would raise the free energy of a protonated ligand species due to repulsion between the positive charges, increasing the observed K k of the surface ligand. Thus A13+adsorption could explain the above results. To verify that the injected moles of A13+are great enough to produce the positive surface potentials determined from fitting of the data, simple calculations were performed to estimate a surface potential for a given ionic strength. The surface charge that arises from a given concentration of A13+ is given by u = zF[A13+]AV/(A,p+m)
(14)
where F is the Faraday constant, AV is the volume of the sample zone, rn is the mass of the derivatized silica sample, and AN3+ is the effective surface area per gram of silica available to A13+ derived from a fractal calculation. The surface potential J.o can then be calculated by using the before-mentioned Gouy-Chapman model from $o = 2 k T / e sinh-l [u/(8DkTNo)]
(15)
where D is the dielectric constant of the solution (equal to 8OX the permittivity of vaccuum) and No is the bulk electrolyte concentration (equal to the ionic strength in this case). It should be mentioned that eq 15 assumes a flat surface, which is not the situation present for porous silica substrates; however, it can be used to give an approximation. The calculated J.o is in Table I; it can be seen that there are sufficient amounts of A13+ present in the FIA injections to produce surface potentials on the order of what is observed from the pH-dependent data. Kinetics of Flow Injection Conditions. Interpretation of the flow injection pH dependence is further clarified by examining the time response of metal chelate fluorescence to an injection of aluminum ion, as shown in Figure 4 along with the fluorescence from an injection of an unretained dye (quinine bisulfate). The formation of aluminum quinolate on the silica surface a t a pH = 4.1 rises rapidly and reaches a plateau following the passage of the injected A13+ sample.
where the concentration of metal ion, [MIt, is approximated by the time-dependent profile of the quinine peak. The reactive ligand concentration [HQ] can be assumed to be constant by a steady-state approximation involving the fast acid-base equilibrium of eq 5. For pH < pKk and [ A P ] lo% excess of the integral expression and provide agreement with the data over its entire range. These results indicate that the reverse rate of the reaction, while small, plays an important role in the response of the immobilized ligand even over the comparatively short duration of the FIA experiment. This conclusion is reinforced by the quadratic dependence of the FIA response on (H+Jin agreement with the prediction of an equilibrium model. If the system were under kinetic control, it would be dominated by the forward rate of reaction. Therefore, any trends in the p H behavior should be related to the forward rate. Complexation rates of aluminum ion in free solution are, however, not generally pH sensitive in this range, do not depend strongly on the identity of the ligand, and are therefore thought to be limited by the slow rate of exchange of water in the inner solvation sphere of the ion (39). Since the rate of reaction for aluminum ion appears fixed, the shift of the surface pH dependence curve a t I = 2.00 is likely related to an increased activity of A13+. It is known in free solution that activity coefficients of cations of high valencies pass through a minimum and begin to rise with increasing ionic strength, attributed to a reduction in the activity of water (40). It would seem reasonable, then, that such an effect would increase the rate of water exchange in the solvation sphere of aluminum and consequentially increase its rate of complexation. The ability of the charged interface to concentrate ions over the bulk concentration would amplify such an effect over its free solution counterpart and would thus explain the trends in Figure 4. Quantitative Applications and Response Saturation. In order to produce reasonable fluorescence sensitivity to dilute aluminum ion in solution, the pH a t which the complexation reaction is carried out must be adjusted to produce a moderate yield of quinolate complex. This requirement, coupled with the slow complexation rate of aluminum ion,
---
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
1008
-
26 21
; '81 7.'
/'
~
lI tk 1
i
t: f
i
I
I'
fl5L.?
2.2
I 4
,
3 8 4
/--
1
e
r
/
I
I
I
a 6
j
1
CI
~
I
d
02 "
4
8
0
I
200
I
,
I
400 bnl.,
I
600
I
I
800
I
I
1000
3.CO*d,
Flgure 5. Time record of quinolate fluorescence intensity upon repeated injections of 180 pM aluminum samples: pH = 4.06; I = 0.05.
makes the reaction nearly irreversible on the time scale of the sample injection. While this reaction, therefore, does not meet the general criterion for optical sensing of reversibility, flow injection methods provide control over reaction conditions, which allows an integrating response to be utilized. As with any analytical method based on a limited quantity of reagent, there exists a trade-off between concentration sensitivity and range of response. The pH and ionic strength of the carrier buffer establish the rate of reaction and determine what fraction of the available ligand will react at a given metal ion concentration over the time duration of sample exposure, the latter being governed by the flow rate and sample volume. While the kinetics of the reverse reaction are too slow to produce a standard flow injection peak, exposure of the reactive silica under controlled conditions to a sample of given concentration of metal ion rises reproducibly to a specific plateau height. This form of response should provide a more precise determination of sample concentration since peak area measurements exhibit improved signal-to-noise ratio compared with measurements of peak height (41) and represent a statistically optimal method of estimating concentration when the detection process is dominated by shot noise, as in fluorescence (42). In addition to regulating the rate of sample integration, flow injection methods provide another element of control required for utilizing an irreversible reaction in an optical sensing application, that is the ability to reset the integrator when the full range of response has been reached. For measurements with silica-immobilized 8-hydroxyquinoline, one can reset the chemical integrator by switching the carrier to a 0.05 M perchloric acid, which accelerates the rate of the decomplexation reaction and removes all the bound aluminum ion from the flow cell within 5 min. T o demonstrate the analytical application of this immobilized reagent operated with an integrating response, the cumulative fluorescence intensity produced at a pH = 3.6 and an ionic strength I = 0.05, a linear calibration curve was generated by multiple injections of 0.2-mL samples, each having an aluminum concentration of 0.8 pM or 16 nmol. The sensitivity is constant over the cumulative exposure of 130 nmol; the detection limit for these conditions is about 0.4 pM, corresponding to a sample concentration of 0.5 ppm for a 0.2-mL injection. A more sensitive response is achieved at a pH = 4.0, yielding a concentration detection limit below 100 ppb, but the detection a t this pH reaches the limit of its dynamic range a t lower levels of metal ion exposure as the response of the immobilized reagent becomes saturated. This behavior is apparent in Figure 5 , where upon continual injections of 180 pM A13+a t a pH = 4.06, the steps of chemical integration become smaller. A second series of injections was also performed a t the same carrier conditions on the same silica material but where the A13+ content of each injection was 36 WMor H factor of 5 smaller concentration per injection.
43+. nrno1..
Flgure 6. Cumulative fluorescence response from silica-immobilized
8-hydroxyquinoline to repeated sample injections. Circles are results for 180 pM AI3+ injections and triangles are 36 pM injections. The rise on each sample step is added as an increment. The smooth curves are the best fit to eq 19. The same general behavior is observed in these results, as shown in Figure 6; however, the absolute response for equivalent moles of A13+for the two experiments differs reproducibly. To account for this difference and the nonlinearity in response, the system was first checked for front surface absorption of the fluorescence intensity. This artifact was ruled out by the following experiment: After maximum fluorescence intensity from A13+was reached, solutions of quinine bisulfate with concentrations ranging from 2.21 X lo4 M to 4.43 X lo" M in buffer were injected, producing fluorescence peaks, on top of the aluminum quinolate fluorescence, which were linear when plotted against concentration. The nonlinear response is, therefore, not spectroscopic in origin but likely relates to a chemical saturation. Saari and Seitz (6) have modeled similar behavior in terms of reagent consumption and have obtained results in agreement with experiment. They derived a linearized equation in terms of the conditional equilibrium constant and the total concentration of immobilized reagent. Since the surface potential was believed to influence the pHdependent equilibrium response, the Seitz theory was modified to account for response saturation with consideration given to surface potential effects. Furthermore, since equilibrium was demonstrated to be achieved on the time scale of the injection, the results of the pH-dependence study can be utilized. The amount of metal complex formed during a single injection can be related through a rearrangement of the mass action expression of eq 4, and the cumulative metal complex concentration obtained during the nth injection can therefore be predicted to be [MQln,t = [MQIn-, + (Kz[HQln[MIn)/IH+l
(18)
The amount of ligand available for complexation is related to the total free ligand concentration by eq 7. For a series of injections of the same concentration where metal ion injection does not depend on n, [MI, = [MI, and for a surface that is initially free of metal complex, [MQ], = 0, the total amount of metal complex formed following nth injection can be obtained from a power series (43) which has a simple algebraic form:
'MQ1n = (KAx QOKPu + (H+))(I+-z)) (19) where [MI is taken to be the average AI3+concentration over the dispersed injection profile. The value of KAxwas taken from eq 13, with the assumption that the surface charge is proportional to the bulk A13+concentration in eq 14, or that the aluminum ion adsorption isotherm is linear. Because of
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
the nature of this assumption, only qualitative behavior described by eq 19 can be discussed, but this provides more insight into the chemical nature of the system during saturation. With the use of the accumulated step height from a series of injections of 36 and 180 pM aluminum, the results shown in Figure 6 were fit to the rise in aluminum quinolate concentration predicted by eq 19. The results are found to follow the shape of the theory, although the values of the apparent equilibrium constant that were obtained from the 36 and 180 MMinjections are K, = 0.7 and 0.45; these differ somewhat from the pH-dependent study of single injections, where K, = 0.8. While most of this discrepancy could be explained by the assumed dependence of surface charge on the total AP+ injected, these results indicate that the series of higher concentration aluminum ion samples are less reactive than the lower concentration samples. The higher concentration aluminum ion affects the ionic strength of the sample zone by less that 2%. The effect of accumulated surface charge on the reaction has already been indicated in the difference between the pH dependnce of the steady-state and FIA results. In the steady-state experiments, it was noted that 90% of the total response for the steady-state experiment was achieved in a few minutes, and the remaining 10% required the balance of the 20 min to reach equilibrium. The slow adsorption of A13+ may explain the greater reactivity of a large number of low-concentration samples. Such a series could allow a greater initial accumulation of adsorbed A13+ on the surface, increasing the surface potential and lowering the surface pKk, of the ligand. When the surface concentration of A13+becomes high, however, the repulsive forces from adsorbed aluminum ion present a barrier to further adsorption, and only higher bulk concentrations of A13+can drive further adsorption. These observations are consistent with the time scale required for A13+adsorption being longer than the time it takes for a single injection to pass over the sample. Calibration of the response of these materials should, therefore, include a range of sample concentrations and history of analyte exposure in order to account for changes in sensitivity. A final indication of the importance of surface potential in these reactions is the significant increase in the rate of the complex back-reaction as more aluminum ion is adsorbed on the surface. At low bound complex concentrations, the product is relatively stable, indicated by the plateau in the fluorescence response as shown in Figure 5. As more product is formed on the surface, the plateau shows an increasingly negative slope, suggesting that the back-reaction rate is increasing. Since the pH of the solution is buffered, the change in stability of the complex must be related to changes in surface environment, the simplest source of which would be the more positive surface potential derived from the charge density of adsorbed aluminum ion. Further studies have been carried out (33)to systematically measure the reaction rates for decomplexation and determine how these respond to the concentration of charged species a t the interface. Conclusions. In situ fluorescence spectroscopy and flow methods have allowed the reactivity of silica-immobilized 8-hydroxyquinoline with A13+ to be investigated. Flow methods provide the means to study the dynamic response of a system, which gives more insight into competing interfacial phenomena. Surface concentrations, equilibrium constants, and reaction rates which govern the response of the surface bound reagent are related to one another and to the surface potential; the relative importance of these depends on the solution and sample conditions and history of surface exposure. In this study, it was found that these effects could be distinguished in part by external control of ionic strength, pH, and sample exposure using flow methods. It would be
1000
difficult to discern between these interactions in a passive chemical sensing environment, which does not allow control of sample exposure or mass transport, for example. It was noted in this work, from the response to small injections versus steady sample flow, that the unusually high charge density of A13+ and its affinity for silica affected the silica surface potential. Since this behavior is independent of the ligand, all reagents immobilized on silica substrates for the detection of A13+will probably exhibit such behavior; one may expect other cations of high valency to behave similarly. In addition, flow injection allows reproducible and more sensitive analytical detection, which is readily adjustable to optimize response for a given environment. Such qualities may be useful in a practical setting, where large numbers of different samples can be analyzed efficiently by resetting the surface reactvity and canceling the past history of the surface exposure, and adjusting the carrier solution accordingly. The results of this study point out several factors that affect the reactivity of surface bound reagents which should be considered in developing their applications as sensing materials, particularly for ionic species. For example, the chemical form of the reactive ligand was found to be dependent on the surface potential, and the tendency of the silica substrate to concentrate ions from the bulk was speculated to alter the observed equilibria. Two points with regard to these factors should be made. First, it may be possible to take advantage of such surface-induced anomalies in ligand chemistries to make optical sensor chemistries more selective than their free solution counterparts. Secondly, while it is commonly acknowledged that the heterogeneous nature of surface chemistries presents problems in constructing practical optical sensors, attention should also be paid to other details that greatly influence sensor performance. For example, use of buffers constructed with a known and constant ionic strength is often neglected in the optical sensor literature, and it was found in this study that ionic strength has a profound effect on the chemical response of the immobilized reagent. Given the potential sensitivity of surface immobilized reagents to solution conditions and history, their application as passive probes of uncontrolled chemical environments could be rather limited, while their use in a controlled flow analysis setting might generally be more successful.
ACKNOWLEDGMENT We gratefully acknowledge Michael Lee Hunnicutt for help with the synthesis of the 8-hydroxyquinoline reagent.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
(17) (18) (19)
Borman, S. A. Anal. Chem. 1981, 5 3 , 1616A. Peterson, J. 1.; Vurek, G. G. Science 1984, 224, 123-127. Seitz, W. R. Anal. Chem. 1984, 56, MA-34A. Wolfbeis, 0. S. TrAC, Trends Anal. Chem. (Pers. Ed.) 1985, 4 , 184. Hirshfeld, T. "The Feasibility of Using Fiber Optics for Monitoring Ground Water Contaminants"; NTIS EPA-600/7-84-067, June 1984. Saari, L.; Seitz, W. R. Anal. Chem. 1983, 55, 667. Ditzler, M.; Pierre-Jacques, H.; Harrington, S. A. Anal. Chem. 1986, 58, 195. Leyden, D. E.; Dill, J. A. I n Silanes, Surfaces, and Interfaces; Leyden, D. E., Ed.; Gordon and Breach Science: New York, 1986; pp 545-565. Janata, J. Anal. Chem. 1987, 59, 1356. Marshall, M.; Motolla. H. Anal. Chem. 1983, 55, 2089-2093. Kolstad, A. K.; Chow, P. Y. T.; Cantwell, F. F. Anal. Chem. 1988, 60. 1565. Chow, P. Y. T.; Cantwell, F. F. Anal. Chem. 1988, 6 0 , 1569. Ballard, R. E.; Edwards, J. W. J . Chem. Soc. 1964, 4868-4874. Lytle, F. E.: Storey, D. R.; Juriclch, M. E. Spectrochlm. Acta 1973, 29A, 1357-1369. Schulman, S. G. fluorescence and Phosphorescence Spectroscopy: U. K., Physicochemical PrJnciples and Practice; Pergamon Press: Oxford, U. K., 1977. Stary, J.; Zolotov, Yu, A,; Petrulchin, 0. M. CritiCalEvaluationInvolving 8-&droxyquinoline and its Metal Chelates; IUPAC Chemical Data Series No. 24; Pergamon Press: Oxford, U. K. 1979. Bowers, L. Anal. Chem. i986, 58, 513A-530A. Plueddemann, E. P. Silane Coupling Reagents; Plenum Press: New York, 1982; p 219. Plueddemann, E. P. US. Patent 3,328,459, 1967.
1010
Anal. Chem. 1989, 61, 1010-1013
(20) ManderJones, 6.; Trikojus, V. M. J . froc. R . SOC.N . S . w . L X V I , 313. (21) Monson, R. S . Advanced Organic Synthesis; Academic Press: New York, 1971; pp 43, 44. (22) Kolthoff, I.M.; Sandell, E.; Meehan, E.; Bruckenstein, S. Quantitative Chemical Analysis, 4th ed.; Macmillan Co.: London, 1969; pp 81, 278. 599. (23) Davies, C. W. J. Chem. SOC. 1938, 2093. (24) Carr, J. W.; Harrls, J. M. Anal. Chem. 1086, 5 8 , 626-631. (25) Lochmuller, C. H.; Colborn, S. A,. Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1983, 5 5 , 1344. (26) Ruzicka, J.; Hansen, E. H.; Ramsing, A. U. Anal. Chim. Acta 1982. 134. 55-74. (27) Stary,-J. The Solvent Extraction of Metal Chelates; Macmillan Co.: New York, 1964. (28) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; John Wiley and Sons: New York, 1982; Chapter V. (29) Bockris, J. O'M; Reddy, A. K. N. Modern Electrochemistry; Plenum Publishing Corp.: New York, 1970; Vol. 2, Chapter 8. (30) Fernandez, M. S.;Fromherz, P. J. fhys. Chem. 1977, 8 1 , 1755. (31) Drummond, C. J.; Greiser. F.: Healy, T. W. Faraday Discuss. Chem. SOC. 1986, No. 81, 95. (32) Avnir, D. J. Am. Chem. SOC. 1987, 109, 2931.
(33) (34) (35) (36) (37) (38) (39) (40)
Weaver, M. R.: Harris, J. M., to be submitted for publication. Ohnesorge, W. E. J. Inorg. Nucl. Chem. 1967, 29, 485. Wolfbeis, 0.; Offenbacher, H. Sens. Actuators 1986. 9. 85-91. Kawabata, Y.; Tsuchida, K.; Imasaka, T.; Ishlbashi, N. Anal. S d . 1987. 3 , 7. Iler, R. K. The Chemistry of Silica; John Wiiey and Sons: New York, 1979. Bousse. L.; de Rooii. N. F.; BeraveM. P. I€€€ Trans. Electron Devicies 1983, ED-30, 1263. Fiat, D.; Connick, R. E. J. Am. Chem. SOC. 1968, 90, 608. Harned, H. S.; Owen, B. B. The fhysical Chemistry of €lectro&ilc Solutions, 3rd ed.; Van Nostrand-Reinhold: Princeton, NJ, 1958. Synovec, R. E.; Yeung, E. S. Anal. Chem. 1985, 57, 2162-2167. Poston, P. E.; Harris, J. M. Anal. Chem. 1987. 59, 1620-1626. Abramowitz, M.; Stegun, I. A. Handbook of Mathematical Functions; Dover: New York, 1972; p 10.
RECEIVED for review November 2, 1987. Resubmitted September 6, 1988. Accepted January 17, 1989. This research was supported in part by the Office of Naval Research, Dow Chemical U.S.A.
Low-Level Detection of Metal Atoms by Multiphoton Ionization in a Low-Pressure Flame Sampling Cell Perry R. Blazewicz, William B. Whitten, and J. Michael Ramsey* Oak Ridge National Laboratory, Analytical Chemistry Division, Oak Ridge, Tennessee 37831 -6142
We are uslng a low-pressure sampling cell to extract specles from an alrlacetylene analytical burner. Slngle-color multiphoton lonlzatlon by a pulsed dye laser Is used for the sensitive detection of atomic specles In the cell. The dye laser excites one of the low-lylng twcbphoton accessible states, and absorptlon of an additlonal photon efficiently ionlres the excled state. Excellent detectlon lknns are reported for sodlum, lithium, calclum, and copper (Na, 20 pptr (parts per trilllon); Li, 100 pptr; Ca, 7 ppb; Cu, 25 pptr). No slgnals were observed for neodymium or alumlnum, presumably due to the consumptlon of the atoms to form molecular specles in the cell. Potasslum shows a much poorer detection llmlt and a quadratic dependence of slgnal on concentration.
Resonance ionization spectroscopy (RIS) is a well-developed method for the sensitive detection of atomic species. Schemes have been developed for the efficient photoionization of most elements (1). Sensitivity has extended to the detection of a single atom of some species ( I , 2). Flame atomization using an analytical burner is a commonly used method for the spectroscopic study of elements contained in solution samples. We have recently demonstrated highresolution spectroscopy using intermodulated fluorescence (3, 4 ) and degenerate four-wave mixing (5,6)on flame-atomized species sampled in a low-pressure cell. The flame from the air/acetylene analytical burner is sucked through an orifice into a vacuum-pumped region where the gaseous species are interrogated by a laser beam. By placing a biased wire inside the low-pressure cell, we can sensitively detect ionization of atomic species via [Z + 11multiphoton ionization (MPI), i.e., by exciting two-photon resonances and then photoionizing them with an additional photon (7). Two-photon absorption
* Author to whom correspondence should be addressed.
excites states of the same parity as the ground state. When absorption of an additional photon is energetically sufficient to cause ionization of the species, strong ion signals result if the laser is tuned to the two-photon resonance. Two-photon transitions are useful because they can be driven to saturation with common lasers and can be used for efficient sub-Doppler excitation (8). They also permit the study of many states at visible and near-UV wavelengths by using available lasers. Ionization detection is advantageous in that there is generally no background in the absence of the laser beam. Furthermore, it is often possible to convert essentially all excited states produced to ions, and the ions can be efficiently collected with only a biased wire, placing few constraints on the accessibility of the ionization region. The use of two-photon transitions in efficient ionization schemes has been discussed earlier ( I , 9,10) and applied to resonance ionization mass spectrometry of several metallic elements (7). Previous flame studies have demonstrated sensitive detection of molecular species by [ 1 + 11 resonant photoionization in atmospheric pressure flames (11, 12). Our experimental configuration allows the convenient study of samples in aqueous solution as is common in analytical experiments. At the same time, the low-pressure sampling technique provides a cleaner environment for a variety of spectroscopic methods. The sampling cell environment gives a reduction in the number of collisions, in stray light, in background ionization, and control over molecular density or even temperature. Ultimately, this sort of configuration is well-suited for Doppler-free spectroscopic methods with isotopic discrimination as we have shown with four-wave mixing (5).
EXPERIMENTAL SECTION Most of the details of the experimental arrangement of the burner and sampling cell have been given previously (4, 6 ) . The air/acetylene burner is of a type commonly used in analytical studies with the slot burner replaced by a cylindrical
0003-2700/89/0361-1010$01.50/0@ 1989 American Chemical Society
